In this review, we synthesize recent conceptual and experimental advances in neuroscience, highlighting selected studies that delineate the roles of reactive microglia and astrocytes in the contexts of developmental inflammatory stress, neurodegenerative diseases, and cellular senescence. Since the characterization of disease-associated glial phenotypes in 2017, building on earlier pioneering discoveries, we focus here on disease-associated microglia (DAM) and disease-associated astrocyte (DAA) to reassess their contributions to glio-inflammation. It is now recognized that the stress-induced glial states are far from uniform; however, the ontogeny, molecular determinants, and functional consequences of this heterogeneity remain incompletely understood, particularly in psychiatric disorders, Alzheimer's disease, and amyotrophic lateral sclerosis. Accordingly, we compare the glial heterogeneity and its underlying mechanisms across translational mouse models and human neuropathology, considering their evolutionary and physiological contexts. While this review does not aim to be exhaustive, we propose an integrative framework that redefines glial stress responses through the combined lenses of inflammation, transcriptomics, mitochondrial dynamics, lipid metabolism, epigenomic regulation, and cellular senescence. Finally, we outline emerging frontiers for AI-enabled multi-omic physiological and pathological approaches, emphasizing their potential to illuminate glial state transitions and accelerate therapeutic discovery in the near future.

Sporadic amyotrophic lateral sclerosis (ALS) is characterized by selective motor neuron vulnerability accompanied by prominent glial responses, yet their molecular heterogeneity and spatial organization in the human central nervous system and spinal cord remain incompletely defined. Here, we delineate ALS-associated glial states in situ by spatially resolved, single-cell proteomic profiling of postmortem human sporadic ALS and non-neurological control tissues across the motor cortex, spinal cord, and cerebellum. We identify subpopulations of astrocytes and microglia co-expressing both canonical ALS markers together with newly implicated ALS-associated proteins, including phosphorylated TDP-43, UBQLN2, TREM2, IL6ST, and glutaminase (GLS). These glial states are consistent with transcriptomic signatures characterized by TP53 and GLS upregulation, indicating convergence on senescence stress- and glutamine metabolism-related pathways. Notably, p53–GLS-associated profiles are selectively enriched in anatomically vulnerable regions in ALS, supporting region-specific glial metabolic alterations in human disease. Together, these findings provide insight into the metabolic alterations in ALS-affected human tissue and highlight the relevance of spatial context in shaping glial responses to neurodegeneration.

The functional alteration of microglia arises in brains exposed to external stress during early development. Pathophysiological findings of neurodevelopmental disorders such as schizophrenia and autism spectrum disorder suggest cerebellar functional deficits. However, the link between stress-induced microglia reactivity and cerebellar dysfunction is missing. Here, we investigate the developmental immune environment in translational mouse models that combine two risk factors: maternal infection and repeated social defeat stress (2HIT). We find the synergy of inflammatory stress insults, leading to microglial increase specifically in the cerebellum of both sexes. Microglial turnover correlates with the Purkinje neuron loss in 2HIT mice. Highly multiplexed imaging-mass-cytometry identifies a cell transition to TREM2(+) stress-associated microglia in the cerebellum. Single-cell-proteomic clustering reveals IL-6- and TGFβ-signaling association with microglial cell transitions. Reduced excitability of remaining Purkinje cells, cerebellum-involved brain-wide functional dysconnectivity, and behavioral abnormalities indicate cerebellar cognitive dysfunctions in 2HIT animals, which are ameliorated by both systemic and cerebellum-specific microglia replacement. Early developmental stress alters microglial function, linked to schizophrenia and autism. In a mouse model, inflammatory 2HIT stress increased cerebellar microglia and its turnover, causing Purkinje neuron loss and cerebellar dysfunction, which was improved by microglia replacement.

Despite the rapid and sustained antidepressant effects of ketamine and its metabolites, their underlying cellular and molecular mechanisms are not fully understood. Here, we demonstrate that the sustained antidepressant-like behavioral effects of (2S,6S)-hydroxynorketamine (HNK) in repeatedly stressed animal models involve neurobiological changes in the anterior paraventricular nucleus of the thalamus (aPVT). Mechanistically, (2S,6S)-HNK induces mRNA expression of extrasynaptic GABAA receptors and subsequently enhances GABAA-receptor-mediated tonic currents, leading to the nuclear export of histone demethylase KDM6 and its replacement by histone methyltransferase EZH2. This process increases H3K27me3 levels, which in turn suppresses the transcription of genes associated with G-protein-coupled receptor signaling. Thus, our findings shed light on the comprehensive cellular and molecular mechanisms in aPVT underlying the sustained antidepressant behavioral effects of ketamine metabolites. This study may support the development of potentially effective next-generation pharmacotherapies to promote sustained remission of stress-related psychiatric disorders.

