Rewiring of the epigenome and chromatin architecture by exogenously induced retinoic acid signaling during zebrafish embryonic development

Retinoic acid (RA) is the ligand of RA receptors (RARs), transcription factors that bind to RA response elements. RA signaling is required for multiple processes during embryonic development, including body axis extension, hindbrain antero-posterior patterning and forelimb bud initiation. Although some RA target genes have been identified, little is known about the genome-wide effects of RA signaling during in vivo embryonic development. Here, we stimulate the RA pathway by treating zebrafish embryos with all-trans-RA (atRA) and use a combination of RNA-seq, ATAC-seq, ChIP-seq and HiChIP to gain insight into the molecular mechanisms by which exogenously induced RA signaling controls gene expression. We find that RA signaling is involved in anterior/posterior patterning, central nervous system development, and the transition from pluripotency to differentiation. AtRA treatment also alters chromatin accessibility during early development and promotes chromatin binding of RARαa and the RA targets Hoxb1b, Meis2b and Sox3, which cooperate in central nervous system development. Finally, we show that exogenous RA induces a rewiring of chromatin architecture, with alterations in chromatin 3D interactions involving target genes. Altogether, our findings identify genome-wide targets of RA signaling and provide a molecular mechanism by which developmental signaling pathways regulate target gene expression by altering chromatin topology.

Unraveling the transcriptomic landscape of eye migration and visual adaptations during flatfish metamorphosis

Flatfish undergo a remarkable metamorphosis from symmetrical pelagic larvae to fully asymmetrical benthic juveniles. The most distinctive features of this transformation is the migration of one eye. The molecular role of thyroid hormone in the metamorphosis process in flatfishes is well established. However, the regulatory network that facilitates eye movement remains enigmatic. This paper presents a morphological investigation of the metamorphic process in turbot eyes, using advanced imaging techniques and a global view of gene expression. The study covers migrant and non-migrant eyes and aims to identify the genes that are active during ocular migration. Our transcriptomic analysis shows a significant up-regulation of immune-related genes. The analysis of eye-specific genes reveals distinct patterns during the metamorphic process. Myosin is highlighted in the non-migrant eye, while ependymin is highlighted in the migrant eye, possibly involved in optic nerve regeneration. Furthermore, a potential association between the alx3 gene and cranial restructuring has been identified. Additionally, it confirmed simultaneous adaptation to low light in both eyes, as described by changes in opsins expression during the metamorphic process. The study also revealed that ocular migration activates systems asynchronously in both eyes, providing insight into multifaceted reorganization processes during metamorphosis of flatfish.

The cellular landscape of the endochondral bone during the transition to extrauterine life

The cellular complexity of the endochondral bone underlies its essential and pleiotropic roles during organismal life. While the adult bone has received significant attention, we still lack a deep understanding of the perinatal bone cellulome. Here, we have profiled the full composition of the murine endochondral bone at the single-cell level during the transition from fetal to newborn life and in comparison with the adult tissue, with particular emphasis on the mesenchymal compartment. The perinatal bone contains different fibroblastic clusters with blastema-like characteristics in organizing and supporting skeletogenesis, angiogenesis and hematopoiesis. Our data also suggest dynamic inter- and intra-compartment interactions, as well as a bone marrow milieu that seems prone to anti-inflammation, which we hypothesize is necessary to ensure the proper program of lymphopoiesis and the establishment of central and peripheral tolerance in early life. Our study provides an integrative roadmap for the future design of genetic and cellular functional assays to validate cellular interactions and lineage relationships within the perinatal bone.

Forkhead transcription factor FKH-8 cooperates with RFX in the direct regulation of sensory cilia in Caenorhabditis elegans

Cilia, either motile or non-motile (a.k.a primary or sensory), are complex evolutionarily conserved eukaryotic structures composed of hundreds of proteins required for their assembly, structure and function that are collectively known as the ciliome. Ciliome gene mutations underlie a group of pleiotropic genetic diseases known as ciliopathies. Proper cilium function requires the tight coregulation of ciliome gene transcription, which is only fragmentarily understood. RFX transcription factors (TF) have an evolutionarily conserved role in the direct activation of ciliome genes both in motile and non-motile cilia cell-types. In vertebrates, FoxJ1 and FoxN4 Forkhead (FKH) TFs work with RFX in the direct activation of ciliome genes, exclusively in motile cilia cell-types. No additional TFs have been described to act together with RFX in primary cilia cell-types in any organism. Here we describe FKH-8, a FKH TF, as a direct regulator of the sensory ciliome genes in Caenorhabditis elegans. FKH-8 is expressed in all ciliated neurons in C. elegans, binds the regulatory regions of ciliome genes, regulates ciliome gene expression, cilium morphology and a wide range of behaviors mediated by sensory ciliated neurons. FKH-8 and DAF-19 (C. elegans RFX) physically interact and synergistically regulate ciliome gene expression. C. elegans FKH-8 function can be replaced by mouse FOXJ1 and FOXN4 but not by other members of other mouse FKH subfamilies. In conclusion, RFX and FKH TF families act jointly as direct regulators of ciliome genes also in sensory ciliated cell types suggesting that this regulatory logic could be an ancient trait predating functional cilia sub-specialization.

Genome-wide chromatin accessibility and gene expression profiling during flatfish metamorphosis

Metamorphosis is a widely studied post-embryonic process in which many tissues undergo dramatic modifications to adapt to the new adult lifestyle. Flatfishes represent a good example of metamorphosis in teleost fishes. During metamorphosis of flatfish, organ regression and neoformation occur, with one of the most notable changes being the migration of one of the eyes to the other side of the body. In order to create a useful and reliable tool to advance the molecular study of metamorphosis in flatfish, we generated a chromatin accessible atlas as well as gene expression profile during four developmental stages ranging from a phylotypic to a post-metamorphic stage. We identified 29,019 differentially accessible chromatin regions and 3,253 differentially expressed genes. We found stage-specific regulatory regions and gene expression profiles, supporting the quality of the results. Our work provides strongly reproducible data for further studies to elucidate the regulatory elements that ensure successful metamorphosis in flatfish species.

