Coupling of the inositol 1,4,5-trisphosphate receptor and chromogranins A and B in secretory granules

New insights into the role of acidocalcisomes in trypanosomatids

Acidocalcisomes are electron-dense organelles rich in polyphosphate and inorganic and organic cations that are acidified by proton pumps, and possess several channels, pumps, and transporters. They are present in bacteria and eukaryotes and have been studied in greater detail in trypanosomatids. Biogenesis studies of trypanosomatid acidocalcisomes found that they share properties with lysosome-related organelles of animal cells. In addition to their described roles in autophagy, cation and phosphorus storage, osmoregulation, pH homeostasis, and pathogenesis, recent studies have defined the role of these organelles in phosphate utilization, calcium ion (Ca2+ ) signaling, and bioenergetics, and will be the main subject of this review.

Inside the Insulin Secretory Granule

The pancreatic β-cell is purpose-built for the production and secretion of insulin, the only hormone that can remove glucose from the bloodstream. Insulin is kept inside miniature membrane-bound storage compartments known as secretory granules (SGs), and these specialized organelles can readily fuse with the plasma membrane upon cellular stimulation to release insulin. Insulin is synthesized in the endoplasmic reticulum (ER) as a biologically inactive precursor, proinsulin, along with several other proteins that will also become members of the insulin SG. Their coordinated synthesis enables synchronized transit through the ER and Golgi apparatus for congregation at the trans-Golgi network, the initiating site of SG biogenesis. Here, proinsulin and its constituents enter the SG where conditions are optimized for proinsulin processing into insulin and subsequent insulin storage. A healthy β-cell is continually generating SGs to supply insulin in vast excess to what is secreted. Conversely, in type 2 diabetes (T2D), the inability of failing β-cells to secrete may be due to the limited biosynthesis of new insulin. Factors that drive the formation and maturation of SGs and thus the production of insulin are therefore critical for systemic glucose control. Here, we detail the formative hours of the insulin SG from the luminal perspective. We do this by mapping the journey of individual members of the SG as they contribute to its genesis.

Chromogranin A in the early steps of the neurosecretory pathway

Chromogranin A (CgA) is a soluble glycoprotein stored with hormones and neuropeptides in secretory granules (SG) of most (neuro)endocrine cells and neurons. Since its discovery in 1967, many studies have reported its structural characteristics, biological roles, and mechanisms of action. Indeed, CgA is both a precursor of various biologically active peptides and a granulogenic protein regulating the storage and secretion of hormones and neuropeptides. This review emphasizes the findings and theoretical concepts around the CgA-linked molecular machinery controlling hormone/neuropeptide aggregation and the interaction of CgA-hormone/neuropeptide aggregates with the trans-Golgi membrane to allow hormone/neuropeptide targeting and SG biogenesis. We will also discuss the intriguing alteration of CgA expression and secretion in various neurological disorders, which could provide insights to elucidate the molecular mechanisms underlying these pathophysiological conditions.

The acidocalcisome inositol-1,4,5-trisphosphate receptor of Trypanosoma brucei is stimulated by luminal polyphosphate hydrolysis products

Acidocalcisomes are acidic calcium stores rich in polyphosphate (polyP) and are present in trypanosomes and also in a diverse range of other organisms. Ca²⁺ is released from these organelles through a channel, inositol 1,4,5-trisphosphate receptor (TbIP₃R), which is essential for growth and infectivity of the parasite Trypanosoma brucei However, the mechanism by which TbIP₃R controls Ca²⁺ release is unclear. In this work, we expressed TbIP₃R in a chicken B lymphocyte cell line in which the genes for all three vertebrate IP₃Rs were stably ablated (DT40-3KO). We show that IP₃-mediated Ca²⁺ release depends on Ca²⁺ but not on ATP concentration and is inhibited by heparin, caffeine, and 2-aminomethoxydiphenyl borate (2-APB). Excised patch clamp recordings from nuclear membranes of DT40 cells expressing only TbIP₃R disclosed that luminal inorganic orthophosphate (P_i) or pyrophosphate (PP_i), and neutral or alkaline pH can stimulate IP₃-generated currents. In contrast, polyP or acidic pH did not induce these currents, and nuclear membranes obtained from cells expressing rat IP₃R were unresponsive to polyP or its hydrolysis products. Our results are consistent with the notion that polyP hydrolysis products within acidocalcisomes or alkalinization of their luminal pH activate TbIP₃R and Ca²⁺ release. We conclude that TbIP₃R is well-adapted to its role as the major Ca²⁺ release channel of acidocalcisomes in T. brucei.

Localization and projected role of phosphatidylinositol 4-kinases IIα and IIβ in inositol 1,4,5-trisphosphate-sensitive nucleoplasmic Ca²⁺ store vesicles

Phosphatidylinositol (PI) kinases are key molecules that participate in the phosphoinositide signaling in the cytoplasm. Despite the accumulating evidence that supports the existence and operation of independent PI signaling system in the nucleus, the exact location of the PI kinases inside the nucleus is not well defined. Here we show that PI4-kinases IIα and IIβ, which play central roles in PI(4,5)P2 synthesis and PI signaling, are localized in numerous small nucleoplasmic vesicles that function as inositol 1,4,5-trisphosphate (Ins(1,4,5)P3)-sensitive Ca(2+) stores. This is in accord with the past results that showed the localization of PI4(P)5-kinases that are essential in PI(4,5)P2 production and PI(4,5)P2 in nuclear matrix. Along with PI(4,5)P2 that also exists on the nucleoplasmic vesicle membranes, the localization of PI4-kinases IIα and IIβ in the nucleoplasmic vesicles strongly implicates the vesicles to the PI signaling as well as the Ins(1,4,5)P3-depenent Ca(2+) signaling in the nucleus. Accordingly, the nucleoplasmic vesicles indeed release Ca(2+) rapidly in response to Ins(1,4,5)P3. Further, the Ins(1,4,5)P3-induced Ca(2+) release studies suggest that PI4KIIα and IIβ are localized near the Ins(1,4,5)P3 receptor (Ins(1,4,5)P3R)/Ca(2+) channels on the Ca(2+) store vesicle membranes. In view of the widespread presence of the Ins(1,4,5)P3-dependent Ca(2+) store vesicles and the need to fine-control the nuclear Ca(2+) concentrations at multiple sites along the chromatin fibers in the nucleus, the existence of the key PI enzymes in the Ins(1,4,5)P3-dependent nucleoplasmic Ca(2+) store vesicles appears to be in perfect harmony with the physiological roles of the PI kinases in the nucleus.

