Electrophysiological characterization of retinal Müller glial cells from mouse during postnatal development: comparison with rabbit cells

Müller glia fused with adult stem cells undergo neural differentiation in human retinal models

Background

Visual impairments are a critical medical hurdle to be addressed in modern society. Müller glia (MG) have regenerative potential in the retina in lower vertebrates, but not in mammals. However, in mice, in vivo cell fusion between MG and adult stem cells forms hybrids that can partially regenerate ablated neurons.

Methods

We used organotypic cultures of human retina and preparations of dissociated cells to test the hypothesis that cell fusion between human MG and adult stem cells can induce neuronal regeneration in human systems. Moreover, we established a microinjection system for transplanting human retinal organoids to demonstrate hybrid differentiation.

Findings

We first found that cell fusion occurs between MG and adult stem cells, in organotypic cultures of human retina as well as in cell cultures. Next, we showed that the resulting hybrids can differentiate and acquire a proto-neural electrophysiology profile when the Wnt/beta-catenin pathway is activated in the adult stem cells prior fusion. Finally, we demonstrated the engraftment and differentiation of these hybrids into human retinal organoids.

Interpretation

We show fusion between human MG and adult stem cells, and demonstrate that the resulting hybrid cells can differentiate towards neural fate in human model systems. Our results suggest that cell fusion-mediated therapy is a potential regenerative approach for treating human retinal dystrophies.

Funding

This work was supported by La Caixa Health (HR17-00231), Velux Stiftung (976a) and the Ministerio de Ciencia e Innovación, (BFU2017-86760-P) (AEI/FEDER, UE), AGAUR (2017 SGR 689, 2017 SGR 926).

Müller Glia in Retinal Development: From Specification to Circuit Integration

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

Müller Glial Cells Participate in Retinal Waves via Glutamate Transporters and AMPA Receptors

Retinal waves, the spontaneous patterned neural activities propagating among developing retinal ganglion cells (RGCs), instruct the activity-dependent refinement of visuotopic maps. Although it is known that the wave is initiated successively by amacrine cells and bipolar cells, the behavior and function of glia in retinal waves remain unclear. Using multiple in vivo methods in larval zebrafish, we found that Müller glial cells (MGCs) display wave-like spontaneous activities, which start at MGC processes within the inner plexiform layer, vertically spread to their somata and endfeet, and horizontally propagate into neighboring MGCs. MGC waves depend on glutamatergic signaling derived from bipolar cells. Moreover, MGCs express both glia-specific glutamate transporters and the AMPA subtype of glutamate receptors. The AMPA receptors mediate MGC calcium activities during retinal waves, whereas the glutamate transporters modulate the occurrence of retinal waves. Thus, MGCs can sense and regulate retinal waves via AMPA receptors and glutamate transporters, respectively.

Comparative electrophysiology of retinal Müller glial cells-A survey on vertebrate species

Müller cells are the dominant macroglial cells in the retina of all vertebrates. They fulfill a variety of functions important for retinal physiology, among them spatial buffering of K⁺ ions and uptake of glutamate and other neurotransmitters. To this end, Müller cells express inwardly rectifying K⁺ channels and electrogenic glutamate transporters. Moreover, a lot of voltage- and ligand-gated ion channels, aquaporin water channels, and electrogenic transporters are expressed in Müller cells, some of them in a species-specific manner. For example, voltage-dependent Na⁺ channels are found exclusively in some but not all mammalian species. Whereas a lot of data exist from amphibians and mammals, the results from other vertebrates are sparse. It is the aim of this review to present a survey on Müller cell electrophysiology covering all classes of vertebrates. The focus is on functional studies, mainly performed using the whole-cell patch-clamp technique. However, data about the expression of membrane channels and transporters from immunohistochemistry are also included. Possible functional roles of membrane channels and transporters are discussed. Obviously, electrophysiological properties involved in the main functions of Müller cells developed early in vertebrate evolution. GLIA 2017;65:533-568.

