A Binary Cre Transgenic Approach Dissects Microglia and CNS Border-Associated Macrophages

Microglia in Health and Diseases: Integrative Hubs of the Central Nervous System (CNS)

Microglia are usually referred to as "the innate immune cells of the brain," "the resident macrophages of the central nervous system" (CNS), or "CNS parenchymal macrophages." These labels allude to their inherent immune function, related to their macrophage lineage. However, beyond their classic innate immune responses, microglia also play physiological roles crucial for proper brain development and maintenance of adult brain homeostasis. Microglia sense both external and local stimuli through a variety of surface receptors. Thus, they might serve as integrative hubs at the interface between the external environment and the CNS, able to decode, filter, and buffer cues from outside, with the aim of preserving and maintaining brain homeostasis. In this perspective, we will cast a critical look at how these multiple microglial functions are acquired and coordinated, and we will speculate on their impact on human brain physiology and pathology.

Endothelial and macrophage interactions in the angiogenic niche

The interactions between vascular cells, especially endothelial cells, and macrophages play a pivotal role in maintaining the subtle balance of vascular biology, which is crucial for angiogenesis in both healthy and diseased states. These cells are central to ensuring a harmonious balance between tissue repair and preventing excessive angiogenic activity, which could lead to pathological conditions. Recent advances in sophisticated genetic engineering vivo models and novel sequencing approaches, such as single-cell RNA-sequencing, in immunobiology have significantly enhanced our understanding of the gene expression and behavior of macrophages. These insights offer new perspectives on the role macrophages play not only in development but also across various health conditions. In this review, we explore the complex interactions between multiple types of macrophages and endothelium, focusing on their impact on new blood vessel formation. By understanding these intricate interactions, we aim to provide insights into new methods for managing angiogenesis in various diseases, thereby offering hope for the development of novel therapeutic approaches.

Multiplexed imaging to reveal tissue dendritic cell spatial localisation and function

Dendritic cells (DCs) play a pivotal role in immune surveillance, acting as sentinels that coordinate immune responses within tissues. Although differences in the identity and functional states of DC subpopulations have been identified through multiparametric flow cytometry and single-cell RNA sequencing, these methods do not provide information about the spatial context in which the cells are located. This knowledge is crucial for understanding tissue organisation and cellular cross-talk. Recent developments in multiplex imaging techniques can now offer insights into this complex spatial and functional landscape. This review provides a concise overview of these imaging methodologies, emphasising their application in identifying DCs to delineate their tissue-specific functions and aiding newcomers in navigating this field.

Transgenic Targeting of Fcrls Creates a Highly Efficient Constitutively Active Microglia Cre Line with Differentiated Specificity

Microglia carry out important functions as the resident macrophages of the brain. To study their role in health and disease, the research community needs tools to genetically modify them with maximum completeness in a manner that distinguishes them from closely related cell types, such as monocytes. While currently available tamoxifen-inducible CreERT2 lines can achieve the differentiation from other cells, the field needs improved and publicly available constitutively active Cre lines, especially ones with favorable efficiency and specificity profiles for studies where high recombination efficiency is imperative and where tamoxifen administration is contraindicated. Here, we leverage the microglia-specific Fcrls gene to generate mice expressing Cre. Using genomic methods, we show correct positioning of the transgene and intact microglia homeostasis in Fcrls-2A-Cre mice. Crossing Fcrls-2A-Cre mice to four different reporters, we demonstrate highly efficient recombination in microglia across differentially sensitive loxP alleles in different genomic contexts, indicating robust applicability of the line. Further, we show that microglia recombine a loxP reporter during early embryonic development, supporting the use of the line for developmental studies. Finally, using immunofluorescence and flow cytometry, we reveal that most border-associated macrophages are also targeted whereas only few liver and spleen macrophages and virtually no white blood cell subsets exhibit Cre activity, distinguishing this line from another publicly available Cre line, Cx3cr1-CreM Fcrls-2A-Cre mice are immediately available (JAX #036591) and serve as a valuable addition to the community's microglia toolbox by providing highly efficient constitutive Cre activity with excellent specificity, particularly for studies where tamoxifen administration is undesirable.

An inducible genetic tool to track and manipulate specific microglial states reveals their plasticity and roles in remyelination

Recent single-cell RNA sequencing studies have revealed distinct microglial states in development and disease. These include proliferative-region-associated microglia (PAMs) in developing white matter and disease-associated microglia (DAMs) prevalent in various neurodegenerative conditions. PAMs and DAMs share a similar core gene signature. However, the extent of the dynamism and plasticity of these microglial states, as well as their functional significance, remains elusive, partly due to the lack of specific tools. Here, we generated an inducible Cre driver line, Clec7a-CreERT2, that targets PAMs and DAMs in the brain parenchyma. Utilizing this tool, we profiled labeled cells during development and in several disease models, uncovering convergence and context-dependent differences in PAM and DAM gene expression. Through long-term tracking, we demonstrated microglial state plasticity. Lastly, we specifically depleted DAMs in demyelination, revealing their roles in disease recovery. Together, we provide a versatile genetic tool to characterize microglial states in CNS development and disease.

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.

Coordinated chemokine expression defines macrophage subsets across tissues

Lung-resident macrophages, which include alveolar macrophages and interstitial macrophages (IMs), exhibit a high degree of diversity, generally attributed to different activation states, and often complicated by the influx of monocytes into the pool of tissue-resident macrophages. To gain a deeper insight into the functional diversity of IMs, here we perform comprehensive transcriptional profiling of resident IMs and reveal ten distinct chemokine-expressing IM subsets at steady state and during inflammation. Similar IM subsets that exhibited coordinated chemokine signatures and differentially expressed genes were observed across various tissues and species, indicating conserved specialized functional roles. Other macrophage types shared specific IM chemokine profiles, while also presenting their own unique chemokine signatures. Depletion of CD206hi IMs in Pf4creR26EYFP+DTR and Pf4creR26EYFPCx3cr1DTR mice led to diminished inflammatory cell recruitment, reduced tertiary lymphoid structure formation and fewer germinal center B cells in models of allergen- and infection-driven inflammation. These observations highlight the specialized roles of IMs, defined by their coordinated chemokine production, in regulating immune cell influx and organizing tertiary lymphoid tissue architecture.

