alpha-Aminoadipate induces progenitor cell properties of Müller glia in adult mice

Advances in the study of Müller glia reprogramming in mammals

Müller cells play an integral role in the development, maintenance, and photopic signal transmission of the retina. While lower vertebrate Müller cells can differentiate into various types of retinal neurons to support retinal repair following damage, there is limited neurogenic potential of mammalian Müller cells. Therefore, it is of great interest to harness the neurogenic potential of mammalian Müller cells to achieve self-repair of the retina. While multiple studies have endeavored to induce neuronal differentiation and proliferation of mammalian Müller cells under defined conditions, the efficiency and feasibility of these methods often fall short, rendering them inadequate for the requisites of retinal repair. As the mechanisms and methodologies of Müller cell reprogramming have been extensively explored, a summary of the reprogramming process of unlocking the neurogenic potential of Müller cells can provide insight into Müller cell fate development and facilitate their therapeutic use in retinal repair. In this review, we comprehensively summarize the progress in reprogramming mammalian Müller cells and discuss strategies for optimizing methods and enhancing efficiency based on the mechanisms of fate regulation.

Establishing Functional Retina in a Dish: Progress and Promises of Induced Pluripotent Stem Cell-Based Retinal Neuron Differentiation

The human eye plays a critical role in vision perception, but various retinal degenerative diseases such as retinitis pigmentosa (RP), glaucoma, and age-related macular degeneration (AMD) can lead to vision loss or blindness. Although progress has been made in understanding retinal development and in clinical research, current treatments remain inadequate for curing or reversing these degenerative conditions. Animal models have limited relevance to humans, and obtaining human eye tissue samples is challenging due to ethical and legal considerations. Consequently, researchers have turned to stem cell-based approaches, specifically induced pluripotent stem cells (iPSCs), to generate distinct retinal cell populations and develop cell replacement therapies. iPSCs offer a novel platform for studying the key stages of human retinogenesis and disease-specific mechanisms. Stem cell technology has facilitated the production of diverse retinal cell types, including retinal ganglion cells (RGCs) and photoreceptors, and the development of retinal organoids has emerged as a valuable in vitro tool for investigating retinal neuron differentiation and modeling retinal diseases. This review focuses on the protocols, culture conditions, and techniques employed in differentiating retinal neurons from iPSCs. Furthermore, it emphasizes the significance of molecular and functional validation of the differentiated cells.

Nestin contributes to laser choroidal and retinal neovascularization

PURPOSE

Choroidal and retinal neovascularization plays an essential role in various ocular diseases. In this study, we examined the role of nestin in this process. Nestin is an intermediate filament protein known to play several roles, including as a marker of neural progenitor and proliferating endothelial cells.

METHODS

We used Brown Norway rats, in which choroidal and retinal neovascularization was induced using intraocular laser impacts. The role of nestin was examined using angiography, western blot from the second to the 14th day after laser impacts, and intraocular injection of nestin siRNA. The localization of the protein was specified by co-immunoreactivity with glial fibrillary protein (GFAP), glutamine synthetase (GS), and von Willebrand factor (vWF).

RESULTS

In the control retina, nestin was found principally in glial structures in the ganglion cell layer, as confirmed by nestin/GFAP immunolabeling. Two days after the laser impacts, the nestin expression extended to numerous radial processes at the site of the impacts. With Bruch's membrane ruptured, these processes penetrated into the choroid. Nestin immunolabeling remained high from the third to the seventh day but appeared reduced on the 14th day. The nature of these processes was not clearly defined, but co-immunolabeling with GFAP suggested that they were principally in activated Müller cells from the third day after the laser impacts. However, the co-immunoreactivity of nestin and GS, a marker of mature functional Müller cells, could be observable only from the seventh day. Nestin was also observed in some vascular cells, as demonstrated by the co-immunoreactivity of the protein with vWF in the choroid and retina. As observed on angiography, the numbers of choroidal and retinal blood vessels were significantly increased (principally on the seventh day) after the laser impacts. An intraocular injection of nestin siRNAs led to a significant decrease in the number of blood vessels.

CONCLUSIONS

Our results confirmed the presence of nestin in glial (e.g., astrocytes), reactive Müller, and endothelial cells. They demonstrated their critical involvement in a rat model of retinal and choroidal neovascularization experimentally induced using ocular laser impacts.

Exploration of biomimetic poly(γ-benzyl-L-glutamate) fibrous scaffolds for corneal nerve regeneration

Poly(γ-benzyl-L-glutamate) (PBG) made biomimetic scaffolds are explored as candidate materials for corneal nerve regeneration and neurotrophic keratopathy treatment. The PBG with built-in neurotransmitter glutamate was synthesized and fabricated into 3D fibrous scaffolds containing aligned fibers using electrospinning. In in vitro experiments, primary mouse trigeminal ganglia (TG) cells were used. Immunohistochemistry (IHC) analysis shows that TG cells cultured on PBG have no cytotoxic response for 21 days. Without any nerve growth factor, TG cells have the longest neurite length of 225.3 μm in the PBG group and 1.3 times the average length as compared with the polycaprolactone and no scaffold groups. Also, aligned fibers guide the neurite growth and extension unidirectionally. In vivo assays were carried out by intracorneal implantation of PBG on clinical New Zealand rabbits. The external eye photos and in vivo confocal microscopy (IVCM) show a low immune response. The corneal neural markers (βIII tubulin and SMI312) in the IHC analysis are consistent with the position stained by glutamate of implanted scaffolds, indicating that PBG induces neurogenesis. PBG exhibits mechanical stiffness to resist material deformation possibly caused by surgical operations. The results of this study demonstrate that PBG is suitable for corneal nerve regeneration and the treatment of neurotrophic keratopathy.

Advances in cell therapies using stem cells/progenitors as a novel approach for neurovascular repair of the diabetic retina

BACKGROUND

Diabetic retinopathy, a major complication of diabetes mellitus, is a leading cause of sigh-loss in working age adults. Progressive loss of integrity of the retinal neurovascular unit is a central element in the disease pathogenesis. Retinal ischemia and inflammatory processes drive interrelated pathologies such as blood retinal barrier disruption, fluid accumulation, gliosis, neuronal loss and/or aberrant neovascularisation. Current treatment options are somewhat limited to late-stages of the disease where there is already significant damage to the retinal architecture arising from degenerative, edematous and proliferative pathology. New preventive and interventional treatments to target early vasodegenerative and neurodegenerative stages of the disease are needed to ensure avoidance of sight-loss.

MAIN BODY

Historically, diabetic retinopathy has been considered a primarily microvascular disease of the retina and clinically it is classified based on the presence and severity of vascular lesions. It is now known that neurodegeneration plays a significant role during the pathogenesis. Loss of neurons has been documented at early stages in pre-clinical models as well as in individuals with diabetes and, in some, even prior to the onset of clinically overt diabetic retinopathy. Recent studies suggest that some patients have a primarily neurodegenerative phenotype. Retinal pigment epithelial cells and the choroid are also affected during the disease pathogenesis and these tissues may also need to be addressed by new regenerative treatments. Most stem cell research for diabetic retinopathy to date has focused on addressing vasculopathy. Pre-clinical and clinical studies aiming to restore damaged vasculature using vasoactive progenitors including mesenchymal stromal/stem cells, adipose stem cells, CD34+ cells, endothelial colony forming cells and induced pluripotent stem cell derived endothelial cells are discussed in this review. Stem cells that could replace dying neurons such as retinal progenitor cells, pluripotent stem cell derived photoreceptors and ganglion cells as well as Müller stem cells are also discussed. Finally, challenges of stem cell therapies relevant to diabetic retinopathy are considered.

CONCLUSION

Stem cell therapies hold great potential to replace dying cells during early and even late stages of diabetic retinopathy. However, due to the presence of different phenotypes, selecting the most suitable stem cell product for individual patients will be crucial for successful treatment.

