Role of hyaluronan in YAP signaling on TMJ health and pathology - Abstract The temporomandibular joint (TMJ) controls jaw movement and function during speech and mastication and comprises the mandibular condyle, articular disc, and glenoid fossa. These structures exhibit a fibrocartilaginous organization whereby the apical tissue layer, rich in stem cells and chondroprogenitor cells, regulates normal growth and remodeling by supplying chondrocytes to underlying cartilage. While these aspects of TMJ biology are well understood, other functions remain unclear, such as how the stem/chondroprogenitor cells are maintained, whether changes in these cells underlie TMJ osteoarthritis (OA), or if other factors instigate pathogenesis, including TMJ lubrication. Hyaluronan/hyaluronic acid (HA) lubricates the TMJ and is used to alleviate OA symptoms, providing only short-term relief. Studies in other fields indicate that HA has additional physiological roles, including cell signaling, which may also apply to the TMJ. Notably, Has1- and Has3-null mice exhibit no significant skeletal abnormalities, suggesting the limited roles of these isoforms in skeletal development. Our preliminary studies reveal that Has2 is prominently expressed in condylar chondroprogenitor cells and chondrocytes. Conditional ablation of Has2 leads to changes resembling TMJ OA, with neither Has1 nor Has3 compensating for its loss. Early phenotypic changes in Has2-mutant condyles include disrupted chondrogenesis, impaired chondrocyte maturation, increased expression of the HA-processing enzyme Tmem2, and elevated nuclear translocation of YAP. This is further supported by siRNA-mediated knockdown of Has2 in primary condylar chondroprogenitor cells, which results in increased YAP nuclear translocation and decreased proteoglycan synthesis compared to controls. This leads to our central hypotheses that (i) HA is not only a TMJ lubricant but is also required for the preservation and functioning of TMJ skeletal stem cells, chondroprogenitors, and their progenies and (ii) pathological HA decreases and fragmentation would alter the phenotype of those cells via aberrant activation of YAP signaling. In Aim 1, we will investigate the regulatory mechanisms of HA synthesis and processing in both healthy TMJ and those with OA using scRNAseq. We will assess the impact of HA depletion on cell function in Has2f/f;AgrCreER TMJs. To gain insights into tissue stiffness, elasticity, and lubrication, we will measure their mechanical properties at the nanoscale and surface boundary lubrication. Additionally, we will test whether HA fragments contribute to YAP signaling, leading to dysregulation of cell function in condylar chondroprogenitor cells isolated from porcine embryos. Aim 2 will test whether YAP ablation ameliorates TMJ OA. To expand treatment options, we will evaluate whether YAP inhibitors can prevent or inhibit TMJ OA. This project is based on new data and novel insights into roles played by HA in TMJ health and disease and will test the efficacy of drug therapy. Given its novelty, the project is in its early stages but will undoubtedly have significant implications for basic research and translational medicine for TMJs and other joint-related musculoskeletal diseases.
