Localization and regulation of Ca2+ entry and exit pathways in exocrine gland cells

Driver Mutations of Pancreatic Cancer Affect Ca2+ Signaling and ATP Production

Glandular pancreatic epithelia of the acinar or ductal phenotype may seem terminally differentiated, but they are characterized by remarkable cell plasticity. Stress-induced trans-differentiation of these cells has been implicated in the mechanisms of carcinogenesis. Current consensus links pancreatic ductal adenocarcinoma with onco-transformation of ductal epithelia, but under the presence of driver mutations in Kras and Trp53, also with trans-differentiation of pancreatic acini. However, we do not know when, in the course of cancer progression, physiological functions are lost by mutant acinar cells, nor can we assess their capacity for the production of pancreatic juice components. Here, we investigated whether two mutations-KrasG12D and Trp53R172H-present simultaneously in acinar cells of KPC mice (model of oncogenesis) influence cytosolic Ca2+ signals. Since Ca2+ signals control the cellular handling of digestive hydrolases, any changes that affect intracellular signaling events and cell bioenergetics might have an impact on the physiology of the pancreas. Our results showed that physiological doses of acetylcholine evoked less regular Ca2+ oscillations in KPC acinar cells compared to the control, whereas responses to supramaximal concentrations were markedly reduced. Menadione elicited Ca2+ signals of different frequencies in KPC cells compared to control cells. Finally, Ca2+ extrusion rates were significantly inhibited in KPC cells, likely due to the lower basal respiration and ATP production. Cumulatively, these findings suggest that driver mutations affect the signaling capacity of pancreatic acinar cells even before the changes in the epithelial cell morphology become apparent.

LPS-induced epithelial barrier disruption via hyperactivation of CACC and ENaC

Gram-negative bacterial lipopolysaccharide (LPS) increases the susceptibility of cells to pathogenic diseases, including inflammatory diseases and septic syndrome. In our experiments, we examined whether LPS induces epithelial barrier disruption in secretory epithelia and further investigated its underlying mechanism. The activities of Ca²⁺-activated Cl^- channels (CACC) and epithelial Na⁺ channels (ENaC) were monitored with a short-circuit current using an Ussing chamber. Epithelial membrane integrity was estimated via transepithelial electrical resistance and paracellular permeability assays. We found that the apical application of LPS evoked short-circuit current (I_sc) through the activation of CACC and ENaC. Although LPS disrupted epithelial barrier integrity, this was restored with the inhibition of CACC and ENaC, indicating the role of CACC and ENaC in the regulation of paracellular pathways. We confirmed that LPS, CACC, or ENaC activation evoked apical membrane depolarization. The exposure to a high-K⁺ buffer increased paracellular permeability. LPS induced the rapid redistribution of zonula occludens-1 (ZO-1) and reduced the expression levels of ZO-1 in tight junctions through apical membrane depolarization and tyrosine phosphorylation. However, the LPS-induced epithelial barrier disruption and degradation of ZO-1 were largely recovered by blocking CACC and ENaC. Furthermore, although LPS-impaired epithelial barrier became vulnerable to secondary bacterial infections, this vulnerability was prevented by inhibiting CACC and ENaC. We concluded that LPS induces the disruption of epithelial barrier integrity through the activation of CACC and ENaC, resulting in apical membrane depolarization and the subsequent tyrosine phosphorylation of ZO-1.

Intracellular Ca2+ Signalling in the Pathogenesis of Acute Pancreatitis: Recent Advances and Translational Perspectives

Intracellular Ca²⁺ signalling is a major signal transductional pathway in non-excitable cells, responsible for the regulation of a variety of physiological functions. In the secretory epithelial cells of the exocrine pancreas, such as acinar and ductal cells, intracellular Ca²⁺ elevation regulates digestive enzyme secretion in acini or fluid and ion secretion in ductal cells. Although Ca²⁺ is a uniquely versatile orchestrator of epithelial physiology, unregulated global elevation of the intracellular Ca²⁺ concentration is an early trigger for the development of acute pancreatitis (AP). Regardless of the aetiology, different forms of AP all exhibit sustained intracellular Ca²⁺ elevation as a common hallmark. The release of endoplasmic reticulum (ER) Ca²⁺ stores by toxins (such as bile acids or fatty acid ethyl esters (FAEEs)) or increased intrapancreatic pressure activates the influx of extracellular Ca²⁺ via the Orai1 Ca²⁺ channel, a process known as store-operated Ca²⁺ entry (SOCE). Intracellular Ca²⁺ overload can lead to premature activation of trypsinogen in pancreatic acinar cells and impaired fluid and HCO₃^- secretion in ductal cells. Increased and unbalanced reactive oxygen species (ROS) production caused by sustained Ca²⁺ elevation further contributes to cell dysfunction, leading to mitochondrial damage and cell death. Translational studies of AP identified several potential target molecules that can be modified to prevent intracellular Ca²⁺ overload. One of the most promising drugs, a selective inhibitor of the Orai1 channel that has been shown to inhibit extracellular Ca²⁺ influx and protect cells from injury, is currently being tested in clinical trials. In this review, we will summarise the recent advances in the field, with a special focus on the translational aspects of the basic findings.

Evaluation of the Relative Performance of Pancreas-Specific MicroRNAs in Rat Plasma as Biomarkers of Pancreas Injury

Drug-induced pancreatic injury (DIPI) has become linked in recent years to many commonly prescribed medications from several pharmacological classes. Diagnosis is currently most often focused on identification of acute pancreatitis and generally based on subjective clinical assessment and serum amylase and lipase enzymatic activity, which have been criticized as being insufficiently sensitive and specific. The lack of novel noninvasive biomarkers of DIPI can impede the advancement of drug candidates through nonclinical development and translation into clinical settings. Pancreas-specific microRNAs (miRNAs) are currently being evaluated as biomarkers of DIPI that may outperform and/or add value to the interpretation of amylase and lipase. To assess the relative performance of these novel miRNAs, a comprehensive evaluation was conducted to determine the sensitivity and specificity of detecting DIPI in rats. Four miRNAs were evaluated (miR-216a-5p, miR-216b-5p, miR-217-5p, and miR-375-3p) in plasma from 10 studies in which rats were treated with known pancreatic toxicants to assess sensitivity, and from 10 different studies in which toxicity was evident in tissues other than pancreas to assess specificity. The candidate miRNA biomarker performance was compared with amylase and lipase, and receiver operator characteristics (ROC) were determined. Analysis of ROCs demonstrated that all four miRNAs outperformed amylase and lipase in monitoring acute pancreatic injury defined as acinar cell degeneration/necrosis. Specifically, miR-217-5p had the highest performance among all biomarkers assessed. The increased sensitivity and specificity of these miRNAs support their use as biomarkers of DIPI, thereby adding value to the interpretation of amylase and lipase measurements in nonclinical studies. The potential for miRNAs to serve as translational biomarkers in the clinic for the monitoring of DIPI is also supported by this investigation.

