Ptf1a triggers GABAergic neuronal cell fates in the retina

Defining morphologically and genetically distinct GABAergic/cholinergic amacrine cell subtypes in the vertebrate retina

In mammals, retinal direction selectivity originates from GABAergic/cholinergic amacrine cells (ACs) specifically expressing the sox2 gene. However, the cellular diversity of GABAergic/cholinergic ACs of other vertebrate species remains largely unexplored. Here, we identified 2 morphologically and genetically distinct GABAergic/cholinergic AC types in zebrafish, a previously undescribed bhlhe22+ type and a mammalian counterpart sox2+ type. Notably, while sole sox2 disruption removed sox2+ type, the codisruption of bhlhe22 and bhlhe23 was required to remove bhlhe22+ type. Also, both types significantly differed in dendritic arbors, lamination, and soma position. Furthermore, in vivo two-photon calcium imaging and the behavior assay suggested the direction selectivity of both AC types. Nevertheless, the 2 types showed preferential responses to moving bars of different sizes. Thus, our findings provide new cellular diversity and functional characteristics of GABAergic/cholinergic ACs in the vertebrate retina.

Cellular and Molecular Determinants of Retinal Cell Fate

The vertebrate retina is regarded as a simple part of the central nervous system (CNS) and thus amenable to investigations of the determinants of cell fate. Its five neuronal cell classes and one glial cell class all derive from a common pool of progenitors. Here we review how each cell class is generated. Retinal progenitors progress through different competence states, in each of which they generate only a small repertoire of cell classes. The intrinsic state of the progenitor is determined by the complement of transcription factors it expresses. Thus, although progenitors are multipotent, there is a bias in the types of fates they generate during any particular time window. Overlying these competence states are stochastic mechanisms that influence fate decisions. These mechanisms are determined by a weighted set of probabilities based on the abundance of a cell class in the retina. Deterministic mechanisms also operate, especially late in development, when preprogrammed progenitors solely generate specific fates.

Notch-mediated re-specification of neuronal identity during central nervous system development

Neuronal identity has long been thought of as immutable, so that once a cell acquires a specific fate, it is maintained for life.¹ Studies using the overexpression of potent transcription factors to experimentally reprogram neuronal fate in the mouse neocortex^2,3 and retina^4,5 have challenged this notion by revealing that post-mitotic neurons can switch their identity. Whether fate reprogramming is part of normal development in the central nervous system (CNS) is unclear. While there are some reports of physiological cell fate reprogramming in invertebrates,^6,7 and in the vertebrate peripheral nervous system,⁸ endogenous fate reprogramming in the vertebrate CNS has not been documented. Here, we demonstrate spontaneous fate re-specification in an interneuron lineage in the zebrafish retina. We show that the visual system homeobox 1 (vsx1)-expressing lineage, which has been associated exclusively with excitatory bipolar cell (BC) interneurons,^9-12 also generates inhibitory amacrine cells (ACs). We identify a role for Notch signaling in conferring plasticity to nascent vsx1 BCs, allowing suitable transcription factor programs to re-specify them to an AC fate. Overstimulating Notch signaling enhances this physiological phenotype so that both daughters of a vsx1 progenitor differentiate into ACs and partially differentiated vsx1 BCs can be converted into ACs. Furthermore, this physiological re-specification can be mimicked to allow experimental induction of an entirely distinct fate, that of retinal projection neurons, from the vsx1 lineage. Our observations reveal unanticipated plasticity of cell fate during retinal development.

Positive autofeedback regulation of Ptf1a transcription generates the levels of PTF1A required to generate itch circuit neurons

In this study, Mona et al. set out to investigate the role of Ptf1a in specifying a subset of dorsal spinal cord inhibitory neurons in mice in vivo. The authors used CRISPR to target multiple noncoding sequences with putative cis-regulatory activity controlling Ptf1a and demonstrate a requirement for positive transcriptional autoregulatory feedback to attain the levels of PTF1A necessary for generating correctly balanced neuronal circuits.

Peripheral somatosensory input is modulated in the dorsal spinal cord by a network of excitatory and inhibitory interneurons. PTF1A is a transcription factor essential in dorsal neural tube progenitors for specification of these inhibitory neurons. Thus, mechanisms regulating Ptf1a expression are key for generating neuronal circuits underlying somatosensory behaviors. Mutations targeted to distinct cis-regulatory elements for Ptf1a in mice, tested the in vivo contribution of each element individually and in combination. Mutations in an autoregulatory enhancer resulted in reduced levels of PTF1A, and reduced numbers of specific dorsal spinal cord inhibitory neurons, particularly those expressing Pdyn and Gal. Although these mutants survive postnatally, at ∼3–5 wk they elicit a severe scratching phenotype. Behaviorally, the mutants have increased sensitivity to itch, but acute sensitivity to other sensory stimuli such as mechanical or thermal pain is unaffected. We demonstrate a requirement for positive transcriptional autoregulatory feedback to attain the level of the neuronal specification factor PTF1A necessary for generating correctly balanced neuronal circuits.

Deterministic fate assignment of Müller glia cells in the zebrafish retina suggests a clonal backbone during development

The optic cup houses multipotent retinal progenitor cells that proliferate and differentiate to form the mature retina, containing five main types of neurons and a single glial cell type, the Müller cell. Progenitors of the zebrafish optic cup generate clones that vary regarding the number and types of neurons, a process we previously showed could be described by stochastic models. Here, we present data indicating that each retinal progenitor cell, in the 24 hrs post‐fertilization optic cup, is predestined to form a single Müller cell. This striking fate assignment of Müller cells reveals a dual nature of retinal lineages where stochastic mechanisms produce variable numbers of neurons while there is a strong deterministic component governing the formation of glia cells. A possible mechanism for this stereotypic fate assignment could be the maintenance of a clonal backbone during retina development, which would be similar to invertebrate and rodent cortical neurogenesis.

Notch Signaling-Induced Oscillatory Gene Expression May Drive Neurogenesis in the Developing Retina

After integrating classic and cutting-edge research, we proposed a unified model that attempts to explain the key steps of mammalian retinal neurogenesis. We proposed that the Notch signaling-induced lateral inhibition mechanism promotes oscillatory expression of Hes1. Oscillating Hes1 inhibitory activity as a result leads to oscillatory expression of Notch signaling inhibitors, activators/inhibitors of retinal neuronal phenotypes, and cell cycle-promoting genes all within a retinal progenitor cell (RPC). We provided a mechanism explaining not only how oscillatory expression prevents the progenitor-to-precursor transition, but also how this transition happens. Our proposal of the mechanism posits that the levels of the above factors not only oscillate but also rise (with the exception of Hes1) as the factors accumulate within a progenitor. Depending on which factors accumulate fastest and reach the required supra-threshold levels (cell cycle activators or Notch signaling inhibitors), the progenitor either proliferates or begins to differentiate without any further proliferation when Notch signaling ceases. Thus, oscillatory gene expression may regulate an RPC's decision to proliferate or differentiate. Meanwhile, a post-mitotic precursor's selection of one retinal neuronal phenotype over many others depends on the expression level of key transcription factors (activators) required for each of these retinal neuronal phenotypes. Because the events described above are stochastic due to oscillatory gene expression and gene product inheritance from a mother RPC after its division, an RPC or precursor's decision requires the assignment of probabilities to specific outcomes in the selection process. While low and sustained (non-oscillatory) Notch signaling activity is required to promote the transition of retinal progenitors into various retinal neuronal phenotypes, we propose that the lateral inhibition mechanism, combined with high expression of the BMP signaling-induced Inhibitor of Differentiation (ID) protein family, promotes high and sustained (non-oscillatory) Hes1 and Hes5 expression. These events facilitate the transition of an RPC into the Müller glia (MG) phenotype at the late stage of retinal development.

