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Complex molecular and functional outcomes of single versus sequential cytokine stimulation of rat microglia
Tamjeed A Siddiqui1,2, Starlee Lively1, Lyanne C Schlichter3,4,5
1Genes and Development Division, Krembil Research Institute, University Health Network, Toronto, Ontario, M5T 2S8, Canada.
Background:
Microglia are the "professional" phagocytes of the CNS. Phagocytosis is crucial for normal CNS development and maintenance, but it can be either beneficial or detrimental after injury or disease. For instance, white matter damage releases myelin debris that must be cleared by microglia in order for re-myelination to occur. However, phagocytosis can also produce damaging reactive oxygen species (ROS). Furthermore, microglia can acquire pro-inflammatory (M1) or anti-inflammatory (M2) activation states that affect cell functions. Although microglia are exposed to a changing cytokine environment after injury or disease, little is known about the molecular and functional consequences. Therefore, we applied several microglial activation paradigms, with or without myelin debris. We assessed (i) gene expression changes reflecting microglial activation and inflammatory states, and receptors and enzymes related to phagocytosis and ROS production, (ii) myelin phagocytosis and production of ROS, and (iii) expression and contributions of several ion channels that are considered potential targets for regulating microglial behavior.
Methods:
Primary rat microglia were exposed to cytokines, individually or sequentially. First, responses to individual M1 or M2 stimuli were compared: IFN-γ plus TNF-α ("I + T"; M1 activation), interleukin-4 (M2a/alternative activation), and interleukin-10 (M2c/acquired deactivation). Second, sequential cytokine addition was used to assess microglia repolarization and cell functions. The paradigms were M2a→M1, M2c→M1, M1→M2a, and M1→M2c.
Results:
M1 stimulation increased pro-inflammatory genes, phagocytosis, and ROS, as well as expression of Kv1.3, KCa3.1, and Kir2.1 channels. M2a stimulation increased anti-inflammatory genes, ROS production, and Kv1.3 and KCa3.1 expression. Myelin phagocytosis enhanced the M1 profile and dampened the M2a profile, and both phagocytosis and ROS production were dependent on NOX enzymes and Kir2.1 and CRAC channels. Importantly, microglia showed some capacity for re-polarization between M1 and M2a states, based on gene expression changes, myelin phagocytosis, and ROS production.
Conclusions:
In response to polarizing and re-polarizing cytokine treatments, microglia display complex changes in gene transcription profiles, phagocytic capacity, NOX-mediated ROS production, and in ion channels involved in microglial activation. Because these changes might affect microglia-mediated CNS inflammation, they should be considered in future experimental, pre-clinical studies.
Insights
Microglia activation states (M1/M2) and myelin debris influence their phagocytosis, reactive oxygen species (ROS) production, and ion channel expression, impacting CNS inflammation.
Area of Science:
- Neuroscience
- Immunology
- Cell Biology
Background:
- Microglia are key phagocytes in the central nervous system (CNS), critical for development and homeostasis.
- Microglial phagocytosis can be beneficial (e.g., clearing myelin debris for remyelination) or detrimental (e.g., producing reactive oxygen species - ROS).
- Microglia exhibit distinct activation states (pro-inflammatory M1, anti-inflammatory M2) influenced by the cytokine environment, with consequences for CNS injury and disease.
Purpose of the Study:
- To investigate the molecular and functional consequences of microglial exposure to different cytokine environments and myelin debris.
- To assess changes in gene expression, phagocytic activity, ROS production, and ion channel expression under various microglial activation paradigms.
- To explore the potential for microglial repolarization between M1 and M2 states.
Main Methods:
- Primary rat microglia were stimulated with individual cytokines (IFN-γ/TNF-α for M1; IL-4 for M2a; IL-10 for M2c) or sequentially to assess repolarization.
- Microglial responses were evaluated based on gene expression, myelin phagocytosis assays, and ROS production measurements.
- Expression of ion channels (Kv1.3, KCa3.1, Kir2.1, CRAC) was analyzed in relation to microglial activation and function.
Main Results:
- M1 stimulation upregulated pro-inflammatory genes, phagocytosis, ROS production, and specific ion channels (Kv1.3, KCa3.1, Kir2.1).
- M2a stimulation increased anti-inflammatory genes, ROS production, and Kv1.3/KCa3.1 expression.
- Myelin phagocytosis amplified the M1 profile, reduced the M2a profile, and was dependent on NOX enzymes and Kir2.1/CRAC channels for phagocytosis and ROS production.
- Microglia demonstrated capacity for repolarization between M1 and M2a states, influencing gene expression, phagocytosis, and ROS production.
Conclusions:
- Microglial responses to polarizing and repolarizing cytokine treatments involve complex shifts in gene expression, phagocytosis, ROS production, and ion channel activity.
- These dynamic changes in microglial behavior can significantly impact CNS inflammation.
- Findings underscore the importance of considering microglial activation states and their associated molecular changes in experimental and pre-clinical CNS studies.
