A potassium channel-linked mechanism of glial cell swelling in the postischemic retina

Triamcinolone does not alter glial cell activation in the experimentally detached rabbit retina

AIMS

Retinal detachment induces neural and photoreceptor cell degeneration and fast activation of micro- (immune) and macroglial cells. Hypoxia caused by increased distance between the choriocapillaris and the neural retina, and retinal oedema during detachment, are factors causing gliotic responses and cell degeneration. Triamcinolone may inhibit some cellular responses that accompany hypoxia. Therefore, we investigated whether triamcinolone acetonide may be effective to reduce the gliotic alterations in the detached retina.

METHODS

Local retinal detachment in rabbit eyes was created by subretinal injection of sodium hyaluronate, and triamcinolone acetonide (8 mg) was applied intravitreally. Wholecell patch-clamp records from Muller cells and Ca2+ imaging from retinal wholemounts were carried out. Microglial/immune cells in the nerve-fiber layer of retinal wholemounts were labeled with Griffonia simplicifolia agglutinin (GSA) isolectin. Additionally, two morphological parameters which characterize microglial activation/immune cell infiltration were estimated: the cross-sectional area of the somata of the cells in the nerve-fiber layer and the number of cell processes which evolve from the soma.

RESULTS

Three days after detachment, gliotic alterations were apparent in Muller cells isolated from both detached and nondetached retinal areas, as indicated by the cellular hypertrophy, by the downregulation of the plasma membrane K+ conductance, and by the upregulation of intracellular Ca2+ responsiveness to stimulation of purinergic P2Y receptors. Intravitreal triamcinolone did not alter these gliotic alterations of Muller cells. Furthermore, triamcinolone could not inhibit the immune cell activation present in detached and attached retinal areas. However, intravitreal triamcinolone led to a strong decrease in the process number of GSA lectin-positive cells from detached retinas.

CONCLUSIONS

The results suggest that triamcinolone is ineffective to inhibit gliotic responses in the detached retina. However, the immune cell activation after detachment was partially influenced by triamcinolone.

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

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

Glutamate-evoked alterations of glial and neuronal cell morphology in the guinea pig retina

Neuronal activity is accompanied by transmembranous ion fluxes that cause cell volume changes. In whole mounts of the guinea pig retina, application of glutamate resulted in fast swelling of neuronal cell bodies in the ganglion cell layer (GCL) and the inner nuclear layer (INL) (by approximately 40%) and a concomitant decrease of the thickness of glial cell processes in the inner plexiform layer (IPL) (by approximately 40%) that was accompanied by an elongation of the glial cells, by a thickening of the whole retinal tissue, and by a shrinkage of the extracellular space (by approximately 18%). The half-maximal effect of glutamate was observed at approximately 250 mum, after approximately 4 min. The swelling was caused predominantly by AMPA-kainate receptor-mediated influx of Na+ into retinal neurons. Similar but transient morphological alterations were induced by high K+ and dopamine, which caused release of endogenous glutamate and subsequent activation of AMPA-kainate receptors. Apparently, retinal glutamatergic transmission is accompanied by neuronal cell swelling that causes compensatory morphological alterations of glial cells. The effect of dopamine was elicitable only during light adaptation but not in the dark, and glutamate and high K+ induced strong ereffects in the dark than in the light. This suggests that not only the endogenous release of dopamine but also the responsiveness of glutamatergic neurons to dopamine is regulated by light-dark adaptation. Similar morphological alterations (neuronal swelling and decreased glial process thickness) were observed in whole mounts isolated immediately after experimental retinal ischemia, suggesting an involvement of AMPA-kainate receptor activation in putative neurotoxic cell swelling in the postischemic retina.

Diversity of aquaporin mRNA expressed by rat and human retinas

Muller glial cells of the sensory retina mediate K+ and water fluxes that are facilitated by aquaporin-4 (AQP4) water channels and by Kir4.1-K+ channels. However, it is not known which subtypes of aquaporins are expressed in the mammalian retina. Using RT-PCR, we found that both human and rat retinas express mRNA for a diversity of water channel proteins. The human retina expresses mRNAs for AQP0 to AQP12 proteins. Using real-time PCR, we found that the mRNAs for AQP4 and Kir4.1 are downregulated in retinas that were obtained from patients with proliferative retinopathy compared with post-mortem controls. The data suggest that the development of proliferative gliosis is accompanied by disturbed transglial water and ion movements.

