Mutant frizzled-4 disrupts retinal angiogenesis in familial exudative vitreoretinopathy

The molecular basis of human retinal and vitreoretinal diseases

During the last two to three decades, a large body of work has revealed the molecular basis of many human disorders, including retinal and vitreoretinal degenerations and dysfunctions. Although belonging to the group of orphan diseases, they affect probably more than two million people worldwide. Most excitingly, treatment of a particular form of congenital retinal degeneration is now possible. A major advantage for treatment is the unique structure and accessibility of the eye and its different components, including the vitreous and retina. Knowledge of the many different eye diseases affecting retinal structure and function (night and colour blindness, retinitis pigmentosa, cone and cone rod dystrophies, photoreceptor dysfunctions, as well as vitreoretinal traits) is critical for future therapeutic development. We have attempted to present a comprehensive picture of these disorders, including biological, clinical, genetic and molecular information. The structural organization of the review leads the reader through non-syndromic and syndromic forms of (i) rod dominated diseases, (ii) cone dominated diseases, (iii) generalized retinal degenerations and (iv) vitreoretinal disorders, caused by mutations in more than 165 genes. Clinical variability and genetic heterogeneity have an important impact on genetic testing and counselling of affected families. As phenotypes do not always correlate with the respective genotypes, it is of utmost importance that clinicians, geneticists, counsellors, diagnostic laboratories and basic researchers understand the relationships between phenotypic manifestations and specific genes, as well as mutations and pathophysiologic mechanisms. We discuss future perspectives.

Mutations in TSPAN12 cause autosomal-dominant familial exudative vitreoretinopathy

Familial exudative vitreoretinopathy (FEVR) is an inherited blinding disorder of the retinal vascular system. Although mutations in three genes (LRP5, FZD4, and NDP) are known to cause FEVR, these account for only a fraction of FEVR cases. The proteins encoded by these FEVR genes form part of a signaling complex that activates the Norrin-beta-catenin signaling pathway. Recently, through a large-scale reverse genetic screen in mice, Junge and colleagues identified an additional member of this signaling complex, Tspan12. Here, we report that mutations in TSPAN12 also cause autosomal-dominant FEVR. We describe seven mutations identified in a cohort of 70 FEVR patients in whom we had already excluded the known FEVR genes. This study provides further evidence for the importance of the Norrin-beta-catenin signaling pathway in the development of the retinal vasculature and also indicates that more FEVR genes remain to be identified.

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.

Controlling destiny through chemistry: small-molecule regulators of cell fate

Controlling cell fate is essential for embryonic development, tissue regeneration, and the prevention of human disease. With each cell in the human body sharing a common genome, achieving the appropriate spectrum of stem cells and their differentiated lineages requires the selective activation of developmental signaling pathways, the expression of specific target genes, and the maintenance of these cellular states through epigenetic mechanisms. Small molecules that target these regulatory processes are therefore valuable tools for probing and manipulating the molecular mechanisms by which stem cells self-renew, differentiate, and arise from somatic cell reprogramming. Pharmacological modulators of cell fate could also help remediate human diseases caused by dysregulated cell proliferation or differentiation, heralding a new era in molecular therapeutics.

Differentiation of the brain vasculature: the answer came blowing by the Wnt

Vascularization of the vertebrate brain takes place during embryonic development from a preformed perineural vascular plexus. As a consequence of the intimate contact with neuroectodermal cells the vessels, which are entering the brain exclusively via sprouting angiogenesis, acquire and maintain unique barrier properties known as the blood-brain barrier (BBB). The endothelial BBB depends upon the close association of endothelial cells with pericytes, astrocytes, neurons and microglia, which are summarized in the term neuro-vascular unit. Although it is known since decades that the CNS tissue provides the cues for BBB induction and differentiation in endothelial cells, the molecular mechanism remained obscure.Only recently, the canonical Wnt/beta-catenin pathway and the Wnt7a/7b growth factors have been implicated in brain angiogenesis on the one hand and in BBB induction on the other. This breakthrough in understanding the differentiation of the brain vasculature prompted us to review these findings embedded in the emerging concepts of Wnt signaling in the vasculature. In particular, interactions with other pathways that are crucial for vascular development such as VEGF, Notch, angiopoietins and Sonic hedgehog are discussed. Finally, we considered the potential role of the Wnt pathway in vascular brain pathologies in which BBB function is hampered, as for example in glioma, stroke and Alzheimer's disease.

Wnt modulators in the biotech pipeline

The purpose of this review is to provide a better understanding for the LRP co-receptor-mediated Wnt pathway signaling. Using proteomics, we have also subdivided the LRP receptor family into six sub-families, encompassing the twelve family members. This review includes a discussion of proteins containing a cystine-knot protein motif (i.e., Sclerostin, Dan, Sostdc1, Vwf, Norrin, Pdgf, Mucin) and discusses how this motif plays a role in mediating Wnt signaling through interactions with LRP.

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.

Is Wnt signalling the final common pathway leading to bone formation?

