Stable mitochondrial CICIII2 supercomplex interactions in reptiles versus homeothermic vertebrates

Differential production of mitochondrial reactive oxygen species between mouse (Mus musculus) and crucian carp (Carassius carassius)

AIM

In most vertebrates, oxygen deprivation and subsequent re-oxygenation are associated with mitochondrial impairment and excess production of reactive oxygen species (ROS) like hydrogen peroxide (H2O2). This in turn triggers a cascade of cell-damaging events in a temperature-dependent manner. The crucian carp (Carassius carassius) is one of few vertebrates that survives months without oxygen at cold temperatures and overcomes oxidative damage during re-oxygenation periods. Mitochondria of this anoxia-tolerant species therefore serve as an excellent model in translational research to study adaptation and resilience to low oxygen conditions and thermal variability.

METHODS

Here, we used high-resolution respirometry on isolated mitochondria from hearts of crucian carp and the anoxia-intolerant mouse (Mus musculus), at 37 and 8°C; two temperatures relevant for transplantation medicine (i.e., graft preservation and subsequent rewarming).

RESULTS

We find: (1) a striking difference in H2O2 release between the two species at 37°C despite comparable mitochondrial efficiency and capacity, (2) a massive H2O2 release after inhibition of complex V in mouse at 37°C that is absent in crucian carp, and prevented in mouse by incubation at 8°C or uncoupling with a protonophore at 37°C, and (3) indications that differences in mitochondrial complex I and II capacity and thermal sensitivity influence the release of mitochondrial H2O2 relative to respiration.

CONCLUSION

Our findings provide comparative insights into a spectrum of mitochondrial adaptations in vertebrates and the importance of thermal variability. Furthermore, the species- and temperature-related changes associated with mitochondria highlighted in this study may help identify mitochondria-based targets for translational medicine.

An evolving roadmap: using mitochondrial physiology to help guide conservation efforts

The crucial role of aerobic energy production in sustaining eukaryotic life positions mitochondrial processes as key determinants of an animal's ability to withstand unpredictable environments. The advent of new techniques facilitating the measurement of mitochondrial function offers an increasingly promising tool for conservation approaches. Herein, we synthesize the current knowledge on the links between mitochondrial bioenergetics, ecophysiology and local adaptation, expanding them to the wider conservation physiology field. We discuss recent findings linking cellular bioenergetics to whole-animal fitness, in the current context of climate change. We summarize topics, questions, methods, pitfalls and caveats to help provide a comprehensive roadmap for studying mitochondria from a conservation perspective. Our overall aim is to help guide conservation in natural populations, outlining the methods and techniques that could be most useful to assess mitochondrial function in the field.

Flies on the rise: acclimation effect on mitochondrial oxidation capacity at normal and high temperatures in Drosophila melanogaster

Increased average temperatures and extreme thermal events (such as heatwaves) brought forth by climate change impose important constraints on aerobic metabolism. Notably, mitochondrial metabolism, which is affected by both long- and short-term temperature changes, has been put forward as an important determinant for thermal tolerance of organisms. This study examined the influence of phenotypic plasticity on metabolic and physiological parameters in Drosophila melanogaster and the link between mitochondrial function and their upper thermal limits. We showed that D. melanogaster acclimated to 15°C have a 0.65°C lower critical thermal maximum (CTmax) compared with those acclimated to 24°C. Drosophila melanogaster acclimated to 15°C exhibited a higher proportion of shorter saturated and monounsaturated fatty acids, concomitant with lower proportions of polyunsaturated fatty acids. No mitochondrial quantitative changes (fractional area and number) were detected between acclimation groups, but changes of mitochondrial oxidation capacities were observed. Specifically, in both 15°C- and 24°C-acclimated flies, complex I-induced respiration was increased when measured between 15 and 24°C, but drastically declined when measured at 40°C. When succinate and glycerol-3-phosphate were added, this decrease was however compensated for in flies acclimated to 24°C, suggesting an important impact of acclimation on mitochondrial function related to thermal tolerance. Our study reveals that the use of oxidative substrates at high temperatures is influenced by acclimation temperature and strongly related to upper thermal tolerance as a difference of 0.65°C in CTmax translates into significant mitochondrial changes.

