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

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.

Transformation of cone precursors to functional rod photoreceptors by bZIP transcription factor NRL

Networks of transcriptional regulatory proteins dictate specification of neural lineages from multipotent retinal progenitors. Rod photoreceptor differentiation requires the basic motif-leucine zipper (bZIP) transcription factor NRL, because loss of Nrl in mice (Nrl-/-) results in complete transformation of rods to functional cones. To examine the role of NRL in cell fate determination, we generated transgenic mice that express Nrl under the control of Crx promoter in postmitotic photoreceptor precursors of WT and Nrl-/- retina. We show that NRL expression, in both genetic backgrounds, leads to a functional retina with only rod photoreceptors. The absence of cones does not alter retinal lamination, although cone synaptic circuitry is now recruited by rods. Ectopic expression of NRL in developing cones can also induce rod-like characteristics and partially suppress cone-specific gene expression. We show that NRL is associated with specific promoter sequences in Thrb (encoding TRbeta2 transcription factor required for M-cone differentiation) and S-opsin and may, therefore, directly participate in transcriptional suppression of cone development. Our studies establish that NRL is not only essential but is sufficient for rod differentiation and that postmitotic photoreceptor precursors are competent to make binary decisions during early retinogenesis.

Intravitreal injection of ciliary neurotrophic factor (CNTF) causes peripheral remodeling and does not prevent photoreceptor loss in canine RPGR mutant retina

Ciliary neurotrophic factor (CNTF) rescues photoreceptors in several animal models of retinal degeneration and is currently being evaluated as a potential treatment for retinitis pigmentosa in humans. This study was conducted to test whether CNTF prevents photoreceptor cell loss in XLPRA2, an early onset canine model of X-linked retinitis pigmentosa caused by a frameshift mutation in RPGR exon ORF15. Four different treatment regimens of CNTF were tested in XLPRA2 dogs. Under anesthesia, the animals received at different ages an intravitreal injection of 12 microg of CNTF in the left eye. The right eye served as a control and was injected with a similar volume of phosphate buffered saline (PBS). Ocular examinations were performed regularly during the treatment periods. At termination, the dogs were euthanatized, eyes collected and the retinas were processed for embedding in optimal cutting temperature (OCT) medium. The outer nuclear layer (ONL) thickness was evaluated on H&E sections and values in both CNTF- and PBS-treated eyes were compared. Morphologic alterations in the peripheral retina were characterized by immunohistochemistry using cell-specific markers. Cell proliferation in the retinas was examined on semi-thin plastic sections, and by BrdU pulse-labeling and Ki67 immunohistochemistry on cryosections. All CNTF-treated eyes showed early clinical signs of corneal epitheliopathy, subcapsular cataracts and uveitis. No statistically significant difference in ONL thickness was seen between the CNTF- and PBS-injected eyes. Prominent retinal remodeling that consisted in an abnormal increase in the number of rods, and in misplacement of some rods, cones, bipolar and Müller cells, was observed in the peripheral retina of CNTF-treated eyes. This was only seen when CNTF was in injected before the age at which the canine retina reaches full maturation. In XLPRA2 dogs, intravitreal injections of CNTF failed to prevent photoreceptors from undergoing cell death in the central and mid-peripheral retina. CNTF also caused ocular side-effects and morphologic alterations in the periphery that were consistent with cell dedifferentiation and proliferation. Our findings suggest that some inherited forms of retinal degeneration may not respond to CNTF's neuroprotective effects.

SOCS3 is required to temporally fine-tune photoreceptor cell differentiation

Suppressor of cytokine signaling 3 (SOCS3) is an intracellular, ligand-induced negative feedback modulator of STAT3 activation that acts during inflammation. Here, we demonstrate that SOCS3 expression is important for normal retinal development in the perinatal period. STAT3 is highly activated in the late-embryonic retina, then downregulated at postnatal day 0 (P0), presumably by the depletion of upstream ligands. We found that SOCS3 was required after P0 to shut down the residual STAT3 activation; this loss of activated STAT3 leads to Rhodopsin expression and rod photoreceptor cell differentiation. SOCS3 deficiency failed to terminate STAT3 activation, thereby delaying expression of Rhodopsin and its upstream transcription factor, crx. Development subsequently continued, but its course was temporally erratic, probably because of faulty compensation. Interestingly, SOCS3 protein expression was first detected postnatally, after STAT3 activation was mostly downregulated. It initially appeared in some of the presumptive photoreceptor cells and gradually spread. SOCS3 mRNA level was constant from the late-embryonic to early-postnatal period. Post-transcriptional inhibition of SOCS3 protein expression maintains a high STAT3 activation during late embryogenesis, and after P0, releasing from the inhibition promptly terminates STAT3 activation. Thus, SOCS3 can act as a temporal fine-tuner of STAT3 activation during photoreceptor cell differentiation.

The CNTF/LIF signaling pathway regulates developmental programmed cell death and differentiation of rod precursor cells in the mouse retina in vivo

Natural cell death is critical for normal development of the nervous system, but the extracellular regulators of developmental cell death remain poorly characterized. Here, we studied the role of the CNTF/LIF signaling pathway during mouse retinal development in vivo. We show that exposure to CNTF during neonatal retinal development in vivo retards rhodopsin expression and results in an important and specific deficit in photoreceptor cells. Detailed analysis revealed that exposure to CNTF during retinal development causes a sharp increase in cell death of postmitotic rod precursor cells. Importantly, we show that blocking the CNTF/LIF signaling pathway during mouse retinal development in vivo results in a significant reduction of naturally occurring cell death. Using retroviral lineage analysis, we demonstrate that exposure to CNTF causes a specific reduction of clones containing only rods without affecting other clone types, whereas blocking the CNTF/LIF receptor complex causes a specific increase of clones containing only rods. In addition, we show that stimulation of the CNTF/LIF pathway positively regulates the expression of the neuronal and endothelial nitric oxide synthase (NOS) genes, and blocking nitric oxide production by pre-treatment with a NOS inhibitor abolishes CNTF-induced cell death. Taken together, these results indicate that the CNTF/LIF signaling pathway acts via regulation of nitric oxide production to modulate developmental programmed cell death of postmitotic rod precursor cells.

Developmentally-regulated expression of tissue-specific splice variant of rat vesicular glutamate transporter 1 in retina and pineal gland

Three distinct subtypes of vesicular glutamate transporters (VGLUTs) have been identified to date that are expressed basically in a cell type-specific manner. We have found a splice variant of VGLUT1 mRNA that is expressed almost exclusively in photosensitive tissues, i.e. the retina and the pineal gland. The variant mRNA, termed VGLUT1v, contains an additional 75 base pair sequence derived from part of a second intron (designated as exon IIa) between exons 2 and 3. The variant accounted for approximately 70% and 25%of VGLUT1 mRNA in the adult retina and pineal gland, respectively. The expression of VGLUT1v was developmentally regulated in both tissues. Organ culture showed that expression of the variant in the retina increased in association with the development of rod cells, suggesting that VGLUT1v is expressed in rod cells. In situ hybridization with variant-specific probes showed expression of VGLUT1v in the inner segment layer of photoreceptor cells. On the other hand, variant expression did not parallel the development of rhodopsin-positive cells in the pineal gland. As rod cells and pinealocytes are known to release glutamate continuously at ribbon synapses, it is possible that the variant has some functional advantage over the wild-type transporter in such a specialized manner of glutamate release.

