NLR-Dependent Regulation of Inflammation in Multiple Sclerosis

Microglial NLRP3 inflammasome activation in multiple sclerosis

Multiple sclerosis (MS) is a chronic, autoimmune and neuroinflammatory disease of the central nervous system (CNS) mediated by autoreactive T cells directed against myelin antigens. Although the crucial role of adaptive immunity is well established in MS, the contribution of innate immunity has only recently been appreciated. Microglia are the main innate immune cells of the CNS. Similar to other myeloid cells, microglia recognize both exogenous and host-derived endogenous danger signals through pattern recognition receptors (PRRs) localized on their cell surface such as Toll Like receptor 4, or in the cytosol such as NLRP3. The second one is the sensor protein of the multi-molecular NLRP3 inflammasome complex in activated microglia that promotes the maturation and secretion of proinflammatory cytokines, interleukin-1β and interleukin-18. Overactivation of microglia and aberrant activation of the NLRP3 inflammasome have been implicated in the pathogenesis of MS. Indeed, experimental data, together with post-mortem and clinical studies have revealed an increased expression of NLRP3 inflammasome complex elements in microglia and other immune cells. In this review, we focus on microglial NLRP3 inflammasome activation in MS. First, we overview the basic knowledge about MS, microglia and the NLRP3 inflammasome. Then, we summarize studies about microglial NLRP3 inflammasome activation in MS and its animal models. We also highlight experimental therapeutic approaches that target different steps of NLRP inflammasome activation. Finally, we discuss future research avenues and new methods in this rapidly evolving area.

NLRX1 Is a Multifaceted and Enigmatic Regulator of Immune System Function

Over the last decade, significant progress has been achieved in defining mechanisms underlying NLR regulation of immune system function. However, several NLR family members continue to defy our best attempts at characterization and routinely exhibit confounding data. This is particularly true for NLR family members that regulate signaling associated with the activation of other pattern recognition receptors. NLRX1 is a member of this NLR sub-group and acts as an enigmatic regulator of immune system function. NLRX1 has been shown to negatively regulate type-I interferon, attenuate pro-inflammatory NF-κB signaling, promote reactive oxygen species production, and modulate autophagy, cell death, and proliferation. However, the mechanism/s associated with NLRX1 modulation of these pathways is not fully understood and there are inconsistencies within the field. Likewise, it is highly likely that the full repertoire of biological functions impacted by NLRX1 are yet to be defined. Recent mouse studies have shown that NLRX1 significantly impacts a multitude of diseases, including cancer, virus infection, osteoarthritis, traumatic brain injury, and inflammatory bowel disease. Thus, it is essential that the underlying mechanism associated with NLRX1 function in each of these diseases be robustly defined. Here, we summarize the current progress in understanding mechanisms associated with NLRX1 function. We also offer insight into both unique and overlapping mechanisms regulated by NLRX1 that likely contribute to disease pathobiology. Ultimately, we believe that an improved understanding of NLRX1 will result in better defined mechanisms associated with immune system attenuation and the resolution of inflammation in a myriad of diseases.

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

Nucleotide-binding, leucine-rich repeat containing X1 (NLRX1) is a mitochondria-located innate immune sensor that inhibits major pro-inflammatory pathways such as type I interferon and nuclear factor-κB signaling. We generated a novel, spontaneous, and rapidly progressing mouse model of multiple sclerosis (MS) by crossing myelin-specific T-cell receptor (TCR) transgenic mice with Nlrx1^−/− mice. About half of the resulting progeny developed spontaneous experimental autoimmune encephalomyelitis (spEAE), which was associated with severe demyelination and inflammation in the central nervous system (CNS). Using lymphocyte-deficient mice and a series of adoptive transfer experiments, we demonstrate that genetic susceptibility to EAE lies within the innate immune compartment. We show that NLRX1 inhibits the subclinical stages of microglial activation and prevents the generation of neurotoxic astrocytes that induce neuronal and oligodendrocyte death in vitro. Moreover, we discovered several mutations within NLRX1 that run in MS-affected families. In summary, our findings highlight the importance of NLRX1 in controlling the early stages of CNS inflammation and preventing the onset of spontaneous autoimmunity.

NLRX1 is a guardian protein that inhibits the inflammatory response of glial cells within the central nervous system and prevents the onset of a spontaneous multiple sclerosis–like disease in mice. This study uses a novel mouse model to provide mechanistic insights into the neurodegenerative origin of multiple sclerosis.

