Shawn Jordan

Rho GTPases in animal cell cytokinesis: An occupation by the one percent

Rho GTPases are molecular switches that elicit distinct effects on the actomyosin cytoskeleton to accurately promote cytokinesis. Although they represent less than 1% of the human genome, Rho GTPases exert disproportionate control over cell division. Crucial to this master regulatory role is their localized occupation of specific domains of the cell to ensure the assembly of a contractile ring at the proper time and place. RhoA occupies the division plane and is the central positive Rho family regulator of cytokinesis. Rac1 is a negative regulator of cytokinesis and is inactivated within the division plane while active Rac1 occupies the cell poles. Cdc42 regulation during cytokinesis is less studied, but thus far a clear role has only been shown during polar body emission. Here we review what is known about the function of Rho family GTPases during cell division, as well as their upstream regulators and known downstream cytokinetic effectors.

High-Resolution Temporal Analysis Reveals a Functional Timeline for the Molecular Regulation of Cytokinesis

To take full advantage of fast-acting temperature-sensitive mutations, thermal control must be extremely rapid. We developed the Therminator, a device capable of shifting sample temperature in ~17 s while simultaneously imaging cell division in vivo. Applying this technology to six key regulators of cytokinesis, we found that each has a distinct temporal requirement in the Caenorhabditis elegans zygote. Specifically, myosin-II is required throughout cytokinesis until contractile ring closure. In contrast, formin-mediated actin nucleation is only required during assembly and early contractile ring constriction. Centralspindlin is required to maintain division after ring closure, although its GAP activity is only required until just prior to closure. Finally, the chromosomal passenger complex is required for cytokinesis only early in mitosis, but not during metaphase or cytokinesis. Together, our results provide a precise functional timeline for molecular regulators of cytokinesis using the Therminator, a powerful tool for ultra-rapid protein inactivation.

Cortical PAR polarity proteins promote robust cytokinesis during asymmetric cell division

In asymmetrically dividing C. elegans embryos, the core cortical PAR proteins are required to retain septin and anillin at the anterior cortex away from the contractile ring and to promote normal F-actin levels at the contractile ring and successful cytokinesis.

Cytokinesis: thinking outside the cell.

How might the extracellular matrix contribute to cytokinesis? In a recent report, evidence is presented that the conserved extracellular matrix protein hemicentin(HIM-4) is required for cytokinesis in worms and mice.

Using fast-acting temperature-sensitive mutants to study cell division in Caenorhabditis elegans.

Fast-acting temperature-sensitive (ts) mutations are powerful conditional tools for studying transient cellular processes such as cytokinesis. Fast-acting ts cytokinesis-defective mutants are functional at the permissive temperature; yet show a fully penetrant loss-of-function cytokinesis failure phenotype when upshifted to the restrictive temperature. Fast-acting ts mutations thus allow functional tunability and rapid and reversible protein inactivation by simply shifting the temperature at precise times throughout cell division. In this chapter, we describe several techniques and discuss various approaches for harnessing the power of fast-acting ts mutants to study cytokinesis in Caenorhabditis elegans using both simple passive heat transfer and more advanced fluidic-based thermal control systems. We also provide detailed protocols for standard dissection, mounting, and imaging of early worm embryos.

CYK-4 regulates Rac, but not Rho, during cytokinesis

The roles of the Rho-family GAP CYK-4 and small GTPase Rac during cytokinesis are examined in Caenorhabditis elegans embryos. CYK-4 opposes Rac (and potentially Cdc42) activity during cytokinesis. There is no evidence that CYK-4 is upstream of Rho activity or that Rac disruption is a general suppressor of cytokinesis failure.

David Spector: Coordinating gene expression in space and time.

Spector’s work focuses on the spatial organization and regulation of gene expression. Growing up in Washington Heights in New York City, David Spector’s interest in science began at a very young age. In the fifth or sixth grade, his class was part of a trial NSF program, called: “Time, Space, and Matter: Investigating the Physical World.” Instead of learning by reading materials, the goal of the program was to learn by experience and discussion. As part of this program, the students were handed a blank notebook and asked to write about what they were learning, essentially writing their own “book.” His first experience in science came around the same time, when he and his classmate Stanley Feldman built an electronic oscillator and entered the science fair. They won first prize in the Borough of Manhattan Science Fair for fifth and sixth graders and won a whopping prize of

Hari Shroff: Taking a closer look

10 each. Spector has come quite a long way since these early days as a young scientist. He now runs his own lab at Cold Spring Harbor Laboratory (spectorlab.cshl.edu), where he studies the ways in which nuclear organization impacts gene expression. We contacted him to learn more.

Claudine Kraft: A hunger for understanding

Shroff works on developing new super-resolution imaging tools and using them to study cell biology.

Megan King: A force to be reckoned with

Kraft’s work focuses on the mechanisms that regulate autophagy in response to nutrient availability.