1
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Lin S, Lim B. Multifaceted effects on even-skipped transcriptional dynamics upon Krüppel dosage changes. Development 2024; 151:dev202132. [PMID: 38345298 PMCID: PMC10948998 DOI: 10.1242/dev.202132] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/27/2023] [Accepted: 02/08/2024] [Indexed: 03/05/2024]
Abstract
Although fluctuations in transcription factor (TF) dosage are often well tolerated, TF dosage modulation can change the target gene expression dynamics and result in significant non-lethal developmental phenotypes. Using MS2/MCP-mediated quantitative live imaging in early Drosophila embryos, we analyzed how changing levels of the gap gene Krüppel (Kr) affects transcriptional dynamics of the pair-rule gene even-skipped (eve). Halving the Kr dosage leads to a transient posterior expansion of the eve stripe 2 and an anterior shift of stripe 5. Surprisingly, the most significant changes are observed in eve stripes 3 and 4, the enhancers of which do not contain Kr-binding sites. In Kr heterozygous embryos, both stripes 3 and 4 display narrower widths, anteriorly shifted boundaries and reduced mRNA production levels. We show that Kr dosage indirectly affects stripe 3 and 4 dynamics by modulating other gap gene dynamics. We quantitatively correlate moderate body segment phenotypes of Kr heterozygotes with spatiotemporal changes in eve expression. Our results indicate that nonlinear relationships between TF dosage and phenotypes underlie direct TF-DNA and indirect TF-TF interactions.
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Affiliation(s)
- Shufan Lin
- Department of Bioengineering, University of Pennsylvania, Philadelphia, PA 19104, USA
| | - Bomyi Lim
- Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA
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2
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Kang CK, Kim AR. Deep molecular learning of transcriptional control of a synthetic CRE enhancer and its variants. iScience 2024; 27:108747. [PMID: 38222110 PMCID: PMC10784702 DOI: 10.1016/j.isci.2023.108747] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/20/2023] [Revised: 08/29/2023] [Accepted: 12/12/2023] [Indexed: 01/16/2024] Open
Abstract
Massively parallel reporter assay measures transcriptional activities of various cis-regulatory modules (CRMs) in a single experiment. We developed a thermodynamic computational model framework that calculates quantitative levels of gene expression directly from regulatory DNA sequences. Using the framework, we investigated the molecular mechanisms of cis-regulatory mutations of a synthetic enhancer that cause abnormal gene expression. We found that, in a human cell line, competitive binding between family transcription factors (TFs) with slightly different binding preferences significantly increases the accuracy of recapitulating the transcriptional effects of thousands of single- or multi-mutations. We also discovered that even if various harmful mutations occurred in an activator binding site, CRM could stably maintain or even increase gene expression through a certain form of competitive binding between family TFs. These findings enhance understanding the effect of SNPs and indels on CRMs and would help building robust custom-designed CRMs for biologics production and gene therapy.
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Affiliation(s)
- Chan-Koo Kang
- School of Life Science, Handong Global University, Pohang, Gyeong-Buk 37554, South Korea
- Department of Advanced Convergence, Handong Global University, Pohang, Gyeong-Buk 37554, South Korea
| | - Ah-Ram Kim
- School of Life Science, Handong Global University, Pohang, Gyeong-Buk 37554, South Korea
- Department of Advanced Convergence, Handong Global University, Pohang, Gyeong-Buk 37554, South Korea
- Computer Science and Artificial Intelligence Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
- School of Applied Artificial Intelligence, Handong Global University, Pohang, Gyeong-Buk 37554, South Korea
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3
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Masuda LHP, Sabino AU, Reinitz J, Ramos AF, Machado-Lima A, Andrioli LP. Global repression by tailless during segmentation. Dev Biol 2024; 505:11-23. [PMID: 37879494 PMCID: PMC10949167 DOI: 10.1016/j.ydbio.2023.09.014] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/30/2022] [Revised: 09/26/2023] [Accepted: 09/28/2023] [Indexed: 10/27/2023]
Abstract
The orphan nuclear receptor Tailless (Tll) exhibits conserved roles in brain formation and maintenance that are shared, for example, with vertebrate orthologous forms (Tlx). However, the early expression of tll in two gap domains in the segmentation cascade of Drosophila is unusual even for most other insects. Here we investigate tll regulation on pair-rule stripes. With ectopic misexpression of tll we detected unexpected repression of almost all pair-rule stripes of hairy (h), even-skipped (eve), runt (run), and fushi-tarazu (ftz). Examining Tll embryonic ChIP-chip data with regions mapped as Cis-Regulatory Modules (CRMs) of pair-rule stripes we verified Tll interactions to these regions. With the ChIP-chip data we also verified Tll interactions to the CRMs of gap domains and in the misexpression assay, Tll-mediated repression on Kruppel (Kr), kni (kni) and giant (gt) according to their differential sensitivity to Tll. These results with gap genes confirmed previous data from the literature and argue against indirect repression roles of Tll in the striped pattern. Moreover, the prediction of Tll binding sites in the CRMs of eve stripes and the mathematical modeling of their removal using an experimentally validated theoretical framework shows effects on eve stripes compatible with the absence of a repressor binding to the CRMs. In addition, modeling increased tll levels in the embryo results in the differential repression of eve stripes, agreeing well with the results of the misexpression assay. In genetic assays we investigated eve 5, that is strongly repressed by the ectopic domain and representative of more central stripes not previously implied to be under direct regulation of tll. While this stripe is little affected in tll-, its posterior border is expanded in gt- but detected with even greater expansion in gt-;tll-. We end up by discussing tll with key roles in combinatorial repression mechanisms to contain the expression of medial patterns of the segmentation cascade in the extremities of the embryo.
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Affiliation(s)
| | - Alan Utsuni Sabino
- Departamento de Radiologia e Oncologia, Instituto do Câncer do Estado de São Paulo, Hospital das Clínicas, Faculdade de Medicina, Universidade de São Paulo, São Paulo, Brazil
| | - John Reinitz
- Departments of Statistics, Ecology and Evolution, Molecular Genetics & Cell Biology, Institute of Genomics and Systems Biology, University of Chicago, Chicago, IL, USA
| | | | - Ariane Machado-Lima
- Escola de Artes, Ciências e Humanidades da Universidade de São Paulo, São Paulo, Brazil
| | - Luiz Paulo Andrioli
- Escola de Artes, Ciências e Humanidades da Universidade de São Paulo, São Paulo, Brazil.
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4
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Fischer MD, Graham P, Pick L. The ftz upstream element drives late ftz stripes but is not required for regulation of Ftz target genes. Dev Biol 2024; 505:141-147. [PMID: 37977522 PMCID: PMC10843599 DOI: 10.1016/j.ydbio.2023.11.004] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/10/2023] [Revised: 11/09/2023] [Accepted: 11/10/2023] [Indexed: 11/19/2023]
Abstract
The regulation of gene expression in precise, rapidly changing spatial patterns is essential for embryonic development. Multiple enhancers have been identified for the evolving expression patterns of the cascade of Drosophila segmentation genes that establish the basic body plan of the fly. Classic reporter transgene experiments identified multiple cis-regulatory elements (CREs) that are sufficient to direct various aspects of the evolving expression pattern of the pair-rule gene fushi tarazu (ftz). These include enhancers that coordinately activate expression in all seven stripes and stripe-specific elements that activate expression in one or more ftz stripes. Of the two 7-stripe enhancers, analysis of reporter transgenes demonstrated that the upstream element (UPS) is autoregulatory, requiring direct binding of Ftz protein to direct striped expression. Here, we asked about the endogenous role of the UPS by precisely deleting this 7-stripe enhancer. In ftzΔUPS7S homozygotes, ftz stripes appear in the same order as wildtype, and all but stripe 4 are expressed at wildtype levels by the end of the cellular blastoderm stage. This suggests that the zebra element and UPS harbor information to direct stripe 4 expression, although previous deletion analyses failed to identify a stripe-specific CRE within these two 7-stripe enhancers. However, the UPS is necessary for late ftz stripe expression, with all 7 stripes decaying earlier than wildtype in ftzΔUPS7S homozygotes. Despite this premature loss of ftz expression, downstream target gene regulation proceeds as in wildtype, and segmentation is unperturbed in the overwhelming majority of animals. We propose that this late-acting enhancer provides a buffer against perturbations in gene expression but is not required for establishment of Ftz cell fates. Overall, our results demonstrate that multiple enhancers, each directing distinct aspects of an overall gene expression pattern, contribute to fine-tuning the complex patterns necessary for embryonic development.
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Affiliation(s)
- Matthew D Fischer
- Department of Pathology and Laboratory Medicine, 3501 Civic Center Boulevard, Children's Hospital of Philadelphia, Philadelphia, PA, 19104, USA
| | - Patricia Graham
- Department of Entomology, 4291 Fieldhouse Drive, University of Maryland, College Park, MD, 20742, USA
| | - Leslie Pick
- Department of Entomology, 4291 Fieldhouse Drive, University of Maryland, College Park, MD, 20742, USA.
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5
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Haroush N, Levo M, Wieschaus EF, Gregor T. Functional analysis of the Drosophila eve locus in response to non-canonical combinations of gap gene expression levels. Dev Cell 2023; 58:2789-2801.e5. [PMID: 37890488 PMCID: PMC10872916 DOI: 10.1016/j.devcel.2023.10.001] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/03/2022] [Revised: 08/10/2023] [Accepted: 10/04/2023] [Indexed: 10/29/2023]
Abstract
Transcription factor combinations play a key role in shaping cellular identity. However, the precise relationship between specific combinations and downstream effects remains elusive. Here, we investigate this relationship within the context of the Drosophila eve locus, which is controlled by gap genes. We measure spatiotemporal levels of four gap genes in heterozygous and homozygous gap mutant embryos and correlate them with the striped eve activity pattern. Although changes in gap gene expression extend beyond the manipulated gene, the spatial patterns of Eve expression closely mirror canonical activation levels in wild type. Interestingly, some combinations deviate from the wild-type repertoire but still drive eve activation. Although in homozygous mutants some Eve stripes exhibit partial penetrance, stripes consistently emerge at reproducible positions, even with varying gap gene levels. Our findings suggest a robust molecular canalization of cell fates in gap mutants and provide insights into the regulatory constraints governing multi-enhancer gene loci.
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Affiliation(s)
- Netta Haroush
- Lewis-Sigler Institute for Integrative Genomics, Princeton University, Princeton, NJ 08544, USA
| | - Michal Levo
- Lewis-Sigler Institute for Integrative Genomics, Princeton University, Princeton, NJ 08544, USA
| | - Eric F Wieschaus
- Lewis-Sigler Institute for Integrative Genomics, Princeton University, Princeton, NJ 08544, USA; Department of Molecular Biology and Howard Hughes Medical Institute, Princeton University, Princeton, NJ 08544, USA
| | - Thomas Gregor
- Lewis-Sigler Institute for Integrative Genomics, Princeton University, Princeton, NJ 08544, USA; Joseph Henry Laboratories of Physics, Princeton University, Princeton, NJ 08544, USA; Department of Stem Cell and Developmental Biology, CNRS UMR3738 Paris Cité, Institut Pasteur, 75015 Paris, France.
