Thermal preconditioning in a reef-building coral alleviates oxidative damage through a BI-1-mediated antioxidant response

Abstract

Global coral reef decline is largely driven by the breakdown of the coral-algal symbiosis during temperature stress. Corals can acclimatize to higher temperatures, but the cellular processes underlying this ability are poorly understood. We show that preconditioning-based improvements in thermal tolerance in Pocillopora acuta are accompanied by increases in host glutathione reductase (GR) activity and gene expression, which prevents DNA damage. A strong correlation between GR and BI-1 expressions in heat-stressed preconditioned corals and the presence of an antioxidant response element (ARE) in the GR promoter suggest BI-1 could regulate GR expression through Nrf2/ARE pathway. To fortify this link, we developed and GFP-validated an siRNA-mediated gene knockdown protocol and targeted the coral BI-1 gene. BI-1 knock-down specifically decreased GR expression and activity and increased oxidative DNA damage in heat-stressed preconditioned corals, showing that a BI-1-mediated, enhanced antioxidant response during acute heat stress is a key mechanism that prevents oxidative DNA damage after preconditioning.

BAK knockdown delays bleaching and alleviates oxidative DNA damage in a reef-building coral

“…The complexity of these processes means there are likely to be off-target consequences of ‘forced’ thermal tolerance, so the evaluation of other, potentially detrimental traits, is critical from a basic and applied perspective. Thermal stress causes the accumulation of oxidative DNA damage markers in coral tissue which can be prevented through preconditioning ( 20 ). We hypothesized that heat-stress-induced DNA damage could trigger PCD and lead to subsequent bleaching ( 5 , 20 ).…”

“…The experiment was conducted as described in ( 20 ) with minor modifications. Freshly cut coral fragments (∼ 3 cm) were placed into 20 ml cultivation vials, completely submerged in a flow-through tank with ambient-temperature seawater, and left to acclimatize overnight.…”

“…DNA was extracted from corals at the 48h timepoint (72h after the transfection) using Quick-DNA/RNA TM Microprep Plus Kit (Zymo Research). The level of 8-Hydroxy-2’-deoxyguanosine (8-OhdG) – a proxy for oxidative DNA damage – was analyzed in corals using DNA Damage Competitive ELISA Kit (Invitrogen) and MyCurveFit.com software as described in ( 20 ).…”

“…Many associated pathways have been proposed to play a role in coral bleaching and improved thermal tolerance ( 5 , 6 , 8 , 20–22 ). This complex physiology leads to a range of cellular and molecular consequences, including the accumulation of (oxidative) DNA damage in heat-stressed corals, which can be exacerbated by thermal preconditioning or by the addition of external antioxidants ( 20 , 23 , 24 ).…”

“…Many associated pathways have been proposed to play a role in coral bleaching and improved thermal tolerance ( 5 , 6 , 8 , 20–22 ). This complex physiology leads to a range of cellular and molecular consequences, including the accumulation of (oxidative) DNA damage in heat-stressed corals, which can be exacerbated by thermal preconditioning or by the addition of external antioxidants ( 20 , 23 , 24 ). The ROS-dependent bleaching theory postulates that during heat stress, symbionts are the main source of reactive oxygen species (ROS) which, unless scavenged by the host antioxidant defense, are detrimental to the host cell and trigger symbiosis disruption (reviewed in ( 25 )).…”