Hormone-stimulated calcium release is inhibited by cytoskeleton-disrupting toxins in AR4-2J cells

Clostridium difficile toxin B differentially affects GPCR-stimulated Ca2+ responses in macrophages: independent roles for Rho and PLA2

Clostridium difficile toxins cause acute colitis by disrupting the enterocyte barrier and promoting inflammation. ToxB from C. difficile inactivates Rho family GTPases and causes release of cytokines and eicosanoids by macrophages. We studied the effects of ToxB on GPCR signaling in murine RAW264.7 macrophages and found that ToxB elevated Ca(2+) responses to Galphai-linked receptors, including the C5aR, but reduced responses to Galphaq-linked receptors, including the UDP receptors. Other Rho inhibitors also reduced UDP Ca(2+) responses, but they did not affect C5a responses, suggesting that ToxB inhibited UDP responses by inhibiting Rho but enhanced C5a responses by other mechanisms. By using PLCbeta isoform-deficient BMDM, we found that ToxB inhibited Ca(2+) signaling through PLCbeta4 but enhanced signaling through PLCbeta3. Effects of ToxB on GPCR Ca(2+) responses correlated with GPCR use of PLCbeta3 versus PLCbeta4. ToxB inhibited UDP Ca(2+) signaling without reducing InsP3 production or the sensitivity of cellular Ca(2+) stores to exogenous InsP3, suggesting that ToxB impairs UDP signaling at the level of InsP3/Ca(2+)coupling. In contrast, ToxB elevated InsP3 production by C5a, and the enhancement of Ca(2+) signaling by C5a was prevented by inhibition of PLA(2) or 5-LOX but not COX, implicating LTs but not prostanoids in the mechanism. In sum, ToxB has opposing, independently regulated effects on Ca(2+) signaling by different GPCR-linked PLCbeta isoforms in macrophages.

Microfilament and microtubule assembly is required for the propagation of inositol trisphosphate receptor-induced Ca2+ waves in bovine aortic endothelial cells

Ca2+ is a highly versatile second messenger that plays a key role in the regulation of numerous cell processes. One-way cells ensure the specificity and reliability of Ca2+ signals is by organizing them spatially in the form of waves that propagate throughout the cell or within a specific subcellular region. In non-excitable cells, the inositol 1,4,5-trisphosphate receptor (IP3R) is responsible for the release of Ca2+ from the endoplasmic reticulum. The spatial aspect of the Ca2+ signal depends on the organization of various elements of the Ca2+ signaling toolkit and varies from tissue to tissue. Ca2+ is implicated in many of endothelium functions that thus depend on the versatility of Ca2+ signaling. In the present study, we showed that the disruption of caveolae microdomains in bovine aortic endothelial cells (BAEC) with methyl-beta-cyclodextrin was not sufficient to disorganize the propagation of Ca2+ waves when the cells were stimulated with ATP or bradykinin. However, disorganizing microfilaments with latrunculin B and microtubules with colchicine both prevented the formation of Ca2+ waves. These results suggest that the organization of the Ca2+ waves mediated by IP3R channels does not depend on the integrity of caveolae in BAEC, but that microtubule and microfilament cytoskeleton assembly is crucial.

Acute actions of marine toxin latrunculin A on the electrophysiological properties of cultured dorsal root ganglion neurones

The effects of latrunculin A, isolated from the nudibranch Chromodoris sp., on the excitability of neonatal rat cultured dorsal root ganglion neurones were investigated using patch-clamp recording and Ca(2+) imaging techniques. Under current-clamp conditions, acute application of latrunculin A (100 microM) reversibly induced multiple action potential firing and significantly increased action potential duration. No significant effects on action potential peak amplitude, threshold of action potential firing, resting membrane potential and input resistance were observed. Under voltage-clamp conditions, significant and dose-dependent suppression of K(+) current was seen with 10-100 microM latrunculin A. Additionally, a significant difference between inhibition of the current measured at the peak and the end of a 100 ms voltage step was seen with 100 microM latrunculin A. Fura-2 fluorescence Ca(2+) imaging revealed that latrunculin A (100 microM) significantly inhibited Ca(2+) transients evoked by KCl-induced depolarisation in all neurones. In 36% of DRG neurones, latrunculin A alone had no effect on intracellular Ca(2+). In 64% of neurones, latrunculin A alone evoked a transient rise in intracellular Ca(2+). Moreover, latrunculin A (10-100 microM) significantly inhibited the mean high voltage-activated Ca(2+) current. The effects of latrunculin A on action potential firing and K(+) currents were attenuated by intracellular phalloidin, an indication that these effects are mediated through actin disruption.

