Cell type-specific activation of actin genes in the early amphibian embryo

Dkk2 promotes neural crest specification by activating Wnt/β-catenin signaling in a GSK3β independent manner

Neural crest progenitors are specified through the modulation of several signaling pathways, among which the activation of Wnt/β-catenin signaling by Wnt8 is especially critical. Glycoproteins of the Dickkopf (Dkk) family are important modulators of Wnt signaling acting primarily as Wnt antagonists. Here we report that Dkk2 is required for neural crest specification functioning as a positive regulator of Wnt/β-catenin signaling. Dkk2 depletion in Xenopus embryos causes a loss of neural crest progenitors, a phenotype that is rescued by expression of Lrp6 or β-catenin. Dkk2 overexpression expands the neural crest territory in a pattern reminiscent of Wnt8, Lrp6 and β-catenin gain-of-function phenotypes. Mechanistically, we show that Dkk2 mediates its neural crest-inducing activity through Lrp6 and β-catenin, however unlike Wnt8, in a GSK3β independent manner. These findings suggest that Wnt8 and Dkk2 converge on β-catenin using distinct transduction pathways both independently required to activate Wnt/β-catenin signaling and induce neural crest cells.

Absence of the Drosophila jump muscle actin Act79B is compensated by up-regulation of Act88F

BACKGROUND

Actins are structural components of the cytoskeleton and muscle, and numerous actin isoforms are found in most organisms. However, many actin isoforms are expressed in distinct patterns allowing each actin to have a specialized function. Numerous studies have demonstrated that actin isoforms both can and cannot compensate for each other under specific circumstances. This allows for an ambiguity of whether isoforms are functionally distinct.

RESULTS

In this study, we analyzed mutants of Drosophila Act79B, the predominant actin expressed in the adult jump muscle. Functional and structural analysis of the Act79B mutants found the flies to have normal jumping ability and sarcomere structure. Analysis of actin gene expression determined that expression of Act88F, an actin gene normally expressed in the flight muscles, was significantly up-regulated in the jump muscles of mutants. This indicated that loss of Act79B caused expansion of Act88F expression. When we created double mutants of Act79B and Act88F, this abolished the jump ability of the flies and resulted in severe defects in myofibril formation.

CONCLUSIONS

These results indicate that Act88F can functionally substitute for Act79B in the jump muscle, and that the functional compensation in actin expression in the jump muscles only occurs through Act88F. Developmental Dynamics 247:642-649, 2018. © 2018 Wiley Periodicals, Inc.

Innate Immune Response and Off-Target Mis-splicing Are Common Morpholino-Induced Side Effects in Xenopus

Antisense morpholino oligomers (MOs) have been indispensable tools for developmental biologists to transiently knock down (KD) genes rather than to knock them out (KO). Here we report on the implications of genetic KO versus MO-mediated KD of the mesoderm-specifying Brachyury paralogs in the frog Xenopus tropicalis. While both KO and KD embryos fail to activate the same core gene regulatory network, resulting in virtually identical morphological defects, embryos injected with control or target MOs also show a systemic GC content-dependent immune response and many off-target splicing defects. Optimization of MO dosage and increasing incubation temperatures can mitigate, but not eliminate, these MO side effects, which are consistent with the high affinity measured between MO and off-target sequence in vitro. We conclude that while MOs can be useful to profile loss-of-function phenotypes at a molecular level, careful attention must be paid to their immunogenic and off-target side effects.

Implication of thyroid hormone signaling in neural crest cells migration: Evidence from thyroid hormone receptor beta knockdown and NH3 antagonist studies

Thyroid hormones (TH) have been mainly associated with post-embryonic development and adult homeostasis but few studies report direct experimental evidence for TH function at very early phases of embryogenesis. We assessed the outcome of altered TH signaling on early embryogenesis using the amphibian Xenopus as a model system. Precocious exposure to the TH antagonist NH-3 or impaired thyroid receptor beta function led to severe malformations related to neurocristopathies. These include pathologies with a broad spectrum of organ dysplasias arising from defects in embryonic neural crest cell (NCC) development. We identified a specific temporal window of sensitivity that encompasses the emergence of NCCs. Although the initial steps in NCC ontogenesis appeared unaffected, their migration properties were severely compromised both in vivo and in vitro. Our data describe a role for TH signaling in NCCs migration ability and suggest severe consequences of altered TH signaling during early phases of embryonic development.

Lessons from a great developmental biologist

The announcement that Sir John Gurdon had been awarded the 2012 Nobel Prize for Medicine or Physiology was received with great joy by developmental biologists. It was a very special occasion because of his total dedication to science and turning the Golden Rule of western civilization - love your neighbor as yourself - into a reality in our field. This essay attempts to explain how John became such a great scientific benefactor, and to review some of his discoveries that are less well known than the nuclear transplantation experiments. A few personal anecdotes are also included to illustrate the profound goodness of this unique man of science.

Leiomodin 3 and tropomodulin 4 have overlapping functions during skeletal myofibrillogenesis

Precise regulation of thin filament length is essential for optimal force generation during muscle contraction. The thin filament capping protein tropomodulin (Tmod) contributes to thin filament length uniformity by regulating elongation and depolymerization at thin filament ends. The leiomodins (Lmod1-3) are structurally related to Tmod1-4 and also localize to actin filament pointed ends, but in vitro biochemical studies indicate that Lmods act instead as robust nucleators. Here, we examined the roles of Tmod4 and Lmod3 during Xenopus skeletal myofibrillogenesis. Loss of Tmod4 or Lmod3 resulted in severe disruption of sarcomere assembly and impaired embryonic movement. Remarkably, when Tmod4-deficient embryos were supplemented with additional Lmod3, and Lmod3-deficient embryos were supplemented with additional Tmod4, sarcomere assembly was rescued and embryonic locomotion improved. These results demonstrate for the first time that appropriate levels of both Tmod4 and Lmod3 are required for embryonic myofibrillogenesis and, unexpectedly, both proteins can function redundantly during in vivo skeletal muscle thin filament assembly. Furthermore, these studies demonstrate the value of Xenopus for the analysis of contractile protein function during de novo myofibril assembly.

In vivo T-box transcription factor profiling reveals joint regulation of embryonic neuromesodermal bipotency

The design of effective cell replacement therapies requires detailed knowledge of how embryonic stem cells form primary tissues, such as mesoderm or neurectoderm that later become skeletal muscle or nervous system. Members of the T-box transcription factor family are key in the formation of these primary tissues, but their underlying molecular activities are poorly understood. Here, we define in vivo genome-wide regulatory inputs of the T-box proteins Brachyury, Eomesodermin, and VegT, which together maintain neuromesodermal stem cells and determine their bipotential fates in frog embryos. These T-box proteins are all recruited to the same genomic recognition sites, from where they activate genes involved in stem cell maintenance and mesoderm formation while repressing neurogenic genes. Consequently, their loss causes embryos to form an oversized neural tube with no mesodermal derivatives. This collaboration between T-box family members thus ensures the continuous formation of correctly proportioned neural and mesodermal tissues in vertebrate embryos during axial elongation.

