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

The origins of the full-field flash electroretinogram b-wave

The electroretinogram (ERG) measures the electrical activity of retinal neurons and glial cells in response to a light stimulus. Amongst other techniques, clinicians utilize the ERG to diagnose various eye diseases, including inherited conditions such as cone-rod dystrophy, rod-cone dystrophy, retinitis pigmentosa and Usher syndrome, and to assess overall retinal health. An ERG measures the scotopic and photopic systems separately and mainly consists of an a-wave and a b-wave. The other major components of the dark-adapted ERG response include the oscillatory potentials, c-wave, and d-wave. The dark-adapted a-wave is the initial corneal negative wave that arises from the outer segments of the rod and cone photoreceptors hyperpolarizing in response to a light stimulus. This is followed by the slower, positive, and prolonged b-wave, whose origins remain elusive. Despite a large body of work, there remains controversy around the mechanisms involved in the generation of the b-wave. Several hypotheses attribute the origins of the b-wave to bipolar or Müller glial cells or a dual contribution from both cell types. This review will discuss the current hypothesis for the cellular origins of the dark-adapted ERG, with a focus on the b-wave.

Activation of retinal glial cells contributes to the degeneration of ganglion cells in experimental glaucoma

Elevation of intraocular pressure (IOP) is a major risk factor for neurodegeneration in glaucoma. Glial cells, which play an important role in normal functioning of retinal neurons, are well involved into retinal ganglion cell (RGC) degeneration in experimental glaucoma animal models generated by elevated IOP. In response to elevated IOP, mGluR I is first activated and Kir4.1 channels are subsequently inhibited, which leads to the activation of Müller cells. Müller cell activation is followed by a complex process, including proliferation, release of inflammatory and growth factors (gliosis). Gliosis is further regulated by several factors. Activated Müller cells contribute to RGC degeneration through generating glutamate receptor-mediated excitotoxicity, releasing cytotoxic factors and inducing microglia activation. Elevated IOP activates microglia, and following morphological and functional changes, these cells, as resident immune cells in the retina, show adaptive immune responses, including an enhanced release of pro-inflammatory factors (tumor neurosis factor-α, interleukins, etc.). These ATP and Toll-like receptor-mediated responses are further regulated by heat shock proteins, CD200R, chemokine receptors, and metabotropic purinergic receptors, may aggravate RGC loss. In the optic nerve head, astrogliosis is initiated and regulated by a complex reaction process, including purines, transmitters, chemokines, growth factors and cytokines, which contributes to RGC axon injury through releasing pro-inflammatory factors and changing extracellular matrix in glaucoma. The effects of activated glial cells on RGCs are further modified by the interplay among different types of glial cells. This review is concluded by presenting an in-depth discussion of possible research directions in this field in the future.

Lipid metabolism and retinal diseases

PURPOSE

The retina has enormous lipids demands and must meet those needs. Retinal lipid homeostasis appears to be based on the symbiosis between neurons, Müller glial cells (MGC), and retinal pigment epithelium (RPE) cells, which can be impacted in several retinal diseases. The current research challenge is to better understand lipid-related mechanisms involved in retinal diseases, such as age-related macular degeneration (AMD) and glaucoma.

RESULTS

In a first axis, in vitro and focus on Müller glial cell, we aimed to characterize whether the 24S-hydroxycholesterol (24S-OHC), an overexpressed end-product of cholesterol elimination pathway in neural tissue and likely produced by suffering retinal ganglion cells in glaucoma, may modulate MGC membrane organization, such as lipid rafts, to trigger cellular signalling pathways related to retinal gliosis. We have found that lipid composition appears to be a key factor of membrane architecture, especially for lipid raft microdomain formation, in MGC. However, 24S-OHC did not appear to trigger retinal gliosis via the modulation of lipid or protein composition within lipid rafts microdomains. This study provided a better understanding of the complex mechanisms involved in the pathophysiology of glaucoma. On a second clinical ax, we focused on the lipid-related mechanisms involved in the dysfunction of aging RPE and the appearance of drusenoid deposits in AMD. Using the Montrachet population-based study, we intend to report the frequency of reticular pseudodrusen (RPD) and its ocular and systemic risk factors, particularly related to lipid metabolisms, such as plasma lipoprotein levels, carotenoids levels, and lipid-lowering drug intake. Our study showed that RPD was less common in subjects taking lipid-lowering drugs. Lipid-lowering drugs, such as statins, may reduce the risk of RPD through their effect on the production and function of lipoproteins. This observation highlights the potential role of retinal lipid trafficking via lipoproteins between photoreceptors and retinal pigment epithelium cells in RPD formation. Those findings have been complemented with preliminary results on the analysis of plasma fatty acid (FA) profile, a surrogate marker of short-term dietary lipid intake, according to the type of predominant drusenoid deposit, soft drusen or RPD, in age-related maculopathy.

CONCLUSION

Further research on lipid metabolism in retinal diseases is warranted to better understand the pathophysiology of retinal diseases and develop new promising diagnostic, prognostic, and therapeutic tools for our patients.

Glia as a key factor in cell volume regulation processes of the central nervous system

Brain edema is a pathological condition with potentially fatal consequences, related to cerebral injuries such as ischemia, chronic renal failure, uremia, and diabetes, among others. Under these pathological states, the cell volume control processes are fully compromised, because brain cells are unable to regulate the movement of water, mainly regulated by osmotic gradients. The processes involved in cell volume regulation are homeostatic mechanisms that depend on the mobilization of osmolytes (ions, organic molecules, and polyols) in the necessary direction to counteract changes in osmolyte concentration in response to water movement. The expression and coordinated function of proteins related to the cell volume regulation process, such as water channels, ion channels, and other cotransport systems in the glial cells, and considering the glial cell proportion compared to neuronal cells, leads to consider the astroglial network the main regulatory unit for water homeostasis in the central nervous system (CNS). In the last decade, several studies highlighted the pivotal role of glia in the cell volume regulation process and water homeostasis in the brain, including the retina; any malfunction of this astroglial network generates a lack of the ability to regulate the osmotic changes and water movements and consequently exacerbates the pathological condition.

