NDP52 associates with phosphorylated tau in brains of an Alzheimer disease mouse model

Molecular Insights into Aggrephagy: Their Cellular Functions in the Context of Neurodegenerative Diseases

Protein homeostasis or proteostasis is an equilibrium of biosynthetic production, folding and transport of proteins, and their timely and efficient degradation. Proteostasis is guaranteed by a network of protein quality control systems aimed at maintaining the proteome function and avoiding accumulation of potentially cytotoxic proteins. Terminal unfolded and dysfunctional proteins can be directly turned over by the ubiquitin-proteasome system (UPS) or first amassed into aggregates prior to degradation. Aggregates can also be disposed into lysosomes by a selective type of autophagy known as aggrephagy, which relies on a set of so-called selective autophagy receptors (SARs) and adaptor proteins. Failure in eliminating aggregates, also due to defects in aggrephagy, can have devastating effects as underscored by several neurodegenerative diseases or proteinopathies, which are characterized by the accumulation of aggregates mostly formed by a specific disease-associated, aggregate-prone protein depending on the clinical pathology. Despite its medical relevance, however, the process of aggrephagy is far from being understood. Here we review the findings that have helped in assigning a possible function to specific SARs and adaptor proteins in aggrephagy in the context of proteinopathies, and also highlight the interplay between aggrephagy and the pathogenesis of proteinopathies.

The autophagy of stress granules

Our understanding of stress granule (SG) biology has deepened considerably in recent years, and with this, increased understanding of links has been made between SGs and numerous neurodegenerative diseases. One of the proposed mechanisms by which SGs and any associated protein aggregates may become pathological is based upon defects in their autophagic clearance, and so the precise processes governing the degradation of SGs are important to understand. Mutations and disease-associated variants implicated in amyotrophic lateral sclerosis, Huntington's disease, Parkinson's disease and frontotemporal lobar dementia compromise autophagy, whilst autophagy-inhibiting drugs or knockdown of essential autophagy proteins result in the persistence of SGs. In this review, we will consider the current knowledge regarding the autophagy of SG.

Alterations of ATG4A and LC3B in neurons derived from Alzheimer's disease patients

We investigated the alterations in autophagy-related molecules in neurons differentiated from induced pluripotent stem cells obtained from patients with Alzheimer's disease (AD). Consistent with our previous microarray data, ATG4A protein was upregulated in the neurons derived from a familial AD patient with an APP-E693Δ mutation who showed accumulation of intracellular amyloid β peptide (Aβ). This upregulation was reversed by inhibiting Aβ production, suggesting that the intracellular Aβ may be responsible for the upregulation of ATG4A. The LC3B-II/LC3B-I ratio, an index of autophagosome formation, was lower in the neurons derived from the AD patient with APP-E693Δ as well as the neurons derived from other familial and sporadic AD patients. These findings indicate that dysregulation of autophagy-related molecules may accelerate the pathogenesis of AD.

Epigenetic and post-translational modifications in autophagy: biological functions and therapeutic targets

Autophagy is a conserved lysosomal degradation pathway where cellular components are dynamically degraded and re-processed to maintain physical homeostasis. However, the physiological effect of autophagy appears to be multifaced. On the one hand, autophagy functions as a cytoprotective mechanism, protecting against multiple diseases, especially tumor, cardiovascular disorders, and neurodegenerative and infectious disease. Conversely, autophagy may also play a detrimental role via pro-survival effects on cancer cells or cell-killing effects on normal body cells. During disorder onset and progression, the expression levels of autophagy-related regulators and proteins encoded by autophagy-related genes (ATGs) are abnormally regulated, giving rise to imbalanced autophagy flux. However, the detailed mechanisms and molecular events of this process are quite complex. Epigenetic, including DNA methylation, histone modifications and miRNAs, and post-translational modifications, including ubiquitination, phosphorylation and acetylation, precisely manipulate gene expression and protein function, and are strongly correlated with the occurrence and development of multiple diseases. There is substantial evidence that autophagy-relevant regulators and machineries are subjected to epigenetic and post-translational modulation, resulting in alterations in autophagy levels, which subsequently induces disease or affects the therapeutic effectiveness to agents. In this review, we focus on the regulatory mechanisms mediated by epigenetic and post-translational modifications in disease-related autophagy to unveil potential therapeutic targets. In addition, the effect of autophagy on the therapeutic effectiveness of epigenetic drugs or drugs targeting post-translational modification have also been discussed, providing insights into the combination with autophagy activators or inhibitors in the treatment of clinical diseases.

