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

Association between promoter polymorphisms in anterior pharynx-defective-1a and sporadic Alzheimer's disease in the North Chinese Han population

Amyloid beta-peptide (Abeta) deposition in brain is important in the development of sporadic Alzheimer's disease (SAD) and Abeta is produced through sequential cleaving of amyloid precursor protein (APP) by beta-secretase and gamma-secrease. Anterior pharynx-defective-1 (APH-1) is an important subunit of the gamma-secretase complex, and its expression level was associated with the activity of gamma-secrease. We hypothesized that alterations in the APH-1 promoter region might alter APH-1 expression and the activity of gamma-secrease, thus be involved in the SAD process. In the present study, we sequenced APH-1a promoter region in 20 randomly selected controls and 20 SAD patients and detected two polymorphisms which were -980C/G (rs3754048) and -21C/A (rs2275780). Then, we investigated genotypes and allele of these two polymorphisms as well as apolipoprotein epsilon4 (APOE epsilon4) status in 256 SAD patients and 276 normal controls with restriction fragment length polymorphisms analysis and sequencing. Results showed the GG genotype and G allele of -980C/G polymorphism were more frequent in the SAD group than that in the controls not only in the whole subjects (genotype P=0.038, allele P=0.01 respectively) but also in the APOE epsilon4+subjects (genotype P=0.048, allele P=0.016 respectively). There was no statistical difference between SAD group and controls regarding to the frequency of alleles and genotypes of -21C/A whenever before or after stratification by APOE epsilon4. Our results suggest that there is an association between -980C/G and the development of SAD in the Northern Han Chinese population and that allele G may interact synergistically with the APOE epsilon4 allele to increase the risk of SAD.

Presenilins: members of the gamma-secretase quartets, but part-time soloists too

The presenilins in combination with other proteins generate different gamma-secretases, which are involved in the regulated intramembrane proteolysis of a variety of proteins. Understanding the specificity and regulation of these proteases will potentially lead to novel therapeutics for Alzheimer's disease and cancer. Presenilins appear also to exert additional functions outside of the gamma-secretase quartets, which needs further investigation.

Deficiency of Aph1B/C-gamma-secretase disturbs Nrg1 cleavage and sensorimotor gating that can be reversed with antipsychotic treatment

Regulated intramembrane proteolysis by gamma-secretase cleaves proteins in their transmembrane domain and is involved in important signaling pathways. At least four different gamma-secretase complexes have been identified, but little is known about their biological role and specificity. Previous work has demonstrated the involvement of the Aph1A-gamma-secretase complex in Notch signaling, but no specific function could be assigned to Aph1B/C-gamma-secretase. We demonstrate here that the Aph1B/C-gamma-secretase complex is expressed in brain areas relevant to schizophrenia pathogenesis and that Aph1B/C deficiency causes pharmacological and behavioral abnormalities that can be reversed by antipsychotic drugs. At the molecular level we find accumulation of Nrg1 fragments in the brain of Aph1BC(-/-) mice. Our observations gain clinical relevance by the demonstration that a Val-to-Leu mutation in the Nrg1 transmembrane domain, associated with increased risk for schizophrenia, affects gamma-secretase cleavage of Nrg1. This finding suggests that dysregulation of intramembrane proteolysis of Nrg1 could increase risk for schizophrenia and related disorders.

Evidence that CD147 modulation of beta-amyloid (Abeta) levels is mediated by extracellular degradation of secreted Abeta

Cerebral deposition of beta-amyloid (Abeta) peptides is a pathological hallmark of Alzheimer disease. Intramembranous proteolysis of amyloid precursor protein by a multiprotein gamma-secretase complex generates Abeta. Previously, it was reported that CD147, a glycoprotein that stimulates production of matrix metalloproteinases (MMPs), is a subunit of gamma-secretase and that the levels of secreted Abeta inversely correlate with CD147 expression. Here, we show that the levels and localization of CD147 in fibroblasts, as well as postnatal expression and distribution in brain, are distinct from those of integral gamma-secretase subunits. Notably, we show that although depletion of CD147 increased extracellular Abeta levels in intact cells, membranes isolated from CD147-depleted cells failed to elevate Abeta production in an in vitro gamma-secretase assay. Consistent with an extracellular source that modulates Abeta metabolism, synthetic Abeta was degraded more rapidly in the conditioned medium of cells overexpressing CD147. Moreover, modulation of CD147 expression had no effect on epsilon-site cleavage of amyloid precursor protein and Notch1 receptor. Collectively, our results demonstrate that CD147 modulates Abeta levels not by regulating gamma-secretase activity, but by stimulating extracellular degradation of Abeta. In view of the known function of CD147 in MMP production, we postulate that CD147 expression influences Abeta levels by an indirect mechanism involving MMPs that can degrade extracellular Abeta.

