Dynein mutations impair autophagic clearance of aggregate-prone proteins

Clearing the brain's cobwebs: the role of autophagy in neuroprotection

Protein aggregates or inclusion bodies are common hallmarks of age-related neurodegenerative disorders. Why these aggregates form remains unclear. Equally debated is whether they are toxic, protective, or simple by-products. Increasing evidence, however, supports the notion that in general aggregates confer toxicity and disturb neuronal function by hampering axonal transport, synaptic integrity, transcriptional regulation, and mitochondrial function. Thus, neuroscientists in search of effective treatments to slow neural loss during neurodegeneration have long been interested in finding new ways to clear inclusion bodies. Intriguingly, two studies using conditional neuron-specific gene ablations of autophagy regulators in mice revealed that autophagy loss elicits inclusion body formation and a neurodegenerative cascade.Such studies indicate autophagy may be a built-in defense mechanism to clear the nervous system of inclusion bodies.This new finding has implications for our understanding of aging and neurodegeneration and the development of new therapies. First, we discuss the pathways underlying autophagy and its controversial role in cell death and survival regulation.We then discuss the physiological role of autophagy in the aging process of the nervous system. In the final portion of this review, we discuss the therapeutic promise of inducing autophagy and the potential side effects of such treatments.

Regulation of amyloid precursor protein processing by the Beclin 1 complex

Autophagy is an intracellular degradation pathway that functions in protein and organelle turnover in response to starvation and cellular stress. Autophagy is initiated by the formation of a complex containing Beclin 1 (BECN1) and its binding partner Phosphoinositide-3-kinase, class 3 (PIK3C3). Recently, BECN1 deficiency was shown to enhance the pathology of a mouse model of Alzheimer Disease (AD). However, the mechanism by which BECN1 or autophagy mediate these effects are unknown. Here, we report that the levels of Amyloid precursor protein (APP) and its metabolites can be reduced through autophagy activation, indicating that they are a substrate for autophagy. Furthermore, we find that knockdown of Becn1 in cell culture increases the levels of APP and its metabolites. Accumulation of APP and APP C-terminal fragments (APP-CTF) are accompanied by impaired autophagosomal clearance. Pharmacological inhibition of autophagosomal-lysosomal degradation causes a comparable accumulation of APP and APP-metabolites in autophagosomes. Becn1 reduction in cell culture leads to lower levels of its binding partner Pik3c3 and increased presence of Microtubule-associated protein 1, light chain 3 (LC3). Overexpression of Becn1, on the other hand, reduces cellular APP levels. In line with these observations, we detected less BECN1 and PIK3C3 but more LC3 protein in brains of AD patients. We conclude that BECN1 regulates APP processing and turnover. BECN1 is involved in autophagy initiation and autophagosome clearance. Accordingly, BECN1 deficiency disrupts cellular autophagy and autophagosomal-lysosomal degradation and alters APP metabolism. Together, our findings suggest that autophagy and the BECN1-PIK3C3 complex regulate APP processing and play an important role in AD pathology.

Regulation of amyloid precursor protein processing by the Beclin 1 complex

Autophagy is an intracellular degradation pathway that functions in protein and organelle turnover in response to starvation and cellular stress. Autophagy is initiated by the formation of a complex containing Beclin 1 (BECN1) and its binding partner Phosphoinositide-3-kinase, class 3 (PIK3C3). Recently, BECN1 deficiency was shown to enhance the pathology of a mouse model of Alzheimer Disease (AD). However, the mechanism by which BECN1 or autophagy mediate these effects are unknown. Here, we report that the levels of Amyloid precursor protein (APP) and its metabolites can be reduced through autophagy activation, indicating that they are a substrate for autophagy. Furthermore, we find that knockdown of Becn1 in cell culture increases the levels of APP and its metabolites. Accumulation of APP and APP C-terminal fragments (APP-CTF) are accompanied by impaired autophagosomal clearance. Pharmacological inhibition of autophagosomal-lysosomal degradation causes a comparable accumulation of APP and APP-metabolites in autophagosomes. Becn1 reduction in cell culture leads to lower levels of its binding partner Pik3c3 and increased presence of Microtubule-associated protein 1, light chain 3 (LC3). Overexpression of Becn1, on the other hand, reduces cellular APP levels. In line with these observations, we detected less BECN1 and PIK3C3 but more LC3 protein in brains of AD patients. We conclude that BECN1 regulates APP processing and turnover. BECN1 is involved in autophagy initiation and autophagosome clearance. Accordingly, BECN1 deficiency disrupts cellular autophagy and autophagosomal-lysosomal degradation and alters APP metabolism. Together, our findings suggest that autophagy and the BECN1-PIK3C3 complex regulate APP processing and play an important role in AD pathology.

Retrograde axonal transport: pathways to cell death?

Active transport along the axon is crucial to the neuron. Motor-driven transport supplies the distal synapse with newly synthesized proteins and lipids, and clears damaged or misfolded proteins. Microtubule motors also drive long-distance signaling along the axon via signaling endosomes. Although positive signaling initiated by neurotrophic factors has been well-studied, recent research has focused on stress-signaling along the axon. Here, the connections between axonal transport alterations and neurodegeneration are discussed, including evidence for defective transport of vesicles, mitochondria, degradative organelles, and signaling endosomes in models of amyotrophic lateral sclerosis, Huntington's, Parkinson's and Alzheimer's disease. Defects in transport are sufficient to induce neurodegeneration, but recent progress suggests that changes in retrograde signaling pathways correlate with rapidly progressive neuronal cell death.

