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

Direct microglia replacement reveals pathologic and therapeutic contributions of brain macrophages to a monogenic neurological disease

Krabbe disease, also named globoid cell (GC) leukodystrophy (GLD) for its distinct lipid-laden macrophages, is a severe leukodystrophy caused by galactosylceramidase (GALC) mutations. Hematopoietic stem cell transplant (HSCT) ameliorates disease and is associated with central nervous system (CNS) engraftment of GALC+ donor macrophages. Yet, the role of macrophages in GLD pathophysiology and HSCT remains unclear. Using single-cell sequencing, we revealed early interferon response signatures that preceded progressively severe macrophage dyshomeostasis and identified a molecular signature of GCs, which we validated in human brain specimens. Genetic depletion and direct microglia replacement by CNS monocyte injection rapidly replaced >80% of endogenous microglia with healthy macrophages in the twitcher (GalcW355∗) mouse model of GLD. Perinatal microglia replacement completely normalized transcriptional signatures, rescued histopathology, and doubled average survival. Overall, we uncovered distinct forms of microglial dysfunction and evidence that direct, CNS-limited microglia replacement improves a monogenic neurodegenerative disease, identifying a promising therapeutic target.

Microglia transcriptional states and their functional significance: Context drives diversity

In the brain, microglia are continuously exposed to a dynamic microenvironment throughout life, requiring them to adapt accordingly to specific developmental or disease-related demands. The advent of single-cell sequencing technologies has revealed the diversity of microglial transcriptional states. In this review, we explore the various contexts that drive transcriptional diversity in microglia and assess the extent to which non-homeostatic conditions induce context-specific signatures. We discuss our current understanding and knowledge gaps regarding the relationship between transcriptional states and microglial function, review the influence of complex microenvironments and prior experiences on microglial state induction, and highlight strategies to bridge the gap between mouse and human studies to advance microglia-targeting therapeutics.

Maternal natural killer cells drive neuroimmune disorders in offspring through aberrant secretion of extracellular granzyme B

Prenatal viral infections can lead to neurodevelopmental disorders in offspring. However, how viral-induced maternal perturbations impact fetal brain macrophages remains insufficiently clear. Here, we demonstrated that inflammatory decidual natural killer (NK) cells, triggered by viral infection-induced maternal immune activation, drove macrophage activation, neurodevelopmental disorders, and behavioral deficits in offspring. Extracellular granzyme B (GzmB), predominantly released by the maternal CD49a+ tissue-resident NK subset under type I interferon stimulation, crossed the maternal-fetal barrier and promoted interferon-stimulated gene (ISG)-expressing fetal macrophage accumulation and microglial activation. Targeting extracellular GzmB through the systemic administration of the serine protease inhibitor Serpina3n or genetic ablation of Gzmb in maternal NK cells mitigated neuroimmune disorders in the fetal brain. These findings suggest that exposure to a perturbed maternal milieu reprograms decidual NK cell immunity, disrupting fetal neuroimmune homeostasis and increasing offspring susceptibility to neurodevelopmental disorders later in life.

Microglial and TREM2 dialogues in the developing brain

From the migration of precursor cells to the refinement of neural circuits, the immune system plays a critical role in the development of the central nervous system. As the brain resident macrophages, microglia are integral to these processes, influencing key developmental stages and contributing to circuit remodeling. Recent years have brought a wealth of new insights into how microglia regulate key stages of brain development, particularly through their continuous crosstalk with various brain cell types. In this review, we synthesize this growing body of literature on microglia and neurodevelopment, highlighting the involvement of the TREM2 receptor, known for its role in aging and neurodegeneration, which profoundly affects the state of microglia and guides target cells by shaping their transcriptional and functional fate. We examine microglial communication with four major cell types-neural precursors, neurons, astrocytes, and oligodendrocytes-also delving into the described mechanisms that underpin these interactions.

Diffuse traumatic brain injury induced stimulator of interferons (STING) signaling in microglia drives cortical neuroinflammation, neuronal dysfunction, and impaired cognition

