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The basic helix-loop-helix (bHLH) transcriptional activator Ptf1a determines inhibitory GABAergic over excitatory glutamatergic neuronal cell fate in progenitors of the vertebrate dorsal spinal cord, cerebellum and retina. In an in situ hybridization expression survey of PR domain containing genes encoding putative chromatin-remodeling zinc finger transcription factors in Xenopus embryos, we identified Prdm13 as a histone methyltransferase belonging to the Ptf1a synexpression group. Gain and loss of Ptf1a function analyses in both frog and mice indicates that Prdm13 is positively regulated by Ptf1a and likely constitutes a direct transcriptional target. We also showed that this regulation requires the formation of the Ptf1a-Rbp-j complex. Prdm13 knockdown in Xenopus embryos and in Ptf1a overexpressing ectodermal explants lead to an upregulation of Tlx3/Hox11L2, which specifies a glutamatergic lineage and a reduction of the GABAergic neuronal marker Pax2. It also leads to an upregulation of Prdm13 transcription, suggesting an autonegative regulation. Conversely, in animal caps, Prdm13 blocks the ability of the bHLH factor Neurog2 to activate Tlx3. Additional gain of function experiments in the chick neural tube confirm that Prdm13 suppresses Tlx3(+)/glutamatergic and induces Pax2(+)/GABAergic neuronal fate. Thus, Prdm13 is a novel crucial component of the Ptf1a regulatory pathway that, by modulating the transcriptional activity of bHLH factors such as Neurog2, controls the balance between GABAergic and glutamatergic neuronal fate in the dorsal and caudal part of the vertebrate neural tube., (Copyright © 2013 Elsevier Inc. All rights reserved.)

The Dmrt (doublesex and mab-3 related transcription factor) genes encode a large family of evolutionarily conserved transcription factors whose function in sex specific differentiation has been well studied in all animal lineages. In vertebrates, their function is not restricted to the developing gonads. For example, Xenopus Dmrt4 is essential for neurogenesis in the olfactory system. Here we have isolated and characterized Xenopus Dmrt5 and found that it is coexpressed with Dmrt4 in the developing olfactory placodes. As Dmrt4, Dmrt5 is positively regulated in the ectoderm by neural inducers and negatively by proneural factors. Both Dmrt5 and Dmrt4 genes are also activated by the combined action of the transcription factor Otx2, broadly transcribed in the head ectoderm and of Notch signaling, activated in the anterior neural ridge. As for Dmrt4, knockdown of Dmrt5 impairs neurogenesis in the embryonic olfactory system and in neuralized animal caps. Conversely, its overexpression promotes neuronal differentiation in animal caps, a property that requires the conserved C-terminal DMA and DMB domains. We also found that the sea anenome Dmrt4/5 related gene NvDmrtb also induces neurogenesis in Xenopus animal caps and that conversely, its knockdown in Nematostella reduces elav-1 positive neurons. Together, our data identify Dmrt5 as a novel important regulator of neurogenesis whose function overlaps with that of Dmrt4 during Xenopus olfactory system development. They also suggest that Dmrt may have had a role in neurogenesis in the last common ancestor of cnidarians and bilaterians., (Copyright © 2012 Elsevier Inc. All rights reserved.)

BBP proteins constitute a subclass of CUL3 interacting BTB proteins whose in vivo function remains unknown. Here, we show that the Xenopus BBP gene BTBD6 and the single Drosophila homologue of mammalian BBP genes lute are strongly expressed in the developing nervous system. In Xenopus, BTBD6 expression responds positively to proneural and negatively to neurogenic gene overexpression. Knockdown of BTBD6 in Xenopus or loss of Drosophila lute result in embryos with strong defects in late neuronal markers and strongly reduced and disorganized axons while early neural development is unaffected. XBTBD6 knockdown in Xenopus also affects muscle development. Together, these data indicate that BTBD6/lute is required for proper embryogenesis and plays an essential evolutionary conserved role during neuronal development., (Copyright (c) 2008 Wiley-Liss, Inc.)

The evolutionarily conserved IA1 (Insm1) gene is strongly expressed in the developing nervous system. Here, we show that IA1 is expressed during Xenopus laevis embryogenesis in neural plate primary neurons as well as in a population of uncharacterized anteroventral cells that form in front of the cement gland and that we identified as noradrenergic neurons. We also show that the formation of those anteroventral cells is dependent on BMPs and inhibited by Notch and that it is regulated by the transcription factors Xash1, Phox2, and Hand2. Finally, we provide functional evidence suggesting that IA1 may also play a role in their formation. Together, our results reveal that IA1 constitutes a novel player downstream of Xash1 in the formation of a previously unidentified population of Xenopus noradrenergic primary neurons., (Copyright (c) 2008 Wiley-Liss, Inc.)

The DNA-binding transcription factor Smad-interacting protein-1 (Sip1) (also named Zfhx1b/ZEB2) plays essential roles in vertebrate embryogenesis. In Xenopus, XSip1 is essential at the gastrula stage for neural tissue formation, but the precise molecular mechanisms that underlie this process have not been fully identified yet. Here we show that XSip1 functions as a transcriptional repressor during neural induction. We observed that constitutive activation of BMP signaling prevents neural induction by XSip1 but not the inhibition of several epidermal genes. We provide evidence that XSip1 binds directly to the BMP4 proximal promoter and modulates its activity. Finally, by deletion and mutational analysis, we show that XSip1 possesses multiple repression domains and that CtBPs contribute to its repression activity. Consistent with this, interference with XCtBP function reduced XSip1 neuralizing activity. These results suggest that Sip1 acts in neural tissue formation through direct repression of BMP4 but that BMP-independent mechanisms are involved as well. Our data also provide the first demonstration of the importance of CtBP binding in Sip1 transcriptional activity in vivo.

We have isolated a Xenopus homologue of the mammalian hairy and Enhancer of split related gene HRT1. XHRT1 expression in late gastrula and early neurula embryos is restricted to two stripes of cells in the medial neural plate and in dorsal endodermal cells. At later stages, XHRT1 is expressed in the floor plate, in hypochord cells and in the somitogenic and anterior presomitic mesoderm. By tailbud stage, XHRT1 is also highly expressed in the dorsal hindbrain, telencephalon and eye vesicles, olfactory placodes, pronephros, branchial arches and tail fin. We also show that XHRT1 expression in medial neural cells is induced by Notch signaling and that there are differences in the way XHRT1 and other H/E(spl) genes are regulated.

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.