Genes Encoding Amyloid Precursor Protein Research Articles

Brown-and-white giant pandas (hereafter brown pandas) are distinct coat color mutants found exclusively in the Qinling Mountains, Shaanxi, China. However, its genetic mechanism has remained unclear since their discovery in 1985. Here, we identified the genetic basis for this coat color variation using a combination of field ecological data, population genomic data, and a CRISPR-Cas9 knockout mouse model. We de novo assembled a long-read-based giant panda genome and resequenced the genomes of 35 giant pandas, including two brown pandas and two family trios associated with a brown panda. We identified a homozygous 25-bp deletion in the first exon of Bace2, a gene encoding amyloid precursor protein cleaving enzyme, as the most likely genetic basis for brown-and-white coat color. This deletion was further validated using PCR and Sanger sequencing of another 192 black giant pandas and CRISPR-Cas9 edited knockout mice. Our investigation revealed that this mutation reduced the number and size of melanosomes of the hairs in knockout mice and possibly in the brown panda, further leading to the hypopigmentation. These findings provide unique insights into the genetic basis of coat color variation in wild animals.

Mutations in genes encoding amyloid precursor protein (APP) and presenilins (PSs) cause familial forms of Alzheimer's disease (AD), a neurodegenerative disorder strongly associated with aging. It is currently unknown whether and how AD risks affect early brain development, and to what extent subtle synaptic pathology may occur prior to overt hallmark AD pathology. Transgenic mutant APP/PS1 over-expression mouse lines are key tools for studying the molecular mechanisms of AD pathogenesis. Among these lines, the 5XFAD mice rapidly develop key features of AD pathology and have proven utility in studying amyloid plaque formation and amyloid β (Aβ)-induced neurodegeneration. We reasoned that transgenic mutant APP/PS1 over-expression in 5XFAD mice may lead to neurodevelopmental defects in early cortical neurons, and performed detailed synaptic physiological characterization of layer 5 (L5) neurons from the prefrontal cortex (PFC) of 5XFAD and wild-type littermate controls. L5 PFC neurons from 5XFAD mice show early APP/Aβ immunolabeling. Whole-cell patch-clamp recording at an early post-weaning age (P22-30) revealed functional impairments; although 5XFAD PFC-L5 neurons exhibited similar membrane properties, they were intrinsically less excitable. In addition, these neurons received smaller amplitude and frequency of miniature excitatory synaptic inputs. These functional disturbances were further corroborated by decreased dendritic spine density and spine head volumes that indicated impaired synapse maturation. Slice biotinylation followed by Western blot analysis of PFC-L5 tissue revealed that 5XFAD mice showed reduced synaptic AMPA receptor subunit GluA1 and decreased synaptic NMDA receptor subunit GluN2A. Consistent with this, patch-clamp recording of the evoked L23>L5 synaptic responses revealed a reduced AMPA/NMDA receptor current ratio, and an increased level of AMPAR-lacking silent synapses. These results suggest that transgenic mutant forms of APP/PS1 overexpression in 5XFAD mice leads to early developmental defects of cortical circuits, which could contribute to the age-dependent synaptic pathology and neurodegeneration later in life.

Alzheimer's disease (AD) is a debilitating dementia characterized by progressive memory loss and aggregation of amyloid-β (Aβ) protein into amyloid plaques in patient brains. Mutations in presenilin (PS) lead to abnormal generation of Aβ, which is the major cause of familial AD (FAD), and apolipoprotein E4 (ApoE4) is the major genetic risk factor for sporadic AD (SAD) onset. However, whether dysfunction of PS is involved in the pathogenesis of SAD is largely unknown. We found that ApoE secretion was completely abolished in PS-deficient cells and markedly decreased by inhibition of γ-secretase activity. Blockade of γ-secretase activity by a γ-secretase inhibitor, DAPT, decreased ApoE secretion, suggesting an important role of γ-secretase activity in ApoE secretion. Reduced ApoE secretion is also observed in nicastrin-deficient cells with reduced γ-secretase activity. PS deficiency enhanced nuclear translocation of ApoE and binding of ApoE to importin α4, a nuclear transport receptor. Moreover, the expression of PS mutants in PS-deficient cells suppressed the restoration effects on ApoE secretion compared with the expression of wild-type PS. Plasma ApoE levels were lower in FAD patients carrying PS1 mutations compared with normal control subjects. Our findings suggest a novel role of PS contributing to the pathogenesis of SAD by regulating ApoE secretion.SIGNIFICANCE STATEMENT Familial AD (FAD) typically results from mutations in the genes encoding amyloid precursor protein, presenilin 1 (PS1), or PS2. Many PS mutants have been found to exert impaired γ-secretase activity and increased amyloid-β 42 (Aβ42)/Aβ40 ratio, which induce early amyloid deposition and FAD. On the other hand, apolipoprotein E4 (ApoE4) is the major genetic risk factor for sporadic AD (SAD) and contributes to AD pathogenesis because it has reduced Aβ clearance capability compared with ApoE3 and ApoE2. FAD and SAD have long been considered to be caused by these two independent mechanisms; however, for the first time, we demonstrated that PS is essential for ApoE secretion and PS mutants affected ApoE secretion in vitro and in human samples, suggesting a novel mechanism by which PS is also involved in SAD pathogenesis.

Down syndrome (DS) is a form of accelerated aging, and people with DS are highly prone to aging-related conditions that include vascular and neurological disorders. Due to the overexpression of several genes on Chromosome 21, for example genes encoding amyloid precursor protein (APP), superoxide dismutase (SOD), and some of the interferon receptors, those with DS exhibit significant accumulation of amyloid, phospho-tau, oxidative stress, neuronal loss, and neuroinflammation in the brain as they age. In this review, we will summarize the major strides in this research field that have been made in the last few decades, as well as discuss where we are now, and which research areas are considered essential for the field in the future. We examine the scientific history of DS bridging these milestones in research to current efforts in the field. We extrapolate on comorbidities associated with this phenotype and highlight clinical networks in the USA and Europe pursuing clinical research, concluding with funding efforts and recent recommendations to the NIH regarding DS research.

Familial forms of Alzheimer's disease (FAD) are caused by mutations in the gene encoding amyloid precursor protein, whose processing can result in formation of β-amyloid (Aβ). FAD can also result from mutations in the presenilin 1/2 (PSEN1/2) genes, whose protein products partially compose the γ-secretase complex that cleaves Aβ from amyloid precursor protein fragments. Psen1 KO mice and knock-in (KI) mice with homozygous FAD-associated L435F mutations (Psen1LF/LF) are embryonic and perinatally lethal, precluding a more rigorous examination of the effect of Alzheimer's disease–causing Psen1 mutations on neurodegeneration. Given that the rat is a more suitable model organism with regard to surgical interventions and behavioral testing, we generated a rat KI model of the Psen1LF mutation. In this study, we focused on young Psen1LF rats to determine potential early pathogenic changes caused by this mutation. We found that, unlike Psen1LF/LF mice, Psen1LF/LF rats survive into adulthood despite loss of γ-secretase activity. Consistent with loss of γ-secretase function, Psen1LF/LF rats exhibited low levels of Aβ38, Aβ40, and Aβ42 peptides. In contrast, levels of Aβ43, a longer and potentially more amyloidogenic Aβ form, were significantly increased in Psen1LF/LF and Psen1LF/w rats. The longer survival of these KI rats affords the opportunity to examine the effect of homozygous Psen1 Alzheimer's disease–associated mutations on neurodegeneration in older animals.

Causative mutations in the genes encoding amyloid precursor protein (APP), presenilin 1 (PSEN1), or presenilin 2 (PSEN2) account for a majority of cases of familial Alzheimer disease (FAD) inherited in an autosomal-dominant pattern. For the sake of characterizing mutations, index patients from 148 families with FAD were enrolled from mainland China. Sanger sequencing of the genes APP, PSEN1, and PSEN2 was performed to characterize the mutation spectrum of the Chinese population. Thirteen of 148 (8.8%) individuals had possible pathogenic APP, PSEN1, or PSEN2 variants, including 2 (15.4%) APP variants, 8 (61.5%) PSEN1 variants, and 3 (23.1%) PSEN2 variants. PSEN1 variants represented the largest proportion in Chinese FAD, and PSEN2 variants are responsible for late-onset FAD in China. Analysis of genetic-clinical correlations permitted the conclusion that FAD phenotypes were mainly influenced by specific genetic defects.

Alzheimer disease (AD) is a debilitating dementia believed to result from the deposition of extracellular amyloid-β (Aβ)-containing plaques followed by the formation of neurofibrillary tangles. Familial AD typically results from mutations in the genes encoding amyloid precursor protein (APP), presenilin 1 or presenilin 2. Variations in triggering receptor expressed on myeloid cells 2 (TREM2), one of several genes for which expression is restricted to microglia in the brain, have now been shown to increase the risk of developing late-onset AD. Microglia have been shown to respond to Aβ accumulation and neurodegenerative lesions, progressively acquiring a unique transcriptional and functional signature and evolving into disease-associated microglia (DAM). DAM attenuate the progression of neurodegeneration in certain mouse models, but inappropriate DAM activation accelerates neurodegenerative disease in other models. TREM2 is essential for maintaining microglial metabolic fitness during stress events, enabling microglial progression to a fully mature DAM profile and ultimately sustaining the microglial response to Aβ-plaque-induced pathology. Here, we review the current data detailing the role of TREM2 in microglial biology and AD.

