Birth is an inflammatory event for the newborn, characterized by elevations in interleukin (IL)-6, IL-10, and tumor necrosis factor (TNF)-α peripherally and/or centrally, as well as changes in brain microglia. However, the mechanism(s) underlying these responses is unknown. Toll-like receptors (TLRs) play crucial roles in innate immunity and initiate inflammatory cascades upon recognition of endogenous or exogenous antigens. Most TLR signaling depends on the adaptor molecule myeloid differentiation primary response 88 (MyD88). We independently varied MyD88 gene status in mouse dams and their offspring to determine whether the inflammatory response to birth depends on MyD88 signaling and, if so, whether that signaling occurs in the offspring, the mother, or both. We find that the perinatal surges in plasma IL-6 and brain expression of TNF-α depend solely on MyD88 gene status of the offspring, whereas postnatal increases in plasma IL-10 and TNF-α depend on MyD88 in both the pup and dam. Interestingly, MyD88 genotype of the dam primarily drives differences in offspring brain microglial density and has robust effects on developmental neuronal cell death. Milk cytokines were evaluated as a possible source of postnatal maternal influence; although we found high levels of CXCL1/GROα and several other cytokines in ingested post-partum milk, their presence did not require MyD88. Thus, the inflammatory response previously described in the late-term fetus and newborn depends on MyD88 (and, by extension, TLRs), with signaling in both the dam and offspring contributing. Unexpectedly, naturally-occuring neuronal cell death in the newborn is modulated primarily by maternal MyD88 gene status.
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Microglial pattern recognition via IL-33 promotes synaptic refinement in developing corticothalamic circuits in mice
Microglia are critical regulators of brain development that engulf synaptic proteins during postnatal synapse remodeling. However, the mechanisms through which microglia sense the brain environment are not well defined. Here, we characterized the regulatory program downstream of interleukin-33 (IL-33), a cytokine that promotes microglial synapse remodeling. Exposing the developing brain to a supraphysiological dose of IL-33 altered the microglial enhancer landscape and increased binding of stimulus-dependent transcription factors including AP-1/FOS. This induced a gene expression program enriched for the expression of pattern recognition receptors, including the scavenger receptor MARCO. CNS-specific deletion of IL-33 led to increased excitatory/inhibitory synaptic balance, spontaneous absence-like epileptiform activity in juvenile mice, and increased seizure susceptibility in response to chemoconvulsants. We found that MARCO promoted synapse engulfment, and Marco-deficient animals had excess thalamic excitatory synapses and increased seizure susceptibility. Taken together, these data define coordinated epigenetic and functional changes in microglia and uncover pattern recognition receptors as potential regulators of postnatal synaptic refinement.
more » « less- NSF-PAR ID:
- 10385670
- Publisher / Repository:
- DOI PREFIX: 10.1084
- Date Published:
- Journal Name:
- Journal of Experimental Medicine
- Volume:
- 220
- Issue:
- 2
- ISSN:
- 0022-1007
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Emerging roles for microglia in modifying normal brain development continue to provide new perspectives on the functions of this resident immune cell in the brain. While the molecular underpinnings driving microglia’s position in regulating developmental programs remain largely an unchartered territory, innate immune signaling lies at the forefront. At least three innate immune receptors expressed on microglia—fractalkine, complement, and triggering receptor expressed on microglia (TREM2)—modulate developmental synaptic pruning to refine brain circuitry. Our laboratory recently published that microglia with a unique amoeboid morphology invade the corpus callosum and engulf oligodendrocyte progenitor cells (OPCs) during early postnatal development before myelination in a fractalkine receptor (CX3CR1)‐dependent manner to modulate ensheathment of axons. Amoeboid microglia are observed in the corpus callosum but not cerebral cortex, and lose their amoeboid shape at the commencement of myelination assuming a resting phenotype. Furthermore, OPCs contacted or engulfed by microglia do not express markers of cell death suggesting a novel homeostatic mechanism facilitating an appropriate OPC:axon ratio for proper myelin ensheathment. The unique morphology of microglia and the restricted window for phagocytic engulfment of OPCs suggest a critical period for OPC engulfment important for action potential propagation during development when activity‐dependent mechanisms regulate synaptic pruning. In this review, we summarize the role of activity‐dependent mechanisms in sculpting brain circuitry, how myelin ensheathment influences action potential propagation, the spatial and temporal relationship of microglia‐dependent elimination of OPCs and synapses, and implications for the synergistic role of microglial phagocytosis in shaping the architecture for neuronal function.
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Key points Nucleotide binding oligomerization domain (Nod)‐like receptors regulate cognition, anxiety and hypothalamic–pituitary–adrenal axis activation.
Nod‐like receptors regulate central and peripheral serotonergic biology.
Nod‐like receptors are important for maintenance of gastrointestinal physiology.
Intestinal epithelial cell expression of Nod1 receptors regulate behaviour.
