View
5
Download
0
Category
Preview:
Citation preview
The University of Manchester Research
Ionic signalling in astroglia beyond calcium
DOI:10.1113/JP277478
Document VersionAccepted author manuscript
Link to publication record in Manchester Research Explorer
Citation for published version (APA):Verkhratsky, A., Untiet, V., & Rose, C. R. (2019). Ionic signalling in astroglia beyond calcium. The Journal ofphysiology. https://doi.org/10.1113/JP277478
Published in:The Journal of physiology
Citing this paperPlease note that where the full-text provided on Manchester Research Explorer is the Author Accepted Manuscriptor Proof version this may differ from the final Published version. If citing, it is advised that you check and use thepublisher's definitive version.
General rightsCopyright and moral rights for the publications made accessible in the Research Explorer are retained by theauthors and/or other copyright owners and it is a condition of accessing publications that users recognise andabide by the legal requirements associated with these rights.
Takedown policyIf you believe that this document breaches copyright please refer to the University of Manchester’s TakedownProcedures [http://man.ac.uk/04Y6Bo] or contact uml.scholarlycommunications@manchester.ac.uk providingrelevant details, so we can investigate your claim.
Download date:29. Dec. 2020
This is an Accepted Article that has been peer-reviewed and approved for publication in the The
Journal of Physiology, but has yet to undergo copy-editing and proof correction. Please cite this
article as an 'Accepted Article'; doi: 10.1113/JP277478.
This article is protected by copyright. All rights reserved.
DOI: 10.1113/JP277478
Ionic signalling in astroglia beyond calcium
Alexei Verkhrastky1,2,3 , Verena Untiet2 & Christine R. Rose4
1Faculty of Biology, Medicine and Health, The University of Manchester, Manchester, M13 9PT, UK; 2Center for Basic and Translational Neuroscience, Faculty of Health and Medical Sciences, University
of Copenhagen, 2200 Copenhagen, Denmark; 3Achucarro Centre for Neuroscience, IKERBASQUE,
Basque Foundation for Science, 48011, Bilbao, Spain;
4Institute of Neurobiology, Faculty of Mathematics and Natural Sciences, Heinrich Heine University
Düsseldorf, Universitätsstrasse 1, D-40225 Düsseldorf, Germany
Running Title: Ionic Signalling in Astroglia
*Corresponding author:
Professor Alexei Verkhratsky, The University of Manchester, Oxford Road, Manchester, M13 9PT, UK,
Telephone +44 (0)161-2757324, e-mail: Alexej.Verkhratsky@manchester.ac.uk
Abstract
Astrocytes are homeostatic and protective cells of the central nervous system. Astroglial
homeostatic responses are tightly coordinated with neuronal activity. Astrocytes maintain neuronal
excitability through regulation of extracellular ion concentrations as well as assisting and modulating
synaptic transmission by uptake and catabolism of major neurotransmitters. Moreover, they support
neuronal metabolism and detoxify ammonium and reactive oxygen species. Astroglial homeostatic
actions are initiated and controlled by intercellular signalling of ions, including Ca2+, Na+, Cl-, H+ and
possibly K+. This treatise summarises current knowledge on ionic signals mediated by the major
monovalent ions, which occur in microdomains, as global events or as propagating intercellular
waves and thereby represent the substrate for astroglial excitability.
This article is protected by copyright. All rights reserved.
2
Key words: Astrocytes, Na+ signalling, Cl- signalling, K+ homoeostasis, H+ homeostasis
Ionic signalling
Ion gradients and the maintenance of ion homeostasis within the cytoplasm is a ubiquitous property
of all life forms on planet Earth. What was the ionic composition of the cytosol of the first cellular
ancestor, to what extent it reflected the ion concentrations of the primeval ocean (Kazmierczak et
al., 2013), what was the role of Donnan forces (Donnan, 1911) and which mechanisms (if any) this
primary cell employed to maintain cytosolic ion concentrations are all questions which remain
unanswered (Plattner & Verkhratsky, 2015, 2016, 2018). Environmental stresses (for example an
increase in osmotic pressure due to evaporation or its decrease due to rain) alter the concentrations
of extracellular ions and therefore trigger changes in intracellular ions. In this way, ions became
messengers coupling environmental challenges with cellular responses required for survival. Thus,
ionic signalling was born and it remains omnipresent in all cellular systems to this day. Cellular
physiology has always balanced the need for cytosolic ionic homeostasis (which, when
compromised, leads to rapid death) and the need to produce relevant intracellular ion changes that
trigger signalling cascades responsible for specific responses. Evolution has therefore selected both
for systems maintaining transmembrane ionic gradients and mechanisms for fast transmembrane
ion transport (which shape ionic signals), with the former enabling and regulating the latter.
