FNDC5 and Parkinson disease: In short, we observed that irisin recovered damaged mitochondria number in dopaminergic neurons (Fig. 4c–f) and elevated the inhibited expression levels of SIRT1, PGC-1α, nuclear respiratory factors 2 (NRF-2) and mitochondrial transcription factor A (TFAM), TOM20 in PD models except nuclear respiratory factors 1 (NRF-1) (Fig. 4g–j, n–q), and mtDNA copy number was renewed (Fig. 4m).