title: Expression and functionality of Toll-like receptor 3 in the megakaryocytic lineage.
abstract: BACKGROUND
  In addition to their key role in hemostasis, platelets and megakaryocytes regulate immune and inflammatory responses, in part through their expression of Toll-like receptors (TLRs). Among the TLRs, TLR3 recognizes dsRNA associated with viral infection. Thrombocytopenia is a frequent complication of viral infection. However, the expression and functionality of TLR3 in megakaryocytes and platelets is not yet well understood.
  
  
  OBJECTIVE
  To study the expression and functionality of TLR3 in the megakaryocytic lineage.
  
  
  RESULTS
  RT-PCR, flow cytometric and immunofluorescence assays showed that TLR3 is expressed in CD34(+) cells, megakaryocytes, and platelets. Immunoblotting assays showed that stimulation of megakaryocytes with two synthetic agonists of TLR3, Poly(I:C) and Poly(A:U), activated the nuclear factor-κB (NF-κB), phosphoinositide 3-kinase (PI3K)/Akt, extracellular signal-related kinase (ERK)1/2 and p38 pathways. TLR3-megakaryocyte activation resulted in reduced platelet production in vitro and interferon-β release through the PI3K-Akt and NF-κB signaling pathways. TLR3 ligands potentiated the aggregation mediated by classic platelet agonists. This effect was also observed for ATP release, but not for P-selectin or CD40L membrane exposure, indicating that TLR3 activation was not involved in α-granule release. In addition, TLR3 agonists induced activation of the NF-κB, PI3K-Akt and ERK1/2 pathways in platelets. Reductions in platelet production and platelet fibrinogen binding mediated by Poly(I:C) or Poly(A:U) were prevented by the presence of an inhibitor of the TLR3-dsRNA complex.
  
  
  CONCLUSIONS
  Our findings indicate that functional TLR3 is expressed in CD34(+) cells, megakaryocytes, and platelets, and suggest a potential role for this receptor in the megakaryopoiesis/thrombopoiesis alterations that occur in viral infections.
authors.affiliation: Laboratory of Experimental Thrombosis, Institute of Experimental Medicine, CONICET-National Academy of Medicine, Buenos Aires, Argentina.
authors.forename: L P
authors.initials: LP
authors.lastname: D'Atri
authors.forename: J
authors.initials: J
authors.lastname: Etulain
authors.forename: L
authors.initials: L
authors.lastname: Rivadeneyra
authors.forename: M J
authors.initials: MJ
authors.lastname: Lapponi
authors.forename: M
authors.initials: M
authors.lastname: Centurion
authors.forename: K
authors.initials: K
authors.lastname: Cheng
authors.forename: H
authors.initials: H
authors.lastname: Yin
authors.forename: M
authors.initials: M
authors.lastname: Schattner
mesh_terms: D001792:Blood Platelets / Q000201:enzymology / Q000378:metabolism; D019070:Cell Lineage; D006412:Hematopoietic Stem Cells / Q000378:metabolism; D006801:Humans; D008533:Megakaryocytes / Q000166:cytology / Q000378:metabolism*; D015398:Signal Transduction; D051196:Toll-Like Receptor 3 / Q000378:metabolism*
chemical_list: C495344:TLR3 protein, human; D051196:Toll-Like Receptor 3
keywords: NF-kappa B; Toll-like receptor 3; interferon-beta; megakaryocytes; platelets