NLRP3 Inflammasome’s Activation in Acute and Chronic Brain Diseases—An Update on Pathogenetic Mechanisms and Therapeutic Perspectives with Respect to Other Inflammasomes

Increasingly prevalent acute and chronic human brain diseases are scourges for the elderly. Besides the lack of therapies, these ailments share a neuroinflammation that is triggered/sustained by different innate immunity-related protein oligomers called inflammasomes. Relevant neuroinflammation players such as microglia/monocytes typically exhibit a strong NLRP3 inflammasome activation. Hence the idea that NLRP3 suppression might solve neurodegenerative ailments. Here we review the recent Literature about this topic. First, we update conditions and mechanisms, including RNAs, extracellular vesicles/exosomes, endogenous compounds, and ethnic/pharmacological agents/extracts regulating NLRP3 function. Second, we pinpoint NLRP3-activating mechanisms and known NLRP3 inhibition effects in acute (ischemia, stroke, hemorrhage), chronic (Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, MS, ALS), and virus-induced (Zika, SARS-CoV-2, and others) human brain diseases. The available data show that (i) disease-specific divergent mechanisms activate the (mainly animal) brains NLRP3; (ii) no evidence proves that NLRP3 inhibition modifies human brain diseases (yet ad hoc trials are ongoing); and (iii) no findings exclude that concurrently activated other-than-NLRP3 inflammasomes might functionally replace the inhibited NLRP3. Finally, we highlight that among the causes of the persistent lack of therapies are the species difference problem in disease models and a preference for symptomatic over etiologic therapeutic approaches. Therefore, we posit that human neural cell-based disease models could drive etiological, pathogenetic, and therapeutic advances, including NLRP3’s and other inflammasomes’ regulation, while minimizing failure risks in candidate drug trials.


An Overall Picture
Acute and chronic human brain diseases have been attracting the increased attention of scientists and the public. This has been due to the concurrence of several factors, i.e., brain illnesses' mounting prevalence, the persistent lack of effective therapies, increasingly huge healthcare and economic costs, hardships in assisting such patients particularly at home, marked psychopathological impacts on patients and relatives, a greater sensitivity to improper lifestyle consequences, and a common aspiration to long-lasting and healthy aging. To this must be added the growing concern about the serious risk that severe acute brain injuries surreptitiously evolve into chronic neuropathologies such as Alzheimer's disease (AD), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS). Worldwide yearly estimates of acute brain injuries total about 42 million cases, while symptomatic AD by itself affects more than 50 million people. It is predicted that such figures will double or treble in twenty/thirty years unless effective therapies become available [1,2]. Yet, the Purinergic receptor signaling DDX3X protein/NLRP3 complexes Heat shock protein 60 (HSP60) and TLR-4-p38 MAPKs axis Oxidized mtDNA and proteins Lysosome-released cathepsin B Mitochondria-released hexokinase, ROS NLRP3 activation Ionic imbalances [25,[109][110][111][112][113][114][115][116][117][118][119] Aging Inflammaging ↑ Membrane attack complexes (MAC) Reduced mitochondrial fission and fusion Declined mitophagy Mitochondrial damage Selective autophagy-mediated mitochondrial homeostasis (in microglia) [33,112,[120][121][122] § ↑ = increased.
Most studies have shown that NLRP3 s canonical activation requires two initiating signals. The "Signal 1" or "priming step" is an endocytosed PAMP or an endogenous DAMP/HAMP evoking the signaling from Toll-like receptor 4 (TLR-4) or a NOD-like receptor (NLR) or the tumor necrosis factor receptor (TNFR). Furthermore, signaling from G-protein-coupled receptors (GPCRs) can affect NLRP3 activity (see Box 1 for further details and references).
CNS neural cells express diverse types of purinergic receptors, i.e., P1, for adenosine G proteincoupled receptors; P2X, for ATP-gated ion channels; and P2Y, for G protein-coupled receptors. Importantly, the intra-brain accumulation of Aβs induces the damaged neural cells to release ATP into the extracellular matrix (ECM). Exogenous ATP and the agonist 4-benzoyl-ATP (BzATP) activate the signaling from P2X 7 purinergic receptors expressed by neural cells. The upshots are an increased synthesis and release of pro-inflammatory cytokines and chemokines, and a decline in the α-secretase activity, causing a plunge in the extracellular shedding of the neurotrophic and neuroprotective soluble amyloid precursor protein (APP)-α. Yet, various (e.g., mechanical) stressing factors awaken the signaling of P2X 7 receptors, making the cells release their endogenous ATP through connexin 43 and pannexin hemichannels (i.e., "pathological pores") [159]. The results are the activation of the NF-κB axis and of the NLRP3•ASC•caspase-1 and IL-1β pathways in both the astrocytes and microglia, triggering the sterile neuroinflammation proper of AD within the brain and of glaucoma within the retina [57,160].
Nuclear receptors too control the NLRP3 inflammasome [167]. Thus, various positive and negative signaling pathways strictly regulate NLRP3's activation to prevent any harm while preserving the host tissues' homeostasis [168]. Various kinases, ubiquitin ligases, a de-ubiquitinase, and other enzymes crucially control both NLRP3's activation and function termination via ad hoc post-translational modifications of its protein components [169]. As an example, Bruton's tyrosine kinase (BTK) directly and positively regulates the NLRP3 inflammasome, which might have therapeutic implications [170]. Usually, sterile, and slowacting DAMPs/HAMPs elicit weaker NLRP3 inflammasome responses than infectious PAMPS do [171]. Finally, inflammasome-interested scientists should note that speciesrelated differences in animal models can crucially affect their results [172].

Noncanonical NLRP3 Activation
Hitherto, we have discussed NLRP3's "canonical activation", a concept valid also for NLRP1, NLRC4, and AIM2 inflammasomes. The more recently discovered "noncanonical activation" of inflammasomes is worth mentioning too. Concerning microglia's NLRP3, the noncanonical process involves the activation of caspase-11 and caspase-8 in mice and of caspase-4 and caspase-5 in humans [173][174][175]. These caspases behave as cytosolic sensors that directly bind and are activated by the lipopolysaccharide (LPS) of Gram-negative bacteria. This drives the secretion of mature IL-1β and IL-18. Additionally, the active caspases detach N-terminal fragments from the GSDMD/GsdmD proteins, which form transmembrane pores promoting K + efflux and thus causing both NLRP3's canonical activation and neurons' pyroptosis [176][177][178][179].
The HMGB1 (high mobility group box 1 protein)/caspase-8 pathway is an added mechanism of noncanonical NLRP3 activation proper of eye glaucoma. An acutely elevated intraocular pressure intensifies HMGB1's signaling, which activates the NLRP3 inflammasome by canonical and noncanonical (via caspase-8) mechanisms, producing higher amounts of mature IL-1β within the ischemic retinal tissue and thereby advancing neuroinflammation [59].

