Group I mGluR-mediated inhibition of Kir channels contributes to retinal Müller cell gliosis in a rat chronic ocular hypertension model

Involvement of P2X7 receptors in retinal ganglion cell apoptosis induced by activated Müller cells

Müller cell reactivation (gliosis) is an early response in glaucomatous retina. Previous studies have demonstrated that activation of P2X₇ receptors results in retinal ganglion cell (RGC) apoptosis. Here, the issues of whether and how activated Müller cells may contribute to RGC apoptosis through P2X₇ receptors were investigated. Either intravitreal injection of (S)-3,5-dihydroxyphenylglycine (DHPG), a group I metabotropic glutamate receptor (mGluR I) agonist, in normal rat retinas, or DHPG treatment of purified cultured rat retinal Müller cells induced an increase in glial fibrillary acidic protein (GFAP) expression, indicative of Müller cell gliosis. In addition, an increase in adenosine triphosphate (ATP) release from purified cultured Müller cells was detected during DHPG treatment (for 10 min to 48 h), which was mediated by the intracellular mGluR5/Gq/PI-PLC/PKC signaling pathway. Intravitreal injection of DHPG mimicked the reduction in the number of fluorogold retrogradely labeled RGCs in chronic ocular hypertension (COH) rats. Treatment with the conditioned culture medium (CM) obtained from the DHPG-activated Müller cell medium induced an increase in the number of TUNEL-positive cells in cultured RGCs, which was mimicked by benzoylbenzoyl adenosine triphosphate (BzATP), a P2X₇ receptor agonist, but was partially blocked by brilliant blue G (BBG), a P2X₇ receptor antagonist. Moreover, the CM treatment of cultured RGCs significantly increased Bax protein level and decreased Bcl-2 protein level, which was also mimicked by BzATP and partially blocked by BBG, respectively. These results suggest that reactivated Müller cells may release excessive ATP, in turn leading to RGC apoptosis through activating P2X₇ receptors in these cells.

Activated Müller Cells Involved in ATP-Induced Upregulation of P2X7 Receptor Expression and Retinal Ganglion Cell Death

P2X₇ receptor (P2X₇R), an ATP-gated ion channel, plays an important role in glaucomatous retinal ganglion cell (RGC) apoptotic death, in which activated retinal Müller glial cells may be involved by releasing ATP. In the present study, we investigated whether and how activated Müller cells may induce changes in P2X₇R expression in RGCs by using immunohistochemistry and Western blot techniques. Intravitreal injection of DHPG, a group I metabotropic glutamate receptor (mGluR I) agonist, induced upregulation of GFAP expression, suggestive of Müller cell activation (gliosis), as we previously reported. Accompanying Müller cell activation, P2X₇R protein expression was upregulated, especially in the cells of ganglion cell layer (GCL), which was reversed by coinjection of brilliant blue G (BBG), a P2X₇R blocker. In addition, intravitreal injection of ATP also induced upregulation of P2X₇R protein expression. Similar results were observed in cultured retinal neurons by ATP treatment. Moreover, both DHPG and ATP intravitreal injection induced a reduction in the number of fluorogold retrogradely labeled RGCs, and the DHPG effect was partially rescued by coinjection of BBG. All these results suggest that activated Müller cells may release ATP and, in turn, induce upregulation of P2X₇R expression in the cells of GCL, thus contributing to RGC death.

The role of glial-specific Kir4.1 in normal and pathological states of the CNS

Kir4.1 is an inwardly rectifying K(+) channel expressed exclusively in glial cells in the central nervous system. In glia, Kir4.1 is implicated in several functions including extracellular K(+) homeostasis, maintenance of astrocyte resting membrane potential, cell volume regulation, and facilitation of glutamate uptake. Knockout of Kir4.1 in rodent models leads to severe neurological deficits, including ataxia, seizures, sensorineural deafness, and early postnatal death. Accumulating evidence indicates that Kir4.1 plays an integral role in the central nervous system, prompting many laboratories to study the potential role that Kir4.1 plays in human disease. In this article, we review the growing evidence implicating Kir4.1 in a wide array of neurological disease. Recent literature suggests Kir4.1 dysfunction facilitates neuronal hyperexcitability and may contribute to epilepsy. Genetic screens demonstrate that mutations of KCNJ10, the gene encoding Kir4.1, causes SeSAME/EAST syndrome, which is characterized by early onset seizures, compromised verbal and motor skills, profound cognitive deficits, and salt-wasting. KCNJ10 has also been linked to developmental disorders including autism. Cerebral trauma, ischemia, and inflammation are all associated with decreased astrocytic Kir4.1 current amplitude and astrocytic dysfunction. Additionally, neurodegenerative diseases such as Alzheimer disease and amyotrophic lateral sclerosis demonstrate loss of Kir4.1. This is particularly exciting in the context of Huntington disease, another neurodegenerative disorder in which restoration of Kir4.1 ameliorated motor deficits, decreased medium spiny neuron hyperexcitability, and extended survival in mouse models. Understanding the expression and regulation of Kir4.1 will be critical in determining if this channel can be exploited for therapeutic benefit.