Myogenic waves and myogenic programs during Xenopus embryonic myogenesis

Although Xenopus is a key model organism in developmental biology, little is known about the myotome formation in this species. Here, we assessed the expression of myogenic regulatory factors of the Myod family (MRFs) during embryonic development and revealed distinct MRF programs.

RESULTS

The expression pattern of each MRF during embryonic development highlights three successive myogenic waves. We showed that a first median and lateral myogenesis initiates before dermomyotome formation: the median cell population expresses Myf5, Myod, and Mrf4, whereas the lateral one expresses Myod, moderate levels of Myogenin and Mrf4. The second wave of myoblasts arising from the dermomyotome is characterized by the full MRF program expression, with high levels of Myogenin. The third wave is revealed by Myf5 expression in the myotome and could contribute to the formation of plurinucleated fibers at larval stages. Furthermore, Myf5- or Myod-expressing anlagen are identified in craniofacial myogenesis.

CONCLUSIONS

The first median and lateral myogenesis and their associated MRF programs have probably disappeared in mammals. However, some aspects of Xenopus myogenesis have been conserved such as the development of somitic muscles by successive myogenic waves and the existence of Myf5-dependent and -independent lineages.

Neuro-mesodermal patterns in artificially deformed embryonic explants: a role for mechano-geometry in tissue differentiation

The mutual arrangement of neural and mesodermal rudiments in artificially bent double explants of Xenopus laevis suprablastoporal areas was compared with that of intact explants. While some of the bent explants straightened or became spherical, most retained and actively reinforced the imposed curvature, creating folds on their concave sides and expanding convex surfaces. In the intact explants, the arrangement of neural and mesodermal rudiments exhibited a distinct antero-posterior polarity, with some variability. In the bent explants, this polarity was lost: the neural rudiments were shifted towards concave while the mesodermal tissues moved towards the convex side, embracing the neural rudiments in a horseshoe-shaped manner. We associate these drastic changes in neuro-mesodermal patterning with the active extension and contraction of the convex and concave sides, respectively, triggered by the imposed deformations. We speculate that similar events are responsible for the establishment of neuro-mesodermal patterns during normal development.

Lymph heart musculature is under distinct developmental control from lymphatic endothelium

Lymph hearts are pulsatile organs, present in lower vertebrates, that function to propel lymph into the venous system. Although they are absent in mammals, the initial veno-lymphatic plexus that forms during mammalian jugular lymph sac development has been described as the vestigial homologue of the nascent stage of ancestral anterior lymph hearts. Despite the widespread presence of lymph hearts among vertebrate species and their unique function, extremely little is known about lymph heart development. We show that Xenopus anterior lymph heart muscle expresses skeletal muscle markers such as myoD and 12/101, rather than cardiac markers. The onset of lymph heart myoblast induction can be visualized by engrailed-1 (en1) staining in anterior trunk somites, which is dependent on Hedgehog (Hh) signaling. In the absence of Hh signaling and upon en1 knockdown, lymph heart muscle fails to develop, despite the normal development of the lymphatic endothelium of the lymph heart, and embryos develop edema. These results suggest a mechanism for the evolutionary transition from anterior lymph hearts to jugular lymph sacs in mammals.

Xmc mediates Xctr1-independent morphogenesis in Xenopus laevis

In the frog, Xenopus laevis, fibroblast growth factor (FGF) signaling is required for both mesoderm formation and the morphogenetic movements that drive the elongation of the notochord, a dorsal mesodermal derivative; the coordination of these distinct roles is mediated by the Xenopus Ctr1 (Xctr1) protein: maternal Xctr1 is required for mesodermal differentiation, while the subsequent loss of Xctr1 promotes morphogenesis. The signaling cascade activated by FGF in the presence of Ctr1 has been well characterized; however, the Xctr1-independent, FGF-responsive network remains poorly defined. We have identified Xenopus Marginal Coil (Xmc) as a gene whose expression is highly enriched following Xctr1 knockdown. Zygotic initiation of Xmc expression in vivo coincides with a decrease in maternal Xctr1 transcripts; moreover, Xmc loss-of-function inhibits Xctr1 knockdown-mediated elongation of FGF-treated animal cap explants, implicating Xmc as a key effector of Xctr1-independent gastrular morphogenesis.

Patterning the embryonic kidney: BMP signaling mediates the differentiation of the pronephric tubules and duct in Xenopus laevis

The Bone morphogenetic proteins (BMPs) mediate a wide range of diverse cellular behaviors throughout development. Previous studies implicated an important role for BMP signaling during the differentiation of the definitive mammalian kidney, the metanephros. In order to examine whether BMP signaling also plays an important role during the patterning of earlier renal systems, we examined the development of the earliest nephric system, the pronephros. Using the amphibian model system Xenopus laevis, in combination with reagents designed to inhibit BMP signaling during specific stages of nephric development, we revealed an evolutionarily conserved role for this signaling pathway during renal morphogenesis. Our results demonstrate that conditional BMP inhibition after specification of the pronephric anlagen is completed, but prior to the onset of morphogenesis and differentiation of renal tissues, results in the severe malformation of both the pronephric duct and tubules. Importantly, the effects of BMP signaling on the developing nephron during this developmental window are specific, only affecting the developing duct and tubules, but not the glomus. These data, combined with previous studies examining metanephric development in mice, provide further support that BMP functions to mediate morphogenesis of the specified renal field during vertebrate embryogenesis. Specifically, BMP signaling is required for the differentiation of two types of nephric structures, the pronephric tubules and duct.

Developmental expression of the alpha-skeletal actin gene

Background

Actin is a cytoskeletal protein which exerts a broad range of functions in almost all eukaryotic cells. In higher vertebrates, six primary actin isoforms can be distinguished: alpha-skeletal, alpha-cardiac, alpha-smooth muscle, gamma-smooth muscle, beta-cytoplasmic and gamma-cytoplasmic isoactin. Expression of these actin isoforms during vertebrate development is highly regulated in a temporal and tissue-specific manner, but the mechanisms and the specific differences are currently not well understood. All members of the actin multigene family are highly conserved, suggesting that there is a high selective pressure on these proteins.

Results

We present here a model for the evolution of the genomic organization of alpha-skeletal actin and by molecular modeling, illustrate the structural differences of actin proteins of different phyla. We further describe and compare alpha-skeletal actin expression in two developmental stages of five vertebrate species (mouse, chicken, snake, salamander and fish). Our findings confirm that alpha-skeletal actin is expressed in skeletal muscle and in the heart of all five species. In addition, we identify many novel non-muscular expression domains including several in the central nervous system.

Conclusion

Our results show that the high sequence homology of alpha-skeletal actins is reflected by similarities of their 3 dimensional protein structures, as well as by conserved gene expression patterns during vertebrate development. Nonetheless, we find here important differences in 3D structures, in gene architectures and identify novel expression domains for this structural and functional important gene.

