Is Targeting the Inflammasome a Way Forward for Neuroscience Drug Discovery?

Targeting inflammasome complexes as a novel therapeutic strategy for mood disorders

INTRODUCTION

Inflammasome complexes, especially NLRP3, have gained great attention as a potential therapeutic target in mood disorders. NLRP3 triggers a caspase 1-dependent release of the inflammatory cytokines IL-1β and IL-18, and seems to interact with purinergic and kynurenine pathways, all of which are implicated in mood disorders development and progression.

AREAS COVERED

Emerging evidence supports NLRP3 inflammasome as a promising pharmacological target for mood disorders. We discussed the available evidence from animal models and human studies and provided a reflection on drawbacks and perspectives for this novel target.

EXPERT OPINION

Several studies have supported the involvement of NLRP3 inflammasome in MDD. However, most of the evidence comes from animal models. The role of NLRP3 inflammasome in BD as well as its anti-manic properties is not very clear and requires further exploration. There is evidence of anti-manic effects of P2×R7 antagonists associated with reduction in the brain levels of IL-1β and TNF-α in a murine model of mania. The involvement of other NLRP3 inflammasome expressing cells besides microglia, like astrocytes, and of other inflammasome complexes in mood disorders also deserves further investigation. Preclinical and clinical characterization of NLRP3 and other inflammasomes in mood disorders is needed before considering translational approaches, including clinical trials.

P2X7R influences tau aggregate burden in human tauopathies and shows distinct signalling in microglia and astrocytes

The purinoceptor P2X7R is a promising therapeutic target for tauopathies, including Alzheimer's disease (AD). Pharmacological inhibition or genetic knockdown of P2X7R ameliorates cognitive deficits and reduces pathological tau burden in mice that model aspects of tauopathy, including mice expressing mutant human frontotemporal dementia (FTD)-causing forms of tau. However, disagreements remain over which glial cell types express P2X7R and therefore the mechanism of action is unresolved. Here, we show that P2X7R protein levels increase in human AD post-mortem brain, in agreement with an upregulation of P2RX7 mRNA observed in transcriptome profiles from the AMP-AD consortium. P2X7R protein increases mirror advancing Braak stage and coincide with synapse loss. Using RNAScope we detect P2RX7 mRNA in microglia and astrocytes in human AD brain, including in the vicinity of senile plaques. In cultured microglia, P2X7R activation modulates the NLRP3 inflammasome pathway by promoting the formation of active complexes and release of IL-1β. In astrocytes, P2X7R activates NFκB signalling and increases production of the cytokines CCL2, CXCL1 and IL-6 together with the acute phase protein Lcn2. To further explore the role of P2X7R in a disease-relevant context, we expressed wild-type or FTD-causing mutant forms of tau in mouse organotypic brain slice cultures. Inhibition of P2X7R reduces insoluble tau levels without altering soluble tau phosphorylation or synaptic localisation, suggesting a non-cell autonomous role of glial P2X7R on pathological tau aggregation. These findings support further investigations into the cell-type specific effects of P2X7R-targeting therapies in tauopathies.

Mechanism of pyroptosis in neurodegenerative diseases and its therapeutic potential by traditional Chinese medicine

Neurodegenerative diseases (NDs) are disorders characterized by degenerative degeneration of neurons and loss of their function. NDs have a complicated pathophysiology, of which neuroinflammation and neuronal death are significant factors. The inflammatory process known as pyroptosis ("fiery death") is caused by a family of pore-forming proteins called Gasdermins (GSDMs), which appears downstream from the activation of the inflammasome. Clear evidence of enhanced pyroptosis-related proteins activity in common NDs has coincided with abnormal aggregation of pathological proteins (such as Aβ, tau, α-synuclein et al.), making pyroptosis an attractive direction for the recent study of NDs. The purpose of this review is to provide an overview of the molecular mechanisms driving pyroptosis, the mechanistic links between pyroptosis and NDs, and emerging therapeutic strategies in Traditional Chinese Medicine (TCM) to inhibit pyroptosis for the treatment of NDs.

