Phosphoproteomic quantitation and causal analysis reveal pathways in GPVI/ITAM-mediated platelet activation programs.

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Platelets engage cues of pending vascular injury through coordinated adhesion, secretion and aggregation responses. These rapid, progressive changes in platelet form and function are orchestrated downstream of specific receptors on the platelet cell surface, and through intracellular signaling mechanisms that remain systematically undefined. This study brings together cell physiological and phosphoproteomics methods incorporating peptide tandem mass tag (TMT) labeling, sample multiplexing, synchronous precursor selection (SPS) and triple stage tandem mass spectrometry (MS3) to profile signaling mechanisms downstream of the immunotyrosine activation motif (ITAM) platelet collagen receptor GPVI. Altogether, >3,000 significant (FDR<0.05) phosphorylation events on >1,300 proteins were detected in initial and progressing conditions of GPVI-mediated platelet activation. With literature-guided causal inference tools, >300 site-specific signaling relations were mapped from phosphoproteomics data among key and emerging GPVI effectors (i.e., FcRg, Syk, PLCg2, PKCd, DAPP1). Through signaling validation studies and functional screening, other less-characterized targets were also considered within the context of GPVI/ITAM pathways, including Ras/MAPK axis proteins (i.e., KSR1, SOS1, STAT1, Hsp27). Networks highly regulated in GPVI/ITAM signaling out of context of curated knowledge were also illuminated, including a system of >40 Rab GTPases and associated regulatory proteins - where TAK1-mediated Rab7 S72 phosphorylation associated with endolysosomal maturation and GPVI-mediated platelet function. In addition to serving as a model for generating and testing hypotheses from omics datasets, this study puts forth a means to identify hemostatic effectors, biomarkers and therapeutic targets relevant to thrombosis, vascular inflammation and other platelet-associated disease states.


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