FAP-1 loss impairs megakaryocyte demarcation membrane system and platelet function with myelofibrosis-like features.

Fas-associated phosphatase 1 (FAP-1), a nonreceptor protein tyrosine phosphatase, has been implicated in multiple signaling pathways, but its in vivo role remains unclear. In this study, we showed that FAP-1-deficient (FAP-1ΔP/ΔP) mice develop early megakaryocyte (MK) hyperplasia with defective platelet function and occasional hemorrhagic manifestations, accompanied by myelofibrosis (MF)-like features, in aged animals. Bone marrow analyses revealed impaired demarcation membrane system (DMS) development with pre-DMS arrest in MK, which led to defective proplatelet formation, impaired platelet function, and prolonged bleeding that was partly associated with reduced clot retraction. Mechanistically, a FAP-1 deficiency induces sustained Src activation and cofilin inactivation, thereby impairing the perinuclear actin remodeling required for DMS expansion and leading to pre-DMS arrest. With aging, approximately half of the mice developed a symptomatic phenotype, characterized by extramedullary hematopoiesis, hepatosplenomegaly, anemia, and thrombocytopenia; among these, most remained in a prefibrotic state, whereas a subset progressed to fibrosis-like changes. A bone marrow transplantation demonstrated that the MK abnormalities and fibrosis-associated changes were hematopoietic cell-intrinsic and partially transferable. Pharmacologic inhibition of Src with dasatinib attenuated these defects in FAP-1-deficient mice, supporting pathway specificity. In patients with primary MF, reduced FAP-1 expression is associated with pre-DMS MK accumulation, abnormal DMS and actin organization, and Src activation, thus supporting clinical relevance. Collectively, we identified a FAP-1-dependent mechanism that governs the Src-cofilin-mediated actin remodeling required for MK maturation and platelet function and suggest this pathway as a potential therapeutic target for platelet dysfunction, hemorrhagic complications, and fibrosis-associated disease.