Analysis of Astroglial K+ Channel Expression in the Developing Hippocampus Reveals a Predominant Role of the Kir4.1 Subunit
ABSTRACT Astrocytes in different brain regions display variable functional properties. In the hippocampus, astrocytes predominantly express time- and voltage-independent currents, but the underlying ion channels are not well defined. This ignorance is partly attributable to abundant intercellular coupling of these cells through gap junctions, impeding quantitative analyses of intrinsic membrane properties. Moreover, distinct types of cells with astroglial properties coexist in a given brain area, a finding that confused previous analyses. In the present study, we investigated expression of inwardly rectifying (Kir) and two-pore-domain (K2P) K+ channels in astrocytes, which are thought to be instrumental in the regulation of K+ homeostasis. Freshly isolated astrocytes were used to improve space-clamp conditions and allow for quantitative assessment of functional parameters. Patch-clamp recordings were combined with immunocytochemistry, Western blot analysis, and semiquantitative transcript analysis. Comparative measurements were performed in different CA1 subregions of astrocyte-targeted transgenic mice. While confirming weak Ba2+ sensitivity in situ, our data demonstrate that in freshly isolated astrocytes, the main proportion of membrane currents is sensitive to micromolar Ba2+ concentrations. Upregulation of Kir4.1 transcripts and protein during the first 10 postnatal days was accompanied by a fourfold increase in astrocyte inward current density. Hippocampal astrocytes from Kir4.1-/- mice lacked Ba2+-sensitive currents. In addition, we report functional expression of K2P channels of the TREK subfamily (TREK1, TREK2), which mediate astroglial outward currents. Together, our findings demonstrate that Kir4.1 constitutes the pivotal K+ channel subunit and that superposition of currents through Kir4.1 and TREK channels underlies the "passive" current pattern of hippocampal astrocytes.
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- "Various attempts to correlate the biochemical and functional properties of discrete astrocytic population were then pursued by using diverse approaches. Differences in the current profile of distinct astrocyte subsets were ascribed to the heterogeneous expression of ion channels, including inward rectifying K + (Kir) channels (Butt and Kalsi, 2006; Kofuji et al., 2000; Neusch et al., 2006; Olsen et al., 2007; Seifert et al., 2009). For instance, the K + channel subunit Kir4.1 was described to be abundantly expressed in astrocytes of the spinal cord ventral horns when compared to those of the dorsal horns. "
ABSTRACT: Recent breakthroughs in neuroscience have led to the awareness that we should revise our traditional mode of thinking and studying the CNS, i.e. by isolating the privileged network of "intelligent" synaptic contacts. We may instead need to contemplate all the variegate communications occurring between the different neural cell types, and centrally involving the astrocytes. Basically, it appears that a single astrocyte should be considered as a core that receives and integrates information from thousands of synapses, other glial cells and the blood vessels. In turn, it generates complex outputs that control the neural circuitry and coordinate it with the local microcirculation. Astrocytes thus emerge as the possible fulcrum of the functional homeostasis of the healthy CNS. Yet, evidence indicates that the bridging properties of the astrocytes can change in parallel with, or as a result of, the morphological, biochemical and functional alterations these cells undergo upon injury or disease. As a consequence, they have the potential to transform from supportive friends and interactive partners for neurons into noxious foes. In this review, we summarize the currently available knowledge on the contribution of astrocytes to the functioning of the CNS and what goes wrong in various pathological conditions, with a particular focus on amyotrophic lateral sclerosis, Alzheimer's disease and ischemia. The observations described convincingly demonstrate that the development and progression of several neurological disorders involve the de-regulation of a finely tuned interplay between multiple cell populations. Thus, it seems that a better understanding of the mechanisms governing the integrated communication and detrimental responses of the astrocytes as well as their impact towards the homeostasis and performance of the CNS is fundamental to open novel therapeutic perspectives. Copyright © 2015. Published by Elsevier Ltd.Progress in Neurobiology 04/2015; 29. DOI:10.1016/j.pneurobio.2015.04.003 · 10.30 Impact Factor
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- "The passive conductance implies an abundant and combined expression of multiple leak-type K ϩ channels , such as inwardly rectifying K ir 4.1, two-pore domain K ϩ channels, and Ca 2ϩ -activated K ϩ channels (Chever et al. 2010; Chu et al. 2010; Djukic et al. 2007; Hwang et al. 2014; Kucheryavykh et al. 2007; Longden et al. 2011; Olsen and Sontheimer 2008; Seifert et al. 2009; Skatchkov et al. 2006; Tong et al. 2014). The extremely low R m shown in single astrocytes also suggests that the transmembrane K "
ABSTRACT: Mature astrocytes exhibit a linear current-to-voltage (I-V) relationship 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 the 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 P21 and older animals. At this animal age, dissociated single astrocytes exhibit a similar passive conductance and resting membrane potential as astrocytes do 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 similarly low RM as syncytial coupled astrocytes. Functionally, the symmetric expression of high K(+) conductance enabled rapid change in the intracellular K(+) concentrations in response to changing K(+) driving 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 the passive behavior and low RM are the intrinsic properties of mature astrocytes. Copyright © 2015, Journal of Neurophysiology.Journal of Neurophysiology 03/2015; 113(10):jn.00206.2015. DOI:10.1152/jn.00206.2015 · 3.04 Impact Factor
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- "ed in water balance and ( functionally related ) potassium buffering . Aquaporin 4 appears after PND20 and its levels increase afterwards , according to single - cell gene expression data ( Rusnakova et al . , 2013 ) . The major potassium channel Kir4 . 1 is downregulated to its stable level in the hippocampus within the first ten postnatal days ( Seifert et al . , 2009 ) . In contrast to most astrocytic receptors , the lev - els of GFAP as well as of the calcium binding protein s100b increase during aging ( Nichols , 1999 ; Sheng et al . , 1996 ) . In accord with these observations , a global increase in GFAP expression in the aged brain is detectable , suggesting an increased number of astrocytes in "
ABSTRACT: Memory formation in the brain is thought to rely on the remodeling of synaptic connections which eventually results in neural network rewiring. This remodeling is likely to involve ultrathin astroglial protrusions which often occur in the immediate vicinity of excitatory synapses. The phenomenology, cellular mechanisms, and causal relationships of such astroglial restructuring remain, however, poorly understood. This is in large part because monitoring and probing of the underpinning molecular machinery on the scale of nanoscopic astroglial compartments remains a challenge. Here we briefly summarize the current knowledge regarding the cellular organisation of astroglia in the synaptic microenvironment and discuss molecular mechanisms potentially involved in use-dependent astroglial morphogenesis. We also discuss recent observations concerning morphological astroglial plasticity, the respective monitoring methods, and some of the newly emerging techniques that might help with conceptual advances in the area. GLIA 2015. © 2015 The Authors. Glia Published by Wiley Periodicals, Inc.Glia 03/2015; DOI:10.1002/glia.22821 · 6.03 Impact Factor