Activation of group I metabotropic glutamate receptors regulates the excitability of rat retinal ganglion cells by suppressing Kir and I h

Group I metabotropic glutamate receptor (mGluR I) activation exerts a slow postsynaptic excitatory effect in the CNS. Here, the issues of whether and how this receptor is involved in regulating retinal ganglion cell (RGC) excitability were investigated in retinal slices using patch-clamp techniques. Under physiological conditions, RGCs displayed spontaneous firing. Extracellular application of LY367385 (10 µM)/MPEP (10 µM), selective mGluR1 and mGluR5 antagonists, respectively, significantly reduced the firing frequency, suggesting that glutamate endogenously released from bipolar cells constantly modulates RGC firing. DHPG (10 µM), an mGluR I agonist, significantly increased the firing and caused depolarization of the cells, which were reversed by LY367385, but not by MPEP, suggesting the involvement of the mGluR1 subtype. Intracellular Ca²⁺-dependent PI-PLC/PKC and calcium/calmodulin-dependent protein kinase II (CaMKII) signaling pathways mediated the DHPG-induced effects. In the presence of cocktail synaptic blockers (CNQX, D-AP5, bicuculline, and strychnine), which terminated the spontaneous firing in both ON and OFF RGCs, DHPG still induced depolarization and triggered the cells to fire. The DHPG-induced depolarization could not be blocked by TTX. In contrast, Ba²⁺, an inwardly rectifying potassium channel (Kir) blocker, and Cs⁺ and ZD7288, hyperpolarization-activated cation channel (I _h) blockers, mimicked the effect of DHPG. Furthermore, in the presence of Ba²⁺/ZD7288, DHPG did not show further effects. Moreover, Kir and I _h currents could be recorded in RGCs, and extracellular application of DHPG indeed suppressed these currents. Our results suggest that activation of mGluR I regulates the excitability of rat RGCs by inhibiting Kir and I _h.

Suppression of outward K(+) currents by activating dopamine D1 receptors in rat retinal ganglion cells through PKA and CaMKII signaling pathways

Dopamine plays an important role in regulating neuronal functions in the central nervous system by activating the specific G-protein coupled receptors. Both D1 and D2 dopamine receptors are extensively distributed in the retinal neurons. In the present study, we investigated the effects of D1 receptor signaling on outward K(+) currents in acutely isolated rat retinal ganglion cells (RGCs) by patch-clamp techniques. Extracellular application of SKF81297 (10 μM), a specific D1 receptor agonist, significantly and reversibly suppressed outward K(+) currents of the cells, which was reversed by SCH23390 (10 μM), a selective D1 receptor antagonist. We further showed that SKF81297 mainly suppressed the glybenclamide (Gb)- and 4-aminopyridine (4-AP)-sensitive K(+) current components, but did not show effect on the tetraethylammonium (TEA)-sensitive one. Both protein kinase A (PKA) and calcium/calmodulin-dependent protein kinase II (CaMKII) signaling pathways were likely involved in the SKF81297-induced suppression of the K(+) currents since either Rp-cAMP (10 μM), a cAMP/PKA signaling inhibitor, or KN-93 (10 μM), a specific CaMKII inhibitor, eliminated the SKF81297 effect. In contrast, neither protein kinase C (PKC) nor mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) signaling pathway seemed likely to be involved because both the PKC inhibitor bisindolylmaleimide IV (Bis IV) (10 μM) and the MAPK/ERK1/2 inhibitor U0126 (10 μM) did not block the SKF81297-induced suppression of the K(+) currents. These results suggest that activation of D1 receptors suppresses the Gb- and 4-AP-sensitive K(+) current components in rat RGCs through the intracellular PKA and CaMKII signaling pathways, thus modulating the RGC excitability.

