Rise of cytosolic Ca2+ and activation of membrane-bound guanylyl cyclase activity in rat enterocytes by heat-stable enterotoxin of Vibrio cholerae non-01

Ca(II) Binding Regulates and Dominates the Reactivity of a Transition-Metal-Ion-Dependent Diesterase from Mycobacterium tuberculosis

The diesterase Rv0805 from Mycobacterium tuberculosis is a dinuclear metallohydrolase that plays an important role in signal transduction by controlling the intracellular levels of cyclic nucleotides. As Rv0805 is essential for mycobacterial growth it is a promising new target for the development of chemotherapeutics to treat tuberculosis. The in vivo metal-ion composition of Rv0805 is subject to debate. Here, we demonstrate that the active site accommodates two divalent transition metal ions with binding affinities ranging from approximately 50 nm for Mn(II) to about 600 nm for Zn(II) . In contrast, the enzyme GpdQ from Enterobacter aerogenes, despite having a coordination sphere identical to that of Rv0805, binds only one metal ion in the absence of substrate, thus demonstrating the significance of the outer sphere to modulate metal-ion binding and enzymatic reactivity. Ca(II) also binds tightly to Rv0805 (Kd ≈40 nm), but kinetic, calorimetric, and spectroscopic data indicate that two Ca(II) ions bind at a site different from the dinuclear transition-metal-ion binding site. Ca(II) acts as an activator of the enzymatic activity but is able to promote the hydrolysis of substrates even in the absence of transition-metal ions, thus providing an effective strategy for the regulation of the enzymatic activity.

Long-range interaction effects on calcium-wave propagation

In this paper, numerical simulation of calcium waves in a network of cells coupled together by a paracrine signaling is investigated. The model takes into account the long-range interaction between cells due to the action of extracellular messengers, which provide links between first-neighbor cells, but also on cells located far away from the excited cell. When considering bidirectional coupling, the long-range interaction influences neither the frequency nor the amplitude of oscillations, contrary to one-directional coupling. The long-range interaction influences the speed of propagation of Ca2+ waves in the network and induces enlargement of the transition zone before the steady regime of propagation is attained. We also investigate the long-range effects on the colonization of a given niche by a pathogenic microorganism signal on calcium wave propagation in the network.

Involvement of protein kinase C in the mechanism of action of Escherichia coli heat-stable enterotoxin (STa) in a human colonic carcinoma cell line, COLO-205

The present study was undertaken to determine the involvement of calcium-protein kinase C pathway in the mechanism of action of Escherichia coli heat stable enterotoxin (STa) apart from STa-induced activation of guanylate cyclase in human colonic carcinoma cell line COLO-205, which was used as a model cultured cell line to study the mechanism of action of E. coli STa. In response to E. coli STa, protein kinase C (PKC) activity was increased in a time-dependent manner with its physical translocation from cytosol to membrane. Inhibition of the PKC activity in membrane fraction and inhibition of its physical translocation in response to IP3-mediated calcium release inhibitor dantrolene suggested the involvement of intracellular store depletion in the regulation of PKC activity. Among different PKC isoforms, predominant involvement of calcium-dependent protein kinase C (PKC(alpha)) was specified using isotype-specific pseudosubstrate, which showed pronounce enzyme activity. Inhibition of enzyme activity by PKC(alpha)-specific inhibitor Gö6976 and immunoblott study employing isotype-specific antibody further demonstrated the involvement of calcium-dependent isoform of PKC in the mechanism of action of E. coli STa. Moreover, inhibition of guanylate cyclase activity by PKC(alpha)-specific inhibitor Gö6976 suggested the involvement of PKC(alpha) in the regulation of guanylate cyclase activity.

Role of nitric oxide in NAG-ST induced store-operated calcium entry in rat intestinal epithelial cells

This study was undertaken to find out the mechanism of non-agglutinable Vibrio cholerae heat-stable enterotoxin (NAG-ST)-induced calcium influx across the plasma membrane. Adriamycin, an inhibitor of IP3-specific 3-kinase, could not inhibit NAG-ST-induced calcium influx in rat intestinal epithelial cells, which suggested that inositol 1,3,4,5-tetrakisphosphate (IP4) had no role in NAG-ST-induced calcium influx. NAG-ST increased intracellular nitric oxide level of rat enterocytes as measured by a fluorimetric method using a fluoroprobe 4,5-diaminofluorescein-2-diacetate (DAF-2DA). N-Nitro-L-arginine, an inhibitor of nitric oxide synthase, inhibited NAG-ST-induced rise in nitric oxide level and also calcium influx. Inhibition of inositol trisphosphate (IP3)-mediated intracellular calcium mobilization by Dantrolene could also inhibit NAG-ST-induced rise in intracellular nitric oxide level. Moreover, inhibition of soluble guanylate cyclase by inhibitors (ODQ, LY83583) could inhibit the NAG-ST-induced rise in cyclic guanosine-3',5'-monophosphate (cGMP) level and calcium influx. From this study, it is evident that NAG-ST causes IP3-mediated calcium release from intracellular calcium store, which then stimulates nitric oxide production by activating nitric oxide synthase and the nitric oxide through cGMP activates calcium influx.