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6
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Bishop TR, Onal P, Xu Z, Zheng M, Gunasinghe H, Nien CY, Small S, Datta RR. Multi-level regulation of even-skipped stripes by the ubiquitous factor Zelda. Development 2023; 150:dev201860. [PMID: 37934130 PMCID: PMC10730019 DOI: 10.1242/dev.201860] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/08/2023] [Accepted: 10/26/2023] [Indexed: 11/08/2023]
Abstract
The zinc-finger protein Zelda (Zld) is a key activator of zygotic transcription in early Drosophila embryos. Here, we study Zld-dependent regulation of the seven-striped pattern of the pair-rule gene even-skipped (eve). Individual stripes are regulated by discrete enhancers that respond to broadly distributed activators; stripe boundaries are formed by localized repressors encoded by the gap genes. The strongest effects of Zld are on stripes 2, 3 and 7, which are regulated by two enhancers in a 3.8 kb genomic fragment that includes the eve basal promoter. We show that Zld facilitates binding of the activator Bicoid and the gap repressors to this fragment, consistent with its proposed role as a pioneer protein. To test whether the effects of Zld are direct, we mutated all canonical Zld sites in the 3.8 kb fragment, which reduced expression but failed to phenocopy the abolishment of stripes caused by removing Zld in trans. We show that Zld also indirectly regulates the eve stripes by establishing specific gap gene expression boundaries, which provides the embryonic spacing required for proper stripe activation.
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Affiliation(s)
- Timothy R. Bishop
- Department of Biology, New York University, 100 Washington Square East, New York, NY 10003, USA
| | - Pinar Onal
- Department of Molecular Biology and Genetics, Ihsan Dogramaci Bilkent University, Universiteler Mahallesi, 06800 Ankara, Turkey
| | - Zhe Xu
- Department of Biology, New York University, 100 Washington Square East, New York, NY 10003, USA
| | - Michael Zheng
- Department of Biology, New York University, 100 Washington Square East, New York, NY 10003, USA
| | - Himari Gunasinghe
- Department of Biology, New York University, 100 Washington Square East, New York, NY 10003, USA
| | - Chung-Yi Nien
- Department of Life Sciences, National Central University, Taoyuan 32001, Taiwan
| | - Stephen Small
- Department of Biology, New York University, 100 Washington Square East, New York, NY 10003, USA
| | - Rhea R. Datta
- Department of Biology, Hamilton College, 198 College Hill Rd., Clinton, NY 13323, USA
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7
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Moudgil A, Sobti RC, Kaur T. In-silico identification and comparison of transcription factor binding sites cluster in anterior-posterior patterning genes in Drosophila melanogaster and Tribolium castaneum. PLoS One 2023; 18:e0290035. [PMID: 37590227 PMCID: PMC10434971 DOI: 10.1371/journal.pone.0290035] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/16/2023] [Accepted: 07/26/2023] [Indexed: 08/19/2023] Open
Abstract
The cis-regulatory data that help in transcriptional regulation is arranged into modular pieces of a few hundred base pairs called CRMs (cis-regulatory modules) and numerous binding sites for multiple transcription factors are prominent characteristics of these cis-regulatory modules. The present study was designed to localize transcription factor binding site (TFBS) clusters on twelve Anterior-posterior (A-P) genes in Tribolium castaneum and compare them to their orthologous gene enhancers in Drosophila melanogaster. Out of the twelve A-P patterning genes, six were gap genes (Kruppel, Knirps, Tailless, Hunchback, Giant, and Caudal) and six were pair rule genes (Hairy, Runt, Even-skipped, Fushi-tarazu, Paired, and Odd-skipped). The genes along with 20 kb upstream and downstream regions were scanned for TFBS clusters using the Motif Cluster Alignment Search Tool (MCAST), a bioinformatics tool that looks for set of nucleotide sequences for statistically significant clusters of non-overlapping occurrence of a given set of motifs. The motifs used in the current study were Hunchback, Caudal, Giant, Kruppel, Knirps, and Even-skipped. The results of the MCAST analysis revealed the maximum number of TFBS for Hunchback, Knirps, Caudal, and Kruppel in both D. melanogaster and T. castaneum, while Bicoid TFBS clusters were found only in D. melanogaster. The size of all the predicted TFBS clusters was less than 1kb in both insect species. These sequences revealed more transversional sites (Tv) than transitional sites (Ti) and the average Ti/Tv ratio was 0.75.
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Affiliation(s)
- Anshika Moudgil
- Department of Zoology, DAV University, Jalandhar, Punjab, India
| | | | - Tejinder Kaur
- Department of Zoology, DAV University, Jalandhar, Punjab, India
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8
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Mechanisms of Interaction between Enhancers and Promoters in Three Drosophila Model Systems. Int J Mol Sci 2023; 24:ijms24032855. [PMID: 36769179 PMCID: PMC9917889 DOI: 10.3390/ijms24032855] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/29/2022] [Revised: 01/26/2023] [Accepted: 01/30/2023] [Indexed: 02/05/2023] Open
Abstract
In higher eukaryotes, the regulation of developmental gene expression is determined by enhancers, which are often located at a large distance from the promoters they regulate. Therefore, the architecture of chromosomes and the mechanisms that determine the functional interaction between enhancers and promoters are of decisive importance in the development of organisms. Mammals and the model animal Drosophila have homologous key architectural proteins and similar mechanisms in the organization of chromosome architecture. This review describes the current progress in understanding the mechanisms of the formation and regulation of long-range interactions between enhancers and promoters at three well-studied key regulatory loci in Drosophila.
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9
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Galupa R, Alvarez-Canales G, Borst NO, Fuqua T, Gandara L, Misunou N, Richter K, Alves MRP, Karumbi E, Perkins ML, Kocijan T, Rushlow CA, Crocker J. Enhancer architecture and chromatin accessibility constrain phenotypic space during Drosophila development. Dev Cell 2023; 58:51-62.e4. [PMID: 36626871 PMCID: PMC9860173 DOI: 10.1016/j.devcel.2022.12.003] [Citation(s) in RCA: 7] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/04/2022] [Revised: 10/18/2022] [Accepted: 12/07/2022] [Indexed: 01/11/2023]
Abstract
Developmental enhancers bind transcription factors and dictate patterns of gene expression during development. Their molecular evolution can underlie phenotypical evolution, but the contributions of the evolutionary pathways involved remain little understood. Here, using mutation libraries in Drosophila melanogaster embryos, we observed that most point mutations in developmental enhancers led to changes in gene expression levels but rarely resulted in novel expression outside of the native pattern. In contrast, random sequences, often acting as developmental enhancers, drove expression across a range of cell types; random sequences including motifs for transcription factors with pioneer activity acted as enhancers even more frequently. Our findings suggest that the phenotypic landscapes of developmental enhancers are constrained by enhancer architecture and chromatin accessibility. We propose that the evolution of existing enhancers is limited in its capacity to generate novel phenotypes, whereas the activity of de novo elements is a primary source of phenotypic novelty.
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Affiliation(s)
- Rafael Galupa
- European Molecular Biology Laboratory, 69117 Heidelberg, Germany.
| | | | | | - Timothy Fuqua
- European Molecular Biology Laboratory, 69117 Heidelberg, Germany
| | - Lautaro Gandara
- European Molecular Biology Laboratory, 69117 Heidelberg, Germany
| | - Natalia Misunou
- European Molecular Biology Laboratory, 69117 Heidelberg, Germany
| | - Kerstin Richter
- European Molecular Biology Laboratory, 69117 Heidelberg, Germany
| | | | - Esther Karumbi
- European Molecular Biology Laboratory, 69117 Heidelberg, Germany
| | | | - Tin Kocijan
- European Molecular Biology Laboratory, 69117 Heidelberg, Germany
| | | | - Justin Crocker
- European Molecular Biology Laboratory, 69117 Heidelberg, Germany.
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10
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Raimundo J, Levine M. How do genomes encode developmental time? Genes Dev 2023; 37:37-40. [PMID: 37061966 PMCID: PMC10046422 DOI: 10.1101/gad.350486.123] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 03/10/2023]
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11
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Saha S, Moon HR, Han B, Mugler A. Deduction of signaling mechanisms from cellular responses to multiple cues. NPJ Syst Biol Appl 2022; 8:48. [PMID: 36450797 PMCID: PMC9712676 DOI: 10.1038/s41540-022-00262-5] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/16/2022] [Accepted: 11/08/2022] [Indexed: 12/05/2022] Open
Abstract
Cell signaling networks are complex and often incompletely characterized, making it difficult to obtain a comprehensive picture of the mechanisms they encode. Mathematical modeling of these networks provides important clues, but the models themselves are often complex, and it is not always clear how to extract falsifiable predictions. Here we take an inverse approach, using experimental data at the cell level to deduce the minimal signaling network. We focus on cells' response to multiple cues, specifically on the surprising case in which the response is antagonistic: the response to multiple cues is weaker than the response to the individual cues. We systematically build candidate signaling networks one node at a time, using the ubiquitous ingredients of (i) up- or down-regulation, (ii) molecular conversion, or (iii) reversible binding. In each case, our method reveals a minimal, interpretable signaling mechanism that explains the antagonistic response. Our work provides a systematic way to deduce molecular mechanisms from cell-level data.
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Affiliation(s)
- Soutick Saha
- grid.169077.e0000 0004 1937 2197Department of Physics and Astronomy, Purdue University, West Lafayette, IN 47907 USA
| | - Hye-ran Moon
- grid.169077.e0000 0004 1937 2197School of Mechanical Engineering, Purdue University, West Lafayette, IN 47907 USA
| | - Bumsoo Han
- grid.169077.e0000 0004 1937 2197School of Mechanical Engineering, Purdue University, West Lafayette, IN 47907 USA ,grid.169077.e0000 0004 1937 2197Purdue Center for Cancer Research, Purdue University, West Lafayette, IN 47907 USA
| | - Andrew Mugler
- grid.169077.e0000 0004 1937 2197Department of Physics and Astronomy, Purdue University, West Lafayette, IN 47907 USA ,grid.169077.e0000 0004 1937 2197Purdue Center for Cancer Research, Purdue University, West Lafayette, IN 47907 USA ,grid.21925.3d0000 0004 1936 9000Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260 USA
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12
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Baltruk LJ, Lavezzo GM, Machado-Lima A, Digiampietri LA, Andrioli LP. An additive repression mechanism sets the anterior limits of anterior pair-rule stripes 1. Cells Dev 2022; 171:203802. [PMID: 35934285 DOI: 10.1016/j.cdev.2022.203802] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/22/2022] [Revised: 07/28/2022] [Accepted: 07/29/2022] [Indexed: 01/25/2023]
Abstract
Segments are repeated anatomical units forming the body of insects. In Drosophila, the specification of the body takes place during the blastoderm through the segmentation cascade. Pair-rule genes such as hairy (h), even-skipped (eve), runt (run), and fushi-tarazu (ftz) are of the intermediate level of the cascade and each pair-rule gene is expressed in seven transversal stripes along the antero-posterior axis of the embryo. Stripes are formed by independent cis-regulatory modules (CRMs) under the regulation of transcription factors of maternal source and of gap proteins of the first level of the cascade. The initial blastoderm of Drosophila is a syncytium and it also coincides with the mid-blastula transition when thousands of zygotic genes are transcribed and their products are able to diffuse in the cytoplasm. Thus, we anticipated a complex regulation of the CRMs of the pair-rule stripes. The CRMs of h 1, eve 1, run 1, ftz 1 are able to be activated by bicoid (bcd) throughout the anterior blastoderm and several lines of evidence indicate that they are repressed by the anterior gap genes slp1 (sloppy-paired 1), tll (tailless) and hkb (huckebein). The modest activity of these repressors led to the premise of a combinatorial mechanism regulating the expression of the CRMs of h 1, eve 1, run 1, ftz 1 in more anterior regions of the embryo. We tested this possibility by progressively removing the repression activities of slp1, tll and hkb. In doing so, we were able to expose a mechanism of additive repression limiting the anterior borders of stripes 1. Stripes 1 respond depending on their distance from the anterior end and repressors operating at different levels.
