The topology of pen-2, a γ-secretase subunit, revisited: evidence for a reentrant loop and a single pass transmembrane domain

Functional and topological analysis of PSENEN, the fourth subunit of the γ-secretase complex

The γ-secretase complexes are intramembrane cleaving proteases involved in the generation of the Aβ peptides in Alzheimer's disease. The complex consists of four subunits, with Presenilin harboring the catalytic site. Here, we study the role of the smallest subunit, PSENEN or Presenilin enhancer 2, encoded by the gene Psenen, in vivo and in vitro. We find a profound Notch deficiency phenotype in Psenen-/- embryos confirming the essential role of PSENEN in the γ-secretase complex. We used Psenen-/- fibroblasts to explore the structure-function of PSENEN by the scanning cysteine accessibility method. Glycine 22 and proline 27, which border the membrane domains 1 and 2 of PSENEN, are involved in complex formation and stabilization of γ-secretase. The hairpin structured hydrophobic membrane domains 1 and 2 are exposed to a water-containing cavity in the complex, while transmembrane domain 3 is not water exposed. We finally demonstrate the essential role of PSENEN for the cleavage activity of the complex. PSENEN is more than a structural component of the γ-secretase complex and might contribute to the catalytic mechanism of the enzyme.

The critical role of γ-secretase and its inhibitors in cancer and cancer therapeutics

As a multi-substrate transmembrane protease, γ-secretase exists widely in various cells. It controls multiple important cellular activities through substrate cleavage. γ-secretase inhibitors (GSIs) play a role in cancer inhibition by blocking Notch cleavage, and are considered as potential therapeutic strategies for cancer. Currently, GSIs have encouraging performance in preclinical models, yet this success does not translate well in clinical trials. In recent years, a number of breakthrough discoveries have shown us the promise of targeting γ-secretase for the treatment of cancer. Here, we integrate a large amount of data from γ-secretase and its inhibitors and cancer in nearly 30 years, comb and discuss the close connection between γ-secretase and cancer, as well as the potential and problems of current GSIs in cancer treatment. We analyze the possible reasons for the failure performance of current GSIs in clinical trials, and make recommendations for future research areas.

BACE1 SUMOylation deregulates phosphorylation and ubiquitination in Alzheimer's disease pathology

BACE1 is essential for the generation of amyloid-β (Aβ) that likely initiates the toxicity in Alzheimer's disease (AD). BACE1 activity is mainly regulated by post-translational modifications, but the relationship between these modifications is not fully characterized. Here, we studied the effects of BACE1 SUMOylation on its phosphorylation and ubiquitination. We demonstrate that SUMOylation of BACE1 inhibits its phosphorylation at S498 and its ubiquitination in vitro. Conversely, BACE1 phosphorylation at S498 suppresses its SUMOylation, which results in promoting BACE1 degradation in vitro. Furthermore, an increase in BACE1 SUMOylation is associated with the progression of AD pathology, while its phosphorylation and ubiquitination are decreased in an AD mouse model. Our findings suggest that BACE1 SUMOylation reciprocally influences its phosphorylation and competes against its ubiquitination, which might provide a new insight into the regulations of BACE1 activity and Aβ accumulation.

Tangeretin Inhibits BACE1 Activity and Attenuates Cognitive Impairments in AD Model Mice

Tangeretin (TAN) exhibits many bioactivities, including neuroprotective effects. However, the efficacy of TAN in Alzheimer's disease (AD) has not been sufficiently investigated. In the present study, we integrated behavioral tests, pathology assessment, and biochemical analyses to elucidate the antidementia activity of TAN in APPswe/PSEN1dE9 transgenic (Tg) mice. At supplementation levels of 100 mg/kg body weight per day, TAN significantly attenuated the cognitive impairment of Tg mice in behavioral tests. These effects were associated with less synaptic impairments and fewer β-amyloid accumulations after TAN administration. Furthermore, our study revealed that TAN possessed powerful inhibitory activity against β-secretase both in vitro and in vivo, which played a crucial role in the process of Aβ generation. These findings indicate that TAN is a potential drug for preventing AD pathology. The key mechanism underlying the antidementia effect of TAN may include its inhibitory activity against β-secretase.

