Impeccable chromosome segregation during mitosis underlies genome stability and integrity. Any errors in this process would result in miscarriage, birth defects and/or aneuploidy, the hallmark of human cancers.

Segregating each sister chromatid towards the opposite poles is implemented by the mitotic spindle, a dynamic ensemble of microtubules, microtubule-associated proteins (MAPs) and motor proteins. Nucleation of microtubules in vivo does not occur spontaneously; instead specialised structures called microtubule organising centres (MTOCs) are required, in which the minus end of microtubules is embedded. In animal cells, the centrosome comprises a major MTOC, whilst in fungi the spindle pole body (SPB) plays an analogous role.

Our laboratory has been uncovering the principles of microtubule structure, function and regulation using the genetically amenable model system, the fission yeast Schizosaccharomyces pombe, and more recently we have been using zebrafish and human culture cells to scrutinise the evolutionary conservation of our findings obtained from work in fission yeast. The long-term goal of our laboratory is simple; to understand the molecular mechanisms of how the mitotic spindle ensures faithful chromosome segregation.

During 2014, we have made seminal progress in the following two areas. First, we have uncovered the conserved molecular mechanism by which the minus end of microtubules is tethered to the centrosome and further identified the physiological defects when this process is perturbed. Second, we have determined the molecular pathway leading to the recruitment of the conserved MAP complex, Alp7/TACC-Alp14/TOG, to the SPB upon mitotic entry and shown that this process is critical for mitotic spindle assembly.

The conserved Msd1 protein family plays a ubiquitous role in the anchoring of the microtubule to the fission yeast SPB and human centrosome

The centrosome plays the multi-layered roles in both yeasts and vertebrates. These include the canonical role in microtubule nucleation as the MTOC, cell cycle transition as a structural hub integrating cell cycle regulators and ciliogenesis as the basal body. In addition, there is at least one more critical role, i.e. anchoring of the microtubule minus end. This mechanism ensures that interphase microtubule arrays emanate from the centrosome and that structure and orientation of mitotic spindles are safeguarded. However, our knowledge of microtubule anchoring to this organelle remains surprisingly limited at the molecular level.

We previously showed that in fission yeast, a mitotic specific SPB component, Msd1 (mitotic spindle disanchored 1), is required for anchoring the minus end of spindle microtubules to the SPB (Toya et al., 2007, Nat. Cell Biol. 9:646-653). Now, we have identified the human and zebrafish Msd1 orthologues (Figure 1A) and characterised their roles in microtubule anchoring. We have unveiled that the human Msd1 (hMsd1) protein is delivered to the centrosome in a centriolar satellite- and dynein-dependent manner, wherein it physically binds the microtubule-nucleating γ-tubulin complex (Figure 1B). siRNA-mediated hMsd1 knockdown results in disorganised interphase microtubules due to the inability of the microtubule to be tethered to the centrosome (Figure 1C). During mitosis, mitotic spindles become abnormally tilted and misoriented, which is attributed to the release of astral microtubules from the centrosome, leading to a faulty interaction between microtubules and the cell cortex (Figure 1C).

Moreover, we have found that in both humans and zebrafish, the Msd1 orthologues are critical for ciliogenesis. Remarkably, the loss of Msd1 leads zebrafish embryos to left-right asymmetry defects (Hori et al., 2014, EMBO Rep. 15:175-184; Hori et al., 2015, Mol. Biol. Cell 26:2005-2019). Collectively, the Msd1 proteins are the first molecules to be identified as the conserved microtubule-anchoring factors.