Wnt Signaling Gradients Research Articles

Temporal pattern of the posterior expression of Wingless in Drosophila blastoderm

In most animals, the antero-posterior (A-P) axis requires a gradient of Wnt signaling. Wnts are expressed posteriorly in many vertebrate and invertebrate embryos, forming a gradient of canonical Wnt/β-Catenin activity that is highest in the posterior and lowest in the anterior. One notable exception to this evolutionary conservation is in the Drosophila embryo, in which the A-P axis is established by early transcription factors of maternal origin. Despite this initial axial establishment, Drosophila still expresses Wingless (Wg), the main Drosophila Wnt homologue, in a strong posterior band early in embryogenesis. Since its discovery 30 years ago this posterior band of Wg has been largely ignored. In this study, we re-examined the onset of expression of the Wg posterior band in relation to the expression of Wg in other segments, and compared the timing of its expression to that of axial regulators such as gap and pair-rule genes. It was found that the posterior band of Wg is first detected in blastoderm at mid nuclear cycle 14, before the segment-polarity stripes of Wg are formed in other segments. The onset of the posterior band of Wg expression was preceded by that of the gap gene products Hunchback (hb) and Krüppel (Kr), and the pair-rule protein Even-skipped (Eve). Although the function of the posterior band of Wg was not analyzed in this study, we note that in temperature-sensitive Wg mutants, in which Wg is not properly secreted, the posterior band of Wg expression is diminished in strength, indicating a positive feedback loop required for Wg robust expression at the cellular blastoderm stage. We propose that this early posterior expression could play a role in the refinement of A-P patterning.

Human and Mouse Gastrointestinal Tumor Distribution is Selected According to a Basal Wnt Signalling Gradient

Introduction It is established that Adenomatous polyposis coli (APC) mutations occur non-randomly with respect to each other, resulting in characteristic genotype-phenotype correlations in colorectal cancer (CRC). We have recently compared two mouse polyposis models ( Apc Min and Apc 1322T mice) to demonstrate that ‘just-right’ levels of wnt signalling underpin the severity and distribution of lesions in mouse intestinal neoplasia. We have also shown that human APC mutation spectra varies according to the anatomical distribution of colorectal lesions. We hypothesise that an intestinal gradient of wnt signalling and stem cell number may influence optimal mutation spectra selection. Methods Wnt target gene activity in individual small intestinal and colonic whole mount crypts was assessed using qRT-PCR. Stem cell number was counted using in-situ hybridization for Lgr5 . The regional intestinal effect of increased wnt signalling was examined in the mouse using a stabilized beta catenin transgenic model ( Villin-CreERT2; Catnb lox(ex3)/lox(ex3) ) and the human by comparing upper gastrointestinal (UGI) and colonic APC mutation spectra in FAP and sporadic CRC patients. Results Wnt target gene activity and stem cell number were greatest in the proximal small intestine (SI) decreasing steadily through to the colon. The BMP2 gradient was inversely correlated. The stabilized beta catenin transgenic mice had a striking phenotype with lesion distribution mirroring this gradient. Human FAP associated UGI APC mutations retained more beta-catenin binding 20 amino-acid repeats (AAR) than the same patient9s colonic lesions. Sporadic CRC lesion distribution tended to vary according to the number of 20AAR9s retained with left sided lesions requiring fewer 20AAR9s than right sided. Conclusion The ‘just right’ hypothesis proposes a threshold of wnt signalling should be reached but not exceeded in order for polyp formation to occur. We have demonstrated a physiological SI to colonic gradient of wnt signal and stem cell number. Thus the wnt signal threshold gap varies throughout the intestinal tract. Beta-catenin stabilization in the mouse provides sufficient wnt signal perturbation to initiate tumour formation in the small intestine but is insufficient in the colon. The physiological wnt gradient determines the distribution of sporadic and polyposis-associated tumours by selecting for the optimal mutation spectra in each lesion – thus left sided lesions select for common mutations that provide a greater wnt perturbation than their more proximal counterparts.