Alzheimer's disease (AD) is the most common form of dementia representing a major problem for public health. In 2017 there were an estimated 50 million patients worldwide and this number is expected to almost double every 20 years, reaching 75 million in 2030 and 131.5 million in 2050 (https://www.alz.co.uk/research/statistics). Clinically there are two forms of the disease: the sporadic form (also called late onset AD, LOAD) and the familial form (FAD). LOAD is the most common form. Its prevalence increases with advancing age from 1% in the 65–70 years old cohort to more that 30% after the age of 85. It is characterized by moderate to extreme severity with the advancing age being the main risk factor for LOAD. Familiar AD represents some 5–10% of all AD cases. FAD is linked to mutations in a specific set of genes, most often in the genes encoding amyloid precursor protein (APP) and the presenilins (PS) 1 and 2. Interestingly, the vast majority of AD related mutations are located on PS1 (Steiner et al., 2008; Mattson, 2010) thus identifying this protein as one of the main targets for FAD-modifying therapies. Here we address the role of AD-related presenilin mutations for Ca2+ dyshomeostasis and in vivo neural network dysfunction in AD. The role of presenilins: Presenilins are transmembrane proteins that harbor the catalytic site of the γ-secretase complex, which mediates the intramembranous cleavage of many type I membrane proteins, including APP (Steiner et al., 2008). Numerous FAD-associated presenilin mutations were shown to affect the cleavage specificity of γ-secretase, thus increasing the production of the aggregation-prone and neurotoxic variant of amyloid β peptide Aβ42 (Steiner et al., 2008). Gradual accumulation of Aβ plays the central role in the so called “amyloid hypothesis” of AD (Selkoe, 2002; Selkoe and Hardy, 2016). Consistently, over the last decades therapeutic strategies mainly focused on decreasing Aβ levels inside the brain by either downregulating its production/accumulation or upregulating its clearance. So far, however, the results obtained were rather disappointing. In several trials using antibodies against Aβ, however, post hoc analyses hinted towards a reduction of cognitive decline in patients with mild, but not moderate, form of AD (Selkoe and Hardy, 2016). Besides playing an important role in the γ-secretase complex, presenilins also have other functions, mostly related to the intracellular Ca2+ homeostasis (Hermes et al., 2010; Mattson, 2010; Briggs et al., 2017; Popugaeva et al., 2017). Accordingly, PS mutations were reported to modify intracellular Ca2+ signaling in various experimental AD models. As illustrated in Figure 1, multiple in vitro studies identified IP3 receptors (IP3 Rs), ryanodine receptors (RyRs), sarco/endoplasmic reticulum Ca2+-ATPases (SERCAs) as well as a membrane-associated enzyme Phospholipase C (PLC) as potential interaction partners of presenilins. Multiple AD-associated PS1 and PS2 mutations (e.g., PS1-M146V, PS2-N141I and PS2-M239V) were shown to potentiate the IP3-mediated Ca2+ release from the intracellular Ca2+ stores, likely through an enhancement of the open probability of the IP3 receptors (reviewed in (Hermes et al., 2010; Briggs et al., 2017; Popugaeva et al., 2017). Similarly, mutant presenilins were reported to upregulate the expression levels of RyRs, and to increase RyR-mediated Ca2+ release from the intracellular Ca2+ stores. Studies in brain slices derived from PS1-M146V KI mice and 3xTg-AD mice have documented a substantial increase in the amplitude of RyR-mediated Ca2+ signals across neuronal subcompartments, with particularly large RyR-mediated Ca2+ signals in dendrites and dendritic spines (Chakroborty and Stutzmann, 2014; Briggs et al., 2017). In addition, FAD-linked PS mutations were reported to cause an increase in the basal activity of PLC (Hermes et al., 2010). In vitro studies have also pointed towards colocalization and physical interaction between PS and SERCA pumps (Figure 1). Depletion of PS1 and PS2 resulted in diminished SERCA function, whereas the expression of mutated presenilin (e.g., PS1-M146V) increased SERCA activity, thus resulting in stronger accumulation of Ca2+ within the intracellular stores. Moreover, the work of Bezprozvanny's group (reviewed in Popugaeva et al., 2017) has suggested that presenilins might themselves represent the Ca2+ leak channels of the endoplasmic reticulum and that the AD-related PS mutations, including M146V, L166P, A246E, E273A, G384A, and P436Q, impair their leak channel function, again leading to the overfilling of the intracellular Ca2+ stores. Such overfilling, in turn, impairs the function of store-operated Ca2+ channels (SOCC) as well as associated STIM and Orai proteins and is believed to play the central role in the AD-related Ca2+ dyshomeostasis, which represents one of the main functional hallmarks of AD (Hermes et al., 2010; Briggs et al., 2017; Popugaeva et al., 2017).Figure 1: Main components regulating store-operated Ca2+ signaling in pre- and postsynaptic neuronal compartments.Schematic representation of the key players regulating store-operated Ca2+ signaling as well as their interactions with presenilins. Arrows indicate the direction of ion flux. ER: Endoplasmic reticulum; RyR: ryanodine receptor; IP3R: inositol trisphosphate receptor; IP3: inositol 1,4,5-trisphosphate; PIP2: phosphatidylinositol 4,5-bisphosphate; SERCA: sarcoplasmic/endoplasmic reticulum Ca2+-ATPase; NMDA: N-methyl-D-aspartate; AMPA: α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid; VGCC: voltage-gated Ca2+ channels; PLC: phospholipase C; SOCC: store-operated Ca2+ entry channel.Synaptic dysfunction in AD: Postmortem studies in AD patients identified the degree of synaptic/spine loss as the best neuropathological correlate of the patient's cognitive impairment (Selkoe and Hardy, 2016). Consistently, several in vitro animal studies have shown that soluble Aβ oligomers inhibit inter-synaptic vesicle trafficking and activity-dependent rapid synaptogenesis, selectively depress glutamatergic synaptic transmission, reduce expression of AMPA and promote endocytosis of NMDA receptors (Briggs et al., 2017). Importantly, Aβ oligomers isolated directly from brains of AD patients caused an impairment of long-term potentiation (LTP) and an enhancement of long-term depression (LTD) in mouse hippocampal slices. Moreover, an injection of these oligomers into the lateral ventricle disrupted the memory of a learned behavior in normal rats (summarized in Selkoe and Hardy, 2016). An impaired synaptic transmission was also observed in predepositing 3xTg-AD mice. A suggested cause of the impairment is an activation of hyperpolarizing Ca2+-dependent SK2 K+ channels, activated by excess of Ca2+ released from the overfilled intracellular Ca2+ stores (Briggs et al., 2017). Additional evidence suggests that FAD-mediated impairment of synaptic plasticity can occur in the absence of amyloid and tau pathologies. In vitro studies in PS1-M146V KI mice, for example, have demonstrated an impairment of both the early and the late phases of LTP (Mattson, 2010). Taken together, these data substantiate the so called “synaptic failure hypothesis” of AD (Selkoe, 2002). Neural network hyperactivity in AD: In contrast to what might have being assumed based on the synaptic failure hypothesis of AD, a growing number of studies in humans and mouse models of AD identify neuronal network hyperactivity as an emerging functional hallmark of AD. Indeed, aberrant cortical network activity is observed in AD patients with the sporadic as well as the familiar forms of the disease, in particular those carrying a mutation in the presenilin genes (reviewed in Palop and Mucke, 2009, 2016). In patients, the aberrant forms of neural network hyperactivity include subclinical epileptiform activity (spikes and sharp waves) as well as clinically apparent seizures. According to recent reports, the incidence of seizures is roughly 7–8-fold higher in AD patients compared to the age-matched control group and severe AD cases are more likely to develop unprovoked seizures (Palop and Mucke, 2009, 2016). Moreover, in AD patients evaluated longitudinally, epileptiform activity was associated with a faster cognitive decline. Whereas previously epileptiform activity was considered as an epiphenomenon of the end-stage neurodegeneration, resent data suggest that neuronal hyperactivity can occur early during disease progression, even 4–7 years before the disease diagnosis is made (Palop and Mucke, 2016). Interestingly, 42% of young (58–68 years old) patients with the sporadic form of AD as well as non-demented carriers of the apolipoprotein E4, a known genetic risk factor of sporadic AD, also develop subclinical epileptiform activity (Palop and Mucke, 2009; Lerdkrai et al., 2018). Studies in transgenic mouse models of AD (hereafter referred to as “AD mice”) have also provided ample evidence for amyloidosis-induced network hyperactivity and seizures (see Supplementary Table 1 in Palop and Mucke, 2016). Multiple mouse lines (over)expressing AD-related human mutations in APP alone or together with mutations in tau protein and/or presenilins are reported to show either neuronal network hyperactivity detectable by in vivo Ca2+ imaging (Busche et al., 2008; Lerdkrai et al., 2018) or spontaneous epileptiform discharges (“spikes”) as well as seizures detectable by EEG recordings (Palop and Mucke, 2016). In concordance with human data, neuronal hyperactivity was also observed in ApoE4-KI mice as well as in PS45 presenilin mutant mice (PSEN1 (G384A)), developing neither amyloid plaques nor neuroinflammation (Palop and Mucke, 2016; Lerdkrai et al., 2018). Moreover, in PS45 mice neuronal hyperactivity was evident even in young adult (6–7-month-old) animals (Lerdkrai et al., 2018), in line with the notion that neuronal hyperactivity emerges during early stages of the disease. Furthermore, our recent in vivo data suggest that neuronal hyperactivity develops also during healthy brain ageing, albeit to a lesser extent (Lerdkrai et al., 2018). Interestingly, the ageing-related neuronal hyperactivity is prominent already in 10–14 months old mice. Such animals are not yet considered old and their age roughly corresponds to humans in their fourth or fifth decade of life. Furthermore, neurons are not the only cell type in the brain getting hyperactive with ageing. Similar trend was previously observed in vivo for cortical microglia (Brawek et al., 2014). Thus, during normal ageing the brain of middle-aged mice (and possibly also humans) reaches a different set point with more active neurons and microglia. Such conditions render the brain vulnerable to both neuroinflammation and seizure development, thus alleviating the development of AD. The role of presynaptic calcium stores for neuronal network hyperactivity: Intracellular Ca2+ stores are part of the endoplasmic reticulum, which in neurons is present both pre- and post-synaptically (Figure 1, see also Mattson, 2010; Briggs et al., 2017). The majority of in vitro data emphasized the involvement of the postsynaptic Ca2+ stores in the pathophysiology of AD. Thus, the postsynaptic Ca2+ stores in somata and dendrites of cortical and hippocampal neurons of AD mice were shown to release more Ca2+ in response to the application of IP3R or RyR agonists and to strongly potentiate synaptic and NMDA-receptor mediated Ca2+ transients (reviewed in Mattson, 2010; Chakroborty and Stutzmann, 2014). Excessive RyR-mediated Ca2+ release was also observed in dendritic spines of AD mice and was suggested to dysregulates the maintenance of memory-associated mushroom spines via inhibition of the Ca2+ store-operated Ca2+ entry channels (SOCCs; Figure 1; Briggs et al., 2017; Popugaeva et al., 2017). Our recent in vivo data, however, revealed a rather minor contribution of store-mediated Ca2+ release both to somatic and synaptic Ca2+ signals in layer 2/3 cortical neurons (Lerdkrai et al., 2018). Although somatic RyR-mediated Ca2+ release signals were somewhat longer in AD compared to age-matched WT mice, we did not observe any increase in their amplitudes, much in contrast to more than 200–300% increase in amplitudes of RyR-mediated Ca2+ release signals in vitro (see above). Among cells with different activity patterns (i.e., silent, normal and hyperactive; for details of cell classification see Busche et al., 2008), hyperactive cells showed the longest RyR-mediated Ca2+ release signals (Lerdkrai et al., 2018). Consistently, only in hyperactive cells spontaneous synaptically-driven dendritic Ca2+ transients showed a store-mediated component, which was blocked by emptying the intracellular stores. We concluded, therefore, that hyperactive cells are the only cells exhibiting AD-related overfilling of postsynaptic Ca2+ stores in vivo. Although for somatic and dendritic Ca2+ stores the degree of such overfilling is rather low, it cannot be excluded that somewhat larger dysfunction might be observed for Ca2+ stores in dendritic spines (Figure 1). Assuming that such dysfunction causes spine destabilization (see above), our recent data suggest that in vivo spine destabilization occurs in hyperactive cells only. In contrast to what is known about the postsynaptic side, the role of presynaptic Ca2+ stores for synaptic and neural network dysfunction in AD is much less clear. Under physiological conditions presenilins seem to modulate the evoked glutamate release in a Ca2+ store-dependent manner, and RyRs of AD mice were shown to mediate an increase in frequency of spontaneous vesicle release from presynaptic terminals (Chakroborty and Stutzmann, 2014). This increase, however, was believed to deplete the pool of readily releasable vesicles and to cause a Ca2+-dependent activation of SK2 K+ channels (see above), both leading to weakening of synaptic transmission (Chakroborty and Stutzmann, 2014; Briggs et al., 2017). In contrast, our in vivo data suggest that an AD-related mutation in PSEN1 gene causes heightened presynaptic release of glutamate already in 6–7 months old mice, thus strongly contributing to AD-related neuronal hyperactivity. Consistently, emptying the Ca2+ stores in AD and presenilin mutant mice normalizes cortical neural network activity in these animals (Lerdkrai et al., 2018). Interestingly, ageing- or APP mutation-induced neuronal hyperactivity are not sensitive to store depletion. Together, these data suggest that a single allele of mutated PS1 is sufficient to induce an early and a profound neuronal hyperactivity, mainly caused by the dysfunction of presynaptic intracellular Ca2+ stores. This early hyperactivity is likely to enhance activity-dependent generation and release of amyloid β and tau as well as formation of amyloid plaques (Palop and Mucke, 2016). By this mechanism a single mutation in the PSEN1 gene can lead to early onset full-blown disease in humans. The validity of this hypothesis is also supported by the fact that drugs which either selectively dampen presynaptic release of neurotransmitters (e.g., levetiracetam) or block RyRs releasing Ca2+ from the intracellular stores (e.g., dantrolene), were recently shown to improve memory and cognition in mice and humans (reviewed in Palop and Mucke, 2016; Popugaeva et al., 2017; Lerdkrai et al., 2018). This work was supported by the VolkswagenStiftung (grant No. 90233) to OG. Additional file: Oper peer reviewer report 1.

