IGF1 Signaling in Temporomandibular Joint Fibrocartilage Stem Cells Regulates Cartilage Growth and Homeostasis in Mice

Objective: Investigate functional roles of Igf1 in fibrocartilage stem cell (FCSC) for temporomandibular joint (TMJ) cartilage growth and homeostasis. Methods: Gli1-CreER+; RosaTdTomato mice were used for validating FCSCs lineage labeling efficiency. In Gli1-/Col2-CreER+; Igf1fl/fl mice, TMJ cartilage morphological and functional changes were characterized at 4 weeks and 5 months after Igf1 deletion. H&E, Safranine O and immuno-histochemistry staining were performed. FCSCs specificity were characterized using EdU and TUNEL staining. A unilateral anterior crossbite (UAC) mouse model was generated for mimicking TMJ osteoarthritis status. Results: In Gli1-CreER+; RosaTdTomato mice, RFP labeled FCSCs showed favorable proliferative capacity. 4 weeks after Igf1 deletion, Gli1+ and Col2+ cell lineages led to distinct pathological changes of TMJ cartilage morphology. A more serious reduction of cartilage thickness and cell density were found in the superficial layers in Gli1-CreER+; Igf1fl/fl mice. 5 months after Igf1 deletion, more severe disordered cell arrangement in TMJ cartilage were found in both groups with Gli1+ and Col2+ specific deletion of Igf1. Immunostaining showed that PI3K/Akt signaling pathway was blocked in the superficial layers of TMJ in Gli1-CreER+; RosaTdTomato mice. Finally, deletion of Igf1 in FCSCs significantly aggravated osteoarthritis (OA) phenotypic changes in TMJ in UAC mice model, characterized in decreased cartilage thickness, cell numbers and loss of extracellular matrix secretion. Conclusion: Igf1 deletion disrupted stem cell functions of FCSCs, leading to disordered cell distribution during TMJ growth, as well as exaggerated the OA process in TMJ under pathological condition. In TMJ cartilage, Igf1 expression in FCSCs is critical for PI3K/Akt activation, which may be involved in regulating FCSCs self-renewal and differentiation.