Microglia are the resident immune cells of the central nervous system that guarantee immune surveillance and exert also a modulating role on neuronal synaptic development and function. Upon injury, microglia get activated and modify their morphology acquiring an ameboid phenotype and pro- or anti-inflammatory features. The active role of microglia in blood–brain barrier (BBB) function and their interaction with different cellular components of the BBB—endothelial cells, astrocytes and pericytes—are described. Here, we report the specific crosstalk of microglia with all the BBB cell types focusing in particular on the involvement of microglia in the modulation of BBB function in neuroinflammatory conditions that occur in conjunction with an acute event, such as a stroke, or in a slow neurodegenerative disease, such as Alzheimer’s disease. The potential of microglia to exert a dual role, either protective or detrimental, depending on disease stages and environmental conditioning factors is also discussed.

Microglia represent a first-line defense in the brain. However, in pathological conditions such as Alzheimer’s disease (AD), a pro-inflammatory switch may occur, leading to loss of protective functions. Using the human microglial cell line HMC3, we showed that exposure to low concentrations of β-amyloid peptide 1-42 (Aβ42; 0.2 μM) initially (6 h) upregulated anti-inflammatory markers interleukin (IL)-4, IL-13, and brain-derived neurotrophic factor (BDNF). BDNF increase was prevented by selective inhibition of SIRT1 with EX527 (2 μM). Accordingly, these early effects were accompanied by a significant Aβ42-induced increase of SIRT1 expression, nuclear localization, and activity. SIRT1 modulation involved adenosine monophosphate-regulated kinase (AMPK), which was promptly (30 min) phosphorylated by Aβ42, while the AMPK inhibitor BML-275 (2 μM) attenuated Aβ42-induced SIRT1 increase. Initially observed microglial responses appeared transient, as microglial features changed when exposure to Aβ42 was prolonged (0.2 μM for 72 h). While SIRT1 and BDNF levels were reduced, the expression of inflammatory markers IL-1β and tumor necrosis factor (TNF)-α increased. This coincided with a rise in NF-kB nuclear localization. The effects of melatonin (1 μM) on prolonged microglial exposure to Aβ42 were analyzed for their protective potential. Melatonin was able to prolong SIRT1 and BDNF upregulation, as well as to prevent NF-kB nuclear translocation and acetylation. These effects were sensitive to the melatonin receptor antagonist, luzindole (25 μM). In conclusion, our data define an early microglial defensive response to Aβ42, featuring SIRT1-mediated BDNF upregulation that can be exogenously modulated by melatonin, thus identifying an important target for neuroprotection.

Microglia, together with astrocytes and pericytes, cooperate to ensure blood–brain barrier (BBB) stability, modulating endothelial responses to inflammatory insults. Agonists of the sphingosine 1 phosphate (S1P) receptors, such as siponimod (BAF-312), are important pharmacological tools in multiple sclerosis and other inflammatory diseases. Modulation of S1P receptors may result in a reduced inflammatory response and increased BBB stability. An in vitro BBB model was reproduced using human-derived endothelial cells, astrocytes and microglia. Co-cultures were exposed to inflammatory cytokines (TNFα, 10 UI and IFNγ, 5 UI) in the presence of BAF-312 (100 nM), and the BBB properties and microglia role were evaluated. The drug facilitated microglial migration towards endothelial/astrocyte co-cultures, involving the activity of the metalloprotease 2 (MMP2). Microglia actively cooperated with astrocytes in the maintenance of endothelial barrier stability: in the triple co-culture, selective treatment of microglial cells with BAF-312 significantly prevented cytokines’ effects on the endothelial barrier. In conclusion, BAF-312, modulating S1P receptors in microglia, may contribute to the reinforcement of the endothelial barrier at the BBB, suggesting an additional effect of the drug in the treatment of multiple sclerosis.