Carbonic anhydrase activity in tissues of the icefish Chionodraco hamatus and of the red-blooded teleosts Trematomus bernacchii and Anguilla anguilla

The physiological significance of plasma-accessible carbonic anhydrase in the respiratory systems of fishes

Carbonic anhydrase (CA) activity is ubiquitously found in all vertebrate species, tissues and cellular compartments. Most species have plasma-accessible CA (paCA) isoforms at the respiratory surfaces, where the enzyme catalyzes the conversion of plasma bicarbonate to carbon dioxide (CO2) that can be excreted by diffusion. A notable exception are the teleost fishes that appear to lack paCA at their gills. The present review: (i) recapitulates the significance of CA activity and distribution in vertebrates; (ii) summarizes the current evidence for the presence or absence of paCA at the gills of fishes, from the basal cyclostomes to the derived teleosts and extremophiles such as the Antarctic icefishes; (iii) explores the contribution of paCA to organismal CO2 excretion in fishes; and (iv) the functional significance of its absence at the gills, for the specialized system of O2 transport in most teleosts; (v) outlines the multiplicity and isoform distribution of membrane-associated CAs in fishes and methodologies to determine their plasma-accessible orientation; and (vi) sketches a tentative time line for the evolutionary dynamics of branchial paCA distribution in the major groups of fishes. Finally, this review highlights current gaps in the knowledge on branchial paCA function and provides recommendations for future work.

A solution to Nature's haemoglobin knockout: a plasma-accessible carbonic anhydrase catalyses CO2 excretion in Antarctic icefish gills

In all vertebrates studied to date, CO₂ excretion depends on the enzyme carbonic anhydrase (CA) that catalyses the rapid conversion of HCO₃ ^- to CO₂ at the gas-exchange organs. The largest pool of CA is present within red blood cells (RBCs) and, in some vertebrates, plasma-accessible CA (paCA) isoforms participate in CO₂ excretion. However, teleost fishes typically do not have paCA at the gills and CO₂ excretion is reliant entirely on RBC CA - a strategy that is not possible in icefishes. As the result of a natural knockout, Antarctic icefishes (Channichthyidae) are the only known vertebrates that do not express haemoglobin (Hb) as adults, and largely lack RBCs in the circulation (haematocrit <1%). Previous work has indicated the presence of high levels of membrane-bound CA activity in the gills of icefishes, but without determining its cellular orientation. Thus, we hypothesised that icefishes express a membrane-bound CA isoform at the gill that is accessible to the blood plasma. The CA distribution was compared in the gills of two closely related notothenioid species, one with Hb and RBCs (Notothenia rossii) and one without (Champsocephalus gunnari). Molecular, biochemical and immunohistochemical markers indicate high levels of a Ca4 isoform in the gills of the icefish (but not the red-blooded N. rossii), in a plasma-accessible location that is consistent with a role in CO₂ excretion. Thus, in the absence of RBC CA, the icefish gill could exclusively provide the catalytic activity necessary for CO₂ excretion - a pathway that is unlike that of any other vertebrate.

Genomic conservation of erythropoietic microRNAs (erythromiRs) in white-blooded Antarctic icefish

