Heterogeneity of chromogranin A-derived peptides in bovine gut, pancreas and adrenal medulla

Abstract

Chromogranin A is produced in many endocrine cell types, and is widely used as a marker in endocrine-cell pathology and secretory-cell biology. There is some evidence that it may be proteolytically processed to yield the putative pancreatic regulatory peptide, pancreastatin, and, in order to characterize the relevant pathways in gastrointestinal and pancreatic endocrine cells, we have used, in radioimmunoassay, site-directed antibodies to pancreastatin itself (L331) and to a sequence of chromogranin A immediately C-terminal to pancreastatin (L300). The latter antibody revealed three major forms of immunoreactivity of 8 kDa and five peptides of approx. 3 kDa in bovine pancreas and gut extracts. The 8 kDa peptides were characterized as chromogranin A-(248-313)-peptides, i.e. C-terminally extended forms of pancreastatin; two of the 8 kDa variants differed in two positions, confirming a polymorphism predicted from cDNA sequencing. One of the 3 kDa peptides was characterized as chromogranin A-(297-313)-peptide, i.e. the C-terminal heptadecapeptide of the 8 kDa peptide that would be liberated after cleavage to yield pancreastatin. On the basis of chromatographic studies, immunohistochemistry and the stoichiometry of different immunoreactive peptides, three different pathways of chromogranin A processing were identified: in adrenal chromaffin cells chromogranin A existed mainly as the unmodified intact protein, in pancreatic islet and gastric antral endocrine cells pancreastatin and the 3 kDa peptides were major products, but in small intestine and gastric corpus endocrine cells there was little nor no pancreastatin and the 8 kDa cleavage product predominated. There are therefore important differences in the distribution of chromogranin A-derived peptides between quite closely related populations of endocrine cells that are attributable not only to variable post-translational cleavage but also to the expression of different primary sequences. It seems possible that in different cell types chromogranin A-derived peptides might subserve a variety of different functions.

Selective expression of TLQP-21 and other VGF peptides in gastric neuroendocrine cells and modulation by feeding

“…A parallel is often drawn between VGF and the chromograninsecretogranin family of proteins, because of their selective localisation in secretory granules, as well as their role as precursors of multiple bioactive peptides (Helle 2004). Chromogranins, too, are differentially processed in gastric endocrine cells (Watkinson et al 1991, Watkinson & Dockray 1992, Portela-Gomes & Stridsberg 2002), although to a profile different from the one we observed for VGF peptides. In fact, G cells showed immunoreactivity to virtually all region-specific antibodies to chromogranin A (Portela-Gomes & Stridsberg 2002), as opposed to the limited reactivity of VGF peptides we could demonstrate in such cell type.…”

Chromogranin A: current status as a precursor for bioactive peptides and a granulogenic/sorting factor in the regulated secretory pathway

“…The bovine CGA gene contains an apparent polymorphism in the pancreastatin coding region, resulting in peptide variants containing either an arginine or a histidine residue. Reports of the presence of both peptides in a single bovine tissue suggest this is an allelic variation [27], since but a single mammalian CGA gene appears to exist [25,26,28], localized in the human genome to chromosome 14 [29,30].…”

“…The extent of chromogranin A processing is highly tissuedependent: processing appears to be least complete in the adrenal medulla and progressively greater in intestine, stomach, peripheral nerves, and pancreas [27,103,[110][111][112]. The endocrine pancreas appears to exhibit the highest level of processing based on the ratio of intact CGA to fully processed /3-granin as well as pancreastatin [21,27,88]. CGA intragranular proteolytic processing in the endocrine pancreas is apparently completely calcium-dependent, and CGA is a much better substrate for PC2 than for PC1/3 kex2-1ike prohormone converting enzymes [88].…”

“…The single exception is parastatin, the existence of which is inferred from the presence of a 14 kDa C-terminal CGA fragment generated in rat insulinoma cells, and the biological activity of the parastatin fragment generated by endoproteinase Lys-C digestion of purified CGA [88,108] The processing of CGA seems to occur preferentially at some but not all pairs of basic amino acids, and at some single basic amino acids, at the C-and N-termini of the molecule [27,103,110,111]. The extent of chromogranin A processing is highly tissuedependent: processing appears to be least complete in the adrenal medulla and progressively greater in intestine, stomach, peripheral nerves, and pancreas [27,103,[110][111][112]. The endocrine pancreas appears to exhibit the highest level of processing based on the ratio of intact CGA to fully processed /3-granin as well as pancreastatin [21,27,88].…”

A new human chromogranin A (CgA) immunoradiometric assay involving monoclonal antibodies raised against the unprocessed central domain (145-245)

“…Furthermore, several studies demonstrated that CgA could be degraded in a tissue-specific manner, which led to important disparities in the tissular distribution of the peptides released (Cetin and Grube, 1991;Curry et al, 1991;Watkinson et al, 1991). It is likely that the proteolysis properties may be responsible for the variability of the fragments found in normal and pathological tissues, blood or urine.…”