Functional differentiation of myoglobin isoforms in hypoxia-tolerant carp indicates tissue-specific protective roles

Rapid physiological and transcriptomic changes associated with oxygen delivery in larval anemonefish suggest a role in adaptation to life on hypoxic coral reefs

Connectivity of coral reef fish populations relies on successful dispersal of a pelagic larval phase. Pelagic larvae must exhibit high swimming abilities to overcome ocean and reef currents, but once settling onto the reef, larvae transition to endure habitats that become hypoxic at night. Therefore, coral reef fish larvae must rapidly and dramatically shift their physiology over a short period of time. Taking an integrative, physiological approach, using swimming respirometry, and examining hypoxia tolerance and transcriptomics, we show that larvae of cinnamon anemonefish (Amphiprion melanopus) rapidly transition between "physiological extremes" at the end of their larval phase. Daily measurements of swimming larval anemonefish over their entire early development show that they initially have very high mass-specific oxygen uptake rates. However, oxygen uptake rates decrease midway through the larval phase. This occurs in conjunction with a switch in haemoglobin gene expression and increased expression of myoglobin, cytoglobin, and neuroglobin, which may all contribute to the observed increase in hypoxia tolerance. Our findings indicate that critical ontogenetic changes in the gene expression of oxygen-binding proteins may underpin the physiological mechanisms needed for successful larval recruitment to reefs.

Nitrite Reductase Activity of Ferrous Nitrobindins: A Comparative Study

Nitrobindins (Nbs) are all-β-barrel heme proteins spanning from bacteria to Homo sapiens. They inactivate reactive nitrogen species by sequestering NO, converting NO to HNO2, and promoting peroxynitrite isomerization to NO3−. Here, the nitrite reductase activity of Nb(II) from Mycobacterium tuberculosis (Mt-Nb(II)), Arabidopsis thaliana (At-Nb(II)), Danio rerio (Dr-Nb(II)), and Homo sapiens (Hs-Nb(II)) is reported. This activity is crucial for the in vivo production of NO, and thus for the regulation of blood pressure, being of the utmost importance for the blood supply to poorly oxygenated tissues, such as the eye retina. At pH 7.3 and 20.0 °C, the values of the second-order rate constants (i.e., kon) for the reduction of NO2− to NO and the concomitant formation of nitrosylated Mt-Nb(II), At-Nb(II), Dr-Nb(II), and Hs-Nb(II) (Nb(II)-NO) were 7.6 M−1 s−1, 9.3 M−1 s−1, 1.4 × 101 M−1 s−1, and 5.8 M−1 s−1, respectively. The values of kon increased linearly with decreasing pH, thus indicating that the NO2−-based conversion of Nb(II) to Nb(II)-NO requires the involvement of one proton. These results represent the first evidence for the NO2 reductase activity of Nbs(II), strongly supporting the view that Nbs are involved in NO metabolism. Interestingly, the nitrite reductase reactivity of all-β-barrel Nbs and of all-α-helical globins (e.g., myoglobin) was very similar despite the very different three-dimensional fold; however, differences between all-α-helical globins and all-β-barrel Nbs suggest that nitrite reductase activity appears to be controlled by distal steric barriers, even though a more complex regulatory mechanism can be also envisaged.

Rainbow trout integrated response after recovery from short-term acute hypoxia

Overcoming a stress situation, such as hypoxia episodes, which involve an allostatic load, will depend on the ability of fish to modulate physiological and biochemical systems to maintain homeostasis. The aim of the study was to determine the integrated stress response after acute hypoxia of the rainbow trout considering the different elements and areas of the stress response: systemic and mucosal, local and global, and from the systemic hypothalamic–pituitary–interrenal axis to skin mucosa. For this purpose, trout were subjected to acute hypoxia (dissolved O2 down to 2 mg/L) for 1 h and then recovered and sampled at 1, 6, and 24 h after reoxygenation. Physiological responses were significantly affected by hypoxic stress and their interaction with time after the challenge, being significant for plasma lactate and cortisol levels, in both plasma and skin mucus. At the central brain level, only trh expression was modulated 1 h after hypoxia which indicates that brain function is not heavily affected by this particular stress. Unlike the brain, the head kidney and skin were more affected by hypoxia and reoxygenation. In the head kidney, an upregulation in the expression of most of the genes studied (gr, il1β, il6, tgfβ1, lysozyme, caspase 3, enolase, hif-1, myoglobin, sod2, gpx, gst, and gsr) took place 6 h after recovery, whereas only hsp70 and il10 were upregulated after 1 h. On the contrary, in the skin, most of the analyzed genes showed a higher upregulation during 1 h after stress suggesting that, in the skin, a local response took place as soon as the stressor was detected, thus indicating the importance of the skin in the building of a stress response, whereas the interrenal tissue participated in a later time point to help prevent further alteration at the central level. The present results also show that, even though the stressor is a physical/environmental stressor, all components of the biological systems participate in the regulation of the response process and the recovery process, including neuroendocrine, metabolism, and immunity.

Expression and characterization of rainbow trout Oncorhynchus mykiss recombinant myoglobin

Recombinant expression system was established for rainbow trout myoglobin (Mb) considering its unique primary structure of having one unusual deletion and two cysteine residues in contrast to the other fish Mbs. The obtained recombinant Mb without His-tag showed non-cooperative thermal denaturation profile. The presence of free cysteine residue(s) in rainbow trout Mb was demonstrated by reacting with a sulfhydryl agent, 4, 4´-dithiodipyridine, which ultimately resulted in the oxidation of Mb with characteristic changes in visible absorption spectra. Besides, the recombinant Mb displayed steady peroxidase reactivity indicating in vivo roles of Mb as a reactive oxygen species scavenger. The findings of the present study indicate that the solitary rainbow trout Mb, which ultimately manifest typical secondary structure pattern and corroborate characteristic functionality, can be over expressed in recombinant system devoid of fusion tag.

