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

Pronephric tubulogenesis requires Daam1-mediated planar cell polarity signaling

Canonical β-catenin-mediated Wnt signaling is essential for the induction of nephron development. Noncanonical Wnt/planar cell polarity (PCP) pathways contribute to processes such as cell polarization and cytoskeletal modulation in several tissues. Although PCP components likely establish the plane of polarization in kidney tubulogenesis, whether PCP effectors directly modulate the actin cytoskeleton in tubulogenesis is unknown. Here, we investigated the roles of Wnt PCP components in cytoskeletal assembly during kidney tubule morphogenesis in Xenopus laevis and zebrafish. We found that during tubulogenesis, the developing pronephric anlagen expresses Daam1 and its interacting Rho-GEF (WGEF), which compose one PCP/noncanonical Wnt pathway branch. Knockdown of Daam1 resulted in reduced expression of late pronephric epithelial markers with no apparent effect upon early markers of patterning and determination. Inhibiting various points in the Daam1 signaling pathway significantly reduced pronephric tubulogenesis. These data indicate that pronephric tubulogenesis requires the Daam1/WGEF/Rho PCP pathway.

Do plants and animals differ in phenotypic plasticity?

This paper compares the flexibility in the nexus between phenotype and genotype in plants and animals. These taxa although considered to be fundamentally different are found to be surprisingly similar in the mechanisms used to achieve plasticity. Although non-cognitive behaviour occurs in plants, its range is limited, while morphological and developmental plasticity also occur to a considerable extent in animals. Yet both plants and animals are subject to unique constraints and thus need to find unique solutions to functional problems. A true comparison between the plant and animal phenotype would be a comparison between plants and sessile photosynthesizing colonial invertebrates. Such comparisons are lacking. However, they would provide important insights into the adaptive significance of plasticity in these groups. It is also suggested that a comparison of inflexible traits in these groups would provide an understanding of the constraints, as well as the costs and benefits,of a plastic versus non-plastic phenotype in plants and animals.

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.

Inhibitor of the tissue-specific transcription factor HNF4, a potential regulator in early Xenopus development

Hepatocyte nuclear factor 4alpha (HNF4alpha) is an orphan receptor of the nuclear receptor superfamily and expressed in vertebrates as a tissue-specific transcription factor in liver, kidney, intestine, stomach, and pancreas. It also plays a crucial role in early embryonic development and has been identified as a maternal component in the Xenopus egg. We now report on an activity present in Xenopus embryos that inhibits the DNA binding of HNF4. This HNF4 inhibitor copurifies with a 25-kDa protein under nondenaturing conditions but can be separated from this protein by sodium dodecyl sulfate treatment. Protease treatment of the inhibitor results in a core fragment of about 5 kDa that retains full inhibitory activity. The activity of the HNF4 inhibitor can also be monitored in the absence of DNA, as it alters the mobility of the HNF4 protein in native polyacrylamide gels and the accessibility of antibodies. Comparing the activity of the HNF4 inhibitor with acyl coenzyme A's, recently proposed to be ligands of HNF4, we observe a more stringent specificity for the HNF4 inhibitor activity. Using deletion constructs of the HNF4 protein, we could show that the potential ligand-binding domain of HNF4 is not required, and thus the HNF4 inhibitor does not represent a classical ligand as defined for the nuclear receptor superfamily. Based on our previous finding that maternal HNF4 is abundantly present in Xenopus embryos but the target gene HNF1alpha is only marginally expressed, we propose that the HNF4 inhibitor functions in the embryo to restrict the activity of the maternal HNF4 proteins.

