A presenilin dimer at the core of the gamma-secretase enzyme: insights from parallel analysis of Notch 1 and APP proteolysis

Hippocampal spatial memory impairments caused by the familial Alzheimer's disease-linked presenilin 1 M146V mutation

Mutations in presenilins (PS) 1 and 2 are the major cause of familial Alzheimer's disease. Conditional inactivation of PS1 in the mouse postnatal forebrain leads to mild deficits in spatial learning and memory, whereas inactivation of both PS1 and PS2 results in severe memory and synaptic plasticity impairments, followed by progressive and substantial neurodegeneration. Here we investigate the effect of a familial Alzheimer's disease-linked PS1 missense mutation using knock-in (KI) mice, in which the wild-type PS1 allele is replaced with the M146V mutant allele. In the Morris water maze task, PS1 KI mice at 3 months of age exhibit reduced quadrant occupancy and platform crossing in the probe trial after 6 days of training, though their performance was normal in the probe trial after 12 days of training. By the age of 9 months, even after 12 days of training, PS1 homozygous KI mice still exhibit reduced platform crossing in the post-training probe trial. ELISA analysis revealed a selective increase in cortical levels of beta-amyloid 42 in PS1 KI mice, whereas production of beta-amyloid 40 was normal. Histological and quantitative real-time RT-PCR analyses showed normal gross hippocampal morphology and unaltered expression of three genes involved in inflammatory responses in PS1 KI mice. These results show hippocampal spatial memory impairments caused by the PS1 M146V mutation and age-related deterioration of the memory impairment, suggesting that PS1 KI mice are a valuable model system for the study of memory loss in AD.

Presenilin-1 Maintains a Nine-Transmembrane Topology throughout the Secretory Pathway

Presenilin-1 is a polytopic membrane protein that assembles with nicastrin, PEN-2, and APH-1 into an active gamma-secretase complex required for intramembrane proteolysis of type I transmembrane proteins. Although essential for a correct understanding of structure-function relationships, its exact topology remains an issue of strong controversy. We revisited presenilin-1 topology by inserting glycosylation consensus sequences in human PS1 and expressing the obtained mutants in a presenilin-1 and 2 knock-out background. Based on the glycosylation status of these variants we provide evidence that presenilin-1 traffics through the Golgi after a conformational change induced by complex assembly. Based on our glycosylation variants of presenilin-1 we hypothesize that complex assembly occurs during transport between the endoplasmic reticulum and the Golgi apparatus. Furthermore, our data indicate that presenilin-1 has a nine-transmembrane domain topology with the COOH terminus exposed to the lumen/extracellular surface. This topology is independently underscored by lysine mutagenesis, cell surface biotinylation, and cysteine derivation strategies and is compatible with the different physiological functions assigned to presenilin-1.

Intra- or intercomplex binding to the gamma-secretase enzyme. A model to differentiate inhibitor classes

Gamma-secretase is one of the critical enzymes required for the generation of amyloid-beta peptides from the beta-amyloid precursor protein. Because amyloid-beta peptides are generally accepted to play a key role in Alzheimer disease, gamma-secretase inhibition holds the promise for a disease-modifying therapy for this neurodegenerative condition. Although recent progress has enhanced the understanding of the biology and composition of the gamma-secretase enzyme complex, less information is available on the actual interaction of various inhibitor classes with the enzyme. Here we show that the two principal classes of inhibitor described in the scientific and patent literature, aspartyl protease transition state analogue and small molecule non-transition state inhibitors, display fundamental differences in the way they interact with the enzyme. Taking advantage of a gamma-secretase enzyme overexpressing cellular system and different radiolabeled gamma-secretase inhibitors, we observed that the maximal binding of non-transition state gamma-secretase inhibitors accounts only for half the number of catalytic sites of the recombinant enzyme complex. This characteristic stoichiometry can be best accommodated with a model whereby the non-transition state inhibitors bind to a unique site at the interface of a dimeric enzyme. Subsequent competition studies confirm that this site appears to be targeted by the main classes of small molecule gamma-secretase inhibitor. In contrast, the non-steroidal anti-inflammatory drug gamma-secretase modulator sulindac sulfide displayed noncompetitive antagonism for all types of inhibitor. This finding suggests that non-steroidal anti-inflammatory drug-type gamma-secretase modulators target an alternative site on the enzyme, thereby changing the conformation of the binding sites for gamma-secretase inhibitors.

Deducing the transmembrane domain organization of presenilin-1 in gamma-secretase by cysteine disulfide cross-linking

Gamma-secretase is a founding member of membrane-embedded aspartyl proteases that cleave substrates within transmembrane domains, and this enzyme is an important target for the development of therapeutics for Alzheimer's disease. The structure of gamma-secretase and its precise catalytic mechanism still remain largely unknown. Gamma-secretase is a complex of four integral membrane proteins, with presenilin (PS) as the catalytic component. To gain structural and functional information about the nine-transmembrane domain (TMD) presenilin, we employed a cysteine mutagenesis/disulfide cross-linking approach. Here we report that native Cys92 is close to both Cys410 and Cys419, strongly implying that TMD1 and TMD8 are adjacent to each other. This structural arrangement also suggests that TMD8 is distorted from an ideal helix. Importantly, binding of an active site directed inhibitor, but not a docking site directed inhibitor, reduces the ability of the native cysteine pairs of PS1 to cross-link upon oxidation. These findings suggest that the conserved cysteines of TMD1 and TMD8 contribute to or allosterically interact with the active site of gamma-secretase.

A γ-secretase-like intramembrane cleavage of TNFα by the GxGD aspartyl protease SPPL2b

Gamma-secretase and signal peptide peptidase (SPP) are unusual GxGD aspartyl proteases, which mediate intramembrane proteolysis. In addition to SPP, a family of SPP-like proteins (SPPLs) of unknown function has been identified. We demonstrate that SPPL2b utilizes multiple intramembrane cleavages to liberate the intracellular domain of tumor necrosis factor alpha (TNFalpha) into the cytosol and the carboxy-terminal counterpart into the extracellular space. These findings suggest common principles for regulated intramembrane proteolysis by GxGD aspartyl proteases.

Conserved residues in juxtamembrane region of the extracellular domain of nicastrin are essential for gamma-secretase complex formation

The Alzheimer's disease-linked protein, presenilin, forms the active site of the gamma-secretase enzyme complex. However, three other proteins, nicastrin (NCT), PEN-2 and APH-1, are required for enzyme activity. This complex is responsible for cleaving the beta-amyloid precursor protein to produce amyloid beta and the intracellular domain (AICD). Although much research has focused on the regions of presenilin that are important for gamma-secretase function, less is known about NCT. To further our understanding of the role of NCT in gamma-secretase activity and complex formation, we have undertaken a systematic evaluation of conserved residues in the juxtamembrane region of the extracellular domain of NCT. Two mutants, S632A and W648A, greatly reduce gamma-secretase activity, as seen by a reduction in amyloid beta and AICD levels. Several lines of evidence suggest that these mutations result in reduced gamma-secretase activity because they affect the ability of NCT to stably associate with the other gamma-secretase components. Since NCT and APH-1 must first bind in order for presenilin and PEN-2 to stably join the complex, we propose that S632 and W648 are essential for a stable interaction with APH-1.