The functional alteration of microglia emerges in the brains exposed to extrinsic stress insults during early development. Pathophysiological findings of psychiatric disorders such as schizophrenia and autism implied a localized deficit of the cerebellum. However, the link between microglia reactivity and cerebellar dysfunction is missing. Here, we investigated the developmental immune environment of translational mouse models that combine two risk factors: maternal infection and repeated social defeat stress (i.e., 2HIT). We found synergy of inflammatory stress insults produced reactive microglia, specifically in the cerebellum in both sexes. The microglial proliferation correlated with the Purkinje neuron loss in 2HIT mice. Highly multiplexed imaging-mass-cytometry demonstrated a TREM2-positive population of stress-associated microglia in the cerebellum. Single-cell-proteomic clustering revealed IL-6- and TGFβ-signaling association with microglial cell transition. Purkinje cells reduced excitability, cerebellum-involved brain-wide functional dysconnectivity, and behavioral anomalies underpin the cerebellar cognitive dysfunctions in 2HIT animals, ameliorated by cerebellum-specific microglia replacement.

A pivotal role of microglia is the maintenance of brain homeostasis, but the function is severely altered in the brains with psychiatric disorders such as schizophrenia spectrum disorders. Previous pathological and physiological findings indicate a localized deficit in the cerebellum of the patients. However, it is unknown if the accumulation of inflammatory stress alters microglia morphology, localization, molecular expression pattern, and phenotype. Here, using translational mouse models that exploit known risk factors for mental disorders: maternal infection and repeated social defeat stress, we investigated the brain immune environment of psychiatric disorder models with multiple immune stress. The synergy of maternal immune activation and repeated defeat stress on mice brought forth reactive microglia specifically in the cerebellum. The increase in the reactive microglia in 2HIT correlated with cerebellar Purkinje neuron loss. Highly-multiplexed imaging mass cytometry with originally-customized 26 markers demonstrated a TREM2-positive immune-cell population of disease-associated microglia/macrophages (DAMIMCs) being close at near-vasculature. Single-cell proteomic clustering indicates sIL-6R- and TGFBR2-signaling with cell transition from progenitor cells to mature DAMIMCs, which implies synergistic disruption of neurophysiological properties and functioning in the 2HIT cerebellum. Our study identifies the distinct immune cells in the cerebellum, which may influence cognition and behavior.

EDITORIAL article Front. Cell. Neurosci., 13 February 2023Sec. Cellular Neurophysiology Volume 17 - 2023 | https://doi.org/10.3389/fncel.2023.1132286

The cerebellum governs motor coordination and motor learning. Infection with external microorganisms, such as viruses, bacteria, and fungi, induces the release and production of inflammatory mediators, which drive acute cerebellar inflammation. The clinical observation of acute cerebellitis is associated with the emergence of cerebellar ataxia. In our animal model of the acute inflammation of the cerebellar cortex, animals did not show any ataxia but hyperexcitability in the cerebellar cortex and depression-like behaviors. In contrast, animal models with neurodegeneration of the cerebellar Purkinje cells and hypoexcitability of the neurons show cerebellar ataxia. The suppression of the Ca2+-activated K+ channels in vivo is associated with a type of ataxia. Therefore, there is a gap in our interpretation between the very early phase of cerebellar inflammation and the emergence of cerebellar ataxia. In this review, we discuss the hypothesized scenario concerning the emergence of cerebellar ataxia. First, compared with genetically induced cerebellar ataxias, we introduce infection and inflammation in the cerebellum via aberrant immunity and glial responses. Especially, we focus on infections with cytomegalovirus, influenza virus, dengue virus, and SARS-CoV-2, potential relevance to mitochondrial DNA, and autoimmunity in infection. Second, we review neurophysiological modulation (intrinsic excitability, excitatory, and inhibitory synaptic transmission) by inflammatory mediators and aberrant immunity. Next, we discuss the cerebellar circuit dysfunction (presumably, via maintaining the homeostatic property). Lastly, we propose the mechanism of the cerebellar ataxia and possible treatments for the ataxia in the cerebellar inflammation.

