Cellular compartmentalization of carbonic anhydrase-C and glutamine synthetase in developing and mature mouse neural retina

Systemic Lipopolysaccharide Exposure Exacerbates Choroidal Neovascularization in Mice

This study aims to investigate the effect of a systemic lipopolysaccharide (LPS) stimulus in the course of laser-induced choroidal neovascularization (CNV) in C57BL/6 J mice. A group of CNV-subjected mice received 1 mg/kg LPS via the tail vein immediately after CNV induction. Mouse eyes were monitored in vivo with fluorescein angiography for 2 weeks. In situ hybridization and flow cytometry were performed in the retina at different time points. LPS led to increased fluorescein leakage 3 days after CNV, correlated with a large influx of monocyte-derived macrophages and increase of pro-inflammatory microglia/macrophages in the retina. Additionally, LPS enhanced Vegfα mRNA expression by Glul-expressing cells but not Aif1 positive microglia/macrophages in the laser lesion. These findings suggest that systemic LPS exposure has transient detrimental effects in the course of CNV through activation of microglia/macrophages to a pro-inflammatory phenotype and supports the important role of these cells in the CNV course.

Müller Cell Metabolic Signatures: Evolutionary Conservation and Disruption in Disease

Müller cells are glia that play important regulatory roles in retinal metabolism. These roles have been evolutionarily conserved across at least 300 million years. Müller cells have a tightly locked metabolic signature in the healthy retina, which rapidly degrades in response to insult and disease. This variation in metabolic signature occurs in a chaotic fashion, involving some central metabolic pathways. The cause of this divergence of Müller cells, from a single class with a unique metabolic signature to numerous separable metabolic classes, is currently unknown and illuminates potential alternative metabolic pathways that may be revealed in disease. Understanding the impacts of this heterogeneity on degenerate retinas and the implications for the metabolic support of surrounding neurons will be critical to long-term integration of retinal therapeutics for the restoration of visual perception following photoreceptor degeneration.

Diabetes Alters pH Control in Rat Retina

Purpose

The purpose of this study was to determine whether the ability of the rat retina to control its pH is affected by diabetes.

Methods

Double-barreled H+-selective microelectrodes were used to measure extracellular [H+] in the dark-adapted retina of intact control and diabetic Long-Evans rats 1 to 6 months after intraperitoneal injection of vehicle or streptozotocin, respectively. Two manipulations-increasing of blood glucose and intravenous injection of the carbonic anhydrase blocker dorzolamide (DZM)-were used to examine their effects on retinal pH regulation.

Results

An increase of retinal acidity was correlated with the diabetes-related increase in blood glucose, but only between 1 and 3 months of diabetes, not earlier or later. Adding intravenous glucose had no noticeable effect on the retinal acidity of control animals. In contrast, similar injections of glucose in diabetic rats significantly increased the acidity of the retina. Again, the largest increase of retinal acidity due to artificially elevated blood glucose was observed at 1 to 3 months of diabetes. Suppression of carbonic anhydrase by DZM dramatically increased the retinal acidity in both control and diabetic retinas to a similar degree. However, in controls, the strongest effect of DZM was recorded within 10 minutes after the injection, but in diabetics, the effect tended to increase with time and after 2 hours could be two to three times larger than at the beginning.

Conclusions

During development of diabetes in rats, the control over retinal pH is partly compromised so that conditions that perturb retinal pH lead to larger and/or more sustained changes than in control animals.

DNER and NFIA are expressed by developing and mature AII amacrine cells in the mouse retina

The present study has taken advantage of publicly available cell type specific mRNA expression databases in order to identify potential genes participating in the development of retinal AII amacrine cells. We profile two such genes, Delta/Notch-like EGF repeat containing (Dner) and nuclear factor I/A (Nfia), that are each heavily expressed in AII amacrine cells in the mature mouse retina, and which conjointly identify this retinal cell population in its entirety when using antibodies to DNER and NFIA. DNER is present on the plasma membrane, while NFIA is confined to the nucleus, consistent with known functions of each of these two proteins. DNER also identifies some other subsets of retinal ganglion and amacrine cell types, along with horizontal cells, while NFIA identifies a subset of bipolar cells as well as Muller glia and astrocytes. During early postnatal development, NFIA labels astrocytes on the day of birth, AII amacrine cells at postnatal (P) day 5, and Muller glia by P10, when horizontal cells also transiently exhibit NFIA immunofluorescence. DNER, by contrast, is present in ganglion and amacrine cells on P1, also labeling the horizontal cells by P10. Developing AII amacrine cells exhibit accumulating DNER labeling at the dendritic stalk, labeling that becomes progressively conspicuous by P10, as it is in maturity. This developmental time course is consistent with a prospective role for each gene in the differentiation of AII amacrine cells.

Mice Homozygous for a Deletion in the Glaucoma Susceptibility Locus INK4 Show Increased Vulnerability of Retinal Ganglion Cells to Elevated Intraocular Pressure

A genomic region located on chromosome 9p21 is associated with primary open-angle glaucoma and normal tension glaucoma in genome-wide association studies. The genomic region contains the gene for a long noncoding RNA called CDKN2B-AS, two genes that code for cyclin-dependent kinase inhibitors 2A and 2B (CDKN2A/p16(INK4A) and CDKN2B/p15(INK4B)) and an additional protein (p14(ARF)). We used a transgenic mouse model in which 70 kb of murine chromosome 4, syntenic to human chromosome 9p21, are deleted to study whether this deletion leads to a discernible phenotype in ocular structures implicated in glaucoma. Homozygous mice of this strain were previously reported to show persistent hyperplastic primary vitreous. Fundus photography and optical coherence tomography confirmed that finding but showed no abnormalities for heterozygous mice. Optokinetic response, eletroretinogram, and histology indicated that the heterozygous and mutant retinas were normal functionally and morphologically, whereas glial cells were activated in the retina and optic nerve head of mutant eyes. In quantitative PCR, CDKN2B expression was reduced by approximately 50% in the heterozygous mice and by 90% in the homozygous mice, which suggested that the CDKN2B knock down had no deleterious consequences for the retina under normal conditions. However, compared with wild-type and heterozygous animals, the homozygous mice are more vulnerable to retinal ganglion cell loss in response to elevated intraocular pressure.

Neuroprotective effect of memantine on the retinal ganglion cells of APPswe/PS1ΔE9 mice and its immunomodulatory mechanisms

Besides the cognitive impairment and degeneration in the brain, vision dysfunction and retina damage are always prevalent in patients with Alzheimer's disease (AD). The uncompetitive antagonist of the N-methyl-d-aspartate receptor, memantine (MEM), has been proven to improve the cognition of patients with AD. However, limited information exists regarding the mechanism of neurodegeneration and the possible neuroprotective mechanisms of MEM on the retinas of patients with AD. In the present study, by using APPswe/PS1ΔE9 double transgenic (dtg) mice, we found that MEM rescued the loss of retinal ganglion cells (RGCs), as well as improved visual impairments, including improving the P50 component in pattern electroretinograms and the latency delay of the P2 component in flash visual evoked potentials of APPswe/PS1ΔE9 dtg mice. The activated microglia in the retinas of APPswe/PS1ΔE9 dtg mice were also inhibited by MEM. Additionally, the level of glutamine synthetase expressed by Müller cells within the RGC layer was upregulated in APPswe/PS1ΔE9 dtg mice, which was inhibited by MEM. Simultaneously, MEM also reduced the apoptosis of choline acetyl transferase-immunoreactive cholinergic amacrine cells within the RGC layer of AD mice. Moreover, the phosphorylation level of extracellular regulated protein kinases 1 and 2 was increased in APPswe/PS1ΔE9 dtg mice, which was blocked by MEM treatment. These findings suggest that MEM protects RGCs in the retinas of APPswe/PS1ΔE9 dtg mice by modulating the immune response of microglia and the adapted response of Müller cells, making MEM a potential ophthalmic treatment alternative in patients with AD.

