Non-catalytic role of carbonic anhydrase in rat intestinal absorption

Carbonic Anhydrase IX and Hypoxia Promote Rat Pulmonary Endothelial Cell Survival During Infection

Low tidal volume ventilation protects the lung in mechanically ventilated patients. The impact of the accompanying permissive hypoxemia and hypercapnia on endothelial cell recovery from injury is poorly understood. Carbonic anhydrase IX (CA IX) is expressed in pulmonary microvascular endothelial cells (PMVECs), where it contributes to CO2 and pH homeostasis, bioenergetics and angiogenesis. We hypothesized that CA IX is important for PMVEC survival, and CA IX expression and release from PMVECs are increased during infection. While plasma CA IX was unchanged in human and rat pneumonia, there was a trend towards increasing CA IX in bronchoalveolar fluid of mechanically ventilated critically ill pneumonia patients and a significant increase in CA IX in lung tissue lysate of rat pneumonia. To investigate functional implications of the lung CA IX increase, we generated PMVEC cell lines harboring domain-specific CA IX mutations. Using these cells, we found that infection promotes intracellular expression, release and metalloproteinase-mediated extracellular cleavage of CA IX in PMVECs. Intracellular domain deletion uniquely impaired CA IX membrane localization. Loss of the CA IX intracellular domain promoted cell death following infection, suggesting the important role of intracellular domain in PMVEC survival. We also found that hypoxia improves survival, whereas hypercapnia reverses the protective effect of hypoxia, during infection. Thus, we report that: (1) CA IX increases in rat pneumonia lung; and, (2) the CA IX intracellular domain and hypoxia promote PMVEC survival during infection.

Effects of acid-base variables and the role of carbonic anhydrase on oxalate secretion by the mouse intestine in vitro

Hyperoxaluria is a major risk factor for calcium oxalate kidney stones and the intestine is recognized as an important extra-renal pathway for eliminating oxalate. The membrane-bound chloride/bicarbonate (Cl(-)/) exchangers are involved in the transcellular movement of oxalate, but little is understood about how they might be regulated. , CO2, and pH are established modulators of intestinal NaCl cotransport, involving Na(+)/H(+) and Cl(-)/ exchange, but their influence on oxalate transport is unknown. Measuring (14)C-oxalate and (36)Cl fluxes across isolated, short-circuited segments of the mouse distal ileum and distal colon we examined the role of these acid-base variables and carbonic anhydrase (CA) in oxalate and Cl(-) transport. In standard buffer both segments performed net oxalate secretion (and Cl(-) absorption), but only the colon, and the secretory pathway were responsive to and CO2. Ethoxzolamide abolished net oxalate secretion by the distal colon, and when used in tandem with an impermeant CA inhibitor, signaled an intracellular CA isozyme was required for secretion. There was a clear dependence on as their removal eliminated secretion, while at 42 mmol/L was also decreased and eradicated. Independent of pH, raising Pco2 from 28 to 64 mmHg acutely stimulated net oxalate secretion 41%. In summary, oxalate secretion by the distal colon was dependent on , CA and specifically modulated by CO2, whereas the ileum was remarkably unresponsive. These findings highlight the distinct segmental heterogeneity along the intestine, providing new insights into the oxalate transport mechanism and how it might be regulated.

Decreased expression of colonic Slc26a3 and carbonic anhydrase iv as a cause of fatal infectious diarrhea in mice

Citrobacter rodentium causes epithelial hyperplasia and colitis and is used as a model for enteropathogenic and enterohemorrhagic Escherichia coli infections. Little or no mortality develops in most inbred strains of mice, but C3H and FVB/N mice exhibit fatal outcomes of infection. Here we test the hypothesis that decreased intestinal transport activity during C. rodentium infection results in fatality in C3H/HeOu and FVB/N mice. Susceptible strains were compared to resistant C57BL/6 mice and to inbred strains SWR and SJL of Swiss origin, which have not been previously characterized for outcomes of C. rodentium infection. Mortality in susceptible strains C3H/HeOu and FVB/N was associated with significant fluid loss in feces, a remarkable downregulation of Slc26a3 and carbonic anhydrase IV (CAIV) message and protein expression, retention of chloride in stool, and hypochloremia, suggesting defects in intestinal chloride absorption. SWR, SJL, and C57BL/6 mice were resistant and survived the infection. Fluid therapy fully prevented mortality in C3H/HeOu and FVB/N mice without affecting clinical disease. Common pathogenic mechanisms, such as decreased levels of expression of Slc26a3 and CAIV, affect intestinal ion transport in C. rodentium-infected FVB and C3H mice, resulting in profound electrolyte loss, dehydration, and mortality. Intestinal chloride absorption pathways are likely a potential target for the treatment of infectious diarrhea.

