Familial Forms Of Alzheimer's Disease Research Articles

AATF protects neural cells against oxidative damage induced by amyloid β-peptide

Extensive loss of neurons and synapses in vulnerable regions of the brain is one of the most important pathological features of Alzheimer's disease (AD). Increased oxidative stress has been shown to contribute to the neurodegenerative process in AD. Aggregation of amyloid β-peptide (Aβ) in amyloid plaques is one of the defining features of Alzheimer's disease. Indeed, Aβ has been shown to induce oxidative stress and apoptosis in many in vivo and in vitro models of AD. We now report that AATF (apoptosis-antagonizing transcription factor), a leucine zipper protein initially identified as an interaction partner of DAP like kinase (Dlk, a member of the pro-apoptotic Death-Associated Protein kinase family), is expressed in cortical neurons and in neural PC12 cells. Aβ induces alterations in AATF expression in cortical neurons. Inhibition of AATF induction sensitizes neurons to Aβ toxicity. Overexpression of AATF suppressed superoxide production, inhibited peroxynitrite formation and membrane lipid peroxidation, and protected against Aβ-induced apoptosis in PC12 cells. These results suggest that AATF is a novel neuroprotective factor and it may protect against Aβ-induced apoptosis through its effects on suppressing the production of reactive oxygen species (ROS). AATF may therefore represent a potential candidate for therapeutic intervention of neurodegeneration in both sporadic and familial forms of AD.

The presenilin 2 M239I mutation associated with familial Alzheimer's disease reduces Ca 2+ release from intracellular stores

Mutations in presenilin ( PS) genes account for the majority of the cases of the familial form of Alzheimer's disease (FAD). PS mutations have been correlated with both over-production of the amyloid-β-42 (Aβ42) peptide and alterations of cellular Ca 2+ homeostasis. We here show, for the first time, the effect of the recently described PS2 FAD-associated M239I mutation on two major parameters of intracellular Ca 2+ homeostasis: the Ca 2+ storing capacity of the endoplasmic reticulum (ER) and the activation level of capacitative Ca 2+ entry (CCE), the Ca 2+ influx pathway activated by depletion of intracellular stores. Ca 2+ release from intracellular stores was significantly reduced in fibroblasts from FAD patients, compared to that found in cells from healthy individuals or patients affected by sporadic forms of Alzheimer's Disease (AD). No significant difference was however found in CCE between FAD and control fibroblasts. Similar results were obtained in two cell lines (HEK293 and HeLa) stably or transiently expressing the PS2 M239I mutation.

Welcome to the complex disease world: Alpha2-macroglobulin and Alzheimer's disease

