Generating neuronal diversity in the retina: one for nearly all

Immortalized human fetal retinal cells retain progenitor characteristics and represent a potential source for the treatment of retinal degenerative disease

Human fetal retinal cells have been widely advocated for the development of cellular replacement therapies in patients with retinal dystrophies and age-related macular degeneration. A major limitation, however, is the lack of an abundant and renewable source of cells to meet therapeutic demand, although theoretically this may be addressed through the use of immortalized retinal progenitor cell lines. Here, we have used the temperature-sensitive tsA58 simian virus SV40 T antigen to conditionally immortalize human retinal progenitor cells isolated from retinal tissue at 10-12 weeks of gestation. We show that immortalized human fetal retinal cells retain their progenitor cell properties over many passages, and are comparable with nonimmortalized human fetal retinal cultures from the same gestational period with regard to expression of certain retinal genes. To evaluate the capacity of these cells to integrate into the diseased retina and to screen for potential tumorigenicity, cells were grafted into neonatal hooded Lister rats and RCS dystrophic rats. Both cell lines exhibited scarce integration into the host retina and failed to express markers of mature differentiated retinal cells. Moreover, although immortalized cells showed a greater propensity to survive, the cell lines demonstrated poor long-term survival. All grafts were infiltrated with host macrophage/microglial cells throughout their duration of survival. This study demonstrates that immortalized human fetal retinal progenitor cells retain their progenitor characteristics and may therefore have therapeutic potential in strategies that demand a renewable and consistent supply of donor cells for the treatment of degenerative retinal diseases.

The transcription factor RBP-J is essential for retinal cell differentiation and lamination

Background

The highly ordered vertebrate retina is composed of seven cell types derived from a common pool of retinal progenitor cells (RPCs), and is a good model for the studies of cell differentiation and interaction during neural development. Notch signaling plays a pivotal role in retinogenesis in mammals, but the full scope of the functions of Notch pathway, and the underlying molecular mechanisms, remain unclear.

Results

In this study, we conditionally knocked out RBP-J, the critical transcription factor downstream to all four Notch receptors, in RPCs of mouse retina at different developmental stages. Disruption of RBP-J at early retinogenesis resulted in accelerated RPCs differentiation, but only photoreceptors and ganglion cells were overrepresented, with other neuronal populations diminished. Similarly, deletion of RBP-J at early postnatal days also led to overproduction of photoreceptors, suggesting that RBP-J governed RPCs specification and differentiation through retinogenesis. In all the RBP-J deletion models, the retinal laminar structures were distorted by the formation of numerous rosette-like structures, reminiscent of β-catenin deficient retina. Indeed, we found that these rosettes aligned with gaps in β-catenin expression at the apical surface of the retina. By in vivo electroporation-mediated transfection, we demonstrated that lamination defects in RBP-J deficient retinae were rescued by overexpressing β-catenin.

Conclusions

Our data indicate that RBP-J-mediated canonical Notch signaling governs retinal cell specification and differentiation, and maintains retinal lamination through the expression of β-catenin.

Evolution of vertebrate rod and cone phototransduction genes

Vertebrate cones and rods in several cases use separate but related components for their signal transduction (opsins, G-proteins, ion channels, etc.). Some of these proteins are also used differentially in other cell types in the retina. Because cones, rods and other retinal cell types originated in early vertebrate evolution, it is of interest to see if their specific genes arose in the extensive gene duplications that took place in the ancestor of the jawed vertebrates (gnathostomes) by two tetraploidizations (genome doublings). The ancestor of teleost fishes subsequently underwent a third tetraploidization. Our previously reported analyses showed that several gene families in the vertebrate visual phototransduction cascade received new members in the basal tetraploidizations. We here expand these data with studies of additional gene families and vertebrate species. We conclude that no less than 10 of the 13 studied phototransduction gene families received additional members in the two basal vertebrate tetraploidizations. Also the remaining three families seem to have undergone duplications during the same time period but it is unclear if this happened as a result of the tetraploidizations. The implications of the many early vertebrate gene duplications for functional specialization of specific retinal cell types, particularly cones and rods, are discussed.

C38, equivalent to BM88, is developmentally expressed in maturing retinal neurons and enhances neuronal maturation

C38 antigen is specifically expressed in neuronal cells of the retina. The purpose of this study was to isolate C38 cDNA and determine its molecular functions. Sequence analysis of C38 cDNA revealed that C38 is equivalent to rat BM88, which has been reported to induce cell-cycle arrest and neuronal differentiation in Neuro2a cells. C38 and Ki67, a marker of proliferating cells, were not colocalized during retinal development. C38 was first detected in the retinal ganglion cells at embryonic day 16, much later than the expression of doublecortin, a marker of immature neurons. Although all the horizontal cells were post-mitotic at this stage, C38 was not detected in horizontal cells until the postnatal period. In addition, C38 over-expression did not induce neuronal differentiation or cell-cycle arrest of pluripotent P19 embryonal carcinoma cells. Instead, C38 promoted maturation during neuronal differentiation of P19 embryonal carcinoma cells by down-regulating Oct-3, a pluripotent cell marker and enhancing the expressions of positive regulators of neurogenesis. In conclusion, during retinal development, C38 is first expressed in post-mitotic retinal neurons and is up-regulated during their maturation. C38 does not induce neuronal competence in pluripotent cells, but does promote maturation in already committed neuronal cells.

Expression and function of P2Y receptors on Müller cells of the postnatal rat retina

In the postnatal and mature retina, many processes are controlled by the action of nucleotides. Their effects are partly mediated via activation of metabotropic P2Y receptors. However, little is known about the developmental regulation and cellular localization of P2Y receptor subtypes. Combining immunohistochemical and neurophysiological methods, we investigated the developmental expression of P2Y receptors on Müller cells, the principal macroglial cells of the retina. The P2Y(1) and the P2Y(4) receptors, but no other subtypes, were unequivocally localized on Müller cells. P2Y(1) was expressed from postnatal day 5 (P5) on and mediated a calcium response to ATP in Müller cells as well as a volume regulatory signaling cascade preventing Müller cells from swelling under hypotonic conditions. Differentiation of Müller cells was accompanied by a change of the calcium response pattern; the calcium responses in Müller cell endfeet persisted, but ATP responsiveness of Müller cell somata disappeared. P2Y(4) immunoreactivity was observed in Müller cell endfeet and synaptic terminals of rod bipolar cells from P20 on. Activated protein kinases were detected by immunohistochemistry; p-ERK occurred in Müller cells and amacrine cells, whereas p-Akt was detected in bipolar cells. Our data indicate that purinergic signaling via P2Y(1) and P2Y(4) receptors might contribute to differentiation processes in the postnatal retina.

Retinal pathology of a patient with Goldmann-Favre syndrome

PURPOSE

To define the retinal pathology in an 88-year-old male affected with Goldmann-Favre syndrome with a 2 bp 5' A>C splice site mutation in the NR2E3 gene.

