Complex N-linked glycosylated nicastrin associates with active gamma-secretase and undergoes tight cellular regulation

Presenilin Deficiency Results in Cellular Cholesterol Accumulation by Impairment of Protein Glycosylation and NPC1 Function

Presenilin proteins (PS1 and PS2) represent the catalytic subunit of γ-secretase and play a critical role in the generation of the amyloid β (Aβ) peptide and the pathogenesis of Alzheimer disease (AD). However, PS proteins also exert multiple functions beyond Aβ generation. In this study, we examine the individual roles of PS1 and PS2 in cellular cholesterol metabolism. Deletion of PS1 or PS2 in mouse models led to cholesterol accumulation in cerebral neurons. Cholesterol accumulation was also observed in the lysosomes of embryonic fibroblasts from Psen1-knockout (PS1-KO) and Psen2-KO (PS2-KO) mice and was associated with decreased expression of the Niemann-Pick type C1 (NPC1) protein involved in intracellular cholesterol transport in late endosomal/lysosomal compartments. Mass spectrometry and complementary biochemical analyses also revealed abnormal N-glycosylation of NPC1 and several other membrane proteins in PS1-KO and PS2-KO cells. Interestingly, pharmacological inhibition of N-glycosylation resulted in intracellular cholesterol accumulation prominently in lysosomes and decreased NPC1, thereby resembling the changes in PS1-KO and PS2-KO cells. In turn, treatment of PS1-KO and PS2-KO mouse embryonic fibroblasts (MEFs) with the chaperone inducer arimoclomol partially normalized NPC1 expression and rescued lysosomal cholesterol accumulation. Additionally, the intracellular cholesterol accumulation in PS1-KO and PS2-KO MEFs was prevented by overexpression of NPC1. Collectively, these data indicate that a loss of PS function results in impaired protein N-glycosylation, which eventually causes decreased expression of NPC1 and intracellular cholesterol accumulation. This mechanism could contribute to the neurodegeneration observed in PS KO mice and potentially to the pathogenesis of AD.

γ-Secretase in Alzheimer's disease

Alzheimer's disease (AD) is caused by synaptic and neuronal loss in the brain. One of the characteristic hallmarks of AD is senile plaques containing amyloid β-peptide (Aβ). Aβ is produced from amyloid precursor protein (APP) by sequential proteolytic cleavages by β-secretase and γ-secretase, and the polymerization of Aβ into amyloid plaques is thought to be a key pathogenic event in AD. Since γ-secretase mediates the final cleavage that liberates Aβ, γ-secretase has been widely studied as a potential drug target for the treatment of AD. γ-Secretase is a transmembrane protein complex containing presenilin, nicastrin, Aph-1, and Pen-2, which are sufficient for γ-secretase activity. γ-Secretase cleaves >140 substrates, including APP and Notch. Previously, γ-secretase inhibitors (GSIs) were shown to cause side effects in clinical trials due to the inhibition of Notch signaling. Therefore, more specific regulation or modulation of γ-secretase is needed. In recent years, γ-secretase modulators (GSMs) have been developed. To modulate γ-secretase and to understand its complex biology, finding the binding sites of GSIs and GSMs on γ-secretase as well as identifying transiently binding γ-secretase modulatory proteins have been of great interest. In this review, decades of findings on γ-secretase in AD are discussed.

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.

Validation of a commercial antibody to detect endogenous human nicastrin by immunoblot

Nicastrin (NCSTN) is a transmembrane glycoprotein that is part of the gamma-secretase complex. Gamma-secretase is a protease complex that cleaves type-I single-pass transmembrane proteins. There are many potential substrates for this complex, including NOTCH receptors and amyloid precursor proteins (APP). There are a number of commercial antibodies to nicastrin, but they do not agree on expected peptide size. We confirmed the specificity of a C-terminal binding rabbit anti-human antibody from Sigma-Aldrich (#N1660) using wildtype HEK293 cells and HEK293 cells deleted for nicastrin. The wildtype cells showed a prominent band at approximately 110 kDa. We confirmed this larger than expected sized was due to glycosylation by treating the lysate with peptide-N-glycosidase F (PNGase F), which reduced the band to less than 75 kDa. These data suggest that this polyclonal is specific for nicastrin and can detect endogenous levels of protein.

Essential requirement for nicastrin in marginal zone and B-1 B cell development

γ-secretase is an intramembrane protease complex that catalyzes the proteolytic cleavage of amyloid precursor protein and Notch. Impaired γ-secretase function is associated with the development of Alzheimer's disease and familial acne inversa in humans. In a forward genetic screen of mice with N-ethyl-N-nitrosourea-induced mutations for defects in adaptive immunity, we identified animals within a single pedigree exhibiting both hypopigmentation of the fur and diminished T cell-independent (TI) antibody responses. The causative mutation was in Ncstn, an essential gene encoding the protein nicastrin (NCSTN), a member of the γ-secretase complex that functions to recruit substrates for proteolysis. The missense mutation severely limits the glycosylation of NCSTN to its mature form and impairs the integrity of the γ-secretase complex as well as its catalytic activity toward its substrate Notch, a critical regulator of B cell and T cell development. Strikingly, however, this missense mutation affects B cell development but not thymocyte or T cell development. The Ncstn allele uncovered in these studies reveals an essential requirement for NCSTN during the type 2 transitional-marginal zone precursor stage and peritoneal B-1 B cell development, the TI antibody response, fur pigmentation, and intestinal homeostasis in mice.

Curcumin Derivative GT863 Inhibits Amyloid-Beta Production via Inhibition of Protein N-Glycosylation

Amyloid-β (Aβ) peptides play a crucial role in the pathogenesis of Alzheimer's disease (AD). Aβ production, aggregation, and clearance are thought to be important therapeutic targets for AD. Curcumin has been known to have an anti-amyloidogenic effect on AD. In the present study, we performed screening analysis using a curcumin derivative library with the aim of finding derivatives effective in suppressing Aβ production with improved bioavailability of curcumin using CHO cells that stably express human amyloid-β precursor protein and using human neuroblastoma SH-SY5Y cells. We found that the curcumin derivative GT863/PE859, which has been shown to have an inhibitory effect on Aβ and tau aggregation in vivo, was more effective than curcumin itself in reducing Aβ secretion. We further found that GT863 inhibited neither β- nor γ-secretase activity, but did suppress γ-secretase-mediated cleavage in a substrate-dependent manner. We further found that GT863 suppressed N-linked glycosylation, including that of the γ-secretase subunit nicastrin. We also found that mannosidase inhibitors that block the mannose trimming step of N-glycosylation suppressed Aβ production in a similar fashion, as was observed as a result of treatment with GT863. Collectively, these results suggest that GT863 downregulates N-glycosylation, resulting in suppression of Aβ production without affecting secretase activity.

