POU domain factor Brn-3b is essential for retinal ganglion cell differentiation and survival but not for initial cell fate specification

Brn-3b enhances the pro-apoptotic effects of p53 but not its induction of cell cycle arrest by cooperating in trans-activation of bax expression

The Brn-3a and Brn-3b transcription factor have opposite and antagonistic effects in neuroblastoma cells since Brn-3a is associated with differentiation whilst Brn-3b enhances proliferation in these cells. In this study, we demonstrate that like Brn-3a, Brn-3b physically interacts with p53. However, whereas Brn-3a repressed p53 mediated Bax expression but cooperated with p53 to increase p21^cip1/waf1, this study demonstrated that co-expression of Brn-3b with p53 increases trans-activation of Bax promoter but not p21^cip1/waf1. Consequently co-expression of Brn-3b with p53 resulted in enhanced apoptosis, which is in contrast to the increased survival and differentiation, when Brn-3a is co-expressed with p53. For Brn-3b to cooperate with p53 on the Bax promoter, it requires binding sites that flank p53 sites on this promoter. Furthermore, neurons from Brn-3b knock-out (KO) mice were resistant to apoptosis and this correlated with reduced Bax expression upon induction of p53 in neurons lacking Brn-3b compared with controls. Thus, the ability of Brn-3b to interact with p53 and modulate Bax expression may demonstrate an important mechanism that helps to determine the fate of cells when p53 is induced.

Modifications of the retina neuronal populations of the heterozygous mutant small eye mouse, the Sey(Dey)

We analyzed the modifications of the retinal neurons in a heterozygous mutant small eye mouse, the Sey(Dey). This mouse presents a mutation in chromosome 2 which affects the gene Pax6 and other nearby genes, such as the Wt1 gene and the gene of the Reticulocalbin. The eyes of these animals do not have lenses and their retinas present important morphological alterations: in the anterior portion they are joined to the cornea, they are found detached from the pigment epithelium, they present folds that form rosettes in some zones and alteration of the lamination can be observed. The partial loss of the genes affected does not prevent the formation of the different layers of the retina, but does affect its thickness, principally of the plexiform layers; moreover, the internal limiting membrane is found disorganized. All the neuronal populations are present in the retina of these animals and express the same neurochemical markers as the control animals, but the number of Pax6(+) cells is notably reduced. In these retinas a marked disorganization of the distribution of the dendrites and axons is observed and a notable reduction in the axons of ganglion cells. These results suggest that, although it does not appear determinant in the differentiation of the distinct neuronal types of the retina, the partial lack of genes of the heterozygotes +/Sey(Dey) provokes important morphological and neurochemical modifications in the cytoarchitecture of the retina.

Requirement for Bhlhb5 in the specification of amacrine and cone bipolar subtypes in mouse retina

The mammalian retina comprises six major neuronal cell types and one glial type that are further classified into multiple subtypes based on their anatomical and functional differences. Nevertheless, how these subtypes arise remains largely unknown at the molecular level. Here, we demonstrate that the expression of Bhlhb5, a bHLH transcription factor of the Olig family, is tightly associated with the generation of selective GABAergic amacrine and Type 2 OFF-cone bipolar subtypes throughout retinogenesis. Targeted deletion of Bhlhb5 results in a significant reduction in the generation of these selective bipolar and amacrine subtypes. Furthermore, although a Bhlhb5-null mutation has no effect on the expression of bHLH-class retinogenic genes, Bhlhb5 expression overlaps with that of the pan-amacrine factor NeuroD and the expression of Bhlhb5 and NeuroD is negatively regulated by ganglion cell-competence factor Math5. Our results reveal that a bHLH transcription factor cascade is involved in regulating retinal cell differentiation and imply that Bhlhb5 functions downstream of retinogenic factors to specify bipolar and amacrine subtypes.

Beta-catenin is essential for lamination but not neurogenesis in mouse retinal development

During vertebrate retinal development, the seven retinal cell types differentiate sequentially from a single population of retinal progenitor cells (RPCs) and organize themselves into a distinct laminar structure. The purpose of this study was to determine whether beta-catenin, which functions both as a nuclear effector for the canonical Wnt signaling pathway and as a regulator of cell adhesion, is required for retinal neurogenesis or lamination. We used the Cre-loxP system to either eliminate beta-catenin or to express a constitutively active form during retinal neurogenesis. Eliminating beta-catenin did not affect cell differentiation, but did result in the loss of the radial arrangement of RPCs and caused abnormal migration of differentiated neurons. As a result, the laminar structure was massively disrupted in beta-catenin-null retinas, although all retinal cell types still formed. In contrast to other neural tissues, eliminating beta-catenin did not significantly reduce the proliferation rate of RPCs; likewise, activating beta-catenin ectopically in RPCs did not result in overproliferation, but loss of neural retinal identity. These results indicate that beta-catenin is essential during retinal neurogenesis as a regulator of cell adhesion but not as a nuclear effector of the canonical Wnt signaling pathway. The results further imply that retinal lamination and retinal cell differentiation are genetically separable processes.

Targeting Brn-3b in breast cancer therapy

The Brn-3b POU domain transcription factor is elevated in a significant proportion of breast cancers and in neuroblastoma tumours, where it is associated with increased proliferation, anchorage-independent growth, faster and larger tumour growth in xenograft models, resistance to growth inhibitory stimuli and increased migratory potential. These effects are associated with the ability of Brn-3b to regulate specific genes associated with these processes. Reducing Brn-3b can reverse many of these effects, suggesting that it may be possible to alter the growth and behaviour of tumour cells by abrogating Brn-3b in these cancers. This review discusses the effect of altering Brn-3b in these cancer cells and possible approaches to targeting Brn-3b as a strategy for therapy in treatment of breast cancers.

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.

