NLRP3 Inflammasome and Neurodegenerative Diseases
An overview of the NLRP3 inflammasome and its role in neurodegenerative diseases, including Alzheimer's disease, Parkinson’s disease, and ALS.
What is the NLRP3 inflammasome?
The NOD-like receptor family pyrin domain-containing protein 3 (NLRP3) inflammasome, the most extensively studied inflammasome, is a multi-protein complex that plays a critical role in regulating the innate immune system and inflammatory signaling, and is involved in the development of various immune and inflammation-related diseases. Its canonical activation pathway is triggered when either pathogen-associated molecular patterns (PAMPs) or host-derived damage-associated molecular patterns (DAMPs), released in response to injury or cellular stress, are detected by pattern recognition receptors (PRRs). NOD-like receptors (NLRs), toll-like receptors (TLRs), and absent in melanoma 2-like receptors (ALRs) are distinct types of PRRs that recognize PAMPs and DAMPs and prime the cell for the subsequent activation of the NLRP3 inflammasome.
The NLRP3 inflammasome is composed of three main components: (1) NLRP3 as the sensor, (2) apoptosis-associated speck-like protein containing a C-terminal caspase recruitment domain (ASC) as the adaptor, and (3) pro-caspase-1 as the effector. The formation of the inflammasome process begins with the detection of PAMPs or DAMPS. ASC then binds to pro-caspase-1, converting it into active caspase-1. In turn, active caspase-1 cleaves pro-interleukin (IL)-1β and pro-IL-18 into their active forms, IL-1β and IL-18. Caspase-1 also activates gasdermin D (GSDMD), which forms membrane pores, enabling the release of these proinflammatory cytokines, and serves as a trigger of pyroptosis.
In addition to immune and inflammation-related diseases, the NLRP3 inflammasome has also been implicated in the pathogenesis of neurodegenerative diseases, such as Alzheimer’s disease (AD), Parkinson’s disease (PD), and amyotrophic lateral sclerosis (ALS) (Holbrook, 2021; Singh, 2023). While microglia and astrocytes initially provide protective immune responses in the CNS, their chronic activation can lead to the spread of neuroinflammation, which in turn contributes to the progression of these neurodegenerative diseases (Wang, 2024). As such, therapies targeting the NLRP3 inflammasome are gaining attention for their potential to reduce chronic inflammation and alleviate disease symptoms.
Recent studies exploring small molecules that modulate NLRP3 activation have shown encouraging results in blocking chronic inflammation and improving disease outcomes (Blevins, 2022). While there are still challenges in developing effective treatments targeting the NLRP3 inflammasome for CNS diseases, such as bypassing the blood-brain barrier (BBB), continued advancements in drug development and targeted therapies will pave the way for effective treatments to come.
The NLRP3 inflammasome is activated through the canonical pathway when NLRP3 detects either PAMPs or DAMPs (not shown). This recognition triggers the recruitment of ASC and pro-caspase-1, leading to the assembly of the NLRP3 inflammasome complex. Once formed, active caspase-1 cleaves pro-IL-1β and pro-IL-18 into their active forms, IL-1β and IL-18, as well as GSDMD (not shown). The cleavage of GSDMD creates membrane pores that allow the release of proinflammatory cytokines, IL-1β and IL-18. Figure and caption adapted from Wang et al. (Wang, 2024) under the Creative Commons Attribution License.
What is the role of the NLRP3 inflammasome in AD, PD, and ALS?
Neurodegenerative diseases, including AD, PD, and ALS, are becoming increasingly prevalent. These disorders are characterized by the progressive accumulation of misfolded proteins in various regions of the CNS, leading to neuronal loss. Key proteins and pathologies involved include amyloid-β (Aβ) plaques, hyperphosphorylated tau neurofibrillary tangles, α-synuclein (α-syn) aggregates, and TAR DNA binding protein-43 (TDP-43) and superoxide dismutase 1 (SOD1) inclusions.
A critical consequence of this protein accumulation is the activation of the NLRP3 inflammasome, which triggers the release of proinflammatory cytokines IL-1β and IL-18, contributing to neuroinflammation. Growing evidence underscores the central role of inflammasomes in disease progression, amplifying inflammation and neuronal damage. As our understanding of inflammasome involvement expands, exploring their activation in these diseases is becoming increasingly important.
Alzheimer’s Disease (AD)
AD is a progressive neurodegenerative disorder marked by cognitive decline, memory impairment, and changes in behavior and mood. The accumulation of Aβ plaques and hyperphosphorylated tau neurofibrillary tangles is central to AD pathogenesis. However, neuroinflammation also plays a significant role in disease progression. In AD, Aβ and tau act as DAMPs, activating the NLRP3 inflammasome, particularly in microglia. This activation leads to the release of proinflammatory cytokines like IL-1β, exacerbating neuroinflammation and accelerating neurodegenerative (Wang, 2024). Accordingly, peripheral immune cells and postmortem brain tissue from AD patients show elevated levels of IL-1β and IL-18, along with increased expression of inflammasome components NLRP3, ASC, and caspase-1, with ASC expression correlating with levels of Aβ and tau (Heneka, 2013; Saresella, 2016; Vontell, 2023).
