TLR4 and Parkinson disease: Regarding studies on the role of microglia–microbiota interactions in PD, it has been demonstrated that FMT from healthy mice to mice injected with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a model of toxin-induced PD, attenuates motor deficits, glial activation in the substantia nigra, and reduces Toll-like receptor 4 (TLR4)/TNF-α signaling pathway in the gut and brain.