BMP- and TGFβ-signaling regulate the formation of Müller glia-derived progenitor cells in the avian retina

Sphingosine-1-phosphate signaling regulates the ability of Müller glia to become neurogenic, proliferating progenitor-like cells

The purpose of these studies is to investigate how Sphingosine-1-phosphate (S1P) signaling regulates glial phenotype, dedifferentiation of Müller glia (MG), reprogramming into proliferating MG-derived progenitor cells (MGPCs), and neuronal differentiation of the progeny of MGPCs. We found that S1P-related genes are highly expressed by retinal neurons and glia, and levels of expression were dynamically regulated following retinal damage. S1PR1 is highly expressed by resting MG and is rapidly downregulated following acute retinal damage. Drug treatments that activate S1PR1 or increase levels of S1P suppressed the formation of MGPCs, whereas treatments that inhibit S1PR1 or decreased levels of S1P stimulated the formation of MGPCs. Inhibition of S1PR1 or SPHK1 significantly enhanced the neuronal differentiation of the progeny of MGPCs. Further, ablation of microglia from the retina, wherein the formation of MGPCs in damaged retinas is impaired, has a significant impact upon expression patterns of S1P-related genes in MG. Inhibition of S1PR1 and SPHK1 partially rescued the formation of MGPCs in damaged retinas missing microglia. Finally, we show that TGFβ/Smad3 signaling in the resting retina maintains S1PR1 expression in MG. We conclude that the S1P signaling is dynamically regulated in MG and MGPCs and activation of S1P signaling depends, in part, on signals produced by reactive microglia.

A single-cell transcriptomic study of heterogeneity in human embryonic tanycytes

Disruptions in energy homeostasis can lead to diseases like obesity and diabetes, affecting millions of people each year. Tanycytes, the adult stem cells in the hypothalamus, play crucial roles in assisting hypothalamic neurons in maintaining energy balance. Although tanycytes have been extensively studied in rodents, our understanding of human tanycytes remains limited. In this study, we utilized single-cell transcriptomics data to explore the heterogeneity of human embryonic tanycytes, investigate their gene regulatory networks, analyze their intercellular communication, and examine their developmental trajectory. Our analysis revealed the presence of two clusters of β tanycytes and three clusters of α tanycytes in our dataset. Surprisingly, human embryonic tanycytes displayed significant similarities to mouse tanycytes in terms of marker gene expression and transcription factor activities. Trajectory analysis indicated that α tanycytes were the first to be generated, giving rise to β tanycytes in a dorsal-ventral direction along the third ventricle. Furthermore, our CellChat analyses demonstrated that tanycytes generated earlier along the developmental lineages exhibited increased intercellular communication compared to those generated later. In summary, we have thoroughly characterized the heterogeneity of human embryonic tanycytes from various angles. We are confident that our findings will serve as a foundation for future research on human tanycytes.

ID factors regulate the ability of Müller glia to become proliferating neurogenic progenitor-like cells

The purpose of this study was to investigate how ID factors regulate the ability of Müller glia (MG) to reprogram into proliferating MG-derived progenitor cells (MGPCs) in the chick retina. We found that ID1 is transiently expressed by maturing MG (mMG), whereas ID4 is maintained in mMG in embryonic retinas. In mature retinas, ID4 was prominently expressed by resting MG, but following retinal damage ID4 was rapidly upregulated and then downregulated in MGPCs. By contrast, ID1, ID2, and ID3 were low in resting MG and then upregulated in MGPCs. Inhibition of ID factors following retinal damage decreased numbers of proliferating MGPCs. Inhibition of IDs, after MGPC proliferation, significantly increased numbers of progeny that differentiated as neurons. In damaged or undamaged retinas inhibition of IDs increased levels of p21Cip1 in MG. In response to damage or insulin+FGF2 levels of CDKN1A message and p21Cip1 protein were decreased, absent in proliferating MGPCs, and elevated in MG returning to a resting phenotype. Inhibition of notch- or gp130/Jak/Stat-signaling in damaged retinas increased levels of ID4 but not p21Cip1 in MG. Although ID4 is the predominant isoform expressed by MG in the chick retina, id1 and id2a are predominantly expressed by resting MG and downregulated in activated MG and MGPCs in zebrafish retinas. We conclude that ID factors have a significant impact on regulating the responses of MG to retinal damage, controlling the ability of MG to proliferate by regulating levels of p21Cip1, and suppressing the neurogenic potential of MGPCs.

Protein phosphatases regulate the formation of Müller glia-derived progenitor cells in the chick retina

Different kinase-dependent cell signaling pathways are known to play important roles in glia-mediated neuroprotection and reprogramming of Müller glia (MG) into Müller glia-derived progenitor cells (MGPCs) in the retina. However, very little is known about the phosphatases that regulate kinase-dependent signaling in MG. Using single-cell RNA-sequencing (scRNA-seq) databases, we investigated patterns of expression of Dual Specificity Phosphatases (DUSP1/6) and other protein phosphatases in normal and damaged chick retinas. We found that DUSP1, DUSP6, PPP3CB, PPP3R1 and PPPM1A/B/D/E/G are widely expressed by many types of retinal neurons and are dynamically expressed by MG and MGPCs in retinas during the process of reprogramming. We find that inhibition of DUSP1/6 and PP2C phosphatases enhances the formation of proliferating MGPCs in damaged retinas and in retinas treated with insulin and FGF2 in the absence of damage. By contrast, inhibition of PP2B phosphatases suppressed the formation of proliferating MGPCs, but increased numbers of proliferating MGPCs in undamaged retinas treated with insulin and FGF2. In damaged retinas, inhibition of DUSP1/6 increased levels of pERK1/2 and cFos in MG whereas inhibition of PP2B's decreased levels of pStat3 and pS6 in MG. Analyses of scRNA-seq libraries identified numerous differentially activated gene modules in MG in damaged retinas versus MG in retinas treated with insulin+FGF2 suggesting significant differences in kinase-dependent signaling pathways that converge on the formation of MGPCs. Inhibition of phosphatases had no significant effects upon numbers of dying cells in damaged retinas. We conclude that the activity of different protein phosphatases acting through retinal neurons and MG "fine-tune" the cell signaling responses of MG in damaged retinas and during the reprogramming of MG into MGPCs.

