Region-specific expression of vesicular glutamate and GABA transporters under various ischaemic conditions in mouse forebrain and retina

There is accumulating evidence that glutamate and GABA release are key mechanisms of ischaemic events in the CNS. However, data on the expression of involved transporters for these mediators are inconsistent, potentially impeding further neuroprotective approaches. Here, we applied immunofluorescence labelling to characterise the expression pattern of vesicular glutamate (VGLUT) and GABA transporters (VGAT) after acute focal cerebral ischaemia and in two models of retinal ischaemia. Mice were subjected to filament-based focal cerebral ischaemia predominantly involving the middle cerebral artery territory, also leading to retinal ischaemia due to central retinal artery occlusion (CRAO). Alternatively, retinal ischaemia was induced by a transient increase of the intraocular pressure (HIOP). One day after ischaemia onset, diminished immunolabelling of neuronal nuclei and microtubule-associated protein 2-positive structures were found in the ipsilateral neocortex, subcortex and the retina, indicating neuronal degeneration. VGLUT1 expression did not change significantly in ischaemic tissues whereas VGLUT2 was down-regulated in specific areas of the brain. VGLUT3 expression was only slightly down-regulated in the ischaemia-affected neocortex, and was found to form clusters on fibrils of unknown origin in the ischaemic lateral hypothalamus. In contrast, retinae subjected to CRAO or HIOP displayed a rapid loss of VGLUT3-immunoreactivity. The expression of VGAT appears resistant to ischaemia as there was no significant alteration in all the regions analysed. In summary, these data indicate a region- and subtype-specific change of VGLUT expression in the ischaemia-affected CNS, whose consideration might help to generate specific neuroprotective strategies.

Morphometric analysis of the retina from horses infected with the Borna disease virus

Borna disease (BD) is a fatal disorder of horses, often characterized by blindness. Although degeneration of retinal neurons has been demonstrated in a rat model, there are controversial data concerning whether a similar degeneration occurs in the retina of infected horses. To investigate whether BD may cause degeneration of photoreceptors and possibly of other neuronal cells at least at later stages of the disease, we performed a detailed quantitative morphologic study of retinal tissue from Borna-diseased horses. BD was diagnosed by detection of pathognomonic Joest-Degen inclusion bodies in the postmortem brains. Paraffin sections of paraformaldehyde-fixed retinae were used for histologic and immunohistochemical stainings. Numbers of neurons and Müller glial cells were counted, and neuron-to-Müller cell ratios were calculated. Among tissues from 9 horses with BD, we found retinae with strongly altered histologic appearance as well as retinae with only minor changes. The neuron-to-Müller cell ratio for the whole retina was significantly smaller in diseased animals (8.5 +/- 0.4; P < .01) as compared with controls (17.6 +/- 0.8). It can be concluded that BD in horses causes alterations of the retinal histology of a variable degree. The study provides new data about the pathogenesis of BD concerning the retina and demonstrates that a loss of photoreceptors may explain the observed blindness in infected horses.

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.

P2 receptors in satellite glial cells in trigeminal ganglia of mice

There is strong evidence for the presence of nucleotide (P2) receptors in sensory neurons, which might play a role in the transmission of pain signals. In contrast, virtually nothing is known about P2 receptors in satellite glial cells (SGCs), which are the main glial cells in sensory ganglia. We investigated the possibility that P2 receptors exist in SGCs in murine trigeminal ganglia, using Ca(2+) imaging, patch-clamp recordings, and immunohistochemistry. We found that ATP caused an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) in SGCs. As adenosine had no effect on [Ca(2+)](i), and the P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid largely blocked the response to ATP we conclude that P1 receptors did not contribute to the responses. We obtained the following evidence that the responses to ATP were mediated by metabotropic P2Y receptors: (i) persistence of the responses in Ca(2+)-free solution, (ii) inhibition of the response by cyclopiazonic acid, (iii) [Ca(2+)](i) increases in response to the P2Y agonists uridine triphosphate, adenosine thiodiphosphate, and 2-methylthio ADP, and (iv) failure of the P2X agonist alpha,beta-methylene ATP to elicit a response. Agonists of P2Y(1) receptors and uridine triphosphate, an agonist at P2Y(2) and P2Y(4) receptors, induced [Ca(2+)](i) increases suggesting that at least these P2Y receptor subtypes are present on SGCs. Using an antibody against the P2Y(4) receptor, we found immunopositive SGCs. Patch-clamp recordings of SGCs did not reveal any inward current due to ATP. Therefore, there was no evidence for the activation of ionotropic P2X receptors under the present conditions. The results indicate the presence of functional nucleotide (P2Y) receptors in SGCs.

Experimental dispase-induced retinopathy causes up-regulation of P2Y receptor-mediated calcium responses in Müller glial cells

During proliferative vitreoretinopathy (PVR) Müller glial cells show an up-regulation of their responsiveness to extracellular adenosine 5'-triphosphate (ATP). In the present study, we investigated if such a glial cell response is also a feature for other retinopathies besides PVR. To this aim, the proteolytic enzyme, dispase (0.1 U), was injected into the vitreous of rabbit eyes. After 3 weeks, a distinct retinopathy had developed which showed no signs of PVR. The retinopathy was characterized by strong alterations of the retinal vasculature in the medullary rays, by photoreceptor degeneration, retinal atrophy, and activation of microglial cells. Müller cells became reactive, as indicated by up-regulation of glial fibrillary acidic protein immunoreactivity and by hypertrophy involving subretinal fibrosis. Müller cell reactivity was also evidenced electrophysiologically by a down-regulation of their inwardly rectifying potassium currents and by an up-regulation of their responsiveness to extracellular ATP. Significantly more Müller cells from dispase-treated eyes showed ATP-evoked calcium (83%) and current responses (69%) when compared with cells from control eyes (13 and 9%, respectively). The results indicate that increased responsiveness to extracellular ATP may be a more general feature of Müller cell gliosis, and is also observed in retinopathies besides PVR.

