Sodium channel β1 subunits are post-translationally modified by tyrosine phosphorylation, S-palmitoylation, and regulated intramembrane proteolysis

Voltage-gated sodium channel (VGSC) β1 subunits are multifunctional proteins that modulate the biophysical properties and cell-surface localization of VGSC α subunits and participate in cell-cell and cell-matrix adhesion, all with important implications for intracellular signal transduction, cell migration, and differentiation. Human loss-of-function variants in SCN1B, the gene encoding the VGSC β1 subunits, are linked to severe diseases with high risk for sudden death, including epileptic encephalopathy and cardiac arrhythmia. We showed previously that β1 subunits are post-translationally modified by tyrosine phosphorylation. We also showed that β1 subunits undergo regulated intramembrane proteolysis via the activity of β-secretase 1 and γ-secretase, resulting in the generation of a soluble intracellular domain, β1-ICD, which modulates transcription. Here, we report that β1 subunits are phosphorylated by FYN kinase. Moreover, we show that β1 subunits are S-palmitoylated. Substitution of a single residue in β1, Cys-162, to alanine prevented palmitoylation, reduced the level of β1 polypeptides at the plasma membrane, and reduced the extent of β1-regulated intramembrane proteolysis, suggesting that the plasma membrane is the site of β1 proteolytic processing. Treatment with the clathrin-mediated endocytosis inhibitor, Dyngo-4a, re-stored the plasma membrane association of β1-p.C162A to WT levels. Despite these observations, palmitoylation-null β1-p.C162A modulated sodium current and sorted to detergent-resistant membrane fractions normally. This is the first demonstration of S-palmitoylation of a VGSC β subunit, establishing precedence for this post-translational modification as a regulatory mechanism in this protein family.

Hidradenitis Suppurativa as a Potential Subtype of Autoinflammatory Keratinization Disease

Hidradenitis suppurativa (HS) is a chronic inflammatory skin condition, clinically characterized by boiled cysts, comedones, abscesses, hypertrophic scars, and/or sinus tracts typically in the apocrine-gland-rich areas such as the axillae, groin, and/or buttocks. Although its precise pathogenic mechanisms remain unknown, I herein emphasize the importance of the following three recent discoveries in the pathogenesis of HS: First, heterozygous loss-of-function mutations in the genes encoding γ-secretase, including NCSTN, PSENEN, and PSEN1, have been identified in some patients with HS. Such genetic alterations result in hyperkeratosis, dysregulated hair follicle differentiation, and cyst formation via aberrant Notch signaling. Furthermore, Psen1-/Psen2-, Psen1-, Ncstn+/-, and Notch1-/Notch2- mice share common phenotypes of human HS, suggesting a role of aberrant keratinization in the development of HS. Second, upregulation of interleukin 1β, interleukin-36, caspase-1, and NLRP3 and dysregulation of the Th17:Treg cell axis have been demonstrated in HS samples, suggesting that autoinflammation is a key event in the pathophysiology of the disease. Notably, HS may be complicated with other autoinflammatory diseases such as inflammatory bowel diseases and pyoderma gangrenosum, again highlighting the importance of autoinflammation in HS. Last, biologics such as adalimumab, infliximab, anakinra, ustekinumab, and secukinumab are reportedly effective for moderate-to-severe HS. These findings collectively suggest that HS is closely linked with aberrant keratinization and autoinflammation, raising the question whether it represents an autoinflammatory keratinization disease, a recently proposed disease entity. In this mini review, I introduce the concept of autoinflammatory keratinization disease and attempt to address this clinically important question.

Substrate-Enzyme Interactions in Intramembrane Proteolysis: γ-Secretase as the Prototype

Intramembrane-cleaving proteases (I-CLiPs) catalyze the hydrolysis of peptide bonds within the transmembrane regions of membrane protein substrates, releasing bioactive fragments that play roles in many physiological and pathological processes. Based on their catalytic mechanism and nucleophile, I-CLiPs are classified into metallo, serine, aspartyl, and glutamyl proteases. Presenilin is the most prominent among I-CLiPs, as the catalytic subunit of γ-secretase (GS) complex responsible for cleaving the amyloid precursor protein (APP) and Notch, as well as many other membrane substrates. Recent cryo-electron microscopy (cryo-EM) structures of GS provide new details on how presenilin recognizes and cleaves APP and Notch. First, presenilin transmembrane helix (TM) 2 and 6 are dynamic. Second, upon binding to GS, the substrate TM helix is unwound from the C-terminus, resulting in an intermolecular β-sheet between the substrate and presenilin. The transition of the substrate C-terminus from α-helix to β-sheet is proposed to expose the scissile peptide bond in an extended conformation, leaving it susceptible to protease cleavage. Despite the astounding new insights in recent years, many crucial questions remain unanswered regarding the inner workings of γ-secretase, however. Key unanswered questions include how the enzyme recognizes and recruits substrates, how substrates are translocated from an initial docking site to the active site, how active site aspartates recruit and coordinate catalytic water, and the nature of the mechanisms of processive trimming of the substrate and product release. Answering these questions will have important implications for drug discovery aimed at selectively reducing the amyloid load in Alzheimer's disease (AD) with minimal side effects.

