Integrating genetic and toxicogenomic information for determining underlying susceptibility to developmental disorders

Developmental exposure to methylmercury alters GAD67 immunoreactivity and morphology of endothelial cells and capillaries of midbrain and hindbrain regions of adult rat offspring

INTRODUCTION

Methylmercury (MeHg) is an environmental contaminant that is of particular concern in Northern Arctic Canadian populations. Specifically, organic mercury compounds such as MeHg are potent toxicants that affect multiple bodily systems including the nervous system. Developmental exposure to MeHg is a major concern, as the developing fetus and neonate are thought to be especially vulnerable to the toxic effects of MeHg. The objective of this study was to examine developmental exposure to low doses of MeHg and effects upon the adult central nervous system (CNS). The doses of MeHg chosen were scaled to be proportional to the concentrations of MeHg that have been reported in human maternal blood samples in Northern Arctic Canadian populations.

METHOD

Offspring were exposed to MeHg maternally where pregnant Sprague Dawley rats were fed cookies that contained MeHg or vehicle (vehicle corn oil; MeHg 0.02 mg/kg/body weight or 2.0 mg/kg/body weight) daily, throughout gestation (21 days) and lactation (21 days). Offspring were not exposed to MeHg after the lactation period and were euthanized on postnatal day 450. Brains were extracted, fixed, frozen, and sectioned for immunohistochemical analysis. A battery of markers of brain structure and function were selected including neuronal GABAergic enzymatic marker glutamic acid decarboxylase-67 (GAD67), apoptotic/necrotic marker cleaved caspase-3 (CC3), catecholamine marker tyrosine hydroxylase (TH), immune inflammatory marker microglia (Cd11b), endothelial cell marker rat endothelial cell antigen-1 (RECA-1), doublecortin (DCX), Bergmann glia (glial fibrillary acidic protein (GFAP)), and general nucleic acid and cellular stains Hoechst, and cresyl violet, respectively. Oxidative stress marker lipofuscin (autofluorescence) was also assessed. Both male and female offspring were included in analysis. Two-way analysis of variance (ANOVA) was utilized where sex and treatment were considered as between-subject factors (p* <0.05). ImageJ was used to assess immunohistochemical results.

RESULTS

In comparison with controls, adult rat offspring exposed to both doses of MeHg were observed to have (1) increased GAD67 in the cerebellum; (2) decreased lipofuscin in the locus coeruleus; and (3) decreased GAD67 in the anterior CA1 region. Furthermore, in the substantia nigra and periaqueductal gray, adult male offspring consistently had a larger endothelial cell and capillary perimeter in comparison to females. The maternal high dose of MeHg influenced RECA-1 immunoreactivity in both the substantia nigra and periaqueductal gray of adult rat offspring, where the latter neuronal region also showed statistically significant decreases in RECA-1 immunoreactivity at the maternal low dose exposure level. Lastly, males exposed to high doses of MeHg during development exhibited a statistically significant increase in the perimeter of endothelial cells and capillaries (RECA-1) in the cerebellum, in comparison to male controls.

CONCLUSION

Findings suggest that in utero and early postnatal exposure to MeHg at environmentally relevant doses leads to long-lasting and selective changes in the CNS. Exposure to MeHg at low doses may affect GABAergic homeostasis and vascular integrity of the CNS. Such changes may contribute to neurological disturbances in learning, cognition, and memory that have been reported in epidemiological studies.

Potential frameworks to support evaluation of mechanistic data for developmental neurotoxicity outcomes: A symposium report

A key challenge in systematically incorporating mechanistic data into human health assessments is that, compared to studies of apical health endpoints, these data are both more abundant (mechanistic studies routinely outnumber other studies by several orders of magnitude) and more heterogeneous (e.g. different species, test system, tissue, cell type, exposure paradigm, or specific assays performed). A structured decision-making process for organizing, integrating, and weighing mechanistic DNT data for use in human health risk assessments will improve the consistency and efficiency of such evaluations. At the Developmental Neurotoxicology Society (DNTS) 2016 annual meeting, a symposium was held to address the application of existing organizing principles and frameworks for evaluation of mechanistic data relevant to interpreting neurotoxicology data. Speakers identified considerations with potential to advance the use of mechanistic DNT data in risk assessment, including considering the context of each exposure, since epigenetics, tissue type, sex, stress, nutrition and other factors can modify toxicity responses in organisms. It was also suggested that, because behavior is a manifestation of complex nervous system function, the presence and absence of behavioral change itself could be used to organize the interpretation of multiple complex simultaneous mechanistic changes. Several challenges were identified with frameworks and their implementation, and ongoing research to develop these approaches represents an early step toward full evaluation of mechanistic DNT data for assessments.

