Session S7 - Molecular Motors and the Physics of Cell Division.
INVITED session, Wednesday afternoon, March 24
516C, Palais des Congres

[S7.001] Mitosis: History and Overview.

Mitosis, the process by which identical copies of the replicated genome are distributed to the daughter products of each cell division, depends upon the action of a microtubule(MT)-based protein machine, the mitotic spindle. Mitosis was discovered in the 1800s and by 1950 the basic events of the process had been documented by light microscopists. Subsequent studies have provided a sophisticated explanation of the molecular mechanism of mitotis in terms of MT polymer ratchets and mitotic motors. Key advances included Inoue's proposal that MT-polymer dynamics could drive chromosome motility, leading to the purification of tubulin and the realization that spindle MTs display dynamic instability and poleward flux; McIntosh's hypothesis that mitotic motility involves a "sliding filament mechanism" leading to the discovery of MT-sliding mitotic motors; Ostergren's proposal that chromosome motility depends upon a balance of antagonistic forces; and Niklas' measurements of the magnitude of spindle forces. Recent studies performed in Drosophila embryos have illuminated how multiple mitotic motors and MT polymer ratchets cooperate to coordinate spindle pole dynamics and chromosome motility, aspects of which can now be described using quantitative models.

[S7.002] Reeling in chromosomes to spindle poles: The roles of microtubule-destabilizing enzymes in mitotic spindle dynamics

The central purpose of mitosis is achieved during anaphase when sister chromatids disjoin and translocate towards opposite poles of a microtubule-based machine termed mitotic spindle. We have identified two functionally distinct microtubule-destabilizing Kin I kinesin enzymes that are responsible for normal chromatid-to-pole motion during anaphase in Drosophila. One of them, KLP59C, is required to depolymerize MTs specifically at their kinetochore-associated “plus-ends” such that chromosomes ‘chew’ their way poleward. The second, KLP10A, is required to depolymerize MTs specifically at their pole-associated “minus-ends” thereby ‘reeling’ chromosomes into spindle poles. These findings provide the first description of the protein machinery that drives anaphase chromatid segregation by actively depolymerizing kinetochore MTs at both ends.

[S7.003] Regulatory mechanisms controlling mitotic spindle assembly

Assembly of the spindle in mitosis is essential for the equal segregation of the cellular genetic material during cell division. The spindle comprises of a bi-polar array of microtubules organized by a numerous scaffold and motor proteins. To ensure the correct spatial and temporal segregation of the genetic material, spindle assembly must be coordinated with the cell cycle so that it only persists through mitosis. Studies have demonstrated that the signals which initiate spindle assembly emanate from both the cytoplasm and the nucleus. We have identified one nuclear derived signaling pathway, required for spindle assembly which involves the generation of Ran, bound to GTP (Ran-GTP) in the vicinity of the condensed chromatin. We are currently investigating the downstream spindle assembly events that are regulated by Ran-GTP.

[S7.004] Molecular motors and physics of cell division

In eukaryotic cells, separation of duplicated chromosomes is executed via the mitotic spindle. There appear to be two distinct pathways for spindle formation: chromosome directed and centrosome directed. We have developed a non-equilibrium thermodynamic physical model to describe the remarkable in vitro chromosome-driven spindle experimental results by R. Heald et al. (``Self-organization of microtubules into bipolar spindles around artificial chromosomes in Xenopus egg extracts", Nature, 382, 420-425, (1996)). Our present model is an extension of previous work that led to excellent quantitative comparisons to the results of motility assays of motor driven microtubule motion (F. Gibbons et al, ``A Dynamical Model of Kinesin-Microtubule Motility Assays." Biophysical Journal, 80, 2515-2526 (2001)). We find that different types of spindle structures form dynamically depending both on the forces acting on microtubules by kinesin and dynein molecular motors and on the motor densities. Our mitotic spindle formation results provide new insights and a biophysical understanding of Heald et al. experiments. We have found that the dynein processivity must be sufficiently large or the spindle will not form. There must also be a continuous supply of dynein motors to the system to nucleate the spindle, or their force actions will overwhelm the forces produced by the kinesin motors. The stability of the formed spindles are studied against variations of other biological parameters, like a random distribution of microtubules lengths. Our results shed new light into the conditions for spindle nucleation and its stability. As a consequence, we propose new experimental in vitro conditions where our predictions can be tested.

Work done in collaboration with Stuart C. Schaffner. Research partially supported by the NSF.

Part S of program listing