Control of late off-center cone bipolar cell differentiation and visual signaling by the homeobox gene Vsx1

Histone demethylase Jmjd3 is required for the development of subsets of retinal bipolar cells

Di- and trimethylation of lysine 27 on histone H3 (H3K27me2/3) is an important gene repression mechanism. H3K27me2/3-specific demethylase, Jmjd3, was expressed in the inner nuclear layer during late retinal development. In contrast, H3K27 methyltransferase, Ezh2, was highly expressed in the embryonic retina but its expression decreased rapidly after birth. Jmjd3 loss of function in the developing retina resulted in failed differentiation of PKC-positive bipolar cell subsets (rod-ON-BP) and reduced transcription factor Bhlhb4 expression, which is critical for the differentiation of rod-ON-BP cells. Overexpression of Bhlhb4, but not of other BP cell-related genes, such as transcription factors Neurod and Chx10, in Jmjd3-knockdown retina rescued loss of PKC-positive BP cells. Populations of other retinal cell subsets were not significantly affected. In addition, proliferation activity and apoptotic cell number during retinal development were not affected by the loss of Jmjd3. Levels of histone H3 trimethyl Lys27 (H3K27me3) in the Bhlhb4 locus were lower in Islet-1-positive BP cells and amacrine cells than in the Islet-1-negative cell fraction. The Islet-1-negative cell fraction consisted mainly of photoreceptors, suggestive of lineage-specific demethylation of H3K27me3 in the Bhlhb4 locus. We propose that lineage-specific H3K27me3 demethylation of critical gene loci by spatiotemporal-specific Jmjd3 expression is required for appropriate maturation of retinal cells.

Birth of cone bipolar cells, but not rod bipolar cells, is associated with existing RGCs

Retinal ganglion cells (RGCs) play important roles in retinogenesis. They are required for normal retinal histogenesis and retinal cell number balance. Developmental RGC loss is typically characterized by initial retinal neuronal number imbalance and subsequent loss of retinal neurons. However, it is not clear whether loss of a specific non-RGC cell type in the RGC-depleted retina is due to reduced cell production or subsequent degeneration. Taking advantage of three knockout mice with varying degrees of RGC depletion, we re-examined bipolar cell production in these retinas from various aspects. Results show that generation of the cone bipolar cells is correlated with the existing number of RGCs. However, generation of the rod bipolar cells is unaffected by RGC shortage. Results report the first observation that RGCs selectively influence the genesis of subsequent retinal cell types.

Islet-1 immunoreactivity in the developing retina of Xenopus laevis

The LIM-homeodomain transcription factor Islet1 (Isl1) has been widely used as a marker of neuronal differentiation in the developing visual system of different classes of vertebrates, including mammals, birds, reptiles, and fish. In the present study, we analyzed the spatial and temporal distribution of Isl1-immunoreactive cells during Xenopus laevis retinal development and its relation to the formation of the retinal layers, and in combination with different markers of cell differentiation. The earliest Isl1 expression appeared at St29-30 in the cell nuclei of sparse differentiating neuroblasts located in the vitreal surface of the undifferentiated retina. At St35-36, abundant Isl1-positive cells accumulated at the vitreal surface of the neuroepithelium. As development proceeded and through the postmetamorphic juveniles, Isl1 expression was identified in subpopulations of ganglion cells and in subsets of amacrine, bipolar, and horizontal cells. These data together suggest a possible role for Isl1 in the early differentiation and maintenance of different retinal cell types, and Isl1 can serve as a specific molecular marker for the study of retinal cell specification in X. laevis.

Neto1 associates with the NMDA receptor/amyloid precursor protein complex

Neuropilin tolloid-like 1 (Neto1), is a CUB domain-containing transmembrane protein that was recently identified as a novel component of the NMDA receptor complex. Here, we have investigated the possible association of Neto1 with the amyloid precursor protein (APP)695/GluN1/GluN2A and APP695/GluN1/GluN2B NMDA receptor trafficking complexes that we have previously identified. Neto1(HA) was shown to co-immunoprecipitate with assembled NMDA receptors via GluN2A or GluN2B subunits; Neto1(HA) did not co-immunoprecipitate APP695(FLAG) . Co-immunoprecipitations from mammalian cells co-transfected with APP695(FLAG) , Neto1(HA) and GluN1/GluN2A or GluN1/GluN2B revealed that all four proteins co-exist within one macromolecular complex. Immunoprecipitations from native brain tissue similarly revealed the existence of a GluN1/GluN2A or GluN2B/APP/Neto1 complex. Neto1(HA) caused a reduction in the surface expression of both NMDA receptor subtypes, but had no effect on APP695(FLAG) - or PSD-95α(c-Myc) enhanced surface receptor expression. The Neto1 binding domain of GluN2A was mapped using GluN1/GluN2A chimeras and GluN2A truncation constructs. The extracellular GluN2A domain does not contribute to association with Neto1(HA) but deletion of the intracellular tail resulted in a loss of Neto-1(HA) co-immunoprecipitation which was paralleled by a loss of association between GluN2A and SAP102. Thus, Neto1 is concluded to be a component of APP/NMDA receptor trafficking complexes.

Screening the visual system homeobox 1 gene in keratoconus and posterior polymorphous dystrophy cohorts identifies a novel variant

PURPOSE

Mutations in the visual system homeobox 1 (VSX1) gene have been described at a low frequency in keratoconus and posterior polymorphous corneal dystrophy (PPCD). The putative role is controversial for several reasons, including a lack of mutations detected in other population cohorts. This study aims to determine whether VSX1 contributes to the genetic pathogenesis of keratoconus and PPCD in a New Zealand population, and includes analysis of a Polynesian population.

METHODS

Recruitment of patients with keratoconus and PPCD, comprehensive clinical examination including corneal topography and pachymetry, and collection of biologic samples for DNA extraction were undertaken. Mutational analysis of VSX1 (exons 1-7) with PCR and sequencing with bioinformatic assessment of variants was performed. Probable pathogenic variants were screened for in a control population using high-resolution melting analysis.

RESULTS

Forty-seven patients with keratoconus, including 15 familial cases, and ten unrelated patients with PPCD were recruited. Two pathogenic changes were detected; a novel change c.173C>T (p.Pro58Leu) was found in a patient with PPCD, predicted to be pathogenic, and not seen in 200 ethnically matched control alleles. The previously reported c.731A>G (p.His244Arg) was detected in a patient with sporadic keratoconus, and not present in the controls. No family members were available for segregation analysis.

