Myasthenic syndrome AChRα C-loop mutant disrupts initiation of channel gating

Congenital myasthenic syndromes (CMSs) are neuromuscular disorders that can be caused by defects in ace­tylcholine receptor (AChR) function. Disease-associated point mutants can reveal the unsuspected functional significance of mutated residues. We identified two pathogenic mutations in the extracellular domain of the AChR α subunit (AChRα) in a patient with myasthenic symptoms since birth: a V188M mutation in the C-loop and a heteroallelic G74C mutation in the main immunogenic region. The G74C mutation markedly reduced surface AChR expression in cultured cells, whereas the V188M mutant was expressed robustly but had severely impaired kinetics. Single-channel patch-clamp analysis indicated that V188M markedly decreased the apparent AChR channel opening rate and gating efficiency. Mutant cycle analysis of energetic coupling among conserved residues within or dispersed around the AChRα C-loop revealed that V188 is functionally linked to Y190 in the C-loop and to D200 in β-strand 10, which connects to the M1 transmembrane domain. Furthermore, V188M weakens inter-residue coupling of K145 in β-strand 7 with Y190 and with D200. Cumulatively, these results indicate that V188 of AChRα is part of an interdependent tetrad that contributes to rearrangement of the C-loop during the initial coupling of agonist binding to channel gating

Choline acetyltransferase mutations cause myasthenic syndrome associated with episodic apnea in humans

Choline acetyltransferase (CHAT; EC 2.3.1.6) catalyzes the reversible synthesis of acetylcholine (ACh) from acetyl CoA and choline at cholinergic synapses. Mutations in genes encoding ChAT affecting motility exist in Caenorhabditis elegans and Drosophila, but no CHAT mutations have been observed in humans to date. Here we report that mutations in CHAT cause a congenital myasthenic syndrome associated with frequently fatal episodes of apnea (CMS-EA). Studies of the neuromuscular junction in this disease show a stimulation-dependent decrease of the amplitude of the miniature endplate potential and no deficiency of the ACh receptor. These findings point to a defect in ACh resynthesis or vesicular filling and to CHAT as one of the candidate genes. Direct sequencing of CHAT reveals 10 recessive mutations in five patients with CMS-EA. One mutation (523insCC) is a frameshifting null mutation. Three mutations (1305T, R420C, and E441K) markedly reduce ChAT expression in COS cells. Kinetic studies of nine bacterially expressed ChAT mutants demonstrate that one mutant (E441K) lacks catalytic activity, and eight mutants (L210P, P211A, 1305T, R420C, R482G, S498L, V506L, and R560H) have significantly impaired catalytic efficiencies.</p

Mutations causing congenital myasthenia reveal principal coupling pathway in the acetylcholine receptor epsilon-subunit

WOS: 000423337400011PubMed ID: 29367459We identify 2 homozygous mutations in the epsilon-subunit of the muscle acetylcholine receptor ( AChR) in 3 patients with severe congenital myasthenia: epsilon R218W in the pre-M1 region in 2 patients and epsilon E184K in the beta 8-beta 9 linker in 1 patient. Arg218 is conserved in all eukaryotic members of the Cysloop receptor superfamily, while Glu184 is conserved in the alpha-, delta-, and epsilon-subunits of AChRs from all species. epsilon R218W reduces channel gating efficiency 338-fold and AChR expression on the cell surface 5-fold, whereas epsilon E184K reduces channel gating efficiency 11-fold but does not alter AChR cell surface expression. Determinations of the effective channel gating rate constants, combined with mutant cycle analyses, demonstrate strong energetic coupling between epsilon R218 and epsilon E184, and between eR218 and epsilon E45 from the beta 1-beta 2 linker, as also observed for equivalent residues in the principal coupling pathway of the alpha-subunit. Thus, efficient and rapid gating of the AChR channel is achieved not only by coupling between conserved residues within the principal coupling pathway of the alpha-subunit, but also between corresponding residues in the epsilon-subunit.NIH grantUnited States Department of Health & Human ServicesNational Institutes of Health (NIH) - USA [NS 6277, NS031744]This work was supported in part by NIH grants NS 6277 to AGE and NS031744 to SMS