Our laboratory has been investigating for some time the nature of the response of T lymphocytes in autoimmunity in the reactions against self-proteins that result in a number of diseases, such as type 1 diabetes, multiple sclerosis, rheumatoid arthritis (RA) and others. T cells recognize peptides generated from proteins that are processed by antigen-presenting cells (APC). The peptides may derive from exogenous proteins or from the normal catabolism of self-proteins. The peptides complexed to major histocompatibility complex (MHC) molecules constitute the chemical entity that is engaged by the antigen-receptor of T cells. An important hypothesis postulates that self-peptides that suffer post-translational modifications in the APC may form neo-antigens that are recognized by the immune system and form the target of autoimmunity. Our interest in citrullination in the context of antigen processing and presentation stemmed from studies suggesting that an immune response to citrullinated self-peptides may be involved in autoimmunity. In a first publication, we found T cells that specifically recognized citrullinated peptides after immunization of inbred mice with standard foreign proteins. We used the small protein hen-egg white lysozyme. These T cells only recognized the citrullinated peptide and not the unmodified one, thus proving that a neo-epitope had been created by this modification. But how this modification took place was not known. Our recent report describes a central role for autophagy in citrullination of peptides by APC.
antigen presentation; autoimmunity; autophagy; citrullination; peptidylarginine deiminase
Inhibition of the autophagic pathway has recently revealed promising results in increasing pro-death activity of multiple cancer therapeutics. Here, we discuss our findings regarding the autophagy-blocking and anti-neoplastic effects of a synthetic sphingosine analog, FTY720, in mantle cell lymphoma (MCL). We also emphasize how FTY720 enhances the pro-death activity of the fully humanized monoclonal antibody milatuzumab by inhibiting the autophagy-lysosome dependent degradation of its therapeutic target, CD74. Our results provide justification for further evaluation of FTY720 and milatuzumab as a combination therapy for this aggressive B-cell malignancy.
autophagy; FTY720; lysosomal cell death; mantle cell lymphoma; milatuzumab
In most animals, during oocyte fertilization the spermatozoon provides DNA and centrioles together with some cytoplasm and organelles, but paternal mitochondria are generally eliminated in the embryo. Using the model animal C. elegans we have shown that paternal organelle degradation is dependent on the formation of autophagosomes a few minutes after fertilization. This macroautophagic process is preceded by an active ubiquitination of some spermatozoon-inherited organelles. Analysis of fertilized mouse embryos suggests that this autophagy event is evolutionarily conserved.
C. elegans; fertilization; LGG-1; LGG-2; mitophagy
Oxygen (O2), while essential for aerobic life, can also cause metabolic toxicity through the excess generation of reactive oxygen species (ROS). Pathological changes in ROS production can originate through the partial reduction of O2 during mitochondrial electron transport, as well as from enzymatic sources. This phenomenon, termed the oxygen paradox, has been implicated in aging and disease, and is especially evident in critical care medicine. Whereas high O2 concentrations are utilized as a life-sustaining therapeutic for respiratory insufficiency, they in turn can cause acute lung injury. Alveolar epithelial cells represent a primary target of hyperoxia-induced lung injury. Recent studies have indicated that epithelial cells exposed to high O2 concentrations die by apoptosis, or necrosis, and can also exhibit mixed-phenotypes of cell death (aponecrosis). Autophagy, a cellular homeostatic process responsible for the lysosomal turnover of organelles and proteins, has been implicated as a general response to oxidative stress in cells and tissues. This evolutionarily conserved process is finely regulated by a complex interplay of protein factors. During autophagy, senescent organelles and cellular proteins are sequestered in autophagic vacuoles (autophagosomes) and subsequently targeted to the lysosome, where they are degraded by lysosomal hydrolases, and the breakdown products released for reutilization in anabolic pathways. Autophagy has been implicated as a cell survival mechanism during nutrient-deficiency states, and more generally, as a determinant of cell fate. However, the mechanisms by which autophagy and/or autophagic proteins potentially interact with and/or regulate cell death pathways during high oxygen stress, remain only partially understood.
acute lung injury; Apoptosis; autophagy; caveolin-1; Fas; hyperoxia; LC3B
During early embryogenesis, before the conceptus forms the placenta, maternal nutrients as well as signaling molecules must reach the embryo proper through a tightly sealed epithelial tissue, the visceral endoderm (VE). The VE serves as a signaling center for embryogenesis, where exocytic and endocytic processes integrate signal production, perception and termination. However, the endocytic process in this important tissue has not been well characterized. We show that endocytic delivery to the lysosomes occurs via RAB7-dependent microautophagy. This process is essential for early mammalian development.
