Molecular mechanism of intramembrane proteolysis by γ-secretase

Transient sleep apnea results in long-lasting increase in β-amyloid generation and tau hyperphosphorylation

Sleep apnea is regarded as an important risk factor in the pathogenesis of Alzheimer disease (AD). Chronic intermittent hypoxia treatment (IHT) given during the sleep period of the circadian cycle in experimental animals is a well-established sleep apnea model. Here we report that transient IHT for 4 days on AD model mice causes Aβ overproduction 2 months after IHT presumably via upregulation of synaptic BACE1, side-by-side with tau hyperphosphorylation. These results suggest that even transient IHT may be sufficient to cause long-lasting changes in the molecules measured as AD biomarkers in the brain.

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.

Microbiota dysbiosis caused by dietetic patterns as a promoter of Alzheimer's disease through metabolic syndrome mechanisms

Microbiota dysbiosis and metabolic syndrome, consequences of a non-adequate diet, generate a feedback pathogenic state implicated in Alzheimer's disease development. The lower production of short chain fatty acids (SCFAs) under dysbiosis status leads to lipid homeostasis deregulation and decreases Angptl4 release and AMPK activation in the adipose tissue, promoting higher lipid storage (adipocyte hypertrophy) and cholesterol levels. Also, low SCFA generation reduces GPR41 and GPR43 receptor activation at the adipose tissue (increasing leptin release and leptin receptor resistance) and intestinal levels, reducing the release of GLP-1 and YPP. Therefore, lower satiety sensation and energy expenditure occur, promoting a weight gaining environment mediated by higher food intake and lipid storage, developing dyslipemia. In this context, higher glucose levels, together with higher free fatty acids in the bloodstream, promote glycolipotoxicity, provoking a reduction in insulin released, insulin receptor resistance, advanced glycation products (AGEs) and type 2 diabetes. Intestinal dysbiosis and low SCFAs reduce bacterial biodiversity, increasing lipopolysaccharide (LPS)-producing bacteria and intestinal barrier permeability. Higher amounts of LPS pass to the bloodstream (endotoxemia), causing a low-grade chronic inflammatory state characterized by higher levels of leptin, IL-1β, IL-6 and TNF-α, together with a reduced release of adiponectin and IL-10. At the brain and neuronal levels, the generated insulin resistance, low-grade chronic inflammation, leptin resistance, AGE production and LPS increase directly impact the secretase enzymes and tau hyperphosphorylation, creating an enabling environment for β-amyloid senile plaque and tau tangled formations and, as a consequence, Alzheimer's initiation, development and maintenance.

A Metalloproteinase Cocktail from the Venom of Protobothrops flavoviridis Cleaves Amyloid Beta Peptides at the α-Cleavage Site

A disintegrin and metalloproteinase (ADAM) family proteins are a major class of membrane-anchored multidomain proteinases that are responsible for the shedding of cell surface protein ectodomains, including amyloid precursor protein (APP). Human ADAM 9, 10, and 17 proteolyze APPs and produce non-amyloid-genic p3 peptides, instead of neurotoxic amyloid-β peptides (Aβs; Aβ40 and Aβ42), which form fibrils and accumulate in the brain of patients with Alzheimer's disease (AD). The ADAM family is closely related to snake venom metalloproteinases (SVMPs), which are derived from ancestral ADAMs but act as soluble proteinases. To test the therapeutic potential of SVMPs, we purified SVMPs from Protobothrops flavoviridis venom using metal ion affinity and pooled into a cocktail. Thus, 9 out of 11 SVMPs in the P. flavoviridis genome were identified in the cocktail. SVMPs inhibited Aβ secretion when added to human cell culture medium without affecting APP proteolysis. SVMPs degraded synthetic Aβ40 and Aβ42 peptides at the same cleavage site (α-site of APP) as ADAM9, 10, and 17. SVMPs did not degrade Aβ fibrils but interfered with their formation, assessed using thioflavin-T. Thus, SVMPs have therapeutic potential for AD as an Aβ-degrading protease, and the finding adds to the discovery of bioactive peptides from venoms as novel therapeutics.

Alzheimer's Disease: Significant Benefit from the Yeast-Based Models

Alzheimer's disease (AD) is an age-related, multifaceted neurological disorder associated with accumulation of aggregated proteins (amyloid Aβ and hyperphosphorylated tau), loss of synapses and neurons, and alterations in microglia. AD was recognized by the World Health Organization as a global public health priority. The pursuit of a better understanding of AD forced researchers to pay attention to well-defined single-celled yeasts. Yeasts, despite obvious limitations in application to neuroscience, show high preservation of basic biological processes with all eukaryotic organisms and offer great advantages over other disease models due to the simplicity, high growth rates on low-cost substrates, relatively simple genetic manipulations, the large knowledge base and data collections, and availability of an unprecedented amount of genomic and proteomic toolboxes and high-throughput screening techniques, inaccessible to higher organisms. Research reviewed above clearly indicates that yeast models, together with other, more simple eukaryotic models including animal models, C. elegans and Drosophila, significantly contributed to understanding Aβ and tau biology. These models allowed high throughput screening of factors and drugs that interfere with Aβ oligomerization, aggregation and toxicity, and tau hyperphosphorylation. In the future, yeast models will remain relevant, with a focus on creating novel high throughput systems to facilitate the identification of the earliest AD biomarkers among different cellular networks in order to achieve the main goal-to develop new promising therapeutic strategies to treat or prevent the disease.

