Glucocorticoid receptors in the retina, Müller glia and the formation of Müller glia-derived progenitors

Biomechanic, proteomic and miRNA transcriptional changes in the trabecular meshwork of primates injected with intravitreal triamcinolone

Although biomechanical changes of the trabecular meshwork (TM) are important to the pathogenesis of glucocorticoids-induced ocular hypertension (GC-OHT), there is a knowledge gap in the underlying molecular mechanisms of the development of it. In this study, we performed intravitreal triamcinolone injection (IVTA) in one eye of 3 rhesus macaques. Following IVTA, we assessed TM stiffness using atomic force microscopy and investigated changes in proteomic and miRNA expression profiles. One of 3 macaques developed GC-OHT with a difference in intraocular pressure of 4.2 mmHg and a stiffer TM with a mean increase in elastic moduli of 0.60 kPa versus the non-injected control eye. In the IVTA-treated eyes, proteins associated with extracellular matrix remodeling, cytoskeletal rearrangement, and mitochondrial oxidoreductation were significantly upregulated. The significantly upregulated miR-29b and downregulated miR-335-5p post-IVTA supported the role of oxidative stress and mitophagy in the GC-mediated biomechanical changes in TM, respectively. The significant upregulation of miR-15/16 cluster post-IVTA may indicate a resultant TM cell apoptosis contributing to the increase in outflow resistance. Despite the small sample size, these results expand our knowledge of GC-mediated responses in the TM and furthermore, may help explain steroid responsiveness in clinical settings.

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.

Evaluation of combined epiretinal membrane removal with intravitreal triamcinolone injection utilizing ectopic inner foveal layer staging scheme

OBJECTIVE

To investigate the macular morphological and visual outcomes of combined idiopathic epiretinal membrane (iERM) removal with triamcinolone acetonide (TA) injection based on consideration of the ectopic inner foveal layer (EIFL) staging scheme.

METHODS

Retrospective case-control study. The clinical data of 84 eyes of 84 patients who underwent vitrectomy for iERM between 2018 and 2022 were reviewed. The enrolled subjects were divided into the TA and non-TA groups. Fifty-one eyes received intravitreal TA injection following vitrectomy and ERM peeling (TA group), and 33 were only treated by standard vitrectomy and ERM peeling (non-TA group). Preoperative and postoperative EIFL stages, central foveal thickness (CFT), and best-corrected visual acuity (BCVA) were compared between both groups.

RESULTS

After a mean follow-up of 7.69 ± 3.68 months, both groups exhibited significant improvement in EIFL stages (P < 0.01), with no discernible advantage observed in the TA group. The TA and non-TA groups demonstrated improvement in the EIFL stages in 56.86 and 63.64% of eyes, respectively (P = 0.43). The CFT and BCVA significantly improved in both groups at the final visit (P < 0.01). However, CFT in the non-TA group displayed a more significant reduction during the follow-up (P < 0.03). Subgroup analysis revealed no significant differences in postoperative CFT and BCVA between the two groups in cases with or without continuous EIFL (P > 0.10).

CONCLUSION

Our findings indicate that combined intravitreal TA injection following ERM removal conferred no significant benefits in alleviating macular thickening or improving visual acuity in iERM.

Circadian rhythm of intraocular pressure

Intraocular pressure (IOP) plays a crucial role in glaucoma development, involving the dynamics of aqueous humor (AH). AH flows in from the ciliary body and exits through the trabecular meshwork (TM). IOP follows a circadian rhythm synchronized with the suprachiasmatic nucleus (SCN), the circadian pacemaker. The SCN resets peripheral clocks through sympathetic nerves or adrenal glucocorticoids (GCs). IOP's circadian rhythm is governed by circadian time signals, sympathetic noradrenaline (NE), and GCs, rather than the local clock. The activity of Na+/K+-ATPase in non-pigmented epithelial cells in the ciliary body can influence the nocturnal increase in IOP by enhancing AH inflow. Conversely, NE, not GCs, can regulate the IOP rhythm by suppressing TM macrophage phagocytosis and AH outflow. The activation of the β1-adrenergic receptor (AR)-mediated EPAC-SHIP1 signal through the ablation of phosphatidylinositol triphosphate may govern phagocytic cup formation. These findings could offer insights for better glaucoma management, such as chronotherapy.

GMFB/AKT/TGF-β3 in Müller cells mediated early retinal degeneration in a streptozotocin-induced rat diabetes model

Retinal degeneration, characterized by Müller cell gliosis and photoreceptor apoptosis, is considered an early event in diabetic retinopathy (DR). Our previous study proposed that GMFB may mediate diabetic retinal degeneration. This study identified GMFB as a sensitive and functional gliosis marker for DR. Compared to the wild type (WT) group, Gmfb knockout (KO) significantly improved visual function, attenuated gliosis, reduced the apoptosis of neurons, and decreased the mRNA levels of tumor necrosis factor α (Tnf-α) and interleukin-1β (Il-1β) in diabetic retinas. Tgf-β3 was enriched by hub genes using RNA sequencing in primary WT and KO Müller cells. Gmfb KO significantly upregulated the transforming growth factor (TGF)-β3 protein level via the AKT pathway. The protective effect of TGF-β3 in the vitreous resulted in significantly improved visual function and decreased the number of apoptotic cells in the diabetic retina. The protection of Gmfb KO in primary Müller cells against high glucose (HG)-induced photoreceptor apoptosis was partially counteracted by TGF-β3 antibody and administration of TGFBR1/2 inhibitors. Nuclear receptor subfamily 3 group C member 1 (NR3C1) binds to the promoter region of Gmfb and regulates Gmfb mRNA at the transcriptional level. NR3C1 was increased in the retinas of early diabetic rats but decreased in the retinas of late diabetic rats. N'-[(1E)-(3-Methoxyphenyl)Methylene]-3-Methyl-1H-Pyrazole-5-Carbohydrazide (DS-5) was identified as an inhibitor of GMFB, having a protective role in DR. We demonstrated that GMFB/AKT/TGF-β3 mediated early diabetic retinal degeneration in diabetic rats. This study provides a novel therapeutic strategy for treating retinal degeneration in patients with DR.

