Cadmium-induced toxicity in Meretrix meretrix ovary is characterized by oxidative damage with changes in cell morphology and apoptosis-related factors

Abstract

Cadmium (Cd) is one of the most common pollutants in the environment. It can cause irreversible tissue damage and apoptosis in invertebrates. This study investigated the relationship between Cd exposure and oxidative damage and apoptosis in the ovarian cell of the clam Meretrix meretrix. The clams were exposed to different concentrations of Cd2+ (0, 1.5, 3, 6 and 12 mg L−1) for 5 days, and the accumulated level of Cd2+ in the ovarian tissue, and the degree of oxidative damage, changes in morphology and the response of apoptosis-related factors in the ovarian cell were determined. The bioaccumulation of Cd2+ and the levels of reactive oxygen species (ROS), malondialdehyde (MDA), protein carbonylation (PCO), and DNA-protein crosslinking (DPC) in the ovary were found to increase significantly when the clams were exposed to increasing concentrations of Cd2+. The structure of the ovarian tissue was severely damaged, and the ovarian cells displayed an irregular arrangement. The results of AO/EB staining and flow cytometry showed that the apoptotic rate of the ovarian cells increased with increasing Cd2+ concentrations. The activities of caspase-3, -8, -9, and the mRNA levels of p53, Bax and Caspase-3 in the ovary were also significantly increased. Furthermore, the level of p53 mRNA was positively correlated with the levels of MDA, PCO, DPC and ROS, but negatively correlated with the levels of total antioxidant capacity (T-AOC) and Bcl-2 mRNA. Taken together, these results indicated that Cd2+ exposure would result in oxidative damage and apoptosis for the ovarian cells, suggesting that Cd2+ toxicity could negatively affect the reproductive capacity of M. meretrix, thus threatening the reproductive development of the shellfish.

Transcriptome Analysis and Identification of Cadmium-Induced Oxidative Stress Response Genes in Different Meretrix meretrix Developmental Stages

“…In some industrialized areas in China, the levels of Cd exceed more than 20-fold the range of the sea water according to the official standard (10 to 500 ng L −1 ) [32,38]. Several studies have shown that Cd generates some adverse effects on M. meretrix, and the acute toxicity varies in different tissues [8][9][10]17,29]. Cd toxicity has caused huge economic losses to molluscan cultivation [1,2].…”

“…Both MT and Nrf2 have previously been shown to be important for cytoprotection because of their roles in regulating the expression of antioxidant and detoxification genes in adult M. meretrix [8,17] and other aquatic animals [16,19]. The role of MT in alleviating oxidative damage induced by Cd has been demonstrated in M. meretrix and other bivalves via the simultaneous upregulation of MT expression and increased levels of oxidative damage, exemplified by increased levels of ROS and protein and lipid oxidation induced by Cd [8,10,25]. Furthermore, earlier studies that showed MT can function as a free radical scavenger in vitro provide further support for its role in alleviating oxidative damage [56].…”

“…Once inside the animals, Cd can induce significant oxidative stress by triggering the production of a large amount of reactive oxygen species (ROS), and these ROS can react with different biomolecules, such as DNA, proteins, and lipids, eventually leading to oxidative stress and cell death [7][8][9]. In order to cope with oxidative stress, aquatic organisms have independently evolved an antioxidant defense system capable of removing ROS and repairing damaged DNA [10,11], as well as sequestering metal ions by metal-binding proteins such as metallothionein [8].…”

“…Many studies on the responsive mechanisms of metal stress in mollusks have mainly focused on individual genes. Examples of such genes that have been commonly studied in mollusks are Hps70, Hps90, MT, Nrf2, p38, p53, Bax, Blc2, and Caspase 3 [8,10,[12][13][14]. Among these genes, the Nrf2 (nuclear factor E2-related factor 2) and MT (metallothionein) genes play an important role in oxidative stress induced by metal exposure [15][16][17][18].…”

“…This clam can accumulate heavy metals and other pollutants at high levels, and it has been gradually used as an indicator and model bivalve to study heavy metal pollution in a marine environment [1,33]. Among the toxic effects exerted by heavy metals on M. meretrix are growth inhibition [22], reproductive impairment [8,10], immunotoxicity, and oxidative damage [9,17]. However, the oxidative stress mechanisms associated with such toxic effects and the changes in gene expression patterns under metal stress in M. meretrix larvae at different developmental stages remain unexplored.…”