Role of Microbiota-Modified Bile Acids in the Regulation of Intracellular Organelles and Neurodegenerative Diseases

Bile acids (BAs) are amphiphilic steroidal molecules generated from cholesterol in the liver and facilitate the digestion and absorption of fat-soluble substances in the gut. Some BAs in the intestine are modified by the gut microbiota. Because BAs are modified in a variety of ways by different types of bacteria present in the gut microbiota, changes in the gut microbiota can affect the metabolism of BAs in the host. Although most BAs absorbed from the gut are transferred to the liver, some are transferred to the systemic circulation. Furthermore, BAs have also been detected in the brain and are thought to migrate into the brain through the systemic circulation. Although BAs are known to affect a variety of physiological functions by acting as ligands for various nuclear and cell-surface receptors, BAs have also been found to act on mitochondria and autophagy in the cell. This review focuses on the BAs modified by the gut microbiota and their roles in intracellular organelles and neurodegenerative diseases.

Specific Mutations near the Amyloid Precursor Protein Cleavage Site Increase γ-Secretase Sensitivity and Modulate Amyloid-β Production

Amyloid-β peptides (Aβs) are produced via cleavage of the transmembrane region of the amyloid precursor protein (APP) by γ-secretase and are responsible for Alzheimer's disease. Familial Alzheimer's disease (FAD) is associated with APP mutations that disrupt the cleavage reaction and increase the production of neurotoxic Aβs, i.e., Aβ42 and Aβ43. Study of the mutations that activate and restore the cleavage of FAD mutants is necessary to understand the mechanism of Aβ production. In this study, using a yeast reconstruction system, we revealed that one of the APP FAD mutations, T714I, severely reduced the cleavage, and identified secondary APP mutations that restored the cleavage of APP T714I. Some mutants were able to modulate Aβ production by changing the proportions of Aβ species when introduced into mammalian cells. Secondary mutations include proline and aspartate residues; proline mutations are thought to act through helical structural destabilization, while aspartate mutations are thought to promote interactions in the substrate binding pocket. Our results elucidate the APP cleavage mechanism and could facilitate drug discovery.

[Identification of novel regulators involved in AD pathogenesis using the CRISPR-Cas9 system]

The production of amyloid β peptide (Aβ) is an important process relating to the pathogenesis of Alzheimer disease (AD). It is widely known that the sequential cleavage of amyloid precursor protein (APP) by β- and γ-secretases lead to the production of Aβ. However, the precise regulatory mechanism for Aβ production remains unclear. We have established a CRISPR-Cas9 based screening system to identify the novel regulators of Aβ production. Calcium and integrin-binding protein 1 (CIB1) was identified as a novel potential negative regulator of Aβ production. The knockdown and knockout of Cib1 significantly increased Aβ levels. In addition, immunoprecipitation showed that CIB1 interacts with the γ-secretase complex but did not alter its enzymatic activity. Moreover, Cib1 disruption specifically reduced the cell-surface localization of the γ-secretase complex. Finally, the single-cell RNA-seq analysis in the human brain demonstrated that early-stage AD patients have lower neuronal CIB1 mRNA levels compared to healthy controls. Taken together, we have shown that CIB1 controls the subcellular localization of γ-secretase, resulting in the regulation of Aβ production, suggesting the involvement of CIB1 in the development of AD pathogenesis.

Knockdown of Amyloid Precursor Protein: Biological Consequences and Clinical Opportunities

Amyloid precursor protein (APP) and its cleavage fragment Amyloid-β (Aβ) have fundamental roles in Alzheimer's disease (AD). Genetic alterations that either increase the overall dosage of APP or alter its processing to favour the generation of longer, more aggregation prone Aβ species, are directly causative of the disease. People living with one copy of APP are asymptomatic and reducing APP has been shown to lower the relative production of aggregation-prone Aβ species in vitro. For these reasons, reducing APP expression is an attractive approach for AD treatment and prevention. In this review, we will describe the structure and the known functions of APP and go on to discuss the biological consequences of APP knockdown and knockout in model systems. We highlight progress in therapeutic strategies to reverse AD pathology via reducing APP expression. We conclude that new technologies that reduce the dosage of APP expression may allow disease modification and slow clinical progression, delaying or even preventing onset.

