| The nicotinic acetylcholine receptor (nAChR) is a ligand-gated ion channel responsible for the transmission of action potentials across cholinergic synapses. The nAChR is a membrane protein and as such lipid-protein interactions are of fundamental importance in the regulation of its function. Alterations in the structure of the nAChR due to mutations may result in modifications in functionality which deviates from that of the wild type (WT) nAChR. In addition to structural alterations, because the nAChR is a membrane-bound protein, alterations in the membrane lipid composition may also lead to modifications in the functionality of the nAChR. Herein, the results of an investigation aimed at providing insight into the mechanisms by which structural alterations in regions of the nAChR which are presumably exposed to the lipid as well as alterations in the cholesterol content of cell membranes affect the functionality of the nAChR. By combining an array of approaches, alterations in channel kinetics, conductance, and expression profiles were studied as potential causes for the increase in macroscopic current which follows membrane cholesterol depletion in oocytes expressing the alphaC418W nAChR, which is the first lipid-exposed nAChR mutation known to produce a congenital myasthenic syndrome (CMS) in humans. In addition, the functional consequences of increasing the membrane cholesterol content of oocytes expressing the electric ray organ nAChR, the muscle-type nAChR or the neuronal nAChR were assessed to determine the relative tolerance of these subtypes of receptors to increases in cholesterol content which parallel those observed in physiological scenarios related to aging. Interestingly, while the electric ray organ and neuronal nAChRs are relatively intolerant to physiologically relevant increases in cholesterol content, the muscle-type nAChR remain functional upon the same increase in cholesterol content. These results suggest that the muscle-type nAChR may have developed a mechanism to remain functional even at the higher cholesterol concentrations that accompany aging. |
