Steady-state phosphorylation of SHIP1 by Lyn restricts macrophage activation in the absence of a phagocytic synapse
Microscale engagement of the hemi-ITAM-containing receptor Dectin-1 by a fungus-derived particle initiates signaling through the Src-family kinases (SFKs) and Syk that leads to downstream Erk and Akt activation and the macrophage anti-microbial response. To minimize local tissue damage in the absence of a true pathogenic threat, macrophages must remain unresponsive to low-valency receptor ligation by soluble ligands from food or remote infection sites. To investigate how SFKs regulate macrophage sensitivity, we compared signaling in murine bone-marrow-derived macrophages (BMDMs) exposed to depleted zymosan, a particulate and highly multivalent Dectin-1 ligand, with signaling after pharmacologically induced SFK activation without receptor engagement or formation of a phagocytic synapse. We show that high-valency Dectin-1 engagement and formation of a phagocytic synapse restrict phosphorylation of the ITIM-associated phosphatase SHIP1 and promote Erk and Akt signaling. In contrast, SFK activation in the absence of a phagocytic synapse induces phosphorylation of SHIP1 and leads to dampened activation of Erk and Akt. Whereas multiple SFKs are capable of phos-phorylating SHIP1 in principle, the SFK Lyn functions uniquely in maintaining steady-state phosphorylation of SHIP1 and setting tonic and induced levels of Erk and Akt phosphorylation. Consequently, Lyn has a special role in suppressing the Akt pathway and maintaining Erk/Akt pathway balance. Interestingly, formation of a phagocytic synapse circumvents this requirement for Lyn to maintain Erk/Akt balance. These findings highlight the unique function of Lyn in maintaining mac-rophage steady-state signaling and limiting pro-inflammatory responses to disorganized pathway activation and soluble microbial components.