102 The Importance of Sodium-Phosphate co-transport for Bone Mineralization

Thursday, March 22, 2012: 8 a.m. - 9:30 a.m.
Presentation Type: Oral Session
A. BOSKEY1, L. SPEVAK1, Y. SABBAGH2, and S. SCHIAVI2, 1Hospital for Special Surgery, New York, NY, 2Research Division, Genzyme, a Sanofi Company, Framingham, MA
Objectives: The sodium-phosphate co-transporter, NaPiIIb is up-regulated during cartilage calcification and odontoblast-mediated mineralization. NaPiIIb is also expressed in osteoblasts and in the intestine where it regulates the absorption of inorganic phosphate.  The objective of this study was to measure the bone mineral properties within the conditional NaPiIIb knockout (cKO) mice.

Methods: : The conditional KO was driven by the CMV promoter and activated by tamoxifen administration by oral gavage  at 11-14 wks. Animals were sacrificed at 18-21 wks (4.5 mos) and bones analyzed by FTIR imaging.  In addition to comparing directly to WT controls by ANOVA, also male and female animals were analyzed separately.

Results: As previously reported, in the cKO, serum FGF23 decreased, serum 1,25-dihydroxy vitamin D3 increased. There were no changes in serum PTH or phosphate. Decreased urinary phosphate excretion correlated with FGF23 and increased protein expression of the renal phosphate cotransporter NaPiIIa. The bones appeared normal by histologic and histomorphometric analyses.  However, the FTIRI analyses revealed reduced mineral content in male cancellous bones and increased HA crystallinity in the cancellous and cortical bones of female mice (p<0.03).  There were no significant alterations in carbonate or acid phosphate levels, or in the collagen maturity.

Conclusions: The 7mm resolved FTIR images demonstrated an “osteomalacic” phenotype, similar to that reported in the hypophosphatemic (HYP) mouse that has a PHEX mutation. Since PHEX is on the X-chromosome, a stronger phenotype in HYP male mice was expected.  The reason for the sex-dependent differences in the cKO is not known although NaPiIIb is also highly expressed in the testes and might impact sex hormones. We are currently confirming knockout of NaPiIIb gene within the bone.  Nonetheless, the FTIRI data reported here suggests that NaPiIIb may play a critical role in the mineralization process of chondrocytes or osteoblasts.

This abstract is based on research that was funded entirely or partially by an outside source: NIH/NIDCR DE04141

Keywords: Biochemistry, Bone, Infrared Imaging and Mineralization
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