Conceptually, changes in the cytosolic concentration of any ion, di- or monovalent, can contribute to
the regulation of cellular events, and hence any ion can act as a second messenger (Orlov & Hamet,
2006). Astrocytes, the electrically non-excitable homeostatic cells of the central nervous system
(CNS), utilise ionic signals as a substrate for their excitability (Verkhratsky et al., 2015; Verkhratsky &
Nedergaard, 2018). While using the same substrate (movement of ions across the plasma
membrane), glial excitability is distinct from the electrical excitability typical to neurones. Neurones
have been traditionally regarded as the only excitable cells of the nervous system because they
generate action potentials, which are conducted over large distances and can be transmitted to
other neurones through synaptic mechanisms. Research performed in the last decades, however,
has clearly established that the generation of specific, fast signals that are transmitted to other cells
is by no means an exclusive property of neurones. While not generating action potentials, astrocytes
do exhibit fast, transient fluctuations in their ionic content; moreover these ionic signals may
propagate passively or actively to neighbouring cells (Verkhratsky & Nedergaard, 2018).
Many of the acknowledged roles of astrocytes directly relate to transient changes in intracellular ion
concentrations, induced by neuronal activity. Most research has focused on Ca2+, and the signalling
role for Ca2+ in astroglia is widely accepted (Verkhratsky et al., 1998; Verkhratsky et al., 2012;
Volterra et al., 2014; Rusakov, 2015; Shigetomi et al., 2016). Astroglial Ca2+ signals result from
release of Ca2+ from intracellular stores, as well as from plasmalemmal Ca2+ influx (Shigetomi et al.,
2013; Bazargani & Attwell, 2016; Bindocci et al., 2017). In addition to Ca2+ signalling, astrocytes
experience transient changes in Na+, Cl-, K+ and H+ in response to physiological stimulation. These ion
This article is protected by copyright. All rights reserved.
3
transients reflect major functions that astrocytes fulfil in the nervous system. Among those is the
uptake of K+ from the extracellular space, which results in a rise in intraglial K+ concentrations ([K+]i).
In this way, astrocytes buffer the rise in extracellular K+ concentration ([K+]o) and sustain neuronal
excitability (Ballanyi et al., 1987; Kofuji & Newman, 2004). Another vital function of astrocytes, the
clearance of neurotransmitters from the extracellular space, is similarly related to ion signalling. The
uptake of glutamate and GABA, for example, is mediated by Na+-dependent transporters, which also
generate transient changes in sodium concentration ([Na+]i) in astrocytes (Rose & Ransom, 1996b;
Chatton et al., 2000; Kirischuk et al., 2007). These astroglial Na+ signals have been proposed to serve
an important role in neuro-metabolic coupling (Pellerin & Magistretti, 2012). Activity-related
transient changes in the intracellular pH (pHi) of astrocytes (Chesler & Kraig, 1989) have been
implicated in the stimulation of astrocyte metabolism in response to neuronal activity (Ruminot et
al., 2011). Finally, changes in Cl- concentration ([Cl-]i), induced by activation of Cl--dependent
transporters and/or Cl- channels are linked to activity-related changes in the cellular volume of
astrocytes (Chen & Sun, 2005; Kimelberg et al., 2006; Wilson & Mongin, 2018). The properties and
functional role of Ca2+ signalling has been extensively reviewed in the past, and in this paper we shall
therefore focus on the signalling role of the other four major monovalent ions: Na+, Cl-, K+ and H+.
