Axon-bearing and axon-less horizontal cell subtypes are generated consecutively during chick retinal development from progenitors that are sensitive to follistatin

Cell numbers, cell ratios, and developmental plasticity in the rod pathway of the mouse retina

The precise specification of cellular fate is thought to ensure the production of the correct number of neurons within a population. Programmed cell death may be an additional mechanism controlling cell number, believed to refine the proper ratio of pre- to post-synaptic neurons for a given species. Here, we consider the size of three different neuronal populations in the rod pathway of the mouse retina: rod photoreceptors, rod bipolar cells, and AII amacrine cells. Across a collection of 28 different strains of mice, large variation in the numbers of all three cell types is present. The variation in their numbers is not correlated, so that the ratio of rods to rod bipolar cells, as well as rod bipolar cells to AII amacrine cells, varies as well. Establishing connectivity between such variable pre- and post-synaptic populations relies upon plasticity that modulates process outgrowth and morphological differentiation, which we explore experimentally for both rod bipolar and AII amacrine cells in a mouse retina with elevated numbers of each cell type. While both rod bipolar dendritic and axonal arbors, along with AII lobular arbors, modulate their areal size in relation to local homotypic cell densities, the dendritic appendages of the AII amacrine cells do not. Rather, these processes exhibit a different form of plasticity, regulating the branching density of their overlapping arbors. Each form of plasticity should ensure uniformity in retinal coverage in the presence of the independent specification of afferent and target cell number.

Retinal Development in a Precocial Bird Species, the Quail (Coturnix coturnix, Linnaeus 1758)

The quail (Coturnix coturnix, Linnaeus 1758), a notable model used in developmental biology, is a precocial bird species in which the processes of retinal cell differentiation and retinal histogenesis have been poorly studied. The purpose of the present research is to examine the retinogenesis in this bird species immunohistochemically and compare the results with those from previous studies in precocial and altricial birds. We found that the first PCNA-negative nuclei are detected at Stage (St) 21 in the vitreal region of the neuroblastic layer, coinciding topographically with the first αTubAc-/Tuj1-/Isl1-immunoreactive differentiating ganglion cells. At St28, the first Prox1-immunoreactive nuclei can be distinguished in the vitreal side of the neuroblastic layer (NbL), but also the first visinin-immunoreactive photoreceptors in the scleral surface. The inner plexiform layer (IPL) emerges at St32, and the outer plexiform layer (OPL) becomes visible at St35—the stage in which the first GS-immunoreactive Müller cells are distinguishable. Newly hatched animals show a well-developed stratified retina in which the PCNA-and pHisH3-immunoreactivies are absent. Therefore, retinal cell differentiation in the quail progresses in the stereotyped order conserved among vertebrates, in which ganglion cells initially appear and are followed by amacrine cells, horizontal cells, and photoreceptors. Müller glia are one of the last cell types to be born. Plexiform layers emerge following a vitreal-to-scleral gradient. Finally, our results suggest that there are no significant differences in the timing of different events involved in retinal maturation between the quail and the chicken, but the same events are delayed in an altricial bird species.

Timing and Distribution of Mitotic Activity in the Retina During Precocial and Altricial Modes of Avian Development

During development of the vertebrate retina, mitotic activity is defined as apical when is located at the external surface of the neuroepithelium or as non-apical when is found in more internal regions. Apical mitoses give rise to all retinal cell types. Non-apical mitoses are linked to committed horizontal cell precursors that subsequently migrate vitreo-sclerally, reaching their final position in the outer surface of the inner nuclear layer, where they differentiate. Previous studies have suggested differences in the timing of retinal maturation between altricial and precocial bird species. In the present study we analyze qualitatively and quantitatively the mitotic activity in the developing retina of an altricial (zebra finch, Taeniopygia guttata) and a precocial (Japanese quail, Coturnix coturnix) bird species. We found that pHisH3-immunoreactive apical and non-apical mitoses were abundant in the T. guttata retina at the hatching stage. In contrast, pHisH3 immunoreactivity almost disappeared from the quail retina at the embryonic day 10 (E10). Furthermore, we also found that the onset of the appearance of non-apical mitoses occurred at later stages in the altricial bird species than in the precocial one. The disappearance of apical mitoses and the spatiotemporal distribution of non-apical mitoses followed central to peripheral and dorsal to ventral gradients, similar to gradients of cell differentiation described in the retina of birds. Therefore, these results suggest that retinal neurogenesis is active at the hatching stage in T. guttata, and that horizontal cell differentiation is delayed in the altricial bird species compared to the precocial one. Together, this study reveals important insights into the timing differences that regulate bird retinal maturation and provides a better understanding of the evolution of avian altriciality and precociality.