Since the discovery of the link between mutations in the LRP5 gene and human bone mass, considerable progress has been made in our understanding of Wnt signalling and bone formation. The connection between canonical Wnt signalling and bone formation is convincing, and there is evidence of interaction between the Wnt signalling pathway and key growth factors, transcriptional factors and systemic hormones. More recently, the role of the non-canonical pathway in bone metabolism has also started to be explored as well as potential bone-gut interactions. This review focuses on the role of the Wnt pathway in osteoblast differentiation as well as the interplay between Wnt signalling and other pathways involved in bone formation.

GRASP65 and GRASP55 sequentially promote the transport of C-terminal valine-bearing cargos to and through the Golgi complex

The Golgi matrix proteins GRASP65 and GRASP55 have recognized roles in maintaining the architecture of the Golgi complex, in mitotic progression and in unconventional protein secretion whereas, surprisingly, they have been shown to be dispensable for the transport of commonly used reporter cargo proteins along the secretory pathway. However, it is becoming increasingly clear that many trafficking machineries operate in a cargo-specific manner, thus we have investigated whether GRASPs may control the trafficking of selected classes of cargo. We have taken into consideration the C-terminal valine-bearing receptors CD8alpha and Frizzled4 that we show bind directly to the PSD95-DlgA-zo-1 (PDZ) domains of GRASP65 and GRASP55. We demonstrate that both GRASPs are needed sequentially for the efficient transport to and through the Golgi complex of these receptors, thus highlighting a novel role for the GRASPs in membrane trafficking. Our results open new perspectives for our understanding of the regulation of surface expression of a class of membrane proteins, and suggests the causal mechanisms of a dominant form of autosomal human familial exudative vitreoretinopathy that arises from the Frizzled4 mutation involving its C-terminal valine.

Wnt/beta-catenin signaling: components, mechanisms, and diseases

Signaling by the Wnt family of secreted glycolipoproteins via the transcriptional coactivator beta-catenin controls embryonic development and adult homeostasis. Here we review recent progress in this so-called canonical Wnt signaling pathway. We discuss Wnt ligands, agonists, and antagonists, and their interactions with Wnt receptors. We also dissect critical events that regulate beta-catenin stability, from Wnt receptors to the cytoplasmic beta-catenin destruction complex, and nuclear machinery that mediates beta-catenin-dependent transcription. Finally, we highlight some key aspects of Wnt/beta-catenin signaling in human diseases including congenital malformations, cancer, and osteoporosis, and discuss potential therapeutic implications.

Wnt/beta-catenin signaling: components, mechanisms, and diseases

Signaling by the Wnt family of secreted glycolipoproteins via the transcriptional coactivator beta-catenin controls embryonic development and adult homeostasis. Here we review recent progress in this so-called canonical Wnt signaling pathway. We discuss Wnt ligands, agonists, and antagonists, and their interactions with Wnt receptors. We also dissect critical events that regulate beta-catenin stability, from Wnt receptors to the cytoplasmic beta-catenin destruction complex, and nuclear machinery that mediates beta-catenin-dependent transcription. Finally, we highlight some key aspects of Wnt/beta-catenin signaling in human diseases including congenital malformations, cancer, and osteoporosis, and discuss potential therapeutic implications.

Wnt/beta-catenin signaling: components, mechanisms, and diseases

Signaling by the Wnt family of secreted glycolipoproteins via the transcriptional coactivator beta-catenin controls embryonic development and adult homeostasis. Here we review recent progress in this so-called canonical Wnt signaling pathway. We discuss Wnt ligands, agonists, and antagonists, and their interactions with Wnt receptors. We also dissect critical events that regulate beta-catenin stability, from Wnt receptors to the cytoplasmic beta-catenin destruction complex, and nuclear machinery that mediates beta-catenin-dependent transcription. Finally, we highlight some key aspects of Wnt/beta-catenin signaling in human diseases including congenital malformations, cancer, and osteoporosis, and discuss potential therapeutic implications.

Genetic Evaluation to Establish the Diagnosis of X-Linked Familial Exudative Vitreoretinopathy

PURPOSE

To determine the usefulness of genetic analysis for confirming the diagnosis of X-linked familial exudative vitreoretinopathy (FEVR) and verifying the mode of inheritance.

METHODS

Twenty-seven consecutive patients diagnosed with FEVR were enrolled for genetic analysis. All patients underwent dilated fundus examination. A complete birth, medical, and family history was obtained at the time of examination. Patients were categorized by gender and family history in an effort to identify X-linked FEVR. Participants provided a blood sample for analysis and were evaluated for a mutation in the Norrie's disease gene (NDP) by direct sequencing.

RESULTS

Of the 27 enrolled patients, four male patients had a pedigree consistent with X-linked inheritance and 12 male patients had little or no family history. Two of these 16 patients were found to have a missense mutation in the NDP gene.