Brn3a-expressing retinal ganglion cells project specifically to thalamocortical and collicular visual pathways

Retinal ganglion cells (RGCs) innervate several specific CNS targets serving cortical and subcortical visual pathways and the entrainment of circadian rhythms. Recent studies have shown that retinal ganglion cells express specific combinations of POU- and LIM-domain transcription factors, but how these factors relate to the subsequent development of the retinofugal pathways and the functional identity of RGCs is mostly unknown. Here, we use targeted expression of an genetic axonal tracer, tau/beta-galactosidase, to examine target innervation by retinal ganglion cells expressing the POU-domain factor Brn3a. Brn3a is expressed in RGCs innervating the principal retinothalamic/retinocollicular pathway mediating cortical vision but is not expressed in RGCs of the accessory optic, pretectal, and hypothalamic pathways serving subcortical visuomotor and circadian functions. In the thalamus, Brn3a ganglion cell fibers are primarily restricted to the outer shell of the dorsal lateral geniculate, providing new evidence for the regionalization of this nucleus in rodents. Brn3a RGC axons have a relative preference for the contralateral hemisphere, but known mediators of the laterality of RGC axons are not repatterned in the absence of Brn3a. Brn3a is coexpressed extensively with the closely related factor Brn3b in the embryonic retina, and the effects of the loss of Brn3a in retinal development are not severe, suggesting partial redundancy of function in this gene class.

Reverse genetic analysis of neurogenesis in the zebrafish retina

To gain an understanding of molecular events that underlie pattern formation in the retina, we evaluated the expression profiles of over 8000 transcripts randomly selected from an embryonic zebrafish library. Detailed analysis of cDNAs that display restricted expression patterns revealed factors that are specifically expressed in single cell classes and are potential regulators of neurogenesis. These cDNAs belong to numerous molecular categories and include cell surface receptors, cytoplasmic enzymes, and transcription factors. To test whether expression patterns that we have uncovered using this approach are indicative of function in neurogenesis, we used morpholino-mediated knockdown approach. The knockdown of soxp, a transcript expressed in the vicinity of the inner plexiform layer, revealed its role in cell type composition of amacrine and ganglion cell layers. Blocking the function of cxcr4b, a chemokine receptor specifically expressed in ganglion cells, suggests a role in ganglion cell survival. These experiments demonstrate that in situ hybridization-based reverse genetic screens can be applied to isolate genetic regulators of neurogenesis. This approach very well complements forward genetic mutagenesis studies previously used to study retinal neurogenesis in zebrafish.

Notch1 functions to suppress cone-photoreceptor fate specification in the developing mouse retina

Notch receptor-mediated cell-cell signaling is known to negatively regulate neurogenesis in both vertebrate and invertebrate species, while being implicated in promoting the acquisition of glial fates. We studied Notch1 function directly during retinal neurogenesis by selective Cre/loxP-triggered Notch1 gene inactivation in peripheral retinal progenitor cells (RPCs) prior to the onset of cell differentiation. Consistent with its previously established role, Notch1 inactivation led to dramatic alteration in the expression profile of multiple basic helix-loop-helix transcription factors, consequently prompting premature cell-cycle exit and neuronal specification. Surprisingly, however, Notch1 inactivation led to a striking change in retinal cell composition, with cone-photoreceptor precursors expanding at the expense of other early- as well as late-born cell fates. Intriguingly, the Notch1-deficient precursors adhered to the normal chronological sequence of the cone-photoreceptor differentiation program. Together, these findings reveal an unexpected role of Notch signaling in directly controlling neuronal cell-type composition, and suggest a model by which, during normal retinogenesis, Notch1 functions to suppress cone-photoreceptor fate, allowing for the specification of the diversity of retinal cell types.

The Optimedin Gene Is a Downstream Target of Pax6

The Optimedin gene, also known as Olfactomedin 3, encodes an olfactomedin domain-containing protein. There are two major splice variants of the Optimedin mRNA, Optimedin A and Optimedin B, transcribed from different promoters. The expression pattern of the Optimedin A variant in the eye and brain overlaps with that for Pax6, which encodes a protein containing the paired and homeobox DNA-binding domains. The Pax6 gene plays a critical role for the development of eyes, central nervous system, and endocrine glands. The proximal promoter of the Optimedin A variant contains a putative Pax6 binding site in position -86/-70. Pax6 binds this site through the paired domain in vitro as judged by electrophoretic mobility shift assay. Mutations in this site eliminate Pax6 binding as well as stimulation of the Optimedin promoter activity by Pax6 in transfection experiments. Pax6 occupies the binding site in the proximal promoter in vivo as demonstrated by the chromatin immunoprecipitation assay. Altogether these results identify the Optimedin gene as a downstream target regulated by Pax6. Although the function of optimedin is still not clear, it is suggested to be involved in cell-cell adhesion and cell attachment to the extracellular matrix. Pax6 regulation of Optimedin in the eye and brain may directly affect multiple developmental processes, including cell migration and axon growth.

Ganglion cells are required for normal progenitor- cell proliferation but not cell-fate determination or patterning in the developing mouse retina

The vertebrate retina develops from an amorphous sheet of dividing retinal progenitor cells (RPCs) through a sequential process that culminates in an exquisitely patterned neural tissue. A current model for retinal development posits that sequential cell-type differentiation is the result of changes in the intrinsic competence state of multipotent RPCs as they advance in time and that the intrinsic changes are influenced by continuous changes in the extracellular environment. Although several studies support the proposition that newly differentiated cells alter the extrinsic state of the developing retina, it is still far from clear what role they play in modifying the extracellular environment and in influencing the properties of RPCs. Here, we specifically ablate retinal ganglion cells (RGCs) as they differentiate, and we determine the impact of RGC absence on retinal development. We find that RGCs are not essential for changing the competence of RPCs, but they are necessary for maintaining sufficient numbers of RPCs by regulating cell proliferation via growth factors. Intrinsic rather than extrinsic factors are likely to play the critical roles in determining retinal cell fate.

