XrelA, a Xenopus maternal and zygotic homologue of the p65 subunit of NF-kappa B. Characterisation of transcriptional properties in the developing embryo and identification of a negative interference mutant

Non-coding RNAs targeting NF-κB pathways in aquatic animals: A review

Nuclear factor-kappa B (NF-κB) pathways have a close relationship with many diseases, especially in terms of the regulation of inflammation and the immune response. Non-coding RNAs (ncRNAs) are a heterogeneous subset of endogenous RNAs that directly affect cellular function in the absence of proteins or peptide products; these include microRNAs (miRNAs), long noncoding RNAs (lncRNAs), circular RNAs (circRNAs), etc. Studies on the roles of ncRNAs in targeting the NF-κB pathways in aquatic animals are scarce. A few research studies have confirmed detailed regulatory mechanisms among ncRNAs and the NF-κB pathways in aquatic animals. This comprehensive review is presented concerning ncRNAs targeting the NF-κB pathway in aquatic animals and provides new insights into NF-κB pathways regulatory mechanisms of aquatic animals. The review discusses new possibilities for developing non-coding-RNA-based antiviral applications in fisheries.

MiR-202-3p determines embryo viability during mid-blastula transition

Developmental growth is an intricate process involving the coordinated regulation of the expression of various genes, and microRNAs (miRNAs) play crucial roles in diverse processes throughout animal development. The mid-blastula transition (MBT) is a developmental milestone when maternal RNAs are cleared and the zygotic genome programmed asynchronous cell division begins to drive embryogenesis. While mechanisms underlying MBT have been intensively revealed, factors regulating cell proliferation at the transition remain largely unknown. We report here a microRNA, miR-202-3p to be a key factor that determines embryonic fate during MBT in zebrafish. A miR-202-3p antagomir specifically terminated embryo development at the mid-blastula stage. In vivo deletion of the miR-202 locus recapitulated the fatal phenotypes, which were rescued only by miR-202-3p or its precursor. Transcriptome comparison revealed >250 RNAs including both maternal and zygotic origins were dysregulated at MBT in the miR-202^−/− embryos, corresponding with arrays of homeostatic disorders leading to massive apoptosis. A trio of genes: nfkbiaa, perp and mgll, known to be intimately involved with cell proliferation and survival, were identified as direct targets of miR-202-3p. Importantly, over- or under-expression of any of the trio led to developmental delay or termination at the blastula or gastrula stages. Furthermore, nfkbiaa and perp were shown to inter-regulate each other. Thus, miR-202-3p mediates a regulatory network whose components interact closely during MBT to determine embryonic viability and development.

NFκB signaling regulates embryonic and adult neurogenesis

Both embryonic and adult neurogenesis involves the self-renewal/proliferation, survival, migration and lineage differentiation of neural stem/progenitor cells. Such dynamic process is tightly regulated by intrinsic and extrinsic factors and complex signaling pathways. Misregulated neurogenesis contributes much to a large range of neurodevelopmental defects and neurodegenerative diseases. The signaling of NFκB regulates many genes important in inflammation, immunity, cell survival and neural plasticity. During neurogenesis, NFκB signaling mediates the effect of numerous niche factors such as cytokines, chemokines, growth factors, extracellular matrix molecules, but also crosstalks with other signaling pathways such as Notch, Shh, Wnt/β-catenin. This review summarizes current progress on the NFκB signaling in all aspects of neurogenesis, focusing on the novel role of NFκB signaling in initiating early neural differentiation of neural stem cells and embryonic stem cells.

