Novel mechanism for high-altitude adaptation in hemoglobin of the Andean frog Telmatobius peruvianus

Selection and Gene Duplication Associated With High-Elevation Diversification in Pristimantis, the Largest Terrestrial Vertebrate Genus

The genus Pristimantis diversified in the tropical Andes mountains and is the most speciose genus of terrestrial vertebrates. Pristimantis are notable among frogs in that they thrive at high elevations (>2,000 m) and are direct developers without a tadpole stage. Despite their ecological significance, little is known about the genetic and physiological traits enabling their success. We conducted transcriptomic analysis on seven Pristimantis species sampled across elevations in the Ecuadorean Andes to explore three hypotheses for their success: (i) unique genes are under selection relative to all other frogs, (ii) common selection occurs across all direct developers, or (iii) common selection occurs across all high-elevation frog clades. Comparative analysis with 34 frog species revealed unique positive selection in Pristimantis genes related to aerobic respiration, hemostasis, signaling, cellular transportation of proteins and ions, and immunity. Additionally, we detected positive selection across all direct developers for genes associated with oxygenase activity and metal ion binding. While many genes under selection in Pristimantis were not positively selected in other high-elevation frog species, we identified some shared genes and pathways linked to lipid metabolism, innate immunity, and cellular redox processes. We observed more positive selection in duplicated- versus single-copy genes, while relaxed purifying selection was prevalent in single-copy genes. Notably, copy number of an innate immunity complement gene was positively correlated with Pristimantis species elevation. Our findings contribute novel insights into the genetic basis of adaptation in Pristimantis and provide a foundation for future studies on the evolutionary mechanisms leading to direct development and coping with high elevations.

Gluconeogenesis in frogs during cooling and dehydration exposure: new insights into tissue plasticity of the gluconeogenic pathway dependent on abiotic factors

Anurans undergo significant physiological changes when exposed to environmental stressors such as low temperatures and humidity. Energy metabolism and substrate management play a crucial role in their survival success. Therefore, understanding the role of the gluconeogenic pathway and demonstrating its existence in amphibians is essential. In this study, we exposed the subtropical frog Boana pulchella to cooling (-2.5°C for 24 h) and dehydration conditions (40% of body water loss), followed by recovery (24 h), and assessed gluconeogenesis activity from alanine, lactate, glycerol and glutamine in the liver, muscle and kidney. We report for the first time that gluconeogenesis activity by 14C-alanine and 14C-lactate conversion to glucose occurs in the muscle tissue of frogs, and this tissue activity is influenced by environmental conditions. Against the control group, liver gluconeogenesis from 14C-lactate and 14C-glycerol was lower during cooling and recovery (P<0.01), and gluconeogenesis from 14C-glutamine in the kidneys was also lower during cooling (P<0.05). In dehydration exposure, gluconeogenesis from 14C-lactate in the liver was lower during recovery, and that from 14C-alanine in the muscle was lower during dehydration (P<0.05). Moreover, we observed that gluconeogenesis activity and substrate preference respond differently to cold and dehydration. These findings highlight tissue-specific plasticity dependent on the nature of the encountered stressor, offering valuable insights for future studies exploring this plasticity, elucidating the importance of the gluconeogenic pathway and characterizing it in anuran physiology.

Convergent High O2 Affinity but Distinct ATP-Mediated Allosteric Regulation of Hemoglobins in Oviparous and Viviparous Eremias Lizards from the Qinghai-Tibet Plateau

The functional adaptation and underlying molecular mechanisms of hemoglobins (Hbs) have primarily concentrated on mammals and birds, with few reports on reptiles. This study aimed to investigate the convergent and species-specific high-altitude adaptation mechanisms of Hbs in two Eremias lizards from the Qinghai-Tibet Plateau. The Hbs of high-altitude E. argus and E. multiocellata were characterized by significantly high overall and intrinsic Hb-O2 affinity compared to their low-altitude populations. Despite the similarly low Cl- sensitivities, the Hbs of high-altitude E. argus exhibited higher ATP sensitivity and ATP-dependent Bohr effects than that of E. multiocellata, which could facilitate O2 unloading in respiring tissues. Eremias lizards Hbs exhibited similarly low temperature sensitivities and relatively high Bohr effects at lower temperatures, which could help to stably deliver and release O2 to cold extremities at low temperatures. The oxygenation properties of Hbs in high-altitude populations might be attributed to varying ratios of β2/β1 globin and substitutions on the β2-type globin. Notably, the Asn12Ala in lowland E. argus could cause localized destabilization of the E-helix in the tetrameric Hb by elimination of hydrogen bonds, thereby resulting in its lowest O2 affinity. This study provides a valuable reference for the high-altitude adaptation mechanisms of hemoglobins in reptiles.

Metabolic changes in the subtropical frog Boana pulchella during experimental cooling and recovery conditions

Frogs have developed biochemical and physiological adaptations to occupy diverse ecological niches on Earth successfully. Survival in frozen states is a fascinating strategy made possible by evolving adaptations to produce cryoprotectant solutes. The hylid frog Boana pulchella thrives in South American regions with cold climates, remaining active while enduring sporadic subzero temperatures during winter. The species' metabolic changes during subzero exposure remain unclear. Therefore, we exposed B. pulchella to cooling and recovery, assessing plasma and tissue metabolite changes. Cooling significantly reduced urea concentrations in plasma (P = 0.033), muscle (P = 0.001), heart (P = 0.009), and brain (P = 0.041) compared to acclimation. Liver glucose oxidation and glycogen synthesis were lower in cooling and recovery than in acclimation (P < 0.0001 and P = 0.0117, respectively). Muscle glycogen synthesis was lower in recovery than acclimation (P = 0.0249). These results demonstrate B. pulchella's physiological strategies during subzero exposure, likely reflecting species-specific evolutionary adaptations for brief subzero exposures that enable winter survival in its natural habitat.

Elevation and blood traits in the mesquite lizard: Are patterns repeatable between mountains?

Ecogeographical patterns describe predictable variation in phenotypic traits between ecological communities. For example, high-altitude animals are expected to show elevated hematological values as an adaptation to the lower oxygen pressure. Mountains act like ecological islands and therefore are considered natural laboratories. However, the majority of ecophysiological studies on blood traits lack replication that would allow us to infer if the pattern reported is a local event or whether it is a widespread pattern resulting from larger-scale ecological processes. In lizards, in fact, the increase of hematological values at high altitudes has received mixed support. Here, for the first time, we compare blood traits in lizards along elevational gradients with replication. We tested the repeatability of blood traits in mesquite lizards between different elevations in three different mountains from the Trans-Mexican Volcanic Belt. We measured hematocrit, hemoglobin concentration, mean corpuscular hemoglobin concentration, and erythrocyte size in blood samples of low, medium, and high-elevation lizards. We obtained similar elevational patterns between mountains, but the blood traits differed among mountains. Middle-altitude populations had greater oxygen-carrying capacity than lizards from low and high altitudes. The differences found between mountain systems could be the result of phenotypic plasticity or genetic differentiation as a consequence of abiotic factors not considered.