Microcalcifications, calcium-sensing receptor, and cancer

Calcium stones and calculi are observed in numerous human tissues. They are the result of deposition of calcium salts and are due to high local calcium concentrations. Prostatic calculi are usually classified as endogenous or extrinsic stones. Endogenous stones are commonly caused by obstruction of the prostatic ducts around an enlarged prostate resulting from benign prostatic hyperplasia or from chronic inflammation. The latter occurs mainly around the urethra and is generally caused by reflux of urine into the prostate. Calcium concentrations higher than in the plasma at sites of infection may induce the chemotactic response that eventually leads to recruitment of inflammatory cells. The calcium sensing receptor (CaSR) may be crucial for this recruitment as its expression and activity are increased by cytokines such as IL-6 and high extracellular calcium concentrations, respectively. The links between calcium calculi, inflammation, calcium supplementation, and CaSR functions in prostate cancer patients will be discussed in this review.

ABT-199 (Venetoclax), a BH3-mimetic Bcl-2 inhibitor, does not cause Ca2+ -signalling dysregulation or toxicity in pancreatic acinar cells

BACKGROUND AND PURPOSE

Many cancer cells depend on anti-apoptotic B-cell lymphoma 2 (Bcl-2) proteins for their survival. Bcl-2 antagonism through Bcl-2 homology 3 (BH3) mimetics has emerged as a novel anti-cancer therapy. ABT-199 (Venetoclax), a recently developed BH3 mimetic that selectively inhibits Bcl-2, was introduced into the clinic for treatment of relapsed chronic lymphocytic leukaemia. Early generations of Bcl-2 inhibitors evoked sustained Ca²⁺ responses in pancreatic acinar cells (PACs) inducing cell death. Therefore, BH3 mimetics could potentially be toxic for the pancreas when used to treat cancer. Although ABT-199 was shown to kill Bcl-2-dependent cancer cells without affecting intracellular Ca²⁺ signalling, its effects on PACs have not yet been determined. Hence, it is essential and timely to assess whether this recently approved anti-leukaemic drug might potentially have pancreatotoxic effects.

EXPERIMENTAL APPROACH

Single-cell Ca²⁺ measurements and cell death analysis were performed on isolated mouse PACs.

KEY RESULTS

Inhibition of Bcl-2 via ABT-199 did not elicit intracellular Ca²⁺ signalling on its own or potentiate Ca²⁺ signalling induced by physiological/pathophysiological stimuli in PACs. Although ABT-199 did not affect cell death in PACs, under conditions that killed ABT-199-sensitive cancer cells, cytosolic Ca²⁺ extrusion was slightly enhanced in the presence of ABT-199. In contrast, inhibition of Bcl-xL potentiated pathophysiological Ca²⁺ responses in PACs, without exacerbating cell death.

CONCLUSION AND IMPLICATIONS

Our results demonstrate that apart from having a modest effect on cytosolic Ca²⁺ extrusion, ABT-199 does not substantially alter intracellular Ca²⁺ homeostasis in normal PACs and should be safe for the pancreas during cancer treatment.

LINKED ARTICLES

This article is part of a themed section on Mitochondrial Pharmacology: Featured Mechanisms and Approaches for Therapy Translation. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.22/issuetoc.

The Different Facets of Extracellular Calcium Sensors: Old and New Concepts in Calcium-Sensing Receptor Signalling and Pharmacology

The current interest of the scientific community for research in the field of calcium sensing in general and on the calcium-sensing Receptor (CaR) in particular is demonstrated by the still increasing number of papers published on this topic. The extracellular calcium-sensing receptor is the best-known G-protein-coupled receptor (GPCR) able to sense external Ca2+ changes. Widely recognized as a fundamental player in systemic Ca2+ homeostasis, the CaR is ubiquitously expressed in the human body where it activates multiple signalling pathways. In this review, old and new notions regarding the mechanisms by which extracellular Ca2+ microdomains are created and the tools available to measure them are analyzed. After a survey of the main signalling pathways triggered by the CaR, a special attention is reserved for the emerging concepts regarding CaR function in the heart, CaR trafficking and pharmacology. Finally, an overview on other Ca2+ sensors is provided.

Recent advances in understanding the extracellular calcium-sensing receptor

The extracellular calcium-sensing receptor (CaR), a ubiquitous class C G-protein-coupled receptor (GPCR), is responsible for the control of calcium homeostasis in body fluids. It integrates information about external Ca ²⁺ and a surfeit of other endogenous ligands into multiple intracellular signals, but how is this achieved? This review will focus on some of the exciting concepts in CaR signaling and pharmacology that have emerged in the last few years.

IP3R deficit underlies loss of salivary fluid secretion in Sjögren's Syndrome

The autoimmune exocrinopathy, Sjögren’s syndrome (SS), is associated with secretory defects in patients, including individuals with mild lymphocytic infiltration and minimal glandular damage. The mechanism(s) underlying the secretory dysfunction is not known. We have used minor salivary gland biopsies from SS patients and healthy individuals to assess acinar cell function in morphologically intact glandular areas. We report that agonist-regulated intracellular Ca²⁺ release, critically required for Ca²⁺ entry and fluid secretion, is defective in acini from SS patients. Importantly, these acini displayed reduction in IP3R2 and IP3R3, but not AQP5 or STIM1. Similar decreases in IP3R and carbachol (CCh)-stimulated [Ca²⁺]_i elevation were detected in acinar cells from lymphotoxin-alpha (LTα) transgenic (TG) mice, a model for (SS). Treatment of salivary glands from healthy individuals with LT α, a cytokine linked to disease progression in SS and IL14α mice, reduced Ca²⁺ signaling. Together, our findings reveal novel IP3R deficits in acinar cells that underlie secretory dysfunction in SS patients.

Ca²⁺ signaling and fluid secretion by secretory cells of the airway epithelium

Cytoplasmic Ca(2+) is a master regulator of airway physiology; it controls fluid, mucus, and antimicrobial peptide secretion, ciliary beating, and smooth muscle contraction. The focus of this review is on the role of cytoplasmic Ca(2+) in fluid secretion by airway exocrine secretory cells. Airway submucosal gland serous acinar cells are the primary fluid secreting cell type of the cartilaginous conducting airways, and this review summarizes the current state of knowledge of the molecular mechanisms of serous cell ion transport, with an emphasis on their regulation by intracellular Ca(2+). Many neurotransmitters that regulate secretion from serous acinar cells utilize Ca(2+) as a second messenger. Changes in intracellular Ca(2+) concentration regulate the activities of ion transporters and channels involved in transepithelial ion transport and fluid secretion, including Ca(2+)-activated K(+) channels and Cl(-) channels. We also review evidence of interactions of Ca(2+) signaling with other signaling pathways (cAMP, NO) that impinge upon different ion transport pathways, including the cAMP/PKA-activated cystic fibrosis (CF) transmembrane conductance regulator (CFTR) anion channel. A better understanding of Ca(2+) signaling and its targets in airway fluid secretion may identify novel strategies to intervene in airway diseases, for example to enhance fluid secretion in CF airways.

Multiple roles for the actin cytoskeleton during regulated exocytosis

Regulated exocytosis is the main mechanism utilized by specialized secretory cells to deliver molecules to the cell surface by virtue of membranous containers (i.e., secretory vesicles). The process involves a series of highly coordinated and sequential steps, which include the biogenesis of the vesicles, their delivery to the cell periphery, their fusion with the plasma membrane, and the release of their content into the extracellular space. Each of these steps is regulated by the actin cytoskeleton. In this review, we summarize the current knowledge regarding the involvement of actin and its associated molecules during each of the exocytic steps in vertebrates, and suggest that the overall role of the actin cytoskeleton during regulated exocytosis is linked to the architecture and the physiology of the secretory cells under examination. Specifically, in neurons, neuroendocrine, endocrine, and hematopoietic cells, which contain small secretory vesicles that undergo rapid exocytosis (on the order of milliseconds), the actin cytoskeleton plays a role in pre-fusion events, where it acts primarily as a functional barrier and facilitates docking. In exocrine and other secretory cells, which contain large secretory vesicles that undergo slow exocytosis (seconds to minutes), the actin cytoskeleton plays a role in post-fusion events, where it regulates the dynamics of the fusion pore, facilitates the integration of the vesicles into the plasma membrane, provides structural support, and promotes the expulsion of large cargo molecules.