Protein phosphatases regulate the formation of Müller glia-derived progenitor cells in the chick retina

Different kinase-dependent cell signaling pathways are known to play important roles in glia-mediated neuroprotection and reprogramming of Müller glia (MG) into Müller glia-derived progenitor cells (MGPCs) in the retina. However, very little is known about the phosphatases that regulate kinase-dependent signaling in MG. Using single-cell RNA-sequencing (scRNA-seq) databases, we investigated patterns of expression of Dual Specificity Phosphatases (DUSP1/6) and other protein phosphatases in normal and damaged chick retinas. We found that DUSP1, DUSP6, PPP3CB, PPP3R1 and PPPM1A/B/D/E/G are dynamically expressed by MG and MGPCs in retinas during the process of reprogramming. We find that inhibition of DUSP1/6 and PP2C phosphatases enhances the formation of proliferating MGPCs in damaged retinas and in retinas treated with insulin in FGF2 in the absence of damage. By contrast, inhibition of PP2B phosphatases suppressed the formation of proliferating MGPCs, but increased numbers of proliferating MGPCs in undamaged retinas treated with insulin and FGF2. In damaged retinas, inhibition of DUSP1/6 increased levels of pERK1/2 and cFos in MG whereas inhibition of PP2B's decreased levels of pStat3 and pS6 in MG. Analyses of scRNA-seq libraries identified numerous differentially activated gene modules in MG in damaged retinas versus MG in retinas treated with insulin+FGF2 suggesting significant differences in kinase-dependent signaling pathways that converge on the formation of MGPCs. Inhibition of phosphatases had no significant effects upon numbers of dying cells in damaged retinas. We conclude that the activity of different protein phosphatases "fine-tune" the cell signaling responses of MG in damaged retinas and during the reprogramming of MG into MGPCs.

Heparin-binding epidermal growth factor and fibroblast growth factor 2 rescue Müller glia-derived progenitor cell formation in microglia- and macrophage-ablated chick retinas

Recent studies have demonstrated the impact of pro-inflammatory signaling and reactive microglia/macrophages on the formation of Müller glial-derived progenitor cells (MGPCs) in the retina. In chick retina, ablation of microglia/macrophages prevents the formation of MGPCs. Analyses of single-cell RNA-sequencing chick retinal libraries revealed that quiescent and activated microglia/macrophages have a significant impact upon the transcriptomic profile of Müller glia (MG). In damaged monocyte-depleted retinas, MG fail to upregulate genes related to different cell signaling pathways, including those related to Wnt, heparin-binding epidermal growth factor (HBEGF), fibroblast growth factor (FGF) and retinoic acid receptors. Inhibition of GSK3β, to simulate Wnt signaling, failed to rescue the deficit in MGPC formation, whereas application of HBEGF or FGF2 completely rescued the formation of MGPCs in monocyte-depleted retinas. Inhibition of Smad3 or activation of retinoic acid receptors partially rescued the formation of MGPCs in monocyte-depleted retinas. We conclude that signals produced by reactive microglia/macrophages in damaged retinas stimulate MG to upregulate cell signaling through HBEGF, FGF and retinoic acid, and downregulate signaling through TGFβ/Smad3 to promote the reprogramming of MG into proliferating MGPCs.

Characterization of Fibrodysplasia Ossificans Progessiva relevant Acvr1/Acvr2 Activin receptors in medaka (Oryzias latipes)

Activin and Bone Morphogenetic Protein (BMP) signaling plays crucial roles in vertebrate organ formation, including osteo- and angiogenesis, and tissue homeostasis, such as neuronal maintenance. Activin and BMP signaling needs to be precisely controlled by restricted expression of shared receptors, stoichiometric composition of receptor-complexes and presence of regulatory proteins. A R206H mutation in the human (hs) BMP type I receptor hsACVR1, on the other hand, leads to excessive phosphorylation of Sons of mothers against decapentaplegic (SMAD) 1/5/8. This in turn causes increased inflammation and heterotopic ossification in soft tissues of patients suffering from Fibrodysplasia Ossificans Progressiva (FOP). Several animal models have been established to understand the spontaneous and progressive nature of FOP, but often have inherent limitations. The Japanese medaka (Oryzias latipes, ola) has recently emerged as popular model for bone research. To assess whether medaka is suitable as a potential FOP animal model, we determined the expression of Activin receptor type I (ACVR1) orthologs olaAcvr1 and olaAcvr1l with that of Activin type II receptors olaAcvr2ab, olaAcvr2ba and olaAcvr2bb in embryonic and adult medaka tissues by in situ hybridization. Further, we showed that Activin A binding properties are conserved in olaAcvr2, as are the mechanistic features in the GS-Box of both olaAcvr1 and olaAcvr1l. This consequently leads to FOP-typical elevated SMAD signaling when the medaka type I receptors carry the R206H equivalent FOP mutation. Together, this study therefore provides experimental groundwork needed to establish a unique medaka model to investigate mechanisms underlying FOP.

Chromatin access regulates the formation of Müller glia-derived progenitor cells in the retina

Chromatin access and epigenetic control over gene expression play important roles in regulating developmental processes. However, little is known about how chromatin access and epigenetic gene silencing influence mature glial cells and retinal regeneration. Herein, we investigate the expression and functions of S-adenosylhomocysteine hydrolase (SAHH; AHCY) and histone methyltransferases (HMTs) during the formation of Müller glia (MG)-derived progenitor cells (MGPCs) in the chick and mouse retinas. In chick, AHCY, AHCYL1 and AHCYL2, and many different HMTs are dynamically expressed by MG and MGPCs in damaged retinas. Inhibition of SAHH reduced levels of H3K27me3 and potently blocks the formation of proliferating MGPCs. By using a combination of single cell RNA-seq and single cell ATAC-seq, we find significant changes in gene expression and chromatin access in MG with SAHH inhibition and NMDA-treatment; many of these genes are associated with glial and neuronal differentiation. A strong correlation across gene expression, chromatin access, and transcription factor motif access in MG was observed for transcription factors known to convey glial identity and promote retinal development. By comparison, in the mouse retina, inhibition of SAHH has no influence on the differentiation of neuron-like cells from Ascl1-overexpressing MG. We conclude that in the chick the activity of SAHH and HMTs are required for the reprogramming of MG into MGPCs by regulating chromatin access to transcription factors associated with glial differentiation and retinal development.

Formation of Müller glia-derived progenitor cells in retinas depleted of microglia

Recent studies have demonstrated the complex coordination of pro-inflammatory signaling and reactive microglia/macrophage on the formation Müller glial-derived progenitor cells (MGPCs) in the retinas of fish, birds and mice. We generated scRNA-seq libraries to identify transcriptional changes in Müller glia (MG) that result from the depletion of microglia from the chick retina. We found significant changes in different networks of genes in MG in normal and damaged retinas when the microglia are ablated. We identified a failure of MG to upregulate Wnt-ligands, Heparin binding epidermal growth factor (HBEGF), Fibroblast growth factor (FGF), retinoic acid receptors and genes related to Notch-signaling. Inhibition of GSK3β, to simulate Wnt-signaling, failed to rescue the deficit in formation of proliferating MGPCs in damaged retinas missing microglia. By comparison, application of HBEGF or FGF2 completely rescued the formation of proliferating MGPCs in microglia-depleted retinas. Similarly, injection of a small molecule inhibitor to Smad3 or agonist to retinoic acid receptors partially rescued the formation of proliferating MGPCs in microglia-depleted damaged retinas. According to scRNA-seq libraries, patterns of expression of ligands, receptors, signal transducers and/or processing enzymes to cell-signaling via HBEGF, FGF, retinoic acid and TGFβ are rapidly and transiently upregulated by MG after neuronal damage, consistent with important roles for these cell-signaling pathways in regulating the formation of MGPCs. We conclude that quiescent and activated microglia have a significant impact upon the transcriptomic profile of MG. We conclude that signals produced by reactive microglia in damaged retinas stimulate MG to upregulate cell signaling through HBEGF, FGF and retinoic acid, and downregulate signaling through TGFβ/Smad3 to promote the reprogramming on MG into proliferating MGPCs.