Translocation of protein kinase-C with IP3-induced calcium mobilization by heat-stable enterotoxin of Vibrio cholerae non-O1 in isolated rat enterocytes

The activity of the calcium- and phospholipid-dependent enzyme protein kinase C (PKC) in response to heat-stable enterotoxin (NAG-ST) of Vibrio cholerae non-O1 was examined in isolated rat enterocytes. Optimal stimulation of the membrane-bound PKC activity (about 4.3-fold) was observed after 1 min of incubation of cells with 10 ng/ml toxin; and the effects were dose dependent. Following NAG-ST treatment an increase in PKC activity in the membrane fraction was found with a concomitant decrease in the cytosolic fraction suggesting the redistribution of the enzyme. The pronounced enzyme activity in presence of a classical pseudosubstrate and its complete inhibition by Gö 6976 suggested the involvement of a calcium-dependent isoform of PKC (PKC-alpha). A time course study employing an immunoblot assay provided evidence that NAG-ST led to almost complete translocation of PKC-alpha to the membrane. A 65% inhibition of enzyme activity in the membrane fraction and inhibition of its translocation to some extent by dantrolene treatment further suggested that the enzyme was translocated with the rise of intracellular calcium ([Ca2+]i). The phosphorylation of three membrane proteins by toxin-induced PKC in vitro and abolition of this phosphorylation by Gö 6976 demonstrated that phosphorylation of these membrane proteins was PKC-alpha mediated and might be involved in the alteration of membrane functions.

Evidence of calcium influx across the plasma membrane depends upon the initial rise of cytosolic calcium with activation of IP(3) in rat enterocytes by heat-stable enterotoxin of Vibrio cholerae non-O1

In response to heat-stable enterotoxin of Vibrio cholerae non-O1, the initial rise of cytosolic Ca(2+) occurred with activation of IP(3). Chelation of extracellular Ca(2+) with EGTA and suspension of cells in Ca(2+) free buffer both demonstrated the involvement of internal stores in the rise of [Ca(2+)]i. Cells pretreated with dantrolene resulted in decrease of [Ca(2+)]i response which suggested that the rise of intracellular level of Ca(2+) was mostly due to the mobilization from IP(3) sensitive stores. When the cytosolic Ca(2+) was chelated by loading the cells with BAPTA, NAG-ST could not induce Ca(2+) entry to the cell as assessed by Mn(2+) quenching of fura-2 fluorescence which suggested that calcium influx across the plasma membrane depends upon initial rise of this bivalent cation that maintained the sustained phase of [Ca(2+)]i response. Addition of toxin to the fura-2-loaded cells, preincubated with lanthanum chloride, resulted in reduction of [Ca(2+)]i level with a short duration of irregular sustained phase further suggesting that the influx of Ca(2+) across the plasma membrane might be through the calcium channel.

Morphological and cytoskeletal changes caused by non-membrane damaging cytotoxin of Vibrio cholerae on int 407 and HeLa cells

Vibrio cholerae produces a non-membrane damaging cytotoxin (NMDCY), also known as cell rounding factor, which causes rapid rounding of cultured cells like HeLa, CHO and Vero and reportedly elicits enterotoxic activity in the rabbit ileal loop assay. Pursuing the concept that NMDCY might be an accessory factor contributing to the diarrhea caused by V. cholerae, we investigated the effect of NMDCY on Int 407 (intestinal cell line) and HeLa (non-intestinal cell line) cells using light, fluorescent and electron microscopy to gain insight into the cellular response evoked by NMDCY. Binding assays showed that NMDCY has affinity for both Int 407 and HeLa cells. Changes in the internal organelles and cytoskeletal structures of the cell lines were documented indicating changes in the secretory and metabolic function of the toxin-treated cells. Toxin-treated cells visualized under the electron microscope revealed retraction of cell body, formation of blebs on cell surface, changes in mitochondria having dilated and rarefied matrix and an extensively developed Golgi apparatus, endoplasmic reticulum and lysosomes compared to those in normal cells. Immunofluorescence study showed restructuring of microfilament network represented by actin, filamin and vinculin, as also of the microtubular component, tubulin and the intermediate filament, vimentin. Immunogold study further revealed that the toxin is internalized even within the nucleus. Moreover, a rise in the intracellular calcium level of the NMDCY-treated cells leads us to hypothesize that a cascade of events results in the final impairment of the cell machinery.

Rise of intracellular free calcium levels with activation of inositol triphosphate in a human colonic carcinoma cell line (COLO 205) by heat-stable enterotoxin of Escherichia coli

The heat-stable enterotoxin (STa) produced by Escherichia coli has been found to increase rapidly two potential intracellular signals, inositol triphosphate and cytosolic free calcium in a human colonic cell line, COLO 205. Addition of STa to COLO 205 cells prelabelled with myo-[2-3H]inositol resulted in a rapid rise of [3H]inositol triphosphate. Using fluorescent indicator, Fura-2AM, intracellular free Ca2+ has been found to increase 5.12-fold compared to control. Suspension of cells in calcium-free buffer demonstrated STa-induced rapid rise of cytosolic Ca2+. The same result was found when extracellular calcium was chelated with EGTA. This effect was not observed with cells that were pretreated with dantrolene which suggest that the intracellular calcium rise might be due to mobilization from intracellular stores. This study demonstrated for the first time a change in cytosolic calcium in cultured human colonic cells by STa, which is accompanied by inositol triphosphate activation.