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. Here, we show that FAP-1-deficient (FAP-1ΔP/ΔP) mice develop early megakaryocyte hyperplasia with defective platelet function and occasional hemorrhagic manifestations, accompanied by myelofibrosis-like features in aged animals. Bone marrow analysis revealed impaired demarcation membrane system (DMS) development with pre-DMS arrest in megakaryocytes, leading to defective proplatelet formation and impaired platelet function, with prolonged bleeding partly associated with reduced clot retraction. Mechanistically, FAP-1 deficiency induces sustained Src activation and cofilin inactivation, impairing perinuclear actin remodeling required for DMS expansion and resulting in pre-DMS arrest. With aging, approximately half of mice develop a symptomatic phenotype characterized by extramedullary hematopoiesis, hepatosplenomegaly, anemia, and thrombocytopenia; among these, most remain in a prefibrotic state, while a subset progresses to fibrosis-like changes. Bone marrow transplantation demonstrates that megakaryocyte abnormalities and fibrosis-associated changes are hematopoietic cell-intrinsic and partially transferable. Pharmacologic inhibition of Src with dasatinib attenuates these defects in FAP-1-deficient mice, supporting pathway specificity. In patients with primary myelofibrosis, reduced FAP-1 expression is associated with pre-DMS megakaryocyte accumulation, abnormal DMS and actin organization, and Src activation, supporting clinical relevance. Collectively, we identify a FAP-1-dependent mechanism governing Src-cofilin-mediated actin remodeling required for megakaryocyte maturation and platelet function, and suggest this pathway as a potential therapeutic target for platelet dysfunction, hemorrhagic complications, and fibrosis-associated disease.