RNA sequencing reveals a diverse and dynamic repertoire of the Xenopus tropicalis transcriptome over development

An Integrated Database for Exploring Alternative Promoters in Animals

Alternative promoter (AP) events, as a major pre-transcriptional mechanism, can initiate different transcription start sites to generate distinct mRNA isoforms and regulate their expression. At present, hundreds of thousands of APs have been identified across human tissues, and a considerable number of APs have been demonstrated to be associated with complex traits and diseases. Recent researches have also proven important effects of APs on animals. However, the landscape of APs in animals has not been fully recognized. In this study, 102,349 AP profiles from 23,077 samples across 12 species were systematically characterized. We further identified tissue-specific APs and investigated trait-related promoters among various species. In addition, we analyzed the associations between APs and enhancer RNAs (eRNA)/transcription factors (TF) as a means of identifying potential regulatory factors. Integrating these findings, we finally developed Animal-APdb, a database for the searching, browsing, and downloading of information related to Animal APs. Animal-APdb is expected to serve as a valuable resource for exploring the functions and mechanisms of APs in animals.

Quantitative proteomics reveals the dynamic proteome landscape of zebrafish embryos during the maternal-to-zygotic transition

Maternal-to-zygotic transition (MZT) is central to early embryogenesis. However, its underlying molecular mechanisms are still not well described. Here, we revealed the expression dynamics of 5,000 proteins across four stages of zebrafish embryos during MZT, representing one of the most systematic surveys of proteome landscape of the zebrafish embryos during MZT. Nearly 700 proteins were differentially expressed and were divided into six clusters according to their expression patterns. The proteome expression profiles accurately reflect the main events that happen during the MZT, i.e., zygotic genome activation (ZGA), clearance of maternal mRNAs, and initiation of cellular differentiation and organogenesis. MZT is modulated by many proteins at multiple levels in a collaborative fashion, i.e., transcription factors, histones, histone-modifying enzymes, RNA helicases, and P-body proteins. Significant discrepancies were discovered between zebrafish proteome and transcriptome profiles during the MZT. The proteome dynamics database will be a valuable resource for bettering our understanding of MZT.

Deep transcriptome profiling reveals limited conservation of A-to-I RNA editing in Xenopus

BACKGROUND

Xenopus has served as a valuable model system for biomedical research over the past decades. Notably, ADAR was first detected in frog oocytes and embryos as an activity that unwinds RNA duplexes. However, the scope of A-to-I RNA editing by the ADAR enzymes in Xenopus remains underexplored.

RESULTS

Here, we identify millions of editing events in Xenopus with high accuracy and systematically map the editome across developmental stages, adult organs, and species. We report diverse spatiotemporal patterns of editing with deamination activity highest in early embryogenesis before zygotic genome activation and in the ovary. Strikingly, editing events are poorly conserved across different Xenopus species. Even sites that are detected in both X. laevis and X. tropicalis show largely divergent editing levels or developmental profiles. In protein-coding regions, only a small subset of sites that are found mostly in the brain are well conserved between frogs and mammals.

CONCLUSIONS

Collectively, our work provides fresh insights into ADAR activity in vertebrates and suggest that species-specific editing may play a role in each animal's unique physiology or environmental adaptation.

Molecular evolution of Keap1 was essential for adaptation of vertebrates to terrestrial life

Reactive oxygen species (ROS) posed a risk for the transition of vertebrates from aquatic to terrestrial life. How ancestral organisms adapted to such ROS exposure has remained a mystery. Here, we show that attenuation of the activity of the ubiquitin ligase CRL3Keap1 for the transcription factor Nrf2 during evolution was key to development of an efficient response to ROS exposure. The Keap1 gene was duplicated in fish to give rise to Keap1A and the only remaining mammalian paralog Keap1B, the latter of which shows a lower affinity for Cul3 and contributes to robust Nrf2 induction in response to ROS exposure. Mutation of mammalian Keap1 to resemble zebrafish Keap1A resulted in an attenuated Nrf2 response, and most knock-in mice expressing such a Keap1 mutant died on exposure as neonates to sunlight-level ultraviolet radiation. Our results suggest that molecular evolution of Keap1 was essential for adaptation to terrestrial life.

A single-cell transcriptome atlas profiles early organogenesis in human embryos

The early window of human embryogenesis is largely a black box for developmental biologists. Here we probed the cellular diversity of 4-6 week human embryos when essentially all organs are just laid out. On the basis of over 180,000 single-cell transcriptomes, we generated a comprehensive atlas of 313 clusters in 18 developmental systems, which were annotated with a collection of ontology and markers from 157 publications. Together with spatial transcriptome on embryonic sections, we characterized the molecule and spatial architecture of previously unappreciated cell types. Combined with data from other vertebrates, the rich information shed light on spatial patterning of axes, systemic temporal regulation of developmental progression and potential human-specific regulation. Our study provides a compendium of early progenitor cells of human organs, which can serve as the root of lineage analysis in organogenesis.

Dynamic Transcriptional Landscape of Grass Carp (Ctenopharyngodon idella) Reveals Key Transcriptional Features Involved in Fish Development

A high-quality baseline transcriptome is a valuable resource for developmental research as well as a useful reference for other studies. We gathered 41 samples representing 11 tissues/organs from 22 important developmental time points within 197 days of fertilization of grass carp eggs in order to systematically examine the role of lncRNAs and alternative splicing in fish development. We created a high-quality grass carp baseline transcriptome with a completeness of up to 93.98 percent by combining strand-specific RNA sequencing and single-molecule real-time RNA sequencing technologies, and we obtained temporal expression profiles of 33,055 genes and 77,582 transcripts during development and tissue differentiation. A family of short interspersed elements was preferentially expressed at the early stage of zygotic activation in grass carp, and its possible regulatory components were discovered through analysis. Additionally, after thoroughly analyzing alternative splicing events, we discovered that retained intron (RI) alternative splicing events change significantly in both zygotic activation and tissue differentiation. During zygotic activation, we also revealed the precise regulatory characteristics of the underlying functional RI events.

Gene Structure Analysis of Chemokines and Their Receptors in Allotetraploid Frog, Xenopus laevis

Chemokines, relatively small secreted proteins, are involved in cell migration and function in various biological events, including immunity, morphogenesis, and disease. Due to their nature, chemokines tend to be a target of hijacking of immunity by virus and therefore show an exceptionally high mutation rate. Xenopus laevis is considered an excellent model to investigate the effect of whole-genome duplication for gene family evolution. Because its allotetraploidization occurred around 17–18 million years ago, ancestral subgenomes L and S were well conserved. Based on the gene model of human and diploid frog Xenopus tropicalis, we identified 52 chemokine genes and 26 chemokine receptors in X. laevis. The retention rate of the gene in the X. laevis L and S subgenomes was 96% (45/47) and 68% (32/47), respectively. We conducted molecular phylogenetic analysis and found clear orthologies in all receptor genes but not in the ligand genes, suggesting rapid divergences of the ligand. dN/dS calculation demonstrated that dN/dS ratio greater than one was observed in the four ligand genes, cxcl8b.1.S, cxcl18.S, ccl21.S, and xcl1.L, but nothing in receptor genes. These results revealed that the whole-genome duplication promotes diversification of chemokine ligands in X. laevis while conserving the genes necessary for homeostasis, suggesting that selective pressure also supports a rapid divergence of the chemokines in amphibians.

