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SNAP25

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. SNAP-25 is a component of the trans-SNARE complex, which accounts for membrane fusion specificity, directly executing fusion by forming a tight complex that brings the synaptic vesicle and plasma membranes together.

Structure and function
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 found in the central linker domain of the molecule. This means that SNAP-25 does not contain a trans-membrane domain.

SNAP-25 has been identified to contribute two α-helices to the SNARE complex, a four-α-helix domain complex. The SNARE complex participates in vesicular 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. The energy needed for fusion to occur, results from the assembly of the SNARE proteins along with additional Sec1/Munc18-like (SM) proteins.

To form the SNARE complex, synaptobrevin, syntaxin-1, and SNAP-25 associate and begin to wrap around each other forming 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. Dissociation of the SNARE complex is driven by ATPase N-ethylmaleimide-sensitive fusion (NSF) protein.

SNAP-25 inhibits presynaptic P-, Q-, and L-type voltage-gated calcium channels and interacts with the synaptotagmin C2B domain in Ca2+-independent fashion. In glutamatergic synapses, SNAP-25 decreases the Ca2+responsiveness, while it is normally absent in GABAergic synapses.

Two isoforms (mRNA splice variants) of SNAP-25 exist, which are SNAP-25a and SNAP25b. The two isoforms differ by 9 amino acid residues, including a re-localization of one of the four palmitoylated cysteine residues involved in membrane attachment. The major characteristics of these two forms are outlined in the table below. 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.

Attention Deficit Hyperactivity Disorder (ADHD)
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 bringing them back to levels similar to wildtype mice. This suggests that low protein levels of SNAP-25 can be a cause of hyper-kinetic behavior. Subsequent studies have suggested that at least some of the SNAP-25 gene mutations in humans might predispose to ADHD. 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.

Schizophrenia
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.

The blind-drunk (Bdr) mouse model which has a point mutation in the SNAP-25b protein has provided a complex phenotype involving behaviors such as an abnormal circadian rhythm, uncoordinated gait, and disinterest in new objects/toys. 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. 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.

Alzheimer's Disease
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. In five distinct regions of the brain, low levels of SNAP-25 can be seen in patients with Alzheimer’s.

Bipolar Disorder
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. 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.

Botulism
A genome wide association study pointed to the rs362584 polymorphism in the gene being possibly associated with the personality trait neuroticism. Botulinum toxins A, C and E cleave SNAP-25, leading to paralysis in clinically developed botulism.

Epilepsy
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.

Learning Disabilities
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. Certain polymorphisms of SNAP-25 (rs363043, rs353016, rs363039, rs363050) affect the cognitive behavior, specifically the Intelligence Quotient (IQ)), of patients without pre-existing neurological diseases.

Neonatal Development
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. 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.