NLRX1 inhibits the early stages of CNS inflammation and prevents the onset of spontaneous autoimmunity

The NLR family of innate immune and cell death sensors

Nucleotide-binding oligomerization domain (NOD)-like receptors, also known as nucleotide-binding leucine-rich repeat receptors (NLRs), are a family of cytosolic pattern recognition receptors that detect a wide variety of pathogenic and sterile triggers. Activation of specific NLRs initiates pro- or anti-inflammatory signaling cascades and the formation of inflammasomes-multi-protein complexes that induce caspase-1 activation to drive inflammatory cytokine maturation and lytic cell death, pyroptosis. Certain NLRs and inflammasomes act as integral components of larger cell death complexes-PANoptosomes-driving another form of lytic cell death, PANoptosis. Here, we review the current understanding of the evolution, structure, and function of NLRs in health and disease. We discuss the concept of NLR networks and their roles in driving cell death and immunity. An improved mechanistic understanding of NLRs may provide therapeutic strategies applicable across infectious and inflammatory diseases and in cancer.

NLRX1: Versatile functions of a mitochondrial NLR protein that controls mitophagy

NLRX1 is a member of the of the Nod-like receptor (NLR) family, and it represents a unique pattern recognition molecule (PRM) as it localizes to the mitochondrial matrix in resting conditions. Over the past fifteen years, NLRX1 has been proposed to regulate multiple cellular processes, including antiviral immunity, apoptosis, reactive oxygen species (ROS) generation and mitochondrial metabolism. Similarly, in vivo models have shown that NLRX1 was associated with the control of a number of diseases, including multiple sclerosis, colorectal cancer and ischemia-reperfusion injury. This apparent versatility in function hinted that a common and general overarching role for NLRX1 may exist. Recent evidence has suggested that NLRX1 controls mitophagy through the detection of a specific "danger signal", namely the defective import of proteins into mitochondria, or mitochondrial protein import stress (MPIS). In this review article, we propose that mitophagy regulation may represent the overarching process detected by NLRX1, which could in turn impact on a number of diseases if dysfunctional.

Protective mechanisms of quercetin in neonatal rat brain injury induced by hypoxic-ischemic brain damage (HIBD)

Neonatal hypoxic-ischemic brain damage (HIBD) is a leading cause of infant mortality worldwide. This study explored whether quercetin (Que) exerts neuroprotective effects in a rat model of HIBD. A total of 36 seven-day-old Sprague-Dawley rats were divided into control, Que, HI, and HI + Que groups. The Rice method was used to establish HIBD in HI and HI + Que rats, which were treated with hypoxia (oxygen concentration of 8%) for 2 h after ligation of the left common carotid artery. The rats in the HI + Que group were intraperitoneally injected with Que (30 mg/kg) 1 h before hypoxia, and the rats in the Que group were only injected with the same amount of Que. Brain tissues were harvested 24 h postoperation and assessed by hematoxylin and eosin staining, 2,3,5-triphenyltetrazolium chloride staining, and terminal deoxynucleotidyl transferase dUTP nick-end labeling assay; relative gene and protein levels were evaluated by RT-qPCR, IHC, or western blot (WB) assay. Brain tissue morphologies were characterized by transmission electron microscopy (TEM); LC3B protein levels were assessed by immunofluorescence staining. Escape latencies and platform crossing times were significantly improved (p < .05) in HI + Que groups; infarct volume significantly decreased (p < .001), whereas the numbers of autophagic bodies and apoptotic cells increased and decreased, respectively. Meanwhile, NLRX1, ATG7, and Beclin1 expressions were significantly upregulated, and mTOR and TIM23 expressions, LC3B protein level, and LC 3II/LC 3I ratio were significantly downregulated. Que exerted neuroprotective effects in a rat model of HIBD by regulating NLRX1 and autophagy.

