Anabolic phenotype in cartilage-specific mitogen-inducible gene-6 knockout mice is independent of transforming growth factor-α.

Background/objective: Osteoarthritis (OA) is a whole joint disorder with no disease modifying treatment currently available. The Epidermal Growth Factor Receptor (EGFR) signaling pathway plays an important role in cartilage/bone development and its ligand transforming growth factor-α (TGFα) is upregulated in OA. In contrast, Mitogen-inducible gene 6 (Mig6) is a negative regulator of EGFR, and cartilage-specific Mig-6 deletion results in anabolic effects on cartilage and formation of chondro-osseus nodules (CON). We aimed to attenuate EGFR signaling by inhibiting TGFα production in cartilage-specific Mig6 deficient mice, to test whether this would prevent the formation of CONs. Methods: We generated double knockout mice by crossing cartilage-specific Mig-6fl/flCol2a1-Cre+/- and whole-body Tgfa± mice to generate experimental and control wild-type mice. Knee and elbow sections were used to examine articular cartilage thickness, cell density, and osteoclast presence. Additionally, immunohistochemistry was completed to analyze phospho-EGFR and SOX9. Results: Mig-6 deficient mice display cartilage thickening and CONs at 12 weeks in both the elbow and knee joints, which is independent of TGFα ligand presence. Similarly, articular cartilage cell density is increased in Mig6-cKO/Tgfa-KO and Mig6-cKOmice, but not Tgfa-KO mice, and displays increased SOX9 and phospho-EGFR staining. Conclusion: The articular cartilage displays increased thickness/cell density and CON formation independent of the presence of TGFα, suggesting the anabolic phenotype in the Mig6-deficient mice is independent of TGFα/EGFR binding. The anabolic phenotype may be due to an alternative EGFR ligand activation, or other non-EGFR specific mechanism. More research is required to elucidate the exact pathway responsible for the anabolic effects.

Emerging therapeutic targets for osteoarthritis.

INTRODUCTION: Osteoarthritis is a heterogeneous joint disorder that lacks a clinically meaningful disease modifying drug. Animal models have been beneficial in understanding basic joint pathology and providing rationale for future clinical trials on identified targets. This review aims to discuss promising therapeutic targets of osteoarthritis that are currently in animal studies or early clinical trials. AREAS COVERED: PubMed was searched for articles published between 2017 and 2021 with the following terms: (osteoarthritis AND autophagy) OR (osteoarthritis AND senescence) OR (osteoarthritis AND TGFbeta) OR (osteoarthritis AND EGFR) OR (osteoarthritis AND Wnt/ß-catenin) OR (osteoarthritis AND inflammation). Specific targets include the PI3/AKT/mTOR pathway, epidermal growth factor receptor, Toll-like receptors, and inflammatory interleukins, among others. EXPERT OPINION: In reviewing these targets, it is clear that the field of therapeutic targets for osteoarthritis has grown tremendously. We have gained a better understanding of previously identified targets, identified new targets, and have the opportunity to explore enhanced drug delivery via viral vectors. Regardless, translation to clinical benefits is still lacking in most cases. We propose that this may be due to the heterogeneous nature of the disease, lack of early diagnostic markers, mismatched preclinical animal models and clinical populations, and the complex role of many targets of interest.

Exposure to the RXR Agonist SR11237 in Early Life Causes Disturbed Skeletal Morphogenesis in a Rat Model.

Longitudinal bone growth occurs through endochondral ossification (EO), controlled by various signaling molecules. Retinoid X Receptor (RXR) is a nuclear receptor with important roles in cell death, development, and metabolism. However, little is known about its role in EO. In this study, the agonist SR11237 was used to evaluate RXR activation in EO. Rats given SR11237 from post-natal day 5 to post-natal day 15 were harvested for micro-computed tomography (microCT) scanning and histology. In parallel, newborn CD1 mouse tibiae were cultured with increasing concentrations of SR11237 for histological and whole-mount evaluation. RXR agonist-treated rats had shorter long bones than the controls and developed dysmorphia of the growth plate. Cells invading the calcified and dysmorphic growth plate appeared pre-hypertrophic in size and shape, in correspondence with p57 immunostaining. Additionally, SOX9-positive cells were found surrounding the calcified tissue. The epiphysis of SR11237-treated bones showed increased TRAP staining and additional TUNEL staining at the osteo-chondral junction. MicroCT revealed morphological disorganization in the long bones of the treated animals. This study suggests that stimulation of RXR causes irregular ossification, premature closure of the growth plate, and disrupted long bone growth in rodent models.