Roles of homeobox and bHLH genes in specification of a retinal cell type

Intrinsic control of mammalian retinogenesis

The generation of appropriate and diverse neuronal and glial types and subtypes during development constitutes the critical first step toward assembling functional neural circuits. During mammalian retinogenesis, all seven neuronal and glial cell types present in the adult retina are specified from multipotent progenitors by the combined action of various intrinsic and extrinsic factors. Tremendous progress has been made over the past two decades in uncovering the complex molecular mechanisms that control retinal cell diversification. Molecular genetic studies coupled with bioinformatic approaches have identified numerous transcription factors and cofactors as major intrinsic regulators leading to the establishment of progenitor multipotency and eventual differentiation of various retinal cell types and subtypes. More recently, non-coding RNAs have emerged as another class of intrinsic factors involved in generating retinal cell diversity. These intrinsic regulatory factors are found to act in different developmental processes to establish progenitor multipotency, define progenitor competence, determine cell fates, and/or specify cell types and subtypes.

Direct reprogramming of adult somatic cells into other lineages: past evidence and future perspectives

Direct reprogramming of an adult cell into another differentiated lineage-such as fibroblasts into neurons, cardiomyocytes, or blood cells-without passage through an undifferentiated pluripotent stage is a new area of research that has recently emerged alongside stem cell technology and induced pluripotent stem cell reprogramming; indeed, this avenue of investigation has begun to play a central role in basic biological research and regenerative medicine. Even though the field seems new, its origins go back to the 1980s when it was demonstrated that differentiated adult cells can be converted into another cell lineage through the overexpression of transcription factors, establishing mature cell plasticity. Here, we retrace transdifferentiation experiments from the discovery of master control genes to recent in vivo reprogramming of one somatic cell into another from the perspective of possible applications for the development of new therapeutic approaches for human diseases.

Computational molecular phenotyping of retinal sheet transplants to rats with retinal degeneration

Retinal progenitor sheet transplants have been shown to extend neuronal processes into a degenerating host retina and to restore visual responses in the brain. The aim of this study was to identify cells involved in transplant signals to retinal degenerate hosts using computational molecular phenotyping (CMP). S334ter line 3 rats received fetal retinal sheet transplants at the age of 24-40 days. Donor tissues were incubated with slow-releasing microspheres containing brain-derived neurotrophic factor or glial cell-derived neurotrophic factor. Up to 265 days after surgery, eyes of selected rats were vibratome-sectioned through the transplant area (some slices stained for donor marker human placental alkaline phosphatase), dehydrated and embedded in Eponate, sectioned into serial ultrathin datasets and probed for rhodopsin, cone opsin, CRALBP (cellular retinaldehyde binding protein), l-glutamate, l-glutamine, glutathione, glycine, taurine, γ-aminobutyric acid (GABA) and DAPI (4',6-diamidino-2-phenylindole). In large transplant areas, photoreceptor outer segments in contact with host retinal pigment epithelium revealed rod and cone opsin immunoreactivity whereas no such staining was found in the degenerate host retina. Transplant photoreceptor layers contained high taurine levels. Glutamate levels in the transplants were higher than in the host retina whereas GABA levels were similar. The transplant inner nuclear layer showed some loss of neurons, but amacrine cells and horizontal cells were not reduced. In many areas, glial hypertrophy between the host and transplant was absent and host and transplant neuropil appeared to intermingle. CMP data indicate that horizontal cells and both glycinergic and GABAergic amacrine cells are involved in a novel circuit between transplant and host, generating alternative signal pathways between transplant and degenerating host retina.

Pax6: a multi-level regulator of ocular development

Eye development has been a paradigm for the study of organogenesis, from the demonstration of lens induction through epithelial tissue morphogenesis, to neuronal specification and differentiation. The transcription factor Pax6 has been shown to play a key role in each of these processes. Pax6 is required for initiation of developmental pathways, patterning of epithelial tissues, activation of tissue-specific genes and interaction with other regulatory pathways. Herein we examine the data accumulated over the last few decades from extensive analyses of biochemical modules and genetic manipulation of the Pax6 gene. Specifically, we describe the regulation of Pax6's expression pattern, the protein's DNA-binding properties, and its specific roles and mechanisms of action at all stages of lens and retinal development. Pax6 functions at multiple levels to integrate extracellular information and execute cell-intrinsic differentiation programs that culminate in the specification and differentiation of a distinct ocular lineage.

Injury-independent induction of reactive gliosis in retina by loss of function of the LIM homeodomain transcription factor Lhx2

Müller glia are the primary glial subtype in the retina and perform a wide range of physiological tasks in support of retinal function, but little is known about the transcriptional network that maintains these cells in their differentiated state. We report that selective deletion of the LIM homeodomain transcription factor Lhx2 from mature Müller glia leads to the induction of reactive retinal gliosis in the absence of injury. Furthermore, Lhx2 expression is also down-regulated in Prph2(Rd2/Rd2) animals immediately before the onset of reactive gliosis. Analysis of conditional Lhx2 knockouts showed that gliosis was hypertrophic but not proliferative. Aging of experimental animals demonstrated that constitutive reactive gliosis induced by deletion of Lhx2 reduced rates of ongoing apoptosis and compromised both rod and cone photoreceptor function. Additionally, these animals showed a dramatically reduced ability to induce expression of secreted neuroprotective factors and displayed enhanced rates of apoptosis in light-damage assays. We provide in vivo evidence that Lhx2 actively maintains mature Müller glia in a nonreactive state, with loss of function initiating a specific program of nonproliferative hypertrophic gliosis.

Requirement for the paired-like homeodomain transcription factor VSX1 in type 3a mouse retinal bipolar cell terminal differentiation

Retinal bipolar cells make up a class of at least 11 distinct interneurons that have been classified through morphological and molecular approaches. Previous work has shown that the paired-like homeodomain transcription factor Vsx1 is essential for the proper development of a subset of these interneurons. In Vsx1-null mice, bipolar cells are properly specified but exhibit terminal differentiation defects characterized by reduced expression of OFF bipolar cell markers and defects in OFF visual signaling. Here, we further examined the role of Vsx1 in OFF bipolar cells using recently identified cell-type-specific markers. In contrast to its previously characterized expression in type 2 OFF bipolar cells, Vsx1 expression was not detected in type 3 OFF bipolar cells, by either immunohistological or transgenic reporter labeling approaches. This observation was unexpected given previous findings that Cabp5 immunolabeling of type 3 bipolar cell axon terminals is reduced in Vsx1-null mice. However, we observed reduced levels of the type 3a bipolar cell marker hyperpolarization-activated and cyclic nucleotide-gated channel 4 (HCN4) in Vsx1-null mice, which is consistent with a requirement for Vsx1 in type 3 bipolar cell differentiation. In contrast, expression of the type 3b bipolar cell marker regulatory subunit RII-beta of protein kinase A was unchanged. Despite the absence of Vsx1 in mature type 3 bipolar cells, colabeling of Vsx1 and HCN4 was observed at postnatal stages. These findings reveal a role for Vsx1 in type 3a bipolar cells and suggest that Vsx1 function is required transiently in this cell type during the postnatal period.

