Inhibition of TREM1 reduces inflammation and oxidative stress after spinal cord injury (SCI) associated with HO-1 expressions

Spinal cord injury (SCI) is a devastating event, leading to the progression of chronic neuropathic pain syndrome. Triggering receptor expressed on myeloid cells 1 (TREM1) is an innate immune receptor expressed on neutrophils and monocytes/macrophages. TREM1 enhances inflammatory response in various models of diseases, but its significance in SCI remains unclear. In the present study, we attempted to explore the effects of TREM1 on the regulation of SCI. Spinal cord contusion injury was performed in wild type (WT) and TREM1-knockout (TREM1KO) mice, and real time-quantitative PCR (RT-qPCR), western blot, and immunofluorescent (IF) staining were used to calculate TREM1, inflammation and oxidative stress in spinal cord tissues 42 days after SPII. In vitro, astrocytes (AST) and BV2 cells were transfected TREM siRNA or the negative control (NC) siRNA to knockdown (KD) TREM1 expressions, followed by lipopolysaccharide (LPS) stimulation to verify the role od TREM1 in modulating SPI. The results suggested that TREM1 was highly expressed in the spinal cord tissues of WT mice after SCI. TREMKO mice exhibited improved locomotor function, mechanical and thermal hypersensitivity in the hindpaws after SCI. In addition, peripheral nerve injury-related biomarkers were down-regulated by TREM1KO in SCI mice. TREM1KO increased NeuN-stained cells, and decreased GFAP and Iba-1 expressions in spinal cord tissues of mice after SCI. TREM1KO mice showed reduced expressions of inflammation-related regulators in the injured spinal cord. Further, toll like receptors (TLR-2, -3, -4, and -9), p-IκBα and p-nuclear factor-kappa B (NF-κB) protein expression levels were markedly decreased by TREM1KO in mice after SCI injury. Moreover, TREM-deficiency suppressed oxidative stress markers, while enhanced anti-oxidants, such as superoxide dismutase-1 (SOD1), NAD(P)H:quinone oxidoreductase-1 (NQO-1), heme oxygenase-1 (HO-1) and nuclear factor E2–related factor 2 (Nrf2) in the injured spinal cord. The in vivo results mentioned above were verified in LPS-stimulated AST and BV2 cells in vitro. Of note, the in vitro results also demonstrated that suppressing HO-1 expressions using Zn-protoporphyrin (ZnPP) abrogated TREM1KD-reduced inflammation, oxidative stress and glial cells activation. The results above demonstrated that suppressing TREM1 expressions markedly improved the outcome of SCI, most likely through reducing inflammation and oxidative stress at least partly regulated by HO-1 expressions. TREM1 inhibition might be therefore has potential as a therapeutic target after SCI.