Concerted type I interferon signaling in microglia and neural cells promotes memory impairment associated with amyloid β plaques

Increased thyroid hormone action alleviates hippocampal damage by down-regulating neuronal type-Ⅰ interferon signaling/necroptosis in diabetes-associated cognitive dysfunction

BACKGROUND

Thyroid dysfunction plays an important role in the pathology of diabetes-associated cognitive dysfunction (DACD). However, thyroid hormone (TH) signaling and action changes in DACD brains remain unknown. This study evaluated the alternations in TH signaling and action in the brains of DACD mice, and explored the beneficial effects of levothyroxine (L-T4) treatment.

METHODS

KK-Ay mice, serving as a spontaneous type-2 diabetes mellitus model, underwent intragastric administration of 10 ng/g and 20 ng/g of L-T4 solution or normal saline for 8 weeks. Age-matched C57BL/6J mice were used as normal controls. Cognitive and memory functions were examined through the open field and Morris water maze tests. Hippocampal TH signaling and pathogenic status were evaluated. The potential signaling pathways involved in the neuroprotective action of L-T4 were investigated through RNA sequencing and further verified through quantitative real-time PCR (qPCR), Western blotting (WB), immunofluorescence (IF), and fluorescent multiplex immunohistochemistry (mIHC) in vivo and vitro.

RESULTS

The expressions of hippocampal TH transporters (Mct8 and Oatp1c1), Dio2, and TH receptor were up-regulated, while Dio3 as well as the TH positive-regulated genes MBP, Enpp2 and Klf9 were down-regulated in DACD mice. Exogenous L-T4 partially alleviated cognitive and memory dysfunction and restored hippocampal neuronal activity by optimizing TH signaling. RNA sequencing provided insights into the role of type-I interferon (IFN-I) signaling and necroptosis on the amelioration of hippocampal damage following L-T4 treatment. WB and qPCR further confirmed that the levels of key proteins for IFN-I signaling and necroptosis (p-STAT1, p-STAT2, IRF9, ZBP1, p-RIP3, and p-MLKL) were increased, but largely returned following L-T4 administration in vivo and T3 treatment in vitro. IF and mIHC revealed that IRF9 and p-MLKL co-localized in neurons, but not in astrocytes or microglia, of the hippocampus in DACD mice. The diabetes mellitus group had an increased number of IRF9+p-MLKL+NeuN+ cells, which decreased after L-T4 treatment. The elevated IFN-I signaling-mediated necroptosis in HT22 cells was also decreased by T3.

CONCLUSION

We demonstrated abnormal hippocampal TH signaling and action in DACD. Promoting TH action with exogenous L-T4 ameliorated hippocampal impairment through inhibiting IFN-I signaling-induced necroptosis.

Microglia signaling in health and disease - Implications in sex-specific brain development and plasticity

Microglia, the intrinsic neuroimmune cells residing in the central nervous system (CNS), exert a pivotal influence on brain development, homeostasis, and functionality, encompassing critical roles during both aging and pathological states. Recent advancements in comprehending brain plasticity and functions have spotlighted conspicuous variances between male and female brains, notably in neurogenesis, neuronal myelination, axon fasciculation, and synaptogenesis. Nevertheless, the precise impact of microglia on sex-specific brain cell plasticity, sculpting diverse neural network architectures and circuits, remains largely unexplored. This article seeks to unravel the present understanding of microglial involvement in brain development, plasticity, and function, with a specific emphasis on microglial signaling in brain sex polymorphism. Commencing with an overview of microglia in the CNS and their associated signaling cascades, we subsequently probe recent revelations regarding molecular signaling by microglia in sex-dependent brain developmental plasticity, functions, and diseases. Notably, C-X3-C motif chemokine receptor 1 (CX3CR1), triggering receptors expressed on myeloid cells 2 (TREM2), calcium (Ca2+), and apolipoprotein E (APOE) emerge as molecular candidates significantly contributing to sex-dependent brain development and plasticity. In conclusion, we address burgeoning inquiries surrounding microglia's pivotal role in the functional diversity of developing and aging brains, contemplating their potential implications for gender-tailored therapeutic strategies in neurodegenerative diseases.

Characteristic Profiling of Soluble Factors in the Cerebrospinal Fluid of Patients With Neurosyphilis

BACKGROUND

Soluble inflammatory factors in the cerebrospinal fluid (CSF) of patients with neurosyphilis have been investigated with low-throughput technology. This study aimed to illustrate the characteristics of soluble factor profiles in CSF of patients with neurosyphilis.

METHODS

We measured the concentrations of 45 cytokines, chemokines, and growth factors in CSF from 112 untreated syphilis cases, including latent syphilis (LS), asymptomatic neurosyphilis (ANS), meningeal neurosyphilis (MNS), meningovascular neurosyphilis (MVNS), paralytic dementia (PD), and ocular syphilis (OS).

RESULTS

Thirty-three differentially expressed soluble factors (DeSFs) were categorized into 3 clusters. DeSF scores of clusters 1 and 2 (DeSFS1 and DeSFS2) were positively correlated with elevated neopterin and neurofilament light subunit (NF-L) concentration, respectively. DeSF scores of cluster 3 were positively correlated with white blood cells, protein, NF-L, and neopterin. Patients with LS, ANS, and OS exhibited an overall lower abundance of DeSFs. Patients with PD exhibited significantly increased levels of clusters 1 and 3, and the highest total DeSF score, whereas patients with MNS and MVNS showed enhanced levels of cluster 2. Receiver operating characteristic analysis revealed that DeSFS1 effectively discriminated PD, and DeSFS2 discriminated MNS/MVNS with high accuracy.

CONCLUSIONS

Patients with neurosyphilis at different stages have distinctive patterns of soluble factors in CSF, which are correlated with immune status and neuronal damage.

Immune Response following FLASH and Conventional Radiation in Diffuse Midline Glioma

PURPOSE

Diffuse midline glioma (DMG) is a fatal tumor traditionally treated with radiation therapy (RT) and previously characterized as having a noninflammatory tumor immune microenvironment (TIME). FLASH is a novel RT technique using ultra-high dose rate that is associated with decreased toxicity and effective tumor control. However, the effect of FLASH and conventional (CONV) RT on the DMG TIME has not yet been explored.

METHODS AND MATERIALS

Here, we performed single-cell RNA sequencing (scRNA-seq) and flow cytometry on immune cells isolated from an orthotopic syngeneic murine model of brainstem DMG after the use of FLASH (90 Gy/sec) or CONV (2 Gy/min) dose-rate RT and compared to unirradiated tumor (SHAM).

RESULTS

At day 4 post-RT, FLASH exerted similar effects as CONV in the predominant microglial (MG) population, including the presence of two activated subtypes. However, at day 10 post-RT, we observed a significant increase in the type 1 interferon α/β receptor (IFNAR+) in MG in CONV and SHAM compared to FLASH. In the non-resident myeloid clusters of macrophages (MACs) and dendritic cells (DCs), we found increased type 1 interferon (IFN1) pathway enrichment for CONV compared to FLASH and SHAM by scRNA-seq. We observed this trend by flow cytometry at day 4 post-RT in IFNAR+ MACs and DCs, which equalized by day 10 post-RT. DMG control and murine survival were equivalent between RT dose rates.

CONCLUSIONS

Our work is the first to map CONV and FLASH immune alterations of the DMG TIME with single-cell resolution. Although DMG tumor control and survival were similar between CONV and FLASH, we found that changes in immune compartments differed over time. Importantly, although both RT modalities increased IFN1, we found that the timing of this response was cell-type and dose-rate dependent. These temporal differences, particularly in the context of tumor control, warrant further study.

