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 catecholamine- and acetylcholine-storing organelles in the context of the biogenesis of secretory granules and synaptic-like microvesicles (SLMVs). SLMVs were found to originate directly from early endosomes. Both early endosomes and SLMVs exhibited uptake and storage of biosynthetic acetylcholine. Surprisingly, however, despite the presence of a reserpine-sensitive vesicular amine transporter in early endosomes, SLMVs lacked detectable uptake and storage of catecholamines. This was confined to two populations of mature secretory granules, referred to as small and large mature secretory granules, which both derived from immature secretory granules. Our result show that PC12 cells lack small dense core vesicles, i.e., the catecholamine-storing, but secretory protein-lacking, vesicles found in sympathetic neurons and imply that the biogenesis of these vesicles requires the expression of a distinct type of vesicular amine transporter and/or a change in endosomal protein sorting.