Muscle-tendon cross talk during muscle wasting.

In organisms from flies to mammals, the initial formation of a functional tendon is completely dependent on chemical signals from muscles (myokines). However, how myokines affect the maturation, maintenance, and regeneration of tendons as a function of age is completely unstudied. Here we discuss the role of four myokines-fibroblast growth factors (FGF), myostatin, the secreted protein acidic and rich in cysteine (SPARC) miR-29-in tendon development and hypothesize a role for these factors in the progressive changes in tendon structure and function as a result of muscle wasting (disuse, aging, and disease). Because of the close relationship between mechanical loading and muscle and tendon regulation, disentangling muscle-tendon cross talk from simple mechanical loading is experimentally quite difficult. Therefore, we propose an experimental framework that hopefully will be useful in demonstrating muscle-tendon cross talk in vivo. Though understudied, the promise of a better understanding of muscle-tendon cross talk is the development of new interventions that will improve tendon development, regeneration, and function throughout the lifespan.