Distribution of planar cell polarity proteins in the developing avian retina

Planar cell polarity (PCP) is evolutionary conserved and play a critical role in proper tissue development and function. During central nervous system development, PCP proteins exhibit specific patterns of distribution and are indispensable for axonal growth, dendritogenesis, neuronal migration, and neuronal differentiation. The retina constitutes an excellent model in which to study molecular mechanisms involved in neural development. The analysis of the spatiotemporal expression of PCP proteins in this model constitutes an useful histological approach in order to identify possible roles of these proteins in retinogenesis. Immunohistochemical techniques revealed that Frz6, Celsr1, Vangl1, Pk1, Pk3, and Fat1 were present in emerging axons from recently differentiated ganglion cells in the chicken retina. Except for Vangl1, they were also asymmetrically distributed in differentiated amacrine cells. Pk1 and Pk3 were restricted in the outer nuclear layer to the outer segment of photoreceptors. Vangl1 was also located in the cell somata of Müller glia. Given these findings together, the distribution of PCP proteins in the developing chicken retina suggest essential roles in axonal guidance during early retinogenesis and a possible involvement in the establishment of cell asymmetry and maintenance of retinal cell phenotypes.

A cell atlas of the chick retina based on single-cell transcriptomics

Retinal structure and function have been studied in many vertebrate orders, but molecular characterization has been largely confined to mammals. We used single-cell RNA sequencing (scRNA-seq) to generate a cell atlas of the chick retina. We identified 136 cell types plus 14 positional or developmental intermediates distributed among the six classes conserved across vertebrates - photoreceptor, horizontal, bipolar, amacrine, retinal ganglion, and glial cells. To assess morphology of molecularly defined types, we adapted a method for CRISPR-based integration of reporters into selectively expressed genes. For Müller glia, we found that transcriptionally distinct cells were regionally localized along the anterior-posterior, dorsal-ventral, and central-peripheral retinal axes. We also identified immature photoreceptor, horizontal cell, and oligodendrocyte types that persist into late embryonic stages. Finally, we analyzed relationships among chick, mouse, and primate retinal cell classes and types. Our results provide a foundation for anatomical, physiological, evolutionary, and developmental studies of the avian visual system.

The retinal basis of vision in chicken

The Avian retina is far less known than that of mammals such as mouse and macaque, and detailed study is overdue. The chicken (Gallus gallus) has potential as a model, in part because research can build on developmental studies of the eye and nervous system. One can expect differences between bird and mammal retinas simply because whereas most mammals have three types of visual photoreceptor birds normally have six. Spectral pathways and colour vision are of particular interest, because filtering by oil droplets narrows cone spectral sensitivities and birds are probably tetrachromatic. The number of receptor inputs is reflected in the retinal circuitry. The chicken probably has four types of horizontal cell, there are at least 11 types of bipolar cell, often with bi- or tri-stratified axon terminals, and there is a high density of ganglion cells, which make complex connections in the inner plexiform layer. In addition, there is likely to be retinal specialisation, for example chicken photoreceptors and ganglion cells have separate peaks of cell density in the central and dorsal retina, which probably serve different types of behaviour.

Semblance: An empirical similarity kernel on probability spaces

In data science, determining proximity between observations is critical to many downstream analyses such as clustering, classification, and prediction. However, when the data's underlying probability distribution is unclear, the function used to compute similarity between data points is often arbitrarily chosen. Here, we present a novel definition of proximity, Semblance, that uses the empirical distribution of a feature to inform the pair-wise similarity between observations. The advantage of Semblance lies in its distribution-free formulation and its ability to place greater emphasis on proximity between observation pairs that fall at the outskirts of the data distribution, as opposed to those toward the center. Semblance is a valid Mercer kernel, allowing its principled use in kernel-based learning algorithms, and for any data modality. We demonstrate its consistently improved performance against conventional methods through simulations and real case studies from diverse applications in single-cell transcriptomics, image reconstruction, and financial forecasting.

