Subpopulation of store-operated Ca2+ channels regulate Ca2+-induced Ca2+ release in non-excitable cells

Human salivary gland cells express bradykinin receptors that modulate the expression of proinflammatory cytokines

Bradykinin is an important peptide modulator that affects the function of neurons and immune cells. However, there is no evidence of the bradykinin receptors and their functions in human salivary glands. Here we have identified and characterized bradykinin receptors on human submandibular gland cells. Both bradykinin B1 and B2 receptors are expressed on human submandibular gland cells, A253 cells, and HSG cells. Bradykinin increased the intracellular Ca²⁺ concentration ([Ca²⁺ ]_i ) in a concentration-dependent manner. Interestingly, a specific agonist of the B1 receptor did not have any effect on [Ca²⁺ ]_i in HSG cells, whereas specific agonists of the B2 receptor had a Ca²⁺ mobilizing effect. Furthermore, application of the B1 receptor antagonist, R715, did not alter the bradykinin-mediated increase in cytosolic Ca²⁺ , whereas the B2 receptor antagonist, HOE140, showed a strong inhibitory effect, which implies that bradykinin B2 receptors are functional in modulating the concentration of cytosolic Ca²⁺ . Bradykinin did not affect a carbachol-induced rise of [Ca²⁺ ]_i and did not modulate translocation of aquaporin-5. However, bradykinin did promote the expression of proinflammatory cytokines, including tumor necrosis factor-α (TNF-α), implying the role of bradykinin in salivary gland inflammation. These data suggest that bradykinin receptors are involved in Ca²⁺ signaling in human submandibular gland cells and serve a unique role, which is separate from that of other salivary gland G protein-coupled receptors.

Activation of ERK1/2 by store-operated calcium entry in rat parotid acinar cells

The regulation of intracellular Ca²⁺ concentration ([Ca²⁺]_i) plays a critical role in a variety of cellular processes, including transcription, protein activation, vesicle trafficking, and ion movement across epithelial cells. In many cells, the activation of phospholipase C-coupled receptors hydrolyzes membrane phosphoinositides and produces the depletion of endoplasmic reticulum Ca²⁺ stores, followed by the sustained elevation of [Ca²⁺]_i from Ca²⁺ entry across the plasma membrane via store-operated Ca²⁺ entry (SOCE). Ca²⁺ entry is also increased in a store-independent manner by arachidonate-regulated Ca²⁺ (ARC) channels. Using rat parotid salivary gland cells, we examined multiple pathways of Ca²⁺ entry/elevation to determine if they activated cell signaling proteins and whether this occurred in a pathway-dependent manner. We observed that SOCE activates extracellular signal-related kinases 1 and 2 (ERK1/2) to ∼3-times basal levels via a receptor-independent mechanism when SOCE was initiated by depleting Ca²⁺ stores using the endoplasmic reticulum Ca²⁺-ATPase inhibitor thapsigargin (TG). TG-initiated ERK1/2 phosphorylation increased as rapidly as that initiated by the muscarinic receptor agonist carbachol, which promoted an increase to ∼5-times basal levels. Notably, ERK1/2 phosphorylation was not increased by the global elevation of [Ca²⁺]_i by Ca²⁺ ionophore or by Ca²⁺ entry via ARC channels in native cells, although ERK1/2 phosphorylation was increased by Ca²⁺ ionophore in Par-C10 and HSY salivary cell lines. Agents and conditions that blocked SOCE in native cells, including 2-aminoethyldiphenyl borate (2-APB), SKF96363, and removal of extracellular Ca²⁺, also reduced TG- and carbachol-stimulated ERK1/2 phosphorylation. TG-promoted ERK1/2 phosphorylation was blocked when SRC and Protein Kinases C (PKC) were inhibited, and it was blocked in cells pretreated with β-adrenergic agonist isoproterenol. These observations demonstrate that ERK1/2 is activated by a selective mechanism of Ca²⁺ entry (SOCE) in these cells, and suggest that ERK1/2 may contribute to events downstream of SOCE.

Initial bradykinin triggers calcium-induced calcium release in C6 glioma cells and its significance

OBJECTIVE

To investigate the underlying mechanism for the selective modulation of the permeability of blood-tumor barrier (BTB) by small dose of bradykinin (BK).

METHODS

C6 glioma cells were treated with BK, and changes of intracellular nitric oxide (NO) and intracellular calcium level were measured with fluorescent spectrophotometer.