The Structure and Function of α, β and γ-Secretase as Therapeutic Target Enzymes in the Development of Alzheimer’s Disease: A Review

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Alzheimer's disease is a progressive neurodegenerative disorder that affects the central nervous system. There are several factors that cause AD, like, intracellular hyperphosphorylated Tau tangles, collection of extracellular Amyloid-β42 and generation of reactive oxygen species due to mitochondrial dysfunction. This review analyses the most active target of AD and both types of AD-like early-onset AD and late-onset AD. BACE1 is a β-secretase involved in the cleavage of amyloid precursor protein and the pathogenesis of Alzheimer's disease. The presenilin proteins play a critical role in the pathogenesis of Alzheimer malady by intervening the intramembranous cleavage of amyloid precursor protein and the generation of amyloid β. The two homologous proteins PS1 and PS2 speak to the reactant subunits of particular γ-secretase edifices that intercede an assortment of cellular processes. Natural products are common molecular platforms in drug development in AD. Many natural products are being tested in various animal model systems for their role as a potential therapeutic target in AD. Presently, there are a few theories clarifying the early mechanisms of AD pathogenesis. Recently, research advancements in the field of nanotechnology, which utilize macromolecular strategies to make drugs in nanoscale measurements, offer nanotechnology-based diagnostic tools and drug carriers which are highly sensitive for effective drug targeting in the treatment of Alzheimer’s disease.

Influence of membrane lipid composition on the structure and activity of γ-secretase

γ-Secretase (GS) is a multi-subunit membrane-embedded aspartyl protease that cleaves more than 80 integral membrane proteins, including the amyloid precursor protein (APP) to produce the amyloid-β (Aβ) peptide. Oligomerization and aggregation of the 42-amino acid length Aβ isoform in the brain has been associated with the development and progression of Alzheimer's disease (AD). Based on recent experimental structural studies and using multiscale computational modeling approaches, the conformational states and protein-membrane interactions of the GS complex embedded in six homogeneous and six heterogeneous lipid bilayers were characterized. In order to identify potential lipid and cholesterol binding sites, GS regions with high lipid/cholesterol occupancy values were analyzed using atomistic and coarse-grained simulations. Long lipid residence times were observed to be correlated with a large number of hydrogen bonds between the charged headgroups and key GS amino acids. This observation provides a plausible explanation for the inhibition of GS by charged lipids observed in previous experimental studies. Computed lateral pressure profiles suggest that higher transmembrane pressures favor active state conformations of the catalytic subunit. A probable mechanism for the regulation of the local stress response in cholesterol-rich multicomponent lipid bilayers is identified. Finally, it is demonstrated that interactions between the nicastrin extracellular domain and lipid headgroups leads to a compact structural conformation of the GS complex. Overall, this study provides valuable insight into the effect of bilayer lipid composition on the GS structural ensemble and its function.

Efficient production of a mature and functional gamma secretase protease

Baculoviral protein expression in insect cells has been previously used to generate large quantities of a protein of interest for subsequent use in biochemical and structural analyses. The MultiBac baculovirus protein expression system has enabled, the use of a single baculovirus to reconstitute a protein complex of interest, resulting in a larger protein yield. Using this system, we aimed to reconstruct the gamma (γ)-secretase complex, a multiprotein enzyme complex essential for the production of amyloid-β (Aβ) protein. A MultiBac vector containing all components of the γ-secretase complex was generated and expression was observed for all components. The complex was active in processing APP and Notch derived γ-secretase substrates and proteolysis could be inhibited with γ-secretase inhibitors, confirming specificity of the recombinant γ-secretase enzyme. Finally, affinity purification was used to purify an active recombinant γ-secretase complex. In this study we demonstrated that the MultiBac protein expression system can be used to generate an active γ-secretase complex and provides a new tool to study γ-secretase enzyme and its variants.

High-efficient production and biophysical characterisation of nicastrin and its interaction with APPC100

Nicastrin, the largest member among the four components of the γ-secretase complex, has been identified to be the substrate recognizer for the proteolytic activity of the complex. Here we report that full-length human nicastrin (hNCT) can be obtained by heterologous expression in E. coli. Milligram quantities of the target protein are purified in a two-step purification protocol using affinity chromatography followed by SEC. The FOS-choline 14 purified tetrameric hNCT exhibits a proper folding with 31% α-helix and 23% β-sheet content. Thermal stability studies reveal stable secondary and tertiary structure of the detergent purified hNCT. A physical interaction between nicastrin and the γ-secretase substrate APPC100 confirmed the functionality of hNCT as a substrate recognizer.

Glycomics and glycoproteomics focused on aging and age-related diseases--Glycans as a potential biomarker for physiological alterations

BACKGROUND

Since glycosylation depends on glycosyltransferases, glycosidases, and sugar nucleotide donors, it is susceptible to the changes associated with physiological and pathological conditions. Therefore, alterations in glycan structures may be good targets and biomarkers for monitoring health conditions. Since human aging and longevity are affected by genetic and environmental factors such as diseases, lifestyle, and social factors, a scale that reflects various environmental factors is required in the study of human aging and longevity.

SCOPE OF REVIEW

We herein focus on glycosylation changes elucidated by glycomic and glycoproteomic studies on aging, longevity, and age-related diseases including cognitive impairment, diabetes mellitus, and frailty. We also consider the potential of glycan structures as biomarkers and/or targets for monitoring physiological and pathophysiological changes.

MAJOR CONCLUSIONS

Glycan structures are altered in age-related diseases. These glycans and glycoproteins may be involved in the pathophysiology of these diseases and, thus, be useful diagnostic markers. Age-dependent changes in N-glycans have been reported previously in cohort studies, and characteristic N-glycans in extreme longevity have been proposed. These findings may lead to a deeper understanding of the mechanisms underlying aging as well as the factors influencing longevity.

GENERAL SIGNIFICANCE

Alterations in glycosylation may be good targets and biomarkers for monitoring health conditions, and be applicable to studies on age-related diseases and healthy aging. This article is part of a Special Issue entitled "Glycans in personalised medicine" Guest Editor: Professor Gordan Lauc.

Nicastrin functions to sterically hinder γ-secretase-substrate interactions driven by substrate transmembrane domain

γ-Secretase is an intramembrane-cleaving protease that processes many type-I integral membrane proteins within the lipid bilayer, an event preceded by shedding of most of the substrate's ectodomain by α- or β-secretases. The mechanism by which γ-secretase selectively recognizes and recruits ectodomain-shed substrates for catalysis remains unclear. In contrast to previous reports that substrate is actively recruited for catalysis when its remaining short ectodomain interacts with the nicastrin component of γ-secretase, we find that substrate ectodomain is entirely dispensable for cleavage. Instead, γ-secretase-substrate binding is driven by an apparent tight-binding interaction derived from substrate transmembrane domain, a mechanism in stark contrast to rhomboid--another family of intramembrane-cleaving proteases. Disruption of the nicastrin fold allows for more efficient cleavage of substrates retaining longer ectodomains, indicating that nicastrin actively excludes larger substrates through steric hindrance, thus serving as a molecular gatekeeper for substrate binding and catalysis.

