The role of maternal CREB in early embryogenesis of Xenopus laevis

Transcription Factors and DNA Repair Enzymes Compete for Damaged Promoter Sites

Transcriptional regulation is a tightly regulated, vital process. The transcription factor cyclic AMP-response element-binding protein 1 (CREB1) controls ∼25% of the mammalian transcriptome by binding the CREB1 binding site consensus sequence (CRE) sequence (TGACGTCA). DNA lesions within CRE modulate CREB1 binding negatively and positively. Because appropriate DNA lesions also interact with base excision repair proteins, we investigated whether CREB1 and repair glycosylases compete with each other. We incubated 39-mer CRE-containing double-stranded oligonucleotides with recombinant CREB1 alone or with UNG2 or OGG1, followed by EMSA. The CpG islet within CRE was modified to contain a G/U or 8-oxoG (°G)/C mispair. OGG1 and CREB1 reversibly competed for CRE containing an °G/C pair. Also, OGG1 blocked CREB1 from dimerizing by 69%, even when total CREB1 binding was reduced only by 20-30%. In contrast, bound CREB1 completely prevented access to G/U-containing CRE by UNG2 and, therefore, to base excision repair, whereas UNG2 exposure prevented CREB1 binding. CREB1 dimerization was unaffected by UNG2 when CREB1 bound to CRE, but was greatly reduced by prior UNG2 exposure. To explore physiological relevance, we microinjected zebrafish embryos with the same oligonucleotides, as a sink for endogenous CREB1. As predicted, microinjection with unmodified or lesion-containing CRE, but not scrambled CRE or scrambled CRE with a G/U mispair, resulted in increased embryo death. However, only the G/U mispair in native CRE resulted in substantial developmental abnormalities, thus confirming the danger of unrepaired G/U mispairs in promoters. In summary, CREB1 and DNA glycosylases compete for damaged CRE in vitro and in vivo, thus blocking DNA repair and resulting in transcriptional misregulation leading to abnormal development.

cAMP-responsive element binding protein: a vital link in embryonic hormonal adaptation

The transcription factor cAMP responsive element-binding protein (CREB) and activating transcription factors (ATFs) are downstream components of the insulin/IGF cascade, playing crucial roles in maintaining cell viability and embryo survival. One of the CREB target genes is adiponectin, which acts synergistically with insulin. We have studied the CREB-ATF-adiponectin network in rabbit preimplantation development in vivo and in vitro. From the blastocyst stage onwards, CREB and ATF1, ATF3, and ATF4 are present with increasing expression for CREB, ATF1, and ATF3 during gastrulation and with a dominant expression in the embryoblast (EB). In vitro stimulation with insulin and IGF-I reduced CREB and ATF1 transcripts by approximately 50%, whereas CREB phosphorylation was increased. Activation of CREB was accompanied by subsequent reduction in adiponectin and adiponectin receptor (adipoR)1 expression. Under in vivo conditions of diabetes type 1, maternal adiponectin levels were up-regulated in serum and endometrium. Embryonic CREB expression was altered in a cell lineage-specific pattern. Although in EB cells CREB localization did not change, it was translocated from the nucleus into the cytosol in trophoblast (TB) cells. In TB, adiponectin expression was increased (diabetic 427.8 ± 59.3 pg/mL vs normoinsulinaemic 143.9 ± 26.5 pg/mL), whereas it was no longer measureable in the EB. Analysis of embryonic adipoRs showed an increased expression of adipoR1 and no changes in adipoR2 transcription. We conclude that the transcription factors CREB and ATFs vitally participate in embryo-maternal cross talk before implantation in a cell lineage-specific manner. Embryonic CREB/ATFs act as insulin/IGF sensors. Lack of insulin is compensated by a CREB-mediated adiponectin expression, which may maintain glucose uptake in blastocysts grown in diabetic mothers.

