Roles of the aryl hydrocarbon receptor (AhR) in the myelin structure of the murine central nervous system

Le récepteur aux hydrocarbures aromatiques (AhR) est un facteur de transcription activé par de nombreux xénobiotiques (molécules étrangères à l’organisme) qui régule l’expression d’enzymes et transporteurs permettant le métabolisme et l’élimination de ces ligands. Cette protéine exprimée dans toutes les cellules chez les vertébrés, joue un rôle majeur dans la détoxication et la protection des organismes vis à vis des molécules toxiques. Des orthologues de celle-ci ont été identifiés chez les invertébrés mais ne semblent pas jouer le même rôle; ils sont exprimés principalement dans des neurones et ne sont pas activés par des polluants. L’absence du AhR chez ces organismes entraîne au niveau cellulaire, des défauts de morphologie dendritique et sur le plan comportemental, des anomalies dans le comportement de nutrition. Malgré ces découvertes, peu de recherches ont été entreprises sur les conséquences d’une invalidation du AhR sur le fonctionnement du système nerveux central chez les vertébrés. Au cours de ma thèse, j’ai étudié ces conséquences au niveau moléculaire, cellulaire et comportemental: les souris AhR KO développent un nystagmus pendulaire horizontal dont l’origine est en partie liée à des défauts structuraux de la gaine de myéline. Au niveau moléculaire, nous avons mis en évidence un changement de la composition lipidique, de l’expression des gènes de la myéline et de l’inflammation, défauts qui sont retrouvés en partie chez des souris dont le AhR a été invalidé spécifiquement dans l’oligodendrocyte, la cellule responsable de la formation de la gaine de myéline. J’ai donc réalisé des études en parallèle sur la lignée murine d’oligodendrocyte, 158N, et montré que l’invalidation du AhR dans cette lignée ainsi que in vivo, modifiait l’expression du gène MAG (Myelin Associated Glycoprotein). Compte tenu du rôle du AhR en tant que récepteur de polluants, nous avons également exposé ou traité nos modèles avec de la TCDD (dioxine de Seveso) et montré que celle-ci modifiait également l’expression du gène MAG. Mes travaux démontrent donc que le AhR joue un rôle au niveau oligodendrocytaire dans la formation de la gaine de myéline. Son rôle connu en tant que récepteur de polluants laisse supposer que certaines contaminations environnementales pourraient jouer un rôle dans l’incidence de pathologies au niveau du système nerveux central, ce qui soulève de nombreuses questions en terme de santé publique.The aryl hydrocarbon receptor (AhR) is a transcription factor activated by many xenobiotics (foreign molecules) that regulates the expression of enzymes and transporters which allow the metabolism and elimination of these ligands. This protein expressed in all cells in vertebrates, plays a major role in detoxication and protection of the organisms against toxic molecules. Some orthologs have been identified in invertebrates but do not seem to play the same role; they are expressed mainly in neurons and are not activated by pollutants. The absence of the AhR in these organisms causes at the cellular level, defects of the dendritic morphology and behaviourally, abnormalities in the feeding behavior. Despite these findings, little research has been conducted on the consequences of the AhR invalidation in the central nervous system of vertebrates. During my PhD, I studied these consequences at the molecular, cellular and behavioral : the AhR knockout mice develop a horizontal pendular nystagmus whose origin is partly related to structural defects in the myelin sheath. At the molecular level, we have shown modifications in the lipid composition, myelin and inflammation gene expression, defects that are found partly in mice whose AhR was invalidated specifically in the oligodendrocytes, the cells involved in myelin sheath formation. I therefore made parallel studies on the murine oligodendrocyte lineage, 158N, and showed that the invalidation of the AhR in this cell line and in vivo, altered MAG (Myelin Associated Glycoprotein) gene expression. Given the role of the AhR as a receptor of pollutants, we have also exposed or treated our models with TCDD (dioxin of Seveso) and showed that it also changed the expression of MAG gene. My works show that the AhR is involved in oligodendrocyte level in the formation of the myelin sheath. As the AhR is also a receptor of pollutants, some environmental contaminants may play a role in the incidence of diseases in the central nervous system, which raises many issues in terms of public health

Alimentation, pesticides et pathologies neurologiques

International audienceA recent report from the Inserm brought out the involvement of occupational exposure to pesticides in the development of several diseases such as cancer, fertility problems and neurological diseases. These epidemiological studies have shown that some pesticides can produce toxic effects on the nervous system with the occurrence of neurodegenerative diseases or the onset of cognitive impairment and depression. Firstly, the aim of this review is to introduce the cellular and molecular mechanisms underlying the neurotoxicity associated with this exposure, through a specific example, Parkinson's disease. In the second part, we discuss the problem of pesticide exposure during the critical period of development (in utero, childhood) by studying the association between organophosphate pesticides and neurodevelopmental disorders. Finally, we discuss the advantages and limitations of these studies (concentrations, route and duration of exposure, effects of mixtures…