Specific Mutations in Aph1 Cause γ-Secretase Activation

Amyloid beta peptides (Aβs) are generated from amyloid precursor protein (APP) through multiple cleavage steps mediated by γ-secretase, including endoproteolysis and carboxypeptidase-like trimming. The generation of neurotoxic Aβ42/43 species is enhanced by familial Alzheimer's disease (FAD) mutations within the catalytic subunit of γ-secretase, presenilin 1 (PS1). FAD mutations of PS1 cause partial loss-of-function and decrease the cleavage activity. Activating mutations, which have the opposite effect of FAD mutations, are important for studying Aβ production. Aph1 is a regulatory subunit of γ-secretase; it is presumed to function as a scaffold of the complex. In this study, we identified Aph1 mutations that are active in the absence of nicastrin (NCT) using a yeast γ-secretase assay. We analyzed these Aph1 mutations in the presence of NCT; we found that the L30F/T164A mutation is activating. When introduced in mouse embryonic fibroblasts, the mutation enhanced cleavage. The Aph1 mutants produced more short and long Aβs than did the wild-type Aph1, without an apparent modulatory function. The mutants did not change the amount of γ-secretase complex, suggesting that L30F/T164A enhances catalytic activity. Our results provide insights into the regulatory function of Aph1 in γ-secretase activity.

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

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

APPsα rescues impaired Ca2+ homeostasis in APP- and APLP2-deficient hippocampal neurons

Alterations in Ca2+ homeostasis have been reported in several in vitro and in vivo studies using mice expressing the Alzheimer's disease-associated transgenes, presenilin and the amyloid precursor protein (APP). While intense research focused on amyloid-β-mediated functions on neuronal Ca2+ handling, the physiological role of APP and its close homolog APLP2 is still not fully clarified. We now elucidate a mechanism to show how APP and its homolog APLP2 control neuronal Ca2+ handling and identify especially the ectodomain APPsα as an essential regulator of Ca2+ homeostasis. Importantly, we demonstrate that the loss of APP and APLP2, but not APLP2 alone, impairs Ca2+ handling, the refill of the endoplasmic reticulum Ca2+ stores, and synaptic plasticity due to altered function and expression of the SERCA-ATPase and expression of store-operated Ca2+ channel-associated proteins Stim1 and Stim2. Long-term AAV-mediated expression of APPsα, but not acute application of the recombinant protein, restored physiological Ca2+ homeostasis and synaptic plasticity in APP/APLP2 cDKO cultures. Overall, our analysis reveals an essential role of the APP family and especially of the ectodomain APPsα in Ca2+ homeostasis, thereby highlighting its therapeutic potential.

Calcium Dyshomeostasis in Alzheimer's Disease Pathogenesis

Alzheimer’s disease (AD) is the most common age-related neurodegenerative disorder that is characterized by amyloid β-protein deposition in senile plaques, neurofibrillary tangles consisting of abnormally phosphorylated tau protein, and neuronal loss leading to cognitive decline and dementia. Despite extensive research, the exact mechanisms underlying AD remain unknown and effective treatment is not available. Many hypotheses have been proposed to explain AD pathophysiology; however, there is general consensus that the abnormal aggregation of the amyloid β peptide (Aβ) is the initial event triggering a pathogenic cascade of degenerating events in cholinergic neurons. The dysregulation of calcium homeostasis has been studied considerably to clarify the mechanisms of neurodegeneration induced by Aβ. Intracellular calcium acts as a second messenger and plays a key role in the regulation of neuronal functions, such as neural growth and differentiation, action potential, and synaptic plasticity. The calcium hypothesis of AD posits that activation of the amyloidogenic pathway affects neuronal Ca²⁺ homeostasis and the mechanisms responsible for learning and memory. Aβ can disrupt Ca²⁺ signaling through several mechanisms, by increasing the influx of Ca²⁺ from the extracellular space and by activating its release from intracellular stores. Here, we review the different molecular mechanisms and receptors involved in calcium dysregulation in AD and possible therapeutic strategies for improving the treatment.

CRISPR-activated patient fibroblasts for modeling of familial Alzheimer's disease

Analyzing an appropriate disease model system is important to conduct disease research. Analyzing cells obtained from patient tissues could not only help elucidate the pathological mechanisms and to develop novel therapy but also lead to personalized medicine in the future. However, it is generally difficult to collect and culture neuronal cells from patients suffering from neurodegenerative disorders. Skin fibroblasts are easier to collect than neurons but may not show the expected pathology when disease-relevant genes are not sufficiently expressed. In this article, I describe an in vitro model system that enables the facile analysis of neurological disease mechanisms in patient fibroblast cultures by CRISPR transcriptional activation of endogenous disease-relevant genes. This system introduces an additional platform to analyze neurodegenerative disorders.

Sequential conformational changes in transmembrane domains of presenilin 1 in Aβ42 downregulation

Alzheimer disease (AD) is the most common neurodegenerative disease worldwide. AD is pathologically characterized by the deposition of senile plaques in the brain, which are composed of an amyloid-β peptide (Aβ) that is produced through the multistep cleavage of amyloid precursor protein (APP) by γ-secretase. γ-Secretase is a membrane protein complex, which includes its catalytic subunit presenilin 1 (PS1). However, much about the structural dynamics of this enzyme remain unclear. We have previously demonstrated that movements of the transmembrane domain (TMD) 1 and TMD3 of PS1 are strongly associated with decreased production of the Aβ peptide ending at the 42nd residue (i.e., Aβ42), which is the aggregation-prone, toxic species. However, the association between these movements as well as the sequence of these TMDs remains unclear. In this study, we raised the possibility that the vertical movement of TMD1 is a prerequisite for expansion of the catalytic cavity around TMD3 of PS1, resulting in reduced Aβ42 production. Our results shed light on the association between the conformational changes of TMDs and the regulation of γ-secretase activity.