Transcription factor Ptf1a in development, diseases and reprogramming

The transcription factor Ptf1a is a crucial helix-loop-helix (bHLH) protein selectively expressed in the pancreas, retina, spinal cord, brain, and enteric nervous system. Ptf1a is preferably assembled into a transcription trimeric complex PTF1 with an E protein and Rbpj (or Rbpjl). In pancreatic development, Ptf1a is indispensable in controlling the expansion of multipotent progenitor cells as well as the specification and maintenance of the acinar cells. In neural tissues, Ptf1a is transiently expressed in the post-mitotic cells and specifies the inhibitory neuronal cell fates, mostly mediated by downstream genes such as Tfap2a/b and Prdm13. Mutations in the coding and non-coding regulatory sequences resulting in Ptf1a gain- or loss-of-function are associated with genetic diseases such as pancreatic and cerebellar agenesis in the rodent and human. Surprisingly, Ptf1a alone is sufficient to reprogram mouse or human fibroblasts into tripotential neural stem cells. Its pleiotropic functions in many biological processes remain to be deciphered in the future.

Eye organogenesis: A hierarchical view of ocular development

This chapter provides an overview of the early developmental origins of six ocular tissues: the cornea, lens, ciliary body, iris, neural retina, and retina pigment epithelium. Many of these tissue types are concurrently specified and undergo a complex set of morphogenetic movements that facilitate their structural interconnection. Within the context of vertebrate eye organogenesis, we also discuss the genetic hierarchies of transcription factors and signaling pathways that regulate growth, patterning, cell type specification and differentiation.

xNgn2 induces expression of predominantly sensory neuron markers in Xenopus whole embryo ectoderm but induces mixed subtype expression in isolated ectoderm explants

Proneural basic-helix-loop-helix (bHLH) proteins, such as Neurogenin2 (Ngn2) and Ascl1, are critical regulators at the onset of neuronal differentiation. Endogenously they have largely complementary expression patterns, and have conserved roles in the specification of distinct neuronal subtypes. InXenopusembryos, xNgn2 is the master regulator of primary neurogenesis forming sensory, inter- and motor neurons within the neural plate, while xAscl1 is the master regulator of autonomic neurogenesis, forming noradrenergic neurons in the antero-ventral region of the embryo. Here we characterise neuronal subtype identity of neurons induced by xNgn2 in the ectoderm of wholeXenopusembryos in comparison with xAscl1, and in ectodermal “animal cap” explants. We find that the transcriptional cascades mediating primary and autonomic neuron formation are distinct, and while xNgn2 and xAscl1 can upregulate genes associated with a non-endogenous cascade, this expression is spatially restricted within the embryo. xNgn2 is more potent than xAscl1 at inducing primary neurogenesis as assayed by neural-β-tubulin. In ectoderm of the intact embryo, these induced primary neurons have sensory characteristics with no upregulation of motor neuron markers. In contrast, xNgn2 is able to up-regulate both sensory and motor neuron markers in naïve ectoderm of animal cap explants, suggesting a non-permissive environment for motor identity in the patterned ectoderm of the whole embryo.

Regulatory cocktail for dopaminergic neurons in a protovertebrate identified by whole-embryo single-cell transcriptomics

The CNS of the protovertebrate Ciona intestinalis contains a single cluster of dopaminergic (DA) neurons, the coronet cells, which have been likened to the hypothalamus of vertebrates. Whole-embryo single-cell RNA sequencing (RNA-seq) assays identified Ptf1a as the most strongly expressed cell-specific transcription factor (TF) in DA/coronet cells. Knockdown of Ptf1a activity results in their loss, while misexpression results in the appearance of supernumerary DA/coronet cells. Photoreceptor cells and ependymal cells are the most susceptible to transformation, and both cell types express high levels of Meis Coexpression of both Ptf1a and Meis caused the wholesale transformation of the entire CNS into DA/coronet cells. We therefore suggest that the reiterative use of functional manipulations and single-cell RNA-seq assays is an effective means for the identification of regulatory cocktails underlying the specification of specific cell identities.

Rbpj direct regulation of Atoh7 transcription in the embryonic mouse retina

In vertebrate retinal progenitor cells, the proneural factor Atoh7 exhibits a dynamic tissue and cellular expression pattern. Although the resulting Atoh7 retinal lineage contains all seven major cell types, only retinal ganglion cells require Atoh7 for proper differentiation. Such specificity necessitates complex regulation of Atoh7 transcription during retina development. The Notch signaling pathway is an evolutionarily conserved suppressor of proneural bHLH factor expression. Previous in vivo mouse genetic studies established the cell autonomous suppression of Atoh7 transcription by Notch1, Rbpj and Hes1. Here we identify four CSL binding sites within the Atoh7 proximal regulatory region and demonstrate Rbpj protein interaction at these sequences by in vitro electromobility shift, calorimetry and luciferase assays and, in vivo via colocalization and chromatin immunoprecipitation. We found that Rbpj simultaneously represses Atoh7 transcription using both Notch-dependent and –independent pathways.

Expression of the inactivating deiodinase, Deiodinase 3, in the pre-metamorphic tadpole retina

Thyroid hormone (TH) orchestrates amphibian metamorphosis. Thus, this developmental phase is often used to study TH-dependent responses in specific tissues. However, TH signaling appears early in development raising the question of the control of TH availability in specific cell types prior to metamorphosis. TH availability is under strict temporal and tissue-specific control by deiodinases. We examined the expression of the TH-inactivating enzyme, deiodinase type 3 (D3), during early retinal development. To this end we created a Xenopus laevis transgenic line expressing GFP from the Xenopus dio3 promoter region (pdio3) and followed pdio3-GFP expression in pre-metamorphic tadpoles. To validate retinal GFP expression in the transgenic line as a function of dio3 promoter activity, we used in situ hybridization to compare endogenous dio3 expression to reporter-driven GFP activity. Retinal expression of dio3 increased during pre-metamorphosis through stages NF41, 45 and 48. Both sets of results show dio3 to have cell-specific, dynamic expression in the pre-metamorphic retina. At stage NF48, dio3 expression co-localised with markers for photoreceptors, rods, Opsin-S cones and bipolar neurons. In contrast, in post-metamorphic juveniles dio3 expression was reduced and spatially confined to certain photoreceptors and amacrine cells. We compared dio3 expression at stages NF41 and NF48 with TH-dependent transcriptional responses using another transgenic reporter line: THbZIP-GFP and by analyzing the expression of T3-regulated genes in distinct TH availability contexts. At stage NF48, the majority of retinal cells expressing dio3 were negative for T3 signaling. Notably, most ganglion cells were virtually both dio3-free and T3-responsive. The results show that dio3 can reduce TH availability at the cellular scale. Further, a reduction in dio3 expression can trigger fine-tuned T3 action in cell-type specific maturation at the right time, as exemplified here in photoreceptor survival in the pre-metamorphic retina.