CONCLUSIONS

We found genetic testing of NDP to be helpful in confirming the diagnosis of X-linked FEVR in male patients, especially when limited family history was available. As genetic diagnostics improve, we feel that confirming diagnoses and informing patients better through genetic evaluation and consultation will become more useful in the clinical practice of ophthalmology.

Familial exudative vitreoretinopathy and DiGeorge syndrome: a new locus for familial exudative vitreoretinopathy on chromosome 22q11.2?

PURPOSE

To describe a patient with DiGeorge syndrome in association with familial exudative vitreoretinopathy (FEVR).

DESIGN

Observational case report.

PARTICIPANTS

A newborn female and her parents.

METHODS

Family members were examined by slit-lamp biomicroscopy and indirect ophthalmoscopy. Deletion mapping was performed by fluorescent in situ hybridization and genotyping. Mutation screening was undertaken by direct sequencing.

MAIN OUTCOME MEASURES

The presence or absence of a microdeletion on chromosome 22q11.2 in the patient and her parents and mutation screening of FZD4 and LRP5 in the patient.

RESULTS

The patient had classical features of DiGeorge syndrome and FEVR. A de novo microdeletion on chromosome 22q11.2 was found in the patient, confirming the diagnosis of DiGeorge syndrome. No mutations were identified in the known FEVR genes.

CONCLUSIONS

Patients with DiGeorge syndrome should have a dilated retinal examination to look for signs of FEVR. Chromosome 22q11.2 may represent a novel locus for FEVR.

A Wnt survival guide: from flies to human disease

It has been two decades since investigators discovered the link between the Drosophila wingless (Wg) gene and the vertebrate oncogene int-1, thus establishing the family of signaling proteins known as Wnts. Since the inception of the Wnt signaling field, there have been 19 Wnt isoforms identified in humans. These secreted glycoproteins can activate at least two distinct signaling pathways in vertebrate cells, leading to cellular changes that regulate a vast array of biological processes, including embryonic development, cell fate, cell proliferation, cell migration, stem cell maintenance, tumor suppression, and oncogenesis. In certain contexts, one subset of Wnt isoforms activates the canonical Wnt/beta-catenin pathway that is characterized by the activation of certain beta-catenin-responsive target genes in response to the binding of Wnt ligand to its cognate receptors. Similarly, a second subset of Wnt isoforms activates beta-catenin-independent pathways, including the Wnt/calcium (Wnt/Ca) pathway and the Wnt/planar cell polarity (Wnt/PCP) pathway, in certain cellular contexts. In addition, research has identified several secreted proteins known to regulate Wnt signaling, including the Dickkopf (DKK) family, secreted Frizzled-related proteins (sFRPs), and Wnt inhibitory factor-1 (WIF-1). The advent of technologies that can provide genome-wide expression data continues to implicate Wnts and proteins that regulate Wnt signaling pathways in a growing number of disease processes. The aim of this review is to provide a context on the Wnt field that will facilitate the interpretation and study of Wnt signaling in the context of human disease.

Wnt genes in the mouse uterus: potential regulation of implantation

Wnt genes are involved in critical developmental and growth processes. The present study comprehensively analyzed temporal and spatial alterations in Wnt and Fzd gene expression in the mouse uterus during peri-implantation of pregnancy. Expression of Wnt4, Wnt5a, Wnt7a, Wnt7b, Wnt11, Wnt16, Fzd2, Fzd4, and Fzd6 was detected in the uterus during implantation. Wnt4 mRNA was most abundant in the decidua, whereas Wnt5a mRNA was restricted to the mesometrial decidua during decidualization. Wnt7a, Wnt7b, and Wnt11 mRNAs were abundantly detected in the endometrial epithelia. The expression of Wnt7b was robust in the luminal epithelium (LE) at the implantation site on Gestational Day 5, whereas Wnt11 mRNA disappeared in the LE adjacent to the embryo in the antimesometrial implantation chamber but remained abundant in the LE. Wnt16 mRNA was localized to the stroma surrounding the LE on Day 4 and remained in the stroma adjacent to the LE but not in areas undergoing the decidual reaction. Fzd2 mRNA was detected in the decidua, Fzd4 mRNA was in the vessels and stroma surrounding the embryo, and Fzd6 mRNA was observed in the endometrial epithelia, stroma, and some blood vessels during implantation. Ovarian steroid hormone treatment was found to regulate Wnt genes and Fzd receptors in ovariectomized mice. Especially, single injections of progesterone stimulated Wnt11 mRNA, and estrogen stimulated Wnt4 and Wnt7b. The temporal and spatial alterations in Wnt genes likely play a critical role during implantation and decidualization in mice.

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.

