Characterization and functional properties of the extracellular coelomic hemoglobins from the deep-sea, hydrothermal vent scalewormBranchipolynoe symmytilida

A single coelomic cell type is involved in both immune and respiratory functions of the coastal bioindicator annelid: Capitella C-Channel1 from the English Channel

The polychaete Capitella is a typical member of the 'thiobiome', and is commonly used as an eutrophication indicator species in environmental assessment studies. To deal with a sulfide-rich and poisonous surrounding, cells in close contact with the environment, and thus able to play a major role in detoxication and survival, are circulating cells. This work aimed to morpho-functionally describe the circulating coelomic cells of Capitella from the English Channel inhabiting the sulfide-rich mud in Roscoff Harbor. In general, worms have three types of circulating cells, granulocytes involved in bacterial clearance and defense against microorganisms, eleocytes with an essentially trophic role and elimination of cellular waste, and erythrocytes which play a role in detoxification and respiration via their intracellular hemoglobin. By combining diverse microscopic and cellular approaches, we provide evidence that Capitella does not possess granulocytes and eleocytes, but rather a single abundant rounded cell type with the morphological characteristics of erythrocytes i.e. small size and production of intracellular hemoglobin. Surprisingly, our data show that in addition to their respiratory function, these red cells could exert phagocytic activities, and produce an antimicrobial peptide. This latter immune role is usually supported by granulocytes. Our data highlight that the erythrocytes of Capitella from the English Channel differ in morphology and bear more functions than the erythrocytes of other annelids. The simplicity of this multi-task (or polyvalent) single-cell type makes Capitella an interesting model for studies of the impact of the environment on the immunity of this bioindicator species.

Metal interactions between the polychaete Branchipolynoe seepensis and the mussel Bathymodiolus azoricus from Mid-Atlantic-Ridge hydrothermal vent fields

The vent blood-red commensal polynoid polychaete Branchipolynoe seepensis is commonly found in the pallial cavity of the vent mussel Bathymodiolus azoricus, the dominant bivalve species along the Mid-Atlantic-Ridge (MAR) and is known to be kleptoparasitic. Mussels were collected from three hydrothermal vent fields in the MAR: Menez Gwen (850 m depth, MG2, MG3 and MG4), Lucky Strike (1700 m depth, Montségur-MS and Eiffel Tower-ET) and Rainbow (2300 m depth). Polychaetes were absent in all Menez Gwen vent mussels, while the highest percentage was detected in mussels from Lucky Strike, where more than 70% of the mussels had at least one polychaete in their mantle cavity, followed by Rainbow with 33% of mussels with polychaetes. Total metal concentrations (Ag, Cd, Co, Cu, Fe, Mn, Ni and Zn) were determined in polychaetes whole body and in the mussel tissues (gills, digestive gland and mantle). To understand the possible metal interactions between symbiont and host, the activity of antioxidant defence (catalase (CAT), metallothioneins (MTs)), biotransformation enzymes (glutathione-s-transferases (GST)) activities and lipid peroxidation (LPO) were determined in polychaete whole soft tissues and in mussel tissues (gills, digestive gland and mantle). Metal concentrations in polychaetes and mussels tissues indicated that the accumulation patterns were species specific and also influenced by, and possibly dependent upon, the inter- and intra-variation of vent physico-chemistry between hydrothermal fields. Despite not detecting any strong correlations between metal and enzymes activities in polychaetes and mussels, when in presence of polychaetes, mussels presented less metal concentrations in the gills and digestive gland and lower activity of enzymatic biomarkers. This leads to infer that the polychaete plays a role on the detoxification process, and the interaction between the polychaete mussel association is probably an adaptation to metals concentrations at the vent sites.

