The Role of MKK3b in the Mechanical Activation of mTORC1, Protein Synthesis, and Skeletal Muscle Growth

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

Project Summary/Abstract Mechanical stimuli play a major role in the regulation of skeletal muscle mass and the maintenance of muscle mass significantly contributes to disease prevention and quality of life. While the link between mechanical stimuli and the regulation of muscle mass has been appreciated for decades the exact mechanisms that control this process remain poorly defined. Current models assert that mechanically induced increases in muscle mass are driven by a positive shift in the balance between protein synthesis and degradation which, in turn, leads to the net accumulation of newly synthesized proteins and muscle growth. Work from our lab and others has identified the mechanistic target of rapamycin complex 1 (mTORC1) as a major regulator of this process, however, little is known about the upstream pathway(s) that control the activation of mTORC1 and/or related events that potentially contribute to the mechanical regulation of protein synthesis and growth. In an effort to identify these pathway(s) our lab recently performed a phosphoproteome-wide analysis of the signaling events that occur in response to endurance and resistance exercise in humans. The rationale behind this study was based on the recognition that resistance and endurance exercise lead to very distinct muscular adaptations with endurance exercise promoting an increase in aerobic capacity whereas resistance exercise induces an increase in protein synthesis and growth. Thus, we reasoned that our analyses would lead to the identification of signaling events that are activated specifically by endurance and resistance exercise. We were particularly interested in the resistance exercise-specific signaling events because these would represent potential members of the upstream pathway(s) that promote the activation of mTORC1, protein synthesis, and growth. The outcomes of these analyses revealed that mitogen-activated protein kinase kinase 3b (MKK3b) undergoes a prolonged activation following resistance but not endurance exercise and that this event is highly correlated with the increase in myofibrillar protein synthesis following resistance exercise (R = 0.87). Follow-up studies also revealed that the prolonged activation of MKK3b is conserved in a mouse model of resistance exercise and that the activation of MKK3b is sufficient to induce mTORC1 signaling, protein synthesis, and muscle growth. Thus, my overarching hypothesis is that MKK3b is a central component of the upstream pathway via which mechanical stimuli induce the activation of these processes. In the proposed project I will determine whether MKK3b is necessary for the activation of mTORC1, protein synthesis, and muscle growth by employing skeletal muscle-specific and tamoxifen-inducible MKK3b knockout mice, two complementary but distinct models of mechanically induced growth, advanced imaging techniques, and our innovative method for visualizing and quantifying the accumulation of newly synthesized proteins. Collectively, these studies will not only address a major gap in knowledge regarding the mechanism(s) whereby mechanical stimuli regulate muscle mass, but they will also propel me toward my goal of becoming a principal investigator at a research-intensive institution.