Differential Expression of Secretogranins II and III in Canine Adrenal Chromaffin Cells and Pheochromocytomas

Dominant Expression of Chromogranin B in Pituitary Corticotrophs and Its Putative Role in Interaction With Secretogranin III

SummaryPrevious studies have suggested that chromogranin A (CgA) is a partner molecule of secretogranin III (SgIII). In mouse pituitary corticotroph-derived AtT-20 cells, SgIII plays a role in sorting CgA/hormone aggregates into secretory granules (SGs). Although CgA expression is equivocal, CgB is clearly detectable in the rat pituitary corticotrophs. Therefore, we hypothesized that CgB shares a function with CgA in pituitary corticotrophs. In the binding assays, CgB, similar to CgA, showed binding activity to SgIII under weakly acidic conditions and in the presence of Ca2+. Considering the differences in animal species, the different abilities of antibodies, and the conditions of tissue fixation and thin sectioning in immunofluorescence histochemistry, we found that CgA was expressed in a small population (approximately 10%), and its expression intensity was weaker than that of CgB (>98%) in rodent pituitary corticotrophs. In addition, similar to CgA, CgB and SgIII were colocalized in adrenocorticotropic hormone (ACTH) granules. The labeling of CgA and CgB was not completely consistent, and CgB colocalized with SgIII in many granules. These results suggest that there are multiple sorting systems for ACTH granules in pituitary corticotrophs and that the SgIII/CgB complex behaves more dominantly than the SgIII/CgA complex, which has somewhat different properties.

Insulin granule morphology and crinosome formation in mice lacking the pancreatic β cell-specific phogrin (PTPRN2) gene

We recently reported that phogrin, also known as IA-2β or PTPRN2, forms a complex with the insulin receptor in pancreatic β cells upon glucose stimulation and stabilizes insulin receptor substrate 2. In β cells of systemic phogrin gene knockout (IA-2β-/-) mice, impaired glucose-induced insulin secretion, decreased insulin granule density, and an increase in the number and size of lysosomes have been reported. Since phogrin is expressed not only in β cells but also in various neuroendocrine cells, the precise impact of phogrin expressed in β cells on these cells remains unclear. In this study, we performed a comprehensive analysis of morphological changes in RIP-Cre+/-Phogrinflox/flox (βKO) mice with β cell-specific phogrin gene knockout. Compared to control RIP-Cre+/- Phogrin+/+ (Ctrl) mice, aged βKO mice exhibited a decreased density of insulin granules, which can be categorized into three subtypes. While no differences were observed in the density and size of lysosomes and crinosomes, organelles involved in insulin granule reduction, significant alterations in the regions of lysosomes responding positively to carbohydrate labeling were evident in young βKO mice. These alterations differed from those in Ctrl mice and continued to change with age. These electron microscopic findings suggest that phogrin expression in pancreatic β cells plays a role in insulin granule homeostasis and crinophagy during aging, potentially through insulin autocrine signaling and other mechanisms.