PDZ-mediated signaling through syndecan-4

跨膜受体的羧基端与接头蛋白的保守的PDZ结合模块之间的相互作用已成为组织信号转导蛋白复合物的主要机制。跨膜蛋白聚糖家族属于这一大群结合PDZ的细胞表面受体。直到最近，人们普遍认为蛋白聚糖的功能集中在它们的硫酸肝素链上，它是参与生长因子和细胞外基质蛋白结合的活性成分。然而，由于新发现表明，syndecan跨膜蛋白聚糖家族的高度保守的细胞质尾部也参与了外向内信号的转导，这一观点正在经历修订。虽然所有的syndecan都有一个结合pdz的羧基端，但广泛表达的syndecan-4在其细胞质尾部含有一个独特的磷脂酰肌醇4,5-二磷酸结合域，并促进蛋白激酶C α的激活。我们实验室最近的研究结果表明，syndecan-4的细胞质尾部与含有PDZ结构域的蛋白之间的相互作用对于细胞对碱性成纤维细胞生长因子的反应至关重要，这种相互作用被破坏时内皮细胞的迁移、增殖和血管网络形成受到损害。本提案的目的是通过PDZ-syndecan-4相互作用阐明信号传导机制的本质，采用最近发现的syndecan-4 PDZ伴侣连接蛋白作为原型。这种相互作用是一种新的FGF信号通路的一部分，并形成了信号转导调节的新范式。我们将重点关注(1)表征syndecan-4与syndecan-4之间的结合机制，(2)识别除syndecan-4以外的其他syndecan-4结合伙伴，以确定syndecan-4介导的信号通路的进一步下游成员，以及(3)通过连接蛋白基因敲除小鼠模型表征连接蛋白的一般功能，特别是内皮细胞特异性功能。

The interaction between the carboxy terminus of transmembrane receptors with the conserved PDZ binding module of adaptor proteins has emerged as a major mechanism of organizing signal-transduction protein complexes. The syndecan family of transmembrane proteoglycans belongs to this large group of PDZ- binding cell surface receptors. Until recently, the generally held concept of proteoglycan functions was centered on their heparan sulfate chains as the active component involved in binding of growth factors and of extracellular matrix proteins. This view is undergoing a revision, however, as a consequence of newly emerging findings, which suggest that the highly conserved cytoplasmic tails of the syndecan family of transmembrane proteoglycans also participate in the transduction of outside-in signals. While all the syndecans possess a PDZ-binding carboxy- terminus, the widely expressed syndecan-4 contains a unique phosphatidylinositol 4,5-bisphosphate binding domain in its cytoplasmic tail, and facilitates the activation of protein kinase C alpha. Recent findings from our laboratory indicate that the interaction between the cytoplasmic tail of syndecan-4 and PDZ domain-containing protein(s) is essential for the cellular response to basic fibroblast growth factor, as demonstrated by the impairment of migration, of proliferation, and of vascular network formation by endothelial cells upon the disruption of this interaction. The objective of this proposal is to elucidate the nature of the signaling mechanism through the PDZ-syndecan-4 interaction, employing the recently identified syndecan-4 PDZ partner synectin as a prototype. This interaction is part of a novel FGF signaling pathway, and forms a new paradigm for the regulation of signal transduction. We will focus on (1) characterizing the binding mechanism between syndecan-4 and synectin, (2) identifying additional synectin binding partners other than syndecan-4, in order to determine further downstream members of the syndecan-4-mediated signaling pathway, and (3) characterize the functions of synectin in general, and the endothelial cell-specific ones in particular, by a synectin gene knockout mouse model.