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Asashima M, Satou-Kobayashi Y. Spemann-Mangold organizer and mesoderm induction. Cells Dev 2024; 178:203903. [PMID: 38295873 DOI: 10.1016/j.cdev.2024.203903] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/13/2023] [Revised: 01/23/2024] [Accepted: 01/24/2024] [Indexed: 02/09/2024]
Abstract
The discovery of the Spemann-Mangold organizer strongly influenced subsequent research on embryonic induction, with research aiming to elucidate the molecular characteristics of organizer activity being currently underway. Herein, we review the history of research on embryonic induction, and describe how the mechanisms of induction phenomena and developmental processes have been investigated. Classical experiments investigating the differentiation capacity and inductive activity of various embryonic regions were conducted by many researchers, and important theories of region-specific induction and the concept for chain of induction were proposed. The transition from experimental embryology to developmental biology has enabled us to understand the mechanisms of embryonic induction at the molecular level. Consequently, many inducing substances and molecules such as transcriptional factors and peptide growth factors involved in the organizer formation were identified. One of peptide growth factors, activin, acts as a mesoderm- and endoderm-inducing substance. Activin induces several tissues and organs from the undifferentiated cell mass of amphibian embryos in a concentration-dependent manner. We review the extent to which we can control in vitro organogenesis from undifferentiated cells, and discuss the application to stem cell-based regenerative medicine based on insights gained from animal experiments, such as in amphibians.
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Affiliation(s)
- Makoto Asashima
- Advanced Comprehensive Research Organization, Teikyo University, 2-11-1 Kaga, Itabashi-ku, Tokyo 173-0003, Japan.
| | - Yumeko Satou-Kobayashi
- Advanced Comprehensive Research Organization, Teikyo University, 2-11-1 Kaga, Itabashi-ku, Tokyo 173-0003, Japan
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2
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Janssen R, Budd GE. New insights into mesoderm and endoderm development, and the nature of the onychophoran blastopore. Front Zool 2024; 21:2. [PMID: 38267986 PMCID: PMC10809584 DOI: 10.1186/s12983-024-00521-7] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/29/2023] [Accepted: 01/09/2024] [Indexed: 01/26/2024] Open
Abstract
BACKGROUND Early during onychophoran development and prior to the formation of the germ band, a posterior tissue thickening forms the posterior pit. Anterior to this thickening forms a groove, the embryonic slit, that marks the anterior-posterior orientation of the developing embryo. This slit is by some authors considered the blastopore, and thus the origin of the endoderm, while others argue that the posterior pit represents the blastopore. This controversy is of evolutionary significance because if the slit represents the blastopore, then this would support the amphistomy hypothesis that suggests that a slit-like blastopore in the bilaterian ancestor evolved into protostomy and deuterostomy. RESULTS In this paper, we summarize our current knowledge about endoderm and mesoderm development in onychophorans and provide additional data on early endoderm- and mesoderm-determining marker genes such as Blimp, Mox, and the T-box genes. CONCLUSION We come to the conclusion that the endoderm of onychophorans forms prior to the development of the embryonic slit, and thus that the slit is not the primary origin of the endoderm. It is thus unlikely that the embryonic slit represents the blastopore. We suggest instead that the posterior pit indeed represents the lips of the blastopore, and that the embryonic slit (and surrounding tissue) represents a morphologically superficial archenteron-like structure. We conclude further that both endoderm and mesoderm development are under control of conserved gene regulatory networks, and that many of the features found in arthropods including the model Drosophila melanogaster are likely derived.
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Affiliation(s)
- Ralf Janssen
- Department of Earth Sciences, Palaeobiology, Uppsala University, Villavägen 16, 75236, Uppsala, Sweden.
| | - Graham E Budd
- Department of Earth Sciences, Palaeobiology, Uppsala University, Villavägen 16, 75236, Uppsala, Sweden
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Trinh LT, Osipovich AB, Sampson L, Wong J, Wright CV, Magnuson MA. Differential regulation of alternate promoter regions in Sox17 during endodermal and vascular endothelial development. iScience 2022; 25:104905. [PMID: 36046192 PMCID: PMC9421400 DOI: 10.1016/j.isci.2022.104905] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/21/2021] [Revised: 04/06/2022] [Accepted: 08/05/2022] [Indexed: 11/20/2022] Open
Abstract
Sox17 gene expression is essential for both endothelial and endodermal cell differentiation. To better understand the genetic basis for the expression of multiple Sox17 mRNA forms, we identified and performed CRISPR/Cas9 mutagenesis of two evolutionarily conserved promoter regions (CRs). The deletion of the upstream and endothelial cell-specific CR1 caused only a modest increase in lympho-vasculogenesis likely via reduced Notch signaling downstream of SOX17. In contrast, the deletion of the downstream CR2 region, which functions in both endothelial and endodermal cells, impairs both vascular and endodermal development causing death by embryonic day 12.5. Analyses of 3D chromatin looping, transcription factor binding, histone modification, and chromatin accessibility data at the Sox17 locus and surrounding region further support differential regulation of the two promoters during the development.
