Serum choline activates mutant acetylcholine receptors that cause slow channel congenital myasthenic syndromes

Abstract

We have found that mutant acetylcholine receptor channels (AChRs) that cause slow-channel congenital myasthenic syndromes are activated by serum and that the high frequency of openings in serum is reduced by treatment with choline oxidase. Thus, slow-channel congenital myasthenic syndrome AChRs at the neuromuscular junction are likely to be activated both by steady exposure to serum choline and by transient exposure to synaptically released transmitter. Single-channel kinetic analyses indicate that the increased response to choline is caused by a reduced intrinsic stability of the closed channel. The results suggest that a mutation that destabilizes the inactive conformation of the AChR, together with the sustained exposure of endplates to serum choline, results in continuous channel activity that contributes to the pathophysiology of the disease.

Function of Interfacial Prolines at the Transmitter-binding Sites of the Neuromuscular Acetylcholine Receptor

“…In effect, the CMS mutation turns the AChR from a two-site to a one-site receptor. The primary effect of many other CMS mutations is to decrease ⌬G 0 (31). The ⑀ProD2-L is an exception to this pattern because it has no effect on ⌬G 0 but a large one on ⌬G B ACh .…”

Arg-274 and Leu-277 of the γ-Aminobutyric Acid Type A Receptor α2 Subunit Define Agonist Efficacy and Potency

“…One laboratory (13) has used single channel analysis of ␣ 1 (K273E)␤ glycine receptors to measure an increase in the closing rate constant (Scheme 1, ␣), whereas other researchers have shown that ␣ 1 (R271Q) in the Gly-R reduces the single-channel conductance (25,40). Naturally occurring mutations of the TM2-TM3 loop in muscle nicotinic receptor subunits, which produce the slow-channel form of myasthenia gravis (41), alter the efficacy of the partial agonist choline by increasing the opening rate of the receptor (Scheme 1, ␤). The TM2-TM3 loop may therefore also play a role in the gating of other ligand-gated ion channels within this gene family.…”

Gain of Function Mutants: Ion Channels and G Protein-Coupled Receptors

“…Slow-Channel Syndrome Recent data suggest that slow-channel congenital myasthenic syndrome arises from a form of constitutive activation (Zhou et al 1999). Slow-channel congenital myasthenic syndrome is caused by dominant mutations in human muscle nicotinic acetylcholine receptors, resulting in muscle weakness and fatigue, especially revealed by staircase summation of the prolonged endplate potentials leading to a depolarization block at moderate rates (20-40 Hz) of stimulation.…”

“…Single-channel analysis from the Sine and Auerbach labs previously disclosed the microscopic bases for these shifted dose-response relations: In general, the pore mutations increase ACh-activated currents by increasing the openchannel duration, whereas mutations at or near the agonist binding site increase the rate of opening , Sine et al 1995, Engel et al 1996, Milone et al 1997. Zhou et al (1999) have shown, additionally, that several slow-channel mutations produce receptors activated by choline. The 50% effective concentration for choline is between 500 lM and 3 mM, which causes detectable activation at choline concentrations normally found in extracellular fluid; indeed, serum does activate the mutant receptors.…”