Left-right asymmetry in gut development: what happens next?

The gastrointestinal tract is an asymmetrically patterned organ system. The signals which initiate left-right asymmetry in the developing embryo have been extensively studied, but the downstream steps required to confer asymmetric morphogenesis on the gut organ primordia are less well understood. In this paper we outline key findings on the tissue mechanics underlying gut asymmetry, across a range of species, and use these to synthesise a conserved model for asymmetric gut morphogenesis. We also discuss the importance of correct establishment of left-right asymmetry for gut development and the consequences of perturbations in this process.

The dynamics of spleen morphogenesis.

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.

Postimplantation Mga expression and embryonic lethality of two gene-trap alleles.

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.

Integrated ß-catenin, BMP, PTEN, and Notch signalling patterns the nephron.

The different segments of the nephron and glomerulus in the kidney balance the processes of water homeostasis, solute recovery, blood filtration, and metabolite excretion. When segment function is disrupted, a range of pathological features are presented. Little is known about nephron patterning during embryogenesis. In this study, we demonstrate that the early nephron is patterned by a gradient in ß-catenin activity along the axis of the nephron tubule. By modifying ß-catenin activity, we force cells within nephrons to differentiate according to the imposed ß-catenin activity level, thereby causing spatial shifts in nephron segments. The ß-catenin signalling gradient interacts with the BMP pathway which, through PTEN/PI3K/AKT signalling, antagonises ß-catenin activity and promotes segment identities associated with low ß-catenin activity. ß-catenin activity and PI3K signalling also integrate with Notch signalling to control segmentation: modulating ß-catenin activity or PI3K rescues segment identities normally lost by inhibition of Notch. Our data therefore identifies a molecular network for nephron patterning.

Detection of ß-galactosidase activity: X-gal staining.

X-gal staining is a rapid and convenient histochemical technique used to detect reporter gene expression. A prerequisite is the creation or acquisition of transgenic reporter mouse lines, in which the bacterial LacZ gene has been knocked into the gene of interest or placed under the control of regulatory elements corresponding to the gene of interest. Expression is marked by a dark blue stain and can be detected at the single cell level, providing a robust visual readout of gene expression in the developing kidney. Here, we describe the methodology, applications, and limitations of this technique.

A wt1-controlled chromatin switching mechanism underpins tissue-specific wnt4 activation and repression.

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.

High-efficiency Rosa26 knock-in vector construction for Cre-regulated overexpression and RNAi.

INTRODUCTION: Rosa26 is a genomic mouse locus commonly used to knock-in cDNA constructs for ubiquitous or conditional gene expression in transgenic mice. However, the vectors generally used to generate Rosa26 knock-in constructs show instability problems, which have a severe impact on the efficiency of the system. RESULTS: We have optimized the cloning procedure to generate targeting vectors for Cre-regulated expression of constructs within several days with minimal hands-on time, thereby enabling high-throughput approaches. We demonstrate that transient expression of Cre still results in expression of the construct, as shown by the expression level and via functional assays. In addition to its well-established possibilities in expressing cDNA constructs, we show that the Rosa26 locus can be used to drive expression of functional miRNA constructs from its endogenous promoter. CONCLUSION: We provide a new high-efficiency cloning system for Rosa26 knock-in constructs to express either cDNA or miRNA fragments. Our system will enable high-throughput approaches for controlled expression of cDNA or miRNA constructs, with the latter providing a potential high-speed alternative for conditional knock-out models.

Regulation of pigmentation in zebrafish melanophores.

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.