title: Collagen promotes sustained glycoprotein VI signaling in platelets and cell lines.
abstract: BACKGROUND
  Glycoprotein (GP)VI is the major signaling receptor for collagen on platelets and signals via the associated FcRgamma-chain, which has an immunoreceptor tyrosine-containing activation motif (ITAM).
  
  
  OBJECTIVE
  To determine why GPVI-FcRgamma signals poorly, or not at all, in response to collagen in hematopoietic cell lines, despite robust responses to the GPVI-reactive snake venom toxin convulxin.
  
  
  RESULTS
  Using a nuclear factor of activated T-cells (NFAT) transcriptional reporter assay, a sensitive readout for sustained ITAM signaling, we demonstrate collagen-induced GPVI-FcRgamma signaling in hematopoietic cell lines. This is accompanied by relatively weak but sustained protein tyrosine phosphorylation, in contrast to the stronger but transient response to convulxin. Sustained signaling by collagen is also observed in platelets and is necessary for the maintenance of spreading on collagen. Finally, in cell lines, the inhibitory collagen receptor leukocyte-associated immunoglobulin-like receptor-1 (LAIR-1), which is not expressed on platelets but is present on most hematopoietic cells, inhibits GPVI responses to collagen but not convulxin.
  
  
  CONCLUSIONS
  The inability of previous studies to readily detect GPVI collagen signaling in cell lines is probably because of the weak but sustained nature of the signal and the presence of the inhibitory collagen receptor LAIR-1. In platelets, we propose that GPVI-FcRgamma has evolved to transmit sustained signals in order to maintain spreading over several hours, as well as facilitating rapid activation through release of feedback agonists and integrin activation. The establishment of a cell line NFAT assay will facilitate the molecular dissection of GPVI signaling and the identification of GPVI antagonists in drug discovery.
authors.affiliation: Centre for Cardiovascular Sciences, Institute of Biomedical Research, The Medical School, University of Birmingham, Birmingham, UK. m.g.tomlinson@bham.ac.uk
authors.forename: M G
authors.initials: MG
authors.lastname: Tomlinson
authors.forename: S D
authors.initials: SD
authors.lastname: Calaminus
authors.forename: O
authors.initials: O
authors.lastname: Berlanga
authors.forename: J M
authors.initials: JM
authors.lastname: Auger
authors.forename: T
authors.initials: T
authors.lastname: Bori-Sanz
authors.forename: L
authors.initials: L
authors.lastname: Meyaard
authors.forename: S P
authors.initials: SP
authors.lastname: Watson
mesh_terms: D000818:Animals; D001773:Blood Cells / Q000378:metabolism*; D001792:Blood Platelets / Q000378:metabolism*; D045744:Cell Line, Tumor; D002478:Cells, Cultured; D003094:Collagen / Q000502:physiology*; D006801:Humans; D010980:Platelet Membrane Glycoproteins / Q000235:genetics / Q000378:metabolism*; D051381:Rats; D017452:Receptors, IgG / Q000378:metabolism*; D011971:Receptors, Immunologic / Q000235:genetics / Q000502:physiology*; D015398:Signal Transduction; D014161:Transduction, Genetic
chemical_list: C489017:FCGR1A protein, human; D010980:Platelet Membrane Glycoproteins; D017452:Receptors, IgG; D011971:Receptors, Immunologic; C108073:leukocyte-associated immunoglobulin-like receptor 1; C098341:platelet membrane glycoprotein VI; D003094:Collagen