Transcriptome dynamics and diversity in the early zebrafish embryo

Automated profiling of gene function during embryonic development

Systematic functional profiling of the gene set that directs embryonic development is an important challenge. To tackle this challenge, we used 4D imaging of C. elegans embryogenesis to capture the effects of 500 gene knockdowns and developed an automated approach to compare developmental phenotypes. The automated approach quantifies features-including germ layer cell numbers, tissue position, and tissue shape-to generate temporal curves whose parameterization yields numerical phenotypic signatures. In conjunction with a new similarity metric that operates across phenotypic space, these signatures enabled the generation of ranked lists of genes predicted to have similar functions, accessible in the PhenoBank web portal, for ∼25% of essential development genes. The approach identified new gene and pathway relationships in cell fate specification and morphogenesis and highlighted the utilization of specialized energy generation pathways during embryogenesis. Collectively, the effort establishes the foundation for comprehensive analysis of the gene set that builds a multicellular organism.

Transcriptomic profiling reveals the immune response mechanism of the Thamnaconus modestus induced by the poly (I:C) and LPS

Aquatic animals immune response to pathogenic is a hotspot and related to high-quality development of aquaculture industry and the conservation of fisheries resources. Thamnaconus modestus is an important commercial and economical species which is suffering from various pathogens but by now lack relevant research about revealing the immune response mechanism to the pathogens invasion. In the study, the polyriboinosinic polyribocytidylic acid [poly (I:C)] and Lipopolysaccharides (LPS), respective mimics of viral and bacterial infections, were used to demonstrate the immune response of the species via transcriptome analysis. The results showed that T. modestus had sensitive responses to the viral analog infection at 6 h and 48 h, and at 6 h, the first five major functional genes were NFKBIA, IL1B, JUN, IGH, FOS, and at 48 h, the genes were NFKBIA, IL1B, JUN, IGH, FOS. The genes IL1B, IRF3, PTGS2, THBS1 could helping the fish to fight against the bacterial infection in both the times. Similarly for the bacterial infection, the species had a sensitive response at 6 h, and the first five major functional genes were NFKBIA, JUN, FOS, L1B, GRIN2C. Our study provided an insight about the immune response mechanism of this species and demonstrated that if need for treatment of the virus and bacteria by the biotechnology, the artificial interferential time would be suggested before 6 h since the pathological features occur and the genes NFKBIA, JUN, IL1B, FOS, TRAF2, IL8, SOCS3, PTGS2 should be payed more attention.

Transcriptome analysis of digestive diverticula of Hong Kong oyster (Crassostrea hongkongesis) infected with Vibrio harveyi

The Hong Kong oyster (Crassostrea hongkongesis), as the main marine aquaculture shellfish in the South China Sea, not only has high economic and ecological value, but also is an ideal model for conducting research on pathogen host interaction. However, diseases caused by Vibrio pose a serious impediment to the culture of C. hongkongesis. In this study, we performed transcriptome analysis of digestive diverticula of C. hongkongesis infected with V. harveyi. A total of 977, 689, 912 high quality reads and 955, 208, 562 valid reads were obtained. At 12, 24, 48 and 72 h post-infection, 1402, 2168, 2727 and 1398 differentially expressed genes (DEGs) were captured, respectively. GO enrichment analysis showed that DEGs were significantly enriched in cellular processes, catalytic activity, cell part and other terms. KEGG enrichment analysis revealed that these DEGs were mainly closely related to Necroptosis, RIG-I-like receptor signaling pathway, NF-kappa B signaling pathway, Toll-like receptor signaling pathway and other pathways are related. The results of WGCNA analysis indicated that THBS1, CA10, Trpm2, THAP12, PTPRT, HSPA12A, and ADAM10 were the hub genes in the gene co-expression network. This study will provide new ideas at the transcriptome level for the immune regulatory mechanisms and adaptability of the C. hongkongesis to V. infection, as well as for achieving selective breeding for Vibrio resistance in the C. hongkongesis.

