Surviving without oxygen involves major tissue specific changes in the proteome of crucian carp (Carassius carassius)

The crucian carp (Carassius carassius) can survive complete oxygen depletion (anoxia) for several months at low temperatures, making it an excellent model for studying molecular adaptations to anoxia. Still, little is known about how its global proteome responds to anoxia and reoxygenation. By applying mass spectrometry-based proteome analyses on brain, heart and liver tissue from crucian carp exposed to normoxia, five days anoxia, and reoxygenation, we found major changes in particularly cardiac and hepatic protein levels in response to anoxia and reoxygenation. These included tissue-specific differences in mitochondrial proteins involved in aerobic respiration and mitochondrial membrane integrity. Enzymes in the electron transport system (ETS) decreased in heart and increased massively in liver during anoxia and reoxygenation but did not change in the brain. Importantly, the data support a special role for the liver in succinate handling upon reoxygenation, as suggested by a drastic increase of components of the ETS and uncoupling protein 2, which could allow for succinate metabolism without excessive formation of reactive oxygen species (ROS). Also during reoxygenation, the levels of proteins involved in the cristae junction organization of the mitochondria changed in the heart, possibly functioning to suppress ROS formation. Furthermore, proteins involved in immune (complement) system activation changed in the anoxic heart compared to normoxic controls. The results emphasize that responses to anoxia are highly tissue-specific and related to organ function.

Review: A history and perspective of mitochondria in the context of anoxia tolerance

Symbiosis is found throughout nature, but perhaps nowhere is it more fundamental than mitochondria in all eukaryotes. Since mitochondria were discovered and mechanisms of oxygen reduction characterized, an understanding gradually emerged that these organelles were involved not just in the combustion of oxygen, but also in the sensing of oxygen. While multiple hypotheses exist to explain the mitochondrial involvement in oxygen sensing, key elements are developing that include potassium channels and reactive oxygen species. To understand how mitochondria contribute to oxygen sensing, it is informative to study a model system which is naturally adapted to survive extended periods without oxygen. Amongst air-breathing vertebrates, the most highly adapted are western painted turtles (Chrysemys picta bellii), which overwinter in ice-covered and anoxic water bodies. Through research of this animal, it was postulated that metabolic rate depression is key to anoxic survival and that mitochondrial regulation is a key aspect. When faced with anoxia, excitatory neurotransmitter receptors in turtle brain are inhibited through mitochondrial calcium release, termed "channel arrest". Simultaneously, inhibitory GABAergic signalling contributes to the "synaptic arrest" of excitatory action potential firing through a pathway dependent on mitochondrial depression of ROS generation. While many pathways are implicated in mitochondrial oxygen sensing in turtles, such as those of adenosine, ATP turnover, and gaseous transmitters, an apparent point of intersection is the mitochondria. In this review we will explore how an organelle that was critical for organismal complexity in an oxygenated world has also become a potentially important oxygen sensor.

Exposure to low temperature prepares the turtle brain to withstand anoxic environments during overwintering

In most vertebrates, anoxia drastically reduces the production of the essential adenosine triphosphate (ATP) to power its many necessary functions, and, consequently, cell death occurs within minutes. However, some vertebrates, such as the painted turtle (Chrysemys picta bellii), have evolved the ability to survive months without oxygen by simultaneously decreasing ATP supply and demand, surviving the anoxic period without any apparent cellular damage. The impact of anoxia on the metabolic function of painted turtles has received a lot of attention. However, the impact of low temperature has received less attention and the interactive effect of anoxia and temperature even less. In the present study, we investigated the interactive impacts of reduced temperature and severe hypoxia on the electrophysiological properties of pyramidal neurons in painted turtle cerebral cortex. Our results show that an acute reduction in temperature from 20 to 5°C decreases membrane potential, action potential width and amplitude, and whole-cell conductance. Importantly, acute exposure to 5°C considerably slows membrane repolarization by voltage-gated K+ channels. Exposing pyramidal cells to severe hypoxia in addition to an acute temperature change slightly depolarized membrane potential but did not alter action potential amplitude or width and whole-cell conductance. These results suggest that acclimation to low temperatures, preceding severe environmental hypoxia, induces cellular responses in pyramidal neurons that facilitate survival under low oxygen concentrations. In particular, our results show that temperature acclimation invokes a change in voltage-gated K+ channel kinetics that overcomes the acute inhibition of the channel.

