Retinal Cell Fate Specification Research Articles

A highly reproducible and efficient method for retinal organoid differentiation from human pluripotent stem cells

Human pluripotent stem cell (hPSC)-derived retinal organoids are three-dimensional cellular aggregates that differentiate and self-organize to closely mimic the spatial and temporal patterning of the developing human retina. Retinal organoid models serve as reliable tools for studying human retinogenesis, yet limitations in the efficiency and reproducibility of current retinal organoid differentiation protocols have reduced the use of these models for more high-throughput applications such as disease modeling and drug screening. To address these shortcomings, the current study aimed to standardize prior differentiation protocols to yield a highly reproducible and efficient method for generating retinal organoids. Results demonstrated that through regulation of organoid size and shape using quick reaggregation methods, retinal organoids were highly reproducible compared to more traditional methods. Additionally, the timed activation of BMP signaling within developing cells generated pure populations of retinal organoids at 100% efficiency from multiple widely used cell lines, with the default forebrain fate resulting from the inhibition of BMP signaling. Furthermore, given the ability to direct retinal or forebrain fates at complete purity, mRNA-seq analyses were then utilized to identify some of the earliest transcriptional changes that occur during the specification of these two lineages from a common progenitor. These improved methods also yielded retinal organoids with expedited differentiation timelines when compared to traditional methods. Taken together, the results of this study demonstrate the development of a highly reproducible and minimally variable method for generating retinal organoids suitable for analyzing the earliest stages of human retinal cell fate specification.

Dissection of Gene Regulatory Networks that control retinal cell fate specification

Gene regulatory networks (GRNs) regulate critical events during development. In complex tissues, such as the mammalian central nervous system (CNS), networks likely provide the complex regulatory interactions needed to direct the specification of the many CNS cell types. To understand these GRNs, we started by dissecting a GRN that regulates a binary fate decision between two siblings in the murine retina, the rod photoreceptor and bipolar interneuron. A simple strategy was employed and developed to dissect the rod‐bipolar GRN. First, we identified enhancers of key transcription factors (TFs) that regulate the specification of rod and bipolar cells. Second, by analyzing enhancers, we uncovered upstream regulatory TFs that regulate these enhancers. Third, we established the relationships among different TFs, and mapped TFs into GRNs via performing gene perturbation assays. Specifically, the rod‐bipolar GRN centers on Blimp1, one of the TFs that regulates the rod versus bipolar cell fate decision. We identified a cis‐regulatory enhancer, B108, which mimics Blimp1 expression. Deletion of genomic B108 by CRISPR/Cas9 in vivo using electroporation abolished the function of Blimp1. Otx2 and RORβ were found to regulate Blimp1 expression via B108, and Blimp1 and Otx2 were shown to form a negative feedback loop that regulates the level of Otx2, which regulates the production of the correct ratio of rods and bipolar cells. Furthermore, based on this study, we developed novel enhancer screen assays and TF pull‐down assays to identify enhancers and regulatory TFs in a more profound and large‐scale fashion to facilitate GRN studies.Support or Funding InformationThis work was supported by HHMI (to C.L.C. and S.W.).

Retinal Stem Cells and Regeneration of Vision System

The vertebrate retina is a well-characterized model for studying neurogenesis. Retinal neurons and glia are generated in a conserved order from a pool of mutlipotent progenitor cells. During retinal development, retinal stem/progenitor cells (RPC) change their competency over time under the influence of intrinsic (such as transcriptional factors) and extrinsic factors (such as growth factors). In this review, we summarize the roles of these factors, together with the understanding of the signaling pathways that regulate eye development. The information about the interactions between intrinsic and extrinsic factors for retinal cell fate specification is useful to regenerate specific retinal neurons from RPCs. Recent studies have identified RPCs in the retina, which may have important implications in health and disease. Despite the recent advances in stem cell biology, our understanding of many aspects of RPCs in the eye remains limited. PRCs are present in the developing eye of all vertebrates and remain active in lower vertebrates throughout life. In mammals, however, PRCs are quiescent and exhibit very little activity and thus have low capacity for retinal regeneration. A number of different cellular sources of RPCs have been identified in the vertebrate retina. These include PRCs at the retinal margin, pigmented cells in the ciliary body, iris, and retinal pigment epithelium, and Müller cells within the retina. Because PRCs can be isolated and expanded from immature and mature eyes, it is possible now to study these cells in culture and after transplantation in the degenerated retinal tissue. We also examine current knowledge of intrinsic RPCs, and human embryonic stems and induced pluripotent stem cells as potential sources for cell transplant therapy to regenerate the diseased retina.