Comparison of hematological traits and oxygenation properties of hemoglobins from highland and lowland Asiatic toad (Bufo gargarizans)

The Asiatic toad (Bufo gargarizans) belonging to the family of Bufonidae (Anura: Amphibia) is successfully residing on the Qinghai-Tibetan Plateau (QTP). To investigate whether the oxygen delivery undergoes adaptive adjustments to high-altitude environments in Asian toads inhabiting the QTP (Zoige County, 3446 m), choosing low-altitude populations (Chengdu City, 500 m) as control, we measured hematological traits, O₂ affinities of whole blood, Hb-O₂ affinities of purified Hbs, their sensitivities to temperature, and allosteric effectors (H⁺, Cl^- and ATP). Our results showed that high-altitude Asiatic toads possessed significantly increased hemoglobin concentration, hematocrit, and red blood cell count, but significantly decreased erythrocyte volume compared with low-altitude toads. The whole blood and purified Hbs of high-altitude Asiatic toads both exhibited significantly higher O₂ affinities compared with low-altitude toads. Substantially increased intrinsic Hb-O₂ affinities of high-altitude Asiatic toads Hbs are likely to be the main reason for its elevated Hb-O₂ affinities given the anionic cofactor sensitivities of high- and low-altitude toads were similar. The Hbs of high-altitude toads were also characterized by distinctly strong Bohr effects at the low temperature and low-temperature sensitivities. The adaptive adjustments of hematological traits could enhance the blood-O₂ carrying capacity of high-altitude Asiatic toads. The increased Hb-O₂ affinities could safeguard the pulmonary O₂ uploading under hypoxia. The strong Bohr effects at the low temperature could help the release of O₂ in metabolic tissues and cold limbs, while low-temperature sensitivity could minimize the effect of temperature fluctuation on the Hb-O₂ affinity.

Effect of NH2-terminal acetylation on the oxygenation properties of vertebrate haemoglobin

In vertebrate haemoglobin (Hb), the NH2-terminal residues of the α- and β-chain subunits are thought to play an important role in the allosteric binding of protons (Bohr effect), CO2 (as carbamino derivatives), chloride ions, and organic phosphates. Accordingly, acetylation of the α- and/or β-chain NH2-termini may have significant effects on the oxygenation properties of Hb. Here we investigate the effect of NH2-terminal acetylation by using a newly developed expression plasmid system that enables us to compare recombinantly expressed Hbs that are structurally identical except for the presence or absence of NH2-terminal acetyl groups. Experiments with native and recombinant Hbs of representative vertebrates reveal that NH2-terminal acetylation does not impair the Bohr effect, nor does it significantly diminish responsiveness to allosteric cofactors, such as chloride ions or organic phosphates. These results suggest that observed variation in the oxygenation properties of vertebrate Hbs is principally explained by amino acid divergence in the constituent globin chains rather than post-translational modifications of the globin chain NH2-termini.

Oxygenation properties and underlying molecular mechanisms of hemoglobins in plateau zokor (Eospalax baileyi)

The plateau zokor (Eospalax baileyi) is a species of subterranean rodent endemic to the Tibetan Plateau. It is well adapted to the cold and hypoxic and hypercapnic burrow. To study the oxygenation properties of plateau zokor hemoglobins (Hbs), we measured intrinsic Hb-O₂ affinities and their sensitivities to pH (Bohr effect); CO₂; Cl^-, 2,3-diphosphoglycerate (DPG); and temperature using purified Hbs from zokor and mouse. The optimal deoxyHb model of plateau zokor was constructed and used to study its structural characteristics by molecular dynamics simulations. O₂ binding results revealed that plateau zokor Hbs exhibit remarkably high intrinsic Hb-O₂ affinity, low CO₂ effects compared with human and the relatively low anion allosteric effector sensitivities (DPG and Cl^-) at normal temperature, which would safeguard the pulmonary Hb-O₂ loading under hypoxic and hypercapnic conditions. Furthermore, the high anion allosteric effector sensitivities at low temperature and low temperature sensitivities of plateau zokor Hbs would facilitate the releasing of O₂ in cold extremities and metabolic tissues. However, the high Hb-O₂ affinity of plateau zokor is not compensated by high pH sensitivity as the Bohr factors of plateau zokor Hbs were as low as those of mouse. The results of molecular dynamics simulations revealed the reduced hydrogen bonding between the α1β1- and α2β2-dimer interface of deoxyHb in zokor compared with mouse. It may be the primary mechanism of the high intrinsic Hb-O₂ affinities in zokor. Specifically, substitution of the 131Ser→Asn in the α2-chain weakened the connection between α1- and β2-subunit.

Playing Hide-and-Seek in Beta-Globin Genes: Gene Conversion Transferring a Beneficial Mutation between Differentially Expressed Gene Duplicates

Increasing evidence suggests that adaptation to diverse environments often involves selection on existing variation rather than new mutations. A previous study identified a nonsynonymous single nucleotide polymorphism (SNP) in exon 2 of two paralogous β-globin genes of the bank vole (Clethrionomys glareolus) in Britain in which the ancestral serine (Ser) and the derived cysteine (Cys) allele represent geographically partitioned functional variation affecting the erythrocyte antioxidative capacity. Here we studied the geographical pattern of the two-locus Ser/Cys polymorphism throughout Europe and tested for the geographic correlation between environmental variables and allele frequency, expected if the polymorphism was under spatially heterogeneous environment-related selection. Although bank vole population history clearly is important in shaping the dispersal of the oxidative stress protective Cys allele, analyses correcting for population structure suggest the Europe-wide pattern is affected by geographical variation in environmental conditions. The β-globin phenotype is encoded by the major paralog HBB-T1 but we found evidence of bidirectional gene conversion of exon 2 with the low-expression paralog HBB-T2. Our data support the model where gene conversion reshuffling genotypes between high- and low- expressed paralogs enables tuning of erythrocyte thiol levels, which may help maintain intracellular redox balance under fluctuating environmental conditions. Therefore, our study suggests a possible role for gene conversion between differentially expressed gene duplicates as a mechanism of physiological adaptation of populations to new or changing environments.

