Functional Mapping of Ca2+ Signaling Complexes in Plasma Membrane Microdomains of Polarized Cells

Xerostomia and Its Cellular Targets

Xerostomia, the subjective feeling of a dry mouth associated with dysfunction of the salivary glands, is mainly caused by radiation and chemotherapy, various systemic and autoimmune diseases, and drugs. As saliva plays numerous essential roles in oral and systemic health, xerostomia significantly reduces quality of life, but its prevalence is increasing. Salivation mainly depends on parasympathetic and sympathetic nerves, and the salivary glands responsible for this secretion move fluid unidirectionally through structural features such as the polarity of acinar cells. Saliva secretion is initiated by the binding of released neurotransmitters from nerves to specific G-protein-coupled receptors (GPCRs) on acinar cells. This signal induces two intracellular calcium (Ca2+) pathways (Ca2+ release from the endoplasmic reticulum and Ca2+ influx across the plasma membrane), and this increased intracellular Ca2+ concentration ([Ca2+]i) causes the translocation of the water channel aquaporin 5 (AQP5) to the apical membrane. Consequently, the GPCR-mediated increased [Ca2+]i in acinar cells promotes saliva secretion, and this saliva moves into the oral cavity through the ducts. In this review, we seek to elucidate the potential of GPCRs, the inositol 1,4,5-trisphosphate receptor (IP3R), store-operated Ca2+ entry (SOCE), and AQP5, which are essential for salivation, as cellular targets in the etiology of xerostomia.

The roles of calcium and ATP in the physiology and pathology of the exocrine pancreas

This review deals with the roles of calcium ions and ATP in the control of the normal functions of the different cell types in the exocrine pancreas as well as the roles of these molecules in the pathophysiology of acute pancreatitis. Repetitive rises in the local cytosolic calcium ion concentration in the apical part of the acinar cells not only activate exocytosis but also, via an increase in the intramitochondrial calcium ion concentration, stimulate the ATP formation that is needed to fuel the energy-requiring secretion process. However, intracellular calcium overload, resulting in a global sustained elevation of the cytosolic calcium ion concentration, has the opposite effect of decreasing mitochondrial ATP production, and this initiates processes that lead to necrosis. In the last few years it has become possible to image calcium signaling events simultaneously in acinar, stellate, and immune cells in intact lobules of the exocrine pancreas. This has disclosed processes by which these cells interact with each other, particularly in relation to the initiation and development of acute pancreatitis. By unraveling the molecular mechanisms underlying this disease, several promising therapeutic intervention sites have been identified. This provides hope that we may soon be able to effectively treat this often fatal disease.

Localization of phospholipase C β3 in the major salivary glands of adult mice

Phospholipase C (PLC)β has a role in saliva secretion by controlling intracellular Ca²⁺via its product, IP₃. The present study was attempted to localize PLCβ isoforms in mouse salivary glands in situ. A single major band was detected for PLCβ3 in immunoblots of the parotid and sublingual glands (PG, SLG), while no such band was seen in the submandibular gland (SMG). No bands were detected for PLCβ1 or 4 in the three glands. In immuno-light microscopy of PG and SLG, substantial immunoreactivity for PLCβ3 was seen in the cytoplasm including the plasmalemma of almost all ductal cells, while no distinct immunoreactivity was discerned in most acinar cells except for sublingual demilune cells. Numerous ductal cells exhibited higher immunoreactivity for PLCβ3 in their apical/supranuclear cell domain including the plasmalemma than in the basal/infranuclear domain, indicating an apico-basal polarity. In immuno-gold electron microscopy of PG ducts and SLG ducts and demilunes, most gold particles were found in association with plasma membranes as well as various intracellular membranes, most of which formed small oblong or flattened vesicles and vacuoles. A few particles were seen without association with any membranous structures. The present finding supports the previous physio-pharmacological result that Ca²⁺-signaling proteins as well as initial intracellular Ca²⁺ changes occur in the apical cell domain including the plasma membranes of the exocrine cells.

