Lgr5⁺ amacrine cells possess regenerative potential in the retina of adult mice

Trilogy Development of Proopiomelanocortin Neurons From Embryonic to Adult Stages in the Mice Retina

Proopiomelanocortin-positive amacrine cells (POMC ACs) were first discovered in adult mouse retinas in 2010; however, the development of POMC-ACs has not been studied. We bred POMC-EGFP mice to label POMC-positive cells and investigated the development of POMC neurons from embryonic to adult stages. We found that POMC neuron development is mainly divided into three stages: the embryonic stage, the closed-eye stage, and the open-eye stage. Each stage has unique characteristics. In the embryonic stage, POMC neurons appeared in the retina at about E13. There was a cell number developmental peak at E15, followed by a steep decline at E16. POMC neurons showed a large soma and increased spine numbers at the closed-eye stage, and two dendritic sublaminas formed in the inner plexiform layer (IPL). The appearance and increased soma size and dendrite numbers did not occur continuously in space. We found that the soma number was asymmetric between the superior and inferior retinas according to the developmental topographic map. Density peaked in the superior retina, which existed persistently in the retinal ganglion cell layer (GCL), but disappeared from the inner nuclear layer (INL) at about P6. At the same time, the soma distribution in the INL was the most regular. At the open-eye stage, the development of POMC neurons was nearly stable only with only an increase in the IPL width, which increased the soma–dendrite distance.

On the Generation and Regeneration of Retinal Ganglion Cells

Retinal development follows a conserved neurogenic program in vertebrates to orchestrate the generation of specific cell types from multipotent progenitors in sequential but overlapping waves. In this program, retinal ganglion cells (RGCs) are the first cell type generated. RGCs are the final output neurons of the retina and are essential for vision and circadian rhythm. Key molecular steps have been defined in multiple vertebrate species to regulate competence, specification, and terminal differentiation of this cell type. This involves neuronal-specific transcription factor networks, regulators of chromatin dynamics and miRNAs. In mammals, RGCs and their optic nerve axons undergo neurodegeneration and loss in glaucoma and other optic neuropathies, resulting in irreversible vision loss. The incapacity of RGCs and axons to regenerate reinforces the need for the design of efficient RGC replacement strategies. Here we describe the essential molecular pathways for the differentiation of RGCs in vertebrates, as well as experimental manipulations that extend the competence window for generation of this early cell type from late progenitors. We discuss recent advances in regeneration of retinal neurons in vivo in both mouse and zebrafish and discuss possible strategies and barriers to achieving RGC regeneration as a therapeutic approach for vision restoration in blinding diseases such as glaucoma.

Limitations and Promise of Retinal Tissue From Human Pluripotent Stem Cells for Developing Therapies of Blindness

The self-formation of retinal tissue from pluripotent stem cells generated a tremendous promise for developing new therapies of retinal degenerative diseases, which previously seemed unattainable. Together with use of induced pluripotent stem cells or/and CRISPR-based recombineering the retinal organoid technology provided an avenue for developing models of human retinal degenerative diseases "in a dish" for studying the pathology, delineating the mechanisms and also establishing a platform for large-scale drug screening. At the same time, retinal organoids, highly resembling developing human fetal retinal tissue, are viewed as source of multipotential retinal progenitors, young photoreceptors and just the whole retinal tissue, which may be transplanted into the subretinal space with a goal of replacing patient's degenerated retina with a new retinal "patch." Both approaches (transplantation and modeling/drug screening) were projected when Yoshiki Sasai demonstrated the feasibility of deriving mammalian retinal tissue from pluripotent stem cells, and generated a lot of excitement. With further work and testing of both approaches in vitro and in vivo, a major implicit limitation has become apparent pretty quickly: the absence of the uniform layer of Retinal Pigment Epithelium (RPE) cells, which is normally present in mammalian retina, surrounds photoreceptor layer and develops and matures first. The RPE layer polarize into apical and basal sides during development and establish microvilli on the apical side, interacting with photoreceptors, nurturing photoreceptor outer segments and participating in the visual cycle by recycling 11-trans retinal (bleached pigment) back to 11-cis retinal. Retinal organoids, however, either do not have RPE layer or carry patches of RPE mostly on one side, thus directly exposing most photoreceptors in the developing organoids to neural medium. Recreation of the critical retinal niche between the apical RPE and photoreceptors, where many retinal disease mechanisms originate, is so far unattainable, imposes clear limitations on both modeling/drug screening and transplantation approaches and is a focus of investigation in many labs. Here we dissect different retinal degenerative diseases and analyze how and where retinal organoid technology can contribute the most to developing therapies even with a current limitation and absence of long and functional outer segments, supported by RPE.

