Lead-induced heat shock protein (HSP70) and metallothionein (MT) gene expression in the embryos of African catfish Clarias gariepinus (Burchell, 1822)

The enhanced expression of heat shock proteins (HSPs) and metallothionein (MT) can be detected in response to many environmental stressors, including exposure to heavy metals. We identified partial mRNA sequences for beta2-microglobulin (ß2m), heat shock cognate 70 kDa protein (HSP70), and MT in embryos of the African catfish Clarias gariepinus (Burchell, 1822). Levels of these transcripts were assessed using reverse-transcription quantitative polymerase chain reaction (RT-QPCR) after embryos of various stages were exposed to different concentrations of lead nitrate. ß2m gene expression exhibited a continuous increase over time. Neither HSP70 nor MT were upregulated, supporting the well-known sensitivity of these organisms to environmental pollutants. Interestingly, the partial MT and HSP70 mRNA sequences revealed an intimate phylogenetic relationship with these proteins in siluriforms. These partial sequences were more closely related to salmoniform proteins than to cypriniform proteins. The expression levels of HSP70 and MT were lower in embryonic tissues exposed to high concentrations of lead (500 µg/L), which impaired the fish's coping strategy via toxication at the cellular level. In conclusion, early developmental stages appear to be particularly vulnerable to lead and fail to activate the typical coping mechanisms. Therefore, these early life stages may not be suitable to characterize concentration-dependent toxicity. Keywords: Expression of stress proteins, RT-QPCR, Fish embryos, Heavy metal pollutio