Skip to main content

Table 1 Examples of the impacts of retinoic acid signalling effects on morphology, phenotype, and/or development. Explanations and abbreviations are at the end of this table

From: Highlighting the gaps in hazard and risk assessment of unregulated Endocrine Active Substances in surface waters: retinoids as a European case study

Apical Effect/Key mechanism/Endpoint Morphological alteration level Developmental stage Test system (species) Main relevant and related pathways, genes, enzymes Endpoint/hallmark affected cross-reactions Substances associated with key mechanism/adverse outcome Associated human pathology Reference(s)
Axial development, Anterior-posterior patterning, osteogenesis Tissue Embryo, larvae Branchiostoma floridae (amphioxus), zebrafish (stocksteif mutant), chicken, mouse ADH/ RALDH, Cyp26b1, Cyp26a1, RARα, RARβ, RARγ, Wnt, FGF, Hox (esp. 1&3), BMP RA acts as an early developmental morphogen along the anterior-posterior axis; it coordinates the position of endoderm-derived organs along the anterior-posterior axis. Cyp26/ RA concentration drives the osteogenesis in the vertebral column (in osteoblasts); posteriorization of gills and mouth in invertebrate chordates. Hyperactive RAR induces higher expression levels of RA-metabolizing Cyp26a1 and acts in a paracrine way RA, retinol   [76, 148, 313,314,315,316,317,318,319,320,321,322]
      Symmetric somite development is mediated by RA    [323,324,325]
    Human, zebrafish, rat, mouse Dhrs3, Cyp26(a1) Axial skeletal and craniofacial defects upon exposure Triazole fungicides (flusilazole, triadimefon)   [165, 326]
Neural tube formation Organ/organism Embryo Quail FGF and Wnt gradients/signaling, CYP26A1, RALDH activity Mesodermal segmentation, somite formation, and neurogenesis in caudal neural tube (future spinal cord) are RA dependent RA   [73, 315, 327,328,329]
    Human, zebrafish, rat Dhrs3, Cyp26(a1) Neural tube (and axial) defects; proposed Adverse Outcome Pathway: “for neural tube and axial defects mediated by modulation of retinoic acid homeostasis” Triazole fungicides (flusilazole)   [165]
Neural differentiation and spinal cord formation Tissue/organ Embryo, (adult) Mouse, zebrafish, Xenopus FGF and Wnt, CYP26A1, RALDH, RARβ, RARα, Hox The nervous system develops sequentially along this axis, starting anteriorly (CNS/brain), continuing via hindbrain to spinal cord. Determination of cell fate and differentiation of ventral neuronal cell types in developing spinal cord. Neurite outgrowth in embryos and adults is dependent on RARβ expression; RARα knockdown abolishes atRA-mediated dendritic growth RA (endogenous)   [76, 196, 314, 327, 330,331,332,333,334,335,336]
Early neural differentiation Tissue/organ system Embryo Mouse RAR, TR, MCT8 Organization of the (central) nervous system; RA signalling precedes TRα expression/TR signaling; RA induces MCT8 expression in the developing brain allowing TH transport RA, TH   [135, 136]
    Zebrafish   RA co-administration (1 nM) prevents adverse effects (behavioural and histological) of ethanol (150 mM) exposure during gastrulation RA, ethanol   [337]
Hindbrain segmentation, Ear development and hearing recovery Tissue/organ Embryo, adult Mouse, rat, zebrafish, VAD quail model, chicken, Xenopus RARα, RARβ, (CRABP), SHH, Wnt, FGF, Hox; CYP26A1, CYP26C1, RALDH activity RA guides the formation of 8 segments (rhombomeres) that give rise to e.g. otic vesicle, sensory tract. RA determines the forebrain-hindbrain and hindbrain-spinal cord boundary (excess leads to posteriorization); midbrain-hindbrain boundary is unaffected (in mouse and Xenopus). RA directly influences the differentiation of branchiomotor neurons (zebrafish) RA Congenital hearing loss [338, 339] [91, 124, 148, 321, 331, 340,341,342,343,344,345,346,347,348,349,350,351,352,353,354,355,356,357,358,359]
