Chromogranin A, an “On/Off” Switch Controlling Dense-Core Secretory Granule Biogenesis

Abstract

We present evidence that regulation of dense-core secretory granule biogenesis and hormone secretion in endocrine cells is dependent on chromogranin A (CGA). Downregulation of CGA expression in a neuroendocrine cell line, PC12, by antisense RNAs led to profound loss of dense-core secretory granules, impairment of regulated secretion of a transfected prohormone, and reduction of secretory granule proteins. Transfection of bovine CGA into a CGA-deficient PC12 clone rescued the regulated secretory phenotype. Stable transfection of CGA into a CGA-deficient pituitary cell line, 6T3, lacking a regulated secretory pathway, restored regulated secretion. Overexpression of CGA induced dense-core granules, immunoreactive for CGA, in nonendocrine fibroblast CV-1 cells. We conclude that CGA is an "on/off" switch that alone is sufficient to drive dense-core secretory granule biogenesis and hormone sequestration in endocrine cells.

Pro-hormone Secretogranin II Regulates Dense Core Secretory Granule Biogenesis in Catecholaminergic Cells

“…1). In line with studies addressing CgA-or CgB-mediated granulogenesis in model cell lines (11,12,47), our results emphasize the necessity of in cella analyses to assess the direct role of granins in the regulation of DCG formation. We found a disproportion between the degree of silencing of SgII expression and the decreased number of DCGs in PC12 cells (ϳ80% versus ϳ40%; Fig.…”

“…3 and 5) that the morphological parameters defining a DCG (63) are met when SgII is expressed in A35C cells. Similar outcomes are obtained upon expression of CgA in secretory-deficient endocrine cells lines (10,12). However, expression of CgA, CgB, or SgII in non-neuroendocrine cells consistently generates larger vesicular structures than PC12 DCGs (11,17,19).…”

“…constitutively secreted or mis-trafficked to the lysosomal pathway in secretory-deficient variants) include human growth hormone, the bovine pro-hormone pro-opiomelanocortin, and carboxypeptidase H (10,12,27,61). Here, we used NPY, routed to the mitochondria in non-neuroendocrine and secretory-deficient neuroendocrine cells (42)(43)(44)(45), as a prototype secretory protein.…”

“…In vitro and in vivo evidence now suggests that CgA plays a critical role in the process. Depletion of CgA in PC12 cells reduces the number of DCGs (10 -12) and the intracellular levels of other granule proteins (12,13). Impaired expression of CgA in transgenic mice decreases the number of DCGs in the adrenal medulla and perturbs the storage and release of other DCG constituents, including CgB, neuropeptide Y (NPY), and catecholamines (13,14).…”

“…However, the observed substantial increase of CgB and SgII expression in knock-out animals suggests that CgB and/or SgII might partly compensate for CgA deficiency (15). Indeed, CgB depletion in PC12 cells may contribute to an 80% decreased number of DCGs (11), although CgB seems unable to rescue a regulated secretory pathway in secretory-deficient neuroendocrine cells (12,16). A granulogenic role for CgA and CgB is also documented across cell lineages, where their expression induces the formation of granule-like structures competent for exocytosis (11,12,(17)(18)(19)(20).…”