Parkinson’s Disease and Gut Microbiota

<b><i>Background:</i></b> Parkinson’s disease (PD) is caused by abnormal aggregation of α-synuclein fibrils, called the Lewy bodies, in the central nervous system. Accumulating knowledge points to the notion that α-synuclein fibrils start from the dorsal vagal nucleus and ascend to the locus ceruleus and the substantia nigra (SN). Even in healthy elderly subjects without motor or cognitive impairment, α-synuclein fibrils are frequently observed in the brain and sometimes in the intestinal neural plexus. Enteroendocrine cells have a direct synapse to the vagal afferents, and the vagal nucleus has synaptic pathways to the SN and the striatum. Intestinal bacteria are likely to be involved in the formation of intestinal α-synuclein fibrils. <b><i>Summary:</i></b> A nonparametric meta-analysis of intestinal microbiota in PD in 5 countries, as well as scrutinization of the other reports from the other countries, indicates that mucin-degrading <i>Akkermansia</i> is increased in PD and that short-chain fatty acid (SCFA)-producing bacteria are decreased in PD. Both dysbiosis should increase the intestinal permeability, which subsequently facilitates exposure of the intestinal neural plexus to toxins like lipopolysaccharide and pesticide, which should lead to abnormal aggregation of α-synuclein fibrils. Decreased SCFA also downregulates regulatory T cells and fails to suppress neuronal inflammation. <b><i>Key Messages:</i></b> Therapeutic intervention may be able to be established against these mechanisms. Additional biochemical, cellular, and animal studies are required to further dissect the direct association between intestinal microbiota and PD.