A complement-microglial axis drives synapse loss during virus-induced memory impairment

Microglia aggravate white matter injury via C3/C3aR pathway after experimental subarachnoid hemorrhage

The activation of glial cells is intimately associated with the pathophysiology of neuroinflammation and white matter injury (WMI) during both acute and chronic phases following subarachnoid hemorrhage (SAH). The complement C3a receptor (C3aR) has a dual role in modulating inflammation and contributes to neurodevelopment, neuroplasticity, and neurodegeneration. However, its impact on WMI in the context of SAH remains unclear. In this study, 175 male C57BL/6J mice underwent SAH through endovascular perforation. Oxyhemoglobin (oxy-Hb) was employed to simulate SAH in vitro. A suite of techniques, including immunohistochemistry, transcriptomic sequencing, and a range of molecular biotechnologies, were utilized to evaluate the activation of the C3-C3aR pathway on microglial polarization and WMI. Results revealed that post-SAH abnormal activation of microglia was accompanied by upregulation of complement C3 and C3aR. The inhibition of C3aR decreased abnormal microglial activation, attenuated neuroinflammation, and ameliorated WMI and cognitive deficits following SAH. RNA-Seq indicated that C3aR inhibition downregulated several immune and inflammatory pathways and mitigated cellular injury by reducing p53-induced death domain protein 1 (Pidd1) and Protein kinase RNA-like ER kinase (Perk) expression, two factors mainly function in sensing and responding to cellular stress and endoplasmic reticulum (ER) stress. The deleterious effects of the C3-C3aR axis in the context of SAH may be related to endoplasmic reticulum (ER) stress-dependent cellular injury and inflammasome formation. Agonists of Perk can exacerbate the cellular injury and neuroinflammation, which was attenuated by C3aR inhibition after SAH. Additionally, intranasal administration of C3a during the subacute phase of SAH was found to decrease astrocyte reactivity and alleviate cognitive deficits post-SAH. This research deepens our understanding of the complex pathophysiology of WMI following SAH and underscores the therapeutic potential of C3a treatment in promoting white matter repair and enhancing functional recovery prognosis. These insights pave the way for future clinical application of C3a-based therapies, promising significant benefits in the treatment of SAH and its related complications.

Accumulation of extracellular elastin-derived peptides disturbed neuronal morphology and neuron-microglia crosstalk in aged brain

Extracellular elastin-derived peptides (EDPs) accumulate in the aging brain and have been associated with vascular dementia and Alzheimer's disease (AD). The activation of inflammatory processes in glial cells with EDP treatment has received attention, but not in neurons. To properly understand EDPs' pathogenic significance, the impact on neuronal function and neuron-microglia crosstalk was explored further. Among the EDP molecules, Val-Gly-Val-Ala-Pro-Gly (VGVAPG) is a typical repeating hexapeptide. Here, we observed that EDPs-VGVAPG influenced neuronal survival and morphology in a dose-dependent manner. High concentrations of VGVAPG induced synapse loss and microglia hyperactivation in vivo and in vitro. Following EDP incubation, galectin 3 (Gal-3) released by neurons served as a chemokine, attracting microglial engulfment. Blocking Gal-3 and EDP binding remedied synapse loss in neurons and phagocytosis in microglia. In response to the accumulation of EDPs, proteomics in matrix remodeling and cytoskeleton dynamics, such as a disintegrin and metalloproteinase (ADAM) family, were engaged. These findings in extracellular EDPs provided more evidence for the relationship between aging and neuron dysfunction, increasing the insight of neuroinflammatory responses and the development of new specialized extracellular matrix remolding-targeted therapy options for dementia or other neurodegenerative disease.

Changes in neuroinflammatory biomarkers correlate with disease severity and neuroimaging alterations in patients with COVID-19 neurological complications

COVID-19 induces acute and persistent neurological symptoms in mild and severe cases. Proposed concomitant mechanisms include direct viral infection and strain, coagulopathy, hypoxia, and neuroinflammation. However, underlying molecular alterations associated with multiple neurological outcomes in both mild and severe cases are majorly unexplored. To illuminate possible mechanisms leading to COVID-19 neurological disease, we retrospectively investigated in detail a cohort of 35 COVID-19 mild and severe hospitalized patients presenting neurological alterations subject to clinically indicated cerebrospinal fluid (CSF) sampling. Clinical and neurological investigation, brain imaging, viral sequencing, and cerebrospinal CSF analyses were carried out. We found that COVID-19 patients presented heterogeneous neurological symptoms dissociated from lung burden. Nasal swab viral sequencing revealed a dominant strain at the time of the study, and we could not detect traces of SARS-CoV-2's spike protein in patients' CSF by multiple reaction monitoring analysis. Patients presented ubiquitous systemic hyper-inflammation and broad alterations in CSF proteomics related to inflammation, innate immunity, and hemostasis, irrespective of COVID-19 severity or neuroimaging alterations. Elevated CSF interleukin-6 (IL6) correlated with disease severity (sex-, age-, and comorbidity-adjusted mean Severe 24.5 pg/ml, 95% confidence interval (CI) 9.62-62.23 vs. Mild 3.91 pg/mL CI 1.5-10.3 patients, p = 0.019). CSF tumor necrosis factor-alpha (TNFα) and IL6 levels were higher in patients presenting pronounced neuroimaging alterations compared to those who did not (sex-, age-, and comorbidity-adjusted mean TNFα Pronounced 3.4, CI 2.4-4.4 vs. Non-Pronounced 2.0, CI 1.4-2.5, p = 0.022; IL6 Pronounced 33.11, CI 8.89-123.31 vs Non-Pronounced 6.22, CI 2.9-13.34, p = 0.046). Collectively, our findings put neuroinflammation as a possible driver of COVID-19 acute neurological disease in mild and severe cases.

Triggering Receptor Expressed on Myeloid Cells 2 Alleviated Sevoflurane-Induced Developmental Neurotoxicity via Microglial Pruning of Dendritic Spines in the CA1 Region of the Hippocampus

Sevoflurane induces developmental neurotoxicity in mice; however, the underlying mechanisms remain unclear. Triggering receptor expressed on myeloid cells 2 (TREM2) is essential for microglia-mediated synaptic refinement during the early stages of brain development. We explored the effects of TREM2 on dendritic spine pruning during sevoflurane-induced developmental neurotoxicity in mice. Mice were anaesthetized with sevoflurane on postnatal days 6, 8, and 10. Behavioral performance was assessed using the open field test and Morris water maze test. Genetic knockdown of TREM2 and overexpression of TREM2 by stereotaxic injection were used for mechanistic experiments. Western blotting, immunofluorescence, electron microscopy, three-dimensional reconstruction, Golgi staining, and whole-cell patch-clamp recordings were performed. Sevoflurane exposures upregulated the protein expression of TREM2, increased microglia-mediated pruning of dendritic spines, and reduced synaptic multiplicity and excitability of CA1 neurons. TREM2 genetic knockdown significantly decreased dendritic spine pruning, and partially aggravated neuronal morphological abnormalities and cognitive impairments in sevoflurane-treated mice. In contrast, TREM2 overexpression enhanced microglia-mediated pruning of dendritic spines and rescued neuronal morphological abnormalities and cognitive dysfunction. TREM2 exerts a protective role against neurocognitive impairments in mice after neonatal exposures to sevoflurane by enhancing microglia-mediated pruning of dendritic spines in CA1 neurons. This provides a potential therapeutic target in the prevention of sevoflurane-induced developmental neurotoxicity.

Orchestration of antiviral responses within the infected central nervous system

Many newly emerging and re-emerging viruses have neuroinvasive potential, underscoring viral encephalitis as a global research priority. Upon entry of the virus into the CNS, severe neurological life-threatening conditions may manifest that are associated with high morbidity and mortality. The currently available therapeutic arsenal against viral encephalitis is rather limited, emphasizing the need to better understand the conditions of local antiviral immunity within the infected CNS. In this review, we discuss new insights into the pathophysiology of viral encephalitis, with a focus on myeloid cells and CD8+ T cells, which critically contribute to protection against viral CNS infection. By illuminating the prerequisites of myeloid and T cell activation, discussing new discoveries regarding their transcriptional signatures, and dissecting the mechanisms of their recruitment to sites of viral replication within the CNS, we aim to further delineate the complexity of antiviral responses within the infected CNS. Moreover, we summarize the current knowledge in the field of virus infection and neurodegeneration and discuss the potential links of some neurotropic viruses with certain pathological hallmarks observed in neurodegeneration.