The regulation of self-tolerance and the role of inflammasome molecules

Inflammasome molecules make up a family of receptors that typically function to initiate a proinflammatory response upon infection by microbial pathogens. Dysregulation of inflammasome activity has been linked to unwanted chronic inflammation, which has also been implicated in certain autoimmune diseases such as multiple sclerosis, rheumatoid arthritis, type 1 diabetes, systemic lupus erythematosus, and related animal models. Classical inflammasome activation-dependent events have intrinsic and extrinsic effects on both innate and adaptive immune effectors, as well as resident cells in the target tissue, which all can contribute to an autoimmune response. Recently, inflammasome molecules have also been found to regulate the differentiation and function of immune effector cells independent of classical inflammasome-activated inflammation. These alternative functions for inflammasome molecules shape the nature of the adaptive immune response, that in turn can either promote or suppress the progression of autoimmunity. In this review we will summarize the roles of inflammasome molecules in regulating self-tolerance and the development of autoimmunity.

Pyroptosis in spinal cord injury

Spinal cord injury (SCI) often brings devastating consequences to patients and their families. Pathophysiologically, the primary insult causes irreversible damage to neurons and glial cells and initiates the secondary damage cascade, further leading to inflammation, ischemia, and cells death. In SCI, the release of various inflammatory mediators aggravates nerve injury. Pyroptosis is a new pro-inflammatory pattern of regulated cell death (RCD), mainly mediated by caspase-1 or caspase-11/4/5. Gasdermins family are pore-forming proteins known as the executor of pyroptosis and the gasdermin D (GSDMD) is best characterized. Pyroptosis occurs in multiple central nervous system (CNS) cell types, especially plays a vital role in the development of SCI. We review here the evidence for pyroptosis in SCI, and focus on the pyroptosis of different cells and the crosstalk between them. In addition, we discuss the interaction between pyroptosis and other forms of RCD in SCI. We also summarize the therapeutic strategies for pyroptosis inhibition, so as to provide novel ideas for improving outcomes following SCI.

Morphine-induced microglial immunosuppression via activation of insufficient mitophagy regulated by NLRX1

Background

Chronic morphine exposure induces immunosuppression in the peripheral and central nervous system, resulting in susceptibility of patients to invading pathogens. Mitophagy is a crucial regulator of inflammation, and dysregulated mitophagy may cause immunosuppression, but whether mitophagy is linked with morphine-induced immunosuppression in the brain remains unknown. NLRX1 is the only mitochondrially localized NOD family receptor protein which serves as a critical regulator in immunity and mitophagy activation, but it remains an enigma how NLRX1 functions in the crosstalk between microglial inflammatory defense and mitophagy in the presence of morphine.

Methods

Primary microglia and astrocytes, BV2 and MA cell lines were utilized. Mice were stimulated with repeated morphine treatment to mimic chronic morphine exposure, and activation of mitophagy, lysosomal functions, and inflammation were assayed in specific brain regions and immune organs with or without NLRX1-silencing.

Results

Morphine induced microglial mitophagy in a LC3 (microtubule-associated proteins light chain 3)-dependent manner, which was mediated by NLRX1. Contrastingly, morphine impaired lysosomal functions, including generation, acidification and mitophagosome–lysosome fusion, thus leading to insufficient mitophagy activation in microglia. NLRX1-silencing inhibited mitophagy activity and rescued lysosomal functions including generation and acidification in microglia. The NLRX1-mediated incomplete mitophagy in microglial cells contributed to immunosuppression and vulnerability towards pathogenic challenge after morphine treatment. In vivo, NLRX1-mediated microglial mitophagy activation by morphine was mainly located in the murine brain cortex, striatum, and cerebellum, where NLRX1 functioned as a negative immune regulator and facilitated septic shock. Collectively, microglial immune responses to septic shock were amenable to NLRX1 silencing in the brain with morphine treatment.

Conclusion

Morphine activated insufficient mitophagy in microglia which was regulated by NLRX1, ultimately leading to host immunosuppression and susceptible conditions in the brain.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02453-7.

