cis-Regulatory activity of randomly chosen genomic fragments from the sea urchin

An optimized Tet-On system for conditional control of gene expression in sea urchins

Sea urchins and other echinoderms are important experimental models for studying developmental processes. The lack of approaches for conditional gene perturbation, however, has made it challenging to investigate the late developmental functions of genes that have essential roles during early embryogenesis and genes that have diverse functions in multiple tissues. The doxycycline-controlled Tet-On system is a widely used molecular tool for temporally and spatially regulated transgene expression. Here, we optimized the Tet-On system to conditionally induce gene expression in sea urchin embryos. Using this approach, we explored the roles the MAPK signaling plays in skeletogenesis by expressing genes that perturb the pathway specifically in primary mesenchyme cells during later stages of development. We demonstrated the wide utility of the Tet-On system by applying it to a second sea urchin species and in cell types other than the primary mesenchyme cells. Our work provides a robust and flexible platform for the spatiotemporal regulation of gene expression in sea urchins, which will considerably enhance the utility of this prominent model system.

Architecture and evolution of the cis-regulatory system of the echinoderm kirrelL gene

The gene regulatory network (GRN) that underlies echinoderm skeletogenesis is a prominent model of GRN architecture and evolution. KirrelL is an essential downstream effector gene in this network and encodes an Ig-superfamily protein required for the fusion of skeletogenic cells and the formation of the skeleton. In this study, we dissected the transcriptional control region of the kirrelL gene of the purple sea urchin, Strongylocentrotus purpuratus. Using plasmid- and bacterial artificial chromosome-based transgenic reporter assays, we identified key cis-regulatory elements (CREs) and transcription factor inputs that regulate Sp-kirrelL, including direct, positive inputs from two key transcription factors in the skeletogenic GRN, Alx1 and Ets1. We next identified kirrelL cis-regulatory regions from seven other echinoderm species that together represent all classes within the phylum. By introducing these heterologous regulatory regions into developing sea urchin embryos we provide evidence of their remarkable conservation across ~500 million years of evolution. We dissected in detail the kirrelL regulatory region of the sea star, Patiria miniata, and demonstrated that it also receives direct inputs from Alx1 and Ets1. Our findings identify kirrelL as a component of the ancestral echinoderm skeletogenic GRN. They support the view that GRN subcircuits, including specific transcription factor–CRE interactions, can remain stable over vast periods of evolutionary history. Lastly, our analysis of kirrelL establishes direct linkages between a developmental GRN and an effector gene that controls a key morphogenetic cell behavior, cell–cell fusion, providing a paradigm for extending the explanatory power of GRNs.

Global patterns of enhancer activity during sea urchin embryogenesis assessed by eRNA profiling

We used capped analysis of gene expression with sequencing (CAGE-seq) to profile eRNA expression and enhancer activity during embryogenesis of a model echinoderm: the sea urchin, Strongylocentrotus purpuratus. We identified more than 18,000 enhancers that were active in mature oocytes and developing embryos and documented a burst of enhancer activation during cleavage and early blastula stages. We found that a large fraction (73.8%) of all enhancers active during the first 48 h of embryogenesis were hyperaccessible no later than the 128-cell stage and possibly even earlier. Most enhancers were located near gene bodies, and temporal patterns of eRNA expression tended to parallel those of nearby genes. Furthermore, enhancers near lineage-specific genes contained signatures of inputs from developmental gene regulatory networks deployed in those lineages. A large fraction (60%) of sea urchin enhancers previously shown to be active in transgenic reporter assays was associated with eRNA expression. Moreover, a large fraction (50%) of a representative subset of enhancers identified by eRNA profiling drove tissue-specific gene expression in isolation when tested by reporter assays. Our findings provide an atlas of developmental enhancers in a model sea urchin and support the utility of eRNA profiling as a tool for enhancer discovery and regulatory biology. The data generated in this study are available at Echinobase, the public database of information related to echinoderm genomics.

