Mutational analysis of Norrin-Frizzled4 recognition

Secreted Frizzled-related protein potentiation versus inhibition of Wnt3a/β-catenin signaling

Wnt signaling regulates a variety of cellular processes during embryonic development and in the adult. Many of these activities are mediated by the Frizzled family of seven-pass transmembrane receptors, which bind Wnts via a conserved cysteine-rich domain (CRD). Secreted Frizzled-related proteins (sFRPs) contain an amino-terminal, Frizzled-like CRD and a carboxyl-terminal, heparin-binding netrin-like domain. Previous studies identified sFRPs as soluble Wnt antagonists that bind directly to Wnts and prevent their interaction with Frizzleds. However, subsequent observations suggested that sFRPs and Frizzleds form homodimers and heterodimers via their respective CRDs, and that sFRPs can stimulate signal transduction. Here, we present evidence that sFRP1 either inhibits or enhances signaling in the Wnt3a/β-catenin pathway, depending on its concentration and the cellular context. Nanomolar concentrations of sFRP1 increased Wnt3a signaling, while higher concentrations blocked it in HEK293 cells expressing a SuperTopFlash reporter. sFRP1 primarily augmented Wnt3a/β-catenin signaling in C57MG cells, but it behaved as an antagonist in L929 fibroblasts. sFRP1 enhanced reporter activity in L cells that were engineered to stably express Frizzled 5, though not Frizzled 2. This implied that the Frizzled expression pattern could determine the response to sFRP1. Similar results were obtained with sFRP2 in HEK293, C57MG and L cell reporter assays. CRDsFRP1 mimicked the potentiating effect of sFRP1 in multiple settings, contradicting initial expectations that this domain would inhibit Wnt signaling. Moreover, CRDsFRP1 showed little avidity for Wnt3a compared to sFRP1, implying that the mechanism for potentiation by CRDsFRP1 probably does not require an interaction with Wnt protein. Together, these findings demonstrate that sFRPs can either promote or suppress Wnt/β-catenin signaling, depending on cellular context, concentration and most likely the expression pattern of Fzd receptors.

Secreted and transmembrane wnt inhibitors and activators

Signaling by the Wnt family of secreted glycoproteins plays important roles in embryonic development and adult homeostasis. Wnt signaling is modulated by a number of evolutionarily conserved inhibitors and activators. Wnt inhibitors belong to small protein families, including sFRP, Dkk, WIF, Wise/SOST, Cerberus, IGFBP, Shisa, Waif1, APCDD1, and Tiki1. Their common feature is to antagonize Wnt signaling by preventing ligand-receptor interactions or Wnt receptor maturation. Conversely, the Wnt activators, R-spondin and Norrin, promote Wnt signaling by binding to Wnt receptors or releasing a Wnt-inhibitory step. With few exceptions, these antagonists and agonists are not pure Wnt modulators, but also affect additional signaling pathways, such as TGF-β and FGF signaling. Here we discuss their interactions with Wnt ligands and Wnt receptors, their role in developmental processes, as well as their implication in disease.

Multi-functional norrin is a ligand for the LGR4 receptor

Mammalian LGR4, 5 and 6 are seven-transmembrane receptors that are important for diverse physiological processes. These receptors are orthologous to DLGR2, a Drosophila receptor activated by the burs/pburs heterodimer important for morphogenesis. Although recent studies indicated that four R-spondin proteins are cognate ligands for LGR4, 5 and 6 receptors, several BMP antagonists in vertebrates have been postulated to be orthologous to burs and pburs. Using newly available genome sequences, we showed that norrin is a vertebrate ortholog for insect burs and pburs and stimulates Wnt signaling mediated by LGR4, but not by LGR5 and 6, in mammalian cells. Although norrin could only activate LGR4, binding studies suggested interactions between norrin and LGR4, 5 and 6. Norrin, the Norrie disease gene product, is also capable of activating Wnt signaling mediated by the Frizzled4 receptor and serves as a BMP antagonist. Mutagenesis studies indicated that different norrin mutations found in patients with Norrie disease can be categorized into subgroups according to defects for signaling through the three distinct binding proteins. Thus, norrin is a rare ligand capable of binding three receptors/binding proteins that are important for BMP and Wnt signaling pathways.

WNT signaling in bone homeostasis and disease: from human mutations to treatments

Low bone mass and strength lead to fragility fractures, for example, in elderly individuals affected by osteoporosis or children with osteogenesis imperfecta. A decade ago, rare human mutations affecting bone negatively (osteoporosis-pseudoglioma syndrome) or positively (high-bone mass phenotype, sclerosteosis and Van Buchem disease) have been identified and found to all reside in components of the canonical WNT signaling machinery. Mouse genetics confirmed the importance of canonical Wnt signaling in the regulation of bone homeostasis, with activation of the pathway leading to increased, and inhibition leading to decreased, bone mass and strength. The importance of WNT signaling for bone has also been highlighted since then in the general population in numerous genome-wide association studies. The pathway is now the target for therapeutic intervention to restore bone strength in millions of patients at risk for fracture. This paper reviews our current understanding of the mechanisms by which WNT signalng regulates bone homeostasis.