autophagy; apical vacuole; microautophagy; RAB7; early mouse development
Although hypoxia can cause cell cycle arrest, it may simultaneously suppress a conversion from this arrest to senescence. Furthermore, hypoxia can suppress senescence caused by diverse stimuli, maintaining reversible quiescence instead. Hypoxia activates autophagy and inhibits MTOR, thus also activating autophagy. What is the relationship between autophagy and cellular senescence? Also, can inhibition of MTOR and stimulation of autophagy explain the gerosuppressive effects of hypoxia?
autophagy; senescence; oxygen; rapamycin; aging
In response to toxic stimuli, BCL2L11 (also known as BIM), a BH3-only protein, is released from its interaction with dynein light chain 1 (DYNLL1 also known as LC8) and can induce apoptosis by inactivating anti-apoptotic BCL2 proteins and by activating BAX-BAK1. Recently, we discovered that BCL2L11 interacts with BECN1 (Beclin 1), and that this interaction is facilitated by DYNLL1. BCL2L11 recruits BECN1 to microtubules by bridging BECN1 and DYNLL1, thereby inhibiting autophagy. In starvation conditions, BCL2L11 is phosphorylated by MAPK8/JNK and this phosphorylation abolishes the BCL2L11-DYNLL1 interaction, allowing dissociation of BCL2L11 and BECN1, thereby ameliorating autophagy inhibition. This finding demonstrates a novel function of BIM beyond its roles in apoptosis, highlighting the crosstalk between autophagy and apoptosis, and suggests that BCL2L11’s dual effects in inhibiting autophagy and promoting apoptosis may have important roles in disease pathogenesis.
BIM; autophagy; apoptosis; BH-3 domain; BECN1
Lysosomal storage diseases are metabolic disorders characterized by the accumulation of acidic vacuoles, and are usually the consequence of the deficiency of an enzyme responsible for the metabolism of vesicular lipids, proteins or carbohydrates. In contrast, mucolipidosis type IV (MLIV), results from the absence of a vesicular Ca2+ release channel called mucolipin 1/transient receptor potential mucolipin 1 (MCOLN1/TRPML1) which is required for the fusion of amphisomes with lysosomes. In Drosophila, ablation of the MCOLN1 homolog (trpml) leads to diminished viability during pupation when the animals rely on autophagy for nutrients. This pupal lethality results from decreased target of rapamycin complex 1 (TORC1) signaling, and is reversed by reactivating TORC1. Our findings indicate that one of the primary causes of toxicity in the absence of TRPML is cellular amino acid starvation, and the resulting decrease in TORC1 activity. Furthermore, our findings raise the intriguing possibility that the neurological dysfunction in MLIV patients may arise from amino acid deprivation in neurons. Therefore, future studies evaluating the levels of amino acids and TORC1 activity in MLIV neurons may aid in the development of novel therapeutic strategies to combat the severe manifestations of MLIV.
TRP channels; mucolipin; mucolipidosis type IV; autophagy; MLIV; TORC1; MTOR; amino acids; endosomal
COPI, a coatomer protein complex of secretory vesicles, is involved in Golgi and endoplasmic reticulum traffic and in early endosome maturation. The loss of COPI results in the fragmentation of Golgi, accumulation of immature autophagosomes, inhibition of autophagy, and cell death. Since COPI is required by all cells, it would appear an unlikely target for cancer treatment. However, our recent function-based genomic screen unexpectedly identified a specific COPI subunit, ζ1, as a cancer-specific target. The existing cancer drugs kill only proliferating but not growth-arrested tumor cells, but the depletion of ζ1 induces cell death in both dividing and nondividing tumor cells, while sparing normal cells. The mechanism of this remarkable tumor selectivity turned out to be surprising and heretofore unprecedented.
autophagy; cancer chemotherapy; cancer targets; COPI; COPZ1; COPZ2; function-based genomics; intracellular traffic; microRNA
Our recent study revealed a new role of nucleus accumbens-1 (NAC1), a transcription factor belonging to the BTB/POZ gene family, in regulating autophagy. Moreover, we found that the high-mobility group box 1 (HMGB1), a chromatin-associated nuclear protein acting as an extracellular damage associated molecular pattern molecule (DAMP), is the downstream executor of NAC1 in modulating autophagy. In response to stress such as therapeutic insults, NAC1 increases the expression, cytosolic translocation and release of HMGB1; elevated level of the cytoplasmic HMGB1 leads to activation of autophagy. The NAC1-HMGB1 partnership may represent a previously unrecognized pathway that regulates autophagy in response to various stresses such as chemotherapy.