Maternal Wnt/β-catenin signaling coactivates transcription through NF-κB binding sites during Xenopus axis formation

Maternal Wnt/β-Catenin signaling establishes a program of dorsal-specific gene expression required for axial patterning in Xenopus. We previously reported that a subset of dorsally expressed genes depends not only on Wnt/β-Catenin stimulation, but also on a MyD88-dependent Toll-like receptor/IL1-receptor (TLR/IL1-R) signaling pathway. Here we show that these two signal transduction cascades converge in the nucleus to coactivate gene transcription in blastulae through a direct interaction between β-Catenin and NF-κB proteins. A transdominant inhibitor of NF-κB, ΔNIκBα, phenocopies loss of MyD88 protein function, implicating Rel/NF-κB proteins as selective activators of dorsal-specific gene expression. Sensitive axis formation assays in the embryo demonstrate that dorsalization by Wnt/β-Catenin requires NF-κB protein activity, and vice versa. Xenopus nodal-related 3 (Xnr3) is one of the genes with dual β-Catenin/NF-κB input, and a proximal NF-κB consensus site contributes to the regional activity of its promoter. We demonstrate in vitro binding of Xenopus β-Catenin to several XRel proteins. This interaction is observed in vivo upon Wnt-stimulation. Finally, we show that a synthetic luciferase reporter gene responds to both endogenous and exogenous β-Catenin levels in an NF-κB motif dependent manner. These results suggest that β-Catenin acts as a transcriptional co-activator of NF-κB-dependent transcription in frog primary embryonic cells.

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.

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

The Xenopus egg has a yolk-laden vegetal hemisphere juxtaposed to a darkly pigmented animal hemisphere. Mesoderm is derived from the marginal zone, located at the interface between the two hemispheres. The vegetal-most cells become endoderm and release TGF-beta-related factors, including the Xenopus Nodal related (Xnr) proteins, which diffuse to induce the marginal zone to form mesoderm. The remaining animal cells become ectoderm, but our understanding of the mechanisms that limit the response to induction is incomplete. In this study, we provide evidence to suggest that Xrel3, a member of the Rel/NF-kappaB family, plays a role in defining the boundary separating induced from uninduced cells by regulating Xnr-responsive gene transcription. Ectopic Xrel3 expressed in prospective mesoderm caused repression of mesoderm-specific genes resulting in loss-of-function phenotypes that were rescued by co-expression of Xnr2. Depletion of Xrel3 from embryos with antisense morpholinos increased Xnr-dependent transcription, broadened expression of the pan-mesoderm marker Xbra and sensitized animal cells to mesoderm induction by Xnr2. We propose that an additional component to the mechanism that differentiates the ectoderm from the mesoderm involves regulation of nodal-dependent gene transcription by Xrel3.

An NF-kappaB and slug regulatory loop active in early vertebrate mesoderm

Background

In both Drosophila and the mouse, the zinc finger transcription factor Snail is required for mesoderm formation; its vertebrate paralog Slug (Snai2) appears to be required for neural crest formation in the chick and the clawed frog Xenopus laevis. Both Slug and Snail act to induce epithelial to mesenchymal transition (EMT) and to suppress apoptosis.

Methodology & Principle Findings

Morpholino-based loss of function studies indicate that Slug is required for the normal expression of both mesodermal and neural crest markers in X. laevis. Both phenotypes are rescued by injection of RNA encoding the anti-apoptotic protein Bcl-xL; Bcl-xL's effects are dependent upon IκB kinase-mediated activation of the bipartite transcription factor NF-κB. NF-κB, in turn, directly up-regulates levels of Slug and Snail RNAs. Slug indirectly up-regulates levels of RNAs encoding the NF-κB subunit proteins RelA, Rel2, and Rel3, and directly down-regulates levels of the pro-apopotic Caspase-9 RNA.

Conclusions/Significance

These studies reveal a Slug/Snail–NF-κB regulatory circuit, analogous to that present in the early Drosophila embryo, active during mesodermal formation in Xenopus. This is a regulatory interaction of significance both in development and in the course of inflammatory and metastatic disease.

