A soluble form of the Mer receptor tyrosine kinase inhibits macrophage clearance of apoptotic cells and platelet aggregation

Department of Pediatrics, University of Colorado at Denver, and Health Sciences Center, Aurora, CO 80045, USA.
Blood (Impact Factor: 10.45). 03/2007; 109(3):1026-33. DOI: 10.1182/blood-2006-05-021634
Source: PubMed


Membrane-bound receptors generate soluble ligand-binding domains either by proteolytic cleavage of the extracellular domain or alternative mRNA splicing yielding a secreted protein. Mertk (Mer) is in a receptor tyrosine kinase family with Axl and Tyro-3, and all 3 receptors share the Gas6 ligand. Mer regulates macrophage activation, promotes apoptotic cell engulfment, and supports platelet aggregation and clot stability in vivo. We have found that the membrane-bound Mer protein is cleaved in the extracellular domain via a metalloproteinase. The cleavage results in the production of a soluble Mer protein released in a constitutive manner from cultured cells. Significant amounts of the soluble Mer protein were also detected in human plasma, suggesting its physiologic relevance. Cleavage of Mer was enhanced by treatment with LPS and PMA and was specifically inhibited by a tumor necrosis factor alpha-converting enzyme metalloproteinase inhibitor. As a decoy receptor for Gas6, soluble Mer prevented Gas6-mediated stimulation of membrane-bound Mer. The inhibition of Gas6 activity by soluble Mer led to defective macrophage-mediated engulfment of apoptotic cells. Furthermore, soluble Mer decreased platelet aggregation in vitro and prevented fatal collagen/epinephrine-induced thromboembolism in mice, suggesting a potential therapeutic use for soluble Mer in the treatment of clotting disorders.

