Macromolecular Composition of Stress Fiber–Plasma Membrane Attachment Sites in Endothelial Cells In Situ

Abstract

Stress fibers (SFs) are present along the apical (apical SF) and the basal (basal SF) portions of cultured cells. We have recently shown that apical SFs are anchored to the apical plasma membrane (PM) in a manner similar to how basal SFs are attached to focal adhesion sites. We propose calling such apical SF–membrane attachment sites “apical plaques.” To study the macromolecular composition of the apical plaque and the focal adhesion in endothelial cells (ECs) in situ, we examined by confocal laser scanning and fluorescence microscopy guinea pig aortae stained with various antibodies against focal adhesion–associated proteins. Basal SFs oriented parallel to the blood flow direction were mainly located in the upstream half of the cell. Thin apical SFs were also observed. Spotty staining patterns were observed in the basal and the apical portions of cells stained with anti-vinculin, anti-talin, anti-paxillin, or anti-fibronectin receptor, indicating the presence of focal adhesions and apical plaques in ECs in situ. Although fibronectin receptors were present in the apical plaque, fibronectin was not detected on the apical cell surface. Our data suggest that the molecules responsible for the SF-PM association are the same between in vitro and in situ cells. Our results appear to support a hypothesis that the SF system is involved in sensing and/or signal transduction of fluid mechanical forces.