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Myeloid cells such as resident retinal microglia (MG) or infiltrating blood-derived macrophages (Mϕ) accumulate in areas of retinal ischemia and neovascularization (RNV) and modulate neovascular eye disease. Their temporospatial distribution and biological function in this process, however, remain unclarified. Using state-of-the-art methods, including cell-specific reporter mice and high-throughput RNA sequencing (RNA Seq), this study determined the extent of MG proliferation and Mϕ infiltration in areas with retinal ischemia and RNV in Cx3cr1CreERT2 :Rosa26-tdTomato mice and examined the transcriptional profile of MG in the mouse model of oxygen-induced retinopathy (OIR). For RNA Seq, tdTomato-positive retinal MG were sorted by flow cytometry followed by Gene ontology (GO) cluster analysis. Furthermore, intraperitoneal injections of the cell proliferation marker 5-ethynyl-2'-deoxyuridine (EdU) were performed from postnatal day (p) 12 to p16. We found that MG is the predominant myeloid cell population while Mϕ rarely appears in areas of RNV. Thirty percent of retinal MG in areas of RNV were EdU-positive indicating a considerable local MG cell expansion. GO cluster analysis revealed an enrichment of clusters related to cell division, tubulin binding, ATPase activity, protein kinase regulatory activity, and chemokine receptor binding in MG in the OIR model compared to untreated controls. In conclusion, activated retinal MG alter their transcriptional profile, exhibit considerable proliferative ability and are by far the most frequent myeloid cell population in areas of ischemia and RNV in the OIR model thus presenting a potential target for future therapeutic approaches.

Objective Recently we demonstrated that amoeboid microglia in white matter regions are essential for proper oligodendrocyte homeostasis and myelinogenesis in the first postnatal week. Amoeboid microglia in the mouse corpus callosum change their activation profile within few days after postnatal day (P)7 with microglia of the cerebellum showing similar features. Here we expanded our previous transcriptional analysis and performed detailed bulk RNA sequencing of microglia from corpus callosum, cortex and cerebellum at P7, P10 and P42. The goal of this study was to identify a specific gene profile for both, white matter and grey matter microglia during development. Results Microglia in white matter regions display unique characteristics in the first postnatal week of murine life. In both the corpus callosum and cerebellum microglia show amoeboid morphology and a similar transcription profile during development including high expression of genes related to priming of microglia, phagocytosis and migration at P7; characteristics which are already lost at P10. Together these data verify our previous transcriptional data obtained by microarray analysis and enable a more complete view into white matter and grey matter microglia at different developmental stages. Electronic supplementary material The online version of this article (10.1186/s13104-019-4410-1) contains supplementary material, which is available to authorized users.

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.

Nat. Immunol. 18, 385–392 (2017); published online 22 March 2017; corrected after print 26 April 2017 In the version of this article initially published, some of the arrows along the right side of Figure 1 were incorrect. The arrow from 'Ly6Chi monocyte' to 'pvMΦ long-living' should be deleted, and an arrow should be added from 'Ly6Chi monocyte' into the boxed area above (top right).

Whereas microglia involvement in virtually all brain diseases is well accepted their role in the control of homeostasis in the central nervous system (CNS) is mainly thought to be the maintenance of neuronal function through the formation, refinement, and monitoring of synapses in both the developing and adult brain. Although the prenatal origin as well as the neuron-centered function of cortical microglia has recently been elucidated, much less is known about a distinct amoeboid microglia population formerly described as the "fountain of microglia" that appears only postnatally in myelinated regions such as corpus callosum and cerebellum. Using large-scale transcriptional profiling, fate mapping, and genetic targeting approaches, we identified a unique molecular signature of this microglia subset that arose from a CNS endogenous microglia pool independent from circulating myeloid cells. Microglia depletion experiments revealed an essential role of postnatal microglia for the proper development and homeostasis of oligodendrocytes and their progenitors. Our data provide new cellular and molecular insights into the myelin-supporting function of microglia in the normal CNS.