An essential role for frizzled 5 in mammalian ocular development

Microphthalmia, coloboma and persistent fetal vasculature within the vitreous cavity are among the most common human congenital ocular anomalies,and each has been associated with a variety of genetic disorders. Here we show that, in the mouse, loss of frizzled 5 (Fz5) - a putative Wnt receptor expressed in the eye field, optic cup and retina - causes all of these defects with high penetrance. In the developing Fz5-/- eye, the sequence of defects, in order of appearance, is: increased cell death in the ventral retina, delayed and/or incomplete closure of the ventral fissure, an excess of mesenchymal cells in the vitreous cavity, an excess of retinal astrocyte precursors and mature astrocytes, and persistence of the hyaloid vasculature in association with a large number of pigment cells. Fz5-/- mice also exhibit a late-onset progressive retinal degeneration by ∼6 months of age, which might be related to the expression of Fz5 in Müller glia in the adult retina. These results demonstrate a central role for frizzled signaling in mammalian eye development and are likely to be relevant to the etiology of congenital human ocular anomalies.

Molecular profile of endothelial invasion of three-dimensional collagen matrices: insights into angiogenic sprout induction in wound healing

Sprouting angiogenesis is a multistep process consisting of basement membrane degradation, endothelial cell (EC) activation, proliferation, invasion, lumen formation, and sprout stabilization. Such complexity is consistent with a requirement for orchestration of individual gene expression alongside multiple signaling pathways. To better understand the mechanisms that direct the transformation of adherent ECs on the surface of collagen matrices to develop multicellular invading sprouts, we analyzed differential gene expression with time using a defined in vitro model of EC invasion driven by the combination of sphingosine-1-phosphate, basic FGF, and VEGF. Gene expression changes were confirmed by real-time PCR and Western blot analyses. A cohort of cell adhesion molecule genes involved in adherens junction and cell-extracellular matrix (ECM) interactions were upregulated, whereas a set of genes associated with tight junctions were downregulated. Numerous genes encoding ECM proteins and proteases were induced, indicating that biosynthesis and remodeling of ECM is indispensable for sprouting angiogenesis. Knockdown of a highly upregulated gene, a disintegrin and metalloproteinase with thrombospondin-type repeats-1 (ADAMTS1), decreased invasion responses, confirming a role for ADAMTS1 in mediating EC invasion. Furthermore, differential expression of multiple members of the Wnt and Notch pathways was observed. Functional experiments indicated that inhibition and activation of the Notch signaling pathway stimulated and inhibited EC invasion responses, respectively. This study has enhanced the molecular road map of gene expression changes that occur during endothelial invasion and highlighted the utility of three-dimensional models to study EC morphogenesis.

Formation of the blood-brain barrier: Wnt signaling seals the deal

Capillaries in the brain are especially selective in determining which blood-borne components gain access to neurons. The structural elements of this blood–brain barrier (BBB) reside at the tight junction, an intercellular protein complex that welds together adjacent endothelial cell membranes in the microvasculature. In this issue, Liebner et al. (Liebner, S., M. Corada, T. Bangsow, J. Babbage, A. Taddei, C.J. Czupalla, M. Reis, A. Felici, H. Wolburg, M. Fruttiger, et al. 2008. J. Cell Biol. 183: 409–417) report that Wnt signaling plays an active role in the development of the BBB by regulating expression of key protein constituents of the tight junction. Such mechanistic insight has implications for a variety of neuropathological states in which the BBB is breached.

Wnt signaling in angiogenesis

Although progress has been made in understanding the role of growth factors and their receptors in angiogenesis, little is known about how the Wnt family of growth factors function in the vasculature. Wnts are multifunctional factors that act through the frizzled receptors to regulate proliferation, apoptosis, branching morphogenesis, inductive processes, and cell polarity. All of these processes must occur as developing vascular structures are formed and maintained. Recent evidence has linked the Wnt/Frizzled signaling pathway to proper vascular growth in murine and human retina. Here we review the literature describing the angiogenic functions for Wnt signaling and focus on a newly discovered angiogenic factor, Norrin, which acts through the Wnt receptor, Frizzled4.

The Wnt signaling regulator R-spondin 3 promotes angioblast and vascular development

The vertebrate embryonic vasculature develops from angioblasts, which are specified from mesodermal precursors and develop in close association with blood cells. The signals that regulate embryonic vasculogenesis and angiogenesis are incompletely understood. Here, we show that R-spondin 3 (Rspo3), a member of a novel family of secreted proteins in vertebrates that activate Wnt/beta-catenin signaling, plays a key role in these processes. In Xenopus embryos, morpholino antisense knockdown of Rspo3 induces vascular defects because Rspo3 is essential for regulating the balance between angioblast and blood cell specification. In mice, targeted disruption of Rspo3 leads to embryonic lethality caused by vascular defects. Specifically in the placenta, remodeling of the vascular plexus is impaired. In human endothelial cells, R-spondin signaling promotes proliferation and sprouting angiogenesis in vitro, indicating that Rspo3 can regulate endothelial cells directly. We show that vascular endothelial growth factor is an immediate early response gene and a mediator of R-spondin signaling. The results identify Rspo3 as a novel, evolutionarily conserved angiogenic factor in embryogenesis.

