The cell biology of autophagy in metazoans: a developing story

Macronutrient composition of the diet affects the feeding-mediated down regulation of autophagy in muscle of rainbow trout (O. mykiss)

Autophagy functions as an important catabolic mechanism by mediating the turnover of intracellular organelles and protein complexes through a lysosome dependent degradative pathway. Although the induction of autophagy by starvation has been extensively studied, we still know very little about how autophagy is regulated under normal nutritional conditions. The purpose of the present study was to characterize both in vivo and in vitro the response of the autophagy-lysosomal degradative pathway to nutrient (amino acids and carbohydrates) availability in the muscle of the carnivorous rainbow trout. We report that meal feeding is accompanied by a rapid activation of Akt, FoxO1 and the Target of Rapamycin (TOR) signaling pathways and a concomitant decrease of autophagosome formation. We also show that this effect occurs only when the proportion of dietary proteins increases at the expense of carbohydrates. Concurrently, our in vitro study on primary culture of trout muscle cells demonstrates an opposite effect of amino acids and glucose on the regulation of autophagy-lysosomal pathways. More specifically, the addition of amino acids in cell culture medium inhibited the formation of autophagosomes, whereas the addition of glucose had an opposite effect. The effect of amino acids was accompanied by an activation of TOR, considered as an important regulator of autophagosomal formation. However, the mechanisms involved in the effect of glucose were independent of Akt, TOR and AMPK and remain to be determined. Together, these results demonstrated the specific role of macronutrients as well as that of their interactions in the regulation of autophagy and highlight the interest to consider the macronutrient composition of the diets in the control of this degradative pathway.

Complex regulation of autophagy in cancer - integrated approaches to discover the networks that hold a double-edged sword

Autophagy, a highly regulated self-degradation process of eukaryotic cells, is a context-dependent tumor-suppressing mechanism that can also promote tumor cell survival upon stress and treatment resistance. Because of this ambiguity, autophagy is considered as a double-edged sword in oncology, making anti-cancer therapeutic approaches highly challenging. In this review, we present how systems-level knowledge on autophagy regulation can help to develop new strategies and efficiently select novel anti-cancer drug targets. We focus on the protein interactors and transcriptional/post-transcriptional regulators of autophagy as the protein and regulatory networks significantly influence the activity of core autophagy proteins during tumor progression. We list several network resources to identify interactors and regulators of autophagy proteins. As in silico analysis of such networks often necessitates experimental validation, we briefly summarize tractable model organisms to examine the role of autophagy in cancer. We also discuss fluorescence techniques for high-throughput monitoring of autophagy in humans. Finally, the challenges of pharmacological modulation of autophagy are reviewed. We suggest network-based concepts to overcome these difficulties. We point out that a context-dependent modulation of autophagy would be favored in anti-cancer therapy, where autophagy is stimulated in normal cells, while inhibited only in stressed cancer cells. To achieve this goal, we introduce the concept of regulo-network drugs targeting specific transcription factors or miRNA families identified with network analysis. The effect of regulo-network drugs propagates indirectly through transcriptional or post-transcriptional regulation of autophagy proteins, and, as a multi-directional intervention tool, they can both activate and inhibit specific proteins in the same time. The future identification and validation of such regulo-network drug targets may serve as novel intervention points, where autophagy can be effectively modulated in cancer therapy.

Drosophila p53-related protein kinase is required for PI3K/TOR pathway-dependent growth

Cell growth and proliferation are pivotal for final organ and body size definition. p53-related protein kinase (Bud32/PRPK) has been identified as a protein involved in proliferation through its effects on transcription in yeast and p53 stabilization in human cell culture. However, the physiological function of Bud32/PRPK in metazoans is not well understood. In this work, we have analyzed the role of PRPK in Drosophila development. Drosophila PRPK is expressed in every tissue analyzed and is required to support proliferation and cell growth. The Prpk knockdown animals show phenotypes similar to those found in mutants for positive regulators of the PI3K/TOR pathway. This pathway has been shown to be fundamental for animal growth, transducing the hormonal and nutritional status into the protein translation machinery. Functional interactions have established that Prpk operates as a transducer of the PI3K/TOR pathway, being essential for TOR kinase activation and for the regulation of its targets (S6K and 4E-BP, autophagy and bulk endocytosis). This suggests that Prpk is crucial for stimulating the basal protein biosynthetic machinery in response to insulin signaling and to changes in nutrient availability.

Potential mechanisms of action of lithium in bipolar disorder. Current understanding

Lithium has been used for over half a century for the treatment of bipolar disorder as the archetypal mood stabilizer, and has a wealth of empirical evidence supporting its efficacy in this role. Despite this, the specific mechanisms by which lithium exerts its mood-stabilizing effects are not well understood. Given the inherently complex nature of the pathophysiology of bipolar disorder, this paper aims to capture what is known about the actions of lithium ranging from macroscopic changes in mood, cognition and brain structure, to its effects at the microscopic level on neurotransmission and intracellular and molecular pathways. A comprehensive literature search of databases including MEDLINE, EMBASE and PsycINFO was conducted using relevant keywords and the findings from the literature were then reviewed and synthesized. Numerous studies report that lithium is effective in the treatment of acute mania and for the long-term maintenance of mood and prophylaxis; in comparison, evidence for its efficacy in depression is modest. However, lithium possesses unique anti-suicidal properties that set it apart from other agents. With respect to cognition, studies suggest that lithium may reduce cognitive decline in patients; however, these findings require further investigation using both neuropsychological and functional neuroimaging probes. Interestingly, lithium appears to preserve or increase the volume of brain structures involved in emotional regulation such as the prefrontal cortex, hippocampus and amygdala, possibly reflecting its neuroprotective effects. At a neuronal level, lithium reduces excitatory (dopamine and glutamate) but increases inhibitory (GABA) neurotransmission; however, these broad effects are underpinned by complex neurotransmitter systems that strive to achieve homeostasis by way of compensatory changes. For example, at an intracellular and molecular level, lithium targets second-messenger systems that further modulate neurotransmission. For instance, the effects of lithium on the adenyl cyclase and phospho-inositide pathways, as well as protein kinase C, may serve to dampen excessive excitatory neurotransmission. In addition to these many putative mechanisms, it has also been proposed that the neuroprotective effects of lithium are key to its therapeutic actions. In this regard, lithium has been shown to reduce the oxidative stress that occurs with multiple episodes of mania and depression. Further, it increases protective proteins such as brain-derived neurotrophic factor and B-cell lymphoma 2, and reduces apoptotic processes through inhibition of glycogen synthase kinase 3 and autophagy. Overall, it is clear that the processes which underpin the therapeutic actions of lithium are sophisticated and most likely inter-related.