Lineage tracing analysis of cone photoreceptor associated cis-regulatory elements in the developing chicken retina

During vertebrate retinal development, transient populations of retinal progenitor cells with restricted cell fate choices are formed. One of these progenitor populations expresses the Thrb gene and can be identified by activity of the ThrbCRM1 cis-regulatory element. Short-term assays have concluded that these cells preferentially generate cone photoreceptors and horizontal cells, however developmental timing has precluded an extensive cell type characterization of their progeny. Here we describe the development and validation of a recombinase-based lineage tracing system for the chicken embryo to further characterize the lineage of these cells. The ThrbCRM1 element was found to preferentially form photoreceptors and horizontal cells, as well as a small number of retinal ganglion cells. The photoreceptor cell progeny are exclusively cone photoreceptors and not rod photoreceptors, confirming that ThrbCRM1 progenitor cells are restricted from the rod fate. In addition, specific subtypes of horizontal cells and retinal ganglion cells were overrepresented, suggesting that ThrbCRM1 progenitor cells are not only restricted for cell type, but for cell subtype as well.

Stochastic single cell migration leads to robust horizontal cell layer formation in the vertebrate retina

Developmental programs that arrange cells and tissues into patterned organs are remarkably robust. In the developing vertebrate retina, for example, neurons reproducibly assemble into distinct layers giving the mature organ its overall structured appearance. This stereotypic neuronal arrangement, termed lamination, is important for efficient neuronal connectivity. Although retinal lamination is conserved in many vertebrates, including humans, how it emerges from single cell behaviour is not fully understood. To shed light on this issue, we here investigated the formation of the retinal horizontal cell layer. Using in vivo light sheet imaging of the developing zebrafish retina, we generated a comprehensive quantitative analysis of horizontal single cell behaviour from birth to final positioning. Interestingly, we find that all parameters analysed, including cell cycle dynamics, migration paths and kinetics, as well as sister cell dispersal, are very heterogeneous. Thus, horizontal cells show individual non-stereotypic behaviour before final positioning. Yet these initially variable cell dynamics always generate the correct laminar pattern. Consequently, our data show that the extent of single cell stochasticity in the lamination of the vertebrate retina is underexplored.

Temporal Requirement of Mab21l2 During Eye Development in Chick Reveals Stage-Dependent Functions for Retinogenesis

Purpose

Different missense mutations in the single exon gene Mab21l2 have been identified in unrelated families with various bilateral eye malformations, including microphthalmia, anophthalmia, and coloboma, but the molecular function of Mab21l2 during eye development still remains largely unknown.

Methods

We have established an in vivo Mab21l2-deficient eye development model in chick, by using a Mab21l2 RNA interference construct that we electroporated in ovo in prospective retinal cells. In addition, we designed a Mab21l2 gain-of-function electroporation vector. Mab21l2-modulated retinas were analyzed on consecutive sections in terms of morphology, and molecular markers for apoptosis, cell proliferation, and retinogenesis.

Results

Our Mab21l2-deficient chick model mimics human ocular phenotypes. When Mab21l2 is downregulated prior to optic vesicle formation, the embryos develop anophthalmia, and Mab21l2 inhibition by optic cup stages results in a microphthalmic colobomatous phenotype. Our results show that inhibition of Mab21l2 affects cell proliferation, cell cycle exit, and the expression of Atoh7/Ath5, NeuroD4/Ath3, Isl1, Pax6, AP-2α, and Prox1. In addition, Mab21l2 overexpression hampers cell cycle exit and differentiation of retinal progenitor cells (RPCs).

Conclusions

Our results highlight the importance of a regulated temporal expression of Mab21l2 during eye development: At early stages, Mab21l2 is required to maintain RPC proliferation and expansion of cell number; before retinogenesis, a decrease in Mab21l2 expression in proliferating RPCs is required for cell cycle exit and differentiation; during retinogenesis, Mab21l2 is chronologically upregulated in RGCs, followed by differentiated horizontal and amacrine cells and cone photoreceptor cells.

