Conditional Inactivation of Pen-2 in the Developing Neocortex Leads to Rapid Switch of Apical Progenitors to Basal Progenitors

The transition of apical progenitors (APs) to basal progenitors (BPs) is an important neurogenic process during cortical expansion. Presenilin enhancer 2 (Pen-2, also named as Psenen) is a key subunit of γ-secretase and has been implicated in neurodevelopmental disease. However, it remains unknown how Pen-2 may regulate the maintenance of APs. To address this question, we generated a conditional KO (cKO) mouse in which Pen-2 is specifically inactivated in neural progenitor cells in the telencephalon. Both male and female embryos were used. We show that Pen-2 cKO cortices display remarkable depletion of Aps, but transient increase on BPs, compared with controls. We demonstrate that the proliferation rate of APs or BPs is not changed, but the switch of APs to BPs is dramatically accelerated in Pen-2 cKO cortices. Molecular analyses reveal decreased levels of Hes1 and Hes5 but increased levels of Ngn2 and NeuroD1 in Pen-2 KO cells. We report that expression of Notch1 intracellular domain in Pen-2 cKO cortices restores the population of APs and BPs. In summary, these findings highlight a central role of the Notch signaling in Pen-2-dependent maintenance of neural stem cells in the developing neocortex.SIGNIFICANCE STATEMENT Presenilin enhancer 2 (Pen-2) has been implicated in neurodevelopmental disease. However, mechanisms by which Pen-2 regulates cortical development are not understood. In this study, we generated neural progenitor cell-specific Pen-2 conditional KO mice. We observe depletion of apical progenitors and transiently increased the number of basal progenitors in the developing neocortex of Pen-2 mutant mice. Mechanistic analyses reveal decreased levels of Hes1 and Hes5, but increased levels of neurogenic transcription factors in Pen-2 mutant cortices, compared with controls. We demonstrate that reintroduction of Notch intracellular domain into mutant mice restores the population of apical progenitors to basal progenitors. The above findings strongly suggest that the Pen-2-Notch pathway plays an essential role in the maintenance of neural stem cells during cortical development.

Increased Foxo3a Nuclear Translocation and Activity is an Early Neuronal Response to βγ-Secretase-Mediated Processing of the Amyloid-β Protein Precursor: Utility of an AβPP-GAL4 Reporter Assay

Sequential cleavage of the amyloid-β protein precursor (AβPP) by BACE1 (β-secretase) followed by theγ-secretase complex, is strongly implicated in Alzheimer's disease (AD) but the initial cellular responses to these cleavage events are not fully defined. β-secretase-mediated AβPP processing yields an extracellular domain (sAβPPβ) and a C-terminal fragment of AβPP of 99 amino acids (C99). Subsequent cleavage by γ-secretase produces amyloid-β (Aβ) and an AβPP intracellular domain (AICD). A cellular screen based on the generation of AICD from an AβPP-Gal4 fusion protein was adapted by introducing familial AD (FAD) mutations into the AβPP sequence and linking the assay to Gal4-UAS driven luciferase and GFP expression, to identify responses immediately downstream of AβPP processing in neurons with a focus on the transcription factor Foxo3a which has been implicated in neurodegeneration. The K670N/M671L, E682K, E693G, and V717I FAD mutations and the A673T protective mutation, were introduced into the AβPP sequence by site directed mutagenesis. When expressed in mouse cortical neurons, AβPP-Gal4-UAS driven luciferase and GFP expression was substantially reduced by γ-secretase inhibitors, lowered by β-secretase inhibitors, and enhanced by α-secretase inhibitors suggesting that AICD is a product of the βγ-secretase pathway. AβPP-Gal4-UAS driven GFP expression was exploited to identify individual neurons undergoing amyloidogenic AβPP processing, revealing increased nuclear localization of Foxo3a and enhanced Foxo3a-mediated transcription downstream of AICD production. Foxo3a translocation was not driven by AICD directly but correlated with reduced Akt phosphorylation. Collectively this suggests that βγ-secretase-mediated AβPP processing couples to Foxo3a which could be an early neuronal signaling response in AD.

γ-Secretase Studied by Atomistic Molecular Dynamics Simulations: Global Dynamics, Enzyme Activation, Water Distribution and Lipid Binding