Liver-selective γ-secretase inhibition ameliorates diet-induced hepatic steatosis, dyslipidemia and atherosclerosis

Hepatic γ-secretase regulates low-density lipoprotein receptor (LDLR) cleavage and degradation, affecting clearance of plasma triglyceride (TG)-rich lipoproteins (TRLs). In this study, we investigated whether γ-secretase inhibition modulates risk of Western (high-fat/sucrose and high-cholesterol)-type diet (WTD)-induced hepatic steatosis, dyslipidemia and atherosclerosis. We evaluated liver and plasma lipids in WTD-fed mice with hepatocyte-specific ablation of the non-redundant γ-secretase-targeting subunit Nicastrin (L-Ncst). In parallel, we investigated the effect of liver-selective Ncst antisense oligonucleotides (ASO) on lipid metabolism and atherosclerosis in wildtype (WT) and ApoE knockout (ApoE^-/-) mice fed normal chow or WTD. WTD-fed L-Ncst and Ncst ASO-treated WT mice showed reduced total cholesterol and LDL-cholesterol (LDL-C), as well as reduced hepatic lipid content as compared to Cre- and control ASO-treated WT mice. Treatment of WTD-fed ApoE^-/- mice with Ncst ASO markedly lowered total and LDL cholesterol, hepatic TG and attenuated atherosclerotic lesions in the aorta, as compared to control ASO-treated mice. L-Ncst and Ncst ASO similarly showed reduced plasma glucose as compared to control mice. In conclusion, inhibition of hepatic γ-secretase reduces plasma glucose, and attenuates WTD-induced dyslipidemia, hepatic fat accumulation and atherosclerosis, suggesting potential pleiotropic application for diet-induced metabolic dysfunction.

Papillomaviruses Go Retro

Human papillomaviruses are important pathogens responsible for approximately 5% of cancer as well as other important human diseases, but many aspects of the papillomavirus life cycle are poorly understood. To undergo genome replication, HPV DNA must traffic from the cell surface to the nucleus. Recent findings have revolutionized our understanding of HPV entry, showing that it requires numerous cellular proteins and proceeds via a series of intracellular membrane-bound vesicles that comprise the retrograde transport pathway. This paper reviews the evidence supporting this unique entry mechanism with a focus on the crucial step by which the incoming virus particle is transferred from the endosome into the retrograde pathway. This new understanding provides novel insights into basic cellular biology and suggests novel rational approaches to inhibit HPV infection.

Computer-Aided Drug Design of β-Secretase, γ-Secretase and Anti-Tau Inhibitors for the Discovery of Novel Alzheimer's Therapeutics

Aging-associated neurodegenerative diseases, which are characterized by progressive neuronal death and synapses loss in human brain, are rapidly growing affecting millions of people globally. Alzheimer's is the most common neurodegenerative disease and it can be caused by genetic and environmental risk factors. This review describes the amyloid-β and Tau hypotheses leading to amyloid plaques and neurofibrillary tangles, respectively which are the predominant pathways for the development of anti-Alzheimer's small molecule inhibitors. The function and structure of the druggable targets of these two pathways including β-secretase, γ-secretase, and Tau are discussed in this review article. Computer-Aided Drug Design including computational structure-based design and ligand-based design have been employed successfully to develop inhibitors for biomolecular targets involved in Alzheimer's. The application of computational molecular modeling for the discovery of small molecule inhibitors and modulators for β-secretase and γ-secretase is summarized. Examples of computational approaches employed for the development of anti-amyloid aggregation and anti-Tau phosphorylation, proteolysis and aggregation inhibitors are also reported.

