Familial Forms Of Alzheimer's Disease Research Articles

Intracellular Calcium Deficits in Drosophila Cholinergic Neurons Expressing Wild Type or FAD-Mutant Presenilin

Much of our current understanding about neurodegenerative diseases can be attributed to the study of inherited forms of these disorders. For example, mutations in the presenilin 1 and 2 genes have been linked to early onset familial forms of Alzheimer's disease (FAD). Using the Drosophila central nervous system as a model we have investigated the role of presenilin in one of the earliest cellular defects associated with Alzheimer's disease, intracellular calcium deregulation. We show that expression of either wild type or FAD-mutant presenilin in Drosophila CNS neurons has no impact on resting calcium levels but does give rise to deficits in intracellular calcium stores. Furthermore, we show that a loss-of-function mutation in calmodulin, a key regulator of intracellular calcium, can suppress presenilin-induced deficits in calcium stores. Our data support a model whereby presenilin plays a role in regulating intracellular calcium stores and demonstrate that Drosophila can be used to study the link between presenilin and calcium deregulation.

Disease-Associated Mutant Ubiquitin Causes Proteasomal Impairment and Enhances the Toxicity of Protein Aggregates

Protein homeostasis is critical for cellular survival and its dysregulation has been implicated in Alzheimer's disease (AD) and other neurodegenerative disorders. Despite the growing appreciation of the pathogenic mechanisms involved in familial forms of AD, much less is known about the sporadic cases. Aggregates found in both familial and sporadic AD often include proteins other than those typically associated with the disease. One such protein is a mutant form of ubiquitin, UBB+1, a frameshift product generated by molecular misreading of a wild-type ubiquitin gene. UBB+1 has been associated with multiple disorders. UBB+1 cannot function as a ubiquitin molecule, and it is itself a substrate for degradation by the ubiquitin/proteasome system (UPS). Accumulation of UBB+1 impairs the proteasome system and enhances toxic protein aggregation, ultimately resulting in cell death. Here, we describe a novel model system to investigate how UBB+1 impairs UPS function and whether it plays a causal role in protein aggregation. We expressed a protein analogous to UBB+1 in yeast (Ubext) and demonstrated that it caused UPS impairment. Blocking ubiquitination of Ubext or weakening its interactions with other ubiquitin-processing proteins reduced the UPS impairment. Expression of Ubext altered the conjugation of wild-type ubiquitin to a UPS substrate. The expression of Ubext markedly enhanced cellular susceptibility to toxic protein aggregates but, surprisingly, did not induce or alter nontoxic protein aggregates in yeast. Taken together, these results suggest that Ubext interacts with more than one protein to elicit impairment of the UPS and affect protein aggregate toxicity. Furthermore, we suggest a model whereby chronic UPS impairment could inflict deleterious consequences on proper protein aggregate sequestration.

Computational Study of Assembly and Toxicity Inhibition of Amyloid Beta-Protein and Its Arctic Mutant

Amyloid b-protein (Ab) exists in two main alloforms, Ab40 and Ab42, of which Ab42 is linked particularly strongly to Alzheimer's disease (AD). Prior computational work demonstrated that the ab initio discrete molecular dynamics approach with an intermediate-resolution protein model captures biologically relevant differences between Ab40 and Ab42 folding and oligomerization. In the present work we apply the same approach to explore the relationship between the structure and toxicity. Assuming that Ab42 oligomers are more toxic than oligomers formed by Ab40, our structural analysis indicates that the solvent accessible surface area (SASA) in the N-terminal region of Ab42 oligomers is significantly higher than that of Ab40 oligomers. We then investigate effects of the C-terminal fragment (CTF), which was shown to attenuate Ab42 oligomer toxicity in a cell culture, on Ab42 oligomerization. Our results indicate that CTFs associate with Ab42 to form heterooligomers, consistent with quasielastic light scattering data. We show that the presence of CTFs significantly reduces SASA in the N- terminal region of Ab42 compared to the same region in Ab42 oligomers formed in the absence of CTFs.We further explore folding and oligomer formation of the Arctic mutants, [E22G]Ab40 and [E22G]Ab42, associated with a familial form of AD. Our results demonstrate that the substitution E22G disrupts the folding structure and oligomerization pathways of both Arctic mutants and results in increased SASA at the N-terminus of the Arctic Ab40 mutant. These findings suggest that Ab oligomer neurotoxicity might be directly or indirectly associated with the degree of solvent exposure of the N-terminal region of Ab.

