Quantification of the collagenolytic activity of isolated osteoclasts by enzyme-linked immunosorbent assay

Difficulties in the geometrical definition and measurement of resorption pits is a major problem for the quantitative analysis of bone resorption by isolated osteoclasts cultured on bone or dentin substrates. In this study we developed an enzyme-linked immunosorbent assay (ELISA) for quantification of bone resorption in vitro, which specifically quantifies type I collagen fragments released into the culture medium by the resorptive action of bone cells cultured on slices of bone or dentin. A consistently high correlation between the formation of resorption pits and the release of antigenic collagen fragments was observed for isolated rabbit osteoclasts seeded at various densities and cultured for various periods on bovine, elephant, and human substrates. In a further support of the osteoclastic nature of the collagenolytic effects, a high consistency between pit formation and collagenolysis was also observed when the rabbit bone cells were cultured in the presence of very differently acting but typical inhibitors of pit formation, i.e., the carbonic anhydrase inhibitor acetazolamide, the cysteine proteinase inhibitor epoxysuccinyl-L-leucylamido-(4-guanodino)butane (E-64), the phosphatidyl-inositol 3-kinase inhibitor wortmannin, and the bisphosphonate ibandronate (BM 21.0955). In conclusion, the ELISA represents a simple, precise, and objective way to dynamically monitor bone resorption in vitro through quantification of the collagenolytic activity of isolated osteoclasts.