Developmental expression of the 25-kDa synaptosomal-associated protein (SNAP-25) in rat brain.

Abstract

The developmental expression and subcellular distribution of the neuron-specific 25-kDa synaptosomal protein (SNAP-25) were investigated by using Northern (RNA) blots, immunoblots, and immunocytochemistry. Both SNAP-25 protein and mRNA were present at low levels in embryonic day 15 rat brain, and levels of both increased during early postnatal maturation. Developmental immunoblots with antipeptide antisera demonstrated that a 25-kDa peptide was the major isoform in brain, and this form increased steadily from embryonic day 15 through adulthood. A second 27-kDa immunoreactive isoform was present in brain only during early development. Immunoblots of two-dimensional SDS/polyacrylamide gels revealed the presence of a predominant 25-kDa isoform of SNAP-25 in adult brain. Immunocytochemical studies indicated that as immunoreactivity for SNAP-25 increased during development, the cellular localization of SNAP-25 immunoreactivity concomitantly shifted from axons and cell bodies to presynaptic terminals. These data suggest that the SNAP-25 protein shifts in subcellular localization during development and may play a role in the establishment and stabilization of specific presynaptic terminals in brain.One goal in neurobiology is to understand the molecular mechanisms that regulate the synthesis, storage, release, and reuptake of neurotransmitters (1). Presynaptic regions of neurons may show considerable plasticity during development and after injury (2), suggesting that molecular mechanisms are present that allow adaptation. Thus, by carefully analyzing the developmental changes in specific molecular components within the presynaptic terminal, a better understanding of the function of developing and regenerating synapses may be elucidated.Recent approaches to understanding the function of the presynaptic region have focused on the isolation and characterization of proteins enriched in presynaptic terminals. Synapsin I (3), synaptophysin (4), and vesicle-associated membrane protein (5) are examples of presynaptic proteins present in most presynaptic neurons; such proteins are used as markers of presynaptic terminals. These proteins presumably modulate the interactions of synaptic vesicles with other proteins in the active zone of the synapse, leading to regulation of vesicle mobility, release, and retrieval. For example, calcium-mediated phosphorylation of synapsin I via calmodulin-dependent protein kinases I and II may initiate movement and release of synaptic vesicle contents (6).We have recently isolated and characterized cDNAs encoding a 25-kDa synaptosomal-associated protein termed SNAP-25 (7). This hydrophilic protein is composed of 206 residues and is tightly associated with presynaptic membranes. Immunocytochemical studies indicated that, unlike synapsin I and synaptophysin, SNAP-25 was expressed and localized in the presynaptic regions of specific, but diverse, populations of central nervous system neurons (7,8). Although the exact function of SNAP-25 is unknown, its pattern of localization and high degree...

SNAP‐25 is differentially expressed by noradrenergic and adrenergic chromaffin cells

“…The subcellular localisation of SNAP-25 in neurones reported from several groups is not consistent with an exclusively exocytotic role for this protein [24,28,32,33]. Also in a parallel study we found at the ultrastructural level that splanchnic nerve terminal synaptic vesicles in adrenal medulla were strongly labelled for SNAP-25, a localisation unexpected for a plasma membrane SNARE (unpublished data).…”

“…While at the present time, reasons for the unexpected differential SNAP-25 expression between the two major chromaffin cell phenotypes are not clear, the cytoplasmic localisation in noradrenergic cells may suggest that in these cells SNAP-25 could also be involved in processes other than regulated exocytosis, such as those associated with morphological plasticity, as has been previously proposed [14][15][16]32]. The subcellular localisation of SNAP-25 in neurones reported from several groups is not consistent with an exclusively exocytotic role for this protein [24,28,32,33].…”

Differential expression of SNAP-25 protein isoforms during divergent vesicle fusion events of neural development.

“…The importance of SNAP-25 in synaptic transmission is demonstrated by its being a specific substrate for botulinum neurotoxins A and E, metalloproteases which effectively block neurotransmitter release (19,20). However, prior to synapse formation SNAP-25 is detected in cell bodies and fibers of the neonatal rat brain which are virtually devoid of immunoreactive protein in the adult (21). Moreover, inhibition of SNAP-25 expression by antisense oligonucleotides significantly diminishes axonal extension in developing neurons (22).…”

N-Terminal Acetylation of the Neuronal Protein SNAP-25 Is Revealed by the SMI81 Monoclonal Antibody

“…It was initially identified as the most abundant methionine-rich protein to be rapidly transported along axons, accumulating at synaptic termini (3,4). Levels of SNAP-25 increase during development, and SNAP-25 expression coincides with the onset of synaptogenesis and neuronal maturation (5,6). SNAP-25 is a hydrophilic protein but is tightly associated with synaptic membranes because of the attachment of hydrophobic palmitate lipids to four clustered cysteines in the center of the molecule; indeed, SNAP-25 is the major palmitoylation target in brain (3,4,7,8).…”