A Role for Ligand-Gated Ion Channels in Rod Photoreceptor Development

Global strategies to integrate the proteome and metabolome

A fundamental goal of proteomics is to assign physiological functions to all proteins encoded by eukaryotic and prokaryotic genomes. Of the many activities performed by proteins, the chemical transformations catalyzed by enzymes form the basis for most, if not all, metabolic and signaling pathways. Elucidation of these pathways and their integration into larger cellular networks require new strategies to rapidly and systematically identify physiological substrates of enzymes. Here, we review emerging technologies that aim to assign endogenous biochemical functions to enzymes by profiling the metabolome.

Comparison of the Proliferation and Differentiation Ability between Adult Rat Retinal Stem Cells and Cerebral Cortex-Derived Neural Stem Cells

Recent studies have demonstrated that retinal stem cells (RSCs) and stem cells of the central nervous system both exhibited the abilities of self-renewal, proliferation and differentiation into multilineage. In the present study, we compared the proliferation and differentiation abilities between RSCs and cerebral corticex-derived neural stem cells (CNSCs) of adult rats. Stem cells isolated from pigmented ciliary margins of eyes and cerebral cortical tissues of adult rats were cultured in 96-well plates that contained serum-free medium with epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF). In contrast to RSCs, which stopped proliferating after the 8th week, the total cell count of neurospheres in CNSCs increased twofold at the 5th week and more than fourfold at the 10th week after in vitro culture. In contrast, RSCs stopped proliferating after 8 weeks of culture. After adding 2% fetal calf serum and withdrawing EGF and bFGF from the culture medium, the percentages of nestin-positive cells(20.6 +/- 2.7%), microtubule-associated-protein-2-positive neurons (33.2 +/- 3.9%) and glial-fibrillary-acidic-protein-positive glial cells(51.3 +/- 6.2%) in the differentiated CNSCs were significantly higher than those in the differentiated RSCs (10.2 +/- 1.9, 22.3 +/- 1.3 and 44.6 +/- 5.1%, respectively; p < 0.05). We also found that the combination of transforming growth factor beta type III with retinoic acid played an important role in the induction of CNSCs to differentiate into opsin-positive cells. Our data demonstrated that CNSCs displayed a higher ability of proliferation and retinal lineage. This report also offers an alternative protocol of cell reproduction for producing retinal cells.

The noncoding RNA taurine upregulated gene 1 is required for differentiation of the murine retina

BACKGROUND

With the advent of genome-wide analyses, it is becoming evident that a large number of noncoding RNAs (ncRNAs) are expressed in vertebrates. However, of the thousands of ncRNAs identified, the functions of relatively few have been established.

RESULTS

In a screen for genes upregulated by taurine in developing retinal cells, we identified a gene that appears to be a ncRNA. Taurine Upregulated Gene 1 (TUG1) is a spliced, polyadenylated RNA that does not encode any open reading frame greater than 82 amino acids in its full-length, 6.7 kilobase (kb) RNA sequence. Analyses of Northern blots and in situ hybridization revealed that TUG1 is expressed in the developing retina and brain, as well as in adult tissues. In the newborn retina, knockdown of TUG1 with RNA interference (RNAi) resulted in malformed or nonexistent outer segments of transfected photoreceptors. Immunofluorescent staining and microarray analyses suggested that this loss of proper photoreceptor differentiation is a result of the disregulation of photoreceptor gene expression.

CONCLUSIONS

A function for a newly identified ncRNA, TUG1, has been established. TUG1 is necessary for the proper formation of photoreceptors in the developing rodent retina.

