The Gas6-Axl Protein Interaction Mediates Endothelial Uptake of Platelet Microparticles

Abstract

Upon activation, platelets release plasma membrane-derived microparticles (PMPs) exposing phosphatidylserine on their surface. The functions and clearance mechanism of these microparticles are incompletely understood. As they are pro-coagulant and potentially pro-inflammatory, rapid clearance from the circulation is essential for prevention of thrombotic diseases. The tyrosine kinase receptors Tyro3, Axl, and Mer (TAMs) and their ligands protein S and Gas6 are involved in the uptake of phosphatidylserine-exposing apoptotic cells in macrophages and dendritic cells. Both TAMs and their ligands are expressed in the vasculature, the functional significance of which is poorly understood. In this study, we investigated how vascular TAMs and their ligands may mediate endothelial uptake of PMPs. PMPs, generated from purified human platelets, were isolated by ultracentrifugation and labeled with biotin or PKH67. The uptake of labeled microparticles in the presence of protein S and Gas6 in human aortic endothelial cells and human umbilical vein endothelial cells was monitored by flow cytometry, Western blotting, and confocal/electron microscopy. We found that both endothelial cell types can phagocytose PMPs, and by using TAM-blocking antibodies or siRNA knockdown of individual TAMs, we show that the uptake is mediated by endothelial Axl and Gas6. As circulating PMP levels were not altered in Gas6 ؊/؊ mice compared with Gas6 ؉/؉ mice, we hypothesize that the Gas6-mediated uptake is not a means to clear the bulk of circulating PMPs but may serve to locally phagocytose PMPs generated at sites of platelet activation and as a way to effect endothelial responses.

Factor V‐short and protein S as synergistic tissue factor pathway inhibitor (TFPIα) cofactors

“…Phosphatidylserine (PS), phosphatidyl ethanolamine (PE) and phosphatidyl choline (PC) were from Avanti Polar Lipids. Phospholipid vesicles were prepared using the LiposoFast basic extruder (Armatis, Germany) as previously described 27. The phospholipid vesicles were used within 2 days.…”

The Role of TAM Family Receptors in Immune Cell Function: Implications for Cancer Therapy

“…Since then, rigorous research has revealed crucial functions of these distinctive receptors in normal physiology and in a variety of diseases. TAM signaling has been implicated in controlling platelet aggregation and thrombus formation [3], erythropoiesis [4], and endothelial and vascular smooth-muscle homeostasis [5,6,7]. In addition, TAM-dependent pathways participate in spermatogenesis [8], functional maintenance of the retina and lactating mammary gland [9,10], bone physiology [11], atherosclerosis [12], nervous-system biology [13], and permeability of the blood–brain barrier [14].…”

Thematic Review Series: Exosomes and Microvesicles: Lipids as Key Components of their Biogenesis and Functions Extracellular vesicles and their content in bioactive lipid mediators: more than a sack of microRNA

“…Whereas lipids form the basis of the EV structure and affect EV stability or clearance in the whole organism (1,27,109,110), EVs also shuttle bioactive lipid mediators and …”