Identification and characterization of functional modules in Lef/Tcf

在过去的几年里，我们证实了我们的假设，即单个LEF/Tcf家族成员调节细胞类型对典型WNT信号的特异性反应。同时，人们普遍认为，简单地将Lef/Tcf转录因子结合为Wnt/b-catenin途径的核转导因子的经典观点忽略了细胞和组织特异性反应的许多方面。虽然我们确定Hic-5(Ghogomu等人，2006)是新的亚型特异性结合伙伴，但亚型特异性靶基因调控的许多方面仍然没有答案。同样，包括HIC1、HBP和PIAS在内的其他新的Tcf结合伙伴仅部分解释了亚型特异性。相反，新的结果表明，表观遗传机制是Tcf亚型、启动子和细胞类型特异性调控的主要调控原则。随着XCIRP被确定为XTcf-3特异性靶基因，XTcf-4通过其启动子上的Lef/Tcf结合位点调控，以及通过添加额外的直接WNT靶基因，我们现在有了系统分析Tcf亚型特异性靶基因调控表观遗传学机制的工具。从进化的角度来看，有趣的是，脊椎动物只表达一个Tcf，而脊椎动物表达四个Lef/Tcf。我们之前对非洲爪哇Tcf亚型特异性功能的分析使我们现在能够将早期胚胎发育中的至少一个特定方面分配给一个不同的Lef/Tcf家族成员。因此，在重建实验中，我们将能够分配古老的LEF/Tcf函数。这种方法将帮助我们确定对不同功能重要的区域/域。应进一步确定这些领域的特征。

In the previous years we confirmed our hypothesis that individual Lef/Tcf family members regulate the cell-type specific response to canonical wnt-signaling. Meanwhile, it is widely accepted that the “classical view” to simply unite Lef/Tcf transcription factors as nuclear transducer of the wnt/b-catenin pathway ignores many aspects of cell- and tissue specific responses. Although we identified Hic-5 (Ghogomu et al., 2006) as novel subtype specific binding partner, many aspects of subtype specific target gene regulation remain unanswered. Similarly, other novel Tcf-binding partners including HIC1, HBP and PIAS explain the subtype-specificity only partially. Instead, novel results indicate epigenetic mechanisms as major regulatory principle for Tcf-subtype-, promoter- and cell-type specific regulation. With the identification of Xcirp as XTcf-3 specific target gene, the regulation of XTcf-4 via a Lef/Tcf binding site on its promoter and by including additional direct wnt target genes we have now the tools in our hands for a systematic analysis of epigenetic mechanisms for Tcf subtype specific target gene regulation.From an evolutionary point of view, it is interesting that evertebrates express only one Tcf while vertebrates express four Lef/Tcfs. Our previous analyses on Tcf-subtype specific functions in Xenopus allows us now to assign at least one specific aspect in early embryogenesis to one distinct Lef/Tcf family member. Thus, in reconstitution experiments we will be able to allocate ancient Lef/Tcf functions. This approach will help us to identify regions/domains important for the distinct functions. These domains shall be further characterized.