Gain-of-function R225Q Mutation in AMP-activated Protein Kinase γ3 Subunit Increases Mitochondrial Biogenesis in Glycolytic Skeletal Muscle

AMP-activated protein kinase (AMPK) is a heterotrimeric complex, composed of a catalytic subunit (alpha) and two regulatory subunits (beta and gamma), that works as a cellular energy sensor. The existence of multiple heterotrimeric complexes provides a molecular basis for the multiple roles of this highly conserved signaling system. The AMPK gamma 3 subunit is predominantly expressed in skeletal muscle, mostly in type II glycolytic fiber types. We determined whether the AMPK gamma 3 subunit has a role in signaling pathways that mediate mitochondrial biogenesis in skeletal muscle. We provide evidence that overexpression or ablation of the AMPK gamma 3 subunit does not appear to play a critical role in defining mitochondrial content in resting skeletal muscle. However, overexpression of a mutant form (R225Q) of the AMPK gamma 3 subunit (Tg-AMPK gamma 3(225Q)) increases mitochondrial biogenesis in glycolytic skeletal muscle. These adaptations are associated with an increase in expression of the co-activator PGC-1 alpha and several transcription factors that regulate mitochondrial biogenesis, including NRF-1, NRF-2, and TFAM. Succinate dehydrogenase staining, a marker of the oxidative profile of individual fibers, was also increased in transversal skeletal muscle sections of white gastrocnemius muscle from Tg-AMPK gamma 3(225Q) mice, independent of changes in fiber type composition. In conclusion, a single nucleotide mutation (R225Q) in the AMPK gamma 3 subunit is associated with mitochondrial biogenesis in glycolytic skeletal muscle, concomitant with increased expression of the co-activator PGC-1 alpha and several transcription factors that regulate mitochondrial proteins, without altering fiber type composition.