CONCLUSIONS

This study reports the presence of pathogenic mutations in VSX1 in PPCD and keratoconus, including a novel disease-causing variant. The affected numbers are small, but given the growing body of evidence of pathogenic segregating changes in VSX1 in disease cohorts, the expression in keratocytes as part of wound healing, and the documented association of PPCD and keratoconus, it seems likely that the role of VSX1 as a genetic factor contributing to disease is real.

Regulation of retinal interneuron subtype identity by the Iroquois homeobox gene Irx6

Interneuronal subtype diversity lies at the heart of the distinct molecular properties and synaptic connections that shape the formation of the neuronal circuits that are necessary for the complex spatial and temporal processing of sensory information. Here, we investigate the role of Irx6, a member of the Iroquois homeodomain transcription factor family, in regulating the development of retinal bipolar interneurons. Using a knock-in reporter approach, we show that, in the mouse retina, Irx6 is expressed in type 2 and 3a OFF bipolar interneurons and is required for the expression of cell type-specific markers in these cells, likely through direct transcriptional regulation. In Irx6 mutant mice, presumptive type 3a bipolar cells exhibit an expansion of their axonal projection domain to the entire OFF region of the inner plexiform layer, and adopt molecular features of both type 2 and 3a bipolar cells, highlighted by the ectopic upregulation of neurokinin 3 receptor (Nk3r) and Vsx1. These findings reveal Irx6 as a key regulator of type 3a bipolar cell identity that prevents these cells from adopting characteristic features of type 2 bipolar cells. Analysis of the Irx6;Vsx1 double null retina suggests that the terminal differentiation of type 2 bipolar cells is dependent on the combined expression of the transcription factors Irx6 and Vsx1, but also points to the existence of Irx6;Vsx1-independent mechanisms in regulating OFF bipolar subtype-specific gene expression. This work provides insight into the generation of neuronal subtypes by revealing a mechanism in which opposing, yet interdependent, transcription factors regulate subtype identity.

Neto2 interacts with the scaffolding protein GRIP and regulates synaptic abundance of kainate receptors

Kainate receptors (KARs) are a class of ionotropic glutamate receptors that are expressed throughout the central nervous system. The function and subcellular localization of KARs are tightly regulated by accessory proteins. We have previously identified the single-pass transmembrane proteins, Neto1 and Neto2, to be associated with native KARs. In the hippocampus, Neto1, but not Neto2, controls the abundance and modulates the kinetics of postsynaptic KARs. Here we evaluated whether Neto2 regulates synaptic KAR levels in the cerebellum where Neto1 expression is limited to the deep cerebellar nuclei. In the cerebellum, where Neto2 is present abundantly, we found a ∼40% decrease in GluK2-KARs at the postsynaptic density (PSD) of Neto2-null mice. No change, however, was observed in total level of GluK2-KARs, thereby suggesting a critical role of Neto2 on the synaptic localization of cerebellar KARs. The presence of a putative class II PDZ binding motif on Neto2 led us to also investigate whether it interacts with PDZ domain-containing proteins previously implicated in regulating synaptic abundance of KARs. We identified a PDZ-dependent interaction between Neto2 and the scaffolding protein GRIP. Furthermore, coexpression of Neto2 significantly increased the amount of GRIP associated with GluK2, suggesting that Neto2 may promote and/or stabilize GluK2:GRIP interactions. Our results demonstrate that Neto2, like Neto1, is an important auxiliary protein for modulating the synaptic levels of KARs. Moreover, we propose that the interactions of Neto1/2 with various scaffolding proteins is a critical mechanism by which KARs are stabilized at diverse synapses.

Gene networks: dissecting pathways in retinal development and disease

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

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.

Sequence divergence in the 3'-untranslated region has an effect on the subfunctionalization of duplicate genes

Recent studies demonstrated that sequence divergence in both transcriptional regulatory region and coding region contributes to the subfunctionalization of duplicate gene. However, whether sequence divergence in the 3'-untranslated region (3'-UTR) has an impact on the subfunctionalization of duplicate genes remains unclear. Here, we identified two diverging duplicate vsx1 (visual system homeobox-1) loci in goldfish, named vsx1A1 and vsx1A2. Phylogenetic analysis suggests that vsx1A1 and vsx1A2 may arise from a duplication of vsx1 after the separation of goldfish and zebrafish. Sequence comparison revealed that divergence in both transcriptional and translational regulatory regions is higher than divergence in the introns. vsx1A2 expresses during blastula and gastrula stages and in adult retina but silences from segmentation stage to hatching stage, vsx1A1 starts expression from segmentation onward. Comparing to that zebrafish vsx1 expresses in all the developmental stages and in the adult retina, it appears that goldfish vsx1A1 and vsx1A2 are under going to share the functions of ancestral vsx1. The different but overlapping temporal expression patterns of vsx1A1 and vsx1A2 suggest that sequence divergence in the promoter region of duplicate vsx1 is not sufficient for partitioning the functions of ancestral vsx1. By comparing vsx1A1 and vsx1A2 3'-UTR-linked green fluorescent protein gene expression patterns, we demonstrated that the 3'-UTR of vsx1A1 remains but the 3'-UTR of vsx1A2 has lost the capability of mediating bipolar cell specific expression during retina development. These results indicate that sequence divergence in the 3'-UTRs has a clear effect on subfunctionalization of the duplicate genes.

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.

Non-coding RNAs in retinal development

Retinal development is dependent on an accurately functioning network of transcriptional and translational regulators. Among the diverse classes of molecules involved, non-coding RNAs (ncRNAs) play a significant role. Members of this family are present in the cell as transcripts, but are not translated into proteins. MicroRNAs (miRNAs) are small ncRNAs that act as post-transcriptional regulators. During the last decade, they have been implicated in a variety of biological processes, including the development of the nervous system. On the other hand, long-ncRNAs (lncRNAs) represent a different class of ncRNAs that act mainly through processes involving chromatin remodeling and epigenetic mechanisms. The visual system is a prominent model to investigate the molecular mechanisms underlying neurogenesis or circuit formation and function, including the differentiation of retinal progenitor cells to generate the seven principal cell classes in the retina, pathfinding decisions of retinal ganglion cell axons in order to establish the correct connectivity from the eye to the brain proper, and activity-dependent mechanisms for the functionality of visual circuits. Recent findings have associated ncRNAs in several of these processes and uncovered a new level of complexity for the existing regulatory mechanisms. This review summarizes and highlights the impact of ncRNAs during the development of the vertebrate visual system, with a specific focus on the role of miRNAs and a synopsis regarding recent findings on lncRNAs in the retina.