Scavenging of reactive oxygen species mimics the anoxic response in goldfish pyramidal neurons

Goldfish are one of a few species able to avoid cellular damage during month-long periods in severely hypoxic environments. By suppressing action potentials in excitatory glutamatergic neurons, the goldfish brain decreases its overall energy expenditure. Coincident with reductions in O2 availability is a natural decrease in cellular reactive oxygen species (ROS) generation, which has been proposed to function as part of a low-oxygen signal transduction pathway. Using live-tissue fluorescence microscopy, we found that ROS production decreased by 10% with the onset of anoxia in goldfish telencephalic brain slices. Employing whole-cell patch-clamp recording, we found that, similar to severe hypoxia, the ROS scavengers N-acetyl cysteine (NAC) and MitoTEMPO, added during normoxic periods, depolarized membrane potential (severe hypoxia -73.6 to -61.4 mV, NAC -76.6 to -66.2 mV and MitoTEMPO -71.5 mV to -62.5 mV) and increased whole-cell conductance (severe hypoxia 5.7 nS to 8.0 nS, NAC 6.0 nS to 7.5 nS and MitoTEMPO 6.0 nS to 7.6 nS). Also, in a subset of active pyramidal neurons, these treatments reduced action potential firing frequency (severe hypoxia 0.18 Hz to 0.03 Hz, NAC 0.27 Hz to 0.06 Hz and MitoTEMPO 0.35 Hz to 0.08 Hz). Neither severe hypoxia nor ROS scavenging impacted action potential threshold. The addition of exogenous hydrogen peroxide could reverse the effects of the antioxidants. Taken together, this supports a role for a reduction in [ROS] as a low-oxygen signal in goldfish brain.

Navigating oxygen deprivation: liver transcriptomic responses of the red eared slider turtle to environmental anoxia

The best facultative anaerobes among vertebrates are members of the genera Trachemys (pond slider turtles) and Chrysemys (painted turtles), and are able to survive without oxygen for up to 12 to 18 weeks at ∼3 °C. In this study, we utilized RNAseq to profile the transcriptomic changes that take place in response to 20 hrs of anoxia at 5 °C in the liver of the red eared slide turtle (Trachemys scripta elegans). Sequencing reads were obtained from at least 18,169 different genes and represented a minimum 49x coverage of the C. picta bellii exome. A total of 3,105 genes showed statistically significant changes in gene expression between the two animal groups, of which 971 also exhibited a fold change equal to or greater than 50% of control normoxic values. This study also highlights a number of anoxia-responsive molecular pathways that are may be important to navigating anoxia survival. These pathways were enriched in mRNA found to significantly increase in response to anoxia and included molecular processes such as DNA damage repair and metabolic reprogramming. For example, our results indicate that the anoxic turtle may utilize succinate metabolism to yield a molecule of GTP in addition to the two molecules that results from lactate production, and agrees with other established models of anoxia tolerance. Collectively, our analysis provides a snapshot of the molecular landscape of the anoxic turtle and may provide hints into the how this animal is capable of surviving this extreme environmental stress.