Apoptosis; autophagy; cisplatin; HMGB1; NAC1
Abnormally swollen regions of axons and dendrites (neurites) filled mainly with autophagy-related organelles represent the highly characteristic and widespread form of “neuritic dystrophy” in Alzheimer disease (AD), which implies dysfunction of autophagy and axonal transport. In this punctum, we discuss our recent findings that autophagic/lysosomal degradation is critical to proper axonal transport of autophagic vacuoles (AVs) and lysosomes. We showed that lysosomal protease inhibition induces defective axonal transport of specific cargoes, causing these cargoes to accumulate in axonal swellings that biochemically and morphologically resemble the dystrophic neurites in AD. Our findings suggest that a cargo-specific failure of axonal transport promotes neuritic dystrophy in AD, which involves a mechanism distinct from the global axonal transport deficits seen in some other neurodegenerative diseases.
Alzheimer disease; autophagy; axonal transport; dystrophic neurites; lysosomes; proteolysis
The autophagy-dependent selective degradation of mitochondria (mitophagy) plays an important role in removing excessive, damaged and dysfunctional mitochondria to maintain a proper cellular homeostasis. Relative to its significance in cell physiology, very little is known about the molecular machinery and regulatory mechanism of mitophagy in mammalian cells or yeast. We found that two mitogen-activated protein kinases (MAPKs), Slt2 and Hog1, are required for mitophagy in Saccharomyces cerevisiae. Slt2 is involved in both mitophagy and pexophagy (the selective degradation of peroxisomes through autophagy), whereas Hog1 functions specifically in mitophagy.
autophagy; kinase; mitochondria; PAS; regulation; vacuole
We recently showed that phagophore biogenesis requires SNAREs. Our data indicate that the exocytic Q/t-SNAREs Sso1/2 and Sec9 are required for one of the earliest steps in autophagosome biogenesis, the homotypic fusion of Atg9-containing vesicles. We propose that this step precedes the formation of Atg9-containing tubulovesicular clusters (TVCs) that is a key step in perivacuolar, phagophore assembly. We also found that the endosomal Q/t-SNARE Tlg2 and the R/v-SNAREs Sec22 and Ykt6 interact with Sso1-Sec9, and are required for normal Atg9 trafficking. Thus, autophagosome biogenesis appears to involve multiple SNARE-mediated fusion events. These findings provide novel insights into the mechanism of autophagosome construction.
Atg9; autophagy; lysosome; phagophore assembly site; SNARE; tubulovesicular clusters; vacuole
Mitochondrial DNA (mtDNA) is different in many ways from nuclear DNA. A key difference is that certain types of DNA damage are not repaired in the mitochondrial genome. What, then, is the fate of such damage? What are the effects? Both questions are important from a health perspective because irreparable mtDNA damage is caused by many common environmental stressors including ultraviolet C radiation (UVC). We found that UVC-induced mtDNA damage is removed slowly in the nematode Caenorhabditis elegans via a mechanism dependent on mitochondrial fusion, fission, and autophagy. However, knockdown or knockout of genes involved in these processes—many of which have homologs involved in human mitochondrial diseases—had very different effects on the organismal response to UVC. Reduced mitochondrial fission and autophagy caused no or small effects, while reduced mitochondrial fusion had dramatic effects.
Caenorhabditis elegans; autophagy; mitochondrial DNA damage; mitochondrial fission; mitochondrial fusion; mitophagy; ultraviolet radiation
Autophagy-mediated major histocompatibility complex (MHC) class I presentation can follow either the conventional MHC class I pathway or a recently described vacuolar pathway. In the vacuolar pathway, protein degradation is effected by lysosomal proteases, peptide exchange takes place with recirculating MHC complexes and the newly formed peptide-MHC complexes reach the cell surface by the endocytic pathway. This pathway is independent of the proteasome and the transporter associated with antigen processing (TAP) complex, but generates the same, or a similar, epitope as that from the conventional MHC class I pathway. Here, we discuss different mechanisms by which autophagy mediates MHC class I-restricted antigen presentation, which is crucial to its role in the control of intracellular pathogens.