Are reactive oxygen species always bad? Lessons from hypoxic ectotherms

Oxygen (O2) is required for aerobic energy metabolism but can produce reactive oxygen species (ROS), which are a wide variety of oxidant molecules with a range of biological functions from causing cell damage (oxidative distress) to cell signalling (oxidative eustress). The balance between the rate and amount of ROS generated and the capacity for scavenging systems to remove them is affected by several biological and environmental factors, including oxygen availability. Ectotherms, and in particular hypoxia-tolerant ectotherms, are hypothesized to avoid oxidative damage caused by hypoxia, although it is unclear whether this translates to an increase in ecological fitness. In this Review, we highlight the differences between oxidative distress and eustress, the current mechanistic understanding of the two and how they may affect ectothermic physiology. We discuss the evidence of occurrence of oxidative damage with hypoxia in ectotherms, and that ectotherms may avoid oxidative damage through (1) high levels of antioxidant and scavenging systems and/or (2) low(ering) levels of ROS generation. We argue that the disagreements in the literature as to how hypoxia affects antioxidant enzyme activity and the variable metabolism of ectotherms makes the latter strategy more amenable to ectotherm physiology. Finally, we argue that observed changes in ROS production and oxidative status with hypoxia may be a signalling mechanism and an adaptive strategy for ectotherms encountering hypoxia.

Structural rather than catalytic role for mitochondrial respiratory chain supercomplexes

Mammalian mitochondrial respiratory chain (MRC) complexes are able to associate into quaternary structures named supercomplexes (SCs), which normally coexist with non-bound individual complexes. The functional significance of SCs has not been fully clarified and the debate has been centered on whether or not they confer catalytic advantages compared with the non-bound individual complexes. Mitochondrial respiratory chain organization does not seem to be conserved in all organisms. In fact, and differently from mammalian species, mitochondria from Drosophila melanogaster tissues are characterized by low amounts of SCs, despite the high metabolic demands and MRC activity shown by these mitochondria. Here, we show that attenuating the biogenesis of individual respiratory chain complexes was accompanied by increased formation of stable SCs, which are missing in Drosophila melanogaster in physiological conditions. This phenomenon was not accompanied by an increase in mitochondrial respiratory activity. Therefore, we conclude that SC formation is necessary to stabilize the complexes in suboptimal biogenesis conditions, but not for the enhancement of respiratory chain catalysis.

Ectotherm mitochondrial economy and responses to global warming

Temperature is a key abiotic factor affecting ecology, biogeography, and evolution of species. Alterations of energy metabolism play an important role in adaptations and plastic responses to temperature shifts on different time scales. Mitochondrial metabolism affects cellular bioenergetics and redox balance making these organelles an important determinant of organismal performances such as growth, locomotion, or development. Here I analyze the impacts of environmental temperature on the mitochondrial functions (including oxidative phosphorylation, proton leak, production of reactive oxygen species(ROS), and ATP synthesis) of ectotherms and discuss the mechanisms underlying negative shifts in the mitochondrial energy economy caused by supraoptimal temperatures. Owing to the differences in the thermal sensitivity of different mitochondrial processes, elevated temperatures (beyond the species- and population-specific optimal range) cause reallocation of the electron flux and the protonmotive force (Δp) in a way that decreases ATP synthesis efficiency, elevates the relative cost of the mitochondrial maintenance, causes excessive production of ROS and raises energy cost for antioxidant defense. These shifts in the mitochondrial energy economy might have negative consequences for the organismal fitness traits such as the thermal tolerance or growth. Correlation between the thermal sensitivity indices of the mitochondria and the whole organism indicate that these traits experience similar selective pressures but further investigations are needed to establish whether there is a cause-effect relationship between the mitochondrial failure and loss of organismal performance during temperature change.