Glial cell-mediated spread of retinal degeneration during detachment: a hypothesis based upon studies in rabbits

In human subjects with peripheral retinal detachments, visual deficits are not restricted to the detached retina but are also present in the non-detached tissue. Based upon studies on a rabbit model of rhegmatogenous retinal detachment, we propose a glial cell-mediated mechanism of spread of retinal degeneration into non-detached retinal areas which may also have importance for the understanding of alterations in the human retina. Both detached and attached portions of the rabbit retina display photoreceptor cell degeneration and cystic degeneration of the innermost layers. An inverse mode of photoreceptor cell degeneration in the attached tissue suggests a disturbed support of the photoreceptor cells by Müller cells which show various indications of gliosis (increased expression of intermediate filaments, cell hypertrophy, decreased plasma membrane K(+) conductance, increased Ca(2+) responsiveness to purinergic stimulation) in both detached and attached tissues. We propose that gliotic alterations of Müller cells contribute to the degeneration of the attached retina, via disturbance of glial homeostasis mechanisms. A down-regulation of the K(+) conductance of Müller cells may prevent effective retinal K(+) and water clearance, and may favor photoreceptor cell degeneration and edema development.

Ocular inflammation alters swelling and membrane characteristics of rat Müller glial cells

Ocular inflammation is a common cause of retinal edema that may involve swelling of Müller glial cells. In order to investigate whether endotoxin-induced ocular inflammation in rats alters the swelling and membrane characteristics of Müller cells, lipopolysaccharide (LPS; 0.5%) was intravitreally injected. At 3 and 7 days after treatment, hypotonic challenge induced swelling of Müller cell somata that was not observed in non-treated control eyes. Müller cells of LPS-treated eyes displayed a downregulation of inward K(+) currents and upregulation of A-type K(+) currents that was associated with a decreased expression of Kir4.1 protein in retinal slices. The data suggest that ocular inflammation induces alterations of both the swelling characteristics and the K(+) channel expression of Müller cells.

Redistribution of the water channel protein aquaporin-4 and the K+ channel protein Kir4.1 differs in low- and high-grade human brain tumors

The blood-brain barrier (BBB) regulation is characterized by an interplay between endothelial cells, subendothelial basal laminae and astrocytic cells. Astroglial cells are highly polarized by the differentiation of perivascular membrane domains. These domains are characterized by the aggregation of, among other molecules, the water channel protein aquaporin-4 (AQP4), the dystrophin-dystroglycan complex, and the inwardly rectifying potassium channel protein Kir4.1. Normally, this ion channel plays an important role in spatial buffering of extracellular K(+) in the central nervous system, which only can be performed due to the non-uniform distribution of Kir4.1 across the surface of the glial cell. In this study, we observed a mislocalization of Kir4.1 in various human brain tumors (low- and high-grade astrocytomas and oligodendrogliomas), suggesting that buffering capacity of glial cells may be compromised, leading to water influx (cytotoxic edema). Interestingly, whereas dystrophin remained regularly restricted at the endfeet membranes in all cases investigated, AQP4 was found to be redistributed only in high-grade astrocytomas, not in low-grade astrocytomas. If the mechanisms of redistribution of AQP4 and Kir4.1 are different in low- and high-grade gliomas, this may suggest that the mechanisms of clustering of AQP4 and Kir4.1 at the glial endfeet membrane domains are also different. The redistribution of AQP4 in glioblastoma cells is discussed as a reaction to the vasogenic edema, as induced by the breakdown of the BBB, to facilitate reabsorption of excess fluid.