Altered glucose metabolism in Alzheimer's disease: Role of mitochondrial dysfunction and oxidative stress

Increasing evidence suggests that abnormal cerebral glucose metabolism is largely present in Alzheimer's disease (AD). The brain utilizes glucose as its main energy source and a decline in its metabolism directly reflects on brain function. Weighing on recent evidence, here we systematically assessed the aberrant glucose metabolism associated with amyloid beta and phosphorylated tau accumulation in AD brain. Interlink between insulin signaling and AD highlighted the involvement of the IRS/PI3K/Akt/AMPK signaling, and GLUTs in the disease progression. While shedding light on the mitochondrial dysfunction in the defective glucose metabolism, we further assessed functional consequences of AGEs (advanced glycation end products) accumulation, polyol activation, and other contributing factors including terminal respiration, ROS (reactive oxygen species), mitochondrial permeability, PINK1/parkin defects, lysosome-mitochondrial crosstalk, and autophagy/mitophagy. Combined with the classic plaque and tangle pathologies, glucose hypometabolism with acquired insulin resistance and mitochondrial dysfunction potentiate these factors to exacerbate AD pathology. To this end, we further reviewed AD and DM (diabetes mellitus) crosstalk in disease progression. Taken together, the present work discusses the emerging role of altered glucose metabolism, contributing impact of insulin signaling, and mitochondrial dysfunction in the defective cerebral glucose utilization in AD.

Isolation of ferret astrocytes reveals their morphological, transcriptional, and functional differences from mouse astrocytes

Astrocytes play key roles in supporting the central nervous system structure, regulating synaptic functions, and maintaining brain homeostasis. The number of astrocytes in the cerebrum has markedly increased through evolution. However, the manner by which astrocytes change their features during evolution remains unknown. Compared with the rodent brain, the brain of the ferret, a carnivorous animal, has a folded cerebral cortex and higher white to gray matter ratio, which are common features of the human brain. To further clarify the features of ferret astrocytes, we isolated astrocytes from ferret neonatal brains, cultured these cells, and compared their morphology, gene expression, calcium response, and proliferating ability with those of mouse astrocytes. The morphology of cultured ferret astrocytes differed from that of mouse astrocytes. Ferret astrocytes had longer and more branched processes, smaller cell bodies, and different calcium responses to glutamate, as well as had a greater ability to proliferate, compared to mouse astrocytes. RNA sequencing analysis revealed novel ferret astrocyte-specific genes, including several genes that were the same as those in humans. Astrocytes in the ferret brains had larger cell size, longer primary processes in larger numbers, and a higher proliferation rate compared to mouse astrocytes. Our study shows that cultured ferret astrocytes have different features from rodent astrocytes and similar features to human astrocytes, suggesting that they are useful in studying the roles of astrocytes in brain evolution and cognitive functions in higher animals.

Autophagy Balances Neuroinflammation in Alzheimer's Disease

Autophagy is a highly evolutionary conserved process that degrades cytosolic macromolecules or damaged organelles (e.g., mitochondria), as well as intracellular pathogens for energy and survival. Dysfunction of autophagy has been associated with the pathologies of Alzheimer's disease (AD), including Aβ plaques and neurofibrillary tangles. Recently, the presence of sustained immune response in the brain has been considered a new core pathology in AD. Accumulating evidence suggests that autophagy activation may suppress inflammation response through degrading inflammasomes or pro-inflammatory cytokines and improving immune system function in both clinical trials and preclinical studies. This review provides an overview of updated information on autophagy and inflammation and their potential mediators in AD. In summary, we believe that understanding the relationship between autophagy and inflammation will provide insightful knowledge for future therapeutic implications in AD.

