Page 184 - Haematologica-April 2018

Hemostasis
Correspondence:
peter.lenting@inserm.fr
Received: June 25, 2017. Accepted: December 27, 2017. Pre-published: January 11, 2018.
doi:10.3324/haematol.2017.175216
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Macrophage scavenger receptor SR-AI contributes to the clearance of
von Willebrand factor
Nikolett Wohner,1* Vincent Muczynski,1* Amel Mohamadi,1
Paulette Legendre,1 Valérie Proulle,1,2 Gabriel Aymé,1 Olivier D. Christophe,1 Peter J. Lenting,1 Cécile V. Denis1 and Caterina Casari1
1Institut National de la Santé et de la Recherche Médicale, UMR_S 1176, Univ. Paris-
Haematologica 2018 Volume 103(4):728-737
2
Sud, Université Paris-Saclay, 94276 Le Kremlin-Bicêtre and Service d’Hématologie
Biologique, Centre Hospitalier Universitaire Bicêtre, Assistance Publique- Hôpitaux de Paris, 94276 Le Kremlin-Bicêtre, France
ABSTRACT
Previously, we found that LDL-receptor related protein-1 on macrophages mediated shear stress-dependent clearance of von Willebrand factor. In control experiments, however, we observed that von Willebrand factor also binds to macrophages independently of this receptor under static conditions, suggesting the existence of addition- al clearance-receptors. In search for such receptors, we focused on the macrophage-specific scavenger-receptor SR-AI. von Willebrand factor displays efficient binding to SR-AI (half-maximum binding 14±5 nM). Binding is calcium-dependent and is inhibited by 72±4% in the combined presence of antibodies against the A1- and D4-domains. Association with SR-AI was confirmed in cell-binding experiments. In addition, binding to bone marrow-derived murine SR-AI-deficient macrophages was strongly reduced compared to binding to wild-type murine macrophages. Following expression via hydrodynamic gene transfer, we determined ratios for von Willebrand factor-propeptide over von Willebrand factor- antigen, a marker of von Willebrand factor clearance. Propeptide/antigen ratios were significantly reduced in SR-AI-deficient mice compared to wild-type mice (0.6±0.2 versus 1.3±0.3; P<0.0001), compatible with a slower clearance of von Willebrand factor in SR-AI-deficient mice. Interestingly, mutants associated with increased clearance (von Willebrand factor/p.R1205H and von Willebrand factor/p.S2179F) had significantly increased binding to purified SR-AI and SR-AI expressed on macrophages. Accordingly, propeptide/antigen ratios for these mutants were reduced in SR-AI-deficient mice. In conclusion, we have identified SR-AI as a novel macrophage-specific receptor for von Willebrand factor. Enhanced binding of von Willebrand factor mutants to SR-AI may con- tribute to the increased clearance of these mutants.
Introduction
Mutations in the gene encoding von Willebrand factor (VWF) may generate pro- teins displaying defects in biosynthesis, secretion and/or clearance. The quintessen- tial representative of VWF clearance mutants is VWF/p.R1205H, also known as the Vicenza variant.1 This mutation is associated with VWF antigen (VWF:Ag) levels that are usually below 20%, which are most likely due to a 5- to 10-fold reduced circula- tory half-life of the mutant protein.1-3 Since the description of the Vicenza variant, additional mutations in VWF have been found to provoke a reduced survival. Such mutants have been identified by analyzing VWF survival after desmopressin treat- ment or by determining ratios between VWF propeptide (VWFpp) and VWF:Ag, with elevated VWFpp/VWF:Ag ratios pointing to increased VWF clearance.4-8
The mechanism by which mutations provoke accelerated clearance is poorly understood. Recently, we showed that gain-of-function mutations in the VWF A1 domain induce spontaneous binding to the clearance receptor LDL-receptor related protein-1 (LRP1).9 In addition, truncation of its N-glycans accelerates LRP1-mediat-
NW and VM contributed equally to this work.
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