This study attempts to identify coding risk variants in genes previously implicated in Alzheimer's disease (AD) pathways, through whole-exome sequencing of subjects (N = 17) with AD, with a positive family history of dementia (familial AD). We attempted to evaluate the mutation burden in genes encoding amyloid precursor protein metabolism and previously linked to risk of dementias. Novel variants were identified in genes involved in amyloid precursor protein metabolism such as PSEN1 (chr 14:73653575, W161C, tgg > tgT), PLAT (chr 8:42039530,G272R), and SORL1 (chr11:121414373,G601D). The mutation burden assessment of dementia-related genes for all 17 cases revealed 45 variants, which were either shared across subjects, or were present in just the 1 patient. The study shows that the clinical characteristics, and genetic correlates, obtained in this sample are broadly comparable to the other studies that have investigated familial forms of AD. Our study identifies rare deleterious genetic variations, in the coding region of genes involved in amyloid signaling, and other dementia-associated pathways.

Alzheimer?s Disease (AD) is the most common cause of sporadic dementia, as it affects 60% of cognitive impaired patients; it commonly affects middle and late life, and it is considered an age-related disease. Early-onset familial AD is associated with mutations of the genes encoding amyloid precursor protein (APP), presenilin 1 (PS-1), or PS-2, resulting in the overproduction of amyloid beta-protein. Epidemiological and case-control studies have led to the identification of several risk factors for sporadic AD. The most concrete genetic risk factor for AD is the epsilon4 allele of apolipoprotein E gene (APOE). In addition, several genes such as CTNNA3, GAB2, PVRL2, TOMM40, and APOC1 are known to be the risk factors that contribute to AD pathogenesis. A direct role of interaction between genetic and environmental determinants has been proposed in an epigenetic dynamic for environmental factors operating during the preconceptual, fetal and infant phases of life. Also the the association between mtDNA inherited variants and multifactorial diseases and AD has been investigated by a number of studies that, however, didn?t reach a general consensus on the correlation between mtDNA haplogroups and AD.

Down syndrome, which arises in individuals carrying an extra copy of chromosome 21, is associated with a greatly increased risk of early-onset Alzheimer disease. It is thought that this risk is conferred by the presence of three copies of the gene encoding amyloid precursor protein (APP)--an Alzheimer disease risk factor--although the possession of extra copies of other chromosome 21 genes may also play a part. Further study of the mechanisms underlying the development of Alzheimer disease in people with Down syndrome could provide insights into the mechanisms that cause dementia in the general population.

Affecting approximately 30 million people worldwide, Alzheimer's disease (AD) is the most common cause of dementia and there is no cure. The cost to the American public for treating AD is currently estimated at $214 billion per year, a concerning statistic given that the disease population is expected to quadruple by 2050 (Holtzman et al., 2011). By 2030, it is estimated that the global burden of AD will reach $1 trillion USD, which does not take into account the emotional, physical, and financial burden of unpaid caregivers. Although these estimations suggest a bleak horizon for AD patients, caregivers, and their families, great strides in basic research and translational neuroscience in recent years offer new hope for those suffering from this debilitating disease. There is a unique set of challenges that face researchers when considering how to approach therapeutic intervention for AD. First, the pathological changes that ultimately lead to cognitive decline and dementia begin to accumulate well before symptoms become evident. The two primary pathologies in AD include plaques, extracellular aggregates of the amyloid-beta (Aβ) peptide, and tangles which refer to the intracellular accumulation of aggregated forms of the tau protein. According to the amyloid hypothesis (Hardy and Selkoe, 2002), 10–15 years prior to the onset of dementia, Aβ begins to accumulate within specific regions of the brain and form amyloid plaques. Plaque growth peaks and plateaus just after the onset of clinical symptoms. Aggregated, hyperphosphorylated tau, a marker of neurodegeneration, also begins to accumulate during this pre-symptomatic period. Tau accumulation appears to occur in regions which also have decreased brain volume, synaptic integrity, and glucose metabolism. By the time patients manifest signs of dementia, the pathogenesis of AD is firmly established within the brain and therefore the most effective treatment for AD must not only stop disease progression but must also reverse decades of damage coinciding with Aβ and tau aggregation. Also by the time of diagnosis, Aβ and tau, which appear to be the primary instigators of disease, have initiated a deleterious cascade of secondary disease mechanisms, such as metabolic dysfunction, oxidative stress, and neuroinflammation, which are as equally pernicious to the brain as the primary insult itself. To date, most treatment strategies have tried to target the production or clearance of Aβ or manipulate tau aggregation, while the secondary consequence of plaques and tangles remains largely unaddressed. These approaches have shown limited success in clinical trials to date, most likely due to the timing of therapeutic intervention and the complexity and heterogeneity of disease mechanisms at play in AD. Although targeting plaques and tangles will most likely be integral to any successful treatment strategy for AD, there are other biological processes, such as neuroinflammation, that require equal consideration as researchers move forward toward a holistic and efficacious treatment for AD. Moreover, early intervention, most likely during the pre-symptomatic period is a likely necessity for the most successful treatment of AD. There have been some important new findings over the last year and a few are highlighted here. One issue that has plagued the AD field is the lack of in vitro or in vivo systems that mimic all aspects of the human disease. Approximately 20 years ago (Scheuner et al., 1996), several mutations in the genes encoding amyloid precursor protein (APP) and presenilin (PSEN) were discovered to give rise to early-onset familial AD (FAD) through the increased production of the plaque forming peptide, Aβ. Mouse models harboring these human mutations were generated but fail to fully recapitulate human AD. In particular, the overexpression of mutant APP and PSEN1 leads to increased Aβ and plaque formation, yet little to no tau pathology is present in these models. Similarly, models that overexpress mutant forms of human microtubule-associated protein tau display hyperphosphorylated, insoluble tau but lack any appreciable amyloid pathology. Therefore, preclinical studies investigating therapeutic efficacy occurred in models lacking the full breadth of clinical disease, which could be one explanation for why these therapies fail to translate to efficacy in Phase III clinical trials. With the recent publication by Kim, Tanzi, and colleagues in Nature, the authors describe the first of its kind in vitro model of AD (Choi et al., 2014). By introducing FAD mutations in human neuronal progenitor cells cultured in a three dimensional gelatin matrix, the authors were able to recapitulate both extracellular amyloid plaque pathology as well as intracellular accumulation of aggregated, hyperphosphorylated tau protein, mimicking more closely the pathogenesis of AD in a dish. Not only does this novel 3D culture system provide researchers with a unique opportunity to study the evolution of AD pathology, it provides a tremendous opportunity for high throughout screening of potential drug candidates that can ameliorate both plaque and tangle pathology. Given the investment of time and resources typically involved in drug development and clinical trials, this new technique might offer a more rapid, streamlined approach to candidate selection. Aside from the importance of producing plaque and tangle pathology in a dish, the findings by Kim, Tanzi, and colleagues also re-emphasize the importance of exploring the genetics underlying AD. Although many FAD mutations in APP and PSEN1 have been well-characterized over the past 20 years, the advent of more sophisticated sequencing techniques allows for the identification of an infinite number of genetic changes associated with AD which could not only give insight into disease pathogenesis but also identify novel therapeutic candidates. Genome-wide association studies (GWAS) have identified at least 20 new loci involved in the increased risk for developing AD (Karch et al., 2014). For example, recent whole-exome and whole-genome wide sequencing strategies identified mutations in the TREM2 gene as conferring an increased risk for AD by 3.4-fold (Guerreiro et al., 2013, Jonsson et al., 2013). TREM2, or triggering receptor expressed on myeloid cells 2, is a transmembrane protein expressed by myeloid cells, including microglia and peripheral monocytes. Although endogenous ligands for TREM2 remain unknown, TREM2 regulates phagocytosis and the neuroinflammatory response to pathology within the brain. The identification of TREM2 mutations provides further confirmation that neuroinflammation, specifically microglial activation, is a significant component of AD pathogenesis and is an important component of the comprehensive treatment of AD. Although the role of TREM2 expression on microglial function in the context of Aβ and tau is still up for debate (Ulrich et al., 2014, Melchior et al., 2010, Kleinberger et al., 2014), the importance of GWAS for furthering both the basic understanding of AD as well as the importance in elucidating new therapeutic avenues to pursue is unequivocal. Tremendous strides in working toward the goal of an Alzheimer's therapeutic have been uncovered by fundamental scientific research. By identifying the timeline of pathological changes in the AD brain, researchers can make more informed decisions on clinical trial design and therapeutic interventions. Development of novel basic science tools, such as the human neural stem-cell-derived 3D culture system, provides an unprecedented new way for studying pathological interactions while simultaneously providing a high throughput screen for possible drug candidates in AD. Using GWAS as a basic method to understand individual risk for developing AD also provides researchers with druggable targets and a more comprehensive understanding of the cascade of disease processes. Taken together, basic and translational scientists are working in concert to bring about changes in the field of AD therapeutics.

