Contributors to Pathologic Depolarization in Myotonia Congenita
Abstract
Myotonia congenita is an inherited skeletal muscle disorder caused by loss-of-function mutation in the CLCN1 gene. This gene encodes the ClC-1 chloride channel, which is almost exclusively expressed in skeletal muscle where it acts to stabilize the resting membrane potential. Loss of this chloride channel leads to skeletal muscle hyperexcitability, resulting in involuntary muscle action potentials (myotonic discharges) seen clinically as muscle stiffness (myotonia). Stiffness affects the limb and facial muscles, though specific muscle involvement can vary between patients. Interestingly, respiratory distress is not part of this disease despite muscles of respiration such as the diaphragm muscle also carrying this mutation. In addition to stiffness, patients experience episodes of transient weakness that remained poorly understood despite years of study. Current treatment focuses on use-dependent block of sodium channels, with off-label use of the antiarrhythmic mexiletine and the antianginal ranolazine. We performed intracellular recordings from muscle of both genetic and pharmacologic mouse models of myotonia congenita to identify the mechanism underlying transient weakness. Our recordings revealed transient depolarizations (plateau potentials) of the membrane potential to –25 to –35 mV in our mouse models. Na+ persistent current (NaP) through Nav1.4 channels is the key trigger of plateau potentials. Inhibition of Nav1.4 with Ranolazine eliminates transient weakness in vivo and prevents the development of plateau potentials. These data suggest that targeting NaP may be an effective v treatment to prevent attacks of transient weakness in myotonia congenita. We also performed intracellular, and ex vivo force recordings on the diaphragm muscle of our myotonic mouse model and found that the diaphragm muscle is immune to myotonic discharges and relatively spared from myotonia. Through comparison with myotonia-affected EDL we propose that some of the resistance of diaphragm to myotonia is due to smaller fiber size. This suggests that muscles affected by myotonia may depend on fiber size and may explain lessening symptoms with age. Lastly, we compared use-dependent block of transient Na+ currents (NaT) via ranolazine and mexiletine with tonic block of NaP with μ-conotoxin GIIIA. Our data suggests the use-dependent block of NaT is not required for treatment and that tonic block of NaP may be superior because it eliminates myotonic discharges without drastically altering action potential characteristics.
