Mature glycosylation and trafficking of nicastrin modulate its binding to presenilins

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.

Quantitative comparison of presenilin protein expression reveals greater activity of PS2-γ-secretase

γ-secretase processing of amyloid precursor protein (APP) has long been of interest in the pathological progression of Alzheimer's disease (AD) due to its role in the generation of amyloid-β. The catalytic component of the enzyme is the presenilins of which there are two homologues, Presenilin-1 (PS1) and Presenilin-2 (PS2). The field has focussed on the PS1 form of this enzyme, as it is typically considered the more active at APP processing. However, much of this work has been completed without appropriate consideration of the specific levels of protein expression of PS1 and PS2. We propose that expression is an important factor in PS1- and PS2-γ-secretase activity, and that when this is considered, PS1 does not have greater activity than PS2. We developed and validated tools for quantitative assessment of PS1 and PS2 protein expression levels to enable the direct comparison of PS in exogenous and endogenous expression systems, in HEK-293 PS1 and/or PS2 knockout cells. We show that exogenous expression of Myc-PS1-NTF is 5.5-times higher than Myc-PS2-NTF. Quantitating endogenous PS protein levels, using a novel PS1/2 fusion standard we developed, showed similar results. When the marked difference in PS1 and PS2 protein levels is considered, we show that compared to PS1-γ-secretase, PS2-γ-secretase has equal or more activity on APP and Notch1. This study has implications for understanding the PS1- and PS2-specific contributions to substrate processing, and their potential influence in AD pathogenesis.

Dessert or Poison? The Roles of Glycosylation in Alzheimer's, Parkinson's, Huntington's Disease, and Amyotrophic Lateral Sclerosis

Ministry of Education and Key Laboratory of Neurons and glial cells of the central nervous system (CNS) are modified by glycosylation and rely on glycosylation to achieve normal neural function. Neurodegenerative disease is a common disease of the elderly, affecting their healthy life span and quality of life, and no effective treatment is currently available. Recent research implies that various glycosylation traits are altered during neurodegenerative diseases, suggesting a potential implication of glycosylation in disease pathology. Herein, we summarized the current knowledge about glycosylation associated with Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), and Amyotrophic lateral sclerosis (ALS) pathogenesis, focusing on their promising functional avenues. Moreover, we collected research aimed at highlighting the need for such studies to provide a wealth of disease-related glycosylation information that will help us better understand the pathophysiological mechanisms and hopefully specific glycosylation information to provide further diagnostic and therapeutic directions for neurodegenerative diseases.

Presenilin 1 Modulates Acetylcholinesterase Trafficking and Maturation

In Alzheimer's disease (AD), the reduction in acetylcholinesterase (AChE) enzymatic activity is not paralleled with changes in its protein levels, suggesting the presence of a considerable enzymatically inactive pool in the brain. In the present study, we validated previous findings, and, since inactive forms could result from post-translational modifications, we analyzed the glycosylation of AChE by lectin binding in brain samples from sporadic and familial AD (sAD and fAD). Most of the enzymatically active AChE was bound to lectins Canavalia ensiformis (Con A) and Lens culinaris agglutinin (LCA) that recognize terminal mannoses, whereas Western blot assays showed a very low percentage of AChE protein being recognized by the lectin. This indicates that active and inactive forms of AChE vary in their glycosylation pattern, particularly in the presence of terminal mannoses in active ones. Moreover, sAD subjects showed reduced binding to terminal mannoses compared to non-demented controls, while, for fAD patients that carry mutations in the PSEN1 gene, the binding was higher. The role of presenilin-1 (PS1) in modulating AChE glycosylation was then studied in a cellular model that overexpresses PS1 (CHO-PS1). In CHO-PS1 cells, binding to LCA indicates that AChE displays more terminal mannoses in oligosaccharides with a fucosylated core. Immunocytochemical assays also demonstrated increased presence of AChE in the trans-Golgi. Moreover, AChE enzymatic activity was higher in plasmatic membrane of CHO-PS1 cells. Thus, our results indicate that PS1 modulates trafficking and maturation of AChE in Golgi regions favoring the presence of active forms in the membrane.

Presenilin 1 deficiency impairs Aβ42-to-Aβ40- and angiotensin-converting activities of ACE

Introduction

Alzheimer's disease (AD) is associated with amyloid β-protein 1-42 (Aβ42) accumulation in the brain. Aβ42 and Aβ40 are the major two species generated from amyloid precursor protein. We found that angiotensin-converting enzyme (ACE) converts neurotoxic Aβ42 to neuroprotective Aβ40 in an ACE domain- and glycosylation-dependent manner. Presenilin 1 (PS1) mutations account for most of cases of familial AD and lead to an increased Aβ42/40 ratio. However, the mechanism by which PSEN1 mutations induce a higher Aβ42/40 ratio is unclear.

Methods

We over expressed human ACE in mouse wild-type and PS1-deficient fibroblasts. The purified ACE protein was used to analysis the Aβ42-to-Aβ40- and angiotensin-converting activities. The distribution of ACE was determined by Immunofluorescence staining.

