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

Abstract

The presynaptic plasma membrane protein SNAP-25 (synaptosome-associated protein of 25 kDa) has been implicated as one of several neural-specific components that direct constitutive fusion mechanisms to the regulated vesicle trafficking and exocytosis of neurotransmitter release.There exist two alternatively spliced isoforms of a and b, which differ in a putative membrane-interacting domain. We show that these two isoforms have distinct quantitative and anatomical patterns of expression during brain development, in neurons, and in neuroendocrine cells and that the proteins localize differently in neurites of transfected PC12 pheochromocytoma cells. These findings indicate that alternative isoforms of SNAP-25 may play distinct roles in vesicular fusion events required for membrane addition during axonal outgrowth and for release of neuromodulatory peptides and neurotransmitters.In the nervous system, the regulated release of neurotransmitters and neuromodulatory peptides and the addition of proteins and other constituents required for neurite outgrowth proceed through intracellular trafficking and fusion of vesicles at the plasma membrane (for review, see refs. 1-4). This regulated fusion of donor vesicle and acceptor (target) membranes is likely mediated by the same ubiquitious ATPdependent machinery, composed of soluble N-ethylmaleimide-sensitive factor (NSF) and NSF-associated proteins (SNAPs), that is utilized in constitutive pathways of all eukaryotic cells (5). To provide specificity for the fusion process, distinct sets of related proteins have been postulated to serve as membrane receptors, or SNAREs (6). These enable correct recognition between vesicle and target membranes at different intracellular compartments, and they engage the general fusion machinery. It is likely that specific SNAREs, together with additional cell-specific auxiliary proteins, are critical for the specialized sorting, targeting, and final recycling of vesicles which are required for the Ca2+-triggered release of neurotransmitter and modulatory peptides unique to neurons and neuroendocrine cells (7-9). Although evidence suggests that membrane addition to the expanding plasmalemma of axonal growth cones is Ca2+ dependent (10), it is thought that there are independent vesicle-targeting and fusion mechanisms that chiefly support either axon outgrowth or neurotransmitter release. Recent studies do suggest, however, that at least one of the SNAREs, SNAP-25, is a key player in membrane fusion events of both developing and mature neurons.The presynaptic nerve terminal protein SNAP-25 (synaptosome-associated protein of 25 kDa; ref. 11) has been identified as a plasma membrane protein that together with syntaxin and the synaptic vesicle proteins VAMP/synaptobrevin and synaptotagmin are thought to constitute an initial SNARE docking complex for regulated exocytosis (5, 12, 13). After docking, this complex can incorporate into a 20S fusion parThe publication costs of this article were defrayed in part by page charge payment. This a...

A SNAP25 promoter variant is associated with early-onset bipolar disorder and a high expression level in brain

“…alternative splicing between tandem exon 5. 36 This results in a developmental switch between 1 and 3 weeks of age from expression of predominantly SNAP25a to SNAP25b transcripts that ultimately constitute more than 80% of SNAP25 mRNA in mouse adult brain. These two isoforms diverge only for nine amino acids in a domain involved in membrane association and disassembly, after exocytosis, of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex.…”

Targeting Synaptic Pathology with a Novel Affinity Mass Spectrometry Approach

“…This is of particular interest when studying the SNARE complex proteins, as it has been shown that different combinations of SNARE protein isoforms interact with dissimilar affinities and are differentially distributed in defined areas of the central nervous system, suggesting specialized functions of these isoforms (29). Other SNARE protein isoforms such as SNAP-25A and -25B have very different expression during embryonic and early postnatal development: SNAP-25A is the predominant species during embryonic and early postnatal development, whereas SNAP-25B transcripts increase dramatically after the first postnatal week to become the majority of SNAP-25 mRNA in adult brain (38,39). Other studies have shown that different SNARE protein isoforms could have specialized roles in the neurosecretory process (40).…”

Old proteins, developing roles: The regulation of calcium channels by synaptic proteins

“…SNARE proteins synaptobrevin, SNAP-25 and syntaxin 1A are expressed in early development in hippocampal cultures 144,145 though more dominant isoforms may exist. For example, cleavage of the mature v-SNARE isoform synaptobrevin by tetanus toxin, completely blocks neurotransmission in mature terminals 146 whereas new synapses express a tetanus toxin-resistant isoform of synaptobrevin 145 In rat brain, SNAP-25 expression switches from SNAP-25a during development, to SNAP-25b in mature terminals 147,148 which is purported to be controlled by developmental patterns of electrical activity (i.e., calcium influx). 149 Finally, a splice isoform of syntaxin 1A, termed syntaxin 1C, has been reported.…”