Calcium transport affinity, ion competition and cholera toxin effects on cytosolic Ca concentration

Cyclic nucleotides, gut physiology and inflammation

Misregulation of gut function and homeostasis impinges on the overall well-being of the entire organism. Diarrheal disease is the second leading cause of death in children under 5 years of age, and globally, 1.7 billion cases of childhood diarrhea are reported every year. Accompanying diarrheal episodes are a number of secondary effects in gut physiology and structure, such as erosion of the mucosal barrier that lines the gut, facilitating further inflammation of the gut in response to the normal microbiome. Here, we focus on pathogenic bacteria-mediated diarrhea, emphasizing the role of cyclic adenosine 3',5'-monophosphate and cyclic guanosine 3',5'-monophosphate in driving signaling outputs that result in the secretion of water and ions from the epithelial cells of the gut. We also speculate on how this aberrant efflux and influx of ions could modulate inflammasome signaling, and therefore cell survival and maintenance of gut architecture and function.

pCLCA1 becomes a cAMP-dependent chloride conductance mediator in Caco-2 cells

Members of the CLCA protein family are expressed in airway and intestinal epithelium, where they may participate in secretory activity as mediators of chloride conductance. A calcium-dependent chloride conductance has been observed upon expression of CLCA proteins in non-epithelial cell lines. The pCLCA1 gene, cloned in our laboratory, codes for a product containing a unique A-kinase consensus acceptor site not found in other CLCA proteins. Calcium-dependent, but not cAMP-dependent, chloride conductance increased when pCLCA1 was expressed in NIH/3T3 fibroblasts. We transfected the Caco-2 human colon carcinoma cell line with pCLCA1 to investigate the regulation of CLCA-associated chloride conductance in this differentiated epithelial cell line. Expression of pCLCA1 in the Caco-2 cell line enhanced cAMP-responsive 36Cl efflux, short circuit current, and whole cell chloride current in these cells. This cAMP-dependent chloride conductance was localized to the apical membrane of polarized Caco-2 cells.

The calcium-dependent chloride conductance mediator pCLCA1

The regulatory behavior, inhibitor sensitivity, and properties of the whole cell chloride conductance observed in cells expressing the cDNA coding for a chloride conductance mediator isoform of the CLCA gene family, pCLCA1, have been studied. Common C-kinase consensus phosphorylation sites between pCLCA1 and the closely related human isoform hCLCA1 are consistent with a role for calcium in channel activation. Both channels are activated rapidly on exposure to the calcium ionophore ionomycin. Direct involvement of calcium in the activation of pCLCA1 was supported by the finding that treatment with the intracellular calcium chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-AM reduced the rate of chloride efflux from NIH/3T3 cells expressing the pCLCA1 channel. No combination of A-kinase activators used was effective in activating chloride efflux via this channel despite the presence of a unique strong A-kinase consensus site in pCLCA1. Notable differences of pCLCA1 from the reported properties of CLCA family members include the failure of phorbol 12-myristate 13-acetate to activate chloride efflux in cells expressing pCLCA1 and a lack of inhibition of chloride efflux from these cells after treatment with DIDS or dithiothreitol. However, selected inhibitors of anionic conductance inhibited pCLCA1-dependent anion efflux. The electrogenic nature of the ionomycin-dependent efflux of chloride from cells expressing pCLCA1 was confirmed by detection of outwardly rectifying chloride current and inhibition of this current by chloride conductance inhibitors in a whole cell patch-clamp study.

Role of calbindin-D9k in buffering cytosolic free Ca2+ ions in pig duodenal enterocytes