The recent explosion in our understanding of Alzheimer’s disease (AD) at both the molecular and biochemical levels is due, in large part, to the successful utilization of genetic and genomic analyses over the past two decades. Initial positional cloning efforts of the 80s and 90s led to the identification of three genes, amyloid precursor protein (APP) and presenilins 1 and 2 (PSEN1 and PSEN2), that can harbor mutations leading to the rare, early-onset ( 60 years) familial form of AD. Phenotypically, the majority of these early-onset mutations ( 150 in total; a complete list of early-onset AD mutations can be found at the Alzheimer’s disease mutation database (http://molgenwww.uia.ac.be/ADMutations) lead to increased generation of the highly amyloid plaque-prone A 42 peptide from APP (Haass and De Strooper, 1999; Hardy, 1997; Price et al., 1998; Tanzi, 1999; Tanzi and Bertram, 2001). Similarly, genetic analysis of the more common, lateonset form of AD holds great promise for expanding our understanding of AD pathogenesis, accelerating means for early prediction of AD, and developing novel therapeutic modalities for the prevention and treatment of this devastating neurodegenerative disorder. Despite intensive efforts, only one gene to date, apolipoprotein E (APOE), has been universally established as a susceptibility gene for late-onset AD. In this case, the 4 variant of APOE only increases age-related risk for AD, but does not guarantee onset (Blacker et al., 1997; Meyer et al., 1998; Saunders et al., 1993). Recent segregation analyses suggest that an additional four to seven AD susceptibility genes, of at least moderate effect, remain to be discovered (Daw et al., 2000). Intensive searches for the additional risk-conferring genes is under way in laboratories worldwide and has already resulted in reports of over 100 different genes tested for genetic association with AD. However, with the exception of APOE, none of these putative AD genetic risk factors have been universally replicated in independent samples. The reason for the dearth of confirmed susceptibility genes is the highly complex genetic nature of AD itself. Unlike Mendelian diseases, risk-conferring genes in complex diseases usually impart modestly increased risk with low penetrance and unknown inheritance patterns. These complexities make unequivocal identification of susceptibility genes extremely challenging. While these challenges are formidable, they are not prohibitive. In fact, the future for analyses of complex genetic disorders can be considered as quite bright due to the ongoing deciphering of the human genome, the accelerating discovery of DNA variants (primarily, single nucleotide polymorphisms; SNPs), and technological advances that continue to make high-throughput genotyping and sequencing increasingly fast and relatively inexpensive. In the search for late-onset AD susceptibility genes, no gene besides APOE has been as widely investigated as that encoding 2-macroglobulin (protein: 2M; gene: A2M). A2M is an ideal positional candidate gene for late-onset AD. Biologically, 2M mediates A toxicity, clearance, and degradation. 2M binds the A peptide specifically and tightly (Du et al., 1998; Hughes et al., 1998) and consequently there are three biological manifestations of A / 2M interactions that are directly relevant to the etiology and pathogenesis of AD. First, the interaction between 2M and A prevents A fibril formation and fibril-associated neurotoxicity (Du et al., 1998; Hughes et al., 1998). Second, 2M is a protease inhibitor; protease activation of 2M/A complexes or protease activation of 2M followed by A binding can promote the protease-mediated degradation of * Corresponding author. Fax: 1-617-724-1949. E-mail address: tanzi@helix.mgh.harvard.edu (R.E. Tanzi). R Available online at www.sciencedirect.com

Vasoactive effects of Aβ in isolated human cerebrovessels and in a transgenic mouse model of Alzheimer's disease: Role of inflammation

Aβ peptides are the major protein constituents of Alzheimer's disease (AD) senile plaques and also form some deposits in the cerebrovasculature leading to cerebral amyloid angiopathy and hemorrhagic stroke. Functional vascular abnormalities are one of the earlier clinical manifestations in both sporadic and familial forms of AD. Most of the cardiovascular risk factors (for instance, diabetes, hypertension, high cholesterol levels, atherosclerosis and smoking) constitute risk factors for AD as well, suggesting that functional vascular abnormalities may contribute to AD pathology. We studied the effect of Aβ on endothelin-1 induced vasoconstriction in isolated human cerebral arteries collected following rapid autopsies. We report that freshly solubilized Aβ enhances endothelin-1 induced vasoconstriction in isolated human middle cerebral and basilar arteries. The vasoactive effect of Aβ in these large human cerebral arteries is inhibited by NS-398, a selective cyclooxygenase-2 inhibitor and by SB202190, a specific p38 Mitogen Activated Protein Kinase inhibitor suggesting the involvement of a pro-inflammatory pathway. Using a scanner laser Doppler imager, we observed that cerebral blood flow is decreased in the double transgenic APPsw Alzheimer mouse (PS1/APPsw) compared to PS1 littermates and can be improved by chronic treatment with either NS-398 or SB202190. Altogether, our data suggest a link between inflammation and the compromised cerebral hemodynamics in AD.

The over-expression of the wild type or mutant forms of the presenilin-1 protein alters glycoprotein processing in a human neuroblastoma cell line

Mutations in the presenilin proteins (PS1 and PS2) are responsible for more than 70% of the cases of the familial form of Alzheimer's disease (FAD). The proteins are expressed in the cell at a low level, primarily in the endoplasmic reticulum and cis Golgi, where they have been proposed to play a role in protein processing. As protein glycosylation is a key post-translational event that occurs within the Golgi, we have investigated the effect of altered PS1 expression levels on the protein glycosylation pattern using the SH-SY5Y human neuroblastoma cell line. In cells over-expressing either the wild type or mutant (M146L) PS1-FAD proteins, there was a decrease in the expression levels of protein-bound α2,3-linked sialic acid residues at the level of the cell membrane. This was particularly manifest as a significant decrease in the expression of the polysialic acid chain that is linked to the core oligosaccharide of the neural cell adhesion molecule in an α2,3 bond. These results suggest that the over-expression of either the wild type or mutant PS1 disturbs glycoprotein processing within the Golgi.