Alzheimer's disease (AD) is the most common form of dementia in the elderly and represents an important and increasing clinical challenge in terms of diagnosis and treatment. Mutations in the genes encoding amyloid precursor protein (APP), presenilin 1 (PSEN1) and presenilin 2 (PSEN2) are responsible for early-onset autosomal dominant AD. The ε4 allele of the apolipoprotein E (APOE) gene has been recognized as a major genetic risk factor for the more common, complex, late-onset AD. Fibrillar deposits by phosphorylated tau are also a key pathological feature of AD. The retromer complex also has been reported to late-onset AD. More recently, genome-wide association studies (GWASs) identified putative novel candidate genes associated with late-onset AD. Lastly, several studies showed that circulating microRNAs (miRNAs) in the cerebrospinal fluid (CSF) and blood serum of AD patients can be used as biomarkers in AD diagnosis. This review addresses the advances and challenges in determining genetic and diagnostic markers for complex AD pathogenesis.

Alzheimer disease (AD)9, the most frequent cause of dementia in Western societies, affects approximately 5.5 million people in the US and more than 35 million people worldwide. Common symptoms of this disease include memory loss, confusion, irritability, and aggression. Normal bodily functions are progressively lost, with AD becoming fatal within 3 to 9 years after diagnosis. The major risk factor for AD is advanced age. After the age of 65 years, the incidence of AD doubles every 5 years. As the size of the elderly population increases, particularly the “baby boomers,” the prevalence will approach 30 to 60 million cases in the US alone by 2050. The pathogenesis of AD is not clear, but the major pathogenetic mechanisms that have been suggested include the accumulation of misfolded proteins in the aging brain, which causes oxidative and inflammatory damage, which in turn leads to energy failure and synaptic dysfunction. A very small proportion (<1%) of AD cases are familial with early onset, and such cases have been linked to mutations in genes encoding amyloid precursor protein (APP) and presenilins 1 and 2. There is also a strong genetic association between one variant of apolipoprotein E (apo E) and the risk for developing AD. Over the last 10 years, there has been increased interest in understanding the pathogenesis of this disease and in the development of new therapies. We interview 4 specialists and examine the latest advances in pathogenesis, diagnosis, and therapy for this debilitating disease. Can you summarize briefly some new insights into the pathogenesis of AD? JoAnne McLaurin: We have understood for years that the biggest risk factor for AD is aging. We are now starting to unravel the effects of aging on memory function and link this back to disease progression. New data have arisen that link …

The number of genes underlying monogenic epilepsies in human and animal models continues to grow as the clinical syndromes for a variety of heritable epilepsies yield to neurogenetic analysis. Interestingly, the diversity of co-morbid phenotypes linked to single epilepsy gene defects is also expanding. While many rare familial idiopathic epilepsy disorders are typically defined as pure seizure syndromes, many occur in the context of more complex clinical symptomatology. This is not surprising, since genes that alter cortical excitability are expressed in diverse brain networks, as well as in other tissues. In this brief review, two such genes are described. The gene encoding amyloid precursor protein along with related genes contributing to the accumulation of an aberrant peptide cleavage product, Aβ42, has now been placed in a molecular pathway mediating both the dementia of Alzheimer's Disease and seizures. A second gene, KCNQ1, encoding one of the voltage-sensitive ion channels contributing to membrane repolarisation in the heart, is co-expressed in both heart and brain where mutations contribute to cardiac arrhythmias, early mortality, and seizures. KCNQ1, a member of the cardiac LQTS gene family, is the first validated candidate gene for sudden unexplained death in epilepsy (SUDEP). The new strategy of searching for epilepsy genes linked to non-epileptic co-morbidity is providing illuminating examples of the role of neuronal network hyperexcitability in explaining the clinical spectrum of complex human neurological disorders that feature seizures.

Alzheimer's disease (AD) is the most common form of dementia. Approximately 0.5 per cent of all AD is caused by single major gene mutations and autosomal dominant inheritance. These familial types with early-onset (EOFAD) usually display dementia before the age of 60. Such mutations have been found in the gene encoding amyloid precursor protein (APP), and in the genes encoding presenilin 1 (PSEN1) or presenilin 2 (PSEN2). We herein report the case of a German patient with a EOFAD and a missense mutation at codon 141 (N141I) of the PSEN2 gene. The patient came to our psychiatric clinic for the first time when she was 49 years old. During the following 3 years, her Mini-Mental-State-Examination (MMSE) score dropped from 14 to 0 points. Positron emission tomography with [18F] Fluorodeoxyglucose (18F-FDG PET) demonstrated glucose reduction left parietal and in the pre-cuneus region. Follow-up 18F-FDG PET studies showed progressive hypometabolism of both temporoparietal lobes and left frontal lobe.

Advanced age and mutations in the genes encoding amyloid precursor protein (APP) and presenilin (PS1) are two serious risk factors for Alzheimer's disease (AD). Finding common pathogenic changes originating from these risks may lead to a new therapeutic strategy. We observed a decline in memory performance and reduction in hippocampal long-term potentiation (LTP) in both mature adult (9–15 months) transgenic APP/PS1 mice and old (19–25 months) non-transgenic (nonTg) mice. By contrast, in the presence of bicuculline, a GABAA receptor antagonist, LTP in adult APP/PS1 mice and old nonTg mice was larger than that in adult nonTg mice. The increased LTP levels in bicuculline-treated slices suggested that GABAA receptor-mediated inhibition in adult APP/PS1 and old nonTg mice was upregulated. Assuming that enhanced inhibition of LTP mediates memory decline in APP/PS1 mice, we rescued memory deficits in adult APP/PS1 mice by treating them with another GABAA receptor antagonist, picrotoxin (PTX), at a non-epileptic dose for 10 days. Among the saline vehicle-treated groups, substantially higher levels of synaptic proteins such as GABAA receptor α1 subunit, PSD95, and NR2B were observed in APP/PS1 mice than in nonTg control mice. This difference was insignificant among PTX-treated groups, suggesting that memory decline in APP/PS1 mice may result from changes in synaptic protein levels through homeostatic mechanisms. Several independent studies reported previously in aged rodents both an increased level of GABAA receptor α1 subunit and improvement of cognitive functions by long term GABAA receptor antagonist treatment. Therefore, reduced LTP linked to enhanced GABAA receptor-mediated inhibition may be triggered by aging and may be accelerated by familial AD-linked gene products like Aβ and mutant PS1, leading to cognitive decline that is pharmacologically treatable at least at this stage of disease progression in mice.

Alzheimer's disease (AD) is characterized by neuronal loss in the hippocampus and entorhinal cortex that is manifested by progressive memory impairment and cognitive decline. Autosomal-dominant, familial forms of AD (FAD) are caused by mutations in genes encoding amyloid precursor protein, presenilin-1 (PS1), and presenilin 2. Although it is established that expression of mutant PS1 variants leads to increased production of highly fibrillogenic amyloidbeta42 (Abeta42) peptides that deposit in the brains of patients with AD, the mechanism(s) by which Abeta deposition and expression of mutant genes induce lamina- and region-specific vulnerability of neuronal populations is not known. We have examined the hypothesis that expression of transgene-encoded FAD-linked mutant PS1 variants in entorhinal cortex neurons exacerbates the vulnerability of these cells to lesion-induced neuronal loss. To test this notion, we transected the perforant pathway (PP) of transgenic mice harboring either wild-type human PS1 (PS1HWT) or the FAD-linked mutant PS1DeltaE9 variant and examined neuronal survival in layer II of the entorhinal cortex (ECL2). Remarkably, PP transections lead to marked reductions in the numbers of ECL2 neurons in the ECL2 of mice expressing mutant PS1, compared with ECL2 neurons in PP-lesioned PS1HWT mice. Finally, and in contrast to studies in nontransgenic mice and in mice expressing PS1HWT, ECL2 neurons that express mutant PS1 and the calcium binding protein calbindin-D28k in ECL2 are also susceptible to lesion-induced neuronal loss. We conclude that expression of FAD-linked mutant PS1 variants enhances the vulnerability of neurons in the entorhinal cortex to PP lesion-induced cytotoxicity.