Brain NLRP3 Inflammasome's Modulation by RNAs
Cells express manifold kinds (ribosomal, messenger, and noncoding) of RNAs, which control most of their functions. Long noncoding (Lnc) RNAs have more than 200 base pairs but encode no or few proteins. However, LncRNAs importantly affect body development, cell differentiation, metabolism, autoimmunity, and immune function, and hence NLRP3 inflammasome activity [180,181]. MicroRNAs (or miRs) are ubiquitous 22-nucleotide-long single-stranded RNAs that post-transcriptionally control gene expression by silencing mRNAs via complementary base-pairing [182]. Notably, miRs abound (>2300 types) inside mammalian cells and are released via extracellular vesicles (EVs) or exosomes (Exos) into cerebrospinal fluid and blood. Circulating miRs are under investigation as biomarkers in various diseases and in the distinct stages of each illness. According to ongoing circumstances, distinct miRs promote or inhibit NLRP3 inflammasome activation.
Among noncoding RNAs, Alu-derived RNAs deserve a brief mention. They result from the transcription of primate-specific transposable "Alu elements" by small interspersed nuclear elements (SINEs). Alu-RNAs are plentiful, involving >10% of the human genome, with 102 to 103 copies released into the cytosol of each cell. Alu-RNAs regulate gene expression by binding and inhibiting RNA polymerase II (P2). Alu-RNAs accumulate in the brains of patients with dementia or sporadic Creutzfeldt-Jacob's disease (CJD), in which they drive neuroinflammation and neuron demise [183]. P3-transcribed Alu-RNAs (P3Alus) may advance NLRP3 inflammasome-driven neuroinflammation/neurodegeneration disorders, AD included [184]. Hence P3Alus may be therapeutic targets for such ailments. Later studies revealed that Alu-RNAs processing rates are elevated in mouse and human AD brains, tightly correlating with the up-regulated expression of HSF1 (heat shock transcription factor 1), a crucial stress response factor. The increased Alu-RNAs processing rates would fix into active mode the HSF1/Alu-RNA/stress response/cell death-promoting genes (e.g., p53) axis in AD patients [185,186].
This topic is bound to undergo further developments in regard not only to LncRNAs, miRs, and Alu-RNAs, but also to the recently discovered circular RNAs [187]. Table 2 reports details about LncRNAs/miRs and NLRP3 interactions. Table 2. RNAs modulating brain NLRP3 inflammasome's function.
In summary, the available evidence about EVs' and Exos' beneficial or harmful roles in NLRP3-mediated neuroinflammation is still scanty. A further limitation is that most studies focused on the RNAs conveyed by EVs or Exos. However, EVs or Exos also transport high numbers of different proteins that either promote or hinder neuroinflammation. In fact, Exos from Aβ 25-35 -exposed human cortical astrocytes conveyed significantly increased amounts of p-Taues [212], while Exos from human AD brains transported Aβ oligomers [213].

Other Brain NLRP3 Inflammasome Regulators
Under any situation, complex sets of endogenous factors control or restrain NLRP3 inflammasome assembly and/or function, trying to reestablish and/or upkeep tissue homeostasis. Zhang et al. [214] strengthened the relevance of the NLRP3 concept by proving that NLRP3 gene knockout or pharmacological blockage improved the course of various inflammatory diseases modeled in rodents. Hereafter we mention relevant NLRP3 regulators.
Osteopontin is a highly phosphorylated ECM sialoprotein expressed during the subacute phase following cerebral infarction. It stimulated microglia's chemotaxis while preventing NLRP3's activation and its sequels [52].
Worth mentioning here is PKR (i.e., protein kinase RNA-activated), a multirole serinethreonine kinase controlling mRNA transcription/translation, protein synthesis, cell proliferation, apoptosis, and brain function, in addition to shielding cells from viral infections. A dysfunctional PKR partook in cancer and neuroinflammation [219]. Moreover, by using wild-type and PKR −/− mouse macrophages, Lu et al. [220] showed that PKR needed to physically interact with NLRP3, NLRC4, and AIM-2 inflammasomes to activate them. However, using LPS-treated PKR −/− bone marrow-derived macrophages isolated from different mouse strains, He et al. [221] reported that following stimuli activating NLRP3, NLRC4, and AIM2 inflammasomes' PKR activity was critical for nitric oxide synthase-2 (NOS-2) induction, yet dispensable for pro-IL-1β and pro-IL-18 cleavage by caspase-1 [172]. Altogether the divergent results of Lu et al. [220] and Healy et al. [172] show that the animal species or strains investigated do significantly affect the kind of mechanisms activating or inactivating the NLRP3 and other inflammasomes. This adds a remarkable degree of complexity to the topic and stresses the importance of investigating corresponding mechanisms in human neural cells models.

Brain NLRP3 Downregulation by Officinal Plant Agents/Herbal Extracts
Since time immemorial, plants were and still are the source of drugs helping human ailments. Although extracts of plant body portions are still in use in Traditional Chinese Medicine (TCM), the current more scientific attitude is to find the specific compound(s) of potential therapeutic use. Table 4 reports the most relevant agents and herbal extracts of interest regarding the brain NLRP3 inflammasome.
It is worth noting that save for ginsenoids, artemisinin, and artesunate, all the other hitherto-reported therapeutically promising plant agents/herbal extracts still need in-depth preclinical studies and well conducted clinical trials prior to becoming FDA-approved drugs. On the other hand, altogether the above-listed agents/extracts represent a treasure trove of future therapeutic assets. Traumatic brain injury-model mouse LPS-stimulated BV-2 microglial cells LPS-treated mouse [286,287] Astragaloside IV from Astragalus membranaceus (i.e., Huangqi) pentacyclic triterpenoid Antioxidant activity Transient cerebral ischemia/reperfusion (I/R)-model mice [288] Baicalin from the root of Scutellaria baicalensis Georgi flavonoid ↓ TLR-4/NF-κB/NLRP3 axis APP/PS1 AD-model mice LPS/Aβ-stimulated BV2 microglial cells [289] Benzyl isothiocyanate from cruciferous vegetables benzene ↓ NLRP3 activation via mitochondriagenerated ROS inhibition ↓ NF-κB signaling

