Ablation of Osteopontin Improves the Skeletal Phenotype of Phospho1−/− Mice. (November 2014)

• Record Type: Journal Article
• Authors: Yadav, Manisha C; Huesa, Carmen; Narisawa, Sonoko; Hoylaerts, Marc F; Moreau, Alain; Farquharson, Colin; Millán, José Luis
• Is Part Of: Journal of bone and mineral research. Volume 29:Number 11(2014:Nov.)
• Page Start: 2369
• Page End: 2381
• Publication Date: 2014-11
• Subjects: Bones -- Metabolism -- Periodicals; Mineral metabolism -- Periodicals; 612.392
• DOI: 10.1002/jbmr.2281
• Languages: English
• ISSNs: 0884-0431

Abstract

<abstract abstract-type="main" xml:lang="en"> <title>ABSTRACT</title> <sec id="jbmr2281-sec-0001" sec-type="section"> <p>PHOSPHO1 and tissue‐nonspecific alkaline phosphatase (TNAP) have nonredundant functions during skeletal mineralization. Although TNAP deficiency (<italic>Alpl</italic><sup><italic>−/−</italic></sup> mice) leads to hypophosphatasia, caused by accumulation of the mineralization inhibitor inorganic pyrophosphate (PP<sub>i</sub>), comparably elevated levels of PP<sub>i</sub> in <italic>Phospho1</italic><sup><italic>−/−</italic></sup> mice do not explain their stunted growth, spontaneous fractures, bowed long bones, osteomalacia, and scoliosis. We have previously shown that elevated PP<sub>i</sub> in <italic>Alpl</italic><sup><italic>−/−</italic></sup> mice is accompanied by elevated osteopontin (OPN), another potent mineralization inhibitor, and that the amount of OPN correlates with the severity of hypophosphatasia in mice. Here we demonstrate that plasma OPN is elevated and OPN expression is upregulated in the skeleton, particularly in the vertebrae, of <italic>Phospho1</italic><sup><italic>−/−</italic></sup> mice. Liquid chromatography/tandem mass spectrometry showed an increased proportion of phosphorylated OPN (p‐OPN) peptides in <italic>Phospho1</italic><sup><italic>−/−</italic></sup> mice, suggesting that accumulation of p‐OPN causes the skeletal abnormalities in <italic>Phospho1</italic><sup><italic>−/−</italic></sup> mice. We also show that ablation of the OPN gene, <italic>Spp1</italic>, leads to improvements in the skeletal phenotype in <italic>Phospho1</italic><sup><italic>−/−</italic></sup> as they age. In particular, their scoliosis is ameliorated at 1 month of age and is completely rescued at 3 months of age. There is also improvement in the long bone defects characteristic of <italic>Phospho1</italic><sup><italic>−/−</italic></sup> mice at 3 months of age. Mineralization assays comparing [<italic>Phospho1</italic><sup><italic>−/−</italic></sup>; <italic>Spp1</italic><sup><italic>−/−</italic></sup>], <italic>Phospho1</italic><sup><italic>−/−</italic></sup>, and <italic>Spp1</italic><sup><italic>−/−</italic></sup> chondrocytes display corrected mineralization by the double knockout cells. Expression of chondrocyte differentiation markers was also normalized in the [<italic>Phospho1</italic><sup><italic>−/−</italic></sup>; <italic>Spp1</italic><sup><italic>−/−</italic></sup>] mice. Thus, although <italic>Alpl</italic> and <italic>Phospho1</italic> deficiencies lead to similar skeletal phenotypes and comparable changes in the expression levels of PP<sub>i</sub> and OPN, there is a clear dissociation in the hierarchical roles of these potent inhibitors of mineralization, with elevated PP<sub>i</sub> and elevated p‐OPN levels causing the respective skeletal phenotypes in <italic>Alpl</italic><sup><italic>−/−</italic></sup> and <italic>Phospho1</italic><sup><italic>−/−</italic></sup> mice. © 2014 American Society for Bone and Mineral Research.</p> </sec> </abstract>