The role and therapeutic targeting of α-, β- and γ-secretase in Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia in the elderly and its prevalence is set to increase rapidly in coming decades. However, there are as yet no available drugs that can halt or even stabilize disease progression. One of the main pathological features of AD is the presence in the brain of senile plaques mainly composed of aggregated β amyloid (Aβ), a derivative of the longer amyloid precursor protein (APP). The amyloid hypothesis proposes that the accumulation of Aβ within neural tissue is the initial event that triggers the disease. Here we review research efforts that have attempted to inhibit the generation of the Aβ peptide through modulation of the activity of the proteolytic secretases that act on APP and discuss whether this is a viable therapeutic strategy for treating AD.

Golgi fragmentation in Alzheimer's disease

The Golgi apparatus is an essential cellular organelle for post-translational modifications, sorting, and trafficking of membrane and secretory proteins. Proper functionality of the Golgi requires the formation of its unique cisternal-stacking morphology. The Golgi structure is disrupted in a variety of neurodegenerative diseases, suggesting a common mechanism and contribution of Golgi defects in neurodegenerative disorders. A recent study on Alzheimer's disease (AD) revealed that phosphorylation of the Golgi stacking protein GRASP65 disrupts its function in Golgi structure formation, resulting in Golgi fragmentation. Inhibiting GRASP65 phosphorylation restores the Golgi morphology from Aβ-induced fragmentation and reduces Aβ production. Perturbing Golgi structure and function in neurons may directly impact trafficking, processing, and sorting of a variety of proteins essential for synaptic and dendritic integrity. Therefore, Golgi defects may ultimately promote the development of AD. In the current review, we focus on the cellular impact of impaired Golgi morphology and its potential relationship to AD disease development.

Production of active glycosylation-deficient γ-secretase complex for crystallization studies

Alzheimer's disease (AD)-associated γ-secretase is a ubiquitously expressed multi-subunit protease complex embedded in the lipid bilayer of cellular compartments including endosomes and the plasma membrane. Although γ-secretase is of crucial interest for AD drug discovery, its atomic structure remains unresolved. γ-Secretase assembly and maturation is a multistep process, which includes extensive glycosylation on nicastrin (NCT), the only γ-secretase subunit having a large extracellular domain. These posttranslational modifications lead to protein heterogeneity that likely prevents the three-dimensional (3D) crystallization of the protease complex. To overcome this issue, we have engineered a Chinese hamster ovary (CHO) cell line deficient in complex sugar modifications (CHO lec1) to overexpress the four subunits of γ-secretase as a functional complex. We purified glycosylation-deficient γ-secretase from this recombinant cell line (CL1-9) and fully glycosylated γ-secretase from a recombinant CHO DG44-derived cell line (SS20). We characterized both complexes biochemically and pharmacologically in vitro. Interestingly, we found that the complex oligosaccharides, which largely decorate the extracellular domain of fully glycosylated NCT, are not involved in the proper assembly and maturation of the complex, and are dispensable for the specific generation, in physiological ratios, of the amyloid precursor protein (APP) cleavage products. In conclusion, we propose a novel bioengineering approach for the production of functional glycosylation-deficient γ-secretase, which may be suitable for crystallization studies. We expect that these findings will contribute both to solving the high-resolution 3D structure of γ-secretase and to structure-based drug discovery for AD.

The stress response neuropeptide CRF increases amyloid-β production by regulating γ-secretase activity

The biological underpinnings linking stress to Alzheimer's disease (AD) risk are poorly understood. We investigated how corticotrophin releasing factor (CRF), a critical stress response mediator, influences amyloid-β (Aβ) production. In cells, CRF treatment increases Aβ production and triggers CRF receptor 1 (CRFR1) and γ-secretase internalization. Co-immunoprecipitation studies establish that γ-secretase associates with CRFR1; this is mediated by β-arrestin binding motifs. Additionally, CRFR1 and γ-secretase co-localize in lipid raft fractions, with increased γ-secretase accumulation upon CRF treatment. CRF treatment also increases γ-secretase activity in vitro, revealing a second, receptor-independent mechanism of action. CRF is the first endogenous neuropeptide that can be shown to directly modulate γ-secretase activity. Unexpectedly, CRFR1 antagonists also increased Aβ. These data collectively link CRF to increased Aβ through γ-secretase and provide mechanistic insight into how stress may increase AD risk. They also suggest that direct targeting of CRF might be necessary to effectively modulate this pathway for therapeutic benefit in AD, as CRFR1 antagonists increase Aβ and in some cases preferentially increase Aβ42 via complex effects on γ-secretase.

Quantitative interaction proteomics of neurodegenerative disease proteins

Several proteins have been linked to neurodegenerative disorders (NDDs), but their molecular function is not completely understood. Here, we used quantitative interaction proteomics to identify binding partners of Amyloid beta precursor protein (APP) and Presenilin-1 (PSEN1) for Alzheimer’s disease (AD), Huntingtin (HTT) for Huntington’s disease, Parkin (PARK2) for Parkinson’s disease, and Ataxin-1 (ATXN1) for spinocerebellar ataxia type 1. Our network reveals common signatures of protein degradation and misfolding and recapitulates known biology. Toxicity modifier screens and comparison to genome-wide association studies show that interaction partners are significantly linked to disease phenotypes in vivo. Direct comparison of wild-type proteins and disease-associated variants identified binders involved in pathogenesis, highlighting the value of differential interactome mapping. Finally, we show that the mitochondrial protein LRPPRC interacts preferentially with an early-onset AD variant of APP. This interaction appears to induce mitochondrial dysfunction, which is an early phenotype of AD.

Hosp et al. show that quantitative interaction proteomics of neurodegenerative disease proteins captures interactions relevant to pathogenesis. Differential interactome mapping reveals preferential binding of the mitochondrial protein LRPPRC with an early-onset Alzheimer’s disease (AD) variant of APP, potentially contributing to mitochondrial dysfunction observed in AD.

Identification of new Presenilin-1 phosphosites: implication for γ-secretase activity and Aβ production

An important pathological hallmark of Alzheimer's disease (AD) is the deposition of amyloid-beta (Aβ) peptides in the brain parenchyma, leading to neuronal death and impaired learning and memory. The protease γ-secretase is responsible for the intramembrane proteolysis of the amyloid-β precursor protein (APP), which leads to the production of the toxic Aβ peptides. Thus, an attractive therapeutic strategy to treat AD is the modulation of the γ-secretase activity, to reduce Aβ42 production. Because phosphorylation of proteins is a post-translational modification known to modulate the activity of many different enzymes, we used electrospray (LC-MS/MS) mass spectrometry to identify new phosphosites on highly purified human γ-secretase. We identified 11 new single or double phosphosites in two well-defined domains of Presenilin-1 (PS1), the catalytic subunit of the γ-secretase complex. Next, mutagenesis and biochemical approaches were used to investigate the role of each phosphosite in the maturation and activity of γ-secretase. Together, our results suggest that the newly identified phosphorylation sites in PS1 do not modulate γ-secretase activity and the production of the Alzheimer's Aβ peptides. Individual PS1 phosphosites shall probably not be considered therapeutic targets for reducing cerebral Aβ plaque formation in AD. In this study, we identified 11 new phosphosites in Presenilin-1 (PS1), the catalytic subunit of the Alzheimer's γ-secretase complex. By combining a mutagenesis approach with cell-based and cell-free γ-secretase assays, we demonstrate that the new phosphosites do not modulate the maturation and activity of γ-secretase. Individual PS1 phosphosites shall thus not be considered therapeutic targets for reducing cerebral Aβ plaque formation in Alzheimer's Disease. Aβ, amyloid beta.

