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 (KATP) 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 KATP 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 Kir6.1V65M or SUR2A478V, to assess KATP 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 KATP channel density in both genotypes, but enhanced Mg-nucleotide activation only in SUR2A478V fibers, consistent with muscle KATP channels being formed predominantly as complexes of SUR2A and Kir6.2 subunits. Ex vivo testing of isolated SUR2A478V, but not Kir6.1V65M, 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 SUR2A478V muscles in the presence of the FDA-approved KATP channel inhibitor glibenclamide, indicating that the increased fatigue of isolated muscles is a direct consequence of overactive sarcolemmal KATP channels. These results shed light on the pathophysiologic relevance of SUR2-dependent KATP channel subunits in skeletal muscle and highlight their role in fatiguing conditions, as well as identifying potential therapeutic benefit of skeletal muscle KATP inhibition in CS.