The presence of multiple homologs of the same yeast ATG genes endows an extra layer of complexity on the autophagic machinery in higher eukaryotes. The physiological function of individual homologs in the autophagy pathway remains poorly understood. Here we characterized the function of the two atg16 homologs, atg-16.1 and atg-16.2, in the autophagy pathway in C. elegans. We showed that atg-16.2 mutants exhibit a stronger autophagic defect than atg-16.1 mutants. atg-16.2; atg-16.1 double mutants display a much more severe defect than either single mutant. ATG-16.1 and ATG-16.2 interact with themselves and each other and also directly associate with ATG-5. atg-16.1 mutant embryos exhibit a wild-type expression and distribution pattern of LGG-1/Atg8, while LGG-1 puncta are markedly fewer in number and weaker in intensity in atg-16.2 mutants. In atg-16.2; atg-16.1 double mutants, the lipidated form of LGG-1 accumulates, but LGG-1 puncta are completely absent. ATG-16.2 ectopically expressed on the plasma membrane provides novel sites of LGG-1 puncta formation. We also demonstrated that the C-terminal WD repeats are dispensable for the role of atg-16.2 in aggrephagy (the degradation of protein aggregates by autophagy). Genetic epistasis analysis placed atg-16.2 upstream of atg-2, epg-6, and atg-18. Our study indicated that C. elegans ATG-16s are involved in specifying LGG-1 puncta formation and the two ATG-16 homologs have partially redundant yet distinct functions in the aggrephagy pathway.
atg-16.1; atg-16.2; aggrephagy; C. elegans
Defective mitochondria exert deleterious effects on host cells. To manage this risk, mitochondria display several lines of quality control mechanisms: mitochondria-specific chaperones and proteases protect against misfolded proteins at the molecular level, and fission/fusion and mitophagy segregate and eliminate damage at the organelle level. An increase in unfolded proteins in mitochondria activates a mitochondrial unfolded protein response (UPRmt) to increase chaperone production, while the mitochondrial kinase PINK1 and the E3 ubiquitin ligase PARK2/Parkin, whose mutations cause familial Parkinson disease, remove depolarized mitochondria through mitophagy. It is unclear, however, if there is a connection between those different levels of quality control (QC). Here, we show that the expression of unfolded proteins in the matrix causes the accumulation of PINK1 on energetically healthy mitochondria, resulting in mitochondrial translocation of PARK2, mitophagy and subsequent reduction of unfolded protein load. Also, PINK1 accumulation is greatly enhanced by the knockdown of the LONP1 protease. We suggest that the accumulation of unfolded proteins in mitochondria is a physiological trigger of mitophagy.
unfolded protein response; mitochondria; PINK1; PARK2/Parkin; mitophagy; LONP
In Dendrobium flowers some tepal mesophyll cells showed cytoplasmic areas devoid of large organelles. Such amorphous areas comprised up to about 40% of the cross-section of a cell. The areas were not bound by a membrane. The origin of these areas is not known. We show data suggesting that they can be formed from vesicle-like organelles. The data imply that these organelles and other material become degraded inside the cytoplasm. This can be regarded as a form of autophagy. The amorphous areas became surrounded by small vacuoles, vesicles or double membranes. These seemed to merge and thereby sequester the areas. Degradation of the amorphous areas therefore seemed to involve macroautophagy.
amorphous area; autophagosome; autolysosome; cytoplasm; engulfment; membrane; mesophyll; plant; plastid; tepal; vacuole
We characterized phosphoinositide binding of the S. cerevisiae PROPPIN Hsv2 qualitatively with density flotation assays and quantitatively through isothermal titration calorimetry (ITC) measurements using liposomes. We discuss the design of these experiments and show with liposome flotation assays that Hsv2 binds with high specificity to both PtdIns3P and PtdIns(3,5)P2. We propose liposome flotation assays as a more accurate alternative to the commonly used PIP strips for the characterization of phosphoinositide-binding specificities of proteins. We further quantitatively characterized PtdIns3P binding of Hsv2 with ITC measurements and determined a dissociation constant of 0.67 µM and a stoichiometry of 2:1 for PtdIns3P binding to Hsv2. PtdIns3P is crucial for the biogenesis of autophagosomes and their precursors. Besides the PROPPINs there are other PtdIns3P binding proteins with a link to autophagy, which includes the FYVE-domain containing proteins ZFYVE1/DFCP1 and WDFY3/ALFY and the PX-domain containing proteins Atg20 and Snx4/Atg24. The methods described could be useful tools for the characterization of these and other phosphoinositide-binding proteins.