Autophagy in health and disease. 5. Mitophagy as a way of life

Our understanding of autophagy has expanded greatly in recent years, largely due to the identification of the many genes involved in the process and to the development of better methods to monitor the process, such as green fluorescent protein-LC3 to visualize autophagosomes in vivo. A number of groups have demonstrated a tight connection between autophagy and mitochondrial turnover. Mitochondrial quality control is the process whereby mitochondria undergo successive rounds of fusion and fission with a dynamic exchange of components to segregate functional and damaged elements. Removal of the mitochondrion that contains damaged components is accomplished via autophagy (mitophagy). Mitophagy also serves to eliminate the subset of mitochondria producing the most reactive oxygen species, and episodic removal of mitochondria will reduce the oxidative burden, thus linking the mitochondrial free radical theory of aging with longevity achieved through caloric restriction. Mitophagy must be balanced by biogenesis to meet tissue energy needs, but the system is tunable and highly dynamic. This process is of greatest importance in long-lived cells such as cardiomyocytes, neurons, and memory T cells. Autophagy is known to decrease with age, and the failure to maintain mitochondrial quality control through mitophagy may explain why the heart, brain, and components of the immune system are most vulnerable to dysfunction as organisms age.

Inhibition of hsp70 by methylene blue affects signaling protein function and ubiquitination and modulates polyglutamine protein degradation

The Hsp90/Hsp70-based chaperone machinery regulates the activity and degradation of many signaling proteins. Cycling with Hsp90 stabilizes client proteins, whereas Hsp70 interacts with chaperone-dependent E3 ubiquitin ligases to promote protein degradation. To probe these actions, small molecule inhibitors of Hsp70 would be extremely useful; however, few have been identified. Here we test the effects of methylene blue, a recently described inhibitor of Hsp70 ATPase activity, in three well established systems of increasing complexity. First, we demonstrate that methylene blue inhibits the ability of the purified Hsp90/Hsp70-based chaperone machinery to enable ligand binding by the glucocorticoid receptor and show that this effect is due to specific inhibition of Hsp70. Next, we establish that ubiquitination of neuronal nitric-oxide synthase by the native ubiquitinating system of reticulocyte lysate is dependent upon both Hsp70 and the E3 ubiquitin ligase CHIP and is blocked by methylene blue. Finally, we demonstrate that methylene blue impairs degradation of the polyglutamine expanded androgen receptor, an Hsp90 client mutated in spinal and bulbar muscular atrophy. In contrast, degradation of an amino-terminal fragment of the receptor, which lacks the ligand binding domain and, therefore, is not a client of the Hsp90/Hsp70-based chaperone machinery, is enhanced through homeostatic induction of autophagy that occurs when Hsp70-dependent proteasomal degradation is inhibited by methylene blue. Our data demonstrate the utility of methylene blue in defining Hsp70-dependent functions and reveal divergent effects on polyglutamine protein degradation depending on whether the substrate is an Hsp90 client.

Parkin-mediated ubiquitin signalling in aggresome formation and autophagy

Understanding how cells handle and dispose of misfolded proteins is of paramount importance because protein misfolding and aggregation underlie the pathogenesis of many neurodegenerative disorders, including PD (Parkinson's disease) and Alzheimer's disease. In addition to the ubiquitin-proteasome system, the aggresome-autophagy pathway has emerged as another crucial cellular defence system against toxic build-up of misfolded proteins. In contrast with basal autophagy that mediates non-selective, bulk clearance of misfolded proteins along with normal cellular proteins and organelles, the aggresome-autophagy pathway is increasingly recognized as a specialized type of induced autophagy that mediates selective clearance of misfolded and aggregated proteins under the conditions of proteotoxic stress. Recent evidence implicates PD-linked E3 ligase parkin as a key regulator of the aggresome-autophagy pathway and indicates a signalling role for Lys(63)-linked polyubiquitination in the regulation of aggresome formation and autophagy. The present review summarizes the current knowledge of the aggresome-autophagy pathway, its regulation by parkin-mediated Lys(63)-linked polyubiquitination, and its dysfunction in neurodegenerative diseases.

Autophagy, a guardian against neurodegeneration

Autophagy is an intracellular degradation process responsible for the clearance of most long-lived proteins and organelles. Cytoplasmic components are enclosed by double-membrane autophagosomes, which subsequently fuse with lysosomes for degradation. Autophagy dysfunction may contribute to the pathology of various neurodegenerative disorders, which manifest abnormal protein accumulation. As autophagy induction enhances the clearance of aggregate-prone intracytoplasmic proteins that cause neurodegeneration (like mutant huntingtin, tau and ataxin 3) and confers cytoprotective roles in cell and animal models, upregulating autophagy may be a tractable therapeutic strategy for diseases caused by such proteins. Here, we will review the molecular machinery of autophagy and its role in neurodegenerative diseases. Drugs and associated signalling pathways that may be targeted for pharmacological induction of autophagy will also be discussed.

Chemical inducers of autophagy that enhance the clearance of mutant proteins in neurodegenerative diseases

Many of the neurodegenerative diseases that afflict people are caused by intracytoplasmic aggregate-prone proteins. These include Parkinson disease, tauopathies, and polyglutamine expansion diseases such as Huntington disease. In Mendelian forms of these diseases, the mutations generally confer toxic novel functions on the relevant proteins. Thus, one potential strategy for dealing with these mutant proteins is to enhance their degradation. This can be achieved by up-regulating macroautophagy, which we will henceforth call autophagy. In this minireview, we will consider the reasons why autophagy up-regulation may be a powerful strategy for these diseases. In addition, we will consider some of the drugs and associated signaling pathways that can be used to induce autophagy with these therapeutic aims in mind.

