Department of Medical & Molecular Genetics, King's College London, London, UK.
Randall Centre for Cell and Molecular Biophysics, King's College London, London, UK; Asthma UK Centre in Allergic Mechanisms of Asthma, London, UK.
J Biol Chem. 2021 Jan-Jun;296:100513. doi: 10.1016/j.jbc.2021.100513. Epub 2021 Mar 4.
The C1q and TNF related 4 (C1QTNF4) protein is a structurally unique member of the C1QTNF family, a family of secreted proteins that have structural homology with both complement C1q and the tumor necrosis factor superfamily. C1QTNF4 has been linked to the autoimmune disease systemic lupus erythematosus through genetic studies; however, its role in immunity and inflammation remains poorly defined and a cell surface receptor of C1QTNF4 has yet to be identified. Here we report identification of nucleolin as a cell surface receptor of C1QTNF4 using mass spectrometric analysis. Additionally, we present evidence that the interaction between C1QTNF4 and nucleolin is mediated by the second C1q-like domain of C1QTNF4 and the C terminus of nucleolin. We show that monocytes and B cells are target cells of C1QTNF4 and observe extensive binding to dead cells. Imaging flow cytometry experiments in monocytes show that C1QTNF4 becomes actively internalized upon cell binding. Our results suggest that nucleolin may serve as a docking molecule for C1QTNF4 and act in a context-dependent manner through coreceptors. Taken together, these findings further our understanding of C1QTNF4's function in the healthy immune system and how dysfunction may contribute to the development of systemic lupus erythematosus.
C1q 和 TNF 相关蛋白 4（C1QTNF4）是 C1QTNF 家族的一个结构独特的成员，C1QTNF 家族是一类具有结构同源性的分泌蛋白，与补体 C1q 和肿瘤坏死因子超家族都有关联。通过遗传研究发现，C1QTNF4 与自身免疫性疾病系统性红斑狼疮有关；然而，其在免疫和炎症中的作用仍未得到明确界定，且 C1QTNF4 的细胞表面受体尚未被鉴定。在这里，我们通过质谱分析报告了核仁素是 C1QTNF4 的细胞表面受体。此外，我们还提供了证据表明，C1QTNF4 与核仁素之间的相互作用是由 C1QTNF4 的第二个 C1q 样结构域和核仁素的 C 末端介导的。我们表明单核细胞和 B 细胞是 C1QTNF4 的靶细胞，并观察到其与死细胞的广泛结合。在单核细胞中的成像流式细胞术实验表明，C1QTNF4 在细胞结合后会被主动内化。我们的结果表明，核仁素可能作为 C1QTNF4 的对接分子，并通过共受体以依赖于上下文的方式发挥作用。总之，这些发现进一步了解了 C1QTNF4 在健康免疫系统中的功能，以及功能障碍如何导致系统性红斑狼疮的发展。