Exome sequencing in multiple sclerosis families identifies 12 candidate genes and nominates biological pathways for the genesis of disease

Multiple sclerosis (MS) is an inflammatory disease of the central nervous system characterized by myelin loss and neuronal dysfunction. Although the majority of patients do not present familial aggregation, Mendelian forms have been described. We performed whole-exome sequencing analysis in 132 patients from 34 multi-incident families, which nominated likely pathogenic variants for MS in 12 genes of the innate immune system that regulate the transcription and activation of inflammatory mediators. Rare missense or nonsense variants were identified in genes of the fibrinolysis and complement pathways (PLAU, MASP1, C2), inflammasome assembly (NLRP12), Wnt signaling (UBR2, CTNNA3, NFATC2, RNF213), nuclear receptor complexes (NCOA3), and cation channels and exchangers (KCNG4, SLC24A6, SLC8B1). These genes suggest a disruption of interconnected immunological and pro-inflammatory pathways as the initial event in the pathophysiology of familial MS, and provide the molecular and biological rationale for the chronic inflammation, demyelination and neurodegeneration observed in MS patients.

NLRX1 Enhances Glutamate Uptake and Inhibits Glutamate Release by Astrocytes

Uptake of glutamate from the extracellular space and glutamate release to neurons are two major processes conducted by astrocytes in the central nervous system (CNS) that protect against glutamate excitotoxicity and strengthen neuronal firing, respectively. During inflammatory conditions in the CNS, astrocytes may lose one or both of these functions, resulting in accumulation of the extracellular glutamate, which eventually leads to excitotoxic neuronal death, which in turn worsens the CNS inflammation. NLRX1 is an innate immune NOD-like receptor that inhibits the major inflammatory pathways. It is localized in the mitochondria and was shown to inhibit cell death, enhance ATP production, and dampen oxidative stress. In the current work, using primary murine astrocyte cultures from WT and Nlrx1^-/- mice, we demonstrate that NLRX1 potentiates astrocytic glutamate uptake by enhancing mitochondrial functions and the functional activity of glutamate transporters. Also, we report that NLRX1 inhibits glutamate release from astrocytes by repressing Ca²⁺-mediated glutamate exocytosis. Our study, for the first time, identified NLRX1 as a potential regulator of glutamate homeostasis in the CNS.

Bacteria-Host Interactions in Multiple Sclerosis

Multiple sclerosis (MS) is caused by a complex interaction of genetic and environmental factors. Numerous causative factors have been identified that play a role in MS, including exposure to bacteria. Mycobacteria, Chlamydia pneumoniae, Helicobacter pylori, and other bacteria have been proposed as risk factors for MS with different mechanisms of action. Conversely, some pathogens may have a protective effect on its etiology. In terms of acquired immunity, molecular mimicry has been hypothesized as the mechanism by which bacterial structures such as DNA, the cell wall, and intracytoplasmic components can activate autoreactive T cells or produce autoantibodies in certain host genetic backgrounds of susceptible individuals. In innate immunity, Toll-like receptors play an essential role in combating invading bacteria, and their activation leads to the release of cytokines or chemokines that mediate effective adaptive immune responses. These receptors may also be involved in central nervous system autoimmunity, and their contribution depends on the infection site and on the pathogen. We have reviewed the current knowledge of the influence of bacteria on MS development, emphasizing the potential mechanisms of action by which bacteria affect MS initiation and/or progression.

Molecular cloning and functional characterisation of NLRX1 in grass carp (Ctenopharyngodon idella)

The nucleotide-binding domain and leucine-rich-repeat-containing (NLR) proteins regulate innate immunity. Although the positive regulatory impact of NLRs is clear, their inhibitory roles are not well defined. In the present study, the NLR family gene NLRX1 from grass carp (Ctenopharyngodon idella) was cloned and characterised. NLRX1 was widely expressed in all tissues examined, albeit at varying levels. After exposure to the grass carp reovirus (GCRV), NLRX1 mRNA expression levels were altered in immune organs, and dramatically altered in liver. Subcellular localisation indicated that NLRX1 protein co-localised with the mitochondria in the transfected cells. Additionally, the bimolecular fluorescence complementation (BiFC) system was introduced to detect the interaction between tumour necrosis factor (TNF) receptor associated factor 6 (TRAF6) and NLRX1. Moreover, deficient of NLRX1 in CIK cells with small interference RNA (siRNA) promoted polyinosinic:polycytidylic acid (poly (I:C))-induced IFN-related genes production, including IRF3, IRF7, and IFN-I, which reveals that NLRX1 is a negative regulator of IFN. Taken together, our results demonstrate that NLRX1 gene plays an important role in innate immune regulation and provide new insights into understanding the functional characteristics of the NLRX1 in teleosts.