The Genetics of Keratoconus: A Review

Keratoconus is the most common ectatic disorder of the corneal. Genetic and environmental factors may contribute to its pathogenesis. The focus of this article is to summarize current research into the complex genetics of keratoconus. We discuss the evidence of genetic etiology including family-based linkage studies, twin studies, genetic mutations, and genome-wide association studies. The genes implicated potentially include VSX1, miR-184, DOCK9, SOD1, RAB3GAP1, and HGF. Besides the coding mutations, we also highlight the potential contribution of DNA copy number variants in the pathogenesis of keratoconus. Finally, we present future directions for genetic research in the understanding of the complex genetics of keratoconus and its clinical significance. As new functional, candidate genes for keratoconus are being discovered at a rapid pace, the molecular genetic mechanisms underlying keratoconus pathogenesis will advance our understanding of keratoconus and promote the development of a novel therapy.

Vsx1 regulates terminal differentiation of type 7 ON bipolar cells

Although retinal bipolar cells represent a morphologically well defined population of retinal interneurons, very little is known about the developmental mechanisms that regulate their processing. Furthermore, the identity of specific bipolar cell types that function in distinct visual circuits remains poorly understood. Here, we show that the homeobox gene Vsx1 is expressed in Type 7 ON bipolar cells. In the absence of Vsx1, Type 7 bipolar cells exhibit proper morphological specification but show defects in terminal gene expression. Vsx1 is required for the repression of bipolar cell-specific markers, including Calcium-binding protein 5 and Chx10. This contrasts its genetic requirement as an activator of gene expression in OFF bipolar cells. To assess possible ON signaling defects in Vsx1-null mice, we recorded specifically from ON-OFF directionally selective ganglion cells (DSGCs), which cofasciculate with Type 7 bipolar cell terminals. Vsx1-null ON-OFF DSGCs received more sustained excitatory synaptic input, possibly due to Type 7 bipolar cell defects. Interestingly, in Vsx1-null mice, the directionally selective circuit is functional but compromised. Together, these findings indicate that Vsx1 regulates terminal gene expression in Type 7 bipolar cells and is necessary for proper ON visual signaling within a directionally selective circuit.

A transgenic mouse line expressing cre recombinase in undifferentiated postmitotic mouse retinal bipolar cell precursors

Approaches for manipulating cell type-specific gene expression during development depend on the identification of novel genetic tools. Here, we report the generation of a transgenic mouse line that utilizes Vsx2 upstream sequences to direct Cre recombinase to developing retinal bipolar cells. In contrast to the endogenous Vsx2 expression pattern, transgene expression was not detected in proliferating retinal progenitor cells and was restricted to post-mitotic bipolar cells. Cre immunolabeling was detected in rod bipolar cells and a subset of ON and OFF cone bipolar cells. Expression was first observed at postnatal day 3 and was detectable between 24 hours and 36 hours after the last S-phase of the cell cycle. The appearance of Cre-immunolabeled cells preceded the expression of bipolar cell type-specific markers such as PKCα and Cabp5 suggesting that transgene expression is initiated prior to terminal differentiation. In the presence of a constitutive conditional reporter transgene, reporter fluorescence was detected in Cre-expressing bipolar cells in the mature retina as expected, but was also observed in Cre-negative Type 2 bipolar cells and occasionally in Cre-negative photoreceptor cells. Together these findings reveal a new transgenic tool for directing gene expression to post-mitotic retinal precursors that are mostly committed to a bipolar cell fate.

Neto1 is an auxiliary subunit of native synaptic kainate receptors

Ionotropic glutamate receptors of AMPA, NMDA, and kainate receptor (KAR) subtypes mediate fast excitatory synaptic transmission in the vertebrate CNS. Auxiliary proteins have been identified for AMPA and NMDA receptor complexes, but little is known about KAR complex proteins. We previously identified the CUB (complement C1r/C1s, Uegf, Bmpl) domain protein, Neto1, as an NMDA receptor-associated polypeptide. Here, we show that Neto1 is also an auxiliary subunit for endogenous synaptic KARs. We found that Neto1 and KARs coimmunoprecipitated from brain lysates, from postsynaptic densities (PSDs) and, in a manner dependent on Neto1 CUB domains, when coexpressed in heterologous cells. In Neto1-null mice, there was an ∼50% reduction in the abundance of GluK2-KARs in hippocampal PSDs. Neto1 strongly localized to CA3 stratum lucidum, and loss of Neto1 resulted in a selective deficit in KAR-mediated neurotransmission at mossy fiber-CA3 pyramidal cell (MF-CA3) synapses: KAR-mediated EPSCs in Neto1-null mice were reduced in amplitude and decayed more rapidly than did those in wild-type mice. In contrast, the loss of Neto2, which also localizes to stratum lucidum and interacts with KARs, had no effect on KAR synaptic abundance or MF-CA3 transmission. Indeed, MF-CA3 KAR deficits in Neto1/Neto2-double-null mutant mice were indistinguishable from Neto1 single-null mice. Thus, our findings establish Neto1 as an auxiliary protein required for synaptic function of KARs. The ability of Neto1 to regulate both NMDARs and KARs reveals a unique dual role in controlling synaptic transmission by serving as an auxiliary protein for these two classes of ionotropic glutamate receptors in a synapse-specific fashion.

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.

Absence of Vsx1 expression in the normal and damaged mouse cornea

PURPOSE

To examine the expression of visual system homeobox 1 (Vsx1) in the mouse cornea and its potential role in the corneal wound response pathway.

METHODS

Expression of Vsx1 was examined by quantitative reverse-transcription PCR (qRT-PCR) in corneal tissue from developing and adult mice and from mice that had undergone alkali-burn corneal wounding. Immunolabeling and Vsx1 knock-in reporter gene expression in wild type and Vsx1 null-mice were used to confirm the qRT-PCR data.

RESULTS

Using qRT-PCR, Vsx1 expression was not detected in either the postnatal or adult mouse cornea or in corneas following wounding. This qRT-PCR data was supported by the absence of specific Vsx1 immunolabeling and Vsx1 knock-in reporter expression in untreated and wounded corneas.

CONCLUSIONS

In mice, Vsx1 mRNA, protein or reporter gene expression is not detected in the normal or damaged cornea. These results make it uncertain what role VSX1/Vsx1 plays in corneal biology. Future experiments examining the pathogenicity of VSX1 mutations associated with corneal dystrophy are required to rule out species differences and possible non-cell autonomous roles for VSX1 in the cornea.