A novel role for Bcl-2 in regulation of cellular calcium extrusion

The antiapoptotic protein Bcl-2 [1, 2] plays important roles in Ca²⁺ signaling [3] by influencing inositol triphosphate receptors and regulating Ca²⁺-induced Ca²⁺ release [4–6]. Here we investigated whether Bcl-2 affects Ca²⁺ extrusion in pancreatic acinar cells. We specifically blocked the Ca²⁺ pumps in the endoplasmic reticulum and assessed the rate at which the cells reduced an elevated cytosolic Ca²⁺ concentration after a period of enhanced Ca²⁺ entry. Because external Ca²⁺ was removed and endoplasmic reticulum Ca²⁺ pumps were blocked, Ca²⁺ extrusion was the only process responsible for recovery. Cells lacking Bcl-2 restored the basal cytosolic Ca²⁺ level much faster than control cells. The enhanced Ca²⁺ extrusion in cells from Bcl-2 knockout (Bcl-2 KO) mice was not due to increased Na⁺/Ca²⁺ exchange activity, because removal of external Na⁺ did not influence the Ca²⁺ extrusion rate. Overexpression of Bcl-2 in the pancreatic acinar cell line AR42J decreased Ca²⁺ extrusion, whereas silencing Bcl-2 expression (siRNA) had the opposite effect. Loss of Bcl-2, while increasing Ca²⁺ extrusion, dramatically decreased necrosis and promoted apoptosis induced by oxidative stress, whereas specific inhibition of Ca²⁺ pumps in the plasma membrane (PMCA) with caloxin 3A1 reduced Ca²⁺ extrusion and increased necrosis. Bcl-2 regulates PMCA function in pancreatic acinar cells and thereby influences cell fate.

A novel function of Noc2 in agonist-induced intracellular Ca2+ increase during zymogen-granule exocytosis in pancreatic acinar cells

Noc2, a putative Rab effector, contributes to secretory-granule exocytosis in neuroendocrine and exocrine cells. Here, using two-photon excitation live-cell imaging, we investigated its role in Ca²⁺-dependent zymogen granule (ZG) exocytosis in pancreatic acinar cells from wild-type (WT) and Noc2-knockout (KO) mice. Imaging of a KO acinar cell revealed an expanded granular area, indicating ZG accumulation. In our spatiotemporal analysis of the ZG exocytosis induced by agonist (cholecystokinin or acetylcholine) stimulation, the location and rate of progress of ZG exocytosis did not differ significantly between the two strains. ZG exocytosis from KO acinar cells was seldom observed at physiological concentrations of agonists, but was normal (vs. WT) at high concentrations. Flash photolysis of a caged calcium compound confirmed the integrity of the fusion step of ZG exocytosis in KO acinar cells. The decreased ZG exocytosis present at physiological concentrations of agonists raised the possibility of impaired elicitation of calcium spikes. When calcium spikes were evoked in KO acinar cells by a high agonist concentration: (a) they always started at the apical portion and traveled to the basal portion, and (b) calcium oscillations over the 10 µM level were observed, as in WT acinar cells. At physiological concentrations of agonists, however, sufficient calcium spikes were not observed, suggesting an impaired [Ca²⁺]_i-increase mechanism in KO acinar cells. We propose that in pancreatic acinar cells, Noc2 is not indispensable for the membrane fusion of ZG per se, but instead performs a novel function favoring agonist-induced physiological [Ca²⁺]_i increases.

Orai channel-dependent activation of phospholipase C-δ: a novel mechanism for the effects of calcium entry on calcium oscillations

The frequency of oscillatory Ca(2+) signals is a major determinant in the selective activation of discrete downstream responses in non-excitable cells. An important modulator of this oscillation frequency is known to be the rate of agonist-activated Ca(2+) entry. However precisely how this is achieved and the respective roles of store-operated versus store-independent Ca(2+) entry pathways in achieving this are unclear. Here, we examine the possibility that a direct stimulation of a phospholipase C (PLC) by the entering Ca(2+) can induce a modulation of Ca(2+) oscillation frequency, and examine the roles of the endogenous store-operated and store-independent Orai channels (CRAC and ARC channels, respectively) in such a mechanism. Using the decline in the magnitude of currents through expressed PIP(2)-dependent Kir2.1 channels as a sensitive assay for PLC activity, we show that simple global increases in Ca(2+) concentrations over the physiological range do not significantly affect PLC activity. Similarly, maximal activation of endogenous CRAC channels also fails to affect PLC activity. In contrast, equivalent activation of endogenous ARC channels resulted in a 10-fold increase in the measured rate of PIP(2) depletion. Further experiments show that this effect is strictly dependent on the Ca(2+) entering via these channels, rather than the gating of the channels or the arachidonic acid used to activate them, and that it reflects the activation of a PLCδ by local Ca(2+) concentrations immediately adjacent to the active channels. Finally, based on the effects of expression of either a dominant-negative mutant Orai3 that is an essential component of the ARC channel, or a catalytically compromised mutant PLCδ, it was shown that this specific action of the store-independent ARC channel-mediated Ca(2+) entry on PLCδ has a significant impact on the oscillation frequency of the Ca(2+) signals activated by low concentrations of agonist.

Dissection of calcium signaling events in exocrine secretion

The secretion of fluid and electrolytes by salivary gland acinar cells requires the coordinated regulation of multiple ion channel and transporter proteins, signaling components, and water transport. Importantly, neurotransmitter stimulated increase in the cytosolic free [Ca(2+)] ([Ca(2+)](i)) is critical for the regulation of salivary gland secretion as it regulates several major ion fluxes that together establish the sustained osmotic gradient to drive fluid secretion. The mechanisms that act to modulate these increases in [Ca(2+)](i) are therefore central to the process of salivary fluid secretion. Such modulation involves membrane receptors for neurotransmitters, as well as mechanisms that mediate intracellular Ca(2+) release, and Ca(2+) entry, as well as those that maintain cellular Ca(2+) homeostasis. Together, these mechanisms determine the spatial and temporal aspects of the [Ca(2+)](i) signals that regulate fluid secretion. Molecular cloning of these transporters and channels as well as development of mice lacking these proteins has established the physiological significance of key components that are involved in regulating [Ca(2+)](i) in salivary glands. This review will discuss these important studies and the findings which have led to resolution of the Ca(2+) signaling mechanisms that determine salivary gland fluid secretion.

