Membrane specific carbonic anhydrase (CAIV) expression in human tissues

Partial amniotic carbon dioxide insufflation for fetal surgery

Partial amniotic carbon dioxide insufflation (PACI) involves insufflating the amniotic sac with carbon dioxide (CO₂ ) and, in some cases, draining some of the amniotic fluid. The creation of a gaseous intra-amniotic compartment improves visualization, even in the presence of limited bleeding, and creates the work space required for complex fetoscopic procedures. Clinically, PACI is mostly used to perform fetoscopic myelomeningocele (MMC) repair, enabling a minimally invasive alternative to open fetal surgery. However, evidence of the fetal safety of PACI is limited. Previous animal experiments in sheep demonstrate that PACI induces fetal hypercapnia and acidosis with largely unknown short and longer term implications. In this review, we examine the literature for the physiological effects of intrauterine insufflation pressure, duration, humidity, and the role of maternal hyperventilation on fetal physiology and well-being.

Carbonic Anhydrases: Role in pH Control and Cancer

The pH of the tumor microenvironment drives the metastatic phenotype and chemotherapeutic resistance of tumors. Understanding the mechanisms underlying this pH-dependent phenomenon will lead to improved drug delivery and allow the identification of new therapeutic targets. This includes an understanding of the role pH plays in primary tumor cells, and the regulatory factors that permit cancer cells to thrive. Over the last decade, carbonic anhydrases (CAs) have been shown to be important mediators of tumor cell pH by modulating the bicarbonate and proton concentrations for cell survival and proliferation. This has prompted an effort to inhibit specific CA isoforms, as an anti-cancer therapeutic strategy. Of the 12 active CA isoforms, two, CA IX and XII, have been considered anti-cancer targets. However, other CA isoforms also show similar activity and tissue distribution in cancers and have not been considered as therapeutic targets for cancer treatment. In this review, we consider all the CA isoforms and their possible role in tumors and their potential as targets for cancer therapy.

Downregulation of carbonic anhydrase IV contributes to promotion of cell proliferation and is associated with poor prognosis in non-small cell lung cancer

The present study aimed to unveil the biological role of carbonic anhydrase IV (CA IV) and its association with clinical pathological characteristics and prognostic significance in non-small cell lung cancer (NSCLC). The relative expression level of CA IV was measured by reverse transcription-quantitative polymerase chain reaction in 114 resected NSCLC tumors and matched adjacent normal tissues and NSCLC cell lines. Overexpression and cell proliferation were assessed in human NSCLC cell lines transfected with CA IV mRNA by lentivirus-mediated technology. The association of CA IV expression with clinical pathological features and overall survival in 114 cases of NSCLC patients was analyzed. It was demonstrated that CA IV expression was significantly downregulated in NSCLC tumors and six cell lines. Reduced expression of CA IV was significantly correlated with lymph node metastasis. The overall survival of NSCLC patients with low CA IV expression was significantly shorter compared with the high expression group. Overexpression of CA IV suppressed cell proliferation in A549 and NCI-H1299 cells. The results indicate that low expression of CA IV promotes cell proliferation and serves as an indicator for poor prognosis in NSCLC.

Membrane associated carbonic anhydrase IV (CA IV): a personal and historical perspective

Carbonic anhydrase IV is one of 12 active human isozymes and one of four expressed on the extracellular surfaces of certain endothelial and epithelial cells. It is unique in being attached to the plasma membrane by a glycosyl-phosphatiydyl-inositol (GPI) anchor rather than by a membrane-spanning domain. It is also uniquely resistant to high concentrations of sodium dodecyl sulfate (SDS), which allows purification from tissues by inhibitor affinity chromatography without contamination by other isozymes. This unique resistance to SDS and recovery following denaturation is explained by the two disulfide bonds. The 35-kDa human CA IV is a "high activity" isozyme in CO2 hydration activity, like CA II, and has higher activity than other isozymes in catalyzing the dehydration of HCO3 (-). Human CA IV is also unique in that it contains no oligosaccharide chains, where all other mammalian CA IVs are glycoproteins with one to several oligosaccharide side chains.Although CA IV has been shown to be active in mediating CO2 and HCO3 (-) transport in many important tissues like kidney and lung, and in isolated cells from brain and muscle, the gene for CA IV appears not to be essential. The CA IV knockout mouse produced by targeted mutagenesis, though slightly smaller and produced in lower than expected numbers, is viable and has no obvious mutant phenotype. Conversely, several dominant negative mutations in humans are associated with one form of reitinitis pigmentosa (RP-17), which we attribute to unfolded protein accumulation in the choreocapillaris, leading to apoptosis of cells in the overlying retina.

