Cell shape changes during epithelial morphogenesis are driven by differential distribution of cell adhesion molecules, cytoskeletal components and polarity factors. Echinoid (Ed) is a homophilic cell adhesion molecule required in normal epithelial morphogenesis during embryonic dorsal closure and ovarian follicular epithelium development. Both of these processes are characterized by loss of Ed from a defined group of cells, which leads to the formation of a supracellular actomyosin cable and smooth contour at the interface of Ed-positive (Ed) and Ed-negative (no-Ed) cells. Ed is also lost from the Ed-positive cells at this interface, due to the absence of a homophilic binding partner in the adjacent cells, resulting in a planar polarized distribution of Ed in these cells. We are testing two alternate hypotheses to explain this phenotype. The first is that planar polarized distribution of Ed guides the formation of actomyosin cable at the interface devoid of Ed. The second hypothesis is that Ed simply functions as a negative regulator of the actomyosin network and absence of Ed at an interface leads to actomyosin enrichment at that particular interface.
Quantification of actin using fluorescently-labeled phalloidin, and myosin using fluorescently labeled heavy or light chain of Myosin II, revealed that significant enrichment of actin and myosin occurs at interfaces between Ed/no-Ed cells as well as no-Ed/no-Ed cells. Cells lacking Ed are also apically constricted, consistent with higher actomyosin contractility. These observations seem to support the negative regulator hypothesis. However, Ed/no-Ed interfaces exclusively display a smooth border, loss/disruption of the polarity protein Par3/Bazooka and enrichment of the actomyosin contractility regulator Rho-Kinase. We also studied distribution of myosin tagged with GFP and RFP–in adjacent wild type (WT) and ed mutant cells respectively. We observed that myosin polarization is stronger in the WT cells at the ed clone border than in the adjacent ed mutant cells, indicating that polarization of Ed has a stronger effect on myosin than the global absence of it. These observations are consistent with the planar polarization hypothesis. Thus, the regulation of actomyosin network by Ed may encompass a variety of interactions between downstream effector molecules involved in cell polarity and cytoskeletal dynamics.