The Dynamics of Autophagy Visualised in Live Cells: from Autophagosome Formation to Fusion with Endo/lysosomes

Autophagy has been implicated in a range of disorders and hence is of major interest. However, imaging autophagy in real time has been hampered by lack of suitable markers. We have compared the potential of monodansylcadaverine, widely used as an autophagosomal marker, and the Atg8 homologue LC3, to follow autophagy by fluorescence microscopy whilst labelling late endosomes and lysosomes simultaneously using EGFP-CD63. Monodansylcadaverine labelled only acidic CD63-positive compartments in response to a range of autophagic inducers in various live or post-fixed cells, staining being identical in atg5+/+ and atg5-/- MEFs in which autophagosome formation is disabled. Monodansylcadaverine staining was essentially indistinguishable from that of LysoTracker Red, LAMP1 or LAMP2. In contrast, 60-90% of EGFP-LC3-positive punctate organelles did not colocalise with LAMP1/LAMP2/CD63 and were monodansylcadaverine-negative while EGFP-LC3 puncta that did colocalise with LAMP1/LAMP2/CD63 were also monodansylcadaverine-positive. Hence monodansylcadaverine is no different from other markers of acidic compartments and it cannot be used to follow autophagosome formation. In contrast, fusion of mRFP-LC3-labelled autophagosomes with EGFP-CD63-positive endosomes and lysosomes and sequestration of dsRed-labelled mitochondria by EGFP-LC3- and EGFP-CD63-positive compartments could be visualised in real time. Moreover, transition of EGFP-LC3-I (45 kDa) to EGFP-LC3-II (43 kDa) - traced by immunoblotting and verified by [3H]ethanolamine labelling - revealed novel insights into the dynamics of autophagosome homeostasis, including the rapid activation of autophagy by the apoptotic inducer staurosporine prior to apoptosis proper. Use of fluorescent LC3 and a counterfluorescent endosomal/lysosomal protein clearly allows the entire autophagic process to be followed by live cell imaging with high fidelity.     

Beclin1-binding UVRAG targets the class C Vps complex to coordinate autophagosome maturation and endocytic trafficking

Autophagic and endocytic pathways are tightly regulated membrane rearrangement processes that are crucial for homeostasis, development and disease. Autophagic cargo is delivered from autophagosomes to lysosomes for degradation through a complex process that topologically resembles endosomal maturation. Here, we report that a Beclin1-binding autophagic tumour suppressor, UVRAG, interacts with the class C Vps complex, a key component of the endosomal fusion machinery. This interaction stimulates Rab7 GTPase activity and autophagosome fusion with late endosomes/lysosomes, thereby enhancing delivery and degradation of autophagic cargo. Furthermore, the UVRAG-class-C-Vps complex accelerates endosome-endosome fusion, resulting in rapid degradation of endocytic cargo. Remarkably, autophagosome/endosome maturation mediated by the UVRAG-class-C-Vps complex is genetically separable from UVRAG-Beclin1-mediated autophagosome formation. This result indicates that UVRAG functions as a multivalent trafficking effector that regulates not only two important steps of autophagy - autophagosome formation and maturation - but also endosomal fusion, which concomitantly promotes transport of autophagic and endocytic cargo to the degradative compartments.     

Dissection of the autophagosome maturation process by a novel reporter protein, tandem fluorescent-tagged LC3

During the process of autophagy, autophagosomes undergo a maturation process consisting of multiple fusions with endosomes and lysosomes, which provide an acidic environment and digestive function to the interior of the autophagosome. Here we found that a fusion protein of monomeric red-fluorescence protein and LC3, the most widely used marker for autophagosomes, exhibits a quite different localization pattern from that of GFP-LC3. GFP-LC3 loses fluorescence due to lysosomal acidic and degradative conditions but mRFP-LC3 does not, indicating that the latter can label the autophagic compartments both before and after fusion with lysosomes. Taking advantage of this property, we devised a novel method for dissecting the maturation process of autophagosomes. mRFP-GFP tandem fluorescent-tagged LC3 (tfLC3) showed a GFP and mRFP signal before the fusion with lysosomes, and exhibited only the mRFP signal subsequently. Using this method, we provided evidence that overexpression of a dominant negative form of Rab7 prevented the fusion of autophagosomes with lysosomes, suggesting that Rab7 is involved in this step. This method will be of general utility for analysis of the autophagosome maturation process.     

