Nicastrin is required for assembly of presenilin/gamma-secretase complexes to mediate Notch signaling and for processing and trafficking of beta-amyloid precursor protein in mammals

Notch signaling without the APH-2/nicastrin subunit of gamma secretase in Caenorhabditis elegans germline stem cells

The final step in Notch signaling activation is the transmembrane cleavage of Notch receptor by γ secretase. Thus far, genetic and biochemical evidence indicates that four subunits are essential for γ secretase activity in vivo: presenilin (the catalytic core), APH-1, PEN-2, and APH-2/nicastrin. Although some γ secretase activity has been detected in APH-2/nicastrin-deficient mammalian cell lines, the lack of biological relevance for this activity has left the quaternary γ secretase model unchallenged. Here, we provide the first example of in vivo Notch signal transduction without APH-2/nicastrin. The surprising dispensability of APH-2/nicastrin is observed in Caenorhabditis elegans germline stem cells (GSCs) and contrasts with its essential role in previously described C. elegans Notch signaling events. Depletion of GLP-1/Notch, presenilin, APH-1, or PEN-2 causes a striking loss of GSCs. In contrast, aph-2/nicastrin mutants maintain GSCs and exhibit robust and localized expression of the downstream Notch target sygl-1. Interestingly, APH-2/nicastrin is normally expressed in GSCs and becomes essential under conditions of compromised Notch function. Further insight is provided by reconstituting the C. elegans γ secretase complex in yeast, where we find that APH-2/nicastrin increases but is not essential for γ secretase activity. Together, our results are most consistent with a revised model of γ secretase in which the APH-2/nicastrin subunit has a modulatory, rather than obligatory role. We propose that a trimeric presenilin-APH-1-PEN-2 γ secretase complex can provide a low level of γ secretase activity, and that cellular context determines whether or not APH-2/nicastrin is essential for effective Notch signal transduction.

Presenilin deficiency enhances tau phosphorylation and its secretion

Alzheimer's disease (AD) is characterized by the accumulation of abnormally folded amyloid β-protein (Aβ) in the brain parenchyma and phosphorylated tau in neurons. Presenilin (PS, PSEN) 1 and PS2 are essential components of γ-secretase, which is responsible for the cleavage of amyloid precursor protein (APP) to generate Aβ. PSEN mutations are associated with tau aggregation in frontotemporal dementia, regardless of the presence or absence of Aβ pathology. However, the mechanism by which PS regulates tau aggregation is still unknown. Here, we found that tau phosphorylation and secretion were significantly increased in PS double-knock-out (PS1/2-/-) fibroblasts compared with wild-type fibroblasts. Tau-positive vesicles in the cytoplasm were significantly increased in PS1/2-/- fibroblasts. Active GSK-3β was increased in PS1/2-/- fibroblasts, and inhibiting GSK3β activity in PS1/2-/- fibroblasts resulted in decreased tau phosphorylation and secretion. Transfection of WT human PS1 and PS2 reduced the secretion of phosphorylated tau and active GSK-3β in PS1/2-/- fibroblasts. However, PS1D257A without γ-secretase activity did not decrease the secretion of phosphorylated tau. Furthermore, nicastrin deficiency also increased tau phosphorylation and secretion. These results suggest that deficient PS complex maturation may increase tau phosphorylation and secretion. Thus, our studies discover a new pathway by which PS regulates tau phosphorylation/secretion and pathology independent of Aβ and suggest that PS serves as a potential therapeutic target for treating neurodegenerative diseases involving tau aggregation.

Caenorhabditis elegans: A transgenic model for studying age-associated neurodegenerative diseases

Neurodegenerative diseases (NDs) are a heterogeneous group of aging-associated ailments characterized by interrupting cellular proteostasic machinery and the misfolding of distinct proteins to form toxic aggregates in neurons. Neurodegenerative diseases, which include Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), Huntington's disease (HD), and others, are becoming an increasing threat to human health worldwide. The degeneration and death of certain specific groups of neurons are the hallmarks of these diseases. Over the past decades, Caenorhabditis eleganshas beenwidely used as a transgenic model to investigate biological processes related to health and disease. The nematode Caenorhabditis elegans (C. elegans) has developed as a powerful tool for studying disease mechanisms due to its ease of genetic handling and instant cultivation while providing a whole-animal system amendable to several molecular and biochemical techniques. In this review, we elucidate the potential of C. elegans as a versatile platform for systematic dissection of the molecular basis of human disease, focusing on neurodegenerative disorders, and may help better our understanding of the disease mechanisms and search for new therapeutics for these devastating diseases.

Drug-tolerant persister B-cell precursor acute lymphoblastic leukemia cells

Reduced responsiveness of precursor B-acute lymphoblastic leukemia (BCP-ALL) to chemotherapy can be inferred when leukemia cells persist after 28 days of initial treatment. Survival of these long-term persister (LTP) / minimal residual disease (MRD) cells is partly due to bone marrow stromal cells that protect them under conditions of chemotherapy stress. We used RNA-seq to analyse BCP-ALL cells that survived a long-term, 30-day vincristine chemotherapy treatment while in co-culture with bone marrow stromal cells. RNAs of as many as 10% of the protein-encoding genes were differentially expressed. There was substantial overlap with genes associated with MRD cell persistence reported in other studies. The top pathway regulated in the LTP cells was that involving p53, a master regulator of a spectrum of responses relevant to drug resistance and cytotoxic drug exposure including control of autophagy. We tested a select number of genes for contribution to BCP-ALL cell survival using Cas9/CRISPR in a 2-step selection, initially for overall effect on cell fitness, followed by 21 days of exposure to vincristine. Many genes involved in autophagy and lysosomal function were found to contribute to survival both at steady-state and during drug treatment. We also identified MYH9, NCSTN and KIAA2013 as specific genes contributing to fitness of BCP-ALL cells. CD44 was not essential for growth under steady state conditions but was needed for survival of vincristine treatment. Finally, although the drug transporter ABCC1/MRP1 is not overexpressed in BCP-ALL, a functional gene was needed for DTP cells to survive treatment with vincristine. This suggests that addition of possible ABCC1 inhibitors during induction therapy could provide benefit in eradication of minimal residual disease in patients treated with a chemotherapy regimen that includes vincristine.

Alpha7 nicotinic acetylcholine receptor agonist PHA-543613 improves memory deficits in presenilin 1 and presenilin 2 conditional double knockout mice

Cholinergic system dysfunction has been considered as a critical feature of neurodegenerative progression in Alzheimer's disease (AD). The α7 nicotinic acetylcholine receptors (α7-nAChRs) are widely expressed in the hippocampus cortex and play an important role in memory formation, considered as potential therapeutic agents targets. However, underlying mechanisms have not been fully elucidated. Here, we combine behavioral, molecular biological methods with in vitro slice and in vivo multichannel electrophysiological recording techniques to investigate the molecular, cellular synaptic and neuronal mechanisms of activating α7-nAChR by PHA-543613 (a selective α7-nAChR agonist), which influences the impaired cognitive function using presenilin 1 (PS1) and presenilin 2 (PS2) conditional double knockout (cDKO) mice. Our results demonstrated that PHA-543613 treatment significantly improved the impaired hippocampus-related memory via recovering the reduced the hippocampal synaptic protein levels of α7-nAChR, NMADAR and AMPAR, thereby restoring the impaired post-tetanic potentiation (PTP), long-term potentiation (LTP), activation of molecular signaling pathway for neuronal protection, theta power and strength of theta-gamma phase-amplitude coupling (PAC) at hippocampus in 6-month-old cDKO mice. For the first time, we systematically reveal the mechanisms by which PHA-543613 improves memory deficits at different levels. Therefore, our findings may be significant for the development of therapeutic strategies for AD.