Focus on the Role of the NLRP3 Inflammasome in Multiple Sclerosis: Pathogenesis, Diagnosis, and Therapeutics

Neuroinflammation is initiated with an aberrant innate immune response in the central nervous system (CNS) and is involved in many neurological diseases. Inflammasomes are intracellular multiprotein complexes that can be used as platforms to induce the maturation and secretion of proinflammatory cytokines and pyroptosis, thus playing a pivotal role in neuroinflammation. Among the inflammasomes, the nucleotide-binding oligomerization domain-, leucine-rich repeat- and pyrin domain-containing 3 (NLRP3) inflammasome is well-characterized and contributes to many neurological diseases, such as multiple sclerosis (MS), Alzheimer's disease (AD), and ischemic stroke. MS is a chronic autoimmune disease of the CNS, and its hallmarks include chronic inflammation, demyelination, and neurodegeneration. Studies have demonstrated a relationship between MS and the NLRP3 inflammasome. To date, the pathogenesis of MS is not fully understood, and clinical studies on novel therapies are still underway. Here, we review the activation mechanism of the NLRP3 inflammasome, its role in MS, and therapies targeting related molecules, which may be beneficial in MS.

Myrtenal and β-caryophyllene oxide screened from Liquidambaris Fructus suppress NLRP3 inflammasome components in rheumatoid arthritis

Background

Liquidambaris Fructus (LF) is the infructescence of Liquidambar formosana. In Traditional Chinese Medicine, LF has been used to treat joint pain, a common symptom of arthritis and rheumatism; however, a lack of pharmacological evidence has limited its applications in modern clinics. Therefore, this study aims to explore the protective effect of LF on rheumatoid arthritis (RA) and to identify its active ingredients.

Methods

Rats with adjuvant-induced arthritis (AIA) were divided into 4 groups and administered petroleum ether extract of LF (PEL), ethyl acetate extract of LF (EEL), water extract of LF (WEL), or piroxicam (PIR) respectively for 3 weeks. Two additional groups were used as normal control (NC) and model control (MC) and administered distilled water as a placebo. The clinical scores for arthritis, bone surface, synovial inflammation and cartilage erosion were used to evaluate the therapeutic efficacy of each treatment. The serum IL-1β and TNF-α level and the expression of NLRP3, IL-1β and caspase-1 p20 in the synovial tissue of AIA rats were evaluated by ELISA and Western blot. The active ingredients of LF were investigated using network pharmacology and molecular docking methods, and their inhibition of NLRP3 inflammasome activation was verified in the human rheumatoid arthritis fibroblast-like synovial cells (RA-FLS) model.

Results

PEL could alleviate paw swelling, bone and joint destruction, synovial inflammation and cartilage erosion in the AIA rats, with significantly superior efficacy to that of EEL and WEL. PEL reduced IL-1β and TNF-α serum levels, and attenuated the upregulation of NLRP3, IL-1β and caspase-1 p20 expression in the synovial tissue of AIA rats. Network pharmacology and molecular docking results indicated that myrtenal and β-caryophyllene oxide were the main two active ingredients of PEL, and these two compounds showed significant inhibition on TNF-α, NLRP3, IL-1β and caspase-1 p20 expression in RA-FLS.

Conclusions

Myrtenal and β-caryophyllene oxide screened from PEL could suppress the activation of NLRP3 inflammasome, thereby alleviating RA symptoms.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12906-021-03410-2.

Beneficial Effects of Exogenous Ketogenic Supplements on Aging Processes and Age-Related Neurodegenerative Diseases

Life expectancy of humans has increased continuously up to the present days, but their health status (healthspan) was not enhanced by similar extent. To decrease enormous medical, economical and psychological burden that arise from this discrepancy, improvement of healthspan is needed that leads to delaying both aging processes and development of age-related diseases, thereby extending lifespan. Thus, development of new therapeutic tools to alleviate aging processes and related diseases and to increase life expectancy is a topic of increasing interest. It is widely accepted that ketosis (increased blood ketone body levels, e.g., β-hydroxybutyrate) can generate neuroprotective effects. Ketosis-evoked neuroprotective effects may lead to improvement in health status and delay both aging and the development of related diseases through improving mitochondrial function, antioxidant and anti-inflammatory effects, histone and non-histone acetylation, β-hydroxybutyrylation of histones, modulation of neurotransmitter systems and RNA functions. Administration of exogenous ketogenic supplements was proven to be an effective method to induce and maintain a healthy state of nutritional ketosis. Consequently, exogenous ketogenic supplements, such as ketone salts and ketone esters, may mitigate aging processes, delay the onset of age-associated diseases and extend lifespan through ketosis. The aim of this review is to summarize the main hallmarks of aging processes and certain signaling pathways in association with (putative) beneficial influences of exogenous ketogenic supplements-evoked ketosis on lifespan, aging processes, the most common age-related neurodegenerative diseases (Alzheimer’s disease, Parkinson’s disease and amyotrophic lateral sclerosis), as well as impaired learning and memory functions.

