Some of the benefits of a rigorous workout might one day be achievable without any physical exertion. A study in mice shows that injecting muscle cells beneath the skin creates a patch of these cells that continuously contracts, delivering some of the benefits of exercise around the clock. While this should never replace physical activity, it may be helpful for people who are unable to exercise, such as those in hospital.
Initially, at the Beijing Institute for Stem Cell and Regenerative Medicine in China and his colleagues injected muscle cells into mice with the aim of increasing their muscle mass, but this had an unexpected result. 鈥淭his thing was contracting under the skin continuously, and we thought, 鈥楾his thing is like non-stop exercise鈥�,鈥� says Shyh-Chang.
They started by taking samples of muscle stem cells from live mice, which they cultured until the cells differentiated into mature, contractile muscle cells known as myocytes. They then injected these just beneath the skin on the backs of the same mice. 鈥淭he original intention was to increase muscle mass by injecting muscle cells,鈥� says Shyh-Chang.
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With an injection of about 6 million myocytes, a patch of cells formed, creating its own network of mini blood vessels. This then began to pulse continuously, even when the mice were asleep.
Various experiments showed the patches had widespread effects around the mice鈥檚 bodies. For instance, the mice given the muscle patches had a higher whole-body muscle mass and better immune function compared with those that got a sham injection. All the mice were free to move around their cages as normal during the study.
What鈥檚 more, in mice that were 18 months old (roughly equivalent to a person in their late 50s or 60s) and those fed to induce obesity, the patches improved muscle mass, bone density, physical strength and endurance.
These mice also showed reversal in signs of liver damage, as well as reduced inflammation and the ability to navigate a maze faster. 鈥淪o, it also has effects on its brain,鈥� says Shyh-Chang. 鈥淭his is known from exercise, but here, we鈥檙e seeing it from a small muscle graft.鈥�
The patch鈥檚 wide-ranging effects may be due to it mimicking how . 鈥淲hat I find particularly interesting is that the effects appear to go beyond the graft itself,鈥� says at Duke-NUS Medical School in Singapore. 鈥淭he study suggests that the contracting muscle can act as an endocrine organ, with changes in circulating factors that may contribute to effects in other tissues.鈥�
Imaging showed that the muscle patches survived for at least 81 days, when the researchers stopped taking measurements, but Shyh-Chang says that follow-up work indicates they are still stable after six months. 鈥淪o far, there鈥檚 no limit to its lifespan as far as I can tell,鈥� he says. There also seems to be very little reduction in volume over time and no evidence of tumour growth, which can be a risk after implanting stem cells.
Shyh-Chang hopes the patch could help people with medical conditions that stop them from exercising. The researchers are talking to several hospitals about starting clinical trials in people with limited mobility, which could begin within a year, he says. 鈥淲e always say we should exercise to keep muscle mass, but the folks that need exercise the most are actually the ones least likely to be able to exercise,鈥� he says.
鈥淭he idea of mimicking exercise for people who can鈥檛 exercise is amazing,鈥� says at King鈥檚 College London, but he questions how comfortable a continuously contracting graft like this would be.
Tang stresses that the patch should be thought of as mimicking some aspects of exercise, rather than replacing it. 鈥淓xercise has many other effects 鈥� including cardiovascular, neuromuscular, mechanical and bone-loading effects 鈥� that a small subcutaneous muscle graft cannot fully reproduce,鈥� he says.
Nature Aging
