Researchers discover secreted protein helps to repair, grow muscles

TOKYO: A protein known as platelet-derived growth factor subunit B (PDGF-B), which is continually released from skeletal muscle cells, has been found to aid in muscle repair by promoting the formation of myoblasts, or muscle stem cells, according to research from Tokyo Metropolitan University.

Unexpectedly, they discovered that PDGF-B promotes the growth of muscle fibres as well. This matched the stronger contraction of the fibres, they found. The medicines they have discovered could revolutionise the way we treat muscle injuries and atrophy.

Myokines are small proteins secreted by skeletal muscle cells. They have a wide range of functions and may act on cells both near and far where they are made.

A comprehensive picture of how myokines affect cellular processes is far from clear, but it is believed that they play an important role in exercise-related bodily functions, particularly the maintenance of muscle tissue.

A team led by Associate Professor Yasuko Manabe at Tokyo Metropolitan University has been studying how myokines affect the behaviour of muscle cells.

Through extensive experiments, they found that a myokine known as platelet-derived growth factor subunit B, or PDGF-B, is secreted by skeletal muscles in a constitutive way i.e. without any stimulus.

To understand what role it plays, they took myoblasts, precursor cells which go on to differentiate into muscle fibres, and exposed them to PDGF-B. They were able to clearly show that PDGF-B induced greater proliferation of myoblasts.

Curiously, they also found that PDGF-B impacted cells which had already differentiated. They took myotubes, a developmental stage of muscle fibres, and exposed them to the same myokine. Myotubes treated in this way exhibited significantly more maturation, visibly increasing in diameter under microscope observation.

They also expressed more Myosin Heavy Chain, a key part of the protein structure of myosin, the molecular motor responsible for muscle contraction. Using a recently developed technique based on observing how myotubes react to an electric pulse, this was shown to directly correspond to increased contractile strength.

Thus, PDGF-B not only helps make more muscle but makes them stronger. But this doesn't mean both processes are accelerated in a haphazard manner.

They noticed subtle differences in PDGF-B signalling pathways between myotubes and myoblasts; the team believe these differences may be involved in cells switching from a proliferating phase to one where they are maturing.