A transient wave of BMP signaling in the retina is necessary for Müller glial differentiation

Müller cells trophism and pathology as the next therapeutic targets for retinal diseases

Müller cells are a crucial retinal cell type involved in multiple regulatory processes and functions that are essential for retinal health and functionality. Acting as structural and functional support for retinal neurons and photoreceptors, Müller cells produce growth factors, regulate ion and fluid homeostasis, and facilitate neuronal signaling. They play a pivotal role in retinal morphogenesis and cell differentiation, significantly contributing to macular development. Due to their radial morphology and unique cytoskeletal organization, Müller cells act as optical fibers, efficiently channeling photons directly to the photoreceptors. In response to retinal damage, Müller cells undergo specific gene expression and functional changes that serve as a first line of defense for neurons, but can also lead to unwarranted cell dysfunction, contributing to cell death and neurodegeneration. In some species, Müller cells can reactivate their developmental program, promoting retinal regeneration and plasticity-a remarkable ability that holds promising therapeutic potential if harnessed in mammals. The crucial and multifaceted roles of Müller cells-that we propose to collectively call "Müller cells trophism"-highlight the necessity of maintaining their functionality. Dysfunction of Müller cells, termed "Müller cells pathology," has been associated with a plethora of retinal diseases, including age-related macular degeneration, diabetic retinopathy, vitreomacular disorders, macular telangiectasia, and inherited retinal dystrophies. In this review, we outline how even subtle disruptions in Müller cells trophism can drive the pathological cascade of Müller cells pathology, emphasizing the need for targeted therapies to preserve retinal health and prevent disease progression.

Experimental Framework for Assessing Mouse Retinal Regeneration Through Single-Cell RNA-Sequencing

Retinal degenerative diseases including age-related macular degeneration and glaucoma are estimated to currently affect more than 14 million people in the United States, with an increased prevalence of retinal degenerations in aged individuals. An expanding aged population who are living longer forecasts an increased prevalence and economic burden of visual impairments. Improvements to visual health and treatment paradigms for progressive retinal degenerations slow vision loss. However, current treatments fail to remedy the root cause of visual impairments caused by retinal degenerations-loss of retinal neurons. Stimulation of retinal regeneration from endogenous cellular sources presents an exciting treatment avenue for replacement of lost retinal cells. In multiple species including zebrafish and Xenopus, Müller glial cells maintain a highly efficient regenerative ability to reconstitute lost cells throughout the organism's lifespan, highlighting potential therapeutic avenues for stimulation of retinal regeneration in humans. Here, we describe how the application of single-cell RNA-sequencing (scRNA-seq) has enhanced our understanding of Müller glial cell-derived retinal regeneration, including the characterization of gene regulatory networks that facilitate/inhibit regenerative responses. Additionally, we provide a validated experimental framework for cellular preparation of mouse retinal cells as input into scRNA-seq experiments, including insights into experimental design and analyses of resulting data.

Identifying Genes that Affect Differentiation of Human Neural Stem Cells and Myelination of Mature Oligodendrocytes

Human neural stem cells (NSCs) are self-renewing, multipotent cells of the central nervous system (CNS). They are characterized by their ability to differentiate into a range of cells, including oligodendrocytes (OLs), neurons, and astrocytes, depending on exogenous stimuli. An efficient and easy directional differentiation method was developed for obtaining large quantities of high-quality of human OL progenitor cells (OPCs) and OLs from NSCs. RNA sequencing, immunofluorescence staining, flow cytometry, western blot, label-free proteomic sequencing, and qPCR were performed in OL lines differentiated from NSC lines. The changes in the positive rate of typical proteins were analyzed expressed by NSCs, neurons, astrocytes, OPCs, and OLs. We assessed Gene Ontology (GO) enrichment and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways of differentially expressed (DE) messenger RNAs (mRNAs) related to the differentiation of NSCs and the maturation of OLs. The percentage of NSCs differentiated into neurons, astrocytes, and OLs was 82.13%, 80.19%, and 90.15%, respectively. We found that nestin, PAX6, Musashi, and vimentin were highly expressed in NSCs; PDGFR-α, A2B5, NG2, OLIG2, SOX10, and NKX2-2 were highly expressed in OPCs; and CNP, GALC, PLP1, and MBP were highly expressed in OLs. RNA sequencing, western blot and qPCR revealed that ERBB4 and SORL1 gradually increased during NSC–OL differentiation. In conclusion, NSCs can differentiate into neurons, astrocytes, and OLs efficiently. PDGFR-α, APC, ID4, PLLP, and other markers were related to NSC differentiation and OL maturation. Moreover, we refined a screening method for ERBB4 and SORL1, which may underlie NSC differentiation and OL maturation.