Norrin/Frizzled4 signaling in retinal vascular development and blood brain barrier plasticity

Norrin/Frizzled4 (Fz4) signaling activates the canonical Wnt pathway to control retinal vascular development. Using genetically engineered mice, we show that precocious Norrin production leads to premature retinal vascular invasion and delayed Norrin production leads to characteristic defects in intraretinal vascular architecture. In genetic mosaics, wild-type endothelial cells (ECs) instruct neighboring Fz4(-/-) ECs to produce an architecturally normal mosaic vasculature, a cell nonautonomous effect. However, over the ensuing weeks, Fz4(-/-) ECs are selectively eliminated from the mosaic vasculature, implying the existence of a quality control program that targets defective ECs. In the adult retina and cerebellum, gain or loss of Norrin/Fz4 signaling results in a cell-autonomous gain or loss, respectively, of blood retina barrier and blood brain barrier function, indicating an ongoing requirement for Frizzled signaling in barrier maintenance and substantial plasticity in mature CNS vascular structure.

Shared cell surface marker expression in mesenchymal stem cells and adult sarcomas

Advanced adult soft-tissue sarcomas (STSs) are rare tumors with a dismal prognosis and limited systemic treatment options. STSs may originate from mesenchymal stem cells (MSCs); the latter have mainly been isolated from adult bone marrow as plastic-adherent cells with differentiation capacity into mesenchymal tissues. Recently, a panel of antibodies has been established that allows for the prospective isolation of primary MSCs with high selectivity. Similar to cancer stem cells in other malignancies, sarcoma stem cells may bear immunophenotypic similarity with the corresponding precursor, that is, MSCs. We therefore set out to establish the expression pattern of MSC markers in sarcoma cell lines and primary tumor samples by flow cytometry. In addition, fibroblasts from different sources were examined. The results document a significant amount of MSC markers shared by sarcoma cells. The expression pattern includes uniformly expressed markers, as well as MSC markers that only stained subpopulations of sarcoma cells. Expression of W5C5, W8B2 (tissue nonspecific alkaline phosphatase [TNAP]), CD344 (frizzled-4), and CD271 marked subpopulations displaying increased proliferation potential. Moreover, CD271+ cells displayed in vitro doxorubicin resistance and an increased capacity to form spheres under serum-free conditions. Interestingly, another set of antigens, including the bona fide progenitor cell markers CD117 and CD133, were not expressed. Comparative expression patterns of novel MSC markers in sarcoma cells, as well as fibroblasts and MSCs, are presented. Our data suggest a hierarchical cytoarchitecture of the most common adult type sarcomas and introduce W5C5, TNAP, CD344, and CD271 as potential sarcoma progenitor cell markers.

The complex world of WNT receptor signalling

30 years after the identification of WNTs, their signal transduction has become increasingly complex, with the discovery of more than 15 receptors and co-receptors in seven protein families. The recent discovery of three receptor classes for the R-spondin family of WNT agonists further adds to this complexity. What emerges is an intricate network of receptors that form higher-order ligand-receptor complexes routing downstream signalling. These are regulated both extracellularly by agonists such as R-spondin and intracellularly by post-translational modifications such as phosphorylation, proteolytic processing and endocytosis.

Structural architecture and functional evolution of Wnts

The crystal structure of a Wnt morphogen bound to its Frizzled receptor ectodomain provides insights into the evolutionary provenance of this complex fold and offers an explanation for why Wnts utilize both lipid- and protein-mediated contacts to engage Frizzleds.

Structural basis of Wnt recognition by Frizzled

Wnts are lipid-modified morphogens that play critical roles in development principally through engagement of Frizzled receptors. The 3.25 angstrom structure of Xenopus Wnt8 (XWnt8) in complex with mouse Frizzled-8 (Fz8) cysteine-rich domain (CRD) reveals an unusual two-domain Wnt structure, not obviously related to known protein folds, resembling a "hand" with "thumb" and "index" fingers extended to grasp the Fz8-CRD at two distinct binding sites. One site is dominated by a palmitoleic acid lipid group projecting from serine 187 at the tip of Wnt's thumb into a deep groove in the Fz8-CRD. In the second binding site, the conserved tip of Wnt's "index finger" forms hydrophobic amino acid contacts with a depression on the opposite side of the Fz8-CRD. The conservation of amino acids in both interfaces appears to facilitate ligand-receptor cross-reactivity, which has important implications for understanding Wnt's functional pleiotropy and for developing Wnt-based drugs for cancer and regenerative medicine.