Pdm and Castor specify late-born motor neuron identity in the NB7-1 lineage

Embryonic development requires generating cell types at the right place (spatial patterning) and the right time (temporal patterning). Drosophila neuroblasts undergo stem cell-like divisions to generate an ordered sequence of neuronal progeny, making them an attractive system to study temporal patterning. Embryonic neuroblasts sequentially express Hunchback, Krüppel, Pdm1/Pdm2 (Pdm), and Castor (Cas) transcription factors. Hunchback and Krüppel specify early-born temporal identity, but the role of Pdm and Cas in specifying temporal identity has never been addressed. Here we show that Pdm and Cas regulate late-born motor neuron identity within the NB7-1 lineage: Pdm specifies fourth-born U4 motor neuron identity, while Pdm/Cas together specify fifth-born U5 motor neuron identity. We conclude that Pdm and Cas specify late-born neuronal identity; that Pdm and Cas act combinatorially to specify a temporal identity distinct from either protein alone, and that Cas repression of pdm expression regulates the generation of neuronal diversity.

Direct and indirect effects of hedgehog pathway activation in the mammalian retina

The morphogen Sonic hedgehog (Shh) is expressed by the projection neurons of the retina, retinal ganglion cells (RGCs) and promotes retinal precursor cell (RPC) proliferation. To distinguish between direct and indirect effects of Hedgehog (Hh) pathway activation in the perinatal mouse retina, we followed the fate of cells that expressed a constitutively active allele of Smoothened (SMO-M2), the signal transduction component of the Hh pathway. SMO-M2 expression promoted a cell-autonomous increase in CyclinD1 expression and RPC proliferation and promoted the development of cells with an inner nuclear layer identity. SMO-M2 expression also inhibited rhodopsin expression in uninfected cells, thus highlighting an unexpected non-cell autonomous effect of Hh pathway activation on photoreceptor development.

TrkB/BDNF signaling regulates photoreceptor progenitor cell fate decisions

Neurotrophins, via activation of Trk receptor tyrosine kinases, serve as mitogens, survival factors and regulators of arborization during retinal development. Brain-derived neurotrophic factor (BDNF) and TrkB regulate neuronal arborization and survival in late retinal development. However, TrkB is expressed during early retinal development where its functions are unclear. To assess TrkB/BDNF actions in the early chick retina, replication-incompetent retroviruses were utilized to over-express a dominant negative truncated form of TrkB (trunc TrkB), or BDNF and effects were assessed at E15. Clones expressing trunc TrkB were smaller than controls, and proliferation and apoptosis assays suggest that decreased clone size correlated with increased cell death when BDNF/TrkB signaling was impaired. Analysis of clonal composition revealed that trunc TrkB over-expression decreased photoreceptor numbers (41%) and increased cell numbers in the middle third of the inner nuclear layer (INL) (23%). Conversely, BDNF over-expression increased photoreceptor numbers (25%) and decreased INL numbers (17%). Photoreceptors over-expressing trunc TrkB demonstrated no increase in apoptosis nor abnormalities in lamination suggesting that TrkB activation is not required for photoreceptor cell survival or migration. These studies suggest that TrkB signaling regulates commitment to and/or differentiation of photoreceptor cells from retinal progenitor cells, identifying a novel role for TrkB/BDNF in regulating cell fate decisions.

Involvement of Pleiotrophin in CNTF-mediated differentiation of the late retinal progenitor cells

Ciliary neurotrophic factor (CNTF) participates in retinal development by inhibiting rod differentiation and promoting bipolar and Müller cell differentiation. In order to identify genes which are regulated by CNTF in the developing retina, we carried out a subtractive hybridization study. By this approach, we identified the Pleiotrophin (Ptn) as an upregulated gene in postnatal day 0 (P0) retinal explants upon addition of CNTF. Correlation of overall expression patterns between different retinal cell markers and Ptn in situ hybridization suggest that Ptn transcripts are initially expressed in progenitor cells then in postmitotic precursors of the INL expressing the Chx10 gene, and later in some differentiated retinal Müller glial (RMG) cells and rod-bipolar cells. Overexpression of Ptn by in vitro electroporation of P0 rat retinal explants partially blocks rod differentiation and promotes bipolar cell production, similar to effects of exogenous CNTF and leukemia inhibitory factor (LIF). Furthermore, in P0 retinal explants from mice lacking Ptn, the inhibitory effect of CNTF and LIF on rod differentiation is partially reduced and the cytokine-induced bipolar cell differentiation is largely prevented. Together, these results demonstrate that influence of CNTF family of cytokines on the differentiation of late retinal progenitor cell population is partially mediated by the release of Ptn.

Stem cells in the mammalian eye: a tool for retinal repair

Degenerative diseases and traumatic injuries of the central nervous system (CNS) are major causes of long-term disability, whether such insults impact the brain, retina, or spinal cord. Substantial tissue destruction can be sustained by these complex structures without loss of life, while the lack of effective CNS regeneration frequently results in a marked degradation in quality of life. Only recently has it become clear that an enormous potential for regeneration is present within the mammalian CNS. The challenge now presented to researchers is to harness this potential to treat disease. Recent studies showing that stem and progenitor cells can be isolated from the mammalian retina have prompted many researchers to develop strategies aimed at restoring function to the diseased retina. This review summarizes a number of issues related to this goal, including retinal development, transplantation immunology, tissue engineering, and large animal studies. The application of these divergent disciplines to stem cell technology is vital to the development of the novel strategies needed to make retinal transplantation a clinical success.

Leukemia inhibitory factor inhibits neuronal development and disrupts synaptic organization in the mouse retina

Leukemia inhibitory factor (LIF) belongs to the interleukin-6 cytokine family, all members of which signal through the common gp130 receptor. Neurotrophic members of this cytokine family are known to arrest photoreceptor maturation and are likely to regulate maturation of other retinal neurons as well. We have used transgenic mice that constitutively express LIF beginning in embryonic development to determine its effects on synaptic organization and molecular maturation of all classes of retinal neurons. LIF reduced the numbers of cells showing markers characteristic of mature cells of all neuronal classes and caused synaptic ectopia. The net effect was disrupted morphological development and disturbed synaptic organization. Our study suggests that cytokines signaling through gp130 are capable of regulating many aspects of neuronal differentiation in the retina, including synaptic targeting.

Polyglutamine expansion causes neurodegeneration by altering the neuronal differentiation program

Huntington's disease (HD) and spinocerebellar ataxia type 7 (SCA7) belong to a group of inherited neurodegenerative diseases caused by polyglutamine (polyQ) expansion in corresponding proteins. Transcriptional alteration is a unifying feature of polyQ disorders; however, the relationship between polyQ-induced gene expression deregulation and degenerative processes remains unclear. R6/2 and R7E mouse models of HD and SCA7, respectively, present a comparable retinal degeneration characterized by progressive reduction of electroretinograph activity and important morphological changes of rod photoreceptors. The retina, which is a simple central nervous system tissue, allows correlating functional, morphological and molecular defects. Taking advantage of comparing polyQ-induced degeneration in two retina models, we combined gene expression profiling and molecular biology techniques to decipher the molecular pathways underlying polyQ expansion toxicity. We show that R7E and R6/2 retinal phenotype strongly correlates with loss of expression of a large cohort of genes specifically involved in phototransduction function and morphogenesis of differentiated rod photoreceptors. Accordingly, three key transcription factors (Nrl, Crx and Nr2e3) controlling rod differentiation genes, hence expression of photoreceptor specific traits, are down-regulated. Interestingly, other transcription factors known to cause inhibitory effects on photoreceptor differentiation when mis-expressed, such as Stat3, are aberrantly re-activated. Thus, our results suggest that independently from the protein context, polyQ expansion overrides the control of neuronal differentiation and maintenance, thereby causing dysfunction and degeneration.