Spatial transcriptomic brain imaging reveals the effects of immunomodulation therapy on specific regional brain cells in a mouse dementia model

Increasing evidence of brain-immune crosstalk raises expectations for the efficacy of novel immunotherapies in Alzheimer's disease (AD), but the lack of methods to examine brain tissues makes it difficult to evaluate therapeutics. Here, we investigated the changes in spatial transcriptomic signatures and brain cell types using the 10x Genomics Visium platform in immune-modulated AD models after various treatments. To proceed with an analysis suitable for barcode-based spatial transcriptomics, we first organized a workflow for segmentation of neuroanatomical regions, establishment of appropriate gene combinations, and comprehensive review of altered brain cell signatures. Ultimately, we investigated spatial transcriptomic changes following administration of immunomodulators, NK cell supplements and an anti-CD4 antibody, which ameliorated behavior impairment, and designated brain cells and regions showing probable associations with behavior changes. We provided the customized analytic pipeline into an application named STquantool. Thus, we anticipate that our approach can help researchers interpret the real action of drug candidates by simultaneously investigating the dynamics of all transcripts for the development of novel AD therapeutics.

Need of orthogonal approaches in neurological disease modeling in mouse

Over the years, advancements in modeling neurological diseases have revealed innovative strategies aimed at gaining deeper insights and developing more effective treatments for these complex conditions. However, these progresses have recently been overshadowed by an increasing number of failures in clinical trials, raising doubts about the reliability and translatability of this type of disease modeling. This mini-review does not aim to provide a comprehensive overview of the current state-of-the-art in disease mouse modeling. Instead, it offers a brief excursus over some recent approaches in modeling neurological diseases to pinpoint a few intriguing strategies applied in the field that may serve as sources of inspiration for improving currently available animal models. In particular, we aim to guide the reader toward the potential success of adopting a more orthogonal approach in the study of human diseases.

Brain border-associated macrophages: common denominators in infection, aging, and Alzheimer's disease?

Mammalian brain border-associated macrophages (BAMs) are strategically positioned to support vital properties and processes: for example, the composition of the brain's perivascular extracellular matrix and cerebrospinal fluid flow via the glymphatic pathway. BAMs also effectively restrict the spread of infectious microbes into the brain. However, while fighting infections, BAMs sustain long-term transcriptomic changes and can be replaced by inflammatory monocytes, potentially leading to a gradual loss of their beneficial homeostatic functions. We hypothesize that by expediting the deterioration of BAMs, multiple infection episodes might be associated with accelerated brain aging and the putative development of neurodegenerative diseases. Our viewpoint is supported by recent studies suggesting that rejuvenating aged BAMs, and counterbalancing their detrimental inflammatory signatures during infections, might hold promise in treating aging-related neurological disorders, including Alzheimer's disease (AD).

Characterization of the responses of brain macrophages to focused ultrasound-mediated blood-brain barrier opening

The opening of the blood-brain barrier (BBB) by focused ultrasound (FUS) coupled with intravenously injected microbubbles can be leveraged as a form of immunotherapy for the treatment of neurodegenerative disorders. However, how FUS BBB opening affects brain macrophages is not well understood. Here by using single-cell sequencing to characterize the distinct responses of microglia and central nervous system-associated macrophages (CAMs) to FUS-mediated BBB opening in mice, we show that the treatment remodels the immune landscape via the recruitment of CAMs and the proliferation of microglia and via population size increases in disease-associated microglia. Both microglia and CAMs showed early and late increases in population sizes, yet only the proliferation of microglia increased at both timepoints. The population of disease-associated microglia also increased, accompanied by the upregulation of genes associated with gliogenesis and phagocytosis, with the depletion of brain macrophages significantly decreasing the duration of BBB opening.

Unraveling the enigma: housekeeping gene Ugt1a7c as a universal biomarker for microglia

Background

Microglia, brain resident macrophages, play multiple roles in maintaining homeostasis, including immunity, surveillance, and protecting the central nervous system through their distinct activation processes. Identifying all types of microglia-driven populations is crucial due to the presence of various phenotypes that differ based on developmental stages or activation states. During embryonic development, the E8.5 yolk sac contains erythromyeloid progenitors that go through different growth phases, eventually resulting in the formation of microglia. In addition, microglia are present in neurological diseases as a diverse population. So far, no individual biomarker for microglia has been discovered that can accurately identify and monitor their development and attributes.

Summary

Here, we highlight the newly defined biomarker of mouse microglia, UGT1A7C, which exhibits superior stability in expression during microglia development and activation compared to other known microglia biomarkers. The UGT1A7C sensing chemical probe labels all microglia in the 3xTG AD mouse model. The expression of Ugt1a7c is stable during development, with only a 4-fold variation, while other microglia biomarkers, such as Csf1r and Cx3cr1, exhibit at least a 10-fold difference. The UGT1A7C expression remains constant throughout its lifespan. In addition, the expression and activity of UGT1A7C are the same in response to different types of inflammatory activators' treatment in vitro.

Conclusion

We propose employing UGT1A7C as the representative biomarker for microglia, irrespective of their developmental state, age, or activation status. Using UGT1A7C can reduce the requirement for using multiple biomarkers, enhance the precision of microglia analysis, and even be utilized as a standard for gene/protein expression.

Deciphering perivascular macrophages and microglia in the retinal ganglion cell layers

Introduction: The classically defined two retinal microglia layers are distributed in inner and outer plexiform layers. Although there are some reports that retinal microglia are also superficially located around the ganglion cell layer (GCL) in contact with the vitreous, there has been a lack of detailed descriptions and not fully understood yet. Methods: We visualized the microglial layers by using CX3CR1-GFP (C57BL6) transgenic mice with both healthy and disease conditions including NaIO3-induced retinal degeneration models and IRBP-induced auto-immune uveitis models. Result: We found the GCL microglia has two subsets; peripheral (pph) microglia located on the retinal parenchyma and BAM (CNS Border Associated Macrophage) which have a special stretched phenotype only located on the surface of large retinal veins. First, in the pph microglia subset, but not in BAM, Galectin-3 and LYVE1 are focally expressed. However, LYVE1 is specifically expressed in the amoeboid or transition forms, except the typical dendritic morphology in the pph microglia. Second, BAM is tightly attached to the surface of the retinal veins and has similar morphology patterns in both the healthy and disease conditions. CD86+ BAM has a longer process which vertically passes the proximal retinal veins. Our data helps decipher the basic anatomy and pathophysiology of the retinal microglia in the GCL. Discussion: Our data helps decipher the basic anatomy and pathophysiology of the retinal microglia in the GCL.

