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Event: 1902

Key Event Title

The KE title should describe a discrete biological change that can be measured. It should generally define the biological object or process being measured and whether it is increased, decreased, or otherwise definably altered relative to a control state. For example “enzyme activity, decreased”, “hormone concentration, increased”, or “growth rate, decreased”, where the specific enzyme or hormone being measured is defined. More help

Repression of Gbx2 expression

Short name
The KE short name should be a reasonable abbreviation of the KE title and is used in labelling this object throughout the AOP-Wiki. The short name should be less than 80 characters in length. More help
Repression of Gbx2 expression

Biological Context

Structured terms, selected from a drop-down menu, are used to identify the level of biological organization for each KE. Note, KEs should be defined within a particular level of biological organization. Only KERs should be used to transition from one level of organization to another. Selection of the level of biological organization defines which structured terms will be available to select when defining the Event Components (below). More help

Cell term

Further information on Event Components and Biological Context may be viewed on the attached pdf.The biological context describes the location/biological environment in which the event takes place.  For molecular/cellular events this would include the cellular context (if known), organ context, and species/life stage/sex for which the event is relevant. For tissue/organ events cellular context is not applicable.  For individual/population events, the organ context is not applicable. More help

Organ term

Further information on Event Components and Biological Context may be viewed on the attached pdf.The biological context describes the location/biological environment in which the event takes place.  For molecular/cellular events this would include the cellular context (if known), organ context, and species/life stage/sex for which the event is relevant. For tissue/organ events cellular context is not applicable.  For individual/population events, the organ context is not applicable. More help

Key Event Components

Further information on Event Components and Biological Context may be viewed on the attached pdf.Because one of the aims of the AOP-KB is to facilitate de facto construction of AOP networks through the use of shared KE and KER elements, authors are also asked to define their KEs using a set of structured ontology terms (Event Components). In the absence of structured terms, the same KE can readily be defined using a number of synonymous titles (read by a computer as character strings). In order to make these synonymous KEs more machine-readable, KEs should also be defined by one or more “event components” consisting of a biological process, object, and action with each term originating from one of 22 biological ontologies (Ives, et al., 2017; See List). Biological process describes dynamics of the underlying biological system (e.g., receptor signalling). The biological object is the subject of the perturbation (e.g., a specific biological receptor that is activated or inhibited). Action represents the direction of perturbation of this system (generally increased or decreased; e.g., ‘decreased’ in the case of a receptor that is inhibited to indicate a decrease in the signalling by that receptor).Note that when editing Event Components, clicking an existing Event Component from the Suggestions menu will autopopulate these fields, along with their source ID and description. To clear any fields before submitting the event component, use the 'Clear process,' 'Clear object,' or 'Clear action' buttons. If a desired term does not exist, a new term request may be made via Term Requests. Event components may not be edited; to edit an event component, remove the existing event component and create a new one using the terms that you wish to add. More help

Key Event Overview

AOPs Including This Key Event

All of the AOPs that are linked to this KE will automatically be listed in this subsection. This table can be particularly useful for derivation of AOP networks including the KE. Clicking on the name of the AOP will bring you to the individual page for that AOP. More help
AOP Name Role of event in AOP Point of Contact Author Status OECD Status
Repression of Gbx2 expression leads to increased mortality MolecularInitiatingEvent Vid Modic (send email) Under development: Not open for comment. Do not cite

Stressors

This is a structured field used to identify specific agents (generally chemicals) that can trigger the KE. Stressors identified in this field will be linked to the KE in a machine-readable manner, such that, for example, a stressor search would identify this as an event the stressor can trigger. NOTE: intermediate or downstream KEs in one AOP may function as MIEs in other AOPs, meaning that stressor information may be added to the KE description, even if it is a downstream KE in the pathway currently under development.Information concerning the stressors that may trigger an MIE can be defined using a combination of structured and unstructured (free-text) fields. For example, structured fields may be used to indicate specific chemicals for which there is evidence of an interaction relevant to this MIE. By linking the KE description to a structured chemical name, it will be increasingly possible to link the MIE to other sources of chemical data and information, enhancing searchability and inter-operability among different data-sources and knowledgebases. The free-text section “Evidence for perturbation of this MIE by stressor” can be used both to identify the supporting evidence for specific stressors triggering the MIE as well as to define broad chemical categories or other properties that classify the stressors able to trigger the MIE for which specific structured terms may not exist. More help

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) can be selected from an ontology. In many cases, individual species identified in these structured fields will be those for which the strongest evidence used in constructing the AOP was available in relation to this KE. More help
Term Scientific Term Evidence Link
zebrafish Danio rerio High NCBI

