title: An acquired defect associated with abnormal signaling of the platelet collagen receptor glycoprotein VI.
abstract: BACKGROUND
  Ligands acting at the platelet collagen receptor, glycoprotein (GP)VI, induce intracellular FcRγ/Syk-dependent signaling pathways and Syk-dependent or Syk-independent generation of intracellular reactive oxygen species (ROS). Additional signaling-dependent or signaling-independent pathways lead to metalloproteinase-mediated shedding of GPVI.
  
  
  OBJECTIVE
  Analysis of platelet GPVI expression and signaling in a patient with a collagen-selective defect associated with myelodysplastic syndrome (MDS) uniquely demonstrates divergent pathways leading to ROS generation and Syk phosphorylation in human platelets.
  
  
  METHODS
  Surface expression of GPVI and ligand-induced ROS generation was quantitated by flow cytometry. GPVI shedding and Syk phosphorylation were analyzed by Western blot.
  
  
  RESULTS
  Despite platelet count/size and GPVI surface expression within normal ranges, platelet-rich plasma showed no aggregation in response to collagen or GPVI-selective agonist collagen-related peptide, but aggregated in response to other agonists, consistent with dysfunctional GPVI signaling. We observed rapid GPVI-dependent Syk-independent ROS generation and disulfide-dependent GPVI homodimerization, but not Syk-dependent ROS or ligand-induced shedding. Temporal analysis showed a gradual decline in platelet count and the appearance of ligand-induced phosphorylation of an ∼40-kDa Syk fragment.
  
  
  CONCLUSIONS
  These studies show that GPVI ligation in platelets induces intracellular ROS production independent of either Syk activation or divergent pathways leading to platelet aggregation or ectodomain shedding.
authors.affiliation: Australian Centre for Blood Diseases, Monash University, Melbourne, Vic., Australia.
authors.forename: Jianlin
authors.initials: J
authors.lastname: Qiao
authors.forename: Jane F
authors.initials: JF
authors.lastname: Arthur
authors.forename: Margaret
authors.initials: M
authors.lastname: Collecutt
authors.forename: Yang
authors.initials: Y
authors.lastname: Shen
authors.forename: Fi-Tjen
authors.initials: FT
authors.lastname: Mu
authors.forename: Michael C
authors.initials: MC
authors.lastname: Berndt
authors.forename: Amanda K
authors.initials: AK
authors.lastname: Davis
authors.forename: Robert K
authors.initials: RK
authors.lastname: Andrews
authors.forename: Elizabeth E
authors.initials: EE
authors.lastname: Gardiner
mesh_terms: D000368:Aged; D005260:Female; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins / Q000378:metabolism; D008024:Ligands; D009190:Myelodysplastic Syndromes / Q000503:physiopathology*; D010980:Platelet Membrane Glycoproteins / Q000502:physiology*; D011505:Protein-Tyrosine Kinases / Q000378:metabolism; D017382:Reactive Oxygen Species / Q000378:metabolism*; D038981:Receptors, Collagen / Q000502:physiology*; D015398:Signal Transduction / Q000502:physiology*; D000072377:Syk Kinase
chemical_list: D047908:Intracellular Signaling Peptides and Proteins; D008024:Ligands; D010980:Platelet Membrane Glycoproteins; D017382:Reactive Oxygen Species; D038981:Receptors, Collagen; C098341:platelet membrane glycoprotein VI; D011505:Protein-Tyrosine Kinases; C000606896:SYK protein, human; D000072377:Syk Kinase