Sabine Fuhrmann - US grants

DESCRIPTION (provided by applicant): The long-term goal of this project is to understand how cellular and tissue-tissue interactions regulate development of the central nervous system. Disruption of tissue-tissue interactions can lead to a wide range of serious birth defects affecting the brain and eye, such as holoprosencephaly, congenital anophthalmia, microphthalmia, and anencephaly. Understanding the normal process of development will allow for better prevention and treatment of such defects. It is proposed to study the role of signaling molecules involved in these interactions using the embryonic eye as a model system for forebrain development, where the same mechanisms most likely regulate regional specification. Classical embryological studies have shown that the extraocular tissues are required for normal eye growth and differentiation. At present there is little information about the signals involved in these interactions. In explant cultures of optic vesicles from chick embryos, removal of the extraocular mesenchyme severely interferes with the formation of the retinal pigmented epithelium (RPE). The TGFbeta family member activin has been shown to be a candidate signal that exactly mimics the effects of the extraocular mesenchyme on RPE development in vitro. It is proposed to test the hypothesis whether the activin signaling pathway is required for RPE formation in the chick embryonic eye (Aim 1). To interfere with the activin signaling pathway in the developing RPE, soluble activin type II receptors will be applied as well as ectopic expression of antagonistic Smads (Smad6, 7) and truncated activin type II receptors using electroporation. It will be determined whether activin or a related signal is produced by the extraocular mesenchyme (Aim 2). Since cranial mesenchyme contains the inducer of RPE development, sufficient amounts of this tissue can be isolated for a degenerate PCR strategy. Subsequently, the identified molecule will be cloned and tested for the RPE-promoting activity in explant cultures and by transfection of chick embryos.

[unreadable] DESCRIPTION (provided by applicant): During development of the vertebrate eye, complex patterning events occur which result in the generation of distinct tissue components. Multiple congenital eye disorders, including anophthalmia or micropthalmia, aniridia, coloboma and retinal dysplasia, stem from disruptions in early eye development. Thus, it is critical to define the mechanisms that lead to the patterning and differentiation of ocular tissues, in particular the retina and retinal pigment epithelium (RPE). The molecular signals that mediate patterning events are, for the most part, unknown. Members of the wnt family of ligands are important regulators of cellular proliferation, cell fate decisions and tissue polarity in multiple tissues. Wnts act by binding to members of the Frizzled family of transmembrane receptors. However, the role of Wnt/Frizzled signaling in vertebrate eye development has not been examined, despite the fact that multiple Wnts and Frizzleds are expressed at various stages of eye development. We provide evidence that Wnt/beta-catenin signaling is active in the optic vesicle and hypothesize that it regulates retinal progenitor proliferation and RPE development. To test this we propose experiments in mouse, chick and Xenopus, since each model system offers unique experimental advantages. We will use a reporter of Wnt/beta catenin signaling in transgenic Xenopus and mouse embryos to define when and where during eye development this signaling pathway is active (Aim 1). We will then test whether Wnt/beta-catenin signaling regulates progenitor proliferation and RPE development by perturbing this signaling pathway at various stages of eye development in both chick and Xenopus (Aims 2). Finally, we will determine whether these effects are mediated by the Frizzled-5 receptor, which is selectively expressed in the developing optic vesicle (Aim 3). Together, these experiments will advance our understanding of the signals that pattern the eye during development and may provide clues about how these patterning events are disrupted in congenital eye disorders. In addition, these studies should provide more general insight into the role of Wnt/beta-catenin signaling during nervous system development. [unreadable] [unreadable]

Project Summary/Abstract The retinal pigment epithelium (RPE) is essential for development and function of the eye as it mediates photoreceptor outer segment renewal, regeneration of visual pigments, trans-epithelial transport, retinoid storage and protection against oxidative damage. Accordingly, alterations in RPE structure or physiology caused by environmental or genetic perturbations can ultimately cause blindness. Importantly, improper RPE development impairs eye growth and can result in severe congenital defects such as microphthalmia. Thus, it is critical to identify the mechanisms underlying normal development of the RPE. While some genes crucial for RPE development and function (e.g. Mitf, Otx2) are known, it is unclear how RPE-specific gene expression is initiated and maintained. Our preliminary data in mouse optic vesicle explants suggests that the surrounding extraocular mesenchyme produces a signal(s) to promote RPE development. Furthermore, we have evidence that the Wnt/-catenin pathway is essential for continuation of embryonic RPE differentiation. We hypothesize that activin receptor activation by the mesenchyme acts as an early signal to induce the RPE, while Wnt/- catenin signaling acts later to ensure RPE maintenance and function. In this project, we propose to examine in mouse the exact temporal requirement of extraocular mesenchyme and its role in activating the activin and Wnt/-catenin pathways using explant cultures and tissue-specific gene disruption (Aim 1). To investigate the role of Wnt/-catenin signaling in maintenance of the peri- and postnatal RPE, we will perform inducible, tissue-specific inactivation of -catenin. We will also determine whether Wnt/-catenin through TCF/LEF transcription factors directly activates RPE-specific gene expression using luciferase and ChIP assays (Aim 2). Using AP2 gene disruption in the mouse embryo and FGF treatment of optic vesicle explants, we propose to test whether ectopic and sustained activation of the Wnt/-catenin pathway is sufficient to block transdifferentiation of RPE into retina (Aim 3). Together, these experiments will advance our understanding of the signals that control RPE differentiation during mammalian eye development and may provide clues for therapeutic treatment of degenerative diseases in the eye.