Astroglial Na+ signalling
Sodium ions are fundamental for electrical signalling in the nervous system and are responsible for
the regenerative depolarising phase of the action potentials: inhibition of neuronal Na+ channels
therefore effectively extinguishes nervous impulse generation and propagation (Hodgkin & Huxley,
1952; Narahashi et al., 1964). In astrocytes, the inwardly directed transmembrane Na+ gradient
provides the main electrochemical driving force for the transport of ions, neurotransmitters,
neurotransmitter precursors and many other molecules, thus supporting astroglial homeostatic
functions. Dynamic changes in astroglial Na+ in response to physiological stimulation were
discovered in late 1990s, both in vitro (Rose & Ransom, 1996a) and in situ (Kirischuk et al., 1997).
Further studies revealed their link to glutamatergic neurotransmission (Kirischuk et al., 2007; Bennay
et al., 2008; Langer & Rose, 2009), and demonstrated intra- and intercellular diffusive spread of Na+
(Bernardinelli et al., 2004; Langer et al., 2012; Langer et al., 2017). The concept of astroglial Na+
signalling, coupling neuronal activity to fast local astroglial homeostatic responses has been
subsequently formalised (Kirischuk et al., 2012; Parpura & Verkhratsky, 2012; Rose & Karus, 2013;
Verkhratsky et al., 2013; Rose & Verkhratsky, 2016).
Molecular physiology of astroglial Na+ dynamics
At rest, the cytoplasmic Na+ concentration ([Na+]i) in astrocytes is maintained at a level of ~15 - 20
mM, being thus somewhat higher than in neurones (Rose & Ransom, 1996a; Kirischuk et al., 2012;
Reyes et al., 2012, 2013; Rose & Chatton, 2016). This resting level is maintained by the balance
between Na+ extrusion, mediated almost solely by the Na+-K+-ATPase (NKA) and Na+ influx, resulting
This article is protected by copyright. All rights reserved.
4
from the background activity of several transporters (for example Na+-2HCO3--co-transport by the
NBCe1, or the action of the Na+-H+ exchanger) and other (yet unidentified) Na+ channels. Inhibition
of NKA with ouabain or by removal of extracellular K+ produces a fast increase in [Na+]i, revealing this
constitutive Na+ influx (Rose & Ransom, 1996a; Golovina et al., 2003a; Illarionava et al., 2014).
Astroglial NKA incorporates the 2 catalytic subunit, which bestows properties distinct from the
neuronal Na+-K+ pump (composed of 1 and 3 subunits). The affinity of the astroglial 2 subunit to
extracellular K+ is much lower as compared to neurones: the [K+]0.5 for NKA composed from 2/1
subunits is ~3.6 mM, whereas [K+]0.5 for (neuronal) NKAs assembled from 1/1, 1/2, 3/1 or
3/2 subunits lies between 0.25 and 0.65 mM (Larsen et al., 2014). As a result of this molecular
composition, the astroglial NKA activity can be increased by physiological increases in the
extracellular K+ concentration, whereas K+ binding sites of the neuronal NKA are saturated and
neuronal NKA activity can be increased solely by an elevation in [Na+]i (Hertz et al., 2015). This
functional difference defines the leading role of astroglial NKA in K+ buffering (Larsen et al., 2014;
Verkhratsky & Nedergaard, 2018). Activity of astroglial NKA is regulated by noradrenergic
innervation via -adrenoceptors (Hajek et al., 1996) and by endogenous ouabain-like molecules
(Hertz et al., 2015; Larsen et al., 2016). Loss-of-function mutations in the 2 subunit are associated
with familial hemiplegic migraine type 2; in this pathology astrocytes are deficient both in K+
buffering and glutamate clearance (Capuani et al., 2016; Stoica et al., 2017).
Sodium influx in astrocytes is mediated by plasmalemmal channels and Na+-dependent solute carrier
transporters (Fig. 1). Depending on the brain region, astrocytes may express several types of
ionotropic receptors including N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-
isoxazolepropionic acid (AMPA) type glutamate receptors, P2X purinoceptors and nicotinic
acetylcholine receptors (Verkhratsky, 2010; Verkhratsky & Nedergaard, 2018). Although some of
these receptors exhibit a Ca2+ permeability, their current is carried mainly by Na+, which dominates
the ionic composition of the extracellular milieu. The regional distribution and functional expression
of voltage-gated Na+ channels (Nav) in astrocytes in vivo remain uncertain, although astroglial
expression of Nav1.2, Nav1.3, Nav1.5, Nav1.6 channel subunits has been detected at the mRNA and
protein level, and voltage-gated Na+ currents have been characterised in a subpopulation of
astrocytes in situ in hippocampal and spinal cord slices (Pappalardo et al., 2016). Astrocytes
throughout the CNS also express transient receptor potential (TRP) channels of TRPA1, TRPC1/4/5
and TRPV1/4 varieties, with some regional differences (Doly et al., 2004; Benfenati et al., 2007a;
Malarkey et al., 2008; Butenko et al., 2012; Shigetomi et al., 2012; Verkhratsky & Parpura, 2014).