Analysis of evolution of carbonic anhydrases IV and XV reveals a rich history of gene duplications and a new group of isozymes

Carbonic anhydrase (CA) isozymes CA IV and CA XV are anchored on the extracellular cell surface via glycosylphosphatidylinositol (GPI) linkage. Analysis of evolution of these isozymes in vertebrates reveals an additional group of GPI-linked CAs, CA XVII, which has been lost in mammals. Our work resolves nomenclature issues in GPI-linked fish CAs. Review of expression data brings forth previously unreported tissue and cancer types in which human CA IV is expressed. Analysis of collective glycosylation patterns of GPI-linked CAs suggests functionally important regions on the protein surface.

Expression and subcellular localization of a 35-kDa carbonic anhydrase IV in a human pancreatic ductal cell line (Capan-1)

The high intraluminal concentrations of HCO(3)(-) in the human pancreatic ducts have suggested the existence of a membrane protein supplying the Cl(-)/HCO(3)(-) exchanger. Membrane-bound carbonic anhydrase IV (CA IV) is one of the potential candidates for this protein. The difficulties in isolating human pancreatic ducts have led the authors to study the molecular mechanisms of HCO(3)(-) secretion in cancerous cell lines. In this work, we have characterized the CA IV expressed in Capan-1 cells. A 35-kDa CA IV was detected in cell homogenates and purified plasma membranes. Treatment of purified plasma membranes with phosphatidylinositol-phospholipase-C indicated that this CA IV was not anchored by a glycosylphosphatidylinositol (GPI). In contrast, its detection on purified plasma membranes by an antibody specifically directed against the carboxyl terminus of human immature GPI-anchored CA IV indicated that it was anchored by a C-terminal hydrophobic segment. Immunoelectron microscopy and double-labeling immunofluorescence revealed that this CA IV was present on apical plasma membranes, and in the rough endoplasmic reticulum, the endoplasmic reticulum-Golgi intermediate compartment, the Golgi complex, and secretory granules, suggesting its transport via the classical biosynthesis/secretory pathway. The expression in Capan-1 cells of a 35-kDa CA IV anchored in the apical plasma membrane through a hydrophobic segment, as is the case in the healthy human pancreas, should make the study of its role in pancreatic HCO(3)(-) secretion easier.

Membrane-bound carbonic anhydrases in osteoclasts

Osteoclasts are multinucleated bone-resorbing cells that use multiple pH regulation mechanisms to create an acidic pH in the resorption lacuna. Carbonic anhydrase II and vacuolar H(+)-ATPases produce and transport protons, while chloride channels provide a Cl(-) flux into the resorption site. These activities are required for inorganic matrix dissolution that precedes enzymatic removal of organic bone matrix. In other cell types it has become evident that carbonic anhydrase isoenzymes interact with AE proteins to form transport metabolons that regulate intracellular pH. Membrane-bound carbonic anhydrase isoenzymes may also compensate for the lack of cytoplasmic carbonic anhydrase II. Therefore, our goal was to explore the expression of membrane-bound carbonic anhydrase (CA) isoenzymes CA IV, CA IX, CA XII and CA XIV in bone-resorbing osteoclasts. Immunohistochemistry and confocal microscopy showed expression of CA IV, CA XII and CA XIV in cultured rat and human osteoclasts. To confirm these results, RT-PCR was used. Immunohistochemistry revealed distinct staining patterns for CA IV, CA XII and CA XIV in rat trabecular bone specimens. A plasma membrane staining was observed in bone lining cells with the CA XII antibody while osteoclast plasma membranes were stained with CA IV and CA XIV antibodies. Confocal microscopy of cultured human osteoclasts showed a punctated intracellular CA IV staining and a perinuclear CA XIV staining while no CA IX or CA XII staining was observed. To evaluate the physiological role of membrane-bound CAs in osteoclasts, we used PCS, a novel membrane-impermeable CA inhibitor. Increased osteoclast number and bone resorption activity was observed in rat osteoclast cultures exposed to a low concentration of PCS while higher concentrations affected cell survival. PCS treatment also disturbed intracellular acidification in osteoclasts, as determined by live cell microscopy. In conclusion, our data shows that membrane-bound carbonic anhydrase isoenzymes CA IV and CA XIV are expressed both at mRNA and protein levels in osteoclasts in vivo and in vitro. In addition, the inhibitor experiments provide novel evidence to support the hypothesis that intracellular pH regulation in osteoclasts may indeed involve transport metabolons.

