SARS-CoV-2 drives NLRP3 inflammasome activation in human microglia through spike protein.

Coronavirus disease-2019 (COVID-19) is primarily a respiratory disease, however, an increasing number of reports indicate that SARS-CoV-2 infection can also cause severe neurological manifestations, including precipitating cases of probable Parkinson's disease. As microglial NLRP3 inflammasome activation is a major driver of neurodegeneration, here we interrogated whether SARS-CoV-2 can promote microglial NLRP3 inflammasome activation. Using SARS-CoV-2 infection of transgenic mice expressing human angiotensin-converting enzyme 2 (hACE2) as a COVID-19 pre-clinical model, we established the presence of virus in the brain together with microglial activation and NLRP3 inflammasome upregulation in comparison to uninfected mice. Next, utilising a model of human monocyte-derived microglia, we identified that SARS-CoV-2 isolates can bind and enter human microglia in the absence of viral replication. This interaction of virus and microglia directly induced robust inflammasome activation, even in the absence of another priming signal. Mechanistically, we demonstrated that purified SARS-CoV-2 spike glycoprotein activated the NLRP3 inflammasome in LPS-primed microglia, in a ACE2-dependent manner. Spike protein also could prime the inflammasome in microglia through NF-κB signalling, allowing for activation through either ATP, nigericin or α-synuclein. Notably, SARS-CoV-2 and spike protein-mediated microglial inflammasome activation was significantly enhanced in the presence of α-synuclein fibrils and was entirely ablated by NLRP3-inhibition. Finally, we demonstrate SARS-CoV-2 infected hACE2 mice treated orally post-infection with the NLRP3 inhibitory drug MCC950, have significantly reduced microglial inflammasome activation, and increased survival in comparison with untreated SARS-CoV-2 infected mice. These results support a possible mechanism of microglial innate immune activation by SARS-CoV-2, which could explain the increased vulnerability to developing neurological symptoms akin to Parkinson's disease in COVID-19 infected individuals, and a potential therapeutic avenue for intervention.

The microglial NLRP3 inflammasome is activated by amyotrophic lateral sclerosis proteins.

Microglial NLRP3 inflammasome activation is emerging as a key contributor to neuroinflammation during neurodegeneration. Pathogenic protein aggregates such as ß-amyloid and α-synuclein trigger microglial NLRP3 activation, leading to caspase-1 activation and IL-1ß secretion. Both caspase-1 and IL-1ß contribute to disease progression in the mouse SOD1G93A model of amyotrophic lateral sclerosis (ALS), suggesting a role for microglial NLRP3. Prior studies, however, suggested SOD1G93A mice microglia do not express NLRP3, and SOD1G93A protein generated IL-1ß in microglia independent to NLRP3. Here, we demonstrate using Nlrp3-GFP gene knock-in mice that microglia express NLRP3 in SOD1G93A mice. We show that both aggregated and soluble SOD1G93A activates inflammasome in primary mouse microglia leading caspase-1 and IL-1ß cleavage, ASC speck formation, and the secretion of IL-1ß in a dose- and time-dependent manner. Importantly, SOD1G93A was unable to induce IL-1ß secretion from microglia deficient for Nlrp3, or pretreated with the specific NLRP3 inhibitor MCC950, confirming NLRP3 as the key inflammasome complex mediating SOD1-induced microglial IL-1ß secretion. Microglial NLRP3 upregulation was also observed in the TDP-43Q331K ALS mouse model, and TDP-43 wild-type and mutant proteins could also activate microglial inflammasomes in a NLRP3-dependent manner. Mechanistically, we identified the generation of reactive oxygen species and ATP as key events required for SOD1G93A -mediated NLRP3 activation. Taken together, our data demonstrate that ALS microglia express NLRP3, and that pathological ALS proteins activate the microglial NLRP3 inflammasome. NLRP3 inhibition may therefore be a potential therapeutic approach to arrest microglial neuroinflammation and ALS disease progression.

