Activation of AMPKα2 is not crucial for mitochondrial uncoupling-induced metabolic effects but required to maintain skeletal muscle integrity.

Mario Ost,Susanne Klaus,Anja Voigt,Franziska Werner,Janine Dokas,Juergen Eckel

doi:10.1371/journal.pone.0094689

Abstract

Transgenic (UCP1-TG) mice with ectopic expression of UCP1 in skeletal muscle (SM) show a phenotype of increased energy expenditure, improved glucose tolerance and increase substrate metabolism in SM. To investigate the potential role of skeletal muscle AMPKα2 activation in the metabolic phenotype of UCP1-TG mice we generated double transgenic (DTG) mice, by crossing of UCP1-TG mice with DN-AMPKα2 mice overexpressing a dominant negative α2 subunit of AMPK in SM which resulted in an impaired AMPKα2 activity by 90±9% in SM of DTG mice. Biometric analysis of young male mice showed decreased body weight, lean and fat mass for both UCP1-TG and DTG compared to WT and DN-AMPKα2 mice. Energy intake and weight-specific total energy expenditure were increased, both in UCP1-TG and DTG mice. Moreover, glucose tolerance, insulin sensitivity and fatty acid oxidation were not altered in DTG compared to UCP1-TG. Also uncoupling induced induction and secretion of fibroblast growth factor 21 (FGF21) from SM was preserved in DTG mice. However, voluntary physical cage activity as well as ad libitum running wheel access during night uncovered a severe activity intolerance of DTG mice. Histological analysis showed a progressive degenerative morphology in SM of DTG mice which was not observed in SM of UCP1-TG mice. Moreover, ATP-depletion related cellular stress response via heat shock protein 70 was highly induced, whereas capillarization regulator VEGF was suppressed in DTG muscle. In addition, AMPKα2-mediated induction of mitophagy regulator ULK1 was suppressed in DTG mice, as well as mitochondrial respiratory capacity and content. In conclusion, we demonstrate that AMPKα2 is dispensable for SM mitochondrial uncoupling induced metabolic effects on whole body energy balance, glucose homeostasis and insulin sensitivity. But strikingly, activation of AMPKα2 seems crucial for maintaining SM function, integrity and the ability to compensate chronic metabolic stress induced by SM mitochondrial uncoupling.

Highlights

Skeletal muscle (SM) as a major body compartment is responsible for about 20% of resting energy expenditure and up to 90% of the energy expenditure during physical activity
In two different transgenic mouse models it was shown that uncoupling protein 1 (UCP1) expression in skeletal muscle led to an increased phosphorylation of AMP-activated protein kinase (AMPK) [4,6], linking the positive metabolic effects of skeletal muscle uncoupling to activation of AMPK
This was preserved in double transgenic (DTG) mice which even had a significantly reduced body length compared to all other groups which was most likely due to a severe kyphosis observed in DTG mice

Summary

Introduction

Skeletal muscle (SM) as a major body compartment is responsible for about 20% of resting energy expenditure and up to 90% of the energy expenditure during physical activity. It is an important determinant of overall substrate metabolism. Muscle specific ectopic expression of uncoupling protein 1 (UCP1), the mitochondrial uncoupling protein of brown adipose tissue (BAT), leads to increased energy expenditure, delayed diet-induced obesity development, improved glucose homeostasis, and increased longevity in these UCP1-TG mice [2,3,4]. UCP1-TG mice show an increased substrate flux through the glycolytic pathway paralleled by increased insulin-stimulated glucose uptake and increased lipid metabolism in skeletal muscle [3,4,5]. In two different transgenic mouse models it was shown that UCP1 expression in skeletal muscle led to an increased phosphorylation of AMP-activated protein kinase (AMPK) [4,6], linking the positive metabolic effects of skeletal muscle uncoupling to activation of AMPK

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