LRRK2 kinase activity restricts NRF2-dependent mitochondrial protection in microglia

Mounting evidence supports a critical role for central nervous system (CNS) glial cells in neuroinflammation and neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's Disease (PD), Multiple Sclerosis (MS), as well as neurovascular ischemic stroke. Previously, we found that loss of the PD-associated gene leucine-rich repeat kinase 2 (Lrrk2) in macrophages, peripheral innate immune cells, induced mitochondrial stress and elevated basal expression of type I interferon (IFN) stimulated genes (ISGs) due to chronic mitochondrial DNA engagement with the cGAS/STING DNA sensing pathway. Here, we report that loss of LRRK2 results in a paradoxical response in microglial cells, a CNS-specific macrophage population. In primary murine microglia and microglial cell lines, loss of Lrrk2 reduces tonic IFN signaling leading to a reduction in ISG expression. Consistent with reduced type I IFN, mitochondria from Lrrk2 KO microglia are protected from stress and have elevated metabolism. These protective phenotypes involve upregulation of NRF2, an important transcription factor in the response to oxidative stress and are restricted by LRRK2 kinase activity. Collectively, these findings illustrate a dichotomous role for LRRK2 within different immune cell populations and give insight into the fundamental differences between immune regulation in the CNS and the periphery.

Intermittent hypoxia exacerbates anxiety in high-fat diet-induced diabetic mice by inhibiting TREM2-regulated IFNAR1 signaling

BACKGROUND

Type 2 diabetes mellitus (T2DM) and obstructive sleep apnea (OSA) are mutual risk factors, with both conditions inducing cognitive impairment and anxiety. However, whether OSA exacerbates cognitive impairment and anxiety in patients with T2DM remains unclear. Moreover, TREM2 upregulation has been suggested to play a protective role in attenuating microglia activation and improving synaptic function in T2DM mice. The aim of this study was to explore the regulatory mechanisms of TREM2 and the cognitive and anxiety-like behavioral changes in mice with OSA combined with T2DM.

METHODS

A T2DM with OSA model was developed by treating mice with a 60% kcal high-fat diet (HFD) combined with intermittent hypoxia (IH). Spatial learning memory capacity and anxiety in mice were investigated. Neuronal damage in the brain was determined by the quantity of synapses density, the number and morphology of brain microglia, and pro-inflammatory factors. For mechanism exploration, an in vitro model of T2DM combined with OSA was generated by co-treating microglia with high glucose (HG) and IH. Regulation of TREM2 on IFNAR1-STAT1 pathway was determined by RNA sequencing and qRT-PCR.

RESULTS

Our results showed that HFD mice exhibited significant cognitive dysfunction and anxiety-like behavior, accompanied by significant synaptic loss. Furthermore, significant activation of brain microglia and enhanced microglial phagocytosis of synapses were observed. Moreover, IH was found to significantly aggravate anxiety in the HFD mice. The mechanism of HG treatment may potentially involve the promotion of TREM2 upregulation, which in turn attenuates the proinflammatory microglia by inhibiting the IFNAR1-STAT1 pathway. Conversely, a significant reduction in TREM2 in IH-co-treated HFD mice and HG-treated microglia resulted in the further activation of the IFNAR1-STAT1 pathway and consequently increased proinflammatory microglial activation.

CONCLUSIONS

HFD upregulated the IFNAR1-STAT1 pathway and induced proinflammatory microglia, leading to synaptic damage and causing anxiety and cognitive deficits. The upregulated TREM2 inT2DM mice brain exerted a negative regulation of the IFNAR1-STAT1 pathway. Mice with T2DM combined with OSA exacerbated anxiety via the downregulation of TREM2, causing heightened IFNAR1-STAT1 pathway activation and consequently increasing proinflammatory microglia.

Fate-mapping and functional dissection reveal perilous influence of type I interferon signaling in mouse brain aging

BACKGROUND

Aging significantly elevates the risk of developing neurodegenerative diseases. Neuroinflammation is a universal hallmark of neurodegeneration as well as normal brain aging. Which branches of age-related neuroinflammation, and how they precondition the brain toward pathological progression, remain ill-understood. The presence of elevated type I interferon (IFN-I) has been documented in the aged brain, but its role in promoting degenerative processes, such as the loss of neurons in vulnerable regions, has not been studied in depth.

METHODS

To comprehend the scope of IFN-I activity in the aging brain, we surveyed IFN-I-responsive reporter mice at multiple ages. We also examined 5- and 24-month-old mice harboring selective ablation of Ifnar1 in microglia to observe the effects of manipulating this pathway during the aging process using bulk RNA sequencing and histological parameters.

RESULTS

We detected age-dependent IFN-I signal escalation in multiple brain cell types from various regions, especially in microglia. Selective ablation of Ifnar1 from microglia in aged mice significantly reduced overall brain IFN-I signature, dampened microglial reactivity, lessened neuronal loss, restored expression of key neuronal genes and pathways, and diminished the accumulation of lipofuscin, a core hallmark of cellular aging in the brain.

CONCLUSIONS

Overall, our study demonstrates pervasive IFN-I activity during normal mouse brain aging and reveals a pathogenic, pro-degenerative role played by microglial IFN-I signaling in perpetuating neuroinflammation, neuronal dysfunction, and molecular aggregation. These findings extend the understanding of a principal axis of age-related inflammation in the brain, one likely shared with multiple neurological disorders, and provide a rationale to modulate aberrant immune activation to mitigate neurodegenerative process at all stages.

The meninges host a unique compartment of regulatory T cells that bulwarks adult hippocampal neurogenesis

Our knowledge about the meningeal immune system has recently burgeoned, particularly our understanding of how innate and adaptive effector cells are mobilized to meet brain challenges. However, information on how meningeal immunocytes guard brain homeostasis in healthy individuals remains sparse. This study highlights the heterogeneous and polyfunctional regulatory-T (Treg) cell compartment in the meninges. A Treg subtype specialized in controlling Th1-cell responses and another known to control responses in B-cell follicles were substantial components of this compartment, foretelling that punctual Treg-cell ablation rapidly unleashed interferon-gamma production by meningeal lymphocytes, unlocked their access to the brain parenchyma, and altered meningeal B-cell profiles. Distally, the hippocampus assumed a reactive state, with morphological and transcriptional changes in multiple glial-cell types; within the dentate gyrus, neural stem cells showed exacerbated death and desisted from further differentiation, associated with inhibition of spatial-reference memory. Thus, meningeal Treg cells are a multifaceted bulwark to brain homeostasis at steady-state.

One sentence summary

A distinct population of regulatory T cells in the murine meninges safeguards homeostasis by keeping local interferon-γ-producing lymphocytes in check, thereby preventing their invasion of the parenchyma, activation of hippocampal glial cells, death of neural stem cells, and memory decay.

Microglia phagocytic mechanisms: Development informing disease

Microglia are tissue-resident macrophages and professional phagocytes of the central nervous system (CNS). In development, microglia-mediated phagocytosis is important for sculpting the cellular architecture. This includes the engulfment of dead/dying cells, pruning extranumerary synapses and axons, and phagocytosing fragments of myelin sheaths. Intriguingly, these developmental phagocytic mechanisms by which microglia sculpt the CNS are now appreciated as important for eliminating synapses, myelin, and proteins during neurodegeneration. Here, we discuss parallels between neurodevelopment and neurodegeneration, which highlights how development is informing disease. We further discuss recent advances and challenges towards therapeutically targeting these phagocytic pathways and how we can leverage development to overcome these challenges.

Phenotypic and spatial heterogeneity of brain myeloid cells after stroke is associated with cell ontogeny, tissue damage, and brain connectivity

Acute stroke triggers extensive changes to myeloid immune cell populations in the brain that may be targets for limiting brain damage and enhancing repair. Immunomodulatory approaches will be most effective with precise manipulation of discrete myeloid cell phenotypes in time and space. Here, we investigate how stroke alters mononuclear myeloid cell composition and phenotypes at single-cell resolution and key spatial patterns. Our results show that multiple reactive microglial states and monocyte-derived populations contribute to an extensive myeloid cell repertoire in post-stroke brains. We identify important overlaps and distinctions among different cell types/states that involve ontogeny- and spatial-related properties. Notably, brain connectivity with infarcted tissue underpins the pattern of local and remote altered cell accumulation and reactivity. Our discoveries suggest a global but anatomically governed brain myeloid cell response to stroke that comprises diverse phenotypes arising through intrinsic cell ontogeny factors interacting with exposure to spatially organized brain damage and neuro-axonal cues.