Activation of TRP channels results in a substantial Na+ influx (Reyes et al., 2013). Additionally,
astroglial TRPC channels contribute to store-operated Ca2+ entry and thus link metabotropic
pathways and filling state of the endoplasmic reticulum with Na+ influx (Verkhratsky & Parpura,
2014). Finally, astrocytes in the subfornical organ express Nax channels activated by an increase in
extracellular Na+ concentration above 140 mM, which makes these astrocytes a part of systemic
control over body Na+ homeostasis (Noda & Hiyama, 2015).
This article is protected by copyright. All rights reserved.
5
Another prominent pathway for Na+ influx is associated with the activity of Na+-dependent solute
carrier (SLC) transporters (Fig. 1, 2) which are abundantly expressed in astrocytes (Verkhratsky &
Nedergaard, 2018). The Na+-dependent neurotransmitter transporters are represented by (i)
excitatory amino acid transporters types 1 and 2 (EAAT1/SLC1A3, EAAT2/SLC1A2) which are
responsible for glutamate uptake; (ii) GABA transporters type 3 (GAT-3/SLC6A12 Na+/Cl-/GABA
symporters); (iii) glycine transporters type 1 (GlyT1/SLC6A9); (iv) concentrative nucleoside
transporters CNT2/SLC28A2 and CNT3/SLC28A3 that co-transport 1 molecule of adenosine with 1
Na+ ion; (v) norepinephrine and dopamine transporters (NET/SLC6A2 and DAT/SLC6A3) and (vi) Na+-
coupled neutral amino acid transporters SNAT3/SLC38A3 and SNAT5/SLC38A5, which export
glutamine, an obligatory precursor for glutamate (Danbolt, 2001; Eulenburg et al., 2005; Eulenburg
et al., 2010; Zhou & Danbolt, 2013; Rose et al., 2018; Verkhratsky & Nedergaard, 2018). Operation of
these transporters is thermodynamically coupled to Na+ gradients and results in considerable Na+
fluxes; for example glutamate uptake through EAAT1/2 elevates [Na+]i by 10 - 20 mM (Rose &
Ransom, 1996a; Chatton et al., 2000; Kirischuk et al., 2007; Bennay et al., 2008; Langer & Rose,
2009). In addition to neurotransmitter transporters, Na+ fluxes are associated with other
homeostatic SLCs such as Na+-H+ exchanger NHE1-8/SLC9A1, Na+-bicarbonate co-transporter
NBCe1/SLC4A4, Na+-K+-Cl- co-transporter NKCC1/SLC12A2, Na+-dependent glucose transporter
SGLT1/SLC5A1 or Na+-dependent vitamin C transporter SVCT2/SLC23A2 (see (Verkhratsky &
Nedergaard, 2018) for details; Fig. 1, 2).
Astroglial Na+ and Ca2+ dynamics are coupled through the Na+-Ca2+ exchanger (NCX); of which all
three subtypes NCX1 - NCX3/SLC8A1 - A3 are operational in astrocytes. The reversal potential of the
NCX (around -80 mV) lies close to the astroglial resting membrane potential and it thus fluctuates
between forward and reverse mode. Depolarisation and increases in [Na+]i favour the reverse mode
(that is an import of Ca2+), while a rise in [Ca2+]i promotes the forward transport and Ca2+ export
(Paluzzi et al., 2007; Rojas et al., 2007; Reyes et al., 2012). The NCX thereby acts as a mechanism
coordinating Ca2+ and Na+ signalling in astroglial cells (Verkhratsky et al., 2018).