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Affiliation(s)
- Linh T. Trinh
- Department of Cell and Developmental Biology, Vanderbilt University, Nashville, TN 37232, USA
- Center for Stem Cell Biology, Vanderbilt University, Nashville, TN 37232, USA
- Program in Developmental Biology, Vanderbilt University, Nashville, TN 37232, USA
| | - Anna B. Osipovich
- Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, TN 37232, USA
- Center for Stem Cell Biology, Vanderbilt University, Nashville, TN 37232, USA
| | - Leesa Sampson
- Center for Stem Cell Biology, Vanderbilt University, Nashville, TN 37232, USA
| | - Jonathan Wong
- College of Arts and Science, Vanderbilt University, Nashville, TN 37232, USA
| | - Chris V.E. Wright
- Department of Cell and Developmental Biology, Vanderbilt University, Nashville, TN 37232, USA
- Center for Stem Cell Biology, Vanderbilt University, Nashville, TN 37232, USA
- Program in Developmental Biology, Vanderbilt University, Nashville, TN 37232, USA
| | - Mark A. Magnuson
- Department of Cell and Developmental Biology, Vanderbilt University, Nashville, TN 37232, USA
- Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, TN 37232, USA
- Center for Stem Cell Biology, Vanderbilt University, Nashville, TN 37232, USA
- Program in Developmental Biology, Vanderbilt University, Nashville, TN 37232, USA
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Borchers A, Pieler T. Programming pluripotent precursor cells derived from Xenopus embryos to generate specific tissues and organs. Genes (Basel) 2010; 1:413-26. [PMID: 24710095 PMCID: PMC3966229 DOI: 10.3390/genes1030413] [Citation(s) in RCA: 116] [Impact Index Per Article: 8.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/08/2010] [Revised: 10/21/2010] [Accepted: 11/05/2010] [Indexed: 11/16/2022] Open
Abstract
Xenopus embryos provide a rich source of pluripotent cells that can be differentiated into functional organs. Since the molecular principles of vertebrate organogenesis appear to be conserved between Xenopus and mammals, this system can provide useful guidelines for the directional manipulation of human embryonic stem cells. Pluripotent Xenopus cells can be easily isolated from the animal pole of blastula stage Xenopus embryos. These so called "animal cap" cells represent prospective ectodermal cells, but give rise to endodermal, mesodermal and neuro-ectodermal derivatives if treated with the appropriate factors. These factors include evolutionary conserved modulators of the key developmental signal transduction pathways that can be supplied either by mRNA microinjection or direct application of recombinant proteins. This relatively simple system has added to our understanding of pancreas, liver, kidney, eye and heart development. In particular, recent studies have used animal cap cells to generate ectopic eyes and hearts, setting the stage for future work aimed at programming pluripotent cells for regenerative medicine.
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Affiliation(s)
- Annette Borchers
- Department of Developmental Biochemistry, Center of Molecular Physiology of the Brain (CMPB), GZMB, University of Goettingen, Justus-von-Liebig-Weg 11, 37077 Goettingen, Germany.
| | - Tomas Pieler
- Department of Developmental Biochemistry, Center of Molecular Physiology of the Brain (CMPB), GZMB, University of Goettingen, Justus-von-Liebig-Weg 11, 37077 Goettingen, Germany.
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Middleton AM, King JR, Loose M. Bistability in a model of mesoderm and anterior mesendoderm specification in Xenopus laevis. J Theor Biol 2009; 260:41-55. [PMID: 19490918 DOI: 10.1016/j.jtbi.2009.05.016] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/24/2008] [Revised: 04/10/2009] [Accepted: 05/16/2009] [Indexed: 11/20/2022]
Abstract
In this paper we develop a model of mesendoderm specification in Xenopus laevis based on an existing gene regulation network. The mesendoderm is a population of cells that may contribute to either the mesoderm or endoderm. The model that we develop encompasses the time evolution of transcription factor concentrations in a single cell and is shown to have stable steady states that correspond to mesoderm and anterior mesendodermal cell types, but not endoderm (except in cells where Goosecoid expression is inhibited). Both in vitro and in vivo versions of the model are developed and analysed, the former indicating how cell fate is determined in large part by the concentration of Activin administered to a cell, with the model results comparing favourably with current quantitative experimental data. A numerical investigation of the in vivo model suggests that cell fate is determined largely by a VegT and beta-Catenin pre-pattern, subsequently being reinforced by Nodal. We argue that this sensitivity of the model to a VegT and beta-Catenin pre-pattern indicates that a key VegT self-limiting mechanism (for which there is experimental evidence) is absent from the model. Furthermore, we find that the lack of a steady state corresponding to endoderm is entirely consistent with current in vivo data, and that the in vivo model corresponds to mesendoderm specification on the dorsal, but not the ventral, side of the embryo.
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Cha SW, Lee JW, Hwang YS, Chae JP, Park KM, Cho HJ, Kim DS, Bae YC, Park MJ. Spatiotemporal regulation of fibroblast growth factor signal blocking for endoderm formation in Xenopus laevis. Exp Mol Med 2009; 40:550-7. [PMID: 18985013 DOI: 10.3858/emm.2008.40.5.550] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023] Open
Abstract
We have previously shown that the inhibition of fibroblast growth factor (FGF) signaling induced endodermal gene expression in the animal cap and caused the expansion of the endodermal mass in Xenopus embryos. However, we still do not know whether or not the alteration of FGF signaling controls embryonic cell fate, or when FGF signal blocking is required for endoderm formation in Xenopus. Here, we show that FGF signal blocking in embryonic cells causes their descendants to move into the endodermal region and to express endodermal genes. It is also interesting that blocking FGF signaling between fertilization and embryonic stage 10.5 promotes endoderm formation, but persistent FGF signaling blocking after stage 10.5 restricts endoderm formation and differentiation.