METHODS

Retinal tissue from the macula and periphery was processed for immunohistochemistry. Perimacular retina was processed for transmission electron microscopy. Cryosections were studied by indirect immunofluorescence, using well-characterized antibodies to rhodopsin, cone cytoplasm, and cone opsins. The affected donor eye was compared to a postmortem matched normal eye.

RESULTS

The retina was highly disorganized without laminar organization. The RPE was discontinuous in some perimacular regions. Large (>1 mm) spherical electrondense melanosomes were observed in the RPE and choroid by TEM. Rods were virtually absent in the affected retina. Cones were present in the macula, but were mostly absent from the retinal periphery. In addition, cone rosettes were observed in the perimacular area. Both red/green and blue cone opsins were distributed along the entire cellular expanse of the cone photoreceptors in the affected eye, but were restricted to the cone outer segments in the control retina.

CONCLUSIONS

The histological data obtained from the retina of an elderly male patient with Goldmann-Favre syndrome showed an absence of rods and abnormal distribution of red/green and blue cone opsins.

A global survey identifies novel upstream components of the Ath5 neurogenic network

Regulators of vertebrate Ath5 expression were identified by high-throughput screening; extending the current gene regulatory model network controlling retinal neurogenesis.

Background

Investigating the architecture of gene regulatory networks (GRNs) is essential to decipher the logic of developmental programs during embryogenesis. In this study we present an upstream survey approach, termed trans-regulation screen, to comprehensively identify the regulatory input converging on endogenous regulatory sequences.

Results

Our dual luciferase-based screen queries transcriptome-scale collections of cDNAs. Using this approach we study the regulation of Ath5, the central node in the GRN controlling retinal ganglion cell (RGC) specification in vertebrates. The Ath5 promoter integrates the input of upstream regulators to enable the transient activation of the gene, which is an essential step for RGC differentiation. We efficiently identified potential Ath5 regulators that were further filtered for true positives by an in situ hybridization screen. Their regulatory activity was validated in vivo by functional assays in medakafish embryos.

Conclusions

Our analysis establishes functional groups of genes controlling different regulatory phases, including the onset of Ath5 expression at cell-cycle exit and its down-regulation prior to terminal RGC differentiation. These results extent the current model of the GRN controlling retinal neurogenesis in vertebrates.

Labelling and targeted ablation of specific bipolar cell types in the zebrafish retina

Background

Development of a functional retina depends on regulated differentiation of several types of neurons and generation of a highly complex network between the different types of neurons. In addition, each type of retinal neuron includes several distinct morphological types. Very little is known about the mechanisms responsible for generating this diversity of retinal neurons, which may also display specific patterns of regional distribution.

Results

In a screen in zebrafish, using a trapping vector carrying an engineered yeast Gal4 transcription activator and a UAS:eGFP reporter cassette, we have identified two transgenic lines of zebrafish co-expressing eGFP and Gal4 in specific subsets of retinal bipolar cells. The eGFP-labelling facilitated analysis of axon terminals within the inner plexiform layer of the adult retina and showed that the fluorescent bipolar cells correspond to previously defined morphological types. Strong regional restriction of eGFP-positive bipolar cells to the central part of the retina surrounding the optic nerve was observed in adult zebrafish. Furthermore, we achieved specific ablation of the labelled bipolar cells in 5 days old larvae, using a bacterial nitroreductase gene under Gal4-UAS control in combination with the prodrug metronidazole. Following prodrug treatment, nitroreductase expressing bipolar cells were efficiently ablated without affecting surrounding retina architecture, and recovery occurred within a few days due to increased generation of new bipolar cells.

Conclusion

This report shows that enhancer trapping can be applied to label distinct morphological types of bipolar cells in the zebrafish retina. The genetic labelling of these cells yielded co-expression of a modified Gal4 transcription activator and the fluorescent marker eGFP. Our work also demonstrates the potential utility of the Gal4-UAS system for induction of other transgenes, including a bacterial nitroreductase fusion gene, which can facilitate analysis of bipolar cell differentiation and how the retina recovers from specific ablation of these cells.

Current Prospects for Adult Stem Cell–Based Therapies in Ocular Repair and Regeneration

Recent advances in stem cell biology have led to the exploration of stem cell-based therapies to treat a wide range of human diseases. In the ophthalmic field, much hope has been placed on the potential use of these cells to restore sight, particularly in those conditions in which other established treatments have failed and in which visual function has been irreversibly damaged by disease or injury. At present, there are many limitations for the immediate use of embryonic stem cells to treat ocular disease, and as more evidence emerges that adult stem cells are present in the adult human eye, it is clear that these cells may have advantages to develop into feasible therapeutic treatments without the problems associated with embryonic research and immune rejection. Here we discuss the current prospects for the application of various adult ocular stem cells to human therapies for restoration of vision.

Genetic evidence for shared mechanisms of epimorphic regeneration in zebrafish

In a microarray-based gene profiling analysis of Müller glia-derived retinal stem cells in light-damaged retinas from adult zebrafish, we found that 2 genes required for regeneration of fin and heart tissues in zebrafish, hspd1 (heat shock 60-kDa protein 1) and mps1 (monopolar spindle 1), were up-regulated. Expression of both genes in the neurogenic Müller glia and progenitors was independently verified by quantitative reverse transcriptase PCR and in situ hybridization. Functional analysis of temperature-sensitive mutants of hspd1 and mps1 revealed that both are necessary for Müller glia-based cone photoreceptor regeneration in adult zebrafish retina. In the amputated fin, hspd1 is required for the induction of mesenchymal stem cells and blastema formation, whereas mps1 is required at a later step for rapid cell proliferation and outgrowth. This temporal sequence of hspd1 and mps1 function is conserved in the regenerating retina. Comparison of gene expression profiles from regenerating zebrafish retina, caudal fin, and heart muscle revealed additional candidate genes potentially implicated in injury-induced epimorphic regeneration in diverse zebrafish tissues.

Stepwise differentiation of pluripotent stem cells into retinal cells

Embryonic stem (ES) cells are pluripotent cells derived from the inner cell mass of blastocyst-stage embryos. They can maintain an undifferentiated state indefinitely and can differentiate into derivatives of all three germ layers, namely ectoderm, endoderm and mesoderm. Although much progress has been made in the propagation and differentiation of ES cells, induction of photoreceptors has generally required coculture with or transplantation into developing retinal tissue. Here, we describe a protocol for generating retinal cells from ES cells by stepwise treatment with defined factors. This method preferentially induces photoreceptor and retinal pigment epithelium (RPE) cells from mouse and human ES cells. In our protocol, differentiation of RPE and photoreceptors from mouse ES cells requires 28 d and the differentiation of human ES cells into mature RPE and photoreceptors requires 120 and 150 d, respectively. This differentiation system and the resulting pluripotent stem cell-derived retinal cells will facilitate the development of transplantation therapies for retinal diseases, drug testing and in vitro disease modeling. It will also improve our understanding of the development of the central nervous system, especially the eye.