Expression of voltage-gated calcium channel α(2)δ(4) subunits in the mouse and rat retina

High-voltage activated Ca channels participate in multiple cellular functions, including transmitter release, excitation, and gene transcription. Ca channels are heteromeric proteins consisting of a pore-forming α(1) subunit and auxiliary α(2)δ and β subunits. Although there are reports of α(2)δ(4) subunit mRNA in the mouse retina and localization of the α(2)δ(4) subunit immunoreactivity to salamander photoreceptor terminals, there is a limited overall understanding of its expression and localization in the retina. α(2)δ(4) subunit expression and distribution in the mouse and rat retina were evaluated by using reverse transcriptase polymerase chain reaction, western blot, and immunohistochemistry with specific primers and a well-characterized antibody to the α(2)δ(4) subunit. α(2)δ(4) subunit mRNA and protein are present in mouse and rat retina, brain, and liver homogenates. Immunostaining for the α(2)δ(4) subunit is mainly localized to Müller cell processes and endfeet, photoreceptor terminals, and photoreceptor outer segments. This subunit is also expressed in a few displaced ganglion cells and bipolar cell dendrites. These findings suggest that the α(2)δ(4) subunit participates in the modulation of L-type Ca(2+) current regulating neurotransmitter release from photoreceptor terminals and Ca(2+)-dependent signaling pathways in bipolar and Müller cells.

Oxidative stress and potential applications of free radical scavengers in glaucoma

Glaucoma is the leading cause of irreversible blindness in industrialized countries and comprises a group of diseases characterized by progressive optic nerve degeneration. Glaucoma is commonly associated with elevated intraocular pressure due to impaired outflow of aqueous humor resulting from abnormalities within the drainage system of the anterior chamber angle (open-angle glaucoma) or impaired access of aqueous humor to the drainage system (angle-closure glaucoma). Oxidative injury and altered antioxidant defense mechanisms in glaucoma appear to play a role in the pathophysiology of glaucomatous neurodegeneration that is characterized by death of retinal ganglion cells. Oxidative protein modifications occurring in glaucoma serve as immunostimulatory signals and alter neurosupportive and immunoregulatory functions of glial cells. Initiation of the apoptotic cascade observed in glaucomatous retinopathy can involve oxidant mechanisms and different agents have been shown to be neuroprotective. This review focuses on the molecular mechanisms of oxidant injury and summarizes studies that have investigated novel free radical scavengers in the treatment of glaucomatous neurodegeneration.

Neuronal programmed cell death-1 ligand expression regulates retinal ganglion cell number in neonatal and adult mice

OBJECTIVES

During mouse retina maturation, the final number of retinal ganglion cells (RGCs) is determined by highly regulated programmed cell death. Previous studies demonstrated that the immunoregulatory receptor programmed cell death-1 (PD-1) promotes developmental RGC death. To identify the functional signaling partner(s) for PD-1, we identified retinal expression of PD-1 ligands and examined the effect of PD-1 ligand expression on RGC number. We also explored the hypothesis that PD-1 signaling promotes the development of functional visual circuitry.

METHODS

Characterization of retinal and brain programmed cell death-1 ligand 1 (PD-L1) expression were examined by immunofluorescence on tissue sections. The contribution of PD-ligands, PD-L1, and programmed cell death-1 ligand 2 (PD-L2) to RGC number was examined in PD-ligand knockout mice lacking 1 or both ligands. Retinal architecture was assessed by spectral-domain optical coherence tomography, and retinal function was analyzed by electroretinography in wild-type and PD-L1/L2 double-deficient mice.

RESULTS

PD-L1 expression is found throughout the neonatal retina and persists in adult RGCs, bipolar interneurons, and Müller glia. In the absence of both PD-ligands, there is a significant numerical increase in RGCs (34% at postnatal day 2 [P2] and 18% in adult), as compared to wild type, and PD-ligands have redundant function in this process. Despite the increased RGC number, adult PD-L1/L2 double-knockout mice have normal retinal architecture and outer retina function.

CONCLUSION

This study demonstrates that PD-L1 and PD-L2 together impact the final number of RGCs in adult mice and supports a novel role for active promotion of neuronal cell death through PD-1 receptor-ligand engagement.

Light-induced degeneration and microglial response in the retina of an epibenthonic pigmented teleost: age-dependent photoreceptor susceptibility to cell death

Constant intense light causes apoptosis of photoreceptors in the retina of albino fish. However, very few studies have been performed on pigmented species. Tench (Tinca tinca) is a teleost inhabiting dimly lit environments that has a predominance of rods within the photoreceptor layer. To test the hypothesis that constant high intensity light can result in retinal damage in such pigmented epibenthonic teleost species, photodegeneration of the retina was investigated in the larvae and in juveniles of tench to assess whether any damage may also be dependent on fish age. We exposed both groups of animals to 5 days of constant darkness, followed by 4 days of constant 20,000 lx light, and then by 6 days of recovery in a 14 h light:10 h dark cycle. The results showed that the retina of the larvae group exhibited abundant photoreceptor cell apoptosis during the time of exposition to intense light, whereas that of juveniles was indifferent to it. Damaged retinas showed a strong TUNEL signal in photoreceptor nuclei, and occasionally a weak cytoplasmic TUNEL signal in Müller glia. Specific labelling of microglial cells with Griffonia simplicifolia lectin (GSL) histochemistry revealed that photoreceptor cell death alerts microglia in the degenerating retina, leading to local proliferation, migration towards the injured outer nuclear layer (ONL), and enhanced phagocytosis of photoreceptor debris. During the first days of intense light treatment, Müller cells phagocytosed dead photoreceptor cells but, once microglial cells became activated, there was a progressive increase in the phagocytic capacity of the microglia.

A novel role for α-tocopherol transfer protein (α-TTP) in protecting against chloroquine toxicity

Chloroquine (CQ) is a widely prescribed anti-malarial agent and is also prescribed to treat autoimmune diseases. Clinical treatment with CQ is often accompanied by serious side effects such as hepatitis and retinopathy. As a weak base, CQ accumulates in intracellular acidic organelles, raises the pH, and induces osmotic swelling and permeabilization of acidic organelles, which account for CQ-induced cytotoxicity. We reported previously that CQ treatment caused α-tocopherol transfer protein (α-TTP), a gene product of familial vitamin E deficiency, to change its location from the cytosol to the surface of acidic organelles. Here we show that α-TTP plays a novel role in protecting against CQ toxicity both in vitro and in vivo. In the presence of CQ, rat hepatoma McARH7777 cells, which do not express α-TTP endogenously, showed more severe cytotoxicity, such as larger vacuolation of acidic organelles and caspase activation, than α-TTP transfectant cells. Similarly, α-TTP knockout mice showed more severe CQ toxicity, such as hepatotoxicity and retinopathy, than wild-type mice. These effects were not ameliorated by vitamin E supplementation. In contrast to bafilomycin A1 treatment, which prevents CQ accumulation in cells by raising the pH of acidic organelles, α-TTP expression prevented CQ accumulation without affecting the pH of acidic organelles. Taken together, our data suggest that α-TTP protects against CQ toxicity by preventing CQ accumulation in acidic organelles through a mechanism distinct from vitamin E transport.