RESULTS

The initial application of BK easily triggered extracellular calcium influx, which resulted in intracellular calcium store release in C6 glioma cells. The above mechanism was also named ryanodine mediated calcium induced calcium release (CICR). We also detected a long-lasting intracellular NO elevation in C6 glioma cells upon BK treatment. Further study showed that ryanodine mediated CICR contributed greatly to the secondary NO elevation induced by BK treatment.

CONCLUSION

These results suggested that BK triggered CICR in C6 glioma cells and the associated NO generation might be the underlying mechanism for the selective modulation of BTB permeability by BK.

Bradykinin selectively modulates the blood-tumor barrier via calcium-induced calcium release

To investigate the underlying mechanism for the selective modulation of permeability of the blood-tumor barrier by small dose of BK, we established cell lines of rat brain microvascular endothelial cells (BMECs) and astrocytes by primary culture from neonatal rats. BMECs, astrocytes, or C6 glioma cells were treated with BK, and changes of intracellular NO and intracellular calcium level were measured with a fluorescent spectrophotometer. Similarly to the observations in astrocytes, although the initial application of BK easily triggered a ryanodine-mediated calcium-induced calcium release (CICR), we also detected a long-lasting intracellular nitric oxide (NO) elevation in C6 glioma cells upon BK treatment. However, BMECs are not the direct target of BK. Further study showed that ryanodine-mediated CICR contributes greatly to the secondary NO elevation induced by BK treatment. With an in vitro blood-tumor barrier (BTB) model, we demonstrated that NO generated in C6 glioma cells might act as an intercellular messenger and play an important role in the selective modulation of permeability of BMECs by BK. In conclusion, BK triggered CICR in C6 glioma cells, and the associated NO generation might be the underlying mechanism for the selective modulation of BTB permeability by BK.

Store-operated Ca2+ influx causes Ca2+ release from the intracellular Ca2+ channels that is required for T cell activation

The precise control of many T cell functions relies on cytosolic Ca(2+) dynamics that is shaped by the Ca(2+) release from the intracellular store and extracellular Ca(2+) influx. The Ca(2+) influx activated following T cell receptor (TCR)-mediated store depletion is considered to be a major mechanism for sustained elevation in cytosolic Ca(2+) concentration ([Ca(2+)](i)) necessary for T cell activation, whereas the role of intracellular Ca(2+) release channels is believed to be minor. We found, however, that in Jurkat T cells [Ca(2+)](i) elevation observed upon activation of the store-operated Ca(2+) entry (SOCE) by passive store depletion with cyclopiazonic acid, a reversible blocker of sarco-endoplasmic reticulum Ca(2+)-ATPase, inversely correlated with store refilling. This indicated that intracellular Ca(2+) release channels were activated in parallel with SOCE and contributed to global [Ca(2+)](i) elevation. Pretreating cells with (-)-xestospongin C (10 microM) or ryanodine (400 microM), the antagonists of inositol 1,4,5-trisphosphate receptor (IP3R) or ryanodine receptor (RyR), respectively, facilitated store refilling and significantly reduced [Ca(2+)](i) elevation evoked by the passive store depletion or TCR ligation. Although the Ca(2+) release from the IP3R can be activated by TCR stimulation, the Ca(2+) release from the RyR was not inducible via TCR engagement and was exclusively activated by the SOCE. We also established that inhibition of IP3R or RyR down-regulated T cell proliferation and T-cell growth factor interleukin 2 production. These studies revealed a new aspect of [Ca(2+)](i) signaling in T cells, that is SOCE-dependent Ca(2+) release via IP3R and/or RyR, and identified the IP3R and RyR as potential targets for manipulation of Ca(2+)-dependent functions of T lymphocytes.

Sources of calcium in agonist-induced contraction of rat distal colon smooth muscle in vitro

AIM: To study the origin of calcium necessary for agonist-induced contraction of the distal colon in rats.

METHODS: The change in intracellular calcium concentration ([Ca²⁺]_i) evoked by elevating external Ca²⁺ was detected by fura 2/AM fluorescence. Contractile activity was measured with a force displacement transducer. Tension was continuously monitored and recorded using a Powerlab 4/25T data acquisition system with an ML110 bridge bioelectric physiographic amplifier.