Temporal Transcriptome Analysis Reveals Dynamic Expression Profiles of Gametes and Embryonic Development in Japanese Flounder (Paralichthys olivaceus)

The maternal-to-zygotic transition (MZT) is a crucial event in embryo development. While the features of the MZT across species are shared, the stage of this transition is different among species. We characterized MZT in a flatfish species, Japanese flounder (Paralichthys olivaceus). In this study, we analyzed the 551.57 GB transcriptome data of two types of gametes (sperms and eggs) and 10 embryo developmental stages in Japanese flounder. We identified 2512 maternal factor-related genes and found that most of those maternal factor-related genes expression decreased at the low blastula (LB) stage and remained silent in the subsequent embryonic development period. Meanwhile, we verified that the zygotic genome transcription might occur at the 128-cell stage and large-scale transcription began at the LB stage, which indicates the LB stage is the major wave zygotic genome activation (ZGA) occurs. In addition, we indicated that the Wnt signaling pathway, playing a diverse role in embryonic development, was involved in the ZGA and the axis formation. The results reported the list of the maternal genes in Japanese flounder and defined the stage of MZT, contributing to the understanding of the details of MZT during Japanese flounder embryonic development.

Temporal transcriptomic profiling reveals dynamic changes in gene expression of Xenopus animal cap upon activin treatment

Activin, a member of the transforming growth factor-β (TGF-β) superfamily of proteins, induces various tissues from the amphibian presumptive ectoderm, called animal cap explants (ACs) in vitro. However, it remains unclear how and to what extent the resulting cells recapitulate in vivo development. To comprehensively understand whether the molecular dynamics during activin-induced ACs differentiation reflect the normal development, we performed time-course transcriptome profiling of Xenopus ACs treated with 50 ng/mL of activin A, which predominantly induced dorsal mesoderm. The number of differentially expressed genes (DEGs) in response to activin A increased over time, and totally 9857 upregulated and 6663 downregulated DEGs were detected. 1861 common upregulated DEGs among all Post_activin samples included several Spemann's organizer genes. In addition, the temporal transcriptomes were clearly classified into four distinct groups in correspondence with specific features, reflecting stepwise differentiation into mesoderm derivatives, and a decline in the regulation of nuclear envelop and golgi. From the set of early responsive genes, we also identified the suppressor of cytokine signaling 3 (socs3) as a novel activin A-inducible gene. Our transcriptome data provide a framework to elucidate the transcriptional dynamics of activin-driven AC differentiation, reflecting the molecular characteristics of early normal embryogenesis.

A novel class III endogenous retrovirus with a class I envelope gene in African frogs with an intact genome and developmentally regulated transcripts in Xenopus tropicalis

BACKGROUND

Retroviruses exist as exogenous infectious agents and as endogenous retroviruses (ERVs) integrated into host chromosomes. Such endogenous retroviruses (ERVs) are grouped into three classes roughly corresponding to the seven genera of infectious retroviruses: class I (gamma-, epsilonretroviruses), class II (alpha-, beta-, delta-, lentiretroviruses) and class III (spumaretroviruses). Some ERVs have counterparts among the known infectious retroviruses, while others represent paleovirological relics of extinct or undiscovered retroviruses.

RESULTS

Here we identify an intact ERV in the Anuran amphibian, Xenopus tropicalis. XtERV-S has open reading frames (ORFs) for gag, pol (polymerase) and env (envelope) genes, with a small additional ORF in pol and a serine tRNA primer binding site. It has unusual features and domain relationships to known retroviruses. Analyses based on phylogeny and functional motifs establish that XtERV-S gag and pol genes are related to the ancient env-less class III ERV-L family but the surface subunit of env is unrelated to known retroviruses while its transmembrane subunit is class I-like. LTR constructs show transcriptional activity, and XtERV-S transcripts are detected in embryos after the maternal to zygotic mid-blastula transition and before the late tailbud stage. Tagged Gag protein shows typical subcellular localization. The presence of ORFs in all three protein-coding regions along with identical 5' and 3' LTRs (long terminal repeats) indicate this is a very recent germline acquisition. There are older, full-length, nonorthologous, defective copies in Xenopus laevis and the distantly related African bullfrog, Pyxicephalus adspersus. Additional older, internally deleted copies in X. tropicalis carry a 300 bp LTR substitution.

CONCLUSIONS

XtERV-S represents a genera-spanning member of the largely env-less class III ERV that has ancient and modern copies in Anurans. This provirus has an env ORF with a surface subunit unrelated to known retroviruses and a transmembrane subunit related to class I gammaretroviruses in sequence and organization, and is expressed in early embryogenesis. Additional XtERV-S-related but defective copies are present in X. tropicalis and other African frog taxa. XtERV-S is an unusual class III ERV variant, and it may represent an important transitional retroviral form that has been spreading in African frogs for tens of millions of years.

Comparative Transcriptomics Reveals Distinct Patterns of Gene Expression Conservation through Vertebrate Embryogenesis

Despite life's diversity, studies of variation often remind us of our shared evolutionary past. Abundant genome sequencing and analyses of gene regulatory networks illustrate that genes and entire pathways are conserved, reused, and elaborated in the evolution of diversity. Predating these discoveries, 19th-century embryologists observed that though morphology at birth varies tremendously, certain stages of vertebrate embryogenesis appear remarkably similar across vertebrates. In the mid to late 20th century, anatomical variability of early and late-stage embryos and conservation of mid-stages embryos (the "phylotypic" stage) was named the hourglass model of diversification. This model has found mixed support in recent analyses comparing gene expression across species possibly owing to differences in species, embryonic stages, and gene sets compared. We compare 186 microarray and RNA-seq data sets covering embryogenesis in six vertebrate species. We use an unbiased clustering approach to group stages of embryogenesis by transcriptomic similarity and ask whether gene expression similarity of clustered embryonic stages deviates from a null expectation. We characterize expression conservation patterns of each gene at each evolutionary node after correcting for phylogenetic nonindependence. We find significant enrichment of genes exhibiting early conservation, hourglass, late conservation patterns in both microarray and RNA-seq data sets. Enrichment of genes showing patterned conservation through embryogenesis indicates diversification of embryogenesis may be temporally constrained. However, the circumstances under which each pattern emerges remain unknown and require both broad evolutionary sampling and systematic examination of embryogenesis across species.