NLRP3 Inflammasome in Brain Acute Injuries
Glial NLRP3's role is controversial in HI/OGD (oxygen-glucose deprivation)-model animals. Denes et al. [336] reported that plasma IL-18 levels and brain infarction volume were alike in both wild-type and NLRP3-shRNA-silenced mice. Therefore, NLRP3's downregulation was not as neuroprotective as expected because other inflammasomes took over and functioned in NLRP3's stead. In fact, after shRNA-induced NLRP3 depletion, OGD significantly increased AIM2 inflammasome's expression while NLRC4's expression did not change in BV-2 microglial cells.
Conversely, Yang et al. [337] showed that in newborn mouse astrocytes HI and OGD activated TRPV1 (transient receptor potential vanilloid 1), a non-selective cation channel of the TRP family. Next, the TRPV1 signaling drove the JAK2-STAT3 pathway, which mediated NLRP3 inflammasome's activation and increased IL-1β levels. Notably, in HIand OGD-exposed TRPV1 −/− mouse astrocytes, JAK2 and STAT3 activation and IL-1β upregulation were less intense. Interestingly, this study revealed different cell type-related timings of NLRP3 activation elicited by HI/OGD. In newborn mouse astrocytes of the hippocampus, striatum, and thalamic habenula, NLRP3's activity increased by 3 h, while in microglia it was insignificant at 3 h but increased remarkably by 72 h. Then again, Schölwer et al. [338] showed that OGD completely inactivated phagocytic activity in wildtype BV-2 cells, while HI restored phagocytic activity in NLRP3-shRNA-depleted BV-2 cells. Therefore, the authors posited that NLRP3 plays a minor replaceable role in the OGD-elicited neuroinflammation, at least in microglia. Conversely, an anti-inflammatory pleiotropic cytokine, IL-10, hindered NLRP3 activation in microglia by increasing STAT-3's function, which stifled the transcription/translation of pro-IL-1β and mature IL-1β production [339].
Relevant to this topic is IL-33, another IL-1 family member playing major pleiotropic roles in normal and pathological conditions [340]. In neonatal mouse astrocytes, IL-33 expression markedly increased by 24 h after a cerebral HI episode. Exogenously administered IL-33 did mitigate brain infarction volume by one week after the HI event. Astrocytes' basal expression of ST2 (or suppressor of tumorigenesis 2), the IL-33 receptor, was intense and after HI exposure increased further. Conversely, a ST2 shortfall worsened the HIelicited brain infarction. The IL-33•ST2 signaling-activated pathways mitigated astrocytes' HI-elicited neuroinflammatory response and apoptosis. Moreover, in vitro IL-33-treated murine astrocytes released neurotrophic factors, which protected HI-and OGD-exposed neurons' viability [341]. Besides, administering IL-33 plus MCC950 and antimalarial drugs improved the outcome in a model of murine cerebral malaria [342] in which the Plasmodium falciparum overgrew inside the cortical capillaries, diffusely obstructing blood flow.
Franke et al. [36] showed that following stroke's onset, the early up-regulation of the NLRP3 inflammasome occurred in neurons, glia, and vascular endothelia, leading to blood-brain barrier (BBB) breakdown. Consistently, NLRP3 inhibition hindered endothelial pyroptosis induced by the thrombolytic agent rt-PA (or tissue plasminogen activator), thus preserving the BBB's integrity [11]. Similarly, NLRP3-inhibitor MCC950 protected brain endothelial cells from rt-PA's toxic effects in an in vitro HI-exposed BBB model [343]. Additionally, NLRP3 s knockout alleviated the NF-κB pathway-mediated brain damage in a middle cerebral artery occlusion (MCAO)-induced focal ischemia mouse model [344]. Moreover, lithium (Li + ), the archetypal mood stabilizer, also impeded HI/R-induced NLRP3 inflammasome activation, and by stimulating STAT3's function improved motor behavior, cognition, and depression [345]. Figure 1 sums up the main signaling pathways involving NLRP3 in acute brain injuries. Finally, electroacupuncture (EA) exerted analgesic effects by suppressing NLRP3 inflammasome function in the spinal dorsal horn of mice [346]. Moreover, EA at the skull's Shenting (DU24) and Baihui (DU20) acupoints attenuated cognitive impairment in rats with brain HI/R injury by regulating endogenous melatonin secretion through alkylamine N-acetyltransferase synthesis in the epiphysis. Next, melatonin acted neuroprotectively by blocking NLRP3 activation via upregulating mitophagy-associated proteins [347].  Figure 1 sums up the main signaling pathways involving NLRP3 in acute brain injuries. Figure 1. Schematic illustration of stressors and factors inducing/modulating NLRP3 inflammasome's activation and its sequels in astrocytes and microglia under acute injuries due to hypoxic ischemia, stroke, and hemorrhage. Left: Astrocyte's prompt response. Acute O2 tension fall activates Ca 2+ influx through TPRV1 channels, triggering the JAK2/STAT3 axis and NLRP3 inflammasome activation. It also increases BACE1 and IL-33 gene expression. Over-released IL-33 binds its STD2 receptor, whose signaling mitigates NLRP3 activity. Later, BACE1 increased activity overproduces Aβs. Extracellularly released excess Aβs bind and activate CaSR signaling, which contributes to NLRP3 inflammasome activation by reducing cAMP levels and activating CaMKII. Aβ•CaSR signaling also increases BACE1 and GSK-3β activities, driving the over production of Aβs from APP and p-Taues, which are both intracellularly accumulated and extracellularly released. CaSR NAM (Calcilytic) NPS2143 and CaMKII inhibitor KN93 suppress Aβs•CaSR signaling noxious effects (see for more details Box 1). Top right: Late wild-type microglia response. The NLRP3 activation is blocked by various agents, which activate via Akt the expression of NRF2 transcription factor. NRF2 activity reduces the M1 (proinflammatory) fraction of microglia. Bottom right: In a model of NLRP3 full-knockout microglia Ca 2+ influx activates in NLRP3 stead the AIM2 inflammasome's signaling, the upshot being the same, i.e., the overproduction/release of IL-1β and IL-18 [336]. A yellow frame encloses the assembled inflammasomes, while nuclear envelopes are orange colored. Abbreviations: Aβs = amyloid-β peptides; AC = adenylyl cyclase; AIM2 = absent in melanoma 2 inflammasome; Akt = protein kinase B; APP = amyloid precursor protein; ASC = apoptosis-associated speck-like protein endowed with a caspase recruitment domain or CARD; BACE1 = β-secretase; BBB = bloodbrain barrier; cAMP = 3′,5′-cyclic adenosine monophosphate; CASP1 = caspase-1; CaMKII = Ca 2+ /calmodulin-dependent protein kinase II; CaSR, calcium-sensing receptor; GdCl3 = gadolinium chloride; GSK-3β = glycogen synthase kinase-3β; JAK2 = Janus kinase 2; KN93 = N-  Figure 1. Schematic illustration of stressors and factors inducing/modulating NLRP3 inflammasome's activation and its sequels in astrocytes and microglia under acute injuries due to hypoxic ischemia, stroke, and hemorrhage. Left: Astrocyte's prompt response. Acute O 2 tension fall activates Ca 2+ influx through TPRV1 channels, triggering the JAK2/STAT3 axis and NLRP3 inflammasome activation. It also increases BACE1 and IL-33 gene expression. Over-released IL-33 binds its STD2 receptor, whose signaling mitigates NLRP3 activity. Later, BACE1 increased activity overproduces Aβs. Extracellularly released excess Aβs bind and activate CaSR signaling, which contributes to NLRP3 inflammasome activation by reducing cAMP levels and activating CaMKII. Aβ•CaSR signaling also increases BACE1 and GSK-3β activities, driving the over production of Aβs from APP and p-Taues, which are both intracellularly accumulated and extracellularly released. CaSR NAM (Calcilytic) NPS2143 and CaMKII inhibitor KN93 suppress Aβs•CaSR signaling noxious effects (see for more details Box 1). Top right: Late wild-type microglia response. The NLRP3 activation is blocked by various agents, which activate via Akt the expression of NRF2 transcription factor. NRF2 activity reduces the M1 (proinflammatory) fraction of microglia. Bottom right: In a model of NLRP3 full-knockout microglia Ca 2+ influx activates in NLRP3 stead the AIM2 inflammasome's signaling, the upshot being the same, i.e., the overproduction/release of IL-1β and IL-18 [336]. A yellow frame encloses the assembled inflammasomes, while nuclear envelopes are orange colored. Abbreviations: Aβs = amyloid-β peptides; AC = adenylyl cyclase; AIM2 = absent in melanoma 2 inflammasome; Akt = protein kinase B; APP = amyloid precursor protein; ASC = apoptosis-associated speck-like protein endowed with a caspase recruitment domain or CARD; BACE1 = β-secretase; BBB = blood-brain barrier; cAMP = 3 ,5 -cyclic adenosine monophosphate; CASP1 = caspase-1; CaMKII = Ca 2+ /calmodulin-dependent protein kinase II; CaSR, calcium-sensing receptor; GdCl 3 = gadolinium chloride; GSK-3β = glycogen synthase kinase-3β; JAK2 = Janus In conclusion, given the consistent risk that an acute brain injury triggers a chronic neurodegenerative disease entailing a lethal outcome, the therapeutic mitigation or better suppression of neuroinflammation within a brief time lag following the harmful event constitutes a quite valid target to be pursued.
Notably, ER stress concurs with the depletion of the anti-aging and cognition-enhancing Klotho, FOXO-1, and mTOR proteins. Moreover, proteins partaking in ER stress development -such as BiP (binding immunoglobulin protein), eIF-2α (eukaryotic initiation factor-2α), and CHOP (C/EBP homology protein)-showed heightened levels of expression in the hippocampi of AD brains. Therefore, altogether TXNIP could link the chronic increases in glucocorticoids elicited by a persistent ER stress with AD's enduring NLRP3 activation and neuroinflammation [67,363].
A newly identified gene associated with the risk of AD is TREML2 (triggering receptor expressed on myeloid cell-like 2), a protein expressed by microglia [364,365]. TREML2 protein expression levels rise along with AD progression in vivo [366] and after LPS stimulation in primary microglia in vitro, both proving TREML2 involvement in microglia-induced neuroinflammation [367]. Then again, Wang et al. [368] showed that LPS stimulation or lentivirus-mediated TREML2 overexpression remarkably upregulated NLRP3 inflammasome activation; IL-1β, IL-6, and TNF-α secretion; and proinflammatory M1-type polarization in microglia of APP/PS1 AD-model mice. Therefore, TREML2 inhibition would be a novel anti-AD therapeutic approach.
Two studies showed that caspase-1-mediated overproduction of IL-1β occurred in brain samples from mild cognitive impairment (MCI) and fully symptomatic AD patients. Hence, in both groups, microglial NLRP3 inflammasome activation advanced AD's persistent neuroinflammation [140,348]. Sokolowska et al. [140] also showed that phagocytosed Aβ 1-42 fibrils damaged human macrophages' lysosomes, which released cathepsin B into the cytosol, triggering the NLRP3•ASC•caspase-1 inflammasome's oligomerization and activation. Moreover, studies conducted on brain tissue samples from AD patients that had died because of intercurrent systemic infections and APP/PS1 AD-model mice revealed that any added proinflammatory insults intensified NLRP3 inflammasome's assembly/activation and IL-1β, IL-6, and various chemokines release from microglia, astrocytes, and neurons while increasing the brain's Aβs and p-Taues load. Hence, any concurring etiologic factor could worsen neuroinflammation and hasten AD progression in humans [71,369,370].

