Sparc Suppresses Microglial Neuroinflammation and Promotes Axonal Regeneration by Interacting With Uba52

Background: After spinal cord injury (SCI), pro-inflammatory microglia accumulate and impede axonal regeneration. We explored whether secreted protein acidic and rich in cysteine (Sparc) restrains microglial inflammation and fosters neurite outgrowth. Methods: Mouse microglial BV2 cells were polarized to a pro-inflammatory phenotype with lipopolysaccharides (LPSs). Sparc mRNA and protein were quantified by reverse transcription quantitative PCR (RT-qPCR). Sparc was overexpressed via plasmid transfection, then inflammatory cytokines, mitochondrial membrane potential (Δψm), reactive oxygen species (ROS), and oxidative-phosphorylation proteins, including voltage-dependent anion channel 1 (VDAC1), cytochrome c oxidase subunit 1 (COX1), and ATP synthase α subunit (ATP5A), were assayed by Western blot, enzyme-linked immunosorbent assay (ELISA), and flow cytometry. Immunoprecipitation plus mass spectrometry, co-immunoprecipitation, and immunofluorescence confirmed the interaction between Sparc and ubiquitin A-52 residue ribosomal protein fusion product 1 (Uba52). Effects of Sparc overexpression alone or combined with Uba52 small interfering RNA (si-Uba52) were compared in LPS-induced BV2 cells. Finally, BV2 cells and a mouse hippocampal neuron (HT-22) were co-cultured in the Transwell chamber, and the changes in proliferation, apoptosis, and III-tubulin content of the latter were detected. Results: In LPS-induced BV2 cells, the tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), and ROS levels were elevated, while the IL-10 and transforming growth factor-β (TGF-β) levels, Δψm, and the proteins levels of the VDAC1, COX1, ATP5A, and Sparc decreased. Sparc overexpression reversed these changes. Mechanistically, Sparc bound Uba52 and upregulated its expression; Uba52 knockdown abolished the anti-inflammatory and mitochondrial-protective effects of Sparc. In co-culture, Sparc overexpression rescued HT-22 neurons apoptosis and enhanced axonal growth, but the effects were also reversed by Uba52 knockdown. Conclusions: Sparc may maintain mitochondrial homeostasis by interacting with Uba52 to inhibit LPS-induced BV2 inflammatory response, thereby promoting neuronal axonal regeneration. This suggests that Sparc may play a potential role in SCI repair.