Research Interests

Cell polarity, gastrulation and organogenesis

The animal body plan is organized during the early stages of embryogenesis, when cells that will form internal tissues become positioned in the interior of the embryo. This reorganization, called gastrulation, occurs through the movement of early embryonic cells. Our laboratory uses the model organism C. elegans to investigate the basic cellular mechanisms of gastrulation. During C. elegans gastrulation, specific cells move ('ingress') from the surface of the embryo into a central cavity called the blastocoel. We are focusing on several questions related to these movements: How do early embryonic cells acquire polarity such that proteins needed for blastocoel formation and ingression become properly localized? What are the mechanisms of cell ingression? How are cell ingressions triggered and coordinated during embryogenesis? The C. elegans embryo is ideally suited for such studies because it contains relatively few cells, the movements of these cells can be followed using time-lapse microscopy, and because the function of specific genes can be determined using genetic analysis.

We have found that a group of conserved cell polarity proteins called PAR proteins is required for the polarization of early embryonic cells. In very early embryos, certain PAR proteins such as PAR-3 are found around the entire cell perimeter. As cells begin to make contacts with one another, PAR-3 disappears from sites of cell-cell contact but remains on the contact-free surface, establishing an apical/basolateral (contact-free/contact) asymmetry. The asymmetry of PAR-3 helps set up cytoskeletal changes that promote cell ingressions during gastrulation.  Ingressing cells change their shape by constricting their apical surfaces as they enter the embryo during gastrulation. Apical constriction appears to result from a local contraction of the actomyosin cytoskeleton, as non-muscle myosin progressively accumulates at apical surfaces of ingressing cells. When PAR-3 is removed from the embryo, cells ingress very slowly, their apical surfaces fail to accumulate non-muscle myosin, and these surfaces do not appear to constrict.

We identified the PAC-1 protein as a critical regulatory protein that connects cell contact polarization signals to PAR-3 asymmetry.  PAC-1 is a RhoGAP protein, which is predicted to negatively regulate Rho GTPases.  Rho GTPases are signaling proteins that play conserved roles in cell polarization and cytoskeletal organization.  PAC-1 is recruited to sites of cell contact and locally excludes PAR proteins by inhibiting Rho GTPases. We are currently determining how PAC-1 is recruited specifically to cell contacts, and identifying the downstream effectors of PAC-1 and Rho GTPases that regulate PAR protein localization. 

In a related project, we are examining the role of PAR proteins in polarizing epithelial cells, which form during organogenesis.  Epithelial cells have a pronounced apicobasal polarity that is important for their function.  Loss of polarity can also lead to cancer.  We have shown that PAR-3 defines the apical domain of epithelial cells during polarization, and that the PAR-6 protein is required after polarization to assemble the junctions that connect epithelial cells to each other.  We are currently identifying downstream targets of PAR-3 and PAR-6 that are required for these two aspects of epithelial cell formation.