BMSC-CM prevents over-differentiation of NSCs to astrocytes by upregulating Smad 6 expression

Background: Mesenchymal stem cells (MSCs) are a promising therapy for spinal cord injury (SCI) as they can provide a favorable environment for the regrowth of neurons and axons by inhibiting receptor-regulated Smads (R-Smads) in endogenous neural stem cells (NSCs). However, their mechanism of action and effect on the expression of inhibitory Smads (I-Smads) remains unclear. Method: Conditioned medium (CM) was collected from bone marrow MSCs (BMSCs) isolated from rats with SCIs, and its effect on the regulation of Smad 6 expression was tested in vitro (in NSCs) and in vivo (SCI rats). Western blot analysis and immunohistochemistry staining were used to investigate the proportion of neurons and astrocytes in vitro and in vivo. BBB scores were used to assess the neurological outcome of SCI rats at different time points. Results: : BMSC-CM could upregulate Smad 6 expression in vitro. BMSC-CM-induced upregulation was suppressed by pre-treatment with the TGF-β type I receptor kinase inhibitor SB431542. BMSC-CM was able to promote the differentiation of NSCs to neurons; Smad 6 knockdown in NSCs partly weakened this effect on neural differentiation. In vivo, Smad 6 expression in the later phase of injury was closely associated with BMSC-CM treatment. Conclusion: BMSC-CM can upregulate Smad 6 expression by the secretion of TGF-β. It promotes the differentiation of NSCs into neurons, partly through upregulation of Smad 6.