Angiotensin Converting Enzyme (ACE) expression in microglia reduces amyloid β deposition and neurodegeneration by increasing SYK signaling and endolysosomal trafficking

Genome-wide association studies (GWAS) have identified many gene polymorphisms associated with an increased risk of developing Late Onset Alzheimer's Disease (LOAD). Many of these LOAD risk-associated alleles alter disease pathogenesis by influencing microglia innate immune responses and lipid metabolism. Angiotensin Converting Enzyme (ACE), a GWAS LOAD risk-associated gene best known for its role in regulating systemic blood pressure, also enhances innate immunity and lipid processing in peripheral myeloid cells, but a role for ACE in modulating the function of myeloid-derived microglia remains unexplored. Using novel mice engineered to express ACE in microglia and CNS associated macrophages (CAMs), we find that ACE expression in microglia reduces Aβ plaque load, preserves vulnerable neurons and excitatory synapses, and greatly reduces learning and memory abnormalities in the 5xFAD amyloid mouse model of Alzheimer's Disease (AD). ACE-expressing microglia show enhanced Aβ phagocytosis and endolysosomal trafficking, increased clustering around amyloid plaques, and increased SYK tyrosine kinase activation downstream of the major Aβ receptors, TREM2 and CLEC7A. Single microglia sequencing and digital spatial profiling identifies downstream SYK signaling modules that are expressed by ACE expression in microglia that mediate endolysosomal biogenesis and trafficking, mTOR and PI3K/AKT signaling, and increased oxidative phosphorylation, while gene silencing or pharmacologic inhibition of SYK activity in ACE-expressing microglia abrogates the potentiated Aβ engulfment and endolysosomal trafficking. These findings establish a role for ACE in enhancing microglial immune function and they identify a potential use for ACE-expressing microglia as a cell-based therapy to augment endogenous microglial responses to Aβ in AD.

Fate-mapping and functional dissection reveal perilous influence of type I interferon signaling in mouse brain aging

Although aging significantly elevates the risk of developing neurodegenerative diseases, how age-related neuroinflammation preconditions the brain toward pathological progression is ill-understood. To comprehend the scope of type I interferon (IFN-I) activity in the aging brain, we surveyed IFN-I-responsive reporter mice and detected age-dependent signal escalation in multiple brain cell types from various regions. Selective ablation of Ifnar1 from microglia in aged mice significantly reduced overall brain IFN-I signature, dampened microglial reactivity, lessened neuronal loss, and diminished the accumulation of lipofuscin, a core hallmark of cellular aging in the brain. Overall, our study demonstrates pervasive IFN-I activity during normal mouse brain aging and reveals a pathogenic role played by microglial IFN-I signaling in perpetuating neuroinflammation, neuronal dysfunction, and molecular aggregation. These findings extend the understanding of a principal axis of age-related inflammation in the brain, and provide a rationale to modulate aberrant immune activation to mitigate neurodegenerative process at all stages.

Sustained type I interferon signaling after human immunodeficiency virus type 1 infection of human iPSC derived microglia and cerebral organoids

Human immunodeficiency virus type-1 (HIV-1)-associated neurocognitive disorder (HAND) affects up to half of people living with HIV-1 and causes long term neurological consequences. The pathophysiology of HIV-1-induced glial and neuronal functional deficits in humans remains enigmatic. To bridge this gap, we established a model simulating HIV-1 infection in the central nervous system using human induced pluripotent stem cell (iPSC)-derived microglia combined with sliced neocortical organoids. Incubation of microglia with two replication-competent macrophage-tropic HIV-1 strains (JRFL and YU2) elicited productive infection and inflammatory activation. RNA sequencing revealed significant and sustained activation of type I interferon signaling pathways. Incorporating microglia into sliced neocortical organoids extended the effects of aberrant type I interferon signaling in a human neural context. Collectively, our results illuminate a role for persistent type I interferon signaling in HIV-1-infected microglia in a human neural model, suggesting its potential significance in the pathogenesis of HAND.

A parasite odyssey: An RNA virus concealed in Toxoplasma gondii

We are entering a 'Platinum Age of Virus Discovery', an era marked by exponential growth in the discovery of virus biodiversity, and driven by advances in metagenomics and computational analysis. In the ecosystem of a human (or any animal) there are more species of viruses than simply those directly infecting the animal cells. Viruses can infect all organisms constituting the microbiome, including bacteria, fungi, and unicellular parasites. Thus the complexity of possible interactions between host, microbe, and viruses is unfathomable. To understand this interaction network we must employ computationally assisted virology as a means of analyzing and interpreting the millions of available samples to make inferences about the ways in which viruses may intersect human health. From a computational viral screen of human neuronal datasets, we identified a novel narnavirus Apocryptovirus odysseus (Ao) which likely infects the neurotropic parasite Toxoplasma gondii. Previously, several parasitic protozoan viruses (PPVs) have been mechanistically established as triggers of host innate responses, and here we present in silico evidence that Ao is a plausible pro-inflammatory factor in human and mouse cells infected by T. gondii. T. gondii infects billions of people worldwide, yet the prognosis of toxoplasmosis disease is highly variable, and PPVs like Ao could function as a hitherto undescribed hypervirulence factor. In a broader screen of over 7.6 million samples, we explored phylogenetically proximal viruses to Ao and discovered nineteen Apocryptovirus species, all found in libraries annotated as vertebrate transcriptome or metatranscriptomes. While samples containing this genus of narnaviruses are derived from sheep, goat, bat, rabbit, chicken, and pigeon samples, the presence of virus is strongly predictive of parasitic Apicomplexa nucleic acid co-occurrence, supporting the fact that Apocryptovirus is a genus of parasite-infecting viruses. This is a computational proof-of-concept study in which we rapidly analyze millions of datasets from which we distilled a mechanistically, ecologically, and phylogenetically refined hypothesis. We predict that this highly diverged Ao RNA virus is biologically a T. gondii infection, and that Ao, and other viruses like it, will modulate this disease which afflicts billions worldwide.

SARS-CoV-2-Induced Type I Interferon Signaling Dysregulation in Olfactory Networks Implications for Alzheimer's Disease

Type I interferon signaling (IFN-I) perturbations are major drivers of COVID-19. Dysregulated IFN-I in the brain, however, has been linked to both reduced cognitive resilience and neurodegenerative diseases such as Alzheimer's. Previous works from our group have proposed a model where peripheral induction of IFN-I may be relayed to the CNS, even in the absence of fulminant infection. The aim of our study was to identify significantly enriched IFN-I signatures and genes along the transolfactory route, utilizing published datasets of the nasal mucosa and olfactory bulb amygdala transcriptomes of COVID-19 patients. We furthermore sought to identify these IFN-I signature gene networks associated with Alzheimer's disease pathology and risk. Gene expression data involving the nasal epithelium, olfactory bulb, and amygdala of COVID-19 patients and transcriptomic data from Alzheimer's disease patients were scrutinized for enriched Type I interferon pathways. Gene set enrichment analyses and gene-Venn approaches were used to determine genes in IFN-I enriched signatures. The Agora web resource was used to identify genes in IFN-I signatures associated with Alzheimer's disease risk based on its aggregated multi-omic data. For all analyses, false discovery rates (FDR) <0.05 were considered statistically significant. Pathways associated with type I interferon signaling were found in all samples tested. Each type I interferon signature was enriched by IFITM and OAS family genes. A 14-gene signature was associated with COVID-19 CNS and the response to Alzheimer's disease pathology, whereas nine genes were associated with increased risk for Alzheimer's disease based on Agora. Our study provides further support to a type I interferon signaling dysregulation along the extended olfactory network as reconstructed herein, ranging from the nasal epithelium and extending to the amygdala. We furthermore identify the 14 genes implicated in this dysregulated pathway with Alzheimer's disease pathology, among which HLA-C, HLA-B, HLA-A, PSMB8, IFITM3, HLA-E, IFITM1, OAS2, and MX1 as genes with associated conferring increased risk for the latter. Further research into its druggability by IFNb therapeutics may be warranted.

In sickness and in health-Type I interferon and the brain

Type I interferons (IFN-I) represent a group of pleiotropic cytokines renowned for their antiviral activity and immune regulatory functions. A multitude of studies have unveiled a critical role of IFN-I in the brain, influencing various neurological processes and diseases. In this mini-review, I highlight recent findings on IFN-I's effects on brain aging, Alzheimer's disease (AD) progression, and central nervous system (CNS) homeostasis. The multifaceted influence of IFN-I on brain health and disease sheds light on the complex interplay between immune responses and neurological processes. Of particular interest is the cGAS-STING-IFN-I axis, which extensively participates in brain aging and various forms of neurodegeneration. Understanding the intricate role of IFN-I and its associated pathways in the CNS not only advances our comprehension of brain health and disease but also presents opportunities for developing interventions to modify the process of neurodegeneration and prevent age-related cognitive decline.

