Members of the NIMA-related kinases (NRK) family are recently emerging as central regulators of various aspects of the cell cycle. However, the cellular roles of the mammalian NRK, Nek7, remain obscure. We show here that the endogenous Nek7 protein is enriched at the centrosome in a microtubuleindependent manner. Overexpression of wt or kinase-defective Nek7 resulted in cells of rounder appearance, and higher proportions of multinuclear and apoptotic cells. Down-regulation of Nek7 using a small interfering RNA approach resulted in a significant increase in mitotic cells presenting multipolar spindle phenotype. These results suggest a role for Nek7 in regulating proper spindle assembly and mitotic progression.

The mammalian NIMA-related kinases (Neks) are commonly referred to as mitotic kinases, although a definitive in vivo verification of this definition is largely missing. Reduction in the activity of Nek7 or its close paralog, Nek6, has previously been shown to arrest cells in mitosis, mainly at metaphase. In this study, we investigate the developmental and cellular roles of Nek7 kinase through the generation and analysis of Nek7-deficient mice. We show that absence of Nek7 leads to lethality in late embryogenesis or at early post-natal stages and to severe growth retardation. Mouse embryonic fibroblasts (MEFs) derived from Nek7 À/À embryos show increase tendency for chromosomal lagging, micronuclei formation and cytokinesis failure. Tetraploidy and aneuploidy were commonly observed and their prevalence arises with MEFs passages. The frequency of multicentrosomal cells in the mutant's MEF cells was higher, and it commonly occurred concurrently with a binuclear phenotype, suggesting cytokinesis failure etiology. Lastly, the percentage of mutant MEF cells bearing primary cilia (PC) was low, whereas a cell population having two cilia appeared in the mutant MEFs. Taken together, these results confirm Nek7 as a regulator of cell division, and reveal it as an essential component for mammalian growth and survival. The intimate connection between tetraploidy, aneuploidy and cancer development suggests that Nek7 deregulation can induce oncogenesis.