A gene network downstream of transcription factor Math5 regulates retinal progenitor cell competence and ganglion cell fate

Math5, a mouse homolog of the Drosophila proneural bHLH transcription factor Atonal, is essential in the developing retina to establish retinal progenitor cell competence for a ganglion cell fate. Elucidating the mechanisms by which Math5 influences progenitor cell competence is crucial for understanding how specification of neuronal cell fate occurs in the retina and it requires knowledge of the downstream target genes that depend on Math5 for their expression. To date, only a handful of genes downstream of Math5 have been identified. To better define the gene network operating downstream of Math5, we used custom-designed microarrays to examine the changes in embryonic retinal gene expression caused by deletion of math5. We identified 270 Math5-dependent genes, including those that were expressed specifically either in progenitor cells or differentiated ganglion cells. The ganglion cell-specific genes included both Brn3b-dependent and Brn3b-independent genes, indicating that Math5 regulates distinct branches of the gene network responsible for retinal ganglion cell differentiation. In math5-null progenitor cells, there was an up-regulation of the proneural genes math3, neuroD, and ngn2, indicating that Math5 suppresses the production of other cell types in addition to promoting retinal ganglion cell formation. The promoter regions of many Math5-dependent genes contained binding sites for REST/NRSF, suggesting that release from general repression in retinal progenitor cells is required for ganglion cell-specific gene activation. The identification of multiple roles for Math5 provides new insights into the gene network that defines progenitor cell competence in the embryonic retina.

Intrinsic neuronal regulation of axon and dendrite growth

Neurons extend long axons and highly branched dendrites, and our understanding of the essential regulators of these processes has advanced in recent years. In the past year, investigators have shown that transcriptional control, posttranslational degradation and signaling cascades may be master regulators of axon and dendrite elongation and branching. Thus, evidence is mounting for the importance of the intrinsic growth state of a neuron as a crucial determinant of its ability to grow, or to regenerate, axons and dendrites.

Functional equivalence of Brn3 POU-domain transcription factors in mouse retinal neurogenesis

POU-domain transcription factors play essential roles in cell proliferation and differentiation. Previous studies have shown that targeted deletion of each of the three POU-domain Brn3 factors in mice leads to the developmental failure and apoptosis of a unique set of sensory neurons in retina, dorsal root ganglia, trigeminal ganglia and inner ear. The specific defects associated with the removal of each Brn3 gene closely reflect their characteristic spatiotemporal expression patterns. Nevertheless, it remains elusive whether Brn3 factors are functionally equivalent and act through a common molecular mechanism to regulate the development and survival of these sensory neurons. By knocking-in Brn3a (Brn3aki)into the Brn3b locus, we showed here that Brn3akiwas expressed in a spatiotemporal manner identical to that of endogenous Brn3b. In addition, Brn3aki functionally restored the normal development and survival of retinal ganglion cells (RGCs) in the absence of Brn3b and fully reinstated the early developmental expression profiles of Brn3b downstream target genes in retina. These results indicate that Brn3 factors are functionally equal and that their unique roles in neurogenesis are determined by the distinctive Brn3 spatiotemporal expression patterns.

Identification of an N-terminal transcriptional activation domain within Brn3b/POU4f2

The POU-domain transcription factor Brn3b/ POU4f2 is an essential regulator of gene expression in mouse retinal ganglion cells. Although Brn3b's importance in the differentiation of these cells has been firmly established, the regions on Brn3b where transcriptional activation and/or repression domains reside are only vaguely defined, and conflicting publications report both activation and repression activities for Brn3b. To clarify its function, we monitored the transcriptional activity of Brn3b and Gal4 DNA-binding domain (DBD)-Brn3b fusion proteins in cotransfection experiments using either Brn3-consensus or Gal4 DNA-binding sites to drive reporter gene expression. At Gal4 DNA-binding sites, transrepression activity mapping to the POU domain within Brn3b's C-terminal region masked any transactivation activity. More detailed experiments revealed that expressing abnormally high levels of POU homeodomain- or other homeodomain-containing sequences caused fortuitous transrepression in the cotransfection assay. To avoid transrepression, Brn3b sequences lacking Brn3b's POU domain were fused to the Gal4 DBD to allow identification of regions that were responsible for transcriptional activation. Considerable transactivation activity was located between amino acid residues 100 and 239, although other regions also had activity. The transactivation domain synergized strongly with another transcription factor, LexA-VP16. At Brn3 DNA-binding sites, full-length Brn3b increased transcription more than 25-fold, and similar activation was observed with the closely related factor Brn3a/POU4f1. No transactivation activity was associated with the C-terminal POU domain-containing portion of Brn3b. The results demonstrate that Brn3b regulates gene expression through the action of a strong transcriptional activation domain within its N-terminal sequence.

A gene regulatory hierarchy for retinal ganglion cell specification and differentiation

Retinal ganglion cells (RGCs) are the first cell type to be specified during vertebrate retinogenesis. Specification and differentiation of the RGC lineage are a stepwise process involving a hierarchical gene regulatory network. During the past decade, a framework of the network has emerged and key transcriptional regulators have been identified. Pax6, Notch, Ath5, and the Brn3 (Pou4f) factors act at different levels of the regulatory hierarchy. In this review, we summarize the current understanding of the functions of these and other transcriptional factors in the specification and differentiation of the RGC lineage. We emphasize the regulatory relationships among transcription factors at different steps of RGC development. We discuss critical issues that need to be addressed before a complete understanding of the gene regulatory network for RGC development can be achieved. Future directions in RGC development will inevitably rely on combined genetic and genomics approaches.

Enhanced retinal ganglion cell differentiation by ath5 and NSCL1 coexpression

PURPOSE

The molecular mechanism underlying retinal ganglion cell (RGC) differentiation is not fully understood. In this study, the role of the basic helix-loop-helix (bHLH) genes ath5 and NSCL1 in RGC differentiation was examined, by testing whether their coexpression would promote RGC differentiation to a greater extent than either gene alone.

METHODS

The replication-competent avian RCAS retrovirus was used to coexpress ath5 and NSCL1 through an internal ribosomal entry site. The effect of the coexpression on RGC differentiation was assayed in vivo in the developing chick retina and in vitro in RPE cell cultures derived from day 6 chick embryos.

