Analysis of the post-translational processing of chromogranin A in rat neuroendocrine tissue employing an N-terminal site-specific antiserum

Granin-derived peptides

The granin family comprises altogether 7 different proteins originating from the diffuse neuroendocrine system and elements of the central and peripheral nervous systems. The family is dominated by three uniquely acidic members, namely chromogranin A (CgA), chromogranin B (CgB) and secretogranin II (SgII). Since the late 1980s it has become evident that these proteins are proteolytically processed, intragranularly and/or extracellularly into a range of biologically active peptides; a number of them with regulatory properties of physiological and/or pathophysiological significance. The aim of this comprehensive overview is to provide an up-to-date insight into the distribution and properties of the well established granin-derived peptides and their putative roles in homeostatic regulations. Hence, focus is directed to peptides derived from the three main granins, e.g. to the chromogranin A derived vasostatins, betagranins, pancreastatin and catestatins, the chromogranin B-derived secretolytin and the secretogranin II-derived secretoneurin (SN). In addition, the distribution and properties of the chromogranin A-derived peptides prochromacin, chromofungin, WE14, parastatin, GE-25 and serpinins, the CgB-peptide PE-11 and the SgII-peptides EM66 and manserin will also be commented on. Finally, the opposing effects of the CgA-derived vasostatin-I and catestatin and the SgII-derived peptide SN on the integrity of the vasculature, myocardial contractility, angiogenesis in wound healing, inflammatory conditions and tumors will be discussed.

Mass spectrometry identification of granins and other proteins secreted by neuroblastoma cells

We used mass spectrometry-based protein identification to determine the presence of granins and other proteins in the mouse neuroblastoma secretome. We detected polypeptides derived from four members of the granin family: chromogranin A, chromogranin B, secretogranin III, and VGF. Many of them are derived from previously described biologically active regions; however, for VGF and CgB, we detected peptides not related to known bioactivities. Along with granins, we identified 115 other proteins secreted by mouse neuroblastoma cells, belonging to different functional categories. Fifty-six out of 119 detected proteins possess the signal fragments required for translocation into endoplasmic reticulum. Sequences of remaining 63 proteins were analyzed using SecretomeP algorithm to determine probability of nonclassical secretion. Identified proteins are involved in the regulation of cell cycle, proliferation, apoptosis, angiogenesis, proteolysis, and cell adhesion.

Inhibition of insulin secretion by betagranin, an N-terminal chromogranin A fragment

Betagranin, an N-terminal fragment of chromogranin A, results from a proteolytic processing, and is co-secreted with insulin. While other chromogranin A-derived peptides negatively modulate hormone secretion, the role of betagranin in pancreatic beta-cells is so far unknown. We have recently shown that pancreatic islet betagranin levels are down-regulated in obese, leptin-deficient mice. In the present study, we have investigated the distribution of betagranin in primary mouse islets and cells of the MIN6 line and have evaluated its effects on insulin secretion. We showed that betagranin co-localizes with insulin within secretory granules and strongly inhibited insulin secretion in response to both glucose and potassium, by blocking the influx of calcium. The data demonstrated a hitherto unknown inhibitory effect of betagranin on insulin secretion.

Characterization of natural vasostatin-containing peptides in rat heart

Chromogranin A (CGA) is a protein that is stored and released together with neurotransmitters and hormones in the nervous, endocrine and diffuse neuroendocrine systems. As human vasostatins I and II [CGA(1-76) and CGA(1-113), respectively] have been reported to affect vessel motility and exert concentration-dependent cardiosuppressive effects on isolated whole heart preparations of eel, frog and rat (i.e. negative inotropism and antiadrenergic activity), we investigated the presence of vasostatin-containing peptides in rat heart. Rat heart extracts were purified by RP-HPLC, and the resulting fractions analyzed for the presence of CGA N-terminal fragments using dot-blot analysis. CGA-immunoreactive fractions were submitted to western blot and MS analysis using the TOF/TOF technique. Four endogenous N-terminal CGA-derived peptides [CGA(4-113), CGA(1-124), CGA(1-135) and CGA(1-199)] containing the vasostatin sequence were characterized. The following post-translational modifications of these fragments were identified: phosphorylation at Ser96, O-glycosylation (trisaccharide, NAcGal-Gal-NeuAc) at Thr126, and oxidation at three methionine residues. This first identification of CGA-derived peptides containing the vasostatin motif in rat heart supports their role in cardiac physiology by an autocrine/paracrine mechanism.

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.

Chromogranin A transcription and gene expression in Folliculostellate (TtT/GF) cells inhibit cell growth

Folliculostellate (FS) cells are present in the anterior pituitary and have important regulatory functions including controlling hormone release from other anterior pituitary cells. FS cells do not usually express neuroendocrine genes such as chromogranin A (CgA). We analyzed transcriptional regulation and gene expression in the TtT/GF FS cell line to better understand the role of FS cells in anterior pituitary function. After transient transfection with a human (h) CgA promoter sequence linked to a luciferase reporter, there was basal level of transcriptional activity, which was two- to fourfold less than that observed in the anterior pituitary neuroendocrine cell lines HP75 and GH3. The transcriptional activity was decreased in all cell lines when a mutant hCgA promoter cyclic AMP response element (CRE) was used for transfection. Sodium butyrate treatment increased the transcriptional activity in all cell lines, but remained two- to fourfold higher in the HP75 and GH3 cell lines than in the TtT/GF cells. Stable transfection of a plasmid expressing bovine (b) CgA in the TtT/GF cells led to inhibition of cell growth as measured by 3H-thymidine incorporation, Ki-67 labeling index, and growth curve analysis. CgA protein and mRNA could be readily demonstrated in the cloned cells but not in the parental cell line or vector control cells. When the CgA expressing cloned cells were injected into SCID mice, there was a decrease in the rate of tumor growth compared to the vector control in vivo. These results indicate that the TtT/GF FS cells are fibroblast-like compared to the neuroendocrine anterior pituitary secretory cells when analyzed by transcriptional activity with a transiently transfected CgA promoter. In TtT/GF cells with a stably transfected bCgA plasmid, CgA has a direct regulatory effect on tumor cell proliferation.

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.