Interaction between presenilin 1 and ubiquilin 1 as detected by fluorescence lifetime imaging microscopy and a high-throughput fluorescent plate reader

Presenilin 1 (PS1) in its active heterodimeric form is the catalytic center of the gamma-secretase complex, an enzymatic activity that cleaves amyloid precursor protein (APP) to produce amyloid beta (Abeta). Ubiquilin 1 is a recently described PS1 interacting protein, the overexpression of which increases PS1 holoprotein levels and leads to reduced levels of functionally active PS1 heterodimer. In addition, it has been suggested that splice variants of the UBQLN1 gene are associated with an increased risk of developing Alzheimer disease (AD). However, it is still unclear whether PS1 and ubiquilin 1 interact when expressed at endogenous levels under normal physiological conditions. Here, we employ three novel fluorescence resonance energy transfer-based techniques to investigate the interaction between PS1 and ubiquilin 1 in intact cells. We consistently find that the ubiquilin 1 N terminus is in close proximity to several epitopes on PS1. We show that ubiquilin 1 interacts both with PS1 holoprotein and heterodimer and that the interaction between PS1 and ubiquilin 1 takes place near the cell surface. Furthermore, we show that the PS1-ubiquilin 1 interaction can be detected between endogenous proteins in primary neurons in vitro as well as in brain tissue of healthy controls and Alzheimer disease patients, providing evidence of its physiological relevance.

Pathological and physiological functions of presenilins

Mutations in PSEN1 and PSEN2 genes account for the majority of cases of early-onset familial Alzheimer disease. Since the first prediction of a genetic link between PSEN1 and PSEN2 with Alzheimer's disease, many research groups from both academia and pharmaceutical industry have sought to unravel how pathogenic mutations in PSEN cause presenile dementia. PSEN genes encode polytopic membrane proteins termed presenilins (PS1 and PS2), which function as the catalytic subunit of γ-secretase, an intramembrane protease that has a wide spectrum of type I membrane protein substrates. Sequential cleavage of amyloid precursor protein by BACE and γ-secretase releases highly fibrillogenic β-amyloid peptides, which accumulate in the brains of aged individuals and patients with Alzheimer's disease. Familial Alzheimer's disease-associated presenilin variants are thought to exert their pathogenic function by selectively elevating the levels of highly amyloidogenic Aβ42 peptides. In addition to Alzheimer's disease, several recent studies have linked PSEN1 to familiar frontotemporal dementia. Here, we review the biology of PS1, its role in γ-secretase activity, and discuss recent developments in the cell biology of PS1 with respect to Alzheimer's disease pathogenesis.

A faster migrating variant masquerades as NICD when performing in vitro gamma-secretase assays with bacterially expressed Notch substrates

Intramembrane proteolysis is a new and rapidly growing field. In vitro assays utilizing recombinant substrates for gamma-secretase, an intramembrane-cleaving enzyme, are critically important in order to characterize the biochemical properties of this unusual enzyme. Several recombinant Notch proteins of varying length are commonly used as in vitro substrates for CHAPSO-solubilized gamma-secretase. Here we report that several recombinant Notch constructs undergo limited or no proteolysis in vitro. Instead, upon incubation with or without gamma-secretase, variants of the intact protein migrate during SDS-PAGE at the location expected for the gamma-secretase specific cleavage products. In addition, we show that addition of aspartyl- and gamma-secretase specific protease inhibitors are able to retard the formation of these variants independent of gamma-secretase, which could lead to the erroneous conclusion that Notch cleavage by solubilized gamma-secretase was achieved in vitro even when no proteolysis occurred. In contrast, substrates produced in mammalian or insect cells are cleaved efficiently in vitro. These observations suggest that in vitro studies reliant on recombinant, bacterially produced Notch TMD should be performed with the inclusion of additional controls able to differentiate between actual cleavage and this potential artifact.

Deletion of presenilin 1 hydrophilic loop sequence leads to impaired gamma-secretase activity and exacerbated amyloid pathology

gamma-Secretase processing of the amyloid precursor protein (APP) generates Abeta40 and Abeta42, peptides that constitute the principal components of the beta-amyloid plaque pathology of Alzheimer's disease (AD). The gamma-secretase activity is executed by a high-molecular-weight complex of which presenilin 1 (PS1) is an essential component. PS1 is a multi-pass membrane protein, and the large hydrophilic loop domain between transmembrane domains 6 and 7 has been shown to interact with various proteins. To determine the physiological function of the loop domain, we created a strain of PS1 knock-in mice in which the exon 10, which encodes most of the hydrophilic loop sequence, was deleted from the endogenous PS1 gene. We report here that the homozygous exon 10-deleted mice are viable but exhibit drastically reduced gamma-secretase cleavage at the Abeta40, but not the Abeta42, site. Surprisingly, this reduction of Abeta40 is associated with exacerbated plaque pathology when expressed on APP transgenic background. Thus, the PS1 loop plays a regulatory role in gamma-secretase processing, and decreased Abeta40, not increased Abeta42 is likely the cause for the accelerated plaque deposition in these animals. Our finding supports a protective role of Abeta40 against amyloid pathology and raises the possibility that impaired gamma-secretase activity could be the basis for AD pathogenesis in general.

Role of Sox2 in the development of the mouse neocortex

The mammalian neocortex is established from neural stem and progenitor cells that utilize specific transcriptional and environmental factors to create functional neurons and astrocytes. Here, we examined the mechanism of Sox2 action during neocortical neurogenesis and gliogenesis. We established a robust Sox2 expression in neural stem and progenitor cells within the ventricular zone, which persisted until the cells exited the cell cycle. Overexpression of constitutively active Sox2 in neural progenitors resulted in upregulation of Notch1, recombination signal-sequence binding protein-J (RBP-J) and hairy enhancer of split 5 (Hes5) transcripts and the Sox2 high mobility group (HMG) domain seemed sufficient to confer these effects. While Sox2 overexpression permitted the differentiation of progenitors into astroglia, it inhibited neurogenesis, unless the Notch pathway was blocked. Moreover, neuronal precursors engaged a serine protease(s) to eliminate the overexpressed Sox2 protein and relieve the repression of neurogenesis. Glial precursors and differentiated astrocytes, on the other hand, maintained Sox2 expression until they reached a quiescent state. Sox2 expression was re-activated by signals that triggered astrocytic proliferation (i.e., injury, mitogenic and gliogenic factors). Taken together, Sox2 appears to act upstream of the Notch signaling pathway to maintain the cell proliferative potential and to ensure the generation of sufficient cell numbers and phenotypes in the developing neocortex.

