3D assessment of intervertebral disc degeneration in zebrafish identifies changes to bone density that prime disc disease

Back pain is a common condition with high social impact, which represents a global health burden. Intervertebral disc disease (IVDD) is one of the major causes of back pain; no therapeutics are currently available to reverse the disease. The impact of bone mineral density (BMD) on IVDD has been controversial, with some studies suggesting osteoporosis as causative and others as protective for IVDD. Functional studies to evaluate the influence of genetic components of BMD in IVDD could highlight opportunities for drug development and repurposing. By taking a holistic 3D approach, we established ageing zebrafish as a model for spontaneous IVDD. Increased BMD in ageing, detected by automated computational analysis, is caused by bone deformities at the endplates. However, aged zebrafish spines showed changes to bone morphology, microstructure, mineral heterogeneity and increased fragility that resembled osteoporosis. Elements of the discs recapitulated IVDD symptoms found in humans: intervertebral ligament (equivalent to the annulus fibrosus) showed disorganised collagen fibres and herniation, while disc centre (nucleus pulposus equivalent) showed dehydration and cellular abnormalities. We manipulated BMD in young zebrafish by mutating sp7 and cathepsin K, leading to low and high-BMD, respectively. Remarkably, we detected IVDD in both groups, demonstrating that low-BMD does not protect against IVDD, and finding strong correlation between high-BMD and IVDD. Deep learning of high resolution synchrotron µCT image data was applied to analyse osteocyte 3D lacunar distribution and morphology, revealing a role of sp7 in controlling osteocyte lacunar 3D profile. Our findings suggest potential avenues of targeting bone quality to identify beneficial therapeutics for IVDD.