Marked phenotypic differences of endurance performance and exercise-induced oxygen consumption between AMPK and LKB1 deficiency in mouse skeletal muscle: changes occurring in the diaphragm

Abstract

O. Marked phenotypic differences of endurance performance and exercise-induced oxygen consumption between AMPK and LKB1 deficiency in mouse skeletal muscle changes occurring in the diaphragm. Am J Physiol Endocrinol Metab 305: E213-E229, 2013. First published May 21, 2013; doi:10.1152/ajpendo.00114.2013.-LKB1 phosphorylates members of the AMP-activated protein kinase (AMPK) family. LKB1 and AMPK in the skeletal muscle are believed to regulate not only fuel oxidation during exercise but also exercise capacity. LKB1 was also required to prevent diaphragm fatigue, which was shown to affect exercise performance. Using mice expressing dominant negative (DN) mutants of LKB1 and AMPK, specifically in the skeletal muscle but not in the heart, we investigated the roles of LKB1 and AMPK activity in exercise performance and the effects of these kinases on the characteristics of respiratory and locomotive muscles. In the diaphragm and gastrocnemius, both AMPK-DN and LKB1-DN mice showed complete loss of AMPK␣2 activity, and LKB1-DN mice showed a reduction in LKB1 activity. Exercise capacity was significantly reduced in LKB1-DN mice, with a marked reduction in oxygen consumption and carbon dioxide production during exercise. The diaphragm from LKB1-DN mice showed an increase in myosin heavy chain IIB and glycolytic enzyme expression. Normal respiratory chain function and CPT I activity were shown in the isolated mitochondria from LKB1-DN locomotive muscle, and the expression of genes related to fiber type, mitochondria function, glucose and lipid metabolism, and capillarization in locomotive muscle was not different between LKB1-DN and AMPK-DN mice. We concluded that LKB1 in the skeletal muscle contributes significantly to exercise capacity and oxygen uptake during exercise. LKB1 mediated the change of fiber-type distribution in the diaphragm independently of AMPK and might be responsible for the phenotypes we observed.liver kinase B1; AMP-activated protein kinase; exercise; diaphragm; oxygen uptake IT IS WELL RECOGNIZED THAT INCREASING PHYSICAL ACTIVITY prevents obesity and diabetes (15,22), but the molecular mechanisms mediating these favorable effects of exercise remain elusive. AMP-activated protein kinase (AMPK) is a master sensor and regulator of energy balance at the cellular level (17, 18). In addition, skeletal muscle AMPK activity is required for maintaining exercise capacity. Several studies have demonstrated that exercise capacity is reduced dramatically in a skeletal musclespecific AMPK-deficient mice (14, 33, 51). However, more experiments have now demonstrated that skeletal muscle AMPK is dispensable in modulating the effects of contraction or pharmacological activation on fuel metabolism, raising the possibility that AMPK-independent pathways may regulate glucose and lipid metabolism (2,11,13,26,28,33,37). Although fuel metabolism is an important factor for endurance performance, it is unclear whether these pathways are involved in the decreased exercise capacity observed in genetically modified AMPK model mic...

Mitochondrial and performance adaptations to exercise training in mice lacking skeletal muscle LKB1

“…Our findings agree with those of Miura, et al (33), who very recently showed a similar impairment in exercise tolerance in a skmLKB1-dominant negative mice. Ambulatory cage activity, however, was not altered in skmLKB1-KO mice, indicating that differences between genotypes in untrained mice are not due to a chronic decrease in physical activity.…”

“…AMPK␣1 activity, on the other hand, was not significantly reduced vs. CON muscle in untreated or AICAR-treated skmLKB1-KO muscles. Our findings are consistent with previous findings from mLKB1-KO mice (25) and with very recent data from skeletal muscle-specific LKB1-dominant negative (skmLKB1-DN) mice (33). The persistent AMPK␣1 activity in skmLKB1-KO muscle is likely due to 1) the presence of nonskeletal muscle cells (endothelial cells, neurons, adipocytes) in the tissue sample that highly express AMPK␣1 and do not lack LKB1 and 2) the potential presence of an alternate AMPK kinase, such as calcium/calmodulindependent protein kinase kinase (CaMKK), that is able to phosphorylate AMPK in the absence of LKB1.…”

“…In resting muscles, expressions of cytochrome b and TFAM were significantly lower in skmLKB1-KO mice, but no other differences between genotypes were observed for the genes that we measured, although there was a nonsignificant trend for decreased expression of VEGF. These findings in resting muscles are in general agreement with those of Miura et al (33), who observed significantly lower gene expressions of COX-2, COX-4, and VEGF and a trend for decreased TFAM but no difference in PGC-1␣ expression in skmLKB1-DN mice.…”

“…Miura et al (33) a reported a similar decrease in glycogen after treadmill running between skmLKB1-DN and WT mice, although in contrast to our findings they observed lower glycogen in resting muscles of skmLKB1-DN vs. WT mice (33). To determine how the impaired gene expression at 3 h postexercise in skmLKB1-KO muscles may have been affected by altered intracellular signaling, we measured phosphorylation of AMPK, p38 MAPK, Erk, and CaMKII.…”

AMPKα is critical for enhancing skeletal muscle fatty acid utilization duringin vivoexercise in mice

“…Studies conducted in different mouse models with a functional ablation of the AMPKα2 subunit have not shown any genotype differences in RER during exercise or in ex vivo contraction‐induced FA oxidation (13, 15‐18). While 2 of these studies observed that the RER was increased with exercise in LKB1‐deficient models, it led to the conclusions that LKB1 but not AMPK is involved in regulating FA oxidation in mouse skeletal muscle with exercise (17, 18). This suggests that exercise‐ and contraction‐induced FA utilization in skeletal muscle is compromised only when either LKB1 or both AMPKα1 and AMPKα2 protein expression is ablated, reinforcing the role of the LKB1‐AMPKα axis in metabolic regulation with exercise and muscle contraction.…”