Metallothionein and Heat-Shock Protein 70 Induction in Caged and Wild Fathead Minnows (Pimephales promelas) Exposed to the Ouachita River, Louisiana

Abstract

The purpose of this study was to explore the changes in mRNA expression levels for metallothionein subtype 2 (MT-2) and heat-shock protein 70 (HSP70) in fathead minnows in response to environmental exposure in a mercury (Hg)-contaminated freshwater ecosystem. It was hypothesized that expression levels of both genes may rise concurrent with the bioaccumulation of Hg and possibly other heavy metals during exposure to the Ouachita River. The experimental design incorporated three distinct populations of fathead minnows: (1) a negative control population of laboratory-raised fathead minnows unexposed to heavy metals or other contaminants, (2) laboratory-raised fatheads placed in cages and exposed to a contaminated ecosystem for 2 wk, and (3) wild-caught (native) fathead minnows captured at the same site where caged fatheads tested positive for Hg bioaccumulation. Study endpoints included growth rates and gross pathology at necropsy. Total Hg levels of the water at the exposure sites as well as in whole fish homogenates were determined by cold vapor atomic absorption spectroscopy (AAS). AAS was also used to assay levels of lead (Pb) and copper (Cu), though these were below detectable limits. Hepatic expression levels of MT and HSP70 mRNA were determined by quantitative real-time reverse-transcription polymerase chain reaction (qRT-PCR). As hypothesized, levels of both transcripts were significantly increased in the caged exposure group and native fish group compared to unexposed control fish. In addition, the native fish group had significantly higher levels of expression for both genes when compared to caged exposed fish.

An inexpensive, temporally integrated system for monitoring occurrence and biological effects of aquatic contaminants in the field

“…It is widely used because of a number of attributes, including broad natural distribution, ready availability of animals for testing from laboratory cultures, ability to tolerate a wide range of water types (and temperatures), comparatively small body size, and relevance to regulatory ecotoxicology . Caged fathead minnows have been used for a variety of purposes, including determining bioaccumulation of metals or organic chemicals ; assessment of the effects of ambient conditions on apical responses such as growth and reproduction ; and determination of histological, biochemical, or molecular alterations indicative of perturbation of different biological pathways by, primarily, chemical stressors . Because of varying objectives and endpoints, previous fathead minnow caging studies have used a wide range of physical test systems and experimental conditions such as, age or size of animals, exposure durations (from a few days to several weeks), and feeding regimes.…”

“…A variety of different fish species have been employed for in situ testing. Among them, the fathead minnow ( Pimephales promelas ) probably has been the most commonly used freshwater species for caged fish studies in North America . Widespread use of the fathead minnow for this type of work stems from a number of considerations, including its long history as a standard model for regulatory ecotoxicology .…”

“…Previous caging studies with the fathead minnow have used a wide variety of test systems, depending on variables such as study objectives, water body characteristics, available materials, etc. . For the GLRI work, we wanted to develop a relatively simple, standardized test system for in situ exposures suitable for the wide range of habitat or deployment situations that might be encountered in the Great Lakes.…”