Myosin VI depletion delays neuromuscular junction maturation and exacerbates muscle performance.

Unconventional myosin VI (MVI) is an ATP-dependent actin-binding molecular motor that participates in numerous cellular and tissue functions, including striated muscle physiology. Lack of MVI expression significantly aberrates myogenesis and skeletal muscle metabolism, and alters myoblast adhesion, fusion, and cytoskeletal organisation. Concomitantly, MVI knockout mice display functional and structural cardiac defects. Here, for the first time, we investigate the impact of MVI on neuromuscular junctions (NMJs), the peripheral synapses crucial for skeletal muscle contraction. We show that MVI is enriched at the postsynaptic machinery of developing and adult NMJs. We analyse the morphology of NMJs of MVI knockout mice (Snell's waltzer, SV) during early developmental remodelling and show that MVI deficiency delays NMJ maturation in fast- and slow-twitch muscles. It also reduces the NMJ size of the soleus muscle, as demonstrated by the decreased morphological parameters of both presynaptic and postsynaptic compartments. Simultaneously, synaptic elimination remains unaffected after MVI knockout, suggesting that the observed phenotypes are innervation-independent. Lastly, depletion of MVI impairs the grip strength of both female and male SV/SV mice. In summary, our studies show that MVI is an important regulator of NMJ size and maturation, controls muscle performance, and its impact is independent of innervation and sex.