High expression of 92-kD type IV collagenase (gelatinase B) in the osteoclast lineage during mouse development

cDNA clones for murine 92 kD type IV collagenase (gelatinase B) were generated for the determination of its primary structure and for analysis of temporal and spatial expression in vivo. The mouse enzyme has 72% sequence identity with the human counterpart, the major difference being the presence of a 16-residue segment absent from the human enzyme. In situ hybridization analyses of embryonic and postnatal mouse tissues revealed intense signals in cells of the osteoclast cell lineage. Clear expression above background was not observed in macrophages, polymorphonuclear leukocytes, monocytes, or epithelial cells which have been shown to express the gene in vitro in cell cultures. Expression of the gene was first observed at early stage of cartilage and tooth development at E13, where signals were seen transiently in surrounding mesenchymal cells. At later developmental stages and postnatally strong expression was seen in large cells at the surface of bones. These cells were presumably osteoclasts as their location correlated with that of TRAP positive cells. Signals above background were not observed in a number of other tissues studied. The results represent the first demonstration of a highly osteoclast specific extracellular proteinase. The results suggest that during normal development of embryonic organs the 92-kD type IV collagenase does not have a major role in basement membrane degradation, but is rather mainly used for the turnover of bone matrix, possibly as a gelatinase required for the removal of denatured collagen fragments (gelatin) generated by interstitial collagenase.