Muscle fatigue arising intrinsically from SUR2- but not Kir6.1-dependent gain-of-function in Cantu syndrome mice

Cantu syndrome (CS) is a rare disease caused by gain-of-function (GOF) mutations of Kir6.1 or SUR2 subunits of ATP-sensitive potassium (K_ATP) channels. CS patients with SUR2 and Kir6.1 variants display a similar constellation of symptoms, including muscle weakness and fatigue. The effects of CS mutations on skeletal muscle K_ATP channels, and any consequent direct effects on contractility, are currently unclear. Here, we used two knock-in mouse models of CS, respectively, carrying GOF mutations Kir6.1[V65M] or SUR2[A478V], to assess K_ATP channel properties and contractility in isolated fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus (SOL) muscles. Electrophysiological recordings in isolated myofibers showed normal resting potentials, and excised patch-clamp recordings showed normal K_ATP channel density in both genotypes, but enhanced Mg-nucleotide activation only in SUR2[A478V] fibers, consistent with muscle K_ATP channels being formed predominantly as complexes of SUR2A and Kir6.2 subunits. Ex vivo testing of isolated SUR2[A478V], but not Kir6.1[V65M], muscles showed an earlier onset of fatigue and a marked intra-tetanic decline of force compared with littermate controls. Importantly, normal contractile behavior was restored ex vivo and in vivo in SUR2[A478V] muscles in the presence of the FDA-approved K_ATP channel inhibitor glibenclamide, indicating that the increased fatigue of isolated muscles is a direct consequence of overactive sarcolemmal K_ATP channels. These results shed light on the pathophysiologic relevance of SUR2-dependent K_ATP channel subunits in skeletal muscle and highlight their role in fatiguing conditions, as well as identifying potential therapeutic benefit of skeletal muscle K_ATP inhibition in CS.