Neuropsychiatric complications including depression and cognitive impairment develop, persist, and worsen in the years after traumatic brain injury (TBI), negatively affecting life and lifespan. Inflammatory responses mediated by microglia are associated with the transition from acute to chronic neuroinflammation after TBI. Moreover, type I interferon (IFN-I) signaling is a key mediator of inflammation during this transition. Thus, the purpose of this study was to determine the degree to which a microglia-specific knockout of the stimulator of interferons (STING) influenced TBI-induced neuroinflammation, neuronal dysfunction, and cognitive impairment. Here, microglial inducible STING knockout (CX₃CR1Cre/ERT2 x STINGfl/fl) mice were created and validated (mSTING-/-). Diffuse brain injury (midline fluid percussion) in male and female mice increased STING expression in microglia, promoted microglial morphological restructuring, and induced robust cortical inflammation and pathology 7 days post injury (dpi). These TBI-associated responses were attenuated in mSTING-/- mice. Increased cortical astrogliosis and rod-shaped microglia induced by TBI were independent of mSTING-/-. 7 dpi, TBI induced 237 differentially expressed genes (DEG) in the cortex of functionally wildtype (STINGfl/fl) associated with STING, NF-κB, and Interferon Alpha signaling and 85% were attenuated by mSTING-/-. Components of neuronal injury including reduced NeuN expression, increased cortical lipofuscin, and increased neurofilament light chain in plasma were increased by TBI and dependent on mSTING. TBI-associated cognitive tasks (novel object recognition/location, NOR/NOL) at 7 dpi were dependent on mSTING. Notably, the TBI-induced cognitive deficits in NOR/NOL and increased cortical inflammation 7 dpi were unaffected in global interferon-α/β receptor 1 knockout (IFNAR1) mice. In the final study, the RNA profile of neurons after TBI in STINGfl/fl and mSTING-/- mice was assessed 7 dpi by single nucleus RNA-sequencing. There was a TBI-dependent suppression of cortical neuronal homeostasis with reductions in CREB signaling, synaptogenesis, and oxytocin signaling and increases in cilium assembly and PTEN signaling. Overall, mSTING-/- prevented 50% of TBI-induced DEGs in cortical neurons. Collectively, ablation of STING in microglia attenuates TBI-induced interferon responses, cortical inflammation, neuronal dysfunction, neuronal pathology, and cognitive impairment.

Type I interferon signalling and interferon-responsive microglia in health and disease

Recent evidence suggests that type I interferon (IFN-I) signalling extends beyond its canonical roles in antiviral defence and immunomodulation. Over the past decade, dysregulated IFN-I signalling has been linked to genetic disorders and neurodegenerative diseases, where it may contribute to neurological impairments. Microglia have emerged as key mediators of IFN-I responses in the central nervous system. A distinct transcriptional state responsive to interferons has recently been identified in microglia. The activation of the IFN-I pathway in these cells is now recognised as pivotal in both development and neurodegeneration. This review is divided into two main sections: the first examines the broader role of IFN-I signalling in the central nervous system, particularly its contribution to neurological dysfunction; the second focuses on the specific state of interferon-responsive microglia, exploring its mechanisms and relevance in neurodegenerative conditions. Finally, we discuss how these areas intersect and their implications for both healthy and diseased states.

Microglia heterogeneity, modeling and cell-state annotation in development and neurodegeneration

Within the CNS, microglia execute various functions associated with brain development, maintenance of homeostasis and elimination of pathogens and protein aggregates. This wide range of activities is closely associated with a plethora of cellular states, which may reciprocally influence or be influenced by their functional dynamics. Advancements in single-cell RNA sequencing have enabled a nuanced exploration of the intricate diversity of microglia, both in health and disease. Here, we review our current understanding of microglial transcriptional heterogeneity. We provide an overview of mouse and human microglial diversity encompassing aspects of development, neurodegeneration, sex and CNS regions. We offer an insight into state-of-the-art technologies and model systems that are poised to improve our understanding of microglial cell states and functions. We also provide suggestions and a tool to annotate microglial cell states on the basis of gene expression.

Modification of Neural Circuit Functions by Microglial P2Y6 Receptors in Health and Neurodegeneration

Neural circuits consisting of neurons and glial cells help to establish all functions of the CNS. Microglia, the resident immunocytes of the CNS, are endowed with UDP-sensitive P2Y6 receptors (P2Y6Rs) which regulate phagocytosis/pruning of excessive synapses during individual development and refine synapses in an activity-dependent manner during adulthood. In addition, this type of receptor plays a decisive role in primary (Alzheimer's disease, Parkinson's disease, neuropathic pain) and secondary (epilepsy, ischemic-, mechanical-, or irradiation-induced) neurodegeneration. A whole range of microglial cytokines controlled by P2Y6Rs, such as the interleukins IL-1β, IL-6, IL-8, and tumor necrosis factor-α (TNF-α), leads to neuroinflammation, resulting in neurodegeneration. Hence, small molecular antagonists of P2Y6Rs and genetic knockdown of this receptor provide feasible ways to alleviate inflammation-induced neurological disorders but might also interfere with the regulation of the synaptic circuitry. The present review aims at investigating this dual role of P2Y6Rs in microglia, both in shaping neural circuits by targeted phagocytosis and promoting neurodegenerative illnesses by fostering neuroinflammation through multiple transduction mechanisms.

Pharmacological targeting of mitophagy via ALT001 improves herpes simplex virus 1 (HSV1)-mediated microglial inflammation and promotes amyloid β phagocytosis by restricting HSV1 infection

