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KEY POINTS: The physiological significance of the developmental switch from fetal to adult acetylcholine receptors in muscle (AChRs) and the functional impact of AChR clustering by rapsyn are still not well studied. Performing patch clamp experiments we show that recovery from desensitisation is faster in the adult AChR isoform. Recovery from desensitisation is determined by the AChR isoform specific cytoplasmic M3-M4 domain. The co-expression of rapsyn in muscle cells induced AChR clustering and facilitated recovery from desensitisation in both fetal and adult AChRs. In fetal AChRs, facilitation of recovery kinetics by rapsyn was independent of AChR clustering. These effects could be crucial adaptations to motor neuron firing rates that in rodents have been shown to increase around the time of birth when AChRs cluster at the developing neuromuscular junctions. ABSTRACT: The neuromuscular junction (NMJ) is the site of a number of autoimmune and genetic disorders, many involving the muscle-type nicotinic acetylcholine receptor (AChR), but there are aspects of normal NMJ development and function that need to be better understood. In particular, there are still questions regarding the implications of the developmental switch from fetal to adult AChRs and how their functions might be modified by rapsyn that clusters the AChRs. Desensitisation of human muscle AChRs was studied using the patch clamp technique to measure whole-cell currents in muscle-type (TE671/CN21) and non-muscle (HEK293) cell lines expressing either fetal or adult AChRs. Desensitisation time constants were similar with both AChR isoforms but recovery time constants were shorter in cells expressing adult compared to fetal AChRs (p<0.0001). Chimeric experiments showed that recovery from desensitisation was determined by the M3-M4 cytoplasmic loops of the γ- and ε-subunits. Expression of rapsyn in TE671/CN21 cells induced AChR aggregation and also, surprisingly, shortened recovery time constants in both fetal and adult AChRs. However, this was not dependent on clustering as rapsyn also facilitated recovery from desensitisation in HEK293 cells expressing a δ-R375H AChR mutant that in C2C12 myotubes did not form clusters. Thus, rapsyn interactions with AChRs lead not only to clustering but also to a clustering-independent faster recovery from desensitisation. Both effects of rapsyn could be a necessary adjustment to the motor neuron firing rates that increase around the time of birth. This article is protected by copyright. All rights reserved.

Original publication

DOI

10.1113/JP277819

Type

Journal article

Journal

J Physiol

Publication Date

03/06/2019