The Aryl Hydrocarbon Receptor and the Nervous System

International audienceThe aryl hydrocarbon receptor (or AhR) is a cytoplasmic receptor of pollutants. It translocates into the nucleus upon binding to its ligands, and forms a heterodimer with ARNT (AhR nuclear translocator). The heterodimer is a transcription factor, which regulates the transcription of xenobiotic metabolizing enzymes. Expressed in many cells in vertebrates, it is mostly present in neuronal cell types in invertebrates, where it regulates dendritic morphology or feeding behavior. Surprisingly, few investigations have been conducted to unravel the function of the AhR in the central or peripheral nervous systems of vertebrates. In this review, we will present how the AhR regulates neural functions in both invertebrates and vertebrates as deduced mainly from the effects of xenobiotics. We will introduce some of the molecular mechanisms triggered by the well-known AhR ligand, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), which impact on neuronal proliferation, differentiation, and survival. Finally, we will point out the common features found in mice that are exposed to pollutants, and in AhR knockout mice

AhR signaling pathways and regulatory functions

International audienceAnimals and humans are exposed each day to a multitude of chemicals in the air, water and food. They have developed a battery of enzymes and transporters that facilitate the biotransformation and elimination of these compounds. Moreover, a majority of these enzymes and transporters are inducible due to the activation of xenobiotic receptors which act as transcription factors for the regulation of their target genes (such as xenobiotic metabolizing enzymes, see below §4 for the AhR). These receptors include several members of the nuclear/steroid receptor family (CAR for Constitutive Androstane Receptor, PXR for Pregnane X Receptor) but also the Aryl hydrocarbon Receptor or AhR, a member of the bHLH-PAS family (basic Helix-Loop-Helix - Period/ARNT/Single minded). In addition to the regulation of xenobiotic metabolism, numerous alternative functions have been characterized for the AhR since its discovery. These alternative functions will be described in this review along with its endogenous functions as revealed by experiments performed on knock-out animals

Hexokinase 2 is a transcriptional target and a positive modulator of AHR signalling

International audienceThe aryl hydrocarbon receptor (AHR) regulates the expression of numerous genes in response to activation by agonists including xenobiotics. Although it is well appreciated that environmental signals and cell intrinsic features may modulate this transcriptional response, how it is mechanistically achieved remains poorly understood. We show that hexokinase 2 (HK2) a metabolic enzyme fuelling cancer cell growth, is a transcriptional target of AHR as well as a modulator of its activity. Expression of HK2 is positively regulated by AHR upon exposure to agonists both in human cells and in mice lung tissues. Conversely, overexpression of HK2 regulates the abundance of many proteins involved in the regulation of AHR signalling and these changes are linked with altered AHR expression levels and transcriptional activity. HK2 expression also shows a negative correlation with AHR promoter methylation in tumours, and these tumours with high HK2 expression and low AHR methylation are associated with a worse overall survival in patients. In sum, our study provides novel insights into how AHR signalling is regulated which may help our understanding of the context-specific effects of this pathway and may have implications in cancer

Age-dependent vulnerability of the ovary to AhR-mediated TCDD action before puberty: Evidence from mouse models

International audienceIn female mammals, puberty and fertility are regulated by the synthesis of estradiol (E2) by the ovaries at the infantile stage and at the approach of puberty, a process which may be affected by endocrine disrupting chemicals (EDC)s acting through the Aryl hydrocarbon receptor (AhR). However, there is no information on AhR-mediated regulation of ovarian estrogenic activity during these developmental periods. Here, we assessed in mouse models, the intrinsic and exogenous ligand-induced AhR action on E2 synthesis at the infantile stage (14 days postnatal (dpn)) and at the approach of puberty (28 dpn). Intrinsic AhR pathway became activated in the ovary at the approach of puberty, as suggested by the decreased intra-ovarian expression in prototypical and steroidogenesis-related AhR targets and E2 contents in Ahr knockout (Ahr(-/-)) mice versus Ahr(+/+) mice exclusively at 28 dpn. Accordingly, AhR nuclear localization in granulosa cells, reflecting its activity in cells responsible for E2 synthesis, was much lower at 14 dpn than at 28 dpn in C57BL/6 mice. However, AhR signaling could be activated by exogenous ligands at both ages, as revealed by FICZ- and TCDD-induced Ahrr and Cyp1a1 expression in C57BL/6 mice. Nevertheless, TCDD impacted ovarian estrogenic activity only at 28 dpn. This age-related AhR action may be ligand-dependent, since FICZ had no effect on E2 synthesis at 28 dpn. In conclusion, AhR would not regulate ovarian estrogenic activity before the approach of puberty. Its activation by EDCs may be more detrimental to reproductive health at this stage than during infancy. (C) 2020 Elsevier Ltd. All rights reserved