Heterogeneous localization of muscarinic cholinoceptor M1 in the salivary ducts of adult mice

We hypothesize variation in expression and localization, along the course of the glandular tubule, of muscarinic cholinergic receptor M₁ which plays as a distinct contribution, though minor in comparison with M₃ receptor, in saliva secretion. Localization of the M₁ receptor was examined using immunohistochemistry in three major salivary glands. Although all glandular cells were more or less M₁-immunoreactive, acinar cells were weakly immunoreactive, while ductal cells exhibited substantial M₁-immunoreactivity. Many ductal cells exhibited clear polarity with higher immunoreactivity in their apical/supra-nuclear domain. However, some exhibited indistinct polarity because of additional higher immunoreactivity in their basal/infra-nuclear domain. A small group of cells with intense immunoreactivity was found, mostly located in the intercalated ducts or in portions of the striated ducts close to the intercalated ducts. In immuno-electron microscopy, the immunoreactive materials were mainly in the cytoplasm including various vesicles and vacuoles. Unexpectedly, distinct immunoreactivity on apical and basal plasma membranes was infrequent in most ductal cells. The heterogeneous localization of M₁-immunoreactivity along the gland tubular system is discussed in view of possible modulatory roles of the M₁ receptor in saliva secretion.

CRAC channels in secretory epithelial cell function and disease

The receptor-evoked Ca²⁺ signal in secretory epithelia mediate many cellular functions essential for cell survival and their most fundamental functions of secretory granules exocytosis and fluid and electrolyte secretion. Ca²⁺ influx is a key component of the receptor-evoked Ca²⁺ signal in secretory cell and is mediated by both TRPC and the STIM1-activated Orai1 channels that mediates the Ca²⁺ release-activated current (CRAC) I_crac. The core components of the receptor-evoked Ca²⁺ signal are assembled at the ER/PM junctions where exchange of materials between the plasma membrane and internal organelles take place, including transfer of lipids and Ca²⁺. The Ca²⁺ signal generated at the confined space of the ER/PM junctions is necessary for activation of the Ca²⁺-regulated proteins and ion channels that mediate exocytosis with high fidelity and tight control. In this review we discuss the general properties of Ca²⁺ signaling, PI(4,5)P₂ and other lipids at the ER/PM junctions with regard to secretory cells function and disease caused by uncontrolled Ca²⁺ influx.

Orai1 and STIM1 in ER/PM junctions: roles in pancreatic cell function and dysfunction

Membrane contact sites (MCS) are critical junctions that form between the endoplasmic reticulum (ER) and membranes of various organelles, including the plasma membrane (PM). Signaling complexes, including mediators of Ca(2+) signaling, are assembled within MCS, such as the ER/PM junction. This is most evident in polarized epithelial cells, such as pancreatic cells. Core Ca(2+) signaling proteins cluster at the apical pole, the site of inositol 1,4,5-trisphosphate-mediated Ca(2+) release and Orai1/transient receptor potential canonical-mediated store-dependent Ca(2+) entry. Recent advances have characterized the proteins that tether the membranes at MCS and the role of these proteins in modulating physiological and pathological intracellular signaling. This review discusses recent advances in the characterization of Ca(2+) signaling at ER/PM junctions and the relation of these junctions to physiological and pathological Ca(2+) signaling in pancreatic acini.

The dynamic complexity of the TRPC1 channelosome

A rise in cytoplasmic [Ca2+] due to store-operated Ca2+ entry (SOCE) triggers a plethora of responses, both acute and long term. This leads to the important question of how this initial signal is decoded to regulate specific cellular functions. It is now clearly established that local [Ca2+] at the site of SOCE can vary significantly from the global [Ca2+] in the cytosol. Such Ca2+ microdomains are generated by the assembly of key Ca2+ signaling proteins within the domains. For example, GPCR, IP 3 receptors, TRPC3 channels, the plasma membrane Ca2+ pump and the endoplasmic reticulum (ER) Ca2+ pump have all been found to be assembled in a complex and all of them contribute to the Ca2+ signal. Recent studies have revealed that two other critical components of SOCE, STIM1 and Orai1, are also recruited to these regions. Thus, the entire machinery for activation and regulation of SOCE is compartmentalized in specific cellular domains which facilitates the specificity and rate of protein-protein interactions that are required for activation of the channels. In the case of TRPC1-SOC channels, it appears that specific lipid domains, lipid raft domains (LRDs), in the plasma membrane, as well as cholesterol-binding scaffolding proteins such as caveolin-1 (Cav-1), are involved in assembly of the TRPC channel complexes. Thus, plasma membrane proteins and lipid domains as well as ER proteins contribute to the SOCE-Ca2+ signaling microdomain and modulation of the Ca2+ signals per se. Of further interest is that modulation of Ca2+ signals, i.e. amplitude and/or frequency, can result in regulation of specific cellular functions. The emerging data reveal a dynamic Ca2+ signaling complex composed of TRPC1/Orai1/STIM1 that is physiologically consistent with the dynamic nature of the Ca2+ signal that is generated. This review will focus on the recent studies which demonstrate critical aspects of the TRPC1 channelosome that are involved in the regulation of TRPC1 function and TRPC1-SOC-generated Ca2+ signals.

