Regulation of the response to Nodal-mediated mesoderm induction by Xrel3

Multiple functions of FADD in apoptosis, NF-κB-related signaling, and heart development in Xenopus embryos

FADD is an adaptor protein that transmits apoptotic signals from death receptors. Additionally, FADD has been shown to play a role in various functions including cell proliferation. However, the physiological role of FADD during embryonic development remains to be delineated. Here, we show the novel roles FADD plays in development and the molecular mechanisms of these roles in Xenopus embryos. By whole-mount in situ hybridization and RT-PCR analysis, we observed that fadd is constantly expressed in early embryos. The upregulation or downregulation of FADD proteins by embryonic manipulation resulted in induction of apoptosis or size changes in the heart during development. Expression of a truncated form of FADD, FADDdd, which lacks pro-apoptotic activity, caused growth retardation of embryos associated with dramatic expressional fluctuations of genes that are regulated by NF-κB. Moreover, we isolated a homolog of mammalian cullin-4 (Cul4), a component of the ubiquitin E3 ligase family, as a FADDdd-interacting molecule in Xenopus embryos. Thus, our study shows that FADD has multiple functions in embryos; it plays a part in the regulation of NF-κB activation and heart formation, in addition to apoptosis. Furthermore, our findings provide new insights into how Cul4-based ligase is related to FADD signaling in embryogenesis.

sizzled function and secreted factor network dynamics

Studies on the role of the E-box binding transcription factor Snail2 (Slug) in the induction of neural crest by mesoderm (Shi et al., 2011) revealed an unexpected increase in the level of sizzled RNA in the dorsolateral mesodermal zone (DMLZ) of morphant Xenopus embryos. sizzled encodes a secreted protein with both Wnt and BMP inhibitor activities. Morpholino-mediated down-regulation of sizzled expression in one cell of two cell embryos or the C2/C3 blastomeres of 32-cell embryos, which give rise to the DLMZ, revealed decreased expression of the mesodermal marker brachyury and subsequent defects in neural crest induction, pronephros formation, and muscle patterning. Loss of sizzled expression led to decreases in RNAs encoding the secreted Wnt inhibitor SFRP2 and the secreted BMP inhibitor Noggin; the sizzled morphant phenotype could be rescued by co-injection of RNAs encoding Noggin and either SFRP2 or Dickkopf (a mechanistically distinct Wnt inhibitor). Together, these observations reveal that sizzled, in addition to its established role in dorsal-ventral patterning, is also part of a dynamic BMP and Wnt signaling network involved in both mesodermal patterning and neural crest induction.

Xrel3/XrelA attenuates β-catenin-mediated transcription during mesoderm formation in Xenopus embryos

In Xenopus laevis embryonic development, activation of the Wnt/β-catenin pathway promotes mesoderm cell fate determination via Xnr (Xenopus nodal-related) expression. We have demonstrated previously that Rel/NF-κB (nuclear factor κB) proteins expressed in presumptive ectoderm limit the activity of Xnrs to the marginal zone of embryos during mesoderm induction, which assists to distinguish mesoderm from ectoderm. The mechanism of this regulation, however, is unknown. In the present study, we investigated whether Rel/NF-κB proteins are able to modulate mesoderm formation by mediating Wnt/β-catenin signalling. We determined that ectopic expression of XrelA or Xrel3 in the dorsal marginal zone perturbed dorsal mesoderm formation by down-regulating multiple Wnt/β-catenin target genes including Xnr3, Xnr5 and Xnr6. Ventral co-expression of XrelA or Xrel3 with either wild-type β-catenin or constitutively active β-cateninS37A abrogated β-catenin-induced axis duplication and attenuated β-catenin-stimulated reporter transcription. Lastly, we provide evidence that Xrel3, but not XrelA, can interact with β-catenin without affecting the association of β-catenin with other transcriptional co-activators in vitro. Both Xrel3 and XrelA, however, prevented the accumulation, in nuclei, of exogenously expressed and endogenous β-catenin in vivo. These results suggest that Rel proteins are able to bind β-catenin and attenuate β-catenin-mediated transcription by nuclear exclusion.

