CHROMOGRANIN-B (SECRETOGRANIN-I), A NEUROENDOCRINE-REGULATED SECRETORY PROTEIN, IS SORTED TO EXOCRINE SECRETORY GRANULES IN TRANSGENIC MICE

Chromogranin B (CgB, secretogranin I) is a secretory granule matrix pr otein expressed in a wide variety of endocrine cells and neurons. Here we generated transgenic mice expressing CgB under the control of the human cytomegalovirus promoter, Northern and immunoblot analyses, in s itu hybridization and immunocytochemistry revealed that the exocrine p ancreas was the tissue with the highest level of ectopic CgB expressio n; Upon subcellular fractionation of the exocrine pancreas, the distri bution of CgB in the various fractions was indistinguishable from that of amylase, an endogenous constituent of zymogen granules. Immunogold electron microscopy of pancreatic acinar cells showed co-localization of CgB with zymogens in Golgi cisternae, condensing vacuoles/immature granules and mature zymogen granules; the ratio of immunoreactivity o f CgB to zymogens being highest in condensing vacuoles/immature granul es. CgB isolated from zymogen granules of the pancreas of the transgen ic mice aggregated in a mildly acidic (pH 5.5) milieu in vitro, sugges ting that low pa-induced aggregation contributed to the observed conce ntration of CgB in condensing vacuoles,;Our results show that a neuroe ndocrine-regulated secretory protein can be sorted to exocrine secreto ry granules in vivo, and imply that a key feature of CgB sorting in th e traits-Golgi network of neuroendocrine cells, i.e. its aggregation-m ediated concentration in the course of immature secretory granule form ation, also occurs in exocrine cells although secretory protein sortin g in these cells is thought to occur largely in the course of secretor y granule maturation.