UC Berkeley

A bipolar spindle functions to segregate chromosomes during cell division in all eukaryotes, yet spindle size and microtubule organization vary dramatically across different species and cell types. Targeting protein for Xklp2 (TPX2) is one candidate factor for modulating spindle architecture through its roles in branching microtubule nucleation, activation of the mitotic kinase Aurora A, and association with the kinesin-5 (Eg5) motor.

Xenopus egg extracts provide an ideal system to study the role of TPX2 in defining spindle architecture since in vitro reactions combining metaphase-arrested extracts and sperm nuclei recapitulate meiotic spindle assembly and can be biochemically manipulated. Here, we characterize a conserved nuclear localization sequence (NLS) motif, 123-KKLK-126, in X. laevis TPX2, which regulates astral microtubule formation and spindle pole morphology in Xenopus egg extracts. Addition of recombinant TPX2 with this sequence mutated to AALA dramatically increased spontaneous formation of microtubule asters and recruitment of phosphorylated Aurora A, Eg5 and pericentrin to meiotic spindle poles.

TPX2 is frequently overexpressed in aggressive human cancers, and it is not well understood how overexpression of TPX2 affects spindle assembly and function. We analyzed the spindle morphology of human cell lines and showed that cells over-expressing the oncogene MYC and TPX2 possess shorter spindles with increased TPX2 localization at spindle poles. These results indicate that TPX2 alters spindle length and morphology in cancer cells, which may contribute their ability to divide despite MYC-induced mitotic stress and are consistent with previous observations in Xenopus egg extract showing that high levels of TPX2 shrink meiotic spindles.

We propose that TPX2 is a lynchpin spindle assembly factor that contributes to the recruitment and activity of microtubule nucleation factors, regulating spindle size and generating distinct spindle architectures.