Tandem-pore domain potassium channels are functionally expressed in retinal (Müller) glial cells

Tandem-pore domain (2P-domain) K+-channels regulate neuronal excitability, but their function in glia, particularly, in retinal glial cells, is unclear. We have previously demonstrated the immunocytochemical localization of the 2P-domain K+ channels TASK-1 and TASK-2 in retinal Müller glial cells of amphibians. The purpose of the present study was to determine whether these channels were functional, by employing whole-cell recording from frog and mammalian (guinea pig, rat and mouse) Müller cells and confocal microscopy to monitor swelling in rat Müller cells. TASK-like immunolabel was localized in these cells. The currents mediated by 2P-domain channels were studied in isolation after blocking Kir, K(A), K(D), and BK channels. The remaining cell conductance was mostly outward and was depressed by acid pH, bupivacaine, methanandamide, quinine, and clofilium, and activated by alkaline pH in a manner consistent with that described for TASK channels. Arachidonic acid (an activator of TREK channels) had no effect on this conductance. Blockade of the conductance with bupivacaine depolarized the Müller cell membrane potential by about 50%. In slices of the rat retina, adenosine inhibited osmotic glial cell swelling via activation of A1 receptors and subsequent opening of 2P-domain K+ channels. The swelling was strongly increased by clofilium and quinine (inhibitors of 2P-domain K+ channels). These data suggest that 2P-domain K+ channels are involved in homeostasis of glial cell volume, in activity-dependent spatial K+ buffering and may play a role in maintenance of a hyperpolarized membrane potential especially in conditions where Kir channels are blocked or downregulated.

Altered membrane physiology in Müller glial cells after transient ischemia of the rat retina

Inwardly rectifying K+ (Kir) channels have been implicated in the mediation of retinal K+ homeostasis by Muller glial cells. To assess possible involvement of altered glial K+ channel expression in ischemia-reperfusion injury, transient retinal ischemia was induced in rat eyes. Acutely isolated Muller cells from postischemic retinae displayed a fast downregulation of their Kir currents, which began within 1 day and reached a maximum at 3 days of reperfusion, with a peak decrease to 20% as compared with control. This strong decrease of Kir currents was accompanied by an increase of the incidence of cells which displayed depolarization-evoked fast transient (A-type) K+ currents. While no cell from untreated control rats expressed A-type K+ currents, all cells investigated from 3- and 7-day postischemic retinae displayed such currents. An increased incidence of cells displaying fast transient Na+ currents was observed at 7 days after ischemia. These results suggest a role of altered glial Kir channel expression in postischemic neuronal degeneration via disturbance of retinal K+ siphoning.

Pathomechanisms of Cystoid Macular Edema

Cystoid macular edema (CME) is a well-known endpoint of various ocular diseases, but the relative pathogenic impact of extra- and intracellular fluid accumulation within the retinal tissue still remains uncertain. While most authors favor an extracellular fluid accumulation as the main causative factor of cyst formation, there are indications that Müller cell swelling may also contribute to CME development (particularly in cases without significant angiographic vascular leakage). Vascular leakage occurs after a breakdown of the blood-retinal barrier during traumatic, vascular, and inflammatory ocular diseases, and allows the serum to get into the retinal interstitium. Since intraretinal fluid distribution is restricted by two diffusion barriers, the inner and outer plexiform layers, serum leakage from intraretinal vessels causes cysts mainly in the inner nuclear layer while leakage from choroid/pigment epithelium generates (in addition to subretinal fluid accumulation) cyst formation in the Henle fiber layer. In the normal healthy retina, the transretinal water fluxes are mediated by glial and pigment epithelial cells. These water fluxes are inevitably coupled to fluxes of osmolytes; in the case of glial (Müller) cells, to K(+) clearance currents. For this purpose, the cells express a complex, microtopographically optimized pattern of transporters and channels for osmolytes and water in their plasma membrane. Ischemic/hypoxic alterations of the retinal microvasculature result in gliotic responses which involve down-regulation of K(+) channels in the perivascular Müller cell end-feet. This means a closure of the main pathway which normally generates the osmotic drive for the redistribution of water from the inner retina into the blood. The result is an intracellular K(+) accumulation which, then, osmotically drives water from the blood into the glial cells (i.e., in the opposite direction) and causes glial cell swelling, edema, and cyst formation. While the underlying mechanisms await further research, it is expected that their improved knowledge will stimulate the development of novel therapeutic approaches to resolve edema in retinal tissue.