Whole Body Deletion of AMP-activated Protein Kinase β2 Reduces Muscle AMPK Activity and Exercise Capacity

Abstract

AMP-activated protein kinase (AMPK) ␤ subunits (␤1 and ␤2) provide scaffolds for binding ␣ and ␥ subunits and contain a carbohydrate-binding module important for regulating enzyme activity. We generated C57Bl/6 mice with germline deletion of AMPK ␤2 (␤2 KO) and examined AMPK expression and activity, exercise capacity, metabolic control during muscle contractions, aminoimidazole carboxamide ribonucleotide (AICAR) sensitivity, and susceptibility to obesity-induced insulin resistance. We find that ␤2 KO mice are viable and breed normally. ␤2 KO mice had a reduction in skeletal muscle AMPK ␣1 and ␣2 expression despite up-regulation of the ␤1 isoform. Heart AMPK ␣2 expression was also reduced but this did not affect resting AMPK ␣1 or ␣2 activities. AMPK ␣1 and ␣2 activities were not changed in liver, fat, or hypothalamus. AICAR-stimulated glucose uptake but not fatty acid oxidation was impaired in ␤2 KO mice. During treadmill running ␤2 KO mice had reduced maximal and endurance exercise capacity, which was associated with lower muscle and heart AMPK activity and reduced levels of muscle and liver glycogen. Reductions in exercise capacity of ␤2 KO mice were not due to lower muscle mitochondrial content or defects in contraction-stimulated glucose uptake or fatty acid oxidation. When challenged with a high-fat diet ␤2 KO mice gained more weight and were more susceptible to the development of hyperinsulinemia and glucose intolerance. In summary these data show that deletion of AMPK ␤2 reduces AMPK activity in skeletal muscle resulting in impaired exercise capacity and the worsening of diet-induced obesity and glucose intolerance.The AMP-activated protein kinase (AMPK) 5 is an evolutionary conserved serine/threonine protein kinase that functions as a metabolic regulatory enzyme at both the intracellular and whole body level (1, 2). As a metabolic stress-sensing enzyme, AMPK is activated through phosphorylation of Thr 172 in the ␣-catalytic subunit by upstream kinases, liver kinase B1 (LKB1) and calcium/calmodulin-dependent kinase kinase in response to physiological processes that consume ATP (exercise) or inhibit ATP production (ischemia or hypoxia) (3). Following activation, AMPK acutely regulates lipid, protein, and carbohydrate metabolism through phosphorylation induced changes that alter enzyme activities by switching off ATP consuming anabolic pathways and switching on ATP generating catabolic pathways (4). In addition to these acute effects, AMPK regulates transcription factors to influence gene expression (4). Modulation of AMPK activity by hormones and cytokines adds a complex layer of regulation allowing energy supply and demand within a cell to be integrated with the energy requirements of the whole organism (5).AMPK functions as an ␣␤␥ heterotrimer where the C terminus of the ␤ isoforms (␤1 and ␤2) contains the subunit-binding sequence that is essential for binding the ␥ and ␣ subunits (6, 7). In addition to their structural role in maintaining the AMPK heterotrimer, AMPK ␤ subunits contain an evolutionary ...

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