The Ca2+ pumps of the endoplasmic reticulum and Golgi apparatus

The various splice variants of the three SERCA- and the two SPCA-pump genes in higher vertebrates encode P-type ATPases of the P(2A) group found respectively in the membranes of the endoplasmic reticulum and the secretory pathway. Of these, SERCA2b and SPCA1a represent the housekeeping isoforms. The SERCA2b form is characterized by a luminal carboxy terminus imposing a higher affinity for cytosolic Ca(2+) compared to the other SERCAs. This is mediated by intramembrane and luminal interactions of this extension with the pump. Other known affinity modulators like phospholamban and sarcolipin decrease the affinity for Ca(2+). The number of proteins reported to interact with SERCA is rapidly growing. Here, we limit the discussion to those for which the interaction site with the ATPase is specified: HAX-1, calumenin, histidine-rich Ca(2+)-binding protein, and indirectly calreticulin, calnexin, and ERp57. The role of the phylogenetically older and structurally simpler SPCAs as transporters of Ca(2+), but also of Mn(2+), is also addressed.

Local Ca²+ entry via Orai1 regulates plasma membrane recruitment of TRPC1 and controls cytosolic Ca²+ signals required for specific cell functions

Store-operated Ca²⁺ entry (SOCE) has been associated with two types of channels: CRAC channels that require Orai1 and STIM1 and SOC channels that involve TRPC1, Orai1, and STIM1. While TRPC1 significantly contributes to SOCE and SOC channel activity, abrogation of Orai1 function eliminates SOCE and activation of TRPC1. The critical role of Orai1 in activation of TRPC1-SOC channels following Ca²⁺ store depletion has not yet been established. Herein we report that TRPC1 and Orai1 are components of distinct channels. We show that TRPC1/Orai1/STIM1-dependent I_SOC, activated in response to Ca²⁺ store depletion, is composed of TRPC1/STIM1-mediated non-selective cation current and Orai1/STIM1-mediated I_CRAC; the latter is detected when TRPC1 function is suppressed by expression of shTRPC1 or a STIM1 mutant that lacks TRPC1 gating, STIM1(⁶⁸⁴EE⁶⁸⁵). In addition to gating TRPC1 and Orai1, STIM1 mediates the recruitment and association of the channels within ER/PM junctional domains, a critical step in TRPC1 activation. Importantly, we show that Ca²⁺ entry via Orai1 triggers plasma membrane insertion of TRPC1, which is prevented by blocking SOCE with 1 µM Gd³⁺, removal of extracellular Ca²⁺, knockdown of Orai1, or expression of dominant negative mutant Orai1 lacking a functional pore, Orai1-E106Q. In cells expressing another pore mutant of Orai1, Orai1-E106D, TRPC1 trafficking is supported in Ca²⁺-containing, but not Ca²⁺-free, medium. Consistent with this, I_CRAC is activated in cells pretreated with thapsigargin in Ca²⁺-free medium while I_SOC is activated in cells pretreated in Ca²⁺-containing medium. Significantly, TRPC1 function is required for sustained K_Ca activity and contributes to NFκB activation while Orai1 is sufficient for NFAT activation. Together, these findings reveal an as-yet unidentified function for Orai1 that explains the critical requirement of the channel in the activation of TRPC1 following Ca²⁺ store depletion. We suggest that coordinated regulation of the surface expression of TRPC1 by Orai1 and gating by STIM1 provides a mechanism for rapidly modulating and maintaining SOCE-generated Ca²⁺ signals. By recruiting ion channels and other signaling pathways, Orai1 and STIM1 concertedly impact a variety of critical cell functions that are initiated by SOCE.

Store-operated Ca²⁺ entry is present in all cell types and determines sustained cytosolic [Ca²⁺] increases that are critical for regulating a wide variety of physiological functions. This Ca²⁺ entry mechanism is activated in response to depletion of Ca²⁺ in the endoplasmic reticulum (ER). When ER [Ca²⁺] is decreased, the Ca²⁺-sensor protein STIM1 aggregates in the ER membrane and moves to regions in the periphery of the cells where it interacts with and activates two major types of channels that contribute to store-operated Ca²⁺ entry: CRAC and SOC. While gating of Orai1 by STIM1 is sufficient for CRAC channel activity, both Orai1 and transient receptor potential channel 1 (TRPC1) contribute to SOC channel function. The molecular composition of SOC channels and the critical role of Orai1 in activation of TRPC1 have not yet been established. In this study, we demonstrate that TRPC1 and Orai1 are components of distinct channels, both of which are regulated by STIM1. Importantly, we show that Orai1-mediated Ca²⁺ entry triggers plasma membrane insertion of TRPC1 which is then gated by STIM1. Ca²⁺ entry via functional TRPC1-STIM1 channels provides additional increase in cytosolic [Ca²⁺] that is required for regulation of specific cell functions such as K_Ca activation. Together, our findings elucidate the critical role of Orai1 in TRPC1 channel function. We suggest that the regulation of TRPC1 trafficking provides a mechanism for rapidly modulating cytosolic [Ca²⁺] following Ca²⁺ store depletion.

Acute pancreatitis

Acute pancreatitis (AP) is an important cause of morbidity and mortality worldwide and the annual incidence appears to be increasing. It presents as a mild self-limiting illness in 80% of patients. However, one-fifth of these develop a severe complicated life-threatening disease requiring intensive and prolonged therapeutic intervention. Alcohol and gallstone disease remain the commonest causes of AP but metabolic abnormalities, obesity and genetic susceptibility are thought be increasingly important aetiological factors. The prompt diagnosis of AP and stratification of disease severity is essential in directing rapid delivery of appropriate therapeutic measures. In this review, the range of diagnostic and prognostic assays, severity scoring systems and radiological investigations used in current clinical practice are described, highlighting their strengths and weaknesses. Increased understanding of the complex pathophysiology of AP has generated an array of new potential diagnostic assays and these are discussed. The multidisciplinary approach to management of severe pancreatitis is outlined, including areas of controversy and novel treatments.

The presence of bestrophin-1 modulates the Ca2+ recruitment from Ca2+ stores in the ER

Bestrophin-1, mainly analyzed in overexpression experiments, functions as Ca(2+)-dependent Cl(-) channel. Analysis of endogenously expressed bestrophin-1 suggested an influence on intracellular Ca(2+). The aim of the study is to analyze the influence of endogenously expressed bestrophin-1 on Ca(2+) homeostasis. Primary cultures of retinal pigment epithelial (RPE) cells were established from wild-type and bestrophin-1-deficient mice. Intracellular free Ca(2+) ([Ca(2+)](i)) was recorded by Ca(2+) imaging; through immunocytochemistry and differential centrifugation, subcellular localization of bestrophin-1 was analyzed. RPE cells of bestrophin-1-deficient mice showed higher levels of resting [Ca(2+)](i) than cells from wild-type mice. In cells from knockout mice and wild-type mice, ATP led to increases in [Ca(2+)](i) subsequent to phospholipase C activation. ATP-induced Ca(2+) in bestrophin-1-deficient mice rose faster and decayed slower. In cells from wild-type mice, ATP led to [Ca(2+)](i) increase via depletion of Ca(2+) from thapsigargin-sensitive stores. In cells from bestrophin-1-deficient mice, ATP-dependent increase in [Ca(2+)](i) resulted in 40% of cells from depletion of bafilomycin-sensitive and in 60% from thapsigargin-sensitive Ca(2+) stores. After differential centrifugation, bestrophin-1 was found in fractions enriched of ClC-3 Cl channel and myosin-7A. Co-localization analysis of bestrophin-1, with beta-catenin or pan-cadherin, in fresh sections of porcine retina, revealed bestrophin-1 in the basolateral membrane. A portion of endogenously expressed bestrophin-1,localized in the endoplasmic reticulum, influenced uptake of Ca(2+) into Ca(2+) stores. Therefore, bestrophin-1 possibly conducts Cl(-) as counter ion for Ca(2+) uptake into cytosolic Ca(2+) stores.