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Affiliation(s)
- Sang-wook Cha
- Department of Anatomy, School of Medicine, Kyungpook National University, Daegu 700-412, Korea
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Yagi Y, Ito Y, Kuhara S, Tashiro K. Cephalic hedgehog expression is regulated directly by Sox17 in endoderm development of Xenopus laevis. Cytotechnology 2008; 57:151-9. [PMID: 19003160 DOI: 10.1007/s10616-008-9127-2] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/27/2007] [Accepted: 01/18/2008] [Indexed: 11/28/2022] Open
Abstract
In early development of animals, hedgehog (Hh) genes function as morphogen in the axis determination and the organ formation. In Xenopus, three hedgehog genes, sonic (shh), banded (bhh), and cephalic (chh), were identified and might organize various tissues and organs in embryogenesis. Here, we report the spatial and temporal regulation of Xchh which is expressed in endoderm cells differentiating to digestive organs. Xchh expression in endoderm was inhibited by ectopic expression of the dominant-negative activin receptor, tAR. Moreover, a maternally inherited transcription factor VegT and its downstream regulators activated Xchh expression. These indicates that Xchh is regulated by the factor involved in the cascade originated from VegT via activin/nodal signals. Using the Sox17alpha-VP16-GR construct, we showed that Xchh expression might be induced directly by transcription factor Sox17.
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Affiliation(s)
- Yumihiko Yagi
- Graduate School of Systems Life Sciences, Kyushu University, Hakozaki, Higashi-ku, Fukuoka, 812-8581, Japan
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Smith J, Wardle F, Loose M, Stanley E, Patient R. Germ layer induction in ESC--following the vertebrate roadmap. ACTA ACUST UNITED AC 2008; Chapter 1:Unit 1D.1. [PMID: 18785165 DOI: 10.1002/9780470151808.sc01d01s1] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022]
Abstract
Controlled differentiation of pluripotential cells takes place routinely and with great success in developing vertebrate embryos. It therefore makes sense to take note of how this is achieved and use this knowledge to control the differentiation of embryonic stem cells (ESCs). An added advantage is that the differentiated cells resulting from this process in embryos have proven functionality and longevity. This unit reviews what is known about the embryonic signals that drive differentiation in one of the most informative of the vertebrate animal models of development, the amphibian Xenopus laevis. It summarizes their identities and the extent to which their activities are dose-dependent. The unit details what is known about the transcription factor responses to these signals, describing the networks of interactions that they generate. It then discusses the target genes of these transcription factors, the effectors of the differentiated state. Finally, how these same developmental programs operate during germ layer formation in the context of ESC differentiation is summarized.
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Affiliation(s)
- Jim Smith
- Wellcome Trust/Cancer Research UK Gurdon Institute, University of Cambridge, Cambridge, United Kingdom
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9
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Woodland HR, Zorn AM. The core endodermal gene network of vertebrates: combining developmental precision with evolutionary flexibility. Bioessays 2008; 30:757-65. [DOI: 10.1002/bies.20785] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023]
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Morris SA, Almeida AD, Tanaka H, Ohta K, Ohnuma SI. Tsukushi modulates Xnr2, FGF and BMP signaling: regulation of Xenopus germ layer formation. PLoS One 2007; 2:e1004. [PMID: 17925852 PMCID: PMC1994590 DOI: 10.1371/journal.pone.0001004] [Citation(s) in RCA: 34] [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: 07/03/2007] [Accepted: 09/11/2007] [Indexed: 11/28/2022] Open
Abstract
Background Cell-cell communication is essential in tissue patterning. In early amphibian development, mesoderm is formed in the blastula-stage embryo through inductive interactions in which vegetal cells act on overlying equatorial cells. Members of the TGF-β family such as activin B, Vg1, derrière and Xenopus nodal-related proteins (Xnrs) are candidate mesoderm inducing factors, with further activity to induce endoderm of the vegetal region. TGF-β-like ligands, including BMP, are also responsible for patterning of germ layers. In addition, FGF signaling is essential for mesoderm formation whereas FGF signal inhibition has been implicated in endoderm induction. Clearly, several signaling pathways are coordinated to produce an appropriate developmental output; although intracellular crosstalk is known to integrate multiple pathways, relatively little is known about extracellular coordination. Methodology/Principal Findings Here, we show that Xenopus Tsukushi (X-TSK), a member of the secreted small leucine rich repeat proteoglycan (SLRP) family, is expressed in ectoderm, endoderm, and the organizer during early development. We have previously reported that X-TSK binds to and inhibits BMP signaling in cooperation with chordin. We now demonstrate two novel interactions: X-TSK binds to and inhibits signaling by FGF8b, in addition to binding to and enhancement of Xnr2 signaling. This signal integration by X-TSK at the extracellular level has an important role in germ layer formation and patterning. Vegetally localized X-TSK potentiates endoderm formation through coordination of BMP, FGF and Xnr2 signaling. In contrast, X-TSK inhibition of FGF-MAPK signaling blocks ventrolateral mesoderm formation, while BMP inhibition enhances organizer formation. These actions of X-TSK are reliant upon its expression in endoderm and dorsal mesoderm, with relative exclusion from ventrolateral mesoderm, in a pattern shaped by FGF signals. Conclusions/Significance Based on our observations, we propose a novel mechanism by which X-TSK refines the field of positional information by integration of multiple pathways in the extracellular space.
