Of mice and men: Relevance of cellular and molecular characterizations of myosin IIA to MYH9-related human disease.

Non-muscle myosin II has diverse functions in cell contractility, morphology, cytokinesis and migration. Mammalian cells have three isoforms of non-muscle myosin II, termed IIA, IIB and IIC, encoded by three different genes. These isoforms share considerable homology and some overlapping functions, yet they exhibit differences in enzymatic properties, subcellular localization, molecular interaction and tissue distribution.(1-6) Our studies have focused on the IIA isoform, and they reveal unique regulatory roles in cell-cell adhesion and cell migration that are associated with cross-talk of the actomyosin system with microtubules. In humans, various mutations in the MYH9 gene that encodes the myosin IIA heavy chain cause autosomal dominant disease, whereas in mice, the complete deficiency is embryonic lethal but heterozygous mice are nearly normal. We discuss here the differences between mouse and human phenotypes and how the wealth of mechanistic knowledge about myosin II based on in vitro studies and mouse models can help us understand the molecular and cellular pathophysiology of myosin IIA deficiency in humans.