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

Differential activation of canonical Wnt signaling determines cranial sutures fate: a novel mechanism for sagittal suture craniosynostosis

Premature closure of cranial sutures, which serve as growth centers for the skull vault, result in craniosynostosis. In the mouse posterior frontal (PF) suture closes by endochondral ossification, whereas sagittal (SAG) remain patent life time, although both are neural crest tissue derived. We therefore, investigated why cranial sutures of same tissue origin adopt a different fate. We demonstrated that closure of the PF suture is tightly regulated by canonical Wnt signaling, whereas patency of the SAG suture is achieved by constantly activated canonical Wnt signaling. Importantly, the fate of PF and SAG sutures can be reversed by manipulating Wnt signaling. Continuous activation of canonical Wnt signaling in the PF suture inhibits endochondral ossification and therefore, suture closure, In contrast, inhibition of canonical Wnt signaling in the SAG suture, upon treatment with Wnt antagonists results in endochondral ossification and suture closure. Thus, inhibition of canonical Wnt signaling in the SAG suture phenocopies craniosynostosis. Moreover, mice haploinsufficient for Twist1, a target gene of canonical Wnt signaling which inhibits chondrogenesis, have sagittal craniosynostosis. We propose that regulation of canonical Wnt signaling is of crucial importance during the physiological patterning of PF and SAG sutures. Importantly, dysregulation of this pathway may lead to craniosynostosis.

Emerging links between CDK cell cycle regulators and Wnt signaling

Wnt/beta-catenin signaling controls many aspects of cell behavior throughout development and in adults. One of its best-known and cancer-relevant functions is to stimulate cell proliferation. Recent work has implicated Wnt components in regulating mitotic events, suggesting that the cell cycle and Wnt signaling are directly linked. This concept has now been substantially strengthened with the finding that the mitotic CDK14/cyclin Y complex promotes Wnt signaling through phosphorylation of the LRP6 co-receptor, a key regulatory nexus in the Wnt/beta-catenin pathway. Thus, an unexpectedly tight collaboration between the mitotic cell cycle machinery and Wnt signaling is emerging, suggesting that this pathway might orchestrate mitotic processes.

rBMP represses Wnt signaling and influences skeletal progenitor cell fate specification during bone repair

Bone morphogenetic proteins (BMPs) participate in multiple stages of the fetal skeletogenic program from promoting cell condensation to regulating chondrogenesis and bone formation through endochondral ossification. Here, we show that these pleiotropic functions are recapitulated when recombinant BMPs are used to augment skeletal tissue repair. In addition to their well-documented ability to stimulate chondrogenesis in a skeletal injury, we show that recombinant BMPs (rBMPs) simultaneously suppress the differentiation of skeletal progenitor cells in the endosteum and bone marrow cavity to an osteoblast lineage. Both the prochondrogenic and antiosteogenic effects are achieved because rBMP inhibits endogenous beta-catenin-dependent Wnt signaling. In the injured periosteum, this repression of Wnt activity results in sox9 upregulation; consequently, cells in the injured periosteum adopt a chondrogenic fate. In the injured endosteum, rBMP also inhibits Wnt signaling, which results in the runx2 and collagen type I downregulation; consequently, cells in this region fail to differentiate into osteoblasts. In muscle surrounding the skeletal injury site, rBMP treatment induces Smad phosphorylation followed by exuberant cell proliferation, an increase in alkaline phosphatase activity, and chondrogenic differentiation. Thus different populations of adult skeletal progenitor cells interpret the same rBMP stimulus in unique ways, and these responses mirror the pleiotropic effects of BMPs during fetal skeletogenesis. These mechanistic insights may be particularly useful for optimizing the reparative potential of rBMPs while simultaneously minimizing their adverse outcomes.