Primary oocyte transcriptional activation of aqp1ab by the nuclear progestin receptor determines the pelagic egg phenotype of marine teleosts

In marine teleosts, the aqp1ab water channel plays a vital role in the development of the pelagic egg phenotype. However, the developmental control of aqp1ab activation during oogenesis remains to be established. Here, we report the isolation of the 5'-flanking region of the teleost gilthead seabream aqp1ab gene, in which we identify conserved cis-regulatory elements for the binding of the nuclear progestin receptor (Pgr) and members of the Sox family of transcription factors. Subcellular localization studies indicated that the Pgr, as well as sox3 and -8b transcripts, are co-expressed in seabream oogonia, whereas in meiosis-arrested primary growth (pre-vitellogenic) oocytes, when aqp1ab mRNA and protein are first synthesized, the Pgr appears to be completely translocated from the ooplasm into the nucleus. By contrast, sox9b is highly expressed in more advanced oocytes, coinciding with a strong depletion of aqp1ab transcripts in the oocyte. Functional characterization of wild-type and mutated aqp1ab promoter constructs, using mammalian cells and Xenopus laevis oocytes, demonstrated that aqp1ab transcription is initiated by the Pgr, which is activated by the progestin 17α,20β-dihydroxy-4-pregnen-3-one (17,20β-P), the natural ligand of the seabream Pgr. In vitro incubation of seabream primary ovarian explants with the follicle-stimulating hormone or 17,20β-P confirmed that progestin-activated Pgr enhanced Aqp1ab synthesis via the aqp1ab promoter. However, transactivation assays in heterologous systems showed that Sox transcription factors can potentially modulate this mechanism. These data uncover the existence of an endocrine pathway involved in the early activation of a water channel necessary for egg formation in marine teleosts.

Neural ectoderm-secreted FGF initiates the expression of Nkx2.5 in cardiac progenitors via a p38 MAPK/CREB pathway

Vertebrate heart development is derived from paired primordia of anterior dorsolateral mesoderm expressing Nkx2.5 and GATA4 transcription factors. Yet growth factors and intracellular pathways specifying heart precursor gene expression are poorly understood. In the present work, we investigated the signaling events initiating Nkx2.5 expression in Xenopus laevis. We describe here that fibroblast growth factor (FGF) initiates the expression of Nkx2.5 without affecting GATA4. At gastrula, FGF3 is expressed in anterior neural ectoderm, and results presented here indicate that this tissue is involved in the induction of Nkx2.5 expression in neighboring lateral tissues. Further studies indicate that the intracellular p38 MAPK and the CREB transcription factor function downstream of FGF to initiate Nkx2.5 expression. Activation of the p38 MAPK pathway and of the CREB protein is both necessary and sufficient for the initial expression of Nkx2.5. Therefore, we would like to suggest that FGF expressed in anterior neural ectoderm is a major inducer of Nkx2.5 expression in neighboring cells. In these cells, FGF activates an intracellular p38 MAPK signaling pathway and its downstream target, the CREB transcription factor, all participating in the expression of Nkx2.5 in cardiac progenitors.

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.

Functional characterization of mouse fetal TnI gene promoters in myocardial cells

Two major troponin I (TnI) genes, fetal TnI (ssTnI) and adult TnI (cTnI), are expressed in the mammalian heart under the control of a developmentally regulated program. In this study, the up-stream domain ( approximately 1,800 bp) of mouse fetal TnI gene has been cloned and characterized. There is a high homology of this region among mouse, rat and human. Analysis of the sequence revealed several putative regulatory domains and binding sites (Sp1 binding sites, GATA binding site, MyoD, CREB, MEF2, AP1, NFkappaB, etc). Transfection assays indicated that conserved GA-rich sequences, CREB and a CCAAT box within the first 300 bp upstream of the transcription start site were critical for the gene expression. Electrophoretic mobility shift assays (EMSAs) and chromatin immunoprecipitation (ChIP) assays revealed binding proteins to CREB site in nuclear extracts from myocardial cells. An inhibitory domain was revealed within the sequence between -1,700 to -1,780. Thyroid hormone (T(3)) caused a significant inhibitory effect on ssTnI expression in myocardial cells whereas this effect was not evident in CHO cells.

