Principles of Chromosome Segregation and Consequences of Defective Segregation
Chromosome segregation, aneuploidy, mitosis, kinetochore, microtubul
During cell division, microtubules of the mitotic spindle impart forces to accomplish two important functions: (i) Spindle microtubules capture chromosomes and pull the DNA apart into two equal sets. (ii) Astral microtubules interact with the cell cortex and rotate the entire spindle towards a predefined axis. These complex microtubule-mediated events of force generation are controlled precisely, during every cell division, to ensure the accurate segregation of the genome and proper orientation of the spindle. The molecular details of how microtubule interaction with chromosomes and the cell cortex are established and monitored remain unclear. To uncover the biochemical principles that govern microtubule-mediated functions during chromosome segregation and spindle orientation, we use a combination of high-resolution cell biology and high-throughput biochemistry tools.
Defects in chromosome segregation can lead to chromosomal instability and aneuploidy, hallmarks of aggressive cancers. Defects in spindle orientation can lead to incorrect plane of cell division and loss of tissue organization, commonly found in solid cancers. So, we are also interested in using the molecular knowledge of microtubule regulation and function to develop targeted cancer therapeutic tools.
Mechanisms of chromosome-microtubule capture: The End-On Conversion Process
Microtubules capture chromosomes at a specalised sub-micron sized multi-protein structure called the ‘kinetochore’. Correct attachment of kinetochores to microtubule-ends is important for translating microtubule growth and shrinkage into pulling and pushing forces that move chromosomes. In a way, the kinetochore acts as a machine-control unit that can regulate microtubule growth and shrinkage phases and thereby, control the powering of chromosome movement (reviewed in Tamura and Draviam, 2012). Thus it is important that the ends of microtubules are tethered properly at the kinetochore; how tethering to microtubule-ends is achieved is not understood.
By developing a high-resolution imaging methodology, we showed that although kinetochores are capable of attaching to both lateral-walls and ends of the microtubule fibre, the attachment to lateral walls is gradually converted to microtubule-ends through a multi-step process (Shrestha and Draviam., 2013). We term this the end-on conversion process wherein distinct sets of proteins are required for tethering the kinetochore to microtubule-walls versus microtubule-ends. We study how these two different modes of kinetochore-tethering are achieved, monitored and controlled using a combination of biochemistry and cell biology tools.
Regulation of cortex-microtubule interaction: Biased spindle rotation and orientation maintenance
The mitotic cell cortex recruits force-generators (Gαi-LGN-NuMA-dynein/dynactin) that directly interact with astral microtubules of the mitotic spindle and mediate the rotation of the spindle. We developed a semi-automated tool to monitor the temporal evolution of spindle movements during mitosis and uncovered a phase of biased spindle rotation in early mitosis. We showed that cortical force generators are required for the biased rotation (Corrigan et al, 2013). However, the cortical force generators are dispensable for stable maintenance of an already oriented spindle, suggesting the presence of unrecognized cortical tethering mechanisms. How is the spindle stably positioned in the absence of LGN and cortical force generators? We are currently investigating this using microtubule and spindle tracking tools in combination with protein depletion and mutant protein expression studies.
3 key publications
- Lateral to End-on conversion of chromosome-microtubule attachment requires kinesins CENP-E and MCAK. Shrestha R R L and Draviam V M. (2013) Current Biology. 23: 1-13.
- Automated tracking of mitotic spindle pole positions shows that LGN is required for spindle rotation but not orientation maintenance. Corrigan A M, Shrestha R L, Zulkipli I, Hiroi N, Liu Y, Tamura N, Yang B, Patel J, Funahashi A, Donald A, Draviam V M. (2013) Cell Cycle. 12: 16
- Microtubule plus-ends within a mitotic cell are ‘moving platforms’ with anchoring, signaling and force-coupling roles. Tamura N and Draviam V M. (2012) Open Biol. 2: 120132