Date of Award

2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Neuroscience

First Advisor

Rand, Matthew

Abstract

Methylmercury (MeHg) is a ubiquitous environmental toxin. Exposure to MeHg in humans occurs primarily through the consumption of contaminated seafood. MeHg has been shown to act most strongly during neural development. Epidemiological data on the effect MeHg exposure through seafood has on children and fetuses is conflicted, with large cohort studies showing both presence and absence of MeHg-induced deficits in achieving developmental milestones. Because of this uncertainty in the literature it is important that we come to understand the mechanisms of MeHg toxicity so that we might advise the public more accurately on the risks of MeHg exposure. Research into the mechanisms of MeHg toxicity has found a number of cellular and molecular effects including disruptions of microtubule formation, Ca2+ homeostasis, and glutamate signaling. However, none of these effects of MeHg fully explains its neurodevelopmental specificity. Previous work in Drosophila neural-derived cell lines has shown that MeHg causes upregulation of the canonical Notch response gene E(spl)m . The Notch pathway is crucial to neural development and perturbation of a Notch target may explain the developmental specificity of MeHg. In this dissertation I describe experiments I performed to test the hypothesis that the observed upregulation of E(spl)m plays an important role in MeHg toxicity in Drosophila. I first describe experimental evidence that E(spl)m is upregulated by MeHg treatment in vivo in Drosophila embryos in addition to cells, as has previously been shown. By contrasting the effects of the toxic inorganic mercurial HgCl2 with MeHg I show that the E(spl)m expression response to MeHg is not simply a stress response and is a likely specific activity of MeHg. I also show that the effect of MeHg on E(spl)m expression is not simply due to a developmental delay induced by the toxin. I also identify two neural phenotypes of MeHg toxicity in Drosophila embryos, in the outgrowth of the intersegmental and segmental motor nerves. Genetic manipulation causing overactivity of the Notch pathway in neurons can mimic these phenotypes. However, induced expression of E(spl)m in neurons does not cause a failure of motor nerve outgrowth. Upon further examination I demonstrate that endogenous expression of E(spl)m occurs in the muscle. Induced E(spl)m expression in the muscle causes a segmental nerve phenotype similar to MeHg treatment, indicating a role for E(spl)m in MeHg toxicity in this system. MeHg treatment and E(spl)m overexpression in the muscle causes a failure of normal muscle development. Yet, this gross developmental abnormality only partially explains the observed motor nerve phenotype. E(spl)m is unique among the E(spl) genes in its ability to cause these muscle and motor nerve phenotypes as shown by contrasting genetic manipulation of the closely related E(spl)m . Overall my findings support the hypothesis that MeHg toxicity in Drosophila is mediated in part by E(spl)m . They also suggest that E(spl)m plays an important role in the formation of the muscle during embryonic development, contributing to the literature describing disparate functions for E(spl) genes despite structural similarities. Finally, my findings suggest that MeHg may be able to impact neural development through toxicity in supporting tissues rather than neurons themselves. This final finding has implications for the study of MeHg toxicity in humans, and is supported by previous findings that describe a role of glia in modulating MeHg neurotoxicity.

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