Gut feelings: Studying enteric nervous system development, function, and disease in the zebrafish model system

The enteric nervous system (ENS) is the largest part of the peripheral nervous system and is entirely neural crest-derived. It provides the intrinsic innervation of the gut, controlling different aspects of gut function, such as motility. In this review, we will discuss key points of Zebrafish ENS development, genes, and signaling pathways regulating ENS development, as well as contributions of the Zebrafish model system to better understand ENS disorders. During their migration, enteric progenitor cells (EPCs) display a gradient of developmental states based on their proliferative and migratory characteristics, and show spatiotemporal heterogeneity based on gene expression patterns. Many genes and signaling pathways that regulate the migration and proliferation of EPCs have been identified, but later stages of ENS development, especially steps of neuronal and glial differentiation, remain poorly understood. In recent years, Zebrafish have become increasingly important to test candidate genes for ENS disorders (e.g., from genome-wide association studies), to identify environmental influences on ENS development (e.g., through large-scale drug screens), and to investigate the role the gut microbiota play in ENS development and disease. With its unique advantages as a model organism, Zebrafish will continue to contribute to a better understanding of ENS development, function, and disease. Developmental Dynamics 247:268-278, 2018. © 2017 Wiley Periodicals, Inc.

Prdm13 forms a feedback loop with Ptf1a and is required for glycinergic amacrine cell genesis in the Xenopus Retina

Background

Amacrine interneurons that modulate synaptic plasticity between bipolar and ganglion cells constitute the most diverse cell type in the retina. Most are inhibitory neurons using either GABA or glycine as neurotransmitters. Although several transcription factors involved in amacrine cell fate determination have been identified, mechanisms underlying amacrine cell subtype specification remain to be further understood. The Prdm13 histone methyltransferase encoding gene is a target of the transcription factor Ptf1a, an essential regulator of inhibitory neuron cell fate in the retina. Here, we have deepened our knowledge on its interaction with Ptf1a and investigated its role in amacrine cell subtype determination in the developing Xenopus retina.

Methods

We performed prdm13 gain and loss of function in Xenopus and assessed the impact on retinal cell fate determination using RT-qPCR, in situ hybridization and immunohistochemistry.

Results

We found that prdm13 in the amphibian Xenopus is expressed in few retinal progenitors and in about 40% of mature amacrine cells, predominantly in glycinergic ones. Clonal analysis in the retina reveals that prdm13 overexpression favours amacrine cell fate determination, with a bias towards glycinergic cells. Conversely, knockdown of prdm13 specifically inhibits glycinergic amacrine cell genesis. We also showed that, as in the neural tube, prdm13 is subjected to a negative autoregulation in the retina. Our data suggest that this is likely due to its ability to repress the expression of its inducer, ptf1a.

Conclusions

Our results demonstrate that Prdm13, downstream of Ptf1a, acts as an important regulator of glycinergic amacrine subtype specification in the Xenopus retina. We also reveal that Prdm13 regulates ptf1a expression through a negative feedback loop.

Horizontal Cells, the Odd Ones Out in the Retina, Give Insights into Development and Disease

Thorough investigation of a neuronal population can help reveal key aspects regarding the nervous system and its development. The retinal horizontal cells have several extraordinary features making them particularly interesting for addressing questions regarding fate assignment and subtype specification. In this review we discuss and summarize data concerning the formation and diversity of horizontal cells, how morphology is correlated to molecular markers, and how fate assignment separates the horizontal lineage from the lineages of other retinal cell types. We discuss the novel and unique features of the final cell cycle of horizontal cell progenitors and how they may relate to retinoblastoma carcinogenesis.

Regulating the dorsal neural tube expression of Ptf1a through a distal 3' enhancer

Generating the correct balance of inhibitory and excitatory neurons in a neural network is essential for normal functioning of a nervous system. The neural network in the dorsal spinal cord functions in somatosensation where it modulates and relays sensory information from the periphery. PTF1A is a key transcriptional regulator present in a specific subset of neural progenitor cells in the dorsal spinal cord, cerebellum and retina that functions to specify an inhibitory neuronal fate while suppressing excitatory neuronal fates. Thus, the regulation of Ptf1a expression is critical for determining mechanisms controlling neuronal diversity in these regions of the nervous system. Here we identify a sequence conserved, tissue-specific enhancer located 10.8kb 3' of the Ptf1a coding region that is sufficient to direct expression to dorsal neural tube progenitors that give rise to neurons in the dorsal spinal cord in chick and mouse. DNA binding motifs for Paired homeodomain (Pd-HD) and zinc finger (ZF) transcription factors are required for enhancer activity. Mutations in these sequences implicate the Pd-HD motif for activator function and the ZF motif for repressor function. Although no repressor transcription factor was identified, both PAX6 and SOX3 can increase enhancer activity in reporter assays. Thus, Ptf1a is regulated by active and repressive inputs integrated through multiple sequence elements within a highly conserved sequence downstream of the Ptf1a gene.

A novel subset of enteric neurons revealed by ptf1a:GFP in the developing zebrafish enteric nervous system

The enteric nervous system, the largest division of the peripheral nervous system, is derived from vagal neural crest cells that invade and populate the entire length of the gut to form diverse neuronal subtypes. Here, we identify a novel population of neurons within the enteric nervous system of zebrafish larvae that express the transgenic marker ptf1a:GFP within the midgut. Genetic lineage analysis reveals that enteric ptf1a:GFP(+) cells are derived from the neural crest and that most ptf1a:GFP(+) neurons express the neurotransmitter 5HT, demonstrating that they are serotonergic. This transgenic line, Tg(ptf1a:GFP), provides a novel neuronal marker for a subpopulation of neurons within the enteric nervous system, and highlights the possibility that Ptf1a may act as an important transcription factor for enteric neuron development.

The Independent Probabilistic Firing of Transcription Factors: A Paradigm for Clonal Variability in the Zebrafish Retina

Early retinal progenitor cells (RPCs) in vertebrates produce lineages that vary greatly both in terms of cell number and fate composition, yet how this variability is achieved remains unknown. One possibility is that these RPCs are individually distinct and that each gives rise to a unique lineage. Another is that stochastic mechanisms play upon the determinative machinery of equipotent early RPCs to drive clonal variability. Here we show that a simple model, based on the independent firing of key fate-influencing transcription factors, can quantitatively account for the intrinsic clonal variance in the zebrafish retina and predict the distributions of neuronal cell types in clones where one or more of these fates are made unavailable.

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A simple quantitative model can explain clonal variability in the retina

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This model is based on the firing probabilities of key transcription factors

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These probabilities are shown to be largely independent of each other

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The environment has only a minor effect on these probabilities

From induction to conduction: how intrinsic transcriptional priming of extrinsic neuronal connectivity shapes neuronal identity

Every behaviour of an organism relies on an intricate and vastly diverse network of neurons whose identity and connectivity must be specified with extreme precision during development. Intrinsically, specification of neuronal identity depends heavily on the expression of powerful transcription factors that direct numerous features of neuronal identity, including especially properties of neuronal connectivity, such as dendritic morphology, axonal targeting or synaptic specificity, ultimately priming the neuron for incorporation into emerging circuitry. As the neuron's early connectivity is established, extrinsic signals from its pre- and postsynaptic partners feedback on the neuron to further refine its unique characteristics. As a result, disruption of one component of the circuitry during development can have vital consequences for the proper identity specification of its synaptic partners. Recent studies have begun to harness the power of various transcription factors that control neuronal cell fate, including those that specify a neuron's subtype-specific identity, seeking insight for future therapeutic strategies that aim to reconstitute damaged circuitry through neuronal reprogramming.

