Retinoic acid levels control tooth morphology in fish

Abstract

Misshapen teeth are highly common in humans. In most instances, they are due to genetic mutations in genes controlling the morphogenesis of teeth. This is the case for the Runx2 gene, which once mutated leads to cleidocranial dysplasia (CCD) an autosomal dominant genetic syndrome presenting with peg‐like teeth in humans. While many factors have been identified in the process of tooth morphogenesis, currently, little is known about how cell signaling participates in establishing organ shape during odontogenesis. Our previous work has identified the signaling molecule retinoic acid (RA) to be one of the main actors of tooth induction in fish and recent data suggest that RA also plays a role during tooth morphogenesis. Fish and zebrafish, in particular, are good models to study the genetics, cell and molecular biology, and organogenesis of the tooth in vertebrates. The objective of the proposed studies is to understand the roles played by retinoic acid during tooth morphogenesis in different fish species, to develop a fish model of CCD, to clarify the role played by Runx2 during tooth morphogenesis and to understand its link with RA signaling. By exposing fish embryos to exogenous RA and to an RA inhibitor during tooth morphogenesis we have been able to quantify changes in tooth shape and better understand the mechanism of action of RA signaling during tooth morphogenesis in fish. Exogenous RA exposure alters the shape of the first developing tooth in zebrafish making it narrower and more elongated resembling other tooth shape observed in the wild. RA exposure to a fish species, like the goldfish, with shorter and wider tooth makes it also narrower and more elongated making it more like a zebrafish tooth. At the molecular levels, the expression of tooth epithelium and mesenchyme markers dlx2a; dlx3b; pitx2a; runx2b and lhx6 is expanded in RA treated zebrafish embryo. Our data reveal that the levels of RA in different cells of the tooth germ is controlled by the timing and level of expression of the RA degrading enzyme cyp26b1 in a subset of cells of the developing tooth germ. We hypothesize that modifying the onset of cyp26b1 expression in the tooth germ will, therefore, change the level of RA available in the tooth germ, ultimately altering the shape of the tooth. Finally, our work has revealed more about how RA regulates Runx2 and controls tooth morphogenesis in the development of CCD and during evolution. Our project has revealed that fine tuning of RA level in the developing tooth is a crucial player in the vast tooth diversity observed in fish species in the wild.