Bacterial toxins and the nervous system: neurotoxins and multipotential toxins interacting with neuronal cells

Toxins are potent molecules used by various bacteria to interact with a host organism. Some of them specifically act on neuronal cells (clostridial neurotoxins) leading to characteristics neurological affections. But many other toxins are multifunctional and recognize a wider range of cell types including neuronal cells. Various enterotoxins interact with the enteric nervous system, for example by stimulating afferent neurons or inducing neurotransmitter release from enterochromaffin cells which result either in vomiting, in amplification of the diarrhea, or in intestinal inflammation process. Other toxins can pass the blood brain barrier and directly act on specific neurons.

CRAC channelopathies

Store-operated Ca2+ entry (SOCE) is an important Ca2+ influx pathway in many non-excitable and some excitable cells. It is regulated by the filling state of intracellular Ca2+ stores, notably the endoplasmic reticulum (ER). Reduction in [Ca2+]ER results in activation of plasma membrane Ca2+ channels that mediate sustained Ca2+ influx which is required for many cell functions as well as refilling of Ca2+ stores. The Ca2+ release activated Ca2+ (CRAC) channel is the best characterized SOC channel with well-defined electrophysiological properties. In recent years, the molecular components of the CRAC channel, long mysterious, have been defined. ORAI1 (or CRACM1) acts as the pore-forming subunit of the CRAC channel in the plasma membrane. Stromal interaction molecule (STIM) 1 is localized in the ER, senses [Ca2+]ER, and activates the CRAC channel upon store depletion by binding to ORAI1. Both proteins are widely expressed in many tissues in both human and mouse consistent with the widespread prevalence of SOCE and CRAC channel currents in many cells types. CRAC channelopathies in human patients with mutations in STIM1 and ORAI1 are characterized by abolished CRAC channel currents, lack of SOCE and-clinically-immunodeficiency, congenital myopathy, and anhydrotic ectodermal dysplasia. This article reviews the role of ORAI and STIM proteins for SOCE and CRAC channel function in a variety of cell types and tissues and compares the phenotypes of ORAI1 and STIM1-deficient human patients and mice with targeted deletion of Orai and Stim genes.

Determination of apparent calcium affinity for endogenously expressed human sarco(endo)plasmic reticulum calcium-ATPase isoform SERCA3

The sarco(endo)plasmic reticulum Ca2+-ATPases (SERCAs) play a crucial role in regulating free cytosolic Ca2+ concentration in diverse cell types. It has been shown that recombinant SERCA3, when measured in heterologous systems, exhibits low apparent affinity for Ca2+; however, Ca2+ affinity of native SERCA3 in an endogenous setting has not been examined. Such a measurement is complicated, because SERCA3 is always coexpressed with the housekeeping isoform SERCA2b. We used a fluorescence-based assay for monitoring continuous Ca2+ uptake into microsomes to examine the properties of endogenous human SERCA3 and SERCA2b. The kinetic parameters were derived using a cooperative two-component uptake model for Ca2+ activation, and the values assigned to SERCA3 were confirmed using the highly specific human SERCA3 inhibitory antibody PL/IM430. First, using recombinant human SERCA3 and SERCA2b proteins transiently expressed in HEK-293 cells, we confirmed the previously observed low apparent Ca2+ affinity for SERCA3 compared with SERCA2b (1.10 ± 0.04 vs. 0.26 ± 0.01 μM), and using mixtures of recombinant protein isoforms, we validated the two-component uptake model. Then we determined apparent Ca2+ affinity for SERCA proteins present endogenously in cultured Jurkat T lymphocytes and freshly isolated human tonsil lymphocytes. The apparent Ca2+ affinity in these two preparations was 1.04 ± 0.07 and 1.1 ± 0.2 μM for SERCA3 and 0.27 ± 0.02 and 0.26 ± 0.01 μM for SERCA2b, respectively. Our data demonstrate, for the first time, that affinity for Ca2+ is inherently lower for SERCA3 expressed in situ than for other SERCA isoforms.

Calnuc plays a role in dynamic distribution of Galphai but not Gbeta subunits and modulates ACTH secretion in AtT-20 neuroendocrine secretory cells

In AtT-20 cells ACTH secretion is regulated by both Ca²⁺ and G proteins. We previously demonstrated that calnuc, an EF-hand Ca²⁺ binding protein which regulates Alzheimer's β-amyloid precursor protein (APP) biogenesis, binds both Ca²⁺ as well as Gα subunits. Here we investigate calnuc's role in G protein-mediated regulation of ACTH secretion in AtT-20 neuroendocrine secretory cells stably overexpressing calnuc-GFP. Similar to endogenous calnuc, calnuc-GFP is mainly found in the Golgi, on the plasma membrane (PM), and associated with regulated secretion granules (RSG). By deconvolution immunofluorescence, calnuc-GFP partially colocalizes with Gαi1/2 and Gαi3 at the PM and on RSG. Cytosolic calnuc(ΔSS)-CFP with the signal sequence deleted also partially colocalizes with RSG and partially cosediments with Gαi1/2 in fractions enriched in RSG. Overexpression of calnuc-GFP specifically increases the distribution of Gαi1/2 on the PM whereas the distribution of Gβ subunits and synaptobrevin 2 (Vamp 2) is unchanged. Overexpression of calnuc-GFP or cytosolic calnuc(ΔSS)-CFP enhances ACTH secretion two-fold triggered by mastoparan or GTPγS but does not significantly affect glycosaminoglycan (GAG) chain secretion along the constitutive pathway or basal secretion of ACTH. Calnuc's facilitating effects on ACTH secretion are decreased after introducing anti-Gαi1/2, Gαi3, Gβ or calnuc IgG into permeabilized cells but not when Gα12 or preimmune IgG is introduced. The results suggest that calnuc binds to Gα subunits on the Golgi and on RSG and that overexpression of calnuc causes redistribution of Gαi subunits to the PM and RSG, indicating that calnuc plays a role in dynamic distribution of only Gα but not Gβ subunits. Thus calnuc may connect G protein signaling and calcium signaling during regulated secretion.

Ca2+ signalling and Ca2+-activated ion channels in exocrine acinar cells

The development of the calcium signalling field, from its early beginnings some 40 years ago to the present, is described. Calcium signalling in exocrine gland acinar cells and the effects of neurotransmitter- or hormone-elicited rises in the cytosolic calcium ion concentration on ion channel gating are reviewed. The highly polarized arrangement of the organelle systems in living acinar cells is described as well as its importance for the physiologically relevant local and polarized calcium signalling events.

Polarized calcium signaling in exocrine gland cells

Cytosolic Ca2+ signals are crucial for the control of fluid and enzyme secretion from exocrine glands. The highly polarized exocrine acinar cells have evolved sophisticated and complex Ca2+ signaling mechanisms that exercise precise control of the secretory events occurring across the apical plasma membrane bordering the gland lumen. Ca2+ stores in the endoplasmic reticulum, the secretory granules, the lysosomes, and the endosomes all play important roles in the generation of the local apical Ca2+ spikes that switch on Cl(-) channels in the apical plasma membrane as well as exocytotic export of enzymes. The mitochondria are crucial not only for ATP generation but also for the physiologically important subcellular compartmentalization of the cytosolic Ca2+ signals.

