Expression of carbonic anhydrase IV in carbonic anhydrase II-deficient mice

Use of a carbonic anhydrase Ca17a knockout to investigate mechanisms of ion uptake in zebrafish (Danio rerio)

In fishes, branchial cytosolic carbonic anhydrase (CA) plays an important role in ion and acid-base regulation. The Ca17a isoform in zebrafish (Danio rerio) is expressed abundantly in Na⁺-absorbing/H⁺-secreting H⁺-ATPase-rich (HR) cells. The present study aimed to identify the role of Ca17a in ion and acid-base regulation across life stages using CRISPR/Cas9 gene editing. However, in preliminary experiments, we established that ca17a knockout is lethal with ca17a^-/- mutants exhibiting a significant decrease in survival beginning at ∼12 days postfertilization (dpf) and with no individuals surviving past 19 dpf. Based on these findings, we hypothesized that ca17a^-/- mutants would display alterations in ion and acid-base balance and that these physiological disturbances might underlie their early demise. Na⁺ uptake rates were significantly increased by up to 300% in homozygous mutants compared with wild-type individuals at 4 and 9 dpf; however, whole body Na⁺ content remained constant. While Cl^- uptake was significantly reduced in ca17a^-/- mutants, Cl^- content was unaffected. Reduction of CA activity by Ca17a morpholino knockdown or ethoxzolamide treatments similarly reduced Cl^- uptake, implicating Ca17a in the mechanism of Cl^- uptake by larval zebrafish. H⁺ secretion, O₂ consumption, CO₂ excretion, and ammonia excretion were generally unaltered in ca17a^-/- mutants. In conclusion, while the loss of Ca17a caused marked changes in ion uptake rates, providing strong evidence for a Ca17a-dependent Cl^- uptake mechanism, the underlying causes of the lethality of this mutation in zebrafish remain unclear.

Oxalate transport by the mouse intestine in vitro is not affected by chronic challenges to systemic acid-base homeostasis

In rats, we recently showed how a chronic metabolic acidosis simultaneously reduced urinary oxalate excretion and promoted oxalate secretion by the distal colon leading to the proposition that acid-base disturbances may trigger changes to renal and intestinal oxalate handling. The present study sought to reproduce and extend these observations using the mouse model, where the availability of targeted gene knockouts (KOs) would offer future opportunities to reveal some of the underlying transporters and mechanisms involved. Mice were provided with a sustained load of acid (NH₄Cl), base (NaHCO₃) or the carbonic anhydrase inhibitor acetazolamide (ATZ) for 7 days after which time the impacts on urinary oxalate excretion and its transport by the intestine were evaluated. Mice consuming NH₄Cl developed a metabolic acidosis but urinary oxalate was only reduced 46% and not statistically different from the control group, while provision of NaHCO₃ provoked a significant 2.6-fold increase in oxalate excretion. For mice receiving ATZ, the rate of urinary oxalate excretion did not change significantly. Critically, none of these treatments altered the fluxes of oxalate (or chloride) across the distal ileum, cecum or distal colon. Hence, we were unable to produce the same effects of a metabolic acidosis in mice that we had previously found in rats, failing to find any evidence of the 'gut-kidney axis' influencing oxalate handling in response to various acid-base challenges. Despite the potential advantages offered by KO mice, this model species is not suitable for exploring how acid-base status regulates oxalate handling between the kidney and intestine.

