NOVEL MECHANISMS GOVERNING SKELETAL MUSCLE MITOCHONDRIAL BIOENERGETICS : OXPHOS EFFICIENCY AND cAMP/PKA SIGNALING

Abstract

Understanding the regulation of cellular metabolism is paramount to treating the growing prevalence of metabolic disease worldwide. In cellular metabolism, mitochondrial oxidative phosphorylation (OXPHOS) plays a key role as it is a primary source of energy and the governor of cellular redox homeostasis. A fundamental aspect of mitochondrial function is that cellular metabolic demand requires a corresponding increase in flux through OXPHOS; however, the regulation of OXPHOS is incompletely understood. Herein, two hypotheses were tested: 1) OXPHOS efficiency increases as a function of metabolic demand to allow mitochondria to maximize ATP synthesis at a given level of O₂ flux and 2) that OXPHOS is regulated by cAMP/PKA signaling within skeletal muscle mitochondria. First, in permeabilized myofibers (PmFBs) from mouse skeletal muscle and myocardium, the data provided herein demonstrate that OXPHOS efficiency increases from ~20% to >70% from resting [ADP] to [ADP] found during exhaustive exercise in skeletal muscle, whereas [ADP] in the myocardium remains static (at ~75-100 [mu]M) regardless of workload. Importantly, in the presence of small changes in [ADP] (e.g. 5-20 [mu]M), ATP synthesis increased independent of an increase in JO₂, suggesting that skeletal muscle mitochondria can accommodate increased metabolic demand without a requisite increase in O₂ flux, suggesting a decrease in proton leak. Second, it was demonstrated that tricarboxylic acid (TCA) cycle flux alone is insufficient to increase cAMP levels in isolated skeletal muscle mitochondria. However, pharmacological inhibition of PKA impairs a multitude of mitochondrial function outcomes in both liver and skeletal muscle that summarily implicate Complex I as a primary target. In conclusion, given the absolute necessity for coupled OXPHOS in the maintenance of energy homeostasis and the variety of diseases linked to decreased Complex I activity, the findings provided herein not only advance our current knowledge of mitochondrial bioenergetics, but provide a multitude of opportunities for future investigations.