The little skate genome and the evolutionary emergence of wing-like fins

Skates are cartilaginous fish whose body plan features enlarged wing-like pectoral fins, enabling them to thrive in benthic environments1,2. However, the molecular underpinnings of this unique trait remain unclear. Here we investigate the origin of this phenotypic innovation by developing the little skate Leucoraja erinacea as a genomically enabled model. Analysis of a high-quality chromosome-scale genome sequence for the little skate shows that it preserves many ancestral jawed vertebrate features compared with other sequenced genomes, including numerous ancient microchromosomes. Combining genome comparisons with extensive regulatory datasets in developing fins-including gene expression, chromatin occupancy and three-dimensional conformation-we find skate-specific genomic rearrangements that alter the three-dimensional regulatory landscape of genes that are involved in the planar cell polarity pathway. Functional inhibition of planar cell polarity signalling resulted in a reduction in anterior fin size, confirming that this pathway is a major contributor to batoid fin morphology. We also identified a fin-specific enhancer that interacts with several hoxa genes, consistent with the redeployment of hox gene expression in anterior pectoral fins, and confirmed its potential to activate transcription in the anterior fin using zebrafish reporter assays. Our findings underscore the central role of genome reorganization and regulatory variation in the evolution of phenotypes, shedding light on the molecular origin of an enigmatic trait.

Parallel evolution of amphioxus and vertebrate small-scale gene duplications

BACKGROUND

Amphioxus are non-vertebrate chordates characterized by a slow morphological and molecular evolution. They share the basic chordate body-plan and genome organization with vertebrates but lack their 2R whole-genome duplications and their developmental complexity. For these reasons, amphioxus are frequently used as an outgroup to study vertebrate genome evolution and Evo-Devo. Aside from whole-genome duplications, genes continuously duplicate on a smaller scale. Small-scale duplicated genes can be found in both amphioxus and vertebrate genomes, while only the vertebrate genomes have duplicated genes product of their 2R whole-genome duplications. Here, we explore the history of small-scale gene duplications in the amphioxus lineage and compare it to small- and large-scale gene duplication history in vertebrates.

RESULTS

We present a study of the European amphioxus (Branchiostoma lanceolatum) gene duplications thanks to a new, high-quality genome reference. We find that, despite its overall slow molecular evolution, the amphioxus lineage has had a history of small-scale duplications similar to the one observed in vertebrates. We find parallel gene duplication profiles between amphioxus and vertebrates and conserved functional constraints in gene duplication. Moreover, amphioxus gene duplicates show levels of expression and patterns of functional specialization similar to the ones observed in vertebrate duplicated genes. We also find strong conservation of gene synteny between two distant amphioxus species, B. lanceolatum and B. floridae, with two major chromosomal rearrangements.

CONCLUSIONS

In contrast to their slower molecular and morphological evolution, amphioxus' small-scale gene duplication history resembles that of the vertebrate lineage both in quantitative and in functional terms.

Multidimensional chromatin profiling of zebrafish pancreas to uncover and investigate disease-relevant enhancers

The pancreas is a central organ for human diseases. Most alleles uncovered by genome-wide association studies of pancreatic dysfunction traits overlap with non-coding sequences of DNA. Many contain epigenetic marks of cis-regulatory elements active in pancreatic cells, suggesting that alterations in these sequences contribute to pancreatic diseases. Animal models greatly help to understand the role of non-coding alterations in disease. However, interspecies identification of equivalent cis-regulatory elements faces fundamental challenges, including lack of sequence conservation. Here we combine epigenetic assays with reporter assays in zebrafish and human pancreatic cells to identify interspecies functionally equivalent cis-regulatory elements, regardless of sequence conservation. Among other potential disease-relevant enhancers, we identify a zebrafish ptf1a distal-enhancer whose deletion causes pancreatic agenesis, a phenotype previously found to be induced by mutations in a distal-enhancer of PTF1A in humans, further supporting the causality of this condition in vivo. This approach helps to uncover interspecies functionally equivalent cis-regulatory elements and their potential role in human disease.

Alterations in cis-regulatory elements (CREs) can contribute to pancreatic diseases. Here the authors combine chromatin profiling and interaction points with in vivo reporter assays in zebrafish to uncover functionally equivalent human CREs, helping to predict disease-relevant enhancers.

Duplications disrupt chromatin architecture and rewire GPR101-enhancer communication in X-linked acrogigantism

X-linked acrogigantism (X-LAG) is the most severe form of pituitary gigantism and is characterized by aggressive growth hormone (GH)-secreting pituitary tumors that occur in early childhood. X-LAG is associated with chromosome Xq26.3 duplications (the X-LAG locus typically includes VGLL1, CD40LG, ARHGEF6, RBMX, and GPR101) that lead to massive pituitary tumoral expression of GPR101, a novel regulator of GH secretion. The mechanism by which the duplications lead to marked pituitary misexpression of GPR101 alone was previously unclear. Using Hi-C and 4C-seq, we characterized the normal chromatin structure at the X-LAG locus. We showed that GPR101 is located within a topologically associating domain (TAD) delineated by a tissue-invariant border that separates it from centromeric genes and regulatory sequences. Next, using 4C-seq with GPR101, RBMX, and VGLL1 viewpoints, we showed that the duplications in multiple X-LAG-affected individuals led to ectopic interactions that crossed the invariant TAD border, indicating the existence of a similar and consistent mechanism of neo-TAD formation in X-LAG. We then identified several pituitary active cis-regulatory elements (CREs) within the neo-TAD and demonstrated in vitro that one of them significantly enhanced reporter gene expression. At the same time, we showed that the GPR101 promoter permits the incorporation of new regulatory information. Our results indicate that X-LAG is a TADopathy of the endocrine system in which Xq26.3 duplications disrupt the local chromatin architecture forming a neo-TAD. Rewiring GPR101-enhancer interaction within the new regulatory unit is likely to cause the high levels of aberrant expression of GPR101 in pituitary tumors caused by X-LAG.

