Apelin-13 is another neuropeptide; it acts as a ligand for the orphan G protein-coupled apelin receptor (APJ) [107] and has shown neuroprotective effects in a cellular model of PD using SH-SY5Y cells treated with 1-methyl-4-phenylpyridine (MPP+) [108]. The gene discussed is APLN; the disease is Parkinson disease.