The glucocorticoid receptor as a master regulator of the Müller cell response to diabetic conditions in mice

Diabetic retinopathy (DR) is considered a primarily microvascular complication of diabetes. Müller glia cells are at the centre of the retinal neurovascular unit and play a critical role in DR. We therefore investigated Müller cell-specific signalling pathways that are altered in DR to identify novel targets for gene therapy. Using a multi-omics approach on purified Müller cells from diabetic db/db mice, we found the mRNA and protein expression of the glucocorticoid receptor (GR) to be significantly decreased, while its target gene cluster was down-regulated. Further, oPOSSUM TF analysis and ATAC- sequencing identified the GR as a master regulator of Müller cell response to diabetic conditions. Cortisol not only increased GR phosphorylation. It also induced changes in the expression of known GR target genes in retinal explants. Finally, retinal functionality was improved by AAV-mediated overexpression of GR in Müller cells. Our study demonstrates an important role of the glial GR in DR and implies that therapeutic approaches targeting this signalling pathway should be aimed at increasing GR expression rather than the addition of more ligand.

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.

Heterogeneity in quiescent Müller glia in the uninjured zebrafish retina drive differential responses following photoreceptor ablation

Introduction

Loss of neurons in the neural retina is a leading cause of vision loss. While humans do not possess the capacity for retinal regeneration, zebrafish can achieve this through activation of resident Müller glia. Remarkably, despite the presence of Müller glia in humans and other mammalian vertebrates, these cells lack an intrinsic ability to contribute to regeneration. Upon activation, zebrafish Müller glia can adopt a stem cell-like state, undergo proliferation and generate new neurons. However, the underlying molecular mechanisms of this activation subsequent retinal regeneration remains unclear.

Methods/Results

To address this, we performed single-cell RNA sequencing (scRNA-seq) and report remarkable heterogeneity in gene expression within quiescent Müller glia across distinct dorsal, central and ventral retina pools of such cells. Next, we utilized a genetically driven, chemically inducible nitroreductase approach to study Müller glia activation following selective ablation of three distinct photoreceptor subtypes: long wavelength sensitive cones, short wavelength sensitive cones, and rods. There, our data revealed that a region-specific bias in activation of Müller glia exists in the zebrafish retina, and this is independent of the distribution of the ablated cell type across retinal regions. Notably, gene ontology analysis revealed that injury-responsive dorsal and central Müller glia express genes related to dorsal/ventral pattern formation, growth factor activity, and regulation of developmental process. Through scRNA-seq analysis, we identify a shared genetic program underlying initial Müller glia activation and cell cycle entry, followed by differences that drive the fate of regenerating neurons. We observed an initial expression of AP-1 and injury-responsive transcription factors, followed by genes involved in Notch signaling, ribosome biogenesis and gliogenesis, and finally expression of cell cycle, chromatin remodeling and microtubule-associated genes.

Discussion

Taken together, our findings document the regional specificity of gene expression within quiescent Müller glia and demonstrate unique Müller glia activation and regeneration features following neural ablation. These findings will improve our understanding of the molecular pathways relevant to neural regeneration in the retina.

Nanoparticle-based targeting of microglia improves the neural regeneration enhancing effects of immunosuppression in the zebrafish retina

Retinal Müller glia function as injury-induced stem-like cells in zebrafish but not mammals. However, insights gleaned from zebrafish have been applied to stimulate nascent regenerative responses in the mammalian retina. For instance, microglia/macrophages regulate Müller glia stem cell activity in the chick, zebrafish, and mouse. We previously showed that post-injury immunosuppression by the glucocorticoid dexamethasone accelerated retinal regeneration kinetics in zebrafish. Similarly, microglia ablation enhances regenerative outcomes in the mouse retina. Targeted immunomodulation of microglia reactivity may therefore enhance the regenerative potential of Müller glia for therapeutic purposes. Here, we investigated potential mechanisms by which post-injury dexamethasone accelerates retinal regeneration kinetics, and the effects of dendrimer-based targeting of dexamethasone to reactive microglia. Intravital time-lapse imaging revealed that post-injury dexamethasone inhibited microglia reactivity. The dendrimer-conjugated formulation: (1) decreased dexamethasone-associated systemic toxicity, (2) targeted dexamethasone to reactive microglia, and (3) improved the regeneration enhancing effects of immunosuppression by increasing stem/progenitor proliferation rates. Lastly, we show that the gene rnf2 is required for the enhanced regeneration effect of D-Dex. These data support the use of dendrimer-based targeting of reactive immune cells to reduce toxicity and enhance the regeneration promoting effects of immunosuppressants in the retina.

Heart failure and cognitive impairment: A narrative review of neuroimaging mechanism from the perspective of brain MRI

Both heart failure (HF) and cognitive impairment (CI) have a significant negative impact on the health of the elderly individuals. Magnetic resonance imaging (MRI) can non-invasively detect functional and structural variations in the heart and brain, making it easier to explore the connection between the heart and brain. According to neuroimaging studies, HF patients have a higher chance of developing CI because they have a variety of different types of brain injuries. To examine how HF and CI are influenced by one another, English-language literature was searched in the Web of Science, PubMed EMBASE (OVID), PsycInfo, and Scopus databases. The search terms included “high-frequency,” “brain function,” “brain injury,” “cognition,” “cognitive impairment,” and “magnetic resonance imaging.” Normal brain function is typically impaired by HF in the form of decreased cerebral perfusion pressure, inflammation, oxidative stress, and damage to the BBB, resulting in CI and subsequent HF. Early pathophysiological alterations in patients’ brains have been widely detected using a range of novel MRI techniques, opening up new avenues for investigating the connection between HF and CI. This review aims to describe the pathogenesis of HF with CI and the early diagnostic role of MRI in the heart-brain domain.

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.