The Sixth Sense: Self-nucleic acid sensing in the brain

Our innate immune system uses pattern recognition receptors (PRRs) as a first line of defense to detect microbial ligands and initiate an immune response. Viral nucleic acids are key ligands for the activation of many PRRs and the induction of downstream inflammatory and antiviral effects. Initially it was thought that endogenous (self) nucleic acids rarely activated these PRRs, however emerging evidence indicates that endogenous nucleic acids are able to activate host PRRs in homeostasis and disease. In fact, many regulatory mechanisms are in place to finely control and regulate sensing of self-nucleic acids by PRRs. Sensing of self-nucleic acids is particularly important in the brain, as perturbations to nucleic acid sensing commonly leads to neuropathology. This review will highlight the role of nucleic acid sensors in the brain, both in disease and homeostasis. We also indicate the source of endogenous stimulatory nucleic acids where known and summarize future directions for the study of this growing field.

Integrated Analysis of Chromatin and Transcriptomic Profiling Identifies PU.1 as a Core Regulatory Factor in Microglial Activation Induced by Chronic Cerebral Hypoperfusion

In addition to causing white matter lesions, chronic cerebral hypoperfusion (CCH) can also cause damage to gray matter, but the underlying molecular mechanisms remain largely unknown. In order to obtain a better understanding of the relationship between gene expression and transcriptional regulation alterations, novel upstream regulators could be identified using integration analysis of the transcriptome and epigenetic approaches. Here, a bilateral common carotid artery stenosis (BCAS) model was established for inducing CCH in mice. The spatial cognitive function of mice was evaluated, and changes in cortical microglia morphology were observed. RNA-sequencing (RNA-seq) and the assay for transposase-accessible chromatin with high-throughput sequencing (ATAC-seq) were performed on isolated mouse cortical brain tissue. Then, a systematic joint analysis of BCAS hypoperfusion-induced cortex-specific RNA-seq and ATAC-seq was conducted in order to assess the extent of the correlation between the two, and PU.1 was found to be greatly enriched through motif analysis and transcription factor annotation. Also, the core regulatory factor PU.1 induced by BCAS hypoperfusion was shown to be colocalized with microglia. Based on the above analysis, PU.1 plays a key regulatory role in microglial activation induced by CCH. And the transcriptome and epigenomic data presented in this study can help identify potential targets for future research exploring chronic hypoperfusion-induced brain injury.

DLK-MAPK Signaling Coupled with DNA Damage Promotes Intrinsic Neurotoxicity Associated with Non-Mutated Tau

Alzheimer's disease (AD) is the most prevalent form of neurodegeneration. Despite the well-established link between tau aggregation and clinical progression, the major pathways driven by this protein to intrinsically damage neurons are incompletely understood. To model AD-relevant neurodegeneration driven by tau, we overexpressed non-mutated human tau in primary mouse neurons and observed substantial axonal degeneration and cell death, a process accompanied by activated caspase 3. Mechanistically, we detected deformation of the nuclear envelope and increased DNA damage response in tau-expressing neurons. Gene profiling analysis further revealed significant alterations in the mitogen-activated protein kinase (MAPK) pathway; moreover, inhibitors of dual leucine zipper kinase (DLK) and c-Jun N-terminal kinase (JNK) were effective in alleviating wild-type human tau-induced neurodegeneration. In contrast, mutant P301L human tau was less toxic to neurons, despite causing comparable DNA damage. Axonal DLK activation induced by wild-type tau potentiated the impact of DNA damage response, resulting in overt neurotoxicity. In summary, we have established a cellular tauopathy model highly relevant to AD and identified a functional synergy between the DLK-MAPK axis and DNA damage response in the neuronal degenerative process.

Microglia as integrators of brain-associated molecular patterns

Microglia are brain-resident macrophages that play key roles in brain development and experience dependent plasticity. In this review we discuss recent findings regarding the molecular mechanisms through which mammalian microglia sense the unique molecular patterns of the homeostatic brain. We propose that microglial function is acutely controlled in response to 'brain-associated molecular patterns' (BAMPs) that function as indicators of neuronal activity and neural circuit remodeling. A further layer of regulation comes from instructive cytokine cues that define unique microglial functional states. A systematic investigation of the receptors and signaling pathways that mediate these two regulatory axes may begin to define a functional code for microglia-neuron interactions.

Type-I-interferon-responsive microglia shape cortical development and behavior

Microglia are brain-resident macrophages that shape neural circuit development and are implicated in neurodevelopmental diseases. Multiple microglial transcriptional states have been defined, but their functional significance is unclear. Here, we identify a type I interferon (IFN-I)-responsive microglial state in the developing somatosensory cortex (postnatal day 5) that is actively engulfing whole neurons. This population expands during cortical remodeling induced by partial whisker deprivation. Global or microglial-specific loss of the IFN-I receptor resulted in microglia with phagolysosomal dysfunction and an accumulation of neurons with nuclear DNA damage. IFN-I gain of function increased neuronal engulfment by microglia in both mouse and zebrafish and restricted the accumulation of DNA-damaged neurons. Finally, IFN-I deficiency resulted in excess cortical excitatory neurons and tactile hypersensitivity. These data define a role for neuron-engulfing microglia during a critical window of brain development and reveal homeostatic functions of a canonical antiviral signaling pathway in the brain.

Single cell analysis reveals the roles and regulatory mechanisms of type-I interferons in Parkinson's disease

The pathogenesis of Parkinson's disease (PD) is strongly associated with neuroinflammation, and type I interferons (IFN-I) play a crucial role in regulating immune and inflammatory responses. However, the specific features of IFN in different cell types and the underlying mechanisms of PD have yet to be fully described. In this study, we analyzed the GSE157783 dataset, which includes 39,024 single-cell RNA sequencing results for five PD patients and six healthy controls from the Gene Expression Omnibus database. After cell type annotation, we intersected differentially expressed genes in each cell subcluster with genes collected in The Interferome database to generate an IFN-I-stimulated gene set (ISGs). Based on this gene set, we used the R package AUCell to score each cell, representing the IFN-I activity. Additionally, we performed monocle trajectory analysis, and single-cell regulatory network inference and clustering (SCENIC) to uncover the underlying mechanisms. In silico gene perturbation and subsequent experiments confirm NFATc2 regulation of type I interferon response and neuroinflammation. Our analysis revealed that microglia, endothelial cells, and pericytes exhibited the highest activity of IFN-I. Furthermore, single-cell trajectory detection demonstrated that microglia in the midbrain of PD patients were in a pro-inflammatory activation state, which was validated in the 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced PD mouse model as well. We identified transcription factors NFATc2, which was significantly up-regulated and involved in the expression of ISGs and activation of microglia in PD. In the 1-Methyl-4-phenylpyridinium (MPP+)-induced BV2 cell model, the suppression of NFATc2 resulted in a reduction in IFN-β levels, impeding the phosphorylation of STAT1, and attenuating the activation of the NF-κB pathway. Furthermore, the downregulation of NFATc2 mitigated the detrimental effects on SH-SY5Y cells co-cultured in conditioned medium. Our study highlights the critical role of microglia in type I interferon responses in PD. Additionally, we identified transcription factors NFATc2 as key regulators of aberrant type I interferon responses and microglial pro-inflammatory activation in PD. These findings provide new insights into the pathogenesis of PD and may have implications for the development of novel therapeutic strategies.

Cytosolic DNA sensors in neurodegenerative diseases: from physiological defenders to pathological culprits

Cytosolic DNA sensors are a group of pattern recognition receptors (PRRs) that vary in structures, molecular mechanisms, and origins but share a common function to detect intracellular microbial DNA and trigger the innate immune response like type 1 interferon production and autophagy. Cytosolic DNA sensors have been proven as indispensable defenders against the invasion of many pathogens; however, growing evidence shows that self-DNA misplacement to cytoplasm also frequently occurs in non-infectious circumstances. Accumulation of cytosolic DNA causes improper activation of cytosolic DNA sensors and triggers an abnormal autoimmune response, that significantly promotes pathological progression. Neurodegenerative diseases are a group of neurological disorders characterized by neuron loss and still lack effective treatments due to a limited understanding of pathogenesis. But current research has found a solid relationship between neurodegenerative diseases and cytosolic DNA sensing pathways. This review summarizes profiles of several major cytosolic DNA sensors and their common adaptor protein STING. It also discusses both the beneficial and detrimental roles of cytosolic DNA sensors in the genesis and progression of neurodegenerative diseases.