A Review of the Familial Alzheimer's Disease Locus PRESENILIN 2 and Its Relationship to PRESENILIN 1

PRESENILIN 1 (PSEN1) and PRESENILIN 2 (PSEN2) genes are loci for mutations causing familial Alzheimer's disease (fAD). However, the function of these genes and how they contribute to fAD pathogenesis has not been fully determined. This review provides a summary of the overlapping and independent functions of the PRESENILINS with a focus on the lesser studied PSEN2. As a core component of the γ-secretase complex, the PSEN2 protein is involved in many γ-secretase-related physiological activities, including innate immunity, Notch signaling, autophagy, and mitochondrial function. These physiological activities have all been associated with AD progression, indicating that PSEN2 plays a particular role in AD pathogenesis.

Decrease in p3-Alcβ37 and p3-Alcβ40, products of Alcadein β generated by γ-secretase cleavages, in aged monkeys and patients with Alzheimer's disease

Introduction

Neuronal p3-Alcβ peptides are generated from the precursor protein Alcadein β (Alcβ) through cleavage by α- and γ-secretases of the amyloid β (Aβ) protein precursor (APP). To reveal whether p3-Alcβ is involved in Alzheimer's disease (AD) contributes for the development of novel therapy and/or drug targets.

Methods

We developed new sandwich enzyme-linked immunosorbent assay (sELISA) systems to quantitate levels of p3-Alcβ in the cerebrospinal fluid (CSF).

Results

In monkeys, CSF p3-Alcβ decreases with age, and the aging is also accompanied by decreased brain expression of Alcβ. In humans, CSF p3-Alcβ levels decrease to a greater extent in those with AD than in age-matched controls. Subjects carrying presenilin gene mutations show a significantly lower CSF p3-Alcβ level. A cell study with an inverse modulator of γ-secretase remarkably reduces the generation of p3-Alcβ37 while increasing the production of Aβ42.

Discussion

Aging decreases the generation of p3-Alcβ, and further significant decrease of p3-Alcβ caused by aberrant γ-secretase activity may accelerate pathogenesis in AD.

Dendrobium alkaloids decrease Aβ by regulating α- and β-secretases in hippocampal neurons of SD rats

Background

Alzheimer's disease (AD) is the primary cause of dementia in the elderly. The imbalance between production and clearance of amyloid β (Aβ) is a very early, often initiating factor in AD. Dendrobium nobile Lindl. alkaloids (DNLA) extracted from a Chinese medicinal herb, which have been shown to have anti-aging effects, protected against neuronal impairment in vivo and in vitro. Moreover, we confirmed that DNLA can improve learning and memory function in elderly normal mice, indicating that DNLA has potential health benefits. However, the underlying mechanism is unclear. Therefore, we further explored the effect of DNLA on neurons, which is closely related to learning and memory, based on Aβ.

Methods

We exposed cultured hippocampal neurons to DNLA to investigate the effect of DNLA on Aβ in vitro. Cell viability was evaluated by MTT assays. Proteins were analyzed by Western blot analysis.

Results

The cell viability of hippocampal neurons was not changed significantly after treatment with DNLA. But DNLA reduced the protein expression of amyloid precursor protein (APP), disintegrin and metalloprotease 10 (ADAM10), β-site APP cleaving enzyme 1 (BACE1) and Aβ_1-42 of hippocampal neurons in rats and increased the protein expression of ADAM17.

Conclusions

DNLA decreases Aβ by regulating α- and β-secretase in hippocampal neurons of SD rats.

Aging-induced Akt activation involves in aging-related pathologies and Aβ-induced toxicity

Multicellular signals are altered in the processes of both aging and neurodegenerative diseases, including Alzheimer's disease (AD). Similarities in behavioral and cellular functional changes suggest a common regulator between aging and AD that remains undetermined. Our genetics and behavioral approaches revealed the regulatory role of Akt in both aging and AD pathogenesis. In this study, we found that the activity of Akt is upregulated during aging through epidermal growth factor receptor activation by using the fruit fly as an in vivo model. Downregulation of Akt in neurons improved cell survival, locomotor activity, and starvation challenge in both aged and Aβ42‐expressing flies. Interestingly, increased cAMP levels attenuated both Akt activation‐induced early death and Aβ42‐induced learning deficit in flies. At the molecular level, overexpression of Akt promoted Notch cleavage, suggesting that Akt is an endogenous activity regulator of γ‐secretase. Taken together, this study revealed that Akt is involved in the aging process and Aβ toxicity, and manipulating Akt can restore both neuronal functions and improve behavioral activity during the processes of aging and AD pathogenesis.