Evolution of Single-Domain Globins in Hydrothermal Vent Scale-Worms

Hypoxia at deep-sea hydrothermal vents represents one of the most basic challenges for metazoans, which then requires specific adaptations to acquire oxygen to meet their metabolic needs. Hydrothermal vent scale-worms (Polychaeta; Polynoidae) express large amounts of extracellular single- and multi-domain hemoglobins, in contrast with their shallow-water relatives that only possess intracellular globins in their nervous system (neuroglobins). We sequenced the gene encoding the single-domain (SD) globin from nine species of polynoids found in various vent and deep-sea reduced microhabitats (and associated constraints) to determine if the Polynoidae SD globins have been the targets of diversifying selection. Although extracellular, all the SD globins (and multi-domain ones) form a monophyletic clade that clusters within the intracellular globin group of other annelids, indicating that these hemoglobins have evolved from an intracellular myoglobin-like form. Positive selection could not be detected at the major ecological changes that the colonization of the deep-sea and hydrothermal vents represents. This suggests that no major structural modification was necessary to allow the globins to function under these conditions. The mere expression of these globins extracellularly may have been sufficiently advantageous for the polynoids living in hypoxic hydrothermal vents. Among hydrothermal vent species, positively selected amino acids were only detected in the phylogenetic lineage leading to the two mussel-commensal species (Branchipolynoe). In this lineage, the multiplicity of hemoglobins could have lessened the selective pressure on the SD hemoglobin, allowing the acquisition of novel functions by positive Darwinian selection. Conversely, the colonization of hotter environments (species of Branchinotogluma) does not seem to have required additional modifications.

Direct comparison of oligochaete erythrocruorins as potential blood substitutes

While many blood substitutes are based on mammalian hemoglobins (e.g., human hemoglobin, HbA), the naturally extracellular hemoglobins of invertebrates (a.k.a. erythrocruorins, Ecs) are intriguing alternative oxygen carriers. Specifically, the erythrocruorin of Lumbricus terrestris has been shown to effectively deliver oxygen in mice and rats without the negative side effects observed with HbA. In this study, the properties of six oligochaete Ecs (Lumbricus terrestris, Eisenia hortensis, Eisenia fetida, Eisenia veneta, Eudrilus eugeniae, and Amynthas gracilis) were compared in vitro to identify the most promising blood substitute candidate(s). Several metrics were used to compare the Ecs, including their oxidation rates, dissociation at physiological pH, thermal stability, and oxygen transport characteristics. Overall, the Ecs of Lumbricus terrestris (LtEc) and Eisenia fetida (EfEc) were identified as promising candidates, since they demonstrated high thermal and oligomeric stability, while also exhibiting relatively low oxidation rates. Interestingly, the O₂ affinity of LtEc (P₅₀ = 26.25 mmHg at 37 °C) was also observed to be uniquely lower than EfEc and all of the other Ecs (P₅₀ = 9.29–13.62 mmHg). Subsequent alignment of the primary sequences of LtEc and EfEc revealed several significant amino acid substitutions within the D subunit interfaces that may be responsible for this significant change in O₂ affinity. Nonetheless, these results show that LtEc and EfEc are promising potential blood substitutes that are resistant to oxidation and denaturation, but additional experiments will need to be conducted to determine their safety, efficacy, and the effects of their disparate oxygen affinities in vivo.

Adaptation and evolution of deep-sea scale worms (Annelida: Polynoidae): insights from transcriptome comparison with a shallow-water species

Polynoid scale worms (Polynoidae, Annelida) invaded deep-sea chemosynthesis-based ecosystems approximately 60 million years ago, but little is known about their genetic adaptation to the extreme deep-sea environment. In this study, we reported the first two transcriptomes of deep-sea polynoids (Branchipolynoe pettiboneae, Lepidonotopodium sp.) and compared them with the transcriptome of a shallow-water polynoid (Harmothoe imbricata). We determined codon and amino acid usage, positive selected genes, highly expressed genes and putative duplicated genes. Transcriptome assembly produced 98,806 to 225,709 contigs in the three species. There were more positively charged amino acids (i.e., histidine and arginine) and less negatively charged amino acids (i.e., aspartic acid and glutamic acid) in the deep-sea species. There were 120 genes showing clear evidence of positive selection. Among the 10% most highly expressed genes, there were more hemoglobin genes with high expression levels in both deep-sea species. The duplicated genes related to DNA recombination and metabolism, and gene expression were only enriched in deep-sea species. Deep-sea scale worms adopted two strategies of adaptation to hypoxia in the chemosynthesis-based habitats (i.e., rapid evolution of tetra-domain hemoglobin in Branchipolynoe or high expression of single-domain hemoglobin in Lepidonotopodium sp.).

