Different functions of fetal and adult AChR subtypes for the formation and maintenance of neuromuscular synapses revealed in ε‐subunit‐deficient mice

Mice deficient in ε‐subunits of the acetylcholine receptor (AChR) channel die prematurely due to severe AChR deficiency that leads to the progressive reduction in AChR density at the neuromuscular endplate [Witzemann, V., Schwarz, H., Koenen, M., Berberich, C., Villarroel, A., Wernig, A., Brenner, H.R. & Sakmann, B. (1996) Proc. Natl Acad. Sci. USA, 93, 13286–13291]. The mice may serve as a model for studying AChR‐related myasthenic diseases. The postnatal development of the subsynaptic apparatus takes place in the absence of the adult type, ε‐subunit‐containing receptors which normally replace the fetal γ‐subunit‐containing receptors. During later development the secondary folds of the postsynaptic membrane disappear concomitant with the decrease in AChR density, so that the flattened‐out membrane with its remaining nicotinic receptors is in close proximity to the subsynaptic cytoplasmatic compartment and the subsynaptic myonuclei. The decrease in AChR concentration is correlated with a decrease of postsynaptic rapsyn, but has less effect on agrin, a neuronally released aggregating factor for AChRs. Thus, despite the presence of agrin at the synapse, AChR expression is not maintained at the level required to stabilize normal synaptic structure comprising secondary postsynaptic membrane folds. Collectively the results suggest that the postnatal switch from the global, activity‐sensitive γ‐subunit gene transcription to the synapse‐specific, activity‐independent ε‐subunit gene transcription is not required for the formation and differentiation of synapses but is essential for the maintenance of the highly organized structure of the neuromuscular endplate.