γ-secretase, an intramembrane-cleaving aspartyl protease is involved in the cleavage of a large number of intramembrane proteins. The most prominent substrate is the amyloid precursor protein, whose proteolytic processing leads to the production of different amyloid Aβ peptides. These peptides are known to form toxic aggregates and may play a key role in Alzheimer's disease (AD). Recently, the three-dimensional structure of γ-secretase has been determined via Cryo-EM, elucidating the spatial geometry of this enzyme complex in different functional states. We have used molecular dynamics (MD) simulations to study the global dynamics and conformational transitions of γ-secretase, as well as the water and lipid distributions in and around the transmembrane domains in atomic detail. Simulations were performed on the full enzyme complex and on the membrane embedded parts alone. The simulations revealed global motions compatible with the experimental enzyme structures and indicated little dependence of the dynamics of the transmembrane domains on the soluble extracellular subunits. During the simulation on the membrane spanning part a transition between an inactive conformation (with catalytic residues far apart) toward a putatively active form (with catalytic residues in close proximity) has been observed. This conformational change is associated with a distinct rearrangement of transmembrane helices, a global compaction of the catalytically active presenilin subunit a change in the water structure near the active site and a rigidification of the protein fold. The observed conformational rearrangement allows the interpretation of the effect of several mutations on the activity of γ-secretase. A number of long-lived lipid binding sites could be identified on the membrane spanning surface of γ-secretase which may coincide with association regions of hydrophobic membrane helices to form putative substrate binding exosites.

Bax inhibitor 1 is a γ-secretase-independent presenilin-binding protein

β-amyloid is regarded by some scientists to be the cause of Alzheimer’s disease (AD). One of the strongest arguments against this hypothesis is the presence of hundreds of AD-causing mutations in presenilin, but none in the other three components of γ-secretase. This observation implies a γ-secretase–independent function of presenilin. To understand such a putative function, discovery of presenilin-binding proteins represents an important first step. In this study, we report the identification of Bax-inhibitor 1 (BI1) as a stable interacting partner of presenilin 1 (PS1), but not the intact γ-secretase. Our results link PS1 to BI1, a protein thought to play a role in apoptosis and calcium channel regulation. This finding opens a range of possibilities for the investigation of PS1 function and AD genesis.

Presenilin is the catalytic subunit of γ-secretase, a four-component intramembrane protease responsible for the generation of β-amyloid (Aβ) peptides. Over 200 Alzheimer’s disease-related mutations have been identified in presenilin 1 (PS1) and PS2. Here, we report that Bax-inhibitor 1 (BI1), an evolutionarily conserved transmembrane protein, stably associates with PS1. BI1 specifically interacts with PS1 in isolation, but not with PS1 in the context of an assembled γ-secretase. The PS1–BI1 complex exhibits no apparent proteolytic activity, as judged by the inability to produce Aβ40 and Aβ42 from the substrate APP-C99. At an equimolar concentration, BI1 has no impact on the proteolytic activity of γ-secretase; at a 200-fold molar excess, BI1 reduces γ-secretase activity nearly by half. Our biochemical study identified BI1 as a PS1-interacting protein, suggesting additional functions of PS1 beyond its involvement in γ-secretase.

Making the final cut: pathogenic amyloid-β peptide generation by γ-secretase

Alzheimer´s disease (AD) is a devastating neurodegenerative disease of the elderly population. Genetic evidence strongly suggests that aberrant generation and/or clearance of the neurotoxic amyloid-β peptide (Aβ) is triggering the disease. Aβ is generated from the amyloid precursor protein (APP) by the sequential cleavages of β- and γ-secretase. The latter cleavage by γ-secretase, a unique and fascinating four-component protease complex, occurs in the APP transmembrane domain thereby releasing Aβ species of 37-43 amino acids in length including the longer, highly pathogenic peptides Aβ42 and Aβ43. The lack of a precise understanding of Aβ generation as well as of the functions of other γ-secretase substrates has been one factor underlying the disappointing failure of γ-secretase inhibitors in clinical trials, but on the other side also been a major driving force for structural and in depth mechanistic studies on this key AD drug target in the past few years. Here we review recent breakthroughs in our understanding of how the γ-secretase complex recognizes substrates, of how it binds and processes β-secretase cleaved APP into different Aβ species, as well as the progress made on a question of outstanding interest, namely how clinical AD mutations in the catalytic subunit presenilin and the γ-secretase cleavage region of APP lead to relative increases of Aβ42/43. Finally, we discuss how the knowledge emerging from these studies could be used to therapeutically target this enzyme in a safe way.

Reduced Aβ secretion by human neurons under conditions of strongly increased BACE activity

The initial step in the amyloidogenic cascade of amyloid precursor protein (APP) processing is catalyzed by beta-site APP-cleaving enzyme (BACE), and this protease has increased activities in affected areas of Alzheimer's disease brains. We hypothesized that altered APP processing, because of augmented BACE activity, would affect the actions of direct and indirect BACE inhibitors. We therefore compared post-mitotic human neurons (LUHMES) with their BACE-overexpressing counterparts (BLUHMES). Although β-cleavage of APP was strongly increased in BLUHMES, they produced less full-length and truncated amyloid beta (Aβ) than LUHMES. Moreover, low concentrations of BACE inhibitors decreased cellular BACE activity as expected, but increased Aβ_1-40 levels. Several other approaches to modulate BACE activity led to a similar, apparently paradoxical, behavior. For instance, reduction in intracellular acidification by bepridil increased Aβ production in parallel with decreased BACE activity. In contrast to BLUHMES, the respective control cells (LUHMES or BLUHMES with catalytically inactive BACE) showed conventional pharmacological responses. Other non-canonical neurochemical responses (so-called 'rebound effects') are well-documented for the Aβ pathway, especially for γ-secretase: a partial block of its activity leads to an increased Aβ secretion by some cell types. We therefore compared LUHMES and BLUHMES regarding rebound effects of γ-secretase inhibitors and found an Aβ rise in LUHMES but not in BLUHMES. Thus, different cellular factors are responsible for the γ-secretase- versus BACE-related Aβ rebound. We conclude that increased BACE activity, possibly accompanied by an altered cellular localization pattern, can dramatically influence Aβ generation in human neurons and affect pharmacological responses to secretase inhibitors. OPEN PRACTICES: Open Science: This manuscript was awarded with the Open Materials Badge. For more information see: https://cos.io/our-services/open-science-badges/.