Cooperative assembly of the mitochondrial respiratory chain

Deep understanding of the pathophysiological role of the mitochondrial respiratory chain (MRC) relies on a well-grounded model explaining how its biogenesis is regulated. The lack of a consistent framework to clarify the modes and mechanisms governing the assembly of the MRC complexes and supercomplexes (SCs) works against progress in the field. The plasticity model was postulated as an attempt to explain the coexistence of mammalian MRC complexes as individual entities and associated in SC species. However, mounting data accumulated throughout the years question the universal validity of the plasticity model as originally proposed. Instead, as we argue here, a cooperative assembly model provides a much better explanation to the phenomena observed when studying MRC biogenesis in physiological and pathological settings.

Hibernation is super complex: distribution, dynamics, and stability of electron transport system supercomplexes in Ictidomys tridecemlineatus

Complexes of the electron transport system can associate with each other to form supercomplexes (SCs) within mitochondrial membranes, perhaps increasing respiratory capacity or reducing reactive oxygen species production. In this study, we determined the abundance, composition, and stability of SCs in a mammalian hibernator, in which both whole-animal and mitochondria metabolism change greatly throughout winter. We isolated mitochondria from thirteen-lined ground squirrels (Ictidomys tridecemlineatus) in different hibernation states, as well as from rats (Rattus norvegicus). We extracted mitochondrial proteins using two non-ionic detergents of different strengths, and quantified SC abundance using two-dimensional gel electrophoresis and immunoblotting. Rat heart and liver had fewer SCs than ground squirrels. Within ground squirrels, SCs are dynamic, changing among hibernation states within a matter of hours. In brown adipose tissue, Complex III composition in different SCs differed between the torpid and interbout euthermic phase of a hibernation bout. In heart and liver, complex III composition changed between winter and summer. We also evaluated the stability of liver SCs using a stronger detergent and found that the stability of SCs differed: torpor SCs were more stable than the SCs of ground squirrels in other states and rats. This study is the first report of SC changes during hibernation, and the first to demonstrate their dynamics on a short timescale.

Enhanced resistance to Ca2+-induced mitochondrial permeability transition in the long-lived red-footed tortoise Chelonoidis carbonaria

The interaction between supraphysiological cytosolic Ca2+ levels and mitochondrial redox imbalance mediates the mitochondrial permeability transition (MPT). The MPT is involved in cell death, diseases and aging. This study compared the liver mitochondrial Ca2+ retention capacity and oxygen consumption in the long-lived red-footed tortoise (Chelonoidis carbonaria) with those in the rat as a reference standard. Mitochondrial Ca2+ retention capacity, a quantitative measure of MPT sensitivity, was remarkably higher in tortoises than in rats. This difference was minimized in the presence of the MPT inhibitors ADP and cyclosporine A. However, the Ca2+ retention capacities of tortoise and rat liver mitochondria were similar when both MPT inhibitors were present simultaneously. NADH-linked phosphorylating respiration rates of tortoise liver mitochondria represented only 30% of the maximal electron transport system capacity, indicating a limitation imposed by the phosphorylation system. These results suggested underlying differences in putative MPT structural components [e.g. ATP synthase, adenine nucleotide translocase (ANT) and cyclophilin D] between tortoises and rats. Indeed, in tortoise mitochondria, titrations of inhibitors of the oxidative phosphorylation components revealed a higher limitation of ANT. Furthermore, cyclophilin D activity was approximately 70% lower in tortoises than in rats. Investigation of critical properties of mitochondrial redox control that affect MPT demonstrated that tortoise and rat liver mitochondria exhibited similar rates of H2O2 release and glutathione redox status. Overall, our findings suggest that constraints imposed by ANT and cyclophilin D, putative components or regulators of the MPT pore, are associated with the enhanced resistance to Ca2+-induced MPT in tortoises.

