Abstract
Marine molluscs, including oysters, can concentrate high levels of cadmium (Cd) in their soft tissues, but the molecular mechanisms of Cd toxicity remain speculative. In this study, Pacific oysters (Crassostrea gigas) were exposed to Cd for 9 days and their gills were subjected to proteomic analysis, which were further confirmed with transcriptomic analysis. A total of 4,964 proteins was quantified and 515 differentially expressed proteins were identified in response to Cd exposure. Gene Ontology enrichment analysis revealed that excess Cd affected the DNA and protein metabolism. Specifically, Cd toxicity resulted in the inhibition of DNA glycosylase and gap-filling and ligation enzymes expressions in base excision repair pathway, which may have decreased DNA repair capacity. At the protein level, Cd induced the heat shock protein response, initiation of protein refolding as well as degradation by ubiquitin proteasome pathway, among other effects. Excess Cd also induced antioxidant responses, particularly glutathione metabolism, which play important roles in Cd chelation and anti-oxidation. This study provided the first molecular mechanisms of Cd toxicity on DNA and protein metabolism at protein levels, and identified molecular biomarkers for Cd toxicity in oysters.
Highlights
Cadmium (Cd) ranks eighth in the priority list of top 20 hazardous substances[1], and industrialization has greatly increased its input into rivers, estuaries and coastal waters[2]
These transcriptional level studies only partially contribute to our understanding of stress adaptation, because not all transcripts can be translated, and messenger RNA abundances may not correspond with the protein expression levels due to translation modifications
The gill concentration of Cd in the oysters measured in this study was about 3.7-fold higher than those measured in the field, and was considered to be environmentally relevant
Summary
Cadmium (Cd) ranks eighth in the priority list of top 20 hazardous substances[1], and industrialization has greatly increased its input into rivers, estuaries and coastal waters[2]. The isobaric tags for relative and absolute quantitation (iTRAQ) proteomic approach quantifies the proteins based on the HPLC-MS method. In this technique, digested peptides are labelled with amine-specific isobaric tags. Relative protein quantification is achieved by comparing the peak areas of the reporter ions[17] This method has previously been used to determine the relative protein expression levels in oysters[18]. Identifying peptides by interpreting MS spectra is a challenging task because there is little information available regarding non-model organisms whose genomes have not been fully sequenced This results in a relatively small number of proteins being identified, and it is difficult to determine the entire protein expression profile of the organisms. The majority of the DEPs were not identified because of a lack of oyster genomic information
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