SELECTIVE STORAGE OF ACETYLCHOLINE, BUT NOT CATECHOLAMINES, IN NEUROENDOCRINE SYNAPTIC-LIKE MICROVESICLES OF EARLY ENDOSOMAL ORIGIN

We have defined, in the neuroendocrine cell line PC12, the catecholami ne- and acetylcholine-storing organelles in the context of the biogene sis of secretory granules and synaptic-like microvesicles (SLMVs). SLM Vs were found to originate directly from early endosomes. Both early e ndosomes and SLMVs exhibited uptake and storage of biosynthetic acetyl choline. Surprisingly, however, despite the presence of a reserpine-se nsitive vesicular amine transporter in early endosomes, SLMVs lacked d etectable uptake and storage of catecholamines. This was confined to t wo populations of mature secretory granules, referred to as small and large mature secretory granules, which both derived from immature secr etory granules. Our results show that PC 12 cells lack small dense cor e vesicles, i.e., the catecholamine-storing, but secretory protein-lac king, vesicles found in sympathetic neurons and imply that the biogene sis of these vesicles requires the expression of a distinct type of ve sicular amine transporter and/or a change in endosomal protein sorting .