The role of carbonic anhydrases in renal physiology

Carbonic anhydrase (CA) catalyzes the reversible hydration of CO(2). CA is expressed in most segments of the kidney. CAII and CAIV predominate in human and rabbit kidneys; in rodent kidneys, CAXII, and CAXIV are also present. CAIX is expressed by renal cell carcinoma (RCC). Most of these isoforms, except for rodent CAIV, have high turnover rates. CAII is a cytoplasmic enzyme, whereas the others are membrane-associated; CAIV is anchored by glycosylphosphatidylinositol linkage. Membrane polarity is apical for CAXIV, basolateral for CAXII, and apical and basolateral for CAIV. Luminal membrane CAs facilitate the dehydration of carbonic acid (H(2)CO(3)) that is formed when secreted protons combine with filtered bicarbonate. Basolateral CA enhances the efflux of bicarbonate via dehydration of H(2)CO(3). CAII and CAIV can associate with bicarbonate transporters (e.g., AE1, kNBC1, NBC3, and SCL26A6), and proton antiporter, NHE1 in a membrane protein complex called a transport metabolon. CAXII and CAXIV may also be associated with transporters in normal kidney and CAIX in RCCs. The multiplicity of CAs implicates their importance in acid-base and other solute transport along the nephron. For example, CAII on the cytoplasmic face and CAIV on the extracellular surface provide the 'push' and 'pull' for bicarbonate transport by supplying and dissipating substrate respectively.

Carbonic anhydrase IV and XIV knockout mice: roles of the respective carbonic anhydrases in buffering the extracellular space in brain

Previous studies have implicated extracellular carbonic anhydrases (CAs) in buffering the alkaline pH shifts that accompany neuronal activity in the rat and mouse hippocampus. CAs IV and XIV both have been proposed to mediate this extracellular buffering. To examine the relative importance of these two isozymes in this and other physiological functions attributed to extracellular CAs, we produced CA IV and CA XIV knockout (KO) mice by targeted mutagenesis and the doubly deficient CA IV/XIV KO mice by intercrossing the individual null mice. Although CA IV and CA XIV null mice both are viable, the CA IV nulls are produced in smaller numbers than predicted, indicating either fetal or postnatal losses, which preferentially affect females. CA IV/XIV double KO mice are also produced in fewer numbers than predicted and are smaller than WT mice, and many females die prematurely before and after weaning. Electrophysiological studies on hippocampal slices on these KO mice showed that either CA can mediate buffering after synaptic transmission in hippocampal slices in the absence of the other, but that eliminating both is nearly as effective as the CA inhibitor, benzolamide, in blocking the buffering seen in the WT mice. Thus, both CA IV and CA XIV contribute to extracellular buffering in the central nervous system, although CA IV appears to be more important in the hippocampus. These individual and double KO mice should be valuable tools in clarifying the relative contributions of each CA to other physiological functions where extracellular CAs have been implicated.

Expression of membrane-associated carbonic anhydrase isoforms IV, IX, XII, and XIV in the rabbit: induction of CA IV and IX during maturation

Several carbonic anhydrase (CA) isoforms are associated with plasma membranes. It is probable that these enzymes interact with anion transporters to facilitate the movement of HCO3- into or out of the cell. A better knowledge of CA isoform expression in a given tissue would facilitate a systematic examination of any associations with such transporters. We examined the expression of CAs IV, IX, XII, and XIV mRNAs in rabbit tissues, including kidney, heart, lung, skeletal muscle, liver, pancreas, gall bladder, stomach, small intestine, colon, and spleen, using quantitative real-time reverse transcription polymerase chain reaction (RT-PCR). CA IV mRNA was mainly in kidney, heart, lung, colon, and gall bladder. CA IX mRNA was restricted to stomach, gall bladder, duodenum, and early jejunum. CA XII mRNA was found in kidney and colon. CA XIV mRNA was localized to heart, lung, skeletal muscle, and liver. The data indicate that there are different patterns of CA expression in various tissues: CA IX was expressed in the proximal gastrointestinal tract, whereas CA XII and CA IV were more distal. CA IV and CA XII are important kidney isoforms. CA XIV was abundant in metabolically active tissues such as liver, heart, lung, and skeletal muscle. Some significant species differences were noted in the expression of some of these isoforms; for example, CA XIV is not expressed in rabbit kidney, despite being abundant in mouse kidney. Maturational studies showed that the expression of CA IX mRNA and protein increased markedly with weaning ( approximately 3-4 postnatal wk) and was well correlated with the maturational expression of the alpha-subunit of the gastric H+,K+-ATPase, suggesting that function of CA IX and the gastric H+ pump might be linked in the digestion of adult foodstuffs. The unique pattern of membrane-bound CA isoforms suggests different functional associations with transporters, depending on the physiological demands on the tissue.

