Osteopathy and resistance to vitamin D toxicity in mice null for vitamin D binding protein
Fayez F. Safadi, … , Stephen A. Liebhaber, Nancy E. Cooke
Fayez F. Safadi, … , Stephen A. Liebhaber, Nancy E. Cooke
Published January 15, 1999
Citation Information: J Clin Invest. 1999;103(2):239-251. https://doi.org/10.1172/JCI5244.
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Article

Osteopathy and resistance to vitamin D toxicity in mice null for vitamin D binding protein

Abstract

A line of mice deficient in vitamin D binding protein (DBP) was generated by targeted mutagenesis to establish a model for analysis of DBP's biological functions in vitamin D metabolism and action. On vitamin D–replete diets, DBP–/– mice had low levels of total serum vitamin D metabolites but were otherwise normal. When maintained on vitamin D–deficient diets for a brief period, the DBP–/–, but not DBP+/+, mice developed secondary hyperparathyroidism and the accompanying bone changes associated with vitamin D deficiency. DBP markedly prolonged the serum half-life of 25(OH)D and less dramatically prolonged the half-life of vitamin D by slowing its hepatic uptake and increasing the efficiency of its conversion to 25(OH)D in the liver. After an overload of vitamin D, DBP–/– mice were unexpectedly less susceptible to hypercalcemia and its toxic effects. Peak steady-state mRNA levels of the vitamin D–dependent calbindin-D9K gene were induced by 1,25(OH)2D more rapidly in the DBP–/– mice. Thus, the role of DBP is to maintain stable serum stores of vitamin D metabolites and modulate the rates of its bioavailability, activation, and end-organ responsiveness. These properties may have evolved to stabilize and maintain serum levels of vitamin D in environments with variable vitamin D availability.

Authors

Fayez F. Safadi, Paul Thornton, Holly Magiera, Bruce W. Hollis, Michael Gentile, John G. Haddad, Stephen A. Liebhaber, Nancy E. Cooke

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Figure 1

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Targeted disruption of the mouse DBP locus. (a) A fragment of mouse geno...
Targeted disruption of the mouse DBP locus. (a) A fragment of mouse genomic DNA containing exons 4–8 of the DBP gene was used to design the targeting vector. A PGK-promoter/neomycin phosphotransferase cassette was inserted at the BamHI (M) site in exon 5 to disrupt the mDBP gene and provide for positive selection. A DTA cassette was ligated to the 5′ HindIII site for selection against random integration. (b) Restriction enzyme mapping distinguished the intact from the disrupted DBP allele. A mouse DBP exon 2 probe, located outside of the targeting vector itself, hybridized to an 8.8-kb EcoRI (R) fragment from the native DBP allele and a 5.7-kb EcoRI fragment from the disrupted mDBP allele. Restriction sites are: S, SalI; H, HindIII; B, BglII; R, EcoRI; C, ClaI; M, BamHI. (c) The targeting vector (a) was transfected into ES cells, G418 selection was applied, and surviving clones were analyzed by Southern blotting. Analysis of 8 representative ES cell lines among the 65 examined is shown. All eight clones contained the native 8.8-kb mouse DBP EcoRI fragment, and one clone (D1) also contained the 5.7-kb fragment, indicative of successful homologous recombination. DBP, vitamin D binding protein; DTA, diphtheria toxin A; ES, embryonic stem.