Effects of anoxia on ATP, water, ion and pH balance in an insect (Locusta migratoria)

When exposed to anoxia, insects rapidly go into a hypometabolic coma from which they can recover when exposed to normoxia again. However, prolonged anoxic bouts eventually lead to death in most insects, although some species are surprisingly tolerant. Anoxia challenges ATP, ion, pH and water homeostasis, but it is not clear how fast and to what degree each of these parameters is disrupted during anoxia, nor how quickly they recover. Further, it has not been investigated which disruptions are the primary source of the tissue damage that ultimately causes death. Here, we show, in the migratory locust (Locusta migratoria), that prolonged anoxic exposures are associated with increased recovery time, decreased survival, rapidly disrupted ATP and pH homeostasis and a slower disruption of ion ([K⁺] and [Na⁺]) and water balance. Locusts could not fully recover after 4 h of anoxia at 30°C, and at this point hemolymph [K⁺] was elevated 5-fold and [Na⁺] was decreased 2-fold, muscle [ATP] was decreased to ≤3% of normoxic values, hemolymph pH had dropped 0.8 units from 7.3 to 6.5, and hemolymph water content was halved. These physiological changes are associated with marked tissue damage in vivo and we show that the isolated and combined effects of hyperkalemia, acidosis and anoxia can all cause muscle tissue damage in vitro to equally large degrees. When locusts were returned to normoxia after a moderate (2 h) exposure of anoxia, ATP recovered rapidly (15 min) and this was quickly followed by recovery of ion balance (30 min), while pH recovery took 2-24 h. Recovery of [K⁺] and [Na⁺] coincided with the animals exiting the comatose state, but recovery to an upright position took ∼90 min and was not related to any of the physiological parameters examined.

Taurine activates glycine and GABAA receptor currents in anoxia-tolerant painted turtle pyramidal neurons

Unlike anoxia-intolerant mammals, painted turtles can survive extended periods without oxygen. This is partly accomplished by an anoxia-mediated increase in gamma-aminobutyric acid (GABA) release, which activates GABA receptors and mediates spike arrest in turtle neurons via shunting inhibition. Extracellular taurine levels also increase during anoxia; why this occurs is unknown but it is speculated that glycine and/or GABA_A/B receptors are involved. Given the general importance of inhibitory neurotransmission in the anoxia-tolerant painted turtle brain, we investigated the function of taurine as an inhibitory neuromodulator in turtle pyramidal neurons. Using whole-cell patch-clamp electrophysiological methods to record from neurons within a cortical brain sheet, we found that taurine depolarized membrane potential by ∼8 mV, increased whole-cell conductance ∼2-fold, and induced an inward current that possessed characteristics similar to GABA- and glycine-evoked currents. These effects were mitigated following glycine receptor antagonism with strychnine and GABA_A receptor antagonism with gabazine, bicuculine or picrotoxin, but were unchanged following GABA_B or glutamatergic receptor inhibition. These data indicate that a high concentration of taurine in vitro mediates its effects through both glycine and GABA_A receptors, and suggests that taurine, in addition to GABA, inhibits neuronal activity during anoxia in the turtle cortex.

Energetics of acclimation to NaCl by submerged, anoxic rice seedlings

BACKGROUND AND AIMS

Our aim was to elucidate how plant tissues under a severe energy crisis cope with imposition of high NaCl, which greatly increases ion fluxes and hence energy demands. The energy requirements for ion regulation during combined salinity and anoxia were assessed to gain insights into ion transport processes in the anoxia-tolerant coleoptile of rice.

METHODS

We studied the combined effects of anoxia plus 50 or 100 mm NaCl on tissue ions and growth of submerged rice (Oryza sativa) seedlings. Excised coleoptiles allowed measurements in aerated or anoxic conditions of ion net fluxes and O2 consumption or ethanol formation and by inference energy production.

KEY RESULTS

Over 80 h of anoxia, coleoptiles of submerged intact seedlings grew at 100 mm NaCl, but excised coleoptiles, with 50 mm exogenous glucose, survived only at 50 mm NaCl, possibly due to lower energy production with glucose than for intact coleoptiles with sucrose as substrate. Rates of net uptake of Na+ and Cl- by coleoptiles in anoxia were about half those in aerated solution. Ethanol formation in anoxia and O2 uptake in aerobic solution were each increased by 13-15 % at 50 mm NaCl, i.e. ATP formation was stimulated. For acclimation to 50 mm NaCl, the anoxic tissues used only 25 % of the energy that was expended by aerobic tissues. Following return of coleoptiles to aerated non-saline solution, rates of net K+ uptake recovered to those in continuously aerated solution, demonstrating there was little injury during anoxia with 50 mm NaCl.

