FNDC5 and depressive disorder: As shown in Figure 2, the serum level of PGC-1α was significantly downregulated in MDD patients (1.070 ± 0.151 in HCs and 0.667 ± 0.253 in depression, p < 0.01), whereas FNDC5 levels remained stable (1.256 ± 0.226 in HCs and 1.509 ± 0.442 in depression, p > 0.05).