Human Nek7-interactor RGS2 is required for mitotic spindle organization

The mitotic spindle apparatus is composed of microtubule (MT) networks attached to kinetochores organized from 2 centrosomes (a.k.a. spindle poles). In addition to this central spindle apparatus, astral MTs assemble at the mitotic spindle pole and attach to the cell cortex to ensure appropriate spindle orientation. We propose that cell cycle-related kinase, Nek7, and its novel interacting protein RGS2, are involved in mitosis regulation and spindle formation. We found that RGS2 localizes to the mitotic spindle in a Nek7-dependent manner, and along with Nek7 contributes to spindle morphology and mitotic spindle pole integrity. RGS2-depletion leads to a mitotic-delay and severe defects in the chromosomes alignment and congression. Importantly, RGS2 or Nek7 depletion or even overexpression of wild-type or kinase-dead Nek7, reduced γ-tubulin from the mitotic spindle poles. In addition to causing a mitotic delay, RGS2 depletion induced mitotic spindle misorientation coinciding with astral MT-reduction. We propose that these phenotypes directly contribute to a failure in mitotic spindle alignment to the substratum. In conclusion, we suggest a molecular mechanism whereupon Nek7 and RGS2 may act cooperatively to ensure proper mitotic spindle organization.

有丝分裂纺锤体装置由附着于动粒的微管（microtubule, MT）网络构成，该网络由两个中心体（亦称纺锤体极）组装而成。除这一核心纺锤体装置外，星状微管（astral MT）会在有丝分裂纺锤体极处组装，并附着于细胞皮层，以确保纺锤体定向的正确性。我们提出，细胞周期相关激酶Nek7及其新型互作蛋白RGS2参与有丝分裂调控与纺锤体形成。研究发现，RGS2以Nek7依赖的方式定位于有丝分裂纺锤体，并与Nek7共同维持纺锤体形态与有丝分裂纺锤体极的完整性。敲低RGS2会导致有丝分裂延迟，并在染色体排列与集积过程中引发严重缺陷。值得注意的是，敲低RGS2或Nek7，甚至过表达野生型或激酶失活性的Nek7，均会减少纺锤体极处的γ-微管蛋白（γ-tubulin）。除引发有丝分裂延迟外，敲低RGS2还会诱导有丝分裂纺锤体定向错误，同时伴随星状微管数量减少。我们认为，这些表型直接导致有丝分裂纺锤体无法正确定向于细胞基质。综上，我们提出一种分子机制：Nek7与RGS2可协同发挥作用，以确保有丝分裂纺锤体的正确组装。