Selective co-expression of synaptic proteins, α-synuclein, cysteine string protein-α, synaptophysin, synaptotagmin-1, and synaptobrevin-2 in vesicular acetylcholine transporter-immunoreactive axons in the guinea-pig ileum
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Parkinson's disease is a neurodegenerative disorder characterized by Lewy bodies and neurites composed mainly of the presynaptic protein α-synuclein. Frequently, Lewy bodies and neurites are identified in the gut of Parkinson's disease patients and may underlie associated gastrointestinal dysfunctions. We recently reported selective expression of α-synuclein in the axons of cholinergic neurons in the guinea pig and human distal gut; however, it is not clear whether α-synuclein expression varies along the gut, nor how closely expression is associated with other synaptic proteins. We used multiple-labeling immunohistochemistry to quantify which neurons in the guinea pig ileum expressed α-synuclein, cysteine string protein-α (CSPα), synaptophysin, synaptotagmin-1, or synaptobrevin-2 in their axons. Among the 10 neurochemically defined axonal populations, a significantly greater proportion of vesicular acetylcholine transporter-immunoreactive (VAChT-IR) varicosities (80% ± 1.7%, n = 4, P < 0.001) contained α-synuclein immunoreactivity, and a significantly greater proportion of α-synuclein-IR axons also contained VAChT immunoreactivity (78% ± 1.3%, n = 4) compared with any of the other nine populations (P < 0.001). Among synaptophysin-, synaptotagmin-1-, synaptobrevin-2-, and CSPα-IR varicosities, 98% ± 0.7%, 96% ± 0.7%, 88% ± 1.6%, and 85% ± 2.9% (n = 4) contained α-synuclein immunoreactivity, respectively. Among α-synuclein-IR varicosities, 96% ± 0.9%, 99% ± 0.6%, 83% ± 1.9%, and 87% ± 2.3% (n = 4) contained synaptophysin-, synaptotagmin-1-, synaptobrevin-2-, and CSPα immunoreactivity, respectively. We report a close association between the expression of α-synuclein and the expression of other synaptic proteins in cholinergic axons in the guinea pig ileum. Selective expression of α-synuclein may relate to the neurotransmitter system utilized and predispose cholinergic enteric neurons to degeneration in Parkinson's disease.
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