Presenilin proteins undergo heterogeneous endoproteolysis between Thr291 and Ala299 and occur as stable N- and C-terminal fragments in normal and Alzheimer brain tissue

Presenilin structure, function and role in Alzheimer disease

Numerous missense mutations in the presenilins are associated with the autosomal dominant form of familial Alzheimer disease. 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, unfolded protein responses and processing of selected proteins including the beta-amyloid precursor protein. Although the underlying mechanism by which presenilin mutations lead to development of Alzheimer disease 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 the physiological and mutational effects are also observed in the catenin signal transduction pathway.

Inhibiting amyloid precursor protein C-terminal cleavage promotes an interaction with presenilin 1

Presenilin 1 (PS1) plays a pivotal role in the production of the amyloid-beta protein, which is central to the pathogenesis of Alzheimer's disease. It has been demonstrated that PS1 regulates the gamma-secretase proteolysis of the amyloid precursor protein (APP) C-terminal fragment (APP-C100), which is the final step in amyloid-beta protein production. The mechanism and detailed pathway of this PS1 activity has yet to be fully resolved, but it may be due to a presenilin-controlled trafficking of the APP fragment or possibly an inherent PS1 proteolytic activity. We have investigated the possibility of a direct interaction of PS1 and the APP-C100 within the high molecular mass presenilin complex. However, the APP-C100 is rapidly degraded, and if it forms, then any PS1.APP complex is likely to be very transitory. To circumvent this problem, we have utilized the protease inhibitor N-acetyl-leucyl-norleucinal (LLnL) and the lysosomotropic agent NH(4)Cl, which inhibits the turnover of the APP-C100. Under these conditions, levels of the fragment increased appreciably, and as shown by glycerol gradient analysis, the APP-C100 shifted to a higher molecular mass complex that overlapped with PS1. Immunoprecipitation studies demonstrated that a significant population of the APP-C100 co-precipitated with PS1. These findings suggest that PS1 may mediate the shuttling of APP fragments and/or facilitate their presentation for gamma-secretase cleavage through a direct interaction.

Transition-state analogue inhibitors of gamma-secretase bind directly to presenilin-1

The beta-amyloid precursor protein (beta-APP), which is involved in the pathogenesis of Alzheimer's disease, and the Notch receptor, which is responsible for critical signalling events during development, both undergo unusual proteolysis within their transmembrane domains by unknown gamma-secretases. Here we show that an affinity reagent designed to interact with the active site of gamma-secretase binds directly and specifically to heterodimeric forms of presenilins, polytopic proteins that are mutated in hereditary Alzheimer's and are known mediators of gamma-secretase cleavage of both beta-APP and Notch. These results provide evidence that heterodimeric presenilins contain the active site of gamma-secretase, and validate presenilins as principal targets for the design of drugs to treat and prevent Alzheimer's disease.

Presenilin mutations in Alzheimer's disease

The presenilins (PS-1 and PS-2) are 2 members of a novel family of genes encoding integral membrane proteins recently implicated in Alzheimer's disease (AD) pathology. To date, 43 mutations have been identified in PS-1 and 2 in PS-2 that lead to familial presenile AD (onset before age 65 years). The normal and pathological functions of the PS proteins (ps-1 and ps-2) are unknown, but their high degree of homology predicts similar biological activities. Homologies with ps from other species suggest that they may play a role in intracellular protein sorting and trafficking, in intercellular cell signaling, or in cell death. Since to date only missense mutations and in-frame deletions were identified, it is believed that mutated ps act through either a gain of (dys-)function or a dominant negative effect. In vivo and in vitro studies have linked PS mutations to amyloid deposition, an early pathological event in AD brains.

The nonconserved hydrophilic loop domain of presenilin (PS) is not required for PS endoproteolysis or enhanced abeta 42 production mediated by familial early onset Alzheimer's disease-linked PS variants

Presenilin 1 (PS1) and presenilin 2 (PS2) are polytopic membrane proteins that are mutated in the majority of early onset familial Alzheimer's disease (FAD) cases. Two lines of evidence establish a critical role for PS in the production of beta-amyloid peptides (Abeta). FAD-linked PS mutations elevate the levels of highly amyloidogenic Abeta ending at residue 42 (Abeta42), and cells with ablated PS1 alleles secrete low levels of Abeta. Several recent reports have shown that the hydrophilic loop (HL) domain, located between transmembrane domains 6 and 7, contains sites for phosphorylation, caspase cleavage, and sequences that bind several PS-interacting proteins. In the present report, we examined the metabolism of PS polypeptides lacking the HL domain and the influence of these molecules on Abeta production. We report that the deletion of the HL domain does not have a deleterious effect on the regulated endoproteolysis of PS, saturable accumulation of PS fragments, or the self-association of PS fragments. Abeta production was not significantly altered in cells expressing HL-deleted PS polypeptides compared with cells expressing full-length PS. Importantly, deletion of the HL domain did not affect FAD mutation-mediated elevation in the production of Abeta42. Furthermore, the deletion of the HL domain did not impair the role of PS1 or PS2 in facilitating Notch processing. Thus, our results argue against a biologically or pathologically relevant role for the HL domain phosphorylation and caspase cleavage and the association of PS HL domain-interacting proteins, in amyloid precursor protein metabolism and Abeta production or Notch cleavage.

Familial Alzheimer's disease-associated mutations block translocation of full-length presenilin 1 to the nuclear envelope

A polyclonal antibody, M5, to the hydrophilic loop domain of human presenilin 1 (PS1) was prepared. Western blot and immunoprecipitation analyses showed that M5 specifically recognized the processed C-terminal fragment, but not the full-length PS1. Epitope mapping analysis revealed that the essential sequence for recognition of the C-terminal fragment by M5 is DPEAQRR (302-308). The recognition of the C-terminal fragment by M5 in a processing-dependent manner was further confirmed by competitive enzyme-linked immunosorbent assay using the synthetic peptide L281 (281-311), which contains the putative processing site and the preceding amino acids to the site. Although L281 contains the epitope sequence for M5, the maximum inhibition was only 14%. Immunocytochemistry using M5 combined with hL312, which recognizes both full-length PS1 and the C-terminal fragment, allowed us to distinguish the localization of the processed C-terminal fragment from that of full-length PS1. Confocal microscopy demonstrated that the full-length form of wild-type PS1 is preferentially located in the nuclear envelope, while the processed C-terminal fragment is mainly present in the endoplasmic reticulum (ER). However, PS1 with familial Alzheimer's disease-associated mutations could not translocate to the nuclear envelope, and both the full-length and processed mutants were co-localized in the ER.

Separation of presenilin function in amyloid beta-peptide generation and endoproteolysis of Notch

Most of the genetically inherited Alzheimer's disease cases are caused by mutations in the presenilin genes, PS1 and PS2. PS mutations result in the enhanced production of the highly amyloidogenic 42/43 amino acid variant of amyloid beta-peptide (Abeta). We have introduced arbitrary mutations at position 286 of PS1, where a naturally occurring PS1 mutation has been described (L286V). Introduction of charged amino acids (L286E or L286R) resulted in an increase of Abeta42/43 production, which reached almost twice the level of the naturally occurring PS1 mutation. Although pathological Abeta production was increased, endoproteolysis of Notch and nuclear transport of its cytoplasmic domain was significantly inhibited. These results demonstrate that the biological function of PS proteins in the endoproteolysis of beta-amyloid precursor protein and Notch can be separated.