BacMam system for high-level expression of recombinant soluble and membrane glycoproteins for structural studies

Baculovirus mediated gene transduction of mammalian cells (BacMam) is an emerging technique for rapid recombinant protein expression in mammalian cells. We constructed two baculovirus transfer vectors that incorporate several mammalian transcriptional regulatory elements necessary for high-level protein expression in mammalian cells. Using these vectors, we show that the BacMam system in combination with the 293 GnTI(-) cell line can be used for production of milligram quantities of soluble glycoproteins. Moreover, for crystallization trials, the purified glycoproteins are sensitive to EndoH treatment resulting in a loss of the bulk of the attached N-linked glycosylation. In addition, we also show that a combination of the BacMam system and 293 GnTI(-) cell line can be used for producing milligram quantities of a GPCR-protein ligand complex suitable for crystallization trials.

Vascular changes in the cerebellum of Norrin /Ndph knockout mice correlate with high expression of Norrin and Frizzled-4

X-linked Norrie disease, familial exudative vitreoretinopathy (FEVR), Coat's disease and retinopathy of prematurity are severe human eye diseases and can all be caused by mutations in the Norrie disease pseudoglioma gene. They all show vascular defects and characteristic features of retinal hypoxia. Only Norrie disease displays additional neurological symptoms, which are sensorineural hearing loss and mental retardation. In the present study, we analysed transcript levels of the ligand Norrin (Ndph) and its two receptors Frizzled-4 (Fzd4) and LDL-related protein receptor 5 (Lrp5) in six different brain regions (cerebellum, cortex, hippocampus, olfactory bulb, pituitary and brain stem) of 6- to 8-month-old wild-type and Ndph knockout mice by quantitative real-time PCR. No effect of the Ndph knockout allele on Fzd4 or Lrp5 receptor expression was found. Furthermore, no alterations of the transcript levels of three hypoxia-regulated angiogenic factors (Vegfa, Itgrb3 and Tie1) were observed in the absence of Norrin. Interestingly, we identified significant differences in Ndph, Fzd4 and Lrp5 transcript levels in brain regions of wild-type mice and observed highest expression of Norrin and frizzled-4 in cerebellum. Transcript analyses were correlated with morphological data obtained from cerebellum and immunohistochemical studies of blood vessels in different brain regions. Vessel density was reduced in the cerebellum of Ndph knockout mice but the number of Purkinje and granular cells was not altered. This provides the first description of a brain phenotype in Ndph knockout mice, which will help to elucidate the role of Norrin in the brain.

Comparative transcriptomics of human multipotent stem cells during adipogenesis and osteoblastogenesis

Background

A reciprocal relationship between bone and fat development in osteoporosis is clinically well established. Some of the key molecular regulators involved in this tissue replacement process have been identified. The detailed mechanisms governing the differentiation of mesenchymal stem cells (MSC) – the key cells involved – are however only now beginning to emerge. In an attempt to address the regulation of the adipocyte/osteoblast balance at the level of gene transcription in a comprehensive and unbiased manner, we performed a large-scale gene expression profiling study using a unique cellular model, human multipotent adipose tissue-derived stem cells (hMADS).

Results

The analysis of 1606 genes that were found to be differentially expressed between adipogenesis and osteoblastogenesis revealed gene repression to be most prevalent prior to commitment in both lineages. Computational analyses suggested that this gene repression is mediated by miRNAs. The transcriptional activation of lineage-specific molecular processes in both cases occurred predominantly after commitment. Analyses of the gene expression data and promoter sequences produced a set of 65 genes that are candidates for genes involved in the process of adipocyte/osteoblast commitment. Four of these genes were studied in more detail: LXRα and phospholipid transfer protein (PLTP) for adipogenesis, the nuclear receptor COUP-TF1 and one uncharacterized gene, TMEM135 for osteoblastogenesis. PLTP was secreted during both early and late time points of hMADS adipocyte differentiation. LXRα, COUP-TF1, and the transmembrane protein TMEM135 were studied in primary cultures of differentiating bone marrow stromal cells from healthy donors and were found to be transcriptionally activated in the corresponding lineages.

Conclusion

Our results reveal gene repression as a predominant early mechanism before final cell commitment. We were moreover able to identify 65 genes as candidates for genes controlling the adipocyte/osteoblast balance and to further evaluate four of these. Additional studies will explore the precise role of these candidate genes in regulating the adipogenesis/osteoblastogenesis switch.

[Mutations of the frizzled-4 gene. Their impact on medical care of patients with autosomal dominant exudative vitreoretinopathy]

PURPOSE

Autosomal dominant (familial) exudative vitreoretinopathy (adEVR) is a rare, congenital disease of the retinal vascular system, which may lead to blindness in severely affected eyes. One of the causative disease genes is located on chromosome 11q13-q23 and codes for "frizzled-4" (FZD4), a protein involved in vascular differentiation.

METHOD

Examination of two families with adEVR over six and four generations and FZD4 mutation analysis.

RESULTS

In family I, 18 examined affected members exhibited a heterozygous missense mutation (p.G492R) in the FZD4 gene. In family II, four examined family members were affected and carried a heterozygous deletion of five nucleotides (c.1286del5). Both mutations are novel and showed 100% penetrance and variable expressivity.

