Wnt signaling in disease and in development

The highly conserved Wnt secreted proteins are critical mediators of cell-to-cell signaling during development of animals. Recent biochemical and genetic analyses have led to significant insight into understanding how Wnt signals work. The catalogue of Wnt signaling components has exploded. We now realize that multiple extracellular, cytoplasmic, and nuclear components modulate Wnt signaling. Moreover, receptor-ligand specificity and multiple feedback loops determine Wnt signaling outputs. It is also clear that Wnt signals are required for adult tissue maintenance. Perturbations in Wnt signaling cause human degenerative diseases as well as cancer.     

Wnt signaling in kidney development and disease

The Wnt gene family, which encodes secreted growth and differentiation factors, has been implicated in kidney organogenesis. The Wnts control both ureteric bud development and signaling, but they also serve as inductive factors to regulate nephrogenesis in the mesenchcymal cells. Several of the Wnt genes are expressed in the developing kidney, and gene knock-out studies have revealed specific developmental functions for these. Consistent with this, changes in Wnt ligands and pathway components are associated with many kidney diseases, including kidney cancers, renal fibrosis, cystic kidney diseases, acute renal failure, diabetic nephropathy and ischaemic injury. It is these associations of the Wnt signaling system with kidney development and kidney diseases that form to topic of this review.Key words: Wnt signaling, tubule induction, ureter development, kidney diseases, kidney cancer     

Wnt/beta-catenin signaling in development and disease

A remarkable interdisciplinary effort has unraveled the WNT (Wingless and INT-1) signal transduction cascade over the last two decades. Wnt genes encode small secreted proteins that are found in all animal genomes. Wnt signaling is involved in virtually every aspect of embryonic development and also controls homeostatic self-renewal in a number of adult tissues. Germline mutations in the Wnt pathway cause several hereditary diseases, and somatic mutations are associated with cancer of the intestine and a variety of other tissues.     

Wnt signaling and skeletal development

Wnt proteins are a family of secreted proteins that regulate many aspects of cellular functions. The discovery that mutations in low-density lipoprotein receptor-related protein 5, a putative Wnt coreceptor, could positively and negatively affect bone mass in humans generated an enormous amount of interest in the possible role of the Wnt signaling pathway in skeletal biology. Over the last decade, considerable progress has been made in determining the role of the canonical Wnt signaling pathway in various aspects of skeletal development. Furthermore, recent evidence indicates the important role of non-canonical Wnt signaling in skeletal development. In this review we discuss the current understanding of the role of Wnt signaling in chondrogenesis, osteoblastogenesis, and osteoclastogenesis.     

Wnt signaling and forebrain development

Components of the Wnt signaling pathway are expressed in a tightly regulated and spatially specific manner during development of the forebrain, and Wnts are key regulators of regional forebrain identity. Wnt signaling from the cortical hem regulates the expansion and cell-type specification of the adjacent neuroepithelium and, in conjunction with Bmp, Fgf, and Shh signaling, controls dorsal-ventral forebrain patterning. Subsequently, Wnt signaling dynamically regulates the behavior of cortical progenitor cells, initially promoting the expansion of radial glia progenitor cells and later inducing neurogenesis by promoting terminal differentiation of intermediate progenitor cells. A role for Wnt signaling in cell-type specification has also been proposed.     

Wnt signaling in cartilage development and degeneration

The Wnt signaling network, which is composed of Wnt ligands, receptors, antagonists, and intracellular signaling molecules, has emerged as a powerful regulator of cell fate, proliferation, and function in multicellular organisms. Over the past two decades, the critical role of Wnt signaling in embryonic cartilage and bone development has been well established, and much has been learnt regarding the role of Wnt signaling in chondrogenesis and cartilage development. However, relatively little is known about the role of Wnt signaling in adult articular cartilage and degenerative cartilage tissue. This review will briefly summarize recent advances in Wnt regulation of chondrogenesis and hypertrophic maturation of chondrocytes, and review data concerning the role of Wnt signaling in the maintenance and degeneration of articular chondrocytes and cartilage.     

Wnt signaling during synaptic development and plasticity

The formation of synaptic connections requires a dialogue between pre and postsynaptic cells to coordinate the assembly of the presynaptic release machinery and the postsynaptic receptive complexes. Signaling molecules of the Wnt family of proteins are central to this trans-synaptic dialogue. At the neuromuscular junction and central synapses, Wnts promote synaptic assembly by signaling to the developing pre and postsynaptic compartments. In addition, new studies reveal that expression of Wnt proteins and localization of their Fz receptors are regulated by neuronal activity. Importantly, Wnts mediates the synaptic changes induced by patterned neuronal activity or sensory experience in mature neurons. Here we review recent findings into the function of Wnt signaling at the synapse and its link to activity-dependent synaptic growth and function.     