Human and mouse gastrointestinal tumour distribution is selected according to a basal intestinal wnt signalling gradient

<h3>ABSTRACT</h3> Genomic imprinting results in gene expression biased by parental chromosome of origin and occurs in genes with important roles during human brain development. However, the cell-type and temporal specificity of imprinting during human neurogenesis is generally unknown. By detecting within-donor allelic biases in chromatin accessibility and gene expression that are unrelated to cross-donor genotype, we inferred imprinting in both primary human neural progenitor cells (phNPCs) and their differentiated neuronal progeny from up to 85 donors. We identified 43/20 putatively imprinted regulatory elements (IREs) in neurons/progenitors, and 133/79 putatively imprinted genes in neurons/progenitors. Though 10 IREs and 42 genes were shared between neurons and progenitors, most imprinting was only detected within specific cell types. In addition to well-known imprinted genes and their promoters, we inferred novel IREs and imprinted genes. We found IREs overlapped with CpG islands more than non-imprinted regulatory elements. Consistent with DNA methylation-based regulation of imprinted expression, some putatively imprinted regulatory elements also overlapped with differentially methylated regions on the maternal germline. Finally, we identified a progenitor-specific putatively imprinted gene overlap with copy number variation that is associated with uniparental disomy-like phenotypes. Our results can therefore be useful in interpreting the function of variants identified in future parent-of-origin association studies.

Wnt Signaling Gradients Establish Planar Cell Polarity by Inducing Vangl2 Phosphorylation through Ror2

It is fundamentally important that signaling gradients provide positional information to govern morphogenesis of multicellular organisms. Morphogen gradients can generate different cell types in specific spatial order at distinct threshold concentrations. However, it is largely unknown whether and how signaling gradients also control cell polarities by acting as global cues. Here, we show that Wnt signaling gradient provides directional information to a field of cells. Vangl2, a core component in planar cell polarity, forms Wnt-induced receptor complex with Ror2 to sense Wnt dosages. Wnts dose-dependently induce Vangl2 phosphorylation of serine/threonine residues and Vangl2 activities depend on its levels of phosphorylation. In the limb bud, Wnt5a signaling gradient controls limb elongation by establishing PCP in chondrocytes along the proximal-distal axis through regulating Vangl2 phosphorylation. Our studies have provided new insight to Robinow syndrome, Brachydactyly Type B1, and spinal bifida which are caused by mutations in human ROR2, WNT5A, or VANGL.

A β-catenin gradient links the clock and wavefront systems in mouse embryo segmentation

Rhythmic production of vertebral precursors, the somites, causes bilateral columns of embryonic segments to form. This process involves a molecular oscillator--the segmentation clock--whose signal is translated into a spatial, periodic pattern by a complex signalling gradient system within the presomitic mesoderm (PSM). In mouse embryos, Wnt signalling has been implicated in both the clock and gradient mechanisms, but how the Wnt pathway can perform these two functions simultaneously remains unclear. Here, we use a yellow fluorescent protein (YFP)-based, real-time imaging system in mouse embryos to demonstrate that clock oscillations are independent of beta-catenin protein levels. In contrast, we show that the Wnt-signalling gradient is established through a nuclear beta-catenin protein gradient in the posterior PSM. This gradient of nuclear beta-catenin defines the size of the oscillatory field and controls key aspects of PSM maturation and segment formation, emphasizing the central role of Wnt signalling in this process.

A dynamic gradient of Wnt signaling controls initiation of neurogenesis in the mammalian cortex and cellular specification in the hippocampus

Neurogenesis in the developing neocortex is a strictly regulated process of cell division and differentiation. Here we report that a gradual retreat of canonical Wnt signaling in the cortex from lateral-to-medial and anterior-to-posterior is a prerequisite of neurogenesis. Ectopic expression of a beta-catenin/LEF1 fusion protein maintains active canonical Wnt signaling in the developing cortex and delays the expression onset of the neurogenic factors Pax6, Ngn2 and Tbr2 and subsequent neurogenesis. Contrary to this, conditional ablation of beta-catenin accelerates expression of the same neurogenic genes. Furthermore, we show that a sustained canonical Wnt activity in the lateral cortex gives rise to cells with hippocampal characteristics in the cortical plate at the expense of the cortical fate, and to cells with dentate gyrus characteristics in the hippocampus. This suggests that the dose of canonical Wnt signaling determines cellular fate in the developing cortex and hippocampus, and that recession of Wnt signaling acts as a morphogenetic gradient regulating neurogenesis in the cortex.