Cell diversity in the retina: more than meets the eye

The ZO-1 protein Polychaetoid as an upstream regulator of the Hippo pathway in Drosophila

The generation of a diversity of photoreceptor (PR) subtypes with different spectral sensitivities is essential for color vision in animals. In the Drosophila eye, the Hippo pathway has been implicated in blue- and green-sensitive PR subtype fate specification. Specifically, Hippo pathway activation promotes green-sensitive PR fate at the expense of blue-sensitive PRs. Here, using a sensitized triple heterozygote-based genetic screening approach, we report the identification of the single Drosophila zonula occludens-1 (ZO-1) protein Polychaetoid (Pyd) as a new regulator of the Hippo pathway during the blue- and green-sensitive PR subtype binary fate choice. We demonstrate that Pyd acts upstream of the core components and the upstream regulator Pez in the Hippo pathway. Furthermore, We found that Pyd represses the activity of Su(dx), a E3 ligase that negatively regulates Pez and can physically interact with Pyd, during PR subtype fate specification. Together, our results identify a new mechanism underlying the Hippo signaling pathway in post-mitotic neuronal fate specification.

The Hippo signaling pathway was originally discovered for its critical role in tissue growth and organ size control. Its evolutionarily conserved roles in various biological processes, including cell differentiation, stem cell regeneration and homeostasis, innate immune biology, as well as tumorigenesis, have been subsequently found in other species. During the development of the Drosophila eye, the Hippo pathway promotes green- and represses blue-sensitive photoreceptor (PR) subtype fate specification. Taking advantage of this binary PR fate choice, we screened Drosophila chromosomal deficiency lines to seek new regulators of the Hippo signaling pathway. We identified the Drosophila membrane-associated ZO-1 protein Pyd as an upstream regulator of the Hippo pathway to specify PR subtypes. Our results have demonstrated that Pyd represses Su(dx)’s activity in the Hippo pathway to specify PR subtypes. Our results demonstrate a new mechanism underlying the Hippo signaling pathway in post-mitotic neuronal fate specification.

Regional Variation of Gap Junctional Connections in the Mammalian Inner Retina

The retinas of many species show regional specialisations that are evident in the differences in the processing of visual input from different parts of the visual field. Regional specialisation is thought to reflect an adaptation to the natural visual environment, optical constraints, and lifestyle of the species. Yet, little is known about regional differences in synaptic circuitry. Here, we were interested in the topographical distribution of connexin-36 (Cx36), the major constituent of electrical synapses in the retina. We compared the retinas of mice, rats, and cats to include species with different patterns of regional specialisations in the analysis. First, we used the density of Prox1-immunoreactive amacrine cells as a marker of any regional specialisation, with higher cell density signifying more central regions. Double-labelling experiments showed that Prox1 is expressed in AII amacrine cells in all three species. Interestingly, large Cx36 plaques were attached to about 8-10% of Prox1-positive amacrine cell somata, suggesting the strong electrical coupling of pairs or small clusters of cell bodies. When analysing the regional changes in the volumetric density of Cx36-immunoreactive plaques, we found a tight correlation with the density of Prox1-expressing amacrine cells in the ON, but not in the OFF sublamina in all three species. The results suggest that the relative contribution of electrical synapses to the ON- and OFF-pathways of the retina changes with retinal location, which may contribute to functional ON/OFF asymmetries across the visual field.

Low availability of choline in utero disrupts development and function of the retina