Measurements of three-dimensional refractive index tomography and membrane deformability of live erythrocytes from Pelophylax nigromaculatus

Unlike mammalian erythrocytes, amphibian erythrocytes have distinct morphological features including large cell sizes and the presence of nuclei. The sizes of the cytoplasm and nuclei of erythrocytes vary significantly over different species, their environments, or pathophysiology, which makes hematological studies important for investigating amphibian species. Here, we present a label-free three-dimensional optical quantification of individual amphibian erythrocytes from frogs Pelophylax nigromaculatus (Rana nigromaculata). Using optical diffraction tomography, we measured three-dimensional refractive index (RI) tomograms of the cells, which clearly distinguished the cytoplasm and nuclei of the erythrocytes. From the measured RI tomograms, we extracted the relevant biochemical parameters of the cells, including hemoglobin contents and hemoglobin concentrations. Furthermore, we measured dynamic membrane fluctuations and investigated the mechanical properties of the cell membrane. From the statistical and correlative analysis of these retrieved parameters, we investigated interspecific differences between frogs and previously studied mammals.

Contribution of both positive selection and relaxation of selective constraints to degeneration of flyability during geese domestication

Flyability is the most discrepant trait between modern-day geese and their wild ancestors, and the degeneration of flyability is a key marker of the successful domestication of wild geese. In light of the relatively short history of domestic geese, intense artificial selection is thought to play an important role in the degeneration of flyability. However, the underlying mechanism behind this phenomenon has seldom been investigated. In this study, we applied a molecular evolutionary approach to the evaluation of partial breeds of domestic geese in order to look for genes involved in the selection pressure toward degeneration of flyability. The haplotype networks, pairwise fixation index (FST) values, and analysis of molecular variance results all clearly illustrated a population variance between Landes geese and partial Chinese domestic geese. We also detected signatures of positive artificial selection in the COX2 and COX3 genes, and related selection in the HBB gene. Our results support the independent origins of partial European domestic geese and Chinese domestic geese. In addition, both positive artificial selection and the relaxation of functional constraints appeared to play important roles in the degeneration of flyability in domestic geese.

O2 binding and CO2 sensitivity in haemoglobins of subterranean African mole rats

Inhabiting deep and sealed subterranean burrows, mole rats exhibit a remarkable suite of specializations, including eusociality (living in colonies with single breeding queens), extraordinary longevity, cancer immunity and poikilothermy, and extreme tolerance of hypoxia and hypercapnia. With little information available on adjustments in haemoglobin (Hb) function that may mitigate the impact of exogenous and endogenous constraints on the uptake and internal transport of O₂, we measured haematological characteristics, as well as Hb-O₂ binding affinity and sensitivity to pH (Bohr effect), CO₂, temperature and 2,3-diphosphoglycerate (DPG, the major allosteric modulator of Hb-O₂ affinity in red blood cells) in four social and two solitary species of African mole rats (family Bathyergidae) originating from different biomes and soil types across Central and Southern Africa. We found no consistent patterns in haematocrit (Hct) and blood and red cell DPG and Hb concentrations or in intrinsic Hb-O₂ affinity and its sensitivity to pH and DPG that correlate with burrowing, sociality and soil type. However, the results reveal low specific (pH independent) effects of CO₂ on Hb-O₂ affinity compared with humans that predictably safeguard pulmonary loading under hypoxic and hypercapnic burrow conditions. The O₂ binding characteristics are discussed in relation to available information on the primary structure of Hbs from adult and developmental stages of mammals subjected to hypoxia and hypercapnia and the molecular mechanisms underlying functional variation in rodent Hbs.

Rich diversity and potency of skin antioxidant peptides revealed a novel molecular basis for high-altitude adaptation of amphibians

Elucidating the mechanisms of high-altitude adaptation is an important research area in modern biology. To date, however, knowledge has been limited to the genetic mechanisms of adaptation to the lower oxygen and temperature levels prevalent at high altitudes, with adaptation to UV radiation largely neglected. Furthermore, few proteomic or peptidomic analyses of these factors have been performed. In this study, the molecular adaptation of high-altitude Odorrana andersonii and cavernicolous O. wuchuanensis to elevated UV radiation was investigated. Compared with O. wuchuanensis, O. andersonii exhibited greater diversity and free radical scavenging potentiality of skin antioxidant peptides to cope with UV radiation. This implied that O. andersonii evolved a much more complicated and powerful skin antioxidant peptide system to survive high-altitude UV levels. Our results provided valuable peptidomic clues for understanding the novel molecular basis for adaptation to high elevation habitats.

Oxygenation properties and isoform diversity of snake hemoglobins

Available data suggest that snake hemoglobins (Hbs) are characterized by a combination of unusual structural and functional properties relative to the Hbs of other amniote vertebrates, including oxygenation-linked tetramer-dimer dissociation. However, standardized comparative data are lacking for snake Hbs, and the Hb isoform composition of snake red blood cells has not been systematically characterized. Here we present the results of an integrated analysis of snake Hbs and the underlying α- and β-type globin genes to characterize 1) Hb isoform composition of definitive erythrocytes, and 2) the oxygenation properties of isolated isoforms as well as composite hemolysates. We used species from three families as subjects for experimental studies of Hb function: South American rattlesnake, Crotalus durissus (Viperidae); Indian python, Python molurus (Pythonidae); and yellow-bellied sea snake, Pelamis platura (Elapidae). We analyzed allosteric properties of snake Hbs in terms of the Monod-Wyman-Changeux model and Adair four-step thermodynamic model. Hbs from each of the three species exhibited high intrinsic O2 affinities, low cooperativities, small Bohr factors in the absence of phosphates, and high sensitivities to ATP. Oxygenation properties of the snake Hbs could be explained entirely by allosteric transitions in the quaternary structure of intact tetramers, suggesting that ligation-dependent dissociation of Hb tetramers into αβ-dimers is not a universal feature of snake Hbs. Surprisingly, the major Hb isoform of the South American rattlesnake is homologous to the minor HbD of other amniotes and, contrary to the pattern of Hb isoform differentiation in birds and turtles, exhibits a lower O2 affinity than the HbA isoform.

Cooperative binding of annexin A2 to cholesterol- and phosphatidylinositol-4,5-bisphosphate-containing bilayers

Biological membranes are organized into dynamic microdomains that serve as sites for signal transduction and membrane trafficking. The formation and expansion of these microdomains are driven by intrinsic properties of membrane lipids and integral as well as membrane-associated proteins. Annexin A2 (AnxA2) is a peripherally associated membrane protein that can support microdomain formation in a Ca(2+)-dependent manner and has been implicated in membrane transport processes. Here, we performed a quantitative analysis of the binding of AnxA2 to solid supported membranes containing the annexin binding lipids phosphatidylinositol-4,5-bisphosphate and phosphatidylserine in different compositions. We show that the binding is of high specificity and affinity with dissociation constants ranging between 22.1 and 32.2 nM. We also analyzed binding parameters of a heterotetrameric complex of AnxA2 with its S100A10 protein ligand and show that this complex has a higher affinity for the same membranes with Kd values of 12 to 16.4 nM. Interestingly, binding of the monomeric AnxA2 and the AnxA2-S100A10 complex are characterized by positive cooperativity. This cooperative binding is mediated by the conserved C-terminal annexin core domain of the protein and requires the presence of cholesterol. Together our results reveal for the first time, to our knowledge, that AnxA2 and its derivatives bind cooperatively to membranes containing cholesterol, phosphatidylserine, and/or phosphatidylinositol-4,5-bisphosphate, thus providing a mechanistic model for the lipid clustering activity of AnxA2.

