Skeletal development of mice lacking bone sialoprotein (BSP)--impairment of long bone growth and progressive establishment of high trabecular bone mass.

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

Impact of the lack of BSP on mother's behavior.

<p>(A) Pictures of BSP+/+ and BSP−/− mothers with pups; note the well formed "nest" in the wild type mouse cage as opposed to the untouched and spread out pieces of soft paper in the mutant mouse cage (arrows). Note: the 129 sv/CD1 background is outbred, leading to hair color variability. (B) Body length (left) and weight (right) increase in the 30 first days of growing male and female BSP+/− mice from BSP+/+ (○, N = 7 pups) and BSP−/− mothers (•, N = 6 pups). (C) Femur and tibia length of 40 days old BSP+/− mice from either BSP+/+ or BSP−/− mothers. (D) Femur and tibia length of 40 days old BSP+/− (N = 6) and BSP−/− mice (N = 6) from BSP−/− mothers crossed with BSP+/− males. (E) Body weight and femur length of 40 days old BSP+/+ (N = 7), BSP−/− (N = 11) and BSP+/− mice (N = 22) from BSP+/− parents. Data are Mean±SEM; *: p<0.05 vs BSP+/+ and/or BSP+/−, Mann-Whitney U Test or (E) Kuskall-Wallis with post-test.</p

Morphology and morphometry of the long bones in newborn BSP+/+ and BSP−/− mice.

<p>(A) 3D µCT reconstruction and morphometry of newborn whole femurs from BSP+/+ (+/+) and −/− newborn mice. (B) Histomorphometry of trabecular bone in newborn femurs from +/+ and −/− mice. Data are Mean±SEM of N = 4 to 10 mice; *:p<0.05, **:p<0.01 Vs +/+, Mann-Whitney U Test. (C) Micrographs of Goldner's trichrome and Von Kossa staining of BSP+/+ and BSP−/− newborn femur sagittal sections. Yellow polygon: ROI for histomorphometry of the trabecular bone. Bar  = 400 µm.</p

Static histomorphometric parameters and cellular activities in femur bones from 3, 10 and 16 week old BSP+/+ (+/+) and BSP−/− (−/−) mice.

<p>Data are Mean±SEM of N = 6 to 10 mice; *:p<0.05, **:p<0.001 vs BSP+/+, Mann-Whitney U Test.</p

Growth plate and osteoblast marker protein expression and blood levels in newborn and adult BSP+/+ and BSP−/− mice.

<p>QRT-PCR was performed 6 days after birth on collected whole femurs and tibias including the growth plates, ground and extracted in Tri-reagent. Messenger RNA expression of growth plate regulators IHH, PTHrP and IGF-1 (A), markers of bone formation Runx2, Osx and Ocn (B) and SIBLING proteins DMP1, MEPE and Opn (C) were assessed in samples from BSP+/+ (N = 5) and BSP−/− (N = 9) mice. Expression levels are normalized on the housekeeping gene GAPDH, and on Runx2 levels for Opn (C). ELISA assay of Opn (D) in the serum of 6 days (N = 5 pools), 35 days and 12 month old (N = 5 male) BSP+/+ and BSP−/− mice. Data are Mean±SEM; *: p<0.05, **: p<0.01, ***: p<0.001 vs BSP+/+, Mann-Whitney U test.</p

Growth plate kinetics of newborn, 3 week old, 10 and 16 week old BSP+/+ and BSP−/− mice.

<p>Data are Mean±SEM of N = 7 to 8 newborn and 4 to 6 adult mice; *:p<0.05, **:p<0.001 vs BSP+/+ Mann-Whitney U Test. Micrographs of the growth plate in the distal femur of BSP+/+ and BSP−/− newborn and 3, 10 and 16 week old mice.</p

Newborn morphology and growth of BSP+/+ and BSP−/− mice.

<p>(A) general aspect of BSP+/+ (+/+) and BSP −/− (−/−) newborn mice. (B) Kinetics of weight gain during the growth of BSP+/+ and BSP−/− mice. M, males; F, females. Data are Mean±SEM of N = 4 to 31 mice. Note the log scale.</p

Impact of the absence of BSP on postnatal development of the skeleton.

<p>The table compares to wild type (+/+), at different ages of developing mutant mouse long bones, the whole growth plate (GP), hypertrophic (HZ) and proliferating zone (PZ) thickness, cortical bone thickness, trabecular bone volume (BV/TV), osteoclast surface/number and osteoid thickness.</p

Skeletal development and mineralization of newborn BSP+/+ and BSP−/− mice.

<p>(A) Skeletal preparation (alcian blue/alizarin red staining) of BSP+/+ (+/+) and BSP −/− (−/−) E19 mice. (B) 3D µCT reconstruction of whole newborn +/+ and −/− mice, showing undermineralized fingers (arrows and black-bordered detailed pictures), and (C) high magnification of the heads showing the widened sutures (arrows).</p

PCR primer sequences in 5′-3′ direction.

<p>The meaning of the abbreviations is explained in the text.</p