NLRP3 and NLRP1 inflammasomes are up-regulated in patients with mesial temporal lobe epilepsy and may contribute to overexpression of caspase-1 and IL-β in sclerotic hippocampi

Inflammation plays a role in the pathophysiology of mesial temporal lobe epilepsy (MTLE). Inflammasome pathways, including the NLRP1 and NLRP3-induced ones, promote neuroinflammation and pyroptosis through interleukin (IL)-1β and caspase-1 action. Evaluation of NLRP1 in sclerotic hippocampi is scarce and there are no data on NLRP3 in human TLE. The aim of this study was to evaluate the expression of these proteins alongside caspase-1 and IL-1β in the hippocampi of patients with TLE compared to control samples. We also sought to investigate peripheral levels of caspase-1 and IL-1β in an independent cohort. Sclerotic and control hippocampi were collected for both histological and immunohistochemical analyses of NLRP1, NLRP3, caspase-1 and IL-1β; plasma was sampled for the measurement of caspase-1 and IL-1β levels through enzyme-linked immunoassay (ELISA) and cytometric bead array (CBA). Sclerotic hippocampi displayed higher expression of the measured proteins than control. Both glia and neurons showed activation of these pathways. Additionally, increased expression of NLRP1 and NLRP3 was associated with elevated plasma levels of IL-1β and in TLE, and increased levels of peripheral caspase-1 were associated with bilateral hippocampal sclerosis (HS). In conclusion, NLRP1 and NLRP3 are up-regulated in sclerotic hippocampi, what may be responsible, at least in part, for the increased hippocampal expression of caspase-1 and IL-1β. Our data suggest a role for inflammasome activation in central and peripheral inflammation in TLE.

Inhibition of the NLRP3 inflammasome by HSP90 inhibitors

Excessive and dysregulated inflammation is known to contribute to disease progression. HSP90 is an intracellular chaperone known to regulate inflammatory processes including the NLRP3 inflammasome and secretion of the pro‐inflammatory cytokine interleukin(IL)‐1β. Here, primarily using an in vitro inflammasome ASC speck assay, and an in vivo model of murine peritonitis, we tested the utility of HSP90 inhibitors as anti‐inflammatory molecules. We report that the HSP90 inhibitor EC144 effectively inhibited inflammatory processes including priming and activation of NLRP3 in vitro and in vivo. A specific inhibitor of the β HSP90 isoform was ineffective suggesting the importance of the α isoform in inflammatory signalling. EC144 inhibited IL‐1β and IL‐6 in vivo when administered orally, and was brain‐penetrant. These data suggest that HSP90 inhibitors may be useful for targeting inflammation in diverse diseases that are worsened by the presence of inflammation.

Selective inhibition of the K+ efflux sensitive NLRP3 pathway by Cl- channel modulation

The NLRP3 inflammasome regulates production of the pro-inflammatory cytokines interleukin-1β (IL-1β) and IL-18, and contributes to inflammation exacerbating disease. Fenamate non-steroidal anti-inflammatory drugs (NSAIDs) were recently described as NLRP3 inflammasome inhibitors via chloride channel inhibition. Fenamate NSAIDs inhibit cyclooxygenase (COX) enzymes, limiting their potential as therapeutics for NLRP3-associated diseases due to established side effects. The aim here was to develop properties of the fenamates that inhibit NLRP3, and at the same time to reduce COX inhibition. We synthesised a library of analogues, with feedback from in silico COX docking potential, and IL-1β release inhibitory activity. Through iterative screening and rational chemical design, we established a collection of chloride channel inhibiting active lead molecules with potent activity at the canonical NLRP3 inflammasome and no activity at COX enzymes, but only in response to stimuli that activated NLRP3 by a K⁺ efflux-dependent mechanism. This study identifies a model for the isolation and removal of unwanted off-target effects, with the enhancement of desired activity, and establishes a new chemical motif for the further development of NLRP3 inflammasome inhibitors.