Inhibition of Notch1 induces population and suppressive activity of regulatory T cell in inflammatory arthritis

Inhibition of Notch signalling has shown anti-inflammatory properties in vivo and in vitro models of rheumatoid arthritis (RA). The objective of this study was to determine whether Notch1 might play a role in regulating T-regulatory cells (Tregs) in animal models of RA.

Methods: Collagen-induced arthritis (CIA) and collagen antibody-induced arthritis (CAIA) were induced in C57BL/6, Notch1 antisense transgenic (NAS) or DBA1/J mice. We examined whether pharmacological inhibitors of γ-secretase (an enzyme required for Notch1 activation) and antisense-mediated knockdown of Notch1 could attenuate the severity of inflammatory arthritis in CIA and CAIA mice. Proportions of CD4⁺CD25⁺Foxp3⁺ Treg cells were measured by flow cytometry. To assess the suppressive capacity of Treg toward responder cells, CFSE-based suppression assay of Treg was performed.

Results: γ-secretase inhibitors and antisense-mediated knockdown of Notch1 reduced the severity of inflammatory arthritis in both CIA and CAIA mice. Pharmacological and genetic inhibition of Notch1 signalling induced significant elevation of Treg cell population in CIA and CAIA mice. We also demonstrated that inhibition of Notch signalling suppressed the progression of inflammatory arthritis through modulating the expansion and suppressive function of regulatory T (Treg) cells.

Conclusion: Pharmacological and genetic inhibition of Notch1 signalling suppresses the progression of inflammatory arthritis through modulating the population and suppressive function of Treg cells in animal models of RA.

Retrograde transport of γ-secretase from endosomes to the trans-Golgi network regulates Aβ42 production

The aberrant metabolism of amyloid-β protein (Aβ) in the human brain has been implicated in the etiology of Alzheimer disease (AD). γ-Secretase is the enzyme that generates various forms of Aβ, such as Aβ40 and Aβ42, the latter being an aggregation-prone toxic peptide that is involved in the pathogenesis of AD. Recently, we found that clathrin-mediated endocytosis of γ-secretase affects the production and deposition of Aβ42 in vivo, suggesting that the membrane trafficking of γ-secretase affects its enzymatic activity. However, the detailed intracellular trafficking pathway of γ-secretase and its contribution to Aβ42 generation remain unclear. Here, we show that Retro-2, which inhibits the retrograde transport, elevated the Aβ42-generating activity both in cultured cells and mice brain. However, the result of in vitro γ-secretase assay using a recombinant substrate suggested that Retro-2 did not elevate the intrinsic Aβ42-production activity of γ-secretase. Immunocytochemistry and cell-surface biotinylation experiments revealed that γ-secretase is recycled via the endosome-to-trans-Golgi network transport. In addition, γ-secretase is retrogradely transported by syntaxin 5/6, known as targets of Retro-2, independent pathway. Conversely, TPT-260, which enhances the trafficking function of retromers, lowered Aβ42 levels and the Aβ42/(Aβ40 + Aβ42) ratio in secreted Aβ from cultured cells. Our results strongly suggest that the endosome-to-trans-Golgi network trafficking of γ-secretase regulates its Aβ42 production activity. Modulation of this trafficking pathway might be a potential target for the development of Aβ42-lowering AD therapeutics.

OPEN PRACTICES

Open Science: This manuscript was awarded with the Open Materials Badge. For more information see: https://cos.io/our-services/open-science-badges/.

