Smoothened-mediated signaling contributes to immune and non-immune functions of microglia

The brain macrophages, or microglia, display essential functions ranging from contributing to brain development to triggering innate immune responses. The different ways microglia operate reflect their varying context-dependent states. However, the mechanisms that control these states remain largely unknown. Here, we identified a small population of microglia that express Smoothened (Smo), the well-known key component of the Hedgehog signaling pathway. Our experiments involving both loss and gain of function demonstrate that the intrinsic activity of microglial Smo is mostly associated with the effective initiation of appropriate innate immune responses to pathogens via the control of microglia phagocytic activities. Furthermore, microglial Smo activity is also involved in triggering the appearance of amoeboid microglia that transiently arise in fiber tracts during the perinatal period. On the other hand, the exogenous and Hedgehog signaling-independent activation of microglial Smo proves to be capable to block the molecular cascades occurring in microglia under inflammatory conditions. All these data indicate previously unrecognized roles for the Smo receptor and could lead to further research to discover a new category of non-canonical Smo agonists that might specifically regulate microglial states.