Ectopic expression of Smurf2 and acceleration of age-related intervertebral disc degeneration in a mouse model

OBJECTIVE Lumbar intervertebral disc degeneration, an age-related process, is a major cause of low-back pain. Although low-back pain is a very common clinical problem in the aging population, no effective treatment is available, largely owing to lack of understanding of the molecular mechanisms underlying disc degeneration. The goal of this study was to characterize how ectopic expression of Smurf2 driven by the collagen Type II alpha 1 ( Col2a1) promoter alters disc cell phenotype and associated cellular events, matrix synthesis, and gene expression during disc degeneration in mice. METHODS To characterize how ectopic expression of Smurf2 in Col2a1-promoter working cells affects the disc degeneration process, the authors performed histological and immunohistochemical analysis of lumbar spine specimens harvested from wild-type (WT) and Col2a1-Smurf2 transgenic mice at various ages (n ≥ 6 in each age group). To elucidate the molecular mechanism underlying Smurf2-mediated disc degeneration, the authors isolated cells from WT and Col2a1-Smurf2 transgenic lumbar intervertebral discs and performed Western blot and real-time RT-PCR (reverse transcription polymerase chain reaction) to examine the protein and mRNA levels of interesting targets. RESULTS The authors demonstrated that approximately 30% of WT mice at 10-12 months of age had started to show disc degeneration and that the disc degeneration process was accelerated by 3-6 months in Col2a1-Smurf2 transgenic mice. Chondrocyte-like cell proliferation, maturation, and fibrotic tissue formation in the inner annulus were often accompanied by fibroblast-to-chondrocyte differentiation in the outer annulus in transgenic discs. The chondrocyte-like cells in transgenic discs expressed higher levels of connective tissue growth factor (CTGF) than were expressed in WT counterparts. CONCLUSIONS The findings that ectopic expression of Smurf2 driven by the Col2a1 promoter accelerated disc degeneration in Col2a1-Smurf2 transgenic mice, and that higher levels of CTGF protein and mRNA were present in Col2a1-Smurf2 transgenic discs, indicate that Smurf2 accelerates disc degeneration via upregulation of CTGF.