Strain: AB Gender: Female Age: 6-8 months Tissue: Whole Ovary
Zebrafish International Resource Center, Eugene, OR
The dietary exposure regimen was designed to expose adult female zebrafish to non-lethal concentrations of TCDD that would yield TCDD egg concentrations ranging from 0 – 13.5 ng TCDD/g egg, which are within the range expected to be toxic to embryos based on the LD50 of 2.5 ng TCDD/g egg (Elonen et al., 1998; Henry et al., 1997). The nominal TCDD concentrations were chosen based on the desired target TCDD concentrations in the eggs following a protocol described by Tietge et al (1998). Tritium-labeled TCDD was synthesized and purified to >99% by the manufacturer (Eagle-Picher, Lenexa, KS, specific activity 47 Ci/mmol). Food yielding a final concentration of 272 ng 3H-TCDD/g food was prepared following a protocol described by Fernandez et al. (1998), with modifications. In brief, the 3H-TCDD stock solution was diluted in acetone, added to trout chow ( Zeigler, Gardner, PA), swirled to ensure homogeneous distribution, and the acetone evaporated from the food in a fume hood overnight. This food was then mixed with uncontaminated trout chow to achieve appropriate concentrations, which were confirmed by liquid scintillation counting to be 0 (acetone only), 10, 40, 100, and 270 ng 3H-TCDD/g food.
Male and female zebrafish were housed separately, and acclimated prior to the initiation of experiments for several weeks at the UWM Marine and Freshwater Biomedical Sciences Center (Milwaukee, WI). Fish were maintained at 26-28ºC on a 14-hour light and 10-hour dark cycle in a flow-through buffered, de-chlorinated water system of the UWM Great Lakes Wisconsin Aquaculture Technology and Environmental Research (WATER) Institute. Fish were spawned once weekly during the experiment. All experimental procedures were approved by the University of Wisconsin-Milwaukee Animal Care and Use Committee.
Following 15 days of dietary exposure, five females from each treatment group were anesthetized in ice-cold water, and fish were killed by cervical spinal cord transection. Using scissors and forceps cleaned with 0.5M NaOH to reduce RNase contamination, ovaries were removed and immediately frozen in liquid nitrogen and stored at -80C. Total RNA was isolated from individual ovaries homogenized in Trizol reagent (Invitrogen) and further purified using an RNeasy MinElute cleanup kit (Qiagen) as per manufacturers’ protocols. Samples were separated by electrophoresis and stained with ethidium bromide to confirm the integrity of the 18s and 28s ribosomal RNAs, and were quantified by UV spectrophotometry at 260nm using a Nanodrop ND-1000 Spectorphotometer (Nanodrop Technologies, Wilmington, DE).
Biotin with streptavidin phycoerythrin
First- and second-strand cDNA was synthesized from 15 µg pooled total RNA (3 µg from each of 5 females), and cRNA was synthesized, labeled and fragmented in accordance with standard Affymetrix protocols (Affymetrix, Santa Clara, CA). Following hybridization biotin-labeled cRNAs were stained with phycoerithrin conjugated to streptavidin (Molecular Probes, Eugene, OR).
Biotin-labeled cRNAs were hybridized to the zebrafish genome array, washed, and stained with phycoerithrin conjugated to streptavidin (Molecular Probes, Eugene, OR) in accordance with standard Affymetrix protocols (Affymetrix, Santa Clara, CA).
Scanned at 10µM using an Affymetrix scanner. Images were analyzed using Microarray Suite 5.0 (MAS5.0).
This study describes the molecular targets of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) within the zebrafish ovary and insights into TCDD’s ovarian toxicity. More details on the reproductive and physiological endpoints can be found in Toxicological Sciences 87(2) 497-507 (2005) and Toxicological Sciences 90(2) 490-499 (2006).