Gene regulatory networks of epidermal and neural fate choice in a chordate

Neurons are a highly specialized cell type only found in metazoans. They can be scattered throughout the body or grouped together, forming ganglia or nerve cords. During embryogenesis, centralized nervous systems develop from the ectoderm, which also forms the epidermis. How pluripotent ectodermal cells are directed toward neural or epidermal fates, and to which extent this process is shared among different animal lineages, are still open questions. Here, by using micromere explants, we were able to define in silico the putative gene regulatory networks (GRNs) underlying the first steps of the epidermis and the central nervous system formation in the cephalochordate amphioxus. We propose that although the signal triggering neural induction in amphioxus (i.e., Nodal) is different from vertebrates, the main transcription factors implicated in this process are conserved. Moreover, our data reveal that transcription factors of the neural program seem to not only activate neural genes but also to potentially have direct inputs into the epidermal GRN, suggesting that the Nodal signal might also contribute to neural fate commitment by repressing the epidermal program. Our functional data on whole embryos support this result and highlight the complex interactions among the transcription factors activated by the signaling pathways that drive ectodermal cell fate choice in chordates.

Brains in Metamorphosis: Temporal Transcriptome Dynamics in Hatchery-Reared Flatfishes

Metamorphosis is a captivating process of change during which the morphology of the larva is completely reshaped to face the new challenges of adult life. In the case of fish, this process initiated in the brain has traditionally been considered to be a critical rearing point and despite the pioneering molecular work carried out in other flatfishes, the underlying molecular basis is still relatively poorly characterized. Turbot brain transcriptome of three developmental stages (pre-metamorphic, climax of metamorphosis and post-metamorphic) were analyzed to study the gene expression dynamics throughout the metamorphic process. A total of 1570 genes were differentially expressed in the three developmental stages and we found a specific pattern of gene expression at each stage. Unexpectedly, at the climax stage of metamorphosis, we found highly expressed genes related to the immune response, while the biological pathway enrichment analysis in pre-metamorphic and post-metamorphic were related to cell differentiation and oxygen carrier activity, respectively. In addition, our results confirm the importance of thyroid stimulating hormone, increasing its expression during metamorphosis. Based on our findings, we assume that immune system activation during the climax of metamorphosis stage could be related to processes of larval tissue inflammation, resorption and replacement, as occurs in other vertebrates.

Topologically Associating Domains and Regulatory Landscapes in Development, Evolution and Disease

Animal genomes are folded in topologically associating domains (TADs) that have been linked to the regulation of the genes they contain by constraining regulatory interactions between cis-regulatory elements and promoters. Therefore, TADs are proposed as structural scaffolds for the establishment of regulatory landscapes (RLs). In this review, we discuss recent advances in the connection between TADs and gene regulation, their relationship with gene RLs and their dynamics during development and differentiation. Moreover, we describe how restructuring TADs may lead to pathological conditions, which explains their high evolutionary conservation, but at the same time it provides a substrate for the emergence of evolutionary innovations that lay at the origin of vertebrates and other phylogenetic clades.

Ancient Genomic Regulatory Blocks Are a Source for Regulatory Gene Deserts in Vertebrates after Whole-Genome Duplications

We investigated how the two rounds of whole-genome duplication that occurred at the base of the vertebrate lineage have impacted ancient microsyntenic associations involving developmental regulators (known as genomic regulatory blocks, GRBs). We showed that the majority of GRBs identified in the last common ancestor of chordates have been maintained as a single copy in humans. We found evidence that dismantling of the duplicated GRB copies occurred early in vertebrate evolution often through the differential retention of the regulatory gene but loss of the bystander gene’s exonic sequences. Despite the large evolutionary scale, the presence of duplicated highly conserved noncoding regions provided unambiguous proof for this scenario for multiple ancient GRBs. Remarkably, the dismantling of ancient GRB duplicates has contributed to the creation of large gene deserts associated with regulatory genes in vertebrates, providing a potentially widespread mechanism for the origin of these enigmatic genomic traits.

Straightforward protocol for allele-specific chromatin conformation capture

A key advance in our understanding of gene regulation came with the finding that the genome undergoes three-dimensional nuclear folding in a genetically determined process. This 3D conformation directly influences the association between enhancers and their target promoters. This complex interplay has been proven to be essential for gene regulation, and genetic variants affecting this process have been associated to human diseases. The development of new technologies that quantify these DNA interactions represented a revolution in the field. High throughput techniques like HiC provide a general picture of chromatin topology. However, they often lack resolution to evidence subtle effects that single nucleotide polymorphisms exert over the contacts between cis-regulatory regions and target promoters. Here we propose a cost-efficient approach to perform allele-specific chromatin conformation analysis. As a proof of concept, we analyzed the impact of a common deletion mapping between SIRPB1 promoter and one of its downstream enhancers.

Functional Conservation of Divergent p63-Bound cis-Regulatory Elements

The transcription factor p63 is an essential regulator of vertebrate ectoderm development, including epidermis, limbs, and craniofacial tissues. Here, we have investigated the evolutionary conservation of p63 binding sites (BSs) between zebrafish and human. First, we have analyzed sequence conservation of p63 BSs by comparing ChIP-seq data from human keratinocytes and zebrafish embryos, observing a very poor conservation. Next, we compared the gene regulatory network orchestrated by p63 in both species and found a high overlap between them, suggesting a high degree of functional conservation during evolution despite sequence divergence and the large evolutionary distance. Finally, we used transgenic reporter assays in zebrafish embryos to functionally validate a set of equivalent p63 BSs from zebrafish and human located close to genes involved in epidermal development. Reporter expression was driven by human and zebrafish BSs to many common tissues related to p63 expression domains. Therefore, we conclude that the gene regulatory network controlled by p63 is highly conserved across vertebrates despite the fact that p63-bound regulatory elements show high divergence.

