MERTK Coordinates Efferocytosis by Regulating Integrin Localization and Activation
Efferocytosis - the phagocytic removal of apoptotic cells - is a central component of tissue homeostasis, and in many tissues is mediated by the efferocytic receptor MERTK expressed by macrophages. Although MERTK is critical for efferocytosis, the mechanism by which it directs the engulfment of apoptotic cells is largely unknown. Using immunoprecipitation, mass spectrometry, and super-resolution microscopy, we have identified a pre-formed receptor complex on the macrophage plasma membrane comprised of [~]180 nm clusters of MERTK, {beta}2 integrins, and multiple signaling molecules including Src-family kinases, PI3-kinases, and the integrin regulatory proteins ILK and FAK. MERTK is unable to mediate efferocytosis in the absence of {beta}2 integrins or their opsonins, while {beta}2 integrins require activation via MERTK signaling to induce the engulfment of apoptotic cells. Using FRET microscopy, we determined that MERTK directly induces the conformational change of {beta}2 integrins from the low to high-affinity form via a PI3-kinase-dependent signaling pathway. MERTK and {beta}2 integrins then form a highly structured synapse in which MERTK is retained by ligand-induced clustering in the synapse centre, while {beta}2 integrins and actin form a Src family kinase-, ILK- and FAK-dependent expanding ring which defines the leading edge of the synapse that ultimately engulfs the apoptotic cell. The identification of the MERTK membrane-proximal signaling pathway and the role of {beta}2 integrins in this pathway provides new insights into the function of this critical homeostatic receptor and provides new insights into how MERTK mutations and signaling defects may contribute to inflammatory and autoimmune diseases.