Extraction (DSX) from Erigeron breviscapus modulates outward potassium currents in rat retinal ganglion cells

AIM

To investigate the effect of DSX, an active component extracted from Erigeron breviscapus, on the voltage-gated outward K(+) channel currents in rat retinal ganglion cells (RGCs) by using electrophysiological method, and to explore the possible mechanisms of DSX on optic nerve protection.

METHODS

Outward K(+) currents were recorded by using whole-cell patch-clamp techniques on acutely isolated rat RGCs. Outward K(+) currents were induced by a series of depolarizing voltage pulses from a holding potential of -70 mV to +20 mV in an increment of 10 mV.

RESULTS

Extracellular application of DSX voltage-dependently suppressed both the steady-state and peak current amplitudes of outward K(+) currents in rat RGCs. Furthermore, DSX reversibly and dose-dependently inhibited the amplitudes of outward K(+) currents of the cells. At +20 mV membrane potential DSX at the concentrations of 0.02 g/L and 0.05 g/L showed no significant effects on the currents. In contrast, DSX at higher concentrations (0.1 g/L, 0.2 g/L and 0.5 g/L) significantly suppressed the current amplitudes.

CONCLUSION

These results suggest that DSX reversibly and dose-dependently suppress outward K(+) channel currents in rat RGCs, which may be one of the possible mechanisms underlying Erigeron breviscapus prevents vision loss and RGC damage caused by glaucoma.

GluA2 trafficking is involved in apoptosis of retinal ganglion cells induced by activation of EphB/EphrinB reverse signaling in a rat chronic ocular hypertension model

EphB1, expressed in Müller cells, and ephrinB2, expressed in both Müller cells and retinal ganglion cells (RGCs), constitute an EphB/ephrinB reverse signaling in RGCs. Whether and how this reverse signaling is involved in RGC apoptosis in a rat chronic ocular hypertension (COH) model was investigated. In the COH model, both EphB1 and ephrinB2 were significantly increased and the reverse signaling was activated, which was accompanied by increased protein levels of phosphorylated (p) src, GluA2, and p-GluA2. Intravitreal injection of EphB2-Fc, an activator of ephrinB2, induced an increase in TUNEL-positive signals in normal retinae. A coimmunoprecipitation assay demonstrated direct interactions among ephrinB2, p-src, and GluA2. Moreover, in COH rats the expression of GluA2 proteins on the surface of retinal cells was decreased. Such GluA2 endocytosis could be prevented by preoperational intravitreal injection of 4-amino-3-(4-chlorophenyl)-1-(t-butyl)-1H-pyrazolo [3,4-d] pyrimidine (PP2), an inhibitor of src family tyrosine kinases, and possibly involved the protein interacting with C kinase 1 and phosphorylation of GluA2. In normal rats, intravitreal injection of EphB2-Fc caused changes in these protein levels similar to those observed in COH rats, which all could be avoided by preinjection of PP2. Patch-clamp experiments further showed that the current-voltage relationship of AMPA receptor-mediated EPSCs of RGCs exhibited stronger inward rectification in EphB2-Fc-injected rats. Furthermore, preinjection of PP2 or N-[3-[[4-[(3-aminopropyl)amino]butyl]amino]propyl]-1-naphthaleneacetamide trihydrochloride) (Naspm), a Ca(2+)-permeable GluA2-lacking AMPA receptor inhibitor, remarkably inhibited RGC apoptosis in either EphB2-Fc-injected or COH rats. Together, elevated GluA2 trafficking induced by activated EphB2/ephrinB2 reverse signaling likely contributes to RGC apoptosis in COH rats.