RESULTS

Coexpression of ath5 and NSCL1 in RPE cells cultured in the presence of bFGF promoted RPE transdifferentiation toward RGCs, and the degree of transdifferentiation was much higher than with either gene alone. Cells expressing RGC markers, including RA4, calretinin, and two neurofilament-associated proteins, displayed processes that were remarkably long and thin and often had numerous branches, characteristics of long-projecting RGCs. In the developing chick retina, retroviral expression of NSCL1 resulted in a moderate increase in the number of RGCs, results similar to retroviral expression of ath5. Coexpression of ath5 and NSCL1 yielded increases in RGCs greater than the sum of their increases when expressed separately.

CONCLUSIONS

Both in vitro and in vivo data indicate that the combination of ath5 and NSCL1 promotes RGC differentiation to a greater degree than either gene alone, suggesting a synergism between ath5 and NSCL1 in advancing RGC development.

A novel basic helix–loop–helix (bHLH) transcriptional repressor, NeuroAB, expressed in bipolar and amacrine cells in the chick retina

Basic helix-loop-helix (bHLH) transcription factors are implicated in cell fate determination and differentiation in neurogenesis. We identified a novel chick bHLH transcription factor, NeuroAB. A phylogenetic tree prepared from bHLH sequences suggested that NeuroAB belongs to the BETA3 group in the Atonal-related protein family (ARPs). In situ hybridization and immunostaining indicated that NeuroAB is expressed predominantly in postmitotic bipolar cells and GABAergic amacrine cells in the retina. Reporter and DNA pull down assays indicated that NeuroAB functions as a transcriptional repressor by binding to the E-box sequence, and its activity is modulated by phosphorylation at a specific serine residue that fits the consensus phosphorylation site for glycogen synthase kinase 3beta (GSK3beta). Since members of the BETA3 group possess this consensus site, it is suggested that their activities are commonly regulated by GSK3beta or other kinases bearing the same substrate specificity. We found that the expression of GSK3beta is spatially and temporally regulated in the developing retina; its strong expression was observed in ganglion cells from E8 and a subset of amacrine cells from E12. These findings suggest that NeuroAB is involved in the maturation and maintenance of bipolar cells and GABAergic amacrine cells and regulation by GSK3beta plays an important role in retinogenesis.

Retinal Ganglion Cell Type, Size, and Spacing Can Be Specified Independent of Homotypic Dendritic Contacts

In Brn3b(-/-) mice, where 80% of retinal ganglion cells degenerate early in development, the remaining 20% include most or all ganglion cell types. Cells of the same type cover the retinal surface evenly but tile it incompletely, indicating that a regular mosaic and normal dendritic field size can be maintained in the absence of contact among homotypic cells. In Math5(-/-) mice, where only approximately 5% of ganglion cells are formed, the dendritic arbors of at least two types among the residual ganglion cells are indistinguishable from normal in shape and size, even though throughout development they are separated by millimeters from the nearest neighboring ganglion cell of the same type. It appears that the primary phenotype of retinal ganglion cells can develop without homotypic contact; dendritic repulsion may be an end-stage mechanism that fine-tunes the dendritic arbors for more efficient coverage of the retinal surface.

Discrete gene sets depend on POU domain transcription factor Brn3b/Brn-3.2/POU4f2 for their expression in the mouse embryonic retina

Brn3b/Brn-3.2/POU4f2 is a POU domain transcription factor that is essential for retinal ganglion cell (RGC) differentiation, axonal outgrowth and survival. Our goal was to establish a link between Brn3b and the downstream events leading to RGC differentiation. We sought to determine both the number and types of genes that depend on Brn3b for their expression. RNA probes from wild-type and Brn3b(-/-) E14.5, E16.5 and E18.5 mouse retinas were hybridized to a microarray containing 18,816 retina-expressed cDNAs. At E14.5, we identified 87 genes whose expression was significantly altered in the absence of Brn3b and verified the results by real-time PCR and in situ hybridization. These genes fell into discrete sets that encoded transcription factors, proteins associated with neuron integrity and function, and secreted signaling molecules. We found that Brn3b influenced gene expression in non RGCs of the retina by controlling the expression of secreted signaling molecules such as sonic hedgehog and myostatin/Gdf8. At later developmental stages, additional alterations in gene expression were secondary consequences of aberrant RGC differentiation caused by the absence of Brn3b. Our results demonstrate that a small but crucial fraction of the RGC transcriptome is dependent on Brn3b. The Brn3b-dependent gene sets therefore provide a unique molecular signature for the developing retina.

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.

In vitro generation of early-born neurons from late retinal progenitors

Evidence suggests that, as development ensues, the competence of neural progenitors is progressively altered, such that they become fated to give rise to neurons of a particular stage. Here, we demonstrate that late retinal progenitors can give rise to retinal ganglion cells (RGCs), an example of an early-born cell type in the retina. A subset of late retinal progenitors in vitro responds to cues that favor RGC differentiation by displaying markers characteristic of RGCs. In addition, mechanisms used during normal RGC differentiation are recruited by these cells toward their differentiation along RGC lineage. Our observations suggest that late neural progenitors may not be irreversibly fated but may appear as such under the constraints dictated by epigenetic cues.

Retinal expression of zebrafish six3.1 and its regulation by Pax6

Homologues of the homeobox genes sine oculis (so) and eyeless (ey) are important regulators of eye development in both vertebrates and invertebrates. A Drosophila paralogue of so, optix, is an orthologue of the vertebrate Six3 gene family. Our analysis of zebrafish six3.1 demonstrated retinal expression in two separate cell layers and the ciliary marginal zone. This pattern is consistent with the observations of Six3 in other vertebrates and indicates functional conservation. We studied the 5(') flanking region of six3.1 and showed that separate enhancing elements are required for expression at different stages of eye development. This analysis also revealed specific binding of zebrafish Pax6.1 protein to an element required for six3.1 expression in ganglion cells. Furthermore, an enhancement of six3.1 transcription by Pax6.1 was observed by co-injection experiments. These results provide evidence for a direct regulatory interaction between vertebrate Pax6 and Six3 genes in eye 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.

