Homocysteine inhibits store-mediated calcium entry in human endothelial cells: Evidence for involvement of membrane potential and actin cytoskeleton

The role of homocysteine for store-operated calcium influx was investigated in human umbilical cord endothelial cell line. Homocysteine significantly decreased thapsigargin-evoked Ca²⁺ entry, membrane hyperpolarization and actin polymerization. GSH and DTT prevented homocysteine-induced inhibition of thapsigargin-evoked Ca²⁺ entry, membrane hyperpolarization and actin polymerization; while GSSG had the opposite effect. Homocysteine blocked large conductance Ca²⁺-activated K⁺ (BK_Ca) channels in a concentration-dependent manner and related to the redox status of the endothelial cells. BK_Ca channels opener NS1619 reversed thapsigargin-evoked Ca²⁺ entry, membrane hyperpolarization and actin polymerization; BK_Ca channels inhibitor iberiotoxin had the opposite effect. The findings suggest that homocysteine is involved in store-regulated Ca²⁺ entry through membrane potential-dependent and actin cytoskeleton-dependent mechanisms, redox status of homocysteine and BK_Ca channels may play a regulatory role in it. (Mol Cell Biochem 269: 37–47, 2005)     

Modelling mechanism of calcium oscillations in pancreatic acinar cells

We present a simple model for calcium oscillations in the pancreatic acinar cells. This model is based on the calcium release from two receptors, inositol trisphosphate receptors (IPR) and ryanodine receptors (RyR) through the process of calcium induced calcium release (CICR). In pancreatic acinar cells, when the Ca²⁺ concentration increases, the mitochondria uptake it very fast to restrict Ca²⁺ response in the cell. Afterwards, a much slower release of Ca²⁺ from the mitochondria serves as a calcium supply in the cytosol which causes calcium oscillations. In this paper we discuss a possible mechanism for calcium oscillations based on the interplay among the three calcium stores in the cell: the endoplasmic reticulum (ER), mitochondria and cytosol. Our model predicts that calcium shuttling between ER and mitochondria is a pacemaker role in the generation of Ca²⁺oscillations. We also consider the calcium dependent production and degradation of (1,4,5) inositol-trisphosphate (IP₃), which is a key source of intracellular calcium oscillations in pancreatic acinar cells. In this study we are able to predict the different patterns of calcium oscillations in the cell from sinusoidal to raised-baseline, high frequency and low-frequency baseline spiking.     

Does Actin Polymerization Status Modulate CaStorage in Human Neutrophils? Release and Coalescence of CaStores by Cytochalasins

The aim of this paper was to establish whether actin polymerization modulated cytosolic Ca²⁺storage in human neutrophils. Over the concentration ranges which inhibit actin polymerization, cytochalasins A, B, and D liberated Ca²⁺from membrane-bound stores within neutrophils. Two Ca²⁺storage sites were identified in neutrophils by the accumulation of the Ca²⁺binding probe, chlortetracycline: one at the center of the cell and the other at the cell periphery. Confocal imaging demonstrated that cytochalasins released Ca²⁺from the neutrophil periphery, but not from the central Ca²⁺store. Ca²⁺store release was coupled to Ca²⁺influx, suggesting that the peripheral site may be a physiological store containing a Ca²⁺influx factor. 3,3′-Dihexyloxacarbocyanine iodide staining organelles, which correlate with Ca²⁺release sites, coalesced in neutrophils after treatment with cytochalasins. We propose that peripheral Ca²⁺storage sites are restricted from coalescence by cortical polymerized actin and that Ca²⁺store coalescence and Ca²⁺release are coupled events.     

Evidence that the ability to respond to a calcium stimulus in exocytosis is determined by the secretory granule membrane: Comparison of exocytosis of injected bovine chromaffin granule membranes and endogenous cortical granules inXenopus laevis oocytes