Step-wise specification of retinal stem cells during normal embryogenesis

The specification of embryonic cells to produce the retina begins at early embryonic stages as a multi-step process that gradually restricts fate potentials. First, a subset of embryonic cells becomes competent to form retina by their lack of expression of endo-mesoderm-specifying genes. From these cells, a more restricted subset is biased to form retina by virtue of their close proximity to sources of bone morphogenetic protein antagonists during neural induction. During gastrulation, the definitive RSCs (retinal stem cells) are specified as the eye field by interactions with underlying mesoderm and the expression of a network of retina-specifying genes. As the eye field is transformed into the optic vesicle and optic cup, a heterogeneous population of RPCs (retinal progenitor cells) forms to give rise to the different domains of the retina: the optic stalk, retinal pigmented epithelium and neural retina. Further diversity of RPCs appears to occur under the influences of cell-cell interactions, cytokines and combinations of regulatory genes, leading to the differentiation of a multitude of different retinal cell types. This review examines what is known about each sequential step in retinal specification during normal vertebrate development, and how that knowledge will be important to understand how RSCs might be manipulated for regenerative therapies to treat retinal diseases.

Brn-3b inhibits generation of amacrine cells by binding to and negatively regulating DLX1/2 in developing retina

During retinogenesis, the basic helix-loop-helix proneural gene math5 (atoh7) initiates the generation of the first-born neurons, retinal ganglion cells (RGCs), by activating a network of RGC transcription factors, including Brn-3b (POU4F2). Herein, we show that the expression of DLX1 and DLX2 is significantly down-regulated in math5-null retina but is markedly increased in Brn-3b-null retina. Interestingly, Brn-3b interacts with DLX1 through its homeodomain, and this interaction represses DLX1 activity. Retrovirus-mediated mis-expression of DLX1 or DLX2 dramatically increases the number of amacrine/bipolar cells and concurrently reduces rod photoreceptors. Conversely, combined ectopic expression of Brn-3b with DLX1 or DLX2 promotes the production of RGCs and inhibits amacrine cell differentiation. Thus, DLX1/2 play an essential role in cell fate selection between amacrine and RGCs. Brn-3b suppresses the role of DLX1/2 through physical interaction and biases the competent precursors toward RGC fates.

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

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

Generation of subtype-specific neurons from postnatal astroglia of the mouse cerebral cortex

Instructing glial cells to generate neurons may prove to be a strategy to replace neurons that have degenerated. Here, we describe a robust protocol for the efficient in vitro conversion of postnatal astroglia from the mouse cerebral cortex into functional, synapse-forming neurons. This protocol involves two steps: (i) expansion of astroglial cells (7 d) and (ii) astroglia-to-neuron conversion induced by persistent and strong retroviral expression of Neurog2 (encoding neurogenin-2) or Mash1 (also referred to as achaete-scute complex homolog 1 or Ascl1) and/or distal-less homeobox 2 (Dlx2) for generation of glutamatergic or GABAergic neurons, respectively (7-21 d for different degrees of maturity). Our protocol of astroglia-to-neuron conversion by a single neurogenic transcription factor provides a stringent experimental system to study the specification of a selective neuronal subtype, thus offering an alternative to the use of embryonic or neural stem cells. Moreover, it can be a useful model for studies of lineage conversion from non-neuronal cells, with potential for brain regenerative medicine.

Comparative analysis of the retinal potential of embryonic stem cells and amniotic fluid-derived stem cells

Photoreceptors have recently been generated from mouse and human embryonic stem cells (ESCs), although ethics concerns impede their utilization for cell replacement therapy for retinal disease. Extra-embryonic tissues have received attention as alternative therapeutic sources of stem cells. Human and mouse amniotic fluid-derived stem cells (AFCs) have been reported to be multipotent and express embryonic and adult stem cell markers. Here, in vitro conditions that generate retinal cells from ESCs were used to analyze and compare the retinal potential of murine AFCs and ESCs. We show that AFCs express pluripotency markers (Nanog, Sox2, and Oct3/4) as well as retinal transcription factor genes (Et, Lhx2, Tll1, Six6, Otx2, Pax6, and Fgf15). AFCs from amniotic fluid of Fgf15.gfp, Nrl.gfp, and Crx.gfp embryos cultured in retinal proliferation and differentiation conditions failed to switch on these retinal transgenes. AFCs cultured in retinal-promoting conditions, effective on ESCs, showed reduced expression of retinal markers. Retinal co-cultures activated retinal genes in ESCs but not in AFCs, and migration assays in retinal explants showed limited migration of AFCs compared with ESCs. Unlike ESCs, AFCs do not express the early embryonic ectodermal gene Utf1 and Western analysis of AFCs identified only the B isoform of Oct3/4, rather than the isoform A present in ESCs. We conclude that AFCs have restricted potential and differ considerably from ESCs and retinal progenitor cells. Reprogramming to induce pluripotency or new differentiation protocols will be required to confer retinal potential to AFCs as expression of a subset of pluripotency and retinal markers is not sufficient.

MATH5 controls the acquisition of multiple retinal cell fates

Math5-null mutation results in the loss of retinal ganglion cells (RGCs) and in a concurrent increase of amacrine and cone cells. However, it remains unclear whether there is a cell fate switch of Math5-lineage cells in the absence of Math5 and whether MATH5 cell-autonomously regulates the differentiation of the above retinal neurons. Here, we performed a lineage analysis of Math5-expressing cells in developing mouse retinas using a conditional GFP reporter (Z/EG) activated by a Math5-Cre knock-in allele. We show that during normal retinogenesis, Math5-lineage cells mostly develop into RGCs, horizontal cells, cone photoreceptors, rod photoreceptors, and amacrine cells. Interestingly, amacrine cells of Math5-lineage cells are predominately of GABAergic, cholinergic, and A2 subtypes, indicating that Math5 plays a role in amacrine subtype specification. In the absence of Math5, more Math5-lineage cells undergo cell fate conversion from RGCs to the above retinal cell subtypes, and occasionally to cone-bipolar cells and Müller cells. This change in cell fate choices is accompanied by an up-regulation of NEUROD1, RXRγ and BHLHB5, the transcription factors essential for the differentiation of retinal cells other than RGCs. Additionally, loss of Math5 causes the failure of early progenitors to exit cell cycle and leads to a significant increase of Math5-lineage cells remaining in cell cycle. Collectively, these data suggest that Math5 regulates the generation of multiple retinal cell types via different mechanisms during retinogenesis.