Misexpression of ptf1a in cortical pyramidal cells in vivo promotes an inhibitory peptidergic identity

The intracellular transcriptional milieu wields considerable influence over the induction of neuronal identity. The transcription factor Ptf1a has been proposed to act as an identity "switch" between developmentally related precursors in the spinal cord (Glasgow et al., 2005; Huang et al., 2008), retina (Fujitani et al., 2006; Dullin et al., 2007; Nakhai et al., 2007; Lelièvre et al., 2011), and cerebellum (Hoshino et al., 2005; Pascual et al., 2007; Yamada et al., 2014), where it promotes an inhibitory over an excitatory neuronal identity. In this study, we investigate the potency of Ptf1a to cell autonomously confer a specific neuronal identity outside of its endogenous environment, using mouse in utero electroporation and a conditional genetic strategy to misexpress Ptf1a exclusively in developing cortical pyramidal cells. Transcriptome profiling of Ptf1a-misexpressing cells using RNA-seq reveals that Ptf1a significantly alters pyramidal cell gene expression, upregulating numerous Ptf1a-dependent inhibitory interneuron markers and ultimately generating a gene expression profile that resembles the transcriptomes of both Ptf1a-expressing spinal interneurons and endogenous cortical interneurons. Using RNA-seq and in situ hybridization analyses, we also show that Ptf1a induces expression of the peptidergic neurotransmitter nociceptin, while minimally affecting the expression of genes linked to other neurotransmitter systems. Moreover, Ptf1a alters neuronal morphology, inducing the radial redistribution and branching of neurites in cortical pyramidal cells. Thus Ptf1a is sufficient, even in a dramatically different neuronal precursor, to cell autonomously promote characteristics of an inhibitory peptidergic identity, providing the first example of a single transcription factor that can direct an inhibitory peptidergic fate.

Tfap2a and 2b act downstream of Ptf1a to promote amacrine cell differentiation during retinogenesis

Retinogenesis is a precisely controlled developmental process during which different types of neurons and glial cells are generated under the influence of intrinsic and extrinsic factors. Three transcription factors, Foxn4, RORβ1 and their downstream effector Ptf1a, have been shown to be indispensable intrinsic regulators for the differentiation of amacrine and horizontal cells. At present, however, it is unclear how Ptf1a specifies these two cell fates from competent retinal precursors. Here, through combined bioinformatic, molecular and genetic approaches in mouse retinas, we identify the Tfap2a and Tfap2b transcription factors as two major downstream effectors of Ptf1a. RNA-seq and immunolabeling analyses show that the expression of Tfap2a and 2b transcripts and proteins is dramatically downregulated in the Ptf1a null mutant retina. Their overexpression is capable of promoting the differentiation of glycinergic and GABAergic amacrine cells at the expense of photoreceptors much as misexpressed Ptf1a is, whereas their simultaneous knockdown has the opposite effect. Given the demonstrated requirement for Tfap2a and 2b in horizontal cell differentiation, our study thus defines a Foxn4/RORβ1-Ptf1a-Tfap2a/2b transcriptional regulatory cascade that underlies the competence, specification and differentiation of amacrine and horizontal cells during retinal development.

Genetic chimeras reveal the autonomy requirements for Vsx2 in embryonic retinal progenitor cells

BACKGROUND

Vertebrate retinal development is a complex process, requiring the specification and maintenance of retinal identity, proliferative expansion of retinal progenitor cells (RPCs), and their differentiation into retinal neurons and glia. The homeobox gene Vsx2 is expressed in RPCs and required for the proper execution of this retinal program. However, our understanding of the mechanisms by which Vsx2 does this is still rudimentary. To define the autonomy requirements for Vsx2 in the regulation of RPC properties, we generated chimeric mouse embryos comprised of wild-type and Vsx2-deficient cells.

RESULTS

We show that Vsx2 maintains retinal identity in part through the cell-autonomous repression of the retinal pigment epithelium determinant Mitf, and that Lhx2 is required cell autonomously for the ectopic Mitf expression in Vsx2-deficient cells. We also found significant cell-nonautonomous contributions to Vsx2-mediated regulation of RPC proliferation, pointing to an important role for Vsx2 in establishing a growth-promoting extracellular environment. Additionally, we report a cell-autonomous requirement for Vsx2 in controlling when neurogenesis is initiated, indicating that Vsx2 is an important mediator of neurogenic competence. Finally, the distribution of wild-type cells shifted away from RPCs and toward retinal ganglion cell precursors in patches of high Vsx2-deficient cell density to potentially compensate for the lack of fated precursors in these areas.

CONCLUSIONS

Through the generation and analysis of genetic chimeras, we demonstrate that Vsx2 utilizes both cell-autonomous and cell-nonautonomous mechanisms to regulate progenitor properties in the embryonic retina. Importantly, Vsx2's role in regulating Mitf is in part separable from its role in promoting proliferation, and proliferation is excluded as the intrinsic timer that determines when neurogenesis is initiated. These findings highlight the complexity of Vsx2 function during retinal development and provide a framework for identifying the molecular mechanisms mediating these functions.

The E3 ubiquitin ligase thyroid hormone receptor-interacting protein 12 targets pancreas transcription factor 1a for proteasomal degradation

Pancreas transcription factor 1a (PTF1a) plays a crucial role in the early development of the pancreas and in the maintenance of the acinar cell phenotype. Several transcriptional mechanisms regulating expression of PTF1a have been identified. However, regulation of PTF1a protein stability and degradation is still unexplored. Here, we report that inhibition of proteasome leads to elevated levels of PTF1a and to the existence of polyubiquitinated forms of PTF1a. We used the Sos recruitment system, an alternative two-hybrid system method to detect protein-protein interactions in the cytoplasm and to map the interactome of PTF1a. We identified TRIP12 (thyroid hormone receptor-interacting protein 12), an E3 ubiquitin-protein ligase as a new partner of PTF1a. We confirmed PTF1a/TRIP12 interaction in acinar cell lines and in co-transfected HEK-293T cells. The protein stability of PTF1a is significantly increased upon decreased expression of TRIP12. It is reduced upon overexpression of TRIP12 but not a catalytically inactive TRIP12-C1959A mutant. We identified a region of TRIP12 required for interaction and identified lysine 312 of PTF1a as essential for proteasomal degradation. We also demonstrate that TRIP12 down-regulates PTF1a transcriptional and antiproliferative activities. Our data suggest that an increase in TRIP12 expression can play a part in PTF1a down-regulation and indicate that PTF1a/TRIP12 functional interaction may regulate pancreatic epithelial cell homeostasis.

Development of Retinal Amacrine Cells and Their Dendritic Stratification

Themammalian retina containsmultiple neurons, each of which contributes differentially to visual processing. Of these retinal neurons, amacrine cells have recently come to prime light since they facilitate majority of visual processing that takes place in the retina. Amacrine cells are also the most diverse group of neurons in the retina, classified majorly based on the neurotransmitter type they express and morphology of their dendritic arbors. Currently, little is known about the molecular basis contributing to this diversity during development. Amacrine cells also contribute to most of the synapses in the inner plexiform layer and mediate visual information input from bipolar cells onto retinal ganglion cells. In this review, we will describe the current understanding of amacrine cell and cell subtype development. Furthermore, we will address the molecular basis of retinal lamination at the inner plexiform layer. Overall, our review will provide a developmental perspective of amacrine cell subtype classification and their dendritic stratification.

