A proposed sequence of events involves age- infection-, or dysbiosis-driven αSyn accumulation in the intestine, leading to trans-synaptic transmission of pathological forms of αSyn from enteric neurons to the vagus nerve, and retrograde axonal transport along the vagus to the brainstem.149-151 This model draws on autopsy studies of Lewy bodies, and observations that onset of gastrointestinal symptoms in PD patients, and detection of pathological α-synuclein (αSyn) in gastrointestinal tissues in animal models of PD, are seen prior to the onset of motor symptoms. Here, SNCA is linked to Parkinson disease.