Developmental effector gene regulation: Multiplexed strategies for functional analysis

The staggering complexity of the genome controls for developmental processes is revealed through massively parallel cis-regulatory analysis using new methods of perturbation and readout. The choice of combinations of these new methods is tailored to the system, question and resources at hand. Our focus is on issues that include the necessity or sufficiency of given cis-regulatory modules, cis-regulatory function in the normal spatial genomic context, and easily accessible high throughput and multiplexed analysis methods. In the sea urchin embryonic model, recombineered BACs offer new opportunities for consecutive modes of cis-regulatory analyses that answer these requirements, as we here demonstrate on a diverse suite of previously unstudied sea urchin effector genes expressed in skeletogenic cells. Positively active cis-regulatory modules were located in single Nanostring experiments per BAC containing the gene of interest, by application of our previously reported "barcode" tag vectors of which> 100 can be analyzed at one time. Computational analysis of DNA sequences that drive expression, based on the known skeletogenic regulatory state, then permitted effective identification of functional target site clusters. Deletion of these sub-regions from the parent BACs revealed module necessity, as simultaneous tests of the same regions in short constructs revealed sufficiency. Predicted functional inputs were then confirmed by site mutations, all generated and tested in multiplex formats. There emerged the simple conclusion that each effector gene utilizes a small subset of inputs from the skeletogenic GRN. These inputs may function to only adjust expression levels or in some cases necessary for expression. Since we know the GRN architecture upstream of the effector genes, we could then conceptually isolate and compare the wiring of the effector gene driver sub-circuits and identify the inputs whose removal abolish expression.

Global analysis of primary mesenchyme cell cis-regulatory modules by chromatin accessibility profiling

Background

The developmental gene regulatory network (GRN) that underlies skeletogenesis in sea urchins and other echinoderms is a paradigm of GRN structure, function, and evolution. This transcriptional network is deployed selectively in skeleton-forming primary mesenchyme cells (PMCs) of the early embryo. To advance our understanding of this model developmental GRN, we used genome-wide chromatin accessibility profiling to identify and characterize PMC cis-regulatory modules (CRMs).

Results

ATAC-seq (Assay for Transposase-Accessible Chromatin using sequencing) analysis of purified PMCs provided a global picture of chromatin accessibility in these cells. We used both ATAC-seq and DNase-seq (DNase I hypersensitive site sequencing) to identify > 3000 sites that exhibited increased accessibility in PMCs relative to other embryonic cell lineages, and provide both computational and experimental evidence that a large fraction of these sites represent bona fide skeletogenic CRMs. Putative PMC CRMs were preferentially located near genes differentially expressed by PMCs and consensus binding sites for two key transcription factors in the PMC GRN, Alx1 and Ets1, were enriched in these CRMs. Moreover, a high proportion of candidate CRMs drove reporter gene expression specifically in PMCs in transgenic embryos. Surprisingly, we found that PMC CRMs were partially open in other embryonic lineages and exhibited hyperaccessibility as early as the 128-cell stage.

Conclusions

Our work provides a comprehensive picture of chromatin accessibility in an early embryonic cell lineage. By identifying thousands of candidate PMC CRMs, we significantly enhance the utility of the sea urchin skeletogenic network as a general model of GRN architecture and evolution. Our work also shows that differential chromatin accessibility, which has been used for the high-throughput identification of enhancers in differentiated cell types, is a powerful approach for the identification of CRMs in early embryonic cells. Lastly, we conclude that in the sea urchin embryo, CRMs that control the cell type-specific expression of effector genes are hyperaccessible several hours in advance of gene activation.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4542-z) contains supplementary material, which is available to authorized users.

