Time Domains of Hypoxia Adaptation-Elephant Seals Stand Out Among Divers

Carbon Monoxide as a Potential Therapeutic Agent: A Molecular Analysis of Its Safety Profiles

Carbon monoxide (CO) is endogenously produced in mammals, with blood concentrations in the high micromolar range in the hemoglobin-bound form. Further, CO has shown therapeutic effects in various animal models. Despite its reputation as a poisonous gas at high concentrations, we show that CO should have a wide enough safety margin for therapeutic applications. The analysis considers a large number of factors including levels of endogenous CO, its safety margin in comparison to commonly encountered biomolecules or drugs, anticipated enhanced safety profiles when delivered via a noninhalation mode, and the large amount of safety data from human clinical trials. It should be emphasized that having a wide enough safety margin for therapeutic use does not mean that it is benign or safe to the general public, even at low doses. We defer the latter to public health experts. Importantly, this Perspective is written for drug discovery professionals and not the general public.

Blood oxygen transport and depletion in diving emperor penguins

Oxygen store management underlies dive performance and is dependent on the slow heart rate and peripheral vasoconstriction of the dive response to control tissue blood flow and oxygen uptake. Prior research has revealed two major patterns of muscle myoglobin saturation profiles during dives of emperor penguins. In Type A profiles, myoglobin desaturated rapidly, consistent with minimal muscle blood flow and low tissue oxygen uptake. Type B profiles, with fluctuating and slower declines in myoglobin saturation, were consistent with variable tissue blood flow patterns and tissue oxygen uptake during dives. We examined arterial and venous blood oxygen profiles to evaluate blood oxygen extraction and found two primary patterns of venous hemoglobin desaturation that complemented corresponding myoglobin saturation profiles. Type A venous profiles had a hemoglobin saturation that (a) increased/plateaued for most of a dive's duration, (b) only declined during the latter stages of ascent, and (c) often became arterialized [arterio-venous (a-v) shunting]. In Type B venous profiles, variable but progressive hemoglobin desaturation profiles were interrupted by inflections in the profile that were consistent with fluctuating tissue blood flow and oxygen uptake. End-of-dive saturation of arterial and Type A venous hemoglobin saturation profiles were not significantly different, but did differ from those of Type B venous profiles. These findings provide further support that the dive response of emperor penguins is a spectrum of cardiac and vascular components (including a-v shunting) that are dependent on the nature and demands of a given dive and even of a given segment of a dive.

Field physiology in the aquatic realm: ecological energetics and diving behavior provide context for elucidating patterns and deviations

Comparative physiology has developed a rich understanding of the physiological adaptations of organisms, from microbes to megafauna. Despite extreme differences in size and a diversity of habitats, general patterns are observed in their physiological adaptations. Yet, many organisms deviate from the general patterns, providing an opportunity to understand the importance of ecology in determining the evolution of unusual adaptations. Aquatic air-breathing vertebrates provide unique study systems in which the interplay between ecology, physiology and behavior is most evident. They must perform breath-hold dives to obtain food underwater, which imposes a physiological constraint on their foraging time as they must resurface to breathe. This separation of two critical resources has led researchers to investigate these organisms' physiological adaptations and trade-offs. Addressing such questions on large marine animals is best done in the field, given the difficulty of replicating the environment of these animals in the lab. This Review examines the long history of research on diving physiology and behavior. We show how innovative technology and the careful selection of research animals have provided a holistic understanding of diving mammals' physiology, behavior and ecology. We explore the role of the aerobic diving limit, body size, oxygen stores, prey distribution and metabolism. We then identify gaps in our knowledge and suggest areas for future research, pointing out how this research will help conserve these unique animals.

Effects of disease on foraging behaviour and success in an individual free-ranging northern elephant seal

