A cytokine that packs a punch

Muscle cells undergo an unusual developmental program in which several partially differentiated cells called myoblasts fuse to form a multinucleated myotube. This nascent myotube undergoes further maturation and growth, which requires the addition of nuclei by fusion of more mononucleated myoblasts with myotubes. Valerie Horsley, Grace Pavlath, and colleagues (Emory University, Atlanta, Georgia) have found that nascent myotubes promote fusion, and thus their own growth, by secreting a cytokine normally associated with immune cells.

The cross-system cytokine is IL-4, which is required in immune cells for macrophage fusion. Not one to throw away a good thing, Nature evidently coopted the system for muscle cells. As in immune cells, IL-4 expression in nascent myotubes is driven by a member of the NFAT transcription factor family. Myotubes lacking either IL-4 or the NFAT factor were smaller...