Treg-mediated immunosuppression involves activation of the Notch-HES1 axis by membrane-bound TGF-beta

Studies in humans and mice show an important role for Tregs in the control of immunological disorders. The mechanisms underlying the immunosuppressive functions of Tregs are not well understood. Here, we show that CD4+ T cells expressing Foxp3 and membrane-bound TGF-beta (TGF-beta(m+)Foxp3+), previously shown to be immunosuppressive in both allergic and autoimmune diseases, activate the Notch1-hairy and enhancer of split 1 (Notch1-HES1) axis in target cells. Soluble TGF-beta and cells secreting similar levels of soluble TGF-beta were unable to trigger Notch1 activation. Inhibition of Notch1 activation in vivo reversed the immunosuppressive functions of TGF-beta(m+)Foxp3+ cells, resulting in severe allergic airway inflammation. Integration of the TGF-beta and Notch1 pathways may be an important mechanism for the maintenance of immune homeostasis in the periphery.

Familial Alzheimer's disease presenilin 1 mutations cause alterations in the conformation of presenilin and interactions with amyloid precursor protein

Presenilin 1 (PS1) is a critical component of the gamma-secretase complex, an enzymatic activity that cleaves amyloid beta (Abeta) from the amyloid precursor protein (APP). More than 100 mutations spread throughout the PS1 molecule are linked to autosomal dominant familial Alzheimer's disease (FAD). All of these mutations lead to a similar phenotype: an increased ratio of Abeta42 to Abeta40, increased plaque deposition, and early age of onset. We use a recently developed microscopy approach, fluorescence lifetime imaging microscopy, to monitor the relative molecular distance between PS1 N and C termini in intact cells. We show that FAD-linked missense mutations located near the N and C termini, in the mid-region of PS1, and the exon 9 deletion mutation all change the spatial relationship between PS1 N and C termini in a similar way, increasing proximity of the two epitopes. This effect is opposite of that observed by treatment with Abeta42-lowering nonsteroidal anti-inflammatory drugs (NSAIDs) (Lleo et al., 2004b). Accordingly, treatment of M146L PS1-overexpressing neurons with high-dose NSAIDs somewhat offsets the conformational change associated with the mutation. Moreover, by monitoring the relative distance between a PS1 loop epitope and the APP C terminus, we demonstrate that the FAD PS1 mutations are also associated with a consistent change in the configuration of the PS1-APP complex. The nonpathogenic E318G PS1 polymorphism had no effect on PS1 N terminus-C terminus proximity or PS1-APP interactions. We propose that the conformational change we observed may therefore provide a shared molecular mechanism for FAD pathogenesis caused by a wide range of PS1 mutations.

Three-dimensional structure of the gamma-secretase complex

Gamma-secretase belongs to an atypical class of aspartic proteases that hydrolyzes peptide bonds within the transmembrane domain of substrates, including amyloid-beta precursor protein and Notch. gamma-Secretase is comprised of presenilin, nicastrin, APH-1, and PEN-2 which form a large multimeric membrane protein complex, the three-dimensional structure of which is unknown. To gain insight into the structure of this complex enzyme, we purified functional gamma-secretase complex reconstituted in Sf9 cells and analyzed it using negative stain electron microscopy and 3D reconstruction techniques. Analysis of 2341 negatively stained particle images resulted in the three-dimensional representation of gamma-secretase at a resolution of 48 angstroms. The structure occupies a volume of 560 x 320 x 240 angstroms and resembles a flat heart comprised of two oppositely faced, dimpled domains. A low density space containing multiple pores resides between the domains. Some of the dimples in the putative transmembrane region may house the catalytic site. The large dimensions are consistent with the observation that gamma-secretase activity resides within a high molecular weight complex.

Presenilin clinical mutations can affect gamma-secretase activity by different mechanisms

Mutations in human presenilin (PS) genes cause aggressive forms of familial Alzheimer's disease. Presenilins are polytopic proteins that harbour the catalytic site of the gamma-secretase complex and cleave many type I transmembrane proteins including beta-amyloid precursor protein (APP), Notch and syndecan 3. Contradictory results have been published concerning whether PS mutations cause 'abnormal' gain or (partial) loss of function of gamma-secretase. To avoid the possibility that wild-type PS confounds the interpretation of the results, we used presenilin-deficient cells to analyse the effects of different clinical mutations on APP, Notch, syndecan 3 and N-cadherin substrate processing, and on gamma-secretase complex formation. A loss in APP and Notch substrate processing at epsilon and S3 cleavage sites was observed with all presenilin mutants, whereas APP processing at the gamma site was affected in variable ways. PS1-Delta9 and PS1-L166P mutations caused a reduction in beta-amyloid peptide Abeta40 production whereas PS1-G384A mutant significantly increased Abeta42. Interestingly PS2, a close homologue of PS1, appeared to be a less efficient producer of Abeta than PS1. Finally, subtle differences in gamma-secretase complex assembly were observed. Overall, our results indicate that the different mutations in PS affect gamma-secretase structure or function in multiple ways.

The cytoplasmic sequence of E-cadherin promotes non-amyloidogenic degradation of A beta precursors

The presenilin (PS)/gamma-secretase system promotes production of the A beta (A beta) peptides by mediating cleavage of amyloid precursor protein (APP) at the gamma-sites. This system is also involved in the processing of type-I transmembrane proteins, including APP, cadherins and Notch1 receptors, at the epsilon-cleavage site, resulting in the production of peptides containing the intracellular domains (ICDs) of the cleaved proteins. Emerging evidence shows that these peptides have important biological functions, raising the possibility that their inhibition by gamma-secretase inhibitors may be detrimental to the cell. Here, we show that peptide E-Cad/CTF2, produced by the PS1/gamma-secretase processing of E-cadherin, promotes the lysosomal/endosomal degradation of the transmembrane APP derivatives, C99 and C83, and inhibits production of the APP ICD (AICD). In addition, E-Cad/CTF2 decreases accumulation of total secreted A beta. These data suggest a novel method to promote the non-amyloidogenic degradation of A beta precursors and to inhibit A beta production.

Regulation of CRE-dependent transcription by presenilins: prospects for therapy of Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disorder and is characterized by memory loss and other cognitive disabilities. Mutations in the presenilin genes are the major cause of familial AD. Analysis of conditional knockout mice has shown that inactivation of presenilins results in progressive memory impairment and age-dependent neurodegeneration, suggesting that reduced presenilin activity might represent an important pathogenic mechanism. Presenilins positively regulate the transcription of cAMP response element (CRE)-containing genes, some of which are known to be important for memory formation and neuronal survival. Phosphodiesterase 4 and histone deacetylase inhibitors, which can enhance CRE-dependent gene expression, have been shown to ameliorate memory deficits and neurodegeneration in animal models. Thus, modulation of CRE-dependent transcription might be beneficial for the treatment of dementia in AD.

