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Signaling “Cross-Talk” Is Integrated by Transcription Factors in the Development of the Anterior Segment in the Eye

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Signaling “Cross-Talk” Is Integrated by Transcription Factors in the Development of the Anterior Segment in the Eye DEVELOPMENTAL DYNAMICS 238:2149–2162, 2009 SPECIAL ISSUE REVIEWS–A PEER REVIEWED FORUM Signaling “Cross-Talk” Is Integrated by Transcription Factors in the Development of the Anterior Segment in the Eye Philip J. Gage* and Amanda L. Zacharias Extracellular signaling “cross-talk” between tissues is an important requirement for development of many organs yet the underlying mechanisms generally remain poorly understood. The anterior segment of the eye, which is constructed from four embryonic lineages, provides a unique opportunity to genetically dissect developmental processes such as signaling “cross-talk” without fear of inducing lethality. In the current review, we summarize recent data showing that PITX2, a homeodomain transcription factor, integrates retinoic acid and canonical Wnt/␤-catenin signaling during anterior segment development. Because the requirements for retinoic acid signaling, canonical Wnt/␤-catenin signaling, and PITX2 are not unique to the eye, this newly identified pathway may have relevance elsewhere during development and in tissue homeostasis. Developmental Dynamics 238:2149–2162, 2009. © 2009 Wiley-Liss, Inc. Key words: retinoic acid signaling; canonical Wnt signaling; anterior segment; homeobox Accepted 8 June 2009 INTRODUCTION primordia are required for correct pat- development within the anterior seg- terning and cell specification, which ment and many transcription factors In 1901, Hans Spemann reported that requires cells to integrate multiple are essential for the process as well. the optic cup had the ability to cause signaling inputs. A significant advan- However, only recently has the iden- overlying head surface ectoderm to tage of using the eye as a model sys- tification of an initial cohesive path- form a lens, the first step in the devel- opment of the ocular anterior segment tem is that it is not required for via- way that integrates cell signaling be- (Spemann, 1901). Spemann termed bility. Using appropriate genetic tween tissues with transcriptional this property “induction” and argu- techniques, one can test developmen- networks emerged from this multi- ably launched the modern era of ex- tal processes in great molecular detail tude of factors. Work from several lab- perimental developmental biology. without fear of inducing lethality. Ad- oratories has now provided evidence More than a century later, the ante- vances in understanding basic devel- linking retinoic acid signaling from rior segment of the eye remains an opmental mechanisms gained while the optic cup, the homeodomain tran- important model for investigating ba- studying the anterior segment are of- scription factor PITX2 within the neu- sic developmental mechanisms. As ten highly relevant when subse- ral crest and canonical Wnt/␤-catenin with other organs, the mature ante- quently applied to the development of signaling within the ocular surface ec- rior segment arises from multiple em- other organs or tissues that are re- toderm. Pitx2 is a critical integration bryonic primordia. Extensive induc- quired for viability. node that links the two signaling tive interactions and signaling “cross- All major signaling pathways have pathways. This network is required talk” between and within the distinct been implicated in various aspects of during early eye development to es- Departments of Ophthalmology and Visual Sciences, and Cell and Developmental Biology, University of Michigan Medical School, Ann Arbor, Michigan Grant sponsor: NEI/NIH; Grant numbers: EY014126, EY007003. *Correspondence to: Philip J. Gage, Department of Ophthalmology and Visual Sciences, University of Michigan Medical School, 305 Kellogg Eye Center, 1000 Wall Street, Ann Arbor, MI 48105. E-mail: [email protected] DOI 10.1002/dvdy.22033 Published online 21 July 2009 in Wiley InterScience (www.interscience.wiley.com). © 2009 Wiley-Liss, Inc. 2150 GAGE AND ZACHARIAS tablish the basic framework of general tion of a network of transcription will eventually develop from the ante- patterning and initial differentiation factor genes marking first a single and rior rim of the optic cup. on which future morphogenesis of the subsequently two “eye fields” within The days following formation of the anterior segment depends. How addi- the anterior neural ectoderm (Chow lens vesicle (e10–13.5) are critically tional cell signaling pathways and and Lang, 2001). The optic pits, rep- important for anterior segment devel- transcription factors may contribute resenting the first morphological ev- opment, because the general frame- to this pathway remains to be deter- idence of eye development, form as work of patterning and initial