Karyopherins in nuclear transport of homeodomain proteins during development

Homeodomain proteins are crucial transcription factors for cell differentiation, cell proliferation and organ development. Interestingly, their homeodomain signature structure is important for both their DNA-binding and their nucleocytoplasmic trafficking. The accurate nucleocytoplasmic distribution of these proteins is essential for their functions. We summarize information on (a) the roles of karyopherins for import and export of homeoproteins, (b) the regulation of their nuclear transport during development, and (c) the corresponding complexity of homeoprotein nucleocytoplasmic transport signals. This article is part of a Special Issue entitled: Regulation of Signaling and Cellular Fate through Modulation of Nuclear Protein Import.

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.

Early B-cell factors are required for specifying multiple retinal cell types and subtypes from postmitotic precursors

The establishment of functional retinal circuits in the mammalian retina depends critically on the proper generation and assembly of six classes of neurons, five of which consist of two or more subtypes that differ in morphologies, physiological properties, and/or sublaminar positions. How these diverse neuronal types and subtypes arise during retinogenesis still remains largely to be defined at the molecular level. Here we show that all four family members of the early B-cell factor (Ebf) helix-loop-helix transcription factors are similarly expressed during mouse retinogenesis in several neuronal types and subtypes including ganglion, amacrine, bipolar, and horizontal cells, and that their expression in ganglion cells depends on the ganglion cell specification factor Brn3b. Misexpressed Ebfs bias retinal precursors toward the fates of non-AII glycinergic amacrine, type 2 OFF-cone bipolar and horizontal cells, whereas a dominant-negative Ebf suppresses the differentiation of these cells as well as ganglion cells. Reducing Ebf1 expression by RNA interference (RNAi) leads to an inhibitory effect similar to that of the dominant-negative Ebf, effectively neutralizes the promotive effect of wild-type Ebf1, but has no impact on the promotive effect of an RNAi-resistant Ebf1. These data indicate that Ebfs are both necessary and sufficient for specifying non-AII glycinergic amacrine, type 2 OFF-cone bipolar and horizontal cells, whereas they are only necessary but not sufficient for specifying ganglion cells; and further suggest that Ebfs may coordinate and cooperate with other retinogenic factors to ensure proper specification and differentiation of diverse retinal cell types and subtypes.

The PPCD1 mouse: characterization of a mouse model for posterior polymorphous corneal dystrophy and identification of a candidate gene

The PPCD1 mouse, a spontaneous mutant that arose in our mouse colony, is characterized by an enlarged anterior chamber resulting from metaplasia of the corneal endothelium and blockage of the iridocorneal angle by epithelialized corneal endothelial cells. The presence of stratified multilayered corneal endothelial cells with abnormal patterns of cytokeratin expression are remarkably similar to those observed in human posterior polymorphous corneal dystrophy (PPCD) and the sporadic condition, iridocorneal endothelial syndrome. Affected eyes exhibit epithelialized corneal endothelial cells, with inappropriate cytokeratin expression and proliferation over the iridocorneal angle and posterior cornea. We have termed this the “mouse PPCD1” phenotype and mapped the mouse locus for this phenotype, designated “Ppcd1”, to a 6.1 Mbp interval on Chromosome 2, which is syntenic to the human Chromosome 20 PPCD1 interval. Inheritance of the mouse PPCD1 phenotype is autosomal dominant, with complete penetrance on the sensitive DBA/2J background and decreased penetrance on the C57BL/6J background. Comparative genome hybridization has identified a hemizygous 78 Kbp duplication in the mapped interval. The endpoints of the duplication are located in positions that disrupt the genes Csrp2bp and 6330439K17Rik and lead to duplication of the pseudogene LOC100043552. Quantitative reverse transcriptase-PCR indicates that expression levels of Csrp2bp and 6330439K17Rik are decreased in eyes of PPCD1 mice. Based on the observations of decreased gene expression levels, association with ZEB1-related pathways, and the report of corneal opacities in Csrp2bp^{tm1a(KOMP)Wtsi} heterozygotes and embryonic lethality in nulls, we postulate that duplication of the 78 Kbp segment leading to haploinsufficiency of Csrp2bp is responsible for the mouse PPCD1 phenotype. Similarly, CSRP2BP haploinsufficiency may lead to human PPCD.

Development of the retina and optic pathway

Our understanding of the development of the retina and visual pathways has seen enormous advances during the past 25years. New imaging technologies, coupled with advances in molecular biology, have permitted a fuller appreciation of the histotypical events associated with proliferation, fate determination, migration, differentiation, pathway navigation, target innervation, synaptogenesis and cell death, and in many instances, in understanding the genetic, molecular, cellular and activity-dependent mechanisms underlying those developmental changes. The present review considers those advances associated with the lineal relationships between retinal nerve cells, the production of retinal nerve cell diversity, the migration, patterning and differentiation of different types of retinal nerve cells, the determinants of the decussation pattern at the optic chiasm, the formation of the retinotopic map, and the establishment of ocular domains within the thalamus.

Gene duplication and the origins of morphological complexity in pancrustacean eyes, a genomic approach

BACKGROUND

Duplication and divergence of genes and genetic networks is hypothesized to be a major driver of the evolution of complexity and novel features. Here, we examine the history of genes and genetic networks in the context of eye evolution by using new approaches to understand patterns of gene duplication during the evolution of metazoan genomes. We hypothesize that 1) genes involved in eye development and phototransduction have duplicated and are retained at higher rates in animal clades that possess more distinct types of optical design; and 2) genes with functional relationships were duplicated and lost together, thereby preserving genetic networks. To test these hypotheses, we examine the rates and patterns of gene duplication and loss evident in 19 metazoan genomes, including that of Daphnia pulex - the first completely sequenced crustacean genome. This is of particular interest because the pancrustaceans (hexapods+crustaceans) have more optical designs than any other major clade of animals, allowing us to test specifically whether the high amount of disparity in pancrustacean eyes is correlated with a higher rate of duplication and retention of vision genes.

RESULTS

Using protein predictions from 19 metazoan whole-genome projects, we found all members of 23 gene families known to be involved in eye development or phototransduction and deduced their phylogenetic relationships. This allowed us to estimate the number and timing of gene duplication and loss events in these gene families during animal evolution. When comparing duplication/retention rates of these genes, we found that the rate was significantly higher in pancrustaceans than in either vertebrates or non-pancrustacean protostomes. Comparing patterns of co-duplication across Metazoa showed that while these eye-genes co-duplicate at a significantly higher rate than those within a randomly shuffled matrix, many genes with known functional relationships in model organisms did not co-duplicate more often than expected by chance.

