Carbonic anhydrase and red blood cell anion exchange in the neotenic aquatic salamander, Necturus maculosus

Characterization of two carbonic anhydrase isoforms in the pulmonate snail (Lymnaea Stagnalis) and their involvement in Molluskan calcification

Calcifying organisms are suffering from negative impacts induced by climate change, such as CO2-induced acidification, which may impair external calcified structures. Freshwater mollusks have the potential to suffer more from CO2-induced acidification than marine calcifiers due to the lower buffering capacity of many freshwater systems. One of the most important enzymes contributing to the biomineralization reaction is carbonic anhydrase (CA), which catalyzes the reversible conversion of CO2 to bicarbonate, the major carbon source of the calcareous structure in calcifiers. In this study we characterized two α-CA isoforms (LsCA1 and LsCA4) from the freshwater snail Lymnaea stagnalis using a combination of gene sequencing, gene expression, phylogenetic analysis and biochemical assays. Both CA isoforms demonstrated high expression levels in the mantle tissue, the major site for biomineralization. Furthermore, expression of LsCA4 during development parallels shell formation. The primary protein structure analysis, active site configuration and the catalytic activity of LsCA4 together suggest that the LsCA4 is embedded in the apical and basolateral membranes of mantle cells; while LsCA1 is proposed to be cytosolic and might play an important role in acid-base regulation. These findings of LsCA isoforms form a strong basis for a more detailed physiological understanding of the effects of elevated CO2 on calcification in freshwater mollusks.

Cytosolic carbonic anhydrase in the Gulf toadfish is important for tolerance to hypersalinity

Carbonic anhydrase (CA) is a ubiquitous enzyme involved in acid-base regulation and osmoregulation. Many studies have demonstrated a role for this enzyme in fish osmoregulation in seawater as well as freshwater. However, to date CA responses of marine fish exposed to salinities exceeding seawater (approximately 35 ppt) have not been examined. Consequently, the aim of the present study was to examine CA expression and activity in osmoregulatory tissues of the Gulf Toadfish, Opsanus beta, following transfer to 60 ppt. A gene coding, for CAc of 1827 bp with an open reading frame of 260 amino acids was cloned and showed high expression in all intestinal segments and gills. CAc showed higher expression in posterior intestine and rectum than in anterior and mid intestine and in gills of fish exposed to 60 ppt for up to 4 days. The enzymatic activity, in contrast, was higher in all examined tissues two weeks following transfer to 60 ppt. Comparing early expression and later activity levels of acclimated fish reveals a very different response to hypersalinity among tissues. Results highlight a key role of CAc in osmoregulation especially in distal regions of the intestine; moreover, CAc play a role in the gill in hypersaline environments possibly supporting elevated branchial acid extrusion seen under such conditions.

Membrane-associated carbonic anhydrase in the respiratory system of the Pacific hagfish (Eptatretus stouti)

Like other agnathans, the Pacific hagfish (Eptatretus stouti) lacks red blood cell (RBC) Cl(-)/HCO(3)(-) exchange. Despite this absence of anion exchange, the majority (86.7+/-1.4%) of the total CO(2) carried in the blood is found within the plasma as HCO(3)(-), and thus presumably is inaccessible to RBC carbonic anhydrase (CA). As such, a branchial plasma-accessible CA isozyme in hagfish would be beneficial for mobilizing the considerable plasma HCO(3)(-) stores for CO(2) excretion and blood acid-base balance. The current study used a combination of molecular and biochemical methods to identify two membrane-associated CA isozymes in the respiratory system of E. stouti. Using homology cloning methods, CA IV and XV-like isozymes were identified in the gill and RBC, respectively. Real-time PCR analysis of relative mRNA expression revealed that CA IV was specific to the gill, while CA XV was found in several tissues including the RBC, gill, liver, heart and muscle. Isolation of subcellular fractions of gill and RBC verified the presence of membrane-associated CA activity in each tissue that persisted after standard washing protocols. Unlike CA activity associated with the cytoplasmic fractions, the activity in gill membranes was not inhibited by sodium dodecyl sulphate, while RBC membrane activity was inhibited to a lesser degree than the cytoplasmic fraction. Additionally, incubation of gill membrane fractions with phosphatidylinositol-specific phospholipase C released significant CA activity into the supernatant indicating the presence of a glycophosphatidyl inositol-linkage to the membrane, as found with other CA IV and XV isozymes. These results demonstrate that Pacific hagfish possess gill and RBC plasma-accessible membrane-associated CA that may play important roles in respiratory gas exchange and acid-base balance.

