Evaluation of Prg4 as a New Therapy for TMJ Disc Degeneration

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The temporo-mandibular joint (TMJ), composed of the mandibular condyle, the glenoid fossa of the temporal bone and the articular disc, is essential for speech and mastication. The disc plays an important role in TMJ movement, by allowing the condyle to freely slide down the slope of the glenoid fossa. TMJ disorders (disruption in the structure, function or physiology of the TMJ) afflict approximately 20% of the U.S. population, and up to 70% of the conditions are due to TMJ disc displacement and/or damage. It is generally believed that aberrations in the lubrication system, which depends primarily on joint lubricants, including hyaluronic acid, phospholipid, and Proteoglycan 4 (Prg4), contribute to TMJ dysfunction. A critical gap in knowledge is to clarify the extent to which joint lubricants establish and maintain the structure and function of the TMJ disc. We found that TMJs of Prg4-null (Prg4-/-) mice exhibited significantly greater degenerative changes compared to other synovial joints, including the knee. We found in Prg4-/- mice that: (a) the TMJ discs failed to develop their biconcave shape and became substantially thickened over time; (b) a large number of the disc cells differentiated into fibrochondrocytes, leading to disc mineralization; and (c) subsequently, the condylar cartilage broke down and the trabecular bone exhibited architectural changes. In addition, the mechanical properties of the Prg4-/- discs were significantly altered compared to controls. These and other novel data lead to our central hypothesis that Prg4 is necessary: 1) to maintain TMJ disc cell phenotype and function, and 2) to protect the TMJ from degenerative changes. In Aim 1, we will determine the signaling pathways, by which PRG4 regulates region- and stage-specific phenotypes of disc cells across multiple anatomical sites, and elucidate how global loss of Prg4 leads to abnormal disc cell function and disc zonal organization. By inducing cell death in Prg-4 expressing cells postnatally, we will also define the roles of Prg4- expressing cells in disc morphogenesis and homeostasis. In Aim 2, we will test if TMJ disc degeneration is treatable, and, if so, also determine if there is a therapeutic window for successful intervention. We will restore Prg4 expression postnatally via virally-driven overexpression of Prg4 and tamoxifen or virally-driven Cre recombinase in ROSA26CreERT2;Prg4GT (Prg4cGT) mice, which have a reversible gene-trap (GT) in Prg4 (Prg4GT), in which Prg4 expression is activated upon Cre-mediated excision of the gene-trap. Phenotypic analyses will include histopathology, gene and protein expression, and biomechanical tests. Orofacial pain will be evaluated by mechanical reflex testing and rodent grimace scale. Our project will provide novel and far- reaching information regarding the role of joint lubricants in TMJ disc maintenance and disease. Our rescue experiments should provide a proof-of-principle that TMJ disc degeneration is amenable to therapeutic intervention. While our experiments use the TMJ as a model system, we believe results from this application will also be directly applicable to other synovial joints.