Underlying Mechanisms and Related Diseases Behind the Complex Regulatory Role of NOD-Like Receptor X1

NLRX1 Inhibits LPS-Induced Microglial Death via Inducing p62-Dependent HO-1 Expression, Inhibiting MLKL and Activating PARP-1

The activation of microglia and the production of cytokines are key factors contributing to progressive neurodegeneration. Despite the well-recognized neuronal programmed cell death regulated by microglial activation, the death of microglia themselves is less investigated. Nucleotide-binding oligomerization domain, leucine-rich repeat-containing X1 (NLRX1) functions as a scaffolding protein and is involved in various central nervous system diseases. In this study, we used the SM826 microglial cells to understand the role of NLRX1 in lipopolysaccharide (LPS)-induced cell death. We found LPS-induced cell death is blocked by necrostatin-1 and zVAD. Meanwhile, LPS can activate poly (ADP-ribose) polymerase-1 (PARP-1) to reduce DNA damage and induce heme oxygenase (HO)-1 expression to counteract cell death. NLRX1 silencing and PARP-1 inhibition by olaparib enhance LPS-induced SM826 microglial cell death in an additive manner. Less PARylation and higher DNA damage are observed in NLRX1-silencing cells. Moreover, LPS-induced HO-1 gene and protein expression through the p62-Keap1-Nrf2 axis are attenuated by NLRX1 silencing. In addition, the Nrf2-mediated positive feedback regulation of p62 is accordingly reduced by NLRX1 silencing. Of note, NLRX1 silencing does not affect LPS-induced cellular reactive oxygen species (ROS) production but increases mixed lineage kinase domain-like pseudokinase (MLKL) activation and cell necroptosis. In addition, NLRX1 silencing blocks bafilomycin A1-induced PARP-1 activation. Taken together, for the first time, we demonstrate the role of NLRX1 in protecting microglia from LPS-induced cell death. The underlying protective mechanisms of NLRX1 include upregulating LPS-induced HO-1 expression via Nrf2-dependent p62 expression and downstream Keap1-Nrf2 axis, mediating PARP-1 activation for DNA repair via ROS- and autophagy-independent pathway, and reducing MLKL activation.

A zebrafish NLRX1 isoform downregulates fish IFN responses by targeting the adaptor STING

In mammals, NLRX1 is a unique member of the nucleotide-binding domain and leucine-rich repeat (NLR) family showing an ability to negatively regulate IFN antiviral immunity. Intron-containing genes, including NLRX1, have more than one transcript due to alternative splicing; however, little is known about the function of its splicing variants. Here, we identified a transcript variant of NLRX1 in zebrafish (Danio rerio), termed NLRX1-tv4, as a negative regulator of fish IFN response. Zebrafish NLRX1-tv4 was slightly induced by viral infection, with an expression pattern similar to the full-length NLRX1. Despite the lack of an N-terminal domain that exists in the full-length NLRX1, overexpression of NLRX1-tv4 still impaired fish IFN antiviral response and promoted viral replication in fish cells, similar to the full-length NLRX1. Mechanistically, NLRX1-tv4 targeted STING for proteasome-dependent protein degradation by recruiting an E3 ubiquitin ligase RNF5 to drive the K48-linked ubiquitination, eventually downregulating the IFN antiviral response. Mapping of NLRX1-tv4 domains showed that its N-terminal and C-terminal regions exhibited a similar potential to inhibit STING-mediated IFN antiviral response. Our findings reveal that like the full-length NLRX1, zebrafish NLRX-tv4 functions as an inhibitor to shape fish IFN antiviral response.IMPORTANCEIn this study, we demonstrate that a transcript variant of zebrafish NLRX1, termed NLRX1-tv4, downregulates fish IFN response and promotes virus replication by targeting STING for protein degradation and impairing the interaction of STING and TBK1 and that its N- and C-terminus exhibit a similar inhibitory potential. Our results are helpful in clarifying the current contradictory understanding of structure and function of vertebrate NLRX1s.

NLRX1 increases human retinal pigment epithelial autophagy and reduces H2O2-induced oxidative stress and inflammation by suppressing FUNDC1 phosphorylation and NLRP3 activation

BACKGROUND

Age-related macular degeneration (AMD) is a leading cause of impaired vision as well as some earlier effects, such as reading and face recognition. Oxidative damage and inflammation of retinal pigment epithelial (RPE) cells are major causes of AMD. Additionally, autophagy in RPE cells can lead to cellular homeostasis under oxidative stress. Nucleotide-binding oligomerization domain (NOD)-like receptor X1 (NLRX1) is a mysterious modulator of the immune system function which inhibits inflammatory response, attenuates reactive oxygen species (ROS) production, and regulates autophagy. This study attempted to explore the role of NLRX1 in oxidative stress, inflammation, and autophagy in AMD.

METHODS

An in vitro model of AMD was built in human retinal pigment epithelial cell line 19 (ARPE-19) treated with H₂O₂. The cell viability, NLRX1 expressions, levels of superoxide dismutase (SOD), glutathione (GHS), and ROS, concentrations of interleukin (IL)-1β, tumor necrosis factor-α (TNF-α), IL-6, and monocyte chemoattractant protein-1 (MCP-1), expressions of NLRX1, p62, LC3-II/LC3-I, FUNDC1, and NOD-like receptor protein 3 (NLRP3) inflammasome were expounded by cell counting kit-8, colorimetric, enzyme-linked immunosorbent serologic assay (ELISA), and Western blot assay.

RESULTS

H₂O₂ treatment notably reduced the relative protein expression of NLRX1. Meanwhile, H₂O₂ incubation decreased cell viability, diminished SOD and GSH concentrations, accompanied with the increased level of ROS, enhanced IL-1β, TNF-α, IL-6, and MCP-1 concentrations, and aggrandized the relative protein expression of p62 with reduced LC3-II/LC3-I ratio. Moreover, these results were further promoted with knockdown of NLRX1 and reversed with overexpression. Mechanically, silencing of NLRX1 further observably enhanced the relative levels of -phosphorylated FUNDC1/FUNDC1, and NLRP3 inflammasome-related proteins, while overexpression of NLRX1 exhibited inverse results in the H₂O₂-induced ARPE-19 cells.

CONCLUSION

NLRX1 suppressed H₂O₂-induced oxidative stress and inflammation, and facilitated autophagy by suppressing FUNDC1 phosphorylation and NLRP3 activation in ARPE-19 cells.