Interference of endocrine disrupting chemicals with aromatase CYP19 expression or activity, and consequences for reproduction of teleost fish

Many natural and synthetic compounds present in the environment exert a number of adverse effects on the exposed organisms, leading to endocrine disruption, for which they were termed endocrine disrupting chemicals (EDCs). A decrease in reproduction success is one of the most well-documented signs of endocrine disruption in fish. Estrogens are steroid hormones involved in the control of important reproduction-related processes, including sexual differentiation, maturation and a variety of others. Careful spatial and temporal balance of estrogens in the body is crucial for proper functioning. At the final step of estrogen biosynthesis, cytochrome P450 aromatase, encoded by the cyp19 gene, converts androgens into estrogens. Modulation of aromatase CYP19 expression and function can dramatically alter the rate of estrogen production, disturbing the local and systemic levels of estrogens. In the present review, the current progress in CYP19 characterization in teleost fish is summarized and the potential of several classes of EDCs to interfere with CYP19 expression and activity is discussed. Two cyp19 genes are present in most teleosts, cyp19a and cyp19b, primarily expressed in the ovary and brain, respectively. Both aromatase CYP19 isoforms are involved in the sexual differentiation and regulation of the reproductive cycle and male reproductive behavior in diverse teleost species. Alteration of aromatase CYP19 expression and/or activity, be it upregulation or downregulation, may lead to diverse disturbances of the above mentioned processes. Prediction of multiple transcriptional regulatory elements in the promoters of teleost cyp19 genes suggests the possibility for several EDC classes to affect cyp19 expression on the transcriptional level. These sites include cAMP responsive elements, a steroidogenic factor 1/adrenal 4 binding protein site, an estrogen-responsive element (ERE), half-EREs, dioxin-responsive elements, and elements related to diverse other nuclear receptors (peroxisome proliferator activated receptor, retinoid X receptor, retinoic acid receptor). Certain compounds including phytoestrogens, xenoestrogens, fungicides and organotins may modulate aromatase CYP19 activity on the post-transcriptional level. As is shown in this review, diverse EDCs may affect the expression and/or activity of aromatase cyp19 genes through a variety of mechanisms, many of which need further characterization in order to improve the prediction of risks posed by a contaminated environment to teleost fish population.

Patterning the early Xenopus embryo

Developmental biology teachers use the example of the frog embryo to introduce young scientists to the wonders of vertebrate development, and to pose the crucial question, 'How does a ball of cells become an exquisitely patterned embryo?'. Classical embryologists also recognized the power of the amphibian model and used extirpation and explant studies to explore early embryo polarity and to define signaling centers in blastula and gastrula stage embryos. This review revisits these early stages of Xenopus development and summarizes the recent explosion of information on the intrinsic and extrinsic factors that are responsible for the first phases of embryonic patterning.

Interrenal and thyroid development in red drum (Sciaenops ocellatus): effects of nursery environment on larval growth and cortisol concentration during settlement

Red drum settle into shallow seagrass meadows during the larval stage. Day-night cycles in these habitats result in marked diel temperature and dissolved oxygen (DO) cycles, and it is possible that extreme fluctuations influence endocrine development and growth of larvae. Here, we described red drum interrenal and thyroid ontogeny and determine responses to environmental stimuli with special emphasis on settlement to explore possible role of hormones as mediator of directive environmental factors. This study detected an early activation of thyroid and interrenal axis during the yolk-sac phase and a second activation of the thyroid starting at settlement size to the end of the larval period. Whole-body l-thyroxine (T4) and 3-5-3'-triiodo-l-thyronine (T3) showed a sharp decline at the juvenile stage. In contrast, cortisol steadily declines during the larval phase to a minimum before the end of the larval period. Older settlement-size larvae exposed to a strong stimulus increased whole body cortisol. In contrast, new settlers showed a minor cortisol rise suggesting changes on stress responsiveness during the ontogeny of the species. Additionally, settlement-size larvae exposed to various environmentally realistic temperature or DO fluctuations showed no difference in growth compared to fish grown under stable conditions (control). However, growth rate was significantly reduced in DO cycled fish with prolonged exposure to hypoxia. No differences were found in whole-body cortisol levels in the reduced growth treatment groups, suggesting that growth retardation was not related to a cortisol-mediated stress response. In moderate DO and temperature treatment groups, cortisol showed wider fluctuations than control groups during the night time that were not related to stress.

Maternal determinants of embryonic cell fate

How important is the contribution of mRNAs and proteins stored in the oocyte for determining the body plan of the Xenopus embryo? Here we review the current understanding of the roles of maternally supplied transcription factors, signaling molecules, and signaling regulators in establishing the ectoderm, mesoderm, and endoderm germ layers and the embryonic axes. Key essential asymmetries of VegT, Wnt11, and Ectodermin are described, as well as the complexity of maternal transcription factors that are involved in the initial expression of early zygotic genes.