Neurosecretases provide strategies to treat sporadic and familial Alzheimer disorders

Recent discoveries on neurosecretases and their trafficking to release fibril-forming neuropeptides or other products, are of interest to pathology, cell signaling and drug discovery. Nomenclature arose from the use of amyloid precursor protein (APP) as a prototypic type-1 substrate leading to the isolation of beta-secretase (BACE), multimeric complexes (gamma-secretase, gamma-SC) for intramembranal cleavage, and attributing a new function to well-characterized metalloproteases of the ADAM family (alpha-secretase) for normal APP turnover. While purified alpha/beta-secretases facilitate drug discovery, gamma-SC presents greater challenges for characterization and mechanisms of catalysis. The review comments on links between mutation or polymorphisms in relation to enzyme mechanisms and disease. The association between lipoprotein receptor LRP11 variants and sporadic Alzheimer's disease (SAD) offers scope to integrate components of pre- and post-Golgi membranes, or brain clathrin-coated vesicles within pathways for trafficking as targets for intervention. The presence of APP and metabolites in brain clathrin-coated vesicles as significant cargo with lipoproteins and adaptors focuses attention as targets for therapeutic intervention. This overview emphasizes the importance to develop new therapies targeting neurosecretases to treat a major neurological disorder that has vast economic and social implications.

Moderate reduction of gamma-secretase attenuates amyloid burden and limits mechanism-based liabilities

Although gamma-secretase is recognized as a therapeutic target for Alzheimer's disease, side effects associated with strong inhibition of this aspartyl protease raised serious concerns regarding this therapeutic strategy. However, it is not known whether moderate inhibition of this enzyme will allow dissociation of beneficial effects in the CNS from mechanism-based toxicities in the periphery. We tested this possibility by using a series of mice with genetic reduction of gamma-secretase (levels ranging from 25 to 64% of control mice). Here, we document that even 30% reduction of gamma-secretase can effectively ameliorate amyloid burden in the CNS. However, global reduction of this enzyme below a threshold level increased the risk of developing squamous cell carcinoma as well as abnormal proliferation of granulocytes in a gamma-secretase dosage-dependent manner. Importantly, we demonstrate that there exists a critical gamma-secretase level that reduces the risk of amyloidosis in the CNS and limits tumorigenesis in epithelia. Our findings suggest that moderate inhibition of gamma-secretase represents an attractive anti-amyloid therapy for Alzheimer's disease.

Epidermal growth factor receptor and notch pathways participate in the tumor suppressor function of gamma-secretase

Gamma-secretase, a unique aspartyl protease, is required for the regulated intramembrane proteolysis of Notch and APP, pathways that are implicated, respectively, in the pathogenesis of cancer and Alzheimer disease. However, the mechanism whereby reduction of gamma-secretase causes tumors such as squamous cell carcinoma (SCC) remains poorly understood. Here, we demonstrate that gamma-secretase functions in epithelia as a tumor suppressor in an enzyme activity-dependent manner. Notch signaling is down-regulated and epidermal growth factor receptor (EGFR) is activated in SCC caused by genetic reduction of gamma-secretase. Moreover, the level of EGFR is inversely correlated with the level of gamma-secretase in fibroblasts, suggesting that the up-regulation of EGFR stimulates hyperproliferation in epithelia of mice with genetic reduction of gamma-secretase. Supporting this notion is our finding that the proliferative response of fibroblasts lacking gamma-secretase activity is more sensitive when challenged by either EGF or an inhibitor of EGFR as ompared with wild type cells. Interestingly, the up-regulation of EGFR is independent of Notch signaling, suggesting that the EGFR pathway functions in parallel with Notch in the tumorigenesis of SCC. Collectively, our results establish a novel mechanism linking the EGFR pathway to the tumor suppressor role of gamma-secretase and that mice with genetic reduction of gamma-secretase represent an excellent rodent model for clarifying pathogenesis of SCC and for testing therapeutic strategy to ameliorate this type of human cancer.

