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Söderholm S, Jauregi-Miguel A, Pagella P, Ghezzi V, Zambanini G, Nordin A, Cantù C. Single-cell response to Wnt signaling activation reveals uncoupling of Wnt target gene expression. Exp Cell Res 2023:113646. [PMID: 37271249 DOI: 10.1016/j.yexcr.2023.113646] [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: 12/21/2022] [Revised: 05/09/2023] [Accepted: 05/14/2023] [Indexed: 06/06/2023]
Abstract
Wnt signaling drives nuclear translocation of β-catenin and its subsequent association with the DNA-bound TCF/LEF transcription factors, which dictate target gene specificity by recognizing Wnt responsive elements across the genome. β-Catenin target genes are therefore thought to be collectively activated upon Wnt pathway stimulation. However, this appears in contrast with the non-overlapping patterns of Wnt target gene expression in several contexts, including early mammalian embryogenesis. Here we followed Wnt target gene expression in human embryonic stem cells after Wnt pathway stimulation at a single-cell resolution. Cells changed gene expression program over time consistent with three key developmental events: i) loss of pluripotency, ii) induction of Wnt target genes, and iii) mesoderm specification. Contrary to our expectation, not all cells displayed equal amplitude of Wnt target gene activation; rather, they distributed in a continuum from strong to weak responders when ranked based on the expression of the target AXIN2. Moreover, high AXIN2 did not always correspond to elevated expression of other Wnt targets, which were activated in different proportions in individual cells. The uncoupling of Wnt target gene expression was also identified in single cell transcriptomics profiling of other Wnt-responding cell types, including HEK293T, murine developing forelimbs, and human colorectal cancer. Our finding underlines the necessity to identify additional mechanisms that explain the heterogeneity of the Wnt/β-catenin-mediated transcriptional outputs in single cells.
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Affiliation(s)
- Simon Söderholm
- Wallenberg Centre for Molecular Medicine, Linköping University, Linköping, Sweden; Department of Biomedical and Clinical Sciences, Division of Molecular Medicine and Virology, Faculty of Medicine and Health Sciences, Linköping University, Linköping, Sweden
| | - Amaia Jauregi-Miguel
- Wallenberg Centre for Molecular Medicine, Linköping University, Linköping, Sweden; Department of Biomedical and Clinical Sciences, Division of Molecular Medicine and Virology, Faculty of Medicine and Health Sciences, Linköping University, Linköping, Sweden
| | - Pierfrancesco Pagella
- Wallenberg Centre for Molecular Medicine, Linköping University, Linköping, Sweden; Department of Biomedical and Clinical Sciences, Division of Molecular Medicine and Virology, Faculty of Medicine and Health Sciences, Linköping University, Linköping, Sweden
| | - Valeria Ghezzi
- Wallenberg Centre for Molecular Medicine, Linköping University, Linköping, Sweden; Department of Biomedical and Clinical Sciences, Division of Molecular Medicine and Virology, Faculty of Medicine and Health Sciences, Linköping University, Linköping, Sweden
| | - Gianluca Zambanini
- Wallenberg Centre for Molecular Medicine, Linköping University, Linköping, Sweden; Department of Biomedical and Clinical Sciences, Division of Molecular Medicine and Virology, Faculty of Medicine and Health Sciences, Linköping University, Linköping, Sweden
| | - Anna Nordin
- Wallenberg Centre for Molecular Medicine, Linköping University, Linköping, Sweden; Department of Biomedical and Clinical Sciences, Division of Molecular Medicine and Virology, Faculty of Medicine and Health Sciences, Linköping University, Linköping, Sweden
| | - Claudio Cantù
- Wallenberg Centre for Molecular Medicine, Linköping University, Linköping, Sweden; Department of Biomedical and Clinical Sciences, Division of Molecular Medicine and Virology, Faculty of Medicine and Health Sciences, Linköping University, Linköping, Sweden.