The Intracellular Cleavage Product of the NG2 Proteoglycan Modulates Translation and Cell-Cycle Kinetics via Effects on mTORC1/FMRP Signaling

The NG2 proteoglycan is expressed by oligodendrocyte precursor cells (OPCs) and is abundantly expressed by tumors such as melanoma and glioblastoma. Functions of NG2 include an influence on proliferation, migration and neuromodulation. Similar to other type-1 membrane proteins, NG2 undergoes proteolysis, generating a large ectodomain, a C-terminal fragment (CTF) and an intracellular domain (ICD) via sequential action of α- and γ-secretases which is enhanced by neuronal activity. Functional roles of NG2 have so far been shown for the full-length protein, the released ectodomain and CTF, but not for the ICD. In this study, we characterized the role of the NG2 ICD in OPC and Human Embryonic Kidney (HEK) cells. Overexpressed ICD is predominantly localized in the cell cytosol, including the distal processes of OPCs. Nuclear localisation of a fraction of the ICD is dependent on Nuclear Localisation Signals. Immunoprecipitation and Mass Spectrometry followed by functional analysis indicated that the NG2 ICD modulates mRNA translation and cell-cycle kinetics. In OPCs and HEK cells, ICD overexpression results in an mTORC1-dependent upregulation of translation, as well as a shift of the cell population toward S-phase. NG2 ICD increases the active (phosphorylated) form of mTOR and modulates downstream signaling cascades, including increased phosphorylation of p70S6K1 and increased expression of eEF2. Strikingly, levels of FMRP, an RNA-binding protein that is regulated by mTOR/p70S6K1/eEF2 were decreased. In neurons, FMRP acts as a translational repressor under activity-dependent control and is mutated in Fragile X Syndrome (FXS). Knock-down of endogenous NG2 in primary OPC reduced translation and mTOR/p70S6K1 phosphorylation in Oli-neu. Here, we identify the NG2 ICD as a regulator of translation in OPCs via modulation of the well-established mTORC1 pathway. We show that FXS-related FMRP signaling is not exclusive to neurons but plays a role in OPCs. This provides a signal cascade in OPC which can be influenced by the neuronal network, since the NG2 ICD has been shown to be generated by constitutive as well as activity-dependent cleavage. Our results also elucidate a possible role of NG2 in tumors exhibiting enhanced rates of translation and rapid cell cycle kinetics.

Selenium and Zinc against Aβ25-35-Induced Cytotoxicity and Tau Phosphorylation in PC12 Cells and Inhibits γ-cleavage of APP

Amyloid beta (Aβ) is the main component of the amyloid plaques that accumulate in the brains of Alzheimer patients. The present study was conducted to investigate whether the combined treatment with selenium (Se) and zinc (Zn) offers more beneficial effects than that provided by either of them alone in reversing Aβ_25-35-induced neurotoxicity in PC12 cells. Cells were pretreated with 0.1 μmol/L of Se and Zn for 4 h, after treated with 10 mmol/L Aβ_25-35 for 24 h. Cells were divided into control and five treated groups, and received either 10 mmol/L Aβ_25-35,10 mmol/L Aβ_25-35 + 0.1 μmol/L Se, 10 mmol/L Aβ_25-35 + 0.1 μmol/L Zn, 10 mmol/LAβ_25-35 + 0.1 μmol/L Se + 0.1 μmol/L Zn, or 0.1 μmol/L Se + 0.1 μmol/L Zn. The result showed that cell viability was decreased in MTT metabolic rate; LDH release and MDA, H₂O₂, and NO levels were increased and the GSK-3β and phosphorylated tau protein level were increased in Aβ_25-35-treated group (P < 0.05 or P < 0.01), which whole changes were attenuated by Se and Zn and Se combined Zn. In order to evaluate whether the Se and Zn have an effect on processing pathway of amyloid precursor protein (APP), we examined the activity of γ-secretase in primary cultured cortical neuron cells. ELISA analysis showed that Se and Zn could inhibit the activity of γ-secretase. Then we also investigated the effect of Se and Zn on the Aβ_1-40 concentration and APP-N-terminal fragment expression from APP695 stably transfected Chinese hamster ovary (CHO) cells. APP695 stably transfected CHO cells were treated with 0.1 μmol/L Se and Zn; cells were divided into control and four treated groups, which received either 0.5 M DAPT, 0.1 μmol/L Se, 0.1 μmol/L Zn, or 0.1 μmol/L Se + 0.1 μmol/L Zn. Se and Zn could decrease Aβ_1-40 production and increase the APP-N-terminal fragment protein expression. These experiments indicate that Se and Zn have a protective effect on AD pathology that a possible mechanism is inhibiting the activity of γ-secretase to decreasing Aβ_1-40 production further influencing the APP processing. Altogether, our findings may provide a novel therapeutic target to treat AD sufferers.

Complexity and Selectivity of γ-Secretase Cleavage on Multiple Substrates: Consequences in Alzheimer's Disease and Cancer

