Postmitotic cells fated to become rod photoreceptors can be respecified by CNTF treatment of the retina

The Rax homeoprotein in Müller glial cells is required for homeostasis maintenance of the postnatal mouse retina

Müller glial cells, which are the most predominant glial subtype in the retina, play multiple important roles, including the maintenance of structural integrity, homeostasis, and physiological functions of the retina. We have previously found that the Rax homeoprotein is expressed in postnatal and mature Müller glial cells in the mouse retina. However, the function of Rax in postnatal and mature Müller glial cells remains to be elucidated. In the current study, we first investigated Rax function in retinal development using retroviral lineage analysis and found that Rax controls the specification of late-born retinal cell types, including Müller glial cells in the postnatal retina. We next generated Rax tamoxifen-induced conditional KO (Rax iCKO) mice, where Rax can be depleted in mTFP-labeled Müller glial cells upon tamoxifen treatment, by crossing Raxflox/flox mice with Rlbp1-CreERT2 mice, which we have produced. Immunohistochemical analysis showed a characteristic of reactive gliosis and enhanced gliosis of Müller glial cells in Rax iCKO retinas under normal and stress conditions, respectively. We performed RNA-seq analysis on mTFP-positive cells purified from the Rax iCKO retina and found significantly reduced expression of suppressor of cytokinesignaling-3 (Socs3). Reporter gene assays showed that Rax directly transactivates the Socs3 promoter. We observed decreased expression of Socs3 in Müller glial cells of Rax iCKO retinas by immunostaining. Taken together, the present results suggest that Rax suppresses inflammation in Müller glial cells by transactivating Socs3. This study sheds light on the transcriptional regulatory mechanisms underlying retinal Müller glial cell homeostasis.

Molecular mechanisms controlling vertebrate retinal patterning, neurogenesis, and cell fate specification

This review covers recent advances in understanding the molecular mechanisms controlling neurogenesis and specification of the developing retina, with a focus on insights obtained from comparative single cell multiomic analysis. We discuss recent advances in understanding the mechanisms by which extrinsic factors trigger transcriptional changes that spatially pattern the optic cup (OC) and control the initiation and progression of retinal neurogenesis. We also discuss progress in unraveling the core evolutionarily conserved gene regulatory networks (GRNs) that specify early- and late-state retinal progenitor cells (RPCs) and neurogenic progenitors and that control the final steps in determining cell identity. Finally, we discuss findings that provide insight into regulation of species-specific aspects of retinal patterning and neurogenesis, including consideration of key outstanding questions in the field.

Increasing cell culture density during a developmental window prevents fated rod precursors derailment toward hybrid rod-glia cells

In proliferating multipotent retinal progenitors, transcription factors dynamics set the fate of postmitotic daughter cells, but postmitotic cell fate plasticity driven by extrinsic factors remains controversial. Transcriptome analysis reveals the concurrent expression by postmitotic rod precursors of genes critical for the Müller glia cell fate, which are rarely generated from terminally-dividing progenitors as a pair with rod precursors. By combining gene expression and functional characterisation in single cultured rod precursors, we identified a time-restricted window where increasing cell culture density switches off the expression of genes critical for Müller glial cells. Intriguingly, rod precursors in low cell culture density maintain the expression of genes of rod and glial cell fate and develop a mixed rod/Muller glial cells electrophysiological fingerprint, revealing rods derailment toward a hybrid rod-glial phenotype. The notion of cell culture density as an extrinsic factor critical for preventing rod-fated cells diversion toward a hybrid cell state may explain the occurrence of hybrid rod/MG cells in the adult retina and provide a strategy to improve engraftment yield in regenerative approaches to retinal degenerative disease by stabilising the fate of grafted rod precursors.

The 3-Phosphoinositide-Dependent Protein Kinase 1 Inhibits Rod Photoreceptor Development

The transition of rod precursor cells to post-mitotic rod photoreceptors can be promoted by extrinsic factors such as insulin-like growth factor 1 (IGF-1), which regulates phosphatidylinositide concentration, and consequently the 3-phosphoinositide-dependent protein kinase-1 (PDPK-1). PDPK-1 is a 63 kDa cytoplasmic kinase that controls cell proliferation and differentiation. In the mouse retina, PDPK-1 and its phosphorylated derivative p-PDPK-1 (Ser241), showed peak expression during the first postnatal (PN) day with a substantial decline by PN7 and in the adult retina. Though initially widely distributed among cell types, PDPK-1 expression decreased first in the inner retina and later in the outer retina. When PDPK-1 is inhibited in neonatal retinal explants by BX795, there is a robust increase in rod photoreceptor numbers. The increase in rods depended on the activity of PKC, as BX795 had no effect when PKC is inhibited. Inhibition of PDPK-1-dependent kinases, such as P70-S6K, but not others, such as mTORC-1, stimulated rod development. The P70-S6K-dependent increase in rods appears to be correlated with phosphorylation of Thr252 and not at Thr389, a substrate of mTORC-1. This pathway is also inactive while PKC activity is inhibited. We also found that inhibition of the kinase mTORC-2, also stimulated by insulin activity, similarly increased rod formation, and this effect appears to be independent of PKC activity. This may represent a novel intracellular signaling pathway that also stimulates photoreceptor development. Consistent with previous studies, stimulation of STAT3 activity is sufficient to prevent any PDPK-1, P70-S6K, or mTORC2-dependent increase in rods. Together the data indicate that PDPK-1 and other intrinsic kinases downstream of IGF-1 are key regulators of rod photoreceptor formation.

Ciliary neurotrophic factor (CNTF) protects retinal cone and rod photoreceptors by suppressing excessive formation of the visual pigments

The retinal pigment epithelium (RPE)-dependent visual cycle provides 11-cis-retinal to opsins in the photoreceptor outer segments to generate functional visual pigments that initiate phototransduction in response to light stimuli. Both RPE65 isomerase of the visual cycle and the rhodopsin visual pigment have recently been identified as critical players in mediating light-induced retinal degeneration. These findings suggest that the expression and function of RPE65 and rhodopsin need to be coordinately controlled to sustain normal vision and to protect the retina from photodamage. However, the mechanism controlling the development of the retinal visual system remains poorly understood. Here, we show that deficiency in ciliary neurotrophic factor (CNTF) up-regulates the levels of rod and cone opsins accompanied by an increase in the thickness of the outer nuclear layers and the lengths of cone and rod outer segments in the mouse retina. Moreover, retinoid isomerase activity, expression levels of RPE65 and lecithin:retinol acyltransferase (LRAT), which synthesizes the RPE65 substrate, were also significantly increased in the Cntf ^-/- RPE. Rod a-wave and cone b-wave amplitudes of electroretinograms were increased in Cntf ^-/- mice, but rod b-wave amplitudes were unchanged compared with those in WT mice. Up-regulated RPE65 and LRAT levels accelerated both the visual cycle rate and recovery rate of rod light sensitivity in Cntf ^-/- mice. Of note, rods and cones in Cntf ^-/- mice exhibited hypersusceptibility to light-induced degeneration. These results indicate that CNTF is a common extracellular factor that prevents excessive production of opsins, the photoreceptor outer segments, and 11-cis-retinal to protect rods and cones from photodamage.

