Loss of autophagy in the central nervous system causes neurodegeneration in mice

Lysosome dysfunction triggers Atg7-dependent neural apoptosis

Macroautophagy (autophagy) is a process wherein bulk cytosolic proteins and damaged organelles are sequestered and degraded via the lysosome. Alterations in autophagy-associated proteins have been shown to cause neural tube closure defects, neurodegeneration, and tumor formation. Normal lysosome function is critical for autophagy completion and when altered may lead to an accumulation of autophagic vacuoles (AVs) and caspase activation. The tumor suppressor p53 is highly expressed in neural precursor cells (NPCs) and has an important role in the regulation of both autophagy and apoptosis. We hypothesized that altered lysosome function would lead to NPC death via an interaction between autophagy- and apoptosis-associated proteins. To test our hypothesis, we utilized FGF2-expanded NPCs and the neural stem cell line, C17.2, in combination with the lysosomotropic agent chloroquine (CQ) and the vacuolar ATPase inhibitor bafilomycin A1 (Baf A1). Both CQ and Baf A1 caused concentration- and time-dependent AV accumulation, p53 phosphorylation, increased damage regulator autophagy modulator levels, caspase-3 activation, and cell death. Short hairpin RNA knockdown of Atg7, but not Beclin1, expression significantly inhibited CQ- and Baf A1-induced cell death, indicating that Atg7 is an upstream mediator of lysosome dysfunction-induced cell death. Cell death and/or caspase-3 activation was also attenuated by protein synthesis inhibition, p53 deficiency, or Bax deficiency, indicating involvement of the intrinsic apoptotic death pathway. In contrast to lysosome dysfunction, starvation-induced AV accumulation was inhibited by either Atg7 or Beclin1 knockdown, and Atg7 knockdown had no effect on starvation-induced death. These findings indicate that Atg7- and Beclin1-induced autophagy plays a cytoprotective role during starvation but that Atg7 has a unique pro-apoptotic function in response to lysosome dysfunction.

Neural-specific deletion of FIP200 leads to cerebellar degeneration caused by increased neuronal death and axon degeneration

FIP200 (FAK family-interacting protein of 200 kDa) is a conserved protein recently identified as a potential mammalian counterpart of yeast autophagy protein Atg17. However, it remains unknown whether mammalian FIP200 regulates autophagy in vivo. Here we show that neural-specific deletion of FIP200 resulted in cerebellar degeneration accompanied by progressive neuronal loss, spongiosis, and neurite degeneration in the cerebellum. Furthermore, deletion of FIP200 led to increased apoptosis in cerebellum as well as accumulation of ubiquitinated protein aggregates without any deficiency in proteasome catalytic functions. We also observed an increased p62/SQSTM1 accumulation in the cerebellum and reduced autophagosome formation as well as accumulation of damaged mitochondria in the mutant mice. Lastly, analysis of cerebellar neurons in vitro showed reduced JNK activation and increased susceptibility to serum deprivation-induced apoptosis in cerebellar neurons from the mutant mice. Taken together, these results provide strong genetic evidence for a role of FIP200 in the regulation of neuronal homeostasis through its function in autophagy in vivo.

Autophagy in hypertensive heart disease

In response to hypertension, the heart manifests robust hypertrophic growth, which offsets load-induced elevations in wall stress. If sustained, this hypertrophic response is a major risk factor for systolic dysfunction and heart failure. Extensive research efforts have focused on the progression from hypertrophy to failure; however, precise understanding of underlying mechanisms remains elusive. Recently, autophagy, a process of cellular cannibalization, has been implicated. Autophagy is activated during ventricular hypertrophy, serving to maintain cellular homeostasis. Excessive autophagy eliminates, however, essential cellular elements and possibly provokes cell death, which together contribute to hypertension-related heart disease.

An inhibitory role of the G-protein regulator AGS3 in mTOR-dependent macroautophagy

Macroautophagy is a cellular process whereby the cell sequesters and recycles cytosolic constituents in a lysosome-dependent manner. It has also been implicated in a number of disorders, including cancer and neurodegeneration. Although a previous report that AGS3 over-expression promotes macroautophagy suggests a stimulatory role of AGS3 in this process, we have found that knock-down of AGS3, unexpectedly, also induces macroautophagy, indicating an inhibitory function of endogenous AGS3 in macroautophagy. Interestingly, AGS3 phosphorylation is decreased upon induction of mammalian target of rapamycin (mTOR)-dependent macroautophagy. Moreover, unlike wild-type AGS3, over-expression of an AGS3 mutant lacking this modification fails to enhance macroautophagic activity. These observations imply that AGS3 phosphorylation may participate in the modulation of macroautophagy.

Proteasome inhibitors activate autophagy as a cytoprotective response in human prostate cancer cells

The ubiquitin-proteasome and lysosome-autophagy pathways are the two major intracellular protein degradation systems that work cooperatively to maintain homeostasis. Proteasome inhibitors (PIs) have clinical activity in hematological tumors, and inhibitors of autophagy are also being evaluated as potential antitumor therapies. In this study, we found that chemical PIs and small interfering RNA-mediated knockdown of the proteasome's enzymatic subunits promoted autophagosome formation, stimulated autophagic flux, and upregulated expression of the autophagy-specific genes (ATGs) (ATG5 and ATG7) in some human prostate cancer cells and immortalized mouse embryonic fibroblasts (MEFs). Upregulation of ATG5 and ATG7 only occurred in cells displaying PI-induced phosphorylation of the eukaryotic translation initiation factor 2 alpha (eIF2alpha), an important component of the unfolded protein responses. Furthermore, PIs did not induce autophagy or upregulate ATG5 in MEFs expressing a phosphorylation-deficient mutant form of eIF2alpha. Combined inhibition of autophagy and the proteasome induced an accumulation of intracellular protein aggregates reminiscent of neuronal inclusion bodies and caused more cancer cell death than blocking either degradation pathway alone. Overall, our data show that proteasome inhibition activates autophagy through a phospho-eIF2alpha-dependent mechanism to eliminate protein aggregates and alleviate proteotoxic stress.