Dissection of the Autophagosome Maturation Process by a Novel Reporter Protein,Tandem Fluorescent-Tagged LC3

During the process of autophagy, autophagosomes undergo a maturation process consisting of multiple fusions with endosomes and lysosomes, which provide an acidic environment and digestive function to the interior of the autophagosome. Here we found that a fusion protein of monomeric Red-fluorescence protein and LC3, the most widely used marker for autophagosomes, exhibits a quite different localization pattern from that of GFP-LC3. GFP-LC3 loses fluorescence due to lysosomal acidic and degradative conditions but mRFP-LC3 does not, indicating that the latter can label the autophagic compartments both before and after fusion with lysosomes. Taking advantage of this property, we devised a novel method for dissecting the maturation process of autophagosomes. mRFP-GFP tandem fluorescent-tagged LC3 (tfLC3) showed a GFP and mRFP signal before the fusion with lysosomes, and exhibited only the mRFP signal subsequently. Using this method, we provided evidence that overexpression of a dominant negative form of Rab7 prevented the fusion of autophagosomes with lysosomes, suggesting that Rab7 is involved in this step. This method will be of general utility for analysis of the autophagosome maturation process.     

HS1BP3 inhibits autophagy by regulation of PLD1

Macroautophagy/autophagy is a membrane trafficking and intracellular degradation process involving the formation of double-membrane autophagosomes and their ultimate fusion with lysosomes. Much is yet to be learned about the regulation of this process, especially at the level of the membranes and lipids involved. We have recently found that the PX domain protein HS1BP3 (HCLS1 binding protein 3) is a negative regulator of autophagosome formation. HS1BP3 depletion increases the formation of LC3-positive autophagosomes both in human cells and zebrafish. HS1BP3 localizes to ATG16L1- and ATG9-positive autophagosome precursors deriving from recycling endosomes, which appear to fuse with LC3-positive phagophores. The HS1BP3 PX domain interacts with phosphatidic acid (PA) and 3’-phosphorylated phosphoinositides. When HS1BP3 is depleted, the total cellular PA content is upregulated stemming from increased activity of the PA-producing enzyme PLD (phospholipase D) and increased localization of PLD1 to ATG16L1-positive membranes. We propose that HS1BP3 negatively regulates autophagy by decreasing the PA content of the ATG16L1-positive autophagosome precursor membranes through inhibition of PLD1 activity and localization.     

Visualizing the autophagy pathway in avian cells and its application to studying infectious bronchitis virus

Autophagy is a highly conserved cellular response to starvation that leads to the degradation of organelles and long-lived proteins in lysosomes and is important for cellular homeostasis, tissue development and as a defense against aggregated proteins, damaged organelles and infectious agents. Although autophagy has been studied in many animal species, reagents to study autophagy in avian systems are lacking. Microtubule-associated protein 1 light chain 3 (MAP1LC3/LC3) is an important marker for autophagy and is used to follow autophagosome formation. Here we report the cloning of avian LC3 paralogs A, B and C from the domestic chicken, Gallus gallus domesticus, and the production of replication-deficient, recombinant adenovirus vectors expressing these avian LC3s tagged with EGFP and FLAG-mCherry. An additional recombinant adenovirus expressing EGFP-tagged LC3B containing a G120A mutation was also generated. These vectors can be used as tools to visualize autophagosome formation and fusion with endosomes/lysosomes in avian cells and provide a valuable resource for studying autophagy in avian cells. We have used them to study autophagy during replication of infectious bronchitis virus (IBV). IBV induced autophagic signaling in mammalian Vero cells but not primary avian chick kidney cells or the avian DF1 cell line. Furthermore, induction or inhibition of autophagy did not affect IBV replication, suggesting that classical autophagy may not be important for virus replication. However, expression of IBV nonstructural protein 6 alone did induce autophagic signaling in avian cells, as seen previously in mammalian cells. This may suggest that IBV can inhibit or control autophagy in avian cells, although IBV did not appear to inhibit autophagy induced by starvation or rapamycin treatment.     