Chronic Helminth Infection Perturbs the Gut-Brain Axis, Promotes Neuropathology, and Alters Behavior.

Helminth infections in children are associated with impaired cognitive development; however, the biological mechanisms for this remain unclear. Using a murine model of gastrointestinal helminth infection, we demonstrate that early-life exposure to helminths promotes local and systemic inflammatory responses and transient changes in the gastrointestinal microbiome. Behavioral and cognitive analyses performed 9-months postinfection revealed deficits in spatial recognition memory and an anxiety-like behavioral phenotype in worm-infected mice, which was associated with neuropathology and increased microglial activation within the brain. This study demonstrates a previously unrecognized mechanism through which helminth infections may influence cognitive function, via perturbations in the gut-immune-brain axis.

Response to Comment on "Inflammasome inhibition prevents α-synuclein pathology and dopaminergic neurodegeneration in mice".

We have replicated our original finding of elevated cleaved caspase-1 in mouse brains and neuroprotection by an NLRP3 inflammasome inhibitor in two mouse models of Parkinson's disease.

Senolytic therapy alleviates physiological human brain aging and COVID-19 neuropathology.

Aging is a major risk factor for neurodegenerative diseases, and coronavirus disease 2019 (COVID-19) is linked to severe neurological manifestations. Senescent cells contribute to brain aging, but the impact of virus-induced senescence on neuropathologies is unknown. Here we show that senescent cells accumulate in aged human brain organoids and that senolytics reduce age-related inflammation and rejuvenate transcriptomic aging clocks. In postmortem brains of patients with severe COVID-19 we observed increased senescent cell accumulation compared with age-matched controls. Exposure of human brain organoids to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) induced cellular senescence, and transcriptomic analysis revealed a unique SARS-CoV-2 inflammatory signature. Senolytic treatment of infected brain organoids blocked viral replication and prevented senescence in distinct neuronal populations. In human-ACE2-overexpressing mice, senolytics improved COVID-19 clinical outcomes, promoted dopaminergic neuron survival and alleviated viral and proinflammatory gene expression. Collectively our results demonstrate an important role for cellular senescence in driving brain aging and SARS-CoV-2-induced neuropathology, and a therapeutic benefit of senolytic treatments.

SARS-CoV-2 triggers complement activation through interactions with heparan sulfate.

Objectives: To determine whether SARS-CoV-2 can trigger complement activation, the pathways that are involved and the functional significance of the resultant effect. Methods: SARS-CoV-2 was inoculated into a human lepirudin-anticoagulated whole blood model, which contains a full repertoire of complement factors and leukocytes that express complement receptors. Complement activation was determined by measuring C5a production with an ELISA, and pretreatment with specific inhibitors was used to identify the pathways involved. The functional significance of this was then assessed by measuring markers of C5a signalling including leukocyte C5aR1 internalisation and CD11b upregulation with flow cytometry. Results: SARS-CoV-2 inoculation in this whole blood model caused progressive C5a production over 24 h, which was significantly reduced by inhibitors for factor B, C3, C5 and heparan sulfate. However, this phenomenon could not be replicated in cell-free plasma, highlighting the requirement for cell surface interactions with heparan sulfate. Functional analysis of this phenomenon revealed that C5aR1 signalling and CD11b upregulation in granulocytes and monocytes was delayed and only occurred after 24 h. Conclusion: SARS-CoV-2 is a noncanonical alternative pathway activator that progressively triggers complement activation through interactions with heparan sulfate.

Nucleocapsid Specific Diagnostics for the Detection of Divergent SARS-CoV-2 Variants.