CONCLUSIONS

Overall, and when accounting for factors such as differential rates of whole-genome duplication in different groups, our results are broadly consistent with the hypothesis that genes involved in eye development and phototransduction duplicate at a higher rate in Pancrustacea, the group with the greatest variety of optical designs. The result that these genes have a significantly high number of co-duplications and co-losses could be influenced by shared functions or other unstudied factors such as synteny. Since we did not observe co-duplication/co-loss of genes for all known functional modules (e.g. specific regulatory networks), the interactions among suites of known co-functioning genes (modules) may be plastic at the temporal scale of analysis performed here. Other factors in addition to gene duplication - such as cis-regulation, heterotopy, and co-option - are also likely to be strong factors in the diversification of eye types.

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.

MicroRNAs couple cell fate and developmental timing in retina

Cell identity is acquired in different brain structures according to a stereotyped timing schedule, by accommodating the proliferation of multipotent progenitor cells and the generation of distinct types of mature nerve cells at precise times. However, the molecular mechanisms coupling the identity of a specific neuron and its birth date are poorly understood. In the neural retina, only late progenitor cells that divide slowly can become bipolar neurons, by the activation of otx2 and vsx1 genes. In Xenopus, we found that Xotx2 and Xvsx1 translation is inhibited in early progenitor cells that divide rapidly by a set of cell cycle-related microRNAs (miRNAs). Through expression and functional screenings, we selected 4 miRNAs--mir-129, mir-155, mir-214, and mir-222--that are highly expressed at early developmental stages in the embryonic retina and bind to the 3' UTR of Xotx2 and Xvsx1 mRNAs inhibiting their translation. The functional inactivation of these miRNAs in vivo releases the inhibition, supporting the generation of additional bipolar cells. We propose a model in which the proliferation rate and the age of a retinal progenitor are linked to each other and determine the progenitor fate through the activity of a set of miRNAs.

The use of X-ray scattering techniques to quantify the orientation and distribution of collagen in the corneal stroma

The bulk of the corneal stroma is comprised of a layered network of fibrillar collagen. Determining the architecture of this unique structure may help us to better understand the cornea's biomechanical and optical function. The analysis of diffraction patterns obtained when X-rays are passed through the regularly arranged collagen molecules and fibrils of the stromal matrix yields quantitative data on fibrillar organisation, including the orientation and distribution of collagen lamellae within the corneal plane. In recent years, by exploiting the radiation from powerful synchrotron sources, techniques have been developed to enable the mapping of collagen fibril, and therefore lamellar, directions across whole corneas. This article aims to summarise the use of X-ray diffraction to map the orientation and distribution of collagen in the corneal stroma. The implications of the knowledge gained so far are discussed in relation to the optical and biomechanical properties of the cornea, and their alteration due to disease and surgical intervention.

Eye evolution at high resolution: the neuron as a unit of homology

Based on differences in morphology, photoreceptor-type usage and lens composition it has been proposed that complex eyes have evolved independently many times. The remarkable observation that different eye types rely on a conserved network of genes (including Pax6/eyeless) for their formation has led to the revised proposal that disparate complex eye types have evolved from a shared and simpler prototype. Did this ancestral eye already contain the neural circuitry required for image processing? And what were the evolutionary events that led to the formation of complex visual systems, such as those found in vertebrates and insects? The recent identification of unexpected cell-type homologies between neurons in the vertebrate and Drosophila visual systems has led to two proposed models for the evolution of complex visual systems from a simple prototype. The first, as an extension of the finding that the neurons of the vertebrate retina share homologies with both insect (rhabdomeric) and vertebrate (ciliary) photoreceptor cell types, suggests that the vertebrate retina is a composite structure, made up of neurons that have evolved from two spatially separate ancestral photoreceptor populations. The second model, based largely on the conserved role for the Vsx homeobox genes in photoreceptor-target neuron development, suggests that the last common ancestor of vertebrates and flies already possessed a relatively sophisticated visual system that contained a mixture of rhabdomeric and ciliary photoreceptors as well as their first- and second-order target neurons. The vertebrate retina and fly visual system would have subsequently evolved by elaborating on this ancestral neural circuit. Here we present evidence for these two cell-type homology-based models and discuss their implications.

Neto1 is a novel CUB-domain NMDA receptor-interacting protein required for synaptic plasticity and learning

The N-methyl-D-aspartate receptor (NMDAR), a major excitatory ligand-gated ion channel in the central nervous system (CNS), is a principal mediator of synaptic plasticity. Here we report that neuropilin tolloid-like 1 (Neto1), a complement C1r/C1s, Uegf, Bmp1 (CUB) domain-containing transmembrane protein, is a novel component of the NMDAR complex critical for maintaining the abundance of NR2A-containing NMDARs in the postsynaptic density. Neto1-null mice have depressed long-term potentiation (LTP) at Schaffer collateral-CA1 synapses, with the subunit dependency of LTP induction switching from the normal predominance of NR2A- to NR2B-NMDARs. NMDAR-dependent spatial learning and memory is depressed in Neto1-null mice, indicating that Neto1 regulates NMDA receptor-dependent synaptic plasticity and cognition. Remarkably, we also found that the deficits in LTP, learning, and memory in Neto1-null mice were rescued by the ampakine CX546 at doses without effect in wild-type. Together, our results establish the principle that auxiliary proteins are required for the normal abundance of NMDAR subunits at synapses, and demonstrate that an inherited learning defect can be rescued pharmacologically, a finding with therapeutic implications for humans.

The fundamental unit for information processing in the brain is the synapse, a highly specialized site of communication between the brain's multitude of individual neurons. The strength of the communication at each synapse changes in response to neuronal activity—a process called synaptic plasticity—allowing networks of neurons to adapt and learn. How synaptic plasticity occurs is a major question in neurobiology. A central player in synaptic plasticity is an assembly of synaptic proteins called the NMDA receptor complex. Here, we discovered that the protein Neto1 is a component of the NMDA receptor complex. Neto1-deficient mice had a dramatic decrease in the number of NMDA receptors at synapses and consequently, synaptic plasticity and learning were impaired. By indirectly enhancing the function of the residual NMDA receptors in Neto1-deficient mice with a small molecule, we restored synaptic plasticity and learning to normal levels. Our findings establish the principle that inherited abnormalities of synaptic plasticity and learning due to NMDA receptor dysfunction can be pharmacologically corrected. Our discoveries also suggest that synaptic proteins that share a molecular signature, called the CUB domain, with Neto1 may be important components of synaptic receptors across species, because several CUB-domain proteins in worms have also been found to regulate synaptic receptors.