Primary and Secondary Cone Cell Death Mechanisms in Inherited Retinal Diseases and Potential Treatment Options

Inherited retinal diseases (IRDs) are a leading cause of blindness. To date, 260 disease-causing genes have been identified, but there is currently a lack of available and effective treatment options. Cone photoreceptors are responsible for daylight vision but are highly susceptible to disease progression, the loss of cone-mediated vision having the highest impact on the quality of life of IRD patients. Cone degeneration can occur either directly via mutations in cone-specific genes (primary cone death), or indirectly via the primary degeneration of rods followed by subsequent degeneration of cones (secondary cone death). How cones degenerate as a result of pathological mutations remains unclear, hindering the development of effective therapies for IRDs. This review aims to highlight similarities and differences between primary and secondary cone cell death in inherited retinal diseases in order to better define cone death mechanisms and further identify potential treatment options.

The role of the metabolite cargo of extracellular vesicles in tumor progression

Metabolomic reprogramming in tumor and stroma cells is a hallmark of cancer but understanding its effects on the metabolite composition and function of tumor-derived extracellular vesicles (EVs) is still in its infancy. EVs are membrane-bound sacs with a complex molecular composition secreted by all living cells. They are key mediators of intercellular communication both in normal and pathological conditions and play a crucial role in tumor development. Although lipids are major components of EVs, most of the EV cargo studies have targeted proteins and nucleic acids. The potential of the EV metabolome as a source for biomarker discovery has gained recognition recently, but knowledge on the biological activity of tumor EV metabolites still remains limited. Therefore, we aimed (i) to compile the list of metabolites identified in tumor EVs isolated from either clinical specimens or in vitro samples and (ii) describe their role in tumor progression through literature search and pathway analysis.

Insights on the Regeneration Potential of Müller Glia in the Mammalian Retina

Müller glia, the major glial cell types in the retina, maintain retinal homeostasis and provide structural support to retinal photoreceptors. They also possess regenerative potential that might be used for retinal repair in response to injury or disease. In teleost fish (such as zebrafish), the Müller glia response to injury involves reprogramming events that result in a population of proliferative neural progenitors that can regenerate the injured retina. Recent studies have revealed several important mechanisms for the regenerative capacity of Müller glia in fish, which may shed more light on the mechanisms of Müller glia reprogramming and regeneration in mammals. Mammalian Müller glia can adopt stem cell characteristics, and in response to special conditions, be persuaded to proliferate and regenerate, although their native regeneration potential is limited. In this review, we consider the work to date revealing the regenerative potential of the mammalian Müller glia and discuss whether they are a potential source for cell regeneration therapy in humans.

Absence of ephrin-A2/A3 increases retinal regenerative potential for Müller cells in Rhodopsin knockout mice

Müller cells (MC) are considered dormant retinal progenitor cells in mammals. Previous studies demonstrated ephrin-As act as negative regulators of neural progenitor cells in the retina and brain. It remains unclear whether the lack of ephrin-A2/A3 is sufficient to promote the neurogenic potential of MC. Here we investigated whether the MC is the primary retinal cell type expressing ephrin-A2/A3 and their role on the neurogenic potential of Müller cells. In this study, we showed that ephrin-A2/A3 and their receptor EphA4 were expressed in retina and especially enriched in MC. The level of ephrinAs/EphA4 expression increased as the retina matured that is correlated with the reduced proliferative and progenitor cell potential of MC. Next, we investigated the proliferation in primary MC cultures isolated from wild-type and A2-/- A3-/- mice by 5-ethynyl-2'-deoxyuridine (EdU) incorporation. We detected a significant increase of EdU+ cells in MC derived from A2-/- A3-/- mice. Next, we investigated the role of ephrin-A2/A3 in mice undergoing photoreceptor degeneration such as Rhodopsin knockout (Rho-/-) mice. To further evaluate the role of ephrin-A2/A3 in MC proliferation in vivo, EdU was injected intraperitoneally to adult wild-type, A2-/- A3-/- , Rho-/- and Rho-/- A2-/- A3-/- mice and the numbers of EdU+ cells distributed among different layers of the retina. EphrinAs/EphA4 expression was upregulated in the retina of Rho-/- mice compared to the wild-type mice. In addition, cultured MC derived from ephrin-A2-/- A3-/- mice also expressed higher levels of progenitor cell markers and exhibited higher proliferation potential than those from wild-type mice. Interestingly, we detected a significant increase of EdU+ cells in the retinas of adult ephrin-A2-/- A3-/- mice mainly in the inner nuclear layer; and these EdU+ cells were co-localized with MC marker, cellular retinaldehyde-binding protein, suggesting some proliferating cells are from MC. In Rhodopsin knockout mice (Rho-/- A2-/- A3-/- mice), a significantly greater amount of EdU+ cells were located in the ciliary body, retina and RPE than that of Rho-/- mice. Comparing between 6 and 12 weeks old Rho-/- A2-/- A3-/- mice, we recorded more EdU+ cells in the outer nuclear layer in the 12-week-old mice undergoing severe retinal degeneration. Taken together, Ephrin-A2/A3 are negative regulators of the proliferative and neurogenic potentials of MC. Absence of ephrin-A2/A3 promotes the migration of proliferating cells into the outer nuclear layer and may lead to retinal cell regeneration. All experimental procedures were approved by the Animal Care and Use Committee at Schepens Eye Research Institute, USA (approval No. S-353-0715) on October 24, 2012.

TNFα activates MAPK and Jak-Stat pathways to promote mouse Müller cell proliferation

Mouse Müller cells, considered as dormant retinal progenitors, often respond to retinal injury by undergoing reactive gliosis rather than displaying neural regenerative responses. Tumor necrosis factor alpha (TNFα) is a key cytokines induced after injury and implicated in mediating inflammatory and neural regenerative responses in zebrafish. To investigate the involvement of TNFα in mouse retinal injury, adult C57BL/6J mice were subjected to light damage for 14 consecutive days. TNFα was elevated in the retina of mice exposed to light damage, which induced Müller cell proliferation in vitro. Affymetrix microarray showed that, in Müller cells, TNFα induces up-regulation of inflammatory and proliferation-related genes, including NFKB2, leukemia inhibitory factor, interleukin-6, janus kinase (Jak) 1, Jak2, signal transducer and activator of transcription (Stat) 1, Stat2, mitogen-activated protein kinase (MAPK) 7, and MAP4K4 but down-regulation of neuroprogenitor genes, including Sox9, Ascl1, Wnt2 and Hes1. Blocking the Jak/Stat and MAPK pathways attenuated TNFα-induced Müller cell proliferation. These results suggest that TNFα may drive the proliferation and inflammatory response, rather than the neural regenerative potential, of mouse Müller cells.

The road to restore vision with photoreceptor regeneration

Neuroretinal diseases are the predominant cause of irreversible blindness worldwide, mainly due to photoreceptor loss. Currently, there are no radical treatments to fully reverse the degeneration or even stop the disease progression. Thus, it is urgent to develop new biological therapeutics for these diseases on the clinical side. Stem cell-based treatments have become a promising therapeutic for neuroretinal diseases through the replacement of damaged cells with photoreceptors and some allied cells. To date, considerable efforts have been made to regenerate the diseased retina based on stem cell technology. In this review, we overview the current status of stem cell-based treatments for photoreceptor regeneration, including the major cell sources derived from different stem cells in pre-clinical or clinical trial stages. Additionally, we discuss herein the major challenges ahead for and potential new strategy toward photoreceptor regeneration.