Purinergic signaling in retinal degeneration and regeneration

Purinergic signaling is centrally involved in mediating the degeneration of the injured and diseased retina, the induction of retinal gliosis, and the protection of the retinal tissue from degeneration. Dysregulated calcium signaling triggered by overactivation of P2X7 receptors is a crucial step in the induction of neuronal and microvascular cell death under pathogenic conditions like ischemia-hypoxia, elevated intraocular pressure, and diabetes, respectively. Overactivation of P2X7 plays also a pathogenic role in inherited and age-related photoreceptor cell death and in the age-related dysfunction and degeneration of the retinal pigment epithelium. Gliosis of micro- and macroglial cells, which is induced and/or modulated by purinergic signaling and associated with an impaired homeostatic support to neurons, and the ATP-mediated propagation of retinal gliosis from a focal injury into the surrounding noninjured tissue are involved in inducing secondary cell death in the retina. On the other hand, alterations in the glial metabolism of extracellular nucleotides, resulting in a decreased level of ATP and an increased level of adenosine, may be neuroprotective in the diseased retina. Purinergic signals stimulate the proliferation of retinal glial cells which contributes to glial scarring which has protective effects on retinal degeneration and adverse effects on retinal regeneration. Pharmacological modulation of purinergic receptors, e.g., inhibition of P2X and activation of adenosine receptors, may have clinical importance for the prevention of photoreceptor, neuronal, and microvascular cell death in diabetic retinopathy, retinitis pigmentosa, age-related macular degeneration, and glaucoma, respectively, for the clearance of retinal edema, and the inhibition of dysregulated cell proliferation in proliferative retinopathies. This article is part of a Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Group I metabotropic glutamate receptor agonist DHPG modulates Kir4.1 protein and mRNA in cultured rat retinal Müller cells

Müller cell gliosis is a general response in a variety of pathological alternations of the retina, which is characterized by the upregulated expression of glial fibrillary acidic protein (GFAP) and the downregulation of membrane K(+) conductance. We have demonstrated that downregulation of Kir K(+) currents in Müller cells in an experimental glaucoma model is due to activation of group I metabotropic glutamate receptor (mGluR I) by glutamate, which contributes to Müller cell gliosis. Here, whether and how activation of mGluR I modulate membrane Kir4.1 protein internalization and Kir4.1 mRNA expression were investigated in purified cultured rat retinal Müller cells using immunocytochemistry, Western blot and real-time PCR techniques. DHPG (10μM, a selective mGluR I agonist) treatment induced Müller cell gliosis, as evidenced by enhanced GFAP expression. Although total Kir4.1 proteins extracted from the DHPG-treated cells kept unchanged, Kir4.1 proteins in the cell membrane compartment were significantly decreased, which was prior to the change of GFAP in time course. In addition, DHPG (10 and 100μM) treatment induced a transient decrease in Kir4.1 mRNA expression in the cells. All these results suggest that activation of mGluR I by DHPG may decrease the number of functional Kir4.1 channels in purified cultured rat retinal Müller cells through modulating Kir4.1 protein and mRNA, thus contributing to Müller cell gliosis.

Cumulative mtDNA damage and mutations contribute to the progressive loss of RGCs in a rat model of glaucoma

Glaucoma is a chronic neurodegenerative disease characterized by the progressive loss of retinal ganglion cells (RGCs). Mitochondrial DNA (mtDNA) alterations have been documented as a key component of many neurodegenerative disorders. However, whether mtDNA alterations contribute to the progressive loss of RGCs and the mechanism whereby this phenomenon could occur are poorly understood. We investigated mtDNA alterations in RGCs using a rat model of chronic intraocular hypertension and explored the mechanisms underlying progressive RGC loss. We demonstrate that the mtDNA damage and mutations triggered by intraocular pressure (IOP) elevation are initiating, crucial events in a cascade leading to progressive RGC loss. Damage to and mutation of mtDNA, mitochondrial dysfunction, reduced levels of mtDNA repair/replication enzymes, and elevated reactive oxygen species form a positive feedback loop that produces irreversible mtDNA damage and mutation and contributes to progressive RGC loss, which occurs even after a return to normal IOP. Furthermore, we demonstrate that mtDNA damage and mutations increase the vulnerability of RGCs to elevated IOP and glutamate levels, which are among the most common glaucoma insults. This study suggests that therapeutic approaches that target mtDNA maintenance and repair and that promote energy production may prevent the progressive death of RGCs.

Astroglial NMDA receptors inhibit expression of Kir4.1 channels in glutamate-overexposed astrocytes in vitro and in the brain of rats with acute liver failure