Electrophysiological alterations and upregulation of ATP receptors in retinal glial Müller cells from rats infected with the Borna disease virus

Infection with the neurotropic Borna disease virus (BDV) causes an immune-mediated neurological disease in a broad range of species. In addition to encephalitis, BDV-infected Lewis rats develop a retinitis histologically characterized by the loss of most retinal neurons. By contrast, the dominating retinal macroglia, the Müller cells, do not degenerate. It is known from several models of neurodegeneration that glial cells may survive but undergo significant alterations of their physiological parameters. This prompted us to study the electrophysiology and ATP-induced changes of intracellular Ca(2+)-concentration ([Ca(2+)](i)) in Müller cells from BDV-infected rat retinae. Freshly isolated cells were used for whole-cell patch-clamp recordings. Whereas neither zero current potentials nor membrane resistances showed significant alterations, the membrane capacitance increased in cells from BDV-infected rats during survival times of up to 8 months. This process was accompanied by a decrease in K(+) current densities. Müller cells from BDV-infected rats were characterized by expression of a prominent fast-inactivating A-type K(+) current which was rarely found in control cells. Moreover, the number of cells displaying Na(+) currents was slightly increased after BDV-infection. ATP evoked increases in [Ca(2+)](i) in Müller cells within retinal wholemounts of both control and BDV-infected animals. However, the number of ATP-responding isolated cells increased from 24% (age-matched controls) to 78% (cells from animals > or =18 weeks after infection). We conclude that in BDV-induced retinopathy, reactive rat Müller cells change their physiological parameters but these changes are different from those in Müller cells during proliferative vitreoretinopathy in man and rabbit.

Changes of the organotypic retinal organization in Borna virus-infected Lewis rats

Retinae of Borna disease virus (BDV)-infected Lewis rats were investigated with emphasis on long-term changes in organotypic tissue organization and glia-neuron relationship. Virus inoculation was attained via intracerebral BDV injection. Following survival times ranging between two and eight months, the retinal thickness was reduced up to one third of that of controls. Photoreceptor segments were completely extinguished and the number of neurons was dramatically reduced. The typical laminar organization of the retina was largely dissolved. Electron microscopy revealed severe spongy degeneration. Large numbers of activated microglia and macrophages were found, both cell types performing very active phagocytosis. The microglial cells expressed an extraordinary phenotype as characterized by large numbers of processes, with some of them penetrating the endfeet of Müller cells and others establishing highly complex interdigitations with vacuolized swellings and endings of neuronal processes. Müller cells were not reduced in number but displayed clear indications of gliosis such as alterations in the immunoreactivity for filament proteins and glutamine synthetase, significantly thickened stem processes, and an altered pattern of K(+) currents in patch-clamp recordings. These findings demonstrate for the first time long-term neuron-glia interactions in the retina of BDV-infected rats. Moreover, the data contribute to our knowledge on structural and functional alterations accompanying persisting virus infection in the central nervous system.

Kir subfamily in frog retina: specific spatial distribution of Kir 6.1 in glial (Müller) cells

We show by immunocytochemistry in frog retina that most members of the Kir subfamily are expressed in specific neuronal compartments. However, Kir 6.1, the pore-forming subunit of K(ATP) channels, is expressed exclusively in glial Müller cells. Müller cell endfeet display strong Kir 6.1 immunolabel throughout the retina, whereas the somata are labeled only in the retinal periphery. This spatial pattern is similar to that of Kir 4.1, of the ratio of inward to outward K+ currents, and of spermine/spermidine immunoreactivity. We suggest that the co-expression of Kir 4.1 and Kir 6.1 subunits may enable the cells to maintain their high K+ conductance and hyperpolarized membrane potentials both at high ATP levels (Kir 4.1) and during ATP deficiency (Kir 6.1).

Electrophysiological properties of rat retinal Müller (glial) cells in postnatally developing and in pathologically altered retinae

Retinal glial Müller cells are characterized by dominant K(+) conductances. The cells may undergo changes of their membrane currents during ontogeny and gliosis as described in rabbit and man. Although the rat retina is often used in physiological experiments, the electrophysiology of rat Müller cells is less well studied. The aim of the present study was to characterize their membrane currents in postnatal development and in two models of retinal degeneration. Freshly isolated cells were subjected to whole-cell patch clamp recordings. During the first 4 weeks after birth of rats, their Müller cells displayed an increase in all membrane currents, particularly in the inward currents elicited at hyperpolarizing potentials. The decrease of the membrane resistance from more than 760 MOmega to less than 50 MOmega was accompanied by a shift of the zero current potential from about -20 mV to -80 mV, similar as earlier observed in developing rabbit Müller cells. These developmental changes were found in pigmented Brown Norway rats as well as in rats with inherited retinal dystrophy (RCS rats). Moreover, an infection of Lewis rats with the Borna disease virus caused substantial neuroretinal degeneration but did not result in a strong reduction of inward currents and of the zero current potential of the Müller cells. Thus, rat Müller cells fail to change their basic membrane properties in two different models of retinal pathology. This is in contrast to human and rabbit Müller cells, which have been shown to undergo dramatic changes of their membrane physiology in response to retinal diseases and injuries.

Electrophysiology of rabbit Müller (glial) cells in experimental retinal detachment and PVR

PURPOSE

To determine the electrophysiological properties of Müller (glial) cells from experimentally detached rabbit retinas.

METHODS

A stable local retinal detachment was induced by subretinal injection of a sodium hyaluronate solution. Müller cells were acutely dissociated and studied by the whole-cell voltage-clamp technique.

RESULTS

The cell membranes of Müller cells from normal retinas were dominated by a large inwardly rectifying potassium ion (K+) conductance that caused a low-input resistance (<100 M(Omega)) and a high resting membrane potential (-82 +/- 6 mV). During the first week after detachment, the Müller cells became reactive as shown by glial fibrillary acidic protein (GFAP) immunoreactivity, and their inward currents were markedly reduced, accompanied by an increased input resistance (>200 M(Omega)). After 3 weeks of detachment, the input resistance increased further (>300 M(Omega)), and some cells displayed significantly depolarized membrane potentials (mean -69 +/- 18 mV). When PVR developed (in 20% of the cases) the inward K+ currents were virtually completely eliminated. The input resistance increased dramatically (>1000 MOmega), and almost all cells displayed strongly depolarized membrane potentials (-44 +/- 16 mV).

