Expression and Cell Type-specific Localization of Inflammasome Sensors in the Spinal Cord of SOD1(G93A) Mice and Sporadic Amyotrophic lateral sclerosis Patients

From inflammatory signaling to neuronal damage: Exploring NLR inflammasomes in ageing neurological disorders

The persistence of neuronal degeneration and damage is a major obstacle in ageing medicine. Nucleotide-binding oligomerization domain (NOD)-like receptors detect environmental stressors and trigger the maturation and secretion of pro-inflammatory cytokines that can cause neuronal damage and accelerate cell death. NLR (NOD-like receptors) inflammasomes are protein complexes that contain NOD-like receptors. Studying the role of NLR inflammasomes in ageing-related neurological disorders can provide valuable insights into the mechanisms of neurodegeneration. This includes investigating their activation of inflammasomes, transcription, and capacity to promote or inhibit inflammatory signaling, as well as exploring strategies to regulate NLR inflammasomes levels. This review summarizes the use of NLR inflammasomes in guiding neuronal degeneration and injury during the ageing process, covering several neurological disorders such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, stroke, and peripheral neuropathies. To improve the quality of life and slow the progression of neurological damage, NLR-based treatment strategies, including inhibitor-related therapies and physical therapy, are presented. Additionally, important connections between age-related neurological disorders and NLR inflammasomes are highlighted to guide future research and facilitate the development of new treatment options.

Inflammasomes in neurological disorders - mechanisms and therapeutic potential

Inflammasomes are molecular scaffolds that are activated by damage-associated and pathogen-associated molecular patterns and form a key element of innate immune responses. Consequently, the involvement of inflammasomes in several diseases that are characterized by inflammatory processes, such as multiple sclerosis, is widely appreciated. However, many other neurological conditions, including Alzheimer disease, Parkinson disease, amyotrophic lateral sclerosis, stroke, epilepsy, traumatic brain injury, sepsis-associated encephalopathy and neurological sequelae of COVID-19, all involve persistent inflammation in the brain, and increasing evidence suggests that inflammasome activation contributes to disease progression in these conditions. Understanding the biology and mechanisms of inflammasome activation is, therefore, crucial for the development of inflammasome-targeted therapies for neurological conditions. In this Review, we present the current evidence for and understanding of inflammasome activation in neurological diseases and discuss current and potential interventional strategies that target inflammasome activation to mitigate its pathological consequences.

Divergent functional outcomes of NLRP3 blockade downstream of multi-inflammasome activation: therapeutic implications for ALS

NOD-Like Receptor Family Pyrin Domain Containing 3 (NLRP3) inflammasome modulation has emerged as a potential therapeutic approach targeting inflammation amplified by pyroptotic innate immune cell death. In diseases characterized by non-cell autonomous neurodegeneration including amyotrophic lateral sclerosis (ALS), the activation of several inflammasomes has been reported. Since functional redundancy can exist among inflammasome pathways, here we investigate the effects of NLRP3 inhibition on NLRP3, NLR family CARD Domain Containing 4 (NLRC4) and non-canonical pathways to understand whether NLRP3 blockade alone can mitigate pro-inflammatory cytokine release and pyroptotic cell death in contexts where single or multiple inflammasome pathways independent of NLRP3 are activated. In this study we do not limit our insights into inflammasome biology by solely relying on the THP-1 monocytic line under the LPS/nigericin-mediated NLRP3 pathway activation paradigm. We assess therapeutic potential and limitations of NLRP3 inhibition in multi-inflammasome activation contexts utilizing various human cellular systems including cell lines expressing gain of function (GoF) mutations for several inflammasomes, primary human monocytes, macrophages, healthy and Amyotrophic Lateral Sclerosis (ALS) patient induced pluripotent stem cells (iPSC)-derived microglia (iMGL) stimulated for canonical and non-canonical inflammasome pathways. We demonstrate that NLRP3 inhibition can modulate the NLRC4 and non-canonical inflammasome pathways; however, these effects differ between immortalized, human primary innate immune cells, and iMGL. We extend our investigation in more complex systems characterized by activation of multiple inflammasomes such as the SOD1G93A mouse model. Through deep immune phenotyping by single-cell mass cytometry we demonstrate that acute NLRP3 inhibition does not ameliorate spinal cord inflammation in this model. Taken together, our data suggests that NLRP3 inhibition alone may not be sufficient to address dynamic and complex neuroinflammatory pathobiological mechanisms including dysregulation of multiple inflammasome pathways in neurodegenerative disease such as ALS.

Acute Circadian Disruption Due to Constant Light Promotes Caspase 1 Activation in the Mouse Hippocampus

In mammals, the circadian system controls various physiological processes to maintain metabolism, behavior, and immune function during a daily 24 h cycle. Although driven by a cell-autonomous core clock in the hypothalamus, rhythmic activities are entrained to external cues, such as environmental lighting conditions. Exposure to artificial light at night (ALAN) can cause circadian disruption and thus is linked to an increased occurrence of civilization diseases in modern society. Moreover, alterations of circadian rhythms and dysregulation of immune responses, including inflammasome activation, are common attributes of neurodegenerative diseases, including Alzheimer', Parkinson's, and Huntington's disease. Although there is evidence that the inflammasome in the hippocampus is activated by stress, the direct effect of circadian disruption on inflammasome activation remains poorly understood. In the present study, we aimed to analyze whether exposure to constant light (LL) affects inflammasome activation in the mouse hippocampus. In addition to decreased circadian power and reduced locomotor activity, we found cleaved caspase 1 significantly elevated in the hippocampus of mice exposed to LL. However, we did not find hallmarks of inflammasome priming or cleavage of pro-interleukins. These findings suggest that acute circadian disruption leads to an assembled "ready to start" inflammasome, which may turn the brain more vulnerable to additional aversive stimuli.

