Perivascular, subdural meningeal and choroid plexus macrophages are non-parenchymal macrophages that mediate immune responses at brain boundaries. Although the origin of parenchymal microglia has ...recently been elucidated, much less is known about the precursors, the underlying transcriptional program and the dynamics of the other macrophages in the central nervous system (CNS). It was assumed that they have a high turnover from blood-borne monocytes. However, using parabiosis and fate-mapping approaches in mice, we found that CNS macrophages arose from hematopoietic precursors during embryonic development and established stable populations, with the notable exception of choroid plexus macrophages, which had dual origins and a shorter life span. The generation of CNS macrophages relied on the transcription factor PU.1, whereas the MYB, BATF3 and NR4A1 transcription factors were not required.
Myeloid cells in the central nervous system (CNS), such as microglia, CNS-associated macrophages (CAMs), dendritic cells and monocytes, are vital for steady-state immune homeostasis as well as the ...resolution of tissue damage during brain development or disease-related pathology. The complementary usage of multimodal high-throughput and high-dimensional single-cell technologies along with recent advances in cell-fate mapping has revealed remarkable myeloid cell heterogeneity in the CNS. Despite the establishment of extensive expression profiles revealing myeloid cell multiplicity, the local anatomical conditions for the temporal- and spatial-dependent cellular engraftment are poorly understood. Here we highlight recent discoveries of the context-dependent mechanisms of myeloid cell migration and settlement into distinct subtissular structures in the CNS. These insights offer better understanding of the factors needed for compartment-specific myeloid cell recruitment, integration and residence during development and perturbation, which may lead to better treatment of CNS diseases.
Microglia and central nervous system (CNS)-associated macrophages (CAMs), such as perivascular and meningeal macrophages, are implicated in virtually all diseases of the CNS. However, little is known ...about their cell-type-specific roles in the absence of suitable tools that would allow for functional discrimination between the ontogenetically closely related microglia and CAMs. To develop a new microglia gene targeting model, we first applied massively parallel single-cell analyses to compare microglia and CAM signatures during homeostasis and disease and identified hexosaminidase subunit beta (Hexb) as a stably expressed microglia core gene, whereas other microglia core genes were substantially downregulated during pathologies. Next, we generated Hexb
mice to stably monitor microglia behavior in vivo. Finally, the Hexb locus was employed for tamoxifen-inducible Cre-mediated gene manipulation in microglia and for fate mapping of microglia but not CAMs. In sum, we provide valuable new genetic tools to specifically study microglia functions in the CNS.
Abstract
The field of macrophage biology has made enormous progress over recent years. This was triggered by the advent of several new techniques such as the establishment of Cre/loxP-based ...transgenic mouse models that allowed for the first time delineation of the ontogeny and function of specific macrophage populations across many tissues. In addition, the introduction of new high-throughput technologies like bulk RNA sequencing and later single-cell RNA sequencing as well as advances in epigenetic analysis have helped to establish gene expression profiles, enhancer landscapes and local signaling cues that define and shape the identity of diverse macrophage populations. Nonetheless, some macrophage populations, like the ones residing in the peripheral nervous system (PNS), have not been studied in such detail yet. Here, we discuss recent studies that shed new light on the ontogeny, heterogeneity and gene expression profiles of resident macrophages in peripheral nerves and described differential activation of macrophage subsets during and after acute sciatic nerve injury.
In an alpha-synuclein (α-syn) model of Parkinson’s disease (PD), Schonhoff and colleagues have shown that central nervous system (CNS)-associated macrophages (CAMs), but not microglia, potentially ...orchestrate CD4+ T cell recruitment and mediate an α-syn-induced inflammatory makeup.
Microglia have critical roles not only in neural development and homeostasis, but also in neurodegenerative and neuroinflammatory diseases of the central nervous system
. These highly diverse and ...specialized functions may be executed by subsets of microglia that already exist in situ, or by specific subsets of microglia that develop from a homogeneous pool of cells on demand. However, little is known about the presence of spatially and temporally restricted subclasses of microglia in the central nervous system during development or disease. Here we combine massively parallel single-cell analysis, single-molecule fluorescence in situ hybridization, advanced immunohistochemistry and computational modelling to comprehensively characterize subclasses of microglia in multiple regions of the central nervous system during development and disease. Single-cell analysis of tissues of the central nervous system during homeostasis in mice revealed specific time- and region-dependent subtypes of microglia. Demyelinating and neurodegenerative diseases evoked context-dependent subtypes of microglia with distinct molecular hallmarks and diverse cellular kinetics. Corresponding clusters of microglia were also identified in healthy human brains, and the brains of patients with multiple sclerosis. Our data provide insights into the endogenous immune system of the central nervous system during development, homeostasis and disease, and may also provide new targets for the treatment of neurodegenerative and neuroinflammatory pathologies.