C-terminal domain of chromogranin B regulates intracellular calcium signaling

The versatility of intracellular calcium as a second messenger is seen in its ability to mediate opposing events such as neuronal cell growth and apoptosis. A leading hypothesis used to explain how calcium regulates such divergent signaling pathways is that molecules responsible for maintaining calcium homeostasis have multiple roles. For example, chromogranin B (CGB), a calcium binding protein found in secretory granules and in the lumen of the endoplasmic reticulum, buffers calcium and also binds to and amplifies the activity of the inositol 1,4,5-trisphosphate receptor (InsP(3)R). Previous studies have identified two conserved domains of CGB, an N-terminal domain (N-CGB) and a C-terminal domain (C-CGB). N-CGB binds to the third intraluminal loop of the InsP(3)R and inhibits binding of full-length CGB. This displacement of CGB decreases InsP(3)R-dependent calcium release and alters normal signaling patterns. In the present study, we further characterized the role of N-CGB and identified roles for C-CGB. The effect of N-CGB on calcium release depended upon endogenous levels of cellular CGB, whereas the regulatory effect of C-CGB was apparent regardless of endogenous levels of CGB. When either full-length CGB or C-CGB was expressed in cells, calcium transients were increased. Additionally, the calcium signal initiation site was altered upon C-CGB expression in neuronally differentiated PC12 and SHSY5Y cells. These results show that CGB has numerous regulatory roles and that CGB is a critical component in modulating InsP(3)R-dependent calcium signaling.

Role of secretory granules in inositol 1,4,5-trisphosphate-dependent Ca(2+) signaling: from phytoplankton to mammals

The majority of secretory cell calcium is stored in secretory granules that serve as the major IP(3)-dependent intracellular Ca(2+) store. Even in unicellular phytoplankton secretory granules are responsible for the IP(3)-induced Ca(2+) release that triggers exocytosis. The number of secretory granules in the cell is directly related not only to the magnitude of IP(3)-induced Ca(2+) release, which accounts for the majority of the IP(3)-induced cytoplasmic Ca(2+) release in neuroendocrine cells, but also to the IP(3) sensitivity of the cytoplasmic IP(3) receptor (IP(3)R)/Ca(2+) channels. Moreover, secretory granules contain the highest IP(3)R concentrations and the largest amounts of IP(3)Rs in any subcellular organelles in neuroendocrine cells. Secretory granules from phytoplankton to mammals contain large amounts of polyanionic molecules, chromogranins being the major molecules in mammals, in addition to acidic intragranular pH and high Ca(2+) concentrations. The polyanionic molecules undergo pH- and Ca(2+)-dependent conformational changes that serve as a molecular basis for condensation-decondensation phase transitions of the intragranular matrix. Likewise, chromogranins undergo pH- and Ca(2+)-dependent conformational changes with increased exposure of the structure and increased interactions with Ca(2+) and other granule components at acidic pH. The unique physico-chemical properties of polyanionic molecules appear to be at the center of biogenesis, and physiological functions of secretory granules in living organisms from primitive to advanced species.

Inositol 1,4,5-trisphosphate receptor in chromaffin secretory granules and its relation to chromogranins

The inositol 1,4,5-trisphosphate (IP(3))-mediated intracellular Ca(2+) releases in secretory cells play vital roles in controlling not only the intracellular Ca(2+) concentrations but also the Ca(2+)-dependent exocytotic processes. Of intracellular organelles that release Ca(2+) in response to IP(3), secretory granules stand out as the most prominent organelle and are responsible for the majority of IP(3)-dependent Ca(2+) releases in the cytoplasm of chromaffin cells. Bovine chromaffin granules were the first granules that demonstrated the IP(3)-mediated Ca(2+) release as well as the presence of the IP(3) receptor (IP(3)R) in granule membranes. Secretory granules contain all three (type 1, 2, and 3) IP(3)R isoforms, and 58-69% of total cellular IP(3)R isoforms are expressed in bovine chromaffin granules. Moreover, secretory granules contain large amounts (2-4 mM) of chromogranins and secretogranins; chromogranins A and B, and secretogranin II being the major species. Chromogranins A and B, and secretogranin II are high-capacity, low-affinity Ca(2+) binding proteins, binding 30-93 mol of Ca(2+)/mol of protein with dissociation constants of 1.5-4.0 mM. Due to this high Ca(2+) storage properties of chromogranins secretory granules contain ~40 mM Ca(2+). Furthermore, chromogranins A and B directly interact with the IP(3)Rs and modulate the IP(3)R/Ca(2+) channels, i.e., increasing the open probability and the mean open time of the channels 8- to 16-fold and 9- to 42-fold, respectively. Coupled chromogranins change the IP(3)R/Ca(2+) channels to a more ordered, release-ready state, whereby making the IP(3)R/Ca(2+) channels significantly more sensitive to IP(3).

Evidence for the existence of secretory granule (dense-core vesicle)-based inositol 1,4,5-trisphosphate-dependent Ca2+ signaling system in astrocytes

BACKGROUND

The gliotransmitters released from astrocytes are deemed to play key roles in the glial cell-neuron communication for normal function of the brain. The gliotransmitters, such as glutamate, ATP, D-serine, neuropeptide Y, are stored in vesicles of astrocytes and secreted following the inositol 1,4,5-trisphosphate (IP3)-induced intracellular Ca2+ releases. Yet studies on the identity of the IP3-dependent intracellular Ca2+ stores remain virtually unexplored.

PRINCIPAL FINDINGS

We have therefore studied the potential existence of the IP3-sensitive intracellular Ca2+ stores in the cytoplasm of astrocytes using human brain tissue samples in contrast to cultured astrocytes that had primarily been used in the past. It was thus found that secretory granule marker proteins chromogranins and secretogranin II localize in the large dense core vesicles of astrocytes, thereby confirming the large dense core vesicles as bona fide secretory granules. Moreover, consistent with the major IP3-dependent intracellular Ca2+ store role of secretory granules in secretory cells, secretory granules of astrocytes also contained all three (types 1, 2, and 3) IP3R isoforms.

SIGNIFICANCE

Given that the secretory granule marker proteins chromogranins and secretogranin II are high-capacity, low-affinity Ca2+ storage proteins and chromogranins interact with the IP3Rs to activate the IP3R/Ca2+ channels, i.e., increase both the mean open time and the open probability of the channels, these results imply that secretory granules of astrocytes function as the IP3-sensitive intracellular Ca2+ store.