Recurrent erosion of COA1/MITRAC15 exemplifies conditional gene dispensability in oxidative phosphorylation

Skeletal muscle fibers rely upon either oxidative phosphorylation or the glycolytic pathway with much less reliance on oxidative phosphorylation to achieve muscular contractions that power mechanical movements. Species with energy-intensive adaptive traits that require sudden bursts of energy have a greater dependency on glycolytic fibers. Glycolytic fibers have decreased reliance on OXPHOS and lower mitochondrial content compared to oxidative fibers. Hence, we hypothesized that gene loss might have occurred within the OXPHOS pathway in lineages that largely depend on glycolytic fibers. The protein encoded by the COA1/MITRAC15 gene with conserved orthologs found in budding yeast to humans promotes mitochondrial translation. We show that gene disrupting mutations have accumulated within the COA1 gene in the cheetah, several species of galliform birds, and rodents. The genomic region containing COA1 is a well-established evolutionary breakpoint region in mammals. Careful inspection of genome assemblies of closely related species of rodents and marsupials suggests two independent COA1 gene loss events co-occurring with chromosomal rearrangements. Besides recurrent gene loss events, we document changes in COA1 exon structure in primates and felids. The detailed evolutionary history presented in this study reveals the intricate link between skeletal muscle fiber composition and the occasional dispensability of the chaperone-like role of the COA1 gene.

Regulation and functional role of the electron transport chain supercomplexes

Mitochondria are one of the most exhaustively investigated organelles in the cell and most attention has been paid to the components of the mitochondrial electron transport chain (ETC) in the last 100 years. The ETC collects electrons from NADH or FADH2 and transfers them through a series of electron carriers within multiprotein respiratory complexes (complex I to IV) to oxygen, therefore generating an electrochemical gradient that can be used by the F1-F0-ATP synthase (also named complex V) in the mitochondrial inner membrane to synthesize ATP. The organization and function of the ETC is a continuous source of surprises. One of the latest is the discovery that the respiratory complexes can assemble to form a variety of larger structures called super-complexes (SCs). This opened an unexpected level of complexity in this well-known and fundamental biological process. This review will focus on the current evidence for the formation of different SCs and will explore how they modulate the ETC organization according to the metabolic state. Since the field is rapidly growing, we also comment on the experimental techniques used to describe these SC and hope that this overview may inspire new technologies that will help to advance the field.

Feeding effects on liver mitochondrial bioenergetics of Boa constrictor (Serpentes: Boidae)

Snakes are interesting examples of taxa that can overcome energy metabolism challenges, as many species can endure long periods without feeding, and their eventual meals are of reasonably large sizes, thus exhibiting dual extreme adaptations. Consequently, metabolic rate increases considerably to attend to the energetic demand of digestion, absorption and protein synthesis. These animals should be adapted to transition from these two opposite states of energy fairly quickly, and therefore we investigated mitochondrial function plasticity in these states. Herein, we compared liver mitochondrial bioenergetics of the boid snake Boa constrictor during fasting and after meal intake. We fasted the snakes for 60 days, and then we fed a subgroup with 30% of their body size and evaluated their maximum postprandial response. We measured liver respiration rates from permeabilized tissue and isolated mitochondria. From isolated mitochondria, we also measured Ca2+ retention capacity and redox status. Mitochondrial respiration rates were maximized after feeding, reaching an approximately 60% increase from fasting levels when energized with complex I-linked substrates. Interestingly, fasting and fed snakes exhibited similar respiratory control ratios and citrate synthase activity. Furthermore, we found no differences in Ca2+ retention capacity, indicating no increase in susceptibility to mitochondrial permeability transition, and no changes in mitochondrial redox state, although fed animals exhibited increases in the release of H2O2. Thus, we conclude that liver mitochondria from B. constrictor snakes increase respiration rates during the postprandial period and quickly improve the bioenergetic capacity without compromising redox balance.