Subcellular localization of presenilins: association with a unique membrane pool in cultured cells

We have investigated the subcellular distribution of presenilin-1 (PS1) and presenilin-2 (PS2) in a variety of mammalian cell lines. In Iodixanol-based density gradients, PS1 derivatives show a biphasic distribution, cofractionating with membranes containing ER-resident proteins and an additional population of membranes with low buoyant density that do not contain markers of the Golgi complex, ERGIC, COP II vesicles, ER exit compartment, COP II receptor, Golgi SNARE, trans-Golgi network, caveolar membranes, or endocytic vesicles. Confocal immunofluorescence and immunoelectron microscopy studies fully supported the fractionation studies. These data suggest that PS1 fragments accumulate in a unique subcompartment(s) of the ER or ER to Golgi trafficking intermediates. Interestingly, the FAD-linked PS1 variants show a marked redistribution toward the heavier region of the gradient. Finally, and in contrast to PS1, PS2 fragments are detected preponderantly in more densely sedimenting membranes, suggesting that the subcellular compartments in which these molecules accumulate are distinct.

Presenilin-1 differentially facilitates endoproteolysis of the beta-amyloid precursor protein and Notch

Mutations in the presenilin-1 (PS1) gene are associated with Alzheimer's disease and cause increased secretion of the neurotoxic amyloid-beta peptide (Abeta). Critical intramembraneous aspartates at residues 257 and 385 are required for the function of PS1 protein. Here we investigate the biological function of a naturally occurring PS1 splice variant (PS1 Deltaexon 8), which lacks the critical aspartate 257. Cell lines that stably express PS1 Deltaexon 8 or a PS1 protein in which aspartate residue 257 is mutated secrete significant levels of Abeta, whereas Abeta generation is severely reduced in cells transfected with PS1 containing a mutation of aspartate 385. In contrast, endoproteolytic processing of Notch is almost completely inhibited in cell lines expressing any of the PS1 variants that lack one of the critical aspartates. These data indicate that PS1 may differentially facilitate gamma-secretase-mediated generation of Abeta and endoproteolysis of Notch.

Proteolytic processing and degradation of human presenilin-1 expressed in yeast

Numerous mutations causing early-onset familial Alzheimer's disease have been identified in the presenilin-1 gene. Presenilin-1 protein is produced as a 47 kDa holoprotein and proteolytically processed to an N-terminal 28 kDa and a C-terminal 19 kDa fragments by unidentified presenilinase in mammalian cells. We have demonstrated that this proteolytic processing also occurs in yeast. We also show that degradation of C-terminal fragment of presenilin-1 is dependent of proteasomal function. This yeast system will be a good tool to identify presenilinase and to study the role of presenilin-1 in amyloid precursor protein processing.

Alzheimer's disease presenilin-1 exon 9 deletion and L250S mutations sensitize SH-SY5Y neuroblastoma cells to hyperosmotic stress-induced apoptosis

Mutations in the presenilin-1 (PS1) and presenilin-2 (PS2) genes account for the majority of early-onset familial Alzheimer's disease cases. Recent studies suggest that presenilin gene mutations predispose cells to apoptosis by mechanisms involving altered calcium homeostasis and oxidative damage. In the present study, we determined whether PS1 mutations also sensitize cells to hyperosmotic stress-induced apoptosis. For this, we established SH-SY5Y neuroblastoma cell lines stably transfected with wild-type PS1 or either the PS1 exon 9 deletion (deltaE9) or PS1 L250S mutants. Cultured cells were exposed to an overnight (17 h) serum deprivation, followed by a 30 min treatment with either 20 mM glucose, 10 nM insulin-like growth factor-1 or 20 mM glucose + 10 nM insulin-like growth factor-1. Cells were then cultured for a further 3, 6 or 24 h and stained for apoptotic condensed nuclei using propidium iodide. Confirmation that cells were undergoing an active apoptotic process was achieved by labelling of DNA strand breaks using the terminal dUTP nick end labelling (TUNEL) technique. We also determined cell viability using 3-(4,5-dimethyl-thiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) reduction. Propidium iodide staining revealed that all cell lines and controls showed an increased number of apoptotic cells appearing with condensed nuclei at 24 h compared with 6 h and 3 h. High glucose-induced hyperosmotic stress resulted in significantly more apoptotic cells in the PS1 deltaE9 and PS1 L250S mutation cell lines at 24 h, compared with the wild-type PS1 lines (P < 0.001, ANOVA for both comparisons). Mean values (+/-S.D.) for the percentage number of apoptotic cells at 24 h following high glucose treatment were 16.1 +/- 3.5%, 26.7 +/- 5.5% and 31.0 +/- 5.7% for the wild-type PS1, PS1 deltaE9 and PS1 L250S lines, respectively. The pro-apoptotic effects of high glucose treatment were reversed by 10 nM insulin-like growth factor-1, although to a lesser extent in the mutation cell lines (5.8 +/- 2.4%, 15.2 +/- 7.3% and 13.2 +/- 2.0% for the wild-type PS1, PS1 deltaE9 (P < 0.01 for comparison with wild-type PS1) and PS1 L250S (P < 0.01 for comparison with wild-type PS1) transfected lines, respectively. TUNEL labelling of cells at 24 h following treatment gave essentially the same results pattern as obtained using propidium iodide. The percentage number of apoptotic cells with DNA strand breaks (means +/- S.D.) following high glucose treatment was 15.4 +/- 2.6% for the wild-type PS1, 26.8 +/- 3.2% for the PS1 deltaE9 (P < 0.001 for comparison with wild-type PS1) and 29.7 +/- 6.1% for the PS1 L250S transfected lines (P < 0.001 for comparison with wild-type PS1). The PS1 deltaE9 and PS1 L250S transfected lines also showed a higher number of apoptotic cells with DNA strand breaks at 24 h following high glucose plus insulin-like growth factor-1 treatment (11.4 +/- 2.0% and 14.3 +/- 2.8%, respectively), compared with values for the wild-type PS1 lines (8.5 +/- 2.4%). These differences were significant (P < 0.01) for the comparison of wild-type PS1 and PS1 L250S, but not PS1 deltaE9 lines. The mutation-related increases in number of apoptotic cells at 24 h following high glucose treatment were not accompanied by significant differences in cell viability at this time-point. Our results indicate that PS1 mutations predispose to hyperosmotic stress-induced apoptosis and that the anti-apoptotic effects of insulin-like growth factor-1 are compromised by these mutations. Perturbations of insulin-like growth factor-1 signalling may be involved in PS1 mutation-related apoptotic neuronal cell death in Alzheimer's disease.

