γ-Secretase structure and activity are modified by alterations in its membrane localization and ambient environment

γ-Secretase cleaves type I transmembrane proteins in a hydrophobic membrane environment following ectodomain shedding. Mutations in PSEN genes, encoding the catalytic subunits of γ-secretase, presenilins, are the most common cause of familial Alzheimer's disease (AD). Pathogenic mutations in PSEN genes increase production of longer and neurotoxic amyloid-β (Aβ) by intramembrane cleavage of membrane-associated amyloid-β protein precursor (APP) carboxy-terminal fragment β (APP CTFβ), which is generated via primary cleavage of APP by β-site APP cleaving enzyme 1. The longer Aβ is prone to aggregate and accumulate in the brain, however, the accumulation of Aβ in brain is also a pathological feature of sporadic AD. Increased pathogenic Aβ generation, even in the absence of pathogenic PSEN gene mutations, is one of proposed mechanisms for sporadic AD pathogenesis. γ-Secretase digests substrates in the transmembrane region, generating Aβ peptide intermediates of various lengths. The end-products, shorter Aβ40 and Aβ38 peptides, are less neurotoxic, whereas PSEN gene mutations increase the production ratio of longer, neurotoxic Aβ species such as Aβ42, an intermediate in Aβ38 production. γ-Secretase activity or structures is altered because of its aberrant membrane localization or changes in the ambient environment such as luminal acidification. Interestingly, γ-secretase has a pH sensor in presenilins.

Anti-Stroke Chinese Herbal Medicines Inhibit Abnormal Amyloid-β Protein Precursor Processing in Alzheimer's Disease

BACKGROUND

Chinese Herbal Medicines (CHMs), as an important and integral part of a larger system of medicine practiced in China, called Traditional Chinese Medicine (TCM), have been used in stroke therapy for centuries. A large body of studies suggest that some Chinese herbs can help reverse cognitive impairment in stroke patients, while whether these herbs also exert therapeutic benefits for Alzheimer's disease remains to be seen.

OBJECTIVE

To address this issue, we selected four types of CHMs that are commonly prescribed for stroke treatment in clinical practice, namely DengZhanXiXin (D1), TongLuoJiuNao (T2), QingKaiLing (Q3), and HuangQinGan (H4), and tested their effects on amyloid-β protein precursor (AβPP) processing in vitro.

METHODS

AβPP, β-secretase (BACE1), and 99-amino acid C-terminal fragment of AβPP (C99) stably transfected cells were used for the tests of AβPP processing. The production of Aβ, activity of BACE1, neprilysin (NEP), and γ-secretase were assessed by ELISA, RT-PCR, and western blot.

RESULTS

By upregulating BACE1 activity, D1 increased Aβ production whereas decreased the ratio of Aβ 42/Aβ 40; by downregulating BACE1 activity and modulating the expression of γ-secretase, T2 decreased Aβ production and the ratio of Aβ 42/Aβ 40; by downregulating BACE1 activity, Q3 decreased Aβ production; H4 did not change Aβ production due to the simultaneously downregulation of BACE1 and NEP activity.

CONCLUSION

Our study indicates that these four anti-stroke CHMs regulate AβPP processing through different mechanisms. Particularly, T2 with relatively simple components and prominent effect on AβPP processing may be a promising candidate for the treatment of AD.

The Membrane-Bound Notch Regulator Mnr Supports Notch Cleavage and Signaling Activity in Drosophila melanogaster

The Notch signaling pathway is pivotal to cellular differentiation. Activation of this pathway involves proteolysis of the Notch receptor and the release of the biologically active Notch intracellular domain, acting as a transcriptional co-activator of Notch target genes. While the regulation of Notch signaling dynamics at the level of ligand-receptor interaction, endocytosis, and transcriptional regulation has been well studied, little is known about factors influencing Notch cleavage. We identified EP555 as a suppressor of the Notch antagonist Hairless (H). EP555 drives expression of CG32521 encoding membrane-bound proteins, which we accordingly rename membrane-bound Notch regulator (mnr). Within the signal-receiving cell, upregulation of Mnr stimulates Notch receptor activation, whereas a knockdown reduces it, without apparent influence on ligand-receptor interaction. We provide evidence that Mnr plays a role in γ-secretase-mediated intramembrane cleavage of the Notch receptor. As revealed by a fly-eye-based reporter system, γ-secretase activity is stimulated by the overexpression of Mnr, and is inhibited by its knockdown. We conclude that Mnr proteins support Notch signaling activity by fostering the cleavage of the Notch receptor. With Mnr, we identified a membrane-bound factor directly augmenting Notch intra-membrane processing, thereby acting as a positive regulator of Notch signaling activity.