CONCLUSION

Rice seedlings survive anoxia, without the coleoptile incurring significant injury, even with the additional energy demands imposed by NaCl (100 mm when intact, 50 mm when excised). Energy savings were achieved in saline anoxia by less coleoptile growth, reduced ion fluxes as compared to aerobic coleoptiles and apparent energy-economic ion transport systems.

Assessment of anoxia tolerance and photoperiod dependence of GABAergic polarity in the pond snail Lymnaea stagnalis

The pond snail Lymnaea stagnalis is reported to be anoxia-tolerant and if the tolerance mechanism is similar to that of the anoxia-tolerant painted turtle, GABA should play an important role. A potentially confounding factor investigating the role of GABA in anoxia tolerance are reports that GABA has both inhibitory and excitatory effects within L. stagnalis central ganglion. We therefore set out to determine if seasonality or photoperiod has an impact on: 1) the anoxia-tolerance of the intact pond snail, and 2) the response of isolated neuroganglia cluster F neurons to exogenous GABA application. L. stagnalis maintained on a natural summer light cycle were unable to survive any period of anoxic exposure, while those maintained on a natural winter light cycle survived a maximum of 4h. Using intracellular sharp electrode recordings from pedal ganglia cluster F neurons we show that there is a photoperiod dependent shift in the response to GABA. Snails exposed to a 16h:8h light:dark cycle in an environmental chamber (induced summer phenotype) exhibited hyperpolarizing inhibitory responses and those exposed to a 8h:16h light:dark cycle (induced winter phenotype) exhibited depolarizing excitatory responses to GABA application. Using gramicidin-perforated patch recordings we also found a photoperiod dependent shift in the reversal potential for GABA. We conclude that the opposing responses of L. stagnalis central neurons to GABA results from a shift in intracellular chloride concentration that is photoperiod dependent and is likely mediated through the relative efficacy of cation chloride co-transporters. Although the physiological ramifications of the photoperiod dependent shift are unknown this work potentially has important implications for the impact of artificial light pollution on animal health.

Introducing a novel mechanism to control heart rate in the ancestral Pacific hagfish

Although neural modulation of heart rate is well established among chordate animals, the Pacific hagfish (Eptatretus stoutii) lacks any cardiac innervation, yet it can increase its heart rate from the steady, depressed heart rate seen in prolonged anoxia to almost double its normal normoxic heart rate, an almost fourfold overall change during the 1-h recovery from anoxia. The present study sought mechanistic explanations for these regulatory changes in heart rate. We provide evidence for a bicarbonate-activated, soluble adenylyl cyclase (sAC)-dependent mechanism to control heart rate, a mechanism never previously implicated in chordate cardiac control.

Glycogen dynamics of crucian carp (Carassius carassius) in prolonged anoxia

Mobilization of glycogen stores was examined in the anoxic crucian carp (Carassius carassius Linnaeus). Winter-acclimatized fish were exposed to anoxia for 1, 3, or 6 weeks at 2 °C, and changes in the size of glycogen deposits were followed. After 1 week of anoxia, a major part of the glycogen stores was mobilized in liver (79.5 %) and heart (75.6 %), and large decreases occurred in gill (46.7 %) and muscle (45.1 %). Brain was an exception in that its glycogen content remained unchanged. The amount of glycogen degraded during the first anoxic week was sufficient for the anaerobic ethanol production for more than 6 weeks of anoxia. After 3 and 6 weeks of anoxia, there was little further degradation of glycogen in other tissues except the brain where the stores were reduced by 30.1 and 49.9 % after 3 and 6 weeks of anoxia, respectively. One week of normoxic recovery following the 6-week anoxia was associated with a complete replenishment of the brain glycogen and partial recovery of liver, heart, and gill glycogen stores. Notably, the resynthesis of glycogen occurred at the expense of the existing energy reserves of the body in fasting fish. These findings indicate that in crucian carp, glycogen stores are quickly mobilized after the onset of anoxia, with the exception of the brain whose glycogen stores may be saved for putative emergency situations.