Border-associated macrophages in the central nervous system

Tissue-resident macrophages play an important role in the local maintenance of homeostasis and immune surveillance. In the central nervous system (CNS), brain macrophages are anatomically divided into parenchymal microglia and non-parenchymal border-associated macrophages (BAMs). Among these immune cell populations, microglia have been well-studied for their roles during development as well as in health and disease. BAMs, mostly located in the choroid plexus, meningeal and perivascular spaces, are now gaining increased attention due to advancements in multi-omics technologies and genetic methodologies. Research on BAMs over the past decade has focused on their ontogeny, immunophenotypes, involvement in various CNS diseases, and potential as therapeutic targets. Unlike microglia, BAMs display mixed origins and distinct self-renewal capacity. BAMs are believed to regulate neuroimmune responses associated with brain barriers and contribute to immune-mediated neuropathology. Notably, BAMs have been observed to function in diverse cerebral pathologies, including Alzheimer's disease, Parkinson's disease, multiple sclerosis, ischemic stroke, and gliomas. The elucidation of the heterogeneity and diverse functions of BAMs during homeostasis and neuroinflammation is mesmerizing, since it may shed light on the precision medicine that emphasizes deep insights into programming cues in the unique brain immune microenvironment. In this review, we delve into the latest findings on BAMs, covering aspects like their origins, self-renewal capacity, adaptability, and implications in different brain disorders.

Arresting the bad seed: HDAC3 regulates proliferation of different microglia after ischemic stroke

The accumulation of self-renewed polarized microglia in the penumbra is a critical neuroinflammatory process after ischemic stroke, leading to secondary demyelination and neuronal loss. Although known to regulate tumor cell proliferation and neuroinflammation, HDAC3's role in microgliosis and microglial polarization remains unclear. We demonstrated that microglial HDAC3 knockout (HDAC3-miKO) ameliorated poststroke long-term functional and histological outcomes. RNA-seq analysis revealed mitosis as the primary process affected in HDAC3-deficent microglia following stroke. Notably, HDAC3-miKO specifically inhibited proliferation of proinflammatory microglia without affecting anti-inflammatory microglia, preventing microglial transition to a proinflammatory state. Moreover, ATAC-seq showed that HDAC3-miKO induced closing of accessible regions enriched with PU.1 motifs. Overexpressing microglial PU.1 via an AAV approach reversed HDAC3-miKO-induced proliferation inhibition and protective effects on ischemic stroke, indicating PU.1 as a downstream molecule that mediates HDAC3's effects on stroke. These findings uncovered that HDAC3/PU.1 axis, which mediated differential proliferation-related reprogramming in different microglia populations, drove poststroke inflammatory state transition, and contributed to pathophysiology of ischemic stroke.

High-Dimensional Methods of Single-Cell Microglial Profiling to Enhance Understanding of Neuropathological Disease

Microglia are the innate myeloid cells of the central nervous system (CNS) parenchyma, functionally implicated in almost every defined neuroinflammatory and neurodegenerative disorder. Current understanding of disease pathogenesis for many neuropathologies is limited and/or lacks reliable diagnostic markers, vaccines, and treatments. With the increasing aging of society and rise in neurogenerative diseases, improving our understanding of their pathogenesis is essential. Analysis of microglia from murine disease models provides an investigative tool to unravel disease processes. In many neuropathologies, bone-marrow-derived monocytes are recruited to the CNS, adopting a phenotype similar to that of microglia. This significantly confounds the accurate identification of cell-type-specific functions and downstream therapeutic targeting. The increased capacity to analyze more phenotypic markers using spectral-cytometry-based technologies allows improved separation of microglia from monocyte-derived cells. Full-spectrum profiling enables enhanced marker resolution, time-efficient analysis of >40 fluorescence parameters, and extraction of cellular autofluorescence parameters. Coupling this system with additional cytometric technologies, including cell sorting and high-parameter imaging, can improve the understanding of microglial phenotypes in disease. To this end, we provide detailed, step-by-step protocols for the analysis of murine brain tissue by high-parameter ex vivo cytometric analysis using the Aurora spectral cytometer (Cytek), including best practices for unmixing and autofluorescence extraction, cell sorting for single-cell RNA analysis, and imaging mass cytometry. Together, this provides a toolkit for researchers to comprehensively investigate microglial disease processes at protein, RNA, and spatial levels for the identification of therapeutic targets in neuropathology. © 2024 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Processing the mouse brain into a single-cell suspension for microglia isolation Basic Protocol 2: Staining single-cell mouse brain suspensions for microglial phenotyping by spectral cytometry Basic Protocol 3: Flow cytometric sorting of mouse microglia for ex vivo analysis Basic Protocol 4: Processing the mouse brain for imaging mass cytometry for spatial microglia analysis.

Established and emerging techniques for the study of microglia: visualization, depletion, and fate mapping

The central nervous system (CNS) is an essential hub for neuronal communication. As a major component of the CNS, glial cells are vital in the maintenance and regulation of neuronal network dynamics. Research on microglia, the resident innate immune cells of the CNS, has advanced considerably in recent years, and our understanding of their diverse functions continues to grow. Microglia play critical roles in the formation and regulation of neuronal synapses, myelination, responses to injury, neurogenesis, inflammation, and many other physiological processes. In parallel with advances in microglial biology, cutting-edge techniques for the characterization of microglial properties have emerged with increasing depth and precision. Labeling tools and reporter models are important for the study of microglial morphology, ultrastructure, and dynamics, but also for microglial isolation, which is required to glean key phenotypic information through single-cell transcriptomics and other emerging approaches. Strategies for selective microglial depletion and modulation can provide novel insights into microglia-targeted treatment strategies in models of neuropsychiatric and neurodegenerative conditions, cancer, and autoimmunity. Finally, fate mapping has emerged as an important tool to answer fundamental questions about microglial biology, including their origin, migration, and proliferation throughout the lifetime of an organism. This review aims to provide a comprehensive discussion of these established and emerging techniques, with applications to the study of microglia in development, homeostasis, and CNS pathologies.

Border-associated macrophages in the central nervous system

Tissue-resident macrophages play an important role in the local maintenance of homeostasis and immune surveillance. In the central nervous system (CNS), brain macrophages are anatomically divided into parenchymal microglia and non-parenchymal border-associated macrophages (BAMs). Among these immune cell populations, microglia have been well-studied for their roles in normal brain development, neurodegeneration, and brain cancers. BAMs, mostly located in the choroid plexus, meningeal and perivascular spaces, are now gaining increased attention due to advancements in multi-omics technologies and genetic methodologies. Research on BAMs over the past decade has focused on their ontogeny, immunophenotypes, involvement in various CNS diseases, and potential as therapeutic targets. Unlike microglia, BAMs display mixed origins and distinct self-renewal capacity. BAMs are believed to regulate neuroimmune responses associated with brain barriers and contribute to immune-mediated neuropathology. Notably, BAMs have been observed to function in diverse cerebral pathologies, including Alzheimer's disease, Parkinson's disease, multiple sclerosis, ischemic stroke, and gliomas. The elucidation of the heterogeneity and diverse functions of BAMs during homeostasis and neuroinflammation is mesmerizing, since it may shed light on the precision medicine that emphasizes deep insights into programming cues in the unique brain immune microenvironment. In this review, we delve into the latest findings on BAMs, covering aspects like their origins, self-renewal capacity, adaptability, and implications in different brain disorders.