Effect of high hydrostatic pressure on the expression of glutamine synthetase in rat retinal Müller cells cultured in vitro

The aim of the present study was to examine the expression of glutamine synthetase (GS) in rat retinal Müller cells induced by different levels of hydrostatic pressure using a novel pressure mechanism. pH, PCO(2) or PO2 in culture medium as determined by gas analysis was used to examine the pressure mechanism. GS expression in the Müller cells at different levels of hydrostatic pressure (0, 20, 40, 60 and 80 mmHg/24 h) was examined using immunofluorescence, real-time PCR and Western blotting. There was no significant difference in pH, PCO(2) or PO(2) in the culture medium by gas analysis at the different hydrostatic pressure levels (p>0.05). Immunofluorescence staining showed that GS was expressed in the Müller cells. The expression of GS in the 40 mmHg/24 h and the 60 mmHg/24 h groups was increased significantly compared to that in the 0 mmHg/24 h group (p<0.05). These results suggest that the pressure mechanism which was constructed was effective and that moderate pressure promotes the up-regulation of GS in active Müller cells.

Cell type-specific and light-dependent expression of Rab1 and Rab6 GTPases in mammalian retinas

The Ras-like Rab1 and Rab6 GTPases modulate protein traffic along the early secretory pathway and are involved in the regulation of maturation of rhodopsin in the outer retina. However, Rab GTPases have not been studied in the inner retinas. Here, we analyzed the anatomatic distribution and expression of Rab1 and Rab6 in the mouse and rat retinas by immunohistochemistry and immunoblotting. We found that Rab1 was specifically expressed in the rod bipolar cells, while Rab6 was expressed in a different cell type(s) from rod bipolar cells in the inner retina. We also demonstrated that expression of Rab1 and Rab6 was increased with light. These data provided the first evidence implicating that Rab1 and Rab6 may be involved in the regulation of the retinal adaptation.

Sox9 is expressed in mouse multipotent retinal progenitor cells and functions in Müller glial cell development

It is widely accepted that the process of retinal cell fate determination is under tight transcriptional control mediated by a combinatorial code of transcription factors. However, the exact repertoire of factors necessary for the genesis of each retinal cell type remains to be fully defined. Here we show that the HMG-box transcription factor, Sox9, is expressed in multipotent mouse retinal progenitor cells throughout retinogenesis. We also find that Sox9 is downregulated in differentiating neuronal populations, yet expression in Müller glial cells persists into adulthood. Furthermore, by employing a conditional knockout approach, we show that Sox9 is essential for the differentiation and/or survival of postnatal Müller glial cells.

Novel distribution of junctional adhesion molecule-C in the neural retina and retinal pigment epithelium

Junction adhesion molecules-A, -B, and -C (Jams) are cell surface glycoproteins that have been shown to play an important role in the assembly and maintenance of tight junctions and in the establishment of epithelial cell polarity. Recent studies reported that Jam-C mRNA was increased threefold in the all-cone retina of the Nrl(-/-) mouse, suggesting that Jam-C is required for maturation and polarization of cone photoreceptors cells. We examined the expression of Jams in the mouse retina by using confocal immunofluorescence localization. Jam-C was detected in tight junctions of retinal pigment epithelium (RPE) and at the outer limiting membrane (OLM) in the specialized adherens junctions between Müller and photoreceptor cells. Additionally, Jam-C labeling was observed in the long apical processes of Müller and RPE cells that extend between the inner segments and outer segments of photoreceptors, respectively. Jam-B was also detected at the OLM. In the developing retina, Jam-B and -C were detected at the apical junctions of embryonic retinal neuroepithelia, suggesting a role for Jams in retinogenesis. In eyes from Jam-C(-/-) mice, retinal lamination, polarity, and photoreceptor morphology appeared normal. Although Jam-A was not detected at the OLM in wild-type retinas, it was present at the OLM in retinas of Jam-C(-/-) mice. These findings indicate that up-regulation of Jam-A in the retina compensates for the loss of Jam-C. The nonclassical distribution of Jam-C in the apical membranes of Müller cells and RPE suggests that Jam-C has a novel function in the retina.

Oxidative stress in glaucomatous neurodegeneration: mechanisms and consequences

Reactive oxygen species (ROS) are generated as by-products of cellular metabolism, primarily in the mitochondria. Although ROS are essential participants in cell signaling and regulation, when their cellular production overwhelms the intrinsic antioxidant capacity, damage to cellular macromolecules such as DNA, proteins, and lipids ensues. Such a state of "oxidative stress" is thought to contribute to the pathogenesis of a number of neurodegenerative diseases. Growing evidence supports the involvement of oxidative stress as a common component of glaucomatous neurodegeneration in different subcellular compartments of retinal ganglion cells (RGCs). Besides the evidence of direct cytotoxic consequences leading to RGC death, it also seems highly possible that ROS are involved in signaling RGC death by acting as a second messenger and/or modulating protein function by redox modifications of downstream effectors through enzymatic oxidation of specific amino acid residues. Different studies provide cumulating evidence, which supports the association of ROS with different aspects of the neurodegenerative process. Oxidative protein modifications during glaucomatous neurodegeneration increase neuronal susceptibility to damage and also lead to glial dysfunction. Oxidative stress-induced dysfunction of glial cells may contribute to spreading neuronal damage by secondary degeneration. Oxidative stress also promotes the accumulation of advanced glycation end products in glaucomatous tissues. In addition, oxidative stress takes part in the activation of immune response during glaucomatous neurodegeneration, as ROS stimulate the antigen presenting ability of glial cells and also function as co-stimulatory molecules during antigen presentation. By discussing current evidence, this review provides a broad perspective on cellular mechanisms and potential consequences of oxidative stress in glaucoma.

Gap junctional coupling and connexin immunoreactivity in rabbit retinal glia

Gap junctions provide a pathway for the direct intercellular exchange of ions and small signaling molecules. Gap junctional coupling between retinal astrocytes and between astrocytes and Müller cells, the principal glia of vertebrate retinas, has been previously demonstrated by the intercellular transfer of gap-junction permeant tracers. However, functional gap junctions have yet to be demonstrated between mammalian Müller cells. In the present study, when the gap-junction permeant tracers Neurobiotin and Lucifer yellow were injected into a Müller cellviaa patch pipette, the tracers transferred to at least one additional cell in more than half of the cases examined. Simultaneous whole-cell recordings from pairs of Müller cells in the isolated rabbit retina revealed electrical coupling between closely neighboring cells, confirming the presence of functional gap junctions between rabbit Müller cells. The limited degree of this coupling suggests that Müller cell–Müller cell gap junctions may coordinate the functions of small ensembles of these glial cells. Immunohistochemistry and immunoblotting were used to identify the connexins in rabbit retinal glia. Connexin30 (Cx30) and connexin43 (Cx43) immunoreactivities were associated with astrocytes in the medullary ray region of the retinas of both pigmented and albino rabbits. Connexin43 was also found in Müller cells, but antibody recognition differed between astrocytic and Müller cell connexin43.

Carbonic anhydrase XIV identified as the membrane CA in mouse retina: strong expression in Müller cells and the RPE

The presence of carbonic anhydrase (CA) activity in the neural retina has been known for several decades. CA-II, a soluble cytoplasmic isoform expressed by Müller cells and a subset of amacrine cells, was thought to be the sole source of CA activity in the neural retina. However, CA-II deficient mice retain CA activity in the neural retina, which implies that another isoform must be present in that tissue. Recently CA-XIV, an integral membrane protein, was cloned and characterized. We, therefore, sought to determine whether CA-XIV is expressed in the neural retina, and hence is responsible for the CA activity observed in CA-II null animals. Immunohistochemical analyses of histological sections from CA-II null, CA-XIV null, and control mice were performed to localize the CA-XIV isoform, as well as other known retinal markers. Immunoblotting and real-time RT-PCR analyses were also performed to test for CA-XIV expression in retina and other mouse tissues. We determined herein that CA-XIV, a approximately 45kDa membrane protein, is expressed in retina, as it is in kidney. In the retina, CA-XIV is expressed on the plasma membrane of Müller cells. CA-XIV is also found on both the apical and basal membranes of the retinal pigmented epithelium. The data presented here indicate that like CA-II, CA-XIV is highly expressed in the neural retina and, like CA-II, more specifically by the Müller cells. The cellular compartmentalization of the two isoforms in the Müller cell-one cytoplasmic and the other on the plasma membrane-suggest that the two enzymes have specific and unique functions. Future studies will be necessary to assign functions to CA-II and CA-XIV in the mouse neural retina.