NDP52 Protects Against Myocardial Infarction-Provoked Cardiac Anomalies Through Promoting Autophagosome-Lysosome Fusion via Recruiting TBK1 and RAB7

Aims: Acute myocardial infarction (MI), caused by acute coronary artery obstruction, is a common cardiovascular event leading to mortality. Nuclear dot protein 52 (NDP52) is an essential selective autophagy adaptor, although its function in MI is still obscure. This study was designed to examine the function of NDP52 in MI and the associated mechanisms. Results: Our results revealed that MI challenge overtly impaired myocardial geometry and systolic function, along with cardiomyocyte apoptosis, myocardial interstitial fibrosis, and mitochondrial damage, and NDP52 nullified such devastating responses. Further studies showed that the blockade of mitochondrial clearance is related to MI-induced buildup of damaged mitochondria. Mechanistic approaches depicted that 7-day MI induced abnormal mitophagy flux, resulting in poor lysosomal clearance of injured mitochondria. NDP52 promoted mitophagy flux through the recruitment of Ras-associated protein RAB7 (RAB7) and TANK-binding kinase 1 (TBK1). On protein co-localization, TBK1 phosphorylated RAB7, in line with the finding that chloroquine or a TBK1 inhibitor reversed NDP52-dependent beneficial responses. Innovation: This study denoted a novel mechanism that NDP52 promotes cardioprotection against ischemic heart diseases through interaction with TBK1 and RAB7, leading to RAB7 phosphorylation, induction of mitophagy to clear ischemia-induced impaired mitochondria, thus preventing cardiomyocyte apoptosis in MI. Conclusion: Our results indicate that NDP52 promotes autophagic flux and clears damaged mitochondria to diminish reactive oxygen species and cell death in a TBK1/RAB7-dependent manner and thus limits MI-induced injury. Antioxid. Redox Signal. 36, 1119-1135.

Macroautophagy in CNS health and disease

Macroautophagy is an evolutionarily conserved process that delivers diverse cellular contents to lysosomes for degradation. As our understanding of this pathway grows, so does our appreciation for its importance in disorders of the CNS. Once implicated primarily in neurodegenerative events owing to acute injury and ageing, macroautophagy is now also linked to disorders of neurodevelopment, indicating that it is essential for both the formation and maintenance of a healthy CNS. In parallel to understanding the significance of macroautophagy across contexts, we have gained a greater mechanistic insight into its physiological regulation and the breadth of cargoes it can degrade. Macroautophagy is a broadly used homeostatic process, giving rise to questions surrounding how defects in this single pathway could cause diseases with distinct clinical and pathological signatures. To address this complexity, we herein review macroautophagy in the mammalian CNS by examining three key features of the process and its relationship to disease: how it functions at a basal level in the discrete cell types of the brain and spinal cord; which cargoes are being degraded in physiological and pathological settings; and how the different stages of the macroautophagy pathway intersect with diseases of neurodevelopment and adult-onset neurodegeneration.

Compromised autophagy and mitophagy in brain ageing and Alzheimer's diseases

Alzheimer's disease (AD) is one of the most persistent and devastating neurodegenerative disorders of old age, and is characterized clinically by an insidious onset and a gradual, progressive deterioration of cognitive abilities, ranging from loss of memory to impairment of judgement and reasoning. Despite years of research, an effective cure is still not available. Autophagy is the cellular 'garbage' clearance system which plays fundamental roles in neurogenesis, neuronal development and activity, and brain health, including memory and learning. A selective sub-type of autophagy is mitophagy which recognizes and degrades damaged or superfluous mitochondria to maintain a healthy and necessary cellular mitochondrial pool. However, emerging evidence from animal models and human samples suggests an age-dependent reduction of autophagy and mitophagy, which are also compromised in AD. Upregulation of autophagy/mitophagy slows down memory loss and ameliorates clinical features in animal models of AD. In this review, we give an overview of autophagy and mitophagy and their link to the progression of AD. We also summarize approaches to upregulate autophagy/mitophagy. We hypothesize that age-dependent compromised autophagy/mitophagy is a cause of brain ageing and a risk factor for AD, while restoration of autophagy/mitophagy to more youthful levels could return the brain to health.