Retinal progenitor cells release extracellular vesicles containing developmental transcription factors, microRNA and membrane proteins

A range of cell types, including embryonic stem cells, neurons and astrocytes have been shown to release extracellular vesicles (EVs) containing molecular cargo. Across cell types, EVs facilitate transfer of mRNA, microRNA and proteins between cells. Here we describe the release kinetics and content of EVs from mouse retinal progenitor cells (mRPCs). Interestingly, mRPC derived EVs contain mRNA, miRNA and proteins associated with multipotency and retinal development. Transcripts enclosed in mRPC EVs, include the transcription factors Pax6, Hes1, and Sox2, a mitotic chromosome stabilizer Ki67, and the neural intermediate filaments Nestin and GFAP. Proteomic analysis of EV content revealed retinogenic growth factors and morphogen proteins. mRPC EVs were shown to transfer GFP mRNA between cell populations. Finally, analysis of EV mediated functional cargo delivery, using the Cre-loxP recombination system, revealed transfer and uptake of Cre+ EVs, which were then internalized by target mRPCs activating responder loxP GFP expression. In summary, the data supports a paradigm of EV genetic material encapsulation and transfer within RPC populations. RPC EV transfer may influence recipient RPC transcriptional and post-transcriptional regulation, representing a novel mechanism of differentiation and fate determination during retinal development.

miR Cluster 143/145 Directly Targets Nrl and Regulates Rod Photoreceptor Development

Retinal histogenesis requires coordinated and temporal functioning of factors by which different cell types are generated from multipotent progenitors. Development of rod photoreceptors is regulated by multiple transcription factors, and Nrl is one of the major factors involved in their fate specification. Presence or absence of Nrl at the postnatal stages decides the generation of cone photoreceptors or other later retinal cells. This suggests the need for regulated expression of Nrl in order to accelerate the generation of other cell types during retinal development. We found that miR cluster 143/145, comprising miR-143 and miR-145, targets and imparts a posttranscriptional inhibition of Nrl. Expression of both miRNAs was differentially regulated during retinal development and showed least expression at PN1 stage in which most of the rod photoreceptors are generated. Downregulation of rod photoreceptor regulators and markers upon miR cluster 143/145 overexpression demonstrated that this cluster indeed negatively regulates rod photoreceptors. Further, we prove that Nrl positively regulates miR cluster 143/145, thus establishing a feedback loop regulatory mechanism. This may be one possible mechanism by which Nrl is posttranscriptionally regulated to facilitate the generation of other cell types in retina.

Ciliary neurotrophic factor (CNTF) delivery to retina: an overview of current research advancements

The intraocular administration of the ciliary neurotrophic factor (CNTF) has been found to attenuate the photoreceptor degeneration and preserve retinal functions in the animal research models of the inherited or induced retinal disease. Studies with the aim of CNTF transfer to the posterior segment inside the eye have been directed to determine the best method for its administration. An ideal delivery method would overcome the eye drug elimination mechanisms or barriers and provide the sustained release of the CNTF into retina in the safest fashion with the minimum harm to the quality of life. This review focuses on the present state of CNTF delivery to retina, also provides an overview of available technologies and their challenges.

Immunocytochemical Profiling of Cultured Mouse Primary Retinal Cells

Primary retinal cell cultures and immunocytochemistry are important experimental platforms in ophthalmic research. Translation of retinal cells from their native environment to the in vitro milieu leads to cellular stress, jeopardizing their in vivo phenotype features. Moreover, the specificity and stability of many retinal immunochemical markers are poorly evaluated in retinal cell cultures. Hence, we here evaluated the expression profile of 17 retinal markers, that is, recoverin, rhodopsin, arrestin, Chx10, PKC, DCX, CRALBP, GS, vimentin, TPRV4, RBPMS, Brn3a, β-tubulin III, NeuN, MAP2, GFAP, and synaptophysin. At 7 and 18 days of culture, the marker expression profiles of mouse postnatal retinal cells were compared with their age-matched in vivo retinas. We demonstrate stable in vitro expression of all markers, except for arrestin and CRALBP. Differences in cellular expression and location of some markers were observed, both over time in culture and compared with the age-matched retina. We hypothesize that these differences are likely culture condition dependent. Taken together, we suggest a thorough evaluation of the antibodies in specific culture settings, before extrapolating the in vitro results to an in vivo setting. Moreover, the identification of specific cell types may require a combination of different genes expressed or markers with structural information.

The dose-dependent macular thickness changes assessed by fd-oct in patients with retinitis pigmentosa treated with ciliary neurotrophic factor

PURPOSE

To evaluate the effect of intravitreal ciliary neurotrophic factor (CNTF) implant on mean macular thickness (MMT) in eyes with retinitis pigmentosa using high-resolution Fourier domain optical coherence tomography imaging.

METHODS

A cohort of 8 patients (CNTF-3: n = 5; CNTF-4: n = 3) enrolled in Neurotech sponsored Phase 2 clinical trial underwent Fourier domain optical coherence tomography imaging. A ≥3% change in MMT from baseline or fellow eye was considered as a measurable change.

RESULTS

Two patients enrolled in the CNTF-3 study received low-dose implant. At 18 months, a change in MMT from -4.47 μm to 6 μm from baseline was noted. Six patients received high-dose implant (CNTF-3: n = 3; CNTF-4: n = 3). In CNTF-3 group, 1 eye showed an increase in MMT by 19.25 μm (+7.6%) from baseline at 18 months. In CNTF-4 group, 1 eye had an increase in MMT of 27.08 μm (+11%) from baseline at 30 months; second eye had increase in MMT of 31.36 μm (+12%) from contralateral eye. Amongst these 3 responsive high-dose implant eyes, overall thickening of the retina could not be attributed to any specific retinal layer.

CONCLUSION

A heterogeneous dose-dependent response on MMT was noted in eyes treated using intravitreal CNTF implant for retinitis pigmentosa. We recommend corroboration of our findings with Neurotech sponsored clinical trial results.

Regulation of rod photoreceptor differentiation by STAT3 is controlled by a tyrosine phosphatase

Signal pathways that reduce the levels of tyrosine-phosphorylated STAT3 (pSTAT3) allow late retinal progenitors to exit the cell cycle and enter a terminal differentiation pathway into rod photoreceptors. In the mouse retina, we previously identified PKC-β1 and PKC-γ isoforms as essential components of a key signal pathway and IGF-1 as a major extrinsic factor regulating rod formation. In this manuscript, we demonstrate that PKC decreases phosphotyrosine but not phosphoserine on STAT3 in neonatal mouse retinas. Neither IGF-1 nor PMA induced a significant change in the levels of STAT3 or in the levels of the key proteins regulating STAT3 degradation, SOCS3, and PIAS3. Treatment of neonatal mouse retinal explants with sodium orthovanadate inhibited the PKC-mediated reduction in pSTAT3, indicating a role for a phosphatase. Addition of the PTEN inhibitor bpV(phen) to explant cultures treated with IGF-1 or PMA had no effect on the reduction in pSTAT3 levels, but the effect of both IGF-1 and PMA was blocked by a concentration of the inhibitor NSC87877 that is selective for the phosphatases Shp1 and Shp2. Inhibition of Shp1/2 phosphatases was also sufficient to abolish the IGF1-mediated induction of rod photoreceptor differentiation in the retina explant cultures. We conclude that one or both of these phosphatases are key components regulating the formation of rod photoreceptors in mouse retina.