Fertilization regulates apoptosis of Ciona intestinalis extra-embryonic cells through thyroxine (T4)-dependent NF-κB pathway activation during early embryonic development

In Ciona intestinalis, the elimination of extra-embryonic test cells during early stage of development is delayed by a fertilization signal. Test cells undergo a caspase-dependent apoptosis event repressed by thyroxine (T4)-activated NF-kappaB. When apoptosis was experimentally blocked, the hatching stage was delayed. The incubation of unfertilized eggs with a 1-h-fertilized egg extract or purified T4 restored apoptosis in test cells at a similar timing than found in fertilized eggs. Ciona expresses specific genes forming a functional IkappaB/NF-kappaB pathway. One, Ci-p65, was transiently induced upon fertilization via T4 and found to exert its anti-apoptotic role in test cells nuclei as well as in a reconstituted cell system. Blocking NF-kappaB activity by dexamethasone-induced overexpression of Ci-IkappaB abrogated the repression of apoptosis in test cells. Overall, the data are consistent for defining a central coupling role of both T4 and NF-kappaB during early embryo development.

RelA/p65 regulation of IkappaBbeta

IkappaB inhibitor proteins are the primary regulators of NF-kappaB. In contrast to the defined regulatory interplay between NF-kappaB and IkappaBalpha, much less is known regarding the regulation of IkappaBbeta by NF-kappaB. Here, we describe in detail the regulation of IkappaBbeta by RelA/p65. Using p65(-/-) fibroblasts, we show that IkappaBbeta is profoundly reduced in these cells, but not in other NF-kappaB subunit knockouts. This regulation prevails during embryonic and postnatal development in a tissue-specific manner. Significantly, in both p65(-/-) cells and tissues, IkappaBalpha is also reduced, but not nearly to the same extent as IkappaBbeta, thus highlighting the degree to which IkappaBbeta is dependent on p65. This dependence is based on the ability of p65 to stabilize IkappaBbeta protein from the 26S proteasome, a process mediated in large part through the p65 carboxyl terminus. Furthermore, IkappaBbeta was found to exist in both a basally phosphorylated and a hyperphosphorylated form. While the hyperphosphorylated form is less abundant, it is also more stable and less dependent on p65 and its carboxyl domain. Finally, we show that in p65(-/-) fibroblasts, expression of a proteolysis-resistant form of IkappaBbeta, but not IkappaBalpha, causes a severe growth defect associated with apoptosis. Based on these findings, we propose that tight control of IkappaBbeta protein by p65 is necessary for the maintenance of cellular homeostasis.

Characterization of NF-kappa B/I kappa B proteins in zebra fish and their involvement in notochord development

Although largely involved in innate and adaptive immunity, NF-kappa B plays an important role in vertebrate development. In chicks, the inactivation of the NF-kappa B pathway induces functional alterations of the apical ectodermal ridge, which mediates limb outgrowth. In mice, the complete absence of NF-kappa B activity leads to prenatal death and neural tube defects. Here, we report the cloning and characterization of NF-kappa B/I kappa B proteins in zebra fish. Despite being ubiquitously expressed among the embryonic tissues, NF-kappa B/I kappa B members present distinct patterns of gene expression during the early zebra fish development. Biochemical assays indicate that zebra fish NF-kappa B proteins are able to bind consensus DNA-binding (kappa B) sites and inhibitory I kappa B alpha proteins from mammals. We show that zebra fish I kappa B alphas are degraded in a time-dependent manner after induction of transduced murine embryo fibroblasts (MEFs) and that these proteins are able to rescue NF-kappa B activity in I kappa B alpha(-/-) MEFs. Expression of a dominant-negative form of the murine I kappa B alpha (mI kappa B alpha M), which is able to block NF-kappa B in zebra fish cells, interferes with the notochord differentiation, generating no tail (ntl)-like embryos. This phenotype can be rescued by coinjection of the T-box gene ntl (Brachyury homologue), which is typically required for the formation of posterior mesoderm and axial development, suggesting that ntl lies downstream of NF-kappa B . We further show that ntl and Brachyury promoter regions contain functional kappa B sites and NF-kappa B can directly modulate ntl expression. Our study illustrates the conservation and compatibility of NF-kappa B/I kappa B proteins among vertebrates and the importance of NF-kappa B pathway in mesoderm formation during early embryogenesis.

