Bone Shaft Revascularization After Marrow Ablation Is Dramatically Accelerated in BSP-/- Mice, Along With Faster Hematopoietic Recolonization.

The bone organ integrates the activity of bone tissue, bone marrow, and blood vessels and the factors ensuring this coordination remain ill defined. Bone sialoprotein (BSP) is with osteopontin (OPN) a member of the small integrin binding ligand N-linked glycoprotein (SIBLING) family, involved in bone formation, hematopoiesis and angiogenesis. In rodents, bone marrow ablation induces a rapid formation of medullary bone which peaks by ∼8 days (d8) and is blunted in BSP-/- mice. We investigated the coordinate hematopoietic and vascular recolonization of the bone shaft after marrow ablation of 2 month old BSP+/+ and BSP-/- mice. At d3, the ablated area in BSP-/- femurs showed higher vessel density (×4) and vascular volume (×7) than BSP+/+. Vessel numbers in the shaft of ablated BSP+/+ mice reached BSP-/- values only by d8, but with a vascular volume which was twice the value in BSP-/-, reflecting smaller vessel size in ablated mutants. At d6, a much higher number of Lin (×3) as well as LSK (Lin IL-7Rα Sca-1 c-Kit , ×2) and hematopoietic stem cells (HSC: Flt3 LSK, ×2) were counted in BSP-/- marrow, indicating a faster recolonization. However, the proportion of LSK and HSC within the Lin was lower in BSP-/- and more differentiated stages were more abundant, as also observed in unablated bone, suggesting that hematopoietic differentiation is favored in the absence of BSP. Interestingly, unablated BSP-/- femur marrow also contains more blood vessels than BSP+/+, and in both intact and ablated shafts expression of VEGF and OPN are higher, and DMP1 lower in the mutants. In conclusion, bone marrow ablation in BSP-/- mice is followed by a faster vascular and hematopoietic recolonization, along with lower medullary bone formation. Thus, lack of BSP affects the interplay between hematopoiesis, angiogenesis, and osteogenesis, maybe in part through higher expression of VEGF and the angiogenic SIBLING, OPN. J. Cell. Physiol. 232: 2528-2537, 2017. © 2016 Wiley Periodicals, Inc.