Differences in Hematological Traits between High- and Low-Altitude Lizards (Genus Phrynocephalus)

Phrynocephalus erythrurus (Lacertilia: Agamidae) is considered to be the highest living reptile in the world (about 4500-5000 m above sea level), whereas Phrynocephalus przewalskii inhabits low altitudes (about 1000-1500 m above sea level). Here, we report the differences in hematological traits between these two different Phrynocephalus species. Compared with P. przewalskii, the results indicated that P. erythrurus own higher oxygen carrying capacity by increasing red blood cell count (RBC), hemoglobin concentration ([Hb]) and hematocrit (Hct) and these elevations could promote oxygen carrying capacity without disadvantage of high viscosity. The lower partial pressure of oxygen in arterial blood (PaO2) of P. erythrurus did not cause the secondary alkalosis, which may be attributed to an efficient pulmonary system for oxygen (O2) loading. The elevated blood-O2 affinity in P. erythrurus may be achieved by increasing intrinsic O2 affinity of isoHbs and balancing the independent effects of potential heterotropic ligands. We detected one α-globin gene and three β-globin genes with 1 and 33 amino acid substitutions between these two species, respectively. Molecular dynamics simulation results showed that amino acids substitutions in β-globin chains could lead to the elimination of hydrogen bonds in T-state Hb models of P. erythrurus. Based on the present data, we suggest that P. erythrurus have evolved an efficient oxygen transport system under the unremitting hypobaric hypoxia.

High affinity and temperature sensitivity of blood oxygen binding in Pangasianodon hypophthalmus due to lack of chloride-hemoglobin allosteric interaction

Air-breathing fishes represent interesting organisms in terms of understanding the physiological changes associated with the terrestrialization of vertebrates, and, further, are of great socio-economic importance for aquaculture in Southeast Asia. To understand how environmental factors, such as high temperature, affect O2 transport in air-breathing fishes, this study assessed the effects of temperature on O2 binding of blood and Hb in the economically important air-breathing fish Pangasianodon hypophthalmus. To determine blood O2 binding properties, blood was drawn from resting cannulated fishes and O2 binding curves made at 25°C and 35°C. To determine the allosteric regulation and thermodynamics of Hb O2 binding, Hb was purified, and O2 equilibria were recorded at five temperatures in the absence and presence of ATP and Cl(-). Whole blood had a high O2 affinity (O2 tension at half saturation P50 = 4.6 mmHg at extracellular pH 7.6 and 25°C), a high temperature sensitivity of O2 binding (apparent heat of oxygenation ΔH(app) = -28.3 kcal/mol), and lacked a Root effect. Further, the data on Hb revealed weak ATP binding and a complete lack of Cl(-) binding to Hb, which, in part, explains the high O2 affinity and high temperature sensitivity of blood O2 binding. This study demonstrates how a potent mechanism for increasing O2 affinity is linked to increased temperature sensitivity of O2 transport and provides a basic framework for a better understanding of how hypoxia-adapted species will react to increasing temperatures.

Integrating evolutionary and functional tests of adaptive hypotheses: a case study of altitudinal differentiation in hemoglobin function in an Andean Sparrow, Zonotrichia capensis

In air-breathing vertebrates, the physiologically optimal blood-O2 affinity is jointly determined by the prevailing partial pressure of atmospheric O2, the efficacy of pulmonary O2 transfer, and internal metabolic demands. Consequently, genetic variation in the oxygenation properties of hemoglobin (Hb) may be subject to spatially varying selection in species with broad elevational distributions. Here we report the results of a combined functional and evolutionary analysis of Hb polymorphism in the rufous-collared sparrow (Zonotrichia capensis), a species that is continuously distributed across a steep elevational gradient on the Pacific slope of the Peruvian Andes. We integrated a population genomic analysis that included all postnatally expressed Hb genes with functional studies of naturally occurring Hb variants, as well as recombinant Hb (rHb) mutants that were engineered through site-directed mutagenesis. We identified three clinally varying amino acid polymorphisms: Two in the α(A)-globin gene, which encodes the α-chain subunits of the major HbA isoform, and one in the α(D)-globin gene, which encodes the α-chain subunits of the minor HbD isoform. We then constructed and experimentally tested single- and double-mutant rHbs representing each of the alternative α(A)-globin genotypes that predominate at different elevations. Although the locus-specific patterns of altitudinal differentiation suggested a history of spatially varying selection acting on Hb polymorphism, the experimental tests demonstrated that the observed amino acid mutations have no discernible effect on respiratory properties of the HbA or HbD isoforms. These results highlight the importance of experimentally validating the hypothesized effects of genetic changes in protein function to avoid the pitfalls of adaptive storytelling.

Genomic organization and differential signature of positive selection in the alpha and beta globin gene clusters in two cetacean species

The hemoglobin of jawed vertebrates is a heterotetramer protein that contains two α- and two β-chains, which are encoded by members of α- and β-globin gene families. Given the hemoglobin role in mediating an adaptive response to chronic hypoxia, it is likely that this molecule may have experienced a selective pressure during the evolution of cetaceans, which have to deal with hypoxia tolerance during prolonged diving. This selective pressure could have generated a complex history of gene turnover in these clusters and/or changes in protein structure themselves. Accordingly, we aimed to characterize the genomic organization of α- and β-globin gene clusters in two cetacean species and to detect a possible role of positive selection on them using a phylogenetic framework. Maximum likelihood and Bayesian phylogeny reconstructions revealed that both cetacean species had retained a similar complement of putatively functional genes. For the α-globin gene cluster, the killer whale presents a complement of genes composed of HBZ, HBK, and two functional copies of HBA and HBQ genes, whereas the dolphin possesses HBZ, HBK, HBA and HBQ genes, and one HBA pseudogene. For the β-globin gene cluster, both species retained a complement of four genes, two early expressed genes—HBE and HBH—and two adult expressed genes—HBD and HBB. Our natural selection analysis detected two positively selected sites in the HBB gene (56 and 62) and four in HBA (15, 21, 49, 120). Interestingly, only the genes that are expressed during the adulthood showed the signature of positive selection.

