KIFC1 overexpression induces monopolar spindles by preventing centrosome separation during rapid cleavage divisions

Bipolar spindle assembly is essential for accurate chromosome segregation. KIFC1, a conserved Ran- regulated minus-end-directed kinesin-14 motor, accumulates in the nucleus during interphase and promotes chromatin-mediated spindle assembly during mitosis and meiosis. In human oocytes, reduced KIFC1 levels destabilize meiotic spindles, a defect that can be rescued by increasing KIFC1 expression. However, how KIFC1 expression levels affect mitotic spindle stability during cleavage divisions in vertebrates remains unclear. Here, we show that whereas an approximately 50% reduction in KIFC1 causes no detectable defects in spindle assembly, approximately 10-fold overexpression of KIFC1 induces monopolar spindle formation, leading to chromosome mis-segregation and embryonic lethality in medaka early embryos. KIFC1 overexpression results in ectopic centrosomal localization during interphase, impairing the separation of duplicated centrosomes before mitotic entry. Analyses of KIFC1 mutants demonstrated that these centrosome separation defects require KIFC1s microtubule-binding and motor activities and are further enhanced by deletion of KIFC1s nuclear localization sequences. Together, our findings demonstrate that tight regulation of KIFC1 expression and its nuclear sequestration is essential for the proper separation and positioning of duplicated centrosomes before mitotic entry, thereby ensuring efficient bipolar spindle assembly during the rapid cleavage divisions of vertebrate embryos. HighlightsO_LIKIFC1 accumulates in the nucleus and at the embryonic spindle midplane via the Ran pathway. C_LIO_LIPartial KIFC1 depletion does not impair spindle assembly in medaka early embryos. C_LIO_LIKIFC1 overexpression induces monopolar spindles by preventing centrosome separation. C_LIO_LICentrosome separation defects require KIFC1 microtubule-binding and motor activity. C_LI