Isoform-resolved transcriptome of the human preimplantation embryo

Human preimplantation development involves extensive remodeling of RNA expression and splicing. However, its transcriptome has been compiled using short-read sequencing data, which fails to capture most full-length mRNAs. Here, we generate an isoform-resolved transcriptome of early human development by performing long- and short-read RNA sequencing on 73 embryos spanning the zygote to blastocyst stages. We identify 110,212 unannotated isoforms transcribed from known genes, including highly conserved protein-coding loci and key developmental regulators. We further identify 17,964 isoforms from 5,239 unannotated genes, which are largely non-coding, primate-specific, and highly associated with transposable elements. These isoforms are widely supported by the integration of published multi-omics datasets, including single-cell 8CLC and blastoid studies. Alternative splicing and gene co-expression network analyses further reveal that embryonic genome activation is associated with splicing disruption and transient upregulation of gene modules. Together, these findings show that the human embryo transcriptome is far more complex than currently known, and will act as a valuable resource to empower future studies exploring development.

Maternal mRNA deadenylation and allocation via Rbm14 condensates facilitate vertebrate blastula development

Early embryonic development depends on proper utilization and clearance of maternal transcriptomes. How these processes are spatiotemporally regulated remains unclear. Here we show that nuclear RNA-binding protein Rbm14 and maternal mRNAs co-phase separate into cytoplasmic condensates to facilitate vertebrate blastula-to-gastrula development. In zebrafish, Rbm14 condensates were highly abundant in blastomeres and markedly reduced after prominent activation of zygotic transcription. They concentrated at spindle poles by associating with centrosomal γ-tubulin puncta and displayed mainly asymmetric divisions with a global symmetry across embryonic midline in 8- and 16-cell embryos. Their formation was dose-dependently stimulated by m6 A, but repressed by m5 C modification of the maternal mRNA. Furthermore, deadenylase Parn co-phase separated with these condensates, and this was required for deadenylation of the mRNAs in early blastomeres. Depletion of Rbm14 impaired embryonic cell differentiations and full activations of the zygotic genome in both zebrafish and mouse and resulted in developmental arrest at the blastula stage. Our results suggest that cytoplasmic Rbm14 condensate formation regulates early embryogenesis by facilitating deadenylation, protection, and mitotic allocation of m6 A-modified maternal mRNAs, and by releasing the poly(A)-less transcripts upon regulated disassembly to allow their re-polyadenylation and translation or clearance.

High-throughput transcriptome sequencing reveals the key stages of cardiovascular development in zebrafish embryos

Background

The cardiovascular developmental process is a tightly regulated network involving multiple genes. The current understanding of the molecular mechanism behind cardiovascular development is insufficient and requires further research.

Results

Transcriptome sequencing of three developmental stages in zebrafish embryos was performed and revealed three key cardiovascular developmental stages. Then, the differentially expressed genes (DEGs) involved in cardiovascular development were screened out. The three developmental stages were 18 (T1), 24 (T2), and 42 h post fertilization (hpf) (T3), and the three stages were confirmed by detecting differences in expression between cardiomyocyte and endothelial marker genes (cmlc2, fli1) using in situ hybridization, which represents the characteristics of cardiovascular development. Thousands of DEGs were identified using transcriptome analysis. Of them, 2605 DEGs were in T1-vs-T2, including 2003 up-regulated and 602 down-regulated genes, 6446 DEGs were in T1-vs-T3, consisting of 4608 up-regulated and 1838 down-regulated genes, and 3275 DEGs were in T2-vs-T3, including 2420 up-regulated and 855 down-regulated genes. There were 644 common DEGs and 167 common five-fold higher differentially expressed genes (HDEGs) identified, and Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses were performed using the Database for Annotation, Visualization and Integrated Discovery (DAVID). Significant differences was observed in the levels of gene expression among different developmental stages in multiple GO terms and KEGG pathways, such as cell migration to the midline involved in heart development, cardiovascular system development, circulatory system process for biological processes of GO terms; and cardiac muscle contraction, adrenergic signaling in cardiomyocytes for KEGG pathways. These results demonstrated that these three stages were important period for the development of the cardiovascular system. Lastly, we used quantitative real-time PCR (qPCR) to validate the reliability of RNA-sequencing by selecting 21 DEGs.