Rhodopsin-positive cell production by intravitreal injection of small molecule compounds in mouse models of retinal degeneration

We aimed to verify whether the intravitreal injection of small molecule compounds alone can create photoreceptor cells in mouse models of retinal degeneration. Primary cultured mouse Müller cells were stimulated in vitro with combinations of candidate compounds and the rhodopsin expression was measured on day 7 using polymerase chain reaction and immunostaining. We used 6-week-old N-methyl-N-nitrosourea-treated and 4-week-old rd10 mice as representative in vivo models of retinal degeneration. The optimal combination of compounds selected via in vitro screening was injected into the vitreous and the changes in rhodopsin expression were investigated on day 7 using polymerase chain reaction and immunostaining. The origin of rhodopsin-positive cells was also analyzed via lineage tracing and the recovery of retinal function was assessed using electroretinography. The in vitro mRNA expression of rhodopsin in Müller cells increased 30-fold, and 25% of the Müller cells expressed rhodopsin protein 7 days after stimulation with a combination of 4 compounds: transforming growth factor-β inhibitor, bone morphogenetic protein inhibitor, glycogen synthase kinase 3 inhibitor, and γ-secretase inhibitor. The in vivo rhodopsin mRNA expression and the number of rhodopsin-positive cells in the outer retina were significantly increased on day 7 after the intravitreal injection of these 4 compounds in both N-methyl-N-nitrosourea-treated and rd10 mice. Lineage tracing in td-Tomato mice treated with N-methyl-N-nitrosourea suggested that the rhodopsin-positive cells originated from endogenous Müller cells, accompanied with the recovery of the rhodopsin-derived scotopic function. It was suggested that rhodopsin-positive cells generated by compound stimulation contributes to the recovery of retinal function impaired by degeneration.

Modification of Müller Glial Cell Fate and Proliferation with the Use of Small Molecules

In recent years, reprogramming Müller glia by overexpressing Ascl1 and other transcription factors has shown promise for the regeneration of postmitotic retinal neurons, primarily bipolar cells, following injury. Müller glial proliferation and efficiency of neuronal differentiation can be modified by the use of small molecules in various systems. The molecules and pathways studied thus far share remarkable consistency with astrocytes. In this mini review, we provide an overview on the modulation of Müller glial proliferation and cell fate using small molecules in injury and reprogramming. We also compare these observations to what has been observed in astrocytes.

Retina regeneration: lessons from vertebrates

Unlike mammals, vertebrates such as fishes and frogs exhibit remarkable tissue regeneration including the central nervous system. Retina being part of the central nervous system has attracted the interest of several research groups to explore its regenerative ability in different vertebrate models including mice. Fishes and frogs completely restore the size, shape and tissue structure of an injured retina. Several studies have unraveled molecular mechanisms underlying retina regeneration. In teleosts, soon after injury, the Müller glial cells of the retina reprogram to form a proliferating population of Müller glia-derived progenitor cells capable of differentiating into various neural cell types and Müller glia. In amphibians, the transdifferentiation of retinal pigment epithelium and differentiation of ciliary marginal zone cells contribute to retina regeneration. In chicks and mice, supplementation with external growth factors or genetic modifications cause a partial regenerative response in the damaged retina. The initiation of retina regeneration is achieved through sequential orchestration of gene expression through controlled modulations in the genetic and epigenetic landscape of the progenitor cells. Several developmental biology pathways are turned on during the Müller glia reprogramming, retinal pigment epithelium transdifferentiation and ciliary marginal zone differentiation. Further, several tumorigenic pathways and gene expression events also contribute to the complete regeneration cascade of events. In this review, we address the various retinal injury paradigms and subsequent gene expression events governed in different vertebrate species. Further, we compared how vertebrates such as teleost fishes and amphibians can achieve excellent regenerative responses in the retina compared with their mammalian counterparts.

NFkB-signaling promotes glial reactivity and suppresses Müller glia-mediated neuron regeneration in the mammalian retina

Müller glia (MG) in mammalian retinas are incapable of regenerating neurons after damage, whereas the MG in lower vertebrates regenerate functional neurons. Identification of cell signaling pathways and gene regulatory networks that regulate MG-mediated regeneration is key to harnessing the regenerative potential of MG. Here, we study how NFkB-signaling influences glial responses to damage and reprogramming of MG into neurons in the rodent retina. We find activation of NFkB and dynamic expression of NFkB-associated genes in MG after damage, however damage-induced NFkB activation is inhibited by microglia ablation. Knockout of NFkB in MG suppressed the accumulation of immune cells after damage. Inhibition of NFkB following NMDA-damage significantly enhanced the reprogramming of Ascl1-overexpressing MG into neuron-like cells. scRNA-seq of retinal glia following inhibition of NFkB reveals coordination with signaling via TGFβ2 and suppression of NFI and Id transcription factors. Inhibition of Smad3 signal transducer or Id transcription factors increased numbers of neuron-like cells produced by Ascl1-overexpressing MG. We conclude that NFkB is a key signaling hub that is activated in MG after damage, mediates the accumulation of immune cells, and suppresses the neurogenic potential of MG.

Fatty acid-binding proteins and fatty acid synthase influence glial reactivity and promote the formation of Müller glia-derived progenitor cells in the chick retina

A recent comparative transcriptomic study of Müller glia (MG) in vertebrate retinas revealed that fatty acid binding proteins (FABPs) are among the most highly expressed genes in chick ( Hoang et al., 2020). Here, we investigate how FABPs and fatty acid synthase (FASN) influence glial cells in the chick retina. During development, FABP7 is highly expressed by retinal progenitor cells and maturing MG, whereas FABP5 is upregulated in maturing MG. PMP2 (FABP8) is expressed by oligodendrocytes and FABP5 is expressed by non-astrocytic inner retinal glial cells, and both of these FABPs are upregulated by activated MG. In addition to suppressing the formation of Müller glia-derived progenitor cells (MGPCs), we find that FABP-inhibition suppresses the proliferation of microglia. FABP-inhibition induces distinct changes in single cell transcriptomic profiles, indicating transitions of MG from resting to reactive states and suppressed MGPC formation, with upregulation of gene modules for gliogenesis and decreases in neurogenesis. FASN-inhibition increases the proliferation of microglia and suppresses the formation of MGPCs. We conclude that fatty acid metabolism and cell signaling involving fatty acids are important in regulating the reactivity and dedifferentiation of MG, and the proliferation of microglia and MGPCs.