Overlapping spatiotemporal patterns of regulatory gene expression are required for neuronal progenitors to specify retinal ganglion cell fate

Retinal progenitor cells (RPCs) are programmed early in development to acquire the competence for specifying the seven retinal cell types. Acquiring competence is a complex spatiotemporal process that is still only vaguely understood. Here, our objective was to more fully understand the mechanisms by which RPCs become competent for specifying a retinal ganglion cell (RGC) fate. RGCs are the first retinal cell type to differentiate and their abnormal development leads to apoptosis and optic nerve degeneration. Previous work demonstrated that the paired domain factor Pax6 and the bHLH factor Atoh7 are required for RPCs to specify RGCs. RGC commitment is marked by the expression of the Pou domain factor Pou4f2 and the Lim domain factor Isl1. We show that three RPC subpopulations can specify RGCs: Atoh7-expressing RPCs, Neurod1-expressing RPCs, and Atoh7-Neurod1-expressing RPCs. All three RPC subpopulations were highly interspersed throughout retinal development, although each subpopulation maintained a distinct temporal pattern. Most, but not all, RPCs from each subpopulation were postmitotic. Atoh7-Neurod1 double knockout mice were generated and double-mutant retinas revealed an unexpected role for Neurod1 in specifying RGC fate. We conclude that RPCs have a complex regulatory gene expression program in which they acquire competence using highly integrated mechanisms.

Loss of LMO4 in the retina leads to reduction of GABAergic amacrine cells and functional deficits

BACKGROUND

LMO4 is a transcription cofactor expressed during retinal development and in amacrine neurons at birth. A previous study in zebrafish reported that morpholino RNA ablation of one of two related genes, LMO4b, increases the size of eyes in embryos. However, the significance of LMO4 in mammalian eye development and function remained unknown since LMO4 null mice die prior to birth.

METHODOLOGY/PRINCIPAL FINDINGS

We observed the presence of a smaller eye and/or coloboma in ∼40% LMO4 null mouse embryos. To investigate the postnatal role of LMO4 in retinal development and function, LMO4 was conditionally ablated in retinal progenitor cells using the Pax6 alpha-enhancer Cre/LMO4flox mice. We found that these mice have fewer Bhlhb5-positive GABAergic amacrine and OFF-cone bipolar cells. The deficit appears to affect the postnatal wave of Bhlhb5+ neurons, suggesting a temporal requirement for LMO4 in retinal neuron development. In contrast, cholinergic and dopaminergic amacrine, rod bipolar and photoreceptor cell numbers were not affected. The selective reduction in these interneurons was accompanied by a functional deficit revealed by electroretinography, with reduced amplitude of b-waves, indicating deficits in the inner nuclear layer of the retina.

CONCLUSIONS/SIGNIFICANCE

Inhibitory GABAergic interneurons play a critical function in controlling retinal image processing, and are important for neural networks in the central nervous system. Our finding of an essential postnatal function of LMO4 in the differentiation of Bhlhb5-expressing inhibitory interneurons in the retina may be a general mechanism whereby LMO4 controls the production of inhibitory interneurons in the nervous system.

Adult ciliary epithelial cells, previously identified as retinal stem cells with potential for retinal repair, fail to differentiate into new rod photoreceptors

The ciliary margin in lower vertebrates is a site of continual retinal neurogenesis and a stem cell niche. By contrast, the human eye ceases retinal neuron production before birth and loss of photoreceptors during life is permanent and a major cause of blindness. The discovery of a proliferative cell population in the ciliary epithelium (CE) of the adult mammalian eye, designated retinal stem cells, raised the possibility that these cells could help to restore sight by replacing lost photoreceptors. We previously demonstrated the feasibility of photoreceptor transplantation using cells from the developing retina. CE cells could provide a renewable source of photoreceptors for transplantation. Several laboratories reported that these cells generate new photoreceptors, whereas a recent report questioned the existence of retinal stem cells. We used Nrl.gfp transgenic mice that express green fluorescent protein in rod photoreceptors to assess definitively the ability of CE cells to generate new photoreceptors. We report that CE cells expanded in monolayer cultures, lose pigmentation, and express a subset of eye field and retinal progenitor cell markers. Simultaneously, they continue to express some markers characteristic of differentiated CE and typically lack a neuronal morphology. Previously reported photoreceptor differentiation conditions used for CE cells, as well as conditions used to differentiate embryonic retinal progenitor cells (RPCs) and embryonic stem cell-derived RPCs, do not effectively activate the Nrl-regulated photoreceptor differentiation program. Therefore, we conclude that CE cells lack potential for photoreceptor differentiation and would require reprogramming to be useful as a source of new photoreceptors.

Maximizing functional photoreceptor differentiation from adult human retinal stem cells

Retinal stem cells (RSCs) are present in the ciliary margin of the adult human eye and can give rise to all retinal cell types. Here we show that modulation of retinal transcription factor gene expression in human RSCs greatly enriches photoreceptor progeny, and that strong enrichment was obtained with the combined transduction of OTX2 and CRX together with the modulation of CHX10. When these genetically modified human RSC progeny are transplanted into mouse eyes, their retinal integration and differentiation is superior to unmodified RSC progeny. Moreover, electrophysiologic and behavioral tests show that these transplanted cells promote functional recovery in transducin mutant mice. This study suggests that gene modulation in human RSCs may provide a source of photoreceptor cells for the treatment of photoreceptor disease.

Directing astroglia from the cerebral cortex into subtype specific functional neurons

Forced expression of single defined transcription factors can selectively and stably convert cultured astroglia into synapse-forming excitatory and inhibitory neurons.