A Two Decade Contribution of Molecular Cell Biology to the Centennial of Alzheimer's Disease: Are We Progressing Toward Therapy?

Alzheimer's disease (AD), described for the first time 100 years ago, is a neurodegenerative disease characterized by two neuropathological hallmarks: neurofibrillary tangles containing hyperphosphorylated tau and senile plaques. These lesions are likely initiated by an imbalance between production and clearance of amyloid beta, leading to increased oligomerization of these peptides, formation of amyloid plaques in the brain of the patient, and final dementia. Amyloid beta is generated from amyloid precursor protein (APP) by subsequent beta- and gamma-secretase cleavage, the latter being a multiprotein complex consisting of presenilin-1 or -2, nicastrin, APH-1, and PEN-2. Alternatively, APP can be cleaved by alpha- and gamma-secretase, precluding the production of Abeta. In this review, we discuss the major breakthroughs during the past two decades of molecular cell biology and the current genetic and cell biological state of the art on APP proteolysis, including structure-function relationships and subcellular localization. Finally, potential directions for cell biological research toward the development of AD therapies are briefly discussed.

Detection of presenilin-1 homodimer formation in intact cells using fluorescent lifetime imaging microscopy

Presenilin-1 (PS1) is a multipass transmembrane domain protein, which is believed to be the catalytic component of the gamma-secretase complex. The complex is comprised of four major components: PS1, nicastrin, Aph-1, and Pen-2. The exact stoichiometric relationship between the four components remains unclear. It has been shown that gamma-secretase exists as high molecular weight complexes, suggesting the possibility of dimer/multimer formation. We combined a biochemical approach with a novel morphological microscopy assay to analyze PS1 dimer formation and subcellular distribution in situ, in intact mammalian cells. Both coimmunoprecipitation and fluorescent lifetime imaging microscopy approaches showed that wildtype PS1 molecules form dimers. Moreover, PS1 holoproteins containing the D257A mutation also come into close enough proximity to form a dimer, suggesting that cleavage within the loop is not necessary for dimer formation. Taken together these data suggest that PS1 dimerization occurs during normal PS1 function.

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.

Analysis of transmembrane domain mutants is consistent with sequential cleavage of Notch by gamma-secretase

gamma-Secretase is a lipid-embedded, intramembrane-cleaving aspartyl protease that cleaves its substrates twice within their transmembrane domains (TMD): once near the cytosolic leaflet (at S3/epsilon) and again in the middle of the TMD (at S4/gamma). To address whether this unusual process occurs in two independent or interdependent steps, we investigated how mutations at the S3/epsilon site in Notch1-based substrates impact proteolysis. We demonstrate that such mutations greatly inhibit not only gamma-secretase-mediated cleavage at S3 but also at S4, independent of their impact on NICD stability. These results, together with our previous observations, suggest that hydrolysis at the center of the Notch transmembrane domain (S4/gamma) is dependent on the S3/epsilon cleavage. Notch (and perhaps all gamma-secretase substrates) may be cleaved by sequential proteolysis starting at S3.

Pen-2 is incorporated into the gamma-secretase complex through binding to transmembrane domain 4 of presenilin 1

gamma-Secretase is a multimeric membrane protein complex comprised of presenilin (PS), nicastrin (Nct), Aph-1, and Pen-2. It is a member of an atypical class of aspartic proteases that hydrolyzes peptide bonds within the membrane. During the biosynthetic process of the gamma-secretase complex, Nct and Aph-1 form a heterodimeric intermediate complex and bind to the C-terminal region of PS, serving as a stabilizing scaffold for the complex. Pen-2 is then recruited into this trimeric complex and triggers endoproteolysis of PS, conferring gamma-secretase activity. Although the Pen-2 accumulation depends on PS, the binding partner of Pen-2 within the gamma-secretase complex remains unknown. We reconstituted PS1 in Psen1/Psen2 deficient cells by expressing a series of PS1 mutants in which one of the N-terminal six transmembrane domains (TMDs) was swapped with those of CD4 (a type I transmembrane protein) or CLAC-P (a type II transmembrane protein). We report that the proximal two-thirds of TMD4 of PS1, including the conserved Trp-Asn-Phe sequence, are required for its interaction with Pen-2. Using a chimeric CD4 molecule harboring PS1 TMD4, we further demonstrate that the PS1 TMD4 bears a direct binding motif to Pen-2. Pen-2 may contribute to the activation of the gamma-secretase complex by directly binding to the TMD4 of PS1.

Longer forms of amyloid beta protein: implications for the mechanism of intramembrane cleavage by gamma-secretase

Gamma-cleavage of beta-amyloid precursor protein (APP) in the middle of the cell membrane generates amyloid beta protein (Abeta), and epsilon-cleavage, approximately 10 residues downstream of the gamma-cleavage site, releases the APP intracellular domain (AICD). A significant link between generation of Abeta and AICD and failure to detect AICD41-99 led us to hypothesize that epsilon-cleavage generates longer Abetas, which are then processed to Abeta40/42. Using newly developed gel systems and an N-end-specific monoclonal antibody, we have identified the longer Abetas (Abeta1-43, Abeta1-45, Abeta1-46, and Abeta1-48) within the cells and in brain tissues. The production of these longer Abetas as well as Abeta40/42 is presenilin dependent and is suppressed by {1S-benzyl-4R-[1S-carbamoyl-2-phenylethylcarbamoyl-1S-3-methylbutylcarbamoyl]-2R-hydroxy-5-phenylpentyl}carbamic acid tert-butyl ester, a transition state analog inhibitor for aspartyl protease. In contrast, N-[N-(3,5-difluorophenacetyl)-L-alanyl]-S-phenylglycine t-butyl ester, a potent dipeptide gamma-secretase inhibitor, builds up Abeta1-43 and Abeta1-46 intracellularly, which was also confirmed by mass spectrometry. Notably, suppression of Abeta40 appeared to lead to an increase in Abeta43, which in turn brings an increase in Abeta46, in a dose-dependent manner. We therefore propose an alpha-helical model in which longer Abeta species generated by epsilon-cleavage is cleaved at every three residues in its carboxyl portion.

A {gamma}-secretase-independent mechanism of signal transduction by the amyloid precursor protein

It has been proposed that gamma-secretase-mediated release of the amyloid precursor protein (APP) intracellular domain (AICD) results in nuclear translocation and signaling through a complex with the adaptor protein Fe65 and the histone acetyltransferase Tip60. Here, we show that APP and Fe65 activate transcription through a Gal4-Tip60 reporter in presenilin-1/2-deficient cells lacking generation of AICD. APP and Fe65 also activated transcription in the presence of gamma-secretase inhibitors that prevent amyloid beta-peptide production in human embryonic kidney 293 and SH-SY5Y cells. In contrast to the transcriptionally active Notch intracellular domain, expression of AICD did not activate transcription. An alternative mechanism for APP signal transduction is suggested by the identification of essential cyclin-dependent kinase (CDK) phosphorylation sites in Tip60. Mutation of these Tip60 phosphorylation sites or treatment with the CDK inhibitor roscovitine blocked the ability of APP to signal through Tip60. Moreover, APP stabilized Tip60 through CDK-dependent phosphorylation. Subcellular fractionation and confocal immunofluorescence showed that APP recruited Tip60 to membrane compartments. Thus, APP may signal to the nucleus by a gamma-secretase-independent mechanism that involves membrane sequestration and phosphorylation of Tip60.

