A loss of function mutant of the presenilin homologue SEL-12 undergoes aberrant endoproteolysis in Caenorhabditis elegans and increases abeta 42 generation in human cells

Activation of PPARα enhances astroglial uptake and degradation of β-amyloid

[Figure: see text].

Identification of Small Peptides in Human Cerebrospinal Fluid upon Amyloid-β Degradation

BACKGROUND

Amyloid-β (Aβ) degradation in brains of Alzheimer disease patients is a crucial focus for the clarification of disease pathogenesis. Nevertheless, the mechanisms underlying Aβ degradation in the human brain remain unclear.

OBJECTIVE

This study aimed to quantify the levels of small C-terminal Aβ fragments generated upon Aβ degradation in human cerebrospinal fluid (CSF).

METHODS

A fraction containing small peptides was isolated and purified from human CSF by high-pressure liquid chromatography. Degradation products of Aβ C termini were identified and measured by liquid chromatography-tandem mass spectrometry. The C-terminal fragments of Aβ in the conditioned medium of cultured cells transfected with the Swedish variant of βAPP (sw βAPP) were analyzed. These fragments in brains of PS1 I213T knock-in transgenic mice, overexpressing sw βAPP, were also analyzed.

RESULTS

The peptide fragments GGVV and GVV, produced by the cleavage of Aβ40, were identified in human CSF as well as in the brains of the transgenic mice and in the conditioned medium of the cultured cells. Relative to Aβ40 levels, GGVV and GVV levels were 7.6 ± 0.81 and 1.5 ± 0.18%, respectively, in human CSF. Levels of the GGVV fragment did not increase by the introduction of genes encoding neprilysin and insulin-degrading enzyme to the cultured cells.

CONCLUSION

Our results indicate that a substantial amount of Aβ40 in human brains is degraded via a neprilysin- or insulin-degrading enzyme-independent pathway.

Screening for Presenilin Inhibitors Using the Free-Living Nematode, Caenorhabditis elegans

Caenorhabditis elegans contains 3 homologs of presenilin genes that are associated with Alzheimer s disease. Loss-of-function mutations in C. elegans genes cause a defect in egg laying. In humans, loss of presenilin-1 (PS1) function reduces amyloid-beta peptide processing from the amyloid protein precursor. Worms were screened for compounds that block egg laying, phenocopying presenilin loss of function. To accommodate even relatively high throughput screening, a semi-automated method to quantify egg laying was devised by measuring the chitinase released into the culture medium. Chitinase is released by hatching eggs, but little is shed into the medium from the body cavity of a hermaphrodite with an egg laying deficient ( egl) phenotype. Assay validation involved measuring chitinase release from wild-type C. elegans (N2 strain), sel-12 presenilin loss-of-function mutants, and 2 strains of C. elegans with mutations in the egl-36K+channel gene. Failure to find specific presenilin inhibitors in this collection likely reflects the small number of compounds tested, rather than a flaw in screening strategy. Absent defined biochemical pathways for presenilin, this screening method, which takes advantage of the genetic system available in C. elegans and its historical use for anthelminthic screening, permits an entry into mechanism-based discovery of drugs for Alzheimer s disease. ( Journal of Biomolecular Screening 2004:147-152)

α-synuclein regulation of dopamine transporter

The development of effective therapeutic interventions for neurodegeneration requires a better understanding of the early events that precede neuronal loss. Recent work in various disease models has begun to emphasize the significance of presynaptic dysfunction as an early event that occurs before manifestation of neurological disorders. Dysregulation of dopamine (DA) homeostasis is implicated in neurodegenerative diseases, drug addiction, and neuropsychiatric disorders. The neuronal plasma membrane dopamine transporter (DAT) is essential for the maintenance of DA homeostasis in the brain. α-synuclein is a 140-amino acid protein that forms a stable complex with DAT and is linked to the pathogenesis of neurodegenerative disease. In this review we will examine the prevailing hypotheses for α-synuclein-regulation of DAT biology.

Drug development for Alzheimer's disease: recent progress

Alzheimer's disease, the most common cause of dementia, is characterized by two major pathological hallmarks: amyloid plaques and neurofibrillary tangles. Based on these two indicators, an amyloid cascade hypothesis was proposed, and accordingly, most current therapeutic approaches are now focused on the removal of β-amyloid peptides (Aβ from the brain. Additionally, strategies for blocking tau hyperphosphorylation and aggregation have been suggested, including the development of drugs that can block the formation of tangles. However, there are no true disease-modifying drugs in the current market, though many drugs based on theories other than Aβ and tau pathology are under development. The purpose of this review was to provide information on the current development of AD drugs and to discuss the issues related to drug development.

