Transcriptional Responses Of Rho-and Alpha-GST Genes In The Liver Of Three Freshwater Fishes Intra-peritoneally Injected With Microcystin-LR

Mass populations of cyanobacteria (blue-green algae) often develop in water bodies due to enrichment with plant nutrients. Microcystin detoxification-related liver genes, such as phaseⅡdetoxification enzyme soluble glutathione S-transferase (sGST) has been supposed to play an important role in microcystin detoxification in fish body. In fish liver, the rho-and alpha-sGST are the main type. In this report, the full-length cDNAs encoding rho-class sGST were cloned and sequenced from the hepatopancreas of silver carp (Hypophthalmichthys molitrix), Nile tilapia (Oreochromis niIotica) and grass carp (Ctenopharyngodon idellus) with 1078 bp, 1104 bp, 904bp in length respectively. In addition, liver rho-class and alpha-class sGST gene expression levels among these freshwater fishes were also studied, which would shed a new light on the detoxification mechanism of microcystins in freshwater fishes. To study the transcriptional responses of liver rho-class and alpha-sGST gene by microcystin treatment, silver carp, Nile tilapia and grass carp were respectively exposed to a single 50μg kg-l body weight (bwt) dose of pure MC-LR, and were then sacrificed at 8h and 24h post-exposure. Using beta-actin as external control, we compared the inducible expression level of the liver sGST gene among three freshwater fishes with different tolerance to microcystins. As a result, in silver carp, a significant increase in sGSTR mRNA expression was observed at 8h (P＜0.05), while there was a significant decrease of sGSTR mRNA expression at 24h (P＜0.05). In sGSTA mRNA expression, a significant increase was observed both at 8h and 24h. In Nile tilapia, the mRNA expression level of sGSTR tended to increase exposed to MC-LR at 8h (P＜0.05), while significantly reduced (P＜0.05) at 24h. There was a significant decrease of sGSTA mRNA expression both at 8h and 24h. In grass carp, there was no significant difference of GSTR mRNA expression both at 8h and at 24h (P＞0.05), whereas the mRNA expression level of sGSTA tended to decrease after MC-LR exposure both at 8h and 24h (P＜0.05). These results implicated that high-resistant fish (phytoplanktivorous silver carp and Nile tilapia) had high sGSTA inducible expression level, while low constitutive expression. The high-sensitive fish (herbivorous grass carp) had low sGSTA inducible expression level and sGSTR had no significant difference after injected with MC-LR. From the results, we suggest that the difference of tolerence to MC-LR in different freshwater fishes is relavent to the induced expression of phaseⅡdetoxified enzyme sGST. Taken together with the reciprocal relationship of inducible liver sGST expression level in some tested fish species to microcystin exposure, the molecular mechanism for different microcystin detoxification abilities of the freshwater fishes was discussed.