Noninvasive Electrical Stimulation Improves Photoreceptor Survival and Retinal Function in Mice with Inherited Photoreceptor Degeneration

Purpose

Neurons carry electrical signals and communicate via electrical activities. The therapeutic potential of electrical stimulation (ES) for the nervous system, including the retina, through improvement of cell survival and function has been noted. Here we investigated the neuroprotective and regenerative potential of ES in a mouse model of inherited retinal degeneration.

Methods

Rhodopsin-deficient (Rho^−/−) mice received one or two sessions of transpalpebral ES or sham treatments for 7 consecutive days. Intraperitoneal injection of 5-ethynyl-2′-deoxyuridine was used to label proliferating cells. Weekly electroretinograms were performed to monitor retinal function. Retinal morphology, photoreceptor survival, and regeneration were evaluated in vivo using immunohistochemistry and genetic fate-mapping techniques. Müller cell (MC) cultures were employed to further define the optimal conditions of ES application.

Results

Noninvasive transpalpebral ES in Rho^−/− mice improved photoreceptor survival and electroretinography function in vivo. ES also triggered residential retinal progenitor-like cells such as MCs to reenter the cell cycle, possibly producing new photoreceptors, as shown by immunohistochemistry and genetic fate-mapping techniques. ES directly stimulated cell proliferation and the expression of progenitor cell markers in MC cultures, at least partially through bFGF signaling.

Conclusions

Our study showed that transpalpebral ES improved photoreceptor survival and retinal function and induced the proliferation, probably photoreceptor regeneration, of MCs; this occurs via stimulation of the bFGF pathways. These results suggest the exciting possibility of applying noninvasive ES as a versatile tool for preventing photoreceptor loss and mobilizing endogenous progenitors for reversing vision loss in patients with photoreceptor degeneration.

Retina in a dish: Cell cultures, retinal explants and animal models for common diseases of the retina

For many retinal diseases, including age-related macular degeneration (AMD), glaucoma, and diabetic retinopathy (DR), the exact pathogenesis is still unclear. Moreover, the currently available therapeutic options are often unsatisfactory. Research designed to remedy this situation heavily relies on experimental animals. However, animal models often do not faithfully reproduce human disease and, currently, there is strong pressure from society to reduce animal research. Overall, this creates a need for improved disease models to understand pathologies and develop treatment options that, at the same time, require fewer or no experimental animals. Here, we review recent advances in the field of in vitro and ex vivo models for AMD, glaucoma, and DR. We highlight the difficulties associated with studies on complex diseases, in which both the initial trigger and the ensuing pathomechanisms are unclear, and then delineate which model systems are optimal for disease modelling. To this end, we present a variety of model systems, ranging from primary cell cultures, over organotypic cultures and whole eye cultures, to animal models. Specific advantages and disadvantages of such models are discussed, with a special focus on their relevance to putative in vivo disease mechanisms. In many cases, a replacement of in vivo research will mean that several different in vitro models are used in conjunction, for instance to analyze and validate causative molecular pathways. Finally, we argue that the analytical decomposition into appropriate cell and tissue model systems will allow making significant progress in our understanding of complex retinal diseases and may furthermore advance the treatment testing.

Effects of Cord Blood Serum (CBS) on viability of retinal Müller glial cells under in vitro injury

Oxidative stress and inflammation determine retinal ganglion cell degeneration, leading to retinal impairment and vision loss. Müller glial cells regulate retinal repair under injury, through gliosis. Meanwhile, reactive gliosis can turn in pathological effects, contributing to neurodegeneration. In the present study, we tested whether Cord Blood Serum (CBS), rich of growth factors, might improve the viability of Müller cells under in vitro damage. BDNF, NGF, TGF-α, GDNF and EGF levels were measured in CBS samples by Human Magnetic Luminex Assay. CBS effects were evaluated on rat (rMC-1) and human (MIO-M1) Müller cells, under H₂O₂ and IL-1β damage. Cells grown with FBS or CBS both at 5% were exposed to stress and analyzed in terms of cell viability, GFAP, IL-6 and TNF-α expression. CBS was also administrated after treatment with K252a, inhibitor of the neurotrophin receptor Trk. Cell viability of rMC-1 and MIO-M1 resulted significantly improved when pretreated with CBS and exposed to H₂O₂ and IL-1β, in comparison to the standard culture with FBS. Accordingly, the gliosis marker GFAP resulted down-regulated following CBS priming. In parallel, we observed a lower expression of the inflammatory mediators in rMC-1 (TNF-α) and MIO-M1 (IL-6, TNF- α), especially in presence of inflammatory damage. Trk inhibition through K252a administration impaired the effects of CBS under stress conditions on MIO-M1 and rMC-1 viability, not significantly different from FBS condition. CBS is enriched with neurotrophins and its administration to rMC-1 and MIO-M1 attenuates the cytotoxic effects of H₂O₂ and IL-1β. Moreover, the decrease of the main markers of gliosis and inflammation suggests a promising use of CBS for neuroprotection aims. This study is a preliminary basis that prompts future investigations to deeply explore and confirm the CBS potential.

VEGF Upregulates EGFR Expression to Stimulate Chemotactic Behaviors in the rMC-1 Model of Müller Glia

Progressive vision loss in adults has become increasingly prevalent worldwide due to retinopathies associated with aging, genetics, and epigenetic factors that damage the retinal microvasculature. Insufficient supply of oxygen and/or nutrients upregulates factors such as vascular endothelial growth factor (VEGF) and epidermal growth factor (EGF), which can induce abnormal angiogenesis and damage the structural arrangement of the retinal blood barrier (BRB). Müller glia (MG) regulate the diffusion of essential compounds across the BRB and respond to retinal insults via reactive gliosis, which includes cell hypertrophy, migration, and/or proliferation near areas of elevated VEGF concentration. Increasing concentrations of exogenous VEGF, upregulated by retinal pigmented epithelium cells, and endogenous epidermal growth factor receptor (EGF-R) stimulation in MG, implicated in MG proliferative and migratory behavior, often lead to progressive and permanent vision loss. Our project examined the chemotactic responses of the rMC-1 cell line, a mammalian MG model, toward VEGF and EGF signaling fields in transwell assays, and within respective concentration gradient fields produced in the glia line (gLL) microfluidic system previously described by our group. rMC-1 receptor expression in defined ligand fields was also evaluated using quantitative polymerase chain reaction (qPCR) and immunocytochemical staining. Results illustrate dramatic increases in rMC-1 chemotactic responses towards EGF gradient fields after pre-treatment with VEGF. In addition, qPCR illustrated significant upregulation of EGF-R upon VEGF pre-treatment, which was higher than that induced by its cognate ligand, EGF. These results suggest interplay of molecular pathways between VEGF and EGF-R that have remained understudied in MG but are significant to the development of effective anti-VEGF treatments needed for a variety of retinopathies.

Enayati K, Xie J, Zhao E, Turunen T, Sehic A, Lu L, Utheim TP, Chen DF. Electrical Stimulation Induces Retinal Müller Cell Proliferation and Their Progenitor Cell Potential

Non-invasive electrical stimulation (ES) is increasingly applied to improve vision in untreatable eye conditions, such as retinitis pigmentosa and age-related macular degeneration. Our previous study suggested that ES promoted retinal function and the proliferation of progenitor-like glial cells in mice with inherited photoreceptor degeneration; however, the underlying mechanism remains obscure. Müller cells (MCs) are thought to be dormant residential progenitor cells that possess a high potential for retinal neuron repair and functional plasticity. Here, we showed that ES with a ramp waveform of 20 Hz and 300 µA of current was effective at inducing mouse MC proliferation and enhancing their expression of progenitor cell markers, such as Crx (cone-rod homeobox) and Wnt7, as well as their production of trophic factors, including ciliary neurotrophic factor. RNA sequencing revealed that calcium signaling pathway activation was a key event, with a false discovery rate of 5.33 × 10-8 (p = 1.78 × 10-10) in ES-mediated gene profiling changes. Moreover, the calcium channel blocker, nifedipine, abolished the observed effects of ES on MC proliferation and progenitor cell gene induction, supporting a central role of ES-induced Ca2+ signaling in the MC changes. Our results suggest that low-current ES may present a convenient tool for manipulating MC behavior toward neuroregeneration and repair.