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

Among nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs), NOD-like receptor X1 (NLRX1) is the only known NLR family member that is targeted to the mitochondria, which contains a C-terminal leucine-rich repeat domain, a central conserved nucleotide-binding domain, and an unconventional N-terminal effector domain. It is unique due to several atypical features, such as mitochondrial localization, noninflammasome forming, and relatively undefined N-terminal domain. NLRX1 has multiple functions, including negative regulation of type-I interferon signaling, attenuation of proinflammatory nuclear factor kappa B (NF-κB) signaling, autophagy induction, modulation of reactive oxygen species production, cell death regulation, and participating in cellular senescence. In addition, due to its diverse functions, NLRX1 has been associated with various human diseases, including respiratory, circulatory, motor, urinary, nervous, and digestive systems, to name but a few. However, the exact regulatory mechanisms of NLRX1 are still unclear in many related diseases since conflicting and controversial topics on NLRX1 in the previous studies remain. In this review, we review recent research advances on the underlying mechanisms and related disorders behind the complex regulatory role of NLRX1, which may provide a promising target to prevent and/or treat the corresponding diseases.

Impact of intracellular innate immune receptors on immunometabolism

Immunometabolism, which is the metabolic reprogramming of anaerobic glycolysis, oxidative phosphorylation, and metabolite synthesis upon immune cell activation, has gained importance as a regulator of the homeostasis, activation, proliferation, and differentiation of innate and adaptive immune cell subsets that function as key factors in immunity. Metabolic changes in epithelial and other stromal cells in response to different stimulatory signals are also crucial in infection, inflammation, cancer, autoimmune diseases, and metabolic disorders. The crosstalk between the PI3K-AKT-mTOR and LKB1-AMPK signaling pathways is critical for modulating both immune and nonimmune cell metabolism. The bidirectional interaction between immune cells and metabolism is a topic of intense study. Toll-like receptors (TLRs), cytokine receptors, and T and B cell receptors have been shown to activate multiple downstream metabolic pathways. However, how intracellular innate immune sensors/receptors intersect with metabolic pathways is less well understood. The goal of this review is to examine the link between immunometabolism and the functions of several intracellular innate immune sensors or receptors, such as nucleotide-binding and leucine-rich repeat-containing receptors (NLRs, or NOD-like receptors), absent in melanoma 2 (AIM2)-like receptors (ALRs), and the cyclic dinucleotide receptor stimulator of interferon genes (STING). We will focus on recent advances and describe the impact of these intracellular innate immune receptors on multiple metabolic pathways. Whenever appropriate, this review will provide a brief contextual connection to pathogenic infections, autoimmune diseases, cancers, metabolic disorders, and/or inflammatory bowel diseases.

LncRNA SNHG12 ameliorates bupivacaine-induced neurotoxicity by sponging miR-497-5p to upregulate NLRX1

Long non-coding RNA (lncRNA) small nucleolar RNA host gene 12 (SNHG12) has been reported to participate in the regulation of various nervous system disorders. Bupivacaine (BV), a commonly used local anesthetic, could generate neurotoxicity in neurons. This work intended to investigate the role and specific mechanism of SNHG12 in BV-induced neurotoxicity. In this study, we established an in vitro cell model of BV-induced neurotoxicity by exposing human neuroblastoma cells (SH-SY5Y) to BV. It was found that SNHG12 and NLRX1 levels were gradually downregulated, while miR-497-5p enrichment was upregulated accordingly with the increase of BV concentration. As indicated by functional assays, SNHG12 overexpression promoted cell viability but inhibited cell apoptosis and oxidative stress in BV-treated SH-SY5Y cells. In addition, it was identified that SNHG12 directly targeted miR-497-5p and attenuated BV-induced neurotoxicity via interaction with miR-497-5p. Besides, it was confirmed that SNHG12 could upregulate NLRX1 expression by absorbing miR-497-5p. Moreover, miR-497-5p decreased cell viability and induced cell apoptosis and oxidative stress, which was partly reversed by NLRX1 upregulation. In conclusion, our findings indicated that SNHG12 might relieve BV-associated neurotoxicity by upregulating NLRX1 via miR-497-5p in vitro, providing novel clues and biomarkers for the treatment and prevention of BV-associated neurotoxicity.