Genomic Control of Retinal Cell Number: Challenges, Protocol, and Results

This chapter considers some of the challenges in obtaining accurate and consistent estimates of neuronal population size in the mouse retina, in order to identify the genetic control of cell number through QTL mapping and candidate gene analysis. We first discuss a variety of best practices for analyzing large numbers of recombinant inbred strains of mice over the course of a year in order to amass a satisfactory dataset for QTL mapping. We then consider the relative merits of using average cell density versus estimated total cell number as the target trait to be assessed, and why estimates of heritability may differ for these two traits when studying the retina in whole-mount preparations. Using our dataset on cell number for 12 different retinal cell types across the AXB/BXA recombinant inbred strain set as an example, we briefly review the QTL identified and their relationship to one another. Finally, we discuss our strategies for parsing QTL in order to identify prospective candidate genes, and how those candidates may in turn be dissected to identify causal regulatory or coding variants. By identifying the genetic determinants of nerve cell number in this fashion, we can then explore their roles in modulating developmental processes that underlie the formation of the retinal architecture.

A regulatory sequence from the retinoid X receptor γ gene directs expression to horizontal cells and photoreceptors in the embryonic chicken retina

PURPOSE

Combining techniques of episomal vector gene-specific Cre expression and genomic integration using the piggyBac transposon system enables studies of gene expression-specific cell lineage tracing in the chicken retina. In this work, we aimed to target the retinal horizontal cell progenitors.

METHODS

A 208 bp gene regulatory sequence from the chicken retinoid X receptor γgene (RXRγ208) was used to drive Cre expression. RXRγ is expressed in progenitors and photoreceptors during development. The vector was combined with a piggyBac "donor" vector containing a floxed STOP sequence followed by enhanced green fluorescent protein (EGFP), as well as a piggyBac helper vector for efficient integration into the host cell genome. The vectors were introduced into the embryonic chicken retina with in ovo electroporation. Tissue electroporation targets specific developmental time points and in specific structures.

RESULTS

Cells that drove Cre expression from the regulatory RXRγ208 sequence excised the floxed STOP-sequence and expressed GFP. The approach generated a stable lineage with robust expression of GFP in retinal cells that have activated transcription from the RXRγ208 sequence. Furthermore, GFP was expressed in cells that express horizontal or photoreceptor markers when electroporation was performed between developmental stages 22 and 28. Electroporation of a stage 12 optic cup gave multiple cell types in accordance with RXRγ gene expression in the early retina.

CONCLUSIONS

In this study, we describe an easy, cost-effective, and time-efficient method for testing regulatory sequences in general. More specifically, our results open up the possibility for further studies of the RXRγ-gene regulatory network governing the formation of photoreceptor and horizontal cells. In addition, the method presents approaches to target the expression of effector genes, such as regulators of cell fate or cell cycle progression, to these cells and their progenitor.

Two Opsin 3-Related Proteins in the Chicken Retina and Brain: A TMT-Type Opsin 3 Is a Blue-Light Sensor in Retinal Horizontal Cells, Hypothalamus, and Cerebellum

Opsin family genes encode G protein-coupled seven-transmembrane proteins that bind a retinaldehyde chromophore in photoreception. Here, we sought potential as yet undescribed avian retinal photoreceptors, focusing on Opsin 3 homologs in the chicken. We found two Opsin 3-related genes in the chicken genome: one corresponding to encephalopsin/panopsin (Opn3) in mammals, and the other belonging to the teleost multiple tissue opsin (TMT) 2 group. Bioluminescence imaging and G protein activation assays demonstrated that the chicken TMT opsin (cTMT) functions as a blue light sensor when forced-expressed in mammalian cultured cells. We did not detect evidence of light sensitivity for the chicken Opn3 (cOpn3). In situ hybridization demonstrated expression of cTMT in subsets of differentiating cells in the inner retina and, as development progressed, predominant localization to retinal horizontal cells (HCs). Immunohistochemistry (IHC) revealed cTMT in HCs as well as in small numbers of cells in the ganglion and inner nuclear layers of the post-hatch chicken retina. In contrast, cOpn3-IR cells were found in distinct subsets of cells in the inner nuclear layer. cTMT-IR cells were also found in subsets of cells in the hypothalamus. Finally, we found differential distribution of cOpn3 and cTMT proteins in specific cells of the cerebellum. The present results suggest that a novel TMT-type opsin 3 may function as a photoreceptor in the chicken retina and brain.

