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

The pathogenicity of PSEN2 variants is tied to Aβ production and homology to PSEN1

INTRODUCTION

Though recognized as a potential cause of Autosomal Dominant Alzheimer's Disease, the pathogenicity of many PSEN2 variants remains uncertain. We compared Aβ production across all missense PSEN2 variants in the Alzforum database and, when possible, to corresponding PSEN1 variants.

METHODS

We expressed 74 PSEN2 variants, 21 of which had homologous PSEN1 variants with the same amino acid substitution, in HEK293 cells lacking PSN1/2. Aβ production was compared to age at symptom onset (AAO) and between homologous PSEN1/2 variants.

RESULTS

Aβ42/40 and Aβ37/42 ratios were associated with AAO across PSEN2 variants, strongly driven by PSEN2 variants with PSEN1 homologs. PSEN2 AAO was 18.3 years later compared to PSEN1 homologs. Aβ ratios from PSEN1 / 2 homologs were highly correlated, suggesting a similar mechanism of γ-secretase dysfunction.

DISCUSSION

The existence of a PSEN1 homolog and patterns of Aβ production are important considerations in assessing the pathogenicity of previously-reported and new PSEN2 variants.

Alzheimer's disease-related presenilins are key to intestinal epithelial cell function and gut immune homoeostasis

OBJECTIVE

Mutations in presenilin genes are the major cause of Alzheimer's disease. However, little is known about their expression and function in the gut. In this study, we identify the presenilins Psen1 and Psen2 as key molecules that maintain intestinal homoeostasis.

DESIGN

Human inflammatory bowel disease (IBD) and control samples were analysed for Psen1 expression. Newly generated intestinal epithelium-specific Psen1-deficient, Psen2-deficient and inducible Psen1/Psen2 double-deficient mice were used to dissect the functional role of presenilins in intestinal homoeostasis.

RESULTS

Psen1 expression was regulated in experimental gut inflammation and in patients with IBD. Induced deletion of Psen1 and Psen2 in mice caused rapid weight loss and spontaneous development of intestinal inflammation. Mice exhibited epithelial barrier disruption with bacterial translocation and deregulation of key pathways for nutrient uptake. Wasting disease was independent of gut inflammation and dysbiosis, as depletion of microbiota rescued Psen-deficient animals from spontaneous colitis development but not from weight loss. On a molecular level, intestinal epithelial cells lacking Psen showed impaired Notch signalling and dysregulated epithelial differentiation.

CONCLUSION

Overall, our study provides evidence that Psen1 and Psen2 are important guardians of intestinal homoeostasis and future targets for barrier-promoting therapeutic strategies in IBD.

C-terminal amino acids in the type I transmembrane domain of L-type lectin VIP36 affect γ-secretase susceptibility

Regulated intramembrane proteolysis (RIP) is a two-step processing mechanism for transmembrane proteins consisting of ectodomain shedding (shedding), which removes the extracellular domain through juxtamembrane processing and intramembrane proteolysis, which processes membrane-anchored shedding products within the transmembrane domain. RIP irreversibly converts one transmembrane protein into multiple soluble proteins that perform various physiological functions. The only requirement for the substrate of γ-secretase, the major enzyme responsible for intramembrane proteolysis of type I transmembrane proteins, is the absence of a large extracellular domain, and it is thought that γ-secretase can process any type I membrane protein as long as it is shed. In the present study, we showed that the shedding susceptible type I membrane protein VIP36 (36 kDa vesicular integral membrane protein) and its homolog, VIPL, have different γ-secretase susceptibilities in their transmembrane domains. Analysis of the substitution mutants suggested that γ-secretase susceptibility is regulated by C-terminal amino acids in the transmembrane domain. We also compared the transmembrane domains of several shedding susceptible membrane proteins and found that each had a different γ-secretase susceptibility. These results suggest that the transmembrane domain is not simply a stretch of hydrophobic amino acids but is an important element that regulates membrane protein function by controlling the lifetime of the membrane-anchored shedding product.

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

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

Alzheimer's disease: The role of proteins in formation, mechanisms, and new therapeutic approaches

Alzheimer's disease (AD) is a progressive neurological disorder that affects the central nervous system (CNS), leading to memory and cognitive decline. In AD, the brain experiences three main structural changes: a significant decrease in the quantity of neurons, the development of neurofibrillary tangles (NFT) composed of hyperphosphorylated tau protein, and the formation of amyloid beta (Aβ) or senile plaques, which are protein deposits found outside cells and surrounded by dystrophic neurites. Genetic studies have identified four genes associated with autosomal dominant or familial early-onset AD (FAD): amyloid precursor protein (APP), presenilin 1 (PS1), presenilin 2 (PS2), and apolipoprotein E (ApoE). The formation of plaques primarily involves the accumulation of Aβ, which can be influenced by mutations in APP, PS1, PS2, or ApoE genes. Mutations in the APP and presenilin (PS) proteins can cause an increased amyloid β peptides production, especially the further form of amyloidogenic known as Aβ42. Apart from genetic factors, environmental factors such as cytokines and neurotoxins may also have a significant impact on the development and progression of AD by influencing the formation of amyloid plaques and intracellular tangles. Exploring the causes and implications of protein aggregation in the brain could lead to innovative therapeutic approaches. Some promising therapy strategies that have reached the clinical stage include using acetylcholinesterase inhibitors, estrogen, nonsteroidal anti-inflammatory drugs (NSAIDs), antioxidants, and antiapoptotic agents. The most hopeful therapeutic strategies involve inhibiting activity of secretase and preventing the β-amyloid oligomers and fibrils formation, which are associated with the β-amyloid fibrils accumulation in AD. Additionally, immunotherapy development holds promise as a progressive therapeutic approach for treatment of AD. Recently, the two primary categories of brain stimulation techniques that have been studied for the treatment of AD are invasive brain stimulation (IBS) and non-invasive brain stimulation (NIBS). In this article, the amyloid proteins that play a significant role in the AD formation, the mechanism of disease formation as well as new drugs utilized to treat of AD will be reviewed.

