Endurance performance and energy metabolism during exercise in mice with a muscle-specific defect in the control of branched-chain amino acid catabolism

<div><p>It is known that the catabolism of branched-chain amino acids (BCAAs) in skeletal muscle is suppressed under normal and sedentary conditions but is promoted by exercise. BCAA catabolism in muscle tissues is regulated by the branched-chain α-keto acid (BCKA) dehydrogenase complex, which is inactivated by phosphorylation by BCKA dehydrogenase kinase (BDK). In the present study, we used muscle-specific BDK deficient mice (BDK-mKO mice) to examine the effect of uncontrolled BCAA catabolism on endurance exercise performance and skeletal muscle energy metabolism. Untrained control and BDK-mKO mice showed the same performance; however, the endurance performance enhanced by 2 weeks of running training was somewhat, but significantly less in BDK-mKO mice than in control mice. Skeletal muscle of BDK-mKO mice had low levels of glycogen. Metabolome analysis showed that BCAA catabolism was greatly enhanced in the muscle of BDK-mKO mice and produced branched-chain acyl-carnitine, which induced perturbation of energy metabolism in the muscle. These results suggest that the tight regulation of BCAA catabolism in muscles is important for homeostasis of muscle energy metabolism and, at least in part, for adaptation to exercise training.</p></div

Plasma BCAA concentrations of control and BDK-mKO mice with and without the running exercise bout.

<p># Significant difference between control and BDK-mKO mice. * Significant difference in the same group of mice with and without the exercise bout.</p

Metabolites in the glycolytic pathway.

<p>Changes in metabolite levels in skeletal muscle of BDK-mKO mice and control mice with and without the exercise bout are shown. # Significant difference between control and BDK-mKO mice. G6P, glucose 6-phosphate; F1,6BP, fructose 1,6-bisphosphate; DHAP, dihydroxyacetone phosphate; 2PG, 2-phosphoglycerate; and PEP, phosphoenolpyruvate.</p

Acetyl-CoA and metabolites in the TCA cycle.

<p>Changes in metabolite levels in skeletal muscle of BDK-mKO mice and control mice with and without the exercise bout are shown. # Significant difference between control and BDK-mKO mice. * Significant difference in the same group of mice with and without exercise bout. αKG, α-ketoglutarate.</p

Citrate synthase and cytochrome c oxidase activities.

<p># Significant difference between control and BDK-mKO mice.</p

Exercise performance of control and BDK-mKO mice.

<p><b>(A)</b> Running distance to exhaustion before and after 2 weeks of training, and (B) swimming time to exhaustion of untrained mice on each of 4 consecutive days. # Significant difference between control and BDK-mKO mice.</p

BCAAs and their metabolites.

<p>Changes in the metabolite levels in the skeletal muscle of control and BDK-mKO mice with and without the exercise bout are shown. # Significant difference between control and BDK-mKO mice. * Significant difference in the same group of mice with and without the exercise bout. PMP, pyridoxamine 5'-phosphate; KIC, α-ketoisocaproate; KMV, α-keto-ß-methylvalerate; KIV, α-ketoisovalerate; ßMB-CAR, ß-methylbutyryl-carnitine; αMB-CAR, α-methylbutyryl-carnitine; IB-CAR, isobutyryl-carnitine; and αKG, α-ketoglutarate.</p

NADH, NAD⁺, and high energy compounds.

<p>NADH, NAD⁺, and high energy compounds.</p

Schematic of whole-body BCAA metabolism.

<p>Ketoacids are formed by reversible transamination catalyzed by the mitochondrial or cytosolic isoforms of branched chain amino acid transaminase (<i>BCAT</i>). The action of the branched chain keto acid dehydrogenase complex (<i>BCKDC</i>) in the mitochondrial matrix leads to the evolution of CO₂ from the 1-carbon of the keto acids including KIC, which was ¹⁴C labeled and measured from the expired air in these studies. Subsequent intramitochondrial metabolism leads to the formation of various acyl-coenzyme A (R-CoA) esters that can reversibly form acylcarnitines (not displayed). Neither FAD and NAD Cofactors nor CO₂ and H₂O substrates are displayed. Bold font indicates metabolites or corresponding acylcarnitines that were detected and measured quantitatively in the 24 h urines (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0059443#pone-0059443-t004" target="_blank">Table 4</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0059443#pone-0059443-t005" target="_blank">5</a>). AA, amino acids.</p

Illustration of experimental set up for measuring Leu flux in rats.

<p>After being filtered through mist (Fuller’s Earth) and CO₂ (Soda lime) absorbent, a constant stream of air flowed through one tube to a closed metabolic cage with a quiet convection fan (Columbus Instruments, Columbus, OH) and out through two outlet tubes which bubbled into a 50 ml conical tube containing Hyamine 10X hydroxide (Perkin Elmer, Waltham, MA)-ethanol (1∶1, vol/vol) for CO₂ fixation. ¹⁴CO₂ samples were collected every 10 min throughout the infusions. The rat was implanted with a jugular vein catheter and infused with [¹⁴C]-NaHCO₃ (Moravek Biochemical, Brea, CA) and then [1-¹⁴C]-Leu (Moravek Biochemical, Brea, CA). [¹⁴C]-NaHCO₃ and [1-¹⁴C]-Leu were each sequentially infused for 2 h. Three blood samples were collected at the 90 and 105 min (0.5 ml) and 120 min (∼3 ml) after start of [1-¹⁴C]-Leu infusion. A carotid catheter (not shown) came through the same sealed cage port as the jugular cannula pair and was used for blood sampling.</p