A novel G protein-coupled receptor, related to GPR4, is required for assembly of the cortical actin skeleton in early Xenopus embryos

As the fertilized Xenopus egg undergoes sequential cell divisions to form a blastula, each cell develops a network of cortical actin that provides shape and skeletal support for the whole embryo. Disruption of this network causes loss of shape and rigidity of the embryo, and disrupts gastrulation movements. We previously showed that lysophosphatidic acid (LPA) signaling controls the change in cortical actin density that occurs at different stages of the cell cycle. Here, we use a gain-of-function screen, using an egg cDNA expression library, to identify an orphan G protein-coupled cell-surface receptor (XFlop) that controls the overall amount of cortical F-actin. Overexpression of XFlop increases the amount of cortical actin, as well as embryo rigidity and wound healing, whereas depletion of maternal XFlop mRNA does the reverse. Both overexpression and depletion of XFlop perturb gastrulation movements. Reciprocal rescue experiments, and comparison of the effects of their depletion in early embryos, show that the XLPA and XFlop signaling pathways play independent roles in cortical actin assembly, and thus that multiple signaling pathways control the actin skeleton in the blastula.

New roles for FoxH1 in patterning the early embryo

FoxH1 (Fast1) was first characterized as the transcriptional partner for Smad proteins. Together with Smad2/4, it forms the activin response factor (ARF) that binds to the Mix.2 promoter in Xenopus embryos. Foxh1 is expressed maternally in Xenopus. Depletion of maternal Foxh1 mRNA results in abnormalities of head and dorsal axis formation. We show that FoxH1 is required, together with XTcf3/beta catenin, to activate the zygotic expression of the nodal gene, Xnr3 in a Smad2-independent manner. In contrast, maternal FoxH1 acts as an inhibitor of Xnr5 and 6 transcription, preventing their upregulation on the ventral side of the embryo, by the maternal T-box transcription factor VegT. We conclude that maternal FoxH1 has essential, context-dependent roles in regulating the pattern of zygotic gene expression in the early embryo.

The human T locus and spina bifida risk

The transcription factor T is essential for mesoderm formation and axial development during embryogenesis. Embryonic genotype for a single-nucleotide polymorphism in intron 7 of T ( TIVS7 T/C) has been associated with the risk of spina bifida in some but not all studies. We developed a novel genotyping assay for the TIVS7 polymorphism using heteroduplex generator methodology. This assay was used to genotype spina bifida case-parent trios and the resulting data were analyzed using the transmission disequilibrium test and log-linear analyses. Analyses of these data demonstrated that heterozygous parents transmit the TIVS7-C allele to their offspring with spina bifida significantly more frequently than expected under the assumption of Mendelian inheritance (63 vs 50%, P=0.02). Moreover, these analyses suggest that the TIVS7-C allele acts in a dominant fashion, such that individuals carrying one or more copies of this allele have a 1.6-fold increased risk of spina bifida compared with individuals with zero copies. In silico analysis of the sequence surrounding this polymorphism revealed a potential target site for olfactory neuron-specific factor-1, a transcription factor expressed in the neural tube during development, spanning the polymorphic site. Several other putative, developmentally important and/or environmentally responsive transcription factor-binding sites were also identified close to the TIVS7 polymorphism. The TIVS7 polymorphism or a variant that is in linkage disequilibrium with the TIVS7 polymorphism may, therefore, play a role in T gene expression and influence the risk of spina bifida.

Four-dimensional imaging of cytoskeletal dynamics in Xenopus oocytes and eggs

The Xenopus laevis (African clawed frog) system has long been popular for studies of both developmental and cell biology, based on a variety of its intrinsic features including the large size of Xenopus oocytes, eggs, and embryos, and the relative ease of manipulation. Unfortunately, the large size has also been considered a serious impediment for high-resolution light microscopy, as has the heavy pigmentation. However, the recent development and exploitation of 4D imaging approaches, and the fact that much of what is of most interest to cell and developmental biologists takes place near the cell surface, indicates that such concerns are no longer valid. Consequently, the Xenopus system in many respects is now as good as other model systems considered to be ideal for microscopy-based studies. Here, 4D imaging and its recent applications to cytoskeletal imaging in Xenopus oocytes and eggs are discussed.