Rationale: One of the hallmarks of Alzheimer's disease (AD) is the accumulation of dysfunctional mitochondria. Herpes simplex virus type 1 (HSV1) may be a risk factor for the neuropathology linked to amyloid β (Aβ) accumulation. However, the mechanisms underlying HSV1-associated mitochondrial dysfunction in AD remain unclear. ALT001 is a novel drug that ameliorates AD-related cognitive impairment via ULK1/Rab9-mediated alternative mitophagy. In this study, we investigated the effects of ALT001 on the neurodegeneration-related microglial signatures associated with HSV1 infection. Methods: Molecular mechanisms and physiological functions of mitophagy was investigated in HSV1-infected microglia, including primary murine and human embryonic stem cell (ESC)-derived microglia (ES-MG), as well as in a microglia-neuron co-culture system. Microglial gene signatures following HSV1 infection in the presence or absence of ALT001 were analyzed using bulk RNA sequencing, and the effects of ALT001 on microglial phagocytosis and microglia-mediated immune responses were further evaluated by flow cytometry and cytokine profiles. Results: HSV1 infection inhibited PINK1/Parkin-mediated mitophagy via HSV1-encoded protein kinase US3, resulting in mitochondrial dysfunction in both human and mouse microglia. Furthermore, transcriptomic analysis of HSV1-infected microglia revealed an upregulation of distinct microglial genes associated with disease-associated microglia (DAM)-like phenotype and pro-inflammatory activity. Pharmacological targeting of mitophagy using ALT001 prevents mitochondrial damage caused by HSV1 through ULK1/Rab9-mediated pathway. Furthermore, ALT001-induced ULK1/Rab9-dependent mitophagy restricts HSV1 infection by activating interferon-mediated antiviral immunity. Consequently, ALT001 reduces HSV1-triggered neuroinflammation, recovers HSV1-altered microglial phagocytosis for Aβ, and efficiently reverses morphological and molecular abnormalities in HSV1-infected microglia by triggering mitophagy in ES-MG. ALT001 also suppressed HSV1-mediated Aβ accumulation and neurodegeneration in the microglia-neuron co-culture and cerebral organoid model. Conclusions: In this study, we identified a critical molecular link between HSV1 and AD-related microglial dysfunction. Furthermore, our findings provide an evidence that therapeutic targeting of alternative mitophagy via ALT001 effectively interfere with HSV1-induced microglial dysfunction and alleviate neurodegeneration.

Microglia regulate myelin clearance and cholesterol metabolism after demyelination via interferon regulatory factor 5

Interferon regulatory factor 5 (IRF5) is a transcription factor that plays a role in orchestrating innate immune responses, particularly in response to viral infections. Notably, IRF5 has been identified as a microglia risk gene linked to multiple sclerosis (MS), but its specific role in MS pathogenesis remains unclear. Through the use of Irf5-/- mice, our study uncovers a non-canonical function of IRF5 in MS recovery. Irf5-/- mice exhibited increased damage in an experimental autoimmune encephalomyelitis (EAE) model and demonstrated impaired oligodendrocyte recruitment into the lesion core following lysolecithin-induced demyelination. Transcriptomic and lipidomic analyses revealed that IRF5 has a role in microglia-mediated myelin phagocytosis, lipid metabolism, and cholesterol homeostasis. Indeed, Irf5-/- microglia phagocytose myelin, but myelin debris is not adequately degraded, leading to an accumulation of lipid droplets, cholesterol esters, and cholesterol crystals within demyelinating lesions. This abnormal buildup can hinder remyelination processes. Importantly, treatments that promote cholesterol transport were found to reduce lipid droplet accumulation and mitigate the exacerbated damage in Irf5-/- mice with EAE. Altogether, our study identified the antiviral transcription factor IRF5 as a key transcriptional regulator of lipid degradation and cholesterol homeostasis and suggest that loss of IRF5 function leads to pathogenic lipid accumulation in microglia, thereby obstructing remyelination. These data and the fact that Irf5 polymorphisms are significantly associated with MS, highlight IRF5 as a potential therapeutic target to promote regenerative responses.

The neuroimmune nexus: unraveling the role of the mtDNA-cGAS-STING signal pathway in Alzheimer's disease

The relationship between Alzheimer's disease (AD) and neuroimmunity has gradually begun to be unveiled. Emerging evidence indicates that cyclic GMP-AMP synthase (cGAS) acts as a cytosolic DNA sensor, recognizing cytosolic damage-associated molecular patterns (DAMPs), and inducing the innate immune response by activating stimulator of interferon genes (STING). Dysregulation of this pathway culminates in AD-related neuroinflammation and neurodegeneration. A substantial body of evidence indicates that mitochondria are involved in the critical pathogenic mechanisms of AD, whose damage leads to the release of mitochondrial DNA (mtDNA) into the extramitochondrial space. This leaked mtDNA serves as a DAMP, activating various pattern recognition receptors and immune defense networks in the brain, including the cGAS-STING pathway, ultimately leading to an imbalance in immune homeostasis. Therefore, modulation of the mtDNA-cGAS-STING pathway to restore neuroimmune homeostasis may offer promising prospects for improving AD treatment outcomes. In this review, we focus on the mechanisms of mtDNA release during stress and the activation of the cGAS-STING pathway. Additionally, we delve into the research progress on this pathway in AD, and further discuss the primary directions and potential hurdles in developing targeted therapeutic drugs, to gain a deeper understanding of the pathogenesis of AD and provide new approaches for its therapy.

