Investigating mechanisms regulating cell adhesion during tissue remodeling

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Project Summary: Precise control of cell-cell adhesion is critical for maintaining tissue integrity during development and in adult tissues. Abnormal activation of signals that regulate adhesion in tumors can result in epithelial-mesenchymal transition (EMT) and cancer metastasis, but we do not fully understand the mechanism through which these regulatory signals cause loss of adhesion. A proposed model for this process is transcriptional repression of cell adhesion genes: the EMT regulator Snail represses expression of the E- cadherin adhesion molecule, a core component of Adherens Junctions (AJs), and is thought to control adhesion in this manner. However, recent observations have shown that Snail regulates the stability and localization of AJs independent of transcriptional regulation of E-Cadherin levels. The specific mechanism by which Snail regulates AJ organization remains unknown, highlighting an important gap in our understanding of the signals that regulate adhesion and govern the cellular decision to undergo EMT. The long-term goal of this project is to determine how cell-cell adhesion is controlled during development to enable tissue morphogenesis and segregation of germ layers. The overall objective of this proposal is to identify mechanisms by which Snail regulates AJ organization during tissue remodeling events during embryonic development in Drosophila, and determine how this regulation contributes to a cell’s decision to undergo EMT. Preliminary data indicate that ectopic Snail expression causes a rapid shift in E-Cadherin protein localization from cell junctions to intracellular structures. Other observations have shown that cells in the Drosophila ventral furrow undergo junctional remodeling through Snail-dependent destabilization of AJs. Remarkably, this regulation occurs prior to depletion of maternally provisioned E-Cadherin protein, and is not inhibited by ectopic E-cadherin expression. Together these data indicate that Snail controls AJ organization through an additional post- transcriptional mechanism. The rationale for this proposed work is to gain insight not only into the nature of these mechanisms, but also the general principles governing cell adhesion during EMT events. Our central hypothesis is that Snail modulates adhesion by altering the stability and localization of junctional cadherin protein complexes through a mechanism independent of E-cadherin transcriptional regulation. This hypothesis will be tested by pursuing two specific aims: we will (1) identify the mechanism through which Snail affects AJ organization, and (2) define the physical conditions in which Snail can promote EMT in Drosophila epithelial tissues. Our approach is innovative because it is one of the first to examine the mechanistic basis of Snail- dependent shifts in cadherin localization, and further because it uses an integrative strategy that combines biochemical and cell biological approaches. The proposed research is significant because it is expected to advance our understanding of the post-transcriptional regulation of cell-cell adhesion, and may open new avenues of research for understanding cancer biology and developmental defects.