Osteopontin Deficiency Increases Mineral Content and Mineral Crystallinity in Mouse Bone

Abstract

Fourier transform infrared microspectroscopy (FTIRM) and infrared imaging (FTIRI) were used to characterize the mineral in bones of two different lines of Opn-deficient (Opn-/-) mice and their background-matched wild-type controls (Opn+/+). Sections of tibia and femur from 12-week-old and 16-week-old mice were evaluated with a spatial resolution between 10 microm (FTIRM) and 7 microm (FTIRI). FTIRI was used to examine 400 microm x 400 microm areas in cortical bone and trabecular bone and FTIRM examined selected 20 microm x 20 microm areas at sites within these anatomically defined areas. Despite the absence of an obvious phenotype in Opn-deficient mice, being undetectable by radiographic and histological methods, FTIRM analyses revealed that the relative amount of mineral in the more mature areas of the bone (central cortical bone) of Opn-knockout mice was significantly increased. Moreover, mineral maturity (mineral crystal size and perfection) throughout all anatomic regions of the Opn-deficient bone was significantly increased. The 2-dimensional, color-coded data (images) produced by FTIRI showed similar increases in mineral maturity in the Opn-/- bone, however, the crystallinity parameters were less sensitive, and significance was not achieved in all areas analyzed. Nonetheless, the findings of increased mineral content and increased crystal size/perfection in both lines of Opn-deficient mice at both ages are consistent with in vitro data indicating that Opn is a potent inhibitor of mineral formation and mineral crystal growth and proliferation, and also support a role for Opn in osteoclast recruitment and function.

Proteolytic processing of osteopontin by PHEX and accumulation of osteopontin fragments in Hyp mouse bone, the murine model of X-linked hypophosphatemia

“…(34) Thus, both the full-length protein and its peptides appear to be important in the inhibition of mineralization. Corroborating the present work, other studies have shown increased levels of OPN in disease models of osteomalacia, suggesting an important role for OPN in regulating normal mineralization.…”

Pyrophosphate: a key inhibitor of mineralisation

“…Osteopontin is a secreted glycoprotein which limits hydroxyapatite formation and deposition [100,101]. Pyrophosphate can induce osteopontin in osteoblasts via the MAPK signalling pathway [28,102] and both Enpp1 -/-and ank/ank mice display reduced osteopontin expression in osteoblasts and decreased serum levels of the protein [28,44,45].…”

“…Firstly, pyrophosphate has to be injected as it is ineffective orally because of hydrolytic destruction within the gut; BPs are effective by mouth despite being poorly absorbed. Secondly, pyrophosphate does not inhibit bone resorption, whereas this is the key pharmacological action of BPs when used to treat clinical disorders characterised by excessive resorption [100]. BPs are very effective drugs with more than 40 years of clinical use, and have proven to be remarkably safe, with an excellent benefit to risk ratio [109].…”

Osteopontin is an argentophilic protein in the bone matrix and in cells of kidney convoluted tubules

“…In vitro studies by several groups have established that OPN is an inhibitor of calcification [17,18]. Bones from OPN-knockout mice have a greater mineral content and contain larger hydroxyapatite crystals than wild type animals [19]. It is probable that osteocytes elaborate OPN to inhibit the growth of hydroxyapatite in the wall of their lacuna and in the canaliculi to avoid burying due to secondary mineralization.…”