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Single-cell transcriptomics of microglia isolated from the hypothalamic/thalamic region of E15.5 brains under control and stressed conditions

ABSTRACT: Microglia are the resident phagocytic immune cells of the central nervous system (CNS). In the embryo, microglia invade the CNS during early embryogenesis and continue to colonize alongside neural development. Microglia also mature across development, altering their morphology and expression signatures from embryonic to adult stages, suggesting differing roles for microglia in the developing brain than in the adult. While microglia were originally thought to be resting immune cells responsible for cleaning up cellular debris, studies are now showing that microglia are highly dynamic cells involved in proper postnatal development of the brain and adult neuronal plasticity. Although reports suggest that microglia in the embryonic brain are responsive to maternal stress, causing changes in neural development that lead to autistic- and schizophrenic-like behaviours later in life, the actual effects of microglia on embryonic developmental programs are just emerging. Our lab studies the hypothalamus, a brain region that controls a variety of physiologies including energy balance, reproduction, and even parenting behaviors, and it is also responsive to maternal stress. Interestingly, my previous work has shown that that the hypothalamus is uniquely comprised of at least two distinct populations of microglia. The first group of microglia are ramified, and appear to be passively surveying the hypothalamic mantle. The second cluster of microglia is amoeboid and activated, and are located adjacent to the ventricular zone (VZ) where hypothalamic neural progenitor cells (NPCs) reside. Given that this second group of microglia is in direct contact with NPCs, we hypothesize that microglia located in the VZ of the hypothalamus play an important role during embryonic development to organize the neurons and neuronal connections required for proper hypothalamic function. In order to identify the expression profiles of microglia in the VZ as compared to microglia present in the mantle of the hypothalamus, we extracted microglia from the brains of CD1 embryos and employed single-cell RNAseq. Specifically, we micro-dissected out the hypothalamic/thalamic region of E15.5 brains, created a single-cell suspension that was stained with Cd11b-PE / Cd45-FITC to mark microglia, and then used FACS to extract microglia from our mixed cell suspension. We then performed single-cell RNAseq on the microglia to determine the transcriptomic phenotype of these two populations either in a control condition or following maternal stress, where pregnant dams were exposed to a mild cold stress for 30 min a day over 5 days.

ORGANISM(S): Mus musculus

PROVIDER: GSE133194 | GEO | 2021/04/05

REPOSITORIES: GEO

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Project description:Microglia are the resident immune cells of the central nervous system (CNS) and are derived from yolk sac progenitors. In the embryo, microglia invade the CNS starting at embryonic day 9.5 (E9.5) and continue to colonize alongside neural development. Microglia also mature across development, altering their morphology and expression signatures from embryonic to adult stages, suggesting differing roles for microglia in the developing brain than in the adult. While microglia were originally thought to be resting immune cells responsible for cleaning up cellular debris, studies are now showing that microglia are highly dynamic cells involved in proper postnatal development of the brain and adult neuronal plasticity. Although recent reports have demonstrated that microglia in the embryonic brain are responsive to maternal stress, causing changes in neural development that lead to autistic- and schizophrenic-like behaviours later in life, the actual effects of microglia on embryonic developmental programs are just emerging. Our lab studies the hypothalamus, a brain region that controls a variety of physiologies including energy balance, reproduction, and even parenting behaviors, and it is also responsive to maternal stress. Interestingly, my previous work has shown that that the hypothalamus uniquely is comprised of at least two distinct populations of microglia. The first group of microglia are ramified, and appear to be passively surveying the hypothalamic mantle as we would expect. The second cluster of microglia is amoeboid and activated, and are located adjacent to the ventricular zone (VZ) where hypothalamic neural progenitor cells (NPCs) reside. Given that this second group of microglia is in direct contact with NPCs, we hypothesize that microglia located in the VZ of the hypothalamus play an important role during embryonic development to organize the neurons and neuronal connections required for proper hypothalamic function. In order to identify the expression profiles of microglia in the VZ as compared to microglia present in the mantle of the hypothalamus, we used Cx3cr1-Cre; Rosa26tdTomato mice to mark microglia. We then micro-dissected the hypothalamic/thalamic region of the E15.5 brains and used flow cytometry to extract tdTomato+ microglia. We then performed single-cell RNAseq on the microglia to determine the transcriptomic phenotype of these two populations.

Project description:Microglia colonize the brain parenchyma at early stages of development and accumulate in specific regions where they actively participate in cell death, angiogenesis, neurogenesis and synapse elimination. A recurring feature of embryonic microglial distribution is their association with developing axon tracts which, together with in vitro data, supports the idea of a physiological role for microglia in neurite development. Yet the demonstration of this role of microglia is still lacking. Here, we have studied the consequences of microglial dysfunction on the formation of the corpus callosum, the largest connective structure in the mammalian brain, which shows consistent microglial accumulation during development. We studied two models of microglial dysfunction: the loss-of-function of DAP12, a key microglial-specific signaling molecule, and a model of maternal inflammation by peritoneal injection of LPS at E15.5. We performed transcriptional profiling of maternally inflamed and Dap12-mutant microglia at E17.5. We found that both treatments principally down-regulated genes involved in nervous system development and function, particularly in neurite formation. We then analyzed the functional consequences of these microglial dysfunctions on the formation of the corpus callosum. We also took advantage of the Pu.1-/- mouse line, which is devoid of microglia. We now show that all three models of altered microglial activity resulted in the same defasciculation phenotype. Our study demonstrates that microglia are actively involved in the fasciculation of corpus callosum axons. To investigate possible roles for microglial during brain development, we challenged microglial function by two complementary approaches. First, we induced maternal inflammation by peritoneal injection of LPS into pregnant dams. Next, we analyzed the consequences of a loss of function of DAP12, a signaling molecule specifically expressed in microglia that is crucial for several aspects of microglia biology (references in Wakselman et al., 2008). We compared the gene expression profiles of microglia from control, maternally-inflamed by LPS (MI), and Dap12-mutated embryos. We isolated RNA from FACS sorted maternally inflamed (by LPS) and Dap12-mutant microglia at E17.5 pooled per pregnant dam; as a control we included PBS treated and untreated (UT) microglia. We compared gene expression between maternally inflamed microlgia (PBSvsLPS) and DAP12-mutant microglia (UTvsDAP12KO).

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Single-cell transcriptomics of microglia isolated from the hypothalamic/thalamic region of E15.5 brains under control and stressed conditions

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