The extracellular adenosine deaminase growth factor, ADGF/CECR1, plays a role in Xenopus embryogenesis via the adenosine/P1 receptor

Adenosine deaminase-related growth factors (ADGF), also known as CECR1 in vertebrates, are a novel family of growth factors with sequence similarity to classical cellular adenosine deaminase. Although genes for ADGF/CECR1 have been identified in both invertebrates as well as vertebrates, their in vivo functions in vertebrates remain unknown. We isolated cDNA clones for two cerc 1s from Xenopus laevis. Both recombinant Xenopus CECR1s exhibited adenosine deaminase and growth factor activity, and the adenosine deaminase activity was found to be indispensable for growth factor activity. The Xenopus cerc 1s are expressed in the somites, pronephros, eyes, cement gland, neural tube, and neural floor plate of the embryos. Knock-down of these two genes using morpholino oligonucleotides caused a reduction in the body size and abnormalities of the body axis in the Xenopus embryos, accompanied by selective changes in the expression of developmental marker genes. Injection of adenosine, agonists for adenosine/P1 receptors, or adenosine deaminase inhibitor into late gastrula archenteron embryos resulted in developmental defects similar to those caused by morpholino oligonucleotide injection. These results show, for the first time, the involvement of CECR1s via the adenosine/P1 receptors in vertebrate embryogenesis via regulation of extracellular adenosine concentrations.

Vertebrate Ctr1 coordinates morphogenesis and progenitor cell fate and regulates embryonic stem cell differentiation

Embryogenesis involves two distinct processes. On the one hand, cells must specialize, acquiring fates appropriate to their positions (differentiation); on the other hand, they must physically construct the embryo through coordinated mechanical activity (morphogenesis). In early vertebrate development, fibroblast growth factor (FGF) regulates multiple embryonic events, including germ layer differentiation and morphogenesis; the cellular components that direct FGF signaling to evoke these different responses remain largely unknown. We show here that the copper transporter 1 (Ctr1) protein is a critical router of FGF signals during early embryogenesis. Ctr1 both promotes the differentiation and inhibits the morphogenesis of mesoderm and neurectoderm in embryos of the frog Xenopus laevis, thereby coordinating normal development. Signal sorting by Ctr1 involves the activation of the Ras-MAP kinase cascade and appears to be independent of its role in copper transport. Mouse embryonic stem (ES) cells deficient for Ctr1 (Ctr1(-/-)) retain characteristics of pluripotency under conditions that favor differentiation in wild-type ES cells, indicating a conserved role for Ctr1 during amphibian and mammalian cell fate determination. Our studies support a model in which vertebrate Ctr1 functions as a key regulator of the differentiation capacity of both stem and progenitor cell populations.

From Nuclear Transfer to Nuclear Reprogramming: The Reversal of Cell Differentiation

This is a personal historical account of events leading from the earliest success in vertebrate nuclear transfer to the current hope that nuclear reprogramming may facilitate cell replacement therapy. Early morphological evidence in Amphibia for the toti- or multipotentiality of some nuclei from differentiated cells first established the principle of the conservation of the genome during cell differentiation. Molecular markers show that many somatic cell nuclei are reprogrammed to an embryonic pattern of gene expression soon after nuclear transplantation to eggs. The germinal vesicles of oocytes in first meiotic prophase have a direct reprogramming activity on mammalian as well as amphibian nuclei and offer a route to identify nuclear reprogramming molecules. Amphibian eggs and oocytes have a truly remarkable ability to transcribe genes as DNA or nuclei, to translate mRNA, and to modify or localize proteins injected into them. The development of nuclear transplant embryos depends on the ability of cells to interpret small concentration changes of signal factors in the community effect and in morphogen gradients. Many difficulties in a career can be overcome by analyzing in increasing depth the same fundamentally interesting and important problem.

Genetic screens for mutations affecting development of Xenopus tropicalis

We present here the results of forward and reverse genetic screens for chemically-induced mutations in Xenopus tropicalis. In our forward genetic screen, we have uncovered 77 candidate phenotypes in diverse organogenesis and differentiation processes. Using a gynogenetic screen design, which minimizes time and husbandry space expenditures, we find that if a phenotype is detected in the gynogenetic F2 of a given F1 female twice, it is highly likely to be a heritable abnormality (29/29 cases). We have also demonstrated the feasibility of reverse genetic approaches for obtaining carriers of mutations in specific genes, and have directly determined an induced mutation rate by sequencing specific exons from a mutagenized population. The Xenopus system, with its well-understood embryology, fate map, and gain-of-function approaches, can now be coupled with efficient loss-of-function genetic strategies for vertebrate functional genomics and developmental genetics.

Organization and developmental expression of an amphibian vascular smooth muscle alpha-actin gene

A gene encoding a putative homologue of the avian and mammalian vascular smooth muscle alpha-actin was isolated from an amphibian, Rana catesbeiana, and characterized in terms of its sequence, organization, and expression pattern. To assess the expression of this gene during amphibian embryonic development, a cDNA encoding the Xenopus homologue of this mRNA was isolated and characterized by in situ hybridization. The expression of this gene was not detected in the enteric smooth muscle cells or, unlike its avian and mammalian homologues, in the somites/skeletal muscle of the Xenopus embryos/tadpoles. Its initial expression coincides with the onset of cardiac muscle differentiation and is coincidental with the expression of the cardiac alpha-actin mRNAs in the heart-forming region of the stage 26/27 embryo. As development proceeds, transcripts from this gene are expressed throughout the developing heart until the formation of the heart chambers is completed and, thereafter, its expression becomes restricted to the outflow tract of the tadpole heart. The subsequent restricted expression of this gene to the vascular system in both of these amphibians identifies it as the amphibian homologue of the avian and mammalian vascular smooth muscle alpha-actin.

Tsukushi controls ectodermal patterning and neural crest specification in Xenopus by direct regulation of BMP4 and X-delta-1 activity

In Xenopus, ectodermal patterning depends on a mediolateral gradient of BMP signaling, higher in the epidermis and lower in the neuroectoderm. Neural crest cells are specified at the border between the neural plate and the epidermis, at intermediate levels of BMP signaling. We recently described a novel secreted protein, Tsukushi (TSK), which works as a BMP antagonist during chick gastrulation. Here, we report on the Xenopus TSK gene (X-TSK), and show that it is involved in neural crest specification. X-TSK expression accumulates after gastrulation at the anterior-lateral edges of the neural plate, including the presumptive neural crest region. In gain-of-function experiments, X-TSK can strongly enhance neural crest specification by the dorsolateral mesoderm or X-Wnt8 in ectodermal explants, while the electroporation of X-TSK mRNA in the lateral ectoderm of embryos after gastrulation can induce the expression of neural crest markers in vivo. By contrast, depletion of X-TSK in explants or embryos impairs neural crest specification. Similarly to its chick homolog, X-TSK works as a BMP antagonist by direct binding to BMP4. However, X-TSK can also indirectly regulate BMP4 mRNA expression at the neural plate border via modulation of the Delta-Notch signaling pathway. We show that X-TSK directly binds to the extracellular region of X-delta-1, and modulates Delta-dependent Notch activity. We propose that X-TSK plays a key role in neural crest formation by directly regulating BMP and Delta activities at the boundary between the neural and the non-neural ectoderm.