Mouse Retinal Cell Atlas: Molecular Identification of over Sixty Amacrine Cell Types

Amacrine cells (ACs) are a diverse class of interneurons that modulate input from photoreceptors to retinal ganglion cells (RGCs), rendering each RGC type selectively sensitive to particular visual features, which are then relayed to the brain. While many AC types have been identified morphologically and physiologically, they have not been comprehensively classified or molecularly characterized. We used high-throughput single-cell RNA sequencing to profile >32,000 ACs from mice of both sexes and applied computational methods to identify 63 AC types. We identified molecular markers for each type and used them to characterize the morphology of multiple types. We show that they include nearly all previously known AC types as well as many that had not been described. Consistent with previous studies, most of the AC types expressed markers for the canonical inhibitory neurotransmitters GABA or glycine, but several expressed neither or both. In addition, many expressed one or more neuropeptides, and two expressed glutamatergic markers. We also explored transcriptomic relationships among AC types and identified transcription factors expressed by individual or multiple closely related types. Noteworthy among these were Meis2 and Tcf4, expressed by most GABAergic and most glycinergic types, respectively. Together, these results provide a foundation for developmental and functional studies of ACs, as well as means for genetically accessing them. Along with previous molecular, physiological, and morphologic analyses, they establish the existence of at least 130 neuronal types and nearly 140 cell types in the mouse retina.SIGNIFICANCE STATEMENT The mouse retina is a leading model for analyzing the development, structure, function, and pathology of neural circuits. A complete molecular atlas of retinal cell types provides an important foundation for these studies. We used high-throughput single-cell RNA sequencing to characterize the most heterogeneous class of retinal interneurons, amacrine cells, identifying 63 distinct types. The atlas includes types identified previously as well as many novel types. We provide evidence for the use of multiple neurotransmitters and neuropeptides, and identify transcription factors expressed by groups of closely related types. Combining these results with those obtained previously, we proposed that the mouse retina contains ∼130 neuronal types and is therefore comparable in complexity to other regions of the brain.

microRNA: The Impact on Cancer Stemness and Therapeutic Resistance

Cancer ranks as the second leading cause of death worldwide, causing a large social and economic burden. However, most anti-cancer treatments face the problems of tumor recurrence and metastasis. Therefore, finding an effective cure for cancer needs to be solved urgently. Recently, the discovery of cancer stem cells (CSCs) provides a new orientation for cancer research and therapy. CSCs share main characteristics with stem cells and are able to generate an entire tumor. Besides, CSCs usually escape from current anti-cancer therapies, which is partly responsible for tumor recurrence and poor prognosis. microRNAs (miRNAs) belong to small noncoding RNA and regulate gene post-transcriptional expression. The dysregulation of miRNAs leads to plenty of diseases, including cancer. The aberrant miRNA expression in CSCs enhances stemness maintenance. In this review, we summarize the role of miRNAs on CSCs in the eight most common cancers, hoping to bridge the research of miRNAs and CSCs with clinical applications. We found that miRNAs can act as tumor promoter or suppressor. The dysregulation of miRNAs enhances cell stemness and contributes to tumor metastasis and therapeutic resistance via the formation of feedback loops and constitutive activation of carcinogenic signaling pathways. More importantly, some miRNAs may be potential targets for diagnosis, prognosis, and cancer treatments.