Presenilins and Alzheimer's disease: biological functions and pathogenic mechanisms

Alzheimer's disease (AD) is the most common cause of dementia in the elderly population. Dementia is associated with massive accumulation of fibrillary aggregates in various cortical and subcortical regions of the brain. These aggregates appear intracellularly as neurofibrillary tangles, extracellularly as amyloid plaques and perivascular amyloid in cerebral blood vessels. The causative factors in AD etiology implicate both, genetic and environmental factors. The large majority of early-onset familial Alzheimer's disease (FAD) cases are linked to mutations in the genes coding for presenilin 1 (PS1) and presenilin 2 (PS2). The corresponding proteins are 467 (PS1) and 448 (PS2) amino-acids long, respectively. Both are membrane proteins with multiple transmembrane regions. Presenilins show a high degree of conservation between species and a presenilin homologue with definite conservation of the hydrophobic structure has been identified even in the plant Arabidopsis thaliana. More than 50 missense mutations in PS1 and two missense mutations in PS2 were identified which are causative for FAD. PS mutations lead to the same functional consequence as mutations on amyloid precursor protein (APP), altering the processing of APP towards the release of the more amyloidogenic form 1-42 of Abeta (Abeta42). In this regard, the physical interaction between APP and presenilins in the endoplasmic reticulum has been demonstrated and might play a key role in Abeta42 production. It was hypothesized that PS1 might directly cleave APP. However, extracellular amyloidogenesis and Abeta production might not be the sole factor involved in AD pathology and several lines of evidence support a role of apoptosis in the massive neuronal loss observed. Presenilins were shown to modify the apoptotic response in several cellular systems including primary neuronal cultures. Some evidence is accumulating which points towards the beta-catenin signaling pathways to be causally involved in presenilin mediated cell death. Increased degradation of beta-catenin has been shown in brain of AD patients with PS1 mutations and reduced beta-catenin signaling increased neuronal vulnerability to apoptosis in cell culture models. The study of presenilin physiological functions and the pathological mechanisms underlying their role in pathogenesis clearly advanced our understanding of cellular mechanisms underlying the neuronal cell death and will contribute to the identification of novel drug targets for the treatment of AD.

Human aspartic protease memapsin 2 cleaves the beta-secretase site of beta-amyloid precursor protein

The cDNAs of two new human membrane-associated aspartic proteases, memapsin 1 and memapsin 2, have been cloned and sequenced. The deduced amino acid sequences show that each contains the typical pre, pro, and aspartic protease regions, but each also has a C-terminal extension of over 80 residues, which includes a single transmembrane domain and a C-terminal cytosolic domain. Memapsin 2 mRNA is abundant in human brain. The protease domain of memapsin 2 cDNA was expressed in Escherichia coli and was purified. Recombinant memapsin 2 specifically hydrolyzed peptides derived from the beta-secretase site of both the wild-type and Swedish mutant beta-amyloid precursor protein (APP) with over 60-fold increase of catalytic efficiency for the latter. Expression of APP and memapsin 2 in HeLa cells showed that memapsin 2 cleaved the beta-secretase site of APP intracellularly. These and other results suggest that memapsin 2 fits all of the criteria of beta-secretase, which catalyzes the rate-limiting step of the in vivo production of the beta-amyloid (Abeta) peptide leading to the progression of Alzheimer's disease. Recombinant memapsin 2 also cleaved a peptide derived from the processing site of presenilin 1, albeit with poor kinetic efficiency. Alignment of cleavage site sequences of peptides indicates that the specificity of memapsin 2 resides mainly at the S(1)' subsite, which prefers small side chains such as Ala, Ser, and Asp.

Mutational analysis of intrinsic regions of presenilin 2 that determine its endoproteolytic cleavage and pathological function

To investigate the significance of endoproteolytic processing of presenilin 2 (PS2) on its pathological function, we constructed PS2 cDNAs causing amino acid substitutions or deletions around the cleavage site. We found that a PS2 mutant (Del3) with a 20-amino acid deletion was not endoproteolytically processed, while other PS2s with amino acid substitutions and short deletions were cleaved. Overproduction of all the mutant proteins led to a compensatory decrease of endogenous PS1 fragments, but did not affect the amyloid beta peptide X-42/Abeta X-40 ratio without the familial Alzheimer's disease mutation. The Del3 mutant did not exhibit significant deficits in gamma-secretase activity. The turnover rate of the Del3 holoprotein was the same as that of full-length PS2. These data suggest that the determinants of the PS2 cleavage site reside within a large region and that the pathological function of PS2 is exerted by familial Alzheimer's disease mutations not related to the cleavage of holoproteins. We also found that PS2 with an 18-amino acid deletion at the C-terminal end was not processed. Its overexpression led neither to diminished accumulation of endogenous PS1 fragments nor to increased production of amyloid beta peptide X-42. The C-terminal end of PS2 seems to possess the signal for entry into the processing pathway.

The transmembrane aspartates in presenilin 1 and 2 are obligatory for gamma-secretase activity and amyloid beta-protein generation

The discovery that a deficiency of presenilin 1 (PS1) decreases the production of amyloid beta-protein (Abeta) identified the presenilins as important mediators of the gamma-secretase cleavage of beta-amyloid precursor protein (APP). Recently, we found that two conserved transmembrane (TM) aspartates in PS1 are critical for Abeta production, providing evidence that PS1 either functions as a required diaspartyl cofactor for gamma-secretase or is itself gamma-secretase. Presenilin 2 (PS2) shares substantial sequence and possibly functional homology with PS1. Here, we show that the two TM aspartates in PS2 are also critical for gamma-secretase activity, providing further evidence that PS2 is functionally homologous to PS1. Cells stably co-expressing TM Asp --> Ala mutations in both PS1 and PS2 show further accumulation of the APP-derived gamma-secretase substrates, C83 and C99. The production of Abeta is reduced to undetectable levels in the conditioned media of these cells. Furthermore, endoproteolysis of the exogenous Asp mutant PS2 is absent, and endogenous PS1 C-terminal fragments are diminished to undetectable levels. Therefore, the co-expression of PS1 and PS2 TM Asp --> Ala mutants suppresses the formation of any detectable PS1 or PS2 heterodimeric fragments and essentially abolishes the production of Abeta. These results explain the residual Abeta production seen in PS1-deficient cells and demonstrate the absolute requirement of functional presenilins for Abeta generation. We conclude that presenilins, and their TM aspartates in particular, are attractive targets for lowering Abeta therapeutically to prevent Alzheimer's disease.

Molecular genetics of Alzheimer's disease

Application of genetic paradigms to Alzheimer's disease (AD) has led to confirmation that genetic factors play a role in this disease. Additionally, researchers now understand that AD is genetically heterogeneous and that some genetic isoforms appear to have similar or related biochemical consequences. Genetic epidemiologic studies indicate that first-degree relatives of AD probands have an age-dependent risk for AD approximately equal to 38% by age 90 years (range 10% to 50%). This incidence strongly suggests that transmission may be more complicated than a simple autosomal dominant trait. Nevertheless, a small proportion of AD cases with unequivocal autosomal dominant transmission have been identified. Studies of these autosomal dominant familial AD (FAD) pedigrees have thus far identified four distinct FAD genes. The beta-amyloid precursor protein (beta APP) gene (on chromosome 21), the presenilin 1 (PS1) gene (on chromosome 14), and the presenilin 2 (PS2) gene (on chromosome 1) gene are all associated with early-onset AD. Missense mutations in these genes cause abnormal beta APP processing with resultant overproduction of A beta 42 peptides. In addition, the epsilon 4 allele of apolipoprotein E (APOE) is associated with a increased risk for late-onset AD. Although attempts to develop symptomatic treatments based on neurotransmitter replacement continue, some laboratories are attempting to design treatments that will modulate production or disposition of A beta peptides.