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2
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Kikuchi A, Takagi J, Takada S, Ishitani T, Minami Y. Wnt 2022 EMBO | the Company of Biologists workshop and Yamada conference. Genes Cells 2023; 28:249-257. [PMID: 36846946 DOI: 10.1111/gtc.13014] [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: 01/16/2023] [Revised: 02/14/2023] [Accepted: 02/15/2023] [Indexed: 03/01/2023]
Abstract
Wnt2022 was held on November 15th-19th, 2022, in Awaji Yumebutai International Conference Center, Hyogo Prefecture, Japan, as an in-person meeting for the first time in last 3 years. Wnt signaling is a highly conserved pathway among various species. Since Wnt1 was discovered in 1982, a number of studies using many model animals and human samples have revealed that Wnt signaling plays crucial roles in embryonic development, tissue morphogenesis, and regeneration, as well as many other physiological and pathological processes. Since the year 2022 marks the 40th anniversary of Wnt research, we aimed to look back at our research progress and discuss the future direction of this field. The scientific program consisted of plenary lectures, invited talks, short talks selected from abstracts, and poster sessions. Whereas several different Wnt meetings have been held almost every year in Europe and the United States, this was the first Wnt meeting convened in Asia. Therefore, Wnt2022 was highly anticipated to bring together leaders and young scientists from Europe, the United States, and especially Asia and Oceania. In fact, 148 researchers from 21 countries attended this meeting. Although there were travel and administrative restrictions due to COVID-19, the meeting was highly successful in enabling face-to-face discussions.
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Affiliation(s)
- Akira Kikuchi
- Center for Infectious Disease Education and Research, Osaka University, Suita, Japan
| | - Junichi Takagi
- Institute for Protein Research, Osaka University, Suita, Japan
| | - Shinji Takada
- National Institutes of Basic Biology, Okazaki, Japan.,Exploratory Research Center on Life and Living Systems (ExCELLS), National Institutes of Natural Sciences, Okazaki, Japan.,The Graduate University for Advanced Studies (SOKENDAI), Okazaki, Japan
| | - Tohru Ishitani
- Reserach Institute for Microbial Diseases, Osaka University, Suita, Japan
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3
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Pluta R, Aragón E, Prescott NA, Ruiz L, Mees RA, Baginski B, Flood JR, Martin-Malpartida P, Massagué J, David Y, Macias MJ. Molecular basis for DNA recognition by the maternal pioneer transcription factor FoxH1. Nat Commun 2022; 13:7279. [PMID: 36435807 PMCID: PMC9701222 DOI: 10.1038/s41467-022-34925-y] [Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/04/2022] [Accepted: 11/10/2022] [Indexed: 11/28/2022] Open
Abstract
Forkhead box H1 (FoxH1) is an essential maternal pioneer factor during embryonic development that binds to specific GG/GT-containing DNA target sequences. Here we have determined high-resolution structures of three FoxH1 proteins (from human, frog and fish species) and four DNAs to clarify the way in which FoxH1 binds to these sites. We found that the protein-DNA interactions extend to both the minor and major DNA grooves and are thus almost twice as extensive as those of other FOX family members. Moreover, we identified two specific amino acid changes in FoxH1 that allowed the recognition of GG/GT motifs. Consistent with the pioneer factor activity of FoxH1, we found that its affinity for nucleosomal DNA is even higher than for linear DNA fragments. The structures reported herein illustrate how FoxH1 binding to distinct DNA sites provides specificity and avoids cross-regulation by other FOX proteins that also operate during the maternal-zygotic transition and select canonical forkhead sites.