The role of voltage-gated calcium channels in neurotransmitter phenotype specification: Coexpression and functional analysis in Xenopus laevis

Calcium activity has been implicated in many neurodevelopmental events, including the specification of neurotransmitter phenotypes. Higher levels of calcium activity lead to an increased number of inhibitory neural phenotypes, whereas lower levels of calcium activity lead to excitatory neural phenotypes. Voltage-gated calcium channels (VGCCs) allow for rapid calcium entry and are expressed during early neural stages, making them likely regulators of activity-dependent neurotransmitter phenotype specification. To test this hypothesis, multiplex fluorescent in situ hybridization was used to characterize the coexpression of eight VGCC α1 subunits with the excitatory and inhibitory neural markers xVGlut1 and xVIAAT in Xenopus laevis embryos. VGCC coexpression was higher with xVGlut1 than xVIAAT, especially in the hindbrain, spinal cord, and cranial nerves. Calcium activity was also analyzed on a single-cell level, and spike frequency was correlated with the expression of VGCC α1 subunits in cell culture. Cells expressing Ca_v2.1 and Ca_v2.2 displayed increased calcium spiking compared with cells not expressing this marker. The VGCC antagonist diltiazem and agonist (−)BayK 8644 were used to manipulate calcium activity. Diltiazem exposure increased the number of glutamatergic cells and decreased the number of γ-aminobutyric acid (GABA)ergic cells, whereas (−)BayK 8644 exposure decreased the number of glutamatergic cells without having an effect on the number of GABAergic cells. Given that the expression and functional manipulation of VGCCs are correlated with neurotransmitter phenotype in some, but not all, experiments, VGCCs likely act in combination with a variety of other signaling factors to determine neuronal phenotype specification. J. Comp. Neurol. 522:2518–2531, 2014.

Spectrum of Fates: a new approach to the study of the developing zebrafish retina

The ability to image cells live and in situ as they proliferate and differentiate has proved to be an invaluable asset to biologists investigating developmental processes. Here, we describe a Spectrum of Fates approach that allows the identification of all the major neuronal subtypes in the zebrafish retina simultaneously. Spectrum of Fates is based on the combinatorial expression of differently coloured fluorescent proteins driven by the promoters of transcription factors that are expressed in overlapping subsets of retinal neurons. Here, we show how a Spectrum of Fates approach can be used to assess various aspects of neural development, such as developmental waves of differentiation, neuropil development, lineage tracing and hierarchies of fates in the developing zebrafish retina.

Mechanisms regulating GABAergic neuron development

Neurons using gamma-aminobutyric acid (GABA) as their neurotransmitter are the main inhibitory neurons in the mature central nervous system (CNS) and show great variation in their form and function. GABAergic neurons are produced in all of the main domains of the CNS, where they develop from discrete regions of the neuroepithelium. Here, we review the gene expression and regulatory mechanisms controlling the main steps of GABAergic neuron development: early patterning of the proliferative neuroepithelium, production of postmitotic neural precursors, establishment of their identity and migration. By comparing the molecular regulation of these events across CNS, we broadly identify three regions utilizing distinct molecular toolkits for GABAergic fate determination: telencephalon-anterior diencephalon (DLX2 type), posterior diencephalon-midbrain (GATA2 type) and hindbrain-spinal cord (PTF1A and TAL1 types). Similarities and differences in the molecular regulatory mechanisms reveal the core determinants of a GABAergic neuron as well as provide insights into generation of the vast diversity of these neurons.

Ascl1 as a novel player in the Ptf1a transcriptional network for GABAergic cell specification in the retina

In contrast with the wealth of data involving bHLH and homeodomain transcription factors in retinal cell type determination, the molecular bases underlying neurotransmitter subtype specification is far less understood. Using both gain and loss of function analyses in Xenopus, we investigated the putative implication of the bHLH factor Ascl1 in this process. We found that in addition to its previously characterized proneural function, Ascl1 also contributes to the specification of the GABAergic phenotype. We showed that it is necessary for retinal GABAergic cell genesis and sufficient in overexpression experiments to bias a subset of retinal precursor cells towards a GABAergic fate. We also analysed the relationships between Ascl1 and a set of other bHLH factors using an in vivo ectopic neurogenic assay. We demonstrated that Ascl1 has unique features as a GABAergic inducer and is epistatic over factors endowed with glutamatergic potentialities such as Neurog2, NeuroD1 or Atoh7. This functional specificity is conferred by the basic DNA binding domain of Ascl1 and involves a specific genetic network, distinct from that underlying its previously demonstrated effects on catecholaminergic differentiation. Our data show that GABAergic inducing activity of Ascl1 requires the direct transcriptional regulation of Ptf1a, providing therefore a new piece of the network governing neurotransmitter subtype specification during retinogenesis.

Reconciling competence and transcriptional hierarchies with stochasticity in retinal lineages

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Problems with a strict retinal competence model are explained.

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The apparent conflict between transcriptional hierarchies and stochasticity is resolved.

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The underlying nature of retinal progenitor cell stochasticity is discussed.

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Key issues that can be addressed in the face of stochasticity are enumerated.

Recent advances suggest that there is a stochastic contribution to the proliferation and fate choice of retinal progenitors. How does this stochasticity fit with the progression of temporal competence and the transcriptional hierarchies that also influence cell division and cell fate in the developing retina? Where may stochasticity arise in the system and how do we make progress in this field when we may never fully explain the behavior of individual progenitor cells?

Making of a retinal cell: insights into retinal cell-fate determination

Understanding the process by which an uncommitted dividing cell produces particular specialized cells within a tissue remains a fundamental question in developmental biology. Many tissues are well suited for cell-fate studies, but perhaps none more so than the developing retina. Traditionally, experiments using the retina have been designed to elucidate the influence that individual environmental signals or transcription factors can have on cell-fate decisions. Despite a substantial amount of information gained through these studies, there is still much that we do not yet understand about how cell fate is controlled on a systems level. In addition, new factors such as noncoding RNAs and regulators of chromatin have been shown to play roles in cell-fate determination and with the advent of "omics" technology more factors will most likely be identified. In this chapter we summarize both the traditional view of retinal cell-fate determination and introduce some new ideas that are providing a challenge to the older way of thinking about the acquisition of cell fates.

The Prdm13 histone methyltransferase encoding gene is a Ptf1a-Rbpj downstream target that suppresses glutamatergic and promotes GABAergic neuronal fate in the dorsal neural tube

The basic helix-loop-helix (bHLH) transcriptional activator Ptf1a determines inhibitory GABAergic over excitatory glutamatergic neuronal cell fate in progenitors of the vertebrate dorsal spinal cord, cerebellum and retina. In an in situ hybridization expression survey of PR domain containing genes encoding putative chromatin-remodeling zinc finger transcription factors in Xenopus embryos, we identified Prdm13 as a histone methyltransferase belonging to the Ptf1a synexpression group. Gain and loss of Ptf1a function analyses in both frog and mice indicates that Prdm13 is positively regulated by Ptf1a and likely constitutes a direct transcriptional target. We also showed that this regulation requires the formation of the Ptf1a-Rbp-j complex. Prdm13 knockdown in Xenopus embryos and in Ptf1a overexpressing ectodermal explants lead to an upregulation of Tlx3/Hox11L2, which specifies a glutamatergic lineage and a reduction of the GABAergic neuronal marker Pax2. It also leads to an upregulation of Prdm13 transcription, suggesting an autonegative regulation. Conversely, in animal caps, Prdm13 blocks the ability of the bHLH factor Neurog2 to activate Tlx3. Additional gain of function experiments in the chick neural tube confirm that Prdm13 suppresses Tlx3(+)/glutamatergic and induces Pax2(+)/GABAergic neuronal fate. Thus, Prdm13 is a novel crucial component of the Ptf1a regulatory pathway that, by modulating the transcriptional activity of bHLH factors such as Neurog2, controls the balance between GABAergic and glutamatergic neuronal fate in the dorsal and caudal part of the vertebrate neural tube.

