31P-NMR measurements of pHi and high-energy phosphates in isolated turtle hearts during anoxia and acidosis

Developmental programming of DNA methylation and gene expression patterns is associated with extreme cardiovascular tolerance to anoxia in the common snapping turtle

Background

Environmental fluctuation during embryonic and fetal development can permanently alter an organism’s morphology, physiology, and behaviour. This phenomenon, known as developmental plasticity, is particularly relevant to reptiles that develop in subterranean nests with variable oxygen tensions. Previous work has shown hypoxia permanently alters the cardiovascular system of snapping turtles and may improve cardiac anoxia tolerance later in life. The mechanisms driving this process are unknown but may involve epigenetic regulation of gene expression via DNA methylation. To test this hypothesis, we assessed in situ cardiac performance during 2 h of acute anoxia in juvenile turtles previously exposed to normoxia (21% oxygen) or hypoxia (10% oxygen) during embryogenesis. Next, we analysed DNA methylation and gene expression patterns in turtles from the same cohorts using whole genome bisulfite sequencing, which represents the first high-resolution investigation of DNA methylation patterns in any reptilian species.

Results

Genome-wide correlations between CpG and CpG island methylation and gene expression patterns in the snapping turtle were consistent with patterns observed in mammals. As hypothesized, developmental hypoxia increased juvenile turtle cardiac anoxia tolerance and programmed DNA methylation and gene expression patterns. Programmed differences in expression of genes such as SCN5A may account for differences in heart rate, while genes such as TNNT2 and TPM3 may underlie differences in calcium sensitivity and contractility of cardiomyocytes and cardiac inotropy. Finally, we identified putative transcription factor-binding sites in promoters and in differentially methylated CpG islands that suggest a model linking programming of DNA methylation during embryogenesis to differential gene expression and cardiovascular physiology later in life. Binding sites for hypoxia inducible factors (HIF1A, ARNT, and EPAS1) and key transcription factors activated by MAPK and BMP signaling (RREB1 and SMAD4) are implicated.

Conclusions

Our data strongly suggests that DNA methylation plays a conserved role in the regulation of gene expression in reptiles. We also show that embryonic hypoxia programs DNA methylation and gene expression patterns and that these changes are associated with enhanced cardiac anoxia tolerance later in life. Programming of cardiac anoxia tolerance has major ecological implications for snapping turtles, because these animals regularly exploit anoxic environments throughout their lifespan.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13072-021-00414-7.

Development-specific transcriptomic profiling suggests new mechanisms for anoxic survival in the ventricle of overwintering turtles

Oxygen deprivation swiftly damages tissues in most animals, yet some species show remarkable abilities to tolerate little or even no oxygen. Painted turtles exhibit a development-dependent tolerance that allows adults to survive anoxia ∼4x longer than hatchlings: adults survive ∼170 days and hatchlings survive ∼40 days at 3°C. We hypothesized this difference is related to development-dependent differences in ventricular gene expression. Using a comparative ontogenetic approach, we examined whole transcriptomic changes before, during, and five days after a 20-day bout of anoxic submergence at 3°C. Ontogeny accounted for more gene expression differences than treatment (anoxia or recovery): 1,175 vs. 237 genes, respectively. Of the 237 differences, 93 could confer protection against anoxia and reperfusion injury, 68 could be injurious, and 20 may be constitutively protective. Especially striking during anoxia was the expression pattern of all 76 annotated ribosomal protein (R-protein) mRNAs, which decreased in anoxia-tolerant adults, but increased in anoxia-sensitive hatchlings, suggesting adult-specific regulation of translational suppression. These genes, along with 60 others that decreased their levels in adults and either increased or remained unchanged in hatchlings, implicate antagonistic pleiotropy as a mechanism to resolve the long-standing question about why hatchling painted turtles overwinter in terrestrial nests, rather than emerge and overwinter in water during their first year. In sum, developmental differences in the transcriptome of the turtle ventricle revealed potentially protective mechanisms that contribute to extraordinary adult-specific anoxia tolerance, and provide a unique perspective on differences between the anoxia-induced molecular responses of anoxia-tolerant or anoxia-sensitive phenotypes within a species.

