Bone marrow laminins influence hematopoietic stem and progenitor cell cycling and homing to the bone marrow.

Hematopoietic stem and progenitor cell (HSPC) functions are regulated by a specialized microenvironment in the bone marrow - the hematopoietic stem cell niche - of which the extracellular matrix (ECM) is an integral component. We describe here the localization of ECM molecules, in particular the laminin α4, α3 and α5 containing isoforms in the bone marrow. Laminin 421 (composed of laminin α4, β2, γ1 chains) is identified as a major component of the bone marrow ECM, occurring abundantly surrounding venous sinuses and in a specialized reticular fiber network of the intersinusoidal spaces of murine bone marrow (BM) in close association with HSPC. Bone marrow from Lama4^-/- mice is significantly less efficient in reconstituting the hematopoietic system of irradiated wildtype (WT) recipients in competitive bone marrow transplantation assays and shows reduced colony formation in vitro. This is partially due to retention of Lin^-c-kit⁺Sca-1⁺CD48^- long-term and short-term hematopoietic stem cells (LT-HSC/ST-HSC) in the G0 phase of the cell cycle in Lama4^-/- bone marrow and hence a more quiescent phenotype. In addition, the extravasation of WT BM cells into Lama4^-/- bone marrow is impaired, influencing the recirculation of HSPC. Our data suggest that these effects are mediated by a compensatory expression of laminin α5 containing isoforms (laminin 521/522) in Lama4^-/- bone marrow. Collectively, these intrinsic and extrinsic effects lead to reduced HSPC numbers in Lama4^-/- bone marrow and reduced hematopoietic potential.

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