Result

We found that ACE purified from PS1-deficient fibroblasts exhibited altered glycosylation and significantly reduced Aβ42-to-Aβ40- and angiotensin-converting activities compared with ACE from wild-type fibroblasts. Overexpression of wild-type PS1 in PS1-deficient fibroblasts restored the Aβ42-to-Aβ40- and angiotensin-converting activities of ACE. Interestingly, PS1 mutants completely restored the angiotensin-converting activity in PS1-deficient fibroblasts, but some PS1 mutants did not restore the Aβ42-to-Aβ40-converting activity. We also found that the glycosylation of ACE in adult mouse brain differed from that of embryonic brain and that the Aβ42-to-Aβ40-converting activity in adult mouse brain was lower than that in embryonic brain.

Conclusion

PS1 deficiency altered ACE glycosylation and impaired its Aβ42-to-Aβ40- and angiotensin-converting activities. Our findings suggest that PS1 deficiency and PSEN1 mutations increase the Aβ42/40 ratio by reducing the Aβ42-to-Aβ40-converting activity of ACE.

MS-based glycomics: An analytical tool to assess nervous system diseases

Neurological diseases affect millions of peopleochemistryorldwide and are continuously increasing due to the globe's aging population. Such diseases affect the nervous system and are characterized by a progressive decline in brain function and progressive cognitive impairment, decreasing the quality of life for those with the disease as well as for their families and loved ones. The increased burden of nervous system diseases demands a deeper insight into the biomolecular mechanisms at work during disease development in order to improve clinical diagnosis and drug design. Recently, evidence has related glycosylation to nervous system diseases. Glycosylation is a vital post-translational modification that mediates many biological functions, and aberrant glycosylation has been associated with a variety of diseases. Thus, the investigation of glycosylation in neurological diseases could provide novel biomarkers and information for disease pathology. During the last decades, many techniques have been developed for facilitation of reliable and efficient glycomic analysis. Among these, mass spectrometry (MS) is considered the most powerful tool for glycan analysis due to its high resolution, high sensitivity, and the ability to acquire adequate structural information for glycan identification. Along with MS, a variety of approaches and strategies are employed to enhance the MS-based identification and quantitation of glycans in neurological samples. Here, we review the advanced glycomic tools used in nervous system disease studies, including separation techniques prior to MS, fragmentation techniques in MS, and corresponding strategies. The glycan markers in common clinical nervous system diseases discovered by utilizing such MS-based glycomic tools are also summarized and discussed.

The mammalian rhomboid protein RHBDL4 protects against endoplasmic reticulum stress by regulating the morphology and distribution of ER sheets

In metazoans, the architecture of the endoplasmic reticulum (ER) differs between cell types and undergoes major changes throughout the cell cycle and according to physiological needs. Although much is known about how the different ER morphologies are generated and maintained, especially ER tubules, how context-dependent changes in ER shape and distribution are regulated and the factors involved are less well characterized, as are the factors that contribute to the positioning of the ER within the cell. By overexpression and KO experiments, we show that the levels of RHBDL4, an ER-resident rhomboid protease, modulate the shape and distribution of the ER, especially during conditions that require rapid changes in the ER sheet distribution, such as ER stress. We demonstrate that RHBDL4 interacts with cytoskeleton-linking membrane protein 63 (CLIMP-63), a protein involved in ER sheet stabilization, as well as with the cytoskeleton. Furthermore, we found that mice lacking RHBDL4 are sensitive to ER stress and develop liver steatosis, a phenotype associated with unresolved ER stress. Taken together, these data suggest a new physiological role for RHBDL4 and also imply that this function does not require its enzymatic activity.

What Can N-glycomics and N-glycoproteomics of Cerebrospinal Fluid Tell Us about Alzheimer Disease?

Proteomics-large-scale studies of proteins-has over the last decade gained an enormous interest for studies aimed at revealing proteins and pathways involved in disease. To fully understand biological and pathological processes it is crucial to also include post-translational modifications in the "omics". To this end, glycomics (identification and quantification of glycans enzymatically or chemically released from proteins) and glycoproteomics (identification and quantification of peptides/proteins with the glycans still attached) is gaining interest. The study of protein glycosylation requires a workflow that involves an array of sample preparation and analysis steps that needs to be carefully considered. Herein, we briefly touch upon important steps such as sample preparation and preconcentration, glycan release, glycan derivatization and quantification and advances in mass spectrometry that today are the work-horse for glycomics and glycoproteomics studies. Several proteins related to Alzheimer disease pathogenesis have altered protein glycosylation, and recent glycomics studies have shown differences in cerebrospinal fluid as well as in brain tissue in Alzheimer disease as compared to controls. In this review, we discuss these techniques and how they have been used to shed light on Alzheimer disease and to find glycan biomarkers in cerebrospinal fluid.

