Pathogenesis of the slow-channel congenital myasthenic syndrome (SCCMS).

In collaboration with Dr. Christopher Gomez at the University of Chicago we investigate nicotinic neuromuscular receptor genes of patients with slow congenital myasthenic syndrome (SCCMS). SCCMS are a group of genetic disorders of neuromuscular transmission characterized by weakness and fatigability. The SCCMS disease is a channelopathy caused by mutations in the muscle acetylcholine receptor that give rise to excitotoxic degeneration of the neuromuscular junction. The SCCMS is characterized by a progressive degeneration of the neuromuscular junction and muscle atrophy leading to fatigability and weakness. In this research effort we are attempting to identify: 1) which spontaneous acetylcholine receptor (AChR) mutations give rise to the slow-channel syndrome (SCS); 2) the nature of the defect in AChR function that causes progressive degeneration of neuromuscular synapses; and 3) potential correlations between structural and functional alterations of neuromuscular junctions with the altered ion channel properties of each of these SCCMS mutations. Although all of the mutations appear to cause delayed closure or excessive opening of the acetylcholine receptor ion channel, the mechanisms responsible for weakness may differ, depending on the molecular and pathological consequences of the mutation. We are using several transgenic mouse lines as models of the SCS by expressing mutant acetylcholine receptor subunits in muscle developed in Dr. Gomez laboratory. One line, expressing a human SCCMS mutation develops the complete spectrum of abnormalities in the disease, including weakness, fatigability, impaired neuromuscular transmission, and calcium ion overload and degeneration of the neuromuscular junctions. We use confocal and two photon microscopy (www.cifupr.org) to look for potential correlations between the temporal course of structural and functional alterations of neuromuscular junctions with the altered ion channel properties of each of these transgenic mice lines (aL251T, dS268F, aV249F and aC418W). Mice are monitored at one week of age when tails are clipped for genotyping.  Mice expressing distinct mutations are compared with respect to clinical, histological, and electrophysiological changes, and AChR content throughout the course of the disease.  Strength testing and behavioral analysis are performed on three age groups 2-4, 6-8, and 9-12 month old. Assessment of AChR density by measuring a-Bungarotoxin binding sites, and sampling muscle for histology are performed on a separate group of mice, as are voltage-clamp recordings of diaphragm MEPCs and quantal content.  The goal of these experiments is to examine for potential correlations between the temporal course of structural and functional alterations of neuromuscular junctions with the altered ion channel properties of each of these SCCMS mutations.

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