Degenerated Intervertebral Discs Contain Increased Proportion Of Alpha-Smooth Muscle Actin Positive Cells

Purpose: Low back pain (LBP) is a common disease which affects 80% of people around the world for at least once in their life time. Our previous study based on Hong Kong cohort demonstrated that intervertebral disc (IVD) degenerates with aging and is a major contribution to LBP. To date, the molecular changes in the IVD during degeneration progress is not fully illustrated. Previously, Melrose et al reported that a proportion of NP (the inner compartment of IVD) and AF (the surrounding lamella tissue surrounding NP) cells expressed α-smooth muscle actin (α-SMA). Here we aim to evaluate if α-smooth muscle actin (α-SMA) expression is associated with disc degeneration and its source of expression. Methods: Human degenerated IVD were collected from patients with disc degeneration (graded IV-V at the Schneiderman scale) undergoing discectomy (n = 5). Human non-degenerated IVD were collected from adolescent scoliosis patients during corrective scoliosis surgery (n = 5). The corresponding informed patient consent and institutional review board (IRB) approval was obtained from the relevant committee. IVD tissues were carefully dissected into NP and AF, embedded into paraffin and cut into 6 um sections. The expression of α-SMA and MMP12 (matrix metalloproteinase 12) was investigated by immunochemical staining. The positivity was determined as the number of positive cells divided by the total number of cells per whole section. Results: α-SMA was mainly located in cell cytoplasm. Its expression was detected in both NP and AF of degenerated and non-degenerated human IVD. Degenerated human NP contained significantly higher proportion of α-SMA positive cells when compared with non-degenerated human NP (D-NP vs. ND-NP: 21.89 ± 3.79% vs. 4.42 ± 1.47%). Similarly, degenerated human AF also contained more α-SMA positive cells compared to non-degenerated human NP (D-AF vs. ND-AF: 29.05 ± 6.55% vs. 7.83 ± 2.46%). Interestingly, human degenerated NP and AF also had significantly higher percentage of MMP12(+) cells than their non-degenerated counterparts. Analysis of the positivity for α-SMA and MMP12 in all samples showed that the positivity for α-SMA is significantly correlated with the positivity for MMP12 (NP: p < 0.01; AF: p < 0.01). Conclusions: In this study, we found that the positivity of α-SMA increased in degenerated human NP and AF. The role of α-SMA in IVD cells is yet not clear. α-SMA is reported to aid wound contraction, it also helps to resist high mechanical stress and maintain cell shape. Besides, α-SMA is a marker characteristically expressed by myofibroblasts, which is a major type of cells contributing to tissue fibrosis. In our study, the correlation of α-SMA expression with MMP12, a pre-fibrotic mediator, may indicate the on-going fibrosis in IVD degeneration.