Graphical Abstract

Potential unreported genes and proteins may regulate differentiation of human neural stem cells into oligodendrocyte lineage. Neural stem cells (NSCs) can differentiate into neurons, astrocytes, and oligodendrocyte (OLs) efficiently. By analyzing the DE mRNAs and proteins of NSCs and OLs lineage, we could identify reported markers and unreported markers of ERBB4 and SORL1 that may underlie regulate NSC differentiation and OL maturation.

Self-Organization of the Retina during Eye Development, Retinal Regeneration In Vivo, and in Retinal 3D Organoids In Vitro

Self-organization is a process that ensures histogenesis of the eye retina. This highly intricate phenomenon is not sufficiently studied due to its biological complexity and genetic heterogeneity. The review aims to summarize the existing central theories and ideas for a better understanding of retinal self-organization, as well as to address various practical problems of retinal biomedicine. The phenomenon of self-organization is discussed in the spatiotemporal context and illustrated by key findings during vertebrate retina development in vivo and retinal regeneration in amphibians in situ. Described also are histotypic 3D structures obtained from the disaggregated retinal progenitor cells of birds and retinal 3D organoids derived from the mouse and human pluripotent stem cells. The review highlights integral parts of retinal development in these conditions. On the cellular level, these include competence, differentiation, proliferation, apoptosis, cooperative movements, and migration. On the physical level, the focus is on the mechanical properties of cell- and cell layer-derived forces and on the molecular level on factors responsible for gene regulation, such as transcription factors, signaling molecules, and epigenetic changes. Finally, the self-organization phenomenon is discussed as a basis for the production of retinal organoids, a promising model for a wide range of basic scientific and medical applications.

Müller Glia in Retinal Development: From Specification to Circuit Integration

Müller glia of the retina share many features with astroglia located throughout the brain including maintenance of homeostasis, modulation of neurotransmitter spillover, and robust response to injury. Here we present the molecular factors and signaling events that govern Müller glial specification, patterning, and differentiation. Next, we discuss the various roles of Müller glia in retinal development, which include maintaining retinal organization and integrity as well as promoting neuronal survival, synaptogenesis, and phagocytosis of debris. Finally, we review the mechanisms by which Müller glia integrate into retinal circuits and actively participate in neuronal signaling during development.

The Role of Small Molecules and Their Effect on the Molecular Mechanisms of Early Retinal Organoid Development

Early in vivo embryonic retinal development is a well-documented and evolutionary conserved process. The specification towards eye development is temporally controlled by consecutive activation or inhibition of multiple key signaling pathways, such as the Wnt and hedgehog signaling pathways. Recently, with the use of retinal organoids, researchers aim to manipulate these pathways to achieve better human representative models for retinal development and disease. To achieve this, a plethora of different small molecules and signaling factors have been used at various time points and concentrations in retinal organoid differentiations, with varying success. Additions differ from protocol to protocol, but their usefulness or efficiency has not yet been systematically reviewed. Interestingly, many of these small molecules affect the same and/or multiple pathways, leading to reduced reproducibility and high variability between studies. In this review, we make an inventory of the key signaling pathways involved in early retinogenesis and their effect on the development of the early retina in vitro. Further, we provide a comprehensive overview of the small molecules and signaling factors that are added to retinal organoid differentiation protocols, documenting the molecular and functional effects of these additions. Lastly, we comparatively evaluate several of these factors using our established retinal organoid methodology.