Individual and combined presenilin 1 and 2 knockouts reveal that both have highly overlapping functions in HEK293T cells

Presenilins 1 and 2 (PS1 and 2) are the catalytic subunits of γ-secretase, a multiprotein protease that cleaves amyloid protein precursor and other type I transmembrane proteins. Previous studies with mouse models or cells have indicated differences in PS1 and PS2 functions. We have recently reported that clinical γ-secretase inhibitors (GSIs), initially developed to manage Alzheimer's disease and now being considered for other therapeutic interventions, are both pharmacologically and functionally distinct. Here, using CRISPR/Cas9-based gene editing, we established human HEK 293T cell lines in which endogenous PS1, PS2, or both have been knocked out. Using these knockout lines to examine differences in PS1- and PS2-mediated cleavage events, we confirmed that PS2 generates more intracellular β-amyloid than does PS1. Moreover, we observed subtle differences in PS1- and PS2-mediated cleavages of select substrates. In exploring the question of whether differences in activity among clinical GSIs could be attributed to differential inhibition of PS1 or PS2, we noted that select GSIs inhibit PS1 and PS2 activities on specific substrates with slightly different potencies. We also found that endoproteolysis of select PS1 FAD-linked variants in human cells is more efficient than what has been previously reported for mouse cell lines. Overall, these results obtained with HEK293T cells suggest that selective PS1 or PS2 inhibition by a given GSI does not explain the previously observed differences in functional and pharmacological properties among various GSIs.

Induction of Amyloid-β42 Production by Fipronil and Other Pyrazole Insecticides

Generation of amyloid-β peptides (Aβs) by proteolytic cleavage of the amyloid-β protein precursor (AβPP), especially increased production of Aβ42/Aβ43 over Aβ40, and their aggregation as oligomers and plaques, represent a characteristic feature of Alzheimer's disease (AD). In familial AD (FAD), altered Aβ production originates from specific mutations of AβPP or presenilins 1/2 (PS1/PS2), the catalytic subunits of γ-secretase. In sporadic AD, the origin of altered production of Aβs remains unknown. We hypothesize that the 'human chemical exposome' contains products able to favor the production of Aβ42/Aβ43 over Aβ40 and shorter Aβs. To detect such products, we screened a library of 3500 + compounds in a cell-based assay for enhanced Aβ42/Aβ43 production. Nine pyrazole insecticides were found to induce a β- and γ-secretase-dependent, 3-10-fold increase in the production of extracellular Aβ42 in various cell lines and neurons differentiated from induced pluripotent stem cells derived from healthy and FAD patients. Immunoprecipitation/mass spectrometry analyses showed increased production of Aβs cleaved at positions 42/43, and reduced production of peptides cleaved at positions 38 and shorter. Strongly supporting a direct effect on γ-secretase activity, pyrazoles shifted the cleavage pattern of another γ-secretase substrate, alcadeinα, and shifted the cleavage of AβPP by highly purified γ-secretase toward Aβ42/Aβ43. Focusing on fipronil, we showed that some of its metabolites, in particular the persistent fipronil sulfone, also favor the production of Aβ42/Aβ43 in both cell-based and cell-free systems. Fipronil administered orally to mice and rats is known to be metabolized rapidly, mostly to fipronil sulfone, which stably accumulates in adipose tissue and brain. In conclusion, several widely used pyrazole insecticides enhance the production of toxic, aggregation prone Aβ42/Aβ43 peptides, suggesting the possible existence of environmental "Alzheimerogens" which may contribute to the initiation and propagation of the amyloidogenic process in sporadic AD.

The vexing complexity of the amyloidogenic pathway

The role of the amyloidogenic pathway in the etiology of Alzheimer's disease (AD), particularly the common sporadic late onset forms of the disease, is controversial. To some degree, this is a consequence of the failure of drug and therapeutic antibody trials based either on targeting the proteases in this pathway or its amyloid end products. Here, we explore the formidable complexity of the biochemistry and cell biology associated with this pathway. For example, we review evidence that the immediate precursor of amyloid-β, the C99 domain of the amyloid precursor protein (APP), may itself be toxic. We also review important new results that appear to finally establish a direct genetic link between mutations in APP and the sporadic forms of AD. Based on the complexity of amyloidogenesis, it seems possible that a major contributor to the failure of related drug trials is that we have an incomplete understanding of this pathway and how it is linked to Alzheimer's pathogenesis. If so, this highlights a need for further characterization of this pathway, not its abandonment.