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Affiliation(s)
- Samantha A. Morris
- Department of Oncology, The Hutchison/Medical Research Council Research Centre, University of Cambridge, Cambridge, United Kingdom
- * To whom correspondence should be addressed. E-mail: (SM); (SO)
| | - Alexandra D. Almeida
- Department of Oncology, The Hutchison/Medical Research Council Research Centre, University of Cambridge, Cambridge, United Kingdom
| | - Hideaki Tanaka
- Department of Developmental Neurobiology, Graduate School of Medical Sciences, Kumamoto University, Kumamoto, Japan
| | - Kunimasa Ohta
- Department of Developmental Neurobiology, Graduate School of Medical Sciences, Kumamoto University, Kumamoto, Japan
| | - Shin-ichi Ohnuma
- Department of Oncology, The Hutchison/Medical Research Council Research Centre, University of Cambridge, Cambridge, United Kingdom
- * To whom correspondence should be addressed. E-mail: (SM); (SO)
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11
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Howard L, Rex M, Clements D, Woodland HR. Regulation of the Xenopus Xsox17alpha(1) promoter by co-operating VegT and Sox17 sites. Dev Biol 2007; 310:402-15. [PMID: 17719026 PMCID: PMC2098691 DOI: 10.1016/j.ydbio.2007.07.028] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/20/2007] [Revised: 07/03/2007] [Accepted: 07/23/2007] [Indexed: 11/23/2022]
Abstract
The gene encoding the Sox F-group transcription factor Xsox17α1 is specifically expressed throughout the entire region of the Xenopus blastula fated to become endoderm, and is important in controlling endodermal development. Xsox17α1 is a direct target of the maternal endodermal determinant VegT and of Sox17 itself. We have analysed the promoter of the Xenopus laevis Xsox17α1 gene by transgenesis, and have identified two important control elements which reside about 9 kb upstream at the start of transcription. These elements individually drive transgenic endodermal expression in the blastula and gastrula. One contains functional, cooperating VegT and Sox-binding consensus sites. The Sox sites in this region are occupied in vivo. The other responds to TGF-β signals like Activin or Nodals that act through Smad2/3. We propose that these two regions co-operate in regulating the early endodermal expression of the Xsox17α1 gene.
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12
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Abstract
Asymmetries in the egg, established during oogenesis, set the stage for a cascade of intercellular signaling events leading to differential gene expression and subsequent tissue and organ formation. Maternally supplied Sox-type transcription factors have recently emerged as key components in the patterning of the early embryo and the regulation of embryonic stem cell differentiation. In deuterostomes, B1-type Soxs are asymmetrically localized to the future animal/ectodermal region where they act to suppress mesendodermal, and favor neuroectodermal differentiation, while vegetally localized F-type Soxs are involved in mesendodermal differentiation. Here, we review past observations and present new data from studies on the clawed frog Xenopus laevis. Animally localized Sox3 acts to inhibit Nodal (Xnr5 and Xnr6) expression, and induces the expression of genes (Ectodermin, Xema, and Coco) whose products repress Nodal signaling. Vegetally localized Sox7 positively regulates Nodal (Xnr4, Xnr5, and Xnr6) expression, as well as the expression of genes involved in mesodermal (Xmenf, Slug, and Snail) and endodermal (Endodermin and Sox17beta) differentiation. Given the evolutionary strategy of using common regulatory networks, it seems likely that a homologous Sox-Axis is active during embryonic development in many metazoans.
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Affiliation(s)
- Chi Zhang
- Department of Molecular, Cellular and Developmental Biology University of Colorado at Boulder Boulder, CO 80309-0347, USA
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Yan B, Moody SA. The competence of Xenopus blastomeres to produce neural and retinal progeny is repressed by two endo-mesoderm promoting pathways. Dev Biol 2007; 305:103-19. [PMID: 17428460 PMCID: PMC1892348 DOI: 10.1016/j.ydbio.2007.01.040] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/08/2006] [Revised: 01/11/2007] [Accepted: 01/31/2007] [Indexed: 10/23/2022]
Abstract
Only a subset of cleavage stage blastomeres in the Xenopus embryo is competent to contribute cells to the retina; ventral vegetal blastomeres do not form retina even when provided with neuralizing factors or transplanted to the most retinogenic position of the embryo. These results suggest that endogenous maternal factors in the vegetal region repress the ability of blastomeres to form retina. Herein we provide three lines of evidence that two vegetal-enriched maternal factors (VegT, Vg1), which are known to promote endo-mesodermal fates, negatively regulate which cells are competent to express anterior neural and retinal fates. First, both molecules can repress the ability of dorsal-animal retinogenic blastomeres to form retina, converting the lineage from neural/retinal to non-neural ectodermal and endo-mesodermal fates. Second, reducing the endogenous levels of either factor in dorsal-animal retinogenic blastomeres expands expression of neural/retinal genes and enlarges the retina. The dorsal-animal repression of neural/retinal fates by VegT and Vg1 is likely mediated by Sox17alpha and Derriere but not by XNr1. VegT and Vg1 likely exert their effects on neural/retinal fates through at least partially independent pathways because Notch1 can reverse the effects of VegT and Derriere but not those of Vg1 or XNr1. Third, reduction of endogenous VegT and/or Vg1 in ventral vegetal blastomeres can induce a neural fate, but only allows expression of a retinal fate when both BMP and Wnt signaling pathways are concomitantly repressed.
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Affiliation(s)
- Bo Yan
- Department of Anatomy and Cell Biology, The George Washington University Medical Center, 2300 Eye Street, N.W., Washington, DC 20037, USA
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14
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Zamparini AL, Watts T, Gardner CE, Tomlinson SR, Johnston GI, Brickman JM. Hex acts with beta-catenin to regulate anteroposterior patterning via a Groucho-related co-repressor and Nodal. Development 2007; 133:3709-22. [PMID: 16936074 DOI: 10.1242/dev.02516] [Citation(s) in RCA: 39] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022]
Abstract
In Xenopus, the establishment of the anteroposterior axis involves two key signalling pathways, canonical Wnt and Nodal-related TGFbeta. There are also a number of transcription factors that feedback upon these pathways. The homeodomain protein Hex, an early marker of anterior positional information, acts as a transcriptional repressor, suppressing induction and propagation of the Spemman organiser while specifying anterior identity. We show that Hex promotes anterior identity by amplifying the activity of canonical Wnt signalling. Hex exerts this activity by inhibiting the expression of Tle4, a member of the Groucho family of transcriptional co-repressors that we identified as a Hex target in embryonic stem (ES) cells and Xenopus embryos. This Hex-mediated enhancement of Wnt signalling results in the upregulation of the Nieuwkoop centre genes Siamois and Xnr3, and the subsequent increased expression of the anterior endodermal marker Cerberus and other mesendodermal genes downstream of Wnt signalling. We also identified Nodal as a Hex target in ES cells. We demonstrate that in Xenopus, the Nodal-related genes Xnr1 and Xnr2, but not Xnr5 and Xnr6, are regulated directly by Hex. The identification of Nodal-related genes as Hex targets explains the ability of Hex to suppress induction and propagation of the organiser. Together, these results support a model in which Hex acts early in development to reinforce a Wnt-mediated, Nieuwkoop-like signal to induce anterior endoderm, and later in this tissue to block further propagation of Nodal-related signals. The ability of Hex to regulate the same targets in both Xenopus and mouse implies this model is conserved.