Induction of the ganglion cell differentiation program in human retinal progenitors before cell cycle exit

BACKGROUND

Despite the disease relevance, understanding of human retinal development lags behind that of other species. We compared the kinetics of gene silencing or induction during ganglion cell development in human and murine retina.

RESULTS

Induction of POU4F2 (BRN3B) marks ganglion cell commitment, and we detected this factor in S-phase progenitors that had already silenced Cyclin D1 and VSX2 (CHX10). This feature was conserved in human and mouse retina, and the fraction of Pou4f2+ murine progenitors labeled with a 30 min pulse of BrdU matched the fraction of ganglion cells predicted to be born in a half-hour period. Additional analysis of 18 markers revealed many with conserved kinetics, such as the POU4F2 pattern above, as well as the surprising maintenance of "cell cycle" proteins KI67, PCNA, and MCM6 well after terminal mitosis. However, four proteins (TUBB3, MTAP1B, UCHL1, and RBFOX3) showed considerably delayed induction in human relative to mouse retina, and two proteins (ISL1, CALB2) showed opposite kinetics, appearing on either side of terminal mitosis depending on the species.

CONCLUSION

With some notable exceptions, human and murine ganglion cell differentiation show similar kinetics, and the data add weight to prior studies supporting the existence of biased ganglion cell progenitors.

Making of a retinal cell: insights into retinal cell-fate determination

Understanding the process by which an uncommitted dividing cell produces particular specialized cells within a tissue remains a fundamental question in developmental biology. Many tissues are well suited for cell-fate studies, but perhaps none more so than the developing retina. Traditionally, experiments using the retina have been designed to elucidate the influence that individual environmental signals or transcription factors can have on cell-fate decisions. Despite a substantial amount of information gained through these studies, there is still much that we do not yet understand about how cell fate is controlled on a systems level. In addition, new factors such as noncoding RNAs and regulators of chromatin have been shown to play roles in cell-fate determination and with the advent of "omics" technology more factors will most likely be identified. In this chapter we summarize both the traditional view of retinal cell-fate determination and introduce some new ideas that are providing a challenge to the older way of thinking about the acquisition of cell fates.

Neural stem cells-trends and advances

For many years, accepted dogma held that brain is a static organ with no possibility of regeneration of cells in injured or diseased human brain. However, recent preclinical reports have shown regenerative potential of neural stem cells using various injury models. This has resulted in renewed hope for those suffering from spinal cord injury and neural damage. As the potential of stem cell therapy gained impact, these claims, in particular, led to widespread enthusiasm that acute and chronic injury of the nervous system would soon be a problem of the past. The devastation caused by injury or diseases of the brain and spinal cord led to wide premature acceptance that "neural stem cells (NSCs)" derived from embryonic, fetal or adult sources would soon be effective in reversing neural and spinal trauma. However, neural therapy with stem cells has not been realized to its fullest extent. Although, discrete population of regenerative stem cells seems to be present in specific areas of human brain, the function of these cells is unclear. However, similar cells in animals seem to play important role in postnatal growth as well as recovery of neural tissue from injury, anoxia, or disease.

Notch1 is required in newly postmitotic cells to inhibit the rod photoreceptor fate

Several models of cell fate determination can be invoked to explain how single retinal progenitor cells (RPCs) produce different cell types in a terminal division. To gain insight into this process, the effects of the removal of a cell fate regulator, Notch1, were studied in newly postmitotic cells using a conditional allele of Notch1 (N1-CKO) in mice. Almost all newly postmitotic N1-CKO cells became rod photoreceptors, whereas wild-type (WT) cells achieved a variety of fates. Single cell profiling of wild-type and N1-CKO retinal cells transitioning from progenitor to differentiated states revealed differential expression of inhibitor of DNA binding factors Id1 and Id3, as well as Notch-regulated ankyrin repeat protein (Nrarp). Misexpression of Id1 and Id3 was found to be sufficient to drive production of Müller glial cells and/or RPCs. Moreover, Id1 and Id3 were shown to partially rescue the production of bipolar and Müller glial cells in the absence of Notch1 in mitotic and newly postmitotic cells. Misexpression of Nrarp, a downstream target gene and inhibitor of the Notch signaling pathway, resulted in the overproduction of rod photoreceptors at the expense of Müller glial cells. These data demonstrate that cell fate decisions can be made in newly postmitotic retinal cells, and reveal some of the regulators downstream of Notch1 that influence the choice of rod and non-rod fates. Taken together, our results begin to address how different signals downstream from a common pathway lead to different fate outcomes.

Neurotrophic molecules in the treatment of neurodegenerative disease with focus on the retina: status and perspectives

Neurotrophic factors are operationally defined as molecules that promote the survival and differentiation of neurons. Chemically, they belong to divergent classes of molecules but most of the classic neurotrophic factors are proteins. Together with stem cells, viral vectors and genetically engineered cells, they constitute important tools in neuroprotective and regenerative neurobiology. Protein neurotrophic molecules signal through receptors located on the cell membrane. Their downstream signaling exploits pathways that are often common to chemically different factors and frequently target a relatively restricted set of transcription factors, RNA interference and diverse molecular machinery involved in the life vs. death decisions of neurons. Application of neurotrophic factors with the aim of curing or, at least, improving the outcome of neurodegenerative diseases requires (1) profound knowledge of the complex molecular pathology of the disease, (2) the development of animal models as closely as possible resembling the human disease, (3) the identification of target cells to be addressed, (4) intense efforts in chemical engineering to ensure the stability of molecules or to design carriers and small analogs with the ability to cross the blood-brain barrier and (5) scrutinity with regard to possible side effects. Last, but not least, engineering efforts to optimize administration, e.g., by designing the right canulae and infusion devices, are important for the successful translation of preclinical advances into clinical benefit. This article presents selected examples of neurotrophic factors that are currently being tested in animal models or developed for transfer to the clinic, with a major focus on factors with the potential of becoming applicable in various forms of retinal degeneration.

Heterochronic misexpression of Ascl1 in the Atoh7 retinal cell lineage blocks cell cycle exit

Retinal neurons and glia arise from a common progenitor pool in a temporal order, with retinal ganglion cells (RGCs) appearing first, and Müller glia last. The transcription factors Atoh7/Math5 and Ascl1/Mash1 represent divergent bHLH clades, and exhibit distinct spatial and temporal retinal expression patterns, with little overlap during early development. Here, we tested the ability of Ascl1 to change the fate of cells in the Atoh7 lineage when misexpressed from the Atoh7 locus, using an Ascl1-IRES-DsRed2 knock-in allele. In Atoh7(Ascl1KI/+) and Atoh7(Ascl1KI/Ascl1KI) embryos, ectopic Ascl1 delayed cell cycle exit and differentiation, even in cells coexpressing Atoh7. The heterozygous retinas recovered, and eventually produced a normal complement of RGCs, while homozygous substitution of Ascl1 for Atoh7 did not promote postnatal retinal fates precociously, nor rescue Atoh7 mutant phenotypes. However, our analyses revealed two unexpected findings. First, ectopic Ascl1 disrupted cell cycle progression within the marked Atoh7 lineage, but also nonautonomously in other retinal cells. Second, the size of the Atoh7 retinal lineage was unaffected, supporting the idea of a compensatory shift of the non-proliferative cohort to maintain lineage size. Overall, we conclude that Ascl1 acts dominantly to block cell cycle exit, but is incapable of redirecting the fates of early RPCs.