Molawi et al. examine the origin and cellular dynamics of macrophages in the heart during postnatal development. Cardiac macrophages derived from CX3CR1+ embryonic progenitors persist into adulthood, but the contribution of these cells to resident macrophages declines after birth with diminished self-renewal as the mice age. Over time, the heart is progressively reconstituted with bone marrow–derived macrophages, even in the absence of inflammation., Cardiac macrophages (cMΦ) are critical for early postnatal heart regeneration and fibrotic repair in the adult heart, but their origins and cellular dynamics during postnatal development have not been well characterized. Tissue macrophages can be derived from embryonic progenitors or from monocytes during inflammation. We report that within the first weeks after birth, the embryo-derived population of resident CX3CR1+ cMΦ diversifies into MHCII+ and MHCII− cells. Genetic fate mapping demonstrated that cMΦ derived from CX3CR1+ embryonic progenitors persisted into adulthood but the initially high contribution to resident cMΦ declined after birth. Consistent with this, the early significant proliferation rate of resident cMΦ decreased with age upon diversification into subpopulations. Bone marrow (BM) reconstitution experiments showed monocyte-dependent quantitative replacement of all cMΦ populations. Furthermore, parabiotic mice and BM chimeras of nonirradiated recipient mice revealed a slow but significant donor contribution to cMΦ. Together, our observations indicate that in the heart, embryo-derived cMΦ show declining self-renewal with age and are progressively substituted by monocyte-derived macrophages, even in the absence of inflammation.

Myeloid cells in the central nervous system (CNS) represent a heterogeneous class of innate immune cells that contribute to the maintenance of tissue homeostasis differentially during development and adulthood. The subsets of CNS myeloid cells identified so far, including parenchymal microglia and non-parenchymal meningeal, perivascular and choroid-plexus macrophages, as well as disease-associated monocytes, have classically been distinguished on the basis of their surface epitope expression, localization and morphology. However, studies using cell-specific targeting, in vivo imaging, single-cell expression analysis and other sophisticated tools have now increased the depth of knowledge of this immune-cell compartment and call for reevaluation of the traditional views on the origin, fate and function of distinct CNS myeloid subsets. The concepts of CNS macrophage biology that are emerging from these new insights have broad implications for the understanding and treatment of CNS diseases.

Recent studies have shown that tissue macrophages (MΦ) arise from embryonic progenitors of the yolk sac (YS) and fetal liver and colonize tissues before birth. Further studies have proposed that developmentally distinct tissue MΦ can be identified based on the differential expression of F4/80 and CD11b, but whether a characteristic transcriptional profile exists is largely unknown. Here, we took advantage of an inducible fate‐mapping system that facilitated the identification of CD45+c‐kit−CX3CR1+F4/80+ (A2) progenitors of the YS as the source of F4/80hi but not CD11bhi MΦ. Large‐scale transcriptional profiling of MΦ precursors from the YS stage to adulthood allowed for building computational models for F4/80hi tissue macrophages being direct descendants of A2 progenitors. We further identified a distinct molecular signature of F4/80hi and CD11bhi MΦ and found that Irf8 was vital for MΦ maturation. Our data provide new cellular and molecular insights into the origin and developmental pathways of tissue MΦ. ![][1] In vivo fate mapping combined with transcriptomics shows the existence of different mouse macrophage subsets, and an important role for the transcription factor Irf8 in regulating their development. The EMBO Journal (2016) 35: 1730–1744 [1]: /embed/graphic-1.gif

