Human myoblast fusion requires expression of functional inward rectifier Kir2.1 channels

Myoblast fusion is essential to skeletal muscle development and repair. We have demonstrated previously that human myoblasts hyperpolarize, before: fu sion, through the sequential expression of two K+ channels:an ether-a-go-go and an inward rectifier. This hyperpolarization is a prerequisite for fusi on, as it sets the resting membrane potential in a range at; which Ca2+ can enter myoblasts and thereby trigger fusion via a window current through al pha 1H T channels. This work was undertaken to elucidate the molecular identity of the inward rectifier (Kir) channel involved in fusion. Using RNase protection assays, we detected mRNA for Kir2.1 and Kir2.2. Transcript levels fbr both Kir cand idates increased during myoblast differentiation. Single-channel recordings of undifferentiated myoblasts overexpressing Kir2.1 or Kir2.2 indicated th at only the conductance of Kir2.1 corresponds to that of the endogenous cha nnel. Inhibition of Kir2.1. expression with an antisense-Kir2.1-RNA express ed from transfected vector drastically reduced the endogenous inward rectif ier current and blocked fusion. In contrast, an antisense-Kir2.2-RNA had no effect on fusion. Taken together, our results demonstrate that functional Kir2.1 channels are required for human myoblast fusion.