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

• Published in: British journal of pharmacology Vol. 178; no. 17; pp. 3395 - 3413
• Main Authors: Rosa, Juliana M., Farré‐Alins, Víctor, Ortega, María Cristina, Navarrete, Marta, Lopez‐Rodriguez, Ana Belen, Palomino‐Antolín, Alejandra, Fernández‐López, Elena, Vila‐del Sol, Virginia, Decouty, Céline, Narros‐Fernández, Paloma, Clemente, Diego, Egea, Javier
• Format: Journal Article
• Language: English
• Published: England Blackwell Publishing Ltd 01-09-2021; John Wiley and Sons Inc
• Subjects: Astrocytes; BBB breakdown; Blood-brain barrier; Calcium imaging; Flow cytometry; Glutamic acid transporter; Immunofluorescence; Inflammation; Microglia; microglia/infiltrated monocytes; Monocytes; Neural networks; Neuroimaging; Research Paper; Research Papers; Signal transduction; Synaptic density; synaptic remodelling; Thrombospondin; TLR4 antagonism; TLR4 protein; Toll-like receptors; Traumatic brain injury; TLR4 antagonism, synaptic remodelling, BBB breakdown; microglia/infiltrated monocytes; traumatic brain injury; astrocytes
• ISSN: 0007-1188; 1476-5381
• DOI: 10.1111/bph.15488

Background and Purpose
Activation of astrocytes contributes to synaptic remodelling, tissue repair and neuronal survival following traumatic brain injury (TBI). The mechanisms by which these cells interact to resident/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 Approach
To determine how functional astrocyte alterations induced by activation of TLR4 pathway in inflammatory cells regulate synapses and neurovascular integrity after TBI, we used pharmacology, genetic approaches, live calcium imaging, immunofluorescence, flow cytometry, blood–brain barrier (BBB) integrity assessment and molecular and behavioural methods.
Key Results
Shortly after a TBI, there is a recruitment of excitable and reactive astrocytes mediated by TLR4 pathway activation with detrimental effects on post‐synaptic density‐95 (PSD‐95)/vesicular glutamate transporter 1 (VGLUT1) synaptic puncta, BBB integrity and neurological outcome. Pharmacological blockage of the TLR4 pathway with resatorvid (TAK‐242) partially reversed many of the observed effects. Synapses and BBB recovery after resatorvid 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 Implications
Our data demonstrate that TLR4‐mediated signalling, most probably through 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.