Novel genomic resources for a climate change sensitive mammal: characterization of the American pika transcriptome

BACKGROUND

When faced with climate change, species must either shift their home range or adapt in situ in order to maintain optimal physiological balance with their environment. The American pika (Ochotona princeps) is a small alpine mammal with limited dispersal capacity and low tolerance for thermal stress. As a result, pikas have become an important system for examining biotic responses to changing climatic conditions. Previous research using amplified fragment length polymorphisms (AFLPs) has revealed evidence for environmental-mediated selection in O. princeps populations distributed along elevation gradients, yet the anonymity of AFLP loci and lack of available genomic resources precluded the identification of associated gene regions. Here, we harnessed next-generation sequencing technology in order to characterize the American pika transcriptome and identify a large suite of single nucleotide polymorphisms (SNPs), which can be used to elucidate elevation- and site-specific patterns of sequence variation.

RESULTS

We constructed pooled cDNA libraries of O. princeps from high (1400 m) and low (300 m) elevation sites along a previously established transect in British Columbia. Transcriptome sequencing using the Roche 454 GS FLX titanium platform generated 780 million base pairs of data, which were assembled into 7,325 high coverage contigs. These contigs were used to identify 24,261 novel SNP loci. Using high resolution melt analysis, we developed 17 of these SNPs into genotyping assays, which were validated with independent DNA samples from British Columbia Canada and Oregon State USA. In addition, we detected haplotypes in the NADH dehydrogenase subunit 5 of the mitochondrial genome that were fixed and different among elevations, suggesting that this may be an informative target gene for studying the role of cellular respiration in local adaptation. We also identified contigs that were unique to each elevation, including a high elevation-specific contig that was a positive match with the hemoglobin alpha chain from the plateau pika, a species restricted to high elevation steppes in Asia. Elevation-specific contigs may represent candidate regions subject to differential levels of gene expression along this elevation gradient.

CONCLUSIONS

To our knowledge, this is the first broad-scale, transcriptome-level study conducted within the Ochotonidae, providing novel genomic resources for studying pika ecology, behaviour and population history.

Novel control of lactate dehydrogenase from the freeze tolerant wood frog: role of posttranslational modifications

Lactate dehydrogenase (LDH), the terminal enzyme of anaerobic glycolysis, plays a crucial role both in sustaining glycolytic ATP production under oxygen-limiting conditions and in facilitating the catabolism of accumulated lactate when stress conditions are relieved. In this study, the effects on LDH of in vivo freezing and dehydration stresses (both of which impose hypoxia/anoxia stress on tissues) were examined in skeletal muscle of the freeze-tolerant wood frog, Rana sylvatica. LDH from muscle of control, frozen and dehydrated wood frogs was purified to homogeneity in a two-step process. The kinetic properties and stability of purified LDH were analyzed, revealing no significant differences in V max, K m and I 50 values between control and frozen LDH. However, control and dehydrated LDH differed significantly in K m values for pyruvate, lactate, and NAD, I 50 urea, and in temperature, glucose, and urea effects on these parameters. The possibility that posttranslational modification of LDH was responsible for the stable differences in enzyme behavior between control and dehydrated states was assessed using ProQ diamond staining to detect phosphorylation and immunoblotting to detect acetylation, methylation, ubiquitination, SUMOylation and nitrosylation of the enzyme. LDH from muscle of dehydrated wood frogs showed significantly lower levels of acetylation, providing one of the first demonstrations of a potential role for protein acetylation in the stress-responsive control of a metabolic enzyme.

Gene duplication and the evolution of hemoglobin isoform differentiation in birds

The majority of bird species co-express two functionally distinct hemoglobin (Hb) isoforms in definitive erythrocytes as follows: HbA (the major adult Hb isoform, with α-chain subunits encoded by the α(A)-globin gene) and HbD (the minor adult Hb isoform, with α-chain subunits encoded by the α(D)-globin gene). The α(D)-globin gene originated via tandem duplication of an embryonic α-like globin gene in the stem lineage of tetrapod vertebrates, which suggests the possibility that functional differentiation between the HbA and HbD isoforms may be attributable to a retained ancestral character state in HbD that harkens back to a primordial, embryonic function. To investigate this possibility, we conducted a combined analysis of protein biochemistry and sequence evolution to characterize the structural and functional basis of Hb isoform differentiation in birds. Functional experiments involving purified HbA and HbD isoforms from 11 different bird species revealed that HbD is characterized by a consistently higher O(2) affinity in the presence of allosteric effectors such as organic phosphates and Cl(-) ions. In the case of both HbA and HbD, analyses of oxygenation properties under the two-state Monod-Wyman-Changeux allosteric model revealed that the pH dependence of Hb-O(2) affinity stems primarily from changes in the O(2) association constant of deoxy (T-state)-Hb. Ancestral sequence reconstructions revealed that the amino acid substitutions that distinguish the adult-expressed Hb isoforms are not attributable to the retention of an ancestral (pre-duplication) character state in the α(D)-globin gene that is shared with the embryonic α-like globin gene.

Structures of haemoglobin from woolly mammoth in liganded and unliganded states

The haemoglobin (Hb) of the extinct woolly mammoth has been recreated using recombinant genes expressed in Escherichia coli. The globin gene sequences were previously determined using DNA recovered from frozen cadavers. Although highly similar to the Hb of existing elephants, the woolly mammoth protein shows rather different responses to chloride ions and temperature. In particular, the heat of oxygenation is found to be much lower in mammoth Hb, which appears to be an adaptation to the harsh high-latitude climates of the Pleistocene Ice Ages and has been linked to heightened sensitivity of the mammoth protein to protons, chloride ions and organic phosphates relative to that of Asian elephants. To elucidate the structural basis for the altered homotropic and heterotropic effects, the crystal structures of mammoth Hb have been determined in the deoxy, carbonmonoxy and aquo-met forms. These models, which are the first structures of Hb from an extinct species, show many features reminiscent of human Hb, but underline how the delicate control of oxygen affinity relies on much more than simple overall quaternary-structure changes.

Genetic and phenotypic divergence between low- and high-altitude populations of two recently diverged cinnamon teal subspecies

Spatial variation in the environment can lead to divergent selection between populations occupying different parts of a species' range, and ultimately lead to population divergence. The colonization of new areas can thus facilitate divergence in beneficial traits, yet with little differentiation at neutral genetic markers. We investigated genetic and phenotypic patterns of divergence between low- and high-altitude populations of cinnamon teal inhabiting normoxic and hypoxic regions in the Andes and adjacent lowlands of South America. Cinnamon teal showed strong divergence in body size (PC1; P(ST) = 0.56) and exhibited significant frequency differences in a single nonsynonymous α-hemoglobin amino acid polymorphism (Asn/Ser-α9; F(ST) = 0.60) between environmental extremes, despite considerable admixture of mtDNA and intron loci (F(ST) = 0.004-0.168). Inferences of strong population segregation were further supported by the observation of few mismatched individuals in either environmental extreme. Coalescent analyses indicated that the highlands were most likely colonized from lowland regions but following divergence, gene flow has been asymmetric from the highlands into the lowlands. Multiple selection pressures associated with high-altitude habitats, including cold and hypoxia, have likely shaped morphological and genetic divergence within South American cinnamon teal populations.