RaxL regulates chick ganglion cell development

RaxL is a paired-like homeobox gene involved in vertebrate eye morphogenesis. We examined RaxL protein expression patterns during chick retinal development in combination with ganglion cell markers including the RA4 antigen, cBrn-3, Islet-1 and neuronal type III beta-tubulin. Double-immunostaining demonstrated that downregulation of RaxL protein correlates with upregulation of ganglion cell markers in the ganglion cell layer (GCL). To explore this correlation in vivo, we performed gain- and loss-of-function experiments by electroporating retroviral vectors encoding wild-type and dominant-negative-RaxL into the optic vesicles of stage 10 chick embryos. Infection with virus expressing RaxL led to a 35% decrease in Islet-1-positive ganglion cells at E5.0 and a complete loss of ganglion cells at E15, with no effect on displaced amacrine cells in the GCL. When dominant-negative RaxL was expressed, the total number of cells in the GCL increased by approximately 40% at E5.0 but was reduced to 40% at E15, due to ectopic apoptosis in the GCL from E9 to E15. These results suggest that RaxL gives an inhibitory effect on ganglion cell development and that the loss of RaxL expression is required for maintenance of ganglion cells.

Multiple roles for u-turn/ventral veinless in the development of Drosophila PNS

Most of the cells in the embryonic peripheral nervous system (PNS) of Drosophila are born in their final location. One known exception is the group of lateral chordotonal organs (lch5) whose precursors form in a dorsal position, yet the mature organs are located in the lateral PNS cluster. Mutations in the u-turn (ut) locus perturb the localization of lch5 neurons and result in a 'dorsal chordotonals' phenotype. We show that ut is allelic to ventral veinless (vvl), also known as drifter. VVL, a POU-domain transcription factor, has been shown to participate in the development of tracheae and CNS in the embryo, and in wing development in the adult; however, its role in PNS development has not been described. Characterization of the 'dorsal chordotonals' phenotype of vvl mutant embryos revealed that in the absence of VVL, cell fates within the lch5 lineage are determined properly and the entire organ is misplaced. Based on the positions of lch5 cells relative to each other in mutant embryos, and in normal embryos at different developmental stages, we propose a two-step model for lch5 localization. lch5 organs must first rotate to assume a correct polarity and are then stretched ventrally to their final position. In this process, VVL function is required in the ectoderm and possibly in the lch5 organs too. VVL is also expressed in developing external sensory organs in the embryo and in the adult. In the embryo, loss of VVL function results in increased apoptosis in specific es organs. Analysis of vvl mutant clones in adults revealed a requirement for VVL in the control of cell number within the bristle lineage.

Direct autoregulation and gene dosage compensation by POU-domain transcription factor Brn3a

Brn3a is a POU-domain transcription factor expressed in peripheral sensory neurons and in specific interneurons of the caudal CNS. Sensory expression of Brn3a is regulated by a specific upstream enhancer, the activity of which is greatly increased in Brn3a knockout mice, implying that Brn3a negatively regulates its own expression. Brn3a binds to highly conserved sites within this enhancer, and alteration of these sites abolishes Brn3a regulation of reporter transgenes. Furthermore, endogenous Brn3a expression levels in the sensory ganglia of Brn3a(+/+) and Brn3a(+/-) mice are similar, demonstrating that autoregulation can compensate for the loss of one allele by increasing transcription of the remaining gene copy. Conversely, transgenic overexpression of Brn3a in the trigeminal ganglion suppresses the expression of the endogenous gene. These findings demonstrate that the Brn3a locus functions as a self-regulating unit to maintain a constant expression level of this key regulator of neural development.

Retinal ganglion cell differentiation in cultured mouse retinal explants

The availability of genetically engineered mice harboring specific mutations in genes affecting one or more retinal cell types affords new opportunities for investigating the genetic regulatory mechanisms of vertebrate retina formation. When identifying critical regulatory genes involved in retina development it is often advantageous to complement in vivo analysis with in vitro characterization. In particular, by combining classical techniques of retinal explant culturing with gene transfer procedures relying on herpes simple virus (HSV) amplicon vectors, gain-of-function analysis with genes of interest can be performed quickly and efficiently. Here, details are provided for isolating and culturing explants containing retinal progenitor cells and for infecting the explants with HSV expression vectors that perturb or rescue retinal ganglion cells, the first cell type to differentiate in the retina. In addition, the availability of sensitive techniques to monitor gene expression, including detection of reporter gene expression using antibodies and detection of endogenous marker gene expression using quantitative RT-PCR, provides an effective means for comparing wild-type and mutant retinas from genetically engineered mice.

BMP signaling is required for development of the ciliary body

The ciliary body in the eye secretes aqueous humor and glycoproteins of the vitreous body and maintains the intraocular pressure. The ciliary muscle controls the shape of the lens through the ciliary zonules to focus the image onto the retina. During embryonic development, the ciliary epithelium is derived from the optic vesicle, but the molecular signals that control morphogenesis of the ciliary body are unknown. We report that lens-specific expression of a transgenic protein, Noggin, can block BMP signaling in the mouse eye and result in failure in formation of the ciliary processes. Co-expression of transgenic BMP7 restores normal development of the ciliary epithelium. Ectopic expression of Noggin also promotes differentiation of retinal ganglion cells. These results indicate that BMP signaling is required for development of the ciliary body and may also play a role in regulation of neuronal differentiation in the developing eye.

The Wilms' tumor gene Wt1 is required for normal development of the retina

The Wilms' tumor gene Wt1 is known for its important functions during genitourinary and mesothelial formation. Here we show that Wt1 is necessary for neuronal development in the vertebrate retina. Mouse embryos with targeted disruption of Wt1 exhibit remarkably thinner retinas than age-matched wild-type animals. A large fraction of retinal ganglion cells is lost by apoptosis, and the growth of optic nerve fibers is severely disturbed. Strikingly, expression of the class IV POU-domain transcription factor Pou4f2 (formerly Brn-3b), which is critical for the survival of most retinal ganglion cells, is lost in Wt1(-/-) retinas. Forced expression of Wt1 in cultured cells causes an up-regulation of Pou4f2 mRNA. Moreover, the Wt1(-KTS) splice variant can activate a reporter construct carrying 5'-regulatory sequences of the human POU4F2. The lack of Pou4f2 and the ocular defects in Wt1(-/-) embryos are rescued by transgenic expression of a 280 kb yeast artificial chromosome carrying the human WT1 gene. Taken together, our findings demonstrate a continuous requirement for Wt1 in normal retina formation with a critical role in Pou4f2-dependent ganglion cell differentiation.