Evidence for a membrane carbonic anhydrase IV anchored by its C-terminal peptide in normal human pancreatic ductal cells

The high concentration of HCO(3)(-) ions (150 mM) in the human pancreatic ducts raises the question of the membrane proteins responsible for their secretion in addition to the Cl(-)/HCO(3)(-) exchanger. In this study, we investigated the expression of carbonic anhydrase IV (CA IV), a possible candidate. Experiments were carried out on specimens of normal human pancreas obtained from brain-dead donors ( n=9) as well as on isolated human ductal cells. Two antibodies were generated: CA IV NH(2) antibody directed against the NH(2) terminal of human glycosyl phosphatidylinositol (GPI)-anchored CA IV and CA IV COOH antibody directed against the COOH terminal of the same protein before its association with a GPI in the rough endoplasmic reticulum. A 35-kDa CA IV was detected in the homogenates of human pancreas. Immunocytochemistry demonstrated the expression of CA IV in centroacinar cells and in intercalated, intralobular, and interlobular ductal cells. The immunoreactivity observed with the CA IV COOH antibody was mainly localized on luminal membranes of ductal cells. Treatment of purified plasma membranes with phosphatidylinositol-phospholipase C indicated that the CA IV expressed in pancreatic ducts was not GPI-anchored. Its detection in the same extracts by the CA IV COOH antibody indicated that it was anchored by a hydrophobic segment at the carboxy terminal. Taken together, these results suggest that normal human pancreatic ductal cells express a 35-kDa CA IV anchored in their luminal plasma membrane by a hydrophobic segment of the COOH terminus. In view of its localization and its mode of anchorage in luminal plasma membranes, this CA IV may participate in the maintenance of luminal pH.

Regulation and modulation of pH in the brain

The regulation of pH is a vital homeostatic function shared by all tissues. Mechanisms that govern H+ in the intracellular and extracellular fluid are especially important in the brain, because electrical activity can elicit rapid pH changes in both compartments. These acid-base transients may in turn influence neural activity by affecting a variety of ion channels. The mechanisms responsible for the regulation of intracellular pH in brain are similar to those of other tissues and are comprised principally of forms of Na+/H+ exchange, Na+-driven Cl-/HCO3- exchange, Na+-HCO3- cotransport, and passive Cl-/HCO3- exchange. Differences in the expression or efficacy of these mechanisms have been noted among the functionally and morphologically diverse neurons and glial cells that have been studied. Molecular identification of transporter isoforms has revealed heterogeneity among brain regions and cell types. Neural activity gives rise to an assortment of extracellular and intracellular pH shifts that originate from a variety of mechanisms. Intracellular pH shifts in neurons and glia have been linked to Ca2+ transport, activation of acid extrusion systems, and the accumulation of metabolic products. Extracellular pH shifts can occur within milliseconds of neural activity, arise from an assortment of mechanisms, and are governed by the activity of extracellular carbonic anhydrase. The functional significance of these compartmental, activity-dependent pH shifts is discussed.

Targeting of carbonic anhydrase IV to plasma membranes is altered in cultured human pancreatic duct cells expressing a mutated (deltaF508) CFTR

