ARTICLE IN BRIEF
In a mouse model, investigators were able to chemically reverse the degeneration of muscle stem cells by inhibiting a pathway that prevents replenishment of satellite cells in aged muscle.
A pathway that prevents replenishment of satellite cells in aged muscle has been discovered and its effects chemically reversed, providing proof of principle for strategies to “rejuvenate” this cell population and possibly develop therapies for muscle repair. But questions remain whether this pathway is too sensitive or too central to manipulate, and whether doing so can alter the multiple effects of aging in such a complex tissue.
“Aging in muscle tissue is a complicated process that is not well understood,” said Bradley Olwin, PhD, senior author on one of two papers in the Feb. 16 online edition of Nature Medicine. “It is clear that the anabolic processes are not maintained as well, and there is a general switch to catabolic pathways,” leading to age-related loss of muscle mass. Past research has implicated systemic factors independent of the muscle themselves, but there remains controversy.
That led Dr. Olwin, and Helen Blau, PhD, senior author on the second paper, to focus on the satellite cells. “The question we both asked was whether there was any long-term deficit in the ability of satellite cells in those aged muscles to maintain themselves.”
Dr. Blau is director of the Baxter Laboratory for Stem Cell Biology and professor of microbiology and immunology at the Stanford University School of Medicine. Dr. Olwin is a professor of molecular, cellular, and developmental biology at the University of Colorado in Boulder.
A NOVEL STUDY TECHNIQUE
Satellite cells are challenging to study because contacting the stiff plastic of a culture dish causes them to terminally differentiate, rather than self-renew. To overcome this, Dr. Blau suspended satellite cells on a hydrogel, five orders of magnitude less stiff than plastic, to mimic the soft surroundings of their native environment. “It made a difference much greater than we ever dreamed,” she said, allowing the cells to maintain their stem cell identity.
Her initial experiments indicated that two thirds of the stem cells isolated from 24-month-old mice, the gerontological equivalent of an 80-year-old human, are dysfunctional, dividing little if at all when transplanted into irradiated muscle in immunodeficient mice.
The hydrogel system also provided a platform for drug screening. When Dr. Blau tested an inhibitor of p38 mitogen activated protein kinase (p38 MAPK), a ubiquitous signaling molecule, she saw a 40-fold increase in proliferation by the satellite cell population. Detailed examination of individual cells over time indicated that many cells were not, in fact, affected by the treatment. Rather, the inhibitor promoted the proliferation of a subpopulation of the cells, “and as a result, the proportion of functional stem cells increased.”
The “ultimate test of functional recovery” was to determine if treated satellite cells could restore strength to injured muscle. Dr. Blau found that injured muscles of aged mice that received these cells “exhibited increases in both twitch and tetanus forces that restored them to the levels found in uninjured muscles of young mice.” She concluded: “Our findings provide evidence of the potent restorative function of the expanded muscle stem cell population derived from aged mice, even in an injured, aged, syngeneic, and immunocompetent recipient.”
THE P38 MAPK INHIBITOR PATHWAY
Dr. Olwin, meanwhile, came to test the effects of p38 MAPK inhibitors based on his previous work showing that the signaling pathway was involved in the exit of satellite cells from quiescence and differentiation of muscle progenitors.
“We also recently discovered that this pathway is implicated in asymmetric division, so we thought that it would likely be involved in loss of self-renewal in the aged cell as well,” he said.
What became clear as he explored inhibiting the pathway is that the dose is central to the effect. With too little inhibitor, signaling continues and the satellite cells remain quiescent. But with too much inhibitor, he found, the cells appear to forego the asymmetric division that retains the stem cell character in one daughter; instead both offspring terminally differentiate. “Incompletely blocking the pathway is the key.” Dr. Olwin hypothesized that the aged muscle environment elevates p38 MAPK signaling in satellite cells, through yet-to-be-determined means, and that hyperactive signaling disrupts asymmetric division.
WHAT IS THE THERAPEUTIC POTENTIAL?
Much remains to be determined about the therapeutic potential of this pathway for muscle aging. Dr. Blau's experiments showed that the rejuvenated pool of cells maintained their proliferative vigor through serial transplantation, suggesting that short-term treatment could lead to long-term changes. But whether treatment can restore muscle in the long term in humans is a different, and more complicated, question.
“No one has yet characterized muscle stem cells from adult humans,” she noted. “That's a major focus of our research. We are keen on translating the results from the mouse into therapies for humans.” Ultimately, she said, drugs acting on this pathway, or further up or downstream, might be used to alter a patient's own stem cells ex vivo, followed by reimplantation of the treated cells.
However, Dr. Olwin cautioned, “I think it is unlikely that this is going to fix the aging phenotype, because there is more involved in aging than just the stem cells.” Nonetheless, he said, drugs are being developed to inhibit p38 MAPK, which is also a major proinflammatory signaling molecule. “It would be interesting to see if these drugs also improve muscle.” Dosing may be complicated by the U-shaped response curve of the cells to the inhibitor, but further work may identify a better target or a practical dose.
“For many years, the field has been drilling down to understand the molecular mechanisms of muscle stem cells,” said Michael Rudnicki, PhD, director of the Regenerative Medicine Program and professor of medicine at the University of Ottawa, Canada. “We are now reaching the point at which we are identifying druggable targets. This is a really exciting transition.” Potential applications include muscle wasting diseases, cachexia, sarcopenia, and perhaps even the muscular dystrophies. The p38 MAPK protein may not be a likely target itself, he said, given its ubiquity, but other molecules in the pathway may be.
Terry Partridge, PhD, associate director of the Children's Research Institute and Center for Genetic Medicine Research at Children's National Medical Center in Washington, DC, was more cautious about the applications of the findings, because of how little is known about satellite cell function in humans.
“We have no real evidence that we maintain our muscle mass throughout our life by satellite cell activity, or that it is the lack of satellite cell activity that is causing the atrophy in older people,” he said. “Theirs are interesting observations, but aging is likely a phenomenon with multiple causes,” including the accumulation of mitochondria defects that are not necessarily going to be reversed by treatment of satellite cells.