The role of type I interferon signaling in myeloid anti-tumor immunity

Tumors often arise in chronically inflamed, and thus immunologically highly active niches. While immune cells are able to recognize and remove transformed cells, tumors eventually escape the control of the immune system by shaping their immediate microenvironment. In this context, macrophages are of major importance, as they initially exert anti-tumor functions before they adopt a tumor-associated phenotype that instead inhibits anti-tumor immune responses and even allows for sustaining a smoldering inflammatory, growth promoting tumor microenvironment (TME). Type I interferons (IFNs) are well established modulators of inflammatory reactions. While they have been shown to directly inhibit tumor growth, there is accumulating evidence that they also play an important role in altering immune cell functions within the TME. In the present review, we focus on the impact of type I IFNs on anti-tumor responses, driven by monocytes and macrophages. Specifically, we will provide an overview of tumor-intrinsic factors, which impinge on IFN-stimulated gene (ISG) expression, like the presence of nucleic acids, metabolites, or hypoxia. We will further summarize the current understanding of the consequences of altered IFN responses on macrophage phenotypes, i.e., differentiation, polarization, and functions. For the latter, we will focus on macrophage-mediated tumor cell killing and phagocytosis, as well as on how macrophages affect their environment by secreting cytokines and directly interacting with immune cells. Finally, we will discuss how type I IFN responses in macrophages might affect and should be considered for current and future tumor therapies.

Glial 'omics in ischemia: Acute stroke and chronic cerebral small vessel disease

Vascular injury and pathologies underlie common diseases including ischemic stroke and cerebral small vessel disease (CSVD). Prior work has identified a key role for glial cells, including microglia, in the multifaceted and temporally evolving neuroimmune response to both stroke and CSVD. Transcriptional profiling has led to important advances including identification of distinct gene expression signatures in ischemia-exposed, flow cytometrically sorted microglia and more recently single cell RNA sequencing-identified microglial subpopulations or clusters. There is a reassuring degree of overlap in the results from these two distinct methodologies with both identifying a proliferative and a separate type I interferon responsive microglial element. Similar patterns were later seen using multimodal and spatial transcriptomal profiling in ischemia-exposed microglia and astrocytes. Methodological advances including enrichment of specific neuroanatomic/functional regions (such as the neurovascular unit) prior to single cell RNA sequencing has led to identification of novel cellular subtypes and generation of new credible hypotheses as to cellular function based on the enhanced cell sub-type specific gene expression patterns. A ribosomal tagging strategy focusing on the cellular translatome analyses carried out in the acute phases post stroke has revealed distinct inflammation-regulating roles for microglia and astrocytes in this setting. Early spatial transcriptomics experiments using cerebral ischemia models have identified regionally distinct microglial cell clusters in ischemic core versus penumbra. There is great potential for combination of these methods for multi-omics approaches to further elucidate glial responses in the context of both acute ischemic stroke and chronic CSVD.

Neuroinflammation in Adaptive Immunodeficient Mice with Colitis-like Symptoms

Emerging evidence suggests that systemic inflammation may play a critical role in neurological disorders. Recent studies have shown the connection between inflammatory bowel diseases (IBD) and neurological disorders, revealing a bidirectional relationship through the gut-brain axis. Immunotherapies, such as Treg cells infusion, have been proposed for IBD. However, the role of adaptive immune cells in IBD-induced neuroinflammation remains unclear. In this study, we established an animal model for IBD in mice with severe combined immune-deficient (SCID), an adaptive immune deficiency, to investigate the role of adaptive immune cells in IBD-induced neuroinflammation. Mice were fed 1%, 3%, or 5% dextran sulfate sodium (DSS) for 5 days. We measured body weight, colon length, disease activity index (DAI), and crypt damage. Pro-inflammatory cytokines were measured in the colon, while microglial morphology, neuronal count, and inflammatory cytokines were analyzed in the brain. In the 3% DSS group, colitis symptoms appeared at day 7, with reduced colon length and increased crypt damage showing colitis-like symptoms. By day 21, colon length and crypt damage persisted, while DAI showed recovery. Although colonic inflammation peaked at day 7, no significant increase in inflammatory cytokines or microglial hyperactivation was observed in the brain. By day 21, neuroinflammation was detected, albeit with a slight delay, in the absence of adaptive immune cells. The colitis-induced neuroinflammation model provides insights into the fundamental immune mechanisms of the gut-brain axis and may contribute to developing immune cell therapies for IBD-induced neuroinflammation.

Mitochondrially Transcribed dsRNA Mediates Manganese-induced Neuroinflammation

Manganese (Mn) is an essential trace element required for various biological functions, but excessive Mn levels are neurotoxic and lead to significant health concerns. The mechanisms underlying Mn-induced neurotoxicity remain poorly understood. Neuropathological studies of affected brain regions reveal astrogliosis, and neuronal loss, along with evidence of neuroinflammation. Here, we present a novel Mn-dependent mechanism linking mitochondrial dysfunction to neuroinflammation. We found that Mn disrupts mitochondrial transcriptome processing, resulting in the accumulation of complementary RNAs that form double-stranded RNA (dsRNA). This dsRNA is released to the cytoplasm, where it activates cytosolic sensor pathways, triggering type I interferon responses and inflammatory cytokine production. This mechanism is present in 100-day human cerebral organoids, where Mn-induced inflammatory responses are observed predominantly in mature astrocytes. Similar effects were observed in vivo in a mouse model carrying mutations in the SLC30A10 gene, which results in Mn accumulation. These findings highlight a previously unrecognized role for mitochondrial dsRNA in Mn-induced neuroinflammation and provide insights into the molecular basis of manganism. We propose that this mitochondrial dsRNA-induced inflammatory pathway has broad implications in for neurodegenerative diseases caused by environmental or genetic insults.