Spatial learning and memory depend on the N-methyl-D-aspartic acid receptor, a synaptic ion channel regulated by Neto1. Impaired cognition due to the absence of Neto1 can be rescued pharmacologically, a finding with implications for the therapy of inherited learning defects in humans.

Distinct roles of transcription factors brn3a and brn3b in controlling the development, morphology, and function of retinal ganglion cells

Transcriptional regulatory networks that control the morphologic and functional diversity of mammalian neurons are still largely undefined. Here we dissect the roles of the highly homologous POU-domain transcription factors Brn3a and Brn3b in retinal ganglion cell (RGC) development and function using conditional Brn3a and Brn3b alleles that permit the visualization of individual wild-type or mutant cells. We show that Brn3a- and Brn3b-expressing RGCs exhibit overlapping but distinct dendritic stratifications and central projections. Deletion of Brn3a alters dendritic stratification and the ratio of monostratified:bistratified RGCs, with little or no change in central projections. In contrast, deletion of Brn3b leads to RGC transdifferentiation and loss, axon defects in the eye and brain, and defects in central projections that differentially compromise a variety of visually driven behaviors. These findings reveal distinct roles for Brn3a and Brn3b in programming RGC diversity, and they illustrate the broad utility of germline methods for genetically manipulating and visualizing individual identified mammalian neurons.

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.

Plasmalemmal and vesicular gamma-aminobutyric acid transporter expression in the developing mouse retina

Plasmalemmal and vesicular gamma-aminobutyric acid (GABA) transporters influence neurotransmission by regulating high-affinity GABA uptake and GABA release into the synaptic cleft and extracellular space. Postnatal expression of the plasmalemmal GABA transporter-1 (GAT-1), GAT-3, and the vesicular GABA/glycine transporter (VGAT) were evaluated in the developing mouse retina by using immunohistochemistry with affinity-purified antibodies. Weak transporter immunoreactivity was observed in the inner retina at postnatal day 0 (P0). GAT-1 immunostaining at P0 and at older ages was in amacrine and displaced amacrine cells in the inner nuclear layer (INL) and ganglion cell layer (GCL), respectively, and in their processes in the inner plexiform layer (IPL). At P10, weak GAT-1 immunostaining was in Müller cell processes. GAT-3 immunostaining at P0 and older ages was in amacrine cells and their processes, as well as in Müller cells and their processes that extended radially across the retina. At P10, Müller cell somata were observed in the middle of the INL. VGAT immunostaining was present at P0 and older ages in amacrine cells in the INL as well as processes in the IPL. At P5, weak VGAT immunostaining was also observed in horizontal cell somata and processes. By P15, the GAT and VGAT immunostaining patterns appear similar to the adult immunostaining patterns; they reached adult levels by about P20. These findings demonstrate that GABA uptake and release are initially established in the inner retina during the first postnatal week and that these systems subsequently mature in the outer retina during the second postnatal week.

Expression profiling of zebrafish sox9 mutants reveals that Sox9 is required for retinal differentiation

The transcription factor gene Sox9 plays various roles in development, including differentiation of the skeleton, gonads, glia, and heart. Other functions of Sox9 remain enigmatic. Because Sox9 protein regulates expression of target genes, the identification of Sox9 targets should facilitate an understanding of the mechanisms of Sox9 action. To help identify Sox9 targets, we used microarray expression profiling to compare wild-type embryos to mutant embryos lacking activity for both sox9a and sox9b, the zebrafish co-orthologs of Sox9. Candidate genes were further evaluated by whole-mount in situ hybridization in wild-type and sox9 single and double mutant embryos. Results identified genes expressed in cartilage (col2a1a and col11a2), retina (calb2a, calb2b, crx, neurod, rs1, sox4a and vsx1) and pectoral fin bud (klf2b and EST AI722369) as candidate targets for Sox9. Cartilage is a well-characterized Sox9 target, which validates this strategy, whereas retina represents a novel Sox9 function. Analysis of mutant phenotypes confirmed that Sox9 helps regulate the number of Müller glia and photoreceptor cells and helps organize the neural retina. These roles in eye development were previously unrecognized and reinforce the multiple functions that Sox9 plays in vertebrate development.

Cone contacts, mosaics, and territories of bipolar cells in the mouse retina

We report a quantitative analysis of the different bipolar cell types of the mouse retina. They were identified in wild-type mice by specific antibodies or in transgenic mouse lines by specific expression of green fluorescent protein or Clomeleon. The bipolar cell densities, their cone contacts, their dendritic coverage, and their axonal tiling were measured in retinal whole mounts. The results show that each and all cones are contacted by at least one member of any given type of bipolar cell (not considering genuine blue cones). Consequently, each cone feeds its light signals into a minimum of 10 different bipolar cells. Parallel processing of an image projected onto the retina, therefore, starts at the first synapse of the retina, the cone pedicle. The quantitative analysis suggests that our proposed catalog of 11 cone bipolar cells and one rod bipolar cell is complete, and all major bipolar cell types of the mouse retina appear to have been discovered.

A core paired-type and POU homeodomain-containing transcription factor program drives retinal bipolar cell gene expression

The diversity of cell types found within the vertebrate CNS arises in part from action of complex transcriptional programs. In the retina, the programs driving diversification of various cell types have not been completely elucidated. To investigate gene regulatory networks that underlie formation and function of one retinal circuit component, the bipolar cell, transcriptional regulation of three bipolar cell-enriched genes was analyzed. Using in vivo retinal DNA transfection and reporter gene constructs, a 200 bp Grm6 enhancer sequence, a 445 bp Cabp5 promoter sequence, and a 164 bp Chx10 enhancer sequence, were defined, each driving reporter expression specifically in distinct but overlapping bipolar cell subtypes. Bioinformatic analysis of sequences revealed the presence of potential paired-type and POU homeodomain-containing transcription factor binding sites, which were shown to be critical for reporter expression through deletion studies. The paired-type homeodomain transcription factors (TFs) Crx and Otx2 and the POU homeodomain factor Brn2 are expressed in bipolar cells and interacted with the predicted binding sequences as assessed by electrophoretic mobility shift assay. Grm6, Cabp5, and Chx10 reporter activity was reduced in Otx2 loss-of-function retinas. Endogenous gene expression of bipolar cell molecular markers was also dependent on paired-type homeodomain-containing TFs, as assessed by RNA in situ hybridization and reverse transcription-PCR in mutant retinas. Cabp5 and Chx10 reporter expression was reduced in dominant-negative Brn2-transfected retinas. The paired-type and POU homeodomain-containing TFs Otx2 and Brn2 together appear to play a common role in regulating gene expression in retinal bipolar cells.

