Fellows' research: Orienting the Spindle: Discovering what regulates error-free cell division

19 Dec 2018

Fellows' research: Orienting the Spindle: Discovering what regulates error-free cell division
Dr. Sachin Kotak, Intermediate Fellow 
IISc, Bangalore
Our recently published research in Life Science Alliance provides further insights into the molecular mechanisms of proper spindle orientation during mitosis. Mitosis is a type of cell division that results in two daughter cells identical to the parent cell, essential for growth and development. During mitosis, replicated chromosomes of the parent are neatly segregated between the two daughter cells during cell division, and this process further ensures that the cells divide in a particular plane. Mitosis is timely orchestrated by a sophisticated molecular machinery inside the cell.
Figure 1. A schematic of cell division in human cells
During mitosis, a diamond-shaped structure called the mitotic spindle is formed. The mitotic spindle is made up of various proteins that help in faithfully segregating newly replicated sister chromatids (Figure 1). The proper orientation of the mitotic spindle helps maintain the relative sizes and spatial organization of the daughter cells. Several types of microtubules, the microscopic hollow structure made up of tubulin dimer are part of this extraordinary dynamic structure, including astral microtubules that grow out and reach to the actin-rich cytoskeleton located beneath the plasma membrane. The astral microtubules contact the cortical machinery containing the motor protein complex, dynein, which drives microtubule movement. Cortical dynein, being a minus-end-directed motor (driving movement towards the minus-end of microtubules i.e. towards the spindle poles), attempts to walk onto the astral microtubules towards the spindle poles. However, since it is cortically anchored, it instead generates pulling forces to orient the mitotic spindle. Correct orientation of the mitotic spindle dictates the proper placement of the cytokinetic furrow, and thus, is essential for determining the relative sizes and spatial organization of the daughter cells within a tissue (Figure 2A).
Figure 2
How dynein is anchored at the cell cortex, and more importantly what are the underlying cellular mechanisms that choreograph cortical levels of dynein during mitosis in time and space? These questions have confounded the cell biology field for some time now.  
Dynein is localized at the cell cortex with the help of evolutionarily conserved dynein adaptor molecule Nuclear mitotic apparatus protein (NuMA). However, the mechanisms that operate to critically orchestrate the cortical levels of NuMA remain elusive. 
Our published research, reveals that loss of an essential mitotic protein named, Polo-like kinase 1 (Plk1), when sister chromatids align at the equatorial plane (metaphase), enriches cortical NuMA, and therefore dynein at the cell cortex. The increased levels of cortical NuMA/dynein, consequently, impact spindle orientation (Figure 2B). We have shown that such an increase in the cortical NuMA is because of the altered molecular mobility of NuMA at the cell cortex. Notably, we further show that Plk1 directly chemically and structurally modifies NuMA, and this modification negatively affects cortical NuMA levels. Plk1 is localized at the spindle pole, and previous reports have linked spindle pole pool of Plk1 in spindle orientation. However, the mechanism by which spindle pole-localized Plk1 affects spindle orientation has remained unknown. In this paradigm, our study identifies NuMA as a primary target of Plk1 and addresses a long-standing puzzle: how spindle pole pool of Plk1 regulates spindle orientation for error-free cell division. Because Plk1 has been implicated in a variety of tumours and spindle mis-orientaion is linked with tumorigenesis, it would be interesting to test if Plk1 mediated spindle orientation defects gives rise to tumour formation.
Plk1 regulates spindle orientation by phosphorylating NuMA in human cells. Sana, S., Keshri, R., Rajeevan, A., Kapoor, S., & Kotak, S. Life Science Alliance, November 2018
Banner image credit: Matthew Daniels. Wellcome Images. Human cells showing the stages of cell division