The processing of the amyloid-β protein precursor (AβPP) by β- and γ-secretases is a pivotal event in the genesis of Alzheimer's disease (AD). Besides familial mutations on the AβPP gene, or upon its overexpression, familial forms of AD are often caused by mutations or deletions in presenilin 1 (PSEN1) and 2 (PSEN2) genes: the catalytic components of the proteolytic enzyme γ-secretase (GS). The "amyloid hypothesis", modified over time, states that the aberrant processing of AβPP by GS induces the formation of specific neurotoxic soluble amyloid-β (Aβ) peptides which, in turn, cause neurodegeneration. This theory, however, has recently evidenced significant limitations and, in particular, the following issues are debated: 1) the concept and significance of presenilin's "gain of function" versus "loss of function"; and 2) the presence of several and various GS substrates, which interact with AβPP and may influence Aβ formation. The latter consideration is suggestive: despite the increasing number of GS substrates so far identified, their reciprocal interaction with AβPP itself, even in the AD field, is significantly unexplored. On the other hand, GS is also an important pharmacological target in the cancer field; inhibitors or GS activity are investigated in clinical trials for treating different tumors. Furthermore, the function of AβPP and PSENs in brain development and in neuronal migration is well known. In this review, we focused on a specific subset of GS substrates that directly interact with AβPP and are involved in its proteolysis and signaling, by evaluating their role in neurodegeneration and in cell motility or proliferation, as a possible connection between AD and cancer.

A role for astrocyte-derived amyloid β peptides in the degeneration of neurons in an animal model of temporal lobe epilepsy

Kainic acid, an analogue of the excitatory neurotransmitter glutamate, can trigger seizures and neurotoxicity in the hippocampus and other limbic structures in a manner that mirrors the neuropathology of human temporal lobe epilepsy (TLE). However, the underlying mechanisms associated with the neurotoxicity remain unclear. Since amyloid-β (Aβ) peptides, which are critical in the development of Alzheimer's disease, can mediate toxicity by activating glutamatergic NMDA receptors, it is likely that the enhanced glutamatergic transmission that renders hippocampal neurons vulnerable to kainic acid treatment may involve Aβ peptides. Thus, we seek to establish what role Aβ plays in kainic acid-induced toxicity using in vivo and in vitro paradigms. Our results show that systemic injection of kainic acid to adult rats triggers seizures, gliosis and loss of hippocampal neurons, along with increased levels/processing of amyloid precursor protein (APP), resulting in the enhanced production of Aβ-related peptides. The changes in APP levels/processing were evident primarily in activated astrocytes, implying a role for astrocytic Aβ in kainic acid-induced toxicity. Accordingly, we showed that treating rat primary cultured astrocytes with kainic acid can lead to increased Aβ production/secretion without any compromise in cell viability. Additionally, we revealed that kainic acid reduces neuronal viability more in neuronal/astrocyte co-cultures than in pure neuronal culture, and this is attenuated by precluding Aβ production. Collectively, these results indicate that increased production/secretion of Aβ-related peptides from activated astrocytes can contribute to neurotoxicity in kainic acid-treated rats. Since kainic acid administration can lead to neuropathological changes resembling TLE, it is likely that APP/Aβ peptides derived from astrocytes may have a role in TLE pathogenesis.

Presenilins as Drug Targets for Alzheimer's Disease-Recent Insights from Cell Biology and Electrophysiology as Novel Opportunities in Drug Development

A major cause underlying familial Alzheimer's disease (AD) are mutations in presenilin proteins, presenilin 1 (PS1) and presenilin 2 (PS2). Presenilins are components of the γ-secretase complex which, when mutated, can affect amyloid precursor protein (APP) processing to toxic forms of amyloid beta (Aβ). Consequently, presenilins have been the target of numerous and varied research efforts to develop therapeutic strategies for AD. The presenilin 1 gene harbors the largest number of AD-causing mutations resulting in the late onset familial form of AD. As a result, the majority of efforts for drug development focused on PS1 and Aβ. Soon after the discovery of the major involvement of PS1 and PS2 in γ-secretase activity, it became clear that neuronal signaling, particularly calcium ion (Ca²⁺) signaling, is regulated by presenilins and impacted by mutations in presenilin genes. Intracellular Ca²⁺ signaling not only controls the activity of neurons, but also gene expression patterns, structural functionality of the cytoskeleton, synaptic connectivity and viability. Here, we will briefly review the role of presenilins in γ-secretase activity, then focus on the regulation of Ca²⁺ signaling, oxidative stress, and cellular viability by presenilins within the context of AD and discuss the relevance of presenilins in AD drug development efforts.

Endosomal-Lysosomal Cholesterol Sequestration by U18666A Differentially Regulates Amyloid Precursor Protein (APP) Metabolism in Normal and APP-Overexpressing Cells

Amyloid β (Aβ) peptide, derived from amyloid precursor protein (APP), plays a critical role in the development of Alzheimer's disease. Current evidence indicates that altered levels or subcellular distribution of cholesterol can regulate Aβ production and clearance, but it remains unclear how cholesterol sequestration within the endosomal-lysosomal (EL) system can influence APP metabolism. Thus, we evaluated the effects of U18666A, which triggers cholesterol redistribution within the EL system, on mouse N2a cells expressing different levels of APP in the presence or absence of extracellular cholesterol and lipids provided by fetal bovine serum (FBS). Our results reveal that U18666A and FBS differentially increase the levels of APP and its cleaved products, the α-, β-, and η-C-terminal fragments, in N2a cells expressing normal levels of mouse APP (N2awt), higher levels of human wild-type APP (APPwt), or "Swedish" mutant APP (APPsw). The cellular levels of Aβ_1-40/Aβ_1-42 were markedly increased in U18666A-treated APPwt and APPsw cells. Our studies further demonstrate that APP and its cleaved products are partly accumulated in the lysosomes, possibly due to decreased clearance. Finally, we show that autophagy inhibition plays a role in mediating U18666A effects. Collectively, these results suggest that altered levels and distribution of cholesterol and lipids can differentially regulate APP metabolism depending on the nature of APP expression.