CONCLUSIONS

Reactive Müller cells are characterized by a severe reduction of their K+ inward conductance, accompanied by depolarized membrane potentials. These changes must impair physiological glial functions, such as neurotransmitter recycling and K+ ion clearance. Furthermore, the open probability of certain types of voltage-dependent ion channels (e.g., Ca2+-dependent K+ maxi channels) increases that may be a precondition for Müller cell proliferation, particularly in PVR when a dramatic downregulation of both inward current density and resting membrane potential occurs.

Upregulation of P2X(7) receptor currents in Müller glial cells during proliferative vitreoretinopathy

PURPOSE

Müller glial cells from the human retina express purinergic P2X(7) receptors. Because extracellular adenosine triphosphate (ATP) is assumed to be a mediator of the induction or maintenance of gliosis, this study was undertaken to determine whether the expression of these receptors is different in human Müller cells obtained from retinas of healthy donors and of patients with choroidal melanoma and proliferative vitreoretinopathy (PVR).

METHODS

Human Müller cells were enzymatically isolated from donor retinas, and whole-cell patch-clamp recordings were made to characterize the density of the P2X(7) currents and the activation of currents through Ca2+-activated K+ channels of big conductance (I:(BK)) that reflects the increase of the intracellular Ca2+ concentration.

RESULTS

Stimulation by external ATP or by benzoylbenzoyl ATP (BzATP) evoked both release of Ca2+ from thapsigargin-sensitive intracellular stores and opening of Ca2+ -permeable P2X(7) channels. These responses caused transient and sustained increases in I:(BK). In Müller cells from patients with PVR, the mean density of the BzATP-evoked cation currents was significantly greater compared with cells from healthy donors. As a consequence, such cells displayed an enlarged I:(BK) during application of purinergic agonists. ATP and BzATP increased the DNA synthesis rate of cultured cells. This effect could be reversed by blocking the I:(BK).

CONCLUSIONS

The increased density of P2X(7) receptor channels may permit a higher level of entry of extracellular Ca2+ into cells from patients with PVR. Enhanced Ca2+ entry and the subsequent stronger activation of I:(BK) may contribute to the induction or maintenance of proliferative activity in gliotic Müller cells during PVR.

Müller glial cells in anuran retina

Whereas in the brain, the activity of the neurons is supported by several types of glial cells such as astrocytes, oligodendrocytes, and ependymal cells, the retina (evolving from the brain during ontogenesis) contains only one type of macroglial cell, the Müller (radial glial) cells, in most vertebrates including the anurans. These cells span the entire thickness of the tissue, and thereby contact and ensheath virtually every type of neuronal cell body and process. This intimate topographical relationship is reflected by a multitude of functional interactions between retinal neurons and Müller glial cells. Müller cells are the principal stores of retinal glycogen, and are thought to fuel retinal neurons with substrate (lactate/pyruvate) for their oxidative metabolism. Furthermore, Müller cells are involved in the control and homeostasis of many constituents of the extracellular space, such as potassium and perhaps other ions, signaling molecules, and of the extracellular pH. They also seem to play important roles in recycling mechanisms of photopigment molecules and neurotransmitter molecules such as glutamate and GABA. By containing the main retinal stores of glutathione, Müller cells may protect retinal neurons against free radicals. Moreover, Müller cells express receptors for many neuroactive substances, and may also release such substances to their neighbouring neurons. Thus, Müller cells exert many functions crucial for signal processing in the normal retina. Moreover, Müller cells change their properties in cases of retinal disease and injury, and may either support the survival of neuronal cells or accelerate the progress of neuronal degeneration.

P2X7 receptors in Müller glial cells from the human retina

ATP has been shown to be an important extracellular signaling molecule. There are two subgroups of receptors for ATP (and other purines and pyrimidines): the ionotropic P2X and the G-protein-coupled P2Y receptors. Different subtypes of these receptors have been identified by molecular biology, but little is known about their functional properties in the nervous system. Here we present data for the existence of P2 receptors in Müller (glial) cells of the human retina. The cells were studied by immunocytochemistry, electrophysiology, Ca(2+)-microfluorimetry, and molecular biology. They displayed both P2Y and P2X receptors. Freshly enzymatically isolated cells were used throughout the study. Although the [Ca(2+)](i) response to ATP was dominated by release from intracellular stores, there is multiple evidence that the ATP-induced membrane currents were caused by an activation of P2X(7) receptors. Immunocytochemistry and single-cell RT-PCR revealed the expression of P2X(7) receptors by Müller cells. In patch-clamp studies, we found that (1) benzoyl-benzoyl ATP (BzATP) was the most effective agonist to evoke large inward currents and (2) the currents were abolished by P2X antagonists; however, (3) long-lasting application of BzATP did not cause an opening of large pores in addition to the cationic channels. By microfluorimetry it was shown that the P2X receptors mediated a Ca(2+) influx that contributed a small component to the total [Ca(2+)](i) response. Activation of P2X receptors may modulate the uptake of neurotransmitters from the extracellular space by Müller cells in the retina.

Cell-cell coupling in cultures of striatal and cortical astrocytes of the monkey Cebus apella

Astrocytes were cultured from striatum and neocortex of fetal (embryonic day 90) monkeys (Cebus apella). The cultures grew well, and the cells retained viability after freeze-storage and thawing. The cells displayed depolarized membrane potentials (-19 and -33 mV, for striatal and cortical cells, respectively) and the vast majority of cells were dye-coupled to a mean of 7 (1-18) neighbouring cells. Cell coupling was blocked by octanol (0.25 and 0.5 mM) but was independent of high K+ (10 and 50 mM) and glutamate (500 microm). Thus, cultures of fetal primate astrocytic cells are established as a model system for studies on astroglial cell-cell coupling.