Recombinant human erythropoietin (EPO) improves cognitive performance in neuropsychiatric diseases ranging from schizophrenia and multiple sclerosis to major depression and bipolar disease. This consistent EPO effect on cognition is independent of its role in hematopoiesis. The cellular mechanisms of action in brain, however, have remained unclear. Here we studied healthy young mice and observed that 3-week EPO administration was associated with an increased number of pyramidal neurons and oligodendrocytes in the hippocampus of ∼20%. Under constant cognitive challenge, neuron numbers remained elevated until >6 months of age. Surprisingly, this increase occurred in absence of altered cell proliferation or apoptosis. After feeding a 15 N-leucine diet, we used nanoscopic secondary ion mass spectrometry, and found that in EPO-treated mice, an equivalent number of neurons was defined by elevated 15 N-leucine incorporation. In EPO-treated NG2-Cre-ERT2 mice, we confirmed enhanced differentiation of preexisting oligodendrocyte precursors in the absence of elevated DNA synthesis. A corresponding analysis of the neuronal lineage awaits the identification of suitable neuronal markers. In cultured neurospheres, EPO reduced Sox9 and stimulated miR124, associated with advanced neuronal differentiation. We are discussing a resulting working model in which EPO drives the differentiation of non-dividing precursors in both (NG2+) oligodendroglial and neuronal lineages. As endogenous EPO expression is induced by brain injury, such a mechanism of adult neurogenesis may be relevant for central nervous system regeneration. Fil: Hassouna, I.. Minufiya University; . Max Planck Institut Fur Experimentelle Medizin; Fil: Ott, C.. Max Planck Institut Fur Experimentelle Medizin; Fil: Wüstefeld, L.. Max Planck Institut Fur Experimentelle Medizin; Fil: Offen, N.. Universität Würzburg; Alemania Fil: Neher, R.A.. Max Planck Institute For Developmental Biology; Fil: Mitkovski, M.. Max Planck Institut Fur Experimentelle Medizin; Fil: Winkler, D.. Max Planck Institut Fur Experimentelle Medizin; Fil: Sperling, S.. Max Planck Institut Fur Experimentelle Medizin; Fil: Fries, L.. Universität Würzburg; Alemania Fil: Goebbels, S.. Max Planck Institut Fur Experimentelle Medizin; Fil: Vreja, I.C.. International Max Planck Research School Molecular Biology; . Universitatsmedizin Gottingen; Fil: Hagemeyer, N.. Max Planck Institut Fur Experimentelle Medizin; Fil: Dittrich, M.. Universität Würzburg; Alemania Fil: Rossetti, María Florencia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Salud y Ambiente del Litoral. Universidad Nacional del Litoral. Instituto de Salud y Ambiente del Litoral; Argentina Fil: Kröhnert, K.. Universitatsmedizin Gottingen; Fil: Hannke, K.. Max Planck Institut Fur Experimentelle Medizin; Fil: Boretius, S.. Christian-albrechts-universitat Zu Kiel; Fil: Zeug, A.. Medizinische Hochschule Hannover (mhh); Fil: Höschen, C.. Universitat Technical Zu Munich; Alemania Fil: Dandekar, T.. Universität Würzburg; Alemania Fil: Dere, E.. Max Planck Institut Fur Experimentelle Medizin; Fil: Neher, E.. Max Planck Institute For Biophysical Chemistry (karl Friedrich Bonhoeffer Institute); . Deutsche Forschungsgemeinschaft (dfg); Fil: Rizzoli, S.O.. Universitatsmedizin Gottingen; . Deutsche Forschungsgemeinschaft (dfg); Fil: Nave, K.-A.. Deutsche Forschungsgemeinschaft (dfg); . Max Planck Institut Fur Experimentelle Medizin; Fil: Sirén, A.-L.. Universität Würzburg; Alemania Fil: Ehrenreich, H.. Max Planck Institut Fur Experimentelle Medizin; . Deutsche Forschungsgemeinschaft (dfg)

Microglia are tissue resident macrophages of the central nervous system (CNS) that maintain homeostasis and respond to immune challenges. New genetic fate mapping tools have revealed a yolk sac origin of microglia. Once established in the CNS, microglia persist throughout the lifetime of the organism behind the blood-brain barrier and maintain themselves by self-renewal. Recent studies uncovered a broad spectrum of microglial functions that are influenced by the dynamism of brain formation and neuronal wiring. This review focuses on current findings concerning microglia origin and formation during development and discusses the factors important for microglia survival and function.