Transcriptome Analysis Did Not Show Endogenous Stem Cell Characteristics in Murine Lgr5+ Retinal Cells

Lgr5, an intestinal adult stem cell marker, was recently also found in neuronal tissues. We investigated whether retinal Lgr5⁺ cells express properties of neural stem cells (NSC) and/or of differentiated interneurons during retinal development. RNA was isolated from Lgr5⁺ and Lgr5⁻ populations from postnatal day 5 (PN5) and adult retinas of Lgr5^{EGFP-Ires-CreERT2} knock-in mice sorted by fluorescence-activated cell sorting (FACS). Transcriptome analyses were performed on two RNA samples of each developmental stage (PN5 and adult). The online platform PANTHER (Protein ANalysis THrough Evolutionary Relationships) was used to determine overrepresented gene ontology (GO) terms of biological processes within the set of differentially expressed genes. The detailed evaluation included gene expression in regard to stem cell maintenance/proliferation, cell cycle, and Wnt signaling but also markers of differentiated retinal neurons. None of the enriched GO terms of upregulated genes of Lgr5⁺ cells showed a positive association to NSC. On the contrary, NSC maintenance and proliferation rather prevail in the Lgr5⁻ cell population. Furthermore, results suggesting that Wnt signaling is not active in the Lgr5⁺ population. Therefore, our transcriptome analysis of Lgr5⁺ retinal cells suggest that these cells are differentiated neurons, specifically glycinergic amacrine cells.

Immunohistological Study of Monkey Foveal Retina

The fovea centralis, an anatomically concave pit located at the center of the macula, is avascular, hypoxic, and characteristic of stem-cell niches of other tissues. We hypothesized that in the fovea, undifferentiated retinal-stem-cell-like cells may exist, and that neurogenesis may occur. Hence, we performed an immunohistological study using cynomolgus monkey retinas. After preparing frozen tissue sections of the retina including the foveal pit, immunostaining was performed for glial fibrillary acidic protein (GFAP), nestin, vimentin, neuron-specific class III β-tubulin (Tuj-1), arrestin 4, neurofilament, CD117, CD44, Ki67, and cellular retinaldehyde-binding protein (CRALBP), followed by fluorescence and/or confocal microscopy examinations. Immunostaining of the tissue sections enabled clear observation of strongly GFAP-positive cells that corresponded to the inner-half layer of the foveolar Müller cell cone. The surface layer of the foveal slope was partially costained with GFAP and vimentin. Tuj-1-positive cells were observed in the innermost layer of the foveolar retina, which spanned to the surrounding ganglion cell layer. Moreover, colocalization of Tuj-1 and GFAP was observed at the foveal pit. The coexpression of CD117 and CD44 was found in the interphotoreceptor matrix of the fovea. The foveolar cone stained positive for both nestin and arrestin 4, however, the photoreceptor layer outside of the foveola displayed weak staining for nestin. Colocalization of nestin and vimentin was observed in the inner half of the Henle layer, while colocalization of nestin and neurofilament was observed in the outer half, predominantly. Scattered Ki67-positive cells were observed in the cellular processes of the outer plexiform layer and the ganglion cell layer around the foveola. Immunostaining for CRALBP was negative in most parts of the GFAP-positive area. The Müller cell cone was divided into GFAP-strongly positive cells, presumably astrocytes, in the inner layer and nestin-positive/GFAP-weakly positive radial glia-like cells in the outer layer. These findings indicated that groups of such undifferentiated cells in the foveola might be involved in maintaining morphology and regeneration.

Adult Stem Cells, Tools for Repairing the Retina

Purpose

Retinal degenerative diseases lead to the death of retinal neurons causing visual impairment and blindness. In lower order vertebrates, the retina and its surrounding tissue contain stem cell niches capable of regenerating damaged tissue. Here we examine these niches and review their capacity to be used as retinal stem/progenitor cells (RSC/RPCs) for retinal repair.