Challenges in the study of neuronal differentiation: a view from the embryonic eye

Progress in the study of the molecular mechanisms that regulate neuronal differentiation has been quite impressive in recent years, and promises to continue to an equally fast pace. This should not lead us into a sense of complacency, however, because there are still significant barriers that cannot be overcome by simply conducting the same type of experiments that we have been performing thus far. This article will describe some of these challenges, while highlighting the conceptual and methodological breakthroughs that will be necessary to overcome them.

Enhanced Neurotrophin Synthesis and Molecular Differentiation in Non-Transformed Human Retinal Progenitor Cells Cultured in a Rotating Bioreactor

One approach to the treatment of retinal diseases, such as retinitis pigmentosa, is to replace diseased or degenerating cells with healthy cells. Even if all of the problems associated with tissue transplant were to be resolved, the availability of tissue would remain an ongoing problem. We have previously shown that transformed human retinal cells can be grown in a NASA-developed horizontally rotating culture vessel (bioreactor) to form three-dimensional-like structures with the expression of several retinal specific proteins. In this study, we have investigated growth of non-transformed human retinal progenitors (retinal stem cells) in a rotating bioreactor. This rotating culture vessel promotes cell-cell interaction between similar and dissimilar cells. We cultured retinal progenitors (Ret 1-4) alone or as a co-culture with human retinal pigment epithelial cells (RPE, D407) in this system to determine if 3D structures can be generated from non-transformed progenitors. Our second goal was to determine if the formation of 3D structures correlates with the upregulation of neurotrophins, basic fibroblast growth factor (bFGF), transforming growth factor alpha (TGFalpha), ciliary neurotrophic factor (CNTF), and brain-delivered neurotrophic factor (BDNF). These factors have been implicated in progenitor cell proliferation, commitment, differentiation, and survival. We also investigated the expression of the following retinal specific proteins in this system: neuron specific enolase (NSE); tyrosine hydroxylase (TH); D(2)D(3), D(4) receptors; protein kinase-C alpha (PKCalpha), and calbindin. The 3D structures generated were characterized by phase and scanning transmission electron microscopy. Retinal progenitors, cultured alone or as a co-culture in the rotating bioreactor, formed 3D structures with some degree of differentiation, accompanied by the upregulation of bFGF, CNTF, and TGFalpha. Brain-derived neurotrophic factor, which is expressed in vivo in RPE (D407), was not expressed in monolayer cultures of RPE but expressed in the rotating bioreactor-cultured RPE and retinal progenitors (Ret 1-4). Upregulation of neurotrophins was noted in all rotating bioreactor-cultured cells. Also, upregulation of D(4) receptor, calbindin, and PKCalpha was noted in the rotating bioreactor-cultured cells. We conclude that non-transformed retinal progenitors can be grown in the rotating bioreactor to form 3D structures with some degree of differentiation. We relied on molecular and biochemical analysis to characterize differentiation in cells grown in the rotating bioreactor.

Roles of cell-intrinsic and microenvironmental factors in photoreceptor cell differentiation

Photoreceptor differentiation requires the coordinated expression of numerous genes. It is unknown whether those genes share common regulatory mechanisms or are independently regulated by distinct mechanisms. To distinguish between these scenarios, we have used in situ hybridization, RT-PCR, and real-time PCR to analyze the expression of visual pigments and other photoreceptor-specific genes during chick embryo retinal development in ovo, as well as in retinal cell cultures treated with molecules that regulate the expression of particular visual pigments. In ovo, onset of gene expression was asynchronous, becoming detectable at the time of photoreceptor generation (ED 5-8) for some photoreceptor genes, but only around the time of outer segment formation (ED 14-16) for others. Treatment of retinal cell cultures with activin, staurosporine, or CNTF selectively induced or down-regulated specific visual pigment genes, but many cognate rod- or cone-specific genes were not affected by the treatments. These results indicate that many photoreceptor genes are independently regulated during development, are consistent with the existence of at least two distinct stages of gene expression during photoreceptor differentiation, suggest that intrinsic, coordinated regulation of a cascade of gene expression triggered by a commitment to the photoreceptor fate is not a general mechanism of photoreceptor differentiation, and imply that using a single photoreceptor-specific "marker" as a proxy to identify photoreceptor cell fate is problematic.

Membrane properties of retinal stem cells/progenitors

The membrane properties of cells help integrate extrinsic information relayed through growth factors, chemokines, extracellular matrix, gap junctions and neurotransmitters towards modulating cell-intrinsic properties, which in turn determine whether cells remain quiescent, proliferate, differentiate, establish contact with other cells or remove themselves by activating programmed cell death. This review highlights some of the membrane properties of early and late retinal stem cells/progenitors, which are likely to be helpful in the identification and enrichment of these cells and in understanding mechanisms underlying their maintenance and differentiation. Understanding of membrane properties of retinal stem cells/progenitors is essential for the successful formulation of approaches to treat retinal degeneration and diseases by cell therapy.

Rhodopsin, Violet and Blue Opsin Expressions in the Chick Are Highly Dependent on Tissue and Serum Conditions

The molecular, cellular or tissue environment can influence the expression of genes and thereby regulate processes of tissue formation. Here we determined the tissue and serum dependence of the expression of all photopigments in the chick by a series of distinct retinal cell cultures, analyzed by RT-PCR using specific primers for all four opsins and rhodopsin followed by quantitative scanning of the respective gel bands. For comparison, we first determined expression of all opsins during normal chick retinogenesis, which began with red and violet opsins at E12, shortly followed by blue and green opsins and finally rhodopsin at E14. This period corresponds to the time of synaptogenesis in the inner retina. All cultures were started with 6-day-old dissociated retinal cells. Cells were kept at low or high cell density (called LoDens or HiDens), or they were reaggregated as retinal spheres, whereby all of them were raised at low (2%) or high serum (12%) levels (called LoSer or HiSer). In LoDens/HiSer cultures, expression of all opsins was weak. At HiDens/LoSer red and green opsin expression was strong, while rhodopsin and violet/blue remained low. In HiDens/HiSer cultures the expression of red and green was strong; rhodopsin was almost normal, while violet and green were low. In reaggregates at high serum the expression came closest to a normal retina, but violet and blue opsins were still below normal. At low serum, however, violet and blue were negligible and rhodopsin was low. This in vitro study shows that rhodopsin, followed by violet and blue opsin expressions is highly dependent on serum, cell density and tissue conditions, while red and green opsins are more autonomous.

STAT3 activation in response to growth factors or cytokines participates in retina precursor proliferation

Growth factors and cytokines play an important role in the development of central nervous systems including neurons of the retina. However, the molecular pathways that trigger cell growth remain unclear in neuronal precursors. In the present studies, we used a retinal explant culture system to investigate the response of signal transducer and activator of transcription factors (STATs) to extrinsic factors during mouse retinal development. Retinas from embryonic and neonatal stages showed that STAT3 but not STAT1 was activated in response to ciliary neurotrophic factor (CNTF), leukemia inhibitory factor (LIF), fibroblast growth factor-1 (FGF1), fibroblast growth factor-2 (FGF2), epidermal growth factor (EGF), interferon-alpha (IFN-alpha) and interferon-gamma (IFN-gamma) in distinct patterns. STAT3 activation was detected in the outermost retina layer in response to CNTF, LIF, FGF1, and IFN-alpha 24 hr after stimulation in postnatal day 1 (PN1) explants, but not FGF2, EGF, IFN-gamma, and retinoic acid (RA). Cytokine stimulation increased the number of cells incorporating BrdU and the labelled cells co-localized with phosphorylated STAT3, indicating that STAT3 may play an essential role in coupling extrinsic factors to retina precursor cell (RPC) proliferation. Furthermore, persistent expression of two neural precursor markers, Hes1 and Otx2 was detected in outer retinal layers and correlated with STAT3 activation by CNTF, suggesting that STAT3 activation may play a critical role in stimulating mitotic precursors. These results strongly support a model that STAT3-mediated signalling regulates precursor populations during mouse retina development.

