Unconventional myosin VI is involved in regulation of muscle energy metabolism

Mitochondria are essential for the regulation of the metabolic state of skeletal muscle, making their structure and function crucial for muscle performance. Myosin VI (MVI), an unconventional minus-end-directed motor, is expressed in skeletal muscle and myogenic cells. To explore its role in mitochondrial function and muscle metabolism, we used MVI knockout mice (<i>Snell's waltzer</i>, <i>SV</i> MVI-KO) and their heterozygous littermates. We analyzed muscle samples from newborn (P0) and adult mice (3- and 12-mo-old) and found that both MVI mRNA and protein levels were highest in newborn muscles and decreased with age. MVI expression also varied by muscle type, being highest in the slow-twitch soleus muscle (SOL) of adult mice. Loss of MVI had the most significant effects on SOL, which contains the highest number of mitochondria compared with fast-twitch muscles. MVI loss resulted in reduced respiratory capacity and adenosine-5'-triphosphate production in myogenic cells, indicating impaired mitochondrial function. Furthermore, MVI deficiency caused a shift from glycolytic to oxidative fiber types, especially in SOL. We also observed increased phospho-AMP-activated protein kinase levels in MVI-KO SOL across all time points, along with downregulation of the mammalian target of rapamycin pathway and upregulation of proteins involved in lipolysis. These findings highlight MVI as a novel regulator of metabolic processes in skeletal muscle.<b>NEW & NOTEWORTHY</b> Myosin VI (MVI), a motor protein, regulates mitochondrial function and metabolism in skeletal muscle. In MVI-knockout mice, its expression peaked in neonatal muscles and remained highest in adult soleus. MVI loss impaired mitochondrial respiration, reduced ATP production, and promoted a shift toward oxidative fibers. It also increased AMPK, suppressed mTOR signaling, and altered lipid metabolism, including reduced triacylglycerol levels. These findings reveal MVI's important role in energy balance, protein synthesis, and lipid regulation in muscle.