Nuclear Factor I in neurons, glia and during the formation of Müller glia-derived progenitor cells in avian, porcine and primate retinas

The regenerative potential of Müller glia (MG) is extraordinary in fish, poor in chick and terrible in mammals. In the chick model, MG readily reprogram into proliferating Müller glia-derived progenitor cells (MGPCs), but neuronal differentiation is very limited. The factors that suppress the neurogenic potential of MGPCs in the chick are slowly being revealed. Isoforms of Nuclear Factor I (NFI) are cell-intrinsic factors that limit neurogenic potential; these factors are required for the formation of MG in the developing mouse retina (Clark et al., 2019) and deletion of these factors reprograms MG into neuron-like cells in mature mouse retina (Hoang et al., 2020). Accordingly, we sought to characterize the patterns of expression NFIs in the developing, mature and damaged chick retina. In addition, we characterized patterns of expression of NFIs in the retinas of large mammals, pigs and monkeys. Using a combination of single cell RNA-sequencing (scRNA-seq) and immunolabeling we probed for patterns of expression. In embryonic chick, levels of NFIs are very low in early E5 (embryonic day 5) retinal progenitor cells (RPCs), up-regulated in E8 RPCs, further up-regulated in differentiating MG at E12 and E15. NFIs are maintained in mature resting MG, microglia and neurons. Levels of NFIs are reduced in activated MG in retinas treated with NMDA and/or insulin+FGF2, and further down-regulated in proliferating MGPCs. However, levels of NFIs in MGPCs were significantly higher than those seen in RPCs. Immunolabeling for NFIA and NFIB closely matched patterns of expression revealed in different types of retinal neurons and glia, consistent with findings from scRNA-seq. In addition, we find expression of NFIA and NFIB through progenitors in the circumferential marginal zone at the far periphery of the retina. We find similar patterns of expression for NFIs in scRNA-seq databases for pig and monkey retinas. Patterns of expression of NFIA and NFIB were validated with immunofluorescence in pig and monkey retinas wherein these factors were predominantly detected in MG and a few types of inner retinal neurons. In summary, NFIA and NFIB are prominently expressed in developing chick retina and by mature neurons and glia in the retinas of chicks, pigs and monkeys. Although levels of NFIs are decreased in chick, in MGPCs these levels remain higher than those seen in neurogenic RPCs. We propose that the neurogenic potential of MGPCs in the chick retina is suppressed by NFIs. This article is protected by copyright. All rights reserved.

Dpp/TGFβ-superfamily play a dual conserved role in mediating the damage response in the retina

Retinal homeostasis relies on intricate coordination of cell death and survival in response to stress and damage. Signaling mechanisms that coordinate this process in the adult retina remain poorly understood. Here we identify Decapentaplegic (Dpp) signaling in Drosophila and its mammalian homologue Transforming Growth Factor-beta (TGFβ) superfamily, that includes TGFβ and Bone Morphogenetic Protein (BMP) signaling arms, as central mediators of retinal neuronal death and tissue survival following acute damage. Using a Drosophila model for UV-induced retinal damage, we show that Dpp released from immune cells promotes tissue loss after UV-induced retinal damage. Interestingly, we find a dynamic response of retinal cells to this signal: in an early phase, Dpp-mediated stimulation of Saxophone/Smox signaling promotes apoptosis, while at a later stage, stimulation of the Thickveins/Mad axis promotes tissue repair and survival. This dual role is conserved in the mammalian retina through the TGFβ/BMP signaling, as supplementation of BMP4 or inhibition of TGFβ using small molecules promotes retinal cell survival, while inhibition of BMP negatively affects cell survival after light-induced photoreceptor damage and NMDA induced inner retinal neuronal damage. Our data identify key evolutionarily conserved mechanisms by which retinal homeostasis is maintained.

An insight on established retinal injury mechanisms and prevalent retinal stem cell activation pathways in vertebrate models

Implementing different tools and injury mechanisms in multiple animal models of retina regeneration, researchers have discovered the existence of retinal stem/progenitor cells. Although they appear to be distributed uniformly across the vertebrate lineage, the reparative potential of the retina is mainly restricted to lower vertebrates. Regenerative repair post-injury requires the creation of a proliferative niche, vital for proper stem cell activation, propagation, and lineage differentiation. This seems to be lacking in mammals. Hence, in this review, we first discuss the many forms of retinal injuries that have been generated using animal models. Next, we discuss how they are utilized to stimulate regeneration and mimic eye disease pathologies. The key to driving stem cell activation in mammals relies on the information we can gather from these models. Lastly, we present a brief update about the genes, growth factors, and signaling pathways that have been brought to light using these models.

Overexpression of BMP4 protects retinal ganglion cells in a mouse model of experimental glaucoma

PURPOSE

Activation of bone morphogenetic protein (BMP) 4 signaling promotes the survival of retinal ganglion cell (RGC) after acute injury. Chordin-like 1 (CHRDL1) is an endogenous BMP antagonist. In this study, we researched whether CHRDL1 was involved in BMP4 signaling and regulation of RGC degeneration in a mouse model of glaucoma.

METHODS

Magnetic microbeads were intracameral injected to induce experimental glaucoma in a mouse model. A recombinant adeno-associated virus (rAAV) system was designed for overexpression of BMP4 or CHRDL1 in mouse retina. Immunohistochemistry and hematoxylin-eosin (HE) stains were performed to identify changes in retinal morphology. Electroretinogram (ERG) recordings were used to assess changes in visual function.

RESULTS

The mRNA expression levels of Bmp4 and its downstream BMPRIa, small mothers against decapentaplegic 1 (Smad1), were significantly upregulated in retinas with glaucoma. RGC survival was significantly enhanced in the beads + AAV-BMP4 group and significantly reduced in the beads + AAV-CHRDL1 group, compared with the beads + AAV-EGFP group. Similar results were observed in retinal explant culture in vitro. Consistent with these findings, the photopic negative response (PhNR)responses in ERG, which indicate RGC function, were restored in mice overexpressing BMP4, whereas a-wave and b-wave responses were not. Activation of CHRLD1 inhibited Smad1/5/8 phosphorylation and exacerbated RGC damage. The expression of Glial fibrillary acidic protein (GFAP) was decreased significantly in beads + AAV-BMP4 group.

CONCLUSIONS

BMP4 promoted RGC survival and visual function in an experimental glaucoma model. Activation of CHRDL1 exaggerated RGC degeneration by inhibiting the BMP4/Smad1/5/8 pathway. The mechanism of BMP4/Smad1/5/8 pathway may be related to the inhibition of glial cell activation. Our studies suggested that BMP4 and CHRLD1 might serve as therapeutic targets in glaucoma.

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.