Abstract Gut–brain axis signalling is critical for maintaining health and homeostasis. Stressful life events can impact gut–brain signalling, leading to altered mood, cognition and intestinal dysfunction. In the present study, we identified nucleotide binding oligomerization domain (Nod)‐like receptors (NLR), Nod1 and Nod2, as novel regulators for gut–brain signalling. NLR are innate immune pattern recognition receptors expressed in the gut and brain, and are important in the regulation of gastrointestinal physiology. We found that mice deficient in both Nod1 and Nod2 (NodDKO) demonstrate signs of stress‐induced anxiety, cognitive impairment and depression in the context of a hyperactive hypothalamic–pituitary–adrenal axis. These deficits were coupled with impairments in the serotonergic pathway in the brain, decreased hippocampal cell proliferation and immature neurons, as well as reduced neural activation. In addition, NodDKO mice had increased gastrointestinal permeability and altered serotonin signalling in the gut following exposure to acute stress. Administration of the selective serotonin reuptake inhibitor, fluoxetine, abrogated behavioural impairments and restored serotonin signalling. We also identified that intestinal epithelial cell‐specific deletion of Nod1 (VilCre+Nod1f/f), but not Nod2, increased susceptibility to stress‐induced anxiety‐like behaviour and cognitive impairment following exposure to stress. Together, these data suggest that intestinal epithelial NLR are novel modulators of gut–brain communication and may serve as potential novel therapeutic targets for the treatment of gut–brain disorders.
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Abstract Huntington’s disease is associated with a reactive microglial response and consequent inflammation. To address the role of these cells in disease pathogenesis, we depleted microglia from R6/2 mice, a rapidly progressing model of Huntington’s disease marked by behavioural impairment, mutant huntingtin (mHTT) accumulation, and early death, through colony-stimulating factor 1 receptor inhibition (CSF1Ri) with pexidartinib (PLX3397) for the duration of disease. Although we observed an interferon gene signature in addition to downregulated neuritogenic and synaptic gene pathways with disease, overt inflammation was not evident by microglial morphology or cytokine transcript levels in R6/2 mice. Nonetheless, CSF1Ri-induced microglial elimination reduced or prevented disease-related grip strength and object recognition deficits, mHTT accumulation, astrogliosis, and striatal volume loss, the latter of which was not associated with reductions in cell number but with the extracellular accumulation of chondroitin sulphate proteoglycans (CSPGs)—a primary component of glial scars. A concurrent loss of proteoglycan-containing perineuronal nets was also evident in R6/2 mice, and microglial elimination not only prevented this but also strikingly increased perineuronal nets in the brains of naïve littermates, suggesting a new role for microglia as homeostatic regulators of perineuronal net formation and integrity.
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Abstract Background Emerging RNA viruses that target the central nervous system (CNS) lead to cognitive sequelae in survivors. Studies in humans and mice infected with West Nile virus (WNV), a re-emerging RNA virus associated with learning and memory deficits, revealed microglial-mediated synapse elimination within the hippocampus. Moreover, CNS-resident memory T (TRM) cells activate microglia, limiting synapse recovery and inducing spatial learning defects in WNV-recovered mice. The signals involved in T cell-microglia interactions are unknown.
Methods Here, we examined immune cells within the murine WNV-recovered forebrain using single-cell RNA sequencing to identify putative ligand-receptor pairs involved in intercellular communication between T cells and microglia. Clustering and differential gene analyses were followed by protein validation and genetic and antibody-based approaches utilizing an established murine model of WNV recovery in which microglia and complement promote ongoing hippocampal synaptic loss.
Results Profiling of host transcriptome immune cells at 25 days post-infection in mice revealed a shift in forebrain homeostatic microglia to activated subpopulations with transcriptional signatures that have previously been observed in studies of neurodegenerative diseases. Importantly, CXCL16/CXCR6, a chemokine signaling pathway involved in TRM cell biology, was identified as critically regulating CXCR6 expressing CD8+TRM cell numbers within the WNV-recovered forebrain. We demonstrate that CXCL16 is highly expressed by all myeloid cells, and its unique receptor, CXCR6, is highly expressed on all CD8+T cells. Using genetic and pharmacological approaches, we demonstrate that CXCL16/CXCR6 not only is required for the maintenance of WNV-specific CD8 TRM cells in the post-infectious CNS, but also contributes to their expression of TRM cell markers. Moreover, CXCR6+CD8+T cells are required for glial activation and ongoing synapse elimination.
Conclusions We provide a comprehensive assessment of the role of CXCL16/CXCR6 as an interaction link between microglia and CD8+T cells that maintains forebrain TRM cells, microglial and astrocyte activation, and ongoing synapse elimination in virally recovered animals. We also show that therapeutic targeting of CXCL16 in mice during recovery may reduce CNS CD8+TRM cells.