isothermal titration calorimetry; liposome flotation assays; multi-angle laser light scattering; PROPPIN; small unilamellar vesicle
Autophagy is a cellular catabolic process in which various cytosolic components are degraded. For example, autophagy can mediate lipolysis of neutral lipid droplets. In contrast, we here report that autophagy is required to facilitate normal levels of neutral lipids in C. elegans. Specifically, by using multiple methods to detect lipid droplets including CARS microscopy, we observed that mutants in the gene bec-1 (VPS30/ATG6/BECN1), a key regulator of autophagy, failed to store substantial neutral lipids in their intestines during development. Moreover, loss of bec-1 resulted in a decline in lipid levels in daf-2 [insulin/IGF-1 receptor (IIR) ortholog] mutants and in germline-less glp-1/Notch animals, both previously recognized to accumulate neutral lipids and have increased autophagy levels. Similarly, inhibition of additional autophagy genes, including unc-51/ULK1/ATG1 and lgg-1/ATG8/MAP1LC3A/LC3 during development, led to a reduction in lipid content. Importantly, the decrease in fat accumulation observed in animals with reduced autophagy did not appear to be due to a change in food uptake or defecation. Taken together, these observations suggest a broader role for autophagy in lipid remodeling in C. elegans.
autophagy; fat storage; lipid metabolism; intestine; Oil-Red-O staining; CARS microscopy; C. elegans
Representatives of all major metazoan lineages form biominerals. The molecular mechanisms that underlie this widespread and evolutionarily ancient ability are gradually being revealed for some lineages. However, until a wider range of metazoan biomineralization strategies are understood, the true diversity, and therefore the evolutionary origins of this process, will remain unknown. We have previously shown that the coralline demosponge, Astrosclera willeyana, in some way employs its endobiotic bacterial community to form its highly calcified skeleton. Here, using in situ hybridization and immunohistochemistry, we show that an ortholog of ATG8 (most likely a GABARAPL2/GATE-16 ortholog) is expressed in cells that construct the individual skeletal elements of the sponge. In TEM sections sponge cells can be observed to contain extensive populations of bacteria, and frequently possesses double-membrane structures which we interpret to be autophagosomes. In combination with our previous work, these findings support the hypothesis that the host sponge actively degrades a proportion of its bacterial community using an autophagy pathway, and uses the prokaryotic organic remains as a framework upon which calcification of the sponge skeleton is initiated.
biomineralization; biocalcification; sponge; autophagy; symbiosis; symbiophagy; evolution; bacteria; GABARAPL2/GATE-16; ATG8
Due in part to the increasing number of links between autophagy malfunction and human diseases, this field has gained tremendous attention over the past decade. Our increased understanding of the molecular machinery involved in macroautophagy (hereafter autophagy) seems to indicate that the most complex step, or at least the stage of the process where the majority of the autophagy-related (Atg) proteins participate, is in the formation of the double-membrane sequestering vesicle. Thus, it is important to establish reliable approaches to monitor this specific process. One of the most commonly used methods is morphological analysis by electron microscopy of the cytosolic vesicles used in the cytoplasm-to-vacuole targeting (Cvt) pathway and autophagy, or the single-membrane intralumenal products, termed Cvt or autophagic bodies, that are formed after the fusion of these vesicles with the yeast vacuole. This method, however, can be costly and time consuming, and reliable analysis requires expert input. Furthermore, it is extremely difficult to detect an incomplete autophagosome by electron microscopy because of the difficulty of obtaining a section that randomly cuts through the open portion of the phagophore. The primary Cvt pathway cargo, precursor amminopeptidase I (prApe1), is enwrapped within either a Cvt vesicle or autophagosome depending on the nutritional conditions. The proteolytic sensitivity of the prApe1 propeptide can therefore serve as a useful tool to determine the completion status of double-membrane Cvt vesicles/autophagosomes in the presence of exogenously added proteinase. Here, we describe an assay that examines the proteinase protection of prApe1 for determining the completion of Cvt vesicles/autophagosomes.
