Influence of the Aryl Hydrocarbon Receptor Activating Environmental Pollutants on Autism Spectrum Disorder

Intercontinental insights into autism spectrum disorder: a synthesis of environmental influences and DNA methylation

Autism Spectrum Disorder (ASD) is a complex neurodevelopmental disorder characterized by a broad range of symptoms. The etiology of ASD is thought to involve complex gene-environment interactions, which are crucial to understanding its various causes and symptoms. DNA methylation is an epigenetic mechanism that potentially links genetic predispositions to environmental factors in the development of ASD. This review provides a global perspective on ASD, focusing on how DNA methylation studies may reveal gene-environment interactions characteristic of specific geographical regions. It delves into the role of DNA methylation in influencing the causes and prevalence of ASD in regions where environmental influences vary significantly. We also address potential explanations for the high ASD prevalence in North America, considering lifestyle factors, environmental toxins, and diagnostic considerations. Asian and European studies offer insights into endocrine-disrupting compounds, persistent organic pollutants, maternal smoking, and their associations with DNA methylation alterations in ASD. In areas with limited data on DNA methylation and ASD, such as Africa, Oceania, and South America, we discuss prevalent environmental factors based on epidemiological studies. Additionally, the review integrates global and country-specific prevalence data from various studies, providing a comprehensive picture of the variables influencing ASD diagnoses over region and year of assessment. This prevalence data, coupled with regional environmental variables and DNA methylation studies, provides a perspective on the complexities of ASD research. Integrating global prevalence data, we underscore the need for a comprehensive global understanding of ASD's complex etiology. Expanded research into epigenetic mechanisms of ASD is needed, particularly in underrepresented populations and locations, to enhance biomarker development for diagnosis and intervention strategies for ASD that reflect the varied environmental and genetic landscapes worldwide.

Deficits in olfactory system neurogenesis in neurodevelopmental disorders

The role of neurogenesis in neurodevelopmental disorders (NDDs) merits much attention. The complex process by which stem cells produce daughter cells that in turn differentiate into neurons, migrate various distances, and form synaptic connections that are then refined by neuronal activity or experience is integral to the development of the nervous system. Given the continued postnatal neurogenesis that occurs in the mammalian olfactory system, it provides an ideal model for understanding how disruptions in distinct stages of neurogenesis contribute to the pathophysiology of various NDDs. This review summarizes and discusses what is currently known about the disruption of neurogenesis within the olfactory system as it pertains to attention-deficit/hyperactivity disorder, autism spectrum disorder, Down syndrome, Fragile X syndrome, and Rett syndrome. Studies included in this review used either human subjects, mouse models, or Drosophila models, and lay a compelling foundation for continued investigation of NDDs by utilizing the olfactory system.

Maternal Inflammation with Elevated Kynurenine Metabolites Is Related to the Risk of Abnormal Brain Development and Behavioral Changes in Autism Spectrum Disorder

Several studies show that genetic and environmental factors contribute to the onset and progression of neurodevelopmental disorders. Maternal immune activation (MIA) during gestation is considered one of the major environmental factors driving this process. The kynurenine pathway (KP) is a major route of the essential amino acid L-tryptophan (Trp) catabolism in mammalian cells. Activation of the KP following neuro-inflammation can generate various endogenous neuroactive metabolites that may impact brain functions and behaviors. Additionally, neurotoxic metabolites and excitotoxicity cause long-term changes in the trophic support, glutamatergic system, and synaptic function following KP activation. Therefore, investigating the role of KP metabolites during neurodevelopment will likely promote further understanding of additional pathophysiology of neurodevelopmental disorders, including autism spectrum disorder (ASD). In this review, we describe the changes in KP metabolism in the brain during pregnancy and represent how maternal inflammation and genetic factors influence the KP during development. We overview the patients with ASD clinical data and animal models designed to verify the role of perinatal KP elevation in long-lasting biochemical, neuropathological, and behavioral deficits later in life. Our review will help shed light on new therapeutic strategies and interventions targeting the KP for neurodevelopmental disorders.