NOD-like receptors in autoimmune diseases

Autoimmune diseases are chronic immune diseases characterized by dysregulation of immune system, which ultimately results in a disruption in self-antigen tolerance. Cumulative data show that nucleotide-binding and oligomerization domain (NOD)-like receptors (NLRs) play essential roles in various autoimmune diseases, such as inflammatory bowel disease (IBD), rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), psoriasis, multiple sclerosis (MS), etc. NLR proteins, consisting of a C-terminal leucine-rich repeat (LRR), a central nucleotide-binding domain, and an N-terminal effector domain, form a group of pattern recognition receptors (PRRs) that mediate the immune response by specifically recognizing cellular pathogen-associated molecular patterns (PAMPs) or damage-associated molecular patterns (DAMPs) and triggering numerous signaling pathways, including RIP2 kinase, caspase-1, nuclear factor kappa B (NF-κB), mitogen-activated protein kinase (MAPK) and so on. Based on their N-terminal domain, NLRs are divided into five subfamilies: NLRA, NLRB, NLRC, NLRP, and NLRX1. In this review, we briefly describe the structures and signaling pathways of NLRs, summarize the recent progress on NLR signaling in the occurrence and development of autoimmune diseases, as well as highlight numerous natural products and synthetic compounds targeting NLRs for the treatment of autoimmune diseases.

Focusing on the Cell Type Specific Regulatory Actions of NLRX1

Cells utilize a diverse repertoire of cell surface and intracellular receptors to detect exogenous or endogenous danger signals and even the changes of their microenvironment. However, some cytosolic NOD-like receptors (NLR), including NLRX1, serve more functions than just being general pattern recognition receptors. The dynamic translocation between the cytosol and the mitochondria allows NLRX1 to interact with many molecules and thereby to control multiple cellular functions. As a regulatory NLR, NLRX1 fine-tunes inflammatory signaling cascades, regulates mitochondria-associated functions, and controls metabolism, autophagy and cell death. Nevertheless, literature data are inconsistent and often contradictory regarding its effects on individual cellular functions. One plausible explanation might be that the regulatory effects of NLRX1 are highly cell type specific and the features of NLRX1 mediated regulation might be determined by the unique functional activity or metabolic profile of the given cell type. Here we review the cell type specific actions of NLRX1 with a special focus on cells of the immune system. NLRX1 has already emerged as a potential therapeutic target in numerous immune-related diseases, thus we aim to highlight which regulatory properties of NLRX1 are manifested in disease-associated dominant immune cells that presumably offer promising therapeutic solutions to treat these disorders.

NLR in eXile: Emerging roles of NLRX1 in immunity and human disease

NLRX1 is a member of the NOD-like receptor family, a set of pattern recognition receptors associated with innate immunity. Interestingly, NLRX1 exists in somewhat of an exile from its NLR counterparts with unique features that mediate atypical functions compared with traditional NOD-like receptors (NLRs). Aside from a mitochondrial targeting sequence, the N-terminal region is yet to be characterized. Mitochondrially located, NLRX1 sits within a subgroup of regulatory NLRs responsible for negatively regulating cellular inflammatory signalling. As well as modulating pathogen response, emerging evidence is implicating NLRX1 as a central homeostatic gatekeeper between mitochondrial biology and immunological response. More recently, NLRX1 has been implicated in a wide range of disease, both pathogen-driven and otherwise. Emerging links of NLRX1 in cancer biology, autoimmunity and other inflammatory conditions are raising the potential of targeting NLRX1 therapeutically, with recent studies in inflammatory bowel disease showing great promise. Within this review, we address the unique features of NLRX1, its roles in innate immune signalling and its involvement in a range of inflammatory, metabolic and oncology disease indications with a focus on areas that could benefit from therapeutic targeting of NLRX1.

