IS COLLAGENASE (MATRIX METALLOPROTEINASE-1) NECESSARY FOR BONE AND OTHER CONNECTIVE-TISSUE REMODELING

In physiologic remodeling of bone and other connective tissues, protei nases such as the matrix metalloproteinases (MMPs) which can cleave Ty pe I collagen play a critical role. In bone, MMP-1 is secreted by stro mal fibroblasts, osteoblasts, and osteoclasts. Only the collagenases ( MMP-1 and MMP-8) cleave native undenatured collagen at neutral pH. The cleavage is site specific at a single locus in the alpha 1(I) chain b etween Gl(775)/Ile(776). The authors have altered the amino acid seque nces around the collagenase cleavage site by site-directed mutagenesis of the murine Col1a-I gene, introducing Pro for Gln(774), Pro for Ala (777), and Met for Ile(776). The mutant Col1a-I gene has been expresse d in Mov13 fibroblasts, and secreted Type I collagen molecules have be en found to be resistant to cleavage at Gly(775)/Ile(776) by MMP-1 or MMP-8. This subtle mutation was introduced recently into the endogenou s Col1a-I gene by homologous recombination in embryonic stem cells to determine the role of collagenase in vivo. Chimaeric mice derived from blastocysts injected with these embryonic stem cells transmitted the mutant Col1a-I gene to their offspring. Surprisingly, homozygous mutan t mice reproduce and appear to develop normally. The mechanisms of col lagen resorption in remodeling of bone and soft tissues in these mice are being examined currently. Information should be derived that will be useful in interpreting human disorders characterized by increased c ollagen deposition, such as osteopetrosis and dermal fibrosis.