Study on the Putative Contribution of Caspases and the Proteasome to the Degradation of Aph-1a and Pen-2

The presenilin-dependent gamma-secretase complex is mainly composed of four distinct proteins, namely presenilin 1 or presenilin 2, nicastrin, anterior pharynx defective-1 (Aph-1) and presenilin enhancer (Pen-2). The mechanisms by which the complex is assembled, how its stoichiometry is controlled and how its catalytic activity is regulated are poorly understood. Recent studies indicated that Aph-1 and Pen-2 undergo proteolysis by the proteasome. We have examined the susceptibility of endogenous and overexpressed Aph-1a and Pen-2 to proteolysis by endogenous and purified proteasome as well as by recombinant caspases. We show that endogenous Aph-1a and Pen-2 resist proteolysis by caspases and by the proteasome. Furthermore, we show that unexpected interference of proteasome inhibitors with the cmv promoter region driving expression of Aph-1a and Pen-2 led to artifactual enhancement of overexpressed Aph-1a and Pen-2-like immunoreactivities but that these proteins also resist to in vitro degradation by endogenous and purified proteasome.

Presenilin/gamma-secretase-dependent processing of beta-amyloid precursor protein regulates EGF receptor expression

Presenilins (PS, PS1/PS2) are necessary for the proteolytic activity of gamma-secretase, which cleaves multiple type I transmembrane proteins including Alzheimer's beta-amyloid precursor protein (APP), Notch, ErbB4, etc. Cleavage by PS/gamma-secretase releases the intracellular domain (ICD) of its substrates. Notch ICD translocates into the nucleus to regulate expression of genes important for development. However, the patho/physiological role of other ICDs, especially APP ICD (AICD), in regulating gene expression remains controversial because evidence supporting this functionality stems mainly from studies performed under supraphysiological conditions. EGF receptor (EGFR) is up-regulated in a wide variety of tumors and hence is a target for cancer therapeutics. Abnormal expression/activation of EGFR contributes to keratinocytic carcinomas, and mice with reduced PS dosages have been shown to develop skin tumors. Here we demonstrate that the levels of PS and EGFR in the skin tumors of PS1(+/-)/ PS2(-/-) mice and the brains of PS1/2 conditional double knockout mice are inversely correlated. Deficiency in PS/gamma-secretase activity or APP expression results in a significant increase of EGFR in fibroblasts. Importantly, we show that AICD mediates transcriptional regulation of EGFR. Furthermore, we provide in vivo evidence demonstrating direct binding of endogenous AICD to the EGFR promoter. Our results indicate an important role of PS/gamma-secretase-generated APP metabolite AICD in gene transcription and in EGFR-mediated tumorigenesis.

Pathological activity of familial Alzheimer's disease-associated mutant presenilin can be executed by six different gamma-secretase complexes

gamma-Secretase is a protease complex, which catalyzes the final of two subsequent cleavages of the beta-amyloid precursor protein (APP) to release the amyloid-beta peptide (Abeta) implicated in Alzheimer's disease (AD) pathogenesis. In human cells, six gamma-secretase complexes exist, which are composed of either presenilin (PS) 1 or 2, the catalytic subunit, nicastrin, PEN-2, and either APH-1a (as S or L splice variants) or its homolog APH-1b. It is not known whether and how different APH-1 species contribute to the pathogenic activity of gamma-secretase complexes with familial AD (FAD)-associated mutant PS. Here we show that all known gamma-secretase complexes are active in APP processing and that all combinations of APH-1 variants with either FAD mutant PS1 or PS2 support pathogenic Abeta(42) production. Since our data suggest that pathogenic gamma-secretase activity cannot be attributed to a discrete gamma-secretase complex, we propose that all gamma-secretase complexes have to be explored and evaluated for their potential as AD drug target.