EphA4 regulates Aβ production via BACE1 expression in neurons

Several lines of evidence suggest that the aggregation and deposition of amyloid-β peptide (Aβ) initiate the pathology of Alzheimer's disease (AD). Recently, a genome-wide association study demonstrated that a single-nucleotide polymorphism proximal to the EPHA4 gene, which encodes a receptor tyrosine kinase, is associated with AD risk. However, the molecular mechanism of EphA4 in the pathogenesis of AD, particularly in Aβ production, remains unknown. Here, we performed several pharmacological and biological experiments both in vitro and in vivo and demonstrated that EphA4 is responsible for the regulation of Aβ production. Pharmacological inhibition of EphA4 signaling and knockdown of Epha4 led to increased Aβ levels accompanied by increased expression of β-site APP cleaving enzyme 1 (BACE1), which is an enzyme responsible for Aβ production. Moreover, EPHA4 overexpression and activation of EphA4 signaling via ephrin ligands decreased Aβ levels. In particular, the sterile-alpha motif domain of EphA4 was necessary for the regulation of Aβ production. Finally, EPHA4 mRNA levels were significantly reduced in the brains of AD patients, and negatively correlated with BACE1 mRNA levels. Our results indicate a novel mechanism of Aβ regulation by EphA4, which is involved in AD pathogenesis.

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.

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.

Distinct anterograde trafficking pathways of BACE1 and amyloid precursor protein from the TGN and the regulation of amyloid-β production

Processing of amyloid precursor protein (APP) by the β-secretase BACE1 is the initial step of the amyloidogenic pathway to generate amyloid-β (Aβ). Although newly synthesized BACE1 and APP are transported along the secretory pathway, it is not known whether BACE1 and APP share the same post-Golgi trafficking pathways or are partitioned into different transport routes. Here we demonstrate that BACE1 exits the Golgi in HeLa cells and primary neurons by a pathway distinct from the trafficking pathway for APP. By using the Retention Using Selective Hooks system, we show that BACE1 is transported from the trans-Golgi network to the plasma membrane in an AP-1- and Arf1/4-dependent manner. Subsequently, BACE1 is endocytosed to early and recycling endosomes. Perturbation of BACE1 post-Golgi trafficking results in an increase in BACE1 cleavage of APP and increased production of both Aβ40 and Aβ42. These findings reveal that Golgi exit of BACE1 and APP in primary neurons is tightly regulated, resulting in their segregation along different transport routes, which limits APP processing.

Effect of atorvastatin on Aβ1-42 -induced alteration of SESN2, SIRT1, LC3II and TPP1 protein expressions in neuronal cell cultures

OBJECTIVES

Sestrins (SESNs) and sirtuins (SIRTs) are antioxidant and antiapoptotic genes and crucial mediators for lysosomal autophagy regulation that play a pivotal role in the Alzheimer's disease (AD). Recently, statins have been linked to the reduced prevalence of AD in statin-prescribed populations yet molecular basis for the neuroprotective action of statins is still under debate.

METHODS

This study was undertaken whether Aβ-induced changes of SESN2 and SIRT1 protein expression, autophagy marker LC3II and lysosomal enzyme TPP1 affected by atorvastatin (Western blot) and its possible role in Aβ neurotoxicity (ELISA).

KEY FINDINGS/RESULTS

We showed that SESN2 and LC3II expressions were elevated, whereas SIRT1 and TPP1 expressions were decreased in the Aβ_1-42 -exposed human neuroblastoma cells (SH-SY5Y). Co-administration of atorvastatin with Aβ_1-42 compensates SESN2 increase and recovers SIRT1 decline by reducing oxidative stress, decreasing SESN2 expression and increasing SIRT1 expression by its neuroprotective action. Atorvastatin induced LC3II but not TPP1 level in the Aβ_1-42 -exposed cells suggested that atorvastatin is effective in the formation of autophagosome but not on the expression of the specific lysosomal enzyme TPP1.

DISCUSSION AND CONCLUSION

Together, these results indicate that atorvastatin induced SESN2, SIRT1 and LC3II levels play a protective role against Aβ_1-42 neurotoxicity.

Conformational Dynamics of Transmembrane Domain 3 of Presenilin 1 Is Associated with the Trimming Activity of γ-Secretase