Cloning, immunolocalization, and functional expression of a GABA transporter from the retina of the skate

Termination of GABA signals within the retina occurs through high-affinity reuptake of the released neurotransmitter by GABA transporters (GATs) present in neurons and glia surrounding the release site. In the present work, we have cloned a novel GAT from the retina of the skate (Raja erinacea). The clone codes for a 622 amino acid protein whose sequence has highest similarity to the GABA/β-alanine transporter of the electric ray (Torpedo marmorata) (88% identity) and the GAT-3 isolated from rat brain (75% identity). The protein was expressed inXenopusoocytes and characterized using the two-electrode voltage-clamp technique. Application of GABA induced a dose-dependent inward current, with 8 μM GABA producing a half-maximal response. The current required the presence of extracellular sodium and was unaffected by the GABA receptor blocker picrotoxin or the GAT-1 specific antagonist NO-711. The high homology between the cloned skate GABA transporter and the GAT-3 equivalents of other species, coupled with the strikingly similar pharmacological profile to GAT-3s of other species, lead us to conclude that we had cloned the GAT-3 homologue for the skate. Polyclonal antibodies specific to GAT-3 and the previously cloned skate GAT-1 transporter were used to examine the distribution of GAT-3 and GAT-1 immunoreactivity in the retina and in isolated cells of the skate. Antibodies for both transporters showed labeling in the outer and inner plexiform layers, and staining extended from the outer to inner limiting membranes. Both GAT-1 and GAT-3 antibodies labeled enzymatically isolated Müller cells, while bipolar cells and horizontal cells did not appear to express either transporter. These results imply that GAT-1 and GAT-3 are both present in Müller cells of the skate retina where they are likely involved in regulating extracellular concentrations of GABA.

Development of cholinergic amacrine cells is visual activity-dependent in the postnatal mouse retina

In the present study, we used immunocytochemistry to study the temporal and spatial arrangement of mouse cholinergic amacrine cells during postnatal retinal development under normal light/dark cycles and during visual deprivation. Choline acetyltransferase (ChAT)-immunolabeled cells were detected in the neuroblastic layer (NBL) and in the ganglion cell layer (GCL) at postnatal day 0 (P0). Between P3-5, two characteristic cholinergic bands were clearly identified in the inner plexiform layer (IPL). The signal intensity of somas and processes progressively increased over the first 2 postnatal weeks. Around eye opening at P12, cholinergic neurons were mature-like. This early developmental process was not altered by visual deprivation. After eye opening, the space between the two cholinergic bands increased continuously and the spatial regularity index changed constantly, indicating that the cholinergic neurons possibly underwent refinement during later postnatal development. The changes occurring following eye opening were retarded by visual deprivation. The morphologies of photoreceptors, horizontal cells, recoverin-positive OFF-cone bipolar cells, rod bipolar cells, dopaminergic amacrine cells, and Müller cells appeared normal. Their stratification in the outer plexiform layer (OPL) and the IPL was not affected by visual deprivation. However, glial cells grew vertically across the entire thickness of dark-reared retinas. Our results suggest that the development of cholinergic neurons before eye opening is independent of the lighting conditions. Their development after eye opening is greatly impeded by visual deprivation. This visual activity-dependent phase of development may be a critical period for the maturation and synaptic wiring of cholinergic amacrine cells in the mammalian retina.

Distribution of Rab GTPases in mouse kidney and comparison with vacuolar H+-ATPase

BACKGROUND

Vacuolar H+-ATPases (V-ATPases) are essential for renal bicarbonate transport in both the proximal and distal nephron. Regulation of proton transport occurs, in part, by vesicle-mediated traffic of V-ATPases between intracellular vacuoles and the plasma membrane. Although the proteins involved in regulated V-ATPase traffic are largely unknown, Rab GTPases have a central role in the traffic and recycling of other membrane proteins.

METHODS

To identify candidate Rab GTPases potentially involved in V-ATPase traffic, immunocytochemical and subcellular fractionation studies were used to evaluate the distribution to sites of abundant V-ATPase of 5 Rab GTPases expressed in kidney, Rab5a, Rab11, Rab13, Rab18, and Rab20.

RESULTS

The immunocytochemical distribution of Rab5a and Rab13 and the subcellular distribution of Rab18 were not compatible with a role in V-ATPase traffic. In contrast, Rab11 colocalized with V-ATPase in apical regions of proximal tubule, and Rab20 colocalized with the enzyme in intercalated cells. Rab11 and Rab20 were enriched in membrane fractions that were also enriched in V-ATPase B2 and B1 subunit isoforms, respectively.

CONCLUSIONS

The immunohistochemical data in combination with the membrane fractionation studies are consistent with a potential role for Rab11 and Rab20 in regulating V-ATPase traffic in specific segments of the nephron.

Membrane-associated guanylate kinase proteins MPP4 and MPP5 associate with Veli3 at distinct intercellular junctions of the neurosensory retina

MPP4 and MPP5 are closely related members of the p55-subfamily of membrane-associated guanylate kinases (MAGUKs) known to mediate the assembly of protein complexes at the plasma membrane of cell-cell junctions. Both MPP4 and MPP5 have been implicated in retinal function; however, their specific roles in the cellular mechanisms underlying vision are largely unknown. Here, we generated specific poly- and monoclonal antibodies against the two proteins and show that MPP4 and MPP5 are localized at distinct sites of cell-cell contact in the mouse retina. While MPP4 is a component of the synaptic terminals of photoreceptors, MPP5 exclusively localizes to apical membrane domains of the outer limiting membrane (OLM) junctions. The vertebrate homologs of Caenorhabditis elegans lin-7, Veli1, -2, and -3, have previously been identified as putative binding partners of MPP5. In this study, we show that MPP4 directly interacts with the Veli proteins via L27 heterodimerization in vitro. In addition, two of the three Veli isoforms, Veli1 and -3, are demonstrated to be expressed in the mouse retina. Immunofluorescence microscopy reveals extensive colocalization of Veli3 with both MPP4 and MPP5. This association of Veli3 with either MPP4 or MPP5 suggests that the MAGUKs recruit Veli3 and its binding partners to different cellular regions of the retina where they may participate in the organization of specialized intercellular junctions.

Under stress, the absence of intermediate filaments from Müller cells in the retina has structural and functional consequences

In epithelial and muscle cells, intermediate filaments (IFs) are important for resistance to mechanical stress. The aim of this study was to elucidate whether IFs are also important for providing resistance to mechanical stress in the Müller cells of the retina and whether this has any pathophysiological consequences. We used mice deficient in IF proteins glial fibrillary acidic protein and/or vimentin (GFAP(-/-), Vim(-/-) and GFAP(-/-) Vim(-/-)), and stress on the retina was applied by excision of the eyes immediately post mortem (compared with in situ fixation) or by inducing a neovascular response to oxygen-induced retinopathy (OIR). The structure of unchallenged retinas was normal, but mechanical stress caused local separation of the inner limiting membrane (ILM) and adjacent tissue from the rest of the retina in GFAP(-/-) Vim(-/-) mice and, to a lesser extent, in Vim(-/-) mice. This detachment occurred within the endfeet of Müller cells, structures normally rich in IFs but IF-free in GFAP(-/-) Vim(-/-) mice. Hypoxia-induced neovascularization was comparable in all groups of mice with respect to the retinal surface area occupied by new vessels. However, the vessels traversed the ILM and penetrated the vitreous body less frequently than in wild-type retinas (31-55% in Vim(-/-), 66-79% in GFAP(-/-) Vim(-/-)). We conclude that IFs are important for maintaining the mechanical integrity of Müller-cell endfeet and the inner retinal layers under a mechanical challenge. Furthermore, the absence of IFs in Müller cells leads to an abnormal response of the vascular system to ischemia, specifically decreased ability of newly formed blood vessels to traverse the ILM.