Posttranslational modification and plasma membrane localization of the Drosophila melanogaster presenilin

Mutations in two genes, presenilin 1 (PS1) and presenilin 2, are linked to early onset cases of familial Alzheimer's disease. The presenilins are thought to contribute to the pathogenesis of Alzheimer's disease by directly or indirectly affecting the proteolytic processing of the amyloid precursor protein. They have also been implicated in the proteolytic processing of Notch. In PS1-deficient mammalian cells, the proteolytic release of the Notch intracellular domain is reduced. Likewise, loss-of-function mutations in Drosophila presenilin (Psn) prevent the production of the intracellular Notch signaling fragment and lead to phenotypes resembling Notch mutants. Here we characterize the Drosophila Psn protein and demonstrate that it undergoes a proteolytic cleavage. We describe Psn expression at different developmental stages of the fly and show Psn localization near both apical and basal plasma membranes. Furthermore, we demonstrate that portions of the Psn protein span the plasma membrane in S2 cells.

Cell surface presenilin-1 participates in the gamma-secretase-like proteolysis of Notch

Presenilin-1 (PS1), a polytopic membrane protein primarily localized to the endoplasmic reticulum, is required for efficient proteolysis of both Notch and beta-amyloid precursor protein (APP) within their trans- membrane domains. The activity that cleaves APP (called gamma-secretase) has properties of an aspartyl protease, and mutation of either of the two aspartate residues located in adjacent transmembrane domains of PS1 inhibits gamma-secretase processing of APP. We show here that these aspartates are required for Notch processing, since mutation of these residues prevents PS1 from inducing the gamma-secretase-like proteolysis of a Notch1 derivative. Thus PS1 might function in Notch cleavage as an aspartyl protease or di-aspartyl protease cofactor. However, the ER localization of PS1 is inconsistent with that hypothesis, since Notch cleavage occurs near the cell surface. Using pulse-chase and biotinylation assays, we provide evidence that PS1 binds Notch in the ER/Golgi and is then co-transported to the plasma membrane as a complex. PS1 aspartate mutants were indistinguishable from wild-type PS1 in their ability to bind Notch or traffic with it to the cell surface, and did not alter the secretion of Notch. Thus, PS1 appears to function specifically in Notch proteolysis near the plasma membrane as an aspartyl protease or cofactor.

The influence of endoproteolytic processing of familial Alzheimer's disease presenilin 2 on abeta42 amyloid peptide formation

Mutant presenilins (PS) contribute to the pathogenesis of familial Alzheimer's disease (FAD) by enhancing the production of Abeta42 from beta-amyloid precursor protein. Presenilins are endoproteolytically processed to N-terminal and C-terminal fragments, which together form a stable 1:1 complex. We have mapped the cleavage site in the PS2 protein by direct sequencing of its C-terminal fragment isolated from mouse liver. Three different N-terminal residues were identified starting at Val-299, Thr-301, and Leu-307 that correspond closely to the previously described N termini of the C-terminal fragment of human PS1. Mutational analysis of the PS2 cleavage site indicates that the principal endoproteolytic cleavage occurs at residues Met-298/Val-299 and that the N terminus is subsequently modified by secondary proteolytic cleavages. We have generated cleavage defective PS2 constructs, which accumulate exclusively as full-length polypeptides in transfected Neuro2a cells. Functional analysis of such cleavage defective PS2 carrying the FAD mutation Asn-141 --> Ile showed that its Abeta42 producing activity was strongly reduced compared with cleavage-competent FAD PS2. In contrast, cleavage defective PS2 was active in rescuing the egg-laying defect of a sel-12 mutant in Caenorhabditis elegans. We conclude that PS2 endoproteolytic cleavage is not an absolute requirement for its activities but may rather selectively enhance or stabilize its functions.

Presenilin-1 forms complexes with the cadherin/catenin cell-cell adhesion system and is recruited to intercellular and synaptic contacts

In MDCK cells, presenilin-1 (PS1) accumulates at intercellular contacts where it colocalizes with components of the cadherin-based adherens junctions. PS1 fragments form complexes with E-cadherin, beta-catenin, and alpha-catenin, all components of adherens junctions. In confluent MDCK cells, PS1 forms complexes with cell surface E-cadherin; disruption of Ca(2+)-dependent cell-cell contacts reduces surface PS1 and the levels of PS1-E-cadherin complexes. PS1 overexpression in human kidney cells enhances cell-cell adhesion. Together, these data show that PS1 incorporates into the cadherin/catenin adhesion system and regulates cell-cell adhesion. PS1 concentrates at intercellular contacts in epithelial tissue; in brain, it forms complexes with both E- and N-cadherin and concentrates at synaptic adhesions. That PS1 is a constituent of the cadherin/catenin complex makes that complex a potential target for PS1 FAD mutations.

Staurosporine-induced activation of caspase-3 is potentiated by presenilin 1 familial Alzheimer's disease mutations in human neuroglioma cells

Familial Alzheimer's disease (FAD) mutant forms of presenilin 1 (PS1) and 2 have been shown to sensitize cells to apoptotic cell death. Here we explore the effects of FAD mutant forms of PS1 on caspase activation during apoptosis. We show that caspase activation leads to increased generation of alternative C-terminal fragments (CTFs) from mutant as compared to wild-type (wt) PS1. For this purpose, very low expression levels of wt, A246E, L286V, and deltaE10 FAD mutant PS1 proteins in stably transfected human H4 neuroglioma cells were used to avoid artifactual induction of spontaneous apoptosis due to overexpression of PS1. Staurosporine treatment of these cells resulted in increased cell death and up to a 10-fold increase in caspase-3 activation in mutant versus wt PS1-expressing cell lines. Correspondingly, relative levels of caspase-cleaved PS1 CTFs were increased by five- to sixfold in the FAD mutant versus wt PS1 cells. Elevated caspase activation and caspase cleavage of FAD mutant PS1 suggest the possibility of either a direct proapoptotic effect of mutant PS1 or interference of mutant PS1 with antiapoptotic effects of wt PS1.

Drosophila presenilin is required for neuronal differentiation and affects notch subcellular localization and signaling

Presenilins are a highly conserved family of proteins first identified as causative genes in early onset familial Alzheimer's disease. Recent studies have suggested a role for presenilins in the Notch-signaling pathway, but their specific function within this pathway remains unclear. Here, we have characterized the Drosophila presenilin gene and protein and studied their interaction with Notch in both mutants and transgenics. We find that the Drosophila presenilin protein is proteolytically cleaved and broadly expressed during development with the highest levels in neurons within the larval CNS. We also show that mutations in Drosophila presenilin (Dps) genetically interact with Notch and result in an early pupal-lethal phenotype characterized by defects in eye and wing development and incomplete neuronal differentiation within the larval CNS. Moreover, we find that processing of Notch in the Golgi by the furin protease is unaffected in Dps mutants and that Notch is present and may even accumulate on the plasma membrane of neuroblasts in the larval CNS of Dps mutants. In contrast, overexpression of Dps in transgenics causes Notch to accumulate in the cytoplasm. Taken together, these results indicate that Drosophila presenilin is required for proper neuronal differentiation and may regulate the subcellular localization of Notch proteins within cells, necessary for their accumulation and subsequent signaling capabilities.