The role of presenilin cofactors in the gamma-secretase complex.

Mutations in presenilin genes account for the majority of the cases of the familial form of Alzheimer's disease (FAD). Presenilin is essential for gamma-secretase activity, a proteolytic activity involved in intramembrane cleavage of Notch and beta-amyloid precursor protein (betaAPP). Cleavage of betaAPP by FAD mutant presenilin results in the overproduction of highly amyloidogenic amyloid beta42 peptides. gamma-Secretase activity requires the formation of a stable, high-molecular-mass protein complex that, in addition to the endoproteolysed fragmented form of presenilin, contains essential cofactors including nicastrin, APH-1 (refs 15-18) and PEN-2 (refs 16, 19). However, the role of each protein in complex formation and the generation of enzymatic activity is unclear. Here we show that Drosophila APH-1 (Aph-1) increases the stability of Drosophila presenilin (Psn) holoprotein in the complex. Depletion of PEN-2 by RNA interference prevents endoproteolysis of presenilin and promotes stabilization of the holoprotein in both Drosophila and mammalian cells, including primary neurons. Co-expression of Drosophila Pen-2 with Aph-1 and nicastrin increases the formation of Psn fragments as well as gamma-secretase activity. Thus, APH-1 stabilizes the presenilin holoprotein in the complex, whereas PEN-2 is required for endoproteolytic processing of presenilin and conferring gamma-secretase activity to the complex.

Proinflammatory and vasoactive effects of Abeta in the cerebrovasculature.

Abeta peptides are thought to be critical molecules in the pathophysiology of Alzheimer's disease (AD) and are the major protein constituents of senile plaques. In most AD cases, Abeta peptides also form some deposits in the cerebrovasculature, leading to cerebral amyloid angiopathy and hemorrhagic stroke. Regional cerebral hypoperfusion is one of the earlier clinical manifestations in both the sporadic and familial forms of AD. In addition, a variety of vascular risk factors of different etiologies (for instance, diabetes, hypertension, high cholesterol level, atherosclerosis, and smoking) constitute risk factors for AD as well, suggesting that functional vascular abnormalities may contribute to AD pathology. We studied the effect of Abeta on constrictor responses elicited by endothelin-1 in isolated human cerebral arteries collected following rapid autopsies. We report that freshly solubilized Abeta potentiates endothelin-1-induced vasoconstriction in isolated human middle cerebral and basilar arteries. The vasoconstriction elicited by Abeta in these large human cerebral arteries appears to be completely antagonized by NS-398, a selective cyclooxygenase-2 inhibitor, or by SB202190, a specific p38 mitogen-activated protein kinase inhibitor, suggesting that Abeta vasoactivity is mediated via the stimulation of a proinflammatory pathway. In addition, a similar proinflammatory response appears to be mediated by Abeta in isolated human brain microvessels, resulting in an increased production of prostaglandin E(2) and F(2alpha). Using a scanner laser Doppler imager, we show a progressive decline with aging in cortical perfusion level in transgenic APPsw mice (line 2576) compared with age-matched control littermates. The relation between the acute proinflammatory and vasoactive properties of Abeta and the chronic progressive hypoperfusion seen in AD (and transgenic models thereof) is yet to be elucidated.

Perturbed endoplasmic reticulum function, synaptic apoptosis and the pathogenesis of Alzheimer's disease.

