Evidence for an Important Role of Smad-7 in Intervertebral Disc Degeneration

Smad-7 inhibited the transforming growth factor beta (TGF-β)-induced proteoglycan synthesis in chondrocytes and completely antagonized the effect of TGF-β on the proliferation of the cells. The aim of this study was to evaluate the contribution of Smad-7 to the pathophysiology of disc degeneration by determining the expression of Smad-7 in the degenerative intervertebral discs and its effect on the extracellular matrix metabolism of disc cells. Instability of the lumbar spine produced by imbalanced dynamic and static forces was used to induce intervertebral disc degeneration in rats. The expression of Smad-7 was assessed by the immunohistochemical method. Disc cell apoptosis was detected by in situ TUNEL staining. The effect of Smad-7 overexpression on the matrix metabolism of disc cells was analyzed in vitro by real-time polymerase chain reaction (PCR) and Western blotting. Finally, intradiscal injection of the Smad-7 overexpression lentivirus was performed to evaluate the in vivo effect of Smad-7 on disc degeneration. Radiographic and histomorphological examinations showed that lumbar disc degeneration became more and more severe in the rats with induced instability. Immunohistochemical observation demonstrated increasing protein expression of Smad-7 in the degenerative discs. A significantly positive correlation was found between Smad-7 expression and the degree of disc degeneration and between Smad-7 expression and disc cell apoptosis. Overexpression of Smad-7 in disc cells inhibited the expression of TGF-β1, collagen type-I, collagen type-II, and aggrecan and promoted the expression of MMP-13, but did not change the expression of ADAMTS-5. The in vivo findings illustrated that intradiscal injection of lentivirus vector with Smad-7 overexpression accelerated the progress of disc degeneration. In conclusion, Smad-7 was highly expressed in the degenerative discs. Overexpression of Smad-7 weakened the protective role of TGF-β and accelerated the progress of disc degeneration. Interference on Smad-7 might be a potential therapeutic method for the prevention and treatment of degenerative disc diseases.