Deposition of the beta-amyloid peptide (Abeta) in neuritic plaques is a hallmark of Alzheimer disease (AD). Mutations in genes encoding amyloid precursor protein (APP) and presenilin 1 and 2 (PSEN1, PSEN2) are associated with increased accumulation of Abeta in neuritic plaques or in the walls of cerebral vessels. Intracerebral hemorrhage occasionally affects patients with AD. A Finnish family with dementia in four generations and with frequent co-occurrence of dementia and intracerebral hemorrhage was identified. Clinical features of 14 family members with a cognitive decline were evaluated. All exons in genes encoding APP, PSEN1, PSEN2, cystatin C, transthyretin, gelsolin, and ITM2B were sequenced, and an association study of APP was conducted by identification of single-nucleotide polymorphisms. Neuropathologic examination revealed Alzheimer-type changes with Abeta in neuritic plaques and vessel walls, but the cognitive profile of the patients differed from that in AD, as the visuoconstructive functions and verbal fluency were well preserved even in the moderate stage of the disease. In addition to cognitive decline, five patients had had lobar intracerebral hemorrhages and one was diagnosed with hemosiderin deposits in MRI, suggesting previous cerebral microbleeds. No causative mutations were identified in candidate genes associated with amyloid diseases, but linkage to APP region could not be entirely excluded. The family presents an autosomal dominant form of beta-amyloidogenic disease that resembles the Italian, Flemish, and Iowa types of AD. No amyloidogenic mutations were identified, but the role of the APP region could not be entirely excluded.