Role of meningeal immunity in brain function and protection against pathogens

The brain and spinal cord collectively referred to as the Central Nervous System (CNS) are protected by the blood-brain barrier that limits molecular, microbial and immunological trafficking. However, in the last decade, many studies have emphasized the protective role of 'border regions' at the surface of the CNS which are highly immunologically active, in contrast with the CNS parenchyma. In the steady-state, lymphoid and myeloid cells residing in the cranial meninges can affect brain function and behavior. Upon infection, they provide a first layer of protection against microbial neuroinvasion. The maturation of border sites over time enables more effective brain protection in adults as compared to neonates. Here, we provide a comprehensive update on the meningeal immune system and its role in physiological brain function and protection against infectious agents.

Dynamic Multiplex Tissue Imaging in Inflammation Research

Inflammation is a highly dynamic process with immune cells that continuously interact with each other and parenchymal components as they migrate through tissue. The dynamic cellular responses and interaction patterns are a function of the complex tissue environment that cannot be fully reconstructed ex vivo, making it necessary to assess cell dynamics and changing spatial patterning in vivo. These dynamics often play out deep within tissues, requiring the optical focus to be placed far below the surface of an opaque organ. With the emergence of commercially available two-photon excitation lasers that can be combined with existing imaging systems, new avenues for imaging deep tissues over long periods of time have become available. We discuss a selected subset of studies illustrating how two-photon microscopy (2PM) has helped to relate the dynamics of immune cells to their in situ function and to understand the molecular patterns that govern their behavior in vivo. We also review some key practical aspects of 2PM methods and point out issues that can confound the results, so that readers can better evaluate the reliability of conclusions drawn using this technology.

A comparative evaluation of the strengths and potential caveats of the microglial inducible CreER mouse models

The recent proliferation of new Cre and CreER recombinase lines provides researchers with a diverse toolkit to study microglial gene function. To determine how best to apply these lines in studies of microglial gene function, a thorough and detailed comparison of their properties is needed. Here, we examined four different microglial CreER lines (Cx3cr1YFP-CreER(Litt), Cx3cr1CreER(Jung), P2ry12CreER, and Tmem119CreER), focusing on (1) recombination specificity, (2) leakiness (the degree of tamoxifen-independent recombination in microglia and other cells), (3) the efficiency of tamoxifen-induced recombination, (4) extraneural recombination (the degree of recombination in cells outside of the CNS, particularly myelo/monocyte lineages), and (5) off-target effects in the context of neonatal brain development. We identify important caveats and strengths for these lines, which will provide broad significance for researchers interested in performing conditional gene deletion in microglia. We also provide data emphasizing the potential of these lines for injury models that result in the recruitment of splenic immune cells.

Distinct microglial transcriptomic signatures within the hippocampus

Microglia, the resident immune cells of the brain, are crucial in the development of the nervous system. Recent evidence demonstrates that microglia modulate adult hippocampal neurogenesis by inhibiting cell proliferation of neural precursors and survival both in vitro and in vivo, thus maintaining a balance between cell division and cell death in the neural stem cell pool. There are increasing reports suggesting these microglia found in neurogenic niches differ from their counterparts in non-neurogenic areas. Here, we present evidence that hippocampal microglia exhibit transcriptomic heterogeneity, with some cells expressing genes associated with neurogenesis. By comprehensively profiling myeloid lineage cells in the hippocampus using single cell RNA-sequencing, we have uncovered a small, yet distinct population of microglia which exhibit depletion in genes associated with homeostatic microglia and enrichment of genes associated with phagocytosis. Intriguingly, this population also expresses a gene signature with substantial overlap with previously characterized phenotypes, including disease associated microglia (DAM), a particularly unique and compelling microglial state.

Brain perivascular macrophages: current understanding and future prospects

Brain perivascular macrophages are specialized populations of macrophages that reside in the space around cerebral vessels, such as penetrating arteries and venules. With the help of cutting-edge technologies, such as cell fate mapping and single-cell multi-omics, their multifaceted, pivotal roles in phagocytosis, antigen presentation, vascular integrity maintenance and metabolic regulation have more recently been further revealed under physiological conditions. Accumulating evidence also implies that perivascular macrophages are involved in the pathogenesis of neurodegenerative disease, cerebrovascular dysfunction, autoimmune disease, traumatic brain injury and epilepsy. They can act in either protective or detrimental ways depending on the disease course and stage. However, the underlying mechanisms of perivascular macrophages remain largely unknown. Therefore, we highlight potential future directions in research on perivascular macrophages, including the utilization of genetic mice and novel therapeutic strategies that target these unique immune cells for neuroprotective purposes. In conclusion, this review provides a comprehensive update on the current knowledge of brain perivascular macrophages, shedding light on their pivotal roles in central nervous system health and disease.

Not just Glia-Dissecting brain macrophages in the mouse

Macrophages have emerged as critical cellular components of the central nervous system (CNS), promoting development, maintenance, and immune defense of the CNS. Here we will review recent advances in our understanding of brain macrophage heterogeneity, including microglia and border-associated macrophages, focusing on the mouse. Emphasis will be given to the discussion of strengths and limitations of the experimental approaches that have led to the recent insights and hold promise to further deepen our mechanistic understanding of brain macrophages that might eventually allow to harness their activities for the management of CNS pathologies.

Fate-Mapping Macrophages: From Ontogeny to Functions

Macrophages are vital to the physiological function of most tissues, but also contribute to disease through a multitude of pathological roles. They are thus highly plastic and heterogeneous. It is now well recognized that macrophages develop from several distinct progenitors from embryogenesis onwards and extending throughout life. Tissue-resident macrophages largely originate from embryonic sources and in many cases self-maintain independently without monocyte input. However, in certain tissues, monocyte-derived macrophages replace these over time or as a result of tissue injury and inflammation. This additional layer of heterogeneity has introduced many questions regarding the influence of origin on fate and function of macrophages in health and disease. To comprehensively address these questions, appropriate methods of tracing macrophage ontogeny are required. This chapter explores why ontogeny is of vital importance in macrophage biology and how to delineate macrophage populations by origin through genetic fate mapping. First, we summarize the current view of macrophage ontogeny and briefly discuss how origin may influence macrophage function in homeostasis and pathology. We go on to make the case for genetic fate mapping as the gold standard and briefly review different fate-mapping models. We then put forward our recommendations for fate-mapping strategies best suited to answer specific research questions and finally discuss the strengths and limitations of currently available models.