Localization of the ammonium transporter proteins RhBG and RhCG in mouse kidney

Ammonia is both produced and transported by renal epithelial cells, and it regulates renal ion transport. Recent studies have identified a family of putative ammonium transporters; mRNA for two members of this family, Rh B-glycoprotein (RhBG) and Rh C-glycoprotein (RhCG), is expressed in the kidney. The purpose of this study was to determine the cellular location of RhBG and RhCG protein in the mouse kidney. We generated RhBG- and RhCG-specific anti-peptide antibodies. Immunoblot analysis confirmed that both proteins were expressed in the mouse kidney. RhBG localization with immunohistochemistry revealed discrete basolateral labeling in the connecting segment (CNT) and in the majority of initial collecting tubule (ICT) and cortical collecting duct (CCD) cells. In the outer medullary collecting duct (OMCD) and inner medullary collecting duct (IMCD) only a subpopulation of cells exhibited basolateral immunoreactivity. Colocalization of RhBG with carbonic anhydrase II, the thiazide-sensitive transporter, and the anion exchangers AE1 and pendrin demonstrated RhBG immunoreactivity in all CNT cells and all CCD and ICT principal cells. In the ICT and CCD, basolateral RhBG immunoreactivity is also present in A-type intercalated cells but not in pendrin-positive CCD intercalated cells. In the OMCD and IMCD, only intercalated cells exhibit RhBG immunoreactivity. Immunoreactivity for a second putative ammonium transporter, RhCG, was present in the apical region of cells with almost the same distribution as RhBG. However, RhCG immunoreactivity was present in all CCD cells, and it was present in outer stripe OMCD principal cells, in addition to OMCD and IMCD intercalated cells. Thus the majority of RhBG and RhCG protein expression is present in the same epithelial cell types in the CNT and collecting duct but with opposite polarity. These findings suggest that RhBG and RhCG may play important and cell-specific roles in ammonium transport and signaling in these regions of the kidney.

Connexin immunoreactivity in glial cells of the rat retina

The rat retina contains two types of macroglial cells, Müller cells, radial glial cells that are the principal macroglial cells of vertebrate retinas, and astrocytes associated with the surface vasculature. In addition to the often-described gap-junctional coupling between astrocytes, coupling also occurs between astrocytes and Müller cells. Immunohistochemistry and confocal microscopy were used to identify connexins in the retinas of pigmented rats. Several antibodies directed against connexin43 stained astrocytes, identified using antibodies directed against glial fibrillary acidic protein (GFAP). In addition, two connexin43 antibodies stained Müller cells, identified with antibodies directed against S100 or glutamine synthetase. Connexin30-immunoreactive puncta were confined to the vitreal surface of the retina and colocalized with GFAP-immunoreactive astrocyte processes. Connexin45 immunoreactivity was associated with both astrocytes and Müller cells. We conclude that retinal glial cells express multiple connexins, and the patterns of immunostaining that we observe in this study are consistent with the expression of connexins30, -43, and possibly -45 by astrocytes and the expression of connexins43 and -45 by Müller cells. As gap-junction channels may be formed by both homotypic and heterotypic hemichannels, and the hemichannels may themselves be homomeric or heteromeric, there exists a multitude of possible gap-junction channels that could underlie the homotypic coupling between retinal astrocytes and the heterotypic coupling between astrocytes and Müller cells.

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

The electrophysiology of murine Müller cells, and of their precursors, during postnatal development was investigated by using the whole-cell patch-clamp technique. Membrane potential, membrane capacitance, and expression of voltage-gated Na+ currents increased during the first 3 postnatal weeks. During the same period, the membrane resistance decreased due to an upregulation of both inward and outward K+ currents. Glutamate transporter-mediated currents increased during the first postnatal weeks, as well. In Müller (precursor) cells from rabbits, these transporter currents already achieved adult levels at postnatal day 6, the earliest stage studied. Together with the developmental time course of K+ currents, this indicates a delay in the maturation of murine Müller cells, as compared with cells from rabbit.

Exocytosed protons feedback to suppress the Ca2+ current in mammalian cone photoreceptors

A proton pump acidifies synaptic vesicles and provides the electrochemical gradient for transmitter uptake. Although external protons can modulate membrane voltage- and ligand-gated conductances, the fate of the protons released when vesicles fuse with the plasma membrane is unclear. In the dark, the glutamate-laden vesicles of cone photoreceptors fuse continuously with the plasma membrane. I now show that vesicular protons feed back to block the nearby calcium channels that mediate release. This local proton-mediated feedback is a novel mechanism through which neurons may regulate the release of transmitter.

Retinal degeneration following failed photoreceptor maturation in 5A11/basigin null mice

5A11/Basigin is a member of the immunoglobulin gene superfamily which plays an important role in cell-cell interactions in the developing neural retina. These studies were initiated to investigate the distribution of 5A11/Basigin within the mouse retina, as well as the cytoarchitectural and biochemical effects on the retina after the inactivation of the 5A11/Basigin gene in a mouse strain. Immunocytochemical analyses indicated that mouse 5A11/Basigin is located on the surface of Müller cells, the apical and basal surfaces of the retinal pigmented epithelium, and blood vessels. Lower expression levels were found on photoreceptor cell bodies and a portion of the inner segments. Inactivation of the 5A11/Basigin gene in mice resulted in the failure of photoreceptor cells to fully mature. This failed development eventually lead to the degeneration, death and removal of most of the photoreceptors several months after birth. Biochemical analyses indicated that expression of Müller cell specific proteins, including glutamine synthetase and carbonic anhydrase-II, was not effected; however, opsin protein expression never achieved normal adult levels in the 5A11/Basigin null mice. Also, 5A11/Basigin null retinas were considered 'reactive' based on elevated glial fibrillary acidic protein expression. The results presented here suggest that 5A11/Basigin expression on Müller cells and/or the retinal pigmented epithelium is necessary for photoreceptor outer segment biochemical development and structural maintenance. However, the exact role that 5A11/Basigin plays during retinal development remains to be determined.

Müller cell differentiation in the zebrafish neural retina: evidence of distinct early and late stages in cell maturation

The vertebrate neural retina is mainly composed of cells of neuroectodermal origin. The primary cell types found in all vertebrate retinas are several categories of neurons and the archetypical retina glial cell the Müller cell. Although the neurons and the single glial cell type of the retina are specialized for very distinct functions, they all have a common developmental origin within the tissue. How the distinctions between cell types, in particular between neurons and glia, arise during embryonic development remains a central issue in neurobiology. In this report, we examine the genesis of Müller glial cells during zebrafish (Danio rerio) eye development. Particular emphasis is placed on the expression of the Müller cell maturation markers carbonic anhydrase and glutamine synthetase. In addition, we report that the HNK-1 monoclonal antibody, which identifies a particular glycoconjugate frequently found on cell surface recognition molecules, also identifies zebrafish retina Müller cells early in development. The expression patterns of these three markers clearly show that the Müller cells mature in stages: HNK-1 labeling and glutamine synthetase arise earlier than carbonic anhydrase expression. In addition, the embryonic zebrafish neural retina is characterized by the presence of amoeboid, carbonic anhydrase-positive microglial cells even before the genesis of retinal neuroectodermal glia. The stepwise maturation of the glia is likely to be indicative of an overall retinal maturational program in which cell differentiation and the expression of certain phenotype-defining gene products may be separately regulated.

