Abstract: BACKGROUND: Indolepropionic acid has been shown to reduce diabetes-induced central nervous system inflammation. However, there is a lack of research on whether to inhibit microglia M1 polarization for the treatment of spinal cord injury.
OBJECTIVE: To investigate the mechanism of indolepropionic acid inhibition of microglial cell M1 polarization for the treatment of spinal cord injury through cell and animal experiments. 
METHODS: (1) In vitro experiments: BV2 cell viability was assessed using the CCK-8 assay to determine optimal concentrations of indolepropionic acid. Subsequently, BV2 cells were categorized into control group, administration group (50 μmol/L indolepropionic acid), lipopolysaccharide group (100 ng/mL lipopolysaccharide), and treatment group (100 ng/mL lipopolysaccharide + 50 μmol/L indolepropionic acid). Nitric oxide content was quantified using the Griess method. Real-time quantitative PCR and western blot assay were employed to measure mRNA and protein levels of pro-inflammatory factors. Cell immunofluorescence staining was conducted to assess inducible nitric oxide synthase expression. The Seahorse assay was employed to assess glycolytic stress levels in BV2 cells. (2) In vivo experiments: 30 SD rats were randomly divided into three groups: sham surgery group, spinal cord injury group, and indolepropionic acid group. Motor function recovery in rats after spinal cord injury was assessed using BBB scoring and the inclined plane test. Immunofluorescence staining of spinal cord tissue was conducted to evaluate the expression of inducible nitric oxide synthase in microglial cells. ELISA was employed to measure protein expression levels of the pro-inflammatory cytokines interleukin-1β and tumor necrosis factor-α in spinal cord tissue.
RESULTS AND CONCLUSION: (1) In vitro experiments: Indolepropionic acid exhibited significant suppression of BV2 cell viability when its concentration exceeded 50 μmol/L. Indolepropionic acid achieved this by inhibiting the activation of the nuclear factor κB signaling pathway, thereby suppressing the mRNA and protein expression levels of pro-inflammatory cytokines (interleukin-1β and tumor necrosis factor-α), as well as the M1 polarization marker, inducible nitric oxide synthase, in BV2 cells. Additionally, indolepropionic acid notably reduced the glycolytic level in BV2 cells induced by lipopolysaccharides. (2) In vivo experiments: Following indolepropionic acid intervention in spinal cord injury rats, there was a noticeable increase in BBB scores and the inclined plane test angle. There was also a significant decrease in the number of M1-polarized microglial cells in spinal cord tissue, accompanied by a marked reduction in the protein expression levels of pro-inflammatory cytokines (interleukin-1β and tumor necrosis factor-α). (3) These results conclude that indolepropionic acid promotes functional recovery after spinal cord injury by improving the inflammatory microenvironment through inhibition of microglia M1 polarization.

Key words: indolepropionic acid, spinal cord injury, microglial cell, M1 polarization, pro-inflammatory factor, signaling pathway