Histone acetylation is a checkpoint in FGF-stimulated mesoderm induction

We have previously demonstrated that the transcription factor, AP-1 (c-jun/c-fos heterodimer), mediates fibroblast growth factor (FGF) signaling during mesoderm induction in Xenopus embryo. In the present studies, we show that histone acetylation is involved in FGF-mediated signaling leading to mesoderm induction. Histone acetylation is a dynamic process regulated by the activities of two histone-modifying enzymes, the histone acetyltransferase(s) and histone deacetylase(s) (HDACs). We found that basal and FGF-regulated activator protein 1 (AP-1) activity in Xenopus embryo is markedly reduced by treatment of trichostatin A (TSA), a specific inhibitor of HDAC. However, activity of another transcription factor, NFkappaB, is enhanced by TSA treatment. AP-1-mediated mesoderm induction in the animal caps is dramatically suppressed by TSA at a dose-dependent manner. This suppression can be rescued by ectopic expression of HDAC3 at early stage. Finally, we found that histone acetylation in animal caps is inhibited by FGF whereas enhanced by TSA (as a control). Therefore, we propose that histone acetylation is a checkpoint for transduction of the FGF/AP-1 signals to induce mesoderm. Published 2000 Wiley-Liss, Inc.

Nuclear translocation of nuclear factor kappa B in early 1-cell mouse embryos

Nuclear factor kappa B (NF-kappaB) is a transcription factor that controls the expression of a number of genes under cellular redox potential. It has recently been found that NF-kappaB plays a pivotal role in morphogenesis and embryonic development, e.g., in formation of Drosophila malanogaster ventral structures and chicken limb buds. However, the role of NF-kappaB in preimplantation development in mammals is not yet understood. In this study, we show that RelA, one of the subunits of NF-kappaB, is expressed in mouse eggs and embryos from the metaphase II (MII) oocyte to the blastocyst stage. Therefore, it is thought that RelA is maternally expressed and that it continues to be expressed during preimplantation development. Immunofluorescence analysis showed that RelA protein was mainly distributed in the cytoplasm of embryos, whereas nuclear translocation of RelA, evidence for NF-kappaB activation, was observed only at the early 1-cell stage. Finally we studied the effects of NF-kappaB inhibitors, pyrrolidine dithiocarbamate and N-acetyl-L-cysteine, on the preimplantation development of mouse embryos. When these inhibitors were added to the culture medium from the early 1-cell stage, subsequent development through the 2-cell stage was inhibited. However, little, if any, influence on the progression through the 2-cell stage was observed when the inhibitors were added at the late 1-cell or the 2-cell stage. Taken together, the results suggest that the activation of NF-kappaB at the early 1-cell stage is required for the development of mouse embryos beyond the 2-cell stage.

Structural and functional analysis of the BMP-4 promoter in early embryos of Xenopus laevis

The Xenopus laevis BMP-4 gene shows an evolutionary conserved structure containing two coding exons and a leader exon. The transcripts which are detected after zygotic activation of the gene in ventral mesoderm of late blastula stage embryos do either contain the leader exon or begin within the first intron. Luciferase reporter/promoter studies revealed multiple elements being required for the activation and for the spatial control of transcription. These elements are located within the upstream region and within the second intron and they interact with a most proximal located basal promoter being indispensable for transcriptional activation. The auto-activatory capacity of BMP-4 is mediated by several enhancer elements being responsive not only to BMP-4 but also to BMP-2 signaling. BMP-2 might thus function as a natural activator of the BMP-4 gene in the early embryo. Since reporter activity obtained with distinct BMP-2/4 responsive promoter deletion mutants is simultaneously inhibited by the dominant negative BMP receptor as well as by chordin, we suggest that down-regulation of the BMP-4 gene by chordin results from an interference with the auto-regulatory loop at the level of protein-protein interactions.