Secretory granules in inositol 1,4,5-trisphosphate-dependent Ca2+ signaling in the cytoplasm of neuroendocrine cells

Of all the intracellular organelles, secretory granules contain by far the highest calcium concentration; secretory granules of typical neuroendocrine chromaffin cells contain approximately 40 mM Ca(2+) and occupy approximately 20% cell volume, accounting for >60% of total cellular calcium. They also contain the majority of cellular inositol 1,4,5-trisphosphate receptors (IP(3)Rs) in addition to the presence of >2 mM of chromogranins A and B that function as high-capacity, low-affinity Ca(2+) storage proteins. Chromogranins A and B also interact with the IP(3)Rs and activate the IP(3)R/Ca(2+) channels. In experiments with both neuroendocrine PC12 and nonneuroendocrine NIH3T3 cells, in which the number of secretory granules present was changed by either suppression or induction of secretory granule formation, secretory granules were demonstrated to account for >70% of the IP(3)-induced Ca(2+) releases in the cytoplasm. Moreover, the IP(3) sensitivity of secretory granule IP(3)R/Ca(2+) channels is at least approximately 6- to 7-fold more sensitive than those of the endoplasmic reticulum, thus enabling secretory granules to release Ca(2+) ahead of the endoplasmic reticulum. Further, there is a direct correlation between the number of secretory granules and the IP(3) sensitivity of cytoplasmic IP(3)R/Ca(2+) channels and the increased ratio of IP(3)-induced cytoplasmic Ca(2+) release, highlighting the importance of secretory granules in the IP(3)-dependent Ca(2+) signaling. Given that secretory granules are present in all secretory cells, these results presage critical roles of secretory granules in the control of cytoplasmic Ca(2+) concentrations in other secretory cells.-Yoo, S. H. Secretory granules in inositol 1,4,5-trisphosphate-dependent Ca(2+) signaling in the cytoplasm of neuroendocrine cells.

A large form of secretogranin III functions as a sorting receptor for chromogranin A aggregates in PC12 cells

Granin-family proteins, including chromogranin A and secretogranin III, are sorted to the secretory granules in neuroendocrine cells. We previously demonstrated that secretogranin III binds chromogranin A and targets it to the secretory granules in pituitary corticotrope-derived AtT-20 cells. However, secretogranin III has not been identified in adrenal chromaffin and PC12 cells, where chromogranin A is correctly sorted to the secretory granules. In this study, low levels of a large and noncleaved secretogranin III have been identified in PC12 cells and rat adrenal glands. Although the secretogranin III expression was limited in PC12 cells, when the FLAG-tagged secretogranin III lacking the secretory granule membrane-binding domain was expressed excessively, hemagglutinin-tagged chromogranin A was unable to target to the secretory granules at the tips and shifted to the constitutive secretory pathway. Secretogranin III was able to bind the aggregated form of chromogranin A, suggesting that a small quantity of secretogranin III is enough to carry a large quantity of chromogranin A. Furthermore, secretogranin III bound adrenomedullin, a major peptide hormone in chromaffin cells. Indeed, small interfering RNA-directed secretogranin III depletion impaired intracellular retention of chromogranin A and adrenomedullin, suggesting that they are constitutively released to the medium. We suggest that the sorting function of secretogranin III for chromogranin A is common in PC12 and chromaffin cells as well as in other endocrine cells, and a small amount of secretogranin III is able to sort chromogranin A aggregates together with adrenomedullin to secretory granules.

Sorting of the neuroendocrine secretory protein Secretogranin II into the regulated secretory pathway: role of N- and C-terminal alpha-helical domains

Secretogranin II (SgII) belongs to the granin family of prohormones widely distributed in dense-core secretory granules (DCGs) of endocrine, neuroendocrine, and neuronal cells, including sympathoadrenal chromaffin cells. The mechanisms by which secretory proteins, and granins in particular, are sorted into the regulated secretory pathway are unsettled. We designed a strategy based on novel chimeric forms of human SgII fused to fluorescent (green fluorescent protein) or chemiluminescent (embryonic alkaline phosphatase) reporters to identify trafficking determinants mediating DCG targeting of SgII in sympathoadrenal cells. Three-dimensional deconvolution fluorescence microscopy and secretagogue-stimulated release studies demonstrate that SgII chimeras are correctly targeted to DCGs and released by exocytosis in PC12 and primary chromaffin cells. Results from a Golgi-retained mutant form of SgII suggest that sorting of SgII into DCGs depends on a saturable sorting machinery at the trans-Golgi/trans-Golgi network. Truncation analyses reveal the presence of DCG-targeting signals within both the N- and C-terminal regions of SgII, with the putative alpha-helix-containing SgII-(25-41) and SgII-(334-348) acting as sufficient, independent sorting domains. This study defines sequence features of SgII mediating vesicular targeting in sympathoadrenal cells and suggests a mechanism by which discrete domains of the molecule function in sorting, perhaps by virtue of a particular arrangement in tertiary structure and/or interaction with a specific component of the DCG membrane.

Chromogranins A and B and secretogranin II: evolutionary and functional aspects

Chromogranins/secretogranins or granins are a class of acidic, secretory proteins that occur in endocrine, neuroendocrine, and neuronal cells. Granins are the precursors of several bioactive peptides and may be involved in secretory granule formation and neurotransmitter/hormone release. Characterization and analysis of chromogranin A (CgA), chromogranin B (CgB), and secretogranin II (SgII) in distant vertebrate species confirmed that CgA and CgB belong to related monophyletic groups, probably evolving from a common ancestral precursor, while SgII sequences constitute a distinct monophyletic group. In particular, selective sequences within these proteins, bounded by potential processing sites, have been remarkably conserved during evolution. Peptides named vasostatin, secretolytin and secretoneurin, which occur in these regions, have been shown to exert various biological activities. These conserved domains may also be involved in the formation of secretory granules in different vertebrates. Other peptides such as catestatin and pancreastatin may have appeared late during evolution. The function of granins as propeptide precursors and granulogenic factors is discussed in the light of recent data obtained in various model species and using knockout mice strains.

IP3 receptor/Ca2+ channel: from discovery to new signaling concepts

Inositol 1,4,5-trisphosphate (IP(3)) is a second messenger that induces the release of Ca(2+) from the endoplasmic reticulum (ER). The IP(3) receptor (IP(3)R) was discovered as a developmentally regulated glyco-phosphoprotein, P400, that was missing in strains of mutant mice. IP(3)R can allosterically and dynamically change its form in a reversible manner. The crystal structures of the IP(3)-binding core and N-terminal suppressor sequence of IP(3)R have been identified. An IP(3) indicator (known as IP(3)R-based IP(3) sensor) was developed from the IP(3)-binding core. The IP(3)-binding core's affinity to IP(3) is very similar among the three isoforms of IP(3)R; instead, the N-terminal IP(3) binding suppressor region is responsible for isoform-specific IP(3)-binding affinity tuning. Various pathways for the trafficking of IP(3)R have been identified; for example, the ER forms a meshwork upon which IP(3)R moves by lateral diffusion, and vesicular ER subcompartments containing IP(3)R move rapidly along microtubles using a kinesin motor. Furthermore, IP(3)R mRNA within mRNA granules also moves along microtubules. IP(3)Rs are involved in exocrine secretion. ERp44 works as a redox sensor in the ER and regulates IP(3)R1 activity. IP(3) has been found to release Ca(2+), but it also releases IRBIT (IP(3)R-binding protein released with IP(3)). IRBIT is a pseudo-ligand for IP(3) that regulates the frequency and amplitude of Ca(2+) oscillations through IP(3)R. IRBIT binds to pancreas-type Na, bicarbonate co-transporter 1, which is important for acid-base balance. The presence of many kinds of binding partners, like homer, protein 4.1N, huntingtin-associated protein-1A, protein phosphatases (PPI and PP2A), RACK1, ankyrin, chromogranin, carbonic anhydrase-related protein, IRBIT, Na,K-ATPase, and ERp44, suggest that IP(3)Rs form a macro signal complex and function as a center for signaling cascades. The structure of IP(3)R1, as revealed by cryoelectron microscopy, fits closely with these molecules.