Evolution of lineage-specific functions in ancient cis-regulatory modules

Morphological evolution is driven both by coding sequence variation and by changes in regulatory sequences. However, how cis-regulatory modules (CRMs) evolve to generate entirely novel expression domains is largely unknown. Here, we reconstruct the evolutionary history of a lens enhancer located within a CRM that not only predates the lens, a vertebrate innovation, but bilaterian animals in general. Alignments of orthologous sequences from different deuterostomes sub-divide the CRM into a deeply conserved core and a more divergent flanking region. We demonstrate that all deuterostome flanking regions, including invertebrate sequences, activate gene expression in the zebrafish lens through the same ancient cluster of activator sites. However, levels of gene expression vary between species due to the presence of repressor motifs in flanking region and core. These repressor motifs are responsible for the relatively weak enhancer activity of tetrapod flanking regions. Ray-finned fish, however, have gained two additional lineage-specific activator motifs which in combination with the ancient cluster of activators and the core constitute a potent lens enhancer. The exploitation and modification of existing regulatory potential in flanking regions but not in the highly conserved core might represent a more general model for the emergence of novel regulatory functions in complex CRMs.

Cis-regulatory control of the nuclear receptor Coup-TF gene in the sea urchin Paracentrotus lividus embryo

Coup-TF, an orphan member of the nuclear receptor super family, has a fundamental role in the development of metazoan embryos. The study of the gene's regulatory circuit in the sea urchin embryo will facilitate the placement of this transcription factor in the well-studied embryonic Gene Regulatory Network (GRN). The Paracentrotus lividus Coup-TF gene (PlCoup-TF) is expressed throughout embryonic development preferentially in the oral ectoderm of the gastrula and the ciliary band of the pluteus stage. Two overlapping λ genomic clones, containing three exons and upstream sequences of PlCoup-TF, were isolated from a genomic library. The transcription initiation site was determined and 5′ deletions and individual segments of a 1930 bp upstream region were placed ahead of a GFP reporter cassette and injected into fertilized P.lividus eggs. Module a (−532 to −232), was necessary and sufficient to confer ciliary band expression to the reporter. Comparison of P.lividus and Strongylocentrotus purpuratus upstream Coup-TF sequences, revealed considerable conservation, but none within module a. 5′ and internal deletions into module a, defined a smaller region that confers ciliary band specific expression. Putative regulatory cis-acting elements (RE1, RE2 and RE3) within module a, were specifically bound by proteins in sea urchin embryonic nuclear extracts. Site-specific mutagenesis of these elements resulted in loss of reporter activity (RE1) or ectopic expression (RE2, RE3). It is proposed that sea urchin transcription factors, which bind these three regulatory sites, are necessary for spatial and quantitative regulation of the PlCoup-TF gene at pluteus stage sea urchin embryos. These findings lead to the future identification of these factors and to the hierarchical positioning of PlCoup-TF within the embryonic GRN.

Developmental cis-regulatory analysis of the cyclin D gene in the sea urchin Strongylocentrotus purpuratus

Cyclin D genes regulate the cell cycle, growth and differentiation in response to intercellular signaling. While the promoters of vertebrate cyclin D genes have been analyzed, the cis-regulatory sequences across an entire cyclin D locus have not. Doing so would increase understanding of how cyclin D genes respond to the regulatory states established by developmental gene regulatory networks, linking cell cycle and growth control to the ontogenetic program. Therefore, we conducted a cis-regulatory analysis on the cyclin D gene, SpcycD, of the sea urchin, Strongylocentrotus purpuratus, during embryogenesis, identifying upstream and intronic sequences, located within six defined regions bearing one or more cis-regulatory modules each.

Enhancers

Transcription of eukaryotic genes is an exceedingly sophisticated and complicated process, orchestrated by layers of control mechanisms involving a myriad of transcription factors and DNA control sequences, with both groups subject to multiple modifications. The availability of various recent genomic approaches has provided previously unforeseen opportunities to examine the cis-regulatory landscape of the entire genome, resulting in the identification of a potentially overwhelming number of enhancers and novel enhancer functions. In this review, we focus on the activities of enhancers in metazoans and discuss how they serve to regulate gene expression during early development.

