Amphicoup-TF, a nuclear orphan receptor of the lancelet Branchiostoma floridae, is implicated in retinoic acid signalling pathways

In vertebrates, the orphan nuclear receptors of the COUP-TF group function as negative transcriptional regulators that inhibit the hormonal induction of target genes mediated by classical members of the nuclear hormone superf amily, such as the retinoic acid receptors (RARs) or the thyroid hormone re ceptors (TRs). To investigate the evolutionary conservation of the roles of COUP-TF receptors as negative regulators in the retinoid and thyroid hormo ne pathways, we have characterized AmphiCOUP-TF, the homologue of COUP-TFI and COUP-TFII, in the chordate amphioxus (Branchiostoma floridae), the clos est living invertebrate relative of the vertebrates. Electrophoretic mobili ty shift assays (EMSA) showed that AmphiCOUP-TF binds to a wide variety of response elements, as do its vertebrate homologues. Furthermore, AmphiCOUP- TF is a transcriptional repressor that strongly inhibits retinoic acid-medi ated transactivation. In situ hybridizations revealed expression of AmphiCO UP-TF in the nerve cord of late larvae, in a region corresponding to hindbr ain and probably anterior spinal cord. Although the amphioxus nerve cord ap pears unsegmented at the gross anatomical level, this pattern reflects segm entation at the cellular level with stripes of expressing cells occurring a djacent to the ends and the centers of each myotomal segment, which may inc lude visceral motor neurons and somatic motor neurons respectively, among o ther cells. A comparison of the expression pattern of AmphiCOUP-TF with tho se of its vertebrate homologues, suggests that the roles of COUP-TF in patt erning of the nerve cord evolved prior to the split between the amphioxus a nd vertebrate lineages. Furthermore, in vitro data also suggest that AmphiC OUP-TF acts as a negative regulator of signalling by other nuclear receptor s such as RAR, TR or ER.