Autophagy Activation Is Involved in 3,4-Methylenedioxymethamphetamine (‘Ecstasy’)—Induced Neurotoxicity in Cultured Cortical Neurons

Autophagic (type II) cell death, characterized by the massive accumulation of autophagic vacuoles in the cytoplasm of cells, has been suggested to play pathogenetic roles in cerebral ischemia, brain trauma, and neurodegenerative disorders. 3,4-Methylenedioxymethamphetamine (MDMA or ecstasy) is an illicit drug causing long-term neurotoxicity in the brain. Apoptotic (type I) and necrotic (type III) cell death have been implicated in MDMA-induced neurotoxicity, while the role of autophagy in MDMA-elicited neurotoxicity has not been investigated. The present study aimed to evaluate the occurrence and contribution of autophagy to neurotoxicity in cultured rat cortical neurons challenged with MDMA. Autophagy activation was monitored by expression of microtubule-associated protein 1 light chain 3 (LC3; an autophagic marker) using immunofluorescence and western blot analysis. Here, we demonstrate that MDMA exposure induced monodansylcadaverine (MDC)- and LC3B-densely stained autophagosome formation and increased conversion of LC3B-I to LC3B-II, coinciding with the neurodegenerative phase of MDMA challenge. Autophagy inhibitor 3-methyladenine (3-MA) pretreatment significantly attenuated MDMA-induced autophagosome accumulation, LC3B-II expression, and ameliorated MDMA-triggered neurite damage and neuronal death. In contrast, enhanced autophagy flux by rapamycin or impaired autophagosome clearance by bafilomycin A1 led to more autophagosome accumulation in neurons and aggravated neurite degeneration, indicating that excessive autophagosome accumulation contributes to MDMA-induced neurotoxicity. Furthermore, MDMA induced phosphorylation of AMP-activated protein kinase (AMPK) and its downstream unc-51-like kinase 1 (ULK1), suggesting the AMPK/ULK1 signaling pathway might be involved in MDMA-induced autophagy activation.     

Bulk autophagy,but not mitophagy,is increased in cellular model of mitochondrial disease

Oxidative phosphorylation system (OXPHOS) deficiencies are rare diseases but constitute the most frequent inborn errors of metabolism. We analyzed the autophagy route in 11 skin fibroblast cultures derived from patients with well characterized and distinct OXPHOS defects. Mitochondrial membrane potential determination revealed a tendency to decrease in 5 patients' cells but reached statistical significance only in 2 of them. The remaining cells showed either no change or a slight increase in this parameter. Colocalization analysis of mitochondria and autophagosomes failed to show evidence of increased selective elimination of mitochondria but revealed more intense autophagosome staining in patients' fibroblasts compared with controls. Despite the absence of increased mitophagy, Parkin recruitment to mitochondria was detected in both controls' and patients' cells and was slightly higher in cells harboring complex I defects. Western blot analysis of the autophagosome marker LC3B, confirmed significantly higher levels of the protein bound to autophagosomes, LC3B-II, in patients' cells, suggesting an increased bulk autophagy in OXPHOS defective fibroblasts. Inhibition of lysosomal proteases caused significant accumulation of LC3B-II in control cells, whereas in patients' cells this phenomenon was less pronounced. Electron microscopy studies showed higher content of late autophagic vacuoles and lysosomes in OXPHOS defective cells, accompanied by higher levels of the lysosomal marker LAMP-1. Our findings suggest that in OXPHOS deficient fibroblasts autophagic flux could be partially hampered leading to an accumulation of autophagic vacuoles and lysosomes.     