APP substitutions V715F and L720P alter PS1 conformation and differentially affect Abeta and AICD generation

The 37-43 amino acid Abeta peptide is the principal component of beta-amyloid deposits in Alzheimer's disease (AD) brain, and is derived by serial proteolysis of the amyloid precursor protein (APP) by beta- and gamma-secretase. gamma-Secretase also cleaves APP at Val50 in the Abeta numbering (epsilon cleavage), resulting in the release of a fragment called APP intracellular domain (AICD). The aim of this study was to determine whether amino acid substitutions in the APP transmembrane domain differentially affect Abeta and AICD generation. We found that the APPV715F substitution, which has been previously shown to dramatically decrease Abeta40 and Abeta42 while increasing Abeta38 levels, does not affect in vitro generation of AICD. Furthermore, we found that the APPL720P substitution, which has been previously shown to prevent in vitro generation of AICD, completely prevents Abeta generation. Using a fluorescence resonance energy transfer (FRET) method, we next found that both the APPV715F and APPL720P substitutions significantly increase the distance between the N- and C-terminus of presenilin 1 (PS1), which has been proposed to contain the catalytic site of gamma-secretase. In conclusion, both APPV715F and APPL720P change PS1 conformation with differential effects on Abeta and AICD production.

APH-1a is the principal mammalian APH-1 isoform present in gamma-secretase complexes during embryonic development

APH-1 (anterior pharynx defective) along with nicastrin and PEN-2 (presenilin enhancer) are essential components of the presenilin (PS)-dependent gamma-secretase complex. There exist three murine Aph-1 alleles termed Aph-1a, Aph-1b, and Aph-1c that encode four distinct APH-1 isoforms: APH-1aL and APH-1aS derived from differential splicing of Aph-1a, APH-1b, and APH-1c. To determine the contributions of mammalian APH-1 homologs in formation of functional gamma-secretase complexes, we generated Aph-1a-/- mice and derived immortalized fibroblasts from these embryos. Compared with littermate controls, the development of Aph-1a-/- embryos was dramatically retarded by embryonic day 9.5 and exhibited patterning defects that resemble, but are not identical to, those of Notch1, nicastrin, or PS null embryos. Moreover, in immortalized Aph-1a-/- fibroblasts, the levels of nicastrin, PS fragments, and PEN-2 were dramatically decreased. Consequently, deletion of Aph-1a resulted in significant reduction in levels of high-molecular-weight gamma-secretase complex and secretion of beta-amyloid (Abeta). Importantly, complementation analysis revealed that all mammalian APH-1 isoforms were capable of restoring the levels of nicastrin, PS, and PEN-2, as well as Abeta secretion in Aph-1a-/- cells. Together, our findings establish that APH-1a is the major mammalian APH-1 homolog present in PS-dependent gamma-secretase complexes during embryogenesis and support the view that mammalian APH-1 isoforms define a set of distinct functional gamma-secretase complexes.

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.

Presenilin function and gamma-secretase activity

Alzheimer's disease (AD) is the most common form of dementia and is characterized pathologically by the accumulation of beta-amyloid (Abeta) plaques and neurofibrillary tangles in the brain. Genetic studies of AD first highlighted the importance of the presenilins (PS). Subsequent functional studies have demonstrated that PS form the catalytic subunit of the gamma-secretase complex that produces the Abeta peptide, confirming the central role of PS in AD biology. Here, we review the studies that have characterized PS function in the gamma-secretase complex in Caenorhabditis elegans, mice and in in vitro cell culture systems, including studies of PS structure, PS interactions with substrates and other gamma-secretase complex members, and the evidence supporting the hypothesis that PS are aspartyl proteases that are active in intramembranous proteolysis. A thorough knowledge of the mechanism of PS cleavage in the context of the gamma-secretase complex will further our understanding of the molecular mechanisms that cause AD, and may allow the development of therapeutics that can alter Abeta production and modify the risk for AD.

Low Density Lipoprotein Receptor-related Protein (LRP) Interacts with Presenilin 1 and Is a Competitive Substrate of the Amyloid Precursor Protein (APP) for γ-Secretase

Presenilin 1 (PS1) is a critical component of the gamma-secretase complex, which is involved in the cleavage of several substrates including the amyloid precursor protein (APP) and the Notch receptor. Recently, the low density receptor-related protein (LRP) has been shown to be cleaved by a gamma-secretase-like activity. We postulated that LRP may interact with PS1 and tested its role as a competitive substrate for gamma-secretase. In this report we show that LRP colocalizes and interacts with endogenous PS1 using coimmunoprecipitation and fluorescence lifetime imaging microscopy. In addition, we found that gamma-secretase active site inhibitors do not disrupt the interaction between LRP and PS1, suggesting that the substrate associates with a gamma-secretase docking site located in close proximity to PS1. This is analogous to APP-gamma-secretase interactions. Finally, we show that LRP competes with APP for gamma-secretase activity. Overexpression of a truncated LRP construct consisting of the C terminus, the transmembrane domain, and a short extracellular portion leads to a reduction in the levels of the Abeta40, Abeta42, and p3 peptides without changing the total level of APP expression. In addition, transfection with the beta-chain of LRP causes an increase in uncleaved APP C-terminal fragments and a concomitant decrease in the signaling effects of the APP intracellular domain. In conclusion, LRP is a PS1 interactor and can compete with APP for gamma-secretase enzymatic activity.

Familial Alzheimer's disease mutations inhibit gamma-secretase-mediated liberation of beta-amyloid precursor protein carboxy-terminal fragment

Cleavage of the beta-secretase processed beta-amyloid precursor protein by gamma-secretase leads to the extracellular release of Abeta42, the more amyloidogenic form of the beta-amyloid peptide, which subsequently forms the amyloid-plaques diagnostic of Alzheimer's disease. Mutations in beta-amyloid precursor protein (APP), presenilin-1 and presenilin-2 associated with familial Alzheimer's disease (FAD) increase release of Abeta42, suggesting that FAD may directly result from increased gamma-secretase activity. Here, we show that familial Alzheimer's disease mutations clustered near the sites of gamma-secretase cleavage actually decrease gamma-secretase-mediated release of the intracellular fragment of APP (CTFgamma). Concordantly, presenilin-1 mutations that result in Alzheimer's disease also decrease the release of CTFgamma. Mutagenesis of the epsilon cleavage site in APP mimicked the effects of the FAD mutations, both decreasing CTFgamma release and increasing Abeta42 production, suggesting that perturbation of this site may account for the observed decrement in gamma-secretase-mediated proteolysis of APP. As CTFgamma has been implicated in transcriptional activation, these data indicate that decreased signaling and transcriptional regulation resulting from FAD mutations in beta-amyloid precursor protein and presenilin-1 may contribute to the pathology of Alzheimer's disease.

