A role for synaptic vesicles in non-neuronal cells: clues from pancreatic beta cells and from chromaffin cells.

Synaptic vesicles, vesicular carriers highly specialized for the secretion of fast nonpeptide neurotransmitters, until recently were considered neuron-specific organelles. The identification and characterization of several of the most abundant synaptic vesicle proteins have led to the identification in peptide-secreting endocrine cells of a class of microvesicles, referred to as synaptic-like microvesicles (SLMVs), which are similar to synaptic vesicles in membrane composition, biogenesis, and life cycle. Studies of pancreatic beta cells and of a chromaffin cell-derived cell line (PC12 cells) have suggested that SLMVs, like synaptic vesicles, store and secrete neurotransmitter-like substances that act as paracrine/endocrine signaling molecules. SLMVs of pancreatic beta cells store GABA; SLMVs of PC12 cells contain acetylcholine. Both synaptic vesicles and SLMVs are recycling organelles that represent a specialized subcompartment of the receptor-mediated recycling pathway of all cells. Recently, homologues of membrane proteins of synaptic vesicles and SLMVs have been identified in the endocytic/recycling compartment of non-neuronal, nonendocrine cells. Tracing the evolution of synaptic vesicles from vesicular carriers present in all cells will help to advance our understanding of neurotransmitter release as well as of molecular mechanisms of vesicular traffic.