When needles look like hay: how to find tissue-specific enhancers in model organism genomes

A major prerequisite for the investigation of tissue-specific processes is the identification of cis-regulatory elements. No generally applicable technique is available to distinguish them from any other type of genomic non-coding sequence. Therefore, researchers often have to identify these elements by elaborate in vivo screens, testing individual regions until the right one is found. Here, based on many examples from the literature, we summarize how functional enhancers have been isolated from other elements in the genome and how they have been characterized in transgenic animals. Covering computational and experimental studies, we provide an overview of the global properties of cis-regulatory elements, like their specific interactions with promoters and target gene distances. We describe conserved non-coding elements (CNEs) and their internal structure, nucleotide composition, binding site clustering and overlap, with a special focus on developmental enhancers. Conflicting data and unresolved questions on the nature of these elements are highlighted. Our comprehensive overview of the experimental shortcuts that have been found in the different model organism communities and the new field of high-throughput assays should help during the preparation phase of a screen for enhancers. The review is accompanied by a list of general guidelines for such a project.

Regulation of Ubx expression by epigenetic enhancer silencing in response to Ubx levels and genetic variation

For gene products that must be present in cells at defined concentrations, expression levels must be tightly controlled to ensure robustness against environmental, genetic, and developmental noise. By studying the regulation of the concentration-sensitive Drosophila melanogaster Hox gene Ultrabithorax (Ubx), we found that Ubx enhancer activities respond to both increases in Ubx levels and genetic background. Large, transient increases in Ubx levels are capable of silencing all enhancer input into Ubx transcription, resulting in the complete silencing of this gene. Small increases in Ubx levels, brought about by duplications of the Ubx locus, cause sporadic silencing of subsets of Ubx enhancers. Ubx enhancer silencing can also be induced by outcrossing laboratory stocks to D. melanogaster strains established from wild flies from around the world. These results suggest that enhancer activities are not rigidly determined, but instead are sensitive to genetic background. Together, these findings suggest that enhancer silencing may be used to maintain gene product levels within the correct range in response to natural genetic variation.

Gene expression is generally governed by cis-regulatory elements, also called enhancers. For genes whose expression levels must be tightly controlled, enhancer activities must be tightly regulated. In this work, we show that enhancers that control the expression of the Hox gene Ultrabithorax (Ubx) in Drosophila are regulated by a negative autoregulatory feedback mechanism. Negative autoregulation can be triggered by less than a two-fold increase in Ubx levels or by varying the genetic background. Together, these data reveal that enhancer activities are not always hardwired, but instead may be sensitive to genetic and environmental variation and, in some cases, to the amount of gene product they regulate. The finding that enhancers are sensitive to genetic background suggests that the regulation of gene expression is more plastic than previously thought and has important implications for how transcription is controlled in vivo.

The cis-regulatory system of the tbrain gene: Alternative use of multiple modules to promote skeletogenic expression in the sea urchin embryo

The genomic cis-regulatory systems controlling regulatory gene expression usually include multiple modules. The regulatory output of such systems at any given time depends on which module is directing the function of the basal transcription apparatus, and ultimately on the transcription factor inputs into that module. Here we examine regulation of the Strongylocentrotus purpuratus tbrain gene, a required activator of the skeletogenic specification state in the lineage descendant from the embryo micromeres. Alternate cis-regulatory modules were found to convey skeletogenic expression in reporter constructs. To determine their relative developmental functions in context, we made use of recombineered BAC constructs containing a GFP reporter and of derivatives from which specific modules had been deleted. The outputs of the various constructs were observed spatially by GFP fluorescence and quantitatively over time by QPCR. In the context of the complete genomic locus, early skeletogenic expression is controlled by an intron enhancer plus a proximal region containing a HesC site as predicted from network analysis. From ingression onward, however, a dedicated distal module utilizing positive Ets1/2 inputs contributes to definitive expression in the skeletogenic mesenchyme. This module also mediates a newly discovered negative Erg input which excludes non-skeletogenic mesodermal expression.

