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

Functional and topological analysis of PSENEN, the fourth subunit of the γ-secretase complex

The γ-secretase complexes are intramembrane cleaving proteases involved in the generation of the Aβ peptides in Alzheimer's disease. The complex consists of four subunits, with Presenilin harboring the catalytic site. Here, we study the role of the smallest subunit, PSENEN or Presenilin enhancer 2, encoded by the gene Psenen, in vivo and in vitro. We find a profound Notch deficiency phenotype in Psenen-/- embryos confirming the essential role of PSENEN in the γ-secretase complex. We used Psenen-/- fibroblasts to explore the structure-function of PSENEN by the scanning cysteine accessibility method. Glycine 22 and proline 27, which border the membrane domains 1 and 2 of PSENEN, are involved in complex formation and stabilization of γ-secretase. The hairpin structured hydrophobic membrane domains 1 and 2 are exposed to a water-containing cavity in the complex, while transmembrane domain 3 is not water exposed. We finally demonstrate the essential role of PSENEN for the cleavage activity of the complex. PSENEN is more than a structural component of the γ-secretase complex and might contribute to the catalytic mechanism of the enzyme.

Flexible and Accurate Substrate Processing with Distinct Presenilin/γ-Secretases in Human Cortical Neurons

Mutations in the presenilin genes (PS1, PS2) have been linked to the majority of familial Alzheimer’s disease (AD). Although great efforts have been made to investigate pathogenic PS mutations, which ultimately cause an increase in the toxic form of β-amyloid (Aβ), the intrinsic physiological functions of PS in human neurons remain to be determined. In this study, to investigate the physiological roles of PS in human neurons, we generated PS1 conditional knock-out (KO) induced pluripotent stem cells (iPSCs), in which PS1 can be selectively abrogated under Cre transduction with or without additional PS2 KO. We showed that iPSC-derived neural progenitor cells (NPCs) do not confer a maintenance ability in the absence of both PS1 and PS2, showing the essential role of PS in Notch signaling. We then generated PS-null human cortical neurons, where PS1 was intact until full neuronal differentiation occurred. Aβ40 production was reduced exclusively in human PS1/PS2-null neurons along with a concomitant accumulation of amyloid β precursor protein (APP)-C-terminal fragments CTFs, whereas Aβ42 was decreased in neurons devoid of PS2. Unlike previous studies in mice, in which APP cleavage is largely attributable to PS1, γ-secretase activity seemed to be comparable between PS1 and PS2. In contrast, cleavage of another substrate, N-cadherin, was impaired only in neurons devoid of PS1. Moreover, PS2/γ-secretase exists largely in late endosomes/lysosomes, as measured by specific antibody against the γ-secretase complex, in which Aβ42 species are supposedly produced. Using this novel stem cell-based platform, we assessed important physiological PS1/PS2 functions in mature human neurons, the dysfunction of which could underlie AD pathogenesis.

Structural characterization of a polysaccharide from Lycium barbarum and its neuroprotective effect against β-amyloid peptide neurotoxicity

A water-soluble polysaccharide, designated as LBP-3, was isolated and purified from Lycium barbarum. Chemical analysis indicated that LBP-3 was composed of arabinose and galactose at a molar ratio of 1.00:1.56. The average molecular weight of LBP-3 was 6.74 × 104 Da. The structural features of LBP-3 were investigated by Fourier-transform infrared spectroscopy (FT-IR), methylation, and nuclear magnetic resonance (NMR). LBP-3 is a highly branched polysaccharide with a backbone of 1, 3-linked β-Galp, which is partially substituted at C-6. The branches contain 1, 5-linked α-Araf, 1, 6-linked β-Galp, 1, 3-linked α-Araf, and 1, 4-linked α-Araf. In vitro studies revealed that LBP-3 induced a concentration-dependent decrease in the levels of Aβ42/Aβ40 in N2a/APP695 cells. Proteomic analysis was conducted to investigate the potential molecular mechanism underlying the neuroprotective effect of LBP-3, and the results suggested that LBP-3 might have the potential for the treatment of AD.

Negligible senescence in naked mole rats may be a consequence of well-maintained splicing regulation

Naked mole-rats (NMRs) have amongst the longest lifespans relative to body size of any known, non-volant mammalian species. They also display an enhanced stress resistance phenotype, negligible senescence and very rarely are they burdened with chronic age-related diseases. Alternative splicing (AS) dysregulation is emerging as a potential driver of senescence and ageing. We hypothesised that the expression of splicing factors, important regulators of patterns of AS, may differ in NMRs when compared to other species with relatively shorter lifespans. We designed assays specific to NMR splicing regulatory factors and also to a panel of pre-selected brain-expressed genes known to demonstrate senescence-related alterations in AS in other species, and measured age-related changes in the transcript expression levels of these using embryonic and neonatal developmental stages through to extreme old age in NMR brain samples. We also compared splicing factor expression in both young mouse and NMR spleen and brain samples. Both NMR tissues showed approximately double the expression levels observed in tissues from similarly sized mice. Furthermore, contrary to observations in other species, following a brief period of labile expression in early life stages, adult NMR splicing factors and patterns of AS for functionally relevant brain genes remained remarkably stable for at least two decades. These findings are consistent with a model whereby the conservation of splicing regulation and stable patterns of AS may contribute to better molecular stress responses and the avoidance of senescence in NMRs, contributing to their exceptional lifespan and prolonged healthspan.

miR-151-5p modulates APH1a expression to participate in contextual fear memory formation

Long-term memory formation requires gene expression and new protein synthesis. MicroRNAs (miRNAs), a family of small non-coding RNAs that inhibit target gene mRNA expression, are involved in new memory formation. In this study, elevated miR-151-5p (miR-151) levels were found to be responsible for hippocampal contextual fear memory formation. Using a luciferase reporter assay, we demonstrated that miR-151 targets APH1a, a protein that has been identified as a key factor in γ-secretase activity, namely APH1a. Blocking miR-151 can upregulate APH1a protein levels and subsequently impair hippocampal fear memory formation. These results indicate that miR-151 is involved in hippocampal contextual fear memory by inhibiting APH1a protein expression. This work provides novel evidence for the role of miRNAs in memory formation and demonstrates the implication of APH1a protein in miRNA processing in the adult brain.