Downstream from calcium signalling: mitochondria, vacuoles and pancreatic acinar cell damage

Ca(2+) is one of the most ancient and ubiquitous second messengers. Highly polarized pancreatic acinar cells serve as an important cellular model for studies of Ca(2+) signalling and homeostasis. Downstream effects of Ca(2+) signalling have been and continue to be an important research avenue. The primary functions regulated by Ca(2+) in pancreatic acinar cells--exocytotic secretion and fluid secretion--have been defined and extensively characterized in the second part of the last century. The role of cytosolic Ca(2+) in cellular pathology and the related question of the interplay between Ca(2+) signalling and bioenergetics are important current research lines in our and other laboratories. Recent findings in these interwoven research areas are discussed in the current review.

Signaling by G-protein-coupled receptor (GPCR): studies on the GnRH receptor

Gonadotropin-releasing hormone (GnRH) is the first key hormone of reproduction. GnRH analogs are extensively used in in vitro fertilization, and treatment of sex hormone-dependent cancers, due to their ability to bring about 'chemical castration'. The interaction of GnRH with its cognate type I receptor (GnRHR) in pituitary gonadotropes results in the activation of Gq/G(11), phospholipase Cbeta (PLCbetaI), PLA(2), and PLD. Sequential activation of the phospholipases generates the second messengers inositol 1, 4, 5-trisphosphate (IP(3)), diacylglycerol (DAG), and arachidonic acid (AA), which are required for Ca(2+) mobilization, the activation of various protein kinase C isoforms (PKCs), and the production of prostaglandin (PG) and other metabolites of AA, respectively. PKC isoforms are the major mediators of the downstream activation of a number of mitogen-activated protein kinase (MAPK) cascades by GnRH, namely: extracellular signal-regulated kinase (ERK), jun-N-terminal kinase (JNK), and p38MAPK. The activated MAPKs phosphorylate both cytosolic and nuclear proteins to initiate the transcriptional activation of the gonadotropin subunit genes and the GnRHR. While Ca(2+) mobilization has been found to initiate rapid gonadotropin secretion, Ca(2+), together with various PKC isoforms, MAPKs and AA metabolites also serve as key nodes, in the GnRH-stimulated signaling network that enables the gonadotropes to decode GnRH pulse frequencies and translating that into differential gonadotropin synthesis and release. Even though pulsatility of GnRH is recognized as a major determinant for differential gonadotropin subunit gene expression and gonadotropin secretion very little is yet known about the signaling circuits governing GnRH action at the 'Systems Biology' level. Direct apoptotic and metastatic effects of GnRH analogs in gonadal steroid-dependent cancers expressing the GnRHR also seem to be mediated by the activation of the PKC/MAPK pathways. However, the mechanisms dictating life (pituitary) vs. death (cancer) decisions made by the same GnRHR remain elusive. Understanding these molecular mechanisms triggered by the GnRHR through biochemical and 'Systems Biology' approaches would provide the basis for the construction of the dynamic connectivity maps, which operate in the various cell types (endocrine, cancer, and immune system) targeted by GnRH. The connectivity maps will open a new vista for exploring the direct effects of GnRH analogs in tumors and the design of novel combined therapies for fertility control, reproductive disorders and cancers.

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.

Synthesis, trafficking, and localization of muscarinic acetylcholine receptors

Muscarinic acetylcholine receptors are members of the G-protein coupled receptor superfamily that are expressed in and regulate the function of neurons, cardiac and smooth muscle, glands, and many other cell types and tissues. The correct trafficking of membrane proteins to the cell surface and their subsequent localization at appropriate sites in polarized cells are required for normal cellular signaling and physiological responses. This review will summarize work on the synthesis and trafficking of muscarinic receptors to the plasma membrane and their localization at the cell surface.

