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Regulation of neuronal autophagy in axon: implication of autophagy in axonal function and dysfunction/degeneration.

Autophagy has recently emerged as potential drug target for prevention of neurodegeneration. However, the details of autophagy process and regulation in the central nervous system (CNS) are unclear. By using a neuronal excitotoxicity model mice, we engineered expression of a fluorescent autophagic marker and systematically investigated autophagic activity under neurodegenerative condition. The study reveals an early response of Purkinje cells to excitotoxic insult by induction of autophagy in axon terminals, and that axonal autophagy is particularly robust in comparison to the cell body and dendrites. The accessibility of axons to rapid autophagy induction suggests local biogenesis of autophagosomes in axons. Characterization of functional interaction between autophagosome protein LC3 and microtubule-associated protein 1B (MAP1B), which is involved in axonal growth, injury and transport provides evidence for neuron or axon-specific regulation of autophagosomes. Furthermore, we propose that p62/SQSTM1, a putative autophagic substrate can serve as a marker for evaluating impairment of autophagic degradation, which helps resolve the controversy over autophagy levels under various pathological conditions. Future study of the relationship between autophagy and axonal function (e.g., transport) will provide insight into the mechanism underlying axonopathy which is directly linked to neurodegeneration.

Adaptor Proteins, Signal Transducing↗

AtATG genes, homologs of yeast autophagy genes, are involved in constitutive autophagy in Arabidopsis root tip cells.

In Arabidopsis root tips cultured in medium containing sufficient nutrients and the membrane-permeable protease inhibitor E-64d, parts of the cytoplasm accumulated in the vacuoles of the cells from the meristematic zone to the elongation zone. Also in barley root tips treated with E-64, parts of the cytoplasm accumulated in autolysosomes and pre-existing central vacuoles. These results suggest that vacuolar and/or lysosomal autophagy occurs constitutively in these regions of cells. 3-Methyladenine, an inhibitor of autophagy, inhibited the accumulation of such inclusions in Arabidopsis root tip cells. Such inclusions were also not observed in root tips prepared from Arabidopsis T-DNA mutants in which AtATG2 or AtATG5, an Arabidopsis homolog of yeast ATG genes essential for autophagy, is disrupted. In contrast, an atatg9 mutant, in which another homolog of ATG is disrupted, accumulated a significant number of vacuolar inclusions in the presence of E-64d. These results suggest that both AtAtg2 and AtAtg5 proteins are essential for autophagy whereas AtAtg9 protein contributes to, but is not essential for, autophagy in Arabidopsis root tip cells. Autophagy that is sensitive to 3-methyladenine and dependent on Atg proteins constitutively occurs in the root tip cells of Arabidopsis.

Adenine↗

Generation of cell lines with tetracycline-regulated autophagy and a role for autophagy in controlling cell size.

Autophagy is an intracellular bulk degradation system. We established mouse fibroblast lines coupling the Tet-off system with an Atg5(-/-) mouse embryonic fibroblast line to artificially regulate autophagic ability. In the presence of doxycycline (Dox), Atg5 expression was completely suppressed and these cells were autophagy-defective. After removal of Dox, autophagic ability was restored within 6h. Very low levels of Atg5 could induce an autophagy competent state. We applied this novel system to examine the contribution of autophagy to controlling cell size. Cell size reduction in response to starvation was significantly inhibited in cells unable to undergo autophagy. The generated cell lines will be useful reagents for future mechanistic studies into the regulation and physiologic significance of autophagy.

Animals↗

Intracellular quality control by autophagy: how does autophagy prevent neurodegeneration?

Autophagy is an intracellular bulk degradation process, through which a portion of cytoplasm is delivered to lysosomes to be degraded. In many organisms, the primary role of autophagy is adaptation to starvation. However, we have found that autophagy is also important for intracellular protein quality control. Atg5(-/-) mice die shortly after birth due, at least in part, to nutrient deficiency. These mice also exhibit an intracellular accumulation of protein aggregates in neurons and hepatocytes. We now report the generation of neural cell-specific Atg5-deficient mice. Atg5( flox/flox);Nestin-Cre mice show progressive deficits in motor function and degeneration of some neural cells. In autophagy-deficient cells, diffuse accumulation of abnormal proteins occurs, followed by the generation of aggregates and inclusions. This study emphasizes the point that basal autophagy is important even in individuals who do not express neurodegenerative disease-associated mutant proteins. Furthermore, the primary targets of autophagy are diffuse cytosolic proteins, not protein aggregates themselves.