Horizontal Cells, the Odd Ones Out in the Retina, Give Insights into Development and Disease

Thorough investigation of a neuronal population can help reveal key aspects regarding the nervous system and its development. The retinal horizontal cells have several extraordinary features making them particularly interesting for addressing questions regarding fate assignment and subtype specification. In this review we discuss and summarize data concerning the formation and diversity of horizontal cells, how morphology is correlated to molecular markers, and how fate assignment separates the horizontal lineage from the lineages of other retinal cell types. We discuss the novel and unique features of the final cell cycle of horizontal cell progenitors and how they may relate to retinoblastoma carcinogenesis.

The heterogenic final cell cycle of chicken retinal Lim1 horizontal cells is not regulated by the DNA damage response pathway

Cells with aberrations in chromosomal ploidy are normally removed by apoptosis. However, aneuploid neurons have been shown to remain functional and active both in the cortex and in the retina. Lim1 horizontal progenitor cells in the chicken retina have a heterogenic final cell cycle, producing some cells that enter S-phase without proceeding into M-phase. The cells become heteroploid but do not undergo developmental cell death. This prompted us to investigate if the final cell cycle of these cells is under the regulation of an active DNA damage response. Our results show that the DNA damage response pathway, including γ-H2AX and Rad51 foci, is not triggered during any phase of the different final cell cycles of horizontal progenitor cells. However, chemically inducing DNA adducts or double-strand breaks in Lim1 horizontal progenitor cells activated the DNA damage response pathway, showing that the cells are capable of a functional response to DNA damage. Moreover, manipulation of the DNA damage response pathway during the final cell cycle using inhibitors of ATM/ATR, Chk1/2, and p38MAPK, neither induced apoptosis nor mitosis in the Lim1 horizontal progenitor cells. We conclude that the DNA damage response pathway is functional in the Lim1 horizontal progenitor cells, but that it is not directly involved in the regulation of the final cell cycle that gives rise to the heteroploid horizontal cell population.

Islet-1 immunoreactivity in the developing retina of Xenopus laevis

The LIM-homeodomain transcription factor Islet1 (Isl1) has been widely used as a marker of neuronal differentiation in the developing visual system of different classes of vertebrates, including mammals, birds, reptiles, and fish. In the present study, we analyzed the spatial and temporal distribution of Isl1-immunoreactive cells during Xenopus laevis retinal development and its relation to the formation of the retinal layers, and in combination with different markers of cell differentiation. The earliest Isl1 expression appeared at St29-30 in the cell nuclei of sparse differentiating neuroblasts located in the vitreal surface of the undifferentiated retina. At St35-36, abundant Isl1-positive cells accumulated at the vitreal surface of the neuroepithelium. As development proceeded and through the postmetamorphic juveniles, Isl1 expression was identified in subpopulations of ganglion cells and in subsets of amacrine, bipolar, and horizontal cells. These data together suggest a possible role for Isl1 in the early differentiation and maintenance of different retinal cell types, and Isl1 can serve as a specific molecular marker for the study of retinal cell specification in X. laevis.

Alternative splicing of the chromodomain protein Morf4l1 pre-mRNA has implications on cell differentiation in the developing chicken retina