Early central nervous system development and neuron regeneration

The nervous system is the most complex system in the human body, and the normal development of the central nervous system (CNS) is essential for maintaining the healthy life activities of the individual. CNS development requires the orchestration of multiple internal or external or direct or indirect factors to regulate neural stem cell fate specification. Here, we provide a broad overview of the regulatory system of nerve cell fate decisions and discuss the latest technological approaches to achieve neural regeneration. Understanding the CNS development and regeneration mechanisms has shifted the paradigm from traditional experiments to high-throughput sequencing.

Exosome-based targeted delivery of NF-κB ameliorates age-related neuroinflammation in the aged mouse brain

Neuroinflammation, a significant contributor to various neurodegenerative diseases, is strongly associated with the aging process; however, to date, no efficacious treatments for neuroinflammation have been developed. In aged mouse brains, the number of infiltrating immune cells increases, and the key transcription factor associated with increased chemokine levels is nuclear factor kappa B (NF-κB). Exosomes are potent therapeutics or drug delivery vehicles for various materials, including proteins and regulatory genes, to target cells. In the present study, we evaluated the therapeutic efficacy of exosomes loaded with a nondegradable form of IκB (Exo-srIκB), which inhibits the nuclear translocation of NF-κB to suppress age-related neuroinflammation. Single-cell RNA sequencing revealed that these anti-inflammatory exosomes targeted macrophages and microglia, reducing the expression of inflammation-related genes. Treatment with Exo-srIκB also suppressed the interactions between macrophages/microglia and T and B cells in the aged brain. We demonstrated that Exo-srIκB successfully alleviates neuroinflammation by primarily targeting activated macrophages and partially modulating the functions of age-related interferon-responsive microglia in the brain. Thus, our findings highlight Exo-srIκB as a potential therapeutic agent for treating age-related neuroinflammation.

Microglia and Astrocytes in Postnatal Neural Circuit Formation

Over the past two decades, microglia and astrocytes have emerged as critical mediators of neural circuit formation. Particularly during the postnatal period, both glial subtypes play essential roles in orchestrating nervous system development through communication with neurons. These functions include regulating synapse elimination, modulating neuronal density and activity, mediating synaptogenesis, facilitating axon guidance and organization, and actively promoting neuronal survival. Despite the vital roles of both microglia and astrocytes in ensuring homeostatic brain development, the extent to which the postnatal functions of these cells are regulated by sex and the manner in which these glial cells communicate with one another to coordinate nervous system development remain less well understood. Here, we review the critical functions of both microglia and astrocytes independently and synergistically in mediating neural circuit formation, focusing our exploration on the postnatal period from birth to early adulthood.

Immune control of brain physiology

The peripheral immune system communicates with the brain through complex anatomical routes involving the skull, the brain borders, circumventricular organs and peripheral nerves. These immune-brain communication pathways were classically considered to be dormant under physiological conditions and active only in cases of infection or damage. Yet, peripheral immune cells and signals are key in brain development, function and maintenance. In this Perspective, we propose an alternative framework for understanding the mechanisms of immune-brain communication. During brain development and in homeostasis, these anatomical structures allow selected elements of the peripheral immune system to affect the brain directly or indirectly, within physiological limits. By contrast, in ageing and pathological settings, detrimental peripheral immune signals hijack the existing communication routes or alter their structure. We discuss why a diversity of communication channels is needed and how they work in relation to one another to maintain homeostasis of the brain.

Mechanisms and environmental factors shaping the ecosystem of brain macrophages

Brain macrophages encompass two major populations: microglia in the parenchyma and border-associated macrophages (BAMs) in the extra-parenchymal compartments. These cells play crucial roles in maintaining brain homeostasis and immune surveillance. Microglia and BAMs are phenotypically and epigenetically distinct and exhibit highly specialized functions tailored to their environmental niches. Intriguingly, recent studies have shown that both microglia and BAMs originate from the same myeloid progenitor during yolk sac hematopoiesis, but their developmental fates diverge within the brain. Several works have partially unveiled the mechanisms orchestrating the development of microglia and BAMs in both mice and humans; however, many questions remain unanswered. Defining the molecular underpinnings controlling the transcriptional and epigenetic programs of microglia and BAMs is one of the upcoming challenges for the field. In this review, we outline current knowledge on ontogeny, phenotypic diversity, and the factors shaping the ecosystem of brain macrophages. We discuss insights garnered from human studies, highlighting similarities and differences compared to mice. Lastly, we address current research gaps and potential future directions in the field. Understanding how brain macrophages communicate with their local environment and how the tissue instructs their developmental trajectories and functional features is essential to fully comprehend brain physiology in homeostasis and disease.