Müller cells in developing rats with inherited retinal dystrophy

Morphology and enzymes of Müller cells in the developing retina of RCS (Royal College of Surgeons) rats were investigated. RCS (rdy/rdy) rats with inherited retinal dystrophy were studied and RCS (-/+) rats served as normal controls. Rats underwent intracardiac perfusion with 4% paraformaldehyde and the eyes were enucleated on postnatal days P1, 4,10, 21, 35, and 100. Eyes were then fixed with 4% paraformaldehyde, and silver enhancing technique was applied to show glutamine synthetase (GS) and glial fibrillary acidic protein (GFAP). For solubilized retinas, Western blot analysis and enzyme-linked immunosorbent assay (ELISA) were performed to detect GS and GFAP in the extracts. Immunohistochemistry showed GS expression first on P10. It increased later in both normal and dystrophic retinas. GFAP was not expressed in normal retinas, but Müller cells of dystrophic retinas were stained on P35 and P100. GS immunoblots were recognized on P21 and later in both normal and dystrophic retinas with similar densities, while GFAP immunoblots were observed only on P35 and P100, and only in dystrophic retinas. ELISA demonstrated increased GS concentrations with the development in both normal and dystrophic retinas, but no significant difference was observed between them. GFAP concentrations had no significant difference on P21 between both groups, those of normal ones remained unchanged later, while those of dystrophic rats were remarkably increased on P35 and P100. Müller cells might be affected following the progressive degeneration of photoreceptor cells and react to the glio-neuronal relationship.

Late proliferation of retinal Müller cell progenitors facilitates preferential targeting with retroviral vectors in vitro

During vertebrate neural retina development, the relationship between mitotic activity in progenitor cells and the acquisition of a mature cell phenotype remains an area of controversy. The Müller glial cell has long been recognized as one of the last cell types of the retina to mature, which occurs under the influence of cell-cell interactions. In this report we examine the acquisition of the Müller cell phenotype in relation to mitotic activity. Using immunohistochemical markers, we demonstrate that a gene product characteristic of mature Müller cells, the 2M6 antigen, is expressed in mitotically active cells, even after all the major retina architectural features have been laid down. Furthermore, we show that retroviral infection, a process that requires mitotically active cells, preferentially targets Müller cell progenitors when late embryonic retina is infected in vitro. The two lines of evidence are consistent with a model for Müller cell differentiation that includes a mitotically active progenitor that has already begun to express specific differentiation gene products.

Glucocorticoid control of glial gene expression

The glucocorticoid signaling pathway is responsive to a considerable number of internal and external signals and can therefore establish diverse patterns of gene expression. A glial-specific pattern, for example, is shown by the glucocorticoid-inducible gene glutamine synthetase. The enzyme is expressed at a particularly high level in glial cells, where it catalyzes the recycling of the neurotransmitter glutamate, and at a low level in most other cells, for housekeeping duties. Glial specificity of glutamine synthetase induction is achieved by the use of positive and negative regulatory elements, a glucocorticoid response element and a neural restrictive silencer element. Though not glial specific by themselves, these elements may establish a glial-specific pattern of expression through their mutual activity and their combined effect. The inductive activity of glucocorticoids is markedly repressed by the c-Jun protein, which is expressed at relatively high levels in proliferating glial cells. The signaling pathway of c-Jun is activated by the disruption of glia-neuron cell contacts, by transformation with v-src, and in proliferating retinal cells of early embryonic ages. The c-Jun protein inhibits the transcriptional activity of the glucocorticoid receptor and thus represses glutamine synthetase expression. This repressive mechanism might also affect the ability of glial cells to cope with glutamate neurotoxicity in injured tissues.

Ability of retinal Müller glial cells to protect neurons against excitotoxicity in vitro depends upon maturation and neuron-glial interactions

Glutamate is the most abundant excitatory amino acid in the central nervous system. It has also been described as a potent toxin when present in high concentrations because excessive stimulation of its receptors leads to neuronal death. Glial influence on neuronal survival has already been shown in the central nervous system, but the mechanisms underlying glial neuroprotection are only partly known. When cells isolated from newborn rat retina were maintained in culture as enriched neuronal populations, 80% of the cells were destroyed by application of excitotoxic concentrations of glutamate. Massive neuronal death was also observed in newborn retinal cultures containing large numbers of glia, or when neurons were seeded onto feeder layers of purified cells prepared from immature (postnatal 8 day) rat retina. When newborn retinal neurons were seeded onto feeder layers of purified glial cells prepared from adult retinas, application of excitotoxic amino acids no longer led to neuronal death. Furthermore, neuronal death was not observed in mixed neuron/glial cultures prepared from adult retina. However, in all cases (newborn and adult) application of kainate led to amacrine cell-specific death. Activity of glutamine synthetase, a key glial enzyme involved in glutamate detoxification, was assayed in these cultures in the presence or absence of exogenous glutamate. Whereas pure glial cultures alone (from young or adult retina) showed low activity that was not stimulated by glutamate addition, mixed or co-cultured neurons and adult glia exhibited up to threefold higher levels of activity following glutamate treatment. These data indicate that two conditions must be satisfied to observe glial neuroprotection: maturation of glutamine synthetase expression, and neuron-glial signalling through glutamate-elicited responses.

Glutamine immunoreactivity in Müller cells of monkey eyes with experimental glaucoma

The action of glutamate in retina is largely terminated through rapid uptake by Müller cells and subsequent conversion primarily to glutamine. Glutamine, transferred from Müller cells to neurons, serves as a precursor for the formation of glutamate in neurons completing the glutamate-glutamine cycle. In a monkey model of high-tension glaucoma, we have examined glutamine immunoreactivity in the Müller cell as well as the number of Müller cells to determine whether the activity of these cells in the glutamate-glutamine cycle is affected, particularly since high vitreal glutamate has been reported in glaucoma. Unilateral glaucoma was induced in three monkeys by argon laser application to the trabecular meshwork. LR White sections of retina from the temporal mid-periphery (about 23 degrees) and the parafovea (central 3 degrees) were immunolabeled for glutamine using immunogold and silver intensification. The percentage difference in labeling intensity (darkness) in the glaucomatous retina was determined relative to the labeling found in the control retina by image analysis. Ganglion cell density was estimated from radial sections in the parafovea and from retinal whole mounts in the mid-periphery. The number of Müller cells was estimated from vibratome sections immunolabeled by vimentin antibodies in the temporal mid-periphery (about 30 degrees). Glutamine immunoreactivity was localized predominately in ganglion cells and Müller cells. However, the intensity of glutamine immunolabeling was greater in Müller cells of glaucomatous eyes than in control eyes. This increase in glutamine immunolabeling was 25-32% in the temporal mid-periphery and 27-48% in the parafovea. Müller cell number in the glaucomatous eye was similar to that of the control in the temporal mid-periphery. The data in this study indicate that the increase in glutamine in Müller cells is not a consequence of their loss and that Müller cell function in the glutamate-glutamine cycle continues in glaucomatous eyes. These findings are consistent with a previous report that extracellular/vitreal glutamate concentration is elevated in high-tension glaucoma.