Global gene expression profiling and cluster analysis in Xenopus laevis

We have undertaken a large-scale microarray gene expression analysis using cDNAs corresponding to 21,000 Xenopus laevis ESTs. mRNAs from 37 samples, including embryos and adult organs, were profiled. Cluster analysis of embryos of different stages was carried out and revealed expected affinities between gastrulae and neurulae, as well as between advanced neurulae and tadpoles, while egg and feeding larvae were clearly separated. Cluster analysis of adult organs showed some unexpected tissue-relatedness, e.g. kidney is more related to endodermal than to mesodermal tissues and the brain is separated from other neuroectodermal derivatives. Cluster analysis of genes revealed major phases of co-ordinate gene expression between egg and adult stages. During the maternal-early embryonic phase, genes maintaining a rapidly dividing cell state are predominantly expressed (cell cycle regulators, chromatin proteins). Genes involved in protein biosynthesis are progressively induced from mid-embryogenesis onwards. The larval-adult phase is characterised by expression of genes involved in metabolism and terminal differentiation. Thirteen potential synexpression groups were identified, which encompass components of diverse molecular processes or supra-molecular structures, including chromatin, RNA processing and nucleolar function, cell cycle, respiratory chain/Krebs cycle, protein biosynthesis, endoplasmic reticulum, vesicle transport, synaptic vesicle, microtubule, intermediate filament, epithelial proteins and collagen. Data filtering identified genes with potential stage-, region- and organ-specific expression. The dataset was assembled in the iChip microarray database, , which allows user-defined queries. The study provides insights into the higher order of vertebrate gene expression, identifies synexpression groups and marker genes, and makes predictions for the biological role of numerous uncharacterized genes.

Inhibition of mesodermal fate by Xenopus HNF3beta/FoxA2

The winged-helix transcription factor HNF3beta/FoxA2 is expressed in embryonic organizing centers of the gastrulating mouse, frog, fish, and chick. In the mouse, HNF3beta is required for the formation of the mammalian node and notochord, and can induce ectopic floor plate formation when misexpressed in the developing neural tube; HNF3beta expression in the extraembryonic endoderm is also necessary for the proper morphogenesis of the mammalian primitive streak. In the frog Xenopus laevis, several lines of evidence suggest that the related winged-helix factor Pintallavis functions as the ortholog of mammalian HNF3beta in both axial mesoderm and neurectoderm; the role of Xenopus HNF3beta itself, however, has not been clearly defined, and is the subject of this study. HNF3beta is widely expressed in the vegetal pole but, as previously suggested, is excluded from the gastrula-stage mesoderm. We find that expression of an HNF3beta-Engrailed repressor fusion protein induces ectopic axes and inhibits head formation in Xenopus embryos, while ectopic HNF3beta inhibits mesoderm and anterior endoderm formation in explant assays and in vivo. Our studies suggest that HNF3beta target genes function to limit the extent of mesoderm formation in the Xenopus gastrula, and point to related roles for Xenopus HNF3beta and the extraembryonic component of mammalian HNF3beta during vertebrate gastrulation.

GATA-6 maintains BMP-4 and Nkx2 expression during cardiomyocyte precursor maturation

GATA-6 is expressed in presumptive cardiac mesoderm before gastrulation, but its role in heart development has been unclear. Here we show that Xenopus and zebrafish embryos, injected with antisense morpholino oligonucleotides designed specifically to knock-down translation of GATA-6 protein, are severely compromised for heart development. Injected embryos express greatly reduced levels of contractile machinery genes and, at the same stage, of regulatory genes such as bone morphogenetic protein-4 (BMP-4) and the Nkx2 family. In contrast, initial BMP and Nkx2 expression is normal, suggesting a maintenance role for GATA-6. Endoderm is critical for heart formation in several vertebrates including Xenopus, and separate perturbation of GATA-6 expression in the deep anterior endoderm and in the overlying heart mesoderm shows that GATA-6 is required in both for cardiogenesis. The GATA-6 requirement in cardiac mesoderm was confirmed in zebrafish, an organism in which endoderm is thought not to be necessary for heart formation. We therefore conclude that proper maturation of cardiac mesoderm requires GATA-6, which functions to maintain BMP-4 and Nkx2 expression.

Xenopus Cyr61 regulates gastrulation movements and modulates Wnt signalling

Cyr61 is a secreted, heparin-binding, extracellular matrix-associated protein whose activities include the promotion of adhesion and chemotaxis, and the stimulation of fibroblast and endothelial cell growth. Many, if not all, of these activities of Cyr61 are mediated through interactions with integrins. We explore the role of Cyr61 in the early development of Xenopus laevis. Gain- and loss-of-function experiments show that Xcyr61 is required for normal gastrulation movements. This role is mediated in part through the adhesive properties of Xcyr61 and its related ability to modulate assembly of the extracellular matrix. In addition, Xcyr61 can, in a context-dependent manner, stimulate or inhibit signalling through the Wnt pathway. These properties of Xcyr61 provide a mechanism for integrating cell signalling, cell adhesion and cell migration during gastrulation.

Xenopus X-box binding protein 1, a leucine zipper transcription factor, is involved in the BMP signaling pathway

We describe a novel basic leucine zipper transcription factor, XXBP-1, which interacts with BMP-4 in a positive feedback loop. It is a maternal factor and is zygotically expressed in the dorsal blastopore lip and ventral ectoderm with the exception of the prospective neural plate during gastrulation. Overexpression of XXBP-1 leads to ventralization of early embryos as described for BMP-4, and inhibits neuralization of dissociated ectoderm. Consistent with mediating BMP signaling, we show that the ectopic expression of XXBP-1 recovers the expression of epidermal keratin and reverses the dorsalization imposed by truncated BMP receptor type I, indicating that it may act downstream of the BMP receptor. Its effects can be partially mimicked by a fusion construct containing the VP16 activator domain and the XXBP-1 DNA-binding domain. In contrast, fusing the DNA-binding domain to the even-skipped repressor domain leads to upregulation of the neural markers NCAM and nrp-1 in animal cap assay. Taken together, the results suggest a role for XXBP-1 in the control of neural differentiation, possibly as an activator.

Mouse Mix gene is activated early during differentiation of ES and F9 stem cells and induces endoderm in frog embryos

In frog and zebrafish, the Mix/Bix family of paired type homeodomain proteins play key roles in specification and differentiation of mesendoderm. However, in mouse, only a single Mix gene (mMix) has been identified to date and its function is unknown. We have analyzed the expression of mouse Mix RNA and protein in embryos, embryoid bodies formed from embryonic stem cells and F9 teratocarcinoma cells, as well as several differentiated cell types. Expression in embryoid bodies in culture mirrors that in embryos, where Mix is transcribed transiently in primitive (visceral) endoderm (VE) and in nascent mesoderm. In F9 cells induced by retinoic acid to differentiate to VE, mMix is coordinately expressed with three other endodermal transcription factors, well before AFP, and its protein product is localized to the nucleus. In a subpopulation of nascent mesodermal cells from embryonic stem cell embryoid bodies, mMix is coexpressed with Brachyury. Intriguingly, mMix mRNA is detected in a population (T+Flk1+) of cells which may contain hemangioblasts, before the onset of hematopoiesis and activation of hematopoietic markers. In vitro and in vivo, mMix expression in nascent mesoderm is rapidly down-regulated and becomes undetectable in differentiated cell types. In the region of the developing gut, mMix expression is confined to the mesoderm of mid- and hindgut but is absent from definitive endoderm. Injection of mouse mMix RNA into early frog embryos results in axial truncation of developing tadpoles and, in animal cap assays, mMix alone is sufficient to activate expression of several endodermal (but not mesodermal) markers. Although these observations do not exclude a possible cell-autonomous function for mMix in mesendodermal progenitor cells, they do suggest an additional, non-cell autonomous role in nascent mesoderm in the formation and/or patterning of adjacent definitive endoderm.