The 28-amino acid form of an APLP1-derived Abeta-like peptide is a surrogate marker for Abeta42 production in the central nervous system

Surrogate markers for the Alzheimer disease (AD)-associated 42-amino acid form of amyloid-β (Aβ42) have been sought because they may aid in the diagnosis of AD and for clarification of disease pathogenesis. Here, we demonstrate that human cerebrospinal fluid (CSF) contains three APLP1-derived Aβ-like peptides (APL1β) that are generated by β- and γ-cleavages at a concentration of ∼4.5 nM. These novel peptides, APL1β25, APL1β27 and APL1β28, were not deposited in AD brains. Interestingly, most γ-secretase modulators (GSMs) and familial AD-associated presenilin1 mutants that up-regulate the relative production of Aβ42 cause a parallel increase in the production of APL1β28 in cultured cells. Moreover, in CSF from patients with pathological mutations in presenilin1 gene, the relative APL1β28 levels are higher than in non-AD controls, while the relative Aβ42 levels are unchanged or lower. Most strikingly, the relative APL1β28 levels are higher in CSF from sporadic AD patients (regardless of whether they are at mild cognitive impairment or AD stage), than those of non-AD controls. Based on these results, we propose the relative level of APL1β28 in the CSF as a candidate surrogate marker for the relative level of Aβ42 production in the brain.

Understanding the molecular basis of Alzheimer's disease using a Caenorhabditis elegans model system

Alzheimer's disease (AD) is the major cause of dementia in the United States. At the cellular level, the brains of AD patients are characterized by extracellular dense plaques and intracellular neurofibrillary tangles whose major components are the beta-amyloid peptide and tau, respectively. The beta-amyloid peptide is a cleavage product of the amyloid precursor protein (APP); mutations in APP have been correlated with a small number of cases of familial Alzheimer's disease. APP is the canonical member of the APP family, whose functions remain unclear. The nematode Caenorhabditis elegans, one of the premier genetic workhorses, is being used in a variety of ways to address the functions of APP and determine how the beta-amyloid peptide and tau can induce toxicity. First, the function of the C. elegans APP-related gene, apl-1, is being examined. Although different organisms may use APP and related proteins, such as APL-1, in different functional contexts, the pathways in which they function and the molecules with which they interact are usually conserved. Second, components of the gamma-secretase complex and their respective functions are being revealed through genetic analyses in C. elegans. Third, to address questions of toxicity, onset of degeneration, and protective mechanisms, different human beta-amyloid peptide and tau variants are being introduced into C. elegans and the resultant transgenic lines examined. Here, we summarize how a simple system such as C. elegans can be used as a model to understand APP function and suppression of beta-amyloid peptide and tau toxicity in higher organisms.

Requirement for small side chain residues within the GxGD-motif of presenilin for gamma-secretase substrate cleavage

gamma-Secretase is a pivotal intramembrane-cleaving protease complex and important drug target for Alzheimer's disease. The protease not only releases small peptides, such as the amyloid-beta peptide, which drives Alzheimer's disease pathogenesis, but also intracellular domains, which can have critical functions in nuclear signaling. Unlike typical aspartyl proteases, gamma-secretase contains a non-classical GxGD active site motif in its catalytic subunit presenilin (PS) 1 or PS2. It is not known whether both glycines are of similar functional relevance and why the glycine residues are invariant elements of the motif. Here we identify the N-terminal glycine of the GxGD motif in PS1, G382, as a critical residue of the active site domain of gamma-secretase. Substitution of G382 by a number of different amino acids abrogated gamma-secretase activity. Only the smallest possible G382A substitution allowed substantial gamma-secretase activity. Depending on the substrate, however, the presence of G382 could become even an absolute functional requirement of gamma-secretase. Very similar results were obtained for the C-terminal glycine residue (G384) of the GxGD motif. Our data thus identify a requirement for small side chain residues in the active site domain of gamma-secretase and suggest that the glycines of the GxGD motif could be evolutionary conserved to allow cleavage of all possible gamma-secretase substrates, including those, which are highly sensitive to minimal alteration of the PS active site domain. These findings broaden our understanding of gamma-secretase substrate recognition and cleavage, which may prove crucial for therapeutic targeting of the enzyme.