The mutated human gene encoding hepatocyte nuclear factor 1beta inhibits kidney formation in developing Xenopus embryos

The transcription factor hepatocyte nuclear factor 1beta (HNF1beta) is a tissue-specific regulator that also plays an essential role in early development of vertebrates. In humans, four heterozygous mutations in the HNF1beta gene have been identified that lead to early onset of diabetes and severe primary renal defects. The degree and type of renal defects seem to depend on the specific mutation. We show that the frameshift mutant P328L329fsdelCCTCT associated with nephron agenesis retains its DNA-binding properties and acts as a gain-of-function mutation with increased transactivation potential in transfection experiments. Expression of this mutated factor in the Xenopus embryo leads to defective development and agenesis of the pronephros, the first kidney form of amphibians. Very similar defects are generated by overexpressing in Xenopus the wild-type HNF1beta, which is consistent with the gain-of-function property of the mutant. In contrast, introduction of the human HNF1beta mutant R137-K161del, which is associated with a reduced number of nephrons with hypertrophy of the remaining ones and which has an impaired DNA binding, shows only a minor effect on pronephros development in Xenopus. Thus, the overexpression of both human mutants has a different effect on renal development in Xenopus, reflecting the variation in renal phenotype seen with these mutations. We conclude that mutations in human HNF1beta can be functionally characterized in Xenopus. Our findings imply that HNF1beta not only is an early marker of kidney development but also is functionally involved in morphogenetic events, and these processes can be investigated in lower vertebrates.

Regulation of the human Na(+)-glucose cotransporter gene, SGLT1, by HNF-1 and Sp1

The Na(+)-glucose cotransporter (SGLT1) is expressed primarily by small intestinal epithelial cells and transports the monosaccharides glucose and galactose across the apical membrane. Here we describe the isolation and characterization of 5.3 kb of the 5'-flanking region of the SGLT1 gene by transiently transfecting reporter constructs into a variety of epithelial cell lines. A fragment (nt -235 to +22) of the promoter showed strong activity in the intestinal cell line Caco-2 but was inactive in a nonintestinal epithelial cell line (Chinese hamster ovary). Within this region, three cis-elements, a hepatocyte nuclear factor-1 (HNF-1) and two GC box sites are critical for maintaining the gene's basal level of expression. The two GC boxes bind to several members of the Sp1 family of transcription factors and, in the presence of HNF-1, synergistically upregulate transactivation of the promoter. A novel 16-bp element just downstream of one GC box was also shown to influence the interaction of Sp1 to its binding site. In summary, we report the identification and characterization of the human SGLT1 minimal promoter and the critical role that HNF-1 and Sp1-multigene members have in enhancing the basal level of its transcription in Caco-2 cells.

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.

Molecular characterization of the Drosophila melanogaster gene encoding the pterin 4alpha-carbinolamine dehydratase

We have isolated and characterized the cDNA and the genomic DNA encoding Drosophila melanogaster pterin 4alpha-carbinolamine dehydratase (PCD). The amino acid sequence deduced from the cDNA sequence was very similar to those of PCDs previously reported in other species (19-57% identity). The protein coding region of the cDNA was expressed in E. coli as a histidine fusion protein, and the expressed protein proved to have PCD activity. The characterization of the Drosophila genomic clone revealed that the Drosophila PCD gene is interrupted by two introns. The potential promoter region, deduced from the determination of the transcription start point (tsp), lacks the distinct TATAAA box consensus sequence.

An alternative splice variant of the tissue specific transcription factor HNF4alpha predominates in undifferentiated murine cell types

The transcription factor hepatocyte nuclear factor 4alpha (HNF4alpha) is a tissue specific transcription factor mainly expressed in the liver, kidney, intestine and the endocrine pancreas, but is also an essential regulator for early embryonic events. Based on its protein structure HNF4alpha is classified as an orphan member of the nuclear receptor superfamily. Comparing HNF4alpha transcription factors in the differentiated and dedifferentiated murine hepatocyte cell line MHSV-12 we identified in dedifferentiated cells the novel splice variant HNF4alpha7. This variant is characterized by an alternative first exon and has a lower transactivation potential in transient transfection assays using HNF4 dependent reporter genes. HNF4alpha7 mRNA and the corresponding protein are expressed in the undifferentiated pluripotent embryonal carcinoma cell line F9, whereas HNF4alpha1 only appears after differentiation of F9 cells to visceral endoderm. HNF4alpha7 mRNA is also found in totipotent embryonic stem cells. However, the function of HNF4alpha7 seems not to be restricted to embryonic cells as the HNF4alpha7 mRNA is also present in adult tissues, most notably the stomach. All these features suggest that the presence of distinct splice variants of HNF4alpha modulates the activity of HNF4alphain a cell type specific way.