The Multifaceted Neurotoxicity of Astrocytes in Ageing and Age-Related Neurodegenerative Diseases: A Translational Perspective

In a healthy physiological context, astrocytes are multitasking cells contributing to central nervous system (CNS) homeostasis, defense, and immunity. In cell culture or rodent models of age-related neurodegenerative diseases (NDDs), such as Alzheimer’s disease (AD) and Parkinson’s disease (PD), numerous studies have shown that astrocytes can adopt neurotoxic phenotypes that could enhance disease progression. Chronic inflammatory responses, oxidative stress, unbalanced phagocytosis, or alteration of their core physiological roles are the main manifestations of their detrimental states. However, if astrocytes are directly involved in brain deterioration by exerting neurotoxic functions in patients with NDDs is still controversial. The large spectrum of NDDs, with often overlapping pathologies, and the technical challenges associated with the study of human brain samples complexify the analysis of astrocyte involvement in specific neurodegenerative cascades. With this review, we aim to provide a translational overview about the multi-facets of astrocyte neurotoxicity ranging from in vitro findings over mouse and human cell-based studies to rodent NDDs research and finally evidence from patient-related research. We also discuss the role of ageing in astrocytes encompassing changes in physiology and response to pathologic stimuli and how this may prime detrimental responses in NDDs. To conclude, we discuss how potentially therapeutic strategies could be adopted to alleviate or reverse astrocytic toxicity and their potential to impact neurodegeneration and dementia progression in patients.

Neuronal accumulation of peroxidated lipids promotes demyelination and neurodegeneration through the activation of the microglial NLRP3 inflammasome

Peroxidated lipids accumulate in the presence of reactive oxygen species and are linked to neurodegenerative diseases. Here we find that neuronal ablation of ARF1, a small GTPase important for lipid homeostasis, promoted accumulation of peroxidated lipids, lipid droplets and ATP in the mouse brain and led to neuroinflammation, demyelination and neurodegeneration, mainly in the spinal cord and hindbrain. Ablation of ARF1 in cultured primary neurons led to an increase in peroxidated lipids in co-cultured microglia, activation of the microglial NLRP3 inflammasome and release of inflammatory cytokines in an Apolipoprotein E-dependent manner. Deleting the Nlrp3 gene rescued the neurodegenerative phenotypes in the neuronal Arf1-ablated mice. We also observed a reduction in ARF1 in human brain tissue from patients with amyotrophic lateral sclerosis and multiple sclerosis. Together, our results uncover a previously unrecognized role of peroxidated lipids released from damaged neurons in activation of a neurotoxic microglial NLRP3 pathway that may play a role in human neurodegeneration.

Sex-specific Regulation of Spine Density and Synaptic Proteins by G-protein-coupled Estrogen Receptor (GPER)1 in Developing Hippocampus

G-protein-coupled-estrogen-receptor 1 (GPER1) is a membrane-bound receptor that mediates estrogen signaling via intracellular signaling cascades. We recently showed that GPER1 promotes the distal dendritic enrichment of hyperpolarization activated and cyclic nucleotide-gated (HCN)1 channels in CA1 stratum lacunosum-moleculare (SLM), suggesting a role of GPER1-mediated signaling in neuronal plasticity. Here we studied whether this role involves processes of structural plasticity, such as the regulation of spine and synapse density in SLM. In organotypic entorhino-hippocampal cultures from mice expressing eGFP, we analyzed spine densities in SLM after treatment with GPER1 agonist G1 (20 nM). G1 significantly increased the density of "non-stubby" spines (maturing spines with a spine head and a neck), but did so only in cultures from female mice. In support of this finding, the expression of synaptic proteins was sex-specifically altered in the cultures: G1 increased the protein (but not mRNA) expression of PSD95 and reduced the p-/n-cofilin ratio only in cultures from females. Application of E2 (2 nM) reproduced the sex-specific effect on spine density in SLM, but only partially on the expression of synaptic proteins. Spine synapse density was, however, not altered after G1-treatment, suggesting that the increased spine density did not translate into an increased spine synapse density in the culture model. Taken together, our results support a role of GPER1 in mediating structural plasticity in CA1 SLM, but suggest that in developing hippocampus, this role is sex-specific.

Targeting Inflammasomes to Treat Neurological Diseases

Inflammasomes are multimeric protein complexes that can sense a plethora of microbe- and damage-associated molecular signals. They play important roles in innate immunity and are key regulators of inflammation in health and disease. Inflammasome-mediated processing and secretion of proinflammatory cytokines such as interleukin (IL) 1β and IL-18 and induction of pyroptosis, a proinflammatory form of cell death, have been associated with the development and progression of common immune-mediated and degenerative central nervous system (CNS) diseases such as Alzheimer disease, multiple sclerosis, brain injury, stroke, epilepsy, Parkinson disease, and amyotrophic lateral sclerosis. A growing number of pharmacological compounds inhibiting inflammasome activation and signaling show therapeutic efficacy in preclinical models of the aforementioned disease conditions. Here, we illustrate regulatory mechanisms of inflammasome activation during CNS homeostasis and tissue injury. We highlight the evidence for inflammasome activation as a mechanistic underpinning in a wide range of CNS diseases and critically discuss the promise and potential limitations of therapeutic strategies that aim to inhibit the inflammasome components in neurological disorders. ANN NEUROL 2021;90:177-188.