Retinoic Acid Influences connexin43 Expression During Joint Formation in the Regenerating Zebrafish Fin.

Background: The regenerating zebrafish fin skeleton is comprised of multiple bony fin rays, each made of alternating bony segments and fin ray joints. This pattern is regulated by the gap junction protein Connexin43 (Cx43), which provides instructional cues to skeletal precursor cells (SPCs). Elevated Cx43 favors osteoblast differentiation and disfavors joint forming cell differentiation. The goal of this article is to test if retinoic acid (RA) contributes to the regulation of cx43 expression.

Materials and methods: Functional studies inhibiting the RA-synthesizing enzyme Adh1a2 were evaluated using in situ hybridization to monitor gene expression and with measurements of the length of fin ray segments to monitor impacts on SPC differentiation and joint formation.

Results: Aldh1a2-knockdown leads to reduced expression of cx43 and increased expression of evx1, a gene required for joint formation. Additionally, inhibition of Aldh1a2 function leads to short fin ray segments. We also find evidence for synergy between aldh1a2 and cx43, suggesting that these genes function in a common molecular pathway to regulate joint formation.

Conclusions: The role of RA is to promote cx43 expression in the regenerating fin to regulate joint formation and the length of bony fin ray segments. We suggest that RA signaling must coordinate with additional pathways that also regulate cx43 transcription.