Role of mitochondrial dynamics in microglial activation and metabolic switch

Abstract

Background Microglia are the endogenous immune cells of the central nervous system (CNS) and act as sensors of injury in the brain, favouring its homeostasis. Their activation and polarization towards a pro-inflammatory phenotype are associated to injury and disease. These processes are linked to a metabolic reprogramming of the cells, characterized by high rates of glycolytic function and suppressed levels of oxidative phosphorylation. This metabolic switch can be reproduced in vitro by stimulation with lipopolysaccharide (LPS) plus Interferon-γ (IFNγ). In an attempt to understand the mechanisms regulating mitochondrial respiration abolishment, we examined potential alterations in mitochondrial features during the metabolic switch. In addition, we studied the possible implication of mitochondrial dynamics in the metabolic switch using the mitochondrial division inhibitor-1 (Mdivi-1), which blocks Drp1-dependent mitochondrial fission. Methods Cultured microglia was treated with LPS + IFNγ to reproduce the metabolic switch under pro-inflammatory stimuli in the absence or in the presence of Mdivi-1 to block mitochondrial fission. Mitochondrial membrane potential and mitochondrial calcium were measured with living cell imaging, and microglial polarization was assessed by immunofluorescence and qRT-PCR. The metabolic profile of the cells was measured using the Seahorse XFe96 Extracellular Flux Analyzer. Results Under conditions of mitochondrial respiration abolishment, microglia did not show any change in mitochondria morphology, nor in mitochondrial membrane potential, indicative of a limited impact in its viability. We provided evidence that reverse operation of F0F1-ATP synthase contributes to mitochondrial membrane potential. On the other hand. mitochondrial fission blockage significantly reduced the expression of pro-inflammatory markers in LPS + IFNγ-treated microglia, such as the inducible nitric oxide synthase (iNOS). However, this inhibition did not lead to a recovery of the oxidative phosphorylation ablation by LPS + IFNγ or to a microglia repolarization. Conclusions Altogether, these results suggest that Drp1-dependent mitochondrial fission, although potentially involved in microglial activation, does not play an essential role in metabolic reprogramming and repolarization of microglia.