Lipopolysaccharide (LPS), an outer component of Gram-negative bacteria, induces a strong response of innate immunity via microglia, which triggers a modulation of the intrinsic excitability of neurons. However, it is unclear whether the modulation of neurophysiological properties is similar among neurons. Here, we found the hypoexcitability of layer 5 (L5) pyramidal neurons after exposure to LPS in the medial prefrontal cortex (mPFC) of juvenile rats. We recorded the firing frequency of L5 pyramidal neurons long-lastingly under in vitro whole-cell patch-clamp, and we found a reduction of the firing frequency after applying LPS. A decrease in the intrinsic excitability against LPS-exposure was also found in L2/3 pyramidal neurons but not in fast-spiking interneurons. The decrease in the excitability by immune-activation was underlain by increased activity of small-conductance Ca2+-activated K+ channels (SK channels) in the pyramidal neurons and tumor necrosis factor (TNF)-α released from microglia. We revealed that the reduction of the firing frequency of L5 pyramidal neurons was dependent on intraneuronal Ca2+ and PP2B. These results suggest the hypoexcitability of pyramidal neurons caused by the upregulation of SK channels via Ca2+-dependent phosphatase during acute inflammation in the mPFC. Such a mechanism is in contrast to that of cerebellar Purkinje cells, in which immune activation induces hyperexcitability via downregulation of SK channels. Further, a decrease in the frequency of spontaneous inhibitory synaptic transmission reflected network hypoactivity. Therefore, our results suggest that the directionality of the intrinsic plasticity by microglia is not consistent, depending on the brain region and the cell type.

Immune cells play numerous roles in the host defense against the invasion of microorganisms and pathogens, which induces the release of inflammatory mediators (e.g., cytokines and chemokines). In the CNS, microglia is the major resident immune cell. Recent efforts have revealed the diversity of the cell types and the heterogeneity of their functions. The refinement of the synapse structure was a hallmark feature of the microglia, while they are also involved in the myelination and capillary dynamics. Another promising feature is the modulation of the synaptic transmission as synaptic plasticity and the intrinsic excitability of neurons as non-synaptic plasticity. Those modulations of physiological properties of neurons are considered induced by both transient and chronic exposures to inflammatory mediators, which cause behavioral disorders seen in mental illness. It is plausible for astrocytes and pericytes other than microglia and macrophage to induce the immune-triggered plasticity of neurons. However, current understanding has yet achieved to unveil what inflammatory mediators from what immune cells or glia induce a form of plasticity modulating pre-, post-synaptic functions and intrinsic excitability of neurons. It is still unclear what ion channels and intracellular signaling of what types of neurons in which brain regions of the CNS are involved. In this review, we introduce the ubiquitous modulation of the synaptic efficacy and the intrinsic excitability across the brain by immune cells and related inflammatory cytokines with the mechanism for induction. Specifically, we compare neuro-modulation mechanisms by microglia of the intrinsic excitability of cerebellar Purkinje neurons with cerebral pyramidal neurons, stressing the inverted directionality of the plasticity. We also discuss the suppression and augmentation of the extent of plasticity by inflammatory mediators, as the meta-plasticity by immunity. Lastly, we sum up forms of immune-triggered plasticity in the different brain regions with disease relevance. Together, brain immunity influences our cognition, sense, memory, and behavior via immune-triggered plasticity.

Prominently accountable for the upsurge of COVID-19 cases as the world attempts to recover from the previous two waves, Omicron has further threatened the conventional therapeutic approaches. Omicron is the fifth variant of concern (VOC), which comprises more than 10 mutations in the receptor-binding domain (RBD) of the spike protein. However, the lack of extensive research regarding Omicron has raised the need to establish correlations to understand this variant by structural comparisons. Here, we evaluate, correlate, and compare its genomic sequences through an immunoinformatic approach with wild and mutant RBD forms of the spike protein to understand its epidemiological characteristics and responses towards existing drugs for better patient management. Our computational analyses provided insights into infectious and pathogenic trails of the Omicron variant. In addition, while the analysis represented South Africa's Omicron variant being similar to the highly-infectious B.1.620 variant, mutations within the prominent proteins are hypothesized to alter its pathogenicity. Moreover, docking evaluations revealed significant differences in binding affinity with human receptors, ACE2 and NRP1. Owing to its characteristics of rendering existing treatments ineffective, we evaluated the drug efficacy against their target protein encoded in the Omicron through molecular docking approach. Most of the tested drugs were proven to be effective. Nirmatrelvir (Paxlovid), MPro 13b, and Lopinavir displayed increased effectiveness and efficacy, while Ivermectin showed the best result against Omicron.

