| Mer (MERTK) is a transmembrane receptor tyrosine kinase of the TAM family, together with AXL and TYRO3, and functions as a surface receptor that controls apoptotic cell clearance, immune homeostasis, and tissue integrity in systems such as the retina and mononuclear phagocyte compartment. The receptor contains an extracellular region with tandem immunoglobulin‑like and fibronectin type III domains that bind vitamin K–dependent ligands, a single transmembrane segment, and a cytoplasmic tyrosine kinase domain with multiple regulatory tyrosines that undergo autophosphorylation after ligand engagement. Gas6 and Protein S are physiological ligands that bind Mer via their laminin G–like domains while their N‑terminal Gla regions contact phosphatidylserine on apoptotic cell membranes; this configuration presents apoptotic targets to Mer‑expressing phagocytes and positions the kinase domain for activation. Ligand‑dependent Mer activation induces autophosphorylation on intracellular tyrosines and creates docking sites for SH2‑containing adaptors such as GRB2, GAB1, and PI3K regulatory subunits, which connect Mer to RAS/RAF/MEK/ERK and PI3K/Akt signaling modules that promote cytoskeletal remodeling, engulfment cup formation, and survival of the phagocytic cell during efferocytosis. Mer signaling also upregulates suppressor of cytokine signaling proteins and other negative regulators of innate immunity, which limit pro‑inflammatory cytokine production during apoptotic cell clearance and maintain a non‑immunogenic environment in tissues engaged in high‑volume efferocytosis such as the retinal pigment epithelium. Retinal pigment epithelial cells express Mer and perform circadian phagocytosis of shed photoreceptor outer segments, and Gas6 and Protein S cooperate to control rhythmic Mer activation, with differential binding to distinct residues in Mer Ig‑like domains and time‑dependent changes in ligand availability contributing to daily peaks of photoreceptor outer‑segment ingestion that are essential for long‑term visual function. Macrophages and dendritic cells express Mer together with AXL and TYRO3, and Mer‑dependent efferocytosis of apoptotic cells exposed to phosphatidylserine contributes to resolution‑phase programming and modulation of macrophage polarization, with TAM signaling showing a characteristic association with anti‑inflammatory and tissue‑repair phenotypes. MERTK upregulation or ectopic expression in diverse epithelial and hematologic malignancies, where Mer activation by Gas6, Protein S, or other ligands such as Tubby, Tulp1, and Galectin‑3 induces receptor autophosphorylation and stimulates downstream PI3K/Akt, MAPK, and STAT signaling pathways that support survival, proliferation, chemoresistance, and motility of tumor cells, and Mer‑driven efferocytosis by tumor or stromal cells correlates with PD‑L1 induction and immune evasion. |