Macroautophagy signaling and regulation

Macroautophagy is a vacuolar degradation pathway that terminates in the lysosomal compartment. Macroautophagy is a multistep process involving: (1) signaling events that occur upstream of the molecular machinery of autophagy; (2) molecular machinery involved in the formation of the autophagosome, the initial multimembrane-bound compartment formed in the autophagic pathway; and (3) maturation of autophagosomes, which acquire acidic and degradative capacities. In this chapter we summarize what is known about the regulation of the different steps involved in autophagy, and we also discuss how macroautophagy can be manipulated using drugs or genetic approaches that affect macroautophagy signaling, and the subsequent formation and maturation of the autophagosomes. Modulating autophagy offers a promising new therapeutic approach to human diseases that involve macroautophagy.

Mechanisms of cross-talk between the ubiquitin-proteasome and autophagy-lysosome systems

The ubiquitin proteasome system (UPS) and macroautophagy (hereafter called autophagy) were, for a long time, regarded as independent degradative pathways with few or no points of interaction. This view started to change recently, in the light of findings that have suggested that ubiquitylation can target substrates for degradation via both pathways. Moreover, perturbations in the flux through either pathway have been reported to affect the activity of the other system, and a number of mechanisms have been proposed to rationalise the link between the UPS and autophagy. Here we critically review these findings and outline some outstanding issues that still await clarification.

Dynein light chain 1 is required for autophagy, protein clearance, and cell death in Drosophila

Autophagy is a catabolic pathway that is important for turnover of long-lived proteins and organelles, and has been implicated in cell survival, tumor progression, protection from infection, neurodegeneration, and cell death. Autophagy and caspases are required for type II autophagic cell death of Drosophila larval salivary glands during development, but the mechanisms that regulate these degradation pathways are not understood. We conducted a forward genetic screen for genes that are required for salivary gland cell death, and here we describe the identification of Drosophila dynein light chain 1 (ddlc1) as a gene that is required for type II cell death. Autophagy is attenuated in ddlc1 mutants, but caspases are active in these cells. ddlc1 mutant salivary glands develop large fibrillar protein inclusions that stain positive for amyloid-specific dyes and ubiquitin. Ectopic expression of Atg1 is sufficient to induce autophagy, clear protein inclusions, and rescue degradation of ddlc1 mutant salivary glands. Furthermore, ddlc1 mutant larvae have decreased motility, and mutations in ddlc1 enhance the impairment of motility that is observed in a Drosophila model of neurodegenerative disease. Significantly, this decrease in larval motility is associated with decreased clearance of protein with polyglutamine expansion, the accumulation of p62 in neurons and muscles, and fewer synaptic boutons. These results indicate that DDLC1 is required for protein clearance by autophagy that is associated with autophagic cell death and neurodegeneration.

Regulation mechanisms and signaling pathways of autophagy

Autophagy is a process of self-degradation of cellular components in which double-membrane autophagosomes sequester organelles or portions of cytosol and fuse with lysosomes or vacuoles for breakdown by resident hydrolases. Autophagy is upregulated in response to extra- or intracellular stress and signals such as starvation, growth factor deprivation, ER stress, and pathogen infection. Defective autophagy plays a significant role in human pathologies, including cancer, neurodegeneration, and infectious diseases. We present our current knowledge on the key genes composing the autophagy machinery in eukaryotes from yeast to mammalian cells and the signaling pathways that sense the status of different types of stress and induce autophagy for cell survival and homeostasis. We also review the recent advances on the molecular mechanisms that regulate the autophagy machinery at various levels, from transcriptional activation to post-translational protein modification.

Regulation mechanisms and signaling pathways of autophagy

Autophagy is a process of self-degradation of cellular components in which double-membrane autophagosomes sequester organelles or portions of cytosol and fuse with lysosomes or vacuoles for breakdown by resident hydrolases. Autophagy is upregulated in response to extra- or intracellular stress and signals such as starvation, growth factor deprivation, ER stress, and pathogen infection. Defective autophagy plays a significant role in human pathologies, including cancer, neurodegeneration, and infectious diseases. We present our current knowledge on the key genes composing the autophagy machinery in eukaryotes from yeast to mammalian cells and the signaling pathways that sense the status of different types of stress and induce autophagy for cell survival and homeostasis. We also review the recent advances on the molecular mechanisms that regulate the autophagy machinery at various levels, from transcriptional activation to post-translational protein modification.

Multiple roles of the cytoskeleton in autophagy

Autophagy is involved in a wide range of physiological processes including cellular remodeling during development, immuno-protection against heterologous invaders and elimination of aberrant or obsolete cellular structures. This conserved degradation pathway also plays a key role in maintaining intracellular nutritional homeostasis and during starvation, for example, it is involved in the recycling of unnecessary cellular components to compensate for the limitation of nutrients. Autophagy is characterized by specific membrane rearrangements that culminate with the formation of large cytosolic double-membrane vesicles called autophagosomes. Autophagosomes sequester cytoplasmic material that is destined for degradation. Once completed, these vesicles dock and fuse with endosomes and/or lysosomes to deliver their contents into the hydrolytically active lumen of the latter organelle where, together with their cargoes, they are broken down into their basic components. Specific structures destined for degradation via autophagy are in many cases selectively targeted and sequestered into autophagosomes. A number of factors required for autophagy have been identified, but numerous questions about the molecular mechanism of this pathway remain unanswered. For instance, it is unclear how membranes are recruited and assembled into autophagosomes. In addition, once completed, these vesicles are transported to cellular locations where endosomes and lysosomes are concentrated. The mechanism employed for this directed movement is not well understood. The cellular cytoskeleton is a large, highly dynamic cellular scaffold that has a crucial role in multiple processes, several of which involve membrane rearrangements and vesicle-mediated events. Relatively little is known about the roles of the cytoskeleton network in autophagy. Nevertheless, some recent studies have revealed the importance of cytoskeletal elements such as actin microfilaments and microtubules in specific aspects of autophagy. In this review, we will highlight the results of this work and discuss their implications, providing possible working models. In particular, we will first describe the findings obtained with the yeast Saccharomyces cerevisiae, for long the leading organism for the study of autophagy, and, successively, those attained in mammalian cells, to emphasize possible differences between eukaryotic organisms.