Type I interferon signaling, cognition and neurodegeneration following COVID-19: update on a mechanistic pathogenetic model with implications for Alzheimer's disease

COVID-19's effects on the human brain reveal a multifactorial impact on cognition and the potential to inflict lasting neuronal damage. Type I interferon signaling, a pathway that represents our defense against pathogens, is primarily affected by COVID-19. Type I interferon signaling, however, is known to mediate cognitive dysfunction upon its dysregulation following synaptopathy, microgliosis and neuronal damage. In previous studies, we proposed a model of outside-in dysregulation of tonic IFN-I signaling in the brain following a COVID-19. This disruption would be mediated by the crosstalk between central and peripheral immunity, and could potentially establish feed-forward IFN-I dysregulation leading to neuroinflammation and potentially, neurodegeneration. We proposed that for the CNS, the second-order mediators would be intrinsic disease-associated molecular patterns (DAMPs) such as proteopathic seeds, without the requirement of neuroinvasion to sustain inflammation. Selective vulnerability of neurogenesis sites to IFN-I dysregulation would then lead to clinical manifestations such as anosmia and cognitive impairment. Since the inception of our model at the beginning of the pandemic, a growing body of studies has provided further evidence for the effects of SARS-CoV-2 infection on the human CNS and cognition. Several preclinical and clinical studies have displayed IFN-I dysregulation and tauopathy in gene expression and neuropathological data in new cases, correspondingly. Furthermore, neurodegeneration identified with a predilection for the extended olfactory network furthermore supports the neuroanatomical concept of our model, and its independence from fulminant neuroinvasion and encephalitis as a cause of CNS damage. In this perspective, we summarize the data on IFN-I as a plausible mechanism of cognitive impairment in this setting, and its potential contribution to Alzheimer's disease and its interplay with COVID-19.

IRF3 regulates neuroinflammatory responses and the expression of genes associated with Alzheimer's disease

The pathological role of interferon signaling is emerging in neuroinflammatory disorders, yet, the specific role of Interferon Regulatory Factor 3 (IRF3) in neuroinflammation remains poorly understood. Here, we show that global IRF3 deficiency delays TLR4-mediated signaling in microglia and attenuates the hallmark features of LPS-induced inflammation such as cytokine release, microglial reactivity, astrocyte activation, myeloid cell infiltration, and inflammasome activation. Moreover, expression of a constitutively active IRF3 (S388D/S390D:IRF3-2D) in microglia induces a transcriptional program reminiscent of the Activated Response Microglia and the expression of genes associated with Alzheimer's Disease, notably apolipoprotein-e. Lastly, using bulk-RNAseq of IRF3-2D brain myeloid cells, we identified Z-DNA binding protein-1 as a target of IRF3 that is relevant across various neuroinflammatory disorders. Together, our results identify IRF3 as an important regulator of LPS-mediated neuroinflammatory responses and highlight IRF3 as a central regulator of disease-specific gene activation in different neuroinflammatory diseases.

Type I Interferon Signalling and Ischemic Stroke: Mechanisms and Therapeutic Potentials

Type I interferon (IFN-I) signalling is intricately involved in the pathogenesis of multiple infectious diseases, autoimmune diseases, and neurological diseases. Acute ischemic stroke provokes overactivation of IFN-I signalling within the injured brain, particularly in microglia. Following cerebral ischemia, damage-associated molecular patterns (DAMPs) released from injured neural cells elicit marked proinflammatory episodes within minutes. Among these, self-nucleic acids, including nuclear DNA and mitochondrial DNA (mtDNA), have been recognized as a critical alarm signal to fan the flames of neuroinflammation, predominantly via inducing IFN-I signalling activation in microglia. The concept of interferon-responsive microglia (IRM), marked by upregulation of a plethora of IFN-stimulated genes, has been emergingly elucidated in ischemic mouse brains, particularly in aged ones. Among the pattern recognition receptors responsible for IFN-I induction, cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) plays integral roles in potentiating microglia-driven neuroinflammation and secondary brain injury after cerebral ischemia. Here, we aim to provide an up-to-date review on the multifaceted roles of IFN-I signalling, the detailed molecular and cellular mechanisms leading to and resulting from aberrant IFN-I signalling activation after cerebral ischemia, and the therapeutic potentials. A thorough exploration of these above points will inform our quest for IFN-based therapies as effective immunomodulatory therapeutics to complement the limited repertoire of thrombolytic agents, thereby facilitating the translation from bench to bedside.

Proteomic and transcriptomic profiling of brainstem, cerebellum and olfactory tissues in early- and late-phase COVID-19

Neurological symptoms, including cognitive impairment and fatigue, can occur in both the acute infection phase of coronavirus disease 2019 (COVID-19) and at later stages, yet the mechanisms that contribute to this remain unclear. Here we profiled single-nucleus transcriptomes and proteomes of brainstem tissue from deceased individuals at various stages of COVID-19. We detected an inflammatory type I interferon response in acute COVID-19 cases, which resolves in the late disease phase. Integrating single-nucleus RNA sequencing and spatial transcriptomics, we could localize two patterns of reaction to severe systemic inflammation, one neuronal with a direct focus on cranial nerve nuclei and a separate diffuse pattern affecting the whole brainstem. The latter reflects a bystander effect of the respiratory infection that spreads throughout the vascular unit and alters the transcriptional state of mainly oligodendrocytes, microglia and astrocytes, while alterations of the brainstem nuclei could reflect the connection of the immune system and the central nervous system via, for example, the vagus nerve. Our results indicate that even without persistence of severe acute respiratory syndrome coronavirus 2 in the central nervous system, local immune reactions are prevailing, potentially causing functional disturbances that contribute to neurological complications of COVID-19.

STING signaling in the brain: Molecular threats, signaling activities, and therapeutic challenges

Stimulator of interferon genes (STING) is an innate immune signaling protein critical to infections, autoimmunity, and cancer. STING signaling is also emerging as an exciting and integral part of many neurological diseases. Here, we discuss recent advances in STING signaling in the brain. We summarize how molecular threats activate STING signaling in the diseased brain and how STING signaling activities in glial and neuronal cells cause neuropathology. We also review human studies of STING neurobiology and consider therapeutic challenges in targeting STING to treat neurological diseases.

The type-I interferon response potentiates seeded tau aggregation and exacerbates tau pathology

INTRODUCTION

Signatures of a type-I interferon (IFN-I) response are observed in the post mortem brain in Alzheimer's disease (AD) and other tauopathies. However, the effect of the IFN-I response on pathological tau accumulation remains unclear.

METHODS

We examined the effects of IFN-I signaling in primary neural culture models of seeded tau aggregation and P301S-tau transgenic mouse models in the context of genetic deletion of the IFN-I receptor (IFNAR).

RESULTS

Polyinosinic:polycytidylic acid (PolyI:C), a synthetic analog of viral nucleic acids, evoked a potent cytokine response that enhanced seeded aggregation of tau in an IFN-I-dependent manner. IFN-I-induced vulnerability could be pharmacologically prevented and was intrinsic to neurons. Aged P301S-tau mice lacking Ifnar1 had significantly reduced tau pathology compared to mice with intact IFN signaling.

DISCUSSION

We identify a critical role for IFN-I in potentiating tau aggregation. IFN-I is therefore identified as a potential therapeutic target in AD and other tauopathies.

HIGHLIGHTS

Type-I IFN (IFN-I) promotes seeded tau aggregation in neural cultures. IFNAR inhibition prevents IFN-I driven sensitivity to tau aggregation. IFN-I driven vulnerability is intrinsic to neurons. Tau pathology is significantly reduced in aged P301S-tau mice lacking IFNAR.

