Multiple Cu-ATPase genes are differentially expressed and transcriptionally regulated by Cu exposure in sea bream, Sparus aurata

Abstract

Copper (Cu) is an essential metal, although in excess is highly toxic due to its redox properties and, therefore intracellular Cu homeostasis is a highly regulated process. Cu-ATPases are pivotal regulatory, proteins of intracellular and bodily Cu homeostasis. Two Cu-ATPases, ATP7A and ATP7B with distinct, functions are found in mammals and herein we report the structure and expression under Cu stress of, homologues of ATP7A and ATP7B in gilthead sea bream (Sparus aurata), the first such report for any, fish. The deduced protein sequences of S. aurata ATP7A (saATP7A) and ATP7B (saATP7B), displayed 63% and 75% identity respectively to their human homologues. All characteristic structural, features of Cu-ATPases were conserved between fish and mammals, although the number of Cu-binding, domains was less in fish ATP7B than in mammalian ATP7B. The tissue expression of sea bream, Cu-ATPases was similar to that observed in mammals, saATP7A being ubiquitously expressed, although low in liver, whilst saATP7B was mainly expressed in the intestine and liver. By analysis of the sequenced genomes of other species we have confirmed the presence of ATP7A and ATP7B genes in fish and propose that the presence of two Cu-ATPase genes in vertebrates represents a retention and neo-functionalization of a duplicated ancestral gene coincident with the development of a closed circulatory system and discrete hepato-biliary system. Expression of Cu-ATPase mRNA was changed after exposure to excess Cu in a manner dependent on exposure route and tissue type. Excess dietary Cu (130mgkg(-1) Cu dry diet) reduced saATP7A mRNA levels in intestine, gill, kidney and liver, and increased hepatic saATP7B mRNA consistent with increased biliary excretion. Whilst after waterborne Cu exposure (0.3mgL(-1) Cu), expression of ATP7A mRNA was increased in intestine and liver and toxic responses were observed in gill and liver. Our results indicate that Cu-ATPases in both fish and mammals have similar functions in maintenance of Cu homeostasis and are consistent with previous physiological evidence from various fish species for the involvement of multiple Cu-ATPases in Cu transport. Furthermore, our evidence suggests that fish can detoxify excess dietary Cu relatively efficiently but are unable to cope with excess dissolved Cu in the water, demonstrating that the exposure route is critical to toxicity.