Endoplasmic reticulum (ER) appears to be a focal point for alterations that result in neuronal dysfunction and death in Alzheimer's disease (AD). Aberrant proteolytic processing and/or trafficking of the beta-amyloid precursor protein (APP) in ER may promote neuronal degeneration by increasing the levels of the neurotoxic forms of beta-amyloid (A beta) and by decreasing the levels of the neuroprotective secreted form of APP (sAPP alpha). Some cases of AD are caused by mutations in the genes encoding presenilin 1 (PS1). When expressed in cultured neuronal cells and transgenic mice, PS1 mutations cause abnormalities in ER calcium homoeostasis, enhancing the calcium responses to stimuli that activate IP3- and ryanodine-sensitive ER calcium pools. Two major consequences of this disrupted ER calcium regulation are altered proteolytic processing of APP and increased vulnerability of neurons to apoptosis and excitotoxicity. The impact of PS1 mutations and aberrant APP processing is particularly great in synaptic terminals. Perturbed synaptic calcium homoeostasis promotes activation of apoptotic cascades involving production of Par-4 (prostate apoptosis response-4), mitochondrial dysfunction and caspase activation. A beta 42 (the 42-amino-acid form of A beta) induces membrane lipid peroxidation in synapses and dendrites resulting in impairment of membrane ion-motive ATPases and glucose and glutamate transporters. This disrupts synaptic ion and energy homoeostasis thereby promoting synaptic degeneration. In contrast, sAPP alpha activates signalling pathways that protect synapses against excitotoxicity and apoptosis. In the more common sporadic forms of AD, the initiating causes of the neurodegenerative cascade are less well defined, but probably involve increased levels of oxidative stress and impaired energy metabolism. Such alterations have been shown to disrupt neuronal calcium homoeostasis in experimental models, and may therefore feed into the same neurodegenerative cascade initiated by mutations in presenilins and APP. Perturbed synaptic ER calcium homoeostasis and consequent alterations in APP processing appear to be pivotal events in both sporadic and familial forms of AD.

Alzheimer's disease: dysfunction of a signalling pathway mediated by the amyloid precursor protein?

All individuals with Alzheimer's disease (AD) experience a progressive loss of cognitive function, resulting from a neurodegenerative process characterized by the deposition of beta-amyloid (A beta) in plaques and in the cerebrovasculature, and by the formation of neurofibrillary tangles in neurons. The cause of the neuronal death is unknown but it is thought to be linked in some way to the beta-amyloid precursor protein (APP), which is the source of the A beta that accumulates in the AD brain. There are two pieces of supporting data for this: first, APP is overexpressed in Down's syndrome, which leads to AD-like neuropathology by the age of 40 in virtually all affected individuals; secondly, specific point mutations in APP cause some forms of familial AD. Our laboratory has focused on a specific aspect of APP and its connection with the neuronal destruction seen in AD. We have hypothesized that AD results from a progressive dysfunction of APP. In addition, on the basis of recent data generated by our laboratory and others, we propose that in the normal brain a percentage of APP is present as an integral protein of the plasma membrane that mediates the transduction of extracellular signals into the cell via its A beta-containing C-terminal tail. In AD, accumulation of abnormal levels of the C-terminus in the neuron disturbs this signal-transduction function of APP, causing disorders in the cell-cycle machinery and consequent apoptosis. Here, we discuss the key findings that support this hypothesis, and discuss its therapeutic implications for AD.

The molecular and genetic basis of AD: the end of the beginning: the 2000 Wartenberg lecture.

The Decade of the Brain—the 1990s—was a time of gains in our knowledge of the molecular and genetic basis of Alzheimer’s disease (AD).1-4 The momentum of discovery and depth of molecular information that developed during that time clearly leads one to predict that it is the end of the beginning of deciphering the pathogenesis of AD leading to effective therapy. Most cases of AD are sporadic; however, the molecular and genetic information of specific genes and other risk factors that cause the familial form of AD will shed light on the etiology of the much more abundant sporadic forms. About 10% of familial disease presents as an autosomal dominant mode of transmission. Epidemiologic studies indicate that about 30% of AD patients have a family history of disease in which at least one first-degree relative is affected. It is clear that early onset autosomal dominant AD is a heterogeneous set of disorders caused in separate families by different genetic mutations. Late-onset sporadic disease is the result of yet-to-be-discovered multiple environmental and genetic influences. In support of this view, only one-third of identical twins are concordant for AD, indicating that nongenetic—perhaps environmental—factors are important in the pathogenesis of AD. Mutations in the amyloid precursor protein gene ( APP ) on chromosome 21q, and the presenilin 1 and 2 genes ( PS1 , PS2 ) on chromosomes 14q and 1q, account for approximately one half of early-onset forms of autosomal dominant inherited disease. An additional locus on chromosome 12 has also been described. It is estimated that 50% of dominantly inherited disease remains yet to be mapped and genetically identified. Although the specific functions of the proteins coded by the APP and the PS1 and PS2 genes are only incompletely known, they share a common effect of abnormally processing APP, resulting in a 50% …

Alzheimer disease: mouse models pave the way for therapeutic opportunities.