NLRX1 Deletion Increases Ischemia-Reperfusion Damage and Activates Glucose Metabolism in Mouse Heart

Background

NOD-like receptors (NLR) are intracellular sensors of the innate immune system, with the NLRP3 being a pro-inflammatory member that modulates cardiac ischemia-reperfusion injury (IRI) and metabolism. No information is available on a possible role of anti-inflammatory NLRs on IRI and metabolism in the intact heart. Here we hypothesize that the constitutively expressed, anti-inflammatory mitochondrial NLRX1, affects IRI and metabolism of the isolated mouse heart.

Methods

Isolated C57Bl/6J and NLRX1 knock-out (KO) mouse hearts were perfused with a physiological mixture of the essential substrates (lactate, glucose, pyruvate, fatty acid, glutamine) and insulin. For the IRI studies, hearts were subjected to either mild (20 min) or severe (35 min) ischemia and IRI was determined at 60 min reperfusion. Inflammatory mediators (IL-6, TNFα) and survival pathways (mito-HKII, p-Akt, p-AMPK, p-STAT3) were analyzed at 5 min of reperfusion. For the metabolism studies, hearts were perfused for 35 min with either 5.5 mM ¹³C-glucose or 0.4 mM ¹³C-palmitate under normoxic conditions, followed by LC-MS analysis and integrated, stepwise, mass-isotopomeric flux analysis (MIMOSA).

Results

NLRX1 KO significantly increased IRI (infarct size from 63% to 73%, end-diastolic pressure from 59 mmHg to 75 mmHg, and rate-pressure-product recovery from 15% to 6%), following severe, but not mild, ischemia. The increased IRI in NLRX1 KO hearts was associated with depressed Akt signaling at early reperfusion; other survival pathways or inflammatory parameters were not affected. Metabolically, NLRX1 KO hearts displayed increased lactate production and glucose oxidation relative to fatty acid oxidation, associated with increased pyruvate dehydrogenase flux and 10% higher cardiac oxygen consumption.

Conclusion

Deletion of the mitochondrially-located NOD-like sensor NLRX1 exacerbates severe cardiac IR injury, possibly through impaired Akt signaling, and increases cardiac glucose metabolism.

Behind the Scenes: Nod-Like Receptor X1 Controls Inflammation and Metabolism

Regulatory Nod-like receptors (NLRs) are a subgroup of the cytosolic NLR family of pathogen recognition receptors (PRRs). These receptors can tune the innate immune responses triggered by the activation of other PRRs by either augmenting or attenuating the activated pro-inflammatory signaling cascades. Nod-like receptor X1 (NLRX1) is the only known mitochondria-associated negative regulatory NLR. NLRX1 attenuates several inflammatory pathways and modulates cellular processes such as autophagy and mitochondrial function following infection or injury. Using both in vitro expression and in vivo experimental models, NLRX1 is extensively described in the context of anti-viral signaling and host-defense against invading pathogens. More recently, NLRX1 has also gained interest in the field of cancer and metabolism where NLRX1 functions to attenuate overzealous inflammation in various inflammatory and autoimmune diseases. However, the exact function of this novel receptor is still under debate and many, often contradictory, mechanisms of action together with cellular localizations have been proposed. Thus, a better understanding of the underlying mechanism is crucial for future research and development of novel therapeutical approaches. Here, we summarize the current findings on NLRX1 and discuss its role in both infectious and inflammatory context.

The emerging molecular mechanisms for mitochondrial dysfunctions in FXTAS

Fragile X-associated tremor/ataxia syndrome (FXTAS) is an inherited neurodegenerative disorder caused by an expansion of 55-200 CGG repeats at 5UTR of FMR1 gene, known as premutation. The main clinical and neuropathological features of FXTAS include progressive intention tremor, gait ataxia, neuronal cell loss and presence of ubiquitin-positive intranuclear inclusions in neurons and astrocytes. Various mitochondrial dysfunctions are reported in in vitro/vivo models of FXTAS; however, the molecular mechanisms underlying such mitochondrial dysfunctions are unclear. CGG expansions are pathogenic through distinct mechanisms involving RNA gain of function, impaired DNA damage repair and FMRpolyG toxicity. Here, we have systematically reviewed the reports of mitochondrial dysfunctions under premutation condition. We have also focused on potential emerging mechanisms to understand mitochondrial associated pathology in FXTAS. This review highlights the important role of mitochondria in FXTAS and other related disorders; and suggests focus of future studies on mitochondrial dysfunction along with other prevailing mechanisms to alleviate neurodegeneration.