Tackling Tissue Macrophage Heterogeneity by SplitCre Transgenesis

Macrophages represent a broad spectrum of distinct, but closely related tissue-resident immune cells. This presents a major challenge for the study of functional aspects of these cells using classical Cre recombinase-mediated conditional mutagenesis in mice, since single promoter-driven Cre transgenic models often display limited specificity toward their intended target. The advent of CRISPR/Cas9 technology has now provided a time- and cost-effective method to explore the full potential of binary transgenic, intersectional genetics. Specifically, the use of two promoters driving inactive Cre fragments that, when co-expressed, dimerize and only then gain recombinase activity allows the characterization and manipulation of genetically defined tissue macrophage subpopulations. Here, we will elaborate on the use of this protocol to capitalize on these recent technological advances in mouse genetics and discuss their strengths and pitfalls to improve the study of tissue macrophage subpopulations in physiology and pathophysiology.

Combined Host-Pathogen Fate Mapping to Investigate Lung Macrophages in Viral Infection

Macrophage identity, as defined by epigenetic, transcriptional, proteomic, and functional programs, is greatly impacted by cues originating from the microenvironment. As a consequence, immunophenotyping based on surface marker expression is established and reliable in homeostatic conditions, whereas environmental challenges, in particular infections, severely hamper the determination of identity states. This has become more evident with recent discoveries that macrophage-inherent plasticity may go beyond limits of lineage-defining immunophenotypes. Therefore, transgenic fate mapping tools, such as the phage-derived loxP-cre-system, are essential for the analysis of macrophage adaptation in the tissue under extreme environmental conditions, for example, upon encounter with pathogens. In this chapter, we describe an advanced application of the loxP-cre-system during infection. Here, the host encodes a cell type-specific cre-recombinase, while the pathogen harbors a STOP-floxed fluorescent reporter gene. As an instructive example for the versatility of the system, we demonstrate that alveolar macrophages are predominantly targeted after respiratory tract infection with mouse cytomegalovirus (MCMV). Combined host-pathogen fate mapping not only enables to distinguish between infected and non-infected (bystander) macrophages but also spurs exploration of phenotypic adaptation and tracing of cellular localization in the context of MCMV infection. Moreover, we provide a gating strategy for resolving the diversity of pulmonary immune cell populations.

Microglia in Central Control of Metabolism

Beyond their role as brain immune cells, microglia act as metabolic sensors in response to changes in nutrient availability, thus playing a role in energy homeostasis. This review highlights the evidence and challenges of studying the role of microglia in metabolism regulation.

Translatome Profiling of Tissue-Resident Macrophages Using the RiboTag Approach

Global gene expression profiling has provided valuable insights into the specific contributions of different cell types to various physiological processes. Notably though, both bulk and single-cell transcriptomics require the prior retrieval of the cells from their tissue context to be analyzed. Isolation protocols for tissue macrophages are, however, notoriously inefficient and, moreover, prone to introduce considerable bias and artifacts. Here, we will discuss a valuable alternative, originally introduced by Amieux and colleagues. This so-called RiboTag approach allows, in combination with respective macrophage-specific Cre transgenic lines, to retrieve macrophage translatomes from crude tissue extracts. We will review our experience with this ingenious method, focusing on the study of brain macrophages, including microglia and border-associated cells. We will elaborate on the advantages of the RiboTag approach that render it a valuable complement to standard cell sorting-based profiling strategies, especially for the investigation of tissue macrophages.

An inducible genetic tool for tracking and manipulating specific microglial states in development and disease

Recent single-cell RNA sequencing studies have revealed distinct microglial states in development and disease. These include proliferative region-associated microglia (PAM) in developing white matter and disease-associated microglia (DAM) prevalent in various neurodegenerative conditions. PAM and DAM share a similar core gene signature and other functional properties. However, the extent of the dynamism and plasticity of these microglial states, as well as their functional significance, remains elusive, partly due to the lack of specific tools. Here, we report the generation of an inducible Cre driver line, Clec7a-CreERT2, designed to target PAM and DAM in the brain parenchyma. Utilizing this tool, we profile labeled cells during development and in several disease models, uncovering convergence and context-dependent differences in PAM/DAM gene expression. Through long-term tracking, we demonstrate surprising levels of plasticity in these microglial states. Lastly, we specifically depleted DAM in cuprizone-induced demyelination, revealing their roles in disease progression and recovery.

Knowledge Mapping of Macrophage in Spinal Cord Injury: A Bibliometric Analysis

BACKGROUND

Spinal cord injury (SCI) is a devastating condition, often leading to significant disability and impairment. As crucial immune cells, macrophages play a critical role in the pathophysiology of SCI. Understanding the current state of knowledge and research trends related to macrophages in SCI is crucial for developing effective therapeutic interventions.

METHODS

Using search strategies, we retrieved relevant articles from the Web of Science Core Collection (WOSCC), resulting in a robust dataset for analysis. VOSviewer, Citespace, and PRISM were employed for analysis and visualization. Various bibliometric indicators, including publication trends, citation analysis, co-authorship networks, and keyword analysis, were utilized to assess the scholarly landscape of macrophage research in SCI.

RESULTS

Our findings revealed a steady increase in publications over the past 33 years, indicating a growing interest in this field. We identified Popovich Phillip G was the most influential author, Ohio State University was the most influential institution, and identification of 2 distinct macrophage subsets with divergent effects causing either neurotoxicity or regeneration in the injured mouse spinal cord was the most influential paper in this field.

CONCLUSIONS

This bibliometric analysis provides a comprehensive overview of the current knowledge landscape and research trends regarding macrophages in SCI. Neuroinflammation and macrophage polarization, transplation and molecular mechanism were emerging research areas and novel directions. Our study serves as a valuable resource for researchers in spinal cord injury research and therapeutic development.

Dermis resident macrophages orchestrate localized ILC2 eosinophil circuitries to promote non-healing cutaneous leishmaniasis

Tissue-resident macrophages are critical for tissue homeostasis and repair. We previously showed that dermis-resident macrophages produce CCL24 which mediates their interaction with IL-4+ eosinophils, required to maintain their M2-like properties in the TH1 environment of the Leishmania major infected skin. Here, we show that thymic stromal lymphopoietin (TSLP) and IL-5+ type 2 innate lymphoid cells are also required to maintain dermis-resident macrophages and promote infection. Single cell RNA sequencing reveals the dermis-resident macrophages as the sole source of TSLP and CCL24. Generation of Ccl24-cre mice permits specific labeling of dermis-resident macrophages and interstitial macrophages from other organs. Selective ablation of TSLP in dermis-resident macrophages reduces the numbers of IL-5+ type 2 innate lymphoid cells, eosinophils and dermis-resident macrophages, and ameliorates infection. Our findings demonstrate that dermis-resident macrophages are self-maintained as a replicative niche for L. major by orchestrating localized type 2 circuitries with type 2 innate lymphoid cells and eosinophils.

