Postsynaptic development of the neuromuscular junction in mice lacking the γ-subunit of muscle nicotinic acetylcholine receptor

The mammalian muscle nicotinic acetylcholine receptor (AChR) is composed of five membrane-spanning subunits and its composition differs between embryonic and adult muscles. In embryonic muscles, it is composed of two α-, one β-, one δ-, and one γ-subunit; the γ-subunit is later replaced by the ε-subunit during postnatal development. This unique temporal expression pattern of the γ-subunit suggests it may play specific roles in embryonic muscles. To address this issue, we examined the formation and function of the neuromuscular junction in mouse embryos deficient in the γ-subunit. At embryonic day 15.5, AChR clusters were absent and the spontaneous miniature endplate potentials were undetectable in the mutant muscles. However, electrical stimulation of the nerves triggered muscle contraction and elicited postsynaptic endplate potential (EPP) in the mutant muscles, although the magnitude of the muscle contraction and the amplitudes of the EPPs were smaller in the mutant compared to the wild-type muscles. Reintroducing a wild-type γ-subunit into the mutant myotubes restored the formation of AChR clusters in vitro. Together, these results have demonstrated that functional AChRs were present in the mutant muscle membrane, but at reduced levels. Thus, in the absence of the γ-subunit, a combination of α, β, and δ subunits may assemble into functional receptors in vivo. These results also suggest that the γ-subunit maybe involved in interacting with rapsyn, a cytoplasmic protein required for AChR clustering.