West Nile Virus-Induced Expression of Senescent Gene Lgals3bp Regulates Microglial Phenotype within Cerebral Cortex

Microglia, the resident macrophages of the central nervous system, exhibit altered gene expression in response to various neurological conditions. This study investigates the relationship between West Nile Virus infection and microglial senescence, focusing on the role of LGALS3BP, a protein implicated in both antiviral responses and aging. Using spatial transcriptomics, RNA sequencing and flow cytometry, we characterized changes in microglial gene signatures in adult and aged mice following recovery from WNV encephalitis. Additionally, we analyzed Lgals3bp expression and generated Lgals3bp-deficient mice to assess the impact on neuroinflammation and microglial phenotypes. Our results show that WNV-activated microglia share transcriptional signatures with aged microglia, including upregulation of genes involved in interferon response and inflammation. Lgals3bp was broadly expressed in the CNS and robustly upregulated during WNV infection and aging. Lgals3bp-deficient mice exhibited reduced neuroinflammation, increased homeostatic microglial numbers, and altered T cell populations without differences in virologic control or survival. These data indicate that LGALS3BP has a role in regulating neuroinflammation and microglial activation and suggest that targeting LGALS3BP might provide a potential route for mitigating neuroinflammation-related cognitive decline in aging and post-viral infections.

A cell autonomous regulator of neuronal excitability modulates tau in Alzheimer's disease vulnerable neurons

Neurons from layer II of the entorhinal cortex (ECII) are the first to accumulate tau protein aggregates and degenerate during prodromal Alzheimer's disease. Gaining insight into the molecular mechanisms underlying this vulnerability will help reveal genes and pathways at play during incipient stages of the disease. Here, we use a data-driven functional genomics approach to model ECII neurons in silico and identify the proto-oncogene DEK as a regulator of tau pathology. We show that epigenetic changes caused by Dek silencing alter activity-induced transcription, with major effects on neuronal excitability. This is accompanied by the gradual accumulation of tau in the somatodendritic compartment of mouse ECII neurons in vivo, reactivity of surrounding microglia, and microglia-mediated neuron loss. These features are all characteristic of early Alzheimer's disease. The existence of a cell-autonomous mechanism linking Alzheimer's disease pathogenic mechanisms in the precise neuron type where the disease starts provides unique evidence that synaptic homeostasis dysregulation is of central importance in the onset of tau pathology in Alzheimer's disease.

CSMD1 regulates brain complement activity and circuit development

Complement proteins facilitate synaptic elimination during neurodevelopmental pruning, but neural complement regulation is not well understood. CUB and Sushi Multiple Domains 1 (CSMD1) can regulate complement activity in vitro, is expressed in the brain, and is associated with increased schizophrenia risk. Beyond this, little is known about CSMD1 including whether it regulates complement activity in the brain or otherwise plays a role in neurodevelopment. We used biochemical, immunohistochemical, and proteomic techniques to examine the regional, cellular, and subcellular distribution as well as protein interactions of CSMD1 in the brain. To evaluate whether CSMD1 is involved in complement-mediated synapse elimination, we examined Csmd1-knockout mice and CSMD1-knockout human stem cell-derived neurons. We interrogated synapse and circuit development of the mouse visual thalamus, a process that involves complement pathway activity. We also quantified complement deposition on synapses in mouse visual thalamus and on cultured human neurons. Finally, we assessed uptake of synaptosomes by cultured microglia. We found that CSMD1 is present at synapses and interacts with complement proteins in the brain. Mice lacking Csmd1 displayed increased levels of complement component C3, an increased colocalization of C3 with presynaptic terminals, fewer retinogeniculate synapses, and aberrant segregation of eye-specific retinal inputs to the visual thalamus during the critical period of complement-dependent refinement of this circuit. Loss of CSMD1 in vivo enhanced synaptosome engulfment by microglia in vitro, and this effect was dependent on activity of the microglial complement receptor, CR3. Finally, human stem cell-derived neurons lacking CSMD1 were more vulnerable to complement deposition. These data suggest that CSMD1 can function as a regulator of complement-mediated synapse elimination in the brain during development.

Vaccination reduces central nervous system IL-1β and memory deficits after COVID-19 in mice

Up to 25% of individuals infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) exhibit postacute cognitive sequelae. Although millions of cases of coronavirus disease 2019 (COVID-19)-mediated memory dysfunction are accumulating worldwide, the underlying mechanisms and how vaccination lowers risk are unknown. Interleukin-1 (IL-1), a key component of innate immune defense against SARS-CoV-2 infection, is elevated in the hippocampi of individuals with COVID-19. Here we show that intranasal infection of C57BL/6J mice with SARS-CoV-2 Beta variant leads to central nervous system infiltration of Ly6Chi monocytes and microglial activation. Accordingly, SARS-CoV-2, but not H1N1 influenza virus, increases levels of brain IL-1β and induces persistent IL-1R1-mediated loss of hippocampal neurogenesis, which promotes postacute cognitive deficits. Vaccination with a low dose of adenoviral-vectored spike protein prevents hippocampal production of IL-1β during breakthrough SARS-CoV-2 infection, loss of neurogenesis and subsequent memory deficits. Our study identifies IL-1β as one potential mechanism driving SARS-CoV-2-induced cognitive impairment in a new mouse model that is prevented by vaccination.

HIV-1 infection promotes neuroinflammation and neuron pathogenesis in novel microglia-containing cerebral organoids

Cerebral organoids (COs) are a valuable tool to study the intricate interplay between glial cells and neurons in brain development and disease, including HIV-associated neuroinflammation. We developed a novel approach to generate microglia containing COs (CO-iMs) by co-culturing hematopoietic progenitors and induced pluripotent stem cells. This approach allowed for the differentiation of microglia within the organoids concomitantly to the neuronal progenitors. CO- iMs exhibited higher efficiency in generation of CD45 + /CD11b + /Iba-1 + microglia cells compared to conventional COs with physiologically relevant proportion of microglia (∼7%). CO-iMs exhibited substantially higher expression of microglial homeostatic and sensome markers as well as markers for the complement cascade. CO-iMs showed susceptibility to HIV infection resulting in a significant increase in several pro-inflammatory cytokines/chemokines and compromised neuronal function, which were abrogated by addition of antiretrovirals. Thus, CO-iM is a robust model to decipher neuropathogenesis, neurological disorders, and viral infections of brain cells in a 3D culture system.

Long COVID: From olfactory dysfunctions to viral Parkinsonism

Neurological and psychiatric complications continue to be a public health concern in long coronavirus disease 2019 (COVID-19). This varies from olfactory dysfunctions such as parosmia to cognitive and emotional challenges. Historically, the surge of neurological disorders followed the viral pandemics, for example, the emergence of Encephalitis Lethargica after the outbreak of Spanish Influenza. During and after COVID-19 infection, the problems associated with the sense of smell and the reports of affected olfactory and limbic brain areas are leading to a growing concern about the similarity with the symptoms and the pattern of degeneration observed at the onset of Parkinson's disease and Alzheimer's disease. These reports reveal the essentiality of long-term studies of olfactory and cognitive functions in the post-COVID era and the experiments using animal models to dissect the neural basis of these complications. In this manuscript, we summarize the research reporting the potential correlation between neurological disorders and viral pandemic outbreaks with a historical perspective. Further, we discuss the studies providing evidence of neurodegeneration due to severe acute respiratory syndrome coronavirus 2 infection by focusing on viral Parkinsonism.

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.

Microglia in Ischemic Stroke: Pathogenesis Insights and Therapeutic Challenges

Ischemic stroke is the most common type of stroke, which is the main cause of death and disability on a global scale. As the primary immune cells in the brain that are crucial for preserving homeostasis of the central nervous system microenvironment, microglia have been found to exhibit dual or even multiple effects at different stages of ischemic stroke. The anti-inflammatory polarization of microglia and release of neurotrophic factors may provide benefits by promoting neurological recovery at the lesion in the early phase after ischemic stroke. However, the pro-inflammatory polarization of microglia and secretion of inflammatory factors in the later phase of injury may exacerbate the ischemic lesion, suggesting the therapeutic potential of modulating the balance of microglial polarization to predispose them to anti-inflammatory transformation in ischemic stroke. Microglia-mediated signaling crosstalk with other cells may also be key to improving functional outcomes following ischemic stroke. Thus, this review provides an overview of microglial functions and responses under physiological and ischemic stroke conditions, including microglial activation, polarization, and interactions with other cells. We focus on approaches that promote anti-inflammatory polarization of microglia, inhibit microglial activation, and enhance beneficial cell-to-cell interactions. These targets may hold promise for the creation of innovative therapeutic strategies.