Localization of the AP-3 adaptor complex defines a novel endosomal exit site for lysosomal membrane proteins

The adaptor protein (AP) 3 adaptor complex has been implicated in the transport of lysosomal membrane proteins, but its precise site of action has remained controversial. Here, we show by immuno-electron microscopy that AP-3 is associated with budding profiles evolving from a tubular endosomal compartment that also exhibits budding profiles positive for AP-1. AP-3 colocalizes with clathrin, but to a lesser extent than does AP-1. The AP-3- and AP-1-bearing tubular compartments contain endocytosed transferrin, transferrin receptor, asialoglycoprotein receptor, and low amounts of the cation-independent mannose 6-phosphate receptor and the lysosome-associated membrane proteins (LAMPs) 1 and 2. Quantitative analysis revealed that of these distinct cargo proteins, only LAMP-1 and LAMP-2 are concentrated in the AP-3-positive membrane domains. Moreover, recycling of endocytosed LAMP-1 and CD63 back to the cell surface is greatly increased in AP-3-deficient cells. Based on these data, we propose that AP-3 defines a novel pathway by which lysosomal membrane proteins are transported from tubular sorting endosomes to lysosomes.     

Disturbed cholesterol traffic but normal proteolytic function in LAMP-1/LAMP-2 double-deficient fibroblasts

Mice double deficient in LAMP-1 and -2 were generated. The embryos died between embryonic days 14.5 and 16.5. An accumulation of autophagic vacuoles was detected in many tissues including endothelial cells and Schwann cells. Fibroblast cell lines derived from the double-deficient embryos accumulated autophagic vacuoles and the autophagy protein LC3II after amino acid starvation. Lysosomal vesicles were larger and more peripherally distributed and showed a lower specific density in Percoll gradients in double deficient when compared with control cells. Lysosomal enzyme activities, cathepsin D processing and mannose-6-phosphate receptor expression levels were not affected by the deficiency of both LAMPs. Surprisingly, LAMP-1 and -2 deficiencies did not affect long-lived protein degradation rates, including proteolysis due to chaperone-mediated autophagy. The LAMP-1/2 double-deficient cells and, to a lesser extent, LAMP-2 single-deficient cells showed an accumulation of unesterified cholesterol in endo/lysosomal, rab7, and NPC1 positive compartments as well as reduced amounts of lipid droplets. The cholesterol accumulation in LAMP-1/2 double-deficient cells could be rescued by overexpression of murine LAMP-2a, but not by LAMP-1, highlighting the more prominent role of LAMP-2. Taken together these findings indicate partially overlapping functions for LAMP-1 and -2 in lysosome biogenesis, autophagy, and cholesterol homeostasis.     

A method to measure cardiac autophagic flux in vivo

Autophagy, a highly conserved cellular mechanism wherein various cellular components are broken down and recycled through lysosomes, has been implicated in the development of heart failure. However, tools to measure autophagic flux in vivo have been limited. Here, we tested whether monodansylcadaverine (MDC) and the lysosomotropic drug chloroquine could be used to measure autophagic flux in both in vitro and in vivo model systems. Using HL-1 cardiac-derived myocytes transfected with GFP-tagged LC3 to track changes in autophagosome formation, autophagy was stimulated by mTOR inhibitor rapamycin. Administration of chloroquine to inhibit lysosomal activity enhanced the rapamycin-induced increase in the number of cells with numerous GFP-LC3-positive autophagosomes. The chloroquine-induced increase of autophagosomes occurred in a dose-dependent manner between 1 microM and 8 microM, and reached a maximum 2 hour after treatment. Chloroquine also enhanced the accumulation of autophagosomes in cells stimulated with hydrogen peroxide, while it attenuated that induced by Bafilomycin A1, an inhibitor of V-ATPase that interferes with fusion of autophagosomes with lysosomes. The accumulation of autophagosomes was inhibited by 3-methyladenine, which is known to inhibit the early phase of the autophagic process. Using transgenic mice expressing 3 mCherry-LC3 exposed to rapamycin for 4 hr, we observed an increase in mCherry-LC3-labeled autophagosomes in myocardium, which was further increased by concurrent administration of chloroquine, thus allowing determination of flux as a more precise measure of autophagic activity in vivo. MDC injected 1 hr before sacrifice colocalized with mCherry-LC3 puncta, validating its use as a marker of autophagosomes. This study describes a method to measure autophagic flux in vivo even in non-transgenic animals, using MDC and chloroquine.     