Autophagy in Alzheimer's disease pathogenesis: Therapeutic potential and future perspectives

Alzheimer's disease (AD) is a complex neurodegenerative disease in the elderly and the most common cause of human dementia. AD is characterized by accumulation of abnormal protein aggregates including amyloid plaques (composed of beta-amyloid (Aβ) peptides) and neurofibrillary tangles (formed by hyper-phosphorylated tau protein). Synaptic plasticity, neuroinflammation, calcium signaling etc. also show dysfunction in AD patients. Autophagy is an evolutionarily conserved lysosome-dependent cellular event in eukaryotes. It is closely linked to modulation of protein metabolism, through which damaged organelles and mis-folded proteins are degraded and then recycled to maintain protein homeostasis. Accumulating evidence has shown that impaired autophagy also contributes to AD pathogenesis. In the present review, we highlight the role of autophagy, including bulk and selective autophagy, in regulating metabolic circuits in AD pathogenesis. We also discuss the potential and future perspectives of autophagy-inducing strategies in AD therapeutics.

Impaired Mitophagy in Neurons and Glial Cells during Aging and Age-Related Disorders

Aging is associated with a decline in cognitive function, which can partly be explained by the accumulation of damage to the brain cells over time. Neurons and glia undergo morphological and ultrastructure changes during aging. Over the past several years, it has become evident that at the cellular level, various hallmarks of an aging brain are closely related to mitophagy. The importance of mitochondria quality and quantity control through mitophagy is highlighted by the contribution that defects in mitochondria-autophagy crosstalk make to aging and age-related diseases. In this review, we analyze some of the more recent findings regarding the study of brain aging and neurodegeneration in the context of mitophagy. We discuss the data on the dynamics of selective autophagy in neurons and glial cells during aging and in the course of neurodegeneration, focusing on three mechanisms of mitophagy: non-receptor-mediated mitophagy, receptor-mediated mitophagy, and transcellular mitophagy. We review the role of mitophagy in neuronal/glial homeostasis and in the molecular pathogenesis of neurodegenerative disorders, such as Parkinson's disease, Alzheimer's disease, and other disorders. Common mechanisms of aging and neurodegeneration that are related to different mitophagy pathways provide a number of promising targets for potential therapeutic agents.

MicroRNA-140 silencing represses the incidence of Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative condition leading to severe disability from progressive impairments in cognitive functions including memory and learning. Non-coding microRNAs (miRNAs or miRs) have been linked to the pathogenesis of AD. The present study aimed to investigate the clinical significance and biological function of miR-140 in AD. First, we examined the expression of miR-140 and PINK1 in brain tissues of the established AD model rats and neurons cultured with Aβ-derived diffusible ligands (AβDDLs). We identified an interaction between miR-140 and PINK1, and measured spatial learning and memory abilities of the model rats using the Morris water maze (MWM) test. After ectopic expression and depletion experiments in neurons and AD rats, we measured the levels of reactive oxygen species (ROS), and mitochondrial membrane potential (MMP), along with mTOR expression and phosphorylation, and autophagy-related factors. Results showed up-regulation of miR-140 and down-regulation of PINK1 in AD model rats and neurons. PINK1 was verified to be a direct target of miR-140, and silencing of miR-140 suppressed mitochondrial dysfunction, and enhanced autophagy in AD model rats and neurons, as supported by decreased levels of mTOR expression and phosphorylation, β-amyloid p-Tau (Ser396), p-Tau (Thr231), Tau and ROS, and increased MMP levels and expression of Beclin 1 expression and LC3-II/LC3-I. Collectively, functional suppression of miR-140 enhanced autophagy and prevented mitochondrial dysfunction by upregulating PINK1, ultimately suggesting a novel therapeutic target for AD.