Spatial properties of astroglial Na+ signalling
Astrocyte Na+ signals can be local, that is, restricted to cellular microdomains. Such localisation was
experimentally detected in Bergmann glial cells and in hippocampal astrocytes upon weak synaptic
stimulation or with local application of glutamate to endfeet (Bennay et al., 2008; Langer & Rose,
2009; Langer et al., 2017). The exact mechanisms of the spatial localisation of Na+ signals in the
astroglial compartments remain elusive. Indeed, the cytosol does not contain known Na+-binding
proteins that may act as specific buffers limiting diffusion. Yet highly localised sub-plasmalemmal Na+
signals have also been proposed for several other cell types including cardiomyocytes (Wendt-
Gallitelli et al., 1993; Barry, 2006) and neurones (Hage & Salkoff, 2012). In astrocytes the EAATs,
NCX, NKA and Na+-permeable ionotropic receptors arguably tend to cluster in perisynaptic
processes, thus creating functional domains which may produce local [Na+]i increases (Chatton et al.,
2000; Cholet et al., 2002; Golovina et al., 2003b; Minelli et al., 2007; Rose et al., 2009). The
This article is protected by copyright. All rights reserved.
6
mechanism of localisation of Na+ (and any cationic) signals may be also provided by negatively
charged “traps” existing on the inner side of plasmalemma, which may promote retention of ions
close to the site of entry (Breslin et al., 2018).
In addition to spatially restricted Na+ transients, there is evidence that stronger synaptic stimulation
– i.e. activating more afferent fibres – results in astrocyte Na+ signals that spread within individual
cells, invading the soma as well as endfeet (Langer et al., 2017). The spread of Na+ is rapid (~100-150
µm/sec) and seems to propagate solely by passive (and un-buffered) Na+ diffusion through gap
junctions (Langer et al., 2012; Langer et al., 2017; Moshrefi-Ravasdjani et al., 2017). Propagating Na+
waves proceed not only through astrocytic networks but also enter other macroglial cells, including
oligodendrocytes and NG-2 cells. This type of co,,unication was identified in grey as well as in white
matter (Augustin et al., 2016; Moshrefi-Ravasdjani et al., 2017).
Na+ signals control astroglial physiological activity
Arguably, Na+ signals in astrocytes may occur in localised nano/microdomains associated with
perisynaptic processes that enwrap synaptic contacts (Bennay et al., 2008; Langer & Rose, 2009;
Langer et al., 2017). These perisynaptic astroglial compartments (known as synaptic cradles -
(Verkhratsky & Nedergaard, 2014)) maintain homeostasis of the synaptic cleft and support the
operation of the glutamate (GABA)-glutamine shuttle, as well as the uptake of other
neurotransmitters. To achieve this end, the membrane of astroglial perisynaptic processes (which
have an exceptionally high surface-to-volume ratio of about 25 m-1 (Grosche et al., 2002;
Reichenbach et al., 2010)) is densely packed with Na+-dependent SLC transporters that in turn are
controlled by the transmembrane Na+ gradient and hence by Na+ signals (Fig. 2).
Several examples of physiologically relevant astroglial Na+ signalling have been accumulated
recently. As described above, increases in Na+ can promote reverse activity of the NCX, resulting in
Ca2+ influx into astrocytes. There is evidence that this Na+-driven Ca2+ signalling regulates
mitochondrial mobility in astrocyte processes (Jackson & Robinson, 2018). Even relatively small
increases in [Na+]i can turn GABA or glycine transporters into the reverse mode (Eulenburg &
Gomeza, 2010; Heja et al., 2012; Unichenko et al., 2013) thus transforming astrocytes from the sink
to the source of inhibitory neurotransmitters. This physiologically relevant reversal of GABA
transporters has been demonstrated not only in vitro but also in situ in neocortical (Unichenko et al.,
2013) and hippocampal (Heja et al., 2012) slices. Of note, Na+-dependent uptake of glutamate is
safeguarded and glutamate transporters do not reverse under physiological conditions (Rose et al.,
2018; Verkhratsky & Nedergaard, 2018). Elevation of [Na+]i in astrocytes in situ directly stimulated
glutamine efflux through SNAT3/SLC38A3 Na+-dependent transporters, thus coordinating supply of
glutamine with neuronal activity (Todd et al, 2017). Influx of Na+ through Na+/HCO3- cotransporter
was recently demonstrated to be a trigger event in responses of chemosensing astrocytes of the
brain stem slices to physiological changes in PCO2/[H+] (Turovsky et al., 2016). Influx of Na+ controls
This article is protected by copyright. All rights reserved.
