Sequential binding of ezrin and moesin to L-selectin regulates monocyte protrusive behaviour during transendothelial migration.

Leukocyte transendothelial migration (TEM) is absolutely fundamental to the inflammatory response, and involves initial pseudopod protrusion and subsequent polarised migration across inflamed endothelium. Ezrin/radixin/moesin (ERM) proteins are expressed in leukocytes and mediate cell shape changes and polarity. The spatio-temporal organisation of ERM proteins with their targets, and their individual contribution to protrusion during TEM, has never been explored. Here, we show that blocking binding of moesin to phosphatidylinositol 4,5-bisphosphate (PIP) reduces its C-terminal phosphorylation during monocyte TEM, and that on-off cycling of ERM activity is essential for pseudopod protrusion into the subendothelial space. Reactivation of ERM proteins within transmigrated pseudopods re-establishes their binding to targets, such as L-selectin. Knockdown of ezrin, but not moesin, severely impaired the recruitment of monocytes to activated endothelial monolayers under flow, suggesting that this protein plays a unique role in the early recruitment process. Ezrin binds preferentially to L-selectin in resting cells and during early TEM. The moesin-L-selectin interaction increases within transmigrated pseudopods as TEM proceeds, facilitating localised L-selectin ectodomain shedding. In contrast, a non-cleavable L-selectin mutant binds selectively to ezrin, driving multi-pseudopodial extensions. Taken together, these results show that ezrin and moesin play mutually exclusive roles in modulating L-selectin signalling and shedding to control protrusion dynamics and polarity during monocyte TEM.