Class II phosphatidylinositol 3-kinase-C2α is essential for Notch signaling by regulating the endocytosis of γ-secretase in endothelial cells

The class II α-isoform of phosphatidylinositol 3-kinase (PI3K-C2α) plays a crucial role in angiogenesis at least in part through participating in endocytosis and, thereby, endosomal signaling of several cell surface receptors including VEGF receptor-2 and TGFβ receptor in vascular endothelial cells (ECs). The Notch signaling cascade regulates many cellular processes including cell proliferation, cell fate specification and differentiation. In the present study, we explored a role of PI3K-C2α in Delta-like 4 (Dll4)-induced Notch signaling in ECs. We found that knockdown of PI3K-C2α inhibited Dll4-induced generation of the signaling molecule Notch intracellular domain 1 (NICD1) and the expression of Notch1 target genes including HEY1, HEY2 and NOTCH3 in ECs but not in vascular smooth muscle cells. PI3K-C2α knockdown did not inhibit Dll4-induced endocytosis of cell surface Notch1. In contrast, PI3K-C2α knockdown as well as clathrin heavy chain knockdown impaired endocytosis of Notch1-cleaving protease, γ-secretase complex, with the accumulation of Notch1 at the perinuclear endolysosomes. Pharmacological blockage of γ-secretase also induced the intracellular accumulation of Notch1. Taken together, we conclude that PI3K-C2α is required for the clathrin-mediated endocytosis of γ-secretase complex, which allows for the cleavage of endocytosed Notch1 by γ-secretase complex at the endolysosomes to generate NICD1 in ECs.

Implications of Glycosylation in Alzheimer's Disease

Alzheimer's disease (AD) is the most common cause of dementia, affecting millions of people worldwide, and no cure is currently available. The major pathological hallmarks of AD are considered to be amyloid beta plaques and neurofibrillary tangles, generated by respectively APP processing and Tau phosphorylation. Recent evidence imply that glycosylation of these proteins, and a number of other AD-related molecules is altered in AD, suggesting a potential implication of this process in disease pathology. In this review we summarize the understanding of glycans in AD pathogenesis, and discuss how glycobiology can contribute to early diagnosis and treatment of AD, serving as potential biomarkers and therapeutic targets. Furthermore, we look into the potential link between the emerging topic neuroinflammation and glycosylation, combining two interesting, and until recent years, understudied topics in the scope of AD. Lastly, we discuss how new model platforms such as induced pluripotent stem cells can be exploited and contribute to a better understanding of a rather unexplored area in AD.

Exploring the Role of PSEN Mutations in the Pathogenesis of Alzheimer's Disease

Alzheimer's disease (AD) is the most common cause of dementia. Mutations of presenilin (PSEN) genes that encode presenilin proteins have been found as the vital causal factors for early-onset familial AD (FAD). AD pathological features such as memory loss, synaptic dysfunction, and formation of plaques have been successfully mimicked in the transgenic mouse models that coexpress FAD-related presenilin and amyloid precursor protein (APP) variants. γ-Secretase (GS) is an enzyme that plays roles in catalyzing intramembranous APP proteolysis to release pathogenic amyloid beta (Aβ). It has been found that presenilins can play a role as the GS's catalytic subunit. FAD-related mutations in presenilins can modify the site of GS cleavage in a way that can elevate the production of longer and highly fibrillogenic Aβ. Presenilins can interact with β-catenin to generate presenilin complexes. Aforesaid interactions have also been studied to observe the mutational and physiological activities in the catenin signal transduction pathway. Along with APP, GS can catalyze intramembrane proteolysis of various substrates that play a vital role in synaptic function. PSEN mutations can cause FAD with autosomal dominant inheritance and early onset of the disease. In this article, we have reviewed the current progress in the analysis of PSENs and the correlation of PSEN mutations and 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.

Stress-Induced Cellular Clearance Is Mediated by the SNARE Protein ykt6 and Disrupted by α-Synuclein

Age-related neurodegenerative disorders are characterized by a slow, persistent accumulation of aggregated proteins. Although cells can elicit physiological responses to enhance cellular clearance and counteract accumulation, it is unclear how pathogenic proteins evade this process in disease. We find that Parkinson's disease α-synuclein perturbs the physiological response to lysosomal stress by impeding the SNARE protein ykt6. Cytosolic ykt6 is normally autoinhibited by a unique farnesyl-mediated regulatory mechanism; however, during lysosomal stress, it activates and redistributes into membranes to preferentially promote hydrolase trafficking and enhance cellular clearance. α-Synuclein aberrantly binds and deactivates ykt6 in patient-derived neurons, thereby disabling the lysosomal stress response and facilitating protein accumulation. Activating ykt6 by small-molecule farnesyltransferase inhibitors restores lysosomal activity and reduces α-synuclein in patient-derived neurons and mice. Our findings indicate that α-synuclein creates a permissive environment for aggregate persistence by inhibiting regulated cellular clearance and provide a therapeutic strategy to restore protein homeostasis by harnessing SNARE activity.