Compositional genome contexts affect gene expression control in sea urchin embryo

Gene expression is widely perceived as exclusively controlled by the information contained in cis-regulatory regions. These are built in a modular way, each module being a cluster of binding sites for the transcription factors that control the level, the location and the time at which gene transcription takes place. On the other hand, results from our laboratory have shown that gene expression is affected by the compositional properties (GC levels) of the isochores in which genes are embedded, i.e. the genome context. To clarify how compositional genomic properties affect the way cis-regulatory information is utilized, we have changed the genome context of a GFP-reporter gene containing the complete cis-regulatory region of the gene spdeadringer (spdri), expressed during sea urchin embryogenesis. We have observed that GC levels higher or lower than those found in the natural genome context can alter the reporter expression pattern. We explain this as the result of an interference with the functionality of specific modules in the gene's cis-regulatory region. From these observations we derive the notion that the compositional properties of the genome context can affect cis-regulatory control of gene expression. Therefore although the way a gene works depends on the information contained in its cis-regulatory region, availability of such information depends on the compositional properties of the genomic context.

Runx expression is mitogenic and mutually linked to Wnt activity in blastula-stage sea urchin embryos

Background

The Runt homology domain (Runx) defines a metazoan family of sequence-specific transcriptional regulatory proteins that are critical for animal development and causally associated with a variety of mammalian cancers. The sea urchin Runx gene SpRunt-1 is expressed throughout the blastula stage embryo, and is required globally during embryogenesis for cell survival and differentiation.

Methodology/Principal Findings

Depletion of SpRunt-1 by morpholino antisense-mediated knockdown causes a blastula stage deficit in cell proliferation, as shown by bromodeoxyuridine (BrdU) incorporation and direct cell counts. Reverse transcription coupled polymerase chain reaction (RT-PCR) studies show that the cell proliferation deficit is presaged by a deficit in the expression of several zygotic wnt genes, including wnt8, a key regulator of endomesoderm development. In addition, SpRunt-1-depleted blastulae underexpress cyclinD, an effector of mitogenic Wnt signaling. Blastula stage cell proliferation is also impeded by knockdown of either wnt8 or cyclinD. Chromatin immunoprecipitation (ChIP) indicates that Runx target sites within 5′ sequences flanking cyclinD, wnt6 and wnt8 are directly bound by SpRunt-1 protein at late blastula stage. Furthermore, experiments using a green fluorescent protein (GFP) reporter transgene show that the blastula-stage operation of a cis-regulatory module previously shown to be required for wnt8 expression (Minokawa et al., Dev. Biol. 288: 545–558, 2005) is dependent on its direct sequence-specific interaction with SpRunt-1. Finally, inhibitor studies and immunoblot analysis show that SpRunt-1 protein levels are negatively regulated by glycogen synthase kinase (GSK)-3.

Conclusions/Significance

These results suggest that Runx expression and Wnt signaling are mutually linked in a feedback circuit that controls cell proliferation during development.

The surprising complexity of the transcriptional regulation of the spdri gene reveals the existence of new linkages inside sea urchin's PMC and Oral Ectoderm Gene Regulatory Networks

