Satya P. Kunapuli

Syk is a non-receptor type protein-tyrosine kinase (PTK), which is associated with platelets surface receptors, glycoprotein VI (GPVI) and C-type lectin-like receptor II-type (CLEC-2). Syk is also expressed in most hematopoietic lineage cells and other cells, such as fibroblasts and neuronal cells. Syk has two tandem SH2 motifs and a C-terminal kinase domain, which are interrupted by interdomains A and B containing multiple tyrosine residues playing a regulatory role upon phosphorylation. This study aims to evaluate the role of Y130 in Syk signaling in platelets. Syk(Y130F) knock-in (KI) mice we generated using the CRISPR-Cas9 technique represent the first in-vivo model harboring this mutation. Using this system, we compared the platelet signaling and responses in wild-type (WT) and Syk(Y130F) littermates. Platelets from homozygous Syk(Y130F) mice showed a decrease in functional responses after activation with CRP, a GPVI agonist, and CLEC-2 crosslinking compared to WT littermates with no significant differences in responses to PAR-4 or purinergic receptor agonists. Key signaling events triggered via both GPVI and CLEC-2, including phosphorylation LAT and PLC γ -2, were also reduced in Syk(Y130F) platelets at low agonist concentrations. Consistent with these findings, the time to occlusion in the FeCl ₃ injury model and bleeding time in the tail bleeding assay were significantly enhanced in Syk(Y130F) mice compared to WT littermates. Thus, phosphorylation of Syk Y130 enhances GPVI- and CLEC-2-mediated signaling and functional responses in platelets affecting thrombosis and hemostasis.

Spleen tyrosine kinase (Syk) is expressed in a variety of hematopoietic cells. Its phosphorylation regulates downstream signaling events upon stimulation of receptors containing an immunoreceptor tyrosine-based activation motif (ITAM), like glycoprotein VI, or a hemITAM, including the C-type lectin-like receptor 2 (CLEC-2). This study focuses on the role of a specific phosphorylation site, tyrosine 317, in the regulation of Syk function. Tyrosine 317 is located in the linker region of Syk that separates the amino-terminal, tandem pair of SH2 domains from the carboxyl-terminal catalytic domain. The amino acid sequence surrounding phosphotyrosine 317 binds to the matching recognition sequence in the tyrosine kinase–binding domain of Cbl, an E3 ubiquitin-protein ligase. To evaluate the function of this phosphorylation site, we generated mice expressing Syk(Y317F) using the CRISPR/Cas9 technique. Platelets from homozygous Syk(Y317F) mice showed enhancement of platelet signaling and physiological responses after stimulation with collagen-related peptide (CRP) and CLEC-2 cross-linking. This enhancement did not occur after stimulation with AYPGKF, a protease-activated receptor 4 agonist, or 2-methylthioadenosine diphosphate, a purinergic agonist. CRP- or CLEC-2 monoclonal antibody–induced downstream signaling events, including phosphorylation of LAT and phospholipase C γ2, were enhanced in Syk(Y317F) platelets compared with platelets from wild-type (WT) littermates. Besides an increase in platelet responses in vitro, the time to occlusion in the FeCl3 injury model was decreased in Syk(Y317F) mice compared with WT littermates. However, there was no significant difference in the tail bleeding times. Taken together, these data reveal that tyrosine 317 negatively regulates Syk signaling and functions in mouse platelets.

The two members of the UBASH3/TULA/STS-protein family have been shown to critically regulate cellular processes in multiple biological systems. The regulatory function of TULA-2 (also known as UBASH3B or STS-1) in platelets is one of the best examples of the involvement of UBASH3/TULA/STS proteins in cellular regulation. TULA-2 negatively regulates platelet signaling mediated by ITAM- and hemITAM-containing membrane receptors that are dependent on the protein tyrosine kinase Syk, which currently represents the best-known dephosphorylation target of TULA-2. The biological responses of platelets to collagen and other physiological agonists are significantly downregulated as a result. The protein structure, enzymatic activity and regulatory functions of UBASH3/TULA/STS proteins in the context of platelet responses and their regulation are discussed in this review.

Platelets are anucleate cells that mediate hemostasis. This occurs via a primary signal that is reinforced by secreted products such as ADP that bind purinergic receptors (P2Y1 and P2Y12) on the platelet surface. We recently identified a human subject, whom we termed platelet defect subject 25 (PDS25) with a platelet functional disorder associated with the P2Y12 receptor. PDS25 has normal blood cell counts and no history of bleeding diathesis. However, platelets from PDS25 have virtually no response to 2-MeSADP (a stable analogue of ADP). Genetic analysis of P2Y12 from PDS25 revealed a heterozygous mutation of D121N within the DRY motif. Rap1b activity was reduced in platelets from PDS25, while VASP phosphorylation was enhanced, suggesting that signaling from the P2Y12 receptor was interrupted by the heterozygous mutation. To explore this further, we produced knock-in mice that mimic our subject. Bleeding failed to cease in homozygous KI mice during tail bleeding assays, while tail bleeding times did not differ between WT and heterozygous KI mice. Furthermore, occlusions failed to form in most homozygous KI mice following carotid artery injury via FeCl₃. These data indicate that the aspartic acid residue found in the DRY motif of P2Y12 is essential for P2Y12 function.

Platelet activation by adenosine diphosphate (ADP) is mediated through two G-protein-coupled receptors, P2Y1 and P2Y12, which signal through Gq and Gi, respectively. P2Y1 stimulation leads to phospholipase C activation and an increase in cytosolic calcium necessary for CalDAG-GEF1 activation. Engagement of P2Y12 inhibits adenylate cyclase, which reduces cAMP, and activation of PI3-kinase, which inhibits RASA3 resulting in sustained activated Rap1b. In this study we activated human platelets with 2-MeSADP in the presence of LY294002, a PI3-kinase inhibitor, AR-C69931MX, a P2Y12 antagonist or MRS2179, a P2Y1 antagonist. We measured the phosphorylation of Akt on Ser473 as an indicator of PI3-kinase activity. As previously shown, LY294002 and ARC69931MX abolished 2MeSADP-induced Akt phosphorylation. MRS2179 reduced ADP-induced Akt phosphorylation but did not abolish it. Rap1b activation, however, was only reduced, but not ablated, using LY294002 and was completely inhibited by ARC69931MX or MRS2179. Furthermore, 2MeSADP-induced Rap1b activation was abolished in either P2Y1 or P2Y12 null platelets. These data suggest that ADP-induced Rap1b activation requires both P2Y1 and P2Y12. In addition, although stimulation of P2Y12 results in PI3-kinase activation leading to Akt phosphorylation and Rap1b activation, Rap1b activation can occur independently of PI3-kinase downstream of P2Y12. Thus, we propose that the P2Y12 receptor can regulate Rap1b, possibly through RASA3, in a pathway independent of PI3-kinase.