Short-term effects of a perinatal exposure to a 16 polycyclic aromatic hydrocarbon mixture in rats: assessment of early motor and sensorial development and cerebral cytochrome oxidase activity in pups

Head-to-Head Study of Developmental Neurotoxicity and Resultant Phenotype in Rats: α-Hexabromocyclododecane versus Valproic Acid, a Recognized Model of Reference for Autism Spectrum Disorders

Evidence is now growing that exposure to environmental pollutants during the critical early-life period of brain development may contribute to the emergence of Autism Spectrum Disorders (ASD). This study seeks to compare the developmental neurotoxicity of the α-isomer of hexabromocyclododecane (α-HBCDD), a persistent brominated flame retardant, to the valproic acid (VPA) model of ASD in rodents. Pregnant Wistar rats were divided into three groups: control, α-HBCDD (100 ng/kg/day p.o., GD0-PND21) and VPA (600 mg/kg i.p., GD12). Male offspring were tested for their neuromotor development from PND2-21. At PND21, brain functionality was assessed by measuring cytochrome oxidase activity (CO). Modifications in neuroglia and synaptic plasticity were evaluated in the cortex. Similar subtle behavioural changes related to neuromotor maturation and noise reaction were observed in both treated groups. At PND21, a reduction in CO activity was measured in the VPA group only, in specific areas including auditory nuclei, visual cortex, cingulate and frontal cortices. At the same age, α-HBCDD pointed out significant overexpression of cortical markers of synaptic plasticity while both treated groups showed a significant under expression of astrocyte proteins (S100-β and GFAP). Early-life exposure to a low dose of α-HBCDD may trigger neurobehavioural alterations in line with ASD.

Phenanthrene induces autism-like behavior by promoting oxidative stress and mTOR pathway activation

Autism is thought to be associated with both environmental and genetic factors. Phenanthrene (Phe) makes up a relatively high proportion of the low-ring polycyclic aromatic hydrocarbons. However, the association between exposure to Phe and Autism remain unclear. In this study, the effect and mechanisms of phenanthrene exposure on autistic behavior were investigated. Three-week-old male Kunming mice were exposed to doses of 5, 50, or 500 μg/kg/d Phe for 22 days. Exposure to phenanthrene induced a marked decrease in the activity of the mice in the central area in the open field test, and caused a significant decrease in communication with unfamiliar mice in the three-chambered social test. The hippocampus of the mice exposed to high concentrations of Phe showed pathological changes. Exposure to phenanthrene induced an increase in the levels of ROS and a decrease in levels of glutathione, and caused a significant decrease in the expression of Shank3 and Beclin1. This also led to an increase in the phosphorylation levels of Akt and mTOR. However, administering Rapamycin or vitamin E, inhibited the oxidative stress and activation of the mTOR pathway induced by Phe exposure, effectively alleviating the above-mentioned autistic-like anxious social behaviors. These results indicate that exposure to phenanthrene will lead to autism-like behavior. The underlying mechanism involves oxidative stress and the mTOR pathway.

N6-Methyladenine in Eukaryotic DNA: Tissue Distribution, Early Embryo Development, and Neuronal Toxicity

DNA methylation is one of the most important epigenetic modifications and is closely related with several biological processes such as regulation of gene transcription and the development of non-malignant diseases. The prevailing dogma states that DNA methylation in eukaryotes occurs essentially through 5-methylcytosine (5mC) but recently adenine methylation was also found to be present in eukaryotes. In mouse embryonic stem cells, 6-methyladenine (6mA) was associated with the repression and silencing of genes, particularly in the X-chromosome, known to play an important role in cell fate determination. Here, we have demonstrated that 6mA is a ubiquitous eukaryotic epigenetic modification that is put in place during epigenetically sensitive periods such as embryogenesis and fetal development. In somatic cells there are clear tissue specificity in 6mA levels, with the highest 6mA levels being observed in the brain. In zebrafish, during the first 120 h of embryo development, from a single pluripotent cell to an almost fully formed individual, 6mA levels steadily increase. An identical pattern was observed over embryonic days 7–21 in the mouse. Furthermore, exposure to a neurotoxic environmental pollutant during the same early life period may led to a decrease in the levels of this modification in female rats. The identification of the periods during which 6mA epigenetic marks are put in place increases our understanding of this mammalian epigenetic modification, and raises the possibility that it may be associated with developmental processes.

