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Isolation of Cortical Microglia with Preserved Immunophenotype and Functionality From Murine Neonates
Published on: January 30, 2014
TLR4-activated microglia require IFN-γ to induce severe neuronal dysfunction and death in situ
Ismini E Papageorgiou1, Andrea Lewen1, Lukas V Galow1
1Institute of Physiology and Pathophysiology, University of Heidelberg, D-69120 Heidelberg, Germany; Interdisciplinary Center for Neurosciences, University of Heidelberg, D-69120 Heidelberg, Germany;
Abstract:
Microglia (tissue-resident macrophages) represent the main cell type of the innate immune system in the CNS; however, the mechanisms that control the activation of microglia are widely unknown. We systematically explored microglial activation and functional microglia-neuron interactions in organotypic hippocampal slice cultures, i.e., postnatal cortical tissue that lacks adaptive immunity. We applied electrophysiological recordings of local field potential and extracellular K(+) concentration, immunohistochemistry, design-based stereology, morphometry, Sholl analysis, and biochemical analyses. We show that chronic activation with either bacterial lipopolysaccharide through Toll-like receptor 4 (TLR4) or leukocyte cytokine IFN-γ induces reactive phenotypes in microglia associated with morphological changes, population expansion, CD11b and CD68 up-regulation, and proinflammatory cytokine (IL-1β, TNF-α, IL-6) and nitric oxide (NO) release. Notably, these reactive phenotypes only moderately alter intrinsic neuronal excitability and gamma oscillations (30-100 Hz), which emerge from precise synaptic communication of glutamatergic pyramidal cells and fast-spiking, parvalbumin-positive GABAergic interneurons, in local hippocampal networks. Short-term synaptic plasticity and extracellular potassium homeostasis during neural excitation, also reflecting astrocyte function, are unaffected. In contrast, the coactivation of TLR4 and IFN-γ receptors results in neuronal dysfunction and death, caused mainly by enhanced microglial inducible nitric oxide synthase (iNOS) expression and NO release, because iNOS inhibition is neuroprotective. Thus, activation of TLR4 in microglia in situ requires concomitant IFN-γ receptor signaling from peripheral immune cells, such as T helper type 1 and natural killer cells, to unleash neurotoxicity and inflammation-induced neurodegeneration. Our findings provide crucial mechanistic insight into the complex process of microglia activation, with relevance to several neurologic and psychiatric disorders.
Insights
Microglia activation requires dual signaling for neurotoxicity. Co-activation of Toll-like receptor 4 and IFN-γ signaling leads to neuronal death, highlighting a key mechanism in neuroinflammation.
Area of Science:
- Neuroscience
- Immunology
- Cell Biology
Background:
- Microglia are the primary immune cells in the central nervous system (CNS).
- Mechanisms controlling microglial activation and their impact on neuronal function are not fully understood.
- Understanding microglial responses is crucial for addressing neuroinflammatory and neurodegenerative diseases.
Purpose of the Study:
- To investigate microglial activation pathways and their functional interactions with neurons in the CNS.
- To elucidate the specific signaling requirements for microglial-induced neurotoxicity and neuroinflammation.
- To explore the role of microglia in maintaining neuronal health and network function.
Main Methods:
- Organotypic hippocampal slice cultures were used to model CNS tissue.
- Electrophysiological recordings assessed neuronal activity and network oscillations.
- Immunohistochemistry, stereology, and biochemical analyses quantified microglial responses and molecular changes.
- Pharmacological inhibition of inducible nitric oxide synthase (iNOS) was employed.
Main Results:
- Activation of Toll-like receptor 4 (TLR4) or IFN-γ alone induced reactive microglia with moderate effects on neuronal excitability and gamma oscillations.
- Co-activation of TLR4 and IFN-γ receptors led to significant neuronal dysfunction and death.
- Enhanced inducible nitric oxide synthase (iNOS) expression and nitric oxide (NO) release by microglia were identified as key mediators of neurotoxicity.
- iNOS inhibition demonstrated neuroprotective effects, confirming its critical role.
Conclusions:
- Microglial neurotoxicity and inflammation-induced neurodegeneration necessitate co-activation of TLR4 and IFN-γ signaling pathways.
- Peripheral immune cell signaling, particularly IFN-γ, is essential for unleashing microglial-mediated neurotoxicity.
- These findings offer critical insights into microglia activation mechanisms relevant to neurological and psychiatric disorders.