The effects of pH and Pi on tension and Ca2+ sensitivity of ventricular myofilaments from the anoxia-tolerant painted turtle

We aimed to determine how increases in intracellular H⁺ and inorganic phosphate (P_i) to levels observed during anoxic submergence affect contractility in ventricular muscle of the anoxia-tolerant Western painted turtle, Chrysemys picta bellii Skinned multicellular preparations were exposed to six treatments with physiologically relevant levels of pH (7.4, 7.0, 6.6) and P_i (3 and 8 mmol l^-1). Each preparation was tested in a range of calcium concentrations (pCa 9.0-4.5) to determine the pCa-tension relationship for each treatment. Acidosis significantly decreased contractility by decreasing Ca²⁺ sensitivity (pCa₅₀) and tension development (P<0.001). Increasing [P_i] also decreased contractility by decreasing tension development at every pH level (P<0.001) but, alone, did not affect Ca²⁺ sensitivity (P=0.689). Simultaneous increases in [H⁺] and [P_i] interacted to attenuate the decreased tension development and Ca²⁺ sensitivity (P<0.001), possibly reflecting a decreased sensitivity to P_i when it is present as the dihydrogen phosphate form, which increases as pH decreases. Compared with that of mammals, the ventricle of turtles exhibits higher Ca²⁺ sensitivity, which is consistent with previous studies of ectothermic vertebrates.

Intrinsic contractile properties of the crucian carp (Carassius carassius) heart during anoxic and acidotic stress

The crucian carp (Carassius carassius) seems unique among vertebrates in its ability to maintain cardiac performance during prolonged anoxia. We investigated whether this phenomenon arises in part from a myocardium tolerant to severe acidosis or because the anoxic crucian carp heart may not experience a severe extracellular acidosis due to the fish's ability to convert lactate to ethanol. Spontaneously contracting heart preparations from cold-acclimated (6-8°C) carp were exposed (at 6.5°C) to graded or ungraded levels of acidosis under normoxic or anoxic conditions and intrinsic contractile performance was assessed. Our results clearly show that the carp heart is tolerant of acidosis as long as oxygen is available. However, heart rate and contraction kinetics of anoxic hearts were severely impaired when extracellular pH was decreased below 7.4. Nevertheless, the crucian carp heart was capable of recovering intrinsic contractile performance upon reoxygenation regardless of the severity of the anoxic + acidotic insult. Finally, we show that increased adrenergic stimulation can ameliorate, to a degree, the negative effects of severe acidosis on the intrinsic contractile properties of the anoxic crucian carp heart. Combined, these findings indicate an avoidance of severe extracellular acidosis and adrenergic stimulation are two important factors protecting the intrinsic contractile properties of the crucian carp heart during prolonged anoxia, and thus likely facilitate the ability of the anoxic crucian carp to maintain cardiac pumping.

Effects of environmental hypoxia on cardiac energy metabolism and performance in tilapia

The ability of an animal to depress ATP turnover while maintaining metabolic energy balance is important for survival during hypoxia. In the present study, we investigated the responses of cardiac energy metabolism and performance in the hypoxia-tolerant tilapia (Oreochromis hybrid sp.) during exposure to environmental hypoxia. Exposure to graded hypoxia (> or =92% to 2.5% air saturation over 3.6 +/- 0.2 h) followed by exposure to 5% air saturation for 8 h caused a depression of whole animal oxygen consumption rate that was accompanied by parallel decreases in heart rate, cardiac output, and cardiac power output (CPO, analogous to ATP demand of the heart). These cardiac parameters remained depressed by 50-60% compared with normoxic values throughout the 8-h exposure. During a 24-h exposure to 5% air saturation, cardiac ATP concentration was unchanged compared with normoxia and anaerobic glycolysis contributed to ATP supply as evidenced by considerable accumulation of lactate in the heart and plasma. Reductions in the provision of aerobic substrates were apparent from a large and rapid (in <1 h) decrease in plasma nonesterified fatty acids concentration and a modest decrease in activity of pyruvate dehydrogenase. Depression of cardiac ATP demand via bradycardia and an associated decrease in CPO appears to be an integral component of hypoxia-induced metabolic rate depression in tilapia and likely contributes to hypoxic survival.

Correlation of cardiac performance with cellular energetic components in the oxygen-deprived turtle heart