Angiogenic signaling in the lungs of a metabolically suppressed hibernating mammal (Ictidomys tridecemlineatus)

To conserve energy in times of limited resource availability, particularly during cold winters, hibernators suppress even the most basic of physiologic processes. Breathing rates decrease from 40 breaths/minute to less than 1 breath/min as they decrease body temperature from 37 °C to ambient. Nevertheless, after months of hibernation, these incredible mammals emerge from torpor unscathed. This study was conducted to better understand the protective and possibly anti-inflammatory adaptations that hibernator lungs may use to prevent damage associated with entering and emerging from natural torpor. We postulated that the differential protein expression of soluble protein receptors (decoy receptors that sequester soluble ligands to inhibit signal transduction) would help identify inhibited inflammatory signaling pathways in metabolically suppressed lungs. Instead, the only two soluble receptors that responded to torpor were sVEGFR1 and sVEGFR2, two receptors whose full-length forms are bound by VEGF-A to regulate endothelial cell function and angiogenesis. Decreased sVEGFR1/2 correlated with increased total VEGFR2 protein levels. Maintained or increased levels of key γ-secretase subunits suggested that decreased sVEGFR1/2 protein levels were not due to decreased levels of intramembrane cleavage complex subunits. VEGF-A protein levels did not change, suggesting that hibernators may regulate VEGFR1/2 signaling at the level of the receptor instead of increasing relative ligand abundance. A panel of angiogenic factors used to identify biomarkers of angiogenesis showed a decrease in FGF-1 and an increase in BMP-9. Torpid lungs may use VEGF and BMP-9 signaling to balance angiogenesis and vascular stability, possibly through the activation of SMAD signaling for adaptive tissue remodeling.

Early Stage Glycosylation Biomarkers in Alzheimer's Disease

Alzheimer's disease (AD) is of great cause for concern in our ageing population, which currently lacks diagnostic tools to permit accurate and timely diagnosis for affected individuals. The development of such tools could enable therapeutic interventions earlier in the disease course and thus potentially reducing the debilitating effects of AD. Glycosylation is a common, and important, post translational modification of proteins implicated in a host of disease states resulting in a complex array of glycans being incorporated into biomolecules. Recent investigations of glycan profiles, in a wide range of conditions, has been made possible due to technological advances in the field enabling accurate glycoanalyses. Amyloid beta (Aβ) peptides, tau protein, and other important proteins involved in AD pathogenesis, have altered glycosylation profiles. Crucially, these abnormalities present early in the disease state, are present in the peripheral blood, and help to distinguish AD from other dementias. This review describes the aberrant glycome in AD, focusing on proteins implicated in development and progression, and elucidates the potential of glycome aberrations as early stage biomarkers of AD.

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.

Fighting the Cause of Alzheimer's and GNE Myopathy

Age is the common risk factor for both neurodegenerative and neuromuscular diseases. Alzheimer disease (AD), a neurodegenerative disorder, causes dementia with age progression while GNE myopathy (GNEM), a neuromuscular disorder, causes muscle degeneration and loss of muscle motor movement with age. Individuals with mutations in presenilin or amyloid precursor protein (APP) gene develop AD while mutations in GNE (UDP N-acetylglucosamine 2 epimerase/N-acetyl Mannosamine kinase), key sialic acid biosynthesis enzyme, cause GNEM. Although GNEM is characterized with degeneration of muscle cells, it is shown to have similar disease hallmarks like aggregation of Aβ and accumulation of phosphorylated tau and other misfolded proteins in muscle cell similar to AD. Similar impairment in cellular functions have been reported in both disorders such as disruption of cytoskeletal network, changes in glycosylation pattern, mitochondrial dysfunction, oxidative stress, upregulation of chaperones, unfolded protein response in ER, autophagic vacuoles, cell death, and apoptosis. Interestingly, AD and GNEM are the two diseases with similar phenotypic condition affecting neuron and muscle, respectively, resulting in entirely different pathology. This review represents a comparative outlook of AD and GNEM that could lead to target common mechanism to find a plausible therapeutic for both the diseases.

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.

Glycosylation status of nicastrin influences catalytic activity and substrate preference of γ-secretase

γ-Secretase complex, the assembly of nicastrin (NCT), Presenilin (PS), Presenilin Enhancer-2 (PEN-2) and Anterior pharynx defective 1 (Aph-1), catalyzes the cleavage of amyloid precursor protein to generate amyloid-β protein (Aβ), the main culprit of Alzheimer's disease. NCT becomes matured through complex glycosylation and play important role in γ-secretase activity by interacting with catalytic subunit PS. However, the role of NCT glycosylation on γ-secretase activity and substrate specificity is still unknown. The purpose of this study is to investigate the effect of NCT glycosylation on γ-secretase activity and substrate specificity in a group of glycosylation mutant lectin resistant CHO (Lec) cells. CHO Lec-1 cells lack glycosyltransferase-I, GnT-I, thus N-glycan on NCT are all oligomannose type, whereas CHO Lec-2 cells synthesize NCT containing sialic acid deficient oligosaccharides due to the impairment of cytidine 5'-monophosphate-sialic acid transporter. Here, we reported that mutant CHO Lec-1 and Lec-2 reduced γ-secretase activity in both cell-based and biochemical assays, and that CHO Lec-1 preferentially reduced Aβ generation. Endogenous level of γ-secretase complex, subcellular distribution of γ-secretase subunits and the level of functional γ-secretase complex remained unchanged in mutants. Interestingly, Coimmunoprecipitation study revealed that mutant γ-secretase could recognize substrate as well as parental γ-secretase. Our data suggests that thorough glycosylation of NCT is critical for enzymatic activity and substrate preference of γ-secretase.

