AQP4 and Parkinson disease: Transforming growth factor‐β1 plays a critical role in the down‐regulation of microglial responses to minimize brain inflammation and efficiently restricts the exacerbation of brain damage in both human PD and MPTP‐induced mouse models of PD.43, 44, 45, 46, 47, 48 AQP4+/+ and AQP4−/− mice showed similar levels of serum TGF‐β1 without MPTP treatment.