Degenerative changes of the intervertebral disc (IVD) are a leading cause of disability affecting humans worldwide and has been attributed primarily to trauma and the accumulation of pathology during aging. While genetic defects have also been associated with disc degeneration, the precise mechanisms driving the initiation and progression of disease have remained elusive due to a paucity of genetic animal models. Here, we discuss a novel conditional mouse genetic model of endplate-oriented disc herniations in adult mice. Using conditional mouse genetics, we show increased mechanical stiffness and reveal dysregulation of typical gene expression profiles of the IVD in adhesion G-protein coupled receptor G6 (Adgrg6) mutant mice prior to the onset of endplate-oriented disc herniations in adult mice. We observed increased STAT3 activation prior to IVD defects and go on to demonstrate that treatment of Adgrg6 conditional mutant mice with a small molecule inhibitor of STAT3 activation ameliorates endplate-oriented herniations. These findings establish ADGRG6 and STAT3 as novel regulators of IVD endplate and growth plate integrity in the mouse, and implicate ADGRG6/STAT3 signaling as promising therapeutic targets for endplate-oriented disc degeneration.

While it is well-known that mechanical signals can either promote or disrupt intervertebral disc (IVD) homeostasis, the molecular mechanisms for transducing mechanical stimuli are not fully understood. The transient receptor potential vanilloid 4 (TRPV4) ion channel activated in isolated IVD cells initiates extracellular matrix (ECM) gene expression, while TRPV4 ablation reduces cytokine production in response to circumferential stretching. However, the role of TRPV4 on ECM maintenance during tissue-level mechanical loading remains unknown. Using an organ culture model, we modulated TRPV4 function over both short- (hours) and long-term (days) and evaluated IVDs' response. Activating TRPV4 with the agonist GSK101 resulted in a Ca2+ flux propagating across the cells within the IVD. NF-kB signaling in the IVD peaked at 6 hours following TRPV4 activation that subsequently resulted in higher IL-6 production at 7 days. These cellular responses were concomitant with the accumulation of glycosaminoglycans and increased hydration in the nucleus pulposus that culminated in higher stiffness of the IVD. Sustained compressive loading of the IVD resulted in elevated NF-kB activity, IL-6 and VEGF-A production, and degenerative changes to the ECM. TRPV4 inhibition using GSK205 during loading mitigated the changes in inflammatory cytokines, protected against IVD degeneration, and but could not prevent ECM disorganization due to mechanical damage in the annulus fibrosus. These results indicate TRPV4 plays an important role in both short- and long-term adaptations of the IVD to mechanical loading. The modulation of TRPV4 may be a possible therapeutic for preventing load-induced IVD degeneration.

Degenerative changes of the intervertebral disc (IVD) are a leading cause of disability affecting humans worldwide. While this is primarily attributed to trauma and aging, genetic variation is associated with disc degeneration in humans. However, the precise mechanisms driving the initiation and progression of disease remain elusive due to a paucity of genetic animal models.Here, we discuss a novel genetic mouse model of endplate-oriented disc degeneration. We show that the adhesion G-protein coupled receptor G6 (ADGRG6) mediates several anabolic and catabolic factors, fibrotic collagen genes, pro-inflammatory pathways, and mechanical properties of the IVD, prior to the onset of overt histopathology of these tissues. Furthermore, we found increased IL-6/STAT3 activation in the IVD and demonstrate that treatment with a chemical inhibitor of STAT3 activation ameliorates disc degeneration in these mutant mice. These findings establish ADGRG6 as a critical regulator of homeostasis of adult disc homeostasis and implicate ADGRG6 and STAT3 as promising therapeutic targets for degenerative joint diseases. Author summaryDegenerative changes of the intervertebral disc (IVD) are a leading cause of disability affecting humans worldwide. While this is primarily attributed to trauma and aging, genetic variation is associated with disc degeneration in humans. However, the precise mechanisms driving the initiation and progression of disease remain elusive due to a paucity of genetic animal models.Here, we discuss a novel genetic mouse model of endplate-oriented disc degeneration. We show that the adhesion G-protein coupled receptor G6 (ADGRG6) mediates fibrotic collagen expression, causing increased mechanical stiffness of the IVD prior to the onset of histopathology in adult mice. Furthermore, we found increased IL-6/STAT3 activation in the IVD and demonstrate that treatment with a chemical inhibitor of STAT3 activation ameliorates disc degeneration in these mutant mice. Our results demonstrate that ADGRG6 regulation of STAT3 signaling is important for IVD homeostasis, indicating potential therapeutic targets for degenerative joint disorders.