Altered astrocyte morphology and vascular development in dystrophin-Dp71-null mice

Understanding retinal vascular development is crucial because many retinal vascular diseases such as diabetic retinopathy (in adults) or retinopathy of prematurity (in children) are among the leading causes of blindness. Given the localization of the protein Dp71 around the retinal vessels in adult mice and its role in maintaining retinal homeostasis, the aim of this study was to determine if Dp71 was involved in astrocyte and vascular development regulation. An experimental study in mouse retinas was conducted. Using a dual immunolabeling with antibodies to Dp71 and anti-GFAP for astrocytes on retinal sections and isolated astrocytes, it was found that Dp71 was expressed in wild-type (WT) mouse astrocytes from early developmental stages to adult stage. In Dp71-null mice, a reduction in GFAP-immunopositive astrocytes was observed as early as postnatal day 6 (P6) compared with WT mice. Using real-time PCR, it was showed that Dp71 mRNA was stable between P1 and P6, in parallel with post-natal vascular development. Regarding morphology in Dp71-null and WT mice, a significant decrease in overall astrocyte process number in Dp71-null retinas at P6 to adult age was found. Using fluorescence-conjugated isolectin Griffonia simplicifolia on whole mount retinas, subsequent delay of developing vascular network at the same age in Dp71-null mice was found. An evidence that the Dystrophin Dp71, a membrane-associated cytoskeletal protein and one of the smaller Duchenne muscular dystrophy gene products, regulates astrocyte morphology and density and is associated with subsequent normal blood vessel development was provided.

Neuron-glia signaling in developing retina mediated by neurotransmitter spillover

Neuron-glia interactions play a critical role in the maturation of neural circuits; however, little is known about the pathways that mediate their communication in the developing CNS. We investigated neuron-glia signaling in the developing retina, where we demonstrate that retinal waves reliably induce calcium transients in Müller glial cells (MCs). During cholinergic waves, MC calcium transients were blocked by muscarinic acetylcholine receptor antagonists, whereas during glutamatergic waves, MC calcium transients were inhibited by ionotropic glutamate receptor antagonists, indicating that the responsiveness of MCs changes to match the neurotransmitter used to support retinal waves. Using an optical glutamate sensor we show that the decline in MC calcium transients is caused by a reduction in the amount of glutamate reaching MCs. Together, these studies indicate that neurons and MCs exhibit correlated activity during a critical period of retinal maturation that is enabled by neurotransmitter spillover from retinal synapses.

Retinal functional alterations in mice lacking intermediate filament proteins glial fibrillary acidic protein and vimentin

Vimentin (Vim) and glial fibrillary acidic protein (GFAP) are important components of the intermediate filament (IF) (or nanofilament) system of astroglial cells. We conducted full-field electroretinogram (ERG) recordings and found that whereas photoreceptor responses (a-wave) were normal in uninjured GFAP(-/-)Vim(-/-) mice, b-wave amplitudes were increased. Moreover, we found that Kir (inward rectifier K(+)) channel protein expression was reduced in the retinas of GFAP(-/-)Vim(-/-) mice and that Kir-mediated current amplitudes were lower in Müller glial cells isolated from these mice. Studies have shown that the IF system, in addition, is involved in the retinal response to injury and that attenuated Müller cell reactivity and reduced photoreceptor cell loss are observed in IF-deficient mice after experimental retinal detachment. We investigated whether the lack of IF proteins would affect cell survival in a retinal ischemia-reperfusion model. We found that although cell loss was induced in both genotypes, the number of surviving cells in the inner retina was lower in IF-deficient mice. Our findings thus show that the inability to produce GFAP and Vim affects normal retinal physiology and that the effect of IF deficiency on retinal cell survival differs, depending on the underlying pathologic condition.

Unidirectional photoreceptor-to-Müller glia coupling and unique K+ channel expression in Caiman retina

Background

Müller cells, the principal glial cells of the vertebrate retina, are fundamental for the maintenance and function of neuronal cells. In most vertebrates, including humans, Müller cells abundantly express Kir4.1 inwardly rectifying potassium channels responsible for hyperpolarized membrane potential and for various vital functions such as potassium buffering and glutamate clearance; inter-species differences in Kir4.1 expression were, however, observed. Localization and function of potassium channels in Müller cells from the retina of crocodiles remain, hitherto, unknown.

Methods

We studied retinae of the Spectacled caiman (Caiman crocodilus fuscus), endowed with both diurnal and nocturnal vision, by (i) immunohistochemistry, (ii) whole-cell voltage-clamp, and (iii) fluorescent dye tracing to investigate K⁺ channel distribution and glia-to-neuron communications.