The relationship between cardiac energy metabolism and the depression of myocardial performance during oxygen deprivation has remained enigmatic. Here, we combine in vivo (31)P-NMR spectroscopy and MRI to provide the first temporal profile of in vivo cardiac energetics and cardiac performance of an anoxia-tolerant vertebrate, the freshwater turtle (Trachemys scripta) during long-term anoxia exposure (approximately 3 h at 21 degrees C and 11 days at 5 degrees C). During anoxia, phosphocreatine (PCr), unbound levels of inorganic phosphate (effective P(i)(2-)), intracellular pH (pH(i)), and free energy of ATP hydrolysis (dG/dxi) exhibited asymptotic patterns of change, indicating that turtle myocardial high-energy phosphate metabolism and energetic state are reset to new, reduced steady states during long-term anoxia exposure. At 21 degrees C, anoxia caused a reduction in pH(i) from 7.40 to 7.01, a 69% decrease in PCr and a doubling of effective P(i)(2-). ATP content remained unchanged, but the free energy of ATP hydrolysis (dG/dxi) decreased from -59.6 to -52.5 kJ/mol. Even so, none of these cellular changes correlated with the anoxic depression of cardiac performance, suggesting that autonomic cardiac regulation may override putative cellular feedback mechanisms. In contrast, during anoxia at 5 degrees C, when autonomic cardiac control is severely blunted, the decrease of pH(i) from 7.66 to 7.12, 1.9-fold increase of effective P(i)(2-), and 6.4 kJ/mol decrease of dG/dxi from -53.8 to -47.4 kJ/mol were significantly correlated to the anoxic depression of cardiac performance. Our results provide the first evidence for a close, long-term coordination of functional cardiac changes with cellular energy status in a vertebrate, with a potential for autonomic control to override these immediate relationships.

Tribute to P. L. Lutz: cardiac performance and cardiovascular regulation during anoxia/hypoxia in freshwater turtles

Freshwater turtles overwintering in ice-covered ponds in North America may be exposed to prolonged anoxia, and survive this hostile environment by metabolic depression. Here, we review their cardiovascular function and regulation, with particular emphasis on the factors limiting cardiac performance. The pronounced anoxia tolerance of the turtle heart is based on the ability to match energy consumption with the low anaerobic ATP production during anoxia. Together with a well-developed temporal and spatial energy buffering by creatine kinase, this allows for cellular energy charge to remain high during anoxia. Furthermore, the turtle heart is well adapted to handle the adverse effects of free phosphate arising when phosphocreatine stores are used. Anoxia causes tenfold reductions in heart rate and blood flows that match the metabolic depression, and blood pressure is largely maintained through increased systemic vascular resistance. Depression of the heart rate is not driven by the autonomic nervous system and seems to arise from direct effects of oxygen lack and the associated hyperkalaemia and acidosis on the cardiac pacemaker. These intra- and extracellular changes also affect cardiac contractility, and both acidosis and hyperkalaemia severely depress cardiac contractility. However, increased levels of adrenaline and calcium may, at least partially, salvage cardiac function under prolonged periods of anoxia.

Effect of temperature and prolonged anoxia exposure on electrophysiological properties of the turtle (Trachemys scripta) heart

Cardiac activity of the turtle (Trachemys scripta) is greatly depressed with cold acclimation and anoxia. We examined what electrophysiological modifications accompany and perhaps facilitate this depression of cardiac activity. Turtles were first acclimated to 21 degrees C or 5 degrees C and held under either normoxic or anoxic (6 h at 21 degrees C; 14 days at 5 degrees C) conditions. We then measured cardiac action potentials (APs) using spontaneously contracting whole heart preparations and whole cell current densities of sarcolemmal ion channels using isolated ventricular myocytes under appropriate normoxic and anoxic conditions. Compared with 21 degrees C-acclimated turtles, 5 degrees C-acclimated turtles exhibited a less negative resting membrane potential (by 18-29 mV), a 4.7- to 6.8-fold slower AP upstroke rate, and a 4.2- to 4.9-fold greater AP duration. Correspondingly, peak densities of ventricular voltage-gated Na(+) (I(Na)) and L-type Ca(2+) currents and inward slope conductances of inward rectifier K(+) (I(K1)) channel current were approximately 1/7th (Q(10) = 3.4), 1/13th (Q(10) = 5.0), and one-half (Q(10) = 1.4) of those of 21 degrees C-acclimated ventricular myocytes, respectively. With anoxia at 21 degrees C, peak I(Na) density doubled and ventricular AP duration increased by 47%, a change proportional to the reported approximately 30% reduction of intrinsic heart rate. In contrast, with anoxia at 5 degrees C, ventricular AP characteristics were unaffected; of the ion currents investigated, only the inward conductance via I(K1) changed significantly (reduced by 46%). The present findings indicate that cold temperature, more so than prolonged anoxia, results in substantial modifications of cardiac APs and reduction of ventricular ion current densities. These changes likely prepare cardiac muscle for winter anoxia conditions.