During sea urchin embryogenesis the spdri gene participates in two separate Gene Regulatory Networks (GRNs): the Primary Mesenchyme Cells' (PMCs) and the Oral Ectoderm's one. In both cases, activation of the gene follows initial specification events [Amore, G., Yavrouian, R., Peterson, K., Ransick, A., McClay, D., Davidson, E., 2003. Spdeadringer, a sea urchin embryo gene required separately in skeletogenic and oral ectoderm gene regulatory networks. Dev. Biol. 261, 55-81.]. We identified a portion of genomic DNA ("4.7IL" -3456;+389) which is sufficient to replicate sdpri's expression pattern in experiments of transgenesis, using a GFP reporter. In our experiments, the activation kinetic of 4.7IL-GFP was similar to that of the endogenous gene and the reporter responded to known spdri's transcriptional regulators (Ets1, Alx1, Gsc and Dri). Here we present a dissection of this regulatory region and a description of the modules involved in spdri's transcriptional regulation. Both in the PMCs' and Oral Ectoderm's expression phases, activation of spdri is obtained through the integration of three kinds of inputs: positive and globally distributed ones; negative ones (that prevent ectopic expression); positive and tissue-specific ones. Our results allow to expand the map of the regulatory connections at the spdri node, both in the PMCs and in the Oral Ectoderm Gene Regulatory Networks (GRNs).

Caught in the evolutionary act: precise cis-regulatory basis of difference in the organization of gene networks of sea stars and sea urchins

The regulatory control of otxbeta1/2 in the sea urchin Strongylocentrotus purpuratus and the sea star Asterina miniata provides an exceptional opportunity to determine the genomic basis of evolutionary change in gene regulatory network (GRN) architectures. Network perturbation analyses in both taxa show that Otx regulates the transcription factors gatae and krox/blimp1 and both of these transcription factors also feed back and regulate otx. The otx gene also autoregulates. This three way interaction is an example of a GRN kernel. It has been conserved for 500 million years since these two taxa last shared a common ancestor. Amid this high level of conservation we show here one significant regulatory change. Tbrain is required for correct otxbeta1/2 expression in the sea star but not in the sea urchin. In sea urchin, tbrain is not co-expressed with otxbeta1/2 and instead has an essential role in specification of the embryonic skeleton. Tbrain in these echinoderms is thus a perfect example of an orthologous gene co-opted for entirely different developmental processes. We isolate and test the sea star otxbeta1/2 cis-regulatory module and demonstrate functional binding sites for each of the predicted inputs, including Tbrain. We compare it to the logic processing operating in the sea urchin otxbeta1/2 cis-regulatory module and present an evolutionary scenario of the change in Tbrain dependence. Finally, inter-specific gene transfer experiments confirm this scenario and demonstrate evolution occurring at the level of sequence changes to the cis-regulatory module.

Expression and function of blimp1/krox, an alternatively transcribed regulatory gene of the sea urchin endomesoderm network

The blimp1/krox gene of Strongylocentrotus purpuratus, formerly krox1, encodes zinc finger transcription factors which play a central role in both early and late endomesoderm specification. Here we show that there are two alternative splice forms transcribed under the control of different regulatory regions. The blimp/krox1b form was previously unknown, and is the form expressed during cleavage, beginning 6-9 h postfertilization. This form is required for the early events of endomesoderm specification. A different splice variant, blimp1/krox1a, is expressed only from gastrula stage onward. During cleavage stages the blimp1/krox gene is expressed in the large micromeres and veg2 descendents. Soon after, it is expressed in the ring of specified mesoderm cells at the vegetal pole of the blastula. Its expression is later restricted to the blastopore region and the posterior of the invaginating archenteron, and finally to the midgut and hindgut of the pluteus larva. The expression of blimp1/krox is dynamic, and involves several distinct spatial territories. A GFP recombinant BAC was created by substituting the GFP coding sequence for that of the second exon (1b), in order to distinguish the expression pattern of the early form from that of the late form. This construct closely mimics blimp1/krox1b expression during early stages of sea urchin development. To expand our knowledge of the downstream linkages of this gene, additional experiments were carried out using antisense morpholino oligos (MASO). We confirmed previously published data that blimp1/krox autoregulates its own expression, but discovered, surprisingly, that this gene represses rather than activates itself. This negative autoregulation is restricted to the mesodermal and probably skeletogenic territories during the blastula stage, as shown by in situ hybridization analysis of MASO injected embryos. The MASO perturbation analysis also revealed blimp1/krox inputs into other genes of the endomesoderm regulatory network.