Toward the structure of presenilin/γ-secretase and presenilin homologs

Presenilin is the catalytic component of the γ-secretase complex, a membrane-embedded aspartyl protease that plays a central role in biology and in the pathogenesis of Alzheimer's disease. Upon assembly with its three protein cofactors (nicastrin, Aph-1 and Pen-2), presenilin undergoes autoproteolysis into two subunits, each of which contributes one of the catalytic aspartates to the active site. A family of presenilin homologs, including signal peptide peptidase, possess proteolytic activity without the need for other protein factors, and these simpler intramembrane aspartyl proteases have given insight into the action of presenilin within the γ-secretase complex. Cellular and molecular studies support a nine-transmembrane topology for presenilins and their homologs, and small-molecule inhibitors and cysteine scanning with crosslinking have suggested certain presenilin residues and regions that contribute to substrate recognition and handling. Identification of partial complexes has also offered clues to protein-protein interactions within the γ-secretase complex. Biophysical methods have allowed 3D views of the γ-secretase complex and presenilins. Most recently, the crystal structure of a microbial presenilin homolog has confirmed a nine-transmembrane topology and intramembranous location and proximity of the two conserved and essential aspartates. The crystal structure also provides a platform for the formulation of specific hypotheses regarding substrate interaction and catalysis as well as the pathogenic mechanism of Alzheimer-causing presenilin mutations. This article is part of a Special Issue entitled: Intramembrane Proteases.

Nicastrin overexpression in transgenic mice induces aberrant behavior and APP processing

Nicastrin (NCT) is a component of the presenilin protein complex, which is involved in the cleavage of β-amyloid precursor protein (βAPP) and Notch. The aim of this study was to determine the manner in which overexpression of wild-type human nicastrin (hNCTw) or mutant human nicastrin (hNCTm, D336A/Y337A) regulates brain functions and amyloid precusor protein (APP) processing. For this, we created transgenic (Tg) mice expressing neuron-specific enolase (NSE)-controlled hNCTw or hNCTm and measured their phenotypes as time passed. The NSE/hNCTw and NSE/hNCTm Tg groups exhibited greater behavioral dysfunction from 10 months of age than the non-Tg group, although their severities differed. Further, activity and component levels of the γ-secretase complex were significantly elevated in NSE/hNCTw Tg mice, expect for PEN-2. These alterations induced stimulation of APP processing, resulting in overproduction of Aβ-42 peptide in the NSE/hNCTw Tg group, whereas the NSE/hNCTm Tg group showed a comparatively weaker effect. Furthermore, the highest expression levels of β-secretase and NICD were observed in the NSE/hNCTw Tg group, similar to other phenotypes. Especially, a significances interference on the interaction between NCT and γ-secretase substrates was detected in NSE/hNCTm Tg groups compare with NSE/hNCTw Tg group. These results indicate that hNCTw overexpression in Tg mice promoted active assembly of the γ-secretase complex through modulation of APP processing and behavior, whereas the lesser effect in NSE/hNCTm Tg mice was due to reduced expression of hNCTm. These Tg mice could be useful for the development and application of therapeutic drugs in an animal model of Alzheimer's disease.

Interactome analyses of mature γ-secretase complexes reveal distinct molecular environments of presenilin (PS) paralogs and preferential binding of signal peptide peptidase to PS2

γ-Secretase plays a pivotal role in the production of neurotoxic amyloid β-peptides (Aβ) in Alzheimer disease (AD) and consists of a heterotetrameric core complex that includes the aspartyl intramembrane protease presenilin (PS). The human genome codes for two presenilin paralogs. To understand the causes for distinct phenotypes of PS paralog-deficient mice and elucidate whether PS mutations associated with early-onset AD affect the molecular environment of mature γ-secretase complexes, quantitative interactome comparisons were undertaken. Brains of mice engineered to express wild-type or mutant PS1, or HEK293 cells stably expressing PS paralogs with N-terminal tandem-affinity purification tags served as biological source materials. The analyses revealed novel interactions of the γ-secretase core complex with a molecular machinery that targets and fuses synaptic vesicles to cellular membranes and with the H⁺-transporting lysosomal ATPase macrocomplex but uncovered no differences in the interactomes of wild-type and mutant PS1. The catenin/cadherin network was almost exclusively found associated with PS1. Another intramembrane protease, signal peptide peptidase, predominantly co-purified with PS2-containing γ-secretase complexes and was observed to influence Aβ production.

Generation of amyloid-β is reduced by the interaction of calreticulin with amyloid precursor protein, presenilin and nicastrin

Dysregulation of the proteolytic processing of amyloid precursor protein by γ-secretase and the ensuing generation of amyloid-β is associated with the pathogenesis of Alzheimer's disease. Thus, the identification of amyloid precursor protein binding proteins involved in regulating processing of amyloid precursor protein by the γ-secretase complex is essential for understanding the mechanisms underlying the molecular pathology of the disease. We identified calreticulin as novel amyloid precursor protein interaction partner that binds to the γ-secretase cleavage site within amyloid precursor protein and showed that this Ca²⁺- and N-glycan-independent interaction is mediated by amino acids 330–344 in the C-terminal C-domain of calreticulin. Co-immunoprecipitation confirmed that calreticulin is not only associated with amyloid precursor protein but also with the γ-secretase complex members presenilin and nicastrin. Calreticulin was detected at the cell surface by surface biotinylation of cells overexpressing amyloid precursor protein and was co-localized by immunostaining with amyloid precursor protein and presenilin at the cell surface of hippocampal neurons. The P-domain of calreticulin located between the N-terminal N-domain and the C-domain interacts with presenilin, the catalytic subunit of the γ-secretase complex. The P- and C-domains also interact with nicastrin, another functionally important subunit of this complex. Transfection of amyloid precursor protein overexpressing cells with full-length calreticulin leads to a decrease in amyloid-β₄₂ levels in culture supernatants, while transfection with the P-domain increases amyloid-β₄₀ levels. Similarly, application of the recombinant P- or C-domains and of a synthetic calreticulin peptide comprising amino acid 330–344 to amyloid precursor protein overexpressing cells result in elevated amyloid-β₄₀ and amyloid-β₄₂ levels, respectively. These findings indicate that the interaction of calreticulin with amyloid precursor protein and the γ-secretase complex regulates the proteolytic processing of amyloid precursor protein by the γ-secretase complex, pointing to calreticulin as a potential target for therapy in Alzheimer's disease.