Animals↗

Membrane recruitment of Aut7p in the autophagy and cytoplasm to vacuole targeting pathways requires Aut1p, Aut2p, and the autophagy conjugation complex.

Autophagy is a degradative pathway by which cells sequester nonessential, bulk cytosol into double-membrane vesicles (autophagosomes) and deliver them to the vacuole for recycling. Using this strategy, eukaryotic cells survive periods of nutritional starvation. Under nutrient-rich conditions, autophagy machinery is required for the delivery of a resident vacuolar hydrolase, aminopeptidase I, by the cytoplasm to vacuole targeting (Cvt) pathway. In both pathways, the vesicle formation process requires the function of the starvation-induced Aut7 protein, which is recruited from the cytosol to the forming Cvt vesicles and autophagosomes. The membrane binding of Aut7p represents an early step in vesicle formation. In this study, we identify several requirements for Aut7p membrane association. After synthesis in the cytosol, Aut7p is proteolytically cleaved in an Aut2p-dependent manner. While this novel processing event is essential for Aut7p membrane binding, Aut7p must undergo additional physical interactions with Aut1p and the autophagy (Apg) conjugation complex before recruitment to the membrane. Lack of these interactions results in a cytosolic distribution of Aut7p rather than localization to forming Cvt vesicles and autophagosomes. This study assigns a functional role for the Apg conjugation system as a mediator of Aut7p membrane recruitment. Further, we demonstrate that Aut1p, which physically interacts with components of the Apg conjugation complex and Aut7p, constitutes an additional factor required for Aut7p membrane recruitment. These findings define a series of steps that results in the modification of Aut7p and its subsequent binding to the sequestering transport vesicles of the autophagy and cytoplasm to vacuole targeting pathways.

Animals↗

4th International Symposium on Autophagy: exploiting the frontiers of autophagy research.

The 4th International Symposium on Autophagy was held in Mishima, a small town between Tokyo and Kyoto, October 1-5, 2006 (http://isa4th.umin.jp/). The meeting was organized by the group of Eiki Kominami. Approximately 150 participants took part in this well-organized meeting in the spacious and comfortable Toray Conference Hall (Fig. 1). The social program offered opportunities for informal discussions, Japanese culture (from karaoke singing to traditional drumming; Fig. 2), history and nature (a visit to a steaming volcano; Fig. 3), as well as delicious Japanese food. The scientific program started with two plenary lectures on Sunday evening. Daniel Klionsky gave an overview of Atg9 cycling in yeast and Shigekazu Nagata talked about apoptosis and engulfment of dead cells by macrophages. The meeting consisted of five oral sessions and two poster sessions covering a wide range of autophagy-related topics. Exciting unpublished results were presented in all sessions, showing how quickly autophagy research is progressing. Two themes were discussed in many sessions during the symposium: the role of autophagy in the degradation of aggregate-prone proteins and protein aggregates, and the possible role of p62 in autophagy.

Aging↗

The actin cytoskeleton is required for selective types of autophagy, but not nonspecific autophagy, in the yeast Saccharomyces cerevisiae.

Autophagy is a catabolic multitask transport route that takes place in all eukaryotic cells. During starvation, cytoplasmic components are randomly sequestered into huge double-membrane vesicles called autophagosomes and delivered into the lysosome/vacuole where they are destroyed. Cells are able to modulate autophagy in response to their needs, and under certain circumstances, cargoes such as aberrant protein aggregates, organelles and bacteria can be selectively and exclusively incorporated into autophagosomes. In the yeast Saccharomyces cerevisiae, for example, double-membrane vesicles are used to transport the Ape1 protease into the vacuole, or for the elimination of superfluous peroxisomes. In the present study we reveal that in this organism, actin plays a role in these two types of selective autophagy but not in the nonselective, bulk process. In particular, we show that precursor Ape1 is not correctly recruited to the PAS, the putative site of double-membrane vesicle biogenesis, and superfluous peroxisomes are not degraded in a conditional actin mutant. These phenomena correlate with a defect in Atg9 trafficking from the mitochondria to the PAS.