Transcription factor NRF2 as a promising therapeutic target for Alzheimer's disease

Oxidative stress is considered as one of the pathogenesis of Alzheimer's disease (AD) and plays an important role in the occurrence and development of AD. Nuclear factor erythroid 2 related factor 2 (NRF2) is a key regulatory of oxidative stress defence. There is growing evidence indicating the relationship between NRF2 and AD. NRF2 activation mitigates multiple pathogenic processes involved in AD by upregulating antioxidative defense, inhibiting neuroinflammation, improving mitochondrial function, maintaining proteostasis, and inhibiting ferroptosis. In addition, several NRF2 activators are currently being evaluated as AD therapeutic agents in clinical trials. Thus, targeting NRF2 has been the focus of a new strategy for prevention and treatment of AD. In this review, the role of NRF2 in AD and the NRF2 activators advanced into clinical and preclinical studies will be summarized.

Melatonin and Autophagy in Aging-Related Neurodegenerative Diseases

With aging, the nervous system gradually undergoes degeneration. Increased oxidative stress, endoplasmic reticulum stress, mitochondrial dysfunction, and cell death are considered to be common pathophysiological mechanisms of various neurodegenerative diseases (NDDs) such as Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), organophosphate-induced delayed neuropathy (OPIDN), and amyotrophic lateral sclerosis (ALS). Autophagy is a cellular basic metabolic process that degrades the aggregated or misfolded proteins and abnormal organelles in cells. The abnormal regulation of neuronal autophagy is accompanied by the accumulation and deposition of irregular proteins, leading to changes in neuron homeostasis and neurodegeneration. Autophagy exhibits both a protective mechanism and a damage pathway related to programmed cell death. Because of its "double-edged sword", autophagy plays an important role in neurological damage and NDDs including AD, PD, HD, OPIDN, and ALS. Melatonin is a neuroendocrine hormone mainly synthesized in the pineal gland and exhibits a wide range of biological functions, such as sleep control, regulating circadian rhythm, immune enhancement, metabolism regulation, antioxidant, anti-aging, and anti-tumor effects. It can prevent cell death, reduce inflammation, block calcium channels, etc. In this review, we briefly discuss the neuroprotective role of melatonin against various NDDs via regulating autophagy, which could be a new field for future translational research and clinical studies to discover preventive or therapeutic agents for many NDDs.

Exploring the bi-directional relationship between autophagy and Alzheimer's disease

Alzheimer's disease (AD) is characterized by β-amyloid (Aβ) deposition and Tau phosphorylation, in which its pathogenesis has not been cleared so far. The metabolism of Aβ and Tau is critically affected by the autophagy. Abnormal autophagy is thought to be involved in the pathogenesis of AD, regulating autophagy may become a new strategy for AD treatment. In the early stage of AD, the presence of Aβ and Tau can induce autophagy to promote their clearance by means of mTOR-dependent and independent manners. As AD progress, the autophagy goes aberrant. As a result, Aβ and Tau generate continually, which aggravates both autophagy dysfunction and AD. Besides, several related genes and proteins of AD can also adapt autophagy to make an effect on the AD development. There seems to be a bi-directional relationship between AD pathology and autophagy. At present, this article reviews this relationship from these aspects: (a) the signaling pathways of regulating autophagy; (b) the relationships between the autophagy and the processing of Aβ; (c) Aβ and Tau cause autophagy dysfunction; (d) normal autophagy promotes the clearance of Aβ and Tau; (e) the relationships between the autophagy and both genes and proteins related to AD: TFEB, miRNAs, Beclin-1, Presenilin, and Nrf2; and (f) the small molecules regulating autophagy on AD therapy. All of the above may help to further elucidate the pathogenesis of AD and provide a theoretical basis for clinical treatment of AD.

Molecular Mechanism of Autophagy: Its Role in the Therapy of Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disorder of progressive dementia that is characterized by the accumulation of beta-amyloid (Aβ)-containing neuritic plaques and intracellular Tau protein tangles. This distinctive pathology indicates that the protein quality control is compromised in AD. Autophagy functions as a "neuronal housekeeper" that eliminates aberrant protein aggregates by wrapping then into autophagosomes and delivering them to lysosomes for degradation. Several studies have suggested that autophagy deficits in autophagy participate in the accumulation and propagation of misfolded proteins (including Aβ and Tau). In this review, we summarize current knowledge of autophagy in the pathogenesis of AD, as well as some pathways targeting the restoration of autophagy. Moreover, we discuss how these aspects can contribute to the development of disease-modifying therapies in AD.

