Progress Reports: Boston University: Estrogen Receptor-Arylhydrocarbon Receptor Interactions in the CNS
Estrogen Receptor-Arylhydrocarbon Receptor Interactions in the CNS
Project Leader: Gloria V. Callard
Year: 2010 2009 2008 2007 2006 2005 2004 2003 2002 2001 2000
In addition to their toxic effects, generally from high dose exposures, many different environmental chemicals have the potential to disrupt critical hormone-regulated processes of development, reproduction, and normal body functions. Termed "hormonally active agents" (HAA), these chemicals are common contaminants at Superfund sites. By virtue of their ability to mimic or block hormone binding to specific receptors, HAA can have effects, even at low doses and transient exposures. During fetal development, effects of HAA may be greatly delayed in onset, or revealed only as an increased susceptibility to disorders associated with genetic factors, nutritional deficits, disease, aging, or subsequent chemical exposure. The effects of lifetime or multigenerational HAA exposures is completely unknown.
To address these issues, Dr. Gloria Callard's research focuses specifically on chemicals such as PCBs and dioxins that can interact to disrupt normal hormonal estrogen signaling during neurodevelopment, and affect neural genes and neural processes in the adult and aging brain. Zebrafish (Danio rerio) are used as a model for conducting controlled experiments in the laboratory, with the objective of establishing clear cause-and-effect relationships between specific chemicals/chemical mixtures and disrupted neural genes/cellular processes (mechanisms). A second, wild caught fish species, the killifish (Fundulus heteroclitus) is collected at a highly polluted Superfund site (New Bedford Harbor, MA) and compared with killifish from a clean site, in order to determine the significance of laboratory findings for animals living long term and over many generations in a highly polluted natural environment.
Callard's lab is using genetic markers to investigate effects of two main classes of HAA (estrogen- and dioxin-like chemicals) on estrogen-regulated neural processes. Results show that each gene/tissue-type has a different chemical sensitivity and responsiveness; effects are cumulative with exposure time and related to life stage (i.e., late > early embryos; embryos >> adults). Fish populations in New Bedford Harbor are exposed to high levels of "estrogenic" contaminants in their environment, possibly PCBs, but they display impaired responses to administered estrogen when compared to fish from a clean site. Whether this partial estrogen "resistance" is physiological (temporary) or genetic (heritable) is under investigation.
To identify effects of HAA on processes of neurodevelopment, neuroplasticity and repair, Callard's group is using known gene markers (e.g., Nurr1, Nur77, Nor1), as well as an unbiased genome-wide search (microarray analysis). To date, 91 estrogen-regulated and 316 dioxin-regulated genes have been identified in zebrafish embryos (of 17,000 total). Of these, 16 genes were co-regulated, and 18 were oppositely regulated, by estrogen and dioxin. A complementary approach (subtractive hybridization) is being used to identify genes involved in neuroplasticity and repair processes in adult fish. This panel of genes will provide valuable reagents for monitoring effects of estrogen and HAA on diverse processes in developing and adult nervous system.
Dr. Callard is also investigating how HAA can subsequently modify the structure and function of normally transcribed genes (splice variants). This is a novel and relatively unstudied, alternative mechanism of action of HAA. Results show that known and suspected HAA, and samples from contaminated sites, can stimulate conversion of a normal (functional) to an abnormal (non-functional) gene product. This mechanism could explain estrogen "resistance" after long-term exposure to an estrogenic environment (New Bedford Harbor).
The Callard group is also conducting a mechanistic study designed to characterize molecular interactions of estrogen and dioxin on the regulatory region of estrogen target genes. Complementary bioinformatic and laboratory approaches are being used to obtain the necessary reagents: e.g., cloned neural gene promoters and estrogen receptors.