What are NLRP3-ASC specks? an experimental progress of 22 years of inflammasome research

Microglial NLRP3 Inflammasomes in Alzheimer's Disease Pathogenesis: From Interaction with Autophagy/Mitophagy to Therapeutics

The nucleotide-binding oligomerization domain-like receptor pyrin domain-containing 3 (NLRP3) inflammasome, discovered 20 years ago, is crucial in controlling innate immune reactions in Alzheimer's disease (AD). By initiating the release of inflammatory molecules (including caspases, IL-1β, and IL-18), the excessively activated inflammasome complex in microglia leads to chronic inflammation and neuronal death, resulting in the progression of cognitive deficiencies. Even though the involvement of NLRP3 has been implicated in neuroinflammation and widely explored in several studies, there are plenty of controversies regarding its precise roles and activation mechanisms in AD. Another prominent feature of AD is impairment in microglial autophagy, which can be either the cause or the consequence of NLRP3 activation and contributes to the aggregation of misfolded proteins and aberrant chronic inflammatory state seen in the disease course. Studies also demonstrate that intracellular buildup of dysfunctional and damaged mitochondria due to defective mitophagy enhances inflammasome activation, further suggesting that restoration of impaired autophagy and mitophagy can effectively suppress it, thereby reducing inflammation and protecting microglia and neurons. This review is primarily focused on the role of NLRP3 inflammasome in the etiopathology of AD, its interactions with microglial autophagy/mitophagy, and the latest developments in NLRP3 inflammasome-targeted therapeutic interventions being implicated for AD treatment.

NLRP3 inflammasome: From drug target to drug discovery

The immune system employs innate and adaptive immunity to combat pathogens and stress stimuli. Innate immunity rapidly detects pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs) via pattern recognition receptors (PRRs), whereas adaptive immunity mediates antigen-specific T/B cell responses. The NLRP3 inflammasome, a key cytoplasmic PRR, consists of leucine-rich repeat, nucleotide-binding, and pyrin domains. Its activation requires priming (signal 1: Toll-like receptors/NOD-like receptors/cytokine receptors) and activation (signal 2: PAMPs/DAMPs/particulates). NLRP3 triggers cytokine storms and neuroinflammation, contributing to inflammatory diseases. Emerging therapies target NLRP3 via nuclear receptors (transcriptional regulation), adeno-associated virus (AAV) vectors (gene delivery), and microRNAs (post-transcriptional modulation). This review highlights NLRP3's signaling cascade, pathological roles, and combinatorial treatments leveraging nuclear receptors, AAVs, and microRNAs for immunomodulation.

Siponimod inhibits microglial inflammasome activation

Siponimod is the first oral drug approved for active secondary progressive multiple sclerosis. It acts as a functional antagonist of sphingosine-1-phosphate (S1P) receptor 1 (S1P1) through S1P1 internalization, and also serves an agonist of S1P5; however, the detailed mechanisms of its therapeutic effects on glial cells have yet to be elucidated. In this study, we investigated the anti-inflammatory mechanism of siponimod in microglia. Pretreatment with either siponimod or the S1P1 antagonist W146 significantly suppressed the production of interleukin-1β in activated microglia stimulated with lipopolysaccharide plus nigericin, an inflammasome activator. Furthermore, siponimod treatment reduced the protein levels of cleaved caspase-1 and inhibited the formation of aggregates of apoptosis-associated speck-like protein containing a C-terminal caspase recruitment domain (ASC specks) in microglia. Our data indicate that siponimod achieves its anti-inflammatory effects by inhibiting inflammasome activation in microglia via S1P1 antagonism. This process is inferred to play a crucial role in mitigating the secondary progression of multiple sclerosis, where microglial activation in the gray matter is considered a key pathological factor.

The intervention of NLRP3 inflammasome inhibitor: oridonin against azoxymethane and dextran sulfate sodium-induced colitis-associated colorectal cancer in male BALB/c mice