autophagy; MHC class I; antigen processing; antigen presentation; human cytomegalovirus
Alzheimer disease (AD) is sometimes referred to as type III diabetes because of the shared risk factors for the two disorders. Insulin resistance, one of the major components of type II diabetes mellitus (T2DM), is a known risk factor for AD. Insulin resistance increases amyloid-β peptide (Aβ) generation, but the exact mechanism underlying the linkage of insulin resistance to increased Aβ generation in the brain is unknown. In this study, we investigated the effect of insulin resistance on amyloid β (A4) precursor protein (APP) processing in mice fed a high-fat diet (HFD), and diabetic db/db mice. We found that insulin resistance promotes Aβ generation in the brain via altered insulin signal transduction, increased BACE1/β-secretase and γ-secretase activities, and accumulation of autophagosomes. Using an in vitro model of insulin resistance, we found that defects in insulin signal transduction affect autophagic flux by inhibiting the mechanistic target of rapamycin (MTOR) pathway. The insulin resistance-induced autophagosome accumulation resulted in alteration of APP processing through enrichment of secretase proteins in autophagosomes. We speculate that the insulin resistance that underlies the pathogenesis of T2DM might alter APP processing through autophagy activation, which might be involved in the pathogenesis of AD. Therefore, we propose that insulin resistance-induced autophagosome accumulation becomes a potential linker between AD and T2DM.
APP processing; Alzheimer disease; Aβ; autophagosome; diabetes; insulin resistance
We recently identified physical exercise as a newly defined inducer of autophagy in vivo. Exercise induced autophagy in multiple organs involved in metabolic regulation, such as muscle, liver, pancreas and adipose tissue. To study the physiological role of exercise-induced autophagy, we generated mice with a knock-in nonphosphorylatable mutation in BCL2 (Thr69Ala, Ser70Ala and Ser84Ala) (BCL2 AAA) that are defective in exercise- and starvation-induced autophagy but not in basal autophagy. We found that BCL2 AAA mice could not run on a treadmill as long as wild-type mice, and did not undergo exercise-mediated increases in skeletal glucose muscle uptake. Unlike wild-type mice, the BCL2 AAA mice failed to reverse high-fat diet-induced glucose intolerance after 8 weeks of exercise training, possibly due to defects in signaling pathways that regulate muscle glucose uptake and metabolism during exercise. Together, these findings suggested a hitherto unknown important role of autophagy in mediating exercise-induced metabolic benefits. In the present addendum, we show that treadmill exercise also induces autophagy in the cerebral cortex of adult mice. This observation raises the intriguing question of whether autophagy may in part mediate the beneficial effects of exercise in neurodegeneration, adult neurogenesis and improved cognitive function.
autophagy; exercise; brain; BCL2; metabolism
Many viruses have evolved elegant strategies to co-opt cellular autophagic responses to facilitate viral propagation and evasion of immune surveillance. Kaposi’s sarcoma-associated herpesvirus (KSHV) establishes a life-long persistent infection in its human host, and is etiologically linked to several cancers. KSHV gene products have been shown to modulate autophagy but their contribution to pathogenesis remains unclear. Our recent study demonstrated that KSHV subversion of autophagy promotes bypass of oncogene-induced senescence (OIS), an important host barrier to tumor initiation. These findings suggest that KSHV has evolved to subvert autophagy, at least in part, to establish an optimal niche for infection, concurrently dampening host antiviral defenses and allowing the ongoing proliferation of infected cells.
KSHV; oncogene; DNA damage; autophagy; oncogene-induced senescence
A growing body of research has connected autophagy to neurodegenerative diseases such as Alzheimer disease (AD). In autopsied AD brain, large multivesicular bodies accumulate in neurons. Knockout mice deficient for key autophagy genes demonstrate age-dependent neurodegeneration. Most neurodegenerative diseases are characterized by accumulation of insoluble protein species; the type of protein and the location of aggregates within the nervous system help to define the type of disorder. It has been hypothesized that the inability to degrade such aggregates is a major causative factor in neuronal dysfunction and eventual neuronal death. As neurons are postmitotic and thus cannot regenerate themselves, mechanisms of protein clearance have received much attention in the field. The function of the ubiquitin-proteasome system (UPS) is impaired in models of neurodegeneration, and overexpression of chaperone proteins, such as those in the HSP70 family, leads to beneficial effects in many models of proteinopathies. Recently, studies of the effects of autophagy as a clearance mechanism have uncovered compelling evidence that inducing autophagy can alleviate many pathogenic and behavioral symptoms in animal and cellular models of neurodegeneration.