The More, the Merrier? Multiple Myoglobin Genes in Fish Species, Especially in Gray Bichir (Polypterus senegalus) and Reedfish (Erpetoichthys calabaricus)

The members of the globin superfamily are a classical model system to investigate gene evolution and their fates as well as the diversity of protein function. One of the best-known globins is myoglobin (Mb), which is mainly expressed in heart muscle and transports oxygen from the sarcolemma to the mitochondria. Most vertebrates harbor a single copy of the myoglobin gene, but some fish species have multiple myoglobin genes. Phylogenetic analyses indicate an independent emergence of multiple myoglobin genes, whereby the origin is mostly the last common ancestor of each order. By analyzing different transcriptome data sets, we found at least 15 multiple myoglobin genes in the polypterid gray bichir (Polypterus senegalus) and reedfish (Erpetoichthys calabaricus). In reedfish, the myoglobin genes are expressed in a broad range of tissues but show very different expression values. In contrast, the Mb genes of the gray bichir show a rather scattered expression pattern; only a few Mb genes were found expressed in the analyzed tissues. Both, gray bichir and reedfish possess lungs which enable them to inhabit shallow and swampy waters throughout tropical Africa with frequently fluctuating and low oxygen concentrations. The myoglobin repertoire probably reflects the molecular adaptation to these conditions. The sequence divergence, the substitution rate, and the different expression pattern of multiple myoglobin genes in gray bichir and reedfish imply different functions, probably through sub- and neofunctionalization during evolution.

Myoglobin primary structure reveals multiple convergent transitions to semi-aquatic life in the world's smallest mammalian divers

The speciose mammalian order Eulipotyphla (moles, shrews, hedgehogs, solenodons) combines an unusual diversity of semi-aquatic, semi-fossorial, and fossorial forms that arose from terrestrial forbearers. However, our understanding of the ecomorphological pathways leading to these lifestyles has been confounded by a fragmentary fossil record, unresolved phylogenetic relationships, and potential morphological convergence, calling for novel approaches. The net surface charge of the oxygen-storing muscle protein myoglobin (ZMb), which can be readily determined from its primary structure, provides an objective target to address this question due to mechanistic linkages with myoglobin concentration. Here, we generate a comprehensive 71 species molecular phylogeny that resolves previously intractable intra-family relationships and then ancestrally reconstruct ZMb evolution to identify ancient lifestyle transitions based on protein sequence alone. Our phylogenetically informed analyses confidently resolve fossorial habits having evolved twice in talpid moles and reveal five independent secondary aquatic transitions in the order housing the world's smallest endothermic divers.

Lessons from the post-genomic era: Globin diversity beyond oxygen binding and transport

Vertebrate hemoglobin (Hb) and myoglobin (Mb) were among the first proteins whose structures and sequences were determined over 50 years ago. In the subsequent pregenomic period, numerous related proteins came to light in plants, invertebrates and bacteria, that shared the myoglobin fold, a signature sequence motif characteristic of a 3-on-3 α-helical sandwich. Concomitantly, eukaryote and bacterial globins with a truncated 2-on-2 α-helical fold were discovered. Genomic information over the last 20 years has dramatically expanded the list of known globins, demonstrating their existence in a limited number of archaeal genomes, a majority of bacterial genomes and an overwhelming majority of eukaryote genomes. In vertebrates, 6 additional globin types were identified, namely neuroglobin (Ngb), cytoglobin (Cygb), globin E (GbE), globin X (GbX), globin Y (GbY) and androglobin (Adgb). Furthermore, functions beyond the familiar oxygen transport and storage have been discovered within the vertebrate globin family, including NO metabolism, peroxidase activity, scavenging of free radicals, and signaling functions. The extension of the knowledge on globin functions suggests that the original roles of bacterial globins must have been enzymatic, involved in defense against NO toxicity, and perhaps also as sensors of O₂, regulating taxis away or towards high O₂ concentrations. In this review, we aimed to discuss the evolution and remarkable functional diversity of vertebrate globins with particular focus on the variety of non-canonical expression sites of mammalian globins and their according impressive variability of atypical functions.

Structural and functional properties of Antarctic fish cytoglobins-1: Cold-reactivity in multi-ligand reactions

While the functions of the recently discovered cytoglobin, ubiquitously expressed in vertebrate tissues, remain uncertain, Antarctic fish provide unparalleled models to study novel protein traits that may arise from cold adaptation. We report here the spectral, ligand-binding and enzymatic properties (peroxynitrite isomerization, nitrite-reductase activity) of cytoglobin-1 from two Antarctic fish, Chaenocephalus aceratus and Dissostichus mawsoni, and present the crystal structure of D. mawsoni cytoglobin-1. The Antarctic cytoglobins-1 display high O₂ affinity, scarcely compatible with an O₂-supply role, a slow rate constant for nitrite-reductase activity, and do not catalyze peroxynitrite isomerization. Compared with mesophilic orthologues, the cold-adapted cytoglobins favor binding of exogenous ligands to the hexa-coordinated bis-histidyl species, a trait related to their higher rate constant for distal-His/heme-Fe dissociation relative to human cytoglobin. At the light of a remarkable 3D-structure conservation, the observed differences in ligand-binding kinetics may reflect Antarctic fish cytoglobin-1 specific features in the dynamics of the heme distal region and of protein matrix cavities, suggesting adaptation to functional requirements posed by the cold environment. Taken together, the biochemical and biophysical data presented suggest that in Antarctic fish, as in humans, cytoglobin-1 unlikely plays a role in O₂ transport, rather it may be involved in processes such as NO detoxification.

