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Chapter 5

‘‘building blocks’’ of muscle proteins, but also directly stimulate intracellular signaling pathways that regulate mRNA translation. Evidence for this is derived from studies in which the provision of single essential amino acids (EAA), such as leucine, is su cient for the stimulation of muscle synthesis, even in the absence of other AA as substrate for peptide synthesis. Leucine, of all the EAA, seems to have the most potent capacity to stimulate muscle synthesis and inhibit lysosome-based muscle protein degradation. Precisely how the EAA stimulate proteostasis-regulating signals remains poorly understood. Unlike peptide hormones and local factors, there does not appear to be a receptor in the plasma membrane as genetic loss of the transporter system (for leucine: system L, a Na1-independent, glutamine-dependent exchanger) abolishes intracellular signaling responses to EAA. Thus EAA presumably act like glucocorticoids, TH and testosterone by binding to proteins once inside the cell. However, there are no known transcription factors or EAA response elements within the DNA and it is not known whether leucine itself or metabolites generated during its catabolism are responsible for the e ects. For example, it remains to be investigated whether deamination of leucine to aKIC via branch chain amino acid transferase or oxidative decarboxylation to isovaleryl CoA via branch chain keto-acid dehydrogenase is required for leucine-induced phosphoproteome activity. Currently, it is believed that AA promote synthesis while also inhibiting degradation, presumably via coordinated action of Akt/ FKHR and mTORc1-mediated inhibition of autophagy (see Section 5.3.1.1 for more detail); though the mechanism is unknown and multiple mechanisms may exist as leucine and aKIC appear to inhibit lysosome-based degradation via distinct pathways.

5.3 Regulation of Muscle Proteostasis in Humans

Since the early 1980s, many of the advances made surrounding the regulation of muscle proteostasis in humans are a consequence of the development of methods for detecting stable isotopically labeled amino acid ‘‘tracers’’ by gas chromatography combustion isotope ratio mass spectrometry (GC-C- IRMS).25 Constant improvements mean that these tracer approaches have extensive application even for studies of muscle proteostasis over periods as short as 30–60 min. Labeling amino acids such as the branch chain amino acid leucine with 13C, for example [1,2-13C2]leucine, allows measurement of fractional synthetic rates (muscle protein synthesis rates) of incorporation of these amino acids into functional muscle proteins from biopsy tissue. Increased sensitivity and precision of gas chromatography mass spectrometry (GC-MS) now allow measurement of the incorporation of deuterium labeled amino acids into protein i.e. d5 phenylalanine. Fractionation of proteins from these tissues further allows measurement of fractional synthetic rates of incorporation into di erent cellular compartments such as mitochondria, sarcoplasm and myofibers. Using these same tracers, the catabolism of the branch chain amino acids (BCAA), the only amino acids that may be oxidized in skeletal muscle, can also