Genomic insights into adaptation to high-altitude environments

Elucidating the molecular genetic basis of adaptive traits is a central goal of evolutionary genetics. The cold, hypoxic conditions of high-altitude habitats impose severe metabolic demands on endothermic vertebrates, and understanding how high-altitude endotherms cope with the combined effects of hypoxia and cold can provide important insights into the process of adaptive evolution. The physiological responses to high-altitude stress have been the subject of over a century of research, and recent advances in genomic technologies have opened up exciting opportunities to explore the molecular genetic basis of adaptive physiological traits. Here, we review recent literature on the use of genomic approaches to study adaptation to high-altitude hypoxia in terrestrial vertebrates, and explore opportunities provided by newly developed technologies to address unanswered questions in high-altitude adaptation at a genomic scale.

Genetic differences in hemoglobin function between highland and lowland deer mice

In high-altitude vertebrates, adaptive changes in blood-O(2) affinity may be mediated by modifications of hemoglobin (Hb) structure that affect intrinsic O(2) affinity and/or responsiveness to allosteric effectors that modulate Hb-O(2) affinity. This mode of genotypic specialization is considered typical of mammalian species that are high-altitude natives. Here we investigated genetically based differences in Hb-O(2) affinity between highland and lowland populations of the deer mouse (Peromyscus maniculatus), a generalist species that has the broadest altitudinal distribution of any North American mammal. The results of a combined genetic and proteomic analysis revealed that deer mice harbor a high level of Hb isoform diversity that is attributable to allelic polymorphism at two tandemly duplicated alpha-globin genes and two tandemly duplicated beta-globin genes. This high level of isoHb diversity translates into a correspondingly high level of interindividual variation in Hb functional properties. O(2) equilibrium experiments revealed that the Hbs of highland mice exhibit slightly higher intrinsic O(2) affinities and significantly lower Cl(-) sensitivities relative to the Hbs of lowland mice. The experiments also revealed distinct biochemical properties of deer mouse Hb related to the anion-dependent allosteric regulation of O(2) affinity. In conjunction with previous findings, our results demonstrate that modifications of Hb structure that alter allosteric anion sensitivity play an important role in the adaptive fine-tuning of blood-O(2) affinity.

Molecular evolution of cytochrome C oxidase underlies high-altitude adaptation in the bar-headed goose

Bar-headed geese (Anser indicus) fly at up to 9,000 m elevation during their migration over the Himalayas, sustaining high metabolic rates in the severe hypoxia at these altitudes. We investigated the evolution of cardiac energy metabolism and O(2) transport in this species to better understand the molecular and physiological mechanisms of high-altitude adaptation. Compared with low-altitude geese (pink-footed geese and barnacle geese), bar-headed geese had larger lungs and higher capillary densities in the left ventricle of the heart, both of which should improve O(2) diffusion during hypoxia. Although myoglobin abundance and the activities of many metabolic enzymes (carnitine palmitoyltransferase, citrate synthase, 3-hydroxyacyl-coA dehydrogenase, lactate dehydrogenase, and pyruvate kinase) showed only minor variation between species, bar-headed geese had a striking alteration in the kinetics of cytochrome c oxidase (COX), the heteromeric enzyme that catalyzes O(2) reduction in oxidative phosphorylation. This was reflected by a lower maximum catalytic activity and a higher affinity for reduced cytochrome c. There were small differences between species in messenger RNA and protein expression of COX subunits 3 and 4, but these were inconsistent with the divergence in enzyme kinetics. However, the COX3 gene of bar-headed geese contained a nonsynonymous substitution at a site that is otherwise conserved across vertebrates and resulted in a major functional change of amino acid class (Trp-116 → Arg). This mutation was predicted by structural modeling to alter the interaction between COX3 and COX1. Adaptations in mitochondrial enzyme kinetics and O(2) transport capacity may therefore contribute to the exceptional ability of bar-headed geese to fly high.

ATP-induced temperature independence of hemoglobin-O2 affinity in heterothermic billfish

The inverse relationship between temperature and hemoglobin-O(2) affinity resulting from the exothermic nature of heme oxygenation favors O(2) unloading from blood to warm, metabolically active tissues. However, this temperature sensitivity is maladaptive, and commonly countered in regional heterotherms, where it may hamper unloading (e.g. in cold extremities of arctic mammals) or increase the diffusive arterio-venous short-circuiting of O(2) (e.g. in counter-current heat exchangers of warm swimming muscles of tuna). We hypothesized analogous blood specializations in heterothermic billfish, whose warm eyes and brains increase the temporal resolution of vision, and measured hemoglobin-O(2) binding properties in three species over a wide pH range, at two temperatures, and in the absence and presence of the major red cell effector, ATP, permitting detailed assessment of overall oxygenation enthalpies (DeltaH') and contributions from oxygenation-linked proton and ATP dissociation. Billfish express multiple isohemoglobins with similar O(2) affinities and pronounced sensitivities to pH and ATP. Compared with the moderate effects associated with proton dissociation upon oxygenation, dissociation of ATP and coupled extra Bohr protons virtually obliterates the temperature sensitivities. At pH 7.4, where this effect is maximal, ATP changes DeltaH' values of blue marlin, striped marlin and shortbill spearfish hemoglobins from -39, -49 and -44 kJ mol(-1) O(2), respectively, to +26, +4 and -7 kJ mol(-1). Thus in addition to allosterically modulating hemoglobin-O(2) affinity, ATP diminishes its temperature sensitivity, reducing deleterious arterio-venous short-circuiting of oxygen in the cranial billfish heat exchangers. The mechanism underlying this reduction in oxygenation enthalpy differs fundamentally from that in tuna, supporting independent evolution of this trait in these scombroid lineages.

Evolutionary and functional insights into the mechanism underlying high-altitude adaptation of deer mouse hemoglobin

Adaptive modifications of heteromeric proteins may involve genetically based changes in single subunit polypeptides or parallel changes in multiple genes that encode distinct, interacting subunits. Here we investigate these possibilities by conducting a combined evolutionary and functional analysis of duplicated globin genes in natural populations of deer mice (Peromyscus maniculatus) that are adapted to different elevational zones. A multilocus analysis of nucleotide polymorphism and linkage disequilibrium revealed that high-altitude adaptation of deer mouse hemoglobin involves parallel functional differentiation at multiple unlinked gene duplicates: two alpha-globin paralogs on chromosome 8 and two beta-globin paralogs on chromosome 1. Differences in O(2)-binding affinity of the alternative beta-chain hemoglobin isoforms were entirely attributable to allelic differences in sensitivity to 2,3-diphosphoglycerate (DPG), an allosteric cofactor that stabilizes the low-affinity, deoxygenated conformation of the hemoglobin tetramer. The two-locus beta-globin haplotype that predominates at high altitude is associated with suppressed DPG-sensitivity (and hence, increased hemoglobin-O(2) affinity), which enhances pulmonary O(2) loading under hypoxia. The discovery that allelic differences in DPG-sensitivity contribute to adaptive variation in hemoglobin-O(2) affinity illustrates the value of integrating evolutionary analyses of sequence variation with mechanistic appraisals of protein function. Investigation into the functional significance of the deer mouse beta-globin polymorphism was motivated by the results of population genetic analyses which revealed evidence for a history of divergent selection between elevational zones. The experimental measures of O(2)-binding properties corroborated the tests of selection by demonstrating a functional difference between the products of alternative alleles.