IPL Sublamination in Chicken Retinal Spheroids Is Initiated via Müller Cells and Cholinergic Differentiation, and Is Disrupted by NMDA Signaling

Purpose

Reaggregates from E6 embryonic chicken retina exhibit areas corresponding to an inner plexiform layer (IPL), which presents an ideal in vitro model to test conditions and constraints of cholinergic and glutamatergic network formation, providing a basis for retinal tissue engineering. Here, we show that ipl formation is regulated by cholinergic starburst amacrine cells (SACs), a glial scaffold and by L-glutamate.

Methods

Rosetted spheroids were cultured in absence or presence of 0.2 to 0.4 mM L-glutamate and analyzed by immuno- and enzyme histochemistry, proliferation, and apoptosis assays.

Results

After 2 days in vitro (div), ipl formation was announced by acetylcholinesterase+ (AChE) and choline acetyltransferase+ (ChAT) cells. Individual vimentin+ or transitin+ Müller glial cell precursors (MCPs) in ipl centers coexpressed ChAT. Comparable to in vivo, pairwise arranged ChAT+ SACs formed two laminar subbands. Projections of calretinin+ amacrine cells (ACs) into ipl associated with MCP processes. In L-glutamate-, or NMDA-treated spheroids ipls were disrupted, including loss of SACs and MCs; coincubation with NMDA receptor inhibitor MK-801 prevented these effects. Also, many Pax6+ cells, comprising most ACs, were lost, while rho4D2+ rod photoreceptors were increased. Cell proliferation was slightly increased, while apoptosis remained unaffected.

Conclusions

This demonstrated: (1) a far-advanced differentiation of an IPL in retinal spheroids, as never described before; (2) ipl sublamination was initiated by cholinergic precursor cells, which-functioning as "ipl founder cells"-(3) gave rise to neurons and glial cells; (4) these SACs and MCPs together organized ipl formation; and (5) this process was counteracted by NMDA-dependent glutamate actions.

Recapitulating developmental mechanisms for retinal regeneration

Degeneration of specific retinal neurons in diseases like glaucoma, age-related macular degeneration, and retinitis pigmentosa is the leading cause of irreversible blindness. Currently, there is no therapy to modify the disease-associated degenerative changes. With the advancement in our knowledge about the mechanisms that regulate the development of the vertebrate retina, the approach to treat blinding diseases through regenerative medicine appears a near possibility. Recapitulation of developmental mechanisms is critical for reproducibly generating cells in either 2D or 3D culture of pluripotent stem cells for retinal repair and disease modeling. It is the key for unlocking the neurogenic potential of Müller glia in the adult retina for therapeutic regeneration. Here, we examine the current status and potential of the regenerative medicine approach for the retina in the backdrop of developmental mechanisms.

Retinal microglia signaling affects Müller cell behavior in the zebrafish following laser injury induction

Microglia are the resident tissue macrophages of the central nervous system including the retina. Under pathophysiological conditions, microglia can signal to Müller cells, the major glial component of the retina, affecting their morphological, molecular, and functional responses. Microglia-Müller cell interactions appear to be bidirectional shaping the overall injury response in the retina. Hence, microglia and Müller cell responses to disease and injury have been ascribed both positive and negative outcomes. However, Müller cell reactivity and survival in the absence of immune cells after injury have not been investigated in detail in adult zebrafish. Here, we develop a model of focal retinal injury combined with pharmacological treatments for immune cell depletion in zebrafish. The retinal injury was induced by a diode laser to damage photoreceptors. Two pharmacological treatments were used to deplete either macrophage-microglia (PLX3397) or selectively eliminate peripheral macrophages (clodronate liposomes). We show that PLX3397 treatment hinders retinal regeneration in zebrafish, which is reversed by microglial repopulation. On the other hand, selective macrophage elimination did not affect the kinetics of retinal regeneration. The absence of retinal microglia and macrophages leads to dysregulated Müller cell behavior. In the untreated fish, Müller cells react after injury induction showing glial fibrillary acidic protein (GFAP), Phospho-p44/42 MAPK (Erk1/2), and PCNA upregulation. However, in the immunosuppressed animals, GFAP and phospho-p44/42 MAPK (Erk1/2) expression was not upregulated overtime and the reentry in the cell cycle was not affected. Thus, microglia and Müller cell signaling is pivotal to unlock the regenerative potential of Müller cells in order to repair the damaged retina.

Müller cells as a target for retinal therapy

Müller cells are specialized glial cells that span the entire retina from the vitreous cavity to the subretinal space. Their functional diversity and unique radial morphology render them particularly interesting targets for new therapeutic approaches. In this review, we reflect on various possibilities for selective Müller cell targeting and describe how some of their cellular mechanisms can be used for retinal neuroprotection. Intriguingly, cross-species investigation of their properties has revealed that Müller cells also have an essential role in retinal regeneration. Although many questions regarding this subject remain, it is clear that Müller cells have unique characteristics that make them suitable targets for the prevention and treatment of numerous retinal diseases.

Inhibition of miRNA-21 promotes retinal ganglion cell survival and visual function by modulating Müller cell gliosis after optic nerve crush

BACKGROUND

Müller cell gliosis not only plays an important physiological role by maintaining retinal neuronal homeostasis but is also associated with multiple pathological events in the retina, including optic nerve crush (ONC) injury. Modulating Müller cell gliosis contributes to the creation of a permissive environment for neuronal survival. However, the underlying mechanism of Müller cell gliosis has remained elusive.

OBJECTIVE

To investigate the underlying mechanism of Müller cell gliosis after ONC.

METHODS

Rats with ONC injury were transfected with miRNA-21 (miR-21) agomir (overexpressing miR-21) or antagomir (inhibiting miR-21) via intravitreous injection. Immunofluorescence and western blotting were performed to confirm the effects of miR-21 on Müller cell gliosis. The retinal nerve fiber layer (RNFL) thickness was measured using optical coherence tomography and the positive scotopic threshold response (pSTR) was recorded using electroretinogram.

RESULTS

In the acute phase (14 days) after ONC, compared with the crushed group, inhibiting miR-21 promoted Müller cell gliosis, exhibiting thicker processes and increased GFAP expression. In the chronic phase (35 days), inhibiting miR-21 ameliorated Müller cell gliosis, which exhibited thicker and denser processes and increased GFAP expression. Retinal ganglion cell (RGC) counts in retinas showed that the number of surviving RGCs increased significantly in the antagomir group. The thickness of the RNFL increased significantly, and pSTR showed significant preservation of the amplitudes in the antagomir group.

CONCLUSIONS

Inhibition of miR-21 promotes RGC survival, RNFL thickness and the recovery of RGC function by modulating Müller cell gliosis after ONC.

