Platelets play a central role in cardiovascular, stroke, and bleeding disease through their quantity, activation, and interaction with the inflammatory and vascular systems. The Wiskott -Aldrich syndrome (WAS) is an X- linked disorder characterized by thrombocytopenia, immunodeficiency, eczema and an increased risk of lymphoma and autoimmunity. The affected gene encodes the cytoskeletal protein, WASP. Through the RhoGTPase, Cdc42, WASp undergoes conformational change that leads to actin polymerization. How WASp results in thrombocytopenia, the most common manifestation of WAS, remains unknown. Cdc42, Src kinases, membrane phospholipids, and SH3 domains control WASP function. Our lab has discovered a new regulator of WASP, CIP4 (Cdc42 interacting protein 4), in a yeast two hybrid screen with the Src kinase Lyn as bait. CIP4 is a member of the F-BAR family of proteins, which remodel the plasma membrane and promote actin polymerization. To determine CIP4's function, we created a CIP4-null mouse. We found that CIP4-/- mice display thrombocytopenia, similar to that observed in WASP-/- mice. The mechanism for thrombocytopenia in WAS is not known. Some studies demonstrate an autoimmune basis; others a defect in proplatelet production. Because our mice do not display signs of autoimmune disease, we favor the hypothesis (defective proplatelet formation hypothesis) that CIP4-WASP forms a signaling pathway that promotes platelet production and that a deficiency of either CIP4 or WASP perturbs actin polymerization in megakaryocytes, which results in defective proplatelet formation. An alternative immune destruction hypothesis states that a defect in this CIP4-WASP pathway promotes autoimmunity and immune-mediated destruction of platelets. We propose to address these hypotheses through the following two specific aims: 1) the defective proplatelet hypothesis by culturing megakaryocyte precursors and megakaryocytes from wild-type, CIP4-/-, WASP-/-, and CIP4-/-WASP-/- mice and analyzing their proplatelet formation. We will use these unique mice strains to analyze proplatelet formation, the interaction of CIP4 with microtubules in megakaryocytes, and ultrastructural features of platelets; and 2) Test the immune destruction hypothesis by labeling platelets and measuring their clearance rates in wild-type, CIP4-/-, WASP-/-, and CIP4-/-WASP-/- mice and measuring their T, B, and NK cell functions. We will use these unique mice strains to measure platelet survival and to correlate with altered immune function.