The pathology of APP transgenic mice: a model of Alzheimer's disease or simply overexpression of APP?

Alzheimer's disease (AD) is characterized by a number of pathological features, notably extracellular senile plaques composed of the beta-amyloid protein (Abeta) and neurofibrillary tangles (NFT's), which are intracellular inclusions of hyperphosphorylated tau protein. In their attempts to generate a model of AD, many laboratories have produced transgenic mice that overexpress the amyloid precursor protein (APP), in particular, mutant APP which is associated with familial forms of AD in man. Histopathological assessment shows that APP transgenic mice demonstrate an accumulation of Abeta in plaques from an early age; these plaques are invariably surrounded by activated inflammatory cells such as astrocytes and microglia, as is common in AD brain. Also, commonly associated with the plaques is hyperphosphorylated tau, although this does not take on the NFT phenotype observed in AD. Atrophy and neurodegenerative pathology are other common features of AD; some neuronal loss is evident in close proximity to plaques in APP transgenic mice although this is not extensive. Consequently, it is evident that APP transgenic mice exhibit, to some degree, many of the pathological features of AD.

GSK-3beta activation mediates Abeta induced tau pathology in multiple transgenic mice

Alzheimer's disease (AD) is a complex disorder neuropathologically characterized by extra- and/or intracellular aggregation of toxic Abeta peptide and tau protein species, neuroinflammation, neuronal and synaptic loss. Toxic Abeta species are considered to be upstream in the etiopathological mechanism. We could mimic all neuropathological characteristics except for the tau pathology in mutant V717I amyloid precursor protein (APP-V717I) overexpressing mice. This mutation causes a familial form of AD in patients. In order to have a more complete animal model we overexpressed APP-V717I and tau-P301L protein in mice. Tau-P301L causes hereditary frontotemporal dementia (FTD) in patients. Whereas the single transgenic APP-V717 and Tau-P301L transgenic mice developed amyloid and tau pathology (intraneuronal aggregation of tau protein), both pathologies were combined in the bigenic (biAT) mice. However, tau pathology was redistributed from the spinal cord and hindbrain to the forebrain in the bigenic mice as compared to the tau-P301L mice. Aggregated tau is invariably hyperphosphorylated. To investigate the contribution of tau phosphorylation in the development of tau pathology tau-P301L and the most important tau kinase, GSK-3beta, were co-overexpressed in mice (biGT mice). Strikingly, the tau pathology in the biGT mice was redistributed in a similar fashion in the biGT mice as in the biAT mice compared to the tau-P301L mice. It could be demonstrated as well that GSK-3beta activity in brain is increased as a consequence of APP overexpression. In a cellular model, tau expressing yeast, we could demonstrate that increased phosphorylation of tau causes a conformational change that starts tau aggregation. Together the data imply that toxic Abeta species cause kinase activation, increased tau phosphorylation, which subsequently leads to tau pathology. The mechanism by which Abeta causes activation of GSK-3beta is unknown. Currently we are investigating the possibility whether neuroinflammation is involved.

