Protein 4.1R, a member of the 4.1 family, functions as a bridge between cytoskeletal and plasma membrane proteins. It is expressed in T cells, where it binds to a linker for activation of T cell (LAT) family member 1 and inhibits its phosphorylation and downstream signaling events after T cell receptor triggering. The role of the 4.1R protein in cell activation through other immunoreceptors is not known. In this study, we used 4.1R-deficient (4.1R-KO) and 4.1R wild-type (WT) mice and explored the role of the 4.1R protein in the high-affinity IgE receptor (FcεRI) signaling in mast cells. We found that bone marrow mast cells (BMMCs) derived from 4.1R-KO mice showed normal growth in vitro and expressed FcεRI and c-KIT at levels comparable to WT cells. However, 4.1R-KO cells exhibited reduced antigen-induced degranulation, calcium response, and secretion of tumor necrosis factor-α. Chemotaxis toward antigen and stem cell factor (SCF) and spreading on fibronectin were also reduced in 4.1R-KO BMMCs, whereas prostaglandin E2-mediated chemotaxis was not affected. Antibody-induced aggregation of tetraspanin CD9 inhibited chemotaxis toward antigen in WT but not 4.1R-KO BMMCs, implying a CD9-4.1R protein cross-talk. Further studies documented that in the absence of 4.1R, antigen-mediated phosphorylation of FcεRI β and γ subunits was not affected, but phosphorylation of SYK and subsequent signaling events such as phosphorylation of LAT1, phospholipase Cγ1, phosphatases (SHP1 and SHIP), MAP family kinases (p38, ERK, JNK), STAT5, CBL, and mTOR were reduced. Immunoprecipitation studies showed the presence of both LAT1 and LAT2 (LAT, family member 2) in 4.1R immunocomplexes. The positive regulatory role of 4.1R protein in FcεRI-triggered activation was supported by in vivo experiments in which 4.1R-KO mice showed the normal presence of mast cells in the ears and peritoneum, but exhibited impaired passive cutaneous anaphylaxis. The combined data indicate that the 4.1R protein functions as a positive regulator in the early activation events after FcεRI triggering in mast cells.

中文翻译：

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细胞骨架蛋白4.1R是肥大细胞中FcεRI信号传导和趋化性的正调节剂。

蛋白质4.1R是4.1家族的成员，在细胞骨架蛋白和质膜蛋白之间起着桥梁的作用。它在T细胞中表达，在此处与激活T细胞（LAT）家族成员1的接头结合，并在触发T细胞受体后抑制其磷酸化和下游信号传导事件。4.1R蛋白在通过其他免疫受体激活细胞中的作用尚不清楚。在这项研究中，我们使用了4.1R缺陷（4.1R-KO）和4.1R野生型（WT）小鼠，并探讨了4.1R蛋白在肥大细胞中高亲和力IgE受体（FcεRI）信号传导中的作用。我们发现，源自4.1R-KO小鼠的骨髓肥大细胞（BMMC）在体外显示正常生长，并以与WT细胞相当的水平表达FcεRI和c-KIT。但是，4.1R-KO细胞显示出降低的抗原诱导的脱粒，钙反应，和肿瘤坏死因子-α的分泌。在4.1R-KO BMMC中，对抗原和干细胞因子（SCF）的趋化性以及在纤连蛋白上的扩散也有所减少，而前列腺素E2介导的趋化性则不受影响。抗体诱导的四跨膜蛋白CD9聚集抑制了WT中抗原的趋化性，但没有抑制4.1R-KO BMMC的趋化性，这意味着CD9-4.1R蛋白相互干扰。进一步的研究表明，在没有4.1R的情况下，FcεRIβ和γ亚基的抗原介导的磷酸化不受影响，但是SYK的磷酸化和随后的信号传递事件，例如LAT1，磷脂酶Cγ1，磷酸酶（SHP1和SHIP），MAP的磷酸化家族激酶（p38，ERK，JNK），STAT5，CBL和mTOR降低。免疫沉淀研究表明在4.1R免疫复合物中同时存在LAT1和LAT2（LAT，家族成员2）。体内实验支持4.1R蛋白在FcεRI触发的激活中的积极调节作用，在该实验中，4.1R-KO小鼠在耳朵和腹膜中显示出肥大细胞的正常存在，但是被动皮肤过敏反应减弱。综合数据表明，在肥大细胞中，FcεRI触发后，4.1R蛋白在早期激活事件中起正调节剂的作用。