A phase I trial of the γ-secretase inhibitor MK-0752 in combination with gemcitabine in patients with pancreatic ductal adenocarcinoma

BACKGROUND

The Notch pathway is frequently activated in cancer. Pathway inhibition by γ-secretase inhibitors has been shown to be effective in pre-clinical models of pancreatic cancer, in combination with gemcitabine.

METHODS

A multi-centre, non-randomised Bayesian adaptive design study of MK-0752, administered per os weekly, in combination with gemcitabine administered intravenously on days 1, 8 and 15 (28 day cycle) at 800 or 1000 mg m-2, was performed to determine the safety of combination treatment and the recommended phase 2 dose (RP2D). Secondary and tertiary objectives included tumour response, plasma and tumour MK-0752 concentration, and inhibition of the Notch pathway in hair follicles and tumour.

RESULTS

Overall, 44 eligible patients (performance status 0 or 1 with adequate organ function) received gemcitabine and MK-0752 as first or second line treatment for pancreatic cancer. RP2Ds of MK-0752 and gemcitabine as single agents could be combined safely. The Bayesian algorithm allowed further dose escalation, but pharmacokinetic analysis showed no increase in MK-0752 AUC (area under the curve) beyond 1800 mg once weekly. Tumour response evaluation was available in 19 patients; 13 achieved stable disease and 1 patient achieved a confirmed partial response.

CONCLUSIONS

Gemcitabine and a γ-secretase inhibitor (MK-0752) can be combined at their full, single-agent RP2Ds.

Inhibition of gamma-secretase in Notch1 signaling pathway as a novel treatment for ovarian cancer

Epithelial ovarian cancer (EOC) is the leading cause of death for gynecological cancer. Most patients are not diagnosed until the cancer is at an advanced stage with poor prognosis. Notch1 signaling pathway plays an oncogenic role in EOC. There have been few studies on enzymatic activity of γ-secretase and the mechanism of how γ-secretase inhibitor works on cancer cell. Here, we show that Jagged1 and NICD were highly expressed in ovarian carcinoma. The expressions of Notch1, Jagged1 and NICD in Notch1 pathway did not correlate with outcome in ovarian cancer. The enzymatic activity of γ-secretase in ovarian cancer cell lines SKOV3, CAOV3 and ES2 is significantly higher than in normal ovarian epithelial cell line T29. DAPT (a γ-secretase inhibitor) reduced the enzymatic activity of γ-secretase, inhibited the proliferation, and increased the apoptosis in ovarian cancer cell lines. Hence, γ-secretase inhibitor may become a highly promising novel therapeutic strategy against ovarian cancer in the field of precision medicine.

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.

γ-Secretase Inhibition Induces Muscle Hypertrophy in a Notch-Independent Mechanism

A wide variety of cellular processes and signaling events are regulated by the proteolytic enzyme γ-secretase. Notch-1 is one of the substrates of γ-secretase and its role in the regulation of muscle differentiation has been well described. Importantly, besides Notch-1, a number of proteins have been identified to undergo proteolysis by γ-secretase. To date, the specific role of γ-secretase during embryonic skeletal muscle differentiation has not been studied. Therefore, we address this question through the analysis of in vitro grown chick myogenic cells during the formation of multinucleated myotubes. The γ-secretase inhibitor DAPT (N-N[-(3,5-Difluorophenacetyl-l-alanyl)]-S-328 phenylglycine-t-butyl-ester) induces muscle hypertrophy. Knockdown of Notch-1 using siRNA specific to chick shows no significant effect in myotube size, suggesting that γ-secretase-dependent effects on muscle hypertrophy in chick myogenic cells are Notch-1-independent. We also investigate the effects of γ-secretase inhibition in the whole proteomic profile of chick myogenic cells. We identified 276 differentially expressed proteins from Label-free proteomic approach. Data overview of interaction network obtained from STRING show that after γ-secretase inhibition cells exhibited imbalance in protein metabolism, cytoskeleton/adhesion, and Sonic Hedgehog signaling. The collection of these results provides new insights into the role of γ-secretase in skeletal muscle hypertrophy.

Effect of dioscin on promoting liver regeneration via activating Notch1/Jagged1 signal pathway

BACKGROUND

Development of novel candidates to promote liver regeneration is critical important after partial hepatectomy (PH). Dioscin, a natural product, shows potent effect on liver protection in our previous works.

PURPOSE

This work aimed to investigate the effect and underlying mechanisms of dioscin on liver regeneration.