Inhibition of the NOTCH and mTOR pathways by nelfinavir as a novel treatment for T cell acute lymphoblastic leukemia

T cell acute lymphoblastic leukemia (T‑ALL), a neoplasm derived from T cell lineage‑committed lymphoblasts, is characterized by genetic alterations that result in activation of oncogenic transcription factors and the NOTCH1 pathway activation. The NOTCH is a transmembrane receptor protein activated by γ‑secretase. γ‑secretase inhibitors (GSIs) are a NOTCH‑targeted therapy for T‑ALL. However, their clinical application has not been successful due to adverse events (primarily gastrointestinal toxicity), limited efficacy, and drug resistance caused by several mechanisms, including activation of the AKT/mTOR pathway. Nelfinavir is an human immunodeficiency virus 1 aspartic protease inhibitor and has been repurposed as an anticancer drug. It acts by inducing endoplasmic reticulum (ER) stress and inhibiting the AKT/mTOR pathway. Thus, it was hypothesized that nelfinavir might inhibit the NOTCH pathway via γ‑secretase inhibition and blockade of aspartic protease presenilin, which would make nelfinavir effective against NOTCH‑associated T‑ALL. The present study assessed the efficacy of nelfinavir against T‑ALL cells and investigated mechanisms of action in vitro and in preclinical treatment studies using a SCL‑LMO1 transgenic mouse model. Nelfinavir blocks presenilin 1 processing and inhibits γ‑secretase activity as well as the NOTCH1 pathway, thus suppressing T‑ALL cell viability. Additionally, microarray analysis of nelfinavir‑treated T‑ALL cells showed that nelfinavir upregulated mRNA levels of CHAC1 (glutathione‑specific γ‑glutamylcyclotransferase 1, a negative regulator of NOTCH) and sestrin 2 (SESN2; a negative regulator of mTOR). As both factors are upregulated by ER stress, this confirmed that nelfinavir induced ER stress in T‑ALL cells. Moreover, nelfinavir suppressed NOTCH1 mRNA expression in microarray analyses. These findings suggest that nelfinavir inhibited the NOTCH1 pathway by downregulating NOTCH1 mRNA expression, upregulating CHAC1 and suppressing γ‑secretase via presenilin 1 inhibition and the mTOR pathway by upregulating SESN2 via ER stress induction. Further, nelfinavir exhibited therapeutic efficacy against T‑ALL in an SCL‑LMO1 transgenic mouse model. Collectively, these findings highlight the potential of nelfinavir as a novel therapeutic candidate for treatment of patients with T‑ALL.

APP-BACE1 Interaction and Intracellular Localization Regulate Aβ Production in iPSC-Derived Cortical Neurons

Alzheimer's disease (AD) is characterized pathologically by amyloid β (Aβ)-containing plaques. Generation of Aβ from amyloid precursor protein (APP) by two enzymes, β- and γ-secretase, has therefore been in the AD research spotlight for decades. Despite this, how the physical interaction of APP with the secretases influences APP processing is not fully understood. Herein, we compared two genetically identical human iPSC-derived neuronal cell types: low Aβ-secreting neuroprogenitor cells (NPCs) and high Aβ-secreting mature neurons, as models of low versus high Aβ production. We investigated levels of substrate, enzymes and products of APP amyloidogenic processing and correlated them with the proximity of APP to β- and γ-secretase in endo-lysosomal organelles. In mature neurons, increased colocalization of full-length APP with the β-secretase BACE1 correlated with increased β-cleavage product sAPPβ. Increased flAPP/BACE1 colocalization was mainly found in early endosomes. In the same way, increased colocalization of APP-derived C-terminal fragment (CTF) with presenilin-1 (PSEN1), the catalytic subunit of γ-secretase, was seen in neurons as compared to NPCs. Furthermore, most of the interaction of APP with BACE1 in low Aβ-secreting NPCs seemed to derive from CTF, the remaining APP part after BACE1 cleavage, indicating a possible novel product-enzyme inhibition. In conclusion, our results suggest that interaction of APP and APP cleavage products with their secretases can regulate Aβ production both positively and negatively. β- and γ-Secretases are difficult targets for AD treatment due to their ubiquitous nature and wide range of substrates. Therefore, targeting APP-secretase interactions could be a novel treatment strategy for AD. Colocalization of APP species with BACE1 in a novel model of low- versus high-Aβ secretion-Two genetically identical human iPSC-derived neuronal cell types: low Aβ-secreting neuroprogenitor cells (NPCs) and high Aβ secreting mature neurons, were compared. Increased full-length APP (flAPP)/BACE1 colocalization in early endosomes was seen in neurons, while APP-CTF/BACE1 colocalization was much higher than flAPP/BACE1 colocalization in NPCs, although the cellular location was not determined.

Glucocorticoid enhances presenilin1-dependent Aβ production at ER's mitochondrial-associated membrane by downregulating Rer1 in neuronal cells

Stress-induced release of glucocorticoid is an important amyloidogenic factor that upregulates amyloid precursor protein (APP) and β secretase 1 (BACE1) levels. Glucocorticoid also contributes to the pathogenesis of Alzheimer's disease (AD) by increasing ER-mitochondria connectivity, in which amyloid β (Aβ) processing occurs rigorously because of its lipid raft-rich characteristics. However, the mechanism by which glucocorticoid enhances γ-secretase activity in the mitochondrial-associated membrane of ER (MAM) and subsequent accumulation of mitochondrial Aβ is unclear. In this study, we determined how glucocorticoid enhances Aβ production in MAM using SH-SY5Y cells and ICR mice. First, we observed that cortisol-induced Aβ accumulation in mitochondria preceded its extracellular apposition by enhancing γ-secretase activity, which was the result of increased presenilin 1 (PSEN1) localization in MAM. Screening data revealed that cortisol selectively downregulated the ER retrieval protein Rer1, which triggered its maturation and subsequent entry into the endocytic secretory pathway of PSEN1. Accordingly, overexpression of RER1 reversed the deleterious effects of mitochondrial Aβ on mitochondrial respiratory function and neuronal cell viability. Notably, we found that cortisol guided the glucocorticoid receptor (GR) to bind directly to the RER1 promoter, thus trans-repressing its expression. Inhibiting GR function reduced Aβ accumulation at mitochondria and improved the outcome of a spatial memory task in mice exposed to corticosterone. Taken together, glucocorticoid enhances PSEN1-mediated Aβ generation at MAM by downregulating Rer1, which is a potential target at early stages of AD pathogenesis.

Converging links between adult-onset neurodegenerative Alzheimer's disease and early life neurodegenerative neuronal ceroid lipofuscinosis?

Evidence from genetics and from analyzing cellular and animal models have converged to suggest links between neurodegenerative disorders of early and late life. Here, we summarize emerging links between the most common late life neurodegenerative disease, Alzheimer's disease, and the most common early life neurodegenerative diseases, neuronal ceroid lipofuscinoses. Genetic studies reported an overlap of clinically diagnosed Alzheimer's disease and mutations in genes known to cause neuronal ceroid lipofuscinoses. Accumulating data strongly suggest dysfunction of intracellular trafficking mechanisms and the autophagy-endolysosome system in both types of neurodegenerative disorders. This suggests shared cytopathological processes underlying these different types of neurodegenerative diseases. A better understanding of the common mechanisms underlying the different diseases is important as this might lead to the identification of novel targets for therapeutic concepts, the transfer of therapeutic strategies from one disease to the other and therapeutic approaches tailored to patients with specific mutations. Here, we review dysfunctions of the endolysosomal autophagy pathway in Alzheimer's disease and neuronal ceroid lipofuscinoses and summarize emerging etiologic and genetic overlaps.