Human pancreatic duct cells secrete HCO3- ions mediated by a Cl-/HCO3- exchanger and a HCO3- channel that may be a carbonic anhydrase IV (CA IV) in a channel-like conformation. This secretion is regulated by CFTR (Cystic Fibrosis Transmembrane conductance Regulator). In CF cells homozygous for the deltaF508 mutation, the defect in targeting of CFTR to plasma membranes leads to a disruption in the secretion of Cl- and HCO3 ions along with a defective targeting of other proteins. In this study, we analyzed the targeting of membrane CA IV in the human pancreatic duct cell line CFPAC-1, which expresses a deltaF508 CFTR, and in the same cells transfected with the wild-type CFTR (CFPAC-PLJ-CFTR6) or with the vector alone (CFPAC-PLJ6). The experiments were conducted on cells in the stationary phase the polarized state of which was checked by the distribution of occludin and actin. We show that both cell lines express a 35-kDa CA IV at comparable levels. Analysis of fractions of plasma membranes purified on a Percoll gradient evidenced lower levels of CA IV (8-fold) in the CFPAC-1 than in the CFPAC-PLJ-CFTR6 cells. Quantitative analyses showed that 6- to 10-fold fewer cells in the CFPAC-1 cell line exhibited membrane CA IV-immunoreactivity than in the CFPAC-PLJ-CFTR6 cell line. Taken together, these results suggest that the targeting of CA IV to apical plasma membranes is impaired in CFPAC-1 cells. CA IV/gamma-adaptin double labeling demonstrated the presence of CA IV in the trans-Golgi network (TGN) of numerous CFPAC-1 cells, indicating that trafficking was disrupted on the exit face of the TGN. The retargeting of CA IV observed in CFPAC-PLJ-CFTR6 cells points to a relationship between the traffic of CFTR and CA IV. On the basis of these observations, we propose that the absence of CA IV in apical plasma membranes due to the impairment in targeting in cells expressing a deltaAF508 CFTR largely contributes to the disruption in HCO3- secretion in CF epithelia.

Biochemical, histological, and inhibitor studies of membrane carbonic anhydrase in frog gastric acid secretion

Gastric acid secretion is dependent on carbonic anhydrase (CA). To define the role of membrane-bound CA, we used biochemical, histochemical, and pharmacological approaches in the frog (Rana pipiens). CA activity and inhibition by membrane-permeant and -impermeant agents were studied in stomach homogenates and microsomal fractions. H(+) secretion in the histamine-stimulated isolated mucosa was measured before and after mucosal addition of a permeant CA inhibitor (methazolamide) and before and after mucosal or serosal addition of two impermeant CA inhibitors of differing molecular mass: a 3,500-kDa polymer linked to aminobenzolamide and p-fluorobenzyl-aminobenzolamide (molecular mass, 454 kDa). Total CA activity of frog gastric mucosa is 2,280 U/g, of which 10% is due to membrane-bound CA. Membrane-bound CA retains detectable activity below pH 4. Histochemically, there is membrane-associated CA in surface epithelial, oxynticopeptic, and capillary endothelial cells. Methazolamide reduced H(+) secretion by 100%, whereas the two impermeant inhibitors equally blocked secretion by 40% when applied to the mucosal side and by 55% when applied to the serosal side. The presence of membrane-bound CA in frog oxynticopeptic cells and its relative resistance to acid inactivation and inhibition by impermeant inhibitors demonstrate that it subserves acid secretion at both the apical and basolateral sides.

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.

Carbonic anhydrase isozymes in the human pancreas

Recently, an increasing number of carbonic anhydrase (CA) isozymes have been discovered in the human pancreas. These isozymes are classified as the CA family with various molecular structures and different subcellular localizations: cytoplasmic CA II, mitochondrial CA VB, secretory CA VI, membrane-bound CA IV, and transmembrane CA IX and XII. However, there is little evidence concerning their pathophysiological roles. Here, we reviewed the expression of CA isozymes in the human pancreas and proposed hypotheses related to their physiological and pathological roles.

Activity-dependent pH shifts in hippocampal slices from normal and carbonic anhydrase II-deficient mice

The type II isoform of carbonic anhydrase is abundant in astrocytes and oligodendroglia. To explore whether the expression of the type II isoform is required for interstitial carbonic anhydrase activity, we studied extracellular pH transients in hippocampal slices from mutant mice devoid of carbonic anhydrase type II and from wild-type littermates. Stimulation of the Schaffer collateral afferents evoked similar extracellular pH transients in the CA1 stratum pyramidale, consisting of a predominant alkaline shift and little or no subsequent acidosis. After 5-s stimulus trains at 10 Hz, alkaline shifts were not significantly different in carbonic anhydrase II-deficient and wild-type preparations, averaging 0.09 +/- 0.04 and 0.08 +/- 0.04 unit pH, respectively. Addition of 1.5 microM benzolamide amplified the alkaline shifts by 385 +/- 146 and 345 +/- 75% in the mutant and wild-type preparations, respectively. Dose response studies with benzolamide displayed similar sensitivity to this carbonic anhydrase inhibitor over a concentration range of 0. 03-10 microM. These data indicate that interstitial carbonic anhydrase activity is effectively unaltered in brains devoid of carbonic anhydrase type II. The results are consistent with the interpretation that a distinct extracellular isoform of carbonic anhydrase exists in brain.

