Subcortical Ca2+ waves sneaking under the plasma membrane in endothelial cells

Relationship between sodium-dependent phosphate transporter (NaPi-IIc) function and cellular vacuole formation in opossum kidney cells

NaPi-IIc/SLC34A3 is a sodium-dependent inorganic phosphate (Pi) transporter in the renal proximal tubules and its mutations cause hereditary hypophosphatemic rickets with hypercalciuria (HHRH). In the present study, we created a specific antibody for opossum SLC34A3, NaPi-IIc (oNaPi-IIc), and analyzed its localization and regulation in opossum kidney cells (a tissue culture model of proximal tubular cells). Immunoreactive oNaPi-IIc protein levels increased during the proliferative phase and decreased during differentiation. Moreover, stimulating cell growth upregulated oNaPi-IIc protein levels, whereas suppressing cell proliferation downregulated oNaPi-IIc protein levels. Immunocytochemistry revealed that endogenous and exogenous oNaPi-IIc proteins localized at the protrusion of the plasma membrane, which is a phosphatidylinositol 4,5-bisphosphate (PIP2) rich-membrane, and at the intracellular vacuolar membrane. Exogenous NaPi-IIc also induced cellular vacuoles and localized in the plasma membrane. The ability to form vacuoles is specific to electroneutral NaPi-IIc, and not electrogenic NaPi-IIa or NaPi-IIb. In addition, mutations of NaPi-IIc (S138F and R468W) in HHRH did not cause cellular PIP2-rich vacuoles. In conclusion, our data anticipate that NaPi-IIc may regulate PIP2 production at the plasma membrane and cellular vesicle formation.

Decoding dynamic Ca(2+) signaling in the vascular endothelium

Although acute and chronic vasoregulation is inherently driven by endothelial Ca(2+), control and targeting of Ca(2+)-dependent signals are poorly understood. Recent studies have revealed localized and dynamic endothelial Ca(2+) events comprising an intricate signaling network along the vascular intima. Discrete Ca(2+) transients emerging from both internal stores and plasmalemmal cation channels couple to specific membrane K(+) channels, promoting endothelial hyperpolarization and vasodilation. The spatiotemporal tuning of these signals, rather than global Ca(2+) elevation, appear to direct endothelial functions under physiologic conditions. In fact, altered patterns of dynamic Ca(2+) signaling may underlie essential endothelial dysfunction in a variety of cardiovascular diseases. Advances in imaging approaches and analyses in recent years have allowed for detailed detection, quantification, and evaluation of Ca(2+) dynamics in intact endothelium. Here, we discuss recent insights into these signals, including their sources of origination and their functional encoding. We also address key aspects of data acquisition and interpretation, including broad applications of automated high-content analysis.

Inhibitors of arachidonate-regulated calcium channel signaling suppress triggered activity induced by the late sodium current

Disturbances in myocyte calcium homeostasis are hypothesized to be one cause for cardiac arrhythmia. The full development of this hypothesis requires (i) the identification of all sources of arrhythmogenic calcium and (ii) an understanding of the mechanism(s) through which calcium initiates arrhythmia. To these ends we superfused rat left atria with the late sodium current activator type II Anemonia sulcata toxin (ATXII). This toxin prolonged atrial action potentials, induced early afterdepolarization, and provoked triggered activity. The calmodulin-dependent protein kinase II (CaMKII) inhibitor KN-93 (N-[2-[[[3-(4-chlorophenyl)-2-propenyl]methylamino]methyl]phenyl]-N-(2-hydroxyethyl)-4-methoxybenzenesulphon-amide) suppressed ATXII triggered activity but its inactive congener KN-92 (2-[N-(4-methoxy benzenesulfonyl)]amino-N-(4-chlorocinnamyl)-N-methylbenzylamine) did not. Neither drug affected normal atrial contractility. Calcium entry via L-type channels or calcium leakage from sarcoplasmic reticulum stores are not critical for this type of ectopy as neither verapamil ((RS)-2-(3,4-dimethoxyphenyl)-5-{[2-(3,4-dimethoxyphenyl)ethyl]-(methyl)amino}-2-prop-2-ylpentanenitrile) nor ryanodine affected ATXII triggered activity. By contrast, inhibitors of the voltage independent arachidonate-regulated calcium (ARC) channel and the store-operated calcium channel specifically suppressed ATXII triggered activity without normalizing action potentials or affecting atrial contractility. Inhibitors of cytosolic calcium-dependent phospholipase A2 also suppressed triggered activity suggesting that this lipase, which generates free arachidonate, plays a key role in ATXII ectopy. Thus, increased left atrial late sodium current appears to activate atrial Orai-linked ARC and store operated calcium channels, and these voltage-independent channels may be unexpected sources for the arrhythmogenic calcium that underlies triggered activity.