Inositol trisphosphate receptor Ca2+ release channels

The inositol 1,4,5-trisphosphate (InsP3) receptors (InsP3Rs) are a family of Ca2+ release channels localized predominately in the endoplasmic reticulum of all cell types. They function to release Ca2+ into the cytoplasm in response to InsP3 produced by diverse stimuli, generating complex local and global Ca2+ signals that regulate numerous cell physiological processes ranging from gene transcription to secretion to learning and memory. The InsP3R is a calcium-selective cation channel whose gating is regulated not only by InsP3, but by other ligands as well, in particular cytoplasmic Ca2+. Over the last decade, detailed quantitative studies of InsP3R channel function and its regulation by ligands and interacting proteins have provided new insights into a remarkable richness of channel regulation and of the structural aspects that underlie signal transduction and permeation. Here, we focus on these developments and review and synthesize the literature regarding the structure and single-channel properties of the InsP3R.

Identification of a novel targeting sequence for regulated secretion in the serine protease inhibitor neuroserpin

Ns (neuroserpin) is a member of the serpin (serine protease inhibitor) gene family that is primarily expressed within the central nervous system. Its principal target protease is tPA (tissue plasminogen activator), which is thought to contribute to synaptic plasticity and to be secreted in a stimulus-dependent manner. In the present study, we demonstrate in primary neuronal cultures that Ns co-localizes in LDCVs (large dense core vesicles) with the regulated secretory protein chromogranin B. We also show that Ns secretion is regulated and can be specifically induced 4-fold by secretagogue treatment. A novel 13-amino-acid sorting signal located at the C-terminus of Ns is identified that is both necessary and sufficient to target Ns to the regulated secretion pathway. Its deletion renders Ns no longer responsive to secretagogue stimulation, whereas PAI-Ns [Ns (neuroserpin)-PAI-1 (plasminogen activator inhibitor-1) chimaera appending the last 13 residues of Ns sequence to the C-terminus of PAI-1] shifts PAI-1 secretion into a regulated secretory pathway.

The inositol 1,4,5-trisphosphate receptor (IP3R) and its regulators: sometimes good and sometimes bad teamwork

In both nonexcitable and excitable cells, the inositol 1,4,5-trisphosphate receptor (IP(3)R) is the primary cytosolic target responsible for the initiation of intracellular calcium (Ca(2+)) signaling. To fulfill this function, the IP(3)R depends on interaction with accessory subunits and regulatory proteins. These include proteins that reside in the lumen of the endoplasmic reticulum (ER), such as chromogranin A and B and ERp44, and cytosolic proteins, such as neuronal Ca(2+) sensor 1, huntingtin, cytochrome c, IP(3)R-binding protein released with inositol 1,4,5-trisphosphate, Homer, and 4.1N. Specific interactions between these modulatory proteins and the IP(3)R have been described, making it clear that the controlled modulation of the IP(3)R by its binding partners is necessary for physiological cell regulation. The functional coupling of these modulators with the IP(3)R can control apoptosis, intracellular pH, the initiation and regulation of neuronal Ca(2+) signaling, exocytosis, and gene expression. The pathophysiological relevance of IP(3)R modulation is apparent when the functional interaction of these proteins is enhanced or abolished by mutation or overexpression. The subsequent deregulation of the IP(3)R leads to pathological changes in Ca(2+) signaling, signal initiation, the amplitude and frequency of Ca(2+) signals, and the duration of the Ca(2+) elevation. Consequences of this deregulation include abnormal growth and apoptosis. Complex regulation of Ca(2+) signaling is required for the cell to live and function, and this difficult task can only be managed when the IP(3)R teams up and acts properly with its numerous binding partners.

Inositol 1,4,5-trisphosphate (IP3) receptors and their role in neuronal cell function

Inositol 1,4,5-trisphosphate (IP(3)) receptor is a Ca(2+) release channel localized on the endoplasmic reticulum (ER) and plays an important role in neuronal function. IP(3) receptor was discovered as a developmentally regulated protein missing in the cerebellar mutant mice. Recent studies indicate that IP(3)Rs are involved in early development and neuronal plasticity. IP(3) works to release IRBIT from the IP(3) binding core in addition to release Ca(2+). IRBIT binds to and activates Na, Bicarbonate cotransporter. Electron microscopic study show the IP(3) receptor has allosteric property to change its form from square to windmill in the presence of Ca(2+). IP(3)R associates with ERp44, a redox sensor, Homer, other proteins and is transported as vesicular ER on microtubules. All these data suggests IP(3) receptor/CA(2+) channel works as a signaling center inside cells.

Interaction between secretogranin III and carboxypeptidase E facilitates prohormone sorting within secretory granules

Secretogranin III (SgIII) and carboxypeptidase E (CPE) bind specifically to cholesterol-rich secretory granule (SG) membranes. We previously showed that SgIII binds chromogranin A (CgA) and targets CgA to the SGs in endocrine cells. We investigated the binding of SgIII and CPE because they frequently localize close to the periphery of SGs, and they bind each other in mouse corticotrope-derived AtT-20 cells. In Cpe fat mouse corticotropes, which have defective CPE, proopiomelanocortin (POMC)-derived adrenocorticotrophin hormone (ACTH)-containing peptides were distributed over the entire surface of the SGs, and displayed a regulated secretion by secretagogues. The Cpe fat pituitary exhibited elevated levels of SgIII and CgA, which suggests that they compensate for a sorting function of CPE for POMC and its intermediates to ACTH. Indeed, both SgIII and CgA were able to bind POMC-derived intermediates. In a competitive pull-down assay, excessive SgIII led to a decrease in CPE-bound POMC-derived intermediate molecules, and SgIII pulled-down by anti-ACTH antibody increased proportionately. We suggest that SgIII and CPE form the separate functional sorting complex by anchoring to cholesterol-rich SG membranes, and POMC-derived peptides are transferred from CPE to SgIII, and subsequently to CgA.

Subcellular distribution of chromogranins A and B in bovine adrenal chromaffin cells

The major secretory granule proteins chromogranins A (CGA) and B (CGB) have recently been shown to play critical roles in inositol 1,4,5-trisphosphate-dependent intracellular Ca(2+) mobilizations. We determined here the subcellular distribution of CGA and CGB based on 3D-images of chromaffin cells, and found that approximately 95% of cellular CGA was present in secretory granules while approximately 5% was in the endoplasmic reticulum (ER), whereas approximately 57% of cellular CGB was in secretory granules while approximately 24% and approximately 19% were in the ER and nucleus, respectively. These results suggest that chromogranins are at the center of intracellular Ca(2+) homeostasis in secretory cells.