NFκB-dependent Control of BACE1 Promoter Transactivation by Aβ42

β-Amyloid (Aβ) peptides that accumulate in Alzheimer disease are generated from the β-amyloid precursor protein (βAPP) by cleavages by β-secretase BACE1 and by presenilin-dependent γ-secretase activities. Very few data document a putative cross-talk between these proteases and the regulatory mechanisms underlying such interaction. We show that presenilin deficiency lowers BACE1 maturation and affects both BACE1 activity and promoter transactivation. The specific γ-secretase inhibitor DFK167 triggers the decrease of BACE1 activity in wild-type but not in presenilin-deficient fibroblasts. This decrease is also elicited by catalytically inactive γ-secretase. The overexpression of APP intracellular domain (AICD), the γ/ϵ-secretase-derived C-terminal product of β-amyloid precursor protein, does not modulate BACE1 activity or promoter transactivation in fibroblasts and does not alter BACE1 expression in AICD transgenic brains of mice. A DFK167-sensitive increase of BACE1 activity is observed in cells overexpressing APPϵ (the N-terminal product of βAPP generated by ϵ-secretase cleavage harboring the Aβ domain but lacking the AICD sequence), suggesting that the production of Aβ could account for the modulation of BACE1. Accordingly, we show that HEK293 cells overexpressing wild-type βAPP exhibit a DFK167-sensitive increase in BACE1 promoter transactivation that is increased by the Aβ-potentiating Swedish mutation. This effect was mimicked by exogenous application of Aβ42 but not Aβ40 or by transient transfection of cDNA encoding Aβ42 sequence. The IκB kinase inhibitor BMS345541 prevents Aβ-induced BACE1 promoter transactivation suggesting that NFκB could mediate this Aβ-associated phenotype. Accordingly, the overexpression of wild-type or Swedish mutated βAPP does not modify the transactivation of BACE1 promoter constructs lacking NFκB-responsive element. Furthermore, APP/β-amyloid precursor protein-like protein deficiency does not affect BACE1 activity and expression. Overall, these data suggest that physiological levels of endogenous Aβ are not sufficient per se to modulate BACE1 promoter transactivation but that exacerbated Aβ production linked to wild-type or Swedish mutated βAPP overexpression modulates BACE1 promoter transactivation and activity via an NFκB-dependent pathway.

Development of transgenic rats producing human beta-amyloid precursor protein as a model for Alzheimer's disease: transgene and endogenous APP genes are regulated tissue-specifically.

BackgroundAlzheimer's disease (AD) is a devastating neurodegenerative disorder that affects a large and growing number of elderly individuals. In addition to idiopathic disease, AD is also associated with autosomal dominant inheritance, which causes a familial form of AD (FAD). Some instances of FAD have been linked to mutations in the β-amyloid protein precursor (APP). Although there are numerous mouse AD models available, few rat AD models, which have several advantages over mice, have been generated.ResultsFischer 344 rats expressing human APP driven by the ubiquitin-C promoter were generated via lentiviral vector infection of Fischer 344 zygotes. We generated two separate APP-transgenic rat lines, APP21 and APP31. Serum levels of human amyloid-beta (Aβ)40 were 298 pg/ml for hemizygous and 486 pg/ml for homozygous APP21 animals. Serum Aβ42 levels in APP21 homozygous rats were 135 pg/ml. Immunohistochemistry in brain showed that the human APP transgene was expressed in neurons, but not in glial cells. These findings were consistent with independent examination of enhanced green fluorescent protein (eGFP) in the brains of eGFP-transgenic rats. APP21 and APP31 rats expressed 7.5- and 3-times more APP mRNA, respectively, than did wild-type rats. Northern blots showed that the human APP transgene, driven by the ubiquitin-C promoter, is expressed significantly more in brain, kidney and lung compared to heart and liver. A similar expression pattern was also seen for the endogenous rat APP. The unexpected similarity in the tissue-specific expression patterns of endogenous rat APP and transgenic human APP mRNAs suggests regulatory elements within the cDNA sequence of APP.ConclusionThis manuscript describes the generation of APP-transgenic inbred Fischer 344 rats. These are the first human AD model rat lines generated by lentiviral infection. The APP21 rat line expresses high levels of human APP and could be a useful model for AD. Tissue-specific expression in the two transgenic rat lines and in wild-type rats contradicts our current understanding of APP gene regulation. Determination of the elements that are responsible for tissue-specific expression of APP may enable new treatment options for AD.