Loss of presenilin function enhances tau phosphorylation and aggregation in mice

Mutations in the presenilin (PS/PSEN) genes encoding the catalytic components of γ-secretase accelerate amyloid-β (Aβ) and tau pathologies in familial Alzheimer’s disease (AD). Although the mechanisms by which these mutations affect Aβ are well defined, the precise role PS/γ-secretase on tau pathology in neurodegeneration independently of Aβ is largely unclear. Here we report that neuronal PS deficiency in conditional knockout (cKO) mice results in age-dependent brain atrophy, inflammatory responses and accumulation of pathological tau in neurons and glial cells. Interestingly, genetic inactivation of presenilin 1 (PS1) or both PS genes in mutant human Tau transgenic mice exacerbates memory deficits by accelerating phosphorylation and aggregation of tau in excitatory neurons of vulnerable AD brain regions (e.g., hippocampus, cortex and amygdala). Remarkably, neurofilament (NF) light chain (NF-L) and phosphorylated NF are abnormally accumulated in the brain of Tau mice lacking PS. Synchrotron infrared microspectroscopy revealed aggregated and oligomeric β-sheet structures in amyloid plaque-free PS-deficient Tau mice. Hippocampal-dependent memory deficits are associated with synaptic tau accumulation and reduction of pre- and post-synaptic proteins in Tau mice. Thus, partial loss of PS/γ-secretase in neurons results in temporal- and spatial-dependent tau aggregation associated with memory deficits and neurodegeneration. Our findings show that tau phosphorylation and aggregation are key pathological processes that may underlie neurodegeneration caused by familial AD-linked PSEN mutations.

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.

Insights from γ-Secretase: Functional Genetics of Hidradenitis Suppurativa

Hidradenitis suppurativa (HS) is a chronic, relapsing, and remitting inflammatory disease of the skin with significant heritability and racial disposition. The pathogenesis of HS remains enigmatic, but occlusion of the terminal hair follicle and dysregulation of the local innate immune response may contribute to pathogenesis. Genetic predisposition might also contribute to disease susceptibility and phenotypic heterogeneity because mutations in γ-secretase have been found to underlie a minor but characteristic subset of patients with HS. In this review, we synthesized the current data on γ-secretase in HS, evaluated its importance in the context of disease pathobiology, and discussed avenues of future studies.

Cimigenoside functions as a novel γ-secretase inhibitor and inhibits the proliferation or metastasis of human breast cancer cells by γ-secretase/Notch axis

Breast cancer (BC) occurrence and development tremendously affect female health. Currently breast cancer targeted drugs are still scarce. Natural products have become the main source of targeted drug for breast cancer due to low toxicity and high efficiency. Cimigenoside, natural compound isolated and purified from Cimicifuga dahurica (Turcz.) Maxim has been suggested to utilize for breast cancer treatment, however the mechanism of action has not been elucidated yet. In this article, the antitumor potential of Cimigenoside against breast cancer in vitro and in vivo study. Moreover, we further predicted the possible binding mode of Cimigenoside with γ-secretase through molecular docking studies. The results show that Cimigenoside has a significant inhibitory effect towards the proliferation or metastasis of breast cancer cells via suppressing the Notch signaling pathway-mediated mitochondrial apoptosis and EMT (epithelial mesenchymal transition). In terms of mechanism, Cimigenoside could inhibit the activation of PSEN-1, the catalytic subunit of γ-secretase, and also by cleaving the Notch protein mediated by PSEN-1. Overall, our findings provide scientific support to utilize Cimigenoside as an effective targeted drug for clinical treatment of BC.