The role of myoglobin in epithelial cancers: Insights from transcriptomics

The muscle-associated respiratory protein myoglobin (MB) is expressed in multiple types of cancer, including breast and prostate tumors. In Kaplan-Meier analyses of the two tumor types, MB positivity is associated with favorable prognoses. Despite its well-characterized function in myocytes, the role of MB in cancer remains unclear. To study the impact of endogenous MB expression, small interfering RNA MB-knockdown cells were engineered using breast, prostate and colon cancer cell lines (MDA-MB468, LNCaP, DLD-1), and their transcriptomes were investigated using RNA-Seq at different oxygen levels. In MB-positive cells, increased expression of glycolytic genes was observed, which was possibly mediated by a higher activity of hypoxia-inducible factor 1α. In addition, the results of the gene set enrichment analysis suggested that MB contributed to fatty acid transport and turnover. MB-positive, wild-type-p53 LNCaP cells also exhibited increased expression of p53 target genes involved in cell cycle checkpoint control and prevention of cell migration. MB-positive cells expressing mutant p53 exhibited upregulation of genes associated with prolonged cancer cell viability and motility. Therefore, it was hypothesized that these transcriptomic differences may result from MB-mediated generation of nitric oxide or reactive oxygen species, thus employing established enzymatic activities of the globin. In summary, the transcriptome comparisons identified potential molecular functions of MB in carcinogenesis by highlighting the interaction of MB with key metabolic and regulatory processes.

Adaptations to environmental change: Globin superfamily evolution in Antarctic fishes

The ancient origins and functional versatility of globins make them ideal subjects for studying physiological adaptation to environmental change. Our goals in this review are to describe the evolution of the vertebrate globin gene superfamily and to explore the structure/function relationships of hemoglobin, myoglobin, neuroglobin and cytoglobin in teleost fishes. We focus on the globins of Antarctic notothenioids, emphasizing their adaptive features as inferred from comparisons with human proteins. We dedicate this review to Guido di Prisco, our co-author, colleague, friend, and husband of C.V. Ever thoughtful, creative, and enthusiastic, Guido spearheaded study of the structure, function, and evolution of the hemoglobins of polar fishes - this review is testimony to his wide-ranging contributions. Throughout his career, Guido inspired younger scientists to embrace polar biological research, and he challenged researchers of all ages to explore evolutionary adaptation in the context of global climate change. Beyond his scientific contributions, we will miss his warmth, his culture, and his great intellect. Guido has left an outstanding legacy, one that will continue to inspire us and our research.

Genetic and functional diversity of the multiple lungfish myoglobins

It is known that the West African lungfish (Protopterus annectens) harbours multiple myoglobin (Mb) genes that are differentially expressed in various tissues and that the Mbs differ in their abilities to confer tolerance towards hypoxia. Here, we show that other lungfish species (Protopterus dolloi, Protopterus aethiopicus and Lepidosiren paradoxa) display a similar diversity of Mb genes and have orthologous Mb genes. To investigate the functional diversification of these genes, we studied the structures, O₂ binding properties and nitrite reductase enzymatic activities of recombinantly expressed P. annectens Mbs (PanMbs). CD spectroscopy and small-angle X-ray scattering revealed the typical globin-fold in all investigated recombinant Mbs, indicating a conserved structure. The highest O₂ affinity was measured for PanMb2 (P₅₀  = 0.88 Torr at 20 °C), which is mainly expressed in the brain, whereas the muscle-specific PanMb1 has the lowest O₂ affinity (P₅₀  = 3.78 Torr at 20 °C), suggesting that tissue-specific O₂ requirements have resulted in the emergence of distinct Mb types. Two of the mainly neuronally expressed Mbs (PanMb3 and PanMb4b) have the highest nitrite reductase rates. These data show different O₂ binding and enzymatic properties of lungfish Mbs, reflecting multiple subfunctionalisation and neofunctionalisation events that occurred early in the evolution of lungfish. Some Mbs may have also taken over the functions of neuroglobin and cytoglobin, which are widely expressed in vertebrates but appear to be missing in lungfish.

Myoglobin as a versatile peroxidase: Implications for a more important role for vertebrate striated muscle in antioxidant defense

Myoglobins (Mb) are ubiquitous proteins found in striated muscle of nearly all vertebrate taxa. Although their function is most commonly associated with facilitating oxygen storage and diffusion, Mb has also been implicated in cellular antioxidant defense. The oxidized (Fe³⁺) form of Mb (metMB) can react with hydrogen peroxide (H₂O₂) to produce ferrylMb. FerrylMb can be reduced back to metMb for another round of reaction with H₂O₂. In the present study, we have shown that horse skeletal muscle Mb displays peroxidase activity using 2,2'-azino-di-(3-ethylbenzothiazoline)-6-sulfonic acid (ABTS) and 3,3',5,5'-tetramethylbenzidine (TMB) as reducing substrates, as well as the biologically-relevant substrates NADH/NADPH, ascorbate, caffeic acid, and resveratrol. We have also shown that ferrylMb can be reduced by both ethanol and acetaldehyde, which are known to accumulate in some vertebrate tissues under anaerobic conditions, such as anoxic goldfish and crucian carp, implying a potential mechanism for ethanol detoxification in striated muscle. We found that metMb peroxidase activity is pH-dependent, increasing as pH decreases from 7.4 to 6.1, which is biologically relevant to anaerobic vertebrate muscle when incurring intracellular lactic acidosis. Finally, we found that metMb reacts with hypochlorite in a heme-dependent fashion, indicating that Mb could play a role in hypochlorite detoxification. Taken together, these data suggest that Mb peroxidase activity might be an important antioxidant mechanism in vertebrate cardiac and skeletal muscle under a variety of physiological conditions, such as those that might occur in contracting skeletal muscle or during hypoxia.