Pulmonary carbonic anhydrase in vertebrate gas exchange organs

Carbonic anhydrase (CA) catalyzes the interconversion of CO(2) and HCO(3)(-). Intracellular (extravascular) and intravascular (extracellular) CA has been identified and localized in the lungs of reptiles and mammals. Less information is known, however, on the presence of intravascular CA in the lungs of amphibians and avians. In the present study, perfusion studies were used to compare the catalytic activity of pulmonary intravascular CA in reptiles and mammals. In addition, SDS-resistant CA activity was examined in microsomal fractions prepared from gill/lung tissue from representative animals in each vertebrate class. Finally, the CNO(-) sensitivity of the microsomal CA activity was compared. No SDS-resistant CA activity was found in gill microsomal fractions of several fish species. In contrast, the data suggest that SDS-resistant, intravascular pulmonary CA activity is present in air-breathing vertebrates with vastly differing lung morphologies and that the kinetics of inhibition is remarkably comparable between the vertebrate classes.

Comparative physiology and molecular evolution of carbonic anhydrase in the erythrocytes of early vertebrates

The different isozymes of carbonic anhydrase (CA) have been the subject of intensive study in mammals, but there is still much to be learned about the early evolution of this enzyme in vertebrates. Erythrocyte CA plays an essential role in the respiratory processes of most vertebrates and is probably the most well studied CA isozyme. The available evidence indicates that there has been a progressive increase in the efficiency of erythrocyte CA during the early evolution of vertebrates. There also appears to be a substantial increase in erythrocyte CA activity during development in some species. At the present time, however, the selective pressures that may be influencing the properties of erythrocyte CA during vertebrate evolution and development have not been clearly determined. When the available molecular sequence information is examined, it is evident that the erythrocyte CAs of early vertebrates have active sites that are more similar to those of mammalian CA VII and II, rather than CA I. We can now also begin to examine the phylogenetic relationships between the different rbc CAs in vertebrates, but more CA sequence information is clearly required from different groups of vertebrates before we have a complete picture of the molecular evolution of erythrocyte CA.

Expression and localization of carbonic anhydrase and ATPases in the symbiotic tubeworm Riftia pachyptila

The symbiotic tubeworm Riftia pachyptila needs to fuel its chemoautotrophic symbiotic bacteria with inorganic carbon. CO(2) is transported from the surrounding water to the bacteriocytes located in the trophosome, through the branchial plume and the body fluids. Previous studies have demonstrated the implication of carbonic anhydrase (CA) and proton pumps (ATPases) at various steps of CO(2) transport. The present study describes the expression pattern of cytosolic CA using an RNA probe and its histochemical and immunocytochemical localization in the trophosome and branchial plume of RIFTIA: Immunolocalization of V-H(+)ATPase and Na(+)K(+)-ATPase were also performed and related to CA localization. In the branchial plume, CA is expressed and localized in the most apical region of the branchial epithelium, close to the surrounding water. V-H(+)ATPase is mostly colocalized with CA and both enzymes probably allow CO(2) entry against the concentration gradient while regulating intracellular pH. Na(+)K(+)-ATPase is mostly restricted to the basal part of epithelial cells and probably participates in CO(2) transport to the body fluids. In the trophosome lobules, cytosolic CA is expressed and found in bacteriocytes and peritoneal cells. Hypotheses on the role of CA in bicarbonate and CO(2) interconversion to fuel the symbiotic bacteria are discussed.