Apolipoprotein E4 inhibits γ-secretase activity via binding to the γ-secretase complex

The mechanisms of amyloid accumulation in familial Alzheimer's disease (FAD) and sporadic AD (SAD) are controversial. In FAD, mutations in presenilin (PSEN) impair γ-secretase activity and lead to abnormal amyloid β-protein (Aβ) production, thereby increasing the Aβ42/40 ratio. SAD is postulated to be caused by decreased Aβ clearance of apolipoprotein E4 (APOE4), the strongest risk factor for SAD. However, whether intracellular APOE4 affects Aβ production is unclear. Using APOE3 and APOE4 knock-in (KI) mouse brain and primary cultured fibroblasts from these mice, in this study, we demonstrated that APOE3 and APOE4 bind to the γ-secretase complex and isoform-dependently regulate its activity and Aβ production. We found that Aβ40 levels and γ-secretase activity were higher in APOE knockout mouse brain than in wild-type mouse brain. APOE4-KI fibroblasts had significant lower Aβ levels and γ-secretase activity but higher Aβ42/40 ratio compared with APOE3-KI cells, indicating that APOE4-KI reduces Aβ production by inhibiting γ-secretase activity. Interestingly, the levels of γ-secretase complex bound to APOE4 are higher than those bound to APOE3, and the levels of γ-secretase complex in the brain and fibroblasts of APOE4-KI mice were higher than those of APOE3-KI mice. Taken together, our findings demonstrate that intracellular APOE4 inhibits Aβ production, more preferentially inhibits Aβ40 production, and thereby induces an increase in the Aβ42/40 ratio via binding to the γ-secretase complex. These results suggest a novel mechanism in which intracellular APOE4 contributes to the pathogenesis of SAD by inhibiting γ-secretase activity.

Revisiting the role of Notch in nephron segmentation confirms a role for proximal fate selection during mouse and human nephrogenesis

Notch signaling promotes maturation of nephron epithelia, but its proposed contribution to nephron segmentation into proximal and distal domains has been called into doubt. We leveraged single cell and bulk RNA-seq, quantitative immunofluorescent lineage/fate tracing, and genetically modified human induced pluripotent stem cells (iPSCs) to revisit this question in developing mouse kidneys and human kidney organoids. We confirmed that Notch signaling is needed for maturation of all nephron lineages, and thus mature lineage markers fail to detect a fate bias. By contrast, early markers identified a distal fate bias in cells lacking Notch2, and a concomitant increase in early proximal and podocyte fates in cells expressing hyperactive Notch1 was observed. Orthogonal support for a conserved role for Notch signaling in the distal/proximal axis segmentation is provided by the demonstration that nicastrin (NCSTN)-deficient human iPSC-derived organoids differentiate into TFA2B+ distal tubule and CDH1+ connecting segment progenitors, but not into HNF4A+ or LTL+ proximal progenitors.

Nanoscale organization of Nicastrin, the substrate receptor of the γ-secretase complex, as independent molecular domains

Alterations in the canonical processing of Amyloid Precursor Protein generate proteoforms that contribute to the onset of Alzheimer’s Disease. Modified composition of γ-secretase or mutations in its subunits has been directly linked to altered generation of Amyloid beta. Despite biochemical evidence about the role of γ-secretase in the generation of APP, the molecular origin of how spatial heterogeneity in the generation of proteoforms arises is not well understood. Here, we evaluated the localization of Nicastrin, a γ-secretase subunit, at nanometer sized functional zones of the synapse. With the help of super resolution microscopy, we confirm that Nicastrin is organized into nanodomains of high molecular density within an excitatory synapse. A similar nanoorganization was also observed for APP and the catalytic subunit of γ-secretase, Presenilin 1, that were discretely associated with Nicastrin nanodomains. Though Nicastrin is a functional subunit of γ-secretase, the Nicastrin and Presenilin1 nanodomains were either colocalized or localized independent of each other. The Nicastrin and Presenilin domains highlight a potential independent regulation of these molecules different from their canonical secretase function. The collisions between secretases and substrate molecules decide the probability and rate of product formation for transmembrane proteolysis. Our observations of secretase nanodomains indicate a spatial difference in the confinement of substrate and secretases, affecting the local probability of product formation by increasing their molecular availability, resulting in differential generation of proteoforms even within single synapses.

Nicastrin-Like, a Novel Transmembrane Protein from Trypanosoma cruzi Associated to the Flagellar Pocket

Nicastrin (NICT) is a transmembrane protein physically associated with the polytypical aspartyl protease presenilin that plays a vital role in the correct localization and stabilization of presenilin to the membrane-bound γ-secretase complex. This complex is involved in the regulation of a wide range of cellular events, including cell signaling and the regulation of endocytosed membrane proteins for their trafficking and protein processing. Methods: In Trypanosoma cruzi, the causal agent of the Chagas disease, a NICT-like protein (Tc/NICT) was identified with a short C-terminus orthologous to the human protein, a large ectodomain (ECD) with numerous glycosylation sites and a single-core transmembrane domain containing a putative TM-domain (457GSVGA461) important for the γ-secretase complex activity. Results: Using the Spot-synthesis strategy with Chagasic patient sera, five extracellular epitopes were identified and synthetic forms were used to generate rabbit anti-Tc/NICT polyclonal serum that recognized a ~72-kDa molecule in immunoblots of T. cruzi epimastigote extracts. Confocal microscopy suggests that Tc/NICT is localized in the flagellar pocket, which is consistent with data from our previous studies with a T. cruzi presenilin-like protein. Phylogenetically, Tc/NICT was localized within a subgroup with the T. rangeli protein that is clearly detached from the other Trypanosomatidae, such as T. brucei. These results, together with a comparative analysis of the selected peptide sequence regions between the T. cruzi and mammalian proteins, suggest a divergence from the human NICT that might be relevant to Chagas disease pathology. As a whole, our data show that a NICT-like protein is expressed in the infective and replicative stages of T. cruzi and may be considered further evidence for a γ-secretase complex in trypanosomatids.