Maternal xNorrin, a canonical Wnt signaling agonist and TGF-β antagonist, controls early neuroectoderm specification in Xenopus

Xenopus maternal Norrin, which activates Wnt signaling but inhibits TGF-β family molecules, is essential for neuroectoderm formation. Loss of TGF-β inhibition in Norrin may contribute to the development of Norrie disease.

Dorsal–ventral specification in the amphibian embryo is controlled by β-catenin, whose activation in all dorsal cells is dependent on maternal Wnt11. However, it remains unknown whether other maternally secreted factors contribute to β-catenin activation in the dorsal ectoderm. Here, we show that maternal Xenopus Norrin (xNorrin) promotes anterior neural tissue formation in ventralized embryos. Conversely, when xNorrin function is inhibited, early canonical Wnt signaling in the dorsal ectoderm and the early expression of the zygotic neural inducers Chordin, Noggin, and Xnr3 are severely suppressed, causing the loss of anterior structures. In addition, xNorrin potently inhibits BMP- and Nodal/Activin-related functions through direct binding to the ligands. Moreover, a subset of Norrin mutants identified in humans with Norrie disease retain Wnt activation but show defective inhibition of Nodal/Activin-related signaling in mesoderm induction, suggesting that this disinhibition causes Norrie disease. Thus, xNorrin is an unusual molecule that acts on two major signaling pathways, Wnt and TGF-β, in opposite ways and is essential for early neuroectoderm specification.

A key step during early embryogenesis is the generation of neural precursors, which later form the central nervous system. In vertebrates, this process requires proper dorsal–ventral axis specification, and we know that the canonical Wnt and BMP signaling pathways help pattern the dorsal ectoderm. In this study, we examine other factors that are involved in neuroectoderm development in the frog species Xenopus laevis. We find that maternal Xenopus Norrin (xNorrin) is required for canonical Wnt signaling in the dorsal ectoderm, functions upstream of neural inducers, and is required for neural formation. We also find that xNorrin not only activates Wnt signaling, but also inhibits BMP/Nodal-related signaling. In humans, mutations in Norrin cause Norrie disease. Using Norrin mutants identified in patients with Norrie disease, we find that some Norrin mutants fail to inhibit BMP/Nodal-related signaling (specifically, TGF-β) but retain the ability to activate the Wnt pathway, suggesting that loss of TGF-β inhibition may contribute to Norrie disease development.

Norrin: molecular and functional properties of an angiogenic and neuroprotective growth factor

Norrin is a secreted signaling molecule with structural and functional characteristics of an autocrine and/or paracrine acting growth factor. In the eye, Norrin is constitutively expressed in Müller cells. Norrin specifically binds to Frizzled-4 receptors and activates the canonical Wnt/β-catenin signaling pathway that is profoundly enhanced when Tspan12 is present at the Norrin/Frizzled-4 receptor complex. In the absence of Norrin or Frizzled-4, intraretinal capillaries are not formed during developmental angiogenesis. As a result there is considerable evidence that Norrin and Frizzled-4 are part of an essential signaling system that controls the formation of the retinal vasculature during eye development. Intriguingly, Norrin promotes vessel regrowth and induces the formation of intraretinal capillaries following oxygen-induced retinopathy in mice, an animal model of retinopathy of prematurity. Moreover, Norrin has pronounced neuroprotective properties on retinal ganglion cells (RGC) with the distinct potential to decrease the damaging effects of excitotoxic NMDA-induced RGC injury. The neuroprotective effects of Norrin similarly involve an activation of Wnt/β-catenin signaling and the subsequent induction of neuroprotective growth factor synthesis in Müller cells, such as that of fibroblast growth factor-2 (FGF2) or ciliary neurotrophic factor (CNTF). Overall, Norrin and the molecules involved in its signaling pathway appear to be promising targets to develop strategies that induce intraretinal vessel formation in patients suffering from ischemic retinopathies, or that increase RGC survival in glaucoma.

Regulation of retinal progenitor expansion by Frizzled receptors: implications for microphthalmia and retinal coloboma

