The role of biomechanical factors in models of intervertebral disc degeneration across multiple length scales

Repetitive strikes loading organ culture model to investigate the biological and biomechanical responses of the intervertebral disc

Background

Disc degeneration is associated with repetitive violent injuries. This study aims to explore the impact of repetitive strikes loading on the biology and biomechanics of intervertebral discs (IVDs) using an organ culture model.

Methods

IVDs from the bovine tail were isolated and cultured in a bioreactor, with exposure to various loading conditions. The control group was subjected to physiological loading, while the model group was exposed to either one strike loading (compression at 38% of IVD height) or repetitive one strike loading (compression at 38% of IVD height). Disc height and dynamic compressive stiffness were measured after overnight swelling and loading. Furthermore, histological morphology, cell viability, and gene expression were analyzed on Day 32. Glycosaminoglycan (GAG) and nitric oxide (NO) release in conditioned medium were also analyzed.

Results

The repetitive one strike group exhibited early disc degeneration, characterized by decreased dynamic compression stiffness, the presence of annulus fibrosus clefts, and degradation of the extracellular matrix. Additionally, this group demonstrated significantly higher levels of cell death (p < 0.05) and glycosaminoglycan (GAG) release (p < 0.05) compared to the control group. Furthermore, upregulation of MMP1, MMP13, and ADAMTS5 was observed in both nucleus pulposus (NP) and annulus fibrosus (AF) tissues of the repetitive one strike group (p < 0.05). The one strike group exhibited annulus fibrosus clefts but showed no gene expression changes compared to the control group.

Conclusions

This study shows that repetitive violent injuries lead to the degeneration of a healthy bovine IVDs, thereby providing new insights into early-stage disc degeneration.

Protective effects of Shensuitongzhi formula on intervertebral disc degeneration via downregulation of NF-κB signaling pathway and inflammatory response

Low back pain (LBP) is a common orthopedic disease over the world. Lumbar intervertebral disc degeneration (IDD) is regarded as an important cause of LBP. Shensuitongzhi formula (SSTZF) is a drug used in clinical treatment for orthopedic diseases. It has been found that SSTZF can have a good treatment for IDD. But the exact mechanism has not been clarified. The results showed that SSTZF protects against LSI-induced degeneration of cartilage endplates and intervertebral discs. Meanwhile, SSTZF treatment dramatically reduces the expression of inflammatory factor as well as the expression of catabolism protein and upregulates the expression of anabolism protein in LSI-induced mice. In addition, SSTZF delayed the progression of LSI-induced IDD via downregulation the level of NF-κB signaling key gene RELA and phosphorylation of key protein P65 in endplate chondrocytes. Our study has illustrated the treatment as well as the latent mechanism of SSTZF in IDD.

Senescent response in inner annulus fibrosus cells in response to TNFα, H2O2, and TNFα-induced nucleus pulposus senescent secretome

Senescence, particularly in the nucleus pulposus (NP) cells, has been implicated in the pathogenesis of disc degeneration, however, the mechanism(s) of annulus fibrosus (AF) cell senescence is still not well understood. Both TNFα and H2O2, have been implicated as contributors to the senescence pathways, and their levels are increased in degenerated discs when compared to healthy discs. Thus, the objective of this study is to identify factor(s) that induces inner AF (iAF) cell senescence. Under TNFα exposure, at a concentration previously shown to induce senescence in NP cells, bovine iAF cells did not undergo senescence, indicated by their ability to continue to proliferate as demonstrated by Ki67 staining and growth curves and lack of expression of the senescent markers, p16 and p21. The lack of senescent response occurred even though iAF express higher levels of TNFR1 than NP cells. Interestingly, iAF cells showed no increase in intracellular ROS or secreted H2O2 in response to TNFα which contrasted to NP cells that did. Following TNFα treatment, only iAF cells had increased expression of the superoxide scavengers SOD1 and SOD2 whereas NP cells had increased NOX4 gene expression, an enzyme that can generate H2O2. Treating iAF cells with low dose H2O2 (50 μM) induced senescence, however unlike TNFα, H2O2 did not induce degenerative-like changes as there was no difference in COL2, ACAN, MMP13, or IL6 gene expression or number of COL2 and ACAN immunopositive cells compared to untreated controls. The latter result suggests that iAF cells may have distinct degenerative and senescent phenotypes. To evaluate paracrine signalling by senescent NP cells, iAF and TNFα-treated NP cells were co-cultured. In contact co-culture the NP cells induced iAF senescence. Thus, senescent NP cells may secrete soluble factors that induce degenerative and senescent changes within the iAF. This may contribute to a positive feedback loop of disc degeneration. It is possible these factors may include H2O2 and cytokines (such as TNFα). Further studies will investigate if human disc cells respond similarly.