Deletion of the γ-secretase subunits Aph1B/C impairs memory and worsens the deficits of knock-in mice modeling the Alzheimer-like familial Danish dementia

Mutations in BRI2/ITM2b genes cause Familial British and Danish Dementias (FBD and FDD), which are pathogenically similar to Familial Alzheimer Disease (FAD). BRI2 inhibits processing of Amyloid precursor protein (APP), a protein involved in FAD pathogenesis. Accumulation of a carboxyl-terminal APP metabolite -ß-CTF- causes memory deficits in a knock-in mouse model of FDD, called FDDKI.We have investigated further the pathogenic function of ß-CTF studying the effect of Aph1B/C deletion on FDDKI mice. This strategy is based on the evidence that deletion of Aph1B/C proteins, which are components of the γ-secretase that cleaves ß-CTF, results in stabilization of ß-CTF and a reduction of Aβ. We found that both the FDD mutation and the Aph1B/C deficiency mildly interfered with spatial long term memory, spatial working/short-term memory and long-term contextual fear memory. In addition, the Aph1BC deficiency induced deficits in long-term cued fear memory. Moreover, the two mutations have additive adverse effects as they compromise the accuracy of spatial long-term memory and induce spatial memory retention deficits in young mice. Overall, the data are consistent with a role for β-CTF in the genesis of memory deficits.

Specific combinations of presenilins and Aph1s affect the substrate specificity and activity of γ-secretase

The γ-secretase complex comprises presenilin (PS), nicastrin (NCT), anterior pharynx-defective 1 (Aph1), and presenilin enhancer 2 (Pen2). PS has two homologues, PS1 and PS2. Aph1 has two isoforms, Aph1a and Aph1b, with the former existing as two splice variants Aph1aL and Aph1aS. Each complex consists of one subunit each, resulting in six different γ-secretases. To better understand the functional differences among the γ-secretases, we reconstituted them using a yeast system and compared Notch1-cleavage and amyloid precursor protein (APP)-cleavage activities. Intriguingly, PS2/Aph1b had a clear substrate specificity: APP-Gal4, but not Notch-Gal4, was cleaved. In HEK cell lines expressing defined γ-secretase subunits, we showed that PS1/Aph1b, PS2/Aph1aL, PS2/Aph1aS and PS2/Aph1b γ-secretase produced amyloid β peptide (Aβ) with a higher Aβ42+Aβ43-to-Aβ40 (Aβ42(43)/Aβ40) ratio than the other γ-secretases. In addition, PS2/Aph1aS γ-secretase produced less Notch intracellular domain (NICD) than did the other 5 γ-secretases. Considering that the Aβ42(43)/Aβ40 ratio is relevant in the pathogenesis of Alzheimer's disease (AD), and that inhibition of Notch cleavage causes severe side effect, these results suggest that the PS2/Aph1aS γ-secretase complex is a potential therapeutic target in AD.

Physiological and pathological roles of the γ-secretase complex

Gamma-secretase (GS) is an enzyme complex that cleaves numerous substrates, and it is best known for cleaving amyloid precursor protein (APP) to form amyloid-beta (Aβ) peptides. Aberrant cleavage of APP can lead to Alzheimer's disease, so much research has been done to better understand GS structure and function in hopes of developing therapeutics for Alzheimer's. Therefore, most of the attention in this field has been focused on developing modulators that reduce pathogenic forms of Aβ while leaving Notch and other GS substrates intact, but GS provides multiple avenues of modulation that could improve AD pathology. GS has complex regulation, through its essential subunits and other associated proteins, providing other targets for AD drugs. Therapeutics can also alter GS trafficking and thereby improve cognition, or move beyond Aβ entirely, effecting Notch and neural stem cells. GS also cleaves substrates that affect synaptic morphology and function, presenting another window by which GS modulation could improve AD pathology. Taken together, GS presents a unique cross road for neural processes and an ideal target for AD therapeutics.

Beyond γ-secretase activity: The multifunctional nature of presenilins in cell signalling pathways

The presenilins are the catalytic subunit of the membrane-embedded tetrameric γ-secretase protease complexes. More that 90 transmembrane proteins have been reported to be γ-secretase substrates, including the widely studied amyloid precursor protein (APP) and the Notch receptor, which are precursors for the generation of amyloid-β peptides and biologically active APP intracellular domain (AICD) and Notch intracellular domain (NICD). The diversity of γ-secretase substrates highlights the importance of presenilin-dependent γ-secretase protease activities as a regulatory mechanism in a range of biological systems. However, there is also a growing body of evidence that supports the existence of γ-secretase-independent functions for the presenilins in the regulation and progression of an array of cell signalling pathways. In this review, we will present an overview of current literature that proposes evolutionarily conserved presenilin functions outside of the γ-secretase complex, with a focus on the suggested role of the presenilins in the regulation of Wnt/β-catenin signalling, protein trafficking and degradation, calcium homeostasis and apoptosis.