Similarly, in the APP/PS1 AD mouse model, increased caspase-1 processing has been observed (Heneka, 2013). When these mice are crossed with NLRP3 or caspase-1 deficient models, memory is preserved, suggesting that NLRP3 and caspase-1 mediate inflammation linked to cognitive dysfunction in AD (Heneka, 2013). Additionally, in these APP/PS1/NLRP3-/- or APP/PS1/Casp-1-/- mice, Aβ phagocytosis is increased, suggesting that NLRP3 inflammasome activation reduces Aβ phagocytosis (Heneka, 2013). Together, these findings support the hypothesis that Aβ-induced NLRP3 inflammasome activation accelerates AD progression by driving an inflammatory response, contributing to cognitive impairment and impeding Aβ clearance.
In addition to Aβ pathology, NLRP3 inflammasome activation also drives tau pathology. In Tau22 mice, increased levels of caspase-1, ASC, and IL-1β have been detected (Ising, 2019). Consistent with a role of NLRP3 inflammasome activation in AD cognitive function, Tau22 mice crossed with ASC or NLRP3 deficiency show lower levels of tau hyperphosphorylation and aggregation, as well as preserved memory (Ising, 2019). Injection of APP/PS1 brain homogenate in Tau22 mice induces tau hyperphosphorylation, but this effect is absent in Tau22/ASC-/- or Tau22/NLRP3-/- mice, suggesting that NLRP3 activity is a critical component of the Aβ-tau cascade (Ising, 2019). Given its key role in both Aβ and tau pathology, targeting the NLRP3 inflammasome is a promising strategy that could yield therapeutic benefits and help slow the progression of AD, as well as other tauopathies (Heneka, 2013; Ising, 2019).
Parkinson’s Disease (PD)
PD, the second most common neurodegenerative disease after AD, is primarily characterized by motor symptoms such as muscle rigidity, bradykinesia, and resting tremor, along with non-motor symptoms, including mood disorders like depression and behavioral disturbances. PD is defined by hallmark features such as the progressive loss of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNc) and the accumulation of Lewy bodies, which are primarily composed of α-syn aggregates. In addition to these hallmark features, inflammation also plays a crucial role in PD pathology (Li, 2021). In post-mortem tissue from PD patients, increased levels of ASC and NLRP3 have been detected, suggesting the activation of the NLRP3 inflammasome in this disease (Anderson, 2021).
In the MPTP-induced mouse model of PD, crossing either NLRP3 or caspase-1 knockout mice results in a reduction in DA neuron loss and improvement in motor function (Qiao, 2017; Lee, 2019). Moreover, in NLRP3-deficient mice, MPTP fails to induce microglial recruitment, IL-1β production, and caspase-1 activation in the SN, suggesting that the activation of the NLRP3 inflammasome in microglia plays a crucial role in the neurodegeneration seen in PD (Lee, 2019). Interestingly, a selective dopamine D2 receptor agonist has been found to inhibit NLRP3 inflammasome activation in the SN of MPTP-treated PD mice (Zhu, 2018). Additionally, it suppressed both caspase-1 and IL-1β expression in primary cultured mouse astrocytes following NLRP3 inflammasome activation (Zhu, 2018). While some studies support NLRP3 inflammasome activation in astrocytes (Freeman, 2017), conflicting evidence exists, suggesting that further research is needed to clarify their involvement.
NLRP3 inflammasome activation may directly modulate α-syn pathology. Caspase-1 can directly cleave α-syn (Wang, 2016), and blocking NLRP3 inflammasome activation with the small molecular inhibitor MCC950 reduces α-syn aggregation, dopaminergic degeneration, neuroinflammation, and motor deficits in both the PFF and the AAV-Syn mouse models of PD (Gordon, 2018; Grotemeyer, 2023). These findings suggest that targeting the NLRP3 inflammasome could offer a promising therapeutic approach for PD.
Amyotrophic Lateral Sclerosis (ALS)
ALS is a neurodegenerative disease characterized by the progressive degeneration of motor neurons in the spinal cord, brainstem, and motor cortex, leading to symptoms such as muscle weakness, difficulty speaking and swallowing, and progressive paralysis. ALS patients have mutations in C9orf72, TARDBP, SOD1, and FUS, with TDP-43 and SOD1 proteins being the most extensively studied. These proteins form abnormal aggregates, leading to impaired protein clearance and neuronal dysfunction. Additionally, the NLRP3 inflammasome may play a significant role, as elevated levels of NLRP3, ASC, IL-18, and caspase-1 have been detected in post-mortem tissue of ALS patients (Johann, 2015).