The distribution and physiological significance of carbonic anhydrase in vertebrate gas exchange organs

The enzyme carbonic anhydrase (CA) catalyzes the reversible hydration/dehydration of CO(2) and water, maintaining a near-instantaneous equilibrium among all chemical species involved in the reaction. CA is found in association with all tissue and organ systems involved in the transport and excretion of CO(2), from the site of CO(2) production, metabolically active tissue such as muscle, to circulating red blood cells in the vasculature, to the various organs of gas exchange, the lungs and gills. The presence of the enzyme in every fluid compartment along the pathway of CO(2) transport appears necessary in order to allow the dehydration of HCO(3)(-) to keep pace with the rapid diffusion of CO(2) across biological membranes. Within the actual organ of gas exchange, CA is compartmentalized in multiple subcellular fractions, with the specific subcellular localization determining the enzyme's physiological function.

Immunochemical evidence for a unique GPI-anchored carbonic anhydrase isozyme in human cardiomyocytes

To clarify the controversial question of cell-specific distribution of carbonic anhydrase (CA) in the heart, endothelial cells and cardiomyocytes were isolated from porcine and human hearts and were characterized with cell-specific markers. CA activity was found in the microsomal fraction of both cell types. It was shown by Triton X-114 phase separation that both cell types possess a membrane-bound form of CA. These CAs share the same mechanism of membrane-anchoring via glycosylphosphatidylinositol (GPI), which excludes identity with transmembrane isoforms CA IX or CA XII. Western blotting analysis of human microsomes with anti-human CA IV antibodies revealed a marked difference in immunoreactivity. Endothelial CA activity resulted in 11-fold stronger CA IV bands compared with identical amounts of myocytic CA activity, indicating that cardiac endothelium and cardiomyocytes possess immunologically distinct forms of CA. We conclude that in human hearts CA IV is associated with the endothelium, whereas most of the CA in myocytes is not identical with one of the known CA isozymes. This suggests that cardiomyocytic CA is a novel isozyme.

Carbonic anhydrase in human pancreas: hypotheses for the pathophysiological roles of CA isozymes

Among more than ten isozymes of the carbonic anhydrase (CA) family, only cytoplasmic CA II and membrane-bound CA IX have been reported to be expressed in human pancreas. To study the mRNA expression of CA isozymes in human pancreas, reverse transcriptase-polymerase chain reaction (RT-PCR)-Southern blot analysis and cDNA sequencing following RT-PCR were employed. CA II, IV, VI, IX, and XII were clearly identified in polyA+ RNA from normal human pancreas by RT-PCR-Southern blotting. Results with cultured pancreatic tumor cell, lines suggest that CA II, IV, IX, and XII are expressed in the ductal cells, and CA VI is expressed in the acinar cells. We propose a hypothesis for the pathophysiological function of CA isozymes in human pancreas; (1) the intraluminal CA isozymes (CA IV, VI, and possibly XII) form a mutually complementary system with cytoplasmic CA II to regulate the luminal pH of the pancreatic duct system and work as a self-defense mechanism against pancreatitis; (2) CA II and other CA isozymes play a pathological role in the autoimmune process of idiopathic chronic pancreatitis.

Gly-63-->Gln substitution adjacent to His-64 in rodent carbonic anhydrase IVs largely explains their reduced activity

Carbonic anhydrase (CA) IV is a glycosyl-phosphatidylinositol-anchored isozyme expressed on plasma membranes of capillary endothelial cells and certain epithelial cells of the nephron, the colon, and the genitourinary tract. CA IVs purified from bovine and rabbit lungs are high-activity enzymes, like human CA IV, while CA IV from mouse and rat lungs had only 10-20% as much catalytic activity. To explain the molecular basis for these differences in activity, we isolated and characterized the full-length cDNAs for bovine and rabbit CA IVs and compared their sequences to those we previously reported for human, murine, and rat CA IVs. These comparisons led us to postulate that a Gly-63-->Gln substitution adjacent to His-64 in the rodent enzymes accounts for their lower activity. To test this hypothesis, we made the Gly-63-->Gln mutants of bovine and rabbit CA IVs and the Gln-63-->Gly mutant of murine CA IV by site-directed mutagenesis, and compared the activities of mutant and wild-type CA IVs expressed in COS-7 cells. In addition, we produced recombinant cDNAs expressing secretory forms of the Gly-63 and Gln-63 forms of each of the three enzymes and compared the activities of the enzymes purified from transfected COS-7 cell secretions with the activities of CA IVs purified from lungs. These studies demonstrated that Gly-63 is important for the high activity of bovine and rabbit CA IVs, and they showed that the low activity of murine CA IV could be improved by the Gln-63-->Gly substitution. We suggest that the lower activity of the rodent CA IVs can be largely explained by the Gln-63 substitution which reduces the efficiency of proton transfer by the adjacent His-64.