Figure 2.
Schematic depiction of stressors and factors inducing/modulating glial cell NLRP3 inflammasome activation and its consequences in AD. Exogenous Aβs, p-Taues, ATP, ASC, IL-1β, and IL-18 interact with cell surface receptors, including CaSR (see Box 1), TL-4, and P2X 7 (see Box 2), or are endocytosed to activate NF-κB and NLRP3 inflammasome signaling. They also induce ER stress, release Cathepsin B from damaged lysosomes, and block autophagy, while overreleasing further amounts of Aβs, p-Taues, and inflammatory cytokines. Altogether, they damage myelin sheaths and cause M1 microglial phenotype polarization and neuron and oligodendrocyte pyroptotic death. NLRP3 and receptor inhibitors mitigate the just-mentioned noxious effects. Additionally, the CaSR NAM NPS-2143 blocks Aβs, p-Taues, and IL-6 over production and release and reactivates autophagy (not shown; [181,199,354]). Regarding the roles of other-than-NLRP3 inflammasomes, see [24]. A yellow frame encloses the assembled NLRP3 inflammasomes, while nuclear envelopes are orange-colored. Immunohistochemical studies conducted on samples of temporal cerebral cortex of AD brains showed that the increased expression of NLRP3 inflammasome's constituents, including pro-caspase-1, and of IL-1β and IL-18, co-localized with glia maturation factor (GMF), APOE-ε4, sequestosome 1 (SQSTM1)/p62, LC3-positive autophagic vesicles, and LAMP1, a lysosomal marker. Notably, clusters of GMF overexpressing reactive astrocytes surrounded the amyloid senile plaques. GMF is a highly conserved proinflammatory protein that activates glial cells advancing human neurodegenerative processes. Conversely, in AD-model animals, GMF suppression mitigated the neurodegeneration. Altogether, these results showed that in humans, GMF could intensify NLRP3-driven neuroinflammation while concurrently hampering the autophagosomal pathway clearing Aβs aggregates [349]. Of note, Ahmed et al. [372] and Ramaswamy et al. [352] posited that GMF may advance neuroinflammation in all neurodegenerative diseases.
By sharp contrast, the results of another human postmortem study negated NLRP3 inflammasome function in the brains of advanced AD cases in which astrocyte activation was instead prominent [132].
In addition to Aβs and neuroinflammation, p-Taues are among AD's main drivers. Stancu et al. [373] and Ising et al. [71] proved that a causal link existed between p-Taues and inflammasomes' activation. They showed that following microglial endocytosis and lysosomal sorting, prion-like Tau seeds activated NLRP3 inflammasome signaling in the THY-Tau22 transgenic mouse line, a tauopathy-model animal. Moreover, the chronic intraventricular administration of NLRP3 inhibitor MCC950 significantly thwarted the neuropathology driven by the exogenous p-Tau seeds. Concurrently, NLRP3 suppression decreased the p-Taues levels and hindered their aggregation into neurofibrillary tangles by restraining Tau kinases' activity while increasing that of p-Tau phosphatases [71]. Then again, Jiang et al. [60] showed that p-Tau paired-helical filaments and p-Taues from human tauopathy brains primed and activated IL-1β production via MyD88 and NLRP3•ASC•caspase-1 pathways in primary human microglia. The authors also showed that p-Taues accumulation concurred with elevated ASC and IL-1β levels in postmortem brains of tauopathies patients.
Autophagy is a conserved process by which lysosomes remove dysfunctional cellular components and relevantly regulate NLRP3's role in inflammatory CNS diseases [10,374]. A reduced biogenesis and function of lysosomes/autophagosomes promotes the NLRP3's inflammasome activation driving the neuroinflammatory response in AD-model animals and cultured neural cells. In keeping with this, Zhou et al. [375] showed that overexpressing the transcription factor EB (TFEB), the primary regulator of lysosomal biogenesis, both improved the autophagosomes/lysosomes function and mitigated the neuroinflammation in AD-model cells.
Summing up, NLRP3 inflammasome targeting might hinder AD's etiopathogenetic tripod, i.e., Aβs, p-Taues, and neuroinflammation, and beneficially affect tauopathies too. This is indeed a sensible proposal, but hitherto its real effectiveness in stopping human AD's progression is unproven. Moreover, it does not consider inflammasomes' plurality, potential functional interchangeability, and their different expression levels in the distinct neural cell types.

