Duelling functions of the V-ATPase

Homotypic vacuole fusion in yeast requires organelle acidification and not the V-ATPase membrane domain

Studies of homotypic vacuole-vacuole fusion in the yeast Saccharomyces cerevisiae have been instrumental in determining the cellular machinery required for eukaryotic membrane fusion and have implicated the vacuolar H(+)-ATPase (V-ATPase). The V-ATPase is a multisubunit, rotary proton pump whose precise role in homotypic fusion is controversial. Models formulated from in vitro studies suggest that it is the proteolipid proton-translocating pore of the V-ATPase that functions in fusion, with further studies in worms, flies, zebrafish, and mice appearing to support this model. We present two in vivo assays and use a mutant V-ATPase subunit to establish that it is the H(+)-translocation/vacuole acidification function, rather than the physical presence of the V-ATPase, that promotes homotypic vacuole fusion in yeast. Furthermore, we show that acidification of the yeast vacuole in the absence of the V-ATPase rescues vacuole-fusion defects. Our results clarify the in vivo requirements of acidification for membrane fusion.

Inhibition of lysosome degradation on autophagosome formation and responses to GMI, an immunomodulatory protein from Ganoderma microsporum

BACKGROUND AND PURPOSE

Autophagic cell death is considered a self-destructive process that results from large amounts of autophagic flux. In our previous study, GMI, a recombinant fungal immunomodulatory protein cloned from Ganoderma microsporum, induced autophagic cell death in lung cancer cells. The aim of this study was to examine the role of autophagosome accumulation in GMI-mediated cell death.

EXPERIMENTAL APPROACH

Western blot analysis, flow cytometry and confocal microscopy were used to evaluate the effects of different treatments, including silencing of ATP6V0A1 by use of short hairpin RNAi, on GMI-mediated cell death, lung cancer cell viability and autophagosome accumulation in vitro.

KEY RESULTS

Lysosome inhibitors bafilomycin-A1 and chloroquine increased GMI-mediated autophagic cell death. GMI and bafilomycin-A1 co-treatment induced the accumulation of large amounts of autophagosomes, but did not significantly induce apoptosis. GMI elicited autophagy through the PKB (Akt)/mammalian target of rapamycin signalling pathway. Silencing of ATP6V0A1, one subunit of vesicular H(+)-ATPases (V-ATPases) that mediates lysosome acidification, spontaneously induced autophagosome accumulation, but did not affect lysosome acidity. GMI-mediated autophagosome accumulation and cytotoxicity was increased in shATP6V0A1 lung cancer cells. Furthermore, ATP6V0A1 silencing decreased autophagosome and lysosome fusion in GMI-treated CaLu-1/GFP-LC3 lung cancer cells.

CONCLUSION AND IMPLICATIONS

We demonstrated that autophagosome accumulation induces autophagic cell death in a GMI treatment model, and ATP6V0A1 plays an important role in mediating autophagosome-lysosome fusion. Our findings provide new insights into the mechanisms involved in the induction of autophagic cell death.