FRET-based sensor analysis reveals caveolae are spatially distinct Ca2+ stores in endothelial cells

Ca2+-regulating and Ca2+-dependent molecules enriched in caveolae are typically shaped as plasmalemmal invaginations or vesicles. Caveolae structure and subcellular distribution are critical for Ca2+ release from endoplasmic reticulum Ca2+ stores and for Ca2+ influx from the extracellular space into the cell. However, Ca2+ dynamics inside caveolae have never been directly measured and remain uncharacterized. To target the fluorescence resonance energy transfer (FRET)-based Ca2+ sensing protein D1, a mutant of cameleon, to the intra-caveolar space, we made a cDNA construct encoding a chimeric protein of lectin-like oxidized low-density lipoprotein receptor 1 (LOX-1) and D1 (LOXD1). Immunofluorescence and immunoelectron microscopy confirmed that a significant portion of LOXD1 was localized with caveolin-1 at morphologically apparent caveolar vesicles in endothelial cells. LOXD1 detected ATP-induced transient Ca2+ decreases by confocal FRET imaging in the presence or absence of extracellular Ca2+. This ATP-induced Ca2+ decrease was abolished following knockdown of caveoin-1, suggesting an association with caveolae. The X-ray spectra obtained by the spot analysis of electron-opaque pyroantimonate precipitates further confirmed that ATP-induced calcium decreases in intra-caveolar vesicles. In conclusion, subplasmalemmal caveolae function as Ca2+-releasable Ca2+ stores in response to ATP. This intracellular local Ca2+ delivery system may contribute to the complex spatiotemporal organization of Ca2+ signaling.

Recruitment of dynamic endothelial Ca2+ signals by the TRPA1 channel activator AITC in rat cerebral arteries

OBJECTIVE

Stimulation of endothelial TRP channels, specifically TRPA1, promotes vasodilation of cerebral arteries through activation of Ca2+ -dependent effectors along the myoendothelial interface. However, presumed TRPA1-triggered endothelial Ca2+ signals have not been described. We investigated whether TRPA1 activation induces specific spatial and temporal changes in Ca2+ signals along the intima that correlates with incremental vasodilation.

METHODS

Confocal imaging, immunofluorescence staining, and custom image analysis were employed.

RESULTS

We found that endothelial cells of rat cerebral arteries exhibit widespread basal Ca2+ dynamics (44 ± 6 events/minute from 26 ± 3 distinct sites in a 3.6 × 10(4) μm2 field). The TRPA1 activator AITC increased Ca2+ signals in a concentration-dependent manner, soliciting new events at distinct sites. Origination of these new events corresponded spatially with TRPA1 densities in IEL holes, and the events were prevented by the TRPA1 inhibitor HC-030031. Concentration-dependent expansion of Ca2+ events in response to AITC correlated precisely with dilation of pressurized cerebral arteries (p = 0.93 by F-test). Correspondingly, AITC caused rapid endothelium-dependent suppression of asynchronous Ca2+ waves in subintimal smooth muscle.

CONCLUSIONS

Our findings indicate that factors that stimulate TRPA1 channels expand Ca2+ signal-effector coupling at discrete sites along the endothelium to evoke graded cerebral artery vasodilation.

Dynamic Ca(2+) signal modalities in the vascular endothelium

The endothelium is vital to normal vasoregulation. Although acute vasodilation associated with broad endothelial Ca(2+) elevation is well known, the control and targeting of Ca(2+) -dependent signals in the endothelium are poorly understood. Recent studies have revealed localized IP(3) -motivated Ca(2+) events occurring basally along the intima that may provide the fundamental basis for various endothelial influences. Here, we provide an overview of dynamic endothelial Ca(2+) signals and discuss the potential role of these signals in constant endothelial control of arterial tone and the titration of functional responses in vivo. In particular, we focus on the functional architecture contributing to the properties and ultimate impact of these signals, and explore new avenues in evaluating their prevalence and specific modalities in intact tissue. Finally, we discuss spatial and temporal effector recruitment through modification of these inherent signals. It is suggested that endothelial Ca(2+) signaling is a continuum in which the specific framework of store-release components and cellular targets along the endothelium allows for differential modes of Ca(2+) signal expansion and distinctive profiles of effector recruitment. The precise composition and distribution of these inherent components may underlie dynamic endothelial control and specialized functions of different vascular beds.

Multilevel complexity of calcium signaling: Modeling angiogenesis

Intracellular calcium signaling is a universal, evolutionary conserved and versatile regulator of cell biochemistry. The complexity of calcium signaling and related cell machinery can be investigated by the use of experimental strategies, as well as by computational approaches. Vascular endothelium is a fascinating model to study the specific properties and roles of calcium signals at multiple biological levels. During the past 20 years, live cell imaging, patch clamp and other techniques have allowed us to detect and interfere with calcium signaling in endothelial cells (ECs), providing a huge amount of information on the regulation of vascularization (angiogenesis) in normal and tumoral tissues. These data range from the spatiotemporal dynamics of calcium within different cell microcompartments to those in entire multicellular and organized EC networks. Beside experimental strategies, in silico endothelial models, specifically designed for simulating calcium signaling, are contributing to our knowledge of vascular physiology and pathology. They help to investigate and predict the quantitative features of proangiogenic events moving through subcellular, cellular and supracellular levels. This review focuses on some recent developments of computational approaches for proangiogenic endothelial calcium signaling. In particular, we discuss the creation of hybrid simulation environments, which combine and integrate discrete Cellular Potts Models. They are able to capture the phenomenological mechanisms of cell morphological reorganization, migration, and intercellular adhesion, with single-cell spatiotemporal models, based on reaction-diffusion equations that describe the agonist-induced intracellular calcium events.