Critical Role of Astrocytic Polyamine and GABA Metabolism in Epileptogenesis

Accumulating evidence indicate that astrocytes are essential players of the excitatory and inhibitory signaling during normal and epileptiform activity via uptake and release of gliotransmitters, ions, and other substances. Polyamines can be regarded as gliotransmitters since they are almost exclusively stored in astrocytes and can be released by various mechanisms. The polyamine putrescine (PUT) is utilized to synthesize GABA, which can also be released from astrocytes and provide tonic inhibition on neurons. The polyamine spermine (SPM), synthesized form PUT through spermidine (SPD), is known to unblock astrocytic Cx43 gap junction channels and therefore facilitate astrocytic synchronization. In addition, SPM released from astrocytes may also modulate neuronal NMDA, AMPA, and kainate receptors. As a consequence, astrocytic polyamines possess the capability to significantly modulate epileptiform activity. In this study, we investigated different steps in polyamine metabolism and coupled GABA release to assess their potential to control seizure generation and maintenance in two different epilepsy models: the low-[Mg2+] model of temporal lobe epilepsy in vitro and in the WAG/Rij rat model of absence epilepsy in vivo. We show that SPM is a gliotransmitter that is released from astrocytes and significantly contributes to network excitation. Importantly, we found that inhibition of SPD synthesis completely prevented seizure generation in WAG/Rij rats. We hypothesize that this antiepileptic effect is attributed to the subsequent enhancement of PUT to GABA conversion in astrocytes, leading to GABA release through GAT-2/3 transporters. This interpretation is supported by the observation that antiepileptic potential of the Food and Drug Administration (FDA)-approved drug levetiracetam can be diminished by specifically blocking astrocytic GAT-2/3 with SNAP-5114, suggesting that levetiracetam exerts its effect by increasing surface expression of GAT-2/3. Our findings conclusively suggest that the major pathway through which astrocytic polyamines contribute to epileptiform activity is the production of GABA. Modulation of astrocytic polyamine levels, therefore, may serve for a more effective antiepileptic drug development in the future.

On the electrical passivity of astrocyte potassium conductance

Predominant expression of leak-type K⁺ channels provides astrocytes a high membrane permeability to K⁺ ions and a hyperpolarized membrane potential that are crucial for astrocyte function in brain homeostasis. In functionally mature astrocytes, the expression of leak K⁺ channels creates a unique membrane K⁺ conductance that lacks voltage-dependent rectification. Accordingly, the conductance is named ohmic or passive K⁺ conductance. Several inwardly rectifying and two-pore domain K⁺ channels have been investigated for their contributions to passive conductance. Meanwhile, gap junctional coupling has been postulated to underlie the passive behavior of membrane conductance. It is now clear that the intrinsic properties of K⁺ channels and gap junctional coupling can each act alone or together to bring about a passive behavior of astrocyte conductance. Additionally, while the passive conductance can generally be viewed as a K⁺ conductance, the actual representation of this conductance is a combined expression of multiple known and unknown K⁺ channels, which has been further modified by the intricate morphology of individual astrocytes and syncytial gap junctional coupling. The expression of the inwardly rectifying K⁺ channels explains the inward-going component of passive conductance disobeying Goldman-Hodgkin-Katz constant field outward rectification. However, the K⁺ channels encoding the outward-going passive currents remain to be determined in the future. Here, we review our current understanding of ion channels and biophysical mechanisms engaged in the passive astrocyte K⁺ conductance, propose new studies to resolve this long-standing puzzle in astrocyte physiology, and discuss the functional implication(s) of passive behavior of K⁺ conductance on astrocyte physiology.

The Na+-K+-ATPase is needed in glia of touch receptors for responses to touch in C. elegans

Four of the five types of mammalian mechanosensors are composed of nerve endings and accessory cells. In Caenorhabditis elegans we showed that glia support the function of nose touch neurons via the activity of glial Na⁺ and K⁺ channels. We show here that a third regulator of Na⁺ and K⁺, the Na⁺-K⁺-ATPase, is needed in glia of nose touch neurons for touch. Importantly, we show that two Na⁺-K⁺-ATPase genes are needed for the function rather than structural integrity and that their ion transport activity is crucial for touch. Finally, when glial Na⁺-K⁺-ATPase genes are knocked out, touch can be restored by activation of a third Na⁺-K⁺-ATPase. Taken together, these data show the requirement in glia of touch neurons of the function of the Na⁺-K⁺-ATPase. These data underscore the importance of the homeostasis of Na⁺ and K⁺, most likely in the space surrounding touch neurons, in touch sensation, a function that might be conserved across species.NEW & NOTEWORTHY Increasing evidence supports that accessory cells in mechanosensors regulate neuronal output; however, the glial molecular mechanisms that control this regulation are not fully understood. We show here in Caenorhabditis elegans that specific glial Na⁺-K⁺-ATPase genes are needed for nose touch-avoidance behavior. Our data support the requirement of these Na⁺-K⁺-ATPases for homeostasis of Na⁺ and K⁺ in nose touch receptors. Our data add to our understanding of glial regulation of mechanosensors.

Investigating the Effects of Mechanical Stimulation on Retinal Ganglion Cell Spontaneous Spiking Activity

Mechanical forces are increasingly recognized as major regulators of several physiological processes at both the molecular and cellular level; therefore, a deep understanding of the sensing of these forces and their conversion into electrical signals are essential for studying the mechanosensitive properties of soft biological tissues. To contribute to this field, we present a dual-purpose device able to mechanically stimulate retinal tissue and to record the spiking activity of retinal ganglion cells (RGCs). This new instrument relies on combining ferrule-top micro-indentation, which provides local measurements of viscoelasticity, with high-density multi-electrode array (HD-MEAs) to simultaneously record the spontaneous activity of the retina. In this paper, we introduce this instrument, describe its technical characteristics, and present a proof-of-concept experiment that shows how RGC spiking activity of explanted mice retinas respond to mechanical micro-stimulations of their photoreceptor layer. The data suggest that, under specific conditions of indentation, the retina perceive the mechanical stimulation as modulation of the visual input, besides the longer time-scale of activation, and the increase in spiking activity is not only localized under the indentation probe, but it propagates across the retinal tissue.

Aquaporin 4 Suppresses Neural Hyperactivity and Synaptic Fatigue and Fine-Tunes Neurotransmission to Regulate Visual Function in the Mouse Retina

The bidirectional water channel aquaporin 4 (AQP4) is abundantly expressed in the neural tissue. The advantages and disadvantages of AQP4 neural tissue deficiency under pathological conditions, such as inflammation, and relationship with neural diseases, such as Alzheimer’s disease, have been previously reported. However, the physiological functions of AQP4 are not fully understood. Here, we evaluated the role of AQP4 in the mouse retina using Aqp4 knockout (KO) mice. Aqp4 was expressed in Müller glial cells surrounding the synaptic area between photoreceptors and bipolar cells. Both scotopic and photopic electroretinograms showed hyperactive visual responses in KO mice, gradually progressing with age. Moreover, the amplitude reduction after frequent stimuli and synaptic fatigue was more severe in KO mice. Glutamine synthetase, glutamate aspartate transporter, synaptophysin, and the inward potassium channel Kir2.1, but not Kir4.1, were downregulated in KO retinas. KIR2.1 colocalized with AQP4 in Müller glial cells at the synaptic area, and its expression was affected by Aqp4 levels in primary Müller glial cell cultures. Intraocular injection of potassium in wild-type mice led to visual function hyperactivity, as observed in Aqp4 KO mice. Mitochondria molecules, such as Pgc1α and CoxIV, were downregulated, while apoptotic markers were upregulated in KO retinas. AQP4 may fine-tune synaptic activity, most likely by regulating potassium metabolism, at least in part, via collaborating with KIR2.1, and possibly indirectly regulating glutamate kinetics, to inhibit neural hyperactivity and synaptic fatigue which finally affect mitochondria and cause neurodegeneration.