MAPT/tau; Alzheimer disease; Drosophila; microtubule; phosphorylation
microRNAs (miRNAs) are a class of small regulatory RNAs that regulate gene expression at the post-transcriptional level. miRNAs play important roles in the regulation of development, growth, and metastasis of cancer, and in determining the response of tumor cells to anticancer therapy. In recent years, they have also emerged as important regulators of autophagy, a lysosomal-mediated pathway that contributes to degradation of a cell's own components. Imatinib, a targeted competitive inhibitor of the BCR-ABL1 tyrosine kinase, has revolutionized the clinical treatment of chronic myelogenous leukemia (CML). We demonstrate that MIR30A-mediated autophagy enhances imatinib resistance against CML including primary stem and progenitor cells. MIR30A, but not MIR101, is a potent inhibitor of autophagy by selectively downregulating BECN1 and ATG5 expression in CML cells. MIR30A mimics, as well as knockdown of BECN1 and ATG5, increases intrinsic apoptotic pathways. In contrast, the antagomir-30A increases autophagy and inhibits intrinsic apoptotic pathways, confirming that autophagy serves to protect against apoptosis. Taken together, these data clarify some of the underlying molecular mechanisms of tyrosine kinase inhibitor-induced autophagy.
Atg5; autophagy; BCR-ABL tyrosine kinase; Beclin 1; chronic myelogenous leukemia; microRNA
Degradation of different cargo by macroautophagy is emerging as a highly selective process which relies upon specific autophagy receptors and adapter molecules that link the cargo with the autophagic molecular machinery. We have recently reported that the large phsophatidylinositol-3-phosphate (PtdIns(3)P)-binding protein Alfy (Autophagy-linked FYVE protein) is required for selective degradation of aggregated proteins. Although depletion of Alfy inhibits Atg5-dependent aggregate degradation, overexpression of Alfy results in Atg5-dependent aggregate clearance and neuroprotection. Alfy-mediated degradation requires the ability of Alfy to directly interact with Atg5. This ability to interact with the core autophagic machinery may cause Alfy to diminish the responsiveness to nonselective autophagic degradation as measured by long-lived protein degradation. Thus, increasing Alfy-mediated protein degradation may be beneficial in some organs, but may be detrimental in others.
autophagy; protein aggregates; neurodegeneration; Alfy; aggregation; degradation
Mutations in the gene for the E3 ubiquitin ligase Parkin are the most prevalent cause of autosomal recessive Parkinson disease (PD), an incurable neurodegenerative disorder. Parkin surveys mitochondrial quality by translocating to depolarized mitochondria and inducing their selective macroautophagic removal (mitophagy). We recently reported that Parkin interacts with Ambra1 (activating molecule in Beclin 1-regulated autophagy), a protein that promotes autophagy in the vertebrate central nervous system. We discovered that prolonged mitochondrial depolarization strongly increases the interaction of Parkin with Ambra1. Ambra1 is recruited in a Parkin-dependent manner to perinuclear clusters of depolarized mitochondria, activates the class III phosphatidylinositol 3-kinase (PtdIns3K) complex around these mitochondria and contributes to their selective autophagic clearance. Here, we discuss these findings and suggest a model where translocated Parkin efficiently triggers mitophagy through combined recruitment of Ambra1 and ubiquitination of outer mitochondrial membrane proteins.
Ambra1; Beclin 1; mitochondria; mitophagy; Parkin; Parkinson disease; PINK1; PtdIns3K; ubiquitin
Starvation induces a protective process of self-cannibalization called autophagy that is thought to mediate nonselective degradation of cytoplasmic material. We recently reported that mitochondria escape autophagosomal degradation through extensive fusion into mitochondrial networks upon certain starvation conditions. The extent of mitochondrial elongation is dependent on the type of nutrient deprivation, with amino acid depletion having a particularly strong effect. Downregulation of the mitochondrial fission protein Drp1 was determined to be important in bringing about starvation-induced mitochondrial fusion. The formation of mitochondrial networks during nutrient depletion selectively blocked their autophagic degradation, presumably allowing cells to sustain efficient ATP production and thereby survive starvation.
autophagy; Drp1; fission; fusion; mitochondria; PKA; starvation
Autophagy is an innate immune defense against bacterial invasion. Recent studies show that two adaptor proteins, p62 and NDP52, are required for autophagy of the bacterial pathogen Salmonella enterica serovar Typhimurium (S. typhimurium). However, it is not known why two different adaptors are required to target the same bacterial cargo to autophagy. Here we show that both adaptors are recruited to bacteria with similar kinetics, that they are recruited to bacteria independently of each other, and that depletion of either adaptor leads to impairment of antibacterial autophagy. Depletion of both adaptors does not synergistically impair autophagy, indicating they act in the same pathway. Remarkably, we observed that these adaptors do not colocalize, but rather form non-overlapping microdomains surrounding bacteria. We conclude that p62 and NDP52 act cooperatively to drive efficient antibacterial autophagy by targeting the protein complexes they coordinate to distinct micro-domains associated with bacteria.
autophagy; Salmonella; ubiquitin; adaptor; p62/SQSTM1; NDP52; ATG8/LC3