Parkinson's Disease (PD)
PD is the second-most-common age-related human neurodegenerative disorder. The progressive spread of PD neuropathology causes motor disturbances and neuropsychiatric disorders (e.g., depression). PD's hallmarks are inclusions rich in misfolded α-synuclein (α-Syn) protein localized at the presynaptic terminals of melanin-rich dopaminergic neurons within the mesencephalic substantia nigra and subcortical corpus striatum. Zhang et al. [376] found the overexpression of IL-1β and IL-18 in cerebrospinal fluid samples from PD patients. Consistently, α-Syn mediated NLRP3 inflammasome activation in cultured human microglia [64]. In PD-model animals, β-hydroxybutyrate, a ketone body, did not inhibit NLRP3 [377] while blocking it in AD [378]. Therefore, α-Syn aggregates trigger chronic neuroinflammation sustained by mitochondrial dysfunction causing ROS over-production and by unrestrained microglia activation advancing dopaminergic neurons' pyroptosis [379][380][381]. Figure 3 sums up the main signaling pathways involving NLRP3 in PD.
progressive spread of PD neuropathology causes motor disturbances and neuropsychiatric disorders (e.g., depression). PD's hallmarks are inclusions rich in misfolded α-synuclein (α-Syn) protein localized at the presynaptic terminals of melanin-rich dopaminergic neurons within the mesencephalic substantia nigra and subcortical corpus striatum. Zhang et al. [376] found the overexpression of IL-1β and IL-18 in cerebrospinal fluid samples from PD patients. Consistently, α-Syn mediated NLRP3 inflammasome activation in cultured human microglia [64]. In PD-model animals, β-hydroxybutyrate, a ketone body, did not inhibit NLRP3 [377] while blocking it in AD [378]. Therefore, α-Syn aggregates trigger chronic neuroinflammation sustained by mitochondrial dysfunction causing ROS overproduction and by unrestrained microglia activation advancing dopaminergic neurons' pyroptosis [379][380][381]. Figure 3 sums up the main signaling pathways involving NLRP3 in PD. Overproduced α-synuclein (α-Syn) forms cytosolic aggregates (named when massive Lewis bodies) that damage lysosomes releasing cathepsin B, a cysteine protease. The latter interferes with mitochondrial activities causing in sequence dysfunctional mitophagy, ROS surpluses, oxidative stress, NF-κB pathway signaling, and overexpression of NLRP3 inflammasome components, the latter's activation, and its downstream consequences. Exogenous ATP from pyroptotic cells helps activate NLRP3 Figure 3. Summary illustration of stressors and factors inducing/modulating dopaminergic neurons' and microglia's NLRP3 inflammasome activation and its consequences in PD. Overproduced α-synuclein (α-Syn) forms cytosolic aggregates (named when massive Lewis bodies) that damage lysosomes releasing cathepsin B, a cysteine protease. The latter interferes with mitochondrial activities causing in sequence dysfunctional mitophagy, ROS surpluses, oxidative stress, NF-κB pathway signaling, and overexpression of NLRP3 inflammasome components, the latter's activation, and its downstream consequences. Exogenous ATP from pyroptotic cells helps activate NLRP3 inflammasome via the P2X 7 purinergic receptor signaling (see Box 2 for more details). The upshots are the release of IL-1β and IL-18 and K + efflux through pores made of GSDMD-N terminal fragments. α-Syn is also released extracellularly within exosomes that spread and are taken up by neighboring neural cells, expanding the neuropathology, or they circulate in the body fluids thus affecting peripheral tissues. Accumulated Cu 2+ ions also harm mitochondria contributing to NLRP3 inflammasome's activation. The toxic α-Syn effects are similar in microglia, in which they are mediated by TLR-2 and TLR-4 receptors too. α-Syn also blocks the chaperone-mediated autophagy (CMA) pathway regulated by the p38 MAPK/TEFB axis. Eventually, both nigrostriatal dopaminergic neurons and microglia undergo pyroptotic death. A yellow frame encloses the assembled inflammasomes, while nuclear envelopes are orange-colored. Abbreviations: ASC = apoptosis-associated speck-like protein endowed with a CARD domain; 5-BDBD = 5- Moreover, Scheiblich et al. [382] reported that the signaling triggered by the binding of α-Syn monomers or, to a lesser extent, α-Syn oligomers to TLR-2 and TLR-5 receptors activated the NLRP3 inflammasome in microglia with no priming needed. Using immunohistochemical and genetic approaches, von Herrmann et al. [383] supplied evidence that dopaminergic neurons are sites of NLRP3 activity in PD. Moreover, increases in NLRP3 inflammasome and NLRP3-dependent pro-inflammatory cytokines were detectable in the peripheral plasma of PD patients, proving NLRP3 inflammasome involvement in PD's pathogenesis [196,384]. The latter authors also showed that miR-7 inhibited NLRP3 gene expression in microglia, thereby reducing microglia activation, neuroinflammation, and nigrostriatal dopaminergic neuron pyroptosis. A patient-based study characterized NLRP3 in the first stages of midbrain nigral neurodegeneration and in the biofluids drawn from PD patients, suggesting that NLRP3 may be both a key inflammation mediator in the degenerating midbrain and a tractable therapeutic target [385]. Moreover, Wang et al. [386] showed that NLRP3 activation and IL-1β and IL-18 maturation occurred in the 6-OHDA (6-hydroxydopamine) neurotoxin-induced PD-model rat. The purinergic P2X 4 -R siRNA-knockdown or block by the specific antagonist 5-BDBD (5-(3-bromophenyl)-1,3dihydro-2H-benzofuro[3,2-e]-1,4-diazepin-2-one) counteracted NLRP3's effects, alleviated neuroinflammation, and reduced dopaminergic neuron pyroptosis. Therefore, the authors posited that the ATP•P2X 4 -R signaling drives NLRP3 inflammasome's activation, which next regulates glial cell activation, nigrostriatal dopaminergic neurodegeneration, and dopamine levels ( Figure 3; see also more details and the literature in Box 2) However, here one should be wary of extrapolating these data to PD patients. In PD-model rat brains, NLRP3 inflammasome's activation is not in fact equivalent to that proper of human PD brains. The present understanding of any beneficial effects of antagonizing ATP•P2X 4 -R's signaling is too limited. Therefore, we need more studies to assess the pathophysiological relevance of nigrostriatal ATP•P2X 4 R signaling in humans.
Finally, despite extensive investigations into the NLRP3 inflammasome-activating mechanisms in the diverse inflammatory brain diseases, their regulatory networks are still unclear in microglia and other neural cell types. Chen et al. [389] showed that NLRP3 is a substrate of chaperone-mediated autophagy (CMA). The p38/TFEB (transcription factor EB) axis regulated NLRP3 inflammasome degradation via CMA, inhibiting the overproduction of proinflammatory cytokines in microglial cells. Furthermore, both p38 and NLRP3 inhibitors could mitigate α-Syn aggregate-induced microglia activation and nigrostriatal dopaminergic neuron pyroptosis. Moreover, Panicker et al. [390] showed that the functional loss of Parkin, an E3 ubiquitin ligase, resulted in the priming and spontaneous activation of the NLRP3 inflammasome in mouse and human dopaminergic neurons, leading to their pyroptosis.
From a clinical standpoint, human PD is quite complex. Therefore, one may conclude that the roles of NLRP3 and other-than-NLRP3 inflammasomes in human PD require further investigations to be fully clarified and integrated to lead to effective therapeutic interventions.

Multiple Sclerosis (MS) and Experimental Autoimmune (or Allergic) Encephalomyelitis (EAE)
MS is a chronic autoimmune disease of unclear etiology affecting both the brain and spinal cord whose hallmarks include focal (plaque) demyelination and chronic neuroinflammation/neurodegeneration. One accredited theory posits that patients' T cells attack myelin sheath antigens, causing MS. The suggested relationship between MS and the NLRP3 inflammasome has linked autoimmunity with innate immunity and neuroinflammation [391][392][393][394][395]. Moreover, as gain-of-function genetic variants of the NLRP3 (e.g., Q705K) and NLRC4 inflammasomes associate with a more severe MS course, a constitutive NLRP3 inflammasome activation could be a risk factor for clinical MS presentation [396]. Moreover, Vidmar et al. [397] highlighted as pathogenetically important for MS patients the increased burden of rare variants in (i) NLRP1 and NLRP3 genes; (ii) genes partaking in inflammasome downregulation via autophagy and IFN-β; and (iii) genes involved in responses to type-1 IFNs (e.g., PTPRC, TYK2) and to DNA virus infections (e.g., DHX58, POLR3A, IFIH1).
Keane et al. [398] and Voet et al. [215] showed that following NLRP3 inflammasome activation, there occurred an increased IL-1β gene expression within MS demyelination plaques coupled with elevated levels of ASC, caspase-1, and IL-18 in the brains and cerebrospinal fluids of MS patients. Moreover, NLRP3 inflammasome pathway-related components were overexpressed in the blood monocytes isolated from the minor fraction of patients suffering from primary progressive (i.e., with no alternation of pauses and relapses) MS (PPMS), so entailing increased IL-1β production [393,394,399]. These results showed IL-1β as a prognostic factor in PPMS patients and the NLRP3 inflammasome as a prospective therapeutic target. Thus, a specific NLRP3 inhibitor may improve MS histopathology and reduce myelin sheath damage.
According to Farooqi et al. [400], EAE is a proper mouse model for pathogenetic and pharmacotherapeutic studies into human MS molecular mechanisms. In EAE-model mice, NLRP3 inflammasome's activation critically induced T-helper cell migration into the CNS. Next, the activated NLRP3 inflammasome of primed T cells (and microglia) drove the release of proinflammatory cytokines, thus partaking in MS pathogenesis [394,401]. In EAEmodel mice the NLRP3 inhibitor MCC950 prevented the conversion of CNS astrocytes to the A1 neurotoxic reactive phenotype otherwise induced via the NF-κB pathway-mediated IL-18 production. Consistently, after the systemic delivery of NLRP3 inhibitor MCC950 axonal injury was mitigated within lysolecithin-induced demyelinated lesions in mice [402,403]. MCC950 also hindered complement C3 protein release from the astrocytes, which would have otherwise impaired hippocampal neuron viability [404]. IFN-β administration did improve this NLRP3-dependent EAE form. Conversely, when ad hoc experimental regimens brought about a NLRP3-independent, more aggressive EAE, the IFN-β treatment was ineffective. A similar NLRP3-independent mechanism might be at work in human MS cases not profiting from IFN-β therapy [405].
In conclusion, there is an intensely felt need to expand the study of NLRP3 and other-than-NLRP3 inflammasomes' role(s) in MS, using human neural cell-based experimental models to achieve a more detailed molecular picture and identify disease-modifying therapeutic targets.