Regulation of spblimp1/krox1a, an alternatively transcribed isoform expressed in midgut and hindgut of the sea urchin gastrula

The sea urchin regulatory gene Spblimp1/krox produces alternatively transcribed and spliced isoforms, 1a and 1b, which have different temporal and spatial patterns of expression. Here we describe a cis-regulatory module that controls the expression of the 1a splice form in the midgut and hindgut at the beginning of gastrulation. Conserved sequence patches revealed by a comparison of the blimp1/krox locus in Strongylocentrotus purpuratus and Lytechinus variegatus genomes were tested by gene transfer, in association with GFP or CAT reporter genes. An expression construct containing a conserved sequence patch immediately 5' of exon 1a included the transcription initiation site for blimp1/krox1a. This construct displays specific mid and hindgut expression, indicating that these are the locations of endogenous blimp1/krox1a transcription during the gastrula stage. Its sequence contains binding sites for Brn1/2/4, Otx, and Blimp1/Krox itself, as predicted in a prior regulatory network analysis.

cis-Regulatory control of cyclophilin, a member of the ETS-DRI skeletogenic gene battery in the sea urchin embryo

The Strongylocentrotus purpuratus cyclophilin1 gene (Sp-cyp1) is expressed exclusively in skeletogenic mesenchyme cells of the embryo, beginning in the micromere lineage of the early blastula stage and continuing after gastrulation during the syncytial deposition of the skeleton. This gene encodes a protein which is a member of the peptidyl-prolyl cis-trans isomerase (PPIase) family. Sp-cyp1 is among the differentiation genes activated in the skeletogenic territory as a terminal function of the endomesodermal gene regulatory network. Network perturbation analysis had predicted the skeletogenic regulators Ets1 and Deadringer (Dri) to be its driver inputs. Here, we show that a 218-bp cis-regulatory DNA fragment recapitulates skeletogenic Sp-cyp1 expression; that elimination of either Ets1 or Dri inputs severely depresses the activity of expression constructs containing this DNA fragment; and that Ets1 and Dri target sites within the 218 bp fragment are required for normal expression. This indicates that the predicted inputs are direct. Other studies indicate that the same inputs are evidently necessary for expression of several other skeletogenic differentiation genes, and these genes probably constitute a skeletogenic gene battery, defined by its Ets plus Dri regulatory inputs.

Decoding cis-regulatory systems in ascidians

Ascidians, or sea squirts, are lower chordates, and share basic gene repertoires and many characteristics, both developmental and physiological, with vertebrates. Therefore, decoding cis-regulatory systems in ascidians will contribute toward elucidating the genetic regulatory systems underlying the developmental and physiological processes of vertebrates. cis-Regulatory DNAs can also be used for tissue-specific genetic manipulation, a powerful tool for studying ascidian development and physiology. Because the ascidian genome is compact compared with vertebrate genomes, both intergenic regions and introns are relatively small in ascidians. Short upstream intergenic regions contain a complete set of cis-regulatory elements for spatially regulated expression of a majority of ascidian genes. These features of the ascidian genome are a great advantage in identifying cis-regulatory sequences and in analyzing their functions. Function of cis-regulatory DNAs has been analyzed for a number of tissue-specific and developmentally regulated genes of ascidians by introducing promoter-reporter fusion constructs into ascidian embryos. The availability of the whole genome sequences of the two Ciona species, Ciona intestinalis and Ciona savignyi, facilitates comparative genomics approaches to identify cis-regulatory DNAs. Recent studies demonstrate that computational methods can help identify cis-regulatory elements in the ascidian genome. This review presents a comprehensive list of ascidian genes whose cis-regulatory regions have been subjected to functional analysis, and highlights the recent advances in bioinformatics and comparative genomics approaches to cis-regulatory systems in ascidians.