Since the start of the COVID-19 pandemic, multiple waves of SARS-CoV-2 variants have emerged. Of particular concern is the omicron variant, which harbors 28 mutations in the spike glycoprotein receptor binding and N-terminal domains relative to the ancestral strain. The high mutability of SARS-CoV-2 therefore poses significant hurdles for development of universal assays that rely on spike-specific immune detection. To address this, more conserved viral antigens need to be targeted. In this work, we comprehensively demonstrate the use of nucleocapsid (N)-specific detection across several assays using previously described nanobodies C2 and E2. We show that these nanobodies are highly sensitive and can detect divergent SARS-CoV-2 ancestral, delta and omicron variants across several assays. By comparison, spike-specific antibodies S309 and CR3022 only disparately detect SARS-CoV-2 variant targets. As such, we conclude that N-specific detection could provide a standardized universal target for detection of current and emerging SARS-CoV-2 variants of concern.

Inflammasomes in CNS Diseases.

Neuroinflammation is a common pathological feature in almost all neurological diseases and is a response triggered as a consequence of the chronic activation of the innate immune response in the CNS against a variety of stimuli, including infection, traumatic brain injury, toxic metabolites, aggregated proteins, or autoimmunity. Crucial mediators of this neurinflammatory process are the intracellular protein complexes known as inflammasomes which can be triggered by pathogens as well as pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs). However, chronic inflammasome activation can eventually result in cellular death and tissue damage, leading to the release of DAMPs that can reactivate the inflammasome, thereby propagating a vicious cycle of inflammation. The primary cells involved in CNS inflammasome activation are the immunocompetent microglia and the infiltrating macrophages into the CNS. However, astrocytes and neurons also express inflammasomes, and the understanding of how they are engaged in the pathogenesis of a variety of neurological diseases is crucial to develop effective therapeutic approaches for CNS pathologies that are propagated by chronic inflammasome activation. This chapter covers the activation mechanisms of relevant inflammasomes in the brain and summarizes their roles in the pathogenesis and progression of different neurological conditions.

Gestational Hypothyroxinemia Imprints a Switch in the Capacity of Astrocytes and Microglial Cells of the Offspring to React in Inflammation.

Hypothyroxinemia (Hpx) is a highly frequent condition characterized by low thyroxine (T4) and normal 3,3',5'-triiodothyronine (T3) and thyroid stimulating hormone (TSH) levels in the blood. Gestational Hpx is closely related to cognitive impairment in the human offspring. In animal models gestational Hpx causes impairment at glutamatergic synapsis, spatial learning, and the susceptibility to suffer strong autoimmune diseases like experimental autoimmune encephalomyelitis (EAE). However, the mechanisms underlying these phenotypes are unknown. On the other hand, it has been shown that astrocytes and microglia affect the outcome of EAE. In fact, the activation of astrocytes and microglia in the central nervous system (CNS) contributes to EAE progression. Thus, in this work, the reactivity of astrocytes and microglia from rats gestated in Hpx was evaluated aiming to understand whether these cells are targets of gestational Hpx. Interestingly, microglia derived from the offspring gestated in Hpx were less reactive compared to microglia derived from offspring gestated in euthyroidism. Instead, astrocytes derived from the offspring gestated in Hpx were significantly more reactive than the astrocytes from the offspring gestated in euthyroidism. This work contributes with novel information regarding the effects of gestational Hpx over astrocytes and microglia in the offspring. It suggests that astrocyte could react strongly to an inflammatory insult inducing neuronal death in the CNS.

Inflammasome inhibition prevents α-synuclein pathology and dopaminergic neurodegeneration in mice.