Induction and patterning of the telencephalon in Xenopus laevis

We report an analysis of the tissue and molecular interplay involved in the early specification of the forebrain, and in particular telencephalic, regions of the Xenopus embryo. In dissection/recombination experiments, different parts of the organizer region were explanted at gastrula stage and tested for their inducing/patterning activities on either naive ectoderm or on midgastrula stage dorsal ectoderm. We show that the anterior dorsal mesendoderm of the organizer region has a weak neural inducing activity compared with the presumptive anterior notochord, but is able to pattern either neuralized stage 10.5 dorsal ectoderm or animal caps injected with BMP inhibitors to a dorsal telencephalic fate. Furthermore, we found that a subset of this tissue, the anterior dorsal endoderm, still retains this patterning activity. At least part of the dorsal telencephalic inducing activities may be reproduced by the anterior endoderm secreted molecule cerberus, but not by simple BMP inhibition, and requires the N-terminal region of cerberus that includes its Wnt-binding domain. Furthermore, we show that FGF action is both necessary and sufficient for ventral forebrain marker expression in neuralized animal caps, and possibly also required for dorsal telencephalic specification. Therefore, integration of organizer secreted molecules and of FGF, may account for patterning of the more rostral part of Xenopus CNS.

Two myogenin-related genes are differentially expressed in Xenopus laevis myogenesis and differ in their ability to transactivate muscle structural genes

Among the myogenic regulatory factors, myogenin is a transcriptional activator situated at a crucial position for terminal differentiation in muscle development. It is unclear at present whether myogenin exhibits unique specificities to transactivate late muscular markers. During Xenopus development, the accumulation of myogenin mRNA is restricted to secondary myogenesis, at the onset of the appearance of adult isoforms of beta-tropomyosin and myosin heavy chain. To determine the role of myogenin in the isoform switch of these contractile proteins, we characterized and directly compared the functional properties of myogenin with other myogenic regulatory factors in Xenopus embryos. Two distinct cDNAs related to myogenin, XmyogU1 and XmyogU2, were differentially expressed during myogenesis and in adult tissues, in which they preferentially accumulated in oxidative myofibers. Animal cap assays in Xenopus embryos revealed that myogenin, but not the other myogenic regulatory factors, induced expression of embryonic/larval isoforms of the beta-tropomyosin and myosin heavy chain genes. Only XmyogU1 induced expression of the adult fast isoform of the myosin heavy chain gene. This is the first demonstration of a specific transactivation of one set of muscle structural genes by myogenin.

Siamois functions in the early blastula to induce Spemann's organiser

In Xenopus, the Spemann organiser is defined as a dorsal territory in the early gastrula that initiates development of the embryonic axis. It has been shown that the early zygotic transcription factor Siamois is essential for Spemann's organiser formation. By the onset of gastrulation, the organiser is patterned into a vegetal head organiser, which induces anterior structures upon transplantation, and a more animal trunk organiser, which induces a posterior neuraxis. However, it is unclear when these distinct organiser domains are initially specified. To shed light on this question, we analysed the temporal activity of Siamois, as this factor induces both head and trunk development, when ectopically expressed via mRNA injection. In this study, we expressed Siamois ectopically at different time points and analysed the extent of axial development. Using a hormone-inducible version of Siamois, we found evidence for a tight window of competence during which ventral cells can respond to Siamois by commencing both the head and the trunk genetic programmes. The competence to form Spemann's organiser was lost 2 h before gastrulation, although partial axis formation could still occur following delayed activation of Siamois. We demonstrate that this late response to Siamois involves a new role for this gene, which can indirectly repress ventral gene expression, in the absence of known organiser genes.

Comparative study of sequential expression of the organizer-related genes in normal Cynops pyrrhogaster embryos and mesodermalized ectoderm

An artificially mesodermalized ectoderm (mE) of early Cynops pyrrhogaster gastrula acquires the organizer property; the mE is able to induce the secondary axis. The expression of organizer-related genes was investigated during the mesodermalizing process of the mE. The expression of C. pyrrhogaster organizer-related genes, such as bra, gsc, lim-1, chd and noggin, were analyzed. Cynops pyrrhogaster shh expression was also investigated. The organizer-related genes were activated by 12 h after the mesoderm-inducing stimulus. It was noted that there was a temporal gap in the expression of each gene. The expression of bra and gsc seemed to be more quickly activated during the mesodermalizing process. While expression of lim-1 and noggin was activated later than that of bra and gsc, lim-1 expression was earlier than chd and noggin expression. Shh expression was activated later than lim-1/noggin. The present study suggests the possibility that the bra/gsc, lim-1, chd, noggin and shh genes are expressed one by one in that order during the mesodermalizing of the presumptive ectoderm. It also indicates that the sequence is not always consistent with that of the whole embryo during normal embryogenesis. The meaning of the discrepancy will be discussed in connection with the cascade of certain genes expressed during the mesodermalizing process.

The casein kinase I family: roles in morphogenesis

Wnt signals play important roles in development and oncogenesis and are transduced through at least two pathways: a canonical beta-catenin-dependent and a beta-catenin-independent cascade. Casein kinase I (CKI) is required in both invertebrates and vertebrates to transduce canonical Wnt signals. However, its role in the beta-catenin-independent pathway was unknown. During vertebrate embryogenesis, the beta-catenin-independent cascade is thought to control cell movements and has been postulated to be analogous to the Drosophila planar cell polarity pathway, which signals through the JNK cascade. Here, we report that blocking CKI function inhibits embryonic morphogenesis and activates JNK in cell lines. These studies suggest that CKI might also act in the beta-catenin-independent pathway and indicate a role for CKI during convergence extension in early vertebrate development.

The neural plate specifies somite size in the Xenopus laevis gastrula

The organizer has traditionally been considered the major source of somite-inducing signals. We show here that signaling from the neural plate specifies somite tissue and regulates somite size in the Xenopus gastrula. Ectopic undifferentiated neural tissue induces massive somite expansion at the expense of intermediate and lateral plate mesoderm. Although the early expanded somite expresses muscle-specific markers, only a portion terminally differentiates, suggesting that myotome development requires additional signals. Explant assays demonstrate that neural tissue induces somite-specific marker expression even in the absence of the organizer. Finally, we demonstrate that neural tissue is required for proper somite development because elimination of neural precursors results in pronounced somite reduction. Thus, an important reciprocal interaction exists between somite and neural tissue that is mutually reinforcing and critical for normal embryonic patterning.