Random walk behavior of migrating cortical interneurons in the marginal zone: time-lapse analysis in flat-mount cortex

Migrating neurons are thought to travel from their origin near the ventricle to distant territories along stereotypical pathways by detecting environmental cues in the extracellular milieu. Here, we report a novel mode of neuronal migration that challenges this view. We performed long-term, time-lapse imaging of medial ganglionic eminence (MGE)-derived cortical interneurons tangentially migrating in the marginal zone (MZ) in flat-mount cortices. We find that they exhibit a diverse range of behaviors in terms of the rate and direction of migration. Curiously, a predominant population of these neurons repeatedly changes its direction of migration in an unpredictable manner. Trajectories of migration vary from one neuron to another. The migration of individual cells lasts for long periods, sometimes up to 2 d. Theoretical analyses reveal that these behaviors can be modeled by a random walk. Furthermore, MZ cells migrate from the cortical subventricular zone to the cortical plate, transiently accumulating in the MZ. These results suggest that MGE-derived cortical interneurons, once arriving at the MZ, are released from regulation by guidance cues and initiate random walk movement, which potentially contributes to their dispersion throughout the cortex.

Retinal neurogenesis and ontogenetic changes in the visual system of the brown banded bamboo shark, Chiloscyllium punctatum (Hemiscyllidae, Elasmobranchii)

The development of the eye of the oviparous brown banded bamboo shark, Chiloscyllium punctatum, was monitored from egg deposition through adulthood. The order and timing of retinal cell differentiation were assessed by light and transmission electron microscopy. As in other vertebrates, the ganglion cells are the first to differentiate, in this case by 81 days post-egg deposition (dpd). The order then deviates from what is typically quoted for vertebrates, with the Müller and amacrine cells differentiating morphologically around the same time, followed by the bipolar cells (101 dpd) and finally the horizontal cells and photoreceptors (124 dpd). The neural retina is fully differentiated and synaptic connections are formed approximately 1 month prior to hatching, which occurs at about 158 dpd. The mature retina is duplex, with a peak rod to cone ratio of approximately 12:1. The eye and lens of C. punctatum continue to grow throughout life and become less aspherical with growth; the equatorial (nasotemporal) lens diameter is 12% larger than the axial (anterior-posterior) lens diameter in embryos and 8% larger in adults. Access to developmental stages and the protracted gestational period of C. punctatum make it a highly valuable model for developmental studies of the visual system. This study also provides an evolutionary perspective on retinal neurogenesis in an elasmobranch.

Identification of pax6-dependent gene regulatory networks in the mouse lens

Lineage-specific DNA-binding transcription factors regulate development by activating and repressing particular set of genes required for the acquisition of a specific cell type. Pax6 is a paired domain and homeodomain-containing transcription factor essential for development of central nervous, olfactory and visual systems, as well as endocrine pancreas. Haploinsufficiency of Pax6 results in perturbed lens development and homeostasis. Loss-of-function of Pax6 is incompatible with lens lineage formation and results in abnormal telencephalic development. Using DNA microarrays, we have identified 559 genes expressed differentially between 1-day old mouse Pax6 heterozygous and wild type lenses. Of these, 178 (31.8%) were similarly increased and decreased in Pax6 homozygous embryonic telencephalon [Holm PC, Mader MT, Haubst N, Wizenmann A, Sigvardsson M, Götz M (2007) Loss- and gain-of-function analyses reveals targets of Pax6 in the developing mouse telencephalon. Mol Cell Neurosci 34: 99–119]. In contrast, 381 (68.2%) genes were differently regulated between the lens and embryonic telencephalon. Differential expression of nine genes implicated in lens development and homeostasis: Cspg2, Igfbp5, Mab21l2, Nrf2f, Olfm3, Spag5, Spock1, Spon1 and Tgfb2, was confirmed by quantitative RT-PCR, with five of these genes: Cspg2, Mab21l2, Olfm3, Spag5 and Tgfb2, identified as candidate direct Pax6 target genes by quantitative chromatin immunoprecipitation (qChIP). In Mab21l2 and Tgfb2 promoter regions, twelve putative individual Pax6-binding sites were tested by electrophoretic mobility shift assays (EMSAs) with recombinant Pax6 proteins. This led to the identification of two and three sites in the respective Mab21l2 and Tgfb2 promoter regions identified by qChIPs. Collectively, the present studies represent an integrative genome-wide approach to identify downstream networks controlled by Pax6 that control mouse lens and forebrain development.

Cell transplantation strategies for retinal repair

Cell transplantation is a novel therapeutic strategy to restore visual responses to the degenerate adult neural retina and represents an exciting area of regenerative neurotherapy. So far, it has been shown that transplanted postmitotic photoreceptor precursors are able to functionally integrate into the adult mouse neural retina. In this review, we discuss the differentiation of photoreceptor cells from both adult and embryonic-derived stem cells and their potential for retinal cell transplantation. We also discuss the strategies used to overcome barriers present in the degenerate neural retina and improve retinal cell integration. Finally, we consider the future translation of retinal cell therapy as a therapeutic strategy to treat retinal degeneration.

The temporal requirement for vitamin A in the developing eye: mechanism of action in optic fissure closure and new roles for the vitamin in regulating cell proliferation and adhesion in the embryonic retina

Mammalian eye development requires vitamin A (retinol, ROL). The role of vitamin A at specific times during eye development was studied in rat fetuses made vitamin A deficient (VAD) after embryonic day (E) 10.5 (late VAD). The optic fissure does not close in late VAD embryos, and severe folding and collapse of the retina is observed at E18.5. Pitx2, a gene required for normal optic fissure closure, is dramatically downregulated in the periocular mesenchyme in late VAD embryos, and dissolution of the basal lamina does not occur at the optic fissure margin. The addition of ROL to late VAD embryos by E12.5 restores Pitx2 expression, supports dissolution of the basal lamina, and prevents coloboma, whereas supplementation at E13.5 does not. Surprisingly, ROL given as late as E13.5 completely prevents folding of the retina despite the presence of an open fetal fissure, showing that coloboma and retinal folding represent distinct VAD-dependent defects. Retinal folding due to VAD is preceded by an overall reduction in the percentage of cyclin D1 positive cells in the developing retina, (initially resulting in retinal thinning), as well as a dramatic reduction in the cell adhesion-related molecules, N-cadherin and beta-catenin. Reduction of retinal cell number combined with a loss of the normal cell-cell adhesion proteins may contribute to the collapse and folding of the retina that occurs in late VAD fetuses.