The Dual Immunoregulatory function of Nlrp12 in T Cell-Mediated Immune Response: Lessons from Experimental Autoimmune Encephalomyelitis

Although the etiology of multiple sclerosis (MS) remains enigmatic, the role of T cells is unquestionably central in this pathology. Immune cells respond to pathogens and danger signals via pattern-recognition receptors (PRR). Several reports implicate Nlrp12, an intracellular PRR, in the development of a mouse MS-like disease, called Experimental Autoimmune Encephalomyelitis (EAE). In this study, we used induced and spontaneous models of EAE, as well as in vitro T cell assays, to test the hypothesis that Nlrp12 inhibits Th1 response and prevents T-cell mediated autoimmunity. We found that Nlrp12 plays a protective role in induced EAE by reducing IFNγ/IL-4 ratio in lymph nodes, whereas it potentiates the development of spontaneous EAE (spEAE) in 2D2 T cell receptor (TCR) transgenic mice. Looking into the mechanism of Nlrp12 activity in T cell response, we found that it inhibits T cell proliferation and suppresses Th1 response by reducing IFNγ and IL-2 production. Following TCR activation, Nlrp12 inhibits Akt and NF-κB phosphorylation, while it has no effect on S6 phosphorylation in the mTOR pathway. In conclusion, we propose a model that can explain the dual immunoregulatory function of Nlrp12 in EAE. We also propose a model explaining the molecular mechanism of Nlrp12-dependent regulation of T cell response.

Flaxseed Oil Attenuates Intestinal Damage and Inflammation by Regulating Necroptosis and TLR4/NOD Signaling Pathways Following Lipopolysaccharide Challenge in a Piglet Model

SCOPE

Flaxseed oil is a rich source of α-linolenic acid (ALA), which is the precursor of the long-chain n-3 polyunsaturated fatty acids (PUFAs), including docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA). This study investigates the protective effect of flaxseed oil against intestinal injury induced by lipopolysaccharide (LPS).

MATERIALS AND RESULTS

Twenty-four weaned pigs were used in a 2 × 2 factorial experiment with dietary treatment (5% corn oil vs 5% flaxseed oil) and LPS challenge (saline vs LPS). On day 21 of the experiment, pigs were administrated with LPS or saline. At 2 h and 4 h post-administration, blood samples were collected. After the blood harvest at 4 h, all piglets were slaughtered and intestinal samples were collected. Flaxseed oil supplementation led to the enrichment of ALA, EPA, and total n-3 PUFAs in intestine. Flaxseed oil improved intestinal morphology, jejunal lactase activity, and claudin-1 protein expression. Flaxseed oil downregulated the mRNA expression of intestinal necroptotic signals. Flaxseed oil also downregulated the mRNA expression of intestinal toll-like receptors 4 (TLR4) and its downstream signals myeloid differentiation factor 88 (MyD88), nuclear factor kappa B (NF-κB), and nucleotide-binding oligomerization domain proteins 1, 2 (NOD1, NOD2) and its adapter molecule, receptor-interacting protein kinase 2 (RIPK2).

CONCLUSION

These results suggest that dietary addition of flaxseed oil enhances intestinal integrity and barrier function, which is involved in modulating necroptosis and TLR4/NOD signaling pathways.

Integrative analysis of Multiple Sclerosis using a systems biology approach

Multiple sclerosis (MS) is a chronic autoimmune disorder characterized by inflammatory-demyelinating events in the central nervous system. Despite more than 40 years of MS research its aetiology remains unknown. This study aims to identify the most frequently reported and consistently regulated molecules in MS in order to generate molecular interaction networks and thereby leading to the identification of deregulated processes and pathways which could give an insight of the underlying molecular mechanisms of MS. Driven by an integrative systems biology approach, gene-expression profiling datasets were combined and stratified into "Non-treated" and "Treated" groups and additionally compared to other disease patterns. Molecular identifiers from dataset comparisons were matched to our Multiple Sclerosis database (MuScle; www.padb.org/muscle ). From 5079 statistically significant molecules, correlation analysis within groups identified a panel of 16 high-confidence genes unique to the naïve MS phenotype, whereas the "Treated" group reflected a common pattern associated with autoimmune disease. Pathway and gene-ontology clustering identified the Interferon gamma signalling pathway as the most relevant amongst all significant molecules, and viral infections as the most likely cause of all down-stream events observed. This hypothesis-free approach revealed the most significant molecular events amongst different MS phenotypes which can be used for further detailed studies.