Research into the molecular mechanisms of Alzheimer disease (AD) continues to clarify important issues in aberrant protein processing while seeking to identify therapeutic targets. Mutations of genes on chromosomes 1, 14 (presenilins 1 and 2), and 21 (the amyloid-beta [Abeta] amyloid precursor protein [APP]) cause the familial forms of AD that often begin before age 65. An allelic polymorphism on chromosome 19 (apolipoprotein E ) affects the age of onset of the more common forms of sporadic AD. Multiple studies in transgenic mice provide strong evidence to support the view that Abeta amyloid formation is an early and critical pathogenic event: mice expressing pathogenic human APP mutations develop Abeta deposits; coexpression of mutant presenilin genes accelerates the rate of Abeta deposition; and apolipoprotein E plays a role in this process. Thus, the 3 established genetic causes or risk factors for AD affect Abeta deposition. The fact that elevation of the Abeta42/Abeta40 ratio (differing only in 2 amino acids in length) is also linked to amyloid deposition in the APP mice and is temporally linked to cognitive impairment suggests that Abeta42 may be a principal inducing factor of AD. The exact sequence of events is still unknown, but the transgenic models generated so far have shown their usefulness in clarifying this complex part of the pathology. The continuing progress in elucidation of the molecular pathogenesis of AD suggests a range of rational pharmacological interventions for this disorder. The most promising strategy involves the development of approaches to retard, halt, or prevent Abeta-mediated disease progression, and these can now be tested in transgenic animals.

The function of presenilin-1 in amyloid beta-peptide generation and brain development.

Several mutations in genes that cause the familial form of Alzheimer's Disease (FAD) have been identified. All mutations in the three FAD genes, i.e., amyloid precursor protein (APP), presenilin 1 (PS-1), and presenilin 2 (PS-2) cause an increased production of a longer, more amyloidogenic form of the amyloid peptide corroborating strongly the idea that abnormal processing of APP is central to the pathogenesis. In PS-1 deficient mice, 80% less amyloid peptide was produced. Instead, membrane associated carboxyterminal fragments generated by (alpha- and beta-secretase accumulated suggesting that PS-1 is involved in the gamma-secretase activity cleaving the transmembrane domain of APP after alpha- and beta-secretase cleavage has occured. The clinical mutations in PS-1 which increase the production of betaA41-42 therefore seem to cause a "selective" gain of its normal function. During cortical plate development in PS-1-deficient mice, neurons do not terminate their movement at the outer margin of the cortical plate, but enter the marginal zone and subarachnoid space. These focal heterotopias closely resemble those occuring, e.g., in human lissencephaly type II. The extracellular matrix of the cortical plate and marginal zone was altered as a consequence of a loss of Cajal-Retzius (CR) neurons from the marginal zone. The pathogenesis of this neuronal migration disorder is associated with a reduction and redistribution of notch- immunoreactivity in CR- and cortical plate neurons, a cell surface receptor operative in cell fate selection, which similar to APP is cleaved in its transmembrane domain during activation by a gamma-secretase like protease.

Apolipoprotein E and α-1-antichymotrypsin allele polymorphism in sporadic and familial Alzheimer's disease

Alzheimer disease (AD) patients with both sporadic and familial forms of AD and non-demented controls were genotyped for common polymorphisms in the signal peptide for α-1-antichymotrypsin (ACT) gene and in two different regions of apolipoprotein E (APOE) gene. The ACT TT genotype was over-represented ( P=0.025) in patients with early onset of sporadic AD. In this patient's group ACT TT genotype conferred a significant crude odds ratio for the disease (OR=2.09; 95% CI=1.09–4.00, P=0.025). After adjustment for the APOE ϵ4 and APOE −491 genotypes, logistic regression analysis confirmed that the ACT TT genotype resulted independently associated with early onset AD (adjusted OR=2.56; 85% C.I.=1.3–5.2, P=0.009). The frequency of APOE ϵ4 allele was increased in AD, as expected (OR=5.92, 95% CI=3.60–9.70, P=0.0001). On the contrary, the APOE −491 A/T genotypes were not associated with AD. No preferential association of the APOE ϵ4 allele or APOE −491 A/T genotypes with ACT A/T alleles was observed in AD. Present findings indicated that subjects with ACT TT genotype had an increased risk of developing AD and suggested that this genotype influenced the risk of an early onset of the disease by affecting the production of ACT molecules.