Characterization of hepatitis C virus core protein multimerization and membrane envelopment: revelation of a cascade of core-membrane interactions

The molecular basis underlying hepatitis C virus (HCV) core protein maturation and morphogenesis remains elusive. We characterized the concerted events associated with core protein multimerization and interaction with membranes. Analyses of core proteins expressed from a subgenomic system showed that the signal sequence located between the core and envelope glycoprotein E1 is critical for core association with endoplasmic reticula (ER)/late endosomes and the core's envelopment by membranes, which was judged by the core's acquisition of resistance to proteinase K digestion. Despite exerting an inhibitory effect on the core's association with membranes, (Z-LL)(2)-ketone, a specific inhibitor of signal peptide peptidase (SPP), did not affect core multimeric complex formation, suggesting that oligomeric core complex formation proceeds prior to or upon core attachment to membranes. Protease-resistant core complexes that contained both innate and processed proteins were detected in the presence of (Z-LL)(2)-ketone, implying that core envelopment occurs after intramembrane cleavage. Mutations of the core that prevent signal peptide cleavage or coexpression with an SPP loss-of-function D219A mutant decreased the core's envelopment, demonstrating that SPP-mediated cleavage is required for core envelopment. Analyses of core mutants with a deletion in domain I revealed that this domain contains sequences crucial for core envelopment. The core proteins expressed by infectious JFH1 and Jc1 RNAs in Huh7 cells also assembled into a multimeric complex, associated with ER/late-endosomal membranes, and were enveloped by membranes. Treatment with (Z-LL)(2)-ketone or coexpression with D219A mutant SPP interfered with both core envelopment and infectious HCV production, indicating a critical role of core envelopment in HCV morphogenesis. The results provide mechanistic insights into the sequential and coordinated processes during the association of the HCV core protein with membranes in the early phase of virus maturation and morphogenesis.

Intramembrane processing by signal peptide peptidase regulates the membrane localization of hepatitis C virus core protein and viral propagation

Hepatitis C virus (HCV) core protein has shown to be localized in the detergent-resistant membrane (DRM), which is distinct from the classical raft fraction including caveolin, although the biological significance of the DRM localization of the core protein has not been determined. The HCV core protein is cleaved off from a precursor polyprotein at the lumen side of Ala(191) by signal peptidase and is then further processed by signal peptide peptidase (SPP) within the transmembrane region. In this study, we examined the role of SPP in the localization of the HCV core protein in the DRM and in viral propagation. The C terminus of the HCV core protein cleaved by SPP in 293T cells was identified as Phe(177) by mass spectrometry. Mutations introduced into two residues (Ile(176) and Phe(177)) upstream of the cleavage site of the core protein abrogated processing by SPP and localization in the DRM fraction. Expression of a dominant-negative SPP or treatment with an SPP inhibitor, L685,458, resulted in reductions in the levels of processed core protein localized in the DRM fraction. The production of HCV RNA in cells persistently infected with strain JFH-1 was impaired by treatment with the SPP inhibitor. Furthermore, mutant JFH-1 viruses bearing SPP-resistant mutations in the core protein failed to propagate in a permissive cell line. These results suggest that intramembrane processing of HCV core protein by SPP is required for the localization of the HCV core protein in the DRM and for viral propagation.

Regulation of Notch signaling by dynamic changes in the precision of S3 cleavage of Notch-1

Intramembrane proteolysis by presenilin-dependent gamma-secretase produces the Notch intracellular cytoplasmic domain (NCID) and Alzheimer disease-associated amyloid-beta. Here, we show that upon Notch signaling the intracellular domain of Notch-1 is cleaved into two distinct types of NICD species due to diversity in the site of S3 cleavage. Consistent with the N-end rule, the S3-V cleavage produces stable NICD with Val at the N terminus, whereas the S3-S/S3-L cleavage generates unstable NICD with Ser/Leu at the N terminus. Moreover, intracellular Notch signal transmission with unstable NICDs is much weaker than that with stable NICD. Importantly, the extent of endocytosis in target cells affects the relative production ratio of the two types of NICD, which changes in parallel with Notch signaling. Surprisingly, substantial amounts of unstable NICD species are generated from the Val-->Gly and the Lys-->Arg mutants, which have been reported to decrease S3 cleavage efficiency in cultured cells. Thus, we suggest that the existence of two distinct types of NICD points to a novel aspect of the intracellular signaling and that changes in the precision of S3 cleavage play an important role in the process of conversion from extracellular to intracellular Notch signaling.