Computational discovery of DNA motifs associated with cell type-specific gene expression in Ciona

Temporally and spatially co-expressed genes are expected to be regulated by common transcription factors and therefore to share cis-regulatory elements. In the ascidian Ciona intestinalis, the whole-genome sequences and genome-scale gene expression profiles allow the use of computational techniques to investigate cis-elements that control transcription. We collected 5' flanking sequences of 50 tissue-specific genes from genome databases of C. intestinalis and a closely related species Ciona savignyi. We searched for DNA motifs over-represented in upstream regions of a group of co-expressed genes. Several motifs were distributed predominantly in upstream regions of photoreceptor, pan-neuronal, or muscle-specific gene groups. One muscle-specific motif, M2, was distributed preferentially in regions from -200 to -100 bp relative to the translational start sites. Promoters of muscle-specific genes of C. intestinalis were isolated, connected with a green fluorescent protein gene (GFP), and introduced into C. intestinalis embryos. In muscle cells, these promoters specifically drove GFP expression, which mutations of the M2 sites greatly reduced. When M2 sites were located upstream of a basal promoter, the reporter GFP was specifically expressed in muscle cells. These results suggest the validity of our computational prediction of cis-regulatory elements. Thus, bioinformatics can help identify cis-regulatory elements involved in chordate development.

R11: a cis-regulatory node of the sea urchin embryo gene network that controls early expression of SpDelta in micromeres

A gene regulatory network (GRN) controls the process by which the endomesoderm of the sea urchin embryo is specified. In this GRN, the program of gene expression unique to the skeletogenic micromere lineage is set in train by activation of the pmar1 gene. Through a double repression system, this gene is responsible for localization of expression of downstream regulatory and signaling genes to cells of this lineage. One of these genes, delta, encodes a Notch ligand, and its expression in the right place and time is crucial to the specification of the endomesoderm. Here we report a cis-regulatory element R11 that is responsible for localizing the expression of delta by means of its response to the pmar1 repression system. R11 was identified as an evolutionarily conserved genomic sequence located about 13 kb downstream of the last exon of the delta gene. We demonstrate here that this cis-regulatory element is able to drive the expression of a reporter gene in the same cells and at the same time that the endogenous delta gene is expressed, and that temporally, spatially, and quantitatively it responds to the pmar1 repression system just as predicted for the delta gene in the endomesoderm GRN. This work illustrates the application of cis-regulatory analysis to the validation of predictions of the GRN model. In addition, we introduce new methodological tools for quantitative measurement of the output of expression constructs that promise to be of general value for cis-regulatory analysis in sea urchin embryos.

Using reporter genes to study cis-regulatory elements

This chapter summarizes four powerful assays for analyzing gene expression in cis-regulatory studies. The enzymatic assays (CAT, luciferase, lacZ) are currently limited by their application to embryo homogenates or fixed samples, but offer more robust analysis of gene activity than GFP. Assays based on CAT enzymatic activity or on CAT mRNA detection by WMISH are laborious but are well established for accurately quantifying gene expression and to determine spatial patterns at defined timepoints during development. LacZ assays are the current standard for spatially visualizing gene products in whole-mount fixed embryos. They are very sensitive but they provide limited temporal or quantitative information due to the perdurance of beta-galactosidase and the subtleties of the staining technique. Recently developed luciferase assays promise to be even more sensitive and accurate than the CAT and lacZ assays, and applicable to living cells and embryos. But, they have not yet been well established in invertebrate deuterostome research. GFP allows visualization of gene expression within living embryos. But because this is not an enzymatic assay, sensitivity can be a problem, particularly for weak promoters. Furthermore, imaging live embryos and quantifying gene expression in space and time (due to scattering of light by tissue, the perdurance of GFP, and other experimental details) is currently fraught with challenges. Ongoing improvements in imaging technology and the advent of multiple fluorescent proteins, as well as fluorescent and luminescent assays for vital imaging, will dramatically facilitate studies of gene expression in the coming decade.