Characterization of the accessible genome in the human malaria parasite Plasmodium falciparum

Human malaria is a devastating disease and a major cause of poverty in resource-limited countries. To develop and adapt within hosts Plasmodium falciparum undergoes drastic switches in gene expression. To identify regulatory regions in the parasite genome, we performed genome-wide profiling of chromatin accessibility in two culture-adapted isogenic subclones at four developmental stages during the intraerythrocytic cycle by using the Assay for Transposase-Accessible Chromatin by sequencing (ATAC-seq). Tn5 transposase hypersensitivity sites (THSSs) localize preferentially at transcriptional start sites (TSSs). Chromatin accessibility by ATAC-seq is predictive of active transcription and of the levels of histone marks H3K9ac and H3K4me3. Our assay allows the identification of novel regulatory regions including TSS and enhancer-like elements. We show that the dynamics in the accessible chromatin profile matches temporal transcription during development. Motif analysis of stage-specific ATAC-seq sites predicts the in vivo binding sites and function of multiple ApiAP2 transcription factors. At last, the alternative expression states of some clonally variant genes (CVGs), including eba, phist, var and clag genes, associate with a differential ATAC-seq signal at their promoters. Altogether, this study identifies genome-wide regulatory regions likely to play an essential function in the developmental transitions and in CVG expression in P. falciparum.

The Repression of Atoh1 by Neurogenin1 during Inner Ear Development

Atonal homolog 1 (Atoh1) and Neurogenin1 (Neurog1) are basic Helix-Loop-Helix (bHLH) transcription factors crucial for the generation of hair cells (HCs) and neurons in the inner ear. Both genes are induced early in development, but the expression of Atoh1 is counteracted by Neurog1. As a result, HC development is prevented during neurogenesis. This work aimed at understanding the molecular basis of this interaction. Atoh1 regulation depends on a 3'Atoh1-enhancer that is the site for Atoh1 autoregulation. Reporter assays on chick embryos and P19 cells show that Neurog1 hampers the autoactivation of Atoh1, the effect being cell autonomous and independent on Notch activity. Assay for Transposase-Accessible Chromatin with high throughput sequencing (ATAC-Seq) analysis shows that the region B of the 3'Atoh1-enhancer is accessible during development and sufficient for both activation and repression. Neurog1 requires the regions flanking the class A E-box to show its repressor effect, however, it does not require binding to DNA for Atoh1 repression. This depends on the dimerization domains Helix-1 and Helix-2 and the reduction of Atoh1 protein levels. The results point towards the acceleration of Atoh1 mRNA degradation as the potential mechanism for the reduction of Atoh1 levels. Such a mechanism dissociates the prevention of Atoh1 expression in neurosensory progenitors from the unfolding of the neurogenic program.

CTCF counter-regulates cardiomyocyte development and maturation programs in the embryonic heart

Cardiac progenitors are specified early in development and progressively differentiate and mature into fully functional cardiomyocytes. This process is controlled by an extensively studied transcriptional program. However, the regulatory events coordinating the progression of such program from development to maturation are largely unknown. Here, we show that the genome organizer CTCF is essential for cardiogenesis and that it mediates genomic interactions to coordinate cardiomyocyte differentiation and maturation in the developing heart. Inactivation of Ctcf in cardiac progenitor cells and their derivatives in vivo during development caused severe cardiac defects and death at embryonic day 12.5. Genome wide expression analysis in Ctcf mutant hearts revealed that genes controlling mitochondrial function and protein production, required for cardiomyocyte maturation, were upregulated. However, mitochondria from mutant cardiomyocytes do not mature properly. In contrast, multiple development regulatory genes near predicted heart enhancers, including genes in the IrxA cluster, were downregulated in Ctcf mutants, suggesting that CTCF promotes cardiomyocyte differentiation by facilitating enhancer-promoter interactions. Accordingly, loss of CTCF disrupts gene expression and chromatin interactions as shown by chromatin conformation capture followed by deep sequencing. Furthermore, CRISPR-mediated deletion of an intergenic CTCF site within the IrxA cluster alters gene expression in the developing heart. Thus, CTCF mediates local regulatory interactions to coordinate transcriptional programs controlling transitions in morphology and function during heart development.

Properly regulated gene expression in time and space during development and differentiation requires not only transcriptional inputs, but also specific structuring of the chromatin. CTCF is a DNA binding factor that is believed to be critical for this process through binding to tens of thousands of sites across the genome. Despite the knowledge gained in recent years on the role of CTCF in genome organization, its functions in vivo are poorly understood. To address this issue, we studied the effect of genetically deleting CTCF in differentiating cardiomyocytes at early stages of mouse development. Surprisingly only a fraction of genes change their expression when CTCF is removed. Importantly, misregulated genes control opposing genetic programs in charge of development and patterning on one hand, and cardiomyocyte maturation on the other. This imbalance leads to faulty mitochondria and incorrect expression of cardiac patterning genes, and subsequent embryonic lethality. Our results suggest that CTCF is not necessary for maintenance of global genome structure, but coordinates dynamic genetic programs controlling phenotypic transitions in developing cells and tissues.

Regulatory landscape fusion in rhabdomyosarcoma through interactions between the PAX3 promoter and FOXO1 regulatory elements

Background

The organisation of vertebrate genomes into topologically associating domains (TADs) is believed to facilitate the regulation of the genes located within them. A remaining question is whether TAD organisation is achieved through the interactions of the regulatory elements within them or if these interactions are favoured by the pre-existence of TADs. If the latter is true, the fusion of two independent TADs should result in the rewiring of the transcriptional landscape and the generation of ectopic contacts.

Results

We show that interactions within the PAX3 and FOXO1 domains are restricted to their respective TADs in normal conditions, while in a patient-derived alveolar rhabdomyosarcoma cell line, harbouring the diagnostic t(2;13)(q35;q14) translocation that brings together the PAX3 and FOXO1 genes, the PAX3 promoter interacts ectopically with FOXO1 sequences. Using a combination of 4C-seq datasets, we have modelled the three-dimensional organisation of the fused landscape in alveolar rhabdomyosarcoma.