Gene networks: dissecting pathways in retinal development and disease

During retinal neurogenesis, diverse cellular subtypes originate from multipotent neural progenitors in a spatiotemporal order leading to a highly specialized laminar structure combined with a distinct mosaic architecture. This is driven by the combinatorial action of transcription factors and signaling molecules which specify cell fate and differentiation. The emerging approach of gene network analysis has allowed a better understanding of the functional relationships between genes expressed in the developing retina. For instance, these gene networks have identified transcriptional hubs that have revealed potential targets and pathways for the development of therapeutic options for retinal diseases. Much of the current knowledge has been informed by targeted gene deletion experiments and gain-of-functional analysis. In this review we will provide an update on retinal development gene networks and address the wider implications for future disease therapeutics.

Defining the integration capacity of embryonic stem cell-derived photoreceptor precursors

Retinal degeneration is a leading cause of irreversible blindness in the developed world. Differentiation of retinal cells, including photoreceptors, from both mouse and human embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs), potentially provide a renewable source of cells for retinal transplantation. Previously, we have shown both the functional integration of transplanted rod photoreceptor precursors, isolated from the postnatal retina, in the adult murine retina, and photoreceptor cell generation by stepwise treatment of ESCs with defined factors. In this study, we assessed the extent to which this protocol recapitulates retinal development and also evaluated differentiation and integration of ESC-derived retinal cells following transplantation using our established procedures. Optimized retinal differentiation via isolation of Rax.GFP retinal progenitors recreated a retinal niche and increased the yield of Crx(+) and Rhodopsin(+) photoreceptors. Rod birth peaked at day 20 of culture and expression of the early photoreceptor markers Crx and Nrl increased until day 28. Nrl levels were low in ESC-derived populations compared with developing retinae. Transplantation of early stage retinal cultures produced large tumors, which were avoided by prolonged retinal differentiation (up to day 28) prior to transplantation. Integrated mature photoreceptors were not observed in the adult retina, even when more than 60% of transplanted ESC-derived cells expressed Crx. We conclude that exclusion of proliferative cells from ESC-derived cultures is essential for effective transplantation. Despite showing expression profiles characteristic of immature photoreceptors, the ESC-derived precursors generated using this protocol did not display transplantation competence equivalent to precursors from the postnatal retina.

Pushing the envelope of retinal ganglion cell genesis: context dependent function of Math5 (Atoh7)

The basic-helix-loop helix factor Math5 (Atoh7) is required for retinal ganglion cell (RGC) development. However, only 10% of Math5-expressing cells adopt the RGC fate, and most become photoreceptors. In principle, Math5 may actively bias progenitors towards RGC fate or passively confer competence to respond to instructive factors. To distinguish these mechanisms, we misexpressed Math5 in a wide population of precursors using a Crx BAC or 2.4 kb promoter, and followed cell fates with Cre recombinase. In mice, the Crx cone-rod homeobox gene and Math5 are expressed shortly after cell cycle exit, in temporally distinct, but overlapping populations of neurogenic cells that give rise to 85% and 3% of the adult retina, respectively. The Crx>Math5 transgenes did not stimulate RGC fate or alter the timing of RGC births. Likewise, retroviral Math5 overexpression in retinal explants did not bias progenitors towards the RGC fate or induce cell cycle exit. The Crx>Math5 transgene did reduce the abundance of early-born (E15.5) photoreceptors two-fold, suggesting a limited cell fate shift. Nonetheless, retinal histology was grossly normal, despite widespread persistent Math5 expression. In an RGC-deficient (Math5 knockout) environment, Crx>Math5 partially rescued RGC and optic nerve development, but the temporal envelope of RGC births was not extended. The number of early-born RGCs (before E13) remained very low, and this was correlated with axon pathfinding defects and cell death. Together, these results suggest that Math5 is not sufficient to stimulate RGC fate. Our findings highlight the robust homeostatic mechanisms, and role of pioneering neurons in RGC development.

Math5 defines the ganglion cell competence state in a subpopulation of retinal progenitor cells exiting the cell cycle

The basic helix-loop-helix (bHLH) transcription factor Math5 (Atoh7) is transiently expressed during early retinal histogenesis and is necessary for retinal ganglion cell (RGC) development. Using nucleoside pulse-chase experiments and clonal analysis, we determined that progenitor cells activate Math5 during or after the terminal division, with progressively later onset as histogenesis proceeds. We have traced the lineage of Math5+ cells using mouse BAC transgenes that express Cre recombinase under strict regulatory control. Quantitative analysis showed that Math5+ progenitors express equivalent levels of Math5 and contribute to every major cell type in the adult retina, but are heavily skewed toward early fates. The Math5>Cre transgene labels 3% of cells in adult retina, including 55% of RGCs. Only 11% of Math5+ progenitors develop into RGCs; the majority become photoreceptors. The fate bias of the Math5 cohort, inferred from the ratio of cone and rod births, changes over time, in parallel with the remaining neurogenic population. Comparable results were obtained using Math5 mutant mice, except that ganglion cells were essentially absent, and late fates were overrepresented within the lineage. We identified Math5-independent RGC precursors in the earliest born (embryonic day 11) retinal cohort, but these precursors require Math5-expressing cells for differentiation. Math5 thus acts permissively to establish RGC competence within a subset of progenitors, but is not sufficient for fate specification. It does not autonomously promote or suppress the determination of non-RGC fates. These data are consistent with progressive and temporal restriction models for retinal neurogenesis, in which environmental factors influence the final histotypic choice.

Manipulation of the recipient retinal environment by ectopic expression of neurotrophic growth factors can improve transplanted photoreceptor integration and survival

Degeneration of the neural retina is the leading cause of untreatable blindness in the developed world. Stem cell replacement therapy offers a novel strategy for retinal repair. Postmitotic photoreceptor precursors derived from the early postnatal (P) retina are able to migrate and integrate into the adult mouse retina following transplantation into the subretinal space, but it is likely that a large number of these cells would be required to restore vision. The adult recipient retina presents a very different environment to that from which photoreceptor precursor donor cells isolated from the developing postnatal retina are derived. Here we considered the possibility that modulation of the recipient environment by ectopic expression of developmentally regulated growth factors, normally present during photoreceptor development, might enhance the migration and integration of transplanted cells into the adult neural retina. Adeno-associated viral (AAV) vectors were used to introduce three growth factors previously reported to play a role in photoreceptor development, IGF1, FGF2, and CNTF, into the adult retina, prior to transplantation of P4 cells derived from the Nrl.GFP(+ve) neural retina. At 3 weeks posttransplantation the number of integrated, differentiated photoreceptor cells present in AAV-mediated neurotrophic factor-treated eyes was assessed and compared to control treated contralateral eyes. We show, firstly, that it is possible to manipulate the recipient retinal microenvironment via rAAV-mediated gene transfer with respect to these developmentally relevant growth factors. Moreover, when combined with cell transplantation, AAV-mediated expression of IGF1 led to significantly increased levels of cell integration, while overexpression of FGF2 had no significant effect on integrated cell number. Conversely, expression of CNTF led to a significant decrease in cell integration and an exacerbated glial response that led to glial scarring. Together, these findings demonstrate the importance of the extrinsic environment of the recipient retina for photoreceptor cell transplantation and show for the first time that it is possible to manipulate this environment using viral vectors to influence photoreceptor transplantation efficiency.

