Genomic structure of the Xenopus laevis liver transcription factor LFB1

The HNF1β transcription factor has several domains involved in nephrogenesis and partially rescues Pax8/lim1-induced kidney malformations

The tissue-specific transcription factors HNF1alpha and HNF1beta are closely related homeodomain proteins conserved in vertebrate evolution. Heterozygous mutations in human HNF1beta but not in HNF1alpha genes are associated with kidney malformations. Overexpression of HNF1beta in Xenopus embryos leads to defective pronephros development, while HNF1alpha has no effect. We have defined the regions responsible for this functional difference between HNF1beta and HNF1alpha in transfected HeLa cells as well as in injected Xenopus embryos. Using domain swapping experiments, we located a nuclear localization signal in the POUH domain of HNF1beta, and showed that the POUS and POUH domains of HNF1beta mediate a high transactivation potential in transfected cells. In injected Xenopus embryos three HNF1beta domains are involved in nephrogenesis. These include the dimerization domain, the 26 amino acid segment specific for splice variant A as well as the POUH domain. As HNF1beta together with Pax8 and lim1 constitute the earliest regulators in the pronephric anlage, it is possible that they cooperate during early nephrogenesis. We have shown here that HNF1beta can overcome the enlargement and the induction of an ectopic pronephros mediated by overexpression of Pax8 and lim1. However, the phenotype induced by Pax8 and lim1 overexpression and characterized by cyst-like structures and thickening of the pronephric tubules was not altered by HNF1beta overexpression. Taken together, HNF1beta acts antagonistically to Pax8 and lim1 in only some processes during nephrogenesis, and a simple antagonistic relationship does not completely describe the functions of these genes. We conclude that HNF1beta has some distinct morphogenetic properties during nephrogenesis.

Distinct promoter elements mediate endodermal and mesodermal expression of the HNF1alpha promoter in transgenic Xenopus

The gene encoding the tissue specific transcription factor HNF1alpha is expressed in vertebrates in tissues of endodermal origin such as the liver and the gut as well as in the kidney, a mesoderm derived organ. Using a 6 kb HNF1alpha promoter fragment linked to GFP we observed green fluorescence in transgenic embryos restricted to the liver and gut as well as to the pronephros, the embryonic kidney. By deletion and mutation analysis of the HNF1alpha promoter we succeeded in dissecting the HNF1alpha promoter into two entities that are either active in the endoderm or the mesoderm. In conclusion, our data establish that the generation of transgenic Xenopus allows the functional dissection of promoters in the context of the entire organism.

Patterning the expression of a tissue-specific transcription factor in embryogenesis: HNF1 alpha gene activation during Xenopus development

Tissue-specific transcription factors play an essential role in establishing cell identity during development. We review our knowledge of the molecular events involved in the activation of the gene encoding the tissue-specific transcription factor HNF1 alpha (LFB1). The available data suggest that the maternal factors OZ-1, HNF4 alpha and HNF4 beta act as initial activators of the HNF1 alpha promoter. We present evidence suggesting that the mesoderm-inducing factor activin A plays a critical role by acting through the HNF4 binding site of the HNF1 alpha promoter. The activity of this embryonic morphogen seems to form a gradient opposing the distribution of the maternal HNF4 proteins that are concentrated at the animal pole of the egg. After zygotic gene transcription the HNF1 alpha-related transcription factor HNF1 beta accumulates faster than HNF1 alpha itself and thus is likely to contribute to the activation of the HNF1 alpha transcription via the HNF1 binding site. The cofactor of the HNF1 proteins (DCoH) is present throughout development and thus cannot limit the activation potential of HNF1 alpha in early development. Our results provide a detailed description of setting up the expression pattern of a tissue-specific transcription factor during embryogenesis.

Checklist: vertebrate homeobox genes

Up to now around 170 different homeobox genes have been cloned from vertebrate genomes. A compilation of the various isolates from mouse, chick, frog, fish and man is presented in the form of a concise checklist, including the designations from the original publications. Putative homologs from different species are aligned, and key characteristics of embryonic or adult expression domains, as well as mutant phenotypes are briefly indicated.

Two independent and interactive DNA-binding subdomains of the Pax6 paired domain are regulated by alternative splicing

Vertebrate Pax proteins share a conserved 128-amino-acid DNA-binding motif, the paired domain. The PAX6 gene, which is mutated in the murine Small eye and human aniridia developmental defects, also encodes a second protein with a 14-amino-acid insertion in the paired domain. This protein, which arises by alternative mRNA splicing, exhibits unique DNA-binding properties. Unlike other paired domains, which bind DNA predominantly by their amino termini, the extended Pax6 paired domain interacts with DNA exclusively through its carboxyl terminus. This property can be stimulated by deletion of 30 amino-terminal residues from the Pax6 or Pax2 paired domains. Thus, the insertion acts as a molecular toggle to unmask the DNA-binding potential of the carboxyl terminus. The functional nonequivalence of the two Pax6 proteins is underscored by a T-->C mutation at position -3 of the alternative splice acceptor site that changes the ratio of the two isoforms and causes a distinct human ocular syndrome.