Conclusions

The chromosomal translocation that leads to alveolar rhabdomyosarcoma development generates a novel TAD that is likely to favour ectopic PAX3:FOXO1 oncogene activation in non-PAX3 territories. Rhabdomyosarcomas may therefore arise from cells which do not normally express PAX3. The borders of this novel TAD correspond to the original 5'- and 3'- borders of the PAX3 and FOXO1 TADs, respectively, suggesting that TAD organisation precedes the formation of regulatory long-range interactions. Our results demonstrate that, upon translocation, novel regulatory landscapes are formed allowing new intra-TAD interactions between the original loci involved.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-017-1225-z) contains supplementary material, which is available to authorized users.

The Dynamic Regulatory Genome of Capsaspora and the Origin of Animal Multicellularity

The unicellular ancestor of animals had a complex repertoire of genes linked to multicellular processes. This suggests that changes in the regulatory genome, rather than in gene innovation, were key to the origin of animals. Here, we carry out multiple functional genomic assays in Capsaspora owczarzaki, the unicellular relative of animals with the largest known gene repertoire for transcriptional regulation. We show that changing chromatin states, differential lincRNA expression, and dynamic cis-regulatory sites are associated with life cycle transitions in Capsaspora. Moreover, we demonstrate conservation of animal developmental transcription-factor networks and extensive network interconnection in this premetazoan organism. In contrast, however, Capsaspora lacks animal promoter types, and its regulatory sites are small, proximal, and lack signatures of animal enhancers. Overall, our results indicate that the emergence of animal multicellularity was linked to a major shift in genome cis-regulatory complexity, most notably the appearance of distal enhancer regulation.

A single three-dimensional chromatin compartment in amphioxus indicates a stepwise evolution of vertebrate Hox bimodal regulation

The HoxA and HoxD gene clusters of jawed vertebrates are organized into bipartite three-dimensional chromatin structures that separate long-range regulatory inputs coming from the anterior and posterior Hox-neighboring regions. This architecture is instrumental in allowing vertebrate Hox genes to pattern disparate parts of the body, including limbs. Almost nothing is known about how these three-dimensional topologies originated. Here we perform extensive 4C-seq profiling of the Hox cluster in embryos of amphioxus, an invertebrate chordate. We find that, in contrast to the architecture in vertebrates, the amphioxus Hox cluster is organized into a single chromatin interaction domain that includes long-range contacts mostly from the anterior side, bringing distant cis-regulatory elements into contact with Hox genes. We infer that the vertebrate Hox bipartite regulatory system is an evolutionary novelty generated by combining ancient long-range regulatory contacts from DNA in the anterior Hox neighborhood with new regulatory inputs from the posterior side.

Favorable genomic environments for cis-regulatory evolution: A novel theoretical framework

Cis-regulatory changes are arguably the primary evolutionary source of animal morphological diversity. With the recent explosion of genome-wide comparisons of the cis-regulatory content in different animal species is now possible to infer general principles underlying enhancer evolution. However, these studies have also revealed numerous discrepancies and paradoxes, suggesting that the mechanistic causes and modes of cis-regulatory evolution are still not well understood and are probably much more complex than generally appreciated. Here, we argue that the mutational mechanisms and genomic regions generating new regulatory activities must comply with the constraints imposed by the molecular properties of cis-regulatory elements (CREs) and the organizational features of long-range chromatin interactions. Accordingly, we propose a new integrative evolutionary framework for cis-regulatory evolution based on two major premises for the origin of novel enhancer activity: (i) an accessible chromatin environment and (ii) compatibility with the 3D structure and interactions of pre-existing CREs. Mechanisms and DNA sequences not fulfilling these premises, will be less likely to have a measurable impact on gene expression and as such, will have a minor contribution to the evolution of gene regulation. Finally, we discuss current comparative cis-regulatory data under the light of this new evolutionary model, and propose that the two most prominent mechanisms for the evolution of cis-regulatory changes are the overprinting of ancestral CREs and the exaptation of transposable elements.

Comparative epigenomics in distantly related teleost species identifies conserved cis-regulatory nodes active during the vertebrate phylotypic period

The complex relationship between ontogeny and phylogeny has been the subject of attention and controversy since von Baer’s formulations in the 19th century. The classic concept that embryogenesis progresses from clade general features to species-specific characters has often been revisited. It has become accepted that embryos from a clade show maximum morphological similarity at the so-called phylotypic period (i.e., during mid-embryogenesis). According to the hourglass model, body plan conservation would depend on constrained molecular mechanisms operating at this period. More recently, comparative transcriptomic analyses have provided conclusive evidence that such molecular constraints exist. Examining cis-regulatory architecture during the phylotypic period is essential to understand the evolutionary source of body plan stability. Here we compare transcriptomes and key epigenetic marks (H3K4me3 and H3K27ac) from medaka (Oryzias latipes) and zebrafish (Danio rerio), two distantly related teleosts separated by an evolutionary distance of 115–200 Myr. We show that comparison of transcriptome profiles correlates with anatomical similarities and heterochronies observed at the phylotypic stage. Through comparative epigenomics, we uncover a pool of conserved regulatory regions (≈700), which are active during the vertebrate phylotypic period in both species. Moreover, we show that their neighboring genes encode mainly transcription factors with fundamental roles in tissue specification. We postulate that these regulatory regions, active in both teleost genomes, represent key constrained nodes of the gene networks that sustain the vertebrate body plan.