Carbonic anhydrases in the mouse harderian gland

The harderian gland is located within the orbit of the eye of most terrestrial vertebrates. It is especially noticeable in rodents, in which it synthesises lipids, porphyrins, and indoles. Various functions have been ascribed to the harderian gland, such as lubrication of the eyes, a site of immune response, and a source of growth factors. Carbonic anhydrases (CAs) are zinc-containing metalloenzymes that catalyse the reaction CO₂ + H₂O <--> H+ + HCO₃. They are involved in the adjustment of pH in the secretions of different glands. Thirteen enzymatically active isozymes have been described in the mammalian α-CA family. Here, we first investigated the mRNA expression of all 13 active CAs in the mouse harderian gland by quantitative real-time PCR. Nine CA mRNAs were detectable in the gland. Car5b and Car13 showed the highest signals. Car4, Car6, and Car12 showed moderate expression levels, whereas Car2, Car3, Car7, and Car15 mRNAs were barely within the detection limits. Immunohistochemical staining was performed to study the expression of Car2, Car4, Car5b, Car12, and Car13 at the protein level. The epithelial cells were intensively stained for CAVB, whereas only weak signal was detected for CAXIII. Positive signals for CAIV and CAXII were observed in the capillary endothelial cells and the basolateral plasma membrane of the epithelial cells, respectively. This study provides an expression profile of all CAs in the mouse harderian gland. These results should improve our understanding of the distribution of CA isozymes and their potential roles in the function of harderian gland. The high expression of mitochondrial CAVB at both mRNA and protein levels suggests a role in lipid synthesis, a key physiological process of the harderian gland.

The most recently discovered carbonic anhydrase, CA XV, is expressed in the thick ascending limb of Henle and in the collecting ducts of mouse kidney

Background

Carbonic anhydrases (CAs) are key enzymes for physiological pH regulation, including the process of urine acidification. Previous studies have identified seven cytosolic or membrane-bound CA isozymes in the kidney. Recently, we showed by in situ hybridization that the mRNA for the most novel CA isozyme, CA XV, is present in the renal cortex. CA XV is a unique isozyme among mammalian CAs, because it has become a pseudogene in primates even though expressed in several other species.

Methodology/Principal Findings

In the present study, we raised a polyclonal antibody against recombinant mouse CA XV that was produced in a baculovirus/insect cell expression system, and the antibody was used for immunohistochemical analysis in different mouse tissues. Positive immunoreactions were found only in the kidney, where the enzyme showed a very limited distribution pattern. Parallel immunostaining experiments with several other anti-CA sera indicated that CA XV is mainly expressed in the thick ascending limb of Henle and collecting ducts, and the reactions were most prominent in the cortex and outer medulla.

Conclusion/Significance

Although other studies have proposed a role for CA XV in cell proliferation, its tightly limited distribution may point to a specialized function in the regulation of acid-base homeostasis.

Carbonic anhydrase XIV in skeletal muscle: subcellular localization and function from wild-type and knockout mice

The expression of carbonic anhydrase (CA) XIV was investigated in mouse skeletal muscles. Sarcoplasmic reticulum (SR) and sarcolemmal (SL) membrane fractions were isolated from wild-type (WT) and CA XIV knockout (KO) mice. The CA XIV protein of 54 kDa was present in SR and SL membrane fractions as shown by Western blot analysis. CA activity measurements of WT and KO membrane fractions showed that CA XIV accounts for approximately 50% and 66% of the total CA activities determined in the SR and SL fractions, respectively. This indicates the presence of at least one other membrane-associated CA isoform in these membranes, e.g., CA IV, CA IX, or CA XII. Muscle fibers of the extensor digitorum longus (EDL) muscle were immunostained with anti-CA XIV/FITC and anti-sarco(endo)plasmic reticulum Ca(2+)-ATPase 1/TRITC, with anti-CA XIV/FITC and anti-ryanodine receptor/TRITC, or with anti-CA XIV/FITC and anti-monocarboxylate transporter-4/TRITC. CA XIV was expressed in the plasma membrane and in the longitudinal SR but not in the terminal SR. Isometric contraction measurements of single twitches and tetani and a fatigue protocol applied to fiber bundles of the fast-twitch EDL and of the slow-twitch soleus muscle from WT and KO mice showed that the lack of SR membrane-associated CA XIV did not affect maximum force, rise and relaxation times, and fatigue behavior. Thus, it is concluded that a reduction of the total SR CA activity by approximately 50% in CA XIV KO mice does not lead to an impairment of SR function.