Ca2+-Operated Transcriptional Networks: Molecular Mechanisms and In Vivo Models

Calcium is the most universal signal used by living organisms to convey information to many different cellular processes. In this review we present well-known and recently identified proteins that sense and decode the calcium signal and are key elements in the nucleus to regulate the activity of various transcriptional networks. When possible, the review also presents in vivo models in which the genes encoding these calcium sensors-transducers have been modified, to emphasize the critical role of these Ca2+-operated mechanisms in many physiological functions.

Hereditary pancreatitis amlodipine trial: a pilot study of a calcium-channel blocker in hereditary pancreatitis

OBJECTIVES

Hereditary pancreatitis (HP) is a form of recurrent acute pancreatitis (AP) mediated by mutations in cationic trypsinogen (PRSS1). Mutations cluster in the calcium-associated regulator regions of PRSS1. In rats, calcium-channel blockers (CCB) prevent hyperstimulation-associated AP. Because of the potential importance of hyperstimulation in triggering episodes of AP in HP, we designed a pilot study to evaluate the safety and potential benefit of CCB use in HP.

METHODS

Subjects 6 years or older had a PRSS1 mutation, recurrent AP, and pain. Total study duration was 16 weeks. Amlodipine was given during weeks 0 to 11. Dose (2.5, 5, or 10 mg) was based on weight (range, 0.08-0.17 mg x kg(-1) x d(-1)). Subjects filled a daily diary including pain (0-10 scale) and blood pressure reading. Clinical assessments occurred at weeks -4, 0, 1, 2, 6, 10, 11, and 12. Subjects filled a Medical Outcomes Study Short-Form Survey version 2 (SF-10 for children <14 years old) at weeks -4, 0, 6, and 10. Data were compared for weeks -4 to 0 and 6 to 10.

RESULTS

Nine subjects signed informed consent (4 males; 12-52 years old). Four were excluded during the screening phase. Drug was discontinued in one due to development of unilateral lower-extremity numbness. Four subjects (12-31 years old) completed the study. Mean blood pressure, laboratory tests, physical findings, and daily pain scores did not clinically significantly differ before and during drug therapy, but all reported reduced symptoms. Three reduced analgesic use. Three had improved scores on the Medical Outcomes Study Short-Form Survey version 2.

CONCLUSIONS

Amlodipine is generally safe in subjects with HP and does not increase pain or episodes of AP. Further research into the mechanism of CCB on pancreatitis would be important to provide a pathophysiologic basis to support further trials in HP.

Mitochondria and Ca(2+) signaling: old guests, new functions

Mitochondria are ancient endosymbiotic guests that joined the cells in the evolution of complex life. While the unique ability of mitochondria to produce adenosine triphosphate (ATP) and their contribution to cellular nutrition metabolism received condign attention, our understanding of the organelle's contribution to Ca(2+) homeostasis was restricted to serve as passive Ca(2+) sinks that accumulate Ca(2+) along the organelle's negative membrane potential. This paradigm has changed radically. Nowadays, mitochondria are known to respond to environmental Ca(2+) and to contribute actively to the regulation of spatial and temporal patterns of intracellular Ca(2+) signaling. Accordingly, mitochondria contribute to many signal transduction pathways and are actively involved in the maintenance of capacitative Ca(2+) entry, the accomplishment of Ca(2+) refilling of the endoplasmic reticulum and Ca(2+)-dependent protein folding. Mitochondrial Ca(2+) homeostasis is complex and regulated by numerous, so far, genetically unidentified Ca(2+) channels, pumps and exchangers that concertedly accomplish the organelle's Ca(2+) demand. Notably, mitochondrial Ca(2+) homeostasis and functions are crucially influenced by the organelle's structural organization and motility that, in turn, is controlled by matrix/cytosolic Ca(2+). This review intends to provide a condensed overview on the molecular mechanisms of mitochondrial Ca(2+) homeostasis (uptake, buffering and storage, extrusion), its modulation by other ions, kinases and small molecules, and its contribution to cellular processes as fundamental basis for the organelle's contribution to signaling pathways. Hence, emphasis is given to the structure-to-function and mobility-to-function relationship of the mitochondria and, thereby, bridging our most recent knowledge on mitochondria with the best-established mitochondrial function: metabolism and ATP production.

Live-cell imaging reveals sequential oligomerization and local plasma membrane targeting of stromal interaction molecule 1 after Ca2+ store depletion

Stromal interaction molecule 1 (STIM1) has recently been identified by our group and others as an endoplasmic reticulum (ER) Ca(2+) sensor that responds to ER Ca(2+) store depletion and activates Ca(2+) channels in the plasma membrane (PM). The molecular mechanism by which STIM1 transduces signals from the ER lumen to the PM is not yet understood. Here we developed a live-cell FRET approach and show that STIM1 forms oligomers within 5 s after Ca(2+) store depletion. These oligomers rapidly dissociated when ER Ca(2+) stores were refilled. We further show that STIM1 formed oligomers before its translocation within the ER network to ER-PM junctions. A mutant STIM1 lacking the C-terminal polybasic PM-targeting motif oligomerized after Ca(2+) store depletion but failed to form puncta at ER-PM junctions. Using fluorescence recovery after photobleaching measurements to monitor STIM1 mobility, we show that STIM1 oligomers translocate on average only 2 mum to reach ER-PM junctions, arguing that STIM1 ER-to-PM signaling is a local process that is suitable for generating cytosolic Ca(2+) gradients. Together, our live-cell measurements dissect the STIM1 ER-to-PM signaling relay into four sequential steps: (i) dissociation of Ca(2+), (ii) rapid oligomerization, (iii) spatially restricted translocation to nearby ER-PM junctions, and (iv) activation of PM Ca(2+) channels.

Novel subcellular locations and functions for secretory pathway Ca2+/Mn2+-ATPases

Secretory pathway Ca2+/Mn2+-ATPases (SPCAs) are important for maintenance of cellular Ca2+and Mn2+homeostasis, and, to date, all SPCAs have been found to localize to the Golgi apparatus. The single Drosophila SPCA gene ( SPoCk) was identified by an in silico screen for novel Ca2+-ATPases. It encoded three SPoCk isoforms with novel, distinct subcellular specificities in the endoplasmic reticulum (ER) and peroxisomes in addition to the Golgi. Furthermore, expression of the peroxisome-associated SPoCk isoform was sexually dimorphic. Overexpression of organelle-specific SPoCk isoforms impacted on cytosolic Ca2+handling in both cultured Drosophila cells and a transporting epithelium, the Drosophila Malpighian (renal) tubule. Specifically, the ER isoform impacted on inositol ( 1 , 4 , 5 )-trisphosphate-mediated Ca2+signaling and the Golgi isoform impacted on diuresis, whereas the peroxisome isoform colocalized with Ca2+“spherites” and impacted on calcium storage and transport. Interfering RNA directed against the common exons of the three SPoCk isoforms resulted in aberrant Ca2+signaling and abolished neuropeptide-stimulated diuresis by the tubule. SPoCk thus contributed to both of the contrasting requirements for Ca2+in transporting epithelia: to transport or store Ca2+in bulk without compromising its use as a signal.