Spatial distribution of spermine/spermidine content and K(+)-current rectification in frog retinal glial (Müller) cells

Previous studies in retinal glial (Müller) cells have suggested that (1) the dominant membrane currents are mediated by K(+) inward-rectifier (Kir) channels (Newman and Reichenbach, Trends Neurosci 19:307-312, 1996), and (2) rectification of these Kir channels is due largely to a block of outward currents by endogenous polyamines such as spermine/spermidine (SPM/SPD) (Lopatin et al., Nature 372:366-369, 1994). In frog Müller cells, the degree of rectification of Kir-mediated currents is significantly higher in the endfoot than in the somatic membrane (Skatchkov et al., Glia 27:171-181, 1999). This article shows that in these cells there is a topographical correlation between the local cytoplasmic SPM/SPD immunoreactivity and the ratio of inward to outward K(+) currents through the surrounding membrane area. Throughout the retina, Müller cell endfeet display a high SPM/SPD immunolabel (assessed by densitometry) and a large inward rectification of K(+) currents, as measured by the ratio of inward to outward current produced by step changes in [K(+)](o). In the retinal periphery, Müller cell somata are characterized by roughly one-half of the SPM/SPD immunoreactivity and K(+)-current rectification as the corresponding endfeet. In the retinal center, Müller cell somata are virtually devoid of both SPM/SPD immunolabel and K(+)-current inward rectification. Comparing one region of the retina with another, we find an exponential correlation between the local K(+) rectification and the local SPM/SPD content. This finding suggests that the degree of inward rectification in a given membrane area is determined by the local cytoplasmic polyamine concentration.

A function of delayed rectifier potassium channels in glial cells: maintenance of an auxiliary membrane potential under pathological conditions

Müller glial cells from human and guinea-pig retinae were investigated using the whole-cell patch-clamp technique. Human Müller cells from eyes with different diseases were characterized by diminished inwardly-rectifying K(+) currents. A comparable reduction of these currents was achieved in guinea pig Müller cells by treatment with iodoacetate to generate ischemia-like conditions. Consequently, the membrane potentials were reduced significantly in both diseased human and iodoacetate-treated guinea-pig Müller cells as compared to normal controls. However, the potentials were still clearly negative. Delayed rectifier currents could still be recorded under these conditions. Application of quinine blocked the delayed rectifier K(+) channels, and resulted in a total breakdown of the membrane potentials. Thus, it becomes apparent that the glial delayed rectifier K(+) channels are necessary to maintain an 'auxiliary' membrane potential under certain pathological conditions that are characterized by an almost total loss of inward rectifier conductance. Therefore, the delayed rectifier K(+) channels of glial cells may become crucial for the support of basic glial functions.

Patch-clamp study of neurons and glial cells in isolated myenteric ganglia

Most of the physiological information on the enteric nervous system has been obtained from studies on preparations of the myenteric ganglia attached to the longitudinal muscle layer. This preparation has a number of disadvantages, e.g., the inability to make patch-clamp recordings and the occurrence of muscle movements. To overcome these limitations we used isolated myenteric ganglia from the guinea pig small intestine. In this preparation movement was eliminated because muscle was completely absent, gigaseals were obtained, and whole cell recordings were made from neurons and glial cells. The morphological identity of cells was verified by injecting a fluorescent dye by micropipette. Neurons displayed voltage-gated inactivating inward Na(+) and Ca(2+) currents as well as delayed-rectifier K(+) currents. Immunohistochemical staining confirmed that most neurons have Na(+) channels. Neurons responded to GABA, indicating that membrane receptors were retained. Glial cells displayed hyperpolarization-induced K(+) inward currents and depolarization-induced K(+) outward currents. Glia showed large "passive" currents that were suppressed by octanol, consistent with coupling by gap junctions among these cells. These results demonstrate the advantages of isolated ganglia for studying myenteric neurons and glial cells.

Ca(2+) channel-mediated currents in retinal glial (Müller) cells of the toad (Bufo marinus)

Whole-cell voltage-clamp recordings were used to detect voltage-gated Ca(2+) channels in freshly isolated retinal glial (Müller) cells of the toad (Bufo marinus). Using Ca(2+) ions (2 mM) as charge carriers (in the presence of 1 mM Mg(2+)), no inwardly directed currents could be observed during the application of depolarizing voltage steps. However, after omitting the divalent cations from the bath solution, large-amplitude inwardly directed currents were evoked that were carried by Na(+) ions, and were mediated by at least two different kinds of Ca(2+) channels, transient low voltage-activated (LVA) channels and sustained high voltage-activated (HVA) channels. While the LVA currents activated at potentials positive to -90 mV and peaked at -40 mV, the HVA currents activated positive to -60 mV and peaked at -20 mV. It is concluded that Müller glial cells of the toad express distinct types of voltage-gated Ca(2+) channels that may be activated, under certain conditions, close to physiological membrane potentials.

Guinea-pig primary cell cultures provide a model to study expression and amyloidogenic processing of endogenous amyloid precursor protein

Until now guinea-pigs have been rarely used to investigate formation and deposition of Alzheimer's disease-associated amyloid beta peptides despite the sequence identity of human and guinea-pig amyloid beta peptides being known, and the overall similarity of human and guinea-pig amyloid precursor protein. We now describe a primary cell culture system of mixed fetal guinea-pig brain cells, which we have applied to characterize endogenous amyloid precursor protein processing and amyloid beta formation. These cell cultures were established at embryonic day 24 of guinea-pigs after comparison of selected stages of guinea-pig ontogenetic development with the known ontogeny of rats, and were characterized by immunocytochemical detection of neuronal and glial marker proteins. Amyloid precursor protein expression, processing and amyloid beta formation increased in parallel with cellular maturation during cultivation and reached a stable phase after approximately 14 days in vitro therefore providing a suitable time for analysis. Aged cultures display strong neuronal amyloid precursor protein immunoreactivity and an altered profile of amyloid precursor protein isoform messenger RNA expression due to glial proliferation as single neurons were shown to retain their typical pattern of amyloid precursor protein expression. We show that amyloid precursor protein in guinea-pig cells is processed by different protease activities which most likely represent alpha- and beta-secretase, leading to the generation of soluble amyloid precursor protein derivatives. Furthermore, endogenous amyloid precursor protein processing leads to production of substantial amounts of amyloid beta-peptides which accumulate in conditioned culture medium. Amyloid beta was readily detectable by western blot analysis and was shown to consist of approximately 80-90% amyloid beta(1-40). We suggest that primary guinea-pig cell cultures provide a valuable tool in amyloid research that resembles amyloid precursor protein processing under physiological concentrations and, therefore, the situation in humans more closely than current rodent models. It should be especially useful in screening experiments for secretase inhibiting compounds.