Distribution profile of inositol 1,4,5-trisphosphate receptor isoforms in adrenal chromaffin cells

Given the importance of inositol 1,4,5-trisphosphate receptor (IP(3)R)/Ca(2+) channels in the control of intracellular Ca(2+) concentrations, we determined the relative concentrations of the IP(3)R isoforms in subcellular organelles, based on serially sectioned electron micrographs. The endoplasmic reticulum (ER) was estimated to contain 15-20% of each of the three IP(3)R isoforms while secretory granules contained 58-69%. The nucleus contained approximately 15% each of IP(3)R-1 and -2, but 25% of IP(3)R-3, whereas the plasma membrane contained approximately 1% or less of each. These suggested that secretory granules, the nucleus and ER are at the center of IP(3)-dependent intracellular Ca(2+) control mechanisms in chromaffin cells.

Role of H+-ATPase-mediated acidification in sorting and release of the regulated secretory protein chromogranin A: evidence for a vesiculogenic function

The constitutive and regulated secretory pathways represent the classical routes for secretion of proteins from neuroendocrine cells. Selective aggregation of secretory granule constituents in an acidic, bivalent cation-rich environment is considered to be a prerequisite for sorting to the regulated secretory pathway. The effect of selective vacuolar H+-ATPase (V-ATPase) inhibitor bafilomycin A1 on the pH gradient along the secretory pathway was used here to study the role of acidification on the trafficking of the regulated secretory protein chromogranin A (CgA) in PC12 cells. Sorting of CgA was assessed by three-dimensional deconvolution microscopy, subcellular fractionation, and secretagogue-stimulated release, examining a series of full-length or truncated domains of human CgA (CgA-(1-115), CgA-(233-439)) fused to either green fluorescent protein or to a novel form of secreted embryonic alkaline phosphatase (EAP). We show that a full-length CgA/EAP chimera is sorted to chromaffin granules for exocytosis. Inhibition of V-ATPase by bafilomycin A1 markedly reduced the secretagogue-stimulated release of CgA-EAP by perturbing sorting of the chimera (at the trans-Golgi network or immature secretory granule) rather than the late steps of exocytosis. The effect of bafilomycin A1 on CgA secretion depends on a sorting determinant located within the amino terminus (CgA-(1-115)) but not the C-terminal region of the granin. Moreover, examination of chromaffin granule abundance in PC12 cells exposed to bafilomycin A1 reveals a substantial decrease in the number of dense-core vesicles. We propose that a V-ATPase-mediated pH gradient in the secretory pathway is an important factor for the formation of dense-core granules by regulating the ability of CgA to form aggregates, a crucial step that may underlie the granulogenic function of the protein.

Presence of the inositol 1,4,5-triphosphate receptor isoforms in the nucleoplasm

Although the inositol 1,4,5-triphosphate (IP(3))-induced nuclear Ca(2+) release has been shown to play key roles in nuclear functions, the presence of IP(3) receptor (IP(3)R)/Ca(2+) channels in the nucleoplasm has not been found. Recently, the IP(3)R/Ca(2+) channels were reported to exist in the nucleoplasmic reticulum structure, an extension of the nuclear envelope. Here we investigated the potential existence of the IP(3)Rs in the nucleoplasm and found the presence of all three IP(3)R isoforms in neuroendocrine and non-neuroendocrine cells. The IP(3)Rs were widely scattered in the nucleoplasm, localizing in both the heterochromatin and euchromatin regions.

Signal transduction, calcium and acute pancreatitis

Evidence consistently suggests that the earliest changes of acute pancreatitis are intracellular, the hallmark of which is premature intracellular activation of digestive zymogens, accompanied by disruption of normal signal transduction and secretion. Principal components of physiological signal transduction include secretagogue-induced activation of G-protein-linked receptors, followed by generation of inositol 1,4,5-trisphosphate, nicotinic acid adenine dinucleotide phosphate and cyclic ADP-ribose. In response, calcium is released from endoplasmic reticulum terminals within the apical, granular pole of the cell, where calcium signals are usually contained by perigranular mitochondria, in turn responding by increased metabolism. When all three intracellular messengers are administered together, even at threshold concentrations, dramatic potentiation results in sustained, global, cytosolic calcium elevation. Prolonged, global elevation of cytosolic calcium is also induced by hyperstimulation, bile salts, alcohol and fatty acid ethyl esters, and depends on continued calcium entry into the cell. Such abnormal calcium signals induce intracellular activation of digestive enzymes, and of nuclear factor kappaB, as well as the morphological changes of acute pancreatitis. Depletion of endoplasmic reticulum calcium and mitochondrial membrane potential may contribute to further cell injury. This review outlines current understanding of signal transduction in the pancreas, and its application to the pathophysiology of acute pancreatitis.

A Functional Interaction between Chromogranin B and the Inositol 1,4,5-Trisphosphate Receptor/Ca2+ Channel

Chromogranins A and B (CGA and CGB) are high capacity, low affinity calcium (Ca2+) storage proteins found in many cell types most often associated with secretory granules of secretory cells but also with the endoplasmic reticulum (ER) lumen of these cells. Both CGA and CGB associate with inositol 1,4,5-trisphosphate receptor (InsP3R) in a pH-dependent manner. At an intraluminal pH of 5.5, as found in secretory vesicles, both CGA and CGB bind to the InsP3R. When the intraluminal pH is 7.5, as found in the ER, CGA totally dissociates from InsP3R, whereas CGB only partially dissociates. To investigate the functional consequences of the interaction between the InsP3R and CGB monomers or CGA/CGB heteromers, purified mouse InsP3R type I were fused to planar lipid bilayers and activated by 2 microM InsP3. In the presence of luminal CGB monomers or CGA/CGB heteromers the InsP3R/Ca2+ channel open probability and mean open time increased significantly. The channel activity remained elevated when the pH was changed to 7.5, a reflection of CGB binding to the InsP3R even at pH 7.5. These results suggest that CGB may play an important modulatory role in the control of Ca2+ release from the ER. Furthermore, the difference in the ability of CGA and CGB to regulate the InsP3R/Ca2+ channel and the variability of CGA/CGB ratios could influence the pattern of InsP3-mediated Ca2+ release.

