Expression and role of the trascription factor POU IV in the differentiation of neurons originating from placodal areas in the Ascidian Ciona intestinalis (Tunicata, Chordata)

Placodes are specialized territories of vertebrate embryos from which sensory organs and their associated ganglia differentiate. Ascidians are considered the closest living relatives of vertebrates. In embryos of the ascidian Ciona intestinalis, two regions sharing homologies with the hypophyseal/olfactory and otic/ lateral line placodes have been described. Transcription factors of the POU IV family are widely expressed in placode derivatives and involved in the differentiation of sensory neurons of zebrafish, chick and mouse. Two alternative transcripts of POU IV with different functions in neuron specification and outgrowth have been described. In C. intestinalis embryos, all sensory neurons express Ci-POU IV gene. The aim of this work was to study how Ci-POU IV drives differentiation of sensory neurons in ascidians. We discovered two alternative transcripts sharing fundamental homologies with those of mouse POU IV gene and analyzed their different expression patterns. By means of a bioinformatic approach, we looked for Ci-POU IV possible target genes containing in the surroundings of their ATG the specific consensus sequence. We selected 8 regions corresponding to 6 genes, whose activity is going to be evaluated. Among the predicted target genes, we found Ci-Pans (neural gene marker) and Ci-TPH (serotonin rate-limiting synthesis enzyme, Tryptophane Hydroxylase). Moreover, we designed Morpholino oligos to perform gene knock-down experiments for the different isoforms of Ci-POU IV. Preliminary results revealed that the expression of Ci-Pans, Ci-TPH and Ci-vGlut (a glutamate transporter) are differently regulated by the two transcripts. Sensory neurons both originating from the hypophyseal/olfactory and the otic/lateral placodal domain were affected. These results indicated that the POU IV isoforms were already involved in placode derivatives specification and terminal differentiation in ascidians as in vertebrates suggesting that the genetic cascade of sensory organs specification was conserved in their last common ancestor.