Gestational arsenic exposure and paternal intergenerational epigenetic inheritance

Transgenerational Effects of Arsenic Exposure on Learning and Memory in Rats: Crosstalk between Arsenic Methylation, Hippocampal Metabolism, and Histone Modifications

Arsenic (As) is widely present in the natural environment, and exposure to it can lead to learning and memory impairment. However, the underlying epigenetic mechanisms are still largely unclear. This study aimed to reveal the role of histone modifications in environmental levels of arsenic (sodium arsenite) exposure-induced learning and memory dysfunction in male rats, and the inter/transgenerational effects of paternal arsenic exposure were also investigated. It was found that arsenic exposure impaired the learning and memory ability of F0 rats and down-regulated the expression of cognition-related genes Bdnf, c-Fos, mGlur1, Nmdar1, and Gria2 in the hippocampus. We also observed that inorganic arsenite was methylated to DMA and histone modification-related metabolites were altered, contributing to the dysregulation of H3K4me1/2/3, H3K9me1/2/3, and H3K4ac in rat hippocampus after exposure. Therefore, it is suggested that arsenic methylation and hippocampal metabolism changes attenuated H3K4me1/2/3 and H3K4ac while enhancing H3K9me1/2/3, which repressed the key gene expressions, leading to cognitive impairment in rats exposed to arsenic. In addition, paternal arsenic exposure induced transgenerational effects of learning and memory disorder in F2 male rats through the regulation of H3K4me2 and H3K9me1/2/3, which inhibited c-Fos, mGlur1, and Nmdar1 expression. These results provide novel insights into the molecular mechanism of arsenic-induced neurotoxicity and highlight the risk of neurological deficits in offspring with paternal exposure to arsenic.

Intergenerational arsenic exposure on the mouse epigenome and metabolic physiology

Inorganic arsenic (iAs) is one of the largest toxic exposures to impact humanity worldwide. Exposure to iAs during pregnancy may disrupt the proper remodeling of the epigenome of F1 developing offspring and potentially their F2 grand-offspring via disruption of fetal primordial germ cells (PGCs). There is a limited understanding between the correlation of disease phenotype and methylation profile within offspring of both generations and whether it persists to adulthood. Our study aims to understand the intergenerational effects of in utero iAs exposure on the epigenetic profile and onset of disease phenotypes within F1 and F2 adult offspring, despite the lifelong absence of direct arsenic exposure within these generations. We exposed F0 female mice (C57BL6/J) to the following doses of iAs in drinking water 2 weeks before pregnancy until the birth of the F1 offspring: 1, 10, 245, and 2300 ppb. We found sex- and dose-specific changes in weight and body composition that persist from early time to adulthood within both generations. Fasting blood glucose challenge suggests iAs exposure causes dysregulation of glucose metabolism, revealing generational, exposure, and sex-specific differences. Toward understanding the mechanism, genome-wide DNA methylation data highlights exposure-specific patterns in liver, finding dysregulation within genes associated with cancer, T2D, and obesity. We also identified regions containing persistently differentially methylated CpG sites between F1 and F2 generations. Our results indicate the F1 developing embryos and their PGCs, which will result in F2 progeny, retain epigenetic damage established during the prenatal period and are associated with adult metabolic dysfunction.

Testicular histone hyperacetylation in mice by valproic acid administration affects the next generation by changes in sperm DNA methylation

Various studies have described epigenetic inheritance through sperms. However, the detailed mechanisms remain unclear. In this study, we focused on DNA methylation in mice treated with valproic acid (VPA), an inducer of epigenomic changes, and analyzed the treatment effects on the sperm from the next generation of mice. The administration of 200 mg/kg/day VPA to mice for 4 weeks caused transient histone hyperacetylation in the testes and DNA methylation changes in the sperm, including promoter CpGs of genes related to brain function. Oocytes fertilized with VPA-treated mouse sperm showed methylation fluctuations at the morula stage. Pups that were fathered by these mice also showed behavioral changes in the light/dark transition test after maturation. Brain RNA-seq of these mice showed that the expression of genes related to neural functions were altered. Comparison of the sperm DNA methylation status of the next generation of mice with that of the parental generation revealed the disappearance of methylation changes observed in the sperm of the parental generation. These findings suggest that VPA-induced histone hyperacetylation may have brain function-related effects on the next generation through changes in sperm DNA methylation.

Epigenomic reprogramming in iAs-mediated carcinogenesis

Arsenic is a naturally occurring metal carcinogen found in the Earth's crust. Millions of people worldwide are chronically exposed to arsenic through drinking water and food. Exposure to inorganic arsenic has been implicated in many diseases ranging from acute toxicities to malignant transformations. Despite the well-known deleterious health effects of arsenic exposure, the molecular mechanisms in arsenic-mediated carcinogenesis are not fully understood. Since arsenic is non-mutagenic, the mechanism by which arsenic causes carcinogenesis is via alterations in epigenetic-regulated gene expression. There are two possible ways by which arsenic may modify the epigenome-indirectly through an arsenic-induced generation of reactive oxygen species which then impacts chromatin remodelers, or directly through interaction and modulation of chromatin remodelers. Whether directly or indirectly, arsenic modulates epigenetic gene regulation and our understanding of the direct effect of this modulation on chromatin structure is limited. In this chapter we will discuss the various ways by which inorganic arsenic affects the epigenome with consequences in health and disease.