Evaluating consequences of stressors on vital rates in marine mammals is of considerable interest to scientific and regulatory bodies. Many of these species face numerous anthropogenic and environmental disturbances. Despite its importance as a critical form of mortality, little is known about disease progression in air-breathing marine megafauna at sea. We examined the movement, diving, foraging behaviour and physiological state of an adult female northern elephant seal (Mirounga angustirostris) who suffered from an infection while at sea. Comparing her to healthy individuals, we identified abnormal behavioural patterns from high-resolution biologging instruments that are likely indicators of diseased and deteriorating condition. We observed continuous extended (3-30 minutes) surface intervals coinciding with almost no foraging attempts (jaw motion) during 2 weeks of acute illness early in her post-breeding foraging trip. Elephant seals typically spend ~ 2 minutes at the surface. There were less frequent but highly extended (30-200 minutes) surface periods across the remainder of the trip. Dive duration declined throughout the trip rather than increasing. This seal returned in the poorest body condition recorded for an adult female elephant seal (18.3% adipose tissue; post-breeding trip average is 30.4%). She was immunocompromised at the end of her foraging trip and has not been seen since that moulting season. The timing and severity of the illness, which began during the end of the energy-intensive lactation fast, forced this animal over a tipping point from which she could not recover. Additional physiological constraints to foraging, including thermoregulation and oxygen consumption, likely exacerbated her already poor condition. These findings improve our understanding of illness in free-ranging air-breathing marine megafauna, demonstrate the vulnerability of individuals at critical points in their life history, highlight the importance of considering individual health when interpreting biologging data and could help differentiate between malnutrition and other causes of at-sea mortality from transmitted data.

Dolphin leukocytes exhibit an attenuated cytokine response and increase heme oxygenase activity upon exposure to lipopolysaccharides

Cetaceans exhibit physiological adaptations that allowed the transition to aquatic life, including a robust antioxidant defense system that prevents injury from repeated exposure to ischemia/reperfusion events associated with breath-hold diving. The signaling cascades that characterize ischemic inflammation in humans are well characterized. In contrast, cetaceans' molecular and biochemical mechanisms that confer tolerance to inflammatory events are poorly understood. Heme oxygenase (HO) is a cytoprotective protein with anti-inflammatory properties. HO catalyzes the first step in the oxidative degradation of heme. The inducible HO-1 isoform is regulated by various stimuli, including hypoxia, oxidant stress, and inflammatory cytokines. The objective of this study was to compare the response of HO-1 and cytokines to a proinflammatory challenge in leukocytes isolated from humans and bottlenose dolphins (Tursiops truncatus). We measured changes in HO activity and expression, and abundance and expression of interleukin 1 beta (IL-1β), interleukin 6 (IL-6), tumor necrosis factor-alpha (TNF-α), and heme oxygenase 1 (HMOX1) in leukocytes treated with lipopolysaccharide (LPS) for 24 and 48 h. HO activity increased (p < 0.05) in dolphin (48 h) but not human cells. TNF-α expression increased in human (24 h, 48 h), but not dolphin cells following LPS stimulation. LPS-induced cytokine expression was lower in dolphin than in human leukocytes, suggesting a blunted cytokine response in bottlenose dolphin leukocytes treated with LPS. Results suggest species-specific regulation of inflammatory cytokines in leukocytes treated with LPS, which may lead to differential responses to a pro-inflammatory challenge between marine and terrestrial mammals.

Time Domains of Hypoxia Responses and -Omics Insights

The ability to respond rapidly to changes in oxygen tension is critical for many forms of life. Challenges to oxygen homeostasis, specifically in the contexts of evolutionary biology and biomedicine, provide important insights into mechanisms of hypoxia adaptation and tolerance. Here we synthesize findings across varying time domains of hypoxia in terms of oxygen delivery, ranging from early animal to modern human evolution and examine the potential impacts of environmental and clinical challenges through emerging multi-omics approaches. We discuss how diverse animal species have adapted to hypoxic environments, how humans vary in their responses to hypoxia (i.e., in the context of high-altitude exposure, cardiopulmonary disease, and sleep apnea), and how findings from each of these fields inform the other and lead to promising new directions in basic and clinical hypoxia research.

What to do with low O2: Redox adaptations in vertebrates native to hypoxic environments

Reactive oxygen species (ROS) are important cellular signalling molecules but sudden changes in redox balance can be deleterious to cells and lethal to the whole organism. ROS production is inherently linked to environmental oxygen availability and many species live in variable oxygen environments that can range in both severity and duration of hypoxic exposure. Given the importance of redox homeostasis to cell and animal viability, it is not surprising that early studies in species adapted to various hypoxic niches have revealed diverse strategies to limit or mitigate deleterious ROS changes. Although research in this area is in its infancy, patterns are beginning to emerge in the suites of adaptations to different hypoxic environments. This review focuses on redox adaptations (i.e., modifications of ROS production and scavenging, and mitigation of oxidative damage) in hypoxia-tolerant vertebrates across a range of hypoxic environments. In general, evidence suggests that animals adapted to chronic lifelong hypoxia are in homeostasis, and do not encounter major oxidative challenges in their homeostatic environment, whereas animals exposed to seasonal chronic anoxia or hypoxia rapidly downregulate redox balance to match a hypometabolic state and employ robust scavenging pathways during seasonal reoxygenation. Conversely, animals adapted to intermittent hypoxia exposure face the greatest degree of ROS imbalance and likely exhibit enhanced ROS-mitigation strategies. Although some progress has been made, research in this field is patchy and further elucidation of mechanisms that are protective against environmental redox challenges is imperative for a more holistic understanding of how animals survive hypoxic environments.