Brain-derived neurotrophic factor from microglia regulates neuronal development in the medial prefrontal cortex and its associated social behavior

Microglia and brain-derived neurotrophic factor (BDNF) are essential for the neuroplasticity that characterizes critical developmental periods. The experience-dependent development of social behaviors-associated with the medial prefrontal cortex (mPFC)-has a critical period during the juvenile period in mice. However, whether microglia and BDNF affect social development remains unclear. Herein, we aimed to elucidate the effects of microglia-derived BDNF on social behaviors and mPFC development. Mice that underwent social isolation during p21-p35 had increased Bdnf in the microglia accompanied by reduced adulthood sociability. Additionally, transgenic mice overexpressing microglial Bdnf-regulated using doxycycline at different time points-underwent behavioral, electrophysiological, and gene expression analyses. In these mice, long-term overexpression of microglial BDNF impaired sociability and excessive mPFC inhibitory neuronal circuit activity. However, administering doxycycline to normalize BDNF from p21 normalized sociability and electrophysiological function in the mPFC, whereas normalizing BDNF from later ages (p45-p50) did not normalize electrophysiological abnormalities in the mPFC, despite the improved sociability. To evaluate the possible role of BDNF in human sociability, we analyzed the relationship between adverse childhood experiences and BDNF expression in human macrophages, a possible proxy for microglia. Results show that adverse childhood experiences positively correlated with BDNF expression in M2 but not M1 macrophages. In summary, our study demonstrated the influence of microglial BDNF on the development of experience-dependent social behaviors in mice, emphasizing its specific impact on the maturation of mPFC function, particularly during the juvenile period. Furthermore, our results propose a translational implication by suggesting a potential link between BDNF secretion from macrophages and childhood experiences in humans.

Basic Science of Neuroinflammation and Involvement of the Inflammatory Response in Disorders of the Nervous System

Neuroinflammation is a key immune response observed in many neurologic diseases. Although an appropriate immune response can be beneficial, aberrant activation of this response recruits excessive proinflammatory cells to cause damage. Because the central nervous system is separated from the periphery by the blood-brain barrier (BBB) that creates an immune-privileged site, it has its own unique immune cells and immune response. Moreover, neuroinflammation can compromise the BBB causing an influx of peripheral immune cells and factors. Recent advances have brought a deeper understanding of neuroinflammation that can be leveraged to develop more potent therapies and improve patient selection.

RIPK3 promotes neuronal survival by suppressing excitatory neurotransmission during CNS viral infection

While recent work has identified roles for immune mediators in the regulation of neural activity, the capacity for cell intrinsic innate immune signaling within neurons to influence neurotransmission remains poorly understood. However, the existing evidence linking immune signaling with neuronal function suggests that modulation of neurotransmission may serve previously undefined roles in host protection during infection of the central nervous system. Here, we identify a specialized function for RIPK3, a kinase traditionally associated with necroptotic cell death, in preserving neuronal survival during neurotropic flavivirus infection through the suppression of excitatory neurotransmission. We show that RIPK3 coordinates transcriptomic changes in neurons that suppress neuronal glutamate signaling, thereby desensitizing neurons to excitotoxic cell death. These effects occur independently of the traditional functions of RIPK3 in promoting necroptosis and inflammatory transcription. Instead, RIPK3 promotes phosphorylation of the key neuronal regulatory kinase CaMKII, which in turn activates the transcription factor CREB to drive a neuroprotective transcriptional program and suppress deleterious glutamatergic signaling. These findings identify an unexpected function for a canonical cell death protein in promoting neuronal survival during viral infection through the modulation of neuronal activity, highlighting new mechanisms of neuroimmune crosstalk.

Levels of neuronal pentraxin 2 in plasma is associated with cognitive function in patients with schizophrenia

RATIONALE

The precise diagnosis and treatment of cognitive impairment remains a major challenge in the field of schizophrenia (SCZ) research. Synaptic dysfunction and loss are thought to be closely related to the occurrence and development of SCZ and may be involved in cognitive dysfunction.

OBJECTIVES

The purpose of this study was to investigate whether neuronal pentraxins (NPTXs) plays a role in the etiology of SCZ and provide evidence of its possible therapeutic value a new target for drug development.

METHODS

We recruited 275 participants, of whom 148 were SCZ from psychiatric hospital and 127 healthy control (HC) subjects from communities. Plasma concentrations of NPTXs were measured in HC and SCZ at baseline and after 8 weeks of antipsychotic treatment. The MATRICS Cognitive Consensus Battery was used to evaluate cognitive function. Furthermore, the brain is parcellated into 246 subregions using the Brainnetome atlas, and we extracted regional white matter volumes from magnetic resonance images of the SCZ groups.

RESULTS

Plasma NPTX2 levels were significantly lower in SCZ compared with HC subjects, but were significantly raised in SCZ after 8 weeks of antipsychotic treatment compared to baseline. In addition, baseline plasma NPTX2 levels were positively correlated with cognitive performance.

CONCLUSIONS

These findings indicate that NPTX2 may reveal novel aspects of disease etiology and act as a promising target for new drug development.

The Influence of Microglia on Neuroplasticity and Long-Term Cognitive Sequelae in Long COVID: Impacts on Brain Development and Beyond

Microglial cells, the immune cells of the central nervous system, are key elements regulating brain development and brain health. These cells are fully responsive to stressors, microenvironmental alterations and are actively involved in the construction of neural circuits in children and the ability to undergo full experience-dependent plasticity in adults. Since neuroinflammation is a known key element in the pathogenesis of COVID-19, one might expect the dysregulation of microglial function to severely impact both functional and structural plasticity, leading to the cognitive sequelae that appear in the pathogenesis of Long COVID. Therefore, understanding this complex scenario is mandatory for establishing the possible molecular mechanisms related to these symptoms. In the present review, we will discuss Long COVID and its association with reduced levels of BDNF, altered crosstalk between circulating immune cells and microglia, increased levels of inflammasomes, cytokines and chemokines, as well as the alterations in signaling pathways that impact neural synaptic remodeling and plasticity, such as fractalkines, the complement system, the expression of SIRPα and CD47 molecules and altered matrix remodeling. Together, these complex mechanisms may help us understand consequences of Long COVID for brain development and its association with altered brain plasticity, impacting learning disabilities, neurodevelopmental disorders, as well as cognitive decline in adults.

Hereditary Connective Tissue Diseases and Risk of Post-Acute SARS-CoV-2

We completed a retrospective review of data collected by the JH-CROWN consortium based on ICD10 codes for a hospitalized cohort. The severity and prevalence of COVID-19 and development of PASC within heritable connective tissue diseases were unknown; however, clinical observation suggested a thorough examination was necessary. We compared rates of disease severity, death, and PASC in connective tissue diseases versus the entire cohort as well as in diabetes and hypertension to determine if connective tissue disease was a risk factor. Of the 15,676 patients in the database, 63 (0.40%) had a connective tissue disease, which is elevated relative to the distribution in the population, suggesting a higher risk of severe disease. Within these 63 patients, 9.52% developed PASC compared to 2.54% in the entire cohort (p < 0.005). Elucidation of populations at high risk for severe disease and development of PASC is integral to improving treatment approaches. Further, no other study to date has examined the risk in those with connective tissue diseases and these data support a need for enhanced awareness among physicians, patients, and the community.