Combinational soluble N-ethylmaleimide-sensitive factor attachment protein receptor proteins VAMP8 and Vti1b mediate fusion of antimicrobial and canonical autophagosomes with lysosomes

Autophagy plays a crucial role in host defense, termed antimicrobial autophagy (xenophagy), as it functions to degrade intracellular foreign microbial invaders such as group A Streptococcus (GAS). Xenophagosomes undergo a stepwise maturation process consisting of a fusion event with lysosomes, after which the cargoes are degraded. However, the molecular mechanism underlying xenophagosome/lysosome fusion remains unclear. We examined the involvement of endocytic soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) in xenophagosome/lysosome fusion. Confocal microscopic analysis showed that SNAREs, including vesicle-associated membrane protein (VAMP)7, VAMP8, and vesicle transport through interaction with t-SNAREs homologue 1B (Vti1b), colocalized with green fluorescent protein-LC3 in xenophagosomes. Knockdown of Vti1b and VAMP8 with small interfering RNAs disturbed the colocalization of LC3 with lysosomal membrane protein (LAMP)1. The invasive efficiency of GAS into cells was not altered by knockdown of VAMP8 or Vti1b, whereas cellular bactericidal efficiency was significantly diminished, indicating that antimicrobial autophagy was functionally impaired. Knockdown of Vti1b and VAMP8 also disturbed colocalization of LC3 with LAMP1 in canonical autophagy, in which LC3-II proteins were negligibly degraded. In contrast, knockdown of Syntaxin 7 and Syntaxin 8 showed little effect on the autophagic fusion event. These findings strongly suggest that the combinational SNARE proteins VAMP8 and Vti1b mediate the fusion of antimicrobial and canonical autophagosomes with lysosomes, an essential event for autophagic degradation.

Autophagy in health and disease. 1. Regulation and significance of autophagy: an overview

Macroautophagy is a vacuolar degradation pathway that terminates in the lysosomal compartment after formation of a cytoplasmic vacuole or autophagosome that engulfs macromolecules and organelles. The identification of ATG (autophagy-related) genes that are involved in the formation of autophagosomes has greatly increased our knowledge of the molecular basis of macroautophagy, and its roles in cell function, which extend far beyond degradation and quality control of the cytoplasm. Macroautophagy, which plays a major role in tissue homeostasis, is now recognized as contributing to innate and adaptive immune responses. Recently, several mediators of apoptosis have been shown to control macroautophagy. Deciphering the cross talk between macroautophagy and apoptosis probably should help increase understanding of the role of macroautophagy in human disease and is likely to be of therapeutic importance.

Atg8-family interacting motif crucial for selective autophagy

Autophagy is a bulk degradation system conserved among most eukaryotes. Recently, autophagy has been shown to mediate selective degradation of various targets such as aggregated proteins and damaged or superfluous organelles. Structural studies have uncovered the conserved specific interactions between autophagic receptors and Atg8-family proteins through WXXL-like sequences, which we term the Atg8-family interacting motif (AIM). AIM functions in various autophagic receptors such as Atg19 in the cytoplasm-to-vacuole targeting pathway, p62 and neighbor of BRCA1 gene 1 (NBR1) in autophagic degradation of protein aggregates, and Atg32 and Nix in mitophagy, and may link the target-receptor complex to autophagic membranes and/or their forming machineries.

Degradative proteomics and disease mechanisms

Protein degradation is a fundamental biological process, which is essential for the maintenance and regulation of normal cellular function. In humans and animals, proteins can be degraded by a number of mechanisms: the ubiquitin-proteasome system, autophagy and intracellular proteases. The advances in contemporary protein analysis means that proteomics is increasingly being used to explore these key pathways and as a means of monitoring protein degradation. The dysfunction of protein degradative pathways has been associated with the development of a number of important diseases including cancer, muscle wasting disorders and neurodegenerative diseases. This review will focus on the role of proteomics to study cellular degradative processes and how these strategies are being applied to understand the molecular basis of diseases arising from disturbances in protein degradation.

HDAC6 controls autophagosome maturation essential for ubiquitin-selective quality-control autophagy

Autophagy is primarily considered a non-selective degradation process induced by starvation. Nutrient-independent basal autophagy, in contrast, imposes intracellular QC by selective disposal of aberrant protein aggregates and damaged organelles, a process critical for suppressing neurodegenerative diseases. The molecular mechanism that distinguishes these two fundamental autophagic responses, however, remains mysterious. Here, we identify the ubiquitin-binding deacetylase, histone deacetylase-6 (HDAC6), as a central component of basal autophagy that targets protein aggregates and damaged mitochondria. Surprisingly, HDAC6 is not required for autophagy activation; rather, it controls the fusion of autophagosomes to lysosomes. HDAC6 promotes autophagy by recruiting a cortactin-dependent, actin-remodelling machinery, which in turn assembles an F-actin network that stimulates autophagosome-lysosome fusion and substrate degradation. Indeed, HDAC6 deficiency leads to autophagosome maturation failure, protein aggregate build-up, and neurodegeneration. Remarkably, HDAC6 and F-actin assembly are completely dispensable for starvation-induced autophagy, uncovering the fundamental difference of these autophagic modes. Our study identifies HDAC6 and the actin cytoskeleton as critical components that define QC autophagy and uncovers a novel regulation of autophagy at the level of autophagosome-lysosome fusion.