Amyotrophic Lateral Sclerosis (ALS)
ALS is a devastatingly progressive multifactorial disorder characterized by the primary degeneration of the cerebral motor cortex, brain stem, and spinal cord motoneurons leading to skeletal muscle atrophy and paralysis. ALS patients may also develop cognitive and behavioral changes due to neurodegeneration-affected subcortical areas, e.g., diencephalon's dorsal thalamus. Typically, 90% of cases occur sporadically, and their etiological factors are poorly defined (smoking, violent sports, military service, exposure to insecticides and pesticides). About 10% of ALS cases are familiar due to heritable mutated genes. SOD1 (superoxide dismutase 1) gene mutations occur in 20% of familiar cases [406].
The current belief is that SOD1 mutations only trigger ALS onset within motoneurons but elicit only delayed and minor harm [407]. However, in astrocytes and/or microglia, SOD1 mutations advance ALS progression [408]. TDP-43 (transactive response DNA binding 43 kDa) protein could be another ALS etiological agent as it accumulates in both sporadic and familial cases [409]. TDP-43 forms toxic ubiquitinated aggregates in the cytoplasm of neural cells of both ALS and frontotemporal lobar degeneration (FTLD) patients [410,411]. Neurons and astrocytes can secrete mutated or oxidized SOD1 and TDP-43 as misfolded proteins, which activate microglia by interacting with CD14, TLR-2, TLR-4, and scavenger receptors [412,413]. Thus, exogenous whole or fragmented, wild-type or mutated TDP-43 bound microglia's CD14 cell surface receptor activating AP1 and NF-κB pathways and upregulating NOX2 (SOD-generating NADPH oxidase 2), TNF-α, NLRP3•ASC•caspase-1, and IL-1β release. Importantly, TDP-43 was toxic to motoneurons only in the presence of microglia presence [414]. Using in situ hybridization and immunocytochemistry, Banerjee et al. [415] showed that an upregulated NLRP3 inflammasome occurred in neurons and glia of cognitively impaired ALS patients. Conversely, no differences were detectable between cognitively resilient ALS and healthy subjects. Figure 4 sums up the main signaling pathways involving NLRP3 in ALS. Johann et al. [127] showed that an activated NLRP3 inflammasome concurred with elevated levels of caspase-1, IL-1β, and IL-18, particularly in the spinal cord astrocytes of the SOD1G93A ALS-model mice and in the serum and spinal cord tissue of sporadic ALS patients-altogether findings confirming NLRP3 inflammasome's involvement in ALS. Moreover, Kadhim et al. [416] found that IL-18 was upregulated in the cerebral tissue of sporadic ALS patients vs. age-matched controls. Furthermore, Gugliandolo et al. [417] strengthened the concept that neuroinflammation plays a crucial role in ALS by confirming NLRP3 inflammasome activation and its sequels in SOD1G93A ALS-model rats. Immunofluorescent studies conducted on symptomatic SOD1G93A ALS-model mice revealed that NLRP3 and ASC expression intensity increased along with ALS progression, proving NLRP3's involvement in neuron death [418]. Moreover, Michaelson et al. [419] suggested a novel ALS pathogenetic mechanism mediated by the amino acid β-N-methylaminol-alanine (BMAA), a Cyanobacteria product. BMAA is not a protein constituent, but a powerful neurotoxin inducing protein misfolding, NLRP3 inflammasome activation, and proinflammatory cytokine overexpression in spinal motoneurons.
In their work, Van Schoor et al. [420] observed increases in the NLRP3 inflammasome, GSDMD-N fragments, and IL-18 in the motor cortex and spinal cord microglia of human ALS patients, which suggested that an activated NLRP3 inflammasome had triggered the cells' pyroptosis. As compared to controls, in human ALS samples, a reduced array of neurons matched with an increased throng of cleaved-GSDMD-positive microglial cells in the underlying white matter of the premotor cortex. No alike findings were obtained in the human spinal cord. Similar findings were made in the cortex of TDP-43A315T transgenic mice in model ALS and FTLD [421]. In addition, these results stressed the relevance of ROS and ATP generation, both potential therapeutic targets, for microglial NLRP3 inflammasome activation and neuronal pyroptosis, which was confirmed in SOD1G93A-induced ALS-model mice. Importantly, both wild-type and mutant TDP-43 proteins activated the overexpressed NLRP3 and its downstream effects in the microglia of SOD1G93A mice. This proved that NLRP3 is the crucial microglial inflammasome mediating SOD1G93A-induced pyroptosis [65].
Lacking a suitable human microglia model, Quek et al. [422] characterized peripheral blood monocyte-derived microglia-like cells (ALS-MDMi) isolated from ALS patients at various stages. Importantly, ALS-MDMi recapitulated ALS neuropathology hallmarks, i.e., abnormal phosphorylated and non-phosphorylated TDP-43 cytoplasmic accumulation and phagocytosis impairment that paralleled ALS progression; altered neuroinflammatory cytology; DNA damage; NLRP3 inflammasome's activation; and microglia pyroptosis.  It is seemly to consider the studies about NLRP3 and other-than-NLRP3 inflammasomes in human ALS are still in a preliminary phase even in the light of the groundbreaking results reported by Van Schoor et al. [420]. The latter should encourage scientists to delve deeper into the pathogenetic mechanisms of this devastating disease to find novel effective therapeutic approaches.