Role of pyroptosis in the pathogenesis of various neurological diseases

Pyroptosis, an inflammatory programmed cell death process, has recently garnered significant attention due to its pivotal role in various neurological diseases. This review delves into the intricate molecular signaling pathways governing pyroptosis, encompassing both caspase-1 dependent and caspase-1 independent routes, while emphasizing the critical role played by the inflammasome machinery in initiating cell death. Notably, we explore the Nucleotide-binding domain leucine-rich repeat (NLR) containing protein family, the Absent in melanoma 2-like receptor family, and the Pyrin receptor family as essential activators of pyroptosis. Additionally, we comprehensively examine the Gasdermin family, renowned for their role as executioner proteins in pyroptosis. Central to our review is the interplay between pyroptosis and various central nervous system (CNS) cell types, including astrocytes, microglia, neurons, and the blood-brain barrier (BBB). Pyroptosis emerges as a significant factor in the pathophysiology of each cell type, highlighting its far-reaching impact on neurological diseases. This review also thoroughly addresses the involvement of pyroptosis in specific neurological conditions, such as HIV infection, drug abuse-mediated pathologies, Alzheimer's disease, and Parkinson's disease. These discussions illuminate the intricate connections between pyroptosis, chronic inflammation, and cell death in the development of these disorders. We also conducted a comparative analysis, contrasting pyroptosis with other cell death mechanisms, thereby shedding light on their unique aspects. This approach helps clarify the distinct contributions of pyroptosis to neuroinflammatory processes. In conclusion, this review offers a comprehensive exploration of the role of pyroptosis in various neurological diseases, emphasizing its multifaceted molecular mechanisms within various CNS cell types. By elucidating the link between pyroptosis and chronic inflammation in the context of neurodegenerative disorders and infections, it provides valuable insights into potential therapeutic targets for mitigating these conditions.

Cognitive impairment in Alzheimer's disease FAD4T mouse model: Synaptic loss facilitated by activated microglia via C1qA

Alzheimer's disease (AD) is a chronic and progressive neurodegenerative disorder characterized by cognitive dysfunction. The connection between neuroinflammation and abnormal synaptic function in AD is recognized, but the underlying mechanisms remain unclear. In this study, we utilized a mouse model of AD, FAD4T mice aged 6-7 months, to investigate the molecular changes affecting cognitive impairment. Behavior tests showed that FAD4T mice exhibited impaired spatial memory compared with their wild-type littermates. Immunofluorescence staining revealed the presence of Aβ plaques and abnormal glial cell activation as well as changes in microglial morphology in the cortex and hippocampus of FAD4T mice. Synaptic function was impaired in FAD4T mice. Patch clamp recordings of hippocampal neurons revealed reduced amplitude of miniature excitatory postsynaptic currents. Additionally, Golgi staining showed decreased dendritic spine density in the cortex and hippocampus of FAD4T mice, indicating aberrant synapse morphology. Moreover, hippocampal PSD-95 and NMDAR1 protein levels decreased in FAD4T mice. RNA-seq analysis revealed elevated expression of immune system and proinflammatory genes, including increased C1qA protein and mRNA levels, as well as higher expression of TNF-α and IL-18. Taken together, our findings suggest that excessive microglia activation mediated by complement factor C1qA may contribute to aberrant synaptic pruning, resulting in synapse loss and disrupted synaptic transmission, ultimately leading to AD pathogenesis and behavioral impairments in the FAD4T mouse model. Our study provides valuable insights into the underlying mechanisms of cognitive impairments and preliminarily explores a potentially effective treatment approach targeting on C1qA for AD.

The role of microglia heterogeneity in synaptic plasticity and brain disorders: Will sequencing shed light on the discovery of new therapeutic targets?

Microglia play a crucial role in interacting with neuronal synapses and modulating synaptic plasticity. This function is particularly significant during postnatal development, as microglia are responsible for removing excessive synapses to prevent neurodevelopmental deficits. Dysregulation of microglial synaptic function has been well-documented in various pathological conditions, notably Alzheimer's disease and multiple sclerosis. The recent application of RNA sequencing has provided a powerful and unbiased means to decipher spatial and temporal microglial heterogeneity. By identifying microglia with varying gene expression profiles, researchers have defined multiple subgroups of microglia associated with specific pathological states, including disease-associated microglia, interferon-responsive microglia, proliferating microglia, and inflamed microglia in multiple sclerosis, among others. However, the functional roles of these distinct subgroups remain inadequately characterized. This review aims to refine our current understanding of the potential roles of heterogeneous microglia in regulating synaptic plasticity and their implications for various brain disorders, drawing from recent sequencing research and functional studies. This knowledge may aid in the identification of pathogenetic biomarkers and potential factors contributing to pathogenesis, shedding new light on the discovery of novel drug targets. The field of sequencing-based data mining is evolving toward a multi-omics approach. With advances in viral tools for precise microglial regulation and the development of brain organoid models, we are poised to elucidate the functional roles of microglial subgroups detected through sequencing analysis, ultimately identifying valuable therapeutic targets.

Complement C3 From Astrocytes Plays Significant Roles in Sustained Activation of Microglia and Cognitive Dysfunctions Triggered by Systemic Inflammation After Laparotomy in Adult Male Mice

Aberrant activation of complement cascades plays an important role in the progress of neurological disorders. Complement C3, the central complement component, has been implicated in synaptic loss and cognitive impairment. Recent study has shown that wound injury-induced systemic inflammation can trigger the increase of C3 in the brain. Our previous studies have demonstrated that laparotomy-triggered systemic inflammation could induce neuroinflammation and cognitive dysfunctions. Furthermore, sustained activation of microglia was observed even 14 days after laparotomy, while most of cytokines had returned to basal levels rapidly at the earlier time point. Although we have demonstrated that anti-inflammatory intervention successfully attenuated cognitive dysfunction by preventing increase of cytokines and activation of microglia, how sustained activation of microglia and cognitive dysfunction occur is still a mystery. In this study, we investigated the role of C3 in mediating activation of microglia and cognitive dysfunction by using laparotomy in adult male mouse only as the experimental model of systemic inflammation and AAV9-C3shRNA. Our data observed that laparotomy induced neurotoxic reactive astrocytes with an increase of C3 in the hippocampus. Furthermore, inhibition of C3 by AAV9-C3shRNA prevented synaptic engulfment by microglia and attenuated cognitive dysfunctions after laparotomy. Inhibition of C3 did not modulate activation of astrocytes and expression of various cytokines. Current findings demonstrated that C3 plays significant roles in sustained activation of microglia and cognitive dysfunctions, which suggests that C3 is the valuable molecule target to attenuate in neurological conditions characterised by neuroinflammation and cognitive dysfunction.

The Auditory Pathway in Congenitally Cytomegalovirus-Infected Human Fetuses

Congenital cytomegalovirus (CMV) infection is the main cause of non-hereditary sensorineural hearing loss (SNHL). In order to shed light on SNHL pathophysiology, we examined the auditory pathway in CMV-infected fetuses; the temporal lobe, in particular the auditory cortex, and the inner ear. We investigated both inner ears and temporal lobes of 20 human CMV-infected fetuses at 21 weeks of gestation. As a negative group, five fetuses from spontaneous miscarriages without CMV infection were studied. Inner ears and temporal lobes were histologically examined, immunohistochemistry for CMV and CMV-PCR were performed. On the auditory cortex, we evaluated the local microglial reaction to the infection. CMV-positive cells were found in 14/20 brains and the damage was classified as severe, moderate, or mild, according to histological features. Fetuses with severe brain damage had a statistically higher temporal lobe viral load and a higher number of activated microglial cells in the auditory cortex compared to fetuses with mild brain damage (p: 0.01; p: 0.01). In the inner ears, the marginal cells of the stria vascularis were the most CMV positive. In our study, CMV affected the auditory pathway, suggesting a tropism for this route. In addition, in the auditory cortex, microglial activation may favor further tissue damage contributing to hearing loss.

Frequency and Focus of in Vitro Studies of Microglia-Expressed Cytokines in Response to Viral Infection: A Systematic Review

It is well known that as part of their response to infectious agents such as viruses, microglia transition from a quiescent state to an activated state that includes proinflammatory and anti-inflammatory phases; this behavior has been described through in vitro studies. However, recent in vivo studies on the function of microglia have questioned the two-phase paradigm; therefore, a change in the frequency of in vitro studies is expected. A systematic review was carried out to identify the microglial cytokine profile against viral infection that has been further evaluated through in vitro studies (pro-inflammatory or anti-inflammatory), along with analysis of its publication frequency over the years. For this review, 531 articles published in the English language were collected from PubMed, Web of Science, EBSCO and ResearchGate. Only 27 papers met the inclusion criteria for this systematic review. In total, 19 cytokines were evaluated in these studies, most of which are proinflammatory; the most common are IL-6, followed by TNF-α and IL-1β. It should be pointed out that half of the studies were published between 2015 and 2022 (raw data available in https://github.com/dadriba05/SystematicReview.git ). In this review, we identified that evaluation of pro-inflammatory cytokines released by microglia against viral infections has been performed more frequently than that of anti-inflammatory cytokines; additionally, a higher frequency of evaluation of the response of microglia cells to viral infection through in vitro studies from 2015 and beyond was noted.