High temperature impairs mitochondrial function in rainbow trout cardiac mitochondria

Mitochondria provide cellular energy through oxidative phosphorylation, and thus temperature-induced constraints on mitochondrial function may be crucial to animal aerobic scope and thermal tolerance. Here, we report the effect of temperature in the range 5–30°C on respiration rates of isolated cardiac mitochondria from rainbow trout (Oncorhynchus mykiss) studied by high-resolution respirometry and spectrophotometric enzyme activity assays. Arrhenius breakpoint temperature analysis indicated that mitochondrial respiration rates under phosphorylating and fully uncoupled conditions increased exponentially up to 20°C, but stopped increasing at higher temperatures. In contrast, respiration rates measured under non-phosphorylating leak conditions continued to increase up to 30°C. The decrease in the ratio between phosphorylating and uncoupled respiration at high temperature indicated that phosphorylation was gradually impaired with increasing temperature, possibly because of the steadily increasing proton leak across the membrane. In addition, we found that complex I (NADH dehydrogenase) activity decreased above 20°C, similarly to mitochondrial respiration, and that complex I was unstable in the presence of detergents, suggesting that it may be particularly sensitive to changes in its interaction with membrane phospholipids. In contrast, complex II (succinate dehydrogenase) maintained activity at temperatures above 20°C, although succinate oxidation was insufficient to compensate for the loss of complex I activity in intact mitochondria. Together, these results indicate that the temperature-induced decrease in cardiac mitochondrial function coincides with the temperature at which trout aerobic scope peaks, and is largely due to impaired phosphorylation and complex I activity.

Mitochondrial respiratory supercomplexes in mammalian cells: structural versus functional role

Mitochondria are recognized as the main source of ATP to meet the energy demands of the cell. ATP production occurs by oxidative phosphorylation when electrons are transported through the electron transport chain (ETC) complexes and develop the proton motive force across the inner mitochondrial membrane that is used for ATP synthesis. Studies since the 1960s have been concentrated on the two models of structural organization of ETC complexes known as "solid-state" and "fluid-state" models. However, advanced new techniques such as blue-native gel electrophoresis, mass spectroscopy, and cryogenic electron microscopy for analysis of macromolecular protein complexes provided new data in favor of the solid-state model. According to this model, individual ETC complexes are assembled into macromolecular structures known as respiratory supercomplexes (SCs). A large number of studies over the last 20 years proposed the potential role of SCs to facilitate substrate channeling, maintain the integrity of individual ETC complexes, reduce electron leakage and production of reactive oxygen species, and prevent excessive and random aggregation of proteins in the inner mitochondrial membrane. However, many other studies have challenged the proposed functional role of SCs. Recently, a third model known as the "plasticity" model was proposed that partly reconciles both "solid-state" and "fluid-state" models. According to the "plasticity" model, respiratory SCs can co-exist with the individual ETC complexes. To date, the physiological role of SCs remains unknown, although several studies using tissue samples of patients or animal/cell models of human diseases revealed an associative link between functional changes and the disintegration of SC assembly. This review summarizes and discusses previous studies on the mechanisms and regulation of SC assembly under physiological and pathological conditions.

Digitonin concentration is determinant for mitochondrial supercomplexes analysis by BlueNative page

The BlueNative page (BNGE) gel has been the reference technique for studying the electron transport chain organization since it was established 20 years ago. Although the migration of supercomplexes has been demonstrated being real, there are still several concerns about its ability to reveal genuine interactions between respiratory complexes. Moreover, the use of different solubilization conditions generates conflicting interpretations. Here, we thoroughly compare the impact of different digitonin concentrations on the liquid dispersions' physical properties and correlate with the respiratory complexes' migration pattern and supercomplexes. Our results demonstrate that digitonin concentration generates liquid dispersions with specific size and variability critical to distinguish between a real association of complexes from being trapped in the same micelle.