The RB protein family in retinal development and retinoblastoma: new insights from new mouse models

The Rb gene was isolated almost 20 years ago, but fundamental questions regarding its role in retinal development and retinoblastoma remain. What is the normal function of RB protein in retinogenesis? What is the cell-of-origin of retinoblastoma? Why do retinoblastoma tumors have recurrent genetic lesions other than Rb inactivation? Why is retinoblastoma not induced by defects in cell cycle regulators other than Rb? Why is the retina so sensitive to Rb loss? Recently developed conditional Rb knockout models provide new insight into some of these issues. The data suggest that RB protein may not control the rate of progenitor division, but is critical for cell cycle exit when dividing retinal progenitors differentiate into postmitotic transition cells. This finding focuses attention on the ectopically dividing transition cell, rather than the progenitor, as the cell-of-origin. Cell-specific analyses in the RB-deficient retina reveal that ectopically dividing photoreceptors, bipolar and ganglion cells die, but amacrine, horizontal and Muller cells survive and stop dividing when they terminally differentiate. Rare amacrine transition cells escape cell cycle exit and generate tumors. These data suggest that post-Rb mutations are required to overcome growth arrest associated with terminal differentiation, rather than apoptosis as previously suggested. To explain why perturbing cell cycle regulators other than RB does not initiate retinoblastoma, we speculate that mutations in other components of the RB pathway perturb cell cycle arrest, but only RB loss triggers genome instability in retinal transition cells, which may be critical to facilitate post-Rb mutations necessary for transformation. Cell-specific differences in the effect of Rb loss on genome stability may contribute to the tremendous sensitivity of retinal transition cells to tumorigenesis. The new mouse models of retinoblastoma will be invaluable for testing these possibilities.

Dedifferentiation of adult human myoblasts induced by ciliary neurotrophic factor in vitro

Ciliary neurotrophic factor (CNTF) is primarily known for its important cellular effects within the nervous system. However, recent studies indicate that its receptor can be highly expressed in denervated skeletal muscle. Here, we investigated the direct effect of CNTF on skeletal myoblasts of adult human. Surprisingly, we found that CNTF induced the myogenic lineage-committed myoblasts at a clonal level to dedifferentiate into multipotent progenitor cells--they not only could proliferate for over 20 passages with the expression absence of myogenic specific factors Myf5 and MyoD, but they were also capable of differentiating into new phenotypes, mainly neurons, glial cells, smooth muscle cells, and adipocytes. These "progenitor cells" retained their myogenic memory and were capable of redifferentiating into myotubes. Furthermore, CNTF could activate the p44/p42 MAPK and down-regulate the expression of myogenic regulatory factors (MRFs). Finally, PD98059, a specific inhibitor of p44/p42 MAPK pathway, was able to abolish the effects of CNTF on both myoblast fate and MRF expression. Our results demonstrate the myogenic lineage-committed human myoblasts can dedifferentiate at a clonal level and CNTF is a novel regulator of skeletal myoblast dedifferentiation via p44/p42 MAPK pathway.

Effects of Ciliary Neurotrophic Factor on Differentiation of Late Retinal Progenitor Cells

Ciliary neurotrophic factor (CNTF) has been shown to be a potent regulator of retinal cell differentiation. The present study was undertaken to investigate the effects of CNTF on in vitro differentiation of expanded late retinal progenitor cells. Retinal progenitor cells used in these studies were isolated from the neural retina of postnatal day-1 green fluorescent protein (GFP) transgenic mice. The resulting GFP-positive neurospheres were dissociated into a single-cell suspension and grown on poly-D-lysine/laminin-coated tissue culture flasks or slides to generate adherent retinal progenitor cells. These adherent cells were treated with 20 ng/ml of CNTF for up to 14 days, and expression of specific retinal cell markers was determined by immunocytochemistry, reverse transcription-polymerase chain reaction (RT-PCR), and immunoblot analysis. In vitro studies showed that CNTF treatment of late retinal progenitor cells resulted in changes in cellular morphology. Immunocytochemical studies showed an increase in the proportion of cells expressing markers of bipolar cells but not rod differentiation. In addition, an increase in the proportion of cells expressing glial cell markers was observed. RT-PCR analysis showed downregulation in Hes1, Nestin, Notch1, and Pax6 transcripts along with a concomitant increase in protein kinase C (PKC) alpha and glial fibrillary acidic protein (GFAP) transcripts. These findings were confirmed by immunoblot analysis, where downregulation in Nestin expression and simultaneous upregulation in PKC alpha and GFAP were observed. The data indicate that CNTF treatment of multipotential late retinal progenitors increases the proportion of cells that express markers of bipolar neurons and glia.

The search for the retinoblastoma cell of origin

The cellular effects of the genetic defects associated with tumorigenesis are context dependent. To better understand the reasons that different cell types require distinct combinations of mutations to form tumours, it is essential to identify and characterize a tumour's 'cell of origin'. Retinoblastoma, a rare childhood cancer of the retina that is caused by RB inactivation, is a good model in which to search for a tumour cell of origin, because retinal development is well understood and the initiating genetic lesion is well characterized. Identifying the cell of origin for this tumour would advance our understanding of how cellular context affects the requirement of specific mutations for cancer initiation and progression.

dazed gene is necessary for late cell type development and retinal cell maintenance in the zebrafish retina

Several molecules, such as growth factors and neurotrophic factors, are required both for the differentiation of specific retinal cell types and the long-term cell survival of all retinal neurons. As diffusible factors, these molecules act non-cell-autonomously. Here, we describe the loss of function phenotype for dazed (dzd), a gene that acts cell-autonomously for retinal cell survival and affects the differentiation of rod photoreceptors and the Muller glia. By 3 days after fertilization, dazed mutant embryos have small eyes and slight heart edema. Acridine orange staining indicated a significant degree of retinal cell death occurring by 48 hr after fertilization, and histological analysis revealed that dying cells were found in the inner and outer nuclear layers and near the marginal zones. Although molecular and morphological differentiation of the inner retina and cone photoreceptors occurred, rod photoreceptors failed to differentiate beyond a small patch in the ventral retina and rod precursors failed to respond to exogenously added retinoic acid, which normally potentiated rod differentiation. Mosaic analysis indicated that the dazed gene acts cell-autonomously for rod production and cell survival, as dazed clones failed to produce rods outside the ventral patch and dazed cells were not maintained in wild-type hosts. Raising mutants under constant light resulted in severe retinal degeneration, whereas raising embryos under constant darkness did not provide any additional protection from cell death. Behavioral analysis showed that a subpopulation of adult fish that were heterozygous for the dazed mutation had elevated visual thresholds and were night blind, suggesting that dazed may also be required for long-term dim-light vision. Taken together, our studies suggest a role for the dazed gene in rod and Muller cell development and overall retinal cell survival and maintenance.

