Selective MAP1LC3C (LC3C) autophagy requires noncanonical regulators and the C-terminal peptide

ATG8 in single membranes: Fresh players of endocytosis and acidic organelle quality control in cancer, neurodegeneration, and inflammation

Ubiquitin-like autophagy-related gene ATG8 proteins are typically associated with degradative quality control via canonical double-membrane macro-autophagosomes in the cell. ATG8 proteins have now stepped forward in non-canonical pathways in single membrane organelles. The growing interest in non-canonical ATG8 roles has been stimulated by recent links to human conditions, especially in the regulation of inflammation, neurodegeneration and cancers. Here, we summarize the evidence linking non-canonical ATG8s to human pathologies and the quality control of acidic V-ATPase-regulated organelles in the cell.

The N-terminal region of the ATG8 autophagy protein LC3C is essential for its membrane fusion properties

Autophagy is a catabolic process in which a double-membrane organelle, the autophagosome (AP), engulfs cellular components that will be degraded in the lysosomes. ATG8 protein family members participate at various stages of AP formation. The present study compares the capacity to induce lipid-vesicle tethering and fusion of two ATG8 family members, LC3B and LC3C, with model membranes. LC3B is the most thoroughly studied ATG8 protein. It is generally considered as an autophagosomal marker and a canonical representative of the LC3 subfamily. LC3C is less studied, but recent data have reported its implication in various processes, crucial to cellular homeostasis. The results in this paper show that LC3C induces higher levels of tethering and of intervesicular lipid mixing than LC3B. As the N-terminus of LC3C is different from that of the other family members, various mutants of the N-terminal region of both LC3B and LC3C were designed, and their activities compared. It was concluded that the N-terminal region of LC3C was responsible for the enhanced vesicle tethering, membrane perturbation and vesicle-vesicle fusion activities of LC3C as compared to LC3B. The results suggest a specialized function of LC3C in the AP expansion process.

SARS-CoV-2-Induced Vasculitic Skin Lesions Are Associated with Massive Spike Protein Depositions in Autophagosomes

In patients infected with severe acute respiratory syndrome coronavirus 2, vasculopathic changes of the skin are associated with a severe prognosis. However, the pathogenesis of this vasculopathy is not conclusively clarified. In this study, 25 prospectively collected skin samples from patients with COVID-19-related skin lesions were examined for vasculopathic changes and, in case of vasculitis, were further analyzed with electron microscopy and immunohistochemistry. Vasculopathy was observed in 76% of all COVID-19-related inflammatory skin lesions. Visual endothelial changes without manifest leukocytoclastic vasculitis were found in 60% of the COVID-19-related skin lesions, whereas leukocytoclastic vasculitis was diagnosed in 16%. In the cases of vasculitis, there were extensive spike protein depositions in microvascular endothelial cells that colocalized with the autophagosome proteins LC3B and LC3C. The autophagy protein complex LC3-associated endocytosis in microvascular endothelial cells seems to be an important pathogenic factor for severe acute respiratory syndrome coronavirus 2-related vasculitis in the skin. On ultrastructural morphology, the vasculitic process was dominated by intracellular vesicle formation and endothelial cell disruption. Direct presence of severe acute respiratory syndrome coronavirus 2 particles in the skin was not observed. Therefore, our results suggest that instead of direct viral infection, dermal vasculitic lesions in COVID-19 are caused by severe acute respiratory syndrome coronavirus 2 spike protein deposition followed by endothelial damage with activation of autophagy.

Microtubule-associated protein MAP1LC3C regulates lysosomal exocytosis and induces zinc reprogramming in renal cancer cells

Microtubule-associated protein 1 light chain 3 gamma (MAP1LC3C or LC3C) is a member of the microtubule-associated family of proteins that are essential in the formation of autophagosomes and lysosomal degradation of cargo. LC3C has tumor-suppressing activity, and its expression is dependent on kidney cancer tumor suppressors, such as von Hippel-Lindau protein and folliculin. Recently, we demonstrated that LC3C autophagy is regulated by noncanonical upstream regulatory complexes and targets for degradation postdivision midbody rings associated with cancer cell stemness. Here, we show that loss of LC3C leads to peripheral positioning of the lysosomes and lysosomal exocytosis (LE). This process is independent of the autophagic activity of LC3C. Analysis of isogenic cells with low and high LE shows substantial transcriptomic reprogramming with altered expression of zinc (Zn)-related genes and activity of polycomb repressor complex 2, accompanied by a robust decrease in intracellular Zn. In addition, metabolomic analysis revealed alterations in amino acid steady-state levels. Cells with augmented LE show increased tumor initiation properties and form aggressive tumors in xenograft models. Immunocytochemistry identified high levels of lysosomal-associated membrane protein 1 on the plasma membrane of cancer cells in human clear cell renal cell carcinoma and reduced levels of Zn, suggesting that LE occurs in clear cell renal cell carcinoma, potentially contributing to the loss of Zn. These data indicate that the reprogramming of lysosomal localization and Zn metabolism with implication for epigenetic remodeling in a subpopulation of tumor-propagating cancer cells is an important aspect of tumor-suppressing activity of LC3C.