Amyloid beta-protein precursor juxtamembrane domain regulates specificity of gamma-secretase-dependent cleavages

Amyloid beta-protein (Abeta), the major component of cerebral plaques associated with Alzheimer disease, is derived from amyloid beta-protein precursor (APP) through sequential proteolytic cleavage involving beta- and gamma-secretase. The intramembrane cleavage of APP by gamma-secretase occurs at two major sites, gamma and epsilon, although the temporal and/or mechanistic relationships between these cleavages remain unknown. In our attempt to address this issue, we uncovered an important regulatory role for the APP luminal juxtamembrane domain. We demonstrated in cell-based assays that domain replacements in this region can greatly reduce secreted Abeta resulting from gamma-cleavage without affecting the epsilon-cleavage product. This Abeta reduction is likely due to impaired proteolysis at the gamma-cleavage site. Further analyses with site-directed mutagenesis identified two juxtamembrane residues, Lys-28 and Ser-26 (Abeta numbering), as the critical determinants for efficient intramembrane proteolysis at the gamma-site. Consistent with the growing evidence that epsilon-cleavage of APP precedes gamma-processing, longer Abeta species derived from the gamma-cleavage-deficient substrates were detected intracellularly. These results indicate that the luminal juxtamembrane region of APP is an important regulatory domain that modulates gamma-secretase-dependent intramembrane proteolysis, particularly in differentiating gamma- and epsilon-cleavages.

Secretion of the Notch-1 Abeta-like peptide during Notch signaling

The canonical pathway of Notch signaling is mediated by regulated intramembrane proteolysis (RIP). In the pathway, ligand binding results in sequential proteolysis of the Notch receptor, and presenilin (PS)-dependent intramembrane proteolysis at the interface between the membrane and cytosol liberates the Notch-1 intracellular domain (NICD), a transcription modifier. Because the degradation of the Notch-1 transmembrane domain is thought to require an additional cleavage near the middle of the transmembrane domain, extracellular small peptides (Notch-1 Abeta-like peptide (Nbeta)) should be produced. Here we showed that Nbeta species are indeed secreted during the process of Notch signaling. We identified mainly two distinct molecular species of novel Nbeta, Nbeta21 and C-terminally elongated Nbeta25, which were produced in an approximately 5:1 ratio. This process is reminiscent of the production of Alzheimer disease-associated Abeta. PS pathogenic mutants increased the production of the longer species of Abeta (Abeta42) from beta-amyloid protein precursor. We revealed that several Alzheimer disease mutants also cause a parallel increase in the secretion of the longer form of Nbeta. Strikingly, chemicals that modify the Abeta42 level caused parallel changes in the Nbeta25 level. These results demonstrated that the characteristics of C-terminal elongation of Nbeta and Abeta are almost identical. In addition, because many other type 1 membrane-bound receptors release intracellular domains by PS-dependent intramembrane proteolysis, we suspect that the release of Abeta- or Nbeta-like peptides is a common feature of the proteolysis during RIP signaling. We anticipate that this study will open the door to searches for markers of RIP signaling and surrogate markers for Abeta42 production.

Effect of valine on the efficiency and precision at S4 cleavage of the Notch-1 transmembrane domain

Presenilin-dependent intramembranous proteolysis mediates the dual cleavage of the Notch-1 protein (S4 and S3) as well as the beta amyloid precursor protein (betaAPP) (gamma40 and epsilon-site). betaAPP has a valine residue just before the gamma40 (amyloid beta [Abeta] numbering) site and after the epsilon-site. Both gamma40 and epsilon have multiple cleavage sites, and the varieties of gamma40 cleavage are associated with Alzheimer's disease (AD). These lines of evidence suggest that valine plays a role in the intramembranous proteolysis. S4 cleavage in the middle of the Notch-1 transmembrane domain (TMD) corresponds to the gamma40 cleavage of betaAPP. The cleavage site is in the center of four sequential alanine residues between Ala1731 and Ala1732, neither of which has a valine residue. To investigate the effects of valine on presenilin-dependent intramembranous proteolysis, we replaced the transmembrane domain residue of Notch-1 with valine and analyzed the efficiency and precision at S4 and S3. We observed that all valine-mutated Notch-1 proteins have a dominant cleavage site (S4) between Ala1731 and Ala1732 with some variations of cleavage precision, suggesting that valine is not indispensable for determining the cleavage site of the Notch-1 transmembrane domain, but affects the efficiency and precision at S4 cleavage of the Notch-1 transmembrane domain.

