Characterization of mice with a platelet-specific deletion of the adapter molecule ADAP.

Abstract

The adhesion and degranulation-promoting adapter protein (ADAP) is expressed in T cells, NK cells, myeloid cells, and platelets. The involvement of ADAP in the regulation of receptor-mediated inside-out signaling leading to integrin activation is well characterized, especially in T cells and in platelets. Due to the fact that animal studies using conventional knock-out mice are limited by the overlapping effects of the different ADAP-expressing cells, we generated conditional ADAP knock-out mice (ADAPfl/fl PF4-Cretg). We observed that loss of ADAP restricted to the megakaryocytic lineage has no impact on other hematopoietic cells even after stimulation conditions. ADAPfl/fl PF4-Cretg mice showed thrombocytopenia in combination with reduced plasma levels of PF4 and TGF-β1. In vitro, platelets from these mice revealed reduced P-selectin expression, lower TGF-β1 release, diminished integrin αIIbβ3 activation and decreased fibrinogen binding after stimulation with podoplanin, the ligand of the C-type lectin-like receptor-2 (CLEC-2). Furthermore, loss of ADAP was associated with impaired CLEC-2-mediated activation of PLCγ2 and Erk1/2. Induction of experimental autoimmune encephalomyelitis (EAE) in mice lacking ADAP expression in platelets caused a more severe disease. In vivo administration of TGF-β1 early after T cell transfer improved EAE severity in mice with loss of ADAP restricted to platelets. Our results reveal a regulatory function of ADAP in platelets in vitro and during autoimmune disease EAE in vivo.