7
astroglial energy metabolism and Na+ signals trigger glucose uptake and lactate production by
astrocytes through aerobic glycolysis (Chatton et al., 2016). Moreover, the spread of Na+ between
astrocytes results in an increase in glucose uptake within the syncytium, coupling glial metabolism
with neuronal activity (Bernardinelli et al., 2014).
In addition, cytosolic Na+ may exert its action through “Na+ sensor” proteins. These may include
signalling cascades (Ono et al., 2010), enzymes (most notably glutamine synthetase (Benjamin,
1987)) or G-proteins (Rishal et al., 2003; Katritch et al., 2014). In addition, Na+ can regulate ion
channels either directly (for example Na+-dependent K+ channels (Bhattacharjee & Kaczmarek,
2005)), indirectly (spermine-induced inhibition of inward rectifier Kir 4.1 channels (Kucheryavykh et
al., 2012)), or by G-dependent inhibition of Ca2+ channels (Blumenstein et al., 2004). On an
integrative level, astroglial Na+ signals can provide long-term regulation of neurotransmission by
acting on all key elements of the glutamate (GABA)-glutamine shuttle – that is the EAATs, the
glutamine synthetase and the SNATs exporting glutamine.
Astroglial Cl- – the link to cell biology and regulator of inhibitory transmission?
Astroglial Cl- homeostasis
Besides bicarbonate (HCO3-), chloride is the main inorganic cytosolic anion, which controls numerous
cellular functions including a contribution to membrane permeability and establishment of the
membrane potential. Studies performed on astrocytes in culture indicated that the cytoplasmic Cl-
concentration in astroglia is rather high, being in the range of 20 to 50 mM, which sets the ECl at ~ -
35 mV; opening of astroglial Cl- channels hence caused Cl- efflux, inward current and depolarisation
(Kettenmann & Schachner, 1985; Bekar & Walz, 2002). In agreement with this, a radioactive Cl-
extrusion assay determined resting [Cl-]i to be 31 - 43 mM (Kimelberg, 1981), while another study
employing ion-sensitive microelectrodes indicated a resting [Cl-]i of 20 - 40 mM in astrocytes
(Kettenmann et al., 1987). Experiments in situ are few and theresults are controversial. Much lower
[Cl-]i, around 3.1-3.9 mM, was estimated in astrocytes in acute hippocampal slices based on
gramicidin patch-clamp recordings (Ma et al., 2012). In contrast, the only direct measurement of [Cl-
]i in situ performed recently performed in cerebellar Bergmann glial cells using the chloride-sensitive
dye MQAE ((6-Methoxyquinolinio)acetic acid ethyl ester bromide) and fluorescence lifetime imaging
microscopy (Untiet et al., 2017) revealed a high resting [Cl-]i of about 50 mM in 5-6 days old mice.
The [Cl-]i was reduced to ~35 mM in older (13 - 100 days old) animals. Such a decrease in glial [Cl-]i
parallels the neuronal Cl- switch seen during the first two postnatal weeks of development, which
enables GABA and glycine to act as inhibitory transmitters in mature brain. The glial Cl- switch is also
correlated with an up-regulation of expression of Cl- channels (Untiet et al., 2017), but the
transporters responsible for accumulation of Cl- in Bergmann glia in situ have not been identified yet.
Whether high or low levels of resting [Cl-]i are common to all astrocytes in the brain or whether
there are differences between astroglial subtypes remains to be elucidated. The only known Cl-
This article is protected by copyright. All rights reserved.
8
accumulating transporter, NKCC1, is highly expressed in cultured astrocytes. Its expression in situ is,
however, controversial, with data both pro (Kelly & Rose, 2010) and contra (Larsen et al., 2014); with
in vivo functional expression of NKCC1 not yet confirmed (Plotkin et al., 1997; Clayton et al., 1998).