Retinal degeneration in Aipl1-deficient mice: a new genetic model of Leber congenital amaurosis

Leber congenital amaurosis (LCA) is the most severe inherited retinopathy, with the earliest age of onset. Because this currently incurable disease is present from birth and is a relatively rare disorder, the development of animal models that closely resemble the phenotype in patients is especially important. Our previous genetic analyses of LCA patients identified mutations in the aryl-hydrocarbon interacting protein-like 1 (AIPL1) gene. Here we present development of an animal model of AIPL1-associated LCA, the Aipl1-deficient mouse. Aipl1 is expressed at low levels throughout human and mouse retinal development and is rapidly upregulated as photoreceptors differentiate. The mouse displays rapid retinal degeneration and massive Müler cell gliosis, resembling the phenotype of the rd mouse, which is caused by a mutation in the gene for the beta-subunit of the rod-specific phosphodiesterase. We confirm that this phenotype is consistent with the human disease using electroretinograms, and document the disease pathogenesis by analyzing the development of all retinal cell types and synaptogenesis during retinal histogenesis. Ectopic expression of AIPL1 led to deregulated retinal progenitor cell proliferation and alterations in cell fate specification; however, no gross abnormalities of proliferation during retinal development were detected. Data from analysis of proliferation and cell fate specification during retinal development of Aipl1-deficient mice suggests that there may be redundancy or compensation for Aipl1 loss by other related proteins. Because this mouse model closely mimics the human retinopathy caused by homozygous mutations in this gene, it provides a preclinical model for testing therapies to rescue the vision of children whose blindness is caused by AIPL1 mutations.

BMP4 and CNTF are neuroprotective and suppress damage-induced proliferation of Müller glia in the retina

In response to acute damage, Müller glia in the chicken retina have been shown to be a source of proliferating progenitor-like cells. The secreted factors and signaling pathways that regulate this process remain unknown. The purpose of this study was to test whether secreted factors, which are known to promote glial differentiation during development, regulate the ability of Müller glia to proliferate and become retinal progenitors in response to acute damage in mature retina. We made intraocular injections of BMP4, BMP7, EGF, NGF, BDNF, or CNTF before or after a single, toxic dose of N-methyl-d-aspartate (NMDA) and assayed for proliferating progenitor-like cells within the retina. We found that injections of BMP4, BMP7, or CNTF, but not EGF, NGF, or BDNF, before NMDA treatment reduced the number of Müller glia that proliferated and gave rise to progenitor-like cells. CNTF and BMP4, but not NGF or BDNF, greatly reduced the number of cells destroyed by toxin treatment indicating that these factors protect retinal neurons from a severe excitotoxic insult. Injections of CNTF 5 days before NMDA treatment prevented neurotoxin-induced cell death and Müller glial proliferation, while injections of BMP4 had no protective effect. In addition, CNTF injected after NMDA treatment suppressed glial proliferation, while BMP4 did not. We conclude that BMP4 and CNTF, when applied before a toxic insult, act as neuroprotective agents and likely suppress the proliferative response of Müller glia to retinal damage by attenuating the retinal damage; protecting bipolar and amacrine neurons from NMDA-induced cell death. When applied after a toxic insult, CNTF suppressed glial proliferation independent of levels of retinal damage.

Foxn4 controls the genesis of amacrine and horizontal cells by retinal progenitors

During vertebrate retinogenesis, seven classes of cells are specified from multipotent progenitors. To date, the mechanisms underlying multipotent cell fate determination by retinal progenitors remain poorly understood. Here, we show that the Foxn4 winged helix/forkhead transcription factor is expressed in a subset of mitotic progenitors during mouse retinogenesis. Targeted disruption of Foxn4 largely eliminates amacrine neurons and completely abolishes horizontal cells, while overexpression of Foxn4 strongly promotes an amacrine cell fate. These results indicate that Foxn4 is both necessary and sufficient for commitment to the amacrine cell fate and is nonredundantly required for the genesis of horizontal cells. Furthermore, we provide evidence that Foxn4 controls the formation of amacrine and horizontal cells by activating the expression of the retinogenic factors Math3, NeuroD1, and Prox1. Our data suggest a model in which Foxn4 cooperates with other key retinogenic factors to mediate the multipotent differentiation of retinal progenitors.

SPECIFICATION OF TEMPORAL IDENTITY IN THE DEVELOPING NERVOUS SYSTEM

The nervous system of higher organisms exhibits extraordinary cellular diversity owing to complex spatial and temporal patterning mechanisms. The role of spatial patterning in generating neuronal diversity is well known; here we discuss how neural progenitors change over time to contribute to cell diversity within the central nervous system (CNS). We focus on five model systems: the vertebrate retina, cortex, hindbrain, spinal cord, and Drosophila neuroblasts. For each, we address the following questions: Do multipotent progenitors generate different neuronal cell types in an invariant order? Do changes in progenitor-intrinsic factors or progenitor-extrinsic signals regulate temporal identity (i.e., the sequence of neuronal cell types produced)? What is the mechanism that regulates temporal identity transitions; i.e., what triggers the switch from one temporal identity to the next? By applying the same criteria to analyze each model system, we try to highlight common themes, point out unique attributes of each system, and identify directions for future research.

Involvement of Ath3 in CNTF-mediated differentiation of the late retinal progenitors

The cellular diversity of the mammalian retina is underpinned by multipotential neural progenitors that generate retinal neurons and glia with temporal and spatial specificity. It is thought, based on studies using a variety of approaches, that the fate of retinal progenitors is determined through interactions between temporally and spatially arrayed epigenetic cues with intrinsic factors that regulate the competence of cells to respond to such cues. Here, we demonstrate interactions between an intrinsic factor Ath3, a neural bHLH protein, and an extrinsic factor CNTF during the differentiation of the late retinal progenitors along the bipolar cell lineage. Expression of Ath3 is predominantly associated with the late stage of retinal histogenesis when bipolar cells are specified, and in adult it is detected in cells expressing bipolar cell-specific markers. We demonstrate that CNTF-induced bipolar cell differentiation is accompanied by an increase in levels of Ath3 transcripts and compromised when Ath3 expression is attenuated. Our study suggests that the influence of CNTF on the differentiation of late retinal progenitors is mediated through Ath3.

Genomic analysis of mouse retinal development

The vertebrate retina is comprised of seven major cell types that are generated in overlapping but well-defined intervals. To identify genes that might regulate retinal development, gene expression in the developing retina was profiled at multiple time points using serial analysis of gene expression (SAGE). The expression patterns of 1,051 genes that showed developmentally dynamic expression by SAGE were investigated using in situ hybridization. A molecular atlas of gene expression in the developing and mature retina was thereby constructed, along with a taxonomic classification of developmental gene expression patterns. Genes were identified that label both temporal and spatial subsets of mitotic progenitor cells. For each developing and mature major retinal cell type, genes selectively expressed in that cell type were identified. The gene expression profiles of retinal Müller glia and mitotic progenitor cells were found to be highly similar, suggesting that Müller glia might serve to produce multiple retinal cell types under the right conditions. In addition, multiple transcripts that were evolutionarily conserved that did not appear to encode open reading frames of more than 100 amino acids in length ("noncoding RNAs") were found to be dynamically and specifically expressed in developing and mature retinal cell types. Finally, many photoreceptor-enriched genes that mapped to chromosomal intervals containing retinal disease genes were identified. These data serve as a starting point for functional investigations of the roles of these genes in retinal development and physiology.

