title: Dual ITAM-mediated proteolytic pathways for irreversible inactivation of platelet receptors: de-ITAM-izing FcgammaRIIa.
abstract: Collagen binding to glycoprotein VI (GPVI) induces signals critical for platelet activation in thrombosis. Both ligand-induced GPVI signaling through its coassociated Fc-receptor gamma-chain (FcRgamma) immunoreceptor tyrosine-activation motif (ITAM) and the calmodulin inhibitor, W7, dissociate calmodulin from GPVI and induce metalloproteinase-mediated GPVI ectodomain shedding. We investigated whether signaling by another ITAM-bearing receptor on platelets, FcgammaRIIa, also down-regulates GPVI expression. Agonists that signal through FcgammaRIIa, the mAbs VM58 or 14A2, potently induced GPVI shedding, inhibitable by the metalloproteinase inhibitor, GM6001. Unexpectedly, FcgammaRIIa also underwent rapid proteolysis in platelets treated with agonists for FcgammaRIIa (VM58/14A2) or GPVI/FcRgamma (the snake toxin, convulxin), generating an approximate 30-kDa fragment. Immunoprecipitation/pull-down experiments showed that FcgammaRIIa also bound calmodulin and W7 induced FcgammaRIIa cleavage. However, unlike GPVI, the approximate 30-kDa FcgammaRIIa fragment remained platelet associated, and proteolysis was unaffected by GM6001 but was inhibited by a membrane-permeable calpain inhibitor, E64d; consistent with this, micro-calpain cleaved an FcgammaRIIa tail-fusion protein at (222)Lys/(223)Ala and (230)Gly/(231)Arg, upstream of the ITAM domain. These findings suggest simultaneous activation of distinct extracellular (metalloproteinase-mediated) and intracellular (calpain-mediated) proteolytic pathways irreversibly inactivating platelet GPVI/FcRgamma and FcgammaRIIa, respectively. Activation of both pathways was observed with immunoglobulin from patients with heparin-induced thrombocytopenia (HIT), suggesting novel mechanisms for platelet dysfunction by FcgammaRIIa after immunologic insult.
authors.affiliation: Department of Immunology, Monash University, Melbourne, Australia.
authors.forename: Elizabeth E
authors.initials: EE
authors.lastname: Gardiner
authors.forename: Denuja
authors.initials: D
authors.lastname: Karunakaran
authors.forename: Jane F
authors.initials: JF
authors.lastname: Arthur
authors.forename: Fi-Tjen
authors.initials: FT
authors.lastname: Mu
authors.forename: Maree S
authors.initials: MS
authors.lastname: Powell
authors.forename: Ross I
authors.initials: RI
authors.lastname: Baker
authors.forename: P Mark
authors.initials: PM
authors.lastname: Hogarth
authors.forename: Mark L
authors.initials: ML
authors.lastname: Kahn
authors.forename: Robert K
authors.initials: RK
authors.lastname: Andrews
authors.forename: Michael C
authors.initials: MC
authors.lastname: Berndt
mesh_terms: D000595:Amino Acid Sequence; D015703:Antigens, CD / Q000737:chemistry / Q000378:metabolism*; D001792:Blood Platelets / Q000378:metabolism*; D002147:Calmodulin / Q000037:antagonists & inhibitors / Q000378:metabolism; D002154:Calpain / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006801:Humans; D007136:Immunoglobulins / Q000097:blood; D008024:Ligands; D045726:Metalloproteases / Q000378:metabolism; D008969:Molecular Sequence Data; D015539:Platelet Activation / Q000187:drug effects / Q000502:physiology*; D010980:Platelet Membrane Glycoproteins / Q000737:chemistry / Q000378:metabolism*; D017434:Protein Structure, Tertiary; D017452:Receptors, IgG / Q000737:chemistry / Q000378:metabolism*; D015398:Signal Transduction / Q000502:physiology; D013449:Sulfonamides / Q000494:pharmacology; D013921:Thrombocytopenia / Q000097:blood
chemical_list: D015703:Antigens, CD; D002147:Calmodulin; D004791:Enzyme Inhibitors; C080568:Fc gamma receptor IIA; D007136:Immunoglobulins; D008024:Ligands; D010980:Platelet Membrane Glycoproteins; D017452:Receptors, IgG; D013449:Sulfonamides; C098341:platelet membrane glycoprotein VI; C017967:W 7; D045726:Metalloproteases; D002154:Calpain