1. The aim of the present study was to test whether the vitamin D-dependent Ca(2+)-binding protein calbindin-D9k could function as an important cytosolic Ca2+ buffer in duodenal enterocytes while facilitating transepithelial active transport of Ca2+ ions. For the investigations we used dual-wavelength, fluorescence ratio imaging, with fura-2 as the Ca(2+)-sensitive dye, to measure changes in cytosolic concentrations of free Ca2+ ions ([Ca2+]i) in isolated pig duodenal enterocytes affected by different cytosolic calbindin-D9k concentrations. 2. Epithelial cells were obtained from weaned piglets with normal calbindin-D9k concentrations (con-piglets), from piglets with low calbindin-D9k levels due to inherited calcitriol deficiency caused by defective renal 25-hydroxycholecalciferol D3-1 alpha-hydroxylase activity (def-piglets), and from piglets with reconstituted calbindin-D9k concentrations, i.e. def-animals treated with high doses of vitamin D3 which elevated plasma calcitriol levels by extrarenal production (def-D3-piglets). Basal levels of [Ca2+]i ranged between 170 and 205 nM and did not differ significantly between the groups. 3. After addition of 5 mM theophylline, the [Ca2+]i in enterocytes from con-piglets doubled during the 10 min incubation. This effect, however, was three times higher in enterocytes from def-piglets compared with those from con-piglets. Similar results were obtained after 4 min incubation of enterocytes from con- and def-piglets in the presence of 1 microM ionomycin. In preparations from def-D3-piglets, ionomycin-induced increases in [Ca2+]i were significantly lower compared with enterocytes from def-piglets and were not different from the control values. 4. From the results, substantial support is given for the hypothesis that one of the major functions of mucosal calbindin-D9k is the effective buffering of Ca2+ ions.

Structure and function of cholera toxin and the related Escherichia coli heat-labile enterotoxin

Cholera and the related Escherichia coli-associated diarrheal disease are important problems confronting Third World nations and any area where water supplies can become contaminated. The disease is extremely debilitating and may be fatal in the absence of treatment. Symptoms are caused by the action of cholera toxin, secreted by the bacterium Vibrio cholerae, or by a closely related heat-labile enterotoxin, produced by Escherichia coli, that causes a milder, more common traveler's diarrhea. Both toxins bind receptors in intestinal epithelial cells and insert an enzymatic subunit that modifies a G protein associated with the adenylate cyclase complex. The consequent stimulated production of cyclic AMP, or other factors such as increased synthesis of prostaglandins by intoxicated cells, initiates a metabolic cascade that results in the excessive secretion of fluid and electrolytes characteristic of the disease. The toxins have a very high degree of structural and functional homology and may be evolutionarily related. Several effective new vaccine formulations have been developed and tested, and a growing family of endogenous cofactors is being discovered in eukaryotic cells. The recent elucidation of the three-dimensional structure of the heat-labile enterotoxin has provided an opportunity to examine and compare the correlations between structure and function of the two toxins. This information may improve our understanding of the disease process itself, as well as illuminate the role of the toxin in studies of signal transduction and G-protein function.

X-ray studies reveal lanthanide binding sites at the A/B5 interface of E. coli heat labile enterotoxin

The crystal structure determination of heat labile enterotoxin (LT) bound to two different lanthanide ions, erbium and samarium, revealed two distinct ion binding sites in the interface of the A subunit and the B pentamer of the toxin. One of the interface sites is conserved in the very similar cholera toxin sequence. These sites may be potential calcium binding sites. Erbium and samarium binding causes a change in the structure of LT: a rotation of the A1 subunit of up to two degrees relative to the B pentamer.

The T-lymphocyte is the primary cellular target for potentiation of the in vitro T-dependent IgM antibody response by the B subunit of cholera toxin

The B (or binding) subunit of cholera toxin (CTB) was reported previously to potentiate the in vitro T-dependent IgM antibody response by a mechanism independent of the cyclic AMP-generating capacity of the intact toxin. In the present report, experiments were designed to determine the immune cell type mediating potentiation by CTB. Firstly, CTB did not potentiate T-independent antibody responses at concentrations that effectively enhanced T-dependent responses. Secondly, separation/reconstitution studies with splenocytes from CTB- and vehicle-treated mice demonstrated potentiation of T-dependent responses by CTB treatment of either the Sephadex G10 non-adherent population or the T-lymphocyte + macrophage population of cells. Potentiation was not observed by CTB treatment of the plastic adherent population or the B-lymphocyte + macrophage population. The evidence indicates that the T-lymphocyte is the primary cellular target for CTB-induced effects on the T-dependent IgM antibody response. Monosialoganglioside GM1, the putative binding site for CTB, is most likely the site of action for CTB on T-lymphocytes. These studies provide new insight on the mechanism of immunomodulation by cholera toxin, and CTB should provide a useful tool for further understanding the role of gangliosides in cellular immune responses.