    Mouse, zebrafish   Development of the olfactory region requires RA. RA stimulates regeneration of auditory hair cells    [360,361,362,363]
        Fetal alcohol syndrome; cerebellar maldevelopment [364, 365]
Head and forebrain development; Eye development Tissue/organ Embryo Mouse, VAD quail model, pig, rabbit, cattle, sheep, rat, zebrafish   Formation of optic vesicle (retina precursor; invagination of neuroepithelium); micro-/anophthalmia in absence of RA. However: head development in general requires absence of RA [76] RA   [90, 125, 127, 129, 163, 363, 366, 367]
    Rat, human RALDH, CYP26, AhR Craniofacial malformations are linked to RA deregulation atRA, 13cRA Cleft palate and lip [368,369,370,371,372]
Telencephalon differentiation Tissue Embryo Mouse   Changed population of ganglia; altered precursor population; RA stimulates production of dopaminergic neurons RA   [373,374,375,376,377]
Cerebral cortex Tissue Embryo, postnatal Mouse RAR Influence on neurogenesis/migration/ differentiation in other brain regions/ at other developmental stages. Sensitivity to RA is retained in the mature cortex RA (endogenous), 13cRA Affective liability and behavioural disinhibition upon 13cRA treatment, depression [177] [73, 374, 378,379,380]
Hippocampus, neuronal plasticity Cell/ tissue Adult Mouse, rat, zebra finch RAR/RXR, esp. RARβ, RXRγ Defects in spatial learning and memory, and recognition working memory (RXRγ) upon vitamin A deprivation; restoration of cognitive impairment with vitamin A supply; cognitive impairment also in excess RA scenario (13cRA). Decreased ability to learn mating song in zebra finches RA, vitamin A Learning and memory impairment, depression [150,151,152,153, 177, 192, 381,382,383]
Hippocampus Tissue/organ Adult Rat, human (Alzheimer’s disease patients), mouse Calbindin D28K, neurogranin, somatostatin, CHAT; RARα, RARβ, RXRγ, amyloid pathway, RBP4, CYP26, RALDH2 (in adjacent meninges) Regulation of memory and spatial learning; RA acts as a proamnesic molecule
Deprivation leads to amyloid-β accumulation, RARα downregulation, CHAT expression loss in forebrain cortical neurons
RA regulates proteins linked to protection from Alzheimer’s disease
Vitamin A Alzheimer’s disease, ageing [82, 151, 178, 192, 205, 211, 384,385,386,387,388,389,390,391,392]
Hippocampus Organ Postnatal Mouse, rat RARs (esp. RARα), RXRs, GR, somatostatin, RALDH2 (in adjacent meninges) RARβ-/- and RXRγ-/-: deficiency in spatial learning and memory, like VAD rats (rescue by RA treatment). Degradation of hippocampal function in aging mice via proliferation/differentiation of hippocampal stem cells
VAD increases GR binding capacity and modulates the somatostatinergic and acetylcholinergic hippocampal system
Vitamin A, atRA, 13cRA Memory/learning impairment, dementia, Alzheimer’s disease, depression [73, 151,152,153, 173, 188, 192, 193, 203, 374, 381, 391, 393,394,395,396,397,398]
    Rat   Hippocampal volume is reduced after 3 weeks of 13cRA treatment    [190]
Learning Organism Embryo Rat RALDH, PC2 Exposure during gestation (day 11–13 in rat) impaired amphetamine-stimulated activity and avoidance learning, but not performance in complex spatial maze or auditory startle response in offspring. A signalling decline is observed in aging and associated with cognitive impairment, decreased acquisition of new memories; reversible by RA administration atRA, 13cRA, 9cRA Possibly: affective disorders, neurodegenerative disorders, schizophrenia, autism [172, 186, 188, 203, 220, 395, 399,400,401]