Summary 1. To understand better the mechanisms which govern the sensitivity of secretory vesicles to a calcium stimulus, we compared the abilities of injected chromaffin granule membranes and of endogenous cortical granules to undergo exocytosis inXenopus laevis oocytes and eggs in response to cytosolic Ca²⁺. Exocytosis of chromaffin granule membranes was detected by the appearance of dopamine--hydroxylase of the chromaffin granule membrane in the oocyte or egg plasma membrane. Cortical granule exocytosis was detected by release of cortical granule lectin, a soluble constituent of cortical granules, from individual cells.2. Injected chromaffin granule membranes undergo exocytosis equally well in frog oocytes and eggs in response to a rise in cytosolic Ca²⁺ induced by incubation with ionomycin.3. Elevated Ca²⁺ triggered cortical granule exocytosis in eggs but not in oocytes.4. Injected chromaffin granule membranes do not contribute factors to the oocyte that allow calcium-dependent exocytosis of the endogenous cortical granules.5. Protein kinase C activation by phorbol esters stimulates cortical granule exocytosis in bothXenopus laevis oocytes andX. laevis eggs (Bement, W. M., and Capco, D. G.,J. Cell Biol. 108, 885–892, 1989). Activation of protein kinase C by phorbol ester also stimulated chromaffin granule membrane exocytosis in oocytes, indicating that although cortical granules and chromaffin granule membranes differ in calcium responsiveness, PKC activation is an effective secretory stimulus for both.6. These results suggest that structural or biochemical characteristics of the chromaffin granule membrane result in its ability to respond to a Ca²⁺ stimulus. In the oocytes, cortical granule components necessary for Ca²⁺-dependent exocytosis may be missing, nonfunctional, or unable to couple to the Ca²⁺ stimulus and downstream events.     

A key role for reverse Na+/Ca2+ exchange influenced by the actin cytoskeleton in store-operated Ca2+ entry in human platelets: Evidence against the de novo conformational coupling hypothesis

We have previously demonstrated a role for the reorganization of the actin cytoskeleton in store-operated calcium entry (SOCE) in human platelets and interpreted this as evidence for a de novo conformational coupling step in SOCE activation involving the type II IP₃ receptor and the platelet hTRPC1-containing store-operated channel (SOC). Here, we present evidence challenging this model. The actin polymerization inhibitors cytochalasin D or latrunculin A significantly reduced Ca²⁺ but not Mn²⁺ or Na⁺ entry into thapsigargin (TG)-treated platelets. Jasplakinolide, which induces actin polymerization, also inhibited Ca²⁺ but not Mn²⁺ or Na⁺ entry. However, an anti-hTRPC1 antibody inhibited TG-evoked entry of all three cations, indicating that they all permeate an hTRPC1-containing store-operated channel (SOC). These results indicate that the reorganization of the actin cytoskeleton is not involved in SOC activation. The inhibitors of the Na⁺/Ca²⁺ exchanger (NCX), KB-R7943 or SN-6, caused a dose-dependent inhibition of Ca²⁺ but not Mn²⁺ or Na⁺ entry into TG-treated platelets. The effects of the NCX inhibitors were not additive with those of actin polymerization inhibitors, suggesting a common point of action. These results indicate a role for two Ca²⁺ permeable pathways activated following Ca²⁺ store depletion in human platelets: A Ca²⁺-permeable, hTRPC1-containing SOC and reverse Na⁺/Ca²⁺ exchange, which is activated following Na⁺ entry through the SOC and requires a functional actin cytoskeleton.     

Identifying the targets of the amplifying pathway for insulin secretion in pancreatic beta-cells by kinetic modeling of granule exocytosis

A kinetic model for insulin secretion in pancreatic β-cells is adapted from a model for fast exocytosis in chromaffin cells. The fusion of primed granules with the plasma membrane is assumed to occur only in the “microdomain” near voltage-sensitive L-type Ca²⁺-channels, where [Ca²⁺] can reach micromolar levels. In contrast, resupply and priming of granules are assumed to depend on the cytosolic [Ca²⁺]. Adding a two-compartment model to handle the temporal distribution of Ca²⁺ between the microdomain and the cytosol, we obtain a unified model that can generate both the fast granule fusion and the slow insulin secretion found experimentally in response to a step of membrane potential. The model can simulate the potentiation induced in islets by preincubation with glucose and the reduction in second-phase insulin secretion induced by blocking R-type Ca²⁺-channels (Ca_V2.3). The model indicates that increased second-phase insulin secretion induced by the amplifying signal is controlled by the “resupply” step of the exocytosis cascade. In contrast, enhancement of priming is a good candidate for amplification of first-phase secretion by glucose, cyclic adenosine 3′:5′-cyclic monophosphate, and protein kinase C. Finally, insulin secretion is enhanced when the amplifying signal oscillates in phase with the triggering Ca²⁺-signal.     