The ubiquitin ligase synoviolin up-regulates amyloid β production by targeting a negative regulator of γ-secretase, Rer1, for degradation

Alzheimer's disease is characterized by the deposition of Aβ, which is generated from the amyloid precursor protein through its cleavage by β- and γ-secretases. The γ-secretase complex component nicastrin (NCT) plays significant roles in the assembly and proper trafficking of the γ-secretase complex and in the recognition of amyloid precursor protein. NCT is incorporated into the γ-secretase complex in the endoplasmic reticulum (ER) and glycosylated in the Golgi. In contrast, unassembled NCT is retrieved or retained in the ER by the protein Retention in endoplasmic reticulum 1 (Rer1). We reported previously that synoviolin (Syvn), an E3 ubiquitin ligase, degrades NCT and affects the generation of Aβ. Here, we examined in more detail the effect of Syvn on the generation of Aβ. We found that overexpression of a dominant negative form of Syvn (C307A mutant) and a Syvn-RNAi decreased the generation of Aβ. These results indicate that the ubiquitin ligase activity of Syvn up-regulates the generation of Aβ. We hypothesized, therefore, that Syvn regulates the assembly or localization of the γ-secretase complex by ubiquitinating Rer1, resulting in its subsequent degradation. Our findings that the level of Rer1 was increased in Syvn knockout fibroblasts because of inhibition of its degradation support this hypothesis. Moreover, we found that Rer1 interacts with Syvn in the ER, is ubiquitinated by Syvn, and is then degraded via the proteasome or lysosomal pathways. Finally, we showed that localization of mature NCT to the plasma membrane as well as γ-secretase complex levels are decreased in fibroblasts of Syvn knockout mice. Thus, it is likely that Syvn regulates the assembly of the γ-secretase complex via the degradation of Rer1, which results in the generation of Aβ.

Generation of monoclonal antibody fragments binding the native γ-secretase complex for use in structural studies

A detailed understanding of γ-secretase structure is crucially needed to elucidate its unique properties of intramembrane protein cleavage and to design therapeutic compounds for the safe treatment of Alzheimer's disease. γ-Secretase is an enzyme complex composed of four membrane proteins, and the scarcity of its supply associated with the challenges of crystallizing membrane proteins is a major hurdle for the determination of its high-resolution structure. This study addresses some of these issues, first by adapting CHO cells overexpressing γ-secretase to growth in suspension, thus yielding multiliter cultures and milligram quantities of highly purified, active γ-secretase. Next, the amounts of γ-secretase were sufficient for immunization of mice and allowed generation of Nicastrin- and Aph-1-specific monoclonal antibodies, from which Fab fragments were proteolytically prepared and subsequently purified. The amounts of γ-secretase produced are compatible with robot-assisted crystallogenesis using nanoliter technologies. In addition, our Fab fragments bind exposed regions of native γ-secretase in a dose-dependent manner without interfering with its catalytic properties and can therefore be used as specific tools to facilitate crystal formation.

The Nicastrin ectodomain adopts a highly thermostable structure

Nicastrin is a type I transmembrane glycoprotein, which is part of the high molecular weight γ-secretase complex. γ-Secretase is one of the key players associated with the generation of Alzheimer's disease pathology, since it liberates the neurotoxic amyloid β-peptide. Four proteins Nicastrin, anterior pharynx-defective-1 (Aph-1), presenilin enhancer-2 (Pen-2) and Presenilin are essential to form the active γ-secretase complex. Recently it has been shown, that Nicastrin has a key function in stabilizing the mature γ-secretase complex and may also be involved in substrate recognition. So far no structural data for the Nicastrin ectodomain or any other γ-secretase component are available. We therefore used Circular Dichroism (CD) spectroscopy to demonstrate that Nicastrin, similar to its homologues, the Streptomyces griseus aminopeptidase (SGAP) and the transferrin receptor (TfR), adopts a thermostable secondary structure. Furthermore, the Nicastrin ectodomain has an exceptionally high propensity to refold after thermal denaturation. These findings provide evidence to further support the hypothesis that Nicastrin may share evolutionary conserved properties with the aminopeptidase and the transferrin receptor family.

Down-regulation of the ATP-binding cassette transporter 2 (Abca2) reduces amyloid-β production by altering Nicastrin maturation and intracellular localization

Clinical, pharmacological, biochemical, and genetic evidence support the notion that alteration of cholesterol homeostasis strongly predisposes to Alzheimer disease (AD). The ATP-binding cassette transporter-2 (Abca2), which plays a role in intracellular sterol trafficking, has been genetically linked to AD. It is unclear how these two processes are related. Here we demonstrate that down-regulation of Abca2 in mammalian cells leads to decreased amyloid-β (Aβ) generation. In vitro studies revealed altered γ-secretase complex formation in Abca2 knock-out cells due to the altered levels, post-translational modification, and subcellular localization of Nicastrin. Reduced Abca2 levels in mammalian cells in vitro, in Drosophila melanogaster and in mice resulted in altered γ-secretase processing of APP, and thus Aβ generation, without affecting Notch cleavage.

Dissociation between the processivity and total activity of γ-secretase: implications for the mechanism of Alzheimer's disease-causing presenilin mutations

The amyloid β-peptide (Aβ), strongly implicated in the pathogenesis of Alzheimer's disease (AD), is produced from the amyloid β-protein precursor (APP) through consecutive proteolysis by β- and γ-secretases. The latter protease contains presenilin as the catalytic component of a membrane-embedded aspartyl protease complex. Missense mutations in presenilin are associated with early-onset familial AD, and these mutations generally both decrease Aβ production and increase the ratio of the aggregation-prone 42-residue form (Aβ42) to the 40-residue form (Aβ40). The connection between these two effects is not understood. Besides Aβ40 and Aβ42, γ-secretase produces a range of Aβ peptides, the result of initial cutting at the ε site to form Aβ48 or Aβ49 and subsequent trimming every three or four residues. Thus, γ-secretase displays both overall proteolytic activity (ε cutting) and processivity (trimming) toward its substrate APP. Here we tested whether a decrease in total activity correlates with decreased processivity using wild-type and AD-mutant presenilin-containing protease complexes. Changes in pH, temperature, and salt concentration that reduced the overall activity of the wild-type enzyme did not consistently result in increased proportions of longer Aβ peptides. Low salt concentrations and acidic pH were notable exceptions that subtly alter the proportion of individual Aβ peptides, suggesting that the charged state of certain residues may influence processivity. Five different AD mutant complexes, representing a broad range of effects on overall activity, Aβ42:Aβ40 ratios, and ages of disease onset, were also tested, revealing again that changes in total activity and processivity can be dissociated. Factors that control initial proteolysis of APP at the ε site apparently differ significantly from factors affecting subsequent trimming and the distribution of Aβ peptides.

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.

Computational insights into the development of novel therapeutic strategies for Alzheimer's disease

BACKGROUND

β-amyloidosis and oxidative stress have been implicated as root causes of Alzheimer's disease (AD). Current potential therapeutic strategies for the treatment of AD include inhibition of amyloid β (Aβ) production, stimulation of Aβ degradation and prevention of Aβ oligomerization. However, efforts in this direction are hindered by the lack of understanding of the biochemical processes occurring at the atomic level in AD.