Developmental program expression of myosin alkali light chain and skeletal actin genes in the rainbow trout Oncorhynchus mykiss

We have isolated MLC1(F) (tMLC1(F)), MLC3(F) (tMLC3(F)) and skeletal actin cDNAs from the teleost Oncorhynchus mykiss. Sequence analysis indicates that tMLC1(F) and tMLC3(F) are not produced from differentially spliced mRNAs as reported in avians and rodents but are encoded by different genes. Results from RNase protection analysis showed that the corresponding transcripts are expressed in fast skeletal muscles. Whole-mount in situ hybridisation revealed distinct expression patterns of the myosin alkali light chains and skeletal actin genes during skeletal muscle development in the embryo.

The amino terminus of Smads permits transcriptional specificity

Smad proteins transduce transforming growth factor-beta signals from the cell surface to the nucleus, regulating a variety of physiologic processes. In the nucleus, Smads control gene expression by binding to both DNA and transcription factors. Individual Smads regulate distinct subsets of target genes. The key residues important for this specificity are thought to reside in the carboxyl-terminal MH2 domain. To further examine Smad specificity in vivo, we undertook structure-function studies in Xenopus laevis embryos and found that truncated Smads containing the MH2 domain activate gene transcription. A striking finding revealed by the in vivo analyses was that the functional truncated Smads all behaved identically and had lost wild-type specificity. For most Smads, wild-type activity required the presence of an MH1 domain, either in cis or in trans. Of note, even heterologous MH1 domains could restore wild-type signaling specificity to effector MH2 domains. We found a possible mechanism to account for these observations, as Smad MH1 domains altered the binding of pathway-specific transcription factors to the MH2 domain. Thus, Smad MH1 domains are important to the regulation of transcriptional specificity.

The polycomb group protein EED interacts with YY1, and both proteins induce neural tissue in Xenopus embryos

Polycomb group (PcG) proteins form multimeric protein complexes which are involved in the heritable stable repression of genes. Previously, we identified two distinct human PcG protein complexes. The EED-EZH protein complex contains the EED and EZH2 PcG proteins, and the HPC-HPH PcG complex contains the HPC, HPH, BMI1, and RING1 PcG proteins. Here we show that YY1, a homolog of the Drosophila PcG protein pleiohomeotic (Pho), interacts specificially with the human PcG protein EED but not with proteins of the HPC-HPH PcG complex. Since YY1 and Pho are DNA-binding proteins, the interaction between YY1 and EED provides a direct link between the chromatin-associated EED-EZH PcG complex and the DNA of target genes. To study the functional significance of the interaction, we expressed the Xenopus homologs of EED and YY1 in Xenopus embryos. Both Xeed and XYY1 induce an ectopic neural axis but do not induce mesodermal tissues. In contrast, members of the HPC-HPH PcG complex do not induce neural tissue. The exclusive, direct neuralizing activity of both the Xeed and XYY1 proteins underlines the significance of the interaction between the two proteins. Our data also indicate a role for chromatin-associated proteins, such as PcG proteins, in Xenopus neural induction.

Xrel3 is required for head development in Xenopus laevis

The Rel/NF-kappa B gene family encodes a large group of transcriptional activators involved in myriad differentiation events, including embryonic development. We have shown previously that Xrel3, a Xenopus Rel/NF-kappa B-related gene, is expressed in the forebrain, dorsal aspect of the mid- and hindbrain, the otocysts and notochord of neurula and larval stage embryos. Overexpression of Xrel3 causes formation of embryonic tumours. We now show that Xrel3-induced tumours and animal caps from embryos injected with Xrel3 RNA express Otx2, Shh and Gli1. Heterodimerisation of a C-terminally deleted mutant of Xrel3 with wild-type Xrel3 inhibits in vitro binding of wild-type Xrel3 to Rel/NF-kappa B consensus DNA sequences. This dominant interference mutant disrupts Shh, Gli1 and Otx2 mRNA patterning and inhibits anterior development when expressed in the dorsal side of zygotes, which is rescued by co-injecting wild-type Xrel3 mRNA. In chick development, Rel activates Shh signalling, which is required for normal limb formation; Shh, Gli1 and Otx2 encode important neural patterning elements in vertebrates. The activation of these genes in tumours by Xrel3 overexpression and the inhibition of their expression and head development by a dominant interference mutant of Xrel3 indicates that Rel/NF-kappa B is required for activation of these genes and for anterior neural patterning in Xenopus.

Serrate and Notch specify cell fates in the heart field by suppressing cardiomyogenesis

Notch signaling mediates numerous developmental cell fate decisions in organisms ranging from flies to humans, resulting in the generation of multiple cell types from equipotential precursors. In this paper, we present evidence that activation of Notch by its ligand Serrate apportions myogenic and non-myogenic cell fates within the early Xenopus heart field. The crescent-shaped field of heart mesoderm is specified initially as cardiomyogenic. While the ventral region of the field forms the myocardial tube, the dorsolateral portions lose myogenic potency and form the dorsal mesocardium and pericardial roof (Raffin, M., Leong, L. M., Rones, M. S., Sparrow, D., Mohun, T. and Mercola, M. (2000) Dev. Biol., 218, 326–340). The local interactions that establish or maintain the distinct myocardial and non-myocardial domains have never been described. Here we show that Xenopus Notch1 (Xotch) and Serrate1 are expressed in overlapping patterns in the early heart field. Conditional activation or inhibition of the Notch pathway with inducible dominant negative or active forms of the RBP-J/Suppressor of Hairless [Su(H)] transcription factor indicated that activation of Notch feeds back on Serrate1 gene expression to localize transcripts more dorsolaterally than those of Notch1, with overlap in the region of the developing mesocardium. Moreover, Notch pathway activation decreased myocardial gene expression and increased expression of a marker of the mesocardium and pericardial roof, whereas inhibition of Notch signaling had the opposite effect. Activation or inhibition of Notch also regulated contribution of individual cells to the myocardium. Importantly, expression of Nkx2. 5 and Gata4 remained largely unaffected, indicating that Notch signaling functions downstream of heart field specification. We conclude that Notch signaling through Su(H) suppresses cardiomyogenesis and that this activity is essential for the correct specification of myocardial and non-myocardial cell fates.

Xoom is required for epibolic movement of animal ectodermal cells in Xenopus laevis gastrulation

Gastrulation is the most dynamic cell movement and initiates the body plan in amphibian development. In contrast to numerous molecular studies on mesodermal induction, the driving force of gastrulation is as yet poorly understood. A novel transmembrane protein, Xoom, was previously reported, which is required for Xenopus gastrulation. In the present study, the role of Xoom during Xenopus gastrulation was further examined in detail. Overexpression and misexpression of Xoom induced overproduction of Xoom protein, but not a changed phenotype. However, Xoom antisense ribonucleic acid (RNA) injection reduced the Xoom protein and caused gastrulation defects without any influence on the involution and translation levels of mesodermal marker genes. Normal migrating activity of dorsal mesodermal cells was recognized in the antisense RNA-injected explant. Morphological examination using artificial exogastrulation showed that convergent extension of mesodermal cells occurred normally, but the ectodermal cell layer significantly shrank in the antisense RNA-injected embryo. Comparison of cell shape among various experimental conditions showed that inhibition of cell spreading occurs specifically in the outer ectodermal layer of the antisense RNA-injected embryo. Cytochemical examination indicated disorganization of F-actin in the ectodermal cells of the antisense RNA-injected embryo. These results suggest that Xoom plays an important role in the epibolic movement of ectodermal cells through some regulation of actin filament organization.