Nineteen Wnt ligands and 10 Frizzled (Fz) receptors mediate multiple distinct cellular events during neuronal development. However, their precise roles in cell-type specification and organogenesis are poorly delineated because of overlapping functions and expression profiles. Here, we have explored the role of two closely related Frizzled receptors, Fz5 and Fz8, in mouse retinal development. We previously showed that Fz5(-/-) mice exhibit mild coloboma and microphthalmia at ~50% penetrance. Fz8 expression overlaps with Fz5 in the neural retina and optic fissure/disc. Mice lacking Fz8 show minimal eye and retinal defects. The embryos lacking both Fz5 and Fz8 die early in development, but a majority of triallelic Fz5(-/-);Fz8(+/-) mutants survive until birth. The triallelic mutant develops severe retinal coloboma and microphthalmia with full penetrance. At the cellular level, impaired neurogenesis is indicated by increased early-born retinal neurons that result from accelerated cell cycle exit of progenitors. Deficiency of apical retinal neuroepithelium is indicated by altered localization of apical junction markers, such as atypical protein kinase C, RhoA and β-catenin. Hes1 expression, which is critical for retinal progenitor expansion, is down-regulated in the triallelic mutant mouse. Furthermore, blocking Frizzled receptors in cultured retinal explants led to basally shifted divisions of retinal progenitors. Together, our studies suggest a dose-dependent regulation of signaling by Fz5 and Fz8 in optic fissure/disc formation and progenitor expansion.

Genetic mosaic analysis reveals a major role for frizzled 4 and frizzled 8 in controlling ureteric growth in the developing kidney

The developing mammalian kidney is an attractive system in which to study the control of organ growth. Targeted mutations in the Wnt receptors frizzled (Fz) 4 and Fz8 lead to reduced ureteric bud growth and a reduction in kidney size, a phenotype previously reported for loss of Wnt11. In cell culture, Fz4 and Fz8 can mediate noncanonical signaling stimulated by Wnt11, but only Fz4 mediates Wnt11-stimulated canonical signaling. In genetically mosaic mouse ureteric buds, competition between phenotypically mutant Fz4(-/-) or Fz4(-/-);Fz8(-/-) cells and adjacent phenotypically wild-type Fz4(+/-) or Fz4(+/-);Fz8(-/-) cells results in under-representation of the mutant cells to an extent far greater than would be predicted from the size reduction of homogeneously mutant kidneys. This discrepancy presumably reflects the compensatory action of a network of growth regulatory systems that minimize developmental perturbations. The present work represents the first description of a kidney phenotype referable to one or more Wnt receptors and demonstrates a general strategy for revealing the contribution of an individual growth regulatory pathway when it is part of a larger homeostatic network.

International Union of Basic and Clinical Pharmacology. LXXX. The class Frizzled receptors

The receptor class Frizzled, which has recently been categorized as a separate group of G protein-coupled receptors by the International Union of Basic and Clinical Pharmacology, consists of 10 Frizzleds (FZD(1-10)) and Smoothened (SMO). The FZDs are activated by secreted lipoglycoproteins of the Wingless/Int-1 (WNT) family, whereas SMO is indirectly activated by the Hedgehog (HH) family of proteins acting on the transmembrane protein Patched (PTCH). Recent years have seen major advances in our knowledge about these seven-transmembrane-spanning proteins, including: receptor function, molecular mechanisms of signal transduction, and the receptor's role in embryonic patterning, physiology, cancer, and other diseases. Despite intense efforts, many question marks and challenges remain in mapping receptor-ligand interaction, signaling routes, mechanisms of specificity and how these molecular details underlie disease and also the receptor's important role in physiology. This review therefore focuses on the molecular aspects of WNT/FZD and HH/SMO signaling discussing receptor structure, mechanisms of signal transduction, accessory proteins, receptor dynamics, and the possibility of targeting these signaling pathways pharmacologically.

An essential role of the cysteine-rich domain of FZD4 in Norrin/Wnt signaling and familial exudative vitreoretinopathy

The Wnt pathway plays important yet diverse roles in health and disease. Mutations in the Wnt receptor FZD4 gene have been confirmed to cause familial exudative vitreoretinopathy (FEVR). FEVR is characterized by incomplete vascularization of the peripheral retina, which can lead to vitreous bleeding, tractional retinal detachment, and blindness. We screened for mutations in the FZD4 gene in five families with FEVR and identified five mutations (C45Y, Y58C, W226X, C204R, and W496X), including three novel mutations (C45Y, Y58C, and W226X). In the retina, Norrin serves as a ligand and binds to FZD4 to activate the Wnt signaling pathway in normal angiogenesis and vascularization. The cysteine-rich domain (CRD) of FZD4 has been shown to play a critical role in Norrin-FZD4 binding. We investigated the effect of mutations in the FZD4 CRD in Norrin binding and signaling in vitro and in vivo. Wild-type and mutant FZD4 proteins were assayed for Norrin binding and Norrin-dependent activation of the canonical Wnt pathway by cell-surface and overlay binding assays and luciferase reporter assays. In HEK293 transfection studies, C45Y, Y58C, and C204R mutants did not bind to Norrin and failed to transduce FZD4-mediated Wnt/β-catenin signaling. In vivo studies using Xenopus embryos showed that these FZD4 mutations disrupt Norrin/β-catenin signaling as evidenced by decreased Siamois and Xnr3 expression. This study identified a new class of FZD4 gene mutations in human disease and demonstrates a critical role of the CRD in Norrin binding and activation of the β-catenin pathway.