Genetic analysis of nucleotide-binding leucine-rich repeat (NLR) receptors in multiple sclerosis

Genetic and functional analyses of the inflammasome suggest a role for this multiprotein complex in the biological mechanisms leading to the onset and progression of multiple sclerosis (MS). Nucleotide-binding, leucine-rich repeat (NLR) receptors trigger the activation and assembly of specific inflammasomes in response to danger signals. Mining exome sequencing data from 326 MS patients identified 17 rare missense or nonsense variants in NLR family pyrin domain containing 1 (NLRP1), NLRP3, NLRP6, NLRP7 and NLR family CARD domain containing 4 (NLRC4). Genotyping these variants in 2503 MS cases and 1076 healthy controls did not result in statistically significant differences between groups, and segregation analysis within MS families was largely unsupportive of co-segregation of these variants with disease. However, the identification of MS patients harboring rare homozygote variants in NLRP1 (p.Ile601Phe and p.Ser1387Ile), a variant in NLRP3 (p.Leu832Ile) resulting in the substitution of a critical amino acid for the formation of its leucine-rich repeat domain, and several MS patients with NLRC4 variants (p.Arg310Ter and p.Glu600Ter) causing protein truncations suggest that rare protein-altering variants in inflammasome-activating NLR receptors may contribute to MS risk.

LRR domain of NLRX1 protein delivery by dNP2 inhibits T cell functions and alleviates autoimmune encephalomyelitis

Multiple sclerosis (MS) is a demyelinating inflammatory disease of the central nervous system (CNS), which is a chronic progressive disease and is caused by uncontrolled activation of myelin antigen specific T cells. It has high unmet medical needs due to the difficulty of efficient drug delivery into the CNS to control tissue inflammation. In this study, we demonstrate that a fusion protein of NOD-like receptor family member X1 (NLRX1) and blood brain barrier (BBB)-permeable peptide, dNP2 ameliorates experimental autoimmune encephalomyelitis (EAE). Methods: We purified recombinant LRR or NBD regions of NLRX1 protein conjugated with dNP2. To examine intracellular delivery efficiency of the recombinant protein, we incubated the proteins with Jurkat T cells or murine splenic T cells and their delivery efficiency was analyzed by flow cytometry. To investigate the therapeutic efficacy in an EAE model, we injected the recombinant protein into mice with 3 different treatment schemes e.g., prevention, semi-therapeutic, and therapeutic. To analyze their functional roles in T cells, we treated MACS-sorted naïve CD4 T cells with the proteins during their activation and differentiation into Th1, Th17, and Treg cells. Results: dNP2-LRR protein treatment showed significantly higher delivery efficiency than TAT-LRR or LRR alone in Jurkat T cells and mouse splenic T cells. In all three treatment schemes of EAE experiments, dNP2-LRR administration showed ameliorated tissue inflammation and disease severity with reduced number of infiltrating T cells producing inflammatory cytokines such as IFNγ. In addition, dNP2-LRR inhibited T cell activation, cytokine production, and Th1 differentiation. Conclusion: These results suggest that dNP2-LRR is a novel agent, which regulates effector T cell functions and could be a promising molecule for the treatment of CNS autoimmune diseases such as multiple sclerosis.

Fiery Cell Death: Pyroptosis in the Central Nervous System

Pyroptosis ('fiery death') is an inflammatory type of regulated cell death (RCD), which occurs downstream of inflammasome activation. Pyroptosis is mediated directly by the recently identified family of pore-forming proteins known as gasdermins, the best characterized of which is gasdermin D (GSDMD). Recent investigations implicate pyroptosis in the pathogenesis of multiple neurological diseases. In this review, we discuss molecular mechanisms that drive pyroptosis, evidence for pyroptosis within the CNS, and emerging therapeutic strategies for its inhibition in the context of neurological disease.