HTRA2 and Parkinson disease: Fibroblasts isolated from the skin biopsies of PD patients with R47X mutations or healthy patient controls and HeLa, SH-SY5Y, and HEK293 cells were cultured at suitable conditions. Metformin (10 mM) was used as a treatment for 24 h. Fibroblasts from TRAP1 or HTRA2 knockout mice were also used for the experiments. Other analysis required vectors containing TRAP1 and HTRA2 cDNA and specific siRNA for TRAP1, HTRA2, and controls.