CONCLUSIONS

With detection of the "family-specific" FZD4 gene mutation, carriers amongst offspring of affected family members can be identified at an early time. The complete penetrance of FZD4 mutations may justify abandoning repeated examinations of offspring of affected family members, if no mutations were detected in FZD4.

From individual Wnt pathways towards a Wnt signalling network

Wnt proteins play important roles during vertebrate and invertebrate development. They obviously have the ability to activate different intracellular signalling pathways. Based on the characteristic intracellular mediators used, these are commonly described as the Wnt/beta-catenin, the Wnt/calcium and the Wnt/Jun N-terminal kinase pathways (also called planar cell polarity pathway). In the past, these different signalling events were mainly described as individual and independent signalling branches. Here, we discuss the possibility that Wnt proteins activate a complex intracellular signalling network rather than individual pathways and suggest a graph representation of this network. Furthermore, we discuss different ways of how to predict the specific outcome of an activation of this network in a particular cell type, which will require the use of mathematical models. We point out that the use of deterministic approaches via the application of differential equations is suitable to model only small aspects of the whole network and that more qualitative approaches are possibly a suitable starting point for the prediction of the global behaviour of such large protein interaction networks.

The canonical Wnt signaling antagonist DKK2 is an essential effector of PITX2 function during normal eye development

Local control of cell signaling activity and integration of inputs from multiple signaling pathways are central for normal development but the underlying mechanisms remain poorly understood. Here we show that Dkk2, encoding an antagonist of canonical Wnt signaling, is an essential downstream target of the PITX2 homeodomain transcription factor in neural crest during eye development. Canonical Wnt signaling is ectopically activated in central ocular surface ectoderm and underlying mesenchyme in Pitx2- and Dkk2-deficient mice. General ocular surface ectoderm identity is maintained during development in Dkk2-deficient mice but peripheral fates, including conjunctival goblet cells and eyelash follicles, are ectopically permitted within more central structures and eyelids are hypomorphic. Loss of DKK2 results in ectopic blood vessels within the periocular mesenchyme and PITX2 expression remains persistently high, providing evidence for a negative feedback loop. Collectively, these data suggest that activation of Dkk2 by PITX2 provides a mechanism to locally suppress canonical Wnt signaling activity during eye development, a paradigm that may be a model for achieving local or transient inhibition of pathway activity elsewhere during embryogenesis. We further propose a model placing PITX2 as an essential integration node between retinoic acid and canonical Wnt signaling during eye development.

Structural diversity of G protein-coupled receptors and significance for drug discovery

G protein-coupled receptors (GPCRs) are the largest family of membrane-bound receptors and also the targets of many drugs. Understanding of the functional significance of the wide structural diversity of GPCRs has been aided considerably in recent years by the sequencing of the human genome and by structural studies, and has important implications for the future therapeutic potential of targeting this receptor family. This article aims to provide a comprehensive overview of the five main human GPCR families--Rhodopsin, Secretin, Adhesion, Glutamate and Frizzled/Taste2--with a focus on gene repertoire, general ligand preference, common and unique structural features, and the potential for future drug discovery.

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.

Recent trends in the management of rhegmatogenous retinal detachment

It has been nearly a century since Jules Gonin performed the first intervention for rhegmatogenous retinal detachment, trans-scleral cautery, achieving successful outcomes in close to 50% of his cases. With the introduction of alternative surgical approaches in the last half-century, including Charles Schepens' scleral buckle technique and Robert Machemer's pars plana vitrectomy, the surgical success rates have risen to close to 90%. Nonetheless, despite dramatic progress in the success of reattachment surgeries, reasonable disagreement exists as to which approach (or combination of approaches) is the best form of surgical intervention for patients with rhegmatogenous retinal detachments. In this review, the authors summarize the current knowledge of retinal detachment, and examine emerging results from the first large scale, prospective, randomized, controlled clinical trials addressing the efficacy of these surgical approaches for retinal detachment, with the hope of identifying the most appropriate (evidence-based) therapeutic intervention for the treatment of rhegmatogenous retinal detachment.

The Wnt signaling pathway: aging gracefully as a protectionist?

No longer considered to be exclusive to cellular developmental pathways, the Wnt family of secreted cysteine-rich glycosylated proteins has emerged as versatile targets for a variety of conditions that involve cardiovascular disease, aging, cancer, diabetes, neurodegeneration, and inflammation. In particular, modulation of Wnt signaling may fill a critical void for the treatment of disorders that impact upon both cellular survival and cellular longevity. Yet, in some scenarios, Wnt signaling can become the catalyst for disease development or promote cell senescence that can compromise clinical utility. This double edge sword in regards to the role of Wnt and its signaling pathways highlights the critical need to further elucidate the cellular mechanisms governed by Wnt in conjunction with the development of robust pharmacological ligands that may open new avenues for disease treatment. Here we discuss the influence of the Wnt pathway during cell survival, metabolism, and aging in order for one to gain a greater insight for the novel role of Wnt signaling as well as exemplify its unique cellular pathways that influence both normal physiology and disease.