The ULK1 complex: sensing nutrient signals for autophagy activation

The Atg1/ULK1 complex plays a central role in starvation-induced autophagy, integrating signals from upstream sensors such as MTOR and AMPK and transducing them to the downstream autophagy pathway. Much progress has been made in the last few years in understanding the mechanisms by which the complex is regulated through protein-protein interactions and post-translational modifications, providing insights into how the cell modulates autophagy, particularly in response to nutrient status. However, how the ULK1 complex transduces upstream signals to the downstream central autophagy pathway is still unclear. Although the protein kinase activity of ULK1 is required for its autophagic function, its protein substrate(s) responsible for autophagy activation has not been identified. Furthermore, examples of potential ULK1-independent autophagy have emerged, indicating that under certain specific contexts, the ULK1 complex might be dispensable for autophagy activation. This raises the question of how the autophagic machinery is activated independent of the ULK1 complex and what are the biological functions of such noncanonical autophagy pathways.

Neuronal synaptobrevin promotes longevity in Drosophila photoreceptors

Neurons have unique challenges relative to other cell types. Unlike most other cells, neurons must remain healthy and functional throughout the lifespan of an animal. Premature neuronal loss underlies many age-related neurodegenerative diseases, including Alzheimer and Parkinson Diseases. Despite previous research aimed at understanding the mechanisms of age-related neurodegenerative diseases, little is known about the mechanisms that allow neurons to remain functional for the lifetime of a healthy animal. Understanding these cellular and biochemical processes is essential to promote healthful aging and reduce the severity of neurodegenerative disease. Here we discuss our recent identification of neuron-specific proteins that regulate endosome fusion events and the role of endosomes in maintaining healthy neurons.

Genes for plant autophagy: functions and interactions

Autophagy, or self-consuming of cytoplasmic constituents in a lytic compartment, plays a crucial role in nutrient recycling, development, cell homeostasis, and defense against pathogens and toxic products. Autophagy in plant cells uses a conserved machinery of core Autophagy-related (Atg) proteins. Recently, research on plant autophagy has been expanding and other components interacting with the core Atg proteins are being revealed. In addition, growing evidence suggests that autophagy communicates with other cellular pathways such as the ubiquitin-proteasome system, protein secretory pathway, and endocytic pathway. An increase in our understanding of plant autophagy will undoubtedly help test the hypothesized functions of plant autophagy in programmed cell death, vacuole biogenesis, and responses to biotic, abiotic, and nutritional stresses. In this review, we summarize recent progress on these topics and suggest topics for future research, after inspecting common phenotypes of current Arabidopsis atg mutants.

A critical role of the nuclear receptor HR3 in regulation of gonadotrophic cycles of the mosquito Aedes aegypti

The orphan nuclear receptor HR3 is essential for developmental switches during insect development and metamorphosis regulated by 20-hydroxyecdysone (20E). Reproduction of female mosquitoes of the major vector of Dengue fever, Aedes aegypti, is cyclic because of its dependence on blood feeding. 20E is an important hormone regulating vitellogenic events in this mosquito; however, any role for HR3 in 20E-driven reproductive events has not been known. Using RNA interference (RNAi) approach, we demonstrated that Aedes HR3 plays a critical role in a timely termination of expression of the vitellogenin (Vg) gene encoding the major yolk protein precursor. It is also important for downregulation of the Target-of-Rapamycin pathway and activation of programmed autophagy in the Aedes fat body at the end of vitellogenesis. HR3 is critical in activating betaFTZ-F1, EcRB and USPA, the expressions of which are highly elevated at the end of vitellogenesis. RNAi depletion of HR3 (iHR3) prior to the first gonadotrophic cycle affects a normal progression of the second gonadotrophic cycle. Most of ovaries 24 h post second blood meal from iHR3 females in the second cycle were small with follicles that were only slightly different in length from of those of resting stage. In addition, these iHR3 females laid a significantly reduced number of eggs per mosquito as compared to those of iMal and the wild type. Our results indicate an important role of HR3 in regulation of 20E-regulated developmental switches during reproductive cycles of A. aegypti females.

8-Amino-adenosine activates p53-independent cell death of metastatic breast cancers

8-Amino-adenosine (8-NH(2)-Ado) is a ribose sugar nucleoside analogue that reduces cellular ATP levels and inhibits mRNA synthesis. Estrogen receptor-negative (ER-) metastatic breast cancers often contain mutant p53; therefore, we asked if 8-NH(2)-Ado could kill breast cancer cells without activating the p53-pathway. Regardless of the breast cancer subtype tested or the p53 status of the cells, 8-NH(2)-Ado was more cytotoxic than either gemcitabine or etoposide. 8-NH(2)-Ado treatment inhibited cell proliferation, activated cell death, and did not activate transcription of the p53 target gene p21 or increase protein levels of either p53 or p21. This occurred in the estrogen receptor-positive (ER+) MCF-7 cells that express wild-type p53, the ER+ T47-D cells that express mutant p53, and the ER- MDA-MB-468 cells or MDA-MB-231 cells that both express mutant p53. 8-NH(2)-Ado induced apoptotic death of MCF-7 cells and apoptosis was not inhibited by knockdown of functional p53. Moreover, the pan-caspase inhibitor Z-VAD blocked the 8-NH(2)-Ado-induced MCF-7 cell death. Interestingly, 8-NH(2)-Ado caused the MDA-MB-231 cells to detach from the plate with only limited evidence of apoptotic cell death markers and the cell death was not inhibited by Z-VAD. Inhibition of MDA-MB-231 cell autophagy, by reduction of ATG7 or 3-methyladenine treatment, did not block this 8-NH(2)-Ado-mediated cytotoxicity. Importantly 8-NH(2)-Ado was highly cytotoxic to triple-negative breast cancer cells and worked through a pathway that did not require wild-type p53 for cytoxicity. Therefore, 8-NH(2)-Ado should be considered for the treatment of triple-negative breast cancers that are chemotherapy resistant.

Probing mechanisms that underlie human neurodegenerative diseases in Drosophila

The fruit fly, Drosophila melanogaster, is an excellent organism for the study of the genetic and molecular basis of metazoan development. Drosophila provides numerous tools and reagents to unravel the molecular and cellular functions of genes that cause human disease, and the past decade has witnessed a significant expansion of the study of neurodegenerative disease mechanisms in flies. Here we review the interplay between oxidative stress and neuronal toxicity. We cover some of the studies that show how proteasome degradation of protein aggregates, autophagy, mitophagy, and lysosomal function affect the quality control mechanisms required for neuronal survival. We discuss how forward genetic screens in flies have led to the isolation of a few loci that cause neurodegeneration, paving the way for large-scale systematic screens to identify such loci in flies as well as promoting gene discovery in humans.