Development of LC3/GABARAP sensors containing a LIR and a hydrophobic domain to monitor autophagy

Macroautophagy allows for bulk degradation of cytosolic components in lysosomes. Overexpression of GFP/RFP-LC3/GABARAP is commonly used to monitor autophagosomes, a hallmark of autophagy, despite artifacts related to their overexpression. Here, we developed new sensors that detect endogenous LC3/GABARAP proteins at the autophagosome using an LC3-interacting region (LIR) and a short hydrophobic domain (HyD). Among HyD-LIR-GFP sensors harboring LIR motifs of 34 known LC3-binding proteins, HyD-LIR(TP)-GFP using the LIR motif from TP53INP2 allowed detection of all LC3/GABARAPs-positive autophagosomes. However, HyD-LIR(TP)-GFP preferentially localized to GABARAP/GABARAPL1-positive autophagosomes in a LIR-dependent manner. In contrast, HyD-LIR(Fy)-GFP using the LIR motif from FYCO1 specifically detected LC3A/B-positive autophagosomes. HyD-LIR(TP)-GFP and HyD-LIR(Fy)-GFP efficiently localized to autophagosomes in the presence of endogenous LC3/GABARAP levels and without affecting autophagic flux. Both sensors also efficiently localized to MitoTracker-positive damaged mitochondria upon mitophagy induction. HyD-LIR(TP)-GFP allowed live-imaging of dynamic autophagosomes upon autophagy induction. These novel autophagosome sensors can thus be widely used in autophagy research.     

Nucleocytoplasmic Distribution and Dynamics of the Autophagosome Marker EGFP-LC3

The process of autophagy involves the formation of autophagosomes, double-membrane structures that encapsulate cytosol. Microtubule-associated protein light chain 3 (LC3) was the first protein shown to specifically label autophagosomal membranes in mammalian cells, and subsequently EGFP-LC3 has become one of the most widely utilized reporters of autophagy. Although LC3 is currently thought to function primarily in the cytosol, the site of autophagosome formation, EGFP-LC3 often appears to be enriched in the nucleoplasm relative to the cytoplasm in published fluorescence images. However, the nuclear pool of EGFP-LC3 has not been specifically studied in previous reports, and mechanisms by which LC3 shuttles between the cytoplasm and nucleoplasm are currently unknown. In this study, we therefore investigated the regulation of the nucleo-cytoplasmic distribution of EGFP-LC3 in living cells. By quantitative fluorescence microscopy analysis, we demonstrate that soluble EGFP-LC3 is indeed enriched in the nucleus relative to the cytoplasm in two commonly studied cell lines, COS-7 and HeLa. Although LC3 contains a putative nuclear export signal (NES), inhibition of active nuclear export or mutation of the NES had no effect on the nucleo-cytoplasmic distribution of EGFP-LC3. Furthermore, FRAP analysis indicates that EGFP-LC3 undergoes limited passive nucleo-cytoplasmic transport under steady state conditions, and that the diffusional mobility of EGFP-LC3 was substantially slower in the nucleus and cytoplasm than predicted for a freely diffusing monomer. Induction of autophagy led to a visible decrease in levels of soluble EGFP-LC3 relative to autophagosome-bound protein, but had only modest effects on the nucleo-cytoplasmic ratio or diffusional mobility of the remaining soluble pools of EGFP-LC3. We conclude that the enrichment of soluble EGFP-LC3 in the nucleus is maintained independently of active nuclear export or induction of autophagy. Instead, incorporation of soluble EGFP-LC3 into large macromolecular complexes within both the cytoplasm and nucleus may prevent its rapid equilibrium between the two compartments.     

