The ribosomal protein QM is expressed differentially during vertebrate endochondral bone development

Endochondral ossification is a carefully coordinated developmental process that converts the cartilaginous model of the embryonic skeleton to bone with accompanying long bone growth. To identify genes that regulate this process we performed a complementary DNA (cDNA) subtractive hybridization of fetal bovine proliferative chondrocyte cDNA from epiphyseal cartilage cDNA. The subtracted product was used to screen a fetal bovine cartilage cDNA library. Ten percent of the clones identified encoded the bovine orthologue of the human ribosomal protein "QM." Northern and western blot analysis confirmed that QM was highly expressed by cells isolated from epiphyseal cartilage as opposed to proliferative chondrocytes. In contrast, no detectable difference in the expression of mRNA for the ribosomal protein S11 was detected. Immunohistochemical analysis of fetal bovine limb sections revealed that QM was not expressed by the majority of the epiphyseal chondrocytes but only by chondrocytes in close proximity to capillaries that had invaded the epiphyseal cartilage. Strongest QM expression was seen in osteoblasts in the diaphyseal region of the bone adjoining the growth plate, within the periosteum covering the growth plate and within secondary centers of ossification. Hypertrophic chondrocytes within the growth plate adjoining the periosteum also were positive for QM as were chondrocytes in the perichondrium adjoining the periosteum. In vitro investigation of the expression of QM revealed higher QM expression in nonmineralizing osteoblast and pericyte cultures as compared with mineralizing cultures. The in vivo and in vitro expression pattern of QM suggests that this protein may have a role in cell differentiation before mineralization.