A gene expression profile for endochondral bone formation: oligonucleotide microarrays establish novel connections between known genes and BMP-2-induced bone formation in mouse quadriceps

Abstract

Endochondral bone formation has been fairly well characterized from a morphological perspective and yet this process remains largely undefined at molecular and biochemical levels. In vitro and in vivo studies have shown that human bone morphogenetic protein-2 (hBMP-2) is an important developmental growth and differentiation factor, capable of inducing ectopic bone formation in vivo. This study evaluated several aspects of the osteogenic effect of hBMP-2 protein injected into quadriceps of female C57B1/6J SCID mice. Mice were euthanized 1, 2, 3, 4, 7, and 14 days postinjection and muscles were collected for several methods of analysis. Hematoxylin and eosin-stained sections of muscles injected with formulation buffer showed no evidence of osteogenesis. In contrast, sections of muscles injected with hBMP-2 showed evidence of endochondral bone formation that progressed to mineralized bone by day 14. In addition, radiographs of mice injected with hBMP-2 showed that much of the quadriceps muscle had undergone mineralization by day 14. Labeled mRNA solutions were prepared and hybridized to oligonucleotide arrays designed to monitor approximately 1300 murine, full-length genes. Changes in gene expression associated with hBMP-2 were determined from time-matched comparisons between buffer and hBMP-2 samples. A gene expression profile was created for 215 genes that showed greater than 4-fold changes at one or more of the indicated time points. One hundred twenty-two of these genes have previously been associated with bone or cartilage metabolism and showed significant increases in expression, e.g., aggrecan (Agc1), runt related transcription factor 2 (Runx2), bone Gla protein 1 (Bglap1), and procollagens type II (Col2a1) and X (Col10a1). In addition, there were 93 genes that have not been explicitly associated with bone or cartilage metabolism. Two of these genes, cytokine receptor-like factor-1 (Crlf1) and matrix metalloproteinase 23 (Mmp23), showed peak changes in gene expression of 15- and 40-fold on days 4 and 7, respectively. In situ hybridizations of muscle sections showed that Mmp23 and Crlf1 mRNAs were expressed in chondrocytes and osteoblasts, suggesting a role for both proteins in some aspect of cartilage or bone formation. In conclusion, oligonucleotide arrays enabled a broader view of endochondral bone formation than has been reported to date. An increased understanding of the roles played by these gene products will improve our understanding of skeletogenesis, fracture repair, and pathological conditions such as osteoporosis.