Astroglia from the postnatal cerebral cortex can be reprogrammed in vitro to generate neurons following forced expression of neurogenic transcription factors, thus opening new avenues towards a potential use of endogenous astroglia for brain repair. However, in previous attempts astroglia-derived neurons failed to establish functional synapses, a severe limitation towards functional neurogenesis. It remained therefore also unknown whether neurons derived from reprogrammed astroglia could be directed towards distinct neuronal subtype identities by selective expression of distinct neurogenic fate determinants. Here we show that strong and persistent expression of neurogenic fate determinants driven by silencing-resistant retroviral vectors instructs astroglia from the postnatal cortex in vitro to mature into fully functional, synapse-forming neurons. Importantly, the neurotransmitter fate choice of astroglia-derived neurons can be controlled by selective expression of distinct neurogenic transcription factors: forced expression of the dorsal telencephalic fate determinant neurogenin-2 (Neurog2) directs cortical astroglia to generate synapse-forming glutamatergic neurons; in contrast, the ventral telencephalic fate determinant Dlx2 induces a GABAergic identity, although the overall efficiency of Dlx2-mediated neuronal reprogramming is much lower compared to Neurog2, suggesting that cortical astroglia possess a higher competence to respond to the dorsal telencephalic fate determinant. Interestingly, however, reprogramming of astroglia towards the generation of GABAergic neurons was greatly facilitated when the astroglial cells were first expanded as neurosphere cells prior to transduction with Dlx2. Importantly, this approach of expansion under neurosphere conditions and subsequent reprogramming with distinct neurogenic transcription factors can also be extended to reactive astroglia isolated from the adult injured cerebral cortex, allowing for the selective generation of glutamatergic or GABAergic neurons. These data provide evidence that cortical astroglia can undergo a conversion across cell lineages by forced expression of a single neurogenic transcription factor, stably generating fully differentiated neurons. Moreover, neuronal reprogramming of astroglia is not restricted to postnatal stages but can also be achieved from terminally differentiated astroglia of the adult cerebral cortex following injury-induced reactivation.

The brain consists of two major cell types: neurons, which transmit information, and glial cells, which support and protect neurons. Interestingly, evidence suggests that some glial cells, including astroglia, can be directly converted into neurons by specific proteins, a transformation that may aid in the functional repair of damaged brain tissue. However, in order for the repaired brain areas to function properly, it is important that astroglia be directed into appropriate neuronal subclasses. In this study, we show that non-neurogenic astroglia from the cerebral cortex can be reprogrammed in vitro using just a single transcription factor to yield fully functional excitatory or inhibitory neurons. We achieved this result through forced expression of the same transcription factors that instruct the genesis of these distinct neuronal subtypes during embryonic forebrain development. Moreover we demonstrate that reactive astroglia isolated from the adult cortex after local injury can be reprogrammed into synapse-forming excitatory or inhibitory neurons following a similar strategy. Our findings provide evidence that endogenous glial cells may prove a promising strategy for replacing neurons that have degenerated due to trauma or disease.

Blimp1 controls photoreceptor versus bipolar cell fate choice during retinal development

Photoreceptors, rods and cones are the most abundant cell type in the mammalian retina. However, the molecules that control their development are not fully understood. In studies of photoreceptor fate determination, we found that Blimp1 (Prdm1) is expressed transiently in developing photoreceptors. We analyzed the function of Blimp1 in the mouse retina using a conditional deletion approach. Developmental analysis of mutants showed that Otx2(+) photoreceptor precursors ectopically express the bipolar cell markers Chx10 (Vsx2) and Vsx1, adopting bipolar instead of photoreceptor fate. However, this fate shift did not occur until the time when bipolar cells are normally specified during development. Most of the excess bipolar cells died around the time of bipolar cell maturation. Our results suggest that Blimp1 expression stabilizes immature photoreceptors by preventing bipolar cell induction. We conclude that Blimp1 regulates the decision between photoreceptor and bipolar cell fates in the Otx2(+) cell population during retinal development.

Neurog2 controls the leading edge of neurogenesis in the mammalian retina

In the mammalian retina, neuronal differentiation begins in the dorso-central optic cup and sweeps peripherally and ventrally. While certain extrinsic factors have been implicated, little is known about the intrinsic factors that direct this process. In this study, we evaluate the expression and function of proneural bHLH transcription factors during the onset of mouse retinal neurogenesis. Dorso-central retinal progenitor cells that give rise to the first postmitotic neurons express Neurog2/Ngn2 and Atoh7/Math5. In the absence of Neurog2, the spread of neurogenesis stalls, along with Atoh7 expression and RGC differentiation. However, neurogenesis is eventually restored, and at birth Neurog2 mutant retinas are reduced in size, with only a slight increase in the retinal ganglion cell population. We find that the re-establishment of neurogenesis coincides with the onset of Ascl1 expression, and that Ascl1 can rescue the early arrest of neural development in the absence of Neurog2. Together, this study supports the hypothesis that the intrinsic factors Neurog2 and Ascl1 regulate the temporal progression of retinal neurogenesis by directing overlapping waves of neuron formation.

Dnmt3 and G9a cooperate for tissue-specific development in zebrafish

Although DNA methylation is critical for proper embryonic and tissue-specific development, how different DNA methyltransferases affect tissue-specific development and their targets remains unknown. We address this issue in zebrafish through antisense-based morpholino knockdown of Dnmt3 and Dnmt1. Our data reveal that Dnmt3 is required for proper neurogenesis, and its absence results in profound defects in brain and retina. Interestingly, other organs such as intestine remain unaffected suggesting tissue-specific requirements of Dnmt3. Further, comparison of Dnmt1 knockdown phenotypes with those of Dnmt3 suggested that these two families have distinct functions. Consistent with this idea, Dnmt1 failed to complement Dnmt3 deficiency, and Dnmt3 failed to complement Dnmt1 deficiency. Downstream of Dnmt3 we identify a neurogenesis regulator, lef1, as a Dnmt3-specific target gene that is demethylated and up-regulated in dnmt3 morphants. Knockdown of lef1 rescued neurogenesis defects resulting from Dnmt3 absence. Mechanistically, we show cooperation between Dnmt3 and an H3K9 methyltransferase G9a in regulating lef1. Further, like Dnmt1-Suv39h1 cooperativity, Dnmt3 and G9a seemed to function together for tissue-specific development. G9a knockdown, but not Suv39h1 loss, phenocopied dnmt3 morphants and G9a overexpression provided a striking rescue of dnmt3 morphant phenotypes, whereas Suv39h1 overexpression failed, supporting the notion of specific DNMT-histone methyltransferase networks. Consistent with this model, H3K9me3 levels on the lef1 promoter were reduced in both dnmt3 and g9a morphants, and its knockdown rescued neurogenesis defects in g9a morphants. We propose a model wherein specific DNMT-histone methyltransferase networks are utilized to silence critical regulators of cell fate in a tissue-specific manner.

