Retinoid signaling in skeletal development: Scoping the system for predictive toxicology
Section snippets
Preface
All-trans retinoic acid (ATRA) is a conserved signal molecule during morphogenesis, growth and differentiation across diverse organ systems. This review of the literature derives from an annex in Detailed Review Paper (DRP) of the OECD Test Guidelines Programme (Project 4.97) that is intended to support recommendations regarding assay development to determine retinoid system toxicants for developmental and reproductive toxicity. Because mutations of the ATRA system tend to be disruptive of
Assessing prenatal developmental toxicity
Current developmental toxicity testing for regulatory purposes adheres largely to protocols suggested in 1966 involving the administration of test compound to pregnant laboratory animals. The skeleton is routinely examined in standard developmental toxicity bioassays (e.g., OECD 414) and has proven to be sensitive to a wide variety of chemical agents [3,4]. The fetal skeleton develops with >200 individual bones anatomically comprising an axial skeleton (vertebral column, ribs, skull) and paired
Overview of the retinoid signaling pathway
Retinoid signaling has a conserved ancestry from gastropods to humans [11]. Recent publications suggest that signaling by ATRA may be an ancestral feature of bilaterians rather than a chordate innovation; however, there is still no conclusive evidence showing that a retinoid is required for development of non-chordates [2].
ATRA is a metabolic derivative from dietary vitamin A, existing in isomers (9-cis, 13-cis, all trans). Apart from retinal, which is involved solely in the visual cycle, in
Altered ATRA signaling
Several lines of study have established functional evidence for retinoid signaling during pregnancy and development. One line of experimentation addresses the developmental consequences of retinoid deficiency, caused either by (i) dietary deficiency in vitamin A, (ii) inhibition of ATRA synthesis by functional inactivation of genes encoding retinaldehyde or alcohol dehydrogenases, (iii) administration of RALDH inhibitors, (iv) deletion of genes encoding RARs, or (v) administration of RAR
ATRA signaling in craniofacial development
Endogenous ATRA is essential for development of the facial bones and branchial arches. Craniofacial malformations induced by retinoid excess, including those of Cyp26(-/-) null mutant mice, have been linked to disruption of craniofacial mesenchyme primarily affecting the formation of bones in the midface. It may also be the case for the malformation of bones derived from the caudal branchial arches under conditions of ‘functional ATRA deficiency’ in RAR null mutant mice. Two migratory cell
Craniofacial teratogenesis
Many teratogenic effects described for exogenous retinoids in laboratory animal models and humans reflect alterations to tissues derived from cranial neural crest cells (CNCs) [61]. Clinical observations following isotretinoin (13-cisRA, Accutane) exposure during pregnancy in humans [62,63] and nonhuman primates [64] have shown a spectrum of malformations including craniofacial defects linked to hypoplasia of the 1st (mandibular) and 2nd (hyoid) branchial arches. Isotretinoin has a low affinity
ATRA signaling in vertebral development
Trunk organization in Vertebrata involves the establishment of a metameric primary body axis leading to formation of the neural tube and paraxial mesoderm (somites). ATRA signaling participates in the organization of both systems (neural tube, vertebral column) although only the vertebral system is considered here. The onset of ATRA signaling in the mouse embryo, based on expression of RDH10, RALDH2, is GD 7.5; however, ectopic or excessive retinoid signaling can disrupt three morphogenetic
Retinoids in Appendicular Development
The appendicular skeleton (upper and lower extremities in bipeds, forelimb and hindlimb in quadrupeds) is defined in three segments along the proximodistal axis: stylopod (humerus, femur), zeugopod (radius-ulna, tibia-fibula), and autopod (hand, foot). A secondary axis defines anterior-posterior asymmetry (e.g., digits I through V in mouse and humans) and a tertiary axis dorsal-ventral asymmetry. The rudimentary ‘limb-bud’ forms as outcroppings of the flank (forelimb bud, hindlimb bud) composed
Limb teratogenesis
Kochhar in 1973 was among the first to identify teratogenic effects of exogenous ATRA on the mouse limb, invoking dose-dependent phocomelia following a single dose in the range of 1- to 100 mg/kg the pregnant dam [127]. The window of vulnerability in mouse (GD 10–12) coincided with the formation of precartilaginous mesenchymal condensations. ATRA caused dose-dependent phocomelia and digital defects when administered to pregnant mice at 20- to 80 mg/kg on GD 11, where the deficiencies to
Adverse Outcome Pathway (AOP) framework
Performance-based models that address the regulation, homeostasis, and biological activity of the retinoid signaling pathway will be useful for predictive toxicology based on alternative (non-mammalian) tests. An AOP framework is necessary to organize the relevant data, information and knowledge on molecular initiating events (MIEs) reflecting a disruption in retinoid signaling at critical stages of gestation, and the ensuing cascade of key events (KEs) and their relationships (KERs) leading to
New Approach Methodologies (NAMs)
Opportunities exist for refining and supplanting current developmental toxicity testing protocols using in vitro data and in silico models in the design and review of revolutionary alternatives to animal testing by experts in the field, alongside their independent validation [241]. New Approach Methodologies (NAMs) is the term adopted as a broadly descriptive reference to any technology, methodology, approach, or combination thereof that can be used to provide information on chemical hazard and
Conclusions
This review underscores the importance of ATRA homeostasis to patterning and differentiation of the fetal skeleton. These pathways are complex and connected directly or indirectly to morphogenetic signaling. A critical role of ATRA signaling during gastrulation and early organogenesis influences regional specification and fate of precursor cell populations in the cranial neural crest, paraxial mesoderm, and lateral plate mesoderm. To date, no OECD test guidelines specifically capture the
Funding
This work was supported by the Chemical Safety for Sustainability (CSS) National Research Program, Virtual Tissue Models (CSS 5.3) project of the U.S. Environmental Protection Agency.
Disclaimer
The views expressed in this article are those of the authors and do not necessarily reflect the views or policies of the U.S. Environmental Protection Agency. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.
Disclosure
The authors contributed this work as an annex of OECD’s Detailed Review Paper on the retinoid system.
Conflict of interest
The authors declare no conflict of interest.
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgements
The authors gratefully acknowledge helpful comments on the DRP draft annex from the following experts: John M. Rogers (US EPA, ORD/CPHEA), Paul C. Brown (US FDA, CDER), Amy C. Nostrandt (US FDA, CDER), Karen L. Hamernik (US EPA, OCSPP), Robert L. Sprando (US FDA, CFSAN), Bruce Blumberg (University of California Irvine, Dept. Developmental and Cell Biology), Shirlee W. Tan (Endocrine Society), Scott M. Belcher (North Carolina State University, Dept. Biological Sciences), Alice Baynes (Institute
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2022, Current Research in ToxicologyCitation Excerpt :A recently published ‘Detailed Review Paper on the Retinoid Signaling Pathway’ (supported by the European Commission and OECD) included an annex focusing on both male and female reproduction (OECD, 2021). But despite these efforts there are still no AOPs for disrupted RA signaling in the AOP-Knowledge Base (AOP-KB) even though several putative AOPs involving retinoid disruption have been suggested (Draskau et al., 2020; Johansson et al., 2020; Knudsen et al., 2021; Tonk et al., 2015). We currently have under development AOP 398 ‘Inhibition of ALDH1A (RALDH) causing reduced all-trans retinoic acid levels leading to impaired fertility’ and here describe a relevant KER that links reduced atRA levels with disrupted meiosis in fetal oocytes.
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