Modification of glutamine synthetase expression by mammalian Müller (glial) cells in retinal organ cultures

One of the key enzymes in glial-neuronal transmitter recycling is glutamine synthetase (GS). In the retina, GS is exclusively expressed by glial (Müller) cells where it serves to convert neuron-released active transmitter substances (glutamate and GABA) into glutamine. Experiments on avian retinae have shown that GS expression is developmentally regulated by glucocorticoid hormones and, to a lesser extent, by a non-hormonal control mechanism(s). Much less is known about GS regulation in mammalian retinae, although either increases or decreases of GS immunoreactivity have been observed in Müller cells in different forms of retinal pathologies. We studied GS expression in postnatal rabbit retinae both in vivo and explanted as wholemounts in vitro, using immunocytochemistry and Western immunoblotting. GS expression was detectable in vivo from the fourth postnatal day, and increased rapidly within the first weeks of life. Levels were lower in vitro than in vivo by an order of magnitude, and could be significantly stimulated (> 60-110%) in vitro by application of hydrocortisone, conditioned medium from cultured retinal pigment epithelium and glutamate or ammonia, but not GABA. It is concluded that GS expression in mammalian Müller cells is dependent on systemic control by glucocorticoid hormones, as observed in birds, but environmental (activity-dependent) factors may play a more important role in mammals.

Relationships between Müller cells and neurons in a primitive tetrapod, the Australian lungfish

We recently proposed a model of cytogenesis which assumes that primitive ancestral mammals and premammalian vertebrates had a retinal composition that consisted of about seven neurons per Müller cell, comprising 1-2 cone photoreceptors, 1-2 rod photoreceptors, 2-3 bipolar cells, 1-2 amacrine cells, less than 1 ganglion cell, and less than 1 horizontal cell (Reichenbach & Robinson, 1995). The Australian lungfish (Neoceratodus forsteri) closely resembles the lobe-finned ancestors of land vertebrates, and has an extremely plesiomorphic nervous system. The present study, therefore, has examined the relative frequencies of retinal neurons and Müller cells (identified by immunolabelling for glutamine synthetase) in the lungfish retina. It was found that for each Müller cell there is an average of 1.9 cone photoreceptors, 1.7 rod photoreceptors, 3.1 amacrine/bipolar/horizontal cells, and 0.6 ganglion cells; amounting to a ratio of 7.3 neurons per Müller cell. These results support our conjecture that the sequence of cytogenesis in mammals is constrained by a developmental program that predates the evolution of mammals. The study also provides the first detailed morphological descriptions of lungfish Müller cells and their relationship with adjacent neurons. It was found that individual Müller cells in lungfish have a volume (more than 12,000 microns3) that is an order of magnitude higher than in mammals, yet the proportion of total retinal volume occupied by these cells (20%) is very similar.

Hepatic retinopathia. Changes in retinal function

In patients suffering from hepatic failure, the brain is subject to defined morphological and functional changes known as hepatic encephalopathia (HE). The morphological changes are dominated by glial cells (Alzheimer-type II astrocytes). It has recently been possible to demonstrate, that the retinal glia (Müller) cells undergo similar morphological changes. The present study was carried out in order to reveal if these Müller cell changes cause any characteristic functional deficits. We examined 11 patients with different stages of HE due to liver cirrhosis. Six patients were at stage 0 or 1 (group I) and five at stage 2 or 3 (group II). They underwent ophthalmological routine examination, colour vision testing and standard ERG recording. None of the patients reported impaired vision, in daylight or at night. There were no fundus abnormalities except very mild changes of the pigment epithelium and abnormal reflexes of the inner limiting membrane, especially in the higher HE stages. The number of confusions in the colour arrangement test increased with the higher stages of HE, preferably in the tritan axis. The scotopic a- and b-waves of the electroretinogram (ERG) were almost unchanged in group I and significantly decreased and delayed in group II. The photopic ERG b-wave amplitudes were changed in a similar fashion. Oscillatory potentials proved to be most sensitive to hepatotoxic changes. Their latencies were significantly delayed even in group I. Amplitudes were decreased significantly only in group II. Patients suffering from hepatic failure and accompanying HE display functional abnormalities of the retina. These are best demonstrated by the ERG, and correlate well with the degree of HE. A hypothesis is presented that relates the observed functional changes to altered neurotransmitter levels and impaired retinal glial-neuronal interaction, due to Müller cell damage caused by elevated ammonia levels.

The Müller cell: a functional element of the retina

Müller cells are the principal glial cells of the retina, assuming many of the functions carried out by astrocytes, oligodendrocytes and ependymal cells in other CNS regions. Müller cells express numerous voltage-gated channels and neurotransmitter receptors, which recognize a variety of neuronal signals and trigger cell depolarization and intracellular Ca2+ waves. In turn, Müller cells modulate neuronal activity by regulating the extracellular concentration of neuroactive substances, including: (1) K+, which is transported via Müller-cell spatial-buffering currents; (2) glutamate and GABA, which are taken up by Müller-cell high-affinity carriers; and (3) H+, which is controlled by the action of Müller-cell Na(+)-HCO3- co-transport and carbonic anhydrase. The two-way communication between Müller cells and retinal neurons indicates that Müller cells play an active role in retinal function.

Coincidence of L-glutamate/L-aspartate transporter (GLAST) and glutamine synthetase (GS) immunoreactions in retinal glia: evidence for coupling of GLAST and GS in transmitter clearance

Our aim was to identify proteins that mediate the uptake and degradation of synaptically released glutamate, focusing on the rat retina with its well-defined glutamatergic pathways. Immunoreactivity against the L-glutamate/L-aspartate transporter (GLAST) is present in Müller cells. Ultrastructurally, even the finest glial processes, particularly those ensheathing identified structures of glutamatergic transmission (rod spherules), are immunoreactive for GLAST. Further light and electron microscopic observations revealed that also retinal astrocytes and pigment epithelial cells are immunoreactive for GLAST. No neuronal or microglial staining was observed. This is in line with uptake of exogenous [3H]glutamate previously localized specifically in Müller cells and pigment epithelium (Ehinger and Falck: Brain Res 33:157-172, 1971). Since endogenous glutamate can only be demonstrated in Müller cells if glutamine synthetase (GS) is inhibited (Pow and Robinson: Neuroscience 60:355-366, 1994), the immunocytochemical localization of GS was determined. GS immunoreactivity was found in all but only those cell types immunoreactive for GLAST. The light and electron microscopic patterns of immunoreactivity were very similar, particularly in the outer plexiform layer. The three cell types containing both GS and GLAST (Müller cells, astrocytes, and retinal pigment epithelium) are related developmentally. In the light of the two references quoted the present data indicate that the proteins mediating retinal uptake and degradation of synaptically released glutamate may be GLAST and GS, respectively, and that they may operate in concert to terminate the neurotransmitter action of glutamate.

Hepatic retinopathy: morphological features of retinal glial (Müller) cells accompanying hepatic failure

More than 80 years ago, Alzheimer described changes in the brains of patients who had suffered hepatic failure. Astrocytes are primarily affected; their nuclei become swollen, their intermediate filament protein composition is altered and their cytoplasm becomes vacuolated. Cells with these features are called Alzheimer type II astrocytes and these changes have been attributed to the toxic effects of elevated ammonia levels. The present study investigates whether the dominant glia of another part of the central nervous system, the Müller cells of the retina, undergo similar changes. Retinae of patients who had died with symptoms of hepatic failure were processed for histology, histochemistry, and immunocytochemistry. Cell nuclei were measured from brain astrocytes (insula cortex), Müller cells, and retinal bipolar neurons. Hepatic failure resulted in the enlargement of nuclei in astrocytes and Müller cells, and the enhanced expression in Müller cells of glial fibrillary acidic protein, cathepsin D, and the beta-subunit of prolyl 4-hydroxylase (glial-p55). In some retinae, signs of gliosis were also observed. We conclude that increased levels of serum ammonia resulting from hepatic insufficiency cause changes in Müller cells that are similar to those seen in brain astrocytes. We term this condition hepatic retinopathy.

