A mutation in the Muscle Nicotinic Receptor Alpha Subunit Leads to Slow Channel Syndrome Through Interaction with Gamma and Not the Epsilon Subunit

Abstract

Slow channel syndrome (SCS), a congenital form of myasthenia, results from a gain of function mutation in the muscle nicotinic acetylcholine receptor (AChR). The mutant line twister represents a zebrafish equivalent of SCS as a result of a L258P mutation in the M2 region of the α-subunit. Zebrafish heterozygous for the twister mutation lack the ability to perform coordinated swimming functions during early development, presumably due to the greatly prolonged end-plate currents (EPC) resulting from altered AChR kinetics. EPCs recorded from fast skeletal muscle of twister decay along a triple exponential time course (1.1, 8.6 and 70.2 ms) compared to a single exponential decay typical of wild-type muscle. Paradoxically, swimming behavior improves during development. The improvement coincides with the selective disappearance of the slowest component of synaptic decay. We recorded single channel properties of heterologously expressed zebrafish subunits αtwi along with β,δ,ε or γ subunits to identify the basis of this slowest component. αtwiβδε receptors exhibit a mean open time that corresponds to the intermediate time constant of twister decay. By contrast, the embryonic receptor isoform αtwiβδγ exhibited a greatly prolonged open time that corresponded to the slowest component of synaptic decay in twister. This result is unexpected given the similarity in mean open times for wild type αβδε (0.5 ms) and αβδγ (1.2 ms) receptors. We propose that behavioral and functional recovery results from the observed developmental decrease in γ subunit mRNA expression, facilitating a switch from αtwiβδγ to αtwiβδε receptors. In support of this idea, morpholino RNA knockdown of γ subunit expression improved swimming performance and reduced the contribution of the slow component to synaptic current decay, presumably through an increase in αtwiβδε AChRs.