Effects of calcitriol on stimulation of ion transport in pig jejunal mucosa

1. Active sodium (Na+) and chloride (Cl-) fluxes were studied in vitro in Ussing-type chambers with stripped jejunal mucosa of piglets which suffered from pseudo-vitamin D deficiency rickets, type I. The piglets are devoid of renal calcitriol (1,25-dihydroxy vitamin D3) production and have only small amounts of calbindin in their jejunal enterocytes. 2. In the presence of 0.01 mM-indomethacin non-stimulated short-circuit current (Isc), transepithelial potential difference (PD), tissue conductance (Gt) and unidirectional Na+ (JNa) and Cl- fluxes (JCl) were not affected by the low calcitriol (LC) concentration in plasma. 3. Adding 10 mM-theophylline to the serosal solution in the presence of 0.01 mM-indomethacin caused significantly greater increases in Isc in LC mucosa than in mucosa of vitamin D3-treated and control piglets with normal calcitriol (NC) concentrations in plasma. Omission of indomethacin significantly increased Isc stimulation provoked by theophylline with LC and NC mucosa. The increase, however, was significantly greater with LC than with NC mucosa. 4. Omission of calcium (Ca2+) from the serosal bathing solution significantly depressed Isc stimulation by 10 mM-theophylline in indomethacin-treated LC and NC mucosa. But depression was greater with LC than with NC mucosa. 5. Blocking Ca2+ entry into the cytosol by adding either 0.1 mM-TMB-8 or 0.5 mM-d,l-verapamil to the serosal bathing solution abolished the difference in Isc response to theophylline between indomethacin-treated LC and NC mucosa due to greater depression of Isc in LC than in NC mucosa. 6. The combined effects of theophylline and A23187 on Isc stimulation were calcitriol dependent. In the presence of indomethacin this dependence was only significant when A23187 was given prior to theophylline. In the absence of indomethacin the combined effects of A23187 and theophylline on Isc were always significantly greater in LC than in NC mucosa, irrespective of the order of adding the two agents. 7. Addition of theophylline stimulated net Na+ and Cl- secretion in indomethacin-treated LC and NC mucosa. The increases of net Na+ and Cl- fluxes fully accounted for the rise of Isc with NC mucosa but accounted only partly for the increase in Isc with LC mucosa. This resulted in significant increase in theophylline-stimulated residual ion flux (JR) in LC mucosa which probably resulted from enhanced secretion of bicarbonate.(ABSTRACT TRUNCATED AT 400 WORDS)

Beta-adrenergic modulation of K+ current in human T lymphocytes

The whole-cell voltage-clamp technique was employed to study the beta-adrenergic modulation of voltage-gated K+ currents in CD8+ human peripheral blood lymphocytes. The beta-receptor agonist, isoproterenol, decreased the peak current amplitude and increased the rate of inactivation of the delayed rectifier K+ current. In addition, isoproterenol decreased the voltage dependence of steady-state inactivation and shifted the steady-state inactivation curve to the left. Isoproterenol, on the other hand, had no significant effect on the steady-state parameters of current activation. The isoproterenol-induced decrease in peak current amplitude was inhibited by the beta-blocker propranolol. Bath application of dibutyryl cAMP (1 mM) mimicked the effects of isoproterenol on both K+ current amplitude and time course of inactivation. Furthermore, the reduction in the peak current amplitude in response to isoproterenol was attenuated when PKI5-24 (2-5 microM), a synthetic peptide inhibitor of cAMP-dependent protein kinase, was present in the pipette solution. The increase in the rate of inactivation of the K+ currents in response to isoproterenol was mimicked by the internal application of GTP-gamma-S (300 microM) and by exposure of the cell to cholera toxin (1 microgram/ml), suggesting the involvement of a G protein. These results demonstrate that the voltage-dependent K+ conductance in T lymphocytes can be modulated by beta-adrenergic stimulation. The effects of beta-agonists, i.e., isoproterenol, appear to be receptor mediated and could involve cAMP-dependent protein kinase as well as G proteins. Since inhibition of the delayed rectifier K+ current has been found to decrease the proliferative response in T lymphocytes, the beta-adrenergic modulation of K+ current may well serve as a feedback control mechanism limiting the extent of cellular proliferation.