Gradual morphogenesis of retinal neurons in the peripheral retinal margin of adult monkeys and humans

The adult mammalian retina has for long been considered to lack a neurogenerative capacity. However, retinal stem/progenitor cells, which can originate retinal neurons in vitro, have been recently reported in the ciliary body of adult mammals. Here we explored the possibility of retinal neurogenesis occurring in vivo in adult monkeys and humans. We found the presence of cells expressing molecular markers of neural and retinal progenitors in the nonlaminated retinal margin and ciliary body pars plana of mature primates. By means of immunohistochemistry and electron microscopy we also observed photoreceptors and other retinal cell types in different stages of morphological differentiation along the peripheral retinal margin. These findings allow us to extend to primates the idea of neurogenesis aimed at retinal cell turnover throughout life.

The protein kinase DYRK1A regulates caspase-9-mediated apoptosis during retina development

The precise regulation of programmed cell death is critical for the normal development of the nervous system. We show here that DYRK1A (minibrain), a protein kinase essential for normal growth, is a negative regulator of the intrinsic apoptotic pathway in the developing retina. We provide evidence that changes in Dyrk1A gene dosage in the mouse strongly alter the cellularity of inner retina layers and result in severe functional alterations. We show that DYRK1A does not affect the proliferation or specification of retina progenitor cells, but rather regulates the number of cells that die by apoptosis. We demonstrate that DYRK1A phosphorylates caspase-9 on threonine residue 125, and that this phosphorylation event is crucial to protect retina cells from apoptotic cell death. Our data suggest a model in which dysregulation of the apoptotic response in differentiating neurons participates in the neuropathology of diseases that display DYRK1A gene-dosage imbalance effects, such as Down's syndrome.

Temporal control of neuronal diversity: common regulatory principles in insects and vertebrates?

It is well established in species as diverse as insects and mammals that different neuronal and glial subtypes are born at distinct times during central nervous system development. In Drosophila, there is now compelling evidence that individual multipotent neuroblasts express a sequence of progenitor transcription factors which, in turn, regulates the postmitotic transcription factors that specify neuronal/glial temporal identities. Here,we examine the hypothesis that the regulatory principles underlying this mode of temporal specification are shared between insects and mammals, even if some of the factors themselves are not. We also propose a general model for birth-order-dependent neural specification and suggest some experiments to test its validity.

Regulation of neurogenesis by interkinetic nuclear migration through an apical-basal notch gradient

The different cell types in the central nervous system develop from a common pool of progenitor cells. The nuclei of progenitors move between the apical and basal surfaces of the neuroepithelium in phase with their cell cycle, a process termed interkinetic nuclear migration (INM). In the retina of zebrafish mikre oko (mok) mutants, in which the motor protein Dynactin-1 is disrupted, interkinetic nuclei migrate more rapidly and deeply to the basal side and more slowly to the apical side. We found that Notch signaling is predominantly activated on the apical side in both mutants and wild-type. Mutant progenitors are, thus, less exposed to Notch and exit the cell cycle prematurely. This leads to an overproduction of early-born retinal ganglion cells (RGCs) at the expense of later-born interneurons and glia. Our data indicate that the function of INM is to balance the exposure of progenitor nuclei to neurogenic versus proliferative signals.

TGIF, a homeodomain transcription factor, regulates retinal progenitor cell differentiation

TG-interacting factor (TGIF) is a TALE homeodomain protein expressed predominantly in the central nervous system and functions as a transcriptional repressor. Several mutations in TGIF have been identified in patients with holoprosencephaly, the most common congenital malformation of the developing human forebrain. However, the precise role of TGIF in neural development is not well understood. We found that TGIF was expressed strongly in the mouse retina during early stages of development, and that its expression gradually decreased as retinal development progressed. In vitro explant cultures of mouse retina mimic the in vivo development of retinal subtypes. Forced expression of TGIF using a retrovirus in explant culture induced the differentiation of amacrine cells from retinal progenitor cells. A TGIF paralog, TGIF2, showed a similar transition in expression during retinal development, and TGIF2 also promoted amacrine cell differentiation in a retinal explant culture system. However, no apparent difference between wild-type and TGIF-knockout mouse retina was observed, suggesting that TGIF and TGIF2 function redundantly in that tissue. Forced expression of TGIF homeodomain (HD)-EnR (repressing) rather than TGIF HD-VP16 (activating) resulted in a phenotype similar to that induced by wild-type TGIF, suggesting that TGIFs may act as transcriptional repressors to induce amacrine genesis.

How autophagy is related to programmed cell death during the development of the nervous system

Programmed cell death, together with proliferation and differentiation, is an essential process during the development of the nervous system. During neurogenesis, neurons and glia are generated in large numbers and, subsequently, they die in a process that depends on trophic signalling that refines the cytoarchitecture and connectivity of the nervous system. In addition, programmed cell death also affects proliferating neuroepithelial cells and recently differentiated neuroblasts. Autophagy is a lysosomal degradative pathway that allows the recycling of cell constituents, and seems to be able to play a dual role. It may serve to protect the cell by preventing the accumulation of deleterious products and organelles and supplying energy and amino acids. On the other hand, it has been considered a type of cell death. The role of autophagy during development is little characterized. The retina provides an excellent model system to study autophagy in the context of neural development, and to establish its relationship with proliferation, differentiation and cell death. In the present review, we summarize recent findings showing that autophagy contributes to the development of the nervous system by providing energy for cell corpse removal after physiological cell death, a process associated with retinal neurogenesis.

Hairy2-Id3 interactions play an essential role in Xenopus neural crest progenitor specification

Loss of function studies have shown that the Xenopus helix-loop-helix transcription factor Hairy2 is essential for neural crest formation and maintains cells in a mitotic undifferentiated state. However, its position in the genetic cascade regulating neural crest formation and its relationship with other neural crest regulators remain largely unknown. Here we find that Hairy2 is regulated by BMP, FGF and Wnt and that it is only required downstream of BMP and FGF for neural crest formation. We show that Hairy2 overexpression represses neural crest and upregulates neural border genes at early stages while it expands a subset of them in later embryos. We show that Hairy2 downregulates Id3, another essential HLH neural crest regulator, through attenuation of BMP signaling. Knockdown and rescue experiments indicate that Id3 protein, which physically interacts with Hairy2, negatively regulates Hairy2 activity. However, Id3 is required to allow Hairy2 to promote neural crest formation. Together, our results provide evidence that Hairy2 acts downstream of FGF and BMP signals at the neural border to maintain cells in an undifferentiated state, and that Hairy2-Id3 interactions play an essential role in neural crest progenitor specification.