A model for familial exudative vitreoretinopathy caused by LPR5 mutations

We have identified a mouse recessive mutation that leads to attenuated and hyperpermeable retinal vessels, recapitulating some pathological features of familial exudative vitreoretinopathy (FEVR) in human patients. DNA sequencing reveals a single nucleotide insertion in the gene encoding the low-density lipoprotein receptor-related protein 5 (LRP5), causing a frame shift and resulting in the replacement of the C-terminal 39 amino acid residues by 20 new amino acids. This change eliminates the last three PPP(S/T)P repeats in the LRP5 cytoplasmic domain that are important for mediating Wnt/beta-catenin signaling. Thus, mutant LRP5 protein is probably unable to mediate its downstream signaling. Immunostaining and three-dimensional reconstructions of retinal vasculature confirm attenuated retinal vessels. Ultrastructural data further reveal that some capillaries lack lumen structure in the mutant retina. We have also verified that LRP5 null mice develop similar alterations in the retinal vasculature. This study provides direct evidence that LRP5 is essential for the development of retinal vasculature, and suggests a novel role played by LRP5 in capillary maturation. LRP5 mutant mice can be a useful model to explore the clinical manifestations of FEVR.

Wnt/Frizzled signaling in angiogenesis

The roles of growth factors such as angiopoietin (Ang) and vascular endothelial growth factor (VEGF) in angiogenesis have been known for some time, yet we have just an incipient appreciation for the contribution of Wnts to this process. Cellular proliferation and polarity, apoptosis, branching morphogenesis, inductive processes, and the maintenance of stem cells in an undifferentiated, proliferative state are all regulated by Wnt signaling. The development and maintenance of vascular structures are dependent on all these processes, and their orchestration has, to some extent, been revealed in studies of VEGF and Ang receptors. Recent evidence links the Wnt/Frizzled signaling pathway to proper vascular growth in mammals but our knowledge of Wnt function in the vasculature is rudimentary. Further insight into vascular development and the process of angiogenesis depends on evaluating the function of novel endothelial regulatory pathways such as Wnt/Frizzled signaling.

The Wnt signaling pathway in familial exudative vitreoretinopathy and Norrie disease

The Wnt signaling pathway is highly conserved among species and has an important role in many cell biological processes throughout the body. This signaling cascade is involved in regulating ocular growth and development, and recent findings indicate that this is particularly true in the retina. Mutations involving different aspects of the Wnt signaling pathway are being linked to several diseases of retinal development. The aim of this article is to first review the Wnt signaling pathway. We will then describe two conditions, familial exudative vitreoretinopathy (FEVR) and Norrie disease (ND), which have been shown to be caused in part by defects in the Wnt signaling cascade.

Characterization of Wnt signaling during photoreceptor degeneration

PURPOSE

The Wnt pathway is an essential signaling cascade that regulates multiple processes in developing and adult tissues, including differentiation, cellular survival, and stem cell proliferation. The authors recently demonstrated altered expression of Wnt pathway genes during photoreceptor death in rd1 mice, suggesting an involvement for Wnt signaling in the disease process. In this study, the authors investigated the role of Wnt signaling in retinal degeneration.

METHODS

The Wnt signaling reporter mouse line Tcf-LacZ was crossed with retinal degeneration rd1 mice, and beta-galactosidase expression was used to localize Wnt signaling during photoreceptor death. To analyze the role of Wnt signaling activation, primary mixed retinal cultures were prepared, and XTT and TUNEL assays were used to quantify cell death. Luciferase reporter assays were used to measure Wnt signaling.

RESULTS

The canonical Wnt signaling pathway was activated in Müller glia and the ganglion cell layer during rod photoreceptor degeneration in rd1/Tcf-LacZ mice. Wnt signaling was confirmed in cultured primary Müller glia. Furthermore, Wnt signaling activators protected photoreceptors in primary retinal cultures from H(2)O(2)-induced oxidative stress. The Wnt ligands Wnt5a, Wnt5b, Wnt10a, and Wnt13 were expressed in the degenerating retina and are candidate Wnt signaling activators in vivo.

CONCLUSIONS

This study is the first demonstration that Wnt signaling is activated in the degenerating retina and that it protects retinal cultures from oxidative stress. These data suggest that Wnt signaling is a component of the glial protective response during photoreceptor injury. Therefore, inducing Wnt activation, alone or in combination with growth factors, may increase the threshold for apoptosis and halt or delay further photoreceptor degeneration.