Autophagy for the avoidance of neurodegeneration

Cellular defense mechanisms, including the unfolded protein response (UPR) and autophagy, attempt to resolve toxic protein aggregates, which are common denominators of neurodegenerative diseases. In this issue of Genes & Development, Hetz and colleagues (pp. 2294-2306) surprisingly show that inhibition of the UPR by knockout of XBP-1 causes a massive increase in autophagy, enhances clearance of superoxide dismutase 1 (SOD1) aggregates, and delays the development of amyotrophic lateral sclerosis. These findings suggest the existence of a homeostatic-if not hormetic-balance between distinct cellular defense mechanisms.

Dynein dysfunction induces endocytic pathology accompanied by an increase in Rab GTPases: a potential mechanism underlying age-dependent endocytic dysfunction

Growing evidence suggests that endocytic dysfunction is intimately involved in early stage Alzheimer disease pathology, such as the accumulation of beta-amyloid precursor protein in enlarged early endosomes. However, it remains unclear how endocytic dysfunction is induced in an age-dependent manner. Cytoplasmic dynein, a microtubule-based motor protein, interacts with another microtubule-associated protein, dynactin. The resulting dynein-dynactin complex mediates minus end-directed vesicle transport, including endosome trafficking. We have previously shown that the interaction between dynein-dynactin complexes is clearly attenuated in aged monkey brains, suggesting that dynein-mediated transport dysfunction exists in aged brains. Our immunohistochemical analyses revealed that age-dependent endocytic pathology was accompanied by an increase in Rab GTPases in aged monkey brains. Here, we demonstrated that siRNA-induced dynein dysfunction reproduced the endocytic pathology accompanied by increased Rab GTPases seen in aged monkey brains and significantly disrupted exosome release. Moreover, it also resulted in endosomal beta-amyloid precursor protein accumulation characterized by increased beta-site cleavage. These findings suggest that dynein dysfunction may underlie age-dependent endocytic dysfunction via the up-regulation of Rab GTPases. In addition, this vicious circle may worsen endocytic dysfunction, ultimately leading to Alzheimer disease pathology.

Equine herpesvirus type 1 (EHV-1) utilizes microtubules, dynein, and ROCK1 to productively infect cells

To initiate infection, equine herpesvirus type 1 (EHV-1) attaches to heparan sulfate on cell surfaces and then interacts with a putative glycoprotein D receptor(s). After attachment, virus entry occurs either by direct fusion of the virus envelope with the plasma membrane or via endocytosis followed by fusion between the virus envelope and an endosomal membrane. Upon fusion, de-enveloped virus particles are deposited into the cytoplasm and travel to the nucleus for viral replication. In this report, we examined the mechanism of EHV-1 intracellular trafficking and investigated the ability of EHV-1 to utilize specific cellular components to efficiently travel to the nucleus post-entry. Using a panel of microtubule-depolymerizing drugs and inhibitors of microtubule motor proteins, we show that EHV-1 infection is dependent on both the integrity of the microtubule network and the minus-end microtubule motor protein, dynein. In addition, we show that EHV-1 actively induces the acetylation of tubulin, a marker of microtubule stabilization, as early as 15 min post-infection. Finally, our data support a role for the cellular kinase, ROCK1, in virus trafficking to the nucleus.

Regulation mechanisms and signaling pathways of autophagy

Autophagy is a process of self-degradation of cellular components in which double-membrane autophagosomes sequester organelles or portions of cytosol and fuse with lysosomes or vacuoles for breakdown by resident hydrolases. Autophagy is upregulated in response to extra- or intracellular stress and signals such as starvation, growth factor deprivation, ER stress, and pathogen infection. Defective autophagy plays a significant role in human pathologies, including cancer, neurodegeneration, and infectious diseases. We present our current knowledge on the key genes composing the autophagy machinery in eukaryotes from yeast to mammalian cells and the signaling pathways that sense the status of different types of stress and induce autophagy for cell survival and homeostasis. We also review the recent advances on the molecular mechanisms that regulate the autophagy machinery at various levels, from transcriptional activation to post-translational protein modification.

Alpha-synuclein-glucocerebrosidase interactions in pharmacological Gaucher models: a biological link between Gaucher disease and parkinsonism

A growing body of experimental and clinical literature indicates an association between Gaucher disease and parkinsonism, raising the possibility that convergent mechanisms may contribute to neurodegeneration in these disorders. The aim of this study was to determine whether there is a relationship between alpha-synuclein (alpha-syn), a key protein in Parkinson's disease pathogenesis, and abnormalities in glucocerebroside (GC) catabolism that lead to the development of Gaucher disease. We inhibited glucocerebrosidase (GCase) with conduritol B epoxide (CBE) in neuroblastoma cells and mice to test whether a biological link exists between GCase activity and alpha-syn. After CBE exposure, enhanced alpha-syn protein was detected in differentiated cells challenged with CBE as compared to vehicle, with no change in alpha-syn mRNA. In the mouse model, after one injection of CBE, elevated nigral alpha-syn levels were also detected. Analyses by Western blot and confocal microscopy revealed that normal alpha-syn distribution was perturbed after CBE exposure with its accumulation apparent within nigral cell bodies as well as astroglia. These findings raise the possibility that alpha-syn may contribute to the cascade of events that promote neuronal dysfunction in Gaucher disease and are the first to implicate this protein as a plausible biological intersection between Gaucher disease and parkinsonism using a pharmacological model.