Genome sequence of walking catfish (Clarias batrachus) provides insights into terrestrial adaptation

Background

Walking catfish (Clarias batrachus) is a freshwater fish capable of air-breathing and locomotion on land. It usually inhabits various low-oxygen habitats, burrows inside the mudflat, and sometimes “walks” to search for suitable environments during summer. It has evolved accessory air-breathing organs for respiring air and corresponding mechanisms to survive in such challenging environments. Thereby, it serves as a great model for understanding adaptations to terrestrial life.

Results

Comparative genomics with channel catfish (Ictalurus punctatus) revealed specific adaptations of C. batrachus in DNA repair, enzyme activator activity, and small GTPase regulator activity. Comparative analysis with 11 non-air-breathing fish species suggested adaptive evolution in gene expression and nitrogenous waste metabolic processes. Further, myoglobin, olfactory receptor related to class A G protein-coupled receptor 1, and sulfotransferase 6b1 genes were found to be expanded in the air-breathing walking catfish genome, with 15, 15, and 12 copies, respectively, compared to non-air-breathing fishes that possess only 1–2 copies of these genes. Additionally, we sequenced and compared the transcriptomes of the gill and the air-breathing organ to characterize the mechanism of aerial respiration involved in elastic fiber formation, oxygen binding and transport, angiogenesis, ion homeostasis and acid-base balance. The hemoglobin genes were expressed dramatically higher in the air-breathing organ than in the gill of walking catfish.

Conclusions

This study provides an important genomic resource for understanding the adaptive mechanisms of walking catfish to terrestrial environments. It is possible that the coupling of enhanced abilities for oxygen storage and oxygen transport through genomic expansion of myoglobin genes and transcriptomic up-regulation of hemoglobin and angiogenesis-related genes are important components of the molecular basis for adaptation of this aquatic species to terrestrial life.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5355-9) contains supplementary material, which is available to authorized users.

Expression levels of myoglobin in muscle and non-muscle tissues of rainbow trout Oncorhynchus mykiss, a hypoxia intolerant species

Myoglobin (Mb) is one of the most intensively studied intracellular respiratory muscle proteins. Since the discovery of the fascinating fact that Mb is not confined only to oxidative muscle tissues but also is co-localized in different non-muscle tissues of cyprinids, hypoxia tolerant cyprinids have been established as the model teleost. Mb both at mRNA and protein levels have been reported in this study for the first time from a number of muscle and non-muscle tissues of rainbow trout Oncorhynchus mykiss, a hypoxia intolerant species. Mb transcript levels were high in the heart and slow skeletal muscle, and were comparatively high in the gonad and gill among the non-muscle tissues. Western-blotting by using anti-rainbow trout Mb peptide rabbit antibody detected Mb protein in the muscles and several non-muscle tissues. By both RNA in situ hybridization and immunofluorescence, Mb was localized in the cardiomyocytes and oxidative muscle fibers. On the other hand, Mb both at mRNA and protein levels was restricted to the lamellar epithelial cells of the gill, epithelial layers of hepato-biliary duct, neurons and endothelial cells of brain, ooplasm of gonad, kidney tubules, endothelial cells, and epithelial layer of intestine. Neuroglobin isoform 1 and 2 mRNAs along with Mb mRNA were localized in the granular layer of cerebellum. Considering the previous data reported for cyprinids, the expression sites of Mb in the muscle and non-muscle tissues of teleost could be universal, where Mb concerted with the other globins might play meaningful physiological roles.

Molecular evolution of myoglobin in the Tibetan Plateau endemic schizothoracine fish (Cyprinidae, Teleostei) and tissue-specific expression changes under hypoxia

Myoglobin (Mb) is an oxygen-binding hemoprotein that was once thought to be exclusively expressed in oxidative myocytes of skeletal and cardiac muscle where it serves in oxygen storage and facilitates intracellular oxygen diffusion. In this study, we cloned the coding sequence of the Mb gene from four species, representing three groups, of the schizothoracine fish endemic to the Qinghai-Tibetan Plateau (QTP), then conducted molecular evolution analyses. We also investigated tissue expression patterns of Mb and the expression response to moderate and severe hypoxia at the mRNA and protein levels in a representative of the highly specialized schizothoracine fish species, Schizopygopsis pylzovi. Molecular evolution analyses showed that Mb from the highly specialized schizothoracine fish have undergone positive selection and one positively selected residue (81L) was identified, which is located in the F helix, close to or in contact with the heme. We present tentative evidence that the Mb duplication event occurred in the ancestor of the schizothoracine and Cyprininae fish (common carp and goldfish), and that the Mb2 paralog was subsequently lost in the schizothoracine fish. In S. pylzovi, Mb mRNA is expressed in various tissues with the exception of the intestine and gill, but all such tissues, including the liver, muscle, kidney, brain, eye, and skin, expressed very low levels of Mb mRNA (< 8.0%) relative to that of the heart. The trace levels of Mb expression in non-muscle tissues are perhaps the major reason why non-muscle Mb remained undiscovered for so long. The expression response of the Mb gene to hypoxia at the mRNA and protein levels was strikingly different in S. pylzovi compared to that found in the common carp, medaka, zebrafish, and goldfish, suggesting that the hypoxia response of Mb in fish may be species and tissue-specific. Notably, severe hypoxia induced significant expression of Mb at the mRNA and protein levels in the S. pylzovi heart, which suggests Mb has a major role in the supply of oxygen to the heart of Tibetan Plateau fish.