Presenilins in the developing, adult, and aging cerebral cortex

Mutations in presenilins are the major cause of familial Alzheimer disease. The involvement of presenilins in the pathogenesis of Alzheimer disease, therefore, has been the subject of intense investigation during the past decade. Genetic analysis of phenotypes associated with presenilin mutations in invertebrate and vertebrate systems has greatly advanced our understanding of the in vivo functions of presenilins. In this review, the authors will summarize the current understanding of presenilin function, with an emphasis on the mammalian cerebral cortex. During development, presenilins play crucial roles in the maintenance of neural progenitor cell proliferation, the temporal control of neuronal differentiation, the survival of Cajal-Retzius neurons, and proper neuronal migration in the developing cerebral cortex. Analysis of presenilin function in the adult cerebral cortex has revealed essential roles for presenilins in synaptic plasticity, long-term memory, and neuronal survival. The authors will also discuss the molecular mechanisms through which presenilins may mediate these functions, including the Notch, CREB, and NMDA receptor-mediated signaling pathways. These diverse functions of presenilins in cortical development and function and neuronal survival have important implications for the pathogenesis of neurodegenerative dementia.

Tau is central in the genetic Alzheimer–frontotemporal dementia spectrum

In contrast to the common and genetically complex senile form of Alzheimer's disease (AD), the molecular genetic dissection of inherited presenile dementias has given important mechanistic insights into the pathogenesis of degenerative brain disease. Here, we focus on recent genotype-phenotype correlative studies in presenile AD and the frontotemporal dementia (FTD) complex of disorders. Together, these studies suggest that AD and FTD are linked in a genetic spectrum of presenile degenerative brain disorders in which tau appears to be the central player.

Two domains within the first putative transmembrane domain of presenilin 1 differentially influence presenilinase and gamma-secretase activity

Presenilins (PS) are thought to contain the active site for presenilinase endoproteolysis of PS and gamma-secretase cleavage of substrates. The structural requirements for PS incorporation into the gamma-secretase enzyme complex, complex stability and maturation, and appropriate presenilinase and gamma-secretase activity are poorly understood. We used rescue assays to identify sequences in transmembrane domain one (TM1) of PS1 required to support presenilinase and gamma-secretase activities. Swap mutations identified an N-terminal TM1 domain that is important for gamma-secretase activity only and a C-terminal TM1 domain that is essential for both presenilinase and gamma-secretase activities. Exchange of residues 95-98 of PS1 (sw95-98) completely abolishes both activities while the familial Alzheimer's disease mutation V96F significantly inhibits both activities. Reversion of residue 96 back to valine in the sw95-98 mutant rescues PS function, identifying V96 as the critical residue in this region. The TM1 mutants do not bind to an aspartyl protease transition state analog gamma-secretase inhibitor, indicating a conformational change induced by the mutations that abrogates catalytic activity. TM1 mutant PS1 molecules retain the ability to interact with gamma-secretase substrates and gamma-secretase complex members, although Nicastrin stability is decreased by the presence of these mutants. gamma-Secretase complexes that contain V96F mutant PS1 molecules display a partial loss of function for gamma-secretase that alters the ratio of amyloid-beta peptide species produced, leading to the amyloid-beta peptide aggregation that causes familial Alzheimer's disease.

The Caenorhabditis elegans IMPAS gene, imp-2, is essential for development and is functionally distinct from related presenilins

Presenilins (PSs) are required for Notch signaling in the development of vertebrates and invertebrates. Mutations in human PS1 and PS2 homologs are a cause of familial Alzheimer's disease (AD). The function of the recently identified ancient family of IMPAS proteins (IMP/SPP/PSH) homologous to PSs is not yet known. We show here that, unlike PSs, IMPs (orthologous C. elegans Ce-imp-2 and human hIMP1/SPP) do not promote Notch (C. elegans lin-12,glp-1) proteolysis or signaling. The knock-down of Ce-imp-2 leads to embryonic death and an abnormal molting phenotype in Caenorhabditis elegans. The molting defect induced by Ce-imp-2 deficiency was mimicked by depleting cholesterol or disrupting Ce-lrp-1 and suppressed, in part, by expression of the Ce-lrp-1 derivate. C. elegans lrp-1 is a homolog of mammalian megalin, lipoprotein receptor-related protein (LRP) receptors essential for cholesterol and lipoprotein endocytosis and signaling. These data suggest that IMPs are functionally distinct from related PSs and implicate IMPs as critical regulators of development that may potentially interact with the lipid-lipoprotein receptor-mediated pathway.