The proliferation, cell cycle exit and differentiation of progenitor cells are controlled by several different factors. The chromodomain protein mortality factor 4-like 1 (Morf4l1) has been ascribed a role in both proliferation and differentiation. Little attention has been given to the existence of alternative splice variants of the Morf4l1 mRNA, which encode two Morf41l isoforms: a short isoform (S-Morf4l1) with an intact chromodomain and a long isoform (L-Morf4l1) with an insertion in or in the vicinity of the chromodomain. The aim of this study was to investigate if this alternative splicing has a function during development. We analysed the temporal and spatial distribution of the two mRNAs and over-expressed both isoforms in the developing retina. The results showed that the S-Morf4l1 mRNA is developmentally regulated. Over-expression of S-Morf4l1 using a retrovirus vector produced a clear phenotype with an increase of early-born neurons: retinal ganglion cells, horizontal cells and cone photoreceptor cells. Over-expression of L-Morf4l1 did not produce any distinguishable phenotype. The over-expression of S-Morf4l1 but not L-Morf4l1 also increased apoptosis in the infected regions. Our results suggest that the two Morf4l1 isoforms have different functions during retinogenesis and that Morf4l1 functions are fine-tuned by developmentally regulated alternative splicing. The data also suggest that Morf4l1 contributes to the regulation of cell genesis in the retina.

Overlapping expression patterns and redundant roles for AP-2 transcription factors in the developing mammalian retina

BACKGROUND

We have previously shown that the transcription factor AP-2α (Tcfap2a) is expressed in postmitotic developing amacrine cells in the mouse retina. Although retina-specific deletion of Tcfap2a did not affect retinogenesis, two other family members, AP-2β and AP-2γ, showed expression patterns similar to AP-2α.

RESULTS

Here we show that, in addition to their highly overlapping expression patterns in amacrine cells, AP-2α and AP-2β are also co-expressed in developing horizontal cells. AP-2γ expression is restricted to amacrine cells, in a subset that is partially distinct from the AP-2α/β-immunopositive population. To address possible redundant roles for AP-2α and AP-2β during retinogenesis, Tcfap2a/b-deficient retinas were examined. These double mutants showed a striking loss of horizontal cells and an altered staining pattern in amacrine cells that were not detected upon deletion of either family member alone.

CONCLUSIONS

These studies have uncovered critical roles for AP-2 activity in retinogenesis, delineating the overlapping expression patterns of Tcfap2a, Tcfap2b, and Tcfap2c in the neural retina, and revealing a redundant requirement for Tcfap2a and Tcfap2b in horizontal and amacrine cell development.

Heterogenic final cell cycle by chicken retinal Lim1 horizontal progenitor cells leads to heteroploid cells with a remaining replicated genome

Retinal progenitor cells undergo apical mitoses during the process of interkinetic nuclear migration and newly generated post-mitotic neurons migrate to their prospective retinal layer. Whereas this is valid for most types of retinal neurons, chicken horizontal cells are generated by delayed non-apical mitoses from dedicated progenitors. The regulation of such final cell cycle is not well understood and we have studied how Lim1 expressing horizontal progenitor cells (HPCs) exit the cell cycle. We have used markers for S- and G2/M-phase in combination with markers for cell cycle regulators Rb1, cyclin B1, cdc25C and p27Kip1 to characterise the final cell cycle of HPCs. The results show that Lim1+ HPCs are heterogenic with regards to when and during what phase they leave the final cell cycle. Not all horizontal cells were generated by a non-apical (basal) mitosis; instead, the HPCs exhibited three different behaviours during the final cell cycle. Thirty-five percent of the Lim1+ horizontal cells was estimated to be generated by non-apical mitoses. The other horizontal cells were either generated by an interkinetic nuclear migration with an apical mitosis or by a cell cycle with an S-phase that was not followed by any mitosis. Such cells remain with replicated DNA and may be regarded as somatic heteroploids. The observed heterogeneity of the final cell cycle was also seen in the expression of Rb1, cyclin B1, cdc25C and p27Kip1. Phosphorylated Rb1-Ser608 was restricted to the Lim1+ cells that entered S-phase while cyclin B1 and cdc25C were exclusively expressed in HPCs having a basal mitosis. Only HPCs that leave the cell cycle after an apical mitosis expressed p27Kip1. We speculate that the cell cycle heterogeneity with formation of heteroploid cells may present a cellular context that contributes to the suggested propensity of these cells to generate cancer when the retinoblastoma gene is mutated.

