The different signaling capabilities of soluble and membrane-bound TWEAK and their relevance for cellular proliferation and differentiation

TWEE(肿瘤坏死因子样的弱诱导细胞凋亡)及其受体Fn14(成纤维细胞生长因子诱导的14)调节多种信号通路的活性，控制一系列细胞功能，特别是在组织损伤的背景下。事实上，TWEAK/Fn14-系统与组织修复和再生有关，但它也会促进过度刺激和慢性炎症/再生反应所产生的不利影响。TWEAK以单跨膜蛋白的形式存在，但也以可溶性配体的形式存在，通过蛋白质降解处理从膜结合的分子中释放出来。在此之前，我们证明了可溶性的TWEEP与Fn14具有高亲和力，并强烈地激活了替代的NF-kappaB途径，增强了TNFR1诱导的细胞死亡，但也拮抗了TRAF2介导的其他肿瘤坏死因子受体的信号转导。有趣的是，我们还注意到，可溶性TWEE并不能刺激经典的核因子-kappaB途径的强烈激活。相反，膜调节可以高效地触发所有与Fn14相关的通路，包括经典的NF-kappaB通路。虽然我们已经有一些初步证据表明，不同的反式激活TRAF2-cIAP1和TRAF2-cIAP2复合体的能力对可溶性和膜TWINE信号能力的不同起着至关重要的作用，但其潜在的机制仍不清楚。此外，这两种调整形式在刺激其他Fn14相关信号通路和细胞功能方面是否也存在差异的问题到目前为止还没有解决。因此，我们将专注于以下问题：1.可溶性TWEEP诱导的Fn14信号复合体与膜TWAKE诱导的Fn14信号复合体有何不同，以及TWEEP/Fn14介导的胞浆TRAF2-cIAP1和TRAF2-cIAP2复合体的缺失如何影响死亡受体诱导的RIP/caspase-8复合体的形成、修饰和稳定性？2.Fn14是否具有独立于TRAF2的信号转导能力？3.到目前为止，可溶性和膜TWAKE对细胞增殖和分化的影响有何不同，哪些信号通路在这些背景下是相关的？我们观察到Fn14在所有表达Fn14的细胞系中对可溶性和膜调节的不同反应性(n&GT；10)和原代细胞类型(n=3)。因此，必须假设潜在的生化机制在大多数细胞中起作用，并对Fn14生物学具有普遍重要性。此外，定义Fn14对可溶性和膜调节的不同反应性的分子机制也可能适用于肿瘤坏死因子家族的其他TRAF2相互作用的受体。因此，在TWEEP/Fn14信号转导实例中发现的分子机制可能对整个肿瘤坏死因子受体超家族具有普遍的相关性。

TWEAK (Tumor necrosis factor (TNF)-like weak inducer of apoptosis) and its receptor Fn14 (fibroblast growth factor (FGF)-induced 14) regulate the activity of a variety of signaling pathways and control a diverse panel of cellular functions especially in the context of tissue injury. Indeed, the TWEAK/Fn14-system has been implicated in tissue repair and regeneration but it also promotes adverse effects that can emerge from overshooting and chronic inflam-matory/regenerative responses.TWEAK occurs as a single spanning transmembrane protein but is also found in form of a soluble ligand that is released from the membrane-bound molecule by proteolytic processing. Previously, we demonstrated that soluble processed TWEAK binds with high affinity to Fn14 and strongly triggers activation of the alternative NF-kappaB pathway and enhancement of TNFR1-induced cell death but also antagonizes TRAF2-mediated signaling by other TNF re-ceptors. Intriguingly, we also noted that soluble TWEAK does not stimulate robust activation of the classical NF-kappaB pathway. In contrast, membrane TWEAK triggers all Fn14-associated pathways with high efficacy including the classical NF-kappaB pathway. Although we have some initial evidence that the different ability to transactivate TRAF2- cIAP1 and TRAF2-cIAP2 complexes plays a crucial role for the varying signaling capacities of soluble and membrane TWEAK, the underlying mechanisms have still to be considered as poorly understood. Moreover, the question whether the two TWEAK forms also differ in their ability to stimulate other Fn14 associated signaling pathways and cellular functions has not been addressed so far. We will therefore focus in the project on the following questions:1. How does the soluble TWEAK-induced Fn14 signaling complex differ from that induced by membrane TWEAK, and how affects TWEAK/Fn14-mediated depletion of cytosolic TRAF2-cIAP1 and TRAF2-cIAP2 complexes the formation, modification and stability of death recep-tor-induced RIP/caspase-8 containing complexes?2. Does Fn14 have the ability to signal independently from TRAF2?3. How do soluble and membrane TWEAK differ in their effects on cellular proliferation and differentiation and which signaling pathways are of relevance in these contexts?So far, we observed the differential responsiveness of Fn14 to soluble and membrane TWEAK in all Fn14-expressing cell lines (n>10) and primary cell types (n=3) we have investi-gated. Thus, it must be assumed that the underlying biochemical mechanisms are operative in most cells and are of general importance for Fn14 biology. Moreover, it is possible that the molecular mechanisms that define the differential responsiveness of Fn14 to soluble and membrane TWEAK also applies to other TRAF2-interacting receptors of the TNF family. The molecular mechanisms that are identified on the example of TWEAK/Fn14 signaling have therefore the potential to be of general relevance for the TNF receptor superfamily as a whole.