Acheate-scute like 1 (Ascl1) is required for normal delta-like (Dll) gene expression and notch signaling during retinal development

Delta gene expression in Drosophila is regulated by proneural basic helix-loop-helix (bHLH) transcription factors, such as acheate-scute. In vertebrates, multiple Delta-like and proneural bHLH genes are expressed during neurogenesis, especially in the retina. We recently uncovered a relationship between Acheate-scute like 1 (Ascl1), Delta-like genes, and Notch in chick retinal progenitors. Here, we report that mammalian retinal progenitors are also the primary source of Delta-like genes, likely signaling through Notch among themselves, while differentiating neurons expressed Jagged2. Ascl1 is coexpressed in Delta-like and Notch active progenitors, and required for normal Delta-like gene expression and Notch signaling. We also reveal a role for Ascl1 in the regulation of Hes6, a proneurogenic factor that inhibits Notch signaling to promote neural rather than glial differentiation. Thus, these results suggest a molecular mechanism whereby attenuated Notch levels coupled with reduced proneurogenic activity in progenitors leads to increased gliogenesis and decreased neurogenesis in the Ascl1-deficient retina.

The homeobox gene CHX10/VSX2 regulates RdCVF promoter activity in the inner retina

Rod-derived Cone Viability Factor (RdCVF) is a trophic factor with therapeutic potential for the treatment of retinitis pigmentosa, a retinal disease that commonly results in blindness. RdCVF is encoded by Nucleoredoxin-like 1 (Nxnl1), a gene homologous with the family of thioredoxins that participate in the defense against oxidative stress. RdCVF expression is lost after rod degeneration in the first phase of retinitis pigmentosa, and this loss has been implicated in the more clinically significant secondary cone degeneration that often occurs. Here, we describe a study of the Nxnl1 promoter using an approach that combines promoter and transcriptomic analysis. By transfection of selected candidate transcription factors, chosen based upon their expression pattern, we identified the homeodomain proteins CHX10/VSX2, VSX1 and PAX4, as well as the zinc finger protein SP3, as factors that can stimulate both the mouse and human Nxnl1 promoter. In addition, CHX10/VSX2 binds to the Nxnl1 promoter in vivo. Since CHX10/VSX2 is expressed predominantly in the inner retina, this finding motivated us to demonstrate that RdCVF is expressed in the inner as well as the outer retina. Interestingly, the loss of rods in the rd1 mouse, a model of retinitis pigmentosa, is associated with decreased expression of RdCVF by inner retinal cells as well as by rods. Based upon these results, we propose an alternative therapeutic strategy aimed at recapitulating RdCVF expression in the inner retina, where cell loss is not significant, to prevent secondary cone death and central vision loss in patients suffering from retinitis pigmentosa.

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.

Proneural gene ash1 promotes amacrine cell production in the chick retina

The diverse types of neurons and Müller glia in the vertebrate retina are believed to arise from common progenitor cells. To better understand how neural diversity is achieved during retinal neurogenesis, we examined the function of ash1, a proneural bHLH gene expressed in progenitor cells throughout retinal neurogenesis. Published studies using retinal explant culture derived from knockout mice concluded that ash1 is required for the production of late-born neurons, including bipolar cells. In this study, gain-of-function experiments were carried out in ovo in embryonic chick retina. In the developing chick retina, expression of ash1 temporally overlapped with, but spatially differed from, the expression of ngn2, also a proneural gene expressed in progenitor cells throughout retinal neurogenesis. Retrovirus-driven overexpression of ash1 in the developing chick retina decreased the progenitor population (BrdU+ or expressing ngn2), expanded the amacrine population (AP2alpha+ or Pax6+), and reduced bipolar (chx10 mRNA+) and Müller glial (vimentin+) populations. Photoreceptor deficiency occurred after the completion of neurogenesis. The number of ganglion cells, which are born first during retinal neurogenesis, remained unchanged. Similar overexpression of ngn2 did not produce discernible changes in retinal neurogenesis, nor in ash1 expression. These results suggest that ash1 promotes the production of amacrine cells and thus may participate in a regulatory network governing neural diversity in the chick retina.

Vsx2 in the zebrafish retina: restricted lineages through derepression

BACKGROUND

The neurons in the vertebrate retina arise from multipotent retinal progenitor cells (RPCs). It is not clear, however, which progenitors are multipotent or why they are multipotent.

RESULTS

In this study we show that the homeodomain transcription factor Vsx2 is initially expressed throughout the retinal epithelium, but later it is downregulated in all but a minor population of bipolar cells and all Müller glia. The Vsx2-negative daughters of Vsx2-positive RPCs divide and give rise to all other cell types in the retina. Vsx2 is a repressor whose targets include transcription factors such as Vsx1, which is expressed in the progenitors of distinct non-Vsx2 bipolars, and the basic helix-loop-helix transcription factor Ath5, which restricts the fate of progenitors to retinal ganglion cells, horizontal cells, amacrine cells and photoreceptors fates. Foxn4, expressed in the progenitors of amacrine and horizontal cells, is also negatively regulated by Vsx2.

CONCLUSION

Our data thus suggest Vsx2-positive RPCs are fully multipotent retinal progenitors and that when Vsx2 is downregulated, Vsx2-negative progenitors escape Vsx2 repression and so are able to express factors that restrict lineage potential.

NeuroD regulates proliferation of photoreceptor progenitors in the retina of the zebrafish

neuroD is a member of the family of proneural genes, which function to regulate the cell cycle, cell fate determination and cellular differentiation. In the retinas of larval and adult teleosts, neuroD is expressed in two populations of post-mitotic cells, a subset of amacrine cells and nascent cone photoreceptors, and proliferating cells in the lineages that give rise exclusively to rod and cone photoreceptors. Based on previous studies of NeuroD function in vitro and the cellular pattern of neuroD expression in the zebrafish retina, we hypothesized that within the mitotic photoreceptor lineages NeuroD selectively regulates aspects of the cell cycle. To test this hypothesis, gain and loss-of-function approaches were employed, relying on the inducible expression of a NeuroD(EGFP) fusion protein and morpholino oligonucleotides to inhibit protein translation, respectively. Conditional expression of neuroD causes cells to withdraw from the cell cycle, upregulate the expression of the cell cycle inhibitors, p27 and p57, and downregulate the cell cycle progression factors, Cyclin B1, Cyclin D1, and Cyclin E2. In the absence of NeuroD, cells specific for the rod and cone photoreceptor lineage fail to exit the cell cycle, and the number of cells expressing Cyclin D1 is increased. When expression is ectopically induced in multipotent progenitors, neuroD promotes the genesis of rod photoreceptors and inhibits the genesis of Müller glia. These data show that in the teleost retina NeuroD plays a fundamental role in photoreceptor genesis by regulating mechanisms that promote rod and cone progenitors to withdraw from the cell cycle. This is the first in vivo demonstration in the retina of cell cycle regulation by NeuroD.

