Functional aspects of the adrenal medullary chromogranins

Chromogranin A regulates renal function by triggering Weibel-Palade body exocytosis

Chromogranin A (CHGA), a protein released from secretory granules of chromaffin cells and sympathetic nerves, triggers endothelin-1 release from endothelial cells. CHGA polymorphisms associate with an increased risk for ESRD, but whether altered CHGA-endothelium interactions may explain this association is unknown. Here, CHGA led to the release of endothelin-1 and Weibel-Palade body exocytosis in cultured human umbilical vein endothelial cells. In addition, CHGA triggered secretion of endothelin-1 from glomerular endothelial cells and TGF-beta1 from mesangial cells cocultured with glomerular endothelial cells. In humans, plasma CHGA correlated positively with endothelin-1 and negatively with GFR. GFR was highly heritable in twin pairs, and common promoter haplotypes of CHGA predicted GFR. In patients with progressive hypertensive renal disease, a CHGA haplotype predicted rate of GFR decline. In conclusion, these data suggest that CHGA acts through the glomerular endothelium to regulate renal function.

New biological aspects of Chromogranin A-derived peptides: Focus on vasostatins

Chromogranin A (CgA), one component of the granin family, represents the major soluble protein co-stored and co-released with catecholamines, within chromaffin cells secretory granules. It is considered a diagnostic and prognostic marker of several diseases, including a variety of tumours and cardiac heart failure. It also represents a precursor of biologically active fragments, generated after proteolytic cleavage at the level of the multiple pairs of dibasic sites which enrich its sequence. CgA, and its derived fragments show an old evolutionary history being ubiquitously present throughout the animal word, from mammals to invertebrates. Their biological functions include control of hormone production, and several paracrine and autocrine actions mainly attributed to its derived peptides. Two N-terminal fragments, named vasostatins 1 (VS-1: CgA(1-76)) and vasostatin 2 (VS-2: CgA(1-113)) due to their ability to dilate pre-constricted vessels, exert a large spectrum of homeostatic actions, including antifungal and antimicrobial effect, modulation of cell adhesion, and inhibition of parathyroid hormone secretion. Recently, on isolated heart preparations from eel, frog and rat they were shown to act as negative inotropic agents able to counteract the effects of beta-adrenergic stimulation. This short note introduces the abstracts of the contributions at the "International Workshop on Vasostatins and Chromogranin A-derived peptides" (Island of Capri, Italy; September 2005). The Workshop was focused on recent findings on the role of vasostatins (VSs) in cardiovascular and gastrointestinal systems, extracellular fluids composition, and innate immunity. Particular attention has been given to the still elusive mechanism of action of these peptides.

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.

Functional Coupling of Chromogranin with the Inositol 1,4,5-Trisphosphate Receptor Shapes Calcium Signaling

Chromogranins A and B are high capacity, low affinity calcium (Ca(2+)) storage proteins that bind to the inositol 1,4,5-trisphosphate-gated receptor (InsP(3) R). Although most commonly associated with secretory granules of neuroendocrine cells, chromogranins have also been found in the lumen of the endoplasmic reticulum (ER) of many cell types. To investigate the functional consequences of the interaction between the InsP(3) R and the chromogranins, we disrupted the interaction between the two proteins by adding a chromogranin fragment, which competed with chromogranin for its binding site on the InsP(3)R. Responses were monitored at the single channel level and in intact cells. When using InsP(3) R type I incorporated into planar lipid bilayers and activated by cytoplasmic InsP(3) and luminal chromogranin, the addition of the fragment reversed the enhancing effect of chromogranin. Moreover, the expression of the fragment in the ER of neuronally differentiated PC12 cells attenuated agonist-induced intracellular Ca(2+) signaling. These results show that the InsP(3)R/chromogranin interaction amplifies Ca(2+) release from the ER and that chromogranin is an essential component of this intracellular channel complex.

Catecholaminergic pathways, chromaffin cells, and human disease

Recent studies demonstrate major effects of adrenal medullary and catecholaminergic pathways on a wide variety of normal physiologic and regulatory events. Alterations in these pathways, involving changes in catecholamines or in proteins and peptides costored and coreleased with catecholamines, may lead to profound changes in autonomic, cardiovascular, neuroendocrine, metabolic, nociceptive, and immune function. These findings have important implications for a variety of human disease states. In addition, molecules associated with catecholaminergic function may provide novel diagnostic and therapeutic strategies for human disease and suggest specific genetic loci as important and fruitful targets for further genetic and pharmacogenetic studies.