miR-27-3p Enhances the Sensitivity of Triple-Negative Breast Cancer Cells to the Antitumor Agent Olaparib by Targeting PSEN-1, the Catalytic Subunit of Γ-Secretase

Olaparib has been used in the treatment of triple-negative breast cancer (TNBC) with BRCA mutations. In the present study, we demonstrated the effect of miR-27-3p on the γ-secretase pathway by regulating the sensitivity of TNBC cells to olaparib. miR-27-3p, a microRNA with the potential to target PSEN-1, the catalytic subunit of γ-secretase mediating the second step of the cleavage of the Notch protein, was identified by the online tool miRDB and found to inhibit the expression of PSEN-1 by directly targeting the 3'-untranslated region (3'-UTR) of PSEN-1. The overexpression of miR-27-3p inhibited the activation of the Notch pathway via the inhibition of the cleavage of the Notch protein, mediated by γ-secretase, and, in turn, enhanced the sensitivity of TNBC cells to the antitumor agent olaparib. Transfection with PSEN-1 containing mutated targeting sites for miR-27-3p or the expression vector of the Notch protein intracellular domain (NICD) almost completely blocked the effect of miR-27-3p on the Notch pathway or the sensitivity of TNBC cells to olaparib, respectively. Therefore, our results suggest that the miR-27-3p/γ-secretase axis participates in the regulation of TNBC and that the overexpression of miR-27-3p represents a potential approach to enhancing the sensitivity of TNBC to olaparib.

Celastrol and Triptolide Suppress Stemness in Triple Negative Breast Cancer: Notch as a Therapeutic Target for Stem Cells

Triple negative breast cancer (TNBC) is observed in ~15% of breast cancers and results in poor survival and increased distant metastases. Within the tumor are present a small portion of cancer stem cells that drive tumorigenesis and metastasis. In this study, we aimed to elucidate whether the two natural compounds, celastrol and triptolide, inhibit stemness in TNBC. MDA-MB-231, BT20, and a patient-derived primary cells (PD-TNBC) were used in the study. Mammosphere assay was performed to assess the stemness. Both celastrol and triptolide treatment suppressed mammosphere formation. Furthermore, the compound suppressed expression of cancer stem cell marker proteins DCLK1, ALDH1, and CD133. Notch signaling plays a critical role in stem cells renewal. Both celastrol or triptolide reduced Notch -1 activation and expression of its downstream target proteins HES-1 and HEY-1. However, when NICD 1 was ectopically overexpressed in the cells, it partially rescued proliferation and mammosphere formation of the cells, supporting the role of notch signaling. Together, these data demonstrate that targeting stem cells and the notch signaling pathway may be an effective strategy for curtailing TNBC progression.

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

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

Is γ-secretase a beneficial inactivating enzyme of the toxic APP C-terminal fragment C99?

Genetic, biochemical, and anatomical grounds led to the proposal of the amyloid cascade hypothesis centered on the accumulation of amyloid beta peptides (Aβ) to explain Alzheimer's disease (AD) etiology. In this context, a bulk of efforts have aimed at developing therapeutic strategies seeking to reduce Aβ levels, either by blocking its production (γ- and β-secretase inhibitors) or by neutralizing it once formed (Aβ-directed immunotherapies). However, so far the vast majority of, if not all, clinical trials based on these strategies have failed, since they have not been able to restore cognitive function in AD patients, and even in many cases, they have worsened the clinical picture. We here propose that AD could be more complex than a simple Aβ-linked pathology and discuss the possibility that a way to reconcile undoubted genetic evidences linking processing of APP to AD and a consistent failure of Aβ-based clinical trials could be to envision the pathological contribution of the direct precursor of Aβ, the β-secretase-derived C-terminal fragment of APP, βCTF, also referred to as C99. In this review, we summarize scientific evidences pointing to C99 as an early contributor to AD and postulate that γ-secretase should be considered as not only an Aβ-generating protease, but also a beneficial C99-inactivating enzyme. In that sense, we discuss the limitations of molecules targeting γ-secretase and propose alternative strategies seeking to reduce C99 levels by other means and notably by enhancing its lysosomal degradation.

