Collagen and proteoglycan abnormalities in the GDF-5-deficient mice and molecular changes when treating disk cells with recombinant growth factor

Design. A magnetic resonance image, histologic, biochemical, and gene expression study was conducted to characterize the effects of growth and development factor-5 (GDF-5) deficiency on the health of the intervertebral disc. Objective. To determine the effect of GDF-5 deficiency on extracellular matrix and gene expression on the intervertebral disc. Summary of Background Data. Developmental and degenerative changes in intervertebral disc are not fully understood. Molecular abnormalities and spontaneous mutations that lead to the deficiency in a normal protein have been useful in understanding the function of certain molecules and the role they play in the structure and health of certain tissues. Although the role of GDF-5 in the disc has not been elucidated, this factor may have an important role in the disc as a result of the well-documented effect of GDF-5 in other chondrogenic tissues. Methods. Intervertebral discs of 20-week-old GDF-5 (-/-) and (+/+) mice were examined radiographically, histologically, biochemically, and with gene expression studies. Cells isolated from GDF-5-deficient mouse discs were treated with recombinant GDF-5 and gene expression was subsequently analyzed. Results. GDF-5 (-/-) mice demonstrated significantly lower T2-weighted signal intensity in the central region of their lumbar discs, and disc histology revealed loss of the normal lamellar architecture of the anulus fibrosus and a shrunken, disorganized nucleus pulposus. Biochemical analysis revealed decreased proteoglycan content but no appreciable change in total collagen content of the discs. Significant downregulation of both aggrecan and type II collagen mRNA, without an appreciable change in type I collagen expression, was noted on gene expression studies. Recombinant GDF-5 treatment of disc cells from the GDF-5-deficient mice resulted in a dose-dependent upregulation of the aggrecan and type II collagen genes. Conclusion. The intervertebral disc is markedly affected by GDF-5 deficiency. This relatively simple (single gene) system with a known molecular defect may be useful in studies designed to define the response of the intervertebral disc to treatment with growth factor in vivo.