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.
"A more hyperpolarized membrane potential compared to neurons can be found in astrocytes which provides the necessary driving force for K + spatial buffering and glutamate transport (Kucheryavykh et al., 2007, 2009; Olsen, 2012). A major ion channel expressed by astrocytes is the inward rectifying potassium channel Kir4.1 (encoded by the gene KCNJ10) (Steinhauser and Seifert, 2002; Seifert et al., 2009). This potassium channel is not only a key player in efficient uptake of K + released by neurons during axon potential propagation (Neusch et al., 2006; Djukic et al., 2007; Chever et al., 2010), but these http://dx.doi.org/10.1016/j.neuroscience.2015.09.044 0306-4522/Ó 2015 IBRO. "
[Show abstract][Hide abstract] ABSTRACT: Diabetics are at risk for a number of serious health complications including an increased incidence of epilepsy and poorer recovery after ischemic stroke. Astrocytes play a critical role in protecting neurons by maintaining extracellular homeostasis and preventing neurotoxicity through glutamate uptake and potassium buffering. These functions are aided by the presence of potassium channels, such as Kir4.1 inwardly rectifying potassium channels, in the membranes of astrocytic glial cells. The purpose of the present study was to determine if hyperglycemia alters Kir4.1 potassium channel expression and homeostatic functions of astrocytes. We used q-PCR, Western blot, patch-clamp electrophysiology studying voltage and potassium step responses and a colorimetric glutamate clearance assay to assess Kir4.1 channel levels and homeostatic functions of astrocytes grown in normal and high glucose conditions. We found that astrocytes grown in high glucose (25mM) had an approximately 50% reduction in Kir4.1 mRNA and protein expression as compared with those grown in normal glucose (5mM). These reductions occurred within 4-7days of exposure to hyperglycemia, whereas reversal occurred between 7 and 14days after return to normal glucose. The decrease in functional Kir channels in the astrocytic membrane was confirmed using barium to block Kir channels. In the presence of 100-μm barium, the currents recorded from astrocytes in response to voltage steps were reduced by 45%. Furthermore, inward currents induced by stepping extracellular [K(+)]o from 3 to 10mM (reflecting potassium uptake) were 50% reduced in astrocytes grown in high glucose. In addition, glutamate clearance by astrocytes grown in high glucose was significantly impaired. Taken together, our results suggest that down-regulation of astrocytic Kir4.1 channels by elevated glucose may contribute to the underlying pathophysiology of diabetes-induced CNS disorders and contribute to the poor prognosis after stroke.
"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. "
"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 "
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