Assessment of 9-OH- and 7,8-diol-benzo[a]pyrene in Blood as Potent Markers of Cognitive Impairment Related to benzo[a]pyrene Exposure: An Animal Model Study

The potent neurotoxicity of benzo[a]pyrene (B[a]P) has been suggested to be a susceptibility factor accelerating the onset of brain tumours and the emergence of neurobehavioural disturbances. B[a]P has been shown to be neurotoxic, acting directly on both the central and peripheral nervous systems, as well as indirectly via peripheral organs like liver and gut. By using a realistic B[a]P exposure scenario (0.02-200 mg/kg/day, 10 days) in mice, we elucidated brain-specific B[a]P metabolism and at identified hydroxylated B[a]P metabolites in serum which could be used as markers of cognitive impairment. Repeated oral administration of B[a]P led to, at the doses of 20 and 200 mg/kg/day, significant overexpression of Cyp1a1/Cyp1b1 in 2 out of the 3 brain regions considered, thereby suggesting the ability of the brain to metabolize B[a]P itself. At the same doses, mice exhibited a reduction in anxiety in both the elevated plus maze and the hole board apparatus. Concomitantly, B[a]P triggered dose-dependent changes in Nmda subunit expression (Nr1 and Nr2a/Nr2b) in areas involved in cognition. We detected 9-OH-B[a]P and 7,8-diol-B[a]P in serum at the level for which cognitive impairment was observed. We suggest that these metabolites may, in the future be exploited as potent biomarkers of B[a]P-induced cognitive impairments.

Long Term Pharmacological Perturbation of Autophagy in Mice: Are HCQ Injections a Relevant Choice?

Macroautophagy (hereafter referred to as autophagy) is an evolutionarily conserved catabolic process whose loss-of-function has been linked to a growing list of pathologies. Knockout mouse models of key autophagy genes have been instrumental in the demonstration of the critical functions of autophagy, but they display early lethality, neurotoxicity and unwanted autophagy-independent phenotypes, limiting their applications for in vivo studies. To avoid problems encountered with autophagy-null transgenic mice, we investigated the possibility of disturbing autophagy pharmacologically in the long term. Hydroxychloroquine (HCQ) ip injections were done in juvenile and adult C57bl/6j mice, at range doses adapted from the human malaria prophylactic treatment. The impact on autophagy was assessed by western-blotting, and juvenile neurodevelopment and adult behaviours were evaluated for four months. Quite surprisingly, our results showed that HCQ treatment in conditions used in this study neither impacted autophagy in the long term in several tissues and organs nor altered neurodevelopment, adult behaviour and motor capabilities. Therefore, we recommend for future long-term in vivo studies of autophagy, to use genetic mouse models allowing conditional inhibition of selected Atg genes in appropriate lineage cells instead of HCQ treatment, until it could be successfully revisited using higher HCQ doses and/or frequencies with acceptable toxicity.

Epigenetic and Neurological Impairments Associated with Early Life Exposure to Persistent Organic Pollutants

The incidence of neurodevelopmental and neurodegenerative diseases worldwide has dramatically increased over the last decades. Although the aetiology remains uncertain, evidence is now growing that exposure to persistent organic pollutants during sensitive neurodevelopmental periods such as early life may be a strong risk factor, predisposing the individual to disease development later in life. Epidemiological studies have associated environmentally persistent organic pollutant exposure to brain disorders including neuropathies, cognitive, motor, and sensory impairments; neurodevelopmental disorders such as autism spectrum disorder (ASD) and attention-deficit hyperactivity disorder (ADHD); and neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis (ALS). In many ways, this expands the classical “Developmental Origins of Health and Disease” paradigm to include exposure to pollutants. This model has been refined over the years to give the current “three-hit” model that considers the individual's genetic factors as a first “hit.” It has an immediate interaction with the early-life exposome (including persistent organic pollutants) that can be considered to be a second “hit.” Together, these first two “hits” produce a quiescent or latent phenotype, most probably encoded in the epigenome, which has become susceptible to a third environmental “hit” in later life. It is only after the third “hit” that the increased risk of disease symptoms is crystallised. However, if the individual is exposed to a different environment in later life, they would be expected to remain healthy. In this review, we examine the effect of exposure to persistent organic pollutants and particulate matters in early life and the relationship to subsequent neurodevelopmental and neurodegenerative disorders. The roles of those environmental factors which may affect epigenetic DNA methylation and therefore influence normal neurodevelopment are then evaluated.

Identification of new tetrahydroxylated metabolites of Polycyclic Aromatic Hydrocarbons in hair as biomarkers of exposure and signature of DNA adduct levels