CNTF and retina

Ciliary neurotrophic factor (CNTF) is one of the most studied neurotrophic factors for neuroprotection of the retina. A large body of evidence demonstrates that CNTF promotes rod photoreceptor survival in almost all animal models. Recent studies indicate that CNTF also promotes cone photoreceptor survival and cone outer segment regeneration in the degenerating retina and improves cone function in dogs with congenital achromotopsia. In addition, CNTF is a neuroprotective factor and an axogenesis factor for retinal ganglion cells (RGCs). This review focuses on the effects of exogenous CNTF on photoreceptors and RGCs in the mammalian retina and the potential clinical application of CNTF for retinal degenerative diseases.

PTEN regulates retinal interneuron morphogenesis and synaptic layer formation

The lipid phosphatase PTEN is a critical negative regulator of extracellular signal-induced PI3K activities, yet the roles of PTEN in the neural retina remain poorly understood. Here, we investigate the function of PTEN during retinal development. Deletion of Pten at the onset of neurogenesis in retinal progenitors results in the reduction of retinal ganglion cells and rod photoreceptors, but increased Müller glial genesis. In addition, PTEN deficiency leads to elevated phosphorylation of Akt, especially in the developing inner plexiform layer, where high levels of PTEN are normally expressed. In Pten mutant retinas, various subtypes of amacrine cells show severe dendritic overgrowth, causing specific expansion of the inner plexiform layer. However, the outer plexiform layer remains relatively undisturbed in the Pten deficient retina. Physiological analysis detects reduced rod function and augmented oscillatory potentials originating from amacrine cells in Pten mutants. Furthermore, deleting Pten or elevating Akt activity in individual amacrine cells is sufficient to disrupt dendritic arborization, indicating that Pten activity is required cell autonomously to control neuronal morphology. Moreover, inhibiting endogenous Akt activity attenuates inner plexiform layer formation in vitro. Together, these findings demonstrate that suppression of PI3K/Akt signaling by PTEN is crucial for proper neuronal differentiation and normal retinal network formation.

MAPK signaling during Müller glial cell development in retina explant cultures

The Müller cell is the only glial cell type generated from the retinal neuroepithelium. This cell type controls normal retina homeostasis and has been suggested to play a neuroprotective role. Recent evidence suggests that mammalian Müller cells can de-differentiate and return to a progenitor or stem cell stage following injury or disease. In vivo exploration of the molecular mechanisms of Müller cell differentiation and proliferation will add essential information to manipulate Müller cell functions. Signal transduction pathways that regulate Müller cell responses and activity are a critical part of their cellular machinery. In this study, we focus on mitogen-activated protein kinase (MAPK) signaling pathway during Müller glial cell differentiation and proliferation. We found that both MAPK and STAT3 signaling pathways are present during Müller glial cell development. Ciliary neurotrophic factor (CNTF)-stimulated Müller glial cell proliferation is associated with early developmental stages. Specific inhibition of MAPK phosphorylation significantly reduced the number of Müller glial cells with or without CNTF stimulation. These results suggested that the MAPK signal transduction pathway is important in the formation of Müller glial cells during retina development.

Ciliary neurotrophic factor induces genes associated with inflammation and gliosis in the retina: a gene profiling study of flow-sorted, Müller cells

Background

Ciliary neurotrophic factor (CNTF), a member of the interleukin-6 cytokine family, has been implicated in the development, differentiation and survival of retinal neurons. The mechanisms of CNTF action as well as its cellular targets in the retina are poorly understood. It has been postulated that some of the biological effects of CNTF are mediated through its action via retinal glial cells; however, molecular changes in retinal glia induced by CNTF have not been elucidated. We have, therefore, examined gene expression dynamics of purified Müller (glial) cells exposed to CNTF in vivo.

Methodology/Principal Findings

Müller cells were flow-sorted from mgfap-egfp transgenic mice one or three days after intravitreal injection of CNTF. Microarray analysis using RNA from purified Müller cells showed differential expression of almost 1,000 transcripts with two- to seventeen-fold change in response to CNTF. A comparison of transcriptional profiles from Müller cells at one or three days after CNTF treatment showed an increase in the number of transcribed genes as well as a change in the expression pattern. Ingenuity Pathway Analysis showed that the differentially regulated genes belong to distinct functional types such as cytokines, growth factors, G-protein coupled receptors, transporters and ion channels. Interestingly, many genes induced by CNTF were also highly expressed in reactive Müller cells from mice with inherited or experimentally induced retinal degeneration. Further analysis of gene profiles revealed 20–30% overlap in the transcription pattern among Müller cells, astrocytes and the RPE.

Conclusions/Significance

Our studies provide novel molecular insights into biological functions of Müller glial cells in mediating cytokine response. We suggest that CNTF remodels the gene expression profile of Müller cells leading to induction of networks associated with transcription, cell cycle regulation and inflammatory response. CNTF also appears to function as an inducer of gliosis in the retina.

Effects of epidermal growth factor and erythropoietin on Müller glial activation and phenotypic plasticity in the adult mammalian retina

Retinal Müller glia have received considerable attention with regard to their potential to function as quiescent retinal precursors. Various activation strategies induce characteristic features of retinal progenitor cells in Müller glia; however, these are often accompanied by hallmark features of reactive gliosis. We investigated the effects of an intravitreal injection of epidermal growth factor (EGF), a known mitogen, and erythropoietin (EPO) on activation and expression of developmental phenotypes within the adult retina. Using thymidine-analogue labeling as well as immunocytochemical and confocal analyses, we assayed the responses of retinal cells exposed to intravitreal administration of either EGF or EPO. We report that adult Müller glia incorporate bromodeoxyuridine (BrdU) and undergo a process of nuclear translocation to ectopic retinal layers following exposure to EGF. These cells survive within the retina for at least 23 days and express the developmental markers Pax6 and Chx10 as well as nestin and glial fibrillary acidic protein. Furthermore, we demonstrate that cotreatment with EGF and EPO suppresses aspects of EGF-induced glial reactivity, alters the retinal distribution of BrdU-positive nuclei, and serves to regulate the expression of developmental phenotypes seen in these cells. These data further our understanding of Müller cell responsiveness to intravitral, combinatorial growth factor treatments.

