During muscle development and regeneration, the threadlike myofibres that make up mammalian skeletal muscles emanate from the progressive fusion of progenitor cells, called myoblasts. Some details of the underlying signalling pathway involved in myoblast fusion are known, but the early triggers have remained elusive. In a ground-breaking study recently published in Nature, a team of scientists from the University of Virginia, USA, led by Kodi Ravichandran came to the startling conclusion that cell death provides a signal that promotes myoblast fusion.
Myoblast fusion involves two proteins (ELMO/Dock180) known to activate the Rac G protein, which in turn induces reorganization of the actin cytoskeleton. Exposure of phosphatidylserine (PS) on the outer leaflet of the myoblast's plasma membrane enhances myoblast fusion. Strikingly, all these molecules are also known to be involved in programmed cell death (apoptosis), specifically in the clearance of apoptotic cells by phagocytosis. During this process, PS exposed on the surface of apoptotic cells acts as a signal to phagocytic cells. PS is then recognized by the G protein-coupled receptor BAI1 and is transmitted in an ELMO/Dock180/Rac-dependent manner to induce remodelling of the cytoskeleton and engulfment of apoptotic cells.
Given the usage of similar pathways in myoblast fusion, the scientists from Virginia wondered whether cell death might be linked to myoblast fusion in a much more direct fashion. They designed a series of sophisticated experiments using myoblast cell cultures that allowed them to induce and quantify myoblast fusions under various experimental conditions.
First, the team focused on the expression of BAI1, which they indeed detected in myoblast cells. Moreover, after they experimentally induced myoblast fusion in the cell culture, they found that BAI1 transcript levels were significantly elevated compared with the control, suggesting a role of BAI1 in myoblast fusion. Motivated by this finding, they went on to artificially overexpress the BAI1 gene in myoblasts. They observed that myoblast fusion was enhanced in these BAI1-overexpressing cells, and furthermore demonstrated that this effect is dependent on ELMO/Dock180 and Rac proteins, as overexpression of a mutant BAI1 version, lacking the docking site for ELMO, failed to enhance fusion. The team also tested their hypothesis in vivo using mice lacking a functional version of the BAI1 gene. In line with their hypothesis, the team found that the myofibres obtained from BAI1-deficient mice were smaller than in control mice and that muscle regeneration was negatively affected.
The team knew from their previous work that a significant number of myoblasts undergo apoptosis during myoblast fusion, so they tested whether blocking either PS signalling or apoptosis would affect myoblast fusion. They added PS-masking proteins or inhibitors of apoptosis to the cell culture and then induced myoblast fusion. Intriguingly, blocking either PS signalling or apoptosis impaired myoblast fusion. By adding back apoptotic cells of different origins, they rescued fusion, suggesting that various types of apoptotic cell can trigger myoblast fusion.
Ravichandran's team has provided compelling evidence that a known cell death signal, PS signalling through BAI1, induces myoblast fusion. But how apoptosis aids fusion and how very similar pathways can result in phagocytosis in some cells and fusion in others remains to be elucidated. Further insights into the underlying mechanism may help to develop new strategies for muscle regeneration in sports and medicine.