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Affiliation(s)
- Radoslaw Pluta
- Institute for Research in Biomedicine (IRB Barcelona), The Barcelona Institute of Science and Technology (BIST), Barcelona, 08028, Spain
| | - Eric Aragón
- Institute for Research in Biomedicine (IRB Barcelona), The Barcelona Institute of Science and Technology (BIST), Barcelona, 08028, Spain
| | - Nicholas A Prescott
- Tri-Institutional PhD Program in Chemical Biology, New York, NY, USA
- Chemical Biology Program, Memorial Sloan Kettering Cancer Center, New York, NY, 10065, USA
| | - Lidia Ruiz
- Institute for Research in Biomedicine (IRB Barcelona), The Barcelona Institute of Science and Technology (BIST), Barcelona, 08028, Spain
| | - Rebeca A Mees
- Institute for Research in Biomedicine (IRB Barcelona), The Barcelona Institute of Science and Technology (BIST), Barcelona, 08028, Spain
| | - Blazej Baginski
- Institute for Research in Biomedicine (IRB Barcelona), The Barcelona Institute of Science and Technology (BIST), Barcelona, 08028, Spain
| | - Julia R Flood
- Chemical Biology Program, Memorial Sloan Kettering Cancer Center, New York, NY, 10065, USA
| | - Pau Martin-Malpartida
- Institute for Research in Biomedicine (IRB Barcelona), The Barcelona Institute of Science and Technology (BIST), Barcelona, 08028, Spain
| | - Joan Massagué
- Cancer Biology and Genetics Program, Memorial Sloan Kettering Cancer Center, New York, NY, 10065, USA
| | - Yael David
- Chemical Biology Program, Memorial Sloan Kettering Cancer Center, New York, NY, 10065, USA
- Department of Pharmacology, Weill Cornell Medicine, New York, NY, 10065, USA
- Department of Physiology, Biophysics and Systems Biology, Weill Cornell Medicine, New York, NY, 10065, USA
| | - Maria J Macias
- Institute for Research in Biomedicine (IRB Barcelona), The Barcelona Institute of Science and Technology (BIST), Barcelona, 08028, Spain.
- Institució Catalana de Recerca i Estudis Avançats (ICREA), Passeig Lluís Companys 23, Barcelona, 08010, Spain.
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4
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Giuraniuc CV, Zain S, Ghafoor S, Hoppler S. A mathematical modelling portrait of Wnt signalling in early vertebrate embryogenesis. J Theor Biol 2022; 551-552:111239. [DOI: 10.1016/j.jtbi.2022.111239] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/07/2022] [Revised: 06/30/2022] [Accepted: 07/29/2022] [Indexed: 11/28/2022]
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5
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Jansen C, Paraiso KD, Zhou JJ, Blitz IL, Fish MB, Charney RM, Cho JS, Yasuoka Y, Sudou N, Bright AR, Wlizla M, Veenstra GJC, Taira M, Zorn AM, Mortazavi A, Cho KWY. Uncovering the mesendoderm gene regulatory network through multi-omic data integration. Cell Rep 2022; 38:110364. [PMID: 35172134 PMCID: PMC8917868 DOI: 10.1016/j.celrep.2022.110364] [Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/18/2020] [Revised: 10/30/2021] [Accepted: 01/19/2022] [Indexed: 01/01/2023] Open
Abstract
Mesendodermal specification is one of the earliest events in embryogenesis, where cells first acquire distinct identities. Cell differentiation is a highly regulated process that involves the function of numerous transcription factors (TFs) and signaling molecules, which can be described with gene regulatory networks (GRNs). Cell differentiation GRNs are difficult to build because existing mechanistic methods are low throughput, and high-throughput methods tend to be non-mechanistic. Additionally, integrating highly dimensional data composed of more than two data types is challenging. Here, we use linked self-organizing maps to combine chromatin immunoprecipitation sequencing (ChIP-seq)/ATAC-seq with temporal, spatial, and perturbation RNA sequencing (RNA-seq) data from Xenopus tropicalis mesendoderm development to build a high-resolution genome scale mechanistic GRN. We recover both known and previously unsuspected TF-DNA/TF-TF interactions validated through reporter assays. Our analysis provides insights into transcriptional regulation of early cell fate decisions and provides a general approach to building GRNs using highly dimensional multi-omic datasets.