Brn3b/Brn3c double knockout mice reveal an unsuspected role for Brn3c in retinal ganglion cell axon outgrowth

In mice, Brn3 POU domain transcription factors play essential roles in the differentiation and survival of projection neurons within the retina, inner ear, dorsal root and trigeminal ganglia. During retinal ganglion cell differentiation, Brn3b is expressed first, followed by Brn3a and Brn3c. Targeted deletion of Brn3b, but not Brn3a or Brn3c, leads to a loss of most retinal ganglion cells before birth. However, as a few retinal ganglion cells are still present in Brn3b–/– mice, Brn3a and Brn3c may partially compensate for the loss of Brn3b. To examine the role of Brn3c in retinal ganglion cell development, we generated Brn3b/Brn3c double knockout mice and analyzed their retinas and optic chiasms. Retinal ganglion cell axons from double knockout mice were more severely affected than were those from Brn3b-deficient mice, indicating that Brn3c was required for retinal ganglion cell differentiation and could partially compensate for the loss of Brn3b. Moreover, Brn3c had functions in retinal ganglion cell differentiation separate from those of Brn3b. Ipsilateral and misrouted projections at the optic chiasm were overproduced in Brn3b–/– mice but missing were entirely in optic chiasms of Brn3b/Brn3c double knockout mice, suggesting that Brn3c controlled ipsilateral axon production. Forced expression of Brn3c in Brn3b–/– retinal explants restored neurite outgrowth, demonstrating that Brn3c could promote axon outgrowth in the absence of Brn3b. Our results reveal a complex genetic relationship between Brn3b and Brn3c in regulating the retinal ganglion cell axon outgrowth.

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.

Gene expression in the developing mouse retina by EST sequencing and microarray analysis

Retinal development occurs in mice between embryonic day E11.5 and post-natal day P8 as uncommitted neuroblasts assume retinal cell fates. The genetic pathways regulating retinal development are being identified but little is understood about the global networks that link these pathways together or the complexity of the expressed gene set required to form the retina. At E14.5, the retina contains mostly uncommitted neuroblasts and newly differentiated neurons. Here we report a sequence analysis of an E14.5 retinal cDNA library. To date, we have archived 15 268 ESTs and have annotated 9035, which represent 5288 genes. The fraction of singly occurring ESTs as a function of total EST accrual suggests that the total number of expressed genes in the library could approach 27 000. The 9035 ESTs were categorized by their known or putative functions. Representation of the genes involved in eye development was significantly higher in the retinal clone set compared with the NIA mouse 15K cDNA clone set. Screening with a microarray containing 864 cDNA clones using wild-type and brn-3b (-/-) retinal cDNA probes revealed a potential regulatory linkage between the transcription factor Brn-3b and expression of GAP-43, a protein associated with axon growth. The retinal EST database will be a valuable platform for gene expression profiling and a new source for gene discovery.

The role of basic helix-loop-helix genes in vertebrate retinogenesis

The developing eye is a favorite model for the study of pattern formation and cell fate determination. Retinal neuron development, in particular, is an approachable system to study molecular and cellular aspects of cell determination and differentiation. Basic helix-loop-helix (bHLH) transcription factors are important regulators of retinal neurogenesis. Proneural bHLH genes have highly defined expression in the developing retina that are influenced by pattern formation and cell specification pathways. Each retinal cell class has unique bHLH requirements, implying that these genes regulate neuronal identity and function. Therefore, proneural genes represent a molecular focal point through which epithelial cells are transformed into a precise neural network. In this review, we focus on the bHLH factor Ath5, an important regulator of retinal ganglion cell development, and discuss factors that regulate its expression in the retina and the target genes through which it may confer specific neuronal properties.

An in situ screen for genes controlling cell proliferation in the optic tectum of the medaka (Oryzias latipes)

The optic tectum is a dorsal, prominent and well corticalised structure of the fish brain. It grows according to a pattern exceptional in the vertebrate central nervous system, by addition of radial columns of cells at its periphery. We took advantage of this peculiar feature to readily identify genes differentially expressed in the tectal proliferative (marginal) vs. post-mitotic (central) zones. Out of 500 medaka cDNA clones screened by WMISH, more than 100 were expressed in one or the other of these zones. Unexpectedly, we also identified a small class of genes expressed between these two zones. All the characterised genes of this class encode down regulators of the cell cycle. Therefore, such a screening strategy allows in particular cases to raise testable hypotheses on the involvement of genes in the control of the cell cycle, in addition to characterising unknown genes with patterned expression related to cell proliferation.

Brn3a is a transcriptional regulator of soma size, target field innervation and axon pathfinding of inner ear sensory neurons

The POU domain transcription factors Brn3a, Brn3b and Brn3c are required for the proper development of sensory ganglia, retinal ganglion cells, and inner ear hair cells, respectively. We have investigated the roles of Brn3a in neuronal differentiation and target innervation in the facial-stato-acoustic ganglion. We show that absence of Brn3a results in a substantial reduction in neuronal size, abnormal neuronal migration and downregulation of gene expression, including that of the neurotrophin receptor TrkC, parvalbumin and Brn3b. Selective loss of TrkC neurons in the spiral ganglion of Brn3a−/− cochlea leads to an innervation defect similar to that of TrkC−/− mice. Most remarkably, our results uncover a novel role for Brn3a in regulating axon pathfinding and target field innervation by spiral and vestibular ganglion neurons. Loss of Brn3a results in severe retardation in development of the axon projections to the cochlea and the posterior vertical canal as early as E13.5. In addition, efferent axons that use the afferent fibers as a scaffold during pathfinding also show severe misrouting. Interestingly, despite the well-established roles of ephrins and EphB receptors in axon pathfinding, expression of these molecules does not appear to be affected in Brn3a−/− mice. Thus, Brn3a must control additional downstream genes that are required for axon pathfinding.