Dual effect of hydrogen peroxide on store-mediated calcium entry in human platelets

Redox regulation is important for the modulation of cytosolic Ca(2+) concentration. Hence, we have investigated the effect of H(2)O(2) on store-mediated Ca(2+) entry (SMCE). In fura-2-loaded human platelets treatment with H(2)O(2) resulted in a concentration-dependent increase in Ca(2+) release from intracellular stores, while the effect on Ca(2+) entry was biphasic. In addition, 1mM H(2)O(2) reduced SMCE induced by agonists. The inhibitory effect of 1mM H(2)O(2) was prevented by inhibition of actin polymerization with cytochalasin D. Consistent with this, we found that 10microM H(2)O(2) and store depletion by treatment with thapsigargin plus ionomycin induced a similar temporal sequence of actin reorganization, while exposure to 1mM H(2)O(2) shifted the dynamics between polymerization and depolymerization in favor of the former. One millimolar H(2)O(2)-induced polymerization was reduced by treatment with methyl 2,5-dihydroxycinnamate and farnesylthioacetic acid, inhibitors of tyrosine kinases and Ras superfamily proteins, respectively. Finally, exposure to 1mM H(2)O(2) significantly increased store depletion-induced p60(src) activation. We conclude that H(2)O(2) exerted a biphasic effect on SMCE. The inhibitory role of high H(2)O(2) concentrations is mediated by an abnormal actin reorganization pattern involving both Ras- and tyrosine kinases-dependent pathways.

NAADP activates a Ca2+ current that is dependent on F-actin cytoskeleton

Nicotinic acid adenine dinucleotide phosphate (NAADP) is involved in the Ca2+ response observed at fertilization in several species, including starfish. In this study, we have employed Ca2+ imaging and the single-electrode voltage-clamp technique to investigate whether the NAADP-mediated Ca2+ entry discovered in our laboratory in starfish oocytes was underlain by a membrane current and whether the response to NAADP required an intact cytoskeleton. Uncaging of preinjected NAADP evoked a cortical Ca2+ flash that was followed by the spreading of the wave to the remainder of the cell. No Ca2+ increase was detected in Ca2+-free sea water. Under voltage-clamp conditions, the photoliberation of NAADP activated an inward rectifying membrane current, which reversed at potentials more positive than +50 mV and was abolished by removal of Ca2+ but not of Na+. The current was affected by preincubation with verapamil, SKF 96356, and thapsigargin but not by preinjection of heparin, 8-NH2- cyclic ADP-ribose, or both antagonists. The membrane current and the Ca2+ wave were inhibited by latrunculin-A and jasplakinolide, which depolymerize and stabilize actin cytoskeleton, respectively. These data offer the first demonstration that NAADP initiates a Ca2+ sweep by activating a Ca2+-permeable membrane current that requires an intact F-actin cytoskeleton as other Ca2+-permeable currents, such as ICRAC and IARC.

Histamine potentiates IP(3)-mediated Ca(2+) release via thapsigargin-sensitive Ca(2+) pumps

We have studied the histamine-induced potentiation of inositol 1,4,5-trisphosphate (IP(3))-mediated Ca(2+) release in HeLa cells. Intracellular IP(3) levels were increased by IP(3) dialysis with the whole-cell configuration of the patch-clamp technique (cell dialysis of IP(3)). Low concentrations of extracellular histamine (1 microM) accelerated the rate of IP(3)-mediated Ca(2+) release, an effect that required the coincidence of both histamine signalling and the increase in IP(3) levels. Our data suggest that the potentiation effect of histamine cannot be explained simply by agonist-induced increase in IP(3) levels. Disordering microfilaments with cytochalasin D and microtubules with colchicine caused a decrease in the histamine-induced Ca(2+) response. Furthermore, both cytochalasin D and colchicine diminished the rate of IP(3)-mediated Ca(2+) release, while only the former reduced slightly the histamine-induced potentiation effect. Remarkably, rapid inhibition of SERCA pumps with thapsigargin to avoid the depletion of internal Ca(2+) stores diminished the histamine-induced potentiation of IP(3)-mediated Ca(2+) release, without affecting the rate of IP(3)-mediated Ca(2+) release. These data indicate that histamine-induced potentiation of Ca(2+) release in HeLa cells requires active SERCA pumps and suggest that SERCA pumps are an important factor in determining the efficiency of agonist-induced Ca(2+) release.