γ-Secretase is an intramembrane-cleaving protease that generates the toxic species of the amyloid-β peptide (Aβ) that is responsible for the pathology of Alzheimer disease. The catalytic subunit of γ-secretase is presenilin 1 (PS1), which is a polytopic membrane protein with a hydrophilic catalytic pore. The length of the C terminus of Aβ is proteolytically determined by its processive trimming by γ-secretase, although the precise mechanism still remains largely unknown. Here, we identified that transmembrane domain (TMD) 3 of human PS1 is involved in the formation of the intramembranous hydrophilic pore. Notably, the water accessibility of TMD3 was greatly altered by point mutations and compounds, which modify γ-secretase activity. The changes in the water accessibility of TMD3 was also correlated with Aβ42 production. Moreover, crosslinking between TMD3 and TMD7 resulted in a loss of sensitivity to a γ-secretase modulator that reduces Aβ42 production. Therefore, our findings indicate that the conformational dynamics of TMD3 is a prerequisite for regulation of the Aβ trimming activity of γ-secretase.SIGNIFICANCE STATEMENT Modulation of γ-secretase activity to reduce the level of toxic amyloid-β species is thought to be a therapeutic strategy for Alzheimer disease. However, the detailed mechanism of the regulation of amyloid-β production, as well as the structure-and-activity relationship of γ-secretase remains unclear. Here we identified that the water accessibility around transmembrane domain 3 in presenilin 1 was increased along with a reduction in toxic amyloid-β production. Our findings demonstrate how the structure of presenilin 1 dynamically changes during amyloid-β production, and provides insights toward the development of treatments against Alzheimer disease.

Insights into membrane-bound presenilin 2 from all-atom molecular dynamics simulations

Presenilins 1 and 2 (PS1 or PS2) are main genetic risk factors of familial Alzheimer's disease (AD) that produce the β-amyloid (Aβ) peptides and also have important stand-alone functions related to, e.g. calcium signaling. Most work so far has focused on PS1, but humans carry both PS1 and PS2, and mutations in both cause AD. Here, we develop a computational model of PS2 in the membrane to address the question how pathogenic PS2 mutations affect the membrane-embedded protein. The models are based on cryo-electron microscopy structures of PS1 translated to PS2, augmented with missing residues and a complete all-atom membrane-water system, and equilibrated using three independent 500-ns simulations of molecular dynamics with a structure-balanced force field. We show that the nine-transmembrane channel structure is substantially controlled by major dynamics in the hydrophilic loop bridging TM6 and TM7, which functions as a 'plug' in the PS2 membrane channel. TM2, TM6, TM7 and TM9 flexibility controls the size of this channel. We find that most pathogenic PS2 mutations significantly reduce stability relative to random mutations, using a statistical ANOVA test with all possible mutations in the affected sites as a control. The associated loss of compactness may also impair calcium affinity. Remarkably, similar properties of the open state are known to impair the binding of substrates to γ-secretase, and we thus argue that the two mechanisms could be functionally related.Communicated by Ramaswamy H. Sarma.

The Biosynthesis, Signaling, and Neurological Functions of Bile Acids

Bile acids (BA) are amphipathic steroid acids synthesized from cholesterol in the liver. They act as detergents to expedite the digestion and absorption of dietary lipids and lipophilic vitamins. BA are also considered to be signaling molecules, being ligands of nuclear and cell-surface receptors, including farnesoid X receptor and Takeda G-protein receptor 5. Moreover, BA also activate ion channels, including the bile acid-sensitive ion channel and epithelial Na⁺ channel. BA regulate glucose and lipid metabolism by activating these receptors in peripheral tissues, such as the liver and brown and white adipose tissue. Recently, 20 different BA have been identified in the central nervous system. Furthermore, BA affect the function of neurotransmitter receptors, such as the muscarinic acetylcholine receptor and γ-aminobutyric acid receptor. BA are also known to be protective against neurodegeneration. Here, we review recent findings regarding the biosynthesis, signaling, and neurological functions of BA.

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.

The role of membrane trafficking in the processing of amyloid precursor protein and production of amyloid peptides in Alzheimer's disease

Alzheimer's disease (AD) is characterized by progressive accumulation of misfolded proteins, which form senile plaques and neurofibrillary tangles, and the release of inflammatory mediators by innate immune responses. β-Amyloid peptide (Aβ) is derived from sequential processing of the amyloid precursor protein (APP) by membrane-bound proteases, namely the β-secretase, BACE1, and γ-secretase. Membrane trafficking plays a key role in the regulation of APP processing as both APP and the processing secretases traffic along distinct pathways. Genome wide sequencing studies have identified several AD susceptibility genes which regulate membrane trafficking events. To understand the pathogenesis of AD it is critical that the cell biology of APP and Aβ production in neurons is well defined. This review discusses recent advances in unravelling the membrane trafficking events associated with the production of Aβ, and how AD susceptible alleles may perturb the sorting and transport of APP and BACE1. Mechanisms whereby inflammation may influence APP processing are also considered.

Advanced Yeast Models of Familial Alzheimer Disease Expressing FAD-Linked Presenilin to Screen Mutations and γ-Secretase Modulators

γ-Secretase is a multisubunit membrane protein complex containing catalytic presenilin (PS1 or PS2) and cofactors such as nicastrin, Aph-1, and Pen2. γ-Secretase hydrolyzes the transmembrane domains of type-I membrane proteins, which include the amyloid precursor protein (APP). APP is cleaved by γ-secretase to produce amyloid β peptide (Aβ), which is deposited in the brains of Alzheimer disease patients. However, the mechanism of this unusual proteolytic process within the lipid bilayer remains unknown. We have established a yeast transcriptional activator Gal4p system with artificial γ-secretase substrates containing APP or Notch fragments to examine the enzymatic properties of γ-secretase. The γ-secretase activities were evaluated by transcriptional activation of reporter genes upon Gal4 release from the membrane bound substrates as assessed by growth of yeast or β-galactosidase assay. We also established an in vitro yeast microsome assay system which identified different Aβ species produced by trimming. The yeast system allows for the screening of mutations and chemicals that inhibit or modulate γ-secretase activity. Herein we describe the genetic and biochemical methods used to analyze γ-secretase activity using the yeast reconstitution system. By studying the loss-of-function properties of PS1 mutants, it is possible to successfully screen FAD suppressor mutations and identify γ-secretase modulators (GSMs), which are promising Alzheimer disease therapeutic agents.

