Endothelial basement membrane laminins - new players in mouse and human myoendothelial junctions and shear stress communication

Basement membranes (BMs) are critical but frequently ignored components of the vascular system. Using high-resolution confocal imaging of whole-mount-stained mesenteric arteries, we identify integrins, vinculin, focal adhesion kinase (FAK) and several BM proteins including laminins as novel components of myoendothelial junctions (MEJs), anatomical microdomains that are emerging as regulators of cross-talk between endothelium and smooth muscle cells (SMCs). Electron microscopy revealed multiple layers of the endothelial BM that surround endothelial projections into the smooth muscle layer as structural characteristics of MEJs. The shear-responsive calcium channel TRPV4 is broadly distributed in endothelial cells and occurs in a proportion of MEJs where it localizes to the tips of the endothelial projections that are in contact with the underlying SMCs. In mice lacking the major endothelial laminin isoform, laminin 411 (Lama4^−/−), which we have previously shown over-dilate in response to shear and exhibit a compensatory laminin 511 upregulation, localization of TRPV4 at the endothelial-SMC interface in MEJs was increased. Endothelial laminins do not affect TRPV4 expression, rather in vitro electrophysiology studies using human umbilical cord arterial endothelial cells revealed enhanced TRPV4 signalling upon culturing on an RGD-motif containing domain of laminin 511. Hence, integrin-mediated interactions with laminin 511 in MEJ structures unique to resistance arteries modulate TRPV4 localization at the endothelial-smooth muscle interface in MEJs and signalling over this shear-response molecule.