Low back pain is a significant socioeconomic burden and intervertebral disc degeneration has been implicated as a cause. A reliable animal model of disc degeneration is necessary to evaluate therapeutics, and functional metrics are essential to quantify their benefit. To this end, needle puncture injuries were created in the caudal intervertebral discs of mice to induce disc degeneration. Compression, torsion, and creep mechanics were assessed both immediately and after eight weeks to distinguish between the effects of injury and the subsequent reparative or degenerative response. Two needle sizes (29 and 26 gauge) were used to determine injury sizedependence. Compressive stiffness (62%), torsional stiffness (60%), and early damping stiffness (84%) decreased immediately after injury with the large needle (26G). These mechanical properties did not change over time despite structural and compositional changes. At 8 weeks following large needle injury, disc height decreased (37%), nucleus pulposus (NP) glycosaminoglycan content decreased (41%), and NP collagen content increased (45%). The small needle size had no significant effect on mechanics and did not initiate degenerative changes in structure and composition. Thus, the injection of therapeutics into the NP with a minimal needle size may limit damage due to the needle insertion. These findings, along with the wide commercial availability of mouse-specific biological probes, indicate that the mouse caudal disc model can be a powerful tool for investigating disc degeneration and therapy.

There is a well-established link between cytokine expression and the progression of intervertebral disc degeneration. Among these cytokines, interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α) are the most commonly studied. To investigate whether systemic hTNF-α overexpression affects intervertebral disc health, we studied the spine phenotype of Tg197 mice, a widely used hTNF-α transgenic line. These mice were studied at 12–16 weeks of age using comprehensive histochemical and immunohistological analysis of the spinal motion segment. Micro-CT analysis was performed to quantify vertebral trabecular bone architecture. The Tg197 mice evidenced spontaneous annular tears and herniation with increased vascularity in subchondral bone and significant immune cell infiltration. The full-thickness annular tear without nucleus pulposus (NP) extrusion resulted in neutrophil, macrophage, and mast cell infiltration into the disc, whereas the disc with full-thickness tear and pronounced NP herniation showed additional presence of CD4+ and CD8+ T cells. While the observed defects involved failure of the annular, endplate, and vertebral junction, there were no obvious alterations in the collagen or aggrecan content in the NP and annulus fibrosus or the maturity of collagen fibers in Tg197 mice. Despite elevated systemic inflammation and pronounced loss of trabecular bone in the vertebrae, intact Tg197 discs were healthy and showed an increase in NP cell number. The NP cells in intact discs preserved expression of phenotypic markers: CAIII, Glut1, and Krt19. In conclusion, elevated systemic TNF-α increases the susceptibility of mice to spontaneous disc herniation and possibly radiculopathy, without adversely affecting intact intervertebral disc health.

IntroductionInflammation plays a key role in the progression of intervertebral disc degeneration, a condition strongly implicated as a cause of lower back pain. The objective of this study was to investigate the therapeutic potential of poly(lactic-co-glycolic acid) (PLGA) microspheres loaded with interleukin-1 receptor antagonist (IL-1ra) for sustained attenuation of interleukin-1 beta (IL-1β) mediated degradative changes in the nucleus pulposus (NP), using an in vitro model.MethodsIL-1ra was encapsulated in PLGA microspheres and release kinetics were determined over 35 days. NP agarose constructs were cultured to functional maturity and treated with combinations of IL-1β and media conditioned with IL-1ra released from microspheres at intervals for up to 20 days. Construct mechanical properties, glycosaminoglycan content, nitrite production and mRNA expression of catabolic mediators were compared to properties for untreated constructs using unpaired Student's t-tests.ResultsIL-1ra release kinetics were characterized by an initial burst release reducing to a linear release over the first 10 days. IL-1ra released from microspheres attenuated the degradative effects of IL-1β as defined by mechanical properties, glycosaminoglycans (GAG) content, nitric oxide production and mRNA expression of inflammatory mediators for 7 days, and continued to limit functional degradation for up to 20 days.ConclusionsIn this study, we successfully demonstrated that IL-1ra microspheres can attenuate the degradative effects of IL-1β on the NP for extended periods. This therapeutic strategy may be appropriate for treating early-stage, cytokine-mediated disc degeneration. Ongoing studies are focusing on testing IL-1ra microspheres in an in vivo model of disc degeneration, as a prelude to clinical translation.

Increased cytokine expression, in particular interleukin‐1β (IL‐1β), is considered a hallmark of intervertebral disc degeneration. However, the causative relationship between IL‐1 and age‐dependent degeneration has not been established. To investigate the role of IL‐1 in driving age‐related disc degeneration, we studied the spine phenotype of global IL‐1α/β double knockout (IL‐1KO) mice at 12 and 20 months. Multiplex ELISA analysis of blood revealed significant reductions in the concentrations of IFN‐γ, IL‐5, IL‐15, TNF‐α, IP‐10, and a trend of reduced concentrations of IL‐10, macrophage inflammatory protein 1α (MIP‐1α), keratinocyte chemoattractant/human growth‐regulated oncogene (KC/GRO), and IL‐6. However, the circulating level of MIP‐2, a neutrophil chemoattractant, was increased in the IL‐1KO. The alterations in systemic cytokine levels coincided with altered bone morphology—IL‐1KO mice exhibited significantly thicker caudal cortical bone at 12 and 20 months. Despite these systemic inflammatory and bony changes, IL‐1 deletion only minimally affected disc health. Both wild‐type (WT) and IL‐1KO mice showed age‐dependent disc degeneration. Unexpectedly, rather than protecting the animals from degeneration, the aging phenotype was more pronounced in IL‐1KO animals: knockout mice evidenced significantly more degenerative changes in the annulus fibrosis (AF) together with alterations in collagen type and maturity. At 20 months, there were no changes in nucleus pulposus (NP) extracellular matrix composition or cellular marker expression; however, the IL‐1KO NP cells occupied a smaller proportion of the NP compartment that those of WT controls. Taken together, these results show that IL‐1 deletion altered the systemic inflammatory environment and vertebral bone morphology. However, instead of protecting discs from age‐related disc degeneration, global IL‐1 deletion amplified the degenerative phenotype. © 2019 American Society for Bone and Mineral Research.