Actin polymerization in human eosinophils,unlike human neutrophils,depends on intracellular calcium mobilization

Eosinophils represent major effector cells in the allergic inflammation. In contrast to neutrophils, the mechanism of eosinophil activation during the inflammatory response is poorly understood. In this study, the relation between calcium fluxes, chemotaxis, and actin polymerization in eosinophils from healthy non-atopic donors was investigated. Pre-incubation of eosinophils with the intracellular calcium chelator BAPTA dose-dependently prevented an increase in the intracellular calcium concentration ([Ca²⁺]_i), whereas the depletion of extracellular calcium in the test medium had no effect. The chemotactic response of eosinophils, which was measured by the modified boyden chamber technique upon stimulation with RANTES, C5a and PAF, was dose-dependently inhibited by the chelation of intracellular calcium as well as inactivation of the cells in Ca²⁺-depleted medium. To evaluate whether other cell functions which are involved in the migratory response of eosinophils might be dependent on intracellular and extracellular calcium, actin polymerization was investigated. Flow-cytometric measurement of F-actin with NBD-phallacidin revealed that actin polymerization in human eosinophils in response to RANTES, C5a, and PAF was dose-dependently inhibited by the intracellular calcium chelator BAPTA. Since it is well known that actin polymerization in neutrophils is not affected by chelation of intracellular calcium, actin polymerization in these cells was investigated under the same conditions as for eosinophils. In contrast to eosinophils, BAPTA did not inhibit actin polymerization in neutrophils. In summary, these data demonstrate that intracellular calcium fluxes represent a prerequisite for eosinophil chemotaxis and actin polymerization in human eosinophils. Furthermore, regulation of actin polymerization in eosinophils differed from that of neutrophils on the level of intracellular calcium fluxes. © 1996 Wiley-Liss, Inc.     

Effect of cAMP-elevating drugs on Ca2+ efflux and actin polymerization in peritoneal macrophages stimulated with N-formyl chemotactic peptide

To investigate intracellular cAMP inhibitory mechanisms related to migration of guinea-pig peritoneal macrophages, we examined the effects of cAMP-elevating drugs on the Ca²⁺ efflux and actin polymerization in macrophages stimulated with fMet-Leu-Phe, a chemotactic peptide. The stimulation with 1·10^?8 M fMet-Leu-Phe enhanced the Ca²⁺ efflux, and induced actin polymerization. Dibutyryl cAMP, theophylline and papaverine, which continuously increased the levels of intracellular cAMP, inhibited the enhancement of Ca²⁺ efflux and induction of actin polymerization by fMet-Leu-Phe. On the other hand, isoproterenol, which transiently increased the cAMP level, inhibited only the early phase of Ca²⁺ efflux and not the actin polymerization. As additions of both cAMP and cAMP-dependent protein kinase did not modify the Ca²⁺ uptake of phagocytic vesicles, the inhibition of Ca²⁺ efflux by these drugs may be due to the inhibition of the Ca²⁺ release from the intracellular store site(s). The cAMP-elevating drugs increased the monomeric actin content without change in the total actin content, indicating an induction of the depolymerization of filamentous actin. From these findings, we conclude that the inhibition of macrophage migration induced by cAMP may be due to the inhibition of both the increase of intracellular Ca²⁺ concentration and actin polymerization. Furthermore, the intracellular levels of cAMP probably play a role in regulating actin states in the macrophages.     

Calcium-Based Interactions of Symbiotic Partners in Legumes: Role of Peribacteroid Membrane

Based on experimental evidence, a concept is formulated that mutualistic relationships between pro- and eukaryotic cells during nitrogen-fixing legume–rhizobia symbiosis rely both on selective transfer of metabolites and ion transport, Ca²⁺ in particular, across the peribacteroid membrane (PBM). PBM in the nitrogen-fixing cells of yellow lupine (Lupinus luteus L.) and broad bean (Vicia faba L.) is endowed with a calcium-translocating ATPase that pumps Ca²⁺ into the symbiosome. This pumping ensures, on the one hand, calcium homeostasis in the cytosol of infected plant cells and, on the other hand, it optimizes Ca²⁺ level in symbiosomes, first of all in the bacteroids, because Ca²⁺ is one of the main factors controlling their nitrogenase activity. The balance between the symbiotic partners and the maintenance of optimal Ca²⁺ level in the bacteroids also depends on passive Ca²⁺ efflux from symbiosomes to the plant cell cytosol via calcium channels. The Ca²⁺-transporting mechanisms residing at PBM are characterized.     