Parkinson's disease (PD) is characterized by a profound loss of dopaminergic neurons in the substantia nigra, accompanied by chronic neuroinflammation, mitochondrial dysfunction, and widespread accumulation of α-synuclein-rich protein aggregates in the form of Lewy bodies. However, the mechanisms linking α-synuclein pathology and dopaminergic neuronal death to chronic microglial neuroinflammation have not been completely elucidated. We show that activation of the microglial NLR family pyrin domain containing 3 (NLRP3) inflammasome is a common pathway triggered by both fibrillar α-synuclein and dopaminergic degeneration in the absence of α-synuclein aggregates. Cleaved caspase-1 and the inflammasome adaptor protein apoptosis-associated speck-like protein containing a C-terminal caspase recruitment domain (ASC) were elevated in the substantia nigra of the brains of patients with PD and in multiple preclinical PD models. NLRP3 activation by fibrillar α-synuclein in mouse microglia resulted in a delayed but robust activation of the NLRP3 inflammasome leading to extracellular interleukin-1ß and ASC release in the absence of pyroptosis. Nanomolar doses of a small-molecule NLRP3 inhibitor, MCC950, abolished fibrillar α-synuclein-mediated inflammasome activation in mouse microglial cells and extracellular ASC release. Furthermore, oral administration of MCC950 in multiple rodent PD models inhibited inflammasome activation and effectively mitigated motor deficits, nigrostriatal dopaminergic degeneration, and accumulation of α-synuclein aggregates. These findings suggest that microglial NLRP3 may be a sustained source of neuroinflammation that could drive progressive dopaminergic neuropathology and highlight NLRP3 as a potential target for disease-modifying treatments for PD.

The Ketone Body ß-Hydroxybutyrate Does Not Inhibit Synuclein Mediated Inflammasome Activation in Microglia.

Parkinson's disease (PD) is recognized as the most common neurodegenerative movement disorder and results in debilitating motor deficits. The accumulation and spread of neurotoxic synuclein aggregates in the form of Lewy bodies is a key pathological feature of PD. Chronic activation of the NLRP3 inflammasome by protein aggregates is emerging as a major pathogenic mechanism in progressive neurodegenerative disorders and is considered an important therapeutic target. Recently the ketone body, ß-hydroxy butyrate (BHB), was shown to efficiently inhibit the NLRP3 inflammasome in macrophages, and in vivo models of inflammatory disease. Furthermore, BHB can readily cross the blood brain barrier suggesting that it could have therapeutic benefits for the management of PD. In this study, we evaluated if BHB could inhibit chronic microglial inflammasome activation induced by pathological fibrillar synuclein aggregates. Interestingly, we found that BHB treatment almost completely blocked all aspects of inflammasome activation and pyroptosis induced by ATP and monosodium urate (MSU) crystals, consistent with previously published reports in macrophages. Surprisingly however, BHB did not inhibit inflammasome activation and release of IL-1ß or caspase-1 induced by synuclein fibrils. Our results demonstrate that BHB does not block the upstream pathways regulating inflammasome activation by synuclein fibrils and suggest that synuclein mediated inflammasome activation proceeds via distinct mechanisms compared to traditional NLRP3 activators such as ATP and MSU.

Gestational hypothyroidism increases the severity of experimental autoimmune encephalomyelitis in adult offspring.

BACKGROUND: Maternal thyroid hormones play a fundamental role in appropriate fetal development during gestation. Offspring that have been gestated under maternal hypothyroidism suffer cognitive impairment. Thyroid hormone deficiency during gestation can significantly impact the central nervous system by altering the migration, differentiation, and function of neurons, oligodendrocytes, and astrocytes. Given that gestational hypothyroidism alters the immune cell ratio in offspring, it is possible that this condition could result in higher sensitivity for the development of autoimmune diseases. METHODS: Adult mice gestated under hypothyroidism were induced with experimental autoimmune encephalomyelitis (EAE). Twenty-one days after EAE induction, the disease score, myelin content, immune cell infiltration, and oligodendrocyte death were evaluated. RESULTS: We observed that mice gestated under hypothyroidism showed higher EAE scores after disease induction during adulthood compared to mice gestated in euthyroidism. In addition, spinal cord sections of mice gestated under hypothyroidism that suffered EAE in adulthood showed higher demyelination, CD4(+) and CD8(+) infiltration, and increased oligodendrocyte death. CONCLUSIONS: These results show for the first time that a deficiency in maternal thyroid hormones during gestation can influence the outcome of a central nervous system inflammatory disease, such as EAE, in their offspring. These data strongly support evaluating thyroid hormones in pregnant women and treating hypothyroidism during pregnancy to prevent increased susceptibility to inflammatory diseases in the central nervous system of offspring.