Retinal ganglion cell genesis requires lakritz, a Zebrafish atonal Homolog

Mutation of the zebrafish lakritz (lak) locus completely eliminates the earliest-born retinal cells, the ganglion cells (RGCs). Instead, excess amacrine, bipolar, and Müller glial cells are generated in the mutant. The extra amacrines are found at ectopic locations in the ganglion cell layer. Cone photoreceptors appear unaffected by the mutation. Molecular analysis reveals that lak encodes Ath5, the zebrafish eye-specific ortholog of the Drosophila basic helix-loop-helix transcription factor Atonal. A combined birth-dating and cell marker analysis demonstrates that lak/ath5 is essential for RGC determination during the first wave of neurogenesis in the retina. Our results suggest that this wave is skipped in the mutant, leading to an accumulation of progenitors for inner nuclear layer cells.

The bHLH factors Xath5 and XNeuroD can upregulate the expression of XBrn3d, a POU-homeodomain transcription factor

The basic helix-loop-helix (bHLH) factor Xath5 promotes retinal ganglion cell differentiation when overexpressed and may do so by regulating the expression of factors involved in the differentiation of these cells. Potential candidates include the Brn3 POU-homeodomain transcription factors, which have been implicated in retinal ganglion cell development. Here we have identified a new member of the Brn3 gene subfamily in Xenopus, XBrn3d. In situ hybridization analysis shows XBrn3d expression in developing sensory neurons and developing ganglion cells of the retina. Using a hormone-inducible Xath5 fusion protein, we have shown that in animal caps Xath5 can directly regulate the expression of XBrn3d. Since XBrn3d is also expressed in sensory populations where Xath5 is not expressed, we examined the regulation of XBrn3d expression by the bHLH factor XNeuroD. A XNeuroD-hGR fusion protein is similarly able to directly induce the expression of XBrn3d in animal caps. In addition, overexpression of XBrn3d by RNA injection promotes the expression of ectopic sensory neuronal markers in the lateral ectoderm, suggesting a role in regulating neuronal development. Therefore, Xath5 and XNeuroD can directly regulate the expression of a neuronal subtype-specific factor, providing a link between neuronal differentiation and cell fate specification.

The Ath5 proneural genes function upstream of Brn3 POU domain transcription factor genes to promote retinal ganglion cell development

During retinogenesis, the Xenopus basic helix-loop-helix transcription factor Xath5 has been shown to promote a ganglion cell fate. In the developing mouse and chicken retinas, gene targeting and overexpression studies have demonstrated critical roles for the Brn3 POU domain transcription factor genes in the promotion of ganglion cell differentiation. However, the genetic relationship between Ath5 and Brn3 genes is unknown. To understand the genetic regulatory network(s) that controls retinal ganglion cell development, we analyzed the relationship between Ath5 and Brn3 genes by using a gain-of-function approach in the chicken embryo. We found that during retinogenesis, the chicken Ath5 gene (Cath5) is expressed in retinal progenitors and in differentiating ganglion cells but is absent in terminally differentiated ganglion cells. Forced expression of both Cath5 and the mouse Ath5 gene (Math5) in retinal progenitors activates the expression of cBrn3c following central-to-peripheral and temporal-to-nasal gradients. As a result, similar to the Xath5 protein, both Cath5 and Math5 proteins have the ability to promote the development of ganglion cells. Moreover, we found that forced expression of all three Brn3 genes also can stimulate the expression of cBrn3c. We further found that Ath5 and Brn3 proteins are capable of transactivating a Brn3b promoter. Thus, these data suggest that the expression of cBrn3c in the chicken and Brn3b in the mouse is initially activated by Ath5 factors in newly generated ganglion cells and later maintained by a feedback loop of Brn3 factors in the differentiated ganglion cells.

Vertebrate neural cell-fate determination: lessons from the retina

Postmitotic neurons are produced from a pool of cycling progenitors in an orderly fashion during development. Studies of cell-fate determination in the vertebrate retina have uncovered several fundamental principles by which this is achieved. Most notably, a model for vertebrate cell-fate determination has been proposed that combines findings on the relative roles of extrinsic and intrinsic regulators in controlling cell-fate choices. At the heart of the model is the proposal that progenitors pass through intrinsically determined competence states, during which they are capable of giving rise to a limited subset of cell types under the influence of extrinsic signals.

Developmental mechanisms in the pathogenesis of neurodegenerative diseases

Cellular genes that are mutated in neurodegenerative diseases code for proteins that are expressed throughout neural development. Genetic analysis suggests that these genes are essential for a broad range of normal neurodevelopmental processes. The proteins they code for interact with numerous other cellular proteins that are components of signaling pathways involved in patterning of the neural tube and in regional specification of neuronal subtypes. Further, pathogenetic mutations of these genes can cause progressive, sublethal alterations in the cellular homeostasis of evolving regional neuronal subpopulations, culminating in late-onset cell death. Therefore, as a consequence of the disease mutations, targeted cell populations may retain molecular traces of abnormal interactions with disease-associated proteins by exhibiting changes in a spectrum of normal cellular functions and enhanced vulnerability to a host of environmental stressors. These observations suggest that the normal functions of these disease-associated proteins are to ensure the fidelity and integration of developmental events associated with the progressive elaboration of neuronal subtypes as well as the maintenance of mature neuronal populations during adult life. The ability to identify alterations within vulnerable neuronal precursors present in pre-symptomatic individuals prior to the onset of irrevocable cellular injury may help foster the development of effective therapeutic interventions using evolving pharmacologic, gene and stem cell technologies.