Functional reconstitution of gamma-secretase through coordinated expression of presenilin, nicastrin, Aph-1, and Pen-2

The gamma-secretase complex has emerged as an unusual membrane-bound aspartyl protease with the ability to cleave certain substrate proteins at peptide bonds believed to be buried within the hydrophobic environment of the lipid bilayer. This cleavage is responsible for a key biochemical step in signaling from several different cell-surface receptors, and it is also crucial in generating the neurotoxic amyloid peptides that are central to the pathogenesis of Alzheimer's disease. Active gamma-secretase is a multimeric protein complex consisting of at least four different proteins, presenilin, nicastrin, Aph-1, and Pen-2, with presenilin serving as the catalytically active core of the aspartyl protease. Presenilin itself undergoes endoproteolytic maturation, a process that is tightly regulated during the assembly and maturation of gamma-secretase, and that depends on the three cofactors nicastrin, Aph-1, and Pen-2. Recent studies have demonstrated that presenilin and its three cofactors are likely to be the major proteins needed for functional reconstitution of active gamma-secretase and have begun to elucidate the specific functions of the cofactors in the ordered assembly of gamma-secretase.

Dying for a cause: invertebrate genetics takes on human neurodegeneration

If invertebrate neurons are injured by hostile environments or aberrant proteins they die much like human neurons, indicating that the powerful advantages of invertebrate molecular genetics might be successfully used for testing specific hypotheses about human neurological diseases, for drug discovery and for non-biased screens for suppressors and enhancers of neurodegeneration. Recent molecular dissection of the genetic requirements for hypoxia, excitotoxicity and death in models of Alzheimer disease, polyglutamine-expansion disorders, Parkinson disease and more, is providing mechanistic insights into neurotoxicity and suggesting new therapeutic interventions. An emerging theme is that neuronal crises of distinct origins might converge to disrupt common cellular functions, such as protein folding and turnover.

The mechanism of gamma-secretase activities through high molecular weight complex formation of presenilins is conserved in Drosophila melanogaster and mammals

Mutations in presenilin 1 (PS1) and PS2 genes contribute to the pathogenesis of early onset familial Alzheimer's disease by increasing secretion of the pathologically relevant Abeta42 polypeptides. PS genes are also implicated in Notch signaling through proteolytic processing of the Notch receptor in Caenorhabditis elegans, Drosophila melanogaster, and mammals. Here we show that Drosophila PS (Psn) protein undergoes endoproteolytic cleavage and forms a stable high molecular weight (HMW) complex in Drosophila S2 or mouse neuro2a (N2a) cells in a similar manner to mammalian PS. The loss-of-function recessive point mutations located in the C-terminal region of Psn, that cause an early pupal-lethal phenotype resembling Notch mutant in vivo, disrupted the HMW complex formation, and abolished gamma-secretase activities in cultured cells. The overexpression of Psn in mouse embryonic fibroblasts lacking PS1 and PS2 genes rescued the Notch processing. Moreover, disruption of the expression of Psn by double-stranded RNA-mediated interference completely abolished the gamma-secretase activity in S2 cells. Surprisingly, gamma-secretase activity dependent on wild-type Psn was associated with a drastic overproduction of Abeta1-42 from human betaAPP in N2a cells, but not in S2 cells. Our data suggest that the mechanism of gamma-secretase activities through formation of HMW PS complex, as well as its abolition by loss-of-function mutations located in the C terminus, are highly conserved features in Drosophila and mammals.