Protein misfolding is now recognized to be a central feature of neurodegenerative diseases. Alzheimer disease (AD), Parkinson disease (PD), amyotrophic lateral sclerosis (ALS), prion diseases, and the polyglutamine (polygln) diseases all represent proteinopathies—diseases in which a particular protein or set of proteins misfolds and aggregates. In the past decade, mutations have been identified in an increasing number of genes underlying specific forms of neurodegeneration—for example, genes encoding amyloid precursor protein (APP) and presenilins 1 and 2 in AD, parkin and alpha-synuclein in PD, Cu/Zn superoxide dismutase in ALS, tau in frontotemporal dementia, prion protein in Creutzfeldt-Jacob and Gerstmann-Straussler diseases, and at least eight different proteins in the polygln diseases (Hardy and Gwinn-Hardy Hardy and Gwinn-Hardy, 1998Hardy J Gwinn-Hardy K Genetic classification of primary neurodegenerative disease.Science. 1998; 282: 1075-1079Crossref PubMed Scopus (293) Google Scholar; Price et al. Price et al., 1998Price DL Sisodia SS Borchelt DR Genetic neurodegenerative diseases: the human illness and transgenic models.Science. 1998; 282: 1079-1083Crossref PubMed Scopus (213) Google Scholar). Identification of these mutations was a first step in understanding the disease process; next, it will be necessary to explain how specific changes in disease proteins may alter the neuron's physiology and to further question how neuronal responses to the mutant protein influence the disease process. This review examines the fate of mutant protein in neurons and the potential role of posttranslational events in pathogenesis, with particular focus on the polygln diseases. Posttranslational events have already been implicated in various neurodegenerative diseases (Hardy and Gwinn-Hardy Hardy and Gwinn-Hardy, 1998Hardy J Gwinn-Hardy K Genetic classification of primary neurodegenerative disease.Science. 1998; 282: 1075-1079Crossref PubMed Scopus (293) Google Scholar; Price et al. Price et al., 1998Price DL Sisodia SS Borchelt DR Genetic neurodegenerative diseases: the human illness and transgenic models.Science. 1998; 282: 1079-1083Crossref PubMed Scopus (213) Google Scholar). For instance, mutations in familial AD—whether in APP or presenilins 1 or 2—influence the processing of amyloid precursor protein, leading to increased production of amyloidogenic forms of A-β peptide (Price et al. Price et al., 1998Price DL Sisodia SS Borchelt DR Genetic neurodegenerative diseases: the human illness and transgenic models.Science. 1998; 282: 1079-1083Crossref PubMed Scopus (213) Google Scholar). In frontotemporal dementia linked to chromosome 17, mutations in the microtubule-associated protein tau reduce its ability to bind to and promote the assembly of microtubules (Hong et al. Hong et al., 1998Hong M Zhukareva V Vogelsberg-Ragaglia V Wszolek Z Reed L Miller B Geschwind D et al.Mutation-specific functional impairments in distinct tau isoforms of hereditary FTDP-17.Science. 1998; 282: 1914-1917Crossref PubMed Scopus (775) Google Scholar). In a recently identified hereditary form of PD, a mutation in the enzyme ubiquitin carboxy-terminal hydrolase L1 implicates the ubiquitin-proteasome–degradation pathway in pathogenesis (Leroy et al. Leroy et al., 1998Leroy E Boyer R Auburger G Leube B Ulm G Mezey E Harta G et al.The ubiquitin pathway in Parkinson's disease.Nature. 1998; 395: 451-452Crossref PubMed Scopus (1329) Google Scholar). Similarly, the amino-terminus of parkin, the disease protein in another form of familial parkinsonism, resembles ubiquitin, suggesting that the molecular mechanism in this disease also involves protein-degradation pathways (Kitada et al. Kitada et al., 1998Kitada T Asakawa S Hattori N Matsumine H Yamamura Y Minoshima S Yokochi M et al.Mutations in the parkin gene cause autosomal recessive juvenile parkinsonism.Nature. 1998; 392: 605-608Crossref PubMed Scopus (3891) Google Scholar). Several posttranslational events may contribute to pathogenesis in polygln diseases, including one or more listed in table 1. Some of these modifications of polygln proteins can be observed in vitro or in cell culture, but evidence from human disease tissue is still lacking (e.g., transglutamination).Table 1Protein Fate in Polyglutamine Diseases: Potential Mediators/Modulators of PathogenesisPosttranslational EventComponent, Enzyme, or PathwayProtein refolding/disaggregationChaperone proteinsDegradation of misfolded polypeptideUbiquitin-proteasome pathwayCleavage of mutant proteinSpecific protease(s)Altered nuclear transportPassive diffusion or active transportCrosslinking of mutant proteinTransglutaminaseAltered binding to normal substratesDisease-specific interacting proteins Open table in a new tab Polygln expansion is now recognized to be a major cause of inherited neurodegenerative disease. To date, eight polygln diseases have been identified (table 2). All eight are caused by expansion of a CAG repeat that encodes an abnormally long glutamine repeat in the otherwise unrelated disease proteins. These progressive, ultimately fatal disorders typically begin in adulthood. The diseases display distinct, yet overlapping, clinical and pathological findings. However, in each disease the phenotype and severity can vary greatly, primarily because of differences in repeat length. Longer repeats cause more severe disease that begins earlier in life, sometimes even in early childhood. With the very longest glutamine repeats, the individual diseases seem to lose some of their specific clinical and pathological features, becoming characterized instead by a more extensive, less selective pattern of neurodegeneration. Very long polygln expansions may be so neurotoxic that they override their specific disease context, causing a more widespread “glutaminopathy.”Table 2Polyglutamine DiseasesDiseaseaDRPLA = dentatorubral-pallidoluysian atrophy; SBMA = spinobulbar muscular atrophy.ProteinRepeat Length in DiseaseEvidence for AggregationHDHuntingtin38–180NI, dystrophic neuritesDRPLAAtrophin49–88NISBMAAndrogen receptor38–65NISCA1Ataxin-139–83NISCA2Ataxin-234–59Dense cytoplasmic staining (?)SCA3/MJDAtaxin-355–84NISCA6Calcium channel21–30?SCA7Ataxin-734–>200NINote.—References for aggregation in disease tissue are the following: DiFiglia et al. (DiFiglia et al., 1997DiFiglia M Sapp E Chase KO Davies SW Bates GP Vonsattel JP Aronin N Aggregation of huntingtin in neuronal intranuclear inclusions and dystrophic neurites in brain.Science. 1997; 277: 1990-1993Crossref PubMed Scopus (2176) Google Scholar); Paulson et al. (Paulson et al., 1997Paulson HL Perez MK Trottier Y Trojanowski JQ Subramony SH Das SS Vig P et al.Intranuclear inclusions of expanded polyglutamine protein in spinocerebellar ataxia type 3.Neuron. 1997; 19: 333-344Abstract Full Text Full Text PDF PubMed Scopus (698) Google Scholar); Becher et al. (Becher et al., 1998Becher MW Kotzuk JA Sharp AH Davies SW Bates GP Price DL Ross CA Intranuclear neuronal inclusions in Huntington's disease and dentatorubral and pallidoluysian atrophy: correlation between the density of inclusions and IT15 CAG triplet repeat length.Neurobiol Dis. 1998; 4: 387-397Crossref PubMed Scopus (357) Google Scholar); Holmberg et al. (Holmberg et al., 1998Holmberg M Duyckaerts C Durr A Cancel G Gourfinkel-An I Damier P Faucheux B et al.Spinocerebellar ataxia type 7 (SCA7): a neurodegenerative disorder with neuronal intranuclear inclusions.Hum Mol Genet. 1998; 7: 913-918Crossref PubMed Scopus (289) Google Scholar); Li et al. (Li et al., 1998Li M Miwa S Kobayashi Y Merry D Yamamoto M Tanaka F Doyu M et al.Nuclear inclusions of the androgen receptor protein in spinal and bulbar muscular atrophy.Ann Neurol. 1998; 44: 249-254Crossref PubMed Scopus (295) Google Scholar); Huynh et al. (Huynh et al., 1999Huynh D Del Bigio MR Ho D Pulst S-M Expression of ataxin-2 in brains from normal individuals and patients with Alzheimer's disease and spinocerebellar ataxia 2.Ann Neurol. 1999; 45: 232-241Crossref PubMed Scopus (142) Google Scholar).a DRPLA = dentatorubral-pallidoluysian atrophy; SBMA = spinobulbar muscular atrophy. Open table in a new tab Note.—References for aggregation in disease tissue are the following: DiFiglia et al. (DiFiglia et al., 1997DiFiglia M Sapp E Chase KO Davies SW Bates GP Vonsattel JP Aronin N Aggregation of huntingtin in neuronal intranuclear inclusions and dystrophic neurites in brain.Science. 1997; 277: 1990-1993Crossref PubMed Scopus (2176) Google Scholar); Paulson et al. (Paulson et al., 1997Paulson HL Perez MK Trottier Y Trojanowski JQ Subramony SH Das SS Vig P et al.Intranuclear inclusions of expanded polyglutamine protein in spinocerebellar ataxia type 3.Neuron. 1997; 19: 333-344Abstract Full Text Full Text PDF PubMed Scopus (698) Google Scholar); Becher et al. (Becher et al., 1998Becher MW Kotzuk JA Sharp AH Davies SW Bates GP Price DL Ross CA Intranuclear neuronal inclusions in Huntington's disease and dentatorubral and pallidoluysian atrophy: correlation between the density of inclusions and IT15 CAG triplet repeat length.Neurobiol Dis. 1998; 4: 387-397Crossref PubMed Scopus (357) Google Scholar); Holmberg et al. (Holmberg et al., 1998Holmberg M Duyckaerts C Durr A Cancel G Gourfinkel-An I Damier P Faucheux B et al.Spinocerebellar ataxia type 7 (SCA7): a neurodegenerative disorder with neuronal intranuclear inclusions.Hum Mol Genet. 1998; 7: 913-918Crossref PubMed Scopus (289) Google Scholar); Li et al. (Li et al., 1998Li M Miwa S Kobayashi Y Merry D Yamamoto M Tanaka F Doyu M et al.Nuclear inclusions of the androgen receptor protein in spinal and bulbar muscular atrophy.Ann Neurol. 1998; 44: 249-254Crossref PubMed Scopus (295) Google Scholar); Huynh et al. (Huynh et al., 1999Huynh D Del Bigio MR Ho D Pulst S-M Expression of ataxin-2 in brains from normal individuals and patients with Alzheimer's disease and spinocerebellar ataxia 2.Ann Neurol. 1999; 45: 232-241Crossref PubMed Scopus (142) Google Scholar). The basis of these diseases is a dominant toxic gain of function that occurs at the protein level and that increases with longer glutamine repeats. The threshold for disease (and the dominant toxic property) is a glutamine repeat of ∼35–40 residues. As shown in table 1, there are exceptions: the much shorter repeat in spinocerebellar ataxia type 6 (SCA6) suggests, perhaps, a different mechanism related to the calcium-channel properties of the disease protein. In SCA3/Machado-Joseph disease (SCA3/MJD), there is a gap between the largest normal repeat (40) and the smallest expanded repeat (55), so that an exact threshold cannot be determined. Evidence increasingly suggests that the novel toxic property is misfolding of the polygln domain. Unique structural features of polygln (Perutz et al. Perutz et al., 1994Perutz MF Johnson T Suzuki M Finch JT Glutamine repeats as polar zippers: their possible role in inherited neurologic diseases.Proc Natl Acad Sci USA. 1994; 91: 5355-5358Crossref PubMed Scopus (922) Google Scholar) likely cause it to adopt an altered conformation when it is expanded (perhaps a β-sheet hairpin structure). Direct evidence for an altered conformation is the existence of antibodies that specifically bind expanded, but not normal, polyglutamine (e.g., Trottier et al. Trottier et al., 1995Trottier Y Lutz Y Stevanin G Imbert G Devys D Cancel G Saudou F et al.Polygln expansion as a pathological epitope in Huntington's disease and four dominant cerebellar ataxias.Nature. 1995; 378: 403-406Crossref PubMed Scopus (568) Google Scholar). A vast amount of indirect evidence also suggests misfolding: numerous studies in transfected cells and transgenic animals have shown that expanded polygln forms insoluble aggregates, presumably derived from misfolded protein (for example, Ikeda et al. Ikeda et al., 1996Ikeda H Yamaguchi M Sugai S Aze Y Narumiya S Kakizuka A Expanded polygln in the Machado-Joseph disease protein induces cell death in vitro and in vivo.Nat Genet. 1996; 13: 196-202Crossref PubMed Scopus (497) Google Scholar; Paulson et al. Paulson et al., 1997Paulson HL Perez MK Trottier Y Trojanowski JQ Subramony SH Das SS Vig P et al.Intranuclear inclusions of expanded polyglutamine protein in spinocerebellar ataxia type 3.Neuron. 1997; 19: 333-344Abstract Full Text Full Text PDF PubMed Scopus (698) Google Scholar; Cooper et al. Cooper et al., 1998Cooper JK Schilling G Peters MF Herring WJ Sharp AH Kaminsky Z Masone J et al.Truncated N-terminal fragments of huntingtin with expanded glutamine repeats form nuclear and cytoplasmic aggregates in cell culture.Hum Mol Genet. 1998; 7: 783-790Crossref PubMed Scopus (302) Google Scholar, Martindale et al. Martindale et al., 1998Martindale D Hackam A Wieczorek A Ellerby L Wellington C McCutcheon K Singaraja R et al.Length of huntingtin and its polyglutamine tract influences localization and frequency of intracellular aggregates.Nat Genet. 1998; 18: 150-154Crossref PubMed Scopus (412) Google Scholar; Merry et al. Merry et al., 1998Merry DE Kobayashi Y Bailey CK Taye AA Fischbeck KH Cleavage, aggregation and toxicity of the expanded androgen receptor in spinal and bulbar muscular atrophy.Hum Mol Genet. 