MP3RNA-seq: Massively parallel 3' end RNA sequencing for high-throughput gene expression profiling and genotyping

Transcriptome deep sequencing (RNA-seq) has become a routine method for global gene expression profiling. However, its application to large-scale experiments remains limited by cost and labor constraints. Here we describe a massively parallel 3' end RNA-seq (MP3RNA-seq) method that introduces unique sample barcodes during reverse transcription to permit sample pooling immediately following this initial step. MP3RNA-seq allows for handling of hundreds of samples in a single experiment, at a cost of about $6 per sample for library construction and sequencing. MP3RNA-seq is effective for not only high-throughput gene expression profiling, but also genotyping. To demonstrate its utility, we applied MP3RNA-seq to 477 double haploid lines of maize. We identified 19,429 genes expressed in at least 50% of the lines and 35,836 high-quality single nucleotide polymorphisms for genotyping analysis. Armed with these data, we performed expression and agronomic trait quantitative trait locus (QTL) mapping and identified 25,797 expression QTLs for 15,335 genes and 21 QTLs for plant height, ear height, and relative ear height. We conclude that MP3RNA-seq is highly reproducible, accurate, and sensitive for high-throughput gene expression profiling and genotyping, and should be generally applicable to most eukaryotic species.

Control of zygotic genome activation in Xenopus

The fertilized frog egg contains all the materials needed to initiate development of a new organism, including stored RNAs and proteins deposited during oogenesis, thus the earliest stages of development do not require transcription. The onset of transcription from the zygotic genome marks the first genetic switch activating the gene regulatory network that programs embryonic development. Zygotic genome activation occurs after an initial phase of transcriptional quiescence that continues until the midblastula stage, a period called the midblastula transition, which was first identified in Xenopus. Activation of transcription is programmed by maternally supplied factors and is regulated at multiple levels. A similar switch exists in most animals and is of great interest both to developmental biologists and to those interested in understanding nuclear reprogramming. Here we review in detail our knowledge on this major switch in transcription in Xenopus and place recent discoveries in the context of a decades old problem.

Global Analysis of Gene Expression Profiles Provides Novel Insights into the Development and Evolution of the Large Crustacean Eriocheir sinensis

Chinese mitten crab (Eriocheir sinensis) is an important aquaculture species in Crustacea. Functional analysis, although essential, has been hindered due to the lack of sufficient genomic or transcriptomic resources. In this study, transcriptome sequencing was conducted on 59 samples representing diverse developmental stages (fertilized eggs, zoea, megalopa, three sub-stages of larvae, juvenile crabs, and adult crabs) and different tissues (eyestalk, hepatopancreas, and muscle from juvenile crabs, and eyestalk, hepatopancreas, muscle, heart, stomach, gill, thoracic ganglia, intestine, ovary, and testis from adult crabs) of E. sinensis. A comprehensive reference transcriptome was assembled, including 19,023 protein-coding genes. Hierarchical clustering based on 128 differentially expressed cuticle-related genes revealed two distinct expression patterns during the early larval developmental stages, demonstrating the distinct roles of these genes in “crab-like” cuticle formation during metamorphosis and cuticle calcification after molting. Phylogenetic analysis of 1406 one-to-one orthologous gene families identified from seven arthropod species and Caenorhabditis elegans strongly supported the hypothesis that Malacostraca and Branchiopoda do not form a monophyletic group. Furthermore, Branchiopoda is more phylogenetically closely related to Hexapoda, and the clade of Hexapoda and Branchiopoda and the clade of Malacostraca belong to the Pancrustacea. This study offers a high-quality transcriptome resource for E. sinensis and demonstrates the evolutionary relationships of major arthropod groups. The differentially expressed genes identified in this study facilitate further investigation of the cuticle-related gene expression networks which are likely associated with “crab-like” cuticle formation during metamorphosis and cuticle calcification after molting.

Foxh1/Nodal Defines Context-Specific Direct Maternal Wnt/β-Catenin Target Gene Regulation in Early Development

Although Wnt/β-catenin signaling is generally conserved and well understood, the regulatory mechanisms controlling context-specific direct Wnt target gene expression in development and disease are still unclear. The onset of zygotic gene transcription in early embryogenesis represents an ideal, accessible experimental system to investigate context-specific direct Wnt target gene regulation. We combine transcriptomics using RNA-seq with genome-wide β-catenin association using ChIP-seq to identify stage-specific direct Wnt target genes. We propose coherent feedforward regulation involving two distinct classes of direct maternal Wnt target genes, which differ both in expression and persistence of β-catenin association. We discover that genomic β-catenin association overlaps with Foxh1-associated regulatory sequences and demonstrate that direct maternal Wnt target gene expression requires Foxh1 function and Nodal/Tgfβ signaling. Our results support a new paradigm for direct Wnt target gene co-regulation with context-specific mechanisms that will inform future studies of embryonic development and more widely stem cell-mediated homeostasis and human disease.

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Combining RNA-seq and β-catenin ChIP-seq identifies direct Wnt target genes

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Two distinct classes of direct maternal Wnt/β-catenin target genes can be discerned

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We propose coherent feedforward regulation of gene expression of the second class

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Maternal Wnt target gene expression of both classes requires Nodal/Foxh1 signaling

FAM46B is a prokaryotic-like cytoplasmic poly(A) polymerase essential in human embryonic stem cells

Family with sequence similarity (FAM46) proteins are newly identified metazoan-specific poly(A) polymerases (PAPs). Although predicted as Gld-2-like eukaryotic non-canonical PAPs, the detailed architecture of FAM46 proteins is still unclear. Exact biological functions for most of FAM46 proteins also remain largely unknown. Here, we report the first crystal structure of a FAM46 protein, FAM46B. FAM46B is composed of a prominently larger N-terminal catalytic domain as compared to known eukaryotic PAPs, and a C-terminal helical domain. FAM46B resembles prokaryotic PAP/CCA-adding enzymes in overall folding as well as certain inter-domain connections, which distinguishes FAM46B from other eukaryotic non-canonical PAPs. Biochemical analysis reveals that FAM46B is an active PAP, and prefers adenosine-rich substrate RNAs. FAM46B is uniquely and highly expressed in human pre-implantation embryos and pluripotent stem cells, but sharply down-regulated following differentiation. FAM46B is localized to both cell nucleus and cytosol, and is indispensable for the viability of human embryonic stem cells. Knock-out of FAM46B is lethal. Knock-down of FAM46B induces apoptosis and restricts protein synthesis. The identification of the bacterial-like FAM46B, as a pluripotent stem cell-specific PAP involved in the maintenance of translational efficiency, provides important clues for further functional studies of this PAP in the early embryonic development of high eukaryotes.