Conclusions

These results demonstrated that these three stages represented the important periods for cardiovascular system development of zebrafish and some candidate genes was obtained and provided a solid foundation for additional functional studies of the DEGs.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08808-x.

Dynamic Expression and Gene Regulation of MicroRNAs During Bighead Carp (Hypophthalmichthys nobilis) Early Development

The early development of fish is regulated through dynamic and complex mechanisms involving the regulation of various genes. Many genes are subjected to post-transcriptional regulation by microRNAs (miRNAs). In the Chinese aquaculture industry, the native species bighead carp (Hypophthalmichthys nobilis) is important. However, the genetic regulation related to the early development of bighead carp is unknown. Here, we generated developmental profiles by miRNA sequencing to study the dynamic regulation of miRNAs during bighead carp early development. This study identified 1 046 miRNAs, comprising 312 known miRNAs and 734 uncharacterized miRNAs. Changes in miRNA expression were identified in the six early development stages. An obviously increased expression trend was detected during the development process, with the main burst of activity occurring after the earliest stage (early blastula, DS1). Investigations revealed that several miRNAs were dominantly expressed during the development process, especially in the later stages (e.g., miR-10b-5p, miR-21, miR-92a-3p, miR-206-3p, and miR-430a-3p), suggesting that these miRNAs exerted important functions during embryonic development. The differentially expressed miRNAs (DEMs) and time-serial analysis (profiles) of DEMs were analyzed. A total of 372 miRNAs were identified as DEMs (fold-change >2, and false discovery rate <0.05), and three expression profiles of the DEMs were detected to have co-expression patterns (r > 0.7, and p < 0.05). The broad negative regulation of target genes by miRNAs was speculated, and many development-related biological processes and pathways were enriched for the targets of the DEMs, which might be associated with maternal genome degradation and embryogenesis processes. In conclusion, we revealed the repertoire of miRNAs that are active during early development of bighead carp. These findings will increase our understanding of the regulatory mechanisms of early development of fish.

Precise Temporal Regulation of Post-transcriptional Repressors Is Required for an Orderly Drosophila Maternal-to-Zygotic Transition

In animal embryos, the maternal-to-zygotic transition (MZT) hands developmental control from maternal to zygotic gene products. We show that the maternal proteome represents more than half of the protein-coding capacity of Drosophila melanogaster’s genome, and that 2% of this proteome is rapidly degraded during the MZT. Cleared proteins include the post-transcriptional repressors Cup, Trailer hitch (TRAL), Maternal expression at 31B (ME31B), and Smaug (SMG). Although the ubiquitin-proteasome system is necessary for clearance of these repressors, distinct E3 ligase complexes target them: the C-terminal to Lis1 Homology (CTLH) complex targets Cup, TRAL, and ME31B for degradation early in the MZT and the Skp/Cullin/F-box-containing (SCF) complex targets SMG at the end of the MZT. Deleting the C-terminal 233 amino acids of SMG abrogates F-box protein interaction and confers immunity to degradation. Persistent SMG downregulates zygotic re-expression of mRNAs whose maternal contribution is degraded by SMG. Thus, clearance of SMG permits an orderly MZT.

Cao et al. show that 2% of the proteome is degraded in early Drosophila embryos, including a repressive ribonucleoprotein complex. Two E3 ubiquitin ligases separately act on distinct components of this complex to phase their clearance. Failure to degrade a key component, the Smaug RNA-binding protein, disrupts an orderly maternal-to-zygotic transition.