Role of glial K(+) channels in ontogeny and gliosis: a hypothesis based upon studies on Müller cells

The electrophysiological properties of Müller cells, the principal glial cells of the retina, are determined by several types of K(+) conductances. Both the absolute and the relative activities of the individual types of K(+) channels undergo important changes in the course of ontogenetic development and during gliosis. Although immature Müller cells express inwardly rectifying K(+) (K(IR)) currents at a very low density, the membrane of normal mature Müller cells is predominated by the K(IR) conductance. The K(IR) channels mediate spatial buffering K(+) currents and maintain a stable hyperpolarized membrane potential necessary for various glial-neuronal interactions. During "conservative" (i.e., non-proliferative) reactive gliosis, the K(IR) conductance of Müller cells is moderately reduced and the cell membrane is slightly depolarized; however, when gliotic Müller cells become proliferative, their K(IR) conductances are dramatically down-regulated; this is accompanied by an increased activity of Ca(2+)-activated K(+) channels and by a conspicuous unstability of their membrane potential. The resultant variations of the membrane potential may increase the activity of depolarization-activated K(+), Na(+) and Ca(2+) channels. It is concluded that in respect to their K(+) current pattern, mature Müller cells pass through a process of dedifferentiation before proliferative activity is initiated.

Expression of Dp71 in Müller glial cells: a comparison with utrophin- and dystrophin-associated proteins

PURPOSE

The abnormal retinal electrophysiology observed in patients with Duchenne muscular dystrophy (DMD) has been attributed to an altered expression of C-terminal products of the dystrophin gene. It has been shown that Dp260 is expressed by photoreceptor cells, whereas Dp71 is present in glial cells. The present study was intended to identify all known members of the dystrophin superfamily and their associated proteins expressed in Müller glial cells (MGC).

METHODS

The expression of the proteins and of their messengers was studied in MGC cultures from 2-week-old rats, by polymerase chain reaction amplification, Western blot analysis, and immunocytochemistry. An immunocytochemical localization of the proteins was also performed on enzymatically dissociated Müller cells from adult rat retinas.

RESULTS

MGCs expressed a spliced isoform of Dp71 called Dp71f, as well as utrophin, beta-dystroglycan, delta and gamma-sarcoglycans, and alpha1-syntrophin. In morphologically preserved differentiated Müller cells, Dp71f was localized in clusters, utrophin was diffusely distributed in the cytoplasm, and dystrophin-associated proteins (DAPs) were membrane-bound. Most of these proteins were preferentially expressed in the vitread portion of the cells. Dp71f and utrophin expression was restricted to MGCs, whereas all DAPs were also present in other retinal cell types.

CONCLUSIONS

The exclusive localization of Dp71f and utrophin in MGCs suggests that these proteins, together with DAPs, play a specific role in these cells. Further knowledge of possible interactions of these proteins within a functional complex may provide new insights into the molecular basis of the electroretinogram phenotype in DMD.

Human Müller glial cells: altered potassium channel activity in proliferative vitreoretinopathy

PURPOSE

To determine differences of K+ channel activity between Müller glial cells obtained from retinas of healthy human donors and of patients with retinal detachment and proliferative vitreoretinopathy.

METHODS

Müller cells were enzymatically isolated from retinas of healthy donors and from excised retinal pieces of patients. The whole-cell and the cell-attached configurations of the patch-clamp technique were used to characterize the current densities of different K+ channel types and the activity of single Ca2+ -activated K+ channels of big conductance (BK).

RESULTS

Cells from patients displayed a less negative mean membrane potential (-52.8 mV) than cells from healthy donors (-80.6 mV). However, the membrane potentials in cells from patients scattered largely between -6 and -99 mV. The inwardly rectifying K+ permeability in cells from patients was strongly reduced (0.3 pA/pF) when compared with cells from healthy donors (6.0 pA/pF). At the resting membrane potential, single BK channels displayed a higher mean activity (open probability, Po, and channel current amplitude) in cells from patients (Po, 0.30) than in cells from healthy donors (Po: 0.03). The variations of BK current amplitudes were correlated with the variations of the membrane potential.

CONCLUSIONS

The dominant expression of inwardly rectifying channels in cells from healthy donors is thought to support important glial cell functions such as the spatial buffering of extracellular K+. The downregulation of these channels and the less negative mean membrane potential in cells from patients should impair spatial buffering currents and neurotransmitter clearance. The increased activity of BK channels may support the proliferative activity of gliotic cells via feedback regulation of Ca2+ entry and membrane potential.

Outwardly rectifying K+ channels display clustering in guinea pig retinal Müller cells

The cell-attached configuration of the patch-clamp technique was used to characterize the outward currents in acutely isolated Müller cells from the guinea pig retina. Sixty-five of 353 patches displayed macroscopic, outwardly rectifying currents due to depolarizing voltage steps. Single channel transitions were found in only two patches. The remaining patches did not reveal any voltage-dependent currents. Tail current analysis revealed a reversal potential close to the resting membrane potential. The currents disappeared if internal K+ was replaced by Cs+ in inside-out patches. From these results we conclude that guinea pig Müller cells possess voltage-dependent K+ channels that are distributed in clusters.