Gamma secretase orthologs are required for lysosomal activity and autophagic degradation in Dictyostelium discoideum, independent of PSEN (presenilin) proteolytic function

Mutations in the γ-secretase complex are strongly associated with familial Alzheimer disease. Both proteolytic and non-proteolytic functions for the γ-secretase complex have been previously described in mammalian model organisms, but their relative contributions to disease pathology remain unclear. Here, we dissect the roles of orthologs of the γ-secretase components in the model system Dictyostelium, focusing on endocytosis, lysosomal activity and autophagy. In this model, we show that the orthologs of PSEN (psenA and psenB), Ncstn (nicastrin) and Aph-1 (gamma-secretase subunit Aph-1), are necessary for optimal fluid-phase uptake by macropinocytosis and in multicellular development under basic pH conditions. Disruption of either psenA/B or Aph-1 proteins also leads to disrupted phagosomal proteolysis as well as decreased autophagosomal acidification and autophagic flux. This indicates a general defect in lysosomal trafficking and degradation, which we show leads to the accumulation of ubiquitinated protein aggregates in cells lacking psenA/B and Aph-1 proteins. Importantly, we find that all the endocytic defects observed in Dictyostelium PSEN ortholog mutants can be fully rescued by proteolytically inactive Dictyostelium psenB and human PSEN1 proteins. Our data therefore demonstrates an evolutionarily conserved non-proteolytic role for presenilin, and γ-secretase component orthologs, in maintaining Dictyostelium lysosomal trafficking and autophagy.

Abbreviations: Atg8: autophagy protein 8a; Aph-1: gamma-secretase subunit Aph-1; crtA: calreticulin; ER: endoplasmic reticulum; GFP: green fluorescent protein; GSK3B: glycogen synthase kinase 3 beta; Ncstn: nicastrin; PSEN1: presenilin 1; psenA and psenB: Dictyostelium presenilin A and B; TRITC; tetramethylrhodamine isothiocyanate.

BACE-1 and γ-Secretase as Therapeutic Targets for Alzheimer's Disease

Alzheimer’s disease (AD) is a growing global health concern with a massive impact on affected individuals and society. Despite the considerable advances achieved in the understanding of AD pathogenesis, researchers have not been successful in fully identifying the mechanisms involved in disease progression. The amyloid hypothesis, currently the prevalent theory for AD, defends the deposition of β-amyloid protein (Aβ) aggregates as the trigger of a series of events leading to neuronal dysfunction and dementia. Hence, several research and development (R&D) programs have been led by the pharmaceutical industry in an effort to discover effective and safety anti-amyloid agents as disease modifying agents for AD. Among 19 drug candidates identified in the AD pipeline, nine have their mechanism of action centered in the activity of β or γ-secretase proteases, covering almost 50% of the identified agents. These drug candidates must fulfill the general rigid prerequisites for a drug aimed for central nervous system (CNS) penetration and selectivity toward different aspartyl proteases. This review presents the classes of γ-secretase and beta-site APP cleaving enzyme 1 (BACE-1) inhibitors under development, highlighting their structure-activity relationship, among other physical-chemistry aspects important for the successful development of new anti-AD pharmacological agents.

Evaluation of the prognostic significances of γ-secretase genes in pancreatic cancer

With the growing requirement for novel prognostic biomarkers for pancreatic cancer, many studies have focused on clinical and/or genomic variables. Although many studies have been performed, carbohydrate antigen 19-9 is the only biomarker in clinical use. Therefore, the present study examined whether γ-secretase genes, including presenilin (PSEN), nicastrin (NCSTN), presenilin enhancer protein 2 (PSENEN), and anterior pharynx-defective 1 (APH1-), could serve as prognostic factors for pancreatic cancer. The cohorts selected included >100 pancreatic cancer patients. Patient data were downloaded from The Cancer Genome Atlas (TCGA) and Gene Expression Omnibus (GSE21501). The prognostic roles of the γ-secretase genes were analyzed by several survival analysis methods. Among the γ-secretase genes, the prognosis tended to be worse in the 2 cohorts with increasing expression of PSEN1, APH1A, and PSENEN, while the remaining genes were the opposite in the 2 cohorts. Notably, although the patient characteristics were quite different, APH1A was statistically significantly associated with prognosis in the 2 cohorts. The hazard ratio of APH1A for overall survival was 1.598 (TCGA) and 2.724 (GSE21501). These results contribute to the study of γ-secretase in pancreatic cancer. We believe that γ-secretase, particularly APH1A, will be a new prognostic biomarker for pancreatic cancer.

