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Sci Rep. 2017 Jan 4;7:39825. doi: 10.1038/srep39825.

Muscle-specific deletion of BDK amplifies loss of myofibrillar protein during protein undernutrition.

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Laboratory of Nutritional Biochemistry, Department of Applied Molecular Biosciences, Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya, Japan.
Animal Resource Center for Infectious Diseases, Research Institute for Microbial Diseases, Osaka University, Suita, Osaka, Japan.
Department of Experimental Genome Research, Research Institute for Microbial Diseases, Osaka University, Suita, Osaka, Japan.
Graduate School of Health Sciences, Matsumoto University, Matsumoto, Nagano, Japan.
School of Human Cultures, University of Shiga Prefecture, Hikone, Shiga, Japan.
Department of Nutrition, Shigakkan University, Obu, Aichi, Japan.
Graduate School of Nutritional and Environmental Sciences, University of Shizuoka, Shizuoka, Japan.
Graduate School of Environmental and Life Science, Kyoto Prefectural University, Kyoto, Japan.


Branched-chain amino acids (BCAAs) are essential amino acids for mammals and play key roles in the regulation of protein metabolism. However, the effect of BCAA deficiency on protein metabolism in skeletal muscle in vivo remains unclear. Here we generated mice with lower BCAA concentrations by specifically accelerating BCAA catabolism in skeletal muscle and heart (BDK-mKO mice). The mice appeared to be healthy without any obvious defects when fed a protein-rich diet; however, bolus ingestion of BCAAs showed that mTORC1 sensitivity in skeletal muscle was enhanced in BDK-mKO mice compared to the corresponding control mice. When these mice were fed a low protein diet, the concentration of myofibrillar protein was significantly decreased (but not soluble protein) and mTORC1 activity was reduced without significant change in autophagy. BCAA supplementation in drinking water attenuated the decreases in myofibrillar protein levels and mTORC1 activity. These results suggest that BCAAs are essential for maintaining myofibrillar proteins during protein undernutrition by keeping mTORC1 activity rather than by inhibiting autophagy and translation. This is the first report to reveal the importance of BCAAs for protein metabolism of skeletal muscle in vivo.

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