A Role of Kindlin-3 in Integrin αMβ2 Outside-In Signaling and the Syk-Vav1-Rac1/Cdc42 Signaling Axis

Integrins mediate cell-cell and cell-extracellular matrix attachments. Integrins are signaling receptors because their cytoplasmic tails are docking sites for cytoskeletal and signaling proteins. Kindlins are a family of band 4.1-ezrin-radixin-moesin-containing intracellular proteins. Apart from regulating integrin ligand-binding affinity, recent evidence suggests that kindlins are involved in integrin outside-in signaling. Kindlin-3 is expressed in platelets, hematopoietic cells and endothelial cells. In humans, loss of kindlin-3 expression accounts for the rare autosomal disease leukocyte adhesion deficiency (LAD) type III that is characterized by bleeding disorders and defective recruitment of leukocytes into sites of infection. Studies have shown that the loss of kindlin-3 expression leads to poor ligand-binding properties of beta 1, beta 2 and beta 3 integrin subfamilies. The leukocyte-restricted beta 2 integrin subfamily comprises four members, namely alpha L beta 2, alpha M beta 2, alpha X beta 2 and alpha D beta 2. Integrin alpha M beta 2 mediates leukocyte adhesion, phagocytosis, degranulation and it is involved in the maintenance of immune tolerance. Here we provide further evidence that kindlin-3 is required for integrin alpha M beta 2-mediated cell adhesion and spreading using transfected K562 cells that expressed endogenous kindlin-3 but not beta 2 integrins. K562 stable cell line expressing si-RNA targeting kindlin-3, but not control-si-RNA, and transfected with constitutively activated integrin alpha M beta 2N329S adhered and spread poorly on iC3b. We also show that kindlin-3 is required for the integrin alpha M beta 2-Syk-Vav1 signaling axis that regulates Rac1 and Cdc42 activities. These findings reinforce a role for kindlin-3 in integrin outside-in signaling.