Bare bones pattern formation: a core regulatory network in varying geometries reproduces major features of vertebrate limb development and evolution

BACKGROUND

Major unresolved questions regarding vertebrate limb development concern how the numbers of skeletal elements along the proximodistal (P-D) and anteroposterior (A-P) axes are determined and how the shape of a growing limb affects skeletal element formation. There is currently no generally accepted model for these patterning processes, but recent work on cartilage development (chondrogenesis) indicates that precartilage tissue self-organizes into nodular patterns by cell-molecular circuitry with local auto-activating and lateral inhibitory (LALI) properties. This process is played out in the developing limb in the context of a gradient of fibroblast growth factor (FGF) emanating from the apical ectodermal ridge (AER).

RESULTS

We have simulated the behavior of the core chondrogenic mechanism of the developing limb in the presence of an FGF gradient using a novel computational environment that permits simulation of LALI systems in domains of varying shape and size. The model predicts the normal proximodistal pattern of skeletogenesis as well as distal truncations resulting from AER removal. Modifications of the model's parameters corresponding to plausible effects of Hox proteins and formins, and of the reshaping of the model limb, bud yielded simulated phenotypes resembling mutational and experimental variants of the limb. Hypothetical developmental scenarios reproduce skeletal morphologies with features of fossil limbs.

CONCLUSIONS

The limb chondrogenic regulatory system operating in the presence of a gradient has an inherent, robust propensity to form limb-like skeletal structures. The bare bones framework can accommodate ancillary gene regulatory networks controlling limb bud shaping and establishment of Hox expression domains. This mechanism accounts for major features of the normal limb pattern and, under variant geometries and different parameter values, those of experimentally manipulated, genetically aberrant and evolutionary early forms, with no requirement for an independent system of positional information.

The temporal dynamics of vertebrate limb development, teratogenesis and evolution

Recent genetic and functional analysis of vertebrate limb development begins to reveal how the functions of particular genes and regulatory hierarchies can drastically change over time. The temporal and spatial interplay of the two instructive signalling centres are part of a larger signalling system that orchestrates limb bud morphogenesis in a rather self-regulatory manner. It appears that mesenchymal cells are specified early and subsequently, the progenitors for the different skeletal elements are expanded and determined progressively during outgrowth. Mutations and teratogens that disrupt distal progression of limb development most often cause death of the early-specified progenitors rather than altering their fates. The proliferative expansion and distal progression of paired appendage development was one of the main driving forces behind the transition from fin to limb buds during paired appendage evolution. Finally, the adaptive diversification or loss of modern tetrapod limbs in particular phyla or species appear to be a consequence of evolutionary tampering with the regulatory systems that control distal progression of limb development.

SOX5 controls cell cycle progression in neural progenitors by interfering with the WNT-beta-catenin pathway

Genes of the SOX family of high-mobility group transcription factors are essential during nervous system development. In this study, we show that SOX5 is expressed by neural progenitors in the chick spinal cord and is turned off as differentiation proceeds. The overexpression of SOX5 in neural progenitors causes premature cell cycle exit and prevents terminal differentiation. Conversely, knocking down SOX5 protein extends the proliferative period of neural progenitors and causes marked cell death in a dorsal interneuron (dI3) population. Furthermore, SOX5 reduces WNT-beta-catenin signalling, thereby triggering the expression of the negative regulator of the pathway axin2. We propose that SOX5 regulates the timing of cell cycle exit by opposing WNT-beta-catenin activity on cell cycle progression.