Epigenetic regulation of retinal development and disease

Epigenetic regulation, including DNA methylation, histone modifications, and chromosomal organization, is emerging as a new layer of transcriptional regulation in retinal development and maintenance. Guided by intrinsic transcription factors and extrinsic signaling molecules, epigenetic regulation can activate and/or repress the expression of specific sets of genes, therefore playing an important role in retinal cell fate specification and terminal differentiation during development as well as maintaining cell function and survival in adults. Here, we review the major findings that have linked these mechanisms to the development and maintenance of retinal structure and function, with a focus on ganglion cells and photoreceptors. The mechanisms of epigenetic regulation are highly complex and vary among different cell types. Understanding the basic principles of these mechanisms and their regulatory pathways may provide new insight into the pathogenesis of retinal diseases associated with transcription dysregulation, and new therapeutic strategies for treatment.

A Systematic Analysis of 1,881,012 Ratios and Differences of the Gene Expression Values on Hippocampus in Alzheimer's Disease Patients Reveals Pairs of Protein-coding and Noncoding Rnas that are Highly Correlated With the Mini-mental State Examination Values

disintegrin and metalloproteinase domain 22’) also observed a 4.58-fold downregulation of Visinin-like 1, correlating with our study in hippocampus in [1], and the loss of expression in the superior frontal gyrus and primary visual cortex in AD patients [3]. Among the long ncRNAs that were upregulated with disease progression, we highlight TUG1 (FLJ20618, taurine upregulated 1, (probe 222244_s_at) and BG251521 (probe 213156_at). TUG1 has been previously reported to have a role in retinal cell fate specification and has not been previously associated with AD. Liang et al. have previously shown that BG251521 was 10.4 fold upregulated in the middle temporal gyrus of AD affected patients relative to controls (p-value < 6.25E-06) [3]. BG251521 is transcribed as an 2 kb long ncRNA that is very highly conserved across vertebrates and also encodes the micro RNA, Mir568. Conclusions: Our results demonstrate the importance of long ncRNAs as markers in disease progression and highlight potential candidates for further understanding the molecular basis of AD.

The long noncoding RNA RNCR2 directs mouse retinal cell specification

BackgroundRecent work has identified that many long mRNA-like noncoding RNAs (lncRNAs) are expressed in the developing nervous system. Despite their abundance, the function of these ncRNAs has remained largely unexplored. We have investigated the highly abundant lncRNA RNCR2 in regulation of mouse retinal cell differentiation.ResultsWe find that the RNCR2 is selectively expressed in a subset of both mitotic progenitors and postmitotic retinal precursor cells. ShRNA-mediated knockdown of RNCR2 results in an increase of both amacrine cells and Müller glia, indicating a role for this lncRNA in regulating retinal cell fate specification. We further report that RNCR2 RNA, which is normally nuclear-retained, can be exported from the nucleus when fused to an IRES-GFP sequence. Overexpression of RNCR2-IRES-GFP phenocopies the effects of shRNA-mediated knockdown of RNCR2, implying that forced mislocalization of RNCR2 induces a dominant-negative phenotype. Finally, we use the IRES-GFP fusion approach to identify specific domains of RNCR2 that are required for repressing both amacrine and Müller glial differentiation.ConclusionThese data demonstrate that the lncRNA RNCR2 plays a critical role in regulating mammalian retinal cell fate specification. Furthermore, we present a novel approach for generating dominant-negative constructs of lncRNAs, which may be generally useful in the functional analysis of this class of molecules.

New meaning in the message: Noncoding RNAs and their role in retinal development

Recent studies have indicated that non-protein-coding RNAs (ncRNAs) may play prominent and diverse roles in the development of the nervous system. These ncRNAs are now known to perform a broad range of cellular functions, and in particular appear to be prominent players in the regulation of transcription and translation. In this review, we discuss recent advances in our understanding of the role of ncRNAs in vertebrate retinal development. Noncoding RNAs that are known or suspected to play a functional role in the specification and maturation of retinal cell subtypes include miRNAs, long noncoding opposite-strand transcripts (OSTs), and other long ncRNAs such as Tug1 and RNCR2. Though the mechanism of action of most of these ncRNAs is still largely unclear, it is likely that these molecules represent a major, and thus far largely unappreciated, component of the molecular machinery involved in retinal cell fate specification.