Mutations in γ-secretase subunit-encoding PSENEN gene alone may not be sufficient for the development of acne inversa

BACKGROUND

The effects of PSENEN mutations in patients with acne inversa (AI) are poorly understood. Hyperproliferation of follicular keratinocytes and resulting occlusion may constitute the initial pathophysiology.

OBJECTIVE

To investigate the effects of PSENEN knockdown on γ-secretase subunits, biological behaviors, and related signaling pathways in keratinocytes.

METHODS

HaCaT cells were divided into an experimental group (PSENEN knock down), a negative control group, and a blank control group. Whole transcriptome sequencing was used to measure differences in mRNA expression of the whole genome; real-time PCR and Western blotting were performed to determine the interference efficiency and the effects of interference on the components of γ-secretase and related molecules. CCK-8 was used to measure cell proliferation, and flow cytometry was used to measure apoptosis and the cell cycle.

RESULTS

A comparison of five healthy controls with three patients with PSENEN mutation (c.66delG, c.279delC, c.229_230insCACC) revealed decreased expression of mRNA and protein in skin lesions of the experimental group. In this group, expression of the other components of γ-secretase presenilin C-terminal fragment decreased, expression of immature nicastrin increased, expression of mature nicastrin decreased, and expression of anterior pharynx defective-1 remained unchanged. KEGG analysis revealed that differentially expressed molecules were enriched in m-TOR signaling pathways. Subsequent verification confirmed that differences in PI3K-AKT-mTOR signaling pathway molecules, cell proliferation, apoptosis, cell cycle and the expression levels of Ki-67, KRT1, and IVL between the groups were not statistically significant.

CONCLUSIONS

PSENEN mutations alone may be insufficient to cause the development of AI, or they may only induce a mild phenotype of AI.

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

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

Unraveling the complexity of γ-secretase

γ-Secretase was initially defined as a proteolytic activity that cleaves within the transmembrane of the amyloid precursor protein (APP) to produce the amyloid β-peptide of Alzheimer's disease. The discovery of mutations in APP and the presenilins associated with familial Alzheimer's disease and their effects on APP processing dovetailed with pharmacological studies on γ-secretase, leading to the revelation that presenilins are unprecedented membrane-embedded aspartyl proteases. Other members of what became known as the γ-secretase complex were subsequently identified. In parallel with these advances, connections between presenilins and Notch receptors essential to metazoan development became evident, resulting in the concurrent realization that γ-secretase also carries out intramembrane proteolysis of Notch as part of its signaling mechanism. Substantial progress has been made toward elucidating how γ-secretase carries out complex processing of transmembrane domains, how it goes awry in familial Alzheimer's disease, the scope of its substrates, and the atomic details of its structure. Critical questions remain for future study, toward further unraveling the complexity of this unique membrane-embedded proteolytic machine and its roles in biology and disease.

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.

Structural and Chemical Biology of Presenilin Complexes

The presenilin proteins are the catalytic subunits of a tetrameric complex containing presenilin 1 or 2, anterior pharynx defective 1 (APH1), nicastrin, and PEN-2. Other components such as TMP21 may exist in a subset of specialized complexes. The presenilin complex is the founding member of a unique class of aspartyl proteases that catalyze the γ, ɛ, ζ site cleavage of the transmembrane domains of Type I membrane proteins including amyloid precursor protein (APP) and Notch. Here, we detail the structural and chemical biology of this unusual enzyme. Taken together, these studies suggest that the complex exists in several conformations, and subtle long-range (allosteric) shifts in the conformation of the complex underpin substrate access to the catalytic site and the mechanism of action for allosteric inhibitors and modulators. Understanding the mechanics of these shifts will facilitate the design of γ-secretase modulator (GSM) compounds that modulate the relative efficiency of γ, ɛ, ζ site cleavage and/or substrate specificity.