Induction of autophagy causes dramatic changes in the subcellular distribution of GFP-Rab24

Rab GTPases comprises a large family of proteins, with more than 50 gene products localized in distinct subcellular compartments. Rab24 is a member of this family whose function is not presently known. In order to elucidate the role of this protein we have generated a GFP-tagged Rab24 and studied the distribution of this chimera by fluorescence microscopy. GFP-Rab24 showed a perinuclear reticular localization that often encircled the nucleus. This reticular pattern partially overlapped with ER markers, cis-Golgi, and the ER-Golgi intermediate compartment. Surprisingly, when GFP-Rab24-transfected cells were starved to induce autophagy the distribution of the protein changed dramatically. GFP-Rab24 localized in large dots, cup-shaped structures and ring-shaped vesicles. Some of these vesicles were labeled with monodansylcadaverine , a specific autophagosome marker. In the presence of vinblastine, an agent that induces the formation of very large autophagic vesicles, GFP-Rab24 accumulated in the large vacuoles that were also labeled by monodansylcadaverine. Furthermore, Rab24 colocalized with LC3, a mammalian homolog of the yeast protein Apg8/Aut7, an essential gene for autophagy. This is the first report indicating that Rab24 localizes on autophagosomes, suggesting that this Rab protein is involved in the autophagic pathway.     

Transiently expressed ATG16L1 inhibits autophagosome biogenesis and aberrantly targets RAB11-positive recycling endosomes

The membrane source for autophagosome biogenesis is an unsolved mystery in the study of autophagy. ATG16L1 forms a complex with ATG12–ATG5 (the ATG16L1 complex). The ATG16L1 complex is recruited to autophagic membranes to convert MAP1LC3B-I to MAP1LC3B-II. The ATG16L1 complex dissociates from the phagophore before autophagosome membrane closure. Thus, ATG16L1 can be used as an early event marker for the study of autophagosome biogenesis. We found that among 3 proteins in the ATG16L1 complex, only ATG16L1 formed puncta-like structures when transiently overexpressed. ATG16L1⁺ puncta formed by transient expression could represent autophagic membrane structures. We thoroughly characterized the transiently expressed ATG16L1 in several mammalian cell lines. We found that transient expression of ATG16L1 not only inhibited autophagosome biogenesis, but also aberrantly targeted RAB11-positive recycling endosomes, resulting in recycling endosome aggregates. We conclude that transient expression of ATG16L1 is not a physiological model for the study of autophagy. Caution is warranted when reviewing findings derived from a transient expression model of ATG16L1.     

4th International Symposium on Autophagy: Exploiting the Frontiers of Autophagy Research

Minocycline has been shown to alleviate several neurological disorders. Unexpectedly, we found that minocycline had opposite effects on glioma cells: minocycline induced nonapoptotic cell death in glioma cells. The glioma cell death was associated with the presence of autophagic vacuoles in the cytoplasm. Minocycline induced autophagy was confirmed by acridine orange, monodansylcadaverine (MDC) stainings of vesicle formation and the conversion of microtubule-associated proteins light chain 3 (LC3-I) to LC3-II. Pretreatment with autophagy inhibitor 3-methyladenine (3-MA) suppressed the induction of acidic vesicular organelles and the accumulation of LC3-II to the autophagosome membrane in glioma cells treated with minocycline. Despite the pretreatment of 3-MA, minocycline induced cell death which could result from the activation of caspase-3. Minocycline effectively inhibited tumor growth and induced autophagy in the xenograft tumor model of C6 glioma cells. These results suggest that minocycline may kill glioma cells by inducing autophagic cell death. When autophagy was inhibited, minocycline still induced cell death through the activation of caspase-3. Thus, minocycline is a promising agent in the treatment of malignant gliomas.     