Hydrophilic loop 1 of Presenilin-1 and the APP GxxxG transmembrane motif regulate γ-secretase function in generating Alzheimer-causing Aβ peptides

γ-Secretase is responsible for the proteolysis of amyloid precursor protein (APP) into amyloid-beta (Aβ) peptides, which are centrally implicated in the pathogenesis of Alzheimer’s disease (AD). The biochemical mechanism of how processing by γ-secretase is regulated, especially as regards the interaction between enzyme and substrate, remains largely unknown. Here, mutagenesis reveals that the hydrophilic loop-1 (HL-1) of presenilin-1 (PS1) is critical for both γ-secretase step-wise cleavages (processivity) and its allosteric modulation by heterocyclic γ-modulatory compounds. Systematic mutagenesis of HL-1, including all of its familial AD mutations and additional engineered variants, and quantification of the resultant Aβ products show that HL-1 is necessary for proper sequential γ-secretase processivity. We identify Y106, L113, and Y115 in HL-1 as key targets for heterocyclic γ-secretase modulators (GSMs) to stimulate processing of pathogenic Aβ peptides. Further, we confirm that the GxxxG domain in the APP transmembrane region functions as a critical substrate motif for γ-secretase processivity: a G29A substitution in APP-C99 mimics the beneficial effects of GSMs. Together, these findings provide a molecular basis for the structural regulation of γ-processivity by enzyme and substrate, facilitating the rational design of new GSMs that lower AD-initiating amyloidogenic Aβ peptides.

Unlocking the Secrets of Cancer Stem Cells with γ-Secretase Inhibitors: A Novel Anticancer Strategy

The dysregulation of Notch signaling is associated with a wide variety of different human cancers. Notch signaling activation mostly relies on the activity of the γ-secretase enzyme that cleaves the Notch receptors and releases the active intracellular domain. It is well-documented that γ-secretase inhibitors (GSIs) block the Notch activity, mainly by inhibiting the oncogenic activity of this pathway. To date, several GSIs have been introduced clinically for the treatment of various diseases, such as Alzheimer's disease and various cancers, and their impacts on Notch inhibition have been found to be promising. Therefore, GSIs are of great interest for cancer therapy. The objective of this review is to provide a systematic review of in vitro and in vivo studies for investigating the effect of GSIs on various cancer stem cells (CSCs), mainly by modulation of the Notch signaling pathway. Various scholarly electronic databases were searched and relevant studies published in the English language were collected up to February 2020. Herein, we conclude that GSIs can be potential candidates for CSC-targeting therapy. The outcome of our study also indicates that GSIs in combination with anticancer drugs have a greater inhibitory effect on CSCs.

SCO-spondin-derived Peptide Protects Neurons from Glutamate-induced Excitotoxicity

Subcommissural organ (SCO)-spondin is a brain-specific glycoprotein produced during embryogenesis, that strongly contributes to neuronal development. The SCO becomes atrophic in adults, halting SCO-spondin production and its neuroprotective functions. Using rat and human neuronal cultures, we evaluated the neuroprotective effect of an innovative peptide derived from SCO-spondin against glutamate excitotoxicity. Primary neurons were exposed to glutamate and treated with the linear (NX210) and cyclic (NX210c) forms of the peptide. Neuronal survival and neurite networks were assessed using immunohistochemistry or biochemistry. The mechanism of action of both peptide forms was investigated by exposing neurons to inhibitors targeting receptors and intracellular mediators that trigger apoptosis, neuronal survival, or neurite growth. NX210c promoted neuronal survival and prevented neurite network retraction in rat cortical and hippocampal neurons, whereas NX210 was efficient only in neuronal survival (cortical neurons) or neurite networks (hippocampal neurons). They triggered neuroprotection via integrin receptors and γ-secretase substrate(s), activation of the PI3K/mTOR pathway and disruption of the apoptotic cascade. The neuroprotective effect of NX210c was confirmed in human cortical neurons via the reduction of lactate dehydrogenase release and recovery of normal basal levels of apoptotic cells. Together, these results show that NX210 and NX210c protect against glutamate neurotoxicity through common and distinct mechanisms of action and that, most often, NX210c is more efficient than NX210. Proof of concept in central nervous system animal models are under investigation to evaluate the neuroprotective action of SCO-spondin-derived peptide.

