Role of hyaluronan in YAP signaling on TMJ health and pathology

The National Institute of Dental and Craniofacial Research (NIDCR) awarded The Children's Hospital of Philadelphia Research Institute a $507,006 Project Grant on August 1, 2025, under the Oral Diseases and Disorders Research program (CFDA 93.121) to investigate the role of hyaluronan in YAP (Yes-associated protein) signaling on temporomandibular joint (TMJ) health and pathology. The research will be conducted over a two-year period through July 31, 2027, and focuses on fundamental mechanistic studies of how hyaluronan—a naturally occurring lubricant in the TMJ—functions not only in joint lubrication but also in cell signaling pathways that regulate stem and chondroprogenitor cell function in the condyle. The project will employ conditional genetic mouse models and in vitro cell studies to elucidate how hyaluronan synthase 2 (HAS2) deficiency leads to osteoarthritis-like pathology and altered YAP nuclear translocation, with the goal of identifying novel therapeutic targets for TMJ osteoarthritis treatment.

This basic science research initiative addresses a critical gap in understanding TMJ biology, as current hyaluronan-based therapeutic interventions provide only short-term symptom relief. By elucidating the signaling mechanisms underlying TMJ pathology, the funded research will generate foundational knowledge to inform the development of more durable and mechanism-based interventions for temporomandibular disorders, a significant oral and craniofacial health problem affecting quality of life and function in affected populations.

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.