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

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

OPEN PRACTICES

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

CRISPR Transcriptional Activation Analysis Unmasks an Occult γ-Secretase Processivity Defect in Familial Alzheimer's Disease Skin Fibroblasts

Mutations in presenilin (PSEN) 1 and 2, which encode components of the γ-secretase (GS) complex, cause familial Alzheimer's disease (FAD). It is hypothesized that altered GS-mediated processing of the amyloid precursor protein (APP) to the Aβ42 fragment, which is accumulated in diseased brain, may be pathogenic. Here, we describe an in vitro model system that enables the facile analysis of neuronal disease mechanisms in non-neuronal patient cells using CRISPR gene activation of endogenous disease-relevant genes. In FAD patient-derived fibroblast cultures, CRISPR activation of APP or BACE unmasked an occult processivity defect in downstream GS-mediated carboxypeptidase cleavage of APP, ultimately leading to higher Aβ42 levels. These data suggest that, selectively in neurons, relatively high levels of BACE1 activity lead to substrate pressure on FAD-mutant GS complexes, promoting CNS Aβ42 accumulation. Our results introduce an additional platform for analysis of neurological disease.

Structure of APP-C991-99 and implications for role of extra-membrane domains in function and oligomerization

The 99 amino acid C-terminal fragment of Amyloid Precursor Protein APP-C99 (C99) is cleaved by γ-secretase to form Aβ peptide, which plays a critical role in the etiology of Alzheimer's Disease (AD). The structure of C99 consists of a single transmembrane domain flanked by intra and intercellular domains. While the structure of the transmembrane domain has been well characterized, little is known about the structure of the flanking domains and their role in C99 processing by γ-secretase. To gain insight into the structure of full-length C99, REMD simulations were performed for monomeric C99 in model membranes of varying thickness. We find equilibrium ensembles of C99 from simulation agree with experimentally-inferred residue insertion depths and protein backbone chemical shifts. In thin membranes, the transmembrane domain structure is correlated with extra-membrane structural states and the extra-membrane domain structural states become less correlated to each other. Mean and variance of the transmembrane and G₃₇G₃₈ hinge angles are found to increase with thinning membrane. The N-terminus of C99 forms β-strands that may seed aggregation of Aβ on the membrane surface, promoting amyloid formation. In thicker membranes the N-terminus forms α-helices that interact with the nicastrin domain of γ-secretase. The C-terminus of C99 becomes more α-helical as the membrane thickens, forming structures that may be suitable for binding by cytoplasmic proteins, while C-terminal residues essential to cytotoxic function become α-helical as the membrane thins. The heterogeneous but discrete extra-membrane domain states analyzed here open the path to new investigations of the role of C99 structure and membrane in amyloidogenesis. This article is part of a Special Issue entitled: Protein Aggregation and Misfolding at the Cell Membrane Interface edited by Ayyalusamy Ramamoorthy.

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

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

Genome-wide screen of gamma-secretase-mediated intramembrane cleavage of receptor tyrosine kinases

Receptor tyrosine kinases (RTKs) have been demonstrated to signal via regulated intramembrane proteolysis, in which ectodomain shedding and subsequent intramembrane cleavage by gamma-secretase leads to release of a soluble intracellular receptor fragment with functional activity. For most RTKs, however, it is unknown whether they can exploit this new signaling mechanism. Here we used a system-wide screen to address the frequency of susceptibility to gamma-secretase cleavage among human RTKs. The screen covering 45 of the 55 human RTKs identified 12 new as well as all nine previously published gamma-secretase substrates. We biochemically validated the screen by demonstrating that the release of a soluble intracellular fragment from endogenous AXL was dependent on the sheddase disintegrin and metalloprotease 10 (ADAM10) and the gamma-secretase component presenilin-1. Functional analysis of the cleavable RTKs indicated that proliferation promoted by overexpression of the TAM family members AXL or TYRO3 depends on gamma-secretase cleavage. Taken together, these data indicate that gamma-secretase-mediated cleavage provides an additional signaling mechanism for numerous human RTKs.

Not just amyloid: physiological functions of the amyloid precursor protein family

Amyloid precursor protein (APP) gives rise to the amyloid-β peptide and thus has a key role in the pathogenesis of Alzheimer disease. By contrast, the physiological functions of APP and the closely related APP-like proteins (APLPs) remain less well understood. Studying these physiological functions has been challenging and has required a careful long-term strategy, including the analysis of different App-knockout and Aplp-knockout mice. In this Review, we summarize these findings, focusing on the in vivo roles of APP family members and their processing products for CNS development, synapse formation and function, brain injury and neuroprotection, as well as ageing. In addition, we discuss the implications of APP physiology for therapeutic approaches.