Depletion of mboa-7, an enzyme that incorporates polyunsaturated fatty acids into phosphatidylinositol (PI), impairs PI 3-phosphate signaling in Caenorhabditis elegans

Phosphatidylinositol (PI) is a constituent of biomembranes and a precursor of all phosphoinositides (PIPs). A prominent characteristic of PI is that its sn-2 position is highly enriched in polyunsaturated fatty acids (PUFAs), such as arachidonic acid or eicosapentaenoic acid. However, the biological significance of PUFA-containing PI remains unknown. We previously identified Caenorhabditis elegans (C. elegans) mboa-7 as an acyltransferase that incorporates PUFAs into the sn-2 position of PI. In this study, we performed an RNAi enhancer screen against PI kinases and phosphatases using mboa-7 mutants that have a reduced PUFA content in PI. Among the genes tested, knockdown of vps-34, a catalytic subunit of class III PI 3-kinase that produces PI 3-phosphate (PI3P) from PI, caused severe growth defects in mboa-7 mutants. In both vps-34 RNAi-treated wild-type worms and mboa-7 mutants, the size of PI3P-positive early endosomes was significantly decreased. We also performed an RNAi enhancer screen against PI3P-related genes and found that, like knockdown of vps-34, knockdown of autophagy-related genes caused severe growth defects in mboa-7 mutants. Finally, we showed that autophagic clearance of protein aggregates is impaired in mboa-7 mutants. Taken together, these results suggest that the PUFA chain in PI has a role in some PI3P signaling.

Atg13 and FIP200 act independently of Ulk1 and Ulk2 in autophagy induction

Under normal growth conditions the mammalian target of rapamycin complex 1 (mTORC1) negatively regulates the central autophagy regulator complex consisting of Unc-51-like kinases 1/2 (Ulk1/2), focal adhesion kinase family-interacting protein of 200 kDa (FIP200) and Atg13. Upon starvation, mTORC1-mediated repression of this complex is released, which then leads to Ulk1/2 activation. In this scenario, Atg13 has been proposed as an adaptor mediating the interaction between Ulk1/2 and FIP200 and enhancing Ulk1/2 kinase activity. Using Atg13-deficient cells, we demonstrate that Atg13 is indispensable for autophagy induction. We further show that Atg13 function strictly depends on FIP200 binding. In contrast, the simultaneous knockout of Ulk1 and Ulk2 did not have a similar effect on autophagy induction. Accordingly, the Ulk1-dependent phosphorylation sites we identified in Atg13 are expendable for this process. This suggests that Atg13 has an additional function independent of Ulk1/2 and that Atg13 and FIP200 act in concert during autophagy induction.

PP2A regulates autophagy in two alternative ways in Drosophila

Protein phosphatase 2A (PP2A) holoenzyme is a heterotrimeric complex, consisting of A, B and C subunits. The catalytic subunit PP2A-C (microtubule star/mts) binds to the C-terminal part of the scaffold protein PP2A-A (PP2A-29B). In Drosophila, there are three different forms of B subunits (widerborst/wdb, twins/tws and PP2A-B'), which determine the subcellular localization and substrate specificity of the holoenzyme. Previous studies demonstrated that PP2A is involved in the control of TOR-dependent autophagy both in yeast and mammals. Furthermore, in Drosophila, wdb genetically interacts with the PtdIns3K/PTEN/Akt signaling cascade, which is a main upstream regulatory system of dTOR. Here we demonstrate that in Drosophila, two different PP2A complexes (containing B' or wdb subunit) play essential roles in the regulation of starvation-induced autophagy. The PP2A-A/wdb/C complex acts upstream of dTOR, whereas the PP2A-A/B'/C complex functions as a target of dTOR and may regulate the elongation of autophagosomes and their subsequent fusion with lysosomes. We also identified three Drosophila Atg orthologs (Atg14, Atg17 and Atg101), which represent potential targets of the PP2A-A/B'/C complex during autophagy.

PED/PEA-15 induces autophagy and mediates TGF-beta1 effect on muscle cell differentiation

TGF-beta1 has been shown to induce autophagy in certain cells but whether and how this action is exerted in muscle and whether this activity relates to TGF-beta1 control of muscle cell differentiation remains unknown. Here, we show that expression of the autophagy-promoting protein phosphoprotein enriched in diabetes/phosphoprotein enriched in astrocytes (PED/PEA-15) progressively declines during L6 and C2C12 skeletal muscle cell differentiation. PED/PEA-15 underwent rapid induction upon TGF-beta1 exposure of L6 and C2C12 myoblasts, accompanied by impaired differentiation into mature myotubes. TGF-beta1 also induced autophagy in the L6 and C2C12 cells through a PP2A/FoxO1-mediated mechanism. Both the TGF-beta1 effect on differentiation and that on autophagy were blocked by specific PED/PEA-15 ShRNAs. Myoblasts stably overexpressing PED/PEA-15 did not differentiate and showed markedly enhanced autophagy. In these same cells, the autophagy inhibitor 3-methyladenine rescued TGF-beta1 effect on both autophagy and myogenesis, indicating that PED/PEA-15 mediates TGF-beta1 effects in muscle. Muscles from transgenic mice overexpressing PED/PEA-15 featured a significant number of atrophic fibers, accompanied by increased light chain 3 (LC3)II to LC3I ratio and reduced PP2A/FoxO1 phosphorylation. Interestingly, these mice showed significantly impaired locomotor activity compared with their non-transgenic littermates. TGF-beta1 causes transcriptional upregulation of the autophagy-promoting gene PED/PEA-15, which in turn is capable to induce atrophic responses in skeletal muscle in vivo.

Maternal control of mouse preimplantation development

Mammalian preimplantation development is a process of dedifferentiation from the terminally differentiated eggs to the totipotent blastomeres at the cleavage stage, and then to the pluripotent cells at the blastocyst stage. Maternal factors that accumulate during oogenesis dominate early preimplantation development until the embryonic factors gain control after the activation of the embryonic genome. Recently, a handful of maternal factors that are encoded by the maternal-effect genes have been characterized in genetically modified mouse models. These factors are shown to participate in many aspects of preimplantation development, such as the degradation of maternal macromolecues, epigenetic modification, protein translation, cellular signaling transduction, and cell compaction. Even so, little is known about the interactions between different maternal factors. In this chapter, we have summarized the functions of known maternal factors and hopefully this will lead to a better understanding of the regulation of preimplantation embryogenesis by the maternal regulatory network.