Role of autophagy in suppression of inflammation and cancer

Autophagy is a crucial component of the cellular stress adaptation response that maintains mammalian homeostasis. Autophagy protects against neurodegenerative and inflammatory conditions, aging, and cancer. This is accomplished by the degradation and intracellular recycling of cellular components to maintain energy metabolism and by damage mitigation through the elimination of damaged proteins and organelles. How autophagy modulates oncogenesis is gradually emerging. Tumor cells induce autophagy in response to metabolic stress to promote survival, suggesting deployment of therapeutic strategies to block autophagy for cancer therapy. By contrast, defects in autophagy lead to cell death, chronic inflammation, and genetic instability. Thus, stimulating autophagy may be a powerful approach for chemoprevention. Analogous to infection or toxins that create persistent tissue damage and chronic inflammation that increases the incidence of cancer, defective autophagy represents a cell-intrinsic mechanism to create the damaging, inflammatory environment that predisposes to cancer. Thus, cellular damage mitigation through autophagy is a novel mechanism of tumor suppression.

Cytoprotective roles for autophagy

Macroautophagy (referred to as autophagy in this review) is a genetically regulated bulk degradation program conserved from yeast to humans, in which cytoplasmic substrates, such as damaged organelles and long-lived proteins, are delivered to lysosomes for degradation. In this review, we consider recent data that highlight possible mechanisms whereby autophagy mediates cytoprotective effects. These include the ability of autophagy to buffer against starvation, protect against apoptotic insults and clear mitochondria, aggregate-prone proteins and pathogens. These effects are pertinent to the roles of autophagy in normal human physiology, including the early neonatal period and ageing, as well as a variety of diseases, including cancer, neurodegenerative conditions and infectious diseases.

Autophagy in disease

Autophagy is a cellular quality control process by which cytoplasmic constituents including proteins, protein aggregates, organelles, and invading pathogens can be delivered to lysosomes for degradation. Autophagy is activated in response to changes in the internal status of the cell and/or changes in the extracellular environment. It is therefore essential for the maintenance of cellular homeostasis and for an efficient response to cellular stresses. As such autophagy has been implicated either in the pathogenesis, or response to a wide variety of diseases, bacterial, and viral infections, and ageing.

Autophagy and aging: lessons from progeria models

Autophagy is an evolutionarily conserved process essential for cellular homeostasis and organismal viability. In fact, this pathway is one of the major protein degradation mechanisms in eukaryotic cells. It has been repeatedly reported that the autophagic activity of living cells decreases with age, probably contributing to the accumulation of damaged macromolecules and organelles during aging. Moreover, autophagy modulation in different model organisms has yielded very promising results suggesting that the maintenance of a proper autophagic activity contributes to extend longevity. On the other hand, recent findings have shown that distinct premature-aging murine models exhibit an extensive basal activation of autophagy instead of the characteristic decline in this process occurring during normal aging. This unexpected autophagic increase in progeroid models is usually associated with a series of metabolic alterations resembling those occurring under calorie restriction or in other situations reported to prolong life-span. In this chapter, we will discuss the current knowledge on the relationship between the autophagy pathway and aging with a special emphasis on the unexpected and novel link between premature aging and autophagy up-regulation.

Cell "self-eating" (autophagy) mechanism in Alzheimer's disease

The autophagy pathway is the major degradation pathway of the cell for long-lived proteins and organelles. Dysfunction of autophagy has been linked to several neurodegenerative disorders that are associated with an accumulation of misfolded protein aggregates. Alzheimer's disease, the most common neurodegenerative disorder, is characterized by 2 aggregate forms, tau tangles and amyloid-beta plaques. Autophagy has been linked to Alzheimer's disease pathogenesis through its merger with the endosomal-lysosomal system, which has been shown to play a role in the formation of the latter amyloid-beta plaques. However, the precise role of autophagy in Alzheimer's disease pathogenesis is still under contention. One hypothesis is that aberrant autophagy induction results in an accumulation of autophagic vacuoles containing amyloid-beta and the components necessary for its generation, whereas other evidence points to impaired autophagic clearance or even an overall reduction in autophagic activity playing a role in Alzheimer's disease pathogenesis. In this review, we discuss the current evidence linking autophagy to Alzheimer's disease as well as the uncertainty over the exact role and level of autophagic regulation in the pathogenic mechanism of Alzheimer's disease.

Role of the ubiquitin–proteasome system and autophagy in polyglutamine neurodegenerative diseases

The accumulation of intracellular protein aggregates is a prominent feature of many late-onset neurodegenerative disorders, including polyglutamine neurodegenerative diseases. Appearance of aggregates of the misfolded mutant disease proteins indicate that the degradative pathways of the cell are failing to efficiently clear them and are being progressively overwhelmed, which could eventually lead to neuronal dysfunction and neurodegeneration. Cellular pathways for degrading misfolded and aggregated-prone proteins include the ubiquitin–proteasome system and autophagy. This article reviews recent studies that have shown a critical role of the ubiquitin–proteasome system and autophagy in the pathogenesis of polyglutamine diseases. Understanding the role of these two pathways in disease pathogenesis could open up a new attractive therapeutic avenue for polyglutamine and other related neurodegenerative disorders.

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.

Loss of autophagy in erythroid cells leads to defective removal of mitochondria and severe anemia in vivo

Timely elimination of damaged mitochondria is essential to protect cells from the potential harm of disordered mitochondrial metabolism and release of proapoptotic proteins. In mammalian red blood cells, the expulsion of the nucleus followed by the removal of other organelles, such as mitochondria, are necessary differentiation steps. Mitochondrial sequestration by autophagosomes, followed by delivery to the lysosomal compartment for degradation (mitophagy), is a major mechanism of mitochondrial turnover. Here we show that mice lacking the essential autophagy gene Atg7 in the hematopoietic system develop severe anemia. Atg7(-/-) erythrocytes accumulate damaged mitochondria with altered membrane potential leading to cell death. We find that mitochondrial loss is initiated in the bone marrow at the Ter119(+)/CD71(High) stage. Proteomic analysis of erythrocyte ghosts suggests that in the absence of autophagy other cellular degradation mechanisms are induced. Importantly, neither the removal of endoplasmic reticulum nor ribosomes is affected by the lack of Atg7. Atg7 deficiency also led to severe lymphopenia as a result of mitochondrial damage followed by apoptosis in mature T lymphocytes. Ex vivo short-lived hematopoietic cells such as monocytes and dendritic cells were not affected by the loss of Atg7. In summary, we show that the selective removal of mitochondria by autophagy, but not other organelles, during erythropoeisis is essential and that this is a necessary developmental step in erythroid cells.