Imaging nascent transcription in wholemount vertebrate embryos to characterize zygotic genome activation

A major event in early embryo development is the awakening of the embryonic genome, a process of large-scale transcriptional induction termed zygotic genome activation (ZGA). To understand how ZGA is controlled temporally and spatially, tools are required to image and quantify nascent transcription in wholemount embryos. In this chapter, we describe a metabolic labeling approach that leverages 5-ethynyl uridine (5-EU) incorporation into newly transcribed RNAs. Subsequently, click chemistry is used to conjugate these nascent transcripts to fluorophores for wholemount confocal imaging or biotin for RNA sequencing. Such an approach facilitates direct visualization of the global transcriptional state of each cell during early embryogenesis and provides a spatial map of gene expression activity. We describe this procedure for imaging nascent transcription in a vertebrate embryo Xenopus laevis, and use it as our model the onset of large-scale ZGA. Unlike cell culture systems in which 5-EU can be added to the media, metabolic labeling in Xenopus embryos requires microinjection in one-cell or two-cell stage embryos. This method is a powerful tool to quantify the nascent transcriptome at a single-cell level and to dissect mechanisms that control ZGA. We propose that this methodology can be applied broadly in other embryonic systems, and demonstrate the feasibility using zebrafish cleavage stage embryos. Finally, we demonstrate how to sequence the nascent transcriptome via 5-EU incorporation and separation of zygotic vs maternal RNAs. Altogether, our generalizable methodology will facilitate new insights into gene regulation and spatial patterning of ZGA during early embryogenesis.

Dynamic transcriptome sequencing and analysis during early development in the bighead carp (Hypophthalmichthys nobilis)

BACKGROUND

Early development is a key process of the life history of fish. However, the relationship between the transcriptome and the dynamic regulation of early development is still uncharacterized in the bighead carp (Hypophthalmichthys nobilis). In the present study, we performed transcriptome analysis of six development stages in H. nobilis, aiming to understand the dynamic molecular regulation of early development in this fish.

RESULTS

A total of 76,573 unigenes were assembled from clean sequence reads, with an average length of 1768 base. Among which, 41,742 (54.54%) unigenes were annotated to public protein databases, and an additional 59,014 simple sequence repeat (SSR) loci were identified among the unigenes. Furthermore, 30,199 differentially expressed transcripts (DETs) (fold change > 4 or < 0.25, and the false discovery rate FDR < 0.01) were observed in comparisons between the adjacent developmental stages, and nine expression patterns (profiles) were simulated using series-cluster analysis across six developmental stages. The unigenes expression level markedly increased after the DS1 stage (early blastula), and the numbers of DETs gradually decreased during subsequent development. The largest transcriptomic change (up- or down-regulated) was detected during the period from DS1 to DS2 (6-somite stage), which was enriched for many biological processes and metabolic pathways related to maternal to zygotic transition (MZT). Distinctly protein-protein interaction (PPI) networks were plotted for DETs during the period from DS1 to DS2. The genes (or proteins) from the same pathways were integrated together, and showed with obvious co-regulation patterns. In the series-cluster analysis, a remarkable profile of gene expression (profile_48) was identified that is probably related to the hatching during H. nobilis development, and the strict co-expression of a hatching enzyme gene (hce1) with 33 other annotated genes was identified from this profile.

CONCLUSIONS

The results indicated that strict dynamic regulation occurs during the early development in H. nobilis, especially in embryogenesis before hatching. This study provides valuable new information and transcriptomic resources related to H. nobilis early development, and for certain events such as MZT and hatching.

