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Key Event Title
Repression of Gbx2 expression
Key Event Components
Key Event Overview
AOPs Including This Key Event
|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 Do not cite|
Key Event Description
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
Domain of Applicability
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
- 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).
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).
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.
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).
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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