Evolution and Expression of Tissue Globins in Ray-Finned Fishes

The globin gene family encodes oxygen-binding hemeproteins conserved across the major branches of multicellular life. The origins and evolutionary histories of complete globin repertoires have been established for many vertebrates, but there remain major knowledge gaps for ray-finned fish. Therefore, we used phylogenetic, comparative genomic and gene expression analyses to discover and characterize canonical “non-blood” globin family members (i.e., myoglobin, cytoglobin, neuroglobin, globin-X, and globin-Y) across multiple ray-finned fish lineages, revealing novel gene duplicates (paralogs) conserved from whole genome duplication (WGD) and small-scale duplication events. Our key findings were that: (1) globin-X paralogs in teleosts have been retained from the teleost-specific WGD, (2) functional paralogs of cytoglobin, neuroglobin, and globin-X, but not myoglobin, have been conserved from the salmonid-specific WGD, (3) triplicate lineage-specific myoglobin paralogs are conserved in arowanas (Osteoglossiformes), which arose by tandem duplication and diverged under positive selection, (4) globin-Y is retained in multiple early branching fish lineages that diverged before teleosts, and (5) marked variation in tissue-specific expression of globin gene repertoires exists across ray-finned fish evolution, including several previously uncharacterized sites of expression. In this respect, our data provide an interesting link between myoglobin expression and the evolution of air breathing in teleosts. Together, our findings demonstrate great-unrecognized diversity in the repertoire and expression of nonblood globins that has arisen during ray-finned fish evolution.

Insulin-Like Growth Factor-Binding Proteins of Teleost Fishes

The insulin-like growth factor (Igf) binding protein (Igfbp) family has a broad range of physiological functions and a fascinating evolutionary history. This review focuses on the Igfbps of teleost fishes, where genome duplication events have diversified gene repertoire, function, and physiological regulation-with six core Igfbps expanded into a family of over twenty genes in some lineages. In addition to briefly summarizing the current state of knowledge on teleost Igfbp evolution, function, and expression-level regulation, we highlight gaps in our understanding and promising areas for future work.

Functional roles of globin proteins in hypoxia-tolerant ectothermic vertebrates

Globins are heme-containing proteins ubiquitously expressed in vertebrates, where they serve a broad range of biological functions, directly or indirectly related to the tight control of oxygen levels and its toxic products in vivo. Perhaps the most investigated of all proteins, hemoglobin and myoglobin are primarily involved in oxygen transport and storage, but also in facilitating arterial vasodilation, suppressing mitochondrial respiration, and preventing tissue oxidative damage via accessory redox enzymatic activities during hypoxia. By contrast, the more recently discovered neuroglobin and cytoglobin do not seem to function as reversible oxygen carriers and are instead involved in redox activities, although their exact biological roles remain to be clarified. In this context, hypoxia-tolerant ectotherms, such as freshwater turtles and members of the carp family that survive winter in extreme hypoxia, have proven as excellent models to appreciate the diversity of biological functions of globin proteins. Unraveling physiological roles of globin proteins in these extreme animals will clarify an important part of the adaptive mechanisms for surviving extreme fluctuations of oxygen availability that are prohibitive to mammals.

Diversification of the duplicated Rab1a genes in a hypoxia-tolerant fish, common carp (Cyprinus carpio)

Common carp is a widely cultivated fish with longer than 2,000 years domestication history, due to its strong environmental adaptabilities, especially hypoxia tolerance. The common carp genome has experienced a very recent whole genome duplication (WGD) event. Among a large number of highly similar duplicated genes, a pair of Ras-associated binding-GTPase 1a (Rab1a) genes were found fast diverging. Four analogous Rab1a genes were identified in the common carp genome. Comparisons of gene structures and sequences indicated Rab1a-1 and Rab1a-2 was a pair of fast diverging duplicates, while Rab1a-3 and Rab1a-4 was a pair of less diverged duplicates. All putative Rab1a proteins shared conserved GTPase domain, which enabled the proteins serve as molecular switches for vesicular trafficking. Rab1a-1 and Rab1a-2 proteins varied in their C-terminal sequences, which were generally considered to encode the membrane localization signals. Differential expression patterns were observed between Rab1a-1 and Rab1a-2 genes. In blood, muscle, spleen, and heart, the mRNA level of Rab1a-1 was higher than that of Rab1a-2. In liver and intestine, the mRNA level of Rab1a-2 was higher. Expression of Rab1a-1 and Rab1a-2 showed distinct hypoxia responses. Under severe hypoxia, Rab1a-1 expression was down-regulated in blood, while Rab1a-2 expression was up-regulated in liver. Compared with the less diverged Rab1a-3/4 gene pair, common carp Rab1a-1/2 gene pair exhibited strong characteristics of sub-functionalization, which might contribute to a sophisticated and efficient Ras-dependent regulating network for the hypoxia-tolerant fish.