Impairment of Store-operated Calcium Entry: Implications in Alzheimer's Neurodegeneration

Alzheimer's disease (AD) is an insidious and progressive neurodegenerative disorder. Dysfunction of central cholinergic neurons, amyloid aggregation and deposition,oxidative stress,and biometal dyshomeostasis has been regarded as the major pathogenic mediators in this devastating disease. However, strategies derived from these hypotheses fail to slow down or stop the progression of AD, warranting a combination of therapies to target multiple etiological factors or examining alternative hypothesis. Store-operated calcium entry (SOCE) is the process by which depletion of calcium in the endoplasmic reticulum (ER) lumen causes an influx of calcium across plasmalemma. Accumulating evidence indicates that neuronal SOCE (nSOCE) is inhibited in family AD (FAD) and the inhibition of which causes instability of dendritic spines and enhances amyloidogenesis. Mutant Presenilin fails to function as an ER calcium leak channel and promotes degradation of stromal interaction molecules (STIM), ER calcium sensors; these effects may account for the repression of nSOCE in FAD. We have demonstrated that activation of autophagy degrades STIM proteins, resulting in a trimming effect on a dendritic arbor, under proteasome inhibition and endoplasmic reticulum stress, which are intimately connected with AD. Thus, we hypothesize that autophagy represses SOCE by degrading STIM proteins, leading to synapse loss in AD. This review article will highlight the roles of SOCE in AD neurodegeneration, the degradative mechanisms of STIM protein, and the therapeutic potential and associated challenge.

Familial Alzheimer's Disease Mutations in PSEN1 Lead to Premature Human Stem Cell Neurogenesis

Mutations in presenilin 1 (PSEN1) or presenilin 2 (PSEN2), the catalytic subunit of γ-secretase, cause familial Alzheimer's disease (fAD). We hypothesized that mutations in PSEN1 reduce Notch signaling and alter neurogenesis. Expression data from developmental and adult neurogenesis show relative enrichment of Notch and γ-secretase expression in stem cells, whereas expression of APP and β-secretase is enriched in neurons. We observe premature neurogenesis in fAD iPSCs harboring PSEN1 mutations using two orthogonal systems: cortical differentiation in 2D and cerebral organoid generation in 3D. This is partly driven by reduced Notch signaling. We extend these studies to adult hippocampal neurogenesis in mutation-confirmed postmortem tissue. fAD cases show mutation-specific effects and a trend toward reduced abundance of newborn neurons, supporting a premature aging phenotype. Altogether, these results support altered neurogenesis as a result of fAD mutations and suggest that neural stem cell biology is affected in aging and disease.

Evaluation of PSEN1 subunit of the γ-secretase gene in patients with psoriasis vulgaris: a pilot study

Introduction

Psoriasis is a chronic autoimmune inflammatory disease, the prevalence of which is 1–3% in the Polish population. Genome testing using single nucleotide polymorphisms revealed more than 50 regions associated with the risk of psoriasis, and most of these genes are associated with the immune system.

Aim

To assess the presence of PSEN1 subunits of the γ-secretase gene polymorphisms in patients with psoriasis and comparison of results with a healthy control group.

Material and methods

We used polymerase chain reaction – restriction fragment length polymorphism (PCR RFLP) method to assess polymorphisms. The starting material for analysis was peripheral blood obtained from the patient.

Results

PSEN1a-positivity was found in 2/52 (2.78%) of patients with psoriasis and 1/36 (3.85%) of healthy controls. PSEN1b positivity was seen in 3/52 (5.77%) of patients with psoriasis and 1/36 (3.85%) of control individuals. Only 3 patients with psoriasis but none of healthy volunteers had a presence of PSEN1c. Four patients were excluded from further statistical analysis.

Conclusions

We have not shown a relationship between PSEN1 polymorphism and the clinical occurrence of psoriasis but now we start the assessment of other subunits of the γ-secretase gene – PSENEN and NCSTN.

