Expression of R-spondins/Lgrs in development of movable craniofacial organs

Wnt signaling plays a critical role in the development of many organs, including the major movable craniofacial organs tongue, lip, and eyelid. Four members of the R-spondin family (Rspo1-4) bind to Lgr4/5/6 to regulate the activation of Wnt signaling. However, it is not fully understood how Rspos/Lgrs regulate Wnt signaling during the development of movable craniofacial organs. To address this question, we examined the expression of Rspos, Lgrs, and Axin2 (major mediator of canonical Wnt signaling) during tongue, lip, and eyelid development. The expression of Axin2, Rspos and Lgrs was observed in many similar regions, suggesting that Rspos likely activate canonical Wnt signaling through the Lgr-dependent pathway in these regions. Lgr expression was not detected in regions where Axin2 and Rspos were expressed, suggesting that Rspos might activate canonical Wnt signaling through the Lgr-independent pathway in these regions. In addition, the expression of Rspos and Lgrs were observed in some other regions where Axin2 was not expressed, suggesting the possibility that Rspos and/or Lgrs are involved in non-canonical Wnt signaling or the Wnt-independent pathway. Thus, we identified a dynamic spatiotemporal expression pattern of Rspos and Lgrs during the development of the eyelid, tongue, and lip.