Migration to Apoptotic “Find-me” Signals Is Mediated via the Phagocyte Receptor G2A

Abstract

Phagocytosis of apoptotic cells is fundamentally important throughout life, because non-cleared cells become secondarily necrotic and release intracellular contents, thus instigating inflammatory and autoimmune responses. Secreted "find-me" and exposed "eat-me" signals displayed by the dying cell in concert with the phagocyte receptors comprise the phagocytic synapse of apoptotic cell clearance. In this scenario, lysophospholipids (lysoPLs) are assumed to act as find-me signals for the attraction of phagocytes. However, both the identity of the lysoPLs released from apoptotic cells and the nature of the phagocyte receptor are largely unknown. By a detailed analysis of the structural requirements we show here that lysophosphatidylcholine (lysoPC), but none of the lysoPC metabolites or other lysoPLs, represents the essential apoptotic attraction signal able to trigger a phagocyte chemotactic response. Furthermore, using RNA interference and expression studies, we demonstrate that the G-protein-coupled receptor G2A, unlike its relative GPR4, is involved in the chemotaxis of monocytic cells. Thus, our study identifies lysoPC and G2A as the crucial receptor/ligand system for the attraction of phagocytes to apoptotic cells and the prevention of autoimmunity.

Do not let death do us part: ‘find-me’ signals in communication between dying cells and the phagocytes

“…Knockdown of G2A could decrease the migratory capacity of phagocytes to apoptotic cell supernatants. 24 However, other phospholipids were also able to neutralize migration to pure LPC in a G2A-dependent manner. It is possible that a balance of different lysophospholipids released from apoptotic cells can affect migration.…”

“…Unlike LPC, other lysophospholipids or metabolic derivatives of LPC were unable to induce phagocyte chemotaxis. 24 This release of LPC was not simply leakage of cellular contents as the membrane integrity was intact. LPC release was dependent on caspase-3-mediated activation of calcium-independent phospholipase A2, 23 which hydrolizes membrane-lipid phosphatidylcholine.…”

“…As shown through investigations on LPC, apoptotic cell supernatants contained a concentration of 200 nM LPC, but the authors used 20-30 μM of purified LPC to detect cell migration. 23,24 Although plasma levels of LPC are known to be considerably high (100 μM-150 μM), 68 extracellular actions of LPC have been reported; 69 an observation likely in part because of the availability of free LPC, as the lipid is thought to be bound to albumin and other carrier proteins. 70 Without detailed knowledge of the receptor for LPC during phagocyte migration, it is difficult to determine if physiological LPC is pertinent.…”

NADPH Oxidase-dependent Generation of Lysophosphatidylserine Enhances Clearance of Activated and Dying Neutrophils via G2A

“…Macrophage G2A Signaling-We have recently reported that lyso-PS can signal through the G proteincoupled receptor, G2A, for calcium flux (20), and G2A has been recently described as a receptor on macrophages responsible for mediating macrophage chemotactic responses (50,51). Accordingly, G2A appeared to be a reasonable candidate for lyso-PS-dependent enhancement of apoptotic cell engulfment.…”

NTE1-encoded Phosphatidylcholine Phospholipase B Regulates Transcription of Phospholipid Biosynthetic Genes

“…These lipases produce numerous second messengers, including arachidonic acid for the synthesis of prostaglandins and leukotrienes to control inflammatory and immune responses, lysophosphatidylcholine to provide a chemotactic signal for macrophages and lymphocytes, and diacylglycerol for the activation of protein kinases (11)(12)(13)(14). Although it is known that the NTE1-encoded phospholipase B catalyzes the deacylation of PC to produce fatty acids and glycerophosphocholine, its cellular role is not known.…”