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Cytokinesis represents the final stage in cell division whereby the actomyosin contractile ring partitions cytoplasm into the daughter cells. We are interested in understanding how the placement and assembly of the contractile ring is coordinated in space and time with chromosome regulation. In an effort to understand these processes, we use the zygotes and early embryos of sea urchins and sand dollars. Urchin eggs have a long history as an experimental system for studying cell division, and we employ a combination of classical embryological manipulations and molecular manipulation of the cell cycle to understand the spatial and temporal regulation of cytokinesis.

 

Cleavage plane specification
In 1961, Ray Rappaport published a landmark paper where he demonstrated that astral microtubules were capable of inducing cleavage furrows in the absence of chromatin and a central spindle (J Exp Zool, 148: 81-89). A modified version of this experiment can be seen in our movie page. These experiments implied that some positional information, or "stimulus" were delivered by growing astral microtubules to the cortical cytoskeleton, where a contractile ring is then assembled. The molecular nature of this positional information is still unknown, but hypotheses range from chromosomal passenger proteins to membrane vesicles to direct microtubule-actin filament interactions. Our current efforts are focused on identifying urchin homologues of genes characterized in yeast that are localized at the spindle pole bodies up until anaphase, at which time they translocate to the actin ring.

 

Timing of cytokinesis
Contractile ring assembly is regulated such that cytoplasmic partitioning never occurs before the onset of anaphase of chromosome segregation. However, with the exception of myosin regulatory light chain, the exact targets of regulation that prevent precocious furrow formation are not known. Previous studies performed in David Burgess' laboratory linked the "timer" for cytokinesis to the mitotic spindle checkpoint (Shuster and Burgess, 2002b), but as mentioned above, we still don't know what occurs at the metaphase-anaphase transition to allow for cleavage furrow formation. Our current efforts are focused on identifying homologues of the Septation Initiation Network, a signaling cascade that drives cytokinesis in fission yeast, and lies downstream of the mitotic spindle checkpoint.
Activation of myosin II contractility
Cytokinesis represents a unique paradigm for nonmuscle myosin II activation, where myosin II contractility is modulated not by extracellular stimuli, but by cell cycle transitions occurring intracellularly. Two signaling cascades may account for myosin regulatory light chain phosphorylation: the calcium/calmodulin-dependent myosin light chain kinase (MLCK), and the small GTPase rho and its effectors. There is considerable evidence that both kinases are required for cytokinesis in animal cells, but the relative contributions of each cascade remain poorly understood. We have identified two contractile events in artificially flattened sea urchin eggs during cell division; one occuring simultaneously with the metaphase-anaphase transition, and the other being the contractile ring itself (See Movie 4 ). Our current efforts are focused on determining whether these contractile events are independently regulated by these two signaling cascades

 

Publications

Siamakpour-Reihani, S., H.J. Argiros, L.J. Weber, L.L. Haas, T.A. Peterson, D.L. Johnson, C.B. Shuster, and B.A. Lyons. 2008. The cell migration protein Grb7 associates with the transcriptional regulator Fhl2 in a Grb7 phosphorylation-dependent manner. Journal of Molecular Recognition. In Press.


George, O., BJ Bryant, R. Chainnasamy, C. Corona, J. Arterburn and C.B. Shuster. 2008. Bisphenol A directly targets tubulin to disrupt spindle organization in embryonic and somatic cells. ACS-Chemical Biology. 3: 167-179.

Preszler, R.W., A. Dawe, C.B. Shuster and M. Shuster. 2007. Assessment of the effects of student response systems on student learning and attitudes over a broad range of biology courses. Cell Biology Education-Life Sciences. In press.

George, O., M. Johnston, and C.B. Shuster. 2006. Aurora B kinase maintains chromatin condensation between meiotic divisions in surf clam oocytes. Cell Cycle. 5: 2648-2656.

Lucero, A.J., C. Stack, A. R. Bresnick, C. B. Shuster. 2006. A global, MLCK-dependent increase in myosin II contractility accompanies the metaphase-anaphase transition in sea urchin eggs. Molecular Biology of the Cell. 17: 4093-4104.

Stack, C., A.J. Lucero, and C.B. Shuster. 2006. Calcium-responsive contractility during fertilization in sea urchin eggs. Developmental Dynamics. 235:1042-1052.

Shuster, C.B., and D.R. Burgess. 2002. Targeted new membrane addition in the cleavage furrow is a late, separate event in cytokinesis. Proceedings of the National Academy of Sciences. 99: 3633-3638.

Shuster, C.B., and D.R. Burgess. 2002. Transitions regulating the timing of cytokinesis in embryonic cells. Current Biology. 12: 854-858.

Shuster, C.B. and D.R. Burgess. 1999. Parameters that specify the timing of cytokinesis. Journal of Cell Biology 146: 981-992.

Shuster, C.B. and I.M. Herman. 1998. The mechanics of vascular cell motility. Microcirculation. 5: 239-57.

Walker, G.R., C. B. Shuster, and D.R. Burgess. 1997. Microtubule-entrained kinase activities associated with the cortical cytoskeleton during cytokinesis. Journal of Cell Science. 110:1373-1386.

Shuster, C.B., A.Y. Lin, R. Nayak, and I.M. Herman. 1996. bcap73: a novel bactin- specific binding protein. Cell Motility and the Cytoskeleton. 35:175-187.

Allen, P.G., C.B. Shuster, J. Kas, C. Chaponnier, P.A. Janmey, and I.M. Herman. 1996. Phalloidin binding and rheological differences among actin isoforms. Biochemistry. 35: 14062-14069.

Shuster, C. B. and I. M. Herman. 1995. Ezrin interactions with F-actin: isoform specificity and calcium sensitivity. Journal of Cell Biology. 128: 837-848.