PPARGC1A and Parkinson disease: Consistent with the results observed in the brain (Figure S2), injection of 6-OHDA reduced the mtDNA copy number and impaired mitochondrial biogenesis in skeletal muscle: the untreated PD group exhibited significantly lower mtDNA copy numbers of Nd1 and Atp6 (Figure 5C), accompanied by lower expression of genes involved in the biogenesis of mitochondria (Pgc1a, Nrf1, and Tfam) (Figure 5D) compared to the NC group.