Expanded high-resolution genetic study of 109 Swedish families with Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disease that affects approximately 20 million persons all over the world. There are both sporadic and familial forms of AD. We have previously reported a genome-wide linkage analysis on 71 Swedish AD families using 365 genotyped microsatellite markers. In this study, we increased the number of individuals included in the original 71 analysed families besides adding 38 new families. These 109 families were genotyped for 1100 novel microsatellite markers. The present study reports on the linkage data generated from the non-overlapping genotypes from the first genome scan and the genotypes of the present scan, which results in a total of 1289 successfully genotyped markers at an average density of 2.85 cM on 468 individuals from 109 AD families. Non-parametric linkage analysis yielded a significant multipoint LOD score in chromosome 19q13, the region harbouring the major susceptibility gene APOE, both for the whole set of families (LOD=5.0) and the APOE varepsilon4-positive subgroup made up of 63 families (LOD=5.3). Other suggestive linkage peaks that were observed in the original genome scan of 71 Swedish AD families were not detected in this extended analysis, and the previously reported linkage signals in chromosomes 9, 10 and 12 were not replicated.

Oxidative stress and cerebrovascular dysfunction in mouse models of Alzheimer's disease

Several factors have been implicated in Alzheimer's disease (AD) but there is no definite conclusion as to the main pathogenic agents. Mutations in the amyloid precursor protein (APP) that lead to increased production of amyloid beta peptide (A beta) are associated with the early-onset, familial forms of AD. However, in addition to ageing, the most common risk factors for the sporadic, prevalent form of AD are hypertension, hypercholesterolaemia, ischaemic stroke, the ApoE4 allele and diabetes, all characterized by a vascular pathology. In AD, the vascular pathology includes accumulation of A beta in the vessel wall, vascular fibrosis, and other ultrastructural changes in constituent endothelial and smooth muscle cells. Moreover, the ensuing chronic cerebral hypoperfusion has been proposed as a determinant factor in the accompanying cognitive deficits. In transgenic mice that overexpress mutated forms of the human APP (APP mice), the increased production of A beta results in vascular oxidative stress and loss of vasodilatory function. The culprit molecule, superoxide, triggers the synthesis of other reactive oxygen species and the sequestration of nitric oxide (NO), thus impairing resting cerebrovascular tone and NO-dependent dilatations. The A beta-induced cerebrovascular dysfunction can be completely abrogated in aged APP mice with antioxidant therapy. In contrast, in mice that overproduce an active form of the cytokine transforming growth factor-beta1 and recapitulate the vascular structural changes seen in AD, antioxidants have no beneficial effect on the accompanying cerebrovascular deficits. This review discusses the beneficial role and limitations of antioxidant therapy in AD cerebrovascular pathology.

Familial Alzheimer's disease mutations alter the stability of the amyloid beta-protein monomer folding nucleus.

Amyloid beta-protein (Abeta) oligomers may be the proximate neurotoxins in Alzheimer's disease (AD). Recently, to elucidate the oligomerization pathway, we studied Abeta monomer folding and identified a decapeptide segment of Abeta, (21)Ala-(22)Glu-(23)Asp-(24)Val-(25)Gly-(26)Ser-(27)Asn-(28)Lys-(29)Gly-(30)Ala, within which turn formation appears to nucleate monomer folding. The turn is stabilized by hydrophobic interactions between Val-24 and Lys-28 and by long-range electrostatic interactions between Lys-28 and either Glu-22 or Asp-23. We hypothesized that turn destabilization might explain the effects of amino acid substitutions at Glu-22 and Asp-23 that cause familial forms of AD and cerebral amyloid angiopathy. To test this hypothesis, limited proteolysis, mass spectrometry, and solution-state NMR spectroscopy were used here to determine and compare the structure and stability of the Abeta(21-30) turn within wild-type Abeta and seven clinically relevant homologues. In addition, we determined the relative differences in folding free energies (DeltaDeltaG(f)) among the mutant peptides. We observed that all of the disease-associated amino acid substitutions at Glu-22 or Asp-23 destabilized the turn and that the magnitude of the destabilization correlated with oligomerization propensity. The Ala21Gly (Flemish) substitution, outside the turn proper (Glu-22-Lys-28), displayed a stability similar to that of the wild-type peptide. The implications of these findings for understanding Abeta monomer folding and disease causation are discussed.

