HISTOMORPHOMETRIC AND IMMUNOCYTOCHEMICAL STUDIES OF SRC-RELATED OSTEOPETROSIS

''Knockout'' of the src proto-oncogene in mice resulted in the unexpected development of osteopetrosis which was shown subsequently in 4-6 week old src-mutants to be due to failure of src-mutant osteoclasts to form ruffled borders. Histomorphometric analysis of vertebrae and humeri from 9-10 month old src-mutant mice has revealed that the osteopetrosis persists and that osteoclasts from these older mice also fail to form ruffled borders. Deficiency of src, however, does not impair formation of other complex cytoplasm extensions, such as respiratory cilia or intestinal villi. Immunocytochemistry using antibodies to pp60src has shown that the src tyrosine kinase is not localized along ruffled borders of normal osteoclasts in human bone, but is expressed diffusely throughout their cytoplasm. Furthermore, pp60src is expressed diffusely throughout the cytoplasm of non-resorbing osteoclasts in a human giant cell tumor of bone. These findings indicate that, unlike some other forms of the disorder, remission does not occur in src-deficiency-related osteopetrosis, at least by ten months of age. They also show that high expression of the pp60src protein is not confined to actively resorbing osteoclasts, but can also occur in osteoclasts unattached to bone surfaces, and suggest that there may be a critical substrate for src that is involved in ruffled border formation.