Group II metabotropic glutamate receptors modulate blood brain barrier function: direct and indirect effect through astrocytes and microglia

To keep brain homeostasis, the highly specialized endothelial cells that constitute the blood-brain barrier (BBB) are also supported by astrocytes and microglia. Glial influence on barrier properties may be controversial in inflammatory conditions as they can either stabilize or compromise BBB functions. The role played by metabotropic glutamate (mGlu) receptors 2 and 3 on BBB properties in inflammatory conditions has not been investigated, and we analyzed whether these receptors represent potential pharmacological targets. We used an in vitro BBB model, where human derived microvascular endothelial cells (TY-10) were cultured either alone or co-cultured with astrocytes (hAST) or in a triple co-culture with astrocytes and microglia (HMC3 cells). Barrier properties were assessed by evaluating transendothelial electrical resistance (TEER), barrier permeability, endothelial expression of junctional proteins and gene expression of inflammatory cytokines and chemokines. Tumor necrosis factor (TNF)α and Interferon (IFN)γ (T&I; 1 ng/ml each) were applied as an inflammatory stimulus in the presence of the mGlu2/3 receptors agonist, LY379268 and the mGlu2 receptor negative allosteric modulator, VU6001966. T&I impaired barrier properties in all experimental settings. In the monoculture model, T&I effect was only slightly counteracted by LY379268 that, instead, prevented T&I effects both in the endothelial/astrocytes co-cultures and in the triple-co-cultures. However, only in the presence of microglia VU6001966 significantly reduced LY379268 effect. Accordingly, LY379268 dampened T&I-induced expression of inflammatory cytokines in microglia cells, and again, co-treatment with VU6001966 contrasted this action. Thus, activation of mGlu2 and mGlu3 receptors preserves BBB functions acting directly on endothelial cells and contrasting the inflammatory response of astrocytes and microglia.