Developmental Regulation of β-Carboline-Induced Inhibition of Glycine-Evoked Responses Depends on Glycine Receptor β Subunit Expression

In this work, we show that beta-carbolines, which are known negative allosteric modulators of GABA(A) receptors, inhibit glycine-induced currents of embryonic mouse spinal cord and hippocampal neurons. In both cell types, beta-carboline-induced inhibition of glycine receptor (GlyR)-mediated responses decreases with time in culture. Single-channel recordings show that the major conductance levels of GlyR unitary currents shifts from high levels (> or = 50 pS) in 2 to 3 days in vitro (DIV) neurons to low levels (<50 pS) in 11 to 14 DIV neurons, assessing the replacement of functional homomeric GlyR by heteromeric GlyR. In cultured spinal cord neurons, the disappearance of beta-carboline inhibition of glycine responses and high conductance levels is almost complete in mature neurons, whereas a weaker decrease in beta-carboline-evoked glycine response inhibition and high conductance level proportion is observed in hippocampal neurons. To confirm the hypothesis that the decreased sensitivity of GlyR to beta-carbolines depends on beta subunit expression, Chinese hamster ovary cells were permanently transfected either with GlyR alpha2 subunit alone or in combination with GlyR beta subunit. Single-channel recordings revealed that the major conductance levels shifted from high levels (> or = 50 pS) in GlyR-alpha2-transfected cells to low levels (<50 pS) in GlyR-alpha2+beta-containing cells. Consistently, both picrotoxin- and beta-carboline-induced inhibition of glycine-gated currents were significantly decreased in GlyR-alpha2+beta-transfected cells compared with GlyR-alpha2-containing cells. In summary, we demonstrate that the incorporation of beta subunits in GlyRs confers resistance not only to picrotoxin but also to beta-carboline-induced inhibition. Furthermore, we also provide evidence that hippocampal neurons undergo in vitro a partial maturation process of their GlyR-mediated responses.

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

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

Control of programmed cell death by neurotransmitters and neuropeptides in the developing mammalian retina

It has long been known that a barrage of signals from neighboring and connecting cells, as well as components of the extracellular matrix, control cell survival. Given the extensive repertoire of retinal neurotransmitters, neuromodulators and neurotrophic factors, and the exhuberant interconnectivity of retinal interneurons, it is likely that various classes of released neuroactive substances may be involved in the control of sensitivity to retinal cell death. The aim of this article is to review evidence that neurotransmitters and neuropeptides control the sensitivity to programmed cell death in the developing retina. Whereas the best understood mechanism of execution of cell death is that of caspase-mediated apoptosis, current evidence shows that not only there are many parallel pathways to apoptotic cell death, but non-apoptotic programs of execution of cell death are also available, and may be triggered either in isolation or combined with apoptosis. The experimental data show that many upstream signaling pathways can modulate cell death, including those dependent on the second messengers cAMP-PKA, calcium and nitric oxide. Evidence for anterograde neurotrophic control is provided by a variety of models of the central nervous system, and the data reviewed here indicate that an early function of certain neurotransmitters, such as glutamate and dopamine, as well as neuropeptides such as pituitary adenylyl cyclase-activating polypeptide and vasoactive intestinal peptide is the trophic support of cell populations in the developing retina. This may have implications both regarding the mechanisms of retinal organogenesis, as well as pathological conditions leading to retinal dystrophies and to dysfunctional cellular behavior.

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

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

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

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

Genomic analysis of mouse retinal development

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

Need Rods? Get Glycine Receptors and Taurine

Taurine, a multifunctional amino acid prevalent in developing nervous tissues, regulates the number of rod photoreceptors in developing postnatal rodent retina. In this issue of Neuron, Young and Cepko show that taurine acts via GlyRalpha2 subunit-containing glycine receptors expressed by retinal progenitor cells at birth.

Synaptic and extrasynaptic neurotransmitter receptors in glial precursors' quest for identity

It is widely established that neurotransmitter receptors are expressed in non-neuronal cells, and particularly in neural progenitor cells in the postnatal central nervous system. The functional role of these receptors during development is unclear, but it needs to be revisited now that cells previously considered restricted to glial lineages have been shown to generate neurons. The present review integrates recent advances, to shed new light on how neurotransmitter receptors may, alternatively, serve as excitable mediators of neuron-glia and neuron-neuroblast interactions.