The chromogranins. Historical perspectives

Untapped Potential: Real-Time Measurements of Opioid Exocytosis at Single Cells

The endogenous opioid system is commonly targeted in pain treatment, but the fundamental nature of neuropeptide release remains poorly understood due to a lack of methods for direct detection of specific opioid neuropeptides in situ. These peptides are concentrated in, and released from, large dense-core vesicles in chromaffin cells. Although catecholamine release from these neuroendocrine cells is well characterized, the direct quantification of opioid peptide exocytosis events has not previously been achieved. In this work, a planar carbon-fiber microelectrode served as a "postsynaptic" sensor for probing catecholamine and neuropeptide release dynamics via amperometric monitoring. A constant potential of 500 mV was employed for quantification of catecholamine release, and a higher potential of 1000 mV was used to drive oxidation of tyrosine, the N-terminal amino acid in the opioid neuropeptides released from chromaffin cells. By discriminating the results collected at the two potentials, the data reveal unique kinetics for these two neurochemical classes at the single-vesicle level. The amplitude of the peptidergic signals decreased with repeat stimulation, as the halfwidth of these signals simultaneously increased. By contrast, the amplitude of catecholamine release events increased with repeat stimulation, but the halfwidth of each event did not vary. The chromogranin dense core was identified as an important mechanistic handle by which separate classes of transmitter can be kinetically modulated when released from the same population of vesicles. Overall, the data provide unprecedented insight into key differences between catecholamine and opioid neuropeptide release from isolated chromaffin cells.

Nutrient Regulation of Pancreatic Islet β-Cell Secretory Capacity and Insulin Production

Pancreatic islet β-cells exhibit tremendous plasticity for secretory adaptations that coordinate insulin production and release with nutritional demands. This essential feature of the β-cell can allow for compensatory changes that increase secretory output to overcome insulin resistance early in Type 2 diabetes (T2D). Nutrient-stimulated increases in proinsulin biosynthesis may initiate this β-cell adaptive compensation; however, the molecular regulators of secretory expansion that accommodate the increased biosynthetic burden of packaging and producing additional insulin granules, such as enhanced ER and Golgi functions, remain poorly defined. As these adaptive mechanisms fail and T2D progresses, the β-cell succumbs to metabolic defects resulting in alterations to glucose metabolism and a decline in nutrient-regulated secretory functions, including impaired proinsulin processing and a deficit in mature insulin-containing secretory granules. In this review, we will discuss how the adaptative plasticity of the pancreatic islet β-cell's secretory program allows insulin production to be carefully matched with nutrient availability and peripheral cues for insulin signaling. Furthermore, we will highlight potential defects in the secretory pathway that limit or delay insulin granule biosynthesis, which may contribute to the decline in β-cell function during the pathogenesis of T2D.

Chromogranin B regulates early-stage insulin granule trafficking from the Golgi in pancreatic islet β-cells

Chromogranin B (CgB, also known as CHGB) is abundantly expressed in dense core secretory granules of multiple endocrine tissues and has been suggested to regulate granule biogenesis in some cell types, including the pancreatic islet β-cell, though the mechanisms are poorly understood. Here, we demonstrate a critical role for CgB in regulating secretory granule trafficking in the β-cell. Loss of CgB impairs glucose-stimulated insulin secretion, impedes proinsulin processing to yield increased proinsulin content, and alters the density of insulin-containing granules. Using an in situ fluorescent pulse-chase strategy to track nascent proinsulin, we show that loss of CgB impairs Golgi budding of proinsulin-containing secretory granules, resulting in a substantial delay in trafficking of nascent granules to the plasma membrane with an overall decrease in total plasma membrane-associated granules. These studies demonstrate that CgB is necessary for efficient trafficking of secretory proteins into the budding granule, which impacts the availability of insulin-containing secretory granules for exocytic release.This article has an associated First Person interview with the first author of the paper.

