Mitochondrial biogenesis in fast skeletal muscle of CK deficient mice.

Creatine kinase (CK) is a phosphotransfer kinase that catalyzes the reversible transfer of a phosphate moiety between ADP and creatine and that is highly expressed in skeletal muscle. In fast glycolytic skeletal muscle, deletion of the cytosolic M isoform of CK in mice (M-CK-/-) leads to a massive increase in the oxidative capacity and of mitochondrial volume. This study was aimed at investigating the transcriptional pathways leading to mitochondrial biogenesis in response to CK deficiency. Wild type and M-CK-/- mice of eleven months of age were used for this study. Gastrocnemius muscles of M-CK-/- mice exhibited a dramatic increase in citrate synthase (+120%) and cytochrome oxidase (COX, +250%) activity, and in mitochondrial DNA (+60%), showing a clear activation of mitochondrial biogenesis. Similarly, mRNA expression of the COXI (mitochondria-encoded) and COXIV (nuclear-encoded) subunits were increased by +103 and +94% respectively. This was accompanied by an increase in the expression of the nuclear respiratory factor (NRF2alpha) and the mitochondrial transcription factor (mtTFA). Expression of the co-activator PGC-1alpha, a master gene in mitochondrial biogenesis was not significantly increased while that of PGC-1beta and PRC, two members of the same family, was moderately increased (+45% and +55% respectively). While the expression of the modulatory calcineurin-interacting protein 1 (MCIP1) was dramatically decreased (-68%) suggesting inactivation of the calcineurin pathway, the metabolic sensor AMPK was activated (+86%) in M-CK-/- mice. These results evidence that mitochondrial biogenesis in response to a metabolic challenge exhibits a unique pattern of regulation, involving activation of the AMPK pathway.