Interaction between the APOE epsilon4 allele and the APH-1b c + 651T > G SNP in Alzheimer's disease

The gamma-secretase complex is a multimeric aspartyl protease which plays a pivotal role in the production of amyloid beta-peptide, the main component of senile plaques in Alzheimer's disease (AD). APH-1a and APH-1b have been recently identified as important subunits of the gamma-secretase complex. We previously studied sequence variations in both genes and their association with AD in a small Italian population. The rare polymorphism c + 651T > G in APH-1b showed a possible interaction with the Apolipoprotein E (APOE) epsilon4 allele in the AD population sample. We extended our genetic analysis to 449 AD patients and 435 controls and, in AD cases, we found a significant interaction (P=0.001) between the allelic variants in the two genes, resulting in a marked increase of the relative risk for AD (OR=28.6). Despite the amino acid substitution does not seem to modify either the intracellular localization or the half-life of APH-1b protein, these data suggest that a cooperative mechanism involving APOE and APH-1b plays a role in the susceptibility to develop AD.

The role of presenilin and its interacting proteins in the biogenesis of Alzheimer's beta amyloid

The biogenesis and accumulation of the beta amyloid protein (Abeta) is a key event in the cascade of oxidative and inflammatory processes that characterises Alzheimer's disease. The presenilins and its interacting proteins play a pivotal role in the generation of Abeta from the amyloid precursor protein (APP). In particular, three proteins (nicastrin, aph-1 and pen-2) interact with presenilins to form a large multi-subunit enzymatic complex (gamma-secretase) that cleaves APP to generate Abeta. Reconstitution studies in yeast and insect cells have provided strong evidence that these four proteins are the major components of the gamma-secretase enzyme. Current research is directed at elucidating the roles that each of these protein play in the function of this enzyme. In addition, a number of presenilin interacting proteins that are not components of gamma-secretase play important roles in modulating Abeta production. This review will discuss the components of the gamma-secretase complex and the role of presenilin interacting proteins on gamma-secretase activity.

Inhibition of gamma-secretase as a therapeutic intervention for Alzheimer's disease: prospects, limitations and strategies

Genetic and experimental evidence points to amyloid-beta (Abeta) peptide as the culprit in Alzheimer's disease pathogenesis. This protein fragment abnormally accumulates in the brain cortex and hippocampus of patients with Alzheimer's disease, and self-aggregates to form toxic oligomers causing neurodegeneration.Abeta is heterogeneous and produced from a precursor protein (amyloid precursor protein [APP]) by two sequential proteolytic cleavages that involve beta- and gamma-secretases. This latter enzyme represents a potentially attractive drug target since it dictates the solubility of the generated Abeta fragment by creating peptides of various lengths, namely Abeta(40) and Abeta(42), the longest being the most aggregating. gamma-Secretase comprises a molecular complex of four integral membrane proteins - presenilin, nicastrin, APH-1 and PEN-2 - and its molecular mechanism remains under extensive scrutiny. The ratio of Abeta(42) over Abeta(40) is increased by familial Alzheimer's disease mutations occurring in the presenilin genes or in APP, near the gamma-secretase cleavage site. Potent gamma-secretase inhibitors have been identified by screening drug libraries or by designing aspartyl protease transition-state analogues based on the APP substrate cleavage site. Most of these compounds are not specific for gamma-secretase cleavage of APP, and equally inhibit the processing of other gamma-secretase substrates, such as Notch and a subset of cell-surface receptors and proteins involved in embryonic development, haematopoiesis, cell adhesion and cell/cell contacts. Therefore, current research aims at finding compounds that show selectivity for APP cleavage, and particularly that inhibit the formation of the aggregating form, Abeta(42). Compounds that target the substrate docking site rather than the enzyme active site are also being investigated as an alternative strategy. The finding that some NSAID analogues preferentially inhibit the formation of Abeta(42) over Abeta(40) and do not affect Notch processing has opened a new therapeutic window. The progress in design of selective inhibitors as well as recent results obtained in animal studies prove that gamma-secretase remains among the best targets for the therapeutic control of amyloid build-up in Alzheimer's disease. The full understanding of gamma-secretase regulation may yet uncover new therapeutic leads.