The immunology of stroke and dementia

Ischemic stroke and vascular cognitive impairment, caused by a sudden arterial occlusion or more subtle but protracted vascular insufficiency, respectively, are leading causes of morbidity and mortality worldwide with limited therapeutic options. Innate and adaptive immunity have long been implicated in neurovascular injury, but recent advances in methodology and new experimental approaches have shed new light on their contributions. A previously unappreciated dynamic interplay of brain-resident, meningeal, and systemic immune cells with the ischemic brain and its vasculature has emerged, and new insights into the frequent overlap between vascular and Alzheimer pathology have been provided. Here, we critically review these recent findings, place them in the context of current concepts on neurovascular pathologies and Alzheimer's disease, and highlight their impact on recent stroke and Alzheimer therapies.

Astrocytes in aging

The mammalian nervous system is impacted by aging. Aging alters brain architecture, is associated with molecular damage, and can manifest with cognitive and motor deficits that diminish the quality of life. Astrocytes are glial cells of the CNS that regulate the development, function, and repair of neural circuits during development and adulthood; however, their functions in aging are less understood. Astrocytes change their transcriptome during aging, with astrocytes in areas such as the cerebellum, the hypothalamus, and white matter-rich regions being the most affected. While numerous studies describe astrocyte transcriptional changes in aging, many questions still remain. For example, how is astrocyte function altered by transcriptional changes that occur during aging? What are the mechanisms promoting astrocyte aged states? How do aged astrocytes impact brain function? This review discusses features of aged astrocytes and their potential triggers and proposes ways in which they may impact brain function and health span.

Inflammatory Stimulation Upregulates the Receptor Transporter Protein 4 (RTP4) in SIM-A9 Microglial Cells

The receptor transporter protein 4 (RTP4) is a receptor chaperone protein that targets class A G-protein coupled receptor (GPCR)s. Recently, it has been found to play a role in peripheral inflammatory regulation, as one of the interferon-stimulated genes (ISGs). However, the detailed role of RTP4 in response to inflammatory stress in the central nervous system has not yet been fully understood. While we have previously examined the role of RTP4 in the brain, particularly in neuronal cells, this study focuses on its role in microglial cells, immunoreactive cells in the brain that are involved in inflammation. For this, we examined the changes in the RTP4 levels in the microglial cells after exposure to inflammatory stress. We found that lipopolysaccharide (LPS) treatment (0.1~1 µg/mL, 24 h) significantly upregulated the RTP4 mRNA levels in the microglial cell line, SIM-A9. Furthermore, the interferon (IFN)-β mRNA levels and extracellular levels of IFN-β were also increased by LPS treatment. This upregulation was reversed by treatment with neutralizing antibodies targeting either the interferon receptor (IFNR) or toll-like receptor 4 (TLR4), and with a TLR4 selective inhibitor, or a Janus kinase (JAK) inhibitor. On the other hand, the mitogen-activated protein kinase kinase (MEK) inhibitor, U0126, significantly enhanced the increase in RTP4 mRNA following LPS treatment, whereas the PKC inhibitor, calphostin C, had no effect. These findings suggest that in microglial cells, LPS-induced inflammatory stress activates TLR4, leading to the production of type I IFN, the activation of IFN receptor and JAK, and finally, the induction of RTP4 gene expression. Based on these results, we speculate that RTP4 functions as an inflammation-responsive molecule in the brain. However, further research is needed to fully understand its role.

Platform influencers-host RNA control of antiviral immunity

Cellular RNAs directly regulate the activity of an antiviral immune signaling complex.

Microglial cathepsin B promotes neuronal efferocytosis during brain development

Half of all newborn neurons in the developing brain are removed via efferocytosis - the phagocytic clearance of apoptotic cells. Microglia are brain-resident professional phagocytes that play important roles in neural circuit development including as primary effectors of efferocytosis. While the mechanisms through which microglia recognize potential phagocytic cargo are widely studied, the lysosomal mechanisms that are necessary for efficient digestion are less well defined. Here we show that the lysosomal protease cathepsin B promotes microglial efferocytosis of neurons and restricts the accumulation of apoptotic cells during brain development. We show that cathepsin B is microglia-specific and enriched in brain regions where neuronal turnover is high in both zebrafish and mouse. Myeloid-specific cathepsin B knockdown in zebrafish led to dysmorphic microglia containing undigested dead cells, as well as an accumulation of dead cells in surrounding tissue. These effects where phenocopied in mice globally deficient for Ctsb using markers for apoptosis. We also observed behavioral impairments in both models. Live imaging studies in zebrafish revealed deficits in phagolysosomal fusion and acidification, and live imaging of cultured mouse microglia reveal delayed phagocytosis consistent with impairments in digestion and resolution of phagocytosis rather than initial uptake. These data reveal a novel role for microglial cathepsin B in mediating neuronal efferocytosis during typical brain development.