Neuronal functional diversity and collective behaviors

A major question in today's neuroscience is how the brain's complex operations and organization emerge from individual components. The robustness of neuronal properties with flexible linkages between regulatory processes conceivably accounts for the adaptive, tunable, multistable dynamics; the coding schemes; and the complexity of neuronal functional (sub)systems. Interneurons and neurotransmitter diversity, resonance phenomena due to properties of the cell, time/frequency-dependent activation of dedicated neuronal assemblies, and code- and frequency-specific oscillations interact in determining the brain functional setup and operations. Such an arrangement would also provide the functional requirements for access to neural mechanisms, dedicated neuronal circuitry and the proper timing allowing for the selective differentiation among cortical neurons due to performing in different tasks. No comprehensive theory or systematic methodological approach appears yet conceivable. The scenario, however incomplete and incompletely characterized, is nevertheless promising and warrants further investigation.

Synaptic activity, visual experience and the maturation of retinal synaptic circuitry

A fundamental feature of the synaptic organization of retina is the laminar-specific structure, in which specific types of retinal neurons form highly selective synapses to transfer distinct synaptic signals. In mature vertebrate retina, the dendrites of most retinal ganglion cells (RGCs) are narrowly stratified and ramified in specific strata of the inner plexiform layer (IPL) of retina to synapse with distinct subtypes of bipolar cells (BCs). However, little is known of how retinal neurons form this laminar-specific synaptic structure during development. Recent studies showed that the formation of retinal synaptic circuitry is regulated by both gene expression and neuronal activity. Here I will briefly discuss the recent advances in our understanding of how synaptic activity modulates the maturation of RGC synaptic connections.

Cadherin-6 function in zebrafish retinal development

Cadherin cell-adhesion molecules play crucial roles in vertebrate development including the development of the visual system. Most studies have focused on examining functions of classical type I cadherins (e.g., cadherin-2) in visual system development. There is little information on the function of classical type II cadherins (e.g., cadherin-6) in the development of the vertebrate visual system. To gain insight into cadherin-6 role in the formation of the retina, we analyzed differentiation of retinal ganglion cells (RGCs), amacrine cells, and photoreceptors in zebrafish embryos injected with cadherin-6 specific antisense morpholino oligonucleotides. Differentiation of the retinal neurons in cadherin-6 knockdown embryos (cdh6 morphants) was analyzed using multiple markers. We found that expression of transcription factors important for retinal development was greatly reduced, and expression of Notch-Delta genes and proneural gene ath5 was altered in the cdh6 morphant retina. The retinal lamination was present in the morphants, although the morphant eyes were significantly smaller than control embryos due mainly to decreased cell proliferation. Differentiation of the RGCs, amacrine cells, and photoreceptors was severely disrupted in the cdh6 morphants due to a significant delay in neural differentiation. Our results suggest that cadherin-6 plays an important role in the normal formation of the zebrafish retina. (c) 2008 Wiley Periodicals, Inc. Develop Neurobiol, 2008.

Dual role of Nr2e3 in photoreceptor development and maintenance

Nr2e3 is a photoreceptor-specific nuclear receptor believed to play a role in photoreceptor development, differentiation, and survival. Much research has focused on the interaction of Nr2e3 with other transcription factors in determining the milieu of target gene expression in photoreceptors of the neonatal and adult retina. To investigate the downstream targets of Nr2e3 and thereby shed light on the functional pathways relevant to photoreceptor development and maintenance, expression profiling was performed on retinas from two different mouse knockout lines, one containing a targeted disruption of the Nr2e3 gene (Nr2e3 -/-), the other containing a spontaneous null allele of the Nr2e3 locus (rd7). Using whole genome microarrays, mRNA expression profiles of retinas from the two mutant strains were compared to those of wildtype C57BL/6 mice over a time course that ranged from postnatal day (p) 2 to 6months of age (p180). Additionally, expression profiling was performed on retinal explants treated with a putative NR2E3 agonist. The molecular profiling of Nr2e3 -/- and rd7/rd7 retinas identified 281 putative Nr2e3-dependent genes that were differentially expressed between wildtype and mutant retinas during at least one time point. Consistent with previous reports that Nr2e3 is necessary for the repression of cone-specific genes, increased expression of cone-specific genes was observed in the mutant samples, thereby providing proof-of-concept for the microarray screen. Further annotation of these data sets revealed ten predominant functional classes involved in the Nr2e3-mediated development and/or maintenance of photoreceptors. Interestingly, differences in the expression of Nr2e3-dependent genes exhibited two distinct temporal patterns. One group of genes showed a sustained difference in expression as compared to wildtype over the entire time course of the study, whereas a second group showed only transient differences which were largest around p10. Comparison of gene expression changes in Nr2e3 -/- and rd7/rd7 retinas with those uncovered by treating retinal explants with a putative NR2E3 agonist revealed four genes that were down-regulated in mutant retinas that lack Nr2e3 function but were up-regulated in agonist-treated explants. These results strongly suggest that the four genes may be direct targets of Nr2e3. Our identification of two sets of Nr2e3-regulated genes provides further evidence of a dual role for Nr2e3 in specification of photoreceptor fate during development as well as photoreceptor maintenance in the adult.

Transient expression of the conserved zinc finger gene INSM1 in progenitors and nascent neurons throughout embryonic and adult neurogenesis

INSM1 is a zinc-finger protein expressed in the developing nervous system and pancreas as well as in medulloblastomas and neuroendocrine tumors. With in situ hybridization combined with immunohistochemistry, we detected INSM1 mRNA in all embryonic to adult neuroproliferative areas examined: embryonic neocortex, ganglionic eminence, midbrain, retina, hindbrain, and spinal cord; autonomic, dorsal root, trigeminal and spiral ganglia; olfactory and vomeronasal organ epithelia; postnatal cerebellum; and juvenile to adult subgranular zone of dentate gyrus, subventricular zone, and rostral migratory stream leading to olfactory bulb. In most of these neurogenic areas, subsets of neuronal progenitors and nascent, but not mature, neurons express INSM1. For example, in developing cerebellum, INSM1 is present in proliferating progenitors of the outer external granule layer (EGL) and in postmitotic cells of the inner EGL, but not in mature granule cell neurons. Also, lining the neural tube from spinal cord to neocortex in mouse as well as human embryos, cells undergoing mitosis apically do not express INSM1. By contrast, nonsurface progenitors located in the basal ventricular and/or subventricular zones express INSM1. Whereas apical progenitors are proliferative and generate one or two additional progenitors, basal progenitors are thought to divide terminally and symmetrically to produce two neurons. The nematode ortholog of INSM1, EGL-46, is expressed during terminal symmetric neurogenic divisions and regulates the termination of proliferation. We propose that, in mice and humans, INSM1 is likewise expressed transiently during terminal neurogenic divisions, from late progenitors to nascent neurons, and particularly during symmetric neuronogenic divisions.

