Study of the mechanism underlying the effect of 5-HT in microglial processes motility

Abstract

Microglia, the brain's resident macrophages, have a distinct ramified morphology orchestrated by repeated cycles of extension and retraction of their long thin processes, allowing them to continuously monitor their environment. Microglia play a role in the homeostasis and functional integrity of the brain parenchyma. Microglia are known to extend their processes in a directional manner in response to tissue damage via activation, by ATP release from damaged sites, of the P2Y12 receptor (P2Y12R) which is expressed by all microglia. Moreover, the laboratory previously reported that the neuromodulator serotonin (5-HT) also has the potential to trigger microglial process outgrowth i.e. “directional motility” toward a focal 5-HT application site. This is allowed sensing of 5-HT by the microglial 5-HT2B receptor (5-HT2BR), which is the main serotonin (5-HT) receptor expressed by microglial cells. Intriguingly, this response to 5-HT also requires microglial P2Y12R and extracellular ATP/ADP, but the mechanism and signaling pathway involved remain elusive. Moreover, it is not known whether all or only a subset of microglia can sense 5-HT. In this work, using the RNA-Scope method, we were able to see the 5-HT2BR expression in different brain regions and notably in a subset of microglial cells. Furthermore, using a fluorescent biosensor for ATP and primary glial cells cultures, we show that 5-HT may trigger the release of ATP by mixed cultures of microglia and astrocytes. Altogether, these findings support the hypothesis that in vivo, 5-HT could have an effect on a large number of microglial cells by activating initially only a few, which in turn would trigger the release of ATP and increase processes motility.