TMBIM6 (transmembrane BAX inhibitor motif containing 6) enhances autophagy through regulation of lysosomal calcium

Lysosomal Ca(2+) contributes to macroautophagy/autophagy, an intracellular process for the degradation of cytoplasmic material and organelles in the lysosomes to protect cells against stress responses. TMBIM6 (transmembrane BAX inhibitor motif containing 6) is a Ca(2+) channel-like protein known to regulate ER stress response and apoptosis. In this study, we examined the as yet unknown role of TMBIM6 in regulating lysosomal Ca(2+) levels. The Ca(2+) efflux from the ER through TMBIM6 was found to increase the resting lysosomal Ca(2+) level, in which ITPR-independent regulation of Ca(2+) status was observed. Further, TMBIM6 regulated the local release of Ca(2+) through lysosomal MCOLN1/TRPML1 channels under nutrient starvation or MTOR inhibition. The local Ca(2+) efflux through MCOLN1 channels was found to activate PPP3/calcineurin, triggering TFEB (transcription factor EB) nuclear translocation, autophagy induction, and lysosome biogenesis. Upon genetic inactivation of TMBIM6, lysosomal Ca(2+) and the associated TFEB nuclear translocation were decreased. Furthermore, autophagy flux was significantly enhanced in the liver or kidney from starved Tmbim6(+/+) mice compared with that in the counter tmbim6(−/-) mice. Together, our observations indicated that under stress conditions, TMBIM6 increases lysosomal Ca(2+) release, leading to PPP3/calcineurin-mediated TFEB activation and subsequently enhanced autophagy. Thus, TMBIM6, an ER membrane protein, is suggested to be a lysosomal Ca(2+) modulator that coordinates with autophagy to alleviate metabolism stress.Abbreviations: AVs: autophagic vacuoles; CEPIA: calcium-measuring organelle-entrapped protein indicator; ER: endoplasmic reticulum; GPN: glycyl-L-phenylalanine-beta-naphthylamide; ITPR/IP3R: inositol 1,4,5-trisphosphate receptor; LAMP1: lysosomal associated membrane protein 1; MCOLN/TRPML: mucolipin; MEF: mouse embryonic fibroblast; ML-SA1: mucolipin synthetic agonist 1; MTORC1: mechanistic target of rapamycin kinase complex 1; RPS6KB1: ribosomal protein S6 kinase B1; SQSTM1: sequestosome 1; TFEB: transcription factor EB; TKO: triple knockout; TMBIM6/BI-1: transmembrane BAX inhibitor motif containing 6