Prdm13 mediates the balance of inhibitory and excitatory neurons in somatosensory circuits

Generating a balanced network of inhibitory and excitatory neurons during development requires precise transcriptional control. In the dorsal spinal cord, Ptf1a, a basic helix-loop-helix (bHLH) transcription activator, maintains this delicate balance by inducing homeodomain (HD) transcription factors such as Pax2 to specify the inhibitory lineage while suppressing HD factors such as Tlx1/3 that specify the excitatory lineage. We uncover the mechanism by which Ptf1a represses excitatory cell fate in the inhibitory lineage. We identify Prdm13 as a direct target of Ptf1a and reveal that Prdm13 actively represses excitatory cell fate by binding to regulatory sequences near the Tlx1 and Tlx3 genes to silence their expression. Prdm13 acts through multiple mechanisms, including interactions with the bHLH factor Ascl1, to repress Ascl1 activation of Tlx3. Thus, Prdm13 is a key component of a highly coordinated transcriptional network that determines the balance of inhibitory versus excitatory neurons in the dorsal spinal cord.

The role of egr1 in early zebrafish retinogenesis

Proper retinal cell differentiation is essential for establishing a functional retina. The purpose of this study is to investigate the role of early growth response 1 (egr1), a transcription factor (TF) that has been reported to control eye development and function, on retinal differentiation in zebrafish. Specifically, cellular changes in the Egr1-knockdown retinas were characterized by immunohistochemistry at 72 and 120 hours post-fertilization (hpf). The results indicate that Egr1 knockdown specifically suppressed the differentiation of subtypes of amacrine cells (ACs) and horizontal cells (HCs), including Parvalbumin- and GABA-positive ACs as well as Islet1-positive HCs. In addition, the knockdown induced a general delay of development of the other retinal cell types. These differentiation problems, particularly the ones with the ACs and HCs, also compromised the integrity of the inner and outer plexiform layers. In the Egr1-knockdown retinas, the expression of ptf1a, a TF that controls the specification of ACs and HCs, was prolonged and found in ectopic locations in the retina up to 72 hpf. Then, it became restricted to the proliferative marginal zone as in the control retinas at 120 hpf. This abnormal and prolonged expression of ptf1a during retinogenesis might affect the differentiation of ACs and HCs in the Egr1-knockdown retinas.

Biasing amacrine subtypes in the Atoh7 lineage through expression of Barhl2

Within the developing vertebrate retina, particular subtypes of amacrine cells (ACs) tend to arise from progenitors expressing the basic helix-loop-helix (bHLH) transcription factor, Atoh7, which is necessary for the early generation of retinal ganglion cells (RGCs). All ACs require the postmitotic expression of the bHLH pancreas transcription factor Ptf1a; however, Ptf1a alone is not sufficient to give subtype identities. Here we use functional and in vivo time-lapse studies in the zebrafish retina to investigate on the developmental programs leading to ACs specification within the subsequent divisions of Atoh7-positive progenitors. We find evidences that the homeobox transcription factor Barhl2 is an AC subtype identity-biasing factor that turns on within Atoh7-positive descendants. In vivo lineage tracing reveals that particular modes of cell division tend to generate Barhl2-positive precursors from sisters of RGCs. Additionally, Atoh7 indirectly impacts these division modes to regulate the right number of barhl2-expressing cells. We finally find that Atoh7 itself influences the subtypes of Barhl2-dependent ACs. Together, the results from our study uncover lineage-related and molecular logic of subtype specification in the vertebrate retina, by showing that specific AC subtypes arise via a particular mode of cell division and a transcriptional network cascade involving the sequential expression of first atoh7 followed by ptf1a and then barhl2.

Dissection, culture, and analysis of Xenopus laevis embryonic retinal tissue

The process by which the anterior region of the neural plate gives rise to the vertebrate retina continues to be a major focus of both clinical and basic research. In addition to the obvious medical relevance for understanding and treating retinal disease, the development of the vertebrate retina continues to serve as an important and elegant model system for understanding neuronal cell type determination and differentiation(1-16). The neural retina consists of six discrete cell types (ganglion, amacrine, horizontal, photoreceptors, bipolar cells, and Müller glial cells) arranged in stereotypical layers, a pattern that is largely conserved among all vertebrates (12,14-18). While studying the retina in the intact developing embryo is clearly required for understanding how this complex organ develops from a protrusion of the forebrain into a layered structure, there are many questions that benefit from employing approaches using primary cell culture of presumptive retinal cells (7,19-23). For example, analyzing cells from tissues removed and dissociated at different stages allows one to discern the state of specification of individual cells at different developmental stages, that is, the fate of the cells in the absence of interactions with neighboring tissues (8,19-22,24-33). Primary cell culture also allows the investigator to treat the culture with specific reagents and analyze the results on a single cell level (5,8,21,24,27-30,33-39). Xenopus laevis, a classic model system for the study of early neural development (19,27,29,31-32,40-42), serves as a particularly suitable system for retinal primary cell culture (10,38,43-45). Presumptive retinal tissue is accessible from the earliest stages of development, immediately following neural induction (25,38,43). In addition, given that each cell in the embryo contains a supply of yolk, retinal cells can be cultured in a very simple defined media consisting of a buffered salt solution, thus removing the confounding effects of incubation or other sera-based products (10,24,44-45). However, the isolation of the retinal tissue from surrounding tissues and the subsequent processing is challenging. Here, we present a method for the dissection and dissociation of retinal cells in Xenopus laevis that will be used to prepare primary cell cultures that will, in turn, be analyzed for calcium activity and gene expression at the resolution of single cells. While the topic presented in this paper is the analysis of spontaneous calcium transients, the technique is broadly applicable to a wide array of research questions and approaches (Figure 1).

The expression of irx7 in the inner nuclear layer of zebrafish retina is essential for a proper retinal development and lamination

Irx7, a member in the zebrafish iroquois transcription factor (TF) family, has been shown to control brain patterning. During retinal development, irx7's expression was found to appear exclusively in the inner nuclear layer (INL) as soon as the prospective INL cells withdraw from the cell cycle and during retinal lamination. In Irx7-deficient retinas, the formation of a proper retinal lamination was disrupted and the differentiation of INL cell types, including amacrine, horizontal, bipolar and Muller cells, was compromised. Despite irx7's exclusive expression in the INL, photoreceptors differentiation was also compromised in Irx7-deficient retinas. Compared with other retinal cell types, ganglion cells differentiated relatively well in these retinas, except for their dendritic projections into the inner plexiform layer (IPL). In fact, the neuronal projections of amacrine and bipolar cells into the IPL were also diminished. These indicate that the retinal lamination issue in the Irx7-deficient retinas is likely caused by the attenuation of the neurite outgrowth. Since the expression of known TFs that can specify specific retinal cell type was also altered in Irx7-deficient retinas, thus the irx7 gene network is possibly a novel regulatory circuit for retinal development and lamination.