Patterns of NFkB activation resulting from damage, reactive microglia, cytokines, and growth factors in the mouse retina

Müller glia are a cellular source for neuronal regeneration in vertebrate retinas. However, the capacity for retinal regeneration varies widely across species. Understanding the mechanisms that regulate the reprogramming of Müller glia into progenitor cells is key to reversing the loss of vision that occurs with retinal diseases. In the mammalian retina, NFkB signaling promotes glial reactivity and represses the reprogramming of Müller glia into progenitor cells. Here we investigate different cytokines, growth factors, cell signaling pathways, and damage paradigms that influence NFkB-signaling in the mouse retina. We find that exogenous TNF and IL1β potently activate NFkB-signaling in Müller glia in undamaged retinas, and this activation is independent of microglia. By comparison, TLR1/2 agonist indirectly activates NFkB-signaling in Müller glia, and this activation depends on the presence of microglia as Tlr2 is predominantly expressed by microglia, but not other types of retinal cells. Exogenous FGF2 did not activate NFkB-signaling, whereas CNTF, Osteopontin, WNT4, or inhibition of GSK3β activated NFkB in Müller glia in the absence of neuronal damage. By comparison, dexamethasone, a glucocorticoid agonist, suppressed NFkB-signaling in Müller glia in damaged retinas, in addition to reducing numbers of dying cells and the accumulation of reactive microglia. Although NMDA-induced retinal damage activated NFkB in Müller glia, optic nerve crush had no effect on NFkB activation within the retina, whereas glial cells within the optic nerve were responsive. We conclude that the NFkB pathway is activated in retinal Müller glia in response to many different cell signaling pathways, and activation often depends on signals produced by reactive microglia.

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.

The role of retinal Müller cells in diabetic retinopathy and related therapeutic advances

Diabetic retinopathy (DR) is a significant complication of diabetes. During the pathogenesis of retinal microangiopathy and neuronopathy, activated retinal Müller cells (RMCs) undergo morphological and structural changes such as increased expression of glial fibrillary acidic protein, disturbance of potassium and water transport regulation, and onset of production of a large number of inflammatory and vascular growth factors as well as chemokines. Evidently, activated RMCs are necessary for the pathogenesis of DR; therefore, exploring the role of RMCs in DR may provide a new target for the treatment thereof. This article reviews the mechanism of RMCs involvement in DR and the progress in related treatments.

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.

Notch Inhibition Promotes Regeneration and Immunosuppression Supports Cone Survival in a Zebrafish Model of Inherited Retinal Dystrophy

Photoreceptor degeneration leads to irreversible vision loss in humans with retinal dystrophies such as retinitis pigmentosa. Whereas photoreceptor loss is permanent in mammals, zebrafish possesses the ability to regenerate retinal neurons and restore visual function. Following acute damage, Müller glia (MG) re-enter the cell cycle and produce multipotent progenitors whose progeny differentiate into mature neurons. Both MG reprogramming and proliferation of retinal progenitor cells require reactive microglia and associated inflammatory signaling. Paradoxically, in zebrafish models of retinal degeneration, photoreceptor death does not induce the MG to reprogram and regenerate lost cells. Here, we used male and female zebrafish cep290 mutants to demonstrate that progressive cone degeneration generates an immune response but does not stimulate MG proliferation. Acute light damage triggered photoreceptor regeneration in cep290 mutants but cones were only restored to prelesion densities. Using irf8 mutant zebrafish, we found that the chronic absence of microglia reduced inflammation and rescued cone degeneration in cep290 mutants. Finally, single-cell RNA-sequencing revealed sustained expression of notch3 in MG of cep290 mutants and inhibition of Notch signaling induced MG to re-enter the cell cycle. Our findings provide new insights on the requirements for MG to proliferate and the potential for immunosuppression to prolong photoreceptor survival.SIGNIFICANCE STATEMENT Inherited retinal degenerations (IRDs) are genetic diseases that lead to the progressive loss of photoreceptors and the permanent loss of vision. Zebrafish can regenerate photoreceptors after acute injury by reprogramming Müller glia (MG) into stem-like cells that produce retinal progenitors, but this regenerative process fails to occur in zebrafish models of IRDs. Here, we show that Notch pathway inhibition can promote photoreceptor regeneration in models of progressive degeneration and that immunosuppression can prevent photoreceptor loss. These results offer insight into the pathways that promote MG-dependent regeneration and the role of inflammation in photoreceptor degeneration.

Regulation of herpes simplex virus type 1 latency-reactivation cycle and ocular disease by cellular signaling pathways

Following acute infection, herpes simplex virus type 1 (HSV-1) establishes life-long latency in sensory and other neurons. Recurrent ocular HSV-1 outbreaks are generally due to reactivation from latency. The HSV-1 latency-reactivation cycle is a complex virus-host relationship. The viral encoded latency-associated transcript (LAT) is abundantly expressed in latency and encodes several micro-RNAs and other small non-coding RNAs, which may regulate expression of key viral and cellular genes. Certain cellular signaling pathways, including Wnt/β-catenin and mTOR pathway, mediate certain aspect of the latency-reactivation cycle. Stress, via activation of the glucocorticoid receptor and other stress induced cellular transcription factors, are predicted to trigger reactivation from latency by stimulating viral gene expression and impairing immune responses and inflammation. These observations suggest stress and certain cellular signaling pathways play key roles in regulating the latency-reactivation cycle and recurrent ocular disease.

Cognitive Impairment in Heart Failure: Landscape, Challenges, and Future Directions

Heart failure (HF) is a major global healthcare problem accounting for substantial deterioration of prognosis. As a complex clinical syndrome, HF often coexists with multi-comorbidities of which cognitive impairment (CI) is particularly important. CI is increasing in prevalence among patients with HF and is present in around 40%, even up to 60%, of elderly patients with HF. As a potent and independent prognostic factor, CI significantly increases the hospitalization and mortality and decreases quality of life in patients with HF. There has been a growing awareness of the complex bidirectional interaction between HF and CI as it shares a number of common pathophysiological pathways including reduced cerebral blood flow, inflammation, and neurohumoral activations. Research that focus on the precise mechanism for CI in HF is still ever insufficient. As the tremendous adverse consequences of CI in HF, effective early diagnosis of CI in HF and interventions for these patients may halt disease progression and improve prognosis. The current clinical guidelines in HF have begun to emphasize the importance of CI. However, nearly half of CI in HF is underdiagnosed, and few recommendations are available to guide clinicians about how to approach CI in patients with HF. This review aims to synthesize knowledge about the link between HF and cognitive dysfunction, issues pertaining to screening, diagnosis and management of CI in patients with HF, and emerging therapies for prevention. Based on data from current studies, critical gaps in knowledge of CI in HF are identified, and future research directions to guide the field forward are proposed.