Life Stages

The structured ontology terms for life-stage are more comprehensive than those for taxa, but may still require further description/development and explanation in the free text section. More help
Life stage Evidence
Embryo High

Sex Applicability

The authors must select from one of the following: Male, female, mixed, asexual, third gender, hermaphrodite, or unspecific. More help
Term Evidence
Unspecific High

Key Event Description

A description of the biological state being observed or measured, the biological compartment in which it is measured, and its general role in the biology should be provided. For example, the biological state being measured could be the activity of an enzyme, the expression of a gene or abundance of an mRNA transcript, the concentration of a hormone or protein, neuronal activity, heart rate, etc. The biological compartment may be a particular cell type, tissue, organ, fluid (e.g., plasma, cerebrospinal fluid), etc. The role in the biology could describe the reaction that an enzyme catalyses and the role of that reaction within a given metabolic pathway; the protein that a gene or mRNA transcript codes for and the function of that protein; the function of a hormone in a given target tissue, physiological function of an organ, etc. Careful attention should be taken to avoid reference to other KEs, KERs or AOPs. Only describe this KE as a single isolated measurable event/state. This will ensure that the KE is modular and can be used by other AOPs, thereby facilitating construction of AOP networks. More help

During vertebrate brain development, the gastrulation brain homeobox 2 gene (gbx2) is expressed in the forebrain (Z. Wang et al., 2018). The genes encoding the Gbx-type homeodomain transcription factors have been identified in a variety of vertebrates, and are primarily implicated in the regulation of various aspects of vertebrate brain development (Nakayama et al., 2017). Gbx2 exhibits DNA-binding transcription factor activity, RNA polymerase II-specific. Involved in cerebellum development; iridophore differentiation; and telencephalon regionalization. Predicted to localize to nucleus. Is expressed in several structures, including midbrain hindbrain boundary neural keel; midbrain hindbrain boundary neural rod; midbrain neural rod; nervous system; and presumptive rhombomere 1. Orthologous to human GBX2 (gastrulation brain homeobox 2) (ZFIN Gene: Gbx2, n.d.)

Retinoids such as retinoic acid (RA) are chemopreventive and chemotherapeutic agents. One source of RA is vitamin A, derived from dietary β-carotene. RA regulates cell proliferation, differentiation, and morphogenesis (X. J. Wang et al., 2007). It inhibits tumorigenesis through suppression of cell growth and stimulation of cellular differentiation (Soprano et al., 2004). Also, RA promotes apoptosis (Atencia et al., 1997; Herget et al., 2000), and this property may contribute to its antitumor properties. The effects of retinoids are mediated by specific nuclear receptors, namely, retinoic acid receptors (RAR-α, -β, and -γ) and retinoid X receptors (RXR- α, - β, and - γ) (Rochette-Egly & Chambon, 2001). RXRs form heterodimers with RARs or other nuclear hormone receptors and function as transcriptional regulators. Retinoids can either activate or repress gene expression through RAR/RXR heterodimers interacting with other transcription factors, such as AP-1, estrogen receptor α, and NF-κB activities (Shaulian & Karin, 2002). Retinoic acid has been shown to repress Gbx2 expression in talencephalon in Zebrafish and other vertebrate models in early stages of development.

How It Is Measured or Detected

One of the primary considerations in evaluating AOPs is the relevance and reliability of the methods with which the KEs can be measured. The aim of this section of the KE description is not to provide detailed protocols, but rather to capture, in a sentence or two, per method, the type(s) of measurements that can be employed to evaluate the KE and the relative level of scientific confidence in those measurements. Methods that can be used to detect or measure the biological state represented in the KE should be briefly described and/or cited. These can range from citation of specific validated test guidelines, citation of specific methods published in the peer reviewed literature, or outlines of a general protocol or approach (e.g., a protein may be measured by ELISA).Key considerations regarding scientific confidence in the measurement approach include whether the assay is fit for purpose, whether it provides a direct or indirect measure of the biological state in question, whether it is repeatable and reproducible, and the extent to which it is accepted in the scientific and/or regulatory community. Information can be obtained from the OECD Test Guidelines website and the EURL ECVAM Database Service on Alternative Methods to Animal Experimentation (DB-ALM). ?