Actins↗

Autophagy as a cell-repair mechanism: activation of chaperone-mediated autophagy during oxidative stress.

Proper removal of oxidized proteins is an important determinant of success when evaluating the ability of cells to handle oxidative stress. The ubiquitin/proteasome system has been considered the main responsible mechanism for the removal of oxidized proteins, as it can discriminate between normal and altered proteins, and selectively target the latter ones for degradation. A possible role for lysosomes, the other major intracellular proteolytic system, in the removal of oxidized proteins has been often refused, mostly on the basis of the lack of selectivity of this system. Although most of the degradation of intracellular components in lysosomes (autophagy) takes place through "in bulk" sequestration of complete cytosolic regions, selective targeting of proteins to lysosomes for their degradation is also possible via what is known as chaperone-mediated autophagy (CMA). In this work, we review recent evidence supporting the participation of CMA in the clearance of oxidized proteins in the forefront of the cellular response to oxidative stress. The consequences of an impairment in CMA activity, observed during aging and in some age-related disorders, are also discussed.

Animals↗

Direct induction of autophagy by Atg1 inhibits cell growth and induces apoptotic cell death.

BACKGROUND: To survive starvation and other forms of stress, eukaryotic cells undergo a lysosomal process of cytoplasmic degradation known as autophagy. Autophagy has been implicated in a number of cellular and developmental processes, including cell-growth control and programmed cell death. However, direct evidence of a causal role for autophagy in these processes is lacking, resulting in part from the pleiotropic effects of signaling molecules such as TOR that regulate autophagy. Here, we circumvent this difficulty by directly manipulating autophagy rates in Drosophila through the autophagy-specific protein kinase Atg1. RESULTS: We find that overexpression of Atg1 is sufficient to induce high levels of autophagy, the first such demonstration among wild-type Atg proteins. In contrast to findings in yeast, induction of autophagy by Atg1 is dependent on its kinase activity. We find that cells with high levels of Atg1-induced autophagy are rapidly eliminated, demonstrating that autophagy is capable of inducing cell death. However, this cell death is caspase dependent and displays DNA fragmentation, suggesting that autophagy represents an alternative induction of apoptosis, rather than a distinct form of cell death. In addition, we demonstrate that Atg1-induced autophagy strongly inhibits cell growth and that Atg1 mutant cells have a relative growth advantage under conditions of reduced TOR signaling. Finally, we show that Atg1 expression results in negative feedback on the activity of TOR itself. CONCLUSIONS: Our results reveal a central role for Atg1 in mounting a coordinated autophagic response and demonstrate that autophagy has the capacity to induce cell death. Furthermore, this work identifies autophagy as a critical mechanism by which inhibition of TOR signaling leads to reduced cell growth.

Animals↗

Respiratory viruses activate autophagy via the IFN-STAT1/STAT5B-SOCS1 axis.

Autophagy is an ancient catabolic process that has emerged as part of innate immunity. Upon infection, autophagy is activated but the key factors responsible remained unclear. Here, we show that interferon (IFN) released during viral infections subsequently activates autophagy via STAT1/5B-mediated upregulation of Suppressor of Cytokine Signaling 1 (SOCS1). Our data show that scavenging of IFNs diminishes autophagy induced by several respiratory viruses. All types of IFN (I, II and III) mediated robust autophagic flux activation in both cell lines and primary human lung fibroblasts in a JAK1-3 dependent manner. Depletion or pharmacological inhibition of individual signal transducer and activator of transcription (STAT) transcription factors demonstrated that both STAT1 and STAT5B are required for IFN-induced autophagy. Upon IFN stimulation STAT1 and STAT5B associate and translocate to the nucleus. Transcriptome analyses revealed that most known anti-viral IFN-stimulated genes (ISGs) remain induced to high levels upon inhibition of STAT5 expect for a small subset of ISGs, among them SOCS1. Overexpression of SOCS1 stimulated autophagy, whereas its depletion impaired IFN-induced autophagy. Successful viruses like measles virus (MeV) or respiratory syncytial virus (RSV) evolved strategies to exploit autophagy to promote their own replication. Uncoupling IFN-mediated ISG defenses from autophagy induction by STAT5 inhibition reduced virus-induced autophagy, and inhibited efficient replication of autophagy-dependent MeV and RSV. Overexpression of SOCS1 upon STAT5 inhibition largely rescued both infection-induced autophagy and viral replication. Taken together, our data show that IFN promotes autophagy via STAT1/STAT5B-SOCS1 in viral infections and reveal that targeting of this axis allows inhibition of autophagy-dependent viruses without compromising innate immune defenses.