Role for the Rana catesbeiana homologue of C/EBP alpha in the reprogramming of gene expression in the liver of metamorphosing tadpoles

During the spontaneous or thyroid hormone (TH)-induced metamorphosis of Rana catesbeiana, developmental changes occur in its liver that are necessary for the transition of this organism from an ammonotelic larva to a ureotelic adult. These changes include the coordinated expression of genes encoding the urea cycle enzymes carbamyl phosphate synthetase (CPS-I) and arnithine transcarbamylase (OTC). Although the expression of these genes is dependent on TH, the mechanisms(s) by which TH initiates this tissue-specific response is thought to be indirect and to involve early TH-induced upregulation of a gene(s), which, in turn, upregulates the coordinated expression of these urea-cycle enzyme genes. Herein, we demonstrate that mRNAs encoding the Rana homologue of the mammalian transcription factor C/EBP alpha (designated RcC/EBP-1) accumulate early in response to TH and that the product of these mRNAs can bind to and transactivate the promoters of both the Rana CPS-1 and OTC genes. These results support the contention that the reprogramming of gene expression in the liver of metamorphosing tadpoles involves a TH-induced cascade of gene activity in which RcC/EBP-1 and, perhaps, other transcription factors coordinate the expression of genes, such as those encoding CPS-I and OTC, whose products are characteristic of the adult liver phenotype.

hsp47 and hsp70 gene expression is differentially regulated in a stress- and tissue-specific manner in zebrafish embryos

We have examined differences in the spatial and temporal regulation of stress-induced hsp47 and hsp70 gene expression following exposure of zebrafish embryos to heat shock or ethanol. Using Northern blot analysis, we found that levels of hsp47 and hsp70 mRNA were dramatically elevated during heat shock in 2-day-old embryos. In contrast, ethanol exposure resulted in strong upregulation of the hsp47 gene whereas hsp70 mRNA levels increased only slightly following the same treatment. Whole-mount in situ hybridization analysis revealed that hsp47 mRNA was expressed predominantly in precartilagenous cells, as well as several other connective tissue cell populations within the embryo following exposure to either stress. hsp70 mRNA displayed a very different cell-specific distribution. For example, neither stress induced hsp70 mRNA accumulation in precartilagenous cells. However, high levels of hsp70 mRNA were detectable in epithelial cells of the developing epidermis following exposure to heat shock, but not to ethanol. These cells did not express the hsp47 gene following exposure to either of these stresses. The results suggest the presence of different inducible regulatory mechanisms for these genes which operate in a cell- and stress-specific manner in zebrafish embryos.

Xenopus msx-1 regulates dorso-ventral axis formation by suppressing the expression of organizer genes

We demonstrated previously that Xmsx-1 is involved in mesoderm patterning along the dorso-ventral axis, under the regulation of BMP-4 signaling. When Xmsx-1 RNA was injected into the dorsal blastomeres, a mass of muscle tissue formed instead of notochord. This activity was similar to that of Xwnt-8 reported previously. In this study, we investigated whether the activity of Xmsx-1 is related to the ventralizing signal and myogenesis promoting factor, Xwnt-8. Whole-mount in situ hybridization showed that Xmsx-1, Xwnt-8, and XmyoD were expressed in overlapping areas, including the ventro-lateral marginal zone at mid-gastrula stage. The expression of XmyoD was induced by the ectopic expression of either Xmsx-1 or Xwnt-8 in dorsal blastomeres, and Xwnt-8 was induced by the ectopic expression of Xmsx-1. On the other hand, the expression of Xmsx-1 was not affected by the loading of pCSKA-Xwnt-8 or dominant-negative Xwnt-8 (DN-Xwnt-8) RNA. In addition, Xmsx-1 RNA did not abrogate the formation of notochord if coinjected with DN-Xwnt-8 RNA. These results suggest that Xmsx-1 functions upstream of the Xwnt-8 signal. Furthermore, the antagonistic function of Xmsx-1 to the expression of organizer genes, such as Xlim-1 and goosecoid, was shown by in situ hybridization analysis and luciferase reporter assay using the goosecoid promoter construct. Finally if Xmsx-1/VP-16 fusion RNA, which was expected to function as a dominant-negative Xmsx-1, was injected into ventral blastomeres, a partial secondary axis formed in a significant number of embryos. In such embryos, the activity of luciferase, under the control of goosecoid promoter sequence, was significantly elevated at gastrula stage. These results led us to conclude that Xmsx-1 plays a central role in establishing dorso-ventral axis in gastrulating embryo, by suppressing the expression of organizer genes.

Phenotypic effects in Xenopus and zebrafish suggest that one-eyed pinhead functions as antagonist of BMP signalling

Zebrafish one-eyed pinhead (oep) is essential for embryonic axis and dorsal midline formation by promoting Nodal signalling and is thought to act as a permissive factor. Here we describe that oep elicits profound phenotypic effects when overexpressed in Xenopus and zebrafish. In Xenopus, wild-type oep inhibits mesoderm induction, disrupts axis formation and neuralizes animal caps. A secreted Oep dorsoanteriorizes and neuralizes Xenopus embryos indicative of BMP inhibition. In zebrafish, misexpression of smad1 in oep mutant embryos also reveals an interaction of oep with BMP signalling. Furthermore, the phenotypic effect of nodal overexpression can be rescued by coexpression of oep both in Xenopus and zebrafish. Taken together, our results support an interaction between oep and nodal but they suggest also (1) that the role of oep in Nodal signalling may include negative as well as positive regulation, (2) that oep is able to function in an active fashion and (3) that oep exerts a regulatory effect on the BMP signalling pathway.

Xwnt11 is a target of Xenopus Brachyury: regulation of gastrulation movements via Dishevelled, but not through the canonical Wnt pathway

Gastrulation in the amphibian embryo is driven by cells of the mesoderm. One of the genes that confers mesodermal identity in Xenopus is Brachyury (Xbra), which is required for normal gastrulation movements and ultimately for posterior mesoderm and notochord differentiation in the development of all vertebrates. Xbra is a transcription activator, and interference with transcription activation leads to an inhibition of morphogenetic movements during gastrulation. To understand this process, we have screened for downstream target genes of Brachyury (Tada, M., Casey, E., Fairclough, L. and Smith, J. C. (1998) Development 125, 3997–4006). This approach has now allowed us to isolate Xwnt11, whose expression pattern is almost identical to that of Xbra at gastrula and early neurula stages. Activation of Xwnt11 is induced in an immediate-early fashion by Xbra and its expression in vivo is abolished by a dominant-interfering form of Xbra, Xbra-En(R). Overexpression of a dominant-negative form of Xwnt11, like overexpression of Xbra-En(R), inhibits convergent extension movements. This inhibition can be rescued by Dsh, a component of the Wnt signalling pathway and also by a truncated form of Dsh which cannot signal through the canonical Wnt pathway involving GSK-3 and (beta)-catenin. Together, our results suggest that the regulation of morphogenetic movements by Xwnt11 occurs through a pathway similar to that involved in planar polarity signalling in Drosophila.