Zeb2 regulates the balance between retinal interneurons and Müller glia by inhibition of BMP-Smad signaling

The interplay between signaling molecules and transcription factors during retinal development is key to controlling the correct number of retinal cell types. Zeb2 (Sip1) is a zinc-finger multidomain transcription factor that plays multiple roles in central and peripheral nervous system development. Haploinsufficiency of ZEB2 causes Mowat-Wilson syndrome, a congenital disease characterized by intellectual disability, epilepsy and Hirschsprung disease. In the developing retina, Zeb2 is required for generation of horizontal cells and the correct number of interneurons; however, its potential function in controlling gliogenic versus neurogenic decisions remains unresolved. Here we present cellular and molecular evidence of the inhibition of Müller glia cell fate by Zeb2 in late stages of retinogenesis. Unbiased transcriptomic profiling of control and Zeb2-deficient early-postnatal retina revealed that Zeb2 functions in inhibiting Id1/2/4 and Hes1 gene expression. These neural progenitor factors normally inhibit neural differentiation and promote Müller glia cell fate. Chromatin immunoprecipitation (ChIP) supported direct regulation of Id1 by Zeb2 in the postnatal retina. Reporter assays and ChIP analyses in differentiating neural progenitors provided further evidence that Zeb2 inhibits Id1 through inhibition of Smad-mediated activation of Id1 transcription. Together, the results suggest that Zeb2 promotes the timely differentiation of retinal interneurons at least in part by repressing BMP-Smad/Notch target genes that inhibit neurogenesis. These findings show that Zeb2 integrates extrinsic cues to regulate the balance between neuronal and glial cell types in the developing murine retina.

Jmjd3 Plays Pivotal Roles in the Proper Development of Early-Born Retinal Lineages: Amacrine, Horizontal, and Retinal Ganglion Cells

Purpose

Trimethylation of histone H3 at lysine 27 (H3K27me3) is a critical mediator of transcriptional gene repression, and Jmjd3 and Utx are the demethylases specific to H3K27me3. Using an in vitro retinal explant culture system, we previously revealed the role of Jmjd3 in the development of rod bipolar cells; however, the roles of Jmjd3 in the development of early-born retinal cells are unknown due to limitations concerning the use of retinal explant culture systems. In this study, we investigated the roles of Jmjd3 in the development of early-born retinal cells.

Methods

We examined retina-specific conditional Jmjd3 knockout (Jmjd3-cKO) mice using immunohistochemistry and quantitative reverse transcription PCR and JMJD3 binding to a target locus by chromatin immunoprecipitation analysis.

Results

We observed reductions in amacrine cells (ACs) and horizontal cells (HCs), as well as lowered expression levels of several transcription factors involved in the development of ACs and HCs in the Jmjd3-cKO mouse retina. JMJD3 bound the promoter regions of these transcription factors. Notably, an elevated number of retinal ganglion cells (RGCs) was observed at embryonic stages, whereas RGCs were moderately reduced at later postnatal stages in the Jmjd3-cKO retina. We also observed reduced expression of Eomes, which is required for the maintenance of RGCs, as well as lower H3K27me3 level and lower JMJD3 binding in the promoter region of Eomes in RGC-enriched cells.

Conclusions

The results indicated that Jmjd3 has critical roles in the development of early-born retinal subtypes, and suggested biphasic roles of Jmjd3 in RGC production and maintenance.

Pluripotent Stem Cells for the Treatment of Retinal Degeneration: Current Strategies and Future Directions

Stem cells have been part of the biomedical landscape since the early 1960s. However, the translation of stem cells to effective therapeutics have met significant challenges, especially for retinal diseases. The retina is a delicate and complex architecture of interconnected cells that are steadfastly interdependent. Degenerative mechanisms caused by acquired or inherited diseases disrupt this interconnectivity, devastating the retina and causing severe vision loss in many patients. Consequently, retinal differentiation of exogenous and endogenous stem cells is currently being explored as replacement therapies in the debilitating diseases. In this review, we will examine the mechanisms involved in exogenous stem cells differentiation and the challenges of effective integration to the host retina. Furthermore, we will explore the current advancements in trans-differentiation of endogenous stem cells, primarily Müller glia.

Developmental changes in the accessible chromatin, transcriptome and Ascl1-binding correlate with the loss in Müller Glial regenerative potential

Diseases and damage to the retina lead to losses in retinal neurons and eventual visual impairment. Although the mammalian retina has no inherent regenerative capabilities, fish have robust regeneration from Müller glia (MG). Recently, we have shown that driving expression of Ascl1 in adult mouse MG stimulates neural regeneration. The regeneration observed in the mouse is limited in the variety of neurons that can be derived from MG; Ascl1-expressing MG primarily generate bipolar cells. To better understand the limits of MG-based regeneration in mouse retinas, we used ATAC- and RNA-seq to compare newborn progenitors, immature MG (P8-P12), and mature MG. Our analysis demonstrated developmental differences in gene expression and accessible chromatin between progenitors and MG, primarily in neurogenic genes. Overexpression of Ascl1 is more effective in reprogramming immature MG, than mature MG, consistent with a more progenitor-like epigenetic landscape in the former. We also used ASCL1 ChIPseq to compare the differences in ASCL1 binding in progenitors and reprogrammed MG. We find that bipolar-specific accessible regions are more frequently linked to bHLH motifs and ASCL1 binding. Overall, our analysis indicates a loss of neurogenic gene expression and motif accessibility during glial maturation that may prevent efficient reprogramming.