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Available from: Douglas K Graham, Oct 16, 2014
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    • "The expression of ApoE mRNA was also significantly higher in Cx3cr1 GFP/GFP peritoneal M/s (prepared from thioglycollate-elicited peritonitis) when compared to WT-M/s cultured for 24 h in the presence of CX3CL1 (Fig 3F). Western blot analysis of equivalent amounts of supernatant protein from peritoneal M/s also showed increased APOE secretion (Fig 3G) in the Cx3cr1 GFP/GFP samples when compared to the soluble Mer receptor tyrosine kinase that is released constitutively from cultured macrophages (Sather et al, 2007) and which served here as a loading control. "
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    ABSTRACT: Physiologically, the retinal pigment epithelium (RPE) expresses immunosuppressive signals such as FAS ligand (FASL), which prevents the accumulation of leukocytes in the subretinal space. Age-related macular degeneration (AMD) is associated with a breakdown of the subretinal immunosuppressive environment and chronic accumulation of mononuclear phagocytes (MPs). We show that subretinal MPs in AMD patients accumulate on the RPE and express high levels of APOE. MPs of Cx3cr1−/− mice that develop MP accumulation on the RPE, photoreceptor degeneration, and increased choroidal neovascularization similarly express high levels of APOE. ApoE deletion in Cx3cr1−/− mice prevents pathogenic age- and stress-induced subretinal MP accumulation. We demonstrate that increased APOE levels induce IL-6 in MPs via the activation of the TLR2-CD14-dependent innate immunity receptor cluster. IL-6 in turn represses RPE FasL expression and prolongs subretinal MP survival. This mechanism may account, in part, for the MP accumulation observed in Cx3cr1−/− mice. Our results underline the inflammatory role of APOE in sterile inflammation in the immunosuppressive subretinal space. They provide rationale for the implication of IL-6 in AMD and open avenues toward therapies inhibiting pathogenic chronic inflammation in late AMD.
    Full-text · Article · Jan 2015 · EMBO Molecular Medicine
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    • "The lupus myeloid DC data included one high-expression outlier; however, even if that data point were excluded from the analysis, the difference remained significant. Because the function of Mer expressed on the surface of peripheral blood leukocytes can be suppressed by the presence of sMer in the blood [29], we examined the levels of sMer in plasma samples from the normal and lupus patients. In agreement with earlier studies [23,25], increased levels of sMer were present in the blood of lupus patients compared with normal healthy individuals (Figure 4D). "
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    ABSTRACT: The requirement for the immunoregulatory Mer tyrosine kinase (Mer) for optimal removal of apoptotic cells prompted us to look at its expression in systemic lupus erythematosus (SLE), where apoptotic cell clearance is abnormal. We compared the levels of expression of Mer in normal human subjects and patients with SLE. We used flow cytometry of isolated peripheral blood mononuclear cells to compare the levels of Mer on leukocyte subsets. We used a Mer-specific ELISA to quantify soluble Mer (sMer) in plasmas. Monocytes, CD1c+ myeloid dendritic cells (mDCs), and plasmacytoid dendritic cells (pDC) from both normal individuals and from SLE patients expressed Mer. In both normal and SLE patients, the CD14++CD16+ subpopulation of monocytes expressed the highest levels of Mer, with somewhat lower levels on the CD14intCD16+ population. Mer levels on CD1c+ mDCs and pDCs, and sMer levels in blood were increased in SLE patients compared to controls. In patients, Mer levels on CD14intCD16+, CD14++CD16- monocytes and CD1c+ dendritic cells correlated positively with type I interferon (IFN I) activity detected in blood. In SLE patients treated with corticosteroids, Mer expression on monocytes correlated with prednisone dose, while CD1c+ myeloid dendritic cells in patients treated with prednisone had higher levels of Mer expression than patients not receiving prednisone. We found no global defect in Mer expression in lupus blood. In contrast, we observed increased levels of Mer expression in DC populations, which could represent a response to increased IFN I in SLE patients. Enhanced Mer expression induced by corticosteroids may contribute to its beneficial effects in SLE.
    Full-text · Article · Mar 2014 · Arthritis research & therapy
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    • "Mer tyrosine kinase (MerTK) is an integral membrane protein that is preferentially expressed in hematopoietic lineages such as monocytes/macrophages, dendritic cells (DCs), and natural killer (NK) cells [1, 2], which is one of the three members of TAM (Tyro3, Axl, Mer) family receptor tyrosine kinases [3, 4]. The proteolytic cleavage of the extracellular domain of transmembrane MerTK (mMer) by A Disintegrin And Metalloproteinases domain 17 (ADAM17) leads to the production of the soluble form of MerTK protein (sMer), which is released into circulation and inhibits efferocytosis and platelet aggregation [5–7]. MerTK is a key molecular for tolerance maintenance of central and peripheral autoimmune responses through multiple mechanisms including recognition and clearance of apoptotic cells (ACs)-derived autoantigens [8–10], downregulation of TLR-induced production of inflammatory cytokines [11, 12], prevention of abnormal activation of antigen presenting cells [13], and expansion of autoreactive B and T cells [14, 15]. "
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    ABSTRACT: Objective . To investigate the expression and clinical significance of trans-membrane MerTK (mMer) on circulating CD14+ monocytes/macrophages and soluble MerTK (sMer) levels in plasma in systemic lupus erythematosus (SLE). Method . 108 SLE patients and 42 healthy controls were recruited in this study. The expression of mMer on the surfaces of CD14+ monocytes/macrophages was evaluated by flow cytometry (FCM). The sMer levels were measured by ELISA. Real-time quantitative PCR was applied to evaluate the mRNA levels of MerTK and ADAM17. Results . Both mMer expression on CD14+ monocytes/macrophages and sMer levels in plasma significantly increased in SLE patients compared to healthy subjects. The frequency of anti-inflammatory MerTK expressing CD14+CD16+ monocytes decreased in SLE. mMer expression was positively correlated with CD163 expression on CD14+ cells. Both the mMer expression on CD14+ monocytes/macrophages and sMer levels in plasma were positively correlated with SLEDAI. Furthermore, more elevated mMer and sMer levels were found in patients with higher SLEDAI, presence of anti-SSA, anti-Sm autoantibodies, and lupus nephritis. Conclusion . Both mMer and sMer levels significantly increased in SLE and positively correlated with disease activity and severity. The upregulation of MerTK expression may serve as a biomarker of the disease activity and severity of SLE.
    Full-text · Article · Mar 2014 · Journal of Immunology Research
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