Abstract
BackgroundThe 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.ResultsWe 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.ConclusionsThe 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.
Highlights
The three distinct types of kidneys, pronephros, mesonephros and metanephros, develop consecutively in vertebrates
Induction of mRNAs encoding the early nephrogenic transcription factors osr1, osr2, hnf1b, lhx1 and pax8 in animal caps treated with retinoic acid and/or activin A Since simultaneous treatment of Xenopus animal caps with 10 ng/ml of activin A and 10-4 M retinoic acid for three hours induces differentiation of pronephric tissues [15], we explored the time dependent induction of the early nephrogenic transcription factors osr1, osr2, hnf1b, lhx1 and pax8 in these embryonic explants
Whereas osr1, osr2, hnf1b and lhx1 mRNAs were induced within 1.5 hours, pax8 mRNA was only increased after thirteen hours
Summary
The three distinct types of kidneys, pronephros, mesonephros and metanephros, develop consecutively in vertebrates. During vertebrate development three kidney types of increasing complexity (pronephros, mesonephros and metanephros) form successively from the intermediate mesoderm, located between the paraxial mesoderm (developing somites) and the lateral plate [1]. All components of the pronephros arise from intermediate mesoderm, but the signals that direct pattering of the presumptive pronephric mesoderm towards pronephric lineages are unknown. Experiments showed that the anterior somites are crucial for pronephros development and provide an essential first signal. If the anterior somites are removed [3] or separated from the presumptive pronephros [4], pronephroi do not form. Anterior somites can induce pronephric tubules in unspecified intermediate mesoderm [3]. The exact timing and nature of the signal provided by the anterior somites are yet unknown, wnt11b expressed throughout the anterior somites has recently been shown as a crucial signal [5]
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