Project Summary/Abstract Myeloproliferative neoplasms (MPNs) are a clonal expansion of hematopoietic cells, and progression leads to deposition of collagen fibers throughout the blood-forming spaces of the bone marrow. In advanced cases, this causes bony replacement of the marrow cavity, blood maturation outside the bone marrow (extramedullary hematopoiesis), and transformation to fatal acute leukemia. Stromal cells normally form a hematopoietic niche to support hematopoietic stem cells, but in MPNs these cells are reprogrammed to produce overwhelming collagen. We dissected the signals from abnormal hematopoietic cells in MPNs that target bone marrow stromal cells, disrupting their ability to support normal hematopoiesis. We recently reported that transforming growth factor-J3 (TGF-J3) promotes collagen deposition and suppresses expression of key niche factors by bone marrow stromal cells. Moreover, blockade of TGF-J3 signaling in stromal cells can reverse bone marrow fibrosis in mouse models of MPN. However, when TGF-J3 signaling is blocked there is still disruption of the hematopoietic niche and development of extramedullary hematopoiesis. To identify additional signals that disrupt the hematopoietic niche in MPNs, I isolated stromal cells from patient bone marrow samples and identified candidate signaling pathways that alter the bone marrow microenvironment. Based on these data, I hypothesize that tumor necrosis factor-alpha (TNF) and platelet-derived growth factor (PDGF) contribute to loss of niche factors. In Aim 1, I will define the role of PDGF receptor signaling in bone marrow stromal populations in MPNs. Using mouse models to abrogate signaling through the two PDGF receptors in bone marrow stromal cells, I will transplant mouse cells with MPL w515 L or JAK2v61 7 F mutations to model MPNs. I will assess hematopoietic stem/progenitor cells, extramedullary hematopoiesis, niche factor expression, fibrosis, and survival. In Aim 2, I will define the role of TNF receptor signaling in bone marrow stromal populations in MPNs. I will abrogate TNF signaling through the two TNF receptors in stromal cells and use MPN transplant models to assess the impact on hematopoietic stem/progenitor cells, niche factor expression, extramedullary hematopoiesis, and fibrosis. Achieving these aims will improve our understanding of alterations in the bone marrow microenvironment through TNF and PDGF that lead to disease progression in MPNs. These insights will refine therapeutic targeting for more effective interventions to improve outcomes for patients. The overall goal is to establish an independent research laboratory studying contributions of the bone marrow microenvironment to normal and malignant hematopoiesis. This proposal outlines a five-year training plan to acquire advanced skills including innovative approaches in hematopoiesis and MPNs. The primary mentor is Dr. Daniel Link, a distinguished scientist in hematopoiesis, bone marrow microenvironment, and m...