Mouse pancreatic acinar/ductular tissue gives rise to epithelial cultures that are morphologically, biochemically, and functionally indistinguishable from interlobular duct cell cultures

Most of the pancreatic exocrine epithelium consists of acinar and intralobular duct (ductular) cells, with the balance consisting of interlobular and main duct cells. Fragments of mouse acinar/ductular epithelium can be isolated by partial digestion with collagenase and purified by Ficoll density gradient centrifugation. We investigated whether previously developed culture conditions used for duct epithelium would result in the selective survival and proliferation of ductular cells from the acinar/ductular fragments. The fragments were cultured on nitrocellulose filters coated with extracellular matrix. After 2 to 4 wk the filters were covered with proliferating cells resembling parallel cultures of duct epithelium by the following criteria: protein/DNA ratio, light and electron microscopic appearance, the presence of duct markers (carbonic anhydrase [CA] activity, CA II mRNA, the cystic fibrosis transmembrane conductance regulator), the near absence of acinar cell markers (amylase and chymotrypsin), a similar polypeptide profile after sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and the presence of spontaneous and secretin-stimulated electrogenic ion transport. Both duct and ductular epithelia formed fluid-filled cysts in collagen gels and both could be subcultured. We conclude that acinar/ductular tissue gives rise to ductular cells in culture by some combination of acinar cell death and/or transdifferentiation to a ductular phenotype, accompanied by proliferation of these cells and preexisting ductular cells. These cultures may be used to investigate the properties of this part of the pancreatic duct system, from which most of the pancreatic juice water and electrolytes probably originates.

Morphogenesis of "duct-like" structures in three-dimensional cultures of human cancerous pancreatic duct cells (Capan-1)

Pancreatic duct cells secrete water and ions, bicarbonate in particular. The study of these secretion processes is hindered by the unavailability of human pancreatic tissue. In this study, pancreatic human cells of the Capan-1 cell line were employed to investigate secretion in vitro. These cells are of ductal origin because in standard culture they polarize spontaneously forming domes in the culture dishes, indicating the existence of transepithelial exchange of water and electrolytes. In culture in suspension, Capan-1 cells form hollow spheroids bounded by a cell monolayer in a radial organization. These three-dimensional structures could be maintained in culture for more than 140 days. In young cultures, the cells of these spheroids grew rapidly (mitotic index = 9.2% on Day 2). Their cytologic features were analyzed by immunocytochemical, cytoenzymatic methods, and by electron microscopy. We showed that they are: a) polarized with an apical pole facing the culture medium; b) organized in a monolayer; c) bound by tight junctions and desmosomes; d) characterized by a particular distribution of enzyme systems known to play a role in ion exchanges, with placental-type alkaline phosphatases and carbonic anhydrases IV on their apical membranes and Ca(2+)-ATPases on their basolateral membranes. Crystalline structures were detected histochemically in the closed cavities and in the intercellular spaces of the spheroids. X-ray emission spectroscopy and electron diffraction showed that they consisted of calcium phosphate in an apatite structure. They were assumed to derive from a raised concentration of Ca2+ and phosphate ions under the impermeable monolayer of the spheroids. In addition, numerous cells secreted M1 gastric-type mucins, and acquired the ability to produce colonic-type M3 mucins. These hollow spheroids swelled during the culture period. Taken together these results suggest that the Capan-1 cells organized in these hollow spheroids exchange ions. Their three-dimensional structure resembles that of human pancreatic ducts, and they may therefore represent a useful model system for investigation of Cl- and HCO3- ion exchange processes in the human pancreas.

pH-dependent association of carbonic anhydrase (CA) with gastric light microsomal membranes isolated from bovine abomasum. Partial characterization of membrane-associated activity

