Chronic hM3Dq signaling in microglia ameliorates neuroinflammation in male mice

William Binning; Aja E. Hogan-Cann; Diana Yae Sakae; Matthew Maksoud; Valeriy Ostapchenko; Mohammed Al-Onaizi; Sara Matovic; Wei-Yang Lu; Marco A. M. Prado; Wataru Inoue; Vania F. Prado

Microglia sense exogenous and endogenous inflammatory signals and orchestrate subsequent neuroinflammation which triggers a number of behavioral changes (Biesmans et al., 2013) . Accumulating evidence indicate that the microglial response to inflammatory stimuli can be very plastic (Neher and Cunningham, 2019) . Upon previously encountering an inflammatory stimulus, microglia can adjust their activation state and elicit amplified responses to a secondary inflammatory insult, a phenomenon referred to as microglial pre-conditioning or "priming" (Neher and Cunningham, 2019) . Microglia can also develop long-lasting molecular reprograming that suppresses microglial activation upon a secondary insult, a phenotype referred to as "innate immune memory" or tolerance (Netea et al., 2016; Netea et al., 2011) . Microglia have recently been shown to display immune training and tolerance in response to repeated daily peripheral stimulation with LPS. One low-dose intraperitoneal injection of LPS led to increased pro-inflammatory cytokine levels in the brain in response to the second injection. Conversely, upon the fourth daily LPS injection microglia barely responded, showing decreased cytokine levels in the brain, an indication of immune tolerance (Wendeln et al., 2018) . We showed that acute activation of hM3Dq in microglia strongly induced brain TNF-α, IL-1β and IL-6 mRNA expression, however, this "proinflammatory" effect was completely lost after repeated (4 days) hM3Dq activation, suggesting that repeated activation of muscarinic M3 receptors in microglia can result in microglial tolerance.