The Notch inhibitor cowanin accelerates nicastrin degradation

Aberrant activation of Notch signaling contributes to the pathogenesis of several different types of cancer, and Notch pathway inhibitors may have significant therapeutic potential. Using a unique cell-based assay system, we isolated twelve compounds, including one new natural product from Garcinia speciosa, that inhibit the Notch signaling pathway. HES1 and HES5 are target genes of the Notch cascade, and compound 2, referred to as cowanin, decreased the protein levels of HES1 and HES5 in assay cells. Furthermore, cowanin (2) showed potent cytotoxicity against human leukemic HPB-ALL cells. The Notch signaling inhibitory activity of cowanin (2) is linked to the increased degradation of nicastrin, which is one of the components of the γ-secretase complex. To the best of our knowledge, this is the first example of a compound with Notch pathway inhibitory activity mediated by nicastrin degradation.

Structural and Chemical Biology of Presenilin Complexes

The presenilin proteins are the catalytic subunits of a tetrameric complex containing presenilin 1 or 2, anterior pharynx defective 1 (APH1), nicastrin, and PEN-2. Other components such as TMP21 may exist in a subset of specialized complexes. The presenilin complex is the founding member of a unique class of aspartyl proteases that catalyze the γ, ɛ, ζ site cleavage of the transmembrane domains of Type I membrane proteins including amyloid precursor protein (APP) and Notch. Here, we detail the structural and chemical biology of this unusual enzyme. Taken together, these studies suggest that the complex exists in several conformations, and subtle long-range (allosteric) shifts in the conformation of the complex underpin substrate access to the catalytic site and the mechanism of action for allosteric inhibitors and modulators. Understanding the mechanics of these shifts will facilitate the design of γ-secretase modulator (GSM) compounds that modulate the relative efficiency of γ, ɛ, ζ site cleavage and/or substrate specificity.

Cholinergic System and Post-translational Modifications: An Insight on the Role in Alzheimer's Disease

BACKGROUND

Alzheimer's disease (AD) is the most common form of old age dementia. The formation of amyloid plaques (Aβ), neurofibrillary tangles and loss of basal forebrain cholinergic neurons are the hallmark events in the pathology of AD.

LITERATURE REVIEW

Cholinergic system is one of the most important neurotransmitter system involved in learning and memory which preferentially degenerates in the initial stages of AD. Activation of cholinergic receptors (muscarinic and nicotinic) activates multiple pathways which result in post translational modifications (PTMs) in multiple proteins which bring changes in nervous system. Cholinergic receptors-mediated PTMs "in-part" substantially affect the biosynthesis, proteolysis, degradation and expression of many proteins and in particular, amyloid precursor protein (APP). APP is subjected to several PTMs (proteolytic processing, glycosylation, sulfation, and phosphorylation) during its course of processing, resulting in Aβ deposition, leading to AD. Aβ also alters the PTMs of tau which is a microtubule associated protein. Therefore, post-translationally modified tau and Aβ collectively aggravate the neuronal loss that leads to cholinergic hypofunction.

CONCLUSION

Despite the accumulating evidences, the interaction between cholinergic neurotransmission and the physiological significance of PTM events remain speculative and still needs further exploration. This review focuses on the role of cholinergic system and discusses the significance of PTMs in pathological progression of AD and highlights some important future directions.

Presenilin 1 and Presenilin 2 Target γ-Secretase Complexes to Distinct Cellular Compartments

γ-Secretase complexes achieve the production of amyloid peptides playing a key role in Alzheimer disease. These proteases have many substrates involved in important physiological functions. They are composed of two constant subunits, nicastrin and PEN2, and two variable ones, presenilin (PS1 or PS2) and APH1 (APH1aL, APH1aS, or APH1b). Whether the composition of a given γ-secretase complex determines a specific cellular targeting remains unsolved. Here we combined a bidirectional inducible promoter and 2A peptide technology to generate constructs for the temporary, stoichiometric co-expression of six different combinations of the four γ-secretase subunits including EGFP-tagged nicastrin. These plasmids allow for the formation of functional γ-secretase complexes displaying specific activities and maturations. We show that PS1-containing γ-secretase complexes were targeted to the plasma membrane, whereas PS2-containing ones were addressed to the trans-Golgi network, to recycling endosomes, and, depending on the APH1-variant, to late endocytic compartments. Overall, these novel constructs unravel a presenilin-dependent subcellular targeting of γ-secretase complexes. These tools should prove useful to determine whether the cellular distribution of γ-secretase complexes contributes to substrate selectivity and to delineate regulations of their trafficking.