Hand2 regulates chondrogenesis in vitro and in vivo

Hand2 is a transcription factor of the basic helix-loop-helix family that plays essential roles during development. Hand2 determines the anterior-posterior axis during limb development and there is also substantial evidence that Hand2 regulates limb skeletogenesis. However, little is known about how Hand2 might regulate skeletogenesis. Here we show that, in a limb bud micromass culture system, over-expression of Hand2 represses chondrogenesis and the expression of chondrocytic genes, Sox9, type II collagen and aggrecan. Furthermore, we show that Hand2 is induced by the activation of canonical Wnt signaling, which strongly represses chondrogenesis. Surprisingly, Hand2 repressed chondrogenesis in a DNA binding- and dimer formation-independent manner. To examine the in vivo role of Hand2 in mice, we targeted the expression of Hand2 to the cartilage using regulatory elements from the collagen II gene. The resulting transgenic mice displayed a dwarf phenotype, with axial, appendicular and craniofacial skeletal deformities. Hand2 strongly inhibited chondrogenesis in the axial and cranial base skeleton. In the sternum, Hand2 inhibited endochondral ossification by slowing chondrocyte maturation. These data support a model of Hand2 regulating endochondral ossification via at least two steps: (1) determination of the site of chondrogenesis by outlining the region of the future cartilage template and (2) regulation of the rate of chondrocyte maturation.

Wnt signaling and its downstream target N-myc regulate basal progenitors in the developing neocortex

Basal progenitors (also called non-surface dividing or intermediate progenitors) have been proposed to regulate the number of neurons during neocortical development through expanding cells committed to a neuronal fate, although the signals that govern this population have remained largely unknown. Here, we show that N-myc mediates the functions of Wnt signaling in promoting neuronal fate commitment and proliferation of neural precursor cells in vitro. Wnt signaling and N-myc also contribute to the production of basal progenitors in vivo. Expression of a stabilized form of beta-catenin, a component of the Wnt signaling pathway, or of N-myc increased the numbers of neocortical basal progenitors, whereas conditional deletion of the N-myc gene reduced these and, as a likely consequence, the number of neocortical neurons. These results reveal that Wnt signaling via N-myc is crucial for the control of neuron number in the developing neocortex.

Ectodermal Wnt6 is an early negative regulator of limb chondrogenesis in the chicken embryo

BACKGROUND

Pattern formation of the limb skeleton is regulated by a complex interplay of signaling centers located in the ectodermal sheath and mesenchymal core of the limb anlagen, which results, in the forelimb, in the coordinate array of humerus, radius, ulna, carpals, metacarpals and digits. Much less understood is why skeletal elements form only in the central mesenchyme of the limb, whereas muscle anlagen develop in the peripheral mesenchyme ensheathing the chondrogenic center. Classical studies have suggested a role of the limb ectoderm as a negative regulator of limb chondrogenesis.

RESULTS

In this paper, we investigated the molecular nature of the inhibitory influence of the ectoderm on limb chondrogenesis in the avian embryo in vivo. We show that ectoderm ablation in the early limb bud leads to increased and ectopic expression of early chondrogenic marker genes like Sox9 and Collagen II, indicating that the limb ectoderm inhibits limb chondrogenesis at an early stage of the chondrogenic cascade. To investigate the molecular nature of the inhibitory influence of the ectoderm, we ectopically expressed Wnt6, which is presently the only known Wnt expressed throughout the avian limb ectoderm, and found that Wnt6 overexpression leads to reduced expression of the early chondrogenic marker genes Sox9 and Collagen II.

CONCLUSION

Our results suggest that the inhibitory influence of the ectoderm on limb chondrogenesis acts on an early stage of chondrogenesis upsteam of Sox9 and Collagen II. We identify Wnt6 as a candidate mediator of ectodermal chondrogenic inhibition in vivo. We propose a model of Wnt-mediated centripetal patterning of the limb by the surface ectoderm.

Cell cycle control of wnt receptor activation

Low-density lipoprotein receptor related proteins 5 and 6 (LRP5/6) are transmembrane receptors that initiate Wnt/beta-catenin signaling. Phosphorylation of PPPSP motifs in the LRP6 cytoplasmic domain is crucial for signal transduction. Using a kinome-wide RNAi screen, we show that PPPSP phosphorylation requires the Drosophila Cyclin-dependent kinase (CDK) L63. L63 and its vertebrate homolog PFTK are regulated by the membrane tethered G2/M Cyclin, Cyclin Y, which mediates binding to and phosphorylation of LRP6. As a consequence, LRP6 phosphorylation and Wnt/beta-catenin signaling are under cell cycle control and peak at G2/M phase; knockdown of the mitotic regulator CDC25/string, which results in G2/M arrest, enhances Wnt signaling in a Cyclin Y-dependent manner. In Xenopus embryos, Cyclin Y is required in vivo for LRP6 phosphorylation, maternal Wnt signaling, and Wnt-dependent anteroposterior embryonic patterning. G2/M priming of LRP6 by a Cyclin/CDK complex introduces an unexpected new layer of regulation of Wnt signaling.