Activation of the inositol 1,4,5-trisphosphate receptor by the calcium storage protein chromogranin A

Secretory granules of neuroendocrine cells are inositol 1,4,5-trisphosphate (InsP(3))-sensitive Ca(2+) stores in which the Ca(2+) storage protein, chromogranin A (CGA), couples with InsP(3)-gated Ca(2+) channels (InsP(3)R) located in the granule membrane. The functional aspect of this coupling has been investigated via release studies and planar lipid bilayer experiments in the presence and absence of CGA. CGA drastically increased the release activity of the InsP(3)R by increasing the channel open probability by 9-fold and the mean open time by 12-fold. Our results show that CGA-coupled InsP(3)Rs are more sensitive to activation than uncoupled receptors. This modulation of InsP(3)R channel activity by CGA appears to be an essential component in the control of intracellular Ca(2+) concentration by secretory granules and may regulate the rate of vesicle fusion and exocytosis.

Relative amounts and molecular forms of NESP55 in various bovine tissues

NESP55 (neuroendocrine secretory protein with Mr 55,000) comprises a novel chromogranin-like protein, which is paternally imprinted at the genomic level. We used antisera raised against GAIPIRRH, a peptide present at the C-terminus of this protein, and against TC-14, a peptide located in the N-terminal half of NESP55. Radioimmunoassay, gel-filtration chromatography and immunoblotting were used to determine the levels and the molecular forms of NESP55 in different bovine organs. The tissues with the highest levels of GAIPIRRH immunoreactivity were, in decreasing order: the adrenal medulla, the anterior pituitary, the posterior pituitary, various brain regions, and the intestine. The degree of proteolytic processing revealed differences among the tissues analyzed. The lowest processing was detected in the anterior pituitary and in the brain where only a peak corresponding to the intact precursor was present. This was also true for cerebrospinal fluid (CSF). In the posterior pituitary and in the intestine, the free peptide GAIPIRRH was the predominant molecular form. GAIPIRRH-IR, as in the CSF, is present in serum mainly as an intact precursor. A relatively high concentration of GAIPIRRH-IR was found in the kidney medulla, probably due to an endocytotic re-uptake of this molecule from the tubuli after filtration in the glomeruli. The present study is consistent with the concept that NESP55, like the other chromogranins, becomes proteolytically processed. The function of this new chromogranin-like protein, therefore, might be to represent a precursor of biologically active peptides.

Novel autocrine feedback control of catecholamine release. A discrete chromogranin a fragment is a noncompetitive nicotinic cholinergic antagonist

Catecholamine secretory vesicle core proteins (chromogranins) contain an activity that inhibits catecholamine release, but the identity of the responsible peptide has been elusive. Size-fractionated chromogranins antagonized nicotinic cholinergic-stimulated catecholamine secretion; the inhibitor was enriched in processed chromogranin fragments, and was liberated from purified chromogranin A. Of 15 synthetic peptides spanning approximately 80% of chromogranin A, one (bovine chromogranin A344-364 [RSMRLSFRARGYGFRGPGLQL], or catestatin) was a potent, dose-dependent (IC50 approximately 200 nM), reversible secretory inhibitor on pheochromocytoma and adrenal chromaffin cells, as well as noradrenergic neurites. An antibody directed against this peptide blocked the inhibitory effect of chromogranin A proteolytic fragments on nicotinic-stimulated catecholamine secretion. This region of chromogranin A is extensively processed within chromaffin vesicles in vivo. The inhibitory effect was specific for nicotinic cholinergic stimulation of catecholamine release, and was shared by this chromogranin A region from several species. Nicotinic cationic (Na+, Ca2+) signal transduction was specifically disrupted by catestatin. Even high-dose nicotine failed to overcome the inhibition, suggesting noncompetitive nicotinic antagonism. This small domain within chromogranin A may contribute to a novel, autocrine, homeostatic (negative-feedback) mechanism controlling catecholamine release from chromaffin cells and neurons.

Evidence that rat stomach ECL cells represent the main source of circulating pancreastatin

Recently, we showed that the ECL cells in the oxyntic mucosa of the rat stomach are an important source of circulating pancreastatin, a fragment of chromogranin A. The present study examined how much the ECL cells contribute to the circulating levels of pancreastatin during omeprazole-evoked hypergastrinemia. Rats received omeprazole (400 mumol kg-1 day-1) by the oral route for 3 weeks. Two weeks after the start of the treatment, the rats were subjected to a sham operation or fundectomy. The concentrations of gastrin and pancreastatin in serum were monitored before and after the operations. The ECL cells were visualized by pancreastatin immunostaining and their number was determined. The activity of oxyntic mucosal histidine decarboxylase (HDC) was measured before and after 2 weeks of omeprazole treatment. Omeprazole-induced hypergastrinemia resulted in elevated serum pancreastatin and increased oxyntic mucosal HDC activity. Pancreastatin-immunoreactive cells were equally numerous before and after 2 weeks of omeprazole treatment. After surgical removal of the ECL cells by fundectomy, the serum gastrin concentration remained high whereas the serum pancreastatin concentration decreased by 90%. We conclude that the ECL cells in omeprazole-treated rats are responsible for 90% of circulating pancreastatin.