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.

Structural biology of presenilin 1 complexes

The presenilin genes were first identified as the site of missense mutations causing early onset autosomal dominant familial Alzheimer's disease. Subsequent work has shown that the presenilin proteins are the catalytic subunits of a hetero-tetrameric complex containing APH1, nicastrin and PEN-2. This complex (variously termed presenilin complex or gamma-secretase complex) performs an unusual type of proteolysis in which the transmembrane domains of Type I proteins are cleaved within the hydrophobic compartment of the membrane. This review describes some of the molecular and structural biology of this unusual enzyme complex. The presenilin complex is a bilobed structure. The head domain contains the ectodomain of nicastrin. The base domain contains a central cavity with a lateral cleft that likely provides the route for access of the substrate to the catalytic cavity within the centre of the base domain. There are reciprocal allosteric interactions between various sites in the complex that affect its function. For instance, binding of Compound E, a peptidomimetic inhibitor to the PS1 N-terminus, induces significant conformational changes that reduces substrate binding at the initial substrate docking site, and thus inhibits substrate cleavage. However, there is a reciprocal allosteric interaction between these sites such that prior binding of the substrate to the initial docking site paradoxically increases the binding of the Compound E peptidomimetic inhibitor. Such reciprocal interactions are likely to form the basis of a gating mechanism that underlies access of substrate to the catalytic site. An increasingly detailed understanding of the structural biology of the presenilin complex is an essential step towards rational design of substrate- and/or cleavage site-specific modulators of presenilin complex function.

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.

Anti-nicastrin monoclonal antibodies elicit pleiotropic anti-tumour pharmacological effects in invasive breast cancer cells

The goal of targeted cancer therapies is to specifically block oncogenic signalling, thus maximising efficacy, while reducing side-effects to patients. The gamma-secretase (GS) complex is an attractive therapeutic target in haematological malignancies and solid tumours with major pharmaceutical activity to identify optimal inhibitors. Within GS, nicastrin (NCSTN) offers an opportunity for therapeutic intervention using blocking monoclonal antibodies (mAbs). Here we explore the role of anti-nicastrin monoclonal antibodies, which we have developed as specific, multi-faceted inhibitors of proliferation and invasive traits of triple-negative breast cancer cells. We use 3D in vitro proliferation and invasion assays as well as an orthotopic and tail vail injection triple-negative breast cancer in vivo xenograft model systems. RNAScope assessed nicastrin in patient samples. Anti-NCSTN mAb clone-2H6 demonstrated a superior anti-tumour efficacy than clone-10C11 and the RO4929097 small molecule GS inhibitor, acting by inhibiting GS enzymatic activity and Notch signalling in vitro and in vivo. Confirming clinical relevance of nicastrin as a target, we report evidence of increased NCSTN mRNA levels by RNA in situ hybridization (RNAScope) in a large cohort of oestrogen receptor negative breast cancers, conferring independent prognostic significance for disease-free survival, in multivariate analysis. We demonstrate here that targeting NCSTN using specific mAbs may represent a novel mode of treatment for invasive triple-negative breast cancer, for which there are few targeted therapeutic options. Furthermore, we propose that measuring NCSTN in patient samples using RNAScope technology may serve as companion diagnostic for anti-NCSTN therapy in the clinic.

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.

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.

The role of protein glycosylation in Alzheimer disease

Glycosylation is one of the most common, and the most complex, forms of post-translational modification of proteins. This review serves to highlight the role of protein glycosylation in Alzheimer disease (AD), a topic that has not been thoroughly investigated, although glycosylation defects have been observed in AD patients. The major pathological hallmarks in AD are neurofibrillary tangles and amyloid plaques. Neurofibrillary tangles are composed of phosphorylated tau, and the plaques are composed of amyloid β-peptide (Aβ), which is generated from amyloid precursor protein (APP). Defects in glycosylation of APP, tau and other proteins have been reported in AD. Another interesting observation is that the two proteases required for the generation of amyloid β-peptide (Aβ), i.e. γ-secretase and β-secretase, also have roles in protein glycosylation. For instance, γ-secretase and β-secretase affect the extent of complex N-glycosylation and sialylation of APP, respectively. These processes may be important in AD pathogenesis, as proper intracellular sorting, processing and export of APP are affected by how it is glycosylated. Furthermore, lack of one of the key components of γ-secretase, presenilin, leads to defective glycosylation of many additional proteins that are related to AD pathogenesis and/or neuronal function, including nicastrin, reelin, butyrylcholinesterase, cholinesterase, neural cell adhesion molecule, v-ATPase, and tyrosine-related kinase B. Improved understanding of the effects of AD on protein glycosylation, and vice versa, may therefore be important for improving the diagnosis and treatment of AD patients.