Impairments in Fast Axonal Transport and Motor Neuron Deficits in Transgenic Mice Expressing Familial Alzheimer's Disease-Linked Mutant Presenilin 1

Presenilins (PS) play a central role in gamma-secretase-mediated processing of beta-amyloid precursor protein (APP) and numerous type I transmembrane proteins. Expression of mutant PS1 variants causes familial forms of Alzheimer's disease (FAD). In cultured mammalian cells that express FAD-linked PS1 variants, the intracellular trafficking of several type 1 membrane proteins is altered. We now report that the anterograde fast axonal transport (FAT) of APP and Trk receptors is impaired in the sciatic nerves of transgenic mice expressing two independent FAD-linked PS1 variants. Furthermore, FAD-linked PS1 mice exhibit a significant increase in phosphorylation of the cytoskeletal proteins tau and neurofilaments in the spinal cord. Reductions in FAT and phosphorylation abnormalities correlated with motor neuron functional deficits. Together, our data suggests that defects in anterograde FAT may underlie FAD-linked PS1-mediated neurodegeneration through a mechanism involving impairments in neurotrophin signaling and synaptic dysfunction.

Calcium-mediated Transient Phosphorylation of Tau and Amyloid Precursor Protein Followed by Intraneuronal Amyloid-β Accumulation

Intraneuronal accumulation of hyperphosphorylated protein tau in paired helical filaments together with amyloid-beta peptide (Abeta) deposits confirm the clinical diagnosis of Alzheimer disease. A common cellular mechanism leading to the production of these potent toxins remains elusive. Here we show that, in cultured neurons, membrane depolarization induced a calcium-mediated transient phosphorylation of both microtubule-associated protein tau and amyloid precursor protein (APP), followed by a dephosphorylation of these proteins. Phosphorylation was mediated by glycogen synthase kinase 3 and cyclin-dependent kinase 5 protein kinases, while calcineurin was responsible for dephosphorylation. Following the transient phosphorylation of APP, intraneuronal Abeta accumulated and induced neurotoxicity. Phosphorylation of APP on Thr-668 was indispensable for intraneuronal accumulation of Abeta. Our data demonstrate that an increase in cytosolic calcium concentration induces modifications of neuronal metabolism of APP and tau, similar to those found in Alzheimer disease.

Cathepsin D expression is decreased in Alzheimer's disease fibroblasts

Cathepsin D (CTSD), a protease detectable in different cell types whose primary function is to degrade proteins by bulk proteolysis in lysosomes, has been suggested to be involved in Alzheimer's disease (AD). In fact, there is increasing evidence that disturbance of the normal balance and localization of cathepsins may contribute to neurodegeneration in AD [Nakanishi H. Neuronal and microglial cathepsins in aging and age-related diseases. Aging Res Rev 2003; 2(4):367–81]. Here, we provide evidence of an altered balance of CTSD in skin fibroblasts from patients affected either by sporadic or familial forms of AD. In particular, we demonstrate that CTSD is down regulated at both transcriptional and translational level and its processing is altered in AD fibroblasts. The oncogene Ras is involved in the regulation of CTSD, as high expression level of the constitutively active form of Ras in normal or AD fibroblasts induces CTSD down-regulation. p38 MAPK signalling pathway also appears to down-modulate CTSD level. Overall results reinforce the hypothesis that a lysosomal impairment may be involved in AD pathogenesis and can be detected not only in the CNS but also at a peripheral level.

Neuroserpin Binds Aβ and Is a Neuroprotective Component of Amyloid Plaques in Alzheimer Disease

Alzheimer disease is characterized by extracellular plaques composed of Abeta peptides. We show here that these plaques also contain the serine protease inhibitor neuroserpin and that neuroserpin forms a 1:1 binary complex with the N-terminal or middle parts of the Abeta(1-42) peptide. This complex inactivates neuroserpin as an inhibitor of tissue plasminogen activator and blocks the loop-sheet polymerization process that is characteristic of members of the serpin superfamily. In contrast neuroserpin accelerates the aggregation of Abeta(1-42) with the resulting species having an appearance that is distinct from the mature amyloid fibril. Neuroserpin reduces the cytotoxicity of Abeta(1-42) when assessed using standard cell assays, and the interaction has been confirmed in vivo in novel Drosophila models of disease. Taken together, these data show that neuroserpin interacts with Abeta(1-42) to form off-pathway non-toxic oligomers and so protects neurons in Alzheimer disease.

