TLR2 Hypersensitivity of Astrocytes as Functional Consequence of Previous Inflammatory Episodes

Abstract

Precedent inflammatory episodes may drastically modify the function and reactivity of cells. We investigated whether priming of astrocytes by microglia-derived cytokines alters their subsequent reaction to pathogen-associated danger signals not recognized in the quiescent state. Resting primary murine astrocytes expressed little TLR2, and neither the TLR2/6 ligand fibroblast-stimulating lipopeptide-1 (FSL1) nor the TLR1/2 ligand Pam3CysSK4 (P3C) triggered NF-κB translocation or IL-6 release. We made use of single-cell detection of NF-κB translocation as easily detectable and sharply regulated upstream indicator of an inflammatory response or of c-Jun phosphorylation to measure restimulation events in astrocytes under varying conditions. Cells prestimulated with IL-1β, with a TLR3 ligand, with a complete cytokine mix consisting of TNF-α, IL-1β, and IFN-γ, or with media conditioned by activated microglia responded strongly to FSL1 or P3C stimulation, whereas the sensitivity of the NF-κB response to other pattern recognition receptors was unchanged. This sensitization to TLR2 ligands was associated with an initial upregulation of TLR2, displayed a “memory” window of several days, and was largely independent of the length of prestimulation. The altered signaling led to altered function, as FSL1 or P3C triggered the release of IL-6, CCL-20, and CXCL-2 in primed cells, but not in resting astrocytes. These data confirmed the hypothesis that astrocytes exposed to activated microglia assume a different functional phenotype involving longer term TLR2 responsiveness, even after the initial stimulation by inflammatory mediators has ended.

Switching from astrocytic neuroprotection to neurodegeneration by cytokine stimulation

“…Data are expressed as mean ± SD of three experiments. Statistics were performed using one-way ANOVA followed by a Dunnett's multiple comparison post hoc test 2004a, b, 2006Henn et al 2011;Kohutnicka et al 1998), and our model recapitulated many of the wellknown features, such as NF-kB translocation, and morphological alterations reminiscent of the one of the migrating activated astrocytes (Buffo et al 2010). The activation of astrocytes is not fully understood.…”

“…To model an inflammatory situation, as it occurs during brain damage and neurodegenerative disease, we used a cytokine mix (TNF-α and IL-1β) known to be produced in neuroinflammatory situations (mainly by microglia) and known to activate astrocytic cells, including IMA (Falsig et al 2006;Henn et al 2011;Kuegler et al 2012;Schildknecht et al 2012). This CM had no effect on the viability and neurite structure of LUHMES monocultures (Fig.…”

“…The different functions of astrocytes may be due to largely different activation states (Avendano et al 2015;Falsig et al 2004a;Henn et al 2011) triggered by neuroinflammatory conditions linked to damage, disease, or infection. The first glial cells to be activated in such conditions are usually microglia.…”

“…These secrete cytokines like IL-1β and TNF-α, which are very potent activators of astrocytes. This large pool of cells then reacts with a second wave of mediators (Falsig et al 2008) and can remain activated and altered in their function for a long time after stimulation (Biesmans et al 2015;Henn et al 2011).…”

Combined Effect of TLR2 Gene Polymorphism and Early Life Stress On the Age at Onset of Bipolar Disorders

“…TLR2, but not TLR4, has also been implicated in the perpetuation of inflammatory responses in the CNS after pro-inflammatory stimulation of astrocytes [70] and involved in hippocampal neurogenesis as mice lacking TLR2 displayed impaired hippocampal and neuronal differentiation, which was not observed for TLR4 [71]. These observations could be related to the TLR2 specificity of our findings especially as hippocampal alterations have been repeatedly reported in adults with a history of childhood traumatic experiences and are suggested to be involved in the pathophysiology of BD [72][73][74].…”