Memantine reduces the production of amyloid-β peptides through modulation of amyloid precursor protein trafficking

Memantine, an uncompetitive glutamatergic N-methyl-D-aspartate (NMDA) receptor antagonist, is widely used as medication for the treatment of Alzheimer's disease (AD). It has been reported that memantine reduces amyloid-β peptide (Aβ) levels in both neuronal cultures and in brains of animal models of AD. However, the underlying mechanism of these effects is unclear. Here we examined the effect of memantine on Aβ production. Memantine was administered to 9-month-old Tg2576 mice, a transgenic mouse model of AD, at 10 or 20mg/kg/day in drinking water for 1 month. Memantine significantly reduced the amounts of both CHAPS-soluble and CHAPS-insoluble Aβ in the brains of Tg2576 mice. Memantine at 10mg/kg/day for 1 month also reduced the levels of insoluble Aβ42 in the brains of aged F344 rats. Moreover, memantine reduced Aβ and sAPPβ levels in conditioned media from rat primary cortical cultures without affecting the enzymatic activities of α-secretase, β-secretase, or γ-secretase. Notably, in a cell-surface biotinylation assay, memantine increased the amount of amyloid precursor protein (APP) at the cell surface without changing the total amount of APP. Collectively, our results indicate that chronic treatment with memantine reduces the levels of Aβ both in AD models and in aged animals, and that memantine affects the endocytosis pathway of APP, which is required for β-secretase-mediated cleavage. This leads to a reduction in Aβ production. These results suggest that memantine reduces Aβ production and plaque deposition through the regulation of intracellular trafficking of APP.

Future Therapeutics in Alzheimer's Disease: Development Status of BACE Inhibitors

Alzheimer's disease (AD) is the primary cause of dementia in the elderly. It remains incurable and poses a huge socio-economic challenge for developed countries with an aging population. AD manifests by progressive decline in cognitive functions and alterations in behaviour, which are the result of the extensive degeneration of brain neurons. The AD pathogenic mechanism involves the accumulation of amyloid beta peptide (Aβ), an aggregating protein fragment that self-associates to form neurotoxic fibrils that trigger a cascade of cellular events leading to neuronal injury and death. Researchers from academia and the pharmaceutical industry have pursued a rational approach to AD drug discovery and targeted the amyloid cascade. Schemes have been devised to prevent the overproduction and accumulation of Aβ in the brain. The extensive efforts of the past 20 years have been translated into bringing new drugs to advanced clinical trials. The most progressed mechanism-based therapies to date consist of immunological interventions to clear Aβ oligomers, and pharmacological drugs to inhibit the secretase enzymes that produce Aβ, namely β-site amyloid precursor-cleaving enzyme (BACE) and γ-secretase. After giving an update on the development and current status of new AD therapeutics, this review will focus on BACE inhibitors and, in particular, will discuss the prospects of verubecestat (MK-8931), which has reached phase III clinical trials.

Aberrant proteolytic processing and therapeutic strategies in Alzheimer disease

Amyloid-β peptide (Aβ) and tau are major components of senile plaques and neurofibrillary tangles, respectively, deposited in the brains of Alzheimer disease (AD) patients. Aβ is derived from amyloid-β precursor protein that is sequentially cleaved by two aspartate proteases, β- and γ-secretases. Secreted Aβ is then catabolized by several proteases. Several lines of evidence suggest that accumulation of Aβ by increased production or decreased degradation induces the tau-mediated neuronal toxicity and symptomatic manifestations of AD. Thus, the dynamics of cerebral Aβ, called as "Aβ economy", would be the mechanistic basis of AD pathogenesis. Partial loss of γ-secretase activity leads to the increased generation of toxic Aβ isoforms, indicating that activation of γ-secretase would provide a beneficial effect for AD. After extensive discovery and development efforts, BACE1, which is a β-secretase enzyme, has emerged as a prime drug target for lowering brain Aβ levels. Recent studies revealed the decreased clearance of Aβ in sporadic AD patients, suggesting the importance of the catabolic mechanism in the pathogenesis of AD. I will discuss with these proteolytic mechanisms involved in the regulation of Aβ economy, and development of effective treatment and diagnostics for AD.

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.

Cell-free methods to produce structurally intact mammalian membrane proteins

The crystal structures of four membrane proteins, from bacteria or a unicellular alga, have been solved with samples produced by cell-free protein synthesis. In this study, for mammalian membrane protein production, we established the precipitating and soluble membrane fragment methods: membrane proteins are synthesized with the Escherichia coli cell-free system in the presence of large and small membrane fragments, respectively, and are simultaneously integrated into the lipid environments. We applied the precipitating membrane fragment method to produce various mammalian membrane proteins, including human claudins, glucosylceramide synthase, and the γ-secretase subunits. These proteins were produced at levels of about 0.1–1.0 mg per ml cell-free reaction under the initial conditions, and were obtained as precipitates by ultracentrifugation. Larger amounts of membrane proteins were produced by the soluble membrane fragment method, collected in the ultracentrifugation supernatants, and purified directly by column chromatography. For several proteins, the conditions of the membrane fragment methods were further optimized, such as by the addition of specific lipids/detergents. The functional and structural integrities of the purified proteins were confirmed by analyses of their ligand binding activities, size-exclusion chromatography profiles, and/or thermal stabilities. We successfully obtained high-quality crystals of the complex of human claudin-4 with an enterotoxin.

