Synaptosome-associated protein release

Some intracellular signal transduction molecules are reduced in schizophrenia. The release of neurotransmitters is regulated by a family of proteins that coordinate vesicular trafficking (see Ch. 9). Of these, the expression of complexin I and II appears to be decreased in prefrontal cortex and subfields of the hippocampal formation, and the ratio of complexin I to complexin II is elevated in the hippocampus [35], SNAP-25 (Synaptosomal Associated Protein, kDa 25) has inconsistently been found to be down-regulated in both these regions. Synapsin expression is also reduced, but more robust decrements have been observed in bipolar disorder (Ch. 55). 

Schematic illustration of a generalized cholinergic junction (not to scale). Choline is transported into the presynaptic nerve terminal by a sodium-dependent choline transporter (CHT). This transporter can be inhibited by hemicholinium drugs. In the cytoplasm, acetylcholine is synthesized from choline and acetyl -A (AcCoA) by the enzyme choline acetyltransferase (ChAT). Acetylcholine is then transported into the storage vesicle by a second carrier, the vesicle-associated transporter (VAT), which can be inhibited by vesamicol. Peptides (P), adenosine triphosphate (ATP), and proteoglycan are also stored in the vesicle. Release of transmitter occurs when voltage-sensitive calcium channels in the terminal membrane are opened, allowing an influx of calcium. The resulting increase in intracellular calcium causes fusion of vesicles with the surface membrane and exocytotic expulsion of acetylcholine and cotransmitters into the junctional cleft (see text). This step can he blocked by botulinum toxin. Acetylcholine s action is terminated by metabolism by the enzyme acetylcholinesterase. Receptors on the presynaptic nerve ending modulate transmitter release. SNAPs, synaptosome-associated proteins VAMPs, vesicle-associated membrane proteins.

Release can be blocked by drugs such as guanethidine and bretylium. After release, norepinephrine diffuses out of the cleft or is transported into the cytoplasm of the terminal (uptake 1 [1], blocked by cocaine, tricyclic antidepressants) or into the postjunctional cell (uptake 2 [2]). Regulatory receptors are present on the presynaptic terminal. (SNAPs, synaptosome-associated proteins VAMPs, vesicle-associated membrane proteins.)... 

Shimazaki Y, Nishiki T, Omori A et al (1996) Phosphorylation of 25-kDa synaptosome-associated protein. Possible involvement in protein kinase C-mediated regulation of neurotransmitter release. J Biol Chem 271 14548-53... 

Stimulus-evoked, calcium-dependent release of acetylcholine (ACh) from the cholinergic synapse normally occurs through the formation of a fusion complex between ACh-containing vesicles and the intracellular leaflet of the nerve terminal membrane (Amon et al., 2001). This synaptic vesicle fusion complex consists of several proteins of the SNARE family, including a 25 kDa synaptosomal associated protein (SNAP-25), vesicle-associated membrane protein (VAMP, or synaptobrevin), and the synaptic membrane protein syntaxin. Other SNARE proteins have been identified as components of membrane transport systems in yeast and mammals but have not been implicated as targets for BoNTs. Meanwhile, type A and E neurotoxins cleave SNAP-25 while types B, D, F, and G act on VAMP and type C1 toxin cleaves both syntaxin and SNAP-25. Neurotoxin-mediated cleavage of any of these substrates disrupts the processes involved in the exocytotic release of ACh and leads to flaccid paralysis of the affected skeletal muscles. 

Inside the cytosol, the L chain of the toxin acts as a proteolytic enzyme whose activity is dependent on the Zn ions present in the molecules. It hydrolyses protein components of the exocytosis apparatus to block the release of the transmitter. It is known that botulinum toxins B, D and G can cleave vesicle associated membrane protein (VAMP), a protein present in the membranes of acetylcholine vesicles. Botulinum toxins A, C and E, on the other hand, act on proteins of the pres maptic plasma membrane. A and E cleave synaptosomal associated protein 25 (SNAP 2s) and C cleaves... 

The LC then targets and cleaves one or more of three soluble NSF attachment protein receptor (SNARE) proteins with exquisite specificity synaptosomal-associated protein 25 (SNAP-25 BoNT/A, /C, /E) vesicle associated membrane protein 1-3 (VAMPl-3 BoNT-/B, /D, /F, /G) or syntaxin (BoNT/C). The SNARE proteins are essential components of the synaptic exocytosis mechanism, and their cleavage prevents functional assembly of the ternary complex, thereby blocking neurotransmitter release. The combination of efficient neuronal targeting and presynaptic activation renders BoNTs the most potent substances known, with estimated human lethal doses as low as 0.1-1 ng/kg. 

---