A-Kinase-Anchoring Protein (AKAP150) is expressed in Astrocytes and Upregulated in Response to Ischemia

A-kinase-anchoring proteins, AKAPs, are scaffolding proteins that associate with kinases and phosphatases, and direct them to a specific submembrane site to coordinate signaling events. AKAP150, a rodent ortholog of human AKAP79, has been extensively studied in neurons, but very little is known about the localization and function of AKAP150 in astrocytes, the major cell type in brain. Thus, in this study, we assessed the localization of AKAP150 in astrocytes and elucidated its role during physiological and ischemic conditions. Herein, we demonstrate that AKAP150 is localized in astrocytes and is up-regulated during ischemia both in vitro and in vivo. Knock-down of AKAP150 by RNAi depolarizes the astrocytic membrane potential and substantially reduces by 80% the ability of astrocytes to take up extracellular potassium during ischemic conditions. Therefore, upregulation of AKAP150 during ischemia preserves potassium conductance and the associated hyperpolarized membrane potential of astrocytes; properties of astrocytes needed to maintain extracellular brain homeostasis. Taken together, these data suggest that AKAP150 may play a pivotal role in the neuroprotective mechanism of astrocytes during pathological conditions.

Adenosine A1 receptor activates background potassium channels and modulates information processing in olfactory bulb mitral cells

KEY POINTS

Adenosine is a widespread neuromodulator in the mammalian brain, but whether it affects information processing in sensory system(s) remains largely unknown. Here we show that adenosine A₁ receptors hyperpolarize mitral cells, one class of principal neurons that propagate odour information from the olfactory bulb to higher brain areas, by activation of background K⁺ channels. The adenosine-modulated background K⁺ channels belong to the family of two-pore domain K⁺ channels. Adenosine reduces spontaneous activity of mitral cells, whereas action potential firing evoked by synaptic input upon stimulation of sensory neurons is not affected, resulting in a higher ratio of evoked firing (signal) over spontaneous firing (noise) and hence an improved signal-to-noise ratio. The study shows for the first time that adenosine influences fine-tuning of the input-output relationship in sensory systems.

ABSTRACT

Neuromodulation by adenosine is of critical importance in many brain regions, but the role of adenosine in olfactory information processing has not been studied so far. We investigated the effects of adenosine on mitral cells, which are projection neurons of the olfactory bulb. Significant expression of A₁ and A_2A receptors was found in mitral cells, as demonstrated by in situ hybridization. Application of adenosine in acute olfactory bulb slices hyperpolarized mitral cells in wild-type but not in adenosine A₁ receptor knockout mice. Adenosine-induced hyperpolarization was mediated by background K⁺ currents that were reduced by halothane and bupivacaine, which are known to inhibit two-pore domain K⁺ (K2P) channels. In mitral cells, electrical stimulation of axons of olfactory sensory neurons evoked synaptic currents, which can be considered as input signals, while spontaneous firing independent of sensory input can be considered as noise. Synaptic currents were not affected by adenosine, while adenosine reduced spontaneous firing, leading to an increase in the signal-to-noise ratio of mitral cell firing. Our findings demonstrate that A₁ adenosine receptors activate two-pore domain K⁺ channels, which increases the signal-to-noise ratio of the input-output relationship in mitral cells and thereby modulates information processing in the olfactory bulb.

An emerging role of astrocytes in vascular contributions to cognitive impairment and dementia

Vascular contributions to cognitive impairment and dementia (VCID) is understood to be the second most common cause of dementia after Alzheimer's disease, and is also a frequent comorbidity with Alzheimer's disease. While VCID is widely acknowledged as a key contributor to dementia, the mechanistic underpinnings of VCID remain poorly understood. In this review, we address the potential role of astrocytes in the pathophysiology of VCID. The vast majority of the blood vessels in the brain are surrounded by astrocytic end-feet. Given that astrocytes make up a significant proportion of the cells in the brain, and that astrocytes are usually passively connected to one another through gap junctions, we hypothesize that astrocytes are key mediators of cognitive impairment because of cerebrovascular disease. In this review, we discuss the existing body of literature regarding the role of astrocytes at the vasculature in the brain, and the known consequences of their dysfunction, as well as our hypotheses regarding the role astrocytes play in VCID. This article is part of the Special Issue "Vascular Dementia".

The SH2B3 and KCNK5 loci may be implicated in regulation of platelet count, volume, and maturity

INTRODUCTION

In recent genome-wide association studies, coronary artery disease (CAD) and myocardial infarction (MI) have been linked to a number of genetic variants, but their role in thrombopoiesis is largely unknown.

AIM

We investigated the association between CAD and MI-associated genetic variants and five thrombopoiesis-related indices: platelet count (PC), mean platelet volume (MPV), immature platelet count (IPC), immature platelet fraction (IPF), and serum thrombopoietin (TPO).

METHODS

We genotyped 45 genome-wide significant CAD/MI-markers in 879 stable CAD patients. A genetic risk score was calculated to assess the combined risk associated with all the genetic variants. Platelet indices were analysed using the Sysmex XE-2100 haematology analyser. TPO was measured by ELISA.

RESULTS

Two variants were nominally associated with several indices; for rs10947789 (KCNK5), the adjusted geometric mean was 2% higher for MPV (95% confidence interval: 1-2%, p=0.002), 6% for IPC (0-12%, p=0.033), and 9% for IPF (3-16%, p=0.004) per CAD risk allele. Moreover, an 11% lower TPO (3-19%, p=0.010) was observed. Rs3184504 (SH2B3) was associated with a higher adjusted geometric mean of 3% (1-6%, p=0.003) per CAD risk allele for PC, and an 11% (5-17%, p<0.001) lower TPO. Furthermore, the adjusted IPC was 5% (0-9%, p=0.037) lower per CAD risk allele for PC, whereas IPF levels did not vary across genotypes.

CONCLUSION

As a novel finding, our study suggests a role for KCNK5 in the regulation of platelet size and maturity. Furthermore, our findings confirm an association between the SH2B3-locus and platelet count.

QUININE TOXICITY: MULTIMODAL RETINAL IMAGING AND ELECTRORETINOGRAPHY FINDINGS

PURPOSE

To report the multimodal retinal imaging and electroretinography (ERG) findings of two cases with quinine toxicity.

METHODS

Retrospective case series describing the retinal imaging and ERG findings in two patients with retinal toxicity due to remote quinine exposure. Color fundus photography, fundus autofluorescence, spectral domain optical coherence tomography, and full-field ERG findings are described.