Transient receptor potential canonical type 3 channels facilitate endothelium-derived hyperpolarization-mediated resistance artery vasodilator activity

AIMS

Microdomain signalling mechanisms underlie key aspects of artery function and the modulation of intracellular calcium, with transient receptor potential (TRP) channels playing an integral role. This study determines the distribution and role of TRP canonical type 3 (C3) channels in the control of endothelium-derived hyperpolarization (EDH)-mediated vasodilator tone in rat mesenteric artery.

METHODS AND RESULTS

TRPC3 antibody specificity was verified using rat tissue, human embryonic kidney (HEK)-293 cells stably transfected with mouse TRPC3 cDNA, and TRPC3 knock-out (KO) mouse tissue using western blotting and confocal and ultrastructural immunohistochemistry. TRPC3-Pyr3 (ethyl-1-(4-(2,3,3-trichloroacrylamide)phenyl)-5-(trifluoromethyl)-1H-pyrazole-4-carboxylate) specificity was verified using patch clamp of mouse mesenteric artery endothelial and TRPC3-transfected HEK cells, and TRPC3 KO and wild-type mouse aortic endothelial cell calcium imaging and mesenteric artery pressure myography. TRPC3 distribution, expression, and role in EDH-mediated function were examined in rat mesenteric artery using immunohistochemistry and western blotting, and pressure myography and endothelial cell membrane potential recordings. In rat mesenteric artery, TRPC3 was diffusely distributed in the endothelium, with approximately five-fold higher expression at potential myoendothelial microdomain contact sites, and immunoelectron microscopy confirmed TRPC3 at these sites. Western blotting and endothelial damage confirmed primary endothelial TRPC3 expression. In rat mesenteric artery endothelial cells, Pyr3 inhibited hyperpolarization generation, and with individual SK(Ca) (apamin) or IK(Ca) (TRAM-34) block, Pyr3 abolished the residual respective IK(Ca)- and SK(Ca)-dependent EDH-mediated vasodilation.

CONCLUSION

The spatial localization of TRPC3 and associated channels, receptors, and calcium stores are integral for myoendothelial microdomain function. TRPC3 facilitates endothelial SK(Ca) and IK(Ca) activation, as key components of EDH-mediated vasodilator activity and for regulating mesenteric artery tone.

Stretch-activated calcium channels relay fast calcium waves propagated by calcium-induced calcium influx

For nearly 30 years, fast calcium waves have been attributed to a regenerative process propagated by CICR (calcium-induced calcium release) from the endoplasmic reticulum. Here, I propose a model containing a new subclass of fast calcium waves which is propagated by CICI (calcium-induced calcium influx) through the plasma membrane. They are called fast CICI waves. These move at the order of 100 to 1000 microm/s (at 20 degrees C), rather than the order of 3 to 30 microm/s found for CICR. Moreover, in this proposed subclass, the calcium influx which drives calcium waves is relayed by stretch-activated calcium channels. This model is based upon reports from approx. 60 various systems. In seven of these reports, calcium waves were imaged, and, in five of these, evidence was presented that these waves were regenerated by CICI. Much of this model involves waves that move along functioning flagella and cilia. In these systems, waves of local calcium influx are thought to cause waves of local contraction by inducing the sliding of dynein or of kinesin past tubulin microtubules. Other cells which are reported to exhibit waves, which move at speeds in the fast CICI range, include ones from a dozen protozoa, three polychaete worms, three molluscs, a bryozoan, two sea urchins, one arthropod, four insects, Amphioxus, frogs, two fish and a vascular plant (Equisetum), together with numerous healthy, as well as cancerous, mammalian cells, including ones from human. In two of these systems, very gentle local mechanical stimulation is reported to initiate waves. In these non-flagellar systems, the calcium influxes are thought to speed the sliding of actinomyosin filaments past each other. Finally, I propose that this mechanochemical model could be tested by seeing if gentle mechanical stimulation induces waves in more of these systems and, more importantly, by imaging the predicted calcium waves in more of them.