1998; 7: 693-701Crossref PubMed Scopus (162) Google Scholar). In vitro studies of recombinant huntingtin fragments have been particularly informative. Scherzinger et al. (Scherzinger et al., 1997Scherzinger E Lurz R Turmaine M Mangiarini L Hollenbach B Hasenbank R Bates GP et al.Huntingtin-encoded polyglutamine expansions form amyloid-like protein aggregates in vitro and in vivo.Cell. 1997; 90: 549-558Abstract Full Text Full Text PDF PubMed Scopus (1051) Google Scholar) demonstrated that huntingtin polygln aggregation is a self-driven process that occurs in a concentration- and repeat length–dependent manner and results in the formation of amyloid-like insoluble fibrils. The threshold repeat length for aggregation closely mirrors the threshold for disease, supporting the view that polygln misfolding is the dominant toxic property. It remains controversial whether the aggregation that results from this misfolding contributes to pathogenesis or is simply a by-product of the disease process. A major clue that protein misfolding actually occurs in vivo was the discovery of neuronal intranuclear inclusions (NIs) containing the mutant protein (fig. 1). These spherical, ubiquitinated aggregates have now been described in all but two polygln diseases and in many transgenic models (table 1) (see also Warrick et al. Warrick et al., 1998Warrick JM Paulson HL Gray-Board G Bui QT Fischbeck KH Pittman RN Bonini NM Expanded polyglutamine protein forms nuclear inclusions and causes neural degeneration in Drosophila.Cell. 1998; 93: 939-949Abstract Full Text Full Text PDF PubMed Scopus (493) Google Scholar; Davies et al. Davies et al., 1997Davies SW Turmaine M Cozens BA DiFiglia M Sharp AH Ross CA Scherzinger E et al.Formation of neuronal intranuclear inclusions underlies the neurological dysfunction in mice transgenic for the HD mutation.Cell. 1997; 90: 537-548Abstract Full Text Full Text PDF PubMed Scopus (1828) Google Scholar); intriguingly, the two exceptions, SCA2 and SCA6, have the shortest glutamine repeats. In addition, aggregates outside the nucleus have been noted in HD (dystrophic neurites or neuropil aggregates), and increased perinuclear staining for ataxin-2 in SCA2 may represent aggregated material (Huynh et al. Huynh et al., 1999Huynh D Del Bigio MR Ho D Pulst S-M Expression of ataxin-2 in brains from normal individuals and patients with Alzheimer's disease and spinocerebellar ataxia 2.Ann Neurol. 1999; 45: 232-241Crossref PubMed Scopus (142) Google Scholar). The discovery of NIs was exciting because they seemed to represent a unifying pathological structure found primarily in the neurons most susceptible to degeneration. This fact suggested that NI formation might be important both to pathogenesis and to the selective vulnerability seen in each disease. On the basis of recent findings, however, the link between NIs and neuronal susceptibility now seems less compelling. For example, certain striatal neurons are among the most vulnerable neurons in HD but reportedly do not develop abundant NIs (C.-A. Gutekunst and S. Hersch, personal communication). Mouse models of disease also suggest a dissociation between NIs and degeneration: transgenic mice expressing a fragment of mutant huntingtin have massive numbers of NIs, with relatively little neurodegeneration (Davies et al. Davies et al., 1997Davies SW Turmaine M Cozens BA DiFiglia M Sharp AH Ross CA Scherzinger E et al.Formation of neuronal intranuclear inclusions underlies the neurological dysfunction in mice transgenic for the HD mutation.Cell. 1997; 90: 537-548Abstract Full Text Full Text PDF PubMed Scopus (1828) Google Scholar), whereas mice expressing full-length mutant huntingtin undergo significant striatal degeneration without many NIs (Reddy et al. Reddy et al., 1998Reddy P Williams M Charles V Garrett L Pike-Buchanan L Whetsell W Miller G et al.Behavioral abnormalities and selective neuronal loss in HD transgenic mice expressing mutated full-length HD cDNA.Nat Genet. 1998; 20: 198-202Crossref PubMed Scopus (345) Google Scholar). An in vitro neuronal model of HD also failed to show a correlation between neuronal death and NI formation, suggesting that NIs may be irrelevant or even beneficial (Saudou et al. Saudou et al., 1998Saudou F Finkbeiner S Devys D Greenberg ME Huntingtin acts in the nucleus to induce apoptosis but death does not correlate with the formation of intranuclear inclusions.Cell. 1998; 95: 55-66Abstract Full Text Full Text PDF PubMed Scopus (1331) Google Scholar). Most importantly, recent results in the study of SCA1 transgenic mice strongly suggest that visible nuclear aggregates are not required for the initiation of pathogenesis (Klement et al. Klement et al., 1998Klement IA Skinner PJ Kaytor MD Yi H Hersch SM Clark HB Zoghbi HY et al.Ataxin-1 nuclear localization and aggregation: role in polyglutamine-induced disease in SCA1 transgenic mice.Cell. 1998; 95: 41-53Abstract Full Text Full Text PDF PubMed Scopus (863) Google Scholar). These findings notwithstanding, it is still too early to conclude that NIs have no role in pathogenesis. The slow progression of polygln disease in humans, together with results from transgenic mouse models, suggest a multistep model of pathogenesis characterized by at least two phases, an early and prolonged period of neuronal dysfunction and a later period of neuronal demise. If NIs do play a role in pathogenesis, it is likely to be late, near the point of neuronal demise. One way that NIs might compromise neuronal function is by sequestering transcription factors or other regulatory nuclear proteins. Many transcription factors contain glutamine-rich or pure polygln domains, and recent studies of transfected cells, transgenic flies, and human tissue all indicate that NIs can sequester certain other polygln-containing proteins, such as the basal transcription-factor TATA-binding protein (Perez et al. Perez et al., 1998Perez M Paulson H Pendse S Saionz S Bonini N Pittman R Recruitment and the role of nuclear localization in polyglutamine-mediated aggregation.J Cell Biol. 1998; 143: 1457-1470Crossref PubMed Scopus (273) Google Scholar). Although the role of NIs remains uncertain, nuclear expression of polygln proteins is clearly implicated in pathogenesis. In SCA1 transgenic mice, neuronal degeneration does not occur when mutant ataxin-1 is targeted to the cytoplasm instead of to its normal nuclear location (Klement et al. Klement et al., 1998Klement IA Skinner PJ Kaytor MD Yi H Hersch SM Clark HB Zoghbi HY et al.Ataxin-1 nuclear localization and aggregation: role in polyglutamine-induced disease in SCA1 transgenic mice.Cell. 1998; 95: 41-53Abstract Full Text Full Text PDF PubMed Scopus (863) Google Scholar). Also, in transfected neurons, mutant huntingtin induces apoptosis only when the protein is localized to the nucleus (Saudou et al. Saudou et al., 1998Saudou F Finkbeiner S Devys D Greenberg ME Huntingtin acts in the nucleus to induce apoptosis but death does not correlate with the formation of intranuclear inclusions.Cell. 1998; 95: 55-66Abstract Full Text Full Text PDF PubMed Scopus (1331) Google Scholar). Several studies further suggest that, compared with the cytoplasm, the nuclear environment favors aggregation (Klement et al. Klement et al., 1998Klement IA Skinner PJ Kaytor MD Yi H Hersch SM Clark HB Zoghbi HY et al.Ataxin-1 nuclear localization and aggregation: role in polyglutamine-induced disease in SCA1 transgenic mice.Cell. 1998; 95: 41-53Abstract Full Text Full Text PDF PubMed Scopus (863) Google Scholar; Perez et al. Perez et al., 1998Perez M Paulson H Pendse S Saionz S Bonini N Pittman R Recruitment and the role of nuclear localization in polyglutamine-mediated aggregation.J Cell Biol. 1998; 143: 1457-1470Crossref PubMed Scopus (273) Google Scholar), perhaps because the nucleus is less efficient than the cytoplasm at degrading, refolding, or disaggregating misfolded protein. Alternatively, the intricate structure of the nucleus might concentrate mutant proteins in subdomains that foster aggregation. Whatever the mechanism, one wonders whether there is a connection between the aggregation-promoting property of the nucleus and its being the site of toxicity. Ways that mutant protein, either as misfolded monomers or as aggregates, might alter vital nuclear functions are easy to imagine. The existence of glutamine-rich transcription factors suggests that mutant protein might bind such factors inappropriately, altering transcription of genes that are critical for neuronal function; other nuclear functions, such as mRNA splicing or the export of proteins and RNA to the cytoplasm, could be disrupted in a similar manner. Altered mRNA processing is a particularly intriguing hypothesis, because it has been suggested to underlie degeneration in the motor-neuron diseases, ALS and spinal muscular atrophy (Lin et al. Lin et al., 1998Lin C-L Bristol L Jin L Dykes-Hoberg M Crawford T Clawson L Rothstein JD Aberrant RNA processing in a neurodegenerative disease: the cause for absent EAAT2, a glutamate transporter, in amyotrophic lateral sclerosis.Neuron. 1998; 20: 589-602Abstract Full Text Full Text PDF PubMed Scopus (561) Google Scholar; Pellizzoni et al. Pellizzoni et al., 1998Pellizzoni L Kataoka N Charroux B Dreyfuss G A novel function for SMN, the spinal muscular atrophy disease gene product, in pre-mRNA splicing.Cell. 1998; 95: 615-624Abstract Full Text Full Text PDF PubMed Scopus (453) Google Scholar) and because at least two polygln disease proteins (ataxin-2 and atrophin) contain motifs found in RNA-splicing proteins. The importance of nuclear localization raises its own set of questions. First, in diseases such as SCA2, in which the protein is apparently cytoplasmic and NIs have not been observed, will the nucleus still be a primary site of pathogenesis? Second, do NIs preferentially form within particular subnuclear domains, such as promyelocytic leukemia protein (PML) nuclear bodies (Hodges et al. Hodges et al., 1998Hodges M Tissot C Howe K Grimwade D Freemont PS Structure, organization and dynamics of promyelocytic leukemia protein nuclear bodies.Am J Hum Genet. 1998; 63: 297-304Abstract Full Text Full Text PDF PubMed Scopus (148) Google Scholar), and, if so, does this disrupt nuclear function in predictable ways? Third, how does a large cytoplasmic protein like huntingtin accumulate in the nucleus, and why does this happen only in the disease state? A relatively small polygln protein like ataxin-3 may be able to enter the nucleus by passive diffusion, but transport of full-length huntingtin and other large polygln proteins would require karyopherin-mediated transit through the nuclear pore. Data from cellular and transgenic models are most consistent with a fragment of huntingtin entering the nucleus (Cooper et al. Cooper et al., 1998Cooper JK Schilling G Peters MF Herring WJ Sharp AH Kaminsky Z Masone J et al.Truncated N-terminal fragments of huntingtin with expanded glutamine repeats form nuclear and cytoplasmic aggregates in cell culture.Hum Mol Genet. 1998; 7: 783-790Crossref PubMed Scopus (302) Google Scholar; Martindale et al. Martindale et al., 1998Martindale D Hackam A Wieczorek A Ellerby L Wellington C McCutcheon K Singaraja R et al.Length of huntingtin and its polyglutamine tract influences localization and frequency of intracellular aggregates.Nat Genet. 1998; 18: 150-154Crossref PubMed Scopus (412) Google Scholar). Whether this fragment is preferentially generated from mutant huntingtin remains to be seen. In AD, the role of proteolytic processing in pathogenesis is widely accepted. In polygln diseases, such a role for proteolysis is also much anticipated but has not yet been established. Several converging lines of evidence make the proteolysis model compelling. First, immunohistochemical studies of at least three diseases indicate that polygln-containing fragments, not the full-length protein, are the constituents of NIs (ataxin-1 inclusions in SCA1 may be an exception). Second, proteolytic fragments of disease protein have been reported in the HD brain (DiFiglia et al. DiFiglia et al., 1997DiFiglia M Sapp E Chase KO Davies SW Bates GP Vonsattel JP Aronin N Aggregation of huntingtin in neuronal intranuclear inclusions and dystrophic neurites in brain.Science. 1997; 277: 1990-1993Crossref PubMed Scopus (2176) Google Scholar) and in cell-based models of several diseases; in some of these cellular models, proteolytic fragments are preferentially seen with expanded polygln protein (Goldberg et al. Goldberg et al., 1996Goldberg YP Nicholson DW Rasper DM Kalchman MA Koide HB Graham RK Bromm M et al.Cleavage of huntingtin by apopain, a proapoptotic cysteine protease, is modulated by the polyglutamine tract.Nat Genet. 1996; 13: 442-449Crossref PubMed Scopus (491) Google Scholar; Merry et al. Merry et al., 1998Merry DE Kobayashi Y Bailey CK Taye AA Fischbeck KH Cleavage, aggregation and toxicity of the expanded androgen receptor in spinal and bulbar muscular atrophy.Hum Mol Genet. 1998; 7: 693-701Crossref PubMed Scopus (162) Google Scholar). Third, studies of recombinant protein and transfected cells have shown that many disease proteins are substrates for caspases (Goldberg et al. Goldberg et al., 1996Goldberg YP Nicholson DW Rasper DM Kalchman MA Koide HB Graham RK Bromm M et al.Cleavage of huntingtin by apopain, a proapoptotic cysteine protease, is modulated by the polyglutamine tract.Nat Genet. 1996; 13: 442-449Crossref PubMed Scopus (491) Google Scholar; Wellington et al. Wellington et al., 1998Wellington CL Ellerby LM Hackam AS Margolis RL Trifiro MA Singaraja R McCutcheon K et al.Caspase cleavage of gene products associated with triplet expansion disorders generates truncated fragments containing the polyglutamine tract.J Biol Chem. 1998; 273: 9158-9167Crossref PubMed Scopus (482) Google Scholar). Finally, polygln-containing fragments that have been freed from their surrounding protein context are particularly potent pathogenic molecules that are more prone than full-length protein to misfold, aggregate and cause cell death in cell-based and transgenic models (Ikeda et al. Ikeda et al., 1996Ikeda H Yamaguchi M Sugai S Aze Y Narumiya S Kakizuka A Expanded polygln in the Machado-Joseph disease protein induces cell death in vitro and in vivo.Nat Genet. 