Intermittent hypoxia leads to functional reorganization of mitochondria and affects cellular bioenergetics in marine molluscs

Abstract

Fluctuations in oxygen (O 2 ) concentrations represent a major challenge to aerobic organisms and can be extremely damaging to their mitochondria. Marine intertidal molluscs are well-adapted to frequent O 2 fluctuations, yet it remains unknown how their mitochondrial functions are regulated to sustain energy metabolism and prevent cellular damage during hypoxia and reoxygenation (H/ R). We used metabolic control analysis to investigate the mechanisms of mitochondrial responses to H/R stress (18 h at <0.1% O 2 followed by 1 h of reoxygenation) using hypoxia-tolerant intertidal clams Mercenaria mercenaria and hypoxia-sensitive subtidal scallops Argopecten irradians as models. We also assessed H/R-induced changes in cellular energy balance, oxidative damage and unfolded protein response to determine the potential links between mitochondrial dysfunction and cellular injury. Mitochondrial responses to H/R in scallops strongly resembled those in other hypoxia-sensitive organisms. Exposure to hypoxia followed by reoxygenation led to a strong decrease in the substrate oxidation (SOX) and phosphorylation (PHOS) capacities as well as partial depolarization of mitochondria of scallops. Elevated mRNA expression of a reactive oxygen speciessensitive enzyme aconitase and Lon protease (responsible for degradation of oxidized mitochondrial proteins) during H/R stress was consistent with elevated levels of oxidative stress in mitochondria of scallops. In hypoxia-tolerant clams, mitochondrial SOX capacity was enhanced during hypoxia and continued rising during the first hour of reoxygenation. In both species, the mitochondrial PHOS capacity was suppressed during hypoxia, likely to prevent ATP wastage by the reverse action of F O ,F 1 -ATPase. The PHOS capacity recovered after 1 h of reoxygenation in clams but not in scallops. Compared with scallops, clams showed a greater suppression of energy-consuming processes (such as protein turnover and ion transport) during hypoxia, indicated by inactivation of the translation initiation factor EIF-2α, suppression of 26S proteasome activity and a dramatic decrease in the activity of Na + /K + -ATPase. The steady-state levels of adenylates were preserved during H/R exposure and AMP-dependent protein kinase was not activated in either species, indicating that the H/R exposure did not lead to severe energy deficiency. Taken together, our findings suggest that mitochondrial reorganizations sustaining high oxidative phosphorylation flux during recovery, combined with the ability to suppress ATP-demanding cellular functions during hypoxia, may contribute to high resilience of clams to H/R stress and help maintain energy homeostasis during frequent H/R cycles in the intertidal zone.

Effects of intermittent hypoxia on oxidative stress and protein degradation in molluscan mitochondria

“…scallops) (Ivanina et al, 2016(Ivanina et al, , 2012Kurochkin et al, 2008). Clams show a notably greater mitochondrial tolerance to H/R stress compared with oysters and especially scallops, as indicated by their ability to upregulate the activity of the ETS and oxidative phosphorylation capacity during hypoxia and subsequent reoxygenation (Ivanina et al, 2016(Ivanina et al, , 2012Kurochkin et al, 2008).…”

“…scallops) (Ivanina et al, 2016(Ivanina et al, , 2012Kurochkin et al, 2008). Clams show a notably greater mitochondrial tolerance to H/R stress compared with oysters and especially scallops, as indicated by their ability to upregulate the activity of the ETS and oxidative phosphorylation capacity during hypoxia and subsequent reoxygenation (Ivanina et al, 2016(Ivanina et al, , 2012Kurochkin et al, 2008). Oysters also showed an upregulation of ETS activity during H/R stress (albeit to a lesser degree than clams), whereas in scallops, H/R stress led to a suppression of the ETS activity, collapse of the oxidative phosphorylation capacity, and a decrease in the mitochondrial membrane potential (Ivanina et al, 2016).…”

“…Clams show a notably greater mitochondrial tolerance to H/R stress compared with oysters and especially scallops, as indicated by their ability to upregulate the activity of the ETS and oxidative phosphorylation capacity during hypoxia and subsequent reoxygenation (Ivanina et al, 2016(Ivanina et al, , 2012Kurochkin et al, 2008). Oysters also showed an upregulation of ETS activity during H/R stress (albeit to a lesser degree than clams), whereas in scallops, H/R stress led to a suppression of the ETS activity, collapse of the oxidative phosphorylation capacity, and a decrease in the mitochondrial membrane potential (Ivanina et al, 2016). Mitochondrial injury during H/R stress is driven by the increased ROS production during oxygen influx combined with the high concentrations of reduced metabolic intermediates that accumulate in hypoxia (Clanton, 2007;Korge et al, 2015).…”

“…However, some animals such as diving birds and mammals or intertidal invertebrates can undergo frequent H/R cycles without any apparent tissue damage. Recent studies showed that hypoxia-tolerant organisms (such as fish and mollusks) maintain mitochondrial integrity during H/R via rapid functional reorganization of mitochondria that upregulates activity of the electron transport system (ETS) and suppresses ATP wastage in hypoxia (Ivanina et al, 2016(Ivanina et al, , 2012Kurochkin et al, 2009). Mitochondrial tolerance to H/R stress in hypoxia-tolerant organisms also requires that the oxidative damage is prevented and/or repaired during the H/R cycles.…”

“…Argopecten irradians is a subtidal species whose distribution is confined to well-oxygenated seagrass beds (Hernández Cordero et al, 2012). It cannot tolerate prolonged hypoxia and experiences mass mortalities after only hours of oxygen deprivation (Ivanina et al, 2016). In contrast, oysters and clams are intertidal species that are regularly exposed to intermitted hypoxia because of the tidal cycles and can survive days to weeks without oxygen, depending on the environmental temperature (Kennedy et al, 1996;Kraeuter and Castagna, 2001).…”