Compartmentalized expression of the alpha- and gamma-subunits of the acetylcholine receptor in recently fused myofibers.

The mRNAs encoding the subunits of the acetylcholine receptor are clustered at the level of the neuromuscular junction in adult muscle fibers. We have followed the distribution of the mRNAs encoding the alpha- and gamma-subunits during development of the diaphragm muscle in the mouse by whole-mount in situ hybridization. We show that the mRNAs encoding both subunits display a nonhomogeneous distribution as early as Day 13.5, when the first neuromuscular contacts are formed. Extrajunctional mRNAs disappear during the following days with a concomitant increase in contrast of the synaptic domains. gamma-subunit mRNAs become undetectable at the end of the first postnatal week, together with the appearance of epsilon-subunit mRNAs. Our results imply that the expression of the acetylcholine receptor genes, including the gamma-subunit gene, is compartmentalized soon after neuromuscular contacts have been established. This has important implications for the understanding of the molecular mechanisms of neuromuscular junction formation.