What the granins tell us about the formation of secretory granules in neuroendocrine cells

The biochemical properties of the granins, studied in vitro and in a perforated TGN system, support the concept that the Sciective aggregation of regulated secretory proteins, promoted by the specific lumenal milieu of the TGN, is a key step in their segregation from constitutive secretory proteins in this compartment. A recently identified membrane-associated form of the granins is likely to also be involved in this aggregation, as well as in the membrane envelopment of the aggregate during the formation of an immature secretory granule.

[Gut hormone secreting cells--unsolved problems]

The gut endocrine cells generally are bipolar paraneurons, extending from the epithelial base to the luminal surface. The cells, also known as the basal-granulated cells, produce peptidic and amine ++ signal substances to release them in response to the luminal stimuli. The substances are released not only in an endocrine fashion, but also, and more importantly, in a paracrine way, thus stimulating nervous, muscular, secretory and other elements in the vicinity. Attention should be paid also to other secretions of the gut endocrine cells, ATP and chromogranins (acidic carrier proteins) which are co-released with the peptides and amines. Various gut hormones have been demonstrated to be released into the gut lumen, besides into the blood and tissue spaces. The possible mechanism of this partial exocrine function of the endocrine cells remains to be explored.

The distribution of chromogranin A-like immunoreactivity in the human hippocampus coincides with the pattern of resistance to epilepsy-induced neuronal damage

The distribution of chromogranin A-like immunoreactivity in the hippocampus of adult humans who were free of neurological disease was examined by immunohistochemical methods. Immunoreactivity was restricted to the cytoplasm of certain neuronal populations, most notably the mossy fibers of denate granule cells (and a subset of their perikarya), and the perikarya of pyramidal cells of the cornu Ammonis 2 (CA2) sector. Additionally, staining was observed in neurons in the stratum oriens, a population of neurons at the periphery of the CA4 sector, scattered, probably short-axon perikarya in the CA1 sector, and fibers in the perforant path and the molecular layer of the dentate gyrus. Pyramidal neurons in the CA1 and CA3 sectors were not immunoreactive. The two prominently immunoreactive neuronal populations, CA2 pyramids and dentate granule cells, are those spared in human and experimental epileptic brain damage, whereas CA1 and CA3 pyramids, lacking chromogranin, are characteristically destroyed in this condition. The known activities of chromogranin in the periphery as a calcium-binding protein and as a precursor of active peptides (autocrine inhibitory modulators) suggest that its distribution in the hippocampus may help to explain the observed pattern of resistance to epileptic brain damage.

Synaptophysin and chromogranins/secretogranins--widespread constituents of distinct types of neuroendocrine vesicles and new tools in tumor diagnosis

Normal and neoplastic neuroendocrine (NE) cells have been identified for many years by morphological criteria only. With the advent of immunocytochemistry, antibodies against NE-specific polypeptides have been used to identify NE cells that had been missed by conventional techniques, thus improving the diagnosis of NE cells. In this review article we discuss (i) the biochemical, cell biological and molecular biological data obtained so far for two major types of NE markers, synaptophysin, which is characteristic of the small "transparent-looking" neurosecretory vesicles, and the chromogranins/secretogranins, which are widespread constituents of the larger "dense-cored" secretory granules; (ii) the immunohistochemical data obtained for these marker proteins in normal and neoplastic human NE cells and tissues; and (iii) future possible developments involving these as well as other proteins that are associated with these two distinct secretory organelles of NE cells and may serve as potential markers in NE cell diagnosis.

Ontogenesis of chromogranin A and B and catecholamines in rat adrenal medulla

The ontogenesis of chromogranins A and B and catecholamines was investigated for rat adrenal medulla. The chromogranins, the major secretory peptides of chromaffin granules, were characterized by one- and two-dimensional immunoblotting. Chromogranin B appeared identical in fetal and postnatal adrenals. On the other hand a significant portion of chromogranin A immunoreactivity in fetal adrenals was present in a component which was identified as a proteoglycan form of chromogranin A. In adult adrenals this proteoglycan chromogranin A was different and much less prominent. Total chromogranins and catecholamines increased parallely from the 17th prenatal day to the adult stage. However in early periods of development chromaffin granules are likely to contain relatively higher catecholamine levels than adult granules. At the end of the gestational period both the ratio of adrenaline to noradrenaline and of chromogranin A vs B increases sharply. These results establish that from the 17th prenatal day onward fetal adrenal glands contain chromaffin granules which are filled with both the major secretory peptides and catecholamines. The relative composition of the secretory content (adrenaline vs noradrenaline and chromogranin A vs chromogranin B) changes at the end of the gestational period in parallel with the development of the corticosteroid-producing adrenal cortex.