Huntington's Disease (HD)
HD is a rare autosomal dominant neurodegenerative disease caused by the unstable CAG repeat expansion in the Huntington (HTT/IT15) gene and presenting with motor, cognitive, and psychiatric symptoms [423] When the HTT/IT15 gene holds 39 to 180 CAG repeats, the translated polyglutamine-containing mutant HTT protein (mHTT) complexes with and disrupts the normal function of several transcription factors, thereby altering the activities of neurons, astrocytes, and microglia. HD's harming mechanisms include mitochondrial dysfunction, excitotoxicity, CREB and BDNF downregulation, and microglia activation, altogether advancing neuronal death by apoptosis, necroptosis, ferroptosis, and NLRP3-linked pyroptosis [424,425].
Various HD-model animals were set up to clarify its molecular mechanisms and to try novel therapeutics for it. The transgenic R6/2 (B6CBA-Tg[HDexon1]62Gpb/1J) mouse line expressing the human HTT gene exon 1 carrying 120 ± 5 CAG repeats is the most popular HD animal model [426]. An upregulated NLRP3 inflammasome and caspase-1 expression already occurred in 13-week-old R6/2 HD-model mice, particularly in striatal parvalbumin interneurons and spiny GABAergic neurons, which preferentially undergo pyroptosis in HD [427]. Poly(ADP-ribose) polymerase-1 (PARP-1) is a nuclear enzyme whose activity is crucial for DNA repair in humans. Olaparib, a PARP-1 inhibitor presently sold as an anti-tumor drug, could also regulate NLRP3 inflammasome activation in the R6/2 HD-model mice. When given from the pre-symptomatic stage onwards, Olaparib mitigated neuronal pyroptosis, neurological symptoms, and neurobehavioral tests results, lengthening the survival of HD-model mice. Therefore, Olaparib could help human HD too [428]. Moreover, Chen et al. [429] showed that NLRP3 inhibitor MCC950 given to R6/2 HD-model mice suppressed IL-1β and ROS overproduction, mitigating neuroinflammation, motor dysfunction, and neuronal pyroptosis, while upregulating PSD-95 and NeuN proteins, and lengthening animals' lifespans. Therefore, inhibition of NLRP3's signaling, and its downstream effects would be therapeutically helpful in HD.
Interestingly, a role in HD etiopathogenesis may be played by galectins (i.e., "S-type lectins")-soluble proteins specifically binding β-galactoside carbohydrates and playing multiple roles in autophagy, immune responses, and inflammation. Siew et al. [430] reported that galectin-3 (Gal-3) plasma levels increased well over healthy controls in HD patients and HD-model mice. In HD-mice, microglia Gal-3 levels increased prior to motor symptom presentation and stayed high while HD progressed. Gal-3 co-localized with microglial lysosomes, blocked the autophagic elimination of damaged endolysosomes, and partook in neuroinflammation via the NF-κB/NLRP3 axis. Gal-3 knockout improved HD-related neuropathology and survival in HD-model mice, showing Gal-3 as a potential therapeutic target. Conversely, Gal-1 and Gal-8 hindered neuroinflammation, promoting neuroprotective effects [431].
HD's rare occurrence is an adjunct hurdle to studies about the roles played in it by NLRP3 and other inflammasomes. However, this circumstance should not discourage attempts to increase our insights in this ailment, both in patients and animal models.

Brain NLRP3 and Neurotropic Viruses Infections
Both DNA and RNA neurotropic viruses activate the brain's NLRP3 inflammasome, causing neuroinflammation and sometimes triggering chronic neurodegenerative diseases [75]. Here, we review a few neurotropic viruses playing NLRP3-linked roles in human neuropathology.

Zika Virus (ZIKV) Encephalitis
The Zika Virus (ZIKV) is a single-stranded positive-sense RNA arbovirus of the Flaviviridae family (Flavivirus genus that also includes Dengue, West Nile, Yellow Fever, and Japanese Encephalitis viruses). ZIKV associates with congenital microcephaly in newborns and Guillain-Barré syndrome, myelopathy, and encephalitis in adults. Tricarico et al. [432] showed that ZIKV infected the U87-MG glioma cell line causing NLRP3 inflammasome activation and IL-1β oversecretion. Consistently, He et al. [82] made the same observations in the brains and sera of ZIKV-infected mice. ZIKV's NS5 protein drove ROS overproduction and NLRP3 inflammasome assembly, both needed for its activation. Conversely, in vitro and in vivo NLRP3 deficiency upregulated type-I IFN and strengthened the host's resistance to ZIKV, confirming NLRP3's role in ZIKV infection [433,434].

West Nile Virus (WNV) Encephalitis
Another Flavivirus, the West Nile Virus (WNV), causes an encephalitis entailing neurons' death and elevated IL-1β plasma levels. In a mouse model, WNV infection briskly induced IL-1β synthesis in cortical neurons. However, by cooperating with type-I IFN, the intensified IL-1β•IL-1β-R (receptor) signaling suppressed neuronal WNV replication, reducing the WNV brain load. Therefore, the NLRP3/IL-1β•IL-1β-R pathway regulated neuronal WNV infection and revealed a novel IL-1β antiviral action [435].

Human Immunodeficiency Virus-1 (HIV-1) Encephalitis
The immunosuppressive Lentiviruses efficiently infect macrophages and lymphoid cells. Human Immunodeficiency Virus-1 (HIV-1) belongs to the Retroviridae family (Lentivirus genus). Burdo et al. [439] showed that during the primary infection, HIV-1 productively infects brain macrophages and microglia. Studies using primary human microglia showed that IL-1β was released after HIV-1 infection. Walsh et al. [440] proved that HIV-1 infection induced an NLRP3 inflammasome-dependent ASC translocation, caspase-1 activation, and mature IL-1β release from cultured microglia. The authors highlighted the need to analyze the inflammasome inhibitors' effectiveness as novel therapeutics for HIV-1/AIDS.

Viroporin Proteins
Various RNA viruses, including Coronaviridae, express the viral-replication-indispensable small viroporin proteins. Being liposoluble, viroporins assemble hydrophilic transmembrane pores, allowing ions and/or small solutes to bidirectionally migrate along their electrochemical gradients. Viroporin activity could act as the "second signal" by increasing [Ca 2+ ] i or lowering the cytosolic pH due to H + -releasing ion channel activity in the lysosomal acidic compartment [441].
Additionally, Ding et al. [457] proved that hypercapnia enhanced NLRP3 inflammasome activation and IL-1β expression only in hypoxic BV-2 microglia cells. Therefore, the hypercapnia resulting from lung-protective ventilatory strategies used in acute respiratory distress syndrome (ARDS) patients may lead to neuroinflammation and cognitive impairment via a microglial NLRP3/IL-1β-dependent mechanism.
Based upon the above findings, Heneka et al. [458] posited that NLRP3 inflammasome activation during COVID-19 heightens the risk for the later development of chronic neurodegenerative diseases. Independent clinical and epidemiological investigations indicated that SARS-CoV-2 infection and the ensuing "long COVID" tightly relate to the onset of AD, PD, prion disease (PrD), and other ailments, particularly in patients in advanced age or suffering from intercurrent illnesses (CVD, T2DM, hypertension, other neurological disorders) or severe/fatal COVID-19 [459][460][461]. Even more alarming, the receptor-binding domain of SARS-CoV-2's S1 spike glycoprotein presents prion-like sequences. The latter diverge among viral variants, show a different affinity for ACE2, and promote immune-evasion, protein clustering, and protein aggregates' "seeding". The upshots would include prion-like proteins spreading, progressive dementia, or fast-evolving CJD [462][463][464].
Obviously, here we have considered only some of the known neurotropic viruses. The field of human brain-infecting viruses is more variegated and might also further expand in the future. Our knowledge about viral neuropathology is, we must admit, limited, particularly because viruses can target all stages of human life, from the uterus onward, with different age-related upshots. There is also a field that for the sake of brevity we omitted considering, i.e., the interactive relations between oncogenic viruses and inflammasomes, which deserves attention because of its potentially significant reflections on therapeutic outcomes.

