TLR4-pathway impairs synaptic number and cerebrovascular functions through astrocyte activation following traumatic brain injury

Background and purposeActivation of astrocytes contributes to synaptic remodeling, tissue repair and neuronal survival following traumatic brain injury (TBI). However, the mechanisms by which these cells interact to infiltrated inflammatory cells to rewire neuronal networks and repair brain functions remain poorly understood. Here, we explored how TLR4-induced astrocyte activation modified synapses and cerebrovascular integrity following TBI. Experimental approachWe used pharmacological and genetic approaches to determine how functional astrocyte alterations induced by activation of TLR4-pathway in inflammatory cells regulate synapses and neurovascular unit after TBI. For that, we used calcium imaging, immunofluorescence, flow cytometry, blood-brain barrier (BBB) integrity assessment and molecular and behavioral tools. Key resultsShortly after a TBI there is a recruitment of excitable and reactive astrocytes mediated by TLR4-pathway activation with detrimental effects on PSD-95/VGlut1 synaptic puncta, BBB integrity and neurological outcome. Pharmacological blockage of the TLR4-pathway with TAK242 partially reverted many of the observed effects. Synapses and BBB recovery after TAK242 administration were not observed in IP3R2-/- mice, indicating that effects of TLR4-inhibition depend on the subsequent astrocyte activation. In addition, TBI increased the astrocytic-protein thrombospondin-1 necessary to induce a synaptic recovery in a sub-acute phase. Conclusions and implicationsOur data demonstrate that TLR4-mediated signaling, most probably though microglia and/or infiltrated monocyte-astrocyte communication, plays a crucial role in the TBI pathophysiology and that its inhibition prevents synaptic loss and BBB damage accelerating tissue recovery/repair, which might represent a therapeutic potential in CNS injuries and disorders. Declaration of transparency and scientific rigourThis Declaration acknowledges that this paper adheres to the principles for transparent reporting and scientific rigour of preclinical research as stated in the BJP guidelines for Design & Analysis, Immunoblotting and Immunochemistry, and Animal Experimentation, and as recommended by funding agencies, publishers and other organisations engaged with supporting research. Bullet point summaryWhat is already known: O_LIAstrocytes and microglia participate in the early cerebral and synaptic response after traumatic brain injury. C_LIO_LITLR4 antagonism exerts neuroprotection in acute brain injuries. C_LI What this study adds: O_LIAcute astrocyte activation contributes to synaptic loss and BBB breakdown in the acute phase of TBI, and synaptic remodeling in the sub-acute phase. C_LIO_LIAstrocyte activation is mediated by microglia/infiltrating-monocytes activation through TLR4 receptors. C_LI Clinical significance: O_LIInhibition of astrocyte activation through TLR4 antagonism could be a promising option for TBI treatment. C_LI