Non-invasive in vivo imaging of brain and retinal microglia in neurodegenerative diseases

Microglia play crucial roles in immune responses and contribute to fundamental biological processes within the central nervous system (CNS). In neurodegenerative diseases, microglia undergo functional changes and can have both protective and pathogenic roles. Microglia in the retina, as an extension of the CNS, have also been shown to be affected in many neurological diseases. While our understanding of how microglia contribute to pathological conditions is incomplete, non-invasive in vivo imaging of brain and retinal microglia in living subjects could provide valuable insights into their role in the neurodegenerative diseases and open new avenues for diagnostic biomarkers. This mini-review provides an overview of the current brain and retinal imaging tools for studying microglia in vivo. We focus on microglia targets, the advantages and limitations of in vivo microglia imaging approaches, and applications for evaluating the pathogenesis of neurological conditions, such as Alzheimer's disease and multiple sclerosis.

Intranasally Administered EVs from hiPSC-derived NSCs Alter the Transcriptomic Profile of Activated Microglia and Conserve Brain Function in an Alzheimer's Model

Antiinflammatory extracellular vesicles (EVs) derived from human induced pluripotent stem cell (hiPSC)-derived neural stem cells (NSCs) hold promise as a disease-modifying biologic for Alzheimer's disease (AD). This study directly addressed this issue by examining the effects of intranasal administrations of hiPSC-NSC-EVs to 3-month-old 5xFAD mice. The EVs were internalized by all microglia, which led to reduced expression of multiple genes associated with disease-associated microglia, inflammasome, and interferon-1 signaling. Furthermore, the effects of hiPSC-NSC-EVs persisted for two months post-treatment in the hippocampus, evident from reduced microglial clusters, inflammasome complexes, and expression of proteins and/or genes linked to the activation of inflammasomes, p38/mitogen-activated protein kinase, and interferon-1 signaling. The amyloid-beta (Aβ) plaques, Aβ-42, and phosphorylated-tau concentrations were also diminished, leading to better cognitive and mood function in 5xFAD mice. Thus, early intervention with hiPSC-NSC-EVs in AD may help maintain better brain function by restraining the progression of adverse neuroinflammatory signaling cascades.

MicroRNA-218-5p-Ddx41 axis restrains microglia-mediated neuroinflammation through downregulating type I interferon response in a mouse model of Parkinson's disease

BACKGROUND

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the loss of dopaminergic (DA) neurons in the substantia nigra (SN). Microglia-mediated neuroinflammation has been largely considered one of main factors to the PD pathology. MicroRNA-218-5p (miR-218-5p) is a microRNA that plays a role in neurodevelopment and function, while its potential function in PD and neuroinflammation remains unclear.

METHODS

We explore the involvement of miR-218-5p in the PD in a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced mouse model. The miR-218-5p agomir used for overexpression was delivered into the substantia nigra (SN) by bilateral stereotaxic infusions. The loss of dopaminergic (DA) neurons and microglial inflammation in the SN was determined using Western blotting and immunofluorescence. Motor function was assessed using the rotarod test. RNA sequencing (RNA-seq) was performed to explore the pathways regulated by miR-218-5p. The target genes of miR-218-5p were predicted using TargetScan and confirmed using dual luciferase reporter assays. The effects of miR-218-5p on microglial inflammation and related pathways were verified in murine microglia-like BV2 cells. To stimulate BV2 cells, SH-SY5Y cells were treated with 1-methyl-4-phenylpyridinium (MPP+) and the conditioned media (CM) were collected.

RESULTS

MiR-218-5p expression was reduced in both the SN of MPTP-induced mice and MPP+-treated BV2 cells. MiR-218-5p overexpression significantly alleviated MPTP-induced microglial inflammation, loss of DA neurons, and motor dysfunction. RNA sequence and gene set enrichment analysis showed that type I interferon (IFN-I) pathways were upregulated in MPTP-induced mice, while this upregulation was reversed by miR-218-5p overexpression. A luciferase reporter assay verified that Ddx41 was a target gene of miR-218-5p. In vitro, miR-218-5p overexpression or Ddx41 knockdown inhibited the IFN-I response and expression of inflammatory cytokines in BV2 cells stimulated with MPP+-CM.

CONCLUSIONS

MiR-218-5p suppresses microglia-mediated neuroinflammation and preserves DA neurons via Ddx41/IFN-I. Hence, miR-218-5p-Ddx41 is a promising therapeutic target for PD.

The Importance of Complement-Mediated Immune Signaling in Alzheimer's Disease Pathogenesis

As an essential component of our innate immune system, the complement system is responsible for our defense against pathogens. The complement cascade has complex roles in the central nervous system (CNS), most of what we know about it stems from its role in brain development. However, in recent years, numerous reports have implicated the classical complement cascade in both brain development and decline. More specifically, complement dysfunction has been implicated in neurodegenerative disorders, such as Alzheimer's disease (AD), which is the most common form of dementia. Synapse loss is one of the main pathological hallmarks of AD and correlates with memory impairment. Throughout the course of AD progression, synapses are tagged with complement proteins and are consequently removed by microglia that express complement receptors. Notably, astrocytes are also capable of secreting signals that induce the expression of complement proteins in the CNS. Both astrocytes and microglia are implicated in neuroinflammation, another hallmark of AD pathogenesis. In this review, we provide an overview of previously known and newly established roles for the complement cascade in the CNS and we explore how complement interactions with microglia, astrocytes, and other risk factors such as TREM2 and ApoE4 modulate the processes of neurodegeneration in both amyloid and tau models of AD.

Dysregulated neuroimmune interactions and sustained type I interferon signaling after human immunodeficiency virus type 1 infection of human iPSC derived microglia and cerebral organoids

Human immunodeficiency virus type-1 (HIV-1) associated neurocognitive disorder (HAND) affects up to half of HIV-1 positive patients with long term neurological consequences, including dementia. There are no effective therapeutics for HAND because the pathophysiology of HIV-1 induced glial and neuronal functional deficits in humans remains enigmatic. To bridge this knowledge gap, we established a model simulating HIV-1 infection in the central nervous system using human induced pluripotent stem cell (iPSC) derived microglia combined with sliced neocortical organoids. Upon incubation with two replication-competent macrophage-tropic HIV-1 strains (JRFL and YU2), we observed that microglia not only became productively infected but also exhibited inflammatory activation. RNA sequencing revealed a significant and sustained activation of type I interferon signaling pathways. Incorporating microglia into sliced neocortical organoids extended the effects of aberrant type I interferon signaling in a human neural context. Collectively, our results illuminate the role of persistent type I interferon signaling in HIV-1 infected microglial in a human neural model, suggesting its potential significance in the pathogenesis of HAND.

Long 3'UTRs predispose neurons to inflammation by promoting immunostimulatory double-stranded RNA formation

Loss of RNA homeostasis underlies numerous neurodegenerative and neuroinflammatory diseases. However, the molecular mechanisms that trigger neuroinflammation are poorly understood. Viral double-stranded RNA (dsRNA) triggers innate immune responses when sensed by host pattern recognition receptors (PRRs) present in all cell types. Here, we report that human neurons intrinsically carry exceptionally high levels of immunostimulatory dsRNAs and identify long 3'UTRs as giving rise to neuronal dsRNA structures. We found that the neuron-enriched ELAVL family of genes (ELAVL2, ELAVL3, and ELAVL4) can increase (i) 3'UTR length, (ii) dsRNA load, and (iii) activation of dsRNA-sensing PRRs such as MDA5, PKR, and TLR3. In wild-type neurons, neuronal dsRNAs signaled through PRRs to induce tonic production of the antiviral type I interferon. Depleting ELAVL2 in WT neurons led to global shortening of 3'UTR length, reduced immunostimulatory dsRNA levels, and rendered WT neurons susceptible to herpes simplex virus and Zika virus infection. Neurons deficient in ADAR1, a dsRNA-editing enzyme mutated in the neuroinflammatory disorder Aicardi-Goutières syndrome, exhibited intolerably high levels of dsRNA that triggered PRR-mediated toxic inflammation and neuronal death. Depleting ELAVL2 in ADAR1 knockout neurons led to prolonged neuron survival by reducing immunostimulatory dsRNA levels. In summary, neurons are specialized cells where PRRs constantly sense "self" dsRNAs to preemptively induce protective antiviral immunity, but maintaining RNA homeostasis is paramount to prevent pathological neuroinflammation.

