Physiologic Hematopoiesis Is Dependent on Stromal Expression of Laminin-γ1

OBJECTIVE: The microenvironment of the bone marrow hematopoietic niche includes: 1) blood vessels; 2) adjacent stromal cells; 3) hematopoietic cells; and 4) cytokines, growth factors, and structural support molecules.  Injury to the bone marrow niche 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.  Here, we examine the role of the extracellular matrix protein laminin-γ1 in adult bone marrow for maintaining the perivascular hematopoietic niche and hematopoiesis. 

METHODS: A global and inducible laminin-γ1 deficient mouse, hereafter referred to as mutant, was generated in which LAMC1 gene recombination could be monitored by a fluorescent reporter transgene. Tamoxifen was used to induce LAMC1 gene recombination and knock-down of laminin-γ1 protein expression in 8-12 week old mice.  Tamoxifen-treated mice lacking the inducible Cre transgene were used as controls for all experiments. Analysis of mutant mice was performed 17-24 days following the first dose of tamoxifen.  Bone marrow samples were examined by immunohistochemistry for the presence of laminin-1 protein, and hematopoietic tissues, including bone marrow, peripheral blood, spleen, thymus and lymph node were analyzed by flow cytometry for hematopoietic stem and progenitor cells, and mature hematopoietic cell populations, and by  ex vivo hematopoietic progenitor cell colony forming assays.  To examine the stromal cell contribution to laminin-γ1 mediated hematopoietic dysfunction, control and mutant mice that had not yet undergone LAMC1 gene recombination were then transplanted with wild type bone marrow. After full hematopoietic reconstitution, chimeric mice were induced with tamoxifen and hematopoietic stem and progenitor cells and mature hematopoietic cell populations were examined. 

RESULTS:  LAMC1 gene recombination in the bone marrow of tamoxifen-treated mutant animals varied between 20-45%.  With LAMC1 gene recombination, laminin-protein in the bone marrow is rapidly depleted.  Bone marrow blood vessels dilate, and the bone marrow becomes hypocellular.  Hematopoietic stem and progenitor cells are reduced in number, as are bone marrow B-cell progenitors, and thymic double-positive T-cells. Erythrocyte and thrombocyte numbers are not changed, nor were NK cell populations, the majority of myeloid cell subtypes, or mature B cells in the bone marrow and blood.  CD8+ T-cells were increased within the bone marrow, as were CD11b⁺, Ly6C^loLy6G⁺ myeloid cells in the peripheral blood.  Examination of gene recombination within each of these cell types was not consistent with cell-intrinsic mechanism of hematopoietic alterations.  Analysis of bone marrow chimeric animals identified loss of stromal cell-produced laminin-γ1 as a significant mediator of these hematopoietic alterations.

CONCLUSIONS:  The rapid depletion of laminin-γ1 from the bone marrow indicates a high basal turnover rate for the extracellular matrix in the bone marrow.  The attendant hematopoietic dysfunction that follows loss of laminin-γ1 production in the bone marrow is specific for immature hematopoietic cell populations, and is dependent on stromal cell production of laminin-γ1.  This study identifies laminin-γ1 as a niche-dependent regulator of hematopoietic stem and progenitor cell populations, and suggests that that regulation of its production and degradation may be new targets for the study of the hematopoietic niche.