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Affiliation(s)
| | - Guilherme Miura Lavezzo
- Program on Bioinformatics, Institute of Mathematics and Statistics, University of São Paulo, São Paulo, Brazil
| | - Ariane Machado-Lima
- Escola de Artes, Ciências e Humanidades, Universidade de São Paulo, São Paulo, SP, Brazil; Program on Bioinformatics, Institute of Mathematics and Statistics, University of São Paulo, São Paulo, Brazil
| | | | - Luiz Paulo Andrioli
- Escola de Artes, Ciências e Humanidades, Universidade de São Paulo, São Paulo, SP, Brazil.
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13
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Single-nucleus RNA-sequencing in pre-cellularization Drosophila melanogaster embryos. PLoS One 2022; 17:e0270471. [PMID: 35749552 PMCID: PMC9232161 DOI: 10.1371/journal.pone.0270471] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/12/2022] [Accepted: 06/10/2022] [Indexed: 12/13/2022] Open
Abstract
Our current understanding of the regulation of gene expression in the early Drosophila melanogaster embryo comes from observations of a few genes at a time, as with in situ hybridizations, or observation of gene expression levels without regards to patterning, as with RNA-sequencing. Single-nucleus RNA-sequencing however, has the potential to provide new insights into the regulation of gene expression for many genes at once while simultaneously retaining information regarding the position of each nucleus prior to dissociation based on patterned gene expression. In order to establish the use of single-nucleus RNA sequencing in Drosophila embryos prior to cellularization, here we look at gene expression in control and insulator protein, dCTCF, maternal null embryos during zygotic genome activation at nuclear cycle 14. We find that early embryonic nuclei can be grouped into distinct clusters according to gene expression. From both virtual and published in situ hybridizations, we also find that these clusters correspond to spatial regions of the embryo. Lastly, we provide a resource of candidate differentially expressed genes that might show local changes in gene expression between control and maternal dCTCF null nuclei with no detectable differential expression in bulk. These results highlight the potential for single-nucleus RNA-sequencing to reveal new insights into the regulation of gene expression in the early Drosophila melanogaster embryo.
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14
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Kondo S, Watanabe M, Miyazawa S. Studies of Turing pattern formation in zebrafish skin. PHILOSOPHICAL TRANSACTIONS. SERIES A, MATHEMATICAL, PHYSICAL, AND ENGINEERING SCIENCES 2021; 379:20200274. [PMID: 34743596 PMCID: PMC8580470 DOI: 10.1098/rsta.2020.0274] [Citation(s) in RCA: 11] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Accepted: 06/16/2021] [Indexed: 05/08/2023]
Abstract
Skin patterns are the first example of the existence of Turing patterns in living organisms. Extensive research on zebrafish, a model organism with stripes on its skin, has revealed the principles of pattern formation at the molecular and cellular levels. Surprisingly, although the networks of cell-cell interactions have been observed to satisfy the 'short-range activation and long-range inhibition' prerequisites for Turing pattern formation, numerous individual reactions were not envisioned based on the classical reaction-diffusion model. For example, in real skin, it is not an alteration in concentrations of chemicals, but autonomous migration and proliferation of pigment cells that establish patterns, and cell-cell interactions are mediated via direct contact through cell protrusions. Therefore, the classical reaction-diffusion mechanism cannot be used as it is for modelling skin pattern formation. Various studies are underway to adapt mathematical models to the experimental findings on research into skin patterns, and the purpose of this review is to organize and present them. These novel theoretical methods could be applied to autonomous pattern formation phenomena other than skin patterns. This article is part of the theme issue 'Recent progress and open frontiers in Turing's theory of morphogenesis'.
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Affiliation(s)
- Shigeru Kondo
- Graduate School of Frontier Biosciences, Osaka University, 1-3 Yamadaoka, Suita, Osaka 565-0871, Japan
| | - Masakatsu Watanabe
- Graduate School of Frontier Biosciences, Osaka University, 1-3 Yamadaoka, Suita, Osaka 565-0871, Japan
| | - Seita Miyazawa
- Graduate School of Frontier Biosciences, Osaka University, 1-3 Yamadaoka, Suita, Osaka 565-0871, Japan
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15
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Ray-Jones H, Spivakov M. Transcriptional enhancers and their communication with gene promoters. Cell Mol Life Sci 2021; 78:6453-6485. [PMID: 34414474 PMCID: PMC8558291 DOI: 10.1007/s00018-021-03903-w] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/12/2021] [Revised: 07/08/2021] [Accepted: 07/19/2021] [Indexed: 12/13/2022]
Abstract
Transcriptional enhancers play a key role in the initiation and maintenance of gene expression programmes, particularly in metazoa. How these elements control their target genes in the right place and time is one of the most pertinent questions in functional genomics, with wide implications for most areas of biology. Here, we synthesise classic and recent evidence on the regulatory logic of enhancers, including the principles of enhancer organisation, factors that facilitate and delimit enhancer-promoter communication, and the joint effects of multiple enhancers. We show how modern approaches building on classic insights have begun to unravel the complexity of enhancer-promoter relationships, paving the way towards a quantitative understanding of gene control.
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Affiliation(s)
- Helen Ray-Jones
- MRC London Institute of Medical Sciences, London, W12 0NN, UK
- Institute of Clinical Sciences, Faculty of Medicine, Imperial College, London, W12 0NN, UK
| | - Mikhail Spivakov
- MRC London Institute of Medical Sciences, London, W12 0NN, UK.
- Institute of Clinical Sciences, Faculty of Medicine, Imperial College, London, W12 0NN, UK.
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16
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Control of vein-forming, striped gene expression by auxin signaling. BMC Biol 2021; 19:213. [PMID: 34556094 PMCID: PMC8461865 DOI: 10.1186/s12915-021-01143-9] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/23/2020] [Accepted: 09/03/2021] [Indexed: 11/10/2022] Open
Abstract
Background Activation of gene expression in striped domains is a key building block of biological patterning, from the recursive formation of veins in plant leaves to that of ribs and vertebrae in our bodies. In animals, gene expression is activated in striped domains by the differential affinity of broadly expressed transcription factors for their target genes and the combinatorial interaction between such target genes. In plants, how gene expression is activated in striped domains is instead unknown. We address this question for the broadly expressed MONOPTEROS (MP) transcription factor and its target gene ARABIDOPSIS THALIANA HOMEOBOX FACTOR8 (ATHB8). Results We find that ATHB8 promotes vein formation and that such vein-forming function depends on both levels of ATHB8 expression and width of ATHB8 expression domains. We further find that ATHB8 expression is activated in striped domains by a combination of (1) activation of ATHB8 expression through binding of peak levels of MP to a low-affinity MP-binding site in the ATHB8 promoter and (2) repression of ATHB8 expression by MP target genes of the AUXIN/INDOLE-3-ACETIC-ACID-INDUCIBLE family. Conclusions Our findings suggest that a common regulatory logic controls activation of gene expression in striped domains in both plants and animals despite the independent evolution of their multicellularity. Supplementary Information The online version contains supplementary material available at 10.1186/s12915-021-01143-9.
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17
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Waymack R, Gad M, Wunderlich Z. Molecular competition can shape enhancer activity in the Drosophila embryo. iScience 2021; 24:103034. [PMID: 34568782 PMCID: PMC8449247 DOI: 10.1016/j.isci.2021.103034] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/07/2021] [Revised: 07/27/2021] [Accepted: 08/20/2021] [Indexed: 01/12/2023] Open
Abstract
Transgenic reporters allow the measurement of regulatory DNA activity in vivo and consequently have long been useful tools for studying enhancers. Despite their utility, few studies have investigated the effects these reporters may have on the expression of other genes. Understanding these effects is required to accurately interpret reporter data and characterize gene regulatory mechanisms. By measuring the expression of Kruppel (Kr) enhancer reporters in live Drosophila embryos, we find reporters inhibit one another’s expression and that of a nearby endogenous gene. Using synthetic transcription factor (TF) binding site arrays, we present evidence that competition for TFs is partially responsible for the observed transcriptional inhibition. We develop a simple thermodynamic model that predicts competition of the measured magnitude specifically when TF binding is restricted to distinct nuclear subregions. Our findings underline an unexpected role of the non-homogenous nature of the nucleus in regulating gene expression. Live tracking of transcription reveals competition between transgenic reporters Transgenic reporters can also depress the expression of a neighboring gene Expression inhibition is in part because of competition for transcription factors (TFs) Competition is predicted with a model that restricts TFs to sub-nuclear “hubs”
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Affiliation(s)
- Rachel Waymack
- Department of Developmental and Cell Biology, University of California, Irvine, CA 92697, USA
| | - Mario Gad
- Department of Developmental and Cell Biology, University of California, Irvine, CA 92697, USA
| | - Zeba Wunderlich
- Department of Developmental and Cell Biology, University of California, Irvine, CA 92697, USA.,Department of Biology, Boston University, 610 Commonwealth Ave., Boston, MA 02215, USA.,Biological Design Center, Boston University, 610 Commonwealth Avenue, Boston, MA 02215, USA
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18
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Graham PL, Fischer MD, Giri A, Pick L. The fushi tarazu zebra element is not required for Drosophila viability or fertility. G3-GENES GENOMES GENETICS 2021; 11:6358135. [PMID: 34518886 PMCID: PMC8527495 DOI: 10.1093/g3journal/jkab300] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 07/13/2021] [Accepted: 08/19/2021] [Indexed: 11/13/2022]
Abstract
Expression of genes in precisely controlled spatiotemporal patterns is essential for embryonic development. Much of our understanding of mechanisms regulating gene expression comes from the study of cis-regulatory elements (CREs) that direct expression of reporter genes in transgenic organisms. This reporter-transgene approach identifies genomic regions sufficient to drive expression but fails to provide information about quantitative and qualitative contributions to endogenous expression, although such conclusions are often inferred. Here we evaluated the endogenous function of a classic Drosophila CRE, the fushi tarazu (ftz) zebra element. ftz is a pair-rule segmentation gene expressed in seven stripes during embryogenesis, necessary for formation of alternate body segments. Reporter transgenes identified the promoter-proximal zebra element as a major driver of the seven ftz stripes. We generated a precise genomic deletion of the zebra element (ftzΔZ) to assess its role in the context of native chromatin and neighboring CREs, expecting large decreases in ftz seven-stripe expression. However, significant reduction in expression was found for only one stripe, ftz stripe 4, expressed at ∼25% of wild type levels in ftzΔZ homozygotes. Defects in corresponding regions of ftzΔZ mutants suggest this level of expression borders the threshold required to promote morphological segmentation. Further, we established true-breeding lines of homozygous ftzΔZ flies, demonstrating that the body segments missing in the mutants are not required for viability or fertility. These results highlight the different types of conclusions drawn from different experimental designs and emphasize the importance of examining transcriptional regulatory mechanisms in the context of the native genomic environment.