Comparative shell buffering properties correlate with anoxia tolerance in freshwater turtles

Freshwater turtles as a group are more resistant to anoxia than other vertebrates, but some species, such as painted turtles, for reasons not fully understood, can remain anoxic at winter temperatures far longer than others. Because buffering of lactic acid by the shell of the painted turtle is crucial to its long-term anoxic survival, we have tested the hypothesis that previously described differences in anoxia tolerance of five species of North American freshwater turtles may be explained at least in part by differences in their shell composition and buffering capacity. All species tested have large mineralized shells. Shell comparisons included 1) total shell CO2concentration, 2) volume of titrated acid required to hold incubating shell powder at pH 7.0 for 3 h (an indication of buffer release from shell), and 3) lactate concentration of shell samples incubated to equilibrium in a standard lactate solution. For each measurement, the more anoxia-tolerant species (painted turtle, Chrysemys picta; snapping turtle, Chelydra serpentina) had higher values than the less anoxia-tolerant species (musk turtle, Sternotherus odoratus; map turtle, Graptemys geographica; red-eared slider, Trachemys scripta). We suggest that greater concentrations of accessible CO2(as carbonate or bicarbonate) in the more tolerant species enable these species, when acidotic, to release more buffer into the extracellular fluid and to take up more lactic acid into their shells. We conclude that the interspecific differences in shell composition and buffering can contribute to, but cannot explain fully, the variations observed in anoxia tolerance among freshwater turtles.

Effects of extracellular changes on spontaneous heart rate of normoxia-and anoxia-acclimated turtles (Trachemys scripta)

Heart rate (fH) of the anoxia-tolerant freshwater turtle (Trachemys scripta) during prolonged anoxia exposure is 2.5-to 5-times lower than the normoxic rate, but whether alterations in blood composition that accompany prolonged anoxia contribute to this bradycardia is unknown. We examined how temperature acclimation, oxygen deprivation,acidosis, hyperkalemia, hypercalcemia and adrenaline affect chronotropy in the turtle myocardium. We monitored spontaneous contraction rates of right-atrial preparations obtained from 21°C- and 5°C-acclimated turtles that had been exposed to either normoxia or anoxia (6 h at 21°C; 2 weeks at 5°C). Sequential exposures to saline solutions were designed to mimic, in a step-wise manner, the shift from a normoxic to anoxic extracellular condition (for normoxia-acclimated preparations) or the reverse (for anoxia-acclimated preparations). Our results clearly show that prolonged anoxia exposure re-sets the intrinsic fH of turtles at both temperatures, with reductions in intrinsic fH in the range of 25%–53% compared with normoxia. This intrinsic change would contribute to the bradycardia observed with prolonged anoxia. Further, we found negative chronotropic effects of extracellular anoxia, acidosis and hyperkalemia, and positive chronotropic effects of hypercalcemia and adrenaline. The exact nature of these extracellular effects depended, however,on the acclimation temperature and the prior exposure of the animal to anoxia. With normoxia-acclimated preparations at 21°C, combined anoxia and acidosis (pH reduced from ∼7.8 to ∼7.2) significantly reduced spontaneous fH by 22% and subsequent exposure to hyperkalemia (3.5 mmol l–1K+) further decreased fH. These negative chronotropic effects were ameliorated by increasing the adrenaline concentration from the tonic level of 1 nmol l–1 to 60 nmol l–1. However, in anoxia-acclimated preparations at 21°C, anoxia alone inhibited fH (by ∼30%). This negative chronotropic effect was counteracted by both hypercalcemia (6 mmol l–1Ca2+) and adrenaline (60 nmol l–1). At 5°C,only the combination of anoxia, acidosis (pH reduced from ∼8.0 to∼7.5) and hyperkalemia (3.5 mmol l–1 K+)significantly reduced spontaneous fH (by 23%) with preparations from normoxia-acclimated turtles. This negative chronotropic effect was fully reversed by hypercalcemia (10 mmol l–1Ca2+). By contrast, spontaneous fH of anoxia-acclimated preparations at 5°C was not affected by any of the extracellular changes. We conclude that prior temperature and anoxia experiences are central to determining fH during prolonged anoxia in Trachemys scripta both as a result of the re-setting of pacemaker rhythm and through the potential influence of extracellular changes.

Effects of temperature and anoxia upon the performance ofin situperfused trout hearts

Rainbow trout (Oncorhynchus mykiss) are likely to experience acute changes in both temperature and oxygen availability and, like many other organisms, exhibit behavioural selection of low temperatures during hypoxia that acts to reduce metabolism and alleviate the demands on the heart. To investigate whether low temperature protects cardiac performance during anoxia, we studied the effects of an acute temperature change, from 10°C to either 5°C, 15°C or 18°C, upon the performance of in situ perfused trout hearts before, during and after exposure to 20 min of anoxia. Routine cardiac workload mimicked in vivo conditions at the given temperatures, and the effects of anoxia were evaluated as maximal cardiac performance before and after 20 min of anoxic perfusion. Functional data were related to maximal activities of glycolytic enzymes and energetic status of the heart at the termination of the experiment.