Domain of Applicability

This free text section should be used to elaborate on the scientific basis for the indicated domains of applicability and the WoE calls (if provided). While structured terms may be selected to define the taxonomic, life stage and sex applicability (see structured applicability terms, above) of the KE, the structured terms may not adequately reflect or capture the overall biological applicability domain (particularly with regard to taxa). Likewise, the structured terms do not provide an explanation or rationale for the selection. The free-text section on evidence for taxonomic, life stage, and sex applicability can be used to elaborate on why the specific structured terms were selected, and provide supporting references and background information.  More help

The gastrulation brain homebox (Gbx) group of transcription factor genes, composed of two genes, gbx1 and gbx2, in vertebrates, is also present in invertebrates (Chiang et al., 1995), and can be regarded as widely conserved among animals (Wang et al., 2018). Gbx2 functions in a variety of developmental processes after midbrain-hindbrain boundary (MHB) establishment. (Burroughs-Garcia et al., 2011) data demonstrate that the role of gbx2 in anterior hindbrain development is functionally conserved between zebrafish and mice. This gene was shown to be required for neural crest (NC) formation in mice (B. Li et al., 2009; Roeseler et al., 2012). In Xenopus gbx2 is the earliest factor for specifying neural crest (NC) cells, and that gbx2 is directly regulated by NC inducing signaling pathways, such as Wnt/β-catenin signaling (Li et al., 2009).

Evidence for Perturbation by Stressor

Overview for Molecular Initiating Event

When a specific MIE can be defined (i.e., the molecular target and nature of interaction is known), in addition to describing the biological state associated with the MIE, how it can be measured, and its taxonomic, life stage, and sex applicability, it is useful to list stressors known to trigger the MIE and provide evidence supporting that initiation. This will often be a list of prototypical compounds demonstrated to interact with the target molecule in the manner detailed in the MIE description to initiate a given pathway (e.g., 2,3,7,8-TCDD as a prototypical AhR agonist; 17α-ethynyl estradiol as a prototypical ER agonist). Depending on the information available, this could also refer to chemical categories (i.e., groups of chemicals with defined structural features known to trigger the MIE). Known stressors should be included in the MIE description, but it is not expected to include a comprehensive list. Rather initially, stressors identified will be exemplary and the stressor list will be expanded over time. For more information on MIE, please see pages 32-33 in the User Handbook.
  • Zebrafish embryos were treated with chemical inhibitors or activators of various signaling pathways, such as the Wnt, FGF, retinoic acid (RA), HH, BMP, Nodal, and Notch pathways, and examined gbx2 expression in the telencephalon. First, embryos were treated with chemicals from 14 hpf to 18 hpf, immediately before the advent of gbx2 expression in the telencephalon, and then gbx2 expression was examined in this brain region . In embryos treated with BIO, a selective GSK3 inhibitor that activates Wnt signaling (Sato et al., 2004), gbx2 expression was specifically repressed in the telencephalon, but was unaffected or weakly activated in the isthmus and otic vesicle (OV). In embryos where FGF signaling was inhibited by SU5402, gbx2 was downregulated in the telencephalon and MHB, but its expression in the OV was little affected. Retinoic acid (RA) treatment strongly repressed gbx2 expression in the telencephalon, but not in the MHB and OV. These results suggest that gbx2-dependent telencephalon development is regulated by Wnt, FGF, and RA signaling (Z. Wang et al., 2018).
  • To clarify the critical stages of previous study for gbx2 regulation in the telencephalon, chemical treatment started between 14 and 17 hpf and gbx2 expression was examined at 18 hpf. Alternatively, chemical treatment was started at 14 hpf and then embryos were washed between 15 and 18 hpf, cultured in the absence of chemicals, and gbx2 expression was examined at 18 hpf. Resuoults showed that the downregulation of gbx2 by BIO grew less significant as the start time was delayed, and the repression of gbx2 by BIO in the telencephalon became less prominent when the chemicals were removed earlier, suggesting that Wnt signaling remains effective throughout the 4-h period (14–18 hpf) and that the repressive effect of BIO is reversible. Similarly, SU5402  mediated repression of gbx2 expression in the telencephalon and MHB became less significant as the treatment start time was delayed from 14 hpf to 17 hpf, and gbx2 expression was gradually restored with earlier removal of the chemical, showing that FGF signaling is continuously required for gbx2 expression in the telencephalon. Essentially the same results were obtained with RA treatment in terms of gbx2 expression in the telencephalon (Z. Wang et al., 2018).

BIO (6-bromoindirubin-3’-oxime)

Embryos were treated with chemicals from 14 hpf to 18 hpf, immediately before the advent of gbx2 expression in the telencephalon, and then gbx2 expression was examined in this brain region . In embryos treated with BIO, a selective GSK3 inhibitor that activates Wnt signaling (Sato et al., 2004), gbx2 expression was specifically repressed in the telencephalon, but was unaffected or weakly activated in the isthmus and otic vesicle (OV).