Programmed autophagy in the fat body of Aedes aegypti is required to maintain egg maturation cycles

Autophagy plays a pivotal role by allowing cells to recycle cellular components under conditions of stress, starvation, development and cancer. In this work, we have demonstrated that programmed autophagy in the mosquito fat body plays a critical role in maintaining of developmental switches required for normal progression of gonadotrophic cycles. Mosquitoes must feed on vertebrate blood for their egg development, with each gonadotrophic cycle being tightly coupled to a separate blood meal. As a consequence, some mosquito species are vectors of pathogens that cause devastating diseases in humans and domestic animals, most importantly malaria and Dengue fever. Hence, deciphering mechanisms to control egg developmental cycles is of paramount importance for devising novel approaches for mosquito control. Central to egg development is vitellogenesis, the production of yolk protein precursors in the fat body, the tissue analogous to a vertebrate liver, and their subsequent specific accumulation in developing oocytes. During each egg developmental cycle, the fat body undergoes a developmental program that includes previtellogenic build-up of biosynthetic machinery, intense production of yolk protein precursors, and termination of vitellogenesis. The importance of autophagy for termination of vitellogenesis was confirmed by RNA interference (RNAi) depletions of several autophagic genes (ATGs), which inhibited autophagy and resulted in untimely hyper activation of TOR and prolonged production of the major yolk protein precursor, vitellogenin (Vg). RNAi depletion of the ecdysone receptor (EcR) demonstrated its activating role of autophagy. Depletion of the autophagic genes and of EcR led to inhibition of the competence factor, betaFTZ-F1, which is required for ecdysone-mediated developmental transitions. Moreover, autophagy-incompetent female mosquitoes were unable to complete the second reproductive cycle and exhibited retardation and abnormalities in egg maturation. Thus, our study has revealed a novel function of programmed autophagy in maintaining egg maturation cycles in mosquitoes.

Cell death by autophagy: facts and apparent artefacts

Autophagy (the process of self-digestion by a cell through the action of enzymes originating within the lysosome of the same cell) is a catabolic process that is generally used by the cell as a mechanism for quality control and survival under nutrient stress conditions. As autophagy is often induced under conditions of stress that could also lead to cell death, there has been a propagation of the idea that autophagy can act as a cell death mechanism. Although there is growing evidence of cell death by autophagy, this type of cell death, often called autophagic cell death, remains poorly defined and somewhat controversial. Merely the presence of autophagic markers in a cell undergoing death does not necessarily equate to autophagic cell death. Nevertheless, studies involving genetic manipulation of autophagy in physiological settings provide evidence for a direct role of autophagy in specific scenarios. This article endeavours to summarise these physiological studies where autophagy has a clear role in mediating the death process and discusses the potential significance of cell death by autophagy.

Role of AMPK-mTOR-Ulk1/2 in the regulation of autophagy: cross talk, shortcuts, and feedbacks

Living cells are adaptive self-sustaining systems. They strictly depend on the sufficient supply of oxygen, energy, and nutrients from the outside in order to sustain their internal organization. However, as autonomous entities they are able to monitor and appropriately adapt to any critical fluctuation in their environment. In the case of insufficient external nutrient supply or augmented energy demands, cells start to extensively digest their own interior. This process, known as macroautophagy, comprises the transport of cytosolic portions and entire organelles to the lysosomal compartment via specific double-membrane vesicles, called autophagosomes. Although extensively upregulated under nutrient restriction, a low level of basal autophagy is likewise crucial in order to sustain the cellular homeostasis. On the other hand, cells have to avoid excessive and enduring self-digestion. The delicate balance between external energy and nutrient supply and internal production and consumption is a demanding task. The complex protein network that senses and precisely reacts to environmental changes is thus mainly regulated by rapid and reversible posttranslational modifications such as phosphorylation. This review focuses on the serine/threonine protein kinases AMP-activated protein kinase, mammalian target of rapamycin (mTOR), and unc-51-like kinase 1/2 (Ulk1/2), three interconnected major junctions within the autophagy regulating signaling network.

Identification and functional analysis of Joka2, a tobacco member of the family of selective autophagy cargo receptors

Two main mechanisms of protein turnover exist in eukaryotic cells: the ubiquitin-proteasome system and the autophagy-lysosomal pathway. Autophagy is an emerging important constituent of many physiological and pathological processes, such as response to nutrient deficiency, programmed cell death and innate immune response. In mammalian cells the selectivity of autophagy is ensured by the presence of cargo receptors, such as p62/SQSTM1 and NBR1, responsible for sequestration of the ubiquitinated proteins. In plants no selective cargo receptors have been identified yet. The present report indicates that structural and functional homologs of p62 and NBR1 proteins exist in plants. The tobacco protein, named Joka2, has been identified in yeast two-hybrid search as a binding partner of a small coiled-coil protein, a member of UP9/LSU family of unknown function, encoded by the UP9C gene strongly and specifically induced during sulfur deficiency. The typical domains of p62 and NBR1 are conserved in Joka2. Similarly to p62, Joka2-YFP has dual localization (cytosolic speckles and the nucleus); it forms homodimers and interacts with a member of the ATG8 family. Increased expression of Joka2 and ATG8f was observed in roots of tobacco plants grown for two days in nutrient-deficient conditions. Constitutive ectopic expression of Joka2-YFP in tobacco resulted in attenuated response (manifested by lesser yellowing of the leaves) to nutrient deficiency. In conclusion, Joka2, and presumably the process of selective autophagy, might constitute an important part of plant response to environmental stresses.

Brain region specific mitophagy capacity could contribute to selective neuronal vulnerability in Parkinson's disease

Parkinson's disease (PD) is histologically well defined by its characteristic degeneration of dopaminergic neurons in the substantia nigra pars compacta. Remarkably, divergent PD-related mutations can generate comparable brain region specific pathologies. This indicates that some intrinsic region-specificity respecting differential neuron vulnerability exists, which codetermines the disease progression. To gain insight into the pathomechanism of PD, we investigated protein expression and protein oxidation patterns of three different brain regions in a PD mouse model, the PINK1 knockout mice (PINK1-KO), in comparison to wild type control mice. The dysfunction of PINK1 presumably affects mitochondrial turnover by disturbing mitochondrial autophagic pathways. The three brain regions investigated are the midbrain, which is the location of substantia nigra; striatum, the major efferent region of substantia nigra; and cerebral cortex, which is more distal to PD pathology. In all three regions, mitochondrial proteins responsible for energy metabolism and membrane potential were significantly altered in the PINK1-KO mice, but with very different region specific accents in terms of up/down-regulations. This suggests that disturbed mitophagy presumably induced by PINK1 knockout has heterogeneous impacts on different brain regions. Specifically, the midbrain tissue seems to be most severely hit by defective mitochondrial turnover, whereas cortex and striatum could compensate for mitophagy nonfunction by feedback stimulation of other catabolic programs. In addition, cerebral cortex tissues showed the mildest level of protein oxidation in both PINK1-KO and wild type mice, indicating either a better oxidative protection or less reactive oxygen species (ROS) pressure in this brain region. Ultra-structural histological examination in normal mouse brain revealed higher incidences of mitophagy vacuoles in cerebral cortex than in striatum and substantia nigra. Taken together, the delicate balance between oxidative protection and mitophagy capacity in different brain regions could contribute to brain region-specific pathological patterns in PD.