Astroglial inward rectifying Kir4.1 potassium channels are fundamental for the maintenance of ion and water homeostasis in the central nervous system (CNS). Down-regulation of Kir4.1 expression is observed in CNS disorders associated with excessive extracellular glutamate (Glu) accumulation, including hepatic encephalopathy related to acute liver failure (ALF). Here we demonstrate that prolonged (3 days) treatment of cultured rat cortical astrocytes with 2 mM Glu or 100 µM NMDA decreases the expression of Kir4.1 mRNA and protein. Inhibition by Glu of Kir4.1 mRNA expression was reversed by NMDA receptor antagonists MK-801 and AP-5 (each at 50 µM), and by a non-transportable inhibitor of Glu uptake TBOA (100 µM). MK-801 reversed the inhibitory effect of Glu on Kir4.1 protein expression. In contrast, transcription of Kir4.1 channels was not affected by: (i) a transportable Glu uptake inhibitor PDC (100 µM); (ii) by group I mGluR antagonist MTEP (100 µM); (iii) by antagonists of oxidative-nitrosative stress (ONS) in astrocytes, including the neuroprotective amino acid taurine (Tau; 10 mM), the NADPH oxidase inhibitor apocyanine (APO; 300 µM), the nitric oxide synthase inhibitor, L-NNA (100 µM), and a membrane permeable glutathione precursor, glutathione-diethyl ester (GEE; 3 mM). Down-regulation of Kir4.1 transcription in rats with ALF was attenuated by intraperitoneal administration of a competitive NMDA receptor antagonist memantine, but not by histidine, which reverses ONS associated with ALF. Collectively, the results indicate that over-activation of astroglial NMDA receptors, aided by as yet undefined effects of Glu entry to astrocytes, is a primary cause of the reduction of Kir4.1 expression in CNS disorders associated with increased exposure to Glu.

Enhanced expression of NR2B subunits of NMDA receptors in the inherited glaucomatous DBA/2J mouse retina

DBA/2J mouse has been used as a model for spontaneous secondary glaucoma. Here, we investigated changes in expression of NMDA receptor (NMDAR) subunits and Cdk5/p35/NMDAR signaling in retinas of DBA/2J mice using Western blot technique. The protein levels of NR1 and NR2A subunits in retinas of DBA/2J mice at all ages (6–12 months) were not different from those in age-matched C57BL/6 mice. In contrast, the protein levels of NR2B subunits, in addition to age-dependent change, significantly increased with elevated intraocular pressure (IOP) in DBA/2J mice at 6 and 9 months as compared with age-matched controls. Moreover, expression of Cdk5, p35 and ratio of p-NR2A^S1232/NR2A progressively increased with time in both strains, suggestive of activated Cdk5/p35 signaling pathway. However, the changes in these proteins were in the same levels in both strain mice, except a significant increase of p35 proteins at 6 months in DBA/2J mice. Meanwhile, the protein levels of Brn-3a, a retinal ganglion cell (RGC) maker, remarkably decreased at 9–12 months in DBA/2J mice, which was in parallel with the changes of NR2B expression. Our results suggest that elevated IOP-induced increase in expression of NR2B subunits of NMDARs may be involved in RGC degeneration of DBA/2J mice.

Suppression of outward K⁺ currents by WIN55212-2 in rat retinal ganglion cells is independent of CB1/CB2 receptors

Cannabinoid CB1 receptor (CB1R) signaling system is extensively distributed in the vertebrate retina. Activation of CB1Rs regulates a variety of functions of retinal neurons through modulating different ion channels. In the present work we studied effects of this receptor signaling on K(+) channels in retinal ganglion cells by patch-clamp techniques. The CB1R agonist WIN55212-2 (WIN) suppressed outward K(+) currents in acutely isolated rat retinal ganglion cells in a dose-dependent manner, with an IC50 of 4.7 μM. We further showed that WIN mainly suppressed the tetraethylammonium (TEA)-sensitive K(+) current component. While CB1Rs were expressed in rat retinal ganglion cells, the WIN effect on K(+) currents was not blocked by either AM251/SR141716, specific CB1R antagonists, or AM630, a selective CB2R antagonist. Consistently, cAMP-protein kinase A (PKA) and mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) signaling pathways were unlikely involved in the WIN-induced suppression of the K(+) currents because both PKA inhibitors H-89/Rp-cAMP and MAPK/ERK1/2 inhibitor U0126 failed to block the WIN effects. WIN-induced suppression of the K(+) currents was not observed when WIN was intracellularly applied. Furthermore, an endogenous ligand of the cannabinoid receptor anandamide, the specific CB1R agonist ACEA and the selective CB2R agonist CB65 also suppressed the K(+) currents, and the effects were not blocked by AM251/SR141716 or AM630 respectively. All these results suggest that the WIN-induced suppression of the outward K(+) currents in rat retinal ganglion cells, thereby regulating the cell excitability, were not through CB1R/CB2R signaling pathways.