Ion channels and transporters in cancer. 6. Vascularizing the tumor: TRP channels as molecular targets

Tumor vascularization is a critical process that determines tumor growth and metastasis. In the last decade new experimental evidence obtained from in vitro and in vivo studies have challenged the classical angiogenesis model forcing us to consider new scenarios for tumor neovascularization. In particular, the genetic stability of tumor-derived endothelial cells (TECs) has been recently questioned in several studies, which show that TECs, as well as pericytes, differ significantly from their normal counterparts at genetic and functional levels. In addition to such an epigenetic action of tumor microenvironment on endothelial cells (ECs) commitment, the distinct characteristics of TECs could be due to differences in their origin compared with preexisting differentiated ECs. Intracellular Ca(2+) signals are involved at different critical phases in the regulation of the complex process of angiogenesis and tumor progression. These signals are generated by a wide variety of intrinsic and extrinsic factors. Several key components of Ca(2+) signaling including Ca(2+) channels in the plasma membrane, endoplasmic reticulum, calcium pumps, and mitochondria contribute to the generation, amplitude, and frequency of these Ca(2+) change. In particular, several members of the transient receptor potential (TRP) family of calcium-permeable channels have profound effects on the function of ECs. Because of its multifaceted role in the control of cell function, proliferation, and motility, TRP channels have been suggested as a potential molecular target for control of tumor neovascularization. Since plasma membrane Ca(2+) channels are easily and directly accessible via the bloodstream, they are potential targets for a number of pharmacological and antibody-targeted therapeutic strategies, with specificity being the main limitation. In this review we discuss recent advances in understanding the role of Ca(2+) channels, with specific reference to TRP channels, in tumor vascularization process.

Na+-Ca2+ exchanger contributes to Ca2+ extrusion in ATP-stimulated endothelium of intact rat aorta

The role of Na(+)-Ca(2+) exchanger (NCX) in vascular endothelium is still matter of debate. Depending on both the endothelial cell (EC) type and the extracellular ligand, NCX has been shown to operate in either the forward (Ca(2+) out)- or the reverse (Ca(2+) in)-mode. In particular, acetylcholine (Ach) has been shown to promote Ca(2+) inflow in the intact endothelium of excised rat aorta. Herein, we assessed the involvement of NCX into the Ca(2+) signals elicited by ATP in such preparation. Removal of extracellular Na(+) (0Na(+)) causes the NCX to switch into the reverse-mode and induced an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)), which disappeared in the absence of extracellular Ca(2+), and in the presence of benzamil, which blocks both modes of NCX, and KB-R 7943, a selective inhibitor of the reverse-mode. ATP induced a transient Ca(2+) signal, whose decay was significantly prolonged by 0Na(+), benzamil, DCB, and monensin while it was unaffected by KB-R 7943. Notably, lowering extracellular Na(+) concentration increased the sensibility to lower doses of ATP. These date suggest that, unlike Ach-stimulated ECs, NCX promotes Ca(2+) extrusion when the stimulus is provided by ATP in intact endothelium of rat aorta. These data show that, within the same preparation, NCX operates in both modes, depending on the chemical nature of the extracellular stimulus.

Aldosterone enhances ligand-stimulated nitric oxide production in endothelial cells

Chronic and acute actions of aldosterone have been shown recently to directly affect the cardiovascular system. However, it is unclear whether the acute effects of aldosterone on vasculature are constrictive or dilatory. Here, to clarify the nongenomic effects of aldosterone on endothelial function, we examined the effects of aldosterone on nitric oxide (NO) production in cultured endothelial cells (ECs) and on vascular tone. The intracellular NO production of bovine aortic ECs loaded with DAF-2 was determined using confocal microscopy. Accumulated NO in the culture medium was quantified by a microplate reader using membrane-impermeable DAF-2. Phosphorylation of endothelial NO synthase (eNOS) at Ser(1179) was assessed by Western blotting. Changes in intracellular Ca(2+) ([Ca(2+)](i)) were determined by confocal microscopy in ECs doubly loaded with fluo-4 and Fura Red. The effects of aldosterone, acetylcholine (ACh), and other signaling molecules on the tension of phenylephrine (PE)-contracted aortas of Sprague-Dawley rats were examined in an ex vivo organ bath chamber system. Short-term pre-exposure to aldosterone (1 x 10(-7) mol/L) enhanced ATP-induced NO production in ECs with increased phosphorylation of eNOS at Ser(1179). These effects were blocked by eplerenone, a mineralocorticoid receptor (MR) antagonist, and LY294002, a phosphatidylinositol 3-kinase (PI3K) inhibitor. Notably, aldosterone alone did not affect ATP-induced [Ca(2+)](i) changes or the Ser(1179) phosphorylation. Similarly, aldosterone (1 x 10(-8) to 1 x 10(-7) mol/L) did not affect the tone of rat aortas pre-contracted by PE, but enhanced ACh-induced vasorelaxation, which was again reversed by eplerenone or LY29400. In contrast, sodium nitroprusside-induced vasorelaxation in endothelium-denuded aortas was not affected by aldosterone. Thus, aldosterone acutely enhances ligand-mediated endothelial NO production by eplerenone-sensitive mechanisms involving a PI3K that may synergize Ca(2+)-dependent eNOS phosphorylation at Ser(1179).