Chromogranin B-induced secretory granule biogenesis: comparison with the similar role of chromogranin A

The two major proteins of secretory granules of secretory cells, chromogranins A (CGA) and B (CGB), have previously been proposed to play key roles in secretory granule biogenesis. Recently, CGA was reported to play an on/off switch role for secretory granule biogenesis. In the present study we found CGB being more effective than CGA in inducing secretory granule formation in non-neuroendocrine NIH3T3 and COS-7 cells. The mean number of dense core granules formed/cell of CGA-transfected NIH3T3 cells was 2.51, whereas that of CGB-transfected cells was 4.02, indicating the formation of 60% more granules in the CGB-transfected cells. Similarly, there were 55% more dense core granules formed in the CGB-transfected COS-7 cells than in the CGA-transfected cells. Moreover, transfection of CGA- and CGB-short interfering RNA (siRNA) into neuroendocrine PC12 cells not only decreased the amount of CGA and CGB expressed but also reduced the number of secretory granules by 41 and 78%, respectively, further suggesting the importance of CGB expression in secretory granule formation.

Biochemical characterization and immunocytochemical localization of EM66, a novel peptide derived from secretogranin II, in the rat pituitary and adrenal glands

Characterization of secretogranin II (SgII) mRNA in various vertebrates has revealed selective conservation of the amino acid sequences of two regions of the protein, i.e., the bioactive peptide secretoneurin and a flanking novel peptide that we named EM66. To help elucidate the possible role of EM66, we examined the occurrence as well as the cellular and subcellular distribution of EM66 in rat pituitary and adrenal glands by using a polyclonal antibody raised against the recombinant human EM66 peptide. High-performance liquid chromatography (HPLC) analysis of rat pituitary and adrenal extracts combined with a radioimmunoassay resolved EM66-immunoreactive material exhibiting the same retention time as recombinant EM66. In the rat pituitary, double-labeling immunohistochemical (IHC) studies showed that EM66 immunoreactivity (IR) was present in gonadotrophs, lactotrophs, thyrotrophs, and melanotrophs, whereas corticotrophs were devoid of labeling. EM66-IR was also observed in nerve endings in the neural lobe. Immunocytochemical staining at the electron microscopic level revealed that EM66-IR is sequestered in the secretory granules within gonadotrophs and lactotrophs. In the adrenal medulla, double IHC labeling showed that EM66-IR occurs exclusively in epinephrine-synthesizing cells. At the ultrastructural level, EM66-IR was seen in chromaffin vesicles of adrenomedullary cells. These results demonstrate that post-translational processing of SgII generates a novel peptide that exhibits a cell-specific distribution in the rat pituitary and adrenal glands where it is stored in secretory granules, supporting the notion that EM66 may play a role in the endocrine system.

Colocalization of chromogranin A and inositol 1,4,5-trisphosphate receptor in ciliated cells of the bovine oviduct

Previous investigations of the expression of chromogranin A (CgA) have been performed primarily in neuroendocrine tissues containing amine and peptide secretory vesicles. More recently it has been shown that CgA, as a high capacity Ca2+ storage protein, interacts with the inositol 1,4,5-trisphosphate receptor/Ca2+ channel (InsP3R) which has been found to be selectively localized in oviductal cells of the mouse. To examine a possible role of this coupling in the Ca2+-dependent ciliary movement, we investigated the topographical and cellular distribution of cells positive for CgA and inositol 1,4,5-trisphosphate receptor type 2 (InsP3R2) in the bovine oviduct at different stages of the oestrous cycle. Using immunohistochemical techniques on paraffin-embedded tissue we have successfully shown that CgA is selectively expressed in ciliated cells of the bovine oviduct. The labelled cells show intense positive staining in the apical surface area in close vicinity to the ciliary apparatus. CgA-positive ciliated cells are most frequently observed at dioestrous while a lower number appears at oestrous. Additionally, secretory and intraepithelial neuroendocrine cells consistently do not stain with the CgA-antiserum. We then investigated whether the reported expression of the InsP3R in oviductal cells of the mouse corresponds to the expression of the InsP3R in bovine oviductal cells. Using a polyclonal antibody to the type 2 InsP3R, we found that the receptor is also selectively expressed in a similar matter to CgA in the apical cytoplasm of ciliated cells. This is the first morphological demonstration of the colocalization of CgA and InsP3R in epithelial ciliated cells of the bovine oviduct. Our results suggest that CgA and InsP3R could be involved in controlling the ciliary activity of oviductal epithelial cells.

IRBIT, a novel inositol 1,4,5-trisphosphate (IP3) receptor-binding protein, is released from the IP3 receptor upon IP3 binding to the receptor

The inositol 1,4,5-trisphosphate (IP(3)) receptors (IP(3)Rs) are IP(3)-gated Ca(2+) channels on intracellular Ca(2+) stores. Herein, we report a novel protein, termed IRBIT (IP(3)R binding protein released with inositol 1,4,5-trisphosphate), which interacts with type 1 IP(3)R (IP(3)R1) and was released upon IP(3) binding to IP(3)R1. IRBIT was purified from a high salt extract of crude rat brain microsomes with IP(3) elution using an affinity column with the huge immobilized N-terminal cytoplasmic region of IP(3)R1 (residues 1-2217). IRBIT, consisting of 530 amino acids, has a domain homologous to S-adenosylhomocysteine hydrolase in the C-terminal and in the N-terminal, a 104 amino acid appendage containing multiple potential phosphorylation sites. In vitro binding experiments showed the N-terminal region of IRBIT to be essential for interaction, and the IRBIT binding region of IP(3)R1 was mapped to the IP(3) binding core. IP(3) dissociated IRBIT from IP(3)R1 with an EC(50) of approximately 0.5 microm, i.e. it was 50 times more potent than other inositol polyphosphates. Moreover, alkaline phosphatase treatment abolished the interaction, suggesting that the interaction was dualistically regulated by IP(3) and phosphorylation. Immunohistochemical studies and co-immunoprecipitation assays showed the relevance of the interaction in a physiological context. These results suggest that IRBIT is released from activated IP(3)R, raising the possibility that IRBIT acts as a signaling molecule downstream from IP(3)R.

The chromogranins: their roles in secretion from neuroendocrine cells and as markers for neuroendocrine neoplasia

Chromogranins are the major components of the secretory granules of most neuroendocrine cells. Within the secretory pathway, chromogranins are involved in granulogenesis, and in sorting and processing of secretory protein cargo prior to secretion. Once secreted, they have hormonal, autocrine, and paracrine activities. The chromogranin family includes chromogranins A (CgA) and B (CgB) and secretogranin II (SgII, once called chromogranin C). The related "granins" NESP55, 7B2, secretogranin III/1B 1075 (SgIII), and secretogranin IV/HISL-19 antigen (SgIV), are also sometimes included when considering the chromogranins. While it is useful to consider the granin proteins as a family with many common features, it is also necessary to examine the distinct features and properties of individual members of the granin family to understand fully their functions, employ them efficiently as tissue, serum, and urinary markers for neuroendocrine neoplasia, and develop an evolutionary-biologic perspective on their contribution to mammalian physiology. Recent advances in chromogranin research include establishing the role of CgA in granulogenesis and the role of CgB in nuclear transcription; new biologic activities for CgA-, CgB-, and SgII-derived peptides; and new marker functions for granins and their proteolytically processed products in endocrine neoplasias.

