ADAP deficiency impairs megakaryocyte polarization with ectopic proplatelet release and causes microthrombocytopenia

Abstract

Bone marrow (BM) megakaryocytes (MKs) produce platelets by extending proplatelets into sinusoidal blood vessels. Defects in thrombopoiesis can lead to thrombocytopenia associated with increased bleeding tendency. Recently, the platelet disorder congenital autosomal-recessive small-platelet thrombocytopenia (CARST) was described; it is caused by mutations in the adhesion and degranulation-promoting adaptor protein (; synonym: ,) gene, and characterized by microthrombocytopenia and bleeding symptoms. In this study, we used constitutive ADAP-deficient mice ( ) as a model to investigate mechanisms underlying the microthrombocytopenia in CARST. We show that mice display several characteristics of human CARST, with moderate thrombocytopenia and smaller-sized platelets. platelets had a shorter life span than control platelets, and macrophage depletion, but not splenectomy, increased platelet counts in mutant mice to control levels. Whole-sternum 3-dimensional confocal imaging and intravital 2-photon microscopy revealed altered morphology of ADAP-deficient MKs with signs of fragmentation and ectopic release of (pro)platelet-like particles into the BM compartment. In addition, cultured BM-derived MKs lacking ADAP showed reduced spreading on extracellular matrix proteins as well as activation of β1 integrins, impaired podosome formation, and displayed defective polarization of the demarcation membrane system in vitro. MK-/platelet-specific ADAP-deficient mice (PF4-cre) also produced fewer and smaller-sized platelets and released platelets ectopically. These data demonstrate that the abnormal platelet production in the mutant mice is an MK-intrinsic defect. Taken together, these results point to an as-yet-unidentified role of ADAP in the process of MK polarization and platelet biogenesis.

Characterization of Mice with a Platelet-Specific Deletion of the Adapter Molecule ADAP

“…Those authors describe defective F-actin organization and impaired podosome formation of megakaryocytes leading to reduced integrin activation and impaired spreading on collagen I and fibronectin. This intrinsic defect of megakaryocytes was shown in conventional ADAP knockout mice and confirmed in conditional ADAP-deficient mice (36). A possible role of podoplaninstimulated CLEC-2 signaling during thrombopoiesis is unsolved and requires further investigation.…”

Inherited thrombocytopenias: history, advances and perspectives

“…Subsequent whole sternum three-dimensional confocal microscopy and intravital two-photon microscopy detected reduced megakaryocyte maturation in Adap −/− mice with signs of ectopic release of platelet-like particles within the bone marrow. 89 Biallelic mutations in PTPRJ , encoding CD148, a master regulator of Src family kinases and the most abundant membrane-bound tyrosine phosphatase in platelets and megakaryocytes, were next shown to result in non-syndromic microthrombocytopenia in two siblings with mild bleeding. 90 Loss of CD148 was accompanied by platelet aggregation defects, particularly in relation to GPVI signaling.…”

New insights into cytoskeletal remodeling during platelet production

“…In other cell types, ADAP indirectly connects with the WASP/ARP2/3 complex, and thus may be critical for F‐actin branching. Mice deficient in ADAP show moderate microthrombocytopenia (Figure ) . Adap−/− MKs exhibit fragmentation and aberrant release of (pro)platelet‐like particles into the BM compartment.…”