Objective. The nucleus pulposus (NP) of the intervertebral disc develops from the notochord. Humans and other species in which notochordal cells (NCs) disappear to be replaced by chondrocyte-like mature NP cells (MNPCs) frequently develop disc degeneration, unlike other species that retain NCs. The reasons for NC disappearance are unknown. In humans, the change in cell phenotype (to MNPCs) coincides with changes that decrease nutrient supply to the avascular disc. We undertook this study to test the hypothesis that the consequent nutrient stress could be associated with NC disappearance.Methods. We measured cell densities and metabolic rates in 3-dimensional cultures of porcine NCs and bovine MNPCs, and we determined survival rates under conditions of nutrient deprivation. We used scanning electron microscopy to examine end plate porosity of discs with NCs and those with MNPCs. Nutrientmetabolite profiles and cell viability were calculated as a function of cell density and disc size in a consumption/ diffusion mathematical model.Results. NCs were more active metabolically and more susceptible to nutrient deprivation than were MNPCs. Hypoxia increased rates of glycolysis in NCs but not in MNPCs. Higher end plate porosity in discs with NCs suggested greater nutrient supply in keeping with higher nutritional demands. Mathematical simulations and experiments using an analog disc diffusion chamber indicated that a fall in nutrient concentrations resulting from increased diffusion distance during growth and/or a fall in blood supply through end plate changes could instigate NC disappearance.Conclusion. NCs demand more energy and are less resistant to nutritional stress than MNPCs, which may shed light on the fate of NCs in humans. This provides important information about prospective NC tissue engineering approaches.The intervertebral discs are cartilaginous structures interspersed between the vertebral bodies, providing flexibility to the spinal column. They consist of 3 regions: the outer annulus fibrosus (AF) surrounding the inner nucleus pulposus (NP) and a thin hyaline cartilaginous end plate lying between the disc and the adjacent vertebral bodies. The AF and NP differ in developmental origin, with the annulus arising from the mesenchyme and the nucleus from the notochord (1,2). During development the highly hydrated NP is populated by clusters of large vacuolated notochordal cells (NCs) of distinct molecular phenotype (2,3). In humans and some other species (e.g., cattle, chondrodystrophoid dogs) but not in others (e.g., rodents, pigs), NCs disappear before maturity to be replaced by chondrocyte-like cells of unknown provenance (here called mature NP cells [MNPCs]), which synthesize a more collagenous and less hydrated matrix (1,4-6).

has been postulated that RAR antagonists may prevent or reverse retinoid-mediated cartilage destruction and a RAR pan antagonist was previously shown to improve clinical scores in the collagen-induced arthritis (CIA) model, albeit with unacceptable adverse effects on testes. We have postulated that the primary beneficial joint effects of RAR antagonists are associated with RARgamma, while the adverse effects on testes are associated with RARalpha. Thus, we have identified a highly selective RARgamma antagonist (LY2813631) to test this hypothesis. Methods: The RAR antagonist LY2813631 demonstrated in vitro selective affinity for RARgamma in a SPA-based binding assay using full length RAR alpha, beta and gamma protein and the synthetic pan agonist 3H-TTNPB. Functional antagonism and selectivity was demonstrated using a GAL4-RAR alpha, beta and gamma and Gal4 response element/Luciferase constructs, co-transfected into HEK 293 cells. Functional activity in chondrocytes was demonstrated using agonist TTNPB and primary bovine chondrocytes, looking at the ability of LY2813631 to regulate OA-relevant genes, such as MMP13, ADAMTS-5 and Type 2 Collagen. In vivo studies utilized Lewis rats to show reversal of RAR gamma agonist induced changes in OA relevant genes in the articular cartilage and reduction in OA-related neoepitopes in the synovial fluid. The collagen induced arthritis (CIA) model was used to determine joint efficacy in rats. Results: In vitro, LY2813631 binds RARgamma (Ki ¼ 0.74 nM) with significantly higher affinity than RARalpha (Ki ¼ 400 nM) and RAR beta (Ki ¼ 25 nM) and shows selective functional antagonism at RAR gamma (Kb ¼ 42 nM) in HEK 293 cells, compared to RARalpha (Kb ¼ 2010 nM) and RAR beta (Kb ¼ 359 nM). Additionally, LY2813631 normalized RAR agonist-induced increases in the catabolic enzyme ADAMTS-5 in primary bovine chondrocytes. In vivo, a selective RARgamma agonist was shown to increase mRNA levels of ADAMTS-5 and decrease mRNA levels of type 2 collagen in the articular cartilage of rats after 3 days of oral dosing. The animals also showed a 5-fold increase in the type 2 collagen neoepitopes 9A4 and CTX-II in synovial fluid. These effects could be blocked by co-dosing RARgamma antagonist LY2813631, demonstrating specific RARgamma-mediated target engagement in the joint space. LY2813631 also improved ankle and knee histopathology scores in the rat CIA model at doses that did not cause testicular degeneration. Conclusions: Our findings support a role for endogenous retinoids in the destruction of articular cartilage in OA and suggest that these effects are primarily mediated through RARgamma signaling. We have also shown that the beneficial effects of RARgamma antagonist LY2813631 can be demonstrated without concomitant adverse effects on the testes, supporting RARgamma as a potentially safe target for disease modification in arthritic diseases.Purpose: Modulation of the BMP pathway remains an attractive target for development of novel anabolic treatments for osteoarthritis. We are aiming to ide...