The Molecular Mechanisms Underlying Differential Cold-induced Apoptosis In Gills Of Tilapia And Zebrafish

Cold stress occurs when a fish is exposed to a lower temperature which it is not acclimated to. Cold stress adversely affects all-tissue performances and may lead to death of the organism if it goes beyond the range of tolerance. There are profound differences in the capacity of cold tolerance among the diverse fish species. So far, little is known on the role of individual tissues play in setting the cold tolerance limit in fish and the molecular mechanisms underlie.Tilapia(Orechromis niloticus) and zebrafish(Danio rerio) are both tropical fishes reared in a same temperature in our laboratory, but they are different in the limit of cold temperature tolerance. Zebrafish can survive at temperatures below 9°C, while tilapia cannot. The grass carp(Ctenopharyngodon idella)on the other hand, is much more eurythermal than zebrafish and tilapia, which tolerates temperatures in a range of 0°C-38°C.In this study, individuals of the three species were exposed to graded cold temperatures, from 28°C to 18°C and 8°C(the lower end of lethal temperature of tilapia). We analyzed tissue damage by quantifying the population of apoptotic cells in the tissues by TUNEL staining. We found: compared to other tissues, apoptosis occurred mainly in the gills of tilapia and zebrafish when the fishes are exposed to 8°C. No other tissues exhibited apoptotic signals, suggesting that apoptosis in gill could be used as a mark for lethal damage induced by cold stresses in fish.In tilapia gill, genes that were involved in the Mdm2-p53 pathway were upregulated at both the mRNA and protein levels; we also observed that genes(dusp1, p38 and erk) involved in the MAPK pathway were up-regulated. We further investigated expression of the PI3K/AKT/GSK-3β pathway and find that in tilapia, the expression of p-PTEN was decreased at cold temperature, whereas the expression of AKT, p-AKT(Ser 473) and p-GSK-3β(Ser 9) was increased at 8°C. Differently, in the gills of the zebrafish and grass carp, the expression of AKT, p-AKT(Ser 473) was repressed at 8°C, whereas the expression of p-GSK-3β(Ser 9) was induced at 8°C. In addition, the induction of p-GSK-3β(Ser 9) at 8°C in tilapia was more significantly than that in other two fishes. These data indicated that the gills of the three fishes are different in the molecular cascade in responding to severe cold stress and the differential regulation may lead to differential apoptosis in the three species.In summary, we found the critical roles of the gill plays in cold stress response. The identification of differential regulatory pathways in the gills of fishes with different capacities of cold tolerance provided us a window to explore the mechanisms underline the regulation of lethal low temperature limits in fishes.