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Affiliation(s)
- Patricia L Graham
- Department of Entomology, University of Maryland, College Park, MD 20742, USA
| | - Matthew D Fischer
- Graduate Program in Molecular & Cell Biology, University of Maryland, College Park, MD 20742, USA
| | - Abhigya Giri
- Department of Entomology, University of Maryland, College Park, MD 20742, USA
| | - Leslie Pick
- Department of Entomology, University of Maryland, College Park, MD 20742, USA.,Graduate Program in Molecular & Cell Biology, University of Maryland, College Park, MD 20742, USA
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19
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Berrocal A, Lammers NC, Garcia HG, Eisen MB. Kinetic sculpting of the seven stripes of the Drosophila even-skipped gene. eLife 2020; 9:61635. [PMID: 33300492 PMCID: PMC7864633 DOI: 10.7554/elife.61635] [Citation(s) in RCA: 17] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/31/2020] [Accepted: 10/09/2020] [Indexed: 12/14/2022] Open
Abstract
We used live imaging to visualize the transcriptional dynamics of the Drosophila melanogaster even-skipped gene at single-cell and high-temporal resolution as its seven stripe expression pattern forms, and developed tools to characterize and visualize how transcriptional bursting varies over time and space. We find that despite being created by the independent activity of five enhancers, even-skipped stripes are sculpted by the same kinetic phenomena: a coupled increase of burst frequency and amplitude. By tracking the position and activity of individual nuclei, we show that stripe movement is driven by the exchange of bursting nuclei from the posterior to anterior stripe flanks. Our work provides a conceptual, theoretical and computational framework for dissecting pattern formation in space and time, and reveals how the coordinated transcriptional activity of individual nuclei shapes complex developmental patterns.
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Affiliation(s)
- Augusto Berrocal
- Department of Molecular & Cell Biology, University of California at Berkeley, Berkeley, United States
| | - Nicholas C Lammers
- Biophysics Graduate Group, University of California at Berkeley, Berkeley, United States
| | - Hernan G Garcia
- Department of Molecular & Cell Biology, University of California at Berkeley, Berkeley, United States.,Biophysics Graduate Group, University of California at Berkeley, Berkeley, United States.,Department of Physics, University of California at Berkeley, Berkeley, United States.,Institute for Quantitative Biosciences-QB3, University of California at Berkeley, Berkeley, United States
| | - Michael B Eisen
- Department of Molecular & Cell Biology, University of California at Berkeley, Berkeley, United States.,Biophysics Graduate Group, University of California at Berkeley, Berkeley, United States.,Institute for Quantitative Biosciences-QB3, University of California at Berkeley, Berkeley, United States.,Department of Integrative Biology, University of California at Berkeley, Berkeley, United States.,Howard Hughes Medical Institute, University of California at Berkeley, Berkeley, United States
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20
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Zeitlinger J. Seven myths of how transcription factors read the cis-regulatory code. CURRENT OPINION IN SYSTEMS BIOLOGY 2020; 23:22-31. [PMID: 33134611 PMCID: PMC7592701 DOI: 10.1016/j.coisb.2020.08.002] [Citation(s) in RCA: 43] [Impact Index Per Article: 10.8] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
Genomics data are now being generated at large quantities, of exquisite high resolution and from single cells. They offer a unique opportunity to develop powerful machine learning algorithms, including neural networks, to uncover the rules of the cis-regulatory code. However, current modeling assumptions are often not based on state-of-the-art knowledge of the cis-regulatory code from transcription, developmental genetics, imaging and structural studies. Here I aim to fill this gap by giving a brief historical overview of the field, describing common misconceptions and providing knowledge that might help to guide computational approaches. I will describe the principles and mechanisms involved in the combinatorial requirement of transcription factor binding motifs for enhancer activity, including the role of chromatin accessibility, repressors and low-affinity motifs in the cis-regulatory code. Deciphering the cis-regulatory code would unlock an enormous amount of regulatory information in the genome and would allow us to locate cis-regulatory genetic variants involved in development and disease.
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Affiliation(s)
- Julia Zeitlinger
- Stowers Institute for Medical Research, Kansas City, MO, USA
- The University of Kansas Medical Center, Kansas City, KS, USA
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21
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Abstract
Key discoveries in Drosophila have shaped our understanding of cellular "enhancers." With a special focus on the fly, this chapter surveys properties of these adaptable cis-regulatory elements, whose actions are critical for the complex spatial/temporal transcriptional regulation of gene expression in metazoa. The powerful combination of genetics, molecular biology, and genomics available in Drosophila has provided an arena in which the developmental role of enhancers can be explored. Enhancers are characterized by diverse low- or high-throughput assays, which are challenging to interpret, as not all of these methods of identifying enhancers produce concordant results. As a model metazoan, the fly offers important advantages to comprehensive analysis of the central functions that enhancers play in gene expression, and their critical role in mediating the production of phenotypes from genotype and environmental inputs. A major challenge moving forward will be obtaining a quantitative understanding of how these cis-regulatory elements operate in development and disease.
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Affiliation(s)
- Stephen Small
- Department of Biology, Developmental Systems Training Program, New York University, 10003 and
| | - David N Arnosti
- Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, Michigan 48824
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22
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Lammers NC, Galstyan V, Reimer A, Medin SA, Wiggins CH, Garcia HG. Multimodal transcriptional control of pattern formation in embryonic development. Proc Natl Acad Sci U S A 2020; 117:836-847. [PMID: 31882445 PMCID: PMC6969519 DOI: 10.1073/pnas.1912500117] [Citation(s) in RCA: 47] [Impact Index Per Article: 11.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/17/2023] Open
Abstract
Predicting how interactions between transcription factors and regulatory DNA sequence dictate rates of transcription and, ultimately, drive developmental outcomes remains an open challenge in physical biology. Using stripe 2 of the even-skipped gene in Drosophila embryos as a case study, we dissect the regulatory forces underpinning a key step along the developmental decision-making cascade: the generation of cytoplasmic mRNA patterns via the control of transcription in individual cells. Using live imaging and computational approaches, we found that the transcriptional burst frequency is modulated across the stripe to control the mRNA production rate. However, we discovered that bursting alone cannot quantitatively recapitulate the formation of the stripe and that control of the window of time over which each nucleus transcribes even-skipped plays a critical role in stripe formation. Theoretical modeling revealed that these regulatory strategies (bursting and the time window) respond in different ways to input transcription factor concentrations, suggesting that the stripe is shaped by the interplay of 2 distinct underlying molecular processes.
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Affiliation(s)
| | - Vahe Galstyan
- Biochemistry and Molecular Biophysics Option, California Institute of Technology, Pasadena, CA 91126
- Department of Physics, Columbia University, New York, NY 10027
| | - Armando Reimer
- Biophysics Graduate Group, University of California, Berkeley, CA 94720
| | - Sean A Medin
- Department of Physics, University of California, Berkeley, CA 94720
| | - Chris H Wiggins
- Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY 10027;
- Data Science Institute, Columbia University, New York, NY 10027
- Department of Systems Biology, Columbia University, New York, NY 10027
- Department of Statistics, Columbia University, New York, NY 10027
| | - Hernan G Garcia
- Biophysics Graduate Group, University of California, Berkeley, CA 94720;
- Department of Physics, University of California, Berkeley, CA 94720
- Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720
- Institute for Quantitative Biosciences-QB3, University of California, Berkeley, CA 94720
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23
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Garcia HG, Berrocal A, Kim YJ, Martini G, Zhao J. Lighting up the central dogma for predictive developmental biology. Curr Top Dev Biol 2019; 137:1-35. [PMID: 32143740 DOI: 10.1016/bs.ctdb.2019.10.010] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
Abstract
Although the last 30years have witnessed the mapping of the wiring diagrams of the gene regulatory networks that dictate cell fate and animal body plans, specific understanding building on such network diagrams that shows how DNA regulatory regions control gene expression lags far behind. These networks have yet to yield the predictive power necessary to, for example, calculate how the concentration dynamics of input transcription factors and DNA regulatory sequence prescribes output patterns of gene expression that, in turn, determine body plans themselves. Here, we argue that reaching a predictive understanding of developmental decision-making calls for an interplay between theory and experiment aimed at revealing how the regulation of the processes of the central dogma dictate network connections and how network topology guides cells toward their ultimate developmental fate. To make this possible, it is crucial to break free from the snapshot-based understanding of embryonic development facilitated by fixed-tissue approaches and embrace new technologies that capture the dynamics of developmental decision-making at the single cell level, in living embryos.
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Affiliation(s)
- Hernan G Garcia
- Department of Molecular and Cell Biology, University of California at Berkeley, Berkeley, CA, United States; Department of Physics, University of California at Berkeley, Berkeley, CA, United States; Biophysics Graduate Group, University of California at Berkeley, Berkeley, CA, United States; Quantitative Biosciences-QB3, University of California at Berkeley, Berkeley, CA, United States.
| | - Augusto Berrocal
- Department of Molecular and Cell Biology, University of California at Berkeley, Berkeley, CA, United States
| | - Yang Joon Kim
- Biophysics Graduate Group, University of California at Berkeley, Berkeley, CA, United States
| | - Gabriella Martini
- Department of Molecular and Cell Biology, University of California at Berkeley, Berkeley, CA, United States
| | - Jiaxi Zhao
- Department of Physics, University of California at Berkeley, Berkeley, CA, United States
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24
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Barr K, Reinitz J, Radulescu O. An in silico analysis of robust but fragile gene regulation links enhancer length to robustness. PLoS Comput Biol 2019; 15:e1007497. [PMID: 31730659 PMCID: PMC6881076 DOI: 10.1371/journal.pcbi.1007497] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/21/2019] [Revised: 11/27/2019] [Accepted: 10/22/2019] [Indexed: 12/31/2022] Open
Abstract
Organisms must ensure that expression of genes is directed to the appropriate tissues at the correct times, while simultaneously ensuring that these gene regulatory systems are robust to perturbation. This idea is captured by a mathematical concept called r-robustness, which says that a system is robust to a perturbation in up to r - 1 randomly chosen parameters. r-robustness implies that the biological system has a small number of sensitive parameters and that this number can be used as a robustness measure. In this work we use this idea to investigate the robustness of gene regulation using a sequence level model of the Drosophila melanogaster gene even-skipped. We consider robustness with respect to mutations of the enhancer sequence and with respect to changes of the transcription factor concentrations. We find that gene regulation is r-robust with respect to mutations in the enhancer sequence and identify a number of sensitive nucleotides. In both natural and in silico predicted enhancers, the number of nucleotides that are sensitive to mutation correlates negatively with the length of the sequence, meaning that longer sequences are more robust. The exact degree of robustness obtained is dependent not only on DNA sequence, but also on the local concentration of regulatory factors. We find that gene regulation can be remarkably sensitive to changes in transcription factor concentrations at the boundaries of expression features, while it is robust to perturbation elsewhere.