Phosphorylation of amyloid precursor carboxy-terminal fragments enhances their processing by a gamma-secretase-dependent mechanism

In Alzheimer's disease, the complex catabolism of amyloid precursor protein (APP) leads to the production of amyloid-beta (Abeta) peptide, the major component of amyloid deposits. APP is cleaved by beta- and alpha-secretases to generate APP carboxy-terminal fragments (CTFs). Abeta peptide and amyloid intracellular domain are resulting from the cleavage of APP-CTFs by the gamma-secretase. In the present study, we hypothesize that post-translational modification of APP-CTFs could modulate their processing by the gamma-secretase. Inhibition of the gamma-secretase was shown to increase the total amount of APP-CTFs. Moreover, we showed that this increase was more marked among the phosphorylated variants and directly related to the activity of the gamma-secretase, as shown by kinetics analyses. Phosphorylated CTFs were shown to associate to presenilin 1, a major protein of the gamma-secretase complex. The phosphorylation of CTFs at the threonine 668 resulting of the c-Jun N-terminal kinase activation was shown to enhance their degradation by the gamma-secretase. Altogether, our results demonstrated that phosphorylated CTFs can be the substrates of the gamma-secretase and that an increase in the phosphorylation of APP-CTFs facilitates their processing by gamma-secretase.

Presenilin endoproteolysis is an intramolecular cleavage

Mutations in the presenilin genes (PS) account for most cases of familial Alzheimer's disease. PS contain the active site of the gamma-secretase complex that cleaves within the transmembrane domain of beta-amyloid precursor protein (APP). Full-length PS undergoes regulated endoproteolysis to produce fragments that comprise the active form of PS. The "presenilinase" responsible for endoproteolysis is unknown but may be the same presenilin-dependent gamma-secretase activity that cleaves APP. To investigate the mechanism of endoproteolysis, we examined sequence specificity at the cleavage site and tested whether PS dimers are important for endoproteolysis as well as gamma-secretase activity. No single point mutation, or a double mutation M292D/V293K, was able to completely abolish endoproteolysis and all mutants supported gamma-secretase activity. When wtPS1 was co-expressed with either M292D/V293K or D257A, it was unable to restore normal endoproteolysis to either mutant. Lack of transcleavage by wtPS1 suggests that PS1 endoproteolysis occurs via intramolecular cleavage and does not require dimerization.

Analysis of Notch Function in Presomitic Mesoderm Suggests a γ-Secretase-Independent Role for Presenilins in Somite Differentiation

The role of Notch signaling in general and presenilin in particular was analyzed during mouse somitogenesis. We visualize cyclical production of activated Notch (NICD) and establish that somitogenesis requires less NICD than any other tissue in early mouse embryos. Indeed, formation of cervical somites proceeds in Notch1; Notch2-deficient embryos. This is in contrast to mice lacking all presenilin alleles, which have no somites. Since Nicastrin-, Pen-2-, and APH-1a-deficient embryos have anterior somites without gamma-secretase, presenilin may have a gamma-secretase-independent role in somitogenesis. Embryos triple homozygous for both presenilin null alleles and a Notch allele that is a poor substrate for presenilin (N1(V-->G)) experience fortuitous cleavage of N1(V-->G) by another protease. This restores NICD, anterior segmentation, and bilateral symmetry but does not rescue rostral/caudal identities. These data clarify multiple roles for Notch signaling during segmentation and suggest that the earliest stages of somitogenesis are regulated by both Notch-dependent and Notch-independent functions of presenilin.

Functional Overexpression of γ-Secretase Reveals Protease-independent Trafficking Functions and a Critical Role of Lipids for Protease Activity

Presenilins appear to form the active center of gamma-secretase but require the presence of the integral membrane proteins nicastrin, anterior pharynx defective 1, and presenilin enhancer 2 for catalytic function. We have simultaneously overexpressed all of these polypeptides, and we demonstrate functional assembly of the enzyme complex, a substantial increase in enzyme activity, and binding of all components to a transition state analogue gamma-secretase inhibitor. Co-localization of all components can be observed in the Golgi compartment, and further trafficking of the individual constituents seems to be dependent on functional assembly. Apart from its catalytic function, gamma-secretase appears to play a role in the trafficking of the beta-amyloid precursor protein, which was changed upon reconstitution of the enzyme but unaffected by pharmacological inhibition. Because the relative molecular mass and stoichiometry of the active enzyme complex remain elusive, we performed size exclusion chromatography of solubilized gamma-secretase, which yielded evidence of a tetrameric form of the complex, yet almost completely abolished enzyme activity. Gamma-secretase activity was reconstituted upon addition of an independent size exclusion chromatography fraction of lower molecular mass and nonproteinaceous nature, which could be replaced by a brain lipid extract. The same treatment was able to restore enzyme activity after immunoaffinity purification of the gamma-secretase complex, demonstrating that lipids play a key role in preserving the catalytic activity of this protease. Furthermore, these data show that it is important to discriminate between intact, inactive gamma-secretase complexes and the active form of the enzyme, indicating the care that must be taken in the study of gamma-secretase.

Presenilin-1 D257A and D385A mutants fail to cleave Notch in their endoproteolyzed forms, but only presenilin-1 D385A mutant can restore its gamma-secretase activity with the compensatory overexpression of normal C-terminal fragment

The enzyme gamma-secretase is involved in the cleavage of several type I membrane proteins, such as Notch 1 and amyloid precursor protein. Presenilin-1 (PS-1) is one of the critical integral membrane protein components of the gamma-secretase complex and is processed endoproteolytically, generating N- and C-terminal fragments (NTF and CTF, respectively). PS-1 is also known to incorporate into a high molecular weight complex by binding to other gamma-secretase components such as Nicastrin, Aph-1, and Pen-2. Mutations on PS-1 can alter the effects of gamma-secretase on its many substrates to different extents. Here, we showed that PS-1 mutants have a different activity for Notch cleavage, which depended on the PS-1 mutation site. We demonstrated that defective PS-1 mutants located in CTF, i.e. D385A and C410Y, could restore their gamma-secretase activities with the compensatory overexpression of wild type CTF or of minimal deleted CTF (amino acids 349-467). However, the defective PS-1 D257A mutant could not restore their gamma-secretase activities with the compensatory overexpression of wild type NTF. In comparison, both D257A NTF and D385A CTF could abolish the gamma-secretase activity of wild type and pathogenic PS-1 mutants. We also showed that PS-1 NTF but not CTF forms strong high molecular weight aggregates in SDS-PAGE. Taken together, results have shown that NTF and CTF integrate differently into high molecular weight aggregates and that PS-1 Asp-257 and Asp-385 have different accessibilities in their unendoproteolyzed conformation.