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.

IKK phosphorylates Huntingtin and targets it for degradation by the proteasome and lysosome

Expansion of the polyglutamine repeat within the protein Huntingtin (Htt) causes Huntington's disease, a neurodegenerative disease associated with aging and the accumulation of mutant Htt in diseased neurons. Understanding the mechanisms that influence Htt cellular degradation may target treatments designed to activate mutant Htt clearance pathways. We find that Htt is phosphorylated by the inflammatory kinase IKK, enhancing its normal clearance by the proteasome and lysosome. Phosphorylation of Htt regulates additional post-translational modifications, including Htt ubiquitination, SUMOylation, and acetylation, and increases Htt nuclear localization, cleavage, and clearance mediated by lysosomal-associated membrane protein 2A and Hsc70. We propose that IKK activates mutant Htt clearance until an age-related loss of proteasome/lysosome function promotes accumulation of toxic post-translationally modified mutant Htt. Thus, IKK activation may modulate mutant Htt neurotoxicity depending on the cell's ability to degrade the modified species.

Oxidative stress promotes autophagic cell death in human neuroblastoma cells with ectopic transfer of mitochondrial PPP2R2B (Bbeta2)

Background

The multifunctional protein phosphatase 2A (PP2A) is a heterotrimeric serine/threonine protein phosphatase composed of a scaffolding, catalytic and regulatory subunits. By modifying various downstream signal transducers, the aberrant expression of the brain-targeted regulatory subunit PPP2R2B is associated with the onset of a panel of neuronal disorders. The alternatively splicing of PPP2R2B encodes two regulatory subunit isoforms that determine cellular distribution of the neuron-specific holoenzyme to mitochondria (Bβ2) and cytoplasm (Bβ1), respectively.

Results

Human neuroblastoma cells were transfected with PPP2R2B constructs encoding the complete sequences of Bβ2 and Bβ1, respectively. The colonies with antibiotic resistance were selected as stable cell lines. Both ectopic Bβ1 and Bβ2 clones exhibited characteristics of autophagy. To test how cells respond to reactive oxygen species generators, the cells were treated with either hydrogen peroxide or t-butyl hydroperoxide and Bβ2 clones induced cell death. Suppression of autophagy using either RNA interference of the essential autophagy gene or pharmacological inhibitor rescued cell death caused by oxidative stress.

Conclusions

Cells with ectopically expressed mitochondria-targeted regulatory subunit PPP2R2B of the holoenzyme PP2A were shown predisposed to autophagy and oxidative stress induced cell death that is related to apoptosis. The results promised a model for studying the mechanism and function of aberrant PPP2R2B expression in neuronal cells. The work provided a new target for understanding and prevention of neuropathogenesis.

The unfolded protein response protects human tumor cells during hypoxia through regulation of the autophagy genes MAP1LC3B and ATG5

Tumor hypoxia is a common microenvironmental factor that adversely influences tumor phenotype and treatment response. Cellular adaptation to hypoxia occurs through multiple mechanisms, including activation of the unfolded protein response (UPR). Recent reports have indicated that hypoxia activates a lysosomal degradation pathway known as autophagy, and here we show that the UPR enhances the capacity of hypoxic tumor cells to carry out autophagy, and that this promotes their survival. In several human cancer cell lines, hypoxia increased transcription of the essential autophagy genes microtubule-associated protein 1 light chain 3beta (MAP1LC3B) and autophagy-related gene 5 (ATG5) through the transcription factors ATF4 and CHOP, respectively, which are regulated by PKR-like ER kinase (PERK, also known as EIF2AK3). MAP1LC3B and ATG5 are not required for initiation of autophagy but mediate phagophore expansion and autophagosome formation. We observed that transcriptional induction of MAP1LC3B replenished MAP1LC3B protein that was turned over during extensive hypoxia-induced autophagy. Correspondingly, cells deficient in PERK signaling failed to transcriptionally induce MAP1LC3B and became rapidly depleted of MAP1LC3B protein during hypoxia. Consistent with these data, autophagy and MAP1LC3B induction occurred preferentially in hypoxic regions of human tumor xenografts. Furthermore, pharmacological inhibition of autophagy sensitized human tumor cells to hypoxia, reduced the fraction of viable hypoxic tumor cells, and sensitized xenografted human tumors to irradiation. Our data therefore demonstrate that the UPR is an important mediator of the hypoxic tumor microenvironment and that it contributes to resistance to treatment through its ability to facilitate autophagy.

Transforming growth factor-beta regulates basal transcriptional regulatory machinery to control cell proliferation and differentiation in cranial neural crest-derived osteoprogenitor cells

Transforming growth factor-beta (Tgf-beta) signaling is crucial for regulating craniofacial development. Loss of Tgf-beta signaling results in defects in cranial neural crest cells (CNCC), but the mechanism by which Tgf-beta signaling regulates bone formation in CNCC-derived osteogenic cells remains largely unknown. In this study, we discovered that Tgf-beta regulates the basal transcriptional regulatory machinery to control intramembranous bone development. Specifically, basal transcription factor Taf4b is down-regulated in the CNCC-derived intramembranous bone in Tgfbr2(fl/fl);Wnt1-Cre mice. Tgf-beta specifically induces Taf4b expression. Moreover, small interfering RNA knockdown of Taf4b results in decreased cell proliferation and altered osteogenic differentiation in primary mouse embryonic maxillary mesenchymal cells, as seen in Tgfbr2 mutant cells. In addition, we show that Taf1 is decreased at the osteogenic initiation stage in the maxilla of Tgfbr2 mutant mice. Furthermore, small interfering RNA knockdown of Taf4b and Taf1 together in primary mouse embryonic maxillary mesenchymal cells results in up-regulated osteogenic initiator Runx2 expression, with decreased cell proliferation and altered osteogenic differentiation. Our results indicate a critical function of Tgf-beta-mediated basal transcriptional factors in regulating osteogenic cell proliferation and differentiation in CNCC-derived osteoprogenitor cells during intramembranous bone formation.