Role of Wnt signaling in the biology of the periodontium

Continuously erupting teeth have associated with them a continuously regenerating periodontal ligament, but the factors that control this amazing regenerative potential are unknown. We used genetic strategies to show that the periodontal ligament arises from the cranial neural crest. Despite their histological similarity, the periodontal ligament of continuously erupting incisor teeth differs dramatically from the periodontal ligament of molar teeth. The most notable difference was in the distribution of Wnt responsive cells in the incisor periodontal ligament, which coincided with regions of periodontal ligament cell proliferation. We discuss these findings in the context of dental tissue regeneration.

Growth factor regulation of prostaglandin-endoperoxide synthase 2 (Ptgs2) expression in colonic mesenchymal stem cells

We previously found that a population of colonic stromal cells that constitutively express high levels of prostaglandin-endoperoxide synthase 2 (Ptgs2, also known as Cox-2) altered their location in the lamina propria in response to injury in a Myd88-dependent manner (Brown, S. L., Riehl, T. E., Walker, M. R., Geske, M. J., Doherty, J. M., Stenson, W. F., and Stappenbeck, T. S. (2007) J. Clin. Invest. 117, 258-269). At the time of this study, the identity of these cells and the mechanism by which they expressed high levels of Ptgs2 were unknown. Here we found that these colonic stromal cells were mesenchymal stem cells (MSCs). These colonic MSCs expressed high Ptgs2 levels not through interaction with bacterial products but instead as a consequence of mRNA stabilization downstream of Fgf9 (fibroblast growth factor 9), a growth factor that is constitutively expressed by the intestinal epithelium. This stabilization was mediated partially through a mechanism involving endogenous CUG-binding protein 2 (CUGbp2). These studies suggest that Fgf9 is an important factor in the regulation of Ptgs2 in colonic MSCs and may be a factor involved in its constitutive expression in vivo.

Vertebrate limb bud development: moving towards integrative analysis of organogenesis

The limb bud is of paradigmatic value to understanding vertebrate organogenesis. Recent genetic analysis in mice has revealed the existence of a largely self-regulatory limb bud signalling system that involves many of the pathways that are known to regulate morphogenesis. These findings contrast with the prevailing view that the main limb bud axes develop largely independently of one another. In this Review, we discuss models of limb development and attempt to integrate the current knowledge of the signalling interactions that govern limb skeletal development into a systems model. The resulting integrative model provides insights into how the specification and proliferative expansion of the anteroposterior and proximodistal limb bud axes are coordinately controlled in time and space.

A directional Wnt/beta-catenin-Sox2-proneural pathway regulates the transition from proliferation to differentiation in the Xenopus retina

Progenitor cells in the central nervous system must leave the cell cycle to become neurons and glia, but the signals that coordinate this transition remain largely unknown. We previously found that Wnt signaling, acting through Sox2, promotes neural competence in the Xenopus retina by activating proneural gene expression. We now report that Wnt and Sox2 inhibit neural differentiation through Notch activation. Independently of Sox2, Wnt stimulates retinal progenitor proliferation and this, when combined with the block on differentiation, maintains retinal progenitor fates. Feedback inhibition by Sox2 on Wnt signaling and by the proneural transcription factors on Sox2 mean that each element of the core pathway activates the next element and inhibits the previous one, providing a directional network that ensures retinal cells make the transition from progenitors to neurons and glia.