Mechanisms of hemoglobin adaptation to high altitude hypoxia

Evidence from a number of vertebrate taxa suggests that modifications of hemoglobin (Hb) function may often play a key role in mediating an adaptive response to high altitude hypoxia. The respiratory functions of Hb are a product of the protein's intrinsic O(2)-binding affinity and its interactions with allosteric effectors such as protons, chloride ions, CO(2), and organic phosphates. Here we review several case studies involving high altitude vertebrates where it has been possible to identify specific mechanisms of Hb adaptation to hypoxia. In addition to comparative studies of Hbs from diverse animal species, functional studies of human Hb mutants also suggest that there is ample scope for evolutionary adjustments in Hb-O(2) affinity through alterations of the equilibrium constants of O(2) binding to deoxy- and oxyHb or through changes in the allosteric equilibrium constants for the transition between the deoxy- and oxyHb quaternary structures. It may be the case that certain evolutionary paths are followed more often than others simply because they are subject to less stringent pleiotropic constraints.

High-altitude adaptations in vertebrate hemoglobins

Vertebrates at high altitude are subjected to hypoxic conditions that challenge aerobic metabolism. O(2) transport from the respiratory surfaces to tissues requires matching between the O(2) loading and unloading tensions and the O(2)-affinity of blood, which is an integrated function of hemoglobin's intrinsic O(2)-affinity and its allosteric interaction with cellular effectors (organic phosphates, protons and chloride). Whereas short-term altitudinal adaptations predominantly involve adjustments in allosteric interactions, long-term, genetically-coded adaptations typically involve changes in the structure of the haemoglobin molecules. The latter commonly comprise substitutions of amino acid residues at the effector binding sites, the heme-protein contacts, or at intersubunit contacts that stabilize either the low-affinity ('Tense') or the high-affinity ('Relaxed') structures of the molecules. Molecular heterogeneity (multiple isoHbs with differentiated oxygenation properties) can further broaden the range of physico-chemical conditions where Hb functions under altitudinal hypoxia. This treatise reviews the molecular and cellular mechanisms that adapt haemoglobin-oxygen affinities in mammals, birds and ectothermic vertebrates at high altitude.

The locus of evolution: evo devo and the genetics of adaptation

An important tenet of evolutionary developmental biology ("evo devo") is that adaptive mutations affecting morphology are more likely to occur in the cis-regulatory regions than in the protein-coding regions of genes. This argument rests on two claims: (1) the modular nature of cis-regulatory elements largely frees them from deleterious pleiotropic effects, and (2) a growing body of empirical evidence appears to support the predominant role of gene regulatory change in adaptation, especially morphological adaptation. Here we discuss and critique these assertions. We first show that there is no theoretical or empirical basis for the evo devo contention that adaptations involving morphology evolve by genetic mechanisms different from those involving physiology and other traits. In addition, some forms of protein evolution can avoid the negative consequences of pleiotropy, most notably via gene duplication. In light of evo devo claims, we then examine the substantial data on the genetic basis of adaptation from both genome-wide surveys and single-locus studies. Genomic studies lend little support to the cis-regulatory theory: many of these have detected adaptation in protein-coding regions, including transcription factors, whereas few have examined regulatory regions. Turning to single-locus studies, we note that the most widely cited examples of adaptive cis-regulatory mutations focus on trait loss rather than gain, and none have yet pinpointed an evolved regulatory site. In contrast, there are many studies that have both identified structural mutations and functionally verified their contribution to adaptation and speciation. Neither the theoretical arguments nor the data from nature, then, support the claim for a predominance of cis-regulatory mutations in evolution. Although this claim may be true, it is at best premature. Adaptation and speciation probably proceed through a combination of cis-regulatory and structural mutations, with a substantial contribution of the latter.

Antagonistic interaction between oxygenation-linked lactate and CO2 binding to human hemoglobin

Oxygen binding to hemoglobin (Hb) depends on allosteric effectors (CO(2), lactate and protons) that may increase drastically in concentration during exercise. The effectors share common binding sites on the Hb molecules, predicting mutual interaction in their effects on Hb (de)oxygenation. We analysed the effects of lactate and CO(2), separately and in combination, on O(2) binding of purified human Hb at 37 degrees C and physiological pH and chloride values. We demonstrate pH-dependent, inhibitory interactions between lactate binding and CO(2) binding (carbamate formation); at pH 7.4, physiological CO(2) tension ( approximately 43 mm Hg) reduced lactate binding more markedly ( approximately 75%), than lactate (50 mM) inhibited carbamate formation ( approximately 25%). In contrast to previous studies on blood and Hb solutions, we moreover find that added lactate neither 'reverses' oxylabile carbamate formation (resulting in lower carbamate levels in deoxyHb than in oxyHb) nor exerts greater allosteric effects on Hb-O(2) affinity than equal increases in chloride ion concentrations.

R-state haemoglobin with low oxygen affinity: crystal structures of deoxy human and carbonmonoxy horse haemoglobin bound to the effector molecule L35

Although detailed crystal structures of haemoglobin (Hb) provide a clear understanding of the basic allosteric mechanism of the protein, and how this in turn controls oxygen affinity, recent experiments with artificial effector molecules have shown a far greater control of oxygen binding than with natural heterotropic effectors. Contrary to the established text-book view, these non-physiological compounds are able to reduce oxygen affinity very strongly without switching the protein to the T (tense) state. In an earlier paper we showed that bezafibrate (BZF) binds to a surface pocket on the alpha subunits of R state Hb, strongly reducing the oxygen affinity of this protein conformation. Here we report the crystallisation of Hb with L35, a related compound, and show that this binds to the central cavity of both R and T state Hb. The mechanism by which L35 reduces oxygen affinity is discussed, in relation to spectroscopic studies of effector binding.

