Aph-2/Nicastrin: an essential component of gamma-secretase and regulator of Notch signaling and Presenilin localization

The Notch signaling pathway plays a role in cell fate specification in many metazoans. A critical aspect of Notch activation involves proteolysis of the Notch receptor. This cleavage event requires Presenilin as a component of a large multiprotein complex, gamma-secretase. This complex mediates a similar cleavage event of the beta-amyloid precursor protein (APP). The transmembrane protein Nicastrin has been found to associate with Presenilin, Notch, and APP. Recent biochemical and genetic studies have focused on elucidating the function of this protein.     

Nicastrin is required for gamma-secretase cleavage of the Drosophila Notch receptor.

Nicastrin is genetically linked to Notch/lin-12 signaling in C. elegans and is part of a large multiprotein complex along with Presenilin. Here we describe the isolation and characterization of Drosophila Nicastrin (Nic) mutants. Nic mutants and tissue clones display characteristic Notch-like phenotypes. Genetic and inhibitor studies indicate a function for Nicastrin in the gamma-secretase step of Notch processing, similar to Presenilin. Further, Nicastrin is genetically required for signaling from membrane-anchored activated Notch. In the absence of Nicastrin, Presenilin is destabilized and mature C-terminal subunits are absent. Nicastrin might recruit gamma-secretase substrates into the proteolytic complex as a prerequisite for Presenilin maturation and active complex assembly.     

Mammalian APH-1 interacts with presenilin and nicastrin and is required for intramembrane proteolysis of amyloid-beta precursor protein and Notch

Presenilin and nicastrin are essential components of the gamma-secretase complex that is required for the intramembrane proteolysis of an increasing number of membrane proteins including the amyloid-beta precursor protein (APP) and Notch. By using co-immunoprecipitation and nickel affinity pull-down approaches, we now show that mammalian APH-1 (mAPH-1), a conserved multipass membrane protein, physically associates with nicastrin and the heterodimers of the presenilin amino- and carboxyl-terminal fragments in human cell lines and in rat brain. Similar to the loss of presenilin or nicastrin, the inactivation of endogenous mAPH-1 using small interfering RNAs results in the decrease of presenilin levels, accumulation of gamma-secretase substrates (APP carboxyl-terminal fragments), and reduction of gamma-secretase products (amyloid-beta peptides and the intracellular domains of APP and Notch). These data indicate that mAPH-1 is probably a functional component of the gamma-secretase complex required for the intramembrane proteolysis of APP and Notch.     

GSK3beta activity modifies the localization and function of presenilin 1

Presenilin 1, a causative gene product of familial Alzheimer disease, has been reported to be localized mainly in the endoplasmic reticulum and Golgi membranes. However, endogenous Presenilin 1 also localizes at the plasma membrane as a biologically active molecule. Presenilin 1 interacts with N-cadherin/beta-catenin to form a trimeric complex at the synaptic site through its loop domain, whose serine residues (serine 353 and 357) can be phosphorylated by glycogen synthase kinase 3beta. Here, we demonstrate that cell-surface expression of Presenilin 1/gamma-secretase is enhanced by N-cadherin-based cell-cell contact. Physical interaction between Presenilin 1 and N-cadherin/beta-catenin plays an important role in this process. Glycogen synthase kinase 3beta-mediated phosphorylation of Presenilin 1 reduces its binding to N-cadherin, thereby down-regulating its cell-surface expression. Moreover, reduction of the Presenilin 1.N-cadherin.beta-catenin complex formation leads to an impaired activation of contact-mediated phosphatidylinositol 3-kinase/Akt cell survival signaling. Furthermore, phosphorylation of Presenilin 1 hinders epsilon-cleavage of N-cadherin, whereas epsilon-cleavage of APP remained unchanged. This is the first report that clarifies the regulatory mechanism of Presenilin 1/gamma-secretase with respect to its subcellular distribution and its differential substrate cleavage. Because the cleavage of various membrane proteins by Presenilin 1/gamma-cleavage is involved in cellular signaling, glycogen synthase kinase 3beta-mediated phosphorylation of Presenilin 1 should be deeply associated with signaling functions. Our findings indicate that the abnormal activation of glycogen synthase kinase 3beta can reduce neuronal viability and synaptic plasticity via modulating Presenilin 1/N-cadherin/beta-catenin interaction and thus have important implications in the pathophysiology of Alzheimer disease.     