Building limb morphology through integration of signalling modules

Growth and patterning of the vertebrate limb relies on signals produced by three discrete signalling centres: the Apical Ectodermal Ridge (AER), the Zone of Polarising Activity (ZPA) and the dorsal ectoderm. The molecular identities of these signals and their associated downstream pathways have begun to be uncovered. In this review, we focus on recent work that has highlighted the importance of cross-talk between these signalling centres and how mesenchymal progenitors integrate multiple signalling inputs. We also discuss recent evidence suggesting how modulations of key signalling pathways have been used to generate the morphological diversity seen between different vertebrate limb appendages.

Embryology of the upper limb

An increased understanding of embryogenesis has advanced our fundamental knowledge of limb anomalies. Animal models with similar limb patterning have been used to dissect and manipulate crucial signaling centers that affect limb development and orientation. Experimental embryologists can produce limb anomalies that are similar to the human phenotype encountered in clinical practice. The evaluating physician must possess a basic comprehension of embryogenesis and limb formation to comprehend congenital limb anomalies and to communicate relevant knowledge to the family. This Current Concepts article is intended to provide an update of limb development that is germane to the clinical scenario.

FGFs in endochondral skeletal development

The mammalian skeleton forms and grows through two developmental pathways: membranous ossification, which gives rise to calvarial bones and the distal clavicle, and endochondral ossification, which is responsible for the bones of the limbs, girdles, vertebrae, face, base of the skull and the medial clavicle. The regulation of both pathways is extremely complex, and the rules that govern it are still emerging. However, it has become clear that fibroblast growth factors (FGFs) and their cognate receptors (FGFRs) play essential roles. This review focuses on the roles of FGFs and FGFRs in endochondral skeletal development, with special attention given to concepts that have emerged in the past few years.

Differentiation of embryonic stem cells into anterior definitive endoderm

Anterior definitive endoderm (ADE) is both an important embryonic signaling center and a unique multipotent precursor of liver, pancreas, and other visceral organs. Here we describe a method for the differentiation of mouse embryonic stem (ES) cells to endoderm with pronounced anterior character. ADE-containing cultures can be produced in vitro by suspension (aggregation or embryoid body) culture and in a serum-free adherent monolayer culture. Purified ES cell-derived ADE cells appear committed to endodermal fates and can undergo further differentiation in vitro towards liver and pancreas with enhanced efficiency.

Genomic response to Wnt signalling is highly context-dependent--evidence from DNA microarray and chromatin immunoprecipitation screens of Wnt/TCF targets

Wnt proteins are important regulators of embryonic development, and dysregulated Wnt signalling is involved in the oncogenesis of several human cancers. Our knowledge of the downstream target genes is limited, however. We used a chromatin immunoprecipitation-based assay to isolate and characterize the actual gene segments through which Wnt-activatable transcription factors, TCFs, regulate transcription and an Affymetrix microarray analysis to study the global transcriptional response to the Wnt3a ligand. The anti-beta-catenin immunoprecipitation of DNA-protein complexes from mouse NIH3T3 fibroblasts expressing a fusion protein of beta-catenin and TCF7 resulted in the identification of 92 genes as putative TCF targets. GeneChip assays of gene expression performed on NIH3T3 cells and the rat pheochromocytoma cell line PC12 revealed 355 genes in NIH3T3 and 129 genes in the PC12 cells with marked changes in expression after Wnt3a stimulus. Only 2 Wnt-regulated genes were shared by both cell lines. Surprisingly, Disabled-2 was the only gene identified by the chromatin immunoprecipitation approach that displayed a marked change in expression in the GeneChip assay. Taken together, our approaches give an insight into the complex context-dependent nature of Wnt pathway transcriptional responses and identify Disabled-2 as a potential new direct target for Wnt signalling.

