Identifying the cellular origins of jaw joint cartilage regeneration in zebrafish

PROJECT SUMMARY Temporomandibular jaw joint osteoarthritis (TMJ OA) and other disorders of the TMJ affect up to 12% of the US population. OA in general is the leading cause of disability in the US and is characterized by the deterioration of the superficial cartilage lining the joints. A contributor to the high incidence of TMJ OA is the fact that humans

are unable to regenerate the specialized superficial cartilage of joints. Current treatments are limited to minimizing pain instead of restoring natural joint structure and function. The Crump lab has recently shown that adult zebrafish can regenerate the superficial cartilage of the synovial jaw joint after injury. This model relies on

transecting a major ligament supporting the jaw, which results in joint degeneration in the short term and full

regeneration of the superficial cartilage lining the jaw joint after the ligament heals. The limitations of this surgical model are that it is labor-intensive, low-throughput, and highly variable. Using an f13a1b enhancer which we discovered drives highly specific activity in superficial cartilage of the jaw and other neural crest-derived joints, I

have generated transgenic lines to specifically ablate joint cartilage. Using this ablation model, I have found that jaw joint superficial cartilage regeneration is highly reproducible, and this will allow me to investigate the source of new cartilage cells. For this proposal, I plan to optimize the transgenic model for joint cartilage ablation, and

then investigate the sources of the regenerating cartilage. To do so, I have identified through single-nuclei chromatin accessibility studies several cell type-specific enhancers that will allow me to mark and lineage trace putative progenitors during joint regeneration. Three potential sources of regenerated joint chondrocytes will be

investigated: 1) The perichondrium surrounding the jaw joint could house progenitor cells that move into the joint upon ablation and form new superficial chondrocytes. 2) Deeper chondrocytes at the joint, which share properties with growth plate chondrocytes, could transdifferentiate into superficial chondrocytes following ablation. 3) Rare

superficial chondrocytes that escape ablation could proliferate and repopulate the joint surface. Using newly identified cell type-specific enhancers, I will create CreERT transgenic lines to trace the contributions of each population during jaw joint cartilage regeneration. In parallel, I will carry out a single-cell multiomic analysis of

the regenerating jaw joint to discover in an unbiased way potential progenitor cells activated by injury that may mediate repair. By identifying the source of jaw joint cartilage regeneration in zebrafish, I will be poised to further explore the molecular basis by which zebrafish can regenerate their joint tissues. In the future, we hope to

uncover whether similar populations exist in the human TMJ that can be activated to counteract OA.