White-blooded Antarctic crocodile icefish are the only vertebrates known to lack functional hemoglobin genes and red blood cells throughout their lives. We do not yet know, however, whether extinction of hemoglobin genes preceded loss of red blood cells or vice versa, nor whether erythropoiesis regulators disappeared along with hemoglobin genes in this erythrocyte-null clade. Several microRNAs, which we here call erythromiRs, are expressed primarily in developing red blood cells in zebrafish, mouse, and humans. Abrogating some erythromiRs, like mir144 and mir451a, leads to profound anemia, demonstrating a functional role in erythropoiesis. Here, we tested two not mutually exclusive hypotheses: 1) that the loss of one or more erythromiR genes extinguished the erythropoietic program of icefish and/or led to the loss of globin gene expression through pseudogenization; and 2) that some erythromiR genes were secondarily lost after the loss of functional hemoglobin and red blood cells in icefish. We explored small RNA transcriptomes generated from the hematopoietic kidney marrow of four Antarctic notothenioids: two red-blooded species (bullhead notothen Notothenia coriiceps and emerald notothen Trematomus bernacchii) and two white-blooded icefish (blackfin icefish Chaenocephalus aceratus and hooknose icefish Chionodraco hamatus). The N. coriiceps genome assembly anchored analyses. Results showed that, like the two red-blooded species, the blackfin icefish genome possessed and the marrow expressed all known erythromiRs. This result indicates that loss of hemoglobin and red blood cells in icefish was not caused by loss of known erythromiR genes. Furthermore, expression of only one erythromiR, mir96, appears to have been lost after the loss of red blood cells and hemoglobin-expression was not detected in the erythropoietic organ of hooknose icefish but was present in blackfin icefish. All other erythromiRs investigated, including mir144 and mir451a, were expressed by all four species and thus are present in the genomes of at least the two white-blooded icefish. Our results rule out the hypothesis that genomic loss of any known erythromiRs extinguished erythropoiesis in icefish, and suggest that after the loss of red blood cells, few erythromiRs experienced secondary loss. Results suggest that functions independent of erythropoiesis maintained erythromiRs, thereby highlighting the evolutionary resilience of miRNA genes in vertebrate genomes.

Phosphorylation increases the catalytic activity of rainbow trout gill cytosolic carbonic anhydrase

Cytoplasmic carbonic anhydrase (CAc) in the gill of teleost fish contributes to ionic regulation and acid–base balance by catalyzing the reversible reaction of CO2 and water, CO2 + H2O ↔ H(+) + HCO3(-). Regulation of CAc abundance and activity therefore is expected to fine-tune responses to ionic or acid–base challenges. The present study investigated the potential for gill CAc of rainbow trout, Oncorhynchus mykiss (tCAc), to undergo reversible phosphorylation. The activity of tCAc was approximately doubled by phosphorylation achieved through in vitro stimulation of endogenous protein kinases; kinase stimulation doubled phospho-threonine content from that observed in tCAc isolated under conditions where both kinases and protein phosphatases were inhibited. In vitro incubation to preferentially stimulate specific kinases implicated protein kinase G (PKG) in mediating the increase in tCAc activity. The kinetic parameters of turnover number (k cat) and substrate affinity (K m) were similarly affected by stimulation of either kinase or phosphatase action. However, phosphorylation via kinase stimulation significantly increased the efficiency of tCAc (V max /K m), and this factor may have contributed to the elevation of tCAc activity. In addition, phosphorylation of tCAc by kinase stimulation significantly increased the inhibition constant (K i) for acetazolamide. These results demonstrate that tCAc is subject to reversible phosphorylation; future work should focus on identifying the physiological situation(s) in which phosphorylation of trout branchial CAc occurs.

Sulfonamide inhibition studies of the γ-carbonic anhydrase from the Antarctic cyanobacterium Nostoc commune

A carbonic anhydrase (CA, EC 4.2.1.1) belonging to the γ-class has been cloned, purified and characterized from the Antarctic cyanobacterium Nostoc commune. The enzyme showed a good catalytic activity for the physiologic reaction (hydration of carbon dioxide to bicarbonate and a proton) with the following kinetic parameters, kcat of 9.5×10(5)s(-1) and kcat/KM of 8.3×10(7)M(-1)s(-1), being the γ-CA with the highest catalytic activity described so far. A range of aromatic/heterocyclic sulfonamides and one sulfamate were investigated as inhibitors of the new enzyme, denominated here NcoCA. The best NcoCA inhibitors were some sulfonylated sulfanilamide derivatives possessing elongated molecules, aminobenzolamide, acetazolamide, benzolamide, dorzolamide, brinzolamide and topiramate, which showed inhibition constants in the range of 40.3-92.3nM. As 1,5-bisphosphate carboxylase/oxygenase (RubisCO) and γ-CAs are closely associated in carboxysomes of cyanobacteria for enhancing the affinity of RubisCO for CO2 and the efficiency of photosynthesis, investigation of this new enzyme and its affinity for modulators of its activity may bring new insights in these crucial processes.