Autophagy: links with the proteasome

The two main protein degradation systems of eukaryotic cells, the ubiquitin-proteasome system and autophagy, have been thought of as quite separate systems. However, recent findings strongly suggest that there is crosstalk and even cooperation between these two degradation pathways. Ubiquitination and degradation of misfolded proteins by the ubiquitin-proteasome system have been investigated for some time, but much less is known about autophagic degradation of misfolded proteins. We will here discuss recent findings that shed some light on the cellular processes deciding when and how misfolded proteins are specifically selected for autophagic degradation in favor of proteasomal degradation.

The role of autophagy in tumour development and cancer therapy

Autophagy is a catabolic membrane-trafficking process that leads to sequestration and degradation of intracellular material within lysosomes. It is executed at basal levels in every cell and promotes cellular homeostasis by regulating organelle and protein turnover. In response to various forms of cellular stress, however, the levels and cargoes of autophagy can be modulated. In nutrient-deprived states, for example, autophagy can be activated to degrade cargoes for cell-autonomous energy production to promote cell survival. In other contexts, in contrast, autophagy has been shown to contribute to cell death. Given these dual effects in regulating cell viability, it is no surprise that autophagy has implications in both the genesis and treatment of malignant disease. In this review, we provide a comprehensive appraisal of the way in which oncogenes and tumour suppressor genes regulate autophagy. In addition, we address the current evidence from human cancer and animal models that has aided our understanding of the role of autophagy in tumour progression. Finally, the potential for targeting autophagy therapeutically is discussed in light of the functions of autophagy at different stages of tumour progression and in normal tissues.

Beclin 1 gene transfer activates autophagy and ameliorates the neurodegenerative pathology in alpha-synuclein models of Parkinson's and Lewy body diseases

Accumulation of the synaptic protein alpha-synuclein (alpha-syn) is a hallmark of Parkinson's disease (PD) and Lewy body disease (LBD), a heterogeneous group of disorders with dementia and parkinsonism, where Alzheimer's disease and PD interact. Accumulation of alpha-syn in these patients might be associated with alterations in the autophagy pathway. Therefore, we postulate that delivery of beclin 1, a regulator of the autophagy pathway, might constitute a strategy toward developing a therapy for LBD/PD. Overexpression of alpha-syn from lentivirus transduction in a neuronal cell line resulted in lysosomal accumulation and alterations in autophagy. Coexpression of beclin 1 activated autophagy, reduced accumulation of alpha-syn, and ameliorated associated neuritic alterations. The effects of beclin 1 overexpression on LC3 and alpha-syn accumulation were partially blocked by 3-MA and completely blocked by bafilomycin A1. In contrast, rapamycin enhanced the effects of beclin 1. To evaluate the potential effects of activating autophagy in vivo, a lentivirus expressing beclin 1 was delivered to the brain of a alpha-syn transgenic mouse. Neuropathological analysis demonstrated that beclin 1 injections ameliorated the synaptic and dendritic pathology in the tg mice and reduced the accumulation of alpha-syn in the limbic system without any significant deleterious effects. This was accompanied by enhanced lysosomal activation and reduced alterations in the autophagy pathway. Thus, beclin 1 plays an important role in the intracellular degradation of alpha-syn either directly or indirectly through the autophagy pathway and may present a novel therapeutic target for LBD/PD.

Autophagy is activated by TGF-beta and potentiates TGF-beta-mediated growth inhibition in human hepatocellular carcinoma cells

Transforming growth factor-beta (TGF-beta) is a multifunctional cytokine that regulates cell growth, differentiation, and apoptosis of various types of cells. Autophagy is emerging as a critical response of normal and cancer cells to environmental changes, but the relationship between TGF-beta signaling and autophagy has been poorly understood. Here, we showed that TGF-beta activates autophagy in human hepatocellular carcinoma cell lines. TGF-beta induced accumulation of autophagosomes and conversion of microtubule-associated protein 1 light chain 3 and enhanced the degradation rate of long-lived proteins. TGF-beta increased the mRNA expression levels of BECLIN1, ATG5, ATG7, and death-associated protein kinase (DAPK). Knockdown of Smad2/3, Smad4, or DAPK, or inhibition of c-Jun NH(2)-terminal kinase, attenuated TGF-beta-induced autophagy, indicating the involvement of both Smad and non-Smad pathway(s). TGF-beta activated autophagy earlier than execution of apoptosis (6-12 versus 48 h), and reduction of autophagy genes by small interfering RNA attenuated TGF-beta-mediated growth inhibition and induction of proapoptotic genes Bim and Bmf, suggesting the contribution of autophagy pathway to the growth-inhibitory effect of TGF-beta. Additionally, TGF-beta also induced autophagy in some mammary carcinoma cells, including MDA-MB-231 cells. These findings show that TGF-beta signaling pathway activates autophagy in certain human cancer cells and that induction of autophagy is a novel aspect of biological functions of TGF-beta.

Cellular and nuclear degradation during apoptosis

Apoptosis ensures quick death and quiet clearance of unwanted or damaged cells, without inducing much, if any, immunological responses from the organism. In metazoan organisms, apoptotic cells are swiftly engulfed by other cells. The degradation of cellular content is initiated in apoptotic cells and completed within engulfing cells. In apoptotic cells, caspase-mediated proteolysis cleaves protein substrates into fragments; nuclear DNA is partially degraded into nucleosomal units; and autophagy potentially contributes to apoptotic cell removal. In engulfing cells, specific signaling pathways promote the sequential fusion of intracellular vesicles with phagosomes and lead to the complete degradation of apoptotic cells in an acidic environment. Phagocytic receptors that initiate the engulfment of apoptotic cells play an additional and crucial role in initiating phagosome maturation through activating these signaling pathways. Here we highlight recent discoveries made in invertebrate models and mammalian systems, focusing on the molecular mechanisms that regulate the efficient degradation of apoptotic cells.

