Notch signaling is involved in nervous system formation in ascidian embryos.

Notch signaling plays crucial roles during embryogenesis in various metazoans. HrNotch, a Notch homologue in the ascidian Halocynthia roretzi, has been previously cloned, and its expression pattern suggests that HrNotch signaling is involved in nervous system formation. To determine the function of HrNotch signaling, in the present study we examined the effects of the constitutively activated forms of HrNotch. Overexpression resulted in larvae with defects in neural tube closure and brain vesicle formation. In embryos expressing the activated HrNotch, the expression of a neural marker gene, HrETR-1, was enhanced and expanded in the central nervous system, although ectopic expression decreased during the tailbud stage. The activated HrNotch also suppressed the formation of the adhesive organ (palps) and the peripheral nervous system, which consists of ciliary mechanosensory neurons, whereas it promoted epidermal differentiation. The suppression and promotion of the formation of these respective cell types were confirmed by examination of the expression of relevant tissue-specific markers. We also cloned Hrdelta, an ascidian homologue of DSL family genes, which encode ligands for which Notch acts as a receptor. The expression of Hrdelta was observed in the precursors of palps and peripheral neurons in addition to the CNS. These results suggest that Notch signaling is important for ascidian nervous system formation and that it affects the fate choice between palps and epidermis and between peripheral neurons and epidermis within the neurogenic regions of the surface ectoderm by suppressing the formations of palps and peripheral neurons and promoting epidermal differentiation.

Notch homologue from Halocynthia roretzi is preferentially expressed in the central nervous system during ascidian embryogenesis.

We describe here the primary structure of HrNotch, an ascidian homologue of the Drosophila neurogenic gene Notch. HrNotch transcripts encode a protein of 2352 amino acids and share the principal features of the Notch gene family: extracellular epidermal growth factor (EGF)-like repeats, three Notch/Lin-12 repeats and six intracellular ankyrin repeats. Yet ascidian Notch contains only 33 EGF repeats in the putative extramembrane domain and specifically lacks the three EGF-like repeats. In situ hybridization shows that maternal HrNotch mRNA is distributed uniformly in the cytoplasm of the unfertilized egg. During cleavage, maternal HrNotch transcripts are ubiquitous in the ectoderm cells of the animal hemisphere, which contain less yolk granules. During gastrulation, maternal transcripts persist in most ectoderm lineage cells. Zygotic expression of HrNotch seems to start at the neural plate stage in both a-line cells (descendants of anterior-animal blastomeres) of the dorsal neuroectoderm and b-line cells (descendants of the posterior-animal blastomeres) that comprise the neural fold. Following this stage, transcripts are most evident in the descendants of these cells, that is, the brain lineage cells, precursors of a larval adhesive organ, and dorsal part of the nerve cord (roof plate). Brain lineage cells include the precursors of sensory pigment cells that are known to comprise an equivalence group in ascidian embryos. During tail elongation, transcripts disappear. Predominant expression of HrNotch in epidermal and neural cells is a common feature of chordate Notch genes. Furthermore, the timing of HrNotch expression in sensory pigment cell precursors suggests involvement in the determinative events in the sensory pigment cell equivalence group.