Control of neurogenic competence in mammalian hypothalamic tanycytes

Hypothalamic tanycytes, radial glial cells that share many features with neuronal progenitors, can generate small numbers of neurons in the postnatal hypothalamus, but the identity of these neurons and the molecular mechanisms that control tanycyte-derived neurogenesis are unknown. In this study, we show that tanycyte-specific disruption of the NFI family of transcription factors (Nfia/b/x) robustly stimulates tanycyte proliferation and tanycyte-derived neurogenesis. Single-cell RNA sequencing (scRNA-seq) and single-cell assay for transposase-accessible chromatin sequencing (scATAC-seq) analysis reveals that NFI (nuclear factor I) factors repress Sonic hedgehog (Shh) and Wnt signaling in tanycytes and modulation of these pathways blocks proliferation and tanycyte-derived neurogenesis in Nfia/b/x-deficient mice. Nfia/b/x-deficient tanycytes give rise to multiple mediobasal hypothalamic neuronal subtypes that can mature, fire action potentials, receive synaptic inputs, and selectively respond to changes in internal states. These findings identify molecular mechanisms that control tanycyte-derived neurogenesis, which can potentially be targeted to selectively remodel the hypothalamic neural circuitry that controls homeostatic physiological processes.

Bone Morphogenetic Proteins and Diabetic Retinopathy

Bone morphogenetic proteins (BMPs) play an important role in bone formation and repair. Recent studies underscored their essential role in the normal development of several organs and vascular homeostasis in health and diseases. Elevated levels of BMPs have been linked to the development of cardiovascular complications of diabetes mellitus. However, their particular role in the pathogenesis of microvascular dysfunction associated with diabetic retinopathy (DR) is still under-investigated. Accumulated evidence from our and others’ studies suggests the involvement of BMP signaling in retinal inflammation, hyperpermeability and pathological neovascularization in DR and age-related macular degeneration (AMD). Therefore, targeting BMP signaling in diabetes is proposed as a potential therapeutic strategy to halt the development of microvascular dysfunction in retinal diseases, particularly in DR. The goal of this review article is to discuss the biological functions of BMPs, their underlying mechanisms and their potential role in the pathogenesis of DR in particular.

Crosstalk between Bone and Nerves within Bone

For the past two decades, the function of intrabony nerves on bone has been a subject of intense research, while the function of bone on intrabony nerves is still hidden in the corner. In the present review, the possible crosstalk between bone and intrabony peripheral nerves will be comprehensively analyzed. Peripheral nerves participate in bone development and repair via a host of signals generated through the secretion of neurotransmitters, neuropeptides, axon guidance factors and neurotrophins, with additional contribution from nerve-resident cells. In return, bone contributes to this microenvironmental rendezvous by housing the nerves within its internal milieu to provide mechanical support and a protective shelf. A large ensemble of chemical, mechanical, and electrical cues works in harmony with bone marrow stromal cells in the regulation of intrabony nerves. The crosstalk between bone and nerves is not limited to the physiological state, but also involved in various bone diseases including osteoporosis, osteoarthritis, heterotopic ossification, psychological stress-related bone abnormalities, and bone related tumors. This crosstalk may be harnessed in the design of tissue engineering scaffolds for repair of bone defects or be targeted for treatment of diseases related to bone and peripheral nerves.

Midkine is neuroprotective and influences glial reactivity and the formation of Müller glia-derived progenitor cells in chick and mouse retinas

Recent studies suggest midkine (MDK) is involved in the development and regeneration of the zebrafish retina. We investigate the expression patterns of MDK and related factors, roles in neuronal survival, and influence upon the formation of Müller glia-derived progenitor cells (MGPCs) in chick and mouse model systems. By using single-cell RNA-sequencing, we find that MDK and pleiotrophin (PTN), a MDK-related cytokine, are upregulated by Müller glia (MG) during later stages of development in chick. While PTN is downregulated, MDK is dramatically upregulated in mature MG after retinal damage or FGF2 and insulin treatment. By comparison, MDK and PTN are downregulated by MG in damaged mouse retinas. In both chick and mouse retinas, exogenous MDK induces expression of cFos and pS6 in MG. In the chick, MDK significantly decreases numbers dying neurons, reactive microglia, and proliferating MGPCs, whereas PTN has no effect. Inhibition of MDK-signaling with Na₃ VO₄ blocks neuroprotective effects with an increase in the number of dying cells and negates the pro-proliferative effects on MGPCs in damaged retinas. Inhibitors of PP2A and Pak1, which are associated with MDK-signaling through integrin β1, suppressed the formation of MGPCs in damaged chick retinas. In mice, MDK promotes a small but significant increase in proliferating MGPCs in damaged retinas and potently decreases the number of dying cells. We conclude that MDK expression is dynamically regulated in Müller glia during embryonic maturation, following retinal injury, and during reprogramming into MGPCs. MDK mediates glial activity, neuronal survival, and the re-programming of Müller glia into proliferating MGPCs.

The complexity of TGFβ/activin signaling in regeneration

The role of transforming growth factor β TGFβ/activin signaling in wound repair and regeneration is highly conserved in the animal kingdom. Various studies have shown that TGF-β/activin signaling can either promote or inhibit different aspects of the regeneration process (i.e., proliferation, differentiation, and re-epithelialization). It has been demonstrated in several biological systems that some of the different cellular responses promoted by TGFβ/activin signaling depend on the activation of Smad-dependent or Smad-independent signal transduction pathways. In the context of regeneration and wound healing, it has been shown that the type of R-Smad stimulated determines the different effects that can be obtained. However, neither the possible roles of Smad-independent pathways nor the interaction of the TGFβ/activin pathway with other complex signaling networks involved in the regenerative process has been studied extensively. Here, we review the important aspects concerning the TGFβ/activin signaling pathway in the regeneration process. We discuss data regarding the role of TGF-β/activin in the most common animal regenerative models to demonstrate how this signaling promotes or inhibits regeneration, depending on the cellular context.

Müller Glia-Mediated Retinal Regeneration

Müller glia originate from neuroepithelium and are the principal glial cells in the retina. During retinal development, Müller glia are one of the last cell types to be born. In lower vertebrates, such as zebrafish, Müller glia possess a remarkable capacity for retinal regeneration following various forms of injury through a reprogramming process in which endogenous Müller glia proliferate and differentiate into all types of retinal cells. In mammals, Müller glia become reactive in response to damage to protect or to further impair retinal function. Although mammalian Müller glia have regenerative potential, it is limited as far as repairing damaged retina. Lessons learned from zebrafish will help reveal the critical mechanisms involved in Müller glia reprogramming. Progress has been made in triggering Müller glia to reprogram and generate functional neurons to restore vision in mammals indicating that Müller glia reprogramming may be a promising therapeutic strategy for human retinal diseases. This review comprehensively summarizes the mechanisms related to retinal regeneration in model animals and the critical advanced progress made in Müller glia reprogramming in mammals.

