Muscle-specific AMPK β1β2-null mice display a myopathy due to loss of capillary density in nonpostural muscles
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AMP-activated protein kinase (AMPK) is a master regulator of metabolism. While muscle-specific AMPK β1β2 double-knockout (β1β2M-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 β1β2M-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, β1β2M-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 β1β2M-KO myopathy in resting muscle resulted from impaired AMPK-nNOSμ signaling, causing increased platelet aggregation, impaired vasodilation, and, ultimately, ischemic injury. Consistent with this hypothesis, AMPK-specific phosphorylation (Ser1446) of nNOSμ was decreased in β1β2M-KO compared to wild-type (WT) mice. The AMPK-nNOSμ relationship was further demonstrated by administration of 5-aminoimidazole-4-carboxamide 1-β-D-ribofuranoside (AICAR) to β1β2-MKO muscles and C2C12 myotubes. AICAR significantly increased nNOSμ phosphorylation and nitric oxide production (P<0.05) within minutes of administration in WT muscles and C2C12 myotubes but not in β1β2M-KO muscles. These findings highlight the importance of the AMPK-nNOSμ pathway in resting skeletal muscle.
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