Insight γ-Secretase: Structure, Function, and Role in Alzheimer's Disease

In neurodegenerative disorders, there is a progressive degeneration of the body, leading to the death of nerve cells. In this state, a patient gets affected day by day with mental weakness, dementia, and ataxia. Alzheimer's disease (AD) is the most common irreversible neurodegenerative brain disorder mainly affecting people over the age of 65. Many types of research suggest that the main culprit for AD is the aggregated form of a (39-43) amino acid peptide called amyloid beta. Amyloid beta (Aβ) is generated by the action of beta-secretase and gamma-secretase on the larger glycoprotein. Gamma (γ) secretase is an intra-membrane protease complex that cleaves the single-- pass transmembrane protein, the amyloid precursor protein, and Notch. The γ-secretase complex contains presenilin, presenilin enhancer-2, anterior pharynx defective-1, and nicastrin. Any mutation in presenilin-1 or the cleavage of amyloid precursor protein by γ-secretase directly or indirectly is associated with AD. Therefore, the prevention of this enzyme is one of the solutions for AD. In this article, we discuss the γ-secretase complex and its inhibitors that can contribute to the prevention of AD.

γ-secretase inhibitors, DAPT and RO4929097, promote the migration of Human Glioma Cells via Smad5-downregulated E-cadherin Expression

Malignant gliomas are a type of central nervous system cancer with extremely high mortality rates in humans. γ-secretase has been becoming a potential target for cancer therapy, including glioma, because of the involvement of its enzymatic activity in regulating the proliferation and metastasis of cancer cells. In this study, we attempted to determine whether γ-secretase activity regulates E-cadherin to affect glioma cell migration. The human glioma cell lines, including LN18 and LN229, and the γ-secretase inhibitors, including N-[N-(3,5-difluorophenacetyl)-L-alanyl]-S-phenylglycine t-butyl ester (DAPT) and RO4929097, were used in this study. It was shown that γ-secretase activity inhibition by DAPT and RO4929097 could promote LN18 and LN229 glioma cell migration via downregulating E-cadherin mRNA and protein expressions, but not via affecting E-cadherin protein processing. In addition, γ-secretase activity inhibition was regulated by bone morphogenetic proteins-independent Smad5 activation in glioma cells. Moreover, endogenous Smad1 in glioma cells was found to play an important role in regulating E-cadherin expression and subsequent cell migration but did not affect DAPT-stimulated effects. These results help further elucidate the molecular mechanisms of γ-secretase activity regulation involved in controlling glioma cell malignancy. Information about a potential role for Smad1/5 activity upregulation and subsequent E-cadherin downregulation during inhibition of γ-secretase activity in the development of gliomas is therefore relevant for future research.

Structural basis of γ-secretase inhibition and modulation by small molecule drugs

Development of γ-secretase inhibitors (GSIs) and modulators (GSMs) represents an attractive therapeutic opportunity for Alzheimer's disease (AD) and cancers. However, how these GSIs and GSMs target γ-secretase has remained largely unknown. Here, we report the cryoelectron microscopy (cryo-EM) structures of human γ-secretase bound individually to two GSI clinical candidates, Semagacestat and Avagacestat, a transition state analog GSI L685,458, and a classic GSM E2012, at overall resolutions of 2.6-3.1 Å. Remarkably, each of the GSIs occupies the same general location on presenilin 1 (PS1) that accommodates the β strand from amyloid precursor protein or Notch, interfering with substrate recruitment. L685,458 directly coordinates the two catalytic aspartate residues of PS1. E2012 binds to an allosteric site of γ-secretase on the extracellular side, potentially explaining its modulating activity. Structural analysis reveals a set of shared themes and variations for inhibitor and modulator recognition that will guide development of the next-generation substrate-selective inhibitors.