Expression of the Norrie disease gene (Ndp) in developing and adult mouse eye, ear, and brain

The Norrie disease gene (Ndp) codes for a secreted protein, Norrin, that activates canonical Wnt signaling by binding to its receptor, Frizzled-4. This signaling system is required for normal vascular development in the retina and for vascular survival in the cochlea. In mammals, the pattern of Ndp expression beyond the retina is poorly defined due to the low abundance of Norrin mRNA and protein. Here, we characterize Ndp expression during mouse development by studying a knock-in mouse that carries the coding sequence of human placental alkaline phosphatase (AP) inserted at the Ndp locus (Ndp(AP)). In the CNS, Ndp(AP) expression is apparent by E10.5 and is dynamic and complex. The anatomically delimited regions of Ndp(AP) expression observed prenatally in the CNS are replaced postnatally by widespread expression in astrocytes in the forebrain and midbrain, Bergman glia in the cerebellum, and Müller glia in the retina. In the developing and adult cochlea, Ndp(AP) expression is closely associated with two densely vascularized regions, the stria vascularis and a capillary plexus between the organ of Corti and the spiral ganglion. These observations suggest the possibility that Norrin may have developmental and/or homeostatic functions beyond the retina and cochlea.

The Norrin/Frizzled4 signaling pathway in retinal vascular development and disease

Disorders of retinal vascular growth and function are responsible for vision loss in a variety of diseases, including diabetic retinopathy, age-related macular degeneration, retinopathy of prematurity and retinal artery or vein occlusion. Over the past decade, a new signaling pathway that controls retinal vascular development has emerged from the study of inherited disorders - in both humans and mice - that are characterized by retinal hypovascularization. This pathway utilizes a glial-derived extracellular ligand, Norrin, that acts on a transmembrane receptor, Frizzled4, a coreceptor, Lrp5, and an auxiliary membrane protein, Tspan12, on the surface of developing endothelial cells. The resulting signal controls a transcriptional program that regulates endothelial growth and maturation. It will be of great interest to determine whether modulating this pathway could represent a therapeutic approach to human retinal vascular disease.

Wnt3-frizzled 1 chimera as a model to study canonical Wnt signaling

Wnt proteins initiate signaling by binding to seven transmembrane spanning receptors of the frizzled (Fz) family together with the members of the low-density lipoprotein receptor-related protein (LRP) 5 and 6. A chimera of human Wnt3 and Fz1 receptor was developed that efficiently activated the TCF-luciferase reporter. Deletion of the cytoplasmic tail and point mutations in the PDZ binding region in the chimera resulted in the loss of Wnt signaling, suggesting a critical role for the Fz cytoplasmic region in Wnt signaling. The Fz CRD is also critical for Wnt signaling, as a deletion of 29 amino acids in the 2nd cysteine loop resulted in the total loss of TCF-luciferase activation. DKK-1 protein blocks upregulation of the TCF-luciferase reporter by the Wnt3-Fz1 chimera, suggesting involvement of LRP in Wnt3-Fz1 signaling. Expression of a Wnt3-Fz1 chimera in C3H10T1/2 cells resulted in the upregulation of alkaline phosphatase activity and inhibition of adipocyte formation, demonstrating that the Wnt3-Fz1 chimera is a potent activator of differentiation of C3H10T1/2 cells into osteoblasts and an inhibitor of their differentiation into the adipocyte lineage. In summary, the Wnt-Fz chimera approach has the potential to better our understanding of the mechanism of Wnt action and its role, particularly in stem cell differentiation. In addition, this methodology can be utilized to identify inhibitors of either Wnt, Fz or interactors of the canonical pathway, which may have potential therapeutic value in the treatment of cancers and other diseases.

Norrin mediates neuroprotective effects on retinal ganglion cells via activation of the Wnt/beta-catenin signaling pathway and the induction of neuroprotective growth factors in Muller cells