differ- mined. localized thickenings of neural ecto- entiation steps on which the future derm within the inner surface of the cornea, limbus, conjunctiva, and eye- OVERVIEW OF ANTERIOR anterior neural folds at the positions lids, will depend are laid down. This SEGMENT initially marked by the two eye fields period is marked by changes in both (Pei and Rhodin, 1970; Kaufman, morphology and gene expression, and MORPHOGENESIS 1992). By embryonic day 9 (e9), the coincides with the time at which the Major structures/tissues of the ma- optic pits deepen to form the optic newly identified RA/PITX2/Wnt regu- ture anterior segment include the vesicles, outgrowths of neural ecto- latory network is first required (see lens, ciliary body, iris, cornea, limbus, derm on either side of the dienceph- below). At the beginning of this phase bulbar conjunctiva, and the eyelid, alon (Fig. 2A) (Pei and Rhodin, 1970; (e10), morphogenesis of the cornea which is comprised of the palpebral Kaufman, 1992). The optic vesicles and other central anterior segment epidermis, palpebral conjunctiva, and continue to expand laterally until tissues starts with migration of eyelid mesenchyme (Fig. 1A). Four they meet the ocular surface ecto- largely neural crest mesenchyme into embryonic tissues contribute the cel- derm (Fig. 2A). the space between the newly formed lular building blocks for anterior seg- The ocular surface ectoderm (OSE) lens vesicle and the OSE (Fig. 2C) (Pei ment structures: neural ectoderm, is a multipotent patch of head surface and Rhodin, 1970; Kaufman, 1992; surface ectoderm, and neural crest ectoderm that, when appropriately Cvekl and Tamm, 2004). The result- and mesoderm mesenchyme (Fig. 1B). patterned, is fated to contribute the ing 7–8-cell-thick layer of mesen- Each of these primordia eventually lens, all epithelia of the eye surface chyme becomes increasingly more or- contributes multiple lineages within (cornea, limbus, conjunctiva), ecto- ganized and condensed, implying a the mature anterior segment (Fig. derm-derived components of the lacri- response to local cues (Fig. 2D). The 1B). In addition, most mature anterior mal and Harderian (in mice) glands, transcription factor genes Foxc1, segment structures are a synthesis of and the eyelid epidermis (Figs. 1, 2A) Foxc2, Lmx1b, and Pitx2 are all ini- both ectoderm and mesenchyme. For (Ashery-Padan et al., 2000). Anterior tially expressed in specific patterns example, in the cornea, the corneal segment development commences when within the mesenchyme at this time epithelium derives from surface ecto- the distal optic vesicle makes direct and are subsequently required for an- derm whereas the corneal stroma and physical contact with the overlying OSE terior segment development (Gage the corneal endothelium derive from on e9.5 in mice, resulting in induction of and Camper, 1997; Kidson et al., mesenchyme (Fig. 1A,B) (Gage et al., the lens placode within the OSE (Fig. 1999; Pressman et al., 2000; Kume et 2005). This is fundamentally different 2A) (Pei and Rhodin, 1970; Kaufman, al., 2001). Beginning shortly after im- from, e.g., retinal development, where 1992). Therefore, the optic vesicle acts migration of the neural crest mesen- all differentiated neurons and the early in anterior segment development chyme, distinct regions of the OSE Mueller glia differentiate from a sin- as an organizing center that is both nec- corresponding to the future corneal, gle, common pool of neural ectoderm essary and sufficient for specifying limbal, and conjunctival ectoderm, precursors. Work from many model where within the ocular surface ecto- and eyelid epidermis can be identified systems has contributed to our cur- derm the lens primordia will form. The based on differential expression and rent understanding of vertebrate eye lens placode invaginates towards the sub-cellular localization of Connexin development, including the anterior head midline, forming a lens pit (Fig. 43, a gap junction protein that is sub- segment (Jacobson and Sater, 1988; 2B), which then separates from the sequently essential for corneal func- Furukawa et al., 1999; Beebe and OSE to form the lens vesicle (e10.5), the tion (Wolosin et al., 2002). In addition, Coats, 2000; Fuhrmann et al., 2000). precursor to the mature lens (Fig. 2C) by e12.5 differential levels of canoni- Because some interspecies differences (Pei and Rhodin, 1970; Kaufman, 1992). cal Wnt/␤-catenin signaling activity exist, mouse will be used as the refer- The molecular cascade leading to lens distinguish future elements arising ence point for summarizing relevant development is known in considerable from the OSE (Smith et al., 2005). The stages of eye development and ante- detail and has been summarized else- appearance of the eyelid grooves rior segment morphogenesis in this where (Chow and Lang, 2001; Lang, within the OSE dorsal and ventral to review as a prelude to mouse genetics 2004).
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