High temperature promotes amyloid β-protein production and γ-secretase complex formation via Hsp90

Alzheimer's disease (AD) is characterized by neuronal loss and accumulation of β-amyloid-protein (Aβ) in the brain parenchyma. Sleep impairment is associated with AD and affects about 25-40% of patients in the mild-to-moderate stages of the disease. Sleep deprivation leads to increased Aβ production; however, its mechanism remains largely unknown. We hypothesized that the increase in core body temperature induced by sleep deprivation may promote Aβ production. Here, we report temperature-dependent regulation of Aβ production. We found that an increase in temperature, from 37 °C to 39 °C, significantly increased Aβ production in amyloid precursor protein-overexpressing cells. We also found that high temperature (39 °C) significantly increased the expression levels of heat shock protein 90 (Hsp90) and the C-terminal fragment of presenilin 1 (PS1-CTF) and promoted γ-secretase complex formation. Interestingly, Hsp90 was associated with the components of the premature γ-secretase complex, anterior pharynx-defective-1 (APH-1), and nicastrin (NCT) but was not associated with PS1-CTF or presenilin enhancer-2. Hsp90 knockdown abolished the increased level of Aβ production and the increased formation of the γ-secretase complex at high temperature in culture. Furthermore, with in vivo experiments, we observed increases in the levels of Hsp90, PS1-CTF, NCT, and the γ-secretase complex in the cortex of mice housed at higher room temperature (30 °C) compared with those housed at standard room temperature (23 °C). Our results suggest that high temperature regulates Aβ production by modulating γ-secretase complex formation through the binding of Hsp90 to NCT/APH-1.

Altered Hinge Conformations in APP Transmembrane Helix Mutants May Affect Enzyme-Substrate Interactions of γ-Secretase

Cleavage of substrates by γ-secretase is an inherently slow process where substrate-enzyme affinities cannot be broken down into specific sequence requirements in contrast to soluble proteases. Nevertheless, despite its apparent sequence tolerance single point mutations in amyloid precursor protein can severely affect cleavage efficiencies and change product line preferences. We have determined by NMR spectroscopy the structures of the transmembrane domain of amyloid precursor protein in TFE/water and compared it to that of four mutants: two FAD mutants, V44M and I45T, and the two diglycine hinge mutants, G38L and G38P. In accordance with previous publications, the transmembrane domain is composed of two helical segments connected by the diglycine hinge. Mutations alter kink angles and structural flexibility. Furthermore, to our surprise, we observe different, but specific mutual orientations of N- and C-terminal helical segments in the four mutants compared to the wildtype. We speculate that the observed orientations for G38L, G38P, V44M, and I45T lead to unfavorable interactions with γ-secretase exosites during substrate movement to the enzyme's active site in presenilin and/or for the accommodation into the substrate-binding cavity of presenilin.

Dual Bioorthogonal Labeling of the Amyloid-β Protein Precursor Facilitates Simultaneous Visualization of the Protein and Its Cleavage Products

The amyloid-β protein precursor (AβPP) is critical in the pathophysiology of Alzheimer’s disease (AD), since two-step proteolytic processing of AβPP generates the neurotoxic amyloid-β peptide (Aβ). We developed a dual fluorescence labeling system to study the exact subcellular location of γ-secretase cleavage of AβPP. The C-terminal tail of AβPP was fluorescently labeled using a SNAP-tag, while the Aβ region of AβPP was fluorescently tagged with a dye at a genetically-encoded noncanonical amino acid (ncAA). The ncAA was introduced at specific positions in AβPP using a genetic code expansion strategy and afterwards, the reactive side-chain of the ncAA was coupled to the dye using a bioorthogonal labeling chemistry. In proof-of-concept experiments, HEK293T cells were transfected with plasmids containing engineered AβPP harboring an amber mutation and an amber codon suppression system with an evolved tRNA synthetase/tRNA pair and grown in the presence of a lysine-derived ncAA. Processing of the AβPP variants was validated with ELISA and immunoblotting, and seven AβPP mutants that showed similar cleavage pattern as wild-type AβPP were identified. The AβPP mutant was fluorescently labeled with 6-methyl-tetrazine-BDP-FL and TMR-Star at the ncAA and SNAP-tag, respectively. Using this approach, AβPP was fluorescently labeled at two sites in living cells with minimal background to allow monitoring of Aβ and C-terminal cleavage products simultaneously. The method described provides a powerful tool to label Aβ with minimal perturbations of its processing, thus enabling studies of the trafficking of the cleavage products of AβPP.