Functional coupling of the downregulated in adenoma Cl-/base exchanger DRA and the apical Na+/H+ exchangers NHE2 and NHE3

Non-nutrient-dependent salt absorption across the brush-border membrane of intestinal epithelial cells is primarily mediated by coupled apical Na(+)/H(+) (aNHE) and anion exchange transport, with the latter suspected of being mediated by DRA (downregulated in adenoma; SLC26A3) that is defective in congenital chloridorrhea. To investigate DRA in greater detail and determine whether DRA and NHE activities can be coupled, we measured (22)Na(+) and (36)Cl(-) uptake in Caco2BBE colon cells infected with the tet-off-inducible DRA transgene. Under basal conditions, DRA activity was low in normal and infected Caco2BBE cells in the presence of tetracycline, whereas NHE activities could be easily detected. When apical NHE activity was increased by transfection or serum-induced expression of the aNHE isoforms NHE2 and NHE3, increased (36)Cl(-) uptake was observed. Inhibition of DRA activity by niflumic acid was greater than that by DIDS as well as by the NHE inhibitor dimethylamiloride and the carbonic anhydrase inhibitor methazolamide. DRA activity was largely aNHE-dependent, whereas a component of DRA-independent aNHE uptake continued to be observed. Coupled aNHE and DRA activities were inhibited by increased cellular cAMP and calcium and were associated with synaptotagmin I-dependent, clathrin-mediated endocytosis. In summary, these data support the role of DRA in electroneutral NaCl absorption involving functional coupling of Cl(-)/base exchange and apical NHE.

Diarrhea as a cause of mortality in a mouse model of infectious colitis

Analysis of gene expression in the colons of Citrobacter rodentium-infected susceptible and resistant mice suggests that mortality is associated with impaired intestinal ion transport.

Background

Comparative characterization of genome-wide transcriptional changes during infection can help elucidate the mechanisms underlying host susceptibility. In this study, transcriptional profiling of the mouse colon was carried out in two cognate lines of mice that differ in their response to Citrobacter rodentium infection; susceptible inbred FVB/N and resistant outbred Swiss Webster mice. Gene expression in the distal colon was determined prior to infection, and at four and nine days post-inoculation using a whole mouse genome Affymetrix array.

Results

Computational analysis identified 462 probe sets more than 2-fold differentially expressed between uninoculated resistant and susceptible mice. In response to C. rodentium infection, 5,123 probe sets were differentially expressed in one or both lines of mice. Microarray data were validated by quantitative real-time RT-PCR for 35 selected genes and were found to have a 94% concordance rate. Transcripts represented by 1,547 probe sets were differentially expressed between susceptible and resistant mice regardless of infection status, a host effect. Genes associated with transport were over-represented to a greater extent than even immune response-related genes. Electrolyte analysis revealed reduction in serum levels of chloride and sodium in susceptible animals.

Conclusion

The results support the hypothesis that mortality in C. rodentium-infected susceptible mice is associated with impaired intestinal ion transport and development of fatal fluid loss and dehydration. These studies contribute to our understanding of the pathogenesis of C. rodentium and suggest novel strategies for the prevention and treatment of diarrhea associated with intestinal bacterial infections.