Meningeal macrophages inhibit chemokine signaling in pre-tumor cells to suppress mouse medulloblastoma initiation

The microenvironment profoundly influences tumor initiation across numerous tissues but remains understudied in brain tumors. In the cerebellum, canonical Wnt signaling controlled by Norrin/Frizzled4 (Fzd4) activation in meningeal endothelial cells is a potent inhibitor of preneoplasia and tumor progression in mouse models of Sonic hedgehog medulloblastoma (Shh-MB). Single-cell transcriptome profiling and phenotyping of the meninges indicate that Norrin/Frizzled4 sustains the activation of meningeal macrophages (mMΦs), characterized by Lyve1 and CXCL4 expression, during the critical preneoplastic period. Depleting mMΦs during this period enhances preneoplasia and tumorigenesis, phenocopying the effects of Norrin loss. The anti-tumorigenic function of mMΦs is derived from the expression of CXCL4, which counters CXCL12/CXCR4 signaling in pre-tumor cells, thereby inhibiting cell-cycle progression and promoting migration away from the pre-tumor niche. These findings identify a pivotal role for mMΦs as key mediators in chemokine-regulated anti-cancer crosstalk between the stroma and pre-tumor cells in the control of MB initiation.

Macrophage and monocyte subsets in response to ischemic stroke

Ischemic stroke is a leading cause of disability and mortality. Despite extensive efforts in stroke research, the only pharmacological treatment currently available is arterial recanalization, which has limited efficacy only in the acute phase of stroke. The neuroinflammatory response to stroke is believed to provide a wider time window than recanalization and has therefore been proposed as an attractive therapeutic target. In this review, we provide an overview of recent advances in the understanding of cellular and molecular responses of distinct macrophage populations following stroke, which may offer potential targets for therapeutic interventions. Specifically, we discuss the role of local responders in neuroinflammation, including the well-studied microglia as well as the emerging players, border-associated macrophages, and macrophages originating from the skull bone marrow. Additionally, we focus on the behavior of monocytes stemming from distant tissues such as the bone marrow and spleen. Finally, we highlight aging as a crucial factor modulating the immune response, which is often neglected in animal studies.

Metabolic adaption of mucosal macrophages: Is metabolism a driver of persistence across tissues?

Macrophages play essential roles in tissue homeostasis, defense, and repair. Their functions are highly tissue-specific, and when damage and inflammation stimulate repopulation by circulating monocytes, the incoming monocytes rapidly acquire the same, tissue-specific functions as the previous, resident macrophages. Several environmental factors are thought to guide the functional differentiation of recruited monocytes, including metabolic pressures imposed by the fuel sources available in each tissue. Here we discuss whether such a model of metabolic determinism can be applied to macrophage differentiation across barrier sites, from the lung to the skin. We suggest an alternative model, in which metabolic phenotype is a consequence of macrophage longevity rather than an early driver of tissue-specific adaption.

A comparative analysis of microglial inducible Cre lines

Cre/loxP technology has revolutionized genetic studies and allowed for spatial and temporal control of gene expression in specific cell types. Microglial biology has particularly benefited because microglia historically have been difficult to transduce with virus or electroporation methods for gene delivery. Here, we investigate five of the most widely available microglial inducible Cre lines. We demonstrate varying degrees of recombination efficiency, cell-type specificity, and spontaneous recombination, depending on the Cre line and inter-loxP distance. We also establish best practice guidelines and protocols to measure recombination efficiency, particularly in microglia. There is increasing evidence that microglia are key regulators of neural circuits and major drivers of a broad range of neurological diseases. Reliable manipulation of their function in vivo is of utmost importance. Identifying caveats and benefits of all tools and implementing the most rigorous protocols are crucial to the growth of the field and the development of microglia-based therapeutics.

A review of tamoxifen administration regimen optimization for Cre/loxp system in mouse bone study

Gene knockout is a technique routinely used in basic experimental research, particularly in mouse skeletal and developmental studies. Tamoxifen-induced Cre/loxp system is known for its temporal and spatial precision and commonly utilized by researchers. However, tamoxifen has been shown its side effects on affecting the phenotype of mouse bone directly. This review aimed to optimize tamoxifen administration regimens including its dosage and duration, to identify an optimal induction strategy that minimizes potential side effects while maintaining recombination efficacy. This study will help researchers in designing gene knockout experiments in bone when using tamoxifen.

Towards an Understanding of Microglia and Border-Associated Macrophages

The central nervous system (CNS) plays a crucial role in regulating bodily functions by sensing and integrating environmental cues and maintaining proper physiological conditions. Recent research has revealed that CNS functions are closely coordinated with the immune system. As even minor disturbances of the immune system in the CNS can lead to various dysfunctions, diseases, or even death, it is highly specialized and segregated from that in peripheral regions. Microglia in the parenchyma and macrophages at the interface between the CNS and peripheral regions are essential immune cells in the CNS that monitor environmental changes. Recent omics analyses have revealed that these cells exhibit highly heterogeneous populations. In this review, we summarize the functions and diversity of microglia in the brain parenchyma and those of macrophages in the border regions, such as the meninges, perivascular spaces, and choroid plexus.

Translatome analysis reveals microglia and astrocytes to be distinct regulators of inflammation in the hyperacute and acute phases after stroke

Neuroinflammation is a hallmark of ischemic stroke, which is a leading cause of death and long-term disability. Understanding the exact cellular signaling pathways that initiate and propagate neuroinflammation after stroke will be critical for developing immunomodulatory stroke therapies. In particular, the precise mechanisms of inflammatory signaling in the clinically relevant hyperacute period, hours after stroke, have not been elucidated. We used the RiboTag technique to obtain microglia and astrocyte-derived mRNA transcripts in a hyperacute (4 h) and acute (3 days) period after stroke, as these two cell types are key modulators of acute neuroinflammation. Microglia initiated a rapid response to stroke at 4 h by adopting an inflammatory profile associated with the recruitment of immune cells. The hyperacute astrocyte profile was marked by stress response genes and transcription factors, such as Fos and Jun, involved in pro-inflammatory pathways such as TNF-α. By 3 days, microglia shift to a proliferative state and astrocytes strengthen their inflammatory response. The astrocyte pro-inflammatory response at 3 days is partially driven by the upregulation of the transcription factors C/EBPβ, Spi1, and Rel, which comprise 25% of upregulated transcription factor-target interactions. Surprisingly, few sex differences across all groups were observed. Expression and log2 fold data for all sequenced genes are available on a user-friendly website for researchers to examine gene changes and generate hypotheses for stroke targets. Taken together, our data comprehensively describe the microglia and astrocyte-specific translatome response in the hyperacute and acute period after stroke and identify pathways critical for initiating neuroinflammation.