Exploring local flexibility/rigidity in psychrophilic and mesophilic carbonic anhydrases

Molecular flexibility and rigidity are required to determine the function and specificity of protein molecules. Some psychrophilic enzymes demonstrate a higher catalytic efficiency at low temperatures, compared to the efficiency demonstrated by their meso/thermophilic homologous. The emerging picture suggests that such enzymes have an improved flexibility of the structural catalytic components, whereas other protein regions far from functional sites may be even more rigid than those of their mesophilic counterparts. To gain a deeper insight in the analysis of the activity-flexibility/rigidity relationship in protein structure, psychrophilic carbonic anhydrase of the Antarctic teleost Chionodraco hamatus has been compared with carbonic anhydrase II of Bos taurus through fluorescence studies, three-dimensional modeling, and activity analyses. Data demonstrated that the cold-adapted enzyme exhibits an increased catalytic efficiency at low and moderate temperatures and, more interestingly, a local flexibility in the region that controls the correct folding of the catalytic architecture, as well as a rigidity in the hydrophobic core. The opposite result was observed in the mesophilic counterpart. These results suggest a clear relationship between the activity and the presence of flexible and rigid protein substructures that may be useful in rational molecular and drug design of a class of enzymes playing a key role in pathologic processes.

Structural features that govern enzymatic activity in carbonic anhydrase from a low-temperature adapted fish, Chionodraco hamatus

The carbonic anhydrase (CA) family of zinc metalloenzymes includes many known isozymes that have different subcellular distributions. The study described here focuses on identification of the structural features that define low-temperature adaptation in a Chionodraco hamatus protein, both for the reaction center, at an atomic level, and for the tertiary structure of the protein. To this aim, an x-ray absorption near-edge spectroscopy/Minuit x-ray absorption near-edge spectroscopy analysis of the reaction center was undertaken for both a structurally characterized human CAII and CA of C. hamatus. Higher structural levels were analyzed by sequence comparison and homology modeling. To establish whether the structural insights acquired in fish CAs are general, theoretical models were generated by homology modeling for three temperate-climate-adapted fish CAs. The measured structural differences between the two proteins are discussed in terms of the differences in the electrostatic potential between human CAII and CA of C. hamatus. We conclude that modulation of the interaction between the catalytic water molecule and the zinc ion could depend on the effect of the electrostatic potential distribution.

Biochemical Characterization of a S-glutathionylated Carbonic Anhydrase Isolated from Gills of the Antarctic Icefish Chionodraco hamatus

Gill cytoplasmic carbonic anhydrase of the haemoglobinless Antarctic icefish Chionodraco hamatus (Ice-CA) was directly sequenced and consists in 259 residues with an acetylated N-terminus. The molecular mass, deduced from the sequence, was 28.45 kDa, while mass spectrometry analysis of the native protein gave higher values. Treatment with dithiothreitol abolished this difference, indicating possible post-translational modifications. Isoelectric focusing analysis of Ice-CA suggested S-thiolation, which was identified as S-glutathionylation by immunostaining. Deglutathionylated Ice-CA maintained the anhydrase activity but showed higher susceptibility to hydrogen peroxide, suggesting that glutathione binding to Cys residues may have a role in the defence against oxidative damage. Ice-CA is characterized by lower thermal stability, higher activity and lower activation energy than its homologue gill CA of the temperate European eel, confirming the adaptation of the catalytic capacity to low temperatures. Alignment of Ice-CA with homologous enzymes from other fish shows high identity; the enzyme is grouped with a previously described fish CA monophyletic clade although Ice-CA shows several characteristics that can increase protein-solvent interaction and structural flexibility.

The structure and function of carbonic anhydrase isozymes in the respiratory system of vertebrates

Carbonic anhydrase is a ubiquitous metalloenzyme that catalyzes the reversible hydration/dehydration of carbon dioxide. To date, 16 different CA isozymes have been identified in mammals, and several novel isozymes have also been identified in non-mammalian vertebrates. These isozymes are involved in many physiological processes; however, one of the most important roles is facilitating the transport and subsequent excretion of carbon dioxide. As such, CA isozymes are found at virtually every step of the process, including the metabolic site of CO(2) production (muscle), the circulating red blood cells, and the primary respiratory surface (gills/lungs). This review will examine the structural characteristics that are integral to CAs participation in respiration, as well as highlight the specific roles and tissues that the different CA isozymes are involved in.

