Comparative effects of direct cadmium contamination on gene expression in gills, liver, skeletal muscles and brain of the zebrafish (Danio rerio) Patrice Gonzalez, Magalie Baudrimont, Alain Boudou & Jean-Paul Bourdineaud* Laboratoire d’Ecophysiologie et Ecotoxicologie des Syste `mes Aquatiques (LEESA), Universite ´ Bordeaux, 1/UMR CNRS 5805, Place du Dr Peyneau, Arcachon, 33120, France; *Author for correspondence (Tel: +33(0)556-22-39-26; Fax: +33(0)556-54-93-83; E-mail: jp.bourdineaud@epoc.u-bordeaux1.fr) Received 22 December 2004; accepted 18 April 2005 Key words: bioaccumulation, cadmium, D. rerio, gene expression, real-time PCR Abstract The effects of cadmium (Cd) on gene expression were examined in four organs (gills, liver, skeletal muscles and brain) of the zebrafish. Adult male fish were subjected to three different water contamination pressures over periods of 7 and 21 days: control medium (C 0 : no Cd added) and two contaminated media (C 1 : 1.9 ± 0.6 lg Cd l )1 , and C 2 : 9.6 ± 2.9 lg Cd l )1 ). Fourteen genes involved in antioxidant defences, metal chelation, active efflux of organic compounds, mitochondrial metabolism, DNA repair and apoptosis were selected and their expression levels investigated by quantitative real-time PCR. Cadmium concentrations were determined in the four organs and metallothionein (MT) protein levels investigated in brain, liver and gills. Although skeletal muscle was a poor Cd-accumulating tissue, many genes were up-regulated at day 7: mt1, cyt, bax, gadd and rad51 genes. Three additional genes, c-jun, pyc and tap, were up-regulated in muscles at day 21 whereas bax, gadd and rad51 had returned to basal levels. Surprisingly, mt1 and c-jun were the only genes displaying a differential induction after 21 days in liver, although this organ accu- mulated the highest cadmium concentration. In brain, only mt1, mt2 and c-jun genes were up-regulated after 21 days. In gills, the highest response was observed after 7 days, featuring the differential expression of oxidative stress-response hsp70 and mitochondrial sod genes, along with genes involved in mitochondrial metabolism and metal detoxification. Then, after 21 days, the expression of almost every genes returned to basal levels while both mt1 and mt2 genes were up-regulated. Introduction Cadmium (Cd) is a non-essential metal whose dispersion in the environment has increased over the past decades due to its widespread industrial use as a colour pigment in paints, in electroplating and galvanising, in batteries, etc. It is also a by- product of zinc and lead mining. Numerous field studies have shown varying degrees of contami- nation of aquatic systems, via direct or indirect Cd inputs (Hutton 1983; Guine´e et al. 1999). In spite of its toxicity, little is known about the effects of Cd on genetic and biochemical adaptive responses of aquatic species under chronic and long-term exposure. Toxicological studies at cellular level have shown that Cd inhibits the mitochondrial electron transfer chain and induces reactive oxy- gen species (ROS) production (Wang et al. 2004). Cd-promoted oxidative stress leads to DNA damage and apoptotic cell death (Bagchi et al. 2000; Chan and Cheng 2003). Different mecha- nisms could be involved in Cd detoxification. Among these, one relies on low molecular weight and cystein-rich proteins called metallothioneins (MT) which are able to bind seven metal ions (zinc or cadmium) in mammals (Chan et al. 2002). The promoters of MT genes contain several metal regulatory elements (MRE) which are bound by a BioMetals (2006) 19:225–235 Ó Springer 2006 DOI 10.1007/s10534-005-5670-x