The Prohormone VGF Regulates β Cell Function via Insulin Secretory Granule Biogenesis

The prohormone VGF is expressed in neuroendocrine and endocrine tissues and regulates nutrient and energy status both centrally and peripherally. We and others have shown that VGF-derived peptides have direct action on the islet β cell as secretagogues and cytoprotective agents; however, the endogenous function of VGF in the β cell has not been described. Here, we demonstrate that VGF regulates secretory granule formation. VGF loss-of-function studies in both isolated islets and conditional knockout mice reveal a profound decrease in stimulus-coupled insulin secretion. Moreover, VGF is necessary to facilitate efficient exit of granule cargo from the trans-Golgi network and proinsulin processing. It also functions to replenish insulin granule stores following nutrient stimulation. Our data support a model in which VGF operates at a critical node of granule biogenesis in the islet β cell to coordinate insulin biosynthesis with β cell secretory capacity.

Secretogranin III: a diabetic retinopathy-selective angiogenic factor

Secretogranin III (Scg3) is a member of the granin protein family that regulates the biogenesis of secretory granules. Scg3 was recently discovered as an angiogenic factor, expanding its functional role to extrinsic regulation. Unlike many other known angiogenic factors, the pro-angiogenic actions of Scg3 are restricted to pathological conditions. Among thousands of quantified endothelial ligands, Scg3 has the highest binding activity ratio to diabetic vs. healthy mouse retinas and lowest background binding to normal vessels. In contrast, vascular endothelial growth factor binds to and stimulates angiogenesis of both diabetic and control vasculature. Consistent with its role in pathological angiogenesis, Scg3-neutralizing antibodies alleviate retinal vascular leakage in mouse models of diabetic retinopathy and retinal neovascularization in oxygen-induced retinopathy mice. This review summarizes our current knowledge of Scg3 as a regulatory protein of secretory granules, highlights its new role as a highly disease-selective angiogenic factor, and envisions Scg3 inhibitors as "selective angiogenesis blockers" for targeted therapy.

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.

Cardiac electrical activity in a genomically "humanized" chromogranin a monogenic mouse model with hyperadrenergic hypertension

The prohormone chromogranin A (CHGA) is ubiquitously found in vesicles of adrenal chromaffin cells and adrenergic neurons, and it is processed to the hypotensive hormone peptide catestatin (CST). Both CHGA and CST regulate blood pressure and cardiac function. This study addresses their role in cardiac electrical activity. We have generated two genomically "humanized" transgenic mouse strains (Tg31CHGA+/+; Chga-/- (HumCHGA31) and Tg19CHGA+/+; Chga-/- (HumCHGA19)) with varied CHGA expression and the ability to rescue the Chga-/- phenotype (hypertensive, hyperadrenergic with dilated cardiomyopathy). The normotensive HumCHGA31 mice express CHGA at levels comparable to wild-type. In contrast, the hypertensive HumCHGA19 mice have low levels of CHGA. EKG recordings revealed that the QT interval, R-amplitude, and QRS time-voltage integral are markedly longer in HumCHGA19 compared to wild-type and HumCHGA31 mice. These differences are accompanied by increased heart rate and QT variability, indicating that ventricular assault happens in a status of low levels of circulating CST.

The extended granin family: structure, function, and biomedical implications

The chromogranins (chromogranin A and chromogranin B), secretogranins (secretogranin II and secretogranin III), and additional related proteins (7B2, NESP55, proSAAS, and VGF) that together comprise the granin family subserve essential roles in the regulated secretory pathway that is responsible for controlled delivery of peptides, hormones, neurotransmitters, and growth factors. Here we review the structure and function of granins and granin-derived peptides and expansive new genetic evidence, including recent single-nucleotide polymorphism mapping, genomic sequence comparisons, and analysis of transgenic and knockout mice, which together support an important and evolutionarily conserved role for these proteins in large dense-core vesicle biogenesis and regulated secretion. Recent data further indicate that their processed peptides function prominently in metabolic and glucose homeostasis, emotional behavior, pain pathways, and blood pressure modulation, suggesting future utility of granins and granin-derived peptides as novel disease biomarkers.

Chromogranins A and B are key proteins in amine accumulation, but the catecholamine secretory pathway is conserved without them

Chromogranins are the main soluble proteins in the large dense core secretory vesicles (LDCVs) found in aminergic neurons and chromaffin cells. We recently demonstrated that chromogranins A and B each regulate the concentration of adrenaline in chromaffin granules and its exocytosis. Here we have further studied the role played by these proteins by generating mice lacking both chromogranins. Surprisingly, these animals are both viable and fertile. Although chromogranins are thought to be essential for their biogenesis, LDCVs were evident in these mice. These vesicles do have a somewhat atypical appearance and larger size. Despite their increased size, single-cell amperometry recordings from chromaffin cells showed that the amine content in these vesicles is reduced by half. These data demonstrate that although chromogranins regulate the amine concentration in LDCVs, they are not completely essential, and other proteins unrelated to neurosecretion, such as fibrinogen, might compensate for their loss to ensure that vesicles are generated and the secretory pathway conserved.