Minocycline inhibits the growth of glioma by inducing autophagy

Minocycline has been shown to alleviate several neurological disorders. Unexpectedly, we found that minocycline had opposite effects on glioma cells: minocycline induced nonapoptotic cell death in glioma cells. The glioma cell death was associated with the presence of autophagic vacuoles in the cytoplasm. Minocycline induced autophagy was confirmed by acridine orange, monodansylcadaverine (MDC) stainings of vesicle formation and the conversion of microtubule-associated proteins light chain 3 (LC3-I) to LC3-II. Pretreatment with autophagy inhibitor 3-methyladenine (3-MA) suppressed the induction of acidic vesicular organelles and the accumulation of LC3-II to the autophagosome membrane in glioma cells treated with minocycline. Despite the pretreatment of 3-MA, minocycline induced cell death which could result from the activation of caspase-3. Minocycline effectively inhibited tumor growth and induced autophagy in the xenograft tumor model of C6 glioma cells. These results suggest that minocycline may kill glioma cells by inducing autophagic cell death. When autophagy was inhibited, minocycline still induced cell death through the activation of caspase-3. Thus, minocycline is a promising agent in the treatment of malignant gliomas.     

SNX18 tubulates recycling endosomes for autophagosome biogenesis

The role of membrane remodeling and phosphoinositide-binding proteins in autophagy remains elusive. PX domain proteins bind phosphoinositides and participate in membrane remodeling and trafficking events and we therefore hypothesized that one or several PX domain proteins are involved in autophagy. Indeed, the PX-BAR protein SNX18 was identified as a positive regulator of autophagosome formation using an image-based siRNA screen. We show that SNX18 interacts with ATG16L1 and LC3, and functions downstream of ATG14 and the class III PtdIns3K complex in autophagosome formation. SNX18 facilitates recruitment of ATG16L1 to perinuclear recycling endosomes, and its overexpression leads to tubulation of ATG16L1- and LC3-positive membranes. We propose that SNX18 promotes LC3 lipidation and tubulation of recycling endosomes to provide membrane for phagophore expansion.     

A method to measure cardiac autophagic flux in vivo

Autophagy, a highly conserved cellular mechanism wherein various cellular components are broken down and recycled through lysosomes, has been implicated in the development of heart failure. However, tools to measure autophagic flux in vivo have been limited. Here, we tested whether monodansylcadaverine (MDC) and the lysosomotropic drug chloroquine could be used to measure autophagic flux in both in vitro and in vivo model systems. Using HL-1 cardiac-derived myocytes transfected with GFP-tagged LC3 to track changes in autophagosome formation, autophagy was stimulated by mTOR inhibitor rapamycin. Administration of chloroquine to inhibit lysosomal activity enhanced the rapamycin-induced increase in the number of cells with numerous GFP-LC3-positive autophagosomes. The chloroquine-induced increase of autophagosomes occurred in a dose-dependent manner between 1 µM and 8 µM, and reached a maximum 2 hour after treatment. Chloroquine also enhanced the accumulation of autophagosomes in cells stimulated with hydrogen peroxide, while it attenuated that induced by Bafilomycin A₁, an inhibitor of V-ATPase that interferes with fusion of autophagosomes with lysosomes. The accumulation of autophagosomes was inhibited by 3-methyladenine, which is known to inhibit the early phase of the autophagic process. Using transgenic mice expressing mCherry-LC3 exposed to rapamycin for 4 hr, we observed an increase in mCherry-LC3-labeled autophagosomes in myocardium, which was further increased by concurrent administration of chloroquine, thus allowing determination of flux as a more precise measure of autophagic activity in vivo. MDC injected 1 hr before sacrifice colocalized with mCherry-LC3 puncta, validating its use as a marker of autophagosomes. This study describes a method to measure autophagic flux in vivo even in non-transgenic animals, using MDC and chloroquine.