The autophagic machinery is necessary for removal of cell corpses from the developing retinal neuroepithelium

Autophagy is a homoeostatic process necessary for the clearance of damaged or superfluous proteins and organelles. The recycling of intracellular constituents also provides energy during periods of metabolic stress, thereby contributing to cell viability. In addition, disruption of autophagic machinery interferes with embryonic development in several species, although the underlying cellular processes affected remain unclear. Here, we investigate the role of autophagy during the early stages of chick retina development, when the retinal neuroepithelium proliferates and starts to generate the first neurons, the retinal ganglion cells. These two developmental processes are accompanied by programmed cell death. Upon treatment with the autophagic inhibitor 3-methyladenine, retinas accumulated numerous TdT-mediated dUTP nick-end labelling-positive cells that correlated with a lack of the 'eat-me' signal phosphatidylserine (PS). In consequence, neighbouring cells did not engulf apoptotic bodies and they persisted as individual cell corpses, a phenotype that was also observed after blockade of phagocytosis with phospho-L-Serine. Supplying the retinas with methylpyruvate, a cell-permeable substrate for ATP production, restored ATP levels and the presentation of PS at the cell surface. Hence, engulfment and lysosomal degradation of apoptotic bodies were also re-established. Together, these data point to a novel role for the autophagic machinery during the development of the central nervous system.

Regulation of retinal cell fate specification by multiple transcription factors

Retinal cell fate specification is strictly regulated by multiple transcription factors. Regarding regulation of cell proliferation and differentiation, basic helix-loop-helix (bHLH) type repressors and activators function in an antagonistic manner. Repressor-type bHLH factors maintain retinal progenitor cells, whereas activator-type bHLH factors promote neuronal cell fate determination. However, bHLH genes alone are not sufficient for acquiring proper neuronal subtype identity. Recent findings have shown that retinal cell fate specification is regulated by combinations of bHLH and homeobox genes. It is conceivable that homeobox genes confer positional identity whereas bHLH genes regulate neuronal determination and differentiation. Moreover, it has been shown that bHLH genes implicated in retinal cell fate determination regulate expression of other bHLH genes, implying that there is a complicated transcription network regulating retinal development.

Iris development in vertebrates; genetic and molecular considerations

The iris plays a key role in visual function. It regulates the amount of light entering the eye and falling on the retina and also operates in focal adjustment of closer objects. The iris is involved in circulation of the aqueous humor and hence functions in regulation of intraocular pressure. Intriguingly, iris pigmented cells possess the ability to transdifferentiate into different ocular cell types of retinal pigmented epithelium, photoreceptors and lens cells. Thus, the iris is considered a potential source for cell-replacement therapies. During embryogenesis, the iris arises from both the optic cup and the periocular mesenchyme. Its interesting mode of development includes specification of the peripheral optic cup to a non-neuronal fate, migration of cells from the surrounding periocular mesenchyme and an atypical formation of smooth muscles from the neuroectoderm. This manner of development raises some interesting general topics concerning the early patterning of the neuroectoderm, the specification and differentiation of diverse cell types and the interactions between intrinsic and extrinsic factors in the process of organogenesis. In this review, we discuss iris anatomy and development, describe major pathologies of the iris and their molecular etiology and finally summarize the recent findings on genes and signaling pathways that are involved in iris development.

Rod differentiation factor NRL activates the expression of nuclear receptor NR2E3 to suppress the development of cone photoreceptors

Neural developmental programs require a high level of coordination between the decision to exit cell cycle and acquisition of cell fate. The Maf-family transcription factor NRL is essential for rod photoreceptor specification in the mammalian retina as its loss of function converts rod precursors to functional cones. Ectopic expression of NRL or a photoreceptor-specific orphan nuclear receptor NR2E3 completely suppresses cone development while concurrently directing the post-mitotic photoreceptor precursors towards rod cell fate. Given that NRL and NR2E3 have overlapping functions and NR2E3 expression is abolished in the Nrl(-/-) retina, we wanted to clarify the distinct roles of NRL and NR2E3 during retinal differentiation. Here, we demonstrate that NRL binds to a sequence element in the Nr2e3 promoter and enhances its activity synergistically with the homeodomain protein CRX. Using transgenic mice, we show that NRL can only partially suppress cone development in the absence of NR2E3. Gene profiling of retinas from transgenic mice that ectopically express NR2E3 or NRL in cone precursors reveals overlapping and unique targets of these two transcription factors. Together with previous reports, our findings establish the hierarchy of transcriptional regulators in determining rod versus cone cell fate in photoreceptor precursors during the development of mammalian retina.

MARCKS-like protein, a membrane protein identified for its expression in developing neural retina, plays a role in regulating retinal cell proliferation

Membrane proteins are expressed in a specific manner in developing tissues, and characterization of these proteins is valuable because it allows them to be used as cell surface markers. Furthermore, they are potentially important for the regulation of organogenesis because some may participate in signal transduction. In the present study, we used proteomics to examine the comprehensive protein expression profile of the membrane fraction in the embryonic and adult mouse retina. We purified the retinal membrane fraction by sucrose-density-gradient centrifugation and analysed total proteins using shotgun analysis on a nanoflow LC-MS/MS (liquid chromatography tandem MS) system. Approximately half of the 326 proteins from the adult retina and a quarter of the 310 proteins from the embryonic retina (day 17) appeared to be membrane-associated proteins. Among these, MLP [MARCKS (myristoylated alanine-rich C-kinase substrate)-like protein], which shares approx. 50% amino acid identity with MARCKS, was selected for further characterization. The mRNA and surface protein expression of MLP decreased as retinal development progressed. Overexpression of MLP by retrovirus-mediated gene transfer enhanced the proliferation of retinal progenitor cells without affecting differentiation or cell migration in a retinal explant culture system. In contrast, MLP overexpression did not promote proliferation in fibroblasts (NIH 3T3 cells). Mutation analysis of MLP demonstrated that myristoylation was necessary to promote proliferation and that phosphorylation inhibited proliferation, indicating the functional importance of membrane localization.

Delineation of multiple subpallial progenitor domains by the combinatorial expression of transcriptional codes

The mammalian telencephalon is considered the most complex of all biological structures. It comprises a large number of functionally and morphologically distinct types of neurons that coordinately control most aspects of cognition and behavior. The subpallium, for example, not only gives rise to multiple neuronal types that form the basal ganglia and parts of the amygdala and septum but also is the origin of an astonishing diversity of cortical interneurons. Despite our detailed knowledge on the molecular, morphological, and physiological properties of most of these neuronal populations, the mechanisms underlying their generation are still poorly understood. Here, we comprehensively analyzed the expression patterns of several transcription factors in the ventricular zone of the developing subpallium in the mouse to generate a detailed molecular map of the different progenitor domains present in this region. Our study demonstrates that the ventricular zone of the mouse subpallium contains at least 18 domains that are uniquely defined by the combinatorial expression of several transcription factors. Furthermore, the results of microtransplantation experiments in vivo corroborate that anatomically defined regions of the mouse subpallium, such as the medial ganglionic eminence, can be subdivided into functionally distinct domains.

