Muscle degeneration and
myotonia are clinical hallmarks of
myotonic dystrophy type 1 (DM1), a multisystemic disorder caused by a CTG repeat expansion in the
3' untranslated region of the
myotonic dystrophy protein kinase (DMPK) gene. Transgenic mice engineered to express
mRNA with expanded (CUG)(250) repeats (HSA(LR) mice) exhibit prominent
myotonia and altered splicing of muscle
chloride channel gene (Clcn1) transcripts. We used whole-cell patch clamp recordings and nonstationary noise analysis to compare and biophysically characterize the magnitude, kinetics, voltage dependence, and single channel properties of the skeletal muscle
chloride channel (ClC-1) in individual flexor digitorum brevis (FDB) muscle fibers isolated from 1-3-wk-old wild-type and HSA(LR) mice. The results indicate that peak ClC-1 current density at -140 mV is reduced >70% (-48.5 +/- 3.6 and -14.0 +/- 1.6 pA/pF, respectively) and the kinetics of channel deactivation increased in FDB fibers obtained from 18-20- d-old HSA(LR) mice. Nonstationary noise analysis revealed that the reduction in ClC-1 current density in HSA(LR) FDB fibers results from a large reduction in
ClC-1 channel density (170 +/- 21 and 58 +/- 11 channels/pF in control and HSA(LR) fibers, respectively) and a modest decrease in maximal channel open probability(0.91 +/- 0.01 and 0.75 +/- 0.03, respectively). Qualitatively similar results were observed for
ClC-1 channel activity in knockout mice for muscleblind-like 1 (Mbnl1(DeltaE3/DeltaE3)), a second murine model of DM1 that exhibits prominent
myotonia and altered Clcn1 splicing (Kanadia et al., 2003). These results support a molecular mechanism for
myotonia in DM1 in which a reduction in both the number of functional sarcolemmal ClC-1 and maximal channel open probability, as well as an acceleration in the kinetics of channel deactivation, results from CUG repeat-containing
mRNA molecules sequestering Mbnl1
proteins required for proper CLCN1
pre-mRNA splicing and
chloride channel function.