RESULTS

Two patients with longstanding decreased vision were found to have bilateral optic disk pallor and retinal vascular attenuation indicative of chronic sequela after acute quinine toxicity. Inner retinal atrophy was identified on spectral domain optical coherence tomography, and full-field ERG was consistent with generalized inner retinal dysfunction in both patients. Fundus autofluorescence failed to demonstrate any evidence of outer retinal or retinal pigment epithelial abnormalities.

CONCLUSION

Quinine toxicity may be associated with vision loss secondary to inner retinal atrophy. We report two cases with a clear history of acute quinine exposure leading to similar retinal imaging findings and distinctive ERG abnormalities. Clinicians should be aware of this uncommon condition as ingestion history may be remote.

Expression and localisation of two-pore domain (K2P) background leak potassium ion channels in the mouse retina

Two-pore domain (K2P) potassium channels perform essential roles in neuronal function. These channels produce background leak type potassium currents that act to regulate resting membrane potential and levels of cellular excitability. 15 different K2P channels have been identified in mammals and these channels perform important roles in a wide number of physiological systems. However, to date there is only limited data available concerning the expression and role of K2P channels in the retina. In this study we conduct the first comprehensive study of K2P channel expression in the retina. Our data show that K2P channels are widely expressed in the mouse retina, with variations in expression detected at different times of day and throughout postnatal development. The highest levels of K2P channel expression are observed for Müller cells (TWIK-1, TASK-3, TRAAK, and TREK-2) and retinal ganglion cells (TASK-1, TREK-1, TWIK-1, TWIK-2 and TWIK-3). These data offer new insight into the channels that regulate the resting membrane potential and electrical activity of retinal cells, and suggests that K2P channels are well placed to act as central regulators of visual signalling pathways. The prominent role of K2P channels in neuroprotection offers novel avenues of research into the treatment of common retinal diseases.

Diversity of astrocyte potassium channels: An update

Astrocyte K⁺ channels and the K⁺ currents they mediate dwarf all other transmembrane conductances in these cells. This defining feature of astrocytes and its functional implications have been investigated intensely over the past decades. Nonetheless, many aspects of astrocyte K⁺ handling and signaling remain incompletely understood. In this review, we provide an update on the diversity of K⁺ channels expressed by astrocytes and new functional implications. We focus on inwardly-rectifying K⁺ channels (particularly Kir4.1), two-pore K⁺ channels and voltage and Ca²⁺-dependent K⁺ channels. We further discuss new insights into the involvement of these K⁺ channels in K⁺ buffering, control of synaptic transmission, regulation of the vasculature and in diseases of the central nervous system.

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

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

Murine K2P5.1 Deficiency Has No Impact on Autoimmune Neuroinflammation due to Compensatory K2P3.1- and KV1.3-Dependent Mechanisms

Lymphocytes express potassium channels that regulate physiological cell functions, such as activation, proliferation and migration. Expression levels of K_2P5.1 (TASK2; KCNK5) channels belonging to the family of two-pore domain potassium channels have previously been correlated to the activity of autoreactive T lymphocytes in patients with multiple sclerosis and rheumatoid arthritis. In humans, K_2P5.1 channels are upregulated upon T cell stimulation and influence T cell effector functions. However, a further clinical translation of targeting K_2P5.1 is currently hampered by a lack of highly selective inhibitors, making it necessary to evaluate the impact of KCNK5 in established preclinical animal disease models. We here demonstrate that K_2P5.1 knockout (K_2P5.1^−/−) mice display no significant alterations concerning T cell cytokine production, proliferation rates, surface marker molecules or signaling pathways. In an experimental model of autoimmune neuroinflammation, K_2P5.1^−/− mice show a comparable disease course to wild-type animals and no major changes in the peripheral immune system or CNS compartment. A compensatory upregulation of the potassium channels K_2P3.1 and K_V1.3 seems to counterbalance the deletion of K_2P5.1. As an alternative model mimicking autoimmune neuroinflammation, experimental autoimmune encephalomyelitis in the common marmoset has been proposed, especially for testing the efficacy of new potential drugs. Initial experiments show that K_2P5.1 is functionally expressed on marmoset T lymphocytes, opening up the possibility for assessing future K_2P5.1-targeting drugs.

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'.

Up-regulation of TREK-2 potassium channels in cultured astrocytes requires de novo protein synthesis: relevance to localization of TREK-2 channels in astrocytes after transient cerebral ischemia

Excitotoxicity due to glutamate receptor over-activation is one of the key mediators of neuronal death after an ischemic insult. Therefore, a major function of astrocytes is to maintain low extracellular levels of glutamate. The ability of astrocytic glutamate transporters to regulate the extracellular glutamate concentration depends upon the hyperpolarized membrane potential of astrocytes conferred by the presence of K+ channels in their membranes. We have previously shown that TREK-2 potassium channels in cultured astrocytes are up-regulated by ischemia and may support glutamate clearance by astrocytes during ischemia. Thus, herein we determine the mechanism leading to this up-regulation and assess the localization of TREK-2 channels in astrocytes after transient middle cerebral artery occlusion. By using a cell surface biotinylation assay we confirmed that functional TREK-2 protein is up-regulated in the astrocytic membrane after ischemic conditions. Using real time RT-PCR, we determined that the levels of TREK-2 mRNA were not increased in response to ischemic conditions. By using Western blot and a variety of protein synthesis inhibitors, we demonstrated that the increase of TREK-2 protein expression requires De novo protein synthesis, while protein degradation pathways do not contribute to TREK-2 up-regulation after ischemic conditions. Immunohistochemical studies revealed TREK-2 localization in astrocytes together with increased expression of the selective glial marker, glial fibrillary acidic protein, in brain 24 hours after transient middle cerebral occlusion. Our data indicate that functional TREK-2 channels are up-regulated in the astrocytic membrane during ischemia through a mechanism requiring De novo protein synthesis. This study provides important information about the mechanisms underlying TREK-2 regulation, which has profound implications in neurological diseases such as ischemia where astrocytes play an important role.

Freshly dissociated mature hippocampal astrocytes exhibit passive membrane conductance and low membrane resistance similarly to syncytial coupled astrocytes

Mature astrocytes exhibit a linear current-to-voltage K(+) membrane conductance (passive conductance) and an extremely low membrane resistance (Rm) in situ. The combination of these electrophysiological characteristics establishes a highly negative and stable membrane potential that is essential for basic functions, such as K(+) spatial buffering and neurotransmitter uptake. However, astrocytes are coupled extensively in situ. It remains to be determined whether the observed passive behavior and low Rm are attributable to the intrinsic properties of membrane ion channels or to gap junction coupling in functionally mature astrocytes. In the present study, freshly dissociated hippocampal tissues were used as a new model to examine this basic question in young adult animals. The morphologically intact single astrocytes could be reliably dissociated from animals postnatal day 21 and older. At this animal age, dissociated single astrocytes exhibit passive conductance and resting membrane potential similar to those exhibited by astrocytes in situ. To precisely measure the Rm from single astrocytes, dual-patch single-astrocyte recording was performed. We show that dissociated single astrocytes exhibit a low Rm similarly to syncytial coupled astrocytes. Functionally, the symmetric expression of high-K(+) conductance enabled rapid change in the intracellular K(+) concentrations in response to changing K(+) drive force. Altogether, we demonstrate that freshly dissociated tissue preparation is a highly useful model for study of the functional expression and regulation of ion channels, receptors, and transporters in astrocytes and that passive behavior and low Rm are the intrinsic properties of mature astrocytes.