The transcription factor, Lmx1b, promotes a neuronal glutamate phenotype and suppresses a GABA one in the embryonic trigeminal brainstem complex

Achieving an appropriate balance between inhibitory and excitatory neuronal fate is critical for development of effective synaptic transmission. However, the molecular mechanisms dictating such phenotypic outcomes are not well understood, especially in the whisker-to-barrel cortex neuraxis, an oft-used model system for revealing developmental mechanisms. In trigeminal nucleus principalis (PrV), the brainstem link in the whisker-barrel pathway, the transcription factor Lmx1b marks glutamatergic cells. In PrV of Lmx1b knockout mice (-/-), initial specification of glutamatergic vs. GABAergic cell fate is normal until embryonic day 14.5. Subsequently, until the day of birth, glutamatergic markers (e.g., VGLUT2) stain significantly fewer PrV neurons, whereas, GABAergic markers (Pax2 and Gad1) stain significantly more PrV cells, notably in Lmx1b null PrV cells. These changes also occurred in Lmx1b/Bax double-/- mice, where PrV cells are rescued from Lmx1b-/- induced apoptosis; thus, effects upon excitatory/inhibitory cell ratios do not reflect a cell death confound. Electroporation-induced ectopic expression of Lmx1b in an array of sites decreases numbers of neurons that express GABAergic markers, but increases VGLUT2+ cell numbers or stain intensity. Thus, Lmx1b is not involved in the initial specification of glutamatergic cell fate, but is essential for maintaining a glutamatergic phenotype. Other experiments suggest that Lmx1b acts to suppress Pax2, a promoter of GABAergic cell fate, in a cell-autonomous manner, which may be a mechanism for maintaining a functional balance of glutamatergic and GABAergic cell types in development.

RNA profiling and chromatin immunoprecipitation-sequencing reveal that PTF1a stabilizes pancreas progenitor identity via the control of MNX1/HLXB9 and a network of other transcription factors

Pancreas development is initiated by the specification and expansion of a small group of endodermal cells. Several transcription factors are crucial for progenitor maintenance and expansion, but their interactions and the downstream targets mediating their activity are poorly understood. Among those factors, PTF1a, a basic helix-loop-helix (bHLH) transcription factor which controls pancreas exocrine cell differentiation, maintenance, and functionality, is also needed for the early specification of pancreas progenitors. We used RNA profiling and chromatin immunoprecipitation (ChIP) sequencing to identify a set of targets in pancreas progenitors. We demonstrate that Mnx1, a gene that is absolutely required in pancreas progenitors, is a major direct target of PTF1a and is regulated by a distant enhancer element. Pdx1, Nkx6.1, and Onecut1 are also direct PTF1a targets whose expression is promoted by PTF1a. These proteins, most of which were previously shown to be necessary for pancreas bud maintenance or formation, form a transcription factor network that allows the maintenance of pancreas progenitors. In addition, we identify Bmp7, Nr5a2, RhoV, and P2rx1 as new targets of PTF1a in pancreas progenitors.

PTEN regulates retinal interneuron morphogenesis and synaptic layer formation

The lipid phosphatase PTEN is a critical negative regulator of extracellular signal-induced PI3K activities, yet the roles of PTEN in the neural retina remain poorly understood. Here, we investigate the function of PTEN during retinal development. Deletion of Pten at the onset of neurogenesis in retinal progenitors results in the reduction of retinal ganglion cells and rod photoreceptors, but increased Müller glial genesis. In addition, PTEN deficiency leads to elevated phosphorylation of Akt, especially in the developing inner plexiform layer, where high levels of PTEN are normally expressed. In Pten mutant retinas, various subtypes of amacrine cells show severe dendritic overgrowth, causing specific expansion of the inner plexiform layer. However, the outer plexiform layer remains relatively undisturbed in the Pten deficient retina. Physiological analysis detects reduced rod function and augmented oscillatory potentials originating from amacrine cells in Pten mutants. Furthermore, deleting Pten or elevating Akt activity in individual amacrine cells is sufficient to disrupt dendritic arborization, indicating that Pten activity is required cell autonomously to control neuronal morphology. Moreover, inhibiting endogenous Akt activity attenuates inner plexiform layer formation in vitro. Together, these findings demonstrate that suppression of PI3K/Akt signaling by PTEN is crucial for proper neuronal differentiation and normal retinal network formation.

Aberrant expression of genes necessary for neuronal development and Notch signaling in an epileptic mind bomb zebrafish

Mutation within an ubiquitin E3 ligase gene can lead to a failure in Notch signaling, excessive neurons, and depletion of neural progenitor cells in mind bomb mutants. Using mib(hi904) zebrafish, we reported seizures and a down-regulation of γ-aminobutyric acid (GABA) signaling pathway genes. A transcriptome analysis also identified differential expression pattern of genes related to Notch signaling and neurodevelopment. Here, we selected nine of these genes (her4.2, hes5, bhlhb5, hoxa5a, hoxb5b, dmbx1a, dbx1a, nxph1, and plxnd1) and performed a more thorough analysis of expression using conventional polymerase chain reaction, real-time polymerase chain reaction and in situ hybridization. Transgenic reporter fish (Gfap:GFP and Dlx5a-6a:GFP) were used to assess early brain morphology in vivo. Down-regulation of many of these genes was prominent throughout key structures of the developing mib(hi904) zebrafish brain including, but not limited to, the pallium, ventral thalamus, and optic tectum. Brain expression of Dlx5a-6a and Gfap was also reduced. In conclusion, these expression studies indicate a general down-regulation of Notch signaling genes necessary for proper brain development and suggest that these mutant fish could provide valuable insights into neurological conditions, such as Angelman syndrome, associated with ubiquitin E3 ligase mutation.

Ptf1a/Rbpj complex inhibits ganglion cell fate and drives the specification of all horizontal cell subtypes in the chick retina

During development, progenitor cells of the retina give rise to six principal classes of neurons and the Müller glial cells found within the adult retina. The pancreas transcription factor 1 subunit a (Ptf1a) encodes a basic-helix-loop-helix transcription factor necessary for the specification of horizontal cells and the majority of amacrine cell subtypes in the mouse retina. The Ptf1a-regulated genes and the regulation of Ptf1a activity by transcription cofactors during retinogenesis have been poorly investigated. Using a retrovirus-mediated gene transfer approach, we reported that Ptf1a was sufficient to promote the fates of amacrine and horizontal cells from retinal progenitors and inhibit retinal ganglion cell and photoreceptor differentiation in the chick retina. Both GABAergic H1 and non-GABAergic H3 horizontal cells were induced following the forced expression of Ptf1a. We describe Ptf1a as a strong, negative regulator of Atoh7 expression. Furthermore, the Rbpj-interacting domains of Ptf1a protein were required for its effects on cell fate specification. Together, these data provide a novel insight into the molecular basis of Ptf1a activity on early cell specification in the chick retina.