In both the SOD1G93A and TDP-43Q331K ALS mouse models, spinal cord tissue shows upregulated expression of NLRP3 inflammasome pathway genes (Deora, 2020). In SOD1G93A mice, spinal cord astrocytes were identified as the major cell type to express NLRP3 components (Johann, 2015). Inhibiting caspase-1 in the SOD1G93A model has been shown to delay disease onset, neurological deterioration, and mortality, further highlighting the role of NLRP3 inflammasome activation in ALS progression (Zhang, 2013). However, other studies indicate that MCC950 treatment does not reduce spinal cord inflammation in this model, suggesting that multiple inflammasomes may be activated in ALS and that NLRP3 inhibition alone may not be sufficient to block inflammation in this disease (Clénet, 2023).
In conclusion, while NLRP3 inflammasome activation has been extensively studied in AD and PD, its role in ALS remains less well understood. In addition to NLRP3, other inflammasome complexes, such as AIM2, NLRC4 and NLRP1, may also contribute to neuropathology in these neurodegenerative diseases. More preclinical studies and clinical trials are needed to unravel the complex role of inflammasomes in neurodegenerative diseases, identify potential therapeutic targets, and develop effective treatments.
Neurodegenerative diseases, including AD, PD and ALS, are commonly characterized by the abnormal aggregation of misfolded proteins such as Aβ, tau, α-syn, TDP-43 and SOD1. These neurotoxic protein aggregates trigger the activation of the NLRP3 inflammasome in microglia and/or astrocytes, leading to the formation of the NLRP3 inflammasome complex. This results in the production and release of proinflammatory cytokines, IL-1β and IL-18, which play a crucial role in mediating neuroinflammation and driving neurodegeneration in these diseases. Figure and caption adapted from Wang et al. (Wang, 2024) under the Creative Commons Attribution License.
What strategies are being explored for therapeutic targeting of the NLRP3 inflammasome pathway in neurodegenerative diseases?
The NLRP3 inflammasome plays a critical role in neurodegenerative diseases by driving neuroinflammation, a key contributor to disease progression. This finding has led to the development of various compounds aimed at targeting the NLRP3 inflammasome pathway to reduce neuroinflammation and its detrimental effects.
One of the most well-known and widely studied NLRP3 inhibitors is MCC950 (also known as CRID3). MCC950 specifically inhibits NLRP3 inflammasome activation, preventing the release of proinflammatory cytokines, like IL-1β, without affecting other inflammasome complexes. It has shown promise in preclinical studies for its ability to alleviate symptoms in mouse models of various neurodegenerative diseases (Blevins, 2022). In addition to MCC950, other NLRP3 inhibitors, including glyburide and OLT1177, are being explored for their therapeutic efficacy. Other inflammasome inhibitors, such as BAY 11-7082 and parthenolide, target multiple inflammasome complexes, but with less specificity for NLRP3.
Inhibiting specific components of the canonical NLRP3 inflammasome signaling pathway is another approach. VX-740 and VX-765, selective caspase-1 inhibitors, block the release of IL-1β and IL-18, reducing inflammation. Preclinical studies show that these inhibitors can improve cognitive function in AD mouse models (Wang, 2024). Similarly, the ASC inhibitor IC100 has demonstrated anti-inflammatory effects in the experimental autoimmune encephalomyelitis (EAE) model (Wang, 2024). Targeting GSDMD is another promising strategy. Disulfiram, a GSDMD pore formation blocker, has shown efficacy in reducing inflammation in PD cell models, and necrosulfonamide, another GSDM inhibitor, has been found to reduce neuroinflammation and protect DA neurons in the MPTP-induced PD model (Wang, 2024).
Despite promising preclinical results, clinical trials investigating NLRP3 inhibitors for treating neurodegenerative diseases remain limited. One notable study is the Phase 2 trial of Usnoflast (ZYIL1), a small molecule NLRP3 inhibitor tested in ALS patients (NCT05981040). This trial followed a successful Phase 1 study showing significant inhibition of IL-1β and IL-18, highlighting the potential of NLRP3 inhibition as a treatment strategy for neurodegenerative diseases (Parmar, 2023).
A key challenge in developing NLRP3-targeted therapies for CNS diseases is crossing the BBB, which is a significant obstacle. Additionally, blocking IL-1β signaling may increase susceptibility to infections, requiring a balance between therapeutic efficacy and safety. These challenges emphasize the need for small-molecule inhibitors of NLRP3 to overcome pharmacokinetic and selectivity limitations (Blevins, 2022).
In summary, while targeting the NLRP3 inflammasome shows promise for treating neurodegenerative diseases, it requires careful consideration of specificity, BBB penetration, and potential adverse effects. Ongoing research and clinical trials will be crucial in determining the therapeutic viability of these strategies.
Our team would be happy to answer any questions about the NLRP3 inflammasome and neurodegenerative diseases or provide specific information about the AD, ALS, and PD models we use for therapeutic efficacy studies.
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