MAP and Kinesin dependent nuclear positioning is required for skeletal muscle function

The basic unit of skeletal muscle in all metazoans is the multinucleate myofiber, within which individual nuclei are regularly positioned(1). The molecular machinery responsible for myonuclear positioning is not known. Improperly positioned nuclei are a hallmark of numerous muscles diseases(2), including centronuclear myopathies(3), but it is unclear whether correct nuclear positioning is necessary for muscle function. Here we identify the microtubule-associated protein Ensconsin(Ens)/MAP7 and Kinesin Heavy Chain (Khc)/Kif5b as essential, evolutionary conserved regulators of myonuclear positioning in Drosophila and cultured mammalian myotubes. We find that these proteins physically interact and that expression of the Kif5b motor domain fused to the MAP7 microtubule binding domain rescues nuclear positioning defects in MAP7 depleted cells. This suggests that MAP7 links Kif5b to the microtubule cytoskeleton to promote nuclear positioning. Finally we demonstrate that myonuclear positioning is physiologically important. Drosophila ens mutant larvae display decreased locomotion and incorrect myonuclear positioning, and these phenotypes are rescued by muscle specific expression of Ens. We conclude that improper nuclear positioning contributes to muscle dysfunction in a cell autonomous fashion.