Muscle-specific AMPK β1β2-null mice display a myopathy due to loss of capillary density in nonpostural muscles

AMP-activated protein kinase (AMPK) is a master regulator of metabolism. While muscle-specific AMPK 12 double-knockout (12M-KO) mice display alterations in metabolic and mitochondrial capacity, their severe exercise intolerance suggested a secondary contributor to the observed phenotype. We find that tibialis anterior (TA), but not soleus, muscles of sedentary 12M-KO mice display a significant myopathy (decreased myofiber areas, increased split and necrotic myofibers, and increased centrally nucleated myofibers. A mitochondrial- and fiber-type-specific etiology to the myopathy was ruled out. However, 12M-KO TA muscles displayed significant (P<0.05) increases in platelet aggregation and apoptosis within myofibers and surrounding interstitium (P<0.05). These changes correlated with a 45% decrease in capillary density (P<0.05). We hypothesized that the 12M-KO myopathy in resting muscle resulted from impaired AMPK-nNOS signaling, causing increased platelet aggregation, impaired vasodilation, and, ultimately, ischemic injury. Consistent with this hypothesis, AMPK-specific phosphorylation (Ser1446) of nNOS was decreased in 12M-KO compared to wild-type (WT) mice. The AMPK-nNOS relationship was further demonstrated by administration of 5-aminoimidazole-4-carboxamide 1--d-ribofuranoside (AICAR) to 12-MKO muscles and C2C12 myotubes. AICAR significantly increased nNOS phosphorylation and nitric oxide production (P<0.05) within minutes of administration in WT muscles and C2C12 myotubes but not in 12M-KO muscles. These findings highlight the importance of the AMPK-nNOS pathway in resting skeletal muscle.Thomas, M. M., Wang, D. C., D'Souza, D. M., Krause, M. P., Layne, A. S., Criswell, D. S., O'Neill, H. M., Connor, M. K., Anderson, J. E., Kemp, B. E., Steinberg, G. R., and Hawke, T. J. Muscle-specific AMPK 12-null mice display a myopathy due to loss of capillary density in nonpostural muscles.