Forkheadbox N4 (FoxN4) triggers context-dependent differentiation in the developing chick retina and neural tube

FoxN4, a forkhead box transcription factor, is expressed in the chicken eye field and in retinal progenitor cells (RPCs) throughout development. FoxN4 labelling overlapped with that of Pax6 and Sox2, two crucial transcription factors for RPCs. Later, during neurogenesis in the retina, some cells were intensely and transiently labelled for FoxN4. These cells co-labelled for Lim1, a transcription factor expressed in early-born horizontal cells. The result suggests that high levels of FoxN4 combined with expression of Lim1 define a population of RPCs committed to the horizontal cell fate prior to their last apical mitosis. As these prospective horizontal cells develop, their FoxN4 expression is down-regulated. Previous results suggested that FoxN4 is important for the generation of horizontal and amacrine cells but that it is not sufficient for the generation of horizontal cells (Li et al., 2004). We found that over-expression of FoxN4 in embryonic day 3 chicken retina could activate horizontal cell markers Prox1 and Lim1, and that it generated numerous and ectopically located horizontal cells of both main subtypes. However, genes expressed in photoreceptors, amacrine and ganglion cells were also activated, indicating that FoxN4 triggered the expression of several differentiation factors. This effect was not exclusive for the retina but was also seen when FoxN4 was over-expressed in the mesencephalic neural tube. Combining the results from over-expression and wild-type expression data we suggest a model where a low level of FoxN4 is maintained in RPCs and that increased levels during a restricted period trigger neurogenesis and commitment of RPCs to the horizontal cell fate.

Alternative splicing of neuroligin and its protein distribution in the outer plexiform layer of the chicken retina

Although synaptogenesis within the retina is obviously essential for vision, mechanisms responsible for the initiation and maintenance of retinal synapses are poorly understood. In addition to its scientific interest, understanding retinal synapse formation is becoming clinically relevant with ongoing efforts to develop transplantation-based approaches for the treatment of retinal degenerative disease. To extend our understanding, we have focused on the chick model system and have studied the neuroligin family of neuronal adhesion factors that has been shown to participate in synapse assembly in the brain. We identified chicken orthologs of neuroligins 1, -3, and -4, but could find no evidence of neuroligin 2. We investigated temporal and spatial patterns of mRNA and protein expression during development using standard polymerase chain reaction (RT-PCR), quantitative PCR (QPCR), laser-capture microdissection (LCM), and confocal microscopy. At the mRNA level, neuroligins were detected at the earliest period tested, embryonic day (ED)5, which precedes the period of inner retina synaptogenesis. Significant alternative splicing was observed through development. While neuroligin gene products were generally detected in the inner retina, low levels of neuroligin 1 mRNA were also detected in the photoreceptor layer. Neuroligin 3 and -4 transcripts, on the other hand, were only detected in the inner retina. At retinal synapses neuroligin 1 protein was detected in the inner plexiform layer, but its highest levels were detected in the outer plexiform layer on the tips of horizontal cell dendrites. This work lays the groundwork for future studies on the functional roles of the neuroligins within the retina.

Ptf1a/Rbpj complex inhibits ganglion cell fate and drives the specification of all horizontal cell subtypes in the chick retina

During development, progenitor cells of the retina give rise to six principal classes of neurons and the Müller glial cells found within the adult retina. The pancreas transcription factor 1 subunit a (Ptf1a) encodes a basic-helix-loop-helix transcription factor necessary for the specification of horizontal cells and the majority of amacrine cell subtypes in the mouse retina. The Ptf1a-regulated genes and the regulation of Ptf1a activity by transcription cofactors during retinogenesis have been poorly investigated. Using a retrovirus-mediated gene transfer approach, we reported that Ptf1a was sufficient to promote the fates of amacrine and horizontal cells from retinal progenitors and inhibit retinal ganglion cell and photoreceptor differentiation in the chick retina. Both GABAergic H1 and non-GABAergic H3 horizontal cells were induced following the forced expression of Ptf1a. We describe Ptf1a as a strong, negative regulator of Atoh7 expression. Furthermore, the Rbpj-interacting domains of Ptf1a protein were required for its effects on cell fate specification. Together, these data provide a novel insight into the molecular basis of Ptf1a activity on early cell specification in the chick retina.