Drosophila nicastrin is essential for the intramembranous cleavage of notch.

The catalytic subunit of gamma-secretase is thought to be Presenilin, which is required for both the cleavage of APP and in the processing of Notch. Presenilin is found in a multisubunit complex that also contains Nicastrin. Nicastrin has been implicated in APP processing, but its role in Notch signaling remains unclear. Here we show that Drosophila Nicastrin is required for Notch signaling, and acts specifically at the S3 cleavage step. Partially processed Notch accumulates apically in nicastrin and presenilin mutant follicle cells. nicastrin and presenilin mutations also disrupt the spectrin cytoskeleton, suggesting that the gamma-secretase complex has another function in Drosophila in addition to its role in processing Notch and APP.     

Notch1 competes with the amyloid precursor protein for gamma-secretase and down-regulates presenilin-1 gene expression

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 Notch1. Based on the fact that APP and Notch are processed by the same gamma-secretase, we postulated that APP and Notch compete for the enzyme activity. In this report, we examined the interactions between APP, Notch, and PS1 using the direct gamma-secretase substrates, Notch 1 Delta extracellular domain (N1DeltaEC) and APP carboxyl-terminal fragment of 99 amino acids, and measured the effects on amyloid-beta protein production and Notch signaling, respectively. Additionally, we tested the hypothesis that downstream effects on PS1 expression may coexist with the competition phenomenon. We observed significant competition between Notch and APP for gamma-secretase activity; transfection with either of two direct substrates of gamma-secretase led to a reduction in the gamma-cleaved products, Notch intracellular domain or amyloid-beta protein. In addition, however, we found that activation of the Notch signaling pathway, by either N1 Delta EC or Notch intracellular domain, induced down-regulation of PS1 gene expression. This finding suggests that Notch activation directly engages gamma-secretase and subsequently leads to diminished PS1 expression, suggesting a complex set of feedback interactions following Notch activation.     

Low density lipoprotein receptor-related protein (LRP) interacts with presenilin 1 and is a competitive substrate of the amyloid precursor protein (APP) for gamma-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.     

The gamma-secretase complex: machinery for intramembrane proteolysis

Gamma-secretase is a membrane protease complex that possesses presenilin as a catalytic subunit. Presenilin generates amyloid beta peptides in the brains of Alzheimer's patients and is indispensable to Notch signaling in tissue development and renewal. Recent studies have revealed how presenilin is assembled with its cofactor proteins and acquires the gamma-secretase activity: Aph-1 and nicastrin initially form a subcomplex to bind and stabilize presenilin, and then Pen-2 confers the gamma-secretase activity and facilitates endoproteolysis of presenilin. Understanding the mechanism of gamma-secretase cleavage will help to clarify how intercellular cell signaling through transmembrane proteins is regulated by intramembrane proteolysis, and will hopefully eventually lead to a cure for Alzheimer's disease.     

Presenilins mutated at Asp-257 or Asp-385 restore Pen-2 expression and Nicastrin glycosylation but remain catalytically inactive in the absence of wild type Presenilin

The Presenilins are part of the gamma-secretase complex that is involved in the regulated intramembrane proteolysis of amyloid precursor protein and other type I integral membrane proteins. Nicastrin, Pen-2, and Aph1 are the other proteins of this complex. The Presenilins probably contribute the catalytic activity to the protease complex. However, several investigators reported normal Abeta-peptide generation in cells expressing Presenilins mutated at the putative catalytic site residue Asp-257, contradicting this hypothesis. Because endogenously expressed wild type Presenilin could contribute to residual gamma-secretase activity in these experiments, we have reinvestigated the problem by expressing mutated Presenilins in a Presenilin-negative cell line. We confirm that Presenilins with mutated Asp residues are catalytically inactive. Unexpectedly, these mutated Presenilins are still partially processed into amino- and carboxyl-terminal fragments by a "Presenilinase"-like activity. They are also able to rescue Pen-2 expression and Nicastrin glycosylation in Presenilin-negative cells and become incorporated into large approximately 440-kDa complexes as assessed by blue native gel electrophoresis. Our study demonstrates that the catalytic activity of Presenilin and its other functions in the generation, stabilization, and transport of the gamma-secretase complex can be separated and extends the concept that Presenilins are multifunctional proteins.     