Polybenzyl Glutamate Biocompatible Scaffold Promotes the Efficiency of Retinal Differentiation toward Retinal Ganglion Cell Lineage from Human-Induced Pluripotent Stem Cells

Optic neuropathy is one of the leading causes of irreversible blindness caused by retinal ganglion cell (RGC) degeneration. The development of induced pluripotent stem cell (iPSC)-based therapy opens a therapeutic window for RGC degeneration, and tissue engineering may further promote the efficiency of differentiation process of iPSCs. The present study was designed to evaluate the effects of a novel biomimetic polybenzyl glutamate (PBG) scaffold on culturing iPSC-derived RGC progenitors. The iPSC-derived neural spheres cultured on PBG scaffold increased the differentiated retinal neurons and promoted the neurite outgrowth in the RGC progenitor layer. Additionally, iPSCs cultured on PBG scaffold formed the organoid-like structures compared to that of iPSCs cultured on cover glass within the same culture period. With RNA-seq, we found that cells of the PBG group were differentiated toward retinal lineage and may be related to the glutamate signaling pathway. Further ontological analysis and the gene network analysis showed that the differentially expressed genes between cells of the PBG group and the control group were mainly associated with neuronal differentiation, neuronal maturation, and more specifically, retinal differentiation and maturation. The novel electrospinning PBG scaffold is beneficial for culturing iPSC-derived RGC progenitors as well as retinal organoids. Cells cultured on PBG scaffold differentiate effectively and shorten the process of RGC differentiation compared to that of cells cultured on coverslip. The new culture system may be helpful in future disease modeling, pharmacological screening, autologous transplantation, as well as narrowing the gap to clinical application.

In Vivo Evaluation of PAX6 Overexpression and NMDA Cytotoxicity to Stimulate Proliferation in the Mouse Retina

Retinal degenerative diseases, due to the lack of regeneration systems and self-renewable cells, often lead to visual impairment. Pax6 is a pleiotropic transcription factor and its expression level determines self-renewal status or differentiation of retinal cells. Here, we investigated the fate of simultaneous induction of retinal ganglion cell death and Pax6 overexpression in retro-differentiation of retinal cells and their commitment to re-enter into the cell cycle. Induction of acute retinal ganglion cell death and generation of mouse experimental model was performed by N-methyl D-aspartic acid (NMDA) injection. Recombinant AAV2 virus harboring PAX6 cDNA and reporter gene was injected into untreated and model mouse eyes. Histological analyses, including IHC and retinal flatmounts immunostaining were performed. The number of Ki67+ cells was clearly increased in model mice, presumably due to NMDA treatment and regardless of Pax6 over-expression. Unlike previous studies, Ki67+ cells were found in GCL layer and interestingly ONL cells expressed Sox2 stemness marker after NMDA cytotoxicity. The potential of retinal cells for robust Ki67 expression, after injury, and expression of Sox2, confirmed their intrinsic plasticity and made a vivid prospect for retinal regenerative medicine.

Dedifferentiation, transdifferentiation and cell fusion: in vivo reprogramming strategies for regenerative medicine

Regenerative capacities vary enormously across the animal kingdom. In contrast to most cold-blooded vertebrates, mammals, including humans, have very limited regenerative capacity when it comes to repairing damaged or degenerating tissues. Here, we review the main mechanisms of tissue regeneration, underlying the importance of cell dedifferentiation and reprogramming. We discuss the significance of cell fate and identity changes in the context of regenerative medicine, with a particular focus on strategies aiming at the promotion of the body's self-repairing mechanisms. We also introduce some of the most recent advances that have resulted in complete reprogramming of cell identity in vivo. Lastly, we discuss the main challenges that need to be addressed in the near future to develop in vivo reprogramming approaches with therapeutic potential.

Small GTPase RIT1 in Mouse Retina; Cellular and Functional Analysis

PURPOSE

Ras-like without CAAX 1 (RIT1/Rit) is a member of the Ras subfamily of small GTP-binding proteins with documented roles in regulating neuronal function, including contributions to neurotrophin signaling, neuronal survival, and neurogenesis. The aim of the study was to (1) examine the expression of RIT1 protein in mouse retina and retinal cell types and (2) determine whether RIT1 contributes to retinal ganglion cell (RGC) survival and synaptic stability following excitotoxic stress.

MATERIALS AND METHODS

Gene expression and immunohistochemical analysis were used to examine RIT1 expression in the mouse retina. Primary RGC and Müller glia cultures were used to validate novel RIT1 lentiviral RNAi silencing reagents, and to demonstrate that RIT1 loss does not alter RGC morphology. Finally, in vitro glutamate exposure identified a role for RIT1 in the adaptation of RGCs to excitotoxic stress.

RESULTS

Gene expression analysis and immunohistochemical studies in whole eyes and primary cell culture demonstrate RIT1 expression throughout the retina, including Müller glia and RGCs. While genetic RIT1 knockout (RIT1-KO) does not affect gross retinal anatomy, including the thickness of constituent retinal layers or RGC cell numbers, RNAi-mediated RIT1 silencing results in increased RGC death and synaptic loss following exposure to excitotoxic stress.

CONCLUSIONS

RIT1 is widely expressed in the murine retina, including both Müller glia and RGCs. While genetic deletion of RIT1 does not result in gross retinal abnormalities, these studies identify a novel role for RIT1 in the adaptation of RGC to excitotoxic stress, with RIT1 promoting both neuronal survival and the retention of PSD-95⁺ synapses.

Endogenous Mobilization of Bone-Marrow Cells Into the Murine Retina Induces Fusion-Mediated Reprogramming of Müller Glia Cells

Müller glial cells (MGCs) represent the most plastic cell type found in the retina. Following injury, zebrafish and avian MGCs can efficiently re-enter the cell cycle, proliferate and generate new functional neurons. The regenerative potential of mammalian MGCs, however, is very limited. Here, we showed that N-methyl-d-aspartate (NMDA) damage stimulates murine MGCs to re-enter the cell cycle and de-differentiate back to a progenitor-like stage. These events are dependent on the recruitment of endogenous bone marrow cells (BMCs), which, in turn, is regulated by the stromal cell-derived factor 1 (SDF1)-C-X-C motif chemokine receptor type 4 (CXCR4) pathway. BMCs mobilized into the damaged retina can fuse with resident MGCs, and the resulting hybrids undergo reprogramming followed by re-differentiation into cells expressing markers of ganglion and amacrine neurons. Our findings constitute an important proof-of-principle that mammalian MGCs retain their regenerative potential, and that such potential can be activated via cell fusion with recruited BMCs. In this perspective, our study could contribute to the development of therapeutic strategies based on the enhancement of mammalian endogenous repair capabilities.

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Endogenous bone marrow cells migrate into NMDA-damaged murine retinae and fuse with retinal Müller glial cells (MGCs).

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MGCs can be reprogrammed to retinal progenitors to then differentiate into ganglion and amacrine neurons.

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Modulation of the SDF1/CXCR4 pathway regulates BMC migration, BMC-MGC fusion, and MGC reprogramming.

Retinal degeneration is present in a large and heterogeneous group of debilitating diseases, often not curable. Cell therapy represents an interesting approach to regenerate injured retinal tissue. However, it comes with some hurdles in terms of engraftment and differentiation of the transplanted cells. Here, we reported that murine Müller glia cells can be converted into retinal neurons after fusion with endogenous bone marrow cells. The efficiency of this mechanism can be enhanced by perturbation of the SDF1/CXCR4 signaling pathway. Our study provides an important proof-of-principle that the limited endogenous regeneration capability of mammals can be enhanced by modulation of specific signaling pathways.

Lin28B promotes Müller glial cell de-differentiation and proliferation in the regenerative rat retinas

Retinal regeneration and repair are severely impeded in higher mammalian animals. Although Müller cells can be activated and show some characteristics of progenitor cells when injured or under pathological conditions, they quickly form gliosis scars. Unfortunately, the basic mechanisms that impede retinal regeneration remain unknown. We studied retinas from Royal College of Surgeon (RCS) rats and found that let-7 family molecules, let-7e and let-7i, were significantly overexpressed in Müller cells of degenerative retinas. It demonstrated that down-regulation of the RNA binding protein Lin28B was one of the key factors leading to the overexpression of let-7e and let-7i. Lin28B ectopic expression in the Müller cells suppressed overexpression of let-7e and let-7i, stimulated and mobilized Müller glia de-differentiation, proliferation, promoted neuronal commitment, and inhibited glial fate acquisition of de-differentiated Müller cells. ERG recordings revealed that the amplitudes of a-wave and b-wave were improved significantly after Lin28B was delivered into the subretinal space of RCS rats. In summary, down-regulation of Lin28B as well as up-regulation of let-7e and let-7i may be the main factors that impede Müller cell de-differentiation and proliferation in the retina of RCS rats.

