Journal Article

Differential TAM receptor–ligand–phospholipid interactions delimit differential TAM bioactivities

Lemke, Greg, Lax, Irit, Través, Paqui G, Lew, Erin D, Schlessinger, Joseph, Burrola, Patrick G, Oh, Jennifer, Zagórska, Anna

2014

Overview

The TAM receptor tyrosine kinases Tyro3, Axl, and Mer regulate key features of cellular physiology, yet the differential activities of the TAM ligands Gas6 and Protein S are poorly understood. We have used biochemical and genetic analyses to delineate the rules for TAM receptor–ligand engagement and find that the TAMs segregate into two groups based on ligand specificity, regulation by phosphatidylserine, and function. Tyro3 and Mer are activated by both ligands but only Gas6 activates Axl. Optimal TAM signaling requires coincident TAM ligand engagement of both its receptor and the phospholipid phosphatidylserine (PtdSer): Gas6 lacking its PtdSer-binding ‘Gla domain’ is significantly weakened as a Tyro3/Mer agonist and is inert as an Axl agonist, even though it binds to Axl with wild-type affinity. In two settings of TAM-dependent homeostatic phagocytosis, Mer plays a predominant role while Axl is dispensable, and activation of Mer by Protein S is sufficient to drive phagocytosis. Cells send out and receive signals to communicate with other cells. Detecting these signals is largely carried out by proteins called receptors that span the cell surface membrane. These proteins typically have extracellular domains outside of the cell that can bind to specific signaling molecules and an intracellular domain inside the cell that relays the information inwards to trigger a response. Three such receptor proteins are collectively known as the TAM receptors. Each day, many billions of cells in the human body die and are engulfed by other cells and broken down so that their building blocks can be reused. TAM receptors are required for this process; and the TAM receptors also help prevent the immune system from going out of control, which would damage the body's own tissues. Two different signaling proteins, called Gas6 and Protein S, can bind to and activate TAM receptors. Both of the signaling proteins can also bind to a phospholipid molecule that is found on the surface membrane of dead cells. However, it is not known if all three TAM receptors bind to both signaling proteins equally, and the importance of the phospholipid-binding domain in the signaling proteins remains unclear. To shed light on the workings of these receptors, Lew et al. created mouse cells that each only express one out of the three TAM receptors. These cells were then exposed to intact Gas6 and Protein S, or shortened versions that lacked the phospholipid-binding domain. Lew et al. found that Gas6 could trigger a response through all three TAM receptors but that Protein S was specific for only two out of the three receptors. Signaling proteins with or without their phospholipid-binding domains bound equally well to the receptors, but the maximum level of response was only triggered when both signaling proteins were intact and the phospholipid molecule was present. This is important since the phospholipid can be thought of as an ‘eat-me’ signal by which the dead cells are recognized by the TAM receptor-expressing cells that will engulf them. Using mice that only produce a TAM receptor called Mer, Lew et al. show that Protein S alone can trigger the process that engulfs and breaks down cells in a living organism. These data and previous work suggest that two TAM receptors—including Mer—are involved in the daily engulfment of dying cells, whereas the third mediates this process during infection and tissue damage. Molecules that inhibit or activate the function of TAM receptors are currently being developed to treat cancer and other diseases. By revealing which receptors respond to which signaling molecules, the findings of Lew et al. will serve to guide these efforts.

• Publisher: eLife Sciences Publications Ltd,eLife Sciences Publications, Ltd
• Subject: Animals / Apoptosis / Axl / Axl protein / Biochemistry / Bone Marrow Cells - cytology / Bone Marrow Cells - drug effects / Bone Marrow Cells - metabolism / c-Mer Tyrosine Kinase / Cell Line / Embryo, Mammalian / Female / Fibroblasts - cytology / Fibroblasts - drug effects / Fibroblasts - metabolism / Flow cytometry / Gas6 / Gene Expression Regulation / HEK293 Cells / Humans / Immune system / Immunology / Intercellular Signaling Peptides and Proteins - genetics / Intercellular Signaling Peptides and Proteins - metabolism / Laboratories / Ligands / Male / Mertk / Mice / Mice, Inbred C57BL / Mice, Knockout / Neurosciences / Phagocytosis / Phagocytosis - genetics / Phosphatidylserine / Phosphatidylserines - metabolism / Phosphatidylserines - pharmacology / Phospholipids / Primary Cell Culture / Pros1 / Protein S / Protein S - genetics / Protein S - metabolism / Protein-tyrosine kinase receptors / Proteins / Proto-Oncogene Proteins - genetics / Proto-Oncogene Proteins - metabolism / Receptor Protein-Tyrosine Kinases - genetics / Receptor Protein-Tyrosine Kinases - metabolism / receptor tyrosine kinase / Recombinant Proteins - genetics / Recombinant Proteins - metabolism / Signal Transduction / Tyro3 / Tyrosine