Adequate supply of choline, an essential nutrient, is necessary to support proper brain development. Whether prenatal choline availability plays a role in development of the visual system is currently unknown. In this study, we addressed the role of in utero choline supply for the development and later function of the retina in a mouse model. We lowered choline availability in the maternal diet during pregnancy and assessed proliferative and differentiation properties of retinal progenitor cells (RPCs) in the developing prenatal retina, as well as visual function in adult offspring. We report that low choline availability during retinogenesis leads to persistent retinal cytoarchitectural defects, ranging from focal lesions with displacement of retinal neurons into subretinal space to severe hypocellularity and ultrastructural defects in photoreceptor organization. We further show that low choline availability impairs timely differentiation of retinal neuronal cells, such that the densities of early-born retinal ganglion cells, amacrine and horizontal cells, as well as cone photoreceptor precursors, are reduced in low choline embryonic d 17.5 retinas. Maintenance of higher proportions of RPCs that fail to exit the cell cycle underlies aberrant neuronal differentiation in low choline embryos. Increased RPC cell cycle length, and associated reduction in neurofibromin 2/Merlin protein, an upstream regulator of the Hippo signaling pathway, at least in part, explain aberrant neurogenesis in low choline retinas. Furthermore, we find that animals exposed to low choline diet in utero exhibit a significant degree of intraindividual variation in vision, characterized by marked functional discrepancy between the 2 eyes in individual animals. Together, our findings demonstrate, for the first time, that choline availability plays an essential role in the regulation of temporal progression of retinogenesis and provide evidence for the importance of adequate supply of choline for proper development of the visual system.-Trujillo-Gonzalez, I., Friday, W. B., Munson, C. A., Bachleda, A., Weiss, E. R., Alam, N. M., Sha, W., Zeisel, S. H., Surzenko, N. Low availability of choline in utero disrupts development and function of the retina.

Prox1 Is a Marker for AII Amacrine Cells in the Mouse Retina

The transcription factor Prox1 is expressed in multiple cells in the retina during eye development. This study has focused on neuronal Prox1 expression in the inner nuclear layer (INL) of the adult mouse retina. Prox1 immunostaining was evaluated in vertical retinal sections and whole mount preparations using a specific antibody directed to the C-terminus of Prox1. Strong immunostaining was observed in numerous amacrine cell bodies and in all horizontal cell bodies in the proximal and distal INL, respectively. Some bipolar cells were also weakly immunostained. Prox1-immunoreactive amacrine cells expressed glycine, and they formed 35 ± 3% of all glycinergic amacrine cells. Intracellular Neurobiotin injections into AII amacrine cells showed that all gap junction-coupled AII amacrine cells express Prox1, and no other Prox1-immunostained amacrine cells were in the immediate area surrounding the injected AII amacrine cell. Prox1-immunoreactive amacrine cell bodies were distributed across the retina, with their highest density (3887 ± 160 cells/mm²) in the central retina, 0.5 mm from the optic nerve head, and their lowest density (3133 ± 350 cells/mm²) in the mid-peripheral retina, 2 mm from the optic nerve head. Prox1-immunoreactive amacrine cell bodies comprised ~9.8% of the total amacrine cell population, and they formed a non-random mosaic with a regularity index (RI) of 3.4, similar to AII amacrine cells in the retinas of other mammals. Together, these findings indicate that AII amacrine cells are the predominant and likely only amacrine cell type strongly expressing Prox1 in the adult mouse retina, and establish Prox1 as a marker of AII amacrine cells.

Mouse retinal development: a dark horse model for systems biology research

The developing retina is an excellent model to study cellular fate determination and differentiation in the context of a complex tissue. Over the last decade, many basic principles and key genes that underlie these processes have been experimentally identified. In this review, we construct network models to summarize known gene interactions that underlie determination and fundamentally affect differentiation of each retinal cell type. These networks can act as a scaffold to assemble subsequent discoveries. In addition, these summary networks provide a rational segue to systems biology approaches necessary to understand the many events leading to appropriate cellular determination and differentiation in the developing retina and other complex tissues.

Prospero and Pax2 combinatorially control neural cell fate decisions by modulating Ras- and Notch-dependent signaling

BACKGROUND

The concept of an equivalence group, a cluster of cells with equal potential to adopt the same specific fate, has served as a useful paradigm to understand neural cell type specification. In the Drosophila eye, a set of five cells, called the 'R7 equivalence group', generates a single photoreceptor neuron and four lens-secreting epithelial cells. This choice between neuronal versus non-neuronal cell fates rests on differential requirements for, and cross-talk between, Notch/Delta- and Ras/mitogen-activated protein kinase (MAPK)-dependent signaling pathways. However, many questions remain unanswered related to how downstream events of these two signaling pathways mediate distinct cell fate decisions.