Calcium transport in human salivary glands: a proposed model of calcium secretion into saliva

Salivary calcium plays a vital role in bio-mineralization of dental enamel and exposed dentin. In order to elucidate the yet unknown cellular and molecular mechanisms of calcium secretion in human salivary glands the presence of various relevant plasma membrane transport systems for calcium were investigated. Using an RT-PCR approach, expression of the epithelial calcium channel (CaT-Like), the calcium binding protein (calbindin-2), the endoplasmic reticulum pumps (SERCA-2 and -3), and the plasma membrane calcium ATPases (PMCA-1, -2, and -4), were found in parotid and submandibular glands. Immunohistochemistry revealed that CaT-Like is located in the basolateral plasma membrane of acinar cells; while calbindin-2, SERCA-2 and SERCA-3 were found inside the acinar cells; and PMCA-2 was found in the apical membrane and in the secretory canaliculi between the cells. Based on these findings, we propose the following model of calcium secretion in human salivary glands: (1) calcium enters the acinar cell at the basolateral side via calcium channel CaT-Like (calcium influx); (2) intracellular calcium is taken up into the endoplasmic reticulum by SERCA-2 and possibly SERCA3 or bound to calbindin-2 (intracellular calcium pool); and (3) calcium is secreted by PMCAs at the apical plasma membrane (calcium efflux).

Immunohistochemical study about the Flt-1/VEGFR1 expression in the gastrointestinal tract of mouse, rat, dog, swine and monkey

Fms-like tyrosine kinase 1 (Flt-1) performs a subordinate effector role in mesenchymal angiogenesis and potentially serves an equally important functional role as a self-contained receptor in epithelial cells. In both endothelial cells and epithelial cells, Flt-1/vascular endothelial growth factor receptor 1 (VEGFR1) downstream signalling is involved in regulating cellular processes such as cytoskeletal changes and cellular survival protection. Cellular renewal of the gastrointestinal mucosa is based on these processes and might involve Flt-1/VEGFR1 pathway activities; the molecular mechanisms regulating these cellular dynamics remain unclear. This study was performed to investigate the presence and distribution of Flt-1/VEGFR1 in epithelial cells of the gastrointestinal tract by immunohistochemistry (IHC). Gastrointestinal tissues were taken from eight anatomical sites from mouse, rat, dog, swine and monkey. Present results revealed a cytosolic Flt-1/VEGFR1 staining pattern in mucosal epithelial cells for all investigated species. Non-epithelial structures also displayed a distinct Flt-1/VEGFR1 positivity and included vascular smooth muscle walls, enteric smooth muscle layers, the enteric nervous system and capillary endothelial cells. Diverse intensities of the Flt-1/VEGFR1 binding reaction within each species were observed in the intestinal mucosa with a strong immunoreaction in enterocytes and with a low protein expression in the ileum in most species. Crypt cells in the large intestine were mostly negative for Flt-1/VEGFR1. A peculiar and mainly intranuclear antibody binding reaction was found in Brunner's gland epithelial cells of mouse and rat whereas Brunner's glands of dog, swine and monkey remained completely negative. These results indicate a potential involvement of Flt-1/VEGFR1 in normal restitution of gastrointestinal structures in the species studied. Additionally, intranuclear Flt-1/VEGFR1 antibody binding in Brunner's glands of rodents may suggest a nuclear translocation of the transmembrane VEGFR1 which has not previously been described.

IP3 receptor types 2 and 3 mediate exocrine secretion underlying energy metabolism

Type 2 and type 3 inositol 1,4,5-trisphosphate receptors (IP3R2 and IP3R3) are intracellular calcium-release channels whose physiological roles are unknown. We show exocrine dysfunction in IP3R2 and IP3R3 double knock-out mice, which caused difficulties in nutrient digestion. Severely impaired calcium signaling in acinar cells of the salivary glands and the pancreas in the double mutants ascribed the secretion deficits to a lack of intracellular calcium release. Despite a normal caloric intake, the double mutants were hypoglycemic and lean. These results reveal IP3R2 and IP3R3 as key molecules in exocrine physiology underlying energy metabolism and animal growth.

Calcium signalling: Past, present and future

Ca2+ is a universal second messenger controlling a wide variety of cellular reactions and adaptive responses. The initial appreciation of Ca2+ as a universal signalling molecule was based on the work of Sydney Ringer and Lewis Heilbrunn. More recent developments in this field were critically influenced by the invention of the patch clamp technique and the generation of fluorescent Ca2+ indicators. Currently the molecular Ca2+ signalling mechanisms are being worked out and we are beginning to assemble a reasonably complete picture of overall Ca2+ homeostasis. Furthermore, investigations of organellar Ca2+ homeostasis have added complexity to our understanding of Ca2+ signalling. The future of the Ca2+ signalling field lies with detailed investigations of the integrative function in vivo and clarification of the pathology associated with malfunctions of Ca2+ signalling cascades.

Another dimension to calcium signaling: a look at extracellular calcium

Cell biologists know the calcium ion best as a vital intracellular second messenger that governs countless cellular functions. However, the recent identification of cell-surface detectors for extracellular Ca(2+) has prompted consideration of whether Ca(2+) also functions as a signaling molecule in the extracellular milieu. The cast of Ca(2+) sensors includes the well-characterized extracellular-Ca(2+)-sensing receptor, a G-protein-coupled receptor originally isolated from the parathyroid gland. In addition, other receptors, channels and membrane proteins, such as gap junction hemichannels, metabotropic glutamate receptors, HERG K(+) channels and the receptor Notch, are all sensitive to external [Ca(2+)] fluctuations. A recently cloned Ca(2+) sensor (CAS) in Arabidopsis extends this concept to the plant kingdom. Emerging evidence indicates that [Ca(2+)] in the local microenvironment outside the cell undergoes alterations potentially sufficient to exert biological actions through these sensor proteins. The extracellular space might therefore constitute a much more dynamic Ca(2+) signaling compartment than previously appreciated.

Apical Localization of a Functional TRPC3/TRPC6-Ca2+-Signaling Complex in Polarized Epithelial Cells

Receptor-coupled [Ca2+]i increase is initiated in the apical region of epithelial cells and has been associated with apically localized Ca2+-signaling proteins. However, localization of Ca2+ channels that are regulated by such Ca2+-signaling events has not yet been established. This study examines the localization of TRPC channels in polarized epithelial cells and demonstrates a role for TRPC3 in apical Ca2+ uptake. Endogenously and exogenously expressed TRPC3 was localized apically in polarized Madin-Darby canine kidney cells (MDCK) and salivary gland epithelial cells. In contrast, TRPC1 was localized basolaterally, whereas TRPC6 was detected in both locations. Localization of Galpha(q/11), inositol 1,4,5-trisphosphate receptor-3, and phospholipase Cbeta1 and -beta2 was also predominantly apical. TRPC3 co-immunoprecipitated with endogenous TRPC6, phospholipase Cbetas, Galpha(q/11), inositol 1,4,5-trisphosphate receptor-3, and syntaxin 3 but not with TRPC1. Furthermore, 1-oleoyl-2-acetyl-sn-glycerol (OAG)-stimulated apical 45Ca2+ uptake was higher in TRPC3-MDCK cells compared with control (MDCK) cells. Bradykinin-stimulated apical 45Ca2+ uptake and transepithelial 45Ca2+ flux were also higher in TRPC3-expressing cells. Consistent with this, OAG induced [Ca2+]i increase in the apical, but not basal, region of TRPC3-MDCK cells that was blocked by EGTA addition to the apical medium. Most importantly, (i) TRPC3 was detected in the apical region of rat submandibular gland ducts, whereas TRPC6 was present in apical as well as basolateral regions of ducts and acini; and (ii) OAG stimulated Ca2+ influx into dispersed ductal cells. These data demonstrate functional localization of TRPC3/TRPC6 channels in the apical region of polarized epithelial cells. In salivary gland ducts this could contribute to the regulation of salivary [Ca2+] and secretion.