Assembly of γ-secretase occurs through stable dimers after exit from the endoplasmic reticulum

γ-Secretase affects many physiological processes through targeting >100 substrates; malfunctioning links γ-secretase to cancer and Alzheimer’s disease. The spatiotemporal regulation of its stoichiometric assembly remains unresolved. Fractionation, biochemical assays, and imaging support prior formation of stable dimers in the ER, which, after ER exit, assemble into full complexes. In vitro ER budding shows that none of the subunits is required for the exit of others. However, knockout of any subunit leads to the accumulation of incomplete subcomplexes in COPII vesicles. Mutating a DPE motif in presenilin 1 (PSEN1) abrogates ER exit of PSEN1 and PEN-2 but not nicastrin. We explain this by the preferential sorting of PSEN1 and nicastrin through Sec24A and Sec24C/D, respectively, arguing against full assembly before ER exit. Thus, dimeric subcomplexes aided by Sec24 paralog selectivity support a stepwise assembly of γ-secretase, controlling final levels in post-Golgi compartments.

Biomarkers for the Clinical Diagnosis of Alzheimer's Disease: Metabolomics Analysis of Brain Tissue and Blood

With an increase in aging populations worldwide, age-related diseases such as Alzheimer's disease (AD) have become a global concern. At present, a cure for neurodegenerative disease is lacking. There is an urgent need for a biomarker that can facilitate the diagnosis, classification, prognosis, and treatment response of AD. The recent emergence of highly sensitive mass-spectrometry platforms and high-throughput technology can be employed to discover and catalog vast datasets of small metabolites, which respond to changed status in the body. Metabolomics analysis provides hope for a better understanding of AD as well as the subsequent identification and analysis of metabolites. Here, we review the state-of-the-art emerging candidate biomarkers for AD.

Angiotensin converting enzyme 2 is a novel target of the γ-secretase complex

Angiotensin converting enzyme 2 (ACE2) is a key regulator of the renin-angiotensin system, but also the functional receptor of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Based on structural similarity with other γ-secretase (γS) targets, we hypothesized that ACE2 may be affected by γS proteolytic activity. We found that after ectodomain shedding, ACE2 is targeted for intramembrane proteolysis by γS, releasing a soluble ACE2 C-terminal fragment. Consistently, chemical or genetic inhibition of γS results in the accumulation of a membrane-bound fragment of ectodomain-deficient ACE2. Although chemical inhibition of γS does not alter SARS-CoV-2 cell entry, these data point to a novel pathway for cellular ACE2 trafficking.

Electroacupuncture Ameliorates Neuroinflammation-Mediated Cognitive Deficits through Inhibition of NLRP3 in Presenilin1/2 Conditional Double Knockout Mice

Neuroinflammation is considered as one of the crucial pathogenesis in promoting neurodegenerative progress of Alzheimer's disease (AD). As complementary and alternative therapy, electroacupuncture (EA) stimulation has been widely used in clinical practice for anti-inflammation. However, whether EA promotes the cognitive deficits resulting from neuroinflammation in AD remains unclear. In this study, the presenilin 1 and 2 conditional double knockout (PS cDKO) mice, exhibited a series of AD-like pathology, robust neuroinflammatory responses, and memory deficits, were used to evaluate the potential neuroprotective effect of EA at Baihui (GV 20) and Shenting (GV 24) by behavioral testing, electrophysiology recording, and molecular biology analyzing. First, we observed that EA improved memory deficits and impaired synaptic plasticity. Moreover, EA possesses an ability to suppress the hyperphosphorylated tau and robust elevated NLRP3, ASC, Caspase-1, IL-1β, and IL-18 in PS cDKO mice. Importantly, MCC950, a potent and selective inhibitor of NLPR3 inflammasome, has similar effects on inhibiting the hyperphosphorylated tau and the robust elevated NLRP3 components and neuroinflammatory responses of PS cDKO mice as well as EA treatment. Furthermore, EA treatment is not able to further improve the AD-like phenotypes of PS cDKO mice in combination with the MCC950 administration. Therefore, EA stimulation at GV 20 and GV 24 acupoints may be a potential alternative therapy for deterring cognitive deficits in AD through suppression of NLRP3 inflammasome activation.

Epidermoid cyst in a patient with Alagille syndrome: Coincidence or connection?

Background:

Alagille syndrome is a rare genetic syndrome, which arises due to defects in the Notch signaling pathway, resulting in liver, cardiopulmonary, renal, skeletal, and ophthalmologic problems, among others. Epidermoid cysts are rare congenital benign lesions that develop from ectopic ectodermal cell rests formed during neurulation.

Case Description:

A 24-year-old Alagille syndrome patient presented with hearing loss and was found to have a sizable posterior fossa mass. He underwent craniotomy for uneventful resection of the lesion, which was found to be an epidermoid cyst.

Conclusion:

While our case may represent a coincidental occurrence of two pathologies presenting together, given that epidermoid cysts arise from aberrant neurulation, and in light of the crucial role of the Notch signaling pathway both in normal neurogenesis and in the pathogenesis of Alagille syndrome, we hypothesize a possible association between these entities.

Angiotensin converting enzyme 2 is a novel target of the γ-secretase complex

Angiotensin converting enzyme 2 (ACE2) is a key regulator of the renin-angiotensin system, but also the functional receptor of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Based on structural similarity with other γ-secretase (γS) targets, we hypothesized that ACE2 may be affected by γS proteolytic activity. We found that after ectodomain shedding, ACE2 is targeted for intramembrane proteolysis by γS, releasing a soluble ACE2 C-terminal fragment. Consistently, chemical or genetic inhibition of γS results in the accumulation of a membrane-bound fragment of ectodomain-deficient ACE2. Although chemical inhibition of γS does not alter SARS-CoV-2 cell entry, these data point to a novel pathway for cellular ACE2 trafficking.

Recent developments of small molecule γ-secretase modulators for Alzheimer's disease

Alzheimer's disease (AD) is the most common form of progressive neurodegenerative disorder, marked by memory loss and a decline in cognitive function. The major hallmarks of AD are the presence of intracellular neurofibrillary tau tangles (NFTs) composed of hyperphosphorylated tau proteins and extracellular plaques composed of amyloid beta peptides (Aβ). The amyloid (Aβ) cascade hypothesis proposes that the AD pathogenesis is initiated by the accumulation of Aβ peptides in the parenchyma of the brain. An aspartyl intramembranal protease called γ-secretase is responsible for the production of Aβ by the cleavage of the amyloid precursor protein (APP). Clinical studies of γ-secretase inhibitors (GSIs) for AD failed due to the lack of substrate specificity. Therefore, γ-secretase modulators (GSMs) have been developed as potential disease modifying agents to modulate the γ-secretase cleavage activity towards the production of toxic Aβ42 peptides. Following the first-generation 'nonsteroidal anti-inflammatory drug' (NSAID) based GSMs, second-generation GSMs (carboxylic acid based NSAID derivatives and non-NSAID derived heterocyclic analogues), as well as natural product-based GSMs, have been developed. In this review, we focus on the recent developments of small molecule-based GSMs that show potential improvements in terms of drug-like properties as well as their current status in human clinical trials and the future perspectives of GSM research.