Temporal and spatial expression of transcription factors FoxN4, Ptf1a, Prox1, Isl1 and Lim1 mRNA in the developing chick retina

Transcription factors are pivotal in regulating cell fate and development. We analyzed five transcription factors - FoxN4, Ptf1a, Prox1, Isl1 and Lim1 - with putative functions in the formation of early-generated retinal interneurons. A full-length chicken FoxN4 cDNA was characterized and in situ as well as RT-PCR showed that FoxN4 expression commenced already in the stage 12-14 optic vesicles. Ptf1a, Prox1, Isl1 and Lim1 expression appeared later by stage 20-24, concomitant with the first post-mitotic ganglion-, amacrine- and horizontal cells. The FoxN4 and Ptf1a expression was transient with peak levels by stage 32-35. Expression disappeared as the retinal progenitor cells differentiated. Prox1, Isl1 and Lim1 expression remained in several differentiated cells including the horizontal cells. The order of expression supports a scheme where Ptf1a and Prox1 is downstream of FoxN4 and that FoxN4 and Ptf1a have transient roles during fate specification while Prox1, Isl1 and Lim1 have roles that are important for the generation of the neuronal subtypes.

Conditional deletion of activating protein 2alpha (AP-2alpha) in the developing retina demonstrates non-cell-autonomous roles for AP-2alpha in optic cup development

Activating protein 2alpha (AP-2alpha) is known to be expressed in the retina, and AP-2alpha-null mice exhibit defects in the developing optic cup, including patterning of the neural retina (NR) and a replacement of the dorsal retinal pigmented epithelium (RPE) with NR. In this study, we analyzed the temporal and spatial retinal expression patterns of AP-2alpha and created a conditional deletion of AP-2alpha in the developing retina. AP-2alpha exhibited a distinct expression pattern in the developing inner nuclear layer of the retina, and colocalization studies indicated that AP-2alpha was exclusively expressed in postmitotic amacrine cell populations. Targeted deletion of AP-2alpha in the developing retina did not result in observable retinal defects. Further examination of AP-2alpha-null mutants revealed that the severity of the RPE defect was variable and, although defects in retinal lamination occur at later embryonic stages, earlier stages showed normal lamination and expression of markers for amacrine and ganglion cells. Together, these data demonstrate that, whereas AP-2alpha alone does not play an intrinsic role in retinogenesis, it has non-cell-autonomous effects on optic cup development. Additional expression analyses showed that multiple AP-2 proteins are present in the developing retina, which will be important to future studies.

BMI1 loss delays photoreceptor degeneration in Rd1 mice. Bmi1 loss and neuroprotection in Rd1 mice

Retinitis pigmentosa (RP) is a heterogeneous group of genetic disorders leading to blindness, which remain untreatable at present. Rd1 mice represent a recognized model of RP, and so far only GDNF treatment provided a slight delay in the retinal degeneration in these mice. Bmi1, a transcriptional repressor, has recently been shown to be essential for neural stem cell (NSC) renewal in the brain, with an increased appearance of glial cells in vivo in Bmi1 knockout (Bmi1-/-) mice. One of the roles of glial cells is to sustain neuronal function and survival. In the view of a role of the retinal Miller glia as a source of neural protection in the retina, the increased astrocytic population in the Bmi1-/- brain led us to investigate the effect of Bmi1 loss in Rd1 mice. We observed an increase of Müller glial cells in Rd1-Bmi1-/- retinas compared to Rd1. Moreover, Rd1-Bmi1-/- mice showed 7-8 rows of photoreceptors at 30 days of age (P30), while in Rd1 littermates there was a complete disruption of the outer nuclear layer (ONL). Preliminary ERG results showed a responsiveness of Rd1-Bmi1-/- mice in scotopic vision at P35. In conclusion, Bmi1 loss prevented, or rescued, photoreceptors from degeneration to an unanticipated extent in Rd1 mice. In this chapter, we will first provide a brief review of our work on the cortical NSCs and introduce the Bmi1 oncogene, thus offering a rational to our observations on the retina.

Late-stage neuronal progenitors in the retina are radial Müller glia that function as retinal stem cells

Neuronal progenitors in the mammalian brain derive from radial glia or specialized astrocytes. In developing neural retina, radial glia-like Müller cells are generated late in neurogenesis and are not considered to be neuronal progenitors, but they do proliferate after injury and can express neuronal markers, suggesting a latent neurogenic capacity. To examine the neurogenic capacity of retinal glial cells, we used lineage tracing in transgenic zebrafish with a glial-specific promoter (gfap, for glial fibrillary acid protein) driving green fluorescent protein in differentiated Müller glia. We found that all Müller glia in the zebrafish retina express low levels of the multipotent progenitor marker Pax6 (paired box gene 6), and they proliferate at a low frequency in the intact, uninjured retina. Müller glia-derived progenitors express Crx (cone rod homeobox) and are late retinal progenitors that generate the rod photoreceptor lineage in the postembryonic retina. These Müller glia-derived progenitors also remain competent to produce earlier neuronal lineages, in that they respond to loss of cone photoreceptors by specifically regenerating the missing neurons. We conclude that zebrafish Müller glia function as multipotent retinal stem cells that generate retinal neurons by homeostatic and regenerative developmental mechanisms.

Alterations of rx1 and pax6 expression levels at neural plate stages differentially affect the production of retinal cell types and maintenance of retinal stem cell qualities

rx1 and pax6 are necessary for the establishment of the vertebrate eye field and for the maintenance of the retinal stem cells that give rise to multiple retinal cell types. They also are differentially expressed in cellular layers in the retina when cell fates are being specified, and their expression levels differentially affect the production of amacrine cell subtypes. To determine whether rx1 and pax6 expression after the eye field is established simply maintains stem cell-like qualities or affects cell type differentiation, we used hormone-inducible constructs to increase or decrease levels/activity of each protein at two different neural plate stages. Our results indicate that rx1 regulates the size of the retinal stem cell pool because it broadly affected all cell types, whereas pax6 regulates more restricted retinal progenitor cells because it selectively affected different cell types in a time-dependent manner. Analysis of rx1 and pax6 effects on proliferation, and expression of stem cell or differentiation markers demonstrates that rx1 maintains cells in a stem cell state by promoting proliferation and delaying expression of neural identity and differentiation markers. Although pax6 also promotes proliferation, it differentially regulates neural identity and differentiation genes. Thus, these two genes work in parallel to regulate different, but overlapping aspects of retinal cell fate determination.