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.

Stimulation of Retinal Pigment Epithelium With an α7 nAChR Agonist Leads to Müller Glia Dependent Neurogenesis in the Adult Mammalian Retina

Purpose

The adult mammalian retina is typically incapable of regeneration when damaged by disease or trauma. Restoration of function would require generation of new adult neurons, something that until recently, mammals were thought to be incapable of doing. However, previous studies from this laboratory have shown that the α7 nicotinic acetylcholine receptor (α7 nAChR) agonist, PNU-282987, induces cell cycle reentry of Müller glia and generation of mature retinal neurons in adult rats, in the absence of detectible injury. This study analyzes how PNU-282987 treatment in RPE leads to robust BrdU incorporation in Müller glia in adult mice and leads to generation of Müller-derived retinal progenitors and neuronal differentiation.

Methods

Retinal BrdU incorporation was examined after eye drop application of PNU-282987 in adult wild-type and transgenic mice that contain tamoxifen-inducible tdTomato Müller glia, or after intraocular injection of conditioned medium from PNU-282987–treated cultured RPE cells.

Results

PNU-282987 induced robust incorporation of BrdU in all layers of the adult mouse retina. The α7 nAChR agonist was found to stimulate cell cycle reentry of Müller glia and their generation of new retinal progenitors indirectly, via the RPE, in an α7 nAChR-dependent fashion.

Conclusions

The results from this study point to RPE as a contributor to Müller glial neurogenic responses. The manipulation of the RPE to stimulate retinal neurogenesis offers a new direction for developing novel and potentially transformative treatments to reverse the loss of neurons associated with neurodegenerative disease, traumatic injury, or aging.

miR-29c regulates neurogliogenesis in the mammalian retina through REST

In the developing central nervous system, including its simple and accessible model retina, neurogenesis is followed by gliogenesis. However, the mechanism underlying the neurogliogenic switch remains poorly understood despite the identification of several regulatory genes, associated with the lineage identity and transition. The mechanism may involve cross talks between regulatory genes, facilitated through microRNAs. Here, we posit miR-29c as one of the regulatory miRNAs that may influence neuronal versus glial differentiation. We observed that the temporal patterns of miR-29c expression corresponded with late retinal histogenesis, the stage in the developing retina when neurogliogenic decision predominantly occurs. Examination of the effects of miR-29c on neurogliogenesis by the perturbation of function approach revealed that miR-29c preferentially facilitated differentiation of late RPCs into rod photoreceptors and bipolar cells, the late-born neurons, at the expense of Müller glia, the sole glia generated by retinal progenitor cells. We further observed that miR-29c facilitated neurogenesis and inhibited gliogenesis by regulating the expression of RE-1 silencing transcription factor (REST), which encodes a transcriptional repressor of cell cycle regulators and neuronal genes. Thus, miR-29c may influence neurogliogenic decision in the developing retina by regulating the instructive out put of a molecular axis helmed by REST.

RBX2 maintains final retinal cell position in a DAB1-dependent and -independent fashion

The laminated structure of the retina is fundamental for the organization of the synaptic circuitry that translates light input into patterns of action potentials. However, the molecular mechanisms underlying cell migration and layering of the retina are poorly understood. Here, we show that RBX2, a core component of the E3 ubiquitin ligase CRL5, is essential for retinal layering and function. RBX2 regulates the final cell position of rod bipolar cells, cone photoreceptors and Muller glia. Our data indicate that sustained RELN/DAB1 signaling, triggered by depletion of RBX2 or SOCS7 - a CRL5 substrate adaptor known to recruit DAB1 - causes rod bipolar cell misposition. Moreover, whereas SOCS7 also controls Muller glia cell lamination, it is not responsible for cone photoreceptor positioning, suggesting that RBX2, most likely through CRL5 activity, controls other signaling pathways required for proper cone localization. Furthermore, RBX2 depletion reduces the number of ribbon synapses and disrupts cone photoreceptor function. Together, these results uncover RBX2 as a crucial molecular regulator of retina morphogenesis and cone photoreceptor function.

