Genetic screens for cell division cycle mutants in the filamentous fungus Aspergillus nidulans led to the discovery of never-in-mitosis A (NIMA), a serine/threonine kinase that is required for mitotic entry. Since that discovery, NIMA-related kinases, or NEKs, have been identified in most eukaryotes, including humans where eleven genetically distinct proteins named NEK1 to NEK11 are expressed. Although there is no evidence that human NEKs are essential for mitotic entry, it is clear that several NEK family members have important roles in cell cycle control. In particular, NEK2, NEK6, NEK7 and NEK9 contribute to the establishment of the microtubule-based mitotic spindle, whereas NEK1, NEK10 and NEK11 have been implicated in the DNA damage response. Roles for NEKs in other aspects of mitotic progression, such as chromatin condensation, nuclear envelope breakdown, spindle assembly checkpoint signalling and cytokinesis have also been proposed. Interestingly, NEK1 and NEK8 also function within cilia, the microtubule-based structures that are nucleated from basal bodies. This has led to the current hypothesis that NEKs have evolved to coordinate microtubule-dependent processes in both dividing and non-dividing cells. Here, we review the functions of the human NEKs, with particular emphasis on those family members that are involved in cell cycle control, and consider their potential as therapeutic targets in cancer.

The NIMA-related kinases represent a family of serine/threonine kinases implicated in cell cycle control. The founding member of this family, the NIMA kinase of Aspergillus nidulans, as well as the fission yeast homologue Fin1, contribute to multiple aspects of mitotic progression including the timing of mitotic entry, chromatin condensation, spindle organization and cytokinesis. Mammals contain a large family of eleven NIMA-related kinases, named Nek1 to Nek11. Of these, there is now substantial evidence that Nek2, Nek6, Nek7 and Nek9 also regulate mitotic events. At least three of these kinases, as well as NIMA and Fin1, have been localized to the microtubule organizing centre of their respective species, namely the centrosome or spindle pole body. Here, they have important functions in microtubule organization and mitotic spindle assembly. Other Nek kinases have been proposed to play microtubule-dependent roles in non-dividing cells, most notably in regulating the axonemal microtubules of cilia and flagella. In this review, we discuss the evidence that NIMA-related kinases make a significant contribution to the orchestration of mitotic progression and thereby protect cells from chromosome instability. Furthermore, we highlight their potential as novel chemotherapeutic targets.

Nek6 and Nek7 are members of the NIMA-related serine/threonine kinase family. Previous work showed that they contribute to mitotic progression downstream of another NIMA-related kinase, Nek9, although the roles of these different kinases remain to be defined. Here, we carried out a comprehensive analysis of the regulation and function of Nek6 and Nek7 in human cells. By generating specific antibodies, we show that both Nek6 and Nek7 are activated in mitosis and that interfering with their activity by either depletion or expression of reduced-activity mutants leads to mitotic arrest and apoptosis. Interestingly, while completely inactive mutants and small interfering RNA-mediated depletion delay cells at metaphase with fragile mitotic spindles, hypomorphic mutants or RNA interference treatment combined with a spindle assembly checkpoint inhibitor delays cells at cytokinesis. Importantly, depletion of either Nek6 or Nek7 leads to defective mitotic progression, indicating that although highly similar, they are not redundant. Indeed, while both kinases localize to spindle poles, only Nek6 obviously localizes to spindle microtubules in metaphase and anaphase and to the midbody during cytokinesis. Together, these data lead us to propose that Nek6 and Nek7 play independent roles not only in robust mitotic spindle formation but also potentially in cytokinesis.

Significance: Histone deacetylase 1 and 2 (HDAC1/2) are sister proteins that regulate access to DNA by modulating chromatin. We have generated the first double knockout (DKO) of Hdac1/2 in embryonic stem (ES) cells and find that gene inactivation causes a loss of cell viability, which is associated with increased abnormal mitotic spindles and chromosome segregation defects. Transcriptome analysis revealed that almost 2,000 genes are deregulated in DKO cells. Significantly for the self-renewal properties of ES cells, this includes down-regulation of the core pluripotent factors, Oct4, Nanog, and Rex1. Furthermore, using the rescue of Hdac1/2 -null cells as a model system to monitor HDAC1/2 activity, we have also shown that mutations that abolish inositol tetraphosphate binding reduce the activity of HDAC1 in vivo.

Significance: Echinoderm microtubule-associated protein (EMAP)-like (EML) proteins normally function in the cytoskeleton. In some lung cancers, genetic abnormalities generate the oncogenic fusion protein EML4-anaplastic lymphoma kinase (ALK) on which the cancer cells depend for survival. We have determined the molecular structure of a conserved, tubulin-binding region of EML1 that reveals an unexpected protein fold. This region is disrupted in ∼70% of EML4-ALK fusions found in patients, causing them to be sensitive to drugs that target Hsp90, a cellular factor that stabilizes misfolded protein. Our findings will potentially enable more effective, stratified therapy of EML4-ALK nonsmall cell lung cancer and suggest that the truncation of a globular domain at the translocation breakpoint may prove generally predictive of Hsp90 inhibitor sensitivity in cancers driven by fusion oncogenes.

We present crystal structures of a trimeric coiled-coil domain found in echinoderm microtubule (MT)-associated protein (EMAP)-like (EML) proteins. This trimerization domain (TD) mediates self-association and interactions between a subset of EML proteins. MT-association of EML proteins requires the TD and an adjacent basic region.