The initiation phase of mRNA translation is a key target for the regulation of protein synthesis in skeletal muscle. Translation initiation is mediated by a number of factors which are collectively referred to as eukaryotic initiation factors (eIF). Our general hypothesis is that the factors governing translational control of muscle protein synthesis are responsive not only to the prevailing plasma concentrations of amino acids, but also to the direction in which concentration of leucine changes. The specific mechanism whereby amino acids affect the rate of muscle protein synthesis in vivo is governed by the interplay of the various initiation factors. We propose that a sufficient amount of amino acids are required to repress the phosphorylation state of eIF2alpha (which reflects charging of tRNA) and to maintain the activity of eukaryotic initiation factor (eIF)2B for the basal rate of muscle protein synthesis to be maintained. Further, responses to potential activation of other pathways will be inadequate to result in increased muscle protein synthesis unless there is sufficient charging of tRNA and activity of eIF2B. We propose that in the circumstance of an adequate basal amount of amino acids the transient stimulation of muscle protein synthesis by an increase in amino acid availability is mediated via an mTOR-dependent signaling pathway resulting in an increased phosphorylation of the eIF4E binding protein 1 (4E-BP1) and the 70KDa ribosomal protein S6 kinase (S6K1). In addition, leucine has a specific stimulatory effect on synthesis by activating an mTOR-independent (ie, rapamycin- insensitive) signaling pathway resulting in an increased phosphorylation of eIF4G. We will investigate a series of specific hypotheses related to these general proposals using a porcine model that enable us to make frequent, rapid measurements of both muscle protein synthesis and the various initiation factors. The results will clarify the mechanisms whereby amino acids regulate muscle protein synthesis, and the ultimate benefit of this information will be in terms of designing appropriate nutritional support in a variety of clinical settings.
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