Microglia as tissue macrophages contribute to the defense and maintenance of central nervous system (CNS) homeostasis. Little is known about the epigenetic signals controlling microglia function in vivo. We employed constitutive and inducible mutagenesis in microglia to delete two class I histone deacetylases, Hdac1 and Hdac2. Prenatal ablation of Hdac1 and Hdac2 impaired microglial development. Mechanistically, the promoters of pro-apoptotic and cell cycle genes were hyperacetylated in absence of Hdac1 and Hdac2, leading to increased apoptosis and reduced survival. In contrast, Hdac1 and Hdac2 were not required for adult microglia survival during homeostasis. In a mouse model of Alzheimer's disease, deletion of Hdac1 and Hdac2 in microglia, but not in neuroectodermal cells, resulted in a decrease in amyloid load and improved cognitive impairment by enhancing microglial amyloid phagocytosis. Collectively, we report a role for epigenetic factors that differentially affect microglia development, homeostasis, and disease that could potentially be utilized therapeutically.

Severe mental illnesses have been linked to white matter abnormalities, documented by postmortem studies. However, cause and effect have remained difficult to distinguish. CNP (2′,3′-cyclic nucleotide 3′-phosphodiesterase) is among the oligodendrocyte/myelin-associated genes most robustly reduced on mRNA and protein level in brains of schizophrenic, bipolar or major depressive patients. This suggests that CNP reduction might be critical for a more general disease process and not restricted to a single diagnostic category. We show here that reduced expression of CNP is the primary cause of a distinct behavioural phenotype, seen only upon aging as an additional ‘pro-inflammatory hit’. This phenotype is strikingly similar in Cnp heterozygous mice and patients with mental disease carrying the AA genotype at CNP SNP rs2070106. The characteristic features in both species with their partial CNP ‘loss-of-function’ genotype are best described as ‘catatonia-depression’ syndrome. As a consequence of perturbed CNP expression, mice show secondary low-grade inflammation/neurodegeneration. Analogously, in man, diffusion tensor imaging points to axonal loss in the frontal corpus callosum. To conclude, subtle white matter abnormalities inducing neurodegenerative changes can cause/amplify psychiatric diseases.

The endogenous microenvironment of the brain is an essential watchdog to guard over myeloid cell function during diseases. Limiting inflammatory reactions of activated microglia and blood-derived monocytes is a key prerequisite for the resolution of tissue insults. So far, however, it was unknown why monocytes but not microglia are able to shift to an anti-inflammatory state during inflammation. In this issue of The EMBO Journal, Cohen and colleagues identified the molecular switch underlying this fundamental functional change. The authors found that the transforming growth factor-β1 (TGFβ1) prevents activated microglia to switch to an anti-inflammatory state by regulating the expression of Irf7.

We have previously shown that high-dose erythropoietin (EPO) treatment improves hippocampal plasticity and cognitive performance in rodents and in patients suffering from neuropsychiatric diseases. It was therefore of interest to explore whether upregulation of endogenous EPO in brain by hypoxia inducible factor (HIF) stabilization would increase hippocampal memory similar to exogenous EPO. HIFs are transcription factors involved in the cellular response to low oxygen, including upregulation of transcripts like vascular endothelial growth factor (VEGF) and EPO. Under normal oxygen, prolylhydroxylases decrease HIF-alpha stability. This is banned by prolylhydroxylase inhibitors, which prevent oxygen dependent degradation and thus prolong HIF-alpha half life. In an experimental set-up identical to the one yielding strong cognitive effects with EPO, healthy male 28-day-old mice received FG-4497, a HIF prolylhydroxylase inhibitor, or placebo intraperitoneally every other day for 3 weeks. Behavioral testing and hematocrit determinations were conducted in independent cohorts at 1, 3, or 4 weeks after treatment completion. Increased EPO and VEGF mRNA expression in hippocampus or primary hippocampal neurons 6h after the application of FG-4497 confirmed its ability to stabilize HIF and upregulate HIF dependent transcription in brain. At 3 and 4 weeks after the last injection, respectively, FG-4497 treated mice compared to placebo mice had improved hippocampal memory in fear conditioning without change in hematocrit. In contrast, no improvement in memory was detected at 1 week, when the hematocrit was increased, indicating that cognitive improvement and hematocrit are not directly related. FG-4497 application for 3 weeks leads to delayed but lasting enhancement of hippocampal memory, making HIF stabilization an attractive target for pharmacological manipulation of cognition.