Huntington disease skeletal muscle is hyperexcitable owing to chloride and potassium channel dysfunction

Abstract

Huntington disease is a progressive and fatal genetic disorder with debilitating motor and cognitive defects. Chorea, rigidity, dystonia, and muscle weakness are characteristic motor defects of the disease that are commonly attributed to central neurodegeneration. However, no previous study has examined the membrane properties that control contraction in Huntington disease muscle. We show primary defects in ex vivo adult skeletal muscle from the R6/2 transgenic mouse model of Huntington disease. Action potentials in diseased fibers are more easily triggered and prolonged than in fibers from WT littermates. Furthermore, some action potentials in the diseased fibers self-trigger. These defects occur because of decreases in the resting chloride and potassium conductances. Consistent with this, the expression of the muscle chloride channel, ClC-1, in Huntington disease muscle was compromised by improper splicing and a corresponding reduction in total Clcn1 (gene for ClC-1) mRNA. Additionally, the total Kcnj2 (gene for the Kir2.1 potassium channel) mRNA was reduced in disease muscle. The resulting muscle hyperexcitability causes involuntary and prolonged contractions that may contribute to the chorea, rigidity, and dystonia that characterize Huntington disease.

Discovery of CLC transport proteins: cloning, structure, function and pathophysiology

“…It results in a loss of function because ClC-1 will now be largely closed at physiological voltages and can therefore neither repolarize action potentials, nor stabilize the resting voltage. Interestingly, abnormal splicing of CLCN1 contributes to myotonia in myotonic dystrophy (Charlet et al 2002;Mankodi et al 2002) and in Huntington's disease (Waters et al 2013), which are caused by trinucleotide repeats in different genes. A recent study reporting that CLCN1 polymorphisms may contribute to epilepsy (Chen et al 2013) has met with skepticism because ClC-1 shows very low expression in brain (Steinmeyer et al 1991b) and because loss of ClC-1 function leads to myotonia, but not epilepsy.…”

Inhibiting persistent inward sodium currents prevents myotonia

“…Because of their small size (400–500μm length × ∼40μm diameter), we were able to control whole FDB fibers under voltage clamp. We have previously used this technique to record larger and faster chloride and potassium currents 23, 24. During ramp depolarizations at a rate of 10mV/s in a normal K + solution with 20μM nifedipine to block Ca v 1.1 channels (Fig 2A), we recorded an inward deflection in current consistent with the activation of a PIC in ClC FDB fibers (see Fig 2B, C).…”

“…Isolation of fibers and recordings were performed as previously described 23, 24. Briefly, muscles were surgically removed, pinned to Sylgard‐bottomed Petri dishes, and enzymatically dissociated at 37°C under mild agitation for ∼1 hour using 1,000U/ml of collagenase type IV (Worthington Biochemical, Lakewood, NJ).…”

The novel KMO inhibitor CHDI-340246 leads to a restoration of electrophysiological alterations in mouse models of Huntington's disease

“…Hyperpolarization, decreased input resistance, and increased rheobase current are all consistent with a CHDI-340246 -induced increase in potassium conductance, key ion channels which underlie pathological alterations in HD cells (Ariano et al, 2005;Cepeda et al, 2003;Tong et al, 2014;Waters et al, 2013). Indeed, clamping R6/2 SPNs at Vh further from putative E K (-90 mV) exacerbated the effects of CHDI-340246 on changes in Rm ( Figure 7A), while clamping the cells at -95 mV, close to E K , abrogated the effect, supporting modulation of an underlying potassium conductance as responsible for the improvement.…”