The γ-secretase complex: from structure to function

One of the most critical pathological features of Alzheimer’s disease (AD) is the accumulation of β-amyloid (Aβ) peptides that form extracellular senile plaques in the brain. Aβ is derived from β-amyloid precursor protein (APP) through sequential cleavage by β- and γ-secretases. γ-secretase is a high molecular weight complex minimally composed of four components: presenilins (PS), nicastrin, anterior pharynx defective 1 (APH-1), and presenilin enhancer 2 (PEN-2). In addition to APP, γ-secretase also cleaves many other type I transmembrane (TM) protein substrates. As a crucial enzyme for Aβ production, γ-secretase is an appealing therapeutic target for AD. Here, we summarize current knowledge on the structure and function of γ-secretase, as well as recent progress in developing γ-secretase targeting drugs for AD treatment.

Allosteric regulation of γ-secretase activity by a phenylimidazole-type γ-secretase modulator

γ-Secretase is an intramembrane-cleaving protease responsible for the generation of amyloid-β (Aβ) peptides. Recently, a series of compounds called γ-secretase modulators (GSMs) has been shown to decrease the levels of long toxic Aβ species (i.e., Aβ42), with a concomitant elevation of the production of shorter Aβ species. In this study, we show that a phenylimidazole-type GSM allosterically induces conformational changes in the catalytic site of γ-secretase to augment the proteolytic activity. Analyses using the photoaffinity labeling technique and systematic mutational studies revealed that the phenylimidazole-type GSM targets a previously unidentified extracellular binding pocket within the N-terminal fragment of presenilin (PS). Collectively, we provide a model for the mechanism of action of the phenylimidazole-type GSM in which binding at the luminal side of PS induces a conformational change in the catalytic center of γ-secretase to modulate Aβ production.

A fast growing spectrum of biological functions of γ-secretase in development and disease

γ-secretase, which assembles as a tetrameric complex, is an aspartyl protease that proteolytically cleaves substrate proteins within their membrane-spanning domain; a process also known as regulated intramembrane proteolysis (RIP). RIP regulates signaling pathways by abrogating or releasing signaling molecules. Since the discovery, already >15 years ago, of its catalytic component, presenilin, and even much earlier with the identification of amyloid precursor protein as its first substrate, γ-secretase has been commonly associated with Alzheimer's disease. However, starting with Notch and thereafter a continuously increasing number of novel substrates, γ-secretase is becoming linked to an equally broader range of biological processes. This review presents an updated overview of the current knowledge on the diverse molecular mechanisms and signaling pathways controlled by γ-secretase, with a focus on organ development, homeostasis and dysfunction. This article is part of a Special Issue entitled: Intramembrane Proteases.

Γ-secretase components as predictors of breast cancer outcome

γ-secretase is a large ubiquitously expressed protease complex composed of four core subunits: presenilin, Aph1, PEN-2, and nicastrin. The function of γ-secretase in the cells is to proteolytically cleave various proteins within their transmembrane domains. Presenilin and Aph1 occur as alternative variants belonging to mutually exclusive γ-secretase complexes and providing the complexes with heterogeneous biochemical and physiological properties. γ-secretase is proposed to have a role in the development and progression of cancer and γ-secretase inhibitors are intensively studied for their probable anti-tumor effects in various types of cancer models. Here, we for the first time determined mRNA expression levels of presenilin-1, presenilin-2, Aph1a, Aph1b, PEN-2, and nicastrin in a set of breast cancer tissue samples (N = 55) by quantitative real-time PCR in order to clarify the clinical significance of the expression of different γ-secretase complex components in breast cancer. We found a high positive correlation between the subunit expression levels implying a common regulation of transcription. Our univariate Kaplan-Meier survival analyses established low expression level of γ-secretase complex as a risk factor for breast cancer specific mortality. The tumors expressing low levels of γ-secretase complex were characterized by high histopathological tumor grade, low or no expression of estrogen and progesterone receptors and consequently high probability to fall into the class of triple negative breast cancer tumors. These results may provide novel tools to further categorize breast cancer tumors, especially the highly aggressive and poorly treatable breast cancer type of triple negative cases, and suggest a significant role for γ-secretase in breast cancer.

The very many faces of presenilins and the γ-secretase complex

Presenilin is a central, catalytic component of the γ-secretase complex which conducts intramembrane cleavage of various protein substrates. Although identified and mainly studied through its role in the development of amyloid plaques in Alzheimer disease, γ-secretase has many other important functions. The complex seems to be evolutionary conserved throughout the Metazoa, but recent findings in plants and Dictyostelium discoideum as well as in archeons suggest that its evolution and functions might be much more diversified than previously expected. In this review, a selective survey of the multitude of functions of presenilins and the γ-secretase complex is presented. Following a brief overview of γ-secretase structure, assembly and maturation, three functional aspects are analyzed: (1) the role of γ-secretase in autophagy and phagocytosis; (2) involvement of the complex in signaling related to endocytosis; and (3) control of calcium fluxes by presenilins.

Presenilins and γ-secretase: structure, function, and role in Alzheimer Disease

Presenilins were first discovered as sites of missense mutations responsible for early-onset Alzheimer disease (AD). The encoded multipass membrane proteins were subsequently found to be the catalytic components of γ-secretases, membrane-embedded aspartyl protease complexes responsible for generating the carboxyl terminus of the amyloid β-protein (Aβ) from the amyloid protein precursor (APP). The protease complex also cleaves a variety of other type I integral membrane proteins, most notably the Notch receptor, signaling from which is involved in many cell differentiation events. Although γ-secretase is a top target for developing disease-modifying AD therapeutics, interference with Notch signaling should be avoided. Compounds that alter Aβ production by γ-secretase without affecting Notch proteolysis and signaling have been identified and are currently at various stages in the drug development pipeline.

