Mechanical stimuli play a major role in the regulation of muscle mass, and the maintenance of muscle mass contributes significantly to disease prevention and the quality of life. Although a link between mechanical stimuli and the regulation of muscle mass has been recognized for decades, the mechanisms involved in converting mechanical information into the molecular events that control this process have not been defined. Nevertheless, significant advancements are being made in this field, and it has recently been established that signaling through a rapamycin-sensitive (RS) pathway is necessary for mechanically-induced growth of skeletal muscle. Since rapamycin is an inhibitor of a protein kinase called the mammalian target of rapamycin (mTOR), many investigators have concluded that mTOR signaling is necessary for mechanically-induced growth. There is, however, a lack of direct evidence that mTOR is the rapamycin-sensitive (RS) element in this pathway, and the mechanisms involved in the mechanical activation of the RS signaling pathway are not known. Therefore, the focus of this application is aimed at defining how mechanical stimuli activate the RS signaling pathway and skeletal muscle growth. The findings of our preliminary studies implicate a role for phospholipase D (PLD) mediated phosphatidic acid (PA) production and led to the following hypothesis: Mechanical stimuli induce a PLD-dependent increase in PA, which subsequently binds and activates mTOR signaling and skeletal muscle growth. To test this hypothesis, a combination of in vivo and ex vivo models in conjunction with a series of pharmacological and molecular interventions will be employed under two specific aims. The goal of the first specific aim is to determine whether PLD-mediated PA production is sufficient for the induction of RS signaling and skeletal muscle growth, and whether PLD-mediated PA production is necessary for mechanically-induced RS signaling and growth. The goal of the second aim is to determine whether mTOR is the RS element that confers mechanically-induced RS signaling and skeletal muscle growth, and also determine whether mTOR kinase activity and binding to PA are necessary for these events. Taken together, these studies will fill an important gap in the mechanistic understanding of RS signaling by mechanical stimulation and elucidate the role of mTOR and the potentially novel functions of PLD and PA in this pathway. These studies have broad application to health-related research and could lead to the development of therapies aimed at preventing skeletal muscle atrophy during conditions such as bed rest, immobilization, spaceflight, aging, cachexia and dystrophy.