Kinetic properties and heme pocket structure of two domains of the polymeric hemoglobin of Artemia in comparison with the native molecule

In this project, we studied some physicochemical properties of two different globin domains of the polymeric hemoglobin of the brine shrimp Artemia salina and compared them with those of the native molecule. Two domains (AsHbC1D1 and AsHbC1D5) were cloned and expressed in BL21(DE3)pLysS strain of Escherichia coli. The recombinant proteins as well as the native hemoglobin (AfHb) were purified from bacteria and frozen Artemia, respectively by standard chromatographic methods and assessed by SDS-PAGE. The heme environment of these proteins was studied by optical spectroscopy and ligand-binding kinetics (e.g. CO association and O2 binding affinity) were measured for the two recombinant proteins and the native hemoglobin. This indicates that the CO association rate for AsHbC1D1 is higher than that of AsHbC1D5 and AfHb, while the calculated P50 value for AsHbC1D1 is lower than that of AsHbC1D5 and AfHb. The geminate and bimolecular rebinding parameters indicate a significant difference between both domains. Moreover, EPR results showed that the heme pocket in AfHb is in a more closed conformation than the heme pocket in myoglobin. Finally, the reduction potential of -0.13V versus the standard hydrogen electrode was determined for AfHb by direct electrochemical measurements. It is about 0.06V higher than the potential of the single domain AsHbC1D5. This work shows that each domain in the hemoglobin of Artemia has different characteristics of ligand binding.

Myoglobin expression in prostate cancer is correlated to androgen receptor expression and markers of tumor hypoxia

Recent studies identified unexpected expression and transcriptional complexity of the hemoprotein myoglobin (MB) in human breast cancer but its role in prostate cancer is still unclear. Expression of MB was immunohistochemically analyzed in three independent cohorts of radical prostatectomy specimens (n = 409, n = 625, and n = 237). MB expression data were correlated with clinicopathological parameters and molecular parameters of androgen and hypoxia signaling. Expression levels of novel tumor-associated MB transcript variants and the VEGF gene as a hypoxia marker were analyzed using qRT-PCR. Fifty-three percent of the prostate cancer cases were MB positive and significantly correlated with androgen receptor (AR) expression (p < 0.001). The positive correlation with CAIX (p < 0.001) and FASN (p = 0.008) as well as the paralleled increased expression of the tumor-associated MB transcript variants and VEGF suggest that hypoxia participates in MB expression regulation. Analogous to breast cancer, MB expression in prostate cancer is associated with steroid hormone signaling and markers of hypoxia. Further studies must elucidate the novel functional roles of MB in human carcinomas, which probably extend beyond its classic intramuscular function in oxygen storage.

Nitrosylation mechanisms of Mycobacterium tuberculosis and Campylobacter jejuni truncated hemoglobins N, O, and P

Truncated hemoglobins (trHbs) are widely distributed in bacteria and plants and have been found in some unicellular eukaryotes. Phylogenetic analysis based on protein sequences shows that trHbs branch into three groups, designated N (or I), O (or II), and P (or III). Most trHbs are involved in the O₂/NO chemistry and/or oxidation/reduction function, permitting the survival of the microorganism in the host. Here, a detailed comparative analysis of kinetics and/or thermodynamics of (i) ferrous Mycobacterium tubertulosis trHbs N and O (Mt-trHbN and Mt-trHbO, respectively), and Campylobacter jejuni trHb (Cj-trHbP) nitrosylation, (ii) nitrite-mediated nitrosylation of ferrous Mt-trHbN, Mt-trHbO, and Cj-trHbP, and (iii) NO-based reductive nitrosylation of ferric Mt-trHbN, Mt-trHbO, and Cj-trHbP is reported. Ferrous and ferric Mt-trHbN and Cj-trHbP display a very high reactivity towards NO; however, the conversion of nitrite to NO is facilitated primarily by ferrous Mt-trHbN. Values of kinetic and/or thermodynamic parameters reflect specific trHb structural features, such as the ligand diffusion pathways to/from the heme, the heme distal pocket structure and polarity, and the ligand stabilization mechanisms. In particular, the high reactivity of Mt-trHbN and Cj-trHbP reflects the great ligand accessibility to the heme center by two protein matrix tunnels and the E7-path, respectively, and the penta-coordination of the heme-Fe atom. In contrast, the heme-Fe atom of Mt-trHbO the ligand accessibility to the heme center of Mt-trHbO needs large conformational readjustments, thus limiting the heme-based reactivity. These results agree with different roles of Mt-trHbN, Mt-trHbO, and Cj-trHbP in vivo.

Function and evolution of vertebrate globins

Globins are haem-proteins that bind O2 and thus play an important role in the animal's respiration and oxidative energy production. However, globins may also have other functions such as the decomposition or production of NO, the detoxification of reactive oxygen species or intracellular signalling. In addition to the well-investigated haemoglobins and myoglobins, genome sequence analyses have led to the identification of six further globin types in vertebrates: androglobin, cytoglobin, globin E, globin X, globin Y and neuroglobin. Here, we review the present state of knowledge on the functions, the taxonomic distribution and evolution of vertebrate globins, drawing conclusions about the functional changes underlying present-day globin diversity.