Distinct tumor architectures and microenvironments for the initiation of breast cancer metastasis in the brain

Brain metastasis, a serious complication of cancer, hinges on the initial survival, microenvironment adaptation, and outgrowth of disseminated cancer cells. To understand the early stages of brain colonization, we investigated two prevalent sources of cerebral relapse, triple-negative (TNBC) and HER2+ (HER2BC) breast cancers. Using mouse models and human tissue samples, we found that these tumor types colonize the brain, with a preference for distinctive tumor architectures, stromal interfaces, and autocrine programs. TNBC models tend to form perivascular sheaths with diffusive contact with astrocytes and microglia. In contrast, HER2BC models tend to form compact spheroids driven by autonomous tenascin C production, segregating stromal cells to the periphery. Single-cell transcriptomics of the tumor microenvironment revealed that these architectures evoke differential Alzheimer's disease-associated microglia (DAM) responses and engagement of the GAS6 receptor AXL. The spatial features of the two modes of brain colonization have relevance for leveraging the stroma to treat brain metastasis.

Cell-Type Resolved Protein Atlas of Brain Lysosomes Identifies SLC45A1-Associated Disease as a Lysosomal Disorder

Mutations in lysosomal genes cause neurodegeneration and neurological lysosomal storage disorders (LSDs). Despite their essential role in brain homeostasis, the cell-type-specific composition and function of lysosomes remain poorly understood. Here, we report a quantitative protein atlas of the lysosome from mouse neurons, astrocytes, oligodendrocytes, and microglia. We identify dozens of novel lysosomal proteins and reveal the diversity of the lysosomal composition across brain cell types. Notably, we discovered SLC45A1, mutations in which cause a monogenic neurological disease, as a neuron-specific lysosomal protein. Loss of SLC45A1 causes lysosomal dysfunction in vitro and in vivo. Mechanistically, SLC45A1 plays a dual role in lysosomal sugar transport and stabilization of V1 subunits of the V-ATPase. SLC45A1 deficiency depletes the V1 subunits, elevates lysosomal pH, and disrupts iron homeostasis causing mitochondrial dysfunction. Altogether, our work redefines SLC45A1-associated disease as a LSD and establishes a comprehensive map to study lysosome biology at cell-type resolution in the brain and its implications for neurodegeneration.

The impact of neuroinflammation on neuronal integrity

Neuroinflammation, characterized by a complex interplay among innate and adaptive immune responses within the central nervous system (CNS), is crucial in responding to infections, injuries, and disease pathologies. However, the dysregulation of the neuroinflammatory response could significantly affect neurons in terms of function and structure, leading to profound health implications. Although tremendous progress has been made in understanding the relationship between neuroinflammatory processes and alterations in neuronal integrity, the specific implications concerning both structure and function have not been extensively covered, with the exception of perspectives on glial activation and neurodegeneration. Thus, this review aims to provide a comprehensive overview of the multifaceted interactions among neurons and key inflammatory players, exploring mechanisms through which inflammation influences neuronal functionality and structural integrity in the CNS. Further, it will discuss how these inflammatory mechanisms lead to impairment in neuronal functions and architecture and highlight the consequences caused by dysregulated neuronal functions, such as cognitive dysfunction and mood disorders. By integrating insights from recent research findings, this review will enhance our understanding of the neuroinflammatory landscape and set the stage for future interventions that could transform current approaches to preserve neuronal integrity and function in CNS-related inflammatory conditions.

Prenatal drivers of microglia vulnerability in the adult

Environmental insults during early development heavily affect brain trajectories. Among these, maternal infections, high-fat diet regimens, and sleep disturbances pose a significant risk for neurodevelopmental derangements in the offspring. Notably, scattered evidence is starting to emerge that also paternal lifestyle habits may impact the offspring development. Given their key role in controlling neurogenesis, synaptogenesis and shaping neuronal circuits, microglia represent the most likely suspects of mediating the detrimental effects of prenatal insults. For some of these environmental triggers, like maternal infections, ample literature evidence demonstrates the central role of microglia, also delineating the specific transcriptomic and proteomic profiles induced by these insults. In other contexts, the analysis of microglia is still in its infancy. Fostering these studies is needed to define microglia as potential therapeutic target in the frame of disorders consequent to maternal immune activation.

Microglia cannibalism during neurodevelopment results in necroptotic cell death

Clearance of dying neurons by microglia is critical to healthy neurodevelopment, but what else do microglia eat? A new study in PLOS Biology demonstrates that microglia not only eat neurons but each other, and that this "microglia cannibalism" causes necroptotic cell death.

Nucleic acid sensing in the central nervous system: Implications for neural circuit development, function, and degeneration

Nucleic acids are a critical trigger for the innate immune response to infection, wherein pathogen-derived RNA and DNA are sensed by nucleic acid sensing receptors. This subsequently drives the production of type I interferon and other inflammatory cytokines to combat infection. While the system is designed such that these receptors should specifically recognize pathogen-derived nucleic acids, it is now clear that self-derived RNA and DNA can also stimulate these receptors to cause aberrant inflammation and autoimmune disease. Intriguingly, similar pathways are now emerging in the central nervous system in neurons and glial cells. As in the periphery, these signaling pathways are active in neurons and glia to present the spread of pathogens in the CNS. They further appear to be active even under steady conditions to regulate neuronal development and function, and they can become activated aberrantly during disease to propagate neuroinflammation and neurodegeneration. Here, we review the emerging new roles for nucleic acid sensing mechanisms in the CNS and raise open questions that we are poised to explore in the future.