Restless legs syndrome-associated intronic common variant in Meis1 alters enhancer function in the developing telencephalon

Genome-wide association studies (GWAS) identified the MEIS1 locus for Restless Legs Syndrome (RLS), but causal single nucleotide polymorphisms (SNPs) and their functional relevance remain unknown. This locus contains a large number of highly conserved noncoding regions (HCNRs) potentially functioning as cis-regulatory modules. We analyzed these HCNRs for allele-dependent enhancer activity in zebrafish and mice and found that the risk allele of the lead SNP rs12469063 reduces enhancer activity in the Meis1 expression domain of the murine embryonic ganglionic eminences (GE). CREB1 binds this enhancer and rs12469063 affects its binding in vitro. In addition, MEIS1 target genes suggest a role in the specification of neuronal progenitors in the GE, and heterozygous Meis1-deficient mice exhibit hyperactivity, resembling the RLS phenotype. Thus, in vivo and in vitro analysis of a common SNP with small effect size showed allele-dependent function in the prospective basal ganglia representing the first neurodevelopmental region implicated in RLS.

Obesity-associated variants within FTO form long-range functional connections with IRX3

Genome-wide association studies (GWAS) have reproducibly associated variants within introns of FTO with increased risk for obesity and type 2 diabetes (T2D). Although the molecular mechanisms linking these noncoding variants with obesity are not immediately obvious, subsequent studies in mice demonstrated that FTO expression levels influence body mass and composition phenotypes. However, no direct connection between the obesity-associated variants and FTO expression or function has been made. Here we show that the obesity-associated noncoding sequences within FTO are functionally connected, at megabase distances, with the homeobox gene IRX3. The obesity-associated FTO region directly interacts with the promoters of IRX3 as well as FTO in the human, mouse and zebrafish genomes. Furthermore, long-range enhancers within this region recapitulate aspects of IRX3 expression, suggesting that the obesity-associated interval belongs to the regulatory landscape of IRX3. Consistent with this, obesity-associated single nucleotide polymorphisms are associated with expression of IRX3, but not FTO, in human brains. A direct link between IRX3 expression and regulation of body mass and composition is demonstrated by a reduction in body weight of 25 to 30% in Irx3-deficient mice, primarily through the loss of fat mass and increase in basal metabolic rate with browning of white adipose tissue. Finally, hypothalamic expression of a dominant-negative form of Irx3 reproduces the metabolic phenotypes of Irx3-deficient mice. Our data suggest that IRX3 is a functional long-range target of obesity-associated variants within FTO and represents a novel determinant of body mass and composition.

Pancreatic islet enhancer clusters enriched in type 2 diabetes risk-associated variants

Type 2 diabetes affects over 300 million people, causing severe complications and premature death, yet the underlying molecular mechanisms are largely unknown. Pancreatic islet dysfunction is central for type 2 diabetes pathogenesis, and therefore understanding islet genome regulation could provide valuable mechanistic insights. We have now mapped and examined the function of human islet cis-regulatory networks. We identify genomic sequences that are targeted by islet transcription factors to drive islet-specific gene activity, and show that most such sequences reside in clusters of enhancers that form physical 3D chromatin domains. We find that sequence variants associated with type 2 diabetes and fasting glycemia are enriched in these clustered islet enhancers, and identify trait-associated variants that disrupt DNA-binding and islet enhancer activity. Our studies illustrate how islet transcription factors interact functionally with the epigenome, and provide systematic evidence that dysregulation of islet enhancers is relevant to the mechanisms underlying type 2 diabetes.

Deep conservation of cis-regulatory elements in metazoans

Despite the vast morphological variation observed across phyla, animals share multiple basic developmental processes orchestrated by a common ancestral gene toolkit. These genes interact with each other building complex gene regulatory networks (GRNs), which are encoded in the genome by cis-regulatory elements (CREs) that serve as computational units of the network. Although GRN subcircuits involved in ancient developmental processes are expected to be at least partially conserved, identification of CREs that are conserved across phyla has remained elusive. Here, we review recent studies that revealed such deeply conserved CREs do exist, discuss the difficulties associated with their identification and describe new approaches that will facilitate this search.

The developmental epigenomics toolbox: ChIP-seq and MethylCap-seq profiling of early zebrafish embryos

Genome-wide profiling of DNA methylation and histone modifications answered many questions as to how the genes are regulated on a global scale and what their epigenetic makeup is. Yet, little is known about the function of these marks during early vertebrate embryogenesis. Here we provide detailed protocols for ChIP-seq and MethylCap-seq procedures applied to zebrafish (Danio rerio) embryonic material at four developmental stages. As a proof of principle, we have profiled on a global scale a number of post-translational histone modifications including H3K4me1, H3K4me3 and H3K27ac. We demonstrate that these marks are dynamic during early development and that such developmental transitions can be detected by ChIP-seq. In addition, we applied MethylCap-seq to show that developmentally-regulated DNA methylation remodeling can be detected by such a procedure. Our MethylCap-seq data concur with previous DNA methylation studies of early zebrafish development rendering this method highly suitable for the global assessment of DNA methylation in early vertebrate embryos.

Dynamics of enhancer chromatin signatures mark the transition from pluripotency to cell specification during embryogenesis

The generation of distinctive cell types that form different tissues and organs requires precise, temporal and spatial control of gene expression. This depends on specific cis-regulatory elements distributed in the noncoding DNA surrounding their target genes. Studies performed on mammalian embryonic stem cells and Drosophila embryos suggest that active enhancers form part of a defined chromatin landscape marked by histone H3 lysine 4 mono-methylation (H3K4me1) and histone H3 lysine 27 acetylation (H3K27ac). Nevertheless, little is known about the dynamics and the potential roles of these marks during vertebrate embryogenesis. Here, we provide genomic maps of H3K4me1/me3 and H3K27ac at four developmental time-points of zebrafish embryogenesis and analyze embryonic enhancer activity. We find that (1) changes in H3K27ac enrichment at enhancers accompany the shift from pluripotency to tissue-specific gene expression, (2) in early embryos, the peaks of H3K27ac enrichment are bound by pluripotent factors such as Nanog, and (3) the degree of evolutionary conservation is higher for enhancers that become marked by H3K27ac at the end of gastrulation, suggesting their implication in the establishment of the most conserved (phylotypic) transcriptome that is known to occur later at the pharyngula stage.