The role of transient receptor potential (TRP) channels in phagocytosis: A comprehensive review

When host cells are exposed to foreign particles, dead cells, or cell hazards, a sophisticated process called phagocytosis begins. During this process, macrophages, dendritic cells, and neutrophils engulf the target by expanding their membranes. Phagocytosis of apoptotic cells is called efferocytosis. This process is of significant importance as billions of cells are eliminated daily without provoking inflammation. Both phagocytosis and efferocytosis depend on Ca2+ signaling. A big family of Ca2+ permeable channels is transient receptor potentials (TRPs) divided into nine subfamilies. We aimed to review their roles in phagocytosis. The present review article shows that various TRP channels such as TRPV1, 2, 3, 4, TRPM2, 4, 7, 8, TRPML1, TRPA1, TRPC1, 3, 5, 6 have roles at various stages of phagocytosis. They are involved in the phagocytosis of amyloid β, α-synuclein, myelin debris, bacteria, and apoptotic cells. In particular, TRPC3 and TRPM7 contribute to efferocytosis. These effects are mediated by changing Ca2+ signaling or targeting intracellular enzymes such as Akt. In addition, they contribute to the chemotaxis of phagocytic cells towards targets. Although a limited number of studies have assessed the role of TRP channels in phagocytosis and efferocytosis, their findings indicate that they have critical roles in these processes. In some cases, their ablation completely abolished the phagocytic function of the cells. As a result, TRP channels are potential targets for developing new therapeutics that modulate phagocytosis.

Amyloid plaques and normal ageing have differential effects on microglial Ca2+ activity in the mouse brain

In microglia, changes in intracellular calcium concentration ([Ca2+]i) may regulate process motility, inflammasome activation, and phagocytosis. However, while neurons and astrocytes exhibit frequent spontaneous Ca2+ activity, microglial Ca2+ signals are much rarer and poorly understood. Here, we studied [Ca2+]i changes of microglia in acute brain slices using Fluo-4-loaded cells and mice expressing GCaMP5g in microglia. Spontaneous Ca2+ transients occurred ~ 5 times more frequently in individual microglial processes than in their somata. We assessed whether microglial Ca2+ responses change in Alzheimer's disease (AD) using AppNL-G-F knock-in mice. Proximity to Aβ plaques strongly affected microglial Ca2+ activity. Although spontaneous Ca2+ transients were unaffected in microglial processes, they were fivefold more frequent in microglial somata near Aβ plaques than in wild-type microglia. Microglia away from Aβ plaques in AD mice showed intermediate properties for morphology and Ca2+ responses, partly resembling those of wild-type microglia. By contrast, somatic Ca2+ responses evoked by tissue damage were less intense in microglia near Aβ plaques than in wild-type microglia, suggesting different mechanisms underlying spontaneous vs. damage-evoked Ca2+ signals. Finally, as similar processes occur in neurodegeneration and old age, we studied whether ageing affected microglial [Ca2+]i. Somatic damage-evoked Ca2+ responses were greatly reduced in microglia from old mice, as in the AD mice. In contrast to AD, however, old age did not alter the occurrence of spontaneous Ca2+ signals in microglial somata but reduced the rate of events in processes. Thus, we demonstrate distinct compartmentalised Ca2+ activity in microglia from healthy, aged and AD-like brains.

Cell death by phagocytosis

Cells can die as a consequence of being phagocytosed by other cells - a form of cell death that has been called phagotrophy, cell cannibalism, programmed cell removal and primary phagocytosis. However, these are all different manifestations of cell death by phagocytosis (termed 'phagoptosis' for short). The engulfed cells die as a result of cytotoxic oxidants, peptides and degradative enzymes within acidic phagolysosomes. Cell death by phagocytosis was discovered by Metchnikov in the 1880s, but was neglected until recently. It is now known to contribute to developmental cell death in nematodes, Drosophila and mammals, and is central to innate and adaptive immunity against pathogens. Cell death by phagocytosis mediates physiological turnover of erythrocytes and other leucocytes, making it the most abundant form of cell death in the mammalian body. Immunity against cancer is also partly mediated by macrophage phagocytosis of cancer cells, but cancer cells can also phagocytose host cells and other cancer cells in order to survive. Recent evidence indicates neurodegeneration and other neuropathologies can be mediated by microglial phagocytosis of stressed neurons. Thus, despite cell death by phagocytosis being poorly recognized, it is one of the oldest, commonest and most important forms of cell death.

Complement-mediated synapse loss in Alzheimer's disease: mechanisms and involvement of risk factors

The complement system is increasingly recognized as a key player in the synapse loss and cognitive impairments observed in Alzheimer's disease (AD). In particular, the process of complement-dependent synaptic pruning through phagocytosis is over-activated in AD brains, driving detrimental excessive synapse elimination and contributing to synapse loss, which is the strongest neurobiological correlate of cognitive impairments in AD. Herein we review recent advances in characterizing complement-mediated synapse loss in AD, summarize the underlying mechanisms, and discuss the possible involvement of AD risk factors such as aging and various risk genes. We conclude with an overview of key questions that remain to be addressed.

A guide to complement biology, pathology and therapeutic opportunity

Complement has long been considered a key innate immune effector system that mediates host defence and tissue homeostasis. Yet, growing evidence has illuminated a broader involvement of complement in fundamental biological processes extending far beyond its traditional realm in innate immunity. Complement engages in intricate crosstalk with multiple pattern-recognition and signalling pathways both in the extracellular and intracellular space. Besides modulating host-pathogen interactions, this crosstalk guides early developmental processes and distinct cell trajectories, shaping tissue immunometabolic and regenerative programmes in different physiological systems. This Review provides a guide to the system-wide functions of complement. It highlights illustrative paradigm shifts that have reshaped our understanding of complement pathobiology, drawing examples from evolution, development of the central nervous system, tissue regeneration and cancer immunity. Despite its tight spatiotemporal regulation, complement activation can be derailed, fuelling inflammatory tissue pathology. The pervasive contribution of complement to disease pathophysiology has inspired a resurgence of complement therapeutics with major clinical developments, some of which have challenged long-held dogmas. We thus highlight major therapeutic concepts and milestones in clinical complement intervention.

Disinhibition of hippocampal parvalbumin interneurons on pyramidal neurons participates in LPS-induced cognitive dysfunction

BACKGROUND

The vulnerability of hippocampal pyramidal (PY) neurons played a key role in the onset of cognitive impairment. Multiple researches revealed that neuroinflammation together with microglia activation and parvalbumin (PV) interneurons participated in the pathogenesis of cognitive dysfunction. However, the underlying mechanism was still unclear. This study aimed to determine whether microglia activation would induce PV interneurons impairment and PY neurons disinhibition, and as a result, promote cognitive dysfunction after lipopolysaccharide (LPS) challenge.

METHODS

Male C57BL/6J mice were injected with LPS to establish systemic inflammation model, and animal behavioral tests were performed. For chemogenetics, the virus was injected bilaterally into the CA1 region. Clozapine N-Oxide (CNO) was used to activate the PV interneurons. Whole-cell patch clamp recording was applied to detect spontaneous inhibitory post synaptic current (sIPSC) and spontaneous excitatory post synaptic current (sEPSC) of PY neurons in the CA1 region.

RESULTS

LPS induced hippocampal dependent memory impairment, which was accompanied with microglia activation. Meanwhile, PV protein level in hippocampus were decreased, and IPSCs of PY neurons in the CA1 were also suppressed. Minocycline reversed all the above changes. In addition, rescuing PV function with CNO improved memory impairment, sIPSCs of PY neurons and perisomatic PV boutons around PY neurons without affecting microglia activation.

CONCLUSION

Disinhibition of hippocampal parvalbumin interneurons on pyramidal neurons participates in LPS-induced cognitive dysfunction.