Deletion of Asrgl1 Leads to Photoreceptor Degeneration in Mice

The asparaginase and isoaspartyl peptidase 1 (ASRGL1) is an L-asparaginase and beta-aspartyl peptidase enzyme that may be involved in the formation of L-aspartate, a neurotransmitter that can operate as an excitatory neurotransmitter in some brain regions. Although variants in ASRGL1 have been reported in retinitis pigmentosa (RP) patients, the in vivo functions and mechanisms of ASRGL in RP remains unknown due to the lack of suitable disease models. To explore the role of ASRGL in RP, we generated an Asrgl1 knockout mouse model (Asrgl1 KO) using the CRISPR/Cas9 technique. Asrgl1 ablation in mice led to an attenuated electroretinogram (ERG) response around 8 months. The thickness of the outer nuclei layer (ONL) started to decrease around 9 months in Asrgl1 KO mice and gradually intensified at 12 and 15 months. Immunostaining revealed thinner inner segment (IS) and thinner outer segment (OS) as well as the progressive degeneration of rod and cone cells in Asrgl1 KO mice. One hundred forty-nine transcriptional differentially expressed genes (DEGs) were found by RNA-seq in Asrgl1 KO retina. These DEGs were linked to a number of biological processes that were considerably enriched, including gastrointestinal disease and organismal injury and abnormalities. By analysis of canonical pathways, glucocorticoid receptor signaling was the most significant canonical pathway altered in Asrgl1 KO retina. Several molecules, including NFE2L2, IL-4, Foxp3, and Fos, were in the central nodes of the interaction network in Asrgl1 KO retina. In summary, our study provided a knockout mouse model for a better understanding of the molecular mechanism for ASRGL1-related RP.

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.

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.

Glucocorticoid agonists enhance retinal stem cell self-renewal and proliferation

BACKGROUND

Adult mammalian retinal stem cells (RSCs) readily proliferate, self-renew, and generate progeny that differentiate into all retinal cell types in vitro. RSC-derived progeny can be induced to differentiate into photoreceptors, making them a potential source for retinal cell transplant therapies. Despite their proliferative propensity in vitro, RSCs in the adult mammalian eye do not proliferate and do not have a regenerative response to injury. Thus, identifying and modulating the mechanisms that regulate RSC proliferation may enhance the capacity to produce RSC-derived progeny in vitro and enable RSC activation in vivo.

METHODS

Here, we used medium-throughput screening to identify small molecules that can expand the number of RSCs and their progeny in culture. In vitro differentiation assays were used to assess the effects of synthetic glucocorticoid agonist dexamethasone on RSC-derived progenitor cell fate. Intravitreal injections of dexamethasone into adult mouse eyes were used to investigate the effects on endogenous RSCs.

RESULTS

We discovered that high-affinity synthetic glucocorticoid agonists increase RSC self-renewal and increase retinal progenitor proliferation up to 6-fold without influencing their differentiation in vitro. Intravitreal injection of synthetic glucocorticoid agonist dexamethasone induced in vivo proliferation in the ciliary epithelium-the niche in which adult RSCs reside.

CONCLUSIONS

Together, our results identify glucocorticoids as novel regulators of retinal stem and progenitor cell proliferation in culture and provide evidence that GCs may activate endogenous RSCs.

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.

Delayed retinal vein recovery responses indicate both non-adaptation to stress as well as increased risk for stroke: the SABPA study

OBJECTIVES

Low or high sympatho-adrenal-medullary axis (SAM) and hypothalamic-pituitary-adrenal axis (HPA) dysregulation reflect chronic stress. Retinal vessel dynamics may relate to SAM, HPA activity and stroke risk. Our objectives were therefore to assess the relationships between retinal vessel, SAM and HPA responses, and to determine stroke risk.

METHODS

A prospective bi-ethnic gender cohort (n = 275, 45 ± 9 years) was included. Urine/serum/saliva samples for SAM [norepinephrine:creatinine ratio (u-NE)] and HPA [adrenocorticotrophic hormone (ACTH), cortisol] were obtained at baseline, three-year follow up and upon flicker light-induced provocation. Diastolic ocular perfusion pressure was measured as a marker of hypo-perfusion. Retinal arterial narrowing and venous widening calibres were quantified from digital images in the mydriatic eye. A validated stress and stroke risk score was applied.

RESULTS

An interaction term was fitted for venous dilation in u-NE tertiles (p ≤ 0.05) and not in u-NE median/quartiles/quintiles. Independent of race or gender, tertile 1 (low u-NE) had a 112% increase in u-NE, decreases in cortisol, and no changes in ACTH over three years (positive feedback). Tertile 3 (high u-NE) contradictorily had decreases in u-NE and cortisol, and increases in ACTH (negative feedback). In tertile 1, reduced arterial dilation, and faster arterial vasoconstriction and narrowing were related to higher SAM activity and hypo-perfusion (p ≤ 0.05), whereas delayed venous dilation, recovery and widening were related to cortisol hypo-secretion (p ≤ 0.05). In tertile 1, delayed venous recovery responses predicted stress and stroke risk [odds ratio 4.8 (1.2-19.6); p = 0.03]. These associations were not found in u-NE tertiles 2 and 3.

CONCLUSIONS

In response to low norepinephrine, a reflex increase in SAM activity occurred, enhancing arterial vasoconstriction and hypo-perfusion. Concomitant HPA dysregulation attenuated retinal vein vasoactivity and tone, reflecting delayed vein recovery responses and non-adaptation to stress. These constrained vein recovery responses are indicative of increased chronic stress and stroke risk.

Maternal separation induces retinal and peripheral blood mononuclear cell alterations across the lifespan of female rats

Early life stress alters the function and feedback regulation of the hypothalamic-pituitaryadrenal (HPA) axis, and can contribute to neuroinflammation and neurodegeneration by modifying peripheral blood mononuclear cell (PBMC) activity. The retina, as part of the nervous system, is sensitive to immune changes induced by stress. However, the consequences of stress experienced at an early age on retinal development have not yet been elucidated. Here we aimed to evaluate the impact of maternal separation (MatSep) across three stages of the lifespan (adolescent, adult, and aged) on the retina, as well as on progression through the cell cycle and mitochondrial activity in PBMCs from female Wistar rats. Newborn pups were separated from their mother from postnatal day (PND) 2 until PND 14 for 3 h/day. Retinal analysis from the MatSep groups showed architectural alterations such as a diminished thickness of retinal layers, as well as increased expression of proinflammatory markers DJ-1, Iba-1, and CD45 and the gliotic marker GFAP. Additionally, MatSep disrupted the cell cycle and caused long-term increases in mitochondrial activity in PBMCs from adolescent and adult rats. Changes in the cell cycle profile of the PBMCs from aged MatSep rats were undetected. However, these PBMCs exhibited increased sensitivity to H₂O₂-induced oxidative stress in vitro. Therefore, these results suggest that early life stress can have long-term effects on retinal structure and function, possibly elicited by neonatal immune preconditioning.