Cross-species genome wide expression analysis during pluripotent cell determination in mouse and rat preimplantation embryos

The transition between morula and blastocyst stage during preimplantation development represents the first differentiation event of embryogenesis. Morula cells undergo the first cellular specialization and produce two well-defined populations of cells, the trophoblast and the inner cell mass (ICM). Embryonic stem cells (ESCs) with unlimited self-renewal capacity are believed to represent the in vitro counterpart of the ICM. Both mouse and rat ESCs can be derived from the ICM cells, but their in vitro stability differs. In this study we performed a microarray analysis in which we compared the transcriptome of mouse and rat morula, blastocyst, and ICM. This cross-species comparison represents a good model for understanding the differences in derivation and cultivation of ESCs observed in the two species. In order to identify alternative regulation of important molecular mechanisms the investigation of differential gene expression between the two species was extended at the level of signaling pathways, gene families, and single selected genes of interest. Some of the genes differentially expressed between the two species are already known to be important factors in the maintenance of pluripotency in ESCs, like for example Sox2 or Stat3, or play a role in reprogramming somatic cells to pluripotency like c-Myc, Klf4 and p53 and therefore represent interesting candidates to further analyze in vitro in the rat ESCs. This is the first study investigating the gene expression changes during the transition from morula to blastocyst in the rat preimplantation development. Our data show that in the pluripotent pool of cells of the rat and mouse preimplantation embryo substantial differential regulation of genes is present, which might explain the difficulties observed for the derivation and culture of rat ESCs using mouse conditions.

Pen-2 is dispensable for endoproteolysis of presenilin 1, and nicastrin-Aph subcomplex is important for both γ-secretase assembly and substrate recruitment

γ-secretase is a protease complex with at least four components: presenilin, nicastrin (NCT), anterior pharynx-defective 1 (Aph-1), and presenilin enhancer 2 (Pen-2). In this study, using knockout cell lines and small interfering RNA technology, our data demonstrated that the disappeared presenilin 1 C-terminal fragment (PS1C) caused by knockdown of pen-2 or knockout of NCT or Aph-1 was recovered by the addition of proteasome inhibitors, indicating that Pen-2, as well as NCT and Aph-1α, is dispensable for presenilin endoproteolysis. Our data also demonstrate that the formation of the nicastrin-Aph-1 subcomplex plays not only an important role in γ-secretase complex assembly but also in recruiting substrate C-terminal fragment of amyloid precursor protein generated by β-cleavage. Ablating any one component resulted in the instability of other components of the γ-secretase complex, and the presence of all three of the other components is required for full maturation of NCT.

An overview of notch signaling in adult tissue renewal and maintenance

The Notch pathway is a critical mediator of short-range cell-cell communication that is reiteratively used to regulate a diverse array of cellular processes during embryonic development and the renewal and maintenance of adult tissues. Most Notch-dependent processes utilize a core signaling mechanism that is dependent on regulated intramembrane proteolysis: Upon ligand binding, Notch receptors undergo ectodomain shedding by ADAM metalloproteases, followed by γ-secretase-mediated intramembrane proteolysis. This releases the Notch intracellular domain, which translocates to the nucleus to activate transcription. In this review, we highlight the roles of Notch signaling particularly in self-renewing tissues in adults and several human diseases and raise some key considerations when targeting ADAMs and γ-secretase as disease-modifying strategies for Alzheimer's Disease.

Increased expression of PS1 is sufficient to elevate the level and activity of γ-secretase in vivo

Increase in the generation and deposition of amyloid-β (Aβ) plays a central role in the development of Alzheimer's Disease (AD). Elevation of the activity of γ-secretase, a key enzyme required for the generation for Aβ, can thus be a potential risk factor in AD. However, it is not known whether γ-secretase can be upregulated in vivo. While in vitro studies showed that expression of all four components of γ-secretase (Nicastrin, Presenilin, Pen-2 and Aph-1) are required for upregulation of γ-secretase, it remains to be established as to whether this is true in vivo. To investigate whether overexpressing a single component of the γ-secretase complex is sufficient to elevate its level and activity in the brain, we analyzed transgenic mice expressing either wild type or familial AD (fAD) associated mutant PS1. In contrast to cell culture studies, overexpression of either wild type or mutant PS1 is sufficient to increase levels of Nicastrin and Pen-2, and elevate the level of active γ-secretase complex, enzymatic activity of γ-secretase and the deposition of Aβ in brains of mice. Importantly, γ-secretase comprised of mutant PS1 is less active than that of wild type PS1-containing γ-secretase; however, γ-secretase comprised of mutant PS1 cleaves at the Aβ42 site of APP-CTFs more efficiently than at the Aβ40 site, resulting in greater accumulation of Aβ deposits in the brain. Our data suggest that whereas fAD-linked PS1 mutants cause early onset disease, upregulation of PS1/γ-secretase activity may be a risk factor for late onset sporadic AD.

Proteolytic processing of Alzheimer's β-amyloid precursor protein

β-Amyloid precursor protein (APP) is a critical factor in the pathogenesis of Alzheimer's disease (AD). APP undergoes post-translational proteolysis/processing to generate the hydrophobic β-amyloid (Aβ) peptides. Deposition of Aβ in the brain, forming oligomeric Aβ and plaques, is identified as one of the key pathological hallmarks of AD. The processing of APP to generate Aβ is executed by β- and γ-secretase and is highly regulated. Aβ toxicity can lead to synaptic dysfunction, neuronal cell death, impaired learning/memory and abnormal behaviors in AD models in vitro and in vivo. Aside from Aβ, proteolytic cleavages of APP can also give rise to the APP intracellular domain, reportedly involved in multiple types of cellular events such as gene transcription and apoptotic cell death. In addition to amyloidogenic processing, APP can also be cleaved by α-secretase to form a soluble or secreted APP ectodomain (sAPP-α) that has been shown to be mostly neuro-protective. In this review, we describe the mechanisms involved in APP metabolism and the likely functions of its various proteolytic products to give a better understanding of the patho/physiological functions of APP.

