Whyburn Lindsey R, Halcomb Kristina E, Contreras Cristina M, Pappu Rajita, Witte Owen N, Chan Andrew C, Satterthwaite Anne B
Simmons Arthritis Research Center, Department of Internal Medicine and Center for Immunology, UT Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75390, USA.
Int Immunol. 2003 Mar;15(3):383-92. doi: 10.1093/intimm/dxg034.
Current models of lymphocyte activation suggest that formation of a signaling complex, or "signalosome", composed of Syk, Bruton's tyrosine kinase (Btk), phospholipase gamma2 and the adaptor protein B cell linker protein (BLNK) is critical for transmission of signals from the BCR. However, impaired B cell development in mice lacking each individual signalosome component has made it difficult to study the functional consequences of the formation of this complex in mature B cells. Sensitized genetic systems, commonly used in Drosophila, define signaling pathways by combining partial loss of function mutations in the components of interest. This allows genetic interactions to be observed in the absence of pleiotropic or lethal effects of complete deficiency of either gene. We used this approach to demonstrate that Btk and BLNK are limiting components of a common signaling pathway that mediates the mitogenic response of mature B cells to antigen. B cells from transgenic mice expressing a limiting dosage of Btk (Btk(lo)) have normal numbers of mature B cells that have reduced, but measurable, responses to BCR cross-linking. Haploinsufficiency of BLNK did not affect the development of Btk(lo) B cells. However, it exacerbated their defects in BCR-induced Ca(2+) flux, IkappaB degradation, and up-regulation of cyclin D2, bcl-x(L) and A1 leading to dramatic impairment of B cell mitogenic responses. In contrast, no effect of reduced Btk and BLNK dosage was observed on extracellular signal-regulated kinase activation. These results suggest that the signals regulating the maintenance and activation of mature B cells are differentially sensitive to the strength of the signal emanating from the signalosome.
目前的淋巴细胞激活模型表明，由脾酪氨酸激酶（Syk）、布鲁顿酪氨酸激酶（Btk）、磷脂酶γ2和衔接蛋白B细胞连接蛋白（BLNK）组成的信号复合物或“信号小体”的形成对于BCR信号的传递至关重要。然而，缺乏单个信号小体成分的小鼠中B细胞发育受损，这使得研究该复合物在成熟B细胞中形成的功能后果变得困难。常用于果蝇的敏感遗传系统通过组合感兴趣成分中的部分功能丧失突变来定义信号通路。这使得在不存在任何一个基因完全缺失的多效性或致死效应的情况下能够观察到基因相互作用。我们使用这种方法来证明Btk和BLNK是共同信号通路的限制成分，该通路介导成熟B细胞对抗原的促有丝分裂反应。表达有限剂量Btk（Btk(lo)）的转基因小鼠的B细胞中成熟B细胞数量正常，但对BCR交联的反应有所降低但仍可测量。BLNK的单倍剂量不足并不影响Btk(lo) B细胞的发育。然而，它加剧了它们在BCR诱导的Ca(2+)通量、IkappaB降解以及细胞周期蛋白D2、bcl-x(L)和A1上调方面的缺陷，导致B细胞促有丝分裂反应严重受损。相比之下，未观察到Btk和BLNK剂量降低对细胞外信号调节激酶激活有影响。这些结果表明，调节成熟B细胞维持和激活的信号对信号小体发出的信号强度具有不同的敏感性。