Current perspectives on microglia-neuron communication in the central nervous system: Direct and indirect modes of interaction

BACKGROUND

The incessant communication that takes place between microglia and neurons is essential the development, maintenance, and pathogenesis of the central nervous system (CNS). As mobile phagocytic cells, microglia serve a critical role in surveilling and scavenging the neuronal milieu to uphold homeostasis.

AIM OF REVIEW

This review aims to discuss the various mechanisms that govern the interaction between microglia and neurons, from the molecular to the organ system level, and to highlight the importance of these interactions in the development, maintenance, and pathogenesis of the CNS.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Recent research has revealed that microglia-neuron interaction is vital for regulating fundamental neuronal functions, such as synaptic pruning, axonal remodeling, and neurogenesis. The review will elucidate the intricate signaling pathways involved in these interactions, both direct and indirect, to provide a better understanding of the fundamental mechanisms of brain function. Furthermore, gaining insights into these signals could lead to the development of innovative therapies for neural disorders.

Antigen non-specific CD8+ T cells accelerate cognitive decline in aged mice following respiratory coronavirus infection

Primarily a respiratory infection, numerous patients infected with SARS-CoV-2 present with neurologic symptoms, some continuing long after viral clearance as a persistent symptomatic phase termed "long COVID". Advanced age increases the risk of severe disease, as well as incidence of long COVID. We hypothesized that perturbations in the aged immune response predispose elderly individuals to severe coronavirus infection and post-infectious sequelae. Using a murine model of respiratory coronavirus, mouse hepatitis virus strain A59 (MHV-A59), we found that aging increased clinical illness and lethality to MHV infection, with aged animals harboring increased virus in the brain during acute infection. This was coupled with an unexpected increase in activated CD8+ T cells within the brains of aged animals but reduced antigen specificity of those CD8+ T cells. Aged animals demonstrated spatial learning impairment following MHV infection, which correlated with increased neuronal cell death and reduced neuronal regeneration in aged hippocampus. Using primary cell culture, we demonstrated that activated CD8+ T cells induce neuronal death, independent of antigen-specificity. Specifically, higher levels of CD8+ T cell-derived IFN-γ correlated with neuronal death. These results support the evidence that CD8+ T cells in the brain directly contribute to cognitive dysfunction following coronavirus infection in aged individuals.

Repurposing of pexidartinib for microglia depletion and renewal

Pexidartinib (PLX3397) is a small molecule receptor tyrosine kinase inhibitor of colony stimulating factor 1 receptor (CSF1R) with moderate selectivity over other members of the platelet derived growth factor receptor family. It is approved for treatment of tenosynovial giant cell tumors (TGCT). CSF1R is highly expressed by microglia, which are macrophages of the central nervous system (CNS) that defend the CNS against injury and pathogens and contribute to synapse development and plasticity. Challenged by pathogens, apoptotic cells, debris, or inflammatory molecules they adopt a responsive state to propagate the inflammation and eventually return to a homeostatic state. The phenotypic switch may fail, and disease-associated microglia contribute to the pathophysiology in neurodegenerative or neuropsychiatric diseases or long-lasting detrimental brain inflammation after brain, spinal cord or nerve injury or ischemia/hemorrhage. Microglia also contribute to the growth permissive tumor microenvironment of glioblastoma (GBM). In rodents, continuous treatment for 1-2 weeks via pexidartinib food pellets leads to a depletion of microglia and subsequent repopulation from the remaining fraction, which is aided by peripheral monocytes that search empty niches for engraftment. The putative therapeutic benefit of such microglia depletion or forced renewal has been assessed in almost any rodent model of CNS disease or injury or GBM with heterogeneous outcomes, but a tendency of partial beneficial effects. So far, microglia monitoring e.g. via positron emission imaging is not standard of care for patients receiving Pexidartinib (e.g. for TGCT), so that the depletion and repopulation efficiency in humans is still largely unknown. Considering the virtuous functions of microglia, continuous depletion is likely no therapeutic option but short-lasting transient partial depletion to stimulate microglia renewal or replace microglia in genetic disease in combination with e.g. stem cell transplantation or as part of a multimodal concept in treatment of glioblastoma appears feasible. The present review provides an overview of the preclinical evidence pro and contra microglia depletion as a therapeutic approach.

Targeting synapse function and loss for treatment of neurodegenerative diseases

Synapse dysfunction and loss are hallmarks of neurodegenerative diseases that correlate with cognitive decline. However, the mechanisms and therapeutic strategies to prevent or reverse synaptic damage remain elusive. In this Review, we discuss recent advances in understanding the molecular and cellular pathways that impair synapses in neurodegenerative diseases, including the effects of protein aggregation and neuroinflammation. We also highlight emerging therapeutic approaches that aim to restore synaptic function and integrity, such as enhancing synaptic plasticity, preventing synaptotoxicity, modulating neuronal network activity and targeting immune signalling. We discuss the preclinical and clinical evidence for each strategy, as well as the challenges and opportunities for developing effective synapse-targeting therapeutics for neurodegenerative diseases.

Long-term neurological dysfunction associated with COVID-19: Lessons from influenza and inflammatory diseases?

As the COVID-19 pandemic persists, SARS-CoV-2 infection is increasingly associated with long-term neurological side effects including cognitive impairment, fatigue, depression, and anxiety, colloquially known as "long-COVID." While the full extent of long-COVID neuropathology across years or even decades is not yet known, we can perhaps take direction from long-standing research into other respiratory diseases, such as influenza, that can present with similar long-term neurological consequences. In this review, we highlight commonalities in the neurological impacts of influenza and COVID-19. We first focus on the common potential mechanisms underlying neurological sequelae of long-COVID and influenza, namely (1) viral neurotropism and (2) dysregulated peripheral inflammation. The latter, namely heightened peripheral inflammation leading to central nervous system dysfunction, is emerging as a shared mechanism in various peripheral inflammatory or inflammation-associated diseases and conditions. We then discuss historical and modern examples of influenza- and COVID-19-associated cognitive impairment, depression, anxiety, and fatigue, revealing key similarities in their neurological sequelae. Although we are learning that the effects of influenza and COVID differ somewhat in terms of their influence on the brain, as the impacts of long-COVID grow, such comparisons will likely prove valuable in guiding ongoing research into long-COVID, and perhaps foreshadow what could be in store for individuals with COVID-19 and their brain health.

Mice deficient in complement C3 are protected against recognition memory deficits and astrogliosis induced by status epilepticus

Introduction

Status epilepticus (SE) can significantly increase the risk of temporal lobe epilepsy (TLE) and cognitive comorbidities. A potential candidate mechanism underlying memory defects in epilepsy may be the immune complement system. The complement cascade, part of the innate immune system, modulates inflammatory and phagocytosis signaling, and has been shown to contribute to learning and memory dysfunctions in neurodegenerative disorders. We previously reported that complement C3 is elevated in brain biopsies from human drug-resistant epilepsy and in experimental rodent models. We also found that SE-induced increases in hippocampal C3 levels paralleled the development of hippocampal-dependent spatial learning and memory deficits in rats. Thus, we hypothesized that SE-induced C3 activation contributes to this pathophysiology in a mouse model of SE and acquired TLE.

Methods

In this study C3 knockout (KO) and wild type (WT) mice were subjected to one hour of pilocarpine-induced SE or sham conditions (control; C). Following a latent period of two weeks, recognition memory was assessed utilizing the novel object recognition (NOR) test. Western blotting was utilized to determine the protein levels of C3 in hippocampal lysates. In addition, we assessed the protein levels and distribution of the astrocyte marker glial fibrillary acidic protein (GFAP).

Results

In the NOR test, control WT + C or C3 KO + C mice spent significantly more time exploring the novel object compared to the familiar object. In contrast, WT+SE mice did not show preference for either object, indicating a memory defect. This deficit was prevented in C3 KO + SE mice, which performed similarly to controls. In addition, we found that SE triggered significant increases in the protein levels of GFAP in hippocampi of WT mice but not in C3 KO mice.

Discussion

These findings suggest that ablation of C3 prevents SE-induced recognition memory deficits and that a C3-astrocyte interplay may play a role. Therefore, it is possible that enhanced C3 signaling contributes to SE-associated cognitive decline during epileptogenesis and may serve as a potential therapeutic target for treating cognitive comorbidities in acquired TLE.