Interaction of Alzheimer's presenilin-1 and presenilin-2 with Bcl-X(L). A potential role in modulating the threshold of cell death

The familial Alzheimer's disease gene products, presenilin-1 and presenilin-2, have been reported to be functionally involved in amyloid precursor protein processing, notch receptor signaling, and programmed cell death or apoptosis. However, the molecular mechanisms by which presenilins regulate these processes remain unknown. With regard to the latter, we describe a molecular link between presenilins and the apoptotic pathway. Bcl-X(L), an anti-apoptotic member of the Bcl-2 family was shown to interact with the carboxyl-terminal fragments of PS1 and PS2 by the yeast two-hybrid system. In vivo interaction analysis revealed that both PS2 and its naturally occurring carboxyl-terminal products, PS2short and PS2Ccas, associated with Bcl-X(L), whereas the caspase-3-generated amino-terminal PS2Ncas fragment did not. This interaction was corroborated by demonstrating that Bcl-X(L) and PS2 partially co-localized to sites of the vesicular transport system. Functional analysis revealed that presenilins can influence mitochondrial-dependent apoptotic activities, such as cytochrome c release and Bax-mediated apoptosis. Together, these data support a possible role of the Alzheimer's presenilins in modulating the anti-apoptotic effects of Bcl-X(L).

Membrane topology of Alzheimer's disease-related presenilin 1. Evidence for the existence of a molecular species with a seven membrane-spanning and one membrane-embedded structure

A significant member of early-onset familial type of Alzheimer's disease cases has been shown to be caused by dominant mutations in either of the two genes encoding presenilin 1 (PS1) and presenilin 2 (PS2). These two proteins are highly homologous to each other and have been reported to be mainly localized to the membranes of intracellular compartments such as the endoplasmic reticulum. Information about the membrane topological structures of these proteins is indispensable for understanding their physiological and pathological roles. Although several models have been proposed previously, their precise membrane topologies remain unknown. In this study, we examined this issue in detail by expressing a series of C-terminally deleted PS1 mutants fused to the hydrophilic portion of Escherichia coli leader peptidase in vitro using a reticulocyte lysate in the presence of microsomal membranes. Our results predict that PS1 exists mainly in a seven membrane-spanning structure with its C-terminal end exposed to the luminal space. This was also confirmed by expressing these fusion proteins in cultured cells. We further showed that a ninth hydrophobic segment is tightly bound to the membrane without spanning it. Based on the above observations, we propose a novel "seven membrane-spanning and one membrane-embedded" topological model for presenilins.

Twenty-nine missense mutations linked with familial Alzheimer's disease alter the processing of presenilin 1

1. Full-length form of human presenilin 1 (PS1) is processed and an N-terminal fragment (28 KD) and C-terminal fragment (19 KD) are generated. To elucidate the possible role of presenilin mutations in Alzheimer's disease (AD), the authors analyze the effects of AD-linked mutations on PS1 processing in cultured cells. 2. Complementary DNAs encoding genes for human PS1 harboring twenty-nine missense mutations linked with familial Alzheimer's disease (FAD) were introduced into PC12 cells. Human PS1 exogenously expressed in the cells was detected by immunoblotting using a monoclonal antibody that recognized the N-terminal region of human PS1. The amounts of full-length form (48 KD) and N-terminal fragment (28 KD) of PS1 was quantified by densitometrical analysis. 3. The ratio of the N-terminal fragment to total PS1 was reduced by twenty-nine mutations. The specific effects on PS1 processing varied according to mutation. 4. These results suggest that AD-linked missense mutations of PS1 are involved in neurodegeneration via inhibition of PS1 processing.

Processing of wild-type and mutant familial Alzheimer's disease-associated presenilin-1 in cultured neurons

Mutations of presenilin (PS)-1, an endoplasmic reticulum/Golgi transmembrane protein, have been associated with early-onset familial Alzheimer's disease (FAD). In mammalian brain, PS1 exists primarily as its processed fragments; however, the role of this cleavage event in PS1 function remains unclear. Although some investigators have shown that mutant PS1 processing is unaltered (with the exception of PS1-deltaE9, which lacks the cleavage site) in stably transfected cells and PS1-FAD transgenic mice, other investigators have reported altered FAD mutant PS1 and PS2 protein processing in transiently transfected cells and human FAD patients. The present study uses recombinant replication-defective adenoviral vectors to transiently express wild-type (WT) or mutant PS1 in various cells, including primary cultured hippocampal neurons. We show that in contrast to PS1-WT, overexpression of mutant PS1 results in an increased ratio of mutant holoprotein to endoproteolytic products that is dependent on cell type and differentiation state. In addition, mutant PS1 overexpression leads to an increase in caspase-type protease derived fragments above that seen with PS1-WT overexpression. Furthermore, overexpression of at least one mutant significantly alters the processing of coexpressed PS1-WT, suggesting that mutant PS1 may affect PS1-WT function. These findings suggest that a defect in PS1 holoprotein stability may be a general defect seen in cells expressing mutant PS1, especially neuronal cells, and may play a critical role in the pathogenesis of FAD.

Translating cell biology into therapeutic advances in Alzheimer's disease

Studies of the molecular basis of Alzheimer's disease exemplify the increasingly blurred distinction between basic and applied biomedical research. The four genes so far implicated in familial Alzheimer's disease have each been shown to elevate brain levels of the self-aggregating amyloid-beta protein, leading gradually to profound neuronal and glial alteration, synaptic loss and dementia. Progress in understanding this cascade has helped to identify specific therapeutic targets and provides a model for elucidating other neurodegenerative disorders.

Analysis of presenilin 1 and presenilin 2 expression and processing by newly developed monoclonal antibodies

Because distinct mutations in presenilin 1 and presenilin 2 are a major cause of early-onset familial Alzheimer's disease, we generated four monoclonal antibodies for the identification, localization, and investigation of presenilins in various cell lines and tissues from patients and controls. We show that these antibodies are specific for the N- and C-terminal domains of human presenilin 1 and presenilin 2. They recognize presenilin full-length proteins and their approximately 28-35 kDa N-terminal fragments and approximately 18-20 kDa C-terminal fragments. None of the antibodies showed cross-reaction in their specific detection ability. We demonstrated that presenilin 1 and presenilin 2 are proteolytically processed in human glioma cell lines, transfected and untransfected human neuroblastoma SH-SY5Y cells, COS-7 cells, rat cerebellar neuronal ST15 cells, mouse and human brain. Remarkably, we observed that presenilin 2 is alternatively cleaved during apoptosis, producing smaller C-terminal fragments. By analyzing the subcellular distribution of presenilins, we found reticular and fine vesicular staining throughout the cell bodies. In addition, staining of Golgi compartments and the perinuclear envelope was observed. Alzheimer's disease brain showed strong immunoreactivity of presenilin 1 in reactive astrocytes and senile plaques. This high expression of presenilin 1 may explain the increased production and accumulation of the amyloid-beta peptide in patients with sporadic Alzheimer's disease in the absence of familial presenilin mutation.