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.

Pancreatitis and calcium signalling: report of an international workshop

Pancreatitis and Calcium Signalling was an international research workshop organized by the authors and held at the Liverpool Medical Institution, Liverpool, United Kingdom, from Sunday 12th to Tuesday 14th November 2006. The overall goal of the workshop was to review progress and explore new opportunities for understanding the mechanisms of acute pancreatitis with an emphasis on the role of pathological calcium signaling. The participants included those with significant interest and expertise in pancreatitis research and others who are in fields outside gastroenterology but with significant expertise in areas of cell biology relevant to pancreatitis. The workshop was designed to enhance interchange of ideas and collaborations, to engage and encourage younger researchers in the field, and promote biomedical research through the participating and supporting organizations and societies. The workshop was divided into 8 topic-oriented sessions. The sessions were: (1) Physiology and pathophysiology of calcium signaling; (2) Interacting signaling mechanisms; (3) Premature digestive enzyme activation; (4) Physiology Society Lecture: Aberrant Ca2+ signaling, bicarbonate secretion, and pancreatitis; (5) NFkappaB, cytokines, and immune mechanisms; (6) Mitochondrial injury; (7) Cell death pathways; and (8) Overview of areas for future research. In each session, speakers presented work appropriate to the topic followed by discussion of the material presented by the group. The publication of these proceedings is intended to provide a platform for enhancing research and therapeutic development for acute pancreatitis.

Extracellular calcium as an integrator of tissue function

The past several decades of research into calcium signaling have focused on intracellular calcium (Ca(i)(2+)), revealing both exquisite spatial and dynamic control of this potent second messenger. Our understanding of Ca(i)(2+) signaling has benefited from the evolution of cell culture methods, development of high affinity fluorescent calcium indicators (both membrane-permeant small molecules and genetically encoded proteins), and high-resolution fluorescence microscopy. As our understanding of single cell calcium dynamics has increased, translational efforts have attempted to push calcium signaling studies back into tissues, organs and whole animals. Emerging results from these more complicated, diffusion-limited systems have begun to define a role for extracellular calcium (Ca(o)(2+)) as an agonist, spurred by the cloning and characterization of a G protein-coupled receptor activated by Ca(o)(2+) (the calcium sensing receptor, CaR). Here, we review the current state-of-the art for measurement of Ca(o)(2+) fluctuations, and the evidence that fluctuations in Ca(o)(2+) can act as primary signals regulating cell function. Current results suggest that Ca(o)(2+) in bone and epidermis may act as a chemotactic homing signal, targeting cells to the appropriate tissue locations prior to initiation of the differentiation program. Ca(i)(2+) signaling-mediated Ca(o)(2+) fluctuations in interstitial spaces may integrate cell signaling responses in multicellular networks through activation of CaR. Appreciation of the importance of Ca(o)(2+) fluctuations in coordinating cell function will likely spur identification of additional, niche-specific Ca(2+) sensors, and provide unique insights into the regulation of multicellular signaling networks.

Ca2+ release dynamics in parotid and pancreatic exocrine acinar cells evoked by spatially limited flash photolysis

Intracellular calcium concentration ([Ca(2+)](i)) signals are central to the mechanisms underlying fluid and protein secretion in pancreatic and parotid acinar cells. Calcium release was studied in natively buffered cells following focal laser photolysis of caged molecules. Focal photolysis of caged-inositol 1,4,5 trisphosphate (InsP(3)) in the apical region resulted in Ca(2+) release from the apical trigger zone and, after a latent period, the initiation of an apical-to-basal Ca(2+) wave. The latency was longer and the wave speed significantly slower in pancreatic compared with parotid cells. Focal photolysis in basal regions evoked only limited Ca(2+) release at the photolysis site and never resulted in a propagating wave. Instead, an apical-to-basal wave was initiated following a latent period. Again, the latent period was significantly longer under all conditions in pancreas than parotid. Although slower in pancreas than parotid, once initiated, the apical-to-basal wave speed was constant in a particular cell type. Photo release of caged-Ca(2+) failed to evoke a propagating Ca(2+) wave in either cell type. However, the kinetics of the Ca(2+) signal evoked following photolysis of caged-InsP(3) were significantly dampened by ryanodine in parotid but not pancreas, indicating a more prominent functional role for ryanodine receptor (RyR) following InsP(3) receptor (InsP(3)R) activation. These data suggest that differing expression levels of InsP(3)R, RyR, and possibly cellular buffering capacity may contribute to the fast kinetics of Ca(2+) signals in parotid compared with pancreas. These properties may represent a specialization of the cell type to effectively stimulate Ca(2+)-dependent effectors important for the differing primary physiological role of each gland.