Different transmembrane domains determine the specificity and efficiency of the cleavage activity of the γ-secretase subunit presenilin

The γ-secretase complex catalyzes the intramembrane cleavage of C99, a carboxy-terminal fragment of the amyloid precursor protein. Two paralogs of its catalytic subunit presenilin (PS1 and PS2) are expressed which are autocatalytically cleaved into an N-terminal and a C-terminal fragment during maturation of γ-secretase. In this study, we compared the efficiency and specificity of C99 cleavage by PS1- and PS2-containing γ-secretases. Mass spectrometric analysis of cleavage products obtained in cell-free and cell-based assays revealed that the previously described lower amyloid-β (Aβ)38 generation by PS2 is accompanied by a reciprocal increase in Aβ37 production. We further found PS1 and PS2 to show different preferences in the choice of the initial cleavage site of C99. However, the differences in Aβ38 and Aβ37 generation appear to mainly result from altered subsequent stepwise cleavage of Aβ peptides. Apart from these differences in cleavage specificity, we confirmed a lower efficiency of initial C99 cleavage by PS2 using a detergent-solubilized γ-secretase system. By investigating chimeric PS1/2 molecules, we show that the membrane-embedded, nonconserved residues of the N-terminal fragment mainly account for the differential cleavage efficiency and specificity of both presenilins. At the level of individual transmembrane domains (TMDs), TMD3 was identified as a major modulator of initial cleavage site specificity. The efficiency of endoproteolysis strongly depends on nonconserved TMD6 residues at the interface to TMD2, i.e., at a putative gate of substrate entry. Taken together, our results highlight the role of individual presenilin TMDs in the cleavage of C99 and the generation of Aβ peptides.

Endosome and Lysosome Membrane Properties Functionally Link to γ-Secretase in Live/Intact Cells

Our unique multiplexed imaging assays employing FRET biosensors have previously detected that γ-secretase processes APP C99 primarily in late endosomes and lysosomes in live/intact neurons. Moreover we have shown that Aβ peptides are enriched in the same subcellular loci. Given that γ-secretase is integrated into the membrane bilayer and functionally links to lipid membrane properties in vitro, it is presumable that γ-secretase function correlates with endosome and lysosome membrane properties in live/intact cells. In the present study, we show using unique live-cell imaging and biochemical assays that the endo-lysosomal membrane in primary neurons is more disordered and, as a result, more permeable than in CHO cells. Interestingly, γ-secretase processivity is decreased in primary neurons, resulting in the predominant production of long Aβ42 instead of short Aβ38. In contrast, CHO cells favor Aβ38 over the Aβ42 generation. Our findings are consistent with the previous in vitro studies, demonstrating the functional interaction between lipid membrane properties and γ-secretase and provide further evidence that γ-secretase acts in late endosomes and lysosomes in live/intact cells.

Regulation of Ryanodine Receptor-Dependent Neurotransmitter Release by AIP, Calstabins, and Presenilins

Ryanodine receptors (RyRs) are Ca2+ release channels located in the endoplasmic reticulum membrane. Presynaptic RyRs play important roles in neurotransmitter release and synaptic plasticity. Recent studies suggest that the proper function of presynaptic RyRs relies on several regulatory proteins, including aryl hydrocarbon receptor-interacting protein, calstabins, and presenilins. Dysfunctions of these regulatory proteins can greatly impact neurotransmitter release and synaptic plasticity by altering the function or expression of RyRs. This chapter aims to describe the interaction between these proteins and RyRs, elucidating their crucial role in regulating synaptic function.

Proteolytically generated soluble Tweak Receptor Fn14 is a blood biomarker for γ-secretase activity

Fn14 is a cell surface receptor with key functions in tissue homeostasis and injury but is also linked to chronic diseases. Despite its physiological and medical importance, the regulation of Fn14 signaling and turnover is only partly understood. Here, we demonstrate that Fn14 is cleaved within its transmembrane domain by the protease γ‐secretase, resulting in secretion of the soluble Fn14 ectodomain (sFn14). Inhibition of γ‐secretase in tumor cells reduced sFn14 secretion, increased full‐length Fn14 at the cell surface, and enhanced TWEAK ligand‐stimulated Fn14 signaling through the NFκB pathway, which led to enhanced release of the cytokine tumor necrosis factor. γ‐Secretase‐dependent sFn14 release was also detected ex vivo in primary tumor cells from glioblastoma patients, in mouse and human plasma and was strongly reduced in blood from human cancer patients dosed with a γ‐secretase inhibitor prior to chimeric antigen receptor (CAR)‐T‐cell treatment. Taken together, our study demonstrates a novel function for γ‐secretase in attenuating TWEAK/Fn14 signaling and suggests the use of sFn14 as an easily measurable pharmacodynamic biomarker to monitor γ‐secretase activity in vivo.

The role of Triggering Receptor Expressed on Myeloid Cells 2 in Parkinson's disease and other neurodegenerative disorders

Parkinson's disease (PD) is a progressive neurological disorder marked by cardinal clinical symptoms such as rigor, tremor, and akinesia. Albeit a loss of dopaminergic neurons from the substantia nigra pars compacta is causative for the movement impairments found in patients, molecular reasoning for this loss is still incomplete. In recent years, triggering factor expressed on myeloid cells (TREM2) gained attention in the field of neurodegeneration as it could be associated with different neurodegenerative disorders. Primarily identified as a risk factor in Alzheimer's disease, variants in TREM2 were linked to PD and multiple sclerosis, too. Expressed on phagocytic cells, such as macrophages and microglia, TREM2 puts the focus on inflammation associated conditions in PD and provides a molecular target that could at least partly explain the role of immune cells in PD. Here, we summarize expression patterns and molecular functions of TREM2, recapitulate on its role in inflammation, phagocytosis and cell survival, before turning to neurodegenerative disorders with an emphasis on PD.