Random mutagenesis of presenilin-1 identifies novel mutants exclusively generating long amyloid beta-peptides

Familial Alzheimer disease-causing mutations in the presenilins increase production of longer pathogenic amyloid beta-peptides (A beta(42/43)) by altering gamma-secretase activity. The mechanism underlying this effect remains unknown, although it has been proposed that heteromeric macromolecular complexes containing presenilins mediate gamma-secretase cleavage of the amyloid beta-precursor protein. Using a random mutagenesis screen of presenilin-1 (PS1) for PS1 endoproteolysis-impairing mutations, we identified five unique mutants, including R278I-PS1 and L435H-PS1, that exclusively generated a high level of A beta43, but did not support physiological PS1 endoproteolysis or A beta40 generation. These mutants did not measurably alter the molecular size or subcellular localization of PS1 complexes. Pharmacological studies indicated that the up-regulation of activity for A beta43 generation by these mutations was not further enhanced by the difluoroketone inhibitor DFK167 and was refractory to inhibition by sulindac sulfide. These results suggest that PS1 mutations can lead to a wide spectrum of changes in the activity and specificity of gamma-secretase and that the effects of PS1 mutations and gamma-secretase inhibitors on the specificity are mediated through a common mechanism.

Coordinated and widespread expression of gamma-secretase in vivo: evidence for size and molecular heterogeneity

Gamma-secretase is a high molecular weight protein complex composed of four subunits, namely, presenilin (PS; 1 or 2), nicastrin, anterior pharynx defective-1 (Aph-1; A or B), and presenilin enhancer-2 (Pen-2), and is responsible for the cleavage of a number of type-1 transmembrane proteins. A fundamental question is whether different gamma-secretase complexes exist in vivo. We demonstrate here by in situ hybridization and by Northern and Western blotting that the gamma-secretase components are widely distributed in all tissues investigated. The expression of the different subunits seems tightly coregulated. However, some variation in the expression of the Aph-1 proteins is observed, Aph-1A being more general and abundantly distributed than Aph-1B. The previously uncharacterized rodent-specific Aph-1C mRNA is highly expressed in the kidney and testis but not in brain or other tissues, indicating some tissue specificity for the Aph-1 component of the gamma-secretase complex. Blue-native electrophoresis revealed size heterogeneity of the mature gamma-secretase complex in various tissues. Using co-immunoprecipitations and blue-native electrophoresis at endogenous protein levels, we find evidence that several independent gamma-secretase complexes can coexist in the same cell type. In conclusion, our results suggest that gamma-secretase is a heterogeneous family of protein complexes widely expressed in the adult organism.

Targeted deletion of numb and numblike in sensory neurons reveals their essential functions in axon arborization

Mouse Numb homologs antagonize Notch1 signaling pathways through largely unknown mechanisms. Here we demonstrate that conditional mouse mutants with deletion of numb and numblike in developing sensory ganglia show a severe reduction in axonal arborization in afferent fibers, but no deficit in neurogenesis. Consistent with these results, expression of Cre recombinase in sensory neurons from numb conditional mutants results in reduced endocytosis, a significant increase in nuclear Notch1, and severe reductions in axon branch points and total axon length. Conversely, overexpression of Numb, but not mutant Numb lacking alpha-adaptin-interacting domain, leads to accumulation of Notch1 in markedly enlarged endocytic-lysosomal vesicles, reduced nuclear Notch1, and dramatic increases in axonal length and branch points. Taken together, our data provide evidence for previously unidentified functions of Numb and Numblike in sensory axon arborization by regulating Notch1 via the endocytic-lysosomal pathways.

Nicastrin, Presenilin, APH-1, and PEN-2 Form Active γ-Secretase Complexes in Mitochondria

Mitochondria are central in the regulation of cell death. Apart from providing the cell with ATP, mitochondria also harbor several death factors that are released upon apoptotic stimuli. Alterations in mitochondrial functions, increased oxidative stress, and neurons dying by apoptosis have been detected in Alzheimer's disease patients. These findings suggest that mitochondria may trigger the abnormal onset of neuronal cell death in Alzheimer's disease. We previously reported that presenilin 1 (PS1), which is often mutated in familial forms of Alzheimer's disease, is located in mitochondria and hypothesized that presenilin mutations may sensitize cells to apoptotic stimuli at the mitochondrial level. Presenilin forms an active gamma-secretase complex together with Nicastrin (NCT), APH-1, and PEN-2, which among other substrates cleaves the beta-amyloid precursor protein (beta-APP) generating the amyloid beta-peptide and the beta-APP intracellular domain. Here we have identified dual targeting sequences (for endoplasmic reticulum and mitochondria) in NCT and showed expression of NCT in mitochondria by immunoelectron microscopy. We also showed that NCT together with APH-1, PEN-2, and PS1 form a high molecular weight complex located in mitochondria. gamma-secretase activity in isolated mitochondria was demonstrated using C83 (alpha-secretase-cleaved C-terminal 83-residue beta-APP fragment from BD8 cells lacking presenilin and thus gamma-secretase activity) or recombinant C100-Flag (C-terminal 100-residue beta-APP fragment) as substrates. Both systems generated an APP intracellular domain, and the activity was inhibited by the gamma-secretase inhibitors l-685,458 or Compound E. This novel localization of NCT, PS1, APH-1, and PEN-2 expands the role and importance of gamma-secretase activity to mitochondria.

Processing of Notch and amyloid precursor protein by gamma-secretase is spatially distinct

gamma-Secretase activity is associated with a presenilin (PS)-containing macromolecular complex. Whether PS contains the active site of gamma-secretase has been controversial. One challenge is to find PS that is engaged in the active gamma-secretase complex at the cell surface, where some substrates appear to be processed. In this study, we developed an intact cell photolabeling technique that allows the direct visualization of active gamma-secretase at the cell surface. We demonstrated that active gamma-secretase is present in the plasma membrane. Moreover, the PS1 heterodimer is specifically photolabeled at the cell surface by a potent inhibitor that binds to only the active gamma-secretase. We also explored the cellular processing sites of gamma-secretase for amyloid precursor protein (APP) and Notch by using small molecular probes. MRL631, a gamma-secretase inhibitor that is unable to penetrate the cell membrane, significantly blocks gamma-secretase-mediated Notch cleavage but has little effect on APP processing. These results indicate that Notch is processed at the cell surface and that the majority of APP is processed by intracellular gamma-secretase. Furthermore, the fact that inhibitors first target gamma-secretase in the plasma membrane for Notch processing, and not for APP, will have important implications for drug development to treat Alzheimer's disease and cancer.

