Cytokinesis is essential for cell proliferation. Its failure leads to aneuploidy, which is often associated with cancer. In spite of its importance, the molecular mechanism of cytokinesis has not yet been fully clarified.We would like to understand cytokinesis more fully, in terms of how molecular machines assemble dynamically.The central spindle is a microtubule-based molecular assembly that forms between the segregating chromosomes during anaphase. During telophase, it associates with the ingressing cleavage furrow and matures into the midbody. These microtubule-based structures play crucial roles in all stages of cytokinesis from initiation to completion.We will address the following questions:

• How is the central spindle/midbody assembled?

• How does the central spindle/midbody contribute to the progression of cytokinesis?

To address these questions, we have been combining various approaches including genetics in Caenorhabditis elegans, biochemistry and live observation in mammalian cultured cells and observation at the single molecule level by total internal reflection fluorescence microscopy (TIRF).

Centralspindlin, a stable protein complex of a kinesin and a RhoGAP, is crucial for the assembly of the central spindle and midbody, and for signalling cleavage furrow formation. We have discovered that centralspindlin travels along microtubules of the central spindle as higher-order clusters and that clustering is essential for both microtubule- bundling and motility along microtubules in vitro, and midbody formation in vivo. Based on these findings, we have proposed a positive feedback loop model to explain the distinct localisation pattern of centralspindlin during cytokinesis. Furthermore, we have revealed that the clustering of centralspindlin is regulated by Aurora B kinase and 14-3-3 protein. This adds another layer of regulation to the positive feedback loop, ensuring co-ordination of cytokinesis with chromosome segregation.

Centralspindlin is an evolutionarily-conserved microtubule-bundling protein complex (A). Both Kinesin-6 and RhoGAP components are essential for in vivo formation of the central spindle and for in vitro microtubule-bundling activity. During early cytokinesis, centralspoindlin sharply accumulates to the centre of the central spindle (B)

Plain English:
Cytokinesis is the process that divides the cytoplasm into two after chromosomes have been separated to opposite ends of the cell. To ensure that the number of chromosomes is the same from one cell to the next, cytokinesis has to happen at the right time and place; failure to do so can result in cells with abnormal numbers of chromosomes - a defect often associated with cancer. At the molecular level, division of the cytoplasm is performed by reorganising both the lipid membranes that surround the cell, and several fibre- or tube-like protein chains. This is achieved by the co-operative action of a number of proteins, including force generating motors. We are trying to understand how these individual molecular "parts" are assembled into more complicated molecular "machines" for cytokinesis in a highly dynamic and coordinated manner.

Selected publications:

• Douglas ME, Mishima M (2010) Still entangled: assembly of the central spindle by multiple microtubule modulators Semin Cell Dev Biol 21, 899-908

• Douglas ME, Davies T, Joseph N, Mishima M (2010) Aurora B and 14-3-3 coordinately regulate clustering of centralspindlin during cytokinesis Curr Biol 20, 927-33

• Hutterer A, Glotzer M, Mishima M (2009) Clustering of centralspindlin is essential for its accumulation to the central spindle and the midbody Curr Biol 19, 2043-9

We have proposed a novel positive feedback mechanism to explain the distinct localisation pattern of centralspindlin. Centralspindlin accumulates to the anti-parallel overlap of microtubules at the equatorial region of a dividing cell. Regulation by 14-3-3 protein and Aurora B ensures spatial co-ordination of this process with chormosome segregation as Aurora B kinase activity peaks between segregating chromosomes.