Presenilin-based genetic screens in Drosophila melanogaster identify novel notch pathway modifiers

Presenilin is the enzymatic component of gamma-secretase, a multisubunit intramembrane protease that processes several transmembrane receptors, such as the amyloid precursor protein (APP). Mutations in human Presenilins lead to altered APP cleavage and early-onset Alzheimer's disease. Presenilins also play an essential role in Notch receptor cleavage and signaling. The Notch pathway is a highly conserved signaling pathway that functions during the development of multicellular organisms, including vertebrates, Drosophila, and C. elegans. Recent studies have shown that Notch signaling is sensitive to perturbations in subcellular trafficking, although the specific mechanisms are largely unknown. To identify genes that regulate Notch pathway function, we have performed two genetic screens in Drosophila for modifiers of Presenilin-dependent Notch phenotypes. We describe here the cloning and identification of 19 modifiers, including nicastrin and several genes with previously undescribed involvement in Notch biology. The predicted functions of these newly identified genes are consistent with extracellular matrix and vesicular trafficking mechanisms in Presenilin and Notch pathway regulation and suggest a novel role for gamma-tubulin in the pathway.     

FAD-linked mutations in presenilin 1 alter the length of Abeta peptides derived from betaAPP transmembrane domain mutants

gamma-Secretase is an enzymatic activity responsible for the final cleavage of the amyloid precursor protein leading to the production of the amyloid beta-peptide (Abeta). gamma-Secretase is likely an aspartyl protease, since its activity can be inhibited by both pepstatin and active-site directed aspartyl protease inhibitors. Recent work has indicated that presenilins 1 and 2 may actually be the gamma-secretase enzymes. Presenilin (PS) mutations, which lead to an increase in the production of a longer form of Abeta, are also the most common cause of familial Alzheimer's disease (FAD). Therefore, in an attempt to better characterize the substrate preferences of gamma-secretase, we performed experiments to determine how FAD-linked mutations in PS1 would affect the generation of Abeta peptides from full length precursor substrates that we have previously demonstrated to be proteolytically cleaved at alternative sites and/or by enzymatic activities that are pharmacologically distinct. Presenilin mutations increased the production of Abeta peptides from sites distal to the primary cleavage site ('longer' peptides) and in several cases also decreased production of 'shorter' peptides. These results support a model in which the FAD-linked mutants subtly alter the conformation of the gamma-secretase complex to favor the production of long Abeta.     

Contribution of presenilin transmembrane domains 6 and 7 to a water-containing cavity in the gamma-secretase complex

Gamma-secretase is a multiprotein complex responsible for the intramembranous cleavage of the amyloid precursor protein and other type I transmembrane proteins. Mutations in Presenilin, the catalytic core of this complex, cause Alzheimer disease. Little is known about the structure of the protein and even less about the catalytic mechanism, which involves proteolytic cleavage in the hydrophobic environment of the cell membrane. It is basically unclear how water, needed to perform hydrolysis, is provided to this reaction. Presenilin transmembrane domains 6 and 7 seem critical in this regard, as each bears a critical aspartate contributing to catalytic activity. Current models imply that both aspartyl groups should closely oppose each other and have access to water. This is, however, still to be experimentally verified. Here, we have performed cysteine-scanning mutagenesis of both domains and have demonstrated that several of the introduced residues are exposed to water, providing experimental evidence for the existence of a water-filled cavity in the catalytic core of Presenilin. In addition, we have demonstrated that the two aspartates reside within this cavity and are opposed to each other in the native complex. We have also identified the conserved tyrosine 389 as a critical partner in the catalytic mechanism. Several additional amino acid substitutions affect differentially the processing of gamma-secretase substrates, implying that they contribute to enzyme specificity. Our data suggest the possibility that more selective gamma-secretase inhibitors could be designed.     