The Extreme C Terminus of Presenilin 1 Is Essential for γ-Secretase Complex Assembly and Activity

The gamma-secretase complex catalyzes the cleavage of the amyloid precursor protein in its transmembrane domain resulting in the formation of the amyloid beta-peptide and the cytoplasmic APP intracellular domain. The active gamma-secretase complex is composed of at least four subunits: presenilin (PS), nicastrin, Aph-1, and Pen-2, where the presence of all components is critically required for gamma-cleavage to occur. The PS proteins are themselves subjected to endoproteolytic cleavage resulting in the generation of an N-terminal and a C-terminal fragment that remain stably associated as a heterodimer. Here we investigated the effects of modifications on the C terminus of PS1 on PS1 endoproteolysis, gamma-secretase complex assembly, and activity in cells devoid of endogenous PS. We report that certain mutations and, in particular, deletions of the PS1 C terminus decrease gamma-secretase activity, PS1 endoproteolysis, and gamma-secretase complex formation. We demonstrate that the N- and C-terminal PS1 fragments can associate with each other in mutants having C-terminal truncations that cause loss of interaction with nicastrin and Aph-1. In addition, we show that the C-terminal fragment of PS1 alone can mediate interaction with nicastrin and Aph-1 in PS null cells expressing only the C-terminal fragment of PS1. Taken together, these data suggest that the PS1 N- and C-terminal fragment intermolecular interactions are independent of an association with nicastrin and Aph-1, and that nicastrin and Aph-1 interact with the C-terminal part of PS1 in the absence of an association with full-length PS1 or the N-terminal fragment.

Amyloid-beta precursor protein processing in neurodegeneration

The amyloid-beta precursor protein is proteolytically cleaved by secretases, resulting in a series of fragments, including the amyloid-beta peptide of Alzheimer's disease. The amyloid precursor protein, when membrane anchored, could operate as a receptor. After cleavage, the soluble ectodomain exerts a trophic function in the subventricular zone. The amyloid-beta peptide itself has a depressant role in synaptic transmission, with both physiological and pathological implications. During the past two years, much time has been invested in determining the molecular pathways that regulate the processing and the signal transduction of the amyloid precursor protein. However, the absence of consistent and informative phenotypes in different loss of function animal models make elucidating the molecular actions of the amyloid-beta precursor protein an ongoing challenge.

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.

Potential role of presenilin-regulated signaling pathways in sporadic neurodegeneration

Neurodegenerative diseases can be genetic or sporadic in origin. Genetic analysis has changed the study of the pathogenesis of these disorders by showing the causative functions of rare mutations. Yet, in the most common age-associated neurodegenerative diseases such as Alzheimer's and Parkinson's diseases, the causes of neurodegeneration remain to be clarified. The observations that presenilin modulates proteolysis and turnover of several signaling molecules have led to speculations that pathways that are important in development may contribute to neurodegeneration. In this article, the possibility that these presenilin-regulated molecules may contribute to neurodegeneration is reviewed.

Intramembrane proteolysis: theme and variations

Proteases that reside in cellular membranes apparently wield water to hydrolyze the peptide bonds of substrates despite their water-excluding environment. Although these intramembrane proteases bear little or no sequence resemblance to classical water-soluble proteases, they have ostensibly converged on similar hydrolytic mechanisms. Identification of essential amino acid residues of these proteases suggests that they use residue combinations for catalysis in the same way as their soluble cousins. In contrast to classical proteases, however, the catalytic residues of intramembrane proteases lie within predicted hydrophobic transmembrane domains. Elucidating the biological functions of intramembrane proteases, identifying their substrates, and understanding how they hydrolyze peptide bonds within membranes will shed light on the ways these proteases regulate crucial biological processes and contribute to disease.

Selected non-steroidal anti-inflammatory drugs and their derivatives target gamma-secretase at a novel site. Evidence for an allosteric mechanism

Gamma-secretase is a multi-component enzyme complex that performs an intramembranous cleavage, releasing amyloid-beta (Abeta) peptides from processing intermediates of the beta-amyloid precursor protein. Because Abeta peptides are thought to be causative for Alzheimer's disease, inhibiting gamma-secretase represents a potential treatment for this neurodegenerative condition. Whereas inhibitors directed at the active center of gamma-secretase inhibit the cleavage of all its substrates, certain non-steroidal antiinflammatory drugs (NSAIDs) have been shown to selectively reduce the production of the more amyloidogenic Abeta(1-42) peptide without inhibiting alternative cleavages. In contrast to the majority of previous studies, however, we demonstrate that in cell-free systems the mode of action of selected NSAIDs and their derivatives, depending on the concentrations used, can either be classified as modulatory or inhibitory. At modulatory concentrations, a selective and, with respect to the substrate, noncompetitive inhibition of Abeta(1-42) production was observed. At inhibitory concentrations, on the other hand, biochemical readouts reminiscent of a nonselective gamma-secretase inhibition were obtained. When these compounds were analyzed for their ability to displace a radiolabeled, transition-state analog inhibitor from solubilized enzyme, noncompetitive antagonism was observed. The allosteric nature of radioligand displacement suggests that NSAID-like inhibitors change the conformation of the gamma-secretase enzyme complex by binding to a novel site, which is discrete from the binding site for transition-state analogs and therefore distinct from the catalytic center. Consequently, drug discovery efforts aimed at this site may identify novel allosteric inhibitors that could benefit from a wider window for inhibition of gamma (42)-cleavage over alternative cleavages in the beta-amyloid precursor protein and, more importantly, alternative substrates.

Functional Implications of the Presenilin Dimerization

Presenilins are the catalytic components of gamma-secretase, an intramembrane-cleaving protease whose substrates include beta-amyloid precursor protein (betaAPP) and the Notch receptors. These type I transmembrane proteins undergo two distinct presenilin-dependent cleavages within the transmembrane region, which result in the production of Abeta and APP intracellular domain (from betaAPP) and the Notch intracellular domain signaling peptide. Most cases of familial Alzheimer's disease are caused by presenilin mutations, which are scattered throughout the coding sequence. Although the underlying molecular mechanism is not yet known, the familial Alzheimer's disease mutations produce a shift in the ratio of the long and short forms of the Abeta peptide generated by the gamma-secretase. We and others have previously shown that presenilin homodimerizes and suggested that a presenilin dimer is at the catalytic core of gamma-secretase. Here, we demonstrate that presenilin transmembrane domains contribute to the formation of the dimer. In-frame substitution of the hydrophilic loop 1, located between transmembranes I and II, which modulates the interactions within the N-terminal fragment/N-terminal fragment dimer, abolishes both presenilinase and gamma-secretase activities. In addition, by reconstituting gamma-secretase activity from two catalytically inactive presenilin aspartic mutants, we provide evidence of an active diaspartyl group assembled at the interface between two presenilin monomers. Under our conditions, this catalytic group mediates the generation of APP intracellular domain and Abeta but not Notch intracellular domain, therefore suggesting that specific diaspartyl groups within the presenilin catalytic core of gamma-secretase mediate the cleavage of different substrates.