autophagy; lysosome; stress; vacuole; yeast
Recently, autophagy has been found to be strongly activated in colon cancer cells, but few studies have addressed the normal colon mucosa. The aim of this study was to characterize autophagy in normal human intestinal cells. We used the expression of LC3-II and BECN1 as well as SQSTM1 as markers of autophagy activity. Using the normal human intestinal epithelial crypt (HIEC) cell experimental model, we found that autophagy was much more active in undifferentiated cells than in differentiated cells. In the normal adult colonic mucosa, BECN1 was found in the proliferative epithelial cells of the lower part of the gland while SQSTM1 was predominantly found in the differentiated cells of the upper part of the gland and surface epithelium. Interestingly, the weak punctate pattern of SQSTM1 expression in the lower gland colocalized with BECN1-labeled autophagosomes. The usefulness of SQSTM1 as an active autophagy marker was confirmed in colon cancer specimens at the protein and transcript levels. In conclusion, our results show that autophagy is active in the colonic gland and is associated with the intestinal proliferative/undifferentiated and progenitor cell populations.
BECN1; p62/SQSTM1; autophagy; colon; epithelium; cancer
Lysosomes contain various hydrolases that can degrade proteins, lipids, nucleic acids and carbohydrates. We recently discovered “RNautophagy,” an autophagic pathway in which RNA is directly taken up by lysosomes and degraded. A lysosomal membrane protein, LAMP2C, a splice variant of LAMP2, binds to RNA and acts as a receptor for this pathway. In the present study, we show that DNA is also directly taken up by lysosomes and degraded. Like RNautophagy, this autophagic pathway, which we term “DNautophagy,” is dependent on ATP. The cytosolic sequence of LAMP2C also directly interacts with DNA, and LAMP2C functions as a receptor for DNautophagy, in addition to RNautophagy. Similarly to RNA, DNA binds to the cytosolic sequences of fly and nematode LAMP orthologs. Together with the findings of our previous study, our present findings suggest that RNautophagy and DNautophagy are evolutionarily conserved systems in Metazoa.
LAMP2; LAMP-2; LAMP2C; LAMP-2C; autophagy; RNA; RNautophagy; DNA; DNautophagy
Autophagy is essential for prolonging yeast survival during nutrient deprivation; however, this report shows that some autophagy proteins may also be accelerating population death in those conditions. While leucine starvation caused YCA1-mediated apoptosis characterized by increased annexin V staining, nitrogen deprivation triggered necrotic death characterized by increased propidium iodide uptake. Although a Δatg8 strain died faster than its parental strain during nitrogen starvation, this mutant died slower than its parent during leucine starvation. Conversely, a Δatg11 strain died slower than its parent during nitrogen starvation, but faster during leucine starvation. Curiously, although GFP-Atg8 complemented the Δatg8 mutation, this protein made ATG8 cells more sensitive to nitrogen starvation, and less sensitive to leucine starvation. These results were difficult to explain if autophagy only extended life but could be an indication that a second form of autophagy could concurrently facilitate either apoptotic or necrotic cell death.