Humans↗

Regulation of autophagy by sphingosine kinase 1 and its role in cell survival during nutrient starvation.

The sphingolipid ceramide induces macroautophagy (here called autophagy) and cell death with autophagic features in cancer cells. Here we show that overexpression of sphingosine kinase 1 (SK1), an enzyme responsible for the production of sphingosine 1-phosphate (S1P), in MCF-7 cells stimulates autophagy by increasing the formation of LC3-positive autophagosomes and the rate of proteolysis sensitive to the autophagy inhibitor 3-methyladenine. Autophagy was blocked in the presence of dimethylsphingosine, an inhibitor of SK activity, and in cells expressing a catalytically inactive form of SK1. In SK1(wt)-overexpressing cells, however, autophagy was not sensitive to fumonisin B1, an inhibitor of ceramide synthase. In contrast to ceramide-induced autophagy, SK1(S1P)-induced autophagy is characterized by (i) the inhibition of mammalian target of rapamycin signaling independently of the Akt/protein kinase B signaling arm and (ii) the lack of robust accumulation of the autophagy protein Beclin 1. In addition, nutrient starvation induced both the stimulation of autophagy and SK activity. Knocking down the expression of the autophagy protein Atg7 or that of SK1 by siRNA abolished starvation-induced autophagy and increased cell death with apoptotic hallmarks. In conclusion, these results show that SK1(S1P)-induced autophagy protects cells from death with apoptotic features during nutrient starvation.

Adenine↗

Accelerated cell death in Podospora autophagy mutants.

Although autophagy is characteristic of type II programmed cell death (PCD), its role in cell death is currently debated. Both cell death-promoting and prosurvival roles of autophagy have been reported depending on the organism and the cell type. In filamentous fungi, a cell death reaction known as an incompatibility reaction occurs when cells of unlike genotype fuse. Cell death by incompatibility is characterized by a dramatic vacuolar enlargement and cell lysis. In Podospora anserina, autophagy is induced early during this cell death reaction. Cell death by incompatibility in Podospora is a model of type II PCD used here to assess the role of autophagy in this type of cell death. We have inactivated PaATG1, the Podospora ortholog of the Saccharomyces cerevisiae ATG1 gene involved in the early steps of autophagy in yeast. The DeltaPaATG1 mutant displays developmental defects characteristic of abrogated autophagy in Podospora. Using the green fluorescent protein-PaATG8 autophagosome marker, we show that autophagy is abolished in this mutant. Neither cell death by incompatibility nor vacuolization are suppressed in DeltaPaATG1 and DeltaPaATG8 autophagy mutants, indicating that a vacuolar cell death reaction without autophagy occurs in Podospora. Our results thus provide a novel example of a type II PCD reaction in which autophagy is not the cause of cell death. In addition, we found that cell death is accelerated in DeltaPaATG null mutants, suggesting that autophagy has a protective role in this type II PCD reaction.

Autophagy↗

Tor, a phosphatidylinositol kinase homologue, controls autophagy in yeast.

Autophagy is a bulk protein degradation process that is induced by starvation. The control mechanism for induction of autophagy is not well understood. We found that Tor, a phosphatidylinositol kinase homologue, is involved in the control of autophagy in the yeast, Saccharomyces cerevisiae. When rapamycin, an inhibitor of Tor function, is added, autophagy is induced even in cells growing in nutrient-rich medium. A temperature-sensitive tor mutant also leads to induction of autophagy at a nonpermissive temperature. These results indicate that Tor negatively regulates the induction of autophagy. Tor is the first molecule that is identified as a pivotal player in the starvation-signaling pathway of autophagy. Furthermore, we found that a high concentration of cAMP is inhibitory for induction of autophagy. APG gene products are involved in autophagy induced by starvation. Autophagy was not induced in apg mutants in the presence of rapamycin, indicating that the site of action of Tor is upstream of those of Apg proteins. In nutrient-rich medium, Apg proteins are involved also in the transport of aminopeptidase I from the cytosol to the vacuole. Tor may act to switch Apg function between autophagy and transport of aminopeptidase I.