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Affiliation(s)
- Andrea L Zamparini
- Centre Development in Stem Cell Biology, Institute for Stem Cell Research, School of Biological Sciences, University of Edinburgh, Edinburgh EH9 3JQ, UK
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15
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Zorn AM, Wells JM. Molecular Basis of Vertebrate Endoderm Development. INTERNATIONAL REVIEW OF CYTOLOGY 2007; 259:49-111. [PMID: 17425939 DOI: 10.1016/s0074-7696(06)59002-3] [Citation(s) in RCA: 114] [Impact Index Per Article: 6.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Abstract
The embryonic endoderm gives rise to the epithelial lining of the digestive and respiratory systems and organs such as the thyroid, lungs, liver, gallbladder, and pancreas. Studies in Xenopus, zebrafish, and mice have revealed a conserved molecular pathway controlling vertebrate endoderm development. The TGFbeta/Nodal signaling pathway is at the top of this molecular hierarchy and controls the expression of a number of key transcription factors including Mix-like homeodomain proteins, Gata zinc finger factors, Sox HMG domain proteins, and Fox forkhead factors. Here we review the function of these molecules comparing and contrasting their roles in each model organism. Finally, we will describe how our understanding of the molecular pathway governing endoderm development in embryos is being used to differentiate embryonic stem cells in vitro along endodermal lineages, with the ultimate goal of making therapeutically useful tissue.
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Affiliation(s)
- Aaron M Zorn
- Division of Developmental Biology, Cincinnati Children's Hospital Research, Foundation and University of Cincinnati College of Medicine, Cincinnati, Ohio 45229, USA
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16
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Sinner D, Kirilenko P, Rankin S, Wei E, Howard L, Kofron M, Heasman J, Woodland HR, Zorn AM. Global analysis of the transcriptional network controlling Xenopus endoderm formation. Development 2006; 133:1955-66. [PMID: 16651540 DOI: 10.1242/dev.02358] [Citation(s) in RCA: 68] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
A conserved molecular pathway has emerged controlling endoderm formation in Xenopus zebrafish and mice. Key genes in this pathway include Nodal ligands and transcription factors of the Mix-like paired homeodomain class, Gata4-6 zinc-finger factors and Sox17 HMG domain proteins. Although a linear epistatic pathway has been proposed, the precise hierarchical relationships between these factors and their downstream targets are largely unresolved. Here, we have used a combination of microarray analysis and loss-of-function experiments to examine the global regulatory network controlling Xenopus endoderm formation. We identified over 300 transcripts enriched in the gastrula endoderm, including most of the known endoderm regulators and over a hundred uncharacterized genes. Surprisingly only 10% of the endoderm transcriptome is regulated as predicted by the current linear model. We find that Nodal genes, Mixer and Sox17 have both shared and distinct sets of downstream targets, and that a number of unexpected autoregulatory loops exist between Sox17 and Gata4-6, between Sox17 and Bix1/Bix2/Bix4, and between Sox17 and Xnr4. Furthermore, we find that Mixer does not function primarily via Sox17 as previously proposed. These data provides new insight into the complexity of endoderm formation and will serve as valuable resource for establishing a complete endoderm gene regulatory network.
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Affiliation(s)
- Débora Sinner
- Division of Developmental Biology, Cincinnati Children's Hospital Research Foundation and Department of Pediatrics, University of Cincinnati College of Medicine, Cincinnati, OH 45299, USA
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Abstract
Mesoderm and endoderm formation in Xenopus involves the coordinated efforts of maternally and zygotically expressed transcription factors together with growth factor signalling, including members of the TGFbeta and wnt families. In this review we discuss our current state of knowledge of these pathways, and describe in more detail some of the transcription factor-DNA interactions that are involved in mesendoderm formation.
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Affiliation(s)
- Fiona C Wardle
- Wellcome Trust/Cancer Research UK Gurdon Institute and Department of Zoology, University of Cambridge, Tennis Court Road, Cambridge CB2 1QN, UK.
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18
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Establishment of mesodermal gene expression patterns in early Xenopus embryos: the role of repression. Dev Dyn 2005; 233:418-29. [PMID: 15779047 DOI: 10.1002/dvdy.20342] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/14/2023] Open
Abstract
In Xenopus, activin-like signals are able to induce and pattern mesoderm in a concentration-dependent manner. Previous experiments demonstrated that discrete gene expression patterns can be formed in animal cap explants as a response to graded activin signals. We analyzed the spatiotemporal appearance of goosecoid (gsc), chordin (chd), and Xbrachyury (Xbra) mRNAs in whole Xenopus embryos ectopically expressing activin or BVg1. To discriminate between direct transcriptional regulation and indirect, protein synthesis-dependent effects of ectopic signals, we combined overexpression studies and cycloheximide treatment. Our experiments revealed long-range signaling of activin/BVg1, but the expression patterns of gsc, chd, and Xbra in response to activin/BVg1 indicated that repressors are essential to establish the proper expression of these genes. Analysis of endogenous gsc, chd, and Xbra transcript distribution in embryos treated with cycloheximide supported this concept. We, therefore, conclude that inhibition is fundamental during early embryonic patterning.