p62, Ref(2)P and ubiquitinated proteins are conserved markers of neuronal aging, aggregate formation and progressive autophagic defects

Suppression of macroautophagy, due to mutations or through processes linked to aging, results in the accumulation of cytoplasmic substrates that are normally eliminated by the pathway. This is a significant problem in long-lived cells like neurons, where pathway defects can result in the accumulation of aggregates containing ubiquitinated proteins. The p62/Ref(2)P family of proteins is involved in the autophagic clearance of cytoplasmic protein bodies or sequestosomes. These unique structures are closely associated with protein inclusions containing ubiquitin as well as key components of the autophagy pathway. In this study we show that detergent fractionation followed by western blot analysis of insoluble ubiquitinated proteins (IUP), mammalian p62 and its Drosophila homologue, Ref(2)P can be used to quantitatively assess the activity level of aggregate clearance (aggrephagy) in complex tissues. Using this technique we show that genetic or age-dependent changes that modify the long-term enhancement or suppression of aggrephagy can be identified. Moreover, using the Drosophila model system this method can be used to establish autophagy-dependent protein clearance profiles that are occurring under a wide range of physiological conditions including developmental, fasting and altered metabolic pathways. This technique can also be used to examine proteopathies that are associated with human disorders such as frontotemporal dementia, Huntington and Alzheimer disease. Our findings indicate that measuring IUP profiles together with an assessment of p62/Ref(2)P proteins can be used as a screening or diagnostic tool to characterize genetic and age-dependent factors that alter the long-term function of autophagy and the clearance of protein aggregates occurring within complex tissues and cells.

The Atg6/Vps30/Beclin 1 ortholog BEC-1 mediates endocytic retrograde transport in addition to autophagy in C. elegans

Autophagy and endocytosis are dynamic and tightly regulated processes that contribute to many fundamental aspects of biology including survival, longevity, and development. However, the molecular links between autophagy and endocytosis are not well understood. Here, we report that BEC-1, the C. elegans ortholog of Atg6/Vps30/Beclin1, a key regulator of the autophagic machinery, also contributes to endosome function. In particular we identify a defect in retrograde transport from endosomes to the Golgi in bec-1 mutants. MIG-14/Wntless is normally recycled from endosomes to the Golgi through the action of the retromer complex and its associated factor RME-8. Lack of retromer or RME-8 activity results in the aberrant transport of MIG-14/Wntless to the lysosome where it is degraded. Similarly, we find that lack of bec-1 also results in mislocalization and degradation of MIG-14::GFP, reduced levels of RME-8 on endosomal membranes, and the accumulation of morphologically abnormal endosomes. A similar phenotype was observed in animals treated with dsRNA against vps-34. We further identify a requirement for BEC-1 in the clearance of apoptotic corpses in the hermaphrodite gonad, suggesting a role for BEC-1 in phagosome maturation, a process that appears to depend upon retrograde transport. In addition, autophagy genes may also be required for cell corpse clearance, as we find that RNAi against atg-18 or unc-51 also results in a lack of cell corpse clearance.

Ethanol enhances susceptibility to apoptotic cell death via down-regulation of autophagy-related proteins

BACKGROUND

Alcohol induces cellular stress and promotes cell death in immune cells. Molecular mechanisms by which ethanol impairs the function of immune cells are largely unknown. Autophagy is a degradation pathway, acting either as a pro-survival or pro-death mechanism activated during stress conditions. We examined whether ethanol influences autophagy in monocytic human U937, CD4 Jurkat, and MCF-7 cells.

METHODS

Effects of ethanol during starvation-induced autophagy were investigated, treating cells with ethanol alone and in combination with activation of autophagy by rapamycin or inhibition by wortmannin. Apoptotic and necrotic cell death features such as the breakdown of the mitochondrial membrane potential, DNA fragmentation, and cell permeability were assessed using FACS analyses. Expression level of Beclin-1, LC3-II, Bcl-2, and the activation of caspase-3, and PARP-1 were determined using Western blot analyses. Influence of ethanol on formation of LC3-II complexes was assessed using fluorescence microscopy in MCF-7 cells stable transfected with a GFP-LC3-II-expression vector.

RESULTS

Ethanol down regulated autophagy proteins such as Beclin-1 and LC3-II. Apoptosis was enhanced as shown by breakdown of mitochondrial potential, up-regulation of cleaved caspase-3 and PARP-1 and down-regulation of anti-apoptotic protein Bcl-2. Formation of LC3-II complexes was inhibited by ethanol in caspase-3 deficient MCF-7 cells. Stimulation of autophagy by rapamycin prevented ethanol-induced apoptotic cell death. Inhibition of autophagy by wortmannin aggravated ethanol-mediated necrotic cell death.

CONCLUSION

Inhibition of autophagy via ethanol enhances susceptibility to cell death.

Nutrient starvation elicits an acute autophagic response mediated by Ulk1 dephosphorylation and its subsequent dissociation from AMPK

Macroautophagy (herein referred to as autophagy) is an evolutionarily conserved self-digestive process cells adapt to starvation and other stress responses. Upon starvation, autophagy is induced, providing cells with needed nutrient supplies. We report here that Unc-51-like kinase 1 (Ulk1), a key initiator for mammalian autophagy, undergoes dramatic dephosphorylation upon starvation, particularly at serine 638 and serine 758. Phosphorylations of Ulk1 are mediated by mammalian target-of-rapamycin (mTOR) kinase and adenosine monophosphate activated protein kinase (AMPK). AMPK interacts with Ulk1 in a nutrient-dependent manner. Proper phosphorylations on Ulk1 are crucial for Ulk1/AMPK association, as a single serine-to-alanine mutation (S758A) at Ulk1 impairs this interaction. Compared to the wild-type ULK1, this Ulk1-S758A mutant initiates starvation-induced autophagy faster at an early time point, but does not alter the maximum capacity of autophagy when starvation prolongs. This study therefore revealed previously unnoticed acute autophagy response to environmental changes.