Assembly of the presenilin γ-/ε-secretase complex

The presenilin complex is composed of four core proteins (presenilin 1 or presenilin 2, APH1, nicastrin, and PEN2). Several endogenous proteins have been reported to selectively modulate the function of the presenilin complexes; these include transmembrane trafficking protein, 21-KD (TMP21), CD147 antigen (basigin), the γ-secretase-activating protein (gSAP), and the orphan G-protein-coupled receptor 3. Because the structure and assembly of these complexes underlies their activity, this review will discuss current work on the assembly of the complex and on presenilin-interacting proteins that regulate secretase activity.

Proteolytic cleavage of Notch: "HIT and RUN"

The Notch pathway is a highly conserved signaling pathway in multicellular eukaryotes essential in controlling spatial patterning, morphogenesis and homeostasis in embryonic and adult tissues. Notch proteins coordinate cell-cell communication through receptor-ligand interactions between adjacent cells. Notch signaling is frequently deregulated by oncogenic mutation or overexpression in many cancer types. Notch activity is controlled by three sequential cleavage steps leading to ectodomain shedding and transcriptional activation. Here we review the key regulatory steps in the activation of Notch, from receptor maturation to receptor activation (HIT) via a rate-limiting proteolytic cascade (RUN) in the context of species-specific differences.

The -980C/G polymorphism in APH-1A promoter confers risk of Alzheimer's disease

We previously described an association between Alzheimer's disease (AD) and a single-nucleotide polymorphism -980C/G (rs3754048) in the promoter of the anterior pharynx-defective-1a (APH-1A) gene. Here, we examine the potential of this -980C/G polymorphism to affect APH-1A transcription and confer a risk of AD. We validated the presence of APH-1A promoter polymorphism -980C/G in other two Chinese cohort sets (450 AD and 450 controls). Subsequently, we measured APH-1A mRNA and protein levels and γ-secretase activity in C or G allele carriers. Finally, we examined the polymorphism's transcriptional function using a dual-luciferase reporter assay and also tracked transcription factor binding to the variant promoter sequence with electrophoretic mobility shift assays (EMSAs). We found that the APH-1A levels and γ-secretase activity were higher in individuals carrying allele G. The G allele increased APH-1A transcriptional activity significantly in both N2A cells and HEK293 cells. The EMSA revealed an increased binding of the transcription factor Yin Yang 1 (YY1) to allele G. Overexpression of YY1 resulted in an activation of the APH-1A promoter (2.7-fold). Specific YY1 siRNA led to decreases in APH-1A promoter activity and mRNA and protein levels. Our data indicate that the APH-1A promoter polymorphism -980C/G might alter the binding ability of YY1 transcription factor, resulting in an increased level of APH-1A and γ-secretase activity. These factors further facilitated β-amyloid (Aβ) 42 generation and ultimately modified patients' susceptibility to AD. The involvement of transcription factor YY1 might be a novel mechanism for the development of AD.

Structural and Functional Determinants of gamma-Secretase, an Intramembrane Protease Implicated in Alzheimer's Disease

Alzheimer’s disease is the most common form of neurodegenerative diseases in humans, characterized by the progressive accumulation and aggregation of amyloid-β peptides (Aβ) in brain regions subserving memory and cognition. These 39-43 amino acids long peptides are generated by the sequential proteolytic cleavages of the amyloid-β precursor protein (APP) by β- and γ-secretases, with the latter being the founding member of a new class of intramembrane-cleaving proteases (I-CliPs) characterized by their intramembranous catalytic residues hydrolyzing the peptide bonds within the transmembrane regions of their respective substrates. These proteases include the S2P family of metalloproteases, the Rhomboid family of serine proteases, and two aspartyl proteases: the signal peptide peptidase (SPP) and γ-secretase. In sharp contrast to Rhomboid and SPP that function as a single component, γ-secretase is a multi-component protease with complex assembly, maturation and activation processes. Recently, two low-resolution three-dimensional structures of γ-secretase and three high-resolution structures of the GlpG rhomboid protease have been obtained almost simultaneously by different laboratories. Although these proteases are unrelated by sequence or evolution, they seem to share common functional and structural mechanisms explaining how they catalyze intramembrane proteolysis. Indeed, a water-containing chamber in the catalytic cores of both γ-secretase and GlpG rhomboid provides the hydrophilic environment required for proteolysis and a lateral gating mechanism controls substrate access to the active site. The studies that have identified and characterized the structural determinants critical for the assembly and activity of the γ-secretase complex are reviewed here.

Functional and topological analysis of Pen-2, the fourth subunit of the gamma-secretase complex

The γ-secretase complex is a member of the family of intramembrane cleaving proteases, involved in the generation of the Aβ peptides in Alzheimer disease. One of the four subunits of the complex, presenilin, harbors the catalytic site, although the role of the other three subunits is less well understood. Here, we studied the role of the smallest subunit, Pen-2, in vivo and in vitro. We found a profound Notch-deficiency phenotype in Pen-2-/- embryos confirming the essential role of Pen-2 in the γ-secretase complex. We used Pen-2-/- fibroblasts to investigate the structure-function relation of Pen-2 by the scanning cysteine accessibility method. We showed that glycine 22 and proline 27 in hydrophobic domain 1 of Pen-2 are essential for complex formation and stability of γ-secretase. We also demonstrated that hydrophobic domain 1 and the loop domain of Pen-2 are located in a water-containing cavity and are in short proximity to the presenilin C-terminal fragment. We finally demonstrated the essential role of Pen-2 for the proteolytic activity of the complex. Our study supports the hypothesis that Pen-2 is more than a structural component of the γ-secretase complex and may contribute to the catalytic mechanism of the enzyme.

