Mice with a Null Allele for NaV1.4 Exhibit Pseudo-Myasthenia, but are not Susceptibile to Periodic Paralysis

Abstract

Functional analysis of disease-associated mutant NaV1.4 channels have revealed defects that often provide a mechanistic basis for derangements in sarcolemma excitability to produce the clinical phenotype. Loss-of-function mutations are less common and the pathogenic basis for muscle dysfunction is less clear than for the gain-of-function NaV1.4 mutations associated with myotonia, paramyotonia congenita or hyperkalemic periodic paralysis. Loss-of-function via enhanced inactivation (both fast and slow) accompanies other channel defects in hypokalemic periodic paralysis (HypoKPP) and is the primary defect in a rare form of congenital myasthenic syndrome. We explored whether a NaV1.4 loss-of-function created in a mouse would produce features of HypoKPP or myasthenia.A null allele of NaV1.4 was generated by excision of exon 12 (174 bp which encodes DIIS2-DIIS4). Heterozygous (+/-) mice were viable, but homozygous nulls (-/-) did not survive beyond P2. Spontaneous movement, feeding, and reproduction were normal in (+/-) mice. Voltage-clamp studies of dissociated fibers showed a 50% reduction of Ina in (+/-) animals. The Na current was completely blocked by 200 nM TTX, suggesting there was no compensatory upregulation of NaV1.5. In vitro contraction testing showed a right shift of the force- stimulus intensity curve and a prominent sag from the early peak force for submaximal stimulus intensities in (+/-) mice. Repetitive nerve stimulation did not elicit a decremental response in the compound muscle AP (CMAP) at baseline, but exposure to low-dose curare (0.5 uM) revealed a larger decrement for (+/-) than WT. These features are consistent with mild myasthenia. Challenge with low K did not diminish maximal tetanic force, nor did insulin & glucose infusion cause a decrease of CMAP amplitude, both of which were observed in our R669H HypoKPP mice.