The functional and signalling role of Cl-
Plasmalemmal Cl- permeability of astrocytes (Fig. 3) is mediated by (i) chloride channels, including
GABAA and glycine receptors; (ii) inwardly rectifying chloride channels ClC-1, -2 and -3; (iii) Ca2+-
dependent Cl- channels; (iv) anion channels of the Bestrophin (Best) family and by (v) volume-
regulated anion channels VRAC or SWELL1 (Parkerson & Sontheimer, 2004; Benfenati et al., 2007b;
Blanz et al., 2007; Park et al., 2009; Park et al., 2013; Verkhratsky & Nedergaard, 2018). In addition,
Cl- is transported across astroglial plasma membrane by SLC transporters including (i) GABA
transporters (GAT1/SLC6A1 and GAT3/SLC6A11) or glycine transporter (GlyT1/SLC6A9) with
stoichiometry of 1GABA(Gly):2Na+:1Cl− (Kavanaugh et al., 1992), (ii) chloride channels associated
with EAAT1/2 transporters (Vandenberg et al., 2008); (iii) Na+-K+-Cl- co-transporter 1
(NKCC1/SLC12A2), which moves Na+, K+ and Cl- into the cell with electroneutral stoichiometry of
1Na+ :1K+ :2Cl- (Macaulay & Zeuthen, 2012) and (iv) possibly members of K+-Cl co-transporter
(KCC/SLC12) family (Ochoa-de la Paz et al., 2005). Of note, the majority of these molecular pathways
mediate diffusional Cl- efflux, with only NKCC1 and GABA transporters capable of a (Na+-dependent)
accumulation of Cl- into the cell. The anion conductance of EAATs is not thermodynamically coupled
to the transport of glutamate but the formation of the anion pore occurs during the glutamate
transport cycle (Machtens et al., 2015).
The signalling role of Cl- is defined by multiple intracellular mechanisms and targets (Fig. 4) which
include (i) regulation of cell proliferation, cell differentiation and cell death mediated by the
apoptotic pathway; (ii) regulation of cell volume and hence morphological plasticity. How tightly
volume-regulation and [Cl-]i are coupled has been demonstrated in glioma cells. These highly
malignant tumour cells derive from astrocytes and have very high [Cl-]i (around 66 – 137 mM (Habela
et al., 2009)). It has been shown that this directly facilitates their migration and infiltration in dense
tissue (Ransom et al., 2001). (iii) modulation of several types of ion channels such as K+ channels and
TRPM7 channels through direct binding to yet uncharacterised channel protein domains and (iv)
regulation of intracellular signalling cascades such as WNK (With No lysine [K+]) and serine/threonine
protein kinases (see (Wilson & Mongin, 2018) for details and relevant literature). At the same time
astroglial [Cl-]i may play an important role in maintenance and regulation of inhibitory transmission
in the CNS. Upon strong activity of GABAergic synapses, Cl- may be depleted from the synaptic cleft.
Astrocytes, however, respond to the same GABA by Cl- efflux, and hence astroglial Cl- secretion can
supplement [Cl-]o thereby preserving the driving force for Cl- to sustain inhibitory neurotransmission
(Kettenmann et al., 1987). The role of Cl- efflux form astrocytes in maintaining GABA-ergic inhibitory
transmission was directly demonstrated in the experiments in CA1 hippocampal slices (Egawa et al.,
2013).
This article is protected by copyright. All rights reserved.
9
Pathophysiological consequences of disturbed Cl- homeostasis
The importance of astroglial Cl- homeostasis for inhibitory synaptic transmission is also suggested
from the analysis of a gain-of-function mutation of EAAT1 anion channels that is associated with
episodic ataxia 6 (EA6), and characterised by ataxia and epileptic seizures (Jen et al., 2005).
Operation of EAATs directly affects Bergmann glial [Cl-]i as has been shown in acute cerebellar slices
(Untiet et al., 2017). The mutated protein has been characterised in a heterologous expression
system. Cells within this system demonstrate a significant increase in anion conductance and a
decrease in glutamate transport rates (Winter et al., 2012). However, true understanding of the
pathological mechanism of this type of episodic ataxia requires further studies.
Investigation of ionic brain oedema in vitro and in situ show activation of NKCC1 mediated Cl-
accumulation followed by water influx and astrocyte swelling. The involvement of NKCC1 in cases of
brain oedema is indicated by several studies that show increased NKCC1 expression, activation and
beneficial effects of blocking NKCC1 using bumetanide. Due to the controversial data about NKCC1
expression in astrocytes and limitation of in vitro and in situ studies investigating fluid redistribution,
the role of astroglial Cl- fluxes during brain oedema need to be verified in vivo (Simard et al., 2007;
Jayakumar et al., 2011; Thrane et al., 2014).