Biosynthesis of large dense-core vesicles in PC12 cells: effects of depolarization and second messengers on the mRNA levels of their constituents

mRNA levels of various constituents of large dense-core vesicles were determined in PC12 cells during depolarization and/or in the presence of BayK 8644, forskolin or phorbolester. For the soluble (secretory) proteins of the vesicles the mRNAs of chromogranin A and B, secretogranin II, neuropeptide Y and VGF were analyzed. Depolarization in the presence of BayK induced a strong up-regulation of the messages for chromogranin B, neuropeptide Y and VGF. Addition of forskolin enhanced this response for neuropeptide Y and VGF, phorbolester did the same only for VGF. Partly membrane-bound and membrane-spanning components analyzed were carboxypeptidase H, dopamine beta-hydroxylase and glycoprotein III (clusterin), peptidylglycine alpha-amidating mono-oxygenase and cytochrome b-561, respectively. Changes of mRNAs for these components were in general smaller and delayed. Six days of depolarization caused an up-regulation of glycoprotein III, peptidylglycine alpha-amidating mono-oxygenase and carboxypeptidase H mRNA levels which were not further increased by cyclic AMP and phorbolester. The dopamine beta-hydroxylase message increased after 6 days of depolarization, however, addition of phorbolester reduced this effect. For cytochrome b-561 there was no change after any of the conditions employed. These in vitro results are compared with those obtained for the biosynthesis regulation of large dense-core vesicles under in vivo conditions. It is suggested that in vivo acetylcholine and vasoactive intestinal polypeptide released from splanchnic nerve induce a differential change in the biosynthesis of large dense-core vesicles by acting via calcium and protein kinase A and C.

Synaptic vesicles and release of transmitters: new insights at the molecular level

Neurotransmitter release from transmitter storage vesicles is a regulated signalling event that has properties in common with other secretory systems. Biochemical characterization of mammalian synaptic vesicle proteins has recently converged with studies of protein traffic in non-neuronal cells and the genetic dissection of the yeast secretory pathway to give us a considerable amount of new data. Many new synaptic vesicle proteins have been characterized together with plasma membrane proteins with which they interact, and it appears that many of the participating components may be part of a general machinery for secretion. The new results significantly improve our understanding of the molecular mechanisms governing transmitter release. This review discusses the recent progress in terms of synaptic vesicle components and the proposed mechanisms for exocytosis.

Circulating pancreastatin is a marker for the enterochromaffin-like cells of the rat stomach

BACKGROUND/AIMS

Peptides of the chromogranin family occur in peptide hormone-producing cells throughout the body. One source of such peptides is the enterochromaffin-like (ECL) cells, which constitute the predominant population of endocrine cells in the fundus (the acid-producing part) of the rat stomach. The purpose of this study was to examine whether ECL cells, which are controlled by gastrin, represent a major source of circulating pancreastatin, a fragment of chromogranin A.

METHODS

Rats underwent surgical procedures and treatments in which the ECL cells could be manipulated. The procedures included antrectomy, fundectomy, and gastrectomy (and adrenalectomy), and the treatments included fasting or feeding, gastrin-17 infusion, and administration of omeprazole or ranitidine. The concentrations of pancreastatin-like immunoreactivity (LI) and gastrin in the serum were determined by radioimmunoassay.

RESULTS

The serum pancreastatin-LI concentration was lowered by about 80% by fundectomy and gastrectomy; both of these procedures eliminated the ECL cell population. Adrenalectomy had no effect on the serum pancreastatin-LI concentration. Gastrin infusion, which activates the ECL cells, promptly increased serum pancreastatin-LI concentration. Refeeding after fasting and administration of omeprazole or ranitidine increased the serum pancreastatin-LI concentrations; these responses were prevented by antrectomy.

CONCLUSIONS

The concentration of circulating pancreastatin-LI reflects the activity of the ECL cells and the size of the ECL cell population in the rat stomach.

Vasoinhibitory activity of synthetic peptides from the amino terminus of chromogranin A

Naturally occurring amino terminal fragments of chromogranin A (CGA), the calcium-binding protein found in all endocrine secretory vesicles, have vasoinhibitory activity when tested in isolated segments of the endothelium-denuded human saphenous vein. Synthetic peptides corresponding to sequences within the first 76 residues of chromogranin A have been made and tested for biological activity. Full length vasostatin I (CGA1-76) (40 nM), but not the truncated vasostatin I, CGA1-40 (100 nM) mimics natural chromogranin A fragments in its inhibition of contractions induced by endothelin-1 (ET-1) in calcium containing medium. CGA1-40 (100 nM) mimics the inhibitory effect of the vasostatins on the contractions induced in the absence of extracellular calcium by high potassium and noradrenaline, but not by ET-1. The iodinated peptides both exhibit saturable binding in an aortic smooth muscle cell line, indicative of a single class of high affinity binding protein ('receptor' with an apparent KD of approximately 45 nM. This binding is not affected by endothelin-1. Iodinated peptides can be crosslinked to a single polypeptide in binding experiments performed on intact calf aortic smooth muscle cells.