Retinoic Acid and POU Genes in Developing Amphioxus: A Focus on Neural Development

POU genes are a family of evolutionarily conserved transcription factors with key functions in cell type specification and neurogenesis. In vitro experiments have indicated that the expression of some POU genes is controlled by the intercellular signaling molecule retinoic acid (RA). In this work, we aimed to characterize the roles of RA signaling in the regulation of POU genes in vivo. To do so, we studied POU genes during the development of the cephalochordate amphioxus, an animal model crucial for understanding the evolutionary origins of vertebrates. The expression patterns of amphioxus POU genes were assessed at different developmental stages by chromogenic in situ hybridization and hybridization chain reaction. Expression was further assessed in embryos subjected to pharmacological manipulation of endogenous RA signaling activity. In addition to a detailed description of the effects of these treatments on amphioxus POU gene expression, our survey included the first description of Pou2 and Pou6 expression in amphioxus embryos. We found that Pit-1, Pou2, Pou3l, and Pou6 expression are not affected by alterations of endogenous RA signaling levels. In contrast, our experiments indicated that Brn1/2/4 and Pou4 expression are regulated by RA signaling in the endoderm and the nerve cord, respectively. The effects of the treatments on Pou4 expression in the nerve cord revealed that, in developing amphioxus, RA signaling plays a dual role by (1) providing anteroposterior patterning information to neural cells and (2) specifying neural cell types. This finding is coherent with a terminal selector function of Pou4 for GABAergic neurons in amphioxus and represents the first description of RA-induced changes in POU gene expression in vivo.