Synaptosomal-Associated Protein, 25kDa (SNAP-25) is a Target Soluble NSF (N-ethylmaleimide-sensitive factor) Attachment Protein Receptor (t-SNARE) protein encoded by the SNAP25 gene found on chromosome 20p12.2 in humans. [5] [6] SNAP-25 is a component of the trans-SNARE complex, which accounts for membrane fusion specificity and directly executes fusion by forming a tight complex that brings the synaptic vesicle and plasma membranes together. [7]
SNAP-25, a Q-SNARE protein, is anchored to the cytosolic face of membranes via palmitoyl side chains covalently bound to cysteine amino acid residues in the central linker domain of the molecule. This means that SNAP-25 does not contain a trans-membrane domain. [9]
SNAP-25 has been identified to contribute two [10] α-helices to the SNARE complex, a four-α-helix domain complex. [11] The SNARE complex participates in vesicle fusion, which involves the docking, priming and merging of a vesicle with the cell membrane to initiate an exocytotic event. Synaptobrevin, a protein that is a part of the vesicle-associated membrane protein (VAMP) family, and syntaxin-1 also help form the SNARE complex by each contributing a single α-helix. SNAP-25 assembles with synaptobrevin and syntaxin-1, and the selective binding of these proteins enables vesicle docking and fusion to occur at active zones on the plasma membrane. [12] The energy needed for fusion to occur, results from the assembly of the SNARE proteins along with additional Sec1/Munc18-like (SM) proteins. [13]
To form the SNARE complex, synaptobrevin, syntaxin-1, and SNAP-25 associate and begin to wrap around each other to form a coiled coil quaternary structure. The α-helices of both synaptobrevin and syntaxin-1 bind to those of SNAP-25. Synaptobrevin binds the α-helix near the C-terminus of SNAP-25, while syntaxin-1 binds the α-helix near the N-terminus. [9] Dissociation of the SNARE complex is driven by ATPase N-ethylmaleimide-sensitive fusion (NSF) protein. [13]
SNAP-25 inhibits presynaptic P-, Q-, and L-type voltage-gated calcium channels [14] and interacts with the synaptotagmin C2B domain in a Ca2+-independent fashion. [15] In glutamatergic synapses, SNAP-25 decreases the Ca2+ responsiveness, while it is normally absent in GABAergic synapses. [16]
Two isoforms (mRNA splice variants) of SNAP-25 exist, which are SNAP-25a and SNAP-25b. The two isoforms differ by nine amino acid residues, including a re-localization of one of the four palmitoylated cysteine residues involved in membrane attachment. [17] The major characteristics of these two forms are outlined in the table below.
| SNAP25a | SNAP25b | |
|---|---|---|
| Structure | N-terminal α-helix Random coil linker region with four cysteines clustered towards the center C-terminal α-helix | N-terminal α-helix Random coil linker region with four cysteines clustered towards the C-terminus C-terminal α-helix |
| Expression | Major SNAP-25 isoform in embryos and developing neural tissue Minimal expression in adult tissue except in pituitary and adrenal gland tissues | Minimal expression during development, major isoform in adult neural tissue [18] |
| Localization | Diffuse | Localized to terminals and varicosities [18] |
SNAP-25 not only plays a role in synaptogenesis and the exocytotic release of neurotransmitters, but it also affects spine morphogenesis and density, post synaptic receptor trafficking and neuronal plasticity. Other non-neuronal processes such as metabolism can also be affected by SNAP-25 protein expression. [19] [20]
Individuals harboring pathogenic heterozygous de novo missense or loss-of-function variants in SNAP-25 often present with an early-onset developmental and epileptic encephalopathy. The core symptoms comprise intellectual disability ranging between mild to profound and early-onset seizures mostly occurring before the age of two years. Further recurrent symptoms include movement disorders, cerebral visual impairment, and brain atrophy. [21] Electrophysiological studies identified aberrant spontaneous neurotransmission as causative and suggest that structurally clustered pathogenic variants lead to similar synaptic phenotypes. [22]
Consistent with the regulation of synaptic Ca2+ responsiveness, heterozygous deletion of the SNAP-25 gene in mice results in a hyperactive phenotype similar to attention deficit hyperactivity disorder (ADHD). In heterozygous mice, a decrease in hyperactivity is observed with dextroamphetamine (or Dexedrine), an active ingredient in the ADHD drug Adderall. Homozygous deletions of the SNAP-25 gene are lethal. An additional study indicated that incorporation of a SNAP-25 transgene back into the heterozygous SNAP-25 mutant mouse can rescue normal activity levels similar to wildtype mice. This suggests that low protein levels of SNAP-25 can be a cause of hyper-kinetic behavior. [23] Subsequent studies have suggested that at least some of the SNAP-25 gene mutations in humans might predispose to ADHD. [24] [25] Identification of polymorphisms in the 3’ untranslated region of the SNAP-25 gene was established in linkage studies with families that had been pre-diagnosed ADHD. [26]