Substantia nigra Smad3 signaling deficiency: relevance to aging and Parkinson's disease and roles of microglia, proinflammatory factors, and MAPK

Background

Smad3 signaling is indicated to regulate microglia activity. Parkinson’s disease (PD) neurodegeneration is shown to be associated with aging and neuroinflammation. However, it remains unclear about the relationship among Smad3 signaling, aging, neuroinflammation, and PD.

Methods

Rats were treated with SIS3 (a specific inhibitor of Smad3, intranigal injection) and/or lipopolysaccharide (intraperitoneal injection). We investigated the effect of SIS3 and lipopolysaccharide and their mechanism of action on motor behavior and nigrostriatal dopaminergic system in the rats. Furthermore, we explored the effect of SIS3 and LPS and their potential signaling mechanism of action on inflammatory response by using primary microglial cultures. Finally, we investigated the relationship among aging, Smad3 signaling, and neuroinflammation using animals of different ages.

Results

Both SIS3 and lipopolysaccharide induced significant behavior deficits and nigrostriatal dopaminergic neurodegeneration in the rats compared with the vehicle-treated (control) rats. Significantly increased behavior deficits and nigrostriatal dopaminergic neurodegeneration were observed in the rats co-treated with SIS3 and lipopolysaccharide compared with the rats treated with vehicle, SIS3, or lipopolysaccharide. Furthermore, both SIS3 and lipopolysaccharide induced significant microglia activation and proinflammatory factor (IL-1β, IL-6, iNOS, and ROS) level increase in the SN of rats compared with the control rats. Significantly enhanced microglial inflammatory response was observed in the rats co-treated with SIS3 and lipopolysaccharide compared with the other three groups. For our in vitro study, both SIS3 and lipopolysaccharide induced significant proinflammatory factor level increase in primary microglia cultures compared with the control cultures. Significantly increased inflammatory response was observed in the cultures co-treated with SIS3 and lipopolysaccharide compared with the other three groups. MAPK (ERK/p38) contributed to microglial inflammatory response induced by co-treatment with SIS3 and lipopolysaccharide. Interestingly, there was decrease in Smad3 and pSmad3 expression (protein) and enhancement of neuroinflammation in the mouse SN with aging. Proinflammatory factor levels were significantly inversely correlated with Smad3 and pSmad3 expression.

Conclusion

Our study strongly indicates the involvement of SN Smad3 signaling deficiency in aging and PD neurodegeneration and provides a novel molecular mechanism underlying the participation of aging in PD and helps to elucidate the mechanisms for the combined effect of multiple factors in PD.

Involvement of HB-EGF/Ascl1/Lin28a Genes in Dedifferentiation of Adult Mammalian Müller Glia

Previous studies from this lab have determined that dedifferentiation of Müller glia occurs after eye drop application of an α7 nicotinic acetylcholine receptor (nAChR) agonist, PNU-282987, to the adult rodent eye. PNU-282987 acts on α7 nAChRs on retinal pigment epithelial cells to stimulate production of Müller-derived progenitor cells (MDPCs) and ultimately lead to neurogenesis. This current study was designed to test the hypothesis that the activation of genes involved in the HB-EGF/Ascl1/Lin28a signaling pathway in Müller glia leads to the genesis of MDPCs. RNA-seq was performed on a Müller glial cell line (rMC-1) following contact with supernatant collected from a retinal pigment epithelial (RPE) cell line treated with PNU-282987. Differentially regulated genes were compared with published literature of Müller glia dedifferentiation that occurs in lower vertebrate regeneration and early mammalian development. HB-EGF was significantly up-regulated by 8 h and expression increased through 12 h. By 48 h, up-regulation of Ascl1 and Lin28a was observed, two genes known to be rapidly induced in dedifferentiating zebrafish Müller glia. Up-regulation of other genes known to be involved in mammalian development and zebrafish regeneration were also observed, as well as down-regulation of some factors necessary for Müller glia cell identity. RNA-seq results were verified using qRT-PCR. Using immunocytochemistry, the presence of markers associated with MDCP identity, Otx2, Nestin, and Vsx2, were found to be expressed in the 48 h treatment group cultures. This study is novel in its demonstration that Müller glia in adult rodents can be induced into regenerative activity by stimulating genes involved in the HB-EGF/Ascl1/Lin28a pathway that leads to MDPCs after introducing conditioned media from PNU-282987 treated RPE. This study furthers our understanding of the mechanism by which Müller glia dedifferentiate in response to PNU-282987 in the adult mammalian retina.

NF-κB signaling regulates the formation of proliferating Müller glia-derived progenitor cells in the avian retina

Retinal regeneration is robust in some cold-blooded vertebrates, but this process is ineffective in warm-blooded vertebrates. Understanding the mechanisms that suppress the reprogramming of Müller glia into neurogenic progenitors is key to harnessing the regenerative potential of the retina. Inflammation and reactive microglia are known to influence the formation of Müller glia-derived progenitor cells (MGPCs), but the mechanisms underlying this interaction are unknown. We used a chick in vivo model to investigate nuclear factor kappa B (NF-κB) signaling, a critical regulator of inflammation, during the reprogramming of Müller glia into proliferating progenitors. We find that components of the NF-κB pathway are dynamically regulated by Müller glia after neuronal damage or treatment with growth factors. Inhibition of NF-κB enhances, whereas activation suppresses, the formation of proliferating MGPCs. Following microglia ablation, the effects of NF-κB-agonists on MGPC-formation are reversed, suggesting that signals provided by reactive microglia influence how NF-κB impacts Müller glia reprogramming. We propose that NF-κB is an important signaling 'hub' that suppresses the reprogramming of Müller glia into proliferating MGPCs and this 'hub' coordinates signals provided by reactive microglia.

Tgfb3 collaborates with PP2A and notch signaling pathways to inhibit retina regeneration

Neuronal degeneration in the zebrafish retina stimulates Müller glia (MG) to proliferate and generate multipotent progenitors for retinal repair. Controlling this proliferation is critical to successful regeneration. Previous studies reported that retinal injury stimulates pSmad3 signaling in injury-responsive MG. Contrary to these findings, we report pSmad3 expression is restricted to quiescent MG and suppressed in injury-responsive MG. Our data indicates that Tgfb3 is the ligand responsible for regulating pSmad3 expression. Remarkably, although overexpression of either Tgfb1b or Tgfb3 can stimulate pSmad3 expression in the injured retina, only Tgfb3 inhibits injury-dependent MG proliferation; suggesting the involvement of a non-canonical Tgfb signaling pathway. Furthermore, inhibition of Alk5, PP2A or Notch signaling rescues MG proliferation in Tgfb3 overexpressing zebrafish. Finally, we report that this Tgfb3 signaling pathway is active in zebrafish MG, but not those in mice, which may contribute to the different regenerative capabilities of MG from fish and mammals.