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).

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.

Effect of reserpine on the generation of the chromogranin A-derived neuropeptide WE-14 in rat oxyntic mucosa

WE-14, a post-translational product of the neuroendocrine protein chromogranin A (CgA), is generated in distinct subpopulations of endocrine cells. The objective of this study was to investigate the generation of WE-14 in the endocrine cell types of the oxyntic mucosa of the stomach, after treatment with reserpine, an irreversible inhibitor of vesicular monoamine uptake 2 (VMAT2). Reserpine (10 mg/kg) was administered subcutaneously and tissue analysed 1, 3, 5 and 18 h following treatment. The oxyntic mucosa was analysed immunohistochemically employing a site-specific WE-14 antiserum, a region-specific CgA antiserum and an antiserum against histidine decarboxylase (HDC), a marker of the histamine-producing ECL cells in the oxyntic mucosa. The number of oxyntic endocrine cells exhibiting WE-14 immunostaining increased more than 100-fold 18 h after reserpine administration relative to vehicle treated controls. Double immunostaining with HDC revealed that most, but not all, of the WE-14 positive cells were ECL cells. These results suggest that reserpine has the ability to influence the post-translational processing of CgA to generate WE-14 in rat stomach ECL cells, presumably as a consequence of reduced VMAT2-driven accumulation of histamine.

Innate immunity: involvement of new neuropeptides

Secretory granules of chromaffin cells from the adrenal medulla store catecholamines and a variety of peptides that are secreted in the extracellular medium during exocytosis. Among these fragments, several natural peptides displaying antimicrobial activities at the micromolar range have been isolated and characterized. We have shown that these peptides, derived from the natural processing of chromogranins (CGs), proenkephalin-A (PEA) and free ubiquitin (Ub), are released into the circulation and display antibacterial and antifungal activities. In this review we focus on three naturally secreted antimicrobial peptides corresponding to CGA1-76 (vasostatin-I), the bisphosphorylated form of PEA209-237 (enkelytin) and Ub. In addition, the antimicrobial properties of the synthetic active domains of vasostatin-I (CGA47-66 or chromofungin) and Ub (Ub65-76 or ubifungin) are reported.

Localisation of WE-14 immunoreactivity in the developing mouse limbo-corneal nerve net

WE-14 is generated in subpopulations of chromogranin A immunopositive endocrine cells and neurons including those innervating the anterior uvea. This study investigated WE-14 in intact sclero-limbo-corneal tissue from embryonic (E17), neonatal (N0-N16), and adult mice using immunocytochemistry and confocal scanning laser microscopy. Weak WE-14 immunostaining was observed at birth in nerve fibre tracts entering the corneal mid-stroma from the limbo-scleral junction. Immunopositive fibre tracts were evident throughout the cornea at N3; by N5 the mid-stromal plexus had begun to generate fibre populations extending toward the developing corneal epithelium, and some varicose fibres terminated amongst the developing epithelium. Immunostaining was evident at N7 in the developing limbo-scleral nerve net and some fibres exhibited a close association with unidentified vascular elements. By N11 and in subsequent neonates, the cornea had developed a distinct stratified nerve net composed of thick mid-stromal and thinner upper stromal nerve fibre bundles; both possessed populations of varicose WE-14 immunopositive fibres. In the adult, a sub-epithelial network of varicose WE-14 immunopositive fibres were evident at the limbo-scleral junction. Some fibres exhibited a close association with unidentified vascular elements, while others extended into the upper peripheral corneal stroma. WE-14 was evident in leashes throughout the basal corneal epithelium and generated fibres ramifying between the stratified epithelium with some fibres terminating amongst the outermost corneal epithelia. This study has demonstrated that WE-14 was evident in the limbo-corneal nerve net at birth and that its detection parallels corneal development to adulthood, where WE-14 is evident in a subpopulation of nerve fibres.