Induced androgenetic development in rainbow trout and transcriptome analysis of irradiated eggs

Ionizing radiation is administered to damage nuclear genome in fish eggs during induced androgenesis. In this study, we examined whether 350 Gy of X-ray applied to damage chromosomes in the rainbow trout eggs affects maternal RNA. Shortly after irradiation, we did not find any symptoms of RNA degradation in the treated eggs. Significant (p < 0.01) differences between non-irradiated and irradiated eggs concerned only a few transcripts including increased expression of immediate early response 2 (IER2) and early growth response 1 (EGR1) genes observed in the irradiated eggs. Both genes belong to the group of “immediate early genes” that respond quickly to the diverse extracellular stimuli. Elevated expression of these genes was accompanied by decreased level of ssa-miR-10b-5p and ssa-miR-21b-5p (p < 0.05), for which IER2 and EGR1 are target genes. The level of RNA in the fertilized irradiated eggs was highly significantly lower than in the non-irradiated eggs (p < 0.001) and in the unfertilized irradiated eggs (p < 0.0001). However, transcriptome profiles of fertilized non-irradiated eggs and fertilized irradiated eggs did not differ significantly. Thus, we assume that reduced abundance of mRNA in the fertilized irradiated eggs was associated with post-translational degradation and clearance of the maternal transcripts rather than from the irradiation of eggs.

High resolution annotation of zebrafish transcriptome using long-read sequencing

With the emergence of zebrafish as an important model organism, a concerted effort has been made to study its transcriptome. This effort is limited, however, by gaps in zebrafish annotation, which are especially pronounced concerning transcripts dynamically expressed during zygotic genome activation (ZGA). To date, short-read sequencing has been the principal technology for zebrafish transcriptome annotation. In part because these sequence reads are too short for assembly methods to resolve the full complexity of the transcriptome, the current annotation is rudimentary. By providing direct observation of full-length transcripts, recently refined long-read sequencing platforms can dramatically improve annotation coverage and accuracy. Here, we leveraged the SMRT platform to study the transcriptome of zebrafish embryos before and after ZGA. Our analysis revealed additional novelty and complexity in the zebrafish transcriptome, identifying 2539 high-confidence novel transcripts that originated from previously unannotated loci and 1835 high-confidence new isoforms in previously annotated genes. We validated these findings using a suite of computational approaches including structural prediction, sequence homology, and functional conservation analyses, as well as by confirmatory transcript quantification with short-read sequencing data. Our analyses provided insight into new homologs and paralogs of functionally important proteins and noncoding RNAs, isoform switching occurrences, and different classes of novel splicing events. Several novel isoforms representing distinct splicing events were validated through PCR experiments, including the discovery and validation of a novel 8-kb transcript spanning multiple mir-430 elements, an important driver of early development. Our study provides a significantly improved zebrafish transcriptome annotation resource.

Acute multi-sgRNA knockdown of KEOPS complex genes reproduces the microcephaly phenotype of the stable knockout zebrafish model

Until recently, morpholino oligonucleotides have been widely employed in zebrafish as an acute and efficient loss-of-function assay. However, off-target effects and reproducibility issues when compared to stable knockout lines have compromised their further use. Here we employed an acute CRISPR/Cas approach using multiple single guide RNAs targeting simultaneously different positions in two exemplar genes (osgep or tprkb) to increase the likelihood of generating mutations on both alleles in the injected F0 generation and to achieve a similar effect as morpholinos but with the reproducibility of stable lines. This multi single guide RNA approach resulted in median likelihoods for at least one mutation on each allele of >99% and sgRNA specific insertion/deletion profiles as revealed by deep-sequencing. Immunoblot showed a significant reduction for Osgep and Tprkb proteins. For both genes, the acute multi-sgRNA knockout recapitulated the microcephaly phenotype and reduction in survival that we observed previously in stable knockout lines, though milder in the acute multi-sgRNA knockout. Finally, we quantify the degree of mutagenesis by deep sequencing, and provide a mathematical model to quantitate the chance for a biallelic loss-of-function mutation. Our findings can be generalized to acute and stable CRISPR/Cas targeting for any zebrafish gene of interest.