Beta-amyloid impairs reelin signaling

Reelin is a signaling protein increasingly associated with the pathogenesis of Alzheimer's disease that relevantly modulates tau phosphorylation. We have previously demonstrated that β-amyloid peptide (Aβ) alters reelin expression. We have now attempted to determine whether abnormal reelin triggered by Aβ will result in signaling malfunction, contributing to the pathogenic process. Here, we show that reelin forms induced by β-amyloid are less capable of down-regulating tau phosphorylation via disabled-1 and GSK3β kinase. We also demonstrate that the scaffold protein 14-3-3 that increases tau phosphorylation by modulating GSK3β activity, is up-regulated during defective reelin signaling. Binding of reelin to its receptor, mainly ApoER2 in the brain, relays the signal into the cell. We associate the impaired reelin signaling with inefficiency of reelin in forming active homodimers and decreased ability to bind efficiently to its receptor, ApoER2. More remarkably, reelin from Alzheimer cortex shows a tendency to form large complexes instead of homodimers, the active form for signaling. Our results suggest that reelin expression is altered by Aβ leading to impaired reelin signaling.

Protein sorting motifs in the cytoplasmic tail of SorCS1 control generation of Alzheimer's amyloid-β peptide

Endosomal sorting of the Alzheimer amyloid precursor protein (APP) plays a key role in the biogenesis of the amyloid-β (Aβ) peptide. Genetic lesions underlying Alzheimer's disease (AD) can act by interfering with this physiological process. Specifically, proteins involved in trafficking between endosomal compartments and the trans-Golgi network (TGN) [including the retromer complex (Vps35, Vps26) and its putative receptors (sortilin, SorL1, SorCS1)] have been implicated in the molecular pathology of late-onset AD. Previously, we demonstrated a role for SorCS1 in APP metabolism and Aβ production and, while we implicated a role for the retromer in this regulation, the underlying mechanism remained poorly understood. Here, we provide evidence for a motif within the SorCS1c cytoplasmic tail that, when manipulated, results in perturbed sorting of APP and/or its fragments to endosomal compartments, decreased retrograde TGN trafficking, and increased Aβ production in H4 neuroglioma cells. These perturbations apparently do not involve turnover of the cell surface APP pool, but rather they involve intracellular APP and/or its fragments, downstream of APP endocytosis.

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.

Leptin regulates amyloid β production via the γ-secretase complex

Alzheimer's disease (AD) is the most common age-related neurodegenerative disease, affecting an estimated 5.3million people in the United States. While many factors likely contribute to AD progression, it is widely accepted that AD is driven by the accumulation of β-amyloid (Aβ), a small, fibrillogenic peptide generated by the sequential proteolysis of the amyloid precursor protein by the β- and γ-secretases. Though the underlying causes of Aβ accumulation in sporadic AD are myriad, it is clear that lifestyle and overall health play a significant role. The adipocyte-derived hormone leptin has varied systemic affects, including neuropeptide release and neuroprotection. A recent study by Lieb et al. (2009) showed that individuals with low plasma leptin levels are at greater risk of developing AD, through unknown mechanisms. In this report, we show that plasma leptin is a strong negative predictor of Aβ levels in the mouse brain, supporting a protective role for the hormone in AD onset. We also show that the inhibition of Aβ accumulation is due to the downregulation of transcription of the γ-secretase components. On the other hand, β-secretase expression is either unchanged (BACE1) or increased (BACE2). Finally, we show that only presenilin 1 (PS1) is negatively correlated with plasma leptin at the protein level (p<0.0001). These data are intriguing and may highlight a role for leptin in regulating the onset of amyloid pathology and AD.

Cellular mechanisms of γ-secretase substrate selection, processing and toxicity

Presenilins (PSs) are catalytic components of the γ-secretase proteolytic complexes that produce Aβ and cell signaling peptides. γ-Secretase substrates are mostly membrane-bound peptides derived following proteolytic cleavage of the extracellular domain of type I transmembrane proteins. Recent work reveals that γ-secretase substrate processing is regulated by proteins termed γ-secretase substrate recruiting factors (γSSRFs) that bridge substrates to γ-secretase complexes. These factors constitute novel targets for pharmacological control of specific γ-secretase products, such as Aβ and signaling peptides. PS familial Alzheimer's disease (FAD) mutants cause a loss of γ-secretase cleavage function at epsilon sites of substrates thus inhibiting production of cell signaling peptides while promoting accumulation of uncleaved toxic substrates. Importantly, γ-secretase inhibitors may cause toxicity in vivo by similar mechanisms. Here we review novel mechanisms that control γ-secretase substrate selection and cleavage and examine their relevance to AD.

