Abstract 434: Formin Dependent Regulation of Adherens Junction Assembly

Adherens junctions are cadherin-dependent structures that mediate intercellular signaling and structural integrity. In endothelial cells, adherens junctions are the primary determiners of vascular permeability and endothelial barrier function. Adherens junctions connect directly to the actin cytoskeleton and this connection is essential for junction formation and maintenance. Formins are a highly conserved family of cytoskeletal remodeling proteins whose activity has been implicated in regulating junction formation in other cell-types. Therefore, we tested the hypothesis that formin activity is essential for adherens junction assembly in endothelial cells. A small-molecule formin inhibitor (smiFH2) was used to determine the effect of formin inhibition on junction formation in Telomerase-Immortalized Microvascular Endothelial (TIME) cells. Using an in vitro vascular permeability assay, we determined that smiFH2 treatment caused a dose-dependent inhibition of junction formation. We used siRNA to knockdown expression of seven of the formins expressed in TIME cells in order to determine which specific formins were required to maintain endothelial barrier function. We determined that individual knockdown of three formins, FHOD1, FHOD3 and Dia1, significantly increased the permeability of the endothelial monolayer. Interestingly, the knockdown of a fourth formin, FMNL2, had the opposite effect and actually potentiated barrier function. Knockdown of the remaining formins had little or no effect on junction formation. Knockdown of FHOD3 had the greatest inhibitory effect on junction assembly and VE-cadherin, β-catenin and α-catenin protein levels were decreased in FHOD3-depleted cells. The FHOD3 knockdown cells were also elongated in comparison to controls and formed thin linear adherens junctions and few focal adherens junctions. In contrast, the morphology of FMNL2-depleted cells did not appear obviously different from controls. In conclusion, our results suggest that multiple formins play diverse roles in endothelial cell adherens junction formation and maintenance.