Presenilin endoproteolysis mediated by an aspartyl protease activity pharmacologically distinct from gamma-secretase

Presenilin (PS)-dependent gamma-secretase cleavage is the final proteolytic step in generating amyloid beta protein (A beta), a key peptide involved in the pathogenesis of Alzheimer's disease. PS undergoes endoproteolysis by an unidentified 'presenilinase' to generate the functional N-terminal and C-terminal fragment heterodimers (NTF/CTF) that may harbor the gamma-secretase active site. To better understand the relationship between presenilinase and gamma-secretase, we characterized the biochemical properties of presenilinase and compared them with those of gamma-secretase. Similar to gamma-secretase, presenilinase was most active at acidic pH 6.3. Aspartyl protease inhibitor pepstatin A blocked presenilinase activity with an IC50 of approximately 1 microM. Difluoroketone aspartyl protease transition state analogue MW167 was relatively selective for presenilinase (IC50 < 1 microM) over gamma-secretase (IC50-16 microM). Importantly, removing the transition state mimicking moiety simultaneously abolished both presenilinase and gamma-secretase inhibition, suggesting that presenilinase, like gamma-secretase, is an aspartyl protease. Interestingly, several of the most potent gamma-secretase inhibitors (IC50 = 0.3 or 20 nM) failed to block presenilinase activity. Although de novo generation of PS1 fragments coincided with production of A beta in vitro, blocking presenilinase activity without reducing pre-existing fragment levels permitted normal de novo generation of A beta and amyloid intracellular domain. Therefore, presenilinase has characteristics of an aspartyl protease, but this activity is distinct from gamma-secretase.     

Signal peptide peptidase and gamma-secretase share equivalent inhibitor binding pharmacology

The enzyme gamma-secretase has long been considered a potential pharmaceutical target for Alzheimer disease. Presenilin (the catalytic subunit of gamma-secretase) and signal peptide peptidase (SPP) are related transmembrane aspartyl proteases that cleave transmembrane substrates. SPP and gamma-secretase are pharmacologically similar in that they are targeted by many of the same small molecules, including transition state analogs, non-transition state inhibitors, and amyloid beta-peptide modulators. One difference between presenilin and SPP is that the proteolytic activity of presenilin functions only within a multisubunit complex, whereas SPP requires no additional protein cofactors for activity. In this study, gamma-secretase inhibitor radioligands were used to evaluate SPP and gamma-secretase inhibitor binding pharmacology. We found that the SPP enzyme exhibited distinct binding sites for transition state analogs, non-transition state inhibitors, and the nonsteroidal anti-inflammatory drug sulindac sulfide, analogous to those reported previously for gamma-secretase. In the course of this study, cultured cells were found to contain an abundance of SPP binding activity, most likely contributed by several of the SPP family proteins. The number of SPP binding sites was in excess of gamma-secretase binding sites, making it essential to use selective radioligands for evaluation of gamma-secretase binding under these conditions. This study provides further support for the idea that SPP is a useful model of inhibitory mechanisms and structure in the SPP/presenilin protein family.     

Gamma-secretase exists on the plasma membrane as an intact complex that accepts substrates and effects intramembrane cleavage

Research on Alzheimer's disease led to the identification of a novel proteolytic mechanism in all metazoans, the presenilin/gamma-secretase complex. This unique intramembrane-cleaving aspartyl protease is required for the normal processing of Notch, Jagged, beta-amyloid precursor protein (APP), E-cadherin, and many other receptor-like proteins. We recently provided indirect evidence of gamma-secretase activity at the cell surface in HeLa cells following inhibition of receptor-mediated endocytosis. Here, we directly identify and isolate gamma-secretase as an intact complex (Presenilin, Nicastrin, Aph-1, and Pen-2) from the plasma membrane, both in overexpressing cell lines and endogenously. Inhibition of its proteolytic activity allowed cell surface gamma-secretase to be captured in association with its plasma membrane-localized APP substrates (C83 and C99). Moreover, non-denaturing isolation of the intact enzyme complex revealed that cell surface gamma-secretase can specifically generate amyloid beta-protein from an APP substrate and similarly cleave a Notch substrate. These data directly establish the proteolytic function of gamma-secretase on the plasma membrane, independent of a hypothesized substrate trafficking role. We conclude that presenilin/gamma-secretase exists as a mature complex at the cell surface, where it interacts with and can cleave its substrates, consistent with an essential function in processing many adhesion molecules and receptors required for cell-cell interaction or intercellular signaling.     