RNA Biology in Retinal Development and Disease

For decades, RNA has served in a supporting role between the genetic carrier (DNA) and the functional molecules (proteins). It is finally time for RNA to take center stage in all aspects of biology. The retina provides a unique opportunity to dissect the molecular underpinnings of neuronal diversity and disease. Transcriptome profiles of the retina and its resident cell types have unraveled unique features of the RNA landscape. The discovery of distinct RNA molecules and the recognition that RNA processing is a major cause of retinal neurodegeneration have prompted the design of biomarkers and novel therapeutic paradigms. We review here RNA biology as it pertains to the retina, emphasizing new avenues for investigations in development and disease.

The Transcription Factor Prdm16 Marks a Single Retinal Ganglion Cell Subtype in the Mouse Retina

Purpose

Retinal ganglion cells (RGC) can be categorized into roughly 30 distinct subtypes. How these subtypes develop is poorly understood, in part because few unique subtype markers have been characterized. We tested whether the Prdm16 transcription factor is expressed by RGCs as a class or within particular ganglion cell subtypes.

Methods

Embryonic and mature retinal sections and flatmount preparations were examined by immunohistochemistry for Prdm16 and several other cell type-specific markers. To visualize the morphology of Prdm16+ cells, we utilized Thy1-YFP-H transgenic mice, where a small random population of RGCs expresses yellow fluorescent protein (YFP) throughout the cytoplasm.

Results

Prdm16 was expressed in the retina starting late in embryogenesis. Prdm16+ cells coexpressed the RGC marker Brn3a. These cells were arranged in an evenly spaced pattern and accounted for 2% of all ganglion cells. Prdm16+ cells coexpressed parvalbumin, but not calretinin, melanopsin, Smi32, or CART. This combination of marker expression and morphology data from Thy1-YFP-H mice suggested that the Prdm16+ cells represented a single ganglion cell subtype. Prdm16 also marked vascular endothelial cells and mural cells of retinal arterioles.

Conclusions

A single subtype of ganglion cell appears to be uniquely marked by Prdm16 expression. While the precise identity of these ganglion cells is unclear, they most resemble the G9 subtype described by Völgyi and colleagues in 2009. Future studies are needed to determine the function of these ganglion cells and whether Prdm16 regulates their development.

Cell lineage tracing in the retina: Could material transfer distort conclusions?

Recent studies reported the transfer of fluorescent labels between grafted and host cells after transplantation of photoreceptor precursor cells in the mouse retina. While clearly impacting the interpretation of transplantation studies in the retina, the potential impact of material transfer in other experimental paradigms using cell-specific labels remains uncertain. Here, we briefly review the evidence supporting material transfer in transplantation studies and discuss whether it might influence retinal cell lineage tracing experiments in developmental and regeneration studies. We also propose ways to control for the possible confounding occurrence of label exchange in such experiments. Developmental Dynamics 247:10-17, 2018. © 2017 Wiley Periodicals, Inc.

Potential of Small Molecule-Mediated Reprogramming of Rod Photoreceptors to Treat Retinitis Pigmentosa

Purpose

Mutations in rod photoreceptor genes can cause retinitis pigmentosa (RP). Rod gene expression is regulated by the nuclear hormone receptor, Nr2e3. Genetic deletion of Nr2e3 reprograms rods into cells that resemble cone photoreceptors, and might therefore prevent their death from some forms of RP. There are no identified ligands for Nr2e3; however, reverse agonists might mimic the genetic rescue effect and may be therapeutically useful for the treatment of RP.

Methods

We screened for small molecule modulators of Nr2e3 using primary retinal cell cultures and characterized the most potent, which we have named photoregulin1 (PR1), in vitro and in vivo. We also tested the ability of PR1 to slow the progression of photoreceptor degeneration in two common mouse models of autosomal dominant RP, the RhoP23H and the Pde6brd1 mutations.

Results

In developing retina, PR1 causes a decrease in rod gene expression and an increase in S opsin+ cones. Photoregulin1 continues to inhibit rod gene expression in adult mice. When applied to two mouse models of RP, PR1 slows the degeneration of photoreceptors.