The role of autophagy in genetic pathways influencing ageing

Autophagy is a conserved cellular degradation pathway for the breakdown of cytosolic macromolecules and organelles. Constitutive autophagy has a housekeeping role and is essential for survival, development and metabolic regulation. Autophagy is also responsive to stress and can degrade damaged proteins and organelles, oxidized lipids and intracellular pathogens. Defects in the autophagic degradation system are linked to disease pathogenesis and ageing. Different signalling pathways converge on autophagy to regulate lifespan in diverse organisms. We discuss recent findings that provide insight into the cross-talk between this critical regulator of metabolic homeostasis and molecular mechanisms that promote longevity.

Cell plasticity in homeostasis and regeneration

Over the past decades, genetic analyses performed in vertebrate and invertebrate organisms deciphered numerous cellular and molecular mechanisms deployed during sexual development and identified genetic circuitries largely shared among bilaterians. In contrast, the functional analysis of the mechanisms that support regenerative processes in species randomly scattered among the animal kingdom, were limited by the lack of genetic tools. Consequently, unifying principles explaining how stress and injury can lead to the reactivation of a complete developmental program with restoration of original shape and function remained beyond reach of understanding. Recent data on cell plasticity suggest that beside the classical developmental approach, the analysis of homeostasis and asexual reproduction in adult organisms provides novel entry points to dissect the regenerative potential of a given species, a given organ or a given tissue. As a clue, both tissue homeostasis and regeneration dynamics rely on the availability of stem cells and/or on the plasticity of differentiated cells to replenish the missing structure. The freshwater Hydra polyp provides us with a unique model system to study the intricate relationships between the mechanisms that regulate the maintenance of homeostasis, even in extreme conditions (starvation and overfeeding) and the reactivation of developmental programs after bisection or during budding. Interestingly head regeneration in Hydra can follow several routes according to the level of amputation, suggesting that indeed the homeostatic background dramatically influences the route taken to bridge injury and regeneration.

SOD2, the principal scavenger of mitochondrial superoxide, is dispensable for embryogenesis and imaginal tissue development but essential for adult survival

Definitive evidence on the impact of MnSOD/SOD2-deficiency and the consequent effects of high flux of mitochondrial reactive oxygen species (ROS) on pre-natal/pre-adult development has yet to be reported for either Drosophila or mice. Here we report that oocytes lacking maternal SOD2 protein develop into adults just like normal SOD2-containing oocytes suggesting that maternal SOD2-mediated protection against mitochondrial ROS is not essential for oocyte viability. However, the capacity of SOD2-null larvae to undergo successful metamorphosis into adults is negatively influenced in the absence of SOD2. We therefore determined the impact of a high superoxide environment on cell size, progression through the cell cycle, cell differentiation, and cell death and found no difference between SOD2-null and SOD2+ larva and pupa. Thus loss of SOD2 activity clearly has no effect on pre-adult imaginal tissues. Instead, we found that the high mitochondrial superoxide environment arising from the absence of SOD2 leads to the induction of autophagy. Such autophagic response may underpin the resistance of pre-adult tissues to unscavenged ROS. Finally, while our data establish that SOD2 activity is less essential for normal development, the mortality of Sod2-/- neonates of both Drosophila and mice suggests that SOD2 activity is indeed essential for the viability of adults. We therefore asked if the early mortality of SOD2-null young adults could be rescued by activation of SOD2 expression. The results support the conclusion that the early mortality of SOD2-null adults is largely attributable to the absence of SOD2 activity in the adult per se. This finding somewhat contradicts the widely held notion that failure to scavenge the high volume of superoxide emanating from the oxidative demands of development would be highly detrimental to developing tissues.

Mycobacterium tuberculosis eis regulates autophagy, inflammation, and cell death through redox-dependent signaling

The “enhanced intracellular survival” (eis) gene of Mycobacterium tuberculosis (Mtb) is involved in the intracellular survival of M. smegmatis. However, its exact effects on host cell function remain elusive. We herein report that Mtb Eis plays essential roles in modulating macrophage autophagy, inflammatory responses, and cell death via a reactive oxygen species (ROS)-dependent pathway. Macrophages infected with an Mtb eis-deletion mutant H37Rv (Mtb-Δeis) displayed markedly increased accumulation of massive autophagic vacuoles and formation of autophagosomes in vitro and in vivo. Infection of macrophages with Mtb-Δeis increased the production of tumor necrosis factor-α and interleukin-6 over the levels produced by infection with wild-type or complemented strains. Elevated ROS generation in macrophages infected with Mtb-Δeis (for which NADPH oxidase and mitochondria were largely responsible) rendered the cells highly sensitive to autophagy activation and cytokine production. Despite considerable activation of autophagy and proinflammatory responses, macrophages infected with Mtb-Δeis underwent caspase-independent cell death. This cell death was significantly inhibited by blockade of autophagy and c-Jun N-terminal kinase-ROS signaling, suggesting that excessive autophagy and oxidative stress are detrimental to cell survival. Finally, artificial over-expression of Eis or pretreatment with recombinant Eis abrogated production of both ROS and proinflammatory cytokines, which depends on the N-acetyltransferase domain of the Eis protein. Collectively, these data indicate that Mtb Eis suppresses host innate immune defenses by modulating autophagy, inflammation, and cell death in a redox-dependent manner.

Tuberculosis is a global health problem: at least one-third of the world's population is infected with Mycobacterium tuberculosis (Mtb). Mtb is a successful pathogen that enhances its own intracellular survival by arresting phagolysosomal fusion. Recently, autophagy has emerged as a host defense strategy against Mtb infection, through stimulation of the fusion of phagosomes and lysosomes. However, excessive and uncontrolled autophagic activity can be detrimental to host cells and can result in their death. The Mtb “enhanced intracellular survival” (eis) gene has been implicated in the intracellular survival of M. smegmatis. However, its exact role and how it regulates host innate immune responses have not been fully explained. Here, we provide evidence that Eis suppresses macrophage autophagy, inflammation, and cell death through the inhibition of reactive oxygen species (ROS) generation. Although it has previously been demonstrated that autophagy is a key host defense response to mycobacterial infections, our data indicate that excessive autophagy, and the resulting cell death, do not significantly affect host defense responses to mycobacteria. Additionally, our data reveal that Eis's ability to regulate ROS generation and proinflammatory responses depends on its N-acetyltransferase domain. These results underscore a previously unrecognized role of Eis in modulating host inflammatory responses, oxidative stress, and cell survival/death during mycobacterial infection.