Contractile properties of skeletal muscle fibre bundles from mice deficient in carbonic anhydrase II

The function of cytosolic carbonic anhydrase (CA) isozyme II is largely unknown in skeletal muscle. Because of this, we compared the in vitro contractile properties of extensor digitorum longus (EDL) and soleus (SOL) fibre bundles from mice deficient in CA II (CAD) to litter mate controls (LM). Twitch rise, 1/2 relaxation time and peak twitch force at 22 degrees C of fibre bundles from CAD EDL [28.4+/-1.4 ms, 31.2+/-2.3 ms, 6.2+/-1.0 Newton/cm(2) (N/cm(2)), respectively] and CAD SOL (54.2+/-7.5 ms, 75.7+/-13.8 ms, 2.9+/-0.5 N/cm(2), respectively) were significantly higher compared to LM EDL (20.5+/-2.2 ms, 21.9+/-3.7 ms, 4.5+/-0.2 N/cm(2)) and LM SOL (42.8+/-3.5 ms, 51.4+/-2.4 ms, 2.1+/-0.4 N/cm(2)). However, in acidic Krebs-Henseleit solution, mimicking the pH, PCO(2), and HCO(3) (-) of arterial blood from CAD mice, twitch rise, 1/2 relaxation time, and peak twitch force of fibre bundles from CAD EDL (19.3+/-0.7 ms, 19.7+/-2.3 ms, 4.8+/-0.8 N/cm(2)) and CAD SOL (41.4+/-3.6 ms, 51.9+/-5.5 ms, 2.2+/-0.7 N/cm(2)) were not significantly different from LM fibre bundles in normal Krebs-Henseleit solution (EDL: 19.7+/-1.1 ms, 21.6+/-0.6 ms, 4.7+/-0.2 N/cm(2); SOL: 42.5+/-3.1 ms, 51.8+/-2.6 ms, 1.8+/-0.3 N/cm(2)). A higher pH(i) during exposure to acidic bathing solution was maintained by CAD EDL (7.37+/-0.02) and CAD SOL (7.33+/-0.05) compared to LM EDL (7.28+/-0.04) and LM SOL (7.22+/-0.02). This suggests that the skeletal muscle of CAD mice possesses an improved defense of pH(i) against elevated pCO(2). In support of this, apparent non-bicarbonate buffer capacity (in mequiv H(+) (pH unit)(-1) (kg cell H(2)O)(-1)) as determined by pH microelectrode was markedly increased in CAD EDL (75.7+/-4.1) and CAD SOL (85.9+/-3.3) compared to LM EDL (39.3+/-4.7) and LM SOL (37.5+/-3.8). Both latter phenomena may be related to the slowed rate of intracellular acidification seen in CAD SOL in comparison with LM SOL upon an increase in PCO(2) of the bath. In conclusion, skeletal muscle from mice deficient in CA II exhibits altered handling of acid-base challenges and shows normal contractile behavior at normal intracellular pH.

Carbonic anhydrase gene expression in CA II-deficient (Car2-/-) and CA IX-deficient (Car9-/-) mice

Using real-time PCR and immunohistochemistry, we have examined the expression of carbonic anhydrase isozymes (CA) I, II, III, IV, IX, XII, XIII and XIV in the brain, kidney, stomach and colon of the wild-type, CA II-deficient (Car2-/-), and CA IX deficient (Car9-/-) mice. The expression of Car4, Car12, Car13 and Car14 mRNAs did not show any significant deviations between the three groups of mice, whereas both groups of CA deficient mice showed decreased expression levels of Car1 in the colon and Car3 in the kidney. The Car2 mRNA level was greatly reduced but not completely abolished in all four tissues from the Car2-/- mice in which no CA II protein was expressed. Sequencing the Car2 cDNA isolated from C57BL6 Car2-/- mice revealed two nucleotide differences from the wild-type C57BL6 mice. One is a silent polymorphism found in Car2 mRNA from wild-type DBA mice, which is the strain that provided the original mutagenized chromosome. The second change is a mutation that causes prematurely terminated translation at codon 155 (Gln155X). Car9 mRNA and CA IX protein expression levels were up-regulated about 2.5- and 3.6-fold, respectively, in the stomach of the Car2-/- mice. These results suggest that the loss of function of cytosolic CA II in the stomach of Car2-/- mice leads to up-regulation of an extracellular CA, namely CA IX, which is expressed on the cell surface of the gastric epithelium.

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.