Microglia mediated neuroinflammation in neurodegenerative diseases: A review on the cell signaling pathways involved in microglial activation

Microglia, the immune sentinels of the central nervous system (CNS), have emerged to be the central players in many neurological and neurodegenerative diseases. Recent studies on large genome databases and omics studies in fact provide support to the idea that microglial cells could be the drivers of these diseases. Microglial cells have the capacity to undergo morphological and phenotypic transformations depending on its microenvironment. From the homeostatic ramified state, they can shift their phenotypes between the two extremes, known as the proinflammatory M1 and anti-inflammatory M2 phenotype, with intermediate transitional states, characterized by different transcriptional signature and release of inflammatory mediators. The temporal regulation of the release of the inflammatory factors are critical for damage control and steering the microglia back towards homeostatic conditions. A dysregulation in these can lead to excessive tissue damage and neuronal death. Therefore, targeting the cell signaling pathways that are the underpinnings of microglial modulations are considered to be an important avenue for treatment of various neurodegenerative diseases. In this review we have discussed various signaling pathways that trigger microglial activation from its ramified state and highlight the mechanisms of microglia-mediated neuroinflammation that are associated with various neurodegenerative diseases. Most of the cellular factors that drive microglia towards a proinflammatory phenotype are components of the immune system signaling pathways and cell proliferation, along with certain ion channels. The anti-inflammatory phenotype is mainly elicited by purinoceptors, metabolic receptors and other receptors that primarily suppress the production proinflammatory mediators.

Emerging roles for ITAM and ITIM receptor signaling in microglial biology and Alzheimer's disease-related amyloidosis

Microglia are critical responders to amyloid beta (Aβ) plaques in Alzheimer's disease (AD). Therefore, the therapeutic targeting of microglia in AD is of high clinical interest. While previous investigation has focused on the innate immune receptors governing microglial functions in response to Aβ plaques, how microglial innate immune responses are regulated is not well understood. Interestingly, many of these microglial innate immune receptors contain unique cytoplasmic motifs, termed immunoreceptor tyrosine-based activating and inhibitory motifs (ITAM/ITIM), that are commonly known to regulate immune activation and inhibition in the periphery. In this review, we summarize the diverse functions employed by microglia in response to Aβ plaques and also discuss the innate immune receptors and intracellular signaling players that guide these functions. Specifically, we focus on the role of ITAM and ITIM signaling cascades in regulating microglia innate immune responses. A better understanding of how microglial innate immune responses are regulated in AD may provide novel therapeutic avenues to tune the microglial innate immune response in AD pathology.

The association of microbial infection and adaptive immune cell activation in Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder and the most common form of dementia. Early symptoms include the loss of memory and mild cognitive ability; however, as the disease progresses, these symptoms can present with increased severity manifesting as mood and behaviour changes, disorientation, and a loss of motor/body control. AD is one of the leading causes of death in the UK, and with an ever-increasing ageing society, patient numbers are predicted to rise posing a significant global health emergency. AD is a complex neurophysiological disorder where pathology is characterized by the deposition and aggregation of misfolded amyloid-beta (Aβ)-protein that in-turn promotes excessive tau-protein production which together drives neuronal cell dysfunction, neuroinflammation, and neurodegeneration. It is widely accepted that AD is driven by a combination of both genetic and immunological processes with recent data suggesting that adaptive immune cell activity within the parenchyma occurs throughout disease. The mechanisms behind these observations remain unclear but suggest that manipulating the adaptive immune response during AD may be an effective therapeutic strategy. Using immunotherapy for AD treatment is not a new concept as the only two approved treatments for AD use antibody-based approaches to target Aβ. However, these have been shown to only temporarily ease symptoms or slow progression highlighting the urgent need for newer treatments. This review discusses the role of the adaptive immune system during AD, how microbial infections may be contributing to inflammatory immune activity and suggests how adaptive immune processes can pose as therapeutic targets for this devastating disease.

Endothelial type I interferon response and brain diseases: identifying STING as a therapeutic target

The endothelium layer lining the inner surface of blood vessels serves relevant physiological functions in all body systems, including the exchanges between blood and extravascular space. However, endothelial cells also participate in innate and adaptive immune response that contribute to the pathophysiology of inflammatory disorders. Type I Interferon (IFN) signaling is an inflammatory response triggered by a variety of pathogens, but it can also be induced by misplaced DNA in the cytosol caused by cell stress or gene mutations. Type I IFN produced by blood leukocytes or by the endothelium itself is well-known to activate the interferon receptor (IFNAR) in endothelial cells. Here, we discuss the induction of type I IFN secretion and signaling in the endothelium, specifically in the brain microvasculature where endothelial cells participate in the tight blood-brain barrier (BBB). This barrier is targeted during neuroinflammatory disorders such as infection, multiple sclerosis, Alzheimer's disease and traumatic brain injury. We focus on type I IFN induction through the cGAS-STING activation pathway in endothelial cells in context of autoinflammatory type I interferonopathies, inflammation and infection. By comparing the pathophysiology of two separate infectious diseases-cerebral malaria induced by Plasmodium infection and COVID-19 caused by SARS-CoV-2 infection-we emphasize the relevance of type I IFN and STING-induced vasculopathy in organ dysfunction. Investigating the role of endothelial cells as active type I IFN producers and responders in disease pathogenesis could lead to new therapeutic targets. Namely, endothelial dysfunction and brain inflammation may be avoided with strategies that target excessive STING activation in endothelial cells.

Mechanistic insights into the role of herpes simplex virus 1 in Alzheimer's disease

Alzheimer's Disease (AD) is an aging-associated neurodegenerative disorder, threatening millions of people worldwide. The onset and progression of AD can be accelerated by environmental risk factors, such as bacterial and viral infections. Human herpesviruses are ubiquitous infectious agents that underpin numerous inflammatory disorders including neurodegenerative diseases. Published studies concerning human herpesviruses in AD imply an active role HSV-1 in the pathogenesis of AD. This review will summarize the current understanding of HSV-1 infection in AD and highlight some barriers to advance this emerging field.

Selective neuroimmune modulation by type I interferon drives neuropathology and neurologic dysfunction following traumatic brain injury

Accumulating evidence suggests that type I interferon (IFN-I) signaling is a key contributor to immune cell-mediated neuropathology in neurodegenerative diseases. Recently, we demonstrated a robust upregulation of type I interferon-stimulated genes in microglia and astrocytes following experimental traumatic brain injury (TBI). The specific molecular and cellular mechanisms by which IFN-I signaling impacts the neuroimmune response and neuropathology following TBI remains unknown. Using the lateral fluid percussion injury model (FPI) in adult male mice, we demonstrated that IFN α/β receptor (IFNAR) deficiency resulted in selective and sustained blockade of type I interferon-stimulated genes following TBI as well as decreased microgliosis and monocyte infiltration. Molecular alteration of reactive microglia also occurred with diminished expression of genes needed for MHC class I antigen processing and presentation following TBI. This was associated with decreased accumulation of cytotoxic T cells in the brain. The IFNAR-dependent modulation of the neuroimmune response was accompanied by protection from secondary neuronal death, white matter disruption, and neurobehavioral dysfunction. These data support further efforts to leverage the IFN-I pathway for novel, targeted therapy of TBI.

MAPK/MAK/MRK overlapping kinase (MOK) controls microglial inflammatory/type-I IFN responses via Brd4 and is involved in ALS

Amyotrophic lateral sclerosis (ALS) is a fatal and incurable neurodegenerative disease affecting motor neurons and characterized by microglia-mediated neurotoxic inflammation whose underlying mechanisms remain incompletely understood. In this work, we reveal that MAPK/MAK/MRK overlapping kinase (MOK), with an unknown physiological substrate, displays an immune function by controlling inflammatory and type-I interferon (IFN) responses in microglia which are detrimental to primary motor neurons. Moreover, we uncover the epigenetic reader bromodomain-containing protein 4 (Brd4) as an effector protein regulated by MOK, by promoting Ser492-phospho-Brd4 levels. We further demonstrate that MOK regulates Brd4 functions by supporting its binding to cytokine gene promoters, therefore enabling innate immune responses. Remarkably, we show that MOK levels are increased in the ALS spinal cord, particularly in microglial cells, and that administration of a chemical MOK inhibitor to ALS model mice can modulate Ser492-phospho-Brd4 levels, suppress microglial activation, and modify the disease course, indicating a pathophysiological role of MOK kinase in ALS and neuroinflammation.