Hypoxia sensing and pathways of cytosolic Ca2+ increases

Oxygen-sensing and reactivity to changes in the concentration of oxygen is a fundamental property of cellular physiology. This central role is determined, mainly, by, to the fact that oxygen represents the final acceptor of electrons, derived from the normal cellular metabolism, at the end of the mitochondrial respiratory chain. Despite significant advances in molecular characterization of various oxygen-sensitive processes, the nature of the oxygen-sensor molecules and the mechanisms that link sensors to effects remains unclear. One such controversy is about the role and nature of reactive oxygen species (ROS) changes during hypoxia. Irrespective of the mechanisms of oxygen sensing, one of the constant early responses to hypoxia in almost all cell types is an increase in intracellular Ca2+ ([Ca2+]i). In many instances, this increase is mediated by the activation of various plasma membrane Ca2+ conductances. Some of these channels have specific Ca2+ permeability (e.g. voltage-operated Ca2+ channels), whereas others have non-specific cation conductances and are activated by a variety of ligands (ligand-operated channels). In the last decade, a large superfamily of channels with significant Ca2+ permeability has been progressively identified and characterised: the TRP channels. Through their properties, some groups of the TRP channels provide a link to the other hypoxia-activated mechanism of [Ca2+]i increase: the release of Ca2+ from intracellular Ca2+ stores. Since the [Ca2+]i signals, depending on their localization and intensity, are important regulators of the subsequent cellular responses to hypoxia, a deeper understanding of the mechanisms through which hypoxia regulate the activity of these pathways that increase intracellular Ca2+ could point the way towards the development of new therapeutic approaches to reduce or suppress the pathological effects of cellular hypoxia, such as those seen in stroke or myocardial ischemia.

Endoplasmic reticulum calcium transport ATPase expression during differentiation of colon cancer and leukaemia cells

The calcium homeostasis of the endoplasmic reticulum (ER) is connected to a multitude of cell functions involved in intracellular signal transduction, control of proliferation, programmed cell death, or the synthesis of mature proteins. Calcium is accumulated in the ER by various biochemically distinct sarco/endoplasmic reticulum calcium transport ATPase isoenzymes (SERCA isoforms). Experimental data indicate that the SERCA composition of some carcinoma and leukaemia cell types undergoes significant changes during differentiation, and that this is accompanied by modifications of SERCA-dependent calcium accumulation in the ER. Because ER calcium homeostasis can also influence cell differentiation, we propose that the modulation of the expression of various SERCA isoforms, and in particular, the induction of the expression of SERCA3-type proteins, is an integral part of the differentiation program of some cancer and leukaemia cell types. The SERCA content of the ER may constitute a new parameter by which the calcium homeostatic characteristics of the organelle are adjusted. The cross-talk between ER calcium homeostasis and cell differentiation may have some implications for the better understanding of the signalling defects involved in the acquisition and maintenance of the malignant phenotype.

Receptor biology and signal transduction in pancreatic acinar cells

PURPOSE OF REVIEW

Secretagogue receptors and their intracellular signaling pathways regulate pancreatic physiology and may be altered in pathophysiology. Therefore, understanding of the continued progress into their nature and function is relevant to both biology and disease.

RECENT FINDINGS

The major secretagogue receptors on acinar cells include those binding cholecystokinin and acetylcholine, whereas secretin receptors regulate duct cells. Two physical models of the cholecystokinin receptor and ligand binding have been proposed through extensive structure-activity studies. Receptor oligomerization has been described for both cholecystokinin and secretin receptors. Ca plays a central role in the control of digestive enzyme secretion and is largely mobilized from intracellular stores. Inositol trisphosphate has been joined by two other Ca-releasing messengers, cyclic ADP ribose and nicotinic acid adenine dinucleotide phosphate, in initiating and coordinating Ca signaling. Progress has also been made in determining the roles of specific organelles in Ca release. Ca triggers secretion, and knowledge of the function and regulation of the proteins involved in exocytosis is accumulating. Continuing advances have also been made in understanding the signaling pathways regulating protein synthesis and growth in adult pancreas. The protein kinase mammalian target of rapamycin and its downstream targets play a central role in protein synthesis, whereas the protein phosphatase calcineurin was recently reported to regulate pancreatic growth. Other signaling molecules include the MAP kinases, PKCs, cytoplasmic tyrosine kinases, and nitric oxide.

SUMMARY

The current findings reviewed here are illuminating the structure and function of receptors on pancreatic acinar and duct cells and the multiple intracellular signaling pathways that they initiate. Understanding of these mechanisms is contributing to knowledge of normal pancreatic functions and alterations in disease such as pancreatitis and pancreatic cancer.

The extracellular calcium-sensing receptor and cell-cell signaling in epithelia

In multicellular organisms, cells are crowded together in organized communities, surrounded by an interstitial fluid of extremely limited volume. Local communication between adjacent cells is known to occur through gap junctions in cells that are physically connected, or through the release of paracrine signaling molecules (e.g. ATP, glutamate, nitric oxide) that diffuse to their target receptors through the extracellular microenvironment. Recent evidence hints that calcium ions may possibly be added to the list of paracrine messengers that allow cells to communicate with one another. Local fluctuations in extracellular [Ca2+] can be generated as a consequence of intracellular Ca2+ signaling events, owing to the activation of Ca2+ influx and efflux pathways at the plasma membrane. In intact tissues, where the interstitial volumes between cells are much smaller than the cells themselves, this can result in significant alterations in external [Ca2+]. This article will explore emerging evidence that these extracellular [Ca2+] changes can be detected by the extracellular calcium-sensing receptor (CaR) on adjacent cells, forming the basis for a paracrine signaling system. Such a mechanism could potentially provide CaR-expressing cells with the means to sense the Ca2+ signaling status of their neighbors, and expand the utility of the intracellular Ca2+ signal to a domain outside the cell.

Morphological and functional changes of dissociated single pancreatic acinar cells: testing the suitability of the single cell as a model for exocytosis and calcium signaling

Isolated single pancreatic acinar cells have long been used as a model for studying many kinds of signaling processes due to their structural and functional polarities, but without significant validation. In this study, we examined the morphological and functional changes of dissociated single pancreatic acinar cells. Acutely isolated single cells showed a collapsed membrane potential and a much reduced secretion of zymogen granules in response to acetylcholine (ACh) stimulation, whereas clustered cells showed a much more negative membrane potential and potent exocytotic secretion. The isolated single cells became vertically flattened due to the loss of supporting adhesions with nearby cells, and the granule-attached luminal membrane was severely reduced versus that of clustered cells. However, polarized Ca(2+) signals and mitochondrial localizations were relatively well preserved in the isolated single cells, in that Ca(2+) release by ACh commenced at the indented luminal membrane. In clusters, the Ca(2+) release site was closest to the lumen where more than three cells met or at the tips of conical regions of the luminal membrane. These findings suggest that the dissociated single pancreatic acinar cells preserve an intact Ca(2+) signaling machinery but alter in shape and have impaired exocytotic functions and resting membrane potentials.