Transcriptome analysis of different developmental stages of amphioxus reveals dynamic changes of distinct classes of genes during development

Vertebrates diverged from other chordates approximately 500 million years ago and have adopted several modifications of developmental processes. Amphioxus is widely used in evolutionary developmental biology research, such as on the basic patterning mechanisms involved in the chordate body plan and the origin of vertebrates. The fast development of next-generation sequencing has advanced knowledge of the genomic organization of amphioxus; however, many aspects of gene regulation during amphioxus development have not been fully characterized. In this study, we applied high-throughput sequencing on the transcriptomes of 13 developmental stages of Chinese amphioxus to gain a comprehensive understanding of transcriptional processes occurring from the fertilized egg to the adult stage. The expression levels of 3,423 genes were significantly changed (FDR ≤ 0.01). All of these genes were included in a clustering analysis, and enrichment of biological functions associated with these clusters was determined. Significant changes were observed in several important processes, including the down-regulation of the cell cycle and the up-regulation of translation. These results should build a foundation for identifying developmentally important genes, especially those regulatory factors involved in amphioxus development, and advance understanding of the developmental dynamics in vertebrates.

Embryonic gene expression of Coregonus palaea (whitefish) under pathogen stress as analyzed by high-throughput RNA-sequencing

Most fishes produce free-living embryos that are exposed to environmental stressors immediately following fertilization, including pathogenic microorganisms. Initial immune protection of embryos involves the chorion, as a protective barrier, and maternally-allocated antimicrobial compounds. At later developmental stages, host-genetic effects influence susceptibility and tolerance, suggesting a direct interaction between embryo genes and pathogens. So far, only a few host genes could be identified that correlate with embryonic survival under pathogen stress in salmonids. Here, we utilized high-throughput RNA-sequencing in order to describe the transcriptional response of a non-model fish, the Alpine whitefish Coregonus palaea, to infection, both in terms of host genes that are likely manipulated by the pathogen, and those involved in an early putative immune response. Embryos were produced in vitro, raised individually, and exposed at the late-eyed stage to a virulent strain of the opportunistic fish pathogen Pseudomonas fluorescens. The pseudomonad increased embryonic mortality and affected gene expression substantially. For example, essential, upregulated metabolic pathways in embryos under pathogen stress included ion binding pathways, aminoacyl-tRNA-biosynthesis, and the production of arginine and proline, most probably mediated by the pathogen for its proliferation. Most prominently downregulated transcripts comprised the biosynthesis of unsaturated fatty acids, the citrate cycle, and various isoforms of b-cell transcription factors. These factors have been shown to play a significant role in host blood cell differentiation and renewal. With regard to specific immune functions, differentially expressed transcripts mapped to the complement cascade, MHC class I and II, TNF-alpha, and T-cell differentiation proteins. The results of this study reveal insights into how P. fluorescens impairs the development of whitefish embryos and set a foundation for future studies investigating host pathogen interactions in fish embryos.

Proteomics of Xenopus development

Modern mass spectrometry-based methods provide an exciting opportunity to characterize protein expression in the developing embryo. We have employed an isotopic labeling technology to quantify the expression dynamics of nearly 6000 proteins across six stages of development in Xenopus laevis from the single stage zygote through the mid-blastula transition and the onset of organogenesis. Approximately 40% of the proteins show significant changes in expression across the development stages. The expression changes for these proteins naturally falls into six clusters corresponding to major events that mark early Xenopus development. A subset of experiments in this study have quantified protein expression differences between single embryos at the same stage of development, showing that, within experimental error, embryos at the same developmental stage have identical protein expression levels.

Cross-species oncogenomics using zebrafish models of cancer

The zebrafish is a relatively recent addition to cancer modeling. These models have now been extensively used in cross-species oncogenomic analyses at both the DNA and RNA levels. The goal of such studies is to identify conserved events that occur in both human and fish tumors which may act as central drivers of tumor phenotypes. Numerous comparisons of somatic DNA changes, using array CGH and exome sequencing, have demonstrated a relatively small set of conserved changes across species. In contrast, striking conservation of RNA expression patterns have been observed between the two species in models such as melanoma, leukemia, and rhabdomyosarcoma. In the future, the zebrafish will increasingly be used to model epigenetic and noncoding aspects of cancer biology.