Autophagy in Drosophila ovaries is induced by starvation and is required for oogenesis

Autophagy, an evolutionarily conserved lysosome-mediated degradation, promotes cell survival under starvation and is controlled by insulin/target of rapamycin (TOR) signaling. In Drosophila, nutrient depletion induces autophagy in the fat body. Interestingly, nutrient availability and insulin/TOR signaling also influence the size and structure of Drosophila ovaries, however, the role of nutrient signaling and autophagy during this process remains to be elucidated. Here, we show that starvation induces autophagy in germline cells (GCs) and in follicle cells (FCs) in Drosophila ovaries. This process is mediated by the ATG machinery and involves the upregulation of Atg genes. We further demonstrate that insulin/TOR signaling controls autophagy in FCs and GCs. The analysis of chimeric females reveals that autophagy in FCs, but not in GCs, is required for egg development. Strikingly, when animals lack Atg gene function in both cell types, ovaries develop normally, suggesting that the incompatibility between autophagy-competent GCs and autophagy-deficient FCs leads to defective egg development. As egg morphogenesis depends on a tightly linked signaling between FCs and GCs, we propose a model in which autophagy is required for the communication between these two cell types. Our data establish an important function for autophagy during oogenesis and contributes to the understanding of the role of autophagy in animal development.

Molecular actions and therapeutic potential of lithium in preclinical and clinical studies of CNS disorders

Lithium has been used clinically to treat bipolar disorder for over half a century, and remains a fundamental pharmacological therapy for patients with this illness. Although lithium's therapeutic mechanisms are not fully understood, substantial in vitro and in vivo evidence suggests that it has neuroprotective/neurotrophic properties against various insults, and considerable clinical potential for the treatment of several neurodegenerative conditions. Evidence from pharmacological and gene manipulation studies support the notion that glycogen synthase kinase-3 inhibition and induction of brain-derived neurotrophic factor-mediated signaling are lithium's main mechanisms of action, leading to enhanced cell survival pathways and alteration of a wide variety of downstream effectors. By inhibiting N-methyl-D-aspartate receptor-mediated calcium influx, lithium also contributes to calcium homeostasis and suppresses calcium-dependent activation of pro-apoptotic signaling pathways. In addition, lithium decreases inositol 1,4,5-trisphosphate by inhibiting phosphoinositol phosphatases, a process recently identified as a novel mechanism for inducing autophagy. Through these mechanisms, therapeutic doses of lithium have been demonstrated to defend neuronal cells against diverse forms of death insults and to improve behavioral as well as cognitive deficits in various animal models of neurodegenerative diseases, including stroke, amyotrophic lateral sclerosis, fragile X syndrome, as well as Huntington's, Alzheimer's, and Parkinson's diseases, among others. Several clinical trials are also underway to assess the therapeutic effects of lithium for treating these disorders. This article reviews the most recent findings regarding the potential targets involved in lithium's neuroprotective effects, and the implication of these findings for the treatment of a variety of diseases.

Autophagic degradation of dBruce controls DNA fragmentation in nurse cells during late Drosophila melanogaster oogenesis

Blocking autophagy protects the apoptosis inhibitor dBruce from destruction and promotes nurse cell survival in developing egg chambers.

Autophagy is an evolutionarily conserved pathway responsible for degradation of cytoplasmic material via the lysosome. Although autophagy has been reported to contribute to cell death, the underlying mechanisms remain largely unknown. In this study, we show that autophagy controls DNA fragmentation during late oogenesis in Drosophila melanogaster. Inhibition of autophagy by genetically removing the function of the autophagy genes atg1, atg13, and vps34 resulted in late stage egg chambers that contained persisting nurse cell nuclei without fragmented DNA and attenuation of caspase-3 cleavage. The Drosophila inhibitor of apoptosis (IAP) dBruce was found to colocalize with the autophagic marker GFP-Atg8a and accumulated in autophagy mutants. Nurse cells lacking Atg1 or Vps34 in addition to dBruce contained persisting nurse cell nuclei with fragmented DNA. This indicates that autophagic degradation of dBruce controls DNA fragmentation in nurse cells. Our results reveal autophagic degradation of an IAP as a novel mechanism of triggering cell death and thereby provide a mechanistic link between autophagy and cell death.

Autophagy shows its animal side

Most autophagy genes have been discovered in the single-celled yeast Saccharomyces cerevisiae, and little is known about autophagy genes that are specific to multicellular animals. In this issue, Tian et al. (2010) now identify four new autophagy genes: one specific to the nematode Caenorhabditis elegans and three conserved from worms to mammals.

C. elegans screen identifies autophagy genes specific to multicellular organisms

The molecular understanding of autophagy has originated almost exclusively from yeast genetic studies. Little is known about essential autophagy components specific to higher eukaryotes. Here we perform genetic screens in C. elegans and identify four metazoan-specific autophagy genes, named epg-2, -3, -4, and -5. Genetic analysis reveals that epg-2, -3, -4, and -5 define discrete genetic steps of the autophagy pathway. epg-2 encodes a coiled-coil protein that functions in specific autophagic cargo recognition. Mammalian homologs of EPG-3/VMP1, EPG-4/EI24, and EPG-5/mEPG5 are essential for starvation-induced autophagy. VMP1 regulates autophagosome formation by controlling the duration of omegasomes. EI24 and mEPG5 are required for formation of degradative autolysosomes. This study establishes C. elegans as a multicellular genetic model to delineate the autophagy pathway and provides mechanistic insights into the metazoan-specific autophagic process.

Autophagy in health and disease. 3. Involvement of autophagy in muscle atrophy

Loss of muscle mass aggravates a variety of diseases, and understanding the molecular mechanisms that control muscle wasting is critical for developing new therapeutic approaches. Weakness is caused by loss of muscle proteins, and recent studies have underlined a major role for the autophagy-lysosome system in regulating muscle mass. Some key components of the autophagy machinery are transcriptionally upregulated during muscle wasting, and their induction precedes muscle loss. However, it is unclear whether autophagy is detrimental, causing atrophy, or beneficial, promoting survival during catabolic conditions. This review discusses recent findings on signaling pathways regulating autophagy.

Maternal control of early mouse development

The hiatus between oocyte and embryonic gene transcription dictates a role for stored maternal factors in early mammalian development. Encoded by maternal-effect genes, these factors accumulate during oogenesis and enable the activation of the embryonic genome, the subsequent cleavage stages of embryogenesis and the initial establishment of embryonic cell lineages. Recent studies in mice have yielded new findings on the role of maternally provided proteins and multi-component complexes in preimplantation development. Nevertheless, significant gaps remain in our mechanistic understanding of the networks that regulate early mammalian embryogenesis, which provide an impetus and opportunities for future investigations.