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.

Spinophilin/neurabin reciprocally regulate signaling intensity by G protein-coupled receptors

Spinophilin (SPL) and neurabin (NRB) are structurally similar scaffolding proteins with several protein binding modules, including actin and PP1 binding motifs and PDZ and coiled-coil domains. SPL also binds regulators of G protein signaling (RGS) proteins and the third intracellular loop (3iL) of G protein-coupled receptors (GPCRs) to reduce the intensity of Ca(2+) signaling by GPCRs. The role of NRB in Ca(2+) signaling is not known. In the present work, we used biochemical and functional assays in model systems and in SPL(-/-) and NRB(-/-) mice to show that SPL and NRB reciprocally regulate Ca(2+) signaling by GPCRs. Thus, SPL and NRB bind all members of the R4 subfamily of RGS proteins tested (RGS1, RGS2, RGS4, RGS16) and GAIP. By contract, SPL, but not NRB, binds the 3iL of the GPCRs alpha(1B)-adrenergic (alpha(1B)AR), dopamine, CCKA, CCKB and the muscarinic M3 receptors. Coexpression of SPL or NRB with the alpha(1B)AR in Xenopus oocytes revealed that SPL reduces, whereas NRB increases, the intensity of Ca(2+) signaling by alpha(1B)AR. Accordingly, deletion of SPL in mice enhanced binding of RGS2 to NRB and Ca(2+) signaling by alphaAR, whereas deletion of NRB enhanced binding of RGS2 to SPL and reduced Ca(2+) signaling by alphaAR. This was due to reciprocal modulation by SPL and NRB of the potency of RGS2 to inhibit Ca(2+) signaling by alphaAR. These findings suggest a novel mechanism of regulation of GPCR-mediated Ca(2+) signaling in which SPL/NRB forms a functional pair of opposing regulators that modulates Ca(2+) signaling intensity by GPCRs by determining the extent of inhibition by the R4 family of RGS proteins.

Organization and function of TRPC channelosomes

TRPC proteins constitute a family of conserved Ca2+-permeable cation channels which are activated in response to agonist-stimulated PIP2 hydrolysis. These channels were initially proposed to be components of the store-operated calcium entry channel (SOC). Subsequent studies have provided substantial evidence that some TRPCs contribute to SOC activity. TRPC proteins have also been shown to form agonist-stimulated calcium entry channels that are not store-operated but are likely regulated by PIP2 or diacylglycerol. Further, and consistent with the presently available data, selective homomeric or heteromeric interactions between TRPC monomers generate distinct agonist-stimulated cation permeable channels. We suggest that interaction between TRPC monomers, as well as the association of these channels with accessory proteins, determines their mode of regulation as well as their cellular localization and function. Currently identified accessory proteins include key Ca2+ signaling proteins as well as proteins involved in vesicle trafficking, cytoskeletal interactions, and scaffolding. Studies reported until now demonstrate that TRPC proteins are segregated into specific Ca2+ signaling complexes which can generate spatially and temporally controlled [Ca2+]i signals. Thus, the functional organization of TRPC channelosomes dictates not only their regulation by extracellular stimuli but also serves as a platform to coordinate specific downstream cellular functions that are regulated as a consequence of Ca2+ entry. This review will focus on the accessory proteins of TRPC channels and discuss the functional implications of TRPC channelosomes and their assembly in microdomains.

