CASP9 and major depressive disorder: The downstream of PI3K/AKT pathway includes caspase 9 (CASP9), glycogen synthase kinase 3 (GSK3), protein 21 (P21), mammalian target of rapamycin (mTOR), and other pathways (Figure 4d), which have been widely reported to play a regulatory role in inflammatory response, depression, and cancer development (Bai et al., 2020; Ren et al., 2021).