title: Factor H binds to washed human platelets.
abstract: BACKGROUND
  Factor H regulates the alternative pathway of complement. The protein has three heparin-binding sites, is synthesized primarily in the liver and copurifies from platelets with thrombospondin-1. Factor H mutations at the C-terminus are associated with atypical hemolytic uremic syndrome, a condition in which platelets are consumed. Objectives The aim of this study was to investigate if factor H interacts with platelets.
  
  
  METHODS
  Binding of factor H, recombinant C- or N-terminus constructs and a C-terminus mutant to washed (plasma and complement-free) platelets was analyzed by flow cytometry. Binding of factor H and constructs to thrombospondin-1 was measured by surface plasmon resonance.
  
  
  RESULTS
  Factor H bound to platelets in a dose-dependent manner. The major binding site was localized to the C-terminus. The interaction was partially blocked by heparin. Inhibition with anti-GPIIb/IIIa, or with fibrinogen, suggested that the platelet GPIIb/IIIa receptor is involved in factor H binding. Factor H binds to thrombospondin-1. Addition of thrombospondin-1 increased factor H binding to platelets. Factor H mutated at the C-terminus also bound to platelets, albeit to a significantly lesser degree.
  
  
  CONCLUSIONS
  This study reports a novel property of factor H, i.e. binding to platelets, either directly via the GPIIb/IIIa receptor or indirectly via thrombospondin-1, in the absence of complement. Binding to platelets was mostly mediated by the C-terminal region of factor H and factor H mutated at the C-terminus exhibited reduced binding.
authors.affiliation: Department of Pediatrics, Lund University, Lund, Sweden.
authors.forename: F
authors.initials: F
authors.lastname: Vaziri-Sani
authors.forename: J
authors.initials: J
authors.lastname: Hellwage
authors.forename: P F
authors.initials: PF
authors.lastname: Zipfel
authors.forename: A G
authors.initials: AG
authors.lastname: Sjöholm
authors.forename: R
authors.initials: R
authors.lastname: Iancu
authors.forename: D
authors.initials: D
authors.lastname: Karpman
mesh_terms: D001665:Binding Sites; D001792:Blood Platelets / Q000166:cytology / Q000378:metabolism*; D017242:Complement Factor H / Q000737:chemistry* / Q000378:metabolism*; D003165:Complement System Proteins / Q000737:chemistry; D004305:Dose-Response Relationship, Drug; D005260:Female; D005434:Flow Cytometry; D006463:Hemolytic-Uremic Syndrome / Q000235:genetics; D006493:Heparin / Q000737:chemistry; D006801:Humans; D007700:Kinetics; D008099:Liver / Q000378:metabolism; D008297:Male; D009154:Mutation; D019039:Platelet Glycoprotein GPIIb-IIIa Complex / Q000737:chemistry; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D011994:Recombinant Proteins / Q000737:chemistry; D020349:Surface Plasmon Resonance; D019700:Thrombospondin 1 / Q000378:metabolism; D013997:Time Factors
chemical_list: C493495:CFH protein, human; D019039:Platelet Glycoprotein GPIIb-IIIa Complex; D011994:Recombinant Proteins; D019700:Thrombospondin 1; D017242:Complement Factor H; D006493:Heparin; D003165:Complement System Proteins