Sall3 plays essential roles in horizontal cell maturation through regulation of neurofilament expression levels

The region-specific homeotic gene spalt (sal) gene plays a critical role in Drosophila development. The mammalian Sal homologous genes contain four members, and Sall3 is mainly expressed in horizontal cells. In the developing retinas of Sall3 knockout (KO) mice until around birth, horizontal precursor cells developed with comparable numbers and position; the horizontal cell marker NF160 was expressed weakly and neurite-like structure had once formed. Since Sall3-KO mice die at postnatal day 1, subsequent retinal development was examined by in vitro retinal explant culture. In the Sall3-KO retina culture, the expression of NF160 was abrogated, and neurite extension was not observed. Furthermore, Sall3-KO horizontal precursors were initially localized at the appropriate horizontal positions, but eventually moved to an abnormal site in the outer nuclear layer. Overexpression of Sall3 in retinal progenitors did not induce differentiation of retinal progenitor cells into the horizontal cell-fate, but enhanced NF160 expression and neurite extension. In addition, differentiation into Müller glia was promoted, and rod cells were severely suppressed without perturbing proliferation. In conclusion, Sall3 may not be involved in horizontal cell-fate determination, but rather functions to instruct terminal differentiation of horizontal cells and to maintain NF160 expression.

Origin and determination of inhibitory cell lineages in the vertebrate retina

Multipotent progenitors in the vertebrate retina often generate clonally related mixtures of excitatory and inhibitory neurons. The postmitotically expressed transcription factor, Ptf1a, is essential for all inhibitory fates in the zebrafish retina, including three types of horizontal and 28 types of amacrine cell. Here, we show that specific types of inhibitory neurons arise from the cell-autonomous influence of Ptf1a in the daughters of fate-restricted progenitors, such as Ath5 or Vsx1/2-expressing progenitors, and that in the absence of Ptf1a, cells that would have become these specific inhibitory subtypes revert to the histogenetically appropriate excitatory subtypes of the same lineage. Altered proportions of amacrine subtypes respecified by the misexpression of Ptf1a in the Ath5 lineage suggest that Ath5-expressing progenitors are biased, favoring the generation of some subtypes more than others. Yet the full array of inhibitory cell subtypes in Ath5 mutants implies the existence of Ath5-independent factors involved in inhibitory cell specification. We also show that an extrinsic negative feedback on the expression of Ptf1a provides a control mechanism by which the number of any and all types of inhibitory cells in the retina can be regulated in this lineage-dependent way.

Notch-independent functions of CSL

Notch-dependent CSL transcription complexes control essential biological processes such as cell proliferation, differentiation, and cell-fate decisions in diverse developmental systems. The orthologous proteins CBF1/Rbpj (mammalian), Su(H) (Drosophila), and Lag-1 (Caenorhabditis elegans) compose the CSL family of sequence-specific DNA-binding transcription factors. The CSL proteins are best known for their role in canonical Notch signaling. However, CSL factors also form transcription complexes that can function independent of Notch signaling and include repression and activation of target gene transcription. Because the different complexes share CSL as a DNA-binding subunit, they can control overlapping sets of genes; but they can also control distinct sets when partnered with tissue-specific cofactors that restrict DNA-sequence recognition or stability of the DNA-bound complex. The Notch-independent functions of CSL and the processes they regulate will be reviewed here with a particular emphasis on the tissue-specific CSL-activator complex with the bHLH factor Ptf1a.

Reconstruction of rat retinal progenitor cell lineages in vitro reveals a surprising degree of stochasticity in cell fate decisions

In vivo cell lineage-tracing studies in the vertebrate retina have revealed that the sizes and cellular compositions of retinal clones are highly variable. It has been challenging to ascertain whether this variability reflects distinct but reproducible lineages among many different retinal progenitor cells (RPCs) or is the product of stochastic fate decisions operating within a population of more equivalent RPCs. To begin to distinguish these possibilities, we developed a method for long-term videomicroscopy to follow the lineages of rat perinatal RPCs cultured at clonal density. In such cultures, cell-cell interactions between two different clones are eliminated and the extracellular environment is kept constant, allowing us to study the cell-intrinsic potential of a given RPC. Quantitative analysis of the reconstructed lineages showed that the mode of division of RPCs is strikingly consistent with a simple stochastic pattern of behavior in which the decision to multiply or differentiate is set by fixed probabilities. The variability seen in the composition and order of cell type genesis within clones is well described by assuming that each of the four different retinal cell types generated at this stage is chosen stochastically by differentiating neurons, with relative probabilities of each type set by their abundance in the mature retina. Although a few of the many possible combinations of cell types within clones occur at frequencies that are incompatible with a fully stochastic model, our results support the notion that stochasticity has a major role during retinal development and therefore possibly in other parts of the central nervous system.

Proneural gene-linked neurogenesis in zebrafish cerebellum

In mammals, cerebellar neurons are categorized as glutamatergic or GABAergic, and are derived from progenitors that express the proneural genes atoh1 or ptf1a, respectively. In zebrafish, three atoh1 genes, atoh1a, atoh1b, and atoh1c, are expressed in overlapping but distinct expression domains in the upper rhombic lip (URL): ptf1a is expressed exclusively in the ventricular zone (VZ). Using transgenic lines expressing fluorescent proteins under the control of the regulatory elements of atoh1a and ptf1a, we traced the lineages of the cerebellar neurons. The atoh1(+) progenitors gave rise not only to granule cells but also to neurons of the anteroventral rhombencephalon. The ptf1a(+) progenitors generated Purkinje cells. The olig2(+) eurydendroid cells, which are glutamatergic, were derived mostly from ptf1a(+) progenitors in the VZ but some originated from the atoh1(+) progenitors in the URL. In the adult cerebellum, atoh1a, atoh1b, and atoh1c are expressed in the molecular layer of the valvula cerebelli and of the medial corpus cerebelli, and ptf1a was detected in the VZ. The proneural gene expression patterns coincided with the sites of proliferating neuronal progenitors in the adult cerebellum. Our data indicate that proneural gene-linked neurogenesis is evolutionarily conserved in the cerebellum among vertebrates, and that the continuously generated neurons help remodel neural circuits in the adult zebrafish cerebellum.

The transcription factor RBP-J is essential for retinal cell differentiation and lamination

Background

The highly ordered vertebrate retina is composed of seven cell types derived from a common pool of retinal progenitor cells (RPCs), and is a good model for the studies of cell differentiation and interaction during neural development. Notch signaling plays a pivotal role in retinogenesis in mammals, but the full scope of the functions of Notch pathway, and the underlying molecular mechanisms, remain unclear.

Results

In this study, we conditionally knocked out RBP-J, the critical transcription factor downstream to all four Notch receptors, in RPCs of mouse retina at different developmental stages. Disruption of RBP-J at early retinogenesis resulted in accelerated RPCs differentiation, but only photoreceptors and ganglion cells were overrepresented, with other neuronal populations diminished. Similarly, deletion of RBP-J at early postnatal days also led to overproduction of photoreceptors, suggesting that RBP-J governed RPCs specification and differentiation through retinogenesis. In all the RBP-J deletion models, the retinal laminar structures were distorted by the formation of numerous rosette-like structures, reminiscent of β-catenin deficient retina. Indeed, we found that these rosettes aligned with gaps in β-catenin expression at the apical surface of the retina. By in vivo electroporation-mediated transfection, we demonstrated that lamination defects in RBP-J deficient retinae were rescued by overexpressing β-catenin.

Conclusions

Our data indicate that RBP-J-mediated canonical Notch signaling governs retinal cell specification and differentiation, and maintains retinal lamination through the expression of β-catenin.