Aging: central role for autophagy and the lysosomal degradative system

The lysosomal network is the major intracellular proteolytic system accounting for more than 98% of long-lived bulk protein degradation and recycling particularly in tissues such as liver and muscles. Lysosomes are the final destination of intracellular damaged structures, identified and sequestered by the processes of macroautophagy and chaperone-mediated autophagy (CMA). In the process of macroautophagy, long-lived proteins and other macromolecular aggregates and damaged intracellular organelles are first engulfed by autophagosomes. Autophagosomes themselves have limited degrading capacity and rely on fusion with lysosomes. Unlike macroautophagy, CMA does not require intermediate vesicle formation and the cytosolic proteins recognized by this pathway are directly translocated to the lysosomal membrane. Aging is a universal phenomenon characterized by progressive deterioration of cells and organs due to accumulation of macromolecular and organelle damage. The continuous removal of worn-out components and replacement with newly synthesized ones ensures cellular homeostasis and delays the aging process. Growing evidence indicate that the rate of autophagosome formation and maturation and the efficiency of autophagosome/lysosome fusion decline with age. In addition, a progressive increase in intralysosomal concentration of free radicals and the age pigment lipofuscin further diminish the efficiency of lysosomal protein degradation. Therefore, integrity of the autophagosomal-lysosomal network appears to be critical in the progression of aging. Discovery of the genes involved in the process of autophagy has provided insight into the various molecular pathways that may be involved in aging and senescence. In this review, we discuss the cellular and molecular mechanisms involved in autophagy and the role of autophagosome/lysosome network in the aging process.

Protein aggregation as a paradigm of aging

The process of physiological decline leading to death of the individual is driven by the deteriorating capacity to withstand extrinsic and intrinsic hazards, resulting in damage accumulation with age. The dynamic changes with time of the network governing the outcome of misfolded proteins, exemplifying as intrinsic hazards, is considered here as a paradigm of aging. The main features of the network, namely, the non-linear increase of damage and the presence of amplifying feedback loops within the system are presented through a survey of the different components of the network and related cellular processes in aging and disease.

The effects of dynein inhibition on the autophagic pathway in glioma cells

Autophagy has multiple physiological functions, including protein degradation, organelle turnover and the response of cancer cells to chemotherapy. Because autophagy is implicated in a number of diseases, a better understanding of the molecular mechanisms of autophagy is needed for therapeutic purposes, including rational design of drugs. Autophagy is a process that occurs in several steps as follows: formation of phagophores, formation of mature autophagosomes, targeting and trafficking of autophagosomes to lysosomes, formation of autolysosomes by fusion between autophagosomes and lysosomes, and finally, degradation of the autophagic bodies within the lysosomes. It has been suggested that autophagosome formation is driven by molecular motor machineries, and, once formed, autophagosomes need to reach lysosomes, enriched perinuclearly around the microtubule-organizing centre. While it is recognized that all these steps require the cytoskeletal network, little is known about the mechanisms involved. Here we assessed the role of cytoplasmic dynein in the autophagic process of human glioma cells to determine the part played by dynein in autophagy. We observed that chemical interference with dynein function led to an accumulation of autophagosomes, suggesting impaired autophagosome-lysosome fusion. In contrast, we found that overexpression of dynamitin, which disrupts the dynein complex, reduced the number of autophagosomes, suggesting the requirement of the dynein-dynactin interaction in the early membrane trafficking step in autophagosome formation. These results suggest that dynein plays a variety of crucial roles during the autophagic process in glioma cells.

Nudel promotes axonal lysosome clearance and endo-lysosome formation via dynein-mediated transport

Axonal transport is critical for neuronal function and survival. Cytoplasmic dynein and its accessory complex dynactin form a microtubule minus end-directed motor in charge of retrograde transport. In this study, we show that Nudel, a dynein regulator, was highly expressed in dorsal root ganglion (DRG) neurons. Microinjection of anti-Nudel antibody into cultured DRG neurons abolished retrograde transport of membranous organelles in the axon and led to dispersions of Golgi cisternae in the soma. As a result, lysosomes, which are normally enriched in the soma, moved persistently into and thus accumulated in axons. Endo-lysosome formation was also markedly delayed. As anterograde motility of mitochondria was not inhibited, the antibody apparently did not abolish retrograde transport by destructing axonal microtubule tracks. Similar results were obtained by microinjecting N-terminal Nudel, anti-dynein antibody or a p150(Glued) mutant capable of abrogating the dynein-dynactin association. These results indicate a critical role of Nudel in dynein-mediated axonal transport. Moreover, the effects of dynein on endolysosome formation and regional sequestration of lysosomes may contribute to defects in the endocytic pathway seen in neurons of patients or animals with malfunction of dynein.

SCAMP5 links endoplasmic reticulum stress to the accumulation of expanded polyglutamine protein aggregates via endocytosis inhibition

Accumulation of expanded polyglutamine proteins is considered to be a major pathogenic biomarker of Huntington disease. We isolated SCAMP5 as a novel regulator of cellular accumulation of expanded polyglutamine track protein using cell-based aggregation assays. Ectopic expression of SCAMP5 augments the formation of ubiquitin-positive and detergent-resistant aggregates of mutant huntingtin (mtHTT). Expression of SCAMP5 is markedly increased in the striatum of Huntington disease patients and is induced in cultured striatal neurons by endoplasmic reticulum (ER) stress or by mtHTT. The increase of SCAMP5 impairs endocytosis, which in turn enhances mtHTT aggregation. On the contrary, down-regulation of SCAMP5 alleviates ER stress-induced mtHTT aggregation and endocytosis inhibition. Moreover, stereotactic injection into the striatum and intraperitoneal injection of tunicamycin significantly increase mtHTT aggregation in the striatum of R6/2 mice and in the cortex of N171-82Q mice, respectively. Taken together, these results suggest that exposure to ER stress increases SCAMP5 in the striatum, which positively regulates mtHTT aggregation via the endocytosis pathway.

