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Abstract
Recent studies suggest that controlled apoptotic response provides an essential mechanism, enabling corals to respond to global warming and ocean acidification. However, the molecules involved and their functions are still unclear. To better characterize the apoptotic response in basal metazoans, we studied the expression profiles of selected genes that encode for putative pro- and anti-apoptotic mediators in the coral Stylophora pistillata under thermal stress and bleaching conditions. Upon thermal stress, as attested by the elevation of the heat-shock protein gene HSP70’s mRNA levels, the expression of all studied genes, including caspase, Bcl-2, Bax, APAF-1 and BI-1, peaked at 6–24 h of thermal stress (hts) and declined at 72 hts. Adversely, the expression levels of the survivin gene showed a shifted pattern, with elevation at 48–72 hts and a return to basal levels at 168 hts. Overall, we show the quantitative anti-apoptotic traits of the coral Bcl-2 protein, which resemble those of its mammalian counterpart. Altogether, our results highlight the similarities between apoptotic networks operating in simple metazoans and in higher animals and clearly demonstrate the activation of pro-cell survival regulators at early stages of the apoptotic response, contributing to the decline of apoptosis and the acclimation to chronic stress.
Highlights
The fine-tuned control of cell death is further regulated through the inhibitor of apoptosis protein (IAP) family, as well as through the anti-apoptotic Bax inhibitor-1 (BI-1) (Fig. 1A)
To better understand the mechanism that underlies apoptotic response in corals and enables them to withstand thermal stress and bleaching[4], the expression profiles of selected genes that encode for putative pro- and anti- apoptotic mediators were studied in the stony coral S. pistillata after it was subjected to extended thermal stress for a duration of 168 h (Fig. 1)
The expression profiles of the seven studied genes varied in the thermal-stressed S. pistillata significantly compared to the controls, indicating the possible role of these genes as mediators of apoptosis in corals (Fig.1B, D–I and Table 1)
Summary
The identification and expression of target genes that encode for apoptotic mediators in S. pistillata. Cascades of anti-oxidant/anti-apoptotic mediators such as HSP70, Bcl-2 and BI-1 are being up-regulated as “first aid” in order to block the progression of apoptosis in other cells This acute response is already clearly evident at 6 hts, but could begin earlier, even at temperatures below the bleaching threshold, as observed in A. aspera[13]. The second stage of the model (starting at 48–72 hts) could include an abatement of the apoptosis-related response (as evidenced by the down regulation of the caspase activity and the expression of caspase, Bax, APAF-1, HSP70, Bcl-2 and BI-1), and the up-regulation of additional anti–apoptotic/pro-cell survival regulators (as indicated by the robust expression of servivin) that could fill an important role in the acclimation of the coral to the sustained stress. These results are in agreement with the qualitative functional analysis carried out using the Bcl-2
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