Chronic hypoxia modulates NMDA-mediated regulation of the hypoxic ventilatory response in an amphibian, Bufo marinus

This study examined whether a hypoxia-tolerant amphibian, the Cane toad, undergoes mammalian-like ventilatory acclimatisation to hypoxia (VAH) and whether chronic hypoxia (CH) alters NMDA-mediated regulation of the acute hypoxic ventilatory response (HVR). Toads were exposed to 10 days of CH (10% O2) followed by acute hypoxic breathing trials or an intra-arterial injection of NaCN. Trials were conducted before and after i.p. treatment with an NMDA-receptor channel blocker (MK801). CH blunted the acute HVR but did not alter resting breathing. MK801 did not alter resting ventilation. In control animals, MK801 augmented breathing frequency (fR) during acute hypoxia by increasing the number of breaths per episode. This effect was attenuated following CH although MK801 did enhance the number of episodes per minute during acute hypoxia. MK801 enhanced the fR response to NaCN in both groups. The results indicate that CH did not produce mammalian-like VAH (i.e. increased resting ventilation and an augmented acute HVR) but did alter MK801-sensitive regulation of breathing pattern and the acute HVR.

Why O2 is required by complex life on habitable planets and the concept of planetary "oxygenation time"

Life is constructed from a limited toolkit: the Periodic Table. The reduction of oxygen provides the largest free energy release per electron transfer, except for the reduction of fluorine and chlorine. However, the bonding of O2 ensures that it is sufficiently stable to accumulate in a planetary atmosphere, whereas the more weakly bonded halogen gases are far too reactive ever to achieve significant abundance. Consequently, an atmosphere rich in O2 provides the largest feasible energy source. This universal uniqueness suggests that abundant O2 is necessary for the high-energy demands of complex life anywhere, i.e., for actively mobile organisms of approximately 10(-1)-10(0) m size scale with specialized, differentiated anatomy comparable to advanced metazoans. On Earth, aerobic metabolism provides about an order of magnitude more energy for a given intake of food than anaerobic metabolism. As a result, anaerobes do not grow beyond the complexity of uniseriate filaments of cells because of prohibitively low growth efficiencies in a food chain. The biomass cumulative number density, n, at a particular mass, m, scales as n (> m) proportional to m(-1) for aquatic aerobes, and we show that for anaerobes the predicted scaling is n proportional to m (-1.5), close to a growth-limited threshold. Even with aerobic metabolism, the partial pressure of atmospheric O2 (P(O2)) must exceed approximately 10(3) Pa to allow organisms that rely on O2 diffusion to evolve to a size approximately 10(3) m x P(O2) in the range approximately 10(3)-10(4) Pa is needed to exceed the threshold of approximately 10(2) m size for complex life with circulatory physiology. In terrestrial life, O(2) also facilitates hundreds of metabolic pathways, including those that make specialized structural molecules found only in animals. The time scale to reach P(O(2)) approximately 10(4) Pa, or "oxygenation time," was long on the Earth (approximately 3.9 billion years), within almost a factor of 2 of the Sun's main sequence lifetime. Consequently, we argue that the oxygenation time is likely to be a key rate-limiting step in the evolution of complex life on other habitable planets. The oxygenation time could preclude complex life on Earth-like planets orbiting short-lived stars that end their main sequence lives before planetary oxygenation takes place. Conversely, Earth-like planets orbiting long-lived stars are potentially favorable habitats for complex life.

Functional adaptation and its molecular basis in vertebrate hemoglobins, neuroglobins and cytoglobins

Hemoglobin (Hb), the paradigm for allosteric proteins through decades, has gained renaissance in recent years following discovery of globins or their genes in all living organisms and in all tissues of higher animals, and of new members of the globin family, such as neuroglobins, Ngb, found predominantly in neural and nerve tissues and cytoglobins, Cygb, that has unprecedented nuclear location. The recent progresses in this field have been prompted by the development of sophisticated techniques to probe molecular structure and functions, which have revealed novel functions, such as the scavenging and release of vasoactive nitric oxide and the regulation of cellular metabolism. This review deals with the functional adaptations and the underlying molecular mechanisms in globins and presents case examples of molecular adaptations encountered in vertebrates and agnathans.

The Bohr effect of haemoglobin in vertebrates: an example of molecular adaptation to different physiological requirements

The Bohr effect, i.e. the pH dependence of the oxygen affinity of haemoglobins (Hbs) from a variety of vertebrates, and its modulation by temperature and other heterotropic effectors has been reviewed. Haemoglobins from vertebrates were not reviewed following the usual classification (i.e. mammals, birds, etc.); instead we have selected several key examples of animals, which are confronted with a similar environmental situation therefore displaying a similar life style. Hence, the paper starts from a description of the general concepts at the basis of the Bohr effect as exemplified by human HbA and goes towards the analysis of the modulation mechanisms which have been observed in different animals in response to the needs induced by: (i) life in cold environments; (ii) diving behaviour; (iii) flight; and (iv) aquatic life. The emerging picture indicates a complex organization of the information contained in the Hb molecule, the oxygen-binding properties of which depend both on the intrinsic characteristics of the protein and on its heterotropic interactions with ligands such as protons (Bohr effect), small anions like chloride and organic phosphates. In addition, each one of the functional effects induced by binding of a given effector appears to be under the strict control of temperature that enhances or decreases its relative weight with respect to all the others. It is just by this sophisticated network of interactions that the Hb molecule is able to satisfy the physiological requirements of a multitude of organisms without changing dramatically its quaternary structure.

Modulation of red cell glycolysis: interactions between vertebrate hemoglobins and cytoplasmic domains of band 3 red cell membrane proteins

Several vital functions/physical characteristics of erythrocytes (including glycolysis, the pentose phosphate pathway, ion fluxes, and cellular deformability) display dependence on the state of hemoglobin oxygenation. The molecular mechanism proposed involves an interaction between deoxyhemoglobin and the cytoplasmic domain of the anion-exchange protein, band 3 (cdB3). Given that band 3 also binds to membrane proteins 4.1 and 4.2, several kinases, hemichromes, and integral membrane proteins, and at least three glycolytic enzymes, it has been suggested that the cdB3-deoxyhemoglobin interaction might modulate the pathways mediated by these associated proteins in an O(2)-dependent manner. We have investigated this mechanism by synthesizing 10-mer peptides corresponding to the NH(2)-terminal fragments of various vertebrate cdB3s, determining their effects on the oxygenation reactions of hemoglobins from the same and different species and examining binding of the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase to the erythrocytic membrane of mouse erythrocytes. The cdB3 interaction is strongly dependent on pH and the number of negative and positive charges of the peptide and at the effector binding site, respectively. It lowers the O(2) association equilibrium constant of the deoxygenated (Tense) state of the hemoglobin and is inhibited by magnesium ions, which neutralize cdB3's charge and by 2,3-diphosphoglycerate, which competes for the cdB3-binding site. The interaction is stronger in humans (whose erythrocytes derive energy predominantly from glycolysis and exhibit higher buffering capacity) than in birds and ectothermic vertebrates (whose erythrocytes metabolize aerobically and are poorly buffered) and is insignificant in fish, suggesting that its role in the regulation of red cell glycolysis increased with phylogenetic development in vertebrates.