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19
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Zhang C, Basta T, Fawcett SR, Klymkowsky MW. SOX7 is an immediate-early target of VegT and regulates Nodal-related gene expression in Xenopus. Dev Biol 2005; 278:526-41. [PMID: 15680368 DOI: 10.1016/j.ydbio.2004.11.008] [Citation(s) in RCA: 37] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/03/2004] [Revised: 10/18/2004] [Accepted: 11/09/2004] [Indexed: 02/06/2023]
Abstract
In zebrafish, the divergent F-type SOX casanova acts downstream of Nodal signaling to specify endoderm. While no casanova orthologs have been identified in tetrapods, the F-type SOX, SOX7, is supplied maternally in Xenopus (Fawcett and Klymkowsky, 2004. GER 4, 29). Subsequent RT-PCR and section-based in situ hybridization analyses indicate that SOX7 mRNA is localized to the vegetal region of the blastula-stage embryo. Overexpression and maternal depletion studies reveal that the T-box transcription factor VegT, which initiates mesoendodermal differentiation, directly regulates SOX7 expression. SOX7, but not SOX17 (another F-type SOX), binds to sites within the Xnr5 promoter and SOX7, but not SOX17, induces expression of the Nodal-related genes Xnr1, Xnr2, Xnr4, Xnr5, and Xnr6, the homeodomain transcription factor Mixer, and the endodermal marker SOX17beta; both SOX7 and SOX17 induce expression of the pan-endodermal marker endodermin. SOX7's induction of Xnr expression in animal caps is independent of Mixer and Nodal signaling. In animal caps, VegT's ability to induce Mixer and Edd appears to depend upon SOX7 activity. Whole embryo experiments suggests that vegetal factors partially compensate for the absence of SOX7. Based on the antagonistic effects of SOX7 and SOX3 (Zhang et al., 2004. Dev. Biol. 273, 23) and their common binding sites in the Xnr5 promoter, we propose a model in which competitive interactions between these two proteins are involved in refining the domain of endodermal differentiation.
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Affiliation(s)
- Chi Zhang
- Molecular, Cellular and Developmental Biology, University of Colorado, Porter Biosci. Building, Boulder, CO 80309-0347, USA
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20
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Koide T, Hayata T, Cho KWY. Xenopus as a model system to study transcriptional regulatory networks. Proc Natl Acad Sci U S A 2005; 102:4943-8. [PMID: 15795378 PMCID: PMC555977 DOI: 10.1073/pnas.0408125102] [Citation(s) in RCA: 75] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/01/2004] [Indexed: 11/18/2022] Open
Abstract
Development is controlled by a complex series of events requiring sequential gene activation. Understanding the logic of gene networks during development is necessary for a complete understanding of how genes contribute to phenotype. Pioneering work initiated in the sea urchin and Drosophila has demonstrated that reasonable transcriptional regulatory network diagrams representing early development in multicellular animals can be generated through use of appropriate genomic, genetic, and biochemical tools. Establishment of similar regulatory network diagrams for vertebrate development is a necessary step. The amphibian Xenopus has long been used as a model for vertebrate early development and has contributed greatly to the elucidation of gene regulation. Because the best and most extensively studied transcriptional regulatory network in Xenopus is that underlying the formation and function of Spemann's organizer, we describe the current status of our understanding of this gene regulatory network and its relationship to mesodermal patterning. Seventy-four transcription factors currently known to be expressed in the mesoendoderm of Xenopus gastrula were characterized according to their modes of action, DNA binding consensus sequences, and target genes. Among them, nineteen transcription factors were characterized sufficiently in detail, allowing us to generate a gene regulatory network diagram. Additionally, we discuss recent amphibian work using a combined DNA microarray and bioinformatics approach that promises to accelerate regulatory network studies.
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Affiliation(s)
- Tetsuya Koide
- Developmental Biology Center and the Department of Developmental and Cell Biology, University of California, Irvine, CA 92697-2300, USA
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21
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Ahmed N, Howard L, Woodland HR. Early endodermal expression of the Xenopus Endodermin gene is driven by regulatory sequences containing essential Sox protein-binding elements. Differentiation 2005; 72:171-84. [PMID: 15157240 DOI: 10.1111/j.1432-0436.2004.07204005.x] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
Abstract
The Endodermin gene is expressed in the early endoderm and the Spemann organizer of Xenopus embryos. It has previously been shown to be a direct target of the early endodermal transcription factor Xsox17 (Clements et al., 2003, Mech Dev 120:337-348). Here we identify two adjacent control elements in the Endodermin promoter; these drive transcription of the gene in late-gastrula endoderm and contain consensus Sox-binding sites. We have analyzed one element in detail and show that it responds directly to Xsox17 and that the Sox sites are essential for endodermal expression in transgenic embryos. However, flanking regions on both sides are also essential, indicating that Xsox17 acts in concert with several DNA-binding partners.