   Postnatal Rat, Mouse RARβ, RXRγ Vitamin A deprivation and RARβ-/- mutants show spatial learning and memory impairment    [151,152,153]
Behavioural changes Organism Adult Mouse, rat   Extended low-dose exposure in mice induced depression-like behaviour. This was partially confirmed in 91 days-old rats, but not in older rats. 13cRA Depression [177, 187, 232, 234]
    Human D2, (D1), Ser1A 13cRA use/treatment is associated with depression and suicidal behaviour with longer onset (~4–8 weeks; long-term effect).   Case studies are reviewed in the reference [177]
Striatum Tissue/organ Adult Mouse, rat, human D2, RAR(α)β/RXRβγ; Nurr1/RXR, RALDH1, RALDH3, neurogranin, GAP43 The striatum shows the highest endogenous RA concentrations in the adult brain. Dopaminergic neurons; autocrine action on neurotransmission, paracrine action on striatal cells; locomotor impairment in RAR/RXR and Nurr1/RXR mutant mice. RXRγ-/- mutants: increased despair behaviour, anhedonia (reversible by re-expression of RXRγ). Induction of Parkinsonism and catatonia by lesions in basal ganglia RA (endogenous), disulphirama Depression, (potentially: Parkinson’s and Huntington’s disease), mood disorders [177, 226,227,228,229,230,231, 374, 395, 402,403,404]
      Extended 13cRA dose (in rat) increases dopamine and serotonin metabolites    [187]
GABAergic (inter-)neurons Cell/ tissue Embryo/foetus Mouse Raldh3 Enhanced differentiation; migration to olfactory bulb and cortex. Raldh3 activity is required for efficient differentiation of GABAergic interneurons, while Raldh2 is not RA (endogenous)   [163, 380]
   Adult    GABAergic interneurons of the olfactory bulb are RA sensitive    [405]
Branchial arches Tissue/organ Embryo Mouse RAR, Hoxa1, Hoxb1, Pax1/9 3rd–6th arch are RA responsive, give rise to: endodermal pouches, thymus, parathyroid glands, aorta and associated large blood vessels, nerves etc. Linked to rhombomeric (hindbrain) origin of mesenchyme/neural crest cells RA DiGeorge syndrome, CATCH22 syndrome [343, 358, 406,407,408,409,410]
Heart development Organ Embryo Mouse, chicken, zebrafish RARs, RXRs (esp. RXRα), Hoxb1, Hoxb5b, Tbx1, RALDH2, STRA6, CYP26A1, FGF8, NR2F5 Congenital heart disease, incl. conotruncal and aortic arch artery malformations (patterning defects); defects in RA synthesis can be, in some cases, partly rescued by maternal RA levels/RA supplementation. STRA6 mutations (vitamin A transport/ cellular uptake) may result in developmental defects in atrial and venous vessels. Later in development, RA is cardiotoxic (in zebrafish) RA Rarely observed/undocumented, maybe due to embryonic death. Matthew-Wood syndrome (STRA6 mutation) [411, 412], DiGeorge syndrome [413] [89, 138, 140, 164, 196, 369, 414,415,416,417,418,419,420,421,422,423,424,425]
    Zebrafish, rat RARα, SHH, AhR   Flame retardants (miTP, TPP, PBDE), TCDD   [169, 170, 370, 426]
Lung development and regeneration Tissue/organ Embryo, adult Mouse; embryonic explants, rat RALDH2, Wnt, TGF-β, FGF10, BMP, SHH Lack of RA/RAR activity prevents induction and growth of primary lung buds; RA induces regeneration of alveoli in rat and rescues lung functionality in experimental hypoplasia. RA is not required for endodermal lung cell fate, but essential for primordial lung bud formation RA   [141,142,143, 427,428,429,430,431,432,433]
     VEGF, FGF18 RA regulates angiogenesis and elastin production in the maturing lung    [434]
