Saikosaponin‐d impedes hippocampal neurogenesis and causes cognitive deficits by inhibiting the survival of neural stem/progenitor cells via neurotrophin receptor signaling in mice

Neural stem/progenitor cells (NPCs) are multipotent stem cells in the central nervous system. Damage to NPCs has been demonstrated to cause adverse effects on neurogenesis and to contribute to neurological diseases. Our previous research suggested that saikosaponin‐d (SSd), a cytostatic drug belonging to the bioactive triterpenoid saponins, exhibited neurotoxicity by inhibiting hippocampal neurogenesis, but the underlying mechanism remained elusive. This study was performed to clarify the role of SSd in cognitive function and the mechanism by which SSd induced damage to hippocampal neurogenesis and NPCs. Our results indicated that SSd caused hippocampus‐dependent cognitive deficits and inhibited hippocampal neurogenesis by reducing the numbers of newborn neurons in mice. RNA sequencing analysis revealed that SSd‐induced neurotoxicity in the hippocampus involved neurotrophin receptor‐interacting MAGE (NRAGE)/neurotrophin receptor interacting factor (NRIF)/p75(NTR)‐associated cell death executor (NADE) cell signaling activated by the p75 neurotrophin receptor (p75(NTR)). Mechanistic studies showed that a short hairpin RNA targeting p75(NTR) intracellular domain reversed SSd‐increased NRAGE/NRIF/NADE signaling and the c‐Jun N‐terminal kinase/caspase apoptotic pathway, subsequently contributing to the survival of NPCs, as well as cell proliferation and differentiation. The addition of recombinant brain‐derived neurotrophic factor (BDNF) ameliorated the SSd‐induced inhibition of BDNF/Tyrosine kinase receptor B (TrkB) neurotrophic signaling, but did not affect SSd‐activated pro‐BDNF/p75(NTR) signaling. Moreover, the SSd‐induced elevation of cytosolic Ca(2+) concentration was responsible for damage to NPCs. The extracellular Ca(2+) chelator ethylene glycol‐bis(2‐aminoethylether)‐N,N,N',N'‐tetraacetic acid (EGTA), rather than the intracellular Ca(2+) chelator 1,2‐bis(2‐aminophenoxy)ethane‐N,N,N',N'‐tetraacetic acid tetrakis(acetoxymethyl ester) (BAPTA/AM), attenuated SSd‐induced cytosolic Ca(2+) dysfunction and SSd‐disordered TrkB/p75(NTR) signaling. Overall, this study demonstrated a new mechanism for the neurotoxic effect of SSd, which has emerging implications for pharmacological research of SSd and provides a better understanding of neurotoxicity induced by cytostatic drugs.