Overexpression of a novel Xenopus rel mRNA gene induces tumors in early embryos

The Rel family of transcriptional activators form a large diverse group of proteins that are involved in the activation of genes involved in immunity, development, apoptosis and cancer. So far, none of the rel genes cloned in mammals appear to be required for embryonic development. We have cloned and characterized a cDNA from an embryonic cDNA library that encodes a novel Xenopus Rel protein, called Xrel3. Xrel3 is a member of the cRel subfamily and is most closely related to but distinct from other Xenopus Rel members. The expression of Xrel3 mRNA was investigated using Northern analysis, RNase protection assay, reverse transcriptase-linked polymerase chain reaction and in situ hybridization. Messages are present maternally and are slightly enriched in the equatorial region of the blastula stage embryo. At gastrulation, the accumulation of Xrel3 messages declines to undetectable levels but then increases after neurulation. In situ RNA hybridization was used to determine the spatial location of Xrel3 messenger RNA in embryos. Messages are localized to the developing forebrain, dorsal mid-hindbrain region, the inner ear primordium, or otocyst, and in the notochord. Overexpression by microinjection of Xrel3 RNA induced tumors in the developing embryo that appeared after gastrulation. The location of the tumors depended on the location of the injection site. These results suggest that Xrel3 might have a generalized role in regulation of cell differentiation in the embryo.

Identification and expression of the Xenopus homolog of mammalian p100-NFkappaB2

We have molecularly cloned a cDNA encoding a new Rel-related protein in Xenopus laevis. The product is most homologous to mammalian p100-NFkappaB2. Furthermore, the putative protein kinase A-phosphorylation site (RRPS), which is found in most of the Rel family proteins and is replaced by KRKR in mammalian p100, is also replaced by KRKK in our clone, indicating that our cDNA most likely encodes the Xenopus p100 (Xp100). Like mammalian p52, a processed product of p100, Xp52 alone binds to the kappaB site but does not activate transcription, while the XRelB/Xp52 heterodimer activates transcription, which is inhibited by the carboxyl-terminal half of Xp100 (XIkappaBdelta). Xp100 transcripts are present at all stages of oocyte maturation and in all adult tissues examined. Xp100 transcripts decrease at the gastrula stage and resume their expression at the neurula stage, which is different from other Xenopus rel family. Xp100 is highly expressed in somitogenic mesoderm at the neurula stage, while in the gastrula and tailbud stages, Xp100 transcripts are not localized to restricted regions. These results suggest that Xp100 could be involved in the late-stage development of Xenopus laevis, especially in the maturation of somites.

HNF4beta, a new gene of the HNF4 family with distinct activation and expression profiles in oogenesis and embryogenesis of Xenopus laevis

The transcription factor hepatocyte nuclear factor 4 (HNF4) is an orphan member of the nuclear receptor superfamily expressed in mammals in liver, kidney, and the digestive tract. Recently, we isolated the Xenopus homolog of mammalian HNF4 and revealed that it is not only a tissue-specific transcription factor but also a maternal component of the Xenopus egg and distributed within an animal-to-vegetal gradient. We speculate that this gradient cooperates with the vegetally localized embryonic induction factor activin A to activate expression of HNF1alpha, a tissue-specific transcription factor with an expression pattern overlapping that of HNF4. We have now identified a second Xenopus HNF4 gene, which is more distantly related to mammalian HNF4 than the previously isolated gene. This new gene was named HNF4beta to distinguish it from the known HNF4 gene, which is now called HNF4alpha. By reverse transcription-PCR, we detected within the 5' untranslated region of HNF4beta two splice variants (HNF4beta2 and HNF4beta3) with additional exons, which seem to affect RNA stability. HNF4beta is a functional transcription factor acting sequence specifically on HNF4 binding sites known for HNF4alpha, but it seems to have a lower DNA binding activity and is a weaker transactivator than the alpha isoform. Furthermore, the two factors differ with respect to tissue distribution in adult frogs: whereas HNF4alpha is expressed in liver and kidney, HNF4beta is expressed in addition in stomach, intestine, lung, ovary, and testis. Both factors are maternal proteins and present at constant levels throughout embryogenesis. However, using reverse transcription-PCR, we found the RNA levels to change substantially: whereas HNF4alpha is expressed early during oogenesis and is absent in the egg, HNF4beta is first detected in the latest stage of oogenesis, and transcripts are present in the egg and early cleavage stages. Furthermore, zygotic HNF4alpha transcripts appear in early gastrula and accumulate during further embryogenesis, whereas HNF4beta mRNA transiently appears during gastrulation before it accumulates again at the tail bud stage. All of these distinct characteristics of the newly identified HNF4 protein imply that the alpha and beta isoform have different functions in development and in adult tissues.