Viruses - a major cause of amyloid deposition in the brain

INTRODUCTION

Clinically, Alzheimer's disease (AD) is a syndrome with a spectrum of various cognitive disorders. There is a complete dissociation between the pathology and the clinical presentation. Therefore, we need a disruptive new approach to be able to prevent and treat AD.

AREAS COVERED

In this review, the authors extensively discuss the evidence why the amyloid beta is not the pathological cause of AD which makes therefore the amyloid hypothesis not sustainable anymore. They review the experimental evidence underlying the role of microbes, especially that of viruses, as a trigger/cause for the production of amyloid beta leading to the establishment of a chronic neuroinflammation as the mediator manifesting decades later by AD as a clinical spectrum. In this context, the emergence and consequences of the infection/antimicrobial protection hypothesis are described. The epidemiological and clinical data supporting this hypothesis are also analyzed.

EXPERT OPINION

For decades, we have known that viruses are involved in the pathogenesis of AD. This discovery was ignored and discarded for a long time. Now we should accept this fact, which is not a hypothesis anymore, and stimulate the research community to come up with new ideas, new treatments, and new concepts.

Selective neuroimmune modulation by type I interferon drives neuropathology and neurologic dysfunction following traumatic brain injury

Accumulating evidence suggests that type I interferon (IFN-I) signaling is a key contributor to immune cell-mediated neuropathology in neurodegenerative diseases. Recently, we demonstrated a robust upregulation of type I interferon-stimulated genes in microglia and astrocytes following experimental traumatic brain injury (TBI). The specific molecular and cellular mechanisms by which IFN-I signaling impacts the neuroimmune response and neuropathology following TBI remains unknown. Using the lateral fluid percussion injury model (FPI) in adult male mice, we demonstrated that IFN α/β receptor (IFNAR) deficiency resulted in selective and sustained blockade of type I interferon-stimulated genes following TBI as well as decreased microgliosis and monocyte infiltration. Phenotypic alteration of reactive microglia also occurred with diminished expression of molecules needed for MHC class I antigen processing and presentation following TBI. This was associated with decreased accumulation of cytotoxic T cells in the brain. The IFNAR-dependent modulation of the neuroimmune response was accompanied by protection from secondary neuronal death, white matter disruption, and neurobehavioral dysfunction. These data support further efforts to leverage the IFN-I pathway for novel, targeted therapy of TBI.

Microglial targeted therapy relieves cognitive impairment caused by Cntnap4 deficiency

Contactin-associated protein-like 4 (Cntnap4) is critical for GABAergic transmission in the brain. Impaired Cntnap4 function is implicated in neurological disorders, such as autism; however, the role of Cntnap4 on memory processing is poorly understood. Here, we demonstrate that hippocampal Cntnap4 deficiency in female mice manifests as impaired cognitive function and synaptic plasticity. The underlying mechanisms may involve effects on the pro-inflammatory response resulting in dysfunctional GABAergic transmission and activated tryptophan metabolism. To efficiently and accurately inhibit the pro-inflammatory reaction, we established a biomimetic microglial nanoparticle strategy to deliver FDA-approved PLX3397 (termed MNPs@PLX). We show MNPs@PLX successfully penetrates the blood brain barrier and facilitates microglial-targeted delivery of PLX3397. Furthermore, MNPs@PLX attenuates cognitive decline, dysfunctional synaptic plasticity, and pro-inflammatory response in female heterozygous Cntnap4 knockout mice. Together, our findings show loss of Cntnap4 causes pro-inflammatory cognitive decline that is effectively prevented by supplementation with microglia-specific inhibitors; thus validating the targeting of microglial function as a therapeutic intervention in neurocognitive disorders.

Novel CH25H+ and OASL+ microglia subclusters play distinct roles in cerebral ischemic stroke

BACKGROUND

Microglial polarization is one of the most promising therapeutic targets for multiple central nervous system (CNS) disorders, including ischemic stroke. However, detailed transcriptional alteration of microglia following cerebral ischemic stroke remains largely unclear.

METHODS

Focal cerebral ischemia was induced by transient middle cerebral artery occlusion (tMCAO) for 60 min in mice. Single-cell RNA sequencing (scRNA-seq) was performed using ischemic brain tissues from tMCAO and sham mice 3 days after surgery. Ch25h^-/- mice were used to investigate the role of specific microglia subcluster on post-stroke infarct volume and neuroinflammation.

RESULTS

We identified a relatively homeostatic subcluster with enhanced antigen processing and three "ischemic stroke associated microglia" (ISAM): MKI67⁺, CH25H⁺ and OASL⁺ subclusters. We found the MKI67⁺ subcluster undergo proliferation and differentiation into CH25H⁺ and OASL⁺ subclusters. CH25H⁺ microglia was a critical subcluster of ISAM that exhibited increased phagocytosis and neuroprotective property after stroke. Ch25h^-/- mice developed significantly increased infarct volume following ischemic stroke compared to Ch25h^+/-. Meanwhile, the OASL⁺ subcluster accumulated in the ischemic brain and was associated with the evolving of neuroinflammation after stroke, which was further aggravated in the aged mice brain.

CONCLUSIONS

Our data reveal previously unrecognized roles of the newly defined CH25H⁺ and OASL⁺ microglia subclusters following ischemic stroke, with novel insights for precise microglia modulation towards stroke therapy.

Cytokines as emerging regulators of central nervous system synapses

Cytokines are key messengers by which immune cells communicate, and they drive many physiological processes, including immune and inflammatory responses. Early discoveries demonstrated that cytokines, such as the interleukin family members and TNF-α, regulate synaptic scaling and plasticity. Still, we continue to learn more about how these traditional immune system cytokines affect neuronal structure and function. Different cytokines shape synaptic function on multiple levels ranging from fine-tuning neurotransmission, to regulating synapse number, to impacting global neuronal networks and complex behavior. These recent findings have cultivated an exciting and growing field centered on the importance of immune system cytokines for regulating synapse and neural network structure and function. Here, we highlight the latest findings related to cytokines in the central nervous system and their regulation of synapse structure and function. Moreover, we explore how these mechanisms are becoming increasingly important to consider in diseases-especially those with a large neuroinflammatory component.

Transforming the understanding of brain immunity

Contemporary studies have completely changed the view of brain immunity from envisioning the brain as isolated and inaccessible to peripheral immune cells to an organ in close physical and functional communication with the immune system for its maintenance, function, and repair. Circulating immune cells reside in special niches in the brain's borders, the choroid plexus, meninges, and perivascular spaces, from which they patrol and sense the brain in a remote manner. These niches, together with the meningeal lymphatic system and skull microchannels, provide multiple routes of interaction between the brain and the immune system, in addition to the blood vasculature. In this Review, we describe current ideas about brain immunity and their implications for brain aging, diseases, and immune-based therapeutic approaches.

An aging, pathology burden, and glial senescence build-up hypothesis for late onset Alzheimer's disease

Alzheimer's disease (AD) predominantly occurs as a late onset (LOAD) form involving neurodegeneration and cognitive decline with progressive memory loss. Risk factors that include aging promote accumulation of AD pathologies, such as amyloid-beta and tau aggregates, as well as inflammation and oxidative stress. Homeostatic glial states regulate and suppress pathology buildup; inflammatory states exacerbate pathology by releasing pro-inflammatory cytokines. Multiple stresses likely induce glial senescence, which could decrease supportive functions and reinforce inflammation. In this perspective, we hypothesize that aging first drives AD pathology burden, whereafter AD pathology putatively induces glial senescence in LOAD. We hypothesize that increasing glial senescence, particularly local senescent microglia accumulation, sustains and drives perpetuating buildup and spread of AD pathologies, glial aging, and further senescence. We predict that increasing glial senescence, particularly local senescent microglia accumulation, also transitions individuals from healthy cognition into mild cognitive impairment and LOAD diagnosis. These pathophysiological underpinnings may centrally contribute to LOAD onset, but require further mechanistic investigation.