Toll-like receptor 4 (TLR4) and triggering receptor expressed on myeloid cells-2(TREM-2) activation balance astrocyte polarization into a proinflammatory phenotype

ROSCISZEWSKI G; LUKIN J; RAMOS A.J; MURTA V; ROGER, THIERRY; CADENA V; VILLARREAL A

Edimburgo

Congreso; Euroglia Meeting; 2017

Euroglia

Astrocytes react to brain injury with a generic response known as reactive gliosis, which involves activationof multiple intracellular pathways including several that may be beneficial for neuronal survival. However, bymechanisms that are not completely understood, reactive astrocytes can polarize into a proinflammatoryphenotype that induces neurodegeneration. In order to study reactive gliosis and astroglial polarization intoa proinflammatory phenotype, we used cortical devascularization-induced brain ischemia in Wistar rats andprimary astroglial cell cultures exposed to oxygen-glucose deprivation (OGD). We analyzed the profile ofTLR4 expression and the consequences of its activation by gain- and loss-of-function studies; and the effectsproduced by the activation of triggering receptor expressed on myeloid cells-2 (TREM-2), a negative regulatorof TLR4 signaling. Both OGD exposure on primary astroglial cell cultures and cortical devascularization brainischemia in rats induced TLR4 expression in astrocytes. In vivo, astroglial TLR4 expression was specificallyobserved in the ischemic penumbra surrounding necrotic core. Functional studies showed that OGDincreased the astroglial response to the TLR4 agonist lipopolysaccharide (LPS) and, conversely, TLR4knockout primary astrocytes had impaired nuclear factor kappa-B (NF-KB) activation when exposed to LPS.In gain of function studies, plasmid-mediated TLR4 over-expression exacerbated astroglial response to LPSas shown by sustained NF-KB activation and increased expression of proinflammatory cytokines IL-1betaand TNF-alpha. TREM-2 expression although present in low level in in naïve primary astrocytes, was inducedby OGD, LPS or high-mobility group box 1 protein (HMGB-1) exposure. TREM-2 activation by antibodycrosslinking partially suppressed LPS-induced NF-KB activation in purified astrocytic cultures. This studydemonstrates that TLR4 expression increases astroglial sensitivity to ligands facilitating astrocyte conversion towards a proinflammatory phenotype, and that astroglial TREM-2 modulates this response reducing thedownstream NF-KB activation. Therefore, the availability of TLR4 and TREM-2 ligands in the ischemicenvironment and the activity of these pathways may control proinflammatory astroglial conversion andsubsequent neurodegeneration.