Extensive conservation of ancient microsynteny across metazoans due to cis-regulatory constraints

The order of genes in eukaryotic genomes has generally been assumed to be neutral, since gene order is largely scrambled over evolutionary time. Only a handful of exceptional examples are known, typically involving deeply conserved clusters of tandemly duplicated genes (e.g., Hox genes and histones). Here we report the first systematic survey of microsynteny conservation across metazoans, utilizing 17 genome sequences. We identified nearly 600 pairs of unrelated genes that have remained tightly physically linked in diverse lineages across over 600 million years of evolution. Integrating sequence conservation, gene expression data, gene function, epigenetic marks, and other genomic features, we provide extensive evidence that many conserved ancient linkages involve (1) the coordinated transcription of neighboring genes, or (2) genomic regulatory blocks (GRBs) in which transcriptional enhancers controlling developmental genes are contained within nearby bystander genes. In addition, we generated ChIP-seq data for key histone modifications in zebrafish embryos, which provided further evidence of putative GRBs in embryonic development. Finally, using chromosome conformation capture (3C) assays and stable transgenic experiments, we demonstrate that enhancers within bystander genes drive the expression of genes such as Otx and Islet, critical regulators of central nervous system development across bilaterians. These results suggest that ancient genomic functional associations are far more common than previously thought—involving ∼12% of the ancestral bilaterian genome—and that cis-regulatory constraints are crucial in determining metazoan genome architecture.

Identification and analysis of conserved cis-regulatory regions of the MEIS1 gene

Meis1, a conserved transcription factor of the TALE-homeodomain class, is expressed in a wide variety of tissues during development. Its complex expression pattern is likely to be controlled by an equally complex regulatory landscape. Here we have scanned the Meis1 locus for regulatory elements and found 13 non-coding regions, highly conserved between humans and teleost fishes, that have enhancer activity in stable transgenic zebrafish lines. All these regions are syntenic in most vertebrates. The composite expression of all these enhancer elements recapitulate most of Meis1 expression during early embryogenesis, indicating they comprise a basic set of regulatory elements of the Meis1 gene. Using bioinformatic tools, we identify a number of potential binding sites for transcription factors that are compatible with the regulation of these enhancers. Specifically, HHc2:066650, which is expressed in the developing retina and optic tectum, harbors several predicted Pax6 sites. Biochemical, functional and transgenic assays indicate that pax6 genes directly regulate HHc2:066650 activity.

An evolutionarily conserved three-dimensional structure in the vertebrate Irx clusters facilitates enhancer sharing and coregulation

Developmental gene clusters are paradigms for the study of gene regulation; however, the mechanisms that mediate phenomena such as coregulation and enhancer sharing remain largely elusive. Here we address this issue by analysing the vertebrate Irx clusters. We first present a deep enhancer screen of a 2-Mbp span covering the IrxA cluster. Using chromosome conformation capture, we show that enhancer sharing is widespread within the cluster, explaining its evolutionarily conserved organization. We also identify a three-dimensional architecture, probably formed through interactions with CCCTC-binding factor, which is present within both Irx clusters of mouse, Xenopus and zebrafish. This architecture brings the promoters of the first two genes together in the same chromatin landscape. We propose that this unique and evolutionarily conserved genomic architecture of the vertebrate Irx clusters is essential for the coregulation of the first two genes and simultaneously maintains the third gene in a partially independent regulatory landscape.

Genome-wide profiling of p63 DNA-binding sites identifies an element that regulates gene expression during limb development in the 7q21 SHFM1 locus

Heterozygous mutations in p63 are associated with split hand/foot malformations (SHFM), orofacial clefting, and ectodermal abnormalities. Elucidation of the p63 gene network that includes target genes and regulatory elements may reveal new genes for other malformation disorders. We performed genome-wide DNA–binding profiling by chromatin immunoprecipitation (ChIP), followed by deep sequencing (ChIP–seq) in primary human keratinocytes, and identified potential target genes and regulatory elements controlled by p63. We show that p63 binds to an enhancer element in the SHFM1 locus on chromosome 7q and that this element controls expression of DLX6 and possibly DLX5, both of which are important for limb development. A unique micro-deletion including this enhancer element, but not the DLX5/DLX6 genes, was identified in a patient with SHFM. Our study strongly indicates disruption of a non-coding cis-regulatory element located more than 250 kb from the DLX5/DLX6 genes as a novel disease mechanism in SHFM1. These data provide a proof-of-concept that the catalogue of p63 binding sites identified in this study may be of relevance to the studies of SHFM and other congenital malformations that resemble the p63-associated phenotypes.

Mammalian embryonic development requires precise control of gene expression in the right place at the right time. One level of control of gene expression is through cis-regulatory elements controlled by transcription factors. Deregulation of gene expression by mutations in such cis-regulatory elements has been described in developmental disorders. Heterozygous mutations in the transcription factor p63 are found in patients with limb malformations, cleft lip/palate, and defects in skin and other epidermal appendages, through disruption of normal ectodermal development during embryogenesis. We reasoned that the identification of target genes and cis-regulatory elements controlled by p63 would provide candidate genes for defects arising from abnormally regulated ectodermal development. To test our hypothesis, we carried out a genome-wide binding site analysis and identified a large number of target genes and regulatory elements regulated by p63. We further showed that one of these regulatory elements controls expression of DLX6 and possibly DLX5 in the apical ectodermal ridge in the developing limbs. Loss of this element through a micro-deletion was associated with split hand foot malformation (SHFM1). The list of p63 binding sites provides a resource for the identification of mutations that cause ectodermal dysplasias and malformations in humans.

