Loss of the F-BAR protein CIP4 reduces platelet production by impairing membrane-cytoskeleton remodeling

Abstract

Key Points• CIP4 affects the remodeling of both plasma membrane and cortical cytoskeleton in megakaryocytes.• CIP4 in platelet biogenesis involves cortical tension, as does WASP, and WASPindependent plasma membrane reorganization.Megakaryocytes generate platelets through extensive reorganization of the cytoskeleton and plasma membrane. Cdc42 interacting protein 4 (CIP4) is an F-BAR protein that localizes to membrane phospholipids through its BAR domain and interacts with WiskottAldrich Syndrome Protein (WASP) via its SRC homology 3 domain. F-BAR proteins promote actin polymerization and membrane tubulation. To study its function, we generated CIP4-null mice that displayed thrombocytopenia similar to that of WAS 2 mice.The number of megakaryocytes and their progenitors was not affected. However, the number of proplatelet protrusions was reduced in CIP4-null, but not WAS 2 , megakaryocytes. Electron micrographs of CIP4-null megakaryocytes showed an altered demarcation membrane system. Silencing of CIP4, not WASP, expression resulted in fewer proplatelet-like extensions. Fluorescence anisotropy studies showed that loss of CIP4 resulted in a more rigid membrane. Micropipette aspiration demonstrated decreased cortical actin tension in megakaryocytic cells with reduced CIP4 or WASP protein. These studies support a new biophysical mechanism for platelet biogenesis whereby CIP4 enhances the complex, dynamic reorganization of the plasma membrane (WASP independent) and actin cortex network (as known for WASP and cortical actin) to reduce the work required for generating proplatelets. CIP4 is a new component in the highly coordinated system of megakaryocytic membrane and cytoskeletal remodeling affecting platelet production. (Blood. 2013;122(10):1695-1706

Cooperative functions of the two F-BAR proteins Cip4 and Nostrin in regulating E-cadherin in epithelial morphogenesis

“…Mice that lack Cip4 are viable and show only a weak endocytosis defect of the insulin-responsive glucose transporter Glut4 (Feng et al, 2010). Mutant animals also display a reduced platelet production and defective integrin-dependent T-cell adhesion, both defects are probably caused by decreased WASP-dependent actin polymerization, rather than impaired endocytosis (Chen et al, 2013). Given the mild phenotypes, the two other Cip4-like subfamily members Toca-1 and FBP17 might have redundant functions and could compensate for the loss of Cip4 function.…”

FlnA binding to PACSIN2 F-BAR domain regulates membrane tubulation in megakaryocytes and platelets

“…8 However, how CIP4 regulates platelet production is unclear, as both Cip4-null and Wasp-null MKs produce platelets of normal morphology. Although Cip4-null, but not Wasp-null, bone marrow MKs have defective DMS formation, 56,57 no association among CIP4, WASp, and the MK DMS and/or platelet OCS has been described.…”

Synthesis and dephosphorylation of MARCKS in the late stages of megakaryocyte maturation drive proplatelet formation

“…[45][46][47][48] In fact, both murine and human WASP KO MKs in culture display proplatelet formation. [45][46][47][48] Therefore, these data are consistent with our model, suggesting WASP is not essential for (pro)platelet formation.…”