Microglia and complement mediate early corticostriatal synapse loss and cognitive dysfunction in Huntington's disease

Huntington's disease (HD) is a devastating monogenic neurodegenerative disease characterized by early, selective pathology in the basal ganglia despite the ubiquitous expression of mutant huntingtin. The molecular mechanisms underlying this region-specific neuronal degeneration and how these relate to the development of early cognitive phenotypes are poorly understood. Here we show that there is selective loss of synaptic connections between the cortex and striatum in postmortem tissue from patients with HD that is associated with the increased activation and localization of complement proteins, innate immune molecules, to these synaptic elements. We also found that levels of these secreted innate immune molecules are elevated in the cerebrospinal fluid of premanifest HD patients and correlate with established measures of disease burden.In preclinical genetic models of HD, we show that complement proteins mediate the selective elimination of corticostriatal synapses at an early stage in disease pathogenesis, marking them for removal by microglia, the brain's resident macrophage population. This process requires mutant huntingtin to be expressed in both cortical and striatal neurons. Inhibition of this complement-dependent elimination mechanism through administration of a therapeutically relevant C1q function-blocking antibody or genetic ablation of a complement receptor on microglia prevented synapse loss, increased excitatory input to the striatum and rescued the early development of visual discrimination learning and cognitive flexibility deficits in these models. Together, our findings implicate microglia and the complement cascade in the selective, early degeneration of corticostriatal synapses and the development of cognitive deficits in presymptomatic HD; they also provide new preclinical data to support complement as a therapeutic target for early intervention.

Oligodendrocyte-derived IL-33 functions as a microglial survival factor during neuroinvasive flavivirus infection

In order to recover from infection, organisms must balance robust immune responses to pathogens with the tolerance of immune-mediated pathology. This balance is particularly critical within the central nervous system, whose complex architecture, essential function, and limited capacity for self-renewal render it susceptible to both pathogen- and immune-mediated pathology. Here, we identify the alarmin IL-33 and its receptor ST2 as critical for host survival to neuroinvasive flavivirus infection. We identify oligodendrocytes as the critical source of IL-33, and microglia as the key cellular responders. Notably, we find that the IL-33/ST2 axis does not impact viral control or adaptive immune responses; rather, it is required to promote the activation and survival of microglia. In the absence of intact IL-33/ST2 signaling in the brain, neuroinvasive flavivirus infection triggered aberrant recruitment of monocyte-derived peripheral immune cells, increased neuronal stress, and neuronal cell death, effects that compromised organismal survival. These findings identify IL-33 as a critical mediator of CNS tolerance to pathogen-initiated immunity and inflammation.

The anti-viral immune response of the adult host robustly modulates neural stem cell activity in spatial, temporal, and sex-specific manners

Viruses induce a wide range of neurological sequelae through the dysfunction and death of infected cells and persistent inflammation in the brain. Neural stem cells (NSCs) are often disturbed during viral infections. Although some viruses directly infect and kill NSCs, the antiviral immune response may also indirectly affect NSCs. To better understand how NSCs are influenced by a productive immune response, where the virus is successfully resolved and the host survives, we used the CD46+ mouse model of neuron-restricted measles virus (MeV) infection. As NSCs are spared from direct infection in this model, they serve as bystanders to the antiviral immune response initiated by selective infection of mature neurons. MeV-infected mice showed distinct regional and temporal changes in NSCs in the primary neurogenic niches of the brain, the hippocampus and subventricular zone (SVZ). Hippocampal NSCs increased throughout the infection (7 and 60 days post-infection; dpi), while mature neurons transiently declined at 7 dpi and then rebounded to basal levels by 60 dpi. In the SVZ, NSC numbers were unchanged, but mature neurons declined even after the infection was controlled at 60 dpi. Further analyses demonstrated sex, temporal, and region-specific changes in NSC proliferation and neurogenesis throughout the infection. A relatively long-term increase in NSC proliferation and neurogenesis was observed in the hippocampus; however, neurogenesis was reduced in the SVZ. This decline in SVZ neurogenesis was associated with increased immature neurons in the olfactory bulb in female, but not male mice, suggesting potential migration of newly-made neurons out of the female SVZ. These sex differences in SVZ neurogenesis were accompanied by higher infiltration of B cells and greater expression of interferon-gamma and interleukin-6 in female mice. Learning, memory, and olfaction tests revealed no overt behavioral changes after the acute infection subsided. These results indicate that antiviral immunity modulates NSC activity in adult mice without inducing gross behavioral deficits among those tested, suggestive of mechanisms to restore neurons and maintain adaptive behavior, but also revealing the potential for robust NSC disruption in subclinical infections.

Molecular and Cellular Mechanisms Underlying Neurologic Manifestations of Mosquito-Borne Flavivirus Infections

Flaviviruses are a family of enveloped viruses with a positive-sense RNA genome, transmitted by arthropod vectors. These viruses are known for their broad cellular tropism leading to infection of multiple body systems, which can include the central nervous system. Neurologic effects of flavivirus infection can arise during both acute and post-acute infectious periods; however, the molecular and cellular mechanisms underlying post-acute sequelae are not fully understood. Here, we review recent studies that have examined molecular and cellular mechanisms that may contribute to neurologic sequelae following infection with the West Nile virus, Japanese encephalitis virus, Zika virus, dengue virus, and St. Louis encephalitis virus. Neuronal death, either from direct infection or due to the resultant inflammatory response, is a common mechanism by which flavivirus infection can lead to neurologic impairment. Other types of cellular damage, such as oxidative stress and DNA damage, appear to be more specific to certain viruses. This article aims to highlight mechanisms of cellular damage that are common across several flavivirus members and mechanisms that are more unique to specific members. Our goal is to inspire further research to improve understanding of this area in the hope of identifying treatment options for flavivirus-associated neurologic changes.

Preferential pruning of inhibitory synapses by microglia contributes to alteration of the balance between excitatory and inhibitory synapses in the hippocampus in temporal lobe epilepsy

BACKGROUND

A consensus has formed that neural circuits in the brain underlie the pathogenesis of temporal lobe epilepsy (TLE). In particular, the synaptic excitation/inhibition balance (E/I balance) has been implicated in shifting towards elevated excitation during the development of TLE.

METHODS

Sprague Dawley (SD) rats were intraperitoneally subjected to kainic acid (KA) to generate a model of TLE. Next, electroencephalography (EEG) recording was applied to verify the stability and detectability of spontaneous recurrent seizures (SRS) in rats. Moreover, hippocampal slices from rats and patients with mesial temporal lobe epilepsy (mTLE) were assessed using immunofluorescence to determine the alterations of excitatory and inhibitory synapses and microglial phagocytosis.

RESULTS

We found that KA induced stable SRSs 14 days after status epilepticus (SE) onset. Furthermore, we discovered a continuous increase in excitatory synapses during epileptogenesis, where the total area of vesicular glutamate transporter 1 (vGluT1) rose considerably in the stratum radiatum (SR) of cornu ammonis 1 (CA1), the stratum lucidum (SL) of CA3, and the polymorphic layer (PML) of the dentate gyrus (DG). In contrast, inhibitory synapses decreased significantly, with the total area of glutamate decarboxylase 65 (GAD65) in the SL and PML diminishing enormously. Moreover, microglia conducted active synaptic phagocytosis after the formation of SRSs, especially in the SL and PML. Finally, microglia preferentially pruned inhibitory synapses during recurrent seizures in both rat and human hippocampal slices, which contributed to the synaptic alteration in hippocampal subregions.

CONCLUSIONS

Our findings elaborately characterize the alteration of neural circuits and demonstrate the selectivity of synaptic phagocytosis mediated by microglia in TLE, which could strengthen the comprehension of the pathogenesis of TLE and inspire potential therapeutic targets for epilepsy treatment.