1. The effect of pH on the association of carbonic anhydrase (CA) with bovine gastric light microsomal membranes (LMMs) was investigated (a) by washing LMMs containing CA activity with solutions of different pHs; (b) by studying the adsorption at various pHs of soluble bovine erythrocyte CA to washed gastric LMMs. In both cases, the association of CA with gastric LMMs was dependent on pH, being lower at neutral or alkaline pH. 2. The amount of soluble CA associated with gastric LMMs at pHs 8.0 and 9.0 was reduced when 140 mM K+/10 mM Na+ was added to the incubation medium. 3. Two sources of CA activity in bovine gastric LMMs were assumed: a loosely- and a firmly-membrane-associated activity. Both CA activities were dose-dependently inhibited by acetazolamide (I50: 3.6 x 10(-9) and 8.4 x 10(-9) M, respectively) and by chloride, acetate, iodide, bromide and nitrate at 100 mM. Firmly-membrane-associated activity appeared to be less sensitive to inhibition by acetazolamide, chloride and iodide. 4. Both activities exhibited different behavior and stability following treatment with alkaline Triton X-100. 5. The possible importance of a membrane-associated CA activity in gastric LMMs related to gastric acid secretion is discussed.

pH transients evoked by excitatory synaptic transmission are increased by inhibition of extracellular carbonic anhydrase

Excitatory synaptic transmission has been associated with a rapid alkalinization of the brain extracellular space. These pH shifts are markedly increased by acetazolamide, an inhibitor of carbonic anhydrase. Although this effect can be readily explained by inhibition of extracellular carbonic anhydrase, this enzyme has been considered strictly intracellular in the central nervous system. To determine whether these alkaline shifts are regulated by extracellular carbonic anhydrase, we studied the effects of a membrane impermeant, dextran-bound inhibitor of this enzyme. Extracellular alkaline transients, measured with pH-sensitive microelectrodes, were generated in the CA1 region of rat hippocampal slices by repetitive electrical stimulation of Schaeffer collateral fibers or by local ejection of glutamate. More direct alkalinizations were elicited by focal ejection of NaOH in the vicinity of a pH microelectrode. These pH transients were reversibly enhanced by addition of the dextran-bound inhibitor. We conclude that there is significant carbonic anhydrase activity in the extracellular space of the brain. We postulate that this enzyme functions in the regulation and modulation of extracellular pH transients associated with neuronal activity.

Immunopurification of gastric parietal cell tubulovesicles

1. The tubulovesicles of hog and rabbit gastric parietal cells were immunopurified from microsomes using monoclonal antibodies against the (H+, K+)-ATPase. 2. The best yields of immunoprecipitation were obtained with an ATPase/mAb molar ratio of 0.3: the immunoprecipitate contained 79 and 90% of the hog and rabbit microsomal PNPPase activity respectively and K(+)-stimulated ATPase specific activity was 221 +/- 29 mumoles Pi per hr and per mg of membrane protein. 3. The immunoprecipitate contained vesicles that were 85% cytoplasmic-side out, like tubulovesicles in vivo, demonstrating that the epitopes were cytoplasmic. 4. The alpha-beta protomer of (H+, K+)-ATPase accounted for 80 +/- 12% of the immunopurified proteins. 5. The major other proteins ran at 80, 75, 69, 57, 47, 44, 39, 34 and 32 kDa on the SDS-PAGE. 6. Comparative analysis between sucrose-gradient purified fractions and immunopurified tubulovesicles demonstrated that carbonic anhydrase and actin were contaminants and that the 53 kDa and presumably the 50 kDa bands of the gradient fraction were alpha and beta subunits of F1 ATPase.

Membrane-bound carbonic anhydrase CA IV in the human kidney

The distribution of membrane-bound carbonic anhydrase, CA IV, was studied in human kidneys by an indirect immunoperoxidase method using a rabbit polyclonal antibody directed against human kidney CA IV. Clear staining of CA IV was found in the apical cell borders of some cells in the cortical and medullary segments of the collecting ducts, presumably the A type of intercalated cells. Weak staining for CA IV was located in the interior of a number of collecting duct cells and in the basolateral regions of the proximal convoluted tubules. However, no staining was found in the brush border of the same tubules. This is a surprising finding, since evidence for carbonic anhydrase activity has been found biochemically and histochemically both in isolated brush-border and baso-lateral membranes. Further work is needed to clarify this matter. The endothelium of the peritubular capillaries also stained for CA IV.