Mitigation of ectopic calcification in osteopontin-deficient mice by exogenous osteopontin

Ectopic calcification is a major cause of bioprosthetic heart valve failure. New therapeutic opportunities are offered by the growing understanding that ectopic calcification is an actively regulated process involving several key gene products. One of these products, osteopontin (OPN), is a glycosylated phosphoprotein previously shown to inhibit apatite crystal formation, induce carbonic anhydrase II, and promote mineral resorption. In this study, OPN-deficient mice (OPN-/-) were utilized as an in vivo model to stimulate the ectopic calcification of glutaraldehyde-fixed bovine pericardium (GFBP) tissue and to examine OPN delivery and structure-function relationships with respect to its anti-calcific activity. Significant calcification of GFBP tissue was obtained within 7 days of subcutaneous implantation in OPN-/- mice. Direct rescue of the calcification phenotype was achieved by the administration of exogenous recombinant rat, histidine-fused OPN (rat His-OPN) to the implant site via soluble injection (up to 72% mitigation achieved) or adsorption onto the implant materials (up to 91% mitigation achieved). Effects were specific, since neither fibronectin nor polyhistidine alone could mitigate calcification of GFBP. The maximum anti-calcific effect was achieved only when rat His-OPN was adequately phosphorylated and contained a functional arginine-glycine-aspartate (RGD) cell adhesive domain. Furthermore, CAII levels in host cells surrounding GFBP were greatest when phosphorylated, RGD-containing rat His-OPN was adsorbed. These data suggest that both physical inhibition, mediated by phosphorylation sites in OPN, as well as the induction of CAII and mineral regression, mediated by the RGD domain, contribute to the unique ability of OPN to mitigate ectopic calcification of bioprosthetic valve tissue.