HCF-2 inhibits cell proliferation and activates differentiation-gene expression programs

HCF-2 is a member of the host-cell-factor protein family, which arose in early vertebrate evolution as a result of gene duplication. Whereas its paralog, HCF-1, is known to act as a versatile chromatin-associated protein required for cell proliferation and differentiation, much less is known about HCF-2. Here, we show that HCF-2 is broadly present in human and mouse cells, and possesses activities distinct from HCF-1. Unlike HCF-1, which is excluded from nucleoli, HCF-2 is nucleolar—an activity conferred by one and a half C-terminal Fibronectin type 3 repeats and inhibited by the HCF-1 nuclear localization signal. Elevated HCF-2 synthesis in HEK-293 cells results in phenotypes reminiscent of HCF-1-depleted cells, including inhibition of cell proliferation and mitotic defects. Furthermore, increased HCF-2 levels in HEK-293 cells lead to inhibition of cell proliferation and metabolism gene-expression programs with parallel activation of differentiation and morphogenesis gene-expression programs. Thus, the HCF ancestor appears to have evolved into a small two-member protein family possessing contrasting nuclear versus nucleolar localization, and cell proliferation and differentiation functions.

Effects of nitrate on development and thyroid hormone signaling pathway during Bufo gargarizans embryogenesis

Nitrate is known to disrupt the thyroid hormone, which is essential for the metamorphosis of amphibians. However, few studies are focused on the effects of nitrate on the maternal thyroid hormone in early amphibian embryos. We aimed to determine the impact of nitrate on maternal thyroid hormone signaling pathway in Bufo gargarizans embryos. B. gargarizans embryos were exposed to different concentrations of nitrate-nitrogen (NO3-N) for 7 days. High concentration of NO3-N (50, 100, and 200 mg/L) could induce embryonic malformation and influence the development of embryos. In addition, maternal T4 and components of the thyroid hormone (TH) signaling pathway were detected by ELISA and RNA-seq, respectively. The expression levels of mRNA related to thyroid hormone and oxidative stress were affected in the early developing embryos in all NO3-N treatment groups. However, the T4 levels and the spatial expression patterns of type II iodothyronine deiodinase (D2), type III iodothyronine deiodinase (D3), thyroid hormone receptor α (TRα), and thyroid hormone receptor β (TRβ) mRNA were not changed by nitrate. In conclusion, the results of our study highlight the crucial role of the maternal thyroid hormone signaling pathway in normal embryonic development, and the adverse effects of nitrate on the expression levels of mRNA related to thyroid hormone signaling pathway and oxidative stress in B. gargarizans embryos.

Origin of new genes after zygotic genome activation in vertebrate

New genes are drivers of evolutionary innovation and phenotypic evolution. Expression of new genes in early development raises the possibility that new genes could originate and be recruited for functions in embryonic development, but this remains undocumented. Here, based on temporal gene expression at different developmental stages in Xenopus tropicalis, we found that young protein-coding genes were significantly enriched for expression in developmental stages occurring after the midblastula transition (MBT), and displayed a decreasing trend in abundance in the subsequent stages after MBT. To complement the finding, we demonstrate essential functional attributes of a young orphan gene, named as Fog2, in morphological development. Our data indicate that new genes could originate after MBT and be recruited for functions in embryonic development, and thus provide insights for better understanding of the origin, evolution, and function of new genes.

A chromosome-scale genome assembly and dense genetic map for Xenopus tropicalis

The Western clawed frog Xenopus tropicalis is a diploid model system for both frog genetics and developmental biology, complementary to the paleotetraploid X. laevis. Here we report a chromosome-scale assembly of the X. tropicalis genome, improving the previously published draft genome assembly through the use of new assembly algorithms, additional sequence data, and the addition of a dense genetic map. The improved genome enables the mapping of specific traits (e.g., the sex locus or Mendelian mutants) and the characterization of chromosome-scale synteny with other tetrapods. We also report an improved annotation of the genome that integrates deep transcriptome sequence from diverse tissues and stages. The exon-intron structures of these genes are highly conserved relative to both X. laevis and human, as are chromosomal linkages ("synteny") and local gene order. A network analysis of developmental gene expression will aid future studies.

Advancing genetic and genomic technologies deepen the pool for discovery in Xenopus tropicalis

Xenopus laevis and Xenopus tropicalis have long been used to drive discovery in developmental, cell, and molecular biology. These dual frog species boast experimental strengths for embryology including large egg sizes that develop externally, well-defined fate maps, and cell-intrinsic sources of nutrients that allow explanted tissues to grow in culture. Development of the Xenopus cell extract system has been used to study cell cycle and DNA replication. Xenopus tadpole tail and limb regeneration have provided fundamental insights into the underlying mechanisms of this processes, and the loss of regenerative competency in adults adds a complexity to the system that can be more directly compared to humans. Moreover, Xenopus genetics and especially disease-causing mutations are highly conserved with humans, making them a tractable system to model human disease. In the last several years, genome editing, expanding genomic resources, and intersectional approaches leveraging the distinct characteristics of each species have generated new frontiers in cell biology. While Xenopus have enduringly represented a leading embryological model, new technologies are generating exciting diversity in the range of discoveries being made in areas from genomics and proteomics to regenerative biology, neurobiology, cell scaling, and human disease modeling.

The Spatiotemporal Control of Zygotic Genome Activation

One of the earliest and most significant events in embryonic development is zygotic genome activation (ZGA). In several species, bulk transcription begins at the midblastula transition (MBT) when, after a certain number of cleavages, the embryo attains a particular nuclear-to-cytoplasmic (N/C) ratio, maternal repressors become sufficiently diluted, and the cell cycle slows down. Here we resolve the frog ZGA in time and space by profiling RNA polymerase II (RNAPII) engagement and its transcriptional readout. We detect a gradual increase in both the quantity and the length of RNAPII elongation before the MBT, revealing that >1,000 zygotic genes disregard the N/C timer for their activation and that the sizes of newly transcribed genes are not necessarily constrained by cell cycle duration. We also find that Wnt, Nodal, and BMP signaling together generate most of the spatiotemporal dynamics of regional ZGA, directing the formation of orthogonal body axes and proportionate germ layers.

The Xenopus animal cap transcriptome: building a mucociliary epithelium

With the availability of deep RNA sequencing, model organisms such as Xenopus offer an outstanding opportunity to investigate the genetic basis of vertebrate organ formation from its embryonic beginnings. Here we investigate dynamics of the RNA landscape during formation of the Xenopus tropicalis larval epidermis. Differentiation of non-neural ectoderm starts at gastrulation and takes about one day to produce a functional mucociliary epithelium, highly related to the one in human airways. To obtain RNA expression data, uncontaminated by non-epidermal tissues of the embryo, we use prospective ectodermal explants called Animal Caps (ACs), which differentiate autonomously into a ciliated epidermis. Their global transcriptome is investigated at three key timepoints, with a cumulative sequencing depth of ∼10⁸ reads per developmental stage. This database is provided as online Web Tool to the scientific community. In this paper, we report on global changes in gene expression, an unanticipated diversity of mRNA splicing isoforms, expression patterns of repetitive DNA Elements, and the complexity of circular RNAs during this process. Computationally we derive transcription factor hubs from this data set, which may help in the future to define novel genetic drivers of epidermal differentiation in vertebrates.