The present study addresses the hypothesis that the concentration of tetrahydroxylated Polycylic Aromatic Hydrocarbons (tetra-OH-PAHs) in hair might be a useful biomarker of human exposure to PAHs, providing quantitative assessment of the internal dose, as well as information on the associated toxicity in relation to individual's specific metabolism. By means of animal models, this work aimed at identifying new tetra-OH-PAHs which can be released from the hydrolysis of DNA-adducts and can also be directly detected in biological matrices usually used in the field of biomonitoring such as hair and urine. Results obtained from a targeted gas chromatography coupled with tandem mass spectrometry (GC-MS/MS) approach, demonstrated the presence of 8 tetrahydroxylated metabolites in DNA and 23 in hairs of rats exposed to mixtures of PAHs, which had never been analyzed before. Ten tetra-OH-PAHs were clearly characterized by using their analytical standards, corresponding to 4 parent PAHs (phenanthrene, chrysene, benz[a]anthracene and benzo[a]pyrene) whereas 13 tetra-OH-PAHs from 3 other parents (anthracene, fluoranthene and benz[k]fluoranthene) were detected but not yet characterized. No tetrahydroxylated metabolite has been clearly identified for naphthalene, fluorene, benzo[b]fluoranthene, benzo[g,h,i]perylene, or dibenzo[a,h]anthracene, which can all potentially form adducts. The relevance of tetra-OH-PAH analysis in hair as biomarkers of PAH exposure was evaluated in a dose-response study conducted on 64 rats (Long Evans females/n = 8 per groups) under repeated exposure (3 times per week) to a mixture of 16 PAHs at low doses (0.01-0.8 mg/kg) for 90 days. Most of the tetra-OH-PAHs targeted in the method were detected in the hairs of the rats, regardless of the dose of exposure. Significant linear relationships (R² ranging from 0.558 to 0.964, p < 0.001) were observed between the administered dose and the tetra-OH-PAH concentrations in the hairs for 20 out of the 23 metabolites. By widening the range of PAH metabolites used as biomarkers of exposure so as to include the analysis of PAH tetrahydroxylated forms (especially those exhibiting more than 5 aromatic rings), the present methodology will enable multi-exposure assessments which are more accurately representative of actual situations of exposure to PAHs.

New insights into urine-based assessment of polycyclic aromatic hydrocarbon-exposure from a rat model: Identification of relevant metabolites and influence of elimination kinetics

A gas chromatography tandem mass-spectrometry method dedicated to the analysis of 50 metabolites of polycyclic aromatic hydrocarbons (OH-PAHs) was applied to urine specimens collected from female Long Evans rats under controlled exposure to a mixture of PAHs (at 7 doses ranging from 0.01 to 0.8 mg/kg, by gavage, 3 times per week for 90 days). On four occasions (day 1, 28, 60 and 90), urine samples were collected over a 24 h period. Among these 50 OH-PAHs, 41 were detected in urine samples. Seven additional OH-PAHs were identified for the first time: 1 corresponding to metabolite of pyrene and 3 of anthracene. Strong linear dose versus urinary concentration relationships were observed for 25 of the 41 OH-PAHs detected in rat urine, confirming their suitability for assessing exposure to their respective parent compound. In addition, some isomers (e.g. 1-OH-pyrene, 3-OH-/4-OH-chrysene, 10-OH-benz[a]anthracene, 8-OH-benzo[k]fluoranthene, 11-OH-benzo[b]fluoranthene and 3-OH-benzo[a]pyrene) that were detected starting from the lowest levels of exposure or even in controls were considered particularly relevant biomarkers compared to metabolites only detected at higher levels of exposure. Finally, on the basis of the excretion profiles (on days 1, 28, 60 and 90) and urinary elimination kinetics of each OH-PAH detected at days 1 and 60, this study highlighted the fact that sampling time may influence the measurement of metabolites in urine. Taken together, these results provide interesting information on the suitability of the analysis of OH-PAHs in urine for the assessment of PAH exposure, which could be taken into consideration for the design of epidemiological studies in the future.

Neurotoxicity of alkylated polycyclic aromatic compounds in human neuroblastoma cells

Polycyclic aromatic compounds (PAC) are ubiquitous environmental pollutants originating from incomplete combustion processes. While the toxicity of parent PAC such as benzo[a]pyrene (BaP) is well characterized, effects of other alkyl-PAC dibenzothiophene (DBT) and retene (Ret) are not well established. The aim of this study was to examine the underlying relative neurotoxic mechanisms attributed to BaP (parent PAH), DBT and Ret (alkyl-PACs) using human neuroblastoma SK-N-SH cells. The lethal concentrations (LC₁₀ and LC₂₀) were found at approximately 10 µM and 40 µM, respectively after 24-h exposure of SK-N-SH cells. It was hypothesized that PAC trigger reactive oxygen species (ROS) production, leading to activation of apoptotic signaling pathways. Differentiated neuronal cells were treated with three compounds at (0.5-40 µM) for 24 h. There was a significant concentration-dependent increase in levels of ROS, even at sub-lethal levels of 1 µM Ret. The mitochondrial membrane potential (MMP) was significantly decreased. Real-time RT-PCR results showed up-regulation of pro-apoptotic genes and down-regulation of antioxidative genes expression in BaP-, DBT-, and Ret-treated SK-N-SH cells. Cytochrome c protein levels and lipid peroxidation (LPO) were also significantly elevated in a concentration-related manner. Data demonstrated that BaP-, DBT-, or Ret-induced neuronal cell damage involved oxidative stress generation through mitochondria-mediated apoptosis pathway. Alkyl-PAC also exhibited higher potency in ROS induction and reduction of MMP than parent PAC. These findings may be important for environmental risk assessment attributed to exposure to PAC.