Norrin is a secreted protein that binds to frizzled 4 and controls development of capillaries in retina and inner ear. We provide evidence that Norrin has distinct neuroprotective properties that are independent from its effects on vascular development. The function of Norrin was investigated in a mouse model of excitotoxic retinal ganglion cell (RGC) damage after intravitreal injection of NMDA, and in cultured Müller glia or immortalized RGC-5 cells. Intravitreal injection of Norrin significantly increased the number of surviving RGC axons in the optic nerve and decreased apoptotic death of retinal neurons following NMDA-mediated damage. This effect could be blocked by adding dickkopf (DKK)-1, an inhibitor of the Wnt/beta-catenin signaling pathway. Treatment of eyes with combined Norrin/NMDA activated Wnt/beta-catenin signaling and increased the retinal expression of leukemia inhibitory factor and endothelin-2, as well as that of neurotrophic growth factors such as fibroblast growth factor-2, brain-derived neurotrophic factor, lens epithelium-derived growth factor, and ciliary neurotrophic factor. A similar activation of Wnt/beta-catenin signaling and an increased expression of neurotrophic factors was observed in cultured Müller cells after treatment with Norrin, effects that again could be blocked by adding DKK-1. In addition, conditioned cell culture medium of Norrin-treated Müller cells increased survival of differentiated RGC-5 cells. We conclude that Norrin has pronounced neuroprotective properties on retinal neurons with the distinct potential to decrease the damaging effects of NMDA-induced RGC loss. The effects of Norrin involve activation of Wnt/beta-catenin signaling and subsequent induction of neurotrophic growth factors in Müller cells.

Norrin promotes vascular regrowth after oxygen-induced retinal vessel loss and suppresses retinopathy in mice

Norrin is a secreted protein that is involved in retinal angiogenesis and activates the Wnt-signaling pathway. We studied the role of Norrin in microvascular endothelial cells in vitro, and in a mouse model of retinopathy characterized by oxygen-induced vascular loss followed by hypoxia-induced pathological neovascularization. Recombinant Norrin significantly increased proliferation, viability, migration, and tube formation in vitro. Two independent transgenic mouse strains with ectopic overexpression of Norrin from the lens (betaB1-Crystallin-Norrin), or the retinal pigment epithelium (Rpe65-Norrin) were generated and exposed to high oxygen. Following oxygen treatment, vascular loss was significantly smaller in retinae of transgenic mice from both strains as compared to wild-type littermates. In addition, the anatomical correct regrowth of vessels was significantly increased, while pathological neovascularization was suppressed. In vitro and in vivo effects of Norrin could be blocked by adding DKK (Dickkopf)-1, an inhibitor of Wnt/beta-catenin signaling. Treatment of microvascular endothelial cells with Norrin caused a substantial increase in the expression of angiopoietin-2 (Ang-2). When inhibitory antibodies against Ang-2 were added to Norrin, the proliferative effects of Norrin were significantly suppressed. We conclude that Norrin is a potent factor to induce angiogenesis in microvascular endothelial cells, which has the distinct potential to suppress the damaging effects of oxygen-induced retinopathy in vivo. The effects of Norrin appear to be mediated, at least partially, via the induction of Ang-2.

TSPAN12 regulates retinal vascular development by promoting Norrin- but not Wnt-induced FZD4/beta-catenin signaling

Mutations in the genes encoding the Wnt receptor Frizzled-4 (FZD4), coreceptor LRP5, or the ligand Norrin disrupt retinal vascular development and cause ophthalmic diseases. Although Norrin is structurally unrelated to Wnts, it binds FZD4 and activates the canonical Wnt pathway. Here we show that the tetraspanin Tspan12 is expressed in the retinal vasculature, and loss of Tspan12 phenocopies defects seen in Fzd4, Lrp5, and Norrin mutant mice. In addition, Tspan12 genetically interacts with Norrin or Lrp5. Overexpressed TSPAN12 associates with the Norrin-receptor complex and significantly increases Norrin/beta-catenin but not Wnt/beta-catenin signaling, whereas Tspan12 siRNA abolishes transcriptional responses to Norrin but not Wnt3A in retinal endothelial cells. Signaling defects caused by Norrin or FZD4 mutations that are predicted to impair receptor multimerization are rescued by overexpression of TSPAN12. Our data indicate that Norrin multimers and TSPAN12 cooperatively promote multimerization of FZD4 and its associated proteins to elicit physiological levels of signaling.

Norrin, frizzled-4, and Lrp5 signaling in endothelial cells controls a genetic program for retinal vascularization

Disorders of vascular structure and function play a central role in a wide variety of CNS diseases. Mutations in the Frizzled-4 (Fz4) receptor, Lrp5 coreceptor, or Norrin ligand cause retinal hypovascularization, but the mechanisms by which Norrin/Fz4/Lrp signaling controls vascular development have not been defined. Using mouse genetic and cell culture models, we show that loss of Fz4 signaling in endothelial cells causes defective vascular growth, which leads to chronic but reversible silencing of retinal neurons. Loss of Fz4 in all endothelial cells disrupts the blood brain barrier in the cerebellum, whereas excessive Fz4 signaling disrupts embryonic angiogenesis. Sox17, a transcription factor that is upregulated by Norrin/Fz4/Lrp signaling, plays a central role in inducing the angiogenic program controlled by Norrin/Fz4/Lrp. These experiments establish a cellular basis for retinal hypovascularization diseases due to insufficient Frizzled signaling, and they suggest a broader role for Frizzled signaling in vascular growth, remodeling, maintenance, and disease.