Apoptosis; Atg8; autophagy; CVT; necrosis; programmed cell death; starvation; yeast
Amino acids, leucine in particular, are known to inhibit autophagy, at least in part by their ability to stimulate MTOR-mediated signaling. Evidence is presented showing that glutamate dehydrogenase, the central enzyme in amino acid catabolism, contributes to leucine sensing in the regulation of autophagy. The data suggest a dual mechanism by which glutamate dehydrogenase activity modulates autophagy, i.e., by activating MTORC1 and by limiting the formation of reactive oxygen species.
reactive oxygen species; NADPH; amino acids; signaling; mitochondria; MTOR; AMPK; transhydrogenase
The role of autophagy in the response of human hepatocytes to oxidative stress remains unknown. Understanding this process may have important implications for the understanding of basic liver epithelial cell biology and the responses of hepatocytes during liver disease. To address this we isolated primary hepatocytes from human liver tissue and exposed them ex vivo to hypoxia and hypoxia-reoxygenation (H-R). We showed that oxidative stress increased hepatocyte autophagy in a reactive oxygen species (ROS) and class III PtdIns3K-dependent manner. Specifically, mitochondrial ROS and NADPH oxidase were found to be key regulators of autophagy. Autophagy involved the upregulation of BECN1, LC3A, Atg7, Atg5 and Atg 12 during hypoxia and H-R. Autophagy was seen to occur within the mitochondria of the hepatocyte and inhibition of autophagy resulted in the lowering a mitochondrial membrane potential and onset of cell death. Autophagic responses were primarily observed in the large peri-venular (PV) hepatocyte subpopulation. Inhibition of autophagy, using 3-methyladenine, increased apoptosis during H-R. Specifically, PV human hepatocytes were more susceptible to apoptosis after inhibition of autophagy. These findings show for the first time that during oxidative stress autophagy serves as a cell survival mechanism for primary human hepatocytes.
apoptosis; autophagy; human hepatocytes; hypoxia; necrosis; reactive oxygen species
Autophagy is a catabolic process that provides the degradation of altered/damaged organelles through the fusion between autophagosomes and lysosomes. Proper regulation of the autophagic flux is fundamental for the homeostasis of skeletal muscles in physiological conditions and in response to stress. Defective as well as excessive autophagy is detrimental for muscle health and has a pathogenic role in several forms of muscle diseases. Recently, we found that defective activation of the autophagic machinery plays a key role in the pathogenesis of muscular dystrophies linked to collagen VI. Impairment of the autophagic flux in collagen VI null (Col6a1–/–) mice causes accumulation of dysfunctional mitochondria and altered sarcoplasmic reticulum, leading to apoptosis and degeneration of muscle fibers. Here we show that physical exercise activates autophagy in skeletal muscles. Notably, physical training exacerbated the dystrophic phenotype of Col6a1–/– mice, where autophagy flux is compromised. Autophagy was not induced in Col6a1–/– muscles after either acute or prolonged exercise, and this led to a marked increase of muscle wasting and apoptosis. These findings indicate that proper activation of autophagy is important for muscle homeostasis during physical activity.
autophagy; muscle; muscular dystrophy; mouse model; collagen VI
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.
Atg13; autophagy; FIP200; Ulk1; Ulk2
Perhaps the most complex step of macroautophagy is the formation of the double-membrane autophagosome. The majority of the autophagy-related (Atg) proteins are thought to participate in nucleation and expansion of the phagophore, and/or the completion of this compartment. Monitoring this part of the process is difficult, and typically involves electron microscopy analysis; however, unless three-dimensional tomography is performed, even this method cannot be used to easily determine if the phagophore is completely enclosed. Accordingly, a complementary approach is to examine the accessibility of sequestered cargo to exogenously added protease. This type of protease protection analysis has been used to monitor the formation of cytoplasm-to-vacuole targeting (Cvt) vesicles and autophagosomes by examining the protease sensitivity of precursor aminopeptidase I (prApe1). For determining the status of autophagosomes formed during nonselective autophagy, however, prApe1 is not the best marker protein. Here, we describe an alternative method for examining autophagosome completion using GFP-Atg8 as a marker for protease protection.