NeuroD1 and Mash1 temporally regulate GnRH receptor gene expression in immortalized mouse gonadotrope cells

Accurate spatial and temporal expression of gonadotrope-specific genes, such as the gonadotropin-releasing hormone receptor (GnRHR) gene, is critical for gonadotrope maturation. Herein, we show that a specific E-box in the mouse GnRHR promoter binds two group A basic-helix-loop-helix (bHLH) transcription factors. Mutation of this E-box decreases expression in mouse gonadotrope-derived alphaT3-1 and LbetaT2 cell lines. Microarray and western blots show that the bHLH transcription factor NeuroD1 is strongly expressed in the gonadotrope progenitor, alphaT3-1, whereas Mash1 is strongly expressed in the more mature gonadotrope, LbetaT2. Over-expression of NeuroD1 or Mash1 increases expression of the GnRHR gene or a multimer of the E-box and this increase is lost upon mutation of the E-box. Electrophoretic mobility shift assays reveal that the GnRHR E-box binds NeuroD1 from alphaT3-1 cells, but binds Mash1 from LbetaT2 cells. The sequential binding of different members of the group A bHLH transcription factor family to mouse GnRHR E-box 3 as the gonadotrope differentiates may represent a mechanism necessary for proper spatial and temporal expression of the GnRHR during gonadotrope development.

Neurogenin3 promotes early retinal neurogenesis

The transcriptional regulatory network governing the establishment of retinal neuron diversity is not well delineated. We report experimental results suggesting proneural gene neurogenin3 (ngn3) participating in this regulatory network. Retinal expression of chick ngn3 was confined to early neurogenesis. Overexpression of ngn3 in chick retina reduced cell proliferation and expanded the population of ganglion cells into the territory normally occupied by amacrine cells. Ngn3 overexpression altered the expression of a number of regulatory genes, including ash1, ath3, ath5, chx10, neuroD, ngn1, ngn2, and NSCL1. Early gene ngn1 was induced, but ash1, ngn2, ath3, and chx10, whose expressions persist through later phases of neurogenesis, were down-regulated. Expression of ath5 was up-regulated at the locale corresponding to young ganglion cells, but was down-regulated at the locale corresponding to progenitor cells. These results suggest that ngn3 regulates retinal neurogenesis by inducing regulatory genes for early-born neurons and repressing those for later-born cells.

Sox9 is expressed in mouse multipotent retinal progenitor cells and functions in Müller glial cell development

It is widely accepted that the process of retinal cell fate determination is under tight transcriptional control mediated by a combinatorial code of transcription factors. However, the exact repertoire of factors necessary for the genesis of each retinal cell type remains to be fully defined. Here we show that the HMG-box transcription factor, Sox9, is expressed in multipotent mouse retinal progenitor cells throughout retinogenesis. We also find that Sox9 is downregulated in differentiating neuronal populations, yet expression in Müller glial cells persists into adulthood. Furthermore, by employing a conditional knockout approach, we show that Sox9 is essential for the differentiation and/or survival of postnatal Müller glial cells.

Vsx2/Chx10 ensures the correct timing and magnitude of Hedgehog signaling in the mouse retina

Vertebrate retinal progenitor cells (RPCs) undergo a robust proliferative expansion to produce enough cells for the retina to form appropriately. Vsx2 (formerly Chx10), a homeodomain protein expressed in RPCs, is required for sufficient proliferation to occur. Sonic Hedgehog protein (SHH), secreted by retinal ganglion cells (RGCs), activates Hedgehog (Hh) signaling in RPCs and is also required for sufficient proliferation to occur. Therefore, we sought to determine if reduced Hh signaling is a contributing factor to the proliferation changes that occur in the absence of Vsx2. To do this, we examined Shh expression and Hh signaling activity in the homozygous ocular retardation J (orJ) mouse, which harbors a recessive null allele in the Vsx2 gene. We found that Shh expression and Hh signaling activity are delayed during early retinal development in orJ mice and this correlates with a delay in the onset of RGC differentiation. At birth, reduced expression of genes regulated by Hh signaling was observed despite the production of SHH ligand. orJ RPCs respond to pre-processed recombinant SHH ligand (SHH-N) in explant culture as evidenced by increased proliferation and expression of Hh target genes. Interestingly, proliferation in the orJ retina is further inhibited by cyclopamine, an antagonist of Hh signaling. Our results suggest that reduced Hh signaling contributes to the reduced level of RPC proliferation in the orJ retina, thereby revealing a role for Vsx2 in mediating mitogen signaling.

Regulation of retinal cell fate specification by multiple transcription factors

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

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.

Expression of the LIM-homeodomain protein Isl1 in the developing and mature mouse retina

The mammalian retina is comprised of six major neuronal cell types and is subdivided into more morphological and physiological subtypes. The transcriptional machinery underlying these subtype fate choices is largely unknown. The LIM-homeodomain protein, Isl1, plays an essential role in central nervous system (CNS) differentiation but its relationship to retinal neurogenesis remains unknown. We report here its dynamic spatiotemporal expression in the mouse retina. Among bipolar interneurons, Isl1 expression commences at postnatal day (P)5 and is later restricted to ON-bipolar cells. The intensity of Isl1 expression is found to segregate the pool of ON-bipolar cells into rod and ON-cone bipolar cells with higher expression in rod bipolar cells. As bipolar cell development proceeds from P5-10 the colocalization of Isl1 and the pan-bipolar cell marker Chx10 reveals the organization of ON-center bipolar cell nuclei to the upper portion of the inner nuclear layer. Further, whereas Isl1 is predominantly a ganglion cell marker prior to embryonic day (E)15.5, at E15.5 and later its expression in nonganglion cells expands. We demonstrate that these Isl1-positive, nonganglion cells acquire the expression of amacrine cell markers embryonically, likely representing nascent cholinergic amacrine cells. Taken together, Isl1 is expressed during the maturation of and is later maintained in retinal ganglion cells and subtypes of amacrine and bipolar cells where it may function in the maintenance of these cells into adulthood.

Directing neurotransmitter identity of neurones derived from expanded adult neural stem cells

In-vitro expanded neural stem cells (NSCs) of the adult subependymal zone (SEZ) may serve as a source for replacing degenerating neurones in disease and trauma. Crucial for the viability of this approach is the ability to selectively generate specific types of neurones from these cells. Here we show that NSCs derived from the adult mouse SEZ and expanded in vitro as neurosphere cells lose their in-vivo specification and generate a mixture of progeny comprising both GABAergic and also, surprisingly, glutamatergic neurones. When forced to express the pro-neural transcription factor neurogenin 2, virtually all progeny of in-vitro expanded adult NSCs acquire a glutamatergic identity, whereas only GABAergic neurones are generated upon expression of the transcription factor Mash1. Respecification of expanded NSCs from the adult SEZ by neurogenin 2 was accompanied by upregulation of the T-box transcription factor Tbr1, suggesting that their progeny had acquired a dorsal telencephalic identity. Thus, in-vitro expanded adult NSCs have the competence to become directed towards distinct functional neurotransmitter phenotypes when the appropriate transcriptional cues are provided.

