Developmental Biology 393 (2014) 195–208 Contents lists available at ScienceDirect Developmental Biology journal homepage: www.elsevier.com/locate/developmentalbiology Review The role of homeobox genes in retinal development and disease Jamie L. Zagozewski a,1, Qi Zhang b,1, Vanessa I. Pinto c, Jeffrey T. Wigle c,d, David D. Eisenstat a,c,e,n a Department of Medical Genetics, University of Alberta, Edmonton, AB, Canada T6G 2H7 b Department of Human Anatomy and Cell Science, University of Manitoba, Winnipeg, MB, Canada R3E 0J9 c Department of Biochemistry and Medical Genetics, University of Manitoba, Winnipeg, MB, Canada R3E 0J9 d Institute of Cardiovascular Sciences, St. Boniface Hospital Research Institute, Winnipeg, MB, Canada R2H 2A6 e Department of Pediatrics, University of Alberta, Edmonton, AB, Canada T6G 1C9 article info abstract Article history: Homeobox genes are an evolutionarily conserved class of transcription factors that are critical for Received 24 March 2014 development of many organ systems, including the brain and eye. During retinogenesis, homeodomain- Received in revised form containing transcription factors, which are encoded by homeobox genes, play essential roles in the 2 July 2014 regionalization and patterning of the optic neuroepithelium, specification of retinal progenitors and Accepted 8 July 2014 differentiation of all seven of the retinal cell classes that derive from a common progenitor. Home- Available online 15 July 2014 odomain transcription factors control retinal cell fate by regulating the expression of target genes Keywords: required for retinal progenitor cell fate decisions and for terminal differentiation of specific retinal cell Retina types. The essential role of homeobox genes during retinal development is demonstrated by the number Vertebrate of human eye diseases, including colobomas and anophthalmia, which are attributed to homeobox gene Homeobox mutations. In the following review, we highlight the role of homeodomain transcription factors during Transcription factor Coloboma retinogenesis and regulation of their gene targets. Understanding the complexities of vertebrate retina Anophthalmia development will enhance our ability to drive differentiation of specific retinal cell types towards novel cell-based replacement therapies for retinal degenerative diseases. & 2014 Elsevier Inc. All rights reserved. Introduction restricted to produce the correct retinal cell types within the appropriate temporal window. Initiation of intrinsic transcription The retina is a remarkably complex, yet structurally simple factor expression in the RPC population plays a critical role in laminar tissue made up of seven distinct retinal cell types (6 directing these multipotent cells to adopt specific retinal cell fates neuronal and 1 glial), that are born sequentially in a temporally (Marquardt, 2003; Ohsawa and Kageyama, 2008). Transcription ordered and overlapping manner (Young, 1985). These cells are factors of the homeodomain, basic helix-loop-helix (bHLH) and organized into three cellular layers, which include the ganglion forkhead box (FOX) families work in concert to direct retinal cell cell layer (GCL), the inner nuclear layer (INL) and the outer nuclear fate specification and differentiation. Homeobox genes encode layer (ONL) (Fig. 1). As mouse embryonic retinal development transcription factors containing a 60 amino acid DNA-binding proceeds, retinal ganglion cells (RGC) are generated first, followed homeodomain. These homeodomains canonically bind to core by cone photoreceptors, horizontal cells and amacrine cells. tetranucelotide TAAT/ATTA motifs (Gehring et al., 1994). Unique During the postnatal period, rod photoreceptors, bipolar cells adjacent consensus motifs impart specificity of binding of indivi- and Müller glia are specified and complete differentiation. Retinal dual transcription factors (Gehring et al., 1994). Homeobox genes cells arise from a common pool of multipotent retinal progenitor can be further classified based on the presence of additional cells (RPC) (Livesey and Cepko, 2001; Turner and Cepko, 1987; conserved domains, such as the paired and LIM domains. Homeo- Wetts and Fraser, 1988). RPC competence gradually becomes box genes play a number of critical roles during vertebrate forebrain (Wigle and Eisenstat, 2008) and eye development including specification of the eye field and the optic stalk, n Corresponding author at: Depatments of Pediatrics, Medical Genetics and progenitor cell fate determination, and retinal cell differentiation Oncology, Room 8-43B, Medical Sciences Building, University of Alberta, Edmonton, and survival. Due to these critical roles, mutations in human AB, Canada T6G 2H7. Fax: þ780 492 1998. E-mail address: [email protected] (D.D. Eisenstat). homeobox genes can lead to a number of ocular abnormalities, 1 JLZ and QZ are co-first authors. ranging from colobomas to anophthalmia (Table 1). We will focus http://dx.doi.org/10.1016/j.ydbio.2014.07.004 0012-1606/& 2014 Elsevier Inc. All rights reserved. 