Colorectal cancer (CRC) ranks third globally in cancer diagnoses. The dysregulation of the NLRP3 inflammasome is prominently linked to several types of cancers. Oridonin, a principal component of Rabdosia rubescens, exhibits inhibitory activity against NLRP3 and is well-recognized for its diverse pharmacological benefits. However, its role in an animal model of colitis-associated colorectal cancer (CACC) remains unexplored. In the present study, the effectiveness of oridonin was investigated against CACC, developed using azoxymethane (AOM), a tumour initiator, and dextran sulphate sodium (DSS), a tumour promoter, in male BALB/c mice. The two-stage murine model of inflammation-associated cancer was established by administering AOM (10 mg/kg b.w.; i.p., once) followed by DSS (2% w/v) in drinking water (3 cycles, 7 days/cycle). Over a span of 10 weeks, the dose-dependent (2.5, 5, and 10 mg/kg, b.w.; i.p.) effects of oridonin were investigated in BALB/c mice. Oridonin significantly alleviated CACC severity, as evidenced by reduced DAI scores and restored body weight. Moreover, it attenuated surrogate markers of inflammation, including myeloperoxidase, nitrite, plasma LPS, TNF-α, IL-1β, and DNA damage. Histopathological examination revealed diminished tumorigenesis and apoptotic cells, corroborated by reduced Ki-67 and TNF-α, along with increased p53 expression in the colon. Following oridonin treatment, IHC/immunofluorescence analyses demonstrated a significantly reduced expression of the components of NLRP3 inflammasome including NLRP3, ASC-1, and caspase-1. Notably, the high dose of oridonin (10 mg/kg) consistently exhibited significant protective effects against CACC by modulating various molecular targets. Present findings confirmed the potential of oridonin in the protection of colitis-associated colorectal cancer, providing valuable insights into its mechanism of action and clinical significance.

Glaesserella parasuis serotype 5 promotes pyroptosis via degrading Caveolin-1 in 3D4/21 cells

Glaesserella parasuis (G. parasuis) is an important pathogen, which can cause systemic inflammatory response in pigs and bring huge economic losses to the global swine industry. G. parasuis can induce a strong inflammatory response in the lungs under environmental changes and certain stress conditions. However, the underlying mechanism of this adverse response has not been thoroughly studied. In this study we demonstrated that G. parasuis serotype 5 strain (GPS5-SQ) has the potential to induce pyroptosis in 3D4/21 cells. GPS5-SQ could degrade the expression of Cav-1. Knockdown or overexpression of Cav-1 promoted or reduced the occurrence of pyroptosis, respectively. These results suggested that Cav-1 is involved in pyroptosis induced by GPS5-SQ in 3D4/21 cells. In addition, overexpression of Cav-1 suppressed the activation of NLRP3 inflammasome by inhibiting ASC oligomerization, resulted in reducing pyroptosis. In general, we found that GPS5-SQ infection could promote pyroptosis by degrading the expression of Cav-1. The results of the study revealed the new mechanism of inflammation induced by GPS5-SQ in 3D4/21 cells.

SIRT3 mitigates neuroinflammation and mitochondrial damage post-hypoxic-ischemic brain injury

OBJECTIVE

We aimed to explore the role of SIRT3 in ameliorating neuroinflammation caused by hypoxia-ischemia (HI).

METHODS

A rat model of HI was established, and 48 hours prior to constructing the HI model, the rats received an intracerebroventricular injection of a recombinant adeno-associated virus type 9 vector. TTC and Nissl staining assessed the effects of SIRT3 on cerebral infarction and brain atrophy in HI rats. Neuroinflammation was evaluated by investigating IL-1β and MPO positive cells, and ELISA for determining inflammatory cytokines. IBA-1, CD68, and iNOS positive microglia and NLRP3 activation-related protein expression were also detected. SIRT3 was overexpressed in oxygen glucose deprivation (OGD)-induced microglia model, where cell morphology and expressions of pro-inflammatory cytokines and NLRP3 inflammasome activation-related proteins were examined. Additionally, neurons co-cultured with SIRT3-overexpressing microglia were analyzed for mitochondrial damage and apoptosis.

RESULTS

SIRT3 alleviated cerebral infarction and atrophy in HI rats. It also inhibited neuroinflammation, reducing IL-1β and MPO positive cells, and lowered the levels of pro-inflammatory cytokines. In both HI rat model and OGD cell model, SIRT3 inhibited excessive activation of microglia and NLRP3 inflammasome. Furthermore, co-culturing neurons with SIRT3-overexpressing microglia resulted in reduced neuronal apoptosis and improved mitochondrial function, evidenced by lower ROS levels, alleviated mitochondrial depolarization and increased ATP production.

CONCLUSION

SIRT3 restrains pro-inflammatory microglia and NLRP3 inflammasome and alleviates neuroinflammation following HI brain injury.