Recent Findings

Exogenous factors can control the in vitro activation of RSCs/PCs found in several niches within the adult eye including cells in the ciliary margin, the retinal pigment epithelium, iris pigment epithelium as well as the inducement of Müller and amacrine cells within the neural retina itself. Recently, factors have been identified for the activation of adult mammalian Müller cells to a RPC state in vivo.

Summary

Whereas cell transplantation still holds potential for retinal repair, activation of the dormant native regeneration process may lead to a more successful process including greater integration efficiency and proper synaptic targeting.

Ndr kinases regulate retinal interneuron proliferation and homeostasis

Ndr2/Stk38l encodes a protein kinase associated with the Hippo tumor suppressor pathway and is mutated in a naturally-occurring canine early retinal degeneration (erd). To elucidate the retinal functions of Ndr2 and its paralog Ndr1/Stk38, we generated Ndr1 and Ndr2 single knockout mice. Although retinal lamination appeared normal in these mice, Ndr deletion caused a subset of Pax6-positive amacrine cells to proliferate in differentiated retinas, while concurrently decreasing the number of GABAergic, HuD and Pax6-positive amacrine cells. Retinal transcriptome analyses revealed that Ndr2 deletion increased expression of neuronal stress genes and decreased expression of synaptic organization genes. Consistent with the latter, Ndr deletion dramatically reduced levels of Aak1, an Ndr substrate that regulates vesicle trafficking. Our findings indicate that Ndr kinases are important regulators of amacrine and photoreceptor cells and suggest that Ndr kinases inhibit the proliferation of a subset of terminally differentiated cells and modulate interneuron synapse function via Aak1.

Leucine-rich G Protein-coupled Receptor-5 Is Significantly Increased in the Aqueous Humor of Human Eye with Proliferative Diabetic Retinopathy

Leucine-rich G protein-coupled receptor-5 (LGR5) is known to be a stem cell marker in many organs. LGR5 may have important roles in proliferative diabetic retinopathy (PDR) because LGR5 potentiate the Wnt/β-catenin pathway, which plays crucial roles in pathologic neovascularization in the retina. The association between LGR5 and retinal pathologic neovascularization has not yet been reported. In the present study, LGR5 was compared in human aqueous humor (AH) between normal control and patients with PDR to confirm the relationship between LGR5 and PDR. AH was collected from 7 naïve PDR patients and 3 control subjects before intravitreal injection and cataract surgery, respectively. LGR5 and key members of Wnt/β-catenin were assessed by western blotting. In the present study, it was confirmed for the first time that LGR5 is detected in AH and it increases in PDR patients. Key members of Wnt/β-catenin pathway were also increased in AH of PDR patients compared to control. These findings might support the hypothesis that LGR5 has important roles in PDR especially considering the roles of the Wnt/β-catenin pathway, which is activated by LGR5, contributing to retinal pathologic neovascularization.

Notch Signaling Regulates Lgr5+ Olfactory Epithelium Progenitor/Stem Cell Turnover and Mediates Recovery of Lesioned Olfactory Epithelium in Mouse Model

The Notch signaling pathway regulates stem cell proliferation and differentiation in multiple tissues and organs, and is required for tissue maintenance. However, the role of Notch in regulation of olfactory epithelium (OE) progenitor/stem cells to maintain tissue function is still not clear. A recent study reported that leucine-rich repeat-containing G-protein-coupled receptor 5 (Lgr5) is expressed in globose basal cells (GBCs) localized in OE. Through lineage tracing in vivo, we found that Lgr5⁺ cells act as progenitor/stem cells in OE. The generation of daughter cells from Lgr5⁺ progenitor/stem cells is delicately regulated by the Notch signaling pathway, which not only controls the proliferation of Lgr5⁺ cells and their immediate progenies but also affects their subsequent terminal differentiation. In conditionally cultured OE organoids in vitro, inhibition of Notch signaling promotes neuronal differentiation. Besides, OE lesion through methimazole administration in mice induces generation of more Notch1⁺ cells in the horizontal basal cell (HBC) layer, and organoids derived from lesioned OE possesses more proliferative Notch1⁺ HBCs. In summary, we concluded that Notch signaling regulates Lgr5⁺ GBCs by controlling cellular proliferation and differentiation as well as maintaining epithelial cell homeostasis in normal OE. Meanwhile, Notch1 also marks HBCs in lesioned OE and Notch1⁺ HBCs are transiently present in OE after injury. This implies that Notch1⁺ cells in OE may have dual roles, functioning as GBCs in early development of OE and HBCs in restoring the lesioned OE. Stem Cells 2018;36:1259-1272.