Negative regulation of dorsal patterning in early embryos by overexpression of XrelA, a Xenopus homologue of NF-kappa B

Recent results by Richardson et al. (Mech. Dev., 52 (1995) 165-177) suggest that the Xenopus Rel gene XrelA may be involved in the formation of the head and tail of the early embryo. We present evidence to suggest that wild-type XrelA also has a role in dorsoventral development. XrelA overexpression in the dorsal side of embryos reduces dorsal development and attenuates in vitro dorsal morphogenetic movements. XrelA also strongly reduces axis duplication caused by overexpression of a dominant negative mutant of Xenopus glycogen synthase kinase-3 beta. Our results indicate that XrelA may have a role in dorsoventral patterning in early embryos.

N-acetyl-cysteine causes a late re-specification of the anteroposterior axis in the Xenopus embryo

N-acetyl cysteine is an agent which has been shown to interrupt signal transduction processes linking a wide range of stimuli to the activation of NF-kappa B in mammalian cells. We have investigated its effect on the early development of Xenopus embryos by injecting it into blastulae, using concentrations comparable to those effective on cultured cells. High concentrations at the late blastula or early gastrula stage suppress posterior and enhance anterior development, yielding embryos with enlarged cement glands and otherwise consisting of little except head in extreme cases. Reducing the amount of N-acetyl cysteine injected leads to progressively more posterior structures developing. Injection into one- or two-cell embryos gives similar phenotypes, but of reduced severity and the cement gland is not so enlarged. Explants of animal cap cells taken several hours after injection develop to give large amounts of cement gland material. We have examined the expression of a number of genes in the anteriorised embryos. Posterior markers and Xsna are reduced. Noggin and Goosecoid mRNA are up-regulated through the gastrula and persist at these levels until at least the late neurula stage, whereas in controls Noggin is much lower and Goosecoid is absent at these stages. The most anteriorised phenotype may be a consequence of this changed expression.

Transcriptional hierarchy in Xenopus embryogenesis: HNF4 a maternal factor involved in the developmental activation of the gene encoding the tissue specific transcription factor HNF1 alpha (LFB1)

The tissue specific transcription factor HNF1 alpha (LFB1) expressed in liver, kidney, stomach and gut gets transcriptionally activated in Xenopus shortly after zygotic transcription starts. By microinjection into fertilized Xenopus eggs, a HNF1 alpha promoter fragment is activated in the middle part of developing larvae, reflecting the activation pattern of the endogenous HNF1 alpha gene. Mutational analysis of the HNF1 alpha promoter shows that HNF1 and HNF4 binding sites are essential for proper embryonic regulation. Since by injecting HNF4 mRNA into fertilized eggs the endogenous HNF1 alpha gene is activated ectopically and HNF4 is present as a maternal protein within an animal to vegetal gradient in the embryo, we assume that HNF4 initiates a transcriptional hierarchy involved in determination of different cell fates.