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Affiliation(s)
- Camden Jansen
- Department of Developmental and Cell Biology, University of California, Irvine, CA, USA; Center for Complex Biological Systems, University of California, Irvine, CA, USA
| | - Kitt D Paraiso
- Department of Developmental and Cell Biology, University of California, Irvine, CA, USA; Center for Complex Biological Systems, University of California, Irvine, CA, USA
| | - Jeff J Zhou
- Department of Developmental and Cell Biology, University of California, Irvine, CA, USA
| | - Ira L Blitz
- Department of Developmental and Cell Biology, University of California, Irvine, CA, USA
| | - Margaret B Fish
- Department of Developmental and Cell Biology, University of California, Irvine, CA, USA
| | - Rebekah M Charney
- Department of Developmental and Cell Biology, University of California, Irvine, CA, USA
| | - Jin Sun Cho
- Department of Developmental and Cell Biology, University of California, Irvine, CA, USA
| | - Yuuri Yasuoka
- Laboratory for Comprehensive Genomic Analysis, RIKEN Center for Integrative Medical Sciences, Yokohama, Japan
| | - Norihiro Sudou
- Department of Anatomy, School of Medicine, Toho University, Tokyo, Japan
| | - Ann Rose Bright
- Department of Molecular Developmental Biology, Radboud University, Nijmegen, the Netherlands
| | - Marcin Wlizla
- Division of Developmental Biology, Department of Pediatrics, Cincinnati Children's Hospital Medical Center, University of Cincinnati College of Medicine, Cincinnati, OH, USA
| | - Gert Jan C Veenstra
- Department of Molecular Developmental Biology, Radboud University, Nijmegen, the Netherlands
| | - Masanori Taira
- Department of Biological Sciences, Chuo University, Tokyo, Japan
| | - Aaron M Zorn
- Division of Developmental Biology, Department of Pediatrics, Cincinnati Children's Hospital Medical Center, University of Cincinnati College of Medicine, Cincinnati, OH, USA
| | - Ali Mortazavi
- Department of Developmental and Cell Biology, University of California, Irvine, CA, USA; Center for Complex Biological Systems, University of California, Irvine, CA, USA.
| | - Ken W Y Cho
- Department of Developmental and Cell Biology, University of California, Irvine, CA, USA; Center for Complex Biological Systems, University of California, Irvine, CA, USA.
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6
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Jones WD, Mullins MC. Cell signaling pathways controlling an axis organizing center in the zebrafish. Curr Top Dev Biol 2022; 150:149-209. [DOI: 10.1016/bs.ctdb.2022.03.005] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/11/2023]
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Wang Y, Yu Z, Fan Z, Fang Y, He L, Peng M, Chen Y, Hu Z, Zhao K, Zhang H, Liu C. Cardiac developmental toxicity and transcriptome analyses of zebrafish (Danio rerio) embryos exposed to Mancozeb. ECOTOXICOLOGY AND ENVIRONMENTAL SAFETY 2021; 226:112798. [PMID: 34592528 DOI: 10.1016/j.ecoenv.2021.112798] [Citation(s) in RCA: 13] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/23/2021] [Revised: 09/12/2021] [Accepted: 09/15/2021] [Indexed: 06/13/2023]
Abstract
Mancozeb (MZ), an antibacterial pesticide, has been linked to reproductive toxicity, neurotoxicity, and endocrine disruption. However, whether MZ has cardiactoxicity is unclear. In this study, the cardiotoxic effects of exposure to environment-related MZ concentrations ranging from 1.88 μM to 7.52 μM were evaluated at the larval stage of zebrafish. Transcriptome sequencing predicted the mechanism of MZ-induced cardiac developmental toxicity in zebrafish by enrichment analysis of Kyoto Encyclopedia of Genes and Genomes (KEGG) and Gene Ontology (GO). Consistent with morphological changes, the osm, pfkfb3, foxh1, stc1, and nrarpb genes may effect normal development of zebrafish heart by activating NOTCH signaling pathways, resulting in pericardial edema, myocardial fibrosis, and congestion in the heart area. Moreover, differential gene expression analysis indicated that cyp-related genes (cyp1c2 and cyp3c3) were significantly upregulated after MZ treatment, which may be related to apoptosis of myocardial cells. These results were verified by real-time quantitative RT-qPCR and acridine orange staining. Our findings suggest that MZ-mediated cardiotoxic development of zebrafish larvae may be related to the activation of Notch and apoptosis-related signaling pathways.