A median third eye: pineal gland retraces evolution of vertebrate photoreceptive organs

In many vertebrates, the pineal gland serves as a photoreceptive neuroendocrine organ. Morphological and functional similarities between the pineal and retinal photoreceptor cells indicate their close evolutionary relationship, and hence the comparative studies on the pineal gland and the retina are the keys to deciphering the evolutionary traces of the vertebrate photoreceptive organs. Several studies have suggested common genetic and molecular mechanisms responsible for their similarities, but largely unknown are those underlying pineal-specific development and physiological functions. Recent studies have identified several cis-acting DNA elements that participate in transcriptional control of the pineal-specific genes. Genetic approaches in the zebrafish have also contributed to elucidating the genetic network regulating the pineal development and neurogenesis. These efforts toward elucidating the molecular instrumentation intrinsic to the pineal gland, back to back with those to the retina, should lead to a comprehensive understanding of the evolutionary history of the vertebrate photoreceptive structures. This article summarizes the current status of research on these topics.

A specific box switches the cell fate determining activity of XOTX2 and XOTX5b in the Xenopus retina

Background

Otx genes, orthologues of the Drosophila orthodenticle gene (otd), play crucial roles in vertebrate brain development. In the Xenopus eye, Xotx2 and Xotx5b promote bipolar and photoreceptor cell fates, respectively. The molecular basis of their differential action is not completely understood, though the carboxyl termini of the two proteins seem to be crucial. To define the molecular domains that make the action of these proteins so different, and to determine whether their retinal abilities are shared by Drosophila OTD, we performed an in vivo molecular dissection of their activity by transfecting retinal progenitors with several wild-type, deletion and chimeric constructs of Xotx2, Xotx5b and otd.

Results

We identified a small 8–10 amino acid divergent region, directly downstream of the homeodomain, that is crucial for the respective activities of XOTX2 and XOTX5b. In lipofection experiments, the exchange of this 'specificity box' completely switches the retinal activity of XOTX5b into that of XOTX2 and vice versa. Moreover, the insertion of this box into Drosophila OTD, which has no effect on retinal cell fate, endows it with the specific activity of either XOTX protein. Significantly, in cell transfection experiments, the diverse ability of XOTX2 and XOTX5b to synergize with NRL, a cofactor essential for vertebrate rod development, to transactivate the rhodopsin promoter is also switched depending on the box. We also show by GST-pull down that XOTX2 and XOTX5b differentially interact with NRL, though this property is not strictly dependent on the box.

Conclusion

Our data provide molecular evidence on how closely related homeodomain gene products can differentiate their functions to regulate distinct cell fates. A small 'specificity box' is both necessary and sufficient to confer on XOTX2 and XOTX5b their distinct activities in the developing frog retina and to convert the neutral orthologous OTD protein of Drosophila into a positive and specific XOTX-like retinal regulator. Relatively little is known of what gives developmental specificity to homeodomain regulators. We propose that this box is a major domain of XOTX proteins that provides them with the appropriate developmental specificity in retinal histogenesis.

Zac1 functions through TGFbetaII to negatively regulate cell number in the developing retina

Background

Organs are programmed to acquire a particular size during development, but the regulatory mechanisms that dictate when dividing progenitor cells should permanently exit the cell cycle and stop producing additional daughter cells are poorly understood. In differentiated tissues, tumor suppressor genes maintain a constant cell number and intact tissue architecture by controlling proliferation, apoptosis and cell dispersal. Here we report a similar role for two tumor suppressor genes, the Zac1 zinc finger transcription factor and that encoding the cytokine TGFβII, in the developing retina.

Results

Using loss and gain-of-function approaches, we show that Zac1 is an essential negative regulator of retinal size. Zac1 mutants develop hypercellular retinae due to increased progenitor cell proliferation and reduced apoptosis at late developmental stages. Consequently, supernumerary rod photoreceptors and amacrine cells are generated, the latter of which form an ectopic cellular layer, while other retinal cells are present in their normal number and location. Strikingly, Zac1 functions as a direct negative regulator of a rod fate, while acting cell non-autonomously to modulate amacrine cell number. We implicate TGFβII, another tumor suppressor and cytokine, as a Zac1-dependent amacrine cell negative feedback signal. TGFβII and phospho-Smad2/3, its downstream effector, are expressed at reduced levels in Zac1 mutant retinae, and exogenous TGFβII relieves the mutant amacrine cell phenotype. Moreover, treatment of wild-type retinae with a soluble TGFβ inhibitor and TGFβ receptor II (TGFβRII) conditional mutants generate excess amacrine cells, phenocopying the Zac1 mutant phenotype.

Conclusion

We show here that Zac1 has an essential role in cell number control during retinal development, akin to its role in tumor surveillance in mature tissues. Furthermore, we demonstrate that Zac1 employs a novel cell non-autonomous strategy to regulate amacrine cell number, acting in cooperation with a second tumor suppressor gene, TGFβII, through a negative feedback pathway. This raises the intriguing possibility that tumorigenicity may also be associated with the loss of feedback inhibition in mature tissues.

Mouse Rab11-FIP4 regulates proliferation and differentiation of retinal progenitors in a Rab11-independent manner

We identified Rab11-family interacting protein 4 (Rab11-FIP4) as a gene strongly expressed in the developing mouse retina. The major transcript encoding a full-length protein, mRab11-FIP4A, was expressed predominantly in neural tissues; whereas an alternative transcript encoding an N-terminally truncated form of the protein, mRab11-FIP4B, was expressed ubiquitously as a minor form. Gain-of-function of mRab11-FIP4A in retina promoted cell cycle exit and increased subpopulations of retinal cells localized in the inner nuclear layer, such as bipolar cells and Müller glia. Reversal of the phenotype was observed in the loss-of-function experiment. Furthermore, Shh signaling was suggested to be involved in these functions. Analysis using truncation mutants revealed the essential role of the N-terminal region containing a conserved EF-hand motif for the retinal phenotypes induced by the expression of mRab11-FIP4A, whereas binding to Rab11 was dispensable, suggesting the involvement of a novel Rab11-independent mechanism for mRab11-FIP4A action in the regulation of retinal development.

