Identification of the Signalosome and Signaling Pathway of the Efferocytic Receptor MERTK
Abstract
MER Tyrosine Kinase (MERTK) is a receptor that mediates efferocytosis - the phagocytosis of apoptotic cells. Efferocytosis is central to homeostasis through removing the ~150 billion apoptotic cells generated each day in the human body. Defects in MERTK function have been shown to contribute to both autoimmune diseases and chronic inflammatory diseases, and MERTK is known to function as the predominant or sole efferocytic receptor in multiple tissues. Despite its clinical importance, very little is understood of how MERTK functions and how it signals. MERTK is known to bind to apoptotic cells via opsonins such as Gas6 and Protein S, with binding activating MERTK’s intrinsic kinase domain. This induces a poorly characterized signaling pathway which is thought to coordinate with integrins to mediate efferocytosis. Using immunoprecipitation and mass spectrometry we have identified a large pre-formed MERTK signalosome on the cell surface containing MERTK, integrins and a number of lipid and protein kinases. Ligation of this complex activates a number of downstream signaling pathways required for efferocytosis, including the canonical Src/Syk/PI3K phagocytic signaling pathway and non-canonical ILK and Erk-dependent signaling pathways. Through ILK and FAK, MERTK signaling activates integrins, and indeed, in macrophages MERTK-mediated efferocytosis requires αxβ2 integrins. Combined, these data demonstrate that MERTK engages in efferocytosis cooperatively with integrins, and mediates this cross-talk through engaging canonical integrin signaling pathways.