Characterizing the structural ensemble of γ-secretase using a multiscale molecular dynamics approach

γ-Secretase is an intramembrane-cleaving aspartyl protease that plays an essential role in the processing of a variety of integral membrane proteins. Its role in the ultimate cleavage step in the processing of amyloid precursor protein to form amyloid-β (Aβ) peptide makes it an important therapeutic target in Alzheimer's disease research. Significant recent advances have been made in structural studies of this critical membrane protein complex. However, details of the mechanism of activation of the enzyme complex remain unclear. Using a multiscale computational modeling approach, combining multiple coarse-grained microsecond dynamic trajectories with all-atom models, the structure and two conformational states of the γ-secretase complex were evaluated. The transition between enzymatic state 1 and state 2 is shown to critically depend on the protonation states of the key catalytic residues Asp257 and Asp385 in the active site domain. The active site formation, related to our γ-secretase state 2, is observed to involve a concerted movement of four transmembrane helices from the catalytic subunit, resulting in the required localization of the catalytic residues. Global analysis of the structural ensemble of the enzyme complex was used to identify collective fluctuations important to the mechanism of substrate recognition and demonstrate that the corresponding fluctuations observed were uncorrelated with structural changes associated with enzyme activation. Overall, this computational study provides essential insight into the role of structure and dynamics in the activation and function of γ-secretase.

Cryo-EM structures of human γ-secretase

γ-secretase, a membrane-embedded aspartate protease, catalyzes peptide bond hydrolysis of a large variety of type I integral membrane proteins exemplified by amyloid precursor protein (APP). Cleavage of APP leads to formation of β-amyloid plaque, which is a hallmark of Alzheimer's disease (AD). Over 200 AD-associated mutations are mapped to presenilin 1 (PS1), the catalytic component of γ-secretase. In the past three years, several cryo-electron microscopy (cryo-EM) structures of human γ-secretase have been determined at near atomic resolutions. Here we summarize the methods involved and discuss structural features of γ-secretase and the associated functional insights.

Physiological and pathological roles of the γ-secretase complex

Gamma-secretase (GS) is an enzyme complex that cleaves numerous substrates, and it is best known for cleaving amyloid precursor protein (APP) to form amyloid-beta (Aβ) peptides. Aberrant cleavage of APP can lead to Alzheimer's disease, so much research has been done to better understand GS structure and function in hopes of developing therapeutics for Alzheimer's. Therefore, most of the attention in this field has been focused on developing modulators that reduce pathogenic forms of Aβ while leaving Notch and other GS substrates intact, but GS provides multiple avenues of modulation that could improve AD pathology. GS has complex regulation, through its essential subunits and other associated proteins, providing other targets for AD drugs. Therapeutics can also alter GS trafficking and thereby improve cognition, or move beyond Aβ entirely, effecting Notch and neural stem cells. GS also cleaves substrates that affect synaptic morphology and function, presenting another window by which GS modulation could improve AD pathology. Taken together, GS presents a unique cross road for neural processes and an ideal target for AD therapeutics.

Implications of GABAergic Neurotransmission in Alzheimer's Disease

Alzheimer's disease (AD) is characterized pathologically by the deposition of β-amyloid peptides (Aβ) and the accumulation of neurofibrillary tangles (NFTs) composed of hyper-phosphorylated tau. Regardless of the pathological hallmarks, synaptic dysfunction is widely accepted as a causal event in AD. Of the two major types of synapses in the central nervous system (CNS): glutamatergic and GABAergic, which provide excitatory and inhibitory outputs respectively, abundant data implicate an impaired glutamatergic system during disease progression. However, emerging evidence supports the notion that disrupted default neuronal network underlies impaired memory, and that alterations of GABAergic circuits, either plays a primary role or as a compensatory response to excitotoxicity, may also contribute to AD by disrupting the overall network function. The goal of this review is to provide an overview of the involvement of Aβ, tau and apolipoprotein E4 (apoE4), the major genetic risk factor in late-onset AD (LOAD), in GABAergic neurotransmission and the potential of modulating the GABAergic function as AD therapy.