Gene Responses in the Central Nervous System of Zebra sh Embryos Exposed to the Neurotoxicant Methyl Mercury Nga Yu Ho, Lixin Yang, Jessica Legradi, Olivier Armant, Masanari Takamiya, Sepand Rastegar, and Uwe Strahle* Institute of Toxicology and Genetics, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany * S Supporting Information ABSTRACT: Methyl mercury (MeHg) is a neurotoxicant with adverse e ects on the development of the nervous system from sh to man. Despite a detailed understanding of the molecular mechanisms by which MeHg a ects cellular homeostasis, it is still not clear how MeHg causes developmental neurotoxicity. We performed here a genome-wide transcriptional analysis of MeHg- exposed zebra sh embryos and combined this with a whole-mount in situ expression analysis of 88 MeHg-a ected genes. The majority of the analyzed genes showed tissue- and region-restricted responses in various organs and tissues. The genes were linked to gene ontology terms like oxidative stress, transport and cell protection. Areas even within the central nervous system (CNS) are a ected di erently resulting in distinct cellular stress responses. Our study revealed an unexpected heterogeneity in gene responses to MeHg exposure in di erent tissues and neuronal subregions, even though the known molecular action of MeHg would predict a similar burden of exposed cells. The overall structure of the developing brain of MeHg-exposed embryos appeared normal, suggesting that the mechanism leading to di erentiation of the CNS is not overtly a ected by exposure to MeHg. We propose that MeHg disturbs the function of the CNS by disturbing the cellular homeostasis. As these cellular stress responses comprise genes that are also involved in normal neuronal activity and learning, MeHg may a ect the developing CNS in a subtle manner that manifests itself in behavioral de cits. INTRODUCTION Mercury is a well-known teratogenic and neurotoxin in humans. 1-3 Both natural geological sources, industrial activities and enrichment of mercury and its organic derivatives like methylmercury (MeHg) through the food chain can contribute to human exposure. 4-6 While Hg levels in surface waters, groundwater, and oceans are usually low in the range of 20 ng/ L, 7 areas heavily polluted by anthropogenic mercury sources can have MeHg levels of more than 2000 ng/L. 8 The concentration of MeHg can increase by accumulation in the food chain million-fold. 9-11 About 95% of MeHg in the diet is absorbed by the gastrointestinal tract 12 and passes to all main tissues even penetrating the blood-brain barrier. 13,14 MeHg poisoning can result in sensory disturbances, de cits in motor coordination, and somatosensory and psychiatric disorders. The developing human brain is more susceptible to MeHg than the adult brain. Children exposed during pregnancy to subacute doses of MeHg showed impairments in motor coordination, speech and involuntary movement due to damage of the brain, particularly the cerebral and cerebellar cortices. 1-3 After the Minamata poisoning, median hair content of mercury was 30 ppm and the maximum was 920 ppm. 1 The EPA regards 5.8 μg Hg/L blood as safe (http://www.epa.gov/hg/exposure.htm). A study of children on the Faroe island suggest that MeHg concentrations below 10 ppm in the hair of the mother correlate with de cient performance in attention, memory and language of her o -spring. 15 MeHg exposure has been shown to result in various cellular changes such as lipid peroxidation, DNA damage, membrane structure alteration, mitochondrial dysfunction, cell cycle alteration, apoptosis, and necrosis. 16 The interactions with sulfhydryl groups, the induction of oxidative stress and the disruption of calcium ion homeostasis have been reported to be the three major and critical mechanisms. 17-24 It is, however, not clear how MeHg acts as a neurotoxicant. Hence vertebrate models are needed to investigate the neurotoxic e ects in detail. The zebra sh embryo is highly sensitive to MeHg. Exposure of zebra sh embryos to 20 and 30 μg/L MeHg led to impaired development of the caudal n and caused an abnormal tail exure. 25-27 Doses between 10 and 20 μg/L MeHg induced faint heartbeats, severe edema, upward exures of the body axis, reduced swimming activity, impaired prey capture performance, a delayed mortality syndrome, behavioral de cits 28 and impaired tail formation. 27 Global transcriptome study on zebra sh embryos showed that acute exposure to 60 μg/L Received: May 30, 2012 Revised: February 28, 2013 Accepted: March 4, 2013 Article pubs.acs.org/est © XXXX American Chemical Society A dx.doi.org/10.1021/es3050967 | Environ. Sci. Technol. XXXX, XXX, XXX-XXX research 1..10 - es3050967 http://pubs.acs.org/doi/pdfplus/10.1021/es3050967 1 of 10 3/21/13 4:27 PM