DESCRIPTION (provided by applicant): Congenital ocular malformations such as anophthalmia, microphthalmia and coloboma are prevalent in ~1 in 3-4,000 individuals and are the cause for over 25% of childhood blindness worldwide. Coloboma alone may account up to 10% of childhood blindness. Therefore, it is vitally important to understand the molecular mechanisms underlying ocular development. Wnts belong to a family of secreted, highly conserved glycoproteins that control key processes during development, disease and regeneration. Wnt signaling is very complex; in mammals, 19 Wnts and 10 Frizzled receptors have been identified that can activate the canonical, Wnt/?-catenin pathway and two less well-defined non-canonical pathways, Wnt/Ca2+ and Planar Cell Polarity. This complexity is reflected by the different roles of Wnt signaling during eye development. In this proposal, we aim to address two important questions; 1) How does non-canonical Wnt signaling regulate early eye development? 2) What is the cellular mechanism by which Wnt signaling controls closure of the optic fissure? To begin to tease this apart, we are focusing on the function of Porcupine (Porcn) that mediates posttranslational modification of Wnts. Porcn is a membrane-bound O-acyltransferase that resides in the endoplasmatic reticulum and mediates palmitoylation critical for the secretion and signaling activity of Wnt ligands. The human disease Focal Dermal Hypoplasia (FDH, Goltz Syndrome) is an X-linked, rare dominant disorder caused by mutations in PORCN. About 20% of FDH patients exhibit microphthalmia, anophthalmia, and coloboma (MAC), among several other severe developmental defects. While Porcn mutations cause MAC, we have no current understanding how depletion of Porcn/Wnt results in severe ocular malformations such as anophthalmia and coloboma. Using temporal and tissue-specific inactivation of Porcn, identification of potential targets and diverse in vitro approaches, we propose to identify the cellular interactions regulating closure of the optic fissure (Aim 1), investigate the role of non-canonical Wnt signaling during eye field and optic vesicle formation (Aim 2). In Aim 3, we will investigate the role of the small GTPase Cdc42 in optic vesicle evagination and optic cup morphogenesis. Our approach will identify novel roles for Wnt signaling during eye development that will be important for treatment and regenerative efforts. The etiology of anophthalmia and coloboma in humans is complex and can result from disruption of several factors. The studies proposed here will advance our knowledge toward an understanding of the cellular and molecular mechanisms controlling eye morphogenesis at the earliest stages and during closure of the optic fissure.

The  retinal  pigment  epithelium  (RPE)  maintains  tissue  homeostasis  through  long-­term  survival,  with  little  evidence of de novo cell production. Due to continued growth of the eye and aging, cell density decreases and  RPE  cells  generally  undergo  hypertrophy.  Degeneration  of  the  central  RPE  causes  the  progressive  chronic  disease  age-­related  macular  degeneration  (AMD),  the  leading  cause  of  irreversible  vision  loss  in  the  elderly  population. Besides some protective strategies, no effective cure for AMD is currently available. Recent studies  revealed that substantial cell heterogeneity may be a feature that exists throughout the RPE. Importantly, certain  subsets of this heterogeneous RPE cell population may contain distinct properties contributing to regeneration,  with an intrinsic capacity for self-­renewal. We and others identified signaling pathways critical for developmental  RPE  growth  and  differentiation,  specifically  the  Hippo  and  Wnt  pathways.  Here  we  propose  to  explore  how  genetic modulation of such signaling pathways can stimulate regenerative repair in the mature mammalian RPE  and  to  identify  RPE  subpopulations  with  regenerative potential.  Our  studies  will  provide  insight  into  potential  novel regulators stimulating an intrinsic, regenerative response in the mature RPE. Identifying candidate targets  for  clinical  therapy  is  a  critical  step  forward  for  treatment  of  RPE-­related  diseases  with  currently  very  limited  therapeutic potential, such as atrophic AMD.