Evidence that intramolecular associations between presenilin domains are obligatory for endoproteolytic processing

Mutations in genes encoding presenilins (PS1 and PS2) cosegregate with the majority of early onset cases of familial Alzheimer's disease. PS1 and PS2 are polytopic membrane proteins that undergo endoproteolytic cleavage to generate stable NH2- and COOH-terminal derivatives (NTF and CTF, respectively). Several lines of evidence suggest that the endoproteolytic derivatives are likely the functional units of PS in vivo. In the present report, we examine the disposition of PS NTF and CTF assemblies in stable mouse N2a neuroblastoma cell lines expressing human PS polypeptides. We show that exogenous expression of PS1 NTFs neither assemble with endogenous CTF nor exhibit dominant negative inhibitory effects on the endogenous PS1 cleavage and the accumulation of derivatives. In cells co-expressing PS1 and PS2, PS1- and PS2-derived fragments do not form mixed assemblies. In contrast, cells expressing a chimeric PS1/PS2 polypeptide form stable PS1 NTF-PS2 CTF assemblies. Moreover, expression of chimeric PS1/PS2 polypeptides harboring a familial early onset AD-linked mutation (M146L) elevates the production of Abeta42 peptides. Our results provide evidence that assembly of structural domains contained within NH2- and COOH-terminal regions of PS occur prior to endoproteolytic cleavage.

Two transmembrane aspartates in presenilin-1 required for presenilin endoproteolysis and gamma-secretase activity

Accumulation of the amyloid-beta protein (Abeta) in the cerebral cortex is an early and invariant event in the pathogenesis of Alzheimer's disease. The final step in the generation of Abeta from the beta-amyloid precursor protein is an apparently intramembranous proteolysis by the elusive gamma-secretase(s). The most common cause of familial Alzheimer's disease is mutation of the genes encoding presenilins 1 and 2, which alters gamma-secretase activity to increase the production of the highly amyloidogenic Abeta42 isoform. Moreover, deletion of presenilin-1 in mice greatly reduces gamma-secretase activity, indicating that presenilin-1 mediates most of this proteolytic event. Here we report that mutation of either of two conserved transmembrane (TM) aspartate residues in presenilin-1, Asp 257 (in TM6) and Asp 385 (in TM7), substantially reduces Abeta production and increases the amounts of the carboxy-terminal fragments of beta-amyloid precursor protein that are the substrates of gamma-secretase. We observed these effects in three different cell lines as well as in cell-free microsomes. Either of the Asp --> Ala mutations also prevented the normal endoproteolysis of presenilin-1 in the TM6 --> TM7 cytoplasmic loop. In a functional presenilin-1 variant (carrying a deletion in exon 9) that is associated with familial Alzheimer's disease and which does not require this cleavage, the Asp 385 --> Ala mutation still inhibited gamma-secretase activity. Our results indicate that the two transmembrane aspartate residues are critical for both presenilin-1 endoproteolysis and gamma-secretase activity, and suggest that presenilin 1 is either a unique diaspartyl cofactor for gamma-secretase or is itself gamma-secretase, an autoactivated intramembranous aspartyl protease.

Direct interaction of Alzheimer's disease-related presenilin 1 with armadillo protein p0071

Alzheimer's disease-related presenilins are thought to be involved in Notch signaling during embryonic development and/or cellular differentiation. Proteins mediating the cellular functions of the presenilins are still unknown. We utilized the yeast two-hybrid system to identify an interacting armadillo protein, termed p0071, that binds specifically to the hydrophilic loop of presenilin 1. In vivo, the presenilins constitutively undergo proteolytic processing, forming two stable fragments. Here, we show that the C-terminal fragment of presenilin 1 directly binds to p0071. Nine out of 10 armadillo repeats in p0071 are essential for mediating this interaction. Since armadillo proteins, like beta-catenin and APC, are known to participate in cellular signaling, p0071 may function as a mediator of presenilin 1 in signaling events.

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.

Characterization of detergent-insoluble complexes containing the familial Alzheimer's disease-associated presenilins

Many cases of early-onset familial Alzheimer's disease have been linked to mutations within two genes encoding the proteins presenilin-1 and presenilin-2. The presenilins are 48-56-kDa proteins that can be proteolytically cleaved to generate an N-terminal fragment (approximately 25-35 kDa) and a C-terminal fragment (approximately 17-20 kDa). The N- and C-terminal fragments of presenilin-1, but not full-length presenilin-1, were readily detected in both human and mouse cerebral cortex and in neuronal and glioma cell lines. In contrast, presenilin-2 was detected almost exclusively in cerebral cortex as the full-length molecule with a molecular mass of 56 kDa. The association of the presenilins with detergent-insoluble, low-density membrane microdomains, following the isolation of these structures from cerebral cortex by solubilization in Triton X-100 and subsequent sucrose density gradient centrifugation, was also examined. A minor fraction (10%) of both the N- and C-terminal fragments of presenilin-1 was associated with the detergent-insoluble, low-density membrane microdomains, whereas a considerably larger proportion of full-length presenilin-2 was present in the same membrane microdomains. In addition, a significant proportion of full-length presenilin-2 was present in a high-density, detergent-insoluble cytoskeletal pellet enriched in beta-actin. The presence of the presenilins in detergent-insoluble, low-density membrane microdomains indicates a possible role for these specialized regions of the membrane in the lateral separation of Alzheimer's disease-associated proteins within the lipid bilayer and/or in the distinct functions of these proteins.

Carboxyl-terminal fragments of presenilin-1 are closely related to cytoskeletal abnormalities in Alzheimer's brains

To clarify the role of presenilin-1 (PS-1) in the pathology of Alzheimer's disease (AD), we tested four antisera to PS-1. The specific antisera to the N-terminus (HSN-2) and C-terminus (HS-C) of PS-1 detected a 44/40kD holoprotein, a 25kD N-terminal fragment (NTF) and a 16kD C-terminal fragment (CTF) of PS-1 in COS-7 cells. The 25kD NTF and 16kD CTF were observed in human brains, and their amounts were not significantly different between the control and AD brains. The antibody HS-C labeled extensive neurofibrillary tangles, dystrophic neurites and curly fibers in the AD brains. In the paired helical filament (PHF) fraction containing A68 protein from AD brains, a smear pattern of CTFs was revealed. Antisera (HS-L292 and HS-L300) to cleavage sites of PS-1 also revealed immunoreactive neurofibrillary tangles in the AD brain sections and the smear pattern of CTFs of A68 protein fraction. The CTFs of PS-1 accumulate with PHF tau, suggesting a close relationship between PS-1 and cytoskeletal abnormalities in AD brains.