Chemico-genetic identification of drebrin as a regulator of calcium responses

Store-operated calcium channels are plasma membrane Ca²⁺ channels that are activated by depletion of intracellular Ca²⁺ stores, resulting in an increase in intracellular Ca²⁺ concentration, which is maintained for prolonged periods in some cell types. Increases in intracellular Ca²⁺ concentration serve as signals that activate a number of cellular processes, however, little is known about the regulation of these channels. We have characterized the immuno-suppressant compound BTP, which blocks store-operated channel mediated calcium influx into cells. Using an affinity purification scheme to identify potential targets of BTP, we identified the actin reorganizing protein, drebrin, and demonstrated that loss of drebrin protein expression prevents store-operated channel mediated Ca²⁺ entry, similar to BTP treatment. BTP also blocks actin rearrangements induced by drebrin. While actin cytoskeletal reorganization has been implicated in store-operated calcium channel regulation, little is known about actin-binding proteins that are involved in this process, or how actin regulates channel function. The identification of drebrin as a mediator of this process should provide new insight into the interaction between actin rearrangement and store-operated channel mediated calcium influx.     

Actin cytoskeleton modulates calcium signaling during maturation of starfish oocytes

Before successful fertilization can occur, oocytes must undergo meiotic maturation. In starfish, this can be achieved in vitro by applying 1-methyladenine (1-MA). The immediate response to 1-MA is the fast Ca²⁺ release in the cell cortex. Here, we show that this Ca²⁺ wave always initiates in the vegetal hemisphere and propagates through the cortex, which is the space immediately under the plasma membrane. We have observed that alteration of the cortical actin cytoskeleton by latrunculin-A and jasplakinolide can potently affect the Ca²⁺ waves triggered by 1-MA. This indicates that the cortical actin cytoskeleton modulates Ca²⁺ release during meiotic maturation. The Ca²⁺ wave was inhibited by the classical antagonists of the InsP₃-linked Ca²⁺ signaling pathway, U73122 and heparin. To our surprise, however, these two inhibitors induced remarkable actin hyper-polymerization in the cell cortex, suggesting that their inhibitory effect on Ca²⁺ release may be attributed to the perturbation of the cortical actin cytoskeleton. In post-meiotic eggs, U73122 and jasplakinolide blocked the elevation of the vitelline layer by uncaged InsP₃, despite the massive release of Ca²⁺, implying that exocytosis of the cortical granules requires not only a Ca²⁺ rise, but also regulation of the cortical actin cytoskeleton. Our results suggest that the cortical actin cytoskeleton of starfish oocytes plays critical roles both in generating Ca²⁺ signals and in regulating cortical granule exocytosis.     

Actin assembly by cadmium ions

Cadmium is a highly toxic metal entering cells by a variety of mechanisms. Its toxic action is far from being completely understood, although specific interaction with the cellular calcium metabolism has been indicated. Metal ions that influence intracellular Ca²⁺ concentrations or compete with Ca²⁺ for protein binding sites may exert an effect on actin filaments, whose assembly and disassembly are both regulated by a number of calcium-dependent factors. Cadmium is such a metal. Much evidence demonstrates that cadmium interferes with the dynamics of actin filaments in various types of cells. Here we show that, at high (0.8–1.0 mM) concentrations, CdCl₂ causes actin denaturation. At such Cd²⁺ concentrations, actin precipitates (really actin, as shown by SDS-PAGE, see Fig. 1B) in the form of irregular, disordered clots, clearly appreciable by electron microscopy. Denaturation seems to be reversible since, after Cd²⁺ removal by dialysis, the polymerizability of sedimented actin is restored almost completely. On the other hand, at concentrations ranging from 0.25 to 0.6 mM, CdCl₂ is more effective as an actin polymerizing agent than both MgCl₂ and CaCl₂. The Cd-related increase in the actin assembly rate is ascribable to an enhanced nucleation rather than to an increased monomer addition to filament growing ends. The latter, in contrast, appears quite slow. Critical concentration measurements revealed that the extent of polymerization of both Mg- and Cd-assembled actin are very close (C_c ranges from 0.25 to 0.5 μM), while Ca-polymerized actin shows a polymerization extent markedly lower (C_c=4.0 μM). By both the fluorescent Ca²⁺ chelator Quin-2 assay and limited proteolysis of actin by trypsin and α-chymotrypsin, the real substitution of G-actin-bound Ca²⁺ by Cd²⁺ has been appreciated. The increase in Quin-2 fluorescence after addition of excess CdCl₂ indicates that, in our experimental conditions, Ca²⁺ tightly-bound to actin is partially (60–70%) replaced by Cd²⁺, forming Cd-actin. Electrophoretic patterns after limited proteolysis reveal that the trypsin cleavage sites in the segment 61–69 of the actin polypeptide chain are less accessible in Cd-actin than in Ca-actin, although the cation-dependent effect is less pronounced in Cd-actin than in Mg-actin. Our results are consistent with some of the consequences on microfilament organization observed in Cd²⁺-treated cells; however, considering the positive effect of Cd²⁺ on actin polymerization in solution we have noticed that this was never observed in vivo. A different indirect effect of Cd²⁺ on some cellular event(s) influencing cytoplasmic actin polymerization appears to be reasonable. © 1997 Elsevier Science B.V. All rights reserved.     

