Wnt Signaling Is Required for Early Development of Zebrafish Swimbladder

10.1371/journal.pone.0018431PLoS ONE63

Profiling subcellular localization of nuclear-encoded mitochondrial gene products in zebrafish

Most mitochondrial proteins are encoded by nuclear genes, synthetized in the cytosol and targeted into the organelle. To characterize the spatial organization of mitochondrial gene products in zebrafish (Danio rerio), we sequenced RNA from different cellular fractions. Our results confirmed the presence of nuclear-encoded mRNAs in the mitochondrial fraction, which in unperturbed conditions, are mainly transcripts encoding large proteins with specific properties, like transmembrane domains. To further explore the principles of mitochondrial protein compartmentalization in zebrafish, we quantified the transcriptomic changes for each subcellular fraction triggered by the chchd4a -/- mutation, causing the disorders in the mitochondrial protein import. Our results indicate that the proteostatic stress further restricts the population of transcripts on the mitochondrial surface, allowing only the largest and the most evolutionary conserved proteins to be synthetized there. We also show that many nuclear-encoded mitochondrial transcripts translated by the cytosolic ribosomes stay resistant to the global translation shutdown. Thus, vertebrates, in contrast to yeast, are not likely to use localized translation to facilitate synthesis of mitochondrial proteins under proteostatic stress conditions.This work was supported by the POLONEZ Fellowship of National Science Centre, Poland, 2016/23/P/NZ3/03730. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 665778 (B Uszczynska-Ratajczak). This work was also supported by EU/FP7: Research Potential FISHMED, grant number 316125. The work was also funded by “Regenerative Mechanisms for Health” project MAB/2017/2 carried out within the International Research Agendas program of the Foundation for Polish Science cofinanced by the European Union under the European Regional Development Fund

Genome Wide Analysis Reveals Zic3 Interaction with Distal Regulatory Elements of Stage Specific Developmental Genes in Zebrafish

<div><p>Zic3 regulates early embryonic patterning in vertebrates. Loss of Zic3 function is known to disrupt gastrulation, left-right patterning, and neurogenesis. However, molecular events downstream of this transcription factor are poorly characterized. Here we use the zebrafish as a model to study the developmental role of Zic3 <i>in vivo</i>, by applying a combination of two powerful genomics approaches – ChIP-seq and microarray. Besides confirming direct regulation of previously implicated Zic3 targets of the Nodal and canonical Wnt pathways, analysis of gastrula stage embryos uncovered a number of novel candidate target genes, among which were members of the non-canonical Wnt pathway and the neural pre-pattern genes. A similar analysis in <i>zic3</i>-expressing cells obtained by FACS at segmentation stage revealed a dramatic shift in Zic3 binding site locations and identified an entirely distinct set of target genes associated with later developmental functions such as neural development. We demonstrate cis-regulation of several of these target genes by Zic3 using <i>in vivo</i> enhancer assay. Analysis of Zic3 binding sites revealed a distribution biased towards distal intergenic regions, indicative of a long distance regulatory mechanism; some of these binding sites are highly conserved during evolution and act as functional enhancers. This demonstrated that Zic3 regulation of developmental genes is achieved predominantly through long distance regulatory mechanism and revealed that developmental transitions could be accompanied by dramatic changes in regulatory landscape.</p></div

Zebrafish mRNA sequencing deciphers novelties in transcriptome dynamics during maternal to zygotic transition

Maternally deposited mRNAs direct early development before the initiation of zygotic transcription during mid-blastula transition (MBT). To study mechanisms regulating this developmental event in zebrafish, we applied mRNA deep sequencing technology and generated comprehensive information and valuable resources on transcriptome dynamics during early embryonic (egg to early gastrulation) stages. Genome-wide transcriptome analysis documented at least 8000 maternal genes and identified the earliest cohort of zygotic transcripts. We determined expression levels of maternal and zygotic transcripts with the highest resolution possible using mRNA-seq and clustered them based on their expression pattern. We unravel delayed polyadenylation in a large cohort of maternal transcripts prior to the MBT for the first time in zebrafish. Blocking polyadenylation of these transcripts confirms their role in regulating development from the MBT onward. Our study also identified a large number of novel transcribed regions in annotated and unannotated regions of the genome, which will facilitate reannotation of the zebrafish genome. We also identified splice variants with an estimated frequency of 50%–60%. Taken together, our data constitute a useful genomic information and valuable transcriptome resource for gene discovery and for understanding the mechanisms of early embryogenesis in zebrafish

Candidate target genes regulated by Zic3 at 8 hpf and 24 hpf developmental stages.

<p>Target genes are grouped based on their signaling pathway or functions. Changes in expression in microarray are represented by red and green backgrounds for up- and down-regulation respectively.</p

Distribution of Zic3 peaks as identified in ChIP-seq experiments according to GREAT algorithm.

<p>A, distribution of peaks located in promoter (within 5 kb upstream of TSS), intragenic, and intergenic regions. B, gene association rule of ‘basal plus 100 kb’ according to GREAT algorithm. C, percentage of peaks associated with none, one, or two genes based on the gene association rule in B. D, number of peaks present in each distance categories along the x-axis, with regards to TSS of associated gene. E, region map showing overlap between genomic locations of peaks in 8 hpf and 24 hpf datasets. F–I, list of biological processes and tissue specific expression terms enriched among Zic3-associated genes at 8 hpf (F, G) and 24 hpf (H, I) according to DAVID GO terms. Light and dark grey bars represent the expected and observed enrichments of functional categories indicated along the y-axis.</p

Zic3 binding sites associated with genes from the Nodal and Wnt pathway genes.

<p>UCSC browser image depicting genomic locations of Zic3 peaks identified near Nodal (A) and Wnt (B) pathway genes at 8 hpf. Single black vertical bars below histogram - peaks called by QuEST algorithm, blue horizontal bars - annotated exons (tall boxes), UTRs (half-sized boxes), introns (lines, arrowheads denote transcript orientation); Zic3 binding sites with negative (red arrows) and positive (green arrows) enhancer activity. Scale bars are indicated by black horizontal line at the top of each panel. C, list of tested fragments associated with Nodal and Wnt pathway genes. Enhancer-driven expression was assayed by qRT-PCR of <i>gfp</i> at 8 hpf and through microscopic observation of GFP expression pattern at 24 hpf. Between 50 to 100 embryos were assayed in each experimental time point. D–G, representative figure of <i>gfp</i> expression driven by selected fragments of Zic3 binding sites in F₀ embryos at 24 hpf, immunostained with anti-GFP antibody. D*, F*, dorsal view; E*, G*, lateral view.</p