Spermine/spermidine is expressed by retinal glial (Müller) cells and controls distinct K+ channels of their membrane

There is recent evidence that polyamines such as spermine (spm) and spermidine (spd) may act as endogenous modulators of the activity of inwardly rectifying K+ channels. This type of K+ channels is abundantly expressed by retinal glial (Müller) cells where they are involved in important glial cell functions such as the clearance of excess extracellular K+ ions. This prompted us to study the following questions, i) do mammalian Müller cells contain endogenous spm/spd?; ii) do Müller cells possess the enzymes (e.g., ornithine decarboxylase, ODC) necessary to produce spm/spd?; and iii) does application of exogenous spm/spd exert specific effects onto inwardly rectifying K+ channels of Müller cells? Immunocytochemical studies were performed on histological sections of guinea-pig, rabbit, porcine, and human retinae, and on enzymatically dissociated Müller cells. Whole-cell and patch-clamp recordings were performed on enzymatically dissociated porcine and guinea-pig Müller cells. All above-mentioned questions could be answered with "yes." Specifically, the majority of Müller cells were labeled with antibodies directed to spm/spd, both within retinal sections and enzymatically isolated from retinal tissue. Müller cells in normal retinae express low levels of ODC but increase this expression markedly in cases of retinal pathology such as experimental epiretinal melanoma. Externally applied polyamines (1 mM) reduce (predominantly inward) whole-cell K+ currents, with the efficacies being spm > spd > put. If applied at the inside of membrane patches, spm (1 mM) blocks completely the outward currents through inwardly rectifying K+ channels but fails to affect the activity of large conductance, Ca2+-activated K+ channels. It is concluded that Müller cells contain endogenous channel-active polyamines, the synthesis of which may be up-regulated in pathological situations, and which may be involved in the control of both glial function and cell proliferation.

The Müller (glial) cell in normal and diseased retina: a case for single-cell electrophysiology

In the retina of most vertebrates there exists only one type of macroglia, the Müller cell. Müller cells express voltage-gated ion channels, neurotransmitter receptors and various uptake carrier systems. These properties enable the Müller cells to control the activity of retinal neurons by regulating the extracellular concentration of neuroactive substances such as K+, GABA and glutamate. We show here how electrophysiological recordings from enzymatically dissociated mammalian Müller cells can be used to study these mechanisms. Müller cells from various species have Na(+)-dependent GABA uptake carriers, but only cells from primates have additional GABA receptors that activate Cl- channels. Application of glutamate analogues causes enhanced membrane currents recorded from Müller cells in situ but not from isolated cells. We show that mammalian Müller cells have no ionotropic glutamate receptors but respond to increased K+ release from glutamate-stimulated retinal neurons. This response is involved in extracellular K+ clearance and is mediated by voltage-gated (inwardly rectifying) K+ channels which are abundantly expressed by healthy Müller cells. In various cases of human retinal pathology, currents through these channels are strongly reduced or even extinguished. Another type of voltage-gated ion channels, observed in Müller cells from many mammalian species, are Na+ channels. In Müller cells from diseased human retinae, voltage-dependent Na+ currents were significantly increased in comparison to cells from control donors. Thus, the expression of glial ion channels seems to be controlled by neuronal signals. This interaction may be involved in the pathogenesis of retinal gliosis which inevitably accompanies any degeneration of retinal neurons. In particular, Müller cell proliferation may be triggered by mechanisms requiring the activation of Ca(2+)-dependent K+ channels. Ca(2+)-dependent K+ currents are easily elicitable in Müller cells from degenerating retinae and can be blocked by 1 mM TEA (tetraethylammonium). In purified Müller cell cultures, the application of 1 mM TEA greatly reduces the proliferative activity of the cells. These data clearly show that Müller cells are altered in cases of neuronal degeneration and may be crucially involved in pathogenetic mechanisms of the retina.

Comparison between functional characteristics of healthy and pathological human retinal Müller glial cells

Enzymatically isolated, noncultured Müller glial cells from human organ donor and patient eyes were studied using the whole-cell-voltage-clamp and the patch-clamp technique. The patients suffered mainly from choroidal melanomas, retinal detachment due to proliferative vitreorentinopathy, glaucomas, and perforating eye injuries. The organ donor eyes were used as a source of corneas for corneal transplantation. Use of the human retinal tissue was approved by the Ethics Committee of the School of Medicine, University of Leipzig. Most of the patient Müller cells showed a marked or complete loss of inwardly rectifying K+ currents, causing a dramatic increase in the input resistance. The zero current potential of the patient Müller cells, which is equivalent to the membrane potential, was significantly reduced (depolarized) as compared with the donor Müller cells. In contrast to the K+ current loss, the Na+ current density was significantly higher in patient Müller cells than in donor Müller cells; the number of Müller cells depicting Na+ currents increased from 33% (3 pA/pF) in donor Müller cells to 85% (about 12pA/pF) in patient Müller cells. Application of glutamate to the Müller cells generated a glutamate-transporter-mediated current, such as that seen in other species. A highly significant increase was noted for the high-affinity Na+-dependent glutamate-transporter-current density in patient Müller cells compared with donor cells. The application of gamma-aminobutyric acid (GABA) evoked, in addition to the GABA transporter currents already known from Müller cells of other mammalian species, GABA(A)-receptor mediated currents in human Müller cells. We found that GABA(A) receptors are expressed in human Müller cells, but not in other nonprimate mammals. Whether a difference exists between the GABA(A) current density in donor and patient Müller cells remains to be seen. The results concerning the disappearance of K+ currents and diminution of the membrane potential may demonstrate early glial changes that may possibly precede pathological neuronal changes, at least in retinas from eyes with choroidal melanomas. In later stages of the diseases, the glial changes may be deleterious for the neurons, because they could diminish glutamate uptake due to the depolarized membrane potential. However, increased extracellular glutamate concentration is toxic for most neurons.

Loss of inwardly rectifying potassium currents by human retinal glial cells in diseases of the eye

We compared the inward K+ currents of Müller glial cells from healthy and pathologically changed human retinas. To this purpose, the whole-cell voltage-clamp technique was performed on noncultured Müller cells acutely isolated from human retinas. Cells originated from retinas of four healthy organ donors and of 24 patients suffering from different vitreoretinal and chorioretinal diseases. Müller cells from organ donors displayed inward K+ currents in the whole-cell mode similar to those found in other species. In contrast, this pattern was clearly changed in the Müller cells from patient retinas. In whole-cell recordings many Müller cells had strongly decreased inward K+ current amplitudes or lost these currents completely. Thus, the mean input resistance of Müller cells from patients was significantly increased to 1,129 +/- 812 M omega, compared to 279 +/- 174 M omega in Müller cells from healthy organ donor retinas. Accordingly, since the membrane potential is mainly determined by the K+ inward conductance in healthy Müller cells, a large amount of Müller cells from patient retinas had a membrane potential which was significantly lower than that of Müller cells from control eyes. The mean membrane potentials were -37 +/- 24 mV and -63 +/- 25 mV for patient and donor Müller cells, respectively. The newly described membrane characteristic changes of Müller cells from patient eyes are assumed to interfere severely with normal retinal function: (1) the retinal K+ homeostasis, which is partly regulated by the Müller cell-mediated spatial buffering, should be disturbed, and (2) the diminished membrane potential should influence voltage-dependent transporter systems of the Müller cells, e.g., the Na(+)-dependent glutamate uptake.