The maternal-to-zygotic transition revisited

The development of animal embryos is initially directed by maternal gene products. Then, during the maternal-to-zygotic transition (MZT), developmental control is handed to the zygotic genome. Extensive research in both vertebrate and invertebrate model organisms has revealed that the MZT can be subdivided into two phases, during which very different modes of gene regulation are implemented: initially, regulation is exclusively post-transcriptional and post-translational, following which gradual activation of the zygotic genome leads to predominance of transcriptional regulation. These changes in the gene expression program of embryos are precisely controlled and highly interconnected. Here, we review current understanding of the mechanisms that underlie handover of developmental control during the MZT.

MALDI-imaging of early stage Xenopus laevis embryos

Xenopus laevis is an important model organism for vertebrate development. An extensive literature has developed on changes in transcript expression during development of this organism, and there is a growing literature on the corresponding protein expression changes during development. In contrast, there is very little information on changes in metabolite expression during development. We present the first MALDI mass-spectrometry images of metabolites within the developing embryo. These images were generated for 142 metabolite ions. The images were subjected to an algorithm that revealed three spatially-resolved clusters of metabolites. One small cluster is localized near the outer membrane of the embryo. A large cluster of metabolites is found in cavities destined to form the neural tube and gut, and contains a number of ceramide species, which are associated with cellular signaling, including differentiation, proliferation, and programmed cell death. Another large cluster of metabolites is found in tissue and is dominated by phosphatidylcholines, which are common components of cell membranes. Surprisingly, no metabolites appear to be homogeneously distributed across the slices; metabolites are localized either within tissue or in cavities, but not both.

RNA Sequencing Analysis to Capture the Transcriptome Landscape during Tenderization in Sea Cucumber Apostichopus japonicus

Sea cucumber (Apostichopus japonicus) is an economically significant species in China having great commercial value. It is challenging to maintain the textural properties during thermal processing due to the distinctive physiochemical structure of the A. japonicus body wall (AJBW). In this study, the gene expression profiles associated with tenderization in AJBW were determined at 0 h (CON), 1 h (T_1h), and 3 h (T_3h) after treatment at 37 °C using Illumina HiSeq™ 4000 platform. Seven-hundred-and-twenty-one and 806 differentially expressed genes (DEGs) were identified in comparisons of T_1h vs. CON and T_3h vs. CON, respectively. Among these DEGs, we found that two endogenous proteases-72 kDa type IV collagenase and matrix metalloproteinase 16 precursor-were significantly upregulated that could directly affect the tenderness of AJBW. In addition, 92 genes controlled four types of physiological and biochemical processes such as oxidative stress response (3), immune system process (55), apoptosis (4), and reorganization of the cytoskeleton and extracellular matrix (30). Further, the RT-qPCR results confirmed the accuracy of RNA-sequencing analysis. Our results showed the dynamic changes in global gene expression during tenderization and provided a series of candidate genes that contributed to tenderization in AJBW. This can help further studies on the genetics/molecular mechanisms associated with tenderization.

The Xenopus tropicalis Model for Studies of Developmental and Reproductive Toxicity

The reproductive cycle encompasses processes such as sex organ differentiation and development in the early life stages and maturation of the gametes in the adult organism. During the early life stages, critical developmental programming of the endocrine and reproductive systems occurs, and exposure to chemicals during these critical developmental windows can result in impaired reproductive function later in life. It is therefore important to evaluate long-term consequences of early life stage exposure to endocrine-disrupting chemicals. The African clawed frog Xenopus tropicalis has several characteristics that facilitate studies of developmental and reproductive toxicity. Here I present a X. tropicalis life cycle test protocol including study design, exposure regimes, and endpoints for chemical disruption of sex differentiation, gonadal and Müllerian duct development, the thyroxin-regulated metamorphosis, estrogen synthesis (activity of the CYP19 aromatase enzyme), spermatogenesis, oogenesis, puberty and fertility.

Zygotic gene activation in the chicken occurs in two waves, the first involving only maternally derived genes

The first wave of transcriptional activation occurs after fertilisation in a species-specific pattern. Despite its importance to initial embryonic development, the characteristics of transcription following fertilisation are poorly understood in Aves. Here, we report detailed insights into the onset of genome activation in chickens. We established that two waves of transcriptional activation occurred, one shortly after fertilisation and another at Eyal-Giladi and Kochav Stage V. We found 1544 single nucleotide polymorphisms across 424 transcripts derived from parents that were expressed in offspring during the early embryonic stages. Surprisingly, only the maternal genome was activated in the zygote, and the paternal genome remained silent until the second-wave, regardless of the presence of a paternal pronucleus or supernumerary sperm in the egg. The identified maternal genes involved in cleavage that were replaced by bi-allelic expression. The results demonstrate that only maternal alleles are activated in the chicken zygote upon fertilisation, which could be essential for early embryogenesis and evolutionary outcomes in birds.

The early stages of animal development involve a handover of genetic control. Initially, the egg cell is maintained by genetic information inherited from the mother, but soon after fertilization it starts to depend on its own genes instead. Activating genes inside the fertilized egg cell (zygote) so that they can take control of development is known as zygotic genome activation.

Despite the fact that birds are often used to study how embryos develop, zygotic genome activation in birds is not well understood. Fertilization in birds, including chickens, is different to mammals in that it requires multiple sperm to fertilize an egg cell. As such, zygotic genome activation in birds is likely to differ from that in mammals.

By examining gene expression in embryos from mixed-breed chickens, Hwang, Seo et al. showed that there are two stages of zygotic genome activation in chickens. The genes derived from the mother become active in the first stage, while genes from the father become active in the second stage. Genome activation in birds is therefore very different to the same process in mammals, which involves genome activation of both parents from the first stage. This extra level of control may help to prevent genetic complications resulting from the presence of multiple sperm, each of which carries a different set of genes from the father.

Genome-wide identification of tissue-specific long non-coding RNA in three farm animal species

Background

Numerous long non-coding RNAs (lncRNAs) have been identified and their roles in gene regulation in humans, mice, and other model organisms studied; however, far less research has been focused on lncRNAs in farm animal species. While previous studies in chickens, cattle, and pigs identified lncRNAs in specific developmental stages or differentially expressed under specific conditions in a limited number of tissues, more comprehensive identification of lncRNAs in these species is needed. The goal of the FAANG Consortium (Functional Annotation of Animal Genomes) is to functionally annotate animal genomes, including the annotation of lncRNAs. As one of the FAANG pilot projects, lncRNAs were identified across eight tissues in two adult male biological replicates from chickens, cattle, and pigs.