The NLRP3 inflammasome regulates production of the pro-inflammatory cytokines interleukin-1β (IL-1β) and IL-18, and contributes to inflammation exacerbating disease.

MCC950, a selective NLPR3 inflammasome inhibitor, improves neurologic function and survival after cardiac arrest and resuscitation

Background

Cardiac arrest (CA) is associated with high morbidity and mortality, even after spontaneous circulation is re-established. This dire situation is partly due to post-CA syndrome for which no specific and effective intervention is available. One key component of post-CA syndrome is sterile inflammation, which affects various organs including the brain. A major effector of sterile inflammation is activated NLRP3 inflammasome, which leads to increased release of interleukin (IL)-1β. However, how NLRP3 inflammasome impacts neuroinflammation and neurologic outcome after CA is largely undefined.

Methods

Mice were subjected to a potassium-based murine CA and cardiopulmonary resuscitation (CPR) model. MCC950 was used to suppress activation of NLRP3 inflammasome after CA/CPR. Levels of protein and mRNA were examined by Western blotting and quantitative PCR, respectively. Immunologic changes were assessed by measuring cytokine expression and immune cell compositions. CA outcomes, including neurologic deficits, bacterial load in the lung, and survival rate, were evaluated.

Results

Using our CA/CPR model, we found that NLRP3 inflammasome was activated in the post-CA brain, and that pro-inflammatory cytokine levels, including IL-1β, were increased. After treatment with MCC950, a potent and selective NLRP3 inflammasome inhibitor, mice exhibited improved functional recovery and survival rate during the 14-day observational period after CA/CPR. In line with these findings, IL-1β mRNA levels in the post-CA brain were significantly suppressed after MCC950 treatment. Interestingly, we also found that in MCC950- vs. vehicle-treated CA mice, immune homeostasis in the spleen was better preserved and bacterial load in the lung was significantly reduced.

Conclusions

Our data demonstrate that activation of NLRP3 inflammasome could be a key event shaping the post-CA immuno- and neuro-pathology, and identify this pathway as a unique and promising therapeutic target to improve outcomes after CA/CPR.

Microglial inflammation and phagocytosis in Alzheimer's disease: Potential therapeutic targets

One of the largest unmet medical needs is a disease-modifying treatment for Alzheimer's disease (AD). Recently, the role of microglia in disease, particularly AD, has gained great interest, following the identification of several disease risk-associated genes that are highly expressed in microglia. Microglia play a critical homeostatic role in the brain, with neuroinflammatory and phagocytic mechanisms being of particular importance. Here, we review the role of NLRP3, the complement system, and the triggering receptor expressed in myeloid cells 2 (TREM2) in modulating microglial functions. We have reviewed the targets, their molecular pathways and the therapeutic interventions aimed at modulating these targets, in the hope of discovering a novel therapeutic approach for the treatment of AD. LINKED ARTICLES: This article is part of a themed section on Therapeutics for Dementia and Alzheimer's Disease: New Directions for Precision Medicine. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.18/issuetoc.

The three cytokines IL-1β, IL-18, and IL-1α share related but distinct secretory routes

Interleukin (IL)-1 family cytokines potently regulate inflammation, with the majority of the IL-1 family proteins being secreted from immune cells via unconventional pathways. In many cases, secretion of IL-1 cytokines appears to be closely coupled to cell death, yet the secretory mechanisms involved remain poorly understood. Here, we studied the secretion of the three best-characterized members of the IL-1 superfamily, IL-1α, IL-1β, and IL-18, in a range of conditions and cell types, including murine bone marrow–derived and peritoneal macrophages, human monocyte–derived macrophages, HeLa cells, and mouse embryonic fibroblasts. We discovered that IL-1β and IL-18 share a common secretory pathway that depends upon membrane permeability and can operate in the absence of complete cell lysis and cell death. We also found that the pathway regulating the trafficking of IL-1α is distinct from the pathway regulating IL-1β and IL-18. Although the release of IL-1α could also be dissociated from cell death, it was independent of the effects of the membrane-stabilizing agent punicalagin, which inhibited both IL-1β and IL-18 release. These results reveal that in addition to their role as danger signals released from dead cells, IL-1 family cytokines can be secreted in the absence of cell death. We propose that models used in the study of IL-1 release should be considered context-dependently.