Identification of differentially expressed genes in early inner ear development

To understand the etiology of congenital hearing loss, a comprehensive understanding of the molecular genetic mechanisms underlying normal ear development is required. We are identifying genes involved in otogenesis, with the longer term goal of studying their mechanisms of action, leading to inner ear induction and patterning. Using Agilent microarrays, we compared the differential expression of a test domain (which consisted of the pre-otic placodal ectoderm with the adjacent hindbrain ectoderm and the underlying mesendodermal tissues) with a rostral control domain (which included tissue that is competent, but not specified, to express inner ear markers in explant assays). We identified 1261 transcripts differentially expressed between the two domains at a 2-fold or greater change: 463 were upregulated and 798 were downregulated in the test domain. We validated the differential expression of several signaling molecules and transcription factors identified in this array using in situ hybridization. Furthermore, the expression patterns of the validated group of genes from the test domain were explored in detail to determine how the timing of their expression relates to specific events of otic induction and development. In conclusion, we identified a number of novel candidate genes for otic placode induction.

von Willebrand factor assembly and secretion

During its life history, von Willebrand factor (VWF) experiences a remarkable sequence of conformational changes that are triggered by differences in pH between the endoplasmic reticulum (ER), Golgi and extracellular environments. VWF subunits dimerize in the ER and assemble into disulfide-linked multimers in the trans-Golgi, which lacks known chaperones and has an acidic pH that inhibits disulfide rearrangement. VWF has circumvented these problems by evolving N-terminal domains that function as an oxidoreductase at the low pH of the Golgi. VWF multimers also condense into tightly packed, tubular arrays for storage in the Weibel-Palade bodies of endothelial cells. Like multimer assembly, tubular packing depends on low pH and Ca2+. Upon secretion, exposure to the neutral pH of the extracellular environment allows enormous VWF multimers to uncoil without tangling, which is crucial for hemostasis. Recent studies have identified some of the biochemical and structural properties that underlie these self-organizing behaviors.

Myocilin is a modulator of Wnt signaling

It is well documented that mutations in the MYOCILIN gene may lead to juvenile- and adult-onset primary open-angle glaucoma. However, the functions of wild-type myocilin are still not well understood. To study the functions of human myocilin and its two proteolytic fragments, these proteins were expressed in HEK293 cells. Conditioned medium from myocilin-expressing cells, as well as purified myocilin, induced the formation of stress fibers in primary cultures of human trabecular meshwork or NIH 3T3 cells. Stress fiber-inducing activity of myocilin was blocked by antibodies against myocilin, as well as secreted inhibitors of Wnt signaling, secreted Frizzled-related protein 1 (sFRP1) or sFRP3, and beta-catenin small interfering RNA. Interaction of myocilin with sFRP1, sFRP3, and several Frizzled receptors was confirmed by immunoprecipitation experiments and by binding of myocilin to the surface of cells expressing cysteine-rich domains of different Frizzled and sFRPs. Treatment of NIH 3T3 cells with myocilin and its fragments induced intracellular redistribution of beta-catenin and its accumulation on the cellular membrane but did not induce nuclear accumulation of beta-catenin. Overexpression of myocilin in the eye angle tissues of transgenic mice stimulated accumulation of beta-catenin in these tissues. Myocilin and Wnt proteins may perform redundant functions in the mammalian eye, since myocilin modulates Wnt signaling by interacting with components of this signaling pathway.

SNPs: impact on gene function and phenotype

Single nucleotide polymorphism (SNP) is the simplest form of DNA variation among individuals. These simple changes can be of transition or transversion type and they occur throughout the genome at a frequency of about one in 1,000 bp. They may be responsible for the diversity among individuals, genome evolution, the most common familial traits such as curly hair, interindividual differences in drug response, and complex and common diseases such as diabetes, obesity, hypertension, and psychiatric disorders. SNPs may change the encoded amino acids (nonsynonymous) or can be silent (synonymous) or simply occur in the noncoding regions. They may influence promoter activity (gene expression), messenger RNA (mRNA) conformation (stability), and subcellular localization of mRNAs and/or proteins and hence may produce disease. Therefore, identification of numerous variations in genes and analysis of their effects may lead to a better understanding of their impact on gene function and health of an individual. This improved knowledge may provide a starting point for the development of new, useful SNP markers for medical testing and a safer individualized medication to treat the most common devastating disorders. This will revolutionize the medical field in the future. To illustrate the effect of SNPs on gene function and phenotype, this minireview focuses on evidences revealing the impact of SNPs on the development and progression of three human eye disorders (Norrie disease, familial exudative vitreoretinopathy, and retinopathy of prematurity) that have overlapping clinical manifestations.