Ontogeny of Carbon Monoxide-Related Gene Expression in a Deep-Diving Marine Mammal

Marine mammals such as northern elephant seals (NES) routinely experience hypoxemia and ischemia-reperfusion events to many tissues during deep dives with no apparent adverse effects. Adaptations to diving include increased antioxidants and elevated oxygen storage capacity associated with high hemoprotein content in blood and muscle. The natural turnover of heme by heme oxygenase enzymes (encoded by HMOX1 and HMOX2) produces endogenous carbon monoxide (CO), which is present at high levels in NES blood and has been shown to have cytoprotective effects in laboratory systems exposed to hypoxia. To understand how pathways associated with endogenous CO production and signaling change across ontogeny in diving mammals, we measured muscle CO and baseline expression of 17 CO-related genes in skeletal muscle and whole blood of three age classes of NES. Muscle CO levels approached those of animals exposed to high exogenous CO, increased with age, and were significantly correlated with gene expression levels. Muscle expression of genes associated with CO production and antioxidant defenses (HMOX1, BVR, GPX3, PRDX1) increased with age and was highest in adult females, while that of genes associated with protection from lipid peroxidation (GPX4, PRDX6, PRDX1, SIRT1) was highest in adult males. In contrast, muscle expression of mitochondrial biogenesis regulators (PGC1A, ESRRA, ESRRG) was highest in pups, while genes associated with inflammation (HMOX2, NRF2, IL1B) did not vary with age or sex. Blood expression of genes involved in regulation of inflammation (IL1B, NRF2, BVR, IL10) was highest in pups, while HMOX1, HMOX2 and pro-inflammatory markers (TLR4, CCL4, PRDX1, TNFA) did not vary with age. We propose that ontogenetic upregulation of baseline HMOX1 expression in skeletal muscle of NES may, in part, underlie increases in CO levels and expression of genes encoding antioxidant enzymes. HMOX2, in turn, may play a role in regulating inflammation related to ischemia and reperfusion in muscle and circulating immune cells. Our data suggest putative ontogenetic mechanisms that may enable phocid pups to transition to a deep-diving lifestyle, including high baseline expression of genes associated with mitochondrial biogenesis and immune system activation during postnatal development and increased expression of genes associated with protection from lipid peroxidation in adulthood.

In silico Characterization of the Heme Oxygenase 1 From Bottlenose Dolphin (Tursiops truncatus): Evidence of Changes in the Active Site and Purifying Selection

Cetacea is a clade well-adapted to the aquatic lifestyle, with diverse adaptations and physiological responses, as well as a robust antioxidant defense system. Serious injuries caused by boats and fishing nets are common in bottlenose dolphins (Tursiops truncatus); however, these animals do not show signs of serious infections. Evidence suggests an adaptive response to tissue damage and associated infections in cetaceans. Heme oxygenase (HO) is a cytoprotective protein that participates in the anti-inflammatory response. HO catalyzes the first step in the oxidative degradation of the heme group. Various stimuli, including inflammatory mediators, regulate the inducible HO-1 isoform. This study aims to characterize HO-1 of the bottlenose dolphin in silico and compare its structure to the terrestrial mammal protein. Upstream HO-1 sequence of the bottlenose dolphin was obtained from NCBI and Ensemble databases, and the gene structure was determined using bioinformatics tools. Five exons and four introns were identified, and proximal regulatory elements were detected in the upstream region. The presence of 10 α-helices, three 310 helices, the heme group lodged between the proximal and distal helices, and a histidine-25 in the proximal helix serving as a ligand to the heme group were inferred for T. truncatus. Amino acid sequence alignment suggests HO-1 is a conserved protein. The HO-1 "fingerprint" and histidine-25 appear to be fully conserved among all species analyzed. Evidence of positive selection within an α-helix configuration without changes in protein configuration and evidence of purifying selection were found, indicating evolutionary conservation of the coding sequence structure.

