PROJECT SUMMARY/ABSTRACT Degenerative temporomandibular joint (TMJ) disorders (TMD) are some of the most challenging oral and maxillofacial problems, and when left untreated, they eventually lead to significant functional deficits, pain, stiffness, and osteoarthritis. As an example of degenerative TMD, the risk for post-traumatic osteoarthritis (PTOA) after serious joint injuries has not improved substantially in the last 40 years despite many refinements in care. This underscores the urgent need for new treatments to prevent PTOA initiated by joint damage. Fibrocartilage is notoriously limited in its intrinsic capacity for repair, focal damage associated with joint injuries seldom heals and often worsens to engulf entire articular surfaces. Cell-based therapies intended to regenerate neocartilage in situ have shown some clinical promise. The two most common, microfracture and autologous chondrocyte implantation (ACI), work well in the short term; however, long term results have been disappointing. The presence of chondrogenic progenitor cells (CPCs) in the TMJ fibrocartilage suggests that a rudimentary self- repair mechanism exists that might be marshaled for fibrocartilage regeneration after traumatic injury. CPC recruitment can be enhanced by exosomes derived from bone marrow stromal cells (BMSC-Exos), which serve an important role in intercellular communication and tissue repair. Our central hypothesis is that BMSC-Exos will stimulate TMJ condylar fibrocartilage repair by promoting CPC chemotaxis and chondrogenesis, intended for the treatment of focal lesions in TMJ fibrocartilage. The main appeal of a CPC-based repair strategy lies in their ability to heal by self-congregating at injury sites without ex vivo cell expansion and the additional surgery needed for grafting in ACI. Specific aims in this proposed study are to characterize BMSC-Exos and determine the effects of BMSC-Exos on CPC migration and fibrocartilage differentiation in vitro. Finally, in vivo fibrocartilage repair will be evaluated in a rabbit TMJ disc perforation model, where direct fibrocartilage contact between the condyle and mandibular fossa will induce degenerative fibrocartilage. BMSC-Exos encapsulated in hydrogel (F-127/hyaluronic acid) will be injected to the damaged TMJs 2 weeks after the perforation surgeries. Six weeks after the injection, integrity of the repaired fibrocartilage will be determined by histological/immunohistochemical staining and evaluated based on a modified OARSI scoring system. At the conclusion of this project, we will be able to define the therapeutic potential of BMSC- Exos for fibrocartilage repair and to identify miRNAs that may regulate cell migration and neotissue formation. As our future research plans, we will validate the candidate key miRNAs and synthesize target microRNA (miRNAs) for CPC migration and neofibrocartilage formation. The target miRNAs will be loaded in engineered exosomes for in vivo delivery, thereby replacing BMSC ...