RESULTS: Store depletion induced Ca²⁺ influx had an effect on [Ca²⁺]_i. In nominally Ca²⁺-free medium, the sarco-endoplasmic reticulum Ca²⁺-ATPase inhibitor thapsigargin (1 &mgr;mol/L) increased [Ca²⁺]_i from 68 to 241 nmol/L, and to 458 (P < 0.01) and 1006 nmol/L (P < 0.01), respectively, when 1.5 mmol/L and 3.0 mmol/L extracellular Ca²⁺ was reintroduced. Furthermore, the change in [Ca²⁺]_i was observed with verapamil (5 &mgr;mol/L), La³⁺ (1 mmol/L) or KCl (40 mmol/L) in the bathing solution. These channels were sensitive to La³⁺ (P < 0.01), insensitive to verapamil, and voltage independent. In isolated distal colons we found that in normal Krebs solution, contraction induced by acetylcholine (ACh) was partially inhibited by verapamil, and the inhibitory rate was 41% (P < 0.05). On the other hand, in Ca²⁺-free Krebs solution, ACh induced transient contraction due to Ca²⁺ release from the intracellular stores. The transient contraction lasted until the Ca²⁺ store was depleted. Restoration of extracellular Ca²⁺ in the presence of atropine produced contraction, mainly due to Ca²⁺ influx. Such contraction was not inhibited by verapamil, but was decreased by La³⁺ (50 &mgr;mol/L) from 0.96 to 0.72 g (P < 0.01).

CONCLUSION: The predominant source of activator Ca²⁺ for the contractile response to agonist is extracellular Ca²⁺, and intracellular Ca²⁺ has little role to play in mediating excitation-contraction coupling by agonists in rat distal colon smooth muscle in vitro. The influx of extracellular Ca²⁺ is mainly mediated through voltage-, receptor- and store-operated Ca²⁺ channels, which can be used as an alternative to develop new drugs targeted on the dysfunction of digestive tract motility.

Functional coupling between ryanodine receptors, mitochondria and Ca(2+) ATPases in rat submandibular acinar cells

Agonist stimulation of exocrine cells leads to the generation of intracellular Ca(2+) signals driven by inositol 1,4,5-trisphosphate receptors (IP(3)Rs) that rapidly become global due to propagation throughout the cell. In many types of excitable cells the intracellular Ca(2+) signal is propagated by a mechanism of Ca(2+)-induced Ca(2+) release (CICR), mediated by ryanodine receptors (RyRs). Expression of RyRs in salivary gland cells has been demonstrated immunocytochemically although their functional role is not clear. We used microfluorimetry to measure Ca(2+) signals in the cytoplasm, in the endoplasmic reticulum (ER) and in mitochondria. In permeabilized acinar cells caffeine induced a dose-dependent, transient decrease of Ca(2+) concentration in the endoplasmic reticulum ([Ca(2+)](ER)). This decrease was inhibited by ryanodine but was insensitive to heparin. Application of caffeine, however, did not elevate cytosolic Ca(2+) concentration ([Ca(2+)](i)) suggesting fast local buffering of Ca(2+) released through RyRs. Indeed, activation of RyRs produced a robust mitochondrial Ca(2+) transient that was prevented by addition of Ca(2+) chelator BAPTA but not EGTA. When mitochondrial Ca(2+) uptake was blocked, activation of RyRs evoked only a non-transient increase in [Ca(2+)](i) and substantially smaller Ca(2+) release from the ER. Upon simultaneous inhibition of mitochondrial Ca(2+) uptake and either plasmalemmal or ER Ca(2+) ATPase, activation of RyRs caused a transient rise in [Ca(2+)](i). Collectively, our data suggest that Ca(2+) released through RyRs is mostly "tunnelled" to mitochondria, while Ca(2+) ATPases are responsible for the fast initial sequestration of Ca(2+). Ca(2+) uptake by mitochondria is critical for maintaining continuous CICR. A complex interplay between RyRs, mitochondria and Ca(2+) ATPases is accomplished through strategic positioning of mitochondria close to both Ca(2+) release sites in the ER and Ca(2+) pumping sites of the plasmalemma and the ER.