Regulation of melanocyte stem cell behavior by the niche microenvironment

Somatic stem cells are regulated by their niches to maintain tissue homeostasis and repair throughout the lifetime of an organism. An excellent example to study stem cell/niche interactions is provided by the regeneration of melanocytes during the hair cycle and in response to various types of injury. These processes are regulated by neighboring stem cells and multiple signaling pathways, including WNT/β-catenin, KITL/KIT, EDNs/EDNRB, TGF-β/TGF-βR, α-MSH/MC1R, and Notch signaling. In this review, we highlight recent studies that have advanced our understanding of the molecular crosstalk between melanocyte stem cells and their neighboring cells, which collectively form the niche microenvironment, and we focus on the question of how McSCs/niche interactions shape the responses to genotoxic damages and mechanical injury.

Dual extra-retinal origins of microglia in the model of retinal microglia repopulation

Elucidating the origin of microglia is crucial for understanding their functions and homeostasis. Previous study has indicated that Nestin-positive progenitor cells differentiate into microglia and replenish the brain after depleting most brain microglia. Microglia have also shown the capacity to repopulate the retina after eliminating all retinal microglia. However, the origin(s) of repopulated retinal microglia is/are unknown. In this study, we aim to investigate the origins of repopulated microglia in the retina. Interestingly, we find that repopulated retinal microglia are not derived from Nestin-positive progenitor cells. Instead, they have two origins: the center-emerging microglia are derived from residual microglia in the optic nerve and the periphery-emerging microglia are derived from macrophages in the ciliary body/iris. Therefore, we have for the first time identified the extra-retinal origins of microglia in the adult mammalian retina by using a model of microglial repopulation, which may shed light on the target exploration of therapeutic interventions for retinal degenerative disorders.

Evidence for a retinal progenitor cell in the postnatal and adult mouse

Progress in cell therapy for retinal disorders has been challenging. Recognized retinal progenitors are a heterogeneous population of cells that lack surface markers for the isolation of live cells for clinical implementation. In the present application, our objective was to use the stem cell factor receptor c-Kit (CD117), a surface marker, to isolate and evaluate a distinct progenitor cell population from retinas of postnatal and adult mice. Here we report that, by combining traditional methods with fate mapping, we have identified a c-Kit-positive (c-Kit⁺) retinal progenitor cell (RPC) that is self-renewing and clonogenic in vitro, and capable of generating many cell types in vitro and in vivo. Based on cell lineage tracing, significant subpopulations of photoreceptors in the outer nuclear layer and bipolar, horizontal, amacrine and Müller cells in the inner nuclear layer are the progeny of c-Kit⁺ cells in vivo. The RPC progeny contributes to retinal neurons and glial cells, which are responsible for the conversion of light into visual signals. The ability to isolate and expand in vitro live c-Kit⁺ RPCs makes them a future therapeutic option for retinal diseases.

Loss of Axin2 Causes Ocular Defects During Mouse Eye Development

Purpose

The scaffold protein Axin2 is an antagonist and universal target of the Wnt/β-catenin pathway. Disruption of Axin2 may lead to developmental eye defects; however, this has not been examined. The purpose of this study was to investigate the role of Axin2 during ocular and extraocular development in mouse.

Methods

Animals heterozygous and homozygous for a Axin2^lacZ knock-in allele were analyzed at different developmental stages for reporter expression, morphology as well as for the presence of ocular and extraocular markers using histologic and immunohistochemical techniques.