Localization and trafficking of endogenous anterior pharynx-defective 1, a component of Alzheimer's disease related gamma-secretase

Anterior pharynx-defective 1 (Aph-1) is a multi-spanning membrane protein and an integral component of the high molecular weight gamma-secretase complex that also contains presenilin, nicastrin, and Pen-2. In order to clarify the existence of an endogenous fragment of Aph-1 and dissect the localization and processing of endogenous Aph-1 proteins, we examined cell lines and primary cell cultures with our own carboxyl terminal-specific antibodies for Aph-1aL. Fractionation and immunofluorescence studies indicated that the endogenous full-length Aph-1aL isoform localizes primarily to the endoplasmic reticulum as well as Golgi intermediate compartment, but small amount of it was detected at Golgi apparatus where most of its carboxyl terminal domain fragment existed. In primary neuronal and glial cultures, Aph-1aL was present in the neurites and glial cell processes. Endogenous Aph-1a and its proteolytic fragment have unique properties for cleavage control that may have implications for gamma-secretase regulation and intracellular distribution.

Inhibition of BACE1 for therapeutic use in Alzheimer's disease

Since BACE1 was reported as the beta-secretase in Alzheimer's disease (AD) over ten years ago, encouraging progress has been made toward understanding the cellular functions of BACE1. Genetic studies have further confirmed that BACE1 is essential for processing amyloid precursor protein (APP) at the beta-secretase site. Only after this cleavage can the membrane-bound APP C-terminal fragment be subsequently cleaved by gamma-secretase to release so-called AD-causing Abeta peptides. Hence, in the past decade, a wide variety of BACE1 inhibitors have been developed for AD therapy. This review will summarize the major historical events during the evolution of BACE1 inhibitors designed through different strategies of drug discovery. Although BACE1 inhibitors are expected to be safe in general, careful titration of drug dosage to avoid undesirable side effects in BACE1-directed AD therapy is also emphasized.

Modeling an anti-amyloid combination therapy for Alzheimer's disease

As only symptomatic treatments are now available for Alzheimer's disease (AD), safe and effective mechanism-based therapies remain a great unmet need for patients with this neurodegenerative disease. Although gamma-secretase and BACE1 [beta-site beta-amyloid (Abeta) precursor protein (APP) cleaving enzyme 1] are well-recognized therapeutic targets for AD, untoward side effects associated with strong inhibition or reductions in amounts of these aspartyl proteases have raised concerns regarding their therapeutic potential. Although moderate decreases of either gamma-secretase or BACE1 are not associated with mechanism-based toxicities, they provide only modest benefits in reducing Abeta in the brains of APPswe/PS1DeltaE9 mice. Because the processing of APP to generate Abeta requires both gamma-secretase and BACE1, it is possible that moderate reductions of both enzymes would provide additive and significant protection against Abeta amyloidosis. Here, we test this hypothesis and assess the value of this novel anti-amyloid combination therapy in mutant mice. We demonstrate that genetic reductions of both BACE1 and gamma-secretase additively attenuate the amyloid burden and ameliorate cognitive deficits occurring in aged APPswe/PS1DeltaE9 animals. No evidence of mechanism-based toxicities was associated with such decreases in amounts of both enzymes. Thus, we propose that targeting both gamma-secretase and BACE1 may be an effective and safe treatment strategy for AD.

Activity-dependent alpha-cleavage of nectin-1 is mediated by a disintegrin and metalloprotease 10 (ADAM10)

Nectin-1 is known to undergo ectodomain shedding by alpha-secretase and subsequent proteolytic processing by gamma-secretase. How secretase-mediated cleavage of nectin-1 is regulated in neuronal cells and how nectin-1 cleavage affects synaptic adhesion is poorly understood. We have investigated alpha-and gamma-secretase-mediated processing of nectin-1 in primary cortical neurons and identified which protease acts as a alpha-secretase. We report here that NMDA receptor activation, but not stimulation of AMPA or metabotropic glutamate receptors, resulted in robust alpha- and gamma-secretase cleavage of nectin-1 in mature cortical neurons. Cleavage of nectin-1 required influx of Ca(2+) through the NMDA receptor, and activation of calmodulin, but was not dependent on calcium/calmodulin-dependent protein kinase II (CaMKII) activation. We found that ADAM10 is the major secretase responsible for nectin-1 ectodomain cleavage in neurons and the brain. These observations suggest that alpha- and gamma-secretase processing of nectin-1 is a Ca(2+)/calmodulin-regulated event that occurs under conditions of activity-dependent synaptic plasticity and ADAM10 and gamma-secretase are responsible for these cleavage events.

Characterization of an atypical gamma-secretase complex from hematopoietic origin

Gamma-secretase is a widely expressed multisubunit enzyme complex which is involved in the pathogenesis of Alzheimer disease and hematopoietic malignancies through its aberrant processing of the amyloid precursor protein (APP) and Notch1, respectively. While gamma-secretase has been extensively studied, there is a dearth of information surrounding the activity, composition, and function of gamma-secretase expressed in distinct cellular populations. Here we show that endogenous gamma-secretase complexes of hematopoietic origin are distinct from epithelial derived gamma-secretase complexes. Hematopoietic gamma-secretase has reduced activity for APP and Notch1 processing compared to epithelial gamma-secretase. Characterization of the active complexes with small molecule affinity probes reveals that hematopoietic gamma-secretase has an atypical subunit composition with significantly altered subunit stoichiometry. Furthermore, we demonstrate that these discrete complexes exhibit cell-line specific substrate selectivity suggesting a possible mechanism of substrate regulation. These data underscore the need for studying endogenous gamma-secretase to fully understand of the biology of gamma-secretase and its complexity as a molecular target for the development of disease therapeutics.