Results

Comprehensive lncRNA annotations for the chicken, cattle, and pig genomes were generated by utilizing RNA-seq from eight tissue types from two biological replicates per species at the adult developmental stage. A total of 9393 lncRNAs in chickens, 7235 lncRNAs in cattle, and 14,429 lncRNAs in pigs were identified. Including novel isoforms and lncRNAs from novel loci, 5288 novel lncRNAs were identified in chickens, 3732 in cattle, and 4870 in pigs. These transcripts match previously known patterns of lncRNAs, such as generally lower expression levels than mRNAs and higher tissue specificity. An analysis of lncRNA conservation across species identified a set of conserved lncRNAs with potential functions associated with chromatin structure and gene regulation. Tissue-specific lncRNAs were identified. Genes proximal to tissue-specific lncRNAs were enriched for GO terms associated with the tissue of origin, such as leukocyte activation in spleen.

Conclusions

LncRNAs were identified in three important farm animal species using eight tissues from adult individuals. About half of the identified lncRNAs were not previously reported in the NCBI annotations for these species. While lncRNAs are less conserved than protein-coding genes, a set of positionally conserved lncRNAs were identified among chickens, cattle, and pigs with potential functions related to chromatin structure and gene regulation. Tissue-specific lncRNAs have potential regulatory functions on genes enriched for tissue-specific GO terms. Future work will include epigenetic data from ChIP-seq experiments to further refine these annotations.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5037-7) contains supplementary material, which is available to authorized users.

PdumBase: a transcriptome database and research tool for Platynereis dumerilii and early development of other metazoans

BACKGROUND

The marine polychaete annelid Platynereis dumerilii has recently emerged as a prominent organism for the study of development, evolution, stem cells, regeneration, marine ecology, chronobiology and neurobiology within metazoans. Its phylogenetic position within the spiralian/ lophotrochozoan clade, the comparatively high conservation of ancestral features in the Platynereis genome, and experimental access to any stage within its life cycle, make Platynereis an important model for elucidating the complex regulatory and functional molecular mechanisms governing early development, later organogenesis, and various features of its larval and adult life. High resolution RNA-seq gene expression data obtained from specific developmental stages can be used to dissect early developmental mechanisms. However, the potential for discovery of these mechanisms relies on tools to search, retrieve, and compare genome-wide information within Platynereis, and across other metazoan taxa.

RESULTS

To facilitate exploration and discovery by the broader scientific community, we have developed a web-based, searchable online research tool, PdumBase, featuring the first comprehensive transcriptome database for Platynereis dumerilii during early stages of development (2 h ~ 14 h). Our database also includes additional stages over the P. dumerilii life cycle and provides access to the expression data of 17,213 genes (31,806 transcripts) along with annotation information sourced from Swiss-Prot, Gene Ontology, KEGG pathways, Pfam domains, TmHMM, SingleP, and EggNOG orthology. Expression data for each gene includes the stage, the normalized FPKM, the raw read counts, and information that can be leveraged for statistical analyses of differential gene expression and the construction of genome-wide co-expression networks. In addition, PdumBase offers early stage transcriptome expression data from five further species as a valuable resource for investigators interested in comparing early development in different organisms. To understand conservation of Platynereis gene models and to validate gene annotation, most Platynereis gene models include a comprehensive phylogenetic analysis across 18 species representing diverse metazoan taxa.

CONCLUSIONS

PdumBase represents the first online resource for the early developmental transcriptome of Platynereis dumerilii. It serves as a research platform for discovery and exploration of gene expression during early stages, throughout the Platynereis life cycle, and enables comparison to other model organisms. PdumBase is freely available at http://pdumbase.gdcb.iastate.edu .

Long non-coding regulatory RNAs in sponges and insights into the origin of animal multicellularity

How animals evolved from a single-celled ancestor over 700 million years ago is poorly understood. Recent transcriptomic and chromatin analyses in the sponge Amphimedon queenslandica, a morphologically-simple representative of one of the oldest animal phyletic lineages, have shed light on what innovations in the genome and its regulation underlie the emergence of animal multicellularity. Comparisons of the regulatory genome of this sponge with those of more complex bilaterian model species and even simpler unicellular relatives have revealed that fundamental changes in genome regulatory complexity accompanied the evolution of animal multicellularity. Here, we review and discuss the results of these recent investigations by specifically focusing on the contribution of long non-coding RNAs to the evolution of the animal regulatory genome.

A Bipartite Network and Resource Transfer-Based Approach to Infer lncRNA-Environmental Factor Associations

Phenotypes and diseases are often determined by the complex interactions between genetic factors and environmental factors (EFs). However, compared with protein-coding genes and microRNAs, there is a paucity of computational methods for understanding the associations between long non-coding RNAs (lncRNAs) and EFs. In this study, we focused on the associations between lncRNA and EFs. By using the common miRNA partners of any pair of lncRNA and EF, based on the competing endogenous RNA (ceRNA) hypothesis and the technique of resources transfer within the experimentally-supported lncRNA-miRNA and miRNA-EF association bipartite networks, we propose an algorithm for predicting new lncRNA-EF associations. Results show that, compared with another recently-proposed method, our approach is capable of predicting more credible lncRNA-EF associations. These results support the validity of our approach to predict biologically significant associations, which could lead to a better understanding of the molecular processes.

The transcriptome of early chicken embryos reveals signaling pathways governing rapid asymmetric cellularization and lineage segregation

The phylogenomics and comparative functional genomics of avian species were investigated in the Bird 10,000 Genomes (B10K) project because of the important evolutionary position of birds and their value as a research model. However, the systematic profiling of transcriptional changes prior to oviposition has not been investigated in avian species because of the practical difficulties in obtaining pre-oviposited eggs. In this study, a total of 137 pre-oviposited embryos were collected from hen ovaries and oviducts and subjected to RNA-sequencing analyses. Two waves of chicken zygotic genome activation (ZGA) were observed. Functionally distinct developmental programs involving Notch, MAPK, Wnt and TGFβ signaling were separately detected during cleavage and area pellucida formation. Furthermore, the early stages of chicken development were compared with the human and mouse counterparts, highlighting chicken-specific signaling pathways and gradually analogous gene expression via ZGA. These findings provide a genome-wide understanding of avian embryogenesis and comparisons among amniotes.