DISCUSSION

A radically different approach to achieve this goal would be the application of comprehensive theoretical and computational techniques such as molecular dynamics, quantum mechanics, hybrid quantum mechanics/molecular mechanics, bioinformatics and rotational spectroscopy to investigate complex chemical and physical processes in β-amyloidosis and the oxidative stress mechanism.

CONCLUSION

Results obtained from these studies will provide an atomic level understanding of biochemical processes occurring in AD and advance efforts to develop effective therapeutic strategies for this disease.

ER-stress-inducible Herp, facilitates the degradation of immature nicastrin

BACKGROUND

Herp is an endoplasmic reticulum (ER)-stress-inducible membrane protein harboring an ubiquitin-like domain (ULD). However, its biological functions are not fully understood. Here, we examined the role of Herp in the degradation of γ-secretase components.

METHODS

Effects of ULD-lacking Herp (ΔUb-Herp) expression on the degradation of γ-secretase components were analyzed.

RESULTS

The cellular expression of ΔUb-Herp was found to inhibit the degradation of overexpressed immature nicastrin and full-length presenilin. The mechanisms underlying Herp-mediated nicastrin degradation was further analyzed. We found that immature nicastrin accumulates in the ER of ΔUb-Herp overexpressing cells or Herp-deficient cells more than that in the ER of wild-type cells. Further, ΔUb-Herp expression inhibited nicastrin ubiquitination, suggesting that the ULD of Herp is likely involved in nicastrin ubiquitination. Co-immunoprecipitation study showed that Herp as well as ΔUb-Herp potentially interacts with nicastrin, mediating nicastrin interaction with p97, which functions in retranslocation of misfolded proteins from the ER to the cytosol.

CONCLUSIONS

Thus, Herp is likely involved in degradation of immature nicastrin by facilitating p97-dependent nicastrin retranslocation and ubiquitination.

GENERAL SIGNIFICANCE

We suggest that Herp could play a role in the elimination of the excess unassembled components of a multimeric complex.

Targeting Notch signaling in pancreatic cancer

IMPORTANCE OF THE FIELD

With some 220,000 new cases/year in the world, pancreatic adenocarcinoma is the fourth highest cause of death by cancers. Among newly diagnosed patients about 210,000 will die within 9 months following diagnosis. Therefore, effective adjuncts to current treatment strategies are necessary. Because embryological signaling pathways are upregulated in pancreatic adenocarcinoma, they represent potential targets for future therapies.

AREAS COVERED IN THIS REVIEW

Our aim is to present the Notch pathway, and to describe its involvement in pancreatic pathophysiology/carcinogenesis. This pathway appeared as a prime target for pancreatic cancer therapy. In the light of the crosstalk of Notch with other survival/embryologic pathways, drugs affecting more than one pathway may have to be combined.

WHAT THE READER WILL GAIN

Drugs against gamma-secretases could thus serve in cancer treatment and can be combined with drugs targeting survival pathways interplaying with Notch such as Hedgehog.

TAKE HOME MESSAGE

Downregulation of Notch contributes to the inhibition and apoptosis of pancreatic cancer cells whereas Hedgehog inhibition will allow for enhanced delivery of drugs to the tumor. Both pathway inhibitors appear to have synergistic effects for future therapeutics for pancreatic adenocarcinoma, once safety issues of compounds are overcome.

Membrane localization of LRRK2 is associated with increased formation of the highly active LRRK2 dimer and changes in its phosphorylation

Autosomal dominant mutations in leucine rich repeat kinase 2 (LRRK2) are the most common genetic cause of Parkinson's disease (PD). Despite the presence of multiple domains, the kinase activity of LRRK2 is thought to represent the primary function of the protein. Alterations in LRRK2 kinase activity are thought to underlie the pathogenesis of its PD-linked mutations; however, many questions regarding basic aspects of LRRK2 function remain unclear, including the cellular mechanisms of LRRK2 regulation and the importance of its unique distribution within the cell. Here, we demonstrate for the first time that the subcellular localization of wild-type LRRK2 is associated with changes in four distinct biochemical properties likely crucial for LRRK2 function. Our data demonstrate for the first time that the wild-type LRRK2 dimer possesses greater kinase activity than its more abundant monomeric counterpart. Importantly, we show that this activated form of LRRK2 is substantially enriched at the membrane of cells expressing endogenous or exogenous LRRK2, and that the membrane-associated fraction of LRRK2 likewise possesses greater kinase activity than cytosolic LRRK2. In addition, membrane-associated LRRK2 binds GTP more efficiently than cytosolic LRRK2 but demonstrates a lower degree of phosphorylation. Our observations suggest that multiple events, including altered protein-protein interactions and post-translational modifications, contribute to the regulation of LRRK2 function, through modulation of membrane association and complex assembly. These findings may have implications for the sites of LRRK2 function within the cell, the identification and localization of bona fide LRRK2 substrates, and efforts to design small molecule inhibitors of LRRK2.

Role of glycans and glycosyltransferases in the regulation of Notch signaling

The evolutionarily conserved Notch signaling pathway plays broad and important roles during embryonic development and in adult tissue homeostasis. Unlike most other pathways used during animal development, Notch signaling does not rely on second messengers and intracellular signaling cascades. Instead, pathway activation results in the cleavage of the Notch intracellular domain and its translocation into the nucleus, where it functions as a transcriptional co-activator of the Notch target genes. To ensure tight spatial and temporal regulation of a pathway with such an unusually direct signaling transduction, animal cells have devised a variety of specialized modulatory mechanisms. One such mechanism takes advantage of decorating the Notch extracellular domain with rare types of O-linked glycans. In this review, we will discuss the genetic and biochemical data supporting the notion that carbohydrate modification is essential for Notch signaling and attempt to provide a brief historical overview of how we have learned what we know about the glycobiology of Notch. We will also summarize what is known about the contribution of specific nucleotide-sugar transporters to Notch biology and the roles-enzymatic and non-enzymatic-played by specific glycosyltransferases in the regulation of this pathway. Mutations in the Notch pathway components cause a variety of human diseases, and manipulation of Notch signaling is emerging as a powerful tool in regenerative medicine. Therefore, studying how sugar modification modulates Notch signaling provides a framework for better understanding the role of glycosylation in animal development and might offer new tools to manipulate Notch signaling for therapeutic purposes.