A genetic method specifically delineates Th1-type Treg cells and their roles in tumor immunity

Regulatory T (Treg) cells expressing the transcription factor (TF) Foxp3 also express other TFs shared by T helper (Th) subsets under certain conditions. Here, to determine the roles of T-bet-expressing Treg cells, we generate a mouse strain, called VeDTR, in which T-bet/Foxp3 double-positive cells are engineered to be specifically labeled and depleted by a combination of Cre- and Flp-recombinase-dependent gene expression control. Characterization of T-bet⁺Foxp3⁺ cells using VeDTR mice reveals high resistance under oxidative stress, which is involved in accumulation of T-bet⁺Foxp3⁺ cells in tumor tissues. Moreover, short-term depletion of T-bet⁺Foxp3⁺ cells leads to anti-tumor immunity but not autoimmunity, whereas that of whole Treg cells does both. Although ablation of T-bet⁺Foxp3⁺ cells during Toxoplasma infection slightly enhances Th1 immune responses, it does not affect the course of the infection. Collectively, the intersectional genetic method reveals the specific roles of T-bet⁺Foxp3⁺ cells in suppressing tumor immunity.

Chemogenetic manipulation of CX3CR1+ cells transiently induces hypolocomotion independent of microglia

Chemogenetic approaches using Designer Receptors Exclusively Activated by Designer Drugs (DREADD, a family of engineered GPCRs) were recently employed in microglia. Here, we used Cx3cr1CreER/+:R26hM4Di/+ mice to express Gi-DREADD (hM4Di) on CX3CR1+ cells, comprising microglia and some peripheral immune cells, and found that activation of hM4Di on long-lived CX3CR1+ cells induced hypolocomotion. Unexpectedly, Gi-DREADD-induced hypolocomotion was preserved when microglia were depleted. Consistently, specific activation of microglial hM4Di cannot induce hypolocomotion in Tmem119CreER/+:R26hM4Di/+ mice. Flow cytometric and histological analysis showed hM4Di expression in peripheral immune cells, which may be responsible for the hypolocomotion. Nevertheless, depletion of splenic macrophages, hepatic macrophages, or CD4+ T cells did not affect Gi-DREADD-induced hypolocomotion. Our study demonstrates that rigorous data analysis and interpretation are needed when using Cx3cr1CreER/+ mouse line to manipulate microglia.

Cardiac resident macrophages: key regulatory mediators in the aftermath of myocardial infarction

Acute myocardial infarction (MI) is a prevalent and highly fatal global disease. Despite significant reduction in mortality rates with standard treatment regimens, the risk of heart failure (HF) remains high, necessitating innovative approaches to protect cardiac function and prevent HF progression. Cardiac resident macrophages (cMacs) have emerged as key regulators of the pathophysiology following MI. cMacs are a heterogeneous population composed of subsets with different lineage origins and gene expression profiles. Several critical aspects of post-MI pathophysiology have been shown to be regulated by cMacs, including recruitment of peripheral immune cells, clearance and replacement of damaged myocardial cells. Furthermore, cMacs play a crucial role in regulating cardiac fibrosis, risk of arrhythmia, energy metabolism, as well as vascular and lymphatic remodeling. Given the multifaceted roles of cMacs in post-MI pathophysiology, targeting cMacs represents a promising therapeutic strategy. Finally, we discuss novel treatment strategies, including using nanocarriers to deliver drugs to cMacs or using cell therapies to introduce exogenous protective cMacs into the heart.

The Neurovasculome: Key Roles in Brain Health and Cognitive Impairment: A Scientific Statement From the American Heart Association/American Stroke Association

BACKGROUND

Preservation of brain health has emerged as a leading public health priority for the aging world population. Advances in neurovascular biology have revealed an intricate relationship among brain cells, meninges, and the hematic and lymphatic vasculature (the neurovasculome) that is highly relevant to the maintenance of cognitive function. In this scientific statement, a multidisciplinary team of experts examines these advances, assesses their relevance to brain health and disease, identifies knowledge gaps, and provides future directions.

METHODS

Authors with relevant expertise were selected in accordance with the American Heart Association conflict-of-interest management policy. They were assigned topics pertaining to their areas of expertise, reviewed the literature, and summarized the available data.

RESULTS

The neurovasculome, composed of extracranial, intracranial, and meningeal vessels, as well as lymphatics and associated cells, subserves critical homeostatic functions vital for brain health. These include delivering O2 and nutrients through blood flow and regulating immune trafficking, as well as clearing pathogenic proteins through perivascular spaces and dural lymphatics. Single-cell omics technologies have unveiled an unprecedented molecular heterogeneity in the cellular components of the neurovasculome and have identified novel reciprocal interactions with brain cells. The evidence suggests a previously unappreciated diversity of the pathogenic mechanisms by which disruption of the neurovasculome contributes to cognitive dysfunction in neurovascular and neurodegenerative diseases, providing new opportunities for the prevention, recognition, and treatment of these conditions.

CONCLUSIONS

These advances shed new light on the symbiotic relationship between the brain and its vessels and promise to provide new diagnostic and therapeutic approaches for brain disorders associated with cognitive dysfunction.

Modulation of microglial metabolism facilitates regeneration in demyelination

Microglia exhibit diverse phenotypes in various central nervous system disorders and metabolic pathways exert crucial effects on microglial activation and effector functions. Here, we discovered two novel distinct microglial clusters, functionally associated with enhanced phagocytosis (PEMs) and myelination (MAMs) respectively, in human patients with multiple sclerosis by integrating public snRNA-seq data. Microglia adopt a PEMs phenotype during the early phase of demyelinated lesions, predominated in pro-inflammatory responses and aggravated glycolysis, while MAMs mainly emerged during the later phase, with regenerative signatures and enhanced oxidative phosphorylation. In addition, microglial triggering receptor expressed on myeloid cells 2 (Trem2) was greatly involved in the phenotype transition in demyelination, but not indispensable for microglia transition toward PEMs. Rosiglitazone could promote microglial phenotype conversion from PEMs to MAMs, thus favoring myelin repair. Taken together, these findings provide insights into therapeutic interventions targeting immunometabolism to switch microglial phenotypes and facilitate regenerative capacity in demyelination.