Developmental regulation of neuronal survival by adenosine in the in vitro and in vivo avian retina depends on a shift of signaling pathways leading to CREB phosphorylation or dephosphorylation

Previous studies have shown a cAMP/protein kinase A-dependent neuroprotective effect of adenosine on glutamate or re-feeding-induced apoptosis in chick retina neuronal cultures. In the present work, we have studied the effect of adenosine on the survival of retinal progenitor cells. Cultures obtained from 6-day-old (E6) or from 8-day-old (E8) chick embryos were challenged 2 h (C0) or 1 day (C1) after seeding and analyzed after 3-4 days in vitro. Surprisingly, treatment with the selective A2a adenosine receptor agonists N(6) -[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)-ethyl]adenosine (DPMA) or 3-[4-[2-[[6-amino-9-[(2R,3R,4S,5S)-5-(ethylcarbamoyl)-3,4-dihydroxy-oxolan-2-yl]purin-2-yl]amino]ethyl]phenyl]propanoic acid (CGS21680) promoted cell death when added at E6C0 but not at E6C1 or E8C0. DPMA-induced cell death involved activation of A2a receptors and the phospholipase C/protein kinase C but not the cAMP/protein kinase A pathway, and was not correlated with early modulation of precursor cells proliferation. Regarding cyclic nucleotide responsive element binding protein (CREB) phosphorylation, cultures from E6 embryos behave in an opposite manner from that from E8 embryos, both in vitro and in vivo. While the phospho-CREB level was high at E6C0 cultures and could be diminished by DPMA, it was lower at E8C0 and could be increased by DPMA. Similar to what was observed in cell survival studies, CREB dephosphorylation induced by DPMA in E6C0 cultures was dependent on the Phospholipase C/protein kinase C pathway. Accordingly, cell death induced by DPMA was inhibited by okadaic acid, a phosphatase blocker. Moreover, DPMA as well as the adenosine uptake blocker nitrobenzyl mercaptopurine riboside (NBMPR) modulate cell survival and CREB phosphorylation in a population of cells in the ganglion cell layer in vivo. These data suggest that A2a adenosine receptors as well as CREB may display a novel and important function by controlling the repertoire of developing retinal neurons.

Cone and rod photoreceptor transplantation in models of the childhood retinopathy Leber congenital amaurosis using flow-sorted Crx-positive donor cells

Retinal degenerative disease causing loss of photoreceptor cells is the leading cause of untreatable blindness in the developed world, with inherited degeneration affecting 1 in 3000 people. Visual acuity deteriorates rapidly once the cone photoreceptors die, as these cells provide daylight and colour vision. Here, in proof-of-principle experiments, we demonstrate the feasibility of cone photoreceptor transplantation into the wild-type and degenerating retina of two genetic models of Leber congenital amaurosis, the Crb1^rd8/rd8 and Gucy2e^−/− mouse. Crx-expressing cells were flow-sorted from the developing retina of CrxGFP transgenic mice and transplanted into adult recipient retinae; CrxGFP is a marker of cone and rod photoreceptor commitment. Only the embryonic-stage Crx-positive donor cells integrated within the outer nuclear layer of the recipient and differentiated into new cones, whereas postnatal cells generated a 10-fold higher number of rods compared with embryonic-stage donors. New cone photoreceptors displayed unambiguous morphological cone features and expressed mature cone markers. Importantly, we found that the adult environment influences the number of integrating cones and favours rod integration. New cones and rods were observed in ratios similar to that of the host retina (1:35) even when the transplanted population consisted primarily of cone precursors. Cone integration efficiency was highest in the cone-deficient Gucy2e^−/− retina suggesting that cone depletion creates a more optimal environment for cone transplantation. This is the first comprehensive study demonstrating the feasibility of cone transplantation into the adult retina. We conclude that flow-sorted embryonic-stage Crx-positive donor cells have the potential to replace lost cones, as well as rods, an important requirement for retinal disease therapy.

Transcriptional regulation of photoreceptor development and homeostasis in the mammalian retina

In the developing vertebrate retina, diverse neuronal subtypes originate from multipotent progenitors in a conserved order and are integrated into an intricate laminated architecture. Recent progress in mammalian photoreceptor development has identified a complex relationship between six key transcription-regulatory factors (RORbeta, OTX2, NRL, CRX, NR2E3 and TRbeta2) that determine rod versus M cone or S cone cell fate. We propose a step-wise 'transcriptional dominance' model of photoreceptor cell fate determination, with the S cone representing the default state of a generic photoreceptor precursor. Elucidation of gene-regulatory networks that dictate photoreceptor genesis and homeostasis will have wider implications for understanding the development of nervous system function and for the treatment of neurodegenerative diseases.

Ciliary neurotrophic factor: a survival and differentiation inducer in human retinal progenitors

Retinitis pigmentosa, age-related macular degeneration, and Parkinson's disease remain major problems in the field of medicine. Some of the strategies being explored for treatment include replacement of damaged tissue by transplantation of healthy tissues or progenitor cells and delivery of neurotrophins to rescue degenerating tissue. One of the neurotrophins with promise is the ciliary neurotrophic factor (CNTF). In this study, we report the role played by CNTF in retinal cell differentiation and survival in retinal progenitors. We found that CNTF is a survival factor for multipotential human retinal cells and increased cell survival by 50%, over a 7-d period, under serum-free conditions, as determined by apoptotic assays (immunohistochemistry and flow cytometry). This effect is dose dependent with a maximum survival at a CNTF concentration of 20 ng/ml. We also report that CNTF might be a cell commitment factor, directing the differentiation mainly toward large multipolar cells with ganglionic and amacrine phenotype. These cells express tyrosine hydroxylase (amacrine cells) as well as, thy 1.1 and neuron-specific enolase (ganglionic cells). Additionally, there was also an increase in protein kinase C alpha, a protein expressed in rod and cone bipolars as well as cone photoreceptors and calbindin, a protein expressed in cone photoreceptors and horizontal cells. In our studies, CNTF doubled the number of cells with ganglionic phenotypes, and basic fibroblast growth factor doubled the number of cells with photoreceptor phenotype. Additionally, CNTF induced a subset of progenitors to undergo multiple rounds of cell division before acquiring the large multipolar ganglionic phenotype. Our conclusion is that CNTF could be an agent that has therapeutic potential and possibly induces differentiation of large multipolar ganglionic phenotype in a subset of progenitors.

Function and mechanism of CNTF/LIF signaling in retinogenesis

Ciliary neurotrophic factor (CNTF) and leukemia inhibitory factor (LIF) exhibit multiple biological effects in the developing vertebrate retina. CNTF/LIF inhibits rod photoreceptor, and promotes bipolar cells and Muller glia differentiation. In addition, CNTF/LIF has been shown to have proliferative and apoptotic effects. Moreover, LIF also inhibits retinal vascular development. CNTF/LIF signaling components CNTFRalpha, LIFRbeta, gp130, and a number of STAT proteins are expressed in the retina. CNTF/LIF activates Jak-STAT, ERK, and Notch pathways during retinal development. Perturbation of CNTF induced signal transduction reveals that different combinations of CNTF/LIF signaling pathways regulate differentiation of retinal neurons and glia. Gene expression studies show that CNTF/LIF affects retinogenesis by regulating various genes involved in transcription, signal transduction, protein modification, apoptosis, protein localization, and cell ion homeostasis. Most past studies have deployed ectopic expression or addition of exogenous CNTF/LIF, thus further ana-lysis of mice with conditional mutations in CNTF/LIF signaling components will allow better understanding of in-vivo functions of CNTF/LIF associated signaling events in retinogenesis.