196 J.L. Zagozewski et al. / Developmental Biology 393 (2014) 195–208 Fig. 1. Expression of homeobox genes in the mature vertebrate retina. Homeobox genes are required for specification and differentiation of retinal cells. RGCs (green, G) are located in the GCL which also contains displaced amacrine cells (not shown). The INL contains the cell bodies of the amacrine cells (pink, A), horizontal cells (purple, H), bipolar cells (gray, B) and the Müller glia (black, M). The ONL contains the light-sensitive rod and cone photoreceptors (red, P). Synaptic connections between the photoreceptors in the ONL and horizontal and bipolar cells are made in the OPL while connections between bipolar cells and horizontal cells are made with the RGC in the IPL. Homeobox genes are expressed in all retinal cell types, including RPC. Specific homeobox genes expressed in the cells of the vertebrate retina are shown on the right and are also color coordinated with the cell type in which they are expressed. In addition, the first letter corresponding to the retinal cell types is indicated next to the homeobox genes expressed in that specific cell type. [GCL, ganglion cell layer; INL, inner nuclear layer; IPL, inner plexiform layer; ONL, outer nuclear layer; OPL, outer plexiform layer; RGC, retinal ganglion cell; RPC, retinal progenitor cell]. our review on members of the homeobox gene family, and the have been associated with both anophthalmia and microphthal- critical role each gene plays in RPC fate specification, retinal cell mia (Table 1)(Abouzeid et al., 2012; Bardakjian and Schneider, type differentiation and inherited eye diseases. 2011; Voronina et al., 2004). Mutations are typically located in either the DNA binding homeodomain or the OAR domain and Rax result in impaired expression of RAX target genes (Lequeux et al., 2008). Retina and anterior neural fold homeobox gene (Rax), pre- In addition to a role in early eye development, Rax regulates viously referred to as the retinal homeobox gene (Rx), belongs to expression of genes required for photoreceptor development the paired-like (Prd-L) homeodomain (HD) family. In addition to (Fig. 2). Knockdown of zebrafish and Xenopus Rax orthologs results the paired-like homeodomain, the RAX protein contains a con- in reduction of photoreceptor-specific gene expression (Nelson et served OAR domain in the C-terminus named for conservation of al., 2009; Pan et al., 2010). Early in vitro studies demonstrated that this region between opt, aristaless and rax (Furukawa et al., 1997). RAX can directly bind to labeled oligonucleotide probes of the Rax genes have been identified in a number of model systems arrestin promoter (a photoreceptor specific gene) through the including mice, zebrafish, chick, Xenopus, and Drosophila with the photoreceptor conserved element (PCE-1) in vitro and activate number of Rax genes varying depending on the species. Both the expression of arrestin in reporter gene assays (Kimura et al., 2000). structure and the expression pattern of the Rax genes are highly More recently, the Xenopus Rax gene was demonstrated to occupy conserved across these species (Chuang and Raymond, 2001; PCE-1 containing promoter elements of rhodopsin and red cone Furukawa et al., 1997; Mathers et al., 1997; Muranishi et al., opsin in vivo (Pan et al., 2010). Rax has also been shown to regulate 2012). Rax is among the earliest genes expressed in the developing the expression of the murine homolog of orthodenticle (Otx2), a retina. During zebrafish gastrulation, rx3 expression in the anterior homeobox gene essential for determination of photoreceptor cell forebrain permits specification of the eye field over telencephalic fate (Muranishi et al., 2011). A conserved enhancer upstream of the fate (Stigloher et al., 2006). Murine Rax is first expressed at E7.5 in OTX2 initiation codon was identified and termed the embryonic the cephalic neural fold (Furukawa et al., 1997; Mathers et al., enhancer locus for photoreceptor Otx2 transcription (EELPOT). 1997). At E9.5, Rax expression initiates in the optic vesicle and is RAX was shown to bind to EELPOT in vivo by chromatin immuno- subsequently localized to the neural retina. Rax is expressed precipitation (ChIP), and significantly activates EELPOT-luciferase uniformly throughout the developing neuroblastic layer (NBL), reporters in vitro. Additionally, Otx2 expression is dramatically with peak expression observed at E16.5. With the onset of retinal decreased with concomitant decrease in cone–rod homeobox (Crx)