Oxygen-linked S-nitrosation in fish myoglobins: a cysteine-specific tertiary allosteric effect

The discovery that cysteine (Cys) S-nitrosation of trout myoglobin (Mb) increases heme O₂ affinity has revealed a novel allosteric effect that may promote hypoxia-induced nitric oxide (NO) delivery in the trout heart and improve myocardial efficiency. To better understand this allosteric effect, we investigated the functional effects and structural origin of S-nitrosation in selected fish Mbs differing by content and position of reactive cysteine (Cys) residues. The Mbs from the Atlantic salmon and the yellowfin tuna, containing two and one reactive Cys, respectively, were S-nitrosated in vitro by reaction with Cys-NO to generate Mb-SNO to a similar yield (∼0.50 SH/heme), suggesting reaction at a specific Cys residue. As found for trout, salmon Mb showed a low O₂ affinity (P₅₀ = 2.7 torr) that was increased by S-nitrosation (P₅₀ = 1.7 torr), whereas in tuna Mb, O₂ affinity (P₅₀ = 0.9 torr) was independent of S-nitrosation. O₂ dissociation rates (k_off) of trout and salmon Mbs were not altered when Cys were in the SNO or N-ethylmaleimide (NEM) forms, suggesting that S-nitrosation should affect O₂ affinity by raising the O₂ association rate (k_on). Taken together, these results indicate that O₂-linked S-nitrosation may occur specifically at Cys107, present in salmon and trout Mb but not in tuna Mb, and that it may relieve protein constraints that limit O₂ entry to the heme pocket of the unmodified Mb by a yet unknown mechanism. UV-Vis and resonance Raman spectra of the NEM-derivative of trout Mb (functionally equivalent to Mb-SNO and not photolabile) were identical to those of the unmodified Mb, indicating that S-nitrosation does not affect the extent or nature of heme-ligand stabilization of the fully ligated protein. The importance of S-nitrosation of Mb in vivo is confirmed by the observation that Mb-SNO is present in trout hearts and that its level can be significantly reduced by anoxic conditions.

Molecular characterization of Clonorchis sinensis secretory myoglobin: delineating its role in anti-oxidative survival

Background

Clonorchiasis is a globally important, neglected food-borne disease caused by Clonorchis sinensis (C. sinensis), and it is highly related to cholangiocarcinoma and hepatocellular carcinoma. Increased molecular evidence has strongly suggested that the adult worm of C. sinensis continuously releases excretory-secretory proteins (ESPs), which play important roles in the parasite-host interactions, to establish successful infection and ensure its own survival. Myoglobin, a hemoprotein, is present in high concentrations in trematodes and ESPs. To further understand the biological function of CsMb and its putative roles in the interactions of C. sinensis with its host, we explored the molecular characterization of CsMb in this paper.

Methods

We expressed CsMb and its mutants in E. coli BL21 and identified its molecular characteristics using bioinformatics analysis and experimental approaches. Reverse transcription PCR analysis was used to measure myoglobin transcripts of C. sinensis with different culture conditions. The peroxidase activity of CsMb was confirmed by spectrophotometry. We co-cultured RAW264.7 cells with recombinant CsMb (rCsMb), and we then measured the production of hydrogen peroxide (H₂O₂) and nitric oxide (NO) in addition to the mRNA levels of inducible nitric oxide synthase (iNOS), Cu-Zn superoxide dismutase (SOD1) and Mn superoxide dismutase (SOD2) in activated RAW264.7 cells.

Results

In the in vitro culture of adult worms, the transcripts of CsMb increased with the increase of oxygen content. Oxidative stress conditions induced by H₂O₂ increased the levels of CsMb transcripts in a dose-dependent manner. Furthermore, CsMb catalyzed oxidation reactions in the presence of H₂O₂, and amino acid 34 of CsMb played an essential role in its reaction with H₂O₂. In addition, CsMb significantly reduced H₂O₂ and NO levels in LPS-activated macrophages, and CsMb downregulated iNOS and SOD expression in activated macrophages.

Conclusion

The present study is the first to investigate the peroxidase activity of CsMb. This investigation suggested that C. sinensis may decrease the redox activation of macrophages by CsMb expression to evade host immune responses. These studies contribute to a better understanding of the role of CsMb in the molecular mechanisms involved in ROS detoxification by C. sinensis.

Nitrite-reductase and peroxynitrite isomerization activities of Methanosarcina acetivorans protoglobin

Within the globin superfamily, protoglobins (Pgb) belong phylogenetically to the same cluster of two-domain globin-coupled sensors and single-domain sensor globins. Multiple functional roles have been postulated for Methanosarcina acetivorans Pgb (Ma-Pgb), since the detoxification of reactive nitrogen and oxygen species might co-exist with enzymatic activity(ies) to facilitate the conversion of CO to methane. Here, the nitrite-reductase and peroxynitrite isomerization activities of the CysE20Ser mutant of Ma-Pgb (Ma-Pgb*) are reported and analyzed in parallel with those of related heme-proteins. Kinetics of nitrite-reductase activity of ferrous Ma-Pgb* (Ma-Pgb*-Fe(II)) is biphasic and values of the second-order rate constant for the reduction of NO₂^– to NO and the concomitant formation of nitrosylated Ma-Pgb*-Fe(II) (Ma-Pgb*-Fe(II)-NO) are k_app1 = 9.6±0.2 M^–1 s^–1 and k_app2 = 1.2±0.1 M^–1 s^–1 (at pH 7.4 and 20°C). The k_app1 and k_app2 values increase by about one order of magnitude for each pH unit decrease, between pH 8.3 and 6.2, indicating that the reaction requires one proton. On the other hand, kinetics of peroxynitrite isomerization catalyzed by ferric Ma-Pgb* (Ma-Pgb*-Fe(III)) is monophasic and values of the second order rate constant for peroxynitrite isomerization by Ma-Pgb*-Fe(III) and of the first order rate constant for the spontaneous conversion of peroxynitrite to nitrate are h_app = 3.8×10⁴ M^–1 s^–1 and h₀ = 2.8×10^–1 s^–1 (at pH 7.4 and 20°C). The pH-dependence of h_on and h₀ values reflects the acid-base equilibrium of peroxynitrite (pK_a = 6.7 and 6.9, respectively; at 20°C), indicating that HOONO is the species that reacts preferentially with the heme-Fe(III) atom. These results highlight the potential role of Pgbs in the biosynthesis and scavenging of reactive nitrogen and oxygen species.