Regulated intramembrane proteolysis of Bri2 (Itm2b) by ADAM10 and SPPL2a/SPPL2b

Presenilin, the catalytic component of the gamma-secretase complex, type IV prepilin peptidases, and signal peptide peptidase (SPP) are the founding members of the family of intramembrane-cleaving GXGD aspartyl proteases. SPP-like (SPPL) proteases, such as SPPL2a, SPPL2b, SPPL2c, and SPPL3, also belong to the GXGD family. In contrast to gamma-secretase, for which numerous substrates have been identified, very few in vivo substrates are known for SPP and SPPLs. Here we demonstrate that Bri2 (Itm2b), a type II-oriented transmembrane protein associated with familial British and Danish dementia, undergoes regulated intramembrane proteolysis. In addition to the previously described ectodomain processing by furin and related proteases, we now describe that the Bri2 protein, similar to gamma-secretase substrates, undergoes an additional cleavage by ADAM10 in its ectodomain. This cleavage releases a soluble variant of Bri2, the BRICHOS domain, which is secreted into the extracellular space. Upon this shedding event, a membrane-bound Bri2 N-terminal fragment remains, which undergoes intramembrane proteolysis to produce an intracellular domain as well as a secreted low molecular weight C-terminal peptide. By expressing all known SPP/SPPL family members as well as their loss of function variants, we demonstrate that selectively SPPL2a and SPPL2b mediate the intramembrane cleavage, whereas neither SPP nor SPPL3 is capable of processing the Bri2 N-terminal fragment.     

Signal peptide peptidase forms a homodimer that is labeled by an active site-directed gamma-secretase inhibitor

Presenilin (PS) is the presumptive catalytic component of the intramembrane aspartyl protease gamma-secretase complex. Recently a family of presenilin homologs was identified. One member of this family, signal peptide peptidase (SPP), has been shown to be a protease, which supports the hypothesis that PS and presenilin homologs are related intramembrane-cleaving aspartyl proteases. SPP has been reported as a glycoprotein of approximately 45 kDa. Our initial characterization of SPP isolated from human brain and cell lines demonstrated that SPP is primarily present as an SDS-stable approximately 95-kDa protein on Western blots. Upon heating or treatment of this approximately 95-kDa SPP band with acid, a approximately 45-kDa band could be resolved. Co-purification of two different epitope-tagged forms of SPP from a stably transfected cell line expressing both tagged versions demonstrated that the approximately 95-kDa band is a homodimer of SPP. Pulse-chase metabolic labeling studies demonstrated that the SPP homodimer assembles rapidly and is metabolically stable. In a glycerol velocity gradient, SPP sedimented from approximately 100-200 kDa. Significantly the SPP homodimer was specifically labeled by an active site-directed photoaffinity probe (III-63) for PS, indicating that the active sites of SPP and PS/gamma-secretase are similar and providing strong evidence that the homodimer is functionally active. Collectively these data suggest that SPP exists in vivo as a functional dimer.     

Loss-of-function presenilin mutations in Alzheimer disease. Talking Point on the role of presenilin mutations in Alzheimer disease

Presenilin mutations are the main cause of familial Alzheimer disease. From a genetic point of view, these mutations seem to result in a gain of toxic function; however, biochemically, they result in a partial loss of function in the gamma-secretase complex, which affects several downstream signalling pathways. Consequently, the current genetic terminology is misleading. In fact, the available data indicate that several clinical presenilin mutations also lead to a decrease in amyloid precursor protein-derived amyloid beta-peptide generation, further implying that presenilin mutations are indeed loss-of-function mutations. The loss of function of presenilin causes incomplete digestion of the amyloid beta-peptide and might contribute to an increased vulnerability of the brain, thereby explaining the early onset of the inherited form of Alzheimer disease. In this review, I evaluate the implications of this model for the amyloid-cascade hypothesis and for the efficacy of presenilin/gamma-secretase as a drug target.