When a muscle grows, because its owner is still growing too or has started exercising regularly, some of the stem cells in this muscle develop into new muscle cells.
The same thing happens when an injured muscle starts to heal. At the same time, however, the muscle stem cells must produce further stem cells – i.e., renew themselves – as their supply would otherwise be depleted very quickly. This requires that the cells involved in muscle growth communicate with each other.
Muscle growth is regulated by the Notch signaling pathway
Two years ago, a team of researchers led by Professor Carmen Birchmeier, head of the Developmental Biology/Signal Transduction Lab at the Berlin-based Max Delbrück Center for Molecular Medicine in the Helmholtz Association (MDC), showed that the development of stem cells into muscle cells is regulated with the help of two proteins, Hes1 and MyoD, which are produced in the progenitor cells in an oscillatory manner – i.e., there are periodic fluctuations in the number of cells produced.
Both proteins are involved in the Notch signaling pathway, a widespread mechanism by which cells respond to external stimuli and communicate with other cells. The signaling pathway is named after its receptor “Notch,” onto which the ligand “Delta,” a cell surface protein, latches.
A third protein, Delta-like1, plays a crucial role
“In our current study, we have provided unequivocal evidence that oscillation in muscle tissue is not just some strange phenomenon of the cells involved, but that these rhythmic fluctuations in gene expression are actually crucial for transforming stem cells into muscle cells in a balanced and controlled manner,” says Birchmeier.
Together with researchers from Japan and France, Birchmeier and four other scientists at the MDC also uncovered the crucial role of a third protein that, along with Hes1 and MyoD, forms a dynamic network within the cells. As the team reports in the journal Nature Communications, this protein is the Notch ligand Delta-like1, or Dll1 for short.