1-Phosphatidylinositol 4-Kinase↗

Autophagy is induced in CD4+ T cells and important for the growth factor-withdrawal cell death.

Autophagy is a tightly regulated catabolic mechanism that degrades proteins and organelles. Autophagy mediates programmed cell death under certain conditions. To determine the role of autophagy in T cells, we examined, in mouse CD4+ T cells, conditions under which autophagy is induced and alterations of the cell fate when autophagy is blocked. We have found that resting naive CD4+ T cells do not contain detectable autophagosomes. Autophagy can be observed in activated CD4+ T cells upon TCR stimulation, cytokine culturing, and prolonged serum starvation. Induction of autophagy in T cells requires JNK and the class III PI3K. Autophagy is inhibited by caspases and mammalian target of rapamycin in T cells. Interestingly, more Th2 cells than Th1 cells undergo autophagy. Th2 cells become more resistant to growth factor-withdrawal cell death when autophagy is blocked using either chemical inhibitors 3-methyladenine, or by RNA interference knockdown of beclin 1 and Atg7. Therefore, autophagy is an important mechanism that controls homeostasis of CD4+ T cells.

Animals↗

Role of autophagy in cancer: management of metabolic stress.

Human breast, ovarian, and prostate tumors display allelic loss of the essential autophagy gene beclin1 with high frequency, and an increase in the incidence of tumor formation is observed in beclin1(+/-) mutant mice. These findings suggest a role for beclin1 and autophagy in tumor suppression; however, the mechanism by which this occurs has been unclear. Autophagy is a bulk degradation process whereby organelles and cytoplasm are engulfed and targeted to lysosomes for proteolysis,(1,2) There is evidence that autophagy sustains cell survival during nutrient deprivation through catabolism, but also that autophagy is a means of achieving cell death when executed to completion. If or how either of these diametrically opposing functions proposed for autophagy may be related to tumor suppression is unknown. We found that metabolic stress is a potent trigger of apoptotic cell death, defects in which enable long-term survival that is dependent on autophagy both in vitro and in tumors in vivo.(3) These findings raise the conundrum whereby inactivation of a survival pathway (autophagy) promotes tumorigenesis. Interestingly, when cells with defects in apoptosis are denied autophagy, this creates the inability to tolerate metabolic stress, reduces cellular fitness, and activates a necrotic pathway to cell death. This necrosis in tumors is associated with inflammation and enhancement of tumor growth, due to the survival of a small population of surviving, but injured, cells in a microenvironment that favors oncogenesis. Thus, by sustaining metabolism through autophagy during periods of metabolic stress, cells can limit energy depletion, cellular damage, and cell death by necrosis, which may explain how autophagy can prevent cancer, and how loss of a survival function can be tumorigenic.

Animals↗

The evolutionarily conserved domain of Beclin 1 is required for Vps34 binding, autophagy and tumor suppressor function.