1996; 13: 196-202Crossref PubMed Scopus (497) Google Scholar; Paulson et al. Paulson et al., 1997Paulson HL Perez MK Trottier Y Trojanowski JQ Subramony SH Das SS Vig P et al.Intranuclear inclusions of expanded polyglutamine protein in spinocerebellar ataxia type 3.Neuron. 1997; 19: 333-344Abstract Full Text Full Text PDF PubMed Scopus (698) Google Scholar; Martindale et al. Martindale et al., 1998Martindale D Hackam A Wieczorek A Ellerby L Wellington C McCutcheon K Singaraja R et al.Length of huntingtin and its polyglutamine tract influences localization and frequency of intracellular aggregates.Nat Genet. 1998; 18: 150-154Crossref PubMed Scopus (412) Google Scholar; Merry et al. Merry et al., 1998Merry DE Kobayashi Y Bailey CK Taye AA Fischbeck KH Cleavage, aggregation and toxicity of the expanded androgen receptor in spinal and bulbar muscular atrophy.Hum Mol Genet. 1998; 7: 693-701Crossref PubMed Scopus (162) Google Scholar). Although this last point does not constitute evidence for proteolysis, it suggests how proteolysis might accelerate the disease process. However, well-characterized, disease-specific proteolytic fragments are still lacking. Moreover, evidence in SCA1 suggests that pathogenesis and aggregation may occur via the full-length protein in this disease. Definitive answers will most likely come from transgenic models, in which potentially relevant proteolytic fragments should be easier to characterize. Cells have a complex array of chaperone proteins that assist in the folding, refolding, and elimination of proteins under both normal and stress conditions such as heat shock. If the central problem in polygln disease is protein misfolding, then the cell might be expected to mount a chaperone stress response that assists in the refolding, elimination, and/or disaggregation of expanded polygln protein. Studies of at least two diseases, SCA1 and SCA3/MJD, suggest that this may indeed be the case. In disease tissue and in transfected cells, specific cellular chaperones are redistributed into NIs and polygln aggregates, most notably heat-shock protein (Hsp) 40 family members (Bailey et al. Bailey et al., 1998Bailey CK McCampbell A Madura K Merry DE Biochemical analysis of high molecular weight protein aggregates containing expanded polyglutamine repeat androgen receptor.Am J Hum Genet Suppl. 1998; 63: A8Google Scholar; Cummings et al. Cummings et al., 1998Cummings CJ Mancini MA Antalffy B DeFranco DB Orr HT Zoghbi HY Chaperone suppression of ataxin-1 aggregation and altered subcellular proteosome localization imply protein misfolding in SCA1.Nat Genet. 1998; 19: 148-154Crossref PubMed Scopus (733) Google Scholar; Paulson et al. Paulson et al., 1998Paulson H Chai Y Gray-Board G Bonini N Misfolding and aggregation in spinocerebellar ataxia type 3: a role for cellular chaperones in glutamine-repeat disease.Am J Hum Genet Suppl. 1998; 63: A8Google Scholar). In certain transfected cell lines, polygln aggregates also cause an upregulation of Hsp70 that colocalizes to the aggregates; however, neurons from the SCA3/MJD brain only rarely show induced Hsp70. Hence, if this response occurs in human disease, it must take place near the point of neuronal death (Paulson et al. Paulson et al., 1998Paulson H Chai Y Gray-Board G Bonini N Misfolding and aggregation in spinocerebellar ataxia type 3: a role for cellular chaperones in glutamine-repeat disease.Am J Hum Genet Suppl. 1998; 63: A8Google Scholar; author's unpublished data). The simplest interpretation of chaperonin redistribution is that it is an appropriate cellular response to handle misfolded and aggregated polypeptides. It is possible, however, that this redistribution into aggregates could titrate out chaperonins that otherwise would serve normal chaperoning functions in the neuron. Importantly, overexpression of the Hsp40 protein, human DnaJ homolog HDJ-2/HSDJ, can reduce the aggregation of polygln proteins (Cummings et al.Cummings et al., 1998Cummings CJ Mancini MA Antalffy B DeFranco DB Orr HT Zoghbi HY Chaperone suppression of ataxin-1 aggregation and altered subcellular proteosome localization imply protein misfolding in SCA1.Nat Genet. 1998; 19: 148-154Crossref PubMed Scopus (733) Google Scholar; Paulson et al. Paulson et al., 1998Paulson H Chai Y Gray-Board G Bonini N Misfolding and aggregation in spinocerebellar ataxia type 3: a role for cellular chaperones in glutamine-repeat disease.Am J Hum Genet Suppl. 1998; 63: A8Google Scholar). This ability to reduce aggregation by overexpressing chaperonins now allows researchers to test directly whether modulating aggregation has a beneficial or deleterious effect. A fly model of polygln disease (Warrick et al. Warrick et al., 1998Warrick JM Paulson HL Gray-Board G Bui QT Fischbeck KH Pittman RN Bonini NM Expanded polyglutamine protein forms nuclear inclusions and causes neural degeneration in Drosophila.Cell. 1998; 93: 939-949Abstract Full Text Full Text PDF PubMed Scopus (493) Google Scholar) that recapitulates many cellular features of polygln disease, including NI formation and late-onset degeneration, will be particularly useful for such experiments. Another major intracellular pathway for the recognition and handling of abnormal protein is the ubiquitin-proteasome pathway, which is also implicated in polygln disease. The proteasome complex is responsible for the ubiquitin-dependent degradation of most cytosolic proteins, including misfolded or damaged proteins. Proteasomes are abundant in neurons, where precisely controlled protein degradation is probably required to maintain neuronal function. Several recent studies have shown redistribution of the proteasome complex into NIs (Bailey et al. Bailey et al., 1998Bailey CK McCampbell A Madura K Merry DE Biochemical analysis of high molecular weight protein aggregates containing expanded polyglutamine repeat androgen receptor.Am J Hum Genet Suppl. 1998; 63: A8Google Scholar; Cummings et al. Cummings et al., 1998Cummings CJ Mancini MA Antalffy B DeFranco DB Orr HT Zoghbi HY Chaperone suppression of ataxin-1 aggregation and altered subcellular proteosome localization imply protein misfolding in SCA1.Nat Genet. 1998; 19: 148-154Crossref PubMed Scopus (733) Google Scholar; Paulson et al. Paulson et al., 1998Paulson H Chai Y Gray-Board G Bonini N Misfolding and aggregation in spinocerebellar ataxia type 3: a role for cellular chaperones in glutamine-repeat disease.Am J Hum Genet Suppl. 1998; 63: A8Google Scholar). As shown in figure 1, the two major components of the proteasome complex, the 20S proteolytic core and the 19S regulatory cap, both redistribute into NIs in SCA3/MJD. It is unclear whether proteasomal redistribution in polygln disease benefits or harms neurons. A working model is that the proteasome represents a first-line cellular defense that recognizes and eliminates misfolded polygln protein before it aggregates. This model of proteasome action is supported by studies showing that proteasome inhibitors promote aggregation of moderately expanded polygln repeats, possibly by increasing the intracellular concentration of misfolded monomers (Chai et al., Chai, in pressChai Y, Koppenhafer S, Shoesmith S, Perez M, Paulson H. Evidence for proteasome involvement in polyglutamine disease: localization to nuclear inclusions in SCA3/MJD and suppression of polyglutamine aggregation in vitro. Hum Mol Genet (in press)Google Scholar). If proteasome redistribution into NIs effectively depletes the neuron of functioning proteasomes, the result would be deleterious to the cell: fewer active proteasomes to degrade abnormal protein, which would lead to a further increase in misfolded polygln protein. It will be important to determine, by means of reconstitution experiments, how well the proteasome degrades expanded polygln protein. To be degraded by the proteasome, a protein must first be unfolded, to enter the central proteolytic chamber of the 20S core. Expanded polygln may adopt an energetically stable structure that resists unfolding and thus blocks entrance to the chamber or that causes incomplete degradation of the polypeptide. Two fundamental characteristics of neurodegenerative proteinopathies are the delay in onset and the selective vulnerability. Polygln diseases are no exception to this general rule. Signs of disease typically do not begin until adulthood, despite expression of the mutant protein throughout life. This delay suggests either an accumulation of a toxic product (possibly protein aggregates) or cumulative damage to one or more critical cellular processes, such as mitochondrial oxidative phosphorylation (Knight et al. Knight et al., 1999Knight SAB Kim R Pain D Dancis A The yeast connection to Friedreich ataxia.Am J Hum Genet. 1999; 64 (in this issue): 365-371Abstract Full Text Full Text PDF PubMed Scopus (39) Google Scholar [in this issue]), axonal transport, subnuclear organization, or proteasome function. In either case, the toxic product or cumulative damage likely reaches a threshold level beyond which the neuron can no longer fully compensate. Despite widespread expression of the various disease proteins, certain populations of neurons are particularly susceptible to degeneration, and the basis of this vulnerability remains a crucial question. Selective vulnerability is not absolute but, rather, represents a continuum of susceptibility in different neuronal populations. For instance, in the most severe early-onset cases due to the longest repeats, degeneration is rather widespread, and, even in typical adult-onset cases, the pathological changes in advanced disease extend beyond the neuronal populations that are known to be most susceptible. Factors contributing to selective vulnerability can be grouped broadly into two categories: those that increase the level of misfolded protein or directly promote misfolding, and those that act downstream of misfolding. The expression level of the disease protein clearly falls into the first category. Although the various disease proteins are widely expressed in the brain, absolute expression levels in different neurons certainly will vary, which will contribute to the differences in intracellular concentration of misfolded monomer. Some of the posttranslational modifications discussed here also would fall under the first category. In susceptible neurons, for example, misfolding and aggregation could be promoted by specific proteolytic events that release a polygln fragment or by aberrant targeting of polygln protein to the nucleus. In addition, specific interacting proteins are likely to contribute to selective vulnerability through both categories. For instance, certain interacting proteins may bind to disease proteins in a way that promotes misfolding and aggregation, as was recently demonstrated for the huntingtin interacting protein, SH3-containing Grb2-like protein (SH3GL3) (Sittler et al. Sittler et al., 1998Sittler A Wälter S Wedemeyer N Hasenbank R Scherzinger E Eickhoff H Bates G et al.SH3GL3 associates with the huntingtin exon 1 protein and promotes the formation of polygln-containing protein aggregates.Mol Cell. 1998; 2: 427-436Abstract Full Text Full Text PDF PubMed Scopus (181) Google Scholar). Other specific interacting proteins are likely to influence events downstream of misfolding, through mechanisms that are linked to the specific functions of the disease proteins. For example, in each disease, a subset of specific interacting proteins may bind more or less avidly to the mutant protein, thereby altering physiological or biochemical properties of one or both proteins. The susceptibility of a neuron to the downstream effects of the mutant protein would depend, in part, on the particular interacting proteins it expresses. The ataxin-1 interacting protein LANP (cerebellar leucine-rich acidic nuclear protein) is just one of a number of recently identified interacting proteins that may contribute in this manner to selective vulnerability (Matilla et al. Matilla et al., 1997Matilla A Koshy BT Cummings CJ Isobe T Orr HT Zoghbi HY The cerebellar leucine-rich acidic nuclear protein interacts with ataxin-1.Nature. 1997; 389: 974-978Crossref PubMed Scopus (228) Google Scholar). The recent profusion of mammalian and invertebrate transgenic models of polygln disease will aid considerably in defining cellular pathways and proteins that mediate or modulate pathogenesis. In particular, genetic screens in Drosophila models should prove especially useful in identifying modifiers of misfolding and degeneration (Jackson Jackson et al., 1998Jackson GR Salecker I Dong X Yao X Arnheim N Faber PW MacDonald ME et al.Polyglutamine-expanded human huntingtin transgenes induce degeneration of Drosophila photoreceptor neurons.Neuron. 1998; 21: 633-642Abstract Full Text Full Text PDF PubMed Scopus (408) Google Scholar; Warrick et al. Warrick et al., 1998Warrick JM Paulson HL Gray-Board G Bui QT Fischbeck KH Pittman RN Bonini NM Expanded polyglutamine protein forms nuclear inclusions and causes neural degeneration in Drosophila.Cell. 1998; 93: 939-949Abstract Full Text Full Text PDF PubMed Scopus (493) Google Scholar). As modifiers are identified, we can expect that some will tie into one or more of the cellular pathways described here, thus leading the way toward novel therapies for these fatal diseases.

The American Journal of Human Genetics

Feb 1, 1999
Henry L. Paulson

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