The biological and pathological function of the presenilin-1 Deltaexon 9 mutation is independent of its defect to undergo proteolytic processing

The two homologous presenilins are key factors for the generation of amyloid beta-peptide (Abeta), since Alzheimer's disease (AD)-associated mutations enhance the production of the pathologically relevant 42-amino acid Abeta (Abeta42), and a gene knockout of presenilin-1 (PS1) significantly inhibits total Abeta production. Presenilins undergo proteolytic processing within the domain encoded by exon 9, a process that may be closely related to their biological and pathological activity. An AD-associated mutation within the PS1 gene deletes exon 9 (PS1Deltaexon9) due to a splicing error and results in the accumulation of the uncleaved full-length protein. We now demonstrate the unexpected finding that the pathological activity of PS1Deltaexon9 is independent of its lack to undergo proteolytic processing, but is rather due to a point mutation (S290C) occurring at the aberrant exon 8/10 splice junction. Mutagenizing the cysteine residue at position 290 to the original serine residue completely inhibits the pathological activity in regard to the elevated production of Abeta42. Like PS1Deltaexon9, the resulting presenilin variant (PS1Deltaexon9 C290S) accumulates as an uncleaved protein and fully replaces endogenous presenilin fragments. Moreover, PS1Deltaexon9 C290S exhibits a significantly increased biological activity in a highly sensitive in vivo assay as compared with the AD-associated mutation. Therefore not only the increased Abeta42 production but also the decreased biological function of PS1Deltaexon9 is due to a point mutation and independent of the lack of proteolytic processing.

Evidence for a physical interaction between presenilin and Notch

Genetic analyses in Caenorhabditis elegans demonstrate that sel-12 and hop-1, homologues of the Alzheimer's disease-associated presenilin genes, modify signaling through LIN-12 and GLP-1, homologues of the Notch cell surface receptor. To gain insight into the biochemical basis of this genetic interaction, we tested the possibility that presenilin-1 (PS1) physically associates with the Notch1 receptor in mammalian cells. Notch1 and PS1 coimmunoprecipitated from transiently transfected human embryonic kidney 293 cell lysates in a detergent-sensitive manner, consistent with a noncovalent physical association between the two proteins. The interaction predominantly occurred early in the secretory pathway prior to Notch cleavage in the Golgi, because PS1 immunoprecipitation preferentially recovered the full-length Notch1 precursor. When PS1 was immunoprecipitated from 293 cells that had been metabolically labeled with [35S]methionine and [35S]cysteine, Notch1 was the primary protein detected in PS1 immunoprecipitates, suggesting that this interaction is specific. Furthermore, endogenous Notch and presenilin coimmunoprecipitated from cultured Drosophila cells, indicating that physical interaction can occur at physiological expression levels. These results suggest that the genetic relationship between presenilins and the Notch signaling pathway derives from a direct physical association between these proteins in the secretory pathway.

Presenilins interact with armadillo proteins including neural-specific plakophilin-related protein and beta-catenin

Missense substitutions in the presenilin 1 (PS1) and presenilin 2 (PS2) proteins are associated with early-onset familial Alzheimer's disease. We have used yeast-two-hybrid and coimmunoprecipitation methods to show that the large cytoplasmic loop domains of PS1 and PS2 interact specifically with three members of the armadillo protein family, including beta-catenin, p0071, and a novel neuronal-specific armadillo protein--neural plakophilin-related armadillo protein (NPRAP). The PS1:NPRAP interaction occurs between the arm repeats of NPRAP and residues 372-399 at the C-terminal end of the large cytoplasmic loop of PS1. The latter residues contain a single arm-like domain and are highly conserved in the presenilins, suggesting that they form a functional armadillo protein binding site for the presenilins.

Effects of presenilin N-terminal fragments on production of amyloid beta peptide and accumulation of endogenous presenilins

To clarify the effects of the proteolytic cleavage of presenilin 1 (PS1) and presenilin 2 (PS2) proteins on their functions, we established stable cell lines which expressed the physiologically cleaved N-terminal fragment (NTF) with or without mutations of familial Alzheimer's disease (FAD). We found that exogenous expression of the PS1-NTF or PS2-NTF harboring FAD mutations was insufficient for increased production of amyloidogenic A beta X-42 peptide and that the overexpressed NTFs had no effect on the accumulation of endogenous presenilin fragments.

Identification of caspases that cleave presenilin-1 and presenilin-2. Five presenilin-1 (PS1) mutations do not alter the sensitivity of PS1 to caspases

Mutations in the presenilin (PS) genes PSI and PS2 are involved in Alzheimer's disease (AD). Recently, apoptosis-associated cleavage of PS proteins was identified. Here we demonstrate that PS1 as well as PS2 are substrates for different members of the caspase protein family. Remarkably, the caspases acting on PS1 could be subdivided in two groups. One group, containing caspase-8, -6 and -11, cleaved PSI after residues ENDD329 and to a lesser extent after residues AQRD341. A second group consisting of caspase-3, -7 and -1 acted uniquely on AQRD341. Importantly, these two cleavage sites were also recognized by caspases in the C-terminal PS1 fragment produced by constitutive proteolysis. In decreasing order of activity, caspase-8, -3, -1, -6 and -7 proteolysed PS2 at the recognition site D326SYD329. Caspase-8 and -3 exhibited the highest proteolytic activity on both PS1 and PS2. PS1 and PS2 were not hydrolyzed by caspase-2 and PS2 also not by caspase-11. None of five missense mutations affected the sensitivity of PSI to caspase-mediated cleavage. This suggests that AD pathogenesis associated with PS1 missense mutations cannot be explained by a change in caspase-dependent processing.

Alzheimer's disease: genetic studies and transgenic models

Recent advances in a variety of areas of research, particularly in genetics and in transgenic (Tg)/gene targeting approaches, have had a substantial impact on our understanding of Alzheimer's disease (AD) and related disorders. After briefly reviewing the progress that has been made in diagnostic assessments of patients with senile dementia and in investigations of the neuropathology of AD, we discuss some of the genes/proteins that are causative or risk factors for this disease, including those encoding amyloid precursor protein, presenilin 1 and 2, and apolipoprotein E. In addition, we comment on several potential new candidate loci/genes. Subsequently, we review selected recent reports of analyses of a variety of lines of Tg mice that show several neuropathological features of AD, including A beta-amyloid deposits and dystrophic neurites. Finally, we discuss the several important issues in future investigations of Tg mice, with particular emphasis on the influences of genetic strains on phenotype, especially behavior, and strategies for making new models of neurodegenerative disorders. We believe that investigations of these Tg models will (a) enhance understanding of the relationships between impaired performance on memory tasks and the pathological/biochemical abnormalities in brain, (b) help to clarify pathogenic mechanisms in vivo, (c) lead to identification of new therapeutic targets, and (d) allow testing of new treatment strategies first in mice and then, if successful, in humans with AD.

Impaired proteolytic processing of presenilin-1 in chromosome 14-linked familial Alzheimer's disease patient lymphocytes

Many cases of early-onset familial Alzheimer's disease (FAD) are caused by mutations in the presenilin 1 (PS1) and PS2 genes. PS1 protein is generated as a 47 kDa protein and is endoproteolytically cleaved into N-terminal 28 kDa and C-terminal 19 kDa fragments in vivo. To examine whether mutated PS1 protein is abnormally metabolized, we performed immunoblot analysis of lymphoblasts from familial Alzheimer's disease patients and controls. More full-length PS1 was apparently detected in samples from PS1 mutants than those from PS2 mutant and controls. This result suggests that impaired proteolysis of PS1 may be associated with the pathogenesis of FAD. Moreover, our simple test using lymphocytes from FAD patients might be useful from a diagnostic point of view.