Autism Spectrum Disorder: Neurodevelopmental Risk Factors, Biological Mechanism, and Precision Therapy

Autism spectrum disorder (ASD) is a heterogeneous, behaviorally defined neurodevelopmental disorder. Over the past two decades, the prevalence of autism spectrum disorders has progressively increased, however, no clear diagnostic markers and specifically targeted medications for autism have emerged. As a result, neurobehavioral abnormalities, neurobiological alterations in ASD, and the development of novel ASD pharmacological therapy necessitate multidisciplinary collaboration. In this review, we discuss the development of multiple animal models of ASD to contribute to the disease mechanisms of ASD, as well as new studies from multiple disciplines to assess the behavioral pathology of ASD. In addition, we summarize and highlight the mechanistic advances regarding gene transcription, RNA and non-coding RNA translation, abnormal synaptic signaling pathways, epigenetic post-translational modifications, brain-gut axis, immune inflammation and neural loop abnormalities in autism to provide a theoretical basis for the next step of precision therapy. Furthermore, we review existing autism therapy tactics and limits and present challenges and opportunities for translating multidisciplinary knowledge of ASD into clinical practice.

Insight into the physiological and pathological roles of the aryl hydrocarbon receptor pathway in glucose homeostasis, insulin resistance, and diabetes development

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcriptional factor that mediates the toxicities of several environmental pollutants. Decades of research have been carried out to understand the role of AhR as a novel mechanism for disease development. Its involvement in the pathogenesis of cancer, cardiovascular diseases, rheumatoid arthritis, and systemic lupus erythematosus have long been known. One of the current hot research topics is investigating the role of AhR activation by environmental pollutants on glucose homeostasis and insulin secretion, and hence the pathogenesis of diabetes mellitus. To date, epidemiological studies have suggested that persistent exposure to environmental contaminants such as dioxins, with subsequent AhR activation increases the risk of specific comorbidities such as obesity and diabetes. The importance of AhR signaling in various molecular pathways highlights that the role of this receptor is far beyond just xenobiotic metabolism. The present review aims at providing significant insight into the physiological and pathological role of AhR and its regulated enzymes, such as cytochrome P450 1A1 (CYP1A1) and CYP1B1 in both types of diabetes. It also provides a comprehensive summary of the current findings of recent research studies investigating the role of the AhR/CYP1A1 pathway in insulin secretion and glucose hemostasis in the pancreas, liver, and adipose tissues. This review further highlights the molecular mechanisms involved, such as gluconeogenesis, hypoxia-inducible factor (HIF), oxidative stress, and inflammation.

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.

Dynamic and Systemic Perspective in Autism Spectrum Disorders: A Change of Gaze in Research Opens to A New Landscape of Needs and Solutions

The first step for a harmonious bio-psycho-social framework in approaching autism spectrum disorders (ASD) is overcoming the conflict between the biological and the psychosocial perspective. Biological research can provide clues for a correct approach to clinical practice, assuming that it would lead to the conceptualization of a pathogenetic paradigm able to account for epidemiologic and clinical findings. The upward trajectory in ASD prevalence and the systemic involvement of other organs besides the brain suggest that the epigenetic paradigm is the most plausible one. The embryo-fetal period is the crucial window of opportunity for keeping neurodevelopment on the right tracks, suggesting that women's health in pregnancy should be a priority. Maladaptive molecular pathways beginning in utero, in particular, a vicious circle between the immune response, oxidative stress/mitochondrial dysfunction, and dysbiosis-impact neurodevelopment and brain functioning across the lifespan and are the basis for progressive multisystemic disorders that account for the substantial health loss and the increased mortality in ASD. Therefore, the biological complexity of ASD and its implications for health requires the enhancement of clinical skills on these topics, to achieve an effective multi-disciplinary healthcare model. Well-balanced training courses could be a promising starting point to make a change.