The bone marrow hematopoietic niche is a complex microenvironment that includes: 1) sinusoid and arteriole blood vessels; 2) adjacent stromal cells; 3) hematopoietic cells; and 4) cytokines, growth factors, and structural support molecules. Injury to the bone marrow vasculature affects hematopoietic cells through loss of either physical contact with stromal cells or niche-derived growth factors. Extracellular matrix factors required to maintain the steady-state bone marrow perivascular niche and hematopoiesis have not been fully identified. In this proposal, we present data that continuous production of the extracellular matrix protein laminin-?1 in adult bone marrow is required to maintain the perivascular hematopoietic niche and hematopoiesis. In the absence of laminin-?1 expression, laminin-protein in the bone marrow is rapidly depleted. Endothelial cells are injured, bone marrow blood vessels dilate, the bone marrow becomes hypocellular, and hematopoietic stem and progenitor cells (HSPCs) are reduced in number. The rapid depletion of laminin-?1 from the bone marrow is a surprising finding and indicates a high basal turnover rate for the extracellular matrix in the bone marrow. Based on these findings, the central hypothesis of this proposal is that extracellular matrix dynamics are a niche-dependent hematopoietic regulatory mechanism. The objective of this proposal is to determine how hematopoiesis and the hematopoietic niches are regulated by extracellular matrix production and degradation, and to determine whether inhibition of laminin-?1 degradation by plasmin is sufficient to rescue the phenotype of laminin-?1-deficiency. Our long-term goal is to identify novel targets for the treatment of bone marrow failure syndromes and hematopoietic malignancies. Laminin-?1 is one such target, and the proposed studies, in addition to providing valuable information about the basic biology of the hematopoietic niche, will characterize new targets for niche-directed therapy. Project Narrative: Hematopoiesis depends upon hematopoietic stem cells and the supportive microenvironment, and abnormalities in either result in abnormal blood cell production. The goal of this proposal i to determine the cellular and molecular mechanisms by which the extracellular matrix protein, laminin-? 1, maintains the perivascular hematopoietic niche and hematopoiesis. The ultimate goal is to identify novel molecular targets for the treatment of diseases of deficient or dysplasti hematopoiesis.