Molecular cloning of cDNA encoding the Xenopus homolog of mammalian RelB

We have molecularly cloned cDNA encoding a new Rel-related protein in Xenopus laevis. Nucleotide sequencing revealed that the product is most homologous to mammalian RelB in its N-terminal region. Furthermore, the putative protein kinase A phosphorylation site (RRPS), found in most of the Rel family proteins, but replaced by QRLT in mammalian RelB, is replaced by QRIT, indicating that our cDNA most likely encodes the Xenopus homolog of mammalian RelB (XrelB). As in the case of mouse RelB, XrelB alone does not bind to DNA efficiently, while XrelB/human p50 heterodimers bind to kappa B sites and activate transcription. XrelB transcripts are present at all stages of oocyte maturation and in adult tissues examined. However, in staged embryos XrelB is undetectable from neurula to stage 28 and resumes expression at stage 47, while Xrel1/XrelA, the Xenopus homolog of p65, has been demonstrated to be expressed throughout embryogenesis. These results raise the possibility that XrelB and Xrel1/XrelA play different roles in the development of X.laevis.

Developmental effects of over-expression of normal and mutated forms of a Xenopus NF-kappa B homologue

High level over-expression of XrelA1, a homologue of the p65 sub-unit of NF-kappa B and of Drosophila dorsal, arrests Xenopus development at the gastrula stage, producing a reduction in the levels of expression of various genes of developmental interest without general reduction in transcription or cessation of cell division. There is little Goosecoid expression, even though a dorsal lip forms. At lower levels XrelA1 mRNA primarily produces disruption of the mid-dorsal axis. A dominant interference gene product, delta 222, produces mainly posterior, but also anterior abnormalities. On the basis of these results we postulate that the role of XrelA1 in the vertebrate embryo is unlikely to be in dorsoventral development, but more likely in the formation of the termini.

Developmental expression of the maternal protein XDCoH, the dimerization cofactor of the homeoprotein LFB1 (HNF1)

The tissue-specific transcription factors LFB1 (HNF1) and LFB3 (vHNF1) mainly expressed in liver, kidney and intestine are homeoproteins that interact with the regulatory element HP1. The HP1 sequence constitutes one of the most important cis-acting elements in liver-specifically expressed genes, while its function in other cell types containing LFB1 and LFB3 is not fully understood. In mammals, LFB1 activity is modulated by DCoH, a cofactor that stimulates the LFB1 transactivation significantly. Using the rat cDNA probe, we cloned the corresponding Xenopus sequence XDCoH, encoding a 104 amino acid protein, that is 85% identical to the rat protein. XDCoH enhances the LFB1-dependent transactivation potential in transfection experiments and interacts in vitro directly with LFB1 and its variant form LFB3. The protein is detectable in liver and kidney extracts of adult frogs and in small amounts also in lung and stomach, organs expressing LFB1 and/or LFB3 protein as well. To investigate the possible involvement of XDCoH in Xenopus development, we analyzed its temporal and spatial expression pattern during early embryogenesis. XDCoH is a maternal factor, although LFB1 is absent in the egg. In early cleavage stages, the protein is detectable in the cytoplasm of each blastomere and enters the nuclei of the cells as early as the zygotic transcription in the Xenopus embryo starts. The amount of XDCoH increases dramatically following neurulation, when the formation of liver, pronephros and other organs takes place. Whole-mount immunostaining demonstrates that, in the developing larvae, XDCoH is localized in the nuclei of the hepatocytes, the gut cells and the pronephric cells, tissues of mesodermal and endodermal origin known to contain LFB1 and LFB3. Surprisingly it is also present in the pigmented epithelium surrounding the eye of the embryo, which is derived from the anterior part of the ectodermal neural plates and lacks LFB1. The tissue distribution of XDCoH during embryogenesis suggests that XDCoH is involved in determination and differentiation of various unrelated cell types. It seems likely that XDCoH interaction is not only essential for the function of LFB1 and LFB3 but also for certain other transcription factors.