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Osteopontin (OPN) and Bone Sialoprotein (BSP), abundantly expressed by osteoblasts and osteoclasts, appear to have important, partly overlapping functions in bone. In gene-knockout (KO, -/-) models of either protein and their double (D)KO in the same CD1/129sv genetic background, we analyzed the morphology, matrix characteristics, and biomechanical properties of femur bone in 2 and 4 month old, male and female mice. OPN−/− mice display inconsistent, perhaps localized hypermineralization, while the BSP−/− are hypomineralized throughout ages and sexes, and the low mineralization of young DKO mice recovers with age. The higher contribution of primary bone remnants in OPN−/− shafts suggests a slow turnover, while their lower percentage in BSP−/− indicates rapid remodeling, despite FTIR-based evidence in this genotype of a high maturity of the mineralized matrix. In 3-point bending assays, OPN−/− bones consistently display higher Maximal Load, Work to Max. Load and in young mice Ultimate Stress, an intrinsic characteristic of the matrix. Young male and old female BSP−/− also display high Work to Max. Load along with low Ultimate Stress. Principal Component Analysis confirms the major role of morphological traits in mechanical competence, and evidences a grouping of the WT phenotype with the OPN−/− and of BSP−/− with DKO, driven by both structural and matrix parameters, suggesting that the presence or absence of BSP has the most profound effects on skeletal properties. Single or double gene KO of OPN and BSP thus have multiple distinct effects on skeletal phenotypes, confirming their importance in bone biology and their interplay in its regulation. [ABSTRACT FROM AUTHOR]

The physiological process of biomineralization (calcification) that occurs in bone tissue takes place throughout an individual's life. Complex biological systems carefully orchestrating crosstalk between skeletal tissue and mineralization modulators include environmental and physiological factors acting as promoters or inhibitors. They consist of local mineralization promoters synthesized by osteoblasts, the phosphatases, namely, tissue-nonspecific alkaline phosphatase (TNAP) and phosphatase orphan 1 (PHOSPHO1). The local inhibitors are produced by osteoblasts and osteocytes and include inorganic pyrophosphate (PPi) and organic non-collagenous proteins or peptides of the extracellular matrix, such as osteopontin. While these modulators act in a paracrine/autocrine manner, locally synthesized fibroblast growth factor 23 (FGF23) regulates systemic phosphate levels by creating bone-kidney-parathyroid feedback loops. Locally, the active role in regulation of mineralization and phosphate homeostasis is played by inorganic forms of phosphate itself, and in particular the Pi/PPi ratio. The upper control of this ratio is mediated by circulating effects of FGF23 on phosphate homeostasis. Bone cell products allow bone to play an active role in coordinating its mineralization with total systemic phosphate regulation. [ABSTRACT FROM AUTHOR]