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Affiliation(s)
- Yongfeng Wang
- Institute of Reproductive Health, Tongji Medical College, Huazhong University of Science and Technology, Hubei 430030, PR China.
| | - Zhiquan Yu
- Institute of Reproductive Health, Tongji Medical College, Huazhong University of Science and Technology, Hubei 430030, PR China.
| | - Zunpan Fan
- Institute of Reproductive Health, Tongji Medical College, Huazhong University of Science and Technology, Hubei 430030, PR China.
| | - Yiwei Fang
- Institute of Reproductive Health, Tongji Medical College, Huazhong University of Science and Technology, Hubei 430030, PR China.
| | - Liting He
- Institute of Reproductive Health, Tongji Medical College, Huazhong University of Science and Technology, Hubei 430030, PR China.
| | - Meili Peng
- Institute of Reproductive Health, Tongji Medical College, Huazhong University of Science and Technology, Hubei 430030, PR China.
| | - Yuanyao Chen
- Institute of Reproductive Health, Tongji Medical College, Huazhong University of Science and Technology, Hubei 430030, PR China.
| | - Zhiyong Hu
- Institute of Reproductive Health, Tongji Medical College, Huazhong University of Science and Technology, Hubei 430030, PR China.
| | - Kai Zhao
- Institute of Reproductive Health, Tongji Medical College, Huazhong University of Science and Technology, Hubei 430030, PR China.
| | - Huiping Zhang
- Institute of Reproductive Health, Tongji Medical College, Huazhong University of Science and Technology, Hubei 430030, PR China.
| | - Chunyan Liu
- Institute of Reproductive Health, Tongji Medical College, Huazhong University of Science and Technology, Hubei 430030, PR China.
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Bou-Rouphael J, Durand BC. T-Cell Factors as Transcriptional Inhibitors: Activities and Regulations in Vertebrate Head Development. Front Cell Dev Biol 2021; 9:784998. [PMID: 34901027 PMCID: PMC8651982 DOI: 10.3389/fcell.2021.784998] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/28/2021] [Accepted: 10/28/2021] [Indexed: 11/22/2022] Open
Abstract
Since its first discovery in the late 90s, Wnt canonical signaling has been demonstrated to affect a large variety of neural developmental processes, including, but not limited to, embryonic axis formation, neural proliferation, fate determination, and maintenance of neural stem cells. For decades, studies have focused on the mechanisms controlling the activity of β-catenin, the sole mediator of Wnt transcriptional response. More recently, the spotlight of research is directed towards the last cascade component, the T-cell factor (TCF)/Lymphoid-Enhancer binding Factor (LEF), and more specifically, the TCF/LEF-mediated switch from transcriptional activation to repression, which in both embryonic blastomeres and mouse embryonic stem cells pushes the balance from pluri/multipotency towards differentiation. It has been long known that Groucho/Transducin-Like Enhancer of split (Gro/TLE) is the main co-repressor partner of TCF/LEF. More recently, other TCF/LEF-interacting partners have been identified, including the pro-neural BarH-Like 2 (BARHL2), which belongs to the evolutionary highly conserved family of homeodomain-containing transcription factors. This review describes the activities and regulatory modes of TCF/LEF as transcriptional repressors, with a specific focus on the functions of Barhl2 in vertebrate brain development. Specific attention is given to the transcriptional events leading to formation of the Organizer, as well as the roles and regulations of Wnt/β-catenin pathway in growth of the caudal forebrain. We present TCF/LEF activities in both embryonic and neural stem cells and discuss how alterations of this pathway could lead to tumors.