The secretases: enzymes with therapeutic potential in Alzheimer disease

The amyloid hypothesis has yielded a series of well-validated candidate drug targets with potential for the treatment of Alzheimer disease (AD). Three proteases that are involved in the processing of amyloid precursor protein-alpha-secretase, beta-secretase and gamma-secretase-are of particular interest as they are central to the generation and modulation of amyloid-beta peptide and can be targeted by small compounds in vitro and in vivo. Given that these proteases also fulfill other important biological roles, inhibiting their activity will clearly be inherently associated with mechanism-based toxicity. Carefully determining a suitable therapeutic window and optimizing the selectivity of the drug treatments towards amyloid precursor protein processing might be ways of overcoming this potential complication. Secretase inhibitors are likely to be the first small-molecule therapies aimed at AD modification that will be fully tested in the clinic. Success or failure of these first-generation AD therapies will have enormous consequences for further drug development efforts for AD and possibly other neurodegenerative conditions.

Specific domains in anterior pharynx-defective 1 determine its intramembrane interactions with nicastrin and presenilin

γ-Secretase, a multisubunit transmembrane protease comprised of presenilin, nicastrin, presenilin enhancer 2, and anterior pharynx-defective one, participates in the regulated intramembrane proteolysis of Type I membrane proteins including the amyloid precursor protein (APP). Although Aph-1 is thought to play a structural role in the assembly of γ-secretase complex and several transmembrane domains (TMDs) of Aph-1 have been shown to be critical for its function, the importance of the other domains of Aph-1 remains elusive. We screened a series of Aph-1 mutants and focused on nine mutations distributed in six different TMDs of human APH-1aS, assessing their ability to complement mouse embryonic fibroblasts lacking Aph-1. We showed that mutations in TMD4 (G126) and TMD5 (H171) of Aph-1aS prevented the formation of the Nct/Aph-1 subcomplex. Importantly, although mutations in TMD3 (Q83/E84/R85) and TMD6 (H197) of APH-1aS did not affect Nct/Aph-1 subcomplex formation, both mutations prevented further association/endoproteolysis of PS1. We propose a model that identifies critical TMDs of Aph-1 for associations with Nct and PS for the stepwise assembly of γ-secretase components.

Proteases and Proteolysis in Alzheimer Disease: A Multifactorial View on the Disease Process

Alzheimer disease is characterized by the accumulation of abnormally folded protein fragments, i.e., amyloid beta peptide (Aβ) and tau that precipitate in amyloid plaques and neuronal tangles, respectively. In this review we discuss the complicated proteolytic pathways that are responsible for the generation and clearance of these fragments, and how disturbances in these pathways interact and provide a background for a novel understanding of Alzheimer disease as a multifactorial disorder. Recent insights evolve from the static view that the morphologically defined plaques and tangles are disease driving towards a more dynamic, biochemical view in which the intermediary soluble Aβ oligomers and soluble tau fragments are considered as the main mediators of neurotoxicity. The relevance of proteolytic pathways, centered on the generation and clearance of toxic Aβ, on the cleavage and nucleation of tau, and on the general proteostasis of the neurons, then becomes obvious. Blocking or stimulating these pathways provide, or have the potential to provide, interesting drug targets, which raises the hope that we will be able to provide a cure for this dreadful disorder.

gamma-Secretase dependent production of intracellular domains is reduced in adult compared to embryonic rat brain membranes

Background

γ-Secretase is an intramembrane aspartyl protease whose cleavage of the amyloid precursor protein (APP) generates the amyloid β-peptide (Aβ) and the APP intracellular domain. Aβ is widely believed to have a causative role in Alzheimer's disease pathogenesis, and therefore modulation of γ-secretase activity has become a therapeutic goal. Besides APP, more than 50 substrates of γ-secretase with different cellular functions during embryogenesis as well as adulthood have been revealed. Prior to γ-secretase cleavage, substrates are ectodomain shedded, producing membrane bound C-terminal fragments (CTFs).

Principal Findings

Here, we investigated γ-secretase cleavage of five substrates; APP, Notch1, N-cadherin, ephrinB and p75 neurotrophin receptor (p75-NTR) in membranes isolated from embryonic, young or old adult rat brain by analyzing the release of the corresponding intracellular domains (ICDs) or Aβ40 by western blot analysis and ELISA respectively. The highest levels of all ICDs and Aβ were produced by embryonic membranes. In adult rat brain only cleavage of APP and Notch1 could be detected and the Aβ40 and ICD production from these substrates was similar in young and old adult rat brain. The CTF levels of Notch1, N-cadherin, ephrinB and p75-NTR were also clearly decreased in the adult brain compared to embryonic brain, whereas the APP CTF levels were only slightly decreased.

Conclusions

In summary our data suggests that γ-secretase dependent ICD production is down-regulated in the adult brain compared to embryonic brain. In addition, the present approach may be useful for evaluating the specificity of γ-secretase inhibitors.

