Modulated microtubule dynamics enable Hklp2/Kif15 to assemble bipolar spindles

Activity of the sliding motor Eg5 and coordinated microtubule dynamics are both essential for mitotic spindle pole separation. It is still a matter of controversy if changes in microtubule dynamics can compensate inhibition of Eg5 activity and re-enable bipolarization. Using a consistent live cell-imaging approach, we show that perturbation of microtubule dynamics can compensate inhibition of Eg5 through a spindle formation process reminiscent of meiosis: In Eg5-inhibited mammalian somatic cells, alteration of microtubule dynamics through depletion of TOGp or low doses of nocodazole induces the formation of multiple acentrosomal spindle poles which pass through an intermediate multipolar state followed by bipolarization. Pole separation depends on Hklp2/Kif15, an otherwise dispensable plus end-directed spindle motor and results in spindles with two centrosomal poles. Once bipolar, spindles do not rely on altered microtubule dynamics to maintain their bipolarity anymore and are functional in chromosome segregation. We conclude that altered microtubule dynamics enable Hklp2/Kif15 to replace Eg5 in pole separation through a mechanism involving the formation of acentrosomal poles. Our observations suggest that combination chemotherapy regimens involving microtubule-targeting drugs and Eg5 inhibitors might be less effective than expected.