RESULTS

Here, we demonstrate that two direct downstream targets of Ras and Notch signaling, the transcription factors Prospero and dPax2, are essential regulators of neuronal versus non-neuronal cell fate decisions in the R7 equivalence group. Prospero controls high activated MAPK levels required for neuronal fate, whereas dPax2 represses Delta expression to prevent neuronal fate. Importantly, activity from both factors is required for proper cell fate decisions to occur.

CONCLUSIONS

These data demonstrate that Ras and Notch signaling are integrated during cell fate decisions within the R7 equivalence group through the combinatorial and opposing activities of Pros and dPax2. Our study provides one of the first examples of how the differential expression and synergistic roles of two independent transcription factors determine cell fate within an equivalence group. Since the integration of Ras and Notch signaling is associated with many developmental and cancer models, these findings should provide new insights into how cell specificity is achieved by ubiquitously used signaling pathways in diverse biological contexts.

Building a fly eye: terminal differentiation events of the retina, corneal lens, and pigmented epithelia

In the past, vast differences in ocular structure, development, and physiology throughout the animal kingdom led to the widely accepted notion that eyes are polyphyletic, that is, they have independently arisen multiple times during evolution. Despite the dissimilarity between vertebrate and invertebrate eyes, it is becoming increasingly evident that the development of the eye in both groups shares more similarity at the genetic level than was previously assumed, forcing a reexamination of eye evolution. Understanding the molecular underpinnings of cell type specification during Drosophila eye development has been a focus of research for many labs over the past 25 years, and many of these findings are nicely reviewed in Chapters 1 and 4. A somewhat less explored area of research, however, considers how these cells, once specified, develop into functional ocular structures. This review aims to summarize the current knowledge related to the terminal differentiation events of the retina, corneal lens, and pigmented epithelia in the fly eye. In addition, we discuss emerging evidence that the different functional components of the fly eye share developmental pathways and functions with the vertebrate eye.

Specification of cell fate in the mammalian cochlea

Mammalian auditory sensation is mediated by the organ of Corti, a specialized sensory epithelium found in the cochlea of the inner ear. Proper auditory function requires that the many different cell types found in the sensory epithelium be precisely ordered within an exquisitely patterned cellular mosaic. The development of this mosaic depends on a series of cell fate decisions that transform the initially nearly uniform cochlear epithelium into the complex structure of the mature organ of Corti. The prosensory domain, which contains the progenitors of both the mechanosensory hair cells and their associated supporting cells, first becomes distinct from both the neural and the nonsensory domains. Further cell fate decisions subdivide prosensory cells into populations of inner and outer hair cells, and several different types of supporting cells. A number of different signaling pathways and transcription factors are known to be necessary for these developmental processes; in this review, we will summarize these results with an emphasis on recent findings.

Prox1 expression in rod precursors and Müller cells

The transcription factor Prox1 acts in rodent retinogenesis, at least in promoting cell cycle withdrawal and horizontal cell production. In the mature retina, this protein is detected at the inner nuclear layer of all vertebrate groups. We have made a neurochemical characterisation of Prox1(+) cell types in two different vertebrate groups: mammals and fish. As well as Prox1(+) horizontal cells, we have observed Prox1(+)/PKC-alpha(+) rod bipolar cells in mouse and cone ON and mixed b bipolar cells in goldfish. In mouse, only some CB(+) and CR(+) amacrine cells are Prox1(+) and the TH(+) and CR(+) amacrine cells are Prox1(-). However, in goldfish all CR(+) amacrine cells and TH(+) interplexiform cells are Prox1(+) and in the GCL displaced amacrine cells are also Prox1(+). Besides its expression in different interneuron subpopulations, we demonstrate, for the first time, the presence of Prox1 in the GS(+) and CRALBP(+) Müller cells in the retina of adult mammals and in developing and mature retina of fish. The presence of Prox1 in these cells appears to be related to survival or maintenance of their phenotype. We also demonstrate that in fish, where retinal formation persists into adulthood, Prox1 is expressed in dividing PCNA(+) cells at the peripheral growing zone, in rod progenitors at the inner and outer nuclear layers as well as in early progenitors during a retinal regeneration process after cryo-lesion of the peripheral growing zone. Therefore, Prox1 functions in vertebrate retinogenesis may be more complex than previously expected.