Evidence against aquaporin-1-dependent CO2 permeability in lung and kidney

AQP1-dependent CO2 transport has been suggested from the increased CO2 permeability in Xenopus oocytes expressing AQP1. Potential implications of this finding include AQP1-facilitated CO2 exchange in mammalian lung and HCO3-/CO2 transport in kidney proximal tubule. We reported previously that: (a) CO2 permeability in erythrocytes was not affected by AQP1 deletion, (b) CO2 permeability in liposomes was not affected by AQP1 reconstitution despite a 100-fold increased water permeability, and (c) CO2 blow-off by the lung in living mice was not impaired by AQP1 deletion. We extend these observations by direct measurement of CO2 permeabilities in lung and kidney. CO2 transport across the air-space-capillary barrier in isolated perfused lungs was measured from changes in air-space fluid pH in response to addition/removal of HCO3-/CO2 from the pulmonary artery perfusate. The pH was measured by pleural surface fluorescence of a pH indicator (BCECF-dextran) in the air-space fluid. Air-space fluid pH equilibrated rapidly (t(1/2) approximately 6 s) in response to addition/removal of HCO3-/CO2. However, the kinetics of pH change was not different in lungs of mice lacking AQP1, AQP5 or AQP1/AQP5 together, despite an up to 30-fold reduction in water permeability. CO2 transport across BCECF-loaded apical membrane vesicles from kidney proximal tubule was measured from the kinetics of intravesicular acidification in response to rapid mixing with a HCO3-/CO2 solution. Vesicles rapidly acidified (t(1/2) approximately 10 ms) in response to HCO3-/CO2 addition. However the acidification rate was not different in kidney vesicles from AQP1-null mice despite a 20-fold reduction in water permeability. The results provide direct evidence against physiologically significant transport of CO2 by AQP1 in mammalian lung and kidney.

Prostaglandin transporter PGT is expressed in cell types that synthesize and release prostanoids

PGT is a broadly expressed transporter of prostaglandins (PGs) and thromboxane that is energetically poised to take up prostanoids across the plasma membrane. To gain insight into the function of PGT, we generated mouse monoclonal antibody 20 against a portion of putative extracellular loop 5 of rat PGT. Immunoblots of endogenous PGT in rat kidney revealed a 65-kDa protein in a zonal pattern corresponding to PG synthesis rates (papilla congruent with medulla > cortex). Immunocytochemically, PGT in rat kidneys was expressed in glomerular endothelial and mesangial cells, arteriolar endothelial and muscularis cells, principal cells of the collecting duct, medullary interstitial cells, medullary vasa rectae endothelia, and papillary surface epithelium. Proximal tubules, which are known to take up and metabolize PGs, were negative. Immunoblotting and immunocytochemistry revealed that rat platelets also express abundant PGT. Coexpression of the PG synthesis apparatus (cyclooxygenase) and PGT by the same cell suggests that prostanoids may undergo release and reuptake.

The plasma membrane carbonic anhydrase in murine hepatocytes identified as isozyme XIV

BACKGROUND

Biochemical and histochemical studies have both previously indicated plasma membrane-associated carbonic anhydrase (CA) activity in hepatocytes which has been assumed to be CA IV. However, immunohistochemical data did not support this assignment. Recent northern blotting results indicated the presence of mRNA for the most recently discovered membrane-bound CA isozyme, CA XIV, in the liver. The present study was designed to examine whether CA XIV could contribute to the CA activity described in the hepatocytes.

METHODS

Tissue samples from mouse liver were subjected to immunohistochemical staining using the antibodies raised against recombinant mouse CA XIV and CA IV. RT-PCR and western blotting were also performed for CA XIV.

RESULTS

A strong immunofluorescent signal was observed in the plasma membrane of mouse hepatocytes. Although CA XIV was expressed on both the apical and basolateral surfaces, the staining was more prominent at the apical (canalicular) membrane domain. The expression of CA XIV in the liver was confirmed by RT-PCR and western blotting.

CONCLUSIONS

The presence of CA XIV in the hepatocyte plasma membrane places this novel enzyme at a strategic site to control pH regulation and ion transport between the hepatocytes, sinusoids and bile canaliculi.