Green Tea Seed Isolated Theasaponin E1 Ameliorates AD Promoting Neurotoxic Pathogenesis by Attenuating Aβ Peptide Levels in SweAPP N2a Cells

Alzheimer’s disease (AD) is the most frequent type of dementia affecting memory, thinking and behaviour. The major hallmark of the disease is pathological neurodegeneration due to abnormal aggregation of Amyloid beta (Aβ) peptides generated by β- and γ-secretases via amyloidogenic pathway. Purpose of the current study was to evaluate the effects of theasaponin E1 on the inhibition of Aβ producing β-, γ-secretases (BACE1, PS1 and NCT) and acetylcholinesterase and activation of the non-amyloidogenic APP processing α-secretase (ADAM10). Additionally, theasaponin E1 effects on Aβ degrading and clearing proteins neprilysin and insulin degrading enzyme (IDE). The effect of theasaponin E1 on these crucial enzymes was investigated by RT-PCR, ELISA, western blotting and fluorometric assays using mouse neuroblastoma cells (SweAPP N2a). theasaponin E1 was extracted and purified from green tea seed extract via HPLC, and N2a cells were treated with different concentrations for 24 h. Gene and protein expression in the cells were measured to determine the effects of activation and/or inhibition of theasaponin E1 on β- and γ-secretases, neprilysin and IDE. Results demonstrated that theasaponin E1 significantly reduced Aβ concentration by activation of the α-secretase and neprilysin. The activities of β- and γ-secretase were reduced in a dose-dependent manner due to downregulation of BACE1, presenilin, and nicastrin. Similarly, theasaponin E1 significantly reduced the activity of acetylcholinesterase. Overall, from the results it is concluded that green tea seed extracted saponin E1 possess therapeutic significance as a neuroprotective natural product recommended for the treatment of Alzheimer’s disease.

Structure-activity relationship of presenilin in γ-secretase-mediated intramembrane cleavage

Genetic research on familial cases of Alzheimer disease have identified presenilin (PS) as an important membrane protein in the pathomechanism of this disease. PS is the catalytic subunit of γ-secretase, which is responsible for the generation of amyloid-β peptide deposited in the brains of Alzheimer disease patients. γ-Secretase is an atypical protease composed of four membrane proteins (i.e., presenilin, nicastrin, anterior pharynx defective-1 (Aph-1), and presenilin enhancer-2 (Pen-2)) and mediates intramembrane proteolysis. Numerous investigations have been conducted toward understanding the structural features of γ-secretase components as well as the cleavage mechanism of γ-secretase. In this review, we summarize our current understanding of the structure and activity relationship of the γ-secretase complex.

Trans-cinnamaldehyde improves neuroinflammation-mediated NMDA receptor dysfunction and memory deficits through blocking NF-κB pathway in presenilin1/2 conditional double knockout mice

A chronic neuroinflammatory response has been considered as a critical pathogenesis promoting neurodegenerative progression in Alzheimer's disease (AD). During neuroinflammatory process, microglia are excessively activated and simultaneously release numerous pro-inflammatory mediators that cause synaptic dysfunction in the forebrain prior to neuronal degeneration and memory deficits in AD. Thus, prevention of neuroinflammation-mediated synaptic dysfunction may be a potential therapeutic approach against neurodegenerative disorders. Trans-cinnamaldehyde (TCA) is a primary bioactive component derived from the stem bark of Cinnamomum cassia, and it possesses potent anti-inflammatory and neuroprotective activities in in vivo and in vitro experiments. However, the in-depth molecular mechanisms of TCA underlying anti-neuroinflammatory and neuroprotective effects on memory deficits in AD are still unclear. The presenilin 1 and 2 conditional double knockout (PS cDKO) mice exhibit AD-like phenotypes including obvious neuroinflammatory responses and synaptic dysfunction and memory deficits. Here, PS cDKO were used to evaluate the potential neuroprotective effects of TCA against neuroinflammation-mediated dementia by performing behavioral tests, electrophysiological recordings and molecular biology analyses. We observed that TCA treatment reversed abnormal expression of synaptic proteins and tau hyperphosphorylation in the hippocampus and prefrontal cortex of PS cDKO mice. TCA treatment also ameliorated NMDA receptor (NMDAR) dysfunction including impaired NMDAR-mediated responses and long-term potentiation (LTP) induction in the hippocampus of PS cDKO mice. Moreover, TCA possesses an ability to suppress neuroinflammatory responses by diminishing microglial activation and levels of pro-inflammatory mediators in the hippocampus and prefrontal cortex of PS cDKO mice. Importantly, improving NMDAR dysfunction and memory deficits of PS cDKO mice was due to the inhibition of neuroinflammatory responses through TCA's interruptive effect on the nuclear factor kappa B (NF-κB) signaling pathway. Therefore, TCA may be a potential anti-neuroinflammatory agent for deterring neurodegenerative progression of AD.

Extracellular interface between APP and Nicastrin regulates Aβ length and response to γ-secretase modulators

γ-Secretase complexes (GSECs) are multimeric membrane proteases involved in a variety of physiological processes and linked to Alzheimer's disease (AD). Presenilin (PSEN, catalytic subunit), Nicastrin (NCT), Presenilin Enhancer 2 (PEN-2), and Anterior Pharynx Defective 1 (APH1) are the essential subunits of GSECs. Mutations in PSEN and the Amyloid Precursor Protein (APP) cause early-onset AD GSECs successively cut APP to generate amyloid-β (Aβ) peptides of various lengths. AD-causing mutations destabilize GSEC-APP/Aβn interactions and thus enhance the production of longer Aβs, which elicit neurotoxic events underlying pathogenesis. Here, we investigated the molecular strategies that anchor GSEC and APP/Aβn during the sequential proteolysis. Our studies reveal that a direct interaction between NCT ectodomain and APPC99 influences the stability of GSEC-Aβn assemblies and thereby modulates Aβ length. The data suggest a potential link between single-nucleotide variants in NCSTN and AD risk. Furthermore, our work indicates that an extracellular interface between the protease (NCT, PSEN) and the substrate (APP) represents the target for compounds (GSMs) modulating Aβ length. Our findings may guide future rationale-based drug discovery efforts.

Pen-2 and Presenilin are Sufficient to Catalyze Notch Processing

Presenilin-1 (PS1) or presenilin-2 (PS2), nicastrin (NCT), anterior pharynx-defective 1 (Aph-1), and presenilin enhancer-2 (Pen-2) have been considered the minimal essential subunits required to form an active γ-secretase complex. Besides PS, which has been widely believed to function as the catalytic subunit of the complex, the functional roles of the other subunits in the γ-secretase complex remain debatable. In the current study, we set out to determine the role of Pen-2 in γ-secretase activity. To this end, using knockout cells in combination with siRNA and immunoprecipitation approaches, our results revealed that Pen-2 together with presenilin are sufficient to form a functionally active enzyme to process Notch. Specifically, our data demonstrated that Pen-2 plays a crucial role in substrate binding, a mechanism by which Pen-2 contributes directly to the catalytic mechanism of γ-secretase activity. Our data also suggested that there may be different requirements for components to process AβPP and Notch. This information would be important for therapeutic strategy aimed at inhibition or modulation of γ-secretase activity.

