Institute for Molecular Bioscience (IMB) and IMB Centre for Inflammation and Disease Research, The University of Queensland; Brisbane, Queensland, Australia.
School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, Queensland, Australia.
J Biol Chem. 2022 May;298(5):101857. doi: 10.1016/j.jbc.2022.101857. Epub 2022 Mar 22.
Innate immune signaling by Toll-like receptors (TLRs) involves receptor phosphorylation, which helps to shape and drive key inflammatory outputs, yet our understanding of the kinases and mechanisms that mediate TLR phosphorylation is incomplete. Spleen tyrosine kinase (Syk) is a nonreceptor protein tyrosine kinase, which is known to relay adaptive and innate immune signaling, including from TLRs. However, TLRs do not contain the conserved dual immunoreceptor tyrosine-based activation motifs that typically recruit Syk to many other receptors. One possibility is that the Syk-TLR association is indirect, relying on an intermediary scaffolding protein. We previously identified a role for the palmitoylated transmembrane adapter protein SCIMP in scaffolding the Src tyrosine kinase Lyn, for TLR phosphorylation, but the role of SCIMP in mediating the interaction between Syk and TLRs has not yet been investigated. Here, we show that SCIMP recruits Syk in response to lipopolysaccharide-mediated TLR4 activation. We also show that Syk contributes to the phosphorylation of SCIMP and TLR4 to enhance their binding. Further evidence pinpoints two specific phosphorylation sites in SCIMP critical for its interaction with Syk-SH2 domains in the absence of immunoreceptor tyrosine-based activation motifs. Finally, using inhibitors and primary macrophages from SCIMP mice, we confirm a functional role for SCIMP-mediated Syk interaction in modulating TLR4 phosphorylation, signaling, and cytokine outputs. In conclusion, we identify SCIMP as a novel, immune-specific Syk scaffold, which can contribute to inflammation through selective TLR-driven inflammatory responses.
Toll 样受体 (TLR) 的先天免疫信号涉及受体磷酸化，这有助于塑造和驱动关键的炎症反应，但我们对介导 TLR 磷酸化的激酶和机制的理解还不完整。Spleen tyrosine kinase (Syk) 是一种非受体蛋白酪氨酸激酶，已知它能传递适应性和先天免疫信号，包括来自 TLR 的信号。然而，TLR 并不包含保守的双免疫受体酪氨酸基激活基序，这些基序通常将 Syk 招募到许多其他受体上。一种可能性是，Syk-TLR 关联是间接的，依赖于中间支架蛋白。我们之前发现了棕榈酰化跨膜衔接蛋白 SCIMP 在支架Src 酪氨酸激酶 Lyn 进行 TLR 磷酸化中的作用，但 SCIMP 在介导 Syk 与 TLR 之间相互作用中的作用尚未得到研究。在这里，我们表明 SCIMP 招募 Syk 以响应脂多糖介导的 TLR4 激活。我们还表明，Syk 有助于 SCIMP 和 TLR4 的磷酸化，以增强它们的结合。进一步的证据指出了 SCIMP 中两个特定的磷酸化位点，对于其在没有免疫受体酪氨酸基激活基序的情况下与 Syk-SH2 结构域相互作用至关重要。最后，使用 SCIMP 小鼠的抑制剂和原代巨噬细胞，我们证实了 SCIMP 介导的 Syk 相互作用在调节 TLR4 磷酸化、信号转导和细胞因子产生中的功能作用。总之，我们将 SCIMP 鉴定为一种新的、免疫特异性的 Syk 支架，它可以通过选择性的 TLR 驱动的炎症反应来促进炎症。