Calcium signaling complexes in microdomains of polarized secretory cells

The highly polarized nature of epithelial cells in exocrine glands necessitates targeting, assembly into complexes and confinement of the molecules comprising the Ca(2+) signaling apparatus, to cellular microdomains. Such high degree of polarized localization has been shown for all Ca(2+) signaling molecules tested, including G protein coupled receptors and their associated proteins, Ca(2+) pumps, Ca(2+) influx channels at the plasma membrane and Ca(2+) release channels in the endoplasmic reticulum. Although the physiological significance of polarized Ca(2+) signaling is clear, little is known about the mechanism of targeting, assembly and retention of Ca(2+) signaling complexes in cellular microdomains. The present review attempts to summarize the evidence in favor of polarized expression of Ca(2+) signaling proteins at the apical pole of secretory cells with emphasis on the role of scaffolding proteins in the assembly and function of the Ca(2+) signaling complexes. The consequence of polarized enrichment of Ca(2+) signaling complexes at the apical pole is generation of an apical to basal pole gradient of cell responsiveness that, at low physiological agonist concentrations, limits Ca(2+) spikes to the apical pole, and when a Ca(2+) wave occurs, it always propagates from the apical to the basal pole. Our understanding of Ca(2+) signaling in microdomains is likely to increase rapidly with the application of techniques to controllably and selectively disrupt components of the complexes and apply high resolution recording techniques, such as TIRF microscopy to this problem.

Functional organization of TRPC-Ca2+ channels and regulation of calcium microdomains

TRP family of proteins are components of unique cation channels that are activated in response to diverse stimuli ranging from growth factor and neurotransmitter stimulation of plasma membrane receptors to a variety of chemical and sensory signals. This review will focus on members of the TRPC sub-family (TRPC1-TRPC7) which currently appear to be the strongest candidates for the enigmatic Ca(2+) influx channels that are activated in response to stimulation of plasma membrane receptors which result in phosphatidyl inositol-(4,5)-bisphosphate (PIP(2)) hydrolysis, generation of IP(3) and DAG, and IP(3)-induced Ca(2+) release from the intracellular Ca(2+) store via inositol trisphosphate receptor (IP(3)R). Homomeric or selective heteromeric interactions between TRPC monomers generate distinct channels that contribute to store-operated as well as store-independent Ca(2+) entry mechanisms. The former is regulated by the emptying/refilling of internal Ca(2+) store(s) while the latter depends on PIP(2) hydrolysis (due to changes in PIP(2) per se or an increase in diacylglycerol, DAG). Although the exact physiological function of TRPC channels and how they are regulated has not yet been conclusively established, it is clear that a variety of cellular functions are controlled by Ca(2+) entry via these channels. Thus, it is critical to understand how cells coordinate the regulation of diverse TRPC channels to elicit specific physiological functions. It is now well established that segregation of TRPC channels mediated by interactions with signaling and scaffolding proteins, determines their localization and regulation in functionally distinct cellular domains. Furthermore, both protein and lipid components of intracellular and plasma membranes contribute to the organization of these microdomains. Such organization serves as a platform for the generation of spatially and temporally dictated [Ca(2+)](i) signals which are critical for precise control of downstream cellular functions.

Regulation of pancreatic acinar cell function

PURPOSE OF REVIEW

Recent investigations into the regulation of pancreatic acinar cell function have led to a more detailed understanding of the mechanisms regulating digestive enzyme synthesis and secretion. This review identifies and puts into context those articles which further our understanding in this area.

RECENT FINDINGS

The secretagogue receptors present on acinar cells, especially muscarinic and cholecystokinin, have been better identified and characterized. The complex control of intracellular Ca by intracellular messengers such as inositol trisphosphate, cellular ion pumps and membrane channels has become more clearly understood, including the identification of organelles sequestering intracellular Ca. In the area of Ca driven exocytosis, progress has been made in understanding the proteins present on the zymogen granules, especially Rabs and SNARE proteins, and the dynamic changes in actin filaments. Secretagogues have also been shown to enhance the translation of new protein by activation of the mammalian target of rapamycin pathway. Finally, considerable progress has been made in understanding the mechanisms regulating pancreatic growth in response to nutrients and following pancreatectomy or pancreatitis.

SUMMARY

Understanding the mechanisms that regulate pancreatic acinar cell function is contributing to our knowledge of normal pancreatic function and alterations in diseases such as pancreatitis and pancreatic cancer.

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.