Stem cells and retinal repair

Retinal stem cells (RSCs) are multipotent central nervous system (CNS) precursors that give rise to the retina during the course of development. RSCs are present in the embryonic eyecup of all vertebrate species and remain active in lower vertebrates throughout life. Mammals, however, exhibit little RSC activity in adulthood and thus little capacity for retinal growth or regeneration. Because CNS precursors can now be isolated from immature and mature mammals and expanded ex vivo, it is possible to study these cells in culture as well as following transplantation to the diseased retina. Such experiments have revealed a wealth of unanticipated findings, both in terms of the instructive cues present in the mature mammalian retina as well as the ability of grafted CNS precursors to respond to them. This review examines current knowledge regarding RSCs, together with other CNS precursors, from the perspective of investigators who wish to isolate, propagate, genetically modify, and transplant these cells as a regenerative strategy with application to retinal disease.

Downregulation of STAT3 activation is required for presumptive rod photoreceptor cells to differentiate in the postnatal retina

Ciliary neurotrophic factor (CNTF) has been known to inhibit the differentiation of presumptive rod photoreceptor cells; however, the underlying mechanisms have remained to be elucidated. We demonstrated that STAT3 activation, but not SHP2 activation, is responsible for the CNTF/gp130 signaling that inhibits expression of Rhodopsin and its upstream activator, crx, in the retinal explants derived from P0 mice (P0 retinal explants), utilizing STAT3-deficient retina and electroporation of dominant-negative form of STAT3 (STAT3F). We also demonstrated that STAT3 activation in presumptive rod photoreceptor cells at E18.5 is rapidly downregulated at P0, when Rhodopsin expression starts during retinal development. Persistent STAT3 activation in the P0 retinal explants prevented Rhodopsin expression and rapid upregulation of crx expression. STAT3-deficient retinas did not exhibit precocious rod photoreceptor cell differentiation as a whole, although they occasionally exhibited precocious upregulation of crx mRNA. Thus, we conclude that downregulation of STAT3 activation is required, but insufficient, for rod photoreceptor cell differentiation in the postnatal retina.

Cell fate decisions and patterning in the vertebrate retina: the importance of timing, asymmetry, polarity and waves

The differentiation of distinct cell populations in the retina is a multi-step process that involves cell cycle exit, migration, and dramatic changes of cell morphology. All these steps are tightly controlled by multiple regulatory pathways, which involve both cell-autonomous networks of transcription factors and cell-cell signaling events. Additional regulatory inputs into cell fate decisions have been recently suggested: accumulating evidence shows that the timing of cell cycle exit, the orientation of the mitotic spindle during the last cell division, and the polarity of neuronal progenitor cells could play important roles in cell fate determination.

Density-dependent differentiation in nontransformed human retinal progenitor cells in response to basic fibroblast growth factor- and transforming growth factor-alpha

Multipotential retinal precursors give rise to all cell types seen in multilayered retina. The generation of differentiation and diversity of neuronal cell types is determined by both extrinsic regulatory signals and endogenous genetic programs. We have previously reported that cell commitment in human retinal precursor cells (SV-40T) can be modified in response to exogenous growth factors, basic fibroblast growth factor, and transforming growth factor alpha (bFGF and TGFalpha). We report in this study that nontransformed human retinal precursors differentiate into photoreceptors by a cell density-dependent mechanism, and the effects were potentiated by bFGF and TGFalpha alone or in combination. A larger proportion of multipotential precursors plated at a density of 1 x 10(4) cells/cm(2) differentiated into neurons (photoreceptors) compared to cells plated at 3-5 x 10(4)/cm(2) and 1 x 10(5) cells/cm(2) under serum-free conditions and the effects were amplified seven- to eightfold in response to growth factors. Basic fibroblast growth factor (bFGF) and TGFalpha can induce 90% of the cells to assume a photoreceptor phenotype at a lower cell density, compared to only 30 and 25% of the cells acquiring a photoreceptor phenotype at intermediate and higher cell densities. Furthermore, at a lower cell density, 60-70% of the cells incorporate Bromodeoxyuridine (Brdu), suggesting that cells in a cell cycle may make a commitment to a specific fate in response to neurotrophins. Neurons with a photoreceptor phenotype were positive for three different sets of antibodies for rods/cones. Cells also exhibited upregulation of other proteins such as a D4 receptor protein expressed in photoreceptors, protein kinase Calpha (PKCalpha) expressed in rod bipolars and blue cones, and some other neuronal cell types. This was also confirmed by Western blot analysis. Newly derived photoreceptors survive for a few days before significant cell death ensues under serum-free conditions. To summarize, differentiation in precursors is density dependent, and growth factors amplify the effects.

Math5 determines the competence state of retinal ganglion cell progenitors

In mice, all of the six retinal neuron types are generated from common multipotent retinal progenitors, and their differentiation from progenitors is regulated by both extrinsic and intrinsic factors. Previously, we showed that targeted deletion of the atonal (ato) homologue math5 blocked the differentiation of most retinal ganglion cells (RGCs), revealing an essential role for math5 in RGC differentiation. In this study, we used the Cre-loxP recombination system to trace the fate of math5-expressing cells in retina. Our results demonstrated that math5 expression was associated with the differentiation of multiple retinal neuron types, including RGCs, photoreceptor, horizontal, and amacrine cells, implying that math5 expression alone is not sufficient to determine the RGC fate. Math5 expression was restricted to postmitotic cells in developing retina, suggesting that cell fate commitment of retinal neurons occurs after the terminal mitosis. The insufficiency of and requirement for math5 in RGC differentiation indicates that, like ato in the development of Drosophila R8 photoreceptors, math5 plays a role in determining the RGC competence state of retinal progenitors and that additional positive and negative factors are required in determining RGC fate. Furthermore, we show that loss of Math5 function severely reduced the RGC expression of the transcription factors Brn-3b, Gfi-1, Isl-1, Isl-2, Nscl-1, Nscl-2, and RPF-1, suggesting that Math5 expression is required to activate a comprehensive transcription network of RGC differentiation.

Otx2 homeobox gene controls retinal photoreceptor cell fate and pineal gland development

Understanding the molecular mechanisms by which distinct cell fate is determined during organogenesis is a central issue in development and disease. Here, using conditional gene ablation in mice, we show that the transcription factor Otx2 is essential for retinal photoreceptor cell fate determination and development of the pineal gland. Otx2-deficiency converted differentiating photoreceptor cells to amacrine-like neurons and led to a total lack of pinealocytes in the pineal gland. We also found that Otx2 transactivates the cone-rod homeobox gene Crx, which is required for terminal differentiation and maintenance of photoreceptor cells. Furthermore, retroviral gene transfer of Otx2 steers retinal progenitor cells toward becoming photoreceptors. Thus, Otx2 is a key regulatory gene for the cell fate determination of retinal photoreceptor cells. Our results reveal the key molecular steps required for photoreceptor cell-fate determination and pinealocyte development.

Transcriptional control of neuronal diversification in the retina

During embryonic development, the array of vastly different neuronal types that are incorporated into the functional architecture of the mature neuroretina derives from a common population of multipotent retinal progenitor cells (RPCs). Retinogenesis proceeds in a precise chronological order, with the seven principal cell classes generated in successive phases. Cell biological experiments established that this histogenetic order, at least in part, reflects intrinsic changes within the RPC pool. In recent years a number of molecules controlling various aspects of cell fate specification from RPCs have been identified. However, few attempts have been made to integrate previous concepts that emerged from cell biological studies and more recent results based on molecular genetic experiments. This review aims at providing an overview of recent advances in our understanding of the cellular and molecular mechanisms underlying retinal neuronal diversification, with a particular focus on cell-intrinsic factors.