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Affiliation(s)
- Kenneth Barr
- Department of Genetic Medicine, University of Chicago, Chicago, Illinois, United States of America
| | - John Reinitz
- Departments of Statistics, Ecology & Evolution, Molecular Genetics & Cell Biology, University of Chicago, Chicago, Illinois, United States of America
| | - Ovidiu Radulescu
- LPHI UMR CNRS 5235, University of Montpellier, Montpellier, France
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25
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Liu LY, Long X, Yang CP, Miyares RL, Sugino K, Singer RH, Lee T. Mamo decodes hierarchical temporal gradients into terminal neuronal fate. eLife 2019; 8:48056. [PMID: 31545163 PMCID: PMC6764822 DOI: 10.7554/elife.48056] [Citation(s) in RCA: 15] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/29/2019] [Accepted: 09/20/2019] [Indexed: 12/20/2022] Open
Abstract
Temporal patterning is a seminal method of expanding neuronal diversity. Here we unravel a mechanism decoding neural stem cell temporal gene expression and transforming it into discrete neuronal fates. This mechanism is characterized by hierarchical gene expression. First, Drosophila neuroblasts express opposing temporal gradients of RNA-binding proteins, Imp and Syp. These proteins promote or inhibit chinmo translation, yielding a descending neuronal gradient. Together, first and second-layer temporal factors define a temporal expression window of BTB-zinc finger nuclear protein, Mamo. The precise temporal induction of Mamo is achieved via both transcriptional and post-transcriptional regulation. Finally, Mamo is essential for the temporally defined, terminal identity of α’/β’ mushroom body neurons and identity maintenance. We describe a straightforward paradigm of temporal fate specification where diverse neuronal fates are defined via integrating multiple layers of gene regulation. The neurodevelopmental roles of orthologous/related mammalian genes suggest a fundamental conservation of this mechanism in brain development.
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Affiliation(s)
- Ling-Yu Liu
- Howard Hughes Medical Institute, Janelia Research Campus, Ashburn, United States
| | - Xi Long
- Howard Hughes Medical Institute, Janelia Research Campus, Ashburn, United States
| | - Ching-Po Yang
- Howard Hughes Medical Institute, Janelia Research Campus, Ashburn, United States
| | - Rosa L Miyares
- Howard Hughes Medical Institute, Janelia Research Campus, Ashburn, United States
| | - Ken Sugino
- Howard Hughes Medical Institute, Janelia Research Campus, Ashburn, United States
| | - Robert H Singer
- Howard Hughes Medical Institute, Janelia Research Campus, Ashburn, United States.,Department of Anatomy and Structural Biology, Gruss Lipper Biophotonics Center, Albert Einstein College of Medicine, New York, United States.,Dominick P Purpura Department of Neuroscience, Gruss Lipper Biophotonics Center, Albert Einstein College of Medicine, New York, United States
| | - Tzumin Lee
- Howard Hughes Medical Institute, Janelia Research Campus, Ashburn, United States
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26
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Repele A, Krueger S, Bhattacharyya T, Tuineau MY. The regulatory control of Cebpa enhancers and silencers in the myeloid and red-blood cell lineages. PLoS One 2019; 14:e0217580. [PMID: 31181110 PMCID: PMC6557489 DOI: 10.1371/journal.pone.0217580] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/18/2019] [Accepted: 05/14/2019] [Indexed: 12/31/2022] Open
Abstract
Cebpa encodes a transcription factor (TF) that plays an instructive role in the development of multiple myeloid lineages. The expression of Cebpa itself is finely modulated, as Cebpa is expressed at high and intermediate levels in neutrophils and macrophages respectively and downregulated in non-myeloid lineages. The cis-regulatory logic underlying the lineage-specific modulation of Cebpa's expression level is yet to be fully characterized. Previously, we had identified 6 new cis-regulatory modules (CRMs) in a 78kb region surrounding Cebpa. We had also inferred the TFs that regulate each CRM by fitting a sequence-based thermodynamic model to a comprehensive reporter activity dataset. Here, we report the cis-regulatory logic of Cebpa CRMs at the resolution of individual binding sites. We tested the binding sites and functional roles of inferred TFs by designing and constructing mutated CRMs and comparing theoretical predictions of their activity against empirical measurements in a myeloid cell line. The enhancers were confirmed to be activated by combinations of PU.1, C/EBP family TFs, Egr1, and Gfi1 as predicted by the model. We show that silencers repress the activity of the proximal promoter in a dominant manner in G1ME cells, which are derived from the red-blood cell lineage. Dominant repression in G1ME cells can be traced to binding sites for GATA and Myb, a motif shared by all of the silencers. Finally, we demonstrate that GATA and Myb act redundantly to silence the proximal promoter. These results indicate that dominant repression is a novel mechanism for resolving hematopoietic lineages. Furthermore, Cebpa has a fail-safe cis-regulatory architecture, featuring several functionally similar CRMs, each of which contains redundant binding sites for multiple TFs. Lastly, by experimentally demonstrating the predictive ability of our sequence-based thermodynamic model, this work highlights the utility of this computational approach for understanding mammalian gene regulation.
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Affiliation(s)
- Andrea Repele
- Department of Biology, University of North Dakota, Grand Forks, ND, United States of America
| | - Shawn Krueger
- Department of Biology, University of North Dakota, Grand Forks, ND, United States of America
| | - Tapas Bhattacharyya
- Department of Biology, University of North Dakota, Grand Forks, ND, United States of America
| | - Michelle Y Tuineau
- Department of Biology, University of North Dakota, Grand Forks, ND, United States of America
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27
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Rothenberg EV. Causal Gene Regulatory Network Modeling and Genomics: Second-Generation Challenges. J Comput Biol 2019; 26:703-718. [PMID: 31063008 DOI: 10.1089/cmb.2019.0098] [Citation(s) in RCA: 15] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022] Open
Abstract
Gene regulatory network modeling has played a major role in advancing the understanding of developmental systems, by crystallizing structures of relevant extant information, by formally posing hypothetical functional relationships between network elements, and by offering clear predictive tests to improve understanding of the mechanisms driving developmental progression. Both ordinary differential equation (ODE)-based and Boolean models have also been highly successful in explaining dynamics within subcircuits of more complex processes. In a very small number of cases, gene regulatory network models of much more global scope have been proposed that successfully predict the dynamics of the processes establishing most of an embryonic body plan. Can such successes be expanded to very different developmental systems, including post-embryonic mammalian systems? This perspective discusses several problems that must be solved in more quantitative and predictive theoretical terms, to make this possible. These problems include: the effects of cellular history on chromatin state and how these affect gene accessibility; the dose dependence of activities of many transcription factors (a problem for Boolean models); stochasticity of some transcriptional outputs (a problem for simple ODE models); response timing delays due to epigenetic remodeling requirements; functionally different kinds of repression; and the regulatory syntax that governs responses of genes with multiple enhancers.
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Affiliation(s)
- Ellen V Rothenberg
- Division of Biology and Biological Engineering, California Institute of Technology, Pasadena, California
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28
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Arnone MI, Oliveri P, Martinez P. A conceptual history of the “regulatory genome”: From Theodor Boveri to Eric Davidson. Mar Genomics 2019; 44:24-31. [DOI: 10.1016/j.margen.2018.11.003] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/16/2018] [Accepted: 11/11/2018] [Indexed: 10/27/2022]
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29
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Petkova MD, Tkačik G, Bialek W, Wieschaus EF, Gregor T. Optimal Decoding of Cellular Identities in a Genetic Network. Cell 2019; 176:844-855.e15. [PMID: 30712870 DOI: 10.1016/j.cell.2019.01.007] [Citation(s) in RCA: 83] [Impact Index Per Article: 16.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/22/2018] [Revised: 11/04/2018] [Accepted: 01/02/2019] [Indexed: 11/24/2022]
Abstract
In developing organisms, spatially prescribed cell identities are thought to be determined by the expression levels of multiple genes. Quantitative tests of this idea, however, require a theoretical framework capable of exposing the rules and precision of cell specification over developmental time. We use the gap gene network in the early fly embryo as an example to show how expression levels of the four gap genes can be jointly decoded into an optimal specification of position with 1% accuracy. The decoder correctly predicts, with no free parameters, the dynamics of pair-rule expression patterns at different developmental time points and in various mutant backgrounds. Precise cellular identities are thus available at the earliest stages of development, contrasting the prevailing view of positional information being slowly refined across successive layers of the patterning network. Our results suggest that developmental enhancers closely approximate a mathematically optimal decoding strategy.
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Affiliation(s)
- Mariela D Petkova
- Joseph Henry Laboratories of Physics and the Lewis-Sigler Institute for Integrative Genomics, Princeton University, Princeton, NJ 08544, USA; Program in Biophysics, Harvard University, Cambridge, MA 02138, USA
| | - Gašper Tkačik
- Institute of Science and Technology Austria, Am Campus 1, 3400 Klosterneuburg, Austria
| | - William Bialek
- Joseph Henry Laboratories of Physics and the Lewis-Sigler Institute for Integrative Genomics, Princeton University, Princeton, NJ 08544, USA
| | - Eric F Wieschaus
- Department of Molecular Biology and Howard Hughes Medical Institute, Princeton University, Princeton, NJ 08544, USA
| | - Thomas Gregor
- Joseph Henry Laboratories of Physics and the Lewis-Sigler Institute for Integrative Genomics, Princeton University, Princeton, NJ 08544, USA; Department of Developmental and Stem Cell Biology, UMR3738, Institut Pasteur, 75015 Paris, France.
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30
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Rothenberg EV. Encounters across networks: Windows into principles of genomic regulation. Mar Genomics 2019; 44:3-12. [PMID: 30661741 DOI: 10.1016/j.margen.2019.01.003] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/23/2018] [Revised: 01/06/2019] [Accepted: 01/06/2019] [Indexed: 12/13/2022]
Abstract
Gene regulatory networks account for the ability of the genome to program development in complex multi-cellular organisms. Such networks are based on principles of gene regulation by combinations of transcription factors that bind to specific cis-regulatory DNA sites to activate transcription. These cis-regulatory regions mediate logic processing at each network node, enabling progressive increases in organismal complexity with development. Gene regulatory network explanations of development have been shown to account for patterning and cell type diversification in fly and sea urchin embryonic systems, where networks are characterized by fast coupling between transcriptional inputs and changes in target gene transcription rates, and crucial cis-regulatory elements are concentrated relatively close to the protein coding sequences of the target genes, thus facilitating their identification. Stem cell-based development in post-embryonic mammalian systems also depends on gene networks, but differs from the fly and sea urchin systems. First, the number of regulatory elements per gene and the distances between regulatory elements and the genes they control are considerably larger, forcing searches via genome-wide transcription factor binding surveys rather than functional assays. Second, the intrinsic timing of network state transitions can be slowed considerably by the need to undo stem-cell chromatin configurations, which presumably add stability to stem-cell states but retard responses to transcription factor changes during differentiation. The dispersed, partially redundant cis-regulatory systems controlling gene expression and the slow state transition kinetics in these systems already reveal new insights and opportunities to extend understanding of the repertoire of gene networks and regulatory system logic.