Conclusions

Chemical compounds identified as modulators of Nr2e3 activity may be useful for the treatment of RP through their effects on expression of disease-causing mutant genes.

Emerging pharmacological approaches to promote neurogenesis from endogenous glial cells

Neurodegenerative disorders are emerging as leading contributors to the global disease burden. While some drug-based approaches have been designed to limit or prevent neuronal loss following acute damage or chronic neurodegeneration, regeneration of functional neurons in the adult Central Nervous System (CNS) still remains an unmet need. In this context, the exploitation of endogenous cell sources has recently gained an unprecedented attention, thanks to the demonstration that, in some CNS regions or under specific circumstances, glial cells can activate spontaneous neurogenesis or can be instructed to produce neurons in the adult mammalian CNS parenchyma. This field of research has greatly advanced in the last years and identified interesting molecular and cellular mechanisms guiding the neurogenic activation/conversion of glia. In this review, we summarize the evolution of the research devoted to understand how resident glia can be directed to produce neurons. We paid particular attention to pharmacologically-relevant approaches exploiting the modulation of niche-associated factors and the application of selected small molecules.

Anatomy and spatial organization of Müller glia in mouse retina

Müller glia, the most abundant glia of vertebrate retina, have an elaborate morphology characterized by a vertical stalk that spans the retina and branches in each retinal layer. Müller glia play diverse, critical roles in retinal homeostasis, which are presumably enabled by their complex anatomy. However, much remains unknown, particularly in mouse, about the anatomical arrangement of Müller cells and their arbors, and how these features arise in development. Here we use membrane-targeted fluorescent proteins to reveal the fine structure of mouse Müller arbors. We find sublayer-specific arbor specializations within the inner plexiform layer (IPL) that occur consistently at defined laminar locations. We then characterize Müller glia spatial patterning, revealing how individual cells collaborate to form a pan-retinal network. Müller cells, unlike neurons, are spread across the retina with homogenous density, and their arbor sizes change little with eccentricity. Using Brainbow methods to label neighboring cells in different colors, we find that Müller glia tile retinal space with minimal overlap. The shape of their arbors is irregular but nonrandom, suggesting that local interactions between neighboring cells determine their territories. Finally, we identify a developmental window at postnatal Days 6 to 9 when Müller arbors first colonize the synaptic layers beginning in stereotyped inner plexiform layer sublaminae. Together, our study defines the anatomical arrangement of mouse Müller glia and their network in the radial and tangential planes of the retina, in development and adulthood. The local precision of Müller glia organization suggests that their morphology is sculpted by specific cell to cell interactions with neurons and each other.

Rapid, Dynamic Activation of Müller Glial Stem Cell Responses in Zebrafish

Purpose

Zebrafish neurons regenerate from Müller glia following retinal lesions. Genes and signaling pathways important for retinal regeneration in zebrafish have been described, but our understanding of how Müller glial stem cell properties are regulated is incomplete. Mammalian Müller glia possess a latent neurogenic capacity that might be enhanced in regenerative therapies to treat degenerative retinal diseases.

Methods

To identify transcriptional changes associated with stem cell properties in zebrafish Müller glia, we performed a comparative transcriptome analysis from isolated cells at 8 and 16 hours following an acute photic lesion, prior to the asymmetric division that produces retinal progenitors.

Results

We report a rapid, dynamic response of zebrafish Müller glia, characterized by activation of pathways related to stress, nuclear factor–κB (NF-κB) signaling, cytokine signaling, immunity, prostaglandin metabolism, circadian rhythm, and pluripotency, and an initial repression of Wnt signaling. When we compared publicly available transcriptomes of isolated mouse Müller glia from two retinal degeneration models, we found that mouse Müller glia showed evidence of oxidative stress, variable responses associated with immune regulation, and repression of pathways associated with pluripotency, development, and proliferation.

Conclusions

Categories of biological processes/pathways activated following photoreceptor loss in regeneration-competent zebrafish Müller glia, which distinguished them from mouse Müller glia in retinal degeneration models, included cytokine signaling (notably NF-κB), prostaglandin E2 synthesis, expression of core clock genes, and pathways/metabolic states associated with pluripotency. These regulatory mechanisms are relatively unexplored as potential mediators of stem cell properties likely to be important in Müller glial cells for successful retinal regeneration.