Modulation of calcium signalling by mitochondria

In this review we will attempt to summarise the complex and sometimes contradictory effects that mitochondria have on different forms of calcium signalling. Mitochondria can influence Ca²⁺ signalling indirectly by changing the concentration of ATP, NAD(P)H, pyruvate and reactive oxygen species — which in turn modulate components of the Ca²⁺ signalling machinery i.e. buffering, release from internal stores, influx from the extracellular solution, uptake into cellular organelles and extrusion by plasma membrane Ca²⁺ pumps. Mitochondria can directly influence the calcium concentration in the cytosol of the cell by importing Ca²⁺ via the mitochondrial Ca²⁺ uniporter or transporting Ca²⁺ from the interior of the organelle into the cytosol by means of Na⁺/Ca²⁺ or H⁺/Ca²⁺ exchangers. Considerable progress in understanding the relationship between Ca²⁺ signalling cascades and mitochondrial physiology has been accumulated over the last few years due to the development of more advanced optical techniques and electrophysiological approaches.     

Calcium Signaling throughout the Toxoplasma gondii Lytic Cycle: A STUDY USING GENETICALLY ENCODED CALCIUM INDICATORS*

Toxoplasma gondii is an obligate intracellular parasite that invades host cells, creating a parasitophorous vacuole where it communicates with the host cell cytosol through the parasitophorous vacuole membrane. The lytic cycle of the parasite starts with its exit from the host cell followed by gliding motility, conoid extrusion, attachment, and invasion of another host cell. Here, we report that Ca²⁺ oscillations occur in the cytosol of the parasite during egress, gliding, and invasion, which are critical steps of the lytic cycle. Extracellular Ca²⁺ enhances each one of these processes. We used tachyzoite clonal lines expressing genetically encoded calcium indicators combined with host cells expressing transiently expressed calcium indicators of different colors, and we measured Ca²⁺ changes in both parasites and host simultaneously during egress. We demonstrated a link between cytosolic Ca²⁺ oscillations in the host and in the parasite. Our approach also allowed us to measure two new features of motile parasites, which were enhanced by Ca²⁺ influx. This is the first study showing, in real time, Ca²⁺ signals preceding egress and their direct link with motility, an essential virulence trait.     

Fluid shear stress induces actin polymerization in human neutrophils

We have previously reported that a physiological range of shear stress induces neutrophil homotypic aggregation mediated by lymphocyte function-associated antigen-1 (LFA-1) and intercellular adhesion molecule-3 (ICAM-3) interactions. To further characterize the homotypic aggregation, actin polymerization was investigated in neutrophils stimulated by shear stress in comparison with formyl-methionyl-leucyl-phenylalanine (fMLP). In fMLP-stimulated neutrophils, actin polymerization was localized in the pseudopods, and this reaction was not mediated by a cytosolic level of Ca²⁺. In contrast to fMLP stimulation, the actin polymerization induced by shear stress in a cone-plate viscometer was localized in cell-cell contact regions, and this polymerization required the increase of intracellular Ca²⁺. This shear stress-induced actin polymerization was not observed when neutrophils were pretreated with anti-LFA-1 or anti-ICAM-3 antibody. In conclusion, LFA-1 and ICAM-3 interaction mediated by the increase of [Ca²⁺]i generated the intercellular signal in order to accumulate F-actin in the cell-cell contact regions. © 1996 Wiley-Liss, Inc.     