Ontogenic reduction of Aph-1b mRNA and gamma-secretase activity in rats with a complex neurodevelopmental phenotype

Selectively bred apomorphine susceptible (APO-SUS) rats display a complex behavioral phenotype remarkably similar to that of human neurodevelopmental disorders, such as schizophrenia. We recently found that the APO-SUS rats have only one or two Aph-1b gene copies (I/I and II/II rats, respectively), whereas their phenotypic counterpart has three copies (III/III). Aph-1b is a component of the gamma-secretase enzyme complex that is involved in multiple (neuro)developmental signaling pathways. Nevertheless, surprisingly little is known about gamma-secretase expression during development. Here, we performed a longitudinal quantitative PCR study in embryos and the hippocampus of I/I, II/II and III/III rats, and found gene-dosage dependent differences in Aph-1b, but not Aph-1a, mRNA expression throughout pre- and post-natal development. On the basis of the developmental mRNA profiles, we assigned relative activities to the various Aph-1a and -1b gene promoters. Furthermore, in the three rat lines, we observed both tissue-specific and temporal alterations in gamma-secretase cleavage activity towards one of its best-known substrates, the amyloid-beta precursor protein APP. We conclude that the low levels of Aph-1b mRNA and gamma-secretase activity observed in the I/I and II/II rats during the entire developmental period may well underlie their complex phenotype.

Gamma-secretase inhibitors for Alzheimer's disease: balancing efficacy and toxicity

The amyloid hypothesis, which states that beta-amyloid (Abeta) aggregates cause the onset and progression of Alzheimer's disease (AD), is a leading proposal to explain AD aetiology. Based on this hypothesis, compounds that inhibit gamma-secretase, one of the enzymes responsible for forming Abeta, are potential therapeutics for AD. Preclinical studies clearly establish that gamma-secretase inhibitors can reduce brain Abeta in rodent models. The initial investigation of the effects of a gamma-secretase inhibitor on Abeta-induced cognitive deficits in transgenic mice showed that modest Abeta reductions (15-30%) are sufficient to reverse Abeta-induced cognitive deficits in Tg2576 mice. Extending these studies to other gamma-secretase inhibitors and other models with Abeta-induced cognitive deficits will be important. Unfortunately, gamma-secretase inhibitors also cause abnormalities in the gastrointestinal tract, thymus and spleen in rodents. These changes likely result from inhibition of Notch cleavage, a transmembrane receptor involved in regulating cell-fate decisions. Two recent studies in rodents suggest that Abeta reduction using gamma-secretase inhibitors can be partially separated from Notch inhibition. Given the uncertain Abeta reduction target and the potential for mechanism-based toxicity, biomarkers for efficacy and toxicity would be helpful in clinical trials. The first report of gamma-secretase inhibitors in clinical trials was recently published. In this study, LY-450139 reduced plasma Abeta, but not cerebrospinal fluid Abeta. Taken together, the results of studies to date suggest that gamma-secretase inhibitors have the potential to address a large unmet medical need if the technical challenges can be overcome.

George-Hyslop P, Checler F. Presenilin-dependent gamma-secretase-mediated control of p53-associated cell death in Alzheimer's disease

Presenilins (PSs) are part of the gamma-secretase complex that produces the amyloid beta-peptide (Abeta) from its precursor [beta-amyloid precursor protein (betaAPP)]. Mutations in PS that cause familial Alzheimer's disease (FAD) increase Abeta production and trigger p53-dependent cell death. We demonstrate that PS deficiency, catalytically inactive PS mutants, gamma-secretase inhibitors, and betaAPP or amyloid precursor protein-like protein 2 (APLP2) depletion all reduce the expression and activity of p53 and lower the transactivation of its promoter and mRNA expression. p53 expression also is diminished in the brains of PS- or betaAPP-deficient mice. The gamma- and epsilon-secretase-derived amyloid intracellular C-terminal domain (AICD) fragments (AICDC59 and AICDC50, respectively) of betaAPP trigger p53-dependent cell death and increase p53 activity and mRNA. Finally, PS1 mutations enhance p53 activity in human embryonic kidney 293 cells and p53 expression in FAD-affected brains. Thus our study shows that AICDs control p53 at a transcriptional level, in vitro and in vivo, and that FAD mutations increase p53 expression and activity in cells and human brains.

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.