Inflammation and matrix metalloproteinase 9 (Mmp-9) regulate photoreceptor regeneration in adult zebrafish

Brain injury activates complex inflammatory signals in dying neurons, surviving neurons, and glia. Here, we establish that inflammation regulates the regeneration of photoreceptors in the zebrafish retina and determine the cellular expression and function of the inflammatory protease, matrix metalloproteinase 9 (Mmp-9), during this regenerative neurogenesis. Following photoreceptor ablation, anti-inflammatory treatment suppresses the number of injury-induced progenitors and regenerated photoreceptors. Upon photoreceptor injury, mmp-9 is induced in Müller glia and Müller glia-derived photoreceptor progenitors. Deleting mmp-9 results in over production of injury-induced progenitors and regenerated photoreceptors, but over time the absence of Mmp-9 compromises the survival of the regenerated cones. At all time-points studied, the levels of tnf-α are significantly elevated in mutant retinas. Anti-inflammatory treatment in mutants rescues the defects in cone survival. These data provide a link between injury-induced inflammation in the vertebrate CNS, Mmp-9 function during neuronal regeneration and the requirement of Mmp-9 for the survival of regenerated cones.

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.

Circadian Regulation of IOP Rhythm by Dual Pathways of Glucocorticoids and the Sympathetic Nervous System

Purpose

Elevated IOP can cause the development of glaucoma. The circadian rhythm of IOP depends on the dynamics of the aqueous humor and is synchronized with the circadian rhythm pacemaker, that is, the suprachiasmatic nucleus. The suprachiasmatic nucleus resets peripheral clocks via sympathetic nerves or adrenal glucocorticoids. However, the detailed mechanisms underlying IOP rhythmicity remain unclear. The purpose of this study was to verify this regulatory pathway.

Methods

Adrenalectomy and/or superior cervical ganglionectomy were performed in C57BL/6J mice. Their IOP rhythms were measured under light/dark cycle and constant dark conditions. Ocular administration of corticosterone or norepinephrine was also performed. Localization of adrenergic receptors, glucocorticoid receptors, and clock proteins Bmal1 and Per1 were analyzed using immunohistochemistry. Period2::luciferase rhythms in the cultured iris/ciliary bodies of adrenalectomized and/or superior cervical ganglionectomized mice were monitored to evaluate the effect of the procedures on the local clock. The IOP rhythm of retina and ciliary epithelium-specific Bmal1 knockout mice were measured to determine the significance of the local clock.

Results

Adrenalectomy and superior cervical ganglionectomy disrupted IOP rhythms and the circadian clock in the iris/ciliary body cultures. Instillation of corticosterone and norepinephrine restored the IOP rhythm. β2-Adrenergic receptors, glucocorticoid receptors, and clock proteins were strongly expressed within the nonpigmented epithelia of the ciliary body. However, tissue-specific Bmal1 knock-out mice maintained their IOP rhythm.

Conclusions

These findings suggest direct driving of the IOP rhythm by the suprachiasmatic nucleus, via the dual corticosterone and norepinephrine pathway, but not the ciliary clock, which may be useful for chronotherapy of glaucoma.

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.

The role of Müller cell glucocorticoid signaling in diabetic retinopathy

Diabetic retinopathy (DR) is a sight-threatening complication associated with the highly prevalent diabetes disorder. Both the microvascular damage and neurodegeneration detected in the retina caused by chronic hyperglycemia have brought special attention to Müller cells, the major macroglia of the retina that are responsible for retinal homeostasis. Given the role of glucocorticoid signaling in anti-inflammatory responses and the almost exclusive expression of glucocorticoid receptors (GRs) in retinal Müller cells, administration of corticosteroid agonists as a potential treatment option has been widely studied. Although these approaches have been moderately efficacious in treating or de-escalating DR pathomechanisms, there are various side effects and gaps of knowledge with regard to introducing exogenous glucocorticoids to the diseased retina. In this paper, we provide a review of the literature concerning the available evidence for the role of Müller cell glucocorticoid signaling in DR and we discuss previously investigated approaches in modulating this system as possible treatment options. Furthermore, we propose a novel alternative to the available choices of treatment by using gene therapy as a tool to regulate the expression of GR in retinal Müller cells. Upregulating GR expression allows for induced glucocorticoid signaling with more enduring effects compared to injection of agonists. Hence, repetitive injections would no longer be required. Lastly, side effects of glucocorticoid therapy such as glucocorticoid resistance of GR following chronic exposure to excess ligands or agonists can be avoided.

Review: The bile acids urso- and tauroursodeoxycholic acid as neuroprotective therapies in retinal disease

Bile acids are produced in the liver and excreted into the intestine, where their main function is to participate in lipid digestion. Ursodeoxycholic acid (UDCA) and tauroursodeoxycholic acid (TUDCA) have shown antiapoptotic, anti-inflammatory, and antioxidant effects in various models of neurodegenerative diseases. However, little is known about signaling pathways and molecular mechanisms through which these bile acids act as neuroprotectors, delaying translation to the clinical setting. We review evidence supporting a potentially therapeutic role for bile acids in retinal disorders, and the mechanisms and pathways involved in the cytoprotective effects of bile acids from the liver and the enterohepatic circulation to the central nervous system and the retina. As secondary bile acids are generated by the microbiota metabolism, bile acids might be a link between neurodegenerative retinal diseases and microbiota.

Avian Adeno-Associated Viral Transduction of the Postembryonic Chicken Retina

Purpose

The posthatching chicken is a valuable animal model for research, but molecular tools needed for altering its gene expression are not yet available. Our purpose here was to adapt the adeno-associated viral (AAV) vector method, used widely in mammalian studies, for use in investigations of the chicken retina. We hypothesized that the recently characterized avian AAV (A3V) vector could effectively transduce chick retinal cells for manipulation of gene expression, after intravitreal or subretinal injection.