Protein kinase C-delta mediates von Willebrand factor secretion from endothelial cells in response to vascular endothelial growth factor (VEGF) but not histamine

BACKGROUND

Vascular endothelial growth factor (VEGF) and histamine induce von Willebrand factor (VWF) release from vascular endothelial cells. Protein kinase C (PKC) is involved in the control of exocytosis in many secretory cell types.

OBJECTIVES

We investigated the role of PKC and the interactions between PKC and Ca2+ signaling in both VEGF-induced and histamine-induced VWF secretion from human umbilical vein endothelial cells (HUVECs).

RESULTS

Several PKC inhibitors (staurosporine, Ro31-8220, myristoylated PKC peptide inhibitor and Go6983) block VEGF-induced but not histamine-induced VWF secretion. PKC-alpha and novel PKCs (PKC-delta, PKC-epsilon, and PKC-eta), but not PKC-beta, are expressed in HUVECs. Both VEGF and histamine activate PKC-delta. However, gene inactivation experiments using small interfering RNA indicate that PKC-delta (but not PKC-alpha) is involved in the regulation of VEGF-induced but not histamine-induced secretion. Both VEGF and histamine induce a rise in cytosolic free Ca2+ ([Ca2+]c), but the response to VEGF is weaker and even absent in a significant subset of cells. Furthermore, VEGF-induced secretion is largely preserved when the rise in [Ca2+]c is prevented by BAPTA-AM.

CONCLUSIONS

Our study identifies striking agonist specificities in signal-secretion coupling. Histamine-induced secretion is dependent on [Ca2+]c but not PKC, whereas VEGF-induced secretion is largely dependent on PKC-delta and significantly less on [Ca2+]c. Our data firmly establish the key role of PKC-delta in VEGF-induced VWF release, but suggest that a third, VEGF-specific, signaling intermediate is required as a PKC-delta coactivator.

The secret marriage between calcium and tumor angiogenesis

Endothelial cell biochemistry and responsiveness to a wide variety of external stimula is regulated by intracellular calcium concentration. During the last twenty years, electrophysiology and functional imaging based on the use of fluorescent probes provided several informations about the dynamics and role of calcium at the single cell level: highly diverse extracellular agonists, such as proangiogenic growth factors and vasoactive compounds, trigger increases in intracellular calcium and specific informations are transduced for proliferation, differentiation, death, movement in physiological and pathological conditions. Obviously, the investigation at multicellular and tissutal levels is much more complex. In this review we discuss the potential specific roles of calcium signaling in tumor angiogenesis progression trying to address two key questions: (i) how can this ion play specific roles in the angiogenesis regulation; and (ii) could it be used as a target to interfere with or prevent tumor vascularization?

Counter regulatory effects of PKCbetaII and PKCdelta on coronary endothelial permeability

OBJECTIVE

The aim of this study was to examine the endothelial distribution and activity of selected PKC isoforms in coronary vessels with respect to their functional impact on endothelial permeability under the experimental conditions relevant to diabetes.

METHODS AND RESULTS

En face immunohistochemistry demonstrated a significant increase of PKC(betaII) and decrease of PKCdelta expression in coronary arterial endothelium of Zucker diabetic rats. To test whether changes in PKC expression alter endothelial barrier properties, we measured the transcellular electric resistance in human coronary microvascular endothelial monolayers and found that either PKC(betaII) overexpression or PKCdelta inhibition disrupted the cell-cell adhesive barrier. Three-dimensional fluorescence microscopy revealed that hyperpermeability was caused by altered PKC activity in association with distinct translocation of PKC(betaII) to the cell-cell junction and PKCdelta localization to the cytosol. Further analyses in fractionated endothelial lysates confirmed the differential redistribution of these isozymes. Additionally, FRET analysis of PKC subcellular dynamics demonstrated a high PKC(betaII) activity at the cell surface and junction, whereas PKCdelta activity is concentrated in intracellular membrane organelles.

CONCLUSIONS

Taken together, these data suggest that PKC(betaII) and PKCdelta counter-regulate coronary endothelial barrier properties by targeting distinctive subcellular sites. Imbalanced PKC(betaII)/PKCdelta expression and activity may contribute to endothelial hyperpermeability and coronary dysfunction in diabetes.

Calcium measurements in organelles with Ca2+-sensitive fluorescent proteins

The recent improvement in the design and use of genetically encoded fluorescent Ca2+ indicators should foster major progress in three aspects of Ca2+ signaling. At the subcellular level, ratiometric probes with expanded dynamics are now available to measure accurately the local Ca2+ changes occurring within specific cell compartments. These tools will allow to determine precisely the role of organelles and of cellular microdomains in Ca2+ handling. At the cellular level, the permanent labeling offered by the genetic probes enables large-scale, long-term Ca2+ measurements with robotic multiplexing technologies such as fluorescence plate readers or automated microscopes. This opens the way to large-scale pharmacological or genetic screens based on organelle-specific functional assays. At the whole animal level, probes with improved dynamics and reduced interference with endogenous proteins will allow to generate transgenic animals bearing Ca2+ sensitive indicators in specific cells and tissues. With this approach, Ca2+ signals can be recorded in neurons, heart, and pancreas of live animals during physiological and pathological stimulations. In this chapter, I will review the progress made in the design and use of genetic Ca2+ indicators, and discuss how these new tools provide an opportunity to challenge several unsolved questions in Ca2+ signaling.