A Cre-deleter specific for embryo-derived brain macrophages reveals distinct features of microglia and border macrophages

Genetic tools to target microglia specifically and efficiently from the early stages of embryonic development are lacking. We generated a constitutive Cre line controlled by the microglia signature gene Crybb1 that produced nearly complete recombination in embryonic brain macrophages (microglia and border-associated macrophages [BAMs]) by the perinatal period, with limited recombination in peripheral myeloid cells. Using this tool in combination with Flt3-Cre lineage tracer, single-cell RNA-sequencing analysis, and confocal imaging, we resolved embryonic-derived versus monocyte-derived BAMs in the mouse cortex. Deletion of the transcription factor SMAD4 in microglia and embryonic-derived BAMs using Crybb1-Cre caused a developmental arrest of microglia, which instead acquired a BAM specification signature. By contrast, the development of genuine BAMs remained unaffected. Our results reveal that SMAD4 drives a transcriptional and epigenetic program that is indispensable for the commitment of brain macrophages to the microglia fate and highlight Crybb1-Cre as a tool for targeting embryonic brain macrophages.

Targeting p53 for neuroinflammation: New therapeutic strategies in ischemic stroke

Ischemic stroke (IS) is characterized by high incidence, high recurrence, and high mortality and places a heavy burden on society and families. The pathological mechanisms of IS are complex, among which secondary neurological impairment mediated by neuroinflammation is considered to be the main factor in cerebral ischemic injury. At present, there is still a lack of specific therapies to treat neuroinflammation. The tumor suppressor protein p53 has long been regarded as a key substance in the regulation of the cell cycle and apoptosis in the past. Recently, studies have found that p53 also plays an important role in neuroinflammatory diseases, such as IS. Therefore, p53 may be a crucial target for the regulation of the neuroinflammatory response. Here, we provide a comprehensive review of the potential of targeting p53 in the treatment of neuroinflammation after IS. We describe the function of p53, the major immune cells involved in neuroinflammation, and the role of p53 in inflammatory responses mediated by these cells. Finally, we summarize the therapeutic strategies of targeting p53 in regulating the neuroinflammatory response after IS to provide new directions and ideas for the treatment of ischemic brain injury.

Deletion of PTEN in microglia ameliorates chronic neuroinflammation following repetitive mTBI

Traumatic brain injury is a leading cause of morbidity and mortality in adults and children in developed nations. Following the primary injury, microglia, the resident innate immune cells of the CNS, initiate several inflammatory signaling cascades and pathophysiological responses that may persist chronically; chronic neuroinflammation following TBI has been closely linked to the development of neurodegeneration and neurological dysfunction. Phosphoinositide 3-kinases (PI3Ks) are a family of lipid kinases that have been shown to regulate several key mechanisms in the inflammatory response to TBI. Increasing evidence has shown that the modulation of the PI3K/AKT signaling pathway has the potential to influence the cellular response to inflammatory stimuli. However, directly targeting PI3K signaling poses several challenges due to its regulatory role in several cell survival pathways. We have previously identified that the phosphatase and tensin homolog deleted on chromosome 10 (PTEN), the major negative regulator of PI3K/AKT signaling, is dysregulated following exposure to repetitive mild traumatic brain injury (r-mTBI). Moreover, this dysregulated PI3K/AKT signaling was correlated with chronic microglial-mediated neuroinflammation. Therefore, we interrogated microglial-specific PTEN as a therapeutic target in TBI by generating a microglial-specific, Tamoxifen inducible conditional PTEN knockout model using a CX3CR1 Cre recombinase mouse line PTEN^fl/fl/CX3CR1^+/CreERT2 (mcg-PTEN^cKO), and exposed them to our 20-hit r-mTBI paradigm. Animals were treated with tamoxifen at 76 days post-last injury, and the effects of microglia PTEN deletion on immune-inflammatory responses were assessed at 90-days post last injury. We observed that the deletion of microglial PTEN ameliorated the proinflammatory response to repetitive brain trauma, not only reducing chronic microglial activation and proinflammatory cytokine production but also rescuing TBI-induced reactive astrogliosis, demonstrating that these effects extended beyond microglia alone. Additionally, we observed that the pharmacological inhibition of PTEN with BpV(HOpic) ameliorated the LPS-induced activation of microglial NFκB signaling in vitro. Together, these data provide support for the role of PTEN as a regulator of chronic neuroinflammation following repetitive mild TBI.

Finding the right tool: a comprehensive evaluation of microglial inducible cre mouse models

The recent proliferation of new Cre and CreER recombinase lines provides researchers with a diverse toolkit to study microglial gene function. To determine how best to apply these lines in studies of microglial gene function, a thorough and detailed comparison of their properties is needed. Here, we examined four different microglial CreER lines (Cx3cr1CreER(Litt), Cx3cr1CreER(Jung), P2ry12CreER, Tmem119CreER), focusing on (1) recombination specificity; (2) leakiness - degree of non-tamoxifen recombination in microglia and other cells; (3) efficiency of tamoxifen-induced recombination; (4) extra-neural recombination -the degree of recombination in cells outside the CNS, particularly myelo/monocyte lineages (5) off-target effects in the context of neonatal brain development. We identify important caveats and strengths for these lines which will provide broad significance for researchers interested in performing conditional gene deletion in microglia. We also provide data emphasizing the potential of these lines for injury models that result in the recruitment of splenic immune cells.

Microglia and macrophages in the neuro-glia-vascular unit: From identity to functions

Although both are myeloid cells located surrounding cerebral vasculature, vessel-associated microglia (VAM) and perivascular macrophages (PVMs) can be distinguished by their distinct morphologies, signatures and microscopic location. As key component of neuro-glia-vascular unit (NGVU), they play prominent roles in neurovasculature development and pathological process of various central nervous system (CNS) diseases, including phagocytosis, angiogenesis, vessel damage/protection and blood flow regulation, therefore serving as potential targets for therapeutics of a broad array of CNS diseases. Herein, we will provide a comprehensive overview of heterogeneity of VAM/PVMs, highlight limitations of current understanding in this field, and discuss possible directions of future investigations.

Role of microglia in HIV-1 infection

The usage of antiretroviral treatment (ART) has considerably decreased the morbidity and mortality related to HIV-1 (human immunodeficiency virus type 1) infection. However, ART is ineffective in eradicating the virus from the persistent cell reservoirs (e.g., microglia), noticeably hindering the cure for HIV-1. Microglia participate in the progression of neuroinflammation, brain aging, and HIV-1-associated neurocognitive disorder (HAND). Some methods have currently been studied as fundamental strategies targeting microglia. The purpose of this study was to comprehend microglia biology and its functions in HIV-1 infection, as well as to look into potential therapeutic approaches targeting microglia.