Trypanosoma cruzi infection affects actin mRNA regulation in heart muscle cells

We have previously described alterations in the cytoskeletal organization of heart muscle cells (HMC) infected with Trypanosoma cruzi in vitro. Our aim was to investigate whether these changes also affect the regulation of the actin mRNAs during HMC differentiation. Northern blot analysis revealed that alpha-cardiac actin mRNA levels increased during cell differentiation while beta-actin mRNA levels declined. Nonmuscle cells displayed beta-actin mRNA signal localized at the cell periphery, while alpha-cardiac actin mRNA had a perinuclear distribution in myocytes. Trypanosoma cruzi-infected cells showed 50% reduction in alpha-cardiac actin mRNA expression after 72 h of infection. In contrast, beta-actin mRNA levels increased approximately 79% after 48 h of infection. In addition, in situ beta-actin mRNA was delocalized from the periphery into the perinuclear region. These observations support the hypothesis that Trypanosoma cruzi affects actin mRNA regulation and localization through its effect on the cytoskeleton of heart muscle cells.

Fibroblast growth factor plays a critical role in SM22alpha expression during Xenopus embryogenesis

Although smooth muscle cells (SMCs) are critical components of the circulatory system, the regulatory mechanisms of SMC differentiation remain largely unknown. In the present study, we examined the mechanism of SMC differentiation by using Xenopus laevis SM22alpha (XSM22alpha) as an SMC-specific marker. XSM22alpha cDNA contained a 600-bp open reading frame, and the predicted amino acid sequences were highly conserved in evolution. XSM22alpha transcripts were first detected in heart anlage, head mesenchyme, and the dorsal side of the lateral plate mesoderm at the tail-bud stage, possibly representing the precursors of muscle lineage. At the tadpole stage, XSM22alpha transcripts were restricted to the vascular and visceral SMCs. XSM22alpha was strongly induced by basic fibroblast growth factor (FGF) in animal caps. Although expressions of Xenopus cardiac actin were not affected by the expression of a dominant-negative FGF receptor, its injection dramatically suppressed the XSM22alpha expression. These results suggest that XSM22alpha is a useful molecular marker for the SMC lineage in Xenopus and that FGF signaling plays an important role in the induction of XSM22alpha and in the differentiation of SMCs.

HNF1(beta) is required for mesoderm induction in the Xenopus embryo

XHNF1(β) is a homeobox-containing gene initially expressed at the blastula stage in the vegetal part of the Xenopus embryo. We investigated its early role by functional ablation, through mRNA injection of an XHNF1(beta)/engrailed repressor fusion construct (XHNF1(beta)/EngR). Dorsal injections of XHNF1(beta)/EngR mRNA abolish dorsal mesoderm formation, leading to axial deficiencies; ventral injections disrupt ventral mesoderm formation without affecting axial development. XHNF1(beta)/EngR phenotypic effects specifically depend on the DNA-binding activity of its homeodomain and are fully rescued by coinjection of XHNF1(beta) mRNA. Vegetal injection of XHNF1(beta)/EngR mRNA blocks the mesoderm-inducing ability of vegetal explants. Both B-Vg1 and VegT maternal determinants trigger XHNF1(beta) expression in animal caps. XHNF1(beta)/EngR mRNA blocks B-Vg1-mediated, but not by eFGF-mediated, mesoderm induction in animals caps. However, wild-type XHNF1(beta) mRNA does not trigger Xbra expression in animal caps. We conclude that XHNF1(beta) function is essential, though not sufficient, for mesoderm induction in the Xenopus embryo.

Distinct effects of XBF-1 in regulating the cell cycle inhibitor p27(XIC1) and imparting a neural fate

XBF-1 is an anterior neural plate-specific, winged helix transcription factor that affects neural development in a concentration-dependent manner. A high concentration of XBF-1 results in suppression of endogenous neuronal differentiation and an expansion of undifferentiated neuroectoderm. Here we investigate the mechanism by which this expansion is achieved. Our findings suggest that XBF-1 converts ectoderm to a neural fate and it does so independently of any effects on the mesoderm. In addition, we show that a high dose of XBF-1 promotes the proliferation of neuroectodermal cells while a low dose inhibits ectodermal proliferation. Thus, the neural expansion observed after high dose XBF-1 misexpression is due both to an increase in the number of ectodermal cells devoted to a neural fate and an increase in their proliferation. We show that the effect on cell proliferation is likely to be mediated by p27(XIC1), a cyclin-dependent kinase (cdk) inhibitor. We show that p27(XIC1) is expressed in a spatially restricted pattern in the embryo, including the anterior neural plate, and when misexpressed it is sufficient to block the cell cycle in vivo. We find that p27(XIC1)is transcriptionally regulated by XBF-1 in a dose-dependent manner such that it is suppressed or ectopically induced by a high or low dose of XBF-1, respectively. However, while a low dose of XBF-1 induces ectopic p27(XIC1)and ectopic neurons, misexpression of p27(XIC1)does not induce ectopic neurons, suggesting that the effects of XBF-1 on cell fate and cell proliferation are distinct. Finally, we show that p27(XIC1)is suppressed by XBF-1 in the absence of protein synthesis, suggesting that at least one component of p27(XIC1)regulation by XBF-1 may be direct. Thus, XBF-1 is a neural-specific transcription factor that can independently affect both the cell fate choice and the proliferative status of the cells in which it is expressed.

Xenopus embryonic E2F is required for the formation of ventral and posterior cell fates during early embryogenesis

Using an expression cloning approach, we have unveiled a novel function for the transcription factor E2F. We demonstrate that Xenopus E2F (xE2F) is required for patterning of the Xenopus embryonic axis. Overexpression of xE2F in embryos induces ectopic expression of ventral and posterior markers, including selected members of the Hox genes, and suppresses the development of dorsoanterior structures. Loss of xE2F function in embryos leads to the elimination of ventral and posterior structures. These observations suggest that xE2F acts as an important regulator of region-specific gene expression and in the formation of the embryonic axis. This study provides evidence for an additional embryonic function for E2F, independent of its well-documented role in cell cycle regulation, and suggests a novel mechanism of region-specific gene expression during vertebrate embryogenesis.

Long-term denervation modulates differentially the accumulation of myogenin and MRF4 mRNA in adult Xenopus muscle

In adult Xenopus laevis, we analyzed, using reverse transcriptase-polymerase chain reaction (RT-PCR) and in situ hybridization, the influence of long-term muscle denervation on the accumulation of MRF4 and myogenin transcripts. The brachial muscle was denervated by cutting the brachial nerve and was examined after 4 months. MRF4 mRNA levels decreased about two-fold in denervated muscle as compared with contralateral muscle. Myogenin mRNA levels, by contrast, were induced about five-fold by denervation. This report shows that muscle denervation persistently reduces the levels of MRF4 transcripts suggesting that MRF4 expression may be induced by innervation and hence may be involved in mediating transcriptional responses to innervation. The up-regulation of myogenin by denervation suggests that myogenin expression may compensate for the down-regulation of MRF4 gene.