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Affiliation(s)
- Ellen V Rothenberg
- Division of Biology & Biological Engineering, California Institute of Technology, Pasadena, CA, USA.
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31
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Combs PA, Fraser HB. Spatially varying cis-regulatory divergence in Drosophila embryos elucidates cis-regulatory logic. PLoS Genet 2018; 14:e1007631. [PMID: 30383747 PMCID: PMC6211617 DOI: 10.1371/journal.pgen.1007631] [Citation(s) in RCA: 15] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/15/2018] [Accepted: 08/14/2018] [Indexed: 12/30/2022] Open
Abstract
Spatial patterning of gene expression is a key process in development, yet how it evolves is still poorly understood. Both cis- and trans-acting changes could participate in complex interactions, so to isolate the cis-regulatory component of patterning evolution, we measured allele-specific spatial gene expression patterns in D. melanogaster × simulans hybrid embryos. RNA-seq of cryo-sectioned slices revealed 66 genes with strong spatially varying allele-specific expression. We found that hunchback, a major regulator of developmental patterning, had reduced expression of the D. simulans allele specifically in the anterior tip of hybrid embryos. Mathematical modeling of hunchback cis-regulation suggested a candidate transcription factor binding site variant, which we verified as causal using CRISPR-Cas9 genome editing. In sum, even comparing morphologically near-identical species we identified surprisingly extensive spatial variation in gene expression, suggesting not only that development is robust to many such changes, but also that natural selection may have ample raw material for evolving new body plans via changes in spatial patterning.
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Affiliation(s)
- Peter A. Combs
- Department of Biology, Stanford University, Stanford, California, United States of America
| | - Hunter B. Fraser
- Department of Biology, Stanford University, Stanford, California, United States of America
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Vincent BJ, Staller MV, Lopez-Rivera F, Bragdon MDJ, Pym ECG, Biette KM, Wunderlich Z, Harden TT, Estrada J, DePace AH. Hunchback is counter-repressed to regulate even-skipped stripe 2 expression in Drosophila embryos. PLoS Genet 2018; 14:e1007644. [PMID: 30192762 PMCID: PMC6145585 DOI: 10.1371/journal.pgen.1007644] [Citation(s) in RCA: 21] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/06/2017] [Revised: 09/19/2018] [Accepted: 08/17/2018] [Indexed: 01/18/2023] Open
Abstract
Hunchback is a bifunctional transcription factor that can activate and repress gene expression in Drosophila development. We investigated the regulatory DNA sequence features that control Hunchback function by perturbing enhancers for one of its target genes, even-skipped (eve). While Hunchback directly represses the eve stripe 3+7 enhancer, we found that in the eve stripe 2+7 enhancer, Hunchback repression is prevented by nearby sequences-this phenomenon is called counter-repression. We also found evidence that Caudal binding sites are responsible for counter-repression, and that this interaction may be a conserved feature of eve stripe 2 enhancers. Our results alter the textbook view of eve stripe 2 regulation wherein Hb is described as a direct activator. Instead, to generate stripe 2, Hunchback repression must be counteracted. We discuss how counter-repression may influence eve stripe 2 regulation and evolution.
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Affiliation(s)
- Ben J. Vincent
- Department of Systems Biology, Harvard Medical School, Boston, Massachusetts, United States of America
| | - Max V. Staller
- Department of Systems Biology, Harvard Medical School, Boston, Massachusetts, United States of America
| | - Francheska Lopez-Rivera
- Department of Systems Biology, Harvard Medical School, Boston, Massachusetts, United States of America
| | - Meghan D. J. Bragdon
- Department of Systems Biology, Harvard Medical School, Boston, Massachusetts, United States of America
| | - Edward C. G. Pym
- Department of Systems Biology, Harvard Medical School, Boston, Massachusetts, United States of America
| | - Kelly M. Biette
- Department of Systems Biology, Harvard Medical School, Boston, Massachusetts, United States of America
| | - Zeba Wunderlich
- Department of Systems Biology, Harvard Medical School, Boston, Massachusetts, United States of America
| | - Timothy T. Harden
- Department of Systems Biology, Harvard Medical School, Boston, Massachusetts, United States of America
| | - Javier Estrada
- Department of Systems Biology, Harvard Medical School, Boston, Massachusetts, United States of America
| | - Angela H. DePace
- Department of Systems Biology, Harvard Medical School, Boston, Massachusetts, United States of America
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Abstract
Traditional studies of gene regulation in the Drosophila embryo centered primarily on the analysis of fixed tissues. These methods provided considerable insight into the spatial control of gene activity, such as the borders of eve stripe 2, but yielded only limited information about temporal dynamics. The advent of quantitative live-imaging and genome-editing methods permits the detailed examination of the temporal control of endogenous gene activity. Here, we present evidence that the pair-rule genes fushi tarazu (ftz) and even-skipped (eve) undergo dynamic shifts in gene expression. We observe sequential anterior shifting of the stripes along the anterior to posterior axis, with stripe 1 exhibiting movement before stripe 2 and the more posterior stripes. Conversely, posterior stripes shift over greater distances (two or three nuclei) than anterior stripes (one or two nuclei). Shifting of the ftz and eve stripes are slightly offset, with ftz moving faster than eve This observation is consistent with previous genetic studies, suggesting that eve is epistatic to ftz The precision of pair-rule temporal dynamics might depend on enhancer-enhancer interactions within the eve locus, since removal of the endogenous eve stripe 1 enhancer via CRISPR/Cas9 genome editing led to precocious and expanded expression of eve stripe 2. These observations raise the possibility of an added layer of complexity in the positional information encoded by the segmentation gene regulatory network.
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Haines JE, Eisen MB. Patterns of chromatin accessibility along the anterior-posterior axis in the early Drosophila embryo. PLoS Genet 2018; 14:e1007367. [PMID: 29727464 PMCID: PMC5955596 DOI: 10.1371/journal.pgen.1007367] [Citation(s) in RCA: 30] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/02/2017] [Revised: 05/16/2018] [Accepted: 04/17/2018] [Indexed: 12/20/2022] Open
Abstract
As the Drosophila embryo transitions from the use of maternal RNAs to zygotic transcription, domains of open chromatin, with relatively low nucleosome density and specific histone marks, are established at promoters and enhancers involved in patterned embryonic transcription. However it remains unclear how regions of activity are established during early embryogenesis, and if they are the product of spatially restricted or ubiquitous processes. To shed light on this question, we probed chromatin accessibility across the anterior-posterior axis (A-P) of early Drosophila melanogaster embryos by applying a transposon based assay for chromatin accessibility (ATAC-seq) to anterior and posterior halves of hand-dissected, cellular blastoderm embryos. We find that genome-wide chromatin accessibility is highly similar between the two halves, with regions that manifest significant accessibility in one half of the embryo almost always accessible in the other half, even for promoters that are active in exclusively one half of the embryo. These data support previous studies that show that chromatin accessibility is not a direct result of activity, and point to a role for ubiquitous factors or processes in establishing chromatin accessibility at promoters in the early embryo. However, in concordance with similar works, we find that at enhancers active exclusively in one half of the embryo, we observe a significant skew towards greater accessibility in the region of their activity, highlighting the role of patterning factors such as Bicoid in this process.
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Affiliation(s)
- Jenna E. Haines
- Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, United States of America
| | - Michael B. Eisen
- Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, United States of America
- Department of Integrative Biology, University of California, Berkeley, Berkeley, United States of America
- Howard Hughes Medical Institute, University of California, Berkeley, Berkeley, United States of America
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Farley EK, Olson KM, Levine MS. Regulatory Principles Governing Tissue Specificity of Developmental Enhancers. COLD SPRING HARBOR SYMPOSIA ON QUANTITATIVE BIOLOGY 2018; 80:27-32. [PMID: 27325706 DOI: 10.1101/sqb.2015.80.027227] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Abstract
Transcriptional enhancers are short segments of genomic DNA (50 bp to 1 kb in length) that can work over long distances (≥1 Mb) to regulate gene expression in specific cells and tissues. Genomic assays have identified on the order of 400,000 to one million putative enhancers in the human genome (e.g., ENCODE Consortium). This suggests that a typical gene is regulated by tens of enhancers, ensuring stringent regulation of gene expression in response to a variety of intrinsic and external signals. Despite the discovery of the first transcriptional enhancer more than 30 years ago, we know surprisingly little about how enhancers regulate gene expression. In particular, the relationship between primary DNA sequence and enhancer specificity remains obscure. Here we summarize recent high-throughput studies in whole embryos aimed at the systematic identification of the sequence and organizational constraints underlying enhancer function and specificity.
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Affiliation(s)
- Emma K Farley
- Lewis-Sigler Institute for Integrative Genomics, Princeton University, Princeton, New Jersey 08544
| | - Katrina M Olson
- Lewis-Sigler Institute for Integrative Genomics, Princeton University, Princeton, New Jersey 08544
| | - Michael S Levine
- Lewis-Sigler Institute for Integrative Genomics, Princeton University, Princeton, New Jersey 08544
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36
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Batut PJ, Gingeras TR. Conserved noncoding transcription and core promoter regulatory code in early Drosophila development. eLife 2017; 6:29005. [PMID: 29260710 PMCID: PMC5754203 DOI: 10.7554/elife.29005] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/30/2017] [Accepted: 12/19/2017] [Indexed: 01/30/2023] Open
Abstract
Multicellular development is driven by regulatory programs that orchestrate the transcription of protein-coding and noncoding genes. To decipher this genomic regulatory code, and to investigate the developmental relevance of noncoding transcription, we compared genome-wide promoter activity throughout embryogenesis in 5 Drosophila species. Core promoters, generally not thought to play a significant regulatory role, in fact impart restrictions on the developmental timing of gene expression on a global scale. We propose a hierarchical regulatory model in which core promoters define broad windows of opportunity for expression, by defining a range of transcription factors from which they can receive regulatory inputs. This two-tiered mechanism globally orchestrates developmental gene expression, including extremely widespread noncoding transcription. The sequence and expression specificity of noncoding RNA promoters are evolutionarily conserved, implying biological relevance. Overall, this work introduces a hierarchical model for developmental gene regulation, and reveals a major role for noncoding transcription in animal development.