autophagy; lysosome; stress; vacuole; yeast
(Macro)autophagy is a membrane-trafficking process that serves to sequester cellular constituents in organelles termed autophagosomes, which target their degradation in the lysosome. Autophagy operates at basal levels in all cells where it serves as a homeostatic mechanism to maintain cellular integrity. The levels and cargoes of autophagy can, however, change in response to a variety of stimuli, and perturbations in autophagy are known to be involved in the etiology of various human diseases. Autophagy must therefore be tightly controlled. We report here that the Drosophila cyclindependent kinase PITSLRE is a modulator of autophagy. Loss of the human PITSLRE ortholog, CDK11, initially appears to induce autophagy, but at later time points CDK11 is critically required for autophagic flux and cargo digestion. Since PITSLRE/CDK11 regulates autophagy in both Drosophila and human cells, this kinase represents a novel phylogenetically conserved component of the autophagy machinery.
PITSLRE; CDK11; cyclin-dependent kinase; autophagy; human; Drosophila
Acute promyelocytic leukemia (APL) results from a blockade of granulocyte differentiation at the promyelocytic stage. All-trans retinoic acid (ATRA) induces clinical remission in APL patients by enhancing the rapid differentiation of APL cells and the clearance of PML-RARα, APL's hallmark oncoprotein. In the present study, we demonstrated that both autophagy and Beclin 1, an autophagic protein, are upregulated during the course of ATRA-induced neutrophil/granulocyte differentiation of an APL-derived cell line named NB4 cells. This induction of autophagy is associated with downregulation of Bcl-2 and inhibition of mTOR activity. Small interfering RNA-mediated knockdown of BECN1 expression enhances apoptosis triggered by ATRA in NB4 cells but does not affect the differentiation process. These results provide evidence that the upregulation of Beclin 1 by ATRA constitutes an anti-apoptotic signal for maintaining the viability of mature APL cells, but has no crucial effect on the granulocytic differentiation. This finding may help to elucidate the mechanisms involved in ATRA resistance of APL patients, and in the ATRA syndrome caused by an accumulation of mature APL cells.
APL; Beclin 1; apoptosis; ATRA; autophagy; differentiation
Modification of target molecules by ubiquitin or ubiquitin-like (Ubl) proteins is generally reversible. Little is known, however, about the physiological function of the reverse reaction, deconjugation. Atg8 is a unique Ubl protein whose conjugation target is the lipid phosphatidylethanolamine (PE). Atg8 functions in the formation of double-membrane autophagosomes, a central step in the well-conserved intracellular degradation pathway of macroautophagy (hereafter autophagy). Here we show that the deconjugation of Atg8−PE by the cysteine protease Atg4 plays dual roles in the formation of autophagosomes. During the early stage of autophagosome formation, deconjugation releases Atg8 from non-autophagosomal membranes to maintain a proper supply of Atg8. At a later stage, the release of Atg8 from intermediate autophagosomal membranes facilitates the maturation of these structures into fusion-capable autophagosomes. These results provide new insights into the functions of Atg8−PE and its deconjugation.
autophagy; ubiquitin-like proteins; deconjugation; Atg4; Atg8
(Macro)Autophagy is a phylogenetically conserved membrane-trafficking process that functions to deliver cytoplasmic cargoes to lysosomes for digestion. The process is a major mechanism for turnover of cellular constituents and is therefore critical for maintaining cellular homeostasis. Macroautophagy is characteristically distinct from other forms of autophagy due to the formation of double-membraned vesicles termed autophagosomes which encapsulate cargoes prior to fusion with lysosomes. Autophagosomes contain an integral membrane-bound form (LC3-II) of the microtubule-associated protein 1 light chain 3 β (MAP1LC3B), which has become a gold-standard marker to detect accumulation of autophagosomes and thereby changes in macroautophagy. Due to the role played by macroautophagy in various diseases, the detection of autophagosomes in tissue sections is frequently desired. To date, however, the detection of endogenous LC3-II on paraffin-embedded tissue sections has proved problematic. We report here a simple, optimized and validated method for the detection of LC3-II by immunohistochemistry in human and mouse tissue samples that we believe will be a useful resource for those wishing to study macroautophagy ex vivo.