C/EBPα-Mediated Transcriptional Activation of PIK3C2A Regulates Autophagy, Matrix Metalloproteinase Expression, and Phenotypic of Vascular Smooth Muscle Cells in Aortic Dissection

Purpose

To investigate the function of C/EBPα in the development of aortic dissection (AD) and the underlying mechanism.

Methods

Aortic vascular smooth muscle cells (VSMCs) were isolated, cultured, and identified from AD rats. Then, C/EBPα and PIK3C2A were knockdown or overexpressed by siRNA or plasmid transfection, respectively. Rapamycin or 3-MA was utilized to stimulate and restrain autophagy of VSMCs, respectively. Western blot was used to evaluate the expression levels of C/EBPα, PIK3C2A, LC3, Beclin-1, p62, MMP-2, MMP-9, α-SMA, SM-MHC, and OPN. The pathological status of aortic ring was evaluated by stretch stress, and ChIP assay was used to analyze the binding between C/EBPα and PIK3C2A. C/EBPα shRNA was injected into tail vein to observe the effect of C/EBPα knockdown in vivo on phenotype, autophagy of aortic vascular tissue by immunohistochemical staining and Western blot.

Results

The protein levels of C/EBPα, PIK3C2A, MMP-2, MMP-9, and LC3 in the aorta of AD rats were all upregulated significantly. C/EBPα and rapamycin promoted notable upregulation of the synthesized proteins (OPN), PIK3C2A, matrix metalloproteinases, LC3, and Beclin-1 in VSMCs, while suppressed contractile proteins (α-SMA and SM-MHC) and p62. The opposite results were observed in the C/EBPα-knockdown VSMCs, PIK3C2A-knockdown VSMCs, or VSMCs treated with 3-MA. C/EBPα, PIK3C2A, and LC3 were dramatically upregulated by the stimulation of 3 g and 5 g stretch stress. The downregulated contractile proteins, upregulated synthetic proteins, activated autophagy, and aggravated pathological state in 5 g stretch stress-treated aortic rings were significantly reversed by the knockdown of C/EBPα. ChIP results indicated that there was a binding site for C/EBPα in the promoter of PIK3C2A. C/EBPα also downregulated α-SMA level and upregulated OPN levels in AD rats in vivo.

Conclusion

Our data indicated that during the development of AD, C/EBPα regulated the transition of VSMC phenotype and extracellular matrix remodeling by activating autophagy through regulating the transcriptional activity of PIK3C2A promoter.

Endosomal LC3C-pathway selectively targets plasma membrane cargo for autophagic degradation

Autophagy selectively targets cargo for degradation, yet mechanistic understanding remains incomplete. The ATG8-family plays key roles in autophagic cargo recruitment. Here by mapping the proximal interactome of ATG8-paralogs, LC3B and LC3C, we uncover a LC3C-Endocytic-Associated-Pathway (LEAP) that selectively recruits plasma-membrane (PM) cargo to autophagosomes. We show that LC3C localizes to peripheral endosomes and engages proteins that traffic between PM, endosomes and autophagosomes, including the SNARE-VAMP3 and ATG9, a transmembrane protein essential for autophagy. We establish that endocytic LC3C binds cargo internalized from the PM, including the Met receptor tyrosine kinase and transferrin receptor, and is necessary for their recruitment into ATG9 vesicles targeted to sites of autophagosome initiation. Structure-function analysis identified that LC3C-endocytic localization and engagement with PM-cargo requires the extended carboxy-tail unique to LC3C, the TBK1 kinase, and TBK1-phosphosites on LC3C. These findings identify LEAP as an unexpected LC3C-dependent pathway, providing new understanding of selective coupling of PM signalling with autophagic degradation.