High-throughput RNAi screen in Drosophila

Genetic and biochemical analyses in model systems such as the fruitfly, Drosophila melanogaster, have successfully identified several genes that play key regulatory roles in fundamental cellular and developmental processes. However, the analyses of the complete genome sequences of Drosophila, as well as of humans, now reveal that traditional methods have ascribed functions to only a fraction of the total predicted genes. Thus, the roles for many, as yet unidentified genes, in normal development and cancer remain to be discovered. The challenge presented by the various large-scale genome projects is how to derive biologically relevant information from the raw sequences. The past few years have witnessed a rapid growth in the development and implementation high-throughput screening (HTS) technologies that researchers are now using to discover "gene-function" in an unbiased, systematic, and time-efficient manner. In fact one of the most promising functional genomic approach that has emerged in the past few years is based on RNA-interference (RNAi), in which the introduction of double-stranded RNA (dsRNA) into cells or whole organisms has been shown to be an effective tool to suppress endogenous gene expression. The RNAi technology has made it feasible to query the function of every gene in the genome for their potential function in a given cell-biological process using cell-based assays. This chapter discusses the application, advantages, and limitations of this powerful technology in the identification of novel modulators of cell-signaling pathways as well as its future scope and utility in designing more efficient genome-scale screens.

The functions and possible significance of Kremen as the gatekeeper of Wnt signalling in development and pathology

Kremen (Krm) was originally discovered as a novel transmembrane protein containing the kringle domain. Both Krm1 (the first identified Krm) and its relative Krm2 were later identified to be the high-affinity receptors for Dickkopf (Dkk), the inhibitor of Wnt/β-catenin signalling. The formation of a ternary complex composed of Krm, Dkk, and Lrp5/6 (the coreceptor of Wnt) inhibits Wnt/β-catenin signalling. In Xenopus gastrula embryos, Wnt/β-catenin signalling regulates anterior-posterior patterning, with low-signalling in anterior regions. Inhibition of Krm1/2 induces embryonic head defects. Together with anterior localization of Krms and Dkks, the inhibition of Wnt signalling by Dkk-Krm action seems to allow anterior embryonic development. During mammalian development, krm1 mRNA expression is low in the early stages, but gradually and continuously increases with developmental progression and differentiation. In contrast with the wide, strong expression of krm1 mRNA in mature tissues, expression of krm1 is diminished in a variety of human tumor cells. Since stem cells and undifferentiated cells rely on Wnt/β-catenin signalling for maintenance in a low differentiation state, the physiological shutdown of Wnt/β-catenin signalling by Dkk-Krm is likely to set cells on a divergent path toward differentiation. In tumour cells, a deficit of Krm may increase the susceptibility to tumourigenic transformation. Both positive and negative regulation of Wnt/β-catenin signalling definitively contributes to diverse developmental and physiological processes, including cell-fate determination, tissue patterning and stem cell regulation. Krm is quite significant in these processes as the gatekeeper of the Wnt/β-catenin signalling pathway.

Genetic susceptibility to retinopathy of prematurity: the evidence from clinical and experimental animal studies

Despite advances in management and treatment, retinopathy of prematurity remains a major cause of childhood blindness. Evidence for a genetic basis for susceptibility to retinopathy of prematurity is examined, including the influences of sex, ethnicity, and ocular pigmentation. The role of polymorphisms is explored in the genes for vascular endothelial growth factor and insulin-like growth factor-1, and of mutations in the Norrie disease gene. Insights into the genetic basis of retinopathy of prematurity provided by the animal model of oxygen induced retinopathy are examined. Evidence for a genetic component for susceptibility to retinopathy of prematurity is strong, although the molecular identity of the gene or genes involved remains uncertain.

Very Low Density Lipoprotein Receptor, a Negative Regulator of the wnt Signaling Pathway and Choroidal Neovascularization

Choroidal neovascularization (CNV) in age-related macular degeneration is a leading cause of blindness. Very low density lipoprotein receptor gene knock-out (Vldlr(-/-)) mice have been shown to develop subretinal neovascularization (NV) with an unknown mechanism. The present study showed that in Vldlr(-/-) mice, NV initiated in the choroid and progressed to penetrate the retinal pigment epithelium layer, proliferating in the subretinal space. This phenotype recapitulated what is seen in wet age-related macular degeneration, suggesting that this is a CNV model. The CNV correlated with overexpression of vascular endothelial growth factor in Vldlr(-/-) eyecups and was blocked by a neutralizing antibody against vascular endothelial growth factor receptor-2. The wnt co-receptor LRP5/6 expression was significantly up-regulated in Vldlr(-/-) eyecups compared with that in wild-type mice. Significantly, Vldlr(-/-) mice showed impaired phosphorylation of downstream effectors of the wnt signaling pathway, glycogen synthase kinase-3beta (GSK-3beta), and beta-catenin, concomitant with increased levels of free GSK-3beta and beta-catenin, suggesting an increased activity of the wnt pathway. Down-regulation of VLDLR by small interference RNA resulted in up-regulation of LRP5/6 expression and activation of beta-catenin in cultured endothelial cells. Furthermore, Dickkopf-1, a specific inhibitor of the wnt pathway, effectively decreased vascular endothelial growth factor and beta-catenin levels in the retinal pigment epithelium of Vldlr(-/-) mice and in cells transfected with the VLDLR small interference RNA. These results suggest that VLDLR functions as a negative regulator of CNV, and this function is mediated through the wnt pathway.