A clinically relevant model of osteoinduction: a process requiring calcium phosphate and BMP/Wnt signalling

In this study, we investigated a clinically relevant model of in vivo ectopic bone formation utilizing human periosteum derived cells (HPDCs) seeded in a Collagraft™ carrier and explored the mechanisms by which this process is driven. Bone formation occurred after eight weeks when a minimum of one million HPDCs was loaded on Collagraft™ carriers and implanted subcutaneously in NMRI nu/nu mice. De novo bone matrix, mainly secreted by the HPDCs, was found juxta-proximal of the calcium phosphate (CaP) granules suggesting that CaP may have triggered the ‘osteoinductive program’. Indeed, removal of the CaP granules by ethylenediaminetetraacetic acid decalcification prior to cell seeding and implantation resulted in loss of bone formation. In addition, inhibition of endogenous bone morphogenetic protein and Wnt signalling by overexpression of the secreted antagonists Noggin and Frzb, respectively, also abrogated osteoinduction. Proliferation of the engrafted HPDCs was strongly reduced in the decalcified scaffolds or when seeded with adenovirus-Noggin/Frzb transduced HPDCs indicating that cell division of the engrafted HPDCs is required for the direct bone formation cascade. These data suggest that this model of bone formation is similar to that observed during physiological intramembranous bone development and may be of importance when investigating tissue engineering strategies.

Growth and patterning in the limb: signaling gradients make the decision

The vertebrate limb bud provides a unique system to investigate the coordinated regulation of growth and patterning, two key processes that govern the formation of a complex multicellular organism from a fertilized egg. Two studies have advanced our understanding of limb development by elucidating that signaling gradients from the limb ectoderm, including the apical ectoderm ridge (AER), act in concert to establish a basic pattern of tissue layers by coordinating cell proliferation and cell fate determination. These studies reveal that cell proliferation and fate determination in development can be two faces of the same coin in that they are regulated by the same signaling pathways. Alterations in the duration and range of the signaling gradients may underlie many of the morphological differences in the evolution of vertebrate limbs.

Neural tube derived Wnt signals cooperate with FGF signaling in the formation and differentiation of the trigeminal placodes

BACKGROUND

Neurogenic placodes are focal thickenings of the embryonic ectoderm that form in the vertebrate head. It is within these structures that the precursors of the majority of the sensory neurons of the cranial ganglia are specified. The trigeminal placodes, the ophthalmic and maxillomandibular, form close to the midbrain-hindbrain boundary and many lines of evidence have shown that signals emanating from this level of the neuraxis are important for the development of the ophthalmic placode.

RESULTS

Here, we provide the first evidence that both the ophthalmic and maxillomandibular placodes form under the influence of isthmic Wnt and FGF signals. Activated Wnt signals direct development of the Pax3 expressing ophthalmic placodal field and induce premature differentiation of both the ophthalmic and the maxillomandibular placodes. Similarly, overexpression of Fgf8 directs premature differentiation of the trigeminal placodes. Wnt signals require FGF receptor activity to initiate Pax3 expression and, subsequently, the expression of neural markers, such as Brn3a, within the cranial ectoderm. Furthermore, fibroblast growth factor signaling via the mitogen activated protein kinase pathway is required to maintain early neuronal differentiation within the trigeminal placodes.

CONCLUSION

We demonstrate the identity of inductive signals that are necessary for trigeminal ganglion formation. This is the first report that describes how isthmic derived Wnt signals act in concert with fibroblast growth factor signaling. Together, both are necessary and sufficient for the establishment and differentiation of the ophthalmic and maxillomandibular placodes and, consequently, the trigeminal ganglion.

Translating insights from development into regenerative medicine: the function of Wnts in bone biology

The Wnt pathway constitutes one of the most attractive candidates for modulating skeletal tissue regeneration based on its functions during skeletal development and homeostasis. Wnts participate in every stage of skeletogenesis, from the self-renewal and proliferation of skeletal stem cells to the specification of osteochondroprogenitor cells and the maturation of chondrocytes and osteoblasts. We propose that the function of Wnts depend upon a skeletogenic cell's state of differentiation. In this review we summarize recent data with a focus on the roles of Wnt signaling in mesenchymal stem cell fate, osteoprogenitor cell differentiation, chondrocyte maturation, bone remodeling, and bone regeneration.