Converging roles of PSENEN/PEN2 and CLN3 in the autophagy-lysosome system

PSENEN/PEN2 is the smallest subunit of the γ-secretase complex, an intramembrane protease that cleaves proteins within their transmembrane domains. Mutations in components of the γ-secretase underlie familial Alzheimer disease. In addition to its proteolytic activity, supplementary, γ-secretase independent, functions in the macroautophagy/autophagy-lysosome system have been proposed. Here, we screened for PSENEN-interacting proteins and identified CLN3. Mutations in CLN3 are causative for juvenile neuronal ceroid lipofuscinosis, a rare lysosomal storage disorder considered the most common neurodegenerative disease in children. As mutations in the PSENEN and CLN3 genes cause different neurodegenerative diseases, understanding shared cellular functions of both proteins might be pertinent for understanding general cellular mechanisms underlying neurodegeneration. We hypothesized that CLN3 modulates γ-secretase activity and that PSENEN and CLN3 play associated roles in the autophagy-lysosome system. We applied CRISPR gene-editing and obtained independent isogenic HeLa knockout cell lines for PSENEN and CLN3. Following previous studies, we demonstrate that PSENEN is essential for forming a functional γ-secretase complex and is indispensable for γ-secretase activity. In contrast, CLN3 does not modulate γ-secretase activity to a significant degree. We observed in PSENEN- and CLN3-knockout cells corresponding alterations in the autophagy-lysosome system. These include reduced activity of lysosomal enzymes and lysosome number, an increased number of autophagosomes, increased lysosome-autophagosome fusion, and elevated levels of TFEB (transcription factor EB). Our study strongly suggests converging roles of PSENEN and CLN3 in the autophagy-lysosome system in a γ-secretase activity-independent manner, supporting the idea of common cytopathological processes underlying different neurodegenerative diseases.

Abbreviations: Aβ, amyloid-beta; AD, Alzheimer disease; APP, amyloid precursor protein; ATP5MC, ATP synthase membrane subunit c; DQ-BSA, dye-quenched bovine serum albumin; ER, endoplasmic reticulum; GFP, green fluorescent protein; ICC, immunocytochemistry; ICD, intracellular domain; JNCL, juvenile neuronal ceroid lipofuscinosis; KO, knockout; LC3, microtubule associated protein 1 light chain 3; NCL, neuronal ceroid lipofuscinoses; PSEN, presenilin; PSENEN/PEN2: presenilin enhancer, gamma-secretase subunit; TAP, tandem affinity purification; TEV, tobacco etch virus; TF, transferrin; WB, Western blot; WT, wild type.

The Potential of Gamma Secretase as a Therapeutic Target for Cardiac Diseases

Heart diseases are some of the most common and pressing threats to human health worldwide. The American Heart Association and the National Institute of Health jointly work to annually update data on cardiac diseases. In 2018, 126.9 million Americans were reported as having some form of cardiac disorder, with an estimated direct and indirect total cost of USD 363.4 billion. This necessitates developing therapeutic interventions for heart diseases to improve human life expectancy and economic relief. In this review, we look into gamma-secretase as a potential therapeutic target for cardiac diseases. Gamma-secretase, an aspartyl protease enzyme, is responsible for the cleavage and activation of a number of substrates that are relevant to normal cardiac development and function as found in mutation studies. Some of these substrates are involved in downstream signaling processes and crosstalk with pathways relevant to heart diseases. Most of the substrates and signaling events we explored were found to be potentially beneficial to maintain cardiac function in diseased conditions. This review presents an updated overview of the current knowledge on gamma-secretase processing of cardiac-relevant substrates and seeks to understand if the modulation of gamma-secretase activity would be beneficial to combat cardiac diseases.

Mechanism of Cellular Formation and In Vivo Seeding Effects of Hexameric β-Amyloid Assemblies

The β-amyloid peptide (Aβ) is found as amyloid fibrils in senile plaques, a typical hallmark of Alzheimer's disease (AD). However, intermediate soluble oligomers of Aβ are now recognized as initiators of the pathogenic cascade leading to AD. Studies using recombinant Aβ have shown that hexameric Aβ in particular acts as a critical nucleus for Aβ self-assembly. We recently isolated hexameric Aβ assemblies from a cellular model, and demonstrated their ability to enhance Aβ aggregation in vitro. Here, we report the presence of similar hexameric-like Aβ assemblies across several cellular models, including neuronal-like cell lines. In order to better understand how they are produced in a cellular context, we investigated the role of presenilin-1 (PS1) and presenilin-2 (PS2) in their formation. PS1 and PS2 are the catalytic subunits of the γ-secretase complex that generates Aβ. Using CRISPR-Cas9 to knockdown each of the two presenilins in neuronal-like cell lines, we observed a direct link between the PS2-dependent processing pathway and the release of hexameric-like Aβ assemblies in extracellular vesicles. Further, we assessed the contribution of hexameric Aβ to the development of amyloid pathology. We report the early presence of hexameric-like Aβ assemblies in both transgenic mice brains exhibiting human Aβ pathology and in the cerebrospinal fluid of AD patients, suggesting hexameric Aβ as a potential early AD biomarker. Finally, cell-derived hexameric Aβ was found to seed other human Aβ forms, resulting in the aggravation of amyloid deposition in vivo and neuronal toxicity in vitro.

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.

Mitochondria-associated endoplasmic reticulum membranes: At the crossroad between familiar and sporadic Alzheimer's disease

Alzheimer's disease (AD) is the leading cause of dementia and is incurable. The widely accepted amyloid hypothesis failed to produce efficient clinical therapies. In contrast, there is increasing evidence suggesting that the disruption of mitochondria-associated endoplasmic reticulum (ER) membranes (MAM) is a critical upstream event of AD pathogenesis. Here, we review MAM's role in some AD symptoms such as plaque formation, tau hyperphosphorylation, synaptic loss, aberrant lipid synthesis, disturbed calcium homeostasis, and abnormal autophagy. At last, we proposed that MAM plays a central role in familial AD (FAD) and sporadic AD (SAD).

Signaling Functions of Intramembrane Aspartyl-Proteases

Intramembrane proteolysis is more than a mechanism to "clean" the membranes from proteins no longer needed. By non-reversibly modifying transmembrane proteins, intramembrane cleaving proteases hold key roles in multiple signaling pathways and often distinguish physiological from pathological conditions. Signal peptide peptidase (SPP) and signal peptide peptidase-like proteases (SPPLs) recently have been associated with multiple functions in the field of signal transduction. SPP/SPPLs together with presenilins (PSs) are the only two families of intramembrane cleaving aspartyl proteases known in mammals. PS1 or PS2 comprise the catalytic center of the γ-secretase complex, which is well-studied in the context of Alzheimer's disease. The mammalian SPP/SPPL family of intramembrane cleaving proteases consists of five members: SPP and its homologous proteins SPPL2a, SPPL2b, SPPL2c, and SPPL3. Although these proteases were discovered due to their homology to PSs, it became evident in the past two decades that no physiological functions are shared between these two families. Based on studies in cell culture models various substrates of SPP/SPPL proteases have been identified in the past years and recently-developed mouse lines lacking individual members of this protease family, will help to further clarify the physiological functions of these proteases. In this review we concentrate on signaling roles of mammalian intramembrane cleaving aspartyl proteases. In particular, we will highlight the signaling roles of PS via its substrates NOTCH, VEGF, and others, mainly focusing on its involvement in vasculature. Delineating also signaling pathways that are affected and/or controlled by SPP/SPPL proteases. From SPP's participation in tumor progression and survival, to SPPL3's regulation of protein glycosylation and SPPL2c's control over cellular calcium stores, various crossovers between proteolytic activity of intramembrane proteases and cell signaling will be described.