Microglia replacement by ER-Hoxb8 conditionally immortalized macrophages provides insight into Aicardi-Goutières Syndrome neuropathology

Microglia, the brain's resident macrophages, can be reconstituted by surrogate cells - a process termed "microglia replacement." To expand the microglia replacement toolkit, we here introduce estrogen-regulated (ER) homeobox B8 (Hoxb8) conditionally immortalized macrophages, a cell model for generation of immune cells from murine bone marrow, as a versatile model for microglia replacement. We find that ER-Hoxb8 macrophages are highly comparable to primary bone marrow-derived (BMD) macrophages in vitro, and, when transplanted into a microglia-free brain, engraft the parenchyma and differentiate into microglia-like cells. Furthermore, ER-Hoxb8 progenitors are readily transducible by virus and easily stored as stable, genetically manipulated cell lines. As a demonstration of this system's power for studying the effects of disease mutations on microglia in vivo, we created stable, Adar1-mutated ER-Hoxb8 lines using CRISPR-Cas9 to study the intrinsic contribution of macrophages to Aicardi-Goutières Syndrome (AGS), an inherited interferonopathy that primarily affects the brain and immune system. We find that Adar1 knockout elicited interferon secretion and impaired macrophage production in vitro, while preventing brain macrophage engraftment in vivo - phenotypes that can be rescued with concurrent mutation of Ifih1 (MDA5) in vitro, but not in vivo. Lastly, we extended these findings by generating ER-Hoxb8 progenitors from mice harboring a patient-specific Adar1 mutation (D1113H). We demonstrated the ability of microglia-specific D1113H mutation to drive interferon production in vivo, suggesting microglia drive AGS neuropathology. In sum, we introduce the ER-Hoxb8 approach to model microglia replacement and use it to clarify macrophage contributions to AGS.

Microglia: The Drunken Gardeners of Early Adversity

Early life adversity (ELA) is a heterogeneous group of negative childhood experiences that can lead to abnormal brain development and more severe psychiatric, neurological, and medical conditions in adulthood. According to the immune hypothesis, ELA leads to an abnormal immune response characterized by high levels of inflammatory cytokines. This abnormal immune response contributes to more severe negative health outcomes and a refractory response to treatment in individuals with a history of ELA. Here, we examine this hypothesis in the context of recent rodent studies that focus on the impact of ELA on microglia, the resident immune cells in the brain. We review recent progress in our ability to mechanistically link molecular alterations in microglial function during a critical period of development with changes in synaptic connectivity, cognition, and stress reactivity later in life. We also examine recent research showing that ELA induces long-term alterations in microglial inflammatory response to "secondary hits" such as traumatic brain injury, substance use, and exposure to additional stress in adulthood. We conclude with a discussion on future directions and unresolved questions regarding the signals that modify microglial function and the clinical significance of rodent studies for humans.

LRRK2 kinase activity restricts NRF2-dependent mitochondrial protection in microglia

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

Dissecting and Visualizing the Functional Diversity of Cardiac Macrophages

Cardiac macrophages represent a functionally diverse population of cells involved in cardiac homeostasis, repair, and remodeling. With recent advancements in single-cell technologies, it is possible to elucidate specific macrophage subsets based on transcriptional signatures and cell surface protein expression to gain a deep understanding of macrophage diversity in the heart. The use of fate-mapping technologies and parabiosis studies have provided insight into the ontogeny and dynamics of macrophages identifying subsets derived from embryonic and adult definitive hematopoietic progenitors that include tissue-resident and bone marrow monocyte-derived macrophages, respectively. Within the heart, these subsets have distinct tissue niches and functional roles in the setting of homeostasis and disease, with cardiac resident macrophages representing a protective cell population while bone marrow monocyte-derived cardiac macrophages have a context-dependent effect, triggering both proinflammatory tissue injury, but also promoting reparative functions. With the increased understanding of the clinical relevance of cardiac macrophage subsets, there has been an increasing need to detect and measure cardiac macrophage compositions in living animals and patients. New molecular tracers compatible with positron emission tomography/computerized tomography and positron emission tomography/ magnetic resonance imaging have enabled investigators to noninvasively and serially visualize cardiac macrophage subsets within the heart to define associations with disease and measure treatment responses. Today, advancements within this thriving field are poised to fuel an era of clinical translation.

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.

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

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

Beyond antiviral: role of IFN-I in brain development

Interferons and central nervous system resident macrophages, microglia, are well-known for their respective roles in antiviral defense and phagocytosis. Using a classic experimental paradigm for examining activity-dependent neural plasticity, Escoubas, Dorman, et al. recently identified a role for microglial type I interferon signaling in the clearance of unwanted neurons during mouse brain development.

Microglia as integrators of brain-associated molecular patterns

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