Ca2+-activated K+ channels in murine endothelial cells: block by intracellular calcium and magnesium

The intermediate (IK(Ca)) and small (SK(Ca)) conductance Ca(2+)-sensitive K(+) channels in endothelial cells (ECs) modulate vascular diameter through regulation of EC membrane potential. However, contribution of IK(Ca) and SK(Ca) channels to membrane current and potential in native endothelial cells remains unclear. In freshly isolated endothelial cells from mouse aorta dialyzed with 3 microM free [Ca(2+)](i) and 1 mM free [Mg(2+)](i), membrane currents reversed at the potassium equilibrium potential and exhibited an inward rectification at positive membrane potentials. Blockers of large-conductance, Ca(2+)-sensitive potassium (BK(Ca)) and strong inward rectifier potassium (K(ir)) channels did not affect the membrane current. However, blockers of IK(Ca) channels, charybdotoxin (ChTX), and of SK(Ca) channels, apamin (Ap), significantly reduced the whole-cell current. Although IK(Ca) and SK(Ca) channels are intrinsically voltage independent, ChTX- and Ap-sensitive currents decreased steeply with membrane potential depolarization. Removal of intracellular Mg(2+) significantly increased these currents. Moreover, concomitant reduction of the [Ca(2+)](i) to 1 microM caused an additional increase in ChTX- and Ap-sensitive currents so that the currents exhibited theoretical outward rectification. Block of IK(Ca) and SK(Ca) channels caused a significant endothelial membrane potential depolarization (approximately 11 mV) and decrease in [Ca(2+)](i) in mesenteric arteries in the absence of an agonist. These results indicate that [Ca(2+)](i) can both activate and block IK(Ca) and SK(Ca) channels in endothelial cells, and that these channels regulate the resting membrane potential and intracellular calcium in native endothelium.

Transport-dependent calcium signaling in spatially segregated cellular caveolar domains

We developed a two-dimensional model of transport-dependent intracellular calcium signaling in endothelial cells (ECs). Our purpose was to evaluate the effects of spatial colocalization of endothelial nitric oxide synthase (eNOS) and capacitative calcium entry (CCE) channels in caveolae on eNOS activation in response to ATP. Caveolae are specialized microdomains of the plasma membrane that contain a variety of signaling molecules to optimize their interactions and regulate their activity. In ECs, these molecules include CCE channels and eNOS. To achieve a quantitative understanding of the mechanisms of microdomain calcium signaling and the preferential sensitivity of eNOS to calcium entering the cell through CCE channels, we constructed a mathematical model incorporating the cell morphology and cellular physiological processes. The model predicts that the spatial segregation of calcium channels in ECs can create transport-dependent sharp gradients in calcium concentration within the cell. The calcium concentration gradient is affected by channel density and cell geometry. This transport-dependent calcium signaling specificity effect is enhanced in ECs by increasing the spatial segregation of the caveolar signaling domains. Our simulation significantly advances the understanding of how Ca2+, despite its many potential actions, can mediate selective activation of signaling pathways. We show that diffusion-limited calcium transport allows functional compartmentalization of signaling pathways based on the spatial arrangements of Ca2+ sources and targets.

Calcium signaling in lizard red blood cells

The ion calcium is a ubiquitous second messenger, present in all eukaryotic cells. It modulates a vast number of cellular events, such as cell division and differentiation, fertilization, cell volume, decodification of external stimuli. To process this variety of information, the cells display a number of calcium pools, which are capable of mobilization for signaling purposes. Here we review the calcium signaling on lizards red blood cells, an interesting model that has been receiving an increasing notice recently. These cells possess a complex machinery to regulate calcium, and display calcium responses to extracellular agonists. Interestingly, the pattern of calcium handling and response are divergent in different lizard families, which enforces the morphological data to their phylogenetic classification, and suggest the radiation of different calcium signaling models in lizards evolution.

Phospholipase C isozymes are differentially distributed in the rat adrenal medulla

Immunohistochemistry has been used to examine the distribution of selected phospholipase C (PLC) isozymes within the adrenal medulla of the rat. PLCbeta isozymes were expressed at moderate levels in the chromaffin cells but more strongly in association with ganglion cell clusters. PLCbeta2 and PLCbeta3 staining of clusters did not overlap suggesting selective PLC isozyme expression in two distinct ganglionic types. The distribution of PLCbeta4 immunoreactivity was very similar to PLCbeta3 with the strongest staining observed in the same cell clusters. Antibodies to PLCbeta1 labelled multiple bands on Western blots and were not therefore used for immunohistochemistry. The chromaffin cells were also immunoreactive for PLCgamma1, although the strongest staining with this antibody was seen in cells surrounding large sinus vessels. PLCdelta1 and PLCdelta2 had quite distinct distributions, with the former selectively localized to an endothelial cell population surrounding the chromaffin cells. This observation was supported by experiments on isolated bovine adrenal medullary cells where PLCdelta1 expression was lost when the cell preparation was enriched for chromaffin cells. Antibodies to PLCdelta2 labelled a network of nerve fibres throughout the medulla and clusters of ganglion cells located primarily at the medullary-cortical boundary. PLCdelta2 immunoreactivity was also present in nerve fibres within the adrenal capsule where it appeared to be co-localized with PLCbeta4 staining.