The Multifunctional Fish Gill: Dominant Site of Gas Exchange, Osmoregulation, Acid-Base Regulation, and Excretion of Nitrogenous Waste

The fish gill is a multipurpose organ that, in addition to providing for aquatic gas exchange, plays dominant roles in osmotic and ionic regulation, acid-base regulation, and excretion of nitrogenous wastes. Thus, despite the fact that all fish groups have functional kidneys, the gill epithelium is the site of many processes that are mediated by renal epithelia in terrestrial vertebrates. Indeed, many of the pathways that mediate these processes in mammalian renal epithelial are expressed in the gill, and many of the extrinsic and intrinsic modulators of these processes are also found in fish endocrine tissues and the gill itself. The basic patterns of gill physiology were outlined over a half century ago, but modern immunological and molecular techniques are bringing new insights into this complicated system. Nevertheless, substantial questions about the evolution of these mechanisms and control remain.

Characterisation of intestinal peptide transporter of the Antarctic haemoglobinless teleost Chionodraco hamatus

H(+)/peptide cotransport was studied in brush-border membrane vesicles (BBMV) from the intestine of the haemoglobinless Antarctic teleost Chionodraco hamatus by monitoring peptide-dependent intravesicular acidification with the pH-sensitive dye Acridine Orange. Diethylpyrocarbonate-inhibited intravesicular acidification was specifically achieved in the presence of extravesicular glycyl-L-proline (Gly-L-Pro) as well as of glycyl-L-alanine (Gly-L-Ala) and D-phenylalanyl-L-alanine (D-Phe-L-Ala). H(+)/Gly-L-Pro cotransport displayed saturable kinetics, involving a single carrier system with an apparent substrate affinity (K(m,app)) of 0.806+/-0.161 mmol l(-1). Using degenerated primers from eel and human (PepT1) transporter sequence, a reverse transcription-polymerase chain reaction (RT-PCR) signal was detected in C. hamatus intestine. RT-PCR paralleled kinetic analysis, confirming the hypothesis of the existence of a PepT1-type transport system in the brush-border membranes of icefish intestine. Functional expression of H(+)/peptide cotransport was successfully performed in Xenopus laevis oocytes after injection of poly(A)(+) RNA (mRNA) isolated from icefish intestinal mucosa. Injection of mRNA stimulated D-Phe-L-Ala uptake in a dose-dependent manner and an excess of glycyl-L-glutamine inhibited this transport. H(+)/peptide cotransport in the Antarctic teleost BBMV exhibited a marked difference in temperature optimum with respect to the temperate teleost Anguilla anguilla, the maximal activity rate occurring at approximately 0 degrees C for the former and 25 degrees C for the latter. Temperature dependence of icefish and eel intestinal mRNA-stimulated uptake in the heterologous system (oocytes) was comparable.

Sensing and transfer of respiratory gases at the fish gill

The gill is both a site of gas transfer and an important location of chemoreception or gas sensing in fish. While often considered separately, these two processes are clearly intricately related because the gases that are transferred between the ventilatory water and blood at the gill are simultaneously sensed by chemoreceptors on, and within, the gill. Modulation of chemoreceptor discharge in response to changes in O(2) and CO(2) levels, in turn, is believed to initiate a series of coordinated cardiorespiratory reflexes aimed at optimising branchial gas transfer. The past decade has yielded numerous advances in terms of our understanding of gas transfer and gas sensing at the fish gill, particularly concerning the transfer and sensing of carbon dioxide. In addition, recent research has moved from striving to construct a single model that covers all fish species, to recognition of the considerable inter-specific variation that exists with respect to the mechanics of gas transfer and the cardiorespiratory responses of fish to changes in O(2) and CO(2) levels. The following review attempts to integrate gas transfer and gas sensing at the fish gill by exploring recent advances in these areas.