Molecular aspects of structure, gating, and physiology of pH-sensitive background K2P and Kir K+-transport channels

K(+) channels fulfill roles spanning from the control of excitability to the regulation of transepithelial transport. Here we review two groups of K(+) channels, pH-regulated K2P channels and the transport group of Kir channels. After considering advances in the molecular aspects of their gating based on structural and functional studies, we examine their participation in certain chosen physiological and pathophysiological scenarios. Crystal structures of K2P and Kir channels reveal rather unique features with important consequences for the gating mechanisms. Important tasks of these channels are discussed in kidney physiology and disease, K(+) homeostasis in the brain by Kir channel-equipped glia, and central functions in the hearing mechanism in the inner ear and in acid secretion by parietal cells in the stomach. K2P channels fulfill a crucial part in central chemoreception probably by virtue of their pH sensitivity and are central to adrenal secretion of aldosterone. Finally, some unorthodox behaviors of the selectivity filters of K2P channels might explain their normal and pathological functions. Although a great deal has been learned about structure, molecular details of gating, and physiological functions of K2P and Kir K(+)-transport channels, this has been only scratching at the surface. More molecular and animal studies are clearly needed to deepen our knowledge.

Nonvesicular release of ATP from rat retinal glial (Müller) cells is differentially mediated in response to osmotic stress and glutamate

Retinal glial (Müller) cells release ATP upon osmotic stress or activation of metabotropic glutamate receptors. ATP inhibits the osmotic Müller cell swelling by activation of P2Y1 receptors. In the present study, we determined the molecular pathways of the ATP release from Müller cells in slices of the rat retina. Administration of the ATP/ADPase apyrase induced a swelling of Müller cells under hypoosmotic conditions, and prevented the swelling-inhibitory effect of glutamate, suggesting that swelling inhibition is mediated by extracellular ATP. A hypoosmotic swelling of Müller cells was also observed in the presence of a blocker of multidrug resistance channels (MK-571), a CFTR inhibitor (glibenclamide), and connexin hemichannel blockers (18-α-glycyrrhetinic acid, 100 µM carbenoxolone). The swelling-inhibitory effect of glutamate was prevented by MK-571, the connexin hemichannel blockers, and a pannexin-1 hemichannel blocker (5 µM carbenoxolone). The p-glycoprotein blocker verapamil had no effect. As revealed by single-cell RT-PCR, subpopulations of Müller cells expressed mRNAs for pannexin-1 and -2, and connexins 30, 30.3, 32, 43, 45, and 46. The data may suggest that rat Müller cells release ATP by multidrug resistance channels, CFTR, and connexin hemichannels in response to osmotic stress, while glutamate induces a release of ATP via multidrug resistance channels, connexin hemichannels, and pannexin-1.

Involvement of potassium channels in the progression of cancer to a more malignant phenotype

Potassium channels are a diverse group of pore-forming transmembrane proteins that selectively facilitate potassium flow through an electrochemical gradient. They participate in the control of the membrane potential and cell excitability in addition to different cell functions such as cell volume regulation, proliferation, cell migration, angiogenesis as well as apoptosis. Because these physiological processes are essential for the correct cell function, K+ channels have been associated with a growing number of diseases including cancer. In fact, different K+ channel families such as the voltage-gated K+ channels, the ether à-go-go K+ channels, the two pore domain K+ channels and the Ca2+-activated K+ channels have been associated to tumor biology. Potassium channels have a role in neoplastic cell-cycle progression and their expression has been found abnormal in many types of tumors and cancer cells. In addition, the expression and activity of specific K+ channels have shown a significant correlation with the tumor malignancy grade. The aim of this overview is to summarize published data on K+ channels that exhibit oncogenic properties and have been linked to a more malignant cancer phenotype. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

Immunocytochemical localization of TASK-3 protein (K2P9.1) in the rat brain

Among all K2P channels, TASK-3 shows the most widespread expression in rat brain, regulating neuronal excitability and transmitter release. Using a recently purified and characterized polyclonal monospecific antibody against TASK-3, the entire rat brain was immunocytochemically analyzed for expression of TASK-3 protein. Besides its well-known strong expression in motoneurons and monoaminergic and cholinergic neurons, TASK-3 expression was found in most neurons throughout the brain. However, it was not detected in certain neuronal populations, and neuropil staining was restricted to few areas. Also, it was absent in adult glial cells. In hypothalamic areas, TASK-3 was particularly strongly expressed in the supraoptic and suprachiasmatic nuclei, whereas other hypothalamic nuclei showed lower protein levels. Immunostaining of hippocampal CA1 and CA3 pyramidal neurons showed strongest expression, together with clear staining of CA3 mossy fibers and marked staining also in the dentate gyrus granule cells. In neocortical areas, most neurons expressed TASK-3 with a somatodendritic localization, most obvious in layer V pyramidal neurons. In the cerebellum, TASK-3 protein was found mainly in neurons and neuropil of the granular cell layer, whereas Purkinje cells were only faintly positive. Particularly weak expression was demonstrated in the forebrain. This report provides a comprehensive overview of TASK-3 protein expression in the rat brain.

Differential effects of P2Y1 deletion on glial activation and survival of photoreceptors and amacrine cells in the ischemic mouse retina

Gliosis of retinal Müller glial cells may have both beneficial and detrimental effects on neurons. To investigate the role of purinergic signaling in ischemia-induced reactive gliosis, transient retinal ischemia was evoked by elevation of the intraocular pressure in wild-type (Wt) mice and in mice deficient in the glia-specific nucleotide receptor P2Y₁ (P2Y₁ receptor-deficient (P2Y1R-KO)). While control retinae of P2Y1R-KO mice displayed reduced cell numbers in the ganglion cell and inner nuclear layers, ischemia induced apoptotic death of cells in all retinal layers in both, Wt and P2Y1R-KO mice, but the damage especially on photoreceptors was more pronounced in retinae of P2Y1R-KO mice. In contrast, gene expression profiling and histological data suggest an increased survival of amacrine cells in the postischemic retina of P2Y1R-KO mice. Interestingly, measuring the ischemia-induced downregulation of inwardly rectifying potassium channel (Kir)-mediated K⁺ currents as an indicator, reactive Müller cell gliosis was found to be weaker in P2Y1R-KO (current amplitude decreased by 18%) than in Wt mice (decrease by 68%). The inner retina harbors those neurons generating action potentials, which strongly rely on an intact ion homeostasis. This may explain why especially these cells appear to benefit from the preserved Kir4.1 expression in Müller cells, which should allow them to keep up their function in the context of spatial buffering of potassium. Especially under ischemic conditions, maintenance of this Müller cell function may dampen cytotoxic neuronal hyperexcitation and subsequent neuronal cell loss. In sum, we found that purinergic signaling modulates the gliotic activation pattern of Müller glia and lack of P2Y₁ has janus-faced effects. In the end, the differential effects of a disrupted P2Y₁ signaling onto neuronal survival in the ischemic retina call the putative therapeutical use of P2Y₁-antagonists into question.