Conformational Changes in Transmembrane Domain 4 of Presenilin 1 Are Associated with Altered Amyloid-β 42 Production

γ-Secretase is an intramembrane-cleaving protease that produces amyloid-β peptide 42 (Aβ42), which is the toxic and aggregation-prone species of Aβ that causes Alzheimer's disease. Here, we used the substituted cysteine accessibility method to analyze the structure of transmembrane domains (TMDs) 4 and 5 of human presenilin 1 (PS1), a catalytic subunit of γ-secretase. We revealed that TMD4 and TMD5 face the intramembranous hydrophilic milieu together with TMD1, TMD6, TMD7, and TMD9 of PS1 to form the catalytic pore structure. Notably, we found a correlation in the distance between the cytosolic sides of TMD4/TMD7 and Aβ42 production levels, suggesting that allosteric conformational changes of the cytosolic side of TMD4 affect Aβ42-generating γ-secretase activity. Our results provide new insights into the relationship between the structure and activity of human PS1.

SIGNIFICANCE STATEMENT

Modulation of γ-secretase activity to reduce toxic amyloid-β peptide species is one plausible therapeutic approaches for Alzheimer's disease. However, precise mechanistic information of γ-secretase still remains unclear. Here we identified the conformational changes in transmembrane domains of presenilin 1 that affect the proteolytic activity of the γ-secretase. Our results highlight the importance of understanding the structural dynamics of presenilin 1 in drug development against Alzheimer's disease.

TRC8-dependent degradation of hepatitis C virus immature core protein regulates viral propagation and pathogenesis

Signal-peptide peptidase (SPP) is an intramembrane protease that participates in the production of the mature core protein of hepatitis C virus (HCV). Here we show that SPP inhibition reduces the production of infectious HCV particles and pathogenesis. The immature core protein produced in SPP-knockout cells or by treatment with an SPP inhibitor is quickly degraded by the ubiquitin–proteasome pathway. Oral administration of the SPP inhibitor to transgenic mice expressing HCV core protein (CoreTg) reduces the expression of core protein and ameliorates insulin resistance and liver steatosis. Moreover, the haploinsufficiency of SPP in CoreTg has similar effects. TRC8, an E3 ubiquitin ligase, is required for the degradation of the immature core protein. The expression of the HCV core protein alters endoplasmic reticulum (ER) distribution and induces ER stress in SPP/TRC8 double-knockout cells. These data suggest that HCV utilizes SPP cleavage to circumvent the induction of ER stress in host cells.

A cellular protease, SPP, participates in production of the mature core protein of hepatitis C virus (HCV). Here, the authors show in mouse models that SPP inhibition reduces viral propagation and pathogenesis via proteasomal degradation of the immature core protein mediated by the E3 ubiquitin ligase TRC8.

Suppressor Mutations for Presenilin 1 Familial Alzheimer Disease Mutants Modulate γ-Secretase Activities

γ-Secretase is a multisubunit membrane protein complex containing presenilin (PS1) as a catalytic subunit. Familial Alzheimer disease (FAD) mutations within PS1 were analyzed in yeast cells artificially expressing membrane-bound substrate, amyloid precursor protein, or Notch fused to Gal4 transcriptional activator. The FAD mutations, L166P and G384A (Leu-166 to Pro and Gly-384 to Ala substitution, respectively), were loss-of-function in yeast. We identified five amino acid substitutions that suppress the FAD mutations. The cleavage of amyloid precursor protein or Notch was recovered by the secondary mutations. We also found that secondary mutations alone activated the γ-secretase activity. FAD mutants with suppressor mutations, L432M or S438P within TMD9 together with a missense mutation in the second or sixth loops, regained γ-secretase activity when introduced into presenilin null mouse fibroblasts. Notably, the cells with suppressor mutants produced a decreased amount of Aβ42, which is responsible for Alzheimer disease. These results indicate that the yeast system is useful to screen for mutations and chemicals that modulate γ-secretase activity.

The Function of Autophagy in Neurodegenerative Diseases

Macroautophagy, hereafter referred to as autophagy, is a bulk degradation process performed by lysosomes in which aggregated and altered proteins as well as dysfunctional organelles are decomposed. Autophagy is a basic cellular process that maintains homeostasis and is crucial for postmitotic neurons. Thus, impaired autophagic processes in neurons lead to improper homeostasis and neurodegeneration. Recent studies have suggested that impairments of the autophagic process are associated with several neurodegenerative diseases, such as Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, amyotrophic lateral sclerosis, and static encephalopathy of childhood with neurodegeneration in adulthood. In this review, we focus on the recent findings regarding the autophagic process and the involvement of autophagy in neurodegenerative diseases.