Zinc finger homeobox is required for the differentiation of serotonergic neurons in the sea urchin embryo

Serotonergicneurons differentiate in the neurogenic animal plate ectoderm of the sea urchinembryo. The regulatory mechanisms that control the specification or differentiation of these neurons in the sea urchinembryo are not yet understood, although, after the genome was sequenced, many genes encoding transcription factors expressed in this region were identified. Here, we report that zinc finger homeobox(zfhx1/z81) is expressed in serotonergicneural precursor cells, using double in situ hybridization screening with a serotonergicneural marker, tryptophan 5-hydroxylase (tph) encoding a serotonin synthase that is required for the differentiation of serotonergicneurons. zfhx1/z81 begins to be expressed at gastrula stage in individual cells in the anterior neuroectoderm, some of which also express delta. zfhx1/z81 expression gradually disappears as neural differentiation begins with tph expression. When the translation of Zfhx1/Z81 is blocked by morpholinoinjection, embryos express neither tph nor the neural marker synaptotagminB in cells of the animal plate, and serotonergicneurons do not differentiate. In contrast, Zfhx1/Z81 morphantsdo express fez, another neural precursor marker, which appears to function in the initial phase of specification/differentiation of serotonergicneurons. In addition, zfhx1/z81 is one of the targets suppressed in the animal plate by anti-neural signals such as Nodal as well as Delta-Notch. We conclude that Zfhx1/Z81 functions during the specification of individual anterior neural precursors and promotes the expression of tph and synaptotagminB, required for the differentiation of serotonergicneurons.