β1 integrins and osteoclast function: Involvement in collagen recognition and bone resorption

The extracellular matrix of bone is composed mainly of type I collagen. In this report we studied the role and collagen binding properties of osteoclast integrins (αv, α2, β1, and β3). Cell adhesion assays with rat osteoclasts and affinity chromatography/SDS-PAGE analysis with purified human osteoclast membranes demonstrated adhesion of osteoclasts to native type I collagen in a divalent cation and Arg-Gly-Asp (RGD)-dependent way via α2β1 integrin, whereas osteoclast adhesion to denatured collagen predominantly involved αvβ3. In receptor-binding assays, the involvement of human recombinant αvβ3 in adhesion to denatured collagen was confirmed. Additionally, osteoclasts adhered to type I collagen fibers and to monomeric types II–V collagen with characteristics similar to those on native monomeric type I collagen. ] Osteoclastic bone resorption in vitro was inhibited (>40%) in the presence of α2 and β1 antibodies. Using scanning laser confocal microscopy, αvβ3, α2, and β1 integrin were detected within podosomes in nonresorbing osteoclasts and in the ruffled border area and basolateral membrane in resorbing osteoclasts, but not in the sealing zone of resorbing osteoclasts. These results demontrate that α2β1, in addition to αvβ3, has an important role in osteoclast function and acts as a receptor for native, but not denatured, collagen.