Microdomains of intracellular Ca2+: molecular determinants and functional consequences

Calcium ions are ubiquitous and versatile signaling molecules, capable of decoding a variety of extracellular stimuli (hormones, neurotransmitters, growth factors, etc.) into markedly different intracellular actions, ranging from contraction to secretion, from proliferation to cell death. The key to this pleiotropic role is the complex spatiotemporal organization of the [Ca(2+)] rise evoked by extracellular agonists, which allows selected effectors to be recruited and specific actions to be initiated. In this review, we discuss the structural and functional bases that generate the subcellular heterogeneity in cellular Ca(2+) levels at rest and under stimulation. This complex choreography requires the concerted action of many different players; the central role is, of course, that of the calcium ion, with the main supporting characters being all the entities responsible for moving Ca(2+) between different compartments, while the cellular architecture provides a determining framework within which all the players have their exits and their entrances. In particular, we concentrate on the molecular mechanisms that lead to the generation of cytoplasmic Ca(2+) microdomains, focusing on their different subcellular location, mechanism of generation, and functional role.

Cyclic ADP ribose-mediated Ca2+ signaling in mediating endothelial nitric oxide production in bovine coronary arteries

The present study tested the hypothesis that cyclic ADP ribose (cADPR) serves as a novel second messenger to mediate intracellular Ca2+ mobilization in coronary arterial endothelial cells (CAECs) and thereby contributes to endothelium-dependent vasodilation. In isolated and perfused small bovine coronary arteries, bradykinin (BK)-induced concentration-dependent vasodilation was significantly attenuated by 8-bromo-cADPR (a cell-permeable cADPR antagonist), ryanodine (an antagonist of ryanodine receptors), or nicotinamide (an ADP-ribosyl cyclase inhibitor). By in situ simultaneously fluorescent monitoring, Ca2+ transient and nitric oxide (NO) levels in the intact coronary arterial endothelium preparation, 8-bromo-cADPR (30 microM), ryanodine (50 microM), and nicotinamide (6 mM) substantially attenuated BK (1 microM)-induced increase in intracellular [Ca2+] by 78%, 80%, and 74%, respectively, whereas these compounds significantly blocked BK-induced NO increase by about 80%, and inositol 1,4,5-trisphosphate receptor blockade with 2-aminethoxydiphenyl borate (50 microM) only blunted BK-induced Ca2+-NO signaling by about 30%. With the use of cADPR-cycling assay, it was found that inhibition of ADP-ribosyl cyclase by nicotinamide substantially blocked BK-induced intracellular cADPR production. Furthermore, HPLC analysis showed that the conversion rate of beta-nicotinamide guanine dinucleotide into cyclic GDP ribose dramatically increased by stimulation with BK, which was blockable by nicotinamide. However, U-73122, a phospholipase C inhibitor, had no effect on this BK-induced increase in ADP-ribosyl cyclase activity for cADPR production. In conclusion, these results suggest that cADPR importantly contributes to BK- and A-23187-induced NO production and vasodilator response in coronary arteries through its Ca2+ signaling mechanism in CAECs.

Intracellular location regulates calcium-calmodulin-dependent activation of organelle-restricted eNOS

Mislocalization of endothelial nitric oxide (NO) synthase (eNOS) in response to oxidized low-density lipoprotein, cholesterol depletion, elevated blood pressure, and bound eNOS interacting protein/NOS traffic inducer is associated with reduced NO release via unknown mechanisms. The proper targeting of eNOS to the plasma membrane or intracellular organelles is an important regulatory step controlling enzyme activity. Previous studies have shown that plasma membrane eNOS is constitutively phosphorylated on serine 1179 and highly active. In contrast, the activity of eNOS targeted to intracellular organelles is more complex. The cis-Golgi eNOS is fully activated by Akt-dependent phosphorylation. However, eNOS targeted to the trans-Golgi is decidedly less active in response to all modes of activation, including mutation to the phosphomimetic aspartic acid. In this study, we establish that when expressed within other intracellular organelles, such as the mitochondria and nucleus, the activity of eNOS is also greatly reduced. To address the mechanisms underlying the impaired catalytic activity of eNOS within these locations, we generated subcellular-targeted constructs that express a calcium-independent NOS isoform, iNOS. With the use of organelle specific (plasma membrane, cis- vs. trans-Golgi, plasma membrane, and Golgi, nucleus, and mitochondria) targeting motifs fused to the wild-type iNOS, we measured NO release from intact cells. With the exception of the Golgi lumen, our results showed no impairment in the ability of targeted iNOS to synthesize NO. Confirmation of correct targeting was obtained through confocal microscopy using identical constructs fused to the green fluorescent protein. We conclude that the reduced activation of eNOS within discrete cytoplasmic regions of the Golgi, the mitochondria and the nucleus is primarily due to insufficient access to calcium-calmodulin.

