Functional retinoid receptors in budding ascidians

A homolog of retinoid X receptors (RXR), named PmRXR, was cloned from the b udding ascidian, Polyandrocarpa misakiensis. Gel-shift assays revealed that PmRXR and a previously identified P. misakiensis retinoic acid receptor (P mRAR) formed a complex to bind vertebrate-type retinoic acid response eleme nt (RARE). Transfection assays were carried out using a reporter gene conta ining a RARE upstream of lacZ. Two chimeric effector genes were constructed by placing PmRXR and PmRAR cDNA fragments (containing the DNA-binding, lig and-binding and ligand-dependent transactivation domains) downstream of the human RXR alpha and RAR alpha cDNA (covering the N-terminal coding region) , respectively. Each chimeric cDNA was ligated to a notochord-specific enha ncer. In case the embryos were transfected with all three transgenes and tr eated with retinoic acid (RA), the reporter gene was activated in the notoc hord cells. The result suggests that the PmRXR/PmRAR complex functions as a n RA-dependent transcriptional activator. The PmRXR mRNA was detected in a mesenchymal cell type, called glomerulocyte, in the developing Polyandrocar pa bud. As this cell type has been shown to express PmRAR mRNA, it seems po ssible that the PmRXR/PmRAR complex mediates RA signaling in this cell type to induce the expression of genes involved in the morphogenesis of the dev eloping bud.