Studies in the post mortem brains of patients with Schizophrenia have shown that altered protein levels of SNAP-25 are specific to regions of the brain. Reduced SNAP-25 protein expression has been observed in the hippocampus as well as an area of the frontal lobe known as Broadman’s area 10 whereas SNAP-25 expression has increased in both the cingulate cortex and prefrontal lobe of Broadman’s area 9. The varying levels of SNAP-25 protein found in different areas of the brain have been thought to contribute to the conflicting psychological behaviors (depressive vs. hyperactive) expressed in some Schizophrenic patients. [27] [28] [29] [30]
The blind-drunk (Bdr) mouse model which has a point mutations in the SNAP-25b protein has provided a complex phenotype involving behaviors such as an abnormal circadian rhythm, [31] uncoordinated gait, and disinterest in new objects/toys. [32] Another mouse model generated from Cre-LoxP recombination, showed that conditional knockout (cKO) of the SNAP-25 gene in the forebrain, showed inactive SNAP-25 gene expression in glutamatergic neurons. However, significant glutamate levels were found in the cortex of these cKO mice. [33] These mice also exhibited deficient social skills, impaired learning and memory, enhanced kinesthetic activity, a reduced startle response, impaired self-care, nursing ability and nest-building skills. Antipsychotic drugs such as Clozapine and Riluzole have been shown to significantly reduce the schizophrenic phenotype expressed in SNAP-25 cKO mice. [33]
Individuals with Alzhiemer’s disease have been shown to have decreased presynaptic protein levels and impaired synaptic function in neurons. SNAP-25 can be used as a biomarker in the cerebral spinal fluid (CSF) of patients exhibiting different variations of Alzheimer's disease (prodromal Alzheimer’s and overt Alzheimer’s). Increased levels of SNAP-25 protein were observed in patients with Alzheimer’s compared to control individuals. Additionally, the presence of truncated SNAP-25 protein can be seen in the CSF of some patients with this disease. [34] In five distinct regions of the brain, low levels of SNAP-25 can be seen in patients with Alzheimer’s. [35]
A single nucleotide polymorphism in the SNAP-25 gene promoter has been shown to influence the expression levels of the SNAP-25b isoform in the prefrontal cortex. Increased levels of SNAP-25b have been shown to impair synaptic transmission and maturation which could lead to early-onset bipolar disorder (EOBD).The most abundant isoform of SNAP-25 is SNAP-25a during the early weeks of development in mice however in adulthood there is a change and the SNAP-25b isoform increases in the brain. This is shown to correlate with adolescent humans being increasingly diagnosed with EOBD during puberty. [36] It has been suggested that early-onset bipolar disorder is more closely linked to Schizophrenia than to Bipolar Disorder itself. The single nucleotide polymorphism of SNAP-25 (rs6039769) associated with EOBD has been shown to increase the risk of patients developing Schizophrenia. [19]
A genome wide association study pointed to the rs362584 polymorphism in the gene as possibly associated with the personality trait neuroticism. [37] Botulinum toxins A, C and E cleave SNAP-25, [38] leading to paralysis in clinically developed botulism.
Deletion of the SNAP-25b isoform has been shown to cause developmental abnormalities and seizures in mice. High levels of SNAP-25a and the protein syntaxin appear to be linked to seizures found in infantile-epilepsy. SNAP-25 knock-in mice have distinct phenotypic behavior similar to the fits and seizures of epileptic patients, as well as anxiety. [39]
In the coloboma hyperactive mutant mouse model where SNAP-25 protein levels are reduced to 50% of the normal level, depolarized neurotransmitter release of dopamine and serotonin were reduced as well as glutamate release. The reduction in glutamate levels can lead to deficient memory and increased learning disabilities. [40] Certain polymorphisms of SNAP-25 (rs363043, rs353016, rs363039, rs363050) have been shown to affect the cognitive behavior, specifically the Intelligence Quotient (IQ)), of patients without pre-existing neurological diseases. [41]
SNAP-25 protein expression can be altered by sex hormone levels in neonatal rats. Male rats that received an antiestrogen drug showed a 30% decrease in SNAP-25 levels and females treated with estrogen or testosterone showed a 30% increase in SNAP-25 levels. [42] This suggests that synaptosomal proteins, such as SNAP-25, may have a dependence on neonatal hormone levels during brain development in rats. An additional study, showed that SNAP-25 levels in the hippocampus of the brain in neonatal mice were altered if the mother had been exposed to human influenza virus during pregnancy. [43]
Loss is lethal to Drosophila , but can be fully substituted by overexpression of the related SNAP-24. [10]
Click on genes, proteins and metabolites below to link to respective articles. [§ 1]
SNAP-25 has been shown to interact with:
In a neuron, synaptic vesicles store various neurotransmitters that are released at the synapse. The release is regulated by a voltage-dependent calcium channel. Vesicles are essential for propagating nerve impulses between neurons and are constantly recreated by the cell. The area in the axon that holds groups of vesicles is an axon terminal or "terminal bouton". Up to 130 vesicles can be released per bouton over a ten-minute period of stimulation at 0.2 Hz. In the visual cortex of the human brain, synaptic vesicles have an average diameter of 39.5 nanometers (nm) with a standard deviation of 5.1 nm.