Atg6/Beclin 1 is an evolutionarily conserved protein family that has been shown to function in vacuolar protein sorting (VPS) in yeast; in autophagy in yeast, Drosophila, Dictyostelium, C.elegans, and mammals; and in tumor suppression in mice. Atg6/Beclin 1 is thought to function as a VPS and autophagy protein as part of a complex with Class III phosphatidylinositol 3'-kinase (PI3K)/Vps34. However, nothing is known about which domains of Atg6/Beclin 1 are required for its functional activity and binding to Vps34. We hypothesized that the most highly conserved region of human Beclin 1 spanning from amino acids 244-337 is essential for Vps34 binding, autophagy, and tumor suppressor function. To investigate this hypothesis, we evaluated the effects of wild-type and mutant beclin 1 gene transfer in autophagy-deficient MCF7 human breast carcinoma cells. We found that, unlike wild-type Beclin 1, a Beclin 1 mutant lacking aa 244-337 (Beclin 1DeltaECD), is unable to enhance starvation-induced autophagy in low Beclin 1-expressing MCF7 human breast carcinoma cells. In contrast to wild-type Beclin 1, mutant Beclin 1DeltaECD is unable to immunoprecipitate Vps34, has no Beclin 1-associated Vps34 kinase activity, and lacks tumor suppressor function in an MCF7 scid mouse xenograft tumor model. The maturation of cathepsin D, which requires intact Vps34-dependent VPS function, is comparable in autophagy-deficient low-Beclin 1 expressing MCF7 cells, autophagy-deficient MCF7 cells transfected with Beclin 1DeltaECD, and autophagy-competent MCF7 cells transfected with wild-type Beclin 1. These findings identify an evolutionarily conserved domain of Beclin 1 that is essential for Vps34 interaction, autophagy function, and tumor suppressor function. Furthermore, they suggest a connection between Beclin 1-associated Class III PI3K/Vps34-dependent autophagy, but not VPS, function and the mechanism of Beclin 1 tumor suppressor action in human breast cancer cells.

Amino Acid Sequence↗

A novel type of autophagy occurs together with vacuole genesis in miniprotoplasts prepared from tobacco culture cells.

Mature plant cells have large vacuoles. But how these vacuoles are formed has not been fully understood. It has been reported that autophagy is involved in the genesis of plant vacuoles. Thus we examined whether autophagy occurs in the vacuole genesis of a plant cell model called miniprotoplasts, in which preexisting large vacuoles have been removed. We prepared miniprotoplasts from tobacco culture cells (BY-2) and observed the formation of vacuoles by light and electron microscopy. The miniprotoplasts had few vacuoles immediately after preparation, but had large vacuoles after 1 to 2 d. When the cysteine protease inhibitor E-64c or E-64d was added to culture media, almost all vacuoles formed contained materials of cytoplasmic origin. This result suggests that autophagy occurs together with the genesis of the vacuoles in miniprotoplasts. 3-Methyladenine and phosphatidylinositol 3-kinase inhibitors such as wortmannin and LY294002, all of which block starvation?induced autophagy in tobacco culture cells and constitutive autophagy in Arabidopsis root cells, did not affect the autophagy in miniprotoplasts. Thus the form of autophagy in miniprotoplasts is probably different from the form of autophagy that arises as a result of sucrose starvation and constitutive autophagy in root tip cells. The causal connection between autophagy and vacuole genesis in miniprotoplasts was not clarified in this study.

Adenine↗

[Inducement of tumor cell autophagy and cell cycle analysis].

BACKGROUND & OBJECTIVE: Autophagy is the main phenomenon of type II programmed cell death which is also named as autophagic cell death, and autophagy has a close relationship with autophagic cell death. The relationship of apoptosis and cell cycle has been explored deeply, but little is known about the relationship of autophagic cell death and cell cycle. This study was to observe the correlation of autophagy induced by different methods to cell cycle. METHODS: Exponentially growing HeLa and SW480 cells, and peripheral blood lymphocytes (PBLs) from healthy donors, with or without 48 h stimulation of phytohemagglutinin (PHA), were treated with Hanks' solution (to produce starvation) or vincristine. Confocal laser microscope and transmission electron microscope (TEM) were used to detect autophagy; flow cytometry (FCM) was innovatively used to detect the cell cycle of autophagic cells with dipl-parameters of microtubule-associated protein 1 light chain 3 (MAP1-LC3-II)/PI. RESULTS: Autophagy of HeLa and SW480 cells induced by starvation or vincristine was observed in G1, S, and G2/M phases and increased along with the inducement time; no autophagy was observed in unstimulated PBLs. The positive rate of LC3-II, indicating the occurrence of autophagy, was lower than 2.62% when induced by starvation in Hanks' solution for 48 h, or 6.16% when induced by vincristine for 48 h. After PBLs were stimulated into cell cycle by PHA, autophagy was markedly detected 2 h after the indicated inducements. CONCLUSIONS: MAP1-LC3-II/DNA dipl-parameter analysis by FCM is a convenient and reliable method for simultaneously analyzing autophagy and cell cycle. Autophagy could be induced when cells are in cell cycle, while the cells in G0 phase are insensitive to the inducers.

Autophagy↗