Ca2+ signaling microdomains:platforms for the assembly and regulation of TRPC channels

The transient receptor potential canonical family (TRPC1-TRPC7) of ion channel proteins, which are activated in response to agonist-stimulated phosphatidylinositol (4,5)-bisphosphate [PtdIns(4,5)P(2)] hydrolysis, are proposed components of the elusive store-operated Ca(2+) (SOC) channel. TRPC channels display distinct properties and interact to form homomeric or heteromeric channels that differ in their function and regulation. Although the exact function of TRPC channels and how they are regulated has not been established, increasing data suggest that they are localized and regulated within Ca(2+) signaling microdomains. TRPC channels contribute to store-operated and store-independent Ca(2+) entry mechanisms, both of which are activated by agonist-stimulated PtdIns(4,5)P(2) hydrolysis. Elucidation of how cells achieve specificity and precise temporal and spatial coordination of channel activation is crucial for understanding the molecular basis of agonist-mediated stimulation of Ca(2+) entry and identifying downstream physiological functions. This review will address the assembly and localization of TRPC channels and how these processes impact their function.

Spatial microenvironment defines Ca2+ entry and Ca2+ release in salivary gland cells

The difference of Ca(2+) mobilization induced by muscarinic receptor activation between parotid acinar and duct cells was examined. Oxotremorine, a muscarinic-cholinergic agonist, induced intracellular Ca(2+) release and extracellular Ca(2+) entry through store-operated Ca(2+) entry (SOC) and non-SOC channels in acinar cells, but it activated only Ca(2+) entry from non-SOC channels in duct cells. RT-PCR experiments showed that both types of cells expressed the same muscarinic receptor, M3. Given that ATP activated the intracellular Ca(2+) stores, the machinery for intracellular Ca(2+) release was intact in the duct cells. By immunocytochemical experiments, IP(3)R2 colocalized with M3 receptors in the plasma membrane area of acinar cells; in duct cells, IP(3)R2 resided in the region on the opposite side of the M3 receptors. On the other hand, purinergic P2Y2 receptors were found in the apical area of duct cells where they colocalized with IP(3)R2. These results suggest that the expression of the IP(3)Rs near G-protein-coupled receptors is necessary for the activation of intracellular Ca(2+) stores. Therefore, the microenvironment probably affects intracellular Ca(2+) release and Ca(2+) entry.

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.

Aberrant localization of intracellular organelles, Ca2+ signaling, and exocytosis in Mist1 null mice

Ca2+ signaling and exocytosis are highly polarized functions of pancreatic acinar cells. The role of cellular architecture in these activities and the capacity of animals to tolerate aberrant acinar cell function are not known. A key regulator of acinar cell polarity is Mist1, a basic helix-loop-helix transcription factor. Ca2+ signaling and amylase release were examined in pancreatic acini of wild type and Mist1 null mice to gain insight into the importance of cellular architecture for Ca2+ signaling and regulated exocytosis. Mist1-/- acinar cells exhibited dramatically altered Ca2+ signaling with up-regulation of the cholecystokinin receptor but minimal effect upon expression of the M3 receptor. However, stimulation of inositol 1,4,5-trisphosphate production by cholecystokinin and carbachol was inefficient in Mist1-/- cells. Although agonist stimulation of Mist1-/- cells evoked a Ca2+ signal, often the Ca2+ increase was not in the form of typical Ca2+ oscillations but rather in the form of a peak/plateau-type response. Mist1-/- cells also displayed distorted apical-to-basal Ca2+ waves. The aberrant Ca2+ signaling was associated with mislocalization and reduced Ca2+ uptake by the mitochondria of stimulated Mist1-/- cells. Deletion of Mist1 also led to mislocalization of the Golgi apparatus and markedly reduced digestive enzyme content. The combination of aberrant Ca2+ signaling and reduced digestive enzyme content resulted in poor secretion of digestive enzymes. Yet, food consumption and growth of Mist1-/- mice were normal for at least 32 weeks. These findings reveal that Mist1 is critical to normal organelle localization in exocrine cells and highlight the critical importance of maintaining cellular architecture and polarized localization of cellular organelles in generating a propagating apical-to-basal Ca2+ wave. The studies also reveal the spare capacity of the exocrine pancreas that allows normal growth and development in the face of compromised exocrine pancreatic function.