MPP+ impairs autophagic clearance of alpha-synuclein by impairing the activity of dynein

Increasing evidence suggests that dynein has an important role in the clearance of misfolded proteins by autophagy. Here we show that treatment of cells with 1-methyl-4-phenylpyridinium (MPP) cause alpha-synuclein overexpression and aggregation, leading to the accumulation of autophagic vacuoles and the recruitment of LC3-II to these vacuoles in the cytoplasm. After MPP treatment, dynein expression decreased and was mainly aggregated at the periphery of cytoplasm and lost its colocalization with alpha-synuclein and lamp1, indicating that dynein lost its function in the aggresome formation and failed to return autophagosome and lysosomes to the center of the cell for degradation. We consider that dynein plays an important role in the autophagic clearance of aggregate-prone proteins.

Role of microtubules in extracellular release of poliovirus

Cellular autophagy, a process that directs cytosolic contents to the endosomal and lysosomal pathways via the formation of double-membraned vesicles, is a crucial aspect of innate immunity to many intracellular pathogens. However, evidence is accumulating that certain RNA viruses, such as poliovirus, subvert this pathway to facilitate viral growth. The autophagosome-like membranes induced during infection with wild-type poliovirus were found to be, unlike cellular autophagosomes, relatively immobile. Their mobility increased upon nocodazole treatment, arguing that vesicular tethering is microtubule dependent. In cells infected with a mutant virus that is defective in its interaction with the host cytoskeleton and secretory pathway, vesicle movement increased, indicating reduced tethering. In all cases, the release of tethering correlated with increased amounts of extracellular virus, which is consistent with the hypothesis that small amounts of cytosol and virus entrapped by double-membraned structures could be released via fusion with the plasma membrane. We propose that this extracellular delivery of cytoplasmic contents be termed autophagosome-mediated exit without lysis (AWOL). This pathway could explain the observed exit, in the apparent absence of cellular lysis, of other cytoplasmic macromolecular complexes, including infectious agents and complexes of aggregated proteins.

All-you-can-eat: autophagy in neurodegeneration and neuroprotection

Autophagy is the major pathway involved in the degradation of proteins and organelles, cellular remodeling, and survival during nutrient starvation. Autophagosomal dysfunction has been implicated in an increasing number of diseases from cancer to bacterial and viral infections and more recently in neurodegeneration. While a decrease in autophagic activity appears to interfere with protein degradation and possibly organelle turnover, increased autophagy has been shown to facilitate the clearance of aggregation-prone proteins and promote neuronal survival in a number of disease models. On the other hand, too much autophagic activity can be detrimental as well and lead to cell death, suggesting the regulation of autophagy has an important role in cell fate decisions. An increasing number of model systems are now available to study the role of autophagy in the central nervous system and how it might be exploited to treat disease. We will review here the current knowledge of autophagy in the central nervous system and provide an overview of the various models that have been used to study acute and chronic neurodegeneration.

Autophagic clearance of aggregate-prone proteins associated with neurodegeneration

Autophagy has emerged as a field of rapidly growing interest with implications in several disease conditions, such as cancer, infectious diseases, and neurodegenerative diseases. Autophagy is a major degradation pathway for aggregate-prone, intracytosolic proteins causing neurodegenerative disorders, such as Huntington's disease and forms of Parkinson's disease. Up-regulating autophagy may be a tractable therapeutic intervention for clearing these disease-causing proteins. The identification of autophagy-enhancing compounds would be beneficial not only in neurodegenerative diseases but also in other conditions where up-regulating autophagy may act as a protective pathway. Furthermore, small molecule modulators of autophagy may also be useful in dissecting pathways governing mammalian autophagy. In this chapter, we highlight assays that can be used for the identification of autophagy regulators, such as measuring the clearance of mutant aggregate-prone proteins or of autophagic flux with bafilomycin A(1). Using these methods, we recently described several mTOR-independent autophagy-enhancing compounds that have protective effects in various models of Huntington's disease.

Okadaic acid increases autophagosomes in rat neurons: implications for Alzheimer's disease

Autophagosomes are accumulated in Alzheimer's disease (AD), but the regulatory pathway of autophagy in AD remains largely unknown. By using electron microscopy, Western blotting, and immunocytochemistry, here we show that autophagosomes are accumulated in rat neurons by okadaic acid (OA), a protein phosphatase-2A inhibitor known to enhance tau phosphorylation, beta-amyloid (Abeta) deposition, and neuronal death, which are the pathological hallmarks of AD. Autophagy can be generally induced via several distinct pathways, such as inhibition of mTOR or activation of beclin-1. Interestingly, OA increased both mTOR and beclin-1 pathways simultaneously, which suggests that autophagy in OA-treated neurons is induced mainly via the beclin-1 pathway, and less so via mTOR inhibition. Finally, inhibition of autophagy by 3MA reduced cytotoxicity in OA-treated neurons. Our novel findings provide new insights into the pathology of and therapeutic intervention for AD.