Experimental icing affects growth, mortality, and flowering in a high Arctic dwarf shrub

Effects of climate change are predicted to be greatest at high latitudes, with more pronounced warming in winter than summer. Extreme mid-winter warm spells and heavy rain-on-snow events are already increasing in frequency in the Arctic, with implications for snow-pack and ground-ice formation. These may in turn affect key components of Arctic ecosystems. However, the fitness consequences of extreme winter weather events for tundra plants are not well understood, especially in the high Arctic. We simulated an extreme mid-winter rain-on-snow event at a field site in high Arctic Svalbard (78°N) by experimentally encasing tundra vegetation in ice. After the subsequent growing season, we measured the effects of icing on growth and fitness indices in the common tundra plant, Arctic bell-heather (Cassiope tetragona). The suitability of this species for retrospective growth analysis enabled us to compare shoot growth in pre and postmanipulation years in icing treatment and control plants, as well as shoot survival and flowering. Plants from icing treatment plots had higher shoot mortality and lower flowering success than controls. At the individual sample level, heavily flowering plants invested less in shoot growth than nonflowering plants, while shoot growth was positively related to the degree of shoot mortality. Therefore, contrary to expectation, undamaged shoots showed enhanced growth in ice treatment plants. This suggests that following damage, aboveground resources were allocated to the few remaining undamaged meristems. The enhanced shoot growth measured in our icing treatment plants has implications for climate studies based on retrospective analyses of Cassiope. As shoot growth in this species responds positively to summer warming, it also highlights a potentially complex interaction between summer and winter conditions. By documenting strong effects of icing on growth and reproduction of a widespread tundra plant, our study contributes to an understanding of Arctic plant responses to projected changes in winter climatic conditions.

Scavenging ROS dramatically increase NMDA receptor whole-cell currents in painted turtle cortical neurons

Oxygen deprivation triggers excitotoxic cell death in mammal neurons through excessive calcium loading via over-activation of N-methyl-d-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. This does not occur in the western painted turtle, which overwinters for months without oxygen. Neurological damage is avoided through anoxia-mediated decreases in NMDA and AMPA receptor currents that are dependent upon a modest rise in intracellular Ca(2+) concentrations ([Ca(2+)]i) originating from mitochondria. Anoxia also blocks mitochondrial reactive oxygen species (ROS) generation, which is another potential signaling mechanism to regulate glutamate receptors. To assess the effects of decreased intracellular [ROS] on NMDA and AMPA receptor currents, we scavenged ROS with N-2-mercaptopropionylglycine (MPG) or N-acetylcysteine (NAC). Unlike anoxia, ROS scavengers increased NMDA receptor whole-cell currents by 100%, while hydrogen peroxide decreased currents. AMPA receptor currents and [Ca(2+)]i concentrations were unaffected by ROS manipulation. Because decreases in [ROS] increased NMDA receptor currents, we next asked whether mitochondrial Ca(2+) release prevents receptor potentiation during anoxia. Normoxic activation of mitochondrial ATP-sensitive potassium (mKATP) channels with diazoxide decreased NMDA receptor currents and was unaffected by subsequent ROS scavenging. Diazoxide application following ROS scavenging did not rescue scavenger-mediated increases in NMDA receptor currents. Fluorescent measurement of [Ca(2+)]i and ROS levels demonstrated that [Ca(2+)]i increases before ROS decreases. We conclude that decreases in ROS concentration are not linked to anoxia-mediated decreases in NMDA/AMPA receptor currents but are rather associated with an increase in NMDA receptor currents that is prevented during anoxia by mitochondrial Ca(2+) release.