Alternative, Non-secretase Processing of Alzheimer's β-Amyloid Precursor Protein during Apoptosis by Caspase-6 and -8

Alzheimer's disease (AD) is a progressive neurodegenerative disorder. Although the pathogenesis of AD is unknown, it is widely accepted that AD is caused by extracellular accumulation of a neurotoxic peptide, known as Abeta. Mutations in the beta-amyloid precursor protein (APP), from which Abeta arises by proteolysis, are associated with some forms of familial AD (FAD) and result in increased Abeta production. Two other FAD genes, presenilin-1 and -2, have also been shown to regulate Abeta production; however, studies examining the biological role of these FAD genes suggest an alternative theory for the pathogenesis of AD. In fact, all three genes have been shown to regulate programmed cell death, hinting at the possibility that dysregulation of apoptosis plays a primary role in causing neuronal loss in AD. In an attempt to reconcile these two hypotheses, we investigated APP processing during apoptosis and found that APP is processed by the cell death proteases caspase-6 and -8. APP is cleaved by caspases in the intracellular portion of the protein, in a site distinct from those processed by secretases. Moreover, it represents a general effect of apoptosis, because it occurs during cell death induced by several stimuli both in T cells and in neuronal cells.

Biology of presenilins as causative molecules for Alzheimer disease.

Many missense mutations in the presenilins are associated with autosomal dominant forms of familial Alzheimer disease (AD). Presenilin genes encode polytopic transmembrane proteins, which are processed by proteolytic cleavage and form high-molecular-weight complexes under physiological conditions. The presenilins have been suggested to be functionally involved in developmental morphogenesis, apoptosis signal pathways, and processing of selected proteins including beta-amyloid precursor protein. Although the underlying mechanism in which presenilin mutations lead to development of AD remains elusive, one consistent mutational effect is an overproduction of long-tailed amyloid beta-peptides. Furthermore, presenilins interact with beta-catenin to form presenilin complexes and presenilin mutations effect beta-catenin signalling pathways.

Alzheimer's Disease-Linked Mutation of Presenilin 2 (N141I-PS2) Drastically Lowers APPα Secretion: Control by the Proteasome

Most of early onset familial forms of Alzheimer's disease (FAD) are due to inherited mutations located on two homologous proteins, presenilins 1 and 2 (PS1 and PS2) encoded by chromosomes 14 and 1, respectively. Here we show that the expression of wild type (wt)-PS2 in human HEK293 cells increases the production of the physiological α-secretase-derived product, APPα. By contrast, APPα secretion is drastically reduced in cells expressing the FAD-linked N141I-PS2. We establish that wt-PS2, N141I-PS2 and their C-terminal maturation fragment are degraded by the enzymatic multicatalytic complex, proteasome. Interestingly, two selective proteasome inhibitors, Z-IE(Ot-Bu)A-Leucinal and lactacystin potentiate the APPα secretion observed in wtPS2-expressing cells and further amplify the N141I-PS2-induced decrease in APPα production. By contrast, a series of pharmacological agents unable to affect the proteasome do not modify PS2 immunoreactivities and APPα recoveries. Altogether, our data indicate that: 1) wtPS2 positively modulates the α-secretase physiological pathway of βAPP maturation in human cells; 2) N141I mutation on PS2 drastically lowers the secretion of APPα; 3) Proteasome inhibitors prevent the degradation of wtPS2, N141I-PS2 and their C-terminal maturation product. This protection against proteasomal degradation directly modulates the APPα secretion response elicited by wt- and FAD-linked PS2 expression in human HEK293 cells.

Alpha-secretase-derived product of beta-amyloid precursor protein is decreased by presenilin 1 mutations linked to familial Alzheimer's disease.