Ciliary neurotrophic factor inhibits differentiation of photoreceptor-like cells in rat pineal glands in vitro

Ciliary neurotrophic factor (CNTF) is a unique member of the interleukin-6 (IL-6) family, whose receptor subunit for ligand binding is exclusively expressed in the nervous system and muscle. The role of CNTF in mammalian development remains unknown. We recently reported the specific expression of CNTF in the pineal gland and eyes. To further examine the expression pattern and role of CNTF in development, we prepared a polyclonal antibody against rat CNTF, performed western blotting with this antibody, and confirmed a strong and specific expression of the CNTF protein in pineal glands and a moderate expression in the eyes among the various tissues examined in newborn rats. In pineal organ cultures of newborn rats, exogenously added recombinant rat CNTF potently inhibited the differentiation of photoreceptor-like cells in a dose-dependent manner, while CNTF did not influence the survival of pineal cells. Among several cell growth factors known to have a similar effect in retinal cultures examined, strong inhibitory effects were seen only with CNTF and the leukemia inhibitory factor (LIF), both of which belong to the IL-6 cytokine family. This inhibitory effect was the strongest during three to 6 days of culture when CNTF was added to these cultures. These results suggest that CNTF plays an inhibitory role in the development of photoreceptor-like cells in early postnatal rat pineal glands.

Distribution of CNTF receptor alpha protein in the central nervous system of the chick embryo

Ciliary neurotrophic factor (CNTF) promotes the survival and differentiation of various neuronal and glial cell populations in the nervous system of vertebrates. In mammals, the ligand-binding alpha-subunit of the CNTF receptor (CNTFRalpha) is expressed in a variety of neuronal populations, including all CNTF-responsive cells. Previous studies suggested that functional differences in the CNTF/CNTF receptor system between chicks and mammals exist. The purpose of the present study was to examine the temporal and spatial expression pattern of the chick CNTFRalpha protein during CNS development. Receptor expression was detectable by immunoblotting in all CNS areas tested but showed area-specific developmental regulation. Interestingly, two variants of CNTFRalpha, 69 and 65 kD, were identified by immunoblotting with a shift from the higher to the lower molecular mass species occurring during development. Immunoreactivity for CNTFRalpha protein was preferentially observed in neuropil and white matter structures of the developing CNS while neuronal somata generally appeared unlabeled. For example, expression was observed in the olfactory system, in the telencephalon, in parts of the somatosensory system, in components of the tectofugal pathway, in the cerebellum, and in auditory brainstem nuclei. Fiber tracts that exhibit CNTFRalpha immunoreactivity were the lateral forebrain bundle, occipitomesencephalic tract, quintofrontal tract, and vestibular nerve. Our study identifies potential new targets of a chick CNTF-related molecule and reveals significant regional differences of CNTFRalpha protein expression between chick and mammals. These results suggest that the CNTF receptor performs distinct developmental functions in different animals.

Lensectomy and vitrectomy decrease the rate of photoreceptor loss in rhodopsin P347L transgenic pigs

BACKGROUND

Photoreceptor degeneration in retinitis pigmentosa (RP) runs an inevitable, gradually progressive course. A wide variety of growth factors of different origins have been shown to slow the rate of degeneration in some rodent models of RP. Recently, lens-derived neurotrophic factors have been shown to rescue degenerating ganglion cells in crush models of the optic nerve. Our objective was to evaluate the potential rescue effect of lensectomy and vitrectomy (L&V) on photoreceptor degeneration in a large-animal model, the rhodopsin P347L transgenic pig.

METHODS

We operated on one eye of each of 49 3-week-old pigs--15 vitrectomies and 34 L&V, 6 of which received steroids. Retinal paraffin sections were prepared for all eyes, in addition to immunohistochemistry in four eyes, 8 weeks after L&V.

RESULTS

At eight weeks after L&V, operated eyes showed significantly more nuclei in the outer nuclear layer (ONL) than the unoperated fellow eyes. The better preservation of the ONL persisted but was less prominent by 20 weeks after surgery. Steroid treatment did not markedly reduce the better preservation of the ONL seen at 8, 10, and 12 weeks after surgery. The significant difference in cell count between operated and unoperated eyes in the L&V group at 8 weeks was due to the difference in the number of rods, not the cones.

CONCLUSION

Lensectomy and vitrectomy delay photoreceptor degeneration in rhodopsin P347L transgenic pigs. Lens-related rescue effect is a probable reason for the delayed degeneration.

Expression and activation of STAT proteins during mouse retina development

Cytokines and growth factors play important roles in mammalian ocular development and maintenance. Recent studies have indicated that some of these ligands can activate signal transducer and activator of transcription factors (STATs) and modulate gene transcription. The purpose of this study was to investigate the expression and activation of STAT proteins in the developing mouse retina. Anti-STAT and anti-phosphorylated STAT antibodies were used to detect the expression and activation of STATs in embryonic and postnatal neuronal retina, ciliary margin, and retinal pigment epithelium (RPE). In situ hybridization and Western blot were also employed. In embryonic stages, all STAT proteins were expressed in the neuronal retina in distinct cell populations at different embryonic stages. For example, Stat3 expression and activation gradually increased in the inner neuroblast layer and ciliary margin during development. In adult retina, Stat3 was detected in the inner nuclear layer and ganglion cells layers. Stat1 was strongly expressed in both outer and inner plexiform layers. Stat5a was clearly expressed in the outer/inner nuclear layer, the ganglion cell layer, and the inner plexiform layer. Strong expression of Stat3, Stat5a, and Stat6 was observed in the RPE. Activated Stat3 and Stat5a were found in the neural retina and the RPE. Distinct STAT proteins were present in different cell populations in neuronal retina and RPE suggesting multiple functions of STATs in mammalian eye development. Studies of STAT signal pathways in the eye may contribute to the understanding of molecular mechanisms in control of ocular development and pathogenesis.

Xrx1 controls proliferation and multipotency of retinal progenitors

We investigated the function of Xrx1 during Xenopus retinogenesis. Xrx1 overexpression lengthens mitotic activity and ectopically activates the expression of markers of undifferentiated progenitors in the developing retina. We assayed Xrx1 ability to support proliferation with a cell-autonomous mechanism by in vivo lipofection of single retinal progenitors. Xrx1 overexpression increases clonal proliferation while Xrx1 functional inactivation exerts the opposite effect. We also compared the effects of Xrx1 with those of the cyclin-dependent kinase cdk2, a strong mitotic promoter. Despite the similar increase in clonal proliferation displayed by both factors, Xrx1 and cdk2 act differently on retinal cell fate determination. cdk2/cyclinA2 lipofected retinas show a decrease in early-born cell types as ganglion cells and cones and an increase in late-born types such as bipolar neurons. On the contrary, Xrx1 lipofected retinas show no changes in the proportions of the different cell types, thus suggesting a role in supporting multipotency of retinal progenitors.

Tissue culture studies of retinal development

Because of a limited number of cell types, a series of well-described cell-type-specific markers and a stereotyped sequence of cell development, the retina has been a valuable model of CNS development. Dissociated and explant cultures have been used to help define some of the requirements for differentiation of each major cell class. In addition to mixed-cell cultures it is now possible to use cell purification or selective growth methods to give cultures of single cell types. Alteration of gene expression by viral infection has proved to be a valuable method to help elucidate developmental pathways.

Cellular diversity in the developing nervous system: a temporal view from Drosophila

This article considers the evidence for temporal transitions in CNS neural precursor cell gene expression during development. In Drosophila, five prospective competence states have so far been identified, characterized by the successive expression of Hb→Kr→Pdm→Cas→Gh in many, but not all, neuroblasts. In each temporal window of transcription factor expression, the neuroblast generates sublineages whose temporal identity is determined by the competence state of the neuroblast at the time of birth of the sublineage. Although similar regulatory programs have not yet been identified in mammals, candidate regulatory genes have been identified. Further investigation of the genetic programs that guide both invertebrate and vertebrate neural precursor cell lineage development will ultimately lead to an understanding of the molecular events that control neuronal diversity.