Dynamic expression of the basic helix-loop-helix transcription factor neuroD in the rod and cone photoreceptor lineages in the retina of the embryonic and larval zebrafish

NeuroD is a basic helix-loop-helix (bHLH) transcription factor critical for determining neuronal cell fate and regulating withdrawal from the cell cycle. We showed previously that, in goldfish, neuroD is expressed in the rod photoreceptor lineage, and we inferred that neuroD is also expressed in a subset of amacrine cells and nascent cone photoreceptors. Here we extended that study by examining the temporal and spatial expression pattern of neuroD in the embryonic and larval zebrafish and by identifying the cell types that express this gene. NeuroD expression in the developing zebrafish retina is dynamic, spanning early retinogenesis and the maturation of cone photoreceptors. In early retinogenesis neuroD expression expands from a small patch in the ventronasal retina, through the remaining retinal neuroepithelium. As retinogenesis progresses, neuroD expression becomes restricted to amacrine cells, immature cones, and cells of rod and cone lineages. This expression achieves an adult pattern by 96 hours postfertilization (hpf), whereupon the temporal pattern of neuroD expression in central retina is spatially recapitulated at the germinative margin. The cellular pattern of expression suggests that neuroD regulates aspects of rod and cone genesis, but through separate cellular lineages. Furthermore, neuroD is coexpressed with the cone-rod-homeobox transcription factor (Crx) in putative cone progenitors and nascent cone photoreceptors, suggesting that, in the zebrafish retina, as in other vertebrate retinas, similar genetic cascades regulate photoreceptor genesis and maturation.

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

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

How to make a mesodiencephalic dopaminergic neuron

Dopaminergic neurons located in the ventral mesodiencephalon are essential for the control of voluntary movement and the regulation of emotion, and are severely affected in neurodegenerative diseases such as Parkinson's disease. Recent advances in molecular biology and mouse genetics have helped to unravel the mechanisms involved in the development of mesodiencephalic dopaminergic (mdDA) neurons, including their specification, migration and differentiation, as well as the processes that govern axonal pathfinding and their specific patterns of connectivity and maintenance. Here, we follow the developmental path of these neurons with the goal of generating a molecular code that could be exploited in cell-replacement strategies to treat diseases such as Parkinson's disease.

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.

Effects of follistatin overexpression on cell differentiation in the chick embryo retina

Although activin is expressed in the embryonic central nervous system (CNS), its possible functions in the regulation of CNS neuronal differentiation remain largely unknown. We have investigated this question in the retina, a well-characterized CNS structure previously shown to respond to activin in vitro, and to express activin subunits and receptors in vivo. RCAS retroviruses were used to overexpress in the chick retina in ovo either follistatin (FS), an activin-binding protein and inhibitor, or alkaline phosphatase (AP), as control. FS-treated retinas appeared normal until ED 8, when they showed a reduction of the inner plexiform layer, accompanied by a marked decrease in the frequency of amacrine cells. The territory lacking amacrine cells showed downregulation of transcription factors necessary for amacrine cell differentiation, such as Pax6 and AP2alpha, accompanied by ectopic expression of transcription factors associated with the development of horizontal or bipolar neurons, such as Prox1, Chx10 and NeuroM. Increases in cell death were also observed in FS-treated retinas. Taken together with previous in vitro studies, our results suggest that activin is a powerful regulator of neuronal differentiation in the central nervous system.

Hes1 and Hes5 regulate the development of the cranial and spinal nerve systems

The basic helix-loop-helix genes Hes1 and Hes5, known Notch effectors, regulate the maintenance of neural stem cells and the development of the central nervous system (CNS). In the absence of Hes1 and Hes5, the size, shape and cytoarchitecture of the CNS are severely disorganized, but the development of the peripheral nervous system remains to be analyzed. Here, we found that in Hes1;Hes5 double-mutant mice, the cranial and spinal nerve systems are also severely disorganized. In these mutant mice, axonal projections from the mesencephalic neurons to the trigeminal (V) ganglion become aberrant and the proximal parts of the glossopharyngeal (IX) and vagus (X) nerves are fused. The hypoglossal (XII) nerve is also formed poorly. Furthermore, the dorsal root ganglia are fused with the spinal cord, and the dorsal and ventral roots of the spinal nerves are lacking in many segments. These results indicate that Hes1 and Hes5 play an important role in the formation of the cranial and spinal nerve systems.

Expression of the basic helix-loop-factor Olig2 in the developing retina: Olig2 as a new marker for retinal progenitors and late-born cells

In this study, we examined the spatiotemporal expression patterns of Olig2, a basic helix-loop-helix transcription factor, in the developing mouse retina. Expression of Olig2 was initially detected on embryonic day 12.5 (E12.5). The majority of Olig2-positive cells were identified as retinal progenitor cells throughout embryogenesis. During later embryonic stages, the number of Olig2-positive retinal progenitor cells increased, and Olig2-positive cells were confined only to the neuroblast layer (NBL). Olig2 expression was not observed in the ganglion cell layer (GCL) nor in the inner nuclear layer (INL) that contain the differentiated retinal cell types, indicating that Olig2 is not expressed in differentiated cells in prenatal retina. In later postnatal stages, Olig2 expression was retained in mature neurons and glial cells, namely retinal ganglion cells (RGCs), amacrine cells (ACs), horizontal cells, bipolar cells and Müller glial cells. Thus, Olig2 is an marker both for retinal progenitor cells during embryonic stages, and also for differentiated retinal subpopulations within the GCL and INL during postnatal stages.

Identification of shared transcriptional targets for the proneural bHLH factors Xath5 and XNeuroD

Proneural basic helix-loop-helix (bHLH) transcription factors are critical positive regulators of neuronal differentiation in a variety of species and are required for proper differentiation of various subtypes of neurons. Although bHLH factors demonstrate some unique functions during neural development, they share the ability to regulate neuronal differentiation, potentially by targeting overlapping sets of genes. To assess this, we performed a screen in ectoderm animal cap tissue to identify direct transcriptional targets shared by two Xenopus ato-related bHLH factors, Xath5 and XNeuroD. Candidate target genes identified in this screen include several transcriptional regulators (Xebf2, Xebf3, XETOR and NKL), an RNA binding protein (elrC), a cell cycle component (Xgadd45gamma) and several novel genes. Overexpression of either Xath5 or XNeuroD induced ectopic in vivo expression of these candidate target genes. Conversely, blocking ato-related bHLH activity prevented endogenous nervous system expression of these genes. Therefore, we have identified a set of genes that can be regulated by multiple ato-related bHLH factors and may function as critical effectors of proneural bHLH-mediated differentiation.