Methods

A3V encoding enhanced green fluorescent protein (EGFP) was injected intravitreally or subretinally into P1-3 chick eye and left for 7 to 10 days. Retinas were then sectioned or flat-mounted and visualized via laser-scanning confocal microscopy for analysis of expression and imaging of retinal cells.

Results

Intravitreal A3V-EGFP injection resulted in EGFP expression in a small percent of retinal cells, primarily those with processes and/or cell bodies near the vitreal surface. In contrast, subretinal injection of A3V-EGFP within confined retinal “blebs” produced high rates of transduction of rods and all types of cones. Some examples of all other major retinal cell types, including horizontal, amacrine, bipolar, ganglion, and Müller cells, were also transduced, although with much lower frequency than photoreceptors.

Conclusions

A3V is a promising tool for investigating chick retinal cells and circuitry in situ. This novel vector can be used for studies in which local photoreceptor transduction is sufficient for meaningful observations.

Translational Relevance

With this vector, the postembryonic chick retina can now be used for preclinical trials of gene therapy for prevention and treatment of human retinal disease.

Oral Selumetinib Does Not Negatively Impact Photoreceptor Survival in Murine Experimental Retinal Detachment

Purpose

Mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) signaling is neuroprotective in some retinal damage models but its role in neuronal survival during retinal detachment (RD) is unclear. In addition, serous RDs are a prevalent side effect of MEK inhibitors (MEKi), blocking MAPK/ERK signaling for treatment of certain cancers. We tested the hypothesis that MEKi treatment in experimental RD would increase photoreceptor death.

Methods

The MEKi selumetinib was delivered daily to C57BL/6 mice at a clinically relevant dose (10 mg/mL) starting 1 day prior to creating RD with subretinal hyaluronic acid injection. Photoreceptor TUNEL and outer nuclear layer (ONL) thickness were analyzed. Phospho-ERK1/2 (pERK) distribution, glial fibrillary acidic protein (GFAP) accumulation, and Iba-1 (microglia/macrophages) were evaluated with immunofluorescence.

Results

pERK accumulated in the Müller glia in detached retinas, but this was effectively blocked by selumetinib. Selumetinib did not induce serous RDs at day 1 and did not increase TUNEL positive photoreceptors or further decrease ONL thickness compared to controls. Retinal gliosis was not altered, but selumetinib did block the increase in intraretinal microglia/macrophage Iba-1 fluorescence intensity and acquisition of amoeboid morphology.

Conclusions

MAPK/ERK is neuroprotective in some retinal damage models; in RD, selumetinib blocked Müller pERK accumulation and changed the retinal microglia/macrophage phenotype but did not alter photoreceptor survival. This is consistent with the relatively good visual acuity seen in patients developing transient retinal detachments on MEK inhibitor therapy. Compensation by other neuroprotective pathways in the retina during retinal detachment may occur in the presence of MEK inhibition.

Glucocorticoids Target Ependymal Glia and Inhibit Repair of the Injured Spinal Cord

Following injury, the mammalian spinal cord forms a glial scar and fails to regenerate. In contrast, vertebrate fish spinal cord tissue regenerates significantly to restore function. Cord transection in zebrafish (Danio rerio) initially causes paralysis and neural cell death. Subsequently, ependymal glia proliferate, bipolar glia extend across the lesion, and new neurons are born; axons from spared and nascent neurons extend along trans-lesional glial bridges to restore functional connectivity. Here we report that glucocorticoids, used in the clinical management of spinal cord injury, directly inhibit neural repair by targeting ependymal glia independently of hematogenous cells and microglia. After transecting injury, the glucocorticoid receptor in ependymal glia is regulated differentially in zebrafish (becoming inactive) vs. the rat (becoming active). Glucocorticoid blockade of neural regeneration via a direct effect on ependymal glia has important therapeutic implications for the putative benefit of corticosteroids in early management of spinal cord injury.

Dexamethasone protects retinal ganglion cells but not Müller glia against hyperglycemia in vitro

Diabetic retinopathy (DR) is a common complication of diabetes, for which hyperglycemia is a major etiological factor. It is known that retinal glia (Müller cells) and retinal ganglion cells (RGCs) are affected by diabetes, and there is evidence that DR is associated with neural degeneration. Dexamethasone is a glucocorticoid used to treat many inflammatory and autoimmune conditions, including several eye diseases like DR. Thus, our goal was to study the effect of dexamethasone on the survival of RGCs and Müller glial cells isolated from rat retinas and maintained in vitro under hyperglycemic conditions. The behavior of primary RGC cell cultures, and of mixed RGC and Müller cell co-cultures, was studied in hyperglycemic conditions (30 mM glucose), both in the presence and absence of Dexamethasone (1 μM). RGC and Müller cell survival was evaluated, and the conditioned media of these cultures was collected to quantify the inflammatory cytokines secreted by these cells using a multiplex assay. The role of IL-1β, IL-6 and TNFα in RGC death was also evaluated by adding these cytokines to the co-cultures. RGC survival decreased significantly when these cells were grown in high glucose conditions, reaching 54% survival when they were grown alone and only 33% when co-cultured with Müller glia. The analysis of the cytokines in the conditioned media revealed an increase in IL-1β, IL-6 and TNFα under hyperglycemic conditions, which reverted to the basal concentration in co-cultures maintained in the presence of dexamethasone. Finally, when these cytokines were added to co-cultures they appeared to have a direct effect on RGC survival. Hence, these cytokines could be implicated in the death of RGCs when glucose concentrations increase and dexamethasone might protect RGCs from the cell death induced in these conditions.

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.

Glucocorticoid receptor signaling in the eye

Glucocorticoids (GCs) are essential steroid hormones that regulate numerous metabolic and homeostatic functions in almost all physiological systems. Synthetic glucocorticoids are among the most commonly prescribed drugs for the treatment of various conditions including autoimmune, allergic and inflammatory diseases. Glucocorticoids are mainly used for their potent anti-inflammatory and immunosuppressive activities mediated through signal transduction by their nuclear receptor, the glucocorticoid receptor (GR). Emerging evidence showing that diverse physiological and therapeutic actions of glucocorticoids are tissue-, cell-, and sex-specific, suggests more complex actions of glucocorticoids than previously anticipated. While several synthetic glucocorticoids are widely used in the ophthalmology clinic for the treatment of several ocular diseases, little is yet known about the mechanism of glucocorticoid signaling in different layers of the eye. GR has been shown to be expressed in different cell types of the eye such as cornea, lens, and retina, suggesting an important role of GR signaling in the physiology of these ocular tissues. In this review, we provide an update on the recent findings from in vitro and in vivo studies reported in the last 5 years that aim at understanding the role of GR signaling specifically in the eye. Advances in studying the physiological effects of glucocorticoids in the eye are vital for the elaboration of optimized and targeted GC therapies with potent anti-inflammatory potential while minimizing adverse effects.