Adipose-specific deletion of autophagy-related gene 7 (atg7) in mice reveals a role in adipogenesis

White adipocytes have a unique structure in which nearly the entire cell volume is occupied by one large lipid droplet. However, the molecular and cellular processes involved in the cytoplasmic remodeling necessary to create this structure are poorly defined. Autophagy is a membrane trafficking process leading to lysosomal degradation. Here, we investigated the effect of the deletion of an essential autophagy gene, autophagy-related gene 7 (atg7), on adipogenesis. A mouse model with a targeted deletion of atg7 in adipose tissue was generated. The mutant mice were slim and contained only 20% of the mass of white adipose tissue (WAT) found in wild-type mice. Interestingly, approximately 50% of the mutant white adipocytes were multilocular. The mutant white adipocytes were smaller with a larger volume of cytosol and contained more mitochondria. These cells exhibited altered fatty acid metabolism with increased rates of beta-oxidation and reduced rates of hormone-induced lipolysis. Consistently, the mutant mice had lower fed plasma concentrations of fatty acids and the levels decreased at faster rates upon insulin stimuli. These mutant mice exhibited increased insulin sensitivity. The mutant mice also exhibited markedly decreased plasma concentrations of leptin but not adiponectin, lower plasma concentrations of triglyceride and cholesterol, and they had higher levels of basal physical activity. Strikingly, these mutant mice were resistant to high-fat-diet-induced obesity. Taken together, our results indicate that atg7, and by inference autophagy, plays an important role in normal adipogenesis and that inhibition of autophagy by disrupting the atg7 gene has a unique anti-obesity and insulin sensitization effect.

A coat of many colors: neuroimmune crosstalk in human immunodeficiency virus infection

The use of antiretroviral therapy has reduced mortality and increased the quality of life of HIV-1-infected people, particularly in more developed countries where access to treatment is more widespread. However, morbidities continue, which include HIV-1-associated neurocognitive disorders (HAND). Subtle cognitive abnormalities and low-level viral replication underlie disease. The balance between robust antiviral adaptive immunity, neuronal homeostatic mechanisms, and neuroprotective factors on one hand and toxicities afforded by dysregulated immune activities on the other govern disease. New insights into the pathobiological processes for neuroimmune-linked disease and ways to modulate such activities for therapeutic gain are discussed. Better understanding of the complexities of immune regulation during HAND can improve diagnosis and disease outcomes but is also relevant for the pathogenesis of a broad range of neurodegenerative disorders.

Oxidative injury triggers autophagy in astrocytes: the role of endogenous zinc

We have recently demonstrated that the accumulation of labile zinc in lysosomes during oxidative stress causes lysosomal membrane permeabilization (LMP) in cultured hippocampal neurons. Since autophagy involves fusion of autophagic vacuoles (AVs) with lysosomes, zinc accumulation may start in AVs. In the present study, we examined the role of endogenous zinc in H2O2-induced autophagy and cell death in mouse astrocyte cultures. Live-cell confocal imaging of astrocytes transfected with GFP-LC3 revealed that the number of AVs positive for LC3 (microtubule-associated protein 1 light chain 3) increased following exposure to H2O2 or ferrous chloride (FeCl2). Staining of RFP-LC3-transfected astrocytes with FluoZin-3 indicated that the levels of labile zinc increased in AVs as well as in the cytosol and nuclei. The majority of AVs were double-stained with LysoTracker, indicating that they were fused with lysosomes. Chelation of zinc with tetrakis [2-pyridylmethyl]ethylenediamine (TPEN) decreased the number of AVs in H2O2-treated astrocytes, whereas exposure to zinc increased their number, suggesting that dysregulation of zinc homeostasis is mechanistically linked to autophagy. Unexpectedly, inhibition of autophagy blocked the rise in labile zinc levels. Astrocytic death induced by H2O2) was ccompanied by LMP. Autophagy inhibitors (3-methyladenine, bafilomycin-1) or TPEN attenuated LMP and cell death in astrocytes. These results support the possibility that endogenous zinc plays a key role in autophagy under oxidative stress in astrocytes, and suggest that autophagy is a necessary preceding event for LMP and cell death in oxidative injury.

Targeted deletion of autophagy-related 5 (atg5) impairs adipogenesis in a cellular model and in mice

Mammalian white adipocytes have a unique structure in which nearly the entire cell volume is occupied by a single large lipid droplet, while the surrounding cytoplasm occupies minimal space. The massive cytoplasmic remodeling processes involved in the formation of this unique cellular structure are poorly defined. Autophagy is a membrane trafficking process leading to lysosomal degradation of cytoplasmic components. Here, we investigated the functional role of atg5, a gene encoding an essential protein required for autophagy, in adipocyte differentiation in a cellular model and in mice. Massive autophagy was activated when wild-type primary mouse embryonic fibroblasts (MEFs) were induced for adipocyte differentiation. Importantly, the autophagy deficient primary atg5(-/-) MEFs exhibited dramatically reduced efficiency in adipogenesis. Time-lapse microscopy revealed that atg5(-/-) MEFs initially appeared to differentiate normally; however, a majority of the differentiating atg5(-/-) cells ultimately failed to undergo further morphological transformation and eventually died, likely through apoptosis. Consistent with these in vitro results, histological analysis revealed that the atg5(-/-) late-stage embryos and neonatal pups had much less subcutaneous perilipin A-positive adipocytes. Consistently, when treated with chloroquine, a functional inhibitor of autophagy, wild-type MEFs exhibited drastically reduced efficiency of adipocyte differentiation. Taken together, these findings demonstrated that Atg5 is involved in normal adipocyte differentiation, suggesting an important role of autophagy in adipogenesis.

