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The Notch signaling pathway plays an important role in many cell-fate decisions during development. Here we investigate the regulation and function of the conserved gene XNAP, which is a member of the Delta-Notch synexpression group in Xenopus. XNAP encodes a small protein with two C-terminal tandem ankyrin repeats which is expressed in the neurectoderm and in the presomitic mesoderm in a pattern that resembles that of other component of the Notch pathway. When a myc-tag form of XNAP is overexpressed in Xenopus or Hela cells, XNAP protein is detected both in the nucleus and the cytoplasm. In embryos and in animal cap assays, XNAP expression is activated, perhaps directly, by the Notch pathway and this activation appears to be Su(H) dependent. Overexpression of XNAP in embryos decreases Notch signaling, which leads to an increase in the number of primary neurons that form within the domains of the neural plate where neurogenesis normally occurs. In culture Hela cells, XNAP overexpression interferes with ICD activation of a Notch regulated reporter gene. Together, these data indicate that XNAP is a novel target of the Notch pathway that may, in a feedback loop, modulate its activity.

We have identified a Xenopus bHLH gene, Xath2, which is the homologue of the murine MATH-2/NEX-1 gene, using a functional expression screening approach. Overexpression of this gene in neurula embryos induces the expression of the N-tubulin neuronal marker but does not stimulate the expression of the X-ngnr-1 and NeuroD proneural genes. Expression of Xath2 begins in stage 32 embryos and is restricted to the dorsal telencephalon. Within the neuroepithelium of the dorsal telencephalon, Xath2 expression is detected in postmitotic cells located more laterally than those expressing several other related bHLH neuronal regulators.

We describe the structural organization of a gene, termed XFDL 141/156, that is transiently activated during early Xenopus development. XFDL 141/156 is first transcribed at the midblastula transition (MBT) and during early gastrulation events. A roughly 200 nucleotide fragment immediately 5' to the transcription start site is sufficient for transient, early zygotic activation of gene expression. The primary transcript is subject to alternative splicing. Corresponding cDNAs encode two structurally related but completely distinct C2H2-type zinc finger proteins of unknown biological function.

Our initial description of the Xenopus gene Eomesodermin (Eomes) indicated that it is expressed largely if not entirely in mesodermal cells of gastrula stage embryos. A more detailed examination, described here, shows that it continues to be expressed in the most anterior (future head) mesoderm during gastrula and neurula stages. However, during tail-bud stages, Eomes expression is re-born in the most anterior part of the brain, becoming strongly transcribed in the olfactory region of the telencephalon. This later Eomes expression marks a very localized region of the forebrain distinct from that of Otx-2, anterior to that of En-1 and overlapping that of Sox-3.