Estrogen receptor beta in astrocytes modulates cognitive function in mid-age female mice

Menopause is associated with cognitive deficits and brain atrophy, but the brain region and cell-specific mechanisms are not fully understood. Here, we identify a sex hormone by age interaction whereby loss of ovarian hormones in female mice at midlife, but not young age, induced hippocampal-dependent cognitive impairment, dorsal hippocampal atrophy, and astrocyte and microglia activation with synaptic loss. Selective deletion of estrogen receptor beta (ERβ) in astrocytes, but not neurons, in gonadally intact female mice induced the same brain effects. RNA sequencing and pathway analyses of gene expression in hippocampal astrocytes from midlife female astrocyte-ERβ conditional knock out (cKO) mice revealed Gluconeogenesis I and Glycolysis I as the most differentially expressed pathways. Enolase 1 gene expression was increased in hippocampi from both astrocyte-ERβ cKO female mice at midlife and from postmenopausal women. Gain of function studies showed that ERβ ligand treatment of midlife female mice reversed dorsal hippocampal neuropathology.

Neuropsychiatric Outcomes and Sleep Dysfunction in COVID-19 Patients: Risk Factors and Mechanisms

The ongoing global health crisis due to the novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has significantly impacted all aspects of life. While the majority of early research following the coronavirus disease caused by SARS-CoV-2 (COVID-19) has focused on the physiological effects of the virus, a substantial body of subsequent studies has shown that the psychological burden of the infection is also considerable. Patients, even without mental illness history, were at increased susceptibility to developing mental health and sleep disturbances during or after the COVID-19 infection. Viral neurotropism and inflammatory storm damaging the blood-brain barrier have been proposed as possible mechanisms for mental health manifestations, along with stressful psychological factors and indirect consequences such as thrombosis and hypoxia. The virus has been found to infect peripheral olfactory neurons and exploit axonal migration pathways, exhibiting metabolic changes in astrocytes that are detrimental to fueling neurons and building neurotransmitters. Patients with COVID-19 present dysregulated and overactive immune responses, resulting in impaired neuronal function and viability, adversely affecting sleep and emotion regulation. Additionally, several risk factors have been associated with the neuropsychiatric sequelae of the infection, such as female sex, age, preexisting neuropathologies, severity of initial disease and sociological status. This review aimed to provide an overview of mental health symptoms and sleep disturbances developed during COVID-19 and to analyze the underlying mechanisms and risk factors of psychological distress and sleep dysfunction.

Blockade of interferon signaling decreases gut barrier integrity and promotes severe West Nile virus disease

The determinants of severe disease caused by West Nile virus (WNV) and why only ~1% of individuals progress to encephalitis remain poorly understood. Here, we use human and mouse enteroids, and a mouse model of pathogenesis, to explore the capacity of WNV to directly infect gastrointestinal (GI) tract cells and contribute to disease severity. At baseline, WNV poorly infects human and mouse enteroid cultures and enterocytes in mice. However, when STAT1 or type I interferon (IFN) responses are absent, GI tract cells become infected, and this is associated with augmented GI tract and blood-brain barrier (BBB) permeability, accumulation of gut-derived molecules in the brain, and more severe WNV disease. The increased gut permeability requires TNF-α signaling, and is absent in WNV-infected IFN-deficient germ-free mice. To link these findings to human disease, we measured auto-antibodies against type I IFNs in serum from WNV-infected human cohorts. A greater frequency of auto- and neutralizing antibodies against IFN-α2 or IFN-ω is present in patients with severe WNV infection, whereas virtually no asymptomatic WNV-infected subjects have such antibodies (odds ratio 24 [95% confidence interval: 3.0 - 192.5; P = 0.003]). Overall, our experiments establish that blockade of type I IFN signaling extends WNV tropism to enterocytes, which correlates with increased gut and BBB permeability, and more severe disease.

Vaccination prevents IL-1β-mediated cognitive deficits after COVID-19

Up to 25% of SARS-CoV-2 patients exhibit post-acute cognitive sequelae. Although millions of cases of COVID-19-mediated memory dysfunction are accumulating worldwide, the underlying mechanisms and how vaccination lowers risk are unknown. Interleukin-1, a key component of innate immune defense against SARS-CoV-2 infection, is elevated in the hippocampi of COVID-19 patients. Here we show that intranasal infection of C57BL/6J mice with SARS-CoV-2 beta variant, leads to CNS infiltration of Ly6Chi monocytes and microglial activation. Accordingly, SARS-CoV-2, but not H1N1 influenza virus, increases levels of brain IL-1β and induces persistent IL-1R1-mediated loss of hippocampal neurogenesis, which promotes post-acute cognitive deficits. Breakthrough infection after vaccination with a low dose of adenoviral vectored Spike protein prevents hippocampal production of IL-1β during breakthrough SARS-CoV-2 infection, loss of neurogenesis, and subsequent memory deficits. Our study identifies IL-1β as one potential mechanism driving SARS-CoV-2-induced cognitive impairment in a new murine model that is prevented by vaccination.

Genetic analysis of potential biomarkers and therapeutic targets in neuroinflammation from sporadic Creutzfeldt-Jakob disease

This study aimed to identify hub genes and pathological mechanisms related to neuroinflammation in Sporadic Creutzfeldt-Jakob disease (SCJD) based on comprehensive bioinformatics. SCJD and normal samples were collected from GSE160208. Weighted gene co-expression network analysis (WGCNA) and Limma R package were used to obtain key genes, which were used for enrichment and immune cell infiltration analyses. Protein-protein interaction (PPI) network, cytoHubba, and machine learning were used to screen the central genes of SCJD. The chemicals related to hub genes were predicted and explored by molecular docking. 88 candidate genes were screened. Enrichment analysis showed they were mainly related to bacterial and viral infection and immune cell activation. Immune cell infiltration analysis suggested that immune cell activation and altered activity of the immune system are involved in the progression of SCJD. After identifying hub genes, KIT and SPP1 had higher diagnostic efficacy for SCJD (AUC > 0.9), so they were identified as central genes. The molecular docking results showed hub genes both docked well with Tretinoin. KIT, SPP1, and Tretinoin are essential in developing neuroinflammation in SCJD and may provide new ideas for diagnosing and treating SCJD.

Biological agents and the aging brain: glial inflammation and neurotoxic signaling

Neuroinflammation is a universal characteristic of brain aging and neurological disorders, irrespective of the disease state. Glial inflammation mediates this signaling, through astrocyte and microglial polarization from neuroprotective to neurotoxic phenotypes. Glial reactivity results in the loss of homeostasis, as these cells no longer provide support to neurons, in addition to the production of chronically toxic pro-inflammatory mediators. These glial changes initiate an inflammatory brain state that injures the central nervous system (CNS) over time. As the brain ages, glia are altered, including increased glial cell numbers, morphological changes, and either a pre-disposition or inability to become reactive. These alterations induce age-related neuropathologies, ultimately leading to neuronal degradation and irreversible damage associated with disorders of the aged brain, including Alzheimer's Disease (AD) and other related diseases. While the complex interactions of these glial cells and the brain are well studied, the role additional stressors, such as infectious agents, play on age-related neuropathology has not been fully elucidated. Both biological agents in the periphery, such as bacterial infections, or in the CNS, including viral infections like SARS-CoV-2, push glia into neuroinflammatory phenotypes that can exacerbate pathology within the aging brain. These biological agents release pattern associated molecular patterns (PAMPs) that bind to pattern recognition receptors (PRRs) on glial cells, beginning an inflammatory cascade. In this review, we will summarize the evidence that biological agents induce reactive glia, which worsens age-related neuropathology.

Vascular Dysfunctions Contribute to the Long-Term Cognitive Deficits Following COVID-19

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a single-stranded RNA virus and a member of the corona virus family, primarily affecting the upper respiratory system and the lungs. Like many other respiratory viruses, SARS-CoV-2 can spread to other organ systems. Apart from causing diarrhea, another very common but debilitating complication caused by SARS-CoV-2 is neurological symptoms and cognitive difficulties, which occur in up to two thirds of hospitalized COVID-19 patients and range from shortness of concentration and overall declined cognitive speed to executive or memory function impairment. Neuro-cognitive dysfunction and "brain fog" are frequently present in COVID-19 cases, which can last several months after the infection, leading to disruption of daily life. Cumulative evidence suggests that SARS-CoV-2 affects vasculature in the extra-pulmonary systems directly or indirectly, leading to impairment of endothelial function and even multi-organ damage. The post COVID-19 long-lasting neurocognitive impairments have not been studied fully and their underlying mechanism remains elusive. In this review, we summarize the current understanding of the effects of COVID-19 on vascular dysfunction and how vascular dysfunction leads to cognitive impairment in patients.