Activation of the aryl hydrocarbon receptor by carcinogenic aromatic amines and modulatory effects of their N-acetylated metabolites

International audienceAromatic amines (AAs) are an important class of chemicals which account for 12 % of known carcinogens. The biological effects of AAs depend mainly on their biotransformation into reactive metabolites or into N-acetylated metabolites which are generally considered as less toxic. Although the activation of the aryl hydrocarbon receptor (AhR) pathway by certain carcinogenic AAs has been reported, the effects of their N-acetylated metabolites on the AhR have not been addressed. Here, we investigated whether carcinogenic AAs and their N-acetylated metabolites may activate/modulate the AhR pathway in the absence and/or the presence of a bona fide AhR ligand (benzo[a]pyrene/B(a)P]. In agreement with previous studies, we found that certain AAs activated the AhR in human liver and lung cells as assessed by an increase in cytochrome P450 1A1 (CYP1A1) expression and activity. Altogether, we report for the first time that these properties can be modulated by the N-acetylation status of the AA. Whereas 2-naphthylamine significantly activated the AhR and induced CYP1A1 expression, its N-acetylated metabolite was less efficient. In contrast, the N-acetylated metabolite of 2-aminofluorene was able to significantly activate AhR, whereas the parent AA, 2-aminofluorene, did not. In the presence of B(a)P, activation of AhR or antagonist effects were observed depending on the AA or its N-acetylated metabolite. Activation and/or modulation of the AhR pathway by AAs and their N-acetylated metabolites may represent a novel mechanism contributing to the toxicological effects of AAs. More broadly, our data suggest biological interactions between AAs and other classes of xenobiotics through the AhR pathway

AhR-deficiency as a cause of demyelinating disease and inflammation

Abstract The Aryl hydrocarbon Receptor(AhR) is among the most important receptors which bind pollutants; however it also regulates signaling pathways independently of such exposure. We previously demonstrated that AhR is expressed during development of the central nervous system(CNS) and that its deletion leads to the occurrence of a congenital nystagmus. Objectives of the present study are to decipher the origin of these deficits, and to identify the role of the AhR in the development of the CNS. We show that the AhR-knockout phenotype develops during early infancy together with deficits in visual-information-processing which are associated with an altered optic nerve myelin sheath, which exhibits modifications in its lipid composition and in the expression of myelin-associated-glycoprotein(MAG), a cell adhesion molecule involved in myelin-maintenance and glia-axon interaction. In addition, we show that the expression of pro-inflammatory cytokines is increased in the impaired optic nerve and confirm that inflammation is causally related with an AhR-dependent decreased expression of MAG. Overall, our findings demonstrate the role of the AhR as a physiological regulator of myelination and inflammatory processes in the developing CNS. It identifies a mechanism by which environmental pollutants might influence CNS myelination and suggest AhR as a relevant drug target for demyelinating diseases

The Involvement of Aryl hydrocarbon receptor in myelination and in human nerve sheath tumorigenesis

International audienceAryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor involved in xenobiotic metabolism. Plexiform neurofibromas (PNFs) can transform into malignant peripheral nerve sheath tumors (MPNSTs) that are resistant to existing therapies. These tumors are primarily composed of Schwann cells. In addition to neurofibroma-tosis type 1 (NF1) gene inactivation, further genetic lesions are required for malignant transformation. We have quantified the mRNA expression levels of AHR and its associated genes in 38 human samples. We report that AHR and the biosynthetic enzymes of its en-dogenous ligand are overexpressed in human biopsies of PNFs and MPNSTs. We also detect a strong nuclear AHR staining in MPNSTs. The inhibition of AHR by siRNA or antagonists, CH-223191 and tri-methoxyflavone, induces apoptosis in human MPNST cells. Since AHR dysregulation is observed in these tumors, we investigate AHR involvement in Schwann cell physiology. Hence, we studied the role of AHR in myelin structure and myelin gene regulation in Ahr −/− mice during myelin development. AHR ablation leads to lo-comotion defects and provokes thinner myelin sheaths around the axons. We observe a dysregulation of myelin gene expression and myelin developmental markers in Ahr −/− mice. Interestingly, AHR does not directly bind to myelin gene promoters. The inhibition of AHR in vitro and in vivo increased β-catenin levels and stimulated the binding of β-catenin on myelin gene promoters. Taken together, our findings reveal an endogenous role of AHR in peripheral myeli-nation and in peripheral nerve sheath tumors. Finally, we suggest a potential therapeutic approach by targeting AHR in nerve tumors. AHR | myelin | nerve | MPNST | neurofibrom