Regulation Of TLRs-triggered Innate Immune Response By RasGRP3,Ca²⁺and Their Mechanisms

Part I Ras guanine nucleotide releasing protein3(RasGRP3) sets the threshold for TLR signaling to limit IL-6production in macrophages by activating Rapl small GTPaseHost immune cells can detect and destruct invading pathogens via pattern-recognition receptors (PRRs) by producing inflammatory cytokines and type I interferons. As conserved molecular switches coupling extracellular signals to various cellular responses, the roles of Rap1small GTPases and their regulators in Toll-like receptor (TLR) signaling have not been fully elucidated. Here we report that Ras guanine nucleotide releasing protein3(RasGRP3), a guanine nucleotide-exchange factor activating Ras and Rap1, limits production of IL-6in macrophages by activating Rap1upon activation by low levels of TLR ligands. We demonstrate that RasGRP3, a dominant member of RasGRPs in macrophages, impairs TLR3/4/9-induced IL-6production and relieves collagen-induced arthritis. Knockdown of RasGRP3decreases GTP-bound Rap1, accelerates dephosphorylation of Akt, but enhances ERK1/2activation, thus increasing IL-6production. Our study suggests that RasGRP3may respond to slight pathogen infection and limits IL-6production by activating Rapl, setting a graded threshold for preventing overwhelmed inflammatory response.Part Ⅱ STIM1-mediated calcium influx guides the feedforward regulation of TLR-triggered innate response via coordinating the activation of Ras and RaplToll-like receptors (TLR)-triggered innate immune response needs feedforward regulation to quickly and accurately instruct the defence against invading microbes. However, the mechanisms underlying feedforward regulation of TLR-triggered innate immune response remain poorly understood. Here we report that stromal interaction molecule1(STIM1)-mediated calcium (Ca2+) influx plays an important role in feedforward regulation of TLR-triggered innate immune response in macrophages by enhancing the activation of small GTPase Ras but inhibiting the activation of small GTPase Rapl. Consequently, Ca2+accelerated the de-phosphorylation of AKT and enhanced the phosphorylation of ERK, and then Ca2+promoted TLR-triggered IL-6production by macrophages in a dose-dependent manner. Therefore, we demonstrate that the second messenger of Ca2+influx can efficiently transmit the strength-information of extracellular TLR stimuli into the innate cells and make the innate cells initiate innate response at the appropriate level. Our data may add mechanistic insight to the feedforward regulation of TLR-triggered innate response.