Drews Christiane, Senkel Sabine, Ryffel Gerhart U
Institut für Zellbiologie (Tumorforschung) Universitätsklinikum Essen, Universität Duisburg-Essen, Hufelandstrasse 55, 45122 Essen, Germany.
BMC Dev Biol. 2011 Jan 31;11:5. doi: 10.1186/1471-213X-11-5.
The three distinct types of kidneys, pronephros, mesonephros and metanephros, develop consecutively in vertebrates. The earliest form of embryonic kidney, the pronephros, is derived from intermediate mesoderm and the first expressed genes localized in the pronephros anlage are the transcription factors osr1, osr2, hnf1b, lhx1 and pax8, here referred to as the early nephrogenic transcription factors. However, the pathway inducing nephrogenesis and the network of theses factors are poorly understood. Treatment of the undifferentiated animal pole explant (animal cap) of Xenopus with activin A and retinoic acid induces pronephros formation providing a powerful tool to analyze key molecular events in nephrogenesis.
We have investigated the expression kinetics of the early nephrogenic transcription factors in activin A and retinoic acid treated animal caps and their potential to induce pronephric differentiation. In treated animal caps, expression of osr1, osr2, hnf1b and lhx1 are induced early, whereas pax8 expression occurs later implying an indirect activation. Activin A alone is able to induce osr2 and lhx1 after three hours treatment in animal caps while retinoic acid fails to induce any of these nephrogenic transcription factors. The early expression of the five transcription factors and their interference with pronephros development when overexpressed in embryos suggest that these factors potentially induce nephrogenesis upon expression in animal caps. But no pronephros development is achieved by either overexpression of OSR1, by HNF1B injection with activin A treatment, or the combined application of LHX1 and PAX8, although they influenced the expression of several early nephrogenic transcription factors in some cases. In an additional approach we could show that HNF1B induces several genes important in nephrogenesis and regulates lhx1 expression by an HNF1 binding site in the lhx1 promoter.
The early nephrogenic transcription factors play an important role in nephrogenesis, but have no pronephros induction potential upon overexpression in animal caps. They activate transcriptional cascades that partially reflect the gene activation initiated by activin A and retinoic acid. Significantly, HNF1B activates the lhx1 promoter directly, thus extending the known activin A regulation of the lhx1 gene via an activin A responsive element.
脊椎动物体内的前肾、中肾和后肾这三种不同类型的肾脏是相继发育形成的。胚胎期最早出现的肾脏形式——前肾，源自中间中胚层，在前肾原基中最早表达的基因是转录因子osr1、osr2、hnf1b、lhx1和pax8，这里将它们称为早期肾发生转录因子。然而，诱导肾发生的信号通路以及这些因子的网络关系目前仍知之甚少。用激活素A和视黄酸处理非洲爪蟾未分化的动物极外植体（动物帽）可诱导前肾形成，这为分析肾发生过程中的关键分子事件提供了一个有力工具。
我们研究了激活素A和视黄酸处理后的动物帽中早期肾发生转录因子的表达动力学及其诱导前肾分化的潜力。在处理后的动物帽中，osr1、osr2、hnf1b和lhx1的表达早期即被诱导，而pax8的表达较晚出现，这意味着其激活是间接的。单独用激活素A处理动物帽3小时后就能诱导osr2和lhx1的表达，而视黄酸则无法诱导这些肾发生转录因子中的任何一个。这五个转录因子的早期表达以及它们在胚胎中过表达时对前肾发育的干扰表明，这些因子在动物帽中表达时可能诱导肾发生。但是，无论是过表达OSR1、注射HNF1B并同时用激活素A处理，还是联合应用LHX1和PAX8，均未实现前肾发育，尽管在某些情况下它们影响了几种早期肾发生转录因子的表达。在另一种方法中，我们发现HNF1B可诱导几个在肾发生中起重要作用的基因，并通过lhx1启动子中的HNF1结合位点调节lhx1的表达。
早期肾发生转录因子在肾发生中起重要作用，但在动物帽中过表达时没有诱导前肾形成的潜力。它们激活转录级联反应，部分反映了由激活素A和视黄酸引发的基因激活。值得注意的是，HNF1B直接激活lhx1启动子，从而扩展了已知的激活素A通过激活素A反应元件对lhx1基因的调控。