N-myc coordinates retinal growth with eye size during mouse development

Myc family members play crucial roles in regulating cell proliferation, size, differentiation, and survival during development. We found that N-myc is expressed in retinal progenitor cells, where it regulates proliferation in a cell-autonomous manner. In addition, N-myc coordinates the growth of the retina and eye. Specifically, the retinas of N-myc-deficient mice are hypocellular but are precisely proportioned to the size of the eye. N-myc represses the expression of the cyclin-dependent kinase inhibitor p27Kip1 but acts independently of cyclin D1, the major D-type cyclin in the developing mouse retina. Acute inactivation of N-myc leads to increased expression of p27Kip1, and simultaneous inactivation of p27Kip1 and N-myc rescues the hypocellular phenotype in N-myc-deficient retinas. N-myc is not required for retinal cell fate specification, differentiation, or survival. These data represent the first example of a role for a Myc family member in retinal development and the first characterization of a mouse model in which the hypocellular retina is properly proportioned to the other ocular structures. We propose that N-myc lies upstream of the cell cycle machinery in the developing mouse retina and thus coordinates the growth of both the retina and eye through extrinsic cues.

Ptf1a triggers GABAergic neuronal cell fates in the retina

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

The role of combinational coding by homeodomain and bHLH transcription factors in retinal cell fate specification

Two major families of transcription factors (TFs), basic helix–loop–helix (bHLH) and homeodomain (HD), are known to be involved in cell fate identity. Some recent findings suggest that these TFs are used combinatorially to code for cellular determination in the retina. However, neither the extent nor the efficiency of such a combinatorial coding mechanism has been tested. To look systematically for interactions between these two TF types that would address these questions, we used a matrix analysis. We co-expressed each of six retinally expressed bHLH TFs (XNeuroD; XNgnr-1; Xath3; Xath5; Xash1; Xash3) with each of eight retinally expressed HD TFs (XRx1; XOptx2; XSix3; XPax6; XOtx2; XOtx5b; XBH; XChx10) in retinal progenitors of Xenopus laevis using targeted lipofection. The effects of each of these combinations were assayed on the six major cell types in the retina: Retinal ganglion cells (GCs), Amacrines (ACs), Bipolars (BCs), Horizontals (HCs), Photoreceptors (PRs), and Muller cells (MCs), creating 288 result categories. Multiple-way ANOVA indicated that in 14 categories, there were interactions between the two TFs that produced significantly more or less of a particular cell type than either of the components alone. However, even the most effective combinations were incapable of generating more than 65% of any particular cell type. We therefore used the same techniques to misexpress selected combinations of three TFs in retinal progenitors, but found no further enhancements of particular cell fates, indicating that other factors are probably involved in cell type specification. To test whether particular combinations were essential for horizontal fates, we made VP16 and EnR fusion constructs of some of the factors to provide dominant negative transcriptional activities. Our results confirmed that normal activities of certain combinations were sufficient, and that individually these activities were important for this fate.

BHLH Genes and Retinal Cell Fate Specification

The various cell types in the vertebrate retina arise from a pool of common progenitors. The way that the cell types are specified has been a long-standing issue. Decades of research have yielded a large body of information regarding the involvement of extrinsic factors, and only recently has the function of intrinsic factors begun to emerge. This article reviews recent studies addressing the role of basic helix-loop-helix (bHLH) factors in specifying retinal cell types, with an emphasis on bHLHhierarchies leading to photoreceptor production. Photoreceptor genesis appears to employ two transcriptional pathways: ngn2-->neuroD-->raxL and ath5-->neuroD-->raxL. ngn2 and ath5 function in progenitors, which can potentially develop into different cell types. neuroD represents one of the central steps in photoreceptor specification. Ath5 is also essential for ganglion cell development. It remains to be demonstrated whether a bHLH gene functions as a key player in specifying the other types of retinal cells. Genetic knockout studies have indicated intricate cross-regulation among bHLH genes. Future studies are expected to unveil the mechanism by which bHLH factors network with intrinsic factors and communicate with extrinsic factors to ensure a balanced production of the various types of retinal cells.

Retinal specification and determination in Drosophila.

Dipteran insects such as Drosophila obtain visual information using compound eyes. In Drosophila,these compound eyes are composed of approximately 800 unit eyes called ommatidia. An ommatidium contains eight distinct photoreceptor cells, each of which projects an axon directly to the optic lobe of the brain. This structure contrasts sharply with the mammalian eye, which contains a single lens and a retina with multiple layers of neurons. In spite of these and other substantial differences in the morphological appearance of insect and vertebrate eyes, work in the last several years has revealed common underlying genetic pathways controlling retinal cell fate specification. This discovery is surprising since the eye was considered an extreme case of convergent evolution, evolving independently as many as 40 different times (reviewed in Land and Fernald 1992). Much of the flurry of molecular and genetic data that has accumulated in recent years challenges this notion and suggests divergence from a single, prototypical visual processing unit. Thus, Drosophila has proven to be an excellent model system for identifying new genes that are conserved in vertebrate retinal development. This chapter will mainly be concerned with describing the factors responsible for specification and determination of retinal cell fate in Drosophila.