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

Abstract

Myoblast fusion is essential to skeletal muscle development and repair. We have demonstrated previously that human myoblasts hyperpolarize, before fusion, through the sequential expression of two K+ channels: an ether-à-go-go and an inward rectifier. This hyperpolarization is a prerequisite for fusion, 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 α1H 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 for both Kir candidates increased during myoblast differentiation. Single-channel recordings of undifferentiated myoblasts overexpressing Kir2.1 or Kir2.2 indicated that only the conductance of Kir2.1 corresponds to that of the endogenous channel. Inhibition of Kir2.1 expression with an antisense-Kir2.1-RNA expressed from transfected vector drastically reduced the endogenous inward rectifier 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.