Identification of a novel sorting determinant for the regulated pathway in the secretory protein chromogranin A

Chromogranin A (CgA) is the index member of the chromogranin/secretogranin (or 'granin') family of regulated secretory proteins that are ubiquitously distributed in amine- and peptide-containing secretory granules of endocrine, neuroendocrine and neuronal cells. Because of their abundance and such widespread occurrence, granins have often been used as prototype proteins to elucidate mechanisms of protein targeting into dense-core secretory granules. In this study, we used a series of full-length, point mutant or truncated CgA-green fluorescent protein (GFP) chimeras to explore routing of CgA in neuroendocrine PC12 cells. Using sucrose gradient fractionation and 3D deconvolution microscopy to determine the subcellular localization of the GFP chimeras, as well as secretagogue-stimulated release, the present study establishes that a CgA-GFP fusion protein expressed in neuroendocrine PC12 cells is trafficked to the dense core secretory granule and thereby sorted to the regulated pathway for exocytosis. We show that information necessary for such trafficking is contained within the N-terminal but not the C-terminal region of CgA. We find that CgA's conserved N-terminal hydrophobic Cys(17)-Cys(38) loop structure may not be sufficient for sorting of CgA into dense-core secretory granules, nor is its stabilization by a disulfide bond necessary for such sorting. Moreover, our data reveal for the first time that the CgA(77-115) domain of the mature protein may be necessary (though perhaps not sufficient) for trafficking CgA into the regulated pathway of secretion.

Large dense-core secretory granule biogenesis is under the control of chromogranin A in neuroendocrine cells

The large dense-core secretory granule is an organelle in neuroendocrine/endocrine cells, where prohormones and proneuropeptides are stored, processed, and secreted in a regulated manner. Here we present evidence that chromogranin A (CgA), one of the most abundant acidic glycoproteins ubiquitously present in neuroendocrine/endocrine cells, regulates dense-core secretory granule biogenesis. Specific depletion of CgA expression by antisense RNAs in PC12 cells led to a profound loss of secretory granule formation. An exogenously expressed prohormone, pro-opiomelanocortin, was neither stored nor secreted in a regulated manner in CgA-deficient PC12 cells. Overexpression of bovine CgA into CgA-deficient PC12 cells rescued regulated secretion. Other secretory granule proteins, such as chromogranin B (CgB), carboxypeptidase E, and synaptotagmin, were rapidly degraded, whereas nongranule proteins were not affected in CgA-deficient PC12 cells. Unlike CgA, another granin protein CgB could not substitute for the role of CgA in secretory granule biogenesis. Thus, we conclude that CgA is a master "on/off" switch regulating the formation of the dense-core secretory granule in neuroendocrine cells.

Inositol 1,4,5-trisphosphate receptor/Ca(2+) channel modulatory role of chromogranins A and B

The secretory granules function as the major IP(3)-sensitive intracellular Ca(2+) store of secretory cells. Recently it was found that the secretory granules contain three isoforms of inositol 1,4,5-trisphosphate receptor (IP(3)R)/Ca(2+) channels and high-capacity, low-affinity Ca(2+) storage proteins chromogranins A (CgA) and B (CgB). The IP(3)R/Ca(2+) channel was shown to directly interact with CgA and CgB at the intragranular pH 5.5, and this coupling led to modulation of the IP(3)R/Ca(2+) channel activity by the coupled chromogranins. These results provide the molecular structural basis of the IP(3)-mediated Ca(2+) release mechanism of secretory granules.

Evidence that zymogen granules do not function as an intracellular Ca2+ store for the generation of the Ca2+ signal in rat parotid acinar cells

Rat parotid acinar cells lacking zymogen granules were obtained by inducing granule discharge with the beta-adrenoceptor agonist isoproterenol. To assess whether zymogen granules are involved in the regulation of Ca(2+) signalling as intracellular Ca(2+) stores, changes in cytosolic free Ca(2+) ion concentration ([Ca(2+)](i)) were studied with imaging microscopy in fura-2-loaded parotid acinar cells lacking zymogen granules. The increase in [Ca(2+)](i) induced by muscarinic receptor stimulation was initiated at the apical pole of the acinar cells, and rapidly spread as a Ca(2+) wave towards the basolateral region. The magnitude of the [Ca(2+)](i) response and the speed of the Ca(2+) wave were essentially similar to those in control acinar cells containing zymogen granules. Western blot analysis of the inositol 1,4,5-trisphosphate receptor (IP(3)R) was performed on zymogen granule membranes and microsomes using anti-IP(3)R antibodies. The immunoreactivity of all three IP(3)Rs was clearly observed in the microsomal preparations. Although a weak band of IP(3)R type-2 was detected in the zymogen granule membranes, this band probably resulted from contamination by the endoplasmic reticulum (ER), because calnexin, a marker protein of the ER, was also detected in the same preparation. Furthermore, Western blotting and reverse transcriptase-PCR analysis failed to provide evidence for the expression of ryanodine receptors in rat parotid acinar cells, whereas expression was clearly detectable in rat skeletal muscle, heart and brain. These results suggest that zymogen granules do not have a critical role in Ca(2+) signalling in rat parotid acinar cells.

Localization of three types of the inositol 1,4,5-trisphosphate receptor/Ca(2+) channel in the secretory granules and coupling with the Ca(2+) storage proteins chromogranins A and B

Although the role of secretory granules as the inositol 1,4,5-trisphosphate (IP(3))-sensitive intracellular Ca(2+) store and the presence of the IP(3) receptor (IP(3)R)/Ca(2+) channel on the secretory granule membrane have been established, the identity of the IP(3)R types present in the secretory granules is not known. We have therefore investigated the presence of different types of IP(3)R in the secretory granules of bovine adrenal medullary chromaffin cells using immunogold electron microscopy and found the existence of all three types of IP(3)R in the secretory granules. To determine whether these IP(3)Rs interact with CGA and CGB, each IP(3)R isoform was co-transfected with CGA or CGB into NIH3T3 or COS-7 cells, and the expressed IP(3)R isoform and CGA or CGB were co-immunoprecipitated. From these studies it was shown that all three types of IP(3)R form complexes with CGA and CGB in the cells. To further confirm whether the IP(3)R isoforms and CGA and CGB form a complex in the secretory granules the potential interaction between all three isoforms of IP(3)R and CGA and CGB was tested by co-immunoprecipitation experiments of the mixture of secretory granule lysates and the granule membrane proteins. The three isoforms of IP(3)R were shown to form complexes with CGA and CGB, indicating the complex formation between the three isoforms of IP(3)R and CGA and CGB in the secretory granules. Moreover, the pH-dependent Ca(2+) binding property of CGB was also studied using purified recombinant CGB, and it was shown that CGB bound 93 mol of Ca(2+)/mol with a dissociation constant (K(d)) of 1.5 mm at pH 5.5 but virtually no Ca(2+) at pH 7.5. The high capacity, low affinity Ca(2+)-binding property of CGB at pH 5.5 is comparable with that of CGA and is in line with its role as a Ca(2+) storage protein in the secretory granules.