Immunohistochemical analyses of oncocytic and chromophobe pituitary adenomas

Immunohistochemical analyses using antibodies against the major pituitary hormones, the alpha-subunit of glycoprotein hormones, chromogranin and synaptophysin were performed on 10 adenomas with oncocytic parts (26%-50% oncocytes, Group I), on 9 oncocytic adenomas with 51%-75% oncocytes (Group II), and on 12 oncocytic adenomas with 76%-100% oncocytes (Group III). Only 11 of the 31 investigated adenomas (35%) showed negative immunostaining for all major anterior pituitary hormones. FSH-content could be shown in 16 of 31 adenomas (52%), LH-content in 12 of 31 adenomas (39%), TSH-content in 3 of 31 adenomas (10%). Comparing all three groups of adenomas, there are no differences in the immunoreactivity to alpha-subunit (24 of 31 adenomas, 77%), chromogranin (26 of 31 adenomas, 84%), and of synaptophysin (13 of 31 adenomas, 42%). Considering the high percentage of cells of oncocytes showing alpha-subunit immunoreactivity we regard oncocytomas as originating very often from TSH-gonadotropin cell complexes of the anterior hypophysis. Alpha-subunit might become a reliable marker for oncocytomas. The finding of immunoreactivity to chromogranin in most cases confirms morphological studies that oncocytes contain some secretory granules. In most cases, the studied oncocytomas did not react to synaptophysin showing different results from other adenomas of the anterior hypophysis.

Secretion from chromaffin cells is controlled by chromogranin A-derived peptides

Chromogranin A (CGA) is the major protein of the secretory granule from chromaffin cells and also is found in a variety of endocrine cells. Although the sequence of this acidic glycoprotein has been elucidated recently, its biological function is unknown. Here we have purified CGA from chromaffin granules; the final preparation contained the 74-kDa native CGA together with two degradation products--three bands near 60 kDa and a single band of 43 kDa. This preparation was found to inhibit (a maximum inhibition of 60% at 1 microM) the nicotine-induced, but not the high K+-evoked, catecholamine secretion from bovine chromaffin cells maintained in primary culture. Spontaneous release was also affected in the nanomolar CGA protein concentration range. The observation that the inhibitory effect is strictly dependent on a preincubation step together with the modification of the CGA protein profile during this preincubation step suggests that the degradation peptide(s) rather than the 74-kDa native CGA--the approximately equal to 60-kDa bands or the 43-kDa singlet band--is actually involved in secretory cell activity. This was demonstrated by using trypsin-generated peptides that were inhibitory without the preincubation period. The finding that unprocessed CGA is not active on chromaffin cell secretion suggests that this molecule is a precursor of a peptide(s) that is able to regulate catecholamine secretion. Thus, the present data suggest that a CGA-derived peptide(s) could exert a feedback control on chromaffin cell secretory activity--a mechanism that might be of importance during stress situations.

Neuroendocrine background of the pathology of the islets of Langerhans. A minireview with particular reference to synaptophysin and chromogranin A as neuroendocrine markers and to the ontogeny of argyrophil insulin immunoreactive cells in the rabbit

As in diseases of other (neuro)endocrine glands, the pathology of the islets of Langerhans comprises both states of hypofunction, notably hypoinsulinism,--diabetes mellitus--and those of hyperfunction, such as hyperinsulinism from either nesidiodysplasia ("nesidioblastosis") or genuine islet-cell neoplasms. The pathogenesis of some characteristic structural lesions of these diseases of the islet parenchymal cells is reviewed against their neuroendocrine background, both phylogenetically and ontogenetically. A preliminary report is given of the appearance of argyrophil insulin cells during the normal foetal development of the islet parenchyma of the rabbit. In addition, the distribution of the newly discovered neuroendocrine markers synaptophysin and chromogranin A has been reviewed immunohistochemically in normal and neoplastic islet parenchyma cells; the insulin cells do not seem to contain chromogranin A.

New suggestions for the physiological role of secretory protein-I

A major endocrine secretory protein of unknown function termed secretory protein-I or, alternatively, chromogranin A, is attracting increasing attention. Because of its abundance and apparently universal presence in endocrine and neuroendocrine cells, many suggestions for a physiological role have been advanced. Recent data provide the first firm bases for some proposed functions that include a regulator of a novel protease capable of converting prohormones to hormones, a hormone precursor, and through its calcium-binding and aggregating properties, a secretory granule condensing protein.

Osmotic properties of the chromogranins and relation to osmotic pressure in catecholamine storage granules

The soluble proteins (chromogranins) of bovine chromaffin granules have been studied by micro-osmometry with semi-permeable membranes (UM2, PM10 and PM30 with cut-offs greater than 1, greater than 10 and greater than 30 kD, respectively) at 1 = 0.15 and pH 5-8 for protein concentrations up to 20 mg X ml-1. After lysis of chromaffin granules in phosphate buffer pH 6, the released chromogranins behaved as aggregating solutes, consistent with an inconspicuous osmotic pressure contribution from the chromogranins at the protein concentration of the intact granules. Thus, in the presence of phosphate about 90% of the molecules behaved as colloids with Mr = 30,300 at c = o. After lysis in phosphate-free buffers the chromogranins behaved as highly non-ideal solutes in a manner which was incompatible with isotonicity at the protein concentration of the intact granules. About two-thirds of the molecules in the lysates in Na-succinate pH 5-6 and K-acetate pH 6 exhibited Mr = 66,000 and 79,000, respectively. In dilute solutions (less than 12 mg protein X ml-1) and ATP/protein ratios corresponding to those in the intact granules, the UM2 pressures were markedly increased, indicating release of polypeptides with Mr 2000-3000 from aggregates. CaCl2 was without specific effect on the colloid osmotic pressures but reduced the ATP-dependent increase in pressure, suggesting release of molecules twice the size of those released by ATP alone. A model is presented for the contribution of the chromogranins to osmotic pressure regulation in the bovine adrenomedullary catecholamine-storing granules.