Differential regulation of the zebrafish orthopedia1 gene during fate determination of diencephalic neurons

Background: The homeodomain transcription factor Orthopedia ( Otp) is essential in restricting the fate of multiple classes of secreting neurons in the neuroendocrine hypothalamus of vertebrates. However, there is little information on the intercellular factors that regulate Otp expression during development. Results: Here, we identified two otp orthologues in zebrafish ( otp1 and otp2) and explored otp1 in the context of the morphogenetic pathways that specify neuroectodermal regions. During forebrain development, otp1 is expressed in anterior groups of diencephalic cells, positioned in the preoptic area ( PO) ( anterior alar plate) and the posterior tuberculum ( PT) ( posterior basal plate). The latter structure is characterized by Tyrosine Hydroxylase ( TH)-positive cells, suggesting a role for otp1 in the lineage restriction of catecholaminergic ( CA) neurons. Disruptions of Hedgehog ( HH) and Fibroblast Growth Factor ( FGF) pathways point to the ability of SHH protein to trigger otp1 expression in PO presumptive neuroblasts, with the attenuating effect of Dzip1 and FGF8. In addition, our data disclose otp1 as a determinant of CA neurons in the PT, where otp1 activity is strictly dependent on Nodal signaling and it is not responsive to SHH and FGF. Conclusion: In this study, we pinpoint the evolutionary importance of otp1 transcription factor in cell states of the diencephalon anlage and early neuronal progenitors. Furthermore, our data indicate that morphogenetic mechanisms differentially regulate otp1 expression in alar and basal plates.