Harnessing temporomandibular disc development to enhance regeneration

The temporomandibular jaw joint (TMJ) is one of the most used joints in the body. It is formed between the mandible bone in the lower jaw and the base of the skull. A disc sits between these elements. This disc is made of a tissue called fibrocartilage and acts as a cushion. Temporomandibular joint disorders (TMDs) are disruptions in the structure, function, or physiology of the jaw joint. Symptoms include chronic joint pain, uncomfortable popping or clicking in the joint, headaches, jaw locking, and difficulty opening the mouth. TMDs are very common, affecting up to 20% of the population. As such TMDs have a negative impact on sufferers' lives, as well as a wider societal and economic impact through management costs, increased access to medical and dental services, and employment days lost. Up to 70% of TMDs are due to TMJ disc displacement, which can lead to perforation of the disc. In turn, disc displacement and damage are associated with destructive osteoarthritis of the joint. Surgical treatment options for these most severe cases of TMD include removal of the damaged disc (discectomy) or joint reconstruction, with either tissue from the patient (typically from the ribs) or synthetic materials. These strategies are both unsatisfactory. Discectomy can lead to degenerative changes to the joint, while reconstruction often results in eventual joint fusion. Therefore, disc repair is an important area for research in order to create new and improved treatments.

In order to repair the disc, there are two potential strategies. Providing a new source of disc cells, or encouraging the existing disc cells to proliferate and repair the damage. In order to achieve the first approach, it is important to be able to create disc cells. For this, we can learn from how the disc cells are formed in the first place in the embryo. This process can then be mimicked to create a new source of disc cells. In addition to molecular signals from the surrounding tissues, the forming disc cells are also subject to mechanical force, which shapes how they form. We will therefore also investigate how these forces shape the disc.
We will carry out this research using mouse embryos, which have a very similar disc to human embryos, and allow us to follow the fate of cells using the latest techniques in mouse genetics.

For the second approach, we will investigate whether there is a resident population of stem cells in the adult mouse disc. We will look at whether the disc cells can be stimulated to repair the damage, either by altering their signalling environment or their mechanical environment or both. This is possible as we can culture mouse discs in a dish and apply force or alter signalling factors and watch the effect on the disc cells.

Our proposed experiments will provide important insight into how the disc forms, the cells that it is created from, and the mechanical and molecular cues that are important to create this unique population of cells. We will also provide new information about the cells of the adult disc and the options for stimulating repair to damaged discs. With our results, we aim to provide the knowledge to allow more effective, less invasive, biology-driven methods to deal with TMJ defects.

Temporomandibular Joint discs have a very poor ability to repair. In order to repair holes a tissue bioengineering approach has been taken in a variety of animal models. The challenge is to create a tissue that does not undergo mineralisation and retains the discs unique properties. These problems are exacerbated by the fact that we know very little about the cells that form the disc, their lineage and genetic signature, how disc identity is regulated by molecular and mechanical signals, and whether the disc contains resident stem cells. This grant aims to address these key issues to ultimately enhance repair strategies.

For this, we will take advantage of our knowledge of mouse craniofacial development, the availability of transgenic mouse lines to lineage trace and knockout specific genes, and the ability to culture disc tissue.

We have three aims
Aim 1: To assess the identity and developmental pathway of TMJ disc cells
Aim 2: To assess how mechanical force impacts TMJ disc cell identity
Aim 3: To investigate the impact of damage and the potential of adult discs to repair

The first two aims focus on disc development. We will here use laser capture dissection to dissect out tissue of interest for comparison by RNASeq.
The first aim will establish the lineage of the TMJ disc and how it relates to the condyle and the formation of sesamoids.

In the second aim, we will compare disc development in mouse embryos with muscle defects, and in explant culture experiments, where we can follow disc development in the absence of mechanical force.

Finally, in the third aim, we move to the adult disc and investigate whether a stem/progenitor cell population is resident in the disc. We will use lineage tracing of alpha-SMA and Gli1 mice and assess whether the disc contains any label-retaining cells. We will turn to culture to investigate how the disc is influenced by injury and whether the disc cells have potential for repair, using an explant model.