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Affiliation(s)
- Philippe J Batut
- Watson School of Biological Sciences, Cold Spring Harbor Laboratory, New York, United States
| | - Thomas R Gingeras
- Watson School of Biological Sciences, Cold Spring Harbor Laboratory, New York, United States
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37
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Crocker J, Ilsley GR. Using synthetic biology to study gene regulatory evolution. Curr Opin Genet Dev 2017; 47:91-101. [DOI: 10.1016/j.gde.2017.09.001] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/05/2017] [Revised: 09/06/2017] [Accepted: 09/11/2017] [Indexed: 12/21/2022]
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Barr KA, Martinez C, Moran JR, Kim AR, Ramos AF, Reinitz J. Synthetic enhancer design by in silico compensatory evolution reveals flexibility and constraint in cis-regulation. BMC SYSTEMS BIOLOGY 2017; 11:116. [PMID: 29187214 PMCID: PMC5708098 DOI: 10.1186/s12918-017-0485-2] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 07/05/2017] [Accepted: 11/09/2017] [Indexed: 11/12/2022]
Abstract
BACKGROUND Models that incorporate specific chemical mechanisms have been successful in describing the activity of Drosophila developmental enhancers as a function of underlying transcription factor binding motifs. Despite this, the minimum set of mechanisms required to reconstruct an enhancer from its constituent parts is not known. Synthetic biology offers the potential to test the sufficiency of known mechanisms to describe the activity of enhancers, as well as to uncover constraints on the number, order, and spacing of motifs. RESULTS Using a functional model and in silico compensatory evolution, we generated putative synthetic even-skipped stripe 2 enhancers with varying degrees of similarity to the natural enhancer. These elements represent the evolutionary trajectories of the natural stripe 2 enhancer towards two synthetic enhancers designed ab initio. In the first trajectory, spatially regulated expression was maintained, even after more than a third of binding sites were lost. In the second, sequences with high similarity to the natural element did not drive expression, but a highly diverged sequence about half the length of the minimal stripe 2 enhancer drove ten times greater expression. Additionally, homotypic clusters of Zelda or Stat92E motifs, but not Bicoid, drove expression in developing embryos. CONCLUSIONS Here, we present a functional model of gene regulation to test the degree to which the known transcription factors and their interactions explain the activity of the Drosophila even-skipped stripe 2 enhancer. Initial success in the first trajectory showed that the gene regulation model explains much of the function of the stripe 2 enhancer. Cases where expression deviated from prediction indicates that undescribed factors likely act to modulate expression. We also showed that activation driven Bicoid and Hunchback is highly sensitive to spatial organization of binding motifs. In contrast, Zelda and Stat92E drive expression from simple homotypic clusters, suggesting that activation driven by these factors is less constrained. Collectively, the 40 sequences generated in this work provides a powerful training set for building future models of gene regulation.
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Affiliation(s)
- Kenneth A Barr
- Committee on Genetics, Genomics, and Systems Biology, University of Chicago, Zoology 111, 1101 E 57th St, Chicago, 60637, Illinois, USA.
- Department of Ecology and Evolution, The University of Chicago, Chicago, 60637, Illinois, USA.
| | - Carlos Martinez
- Department Biochemistry and Molecular Genetics, Northwestern University, Chicago, 60611, Illinois, USA
| | - Jennifer R Moran
- Department Human Genetics, The University of Chicago, Chicago, 60637, Illinois, USA
- Institute for Genomics & Systems Biology, The University of Chicago, Chicago, 60637, Illinois, USA
| | - Ah-Ram Kim
- School of Life Science, Handong Global University, Pohang, 37554, Gyeongbuk, South Korea
| | - Alexandre F Ramos
- Departamento de Radiologia - Faculdade de Medicina, Universidade de São Paulo & Instituto do Câncer do Estado de São Paulo, São Paulo, SP CEP, 05403-911, Brazil
- Escola de Artes, Ciências e Humanidades & Núcleo de Estudos Interdisciplinares em Sistemas Complexos, Universidade de São Paulo, Av. Arlindo Béttio, São Paulo, 1000 CEP 03828-000, SP, Brazil
| | - John Reinitz
- Committee on Genetics, Genomics, and Systems Biology, University of Chicago, Zoology 111, 1101 E 57th St, Chicago, 60637, Illinois, USA
- Department of Ecology and Evolution, The University of Chicago, Chicago, 60637, Illinois, USA
- Institute for Genomics & Systems Biology, The University of Chicago, Chicago, 60637, Illinois, USA
- Department Statistics, The University of Chicago, 5747 S. Ellis Avenue Jones 312, Chicago, 60637, IL, USA
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Clark E. Dynamic patterning by the Drosophila pair-rule network reconciles long-germ and short-germ segmentation. PLoS Biol 2017; 15:e2002439. [PMID: 28953896 PMCID: PMC5633203 DOI: 10.1371/journal.pbio.2002439] [Citation(s) in RCA: 71] [Impact Index Per Article: 10.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/13/2017] [Revised: 10/09/2017] [Accepted: 09/07/2017] [Indexed: 02/07/2023] Open
Abstract
Drosophila segmentation is a well-established paradigm for developmental pattern formation. However, the later stages of segment patterning, regulated by the "pair-rule" genes, are still not well understood at the system level. Building on established genetic interactions, I construct a logical model of the Drosophila pair-rule system that takes into account the demonstrated stage-specific architecture of the pair-rule gene network. Simulation of this model can accurately recapitulate the observed spatiotemporal expression of the pair-rule genes, but only when the system is provided with dynamic "gap" inputs. This result suggests that dynamic shifts of pair-rule stripes are essential for segment patterning in the trunk and provides a functional role for observed posterior-to-anterior gap domain shifts that occur during cellularisation. The model also suggests revised patterning mechanisms for the parasegment boundaries and explains the aetiology of the even-skipped null mutant phenotype. Strikingly, a slightly modified version of the model is able to pattern segments in either simultaneous or sequential modes, depending only on initial conditions. This suggests that fundamentally similar mechanisms may underlie segmentation in short-germ and long-germ arthropods.
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Affiliation(s)
- Erik Clark
- Laboratory for Development and Evolution, Department of Zoology, University of Cambridge, Cambridge, United Kingdom
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40
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Crocker J, Stern DL. Functional regulatory evolution outside of the minimal even-skipped stripe 2 enhancer. Development 2017; 144:3095-3101. [PMID: 28760812 DOI: 10.1242/dev.149427] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/19/2017] [Accepted: 07/19/2017] [Indexed: 12/27/2022]
Abstract
Transcriptional enhancers are regions of DNA that drive precise patterns of gene expression. Although many studies have elucidated how individual enhancers can evolve, most of this work has focused on what are called 'minimal' enhancers, the smallest DNA regions that drive expression that approximates an aspect of native gene expression. Here, we explore how the Drosophila erecta even-skipped (eve) locus has evolved by testing its activity in the divergent D. melanogaster genome. We found, as has been reported previously, that the D. erecta eve stripe 2 enhancer (eveS2) fails to drive appreciable expression in D. melanogaster However, we found that a large transgene carrying the entire D. erecta eve locus drives normal eve expression, including in stripe 2. We performed a functional dissection of the region upstream of the D. erecta eveS2 region and found multiple Zelda motifs that are required for normal expression. Our results illustrate how sequences outside of minimal enhancer regions can evolve functionally through mechanisms other than changes in transcription factor-binding sites that drive patterning.
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Affiliation(s)
- Justin Crocker
- Janelia Research Campus, Howard Hughes Medical Institute, 19700 Helix Drive, Ashburn, VA 20147, USA
| | - David L Stern
- Janelia Research Campus, Howard Hughes Medical Institute, 19700 Helix Drive, Ashburn, VA 20147, USA
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41
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Barr KA, Reinitz J. A sequence level model of an intact locus predicts the location and function of nonadditive enhancers. PLoS One 2017; 12:e0180861. [PMID: 28715438 PMCID: PMC5513433 DOI: 10.1371/journal.pone.0180861] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/07/2016] [Accepted: 06/22/2017] [Indexed: 01/24/2023] Open
Abstract
Metazoan gene expression is controlled through the action of long stretches of noncoding DNA that contain enhancers-shorter sequences responsible for controlling a single aspect of a gene's expression pattern. Models built on thermodynamics have shown how enhancers interpret protein concentration in order to determine specific levels of gene expression, but the emergent regulatory logic of a complete regulatory locus shows qualitative and quantitative differences from isolated enhancers. Such differences may arise from steric competition limiting the quantity of DNA that can simultaneously influence the transcription machinery. We incorporated this competition into a mechanistic model of gene regulation, generated efficient algorithms for this computation, and applied it to the regulation of Drosophila even-skipped (eve). This model finds the location of enhancers and identifies which factors control the boundaries of eve expression. This model predicts a new enhancer that, when assayed in vivo, drives expression in a non-eve pattern. Incorporation of chromatin accessibility eliminates this inconsistency.
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Affiliation(s)
- Kenneth A. Barr
- Committee on Genetics, Genomics, and Systems Biology, University of Chicago, Chicago, Illinois, United States of America
| | - John Reinitz
- Committee on Genetics, Genomics, and Systems Biology, University of Chicago, Chicago, Illinois, United States of America
- Department of Statistics, University of Chicago, Chicago, Illinois, United States of America
- Department of Ecology and Evolution, University of Chicago, Chicago, Illinois, United States of America
- Department of Molecular Genetics and Cell Biology, University of Chicago, Chicago, Illinois, United States of America
- Institute for Genomics and Systems Biology, University of Chicago, Chicago, Illinois, United States of America
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42
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Holloway DM, Spirov AV. Transcriptional bursting in Drosophila development: Stochastic dynamics of eve stripe 2 expression. PLoS One 2017; 12:e0176228. [PMID: 28437444 PMCID: PMC5402966 DOI: 10.1371/journal.pone.0176228] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/21/2016] [Accepted: 04/08/2017] [Indexed: 01/17/2023] Open
Abstract
Anterior-posterior (AP) body segmentation of the fruit fly (Drosophila) is first seen in the 7-stripe spatial expression patterns of the pair-rule genes, which regulate downstream genes determining specific segment identities. Regulation of pair-rule expression has been extensively studied for the even-skipped (eve) gene. Recent live imaging, of a reporter for the 2ndeve stripe, has demonstrated the stochastic nature of this process, with ‘bursts’ in the number of RNA transcripts being made over time. We developed a stochastic model of the spatial and temporal expression of eve stripe 2 (binding by transcriptional activators (Bicoid and Hunchback proteins) and repressors (Giant and Krüppel proteins), transcriptional initiation and termination; with all rate parameters constrained by features of the experimental data) in order to analyze the noisy experimental time series and test hypotheses for how eve transcription is regulated. These include whether eve transcription is simply OFF or ON, with a single ON rate, or whether it proceeds by a more complex mechanism, with multiple ON rates. We find that both mechanisms can produce long (multi-minute) RNA bursts, but that the short-time (minute-to-minute) statistics of the data is indicative of eve being transcribed with at least two distinct ON rates, consistent with data on the joint activation of eve by Bicoid and Hunchback. We also predict distinct statistical signatures for cases in which eve is repressed (e.g. along the edges of the stripe) vs. cases in which activation is reduced (e.g. by mutagenesis of transcription factor binding sites). Fundamental developmental processes such as gene transcription are intrinsically noisy; our approach presents a new way to quantify and analyze time series data during developmental patterning in order to understand regulatory mechanisms and how they propagate noise and impact embryonic robustness.
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Affiliation(s)
- David M. Holloway
- Mathematics Department, British Columbia Institute of Technology, Burnaby, B.C., Canada
- Biology Department, University of Victoria, Victoria, B.C., Canada
- * E-mail:
| | - Alexander V. Spirov
- Computer Science, and Center of Excellence in Wireless and Information Technology, State University of New York, Stony Brook, New York, United States of America
- Sechenov Institute of Evolutionary Physiology and Biochemistry, St. Petersburg, Russia
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43
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Combs PA, Eisen MB. Genome-wide measurement of spatial expression in patterning mutants of Drosophila melanogaster. F1000Res 2017; 6:41. [PMID: 28299188 PMCID: PMC5325077 DOI: 10.12688/f1000research.9720.1] [Citation(s) in RCA: 19] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Accepted: 10/26/2016] [Indexed: 11/20/2022] Open
Abstract
Patterning in the
Drosophila melanogaster embryo is affected by multiple maternal factors, but the effect of these factors on spatial gene expression has not been systematically analyzed. Here we characterize the effect of the maternal factors Zelda, Hunchback and Bicoid by cryosectioning wildtype and mutant blastoderm stage embryos and sequencing mRNA from each slice. The resulting atlas of spatial gene expression highlights the intersecting roles of these factors in regulating spatial patterns, and serves as a resource for researchers studying spatial patterning in the early embryo. We identify a large number of genes with both expected and unexpected patterning changes, and through integrated analysis of transcription factor binding data identify common themes in genes with complex dependence on these transcription factors.