Interstitial carbonic anhydrase (CA) activity in brain is attributable to membrane-bound CA type IV

We tested the hypothesis that extracellular membrane-bound carbonic anhydrase (CA) type IV is responsible for the regulation of interstitial pH (pH(o)) transients in brain. Rat hippocampal slices were incubated in phosphatidylinositol-specific phospholipase C (PI-PLC), which cleaves the link of CA IV to the external face of plasma membranes. Then evoked alkaline pH(o) shifts were studied in a recording chamber, using pH microelectrodes. Incubation fluid was saved for later analysis. The ability to buffer a rapid alkaline load was reduced markedly in PI-PLC-treated tissue as compared with adjacent, paired control slices. The effect of benzolamide (a poorly permeant CA inhibitor) on evoked pH(o) shifts was diminished greatly in the PI-PLC-treated tissue, consistent with the washout of interstitial CA. Treatment of the incubation fluid with SDS abolished nearly all of the CA activity in fluid from controls, whereas an SDS-insensitive component remained in the fluid from PI-PLC-treated slices. These data suggested that CA type II (which is blocked by SDS) leaked from injured glial cells in both slice preparations, whereas CA type IV (which is insensitive to SDS) was liberated selectively into the fluid from PI-PLC-treated tissue. Western blot analysis was consistent with this interpretation, demonstrating a predominance of CA IV in the incubation fluid from PI-PLC-treated tissue and variable amounts of CA II in fluid from PI-PLC-treated and control slices. These results demonstrate that interstitial CA activity brain is attributable principally to membrane-bound CA IV.

Expression of carbonic anhydrase IV in mouse placenta

Carbonic anhydrase (CA) facilitates acid-base transport in several tissues. Acidosis upregulates membrane-bound SDS-resistant hydratase activity in various tissues and CA IV mRNA in rabbit kidney. This study was designed to assess whether the expression of membrane-bound CA IV isozyme in mouse placenta is regulated developmentally and by maternal ammonium chloride loading at the end of pregnancy. For this purpose we used Northern blot analysis, Western blots of microsomal membranes, and immunocytochemistry. The expression of CA IV mRNA on Northern blots tripled from day 11 to day 15 and then remained stable until the end of pregnancy. Expression of CA IV immunoreactive protein on Western blot tripled from day 11 to day 15 and decreased almost to baseline by day 19. Strong staining for CA IV was detected by immunocytochemistry in labyrinthine trophoblast, in the endodermal layer of the yolk sac (both intra- and extraplacental) and in the uterine epithelium. Weak staining was observed in most fetal endothelial cells at 11 days but not later in gestation. Maternal acidosis did not upregulate the expression of CA IV mRNA or CA IV immunoreactive protein. Thus CA IV expression in mouse placenta is developmentally regulated. Maternal acidosis during the last quarter of pregnancy does not upregulate CA IV mRNA or CA IV immunoreactive protein.

Acid-base effects on electrolyte transport in CA II-deficient mouse colon

To determine the role of carbonic anhydrase (CA) in colonic electrolyte transport, we studied Car-2(0) mice, mutants deficient in cytosolic CA II. Ion fluxes were measured under short-circuit conditions in an Ussing chamber. CA was analyzed by assay and Western blots. In Car-2(0) mouse colonic mucosa, total CA activity was reduced 80% and cytosolic CA I and membrane-bound CA IV activities were not increased. Western blots confirmed the absence of CA II in Car-2(0) mice. Normal mouse distal colon exhibited net Na(+) and Cl(-) absorption, a serosa-positive PD, and was specifically sensitive to pH. Decrease in pH stimulated active Na(+) and Cl(-) absorption whether it was caused by increasing solution PCO(2), reducing HCO(-)(3) concentration, or reducing pH in CO(2)/HCO(-)(3)-free HEPES-Ringer solution. Membrane-permeant methazolamide, but not impermeant benzolamide, at 0.1 mM prevented the effects of pH. Car-2(0) mice exhibited similar basal transport rates and responses to pH and CA inhibitors. We conclude that basal and pH-stimulated colonic electrolyte absorption in mice requires CA I. CA II and IV may have accessory roles.