Wnt Regulates Proliferation and Neurogenic Potential of Müller Glial Cells via a Lin28/let-7 miRNA-Dependent Pathway in Adult Mammalian Retinas

In cold-blooded vertebrates such as zebrafish, Müller glial cells (MGs) readily proliferate to replenish lost retinal neurons. In mammals, however, MGs lack regenerative capability as they do not spontaneously re-enter the cell cycle unless the retina is injured. Here, we show that gene transfer of β-catenin in adult mouse retinas activates Wnt signaling and MG proliferation without retinal injury. Upstream of Wnt, deletion of GSK3β stabilizes β-catenin and activates MG proliferation. Downstream of Wnt, β-catenin binds to the Lin28 promoter and activates transcription. Deletion of Lin28 abolishes β-catenin-mediated effects on MG proliferation, and Lin28 gene transfer stimulates MG proliferation. We further demonstrate that let-7 miRNAs are critically involved in Wnt/Lin28-regulated MG proliferation. Intriguingly, a subset of cell-cycle-reactivated MGs express markers for amacrine cells. Together, these results reveal a key role of Wnt-Lin28-let7 miRNA signaling in regulating proliferation and neurogenic potential of MGs in the adult mammalian retina.

MicroRNA-28 potentially regulates the photoreceptor lineage commitment of Müller glia-derived progenitors

Retinal degenerative diseases ultimately result into irreversible photoreceptor death or loss. At present, the most promising treatment for these diseases is cell replacement therapy. Müller glia are the major glia in the retina, displaying cardinal features of retinal progenitor cells, and can be candidate of seed cells for retinal degenerative diseases. Here, mouse retinal Müller glia dissociated and cultured in vitro amplified and were dedifferentiated into Müller glia-derived progenitors (MGDPs), demonstrating expression of stem/progenitor cell markers Nestin, Sox2 and self-renewal capacity. MicroRNAs (miRNAs) play unique roles in the retinogenesis, so we hypothesized miRNAs would contribute to photoreceptor lineage commitment of MGDPs. By TargetScan, Miranda, and Pictar bioinformatics, gain/loss-of-function models, dual luciferase assay, we identified and validated that miR-28 targeted the photoreceptor-specific CRX transcription factor. Anti-miR-28 could induce MGDPs to differentiate into neurons strongly expressing CRX and Rhodopsin, while miR-28 mimic suppressed CRX and Rhodopsin expression. Knockdown of CRX by siRNA blocked the expression of CRX and Rhodospin upregulated by anti-miR-28, indicating that anti-miR-28 potentially induced photoreceptor commitment of MGDPs by targeting CRX, but more experiments are necessary to confirm their role in differentiation.

Emerging pharmacological approaches to promote neurogenesis from endogenous glial cells

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

Müller glial cell reactivation in Xenopus models of retinal degeneration

A striking aspect of tissue regeneration is its uneven distribution among different animal classes, both in terms of modalities and efficiency. The retina does not escape the rule, exhibiting extraordinary self-repair properties in anamniote species but extremely limited ones in mammals. Among cellular sources prone to contribute to retinal regeneration are Müller glial cells, which in teleosts have been known for a decade to re-acquire a stem/progenitor state and regenerate retinal neurons following injury. As their regenerative potential was hitherto unexplored in amphibians, we tackled this issue using two Xenopus retinal injury paradigms we implemented: a mechanical needle poke injury and a transgenic model allowing for conditional photoreceptor cell ablation. These models revealed that Müller cells are indeed able to proliferate and replace lost cells following damage/degeneration in the retina. Interestingly, the extent of cell cycle re-entry appears dependent on the age of the animal, with a refractory period in early tadpole stages. Our findings pave the way for future studies aimed at identifying the molecular cues that either sustain or constrain the recruitment of Müller glia, an issue of utmost importance to set up therapeutic strategies for eye regenerative medicine.

Sphere-Induced Rejuvenation of Swine and Human Müller Glia Is Primarily Caused by Telomere Elongation

Müller cells are the major supportive and protective glial cells in the retina with important functions in histogenesis and synaptogenesis during development, and in maintenance of mature neurons as they show to secrete various cytokines and manifest potentials of self-renewal and transdifferentiation into retinal neurons following injury in the vertebrate retinas. The swine retina has a visual streak structure similar to the human macular where cone photoreceptors are highly concentrated, thereby can serve as a better model for studying retinal diseases and for formulating cell-based therapeutics than the rodent retinas. Like most differentiated somatic mammalian cells, the isolated swine and human Müller glia become senescent over passages in culture, which restricts their potential application in basic and clinic researches. Here, we demonstrate that the senescence of swine and human Müller cells is caused by telomere attrition upon multiplications in vitro; and the senescent cells can be rejuvenated by sphere suspension culture. We also provide evidence that sphere-induced extension of telomeres in swine and human Müller glia is achieved by alternative lengthening of telomeres or/and by telomerase activation. Stem Cells 2017;35:1579-1591.

Prospective purification and characterization of Müller glia in the mouse retina regeneration assay

Reactive gliosis is an umbrella term for various glia functions in neurodegenerative diseases and upon injury. Specifically, Müller glia (MG) in some species readily regenerate retinal neurons to restore vision loss after insult, whereas mammalian MG respond by reactive gliosis-a heterogeneous response which frequently includes cell hypertrophy and proliferation. Limited regeneration has been stimulated in mammals, with a higher propensity in young MG, and in vitro compared to in vivo, but the underlying processes are unknown. To facilitate studies on the mechanisms regulating and limiting glia functions, we developed a strategy to purify glia and their progeny by fluorescence-activated cell sorting. Dual-transgenic nuclear reporter mice, which label neurons and glia with red and green fluorescent proteins, respectively, have enabled MG enrichment up to 93% purity. We applied this approach to MG in a mouse retina regeneration ex vivo assay. Combined cell size and cell cycle analysis indicates that most MG hypertrophy and a subpopulation proliferates which, over time, become even larger in cell size than the ones that do not proliferate. MG undergo timed differential genomic changes in genes controlling stemness and neurogenic competence; and glial markers are downregulated. Genes that are potentially required for, or associated with, regeneration and reactive gliosis are differentially regulated by retina explant culture time, epidermal growth factor stimulation, and animal age. Thus, MG enrichment facilitates cellular and molecular studies which, in combination with the mouse retina regeneration assay, provide an experimental approach for deciphering mechanisms that possibly regulate reactive gliosis and limit regeneration in mammals.

Regulation of Stem Cell Properties of Müller Glia by JAK/STAT and MAPK Signaling in the Mammalian Retina

In humans and other mammals, the neural retina does not spontaneously regenerate, and damage to the retina that kills retinal neurons results in permanent blindness. In contrast to embryonic stem cells, induced pluripotent stem cells, and embryonic/fetal retinal stem cells, Müller glia offer an intrinsic cellular source for regenerative strategies in the retina. Müller glia are radial glial cells within the retina that maintain retinal homeostasis, buffer ion flux associated with phototransduction, and form the blood/retinal barrier within the retina proper. In injured or degenerating retinas, Müller glia contribute to gliotic responses and scar formation but also show regenerative capabilities that vary across species. In the mammalian retina, regenerative responses achieved to date remain insufficient for potential clinical applications. Activation of JAK/STAT and MAPK signaling by CNTF, EGF, and FGFs can promote proliferation and modulate the glial/neurogenic switch. However, to achieve clinical relevance, additional intrinsic and extrinsic factors that restrict or promote regenerative responses of Müller glia in the mammalian retina must be identified. This review focuses on Müller glia and Müller glial-derived stem cells in the retina and phylogenetic differences among model vertebrate species and highlights some of the current progress towards understanding the cellular mechanisms regulating their regenerative response.

