Identification of a chronic non-neurodegenerative microglia activation state in a mouse model of peroxisomal
Simon Verheijden1, Lien Beckers1, Andrea Casazza2,3
1Department of Pharmaceutical and Pharmacological Sciences, KU Leuven-University of Leuven, Cell Metabolism, Leuven, Belgium.
Abstract:
The functional diversity and molecular adaptations of reactive microglia in the chronically inflamed central nervous system (CNS) are poorly understood. We previously showed that mice lacking multifunctional protein 2 (MFP2), a pivotal enzyme in peroxisomal β-oxidation, persistently accumulate reactive myeloid cells in the gray matter of the CNS. Here, we show that the increased numbers of myeloid cells solely derive from the proliferation of resident microglia and not from infiltrating monocytes. We defined the signature of Mfp2(-/-) microglia by gene expression profiling after acute isolation, which was validated by quantitative polymerase reaction (qPCR), immunohistochemical, and flow cytometric analysis. The features of Mfp2(-/-) microglia were compared with those from SOD1(G93A) mice, an amyotrophic lateral sclerosis model. In contrast to the neurodegenerative milieu of SOD1(G93A) spinal cord, neurons were intact in Mfp2(-/-) brain and Mfp2(-/-) microglia lacked signs of phagocytic and neurotoxic activity. The chronically reactive state of Mfp2(-/-) microglia was accompanied by the downregulation of markers that specify the unique microglial signature in homeostatic conditions. In contrast, mammalian target of rapamycin (mTOR) and downstream glycolytic and protein translation pathways were induced, indicative of metabolic adaptations. Mfp2(-/-) microglia were immunologically activated but not polarized to a pro- or anti-inflammatory phenotype. A peripheral lipopolysaccharide challenge provoked an exaggerated inflammatory response in Mfp2(-/-) brain, consistent with a primed state. Taken together, we demonstrate that chronic activation of resident microglia does not necessarily lead to phagocytosis nor overt neurotoxicity.
Insights
Mice lacking multifunctional protein 2 (MFP2) show chronic microglial activation without neurotoxicity. These reactive microglia exhibit metabolic adaptations and a primed inflammatory state, challenging assumptions about chronic microglial responses in the central nervous system (CNS).
Area of Science:
- Neuroimmunology
- Cellular and Molecular Neuroscience
- Central Nervous System (CNS) Inflammation
Background:
- The roles and molecular underpinnings of reactive microglia in chronic CNS inflammation remain unclear.
- Previous studies indicated that mice deficient in multifunctional protein 2 (MFP2) accumulate myeloid cells in the CNS.
- It was unknown whether these myeloid cells originate from resident microglia or infiltrating monocytes.
Purpose of the Study:
- To investigate the origin and characteristics of myeloid cell accumulation in Mfp2-deficient mice.
- To define the molecular signature and functional state of chronically activated microglia.
- To compare Mfp2(-/-) microglia with those in a neurodegenerative model (SOD1(G93A) mice).
Main Methods:
- Gene expression profiling of acutely isolated microglia from Mfp2(-/-) mice.
- Validation using quantitative polymerase reaction (qPCR), immunohistochemistry, and flow cytometry.
- Comparison with microglia from SOD1(G93A) amyotrophic lateral sclerosis model mice.
- Assessment of microglial response to peripheral lipopolysaccharide challenge.
Main Results:
- Myeloid cell accumulation in Mfp2(-/-) CNS results from resident microglia proliferation, not monocyte infiltration.
- Mfp2(-/-) microglia display downregulated homeostatic markers but upregulated metabolic pathways (mTOR, glycolysis, protein translation).
- Despite chronic activation, Mfp2(-/-) microglia lack phagocytic and neurotoxic activity and do not adopt classical inflammatory phenotypes.
- Mfp2(-/-) mice exhibit an exaggerated inflammatory response to peripheral immune challenge, indicating a primed state.
Conclusions:
- Chronic activation of resident microglia does not inevitably lead to phagocytosis or overt neurotoxicity.
- MFP2 deficiency induces unique metabolic adaptations and a primed inflammatory state in microglia.
- These findings provide novel insights into the functional diversity of reactive microglia in the CNS.
