University of Illinois professor of Kinesiology and Community Health, Marni Boppart and her colleagues have published experiments that demonstrate that mesenchymal stem cells (MSCs) rejuvenate skeletal muscle after resistance exercise. These new findings, which were published in the journal Medicine and Science in Sports and Exercise, might be the impetus for new medical interventions to combat age-related declines in muscle structure and function.
Injecting MSCs into mouse leg muscles before several bouts of exercise that mimic resistance training in humans and result in mild muscle damage caused increases in the rate of muscle repair and enhanced the growth and strength of those muscles in exercising mice.
“We have an interest in understanding how muscle responds to exercise, and which cellular components contribute to the increase in repair and growth with exercise,” Boppart said. “But the primary goal of our lab really is to have some understanding of how we can rejuvenate the aged muscle to prevent the physical disability that occurs with age, and to increase quality of life in general as well.”
MSCs are found throughout the body, but several studies have established that MSCs from different tissue sources have distinct biological properties. Typically, MSCs can readily differentiate into bone, fat, and cartilage cells, but coaxing MSCs to form skeletal muscle has proven to be very difficult. MSCs usually form part of the stroma, which is the connective tissue that supports organs and other tissues.
Because of their inability to readily differentiate into skeletal muscle, MSCs probably potentiate muscle repair by “paracrine” mechanisms. Paracrine mechanisms refer to molecules secreted by cells that induce responses in nearby cells. Not surprisingly, MSCs excrete a wide variety of growth factors, cytokines, and other molecules that, according to this new study, stimulate the growth of muscle precursor cells, otherwise known as “satellite cells.” The growth of satellite cells expands muscle tissue and contributes to repair following muscle injury. Once activated, satellite cells fuse with damaged muscle fibers and form new fibers to reconstruct the muscle and enhance strength and restore muscle function.
“Satellite cells are a primary target for the rejuvenation of aged muscle, since activation becomes increasingly impaired and recovery from injury is delayed over the lifespan,” Boppart said. “MSC transplantation may provide a viable solution to reawaken the aged satellite cell.”
Unfortunately, satellite cells, even though they can be isolated from muscle biopsies and grown in culture, will probably not be used therapeutically to enhance repair or strength in young or aged muscle “because they cause an immune response and rejection within the tissue,” Boppart said. But MSCs are “immunoprivileged,” which simply means that they can be transplanted from one individual to another without sparking an immune response.
“Skeletal muscle is a very complex organ that is highly innervated and vascularized, and unfortunately all of these different tissues become dysfunctional with age,” Boppart said. “Therefore, development of an intervention that can heal multiple tissues is ideally required to reverse age-related declines in muscle mass and function. MSCs, because of their ability to repair a variety of different tissue types, are perfectly suited for this task.”