Increase of Myoglobin in Rat Gastrocnemius Muscles with Immobilization-induced Atrophy

[Purpose] Atrophy is a common phenomenon caused by prolonged muscle disuse associated with bed-rest, aging, and immobilization. However, changes in the expression of atrophy-related myoglobin are still poorly understood. In the present study, we examined whether or not myoglobin expression is altered in the gastrocnemius muscles of rats after seven days of cast immobilization. [Methods] We conducted a protein expression and high-resolution differential proteomic analysis using, two-dimensional gel electrophoresis and matrix-assisted laser desorption ionization time-of-flight/time-of-flight mass spectrometry, and western blotting. [Results] The density and expression of myoglobin increased significantly more in atrophic gastrocnemius muscle strips than they did in the control group. [Conclusion] The results suggest that cast immobilization-induced atrophy may be related to changes in the expression of myoglobin in rat gastrocnemius muscles.

Evolution of mammalian diving capacity traced by myoglobin net surface charge

Extended breath-hold endurance enables the exploitation of the aquatic niche by numerous mammalian lineages and is accomplished by elevated body oxygen stores and adaptations that promote their economical use. However, little is known regarding the molecular and evolutionary underpinnings of the high muscle myoglobin concentration phenotype of divers. We used ancestral sequence reconstruction to trace the evolution of this oxygen-storing protein across a 130-species mammalian phylogeny and reveal an adaptive molecular signature of elevated myoglobin net surface charge in diving species that is mechanistically linked with maximal myoglobin concentration. This observation provides insights into the tempo and routes to enhanced dive capacity evolution within the ancestors of each major mammalian aquatic lineage and infers amphibious ancestries of echidnas, moles, hyraxes, and elephants, offering a fresh perspective on the evolution of this iconic respiratory pigment.

Expression patterns and adaptive functional diversity of vertebrate myoglobins

Recent years have witnessed a new round of research on one of the most studied proteins - myoglobin (Mb), the oxygen (O2) carrier of skeletal and heart muscle. Two major discoveries have stimulated research in this field: 1) that Mb has additional protecting functions, such as the regulation of in vivo levels of the signaling molecule nitric oxide (NO) by scavenging and generating NO during normoxia and hypoxia, respectively; and 2) that Mb in vertebrates (particularly fish) is expressed as tissue-specific isoforms in other tissues than heart and skeletal muscle, such as vessel endothelium, liver and brain, as found in cyprinid fish. Furthermore, Mb has also been found to protect against oxidative stress after hypoxia and reoxygenation and to undergo allosteric, O2-linked S-nitrosation, as in rainbow trout. Overall, the emerging evidence, particularly from fish species, indicates that Mb fulfills a broader array of physiological functions in a wider range of different tissues than hitherto appreciated. This new knowledge helps to better understand how variations in Mb structure and function may correlate with differences in animals' lifestyles and hypoxia-tolerance. This review integrates old and new results on Mb expression patterns and functional properties amongst vertebrates and discusses how these may relate to adaptive variations in different species. This article is part of a special issue entitled: Oxygen Binding and Sensing Proteins.

Isolation of heat-tolerant myoglobin from Asian swamp eel Monopterus albus

Myoglobin from Asian swamp eel Monopterus albus was purified from fish muscle using salt fractionation followed by column chromatography and molecular filtration. The purified Mb of 0.68 mg/g wet weight of muscle was determined for its molecular mass by MALDI-TOF-MS to be 15,525.18 Da. Using isoelectric focusing technique, the purified Mb showed two derivatives with pI of 6.40 and 7.12. Six peptide fragments of this protein identified by LC-MS/MS were homologous to Mbs of sea raven Hemitripterus americanus, yellowfin tuna Thunnus albacores, blue marlin Makaira nigicans, common carp Cyprinus carpio, and goldfish Carassius auratus. According to the Mb denaturation, the swamp eel Mb had thermal stability higher than walking catfish Clarias batrachus Mb and striped catfish Pangasius hypophthalmus Mb, between 30 and 60 (°)C. For the thermal stability of Mb, the swamp eel Mb showed a biphasic behavior due to the O(2) dissociation and the heme orientation disorder, with the lowest increase in both Kd(f) and Kd(s). The thermal sensitivity of swamp eel Mb was lower than those of the other Mbs for both of fast and slow reaction stages. These results suggest that the swamp eel Mb globin structure is thermally stable, which is consistent with heat-tolerant behavior of the swamp eel particularly in drought habitat.

Integrating nitric oxide, nitrite and hydrogen sulfide signaling in the physiological adaptations to hypoxia: A comparative approach

Hydrogen sulfide (H(2)S), nitric oxide (NO) and nitrite (NO(2)(-)) are formed in vivo and are of crucial importance in the tissue response to hypoxia, particularly in the cardiovascular system, where these signaling molecules are involved in a multitude of processes including the regulation of vascular tone, cellular metabolic function and cytoprotection. This report summarizes current advances on the mechanisms by which these signaling pathways act and may have evolved in animals with different tolerance to hypoxia, as presented and discussed during the scientific sessions of the annual meeting of the Society for Experimental Biology in 2011 in Glasgow. It also highlights the need and potential for a comparative approach of study and collaborative effort to identify potential link(s) between the signaling pathways involving NO, nitrite and H(2)S in the whole-body responses to hypoxia.