Comparative sorting of neuroendocrine secretory proteins: a search for common ground in a mosaic of sorting models and mechanisms

Endocrine, neuroendocrine and exocrine cells store regulated secretory proteins in secretory granules, while constitutive and constitutive-like secretory proteins are secreted directly without storage. Sorting of secretory proteins takes place in the trans-Golgi network (sorting for entry) or immature secretory granules (sorting by retention). The relative contribution of these sorting steps and the sorting signals and mechanisms involved in each step has been the subject of intense studies and debate in recent years. New evidence now suggests that: (1) two proteins with structurally similar sorting signals can use different sorting mechanisms; (2) one protein with multiple sorting signals can be sorted differently in different cell types; and (3) one cell type can recognize different sorting signals and use different sorting mechanisms. The latter finding suggests that sorting must be a regulated event. While the current image of sorting is complex, recent findings are pointing to common features that form a mosaic of related sorting mechanisms.

Calcium-dependent enzyme activation and vacuole formation in the apical granular region of pancreatic acinar cells

The pancreatic acinar cell produces powerful digestive enzymes packaged in zymogen granules in the apical pole. Ca(2+) signals elicited by acetylcholine or cholecystokinin (CCK) initiate enzyme secretion by exocytosis through the apical membrane. Intracellular enzyme activation is normally kept to a minimum, but in the often-fatal human disease acute pancreatitis, autodigestion occurs. How the enzymes become inappropriately activated is unknown. We monitored the cytosolic Ca(2+) concentration ([Ca(2+)](i)), intracellular trypsin activation, and its localization in isolated living cells with specific fluorescent probes and studied intracellular vacuole formation by electron microscopy as well as quantitative image analysis (light microscopy). A physiological CCK level (10 pM) eliciting regular Ca(2+) spiking did not evoke intracellular trypsin activation or vacuole formation. However, stimulation with 10 nM CCK, evoking a sustained rise in [Ca(2+)](i), induced pronounced trypsin activation and extensive vacuole formation, both localized in the apical pole. Both processes were abolished by preventing abnormal [Ca(2+)](i) elevation, either by preincubation with the specific Ca(2+) chelator 1, 2-bis(O-aminophenoxy)ethane-N,N-N',N'-tetraacetic acid (BAPTA) or by removal of external Ca(2+). CCK hyperstimulation evokes intracellular trypsin activation and vacuole formation in the apical granular pole. Both of these processes are mediated by an abnormal sustained rise in [Ca(2+)](i).

Interaction of chromogranin B and the near N-terminal region of chromogranin B with an intraluminal loop peptide of the inositol 1,4, 5-trisphosphate receptor

Given the interaction of the inositol 1,4,5-trisphosphate receptor (IP(3)R) with chromogranins A (CGA) and B (CGB), two major Ca(2+) storage proteins of secretory granules that have been shown to be IP(3)-sensitive intracellular Ca(2+) store of neuroendocrine cells, we have investigated the potential interaction of the intraluminal loop regions of the IP(3)R with both intact CGB and the conserved near N-terminal region of CGB. The interaction studies carried out with CGB and glutathione S-transferase fusion proteins of intraluminal loop regions of bovine type 1 IP(3)R showed that CGB interacts with intraluminal loop 3-2 (the second loop formed between transmembrane regions 5 and 6) of the IP(3)R at both pH 5.5 and 7.5. Analytical ultracentrifugation studies also indicated that CGB interacts with the same intraluminal loop region of the IP(3)R and the interaction was much stronger than that between CGA and the loop. Moreover, the conserved near N-terminal region of CGB also interacted with the intraluminal loop region of the IP(3)R. The CGB interaction with the IP(3)R intraluminal loop peptide at pH 7.5 showed a DeltaG(0) value of -8.1 kcal/mol at 37 degrees C for a 1:1 stoichiometry, indicating a K(d) of approximately 1.9 micrometer. These results give insight into the molecular organization of the IP(3)-sensitive Ca(2+) store.

Coupling of the IP3 receptor/Ca2+ channel with Ca2+ storage proteins chromogranins A and B in secretory granules

The secretory granules of neuroendocrine cells, which function as an inositol (1,4,5)-trisphosphate-sensitive intracellular Ca2+ store, contain both the inositol (1,4,5)-trisphosphate receptor/Ca2+ channel and the high-capacity low-affinity Ca2+ storage proteins, chromogranins A and B. Chromogranins A and B, which exist in approximately 2 mm range in the secretory granules, can bind 50-100 mol of Ca2+/mol with dissociation constants of 2-4 mm. These proteins interact directly with the inositol (1,4,5)-trisphosphate receptor/ Ca2+ channel at the intragranular pH 5.5, not only changing the conformation of the inositol (1,4,5)-trisphosphate receptor/Ca2+ channel but also modulating the channel activity. Given the homo- and heterotetrameric existence of both the inositol (1,4,5)-trisphosphate receptor/Ca2+ channel and chromogranins A and B, these tetrameric proteins appear to interact, thus controlling the intracellular Ca2+ concentration.

Inositol 1,4,5-trisphosphate receptor/Ca2+ channel modulatory role of chromogranin A, a Ca2+ storage protein of secretory granules

The secretory granules of neuroendocrine cells, which contain large amounts of Ca(2+) and chromogranins, have been demonstrated to release Ca(2+) in response to inositol 1,4,5-trisphosphate (IP(3)), indicating the IP(3)-sensitive intracellular Ca(2+) store role of secretory granules. In our previous study, chromogranin A (CGA) was shown to interact with several secretory granule membrane proteins, including the IP(3) receptor (IP(3)R), at the intravesicular pH 5.5 (Yoo, S. H. (1994) J. Biol. Chem. 269, 12001-12006). To examine the functional aspect of this coupling, we measured the IP(3)-mediated Ca(2+) release property of the IP(3)R reconstituted into liposomes in the presence and absence of CGA. Presence of CGA in the IP(3)R-reconstituted liposome significantly enhanced the IP(3)-mediated Ca(2+) release from the liposomes. Moreover, the number of IP(3) bound to the reconstituted IP(3)R increased. The fluorescence energy transfer and IP(3)R Trp fluorescence quenching studies indicated that the structure of reconstituted IP(3)R becomes more ordered and exposed in the presence of CGA, suggesting that the coupled CGA in the liposome caused structural changes of the IP(3)R, changing it to a structure that is better suited to IP(3) binding and subsequent Ca(2+) release. These results appear to underscore the physiological significance of IP(3)R-CGA coupling in the secretory granules.