Results

Immunohistochemistry revealed that caiman Müller cells, similarly to other vertebrates, express vimentin, GFAP, S100β, and glutamine synthetase. In contrast, Kir4.1 channel protein was not found in Müller cells but was localized in photoreceptor cells. Instead, 2P-domain TASK-1 channels were expressed in Müller cells. Electrophysiological properties of enzymatically dissociated Müller cells without photoreceptors and isolated Müller cells with adhering photoreceptors were significantly different. This suggests ion coupling between Müller cells and photoreceptors in the caiman retina. Sulforhodamine-B injected into cones permeated to adhering Müller cells thus revealing a uni-directional dye coupling.

Conclusion

Our data indicate that caiman Müller glial cells are unique among vertebrates studied so far by predominantly expressing TASK-1 rather than Kir4.1 K⁺ channels and by bi-directional ion and uni-directional dye coupling to photoreceptor cells. This coupling may play an important role in specific glia-neuron signaling pathways and in a new type of K⁺ buffering.

The changes of potassium currents in RCS rat Müller cell during retinal degeneration

Müller cells are the principal glial cells expressing membrane-bound potassium channel and predominantly mediating the homeostatic regulation of extracellular K+ produced by neuronal activity in retina. It's well known that Müller cells can be activated in many pathological conditions, but little is known about the change of potassium currents of Müller cells during the progression of retinitis pigmentosa. Herein, the Royal College of Surgeons rats (RCS rat) were employed to investigate some phenotypic and functional changes of Müller cells during retinal degeneration such as the expression of Kir4.1, membrane properties and K+ channel currents by using immunohistochemistry, RT-PCR, western blot and whole-cell patch clamping respectively. Compared with Müller cells in control retina, increased glutamine synthetase (GS) mRNA levels were seen at P30 and P60, and then decreased gradually in RCS rat retina. Morphologically, Müller cells showed significant hypertrophy and proliferation after p60. The increased expression of intermediate filament, glial fibrillary acidic protein (GFAP) and vimentin began at P30 and reached a peak at p60. Kir4.1 channels presented a peak expression at P30. Concomitantly, K(+) currents of Müller cells increased at P30 and decreased at P90 significantly. We concluded that retinal Müller cells of RCS rats underwent an activation initiated by the onset of retinal degeneration before p60 and then an obvious reactive gliosis, which led the basic membrane properties to suffer marked changes, and caused the Kir4.1 channels of Müller cells to occur a clear functional shift, even lose their normal electrophysiological properties. This process aggravates the impairment caused by the initial photoreceptor degeneration.

Genetic deletion of laminin isoforms β2 and γ3 induces a reduction in Kir4.1 and aquaporin-4 expression and function in the retina

BACKGROUND

Glial cells such as retinal Müller glial cells are involved in potassium ion and water homeostasis of the neural tissue. In these cells, inwardly rectifying potassium (Kir) channels and aquaporin-4 water channels play an important role in the process of spatial potassium buffering and water drainage. Moreover, Kir4.1 channels are involved in the maintenance of the negative Müller cell membrane potential. The subcellular distribution of Kir4.1 and aquaporin-4 channels appears to be maintained by interactions with extracellular and intracellular molecules. Laminins in the extracellular matrix, dystroglycan in the membrane, and dystrophins in the cytomatrix form a complex mediating the polarized expression of Kir4.1 and aquaporin-4 in Müller cells.

METHODOLOGY/PRINCIPAL FINDINGS

The aim of the present study was to test the function of the β2 and γ3 containing laminins in murine Müller cells. We used knockout mice with genetic deletion of both β2 and γ3 laminin genes to assay the effects on Kir4.1 and aquaporin-4. We studied protein and mRNA expression by immunohistochemistry, Western Blot, and quantitative RT-PCR, respectively, and membrane currents of isolated cells by patch-clamp experiments. We found a down-regulation of mRNA and protein of Kir4.1 as well as of aquaporin-4 protein in laminin knockout mice. Moreover, Müller cells from laminin β2 and γ3 knockout mice had reduced Kir-mediated inward currents and their membrane potentials were more positive than those in age-matched wild-type mice.

CONCLUSION

These findings demonstrate a strong impact of laminin β2 and γ3 subunits on the expression and function of both aquaporin-4 and Kir4.1, two important membrane proteins in Müller cells.

