Functional retinoid receptors in budding ascidians

A homolog of retinoid X receptors (RXR), named PmRXR, was cloned from the budding ascidian, Polyandrocarpa misakiensis. Gel-shift assays revealed that PmRXR and a previously identified P. misakiensis retinoic acid receptor (PmRAR) formed a complex to bind vertebrate-type retinoic acid response element (RARE). Transfection assays were carried out using a reporter gene containing a RARE upstream of lacZ. Two chimeric effector genes were constructed by placing PmRXR and PmRAR cDNA fragments (containing the DNA-binding, ligand-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 enhancer. In case the embryos were transfected with all three transgenes and treated with retinoic acid (RA), the reporter gene was activated in the notochord cells. The result suggests that the PmRXR/PmRAR complex functions as an RA-dependent transcriptional activator. The PmRXR mRNA was detected in a mesenchymal cell type, called glomerulocyte, in the developing Polyandrocarpa bud. As this cell type has been shown to express PmRAR mRNA, it seems possible that the PmRXR/PmRAR complex mediates RA signaling in this cell type to induce the expression of genes involved in the morphogenesis of the developing bud.