Genetic modulation of horizontal cell number in the mouse retina

Neuronal populations display conspicuous variability in their size among individuals, but the genetic sources of this variation are largely undefined. We demonstrate a large and highly heritable variation in neuron number within the mouse retina, affecting a critical population of interneurons, the horizontal cells. Variation in the size of this population maps to the distal end of chromosome (Chr) 13, a region homologous to human Chr 5q11.1-11.2. This region contains two genes known to modulate retinal cell number. Using conditional knock-out mice, we demonstrate that one of these genes, the LIM homeodomain gene Islet-1 (Isl1), plays a role in regulating horizontal cell number. Genetic differences in Isl1 expression are high during the period of horizontal cell production, and cis-regulation of Isl1 expression within the retina is demonstrated directly. We identify a single nucleotide polymorphism in the 5' UTR of Isl1 that creates an E-box sequence as a candidate causal variant contributing to this variation in horizontal cell number.

Development of the retina and optic pathway

Our understanding of the development of the retina and visual pathways has seen enormous advances during the past 25years. New imaging technologies, coupled with advances in molecular biology, have permitted a fuller appreciation of the histotypical events associated with proliferation, fate determination, migration, differentiation, pathway navigation, target innervation, synaptogenesis and cell death, and in many instances, in understanding the genetic, molecular, cellular and activity-dependent mechanisms underlying those developmental changes. The present review considers those advances associated with the lineal relationships between retinal nerve cells, the production of retinal nerve cell diversity, the migration, patterning and differentiation of different types of retinal nerve cells, the determinants of the decussation pattern at the optic chiasm, the formation of the retinotopic map, and the establishment of ocular domains within the thalamus.

LIM family transcription factors regulate the subtype-specific morphogenesis of retinal horizontal cells at post-migratory stages

In the nervous system, transcription factor expression in progenitor and/or nascent neurons regulates cell type specification. Although the functions of these transcription factors at early stages are well established, whether or not they are required during late developmental periods remains an open question. To address this issue, we conditionally manipulated gene expression using a recently developed transposon-mediated gene transfer system combined with in ovo electroporation. In chicken retinas, horizontal cells are classified into three subtypes according to their characteristic neuronal morphology. Two LIM family transcription factors, Lim1 and Isl1, begin to be expressed in a distinct subset of nascent retinal neurons, which results in complementary expression of these genes in mature retinas in type I and type II/III horizontal cells, respectively. Overexpression of Isl1 in post-migratory horizontal cells represses endogenous Lim1 expression and increases the number of neurons with a dendritic morphology characteristic of type II horizontal cells, which normally express Isl1. Inhibition of Lim1 function by expression of a dominant negative form Lim1 perturbs axonal morphogenesis of type I horizontal cells. Therefore, we propose that LIM family transcription factors are required for subtype-specific morphogenesis of horizontal cells at later stages of retinal development.

Horizontal cell progenitors arrest in G2-phase and undergo terminal mitosis on the vitreal side of the chick retina

We have addressed the question when horizontal cells in the chick retina are generated and undergo their terminal mitosis. Horizontal cell progenitors replicate their DNA early and migrate bi-directionally to the horizontal cell layer. It was hypothesized that the cells undergo mitosis directly after replication and migrate as post-mitotic transition cells before differentiating to horizontal cells. However, our results show that cells expressing markers for the axon-bearing and the axon-less subtypes of horizontal cells undergo terminal mitosis while residing on the vitreal side of the retina. By combining horizontal cell transcription factors Lim1, Isl1 and Prox1 labeling with phospho-histone H3, a marker for mitosis, we demonstrate that all or a clear majority of vitreal mitoses are undertaken by the horizontal cell committed progenitors. The pattern of cells that incorporated the thymidine analogue EdU implied that the progenitors replicated their genome while migrating towards the vitreal side. Upon arrival to the vitreal retina they become arrested for about two days prior to mitosis. Hence, cells expressing horizontal cell markers are arrested in G2-phase on the vitreal side of the retina. These results support the existence of committed progenitors that give rise to horizontal cells and that those cells become arrested in G2-phase before undergoing terminal mitosis on the vitreal side of the retina followed by migration to the horizontal cell layer. The results also indicate that the regulation of the transition from G2-phase to mitosis is important for the development of these committed progenitor cells.