Results

During early eye development, the Axin2^lacZ reporter was expressed in the periocular mesenchyme, RPE, and optic stalk. In the developing retina, Axin2^lacZ reporter expression was initiated in ganglion cells at late embryonic stages and robustly expressed in subpopulations of amacrine and horizontal cells postnatally. Activation of the Axin2^lacZ reporter overlapped with labeling of POU4F1, PAX6, and Calbindin. Germline deletion of Axin2 led to variable ocular phenotypes ranging from normal to severely defective eyes exhibiting microphthalmia, coloboma, lens defects, and expanded ciliary margin. These defects were correlated with abnormal tissue patterning in individual affected tissues, such as the optic fissure margins in the ventral optic cup and in the expanded ciliary margin.

Conclusions

Our results reveal a critical role for Axin2 during ocular development, likely by restricting the activity of the Wnt/β-catenin pathway.

Progress of stem/progenitor cell-based therapy for retinal degeneration

Retinal degeneration (RD), such as age-related macular degeneration (AMD) and retinitis pigmentosa, is one of the leading causes of blindness. Presently, no satisfactory therapeutic options are available for these diseases principally because the retina and retinal pigmented epithelium (RPE) do not regenerate, although wet AMD can be prevented from further progression by anti-vascular endothelial growth factor therapy. Nevertheless, stem/progenitor cell approaches exhibit enormous potential for RD treatment using strategies mainly aimed at the rescue and replacement of photoreceptors and RPE. The sources of stem/progenitor cells are classified into two broad categories in this review, which are (1) ocular-derived progenitor cells, such as retinal progenitor cells, and (2) non-ocular-derived stem cells, including embryonic stem cells, induced pluripotent stem cells, and mesenchymal stromal cells. Here, we discuss in detail the progress in the study of four predominant stem/progenitor cell types used in animal models of RD. A short overview of clinical trials involving the stem/progenitor cells is also presented. Currently, stem/progenitor cell therapies for RD still have some drawbacks such as inhibited proliferation and/or differentiation in vitro (with the exception of the RPE) and limited long-term survival and function of grafts in vivo. Despite these challenges, stem/progenitor cells represent the most promising strategy for RD treatment in the near future.

Report on the National Eye Institute Audacious Goals Initiative: Replacement of Retinal Ganglion Cells from Endogenous Cell Sources

This report emerges from a workshop convened by the National Eye Institute (NEI) as part of the "Audacious Goals Initiative" (AGI). The workshop addressed the replacement of retinal ganglion cells (RGCs) from exogenous and endogenous sources, and sought to identify the gaps in our knowledge and barriers to progress in devising cellular replacement therapies for diseases where RGCs die. Here, we briefly review relevant literature regarding common diseases associated with RGC death, the genesis of RGCs in vivo, strategies for generating transplantable RGCs in vitro, and potential endogenous cellular sources to regenerate these cells. These topics provided the clinical and scientific context for the discussion among the workshop participants and are relevant to efforts that may lead to therapeutic approaches for replacing RGCs. This report also summarizes the content of the workshop discussion, which focused on: (1) cell sources for RGC replacement and regeneration, (2) optimizing integration, survival, and synaptogenesis of new RGCs, and (3) approaches for assessing the outcomes of RGC replacement therapies. We conclude this report with a summary of recommendations, based on the workshop discussions, which may guide vision scientists seeking to develop therapies for replacing RGCs in humans.