Activating the regenerative potential of Müller glia cells in a regeneration-deficient retina

Regeneration responses in animals are widespread across phyla. To identify molecular players that confer regenerative capacities to non-regenerative species is of key relevance for basic research and translational approaches. Here, we report a differential response in retinal regeneration between medaka (Oryzias latipes) and zebrafish (Danio rerio). In contrast to zebrafish, medaka Müller glia (olMG) cells behave like progenitors and exhibit a restricted capacity to regenerate the retina. After injury, olMG cells proliferate but fail to self-renew and ultimately only restore photoreceptors. In our injury paradigm, we observed that in contrast to zebrafish, proliferating olMG cells do not maintain sox2 expression. Sustained sox2 expression in olMG cells confers regenerative responses similar to those of zebrafish MG (drMG) cells. We show that a single, cell-autonomous factor reprograms olMG cells and establishes a regeneration-like mode. Our results position medaka as an attractive model to delineate key regeneration factors with translational potential.

All animals have at least some ability to repair their bodies after injury. But certain species can regenerate entire body parts and even internal organs. Salamanders, for example, can regrow their tail and limbs, as well as their eyes and heart. Many species of fish can also regenerate organs and tissues. In comparison, mammals have only limited regenerative capacity. Why does regeneration vary between species, and is it possible to convert a non-regenerating system into a regenerating one?

Laboratory studies of regeneration often use the model organism, zebrafish. Zebrafish can restore their sight after an eye injury by regenerating the retina, the light-sensitive tissue at the back of the eye. They are able to do this thanks to cells in the retina called Müller glial cells. These behave like stem cells. They divide to produce identical copies of themselves, which then transform into all of the different cell types necessary to produce a new retina.

Lust and Wittbrodt now show that a distant relative of the zebrafish, the Japanese ricefish ‘medaka’, lacks these regenerative skills. Although Müller glial cells in medaka also divide after injury, they give rise to only a single type of retinal cell. This means that these fish cannot regenerate an entire retina. Lust and Wittbrodt demonstrate that in medaka, but not zebrafish, levels of a protein called Sox2 fall after eye injury. As Sox2 has been shown to be important for regeneration in zebrafish Müller glial cells, the loss of Sox2 may be preventing regeneration in medaka. Consistent with this, restoring Sox2 levels in medaka Müller glial cells enabled them to turn into several different types of retinal cell.

Sox2 is also present in the Müller glial cells of other species with backbones, including chickens, mice, and humans. Future experiments should test whether loss of Sox2 after injury contributes to the lack of regeneration in these species. If it does, the next question will be whether restoring Sox2 can drive a regenerative response.

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.

The chick eye in vision research: An excellent model for the study of ocular disease

The domestic chicken, Gallus gallus, serves as an excellent model for the study of a wide range of ocular diseases and conditions. The purpose of this manuscript is to outline some anatomic, physiologic, and genetic features of this organism as a robust animal model for vision research, particularly for modeling human retinal disease. Advantages include a sequenced genome, a large eye, relative ease of handling and maintenance, and ready availability. Relevant similarities and differences to humans are highlighted for ocular structures as well as for general physiologic processes. Current research applications for various ocular diseases and conditions, including ocular imaging with spectral domain optical coherence tomography, are discussed. Several genetic and non-genetic ocular disease models are outlined, including for pathologic myopia, keratoconus, glaucoma, retinal detachment, retinal degeneration, ocular albinism, and ocular tumors. Finally, the use of stem cell technology to study the repair of damaged tissues in the chick eye is discussed. Overall, the chick model provides opportunities for high-throughput translational studies to more effectively prevent or treat blinding ocular diseases.

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

Müller glia-derived progenitor cells (MGPCs) have the capability to regenerate neurons in the retinas of different vertebrate orders. The formation of MGPCs is regulated by a network of cell-signaling pathways. The purpose of this study was to investigate how BMP/Smad1/5/8- and TGFβ/Smad2/3-signaling are coordinated to influence the formation of MGPCs in the chick model system. We find that pSmad1/5/8 is selectively up-regulated in the nuclei of Müller glia following treatment with BMP4, FGF2, or NMDA-induced damage, and this up-regulation is blocked by a dorsomorphin analogue DMH1. By comparison, Smad2/3 is found in the nuclei of Müller glia in untreated retinas, and becomes localized to the cytoplasm following NMDA- or FGF2-treatment. These findings suggest a decrease in TGFβ- and increase in BMP-signaling when MGPCs are known to form. In both NMDA-damaged and FGF2-treated retinas, inhibition of BMP-signaling suppressed the proliferation of MGPCs, whereas inhibition of TGFβ-signaling stimulated the proliferation of MGPCs. Consistent with these findings, TGFβ2 suppressed the formation of MGPCs in NMDA-damaged retinas. Our findings indicate that BMP/TGFβ/Smad-signaling is recruited into the network of signaling pathways that controls the formation of proliferating MGPCs. We conclude that signaling through BMP4/Smad1/5/8 promotes the formation of MGPCs, whereas signaling through TGFβ/Smad2/3 suppresses the formation of MGPCs.

Upregulation of matrix metalloproteinase triggers transdifferentiation of retinal pigmented epithelial cells in Xenopus laevis: A Link between inflammatory response and regeneration

In adult Xenopus eyes, when the whole retina is removed, retinal pigmented epithelial (RPE) cells become activated to be retinal stem cells and regenerate the whole retina. In the present study, using a tissue culture model, it was examined whether upregulation of matrix metalloproteinases (Mmps) triggers retinal regeneration. Soon after retinal removal, Xmmp9 and Xmmp18 were strongly upregulated in the tissues of the RPE and the choroid. In the culture, Mmp expression in the RPE cells corresponded with their migration from the choroid. A potent MMP inhibitor, 1,10-PNTL, suppressed RPE cell migration, proliferation, and formation of an epithelial structure in vitro. The mechanism involved in upregulation of Mmps was further investigated. After retinal removal, inflammatory cytokine genes, IL-1β and TNF-α, were upregulated both in vivo and in vitro. When the inflammation inhibitors dexamethasone or Withaferin A were applied in vitro, RPE cell migration was severely affected, suppressing transdifferentiation. These results demonstrate that Mmps play a pivotal role in retinal regeneration, and suggest that inflammatory cytokines trigger Mmp upregulation, indicating a direct link between the inflammatory reaction and retinal regeneration. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 77: 1086-1100, 2017.