Evaluation of autophagy using mouse models of brain injury

Autophagy is a homeostatic, carefully regulated, and dynamic process for intracellular recycling of bulk proteins, aging organelles, and lipids. Autophagy occurs in all tissues and cell types, including the brain and neurons. Alteration in the dynamics of autophagy has been observed in many diseases of the central nervous system. Disruption of autophagy for an extended period of time results in accumulation of unwanted proteins and neurodegeneration. However, the role of enhanced autophagy after acute brain injury remains undefined. Established mouse models of brain injury will be valuable in clarifying the role of autophagy after brain injury and are the topic of discussion in this review.

Dining in with BCL-2: new guests at the autophagy table

BCL-2 homologues are major regulators of apoptosis and, as such, play an active role in the survival of adult neurons following injury. In recent years, these proteins have also been associated with the regulation of autophagy, a catabolic process involved in the recycling of nutrients upon starvation. Basal levels of autophagy are also required to eliminate damaged proteins and organelles. This is illustrated by the accumulation of ubiquitin-positive aggregates in cells deficient in autophagy and, in the nervous system, this is associated with progressive cell loss and signs of neurodegeneration. Given the importance of both apoptosis and autophagy for neuronal survival in adult neurons, understanding how BCL-2 homologues co-ordinately regulate these processes will allow a better understanding of the cellular processes leading to neurodegeneration. In the present review, we will discuss the roles of BCL-2 homologues in the regulation of apoptosis and autophagy, focussing on their impact on adult neurons.

Matrix protein 2 of influenza A virus blocks autophagosome fusion with lysosomes

Influenza A virus is an important human pathogen causing significant morbidity and mortality every year and threatening the human population with epidemics and pandemics. Therefore, it is important to understand the biology of this virus to develop strategies to control its pathogenicity. Here, we demonstrate that influenza A virus inhibits macroautophagy, a cellular process known to be manipulated by diverse pathogens. Influenza A virus infection causes accumulation of autophagosomes by blocking their fusion with lysosomes, and one viral protein, matrix protein 2, is necessary and sufficient for this inhibition of autophagosome degradation. Macroautophagy inhibition by matrix protein 2 compromises survival of influenza virus-infected cells but does not influence viral replication. We propose that influenza A virus, which also encodes proapoptotic proteins, is able to determine the death of its host cell by inducing apoptosis and also by blocking macroautophagy.

Autophagy in immune cell regulation and dysregulation

Autophagy is an ancient pathway required for cell and tissue homeostasis and differentiation. Initially thought to be a process leading to cell death, autophagy is currently viewed as a beneficial catabolic process that promotes cell survival under starvation conditions by sequestering components of the cytoplasm, including misfolded proteins, protein aggregates, and damaged organelles, and targeting them for lysosome-mediated degradation. In this way, autophagy plays a role in maintaining a balance between degradation and recycling of cellular material. The importance of autophagy is underscored by the fact that malfunctioning of this pathway results in neurodegeneration, cancer, susceptibility to microbial infection, and premature aging. Autophagy occurs in almost all cell types, including immune cells. Recent advances in the field suggest that autophagy plays a central role in regulating the immune system at multiple levels. In this review, we focus on recent developments in the area of autophagy-mediated modulation of immune responses.

Mitochondrial dysfunction is a trigger of Alzheimer's disease pathophysiology

Mitochondria are uniquely poised to play a pivotal role in neuronal cell survival or death because they are regulators of both energy metabolism and cell death pathways. Extensive literature exists supporting a role for mitochondrial dysfunction and oxidative damage in the pathogenesis of Alzheimer's disease. This review discusses evidence indicating that mitochondrial dysfunction has an early and preponderant role in Alzheimer's disease. Furthermore, the link between mitochondrial dysfunction and autophagy in Alzheimer's disease is also discussed. As a result of insufficient digestion of oxidatively damaged macromolecules and organelles by autophagy, neurons progressively accumulate lipofuscin that could exacerbate neuronal dysfunction. Since autophagy is the major pathway involved in the degradation of protein aggregates and defective organelles, an intense interest in developing autophagy-related therapies is growing among the scientific community. The final part of this review is devoted to discuss autophagy as a potential target of therapeutic interventions in Alzheimer's disease pathophysiology.

Lysosomal degradation of membrane lipids

The constitutive degradation of membrane components takes place in the acidic compartments of a cell, the endosomes and lysosomes. Sites of lipid degradation are intralysosomal membranes that are formed in endosomes, where the lipid composition is adjusted for degradation. Cholesterol is sorted out of the inner membranes, their content in bis(monoacylglycero)phosphate increases, and, most likely, sphingomyelin is degraded to ceramide. Together with endosomal and lysosomal lipid-binding proteins, the Niemann-Pick disease, type C2-protein, the GM2-activator, and the saposins sap-A, -B, -C, and -D, a suitable membrane lipid composition is required for degradation of complex lipids by hydrolytic enzymes.

Discovery of Atg5/Atg7-independent alternative macroautophagy

Macroautophagy is a process that leads to the bulk degradation of subcellular constituents by producing autophagosomes/autolysosomes. It is believed that Atg5 (ref. 4) and Atg7 (ref. 5) are essential genes for mammalian macroautophagy. Here we show, however, that mouse cells lacking Atg5 or Atg7 can still form autophagosomes/autolysosomes and perform autophagy-mediated protein degradation when subjected to certain stressors. Although lipidation of the microtubule-associated protein light chain 3 (LC3, also known as Map1lc3a) to form LC3-II is generally considered to be a good indicator of macroautophagy, it did not occur during the Atg5/Atg7-independent alternative process of macroautophagy. We also found that this alternative process of macroautophagy was regulated by several autophagic proteins, including Unc-51-like kinase 1 (Ulk1) and beclin 1. Unlike conventional macroautophagy, autophagosomes seemed to be generated in a Rab9-dependent manner by the fusion of isolation membranes with vesicles derived from the trans-Golgi and late endosomes. In vivo, Atg5-independent alternative macroautophagy was detected in several embryonic tissues. It also had a function in clearing mitochondria during erythroid maturation. These results indicate that mammalian macroautophagy can occur through at least two different pathways: an Atg5/Atg7-dependent conventional pathway and an Atg5/Atg7-independent alternative pathway.