Changes in metabolic proteins in ex vivo rat retina during glutamate-induced neural progenitor cell induction

Understanding how energy metabolism and related proteins influence neural progenitor cells in adult tissues is critical for developing new strategies in clinical tissue regeneration therapy. We have recently reported that a subtoxic concentration of glutamate-induced neural progenitor cells in the mature ex vivo rat retina. We herein explore changes in the metabolic pathways during the process. We firstly observed an increase in lactate and lactate dehydrogenase concentration in the glutamate-treated retina. We then investigated the levels of glycolytic enzymes and confirmed significant upregulation of pyruvate kinase M type (PKM), especially PKM2, enolase, phosphoglycerate mutase 1 (PGAM1), and inosine-5'-monophosphate dehydrogenase (IMPDH1) in the glutamate-treated retina compared to the untreated retina. An analysis of the subcellular localization of PKM2 revealed nuclear translocation in the treated retina, which has been reported to regulate cell cycle proliferation and glycolytic enzymes. Our findings indicate that the mature rat retina undergoes an increase in aerobic glycolysis. PKM2, both in the cytoplasm and in the nucleus, may thus play an important role during neural progenitor cell induction, as it does in other proliferating cells.

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.

Perturbational Profiling of Metabolites in Patient Fibroblasts Implicates α-Aminoadipate as a Potential Biomarker for Bipolar Disorder

Many studies suggest the presence of aberrations in cellular metabolism in bipolar disorder. We studied the metabolome in bipolar disorder to gain insight into cellular pathways that may be dysregulated in bipolar disorder and to discover evidence of novel biomarkers. We measured polar and nonpolar metabolites in fibroblasts from subjects with bipolar I disorder and matched healthy control subjects, under normal conditions and with two physiologic perturbations: low-glucose media and exposure to the stress-mediating hormone dexamethasone. Metabolites that were significantly different between bipolar and control subjects showed distinct separation by principal components analysis methods. The most statistically significant findings were observed in the perturbation experiments. The metabolite with the lowest p value in both the low-glucose and dexamethasone experiments was α-aminoadipate, whose intracellular level was consistently lower in bipolar subjects. Our study implicates α-aminoadipate as a possible biomarker in bipolar disorder that manifests under cellular stress. This is an intriguing finding given the known role of α-aminoadipate in the modulation of kynurenic acid in the brain, especially as abnormal kynurenic acid levels have been implicated in bipolar disorder.

Retinal Macroglial Responses in Health and Disease

Due to their permanent and close proximity to neurons, glial cells perform essential tasks for the normal physiology of the retina. Astrocytes and Müller cells (retinal macroglia) provide physical support to neurons and supplement them with several metabolites and growth factors. Macroglia are involved in maintaining the homeostasis of extracellular ions and neurotransmitters, are essential for information processing in neural circuits, participate in retinal glucose metabolism and in removing metabolic waste products, regulate local blood flow, induce the blood-retinal barrier (BRB), play fundamental roles in local immune response, and protect neurons from oxidative damage. In response to polyetiological insults, glia cells react with a process called reactive gliosis, seeking to maintain retinal homeostasis. When malfunctioning, macroglial cells can become primary pathogenic elements. A reactive gliosis has been described in different retinal pathologies, including age-related macular degeneration (AMD), diabetes, glaucoma, retinal detachment, or retinitis pigmentosa. A better understanding of the dual, neuroprotective, or cytotoxic effect of macroglial involvement in retinal pathologies would help in treating the physiopathology of these diseases. The extensive participation of the macroglia in retinal diseases points to these cells as innovative targets for new drug therapies.

Interneuron regeneration after ouabain treatment in the adult mammalian retina

The mammalian retina has the potential to regenerate rod cells, bipolar cells, and amacrine cells in vivo to repair damaged nervous tissue through the Müller glial cell (MGC)-mediated response. Both horizontal cell (HC) and amacrine cell are interneurons in the inner nuclear layer (INL) and are generated under the control of some common transcription factors during retinal development. However, to date, the ability of HC regeneration in vivo in mammals remains unclear. Here, ouabain (a Na/K-ATPase inhibitor) was injected into rat eyes to induce an obvious cell loss in the INL. The proliferation, dedifferentiation of MGC and production of new neurons after ouabain injection were examined by BrdU incorporation and immunohistochemistry. Our results showed that 2 days after ouabain treatment, MGCs incorporated BrdU and upregulated the expression of Nestin, which is a marker for retinal progenitor cells. Several weeks after ouabain injection, the BrdU-positive cells in the outer border of the INL expressed Prox1 and Calbindin D-28k, which are specific markers for HC. Taken together, these results suggest that the mammalian retina can regenerate new type of interneurons (HC) in vivo, which advances our understanding of mammalian retinal regeneration after damage.

Notch Signaling Activates Stem Cell Properties of Müller Glia through Transcriptional Regulation and Skp2-mediated Degradation of p27Kip1

Müller glia (MG), the sole glial cells generated by retinal progenitors, have emerged as a viable cellular target for therapeutic regeneration in degenerative blinding diseases, as they possess dormant stem cell properties. However, the mammalian MG does not display the neurogenic potential of their lower vertebrate counterparts, precluding their practical clinical use. The answer to this barrier may be found in two interlinked processes underlying the neurogenic potential, i.e., the activation of the dormant stem cell properties of MG and their differentiation along the neuronal lineage. Here, we have focused on the former and examined Notch signaling-mediated activation of MG. We demonstrate that one of the targets of Notch signaling is the cyclin-dependent kinase inhibitor (CKI), p27^Kip1, which is highly expressed in quiescent MG. Notch signaling facilitates the activation of MG by inhibiting p27^Kip1 expression. This is likely achieved through the Notch- p27^Kip1 and Notch-Skp2-p27^Kip1 axes, the former inhibiting the expression of p27^Kip1 transcripts and the latter levels of p27^Kip1 proteins by Skp2-mediated proteasomal degradation. Thus, Notch signaling may facilitate re-entry of MG into the cell cycle by inhibiting p27^Kip1 expression both transcriptionally and post-translationally.

Feedback from each retinal neuron population drives expression of subsequent fate determinant genes without influencing the cell cycle exit timing

During neurogenesis, progenitors balance proliferation and cell cycle exit together with expression of fate determinant genes to ensure that the correct number of each of these neuron types is generated. Although intrinsic gene expression acting cell autonomously within each progenitor drives these processes, the final number of neurons generated is also influenced by extrinsic cues, representing a potential avenue to direct neurogenesis in developmental disorders or regenerative settings without the requirement to change intrinsic gene expression. Thus, it is important to understand which of these stages of neurogenesis are amenable to such extrinsic influences. Additionally, all types of neurons are specified in a highly conserved histogenic order, although its significance is unknown. This study makes use of conserved patterns of neurogenesis in the relatively simple yet highly organized zebrafish retina model, in which such histogenic birth order is well characterized. We directly visualize and quantify birth dates and cell fate determinant expression in WT vs. environments lacking different neuronal populations. This study shows that extrinsic feedback from developing retinal neurons is important for the temporal expression of intrinsic fate determinants but not for the timing of birth dates. We found no changes in cell cycle exit timing but did find a significant delay in the expression of genes driving the generation only of later- but not earlier-born cells, suggesting that the robustness of this process depends on continuous feedback from earlier-formed cell types. Thus, extrinsic cues selectively influence cell fate determinant progression, which may explain the function of the retinal histogenic order observed. J. Comp. Neurol. 524:2553-2566, 2016. © 2016 Wiley Periodicals, Inc.