Pancreas formation Tissue/organ Embryo Zebrafish, Xenopus, mouse Cyp26a1, Cdx4, RALDH Formation of dorsal pancreatic bud (pancreatic and hepatic endoderm); specification of pancreatic endocrine cell lineages. RA is required for ventral pancreas patterning. In mouse and human pancreas, β-cell differentiation may be influenced by RA RA   [148, 334, 435,436,437,438,439,440,441,442]
Kidney formation Organ Embryo Xenopus, zebrafish, mouse RARα, RARβ, RALDH2, Notch signalling, mecom Inactivation of RARα and RARβ results in renal malformation (mouse); ureteric bud cell signalling depends mainly on RALDH2-generated atRA. Specification of renal progenitor cells depends on RA signaling (Xenopus, zebrafish) RA   [149, 443,444,445,446,447,448]
      Competition for RALDH2 Ethanol   [449, 450]
Limb (and tail) development and regeneration Organ Embryo, postnatal Amphibians, chicken, mouse, zebrafish FGF8, SHH, FGF4, RALDH2, Cyp26(b1), Hoxb8 RA can induce development (embryonic) or regeneration (postnatal) of supernumerary limbs or digits, when locally applied to the limb bud. In mice, RA exposure on gestational day 12 (33–41 somite pairs) affected rather forelimbs, on gestational day 13 (40–51 somite pairs) rather hindlimbs. Also, tail and tail vertebrae development is impaired by RA exposure RA, 9cRA   [130, 134, 363, 451,452,453,454,455,456,457,458,459]
Gene expression repression by unliganded receptors (RARs) Cellular Embryo Xenopus, mouse RAR, NCoR-1/2, SMRT, CYP26. (further: Fgf, Wnt, Hox genes) Head development in Xenopus and skeletal development in mice requires gene repression by unliganded RAR (otherwise: malformation of the head, anterior/posterior shift) RA   [460,461,462,463]
        Acute promyelocytic leukemia [462]
Invertebrate development Organ/organism   Chordata, Arthropoda, Mollusca, Porifera RAR, RXR, CYP26, RALDH Conservation of the retinoid signalling pathway and involvement in invertebrate development; incl.: all body, digestive glands, gonads, limb buds, regeneration of body parts RA, retinol   Reviewed in [322]
    Crustacea, Drosophila EcR, USP Ecdysone is the driver of invertebrate molting. EcR dimerizes with USP (RXR-homolog), which increases dimer stability and affinity towards target DNA sequences Ecdysone, tributyltin   [464,465,466,467]
  Organ   Mollusca, Gastropoda RXR Imposex (gastropods), shell thickening, reproduction perturbation (Crassostrea gigas); RAR/RXR heterodimers have a repressive function (instead of activating) Tributyltin, HX630 (only linked to imposex in gastropods), 9cRA   [111, 468,469,470]
Reproductive tract development Organ   Molluscs RXR; potential crosstalk with PPARγ Induction of outgrowth of male reproductive structures (T. clavigera), regulation of male/female seasonality (reproductive tract recrudescence; Ilyanassa obsolete) 9cRA   [111, 144, 322, 469, 471,472,473]
Reproductive organ development, testes; male fertility Organ/organism Embryo, adult Mouse RARα, RARγ, RXRβ, RALDH2, Cyp26b1, SHH, BMP4, STRA8 Degeneration of testes after knockout of all RARα isoforms; sterility of male mice after knockout of RARγ; male sterility in RXRβ knockout mice. RA is required for spermatogenesis RA Sterility in adult men in absence of RA [124, 160] [13, 76, 145,146,147, 474,475,476,477,478]
    Mouse, in vitro (P19 and C3H10T1/2), human RALDH, Cyp26 Hoxa1, HDAC-I, AR (via SHH)   Phthalate esters (esp. containing aryl and cyclohexane groups), valproic acid   [168, 479,480,481,482]