Conserved role of the Vsx genes supports a monophyletic origin for bilaterian visual systems

BACKGROUND

Components of the genetic network specifying eye development are conserved from flies to humans, but homologies between individual neuronal cell types have been difficult to identify. In the vertebrate retina, the homeodomain-containing transcription factor Chx10 is required for both progenitor cell proliferation and the development of the bipolar interneurons, which transmit visual signals from photoreceptors to ganglion cells.

RESULTS

We show that dVsx1 and dVsx2, the two Drosophila homologs of Chx10, play a conserved role in visual-system development. DVSX1 is expressed in optic-lobe progenitor cells, and, in dVsx1 mutants, progenitor cell proliferation is defective, leading to hypocellularity. Subsequently, DVSX1 and DVSX2 are coexpressed in a subset of neurons in the medulla, including the transmedullary neurons that transmit visual information from photoreceptors to deeper layers of the visual system. In dVsx mutant adults, the optic lobe is reduced in size, and the medulla is small or absent. These results suggest that the progenitor cells and photoreceptor target neurons of the vertebrate retina and fly optic lobe are ancestrally related. Genetic and functional homology may extend to the neurons directly downstream of the bipolar and transmedullary neurons, the vertebrate ganglion cells and fly lobula projection neurons. Both cell types project to visual-processing centers in the brain, and both sequentially express the Math5/ATO and Brn3b/ACJ6 transcription factors during their development.

CONCLUSIONS

Our findings support a monophyletic origin for the bilaterian visual system in which the last common ancestor of flies and vertebrates already contained a primordial visual system with photoreceptors, interneurons, and projection neurons.

Identification of molecular markers of bipolar cells in the murine retina

Retinal bipolar neurons serve as relay interneurons that connect rod and cone photoreceptor cells to amacrine and ganglion cells. They exhibit diverse morphologies essential for correct routing of photoreceptor cell signals to specific postsynaptic amacrine and ganglion cells. The development and physiology of these interneurons have not been completely defined molecularly. Despite previous identification of genes expressed in several bipolar cell subtypes, molecules that mark each bipolar cell type still await discovery. In this report, novel genetic markers of murine bipolar cells were found. Candidates were initially generated by using microarray analysis of single bipolar cells and mining of retinal serial analysis of gene expression (SAGE) data. These candidates were subsequently tested for expression in bipolar cells by RNA in situ hybridization. Ten new molecular markers were identified, five of which are highly enriched in their expression in bipolar cells within the adult retina. Double-labeling experiments using probes for previously characterized subsets of bipolar cells were performed to identify the subtypes of bipolar cells that express the novel markers. Additionally, the expression of bipolar cell genes was analyzed in Bhlhb4 knockout retinas, in which rod bipolar cells degenerate postnatally, to delineate further the identity of bipolar cells in which novel markers are found. From the analysis of Bhlhb4 mutant retinas, cone bipolar cell gene expression appears to be relatively unaffected by the degeneration of rod bipolar cells. Identification of molecular markers for the various subtypes of bipolar cells will lead to greater insights into the development and function of these diverse interneurons.

Genetic control of circuit function: Vsx1 and Irx5 transcription factors regulate contrast adaptation in the mouse retina

Transcriptional programs guide the specification of neural cell types in the developing nervous system. However, it is unclear whether such programs also control specific aspects of neural circuit function at maturity. In the mammalian retina, Vsx1 and Irx5 transcription factors are present in a subset of bipolar interneurons that convey signals from photoreceptors to ganglion cells. The biased expression of Vsx1 and Irx5 in hyperpolarizing OFF compared with depolarizing ON bipolar cells suggests that these transcription factors may selectively regulate signal processing in OFF circuits. To test this hypothesis, we generated mice lacking both Vsx1 and Irx5. Bipolar cells in these mice were morphologically normal, but the expression of cell-specific markers in some OFF but not ON bipolar cells was reduced or absent. To assess visual function in Vsx1(-/-)Irx5(-/-) retinas, we recorded light responses from ensembles of retinal ganglion cells (RGCs). We first identified functional RGC types in control mice and describe their response properties and adaptation to temporal contrast using a simple linear-nonlinear model. We found that space-time receptive fields of RGCs are unchanged in Vsx1(-/-)Irx5(-/-) mice compared with control retinas. In contrast, response threshold, gain, and range were lowered in a cell-type-specific manner in OFF but not ON RGCs in Vsx1(-/-)Irx5(-/-) retinas. Finally, we discovered that the ability to adapt to temporal contrast is greatly reduced in OFF RGCs in the double mutant, suggesting that Vsx1 and Irx5 control specific aspects of visual function in circuits of the mammalian retina.

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.

Genetic analysis of chromosome 20-related posterior polymorphous corneal dystrophy: genetic heterogeneity and exclusion of three candidate genes

PURPOSE

Posterior polymorphous corneal dystrophy (PPCD) is a genetically heterogeneous autosomal dominant condition which maps to the pericentromeric region of chromosome 20. Mutations in the VSX1 transcription factor have been reported in patients affected with PPCD, keratoconus, or a combination of both phenotypes. However, no mutation was identified in the coding region of VSX1 in the family used for the original mapping. To clarify the genetic basis of PPCD1, a thorough analysis was performed on the original PPCD1 family and two other PPCD1-linked families. As part of the analysis, the expression profile, transcript variants, and evolutionary conserved regions of VSX1, a key candidate gene within the linkage interval, were characterized.

METHODS

Haplotype analysis was performed using highly informative markers on the pericentromeric region of chromosome 20. VSX1 transcript variants were identified using RT-PCR and characterized by 3'RACE assay. Temporal expression profile of VSX1 was evaluated using semi-quantitative real-time RT-PCR on human tissues. Evolutionary conserved regions (ECRs) were identified in the vicinity of VSX1 using publicly available sequence alignments (UCSC and rVista) and sequenced for mutation analysis.