A co-dependent requirement of xBcl9 and Pygopus for embryonic body axis development in Xenopus

The Wnt/beta-catenin transcriptional activation complex requires the adapter protein Pygopus (Pygo), which links the basal transcription machinery to beta-catenin, by its association with legless (Lgs)/ B-cell lymphoma-9 (Bcl9). Pygo was shown to be required for development in vertebrates, but the role of Lgs/Bcl9 is unknown. We identified an amphibian orthologue of Lgs/Bcl9, XBcl9, which interacted biochemically with Xbeta-catenin and XPygo2. The body axis promoting ability of Xbeta-catenin was diminished when residues required for its interaction with XBcl9 were mutated. In blastula embryos, XBcl9 was transiently preferentially expressed in nuclei of dorsoanterior cells and ectopically expressed XBcl9 required XPygo2 to localize to nuclei. Furthermore, while neither XBcl9 nor XPygo2 alone affected development when ectopically expressed, both were required to induce supernumerary axis and dorsal gene activation. Like XPygo2, depletion of maternal XBcl9 alone caused dorsal defects. These results indicated an essential role of the Pygo-Bcl9 duet in vertebrate body axis formation.

Unexpected functional redundancy between Twist and Slug (Snail2) and their feedback regulation of NF-kappaB via Nodal and Cerberus

A NF-kappaB-Twist-Snail network controls axis and mesoderm formation in Drosophila. Using translation-blocking morpholinos and hormone-regulated proteins, we demonstrate the presence of an analogous network in the early Xenopus embryo. Loss of twist (twist1) function leads to a reduction of mesoderm and neural crest markers, an increase in apoptosis, and a decrease in snail1 (snail) and snail2 (slug) mRNA levels. Injection of snail2 mRNA rescues twist's loss of function phenotypes and visa versa. In the early embryo NF-kappaB/RelA regulates twist, snail2, and snail1 mRNA levels; similarly Nodal/Smad2 regulate twist, snail2, snail1, and relA RNA levels. Both Twist and Snail2 negatively regulate levels of cerberus RNA, which encodes a Nodal, bone morphogenic protein (BMP), and Wnt inhibitor. Cerberus's anti-Nodal activity inhibits NF-kappaB activity and decreases relA RNA levels. These results reveal both conserved and unexpected regulatory interactions at the core of a vertebrate's mesodermal specification network.

A p38 MAPK-CREB pathway functions to pattern mesoderm in Xenopus

Dorsal-ventral patterning is specified by signaling centers secreting antagonizing morphogens that form a signaling gradient. Yet, how morphogen gradient is translated intracellularly into fate decisions remains largely unknown. Here, we report that p38 MAPK and CREB function along the dorsal-ventral axis in mesoderm patterning. We find that the phosphorylated form of CREB (S133) is distributed in a gradient along the dorsal-ventral mesoderm axis and that the p38 MAPK pathway mediates the phosphorylation of CREB. Knockdown of CREB prevents chordin expression and mesoderm dorsalization by the Spemann organizer, whereas ectopic expression of activated CREB-VP16 chimera induces chordin expression and dorsalizes mesoderm. Expression of high levels of p38 activator, MKK6E or CREB-VP16 in embryos converts ventral mesoderm into a dorsal organizing center. p38 MAPK and CREB function downstream of maternal Wnt/beta-catenin and the organizer-specific genes siamois and goosecoid. At low expression levels, MKK6E induces expression of lateral genes without inducing the expression of dorsal genes. Loss of CREB or p38 MAPK activity enables the expansion of the ventral homeobox gene vent1 into the dorsal marginal region, preventing the lateral expression of Xmyf5. Overall, these data indicate that dorsal-ventral mesoderm patterning is regulated by differential p38/CREB activities along the axis.