Echinacoside Suppresses Amyloidogenesis and Modulates F-actin Remodeling by Targeting the ER Stress Sensor PERK in a Mouse Model of Alzheimer's Disease

Endoplasmic reticulum stress (ERS) plays a vital and pathogenic role in the onset and progression of Alzheimer’s disease (AD). Phosphorylation of PKR-like endoplasmic reticulum kinase (PERK) induced by ERS depresses the interaction between actin-binding protein filamin-A (FLNA) and PERK, which promotes F-actin accumulation and reduces ER-plasma membrane (PM) communication. Echinacoside (ECH), a pharmacologically active component purified from Cistanche tubulosa, exhibits multiple neuroprotective activities, but the effects of ECH on ERS and F-actin remodeling remain elusive. Here, we found ECH could inhibit the phosphorylation of PERK. Firstly ECH can promote PERK-FLNA combination and modulate F-actin remodeling. Secondly, ECH dramatically decreased cerebral Aβ production and accumulation by inhibiting the translation of BACE1, and significantly ameliorated memory impairment in 2 × Tg-AD mice. Furthermore, ECH exhibited high affinity to either mouse PERK or human PERK. These findings provide novel insights into the neuroprotective actions of ECH against AD, indicating that ECH is a potential therapeutic agent for halting and preventing the progression of AD.

Presenilin-2 and Calcium Handling: Molecules, Organelles, Cells and Brain Networks

Presenilin-2 (PS2) is one of the three proteins that are dominantly mutated in familial Alzheimer's disease (FAD). It forms the catalytic core of the γ-secretase complex-a function shared with its homolog presenilin-1 (PS1)-the enzyme ultimately responsible of amyloid-β (Aβ) formation. Besides its enzymatic activity, PS2 is a multifunctional protein, being specifically involved, independently of γ-secretase activity, in the modulation of several cellular processes, such as Ca2+ signalling, mitochondrial function, inter-organelle communication, and autophagy. As for the former, evidence has accumulated that supports the involvement of PS2 at different levels, ranging from organelle Ca2+ handling to Ca2+ entry through plasma membrane channels. Thus FAD-linked PS2 mutations impact on multiple aspects of cell and tissue physiology, including bioenergetics and brain network excitability. In this contribution, we summarize the main findings on PS2, primarily as a modulator of Ca2+ homeostasis, with particular emphasis on the role of its mutations in the pathogenesis of FAD. Identification of cell pathways and molecules that are specifically targeted by PS2 mutants, as well as of common targets shared with PS1 mutants, will be fundamental to disentangle the complexity of memory loss and brain degeneration that occurs in Alzheimer's disease (AD).

The substrate repertoire of γ-secretase/presenilin

The intramembrane protease γ-secretase is a hetero-tetrameric protein complex with presenilin as the catalytic subunit and cleaves its membrane protein substrates within their single transmembrane domains. γ-Secretase is well known for its role in Notch signalling and in Alzheimer's disease, where it catalyzes the formation of the pathogenic amyloid β (Aβ) peptide. However, in the 21 years since its discovery many more substrates and substrate candidates of γ-secretase were identified. Although the physiological relevance of the cleavage of many substrates remains to be studied in more detail, the substrates demonstrate a broad role for γ-secretase in embryonic development, adult tissue homeostasis, signal transduction and protein degradation. Consequently, chronic γ-secretase inhibition may cause significant side effects due to inhibition of cleavage of multiple substrates. This review provides a list of 149 γ-secretase substrates identified to date and highlights by which expeirmental approach substrate cleavage was validated. Additionally, the review lists the cleavage sites where they are known and discusses the functional implications of γ-secretase cleavage with a focus on substrates identified in the recent past, such as CHL1, TREM2 and TNFR1. A comparative analysis demonstrates that γ-secretase substrates mostly have a long extracellular domain and require ectodomain shedding before γ-secretase cleavage, but that γ-secretase is also able to cleave naturally short substrates, such as the B cell maturation antigen. Taken together, the list of substrates provides a resource that may help in the future development of drugs inhibiting or modulating γ-secretase activity in a substrate-specific manner.

Presynaptic failure in Alzheimer's disease

Synaptic loss is the best correlate of cognitive deficits in Alzheimer's disease (AD). Extensive experimental evidence also indicates alterations of synaptic properties at the early stages of disease progression, before synapse loss and neuronal degeneration. A majority of studies in mouse models of AD have focused on post-synaptic mechanisms, including impairment of long-term plasticity, spine structure and glutamate receptor-mediated transmission. Here we review the literature indicating that the synaptic pathology in AD includes a strong presynaptic component. We describe the evidence indicating presynaptic physiological functions of the major molecular players in AD. These include the amyloid precursor protein (APP) and the two presenilin (PS) paralogs PS1 or PS2, genetically linked to the early-onset form of AD, in addition to tau which accumulates in a pathological form in the AD brain. Three main mechanisms participating in presynaptic functions are highlighted. APP fragments bind to presynaptic receptors (e.g. nAChRs and GABA_B receptors), presenilins control Ca²⁺ homeostasis and Ca²⁺-sensors, and tau regulates the localization of presynaptic molecules and synaptic vesicles. We then discuss how impairment of these presynaptic physiological functions can explain or forecast the hallmarks of synaptic impairment and associated dysfunction of neuronal circuits in AD. Beyond the physiological roles of the AD-related proteins, studies in AD brains also support preferential presynaptic alteration. This review features presynaptic failure as a strong component of pathological mechanisms in AD.

Importance of γ-secretase in the regulation of liver X receptor and cellular lipid metabolism

Inhibition of the Alzheimer associated γ-secretase impairs the regulation of cellular lipid droplet homeostasis.