Selective forces acting during multi-domain protein evolution: the case of multi-domain globins

Multi-domain proteins form the majority of proteins in eukaryotes. During their formation by tandem duplication or gene fusion, new interactions between domains may arise as a result of the structurally-forced proximity of domains. The proper function of the formed proteins likely required the molecular adjustment of these stress zones by specific amino acid replacements, which should be detectable by the molecular signature of selection that governed their changes. We used multi-domain globins from three different invertebrate lineages to investigate the selective forces that acted throughout the evolution of these molecules. In the youngest of these molecules [Branchipolynoe scaleworm; original duplication ca. 60 million years (Ma)], we were able to detect some amino acids under positive selection corresponding to the initial duplication event. In older lineages (didomain globin from bivalve mollusks and nematodes), there was no evidence of amino acid positions under positive selection, possibly the result of accumulated non-adaptative mutations since the original duplication event (165 and 245 Ma, respectively). Some amino acids under positive selection were sometimes detected in later branches, either after speciation events, or after the initial duplication event. In Branchipolynoe, the position of the amino acids under positive selection on a 3D model suggests some of them are located at the interface between two domains; while others are locate in the heme pocket.

Origin and evolution of the unique tetra-domain hemoglobin from the hydrothermal vent scale worm Branchipolynoe

Hemoglobin is the most common respiratory pigment in annelids. It can be intra or extracellular, and this latter type can form large multimeric complexes. The hydrothermal vent scale worms Branchipolynoe symmytilida and Branchipolynoe seepensis express an extracellular tetra-domain hemoglobin (Hb) that is unique in annelids. We sequenced the gene for the single-domain and tetra-domain globins in these two species. The single-domain gene codes for a mature protein of 137 amino acids, and the tetra-domain gene codes for a mature protein of 552 amino acids. The single-domain gene has a typical three exon/two intron structure, with introns located at their typical positions (B12.2 and G7.0). This structure is repeated four times in the tetra-domain gene, with no bridge introns or linker sequences between domains. The phylogenetic position of Branchipolynoe globins among known annelid globins revealed that, although extracellular, they cluster within the annelid intracellular globins clade, suggesting that the extracellular state of these Hbs is the result of convergent evolution. The tetra-domain structure likely resulted from two tandem duplications, domain 1 giving rise to domain 2 and after this the two-domain gene duplicated to produce domains 3 and 4. The high O(2) affinity of Branchipolynoe extracellular globins may be explained by the two key residues (B10Y and E7Q) in the heme pocket in each of the domains of the single and tetra-domain globins, which have been shown to be essential in the oxygen-avid Hb from the nematode Ascaris suum. This peculiar globin evolutionary path seems to be very different from other annelid extracellular globins and is most likely the product of evolutionary tinkering associated with the strong selective pressure to adapt to chronic hypoxia that characterizes hydrothermal vents.

Hemoglobin function in deep-sea and hydrothermal-vent endemic fish: Symenchelis parasitica (Anguillidae) and Thermarces cerberus (Zoarcidae)

Deep-sea hydrothermal vents probably provide the harshest physico-chemical conditions confronting metazoan animals in nature. Given the absence of information on hemoglobin (Hb) function in hydrothermal-vent vertebrates, and the complex molecular and functional adaptations observed in hydrothermal-vent invertebrates, we investigated the oxygenation reactions of Hbs from the vent-endemic zoarcid Thermarces cerberus and the deep-sea anguillid Symenchelis parasitica from adjacent habitats. Electrophoretically cathodic and anodic isoHbs from S. parasitica exhibit radical differences in O(2) affinity and pH and organic phosphate (ATP) sensitivities, reflecting a division of labor as in other 'class II' fish that express both Hb types. Remarkably, the cathodic Hb (I) lacks chloride sensitivity, and the anodic Hb (II) shows anticooperativity near half-saturation at low temperature. T. cerberus isoHbs exhibit similar affinities and pH sensitivities ('class I' pattern) but much higher O(2) affinities than those observed in Hbs of the temperate, shallow-water zoarcid Zoarces viviparus, which, unless compensated, reveals markedly higher blood O(2) affinities in the former species. The temperature sensitivity of O(2) binding to T. cerberus Hbs and the anodic S. parasitica Hb, which have normal Bohr effects, is decreased by endothermic proton dissociation, which reduces the effects of ambient temperature variations on O(2) affinity. In the cathodic S. parasitica Hb, similar reduction appears to be associated with endothermic conformational changes that accompany the oxygenation reaction.

