Neurons have at least two pathways of regulated secretion, which involve two classes of secretory organelles: typical synaptic vesicles (SVs) and large dense-core vesicles. Large dense-core vesicles store and secrete peptide neurotransmitters and amines, and may be seen as the neuronal counterpart of secretory granules of endocrine cells. SVs are highly specialized secretory organelles, which store and secrete non-peptide hormones and play a dominant role in the fast, point-to-point signalling typical of the nervous system. Microvesicles that share a variety of biochemical and functional similarities with SVs (synaptic-like microvesicles) have recently been described in endocrine cells. SVs and synaptic-like microvesicles are closely related to vesicular carriers of the receptor-mediated recycling pathway. They undergo repeated cycles of exo-endocytosis, which are thought to involve endosomal intermediates.
In mature neurons, SVs are concentrated in axon endings. To gain insight into the mechanisms responsible for SV targeting, we have studied the traffic of SV proteins in both endocrine cells and developing hippocampal neurons in primary culture at different stages of differentiation. Additionally, the distribution of the SV protein synaptophysin, when expressed by transfection in fibroblastic cells or in polarized epithelial cells (MDCK cells), was investigated.
SV proteins are already present in developing neurons at stages preceding the establishment of neuronal polarity. As axons and dendrites form, SV proteins are found in both types of processes, although they become progressively more concentrated in the axon. Throughout these developmental stages SVs undergo active exo-endocytotic recycling. The nonpolarized distribution of SV proteins is observed even at stages when the transferrin receptor, a protein that is present in epithelial cells only at the basolateral surface, is already restricted to dendrites. This indicates that, in immature neurons, SV proteins are not selectively targeted to axons and that the accumulation in axons may at least partially result from a specific retention. In agreement with this finding, synaptophysin, when transfected into MDCK cells, was targeted to both the basolateral and the apical plasma membrane. Brefeldin A, a fungal metabolite that induces a modification of the steady-state localization of endosomal proteins in a variety of cell types, was found to have a different effect on the distribution of SV proteins in dendrites and in axons. Taken together, these observations support the existence of two separate endosomal systems in axons and dendrites, which have differential properties, are enriched in different proteins, and may be related to the basolateral and apical endosomes of epithelial cells.
- © The Company of Biologists Limited 1993