An engineering perspective on the development and evolution of implantable cardiac monitors in free-living animals

The latest technologies associated with implantable physiological monitoring devices can record multiple channels of data (including: heart rates and rhythms, activity, temperature, impedance and posture), and coupled with powerful software applications, have provided novel insights into the physiology of animals in the wild. This perspective details past challenges and lessons learned from the uses and developments of implanted biologgers designed for human clinical application in our research on free-ranging American black bears (Ursus americanus). In addition, we reference other research by colleagues and collaborators who have leveraged these devices in their work, including: brown bears (Ursus arctos), grey wolves (Canis lupus), moose (Alces alces), maned wolves (Chrysocyon brachyurus) and southern elephant seals (Mirounga leonina). We also discuss the potentials for applications of such devices across a range of other species. To date, the devices described have been used in fifteen different wild species, with publications pending in many instances. We have focused our physiological research on the analyses of heart rates and rhythms and thus special attention will be paid to this topic. We then discuss some major expected step changes such as improvements in sensing algorithms, data storage, and the incorporation of next-generation short-range wireless telemetry. The latter provides new avenues for data transfer, and when combined with cloud-based computing, it not only provides means for big data storage but also the ability to readily leverage high-performance computing platforms using artificial intelligence and machine learning algorithms. These advances will dramatically increase both data quantity and quality and will facilitate the development of automated recognition of extreme physiological events or key behaviours of interest in a broad array of environments, thus further aiding wildlife monitoring and management.

This article is part of the theme issue ‘Measuring physiology in free-living animals (Part I)’.

Forced into an ecological corner: Round-the-clock deep foraging on small prey by elephant seals

Small mesopelagic fishes dominate the world's total fish biomass, yet their ecological importance as prey for large marine animals is poorly understood. To reveal the little-known ecosystem dynamics, we identified prey, measured feeding events, and quantified the daily energy balance of 48 deep-diving elephant seals throughout their oceanic migrations by leveraging innovative technologies: animal-borne smart accelerometers and video cameras. Seals only attained positive energy balance after feeding 1000 to 2000 times per day on small fishes, which required continuous deep diving (80 to 100% of each day). Interspecies allometry suggests that female elephant seals have exceptional diving abilities relative to their body size, enabling them to exploit a unique foraging niche on small but abundant mesopelagic fish. This unique foraging niche requires extreme round-the-clock deep diving, limiting the behavioral plasticity of elephant seals to a changing mesopelagic ecosystem.

Evolved increases in hemoglobin-oxygen affinity and the Bohr effect coincided with the aquatic specialization of penguins

In diving birds like penguins, physiologic considerations suggest that increased hemoglobin (Hb)-O₂ affinity may improve pulmonary O₂ extraction and enhance dive capacity. We integrated experimental tests on whole-blood and native Hbs of penguins with protein engineering experiments on reconstructed ancestral Hbs. The experiments involving ancestral protein resurrection enabled us to test for evolved changes in Hb function in the stem lineage of penguins after divergence from their closest nondiving relatives. We demonstrate that penguins evolved an increased Hb-O₂ affinity in conjunction with a greatly augmented Bohr effect (i.e., reduction in Hb-O₂ affinity at low pH) that should maximize pulmonary O₂ extraction without compromising O₂ delivery at systemic capillaries.

Dive capacities of air-breathing vertebrates are dictated by onboard O₂ stores, suggesting that physiologic specialization of diving birds such as penguins may have involved adaptive changes in convective O₂ transport. It has been hypothesized that increased hemoglobin (Hb)-O₂ affinity improves pulmonary O₂ extraction and enhances the capacity for breath-hold diving. To investigate evolved changes in Hb function associated with the aquatic specialization of penguins, we integrated comparative measurements of whole-blood and purified native Hb with protein engineering experiments based on site-directed mutagenesis. We reconstructed and resurrected ancestral Hb representing the common ancestor of penguins and the more ancient ancestor shared by penguins and their closest nondiving relatives (order Procellariiformes, which includes albatrosses, shearwaters, petrels, and storm petrels). These two ancestors bracket the phylogenetic interval in which penguin-specific changes in Hb function would have evolved. The experiments revealed that penguins evolved a derived increase in Hb-O₂ affinity and a greatly augmented Bohr effect (i.e., reduced Hb-O₂ affinity at low pH). Although an increased Hb-O₂ affinity reduces the gradient for O₂ diffusion from systemic capillaries to metabolizing cells, this can be compensated by a concomitant enhancement of the Bohr effect, thereby promoting O₂ unloading in acidified tissues. We suggest that the evolved increase in Hb-O₂ affinity in combination with the augmented Bohr effect maximizes both O₂ extraction from the lungs and O₂ unloading from the blood, allowing penguins to fully utilize their onboard O₂ stores and maximize underwater foraging time.