Expression of a Familial Alzheimer's Disease-Linked Presenilin-1 Variant Enhances Perforant Pathway Lesion-Induced Neuronal Loss in the Entorhinal Cortex

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.

Frameshift proteins in autosomal dominant forms of Alzheimer disease and other tauopathies

Frameshift (+1) proteins such as APP(+1) and UBB(+1) accumulate in sporadic cases of Alzheimer disease (AD) and in older subjects with Down syndrome (DS). We investigated whether these proteins also accumulate at an early stage of neuropathogenesis in young DS individuals without neuropathology and in early-onset familial forms of AD (FAD), as well as in other tauopathies, such as Pick disease (PiD) or progressive supranuclear palsy (PSP). APP(+1) is present in many neurons and beaded neurites in very young cases of DS, which suggests that it is axonally transported. In older DS patients (>37 years), a mixed pattern of APP(+1) immunoreactivity was observed in healthy looking neurons and neurites, dystrophic neurites, in association with neuritic plaques, as well as neurofibrillary tangles. UBB(+1) immunoreactivity was exclusively present in AD type of neuropathology. A similar pattern of APP(+1) and UBB(+1) immunoreactivity was also observed for FAD and much less explicit in nondemented controls after the age of 51 years. Furthermore, we observed accumulation of +1 proteins in other types of tauopathies, such as PiD, frontotemporal dementia, PSP and argyrophylic grain disease. These data suggest that accumulation of +1 proteins contributes to the early stages of dementia and plays a pathogenic role in a number of diseases that involve the accumulation of tau.

Panel of aneuploid cell lines for physical mapping of the proximal long arm of human chromosome 21

Some forms of familial Alzheimer disease (FAD) have shown linkage to DNA probes for the loci D21S1 and D21S16 on chromosome 21. To investigate the physical location of these DNA probes, we have constructed a physical map of this region of chromosome 21 by using quantitative Southern blot analysis of DNA sequences unique to chromosome 21 and a series of cell lines aneuploid for parts of chromosome 21. We now show that both D21S16 and D21S1/S11 are located in the same region of chromosome 21, in q11.2-q21.05. This new panel of aneuploid cell lines will allow the rapid mapping of new DNA probes in the vicinity of the FAD gene.

Presenilin-1-Dependent Transcriptome Changes

Familial forms of Alzheimer's disease (FADs) are caused by the expression of mutant presenilin 1 (PS1) or presenilin 2. Using DNA microarrays, we explored the brain transcription profiles of mice with conditional knock-out of PS1 (cKO PS1) in the forebrain. In parallel, we performed a transcription profiling of the hippocampus and frontal cortex of the FAD-linked DeltaE9 mutant transgenic (TG) mice and matched controls [TG mice expressing wild-type human PS1 (hPS1)]. When the TG and cKO datasets were cross-compared, the majority of the 30 common expression alterations were in opposite direction, suggesting that the FAD-linked PS1 variant produces transcriptome changes primarily by gain of aberrant function. Our microarray studies also revealed an unanticipated inverse correlation of transcript levels between the brains of mice that coexpress DeltaE9 hPS1+ amyloid precursor protein (APP)695 Swe and DeltaE9 hPS1 single transgenic mice. The opposite directionality of these changes in transcript levels must be a function of APP and/or APP derivatives.

