An apical actin-rich domain drives the establishment of cell polarity during cell adhesion

One of the most important questions in cell biology concerns how cells reorganize after sensing polarity cues. In the present study, we describe the formation of an actin-rich domain on the apical surface of human primary endothelial cells adhering to the substrate and investigate its role in cell polarity. We used confocal immunofluorescence procedures to follow the redistribution of proteins required for endothelial cell polarity during spreading initiation. Activated Moesin, vascular endothelial cadherin and partitioning defective 3 were found to be localized in the apical domain, whereas podocalyxin and caveolin-1 were distributed along the microtubule cytoskeleton axis, oriented from the centrosome to the cortical actin-rich domain. Moreover, activated signaling molecules were localized in the core of the apical domain in tight association with filamentous actin. During cell attachment, loss of the apical domain by Moesin silencing or drug disruption of the actin cytoskeleton caused irregular cell spreading and mislocalization of polarity markers. In conclusion, our results suggest that the apical domain that forms during the spreading process is a structural organizer of cell polarity by regulating trafficking and activation of signaling proteins.