Embryonic Arsenic Exposure Triggers Long-Term Behavioral Impairment with Metabolite Alterations in Zebrafish

Arsenic trioxide (As₂O₃) is a ubiquitous heavy metal in the environment. Exposure to this toxin at low concentrations is unremarkable in developing organisms. Nevertheless, understanding the underlying mechanism of its long-term adverse effects remains a challenge. In this study, embryos were initially exposed to As₂O₃ from gastrulation to hatching under semi-static conditions. Results showed dose-dependent increased mortality, with exposure to 30-40 µM As₂O₃ significantly reducing tail-coiling and heart rate at early larval stages. Surviving larvae after 30 µM As₂O₃ exposure showed deficits in motor behavior without impairment of anxiety-like responses at 6 dpf and a slight impairment in color preference behavior at 11 dpf, which was later evident in adulthood. As₂O₃ also altered locomotor function, with a loss of directional and color preference in adult zebrafish, which correlated with changes in transcriptional regulation of adsl, shank3a, and tsc1b genes. During these processes, As₂O₃ mainly induced metabolic changes in lipids, particularly arachidonic acid, docosahexaenoic acid, prostaglandin, and sphinganine-1-phosphate in the post-hatching period of zebrafish. Overall, this study provides new insight into the potential mechanism of arsenic toxicity leading to long-term learning impairment in zebrafish and may benefit future risk assessments of other environmental toxins of concern.

Mechanistic understanding of the toxic effects of arsenic and warfare arsenicals on human health and environment

Worldwide, more than 200 million people are estimated to be exposed to unsafe levels of arsenic. Chronic exposure to unsafe levels of groundwater arsenic is responsible for multiple human disorders, including dermal, cardiovascular, neurological, pulmonary, renal, and metabolic conditions. Consumption of rice and seafood (where high levels of arsenic are accumulated) is also responsible for human exposure to arsenic. The toxicity of arsenic compounds varies greatly and may depend on their chemical form, solubility, and concentration. Surprisingly, synthetic organoarsenicals are extremely toxic molecules which created interest in their development as chemical warfare agents (CWAs) during World War I (WWI). Among these CWAs, adamsite, Clark I, Clark II, and lewisite are of critical importance, as stockpiles of these agents still exist worldwide. In addition, unused WWII weaponized arsenicals discarded in water bodies or buried in many parts of the world continue to pose a serious threat to the environment and human health. Metabolic inhibition, oxidative stress, genotoxicity, and epigenetic alterations including micro-RNA-dependent regulation are some of the underlying mechanisms of arsenic toxicity. Mechanistic understanding of the toxicity of organoarsenicals is also critical for the development of effective therapeutic interventions. This review provides comprehensive details and a critical assessment of recently published data on various chemical forms of arsenic, their exposure, and implications on human and environmental health.

Health Effects Associated With Pre- and Perinatal Exposure to Arsenic

Inorganic arsenic is a well-established human carcinogen, able to induce genetic and epigenetic alterations. More than 200 million people worldwide are exposed to arsenic concentrations in drinking water exceeding the recommended WHO threshold (10μg/l). Additionally, chronic exposure to levels below this threshold is known to result in long-term health effects in humans. The arsenic-related health effects in humans are associated with its biotransformation process, whereby the resulting metabolites can induce molecular damage that accumulates over time. The effects derived from these alterations include genomic instability associated with oxidative damage, alteration of gene expression (including coding and non-coding RNAs), global and localized epigenetic reprogramming, and histone posttranslational modifications. These alterations directly affect molecular pathways involved in the onset and progression of many conditions that can arise even decades after the exposure occurs. Importantly, arsenic metabolites generated during its biotransformation can also pass through the placental barrier, resulting in fetal exposure to this carcinogen at similar levels to those of the mother. As such, more immediate effects of the arsenic-induced molecular damage can be observed as detrimental effects on fetal development, pregnancy, and birth outcomes. In this review, we focus on the genetic and epigenetic damage associated with exposure to low levels of arsenic, particularly those affecting early developmental stages. We also present how these alterations occurring during early life can impact the development of certain diseases in adult life.

Epigenetic transgenerational inheritance, gametogenesis and germline development†

One of the most important developing cell types in any biological system is the gamete (sperm and egg). The transmission of phenotypes and optimally adapted physiology to subsequent generations is in large part controlled by gametogenesis. In contrast to genetics, the environment actively regulates epigenetics to impact the physiology and phenotype of cellular and biological systems. The integration of epigenetics and genetics is critical for all developmental biology systems at the cellular and organism level. The current review is focused on the role of epigenetics during gametogenesis for both the spermatogenesis system in the male and oogenesis system in the female. The developmental stages from the initial primordial germ cell through gametogenesis to the mature sperm and egg are presented. How environmental factors can influence the epigenetics of gametogenesis to impact the epigenetic transgenerational inheritance of phenotypic and physiological change in subsequent generations is reviewed.