Retinal horizontal cells: challenging paradigms of neural development and cancer biology

A group of retinal interneurons known as horizontal cells has recently been shown to exhibit a variety of unique biological properties, as compared with other nerve cells, that challenge many long-standing assumptions in the fields of neural development and cancer biology. These features include their unusual migratory behavior, their unique morphological plasticity, and their propensity to divide at a relatively late stage during development. Here, we review these novel features, discuss their relevance for other cell types, outline open questions in our understanding of horizontal cell development and consider their implications.

Protein tyrosine phosphatases expression during development of mouse superior colliculus

Protein tyrosine phosphatases (PTPs) are key regulators of different processes during development of the central nervous system. However, expression patterns and potential roles of PTPs in the developing superior colliculus remain poorly investigated. In this study, a degenerate primer-based reverse transcription-polymerase chain reaction (RT-PCR) approach was used to isolate seven different intracellular PTPs and nine different receptor-type PTPs (RPTPs) from embryonic E15 mouse superior colliculus. Subsequently, the expression patterns of 11 PTPs (TC-PTP, PTP1C, PTP1D, PTP-MEG2, PTP-PEST, RPTPJ, RPTPε, RPTPRR, RPTPσ, RPTPκ and RPTPγ) were further analyzed in detail in superior colliculus from embryonic E13 to postnatal P20 stages by quantitative real-time RT-PCR, Western blotting and immunohistochemistry. Each of the 11 PTPs exhibits distinct spatiotemporal regulation of mRNAs and proteins in the developing superior colliculus suggesting their versatile roles in genesis of neuronal and glial cells and retinocollicular topographic mapping. At E13, additional double-immunohistochemical analysis revealed the expression of PTPs in collicular nestin-positive neural progenitor cells and RC-2-immunoreactive radial glia cells, indicating the potential functional importance of PTPs in neurogenesis and gliogenesis.

Eye evolution at high resolution: the neuron as a unit of homology

Based on differences in morphology, photoreceptor-type usage and lens composition it has been proposed that complex eyes have evolved independently many times. The remarkable observation that different eye types rely on a conserved network of genes (including Pax6/eyeless) for their formation has led to the revised proposal that disparate complex eye types have evolved from a shared and simpler prototype. Did this ancestral eye already contain the neural circuitry required for image processing? And what were the evolutionary events that led to the formation of complex visual systems, such as those found in vertebrates and insects? The recent identification of unexpected cell-type homologies between neurons in the vertebrate and Drosophila visual systems has led to two proposed models for the evolution of complex visual systems from a simple prototype. The first, as an extension of the finding that the neurons of the vertebrate retina share homologies with both insect (rhabdomeric) and vertebrate (ciliary) photoreceptor cell types, suggests that the vertebrate retina is a composite structure, made up of neurons that have evolved from two spatially separate ancestral photoreceptor populations. The second model, based largely on the conserved role for the Vsx homeobox genes in photoreceptor-target neuron development, suggests that the last common ancestor of vertebrates and flies already possessed a relatively sophisticated visual system that contained a mixture of rhabdomeric and ciliary photoreceptors as well as their first- and second-order target neurons. The vertebrate retina and fly visual system would have subsequently evolved by elaborating on this ancestral neural circuit. Here we present evidence for these two cell-type homology-based models and discuss their implications.

Conserved role of the Vsx genes supports a monophyletic origin for bilaterian visual systems

BACKGROUND

Components of the genetic network specifying eye development are conserved from flies to humans, but homologies between individual neuronal cell types have been difficult to identify. In the vertebrate retina, the homeodomain-containing transcription factor Chx10 is required for both progenitor cell proliferation and the development of the bipolar interneurons, which transmit visual signals from photoreceptors to ganglion cells.