SNARE proteins – "SNAPREceptors" – are a large protein family consisting of at least 24 members in yeasts, more than 60 members in mammalian cells, and some numbers in plants. The primary role of SNARE proteins is to mediate the fusion of vesicles with the target membrane; this notably mediates exocytosis, but can also mediate the fusion of vesicles with membrane-bound compartments. The best studied SNAREs are those that mediate the release of synaptic vesicles containing neurotransmitters in neurons. These neuronal SNAREs are the targets of the neurotoxins responsible for botulism and tetanus produced by certain bacteria.
Synaptobrevins are small integral membrane proteins of secretory vesicles with molecular weight of 18 kilodalton (kDa) that are part of the vesicle-associated membrane protein (VAMP) family.
Syntaxin-1A is a protein that in humans is encoded by the STX1A gene.
Synaptosomal-associated protein 23 is a protein that in humans is encoded by the SNAP23 gene. Two alternative transcript variants encoding different protein isoforms have been described for this gene.
Syntaxin-4 is a protein that in humans is encoded by the STX4 gene.
Synaptotagmin-1 is a protein that in humans is encoded by the SYT1 gene.
Vesicle-associated membrane protein 2 (VAMP2) is a protein that in humans is encoded by the VAMP2 gene.
Syntaxin-binding protein 1 is a protein that in humans is encoded by the STXBP1 gene. This gene encodes a syntaxin-binding protein. The encoded protein appears to play a role in release of neurotransmitters via regulation of syntaxin, a transmembrane attachment protein receptor. Mutations in this gene have been associated with neurological disorders including epilepsy, intellectual disability, and movement disorders.
Syntaxin-7 is a protein that in humans is encoded by the STX7 gene.
Syntaxin-6 is a protein that in humans is encoded by the STX6 gene.
Vesicle-associated membrane protein 3 is a protein that in humans is encoded by the VAMP3 gene.
Syntaxin-5 is a protein that in humans is encoded by the STX5 gene.
Syntaxin-2, also known as epimorphin, is a protein that in humans is encoded by the STX2 gene.
Vesicle-associated membrane protein 4 is a protein that in humans is encoded by the VAMP4 gene.
Vesicle-associated membrane protein 1 (VAMP1) is a protein that in humans is encoded by the VAMP1 gene.
Syntaxin 3, also known as STX3, is a protein which in humans is encoded by the STX3 gene.
Munc-18 proteins are the mammalian homologue of UNC-18 and are a member of the Sec1/Munc18-like (SM) protein family. Munc-18 proteins have been identified as essential components of the synaptic vesicle fusion protein complex and are crucial for the regulated exocytosis of neurons and neuroendocrine cells.
Soluble N-ethylmaleimide-Sensitive Factor Attachment Proteins are a family of cytosolic adaptor proteins involved in vesicular fusion at membranes during intracellular transport and exocytosis. SNAPs interact with proteins of the SNARE complex and NSF to play a key role in recycling the components of the fusion complex. SNAPs are involved in the priming of the vesicle fusion complex during assembly, as well as in the disassembly following a vesicle fusion event. Following membrane fusion, the tethering SNARE proteins complex disassembles in response to steric changes originating from the ATPase NSF. The energy provided by NSF is transferred throughout the SNARE complex and SNAP, allowing the proteins to untangle, and recycled for future fusion events. Mammals have three SNAP genes: α-SNAP, β-SNAP, and γ-SNAP. α- and γ-SNAP are expressed throughout the body, while β-SNAP is specific to the brain. The yeast homolog of the human SNAP is Sec17, the structural diagram of which is included on this page.
Synaptosome-associated protein, 47 kDal (SNAP47) is a human protein encoded by the SNAP47 gene. Other aliases of this gene are SVAP1, HEL170, ESFI5812, and HEL-S-290. SNAP47 is a synaptosome protein which is associated with the protein coding in multiple diseases, including non small cell lung cancer and schizophrenia. SNAP47 is a member of the SNAP protein family. SNAP proteins are t-snare proteins that are a component of SNARE complex. The SNARE complex mediates vesicle fusion by creating tight complex that brings vesicle and membrane together. This protein causes ubiquitous expression in testis, ovary, and many other tissues
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