Retinoic acid

Zebrafish embryos were treated with chemicals from 14 hpf to 18 hpf, immediately before the advent of gbx2 expression in the telencephalon, and then gbx2 expression was examined in this brain region. Retinoic acid (RA) treatment strongly repressed gbx2 expression in the telencephalon, but not in the MHB and OV.

su5402

Zebrafish embryos were treated with chemicals from 14 hpf to 18 hpf, immediately before the advent of gbx2 expression in the telencephalon, and then gbx2 expression was examined in this brain region. In embryos where FGF signaling was inhibited by SU5402, gbx2 was downregulated in the telencephalon and MHB, but its expression in the OV was little affected (Z. Wang et al., 2018).

References

List of the literature that was cited for this KE description. Ideally, the list of references, should conform, to the extent possible, with the OECD Style Guide (https://www.oecd.org/about/publishing/OECD-Style-Guide-Third-Edition.pdf) (OECD, 2015). More help

Atencia, R., García-Sanz, M., Pérez-Yarza, G., Asumendi, A., Hilario, E., & Aréchaga, J. (1997). A structural analysis of cytoskeleton components during the execution phase of apoptosis. Protoplasma, 198(3–4), 163–169. https://doi.org/10.1007/BF01287565

Chiang, C., Young, K. E., & Beachy, P. A. (1995). Control of Drosophila tracheal branching by the novel homeodomain gene unplugged, a regulatory target for genes of the bithorax complex. Development, 121(11), 3901–3912.

Herget, T., Esdar, C., Oehrlein, S. A., Heinrich, M., Schützei, S., Maelicke, A., & Van Echten-Deckert, G. (2000). Production of ceramides causes apoptosis during early neural differentiation in vitro. Journal of Biological Chemistry, 275(39), 30344–30354. https://doi.org/10.1074/jbc.M000714200

Li, B., Kuriyama, S., Moreno, M., & Mayor, R. (2009). The posteriorizing gene Gbx2 is a direct target of Wnt signalling and the earliest factor in neural crest induction. Development, 136(19), 3267–3278. https://doi.org/10.1242/dev.036954

Luu, B., Ellisor, D., & Zervas, M. (2011). The Lineage Contribution and Role of Gbx2 in Spinal Cord Development. PLoS ONE, 6. https://doi.org/10.1371/journal.pone.0020940

Nakayama, Y., Inomata, C., Yuikawa, T., Tsuda, S., & Yamasu, K. (2017). Comprehensive analysis of target genes in zebrafish embryos reveals gbx2 involvement in neurogenesis. Developmental Biology, 430(1), 237–248. https://doi.org/10.1016/j.ydbio.2017.07.015

Rochette-Egly, C., & Chambon, P. (2001). F9 embryocarcinoma cells: A cell autonomous model to study the functional selectivity of RARs and RXRs in retinoid signaling. Histology and Histopathology, 16(3), 909–922. https://doi.org/10.14670/HH-16.909

Roeseler, D. A., Sachdev, S., Buckley, D. M., Joshi, T., & Wu, D. K. (2012). Elongation Factor 1 alpha1 and Genes Associated with Usher Syndromes Are Downstream Targets of GBX2. PLoS ONE, 7(11), 47366. https://doi.org/10.1371/journal.pone.0047366

Shaulian, E., & Karin, M. (2002). AP-1 as a regulator of cell life and death. Nature Cell Biology, 4(5), E131–E136. https://doi.org/10.1038/ncb0502-e131

Soprano, D. R., Qin, P., & Soprano, K. J. (2004). Retinoic acid receptors and cancers. Annual Review of Nutrition, 24, 201–221. https://doi.org/10.1146/annurev.nutr.24.012003.132407

Wang, X. J., Hayes, J. D., Henderson, C. J., & Roland Wolf, C. (2007). Identification of retinoic acid as an inhibitor of transcription factor Nrf2 through activation of retinoic acid receptor alpha. Proc Natl Acad Sci U S A, 104(49), 19589–19594. www.pnas.org/cgi/content/full/

Wang, Z., Nakayama, Y., Tsuda, S., & Yamasu, K. (2018). The role of gastrulation brain homeobox 2 (gbx2) in the development of the ventral telencephalon in zebrafish embryos. Differentiation, 99(December 2017), 28–40. https://doi.org/10.1016/j.diff.2017.12.005

         ZFIN Gene: gbx2. (n.d.). Retrieved April 12, 2021, from https://zfin.org/ZDB-GENE-020509-2