Wnt5 signaling in vertebrate pancreas development

Background  

Signaling by the Wnt family of secreted glycoproteins through their receptors, the frizzled (Fz) family of seven-pass transmembrane proteins, is critical for numerous cell fate and tissue polarity decisions during development.     

CFTR and Wnt/beta-catenin signaling in lung development

Background  

Cystic fibrosis transmembrane conductance regulator (CFTR) was shown previously to modify stretch induced differentiation in the lung. The mechanism for CFTR modulation of lung development was examined by in utero gene transfer of either a sense or antisense construct to alter CFTR expression levels.     

Sequential roles of Hedgehog and Wnt signaling in osteoblast development

Signals that govern development of the osteoblast lineage are not well understood. Indian hedgehog (Ihh), a member of the hedgehog (Hh) family of proteins, is essential for osteogenesis in the endochondral skeleton during embryogenesis. The canonical pathway of Wnt signaling has been implicated by studies of Lrp5, a co-receptor for Wnt proteins, in postnatal bone mass homeostasis. In the present study we demonstrate that beta-catenin, a central player in the canonical Wnt pathway, is indispensable for osteoblast differentiation in the mouse embryo. Moreover, we present evidence that Wnt signaling functions downstream of Ihh in development of the osteoblast lineage. Finally Wnt7b is identified as a potential endogenous ligand regulating osteogenesis. These data support a model that integrates Hh and Wnt signaling in the regulation of osteoblast development.     

Wnt signaling in heart valve development and osteogenic gene induction

Wnt signaling mediated by β-catenin has been implicated in early endocardial cushion development, but its roles in later stages of heart valve maturation and homeostasis have not been identified. Multiple Wnt ligands and pathway genes are differentially expressed during heart valve development. At E12.5, Wnt2 is expressed in cushion mesenchyme, whereas Wnt4 and Wnt9b are predominant in overlying endothelial cells. At E17.5, both Wnt3a and Wnt7b are expressed in the remodeling atrioventricular (AV) and semilunar valves. In addition, the TOPGAL Wnt reporter transgene is active throughout the developing AV and semilunar valves at E16.5, with more localized expression in the stratified valve leaflets after birth. In chicken embryo aortic valves, genes characteristic of osteogenic cell lineages including periostin, osteonectin, and Id2 are expressed specifically in the collagen-rich fibrosa layer at E14. Treatment of E14 aortic valve interstitial cells (VICs) in culture with osteogenic media results in increased expression of multiple genes associated with bone formation. Treatment of VIC with Wnt3a leads to nuclear localization of β-catenin and induction of periostin and matrix gla protein but does not induce genes associated with later stages of osteogenesis. Together, these studies provide evidence for Wnt signaling as a regulator of endocardial cushion maturation as well as valve leaflet stratification, homeostasis, and pathogenesis.     

Wnt and planar cell polarity signaling in cystic renal disease

Cystic kidney diseases can cause end stage renal disease, affecting millions of individuals worldwide. They may arise early or later in life, are characterized by a spectrum of symptoms and can be caused by diverse genetic defects. The primary cilium, a microtubule-based organelle that can serve as a signaling antenna, has been demonstrated to have a significant role in ensuring correct kidney development and function. In the kidney, one of the signaling pathways that requires the cilium for normal development is Wnt signaling. In this review, the roles of primary cilia in relation to canonical and non-canonical Wnt/PCP signaling in cystic renal disease are described. The evidence of the associations between cilia, Wnt signaling and cystic renal disease is discussed and the significance of planar cell polarity-related mechanisms in cystic kidney disease is presented. Although defective Wnt signaling is not the only cause of renal disease, research is increasingly highlighting its importance, encouraging the development of Wnt-associated diagnostic and prognostic tools for cystic renal disease.     

Reiterated Wnt signaling during zebrafish neural crest development

While Wnt/beta-catenin signaling is known to be involved in the development of neural crest cells in zebrafish, it is unclear which Wnts are involved, and when they are required. To address these issues we employed a zebrafish line that was transgenic for an inducible inhibitor of Wnt/beta-catenin signaling, and inhibited endogenous Wnt/beta-catenin signaling at discrete times in development. Using this approach, we defined a critical period for Wnt signaling in the initial induction of neural crest, which is distinct from the later period of development when pigment cells are specified from neural crest. Blocking Wnt signaling during this early period interfered with neural crest formation without blocking development of dorsal spinal neurons. Transplantation experiments suggest that neural crest precursors must directly transduce a Wnt signal. With regard to identifying which endogenous Wnt is responsible for this initial critical period, we established that wnt8 is expressed in the appropriate time and place to participate in this process. Supporting a role for Wnt8, blocking its function with antisense morpholino oligonucleotides eliminates initial expression of neural crest markers. Taken together, these results demonstrate that Wnt signals are critical for the initial induction of zebrafish neural crest and suggest that this signaling pathway plays reiterated roles in its development.     