Activation of the γ-secretase complex and presence of γ-secretase-activating protein may contribute to Aβ42 production in sporadic inclusion-body myositis muscle fibers

The muscle-fiber phenotype of sporadic inclusion-body myositis (s-IBM), the most common muscle disease associated with aging, shares several pathological abnormalities with Alzheimer disease (AD) brain, including accumulation of amyloid-β 42 (Aβ42) and its cytotoxic oligomers. The exact mechanisms leading to Aβ42 production within s-IBM muscle fibers are not known. Aβ42 and Aβ40 are generated after the amyloid-precursor protein (AβPP) is cleaved by β-secretase and the γ-secretase complex. Aβ42 is considered more cytotoxic than Aβ40, and it has a higher propensity to oligomerize, form amyloid fibrils, and aggregate. Recently, we have demonstrated in cultured human muscle fibers that experimental inhibition of lysosomal enzyme activities leads to Aβ42 oligomerization. In s-IBM muscle, we here demonstrate prominent abnormalities of the γ-secretase complex, as evidenced by: a) increase of γ-secretase components, namely active presenilin 1, presenilin enhancer 2, nicastrin, and presence of its mature, glycosylated form; b) increase of mRNAs of these γ-secretase components; c) increase of γ-secretase activity; d) presence of an active form of a newly-discovered γ-secretase activating protein (GSAP); and e) increase of GSAP mRNA. Furthermore, we demonstrate that experimental inhibition of lysosomal autophagic enzymes in cultured human muscle fibers a) activates γ-secretase, and b) leads to posttranslational modifications of AβPP and increase of Aβ42. Since autophagy is impaired in biopsied s-IBM muscle, the same mechanism might be responsible for its having increased γ-secretase activity and Aβ42 production. Accordingly, improving lysosomal function might be a therapeutic strategy for s-IBM patients.

Phosphorylation of nicastrin by SGK1 leads to its degradation through lysosomal and proteasomal pathways

The gamma-secretase complex is involved in the intramembranous proteolysis of a variety of substrates, including the amyloid precursor protein and the Notch receptor. Nicastrin (NCT) is an essential component of the gamma-secretase complex and functions as a receptor for gamma-secretase substrates. In this study, we determined that serum- and glucocorticoid-induced protein kinase 1 (SGK1) markedly reduced the protein stability of NCT. The SGK1 kinase activity was decisive for NCT degradation and endogenous SGK1 inhibited gamma-secretase activity. SGK1 downregulates NCT protein levels via proteasomal and lysosomal pathways. Furthermore, SGK1 directly bound to and phosphorylated NCT on Ser437, thereby promoting protein degradation. Collectively, our findings indicate that SGK1 is a gamma-secretase regulator presumably effective through phosphorylation and degradation of NCT.

Altered γ-secretase activity in mild cognitive impairment and Alzheimer's disease

We investigated why the cerebrospinal fluid (CSF) concentrations of Aβ42 are lower in mild cognitive impairment (MCI) and Alzheimer's disease (AD) patients. Because Aβ38/42 and Aβ40/43 are distinct product/precursor pairs, these four species in the CSF together should faithfully reflect the status of brain γ-secretase activity, and were quantified by specific enzyme-linked immunosorbent assays in the CSF from controls and MCI/AD patients. Decreases in the levels of the precursors, Aβ42 and 43, in MCI/AD CSF tended to accompany increases in the levels of the products, Aβ38 and 40, respectively. The ratios Aβ40/43 versus Aβ38/42 in CSF (each representing cleavage efficiency of Aβ43 or Aβ42) were largely proportional to each other but generally higher in MCI/AD patients compared to control subjects. These data suggest that γ-secretase activity in MCI/AD patients is enhanced at the conversion of Aβ43 and 42 to Aβ40 and 38, respectively. Consequently, we measured the in vitro activity of raft-associated γ-secretase isolated from control as well as MCI/AD brains and found the same, significant alterations in the γ-secretase activity in MCI/AD brains.

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.

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.

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.

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.

GlycoFish: a database of zebrafish N-linked glycoproteins identified using SPEG method coupled with LC/MS

Zebrafish (Danio rerio) is a model organism that is used to study the mechanisms and pathways of human disorders. Many dysfunctions in neurological, development, and neuromuscular systems are due to glycosylation deficiencies, but the glycoproteins involved in zebrafish embryonic development have not been established. In this study, a mass spectrometry-based glycoproteomic characterization of zebrafish embryos was performed to identify the N-linked glycoproteins and N-linked glycosylation sites. To increase the number of glycopeptides, proteins from zebrafish were digested with two different proteases--chymotrypsin and trypsin--into peptides of different length. The N-glycosylated peptides of zebrafish were then captured by the solid-phase extraction of N-linked glycopeptides (SPEG) method and the peptides were identified with an LTQ OrbiTrap Velos mass spectrometer. From 265 unique glycopeptides, including 269 consensus NXT/S glycosites, we identified 169 different N-glycosylated proteins. The identified glycoproteins were highly abundant in proteins belonging to the transporter, cell adhesion, and ion channel/ion binding categories, which are important to embryonic, organ, and central nervous system development. This proteomics data will expand our knowledge about glycoproteins in zebrafish and may be used to elucidate the role that glycosylation plays in cellular processes and disease. The glycoprotein data are available through the GlycoFish database (http://betenbaugh.jhu.edu/GlycoFish) introduced in this paper.