Selective Autophagy Receptors in Neuronal Health and Disease

Neurons are electrically excitable, postmitotic cells that perform sensory, relaying, and motor functions. Because of their unique morphological and functional specialization, cells of this type are sensitive to the stress caused by accumulation of misfolded proteins or damaged organelles. Autophagy is the fundamental mechanism that ensures sequestration of cytosolic material and its subsequent degradation in lysosomes of eukaryotic cells, thereby providing cell-autonomous nutrients and removing harmful cargos. Strikingly, mice and flies lacking functional autophagy develop early onset progressive neurodegeneration. Like in human neurodegenerative diseases (NDDs)-Alzheimer's disease, Parkinson's disease, frontotemporal dementia, Huntington's disease, and amyotrophic lateral sclerosis-characteristic protein aggregates observed in autophagy-deficient neurons in the animal models are indicators of the ongoing neuronal pathology. A number of selective autophagy receptors (SARs) have been characterized that interact both with the cargo and components of the autophagic machinery, thus providing the molecular basis for selective degradation of sizable cytosolic components. Interference with autophagy in experimental models, but also during the pathological vagaries in neurons, will thus have far-reaching consequences for a range of selective autophagy pathways critical for the normal functioning of the nervous system. Here, we review the key principles behind the selective autophagy and discuss how the SARs may be involved in the pathogenesis of NDDs. Using recently published examples, we also examine the emerging role of less well studied selective autophagy pathways in neuronal health and disease. We conclude by discussing targeting selective autophagy as an emerging therapeutic modality in NDDs.

Tau Clearance Mechanisms

Efficient quality control mechanisms are essential for a healthy, functional neuron. Recognition and degradation of misfolded, damaged, or potentially toxic proteins, is a crucial aspect of protein quality control. Tau is a protein that is highly expressed in neurons, and plays an important role in modulating a number of physiological processes. Maintaining appropriate levels of tau is key for neuronal health; hence perturbations in tau clearance mechanisms are likely significant contributors to neurodegenerative diseases such as Alzheimer's disease and frontotemporal lobar degeneration. In this chapter we will first briefly review the two primary degradative mechanisms that mediate tau clearance: the proteasome system and the autophagy-lysosome pathway. This will be followed by a discussion about what is known about the contribution of each of these pathways to tau clearance. We will also present recent findings on tau degradation through the endolysosomal system. Further, how deficits in these degradative systems may contribute to the accumulation of dysfunctional or toxic forms of tau in neurodegenerative conditions is considered.

Dendrobium nobile Lindl. Alkaloids Decreases the Level of Intracellular β-Amyloid by Improving Impaired Autolysosomal Proteolysis in APP/PS1 Mice

As the major degradation pathway for long-lived proteins and organelles, macroautophagy is a decisive factor for the survival and longevity of cells. The existing evidence indicates that the disruption of substrate proteolysis in autolysosomes is the main mechanism underlying autophagy failure in Alzheimer’s disease (AD). Thus, the restoration of normal lysosomal proteolysis and autophagy efficiency is a novel therapeutic strategy in the treatment of AD. In this study, 9-month-old APPswe/PS1ΔE9 transgenic (APP/PS1) mice were administered Dendrobium nobile Lindl. alkaloids (DNLA, 40 and 80 mg/kg) or Metformin (80 mg/kg), and age-matched wild-type mice were administered an isovolumic vehicle orally once a day for 4 months. The results demonstrated that DNLA significantly improved learning and memory function in APP/PS1 transgenic mice in the Morris water maze. Furthermore, DNLA could increase the expression of the v-ATPase A1 subunit to facilitate lysosomal acidification, prompt the dissociation of the cation independent-mannose-phosphate receptor from cathepsin (cat) D, promote the proteolytic maturation of cat D, increase the degradation of accumulated autophagic vacuoles (AVs) and β-amyloid (Aβ) contained in the AVs, and alleviate neuronal and synaptic injury. These findings demonstrate that DNLA improves learning and memory function in APP/PS1 mice, and the mechanisms appear to be due to the promotion of intracellular Aβ degradation by increasing the protein level of v-ATPase A1 and then improving autolysosomal acidification and proteolysis.