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

Background

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

Methods

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

Results

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

Conclusion

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

The contribution of TWIK-1 channels to astrocyte K(+) current is limited by retention in intracellular compartments

TWIK-1 two-pore domain K⁺ channels are expressed abundantly in astrocytes. In the present study, we examined the extent to which TWIK-1 contributes to the linear current-voltage (I–V) relationship (passive) K⁺ membrane conductance, a dominant electrophysiological feature of mature hippocampal astrocytes. Astrocytes from TWIK-1 knockout mice have a more negative resting potential than those from wild type animals and a reduction in both inward rectification and Cs⁺ permeability. Nevertheless, the overall whole-cell passive conductance is not altered significantly in TWIK-1 knockout astrocytes. The expression of K_ir4.1 and TREK-1, two other major astrocytic K⁺ channels, or of other two-pore K⁺ channels is not altered in TWIK-1 knockout mice, suggesting that the mild effect of TWIK-1 knockout does not result from compensation by these channels. Fractionation experiments showed that TWIK-1 is primarily localized in intracellular cytoplasmic fractions (55%) and mildly hydrophobic internal compartment fractions (41%), with only 5% in fractions containing plasma membranes. Our study revealed that TWIK-1 proteins are mainly located in the intracellular compartments of hippocampal astrocyte under physiological condition, therefore a minimal contribution of TWIK-1 channels to whole-cell currents is likely attributable to a relatively low level presence of channels in the plasma membrane.

TASK-2: a K2P K(+) channel with complex regulation and diverse physiological functions

TASK-2 (K2P5.1) is a two-pore domain K(+) channel belonging to the TALK subgroup of the K2P family of proteins. TASK-2 has been shown to be activated by extra- and intracellular alkalinization. Extra- and intracellular pH-sensors reside at arginine 224 and lysine 245 and might affect separate selectivity filter and inner gates respectively. TASK-2 is modulated by changes in cell volume and a regulation by direct G-protein interaction has also been proposed. Activation by extracellular alkalinization has been associated with a role of TASK-2 in kidney proximal tubule bicarbonate reabsorption, whilst intracellular pH-sensitivity might be the mechanism for its participation in central chemosensitive neurons. In addition to these functions TASK-2 has been proposed to play a part in apoptotic volume decrease in kidney cells and in volume regulation of glial cells and T-lymphocytes. TASK-2 is present in chondrocytes of hyaline cartilage, where it is proposed to play a central role in stabilizing the membrane potential. Additional sites of expression are dorsal root ganglion neurons, endocrine and exocrine pancreas and intestinal smooth muscle cells. TASK-2 has been associated with the regulation of proliferation of breast cancer cells and could become target for breast cancer therapeutics. Further work in native tissues and cells together with genetic modification will no doubt reveal the details of TASK-2 functions that we are only starting to suspect.

TASK-1 channels in oligodendrocytes: a role in ischemia mediated disruption

Oligodendrocytes are the myelinating cells of the CNS and, like neurons, are highly sensitive to ischemic damage. However, the mechanisms underlying cytotoxicity in oligodendrocytes during hypoxic/ischemic episodes are not fully understood. TASK-1 is a K(+) leak channel that mediates hypoxic depolarisation in neurons. The expression and function of TASK-1 in oligodendrocytes had not previously been addressed. In this study, we investigate the expression of TASK-1 in oligodendrocytes and its role in white matter ischemic damage. Expression of TASK-1 in oligodendrocytes was investigated in the mouse brain using immunostaining. TASK-1 channel function was identified by established pharmacological and electrophysiological strategies, using the whole-cell patch clamp technique in cell cultures of oligodendrocytes from the optic nerve, a typical white matter tract. The role of TASK-1 in hypoxia was examined in isolated intact optic nerves subjected to oxygen glucose deprivation (OGD). Oligodendrocytes are strongly immunopositive for TASK-1 throughout the brain. Patch-clamp identified functional TASK-1-like leak currents in oligodendrocytes using two recognised means of inhibiting TASK-1, decreasing extracellular pH to 6.4 and exposure to the TASK-1 selective inhibitor anandamide. Incubation of optic nerves with methanandamide, a non-hydrolysable form of anandamide, significantly protected oligodendrocytes against hypoxic disruption and death in OGD. Our data demonstrate for the first time that oligodendrocytes express functional TASK-1 channels and provide compelling evidence they contribute to oligodendrocyte damage in hypoxia. Since oligodendrocyte damage is a key factor in ischemic episodes, TASK-1 may provide a potential therapeutic target in stroke and white matter disease.

New functions of Müller cells

Müller cells, the major type of glial cells in the retina, are responsible for the homeostatic and metabolic support of retinal neurons. By mediating transcellular ion, water, and bicarbonate transport, Müller cells control the composition of the extracellular space fluid. Müller cells provide trophic and anti-oxidative support of photoreceptors and neurons and regulate the tightness of the blood-retinal barrier. By the uptake of glutamate, Müller cells are more directly involved in the regulation of the synaptic activity in the inner retina. This review gives a survey of recently discoved new functions of Müller cells. Müller cells are living optical fibers that guide light through the inner retinal tissue. Thereby they enhance the signal/noise ratio by minimizing intraretinal light scattering and conserve the spatial distribution of light patterns in the propagating image. Müller cells act as soft, compliant embedding for neurons, protecting them in case of mechanical trauma, and also as soft substrate required for neurite growth and neuronal plasticity. Müller cells release neuroactive signaling molecules which modulate neuronal activity, are implicated in the mediation of neurovascular coupling, and mediate the homeostasis of the extracellular space volume under hypoosmotic conditions which are a characteristic of intense neuronal activity. Under pathological conditions, a subset of Müller cells may differentiate to neural progenitor/stem cells which regenerate lost photoreceptors and neurons. Increasing knowledge of Müller cell function and responses in the normal and diseased retina will have great impact for the development of new therapeutic approaches for retinal diseases.

