Transcriptomic profile of oyster Crassostrea gigas hemocyte after short-term cadmium exposure and bacteria stimulation

Oyster Crassostrea gigas, is considered as a useful environmental indicator since it is widely distributed along the intertidal zone whereby it tends to accumulate cadmium and is always exposed to various pathogen agents. However, its molecular responses to both cadmium and pathogen stimulation remain unclear. In the present study, transcriptome data of hemocytes from oysters were analyzed to reveal specific molecular responses of oyster to cadmium or cadmium/bacteria stimulation. A total of 21591, 22872 and 20107 genes were detected in the BLANK, Cd24h and Cd/Bac24h group, respectively. Among them, there were 685 differentially expressed genes collected in the comparison of Cd24h versus BLANK. GO analysis of these genes found that sixteen terms into the Molecular Function category displayed transporter activities, and were all over-enrichment by cadmium exposure, whereas twelve terms into Biological Process category involved mainly in metabolic process of the various cellular components and two terms into Cellular Component category were all under-enrichment. The 330 immune responsive genes were shared by two gene lists of CdBac24h versus BLANK and CdBac24h versus Cd24h, and seven out of thirty terms in GO analysis were related to the immune process. Further annotation of these genes from the KEGG database revealed fourteen pathways, including two nervous system related pathways, arachidonic acid pathway, four immune pathways, MAPK cascade and other four cell signaling pathways, and two energy related pathways. Twenty-two differentially expressed genes were identified to responsive to both cadmium exposure and bacteria stimulation, but in different expression patterns, suggesting that bilateral responsive genes, such as alkaline phosphatase and sodium and chloride-dependent glycine transporter gene, could be candidate biomarkers for early warning of cadmium pollution. The present results collectively indicated that a profound neuro-endocrine-immune regulatory network was activated in response to cadmium and bacteria stimulation in oyster C. gigas, and the expression pattern of some cadmium responsive genes may be either reversed or strengthened by bacteria stimulation. The results provide knowledge on the transcriptomic response profile of oyster after short-term cadmium exposure and bacteria stimulation, which would be useful for future studies on stress response mechanism of mollusc, and some cadmium-bacteria responsive genes may be explored as potential biomarkers for monitoring marine pollution.