Studying the impact of a low-grade chronic inflammation in a cellular model of microglia

Microglia are the resident immune cells of the central nervous system (CNS), and regulate brain inflammation, synaptic plasticity, and the formation of neural networks, all of which are affected in CNS disorders. The abnormal activation of microglia supports neuroinflammation, that is a key player in several diseases ranging from depressive to neurodevelopmental and neurodegenerative disorders. Moreover, a chronic low-grade inflammation could favor immune senescence and inflammaging, with a further decline in microglia function. In view of these considerations, we tried to set up an in vitro model of chronic low-grade inflamed microglia by treating an immortalized murine microglial cell line, the BV2 cells, with a 4h-pulse of LPS or TNF-α every 48h for 5/7 days. Expression levels of the main inflammatory cytokines IL-1β and IL-6 were about 2.5 times higher in chronically LPS-treated cells compared to control cells, thus confirming the validity of our model. However, we did not detect changes in both the activity of the β-galactosidase enzyme and in the protein expression of Lamin B1 (LMNB-1), two widely used markers of senescence. At variance with LPS, TNF-α was able to induce senescence, as shown by the activation of the β-galactosidase enzyme, but not sustained increases in the levels of IL-1β and IL-6. Efficient protein turnover is essential for cellular homeostasis and functions, and defeats in proteostasis may reduce the ability of cells to cope environmental stress. Using puromycin, a blocker of protein synthesis, we found a fast decrease in the expression of the structural proteins LMNB-1 and tubulin in LPS-treated BV2 cells, but not in control cells. Also, the protein levels of mTOR, a central regulator of cell fate and metabolism, including proteostasis, were decreased in LPS-treated cells, suggesting that changes in protein turnover may represent a possible adaptative mechanism in microglial cells exposed to low-grade chronic inflammation.