Nrf2 mediates the expression of BAG3 and autophagy cargo adaptor proteins and tau clearance in an age-dependent manner

During aging, decreased efficiency of nuclear factor (erythroid-derived 2)-like 2 (Nrf2) activation and autophagic processes in the brain may be a contributing factor in the pathogenesis of neurodegenerative diseases such as Alzheimer's disease. Therefore, we analyzed the expression of Bcl-2-associated athanogene 3, a cochaperone that mediates autophagy, and the autophagy adaptors NBR1, NDP52, and sequestosome 1/p62 in the brains of 4-, 8-, and 12-month-old wild-type and Nrf2 knockout (-/-) mice. We also analyzed the levels of total tau and phospho-tau species. There were minimal differences in the expression of autophagy-related genes or tau species in 4-month-old animals; however, by 12 months, all of these autophagy-associated genes were expressed at significantly lower levels in the Nrf2 (-/-) mice. The decreases in the autophagy-associated genes were accompanied by significantly elevated levels of phospho-tau species in the 12-month-old Nrf2 (-/-) brains. These findings indicate that Nrf2 regulation of autophagy-related genes likely plays a greater role in mediating the clearance of tau as an organism ages.

Autophagy in neurodegenerative diseases: pathogenesis and therapy

The most prevalent pathological features of many neurodegenerative diseases are the aggregation of misfolded proteins and the loss of certain neuronal populations. Autophagy, as major intracellular machinery for degrading aggregated proteins and damaged organelles, has been reported to be involved in the occurrence of pathological changes in many neurodegenerative disorders, including Alzheimer's disease, Parkinson's disease, Huntington's disease and amyotrophic lateral sclerosis. In this review, we summarize most recent research progress in this topic and provide a new perspective regarding autophagy regulation on the pathogenesis of neurodegenerative diseases. Finally, we discuss the signaling molecules in autophagy-related pathways as therapeutic targets for the treatment of these diseases.

The fine-tuning of proteolytic pathways in Alzheimer's disease

Several integrated proteolytic systems contribute to the maintenance of cellular homeostasis through the continuous removal of misfolded, aggregated or oxidized proteins and damaged organelles. Among these systems, the proteasome and autophagy play the major role in protein quality control, which is a fundamental issue in non-proliferative cells such as neurons. Disturbances in the functionality of these two pathways are frequently observed in neurodegenerative diseases, like Alzheimer's disease, and reflect the accumulation of protease-resistant, deleterious protein aggregates. In this review, we explored the sophisticated crosstalk between the ubiquitin-proteasome system and autophagy in the removal of the harmful structures that characterize Alzheimer's disease neurons. We also dissected the role of the numerous shuttle factors and chaperones that, directly or indirectly interacting with ubiquitin and LC3, are used for cargo selection and delivery to one pathway or the other.

Alzheimer's disease genes and autophagy

Autophagy is a process to degrade and recycle cellular constituents via the lysosome for regulating cellular homeostasis. Its dysfunction is now considered to be involved in many diseases, including neurodegenerative diseases. Many features reflecting autophagy impairment, such as autophagosome accumulation and lysosomal dysfunction, have been also revealed to be involved in Alzheimer's disease (AD). Recent genetic studies such as genome-wide association studies in AD have identified a number of novel genes associated with AD. Some of the identified genes have demonstrated dysfunction in autophagic processes in AD, while others remain under investigation. Since autophagy is strongly regarded to be one of the major pathogenic mechanisms of AD, it is necessary to review how the AD-associated genes are related to autophagy. We anticipate our current review to be a starting point for future studies regarding AD-associated genes and autophagy. This article is part of a Special Issue entitled SI:Autophagy.