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 and directly executes 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 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 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. 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 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. 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 a 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 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. 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.
Clinical significance
Developmental and epileptic encephalopathies (DEEs) 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. Electrophysiological studies identified aberrant spontaneous neurotransmission as causative and suggest that structurally clustered pathogenic variants lead to similar synaptic phenotypes.
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 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 mutations 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.
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