Effects of Müller cell disruption on mouse photoreceptor cell development

Müller cells have been proposed to play an important role in photoreceptor cell development during the final stages of retinal maturation. The effect of disrupting Müller cells during mouse retinal development was investigated using the specific glial cell toxin, DL-alpha-aminoadipic acid (AAA). By giving multiple systemic injections over several days, impairment of Müller cell function was maintained during the period of photoreceptor migration and differentiation. Following three consecutive days of AAA treatment [commencing on post-natal (P) day 3, 5, 7 or 9, and examined at P8-P14], clumps of photoreceptor nuclei were displaced through the inner segments, lying immediately beneath the retinal pigment epithelium (RPE). Apart from the scalloped appearance of the outer retina, the overall lamination pattern of the retina was relatively well preserved. Even when AAA treatment commenced as early as P3, several days prior to the formation of the outer nuclear layer, the majority of photoreceptors migrated to their correct position and formed inner and outer segments. Therefore, the signals for photoreceptor migration are either provided by the Müller cells prior to P3, or, alternatively, are derived from different intrinsic or extrinsic cues. Disruption of Müller cell function was evidenced by decreased glutamine synthetase activity as well as by increased glial fibrillary acidic protein (GFAP) and decreased cellular retinaldehyde-binding protein (CRALBP) immunoreactivity. Immunocytochemistry with an antibody to CD44, which labels the microvilli of Müller cells at the outer limiting membrane, coupled with electron microscopic analysis, demonstrated that the zonulae adherentes between Müller cells and photoreceptors were either irregular or absent in areas adjacent to displaced clumps of photoreceptors. Thus AAA treatment of early post-natal mice results in localized disruption of the contacts between Müller cells and photoreceptors. These pathologic changes persist into adulthood since at P28, while short stretches of photoreceptors appeared relatively normal with fully developed outer segments, periodic clumps of displaced photoreceptor nuclei were still present adjacent to the RPE. In conclusion, Müller cell processes at the outer limiting membrane appear to play a critical role in providing a barrier to aberrant photoreceptor migration into the subretinal space.

Alterations of Müller (glial) cells in dystrophic retinae of RCS rats

We have carried out a light microscopical study of Müller cells in the retinae of rats with inherited retinal dystrophy (Royal College of Surgeons rats). Isolated retinae of both control and Royal College of Surgeons rats were exposed to a Procion Yellow solution which is taken up selectively into Müller cells. The shape of the cells was then studied by confocal microscopy. Enzymatically isolated Müller cells were studied immunocytochemically with antibodies against glial fibrillary acidic protein, cathepsin D, beta-amyloid precursor protein, bcl-2 protooncogene product, and glutamine synthetase. Müller cells from RCS retinae were shorter than those from control retinae, and showed a coarse hypertrophy of their distal (sclerad) processes. In Müller cells isolated from the retinae of Royal College of Surgeon's rats, the expression of glial fibrillary acidic protein, cathepsin D, beta-amyloid precursor protein and bcl-2 protooncogene product was increased, and the expression of glutamine synthetase was reduced. Obviously, loss of neighbouring neurons leads to major alterations of both the shape and metabolism of Müller cells. The expression of enzymes that serve functional glio-neuronal interactions, such as glutamine synthetase, seems to be down-regulated, whereas proteins involved in cell reconstruction (cathepsin D), cell repair (possibly beta-amyloid precursor protein), and protection against apoptotic cell death (bcl-2 protooncogene product), are up-regulated, together with the 'pathological marker' glial fibrillary acidic protein.

Effects of enhanced extracellular ammonia concentration on cultured mammalian retinal glial (Müller) cells

Müller (glial) cells of the neonatal rabbit retina were cultured as confluent monolayers and exposed to enhanced concentrations of ammonia (0.25, 0.5, 1, 3, 7, and 10 mM) in medium for various periods (30 min to 10 d). This caused, in a time- and dose-dependent manner, similar changes in the Müller cells as had previously been described in cultured astrocytes. The most conspicuous events were 1) an increasing size of cell nuclei, 2) an accumulation of phagocytotic vacuoles, and 3) a rearrangement of intermediate filaments. 4) A considerable number of cells died when higher ammonia concentrations were applied for more than 1 h. Simultaneous application of dibutyryl-cyclic adenosine monophosphate (dBcAMP) prevented almost completely both the increase in cell nucleus size and the changes of intermediate filaments, but only partly the early cell death of a subpopulation of cells, and the accumulation of phagocytotic vacuoles. Further changes evoked by enhanced ammonia concentration were 5) an accumulation of lipofuscin-like material ("fatty degeneration") revealed by lipophilic stain, 6) reduced immunoreactivity for cathepsin D, and increased immunoreactivity for 7) glial fibrillary acidic protein, 8) glutamine synthetase, and 9) bcl-2 protooncogene protein. These findings are discussed in respect to the possible underlying pathophysiological mechanisms.

A physiological measure of carbonic anhydrase in Müller cells

Carbonic anhydrase activity was characterized in freshly dissociated Müller cells of the salamander retina. Intracellular pH was monitored using ratio imaging of the indicator dye BCECF as extracellular PCO2 was varied. The extracellular solution was switched rapidly (141 ms rise time) from a HEPES buffered to a CO2-HCO3- buffered solution (both pH 7.4). Introduction of CO2-HCO3- produced a rapid cell acidification. Cell pH dropped from a steady-state pH of 7.02 in HEPES solution to pH 6.81 in CO2-HCO3-. Methazolamide, a carbonic anhydrase inhibitor, dramatically reduced the initial rate of acidification, demonstrating that the acidification was produced by the carbonic anhydrase-catalyzed hydration of CO2. The initial rate of acidification, 52.6 pH units per min (0.88 pH units per s), was reduced approximately 150-fold to 0.36 pH units per min by 10(-3) M methazolamide. Half-maximal inhibition occurred at a methazolamide concentration of 5.6.10(-7) M. The carbonic anhydrase inhibitor acetazolamide (10(-3) M) also greatly reduced the rate of cell acidification. The latency to the onset of carbonic anhydrase inhibition was 660 ms for methazolamide and 7.5 s for acetazolamide. The carbonic anhydrase inhibitor benzolamide (10(-4) M, 4 min exposure), which is poorly membrane permeant, had little effect on the rate of cell acidification, indicating that the site of carbonic anhydrase action was intracellular. The activity of Müller cell carbonic anhydrase may help to buffer extracellular CO2 variations in the retina.

Glutamate in some retinal neurons is derived solely from glia

Glutamate is the most abundant excitatory neurotransmitter in the vertebrate central nervous system. It is widely assumed that neurons using this transmitter derive it from several sources: (i) synthesizing it themselves from alpha-ketoglutarate or aspartate, (ii) synthesize it from glial-derived glutamine, or (iii) take up glutamate from the extracellular space. By use of immunocytochemistry we show that glutamate is abundant in the retinal ganglion and bipolar cells of the rabbit, but that immunoreactivity for glutamate in these neurons is reduced below immunocytochemical detection limits after the specific inhibition of glutamine synthesis in glial cells by D,L-methionine D,L-sulphoximine. GABA immunoreactivity in retinal amacrine cells was also reduced after inhibition of glutamine synthetase but the patterns and densities of immunoreactivity for taurine and glycine were unaffected. Therefore, this experimental paradigm does not induce generalized metabolic changes in neurons or glia. This study demonstrates that some glutamatergic neurons are dependent on the synthetic processes in glia for their neurotransmitter content.