The temporomandibular joint (TMJ) is different from other synovial joints, in that it contains an articular disc that separates the joint space into tw synovial cavities. These distinctive features enable complex anatomical rearrangements of TMJ that take place during mastication and speech. These movements also impart shearing and frictional loads and a near dislocation of the joint. To protect joint integrity from destructive mechanical friction, superficial and disc cells produce boundary lubricants including hyaluronic acid (HA) and lubricin. Thus, deterioration of joint lubrication with age or by other insults may underlie disc adhesion and degenerative TMJ disorders. We propose to study currently poorly understood developmental aspects of TMJ biology that are critically important for TMJ functions and its long-term health. Our preliminary data showed that a secreted signaling factor, Indian hedgehog (Ihh), plays pivotal roles in TMJ development. Global Ihh ablation led to a failure of disc and synovial cavity formation, and loss of TGF-?1, PTHrP and lubricin expression. Postnatal ablation of Ihh caused markedly reduced lubricin expression and disc adhesions. These findings led to our central hypothesis that Ihh signaling is required for both TMJ formation and postnatal function.
In Aim 1, the roles of Ihh signaling in TMJ development will be further defined. Initially, we will relate the spatio-temporal patterns of Ihh signaling (Gli1 activation) o the expression of hedgehog receptors, signal transducers and phenotype defining genes during TMJ development. We will then use loss and gain of function approaches to delete Smoothened or Patched1 in TMJ cells at critical developmental stages.
In Aim 2, we will test the hypothesis that TGF-?1 and PTHrP mediate Ihh action on its target cells in TMJ by asking whether TGF-?1 and/or PTHrP are sufficient to rescue Ihh-null TMJ phenotype and whether conditional deletion of these genes in TMJ produce defects similar to those seen in Ihh-null mice. We will further delineate Ihh-TGF-? and Ihh- PTHrP signaling pathways and their potential interactions in the regulation of joint lubricant production and development of relevant cell types.
In Aim 3, we will study the roles of joint lubricants in the Ihh- dependent postnatal TMJ maintenance. Respective roles of HA and lubricin in the protection of the TMJ will be determined by analyzing lubricin (Prg4)-null and conditional HA synthase 2 (HAS2)-deficient mice. We will also test whether TMJ defects in Ihh-null mice can be rescued by systemic or local injection of a hedgehog agonist, PTHrP/PTH, TGF-?1 or local delivery of lubricin- and/or HAS2-expression vectors. In sum, this project will provide novel, important and far-reaching information on hedgehog signaling in TMJ formation and maintenance. Our rescue experiments should provide a proof-of-principle that TMJ defects, including disc adhesions, are amenable to therapeutic intervention.

Public Health Relevance

Temporomandibular joint (TMJ) is essential for function and quality of life and boundary lubricants in synovial fluid have important roles for friction-less TM movement. This project will continue to clarify mechanisms by which the joints form and what mechanisms regulate synovial fluid production. It will thus generate fundamental information that can be used in the future conceive and test possible therapeutic strategies.

National Institute of Health (NIH)
National Institute of Dental & Craniofacial Research (NIDCR)
Research Project (R01)
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Skeletal Biology Structure and Regeneration Study Section (SBSR)
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Vallejo, Yolanda F
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Children's Hospital of Philadelphia
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Kurio, Naito; Saunders, Cheri; Bechtold, Till E et al. (2018) Roles of Ihh signaling in chondroprogenitor function in postnatal condylar cartilage. Matrix Biol 67:15-31
Chandrasekaran, Prashant; Doyran, Basak; Li, Qing et al. (2017) Biomechanical properties of murine TMJ articular disc and condyle cartilage via AFM-nanoindentation. J Biomech 60:134-141
Bechtold, Till E; Saunders, Cheri; Decker, Rebekah S et al. (2016) Osteophyte formation and matrix mineralization in a TMJ osteoarthritis mouse model are associated with ectopic hedgehog signaling. Matrix Biol 52-54:339-354
Bechtold, T E; Saunders, C; Mundy, C et al. (2016) Excess BMP Signaling in Heterotopic Cartilage Forming in Prg4-null TMJ Discs. J Dent Res 95:292-301