Wnt/Notch signalling and information processing during development

The Wnt and Notch signalling pathways represent two major channels of communication used by animal cells to control their identities and behaviour during development. A number of reports indicate that their activities are closely intertwined during embryonic development. Here, we review the evidence for this relationship and suggest that Wnt and Notch ('Wntch') signalling act as components of an integrated device that, rather than defining the fate of a cell, determines the probability that a cell will adopt that fate.     

Expression of Wnt signalling pathway genes during tooth development.

We have carried out comparative in situ hybridisation analysis of six Wnt genes Wnts-3, -4, -5a, -6, -7b, and 10b together with Wnt receptor MFz6 and receptor agonist/antagonists MFrzb1 and Mfrp2 during murine odontogenesis from the earliest formation of the epithelial thickening to the early bell stage. Expression of Wnt-4, Wnt-6, and one Wnt receptor MFz6 was observed in the facial, oral and dental epithelium. Wnt10b was localised specifically to the presumptive dental epithelium. Wnts-3 and -7b were expressed in oral epithelium but showed no expression in the presumptive dental epithelium. Wnt-3 also showed no expression in the epithelial cells of the molar bud stage tooth germs, but showed restricted expression in the enamel knots which are signalling centres believed to be involved in regulating tooth shape. Wnts -6, -10b and MFz6 were also detected in the primary and secondary enamel knots. Wnt-5a and agonist/antagonists MFrzb1 and Mfrp2 were expressed in a graded proximo-distal (P-D) manner in mesenchymal cells during the early stages of tooth development with no overlying expression in the oral or dental epithelium. Wnt-5a and MFrzb1 show strong expression in the dental papilla mesenchyme.     

Bmp,Fgf and Wnt signalling in programmed cell death and chondrogenesis during vertebrate limb development: the role of Dickkopf-1

Dickkopf-1 (Dkk-1) is a potent head inducer in Xenopus. This effect can be attributed to its capability to specifically inhibit Wnt/beta-catenin signalling. Recent data point to a crucial role for Dkk-1 in the control of programmed cell death during vertebrate limb development. In this paper, we present a comparative expression analysis of Dkk-1, Bmp-4 and Sox-9 as well as data on the regulation of Dkk-1 by Wnt. Finally, we summarize the current knowledge of its potential function in the developing limb and present a model how the interplay of the Bmp, Fgf and Wnt signalling pathways might differentially regulate programmed cell death versus chondrogenic differentiation in limb mesodermal cells.     

Extracellular modulation of the Hedgehog, Wnt and TGF-beta signalling pathways during embryonic development.

Localized embryonic expression of members of the Hedgehog, Wnt and TGF-beta families of secreted factors has been shown to organize pattern and provide positional information in many developing systems. Recently, several extracellular factors have been described that act either as facilitators or inhibitors of the activities of those secreted proteins. The variety of molecular strategies involved in the extracellular modulation of signalling activities in the embryo underscores the importance of maintaining a tight spatial and temporal control of the activities of organizing centers during development.     

Wnt signaling during cochlear development

Wnt signaling is a hallmark of all embryonic development with multiple roles at multiple developmental time points. Wnt signaling is also important in the development of several organs, one of which is the inner ear, where it participates in otic specification, the formation of vestibular structures, and the development of the cochlea. In particular, we focus on Wnt signaling in the auditory organ, the cochlea. Attempting to dissect the multiple Wnt signaling pathways in the mammalian cochlea is a challenging task due to limited expression data, particularly at proliferating stages. To offer predictions about Wnt activity, we compare cochlear development with that of other biological systems such as Xenopus retina, brain, cancer cells and osteoblasts. Wnts are likely to regulate development through crosstalk with other signaling pathways, particularly Notch and FGF, leading to changes in the expression of Sox2 and proneural (pro-hair cell) genes. In this review we have consolidated the known signaling pathways in the cochlea with known developmental roles of Wnts from other systems to generate a potential timeline of cochlear development.     

Wnt and FGF signals interact to coordinate growth with cell fate specification during limb development

A fundamental question in developmental biology is how does an undifferentiated field of cells acquire spatial pattern and undergo coordinated differentiation? The development of the vertebrate limb is an important paradigm for understanding these processes. The skeletal and connective tissues of the developing limb all derive from a population of multipotent progenitor cells located in its distal tip. During limb outgrowth, these progenitors segregate into a chondrogenic lineage, located in the center of the limb bud, and soft connective tissue lineages located in its periphery. We report that the interplay of two families of signaling proteins, fibroblast growth factors (FGFs) and Wnts, coordinate the growth of the multipotent progenitor cells with their simultaneous segregation into these lineages. FGF and Wnt signals act together to synergistically promote proliferation while maintaining the cells in an undifferentiated, multipotent state, but act separately to determine cell lineage specification. Withdrawal of both signals results in cell cycle withdrawal and chondrogenic differentiation. Continued exposure to Wnt, however, maintains proliferation and re-specifies the cells towards the soft connective tissue lineages. We have identified target genes that are synergistically regulated by Wnts and FGFs, and show how these factors actively suppress differentiation and promote growth. Finally, we show how the spatial restriction of Wnt and FGF signals to the limb ectoderm, and to a specialized region of it, the apical ectodermal ridge, controls the distribution of cell behaviors within the growing limb, and guides the proper spatial organization of the differentiating tissues.     

