The nuclear receptors COUP-TF: a long-lasting experience in forebrain assembly.

Chicken ovalbumin upstream promoter transcription factors (COUP-TFs) are nuclear receptors belonging to the superfamily of the steroid/thyroid hormone receptors. Members of this family are internalized to the nucleus both in a ligand-dependent or -independent manner and act as strong transcriptional regulators by binding to the DNA of their target genes. COUP-TFs are defined as orphan receptors, since ligands regulating their activity have not so far been identified. From the very beginning of metazoan evolution, these molecules have been involved in various key events during embryonic development and organogenesis. In this review, we will mainly focus on their function during development and maturation of the central nervous system, which has been well characterized in various animal classes ranging from ctenophores to mammals. We will start by introducing the current knowledge on COUP-TF mechanisms of action and then focus our discussion on the crucial processes underlying forebrain ontogenesis, with special emphasis on mammalian development. Finally, the conserved roles of COUP-TFs along phylogenesis will be highlighted, and some hypotheses, worth exploring in future years to gain more insight into the mechanisms controlled by these factors, will be proposed.