Canonical Wnt signaling activity during synovial joint development

Wnt signaling plays important roles in skeletal development. However, the activation and function of canonical Wnt signaling in joint development remains unclear. We analyzed the lineage identity and developmental changes of the Wnt-responsive cells during synovial joint formation as well as adulthood in the Wnt signaling reporter TOPgal transgenic mice. At embryonic day (E) 12.5, we found that the TOPgal was inactivated in the presumptive joint forming interzone, but it was intensively activated in the cartilage anlage of developing long bones and digits. At E14.5, the TOPgal activity was found in a subgroup of the articular chondrocyte lineage cells, which were co-immunolabeled with Doublecortin intensively and with Vinculin weakly. At E18.5, the TOPgal/Doublecortin co-immunolabeled cells were found in the superficial layer of the developing articular cartilage. During postnatal development, the TOPgal(+) articular chondrocytes were abundant at P7 and decreased from P10. A small number of TOPgal(+) articular chondrocytes were also found in adult joints. Our study suggests an age- and lineage-specific role of canonical Wnt signaling in joint development and maintenance.     

Retinoid signaling can repress blastula Wnt signaling and impair dorsal development in Xenopus embryo

Vitamin A derivatives (retinoids) are actively involved during vertebrate embryogenesis. However, exogenous retinoids have also long been known as potent teratogens. The defects caused by retinoid treatment are complex. Here, we provided evidence that RAR-mediated retinoid signaling can repress Xenopus blastula Wnt signaling and impair dorsal development. Exogenous retinoic acid (RA) could antagonize the dorsalizing effects of lithium chloride-mediated Wnt activation in blastula embryos. The Wnt-responsive reporter gene transgenesis and luciferase assay showed that excess RA can repress the Wnt signaling in blastula embryos. In addition, the downstream target genes of the Wnt signaling that direct embryonic dorsal development, were also down-regulated in the RA-treated embryos. Mechanically, RA did not interfere with the stability of beta-catenin, but promoted its nuclear accumulation. The inverse agonist of retinoic acid receptors (RAR) rescued the Wnt signaling repression by RA and relieved the RA-induced nuclear accumulation of beta-catenin. Our results explain one of the reasons for the complicated teratogenic effects of retinoids and shed light on the endogenous way of interactions between two developmentally important signaling pathways.     

The role of Wnt signaling in hematopoietic stem cell development

Hematopoietic stem cells (HSCs) can self-renew and differentiate into all cell types of the blood. This is therapeutically important as HSC transplants can provide a curative effect for blood cancers and disorders. The process by which HSCs develop has been the subject of extensive research in a variety of model organisms; however, efforts to produce bonafide HSCs from pluripotent precursors capable of long-term multilineage reconstitution have fallen short. Studies in zebrafish, chicken, and mice have been instrumental in guiding efforts to derive HSCs from human pluripotent stem cells and have identified a complex set of molecular signals and cellular interactions mediated by such developmental regulators as fibroblast growth factor, Notch, transforming growth factor beta (TGFβ), and Wnt, which collectively promote the stepwise developmental progression toward mature HSCs. Tight temporal and spatial control of these signals is critical to generate the appropriate numbers of HSCs needed for the life of the organism. The role of the Wnt family of signaling proteins in hematopoietic development has been the subject of many studies owing in part to the complex nature of its signaling mechanisms. By integrating cell fate specification with cell polarity establishment, Wnt is uniquely capable of controlling complex biological processes, including at multiple stages of embryonic HSC development, from HSC specification to emergence from the hemogenic epithelium to subsequent expansion. This review highlights key signaling events where specific Wnt signals instruct and guide hematopoietic development in both zebrafish and mice and extend these findings to current efforts of generating HSCs in vitro.     

Update on Wnt signaling in bone cell biology and bone disease

For more than a decade, Wnt signaling pathways have been the focus of intense research activity in bone biology laboratories because of their importance in skeletal development, bone mass maintenance, and therapeutic potential for regenerative medicine. It is evident that even subtle alterations in the intensity, amplitude, location, and duration of Wnt signaling pathways affects skeletal development, as well as bone remodeling, regeneration, and repair during a lifespan. Here we review recent advances and discrepancies in how Wnt/Lrp5 signaling regulates osteoblasts and osteocytes, introduce new players in Wnt signaling pathways that have important roles in bone development, discuss emerging areas such as the role of Wnt signaling in osteoclastogenesis, and summarize progress made in translating basic studies to clinical therapeutics and diagnostics centered around inhibiting Wnt pathway antagonists, such as sclerostin, Dkk1 and Sfrp1. Emphasis is placed on the plethora of genetic studies in mouse models and genome wide association studies that reveal the requirement for and crucial roles of Wnt pathway components during skeletal development and disease.