Development and neurogenic potential of Müller glial cells in the vertebrate retina

Considerable research on normal and diseased states within the retina has focused on neurons. Recent research on glia throughout the central nervous system, including within the retina where Müller glia are the main type of glia, has provided a more in depth view of glial functions in health and disease. Glial cells have been recognized as being vital for the maintenance of a healthy tissue environment, where they actively participate in neuronal activity. More recently, Müller glia have been recognized as being very similar to retinal progenitor cells, particularly when compared at the molecular level using comprehensive expression profiling techniques. The molecular similarities, as well as the developmental events that occur at the end of the genesis period of retinal cells, have led us to propose that Müller glia are a form of late stage retinal progenitor cells. These late stage progenitor cells acquire some specialized glial functions, but do not irreversibly leave the progenitor state. Indeed, Müller glia appear to be able to behave as a progenitor in that they have been shown to proliferate and produce neurons in several instances when an acute injury has been applied to the retina. Enhancement of this response is thus an exciting strategy for retinal repair.

Tetrodotoxin-resistant voltage-gated sodium channels Na(v)1.8 and Na(v)1.9 are expressed in the retina

Voltage-gated sodium channels (VGSCs) are one of the fundamental building blocks of electrically excitable cells in the nervous system. These channels are responsible for the generation of action potentials that are required for the communication of neuronal signals over long distances within a cell. VGSCs are encoded by a family of nine genes whose products have widely varying biophysical properties. In this study, we have detected the expression of two atypical VGSCs (Na(v)1.8 and Na(v)1.9) in the retina. Compared with more common VGSCs, Na(v)1.8 and Na(v)1.9 have unusual biophysical and pharmacological properties, including persistent sodium currents and resistance to the canonical sodium channel blocker tetrodotoxin (TTX). Our molecular biological and immunohistochemical data derived from mouse (Mus musculus) retina demonstrate expression of Na(v)1.8 by retinal amacrine and ganglion cells, whereas Na(v)1.9 is expressed by photoreceptors and Müller glia. The fact that these channels exist in the central nervous system (CNS) and exhibit robust TTX resistance requires a re-evaluation of prior physiological, pharmacological, and developmental data in the visual system, in which the diversity of VGSCs has been previously underestimated.

Atypical gliosis in Müller cells of the slowly degenerating rds mutant mouse retina

Retinal Müller glial cells are known to undergo reactive changes (gliosis) in various retinal diseases. In virtually all cases studied, an upregulation of glial fibrillary acidic protein (GFAP) and a hypertrophy can be observed. Physiological alterations, such as a strong downregulation of inwardly rectifying K+ (Kir) currents, were found after retinal detachment (man, rabbit) and after ischemia/reperfusion (rat) but not in more slowly progressing retinal degenerations (Borna Disease Virus-infected rats, RCS rats). This led us to hypothesize that Müller cells respond with 'typical' reactive gliosis only to rapid but not to slow retinal degeneration. To test this hypothesis, we studied Müller cells from rds mutant mice (PrphRd2), which show a retinal degeneration of early onset and slow progression, resulting in a complete loss of photoreceptors after 9-12 months. In Müller cells of rds mice, we found immunoreactivity for GFAP, a marker of gliosis in Müller cells, from postnatal day 21 on, accompanied by a moderately increased membrane capacitance (taken as an indicator of hypertrophy), whereas no change in the expression of the Kir4.1 protein occurred in adult rds mice. We failed to observe significant changes in the membrane resistance and the membrane potential of cells from rds mice from first week after birth until 1 year of age. Current densities were decreased in cells from 3- and 5-week old rds mice. Furthermore, as in control cells from wildtype animals, these cells displayed dominant Kir currents, voltage-dependent Na+ currents, and glutamate uptake currents. These data support the idea that in mice as well as previously shown in rats, slow retinal degeneration induces an atypical gliosis of Müller cells.

The developmental expression of K+ channels in retinal glial cells is associated with a decrease of osmotic cell swelling

A major function of glial cells is the control of osmotic and ionic homeostasis, mediated by K+ and water movements predominantly through inwardly rectifying K+ (Kir) and aquaporin water channels. It has been suggested that K+ currents through Kir channels are implicated in the regulation of glial cell volume. Here, we investigated whether the developmental increase in Kir channel expression in Müller glial cells of the rat retina is associated with an alteration of cell volume regulation under anisoosmotic conditions. Around the time of eye opening at postnatal day (P) 15, developing retinal glial cells fully alter the profile of their membrane conductances, from a current pattern with prominent fast transient K+ and Na+ currents to a pattern of noninactivating currents through Kir and delayed rectifier K+ channels. Concomitantly, aquaporins-1 and -4 are expressed in the developing retina. This is accompanied by a conspicuous alteration of the swelling characteristics of cells; somata of immature glial cells in early postnatal retinas (P5-P15) swell under hypotonic stress but no swelling is inducible in mature cells at P18 and thereafter. However, glial cells at all developmental stages swell when their Kir channels are blocked by Ba2+. The postnatal maturation of Kir channel currents and volume regulation in retinal glial cells is delayed by visual deprivation. The data suggest that Kir channels are crucially involved in osmotic volume homeostasis of mature glial cells, and that the absence of Kir channels in immature cells is a major cause of their insufficient volume regulation.