The Netrin-related domain of Sfrp1 interacts with Wnt ligands and antagonizes their activity in the anterior neural plate

BACKGROUND

Secreted frizzled related proteins (SFRPs) are multifunctional modulators of Wnt and BMP (Bone Morphogenetic Protein) signalling necessary for the development of most organs and the homeostasis of different adult tissues. SFRPs fold in two independent domains: the cysteine rich domain (SfrpCRD) related to the extracellular portion of Frizzled (Fz, Wnt receptors) and the Netrin module (SfrpNTR) defined by homologies with molecules such as Netrin-1, inhibitors of metalloproteinases and complement proteins. Due to its structural relationship with Fz, it is believed that SfrpCRD interferes with Wnt signalling by binding and sequestering the ligand. In contrast, the functional relevance of the SfrpNTR has been barely addressed.

RESULTS

Here, we combine biochemical studies, mutational analysis and functional assays in cell culture and medaka-fish embryos to show that the Sfrp1NTR mimics the function of the entire molecule, binds to Wnt8 and antagonizes Wnt canonical signalling. This activity requires intact tertiary structure and is shared by the distantly related Netrin-1NTR. In contrast, the Sfrp1CRD cannot mirror the function of the entire molecule in vivo but interacts with Fz receptors and antagonizes Wnt8-mediated beta-catenin transcriptional activity.

CONCLUSION

On the basis of these results, we propose that SFRP modulation of Wnt signalling may involve multiple and differential interactions among Wnt, Fz and SFRPs.

An essential role for Frizzled5 in neuronal survival in the parafascicular nucleus of the thalamus

Frizzled5 (Fz5), a putative Wnt receptor, is expressed in the retina, hypothalamus, and the parafascicular nucleus (PFN) of the thalamus. By constructing Fz5 alleles in which beta-galactosidase replaces Fz5 or in which Cre-mediated recombination replaces Fz5 with alkaline phosphatase, we observe that Fz5 is required continuously and in a cell autonomous manner for the survival of adult PFN neurons, but is not required for proliferation, migration, or axonal growth and targeting of developing PFN neurons. A motor phenotype associated with loss of Fz5 establishes a role for the PFN in sensorimotor coordination. Transcripts coding for Wnt9b, the likely Fz5 ligand in vivo, and beta-catenin, a mediator of canonical Wnt signaling, are both downregulated in the Fz5(-/-) PFN, implying a positive feedback mechanism in which Wnt signaling is required to maintain the expression of Wnt signaling components. These data suggest that defects in Wnt-Frizzled signaling could be the cause of neuronal loss in degenerative CNS diseases.

Non-conventional Frizzled ligands and Wnt receptors

The Wnt family of secreted signaling factors plays numerous roles in embryonic development and in stem cell biology. In the adult, Wnt signaling is involved in tissue homeostasis and mutations that lead to the overexpression of Wnt can be linked to cancer. Wnt signaling is transduced intracellularly by the Frizzled (Fzd) family of receptors. In the canonical pathway, accumulation of beta-catenin and the subsequent formation of a complex with T cell factors (TCF) or lymphoid enhancing factors (Lef) lead to target gene activation. The identification of Ryk as an alternative Wnt receptor and the discovery of the novel Fzd ligands Norrie disease protein (NDP) and R-Spondin, changed the traditional view of Wnts binding to Fzd receptors. Mouse R-Spondin cooperates with Wnt signaling and Low density lipoprotein (LDL) receptor related protein (LRP) to activate beta-catenin dependent gene expression and is involved in processes such as limb and placental development in the mouse. NDP is the product of the Norrie disease gene and controls vascular development in the retina, inner ear and in the female reproductive system during pregnancy. In this review a functional overview of the interactions of the different Wnt and non-Wnt ligands with the Fzd receptors is given as well as a survey of Wnts binding to Ryk and we discuss the biological significance of these interactions.

The Frizzled family of unconventional G-protein-coupled receptors

The Frizzled (FZD) family of receptors is critically involved in embryogenesis, and there is substantial evidence that members of this family also regulate tissue homeostasis in many different organs in the adult. FZD receptors have seven transmembrane-spanning domains and are activated by the WNT family of lipoglycoproteins. Many aspects of FZD signal transduction and pharmacology are still unclear. In this review, we summarize recent advances and some of the key questions about the molecular pharmacology of FZDs, FZD-associated proteins and signal transduction. We also discuss what little is known about the pharmacological binding profiles and the degree of selectivity of WNTs and other extracellular ligands for FZDs. Finally, we focus on signaling events that occur as a direct consequence of FZD activation, signaling via the central phosphoprotein Dishevelled (DVL) and FZD coupling to heterotrimeric G proteins. Here, we outline the current state of knowledge on FZDs and FZD signal transduction and pinpoint aspects of debate and future directions.