MUC1 is a substrate for gamma-secretase

Understanding the underlying mechanisms by which a normal cell avoids the oncogenic potential of MUC1 signaling requires further definition of the pathways by which the MUC1 cytoplasmic tail is processed in both normal and tumor-derived cells. In the present study we describe the processing pathway initiated by TACE/ADAM17 cleavage of MUC1. Utilizing the human uterine epithelial cell line, HES, derived from normal endometrium, we show that endogenous full length MUC1 undergoes regulated intramembranous proteolysis mediated by presenillin-dependent gamma-secretase. Cytokine-stimulated HES cells exposed to gamma-secretase inhibitors accumulated a membrane-associated 15 kDa fragment of the MUC1 C-terminal subunit (CTF15). Inhibitors of TACE/ADAM17-mediated shedding inhibited accumulation of MUC1-CTF15 and MUC1 ectodomain release to a similar extent consistent with MUC1-CTF15 being a product of TACE/ADAM17 action. Reduction of catalytically active gamma-secretase complex by nicastrin siRNA treatment also resulted in CTF15 accumulation. Furthermore, mature nicastrin, the substrate receptor for gamma-secretase, co-immunoprecipitated with CTF15 in the presence of gamma-secretase inhibitors indicating the formation of CTF15: nicastrin complexes. MUC1-CTF15 accumulation in response to gamma-secretase inhibition was demonstrated in both normal and tumor-derived cells from humans and mice indicating that this processing pathway exists in many cell contexts. We did not detect products of MUC1 cleavage by gamma-secretase in the presence of various proteasomal inhibitors indicating that subsequent degradation is either non-proteasomal or extremely efficient. We suggest that this efficient pathway attenuates potential signaling mediated by cytoplasmic tail fragments.

An E3 ubiquitin ligase, Synoviolin, is involved in the degradation of immature nicastrin, and regulates the production of amyloid beta-protein

The presenilin complex, consisting of presenilin, nicastrin, anterior pharynx defective-1 and presenilin enhancer-2, constitutes gamma-secretase, which is required for the generation of amyloid beta-protein. In this article, we show that Synoviolin (also called Hrd1), which is an E3 ubiquitin ligase implicated in endoplasmic reticulum-associated degradation, is involved in the degradation of endogenous immature nicastrin, and affects amyloid beta-protein generation. It was found that the level of immature nicastrin was dramatically increased in synoviolin-null cells as a result of the inhibition of degradation, but the accumulation of endogenous presenilin, anterior pharynx defective-1 and presenilin enhancer-2 was not changed. This was abolished by the transfection of exogenous Synoviolin. Moreover, nicastrin was co-immunoprecipitated with Synoviolin, strongly suggesting that nicastrin is the substrate of Synoviolin. Interestingly, amyloid beta-protein generation was increased by the overexpression of Synoviolin, although the nicastrin level was decreased. Thus, Synoviolin-mediated ubiquitination is involved in the degradation of immature nicastrin, and probably regulates amyloid beta-protein generation. Structured digital abstract: * MINT-7255352: Synoviolin (uniprotkb:Q9DBY1) physically interacts (MI:0915) with NCT (uniprotkb:P57716) by anti tag coimmunoprecipitation (MI:0007) * MINT-7255377: Ubiquitin (uniprotkb:P62991) physically interacts (MI:0915) with NCT (uniprotkb:P57716) by anti bait coimmunoprecipitation (MI:0006) * MINT-7255363: NCT (uniprotkb:P57716) physically interacts (MI:0915) with Synoviolin (uniprotkb:Q9DBY1) by anti bait coimmunoprecipitation (MI:0006).

Single chain variable fragment against nicastrin inhibits the gamma-secretase activity

Gamma-secretase is a membrane protein complex that catalyzes intramembrane proteolysis of a variety of substrates including the amyloid beta precursor protein of Alzheimer disease. Nicastrin (NCT), a single-pass membrane glycoprotein that harbors a large extracellular domain, is an essential component of the gamma-secretase complex. Here we report that overexpression of a single chain variable fragment (scFv) against NCT as an intrabody suppressed the gamma-secretase activity. Biochemical analyses revealed that the scFv disrupted the proper folding and the appropriate glycosyl maturation of the endogenous NCT, which are required for the stability of the gamma-secretase complex and the intrinsic proteolytic activity, respectively, implicating the dual role of NCT in the gamma-secretase complex. Our results also highlight the importance of the calnexin cycle in the functional maturation of the gamma-secretase complex. The engineered intrabodies may serve as rationally designed, molecular targeting tools for the discovery of novel actions of the membrane proteins.

An alternative spliced mouse presenilin-2 mRNA encodes a novel gamma-secretase inhibitor

The gamma-secretase, composed of presenilin-1 (PS1) or presenilin-2 (PS2), nicastrin (NCT), anterior pharynx-defective phenotype 1 (APH-1), and PEN-2, is critical for the development of Alzheimer's disease (AD). PSs are autoproteolytically cleaved, producing an N-terminal fragment (NTF) and a hydrophilic loop domain-containing C-terminal fragment. However, the role of the loop domain in the gamma-secretase complex assembly remains unknown. Here, we report a novel PS2 isoform generated by alternative splicing, named PS2beta, which is composed of an NTF with a hydrophilic loop domain. PS2beta disturbed the interaction between NCT and APH-1, resulting in the inhibition of amyloid-beta production. We concluded that PS2beta may inhibit gamma-secretase activity by affecting the gamma-secretase complex assembly.

Alzheimer's disease: cholesterol, membrane rafts, isoprenoids and statins

Alzheimer's disease (AD) is a heterogeneous neurodegenerative disorder and the most prevalent form of dementia worldwide. AD is characterized pathologically by amyloid-β plaques, neurofibrillary tangles and neuronal loss, and clinically by a progressive loss of cognitive abilities. At present, the fundamental molecular mechanisms underlying the disease are unclear and no treatment for AD is known. Epidemiological evidence continues to mount linking vascular diseases, such as hypertension and diabetes, and hypercholesterolaemia with an increased risk for developing AD. A growing amount of evidence suggests a mechanistic link between cholesterol metabolism in the brain and the formation of amyloid plaques in AD development. Cholesterol and statins clearly modulate β-amyloid precursor protein (βAPP) processing in cell culture and animal models. Statins not only reduce endogenous cholesterol synthesis but also exert other various pleiotrophic effects, such as the reduction in protein isoprenylation. Through these effects statins modulate a variety of cellular functions involving both cholesterol (and membrane rafts) and isoprenylation. Although clearly other factors, such as vascular inflammation, oxidative stress and genetic factors, are intimately linked with the progression of AD, this review focuses on the present research findings describing the effect of cholesterol, membrane rafts and isoprenylation in regulating βAPP processing and in particular γ-secretase complex assembly and function and AD progression, along with consideration for the potential role statins may play in modulating these events.

Rapid purification of active gamma-secretase, an intramembrane protease implicated in Alzheimer's disease

Gamma-secretase is an unconventional aspartyl protease that processes many type 1 membrane proteins within the lipid bilayer. Because its cleavage of amyloid-beta precursor protein generates the amyloid-beta protein (Abeta) of Alzheimer's disease, partially inhibiting gamma-secretase is an attractive therapeutic strategy, but the structure of the protease remains poorly understood. We recently used electron microscopy and single particle image analysis on the purified enzyme to generate the first 3D reconstruction of gamma-secretase, but at low resolution (15 A). The limited amount of purified gamma-secretase that can be produced using currently available cell lines and procedures has prevented the achievement of a high resolution crystal structure by X-ray crystallography or 2D crystallization. We report here the generation and characterization of a new mammalian cell line (S-20) that overexpresses strikingly high levels of all four gamma-secretase components (presenilin, nicastrin, Aph-1 and Pen-2). We then used these cells to develop a rapid protocol for the high-grade purification of proteolytically active gamma-secretase. The cells and purification methods detailed here provide a key step towards crystallographic studies of this ubiquitous enzyme.

