Hyperkalemic periodic paralysis (HyperKPP) produces myotonia and attacks of muscle weakness triggered by rest after exercise or by K+ ingestion. We introduced a missense substitution corresponding to a human familial HyperKPP mutation (Met1592Val) into the mouse gene encoding the skeletal muscle voltage-gated Na+ channel NaV1.4. Mice heterozygous for this mutation exhibited prominent myotonia at rest and muscle fiber-type switching to a more oxidative phenotype compared with controls. Isolated mutant extensor digitorum longus muscles were abnormally sensitive to the Na+/K+ pump inhibitor ouabain and exhibited age-dependent changes, including delayed relaxation and altered generation of tetanic force. Moreover, rapid and sustained weakness of isolated mutant muscles was induced when the extracellular K+ concentration was increased from 4 mM to 10 mM, a level observed in the muscle interstitium of humans during exercise. Mutant muscle recovered from stimulation-induced fatigue more slowly than did control muscle, and the extent of recovery was decreased in the presence of high extracellular K+ levels. These findings demonstrate that expression of the Met1592Val Na+ channel in mouse muscle is sufficient to produce important features of HyperKPP, including myotonia, K+-sensitive paralysis, and susceptibility to delayed weakness during recovery from fatigue.
Lawrence J. Hayward, Joanna S. Kim, Ming-Yang Lee, Hongru Zhou, Ji W. Kim, Kumudini Misra, Mohammad Salajegheh, Fen-fen Wu, Chie Matsuda, Valerie Reid, Didier Cros, Eric P. Hoffman, Jean-Marc Renaud, Stephen C. Cannon, Robert H. Brown Jr.
The intracellular signals that mediate skeletal muscle protein loss and functional deficits due to muscular disuse are just beginning to be elucidated. Previously we showed that the activity of an NF-κB–dependent reporter gene was markedly increased in unloaded muscles, and p50 and Bcl-3 proteins were implicated in this induction. In the present study, mice with a knockout of the p105/p50 (Nfkb1) gene are shown to be resistant to the decrease in soleus fiber cross-sectional area that results from 10 days of hindlimb unloading. Furthermore, the marked unloading-induced activation of the NF-κB reporter gene in soleus muscles from WT mice was completely abolished in soleus muscles from Nfkb1 knockout mice. Knockout of the B cell lymphoma 3 (Bcl3) gene also showed an inhibition of fiber atrophy and an abolition of NF-κB reporter activity. With unloading, fast fibers from WT mice atrophied to a greater extent than slow fibers. Resistance to atrophy in both strains of knockout mice was demonstrated clearly in fast fibers, while slow fibers from only the Bcl3–/– mice showed atrophy inhibition. The slow-to-fast shift in myosin isoform expression due to unloading was also abolished in both Nfkb1 and Bcl3 knockout mice. Like the soleus muscles, plantaris muscles from Nfkb1–/– and Bcl3–/– mice also showed inhibition of atrophy with unloading. Thus both the Nfkb1 and the Bcl3 genes are necessary for unloading-induced atrophy and the associated phenotype transition.
R. Bridge Hunter, Susan C. Kandarian