TLR4 and Parkinson disease: However, evidence implicating gut-related TLR2 and TLR4 signalling in the pathogenesis of ‘body-first’ PD pathology is growing, in light of: 1) altered expression and signalling of both TLR2 and TLR4 in people with PD, especially in the gastrointestinal system; 2) the role of TLR2 and TLR4 in gut homeostasis and thus leaky gut and inflammation; and 3) the proposed role of α-synuclein as an immune signalling molecule and its ability to activate TLR2 and TLR4.