The lymphatic system is important for antigen presentation and immune surveillance. The lymphatic system in the brain was originally introduced by Giovanni Mascagni in 1787, while the rediscovery of it by Jonathan Kipnis and Kari Kustaa Alitalo now opens the door for a new interpretation of neurological diseases and therapeutic applications. The glymphatic system for the exchanges of cerebrospinal fluid (CSF) and interstitial fluid (ISF) is associated with the blood-brain barrier (BBB), which is involved in the maintenance of immune privilege and homeostasis in the brain. Recent notions from studies of postmortem brains and clinical studies of neurodegenerative diseases, infection, and cerebral hemorrhage, implied that the breakdown of those barrier systems and infiltration of activated immune cells disrupt the function of both neurons and glia in the parenchyma (e.g., modulation of neurophysiological properties and maturation of myelination), which causes the abnormality in the functional connectivity of the entire brain network. Due to the vulnerability, such dysfunction may occur in developing brains as well as in senile or neurodegenerative diseases and may raise the risk of emergence of psychosis symptoms. Here, we introduce this hypothesis with a series of studies and cellular mechanisms.

Neuroinflammation in the brain is thought related to the emergence of various psychoses, although the identifying regional significance, the involvement of immune-cells and lymphocytic activity, and ways for the therapeutic recovery are under the effort of researchers. We recently revealed that the cerebellar acute inflammation causes the symptoms manifested in mood disorders or developmental disorders, which were associated with hyperexcitability due to immune-triggered plasticity and the overconnectivity between the inflamed cerebellum and prefrontal cortex (Yamamoto et al., 2019). Here, we give a perspective regarding the emergence of psychoses focusing on the aberrant immunity, the relevance of dysfunctions in brain regions including the cerebellum, and potential therapeutic ways via metabolism.

The vasculature system with a lymphatic function in the brain is manifested as meningeal lymphatic vessels and the glymphatic system, which drain waste products from cerebrospinal fluid (CSF) and produce interstitial fluid. Invasion of stimulated immune cells or inflammatory cytokines during the maturation is regarded as a sign of the disruption of brain functions through excessive immune stresses. However, it is unclear when the lymphatic system is functionally completed and in which parts of the organ the brain immunity is privileged (i.e., the blood-brain barrier is fully established). Here, we visualized the whole-head signal of gadolinium (Gd) contrast agents in mouse brains to investigate the CSF flow from the signals of Gd-contrast agents using magnetic resonance imaging (MRI) in the course of developmental stages. We found higher Gd-signals in the olfactory bulbs, prefrontal cortex, parietal surface regions of the neocortex along with dorsal meningeal lymphatic vessels, ventral midbrain, and a part of cerebellum, as well as in the basal brain regions, at the immature stage of postnatal (P) 4 weeks, compared to the P8-12 weeks. Our results suggest that the barrier of the vasculature system in mouse brains is still permeable until P1-month and small molecules can leak into the parenchyma.

The cerebellum, a universal processor for sensory acquisition and internal models, and its association with synaptic and nonsynaptic plasticity have been envisioned as the biological correlates of learning, perception, and even thought. Indeed, the cerebellum is no longer considered merely as the locus of motor coordination and its learning. Here, we introduce the mechanisms underlying the induction of multiple types of plasticity in cerebellar circuit and give an overview focusing on the plasticity of nonsynaptic intrinsic excitability. The discovery of long-term potentiation of synaptic responsiveness in hippocampal neurons led investigations into changes of their intrinsic excitability. This activity-dependent potentiation of neuronal excitability is distinct from that of synaptic efficacy. Systematic examination of excitability plasticity has indicated that the modulation of various types of Ca2+- and voltage-dependent K(+)channels underlies the phenomenon, which is also triggered by immune activity. Intrinsic plasticity is expressed specifically on dendrites and modifies the integrative processing and filtering effect. In Purkinje cells, modulation of the discordance of synaptic current on soma and dendrite suggested a novel type of cellular learning mechanism. This property enables a plausible synergy between synaptic efficacy and intrinsic excitability, by amplifying electrical conductivity and influencing the polarity of bidirectional synaptic plasticity. Furthermore, the induction of intrinsic plasticity in the cerebellum correlates with motor performance and cognitive processes, through functional connections from the cerebellar nuclei to neocortex and associated regions: for example, thalamus and midbrain. Taken together, recent advances in neuroscience have begun to shed light on the complex functioning of nonsynaptic excitability and the synergy.