Chromogranin A proteolysis to generate beta-granin and WE-14 in the adenohypophysis during the rat oestrous cycle

Immunohistochemical analysis of the male and female rat adenohypophysis revealed that chromogranin A (CgA), beta-granin and WE-14 immunostaining was localised to follicle stimulating hormone (FSH) producing cells, while luteinizing hormone (LH) producing cells exhibited chromogranin A and beta-granin immunostaining. The intensity of chromogranin A, beta-granin and WE-14 immunostaining exhibited variation during the oestrous cycle; weak immunostaining was observed during proestrous and oestrous, corresponding with the lowest cellular concentration of luteinizing and follicle stimulating hormone. Chromogranin A and beta-granin immunostaining were similar in both the male and female (at dioestrous), however, a larger number of more intense WE-14 immunopositive cells were evident in the male adenohypophysis relative to the female at any stage of the cycle. The tissue and plasma concentrations of beta-granin and WE-14 immunoreactivity fluctuated throughout the oestrous cycle. Maximum and minimum beta-granin and WE-14 tissue concentration counterpoised the latent maximum and minimum plasma concentration. Chromatographic analysis of adenohypophysis extracts revealed the degree of chromogranin A proteolysis throughout the oestrous cycle; in contrast, plasma profiles consistently possessed a large chromogranin A-like immunoreactant. This data suggests that chromogranin A biosynthesis, proteolysis and the secretion of its derived peptides parallels that of the gonadotroph hormones throughout the oestrous cycle.

Characterisation of WE-14 in porcine ocular tissue

WE-14 is derived from the cell-specific posttranslational processing of chromogranin A (CgA) in subpopulations of neuroendocrine cells and neurons. Region- and site-specific chromogranin A, pancreastatin and WE-14 antisera were employed to study the generation of WE-14 in porcine ocular tissues. No chromogranin A or pancreastatin immunostaining was detected in ocular tissue. Immunohistochemistry detected WE-14 immunostaining in a network of nerve fibre bundles and nerve fibres throughout the limbus, cornea, iris and ciliary body with sparse nerve fibres detected throughout the choroid and sclera. Retinal analysis detected intense WE-14 immunostaining in large ovoid cells in the ganglion cell layer with weak immunostaining in a population of small cells in the inner nuclear layer; weak immunostaining was detected within the fibre layers in the inner plexiform layer. Quantitatively, the highest WE-14 tissue concentration was recorded in aqueous retinal and corneal extracts with lower concentrations in the sclera, choroid and anterior uveal tissues. Chromatographic profiling resolved a minor chromogranin A-like immunoreactant and a predominant immunoreactant co-eluting with synthetic human WE-14. This is the first study to demonstrate that WE-14 is generated in neuronal fibres primarily innervating the anterior chamber and in select cell populations in the retina.

Cleavage of Chromogranin A N-terminal Domain by Plasmin Provides a New Mechanism for Regulating Cell Adhesion

It has been proposed that chromogranin A (CgA), a protein secreted by many normal and neoplastic neuroendocrine cells, can play a role as a positive or a negative modulator of cell adhesion. The mechanisms that regulate these extracellular functions of CgA are unknown. We show here that plasmin can regulate the anti/pro-adhesive activity of CgA by proteolytic cleavage of the N-terminal domain. Limited proteolytic processing decreased its anti-adhesive activity and induced pro-adhesive effects in fibronectin or serum-dependent fibroblast adhesion assays. Cleavage of Lys(77)-Lys(78) dibasic site in CgA(1-115) was relatively rapid and associated with an increase of pro-adhesive effect. In contrast, antibodies against the region 53-90 enhanced the anti-adhesive activity of CgA and CgA(1-115). Structure-activity relationship studies showed that the conserved region 47-64 (RILSILRHQNLLKELQDL) is critical for both pro- and anti-adhesive activity. These findings suggest that CgA might work on one hand as a negative modulator of cell adhesion and on the other hand as a precursor of positive modulators, the latter requiring proteolytic processing for activation. Given the importance of plasminogen activation in tissue invasion and remodeling, the interplay between CgA and plasmin could provide a novel mechanism for regulating fibroblast adhesion and function in neuroendocrine tumors.