Preconditioning-induced protection of photoreceptors requires activation of the signal-transducing receptor gp130 in photoreceptors

Retinal degenerations are a class of neurodegenerative disorders that ultimately lead to blindness due to the death of retinal photoreceptors. In most cases, death is the result of long-term exposure to environmental, inflammatory, and genetic insults. In age-related macular degeneration, significant vision loss may take up to 70-80 years to develop. The protracted time to develop blindness suggests that retinal neurons have an endogenous mechanism for protection from chronic injury. Previous studies have shown that endogenous protective mechanisms can be induced by preconditioning animals with sublethal bright cyclic light. Such preconditioning can protect photoreceptors from a subsequent damaging insult and is thought to be accomplished through induced expression of protective factors. Some of the factors shown to be associated with protection bind and activate the signal transducing receptor gp130. To determine whether stress-induced endogenous protection of photoreceptors requires gp130, we generated conditional gp130 knockout (KO) mice with the Cre/lox system and used light-preconditioning to induce neuroprotection in these mice. Functional and morphological analyses demonstrated that the retina-specific gp130 KO impaired preconditioning-induced endogenous protection. Photoreceptor-specific gp130 KO mice had reduced protection, although the Müller cell KO mice did not, thus gp130-induced protection was restricted to photoreceptors. Using an animal model of retinitis pigmentosa, we found that the photoreceptor-specific gp130 KO increased sensitivity to genetically induced photoreceptor cell death, demonstrating that gp130 activation in photoreceptors had a general protective role independent of whether stress was caused by light or genetic mutations.

Development and neurogenic potential of Müller glial cells in the vertebrate retina

Considerable research on normal and diseased states within the retina has focused on neurons. Recent research on glia throughout the central nervous system, including within the retina where Müller glia are the main type of glia, has provided a more in depth view of glial functions in health and disease. Glial cells have been recognized as being vital for the maintenance of a healthy tissue environment, where they actively participate in neuronal activity. More recently, Müller glia have been recognized as being very similar to retinal progenitor cells, particularly when compared at the molecular level using comprehensive expression profiling techniques. The molecular similarities, as well as the developmental events that occur at the end of the genesis period of retinal cells, have led us to propose that Müller glia are a form of late stage retinal progenitor cells. These late stage progenitor cells acquire some specialized glial functions, but do not irreversibly leave the progenitor state. Indeed, Müller glia appear to be able to behave as a progenitor in that they have been shown to proliferate and produce neurons in several instances when an acute injury has been applied to the retina. Enhancement of this response is thus an exciting strategy for retinal repair.

Regulation of cytokine signaling components in developing rat retina correlates with transient inhibition of rod differentiation by CNTF

Ciliary neurotrophic factor (CNTF) is known to inhibit the differentiation of rod photoreceptors from postmitotic precursor cells. During early postnatal development, photoreceptor precursors lose their responsiveness to CNTF. The underlying events causing this change in responsiveness are unknown. Moreover, whether rods express CNTF receptor alpha, a prerequisite for a direct response to the factor, is controversial. Since morphological studies have previously produced conflicting results, we have analyzed the expression of cytokine receptor components and potential ligands in the rat photoreceptor layer by real-time reverse transcription with the polymerase chain reaction after laser microdissection and by immunoblotting. Cytokine effects on rods were studied in explant cultures from newborn rat retina. CNTF receptor alpha (CNTFR alpha) and leukemia inhibitory factor receptor ss (LIFRss) were expressed in immature photoreceptors. Expression of the CNTF-specific alpha-subunit (but not of LIFRss) was downregulated specifically in the photoreceptor layer in parallel with the appearance of opsin-positive rods. The decrease of CNTFR alpha levels in explant cultures was closely correlated with the loss of precursor cell responsiveness to CNTF. Increasing the CNTF concentration in the culture medium led to prolonged CNTFR alpha expression and, concomitantly, to persistent inhibition of rod differentiation. Application of CNTF and LIF in vitro induced phosphorylation of STAT3. Inducibility of STAT3 activation by CNTF decreased with photoreceptor maturation, whereas the LIF effect persisted. Our results thus indicate that CNTF acts directly on photoreceptor precursors inhibiting their differentiation via activation of the JAK/STAT3 signal transduction pathway, and that this effect is temporally limited because of the downregulation of CNTFR alpha.

Roles of STAT3/SOCS3 pathway in regulating the visual function and ubiquitin-proteasome-dependent degradation of rhodopsin during retinal inflammation

Inflammatory cytokines cause tissue dysfunction. We previously reported that retinal inflammation down-regulates rhodopsin expression and impairs visual function by an unknown mechanism. Here, we demonstrate that rhodopsin levels were preserved by suppressor of cytokine signaling 3 (SOCS3), a negative feedback regulator of STAT3 activation. SOCS3 was expressed mainly in photoreceptor cells in the retina. In the SOCS3-deficient retinas, rhodopsin protein levels dropped sooner, and the reduction was more profound than in the wild type. Visual dysfunction, measured by electroretinogram, was prolonged in retina-specific SOCS3 conditional knock-out mice. Visual dysfunction and decreased rhodopsin levels both correlated with increased STAT3 activation enhanced by SOCS3 deficiency. Interleukin 6, one of the inflammatory cytokines found during retinal inflammation, activated STAT3 and decreased rhodopsin protein in adult retinal explants. This was enhanced by inhibiting SOCS3 function in vitro, indicating that rhodopsin reduction was not a secondary effect in the mutant mice. Interestingly, in the inflamed SOCS3-deficient adult retina, rhodopsin decreased post-transcriptionally at least partly through ubiquitin-proteasome-dependent degradation accelerated by STAT3 activation and not transcriptionally as in the developing retina, on which we reported previously. A STAT3-dependent E3 ubiquitin ligase, Ubr1, was responsible for rhodopsin degradation and was up-regulated in the inflamed SOCS3-deficient retinas. These results indicate that in wild-type animals, a decrease in rhodopsin during inflammation is minimized by endogenous SOCS3. However, when STAT3 activation exceeds some threshold beyond the compensatory activity of endogenous SOCS3, rhodopsin levels decrease. These findings suggest SOCS3 as a potential therapeutic target molecule for protecting photoreceptor cell function during inflammation.

Individual retinal progenitor cells display extensive heterogeneity of gene expression

The development of complex tissues requires that mitotic progenitor cells integrate information from the environment. The highly varied outcomes of such integration processes undoubtedly depend at least in part upon variations among the gene expression programs of individual progenitor cells. To date, there has not been a comprehensive examination of these differences among progenitor cells of a particular tissue. Here, we used comprehensive gene expression profiling to define these differences among individual progenitor cells of the vertebrate retina. Retinal progenitor cells (RPCs) have been shown by lineage analysis to be multipotent throughout development and to produce distinct types of daughter cells in a temporal, conserved order. A total of 42 single RPCs were profiled on Affymetrix arrays. In situ hybridizations performed on both retinal sections and dissociated retinal cells were used to validate the results of the microarrays. An extensive amount of heterogeneity in gene expression among RPCs, even among cells isolated from the same developmental time point, was observed. While many classes of genes displayed heterogeneity of gene expression, the expression of transcription factors constituted a significant amount of the observed heterogeneity. In contrast to previous findings, individual RPCs were found to express multiple bHLH transcription factors, suggesting alternative models to those previously developed concerning how these factors may be coordinated. Additionally, the expression of cell cycle related transcripts showed differences among those associated with G2 and M, versus G1 and S phase, suggesting different levels of regulation for these genes. These data provide insights into the types of processes and genes that are fundamental to cell fate choices, proliferation decisions, and, for cells of the central nervous system, the underpinnings of the formation of complex circuitry.