New directions in neuroprotection: Basic mechanisms, molecular targets and treatment strategies

Few diseases are as devastating as the neurodegenerative ones including: Alzheimer’s disease (AD), Parkinson’s disease (PD) and Amyotrophic Lateral Sclerosis (ALS), to name a few. Genetic risk-factors influence these age-associated, chronic illness but, by and large, the causes of these diseases remain unknown. There are known familial forms of AD, ALS and PD and the identification of mutations in specific genes causing each of these degenerative disorders has provided opportunities to investigate the molecular participants in the disease process and pathogenic mechanisms. However, the familial forms of these diseases account for only a small percentage of observed cases; the sporadic forms of the diseases are either caused by undiscovered genes or, molecular events and multifactorial risk factors and pathways that we do not completely understand. The neurodegenerative diseases are generally characterized by the dysfunction and death of specific populations of neurons and in many cases by the presence, in brain, of intracellular or extracellular aggregates of misfolded protein(s). Neuronal death occurs either chronically, such as in Alzheimer’s disease, which involves neuronal loss over years to decades or, acutely within days, such as in, head trauma, stroke and spinal cord injury. Broadly speaking there are three types of neuronal death: necrosis which can trigger neuroinflammation and involves secondary damage to healthy neighboring neurons, apoptosis which is non-inflammatory and is less damaging to neighboring neurons and, paraotosis which represents an intermediate between apopotosis and necrosis. Each type of cell death may have a unique set of pathways and mechanisms leading to neuronal death. However, many of the underlying neurotoxic mechanisms are shared across a wide variety of neurodegenerative diseases (e.g. Ca excess, generation of reactive oxygen species, chronic inflammation, caspase activation). The development of therapies for the neurodegenerative disease represents a major challenge to academic, biotechnology and pharmaceutical scientists. Current therapies for the neurodegenerative diseases provide effective symptomatic relief, particularly in early stages of disease (e.g., memantine and cholinesterase inhibitors for AD, dopaminergic drugs for PD, Rilutek for ALS). However, there are too few, if any therapies that affect the underlying disease processes. Therefore, diseasemodifying therapies that halt, slow or reverse disease progression are sorely needed. It is expected that, next generation of neuroprotective therapies will stop and/or slow neuronal cell death and will therefore be disease modifying. Using the tools of modern neuroscience, neurology, pharmacology, chemistry, genomics, proteomics, psychometrics and bioinformatics,neuroscientists have made good progress in not only in understanding some of the mechanisms of neurodegeneration but, also in identifying and validating new drug targets which are the basis for novel therapeutic strategies. For example, a number the molecular and cellular factors that trigger the different types of neuronal cell death have been revealed; represent targets for neuroprotective strategies. Cellular factors include oligodendrocytes, activated microglia and astroglia. These generate molec-

So what if tangles precede plaques?

As proposed by Hardy and Higgins in 1992 [8], the amyloid hypothesis had two tenets. First, that deposition of -amyloid (A ) is the causative agent of Alzheimer’s disease (AD) pathology, and secondly that other lesions (tangles, dystrophic neurites, synaptic and cell loss, vascular degeneration) follow directly from this deposition. In their paper analyzing early formation of amyloid plaques and neurofibrillary tangles, Schonheit et al. aim to refute the second tenet, arguing that tangles form without or prior to the presence of plaques, and therefore that A cannot be the causative agent for neurofibrillary lesions. As discussed below, their argument is consistent with earlier observations that at least the initial formation of tangles is independent of A . On the other hand, the first tenet of the amyloid hypothesis is now supported so strongly by several lines of evidence that it is difficult to refute it. The challenge is how to reconcile these seemingly contradictory conclusions. Probably, the most compelling evidence that A is the causative agent of AD comes from observations on genetic mutations or other conditions that cause familial forms of AD, all of which produce an elevation in A or its more toxic form, A 1–42 [21,22]. These mutations occur directly in the gene for amyloid precursor protein (APP), and produce excess A , or occur in the genes for the Presenilin proteins, which form an important part of the -secretase complex that cuts APP to liberate A . Down’s Syndrome, with triplication of chromosome 21, also results almost invariably in AD, beginning at ages at which AD is extremely rare. Because the gene for APP is on the chromosome 21, an excess amount of A is generated. Amyloid plaques develop in Down’s Syndrome brains at about the age of 20–30 years, and large numbers of tangles appear in these cases during the third to fifth decades of life [9,11,12,26]. Although these genetic conditions account for only a small fraction of the total number of AD cases, the associated pattern of