Physiological resiliency in diving mammals: Insights on hypoxia protection using the Krogh principle to understand COVID-19 symptoms

Sequential diving by wild marine mammals results in a lifetime of rapid physiological transitions between lung collapse-reinflation, bradycardia-tachycardia, vasoconstriction-vasodilation, and oxygen store depletion-restoration. The result is a cycle of normoxia and hypoxia in which blood oxygen partial pressures can decline to <20–30 mmHg during a dive, a level considered injurious to oxygen-dependent human tissues (i.e., brain, heart). Safeguards in the form of enhanced on-board oxygen stores, selective oxygen transport, and unique tissue buffering capacities enable marine-adapted mammals to maintain physiological homeostasis and energy metabolism even when breathing and pulmonary gas exchange cease. This stands in stark contrast to the vulnerability of oxygen-sensitive tissues in humans that may undergo irreversible damage within minutes of ischemia and tissue hypoxia. Recently, these differences in protection against hypoxic injury have become evident in the systemic, multi-organ physiological failure during COVID-19 infection in humans. Prolonged recoveries in some patients have led to delays in the return to normal exercise levels and cognitive function even months later. Rather than a single solution to this problem, we find that marine mammals rely on a unique, integrative assemblage of protections to avoid the deleterious impacts of hypoxia on tissues. Built across evolutionary time, these solutions provide a natural template for identifying the potential for tissue damage when oxygen is lacking, and for guiding management decisions to support oxygen-deprived tissues in other mammalian species, including humans, challenged by hypoxia.

Adaptive Potential of the Heme Oxygenase/Carbon Monoxide Pathway During Hypoxia

Heme oxygenase (HO) enzymes catalyze heme into biliverdin, releasing carbon monoxide (CO) and iron into circulation. These byproducts of heme degradation can have potent cytoprotective effects in the face of stressors such as hypoxia and ischemia-reperfusion events. The potential for exogenous use of CO as a therapeutic agent has received increasing attention throughout the past few decades. Further, HO and CO are noted as putatively adaptive in diving mammals and certain high-altitude human populations that are frequently exposed to hypoxia and/or ischemia-reperfusion events, suggesting that HO and endogenous CO afford an evolutionary advantage for hypoxia tolerance and are critical in cell survival and injury avoidance. Our goal is to describe the importance of examining HO and CO in several systems, the physiological links, and the genetic factors that underlie variation in the HO/CO pathway. Finally, we emphasize the ways in which evolutionary perspectives may enhance our understanding of the HO/CO pathway in the context of diverse clinical settings.

The Mammalian Diving Response: Inroads to Its Neural Control

The mammalian diving response (DR) is a remarkable behavior that was first formally studied by Laurence Irving and Per Scholander in the late 1930s. The DR is called such because it is most prominent in marine mammals such as seals, whales, and dolphins, but nevertheless is found in all mammals studied. It consists generally of breathing cessation (apnea), a dramatic slowing of heart rate (bradycardia), and an increase in peripheral vasoconstriction. The DR is thought to conserve vital oxygen stores and thus maintain life by directing perfusion to the two organs most essential for life-the heart and the brain. The DR is important, not only for its dramatic power over autonomic function, but also because it alters normal homeostatic reflexes such as the baroreceptor reflex and respiratory chemoreceptor reflex. The neurons driving the reflex circuits for the DR are contained within the medulla and spinal cord since the response remains after the brainstem transection at the pontomedullary junction. Neuroanatomical and physiological data suggesting brainstem areas important for the apnea, bradycardia, and peripheral vasoconstriction induced by underwater submersion are reviewed. Defining the brainstem circuit for the DR may open broad avenues for understanding the mechanisms of suprabulbar control of autonomic function in general, as well as implicate its role in some clinical states. Knowledge of the proposed diving circuit should facilitate studies on elite human divers performing breath-holding dives as well as investigations on sudden infant death syndrome (SIDS), stroke, migraine headache, and arrhythmias. We have speculated that the DR is the most powerful autonomic reflex known.