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Background The dual-specificity T-box/basic helix-loop-helix leucine zipper transcription factor MGA is part of the MAX-interacting network of proteins. In the mouse, MGA is necessary for the survival of the pluripotent epiblast cells of the peri-implantation embryo and a null, gene-trap allele MgaGt results in embryonic lethality shortly after implantation. We have used this allele to document expression of Mga in postimplantation embryos and also investigated a second, hypomorphic gene-trap allele, MgaInv. Results Compound heterozygotes, MgaGt/MgaInv, die prior to midgestation. The extraembryonic portion of the embryos appears to develop relatively normally while the embryonic portion, including the pluripotent cells of the epiblast, is severely retarded by E7.5. Mga expression is initially limited to the pluripotent inner cell mass of the blastocyst and epiblast, but during organogenesis it is widely expressed, notably in the central nervous system and sensory organs, reproductive and excretory systems, heart, somites and limbs. Conclusions Widespread yet specific areas of expression of Mga during organogenesis raise the possibility that the transcription factor may play roles in controlling proliferation and potency in the progenitor cell populations of different organ systems. Documentation of these patterns sets the stage for the investigation of specific progenitor cell types.

The mammalian spleen has important functions in immunity and haematopoiesis but little is known about the events that occur during its early embryonic development. Here we analyse the origin of the cells that gives rise to the splenic mesenchyme and the process by which the precursors assume their position along the left lateral side of the stomach. We report a highly conserved regulatory element that regulates the Nkx2-5 gene throughout early spleen development. A transgenic mouse line carrying this element driving a reporter gene was used to show that morphogenesis of the spleen initiates bilaterally and posterior to the stomach, before the splenic precursors grow preferentially leftward. In addition the transgenic line was used in an organ culture system to track spleen precursor cells during development. Spleen cells were shown to move from the posterior mesenchyme and track along the left side of the stomach. Removal of tissue from the anterior stomach resulted in splenic cells randomly scattering suggesting a guidance role for the anterior stomach. Using a mouse line carrying a conditional Cre recombinase to mark early precursor cell populations, the spleen was found to derive from posterior mesenchyme distinct from the closely adjacent stomach mesenchyme.

In comparison with the molecular genetics of melanogenesis in mammals, the regulation of pigmentation in poikilothermic vertebrates is poorly understood. Mammals undergo morphological colour change under hormonal control, but strikingly, many lower vertebrates display a rapid physiological colour change in response to the same hormones. The recent provision of extensive genome sequencing data from teleost zebrafish, Danio rerio, provides the opportunity to define the genes and proteins mediating this physiological pigment response and characterise their function biologically. Here, we illustrate the background adaptation process in adults and larvae and describe a novel assay to visualize and directly quantify the rate of zebrafish melanophore pigment translocation in unprecedented detail. We demonstrate the resolution of this assay system; quantifying the zebrafish melanophore response to melanin-concentrating and melanocyte-stimulating hormones. Furthermore, we investigate the intracellular signalling downstream of hormone stimulation and the biomechanical processes involved in zebrafish pigment translocation, confirming the importance of cyclic adenosine monophosphate (cAMP) as a mediator of pigment translocation and finding intact microtubules are essential for both melanin dispersion and aggregation in zebrafish, but that microfilament disruption affects aggregation only. In conclusion, we propose these data establish the zebrafish as an experimental model for studying both physiological colour change and the molecular basis of pigment translocation.

A number of Wnt genes are expressed during, and are known to be essential for, early kidney development. It is typically assumed that their products will act through the canonical β-catenin signalling pathway. We have found evidence that suggests canonical Wnt signalling is not active in the early nephrogenic metanephric mesenchyme, but instead provide expressional and functional evidence that implicates the non-canonical Calcium/NFAT Wnt signalling pathway in nephrogenesis. Members of the NFAT (Nuclear Factor Activated in T cells) transcription factor gene family are expressed throughout murine kidney morphogenesis and NFATc3 is localised to the developing nephrons. Treatment of kidney rudiments with Cyclosporin A (CSA), an inhibitor of Calcium/NFAT signalling, decreases nephron formation — a phenotype similar to that in Wnt4−/− embryos. Treatment of Wnt4−/− kidneys with Ionomycin, an activator of the pathway, partially rescues the phenotype. We propose that the non-canonical Calcium/NFAT Wnt signalling pathway plays an important role in early mammalian renal development and is required for complete MET during nephrogenesis, potentially acting downstream of Wnt4., Research Highlights ► No β-catenin activity can be detected in the early nephrogenic mesenchyme. ► Expressional and functional data implicate Ca2+/NFAT Wnt signalling in nephrogenesis. ► Disruption of the Ca2+/NFAT pathway disrupts kidney morphogenesis. ► Increased Ca2+ signalling partially rescues nephrogenesis in Wnt4 knockout mice. ► We propose Ca2+/NFAT signalling as a novel regulator of nephrogenesis.

Signaling by GDNF through the Ret receptor tyrosine kinase is required for the normal formation, growth and branching of the ureteric bud (UB) during kidney development. However, the precise role of GDNF/Ret signaling in this process, and the specific responses of UB cells to GDNF, remain to be fully elucidated. Recent studies provide new insight into the effects of Ret signaling on cell behavior, the functional overlap between GDNF and other growth factors, and the genes functioning downstream of Ret. Lineage studies show that the UB tip cells, which express Ret, are the progenitors for UB growth, while GDNF-expressing mesenchymal cells are the progenitors of nephron epithelia. Time-lapse studies of chimeric embryos reveal that the earliest role of Ret signaling is in the Wolffian duct, where it promotes cell movements that give rise to the first ureteric bud tip. The requirement for GDNF/Ret signaling can be largely relieved by removing the negative regulator Sprouty1, implicating other growth factors, in particular FGF10, in the support of UB growth and branching. However, the kidneys that develop in the absence of GDNF/Ret and Sprouty1 display branching abnormalities, suggesting a unique role for GDNF in determining UB branching pattern. A number of genes whose expression is induced in the UB by GDNF has been identified, including two ETS transcription factors Etv4 and Etv5. These genes are required downstream of Ret for the Wolffian duct cell movements that form the UB tip domain, as well as for later UB growth and branching.

Summary Wt1 regulates the epithelial-mesenchymal transition (EMT) in the epicardium and the reverse process (MET) in kidney mesenchyme. The mechanisms underlying these reciprocal functions are unknown. Here, we show in both embryos and cultured cells that Wt1 regulates Wnt4 expression dichotomously. In kidney cells, Wt1 recruits Cbp and p300 as coactivators; in epicardial cells it enlists Basp1 as a corepressor. Surprisingly, in both tissues, Wt1 loss reciprocally switches the chromatin architecture of the entire Ctcf-bounded Wnt4 locus, but not the flanking regions; we term this mode of action "chromatin flip-flop." Ctcf and cohesin are dispensable for Wt1-mediated chromatin flip-flop but essential for maintaining the insulating boundaries. This work demonstrates that a developmental regulator coordinates chromatin boundaries with the transcriptional competence of the flanked region. These findings also have implications for hierarchical transcriptional regulation in development and disease.