Moonlighting activity of presenilin in plants is independent of gamma-secretase and evolutionarily conserved

Presenilins (PS) provide the catalytic activity for gamma-secretase, which cleaves physiologically relevant substrates including Notch, ErbB4, and APP. Recent genetic studies indicated that the contribution of PS1 to mouse development includes gamma-secretase-independent functions that cannot be easily explained by any of the demonstrated or hypothesized functions of this protein. To begin a nonbiased analysis of PS1 activity unencumbered by the dominant effect stemming from loss of Notch function, we characterized PS functions in the early land plant Physcomitrella patens, which lacks Notch, ErbB4, and APP. Removal of P. patens PS resulted in phenotypic abnormalities. Further assays performed to delineate the defective pathways in PS-deficient P. patens implicated improper function of the cytoskeletal network. Importantly, this characterization of a nonmetazoan PS uncovered a previously undescribed, evolutionarily conserved function (human PS1 can rescue the growth and light responses) that is gamma-secretase-independent (mutants with substitutions of the catalytic aspartyl residues retain the activity). Introduction of PpPS into PS-deficient mouse embryonic fibroblasts rescues normal growth rates, demonstrating that at least some metazoan functions of PS are evolutionarily conserved.

Regulation of γ-Secretase Activity in Alzheimer's Disease

The gamma-secretase complex is an intramembrane aspartyl protease that cleaves its substrates along their transmembrane regions. Sequential proteolytic processing of amyloid precursor protein by beta- and gamma-secretase produces amyloid beta-peptides, which are the major components of amyloid plaques in the brains of Alzheimer's disease patients. The gamma-secretase complex is therefore believed to be critical in the pathogenesis of Alzheimer's disease. Here we review the range of factors found to affect the nature and degree of gamma-secretase complex activity; these include gamma-secretase complex assembly and activation, the integral regulatory subunit CD147, transient or weak binding partners, the levels of cholesterol and sphingolipids in cell membranes, and inflammatory cytokines. Integrated knowledge of the molecular mechanisms supporting the actions of these factors is expected to lead to a comprehensive understanding of the functional regulation of the gamma-secretase complex, and this, in turn, should facilitate the development of novel therapeutic strategies for the treatment of Alzheimer's disease.

Differential role of Presenilin-1 and -2 on mitochondrial membrane potential and oxygen consumption in mouse embryonic fibroblasts

Increasing evidence indicates that mitochondrial alterations contribute to the neuronal death in Alzheimer's disease (AD). Presenilin 1 (PS1) and Presenilin 2 (PS2) mutations have been shown to sensitize cells to apoptosis by mechanisms suggested to involve impaired mitochondrial function. We have previously detected active gamma-secretase complexes in mitochondria. We investigated the impact of PS/gamma-secretase on mitochondrial function using mouse embryonal fibroblasts derived from wild-type, PS1-/-, PS2-/- and PS double knock-out (PSKO) embryos. Measurements of mitochondrial membrane potential (DeltaPsim) showed a higher percentage of fully functional mitochondria in PS1-/- and PSwt as compared to PS2-/- and PSKO cells. This result was evident both in whole cell preparations and in isolated mitochondria. Interestingly, pre-treatment of isolated mitochondria with the gamma-secretase inhibitor L-685,458 resulted in a decreased population of mitochondria with high DeltaPsim in PSwt and PS1-/- cells, indicating that PS2/gamma-secretase activity can modify DeltaPsim. PS2-/- cells showed a significantly lower basal respiratory rate as compared to other cell lines. However, all cell lines demonstrated competent bioenergetic function. These data point toward a specific role of PS2/gamma-secretase activity for proper mitochondrial function and indicate interplay between PS1 and PS2 in mitochondrial functionality.

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.

The γ-Secretase Complex: Membrane-Embedded Proteolytic Ensemble

Gamma-secretase is responsible for the proteolytic processing of a variety of membrane-associated fragments derived from type I integral membrane proteins, including the amyloid beta-protein precursor and the Notch receptor. This enzyme is composed of four different integral membrane proteins: presenilin, nicastrin, Aph-1, and Pen-2. During assembly and maturation of the protease complex, presenilin is endoproteolyzed into two subunits, each of which contributes one aspartate to the active site of an aspartyl protease. Substrate apparently interacts with an initial docking site before passing in whole or in part between the two presenilin subunits to the internal water-containing active site. The ectodomain of nicastrin also interacts with the N-terminus of the substrate as an essential step in substrate recognition and processing. Sites for allosteric regulation on the protease complex allow selective inhibition or modulation of APP processing without interfering with Notch signaling, and such selective agents may represent promising leads for the development of Alzheimer's disease therapeutics. Elucidation of detailed structural features of gamma-secretase and other membrane-embedded proteases is the next frontier in understanding how these enzymes carry out hydrolysis within the lipid bilayer.

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.

Equimolar production of amyloid beta-protein and amyloid precursor protein intracellular domain from beta-carboxyl-terminal fragment by gamma-secretase

We showed previously that cells expressing wild-type (WT) beta-amyloid precursor protein (APP) or coexpressing WTAPP and WT presenilin (PS) 1/2 produced APP intracellular domains (AICD) 49-99 and 50-99, with the latter predominating. On the other hand, the cells expressing mutant (MT) APP or coexpressing WTAPP and MTPS1/2 produced a greater proportion of AICD-(49-99) than AICD-(50-99). In addition, the expression of amyloid beta-protein (Abeta) 49 in cells resulted in predominant production of Abeta40 and that of Abeta48 leads to preferential production of Abeta42. These observations suggest that epsilon-cleavage and gamma-cleavage are interrelated. To determine the stoichiometry between Abeta and AICD, we have established a 3-[(3-cholamidopropyl)dimethylammonio]-2-hydroxy-1-propanesulfonic acid-solubilized gamma-secretase assay system that exhibits high specific activity. By using this assay system, we have shown that equal amounts of Abeta and AICD are produced from beta-carboxyl-terminal fragment (C99) by gamma-secretase, irrespective of WT or MTAPP and PS1/2. Although various Abeta species, including Abeta40, Abeta42, Abeta43, Abeta45, Abeta48, and Abeta49, are generated, only two species of AICD, AICD-(49-99) and AICD-(50-99), are detected. We also have found that M233T MTPS1 produced only one species of AICD, AICD-(49-99), and only one for its counterpart, Abeta48, in contrast to WT and other MTPS1s. These strongly suggest that epsilon-cleavage is the primary event, and the produced Abeta48 and Abeta49 rapidly undergo gamma-cleavage, resulting in generation of various Abeta species.