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 months old BSP+/+ and BSP−/− mice.
Bone marrow was aspirated through the right femur epiphysis. Hematopoietic recolonization was analyzed by flow cytometry after labeling with specific antibodies. The bone vascular network was contrasted by barium sulfate infusion. RT-PCR was performed on tri-reagent flushed bone marrow.
At d3, the ablated area in BSP−/− femurs showed higher vessel density (99.2±4.9 vs 23.2±8.5 mm2) and vascular volume than BSP+/+, along with higher VEGF (×1.95) and OPN (×1.8) expression. Interestingly, unablated BSP−/− femur marrow also contains more blood vessels than BSP+/+ (74.2±8.4 vs 58.0±7.6 mm2). 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 mutants. At d6, a much higher number of Linas well as LSK (Lin- IL-7Rα- Sca-1hi c-Kithi,) and hematopoietic stem cells (HSC: Flt3- LSK) were counted in BSP−/− marrow, indicating a faster recolonization.
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. The higher expression OPN an angiogenic SIBLING, concomitant with massive revascularization in BSP−/− mice suggests a functional role, presently investigated with siRNA targeting OPN.
17 May 2014 - 20 May 2014