Stem Cells Aid Muscle Strengthing and Repair After Resistance Exercise

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.

Marni Boppart
Marni Boppart

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.”

Making Heart Muscle from Skeletal Muscle Stem Cells

Several experiments in animals and a few clinical trials in human patients have shown that implanting skeletal muscle cells isolated from muscle biopsies into the heart after a heart attack can help the heart to some degree, but the implanted skeletal muscle cells do not integrate into the existing heart muscle mass and the skeletal muscle cells do not differentiate into heart muscle cells.

Experiments like those mentioned above utilized muscle satellite cells. Muscle satellite cells are a resident stem cell population that respond to muscle damage and divide to form skeletal muscle cells form new muscle. Satellite cells are a perfect example of a unipotent stem cell, which is to say a cell that makes one type of terminally differentiated cell type.

Skeletal muscles, however, have another cell population called muscle-derived stem cells or MDSCs. MDSCs express an entirely different set of cell surface proteins than satellite cells, and have the capacity to differentiate into skeletal muscle, smooth muscle, bone, tendon, nerve, endothelial and hematopoietic cells. MDSCs grow well in culture, tolerate low oxygen conditions quite well, and show excellent regenerative potential.

Other laboratories have managed to culture MDSCs in collagen and produce beating heart muscle cells. Others have observed MDSCs forming a proper myocardium under certain conditions. Several studies have established the ability to MDSCs to treat laboratory animals that have suffered a heart attack. The most recent work from Sekiya and others has established that cell sheets made from MDSCs can reduce dilation of the left ventricle, increased capillary density, and promoted recovery without causing erratic heat beat patterns.

Despite their obvious efficacy. MDSCs remain difficult to isolate in high enough numbers to therapeutic purposes. None of the cell surface molecules sported by MDSCs are unique to those cells. Therefore, getting clean cultures of MDSCs remains a challenge. Still, these cells represent some of the best hopes for regenerative medicine in the heart. These cells do form heart muscle cells and heal ailing hearts. They can be grown in bioreactors to high numbers and can also be combined with engineered materials to shore up a damaged heart and mediate its regeneration. While the use of MDSCs is still in its infancy, the promise certainly is there.

Injected Wnt Protein Helps With Muscular Dystrophy

Duchenne muscular dystrophy is a genetic disease that affects one of every 3,500 newborn males. Because the DMD gene is located on the X chromosome, loss-of-function mutations that cause Duchenne muscular dystrophy (DMD) tend to occur in males.

Muscular dystrophy or MS affects skeletal muscles and causes muscle weakness and muscle loss, and unfortunately, the disease often progresses to a state were the muscles are so weak and damaged that even the diaphragm, which is a voluntary muscle, becomes nonfunctional, and the patients dies from an inability to breath.

Recently, Michael Rudnicki, a MS researcher from the Ottawa Hospital Research Institute in Canada, has led a research team that discovered that injections of a protein called “WNT7a” into muscles can increase the size and strength of muscles in MS mice.

Rudnicki is the director of the Regenerative Medicine Program at Ottawa Hospital Research Institute (OHRI), Canada. The results of this work were published on the Nov. 26, 2012, in the Proceedings of the National Academy of Sciences (PNAS).

For these experiments, Rudnicki collaborated with a San Diego-based biotechnology firm known as Fate Therapeutics. Fate Therapeutics specializes in developing pharmaceuticals that are based on stem cell biology, and Rudnicki is one of the founders of this company. Rudnicki hopes to begin a clinical trial of WNT7a for DMD in the near future.

In 2009, Rudnicki and co-workers showed that WNT7a protein is able to stimulate muscle repair by increasing the available supply of a population of muscle stem cells known as “muscle satellite cells.” Muscle satellite cells are located near muscle fibers but they are dormant until they are needed for muscle repair or muscle fiber regeneration. When the muscle is stressed or damaged, the satellite cells increase in number (proliferate) and mature (differentiate).

Muscle Satellite Cells

These newly published findings build on these earlier results. Once injected into the muscles of mice afflicted with DMD, the WNT7a-injected muscles showed significant increases in fiber strength and size. However, Rudnicki and others also found that WNT7a stimulated a two-fold increase in the number of satellite cells in the injected mouse muscles.

Rudnicki was worried that WNT7a was pushing satellite cells to differentiate prematurely, which was disconcerting because such premature differentiation would deplete the muscle satellite population. However, no evidence of premature differentiation was observed. Additionally, WNT7a-injected mouse muscles showed far less contraction-related injury, suggesting that WNT7a has a kind of protective effect on the muscle.

Even though these experiments were done in a mouse model of DMD, would WNT7a also work in a similar fashion in human muscles? To answer this questions, Rudnicki and his colleagues analyzed human muscle tissue from healthy male donors that had been treated with WNT7a. The results showed that the effects of this protein in skeletal muscle are the same in humans as in mice.

To summarize from their own paper: “Our experiments provide compelling evidence that WNT7a treatment counteracts the significant hallmarks of DMD, including muscle weakness, making WNT7a a promising candidate for development as an ameliorative treatment for DMD.”

The remarkable conclusion is that increasing muscle strength by injecting WNT7a into specific, vital muscle groups, such as those involved in breathing, should be considered as a therapeutic approach for this debilitating disease.