METHODS

The promoting proliferation effects of dioscin on mouse hepatocytem AML12 cells, rat primary hepatocytes, rats and mice after 70% PH were evaluated.

RESULTS

Dioscin significantly promoted proliferation of rat primary hepatocytes and AML12 cells through MTT, BrdU and PCNA staining assays. Meanwhile, dioscin rapidly recovered the liver to body weight ratios, declined ALT and AST levels, and relieved hepatocytes necrosis compared with 70% PH operation groups in rats and mice. Mechanistic test showed that dioscin significantly increased Notch1 and Jagged1 levels, and accelerated γ-secretase activity by up-regulating PS1 expression, leading to nuclear translocation of Notch1 intracellular domain (NICD1). Subsequently, the significant activation of Notch-dependent target genes (Hey1, Hes1, EGFR, VEGF), and cell-cycle regulatory proteins (CyclinD1, CyclinE1, CDK4 and CDK2) were all recognized. In addition, these results were further confirmed by Notch1 siRNA silencing and inhibition of γ-secretase by DAPT (a well-characterized γ-secretase inhibitor) in vitro.

CONCLUSIONS

Dioscin, as a novel efficient γ-secretase activator, NICD1 nucleus translocation promoter and cell cycle regulator, markedly activated Notch1/Jagged1 pathway to promote hepato-proliferation. Our findings provide novel insights into dioscin as a natural product with facilitating liver regeneration after PH.

Dominant negative effect of the loss-of-function γ-secretase mutants on the wild-type enzyme through heterooligomerization

γ-secretase is an intramembrane protease complex consisting of nicastrin, presenilin-1/2, APH-1a/b, and Pen-2. Hydrolysis of the 99-residue transmembrane fragment of amyloid precursor protein (APP-C99) by γ-secretase produces β-amyloid (Aβ) peptides. Pathogenic mutations in PSEN1 and PSEN2, which encode the catalytic subunit presenilin-1/2 of γ-secretase, lead to familial Alzheimer's disease in an autosomal dominant manner. However, the underlying mechanism of how the mutant PSEN gene may affect the function of the WT allele remains to be elucidated. Here we report that each of the loss-of-function γ-secretase variants that carries a PSEN1 mutation suppresses the protease activity of the WT γ-secretase on Aβ production. Each of these γ-secretase variants forms a stable oligomer with the WT γ-secretase in vitro in the presence of the detergent CHAPSO {3-[(3-cholamidopropyl)dimethylammonio]-2-hydroxy-1-propanesulfonate}, but not digitonin. Importantly, robust protease activity of γ-secretase is detectable in the presence of CHAPSO, but not digitonin. These experimental observations suggest a dominant negative effect of the γ-secretase, in which the protease activity of WT γ-secretase is suppressed by the loss-of-function γ-secretase variants through hetero-oligomerization. The relevance of this finding to the genesis of Alzheimer's disease is critically evaluated.

Low-dose γ-secretase inhibition increases secretion of Aβ peptides and intracellular oligomeric Aβ

γ-Secretase inhibitors have been considered promising drug candidates against Alzheimer's disease (AD) due to their ability to reduce amyloid-β (Aβ) production. However, clinical trials have been halted due to lack of clinical efficacy and/or side effects. Recent in vitro studies suggest that low doses of γ-secretase inhibitors may instead increase Aβ production. Using a stem cell-derived human model of cortical neurons and low doses of the γ-secretase inhibitor DAPT, the effects on a variety of Aβ peptides were studied using mass spectrometry. One major focus was to develop a novel method for specific detection of oligomeric Aβ (oAβ), and this was used to study the effects of low-dose γ-secretase inhibitor treatment on intracellular oAβ accumulation. Low-dose treatment (2 and 20nM) with DAPT increased the secretion of several Aβ peptides, especially Aβx-42. Furthermore, using the novel method for oAβ detection, we found that 2nM DAPT treatment of cortical neurons resulted in increased oAβ accumulation. Thus, low dose-treatment with DAPT causes both increased production of long, aggregation-prone Aβ peptides and accumulation of intracellular Aβ oligomers, both believed to contribute to AD pathology.

Activation of γ-Secretase Trimming Activity by Topological Changes of Transmembrane Domain 1 of Presenilin 1