Control of Blood Vessel Formation by Notch Signaling

Blood vessels span throughout the body to nourish tissue cells and to provide gateways for immune surveillance. Endothelial cells that line capillaries have the remarkable capacity to be quiescent for years but to switch rapidly into the activated state once new blood vessels need to be formed. In addition, endothelial cells generate niches for progenitor and tumor cells and provide organ-specific paracrine (angiocrine) factors that control organ development and regeneration, maintenance of homeostasis and tumor progression. Recent data indicate a pivotal role for blood vessels in responding to metabolic changes and that endothelial cell metabolism is a novel regulator of angiogenesis. The Notch pathway is the central signaling mode that cooperates with VEGF, WNT, BMP, TGF-β, angiopoietin signaling and cell metabolism to orchestrate angiogenesis, tip/stalk cell selection and arteriovenous specification. Here, we summarize the current knowledge and implications regarding the complex roles of Notch signaling during physiological and tumor angiogenesis, the dynamic nature of tip/stalk cell selection in the nascent vessel sprout and arteriovenous differentiation. Furthermore, we shed light on recent work on endothelial cell metabolism, perfusion-independent angiocrine functions of endothelial cells in organ-specific vascular beds and how manipulation of Notch signaling may be used to target the tumor vasculature.

BARHL1 Is Downregulated in Alzheimer's Disease and May Regulate Cognitive Functions through ESR1 and Multiple Pathways

The Transcription factor BarH like homeobox 1 (BARHL1) is overexpressed in medulloblastoma and plays a role in neurogenesis. However, much about the BARHL1 regulatory networks and their functions in neurodegenerative and neoplastic disorders is not yet known. In this study, using a tissue microarray (TMA), we report for the first time that BARHL1 is downregulated in hormone-negative breast cancers and Alzheimer’s disease (AD). Furthermore, using an integrative bioinformatics approach and mining knockout mouse data, we show that: (i) BARHL1 and Estrogen Receptor 1 (ESR1) may constitute a network that regulates Neurotrophin 3 (NTF3)- and Brain Derived Neurotrophic Factor (BDNF)-mediated neurogenesis and neural survival; (ii) this is probably linked to AD pathways affecting aberrant post-translational modifications including SUMOylation and ubiquitination; (iii) the BARHL1-ESR1 network possibly regulates β-amyloid metabolism and memory; and (iv) hsa-mir-18a, having common key targets in the BARHL1-ESR1 network and AD pathway, may modulate neuron death, reduce β-amyloid processing and might also be involved in hearing and cognitive decline associated with AD. We have also hypothesized why estrogen replacement therapy improves AD condition. In addition, we have provided a feasible new mechanism to explain the abnormal function of mossy fibers and cerebellar granule cells related to memory and cognitive decline in AD apart from the Tau and amyloid pathogenesis through our BARHL1-ESR1 axis.

The neuritic plaque facilitates pathological conversion of tau in an Alzheimer's disease mouse model

A central question in Alzheimer's Disease (AD) is whether the neuritic plaque is necessary and sufficient for the development of tau pathology. Hyperphosphorylation of tau is found within dystrophic neurites surrounding β-amyloid deposits in AD mouse models but the pathological conversion of tau is absent. Likewise, expression of a human tau repeat domain in mice is insufficient to drive the pathological conversion of tau. Here we developed an Aβ-amyloidosis mouse model that expresses the human tau repeat domain and show that in these mice, the neuritic plaque facilitates the pathological conversion of wild-type tau. We show that this tau fragment seeds the neuritic plaque-dependent pathological conversion of wild-type tau that spreads from the cortex and hippocampus to the brain stem. These results establish that in addition to the neuritic plaque, a second determinant is required to drive the conversion of wild-type tau.

Evidence That the "Lid" Domain of Nicastrin Is Not Essential for Regulating γ-Secretase Activity

Understanding of the structure of the γ-secretase complex consisting of presenilin (PS), anterior pharynx-defective 1 (APH-1), nicastrin (NCT), and presenilin enhancer 2 (PEN-2) is of significant therapeutic interest for the design of γ-secretase modulators for Alzheimer disease. The structure of γ-secretase revealed by cryo-EM approaches suggested a substrate binding mechanism for NCT, a bilobar structure that involved rotation of the two lobes around a central pivot and opening of a "lid" region that facilitates substrate recruitment. To validate this proposal, we expressed NCT that lacks the lid entirely, or a variety of NCT variants that harbor mutations at highly conserved residues in the lid region inNCT-deficient cells, and then assessed their impact on γ-secretase assembly, activity, and stability. In addition, we assessed the impact of mutating a critical residue proposed to be a pivot around which the two lobes of NCT rotate. Our results show that neither the mutations on the lid tested here nor the entire lid deletion has any significant impact on γ-secretase assembly, activity, and stability, and that NCT with the mutation of the proposed pivot rescues γ-secretase activity inNCT-deficient cells in a manner indistinguishable from WT NCT. These findings indicate that the NCT lid is not an essential element necessary for γ-secretase assembly, activity, and stability, and that rotation of the two lobes appears not to be a prerequisite for substrate binding and γ-secretase function.

Nicastrin is required for amyloid precursor protein (APP) but not Notch processing, while anterior pharynx-defective 1 is dispensable for processing of both APP and Notch

The γ-secretase complex is composed of at least four components: presenilin 1 or presenilin-2, nicastrin (NCT), anterior pharynx-defective 1 (Aph-1), and presenilin enhancer 2. In this study, using knockout cell lines, our data demonstrated that knockout of NCT, as well as knockout of presenilin enhancer 2, completely blocked γ-secretase-catalyzed processing of C-terminal fragment (CTF)α and CTFβ, the C-terminal fragments of β-amyloid precursor protein (APP) produced by α-secretase and β-secretase cleavages, respectively. Interestingly, in Aph-1-knockout cells, CTFα and CTFβ were still processed by γ-secretase, indicating Aph-1 is dispensable for APP processing. Furthermore, our results indicate that Aph-1 as well as NCT is not absolutely required for Notch processing, suggesting that NCT is differentially required for APP and Notch processing. In addition, our data revealed that components of the γ-secretase complex are also important for proteasome- and lysosome-dependent degradation of APP and that endogenous APP is mostly degraded by lysosome while exogenous APP is mainly degraded by proteasome. There are unanswered questions regarding the roles of each component of the γ-secretase complex in amyloid precursor protein (APP) and Notch processing. The most relevant, novel finding of this study is that nicastrin (NCT) is required for APP but not Notch processing, while Aph-1 is not essential for processing of both APP and Notch, suggesting NCT as a therapeutic target to restrict Aβ formation without impairing Notch signaling.

Adipocyte-specific blockade of gamma-secretase, but not inhibition of Notch activity, reduces adipose insulin sensitivity

Objective

As the obesity pandemic continues to expand, novel molecular targets to reduce obesity-related insulin resistance and Type 2 Diabetes (T2D) continue to be needed. We have recently shown that obesity is associated with reactivated liver Notch signaling, which, in turn, increases hepatic insulin resistance, opening up therapeutic avenues for Notch inhibitors to be repurposed for T2D. Herein, we tested the systemic effects of γ-secretase inhibitors (GSIs), which prevent endogenous Notch activation, and confirmed these effects through creation and characterization of two different adipocyte-specific Notch loss-of-function mouse models through genetic ablation of the Notch transcriptional effector Rbp-Jk (A-Rbpj) and the obligate γ-secretase component Nicastrin (A-Nicastrin).