Dual roles of proteasome in the metabolism of presenilin 1

Presenilin 1 (PS1) has been identified as a causative gene for most early-onset familial Alzheimer's disease. Biochemical studies revealed that PS1 exists predominantly as two processed fragments in cells and brain tissues. We prepared stably transfected cells expressing the wild-type and familial Alzheimer's disease-associated mutants of PS1 and investigated the enzyme that participates in the metabolism of PS1. After treatment of the cells with proteasome inhibitors, the full-length PS1 was significantly accumulated. The levels of N- and C-terminal fragments were also increased. The accumulation of PS1 with a deletion of exon 10, which is unable to be processed, on treatment of the transfected cells with lactacystin indicated that proteasome can degrade full-length PS1. A synthetic peptide that includes the processing region of PS1 was cleaved by 20S proteasome at the putative processing sites after Met288 and Glu299. Metabolic labeling experiments showed that the appearance of the N-terminal fragment was attenuated by the inhibitor. Finally, 28-kDa N- and 20-kDa C-terminal fragments were generated by purified PS1 in vitro. These data indicated that the proteasome pathway is involved in PS1 processing. These results demonstrate that the proteasome pathway plays dual roles in processing and degradation of PS1.

Abrogation of the presenilin 1/beta-catenin interaction and preservation of the heterodimeric presenilin 1 complex following caspase activation

beta-Catenin has previously been shown to interact with presenilin 1 (PS1) in transfected cells. Here we report that beta-catenin co-immunoprecipitates with the endogenous C-terminal fragment of presenilin 1 (PS1-CTF) but not with the endogenous CTF of presenilin 2 (PS2-CTF) in H4 human neuroglioma cells. During staurosporine (STS)-induced cell death, beta-catenin and PS1-CTF undergo a caspase-mediated cleavage. After 12 h of STS treatment, the beta-catenin.PS1-CTF interaction is abrogated. While PS1-CTF immunoprecipitated with all caspase-cleaved species of beta-catenin, beta-catenin holoprotein did not co-immunoprecipitate with the "alternative" caspase-derived PS1-CTF (PS1-aCTF). Thus, the abrogation of the beta-catenin.PS1-CTF complex was due to caspase cleavage of PS1-CTF. beta-Catenin co-immunoprecipitated with PS1-NTF, but only when PS1-NTF was associated with PS1-CTF. Even though PS1-NTF.CTF complex stability was not altered by caspase cleavage, its ability to bind beta-catenin was abolished. Thus, while the PS1-NTF.CTF complex is preserved after caspase cleavage, it may no longer be fully functional.

Expression of Alzheimer's disease-associated presenilin-1 is controlled by proteolytic degradation and complex formation

Numerous mutations causing early onset Alzheimer's disease have been identified in the presenilin (PS) genes, particularly the PS1 gene. Like the mutations identified within the beta-amyloid precursor protein gene, PS mutations cause the increased generation of a highly neurotoxic variant of amyloid beta-peptide. PS proteins are proteolytically processed to an N-terminal approximately 30-kDa (NTF) and a C-terminal approximately 20-kDa fragment (CTF20) that form a heterodimeric complex. We demonstrate that this complex is resistant to proteolytic degradation, whereas the full-length precursor is rapidly degraded. Degradation of the PS1 holoprotein is sensitive to inhibitors of the proteasome. Formation of a heterodimeric complex is required for the stability of both PS1 fragments, since fragments that do not co-immunoprecipitate with the PS complex are rapidly degraded by the proteasome. Mutant PS fragments not incorporated into the heterodimeric complex lose their pathological activity in abnormal amyloid beta-peptide generation even after inhibition of their proteolytic degradation. The PS1 heterodimeric complex can be attacked by proteinases of the caspase superfamily that generate an approximately 10-kDa proteolytic fragment (CTF10) from CTF20. CTF10 is rapidly degraded most likely by a calpain-like cysteine proteinase. From these data we conclude that PS1 metabolism is highly controlled by multiple proteolytic activities indicating that subtle changes in fragment generation/degradation might be important for Alzheimer's disease-associated pathology.

Genetic neurodegenerative diseases: the human illness and transgenic models

Review The neurodegenerative disorders, a heterogeneous group of chronic progressive diseases, are among the most puzzling and devastating illnesses in medicine. Some of these disorders, such as Alzheimer's disease, amyotrophic lateral sclerosis, the prion diseases, and Parkinson's disease, can occur sporadically and, in some instances, are caused by inheritance of gene mutations. Huntington's disease is acquired in an entirely genetic manner. Transgenic mice that express disease-causing genes recapitulate many features of these diseases. This review provides an overview of transgenic mouse models of familial amyotrophic lateral sclerosis, familial Alzheimer's disease, and Huntington's disease and the emerging insights relevant to the underlying molecular mechanisms of these diseases.

Transgenic mouse models of Alzheimer's disease and amyotrophic lateral sclerosis

Over the past several years, there has been enormous progress in generating transgenic mice that model aspects of human neurodegenerative diseases. These studies build upon the efforts of molecular geneticists who have identified a number of genes that, when mutated, cause familial forms of these diseases. In this review, we focus on the mutations that cause familial forms of Alzheimer's disease (AD) and amyotrophic lateral sclerosis (ALS), and transgenic mouse models that develop clinical and pathological abnormalities resembling those occurring in the human diseases.

Molecular dissection of domains in mutant presenilin 2 that mediate overproduction of amyloidogenic forms of amyloid beta peptides. Inability of truncated forms of PS2 with familial Alzheimer's disease mutation to increase secretion of Abeta42

Mutations in presenilin (PS) 1 or PS2 genes account for the majority of early-onset familial Alzheimer's disease, and these mutations have been shown to increase production of species of amyloid beta peptide (Abeta) ending at residue 42, i.e. the most amyloidogenic form of Abeta. To gain insight into the molecular mechanisms whereby mutant PS induces overproduction of Abeta42, we constructed cDNAs encoding mutant and/or truncated forms of PS2 and examined the secretion of Abeta42 from COS or neuro2a cells transfected with these genes. Cells expressing full-length PS2 harboring both N141I and M239V mutations in the same polypeptide induced overproduction of Abeta42, although the levels of Abeta42 were comparable with those in cells engineered to express PS2 with one or the other of these PS2 mutations. In contrast, cells engineered to express partially truncated PS2 (eliminating the COOH-terminal third of PS2 while retaining the endoproteolytic NH2-terminal fragment) and harboring a N141I mutation, as well as cells expressing COOH-terminal fragments of PS2, did not overproduce Abeta42, and the levels of Abeta42 were comparable with those in cells that expressed full-length, wild-type PS2 or fragments thereof. These data indicate that: (i) the Abeta42-promoting effects of mutant PS2 proteins reach the maximum level with a given single amino acid substitution (i.e. N141I or M239V); and (ii) the expression of full-length mutant PS2 is required for the overproduction of Abeta42. Hence, cooperative interactions of NH2- and COOH-terminal fragments generated from full-length mutant PS2 may be important for the overproduction of Abeta42 that may underlie familial Alzheimer's disease.