Skeletal Development of Mice Lacking Bone Sialoprotein (BSP) - Impairment of Long Bone Growth and Progressive Establishment of High Trabecular Bone Mass

Abstract

Adult Ibsp-knockout mice (BSP−/−) display shorter stature, lower bone turnover and higher trabecular bone mass than wild type, the latter resulting from impaired bone resorption. Unexpectedly, BSP knockout also affects reproductive behavior, as female mice do not construct a proper "nest" for their offsprings. Multiple crossing experiments nonetheless indicated that the shorter stature and lower weight of BSP−/− mice, since birth and throughout life, as well as their shorter femur and tibia bones are independent of the genotype of the mothers, and thus reflect genetic inheritance. In BSP−/− newborns, µCT analysis revealed a delay in membranous primary ossification, with wider cranial sutures, as well as thinner femoral cortical bone and lower tissue mineral density, reflected in lower expression of bone formation markers. However, trabecular bone volume and osteoclast parameters of long bones do not differ between genotypes. Three weeks after birth, osteoclast number and surface drop in the mutants, concomitant with trabecular bone accumulation. The growth plates present a thinner hypertrophic zone in newborns with lower whole bone expression of IGF-1 and higher IHH in 6 days old BSP−/− mice. At 3 weeks the proliferating zone is thinner and the hypertrophic zone thicker in BSP−/− than in BSP+/+ mice of either sex, maybe reflecting a combination of lower chondrocyte proliferation and impaired cartilage resorption. Six days old BSP−/− mice display lower osteoblast marker expression but higher MEPE and higher osteopontin(Opn)/Runx2 ratio. Serum Opn is higher in mutants at day 6 and in adults. Thus, lack of BSP alters long bone growth and membranous/cortical primary bone formation and mineralization. Endochondral development is however normal in mutant mice and the accumulation of trabecular bone observed in adults develops progressively in the weeks following birth. Compensatory high Opn may allow normal endochondral development in BSP−/− mice, while impairing primary mineralization.

Loss of bone sialoprotein leads to impaired endochondral bone development and mineralization

“…While there was no difference in the overall length of the growth plate in newborn Bsp −/− versus WT mice, in agreement with the first published work on the Bsp −/− mice by the Malaval group [25], we observed the resting and proliferative zone lengths were increased and decreased, respectively, with no change in hypertrophic zone length in the P0.5 Bsp −/− growth plates. This differs from the recent work published by Malaval's group 1 where they observed reduced thickness of both the growth plate and the hypertrophic zone length in newborn Bsp −/− mice [35]. At present, we cannot explain these apparent differences; however, differences in housing conditions, animal chow, and/or breeding strategies could be responsible.…”

“…In our study, the mineralized regions of neonatal tibiae from the Bsp −/− mice show a different picture of osteogenic marker expression during development: a normal expression of Spp1 but the decreased expression of other osteogenic cell markers (Alp1, Col1a1, Sp7, Runx2, Bglap) suggesting fewer mature osteoblasts in the Bsp −/− mouse. This is similar to the recently published work on young P6 whole bone extracts from the Bsp −/− mouse [35]. This discrepancy in expression patterns between developing and mature bone suggests multiple roles for BSP at different stages of development.…”

“…Drawing a parallel between the chondrocytes and osteoblasts, we propose the following model for BSP's role in chondrocyte maturation within the growth plate. Lowlevel BSP expression in the chondrocytes initiates cell proliferation 1 After submission of this manuscript, Reference [35] by the Malaval group was published online. from the resting phase while also helping to maintain their proliferative capacity as they mature.…”

The effects of bisphosphonates on ectopic soft tissue mineralization caused by mutations in the ABCC6 gene

“…Secondly, morphometric microarchitecture of the bones in KO mice on standard diet or those supplemented either with 12xETD or 12xAST was evaluated by mCT followed by 3D reconstruction and segmentation. In agreement with previous publications, 29,30 there were significant differences between the male and female mice on bone microarchitecture (Fig. 3).…”

Mineralization defects in cementum and craniofacial bone from loss of bone sialoprotein

“…In a femur cortical bone defect model, Bsp −/− mice displayed delayed bone repair (Malaval et al, 2009b; Monfoulet et al, 2010), and in a femur bone marrow ablation model, absence of BSP caused both reduced medullary trabecular bone formation and delayed osteoclastic resorption (Wade-Gueye et al, 2012). Investigations focusing on early stages of long bone development identified alterations in the growth plate, delayed initiation of mineralization, and a reduction in expression of some osteogenic markers in Bsp −/− vs. WT mice (Bouleftour et al, 2014; Holm et al, 2015). …”