The influence of buffer composition on tissue integrity during permeability experiments "in vitro"

A well-balanced incubation saline is necessary for permeability experiments with the rat jejunal tissue in the diffusion chambers. At the same time the investigated substance must be chemically stable and sufficiently soluble in this incubation saline. To investigate whether the absence of some ions in incubation salines influences the tissue viability and integrity or the diffusional characteristics of the epithelial membrane the electrical parameters were monitored and the permeability of fluorescein and acyclovir was evaluated during the experiments in side-by-side diffusion chambers. Our results show that the tissue integrity and viability are seriously impaired when Ca(2+) and Mg(2+)-free conditions are applied on both sides of the diffusion chambers, but not when only mucosal or only serosal side is Ca(2+) and Mg(2+)-free. Bicarbonate-free incubation salines can also alter the measured apparent permeability coefficients even though the tissue viability and integrity do not change. This change in the apparent permeability is most likely due to a change in the pH of the mucosal surface and can be prevented if the buffer capacity of the incubation saline is increased.

Acid-base effects on intestinal Cl- absorption and vesicular trafficking

In rat ileum and colon, apical membrane Cl(-)/HCO(3)(-) exchange and net Cl(-) absorption are stimulated by increases in Pco(2) or [HCO(3)(-)]. Because changes in Pco(2) stimulate colonic Na(+) absorption, in part, by modulating vesicular trafficking of the Na(+)/H(+) exchanger type 3 isoform to and from the apical membrane, we examined whether changes in Pco(2) affect net Cl(-) absorption by modulating vesicular trafficking of the Cl(-)/HCO(3)(-) exchanger anion exchanger (AE)1. Cl(-) transport across rat distal ileum and colon was measured in the Ussing chamber, and apical membrane protein biotinylation of these segments and Western blots of recovered proteins were performed. In colonic epithelial apical membranes, AE1 protein content was greater at Pco(2) 70 mmHg than at Pco(2) 21 mmHg but was not affected by pH changes in the absence of CO(2). AE1 was internalized when Pco(2) was reduced and exocytosed when Pco(2) was increased, and both mucosal wortmannin and methazolamide inhibited exocytosis. Wortmannin also inhibited the increase in colonic Cl(-) absorption caused by an increase in Pco(2). Increases in Pco(2) stimulated ileal Cl(-) absorption, but wortmannin was without effect. Ileal epithelial apical membrane AE1 content was not affected by Pco(2). We conclude that CO(2) modulation of colonic, but not ileal, Cl(-) absorption involves effects on vesicular trafficking of AE1.

Apical NHE isoforms differentially regulate butyrate-stimulated Na absorption in rat distal colon

Bicarbonate and butyrate stimulate electroneutral Na absorption via apical membrane Na-H exchange (NHE) in rat distal colon. cAMP downregulates NHE-3 isoform and inhibits HCO3-dependent, but not butyrate-dependent, Na absorption. This study sought to determine whether 1) the apical membrane NHE-2 and NHE-3 isoforms differentially mediated HCO3- and butyrate-dependent Na absorption, and 2) cAMP had different effects on NHE-2 and NHE-3 isoforms. The effect of specific inhibitors of NHE-2 and NHE-3 isoforms (50 microM HOE 694 and 2 microM S3226, respectively) on unidirectional 22Na transepithelial fluxes performed across isolated mucosa from rat distal colon under voltage-clamp conditions was examined. HCO3 stimulation of Na absorption was inhibited by EIPA, a nonspecific inhibitor of all NHE isoforms, by S3226 and dibutyryl cAMP but not by HOE 694. In contrast, butyrate stimulation of Na absorption was not altered by dibutyryl cAMP and was not inhibited by HOE 694 in the absence of dibutyryl cAMP, but in the presence of dibutyryl cAMP was HOE694 sensitive. In contrast, S3226 inhibited butyrate-stimulated Na absorption in the absence of dibutyryl cAMP, but not in its presence. We conclude that 1) HCO3-stimulated Na absorption is mediated solely by NHE-3 isoform, whereas butyrate-stimulated Na absorption is mediated by either NHE-3 or NHE-2 isoform, and 2) dibutyryl cAMP selectively inhibits NHE-3 isoform but stimulates NHE-2 isoform. Dibutyryl cAMP does not inhibit butyrate-stimulated Na absorption as a result of its differential effects on NHE-2 and NHE-3 isoforms.