Self-organising aggregates of zebrafish retinal cells for investigating mechanisms of neural lamination

To investigate the cell-cell interactions necessary for the formation of retinal layers, we cultured dissociated zebrafish retinal progenitors in agarose microwells. Within these wells, the cells re-aggregated within hours, forming tight retinal organoids. Using a Spectrum of Fates zebrafish line, in which all different types of retinal neurons show distinct fluorescent spectra, we found that by 48 h in culture, the retinal organoids acquire a distinct spatial organisation, i.e. they became coarsely but clearly laminated. Retinal pigment epithelium cells were in the centre, photoreceptors and bipolar cells were next most central and amacrine cells and retinal ganglion cells were on the outside. Image analysis allowed us to derive quantitative measures of lamination, which we then used to find that Müller glia, but not RPE cells, are essential for this process.

BMP-SMAD signalling output is highly regionalized in cardiovascular and lymphatic endothelial networks

BACKGROUND

Bone morphogenetic protein (BMP) signalling has emerged as a fundamental pathway in endothelial cell biology and deregulation of this pathway is implicated in several vascular disorders. BMP signalling output in endothelial cells is highly context- and dose-dependent. Phosphorylation of the BMP intracellular effectors, SMAD1/5/9, is routinely used to monitor BMP signalling activity. To better understand the in vivo context-dependency of BMP-SMAD signalling, we investigated differences in BMP-SMAD transcriptional activity in different vascular beds during mouse embryonic and postnatal stages. For this, we used the BRE::gfp BMP signalling reporter mouse in which the BMP response element (BRE) from the ID1-promotor, a SMAD1/5/9 target gene, drives the expression of GFP.

RESULTS

A mosaic pattern of GFP was present in various angiogenic sprouting plexuses and in endocardium of cardiac cushions and trabeculae in the heart. High calibre veins seemed to be more BRE::gfp transcriptionally active than arteries, and ubiquitous activity was present in embryonic lymphatic vasculature. Postnatal lymphatic vessels showed however only discrete micro-domains of transcriptional activity. Dynamic shifts in transcriptional activity were also observed in the endocardium of the developing heart, with a general decrease in activity over time. Surprisingly, proliferative endothelial cells were almost never GFP-positive. Patches of transcriptional activity seemed to correlate with vasculature undergoing hemodynamic alterations.

CONCLUSION

The BRE::gfp mouse allows to investigate selective context-dependent aspects of BMP-SMAD signalling. Our data reveals the highly dynamic nature of BMP-SMAD mediated transcriptional regulation in time and space throughout the vascular tree, supporting that BMP-SMAD signalling can be a source of phenotypic diversity in some, but not all, healthy endothelium. This knowledge can provide insight in vascular bed or organ-specific diseases and phenotypic heterogeneity within an endothelial cell population.

Retinal regeneration in birds and mice

Retinal regeneration from resident Müller glia cells is robust in teleost fish, but is severely limited in birds and mammals. After neurotoxic injury, chick Müller glia can proliferate, and activate neurogenic genes, but they display limited capacity to differentiate into neurons. Developmental signaling molecules enhance this process. Regeneration of retinal neurons in rodents is even more limited. However, studies show evidence of proliferation and neurogenic gene expression after injury, with stronger effects in rats than mice, and differences between mouse strains. Mitogenic growth factors and Wnt signaling potentiate the proliferative response, while misexpression of the proneural transcription factor, Ascl1, reprograms to generate neurons from Müller glial in vitro, and stimulates neuronal regeneration in young mice, in vivo.

Next generation sequencing technology and genomewide data analysis: Perspectives for retinal research

The advent of high throughput next generation sequencing (NGS) has accelerated the pace of discovery of disease-associated genetic variants and genomewide profiling of expressed sequences and epigenetic marks, thereby permitting systems-based analyses of ocular development and disease. Rapid evolution of NGS and associated methodologies presents significant challenges in acquisition, management, and analysis of large data sets and for extracting biologically or clinically relevant information. Here we illustrate the basic design of commonly used NGS-based methods, specifically whole exome sequencing, transcriptome, and epigenome profiling, and provide recommendations for data analyses. We briefly discuss systems biology approaches for integrating multiple data sets to elucidate gene regulatory or disease networks. While we provide examples from the retina, the NGS guidelines reviewed here are applicable to other tissues/cell types as well.