Dendritic cells activate pyroptosis and effector-triggered apoptosis to restrict Legionella infection

The innate immune system relies on pattern recognition receptors (PRRs) to detect pathogen-associated molecular patterns (PAMPs) and guard proteins to monitor pathogen disruption of host cell processes. How different immune cell types engage PRR- and guard protein-dependent defenses in response to infection is poorly understood. Here, we show that macrophages and dendritic cells (DCs) respond in distinct ways to bacterial virulence activities. In macrophages, the bacterial pathogen Legionella pneumophila deploys its Dot/Icm type IV secretion system (T4SS) to deliver effector proteins that facilitate its robust intracellular replication. In contrast, T4SS activity triggers rapid DC death that potently restricts Legionella replication within this cell type. Intriguingly, we found that infected DCs exhibit considerable heterogeneity at the single cell level. Initially, a subset of DCs activate caspase-11 and NLRP3 inflammasome-dependent pyroptosis and release IL-1 β early during infection. At later timepoints, a separate DC population undergoes apoptosis driven by T4SS effectors that block host protein synthesis, thereby depleting the levels of the pro-survival proteins Mcl-1 and cFLIP. Together, pyroptosis and effector-triggered apoptosis robustly restrict Legionella replication in DCs. Collectively, our work suggests a model where Mcl-1 and cFLIP guard host translation in DCs, and that macrophages and DCs distinctly employ innate immune sensors and guard proteins to mount divergent responses to Legionella infection.

Cellular takeover: How new world alphaviruses impact host organelle function

Alphavirus replication is dependent on host cell organelles to facilitate multiple steps of the viral life cycle. New world alphaviruses (NWA) consisting of eastern, western and Venezuelan equine encephalitis viruses are a subgroup of alphaviruses associated with central nervous system disease. Despite differing morbidity and mortality amongst these viruses, all are important human pathogens due to their transmission through viral aerosolization and mosquito transmission. In this review, we summarize the utilization of host organelles for NWA replication and the subversion of the host innate immune responses. The impact of viral proteins and replication processes on organelle function is also discussed. Literature involving old world alphaviruses (OWA), such as chikungunya virus and Sindbis virus, is included to compare and contrast between OWA and NWA and highlight gaps in knowledge for NWA. Finally, potential targets for therapeutics or vaccine candidates are highlighted with a focus on host-directed therapeutics.

Discovery of Potent, Specific, and Orally Available NLRP3 Inflammasome Inhibitors Based on Pyridazine Scaffolds for the Treatment of Septic Shock and Peritonitis

The NLRP3 inflammasome is a multiprotein complex that is a component of the innate immune system, involved in the production of pro-inflammatory cytokines. Its abnormal activation is associated with many inflammatory diseases. In this study, we designed and synthesized a series of NLRP3 inflammasome inhibitors based on pyridazine scaffolds. Among them, P33 exhibited significant inhibitory effects against nigericin-induced IL-1β release in THP-1 cells, BMDMs, and PBMCs, with IC50 values of 2.7, 15.3, and 2.9 nM, respectively. Mechanism studies indicated that P33 directly binds to NLRP3 protein (KD = 17.5 nM), inhibiting NLRP3 inflammasome activation and pyroptosis by suppressing ASC oligomerization during NLRP3 assembly. Additionally, P33 displayed excellent pharmacokinetic properties, with an oral bioavailability of 62%. In vivo efficacy studies revealed that P33 significantly ameliorated LPS-induced septic shock and MSU crystal-induced peritonitis in mice. These results indicate that P33 has great potential for further development as a candidate for treating NLRP3 inflammasome-mediated diseases.

ML365 ameliorates postoperative cognitive impairment in aged mice by inhibiting NLRP3 inflammasome activation in the hippocampus

The aim of this study was to examine the effects of ML365, a two-pore potassium channel (K2P) inhibitor, on postoperative cognitive impairment (POCD). A mouse model of POCD was constructed by subjecting aged C57BL/6 mice to exploratory laparotomy. Changes in cognitive function were assessed using the Morris water maze test. Western blotting and qPCR were used to detect hippocampal NLRP3, Caspase-1 and IL-1β expression levels on days 3 and 7 post-surgery. Apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC) expression level was also assessed by western blotting. Pathological changes and nerve damage in the hippocampal CA1 and CA3 regions were detected by H&E staining, while the concentration of malondialdehyde (MDA) in the plasma was measured. We found that pretreatment with ML365 (administered intraperitoneally at a dose of 10 mg/kg) 30 min prior to exploratory laparotomy effectively ameliorated POCD in mice. ML365 pretreatment also reduced NLRP3, Caspase-1, ASC and IL-1β expression levels in the hippocampus, improved POCD-induced pathological changes in the hippocampal CA1 and CA3 areas of aged mice, and decreased levels of plasma MDA and oxidative stress. Together, our findings indicate that ML365 can alleviate POCD in mice by inhibiting NLRP3 inflammasome activation in the hippocampus.