The Drosophila BEACH family protein, blue cheese, links lysosomal axon transport with motor neuron degeneration

Impaired axon transport is one of the earliest pathological manifestations of several neurodegenerative diseases, and mutations in motor proteins can exacerbate or cause degeneration (Williamson and Cleveland, 1999; Gunawardena and Goldstein, 2004; Stokin and Goldstein, 2006). Compromised function in lysosomes and other degradative organelles that intersect with the lysosomal pathway are also strongly implicated in neurodegenerative disease pathology (Nixon and Cataldo, 2006; Rubinsztein, 2006). However, any functional link between these two phenomena has not as yet been recognized. Drosophila mutants in blue cheese (bchs) undergo progressive brain degeneration as adults and have shortened life span (Finley et al., 2003), but the cellular function of Bchs and the cause of degeneration have not been identified. A role in lysosomal trafficking is suggested by the homology of Bchs with the vesicle trafficking-associated BEACH (Beige and Chediak-Higashi) domain protein family (Wang et al., 2002; De Lozanne, 2003) and by its genetic interaction with a lysosomal transport pathway (Simonsen et al., 2007). Here, we describe the degeneration of a population of identified larval motor neurons in bchs mutants. We present evidence that Bchs is primarily lysosomal in those motor neurons in wild type and, using live fluorescence imaging of individual motor neurons in intact larvae, show that lysosomal vesicles fail to be transported toward motor neuron termini in bchs mutant and Bchs-overexpressing larvae. We suggest therefore that anterograde transport of lysosomes toward synaptic termini is a key factor in preventing motor neuron degeneration and that Bchs reveals a functional link between the lysosomal degradative pathway and transport.

Autophagy contributes to degradation of Hirano bodies

Hirano bodies are actin-rich inclusions reported most frequently in the hippocampus in association with a variety of conditions including neurodegenerative diseases, and aging. We have developed a model system for formation of Hirano bodies in Dictyostelium and cultured mammalian cells to permit detailed studies of the dynamics of these structures in living cells. Model Hirano bodies are frequently observed in membrane-enclosed vesicles in mammalian cells consistent with a role of autophagy in the degradation of these structures. Clearance of Hirano bodies by an exocytotic process is supported by images from electron microscopy showing extracellular release of Hirano bodies, and observation of Hirano bodies in the culture medium of Dictyostelium and mammalian cells. An autophagosome marker protein Atg8-GFP, was co-localized with model Hirano bodies in wild type Dictyostelium cells, but not in atg5(-) or atg1-1 autophagy mutant strains. Induction of model Hirano bodies in Dictyostelium with a high level expression of 34 kDa DeltaEF1 from the inducible discoidin promoter resulted in larger Hirano bodies and a cessation of cell doubling. The degradation of model Hirano bodies still occurred rapidly in autophagy mutant (atg5(-)) Dictyostelium, suggesting that other mechanisms such as the ubiquitin-mediated proteasome pathway could contribute to the degradation of Hirano bodies. Chemical inhibition of the proteasome pathway with lactacystin, significantly decreased the turnover of Hirano bodies in Dictyostelium providing direct evidence that autophagy and the proteasome can both contribute to degradation of Hirano bodies. Short term treatment of mammalian cells with either lactacystin or 3-methyl adenine results in higher levels of Hirano bodies and a lower level of viable cells in the cultures, supporting the conclusion that both autophagy and the proteasome contribute to degradation of Hirano bodies.

Dynein motor contributes to stress granule dynamics in primary neurons

Mobilization and translation of mRNAs, two important events believed to involve stress granules (SGs), in neurons are important for their survival and activities. However, the formation and disassembly of SGs in neurons remains unclear. By using an arsenite-induced neuronal stress model of rat primary spinal cord neuron cultures, we demonstrate the formation of SGs that contain common SG components and RNAs in both stressed neuronal cell bodies and their neurites. By employing small interfering RNA (siRNA) knockdown, we discovered that dynein motor subunit localizes in SG, and is important for SG assembly in neurons. Under stress, dynein motor subunit also facilitates translational repression and enhances the formation and integrity of SG in neurons. By blocking the energy source of dynein motor, both the formation and disassembly of SG are attenuated. These findings demonstrate, for the first time, that dynein motor complex plays a critical role in the dynamics of neuronal SGs, as well as translation of certain mRNAs.

Physiological functions of autophagy

The field of autophagy research has advanced rapidly in recent years, with important discoveries made in relation to both molecular mechanisms and physiological functions. Initially, autophagy was thought to be primarily a response to starvation. Although this might be true in lower eukaryotes, this catabolic process exerts various physiological functions in higher eukaryotes. This review summarizes the physiological roles of autophagy in amino acid pool maintenance, intracellular quality control, development, cell death, tumor suppression and anti-aging.

Tau deletion exacerbates the phenotype of Niemann-Pick type C mice and implicates autophagy in pathogenesis

Hyperphosphorylation and aggregation of the microtubule-binding protein tau characterize a diverse array of neurodegenerative disorders. Most of these lack mutations in the encoding MAPT gene, and the role of tau in disease pathogenesis remains controversial. Among these tauopathies is Niemann-Pick type C disease (NPC), a lysosomal storage disorder characterized by progressive neurodegeneration and premature death, most often caused by an inherited deficiency in the intracellular lipid trafficking protein NPC1. To determine the extent to which tau affects NPC pathogenesis, we generated Npc1-/- mice deficient in tau. Unexpectedly, NPC1/tau double null mutants are generated in markedly smaller litters, exhibit an enhanced systemic phenotype and die significantly earlier than NPC1 single null mutants. As autophagy is up-regulated in NPC and protein degradation through this pathway depends on movement along microtubules, we knocked down MAPT expression in NPC1-deficient human fibroblasts and examined effects on this pathway. We show that an acute reduction of tau expression in a cellular model of NPC decreases induction and flux through the autophagic pathway. Our data establish that MAPT deletion exacerbates the NPC phenotype through a mechanism independent of tau protein aggregation and identifies a critical role for tau in the regulation of autophagy in NPC1-deficient cells.