Low dose of bradykinin selectively increases intracellular calcium in glioma cells

To investigate the underlying basis for the selective modulation of the permeability of the blood-brain barrier by small doses of bradykinin, we first established cell lines of rat brain microvascular endothelial cells (BMECs) and astrocytes by primary cultures from neonatal rats. BMECs, astrocytes and C6 glioma cells were treated with different concentrations of BK (range from 10(-8) M to 10(-4) M), and changes of intracellular calcium levels were measured by fluorescence spectrophotometry. Expression levels of B2 receptors were analyzed by Western blot analysis. We found that a low dose (10(-6) M) of BK could trigger an elevation of intracellular calcium level in C6 glioma cells, whereas astrocytes responded to a higher concentration of BK (10(-5) M), and the BMECs remained unresponsive to BK. Western blot results showed that C6 glioma cells expressed the highest level of B2 receptors compared with primary astrocytes or BMECs. B2 receptors are highly expressed in glioma cells and a low dose of BK selectively increases the intracellular calcium level in tumor cells which, in turn, could contribute to the selective increase in the permeability of the blood-tumor barrier by small doses of BK.

Relocalization of STIM1 for activation of store-operated Ca(2+) entry is determined by the depletion of subplasma membrane endoplasmic reticulum Ca(2+) store

STIM1 (stromal interacting molecule 1), an endoplasmic reticulum (ER) protein that controls store-operated Ca(2+) entry (SOCE), redistributes into punctae at the cell periphery after store depletion. This redistribution is suggested to have a causal role in activation of SOCE. However, whether peripheral STIM1 punctae that are involved in regulation of SOCE are determined by depletion of peripheral or more internal ER has not yet been demonstrated. Here we show that Ca(2+) depletion in subplasma membrane ER is sufficient for peripheral redistribution of STIM1 and activation of SOCE. 1 microM thapsigargin (Tg) induced substantial depletion of intracellular Ca(2+) stores and rapidly activated SOCE. In comparison, 1 nM Tg induced slower, about 60-70% less Ca(2+) depletion but similar SOCE. SOCE was confirmed by measuring I(SOC) in addition to Ca(2+), Mn(2+), and Ba(2+) entry. Importantly, 1 nM Tg caused redistribution of STIM1 only in the ER-plasma membrane junction, whereas 1 microM Tg caused a relatively global relocalization of STIM1 in the cell. During the time taken for STIM1 relocalization and SOCE activation, 1 nM Bodipy-fluorescein Tg primarily labeled the subplasma membrane region, whereas 1 microM Tg labeled the entire cell. The localization of Tg in the subplasma membrane region was associated with depletion of ER in this region and activation of SOCE. Together, these data suggest that peripheral STIM1 relocalization that is causal in regulation of SOCE is determined by the status of [Ca(2+)] in the ER in close proximity to the plasma membrane. Thus, the mechanism involved in regulation of SOCE is contained within the ER-plasma membrane junctional region.

Measurement of Ca2+ signaling dynamics in exocrine cells with total internal reflection microscopy

In nonexcitable cells, such as exocrine cells from the pancreas and salivary glands, agonist-stimulated Ca2+ signals consist of both Ca2+ release and Ca2+ influx. We have investigated the contribution of these processes to membrane-localized Ca2+ signals in pancreatic and parotid acinar cells using total internal reflection fluorescence (TIRF) microscopy (TIRFM). This technique allows imaging with unsurpassed resolution in a limited zone at the interface of the plasma membrane and the coverslip. In TIRFM mode, physiological agonist stimulation resulted in Ca2+ oscillations in both pancreas and parotid with qualitatively similar characteristics to those reported using conventional wide-field microscopy (WFM). Because local Ca2+ release in the TIRF zone would be expected to saturate the Ca2+ indicator (Fluo-4), these data suggest that Ca2+ release is occurring some distance from the area subjected to the measurement. When acini were stimulated with supermaximal concentrations of agonists, an initial peak, largely due to Ca2+ release, followed by a substantial, maintained plateau phase indicative of Ca2+ entry, was observed. The contribution of Ca2+ influx and Ca2+ release in isolation to these near-plasma membrane Ca2+ signals was investigated by using a Ca2+ readmission protocol. In the absence of extracellular Ca2+, the profile and magnitude of the initial Ca2+ release following stimulation with maximal concentrations of agonist or after SERCA pump inhibition were similar to those obtained with WFM in both pancreas and parotid acini. In contrast, when Ca2+ influx was isolated by subsequent Ca2+ readmission, the Ca2+ signals evoked were more robust than those measured with WFM. Furthermore, in parotid acinar cells, Ca2+ readdition often resulted in the apparent saturation of Fluo-4 but not of the low-affinity dye Fluo-4-FF. Interestingly, Ca2+ influx as measured by this protocol in parotid acinar cells was substantially greater than that initiated in pancreatic acinar cells. Indeed, robust Ca2+ influx was observed in parotid acinar cells even at low physiological concentrations of agonist. These data indicate that TIRFM is a useful tool to monitor agonist-stimulated near-membrane Ca2+ signals mediated by Ca2+ influx in exocrine acinar cells. In addition, TIRFM reveals that the extent of Ca2+ influx in parotid acinar cells is greater than pancreatic acinar cells when compared using identical methodologies.

