Chromogranin B (CHGB) is the major matrix protein in human catecholamine storage vesicles. CHGB genetic variation alters catecholamine secretion and blood pressure. Here, effective Chgb protein under-expression was achieved by siRNA in PC12 cells, resulting in ~ 48% fewer secretory granules on electron microscopy, diminished capacity for catecholamine uptake (by ~ 79%), and a ~ 73% decline in stores available for nicotinic cholinergic-stimulated secretion. In vivo, loss of Chgb in knockout mice resulted in a ~ 35% decline in chromaffin granule abundance and ~ 44% decline in granule diameter, accompanied by unregulated catecholamine release into plasma. Over-expression of CHGB was achieved by transduction of a CHGB-expressing lentivirus, resulting in ~ 127% elevation in CHGB protein, with ~ 122% greater abundance of secretory granules, but only ~ 14% increased uptake of catecholamines, and no effect on nicotinic-triggered secretion. Human CHGB protein and its proteolytic fragments inhibited nicotinic-stimulated catecholamine release by ~ 72%. One conserved-region CHGB peptide inhibited nicotinic-triggered secretion by up to ~ 41%, with partial blockade of cationic signal transduction. We conclude that bi-directional quantitative derangements in CHGB abundance result in profound changes in vesicular storage and release of catecholamines. When processed and released extra-cellularly, CHGB proteolytic fragments exert a feedback effect to inhibit catecholamine secretion, especially during nicotinic cholinergic stimulation.
Zhang K, Biswas N, Gayen JR, et al., J Neurochem. 2014 Apr;129(1):48-59. doi: 10.1111/jnc.12527. Epub 2013 Nov 25.
The versatility of intracellular calcium as a second messenger is seen in its ability to mediate opposing events such as neuronal cell growth and apoptosis. A leading hypothesis used to explain how calcium regulates such divergent signaling pathways is that molecules responsible for maintaining calcium homeostasis have multiple roles. For example, chromogranin B (CGB), a calcium binding protein found in secretory granules and in the lumen of the endoplasmic reticulum, buffers calcium and also binds to and amplifies the activity of the inositol 1,4,5-trisphosphate receptor (InsP(3)R). Previous studies have identified two conserved domains of CGB, an N-terminal domain (N-CGB) and a C-terminal domain (C-CGB). N-CGB binds to the third intraluminal loop of the InsP(3)R and inhibits binding of full-length CGB. This displacement of CGB decreases InsP(3)R-dependent calcium release and alters normal signaling patterns. In the present study, we further characterized the role of N-CGB and identified roles for C-CGB. The effect of N-CGB on calcium release depended upon endogenous levels of cellular CGB, whereas the regulatory effect of C-CGB was apparent regardless of endogenous levels of CGB. When either full-length CGB or C-CGB was expressed in cells, calcium transients were increased. Additionally, the calcium signal initiation site was altered upon C-CGB expression in neuronally differentiated PC12 and SHSY5Y cells. These results show that CGB has numerous regulatory roles and that CGB is a critical component in modulating InsP(3)R-dependent calcium signaling.
Schmidt S, Mo M, Heidrich FM, Celic A, Ehrlich BE. J Biol Chem. 2011 Dec 30;286(52):44888-96. doi: 10.1074/jbc.M111.251330. Epub 2011 Oct 20.