At high oxygenation, maximum cardiac output and power output increased with temperature (Q10 values of 1.8 and 2.1, respectively) as a result of increased heart rate. Hypoxia tolerance was inversely related to temperature. At 5°C, the hearts maintained routine cardiac output throughout the 20 min period of anoxia, and maximal cardiac performance was fully restored following reoxygenation. By contrast, cardiac function failed sooner during anoxia as temperature was increased and maximal performance after reoxygenation was reduced by 25%, 35% and 55% at 10°C, 15°C and 18°C, respectively. Increased functional impairment following anoxic exposure at elevated temperature occurred even though both cardiac glycolytic enzyme activity and the rate of lactate production were increased proportionally with cardiac work. Nonetheless, there was no indication of myocardial necrosis, as biochemical and energetic parameters were generally unaffected by anoxia.

Expression of heat shock proteins in turtle and mammal hearts: relationship to anoxia tolerance

Heat shock proteins (HSPs) may play a cardioprotective role during hypoxia or ischemia. We hypothesized that cardiac tissue from hypoxia-tolerant animals might have high levels of specific HSPs. We measured myocardial HSP60 and HSP72/73 in painted and softshell turtles during normoxia and anoxia (12 h) and after recovery (12 or 24 h). We also measured myocardial HSPs in normoxic rats and rabbits. During normoxia, hearts from the most highly anoxia-tolerant species, the painted turtle, expressed the highest levels of HSP60 (22.6+/-2.0 mg/g total protein) followed by softshells (11.5+/-0.8 mg/g), rabbits (6.8+/-0.9 mg/g), and rats (4.5+/-0.5 mg/g). HSP72/73 levels, however, were not significantly different. HSP60 levels in hearts from both painted and softshell turtles did not deviate significantly from control values after either 12 h of anoxia or 12 or 24 h of recovery. The pattern of changes observed in HSP72/73 was quite different in the two turtle species. In painted turtles anoxia induced a significant increase in myocardial HSP72/73 (from 2.8+/-0.1 mg/g normoxic to 3.9+/-0.2 mg/g anoxic, P<0.05). By 12 h of recovery, HSP72/73 had returned to control levels (2.7+/-0.1 mg/g) and remained there through 24 h (2.6+/-0.2 mg/g). In softshell turtles, HSP72/73 decreased significantly after 12 h of anoxia (from 2.4+/-0.4 mg/g normoxic to 1.3+/-0.2 mg/g anoxic, P<0.05). HSP72/73 levels were still slightly below control after 12 h of recovery (2.1+/-0.1 mg/g) and then rose to significantly above control after 24 h of recovery (4.1+/-0.7 mg/g, P<0.05). We also conclude that anoxia-tolerant and anoxia-sensitive turtles exhibit different patterns of myocardial HSP changes during anoxia and recovery. Whether these changes correlate with their relative degrees of anoxia tolerance remains to be determined.

The effect of prolonged anoxia at 3 degrees C on tissue high energy phosphates and phosphodiesters in turtles: a 31P-NMR study

Selected tissues (skeletal muscle, heart ventrical, and liver), sampled from turtles (Chrysemys picta bellii) at 3 degrees C either under normoxic conditions or after 12 weeks of anoxic submergence were quantitatively analysed for intracellular pH and phosphorus metabolites using 31P-NMR. Plasma was tested for osmolality and for the concentrations of lactate, calcium, and magnesium to confirm anoxic stress. We hypothesized that, in the anoxic animals, tissue ATP levels would be maintained and that the increased osmolality of the body fluids of anoxic turtles would be accounted for by a corresponding increase in the concentrations of phosphodiesters. The responses observed differed among the three tissues. In muscle, ATP was unchanged by anoxia but phosphocreatine was reduced by 80%; in heart, both ATP and phosphocreatine fell by 35-40%. The reduction in phosphocreatine in heart tissue at 3 degrees C was similar to that observed in isolated, perfused working hearts from turtles maintained at 20 degrees C but no decrease in ATP occurred in the latter tissues. In liver, although analyses of several specimens were confounded by line-broadening, neither ATP nor phosphocreatine was detectable in anoxic samples. Phosphosdiesters were detected in amounts sufficient to account for 30% of normoxic cell osmotic concentration in heart and 11% and 12% in liver and muscle, respectively. The phosphodiester levels did not change in anoxia. Heart ventricular phosphodiester levels in turtles at 3 degrees C were significantly higher than those determined for whole hearts from turtles at 20 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)