Methods

Glucose homeostasis and both local adipose and systemic insulin sensitivity were examined in GSI-treated, A-Rbpj and A-Nicastrin mice, as well as vehicle-treated or control littermates, with complementary in vitro studies in primary hepatocytes and 3T3-L1 adipocytes.

Results

GSI-treatment increases hepatic insulin sensitivity in obese mice but leads to reciprocal lowering of adipose glucose disposal. While A-Rbpj mice show normal body weight, adipose development and mass and unchanged adipose insulin sensitivity as control littermates, A-Nicastrin mice are relatively insulin-resistant, mirroring the GSI effect on adipose insulin action.

Conclusions

Notch signaling is dispensable for normal adipocyte function, but adipocyte-specific γ-secretase blockade reduces adipose insulin sensitivity, suggesting that specific Notch inhibitors would be preferable to GSIs for application in T2D.

•

γ-secretase inhibitors (GSIs) are non-specific inhibitors of Notch signaling.

•

GSI-treatment of obese mice increases hepatic, but lowers adipose insulin sensitivity.

•

Adipocyte-specific Notch inhibition does not affect adipose mass or glucose homeostasis.

•

Adipocyte-specific γ-secretase blockade reduces adipose insulin sensitivity.

•

Specific Notch inhibitors may be preferable to GSIs for treatment of Type 2 Diabetes.

Oxidative stress, mitochondrial dysfunction and neurodegenerative diseases; a mechanistic insight

Mitochondria is one of the main source of oxidative stress (ROS), as it utilizes the oxygen for the energy production. ROS and RNS are normally generated by tightly regulated enzymes. Excessive stimulation of NAD(P)H and electron transport chain leads to the overproduction of ROS, results in oxidative stress, which is a good mediator to injure the cell structures, lipids, proteins, and DNA. Various oxidative events implicated in many diseases due to oxidative stress include alteration in mitochondrial proteins, mitochondrial lipids and mitochondrial DNA, Which in turn leads to the damage to nerve cell as they are metabolically very active. ROS/RNS at moderate concentrations also play roles in normal physiology of many processes like signaling pathways, induction of mitogenic response and in defense against infectious pathogens. Oxidative stress has been considered to be the main cause in the etiology of many diseases, which includes Parkinson's and Alzheimer diseases. Several PD associated genes have been found to be involved in mitochondrial function, dynamics and morphology as well. This review includes source of free radical generation, chemistry and biochemistry of ROS/RNS and mitochondrial dysfunction and the mechanism involved in neurodegenerative diseases.

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.

Glycomic analysis of membrane glycoproteins with bisecting glycosylation from ovarian cancer tissues reveals novel structures and functions

Biomarkers capable of detecting and targeting epithelial ovarian cancer cells for diagnostics and therapeutics would be extremely valuable. Ovarian cancer is the deadliest reproductive malignancy among women in the U.S., killing over 14 000 women each year. Both the lack of presenting symptoms and high mortality rates illustrate the need for earlier diagnosis and improved treatment of this disease. The glycosyltransferase enzyme GnT-III encoded by the Mgat3 gene is responsible for the addition of GlcNAc (N-acetylglucosamine) to form bisecting N-linked glycan structures. GnT-III mRNA expression is amplified in ovarian cancer tissues compared with normal ovarian tissue. We use a lectin capture strategy coupled to nano-ESI–RPLC–MS/MS to isolate and identify the membrane glycoproteins and unique glycan structures associated with GnT-III amplification in human ovarian cancer tissues. Our data illustrate that the majority of membrane glycoproteins with bisecting glycosylation are common to both serous and endometrioid histological subtypes of ovarian cancer, and several have been reported to participate in signaling pathways such as Notch, Wnt, and TGFβ.

Reduction of β-amyloid and γ-secretase by calorie restriction in female Tg2576 mice

Research indicates that female risk of developing Alzheimer's disease (AD) is greater than that of males. Moderate reduction of calorie intake, known as calorie restriction (CR), reduces pathology in AD mouse models and is a potentially translatable prevention measure for individuals at-risk for AD, as well as an important tool for understanding how the brain endogenously attenuates age-related pathology. Whether sex influences the response to CR remains unknown. In this study, we assessed the effect of CR on beta-amyloid peptide (Aβ) pathology and hippocampal CA1 neuron specific gene expression in the Tg2576 mouse model of cerebral amyloidosis. Relative to ad libitum (AL) feeding, CR feeding significantly reduced hippocampal Aβ burden in 15-month-old female, but not age-matched male, Tg2576 mice. Sustained CR also significantly reduced expression of presenilin enhancer 2 (Psenen) and presenilin 1, components of the γ-secretase complex, in Tg2576 females. These results indicate that long-term CR significantly reduces age-dependent female Tg2576 Aβ pathology, which is likely to involve CR-mediated reductions in γ-secretase-dependent amyloid precursor protein (APP) metabolism.

Synaptic function of nicastrin in hippocampal neurons

Synaptic dysfunction is widely thought to play a key role in the pathogenesis of Alzheimer's disease (AD). Presenilins, the major gene products involved in familial AD, are essential for short- and long-term synaptic plasticity in mature neurons as well as for the survival of cortical neurons during aging. Presenilin and nicastrin are both indispensable components of the γ-secretase complex, but it remains unknown whether presenilin regulates synaptic function in a γ-secretase-dependent or γ-secretase-independent manner and whether nicastrin plays similar roles in central synapses. In the current study, we address these questions using an electrophysiological approach to analyze nicastrin conditional knockout (cKO) mice in the hippocampal Schaffer collateral pathway. In these mice, we found that, even at 2 mo of age, deletion of nicastrin in excitatory neurons of the postnatal forebrain using Cre recombinase expressed under the control of the αCaMKII promoter led to deficits in presynaptic short-term plasticity including paired-pulse facilitation and frequency facilitation. Depletion of Ca(2+) in the endoplasmic reticulum mimics and occludes the presynaptic facilitation deficits in nicastrin cKO mice, suggesting that disrupted intracellular Ca(2+) homeostasis underlies the presynaptic deficits. In addition, NMDA receptor-mediated responses and long-term potentiation induced by theta-burst stimulation were decreased in nicastrin cKO mice at 3 mo but not at 2 mo of age. Together, these findings show that, similar to presenilins, nicastrin plays essential roles in the regulation of short- and long-term synaptic plasticity, highlighting the importance of γ-secretase in the function of mature synapses.