Recent reports indicate that missense mutations on presenilin (PS) 1 are likely responsible for the main early-onset familial forms of Alzheimer's disease (FAD). Consensual data obtained through distinct histopathological, cell biology, and molecular biology approaches have led to the conclusion that these PS1 mutations clearly trigger an increased production of the 42-amino-acid-long species of beta-amyloid peptide (A beta). Here we show that overexpression of wild-type PS1 in HK293 cells increases A beta40 secretion. By contrast, FAD-linked mutants of PS1 trigger increased secretion of both A beta40 and A beta42 but clearly favor the production of the latter species. We also demonstrate that overexpression of the wild-type PS1 augments the alpha-secretase-derived C-terminally truncated fragment of beta-amyloid precursor protein (APP alpha) recovery, whereas transfectants expressing mutated PS1 secrete drastically lower amounts of APP alpha when compared with cells expressing wild-type PS1. This decrease was also observed when comparing double transfectants overexpressing wild-type beta-amyloid precursor protein and either PS1 or its mutated congener M146V-PS1. Altogether, our data indicate that PS mutations linked to FAD not only trigger an increased ratio of A beta42 over total A beta secretion but concomitantly down-regulate the production of APP alpha.

Exploring the Etiology of Alzheimer Disease Using Molecular Genetics

Alzheimer disease (AD), the most common cause of dementia in the elderly, exists in both familial and sporadic forms. Genetic studies have led to the identification of 3 genes, beta-amyloid precursor protein (APP), presenilin-1 (PS1), and presenilin-2 (PS2), which, when mutated, can cause familial forms of AD. Mutations in each of these genes result in elevated levels of A beta42/43, a proteolytic processing fragment of APP that is deposited in brains of AD patients. Transgenic mice carrying AD-causing mutations in APP develop spontaneous age-related beta-amyloid (A beta) deposition and memory impairment. Genetic linkage and association studies have also shown that the epsilon4 allele of the apolipoprotein E (APOE) gene increases risk for AD in a dose-dependent manner in both familial and sporadic forms of AD and may account for as much as 50% of the attributable risk. APOE genotyping may be useful both as an adjunct to diagnosis and in identifying patient groups for therapeutic intervention. While the biological basis for the association of APOE epsilon4 with AD is not known, age of onset and A beta deposition are positively correlated with epsilon4 allele dosage in some cases, suggesting that this risk may also be mediated directly or indirectly through A beta. Because 50% of AD cases have no APOE epsilon4 alleles and families showing mendelian inheritance of AD exist in whom there are no mutations in any of the APP, PS1, or PS2 genes, it is likely that there are additional AD risk factors, both genetic and environmental, still to be identified.

In situ hybridization analysis of presenilin 1 mRNA in Alzheimer disease and in lesioned rat brain.

Presenilin-1 (PS-1) gene mutations are responsible for the majority of the early onset familial forms of Alzheimer disease (AD). Neither PS-1's anatomic distribution in brain nor expression in AD have been reported. Using in situ hybridization in the rat forebrain, we show that PS-1 mRNA expression is primarily in cortical and hippocampal neurons, with less expression in subcortical structures, in a regional pattern similar to APP695. Excitotoxic lesions lead to loss of PS-1 signal. A neuronal pattern of expression of PS-1 mRNA was also observed in the human hippocampal formation. AD and control levels did not differ. PS-1 is expressed in brain areas vulnerable to AD changes more so than in areas spared in AD; however, PS-1 expression is not sufficient to mark vulnerable regions. Collectively, these data suggest that the neuropathogenic process consequent to PS-1 mutations begins in neuronal cell populations.

Characterization of human presenilin 1 using N-terminal specific monoclonal antibodies: Evidence that Alzheimer mutations affect proteolytic processing

The majority of cases of early-onset familial Alzheimer disease are caused by mutations in the recently identified presenilin 1 (PS1) gene, located on chromosome 14. PS1, a 467 amino acid protein, is predicted to be an integral membrane protein containing seven putative transmembrane domains and a large hydrophilic loop between the sixth and seventh membrane-spanning domain. We produced 7 monoclonal antibodies that react with 3 non-overlapping epitopes on the N-terminal hydrophilic tail of PS1. The monoclonal antibodies can detect the full-size PS1 at M r 47 000 and a more abundant M r 28 000 product in membrane extracts from human brain and human cell lines. PC12 cells transiently transfected with PS1 constructs containing two different Alzheimer mutations fail to generate the 28 kDa degradation product in contrast to PC12 cells transfected with wild-type PS1. Our results indicate that missense mutations in this form of familial Alzheimer disease may act via a mechanism of impaired proteolytic processing of PS1.