The Wnt antagonist Frzb-1 regulates chondrocyte maturation and long bone development during limb skeletogenesis

The Wnt antagonist Frzb-1 is expressed during limb skeletogenesis, but its roles in this complex multistep process are not fully understood. To address this issue, we determined Frzb-1 gene expression patterns during chick long bone development and carried out gain- and loss-of-function studies by misexpression of Frzb-1, Wnt-8 (a known Frzb-1 target), or different forms of the intracellular Wnt mediator LEF-1 in developing limbs and cultured chondrocytes. Frzb-1 expression was quite strong in mesenchymal prechondrogenic condensations and then characterized epiphyseal articular chondrocytes and prehypertrophic chondrocytes in growth plates. Virally driven Frzb-1 misexpression caused shortening of skeletal elements, joint fusion, and delayed chondrocyte maturation, with consequent inhibition of matrix mineralization, metalloprotease expression, and marrow/bone formation. In good agreement, misexpression of Frzb-1 or a dominant-negative form of LEF-1 in cultured chondrocytes maintained the cells at an immature stage. Instead, misexpression of Wnt-8 or a constitutively active LEF-1 strongly promoted chondrocyte maturation, hypertrophy, and calcification. Immunostaining revealed that the distribution of endogenous Wnt mediator beta-catenin changes dramatically in vivo and in vitro, from largely cytoplasmic in immature proliferating and prehypertrophic chondrocytes to nuclear in hypertrophic mineralizing chondrocytes. Misexpression of Frzb-1 prevented beta-catenin nuclear relocalization in chondrocytes in vivo or in vitro. The data demonstrate that Frzb-1 exerts a strong influence on limb skeletogenesis and is a powerful and direct modulator of chondrocyte maturation, phenotype, and function. Phases of skeletogenesis, such as terminal chondrocyte maturation and joint formation, appear to be particularly dependent on Wnt signaling and thus very sensitive to Frzb-1 antagonistic action.     

Calcium signalling during embryonic development

Consider a hypothetical design specification for an integrated communication-control system within an embryo. It would require short-range (subcellular) and long-range (pan-embryonic) abilities, it would have to be flexible and, at the same time, robust enough to operate in a dynamically changing environment without information being lost or misinterpreted. Although many signalling elements appear, disappear and sometimes reappear during development, it is becoming clear that embryos also depend on a ubiquitous, persistent and highly versatile signalling system that is based around a single messenger, Ca2+.     

En1 and Wnt7a interact with Dkk1 during limb development in the mouse

Wnt signaling plays an essential role in induction and development of the limb. Missing digits are one consequence of the reduced Wnt signaling in Wnt7a null mice, while extra digits result from excess Wnt signaling in mice null for the Wnt antagonist Dkk1. The extra digits and expanded apical ectodermal ridge (AER) of Dkk1-deficient mice closely resemble En1 null mice. To evaluate the in vivo interaction between En1 and the canonical Wnt signaling pathway, we generated double and triple mutants combining the hypomorphic doubleridge allele of Dkk1 with null alleles of En1 and Wnt7a. Reducing Dkk1 expression in Dkk1d/+Wnt7a-/- double mutants prevented digit loss, indicating that Wnt7a acts through the canonical pathway during limb development. Reducing Dkk1 levels in Dkk1d/dEn1-/- double mutants resulted in severe phenotypes not seen in either single mutant, including fused bones in the autopod, extensive defects of the zeugopod, and loss of the ischial bone. The subsequent elimination of Wnt7a in Dkk1d/dEn1-/-Wnt7a-/- triple mutants resulted in correction of most, but not all, of these defects. The failure of Wnt7a inactivation to completely correct the limb defects of Dkk1d/dEn1-/- double mutants indicates that Wnt7a is not the only gene regulated by En1 during development of the mouse limb.     

Wnt signalling escapes to cilia

The primary cilium is proposed to restrain the level of canonical Wnt signalling, but it was unknown how the cilium achieves this. β-catenin, a component of the canonical Wnt signalling pathway, is now shown to be sequestered to the cilium by the Wnt signalling modulator Jouberin (Jbn) to restrain Wnt responses.