Decreased CALM expression reduces Aβ42 to total Aβ ratio through clathrin-mediated endocytosis of γ-secretase

A body of evidence suggests that aberrant metabolism of amyloid-β peptide (Aβ) underlies the aetiology of Alzheimer disease (AD). Recently, a single-nucleotide polymorphism in phosphatidylinositol binding clathrin assembly protein (PICALM/CALM) gene, which encodes a protein implicated in the clathrin-mediated endocytosis, was identified as a genetic protective factor for AD, although its mechanistic details have little been explored. Here we show that loss of CALM leads to the selective decrease in the production ratio of the pathogenic Aβ species, Aβ42. Active form of γ-secretase is constitutively endocytosed via the clathrin-mediated pathway in a CALM dependent manner. Alteration in the rate of clathrin-mediated endocytosis of γ-secretase causes a shift in its steady-state localization, which consequently impacts on the production ratio of Aβ42. Our study identifies CALM as an endogenous modulator of γ-secretase activity by regulating its endocytosis and also as an excellent target for Aβ42-lowering AD therapeutics.

Early onset Alzheimer's disease and oxidative stress

Alzheimer's disease (AD) is the most common cause of dementia in elderly adults. It is estimated that 10% of the world's population aged more than 60-65 years could currently be affected by AD, and that in the next 20 years, there could be more than 30 million people affected by this pathology. One of the great challenges in this regard is that AD is not just a scientific problem; it is associated with major psychosocial and ethical dilemmas and has a negative impact on national economies. The neurodegenerative process that occurs in AD involves a specific nervous cell dysfunction, which leads to neuronal death. Mutations in APP, PS1, and PS2 genes are causes for early onset AD. Several animal models have demonstrated that alterations in these proteins are able to induce oxidative damage, which in turn favors the development of AD. This paper provides a review of many, although not all, of the mutations present in patients with familial Alzheimer's disease and the association between some of these mutations with both oxidative damage and the development of the pathology.

Conditional inactivation of nicastrin restricts amyloid deposition in an Alzheimer's disease mouse model

Production of Aβ by γ-secretase is a key event in Alzheimer's disease (AD). The γ-secretase complex consists of presenilin (PS) 1 or 2, nicastrin (ncstn), Pen-2, and Aph-1 and cleaves type I transmembrane proteins, including the amyloid precursor protein (APP). Although ncstn is widely accepted as an essential component of the complex required for γ-secretase activity, recent in vitro studies have suggested that ncstn is dispensable for APP processing and Aβ production. The focus of this study was to answer this controversy and evaluate the role of ncstn in Aβ generation and the development of the amyloid-related phenotype in the mouse brain. To eliminate ncstn expression in the mouse brain, we used a ncstn conditional knockout mouse that we mated with an established AD transgenic mouse model (5XFAD) and a neuronal Cre-expressing transgenic mouse (CamKIIα-iCre), to generate AD mice (5XFAD/CamKIIα-iCre/ncstn(f/f) mice) where ncstn was conditionally inactivated in the brain. 5XFAD/CamKIIα-iCre/ncstn(f/f) mice at 10 week of age developed a neurodegenerative phenotype with a significant reduction in Aβ production and formation of Aβ aggregates and the absence of amyloid plaques. Inactivation of nctsn resulted in substantial accumulation of APP-CTFs and altered PS1 expression. These results reveal a key role for ncstn in modulating Aβ production and amyloid plaque formation in vivo and suggest ncstn as a target in AD therapeutics.

Presenilins regulate neurotrypsin gene expression and neurotrypsin-dependent agrin cleavage via cyclic AMP response element-binding protein (CREB) modulation

Presenilins, the catalytic components of the γ-secretase complex, are upstream regulators of multiple cellular pathways via regulation of gene transcription. However, the underlying mechanisms and the genes regulated by these pathways are poorly characterized. In this study, we identify Tequila and its mammalian ortholog Prss12 as genes negatively regulated by presenilins in Drosophila larval brains and mouse embryonic fibroblasts, respectively. Prss12 encodes the serine protease neurotrypsin, which cleaves the heparan sulfate proteoglycan agrin. Altered neurotrypsin activity causes serious synaptic and cognitive defects; despite this, the molecular processes regulating neurotrypsin expression and activity are poorly understood. Using γ-secretase drug inhibitors and presenilin mutants in mouse embryonic fibroblasts, we found that a mature γ-secretase complex was required to repress neurotrypsin expression and agrin cleavage. We also determined that PSEN1 endoproteolysis or processing of well known γ-secretase substrates was not essential for this process. At the transcriptional level, PSEN1/2 removal induced cyclic AMP response element-binding protein (CREB)/CREB-binding protein binding, accumulation of activating histone marks at the neurotrypsin promoter, and neurotrypsin transcriptional and functional up-regulation that was dependent on GSK3 activity. Upon PSEN1/2 reintroduction, this active epigenetic state was replaced by a methyl CpG-binding protein 2 (MeCP2)-containing repressive state and reduced neurotrypsin expression. Genome-wide analysis revealed hundreds of other mouse promoters in which CREB binding is similarly modulated by the presence/absence of presenilins. Our study thus identifies Tequila and neurotrypsin as new genes repressed by presenilins and reveals a novel mechanism used by presenilins to modulate CREB signaling based on controlling CREB recruitment.

Age-related neuropsychiatric symptoms in presenilins conditional double knockout mice

Alzheimer's disease (AD) is the most common form of dementia and causes impairments of memory, cognition and behavior. Remarkably, most AD patients exhibit personality changes that often precede other early clinical manifestations. Conditional presenilin1 (PS1) and presenilin2 (PS2) double knockout (DKO) mice have age-related forebrain atrophy, tau hyperphosphorylation, synaptic dysfunction, cognitive deficits and increased inflammatory responses in both the periphery and the brain. Whether these mice have age-related emotional changes have not yet been investigated. In the present study, we used 2-, 6- and 11-month-old DKO and littermate control (CON) mice to examine their age-related emotional conditions. Our results indicate that DKO mice have observable age-related neuropsychiatric symptoms, such as anxiety, irritability, depression, apathy, aggressivity, anhedonia and aberrant motor behavior when compared with other AD-like mouse models. In summary, our results not only indicate that DKO mice may be a valuable model for probing age-related AD diagnoses but also suggest a new pathogenesis of neurodegenerative diseases that is worth further investigation.

Visualizing active enzyme complexes using a photoreactive inhibitor for proximity ligation--application on γ-secretase

Here, we present a highly sensitive method to study protein-protein interactions and subcellular location selectively for active multicomponent enzymes. We apply the method on γ-secretase, the enzyme complex that catalyzes the cleavage of the amyloid precursor protein (APP) to generate amyloid β-peptide (Aβ), the major causative agent in Alzheimer disease (AD). The novel assay is based on proximity ligation, which can be used to study protein interactions in situ with very high sensitivity. In traditional proximity ligation assay (PLA), primary antibody recognition is typically accompanied by oligonucleotide-conjugated secondary antibodies as detection probes. Here, we first performed PLA experiments using antibodies against the γ-secretase components presenilin 1 (PS1), containing the catalytic site residues, and nicastrin, suggested to be involved in substrate recognition. To selectively study the interactions of active γ-secretase, we replaced one of the primary antibodies with a photoreactive γ-secretase inhibitor containing a PEG linker and a biotin group (GTB), and used oligonucleotide-conjugated streptavidin as a probe. Interestingly, significantly fewer interactions were detected with the latter, novel, assay, which is a reasonable finding considering that a substantial portion of PS1 is inactive. In addition, the PLA signals were located more peripherally when GTB was used instead of a PS1 antibody, suggesting that γ-secretase matures distal from the perinuclear ER region. This novel technique thus enables highly sensitive protein interaction studies, determines the subcellular location of the interactions, and differentiates between active and inactive γ-secretase in intact cells. We suggest that similar PLA assays using enzyme inhibitors could be useful also for other enzyme interaction studies.

