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Muscles keep us moving — they are the engine of the body. But as we age, we naturally begin to lose muscle mass, and with it, strength. Slowing this process down is key to a healthy and long life. A recent study offers exciting new insights into the dynamics of muscle regeneration. The findings highlight the role of a protein called platelet-derived growth factor subunit B (PDGF-B), which is produced by muscle cells and helps stimulate both repair and growth. We may be able to harness the protein to develop therapies for muscular injuries and age-related muscle loss.
Myokines: Muscle Messengers
Myokines are small proteins produced by muscle cells, usually during exercise. They work as messengers, sending important signals that regulate not only muscle metabolism but also other tissues and organs, including the liver and the brain. So despite their origins in skeletal muscle, their impact is widespread, even influencing anti-inflammatory and immunological processes.
To date, scientists have discovered more than 3,000 myokines. Although we are still unsure exactly what most of them do, it is becoming increasingly clear that these myokines hold the secret to exercise's health benefits. The more we learn about them, the better our chances of finding interventions that mimic the effects of a workout (or two).
Stimulating Tissue Repair
Platelet-derived growth factor subunit B is one of the aforementioned known myokines. What sets it apart from many others —though not all— is the fact that it is secreted “constitutively,” or without any external stimulus. So, even if you never exercise, your muscles will still produce platelet-derived growth factor subunit B. This caught the researchers’ attention; if it weren’t important, it likely wouldn’t need to be produced continuously.
To figure out its function, the team turned to a special group of precursor cells, called myoblasts, that eventually go on to develop into muscle fiber. They exposed these cells to platelet-derived growth factor subunit B and waited. They soon noticed that the addition of the protein helped boost the proliferation of the myoblasts. Downstream, this meant a boost in overall muscle growth.
Muscle doesn’t just appear overnight. It develops in stages. Proliferation of myoblasts is one of the earliest stages of muscle growth. Another, later stage is the formation of myotubes. These are produced when a bunch of muscle cells fuse together to form a larger, multicellular structure — strength in numbers. This fusion forms the basis for all full-fledged muscle fiber, which itself is just a collection of many myotubes. The formation of myotubes is also what allows for the coordinated contraction required for movement and force.
Given the importance of myotubes to muscle contraction and strength, the researchers checked to see if exposing them to platelet-derived growth factor subunit B had any impact. Surprisingly, adding the protein to myotubes resulted in a significant increase in their diameter, as visible under a microscope. Although not the sole factor in determining strength, larger myotubes usually mean larger muscles, which generally means stronger muscles.
Aside from growing in diameter, myotubes exposed to platelet-derived growth factor subunit B also expressed more Myosin Heavy Chain (MHC). This is a key component of myosin, which is the molecular motor that converts chemical energy into mechanical energy. Without myosin heavy chain, our muscles would not be able to contract.
Testing the treated myotubes for contractile force proved that the changes were not only cosmetic: they noticeably outperformed their untreated counterparts. This suggests that platelet-derived growth factor subunit B not only helps boost muscle formation, it also helps boost muscle strength.
Implications
Age-related muscle loss is a serious threat to well-being, both physical and emotional. It can cause a drop in quality of life, and once it sets in, it can be difficult to reverse. This often sets off a vicious cycle where loss of muscle mass reduces mobility, which in turn reduces the ability to maintain muscle mass.
Myokines like platelet-derived growth factor subunit B may help provide new treatment options that replicate the benefits of regular exercise. This would be a game-changer for many older adults who, due to mobility issues, struggle to work out. Harnessing such proteins could allow us to artificially stimulate muscle growth and strength, curbing the usual muscular problems associated with aging.