The Caenorhabditis elegans presenilin sel-12 is required for mesodermal patterning and muscle function

Mutations in presenilin genes impair Notch signalling and, in humans, have been implicated in the development of familial Alzheimer's disease. We show here that a reduction of the activity of the Caenorhabditis elegans presenilin sel-12 results in a late defect during sex muscle development. The morphological abnormalities and functional deficits in the sex muscles contribute to the egg-laying defects seen in sel-12 hermaphrodites and to the severely reduced mating efficiency of sel-12 males. Both defects can be rescued by expressing sel-12 from the hlh-8 promoter that is active during the development of the sex muscle-specific M lineage, but not by expressing sel-12 from late muscle-specific promoters. Both weak and strong sel-12 mutations cause defects in the sex muscles that resemble the defects we found in lin-12 hypomorphic alleles, suggesting a previously uncharacterised LIN-12 signalling event late in postembryonic mesoderm development. Together with a previous study indicating a role of lin-12 and sel-12 during the specification of the pi cell lineage required for proper vulva-uterine connection, our data suggest that the failure of sel-12 animals to lay eggs properly is caused by defects in at least two independent signalling events in different tissues during development.

Presenilins mediate a dual intramembranous gamma-secretase cleavage of Notch-1

Following ectodomain shedding, Notch-1 undergoes presenilin (PS)-dependent constitutive intramembranous endoproteolysis at site-3. This cleavage is similar to the PS-dependent gamma-secretase cleavage of the beta-amyloid precursor protein (betaAPP). However, topological differences in cleavage resulting in amyloid beta-peptide (Abeta) or the Notch-1 intracellular domain (NICD) indicated independent mechanisms of proteolytic cleavage. We now demonstrate the secretion of an N-terminal Notch-1 Abeta-like fragment (Nbeta). Analysis of Nbeta by MALDI-TOF MS revealed that Nbeta is cleaved at a novel site (site-4, S4) near the middle of the transmembrane domain. Like the corresponding cleavage of betaAPP at position 40 and 42 of the Abeta domain, S4 cleavage is PS dependent. The precision of this cleavage is affected by familial Alzheimer's disease-associated PS1 mutations similar to the pathological endoproteolysis of betaAPP. Considering these similarities between intramembranous processing of Notch and betaAPP, we conclude that these proteins are cleaved by a common mechanism utilizing the same protease, i.e. PS/gamma-secretase.

Presenilin 1 mutations activate gamma 42-secretase but reciprocally inhibit epsilon-secretase cleavage of amyloid precursor protein (APP) and S3-cleavage of notch

The presenilin 1 (PS1) and presenilin 2 (PS2) proteins are necessary for proteolytic cleavage of the amyloid precursor protein (APP) within its transmembrane domain. One of these cleavage events (termed gamma-secretase) generates the C-terminal end of the Abeta-peptide by proteolysis near residue 710 or 712 of APP(770). Another event (termed gamma-like or epsilon-secretase cleavage) cleaves near residue 721 at approximately 2-5 residues inside the cytoplasmic membrane boundary to generate a series of stable, C-terminal APP fragments. This latter cleavage is analogous to S3-cleavage of Notch. We report here that specific mutations in the N terminus, loop, or C terminus of PS1 all increase the production of Abeta(42) but cause inhibition of both epsilon-secretase cleavage of APP and S3-cleavage of Notch. These data support the hypothesis that epsilon-cleavage of APP and S3-cleavage of Notch are similar events. They also argue that, although both the gamma-site and the epsilon-site cleavage of APP are presenilin-dependent, they are likely to be independent catalytic events.

Gamma-secretase-mediated proteolysis in cell-surface-receptor signalling

Many cell-surface receptors transmit signals to the nucleus through complex protein cascades. By contrast, the Notch signalling pathway uses a relatively direct mechanism, in which the intracellular domain of the receptor is liberated by intramembrane cleavage and translocates to the nucleus. This critical cleavage is mediated by the gamma-secretase complex, and new findings reveal that this mechanism is used by various receptors, although many questions remain about the biochemical details.

The presenilins

The presenilins are evolutionarily conserved transmembrane proteins that regulate cleavage of certain other proteins in their transmembrane domains. The clinical significance of this regulation is shown by the contribution of presenilin mutations to 20-50% of early-onset cases of inherited Alzheimer's disease. Although the precise molecular mechanism underlying presenilin function or dysfunction remains elusive, presenilins are thought to be part of a complex of proteins that has 'gamma-secretase cleavage' activity, which is clearly central in the pathogenesis of Alzheimer's disease. Mutations in presenilins increase the production of the longer isoforms of amyloid beta peptide, which are neurotoxic and prone to self-aggregation. Biochemical studies indicate that the presenilins do not act alone but operate within large heteromeric protein complexes, whose components and enzymatic core are the subject of much study and controversy; one essential component is nicastrin. The presenilin primary sequence is remarkably well conserved in eukaryotes, suggesting some functional conservation; indeed, defects caused by mutations in the nemotode presenilin homolog can be rescued by human presenilin.

