Cyclophilin-D: a resident regulator of mitochondrial gene expression.

Cyclophilin-D (Cyp-D) is a mitochondrial matrix peptidyl-prolyl isomerase. Because cyclophilins can regulate nuclear gene expression, we examined whether Cyp-D could regulate mitochondrial gene expression. We demonstrated in HEK 293T cells that transfected Cyp-D interacts with mitochondrial transcription factors B1 and B2 (TFB2M) but not with mitochondrial transcription factor A. We also demonstrated that Cyp-D interacts in vivo with TFB2M. Genetic silencing of Cyp-D and pharmacologic inhibition of Cyp-D markedly reduced mitochondrial transcription to 18 ± 5% (P < 0.05) and 24 ± 3% (P < 0.05) of respective controls. The level of interaction between Cyp-D and TFB2M correlated with the level of nascent mitochondrial RNA intensity (r = 0.896; P = 0.0156). Cyp-D silencing down-regulated mitochondrial transcripts initiated from the heavy strand promoter 2 [i.e., NADH dehydrogenase 1 (ND1) by 11-fold, P < 0.005; cytochrome oxidase 1 (COX1) by 4-fold, P < 0.001; and ATP synthase subunit 6 (ATP6) by 6.5-fold, P < 0.005); but not NADH dehydrogenase 6 (ND6)], which is initiated from the light strand promoter. Cyp-D silencing reduced mitochondrial membrane potential and cellular oxygen consumption (from 59 ± 5 to 34 ± 1 µmol oxygen/min/10(6) cells, P < 0.001); the latter without a statistically significant reversal after uncoupling electron transport from ATP synthesis, consistent with down-regulation of electron transport complexes. Accordingly, these studies provide novel evidence that Cyp-D could play a key role in regulating mitochondrial gene expression.