Effect and mechanism of VSIG4 gene mutation on the function of microglia in retinitis pigmentosa

Objective  To investigate the effect and mechanism of the V-set and immunoglobulin domain-containing 4 ( VSIG4 ) gene mutation on the function of microglia in retinitis pigmentosa (RP).

Methods  Localization of VSIG4 in the retina was detected by immunofluorescence.HMC3 cells (human microglial cells) were transfected with wild-type (Len-WT) VSIG4 gene, mutant type (Len-Mut) VSIG4 gene and empty vector virus (Len-Cont) and stimulated by the presence or absence of lipopolysaccharide (LPS), then divided into control group, LPS-Len-Mut group, LPS-Len-WT group, LPS-Len-Cont group, Len-Mut group, Len-WT group and Len-Cont group.The mRNA expression levels of the inflammatory factors interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α) were detected by real-time fluorescence quantitative PCR.Protein expression levels of nuclear factor erythroid 2-related factor 2 (Nrf2), heme oxygenase 1 (HO-1), glutathione peroxidase 4 (GPX4), nuclear transcription factor-κB (NF-κB) p65 subunit (P65), and phosphorylated P65 (PP65) were detected by Western blot.Cell phagocytic function was detected by phagocytosis assay.Cell migration ability was detected by cell scratch and transwell migration assay.LPS- stimulated HMC3 cells were co-cultured with 661W cells (mouse retinal photoreceptor cells), and the expression levels of B-cell lymphoma-2 (Bcl-2) and Bcl-2-associated X (Bax) proteins of the cells were detected by Western blot.The number of apoptotic cells was determined by apoptosis assay.

Results  VSIG4 was localized to microglia in mouse retina.The real-time fluorescence quantitative PCR results showed that compared with LPS-Len-WT group, the relative expression levels of IL-1β and TNF-α mRNA in HMC3 cells were significantly increased in LPS-Len-Mut group (both at P<0.05).The Western blot results showed that compared with LPS- Len-WT group, the protein expression levels of Nrf2, HO-1, and GPX4 in HMC3 cells were significantly reduced in LPS-Len-Mut group, and the PP65/P65 ratio was significantly increased (all at P<0.05).The phagocytic experiment results showed that the phagocytic rates of HMC3 cells in Len-Cont group, LPS-Len-Cont group, LPS-Len-WT group, and LPS-Len-Mut group were (35.67±3.22)%, (63.67±10.07)%, (84.00±3.46)%, and (64.67±2.31)%, respectively, showing a statistically significant difference ( F=59.06, P<0.001).Compared with LPS-Len-WT group, the phagocytic rate of HMC3 cells was significantly reduced in LPS-Len-Mut group ( P<0.05).The results of cell scratch and transwell migration assay showed that compared with LPS-Len-WT group, the migration rate of HMC3 cells at 24 and 48 hours and the number of invading cells per unit area at 24 hours were significantly reduced in LPS-Len-Mut group (all at P<0.05).Compared with LPS-Len-WT group, the expression ratio of Bax/Bcl-2 protein and the number of cell apoptosis were significantly increased in the LPS-Len-Mut group under the co-culture system (both at P<0.05).

Conclusions  VSIG4 is localized to mouse retinal microglia.When the VSIG4 gene in RP mutates, HMC3 cells under LPS stimulation exhibit a series of changes, including activation of the NF-κB signaling pathway, decreased activation of the Nrf2/HO-1 signaling pathway, increased secretion of inflammatory cytokines, reduced phagocytic and migratory abilities, and increased cell apoptosis in co-culture systems.

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