Notch signaling maintains neural rosette polarity

Formation of the metazoan body plan requires a complex interplay of morphological changes and patterning, and central to these processes is the establishment of apical/basal cell polarity. In the developing nervous system, apical/basal cell polarity is essential for neural tube closure and maintenance of the neural stem cell population. In this report we explore how a signaling pathway important for nervous system development, Notch signaling, impacts on apical/basal cell polarity in neural differentiation. CSL(-/-) mouse embryos, which are devoid of canonical Notch signaling, demonstrated a neural tube phenotype consistent with cell polarity and convergent extension defects, including deficiencies in the restricted expression of apical polarity markers in the neuroepithelium. CSL(-/-) mouse embryonic stem (ES) cells, cultured at low density, behaved as wild-type in the establishment of neural progenitors and apical specification, though progression through rosette formation, an in vitro correlate of neurulation, required CSL for correct maintenance of rosette structure and regulation of neuronal differentiation. Similarly, acute pharmacological inhibition of Notch signaling led to the breakdown of neural rosettes and accelerated neuronal differentiation. In addition to functional Notch signaling, rosette integrity was found to require actin polymerization and Rho kinase (ROCK) activity. Disruption of rosettes through inhibition of actin polymerization or ROCK activity, however, had no effect on neuronal differentiation, indicating that rosette maintenance is not a prerequisite for normal neuronal differentiation. In conclusion, our data indicate that Notch signaling plays a role not only in differentiation, but also in organization and maintenance of polarity during development of the early nervous system.

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.

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

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

The APP intracellular domain (AICD) inhibits Wnt signalling and promotes neurite outgrowth

β- and γ-secretase cleave the amyloid precursor protein (APP) to release the amyloidogenic β-amyloid peptides (Aβ) and the APP intracellular domain (AICD). Aβ has been widely believed to initiate pathogenic cascades culminating in Alzheimer's disease (AD). However, the physiological functions of the AICD remain elusive. In this study, we found the AICD to strongly inhibit Wnt-induced transcriptional reporter activity, and to counteract Wnt-induced c-Myc expression. Loss of the AICD resulted in an increased responsiveness to Wnt/β-catenin-mediated transcription. Mechanically, the AICD was found to interact with glycogen synthase kinase 3 beta (GSK3β) and promote its kinase activity. The subsequent AICD-strengthened Axin-GSK3β complex potentiates β-catenin poly-ubiquitination. Functional studies in N(2)a mouse neuroblastoma cells, rat pheochromocytoma PC12 cells and primary neurons showed that the AICD facilitated neurite outgrowth. And AICD antagonised Wnt3a-suppressed growth arrest and neurite outgrowth in N2a and PC12 cells. Taken together, our results identify the AICD as a novel inhibitory factor of the canonical Wnt signalling pathway and suggest its regulatory role in neuronal cell proliferation and differentiation.

An overview of notch signaling in adult tissue renewal and maintenance

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

Mastermind-like 1 (MamL1) and mastermind-like 3 (MamL3) are essential for Notch signaling in vivo

Mastermind (Mam) is one of the elements of Notch signaling, a system that plays a pivotal role in metazoan development. Mam proteins form transcriptionally activating complexes with the intracellular domains of Notch, which are generated in response to the ligand-receptor interaction, and CSL DNA-binding proteins. In mammals, three structurally divergent Mam isoforms (MamL1, MamL2 and MamL3) have been identified. There have also been indications that Mam interacts functionally with various other transcription factors, including the p53 tumor suppressor, β-catenin and NF-κB. We have demonstrated previously that disruption of MamL1 causes partial deficiency of Notch signaling in vivo. However, MamL1-deficient mice did not recapitulate total loss of Notch signaling, suggesting that other members could compensate for the loss or that Notch signaling could proceed in the absence of Mam in certain contexts. Here, we report the generation of lines of mice null for MamL3. Although MamL3-null mice showed no apparent abnormalities, mice null for both MamL1 and MamL3 died during the early organogenic period with classic pan-Notch defects. Furthermore, expression of the lunatic fringe gene, which is strictly controlled by Notch signaling in the posterior presomitic mesoderm, was undetectable in this tissue of the double-null embryos. Neither of the single-null embryos exhibited any of these phenotypes. These various roles of the three Mam proteins could be due to their differential physical characteristics and/or their spatiotemporal distributions. These results indicate that engagement of Mam is essential for Notch signaling, and that the three Mam isoforms have distinct roles in vivo.

Arc/Arg3.1 regulates an endosomal pathway essential for activity-dependent β-amyloid generation

Assemblies of β-amyloid (Aβ) peptides are pathological mediators of Alzheimer's Disease (AD) and are produced by the sequential cleavages of amyloid precursor protein (APP) by β-secretase (BACE1) and γ-secretase. The generation of Aβ is coupled to neuronal activity, but the molecular basis is unknown. Here, we report that the immediate early gene Arc is required for activity-dependent generation of Aβ. Arc is a postsynaptic protein that recruits endophilin2/3 and dynamin to early/recycling endosomes that traffic AMPA receptors to reduce synaptic strength in both hebbian and non-hebbian forms of plasticity. The Arc-endosome also traffics APP and BACE1, and Arc physically associates with presenilin1 (PS1) to regulate γ-secretase trafficking and confer activity dependence. Genetic deletion of Arc reduces Aβ load in a transgenic mouse model of AD. In concert with the finding that patients with AD can express anomalously high levels of Arc, we hypothesize that Arc participates in the pathogenesis of AD.

Tumor suppressor cell adhesion molecule 1 (CADM1) is cleaved by a disintegrin and metalloprotease 10 (ADAM10) and subsequently cleaved by γ-secretase complex

Cell adhesion molecule 1 (CADM1) is a type I transmembrane glycoprotein expressed in various tissues. CADM1 is a cell adhesion molecule with many functions, including roles in tumor suppression, apoptosis, mast cell survival, synapse formation, and spermatogenesis. CADM1 undergoes membrane-proximal cleavage called shedding, but the sheddase and mechanisms of CADM1 proteolysis have not been reported. We determined the cleavage site involved in CADM1 shedding by LC/MS/MS and showed that CADM1 shedding occurred in the membrane fraction and was inhibited by tumor necrosis factor-α protease inhibitor-1 (TAPI-1). An siRNA experiment revealed that ADAM10 mediates endogenous CADM1 shedding. In addition, the membrane-bound fragment generated by shedding was further cleaved by γ-secretase and generated CADM1-intracellular domain (ICD) in a mechanism called regulated intramembrane proteolysis (RIP). These results clarify the detailed mechanism of membrane-proximal cleavage of CADM1, suggesting the possibility of RIP-mediated CADM1 signaling.

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

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