Comments and Future Perspectives
An old dictum states that every disease starts with an inflammation. The prevalence of neuroinflammatory disease has been epidemically rising because of a lengthened lifespan and of little-appreciated toxic, environmental, and lifestyle-linked factors. To worsen this bleak situation, acute brain illnesses (e.g., stroke, hemorrhage, infection) too can trigger chronic neuroinflammation/neurodegeneration in a significant fraction of patients [465]. A steadily growing literature attests that NLRP3 inflammasome activation in CNS microglia and circulating monocytes plays a pivotal role in promoting the neuroinflammation driven by a host of etiologic factors (q.v. Table 1), potentially advancing the progression of neurodegenerative diseases [27,466,467]. Conversely, NLRP3's roles in the other neural cell types (i.e., neurons, astrocytes, and oligodendrocytes) [3,[468][469][470] and in CNS pericytes and endothelial cells [126,471] have received less attention, probably because such cells preferentially express other types of inflammasomes. In fact, NLRP3 activity in such cells is modest and/or is the object of controversy, particularly in astrocytes, although NLRP3's inhibition still gives some therapeutic advantage. Moreover, these same neural cell types more intensely express various other-than-NLRP3 inflammasomes. The latter can also exert significant neuroinflammation-sustaining effects, as specific NLRP3 inhibitors do not hinder other-than-NLRP3 inflammasomes' activities [24]. We previously reviewed the known roles of various other-than-NLRP3 inflammasomes in human brain disease [24]. That work inspired us to delve deeply also into the role(s) of the brain's NLRP3 inflammasome. Indeed, the NLRP3-related extensive research works herein reviewed shows the high complexity of both the regulatory mechanisms involved and of the physiological, pathological, and ethnic/pharmacological factors that promote or hinder its activation. Particularly the abundance of blocking or preventative factors, many of them identified over millennia by TCM, bodes well for future therapeutic modulations of NLRP3 activity in various pathological settings. Various reports showed that particularly inhibiting microglial NLRP3 function exerted beneficial effects in rodent experimental models of human neurodegenerative illnesses. These favorable outcomes inspired and still inspire the opinion that therapeutically targeting the NLRP3 inflammasome will mitigate or stop both acute and progressive human neuroinflammatory diseases [472][473][474]. As just mentioned, despite or thanks to the intricacies of NLRP3 inflammasome's activating mechanisms, there are plenty of agents modulating its activity (Tables 2-4). At present, many small molecules are undergoing pharmaceutical research/development as novel candidate drugs targeting the NLRP3 inflammasome in various diseases [274]. At least five companies have started ad hoc clinical trials, of which Inflazome and NodThera have reported Phase I positive results of their brain-penetrating NLRP3 inflammasome inhibitors (Inzomelid [251] and NT-0796 [274], respectively), expecting to use them to treat central and peripheral nervous inflammatory diseases. These discoveries have even raised the possibility of a common cure for all or at least some human brain diseases. Moreover, Lupfer and Kanneganti [21] reported the existence of inflammasomes, such as NLRC3, NLRP6, NLRP12, and NLRX1, which hinder NF-κB pathway activation, thereby mitigating or switching off the incumbent or ongoing neuroinflammation. Such "anti-inflammasomes" deserve more consideration because in a hopefully not too far future, their pharmacological activation by proper means (yet to be established) could be a valuable therapeutic asset that will switch off neuroinflammation through physiological mechanisms. Therefore, the intuitive conclusion is that reality is more intricate than it might appear at first sight. Furthermore, uncertainties and controversies about the etiological mechanisms driving human neurodegenerative diseases help confound the picture, as do other problems that we will briefly discuss below.
(i) Are inflammasomes functionally interchangeable? Hitherto the interplays that might occur between or among the distinct inflammasomes expressed by each human neural cell type remain mostly undefined. Yet, it is necessary to clarify them to better assess the therapeutic impact of NLRP3 inflammasome inhibitors. Denes et al.'s [336] study results in mice called for caution, as they showed that inflammasomes (e.g., AIM2) can functionally overtake a blocked NLRP3 (Figure 1). A (partial) solution to this problem might entail targeting the ASC protein, which would hinder the activation of all canonical inflammasomes instead of those of NLRP3s only [475]. The inflammasomes' noncanonical activation problem will persist but might be a minor one.
(ii) The species difference problem. Significant genomic differences apart, not all organs of humans and mammals are morpho-functionally alike. Acceptable similarities exist with liver, kidneys, and lungs. Yet, considering the CNS, while the human cerebral cortex consists mostly of a non-olfactory six-layered neocortex, the widely used rodent models have a less developed, structurally simpler, and mostly olfactory cortex. Moreover, fundamental cytological divergences in size, shape, connections, and functions distinguish the diverse types of neural cells of the human cortex from their rodent counterparts [476]. Human brain's molecular regulatory mechanisms, e.g., those involved in receptor signal transduction [133] and inflammasome regulation [24,27,477] (see also Boxes 1 and 2), also remarkably diverge from those of rodents. Moreover, human neurodegenerative diseases do not plague rodents in nature. Importantly, in rodent models of human neurodegenerative diseases, the astrocytes undergo an early death-which justifies the often-little attention paid to themwhile neurons keep surviving. Conversely, human neurodegenerative diseases kill neurons first, while astrocytes survive and help advance the neuropathologies. Hence, a tight genomic, proteomic, and bio-pathological conformity between animal and human brains is lacking [478,479]. Although brilliant and highly praiseworthy, the manifold animal models of human neurodegenerative diseases in existence cannot surmount such inter-species differences [480]. A quite low animal-to-human translation rate of brain disease-targeting drugs has been persisting for decades, being ascribed to preclinical studies' faults in "internal consistency" (e.g., design flaws, uncontrolled bias) and/or "external consistency (i.e., animal models pre-testing). As a long trail of clinical trial failures shows, it is difficult to safely predict the effectiveness in humans of drugs pre-tested with favorable results in transgenic animal models [481]. Procedures involving animal models were necessary when nothing or truly little was known about human brain diseases. Now we know much more, albeit not yet enough. Moreover, in recent decades, the legislative/bureaucratic requirements to evaluate novel drugs have become increasingly burdensome to hinder the use of inadequately tested therapeutics. This trend has become stronger after rare events in which properly approved drugs unexpectedly elicited adverse reactions in the patients [482]. Moreover, the repurposing for neurodegenerative diseases of drugs previously evaluated for other ailments in clinical trials is not so easy to do, which precludes the faster testing of potentially useful drugs [483]. Hence, it would be wise to introduce some procedural changes. Animal and/or in silico studies should still help preselect lead drugs. Next, preclinical human untransformed neural cell models in vitro would allow for the assessment of the latter [24,141,212,484] prior to any clinical trial assessment. On rare occasions, animal studies might even be skipped in favor of preclinical human model studies [24,141,212,484]. Human neural cells models will help clarify specific etiopathogenetic mechanisms while supplying safer predicting information about effective drug benefits in clinical settings.
(iii) Symptomatic and/or etiologic therapies? Hitherto, no causal "brain disease modifying" therapies are available for human neurodegenerative diseases. An exception may be the just reported promising effects of Lecanemab, a humanized IgG1 monoclonal antibody binding soluble Aβ protofibrils. After 18 months, Lecanemab reduced brain amyloidosis and slowed cognition decline in early-stage AD patients vs. the placebo-given group. However, Lecanemab also caused collateral brain swelling and/or hemorrhage in some patients, particularly in case of APOE-ε4 homozygotes or anticoagulant therapy [485]. Hence, while Lecanemab's results confirm that Aβs play a key pathogenetic role in human AD, further studies will prove its etiologic or symptomatic value regarding Aβs/p-Taues' overproduction and accumulation and inflammasomes' activity.

Conclusions
In recent years, neuroinflammation has been attracting a lot of attention, particularly concerning one of its mediators, i.e., the NLRP3 inflammasome. In the present work, we systematically review the huge and still mounting evidence related to both NLRP3's involvement in human and animal models of acute and chronic brain diseases, and its many functional activators and inhibitors so far known. Unquestionably, no field expert should disregard the NLRP3 inflammasome, as it is intensely expressed by microglia and circulating monocytes. However, here we wish to stress the indisputable fact that human and animal neural cells of all types, whose morphologies and functions significantly diverge, also express many other inflammasomes and various "anti-inflammasomes"-the latter being tasked with mitigating neuroinflammation. Moreover, the so-called primary drivers of the distinct brain diseases should also be taken into due account because they can simultaneously trigger neurotoxicity and neuroinflammation. Hence, a more comprehensive view of the underlying molecular mechanisms of each brain disease would be beneficial. Importantly, the yet available data on the several inflammasomes' roles in human brain diseases are limited and controversial. Therefore, this is a field widely open to groundbreaking investigations. We are confident that choosing human untransformed neural cells as models for pathogenetic and pharmacological studies will advance our knowledge about each neuropathology and hasten the achievement of effective etiological therapies.