Characterization and Distribution of SNARE Proteins at Neuroendocrine Nerve Endings

Abstract

Substantial evidence now exists to support a defined complex of interacting proteins, comprised of soluble, vesicle and plasma membrane components, as the core of a general membrane fusion mechanism. Specializations to the general secretory model occur based on cell-specific differences in Ca²⁺ regulation, secretory organelle types and secretory dynamics. The variation in secretory properties may also result, in part, from isoform diversity and selective-pairing of the molecular components of the core complex. The present report attempts to identify the SNARE proteins found in isolated peptidergic nerve endings of the rat neurohypophysis. The results demonstrate the presence of synaptosomal-associated protein of 25 kD, syntaxin and synaptobrevin as membrane-associated proteins in these nerve endings. Furthermore, we have utilized sucrose density gradient subcellular fractionation and immunoprecipitation protocols to investigate the synaptobrevin isotypes present on secretory granules and to probe using electrophysiological methods their functional relationship to secretion. Secretory granules were found to contain only the synaptobrevin 2 isoform, although the nerve endings themselves were found to possess in addition, synaptobrevin 1 and the closely related protein cellubrevin. Analysis of the secretory characteristics of single nerve endings using membrane capacitance measurements together with Botulinum B toxin dialysis demonstrated the critical importance of synaptobrevin 2 to both the rapid exocytotic release and a slower secretory process, that perhaps includes secretory granule recruitment and priming, in these peptidergic nerve endings.

Robo2 regulates synaptic oxytocin content by affecting actin state

“…Conversely, expression of light chain of botulinum toxin serotype B specifically in the OXT neurons, led to increased synaptic OXT content. This is in agreement with the fact that neurohypophyseal neurons exhibit spontaneous activity and SNARE-mediated synaptic release, which is blocked by the botulinum toxin serotype B (Jurgutis et al, 1996;Tobin et al, 2012).…”

Heterogeneous expression of SNAP‐25 and synaptic vesicle proteins by central and peripheral inputs to sympathetic neurons

“…Large vesicles storing neuropeptides express lower levels of v‐SNAREs than do small vesicles (Jurgutis et al, 1996; Langley and Grant, 1997; Winkler, 1997; Berg et al, 2000), and their exocytosis apparently requires a specific protein (calcium‐dependent activator protein for secretion, mammalian UNC‐31; Berwin et al, 1998; Rupnik et al, 2000) that is unlikely to be involved directly in small vesicle exocytosis. The involvement of these other proteins may contribute to the relative resistance of peptidergic transmission to BoNT/A blockade seen in at least two functional classes of autonomic neurons (Morris et al, 2001, 2002).…”

Dendritic Release of Neurotransmitters

“…Sensitivity of somatodendritic release to TeTX has been described in isolated MCNs (53), implying involvement of VAMP-2 in dendritic release of oxytocin and vasopressin, as well as in transmitter release at synapses. Although many SNARE proteins have already been identified in the terminals of the posterior pituitary (96, 263), immunofluorescence studies have failed to detect core proteins, including VAMP-2 and SNAP-25, in the somata and dendrites of the SON. Somatodendritic peptide release from MCNs thus appears to occur without all of the machinery that is needed for regulated exocytosis in other cell types (235), but it is probable that the functions of the missing protein components are fulfilled by other variants.…”