Matrix remodeling expression in anulus cells subjected to increased compressive load.

STUDY DESIGN: Mechanobiology study of gene expression changes as a result of compressive overload of anular fibrochondrocytes. OBJECTIVE: To test hypotheses regarding phenotype shift in genes coding for representative extracellular matrix (ECM) proteins and matrix modulators. SUMMARY OF THE BACKGROUND DATA: In degenerative disc disease, the transfer of compressive load through the disc shifts largely from the nucleus onto the anulus. In vivo models simulating this condition have shown derangement of the collagenous ultrastructure in the anulus. In vitro models of cultured anulus cells subjected to static compressive stress generally suggest a down-regulation of synthesis. This study evaluated the expression of specific isomers of genes responsible for mechanical viability and metabolism of the disc under cyclic compressive loads. METHODS: Fibrochondrocytes were digested from the anuli of 3, 2-week-old pigs, embedded in 1.5% alginate gel, and hydrostatically compressed at 0.5 Hz for 3 hours to amplitudes of 10 and 30 atm. These levels represented nominal load transfer through the healthy disc and high load transfer through the degenerative disc. Ribonucleic acid was isolated, reverse transcribed, and evaluated by real-time polymerase chain reaction for expression of type I (C-I) and type II (C-II) collagen, aggrecan, the matrix metalloproteinase (MMP-1), and the transforming growth factor beta (TGFbeta-1). Results were expressed at percentages of uncompressed controls. RESULTS: The lower pressure of 10 atm resulted in up-regulation of all ECM protein genes. C-I and C-II both averaged 141%, and aggrecan 121% of controls (P < 0.05). MMP-1 and TGFbeta-1 were essentially unchanged. With the pressure increased to 30 atm, C-II remained approximately at the level expressed under lower pressure, but C-I was reduced to 42% of controls (P < 0.05), indicating a phenotype shift. MMP-1 and TGFbeta-1 also were down-regulated to 71% and 54% of controls, respectively (P < 0.05). CONCLUSIONS: The up-regulation of the ECM genes with nominal pressure highlights the mechanobiological importance of common activity in fibrocartilage homeostasis. Differential regulation of the 2 primary collagen types with high pressure indicates a capacity of the anulus to remodel according to pathomechanical conditions.