Transcriptional expression of voltage-gated Na⁺ and voltage-independent K⁺ channels in the developing rat superficial dorsal horn

Neurons within the superficial dorsal horn (SDH) of the rodent spinal cord exhibit distinct firing properties during early life. While this may reflect a unique combination of voltage-gated Na(+) (Na(v)) and voltage-independent (i.e. "leak'') K(+) channels which strongly influence neuronal excitability across the CNS, surprisingly little is known about which genes encoding for Na(v) and leak K(+) channels are expressed within developing spinal pain circuits. The goal of the present study was therefore to characterize the transcriptional expression of these channels within the rat SDH at postnatal days (P) 3, 10, 21 or adulthood using quantitative real-time polymerase chain reaction. The results demonstrate that Na(v) isoforms are developmentally regulated at the mRNA level in a subtype-specific manner, as Na(v)1.2 and Na(v)1.3 decreased significantly from P3 to adulthood, while Na(v)1.1 was up-regulated during this period. The data also indicate selective, age-dependent changes in the mRNA expression of two-pore domain (K(2P)) K(+) channels, as TWIK-related acid-sensitive K(+) channels TASK-1 (KCNK3) and TASK-3 (KCNK9) were down-regulated during postnatal development in the absence of any changes in the tandem of pore domains in a weak inward rectifying K(+) channel (TWIK) isoforms examined (KCNK1 and KCNK6). In addition, a developmental shift occurred within the TREK subfamily due to decreased TREK-2 (KCNK10) mRNA within the mature SDH. Meanwhile, G-protein-coupled inward rectifying K(+) channels (K(ir)3.1 and K(ir)3.2) were expressed in the SDH at mature levels from birth. Overall, the results suggest that the transcription of ion channel genes occurs in a highly age-dependent manner within the SDH, raising the possibility that manipulating the expression or function of ion channels which are preferentially expressed within immature nociceptive networks could yield novel approaches to relieving pain in infants and children.

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

Müller cell gliosis, which is characterized by upregulated expression of glial fibrillary acidic protein (GFAP), is a universal response in many retinal pathological conditions. Whether down-regulation of inward rectifying K+ (Kir) channels, which commonly accompanies the enhanced GFAP expression, could contribute to Müller cell gliosis is poorly understood. We investigated changes of Kir currents, GFAP and Kir4.1 protein expression in Müller cells in a rat chronic ocular hypertension (COH) model, and explored the mechanisms underlying Müller cell gliosis. We show that Kir currents and Kir4.1 protein expression in Müller cells were reduced significantly, while GFAP expression was increased in COH rats, and these changes were eliminated by MPEP, a group I metabotropic glutamate receptors (mGluR I) subtype mGluR5 antagonist. In normal isolated Müller cells, the mGluR I agonist (S)-3,5-dihydroxyphenylglycine (DHPG) suppressed the Kir currents and the suppression was blocked by MPEP. The DHPG effect was mediated by the intracellular Ca2+ -dependent PLC/IP3-ryanodine/PKC signaling pathway, but the cAMP-PKA pathway was not involved. Moreover, intravitreal injection of DHPG in normal rats induced changes in Müller cells, similar to those observed in COH rats. The DHPG-induced increase of GFAP expression in Müller cells was obstructed by Ba2+, suggesting the involvement of Kir channels. We conclude that overactivation of mGluR5 by excessive extracellular glutamate in COH rats could contribute to Müller cell gliosis by suppressing Kir channels.

Distinct expression/function of potassium and chloride channels contributes to the diverse volume regulation in cortical astrocytes of GFAP/EGFP mice

Recently, we have identified two astrocytic subpopulations in the cortex of GFAP-EGFP mice, in which the astrocytes are visualized by the enhanced green-fluorescent protein (EGFP) under the control of the human glial fibrillary acidic protein (GFAP) promotor. These astrocytic subpopulations, termed high response- (HR-) and low response- (LR-) astrocytes, differed in the extent of their swelling during oxygen-glucose deprivation (OGD). In the present study we focused on identifying the ion channels or transporters that might underlie the different capabilities of these two astrocytic subpopulations to regulate their volume during OGD. Using three-dimensional confocal morphometry, which enables quantification of the total astrocytic volume, the effects of selected inhibitors of K⁺ and Cl⁻ channels/transporters or glutamate transporters on astrocyte volume changes were determined during 20 minute-OGD in situ. The inhibition of volume regulated anion channels (VRACs) and two-pore domain potassium channels (K(2P)) highlighted their distinct contributions to volume regulation in HR-/LR-astrocytes. While the inhibition of VRACs or K(2P) channels revealed their contribution to the swelling of HR-astrocytes, in LR-astrocytes they were both involved in anion/K⁺ effluxes. Additionally, the inhibition of Na⁺-K⁺-Cl⁻ co-transporters in HR-astrocytes led to a reduction of cell swelling, but it had no effect on LR-astrocyte volume. Moreover, employing real-time single-cell quantitative polymerase chain reaction (PCR), we characterized the expression profiles of EGFP-positive astrocytes with a focus on those ion channels and transporters participating in astrocyte swelling and volume regulation. The PCR data revealed the existence of two astrocytic subpopulations markedly differing in their gene expression levels for inwardly rectifying K⁺ channels (Kir4.1), K(2P) channels (TREK-1 and TWIK-1) and Cl⁻ channels (ClC2). Thus, we propose that the diverse volume changes displayed by cortical astrocytes during OGD mainly result from their distinct expression patterns of ClC2 and K(2P) channels.

On the somatosensation of vision

The interconnection between vision and somatosensation is already well-established and is further supplemented by the evolutionary link between eyes and photoreceptors, and the functional connection between photosensation and thermoreception. However, our analysis shows that the relation between vision and somatosensation is much deeper and suggests that somatosensation may possibly be the basis of vision. Surprisingly, our photoreceptor itself needs somatosensory proteins for its functioning, and our entire visual pathway depends on somatosensory cues for its functioning.

Mechanisms of VEGF- and glutamate-induced inhibition of osmotic swelling of murine retinal glial (Müller) cells: indications for the involvement of vesicular glutamate release and connexin-mediated ATP release

We determined the mechanisms of glutamate and ATP release from murine retinal glial (Müller) cells by pharmacological manipulation of the vascular endothelial growth factor (VEGF)- and glutamate-induced inhibition of cellular swelling under hypoosmotic conditions. It has been shown that exogenous glutamate inhibits hypoosmotic swelling of rat Müller cells via the induction of the release of ATP (Uckermann et al. in J Neurosci Res 83:538-550, 53). VEGF was shown to inhibit hypoosmotic swelling of rat Müller cells by inducing the release of glutamate (Wurm et al. in J Neurochem 104:386-399, 55). The swelling-inhibitory effect of VEGF in murine Müller cells was blocked by an inhibitor of vesicular exocytosis, by a modulator of the allosteric site of vesicular glutamate transporters, and by inhibitors of phospholipase C and protein kinase C. The swelling-inhibitory effect of glutamate in murine Müller cells was prevented by inhibitors of connexin hemichannels. The effects of both VEGF and glutamate were blocked by tetrodotoxin and by an inhibitor of T-type voltage-gated calcium channels. Murine Müller cells display connexin-43 immunoreactivity. The data suggest that Müller cells of the murine retina may release glutamate by vesicular exocytosis, whereas ATP is released through connexin hemichannels.

