TLR4 and Parkinson disease: BBB, blood–brain barrier; bSMase, bacterial sphingomyelinase; CSF, cerebrospinal fluid; FTY720, fingolimod; GD, Gaucher's disease; GlcCer, glucosylceramide; iNKT, invariant natural killer T cell; LacCer, lactosylceramide; LPS, lipopolysaccharide; MINCLE, macrophage‐inducible C‐type lectin; PD, Parkinson's disease; S1P, sphingosine‐1‐phosphate; SL, sphingolipid; SN, substantia nigra; SPHK1, sphingosine kinase 1; TLR4, toll‐like receptor 4.