Comparative studies on mammalian Müller (retinal glial) cells

Müller cells from 22 mammalian species were subjected to morphological and electrophysiological studies. In the 'midperiphery' of retinae immunocytochemically labeled for vimentin, estimates of Müller cell densities per unit retinal surface area, and of neuron-to-(Müller) glia indices were performed. Müller cell densities were strikingly similar among the species studied (around 8000-11,000 mm-2) with the extremes of the horse (< or = 5000 mm-2) and the tree shrew (> or = 20,000 mm-2). By contrast, the number of neurons per Müller cell varied widely, being clustered at 6-8 (in retinae with many cones), at about 16, and at up to more than 30 (in strongly rod-dominated retinae). Isolated Müller cell volumes were estimated morphometrically, and cell surface areas were calculated from membrane capacities. Müller cells isolated from thick vascularized retinae (carnivores, rats, mice, ungulates) were longer and thinner, and had smaller volumes but higher surface-to-volume ratios than cells from thin paurangiotic (i.e. with blood vessels only near the optic disc) or avascular retinae (rabbits, guinea pigs, horses, zebras). In whole-cell voltage-clamp studies, Müller cells from all mammals studied displayed two dominant K+ conductances, inwardly rectifying currents and delayed rectifier currents. TTX-sensitive Na+ currents were recorded only in some species. Based on these data, the following hypotheses are presented, (a) neuron-to-(Müller) glia indices are determined by precursor cell proliferation rather than by metabolic demands; (b) Müller cell volumes depend on available space rather than on the number of supported neurons; and (c) it follows that, the specific metabolic activities of Müller cells must differ greatly between species, a difference that may contribute to distinct patterns of retinal vascularization.

The glutathione level of retinal Müller glial cells is dependent on the high-affinity sodium-dependent uptake of glutamate

The dependence of intracellular glutathione, an important radical scavenger, on the extracellular glutamate and cystine concentration and the velocity of the high affinity sodium/glutamate transporter was studied in freshly-isolated Müller glial cells of the guinea-pig, kept in vitro for up to 11 h. To this end the relative Müller cell glutathione levels were measured using the fluorescent dye monochlorobimane, using different concentrations of glutamate and cystine in Ringer solution. In some experiments L-buthionine-[S,R]-sulfoximine, a blocker of glutathione synthesis, or L-trans-pyrrolidine-2,4-dicarboxylic acid and L-alpha-aminoadipic acid, inhibitors of glutamate uptake, were added. The Müller cells maintained about 80% of the normal glutathione level when maintained in Ringer solution containing 100 microM glutamate for 11 h. When under these conditions 100 microM cystine was added, the glutathione level increased to values, which were even higher than those at the beginning of the incubation period. Addition of cystine without glutamate caused a run down of the glutathione level to about 45% of the normal level, which is comparable to the run down in pure Ringer solution. Likewise, application of L-buthionine-[S,R]-sulfoximine (5 mM) lead to a strong run down of the glutathione level even in glutamate/cystine (100 microM)-containing solution. A similar suppressing effect was observed using L-trans-pyrrolidine-2,4-dicarboxylic acid and L-alpha-aminoadipic acid in the presence of 100 microM cystine and glutamate. We conclude that the intracellular glutamate concentration of the Müller cells is determined by the extracellular glutamate concentration and the velocity of the sodium/glutamate uptake. Consequently, cystine uptake into Müller cells, which is performed by the cystine/glutamate antiporter, is fueled by the sodium/glutamate transporter with intracellular glutamate. Both glutamate and cystine are also substrates for glutathione synthesis. The glutathione level is logically limited by the capacity of the sodium/glutamate transporter to provide glutamate intracellularly for, first, cystine uptake and, second, direct insertion into glutathione. Accordingly, the glutathione level is reduced when the sodium/glutamate transporter is blocked. Thus, a diminution of the glutathione level should be taken into consideration when the effects of sodium/glutamate uptake failure and reduced intracellular glutamate concentrations are discussed.

Sodium current amplitude increases dramatically in human retinal glial cells during diseases of the eye

Müller cells, the main macroglial cells of the retina, express several types of voltage and ligand-activated ion channels, including Na+ channels. Using the whole-cell voltage-clamp technique, we studied the expression of Na+ currents in acutely isolated, non-cultivated human Müller cells from retinas of healthy organ donors and patients suffering from different eye diseases. In both types of retinas transient Na+ currents could be recorded from Müller cells. The tetrodotoxin-resistant Na+ currents, which were not completely blocked even at a concentration of 10 microM tetrodotoxin, had a mean current density of 3.0 +/- 3.0 pA/pF (mean +/- SD, n = 10) in Müller cells from donor retinas and of 12.2 +/- 9.6 pA/pF (n = 74) in Müller cells from patient retinas. Only 33.3% of healthy but 88.4% of pathological Müller cells depicted such currents. The GNa+/GK+ ratio was very high in several Müller cells from patient retinas, such that action potential-like activity could be generated after prehyperpolarizing current injection in some of these cells. Apparently, the Na+ channels, due to their negative steady-state inactivation curve (Vh = -84.5 mV), do not influence the lowered membrane potential of the pathological cells, since they are inactivated at these voltages. Currently, we do not have an explanation for the increase in amplitude and frequency of Na+ currents in human Müller cells under pathological conditions. However, the up-regulation of Na+ channels may mirror a basic glial response to pathological conditions, since it has also been found previously in human hippocampal astrocytes from epileptic foci and in rat cortex stab wounds lined by an astrocytic scar.