RESULTS

We show that dVsx1 and dVsx2, the two Drosophila homologs of Chx10, play a conserved role in visual-system development. DVSX1 is expressed in optic-lobe progenitor cells, and, in dVsx1 mutants, progenitor cell proliferation is defective, leading to hypocellularity. Subsequently, DVSX1 and DVSX2 are coexpressed in a subset of neurons in the medulla, including the transmedullary neurons that transmit visual information from photoreceptors to deeper layers of the visual system. In dVsx mutant adults, the optic lobe is reduced in size, and the medulla is small or absent. These results suggest that the progenitor cells and photoreceptor target neurons of the vertebrate retina and fly optic lobe are ancestrally related. Genetic and functional homology may extend to the neurons directly downstream of the bipolar and transmedullary neurons, the vertebrate ganglion cells and fly lobula projection neurons. Both cell types project to visual-processing centers in the brain, and both sequentially express the Math5/ATO and Brn3b/ACJ6 transcription factors during their development.

CONCLUSIONS

Our findings support a monophyletic origin for the bilaterian visual system in which the last common ancestor of flies and vertebrates already contained a primordial visual system with photoreceptors, interneurons, and projection neurons.

Nr2e3 and Nrl can reprogram retinal precursors to the rod fate in Xenopus retina

Transformation of undifferentiated progenitors into specific cell types is largely dependent on temporal and spatial expression of a complex network of transcription factors. Here, we examined whether neural retina leucine zipper (Nrl) and photoreceptor-specific nuclear receptor Nr2e3 transcription factors contribute to cell fate determination. We cloned the Xenopus Nr2e3 gene and showed that its temporal and spatial expression is similar to its mammalian ortholog. We tested its in vivo function by misexpressing these transcription factors in Xenopus eye primordia, demonstrating that either human Nr2e3 or Nrl directed photoreceptor precursors to become rods at the expense of cones. Furthermore, overexpression of Xenopus Nrl dramatically increased the number of lens fibers, whereas human Nrl did not, suggesting evolutionary divergence of function of the Nrl gene family. Misexpression of Nrl and Nr2e3 together were more effective than either transcription factor alone in directing precursors to the rod fate.

Prox1 expression patterns in the developing and adult murine brain

Prox1, a homeobox gene related to the Drosophila gene prospero, is necessary for retina, lens, liver, pancreas, and lymphatics development. However, not much is yet known about Prox1 expression during central nervous system development. Here we provide a detailed analysis of Prox1 mRNA and protein expression during prenatal and postnatal murine brain development. Prenatally, Prox1 is expressed in the subventricular zone or in early differentiating regions of the brain. At these stages, Prox1 mRNA, but not Prox1 protein, was also detected in several regions of the prethalamus and hypothalamus. At an early postnatal stage, Prox1 expression is mainly detected in several nuclei of the thalamus, the cerebellum, and the hippocampus. In adulthood, Prox1 expression remains only in the hippocampus and cerebellum. These complex patterns of expression suggest that Prox1 activity is differentially required during brain development and adulthood.

Characterization of a novel prospero-related homeobox gene, Prox2

Prospero-related homeobox genes have been identified from various multi-cellular organisms and play important roles in development as a cell fate determinant. Mouse Prox1 is essential for embryogenesis and is required to differentiate horizontal cells in the retina. Here we describe a novel prospero family member, Prox2. Transcriptional reporter assays demonstrated that mouse Prox2 is a transcriptional activator and the N-terminal region has been identified as an activation domain. The expression of mouse Prox2 was detected in postnatal eyes and adult testes as well as embryos. To investigate the in vivo role of Prox2, we generated the Prox2 mutant allele, Prox2-, by homologous recombination in mouse ES cells. Prox2- lacks the first coding exon that encodes a translational start site and a part of homeodomain. In spite of the Prox2 expression during embryogenesis, Prox2- homozygous mutant mice were born at the expected Mendelian ratio without overt abnormalities. Histological analyses revealed that Prox2- homozygous eyes retained the organized layer structure including three nuclear layers and differentiated horizontal cells. Prox2- homozygous mutant males produced elongated spermatids and were fertile. These results demonstrate that mouse Prox2 is dispensable for embryonic development, horizontal cell generation and fertility in contrast to mouse Prox1.

