Increased expression of matrix metalloproteinase-10, nerve growth factor and substance P in the painful degenerate intervertebral disc
The human intervertebral disc is an avascular and aneural tissue comprising a central gelatinous region (the nucleus pulposus), surrounded by a fibrous ring of highly organised collagen fibres (the annulus fibrosus). The extracellular matrix of the nucleus pulposus is rich in type II collagen and proteoglycans, predominantly aggrecan, which produces a highly hydrated matrix capable of withstanding the loads experienced within the spine. This extracellular matrix is constantly being remodelled in a process driven by the constituent nucleus pulposus cells.
During intervertebral disc degeneration there is an imbalance in the normal homeostatic mechanisms, which favours matrix catabolism and leads to a loss of disc height,
coupled with ingrowth of both nerves and blood vessels into both the annulus fibrosus and nucleus pulposus. The authors have previously demonstrated that this ingrowth of nerves into the degenerate intervertebral disc is associated with low back pain. While low back pain is multifactorial, studies have shown that this debilitating condition affecting around 80% of adults at some stage of life is associated with intervertebral disc degeneration in approximately 40% of cases. Indeed in a recent study by Cheung et al (2009) it has been shown that there is a significant association of lumber disc degeneration imaged by MRI with low back pain.
The nucleus pulposus of the normal human intervertebral disc is an avascular and aneural environment, comprising of chondrocyte like cells embedded within an extracellular matrix rich in proteoglycans and collagens. This matrix is continuously remodelled in a process controlled by the nucleus pulposus cells and closely regulated by anabolic growth factors and catabolic cytokines. In intervertebral disc degeneration there is disregulation in this finely balanced homeostatic matrix turnover mechanism, leading to an increase in catabolic processes over anabolic matrix formation. Over time this results in breakdown of matrix, until the disc loses both height and function and in a large proportion of cases there is innervation and initiation of the pain response which leads to low back pain.
Matrix metalloproteinases (MMPs) are known to be involved in the degradation of the nucleus pulposus during intervertebral disc degeneration. This study investigated MMP-10 (stromelysin-2) expression in the nucleus pulposus during intervertebral disc degeneration and correlated its expression with proinflammatory cytokines and molecules involved in innervation and nociception during degeneration which results in low back pain.
Human nucleus pulposus tissue was obtained at post-mortem from patients without a history of back pain and graded as histologically normal or degenerate. Symptomatic degenerate nucleus pulposus samples were also obtained at surgery for low back pain. Expression of MMP-10 mRNA and protein was analysed using real time PCR and immunohistochemistry. Gene expression for proinflammatory cytokines IL-1 and TNF-alpha, nerve growth factor and the pain associated neuropeptide Substance P were also analysed. Correlations between MMP-10 and IL-1, TNF-alpha and nerve growth factor were assessed along with nerve growth factor with Substance P.
MMP-10 mRNA was significantly increased in surgical degenerate nucleus pulposus when compared to post-mortem normal and post-mortem degenerate samples. MMP-10 protein was also significantly higher in degenerate surgical nucleus pulposus samples compared to post-mortem normal. IL-1 and MMP-10 mRNA demonstrated a significant correlation in surgical degenerate samples, while TNF-alpha was not correlated with MMP-10 mRNA. Nerve growth factor was significantly correlated with both MMP-10 and Substance P mRNA in surgical degenerate nucleus pulposus samples.
MMP-10 expression is increased in the symptomatic degenerate intervertebral disc, where it may contribute to matrix degradation and initiation of nociception. Importantly, this study suggests differences in the pathways involved in matrix degradation between painful and pain free intervertebral disc degeneration.
This study has demonstrated, for the first time, increased MMP-10 expression in the symptomatic degenerate intervertebral disc when compared to non-degenerate or asymptomatic degenerate intervertebral disc . The correlation of MMP-10 with IL-1 and nerve growth factor, combined with the correlation between nerve growth factor and Substance P in symptomatic degenerate intervertebral disc’s suggests differences in the catabolic pathways between painful and pain free intervertebral disc degeneration. While this study focused on gene and protein expression profiling, it emphasizes the importance of MMP-10 in symptomatic intervertebral disc degeneration and highlights that a more detailed investigation into these pathways, including analysis of enzyme activities is required to better understand the underlying pathogenesis.