Reprogramming Müller Glia to Regenerate Retinal Neurons

In humans, various genetic defects or age-related diseases, such as diabetic retinopathies, glaucoma, and macular degeneration, cause the death of retinal neurons and profound vision loss. One approach to treating these diseases is to utilize stem and progenitor cells to replace neurons in situ, with the expectation that new neurons will create new synaptic circuits or integrate into existing ones. Reprogramming non-neuronal cells in vivo into stem or progenitor cells is one strategy for replacing lost neurons. Zebrafish have become a valuable model for investigating cellular reprogramming and retinal regeneration. This review summarizes our current knowledge regarding spontaneous reprogramming of Müller glia in zebrafish and compares this knowledge to research efforts directed toward reprogramming Müller glia in mammals. Intensive research using these animal models has revealed shared molecular mechanisms that make Müller glia attractive targets for cellular reprogramming and highlighted the potential for curing degenerative retinal diseases from intrinsic cellular sources.

Growth Hormone Neuroprotection Against Kainate Excitotoxicity in the Retina is Mediated by Notch/PTEN/Akt Signaling

Purpose

In the retina, growth hormone (GH) promotes axonal growth, synaptic restoration, and protective actions against excitotoxicity. Notch signaling pathway is critical for neural development and participates in the retinal neuroregenerative process. We investigated the interaction of GH with Notch signaling pathway during its neuroprotective effect against excitotoxic damage in the chicken retina.

Methods

Kainate (KA) was used as excitotoxic agent and changes in the mRNA expression of several signaling markers were determined by qPCR. Also, changes in phosphorylation and immunoreactivity were determined by Western blotting. Histology and immunohistochemistry were performed for morphometric analysis. Overexpression of GH was performed in the quail neuroretinal-derived immortalized cell line (QNR/D) cell line. Exogenous GH was administered to retinal primary cell cultures to study the activation of signaling pathways.

Results

KA disrupted the retinal cytoarchitecture and induced significant cell loss in several retinal layers, but the coaddition of GH effectively prevented these adverse effects. We showed that GH upregulates the Notch signaling pathway during neuroprotection leading to phosphorylation of the PI3K/Akt signaling pathways through downregulation of PTEN. In contrast, cotreatment of GH with the Notch signaling inhibitor, DAPT, prevented its neuroprotective effect against KA. We identified binding sites in Notch1 and Notch2 genes for STAT5. Also, GH prevented Müller cell transdifferentiation and downregulated Sox2, FGF2, and PCNA after cotreatment with KA. Additionally, GH modified TNF receptors immunoreactivity suggesting anti-inflammatory actions.

Conclusions

Our data indicate that the neuroprotective effects of GH against KA injury in the retina are mediated through the regulation of Notch signaling. Additionally, anti-inflammatory and antiproliferative effects were observed.

Biphasic Role of Tgf-β Signaling during Müller Glia Reprogramming and Retinal Regeneration in Zebrafish

Tgf-β signaling is a major antiproliferative pathway governing different biological functions, including cellular reprogramming. Upon injury, Müller glial cells of zebrafish retina reprogram to form progenitors (MGPCs) essential for regeneration. Here, the significance of Tgf-β signaling for inducing MGPCs is explored. Notably, Tgf-β signaling not only performs a pro-proliferative function but also is necessary for the expression of several regeneration-associated, essential transcription factor genes such as ascl1a, lin28a, oct4, sox2, and zebs and various microRNAs, namely, miR-200a, miR-200b, miR-143, and miR-145 during different phases of retinal regeneration. This study also found the indispensable role played by Mmp2/Mmp9 in the efficacy of Tgf-β signaling. Furthermore, the Tgf-β signaling is essential to cause cell cycle exit of MGPCs towards later phases of regeneration. Finally, the Delta-Notch signaling in collaboration with Tgf-β signaling regulates the critical factor, Her4.1. This study provides novel insights into the biphasic roles of Tgf-β signaling in zebrafish during retinal regeneration.

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Tgf-β signaling is essential for retinal progenitor proliferation and cell cycle exit

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pSmad3 binds to 5GC and TIE elements to cause gene activations and repressions

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Tgf-β signaling regulates Zebs and various miRNAs for cellular reprograming

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Translation of Tgf-β signaling requires Mmp2/Mmp9 activity

Oct4 mediates Müller glia reprogramming and cell cycle exit during retina regeneration in zebrafish

The rapid induction of pluripotency-inducing factor Oct4 in the injured retina necessitates the de novo induction of stem cells and their subsequent cell cycle exit.

Octamer-binding transcription factor 4 (Oct4, also known as Pou5F3) is an essential pluripotency-inducing factor, governing a plethora of biological functions during cellular reprogramming. Retina regeneration in zebrafish involves reprogramming of Müller glia (MG) into a proliferating population of progenitors (MGPCs) with stem cell–like characteristics, along with up-regulation of pluripotency-inducing factors. However, the significance of Oct4 during retina regeneration remains elusive. In this study, we show an early panretinal induction of Oct4, which is essential for MG reprogramming through the regulation of several regeneration-associated factors such as Ascl1a, Lin28a, Sox2, Zeb, E-cadherin, and various miRNAs, namely, let-7a, miR-200a/miR-200b, and miR-143/miR-145. We also show the crucial roles played by Oct4 during cell cycle exit of MGPCs in collaboration with members of nucleosome remodeling and deacetylase complex such as Hdac1. Notably, Oct4 regulates Tgf-β signaling negatively during MG reprogramming, and positively to cause cycle exit of MGPCs. Our study reveals unique mechanistic involvement of Oct4, during MG reprogramming and cell cycle exit in zebrafish, which may also account for the inefficient retina regeneration in mammals.

Regenerative Effect of Growth Hormone (GH) in the Retina after Kainic Acid Excitotoxic Damage

In addition to its role as an endocrine messenger, growth hormone (GH) also acts as a neurotrophic factor in the central nervous system (CNS), whose effects are involved in neuroprotection, axonal growth, and synaptogenic modulation. An increasing amount of clinical evidence shows a beneficial effect of GH treatment in patients with brain trauma, stroke, spinal cord injury, impaired cognitive function, and neurodegenerative processes. In response to injury, Müller cells transdifferentiate into neural progenitors and proliferate, which constitutes an early regenerative process in the chicken retina. In this work, we studied the long-term protective effect of GH after causing severe excitotoxic damage in the retina. Thus, an acute neural injury was induced via the intravitreal injection of kainic acid (KA, 20 µg), which was followed by chronic administration of GH (10 injections [300 ng] over 21 days). Damage provoked a severe disruption of several retinal layers. However, in KA-damaged retinas treated with GH, we observed a significant restoration of the inner plexiform layer (IPL, 2.4-fold) and inner nuclear layer (INL, 1.5-fold) thickness and a general improvement of the retinal structure. In addition, we also observed an increase in the expression of several genes involved in important regenerative pathways, including: synaptogenic markers (DLG1, NRXN1, GAP43); glutamate receptor subunits (NR1 and GRIK4); pro-survival factors (BDNF, Bcl-2 and TNF-R2); and Notch signaling proteins (Notch1 and Hes5). Interestingly, Müller cell transdifferentiation markers (Sox2 and FGF2) were upregulated by this long-term chronic GH treatment. These results are consistent with a significant increase in the number of BrdU-positive cells observed in the KA-damaged retina, which was induced by GH administration. Our data suggest that GH is able to facilitate the early proliferative response of the injured retina and enhance the regeneration of neurite interconnections.