Exposure to metals and congenital anomalies: a biomonitoring study of pregnant Bedouin-Arab women

BACKGROUND

The Bedouin-Arab population in Israel comprises a low socio-economic society in transition. Smoking among males and consanguineous marriages are frequent. A previous study showed elevated rates of major malformations within groups from this population residing near an industrial park, where high ambient values of arsenic (As) and nickel (Ni) were detected, compared to groups living in remote localities.

OBJECTIVES

We estimated the extent of exposure to metals in pregnant Bedouin-Arab women in relation to congenital malformations.

METHODS

We collected maternal urine samples from 140 Bedouin women who gave birth in a local hospital. Patient medical history, type of marriage (consanguineous or non-consanguineous), and parental exposure history were collected by interview and medical records.

RESULTS

Aluminum (Al) was detected in 37 women (26.4%), cadmium (Cd) in 2 (1.4%), As in 10 (7.1%), and Ni in 1 woman (0.7%). The detected rate of Cd exposure was low, though more than 92% of the fathers reported smoking. Concentrations of Al were higher for women residing within 10 km of the local industrial park (Prevalence Ratio (PR)=1.12, p-value=0.012) or who reported using a wood burning stove (PR=1.37, p-value=0.011) and cooking over an open fire (PR=1.16, p-value=0.076). Exposure to Al was adversely associated with minor anomalies (OR=3.8, p-value=0.046) after adjusting for history of abortions (OR=6.1, p-value=0.007). Fetuses prenatally exposed to As were born prematurely (p-value=0.001) and at lower weights (pv=0.023).

CONCLUSIONS

The study population of pregnant women is exposed to high levels of metals mainly of household origin. Our findings may be generalized to similar populations in developing countries.

Toxicogenomic approaches in developmental toxicology testing

The emergence of toxicogenomic applications provides new tools to characterize, classify, and potentially predict teratogens. However, due to the vast number of experimental and statistical procedural steps, toxicogenomic studies are challenging. Here, we guide researchers through the basic framework of conducting toxicogenomic investigations in the field of developmental toxicology, providing examples of biological and technical factors that may influence response and interpretation. Furthermore, we review current, diverse applications of toxicogenomic-based approaches in teratology testing, including exposure-response characterization (dose and duration), chemical classification studies, and cross-model comparisons study designs. This review is intended to guide scientists through the challenging and complex structure of conducting toxicogenomic analyses, while considering the many applications of using toxicogenomics in study designs and the future of these types of "omics" approaches in developmental toxicology.

Genome-Wide Analyses of Metal Responsive Genes in Caenorhabditis elegans

Metals are major contaminants that influence human health. Many metals have physiologic roles, but excessive levels can be harmful. Advances in technology have made toxicogenomic analyses possible to characterize the effects of metal exposure on the entire genome. Much of what is known about cellular responses to metals has come from mammalian systems; however the use of non-mammalian species is gaining wider attention. Caenorhabditis elegans is a small round worm whose genome has been fully sequenced and its development from egg to adult is well characterized. It is an attractive model for high throughput screens due to its short lifespan, ease of genetic mutability, low cost, and high homology with humans. Research performed in C. elegans has led to insights in apoptosis, gene expression, and neurodegeneration, all of which can be altered by metal exposure. Additionally, by using worms one can potentially study mechanisms that underline differential responses to metals in nematodes and humans, allowing for identification of novel pathways and therapeutic targets. In this review, toxicogenomic studies performed in C. elegans exposed to various metals will be discussed, highlighting how this non-mammalian system can be utilized to study cellular processes and pathways induced by metals. Recent work focusing on neurodegeneration in Parkinson’s disease will be discussed as an example of the usefulness of genetic screens in C. elegans and the novel findings that can be produced.

A review of toxicogenomic approaches in developmental toxicology

Over the past decade, the use of gene expression profiling (i.e., toxicogenomics or transcriptomics) has been established as the vanguard "omics" technology to investigate exposure-induced molecular changes that underlie the development of disease. As this technology quickly advances, researchers are striving to keep pace in grasping the complexity of toxicogenomic response while at the same time determine its applicability for the field of developmental toxicology. Initial studies suggest toxicogenomics to be a promising tool for multiple types of study designs, including exposure-response investigations (dose and duration), chemical classification, and model comparisons. In this review, we examine the use of toxicogenomics in developmental toxicology, discussing biological and technical factors that influence response and interpretation. Additionally, we provide a framework to guide toxicogenomic investigations in the field of developmental toxicology.