Formins (FHOS) in CD21-mediated signaling & virus entry

描述（由申请人提供）：人CD21是一种先天免疫系统受体（C3dg受体或补体受体2，CR2），是获得性免疫反应的关键调节剂。CD21也是EBV的高亲和力受体。CD21在广泛的人类疾病的发病机制中起直接作用，包括自身免疫性疾病、传染病（EBV和HIV）和癌症。在B细胞上，CD21与主要信号转导调节因子CD19复合物，CD21是否能独立于CD19发出信号尚存争议。尽管CD21早在20多年前就被发现，并且作为C3dg， CD23和EBV的附着蛋白的独特作用已经被发现，但使CD21在细胞外部和内部之间直接传递信息的确切机制尚不清楚。在初步研究中，FHOS/FHQD是formin家族的一员，在体外和体内都被发现与人CD21的细胞质结构域结合。当在上皮细胞（CD19阴性）中表达时，EGFP-FHOS定位于细胞质并与肌动蛋白聚集在膜突起中。值得注意的是，当上皮细胞上的CD21与EBV或抗CD21连接时，FHOS易位到质膜上，并与CD21聚集在一起共定位。Formins是一种支架蛋白，通过将信号转导与细胞骨架重排和基因转录联系起来，调节细胞极性和形态发生。最近，形成蛋白被发现通过不同于经典的arp2,3复合物的途径直接成核肌动蛋白。基于这些观察结果，我们提出配体刺激CD21-FHOS相互作用通过双胍（fhov介导的细胞骨架重组）指导的途径传导细胞内信号。Aim I的目标是鉴定介导CD21和FHOS相互作用的氨基酸残基，并筛选新的FHOS/CD21结合蛋白作为该途径的潜在下游效应物。Aim II的重点是确定FHOS是否直接成核肌动蛋白，并评估CD21在FHOS激活、肌动蛋白聚合和信号传导中的作用。Aim III的目的是分离和表征脾边缘细胞，这是一种在人红髓中丰富的特化细胞，高度表达FHOS， CD21，并被认为在抗原过滤中起作用。这些目标的实现将提供关于CD21如何发出信号的重要新知识，并将阐明最近发现的双胍蛋白（FHOS）在免疫系统中的作用。这些研究可能会导致发现治疗自身免疫性疾病和癌症的新靶点，开发疫苗和根除被CD21隔离的持久性病毒和细菌病原体。

DESCRIPTION (provided by applicant): Human CD21 is an innate immune system receptor (C3dg receptor or complement receptor type 2, CR2) and is a key modulator of the acquired immune response. CD21 is also the high affinity receptor for EBV. CD21 plays a direct role in the pathogenesis of a broad spectrum of human disease, including autoimmune disease, infectious disease (EBV and HIV) and cancer. On B cells, CD21 complexes with the major signal transduction regulator CD19, and it is controversial whether CD21 can signal independently of CD19. Although CD21 was identified more than 20 years ago and distinct roles as an attachment protein for C3dg, CD23 and EBV have been discovered, the precise mechanisms enabling CD21 to directly relay information between the exterior and the interior of the cell are unresolved. In Preliminary Studies, FHOS/FHQD a member of the formin family, was found to bind the cytoplasmic domain of human CD21 both in vitro and in vivo. When expressed in epithelial cells (CD19 negative), EGFP-FHOS localized to the cytoplasm and accumulated with actin in membrane protrusions. Remarkably, when CD21 on epithelial cells was ligated with either EBV or anti-CD21, FHOS translocated to the plasma membrane, where it co-localized in aggregates with CD21. Formins are scaffolding proteins that regulate cell polarity and morphogenesis by linking signal transduction to cytoskeletal rearrangement and gene transcription. Recently, formins were discovered to directly nucleate actin by a pathway distinct from the classic Arp 2,3 complex. Based on these observations it is proposed that ligand stimulation of CD21-FHOS interaction transduces intracellular signals through a pathway directed by formin (FHOSV-mediated cytoskeletal reorganization). The goals of Aim I are to identify the amino acid residues that mediate interaction between CD21 and FHOS and to screen for novel FHOS/CD21-binding proteins as potential downstream effectors of this pathway. Aim II is focused on determining whether FHOS directly nucleates actin, and assessing the role of CD21 in FHOS activation, actin polymerization, and signaling. The goals of Aim III are to isolate and characterize the splenic littoral cell, a specialized cell abundant in human red pulp that highly expresses FHOS, CD21 and is believed to function in antigen filtration. Achievement of these Aims will provide important new knowledge about how CD21 signals and will elucidate the role(s) of the recently discovered formin protein, FHOS, in the immune system. These investigations may lead to identification of novel targets for treatment of autoimmune disease and cancer, for development of vaccines and for eradication of persistent viral and bacterial pathogens that are sequestered by CD21.