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Affiliation(s)
| | - Béatrice C. Durand
- Sorbonne Université, CNRS UMR7622, IBPS Developmental Biology Laboratory, Campus Pierre et Marie Curie, Paris, France
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Koch S. Regulation of Wnt Signaling by FOX Transcription Factors in Cancer. Cancers (Basel) 2021; 13:cancers13143446. [PMID: 34298659 PMCID: PMC8307807 DOI: 10.3390/cancers13143446] [Citation(s) in RCA: 10] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/16/2021] [Revised: 07/04/2021] [Accepted: 07/07/2021] [Indexed: 12/29/2022] Open
Abstract
Simple Summary Cancer is caused by a breakdown of cell-to-cell communication, which results in the unrestricted expansion of cells within a tissue. In many cases, tumor growth is maintained by the continuous activation of cell signaling programs that normally drive embryonic development and wound repair. In this review article, I discuss how one of the largest human protein families, namely FOX proteins, controls the activity of the Wnt pathway, a major regulatory signaling cascade in developing organisms and adult stem cells. Evidence suggests that there is considerable crosstalk between FOX proteins and the Wnt pathway, which contributes to cancer initiation and progression. A better understanding of FOX biology may therefore lead to the development of new targeted treatments for many types of cancer. Abstract Aberrant activation of the oncogenic Wnt signaling pathway is a hallmark of numerous types of cancer. However, in many cases, it is unclear how a chronically high Wnt signaling tone is maintained in the absence of activating pathway mutations. Forkhead box (FOX) family transcription factors are key regulators of embryonic development and tissue homeostasis, and there is mounting evidence that they act in part by fine-tuning the Wnt signaling output in a tissue-specific and context-dependent manner. Here, I review the diverse ways in which FOX transcription factors interact with the Wnt pathway, and how the ectopic reactivation of FOX proteins may affect Wnt signaling activity in various types of cancer. Many FOX transcription factors are partially functionally redundant and exhibit a highly restricted expression pattern, especially in adults. Thus, precision targeting of individual FOX proteins may lead to safe treatment options for Wnt-dependent cancers.
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Affiliation(s)
- Stefan Koch
- Wallenberg Centre for Molecular Medicine (WCMM), Linköping University, 58185 Linköping, Sweden; ; Tel.: +46-132-829-69
- Department of Biomedical and Clinical Sciences (BKV), Linköping University, 58185 Linköping, Sweden
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10
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Abstract
The endoderm is the innermost germ layer that forms the linings of the respiratory and gastrointestinal tracts, and their associated organs, during embryonic development. Xenopus embryology experiments have provided fundamental insights into how the endoderm develops in vertebrates, including the critical role of TGFβ-signaling in endoderm induction,elucidating the gene regulatory networks controlling germ layer development and the key molecular mechanisms regulating endoderm patterning and morphogenesis. With new genetic, genomic, and imaging approaches, Xenopus is now routinely used to model human disease, discover mechanisms underlying endoderm organogenesis, and inform differentiation protocols for pluripotent stem cell differentiation and regenerative medicine applications. In this chapter, we review historical and current discoveries of endoderm development in Xenopus, then provide examples of modeling human disease and congenital defects of endoderm-derived organs using Xenopus.
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Affiliation(s)
- Nicole A Edwards
- Division of Developmental Biology, Center for Stem Cell and Organoid Medicine, Perinatal Institute, Cincinnati Children's Hospital Medical Center, Cincinnati, OH, United States.
| | - Aaron M Zorn
- Division of Developmental Biology, Center for Stem Cell and Organoid Medicine, Perinatal Institute, Cincinnati Children's Hospital Medical Center, Cincinnati, OH, United States; Department of Pediatrics, University of Cincinnati College of Medicine, Cincinnati, OH, United States.
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Abstract
The fertilized frog egg contains all the materials needed to initiate development of a new organism, including stored RNAs and proteins deposited during oogenesis, thus the earliest stages of development do not require transcription. The onset of transcription from the zygotic genome marks the first genetic switch activating the gene regulatory network that programs embryonic development. Zygotic genome activation occurs after an initial phase of transcriptional quiescence that continues until the midblastula stage, a period called the midblastula transition, which was first identified in Xenopus. Activation of transcription is programmed by maternally supplied factors and is regulated at multiple levels. A similar switch exists in most animals and is of great interest both to developmental biologists and to those interested in understanding nuclear reprogramming. Here we review in detail our knowledge on this major switch in transcription in Xenopus and place recent discoveries in the context of a decades old problem.
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