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.

Matrix-metalloproteinase expression and gelatinase activity in the avian retina and their influence on Müller glia proliferation

Gelatinases are a class of matrix metalloproteinases (MMPs) that degrade the extracellular matrix (ECM) to regulate intercellular signaling and cell migration. Gelatinase activity is tightly regulated via proteolytic activation and through the expression of tissue inhibitors of matrix metalloproteinases (TIMPs). Gelatinase activity has been implicated in retinal pathophysiology in different animal models and human disease. However, the role of gelatinases in retinal regeneration remains uncertain. In this study we investigated the dynamic changes in gelatinase activity in response to excitotoxic damage and how this enzymatic activity influenced the formation of Müller glia progenitor cells (MGPCs) in the avian retina. This study used hydrogels containing a gelatinase-degradable fluorescent peptide to measure gelatinase activity in vitro and dye quenched gelatin to localize enzymatic activity in situ. These data were corroborated by using single cell RNA sequencing (scRNA-seq). Gelatinase mRNA, specifically MMP2, was detected in oligodendrocytes and Non-Astrocytic Inner Retinal Glia (NIRG). Total retinal gelatinase activity was reduced following NMDA-treatment, and sustained inhibition of MMP2 prior to damage or growth factor treatment increased the formation of proliferating MGPCs and c-fos signaling. We observed that microglia, Müller glia (MG), and NIRG cells were involved in regulating changes in gelatinase activity through TIMP2 and TIMP3. Collectively, these findings implicate MMP2 in reprogramming of Muller glia into MGPCs.

Cellular Signaling in Müller Glia: Progenitor Cells for Regenerative and Neuroprotective Responses in Pharmacological Models of Retinal Degeneration

Retinal degenerative diseases are a leading cause of visual impairment or blindness. There are many therapies for delaying the progression of vision loss but no curative strategies currently. Stimulating intrinsic neuronal regeneration is a potential approach to therapy in retinal degenerative diseases. In contrast to stem cells, as embryonic/pluripotent stem cell-derived retinal progenitor cell or mesenchymal stem cells, Müller glia provided an endogenous cellular source for regenerative therapy in the retina. Müller glia are a major component of the retina and considerable evidence suggested these cells can be induced to produce the lost neurons in several species. Understanding the specific characteristic of Müller glia to generate lost neurons will inspire an attractive and alternative therapeutic strategy for treating visual impairment with regenerative research. This review briefly provides the different signal transduction mechanisms which are underlying Müller cell-mediated neuroprotection and neuron regeneration and discusses recent advances about regeneration from Müller glia-derived progenitors.

A new multichannel method quantitating TUNEL in detached photoreceptor nuclei

Nuclear co-localization labels are critical to ocular research. Among these, the TUNEL assay has been established as a gold standard of cell death and apoptosis. While several validated computer-based methods exist to quantitate these markers, including ImageJ Retina Analysis (RA) Toolkit and ImagePro, none verify the count with the nuclear counter stain to confirm nuclear co-localization. We established a new ImageJ-based automated multichannel thresholding (MCT) method to quantitate nuclear co-localized labeling. The MCT method was validated by comparing it with the two published TUNEL analysis in TUNEL-positive photoreceptors in an experimental retinal detachment (RD) model. RDs were induced in murine eyes and cross-sectional images of TUNEL and DAPI counter stain were obtained. Images were classified as "typical" or high density "hotspot" TUNEL regions (n = 10/group). Images were analyzed and compared between the MCT method and the published techniques including both "standard" and "high" settings of the RA Toolkit for detecting lower or higher TUNEL densities, respectively. Additional testing of the MCT method with built-in ImageJ thresholding algorithms was performed to produce fully automated measurements. All images were compared with Bland-Altman mean difference plots to assess the difference in counts and linear regression plots to assess correlation. Comparison between the MCT method and the ImagePro method were found to be well correlated (typical: R² = 0.8972, hotspot: R² = 0.9000) with minor to non-significant differences. The RA Toolkit settings were found to be mostly well correlated as well (standard/typical: R² = 0.8036, standard/hotspot: R² = 0.4309, high/typical: R² = 0.7895, high/hotspot: R² = 0.8738) but were often found to have significantly higher counts than the MCT. In conclusion, the MCT method compared favorably with validated computer-based methods of nuclear marker immunofluorescence quantitation and avoids staining artifacts through the incorporation of the nuclear counter stain to confirm positive cells.

Regulation and function of neurogenesis in the adult mammalian hypothalamus

Over the past two decades, evidence has accumulated that neurogenesis can occur in both the juvenile and adult mammalian hypothalamus. Levels of hypothalamic neurogenesis can be regulated by dietary, environmental and hormonal signals. Since the hypothalamus has a central role in controlling a broad range of homeostatic physiological processes, these findings may have far ranging behavioral and medical implications. However, many questions in the field remain unresolved, including the cells of origin of newborn hypothalamic neurons and the extent to which these cells actually regulate hypothalamic-controlled behaviors. In this manuscript, we conduct a critical review of the literature on postnatal hypothalamic neurogenesis in mammals, lay out the main outstanding controversies in the field, and discuss how best to advance our knowledge of this fascinating but still poorly understood process.

Retinoic Acid-Signaling Regulates the Proliferative and Neurogenic Capacity of Müller Glia-Derived Progenitor Cells in the Avian Retina

In the retina, Müller glia have the potential to become progenitor cells with the ability to proliferate and regenerate neurons. However, the ability of Müller glia-derived progenitor cells (MGPCs) to proliferate and produce neurons is limited in higher vertebrates. Using the chick model system, we investigate how retinoic acid (RA)-signaling influences the proliferation and the formation of MGPCs. We observed an upregulation of cellular RA binding proteins (CRABP) in the Müller glia of damaged retinas where the formation of MGPCs is known to occur. Activation of RA-signaling was stimulated, whereas inhibition suppressed the proliferation of MGPCs in damaged retinas and in fibroblast growth factor 2-treated undamaged retinas. Furthermore, inhibition of RA-degradation stimulated the proliferation of MGPCs. Levels of Pax6, Klf4, and cFos were upregulated in MGPCs by RA agonists and downregulated in MGPCs by RA antagonists. Activation of RA-signaling following MGPC proliferation increased the percentage of progeny that differentiated as neurons. Similarly, the combination of RA and insulin-like growth factor 1 (IGF1) significantly increased neurogenesis from retinal progenitors in the circumferential marginal zone (CMZ). In summary, RA-signaling stimulates the formation of proliferating MGPCs and enhances the neurogenic potential of MGPCs and stem cells in the CMZ. Stem Cells 2018;36:392-405.