Wnt signaling promotes regeneration in the retina of adult mammals

Regeneration in the mammalian CNS is severely limited. Unlike in the chick, current models hold that retinal neurons are never regenerated. Previously we demonstrated that, in the adult mammalian retina, Müller glia dedifferentiate and produce retinal cells, including photoreceptors, after acute neurotoxic injury in vivo. However, the number of newly generated retinal neurons is very limited. Here we demonstrate that Wnt (wingless-type MMTV integration site family)/beta-catenin signaling promotes proliferation of Müller glia-derived retinal progenitors and neural regeneration after damage or during degeneration. Wnt3a treatment increases proliferation of dedifferentiated Müller glia >20-fold in the photoreceptor-damaged retina. Supplementation with retinoic acid or valproic acid induces differentiation of these cells primarily into Crx (cone rod homeobox)-positive and rhodopsin-positive photoreceptors. Notably, injury induces nuclear accumulation of beta-catenin, cyclin D1 upregulation, and Wnt/beta-catenin reporter activity. Activation of Wnt signaling by glycogen synthase kinase-3beta inhibitors promotes retinal regeneration, and, conversely, inhibition of the signaling attenuates regeneration. This Wnt3a-mediated regeneration of retinal cells also occurs in rd mice, a model of retinal degeneration. These results provide evidence that Wnt/beta-catenin signaling contributes to CNS regeneration in the adult mammal.

Neurogenesis of GABAergic cells in the retina of malnourished rats

The present study investigated how prenatal protein malnutrition affects the neurogenesis of GABAergic cells in the retina. Rats were treated with a multi-deficient diet, with only 8% of protein that was administered during the gestational and suckling periods. Pregnant mothers and pups from malnourished and control (fed with 22% protein) groups received a single intra-peritoneal injection of [3H]-thymidine at six developmental ages, from E14 to PN4, and the pups were sacrificed at PN18. Eyes were enucleated and cryosections of the retina were double labeled for GABA-immunocytochemistry and for autoradiography. The percentage of double labeled cells, in the retinal inner nuclear and ganglion cell layers, was determined for both groups. Qualitative and quantitative results showed that double labeled cells [GABA+/thymidine+] were present since E14, when mitotic activity for GABAergic cells starts, in both GCL and INL layers. The peak rate of GABAergic cell generation was reached in control animals injected with [3H]-thymidine at E18 in both central and peripheral sectors of the retina, but only at E20 in the malnourished group. The generation of cells of GABA phenotype showed a significant delay in both layers of the retina in the malnourished group. At PN4, close to the age that GABAergic mitotic activity ends in the control group, double labeled cells were significantly higher in the malnourished group. Our data showed a delay in GABAergic cell generation in the malnourished group when compared to the control group that might result in significant functional consequences in the developing retina.

Molecular regulation of visual system development: more than meets the eye

Vertebrate eye development has been an excellent model system to investigate basic concepts of developmental biology ranging from mechanisms of tissue induction to the complex patterning and bidimensional orientation of the highly specialized retina. Recent advances have shed light on the interplay between numerous transcriptional networks and growth factors that are involved in the specific stages of retinogenesis, optic nerve formation, and topographic mapping. In this review, we summarize this recent progress on the molecular mechanisms underlying the development of the eye, visual system, and embryonic tumors that arise in the optic system.

Mechanisms controlling Pax6 isoform expression in the retina have been conserved between teleosts and mammals

The Pax6 gene plays several roles in retinal development, including control of cell proliferation, maintenance of the retinogenic potential of progenitor cells, and cell fate specification. Emerging evidence suggests that these different aspects of Pax6 gene function are mediated by different isoforms of the Pax6 protein; however, relatively little is known about the spatiotemporal expression of Pax6 isoforms in the vertebrate retina. Using bacterial artificial chromosome (BAC) technology, we modified a zebrafish Pax6a BAC such that we could distinguish paired-containing Pax6a transcripts from paired-less Pax6a transcripts. In the zebrafish, the spatial and temporal onset of expression of these transcripts suggests that the paired-less isoform is involved in the cell fate decision leading to the generation of amacrine cells; however, because of limitations associated with transient transgenic analysis, it was not feasible to establish whether this promoter was active in all amacrine cells or in a specific population of amacrine cells. By making mice transgenic for the zebrafish Pax6a BAC reporter transgene, we were able to show that paired-containing and paired-less Pax6a transcripts were differentially expressed in amacrine subpopulations. Our study also directly demonstrates the functional conservation of the regulatory mechanisms governing Pax6 transcription in teleosts and mammals.

Have we achieved a unified model of photoreceptor cell fate specification in vertebrates?

How does a retinal progenitor choose to differentiate as a rod or a cone and, if it becomes a cone, which one of their different subtypes? The mechanisms of photoreceptor cell fate specification and differentiation have been extensively investigated in a variety of animal model systems, including human and non-human primates, rodents (mice and rats), chickens, frogs (Xenopus) and fish. It appears timely to discuss whether it is possible to synthesize the resulting information into a unified model applicable to all vertebrates. In this review we focus on several widely used experimental animal model systems to highlight differences in photoreceptor properties among species, the diversity of developmental strategies and solutions that vertebrates use to create retinas with photoreceptors that are adapted to the visual needs of their species, and the limitations of the methods currently available for the investigation of photoreceptor cell fate specification. Based on these considerations, we conclude that we are not yet ready to construct a unified model of photoreceptor cell fate specification in the developing vertebrate retina.

Anti-apoptotic effect of retinoic acid on retinal progenitor cells mediated by a protein kinase A-dependent mechanism

Retinal progenitor cells (RPCs) are neural stem cells able to differentiate into any normal adult retinal cell type, except for pigment epithelial cells. Retinoic acid (RA) is a powerful growth/differentiation factor that generally causes growth inhibition, differentiation and/or apoptosis. In this study, we demonstrate that RA not only affects mouse RPC differentiation but also improves cell survival by reducing spontaneous apoptotic rate without affecting RPC proliferation. The enhanced cell survival was accompanied by a significant upregulation of the expression of protein kinase A (PKA) and several protein kinase C (PKC) isoforms. Treatment of cells grown in RA-free media with 8-bromoadenosine3',5'-cyclic monophosphate, a known activator of PKA, resulted in an anti-apoptotic effect similar to that caused by RA; whereas the PKA inhibitor N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide dihydrochloride led to a significant ( approximately 32%) increase in apoptosis. In contrast, treatment of RPCs with any of two PKC selective inhibitors, 2,2',3,3',4,4'-hexahydroxy-1,1'-biphenyl-6,6'-dimethanol dimethyl ether and bisindolylmaleimide XI, led to diminished apoptosis; while a PKC activator, phorbol 12-myristate 13-acetate, increased apoptosis. These and other data suggest that the effect of RA on RPC survival is mostly due to the increased anti-apoptotic activity elicited by PKA, which might in turn be antagonized by PKC. Such a mechanism is a new example of tight regulation of important biological processes triggered by RA. Although the detailed mechanisms remain to be elucidated, we provide evidence that the pro-survival effect of RA on RPCs is not mediated by changed expression of p53 or bcl-2, and appears to be independent of beta-amyloid, Fas ligand, TNF-alpha, ganglioside GM1 and ceramide C16-induced apoptotic pathways.