Interaction with telencephalin and the amyloid precursor protein predicts a ring structure for presenilins

The carboxyl terminus of presenilin 1 and 2 (PS1 and PS2) binds to the neuron-specific cell adhesion molecule telencephalin (TLN) in the brain. PS1 deficiency results in the abnormal accumulation of TLN in a yet unidentified intracellular compartment. The first transmembrane domain and carboxyl terminus of PS1 form a binding pocket with the transmembrane domain of TLN. Remarkably, APP binds to the same regions via part of its transmembrane domain encompassing the critical residues mutated in familial Alzheimer's disease. Our data surprisingly indicate a spatial dissociation between the binding site and the proposed catalytic site near the critical aspartates in PSs. They provide important experimental evidence to support a ring structure model for PS.

Metabolism of presenilins

Understanding mechanisms involved in the production of Abeta has long been the central focus of cell biologists engaged in molecular AD research. The discovery of two genes that encode homologous polytopic membrane proteins termed Presenilins (PS), has lead to several exciting recent findings on the proteolytic processes responsible for generating the COOH-terminus of Abeta. What we now know is that PS proteins play an important role in Abeta production and are considered one of the therapeutic targets. Here I have reviewed the vast literature on the biology of PS, especially focusing on PS endoproteolysis and the accumulation of stable PS derivatives that are likely the functional units.

The first proline of PALP motif at the C terminus of presenilins is obligatory for stabilization, complex formation, and gamma-secretase activities of presenilins

Mutations in presenilin (PS) genes cause early-onset familial Alzheimer's disease by increasing production of the amyloidogenic form of amyloid beta peptides ending at residue 42 (Abeta42). PS is an evolutionarily conserved multipass transmembrane protein, and all known PS proteins contain a proline-alanine-leucine-proline (PALP) motif starting at proline (P) 414 (amino acid numbering based on human PS2) at the C terminus. Furthermore, missense mutations that replace the first proline of PALP with leucine (P414L) lead to a loss-of-function of PS in Drosophila melanogaster and Caenorhabditis elegans. To elucidate the roles of the PALP motif in PS structure and function, we analyzed neuro2a as well as PS1/2 null fibroblast cell lines transfected with human PS harboring mutations at the PALP motif. P414L mutation in PS2 (and its equivalent in PS1) abrogated stabilization, high molecular weight complex formation, and entry to Golgi/trans-Golgi network of PS proteins, resulting in failure of Abeta42 overproduction on familial Alzheimer's disease mutant basis as well as of site-3 cleavage of Notch. These data suggest that the first proline of the PALP motif plays a crucial role in the stabilization and formation of the high molecular weight complex of PS, the latter being the active form with intramembrane proteolytic activities.

Human wild presenilin-1 mimics the effect of the mutant presenilin-1 on the processing of Alzheimer's amyloid precursor protein in PC12D cells

Most familial early-onset Alzheimer's disease (FAD) is caused by mutations in the presenilin-1 (PS1) gene. Abeta 42 is derived from amyloid precursor protein (APP) and increased concentrations are widely believed to be a pathological hallmark of abnormal PS function. Thus, the interaction between PS1 and APP is central to the molecular mechanism of AD. To examine the effect of wild-type human PS1 on rat APP metabolism, we made several PC12D cell lines that expressed human wild or mutant PS1, and analyzed the processing of endogenous rat APP and the intracellular gamma-secretase activity. We found the ratio of Abeta 42/Abeta 40 increased in PC12D cells expressing wild-type human PS1. These changes were identical to those found in PC12D cells expressing human PS1 bearing the A260V mutation. These results suggest that APP metabolism is physiologically regulated by the PS1 and that loss of normal PS1 affects gamma-secretase activity.

Presenilin mutations line up along transmembrane alpha-helices

Presenilin 1 mutations are the major cause of autosomal dominant Alzheimer's disease. Here we present evidence that pathogenic mutations in putative transmembrane domains 1, 2, 3, 4 and 6 align along helical faces, thus supporting the view that these are indeed transmembrane domains, and suggesting that disruption of the alignment of these domains is responsible for the pathogenicity of the mutations.