The role of dendrites in the integration of widespread synaptic activity has been studied in experiments and theories (Johnston et al., 1996; Magee, 2007). However, whether the conduction of synaptic currents from dendrites to the soma depends on excitability of those dendritic branches is unclear. How modulation of the branch excitability affects the conduction of synaptic inputs and their selection on dendrites is also elusive. Here, I performed simultaneous voltage-clamp recordings from the soma and dendrites of single cerebellar Purkinje neurons in male Sprague-Dawley rats and analyzed the relationship between spontaneous EPSCs on both sides. I found that EPSCs on distal dendrites have a salient discordance in amplitude compared with those on the soma. Furthermore, individual ratios of the EPSC concurrently recorded on the soma and dendrites were not unique, but discrete, suggesting the occurrence of various attenuations in different paths of dendritic branches to the soma. The obtained data and simulations indicate several distinct groups (4.5 ± 0.3, n = 22 somatodendritic recordings) of co-occurred synaptic inputs in Purkinje cell dendrites. This clustering of synaptic currents was suggested to emerge at farther distances than the secondary bifurcations. Finally, ratios of the co-EPSCs were uniformly distributed after either intrinsic plasticity induction or SK-channel blockade. Overall, results suggest that in Purkinje cells the excitability along the dendrite processes modulates the conduction of EPSCs and makes active inputs heterogeneous through SK channel activity, intrinsic plasticity, and dendritic branching. These properties of dendrites may confer branch-specific computational power to neurons. SIGNIFICANCE STATEMENT I have previously studied the “non-synaptic” plasticity of the intrinsic excitability in the cerebellar Purkinje cells (Belmeguenai et al., 2010), and branch-specific increase of intrinsic excitability of the dendrites (Ohtsuki et al., 2012b; Ohtsuki and Hansel, 2018) through the downregulation of SK (small conductance Ca2+-activated K+) channels. In this study, I show that a dendritic filtering of synaptic electroconductivity is heterogeneous among the branches on distal dendrites and that the increase in the dendritic excitability accompanied with the intrinsic plasticity alters a state with the heterogeneity to a globally excitable state in Purkinje neurons. My findings propose a new learning model relying on the intrinsic excitability plasticity of the dendritic branch fields.

Cerebellar dysfunction relates to various psychiatric disorders, including autism spectrum and depressive disorders. However, the physiological aspect is less advanced. Here, we investigate the immune-triggered hyperexcitability in the cerebellum on a wider scope. Activated microglia via exposure to bacterial endotoxin lipopolysaccharide or heat-killed Gram-negative bacteria induce a potentiation of the intrinsic excitability in Purkinje neurons, which is suppressed by microglia-activity inhibitor and microglia depletion. An inflammatory cytokine, tumor necrosis factor alpha (TNF-α), released from microglia via toll-like receptor 4, triggers this plasticity. Our two-photon FRET ATP imaging shows an increase in ATP concentration following endotoxin exposure. Both TNF-α and ATP secretion facilitate synaptic transmission. Region-specific inflammation in the cerebellum in vivo shows depression- and autistic-like behaviors. Furthermore, both TNF-α inhibition and microglia depletion revert such behavioral abnormality. Resting-state functional MRI reveals overconnectivity between the inflamed cerebellum and the prefrontal neocortical regions. Thus, immune activity in the cerebellum induces neuronal hyperexcitability and disruption of psychomotor behaviors in animals.

Neurons store information and participate in memory engrams as a result of experience-dependent changes in synaptic weights and in membrane excitability. Here, we examine excitatory postsynaptic potential (EPSP) amplitude and neuronal excitability in relation to these two mechanisms of plasticity. We analyze somato-dendritic double-patch recordings from cerebellar Purkinje cells while inducing intrinsic, SK2 channel-dependent plasticity or blocking SK channels with bath application of apamin. Both manipulations increase the build-up of EPSP amplitudes during an EPSP train and enhance the number of EPSP-evoked spikes, yielding insights into the mechanistic contribution of EPSP amplitude to single spikes and spike bursts. EPSP amplitude has an impact on whether spikes are fired or not, but direct measures of excitability (spike threshold/AHP) are better predictors of whether individual spikes or spike bursts are fired. Our findings show that Purkinje cell spiking is synaptically driven but that burst firing is gated by SK2 channel modulation and plasticity.