Repetitive depletion and recovery of intracellular K+ in retinal Müller glial cells during whole-cell voltage-clamp

A procedure was developed allowing repetitive depletion and recovery of K+ from the cell interior of Müller glial cells without patch pipette perfusion. To this end the whole-cell voltage-clamp technique using tight seal pipettes was applied to enzymatically isolated Müller cells of the guinea pig. When K+ was replaced by Cs+ and/or NMDG in the pipette solution voltage-activated outward currents could be generated similar to those found with normal K(+)-containing intracellular solution. This was the result of the distribution of K+ across the cell membrane due to the negative holding potential, i.e., K+ accumulated at the intracellular side of the cell membrane. This distribution should be favored by the low input resistance of Müller cells. K+ could also be removed from the cell interior by depolarizing voltage steps. Simultaneously, K+ influx had to be cut off by using a K(+)-free extracellular solution or blocking K+ channels using Ba2+. This procedure lead to reduction and eventually cessation of the K+ outward currents, indicating extinction of the intracellular K+ pool. The process is reversible, i.e., outward currents can be evoked again by depolarizing voltage steps after renewed application of extracellular K+ and/or removal of Ba2+. Hence, the intracellular K+ pool is filled up again. The described method was applied to demonstrate the dependence of the Müller cell Na+/glutamate transporter on the intracellular K+ concentration.

alpha-Aminoadipic acid blocks the Na(+)-dependent glutamate transport into acutely isolated Müller glial cells from guinea pig retina

The effect of the glial toxin alpha-aminoadipic acid (AAA) upon the Na+/glutamate cotransporter of acutely isolated guinea pig retinal glial cells was studied using the whole-cell voltage-clamp technique. Glutamate evoked an inward current in these cells at negative holding potentials dependent on the presence of extracellular Na+ and intracellular K+. A reversal potential could not be found for the current. L-trans-Pyrrolidine-2.4-dicarboxylic acid (PDC), a blocker of Na(+)-dependent glutamate uptake, diminished the glutamate current also in our cells. Application of L-AAA also generated an inward current at negative holding potentials, without a reversal potential, being suppressed if extracellular Na+ or intracellular K+ was removed. The glutamate uptake blocker, PDC (200 microM), blocked the L-AAA (1 mM) current. Thus, L-AAA proved to be transported by the Na+/glutamate transporter of Müller cells. Hence, glutamate currents were diminished by L-AAA competitively with a Km of 499 microM at a glutamate concentration of 10 microM. The Na+/glutamate uptake was less sensitive to DL- and D-AAA block. It is suggested that the blocking effect of AAA on Na(+)-dependent glutamate uptake into glial cells might be involved in the well known glia toxicity of this compound.

Patch-clamp recording from Müller (glial) cell endfeet in the intact isolated retina and acutely isolated Müller cells of mouse and guinea-pig

Müller cells span through the entire retina and terminate with the formation of endfeet at the vitreous body. These endfeet are thought to be specialized for maintaining the K+ homeostasis in the retina based on the assumption that voltage signals can passively spread from the cell body to the endfeet. We employed the patch-clamp technique to study the physiological properties of these endfeet in a retinal wholemount preparation from guinea-pig or mouse. After assessing one endfoot with the patch pipette and establishing the whole cell recording configuration, a membrane area which approximately matched the size of one endfoot and proximal process could be voltage-clamped. This morphological correlation could be established by filling the cytoplasm with the fluorescent dye Lucifer Yellow via the patch-pipette. The morphological, immunocytochemical and ultrastructural inspection of the recorded cells revealed that mouse Müller cell endfeet were connected by only a thin stalk to the proximal process. In contrast, guinea-pig endfeet were connected by thick stalks. The endfoot current in the mouse was dominated by a voltage and time-independent K+ conductance. In contrast, in some of the recordings from guinea-pig, delayed and inwardly rectifying K+ currents were observed. These voltage-gated currents were more frequently observed or were facilitated when the membrane area under voltage clamp was increased, blocking the passive K+ currents by Ba2+ in both, mouse and guinea-pig. We thus assume that the voltage-gated currents were not in the endfeet membrane, but rather in the proximal process and could thus be better activated in the guinea-pig with its thicker stalk or after increasing the membrane area under voltage clamp control. Similar results were obtained in freshly isolated Müller cells; in contrast to the cells from the wholemount the voltage-gated currents were more frequently observed. These studies demonstrate that the Müller cell endfoot of the mouse with its vascularized retina is an electrically isolated unit and that voltage signals do not spread to the proximal process. Such a property would, however, be required for the redistribution of K+ via spatial buffer currents. In contrast, guinea-pig Müller glial cells with their stout morphological connection between endfoot and proximal process are better suited to fulfil this task.

Voltage-dependent K+ currents in guinea pig Müller (glia) cells show different sensitivities to blockade by Ba2+

The effect of externally applied Ba2+ and Na+ on K+ currents was investigated by means of whole-cell patch-clamp in isolated and in situ Müller cells from guinea pig retina. Müller cells express a typical set of K+ currents, i.e. an ohmic current, an inactivating inward current (IK(IR)), a delayed rectifier (IK(DR)) and an inactivating outward current (IK(A)). Inactivation of the inward current did not occur when extracellular Na+ was replaced by choline. When administered in increasing concentrations, Ba2+ blocked these K+ currents in a typical sequence: the ohmic current and IK(A) were most sensitive, followed by IK(IR), whereas IK(DR) was not completely blocked even in 1 mM Ba2+. The differential sensitivity of Müller cell K+ currents to external Ba2+ may be a tool which can be used to improve our understanding of the Müller cell response to physiological stimulation of the retina.

Na+ channels are expressed by mammalian retinal glial (Müller) cells

Müller cells constitute the principal glia of the vertebrate retina. Unlike other types of neuroglial cells such as astrocytes and Schwann cells, Müller cells have not yet been demonstrated to express Na+ channels. Here we present first evidence of Müller cell Na+ currents from voltage-clamp studies in enzymatically isolated cells. Some cells from retinae of cats and dogs, but none from rabbit or guinea-pig retinae, revealed fast and rapidly inactivating inward currents in response to depolarizing voltage steps. The currents reversibly disappeared in Na+ free solutions or under tetrodotoxin (TTX, 1 microM). Activation and inactivation characteristics of these currents were strikingly similar to those of neurone-type Na+ channels.