Toll-Like Receptor 4 (TLR4) and Triggering Receptor Expressed on Myeloid Cells-2 (TREM-2) Activation Balance Astrocyte Polarization into a Proinflammatory Phenotype

Abstract

Astrocytes react to brain injury with a generic response known as reactive gliosis, which involves activation of multiple intracellular pathways including several that may be beneficial for neuronal survival. However, by unknown mechanisms, reactive astrocytes can polarize into a proinflammatory phenotype that induces neurodegeneration. In order to study reactive gliosis and astroglial polarization into a proinflammatory phenotype, we used cortical devascularization-induced brain ischemia in Wistar rats and primary astroglial cell cultures exposed to oxygen-glucose deprivation (OGD). We analyzed the profile of TLR4 expression and the consequences of its activation by gain- and loss-of-function studies, and the effects produced by the activation of triggering receptor expressed on myeloid cells-2 (TREM-2), a negative regulator of TLR4 signaling. Both OGD exposure on primary astroglial cell cultures and cortical devascularization brain ischemia in rats induced TLR4 expression in astrocytes. In vivo, astroglial TLR4 expression was specifically observed in the ischemic penumbra surrounding necrotic core. Functional studies showed that OGD increased the astroglial response to the TLR4 agonist lipopolysaccharide (LPS), and conversely, TLR4 knockout primary astrocytes had impaired nuclear factor kappa-B (NF-κB) activation when exposed to LPS. In gain-of-function studies, plasmid-mediated TLR4 over-expression exacerbated astroglial response to LPS as shown by sustained NF-κB activation and increased expression of proinflammatory cytokines IL-1β and TNFα. TREM-2 expression, although present in naïve primary astrocytes, was induced by OGD, LPS, or high-mobility group box 1 protein (HMGB-1) exposure. TREM-2 activation by antibody cross-linking or the overexpression of TREM-2 intracellular adaptor, DAP12, partially suppressed LPS-induced NF-κB activation in purified astrocytic cultures. In vivo, TREM-2 expression was observed in macrophages and astrocytes located in the ischemic penumbra. While TREM-2+ macrophages were abundant at 3 days post-lesion (DPL) in the ischemic core, TREM-2+ astrocytes persisted in the penumbra until 14DPL. This study demonstrates that TLR4 expression increases astroglial sensitivity to ligands facilitating astrocyte conversion towards a proinflammatory phenotype, and that astroglial TREM-2 modulates this response reducing the downstream NF-κB activation. Therefore, the availability of TLR4 and TREM-2 ligands in the ischemic environment may control proinflammatory astroglial conversion to the neurodegenerative phenotype.

Exogenous activation of cannabinoid-2 receptor modulates TLR4/MMP9 expression in a spinal cord ischemia reperfusion rat model

“…TLR4 and MMP9 are the important mediators in inflammation-induced BSCB breakdown [5,15,28]. TLR4 elevation induces astrocytes polarization [24]. The reactive astrocytes around microvessels activate endothelial MMP9 [25] or release inflammatory mediators such as cytokines IL-1β, IL-6, and TNFα to regulate blood-brain barrier permeability in inflammation [42].…”

“…Various studies have reported that toll-like receptors 4 (TLR4) are strongly associated with the inflammatory responses after I/R injury and mediate the motor dysfunction and BSCB disruption [7,15,23]. It has also been reported that brain ischemia induces TLR4 expressions in astrocytes, and that TLR4 activation leads to an astroglial polarization towards an inflammatory phenotype [24]. Astrocytes contribute to blood-brain barrier damage through activation of endothelial matrix metalloproteinase 9 (MMP9) [25].…”

The Emerging Role of Curcumin in the Modulation of TLR-4 Signaling Pathway: Focus on Neuroprotective and Anti-Rheumatic Properties

“…Recent studies have shown that Triggering Receptor Expressed on Myeloid cells 2, also known as (TREM2), is an efficient negative regulator of TLR-4 signaling [145][146][147]. TREM2 is a membrane receptor that mediates critical functions of microglia, such as suppression of pro-inflammatory cytokines and promotion of phagocytosis of apoptotic neurons and cell debris [148].…”

HMGB1 is a Potential Mediator of Astrocytic TLR4 Signaling Activation following Acute and Chronic Focal Cerebral Ischemia

“…ese studies thereby suggest that TLR4 protein is not expressed in detectable amounts, using immunohistochemistry, under basal conditions in normal tissue. However, during injury, there appears to be at least a small amount of TLR4 able to initiate ligand binding of various danger-associated molecular patterns (DAMPs) and subsequent signal amplification [10,30]. However, we did observe baseline TLR4 message levels in our cultured astrocyte cell line stimulated with PBS; Figure 5(a).…”

“…Previous studies have shown TLR4 expression in penumbral astrocytes during acute focal cerebral ischemia [5]. More recent studies also show that TLR4 is also expressed by penumbral astrocytes in a model of cortical devascularization [10]. However, it is not known whether TLR4 signaling occurs in these astrocytes during both the acute and chronic phases of focal cerebral ischemia.…”