Jun proteins are starvation-regulated inhibitors of autophagy

The growing number of biological functions affected by autophagy ascribes a special significance to identification of factors regulating it. The activator protein-1 (AP-1) transcription factors are involved in most aspects of cellular proliferation, death, or survival, yet no information regarding their involvement in autophagy is available. Here, we show that the AP-1 proteins JunB and c-Jun, but not JunD, c-Fos, or Fra-1, inhibit autophagy. JunB inhibits autophagy induced by starvation, overexpression of a short form of ARF (smARF), a potent inducer of autophagy, or even after rapamycin treatment. In agreement, acute repression of JunB expression, by JunB knockdown, potently induces autophagy. As expected from autophagy-inhibiting proteins, Jun B and c-Jun expression is reduced by starvation. Decrease in JunB mRNA expression and posttranscriptional events downregulate JunB protein expression after starvation. The inhibition of autophagy by JunB is not mediated by mammalian target of rapamycin (mTOR) regulation, as it occurs also in the absence of mTOR activity, and autophagy induced by JunB knockdown is not correlated with changes in mTOR activity. Nevertheless, the transcriptional activities of c-Jun and JunB are required for autophagy inhibition, and JunB incapable of heterodimerizing is a less effective inhibitor of autophagy. Most importantly, inhibition of autophagy in starved HeLa cells by JunB enhances apoptotic cell death. We suggest that JunB and c-Jun are regulators of autophagy whose expression responds to autophagy-inducing signals.

New insights into the function of Atg9

Autophagy is a lysosomal degradation pathway that is essential for cellular homeostasis. Identification of more than 30 autophagy related proteins including a multi-spanning membrane protein, Atg9, has increased our understanding of the molecular mechanisms involved in autophagy. Atg9 is required for autophagy in several eukaryotic organisms although its function is unknown. Recently, we identified a novel interacting partner of mAtg9, p38 MAPK interacting protein, p38IP. We summarise recent data on the role of Atg9 trafficking in yeast and mammalian autophagy and discuss the role of p38IP and p38 MAPK in regulation of mAtg9 trafficking and autophagy.

Regulation of tissue growth through nutrient sensing

Nutrition is a key regulator of tissue growth. In animals, nutritional status is monitored and signaled at both the cellular and systemic levels. The main mediator of cellular nutrient sensing is the protein kinase TOR (target of rapamycin). TOR receives information from levels of cellular amino acids and energy, and it regulates the activity of processes involved in cell growth, such as protein synthesis and autophagy. Insulin-like signaling is the main mechanism of systemic nutrient sensing and mediates its growth-regulatory functions largely through the phosphatidylinositol 3-kinase (PI3K)/AKT protein kinase pathway. Other nutrition-regulated hormonal mechanisms contribute to growth control by modulating the activity of insulin-like signaling. The pathways mediating signals from systemic and cellular levels converge, allowing cells to combine information from both sources. Here we give an overview of the mechanisms that adjust animal tissue growth in response to nutrition and highlight some general features of the signaling pathways involved.

The nuclear cofactor DOR regulates autophagy in mammalian and Drosophila cells

The regulation of autophagy in metazoans is only partly understood, and there is a need to identify the proteins that control this process. The diabetes- and obesity-regulated gene (DOR), a recently reported nuclear cofactor of thyroid hormone receptors, is expressed abundantly in metabolically active tissues such as muscle. Here, we show that DOR shuttles between the nucleus and the cytoplasm, depending on cellular stress conditions, and re-localizes to autophagosomes on autophagy activation. We demonstrate that DOR interacts physically with autophagic proteins Golgi-associated ATPase enhancer of 16 kDa (GATE16) and microtubule-associated protein 1A/1B-light chain 3. Gain-of-function and loss-of-function studies indicate that DOR stimulates autophagosome formation and accelerates the degradation of stable proteins. CG11347, the DOR Drosophila homologue, has been predicted to interact with the Drosophila Atg8 homologues, which suggests functional conservation in autophagy. Flies lacking CG11347 show reduced autophagy in the fat body during pupal development. All together, our data indicate that DOR regulates autophagosome formation and protein degradation in mammalian and Drosophila cells.

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.

Cracking open cell death in the Drosophila ovary

The Drosophila melanogaster ovary is a powerful yet simple system with only a few cell types. Cell death in the ovary can be induced in response to multiple developmental and environmental signals. These cell deaths occur at distinct stages of oogenesis and involve unique mechanisms utilizing apoptotic, autophagic and perhaps necrotic processes. In this review, we summarize recent progress characterizing cell death mechanisms in the fly ovary.

FoxO transcription factors promote autophagy in cardiomyocytes

In the heart, autophagy is required for normal cardiac function and also has been implicated in cardiovascular disease. FoxO transcription factors promote autophagy in skeletal muscle and have additional roles in regulation of cell size, proliferation, and metabolism. Here we investigate the role of FoxO transcription factors in regulating autophagy and cell size in cardiomyocytes. In cultured rat neonatal cardiomyocytes, glucose deprivation leads to decreased cell size and induction of autophagy pathway genes LC3, Gabarapl1, and Atg12. Likewise, overexpression of either FoxO1 or FoxO3 reduces cardiomyocyte cell size and induces expression of autophagy pathway genes. Moreover, inhibition of FoxO activity by dominant negative FoxO1 (Delta256) blocks cardiomyocyte cell size reduction upon starvation, suggesting the necessity of FoxO function in cardiomyocyte cell size regulation. Under starvation conditions, endogenous FoxO1 and FoxO3 are localized to the nucleus and bind to promoter sequences of Gabarapl1 and Atg12. In vivo studies show that cellular stress, such as starvation or ischemia/reperfusion in mice, results in induction of autophagy in the heart with concomitant dephosphorylation of FoxO, consistent with increased activity of nuclear FoxO transcription factors. Together these results provide evidence for an important role for FoxO1 and FoxO3 in regulating autophagy and cell size in cardiomyocytes.

Intracellular protein degradation in mammalian cells: recent developments

In higher organisms, dietary proteins are broken down into amino acids within the digestive tract but outside the cells, which incorporate the resulting amino acids into their metabolism. However, under certain conditions, an organism loses more nitrogen than is assimilated in the diet. This additional loss was found in the past century to come from intracellular proteins and started an intensive research that produced an enormous expansion of the field and a dispersed literature. Therefore, our purpose is to provide an updated summary of the current knowledge on the proteolytic machinery involved in intracellular protein degradation and its physiological and pathological relevance, especially addressed to newcomers in the field who may find further details in more specialized reviews. However, even providing a general overview, this is an extremely wide field and, therefore, we mainly focus on mammalian cells, while other cells will be mentioned only for comparison purposes.