Transcriptional regulation of APH-1A and increased gamma-secretase cleavage of APP and Notch by HIF-1 and hypoxia

The proteolytic cleavage of Alzheimer beta-amyloid precursor protein (APP) and signaling receptor Notch is mediated by the PS/gamma-secretase complex, which consists of presenilins, nicastrin, APH-1, and PEN-2. Although the four components are known to coordinately regulate each other at the protein level, information regarding their transcription regulation is scarce. Here we characterized the 5'-flanking region of the human APH-1A gene and identified a 271-bp fragment containing the transcription initiation site for the promoter activity. Sequence analysis, mutagenesis, and gel shift studies revealed a binding of AP4 and HIF-1 to the promoter, which affects the promoter activity. Activation of HIF-1 by short-term NiCl2 treatments (a condition of chemical hypoxia) dramatically increased APH-1A mRNA and protein expression, accompanied by increased secretion of Abeta and decreased APP CTFs formation, indicative of an increase in gamma-secretase activity. NiCl2 treatments had little effect on APP and the other three components of the gamma-secretase complex. The cellular concentration of Notch intracellular domain (NICD) was also increased by the hypoxic treatment. Our results demonstrate that APH-1A expression and the gamma-secretase mediated Abeta and Notch NICD generation are regulated by HIF-1, and the specific control of APH-1A expression may imply physiological functions uniquely assigned to APH-1A.

Loss of nicastrin elicits an apoptotic phenotype in mouse embryos

Nicastrin is a member of the high molecular weight presenilin complex that plays a central role in gamma-secretase cleavage of numerous type-1 membrane-associated proteins required for cell signaling, proliferation and lineage development. We have generated a nicastrin-null mouse line by disruption of exon 3. Similar to previously described nicastrin-null mice, these animals demonstrate severe growth retardation, mortality beginning at embryonic age 10.5 days, and marked developmental abnormalities indicative of a severe Notch phenotype. Preceding their mortality, 10.5-day-old nicastrin-null embryos were found to also exhibit specific apoptosis within regions showing profound deformities, particularly in the developing heart and brain. This result suggests that complete disruption of presenilin complexes elicits programmed cell death, in addition to a Notch phenotype, which may contribute to the developmental abnormalities and embryonic mortality of nicastrin-null mice and possibly neurodegeneration in Alzheimer's disease.

Transcriptional regulation of PEN-2, a key component of the gamma-secretase complex, by CREB

Gamma-secretase, which is responsible for the intramembranous cleavage of Alzheimer's beta-amyloid precursor protein (APP), the signaling receptor Notch, and many other substrates, is a multiprotein complex consisting of at least four components: presenilin (PS), nicastrin, APH-1, and PEN-2. Despite the fact that PEN-2 is known to mediate endoproteolytic cleavage of full-length PS and APH-1 and nicastrin are required for maintaining the stability of the complex, the detailed physiological function of each component remain elusive. Unlike that of PS, the transcriptional regulation of PEN-2, APH-1, and nicastrin has not been investigated. Here, we characterized the upstream regions of the human PEN-2 gene and identified a 238-bp fragment located 353 bp upstream of the translational start codon as the key region necessary for the promoter activity. Further analysis revealed a CREB binding site located in the 238-bp region that is essential for the transcriptional activity of the PEN-2 promoter. Mutation of the CREB site abolished the transcriptional activity of the PEN-2 promoter. Electrophoretic mobility shift assays and chromatin immunoprecipitation analysis showed the binding of CREB to the PEN-2 promoter region both in vitro and in vivo. Activation of the CREB transcriptional factor by forskolin dramatically promoted the expression of PEN-2 mRNA and protein, whereas the other components of the gamma-secretase complex remained unaffected. Forskolin treatment slightly increases the secretion of soluble APPalpha and Abeta without affecting Notch cleavage. These results demonstrate that expression of PEN-2 is regulated by CREB and suggest that the specific control of PEN-2 expression may imply additional physiological functions uniquely assigned to PEN-2.

