HS1 regulates chemokine‐induced Rap‐1 activation via PKA in neutrophils to facilitate extravasation

Neutrophil extravasation is a critical step in innate immunity in response to tissue injury or invading pathogens. Inflammatory signals activate β2‐integrins and facilitate neutrophil adhesion onto the endothelial apical surface and intraluminal crawling to the site of diapedesis. Hematopoietic cell‐specific lyn substrate (HS1) regulates actin dynamics at cell contacts and may thus be important for extravasation. Indeed, intravital microscopy of the cremaster revealed an increased rolling velocity and an inhibition of neutrophil adhesion and transmigration in HS1‐deficient mice. This effect can be explained by disturbed Rap1 activation causing reduced neutrophil adhesion in response to chemokine treatment. Interestingly, this process is dependent on PKA activation since PKA inhibition blocked chemokine‐induced Rap1 activation. The importance of PKA for HS1‐mediated support of extravasation is corroborated by the fact that PKA activation increases whereas inhibition reduces transmigration of WT neutrophils but not of HS1‐KO neutrophils. However, HS1 is not a direct substrate of PKA but it interacts with phosphorylated VASP. This interaction is also inhibited after PKA inhibition and may thus provide an important scaffold for Rap1 activation. Our results establish HS1 and PKA as critical signal mediators that coordinate the molecules required for Rap1 activation and neutrophil transmigration.