Spatial and temporal expression profile of pinin during mouse development

SR and SR-related proteins are splicing regulators involved in embryo development in higher eukaryotes. Pinin (pnn) is a SR-related protein localized both within nucleus (nuclear pnn, N-pnn) and at desmosome of cell-cell adhesion (desmosomal pnn, D-pnn). To investigate the role of N-pnn during mouse embryo development, we examined its expression using Northern blot, real-time RT-PCR, immunostaining, and mRNA in situ hybridization (ISH). On Northern analysis, we found that pnn transcripts display two isoforms due to differential utilization of a polyadenylation site and exhibit tissue variable expression with thymus expressing the highest level of transcript. Analysis of pnn expression in mouse embryos revealed N-pnn expression starts from the two-cell fertilized egg stage and is ubiquitous at all stages of mouse embryo development. ISH and immunofluorescent staining of embryo cryosections showed that during mouse organogenesis N-pnn is highly expressed in the central nervous system. In addition, N-pnn was found to be highly expressed in the cortex region of thymus of E16.5 mouse fetus, while in the hepatic primordium the strongest signals were noted at E13.5 to E14.5 rather than at later developmental stages. Finally, we also determined the subcellular location of N-pnn in photoreceptors of developing retinas by nuclear fractionation and Western blot, because N-pnn displayed a staining pattern reminiscent of cytoplasmic proteins at the microscopic level in developing mouse photoreceptors. Altogether these data provide us with a better understanding of the tissue distribution pattern of N-pnn during mouse development.

A postulated role for microRNA in cellular differentiation

Over the past two decades a variety of mechanisms regulating cellular differentiation have been uncovered. These include signaling by morphogens or membrane-associated ligands and asymmetric segregation of cytoplasmic components. Most of these processes are driven by protein coding genes. Here I describe another possible cellular differentiation mechanism that involves asymmetric segregation of microRNAs, a group of recently discovered non-protein coding genes that have been shown to be involved in differentiation.

Inherited eye disease: cause and late effect

Molecular genetics has provided relatively few insights into late-onset eye disorders, but epidemiological data indicate that genetic factors are important in some late-onset eye disorders that cause major health burdens. Much clinical genetic research is based on the belief that developmental and late-onset disorders are not necessarily the result of defects in different genes, but are often caused by different mutations in the same collection of genes. Thus, mutations that either abolish or radically change gene function might cause early-onset disorders, whereas more-subtle changes in gene expression might underlie late-onset diseases. We present arguments and examples that indicate that this principle might be a fruitful guide to investigating the causes of late-onset eye disorders.

Conserved transcriptional activators of the Xenopus rhodopsin gene

Vertebrate rhodopsin promoters exhibit striking sequence identities proximal to the initiation site, suggesting that conserved transcription factors regulate rhodopsin expression in these animals. We identify and characterize two transcriptional activators of the Xenopus rhodopsin gene: homologs of the mammalian Crx and Nrl transcription factors, XOtx5 and XL-Nrl (originally named XL-maf), respectively. XOtx5 stimulated transcription approximately 10-fold in human 293 cells co-transfected with a plasmid containing the rhodopsin promoter (-508 to +41) upstream of luciferase, similar to the approximately 6-fold stimulation with human Crx. XL-Nrl stimulated transcription approximately 27-fold in mammalian 293 cells co-transfected with the rhodopsin luciferase reporter, slightly more than the approximately 17-fold stimulation with Nrl. Together, the Xenopus transcription factors synergistically activated the rhodopsin promoter (approximately 140-fold), as well as in combination with mammalian homologs. Deletion of the Nrl-response element, TGCTGA, eliminated the synergistic activation by both mammalian and Xenopus transcription factors. Deletion of the conserved ATTA sequences (Ret-1 or BAT-1), binding sites for Crx, did not significantly decrease activation by Crx/XOtx5. However, there was increased activation by Nrl/XL-Nrl and an increased synergy when the Ret-1 site was disrupted. These results illustrate conservation of mechanisms of retinal gene expression among vertebrates. In transgenic tadpoles, XOtx5 and XL-Nrl directed premature and ectopic expression from the Xenopus rhodopsin promoter-GFP transgene. Furthermore, activation of the endogenous rhodopsin gene was also observed in some animals, showing that XOtx5 and XL-Nrl can activate the promoter in native chromatin environment.