George-Hyslop P, Alves Da Costa C, Checler F. Catabolism of endogenous and overexpressed APH1a and PEN2: evidence for artifactual involvement of the proteasome in the degradation of overexpressed proteins

PS (presenilin)-dependent gamma-secretase occurs as a high-molecular-mass complex composed of either PS1 or PS2 associated with Nct (nicastrin), PEN2 (presenilin enhancer 2 homologue) and APH1 (anterior pharynx defective 1 homologue). Numerous reports have documented the very complicated physical and functional cross-talk between these proteins that ultimately governs the biological activity of the gamma-secretase, but very few studies examined the fate of the components of the complex. We show that, in both HEK-293 cells and the TSM1 neuronal cell line, the immunoreactivities of overexpressed myc-tagged-APH1a and -PEN2 were enhanced by the proteasome inhibitors ZIE and lactacystin, whereas a broad range of protease inhibitors had no effect. By contrast, proteasome inhibitors were totally unable to affect the cellular expression of endogenous APH1aL and PEN2 in HEK-293 cells, TSM1 and primary cultured cortical neurons. To explain this apparent discrepancy, we examined the degradation of myc-tagged-APH1a and -PEN2, in vitro, by cell extracts containing endogenous proteasome and by purified 20S proteasome. Strikingly, myc-tagged-APH1a and -PEN2 resist proteolysis by endogenous proteasome and purified 20S proteasome. We also show that endogenous PEN2 expression was drastically higher in wild-type than in PS- and Nct-deficient fibroblasts and was enhanced by proteasome inhibitors only in the two deficient cell systems. However, here again, purified 20S proteasome appeared unable to cleave endogenous PEN2 present in PS-deficient fibroblasts. The levels of endogenous APH1aL-like immunoreactivity were not modified by proteasome inhibitors and were unaffected by PS deficiency. Altogether, our results indicate that endogenous PEN2 and APH1aL do not undergo proteasomal degradation under physiological conditions in HEK-293 cells, TSM1 cells and fibroblasts and that the clearance of PEN2 in PS- and Nct-deficient fibroblasts is not mediated by 20S proteasome. Whether the 26S proteasome participates to PEN2 proteolysis in deficient fibroblasts remains to be established.

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.

Differential Localization and Identification of a Critical Aspartate Suggest Non-redundant Proteolytic Functions of the Presenilin Homologues SPPL2b and SPPL3

Signal peptide peptidase (SPP) is an unusual aspartyl protease that mediates clearance of signal peptides by proteolysis within the endoplasmic reticulum (ER). Like presenilins, which provide the proteolytically active subunit of the gamma-secretase complex, SPP contains a critical GXGD motif in its C-terminal catalytic center. Although SPP is known to be an aspartyl protease of the GXGD type, several presenilin homologues/SPP-like proteins (PSHs/SPPL) of unknown function have been identified by data base searches. We now investigated the subcellular localization and a putative proteolytic activity of PSHs/SPPLs in cultured cells and in an in vivo model. We demonstrate that SPPL2b is targeted through the secretory pathway to endosomes/lysosomes, whereas SPP and SPPL3 are restricted to the ER. As suggested by the differential subcellular localization of SPPL2b compared with SPP and SPPL3, we found distinct phenotypes upon antisense gripNA-mediated knockdown in zebrafish. spp and sppl3 knockdowns in zebrafish result in cell death within the central nervous system, whereas reduction of sppl2b expression causes erythrocyte accumulation in an enlarged caudal vein. Moreover, expression of D/A mutations of the putative C-terminal active sites of spp, sppl2, and sppl3 produced phenocopies of the respective knockdown phenotypes. Thus, our data suggest that all investigated PSHs/SPPLs are members of the novel family of GXGD aspartyl proteases. Furthermore, SPPL2b is shown to be the first member of the SPP/PSH/SPPL family that is not located within the ER but in endosomal/lysosomal vesicles.

Protease inhibitors as potential disease-modifying therapeutics for Alzheimer’s disease

The current lack of an effective treatment for Alzheimer's disease (AD) has fuelled an intense search for novel therapies for this neurodegenerative condition. Aberrant production or decreased clearance of amyloid-beta peptides is widely accepted to be causative for AD. Amyloid-beta peptides are produced by sequential processing of the beta-amyloid precursor protein by the two aspartyl-type proteases beta-secretase and gamma-secretase. Because proteases are generally classified as druggable, these secretases are a centre of attraction for various drug discovery efforts. Although a large number of specific drug-like gamma-secretase inhibitors have been discovered, progress towards the clinic has been slowed by the broad substrate specificity of this unusual intramembrane-cleaving enzyme. In particular, the Notch receptor depends on gamma-secretase for its signalling function and, thus, gamma-secretase inhibition produces distinct phenotypes related to a disturbance of this pathway in preclinical animal models. The main task now is to define the therapeutic window in man between desired central efficacy and Notch-related side effects. In contrast, most studies with knockout animals have indicated that beta-secretase inhibition may have minimal adverse effects; however, the properties of the active site of this enzyme make it difficult to find small-molecule inhibitors that bind with high affinity. In most instances, inhibitors are large and peptidic in nature and, therefore, unsuitable as drug candidates. Thus, there are many issues associated with the development of protease inhibitors for AD that must be addressed before they can be used to test the 'amyloid cascade hypothesis' in the clinic. The outcomes of such trials will provide new directions to the scientific community and hopefully new treatment options for AD patients.

