Intracellular Signaling by the Precursor B Cell Receptor

描述(申请人提供)：由前体B细胞受体(PreBCR)转导的信号是B细胞发育所必需的。前BCR组分遗传缺陷的人类和小鼠患有缺乏或缺乏前B细胞和更成熟的B细胞，这是导致无丙种球蛋白血症和由此导致的严重免疫缺陷的原因。下游前BCR信号转导分子失控或功能丧失突变与B细胞白血病和淋巴瘤有关。前BCR信号的一个神秘方面是受体信号被激活的生理方式。在细胞培养系统中已经证实了配体依赖和配体非依赖机制的证据，并被认为是生理学的。这两个模型的共同点是，受体在细胞表面的定位是至关重要的，无论是与那里的配体或关键信号转导分子结合，因为表面表达与受体活性直接相关。与这个模型相反，我们最近证明了前BCR复合体离开细胞膜，重新分布在细胞内，内质网后的隔间，与野生型前BCR一样或几乎一样活跃。这是通过在形成前BCR的免疫球蛋白MU重链(Ig HC)的细胞质尾部附加内膜定位序列，并在前BCR缺陷的转化和原代PRO-B细胞中测试由此产生的重定向的前BCR活性来完成的。这些数据支持这样的模型，即前BCR不需要细胞表面的细胞外配体来进行信号传递，并且前BCR在内质网后隔区(如高尔基体)是活跃的。然而，一些与前BCR信号相关的发育重要过程，如等位基因排斥，无法评估，而且这些过程仍然可能需要依赖于前BCR表面表达的活动。因此，这个R03小提案中的实验目的是通过评估这些重定向的前BCR复合体在体内的生物活性来建立这些发现的基础上。这将通过建立表达重定向免疫球蛋白HCS的转基因小鼠并评估它们支持B细胞发育的能力以及在遗传前BCR缺陷和前BCR充足的小鼠中实现等位基因排斥的能力来实现。这些信息对于更好地理解在正常免疫发育和B细胞病理中决定受体活性的分子机制将是重要的。这项计划中的实验旨在了解一种被称为前BCR的关键分子是如何控制B细胞的发育的，B细胞是制造免疫反应必不可少的抗体的细胞。遗传上不能产生前BCR的人是严重的免疫缺陷。所获得的信息是更好地理解一些B细胞白血病的起源所必需的，这些B细胞白血病可以追溯到影响前BCR功能的突变。

DESCRIPTION (provided by applicant): Signals transduced by the precursor B cell receptor (preBCR) are necessary for B cell development. Humans and mice genetically deficient in preBCR components suffer from an absence or paucity of pre-B and more mature B cells that is responsible for a state of agammaglobulinemia and resultant severe immune deficiency. Deregulating or loss-of-function mutations of downstream preBCR signal transduction molecules are associated with B cell leukemia and lymphoma. An enigmatic aspect of preBCR signaling is the physiological means by which receptor signaling is activated. Evidence for both ligand-dependent and ligand-independent mechanisms has been demonstrated in cell culture systems and suggested to be physiological. Common to both models has been the idea that receptor localization to the cell surface is critical, either to engage ligands or key signal transduction molecules that reside there, because surface expression has directly correlated with receptor activity. Contrary to this model, we have recently demonstrated that preBCR complexes directed away from the cell membrane and redistributed among intracellular, post-ER compartments were as or nearly as active as wild-type preBCRs. This was accomplished by appending endomembrane localization sequences to the cytoplasmic tail of a preBCR-forming immunoglobulin mu heavy chain (Ig HC), and testing the resulting "redirected" preBCR activity in preBCR-deficient transformed and primary pro-B cells. These data support the model that preBCRs do not require extracellular ligands at the cell surface for signaling, and that the preBCR is active in post-ER compartments like the Golgi. However, several developmentally important processes linked to preBCR signaling, such as allelic exclusion, could not be evaluated, and it remains possible that these require activities dependent on preBCR surface expression. Therefore, the purpose of the experiments in this small R03 proposal is to build on these findings by evaluating the biological activity of these redirected preBCR complexes in vivo. This will be accomplished in one Aim by establishing transgenic mice that express the redirected Ig HCs and evaluating their ability to support B cell development and effect allelic exclusion in genetically preBCR-deficient and preBCR-sufficient mice. This information will be important to better understand the molecular mechanisms that determine receptor activity in normal immune development and in B cell pathologies. The experiments in this proposal aim to understand how a critical molecule known as the preBCR controls the development of B cells, the cells that make antibodies essential for an immune response. People that are genetically unable to make a preBCR are severely immune deficient. The information obtained is needed to better understand the origin of some B cell leukemias that can be traced to mutations that affect the functioning of the preBCR.