Pathways of methamphetamine toxicity

Methamphetamine (METH) is a drug of abuse which is neurotoxic for the nigrostriatal system. METH-induced neurodegeneration involves production of reactive oxygen species, triggering autophagic vacuoles within nigral neurons of chronic abusers of METH. In fact, Cu,Zn-superoxide dismutase 1 (SOD1) is a critical protein for the neurotoxic effects of METH on DA neurons. Moreover, mutations in the SOD1 gene cause amyotrophic lateral sclerosis, a dramatic neurodegenerative disorder. In the present paper we demonstrate that in G93A transgenic mice, overexpressing the ALS-linked mutant form of SOD1, surviving motor neurons share common intracellular alterations with METH-exposed DA neurons. We hypothesize that in mutant SOD1 transgenic mice, a defective autophagy might be responsible for the neurotoxic effects seen with in nigral neurons during METH toxicity.

ENU mutagenesis, a way forward to understand gene function

Arguably, the main challenge for contemporary genetics is to understand the function of every gene in a mammalian genome. The mouse has emerged as a model for this task because its genome can be manipulated in a number of ways to study gene function or mimic disease states. Two complementary genetic approaches can be used to generate mouse models. A reverse genetics or gene-driven approach (gene to phenotype) starts from a known gene and manipulates the genome to create genetically modified mice, such as knockouts. Alternatively, a forward genetics or phenotype-driven approach (phenotype to gene) involves screening mice for mutant phenotypes without previous knowledge of the genetic basis of the mutation. N-ethyl-N-nitrosourea (ENU) mutagenesis has been widely used for both approaches to generate mouse mutants. Here we review progress in ENU mutagenesis screening, with an emphasis on creating mouse models for human disorders.

Thiopurine S-methyltransferase pharmacogenetics: autophagy as a mechanism for variant allozyme degradation

OBJECTIVE

Thiopurine S-methyltransferase (TPMT)*3A is degraded much more rapidly than is the 'wild-type' enzyme through a ubiquitin-proteasome-dependent process. It also forms aggresomes, suggesting a possible dynamic balance between degradation and aggregation. We set out to identify genes encoding proteins participating in these processes.

METHODS

Green fluorescent protein tagged TPMT*3A was expressed in a Saccharomyces cerevisiae gene deletion library, and flow cytometry was used to screen for cells with high fluorescence intensity, indicating the loss of a gene essential for TPMT*3A degradation.

RESULTS

Twenty-four yeast genes were identified in functional categories that included ubiquitin-dependent protein degradation, vesicle trafficking, and vacuolar degradation. The presence of genes encoding proteins involved in vesicular transport and vacuolar degradation suggested a possible role in TPMT*3A degradation for autophagy--a process not previously identified as a pharmacogenomic mechanism. In support of that hypothesis, TPMT*3A aggregates increased dramatically in mutants for vacuolar protease and autophagy-related genes. Furthermore, TPMT*3A expression in human cells induced autophagy, and small interfering RNA-mediated knockdown of ATG7, an autophagy-related human protein, enhanced TPMT*3A aggregation but not that of TPMT*3C or wild-type TPMT, indicating that autophagy contributes to TPMT*3A degradation in mammalian cells. We also demonstrated that UBE2G2, the human homologue of the E2 ubiquitin-conjugating enzyme identified during the yeast genetic screen, was involved in TPMT*3A degradation in human cells.

CONCLUSION

These results indicate that autophagy should be considered among mechanisms responsible for the effects of pharmacogenetically significant polymorphisms that alter encoded amino acids.

Nutrient deprivation induces neuronal autophagy and implicates reduced insulin signaling in neuroprotective autophagy activation

Although autophagy maintains normal neural function by degrading misfolded proteins, little is known about how neurons activate this integral response. Furthermore, classical methods of autophagy induction used with nonneural cells, such as starvation, simply result in neuron death. To study neuronal autophagy, we cultured primary cortical neurons from transgenic mice that ubiquitously express green fluorescent protein-tagged LC3 and monitored LC3-I to LC3-II conversion by immunohistochemistry and immunoblotting. Evaluation of different culture media led us to discover that culturing primary neurons in Dulbecco's modified Eagle's medium without B27 supplementation robustly activates autophagy. We validated this nutrient-limited media approach for inducing autophagy by showing that 3-methyl-adenine treatment and Atg5 RNA interference knockdown each inhibits LC3-I to LC3-II conversion. Evaluation of B27 supplement components yielded insulin as the factor whose absence induced autophagy in primary neurons, and this activation was mammalian target of rapamycin-dependent. When we tested if nutrient-limited media could protect neurons expressing polyglutamine-expanded proteins against cell death, we observed a strong protective effect, probably due to autophagy activation. Our results indicate that nutrient deprivation can be used to understand the regulatory basis of neuronal autophagy and implicate diminished insulin signaling in the activation of neuronal autophagy.

Role of axonal transport in neurodegenerative diseases

Many major human neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis (ALS), display axonal pathologies including abnormal accumulations of proteins and organelles. Such pathologies highlight damage to the axon as part of the pathogenic process and, in particular, damage to transport of cargoes through axons. Indeed, we now know that disruption of axonal transport is an early and perhaps causative event in many of these diseases. Here, we review the role of axonal transport in neurodegenerative disease.

The role of autophagy in mammalian development: cell makeover rather than cell death

Autophagy is important for the degradation of bulk cytoplasm, long-lived proteins, and entire organelles. In lower eukaryotes, autophagy functions as a cell death mechanism or as a stress response during development. However, autophagy's significance in vertebrate development, and the role (if any) of vertebrate-specific factors in its regulation, remains unexplained. Through careful analysis of the current autophagy gene mutant mouse models, we propose that in mammals, autophagy may be involved in specific cytosolic rearrangements needed for proliferation, death, and differentiation during embryogenesis and postnatal development. Thus, autophagy is a process of cytosolic "renovation," crucial in cell fate decisions.