Respiratory adaptations in a deep-sea orbiniid polychaete from Gulf of Mexico brine pool NR-1: metabolic rates and hemoglobin structure/function relationships

Methanoaricia dendrobranchiata Blake (Polychaeta; Orbiniidae)occurs in large numbers in association with communities of the mussel Bathymodiolus childressi at hydrocarbon seeps on the Louisiana Slope of the Gulf of Mexico. Its microhabitat can be strongly hypoxic (oxygen is often undetectable) and sulfidic (sulfide concentrations can reach millimolar levels), which may seriously challenge aerobic metabolism. We describe a suite of adaptations to its low-oxygen environment. The worms are capable of regulating their rate of oxygen consumption down to partial pressures of approximately 870 Pa oxygen. This capability correlates with a large gill surface area, a small diffusion distance from sea water to blood, a very high hemoglobin oxygen-affinity (P50=27.8 Pa at 10°C and pH 7.6) and a Bohr effect that is pronounced at high oxygen saturations. When fully saturated, the hemoglobin binds sufficient oxygen for only 31 min of aerobic metabolism. However, these polychaetes can withstand extended periods of anoxia both in the absence and presence of 1 mmoll-1 sulfide(TL50=approx. 5.5 and 4 days, respectively).

Nonvertebrate hemoglobins: functions and molecular adaptations

Hemoglobin (Hb) occurs in all the kingdoms of living organisms. Its distribution is episodic among the nonvertebrate groups in contrast to vertebrates. Nonvertebrate Hbs range from single-chain globins found in bacteria, algae, protozoa, and plants to large, multisubunit, multidomain Hbs found in nematodes, molluscs and crustaceans, and the giant annelid and vestimentiferan Hbs comprised of globin and nonglobin subunits. Chimeric hemoglobins have been found recently in bacteria and fungi. Hb occurs intracellularly in specific tissues and in circulating red blood cells (RBCs) and freely dissolved in various body fluids. In addition to transporting and storing O(2) and facilitating its diffusion, several novel Hb functions have emerged, including control of nitric oxide (NO) levels in microorganisms, use of NO to control the level of O(2) in nematodes, binding and transport of sulfide in endosymbiont-harboring species and protection against sulfide, scavenging of O(2 )in symbiotic leguminous plants, O(2 )sensing in bacteria and archaebacteria, and dehaloperoxidase activity useful in detoxification of chlorinated materials. This review focuses on the extensive variation in the functional properties of nonvertebrate Hbs, their O(2 )binding affinities, their homotropic interactions (cooperativity), and the sensitivities of these parameters to temperature and heterotropic effectors such as protons and cations. Whenever possible, it attempts to relate the ligand binding properties to the known molecular structures. The divergent and convergent evolutionary trends evident in the structures and functions of nonvertebrate Hbs appear to be adaptive in extending the inhabitable environment available to Hb-containing organisms.

Gas transfer system in Alvinella pompejana (Annelida polychaeta, Terebellida): functional properties of intracellular and extracellular hemoglobins

Alvinella pompejana is a tubicolous polychaete that dwells in the hottest part of the hydrothermal vent ecosystem in a highly variable mixture of vent (350 degrees C, anoxic, CO(2)- and sulfide-rich) and deep-sea (2 degrees C, mildly hypoxic) waters. This species has developed distinct-and specifically respiratory-adaptations to this challenging environment. An internal gas exchange system has recently been described, along with the report of an intracellular coelomic hemoglobin, in addition to the previously known extracellular vascular hemoglobin. This article reports the structure of coelomic hemoglobin and the functional properties of both hemoglobins in order to assess possible oxygen transfer. Coelomocytes contain a unique monomeric hemoglobin with a molecular weight of 14,810+/-1.5 Da, as determined by mass spectrometry. The functional properties of both hemoglobins are unexpectedly very similar under the same conditions of pH (6.1-8.2) and temperature (10 degrees -40 degrees C). The oxygen affinity of both proteins is relatively high (P50=0.66 Torr at 20 degrees C and pH 7), which facilitates oxygen uptake from the hypoxic environment. A strong Bohr effect (Phi ranging from -0.8 to -1.0) allows the release of oxygen to acidic tissues. Such similar properties imply a possible bidirectional transfer of oxygen between the two hemoglobins in the perioesophagal pouch, a mechanism that could moderate environmental variations of oxygen concentration and maintain brain oxygenation.