Evidence for secretion-like coupling involving pp60src in the activation and maintenance of store-mediated Ca2+ entry in mouse pancreatic acinar cells

Store-mediated Ca2+ entry (SMCE) is one of the main pathways for Ca2+ influx in non-excitable cells. Recent studies favour a secretion-like coupling mechanism to explain SMCE, where Ca2+ entry is mediated by an interaction of the endoplasmic reticulum (ER) with the plasma membrane (PM) and is modulated by the actin cytoskeleton. To explore this possibility further we have now investigated the role of the actin cytoskeleton in the activation and maintenance of SMCE in pancreatic acinar cells, a more specialized secretory cell type which might be an ideal cellular model to investigate further the properties of the secretion-like coupling model. In these cells, the cytoskeletal disrupters cytochalasin D and latrunculin A inhibited both the activation and maintenance of SMCE. In addition, stabilization of a cortical actin barrier by jasplakinolide prevented the activation, but not the maintenance, of SMCE, suggesting that, as for secretion, the actin cytoskeleton plays a double role in SMCE as a negative modulator of the interaction between the ER and PM, but is also required for this mechanism, since the cytoskeleton disrupters impaired Ca2+ entry. Finally, depletion of the intracellular Ca2+ stores induces cytoskeletal association and activation of pp60(src), which is independent on Ca2+ entry. pp60(src) activation requires the integrity of the actin cytoskeleton and participates in the initial phase of the activation of SMCE in pancreatic acinar cells.

Integrity of actin-network is involved in uridine 5'-triphosphate evoked store-operated Ca2+ entry in bovine adrenocortical fasciculata cells

Store-operated Ca(2+) entry channels (SOCs) play an important role in the regulation of diverse non-excitable cell functions. However, the precise mechanism of SOCs activation is still controversial. Uridine 5'-triphosphate (UTP) was shown to induce Ca(2+) entry in a dihydropyridines-insensitive manner and accelerated steroidogenesis in bovine adrenocortical fasciculata cells (BAFCs) via the Gq/11 protein-coupled P2Y(2) receptor. Therefore we investigated whether UTP is involved in SOCs activation and the mechanism of UTP-induced SOCs activation. Fura 2-loaded BAFCs were used for the measurement of intracellular concentration of Ca(2+) ([Ca(2+)](i)) mobilization. Extracellular UTP evoked Ca(2+) release from intracellular stores followed by an increase in Ca(2+) entry. The Ca(2+) influx elicited by UTP was inhibited not by nifedipine, but by Zn(2+), Cd(2+), and Ni(2+) (potency order: Zn(2+) > Cd(2+) >> Ni(2+)), and the effect of UTP was also attenuated by a phospholipase C inhibitor (U73122). These results indicate that UTP activates SOCs in BAFCs. The increase in [Ca(2+)](i) by UTP was attenuated by ML-9, a myosin-light chain kinase inhibitor, and calmodulin inhibitors, W-7 and E6 berbamine, in a concentration-dependent manner. These reagents depolymerized actin filaments with rhodamine staining in BAFCs. Cytochalasin D also inhibited UTP-activated SOCs and depolymerized actin filaments. From these results, we proposed that calcium/calmodulin dependent myosin-light chain kinase is involved in the mobilization of actin filaments and the integrity of actin-network plays an important role in UTP-induced SOCs activation in BAFCs.

Properties of long myosin light chain kinase binding to F-actin in vitro and in vivo

Short and long myosin light chain kinases (MLCKs) are Ca(2+)/calmodulin-dependent enzymes that phosphorylate the regulatory light chain of myosin II in thick filaments but bind with high affinity to actin thin filaments. Three repeats of a motif made up of the sequence DFRXXL at the N terminus of short MLCK are necessary for actin binding (Smith, L., Su, X., Lin, P., Zhi, G., and Stull, J. T. (1999) J. Biol. Chem. 274, 29433-29438). The long MLCK has two additional DFRXXL motifs and six Ig-like modules in an N-terminal extension, which may confer unique binding properties for cellular localization. Two peptides containing either five or three DFRXXL motifs bound to F-actin and smooth muscle myofilaments with maximal binding stoichiometries consistent with each motif binding to an actin monomer in the filaments. Both peptides cross-linked F-actin and bound to stress fibers in cells. Long MLCK with an internal deletion of the five DFRXXL motifs and the unique NH(2)-terminal fragment containing six Ig-like motifs showed weak binding. Cell fractionation and extractions with MgCl(2) indicate that the long MLCK has a greater affinity for actin-containing filaments than short MLCK in vitro and in vivo. Whereas DFRXXL motifs are necessary and sufficient for short MLCK binding to actin-containing filaments, the DFRXXL motifs and the N-terminal extension of long MLCK confer high affinity binding to stress fibers in cells.