Microglial Pro-Inflammatory and Anti-Inflammatory Phenotypes Are Modulated by Translocator Protein Activation

Abstract

A key role of the mitochondrial Translocator Protein 18 KDa (TSPO) in neuroinflammation has been recently proposed. However, little is known about TSPO-activated pathways underlying the modulation of reactive microglia. In the present work, the TSPO activation was explored in an in vitro human primary microglia model (immortalized C20 cells) under inflammatory stimulus. Two different approaches were used with the aim to (i) pharmacologically amplify or (ii) silence, by the lentiviral short hairpin RNA, the TSPO physiological function. In the TSPO pharmacological stimulation model, the synthetic steroidogenic selective ligand XBD-173 attenuated the activation of microglia. Indeed, it reduces and increases the release of pro-inflammatory and anti-inflammatory cytokines, respectively. Such ligand-induced effects were abolished when C20 cells were treated with the steroidogenesis inhibitor aminoglutethimide. This suggests a role for neurosteroids in modulating the interleukin production. The highly steroidogenic ligand XBD-173 attenuated the neuroinflammatory response more effectively than the poorly steroidogenic ones, which suggests that the observed modulation on the cytokine release may be influenced by the levels of produced neurosteroids. In the TSPO silencing model, the reduction of TSPO caused a more inflamed phenotype with respect to scrambled cells. Similarly, during the inflammatory response, the TSPO silencing increased and reduced the release of pro-inflammatory and anti-inflammatory cytokines, respectively. In conclusion, the obtained results are in favor of a homeostatic role for TSPO in the context of dynamic balance between anti-inflammatory and pro-inflammatory mediators in the human microglia-mediated inflammatory response. Interestingly, our preliminary results propose that the TSPO expression could be stimulated by NF-κB during activation of the inflammatory response.

The Translocator Protein (TSPO) in Mitochondrial Bioenergetics and Immune Processes

“…Differential secretion of pro-and anti-inflammatory cytokines could also be modulated by TSPO ligands and genetic knockdown. This was underpinned by mitochondrial ROS modulation, and NF-κB pathway expression, thereby underscoring TSPO in the mitochondrial processes of inflammatory responses [82]. Similarly, lipopolysaccharide (LPS) stimulation of mouse primary microglia resulted in increased TSPO expression in M1 microglia, though not in IL-4 treated M2 microglia.…”

The Role of Translocator Protein TSPO in Hallmarks of Glioblastoma

“…TSPO silencing in human microglial C20 cells resulted in a more inflamed phenotype and increased the release of pro-inflammatory cytokines. In contrast, exposure of C20 cells to the TSPO ligand XBD173 attenuated the neuroinflammatory response [ 92 ]. Notably, TSPO mRNA expression was increased after stimulation with pro-inflammatory cytokines.…”

“…Notably, TSPO mRNA expression was increased after stimulation with pro-inflammatory cytokines. These observations were accompanied by modulation of mitochondrial ROS and activation of the NF-κB signaling pathway which could then stimulate TSPO expression during the inflammatory response [ 92 ] and could link inflammation to another hallmark, namely cellular energetics.…”

Antidepressant effects of 3‐(3,4‐methylenedioxy‐5‐trifluoromethyl phenyl)‐2E‐propenoic acid isobutyl amide involve TSPO‐mediated mitophagy signalling pathway

“…Increased TSPO expression of TSPO occurs when microglia are activated during neuroinflammation, therefore making it be regarded as a neuroinflammatory marker. TSPO can both reduce and increase the release of pro‐inflammatory and anti‐inflammatory cytokines, respectively, considering TSPO’s homeostatic function associated with neurodegeneration and neuroinflammatory responses 35 …”