ER stress disrupts Ca2+-signaling complexes and Ca2+ regulation in secretory and muscle cells from PERK-knockout mice

Disruption of protein synthesis and folding results in ER stress, which is associated with the pathophysiology of diverse diseases affecting secretory and muscle cells. Cells are protected against ER stress by activation of the unfolded protein response (UPR) that is regulated by the protein kinase PERK, which phosphorylates the translation initiation factor 2 eIF2alpha to attenuate protein synthesis. PERK-/- cells are unable to modulate ER protein load and experience high levels of ER stress. In addition to its role in protein synthesis, the ER also orchestrates many signaling events essential for cell survival, prominent among which is Ca2+ signaling. It is not known, however, whether there is a relationship between ER stress and the function of the Ca2+-signaling pathway in muscle and non-muscle cells. To directly address this question we characterized Ca2+ signaling in the secretory pancreatic and parotid acinar cells and in urinary bladder smooth muscle (UBSM) cells obtained from PERK-/- and wild-type mice. Deletion of PERK that results in high levels of ER stress, and distention and fragmentation of the ER slowed the rate of agonist-mediated Ca2+ release from the ER and reduced Ca2+-induced Ca2+ release, although IP3 production, localization of the IP3 receptors, IP3-mediated Ca2+ release, Ca(v)1.2 current and RyRs activity remained unaltered. On the other hand, ER stress disrupted the integrity of the Ca2+-signaling complexes in both secretory and UBSM cells, as revealed by markedly reduced co-immunoprecipitation of plasma membrane- and ER-resident Ca2+-signaling proteins. These findings establish a relationship between the unfolding protein response, ER stress and Ca2+ signaling and highlight the importance of communication within the terminal ER-plasma membrane microdomain for propagation of the Ca2+ signal from the plasma membrane into the cell.

Protein-protein interaction and functionTRPC channels

Since their identification in the concluding years of the last century, the mammalian transient receptor potential (canonical) (TRPC) channels have remained in the limelight as the primary candidates for the Ca(2+) entry pathway activated by the hormones, growth factors, and neurotransmitters that exert their effect through activation of PLC. Although TRPC channels have been shown clearly to mediate, at least in part, receptor-activated Ca(2+) entry in literally all cell types, several of their central characteristics, as recorded in expression systems using recombinant channels, differ from those of the native receptor-dependent Ca(2+) influx channels. The present review attempts to highlight the interaction of TRPC channels with other proteins, which may explain the variability of TRPC channel activation and regulatory mechanisms observed with the native and recombinant channels. These include the homologous and heterotopous interactions of TRPC channel isoforms, the interaction of TRPC channels with calmodulin, PLCgamma, IP(3) receptors, and with scaffolding proteins like InaD, EBP50/NEHRF, caveolin, Janctate and Homers.

Regulation of fluid and electrolyte secretion in salivary gland acinar cells

The secretion of fluid and electrolytes by salivary gland acinar cells requires the coordinated regulation of multiple water and ion transporter and channel proteins. Notably, all the key transporter and channel proteins in this process appear to be activated, or are up-regulated, by an increase in the intracellular Ca2+ concentration ([Ca2+]i). Consequently, salivation occurs in response to agonists that generate an increase in [Ca2+]i. The mechanisms that act to modulate these increases in [Ca2+]i obviously influence the secretion of salivary fluid. Such modulation may involve effects on mechanisms of both Ca2+ release and Ca2+ entry and the resulting spatial and temporal aspects of the [Ca2+]i signal, as well as interactions with other signaling pathways in the cells. The molecular cloning of many of the transporter and regulatory molecules involved in fluid and electrolyte secretion has yielded a better understanding of this process at the cellular level. The subsequent characterization of mice with null mutations in many of these genes has demonstrated the physiological roles of individual proteins. This review focuses on recent developments in determining the molecular identification of the proteins that regulate the fluid secretion process.