Morphogenesis is the amazing process of forming tissues and organs to build an animal. Coordinating cell-cell adhesion and cell shape change are both essential for morphogenesis. Adherens junctions(AJs) are thought to form mechanical attachments between cells by linking the cytoskeletons of neighboring cells via the cadherin-catenin complex. This linkage was long thought to be direct, but recent evidence called this into question. The nectin-afadin complex has also been proposed to mediate linkages between AJs and the cytoskeleton. In my dissertation research, I investigated the role of Canoe(Cno)/Afadin in Drosophila morphogenesis.

First, I found that Cno is not required for the establishment of adhesion or polarity. However, loss of Cno impairs morphogenesis from the start. Cno is required for the first step of gastrulation, a process requiring apical constriction. Apical constriction initiates, but is incomplete. In the absence of Cno, the actomyosin network disconnects from AJs, uncoupling cell shape change and actomyosin constriction. Cno is also required for the elongation of the body axis, a process requiring cell intercalation. Planar polarity of junctional proteins along the dorsal-ventral(DV) axis and cytoskeleton proteins along the anterior-posterior(AP) axis is thought to be an important driving force for intercalation and axis elongation. In the absence of Cno, axis elongation is slowed. Planar polarity of junctional proteins, but not cytoskeletal proteins is enhanced. Cno is planar polarized on the AP axis with cytoskeletal proteins, suggesting that Cno restrains planar polarity by facilitating connections between AJs and the actomyosin network along the AP axis.

I next investigated where Cno acts to regulate AJ-actomyosin linkages. Cno localizes to AJs and is enriched at tricellular junctions with a subpool of actin, suggesting these structures may play key roles in apical constriction and in restraining planar polarity. Cno has multiple direct interactions with AJ proteins, but is not a core part of the cadherin-catenin complex. Cno does not require either the cadherin-catenin complex or the nectin Echinoid for its cortical localization. Instead, Cno localizes to AJs by a Rap1 and actin-dependent mechanism. Taken together, these data suggest that Cno is required to regulate AJ-actomyosin linkages during dynamic morphogenesis.