The 18-kDa TSPO (translocator protein) localizes on the outer mitochondrial membrane (OMM) and participates in cholesterol transport. Here, we report that TSPO inhibits mitochondrial autophagy downstream of the PINK1-PARK2 pathway, preventing essential ubiquitination of proteins. TSPO abolishes mitochondrial relocation of SQSTM1/p62 (sequestosome 1), and consequently that of the autophagic marker LC3 (microtubule-associated protein 1 light chain 3), thus leading to an accumulation of dysfunctional mitochondria, altering the appearance of the network. Independent of cholesterol regulation, the modulation of mitophagy by TSPO is instead dependent on VDAC1 (voltage-dependent anion channel 1), to which TSPO binds, reducing mitochondrial coupling and promoting an overproduction of reactive oxygen species (ROS) that counteracts PARK2-mediated ubiquitination of proteins. These data identify TSPO as a novel element in the regulation of mitochondrial quality control by autophagy, and demonstrate the importance for cell homeostasis of its expression ratio with VDAC1.
Keywords: ATP5B, ATP synthase, H+ transporting, mitochondrial F1 complex, β subunit; DAPI, 4’, 6-diamidino-2-phenylindole; DHE, dihydroethidium; DNM1L, dynamin 1-like; FCCP, carbonyl cyanide-4-(trifluoromethoxy)phenylhydrazone; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; GSH, glutathione; MAP1LC3/LC3, microtubule-associated protein 1 light chain 3; MCB, monochlorobimane; MEFs, mouse embryonic fibroblasts; MnTBAP, manganese [III] tetrakis (4-benzoic acid) porphyrin; MβCD, methyl-β-cyclodextrin; NRF1, nuclear respiratory factor 1; OMM, outer mitochondrial membrane; PARK2; PBS, phosphate-buffered saline; PINK1, PTEN-induced putative kinase 1; PRKCE, protein kinase C, epsilon; RM, recording medium; ROS; ROS, reactive oxygen species; RT, room temperature; SQSTM1, sequestosome 1; TFAM, transcription factor A, mitochondrial; TMRM, tetramethylrhodamine methyl ester; TSPO; TSPO, translocator protein; VDAC1, voltage-dependent anion channel 1; YFP, yellow fluorescent protein; mitochondria; mitophagy; mtRFP, mitochondrially targeted red fluorescent protein; nsc, nonsilencing control; siRNA, small interfering ribonucleic acid; ubiquitin; Δψm, mitochondrial membrane potential.