Lgr5 regulates the regeneration of lesioned nasal respiratory epithelium

Nasal respiratory epithelium is a ciliated pseudostratified columnar epithelium. The cellular components of nasal respiratory epithelium include ciliated cells, goblet cells, and basal cells. Until now, our knowledge in the development of nasal respiratory epithelium is still limited and the cellular mechanism of regeneration is still elusive. In this study, we found that adult stem cell marker leucine-rich repeat-containing G-protein coupled receptor 5 (Lgr5) is expressed in the mice nasal respiratory epithelium. Both immunostaining and lineage tracing analysis indicated Lgr5 positive cells in the nasal respiratory epithelium are proliferative stem/progenitor cells. Using the Rosa-Tdtomato and Rosa26-DTR mice, we elucidated that Lgr5⁺ cells participate in the regeneration of lesioned nasal respiratory epithelium, and this group of cells is necessary in the process of epithelium recovery. Using the in vitro culture system, we observed the formation of spheres from Lgr5⁺ cells and these spheres have the capacity to generate other types of cells. Above all, this study reported a group of previously unidentified progenitor/stem cells in nasal respiratory epithelium, unveiling the potential cellular mechanism in nasal respiratory epithelium regeneration.

LGR4 and LGR5 Regulate Hair Cell Differentiation in the Sensory Epithelium of the Developing Mouse Cochlea

In the developing cochlea, Wnt/β-catenin signaling positively regulates the proliferation of precursors and promotes the formation of hair cells by up-regulating Atoh1 expression. Not much, however, is known about the regulation of Wnt/β-catenin activity in the cochlea. In multiple tissues, the activity of Wnt/β-catenin signaling is modulated by an interaction between LGR receptors and their ligands from the R-spondin family. The deficiency in Lgr4 and Lgr5 genes leads to developmental malformations and lethality. Using the Lgr5 knock-in mouse line we show that loss of LGR5 function increases Wnt/β-catenin activity in the embryonic cochlea, resulting in a mild overproduction of inner and outer hair cells (OHC). Supernumerary hair cells are likely formed due to an up-regulation of the “pro-hair cell” transcription factors Atoh1, Nhlh1, and Pou4f3. Using a hypomorphic Lgr4 mouse model we showed a mild overproduction of OHCs in the heterozygous and homozygous Lgr4 mice. The loss of LGR4 function prolonged the proliferation in the mid-basal turn of E13 cochleae, causing an increase in the number of SOX2-positive precursor cells within the pro-sensory domain. The premature differentiation of hair cells progressed in a medial to lateral gradient in Lgr4 deficient embryos. No significant up-regulation of Atoh1 was observed following Lgr4 deletion. Altogether, our findings suggest that LGR4 and LGR5 play an important role in the regulation of hair cell differentiation in the embryonic cochlea.

Transitional Progenitors during Vertebrate Retinogenesis

The retina is a delicate neural tissue responsible for light signal capturing, modulating, and passing to mid-brain. The brain then translated the signals into three-dimensional vision. The mature retina is composed of more than 50 subtypes of cells, all of which are developed from a pool of early multipotent retinal progenitors, which pass through sequential statuses of oligopotent, bipotent, and unipotent progenitors, and finally become terminally differentiated retinal cells. A transitional progenitor model is proposed here to describe how intrinsic developmental programs, along with environmental cues, control the step-by-step differentiation during retinogenesis. The model could elegantly explain many current findings as well as predict roles of intrinsic factors during retinal development.

Ocular Stem Cell Research from Basic Science to Clinical Application: A Report from Zhongshan Ophthalmic Center Ocular Stem Cell Symposium

Stem cells hold promise for treating a wide variety of diseases, including degenerative disorders of the eye. The eye is an ideal organ for stem cell therapy because of its relative immunological privilege, surgical accessibility, and its being a self-contained system. The eye also has many potential target diseases amenable to stem cell-based treatment, such as corneal limbal stem cell deficiency, glaucoma, age-related macular degeneration (AMD), and retinitis pigmentosa (RP). Among them, AMD and glaucoma are the two most common diseases, affecting over 200 million people worldwide. Recent results on the clinical trial of retinal pigment epithelial (RPE) cells from human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) in treating dry AMD and Stargardt’s disease in the US, Japan, England, and China have generated great excitement and hope. This marks the beginning of the ocular stem cell therapy era. The recent Zhongshan Ophthalmic Center Ocular Stem Cell Symposium discussed the potential applications of various stem cell types in stem cell-based therapies, drug discoveries and tissue engineering for treating ocular diseases.