Plasma membrane calcium pump activity is affected by the membrane protein concentration: Evidence for the involvement of the actin cytoskeleton

Plasma membrane calcium pumps (PMCAs) are integral membrane proteins that actively expel Ca²⁺ from the cell. Specific Ca²⁺-ATPase activity of erythrocyte membranes increased steeply up to 1.5-5 times when the membrane protein concentration decreased from 50 μg/ml to 1 μg/ml. The activation by dilution was also observed for ATP-dependent Ca²⁺ uptake into vesicles from Sf9 cells over-expressing the PMCA 4b isoform, confirming that it is a property of the PMCA. Dilution of the protein did not modify the activation by ATP, Ca²⁺ or Ca²⁺-calmodulin. Treatment with non-ionic detergents did not abolish the dilution effect, suggesting that it was not due to resealing of the membrane vesicles. Pre-incubation of erythrocyte membranes with Cytochalasin D under conditions that promote actin polymerization abolished the dilution effect. Highly-purified, micellar PMCA showed no dilution effect and was not affected by Cytochalasin D. Taken together, these results suggest that the concentration-dependent behavior of the PMCA activity was due to interactions with cytoskeletal proteins. The dilution effect was also observed with different PMCA isoforms, indicating that this is a general phenomenon for all PMCAs.     

Pattern of rise in subplasma membrane Ca concentration determines type of fusing insulin granules in pancreatic β cells

We simultaneously analyzed insulin granule fusion with insulin fused to green fluorescent protein and the subplasma membrane Ca²⁺ concentration ([Ca²⁺]_PM) with the Ca²⁺ indicator Fura Red in rat β cells by dual-color total internal reflection fluorescence microscopy. We found that rapid and marked elevation in [Ca²⁺]_PM caused insulin granule fusion mostly from previously docked granules during the high KCl-evoked release and high glucose-evoked first phase release. In contrast, the slow and sustained elevation in [Ca²⁺]_PM induced fusion from newcomers translocated from the internal pool during the low KCl-evoked release and glucose-evoked second phase release. These data suggest that the pattern of the [Ca²⁺]_PM rise directly determines the types of fusing granules.     

Secretory vesicle — Cytosol interactions in exocytosis: Isolation by Ca2+-dependent affinity chromatography of proteins that bind to the chromaffin granule membrane

Proteins from adrenal medullary cytosol that bind to chromaffin granule membranes in the presence of Ca²⁺ were isolated by affinity chromatography on granule membranes coupled to Sepharose 4B. Cytosol was applied to the affinity column in the presence of 2 mM free Ca²⁺. One group of proteins was eluted at 50 μM Ca²⁺ and had molecular weights of 60,000, 46,000, 36,000, 34,000, 32,000 and 26,000. At 0.1 μM Ca²⁺ additional proteins of molecular weights 70,000, 44,000 and 33,000 were eluted. Both groups of proteins aggregated isolated chromaffin granules in the presence of Ca²⁺. Since exocytosis involves cytosol-membrane interactions regulated by Ca²⁺, these proteins may have functional roles in this process. The term “chromobindins” is introduced to describe these proteins.     

Adducin Is Involved in Endothelial Barrier Stabilization

Adducins tightly regulate actin dynamics which is critical for endothelial barrier function. Adducins were reported to regulate epithelial junctional remodeling by controlling the assembly of actin filaments at areas of cell-cell contact. Here, we investigated the role of α-adducin for endothelial barrier regulation by using microvascular human dermal and myocardial murine endothelial cells. Parallel transendothelial electrical resistance (TER) measurements and immunofluorescence analysis revealed that siRNA-mediated adducin depletion impaired endothelial barrier formation and led to severe fragmentation of VE-cadherin immunostaining at cell-cell borders. To further test whether the peripheral localization of α-adducin is functionally linked with the integrity of endothelial adherens junctions, junctional remodeling was induced by a Ca²⁺-switch assay. Ca²⁺-depletion disturbed both linear vascular endothelial (VE)-cadherin and adducin location along cell junctions, whereas their localization was restored following Ca²⁺-repletion. Similar results were obtained for α-adducin phosphorylated at a site typical for PKA (pSer481). To verify that endothelial barrier properties and junction reorganization can be effectively modulated by altering Ca²⁺-concentration, TER measurements were performed. Thus, Ca²⁺-depletion drastically reduced TER, whereas Ca²⁺-repletion led to recovery of endothelial barrier properties resulting in increased TER. Interestingly, the Ca²⁺-dependent increase in TER was also significantly reduced after efficient α-adducin downregulation. Finally, we report that inflammatory mediator-induced endothelial barrier breakdown is associated with loss of α-adducin from the cell membrane. Taken together, our results indicate that α-adducin is involved in remodeling of endothelial adhesion junctions and thereby contributes to endothelial barrier regulation.