Immunomodulation-accelerated neuronal regeneration following selective rod photoreceptor cell ablation in the zebrafish retina

Müller glia (MG) function as inducible retinal stem cells in zebrafish, completely repairing the eye after damage. The innate immune system has recently been shown to promote tissue regeneration in which classic wound-healing responses predominate. However, regulatory roles for leukocytes during cellular regeneration-i.e., selective cell-loss paradigms akin to degenerative disease-are less well defined. To investigate possible roles innate immune cells play during retinal cell regeneration, we used intravital microscopy to visualize neutrophil, macrophage, and retinal microglia responses to induced rod photoreceptor apoptosis. Neutrophils displayed no reactivity to rod cell loss. Peripheral macrophage cells responded to rod cell loss, as evidenced by morphological transitions and increased migration, but did not enter the retina. Retinal microglia displayed multiple hallmarks of immune cell activation: increased migration, translocation to the photoreceptor cell layer, proliferation, and phagocytosis of dying cells. To test function during rod cell regeneration, we coablated microglia and rod cells or applied immune suppression and quantified the kinetics of (i) rod cell clearance, (ii) MG/progenitor cell proliferation, and (iii) rod cell replacement. Coablation and immune suppressants applied before cell loss caused delays in MG/progenitor proliferation rates and slowed the rate of rod cell replacement. Conversely, immune suppressants applied after cell loss had been initiated led to accelerated photoreceptor regeneration kinetics, possibly by promoting rapid resolution of an acute immune response. Our findings suggest that microglia control MG responsiveness to photoreceptor loss and support the development of immune-targeted therapeutic strategies for reversing cell loss associated with degenerative retinal conditions.

Jak/Stat signaling regulates the proliferation and neurogenic potential of Müller glia-derived progenitor cells in the avian retina

Müller glia are capable of de-differentiating and proliferating to become Müller glia-derived progenitor cells (MGPCs) with the ability to regenerate retinal neurons. One of the cell-signaling pathways that drives the reprogramming of Müller glia into MGPCs in the zebrafish retina is the Jak/Stat-pathway. However, nothing is known about the influence of Jak/Stat-signaling during the formation of MGPCs in the retinas of warm-blooded vertebrates. Accordingly, we examined whether Jak/Stat-signaling influences the formation of MGPCs and differentiation of progeny in the avian retina. We found that Jak/Stat-signaling is activated in Müller glia in response to NMDA-induced retinal damage or by CNTF or FGF2 in the absence of retinal damage. Inhibition of gp130, Jak2, or Stat3 suppressed the formation of proliferating MGPCs in NMDA-damaged and FGF2-treated retinas. Additionally, CNTF combined with FGF2 enhanced the formation of proliferating MGPCs in the absence of retinal damage. In contrast to the zebrafish model, where activation of gp130/Jak/Stat is sufficient to drive neural regeneration from MGPCs, signaling through gp130 inhibits the neurogenic potential of MGPCs and promotes glial differentiation. We conclude that gp130/Jak/Stat-signaling plays an important role in the network of pathways that drives the formation of proliferating MGPCs; however, this pathway inhibits the neural differentiation of the progeny.

Corticosteroids and the retina: a role for the mineralocorticoid receptor

PURPOSE OF REVIEW

Long-lasting devices releasing steroids have been approved recently for macular edema of various origins. Identification of the retina as a novel mineralo-sensitive tissue also raises new therapeutic options.

RECENT FINDINGS

Recently, the over activation of the mineralocorticoid receptor (MR) pathway has been shown to cause fluid accumulation in the retina, choroidal vasodilation, and to promote retinal neovascularization in hypoxic conditions. These findings indicate that MR antagonists could have beneficial effects in the treatment of retinal diseases. Central serous chorioretinopathy is a retinal disease associated with choroidal vasodilation and subretinal fluid that affects mostly men with type A personality and occurrence has been associated with steroid intake. In several independent studies, MR antagonists have shown beneficial effects, significantly reducing subretinal fluid in eyes of chronic central serous chorioretinopathy patients.

SUMMARY

The role of MR in retinal disorder is emerging and the potential association with psychological traits is considered. The place of MR antagonists for retinal diseases treatment is discussed.

Retinal regeneration in birds and mice

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

mTor signaling is required for the formation of proliferating Müller glia-derived progenitor cells in the chick retina

We investigate the roles of mTor signaling in the formation of Müller glia-derived progenitor cells (MGPCs) in the chick retina. During embryonic development, pS6 (a readout of active mTor signaling) is present in early-stage retinal progenitors, differentiating amacrine and ganglion cells, and late-stage progenitors or maturing Müller glia. By contrast, pS6 is present at low levels in a few scattered cell types in mature, healthy retina. Following retinal damage, in which MGPCs are known to form, mTor signaling is rapidly activated in Müller glia. Inhibition of mTor in damaged retinas prevented the accumulation of pS6 in Müller glia and reduced numbers of proliferating MGPCs. Inhibition of mTor had no effect on MAPK signaling or on upregulation of the stem cell factor Klf4, whereas Pax6 upregulation was significantly reduced. Inhibition of mTor potently blocked the MGPC-promoting effects of Hedgehog, Wnt and glucocorticoid signaling in damaged retinas. In the absence of retinal damage, insulin, IGF1 and FGF2 induced pS6 in Müller glia, and this was blocked by mTor inhibitor. In FGF2-treated retinas, in which MGPCs are known to form, inhibition of mTor blocked the accumulation of pS6, the upregulation of Pax6 and the formation of proliferating MGPCs. We conclude that mTor signaling is required, but not sufficient, to stimulate Müller glia to give rise to proliferating progenitors, and the network of signaling pathways that drive the formation of MGPCs requires activation of mTor.