Gamma-secretase composed of PS1/Pen2/Aph1a can cleave notch and amyloid precursor protein in the absence of nicastrin

Gamma-secretase is a multiprotein, intramembrane-cleaving protease with a growing list of protein substrates, including the Notch receptors and the amyloid precursor protein. The four components of gamma-secretase complex--presenilin (PS), nicastrin (NCT), Pen2, and Aph1--are all thought to be essential for activity. The catalytic domain resides within PS proteins, NCT has been suggested to be critical for substrate recognition, and the contributions of Pen2 and Aph1 remain unclear. The role of NCT has been challenged recently by the observation that a critical residue (E332) in NCT, which had been thought to be essential for gamma-secretase activity, is instead involved in complex maturation. Here, we report that NCT is dispensable for gamma-secretase activity. NCT-independent gamma-secretase activity can be detected in two independent NCT-deficient mouse embryonic fibroblast lines and blocked by the gamma-secretase inhibitors N-[N-(3,5-difluorophenacetyl-L-alanyl)]-S-phenylglycine t-butyl ester and L-685,458. This catalytic activity requires prior ectodomain shedding of the substrate and can cleave ligand-activated endogenous Notch receptors, indicating presence of this activity at the plasma membrane. Small interfering RNA knockdown experiments demonstrated that NCT-independent gamma-secretase activity requires the presence of PS1, Pen2, and Aph1a but can tolerate knockdown of PS2 or Aph1b. We conclude that a PS1/Pen2/Aph1a trimeric complex is an active enzyme, displaying biochemical properties similar to those of gamma-secretase and roughly 50% of its activity when normalized to PS1 N-terminal fragment levels. This PS1/Pen2/Aph1a complex, however, is highly unstable. Thus, NCT acts to stabilize gamma-secretase but is not required for substrate recognition.

gamma-secretases: from cell biology to therapeutic strategies

Presenilins form the catalytic part of the gamma-secretases, protein complexes that are responsible for the intramembranous cleavage of transmembrane proteins. The presenilins are involved in several biological functions, but are best known for their role in the generation of the beta-amyloid (Abeta) peptide in Alzheimer's disease and are therefore thought to be important drug targets for this disorder. Mutations in the presenilin genes cause early-onset familial Alzheimer's disease, but mutation carriers have substantial phenotypic heterogeneity. Recent evidence implicating presenilin mutations in non-Alzheimer's dementias, including frontotemporal dementia and Lewy body dementia, warrants further investigation. An increased understanding of the diversity of the molecular cell biology of the gamma-secretase complex and the effects of clinical mutations in the presenilin genes might help pave the way for improved development of drugs that are designed to target gamma-secretase enzymatic activity in Alzheimer's disease and potentially in other neurological diseases.

BACE and gamma-secretase characterization and their sorting as therapeutic targets to reduce amyloidogenesis

Secretases are named for enzymes processing amyloid precursor protein (APP), a prototypic type-1 membrane protein. This led directly to discovery of novel Aspartyl proteases (beta-secretases or BACE), a tetramer complex gamma-secretase (gamma-SC) containing presenilins, nicastrin, aph-1 and pen-2, and a new role for metalloprotease(s) of the ADAM family as a alpha-secretases. Recent advances in defining pathways that mediate endosomal-lysosomal-autophagic-exosomal trafficking now provide targets for new drugs to attenuate abnormal production of fibril forming products characteristic of AD. A key to success includes not only characterization of relevant secretases but mechanisms for sorting and transport of key metabolites to abnormal vesicles or sites for assembly of fibrils. New developments we highlight include an important role for an 'early recycling endosome' coated in retromer complex containing lipoprotein receptor LRP-II (SorLA) for switching APP to a non-amyloidogenic pathway for alpha-secretases processing, or to shuttle APP to a 'late endosome compartment' to form Abeta or AICD. LRP11 (SorLA) is of particular importance since it decreases in sporadic AD whose etiology otherwise is unknown.

APH1 polar transmembrane residues regulate the assembly and activity of presenilin complexes

Complexes involved in the gamma/epsilon-secretase-regulated intramembranous proteolysis of substrates such as the amyloid-beta precursor protein are composed primarily of presenilin (PS1 or PS2), nicastrin, anterior pharynx defective-1 (APH1), and PEN2. The presenilin aspartyl residues form the catalytic site, and similar potentially functional polar transmembrane residues in APH1 have been identified. Substitution of charged (E84A, R87A) or polar (Q83A) residues in TM3 had no effect on complex assembly or activity. In contrast, changes to either of two highly conserved histidines (H171A, H197A) located in TM5 and TM6 negatively affected PS1 cleavage and altered binding to other secretase components, resulting in decreased amyloid generating activity. Charge replacement with His-to-Lys substitutions rescued nicastrin maturation and PS1 endoproteolysis leading to assembly of the formation of structurally normal but proteolytically inactive gamma-secretase complexes. Substitution with a negatively charged side chain (His-to-Asp) or altering the structural location of the histidines also disrupted gamma-secretase binding and abolished functionality of APH1. These results suggest that the conserved transmembrane histidine residues contribute to APH1 function and can affect presenilin catalytic activity.

gamma-Secretase heterogeneity in the Aph1 subunit: relevance for Alzheimer's disease

The gamma-secretase complex plays a role in Alzheimer's disease and cancer progression. The development of clinically useful inhibitors, however, is complicated by the role of the gamma-secretase complex in regulated intramembrane proteolysis of Notch and other essential proteins. Different gamma-secretase complexes containing different Presenilin or Aph1 protein subunits are present in various tissues. Here we show that these complexes have heterogeneous biochemical and physiological properties. Specific inactivation of the Aph1B gamma-secretase in a mouse Alzheimer's disease model led to improvements of Alzheimer's disease-relevant phenotypic features without any Notch-related side effects. The Aph1B complex contributes to total gamma-secretase activity in the human brain, and thus specific targeting of Aph1B-containing gamma-secretase complexes may help generate less toxic therapies for Alzheimer's disease.