Mitochondrial damage-associated molecular patterns stimulate reactive oxygen species production in human microglia

Microglia are the resident innate immune cells of the central nervous system and exert functions of host defense and maintenance of normal tissue homeostasis, along with support of neuronal processes in the healthy brain. Chronic and dysregulated microglial cell activation has increasingly been linked to the status of neuroinflammation underlying many neurodegenerative diseases, including multiple sclerosis (MS). However, the stimulus (or stimuli) and mechanisms by which microglial activation is initiated and maintained MS are still debated. The purpose of our research was to investigate whether the endogenous mitochondrial (mt)-derived damage-associated molecular patterns (MTDs) mtDNA, N-formyl peptides and cardiolipin (CL) contribute to these phenomena. We characterized the effects of the abovementioned MTDs on microglia activation in vitro (i.e. using HMC3 cells) by evaluating the expression of gene coding for proteins involved in their binding and coupled to downstream signaling pathways, the up-regulation of markers of activation on the cell surface and the production of pro-inflammatory cytokines and reactive oxygen species. At the transcriptional level, significant variations in the mRNA relative expression of five of eleven selected genes were observed in response to stimulation. No changes in activation of antigenic profile or functional properties of HMC3 cells were observed; there was no up-regulation of HLA-DR expression or increased secretion of tumor necrosis factor-α and interleukin-6. However, after stimulation with mtDNA and CL, an increase in cellular oxidative stress, but not in the mt ROS O₂^-, compared to control cells, were observed. There were no effects on cell viability. Overall, our data suggest that MTDs could cause a failure in microglial activation toward a pro-inflammatory phenotype, possibly triggering an endogenous regulatory mechanism for the resolution of neuroinflammation. This could open a door for the development of drugs selectively targeting microglia and modulating its functionality to treat MS and/or other neurodegenerative conditions in which MTDs have a pathogenic relevance.