Myosin 10 supports mitotic spindle bipolarity by promoting PCM integrity and supernumerary centrosome clustering

Myosin 10 (Myo10) is a member of the MyTH4/FERM domain family of unconventional, actin-based motor proteins. Studies have implicated Myo10 in supporting cell adhesion via its integrin-binding FERM domain, and spindle positioning and spindle pole integrity via its microtubule-binding MyTH4 domain. Here we characterized Myo10’s contribution to mitosis using Myo10 knockout HeLa cells and MEFs isolated from a Myo10 knockout mouse. Most notably, both of these knockout cells exhibit a pronounced increase in the frequency of multipolar spindles. Staining of unsynchronized metaphase cells showed that the primary driver of spindle multipolarity in knockout MEFs and knockout HeLa cells lacking supernumerary centrosomes is PCM fragmentation, which creates y-tubulin-positive, centriole-negative microtubule asters that serve as additional spindle poles. For HeLa cells possessing supernumerary centrosomes, Myo10 depletion further accentuates spindle multipolarity by impairing centrosome clustering. These results argue, therefore, that Myo10 supports spindle bipolarity by maintaining PCM integrity in both normal and cancer cells, and by promoting supernumerary centrosome clustering in cancer cells. Finally, we present evidence that the defect in spindle pole integrity in Myo10 knockout cells is likely due to a defect in pole stability rather than pole maturation, and that Myo10 promotes supernumerary centrosome clustering at least in part by promoting cell adhesion during mitosis.