Effects of adeno-associated virus-vectored ciliary neurotrophic factor on retinal structure and function in mice with a P216L rds/peripherin mutation

Past studies have shown that acute administration of ciliary neurotrophic factor (CNTF) can prolong the survival of retinal photoreceptor cells that have undergone phototoxic injury or that express gene mutations. Adenovirus-vectored CNTF has also been effective but for all of these treatments, the effect has been transient. On the other hand, adeno-associated virus-vectored minigenes offer considerable promise for long-term survival. The authors sought to provide long-term, CNTF-based protection of mouse photoreceptors expressing a dominant-negative point mutation in the rds gene by using recombinant adeno-associated virus (rAAV) to deliver minigenes that code for a secreted form of CNTF.Secreted CNTF, under control of a cytomegalovirus (CMV) or chick beta actin (CBA) promoter provided long-term, panretinal rescue of photoreceptors following single injections of rAAV vectors into the subretinal compartment. Rescue was much less effective and less reproducible when the vectors were placed in the vitreous compartment. However, there were unexpected side effects that appeared to be dose-related. One side effect was a change in rod photoreceptor nucleus phenotype, featuring an increase in euchromatin and an increase in nuclear size following subretinal injections but not intravitreal injections. These nuclear changes were panretinal when the putatively stronger CBA promoter was used but not panretinal when the CMV promoter was used. In the latter case, the nuclear changes were much more pronounced at the site of injection. Thus, chronic hyperstimulation of retinal cells with CNTF may up-regulate gene expression in photoreceptors. Based on current knowledge of retinal cell targets for CNTF, this effect may be indirect and may not represent direct stimulation of photoreceptors by CNTF.A second side effect was a paradoxical decrease in scotopic a- and b-wave amplitudes and a decrease in photopic b-wave amplitudes in the injected, rescued retina when compared to its contralateral, uninjected counterpart, in spite of the fact that these retinas had more photoreceptors than their untreated mates. The basis for these decreased ERG amplitudes may be related to changes in gene expression. The mechanisms for these side effects and proper doses of CNTF administration should be determined before human clinical trials are considered for the amelioration of inherited retinal degenerations with CNTF.

The role of tangential dispersion in retinal mosaic formation

Individual types of retinal nerve cell are spaced across the retina in an orderly manner, ensuring a uniform sampling of the visual field. This regularity in cellular spacing has been commonly attributed to fate determination mechanisms operating around the time of cell birth, an hypothesis presuming that the position of a nerve cell is fixed within the plane of the retina from the time of its determination. At odds with this view, recent results from X-inactivation mosaic mice indicate that certain classes of retinal nerve cell, those known to form orderly mosaics in the adult retina, disperse tangentially during development. Furthermore, studies defining the spatial characteristics of developing and mature retinal mosaics suggest that cell-cell interactions around the time of morphological differentiation lead to mutual repulsion. Modelling studies in turn show that nothing more than a simple minimal spacing rule between neighboring cells of the same type is sufficient for the creation of the global patterning observed in biological retinal mosaics. For some cell types, the size of this "exclusion zone" surrounding individual cells is shown to be an intrinsic characteristic of each cell type, invariant across the retina, and accounting for the variation in mosaic regularity across changes in cell density. These results show how short-distance movements driven by intercellular interactions at the local level may mediate the emergence of the global patterning characteristic of retinal mosaics during development.

Genetic analysis of photoreceptor cell development in the zebrafish retina

To gain insight into the genetic mechanisms of photoreceptor development, we analyzed a collection of zebrafish mutations characterized by early photoreceptor cell loss. The mutant defects impair outer segment formation and are accompanied by an abnormal distribution of visual pigments. Rods and different cone types display defects of similar severity suggesting that genetic pathways common to all photoreceptors are affected. To investigate whether these phenotypes involve cell-cell interaction defects, we analyzed genetically mosaic animals. Interaction of niezerka photoreceptors with wild-type tissues improves the survival of mutant cells and restores their elongated morphology. In contrast, cells carrying mutations in the loci brudas, elipsa, fleer, and oval retain their defective phenotypes in a wild-type environment indicating cell-autonomy. These experiments identify distinct phenotypic categories of photoreceptor mutants and indicate that zebrafish photoreceptor defects involve both cell-autonomous and cell-nonautonomous mechanisms.

Generating neuronal diversity in the retina: one for nearly all

Visual perception of our environment essentially depends on the correct assembly of seven principal cell types into the functional architecture of the neuroretina. During retinogenesis these cell types derive from a common population of multipotent retinal progenitor cells (RPCs) residing in the inner layer of the optic cup. In contrast to other well studied regions of the developing CNS, retinal cell diversification is apparently not achieved by spatial prepatterning into distinct progenitor domains, but rather by the sequential production of cell types in a defined histogenetic order. Several lines of evidence suggest that this observation reflects substantial intrinsic changes in the retinogenic potential of RPCs. Recent advances, however, point at the existence of a common molecular framework underlying the retinogenic potential of RPCs throughout retinal neurogenesis.

neurogenin2 elicits the genesis of retinal neurons from cultures of nonneural cells

neurogenin2 (ngn2) encodes a basic helix-loop-helix transcription factor and plays an important role in neurogenesis from migratory neural crest cells. Its role in retinal development is poorly understood. We observed that in the developing chick retina, ngn2 was expressed in a subpopulation of proliferating progenitor cells. Ectopic expression of ngn2 in nonneural, retinal pigment epithelial cell culture triggered de novo generation of cells that expressed neural-specific markers and exhibited neuronal morphologies. Further molecular and morphological analyses showed that the main products of the induced neurogenesis were cells resembling young photoreceptor cells and cells resembling retinal ganglion cells. The generation of multiple cell types suggests that ngn2 induces various retinal pathways. Thus, unlike in the peripheral nervous system where ngn2 specifies one type of sensory neuron, ngn2 in the retina is likely involved in a common step leading to different cellular pathways. Our finding that ngn2 can instruct nonneural retinal pigment epithelial cells to differentiate toward retinal neurons demonstrates one possible way to induce de novo retinal neurogenesis.

Cellular competence plays a role in photoreceptor differentiation in the developing Xenopus retina

Factors in the environment appear to be responsible for inducing many of the cell fates in the retina, including, for example, photoreceptors. Further, there is a conserved order of histogenesis in the vertebrate retina, suggesting that a temporal mechanism interacts in the control of cellular determination. The temporal mechanism involved could result from different inducing signals being released at different times. Alternatively, the inducing signals might be present at many stages, but an autonomous clock could regulate the competence of cells to respond to them. To differentiate between these mechanisms, cells from young embryonic retinas were dissociated and grown together with those from older embryos, and the timing of photoreceptor determination assayed. Young cells appeared uninfluenced by older cells, expressing photoreceptor markers on the same time schedule as when cultured alone. A similar result was obtained when the heterochronic mixing was done in vivo by grafting a small plug of optic vesicle from younger embryos into older hosts. Even the graft cells at the immediate margin of the transplant failed to express photoreceptor markers earlier than normal, despite their being in contact with older, strongly expressing host cells. We conclude that retinal progenitors intrinsically acquire the ability to respond to photoreceptor-inducing cues by a mechanism that runs on a cell autonomous schedule, and that the conserved order of histogenesis is based in part on this competence clock.