Retinal Remodeling: Concerns, Emerging Remedies and Future Prospects

Deafferentation results not only in sensory loss, but also in a variety of alterations in the postsynaptic circuitry. These alterations may have detrimental impact on potential treatment strategies. Progressive loss of photoreceptors in retinal degenerative diseases, such as retinitis pigmentosa and age-related macular degeneration, leads to several changes in the remnant retinal circuitry. Müller glial cells undergo hypertrophy and form a glial seal. The second- and third-order retinal neurons undergo morphological, biochemical and physiological alterations. A result of these alterations is that retinal ganglion cells (RGCs), the output neurons of the retina, become hyperactive and exhibit spontaneous, oscillatory bursts of spikes. This aberrant electrical activity degrades the signal-to-noise ratio in RGC responses, and thus the quality of information they transmit to the brain. These changes in the remnant retina, collectively termed “retinal remodeling”, pose challenges for genetic, cellular and bionic approaches to restore vision. It is therefore crucial to understand the nature of retinal remodeling, how it affects the ability of remnant retina to respond to novel therapeutic strategies, and how to ameliorate its effects. In this article, we discuss these topics, and suggest that the pathological state of the retinal output following photoreceptor loss is reversible, and therefore, amenable to restorative strategies.

Müller glial cell-dependent regeneration of the neural retina: An overview across vertebrate model systems

Retinal dystrophies are a major cause of blindness for which there are currently no curative treatments. Transplantation of stem cell‐derived neuronal progenitors to replace lost cells has been widely investigated as a therapeutic option. Another promising strategy would be to trigger self‐repair mechanisms in patients, through the recruitment of endogenous cells with stemness properties. Accumulating evidence in the past 15 year0s has revealed that several retinal cell types possess neurogenic potential, thus opening new avenues for regenerative medicine. Among them, Müller glial cells have been shown to be able to undergo a reprogramming process to re‐acquire a stem/progenitor state, allowing them to proliferate and generate new neurons for repair following retinal damages. Although Müller cell–dependent spontaneous regeneration is remarkable in some species such as the fish, it is extremely limited and ineffective in mammals. Understanding the cellular events and molecular mechanisms underlying Müller cell activities in species endowed with regenerative capacities could provide knowledge to unlock the restricted potential of their mammalian counterparts. In this context, the present review provides an overview of Müller cell responses to injury across vertebrate model systems and summarizes recent advances in this rapidly evolving field. Developmental Dynamics 245:727–738, 2016. © 2015 The Authors. Developmental Dynamics published by Wiley Periodicals, Inc.

The present review provides an overview of Müller cell responses to injury across vertebrate model systems and summarizes recent advances in this rapidly evolving field.

Glia-neuron interactions in the mammalian retina

The mammalian retina provides an excellent opportunity to study glia-neuron interactions and the interactions of glia with blood vessels. Three main types of glial cells are found in the mammalian retina that serve to maintain retinal homeostasis: astrocytes, Müller cells and resident microglia. Müller cells, astrocytes and microglia not only provide structural support but they are also involved in metabolism, the phagocytosis of neuronal debris, the release of certain transmitters and trophic factors and K(+) uptake. Astrocytes are mostly located in the nerve fibre layer and they accompany the blood vessels in the inner nuclear layer. Indeed, like Müller cells, astrocytic processes cover the blood vessels forming the retinal blood barrier and they fulfil a significant role in ion homeostasis. Among other activities, microglia can be stimulated to fulfil a macrophage function, as well as to interact with other glial cells and neurons by secreting growth factors. This review summarizes the main functional relationships between retinal glial cells and neurons, presenting a general picture of the retina recently modified based on experimental observations. The preferential involvement of the distinct glia cells in terms of the activity in the retina is discussed, for example, while Müller cells may serve as progenitors of retinal neurons, astrocytes and microglia are responsible for synaptic pruning. Since different types of glia participate together in certain activities in the retina, it is imperative to explore the order of redundancy and to explore the heterogeneity among these cells. Recent studies revealed the association of glia cell heterogeneity with specific functions. Finally, the neuroprotective effects of glia on photoreceptors and ganglion cells under normal and adverse conditions will also be explored.

Age-dependent Müller glia neurogenic competence in the mouse retina

The mechanisms limiting neuronal regeneration in mammals and their relationship with reactive gliosis are unknown. Müller glia (MG), common to all vertebrate retinas, readily regenerate neuron loss in some species, but normally not in mammals. However, experimental stimulation of limited mammalian retina regeneration has been reported. Here, we use a mouse retina organ culture approach to investigate the MG responses at different mouse ages. We found that MG undergo defined spatio-temporal changes upon stimulation. In EGF-stimulated juvenile postmitotic retinas, most MG upregulate cell-cycle regulators (Mcm6, Pcna, Ki67, Ccnd1) within 48 h ex vivo; some also express the neurogenic factors Ascl1, Pax6, and Vsx2; up to 60% re-enter the cell cycle, some of which delaminate to divide mostly apically; and the majority cease to proliferate after stimulation. A subpopulation of MG progeny starts to express transcription factors (Ptf1a, Nr4a2) and neuronal (Calb1, Calb2, Rbfox3), but not glial, markers, indicating neurogenesis. BrdU-tracking, genetic lineage-tracing, and transgenic-reporter experiments suggest that MG reprogram to a neurogenic stage and proliferate; and that some MG progeny differentiate into neuronal-like cells, most likely amacrines, no photoreceptors; most others remain in a de-differentiated state. The mouse MG regeneration potential becomes restricted, dependent on the age of the animal, as observed by limited activation of the cell cycle and neurogenic factors. The stage-dependent analysis of mouse MG revealed similarities and differences when compared with MG-derived regeneration in fish and chicks. Therefore, the mouse retina ex vivo approach is a potential assay for understanding and overcoming the limitations of mammalian MG-derived neuronal regeneration. Postmitotic MG in mouse retina ex vivo can be stimulated to proliferate, express neurogenic factors, and generate progeny expressing neuronal or glial markers. This potential regenerative competence becomes limited with increasing mouse age.

Acute retinal injury and the relationship between nerve growth factor, Notch1 transcription and short-lived dedifferentiation transient changes of mammalian Müller cells

Our aim is to define related molecular events on how dormant Müller glia cells re-enter the cell cycle, proliferate and produce new retinal neurons from initial injury to glial scar formation. Sodium iodate (NaIO3) was used to induce acute retinal injury. Long-Evans rats were administered with NaIO3 or phosphate-buffered saline by intraperitoneal injection. The proliferation, dedifferentiation and neurogenesis of Müller cells were analyzed by double-labeled fluorescence immunohistochemistry with primary antibodies - against Müller cells and specific cell markers. Possible molecules that limit the regenerative potential of Müller cells were also determined by immunofluorescence staining, quantitative RT-PCR, protein array, ELISA and Western blot. In the first 3-7days after NaIO3 administration, Müller cells were activated and underwent a fate switch, including transient proliferation, dedifferentiation and neurogenesis. Nerve growth factor (NGF) signaling concomitantly increased with the downregulation of p27(Kip1) in Müller cells, which may promote Müller cells to re-enter the cell cycle. The transient increase of NGF signaling and the transient decrease of Notch signaling inhibited Hes1, which might enhance the neuronal differentiation of dedifferentiated Müller cells and suppress gliosis. Upregulated Notch and decreased NGF expressions limit dedifferentiation and neurogenesis, but induces retinal Müller cell gliosis at a later stage. We conclude that transient NGF upregulation and Notch1 downregulation may activate the transient proliferation, dedifferentiation and neurogenesis of Müller cells during NaIO3-induced acute retinal injury; which could be a therapeutic target for overcoming Müller cell gliosis. Such therapy could be potentially used for treating retinal-related diseases.