Physiological properties of retinal Muller glial cells from the cynomolgus monkey, Macaca fascicularis--a comparison to human Muller cells

Retinae from rabbits and laboratory rodents are often used as 'models' of the human retina, although there are anatomical differences. To test whether monkey eyes provide a better model, a physiological study of Muller glial cells was performed comparing isolated cells and retinal wholemounts from the cynomolgus monkey, Macaca fascicularis and from man. The membrane conductance of Muller cells from both species was dominated by inward and outward K(+) currents. Cells displayed glutamate uptake currents and responded to nucleotides by intracellular Ca(2+) increases. However, there were also species differences, such as a lack of GABA(A) receptors and of Ca(2+)-dependent K(+) currents in monkey cells. Thus, the use of Muller cells from cynomolgus monkeys may be advantageous for investigating a few specific properties; in general, monkey cells are no more similar to human cells than those from standard laboratory animals.

Glutamate transport by retinal Müller cells in glutamate/aspartate transporter-knockout mice

Glutamate transporters are involved in maintaining extracellular glutamate at a low level to ensure a high signal-to-noise ratio for glutamatergic neurotransmission and to protect neurons from excitotoxic damage. The mammalian retina is known to express the excitatory amino acid transporters, EAAT1-5; however, their specific role in glutamate homeostasis is poorly understood. To examine the role of the glial glutamate/aspartate transporter (GLAST) in the retina, we have studied glutamate transport by Muller cells in GLAST-/- mice, using biochemical, electrophysiological, and immunocytochemical techniques. Glutamate uptake assays indicated that the Km value for glutamate uptake was similar in wild-type and GLAST-/- mouse retinas, but the Vmax was approximately 50% lower in the mutant. In Na+-free medium, the Vmax was further reduced by 40%. In patch-clamp recordings of dissociated Muller cells from GLAST-/- mice, application of 0.1 mM glutamate evoked no current showing that the cells lacked functional electrogenic glutamate transporters. The result also indicated that there was no compensatory upregulation of EAATs in Muller cells. [3H]D-Aspartate uptake autoradiography, however, showed that Na+-dependent, high-affinity transporters account for most of the glutamate uptake by Muller cells, and that Na+-independent glutamate transport is negligible. Additional experiments showed that the residual glutamate uptake in Muller cells in the GLAST-/- mouse retina is not due to known glutamate transporters-cystine-glutamate exchanger, ASCT-1, AGT-1, or other heteroexchangers. The present study shows that while several known glutamate transporters are expressed by mammalian Muller cells, new Na+-dependent, high-affinity glutamate transporters remain to be identified.

Electrophysiological properties of retinal Müller glial cells from myelin mutant rat

The structural and functional similarities between Müller cells and oligodendrocytes prompted the present study of the electrophysiological properties of Müller (glia) cells obtained from the retinae of control and myelin mutant taiep rats during the postnatal developmental period (P12-P180). The whole-cell configuration of the patch-clamp technique was used to characterize the general properties and the K+ currents from dissociated Müller cells. During the first 3 weeks of life, a decrease of the membrane resistance and an increase of the membrane potential were observed in Müller cells from both control and taiep rats. However, Müller cells from taiep rats never achieved the very negative membrane potential (-50 mV vs -80 mV) and the low membrane resistance characteristic for control cells. Furthermore, Müller cells displayed increased inward and outward K+ currents during postnatal development up to P30/60 in controls; however, in taiep rats, this increase ceased at P20/30, and low-amplitude currents persisted into adulthood. These results provide first evidence of physiological changes in retinal Müller cells as a consequence of a myelin mutation causing a progressive deterioration of the central nervous system (CNS) due to a disturbance of the microtubule network of oligodendrocytes. We hypothesize that the progressive dysmyelination process of the optic nerve, accompanied by functional deficits of retinal neurons (e.g., ganglion cells), induces physiological alterations of Müller cells.