Peripheral nervous system, regeneration Organ system Adult Human RARα, RARβ, RBP Regeneration of spinal cord motor neurons depends on RA-induced RARβ(2) expression; RA peaks 4-7 days after encountering the injury RA, 9cRA   [82, 154,155,156, 158, 483]
Nervous system Organ system Adult Human RARα, RALDH2, RBP(1) Motor neurons: neurofilament accumulation, astrocytosis, decrease in neuron number, elimination of RARα and reduction of RALDH2 expression, reduction in retinol binding protein levels in spinal cord Vitamin A, RA Amyotrophic lateral sclerosis (ALS) [82, 484,485,486]
   Embryo Rat   Gestational exposure (day 11–13) led to difficulty to swallow milk (motor control); delayed righting reflex at 35 days. Decreased locomotor activity, motor coordination, and learning (90 days)   Depression, suicidal behaviour [223] [177, 186, 395, 487,488,489,490]
Vision   Pregnancy Human (pregnant women)    Vitamin A Night blindness; associated with miscarriage [131]
   Pre-school age children, adult Human   Retinal is required in the retina Vitamin A, RA Blindness; 50% fatal in children within 1 year if untreated; impaired vision [131, 133, 491]
Keratinization of epithelia Tissue Adult, embryo Human, rat   Mucous epithelia (as with the trachea/respiratory/gastrointestinal tract) become keratinized in absence of RA; RA is required for continuous renewal of skin epithelia.
Treatment of cystic and nodular acne with “Accutane” (13cRA)
RA, 13cRA   [124, 139, 160, 221, 492]
    Human (in vitro) AhR, RA-signaling TCDD alters matrix protein (esp. collagen) deposition; atRA shows an additive effect. The increased protein deposition is due to promoter activation and increased mRNA stability TCDD, atRA   [29]
Immune function Organ system Adult Human, mouse RARα Immune function severely compromised in absence of RA. RA is required for (CD4+) T cells in the thymus RA   [124, 126, 157,158,159,160, 493,494,495,496]
  1. Bold indicates high level of confidence. Gene and protein nomenclature has been adapted to human, though homologous genes/proteins in other species may have been assessed in the original studies
  2. ADH: alcohol dehydrogenase, AR: androgen receptor, BMP: borne morphogenetic protein, 9cRA: 9-cis retinoic acid, 13cRA: 13-cis retinoic acid, Cdx4: Homeobox protein transcription factor, CHAT: choline acetyl transferase, CRABP: cellular retinoic acid-binding protein, Dhrs3: short-chain dehydrogenase/reductase 3, EcR: ecdysone receptor (homologous to vertebrate farnesoid X receptor, FXR, though endogenous ligand is the ecdysone steroid), FGF: fibroblast growth factor, GR: glucocorticoid receptor, HDAC-I: class I histone deacetylase, Hox: Homeobox gene family, HC630: selective RXR agonist, MCT8: monocarboxylate transporter 8, mecom: mds1/evi1 complex transcription factor, miTP: monosubstituted isopropylated triaryl phosphate, NCoR-1/2: nuclear receptor corepressor 1/2, NR2F5: nuclear receptor 2F5 (COUP-transcription factor family), PBDE: polybrominated diphenyl ether, RA: retinoic acid, PPARγ: peroxisome proliferator activator receptor γ, RALDH: retinal dehydrogenase, RAR: retinoic acid receptor, RBP: retinol-binding protein, RXR: retinoid X receptor, Ser1A: serotonin receptor 1A, SHH: sonic hedgehog gene family, SMRT: silencing mediator of RAR and thyroid hormone receptor, TCDD: 2,3,7,8-tetrachlorodibenzo-p-dioxin, TH: thyroid hormone, TPP: triphenylphosphate, TR: thyroid hormone receptor, USP: ultraspiracle (nuclear receptor in Drosophila, homologous to vertebrate RXR), VAD: vitamin A deficiency
  3. aDisulfiram acts a selective antagonist to RALDH1, hence prevents endogenous RA synthesis. Therefore, effects are often reported as RA-dependent rather than disulfiram-sensitive