Inhibition of neural crest migration in Xenopus using antisense slug RNA

p68, a DEAD-box RNA helicase, is expressed in chordate embryo neural and mesodermal tissues

The p68 DEAD-box RNA helicases have been identified in diverse organisms, including yeast, invertebrates, and mammals. DEAD-box RNA helicases are thought to unwind duplexed RNAs, and the p68 family may participate in initiating nucleolar assembly. Recent evidence also suggests that they are developmentally regulated in chordate embryos. bobcat, a newly described member of this gene family, has been found in eggs and developing embryos of the ascidian urochordate, Molgula oculata. Antisense RNA experiments have implicated this gene in establishing basic chordate features, including the notochord and neural tube in ascidians (Swalla et al. 1999). We have isolated p68 homologs from chick and Xenopus in order to investigate their possible role in vertebrate development. We show that embryonic expression of p68 in chick, frog, and ascidian embryos is high in the developing brain and spinal cord as well as in the sensory vesicles. In frog embryos, p68 expression also marks the streams of migrating cranial neural crest cells throughout neural tube development and in tailbud stages, but neural crest expression is faint in chick embryos. Ascidian embryos also show mesodermal p68 expression during gastrulation and neurulation, and we document some p68 mesodermal expression in both chick and frog. Thus, as shown in these studies, p68 is expressed in early neural development and in various mesodermal tissues in a variety of chordate embryos, including chick, frog, and ascidian. Further functional experiments will be necessary to understand the role(s) p68 may play in vertebrate development.

A novel function for the Xslug gene: control of dorsal mesendoderm development by repressing BMP-4

The Snail family of genes comprise a group of transcription factors with characteristic zinc finger motifs. One of the members of this family is the Slug gene. Slug has been implicated in the development of neural crest in chick and Xenopus by antisense loss of function experiments. Here, we have generated functional derivatives of Xslug by constructing cDNAs that encode the Xslug protein fused with the transactivation domain of the virus-derived VP16 activator or with the repressor domain of the Drosophila Engrailed protein. Our results suggest that Xslug normally functions as a transcriptional repressor and that Xslug-VP16 behaves as a dominant negative of Xslug. In the present work, we confirm and extend previous results that suggest that Xslug has an important function in neural crest development, by controlling its own transcription. In addition we have uncovered a new function for Xslug. We show that Xslug is expressed in the dorsal mesendoderm at the beginning of gastrulation, where is it able to upregulate the expression of dorsal genes. On the other hand when Xslug is expressed outside of the organizer it represses the expression of ventral genes. Our results indicate that this effect on mesodermal patterning depends on BMP activity, showing that Xslug can directly control the transcription of BMP-4.

Human Slug is a repressor that localizes to sites of active transcription

Snail/Slug family proteins have been identified in diverse species of both vertebrates and invertebrates. The proteins contain four to six zinc fingers and function as DNA-binding transcriptional regulators. Various members of the family have been demonstrated to regulate cell movement, neural cell fate, left-right asymmetry, cell cycle, and apoptosis. However, the molecular mechanisms of how these regulators function and the target genes involved are largely unknown. In this report, we demonstrate that human Slug (hSlug) is a repressor and modulates both activator-dependent and basal transcription. The repression depends on the C-terminal DNA-binding zinc fingers and on a separable repression domain located in the N terminus. This domain may recruit histone deacetylases to modify the chromatin and effect repression. Protein localization study demonstrates that hSlug is present in discrete foci in the nucleus. This subnuclear pattern does not colocalize with the PML foci or the coiled bodies. Instead, the hSlug foci overlap extensively with areas of the SC-35 staining, some of which have been suggested to be sites of active splicing or transcription. These results lead us to postulate that hSlug localizes to target promoters, where activation occurs, to repress basal and activator-mediated transcription.

Snail-related transcriptional repressors are required in Xenopus for both the induction of the neural crest and its subsequent migration

The neural crest is a transient population of precursor cells that arises at the border between the neural plate and prospective epidermis in vertebrate embryos. The earliest known response to neural-crest-inducing signals is the expression of the zinc-finger transcription factors slug and snail. Although it is widely believed that these transcription factors play an essential role in neural crest development, relatively little is understood about their mechanism of action during this process. We have previously shown that overexpression of XSlug leads to expanded expression of neural crest markers and an excess of at least one neural crest derivative, melanocytes. In order to further investigate XSlug function, we overexpressed mutant constructs in which the DNA-binding domain was fused to either the activation domain from Gal4 or the repressor domain from Drosophila Engrailed. The Engrailed repressor fusion was found to mimic the effects of wild-type XSlug, indicating that XSlug functions as a transcriptional repressor during neural crest formation. In contrast, overexpression of either the activation domain fusion or the DNA-binding domain alone was found to inhibit XSlug function. Using a hormone-inducible inhibitory mutant, we show that inhibition of XSlug function at early stages prevents the formation of neural crest precursors, while inhibition at later stages interferes with neural crest migration, demonstrating for the first time that this transcriptional repressor is required during multiple stages of neural crest development.

Membrane-anchored plakoglobins have multiple mechanisms of action in Wnt signaling

In Wnt signaling, beta-catenin and plakoglobin transduce signals to the nucleus through interactions with TCF-type transcription factors. However, when plakoglobin is artificially engineered to restrict it to the cytoplasm by fusion with the transmembrane domain of connexin (cnxPg), it efficiently induces a Wnt-like axis duplication phenotype in Xenopus. In Xenopus embryos, maternal XTCF3 normally represses ventral expression of the dorsalizing gene Siamois. Two models have been proposed to explain the Wnt-like activity of cnxPg: 1) that cnxPg inhibits the machinery involved in the turnover of cytosolic beta-catenin, which then accumulates and inhibits maternal XTCF3, and 2) that cnxPg directly acts to inhibit XTCF3 activity. To distinguish between these models, we created a series of N-terminal deletion mutations of cnxPg and examined their ability to induce an ectopic axis in Xenopus, activate a TCF-responsive reporter (OT), stabilize beta-catenin, and colocalize with components of the Wnt signaling pathway. cnxPg does not colocalize with the Wnt pathway component Dishevelled, but it does lead to the redistribution of APC and Axin, two proteins involved in the regulation of beta-catenin turnover. Expression of cnxPg increases levels of cytosolic beta-catenin; however, this effect does not completely explain its signaling activity. Although cnxPg and Wnt-1 stabilize beta-catenin to similar extents, cnxPg activates OT to 10- to 20-fold higher levels than Wnt-1. Moreover, although LEF1 and TCF4 synergize with beta-catenin and plakoglobin to activate OT, both suppress the signaling activity of cnxPg. In contrast, XTCF3 suppresses the signaling activity of both beta-catenin and cnxPg. Both exogenous XLEF1 and XTCF3 are sequestered in the cytoplasm of Xenopus cells by cnxPg. Based on these data, we conclude that, in addition to its effects on beta-catenin, cnxPg interacts with other components of the Wnt pathway, perhaps TCFs, and that these interactions contribute to its signaling activity.