Aberrant development of motor axons and neuromuscular synapses in MyoD-null mice.

Myogenic regulatory factors (MRFs), muscle-specific transcription factors, are implicated in the activity-dependent regulation of nicotinic acetylcholine receptor (AChR) subunit genes. Here we show, with immunohistochemistry, Western blotting, and electron microscopy that MyoD, a member of the MRF family, also plays a role in fetal synapse formation. In the diaphragm of 14.5 d gestation (E14.5) wild-type and MyoD-/- mice, AChR clusters (the formation of which is under a muscle intrinsic program) are confined to a centrally located endplate zone. This distribution persists in wild-type adult muscles. However, beginning at E15.5 and extending to the adult, innervated AChR clusters are distributed all over the diaphragm of MyoD-/- mice, extending as far as the insertion of the diaphragm into the ribs. In wild-type muscle, motor axons terminate on clusters adjacent to the main intramuscular nerve; in MyoD-/- muscle, axonal bundles form extensive secondary branches that terminate on the widely distributed clusters. The number of AChR clusters on adult MyoD-/- and wild-type diaphram muscles is similar. Junctional fold density is reduced at MyoD-/- endplates, and the transition from the fetal (alpha, beta, gamma, delta) to adult-type (alpha, beta, delta, epsilon) AChRs is markedly delayed. However, MyoD-/- mice assemble a complex postsynaptic apparatus that includes muscle-specific kinase (MuSK), rapsyn, erbB, and utrophin.