While CNS microglia have been extensively studied, relatively little is known about macrophages populating the peripheral nervous system. Here we performed ontogenic, transcriptomic and spatial ...characterization of sciatic nerve macrophages (snMacs). Using multiple fate-mapping systems, we show that snMacs do not derive from the early embryonic precursors colonizing the CNS, but originate primarily from late embryonic precursors and become replaced by bone-marrow-derived macrophages over time. Using single-cell transcriptomics, we identified a tissue-specific core signature of snMacs and two spatially separated snMacs: Relmα
Mgl1
snMacs in the epineurium and Relmα
Mgl1
snMacs in the endoneurium. Globally, snMacs lack most of the core signature genes of microglia, with only the endoneurial subset expressing a restricted number of these genes. In response to nerve injury, the two resident snMac populations respond differently. Moreover, and unlike in the CNS, monocyte-derived macrophages that develop during injury can engraft efficiently in the pool of resident peripheral nervous system macrophages.
All tissue-resident macrophages of the central nervous system (CNS)-including parenchymal microglia, as well as CNS-associated macrophages (CAMs
) such as meningeal and perivascular macrophages
-are ...part of the CNS endogenous innate immune system that acts as the first line of defence during infections or trauma
. It has been suggested that microglia and all subsets of CAMs are derived from prenatal cellular sources in the yolk sac that were defined as early erythromyeloid progenitors
. However, the precise ontogenetic relationships, the underlying transcriptional programs and the molecular signals that drive the development of distinct CAM subsets in situ are poorly understood. Here we show, using fate-mapping systems, single-cell profiling and cell-specific mutants, that only meningeal macrophages and microglia share a common prenatal progenitor. By contrast, perivascular macrophages originate from perinatal meningeal macrophages only after birth in an integrin-dependent manner. The establishment of perivascular macrophages critically requires the presence of arterial vascular smooth muscle cells. Together, our data reveal a precisely timed process in distinct anatomical niches for the establishment of macrophage subsets in the CNS.
Similar to the brain, the eye is considered an immune‐privileged organ where tissue‐resident macrophages provide the major immune cell constituents. However, little is known about spatially ...restricted macrophage subsets within different eye compartments with regard to their origin, function, and fate during health and disease. Here, we combined single‐cell analysis, fate mapping, parabiosis, and computational modeling to comprehensively examine myeloid subsets in distinct parts of the eye during homeostasis. This approach allowed us to identify myeloid subsets displaying diverse transcriptional states. During choroidal neovascularization, a typical hallmark of neovascular age‐related macular degeneration (AMD), we recognized disease‐specific macrophage subpopulations with distinct molecular signatures. Our results highlight the heterogeneity of myeloid subsets and their dynamics in the eye that provide new insights into the innate immune system in this organ which may offer new therapeutic targets for ophthalmological diseases.
SYNOPSIS
The study reveals a substantial heterogeneity of macrophages in distinct compartments of the eye. Microglia represent the predominant cell type in a model of choroidal neovascularization and disease‐associated subpopulations quickly emerge complemented by infiltrating periphery‐derived cells.
•Single‐cell RNA‐sequencing reveals several macrophage subsets in the cornea, ciliary body and retina under homeostatic conditions.
•Embryonic fate mapping reveals prenatal origin of macrophages in the retina, ciliary body and cornea.
•Significant homeostatic turnover was only detectable for cornea macrophages by the use of several models targeting the definitive hematopoiesis.
•Microglia are the predominant myeloid cell type in choroidal neovascularization and create context‐specific disease‐associated microglia subsets.
Single‐cell RNA‐sequencing reveals heterogeneity of macrophages in the cornea, ciliary body and retina under homeostatic conditions.
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.
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•Cx3cr1CreER lines efficiently recombine but have more spontaneous recombination•Tmem119CreER, P2ry12CreER, and HexbCreER are less efficient but are microglia specific•Inter-loxP distance is a key determinant of recombination efficiency•Best practices are provided for measuring recombination in microglia
Microglia are key regulators of neural circuits and neuroinflammation. Tools to specifically manipulate their function in vivo are critical. Faust et al. show that recombination efficiency and specificity differ when comparing different microglial CreER mouse lines. They then provide methods and best practices for using these mice going forward.
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