Presenilins (PS) are the catalytic components of γ-secretase complexes that mediate intramembrane proteolysis. Mutations in the PS genes are a major cause of familial early-onset Alzheimer disease and affect the cleavage of the amyloid precursor protein, thereby altering the production of the amyloid β-peptide. However, multiple additional protein substrates have been identified, suggesting pleiotropic functions of γ-secretase. Here, we demonstrate that inhibition of γ-secretase causes dysregulation of cellular lipid homeostasis, including up-regulation of liver X receptors, and complex changes in the cellular lipid composition. Genetic and pharmacological inhibition of γsecretase leads to strong accumulation of cytoplasmic lipid droplets, associated with increased levels of acylglycerols, but lowered cholesteryl esters. Furthermore, accumulation of lipid droplets was augmented by increasing levels of amyloid precursor protein C-terminal fragments, indicating a critical involvement of this γ-secretase substrate. Together, these data provide a mechanism that functionally connects γ-secretase activity to cellular lipid metabolism. These effects were also observed in human astrocytic cells, indicating an important function of γ-secretase in cells critical for lipid homeostasis in the brain.

Loss of presenilin 2 age-dependently alters susceptibility to acute seizures and kindling acquisition

Patients with Alzheimer's disease (AD) experience seizures at higher rates than the general population of that age, suggesting an underexplored role of hyperexcitability in AD. Genetic variants in presenilin (PSEN) 1 and 2 genes lead to autosomal dominant early-onset AD (ADAD); patients with PSEN gene variants also report seizures. Pharmacological control of seizures in AD may be disease-modifying. Preclinical efficacy of FDA-approved antiseizure drugs (ASDs) is well defined in young adult rodents; however, the efficacy of ASDs in aged rodents with chronic seizures is less clear. The mechanism by which ADAD genes lead to AD remains unclear, and even less studied is the pathogenesis of epilepsy in AD. PSEN variants generally all result in a biochemical loss of function (De Strooper, 2007). We herein determined whether well-established models of acute and chronic seizure could be used to explore the relationship between AD genes and seizures through investigating whether loss of normal PSEN2 function age-dependently influenced susceptibility to seizures and/or corneal kindling acquisition. PSEN2 knockout (KO) and age-matched wild-type (WT) mice were screened from 2- to 10-months-old to establish age-dependent focal seizure threshold. Additionally, PSEN2 KO and WT mice aged 2- and 8-months-old underwent corneal kindling such that mice were aged 3- and 9-months old at the beginning of ASD efficacy testing. We then defined the dose-dependent efficacy of mechanistically distinct ASDs on kindled seizures of young versus aged mice to better understand the applicability of corneal kindling to real-world use for geriatric patients. PSEN2 KO mice demonstrated early-life reductions in seizure threshold. However, kindling acquisition was delayed in 2-month-old PSEN2 KO versus WT mice. Young male WT mice took 24.3 ± 1.3 (S.E.M.) stimulations to achieve kindling criterion, whereas age-matched PSEN2 KO male mice took 41.2 ± 1.1 stimulations (p < .0001). The rate of kindling acquisition of 8-month-old mice was no longer different from WT. This study demonstrates that loss of normal PSEN2 function is associated with age-dependent changes in the in vivo susceptibility to acute seizures and kindling. Loss of normal PSEN2 function may be an underexplored molecular contributor to seizures. The use of validated models of chronic seizures in aged rodents may uncover age-related changes in susceptibility to epileptogenesis and/or ASD efficacy in mice with AD-associated genotypes, which may benefit the management of seizures in AD.

Discovery of Cellular Roles of Intramembrane Proteases

Intramembrane proteases (IMPs) are localized within lipid bilayers of membranes-either the cell membrane or membranes of various organelles. Cleavage of substrates often results in release from the membrane, leading to a downstream biological effect. This mechanism allows different signaling events to happen through intramembrane proteolysis. Over the years, various mechanistically distinct families of IMPs have been discovered, but the research progress has generally been slower than for soluble proteases due to the challenges associated with membrane proteins. In this review we summarize how each mechanistic family of IMPs was discovered, which chemical tools are available for the study of IMPs, and which techniques have been developed for the discovery of IMP substrates. Finally, we discuss the various roles in cellular physiology of some of these IMPs.

RIP at the Synapse and the Role of Intracellular Domains in Neurons

Regulated intramembrane proteolysis (RIP) occurs in a cell when transmembrane proteins are cleaved by intramembrane proteases such as secretases to generate soluble protein fragments in the extracellular environment and the cytosol. In the cytosol, these soluble intracellular domains (ICDs) have local functions near the site of cleavage or in many cases, translocate to the nucleus to modulate gene expression. While the mechanism of RIP is relatively well studied, the fate and function of ICDs for most substrate proteins remain poorly characterized. In neurons, RIP occurs in various subcellular compartments including at the synapse. In this review, we summarize current research on RIP in neurons, focusing specifically on synaptic proteins where the presence and function of the ICDs have been reported. We also briefly discuss activity-driven processing of RIP substrates at the synapse and the cellular machinery that support long-distance transport of ICDs from the synapse to the nucleus. Finally, we describe future challenges in this field of research in the context of understanding the contribution of ICDs in neuronal function.

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.

Molecular dynamics of C99-bound γ-secretase reveal two binding modes with distinct compactness, stability, and active-site retention: implications for Aβ production

The membrane protease γ-secretase cleaves the C99 fragment of the amyloid precursor protein, thus producing the Aβ peptides central to Alzheimer's disease. Cryo-electron microscopy has provided the topology but misses the membrane and loop parts that contribute to substrate binding. We report here an essentially complete atomic model of C99 within wild-type γ-secretase that respects all the experimental constraints and additionally describes loop, helix, and C99 substrate dynamics in a realistic all-atom membrane. Our model represents the matured auto-cleaved state required for catalysis. From two independent 500-ns molecular dynamic simulations, we identify two conformation states of C99 in equilibrium, a compact and a loose state. Our simulations provide a basis for C99 processing and Aβ formation and explain the production of longer and shorter Aβ, as the compact state retains C99 for longer and thus probably trims to shorter Aβ peptides. We expect pathogenic presenilin mutations to stabilize the loose over the compact state. The simulations detail the role of the Lys53-Lys54-Lys55 anchor for C99 binding, a loss of helicity of bound C99, and positioning of Thr48 and Leu49 leading to alternative trimming pathways on opposite sides of the C99 helix in three amino acid steps. The C99 binding topology resembles that of C83-bound γ-secretase without membrane but lacks a presenilin 1-C99 β-sheet, which could be induced by C83's stronger binding. The loose state should be selectively disfavored by γ-secretase modulators to increase C99 trimming and reduce the formation of longer Aβ, a strategy that is currently much explored but has lacked a structural basis.

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.

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.