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Affiliation(s)
- Nadeem Ahmed
- Department of Biological Sciences, University of Warwick, Coventry, CV4 7AL, UK
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22
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Afouda BA, Ciau-Uitz A, Patient R. GATA4, 5 and 6 mediate TGFbeta maintenance of endodermal gene expression in Xenopus embryos. Development 2005; 132:763-74. [PMID: 15659482 DOI: 10.1242/dev.01647] [Citation(s) in RCA: 74] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022]
Abstract
The individual contributions of the three vertebrate GATA factors to endoderm formation have been unclear. Here we detail the early expression of GATA4, 5 and 6 in presumptive endoderm in Xenopus embryos and their induction of endodermal markers in presumptive ectoderm. Induction of HNF3beta by all three GATA factors was abolished when protein synthesis was inhibited, showing that these inductions are indirect. In contrast, whereas induction of Sox17alpha and HNF1beta by GATA4 and 5 was substantially reduced when protein synthesis was inhibited, induction by GATA6 was minimally affected, suggesting that GATA6 is a direct activator of these early endodermal genes. GATA4 induced GATA6 expression in the same assay and antisense morpholino oligonucleotides (MOs), designed to knock down translation of GATA6, blocked induction of Sox17alpha and HNF1beta by GATA4, suggesting that GATA4 induces these genes via GATA6 in this assay. All three GATA factors were induced by activin, although GATA4 and 6 required lower concentrations. GATA MOs inhibited Sox17alpha and HNF1beta induction by activin at low and high concentrations in the order: GATA6>GATA4>GATA5. Together with the timing of their expression and the effects of GATA MOs in vivo, these observations identify GATA6 as the predominant GATA factor in the maintenance of endodermal gene expression by TGFbeta signaling in gastrulating embryos. In addition, examination of gene expression and morphology in later embryos, revealed GATA5 and 6 as the most critical for the development of the gut and the liver.
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Affiliation(s)
- Boni Anatole Afouda
- Institute of Genetics, University of Nottingham, Queen's Medical Centre, Nottingham NG7 2UH, UK
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23
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Nath K, Boorech JL, Beckham YM, Burns MM, Elinson RP. Status of RNAs, localized inXenopus laevis oocytes, in the frogsRana pipiens andEleutherodactylus coqui. JOURNAL OF EXPERIMENTAL ZOOLOGY PART B-MOLECULAR AND DEVELOPMENTAL EVOLUTION 2005; 304:28-39. [PMID: 15515051 DOI: 10.1002/jez.b.21020] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
Early development in the frog model, Xenopus laevis, is governed by RNAs, localized to the vegetal cortex of the oocyte. These RNAs include Xdazl RNA, which is involved in primordial germ cell formation, and VegT RNA, which specifies the mesoderm and endoderm. In order to determine whether orthologues of these RNAs are localized and have similar functions in other frogs, we cloned RpDazl and RpVegT from Rana pipiens, a frog that is phylogenetically distant from X. laevis. RNAs from both genes are localized to the vegetal cortex of the R. pipiens oocyte, indicating that the vegetal localization is likely the basal state. The animal location of EcVegT RNA in Eleutherodactylus coqui that we found previously (Beckham et al., 2003) is then a derived state, probably due to the great increase in egg size required for direct development of this species. To answer the question of function, we injected RpVegT or EcVegT RNAs into X. laevis embryos, and assayed animal caps for gene expression. Both of these RNAs induced the expression of endodermal, mesodermal, and organizer genes, showing that the function of RpVegT and EcVegT as meso-endodermal determinants is conserved in frogs. The RNA localizations and the function of VegT orthologues in germ layer specification may be synapomorphies for anuran amphibians.
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Affiliation(s)
- Kimberly Nath
- Department of Biological Sciences, Duquesne University, Pittsburgh, Pennsylvania 15282, USA
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24
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Zhang C, Basta T, Hernandez-Lagunas L, Simpson P, Stemple DL, Artinger KB, Klymkowsky MW. Repression of nodal expression by maternal B1-type SOXs regulates germ layer formation in Xenopus and zebrafish. Dev Biol 2004; 273:23-37. [PMID: 15302595 DOI: 10.1016/j.ydbio.2004.05.019] [Citation(s) in RCA: 50] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/27/2004] [Revised: 05/24/2004] [Accepted: 05/26/2004] [Indexed: 10/26/2022]
Abstract
B1-type SOXs (SOXs 1, 2, and 3) are the most evolutionarily conserved subgroup of the SOX transcription factor family. To study their maternal functions, we used the affinity-purified antibody antiSOX3c, which inhibits the binding of Xenopus SOX3 to target DNA sequences [Development. 130(2003)5609]. The antibody also cross-reacts with zebrafish embryos. When injected into fertilized Xenopus or zebrafish eggs, antiSOX3c caused a profound gastrulation defect; this defect could be rescued by the injection of RNA encoding SOX3DeltaC-EnR, a SOX3-engrailed repression domain chimera. In antiSOX3c-injected Xenopus embryos, normal animal-vegetal patterning of mesodermal and endodermal markers was disrupted, expression domains were shifted toward the animal pole, and the levels of the endodermal markers SOX17 and endodermin increased. In Xenopus, SOX3 acts as a negative regulator of Xnr5, which encodes a nodal-related TGFbeta-family protein. Two nodal-related proteins are expressed in the early zebrafish embryo, squint and cyclops; antiSOX3c-injection leads to an increase in the level of cyclops expression. In both Xenopus and zebrafish, the antiSOX3c phenotype was rescued by the injection of RNA encoding the nodal inhibitor Cerberus-short (CerS). In Xenopus, antiSOX3c's effects on endodermin expression were suppressed by injection of RNA encoding a dominant negative version of Mixer or a morpholino against SOX17alpha2, both of which act downstream of nodal signaling in the endoderm specification pathway. Based on these data, it appears that maternal B1-type SOX functions together with the VegT/beta-catenin system to regulate nodal expression and to establish the normal pattern of germ layer formation in Xenopus. A mechanistically conserved system appears to act in a similar manner in the zebrafish.