Presenilins and γ-Secretase in Membrane Proteostasis

The presenilin (PS) proteins exert a crucial role in the pathogenesis of Alzheimer disease (AD) by mediating the intramembranous cleavage of amyloid precursor protein (APP) and the generation of amyloid β-protein (Aβ). The two homologous proteins PS1 and PS2 represent the catalytic subunits of distinct γ-secretase complexes that mediate a variety of cellular processes, including membrane protein metabolism, signal transduction, and cell differentiation. While the intramembrane cleavage of select proteins by γ-secretase is critical in the regulation of intracellular signaling pathways, the plethora of identified protein substrates could also indicate an important role of these enzyme complexes in membrane protein homeostasis. In line with this notion, PS proteins and/or γ-secretase has also been implicated in autophagy, a fundamental process for the maintenance of cellular functions and homeostasis. Dysfunction in the clearance of proteins in the lysosome and during autophagy has been shown to contribute to neurodegeneration. This review summarizes the recent knowledge about the role of PS proteins and γ-secretase in membrane protein metabolism and trafficking, and the functional relation to lysosomal activity and autophagy.

A novel triazole, NMK-T-057, induces autophagic cell death in breast cancer cells by inhibiting γ-secretase-mediated activation of Notch signaling

Notch signaling is reported to be deregulated in several malignancies, including breast, and the enzyme γ-secretase plays an important role in the activation and nuclear translocation of Notch intracellular domain (NICD). Hence, pharmacological inhibition of γ-secretase might lead to the subsequent inhibition of Notch signaling in cancer cells. In search of novel γ-secretase inhibitors (GSIs), we screened a series of triazole-based compounds for their potential to bind γ-secretase and observed that 3-(3'4',5'-trimethoxyphenyl)-5-(N-methyl-3'-indolyl)-1,2,4-triazole compound (also known as NMK-T-057) can bind to γ-secretase complex. Very interestingly, NMK-T-057 was found to inhibit proliferation, colony-forming ability, and motility in various breast cancer (BC) cells such as MDA-MB-231, MDA-MB-468, 4T1 (triple-negative cells), and MCF-7 (estrogen receptor (ER)/progesterone receptor (PR)-positive cell line) with negligible cytotoxicity against noncancerous cells (MCF-10A and peripheral blood mononuclear cells). Furthermore, significant induction of apoptosis and inhibition of epithelial-to-mesenchymal transition (EMT) and stemness were also observed in NMK-T-057-treated BC cells. The in silico study revealing the affinity of NMK-T-057 toward γ-secretase was further validated by a fluorescence-based γ-secretase activity assay, which confirmed inhibition of γ-secretase activity in NMK-T-057-treated BC cells. Interestingly, it was observed that NMK-T-057 induced significant autophagic responses in BC cells, which led to apoptosis. Moreover, NMK-T-057 was found to inhibit tumor progression in a 4T1-BALB/c mouse model. Hence, it may be concluded that NMK-T-057 could be a potential drug candidate against BC that can trigger autophagy-mediated cell death by inhibiting γ-secretase-mediated activation of Notch signaling.

Supraphysiologic-dose anabolic-androgenic steroid use: A risk factor for dementia?

Supraphysiologic-dose anabolic-androgenic steroid (AAS) use is associated with physiologic, cognitive, and brain abnormalities similar to those found in people at risk for developing Alzheimer's Disease and its related dementias (AD/ADRD), which are associated with high brain β-amyloid (Aβ) and hyperphosphorylated tau (tau-P) protein levels. Supraphysiologic-dose AAS induces androgen abnormalities and excess oxidative stress, which have been linked to increased and decreased expression or activity of proteins that synthesize and eliminate, respectively, Aβ and tau-P. Aβ and tau-P accumulation may begin soon after initiating supraphysiologic-dose AAS use, which typically occurs in the early 20s, and their accumulation may be accelerated by other psychoactive substance use, which is common among non-medical AAS users. Accordingly, the widespread use of supraphysiologic-dose AAS may increase the numbers of people who develop dementia. Early diagnosis and correction of sex-steroid level abnormalities and excess oxidative stress could attenuate risk for developing AD/ADRD in supraphysiologic-dose AAS users, in people with other substance use disorders, and in people with low sex-steroid levels or excess oxidative stress associated with aging.

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/.