Gamma-secretase-dependent signaling of receptor tyrosine kinases

Human genome harbors 55 receptor tyrosine kinases (RTK). At least half of the RTKs have been reported to be cleaved by gamma-secretase-mediated regulated intramembrane proteolysis. The two-step process involves releasing the RTK ectodomain to the extracellular space by proteolytic cleavage called shedding, followed by cleavage in the RTK transmembrane domain by the gamma-secretase complex resulting in release of a soluble RTK intracellular domain. This intracellular domain, including the tyrosine kinase domain, can in turn translocate to various cellular compartments, such as the nucleus or proteasome. The soluble intracellular domain may interact with transcriptional regulators and other proteins to induce specific effects on cell survival, proliferation, and differentiation, establishing an additional signaling mode for the cleavable RTKs. On the other hand, the same process can facilitate RTK turnover and proteasomal degradation. In this review we focus on the regulation of RTK shedding and gamma-secretase cleavage, as well as signaling promoted by the soluble RTK ICDs. In addition, therapeutic implications of increased knowledge on RTK cleavage on cancer drug development are discussed.

Are N- and C-terminally truncated Aβ species key pathological triggers in Alzheimer's disease?

The histopathology of Alzheimer's disease (AD) is characterized by neuronal loss, neurofibrillary tangles, and senile plaque formation. The latter results from an exacerbated production (familial AD cases) or altered degradation (sporadic cases) of 40/42-amino acid-long β-amyloid peptides (Aβ peptides) that are produced by sequential cleavages of Aβ precursor protein (βAPP) by β- and γ-secretases. The amyloid cascade hypothesis proposes a key role for the full-length Aβ42 and the Aβ40/42 ratio in AD etiology, in which soluble Aβ oligomers lead to neurotoxicity, tau hyperphosphorylation, aggregation, and, ultimately, cognitive defects. However, following this postulate, during the last decade, several clinical approaches aimed at decreasing full-length Aβ42 production or neutralizing it by immunotherapy have failed to reduce or even stabilize AD-related decline. Thus, the Aβ peptide (Aβ40/42)-centric hypothesis is probably a simplified view of a much more complex situation involving a multiplicity of APP fragments and Aβ catabolites. Indeed, biochemical analyses of AD brain deposits and fluids have unraveled an Aβ peptidome consisting of additional Aβ-related species. Such Aβ catabolites could be due to either primary enzymatic cleavages of βAPP or secondary processing of Aβ itself by exopeptidases. Here, we review the diversity of N- and C-terminally truncated Aβ peptides and their biosynthesis and outline their potential function/toxicity. We also highlight their potential as new pharmaceutical targets and biomarkers.

Proteolytic ectodomain shedding of membrane proteins in mammals-hardware, concepts, and recent developments

Proteolytic removal of membrane protein ectodomains (ectodomain shedding) is a post-translational modification that controls levels and function of hundreds of membrane proteins. The contributing proteases, referred to as sheddases, act as important molecular switches in processes ranging from signaling to cell adhesion. When deregulated, ectodomain shedding is linked to pathologies such as inflammation and Alzheimer's disease. While proteases of the "a disintegrin and metalloprotease" (ADAM) and "beta-site APP cleaving enzyme" (BACE) families are widely considered as sheddases, in recent years a much broader range of proteases, including intramembrane and soluble proteases, were shown to catalyze similar cleavage reactions. This review demonstrates that shedding is a fundamental process in cell biology and discusses the current understanding of sheddases and their substrates, molecular mechanisms and cellular localizations, as well as physiological functions of protein ectodomain shedding. Moreover, we provide an operational definition of shedding and highlight recent conceptual advances in the field. While new developments in proteomics facilitate substrate discovery, we expect that shedding is not a rare exception, but rather the rule for many membrane proteins, and that many more interesting shedding functions await discovery.

Modulation of γ- and β-Secretases as Early Prevention Against Alzheimer's Disease

The genetic evidence implicating amyloid-β in the initial stage of Alzheimer's disease is unequivocal. However, the long biochemical and cellular prodromal phases of the disease suggest that dementia is the result of a series of molecular and cellular cascades whose nature and connections remain unknown. Therefore, it is unlikely that treatments directed at amyloid-β will have major clinical effects in the later stages of the disease. We discuss the two major candidate therapeutic targets to lower amyloid-β in a preventive mode, i.e., γ- and β-secretase; the rationale behind these two targets; and the current state of the field.

Dimerization leads to changes in APP (amyloid precursor protein) trafficking mediated by LRP1 and SorLA

Proteolytic cleavage of the amyloid precursor protein (APP) by α-, β- and γ-secretases is a determining factor in Alzheimer's disease (AD). Imbalances in the activity of all three enzymes can result in alterations towards pathogenic Aβ production. Proteolysis of APP is strongly linked to its subcellular localization as the secretases involved are distributed in different cellular compartments. APP has been shown to dimerize in cis-orientation, affecting Aβ production. This might be explained by different substrate properties defined by the APP oligomerization state or alternatively by altered APP monomer/dimer localization. We investigated the latter hypothesis using two different APP dimerization systems in HeLa cells. Dimerization caused a decreased localization of APP to the Golgi and at the plasma membrane, whereas the levels in the ER and in endosomes were increased. Furthermore, we observed via live cell imaging and biochemical analyses that APP dimerization affects its interaction with LRP1 and SorLA, suggesting that APP dimerization modulates its interplay with sorting molecules and in turn its localization and processing. Thus, pharmacological approaches targeting APP oligomerization properties might open novel strategies for treatment of AD.

Disruption of amyloid precursor protein ubiquitination selectively increases amyloid β (Aβ) 40 levels via presenilin 2-mediated cleavage

Amyloid plaques, a neuropathological hallmark of Alzheimer's disease, are largely composed of amyloid β (Aβ) peptide, derived from cleavage of amyloid precursor protein (APP) by β- and γ-secretases. The endosome is increasingly recognized as an important crossroad for APP and these secretases, with major implications for APP processing and amyloidogenesis. Among various post-translational modifications affecting APP accumulation, ubiquitination of cytodomain lysines may represent a key signal controlling APP endosomal sorting. Here, we show that substitution of APP C-terminal lysines with arginine disrupts APP ubiquitination and that an increase in the number of substituted lysines tends to increase APP metabolism. An APP mutant lacking all C-terminal lysines underwent the most pronounced increase in processing, leading to accumulation of both secreted and intracellular Aβ40. Artificial APP ubiquitination with rapalog-mediated proximity inducers reduced Aβ40 generation. A lack of APP C-terminal lysines caused APP redistribution from endosomal intraluminal vesicles (ILVs) to the endosomal limiting membrane, with a subsequent decrease in APP C-terminal fragment (CTF) content in secreted exosomes, but had minimal effects on APP lysosomal degradation. Both the increases in secreted and intracellular Aβ40 were abolished by depletion of presenilin 2 (PSEN2), recently shown to be enriched on the endosomal limiting membrane compared with PSEN1. Our findings demonstrate that ubiquitin can act as a signal at five cytodomain-located lysines for endosomal sorting of APP. They further suggest that disruption of APP endosomal sorting reduces its sequestration in ILVs and results in PSEN2-mediated processing of a larger pool of APP-CTF on the endosomal membrane.