Reduced Aph‐1b expression causes tissue‐ and substrate‐specific changes in γ‐secretase activity in rats with a complex phenotype

The gamma-secretase enzyme complex displays intramembrane catalytic activity toward many type I transmembrane proteins, including the Alzheimer-linked amyloid-beta-protein precursor (APP) and the neuregulin receptor ErbB4. Active gamma-secretase is a tetrameric protein complex consisting of presenilin-1 (or -2), nicastrin, PEN-2, and Aph-1a (or -1b). We have recently discovered that pharmacogenetically bred apomorphine-susceptible Wistar rats (APO-SUS) have only one or two copies of the Aph-1b gene (termed I/I and II/II rats, respectively), whereas their phenotypic counterparts (APO-UNSUS) have three copies (III/III). As a result, APO-SUS rats display reduced Aph-1b expression and a complex phenotype reminiscent of neurodevelopmental disorders. Here we determined in the I/I and III/III rats the gamma-secretase cleavage activity toward the three APP superfamily members, p75 neurotrophin receptor, ErbB4, and neuregulin-2, and found that the cleavage of only a subset of the substrates was changed. Furthermore, the observed differences were restricted to tissues that normally express relatively high Aph-1b compared with Aph-1a levels. Thus, we provide in vivo evidence that subtle alterations in gamma-secretase subunit composition may lead to a variety of affected (neuro)developmental signaling pathways and, consequently, a complex phenotype.

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.

Dissociated phenotypes in presenilin transgenic mice define functionally distinct gamma-secretases

Gamma-secretase depends on presence of presenilins (PS), Nct, Aph-1, and PEN-2 within a core complex. This endoproteolytic activity cleaves within transmembrane domains of amyloid-beta precursor protein (APP) and Notch, and familial Alzheimer's disease (FAD) mutations in PS1 or PS2 genes shift APP cleavage from production of amyloid-beta (Abeta) 40 peptide to greater production of Abeta42. Although studies in PS1/PS2-deficient embryonic cells define overlapping activities for these proteins, in vivo complementation of PS1-deficient animals described here reveals an unexpected spectrum of activities dictated by PS1 and PS2 alleles. Unlike PS1 transgenes, wild-type PS2 transgenes expressed in the mouse CNS support little Abeta40 or Abeta42 production, and FAD PS2 alleles support robust production of only Abeta42. Although wild-type PS2 transgenes failed to rescue Notch-associated skeletal defects in PS1 hypomorphs, a "gained" competence in this regard was apparent for FAD alleles of PS2. The range of discrete and divergent processing activities in mice reconstituted with different PS genes and alleles argues against gamma-secretase being a single enzyme with intrinsically relaxed substrate and cleavage site specificities. Instead, our studies define functionally distinct gamma-secretase variants. We speculate that extrinsic components, in combination with core complexes, may tailor functional variants of this enzyme to their preferred substrates.

Identification and characterization of a novel human APH-1b splice variant lacking exon 4

APH-1 is one of the four essential components of the presenilin-gamma-secretase complex and has two human homologs, APH-1a, and APH-1b, both of which are seven-pass membrane proteins. Here, we identified a novel splice variant of human APH-1b. This variant lacks exon 4, which encodes the entire fourth transmembrane domain. The mRNA expression of this variant was detected in most tissues at low levels. In transiently transfected cells, protein expression of the APH-1b variant was much lower than that of the wild-type. Furthermore, exogenous expression of the APH-1-interacting protein, nicastrin, significantly increased the variant protein levels. These data suggest that the APH-1b variant protein is destabilized, and implies that the fourth transmembrane domain plays an important role in the protein stability and function of APH-1.

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.