Osteopathy and resistance to vitamin D toxicity in mice null for vitamin D binding protein
Fayez F. Safadi, Paul Thornton, Holly Magiera, Bruce W. Hollis, Michael Gentile, John G. Haddad, Stephen A. Liebhaber, Nancy E. Cooke
Fayez F. Safadi, Paul Thornton, Holly Magiera, Bruce W. Hollis, Michael Gentile, John G. Haddad, Stephen A. Liebhaber, Nancy E. Cooke
View: Text | PDF
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

×

Figure 1

Options: View larger image (or click on image) Download as PowerPoint
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.