A chronically implantable device for the controlled delivery of substances, and stimulation and recording of activity in severed nerves

We describe the use of an implantable device for peripheral nerves that allows chronic simultaneous delivery of small volumes of solution, recording of both field and multiunit potentials, and electrical stimulation. This custom-made multifunctional device was attached to the cut end of the abducens (VIth) nerve for stimulation, recording and injection purposes. Our device consists of a polyethylene chamber with two electrodes that can be used for stimulation and recording and two Teflon tubes that serve as inlet and outlet for administering chemicals to the nerve fitted inside. Since the device is implanted in a retro-orbital position, we herein will refer to it as an intraorbitary device (IOD). The applicability of the IOD is demonstrated with an electrophysiological and anatomical account of the properties of the abducens nerve. Furthermore, it is shown that certain neuronal discharge properties can be inferred from the nerve recordings. The IOD can also be efficiently used for the delivery of small volume of pharmacological substances or conventional retrograde markers.

Cracking the genome's second code: enhancer detection by combined phylogenetic footprinting and transgenic fish and frog embryos

Genes involved in vertebrate development are unusually enriched for highly conserved non-coding sequence elements. These regions are readily detected in silico, by genome-wide sequence comparisons between different vertebrates, from mammals to fish (phylogenetic footprinting). It follows that sequence conservation must be the result of positive selection for an essential physiological role. An obvious possibility is that these conserved sequences possess regulatory or structural functions important for gene expression and, thus, an in vivo assay becomes necessary. We have developed a rapid testing system using zebrafish and Xenopus laevis embryos that allows us to assign transcriptional regulatory functions to conserved non-coding sequence elements. The sequences are cloned into a vector containing a minimal promoter and the GFP reporter, and are assayed for their putative cis-regulatory activity in zebrafish or Xenopus transgenic experiments. Vectors used include plasmid DNA and the Tol2 transposon system in fish and X. laevis. We have followed this logic to detect and analyze conserved elements in an intergenic region present in the Iroquois (Irx) gene clusters of zebrafish, Xenopus tropicalis, Fugu rubripes and mouse. We have assayed approximately 50 of these conserved elements and shown that the majority behave as modular positive regulatory elements (enhancers) that contribute to specific temporal and spatial domains that are part of the endogenous gene expression pattern. Moreover, comparison of the activity of cognate Irx enhancers from different organisms demonstrates that conservation of sequence is accompanied by in vivo functional conservation across species. Finally, for some of the most conserved elements, we have been able to identify a critical core sequence, essential for correct enhancer function.

Odd-skipped genes encode repressors that control kidney development

Odd-skipped family of proteins (Odd in Drosophila and Osr in vertebrates) are evolutionarily conserved zinc finger transcription factors. Two Osr genes are present in mammalian genomes, and it was recently reported that Osr1, but not Osr2, is required for murine kidney development. Here, we show that in Xenopus and zebrafish both Osr1 and Osr2 are necessary and sufficient for the development of the pronephros. Osr genes are expressed in early prospective pronephric territories, and morphants for either of the two genes show severely impaired kidney development. Conversely, overexpression of Osr genes promotes formation of ectopic kidney tissue. Molecularly, Osr proteins function as transcriptional repressors during kidney formation. We also show that Drosophila Odd induces kidney tissue in Xenopus. This might be accomplished through recruitment of Groucho-like co-repressors. Odd genes may also be required for proper development of the Malpighian tubules, the Drosophila renal organs. Our results highlight the evolutionary conserved involvement of Odd-skipped transcription factors in the development of kidneys.

A functional survey of the enhancer activity of conserved non-coding sequences from vertebrate Iroquois cluster gene deserts

Recent studies of the genome architecture of vertebrates have uncovered two unforeseen aspects of its organization. First, large regions of the genome, called gene deserts, are devoid of protein-coding sequences and have no obvious biological role. Second, comparative genomics has highlighted the existence of an array of highly conserved non-coding regions (HCNRs) in all vertebrates. Most surprisingly, these structural features are strongly associated with genes that have essential functions during development. Among these, the vertebrate Iroquois (Irx) genes stand out on both fronts. Mammalian Irx genes are organized in two clusters (IrxA and IrxB) that span >1 Mb each with no other genes interspersed. Additionally, a large number of HCNRs exist within Irx clusters. We have systematically examined the enhancer activity of HCNRs from the IrxB cluster using transgenic Xenopus and zebrafish embryos. Most of these HCNRs are active in subdomains of endogenous Irx expression, and some are candidates to contain shared enhancers of neighboring genes, which could explain the evolutionary conservation of Irx clusters. Furthermore, HCNRs present in tetrapod IrxB but not in fish may be responsible for novel Irx expression domains that appeared after their divergence. Finally, we have performed a more detailed analysis on two IrxB ultraconserved non-coding regions (UCRs) duplicated in IrxA clusters in similar relative positions. These four regions share a core region highly conserved among all of them and drive expression in similar domains. However, inter-species conserved sequences surrounding the core, specific for each of these UCRs, are able to modulate their expression.

Cerebellar grafting in the oculomotor system as a model to study target influence on adult neurons

In the last decades, there have been many efforts directed to gain a better understanding on adult neuron-target cell relationships. Embryonic grafts have been used for the study of neural circuit rewiring. Thus, using several donor neuronal tissues, such as cerebellum or striatum, developing grafted cells have been shown to have the capability of substituting neural cell populations and establishing reciprocal connections with the host. In addition, different lesion paradigms have also led to a better understanding of target dependence in neuronal cells. Thus, for example, axotomy induces profound morphofunctional changes in adult neurons, including the loss of synaptic inputs and discharge alterations. These alterations are probably due to trophic factor loss in response to target disconnection. In this review, we summarize the different strategies performed to disconnect neurons from their targets, and the effects of target substitution, performed by tissue grafting, upon neural properties. Using the oculomotor system-and more precisely the abducens internuclear neurons-as a model, we describe herein the effects of disconnecting a population of central neurons from its natural target (i.e., the medial rectus motoneurons at the mesencephalic oculomotor nucleus). We also analyze target-derived influences in the structure and physiology of these neurons by using cerebellar embryonic grafts as a new target for the axotomized abducens internuclear neurons.