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Affiliation(s)
- Chi Zhang
- Molecular, Cellular and Developmental Biology, University of Colorado, Boulder, 80309-0347, USA
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25
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Loose M, Patient R. A genetic regulatory network for Xenopus mesendoderm formation. Dev Biol 2004; 271:467-78. [PMID: 15223347 DOI: 10.1016/j.ydbio.2004.04.014] [Citation(s) in RCA: 101] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/18/2004] [Revised: 04/05/2004] [Accepted: 04/19/2004] [Indexed: 10/26/2022]
Abstract
We have constructed a genetic regulatory network (GRN) summarising the functional relationships between the transcription factors (TFs) and embryonic signals involved in Xenopus mesendoderm formation. It is supported by a relational database containing the experimental evidence and both are available in interactive form via the World Wide Web. This network highlights areas for further study and provides a framework for systematic interrogation of new data. Comparison with the equivalent network for the sea urchin identifies conserved features of the deuterostome ancestral pathway, including positive feedback loops, GATA factors, SoxB, Brachyury and a previously underemphasised role for beta-catenin. In contrast, some features central to one species have not yet been found in the other, for example, Krox and Otx in sea urchin, and Mix and Nodal in Xenopus. Such differences may represent evolved features or may eventually be resolved. For example, in Xenopus, Nodal-related genes are positively regulated by beta-catenin and at least one of them is repressed by Sox3, as is the uncharacterised early signal (ES) inducing endomesoderm in the sea urchin, suggesting that ES may be a Nodal-like TGF-beta. Wider comparisons of such networks will inform our understanding of developmental evolution.
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Affiliation(s)
- Matthew Loose
- Institute of Genetics, University of Nottingham, Queen's Medical Centre, Nottingham NG7 2UH, UK
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26
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Beckham YM, Nath K, Elinson RP. Localization of RNAs in oocytes of Eleutherodactylus coqui, a direct developing frog, differs from Xenopus laevis. Evol Dev 2004; 5:562-71. [PMID: 14984038 DOI: 10.1046/j.1525-142x.2003.03061.x] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Eleutherodactylus coqui develops directly on land to a frog. The large 3.5-mm oocyte of E. coqui has enough yolk to allow development without a feeding tadpole. In the smaller Xenopus laevis oocyte, 1.3 mm in diameter, mRNAs involved in germ layer formation, such as VegT and Vg1, are localized to the vegetal cortex of the oocyte. We hypothesized that an animal shift has occurred in the localization of the E. coqui Orthologs of VegT and Vg1 due to the large egg size. Through a combination of degenerate reverse transcriptase polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE), we cloned 1634 bp of EcVegT and 1377 bp of EcVg1. Northern blot analysis shows that the lengths of these transcripts are 2.5 kb and 1.3 kb, respectively. This result suggests that we have obtained the complete Vg1 transcript, although this transcript has an extremely short 3' untranslated region compared with X. laevis, 256 bp and 1268 bp, respectively. Zygotic expression of EcVegT closely resembles that of VegT, supporting their orthology. Radioactive RT-PCR and in situ hybridization demonstrated the presence of EcVegT and EcVg1 predominantly near the animal pole of the oocyte. RT-PCR showed that the animal blastomeres, formed from the first horizontal cleavage, inherit half of the EcVegT and EcVg1 transcripts, although they contain only about 1% of the embryo volume. Our results indicate major differences between the molecular organization of the eggs of X. laevis and E. coqui.
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Affiliation(s)
- Yvonne M Beckham
- Department of Biological Sciences, Duquesne University, 600 Forbes Avenue, Pittsburgh, PA 15282, USA
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27
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Cha SW, Hwang YS, Chae JP, Lee SY, Lee HS, Daar I, Park MJ, Kim J. Inhibition of FGF signaling causes expansion of the endoderm in Xenopus. Biochem Biophys Res Commun 2004; 315:100-6. [PMID: 15013431 DOI: 10.1016/j.bbrc.2004.01.019] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/01/2003] [Indexed: 02/08/2023]
Abstract
Fibroblast growth factor (FGF) is established as an initiator of signaling events critical for neurogenesis and mesoderm formation during early Xenopus embryogenesis. However, less is known about the role FGF signaling plays in endoderm specification. Here, we show for the first time that endoderm-specific genes are induced when FGF signaling is blocked in animal cap explants. This block of FGF signaling is also responsible for a significant enhancement of endodermal gene expression in animal cap explants that are injected with a dominant-negative BMP-4 receptor (DNBR) RNA or treated with activin, however, neural and mesoderm gene expression is diminished. Consistent with these results, the injection of dominant-negative FGF receptor (DNFR) RNA expands endodermal cell fate boundaries while FGF treatment dramatically reduces endoderm in whole embryos. Taken together, these results indicate that inhibition of FGF signaling promotes endoderm formation, whereas the presence of active FGF signaling is necessary for neurogenesis/mesoderm formation.
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Affiliation(s)
- Sang-Wook Cha
- Department of Anatomy, School of Medicine, Kyungpook National University, Daegu 700-422, Republic of Korea
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28
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Abstract
The T-box gene family, encoding related DNA-binding transcriptional regulators, plays an essential role in controlling many aspects of embryogenesis in a wide variety of organisms. The T-box genes exhibit diverse patterns of spatial and temporal expression in the developing embryo, and both genetic and molecular embryological studies have demonstrated their importance in regulating cell fate decisions that establish the early body plan, and in later processes underlying organogenesis. Despite these studies, little is known of either the regulation of the T-box genes or the identities of their transcriptional targets. The aim of this review is to examine the diverse yet conserved roles of several T-box genes in regulating early patterning in chordates and to discuss possible mechanisms through which this functional diversity might arise. Developmental Dynamics 229:201-218, 2004.
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Affiliation(s)
- Chris Showell
- Department of Genetics, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, USA
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