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Affiliation(s)
- Peter A Combs
- Graduate Program in Biophysics, University of California, Berkley, USA
| | - Michael B Eisen
- Department of Molecular and Cell Biology and Howard Hughes Medical Institute, University of California, Berkeley, USA
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44
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Crocker J, Tsai A, Stern DL. A Fully Synthetic Transcriptional Platform for a Multicellular Eukaryote. Cell Rep 2017; 18:287-296. [DOI: 10.1016/j.celrep.2016.12.025] [Citation(s) in RCA: 21] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/09/2015] [Revised: 12/14/2015] [Accepted: 12/07/2016] [Indexed: 01/12/2023] Open
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45
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Rothenberg EV, Kueh HY, Yui MA, Zhang JA. Hematopoiesis and T-cell specification as a model developmental system. Immunol Rev 2016; 271:72-97. [PMID: 27088908 DOI: 10.1111/imr.12417] [Citation(s) in RCA: 25] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022]
Abstract
The pathway to generate T cells from hematopoietic stem cells guides progenitors through a succession of fate choices while balancing differentiation progression against proliferation, stage to stage. Many elements of the regulatory system that controls this process are known, but the requirement for multiple, functionally distinct transcription factors needs clarification in terms of gene network architecture. Here, we compare the features of the T-cell specification system with the rule sets underlying two other influential types of gene network models: first, the combinatorial, hierarchical regulatory systems that generate the orderly, synchronized increases in complexity in most invertebrate embryos; second, the dueling 'master regulator' systems that are commonly used to explain bistability in microbial systems and in many fate choices in terminal differentiation. The T-cell specification process shares certain features with each of these prevalent models but differs from both of them in central respects. The T-cell system is highly combinatorial but also highly dose-sensitive in its use of crucial regulatory factors. The roles of these factors are not always T-lineage-specific, but they balance and modulate each other's activities long before any mutually exclusive silencing occurs. T-cell specification may provide a new hybrid model for gene networks in vertebrate developmental systems.
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Affiliation(s)
- Ellen V Rothenberg
- Division of Biology & Biological Engineering, California Institute of Technology, Pasadena, CA, USA
| | - Hao Yuan Kueh
- Division of Biology & Biological Engineering, California Institute of Technology, Pasadena, CA, USA
| | - Mary A Yui
- Division of Biology & Biological Engineering, California Institute of Technology, Pasadena, CA, USA
| | - Jingli A Zhang
- Division of Biology & Biological Engineering, California Institute of Technology, Pasadena, CA, USA
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46
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Kong D, Wolf F, Großhans J. Forces directing germ-band extension in Drosophila embryos. Mech Dev 2016; 144:11-22. [PMID: 28013027 DOI: 10.1016/j.mod.2016.12.001] [Citation(s) in RCA: 48] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/28/2016] [Revised: 12/12/2016] [Accepted: 12/13/2016] [Indexed: 01/06/2023]
Abstract
Body axis elongation by convergent extension is a conserved developmental process found in all metazoans. Drosophila embryonic germ-band extension is an important morphogenetic process during embryogenesis, by which the length of the germ-band is more than doubled along the anterior-posterior axis. This lengthening is achieved by typical convergent extension, i.e. narrowing the lateral epidermis along the dorsal-ventral axis and simultaneous extension along the anterior-posterior axis. Germ-band extension is largely driven by cell intercalation, whose directionality is determined by the planar polarity of the tissue and ultimately by the anterior-posterior patterning system. In addition, extrinsic tensile forces originating from the invaginating endoderm induce cell shape changes, which transiently contribute to germ-band extension. Here, we review recent progress in understanding of the role of mechanical forces in germ-band extension.
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Affiliation(s)
- Deqing Kong
- Institute for Developmental Biochemistry, Medical School, University of Göttingen, Justus-von-Liebig Weg 11, 37077 Göttingen, Germany
| | - Fred Wolf
- Department of Nonlinear Dynamics, Max Planck Institute for Dynamics and Self-Organisation, Faculty of Physics, Bernstein Center for Computational Neuroscience, University of Göttingen, Am Faßberg 17, 37077 Göttingen, Germany
| | - Jörg Großhans
- Institute for Developmental Biochemistry, Medical School, University of Göttingen, Justus-von-Liebig Weg 11, 37077 Göttingen, Germany.
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47
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Preger-Ben Noon E, Davis FP, Stern DL. Evolved Repression Overcomes Enhancer Robustness. Dev Cell 2016; 39:572-584. [PMID: 27840106 DOI: 10.1016/j.devcel.2016.10.010] [Citation(s) in RCA: 25] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/30/2016] [Revised: 07/26/2016] [Accepted: 10/14/2016] [Indexed: 12/18/2022]
Abstract
Biological systems display extraordinary robustness. Robustness of transcriptional enhancers results mainly from clusters of binding sites for the same transcription factor, and it is not clear how robust enhancers can evolve loss of expression through point mutations. Here, we report the high-resolution functional dissection of a robust enhancer of the shavenbaby gene that has contributed to morphological evolution. We found that robustness is encoded by many binding sites for the transcriptional activator Arrowhead and that, during evolution, some of these activator sites were lost, weakening enhancer activity. Complete silencing of enhancer function, however, required evolution of a binding site for the spatially restricted potent repressor Abrupt. These findings illustrate that recruitment of repressor binding sites can overcome enhancer robustness and may minimize pleiotropic consequences of enhancer evolution. Recruitment of repression may be a general mode of evolution to break robust regulatory linkages.
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Affiliation(s)
- Ella Preger-Ben Noon
- Janelia Research Campus, Howard Hughes Medical Institute, 19700 Helix Drive, Ashburn, VA 20147, USA.
| | - Fred P Davis
- Janelia Research Campus, Howard Hughes Medical Institute, 19700 Helix Drive, Ashburn, VA 20147, USA
| | - David L Stern
- Janelia Research Campus, Howard Hughes Medical Institute, 19700 Helix Drive, Ashburn, VA 20147, USA.
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48
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Andrioli LP, Dos Santos WS, Aguiar FDS, Digiampietri LA. Repression activity of Tailless on h 1 and eve 1 pair-rule stripes. Mech Dev 2016; 144:156-162. [PMID: 27773632 DOI: 10.1016/j.mod.2016.10.002] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/28/2016] [Revised: 10/18/2016] [Accepted: 10/19/2016] [Indexed: 10/20/2022]
Abstract
We investigated the hypothesis that several transcriptional repressors are necessary to set the boundaries of anterior pair-rule stripes in Drosophila. Specifically, we tested whether Tailless (Tll) is part of a repression mechanism that correctly sets the anterior boundaries of hairy 1 (h 1) and even-skipped 1 (eve 1) stripes. Single mutant tll embryos displayed subtle deviations from the normal positions of h 1 and eve 1 stripes. Moreover, we observed stronger stripe deviations in embryos lacking both Tll and Sloppy-paired 1 (Slp 1), a common repressor for anterior pair-rule stripes. Using h 1 and eve 1 reporter constructs in the genetic assays, we provided further evidence that interference with normal mechanisms of stripe expression is mediated by Tll repression. Indeed, Tll represses both h 1 and eve 1 reporter stripes when misexpressed. Investigating the expression of other anterior gap genes in different genetic backgrounds and in the misexpression assays strengthened Tll direct repression in the regulation of h 1 and eve 1. Our results are consistent with tll being a newly-identified component of a combinatorial network of repressor genes that control pair-rule stripe formation in the anterior blastoderm of Drosophila.
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Affiliation(s)
- Luiz Paulo Andrioli
- Escola de Artes, Ciências e Humanidades, Universidade de São Paulo, R. Arlindo Béttio 1000, Ermelino Matarazzo, 03828-000 São Paulo, SP, Brazil.
| | - Wesley Silva Dos Santos
- Escola de Artes, Ciências e Humanidades, Universidade de São Paulo, R. Arlindo Béttio 1000, Ermelino Matarazzo, 03828-000 São Paulo, SP, Brazil
| | - Francisco Dos Santos Aguiar
- Escola de Artes, Ciências e Humanidades, Universidade de São Paulo, R. Arlindo Béttio 1000, Ermelino Matarazzo, 03828-000 São Paulo, SP, Brazil
| | - Luciano Antonio Digiampietri
- Escola de Artes, Ciências e Humanidades, Universidade de São Paulo, R. Arlindo Béttio 1000, Ermelino Matarazzo, 03828-000 São Paulo, SP, Brazil
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49
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Surkova SY, Golubkova EV, Mamon LA, Samsonova MG. Morphogenetic networks which determine the spatial expression of zygotic genes in early Drosophila embryo. Russ J Dev Biol 2016. [DOI: 10.1134/s1062360416040093] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
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50
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Esposito E, Lim B, Guessous G, Falahati H, Levine M. Mitosis-associated repression in development. Genes Dev 2016; 30:1503-8. [PMID: 27401553 PMCID: PMC4949323 DOI: 10.1101/gad.281188.116] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/19/2016] [Accepted: 06/13/2016] [Indexed: 11/25/2022]
Abstract
Transcriptional repression is a pervasive feature of animal development. Here, we employ live-imaging methods to visualize the Snail repressor, which establishes the boundary between the presumptive mesoderm and neurogenic ectoderm of early Drosophila embryos. Snail target enhancers were attached to an MS2 reporter gene, permitting detection of nascent transcripts in living embryos. The transgenes exhibit initially broad patterns of transcription but are refined by repression in the mesoderm following mitosis. These observations reveal a correlation between mitotic silencing and Snail repression. We propose that mitosis and other inherent discontinuities in transcription boost the activities of sequence-specific repressors, such as Snail.
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Affiliation(s)
- Emilia Esposito
- Lewis-Sigler Institute for Integrative Genomics, Princeton University, Princeton, New Jersey 08544, USA; Department of Molecular and Cell Biology, Division of Genetics, Genomics, and Development, University of California at Berkeley, Berkeley, California 94720, USA
| | - Bomyi Lim
- Lewis-Sigler Institute for Integrative Genomics, Princeton University, Princeton, New Jersey 08544, USA
| | - Ghita Guessous
- Department of Physics, Princeton University, Princeton, New Jersey 08544, USA
| | - Hanieh Falahati
- Lewis-Sigler Institute for Integrative Genomics, Princeton University, Princeton, New Jersey 08544, USA
| | - Michael Levine
- Lewis-Sigler Institute for Integrative Genomics, Princeton University, Princeton, New Jersey 08544, USA; Department of Molecular Biology, Princeton University, Princeton, New Jersey 08544, USA
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