Requirement for math5 in the development of retinal ganglion cells

math5 is a murine orthologue of atonal, a bHLH proneural gene essential for the formation of photoreceptors and chordotonal organs in Drosophila. The expression of math5 coincides with the onset of retinal ganglion cell (RGC) differentiation. Targeted deletion of math5 blocks the initial differentiation of 80% of RGCs and results in an increase in differentiated amacrine cells. Furthermore, the absence of math5 abolishes the retinal expression of brn-3b and the formation of virtually all brn-3b-expressing RGCs. These results imply that math5 is a proneural gene essential for RGC differentiation and that math5 acts upstream to activate brn-3b-dependent differentiation processes in RGCs.

A POU domain transcription factor-dependent program regulates axon pathfinding in the vertebrate visual system

Axon pathfinding relies on the ability of the growth cone to detect and interpret guidance cues and to modulate cytoskeletal changes in response to these signals. We report that the murine POU domain transcription factor Brn-3.2 regulates pathfinding in retinal ganglion cell (RGC) axons at multiple points along their pathways and the establishment of topographic order in the superior colliculus. Using representational difference analysis, we identified Brn-3.2 gene targets likely to act on axon guidance at the levels of transcription, cell-cell interaction, and signal transduction, including the actin-binding LIM domain protein abLIM. We present evidence that abLIM plays a crucial role in RGC axon pathfinding, sharing functional similarity with its C. elegans homolog, UNC-115. Our findings provide insights into a Brn-3.2-directed hierarchical program linking signaling events to cytoskeletal changes required for axon pathfinding.

Abnormal polarization and axon outgrowth in retinal ganglion cells lacking the POU-domain transcription factor Brn-3b

The POU domain transcription factor Brn-3b (also called Brn-3.2) is essential for the normal development of retinal ganglion cells (RGCs) in the mouse. Without Brn-3b, RGCs commit to their fate and migrate to the ganglion cell layer, but most cells die during fetal development. An earlier report (L. Gan et al., 1999, Dev. Biol. 210, 469-480) suggested that cell death was caused by abnormal axon formation. Here, we use retinal explants from wild-type and mutant embryos to show that brn-3b-deficient RGCs are not properly polarized and tend to form dendrites rather than axons. Compared with wild-type explants, neurites of RGCs from brn-3b-deficient retinal explants grew slower, were shorter, and did not fasciculate properly. Mutant neurites had more microtubules than wild-type controls, and the arrangement of microtubules and neurofilaments was characteristic of dendrites rather than axons. Neurites from individual mutant RGCs displayed abnormal polarity and had dendrite-like branches extending outward from their main axis. Most mutant RGCs exhibited abnormal migratory behavior, and their neurites labeled intensely with the dendrite marker MAP-2. A small number of mutant RGCs were not migratory, and their neurites were longer and labeled positively for the axon marker tau-1, suggesting that some RGCs were not as severely affected by the absence of Brn-3b as others. Although tau-1 was not observed in most mutant neurites, it did accumulate in mutant cell bodies, implying that the absence of Brn-3b caused a defect in axon transport. Thus, Brn-3b appears to control the activity of genes that function in establishing RGC polarity, and without Brn-3b, RGCs cannot extend normal axons.

All Brn3 genes can promote retinal ganglion cell differentiation in the chick

Targeted gene disruption studies in the mouse have demonstrated crucial roles for the Brn3 POU domain transcription factor genes, Brn3a, Brn3b, Brn3c (now called Pou4f1, Pou4f2, Pou4f3, respectively) in sensorineural development and survival. During mouse retinogenesis, the Brn3b gene is expressed in a large set of postmitotic ganglion cell precursors and is required for their early and terminal differentiation. In contrast, the Brn3a and Brn3c genes, which are expressed later in ganglion cells, appear to be dispensable for ganglion cell development. To understand the mechanism that causes the functional differences of Brn3 genes in retinal development, we employed a gain-of-function approach in the chick embryo. We find that Brn3b(l) and Brn3b(s), the two isoforms encoded by the Brn3b gene, as well as Brn3a and Brn3c all have similar DNA-binding and transactivating activities. We further find that the POU domain is minimally required for these activities. Consequently, we show that all these Brn3 proteins have a similar ability to promote development of ganglion cells when ectopically expressed in retinal progenitors. During chick retinogenesis, cBrn3c instead of cBrn3b exhibits a spatial and temporal expression pattern characteristic of ganglion cell genesis and its misexpression can also increase ganglion cell production. Based on these data, we propose that all Brn3 factors are capable of promoting retinal ganglion cell development, and that this potential may be limited by the order of expression in vivo.

The zebrafish young mutation acts non-cell-autonomously to uncouple differentiation from specification for all retinal cells

Embryos from mutagenized zebrafish were screened for disruptions in retinal lamination to identify factors involved in vertebrate retinal cell specification and differentiation. Two alleles of a recessive mutation, young, were isolated in which final differentiation and normal lamination of retinal cells were blocked. Early aspects of retinogenesis including the specification of cells along the inner optic cup as retinal tissue, polarity of the retinal neuroepithelium, and confinement of cell divisions to the apical pigmented epithelial boarder were normal in young mutants. BrdU incorporation experiments showed that the initiation and pattern of cell cycle withdrawal across the retina was comparable to wild-type siblings; however, this process took longer in the mutant. Analysis of early markers for cell type differentiation revealed that each of the major classes of retinal neurons, as well as non-neural Muller glial cells, are specified in young embryos. However, the retinal cells fail to elaborate morphological specializations, and analysis of late cell-type-specific markers suggests that the retinal cells were inhibited from fully differentiating. Other regions of the nervous system showed no obvious defects in young mutants. Mosaic analysis demonstrated that the young mutation acts non-cell-autonomously within the retina, as final morphological and molecular differentiation was rescued when genetically mutant cells were transplanted into wild-type hosts. Conversely, differentiation was prevented in wild-type cells when placed in young mutant retinas. Mosaic experiments also suggest that young functions at or near the cell surface and is not freely diffusible. We conclude that the young mutation disrupts the post-specification development of all retinal neurons and glia cells.