Two-pore domain K⁺ channels regulate membrane potential of isolated human articular chondrocytes

Potassium channels that regulate resting membrane potential (RMP) of human articular chondrocytes (HACs) of the tibial joint maintained in short-term (0-3 days) non-confluent cell culture were studied using patch-clamp techniques. Quantitative PCR showed that transcripts of genes for two-pore domain K(+) channels (KCNK1, KCNK5 and KCNK6), and 'BK' Ca(2+)-activated K(+) channels (KCNMA1) were abundantly expressed. Immunocytological methods detected α-subunits for BK and K(2p)5.1 (TASK-2) K(+) channels. Electrophysiological recordings identified three distinct K(+) currents in isolated HACs: (i) a voltage- and time-dependent 'delayed rectifier', blocked by 100 nM α-dendrotoxin, (ii) a large 'noisy' voltage-dependent current that was blocked by low concentrations of tetraethylammonium (TEA; 50% blocking dose = 0.15 mM) and iberiotoxin (52% block, 100 nM) and (iii) a voltage-independent 'background' K(+) current that was blocked by acidic pH (5.5-6), was increased by alkaline pH (8.5), and was not blocked by TEA, but was blocked by the local anaesthetic bupivacaine (0.25 mM). The RMP of isolated HACs was very slightly affected by 5 mM TEA, which was sufficient to block both voltage-dependent K(+) currents, suggesting that these currents probably contributed little to maintaining RMP under 'resting' conditions (i.e. low internal [Ca(2+)]). Increases in external K(+) concentration depolarized HACs by 30 mV in response to a 10-fold increase in [K(+)], indicating a significant but not exclusive role for K(+) current in determining RMP. Increases in external [K(+)] in voltage-clamped HACs revealed a voltage-independent K(+) current whose inward current magnitude increased with external [K(+)]. Block of this current by bupivacaine (0.25-1 mM) in 5 and 25 mM external [K(+)] resulted in a large (8-25 mV) depolarization of RMP. The biophysical and pharmacological properties of the background K(+) current, together with expression of mRNA and α-subunit protein for TASK-2, strongly suggest that these two-pore domain K(+) channels contribute significantly to stabilizing the RMP of HACs.

Cerebrovascular responses in mice deficient in the potassium channel, TREK-1

We tested the hypothesis that TREK-1, a two-pore domain K channel, is involved with dilations in arteries. Because there are no selective activators or inhibitors of TREK-1, we generated a mouse line deficient in TREK-1. Endothelium-mediated dilations were not different in arteries from wild-type (WT) and TREK-1 knockout (KO) mice. This includes dilations of the middle cerebral artery to ATP, dilations of the basilar artery to ACh, and relaxations of the aorta to carbachol, a cholinergic agonist. The nitric oxide (NO) and endothelium-dependent hyperpolarizing factor components of ATP dilations were identical in the middle cerebral arteries of WT and TREK-1 KO mice. Furthermore, the NO and cyclooxygenase-dependent components were identical in the basilar arteries of the different genotypes. Dilations of the basilar artery to alpha-linolenic acid, an activator of TREK-1, were not affected by the absence of TREK-1. Whole cell currents recorded using patch-clamp techniques were similar in cerebrovascular smooth muscle cells (CVSMCs) from WT and TREK-1 KO mice. alpha-linolenic acid or arachidonic acid increased whole cell currents in CVSMCs from both WT and TREK-1 KO mice. The selective blockers of large-conductance Ca-activated K channels, penitrem A and iberiotoxin, blocked the increased currents elicited by either alpha-linolenic or arachidonic acid. In summary, dilations were similar in arteries from WT and TREK-1 KO mice. There was no sign of TREK-1-like currents in CVSMCs from WT mice, and there were no major differences in currents between the genotypes. We conclude that regulation of arterial diameter is not altered in mice lacking TREK-1.

Functions of mature mammalian astrocytes: a current view

Before the roles of normal, mature astrocytes in the mammalian CNS can be discussed, we first need to define these cells. A definition proposed here is that such a class is best defined as consisting of the protoplasmic and fibrous astrocytes of the gray and white matter, respectively, the Bergmann glia of the molecular layer of the cerebellum, and the Muller cells of the retina. It is concluded that the established properties and functions of these mature astrocytes are essential support for neuronal activity, in the sense of Claude Bernard's principle of maintaining "la fixité du milieu intérieur." This milieu would be the extracellular space common to astrocytes and neurons. More specialized roles, such as the recently described "light guides" for retinal Muller cells can also be viewed as support and facilitation. The ECS is also, of course, common to all other neural cells, but here, I limit the discussion to perturbations of the ECS caused only by neuronal activities and the resolution of these perturbations by astrocytes, such as control of increases in extracellular K(+), uptake of excitatory amino acids, and alterations in blood vessel diameter and therefore blood flow. It is also proposed how this fits into the current morphological picture for the protoplasmic astrocytes as having small cell bodies with up to 100,000 process endings that occupy separate territories on which the processes of neighboring astrocytes scarcely intrude.

Serum albumin induces osmotic swelling of rat retinal glial cells

Edema in the ischemic neural tissue develops by increased vascular permeability associated with extravasation of albumin, and by glial swelling. Here, we show that bovine serum albumin acutely administered to slices of the rat retina causes swelling of glial somata under hypoosmotic conditions. The effect of albumin was dose-dependent, with half-maximal and maximal effects at 10 nM and 1 microM, respectively, and was mediated by activation of transforming growth factor-beta receptor type II, oxidative stress, and the production of arachidonic acid and prostaglandins. Albumin-induced glial swelling was prevented by glutamate and purinergic receptor agonists. The data suggest that serum albumin may induce glial swelling in the presence of osmotic gradients.

Porcine Müller Glial Cells Increase Expression of BKCaChannels in Retinal Detachment

PURPOSE

To determine whether experimental retinal detachment causes an alteration in Ca2 +-activated, big conductance K+ (BK) currents of Müller glial cells.

METHODS

Rhegmatogenous retinal detachment was induced in porcine eyes. Müller cells were acutely isolated from control retinas and from retinas that were detached for 7 days. BK currents were detected by using the BK channel opener and the blocker phloretin and tetraethylammonium, respectively.

RESULTS

In addition to cellular hypertrophy and a decrease in inward rectifier K+ currents, Müller cells from detached retinas showed an increase in the amplitude of currents mediated by BK channels (850 +/- 105 pA) when compared with cells from control retinas (228 +/- 60 pA; p < 0.001). Similarly, the density of the BK channel-mediated currents was greater in cells from detached retinas (12.32 +/- 1.52 pA/pF) compared with control cells (4.07 +/- 1.07 pA/pF; p < 0.001). The increase in BK currents was correlated with the decrease of the inward rectifier K+ currents.

CONCLUSIONS

It is suggested that an increase in the expression of functional BK channels may be involved in gliotic responses of Müller cells after retinal detachment (e.g., in mitogen-induced Ca2+ responses and cellular proliferation).