B-type plexins regulate mitosis via RanGTPase.

Aberrant mitosis can result in aneuploidy and cancer. The small GTPase, Ran, is a key regulator of mitosis. B-type Plexins regulate Ran activity by acting as RanGTPase activating proteins (GAPs) and have been implicated in cancer progression. However, whether B-type plexins have a role in mitosis has not so far been investigated. We show here that PlexinB1 functions in the control of mitosis. Depletion of PlexinB1 resulted in defects in chromosomal segregation, increasing cells with multipolar spindles and prolonging metaphase, while activation of PlexinB1/B2 with Sema4D produced aberrant spindles. An increase in MAD1-positive kinetochores in PlexinB1/B2 depleted cells indicated application of spindle assembly checkpoint (SAC), possibly resulting from faulty spindle-microtubule assembly, and PlexinB1 depletion promoted regrowth of acentrosomal microtubules and defects in spindle pole refocussing. Depleted cells that had bypassed SAC possessed lagging chromosomes and chromosomal bridges. The mitotic defects observed upon PlexinB1 depletion were rescued by an RCC1 inhibitor, indicating that PlexinB1 signals, at least in part, via Ran to affect mitosis. These errors in mitosis generated multinucleate cells, and nuclei of altered morphology and abnormal karyotype. Furthermore, Sema4D-treatment increased the percentage of cells with micronuclei, precursors of chromothripsis. Defects in B-type plexins may contribute to the well-established role of plexins in cancer progression by inducing chromosomal instability.