Sponge Long Non-Coding RNAs Are Expressed in Specific Cell Types and Conserved Networks

Although developmental regulation by long non-coding RNAs (lncRNAs) appears to be a widespread feature amongst animals, the origin and level of evolutionary conservation of this mode of regulation remain unclear. We have previously demonstrated that the sponge Amphimedon queenslandica—a morphologically-simple animal—developmentally expresses an array of lncRNAs in manner akin to more complex bilaterians (insects + vertebrates). Here, we first show that Amphimedon lncRNAs are expressed in specific cell types in larvae, juveniles and adults. Thus, as in bilaterians, sponge developmental regulation involves the dynamic, cell type- and context-specific regulation of specific lncRNAs. Second, by comparing gene co-expression networks between Amphimedon queenslandica and Sycon ciliatum—a distantly-related calcisponge—we identify several putative co-expression modules that appear to be shared in sponges; these network-embedded sponge lncRNAs have no discernable sequence similarity. Together, these results suggest sponge lncRNAs are developmentally regulated and operate in conserved gene regulatory networks, as appears to be the case in more complex bilaterians.

A high-resolution mRNA expression time course of embryonic development in zebrafish

We have produced an mRNA expression time course of zebrafish development across 18 time points from 1 cell to 5 days post-fertilisation sampling individual and pools of embryos. Using poly(A) pulldown stranded RNA-seq and a 3′ end transcript counting method we characterise temporal expression profiles of 23,642 genes. We identify temporal and functional transcript co-variance that associates 5024 unnamed genes with distinct developmental time points. Specifically, a class of over 100 previously uncharacterised zinc finger domain containing genes, located on the long arm of chromosome 4, is expressed in a sharp peak during zygotic genome activation. In addition, the data reveal new genes and transcripts, differential use of exons and previously unidentified 3′ ends across development, new primary microRNAs and temporal divergence of gene paralogues generated in the teleost genome duplication. To make this dataset a useful baseline reference, the data can be browsed and downloaded at Expression Atlas and Ensembl.

The North American bullfrog draft genome provides insight into hormonal regulation of long noncoding RNA

Frogs play important ecological roles, and several species are important model organisms for scientific research. The globally distributed Ranidae (true frogs) are the largest frog family, and have substantial evolutionary distance from the model laboratory Xenopus frog species. Unfortunately, there are currently no genomic resources for the former, important group of amphibians. More widely applicable amphibian genomic data is urgently needed as more than two-thirds of known species are currently threatened or are undergoing population declines. We report a 5.8 Gbp (NG50 = 69 kbp) genome assembly of a representative North American bullfrog (Rana [Lithobates] catesbeiana). The genome contains over 22,000 predicted protein-coding genes and 6,223 candidate long noncoding RNAs (lncRNAs). RNA-Seq experiments show thyroid hormone causes widespread transcriptional change among protein-coding and putative lncRNA genes. This initial bullfrog draft genome will serve as a key resource with broad utility including amphibian research, developmental biology, and environmental research.

Dynamic landscape and regulation of RNA editing in mammals

Adenosine-to-inosine (A-to-I) RNA editing is a conserved post-transcriptional mechanism mediated by ADAR enzymes that diversifies the transcriptome by altering selected nucleotides in RNA molecules. Although many editing sites have recently been discovered, the extent to which most sites are edited and how the editing is regulated in different biological contexts are not fully understood. Here we report dynamic spatiotemporal patterns and new regulators of RNA editing, discovered through an extensive profiling of A-to-I RNA editing in 8,551 human samples (representing 53 body sites from 552 individuals) from the Genotype-Tissue Expression (GTEx) project and in hundreds of other primate and mouse samples. We show that editing levels in non-repetitive coding regions vary more between tissues than editing levels in repetitive regions. Globally, ADAR1 is the primary editor of repetitive sites and ADAR2 is the primary editor of non-repetitive coding sites, whereas the catalytically inactive ADAR3 predominantly acts as an inhibitor of editing. Cross-species analysis of RNA editing in several tissues revealed that species, rather than tissue type, is the primary determinant of editing levels, suggesting stronger cis-directed regulation of RNA editing for most sites, although the small set of conserved coding sites is under stronger trans-regulation. In addition, we curated an extensive set of ADAR1 and ADAR2 targets and showed that many editing sites display distinct tissue-specific regulation by the ADAR enzymes in vivo. Further analysis of the GTEx data revealed several potential regulators of editing, such as AIMP2, which reduces editing in muscles by enhancing the degradation of the ADAR proteins. Collectively, our work provides insights into the complex cis- and trans-regulation of A-to-I editing.

Zygotic Genome Activation in Vertebrates

The first major developmental transition in vertebrate embryos is the maternal-to-zygotic transition (MZT) when maternal mRNAs are degraded and zygotic transcription begins. During the MZT, the embryo takes charge of gene expression to control cell differentiation and further development. This spectacular organismal transition requires nuclear reprogramming and the initiation of RNAPII at thousands of promoters. Zygotic genome activation (ZGA) is mechanistically coordinated with other embryonic events, including changes in the cell cycle, chromatin state, and nuclear-to-cytoplasmic component ratios. Here, we review progress in understanding vertebrate ZGA dynamics in frogs, fish, mice, and humans to explore differences and emphasize common features.

High resolution temporal transcriptomics of mouse embryoid body development reveals complex expression dynamics of coding and noncoding loci

Cellular responses to stimuli are rapid and continuous and yet the vast majority of investigations of transcriptional responses during developmental transitions typically use long interval time courses; limiting the available interpretive power. Moreover, such experiments typically focus on protein-coding transcripts, ignoring the important impact of long noncoding RNAs. We therefore evaluated coding and noncoding expression dynamics at unprecedented temporal resolution (6-hourly) in differentiating mouse embryonic stem cells and report new insight into molecular processes and genome organization. We present a highly resolved differentiation cascade that exhibits coding and noncoding transcriptional alterations, transcription factor network interactions and alternative splicing events, little of which can be resolved by long-interval developmental time-courses. We describe novel short lived and cycling patterns of gene expression and dissect temporally ordered gene expression changes in response to transcription factors. We elucidate patterns in gene co-expression across the genome, describe asynchronous transcription at bidirectional promoters and functionally annotate known and novel regulatory lncRNAs. These findings highlight the complex and dynamic molecular events underlying mammalian differentiation that can only be observed though a temporally resolved time course.

Brg1 chromatin remodeling ATPase balances germ layer patterning by amplifying the transcriptional burst at midblastula transition

Zygotic gene expression programs control cell differentiation in vertebrate development. In Xenopus, these programs are initiated by local induction of regulatory genes through maternal signaling activities in the wake of zygotic genome activation (ZGA) at the midblastula transition (MBT). These programs lay down the vertebrate body plan through gastrulation and neurulation, and are accompanied by massive changes in chromatin structure, which increasingly constrain cellular plasticity. Here we report on developmental functions for Brahma related gene 1 (Brg1), a key component of embyronic SWI/SNF chromatin remodeling complexes. Carefully controlled, global Brg1 protein depletion in X. tropicalis and X. laevis causes embryonic lethality or developmental arrest from gastrulation on. Transcriptome analysis at late blastula, before development becomes arrested, indicates predominantly a role for Brg1 in transcriptional activation of a limited set of genes involved in pattern specification processes and nervous system development. Mosaic analysis by targeted microinjection defines Brg1 as an essential amplifier of gene expression in dorsal (BCNE/Nieuwkoop Center) and ventral (BMP/Vent) signaling centers. Moreover, Brg1 is required and sufficient for initiating axial patterning in cooperation with maternal Wnt signaling. In search for a common denominator of Brg1 impact on development, we have quantitatively filtered global mRNA fluctuations at MBT. The results indicate that Brg1 is predominantly required for genes with the highest burst of transcriptional activity. Since this group contains many key developmental regulators, we propose Brg1 to be responsible for raising their expression above threshold levels in preparation for embryonic patterning.

