Human bone marrow-derived mesenchymal stem cells alleviate fluoxetine-resistant depression-like behaviors in mice by multi-target effects in the hippocampus

BACKGROUND Treatment-resistant depression (TRD) remains a major clinical challenge due to the limited efficacy of conventional first-line antidepressants. Mesenchymal stem cells (MSCs), which possess potent immunomodulatory and neurotrophic properties, have emerged as promising therapeutic candidates for neuropsychiatric disorders. Human bone marrow-derived MSCs (hBMSCs) are one of the most widely used sources. However, their therapeutic potential and their underlying mechanisms in the context of TRD have yet to be fully elucidated. AIM To investigate the antidepressant effects of hBMSCs in a TRD-like mouse model and to explore the potential molecular mechanisms involved. METHODS A mouse model of fluoxetine-resistant depression (FRD), defined as an early stage of TRD, was established by combining chronic unpredictable restraint stress with corticosterone administration and validated by the lack of behavioral response to fluoxetine (FLX). hBMSCs (2 × 106 cells) were administered via the tail vein twice weekly for 4 weeks. A battery of behavioral tests was conducted to evaluate depressive- and anxiety-like behaviors. Immunofluorescence staining and western blotting were performed to assess microglial activation, astrocytic integrity, neuronal survival, and hippocampal neurogenesis. RESULTS hBMSC treatment significantly ameliorated depressive- and anxiety-like behaviors in FRD model mice, whereas FLX failed to produce comparable behavioral improvements. hBMSCs markedly attenuated microglial activation and reduced neuroinflammatory responses in the hippocampus. In addition, hBMSCs restored astrocytic structure, protected neuronal integrity, and promoted adult hippocampal neurogenesis. These neuroprotective effects were associated with the brain-derived neurotrophic factor/tropomyosin receptor kinase B/cAMP response element-binding signaling pathway. hBMSCs labeled with 1,1’-dioctadecyl-3,3,3’,3’-tetramethylindocarbocyanine perchlorate were not detected in brain tissue, suggesting that their therapeutic effects may primarily rely on indirect or paracrine mechanisms. CONCLUSION hBMSCs exert robust antidepressant-like effects in an FRD mouse model by reducing neuroinflammation, restoring glial function, enhancing neurogenesis, and activating neurotrophic signaling, supporting hBMSCs as a promising therapy for FRD.