The Src family kinases (SFKs) Src, Lyn and Fyn are essential for platelet activation and also involved in megakaryocyte (MK) development and platelet production. Platelet SFKs are inhibited by C-terminal Src kinase (Csk), which phosphorylates a conserved tyrosine in their C-terminal tail, and are activated by the receptor-type tyrosine phosphatase PTPRJ (CD148, DEP-1), which dephosphorylates the same residue. Deletion of Csk and PTPRJ in the MK lineage in mice results in increased SFK activity, but paradoxically hypoactive platelets due to negative feedback mechanisms, including upregulation of Csk homologous kinase (Chk) expression. Here, we investigate the role of Chk in platelets, functional redundancy with Csk and the physiological consequences of ablating Chk, Csk and PTPRJ in mice. Platelet count was normal in Chk knockout (KO) mice, reduced by 92% in Chk;Csk double KO (DKO) mice, and partially rescued in Chk;Csk;Ptprj triple KO (TKO) mice. Megakaryocyte numbers were significantly increased in both DKO and TKO mice. Phosphorylation of the inhibitory tyrosine of SFKs was almost completely abolished in DKO platelets, which was partially rescued in Src and Fyn in TKO platelets. This residual phosphorylation was abolished by Src inhibitors, revealing an unexpected mechanism in which SFKs auto-inhibit their activity by phosphorylating their C-terminal tyrosine residues. We demonstrate that reduced inhibitory phosphorylation of SFKs leads to thrombocytopenia with Csk being the dominant inhibitor in platelets, and Chk having an auxiliary role. PTPRJ deletion in addition to Chk and Csk ameliorates the extent of thrombocytopenia, suggesting targeting it may have therapeutic benefits in such conditions.

中文翻译：

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酪氨酸激酶 Chk、Csk 和磷酸酶 PTPRJ 之间的相互作用对于调节小鼠血小板至关重要

Src 家族激酶 (SFK) Src、Lyn 和 Fyn 对血小板活化至关重要，并且还参与巨核细胞 (MK) 发育和血小板生成。血小板 SFK 被 C 端 Src 激酶 (Csk) 抑制，C 端 Src 激酶 (Csk) 磷酸化其 C 端尾部的保守酪氨酸，并被受体型酪氨酸磷酸酶 PTPRJ（CD148、DEP-1）激活，使相同的残基去磷酸化. 小鼠 MK 谱系中 Csk 和 PTPRJ 的缺失会导致 SFK 活性增加，但由于负反馈机制，包括 Csk 同源激酶 (Chk) 表达的上调，血小板活性低下。在这里，我们研究了 Chk 在血小板中的作用、与 Csk 的功能冗余以及在小鼠中消融 Chk、Csk 和 PTPRJ 的生理后果。Chk 敲除 (KO) 小鼠的血小板计数正常，在 Chk;Csk 双 KO (DKO) 小鼠中减少了 92%，并在 Chk;Csk;Ptprj 三重 KO (TKO) 小鼠中部分获救。在 DKO 和 TKO 小鼠中，巨核细胞数量显着增加。SFKs 抑制性酪氨酸的磷酸化在 DKO 血小板中几乎完全消除，在 TKO 血小板中的 Src 和 Fyn 中部分挽救。这种残留的磷酸化被 Src 抑制剂消除，揭示了一种意想不到的机制，即 SFK 通过磷酸化其 C 端酪氨酸残基来自动抑制其活性。我们证明 SFK 的抑制性磷酸化降低导致血小板减少症，其中 Csk 是血小板中的主要抑制剂，而 Chk 具有辅助作用。除 Chk 和 Csk 外，PTPRJ 缺失可改善血小板减少症的程度，