Brahma-related-gene-1 (Brg1) is a catalytic subunit of mammalian SWI/SNF chromatin remodeling complexes. Loss of maternal Brg1 protein arrests development in mice at the 2-cell stage, while null homozygotes die at the blastocyst stage. These early requirements have precluded any analysis of Brg1’s embryonic functions. Here we present data from X. laevis and X. tropicalis, which for the first time describe a role for Brg1 during germ layer patterning and axis formation. Brg1-depleted embryos fail to develop past gastrulation. Genome-wide transcriptome analysis at late blastula stage, before the developmental arrest, shows that Brg1 is required predominantly for transcriptional activation of a limited set of genes involved in pattern specification processes and nervous system development shortly after midblastula transition. Mosaic analysis by targeted microinjection defines Brg1 as an essential amplifier of gene expression in dorsal (BCNE and Nieuwkoop center) and ventral (BMP/Vent) signaling centers, being required and sufficient to initiate axial patterning by cooperating with canonical Wnt signaling. Since Brg1-dependent genes share a high burst of transcriptional activation before gastrulation, we propose a systemic role for Brg1 as transcriptional amplifier, which balances the embryonic patterning process.

Competition between histone and transcription factor binding regulates the onset of transcription in zebrafish embryos

Upon fertilization, the genome of animal embryos remains transcriptionally inactive until the maternal-to-zygotic transition. At this time, the embryo takes control of its development and transcription begins. How the onset of zygotic transcription is regulated remains unclear. Here, we show that a dynamic competition for DNA binding between nucleosome-forming histones and transcription factors regulates zebrafish genome activation. Taking a quantitative approach, we found that the concentration of non-DNA-bound core histones sets the time for the onset of transcription. The reduction in nuclear histone concentration that coincides with genome activation does not affect nucleosome density on DNA, but allows transcription factors to compete successfully for DNA binding. In agreement with this, transcription factor binding is sensitive to histone levels and the concentration of transcription factors also affects the time of transcription. Our results demonstrate that the relative levels of histones and transcription factors regulate the onset of transcription in the embryo.

DOI:http://dx.doi.org/10.7554/eLife.23326.001

The DNA in a fertilized egg contains all the information required to form an animal’s body. In order for the animal to develop properly, particular genes encoded in the DNA are only active at specific times. The DNA is wrapped around proteins called histones, which allows the DNA to be tightly packed inside the cell. However, histones can block other proteins called transcription factors from binding to the DNA to activate the genes. Young embryos initially develop with all of their genes switched off, relying on the nutrients and other molecules provided by their mother. After some time, the embryo starts to switch on its own genes to take control of its own development, but it was not clear how this happens.

Joseph et al. investigated how genes are activated in zebrafish embryos, which are often used as models to study how animals develop. The experiments show that competition between histones and transcription factors for binding to DNA controls when genes are switched on. In young fish embryos, there are so many histones present that transcription factors have no opportunity to bind to DNA. Over time, however, the numbers of histones decrease, allowing transcription factors to bind to DNA and switch on genes.

Histones and transcription factors regulate the activity of genes throughout the life of the animal. Therefore, competition between these two types of protein may also control gene activity in other situations. A better understanding of how gene activity is controlled could allow researchers to more easily grow different types of cell in the laboratory or to reprogram specific cells in the body. As such, these new findings may aid the development of therapies to regenerate organs or tissues that have been damaged by injury or disease.

DOI:http://dx.doi.org/10.7554/eLife.23326.002

A gene regulatory program controlling early Xenopus mesendoderm formation: Network conservation and motifs

Germ layer formation is among the earliest differentiation events in metazoan embryos. In triploblasts, three germ layers are formed, among which the endoderm gives rise to the epithelial lining of the gut tube and associated organs including the liver, pancreas and lungs. In frogs (Xenopus), where early germ layer formation has been studied extensively, the process of endoderm specification involves the interplay of dozens of transcription factors. Here, we review the interactions between these factors, summarized in a transcriptional gene regulatory network (GRN). We highlight regulatory connections conserved between frog, fish, mouse, and human endodermal lineages. Especially prominent is the conserved role and regulatory targets of the Nodal signaling pathway and the T-box transcription factors, Vegt and Eomes. Additionally, we highlight network topologies and motifs, and speculate on their possible roles in development.

Characterization of the Transcriptome and Gene Expression of Brain Tissue in Sevenband Grouper (Hyporthodus septemfasciatus) in Response to NNV Infection

Grouper is one of the favorite sea food resources in Southeast Asia. However, the outbreaks of the viral nervous necrosis (VNN) disease due to nervous necrosis virus (NNV) infection have caused mass mortality of grouper larvae. Many aqua-farms have suffered substantial financial loss due to the occurrence of VNN. To better understand the infection mechanism of NNV, we performed the transcriptome analysis of sevenband grouper brain tissue, the main target of NNV infection. After artificial NNV challenge, transcriptome of brain tissues of sevenband grouper was subjected to next generation sequencing (NGS) using an Illumina Hi-seq 2500 system. Both mRNAs from pooled samples of mock and NNV-infected sevenband grouper brains were sequenced. Clean reads of mock and NNV-infected samples were de novo assembled and obtained 104,348 unigenes. In addition, 628 differentially expressed genes (DEGs) in response to NNV infection were identified. This result could provide critical information not only for the identification of genes involved in NNV infection, but for the understanding of the response of sevenband groupers to NNV infection.

Deficiency of microRNA miR-34a expands cell fate potential in pluripotent stem cells

Embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) efficiently generate all embryonic cell lineages but rarely generate extraembryonic cell types. We found that microRNA miR-34a deficiency expands the developmental potential of mouse pluripotent stem cells, yielding both embryonic and extraembryonic lineages and strongly inducing MuERV-L (MERVL) endogenous retroviruses, similar to what is seen with features of totipotent two-cell blastomeres. miR-34a restricts the acquisition of expanded cell fate potential in pluripotent stem cells, and it represses MERVL expression through transcriptional regulation, at least in part by targeting the transcription factor Gata2. Our studies reveal a complex molecular network that defines and restricts pluripotent developmental potential in cultured ESCs and iPSCs.