RESULTS

Recombination events were identified that narrow the PPCD1-disease interval from 20 to 16.44 cM. This smaller interval includes the CHED1 locus and a recently described PPCD locus in Czech families. The three strongest candidate genes of the PPCD1-CHED1 overlap region (RBBP9, ZNF133, SLC24A3) did not show any mutations in our PPCD1-linked families. Semi-quantitative real-time RT-PCR detected VSX1 expression in neonatal human cornea. Six transcript variants of VSX1 were characterized. Four of the transcript variants spliced to two novel exons downstream of the gene. Mutation analysis of the PPCD1-linked families did not reveal any mutations in the full genomic sequence of VSX1 (considering all splice variants) or in the six cis- regulatory modules predicted in the vicinity of VSX1 (100 kb).

CONCLUSIONS

This is the first documentation of VSX1 expression in human neonatal cornea. We provide evidence for genetic heterogeneity of chromosome 20-related PPCD and refinement of the original PPCD1 interval. The full genomic sequence of VSX1 and coding exons of three other candidate genes were excluded from being pathogenic in the original PPCD1 family.

Islet-1 controls the differentiation of retinal bipolar and cholinergic amacrine cells

Whereas the mammalian retina possesses a repertoire of factors known to establish general retinal cell types, these factors alone cannot explain the vast diversity of neuronal subtypes. In other CNS regions, the differentiation of diverse neuronal pools is governed by coordinately acting LIM-homeodomain proteins including the Islet-class factor Islet-1 (Isl1). We report that deletion of Isl1 profoundly disrupts retinal function as assessed by electroretinograms and vision as assessed by optomotor behavior. These deficits are coupled with marked reductions in mature ON- and OFF-bipolar (>76%), cholinergic amacrine (93%), and ganglion (71%) cells. Mosaic deletion of Isl1 permitted a chimeric analysis of "wild-type" cells in a predominantly Isl1-null environment, demonstrating a cell-autonomous role for Isl1 in rod bipolar and cholinergic amacrine development. Furthermore, the effects on bipolar cell development appear to be dissociable from the preceding retinal ganglion cell loss, because Pou4f2-null mice are devoid of similar defects in bipolar cell marker expression. Expression of the ON- and OFF-bipolar cell differentiation factors Bhlhb4 and Vsx1, respectively, requires the presence of Isl1, whereas the early bipolar cell marker Prox1 initially did not. Thus, Isl1 is required for engaging bipolar differentiation pathways but not for general bipolar cell specification. Spatiotemporal expression analysis of additional LIM-homeobox genes identifies a LIM-homeobox gene network during bipolar cell development that includes Lhx3 and Lhx4. We conclude that Isl1 has an indispensable role in retinal neuron differentiation within restricted cell populations and this function may reflect a broader role for other LIM-homeobox genes in retinal development, and perhaps in establishing neuronal subtypes.

Molecular analysis of the VSX1 gene in familial keratoconus

PURPOSE

To evaluate the role of the visual system homeobox gene 1 (VSX1) in the pathogenesis of familial keratoconus.

METHODS

Families with two or more individuals with keratoconus were recruited and their members examined. The coding region and intron-exon junctions of the VSX1 gene were sequenced in affected individuals. In cases where there were possible pathogenic changes, segregation within the pedigree was analyzed. Meta analysis of reports on an association of p.D144E change with keratoconus phenotype was performed.

RESULTS

Probands from a panel of 85 apparently unrelated keratoconus families were included. Eleven sequence variants were observed, including the previously reported c.432C>G (p.D144E) change and two novel intronic single nucleotide polymorphisms. However, these three changes did not cosegregate with the disease phenotype.

CONCLUSIONS

We excluded the c.432C>G sequence alteration as the direct cause of the disease. Lack of possibly pathogenic VSX1 sequence variants in the familial panel suggests that involvement of this gene in the pathogenesis of keratoconus is likely to be confined to a small number of pedigrees, at least in the population studied.

Ptf1a triggers GABAergic neuronal cell fates in the retina

Background

In recent years, considerable knowledge has been gained on the molecular mechanisms underlying retinal cell fate specification. However, hitherto studies focused primarily on the six major retinal cell classes (five types of neurons of one type of glial cell), and paid little attention to the specification of different neuronal subtypes within the same cell class. In particular, the molecular machinery governing the specification of the two most abundant neurotransmitter phenotypes in the retina, GABAergic and glutamatergic, is largely unknown. In the spinal cord and cerebellum, the transcription factor Ptf1a is essential for GABAergic neuron production. In the mouse retina, Ptf1a has been shown to be involved in horizontal and most amacrine neurons differentiation.

Results

In this study, we examined the distribution of neurotransmitter subtypes following Ptf1a gain and loss of function in the Xenopus retina. We found cell-autonomous dramatic switches between GABAergic and glutamatergic neuron production, concomitant with profound defects in the genesis of amacrine and horizontal cells, which are mainly GABAergic. Therefore, we investigated whether Ptf1a promotes the fate of these two cell types or acts directly as a GABAergic subtype determination factor. In ectodermal explant assays, Ptf1a was found to be a potent inducer of the GABAergic subtype. Moreover, clonal analysis in the retina revealed that Ptf1a overexpression leads to an increased ratio of GABAergic subtypes among the whole amacrine and horizontal cell population, highlighting its instructive capacity to promote this specific subtype of inhibitory neurons. Finally, we also found that within bipolar cells, which are typically glutamatergic interneurons, Ptf1a is able to trigger a GABAergic fate.

Conclusion

Altogether, our results reveal for the first time in the retina a major player in the GABAergic versus glutamatergic cell specification genetic pathway.

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.

Light deprivation delays morphological differentiation of bipolar cells in the rabbit retina

Bipolar cells are responsible for transmitting light signals from the photoreceptors to the ganglion cells in the vertebrate retina. Their maturation process is not only important for establishing normal visual function, but may also underlie the dendritic remodeling of ganglion cells during development. It is known that light deprivation affects the synaptic connections of ganglion cells in the mammalian retina, but little is known about impact of visual experience on bipolar cell development. We used dye injection and gene gun labeling to identify bipolar cells, and characterized their morphological differentiation in normal-reared and dark-reared rabbits. Our results show that immature bipolar cells can be found as early as P1-3, and most characteristic bipolar cells can be identified during P4-6. More importantly, we found that light deprivation causes a delay rather than a permanent arrest of bipolar cell maturation in the rabbit retina. By eye opening at P10-11, both normal-reared and dark-reared rabbits possessed adult-like bipolar cells. This suggests that visual experience has a facilitating effect on the morphological differentiation of bipolar cells.

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.