autophagy; LC3; tissue sections; immunohistochemistry
Reprogramming energy metabolism from oxidative phosphorylation to aerobic glycolysis, a common feature of human cancer, is associated with a relative acidic tumor microenvironment which can sometimes be further accentuated by hypoxia operating within most solid tumors. We found that alteration of extracellular pH induces marked and rapid changes of autophagic activity. Interestingly, acidic and basic conditions induced completely opposite effect on autophagy, with its activity suppressed at lower pH whereas stimulated at higher pH. Gene knockdown experiments indicated that pH induced-autophagy requires Beclin 1, Vps34 and Atg5, key components of the autophagy pathway. Of note, an acidic condition not only inhibits the basal but also blocks the starvation-induced autophagy activity. Significantly, examination of different areas of tumor mass revealed a lower autophagic activity within the inner region than the outer region. These findings have important implications on the connections between autophagy and cancer as well as a wide range of other physiological and pathological processes.
autophagy; extracellular pH; cancer; microenvironment; Beclin 1; VPS34; Atg5
Bnip3 is a pro-apoptotic BH3-only protein which is associated with mitochondrial dysfunction and cell death. Bnip3 is also a potent inducer of autophagy in many cells. In this study, we have investigated the mechanism by which Bnip3 induces autophagy in adult cardiac myocytes. Overexpression of Bnip3 induced extensive autophagy in adult cardiac myocytes. Fluorescent microscopy studies and ultrastructural analysis revealed selective degradation of mitochondria by autophagy in myocytes overexpressing Bnip3. Oxidative stress and increased levels of intracellular Ca2+ have been reported by others to induce autophagy, but Bnip3-induced autophagy was not abolished by antioxidant treatment or the Ca2+ chelator BAPTA-AM. We also investigated the role of the mitochondrial permeability transition pore (mPTP) in Bnip3-induced autophagy. Although the mPTP has previously been implicated in the induction of autophagy and selective removal of damaged mitochondria by autophagosomes, mitochondria sequestered by autophagosomes in Bnip3-treated cardiac myocytes had not undergone permeability transition and treatment with the mPTP inhibitor cyclosporine A did not inhibit mitochondrial autophagy in cardiac myocytes. Moreover, cyclophilin D (cypD) is an essential component of the mPTP and Bnip3 induced autophagy to the same extent in embryonic fibroblasts isolated from wild-type and cypD-deficient mice. These results support a model where Bnip3 induces selective removal of the mitochondria in cardiac myocytes and that Bnip3 triggers induction of autophagy independent of Ca2+, ROS generation and mPTP opening.
Bnip3; autophagy; cardiac myocytes; mitochondria; permeability transition pore; cyclophilin D
Autophagy is an evolutionarily conserved physiological process of self-digestion by a cell to adapt to various stresses, including starvation. Its molecular basis involves the concerted activation of proteins encoded by the family of autophagy-related (Atg) genes. The best characterized is the serine/threonine protein kinase Atg1 in yeast which appears to be essential at the early stage of autophagy. In mammals, five Atg1 homologues have been identified as uncoordinated (UNC) 51-like kinase 1 to 4 and STK36. ULK1 and ULK2 are the most closely related members of the family, sharing 78% homology within their protein kinase domains. However, the specific function of ULK1 and ULK2 in mammalian autophagy is not fully understood. Here, we demonstrate that ULK1 and ULK2 are functionally redundant protein kinases required to mediate autophagy under nutrient-deprived conditions in fibroblasts. In contrast, ULK1, but not ULK2, is critical to induce the autophagic response of cerebellar granule neurons (CGN) to low potassium concentration in serum-free conditions. Furthermore, we found that ULK1 has a cytoprotective function in neurons. Together, these results provide strong genetic evidence that ULK1 is an essential component of the autophagic signaling pathway. The ability of ULK2 to compensate for the loss of ULK1 function is cell-type specific.
Atg1; ULK1; ULK2; mTOR; autophagy; apoptosis; neurons; MEFs