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.

Expanding roles for AMP-activated protein kinase in neuronal survival and autophagy

AMP-activated protein kinase (AMPK) is an evolutionarily conserved cellular switch that activates catabolic pathways and turns off anabolic processes. In this way, AMPK activation can restore the perturbation of cellular energy levels. In physiological situations, AMPK senses energy deficiency (in the form of an increased AMP/ATP ratio), but it is also activated by metabolic insults, such as glucose or oxygen deprivation. Metformin, one of the most widely prescribed anti-diabetic drugs, exerts its actions by AMPK activation. However, while the functions of AMPK as a metabolic regulator are fairly well understood, its actions in neuronal cells only recently gained attention. This review will discuss newly emerged functions of AMPK in neuroprotection and neurodegeneration. Additionally, recent views on the role of AMPK in autophagy, an important catabolic process that is also involved in neurodegeneration and cancer, will be highlighted.

Proteinopathy-induced neuronal senescence: a hypothesis for brain failure in Alzheimer's and other neurodegenerative diseases

Background

Alzheimer's disease (AD) and a host of other neurodegenerative central nervous system (CNS) proteinopathies are characterized by the accumulation of misfolded protein aggregates. Simplistically, these aggregates can be divided into smaller, soluble, oligomeric and larger, less-soluble or insoluble, fibrillar forms. Perhaps the major ongoing debate in the neurodegenerative disease field is whether the smaller oligomeric or larger fibrillar aggregates are the primary neurotoxin. Herein, we propose an integrative hypothesis that provides new insights into how a variety of misfolded protein aggregates can result in neurodegeneration.

Results

We introduce the concept that a wide range of highly stable misfolded protein aggregates in AD and other neurodegenerative proteinopathies are recognized as non-self and chronically activate the innate immune system. This pro-inflammatory state leads to physiological senescence of CNS cells. Once CNS cells undergo physiological senescence, they secrete a variety of pro-inflammatory molecules. Thus, the senescence of cells, which was initially triggered by inflammatory stimuli, becomes a self-reinforcing stimulus for further inflammation and senescence. Ultimately, senescent CNS cells become functionally impaired and eventually die, and this neurodegeneration leads to brain organ failure.

Conclusion

This integrative hypothesis, which we will refer to as the proteinopathy-induced senescent cell hypothesis of AD and other neurodegenerative diseases, links CNS proteinopathies to inflammation, physiological senescence, cellular dysfunction, and ultimately neurodegeneration. Future studies characterizing the senescent phenotype of CNS cells in AD and other neurodegenerative diseases will test the validity of this hypothesis. The implications of CNS senescence as a contributing factor to the neurodegenerative cascade and its implications for therapy are discussed.

XBP-1 deficiency in the nervous system protects against amyotrophic lateral sclerosis by increasing autophagy

Mutations in superoxide dismutase-1 (SOD1) cause familial amyotrophic lateral sclerosis (fALS). Recent evidence implicates adaptive responses to endoplasmic reticulum (ER) stress in the disease process via a pathway known as the unfolded protein response (UPR). Here, we investigated the contribution to fALS of X-box-binding protein-1 (XBP-1), a key UPR transcription factor that regulates genes involved in protein folding and quality control. Despite expectations that XBP-1 deficiency would enhance the pathogenesis of mutant SOD1, we observed a dramatic decrease in its toxicity due to an enhanced clearance of mutant SOD1 aggregates by macroautophagy, a cellular pathway involved in lysosome-mediated protein degradation. To validate these observations in vivo, we generated mutant SOD1 transgenic mice with specific deletion of XBP-1 in the nervous system. XBP-1-deficient mice were more resistant to developing disease, correlating with increased levels of autophagy in motoneurons and reduced accumulation of mutant SOD1 aggregates in the spinal cord. Post-mortem spinal cord samples from patients with sporadic ALS and fALS displayed a marked activation of both the UPR and autophagy. Our results reveal a new function of XBP-1 in the control of autophagy and indicate critical cross-talk between these two signaling pathways that can provide protection against neurodegeneration.

Abnormal localization of leucine-rich repeat kinase 2 to the endosomal-lysosomal compartment in lewy body disease

Missense mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are the most common causes of both familial and sporadic forms of Parkinson disease and are also associated with diverse pathological alterations. The mechanisms whereby LRRK2 mutations cause these pathological phenotypes are unknown. We used immunohistochemistry with 3 distinct anti-LRRK2 antibodies to characterize the expression of LRRK2 in the brains of 21 subjects with various neurodegenerative disorders and 7 controls. The immunoreactivity of LRRK2 was localized in a subset of brainstem-type Lewy bodies (LBs) but not in cortical-type LBs, tau-positive inclusions, or TAR-DNA-binding protein-43-positive inclusions. The immunoreactivity of LRRK2 frequently appeared as enlarged granules or vacuoles within neurons of affected brain regions, including the substantia nigra, amygdala, and entorhinal cortex in patients with Parkinson disease or dementia with LBs. The volumes of LRRK2-positive granular structures in neurons of the entorhinal cortex were significantly increased in dementia with LBs brains compared with age-matched control brains (p < 0.05). Double immunolabeling demonstrated that these LRRK2-positive granular structures frequently colocalized with the late-endosomal marker Rab7B and occasionally with the lysosomal marker, the lysosomal-associated membrane protein 2. These results suggest that LRRK2 normally localizes to the endosomal-lysosomal compartment within morphologically altered neurons in neurodegenerative diseases, particularly in the brains of patients with LB diseases.