Trafficking in Alzheimer's Disease: Modulation of APP Transport and Processing by the Transmembrane Proteins LRP1, SorLA, SorCS1c, Sortilin, and Calsyntenin

The amyloid precursor protein (APP), one key player in Alzheimer's disease (AD), is extensively processed by different proteases. This leads to the generation of diverging fragments including the amyloid β (Aβ) peptide, which accumulates in brains of AD patients. Subcellular trafficking of APP is an important aspect for its proteolytic conversion, since the various secretases which cleave APP are located in different cellular compartments. As a consequence, altered subcellular targeting of APP is thought to directly affect the degree to which Aβ is generated. The mechanisms underlying intracellular APP transport are critical to understand AD pathogenesis and can serve as a target for future pharmacological interventions. In the recent years, a number of APP interacting proteins were identified which are implicated in sorting of APP, thereby influencing APP processing at different angles of the secretory or endocytic pathway. This review provides an update on the proteolytic processing of APP and the interplay of the transmembrane proteins low-density lipoprotein receptor-related protein 1, sortilin-receptor with A-type repeats, SorCS1c, sortilin, and calsyntenin. We discuss the specific interactions with APP, the capacity to modulate the intracellular itinerary and the proteolytic conversion of APP, a possible involvement in the clearance of Aβ, and the implications of these transmembrane proteins in AD and other neurodegenerative diseases.

Presenilin 2-Dependent Maintenance of Mitochondrial Oxidative Capacity and Morphology

Mitochondrial dysfunction plays a pivotal role in the progression of Alzheimer's disease (AD), and yet the mechanisms underlying the impairment of mitochondrial function in AD remain elusive. Recent evidence suggested a role for Presenilins (PS1 or PS2) in mitochondrial function. Mutations of PSs, the catalytic subunits of the γ-secretase complex, are responsible for the majority of inherited AD cases (FAD). PSs were shown to be present in mitochondria and particularly enriched in mitochondria-associated membranes (MAM), where PS2 is involved in the calcium shuttling between mitochondria and the endoplasmic reticulum (ER). We investigated the precise contribution of PS1 and PS2 to the bioenergetics of the cell and to mitochondrial morphology in cell lines derived from wild type (PS+/+), PS1/2 double knock-out (PSdKO), PS2KO and PS1KO embryos. Our results showed a significant impairment in the respiratory capacity of PSdKO and PS2KO cells with reduction of basal oxygen consumption, oxygen utilization dedicated to ATP production and spare respiratory capacity. In line with these functional defects, we found a decrease in the expression of subunits responsible for mitochondrial oxidative phosphorylation (OXPHOS) associated with an altered morphology of the mitochondrial cristae. This OXPHOS disruption was accompanied by a reduction of the NAD⁺/NADH ratio. Still, neither ADP/ATP ratio nor mitochondrial membrane potential (ΔΨ) were affected, suggesting the existence of a compensatory mechanism for energetic balance. We observed indeed an increase in glycolytic flux in PSdKO and PS2KO cells. All these effects were truly dependent on PS2 since its stable re-expression in a PS2KO background led to a complete restoration of the parameters impaired in the absence of PS2. Our data clearly demonstrate here the crucial role of PS2 in mitochondrial function and cellular bioenergetics, pointing toward its peculiar role in the formation and integrity of the electron transport chain.

Specificity of presenilin-1- and presenilin-2-dependent γ-secretases towards substrate processing

The two presenilin‐1 (PS1) and presenilin‐2 (PS2) homologs are the catalytic core of the γ‐secretase complex, which has a major role in cell fate decision and Alzheimer's disease (AD) progression. Understanding the precise contribution of PS1‐ and PS2‐dependent γ‐secretases to the production of β‐amyloid peptide (Aβ) from amyloid precursor protein (APP) remains an important challenge to design molecules efficiently modulating Aβ release without affecting the processing of other γ‐secretase substrates. To that end, we studied PS1‐ and PS2‐dependent substrate processing in murine cells lacking presenilins (PSs) (PS1KO, PS2KO or PS1‐PS2 double‐KO noted PSdKO) or stably re‐expressing human PS1 or PS2 in an endogenous PS‐null (PSdKO) background. We characterized the processing of APP and Notch on both endogenous and exogenous substrates, and we investigated the effect of pharmacological inhibitors targeting the PSs activity (DAPT and L‐685,458). We found that murine PS1 γ‐secretase plays a predominant role in APP and Notch processing when compared to murine PS2 γ‐secretase. The inhibitors blocked more efficiently murine PS2‐ than murine PS1‐dependent processing. Human PSs, especially human PS1, expression in a PS‐null background efficiently restored APP and Notch processing. Strikingly, and contrary to the results obtained on murine PSs, pharmacological inhibitors appear to preferentially target human PS1‐ than human PS2‐dependent γ‐secretase activity.

Substrate processing in intramembrane proteolysis by γ-secretase - the role of protein dynamics

Intramembrane proteases comprise a number of different membrane proteins with different types of catalytic sites. Their common denominator is cleavage within the plane of the membrane, which usually results in peptide bond scission within the transmembrane helices of their substrates. Despite recent progress in the determination of high-resolution structures, as illustrated here for the γ-secretase complex and its substrate C99, it is still unknown how these enzymes function and how they distinguish between substrates and non-substrates. In principle, substrate/non-substrate discrimination could occur at the level of substrate binding and/or cleavage. Focusing on the γ-secretase/C99 pair, we will discuss recent observations suggesting that global motions within a substrate transmembrane helix may be much more important for defining a substrate than local unraveling at cleavage sites.

Membrane trafficking and proteolytic activity of γ-secretase in Alzheimer’s disease

γ-Secretase is an intramembrane-cleaving protease that generates various forms of amyloid-β peptides (Aβ) that accumulate in the brains of Alzheimer’s disease (AD) patients. The intracellular trafficking and subcellular localization of γ-secretase are linked to both qualitative and quantitative changes in Aβ production. However, the precise intracellular localization of γ-secretase as well as its detailed regulatory mechanisms have remained elusive. Recent genetic studies on AD provide ample evidence that alteration of the subcellular localization of γ-secretase contributes to the pathogenesis of AD. Here we review our current understanding of the intracellular membrane trafficking of γ-secretase, the association between its localization and proteolytic activity, and the possibility of γ-secretase as a therapeutic target against AD.