γ-Secretase is an intramembrane cleaving protease that is responsible for the generation of amyloid-β peptides, which are linked to the pathogenesis of Alzheimer disease. Recently, γ-secretase modulators (GSMs) have been shown to specifically decrease production of the aggregation-prone and toxic longer Aβ species, and concomitantly increase the levels of shorter Aβ. We previously found that phenylimidazole-type GSMs bind to presenilin 1 (PS1), the catalytic subunit of the γ-secretase, and allosterically modulate γ-secretase activity. However, the precise conformational alterations in PS1 remained unclear. Here we mapped the amino acid residues in PS1 that is crucial for the binding and pharmacological actions of E2012, a phenylimidazole-type GSM, using photoaffinity labeling and the substituted cysteine accessibility method. We also demonstrated that a piston-like vertical motion of transmembrane domain (TMD) 1 occurs during modulation of Aβ production. Taking these results together, we propose a model for the molecular mechanism of phenylimidazole-type GSMs, in which the trimming activity of γ-secretase is modulated by the position of the TMD1 of PS1 in the lipid bilayer.SIGNIFICANCE STATEMENT Reduction of the toxic longer amyloid-β peptide is one of the therapeutic approaches for Alzheimer disease. A subset of small compounds called γ-secretase modulators specifically decreases the longer amyloid-β production, although its mechanistic action remains unclear. Here we found that the modulator compound E2012 targets to the hydrophilic loop 1 of presenilin 1, which is a catalytic subunit of the γ-secretase. Moreover, E2012 triggers the piston movement of the transmembrane domain 1 of presenilin 1, which impacts on the γ-secretase activity. These results illuminate how γ-secretase modulators allosterically affect the proteolytic activity, and highlight the importance of the structural dynamics of presenilin 1 in the complexed process of the intramembrane cleavage.

Recruitment of the amyloid precursor protein by γ-secretase at the synaptic plasma membrane

Γ-secretase is a membrane-embedded protease that cleaves single transmembrane helical domains of various integral membrane proteins. The amyloid precursor protein (APP) is an important substrate due to its pathological relevance to Alzheimer's disease. The mechanism of the cleavage of APP by γ-secretase that leads to accumulation of Alzheimer's disease causing amyloid-β (Aβ) is still unknown. Coarse-grained molecular dynamics simulations in this study reveal initial lipids raft formation near the catalytic site of γ-secretase as well as changes in dynamic behavior of γ-secretase once interacting with APP. The results suggest a precursor of the APP binding mode and hint at conformational changes of γ-secretase in the nicastrin (NCT) domain upon APP binding.

Regulated intramembrane proteolysis: emergent role in cell signalling pathways

Receptor signalling events including those initiated following activation of cytokine and growth factor receptors and the well-characterised death receptors (tumour necrosis factor receptor, type 1, FasR and TRAIL-R1/2) are initiated at the cell surface through the recruitment and formation of intracellular multiprotein signalling complexes that activate divergent signalling pathways. Over the past decade, research studies reveal that many of these receptor-initiated signalling events involve the sequential proteolysis of specific receptors by membrane-bound proteases and the γ-secretase protease complexes. Proteolysis enables the liberation of soluble receptor ectodomains and the generation of intracellular receptor cytoplasmic domain fragments. The combined and sequential enzymatic activity has been defined as regulated intramembrane proteolysis and is now a fundamental signal transduction process involved in the termination or propagation of receptor signalling events. In this review, we discuss emerging evidence for a role of the γ-secretase protease complexes and regulated intramembrane proteolysis in cell- and immune-signalling pathways.

Distinct deposition of amyloid-β species in brains with Alzheimer's disease pathology visualized with MALDI imaging mass spectrometry

Amyloid β (Aβ) deposition in the brain is an early and invariable feature of Alzheimer's disease (AD). The Aβ peptides are composed of about 40 amino acids and are generated from amyloid precursor proteins (APP), by β- and γ-secretases. The distribution of individual Aβ peptides in the brains of aged people, and those suffering from AD and cerebral amyloid angiopathy (CAA), is not fully characterized. We employed the matrix-assisted laser desorption/ionization-imaging mass spectrometry (MALDI-IMS) to illustrate the spatial distribution of a broad range of Aβ species in human autopsied brains. With technical advancements such as formic acid pretreatment of frozen autopsied brain samples, we have: i) demonstrated that Aβ1-42 and Aβ1-43 were selectively deposited in senile plaques while full-length Aβ peptides such as Aβ1-36, 1-37, 1-38, 1-39, 1-40, and Aβ1-41 were deposited in leptomeningeal blood vessels. ii) Visualized distinct depositions of N-terminal truncated Aβ40 and Aβ42, including pyroglutamate modified at Glu-3 (N3pE), only with IMS for the first time. iii) Demonstrated that one single amino acid alteration at the C-terminus between Aβ1-42 and Aβ1-41 results in profound changes in their distribution pattern. In vitro, this can be attributed to the difference in the self-aggregation ability amongst Aβ1-40, Aβ1-41, and Aβ1-42. These observations were further confirmed with immunohistochemistry (IHC), using the newly developed anti-Aβ1-41 antibody. Here, distinct depositions of truncated and/or modified C- and N-terminal fragments of Aβs in AD and CAA brains with MALDI-IMS were visualized in a spacio-temporal specific manner. Specifically, Aβ1-41 was detected both with MALDI-IMS and IHC suggesting that a single amino acid alteration at the C-terminus of Aβ results in drastic distribution changes. These results suggest that MALDI-IMS could be used as a standard approach in combination with clinical, genetic, and pathological observations in understanding the pathology of AD and CAA.