Ultrastructural features of smooth muscle and endothelial cells of isolated isobaric human placental and maternal arteries

The ability of a blood vessel to develop tone is dependent upon morphological parameters of the smooth muscle cells (SMC), including density, relationship with the endothelium and subcellular distribution of myofilaments and intracellular organelles. Consequently, wall ultrastructure of isolated human placental chorionic plate arteries (n=12), fixed when pressurised to mimic their in vivo geometry, was examined qualitatively using electron microscopy, and compared with maternal arteries (omental, n=10, myometrial, n=6). Arteries from women with uncomplicated pregnancy were tested for contractile viability before fixing, with some vessels post-fixed in osmium-ferricyanide for sarcoplasmic reticulum (SR) identification. In contrast to maternal arteries, placental arteries had no internal elastic lamina but exhibited considerable extracellular matrix separating circularly orientated SMC. Human SMC contained tightly packed arrays of myofilaments running parallel to the plasma membrane, enveloping cellular organelles. Synthetic SMC, with few myofilaments and much rough SR, were observed in placental arteries only. SR in SMC from maternal arteries was located centrally, often encircling mitochondria, and also near the plasma membrane associated with caveolae. Positive SR staining was rarely observed in SMC of placental arteries. This study highlights ultrastructural differences between placental and maternal arteries that may underlie specialised mechanisms of regulating vascular tone in the placenta.

Na/K-ATPase tethers phospholipase C and IP3 receptor into a calcium-regulatory complex

We have shown that the caveolar Na/K-ATPase transmits ouabain signals via multiple signalplexes. To obtain the information on the composition of such complexes, we separated the Na/K-ATPase from the outer medulla of rat kidney into two different fractions by detergent treatment and density gradient centrifugation. Analysis of the light fraction indicated that both PLC-gamma1 and IP3 receptors (isoforms 2 and 3, IP3R2 and IP3R3) were coenriched with the Na/K-ATPase, caveolin-1 and Src. GST pulldown assays revealed that the central loop of the Na/K-ATPase alpha1 subunit interacts with PLC-gamma1, whereas the N-terminus binds IP3R2 and IP3R3, suggesting that the signaling Na/K-ATPase may tether PLC-gamma1 and IP3 receptors together to form a Ca(2+)-regulatory complex. This notion is supported by the following findings. First, both PLC-gamma1 and IP3R2 coimmunoprecipitated with the Na/K-ATPase and ouabain increased this interaction in a dose- and time-dependent manner in LLC-PK1 cells. Depletion of cholesterol abolished the effects of ouabain on this interaction. Second, ouabain induced phosphorylation of PLC-gamma1 at Tyr(783) and activated PLC-gamma1 in a Src-dependent manner, resulting in increased hydrolysis of PIP2. It also stimulated Src-dependent tyrosine phosphorylation of the IP3R2. Finally, ouabain induced Ca(2+) release from the intracellular stores via the activation of IP3 receptors in LLC-PK1 cells. This effect required the ouabain-induced activation of PLC-gamma1. Inhibition of Src or depletion of cholesterol also abolished the effect of ouabain on intracellular Ca(2+).

Essential role of a Ca2+-selective, store-operated current (ISOC) in endothelial cell permeability: determinants of the vascular leak site

Store-operated calcium (SOC) entry is sufficient to disrupt the extra-alveolar, but not the alveolar, endothelial cell barrier. Mechanism(s) underlying such insensitivity to transitions in cytosolic calcium ([Ca2+]i) in microvascular endothelial cells are unknown. Depletion of stored Ca2+ activates a larger SOC entry response in extra-alveolar (pulmonary artery; PAECs) than alveolar (pulmonary microvascular; PMVECs) endothelial cells. In vivo permeation studies revealed that Ca2+ store depletion activates similar nonselective cationic conductances in PAECs and PMVECs, while only PAECs possess the calcium-selective, store-operated Ca2+ entry current, I(SOC). Pretreatment with the type 4 phosphodiesterase inhibitor, rolipram, abolished thapsigargin-activated I(SOC) in PAECs, and revealed I(SOC) in PMVECs. Rolipram pretreatment shifted the thapsigargin-induced fluid leak site from extra-alveolar to alveolar vessels in the intact pulmonary circulation. Thus, our results indicate I(SOC) provides a [Ca2+]i source that is needed to disrupt the endothelial cell barrier, and demonstrate that intracellular events controlling I(SOC) activation coordinate the site-specific vascular response to inflammation.