Proliferative and cell fate effects of Hedgehog signaling in the vertebrate retina

The retina is an excellent system for delving into the question of how cell fate, number and organization are regulated in the central nervous system. Multipotential progenitor cells in the immature retina proliferate, exit the cell cycle and generate neurons and one glial cell type in a prescribed temporal sequence. While some aspects of progenitor behavior are controlled cell intrinsically, extrinsic signals present in the retina environment have been shown to impact on proliferation, differentiation and cell fate of progenitors. Intercellular signaling proteins of the Hedgehog (Hh) family regulate several aspects of visual system development in vertebrates--ranging from early eye field patterning to retinal and optic nerve development. This review highlights the role of Hh signaling on retinal progenitor proliferation and diversification.

Preparation and square wave electroporation of retinal explant cultures

This protocol details organotypic cultures of developing mouse, monkey and human retinas, which can be maintained for up to 2 weeks. Intact retinas are placed on polycarbonate filters floating on explant culture medium and fed every day with previously prepared retinal conditioned medium. Developing mouse retinas from E12.5 to P12 have been successfully cultured using this protocol as well as retinas from the equivalent stages of human and monkey development. Although this protocol does not require any special equipment, it provides a relatively high throughput. Retinal explant cultures lend themselves to complex pharmacological and genetic manipulations that are currently not feasible in vivo. A detailed procedure for square wave electroporation of retinal explants is also included to provide a high-throughput means to alter gene expression in the developing retina. This protocol for the preparation of retinal conditioned explant medium requires 4 d. Other steps of this protocol can be completed in 2 h.

Bioinformatic identification of novel putative photoreceptor specific cis-elements

Background

Cell specific gene expression is largely regulated by different combinations of transcription factors that bind cis-elements in the upstream promoter sequence. However, experimental detection of cis-elements is difficult, expensive, and time-consuming. This provides a motivation for developing bioinformatic methods to identify cis-elements that could prioritize future experimental studies. Here, we use motif discovery algorithms to predict transcription factor binding sites involved in regulating the differences between murine rod and cone photoreceptor populations.

Results

To identify highly conserved motifs enriched in promoters that drive expression in either rod or cone photoreceptors, we assembled a set of murine rod-specific, cone-specific, and non-photoreceptor background promoter sequences. These sets were used as input to a newly devised motif discovery algorithm called Iterative Alignment/Modular Motif Selection (IAMMS). Using IAMMS, we predicted 34 motifs that may contribute to rod-specific (19 motifs) or cone-specific (15 motifs) expression patterns. Of these, 16 rod- and 12 cone-specific motifs were found in clusters near the transcription start site. New findings include the observation that cone promoters tend to contain TATA boxes, while rod promoters tend to be TATA-less (exempting Rho and Cnga1). Additionally, we identify putative sites for IL-6 effectors (in rods) and RXR family members (in cones) that can explain experimental data showing changes to cell-fate by activating these signaling pathways during rod/cone development. Two of the predicted motifs (NRE and ROP2) have been confirmed experimentally to be involved in cell-specific expression patterns. We provide a full database of predictions as additional data that may contain further valuable information. IAMMS predictions are compared with existing motif discovery algorithms, DME and BioProspector. We find that over 60% of IAMMS predictions are confirmed by at least one other motif discovery algorithm.

Conclusion

We predict novel, putative cis-elements enriched in the promoter of rod-specific or cone-specific genes. These are candidate binding sites for transcription factors involved in maintaining functional differences between rod and cone photoreceptor populations.

The increase in retinal cells proliferation induced by FGF2 is mediated by tyrosine and PI3 kinases

Since 1973, multiple effects of basic fibroblast growth factor have been described in a large number of cells. These effects include proliferation, survival and differentiation. The aim of this work was to study the intracellular pathways involved in the basic fibroblast growth factor (FGF2) effect on rat retinal cells proliferation in vitro. Our data show that treatment with FGF2 increases proliferation in a concentration- and time-dependent manner. The effect of 25 ng/ml FGF2 was blocked by 10 microM genistein, a tyrosine kinase inhibitor and by 25 microM LY294002, a PI3 kinase inhibitor. The concomitant treatment with 0.3 microM chelerythrine chloride, a protein kinase C inhibitor, and 6.25 microM LY294002 also inhibited the effect of FGF2. Our results suggest that the proliferative effect of FGF2 on retinal cell cultures involves the activation of distinct kinases.

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

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

Retinal stem cells: promising candidates for retina transplantation

Stem cell transplantation is widely considered as a promising therapeutic approach for photoreceptor degeneration, one of the major causes of blindness. In this review, we focus on the biology of retinal stem cells (RSCs) and progenitor cells (RPCs) isolated from fetal, postnatal, and adult animals, with emphasis on those from rodents and humans. We discuss the origin of RSCs/RPCs, the markers expressed by these cells and the conditions for the isolation, culture, and differentiation of these cells in vitro or in vivo by induction with exogenous stimulation.

Functional roles of Otx2 transcription factor in postnatal mouse retinal development

We previously reported that Otx2 is essential for photoreceptor cell fate determination; however, the functional role of Otx2 in postnatal retinal development is still unclear although it has been reported to be expressed in retinal bipolar cells and photoreceptors at postnatal stages. In this study, we first examined the roles of Otx2 in the terminal differentiation of photoreceptors by analyzing Otx2; Crx double-knockout mice. In Otx2+/-; Crx-/- retinas, photoreceptor degeneration and downregulation of photoreceptor-specific genes were much more prominent than in Crx-/- retinas, suggesting that Otx2 has a role in the terminal differentiation of the photoreceptors. Moreover, bipolar cells decreased in the Otx2+/-; Crx-/- retina, suggesting that Otx2 is also involved in retinal bipolar-cell development. To further investigate the role of Otx2 in bipolar-cell development, we generated a postnatal bipolar-cell-specific Otx2 conditional-knockout mouse line. Immunohistochemical analysis of this line showed that the expression of protein kinase C, a marker of mature bipolar cells, was significantly downregulated in the retina. Electroretinograms revealed that the electrophysiological function of retinal bipolar cells was impaired as a result of Otx2 ablation. These data suggest that Otx2 plays a functional role in the maturation of retinal photoreceptor and bipolar cells.

A typology of photoreceptor gene expression patterns in the mouse

Mutations in photoreceptor-enriched genes have been implicated in dozens of human retinal diseases, yet no systematic analysis of rod and cone gene expression patterns has been carried out. In addition, although cone photoreceptor loss accounts for much of the morbidity of retinal disease, relatively few cone-specific genes are known. In this study, we carried out microarray and in situ hybridization analyses of the mouse Neural retina leucine zipper gene (Nrl) mutant, which shows an en masse conversion of rods into cones, to establish a typology of photoreceptor gene expression and to identify novel cone-specific genes. We found a total of 18 new cone-enriched genes, some of which map near uncloned retinal disease loci. Several of these genes have a dorsal-ventral (D-V) pattern of expression similar to that of short- or medium-wavelength opsins. We carried out microarray analysis of dorsal and ventral microdissected WT retina and found additional photoreceptor genes with an asymmetric distribution. Overall, we found that photoreceptor genes fall on an expression spectrum from rod-specific to cone-specific, with many showing varying degrees of rod and cone coexpression. These expression patterns can be reliably predicted from microarray data alone. Our results demonstrate definitive molecular differences between rods and cones that may underlie the physiological differences between these two classes of photoreceptors.

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.