Astroglial potassium microdomains: can they bear the signalling role?
Homeostatic regulation of the interstitial concentration of potassium ions ([K+]o) is the archetypal
function of astrocytes (Larsen et al., 2016). This astroglial function (also known as K+ buffering) was
discovered in 1960, when both diffusional (Orkand et al., 1966) and energy/NKA-dependent
mechanisms were proposed (Hertz, 1965). Potassium membrane permeability of astrocytes (Fig. 5) is
determined by NKA, the main importer of K+ ions, by NKCC1, which seemingly works only in
conditions of K+ overload and may not be even functionally expressed in astrocytes in the in vivo
brain ((Larsen et al., 2014), but see (Kelly & Rose, 2010)) and by diffusion (mostly efflux) through K+
channels of which the inward rectifying Kir4.1 channel is predominant. In the physiological context,
astrocytes operate like a “K+ shuttle” accumulating K+ via NKA at the peak of neuronal activity and
releasing K+ back to neurones through Kir4.1 in the aftermath (Chever et al., 2010; Rimmele &
Chatton, 2014) and thus supporting restoration of neuronal ion gradients (Verkhratsky &
Nedergaard, 2018).
While this is a widely accepted scenario, direct experimental data on K+ signalling in astrocytes are
extremely sparse. This is largely because the available chemical indicator dyes have poor signal-to-
noise-ratio and are difficult to use. A recent study, employing the novel indicator Asante Potassium-
Green-1, reported a decrease in astrocyte [K+] by several mM in response to bath application of
This article is protected by copyright. All rights reserved.
10
glutamate; increasing the [K+]o on the other hand, caused an increase in astrocyte [K+]i (Rimmele &
Chatton, 2014). The proposed K+ shuttle ultimately requires retention of K+ in the perisynaptic
compartments with emergence of K+ microdomains, which may be supported by intramembrane
negative charges preventing K+ diffusion (Breslin et al., 2018). Whether these microdomains may
have any further signalling role remains an open question.
Astroglial protons – what can they do?
Regulation of astroglial pH
Astroglial pH (pHi) is usually within a range of 7.0-7.3, which is slightly more acidic than pH values
commonly found in the extracellular spaces of the brain (pHe) (Deitmer & Rose, 1996; Chesler, 2003;
Obara et al., 2008). Notably, at a pHi between 7.0 and 7.3, the free proton (H+) concentration is only
between 40 and 80 nM, and thus much lower than the free concentrations found for other main
monovalent ions, including Na+, K+ and Cl-. Astrocytes have an exceptionally high intracellular H+
buffering power of about 200 mM, provided by the enzyme carbonic anhydrase (CA), which converts
CO2 to HCO3- and H+, as well as by efficient plasma membrane transport of acid-base equivalents
(HCO3-) through the Na+-HCO3
- co-transporter NBCe1/SLC4A4 (Deitmer & Rose, 1996; Chesler, 2003;
Theparambil & Deitmer, 2015).
In the absence of a strong chemical gradient, EH+ is around -10 mV, resulting in an inwardly directed
driving force for the movement of H+ at the typical high negative membrane potential of astrocytes.
At the same time, protons may not only passively enter astrocytes, but they are also generated as a
product of cellular metabolism and ATP production, for example as a consequence of mitochondrial
respiration and generation of CO2. The latter is converted to HCO3- and H+ by CA. Astrocytes have
particularly high CA activity and it was suggested that CO2 which diffuses out of neurones is
preferentially converted into HCO3- and H+ by glial cells (Deitmer, 2002). Cellular acidification is also
promoted by ion channels, directly permeable to H+, as well as by membrane transporters that
mediate the influx of H+ such as the PMCA or Na+-dependent glutamate transporters (EAATs) (Fig. 6).
The latter couple the uptake of 1 molecule of glutamate to the influx of 3 Na+ and 1 H+ as well as to
the export of 1 K+, thereby acidifying astrocytes (Levy et al., 1998).
Astrocytes must, therefore, actively extrude H+ to counteract acidification of the cytosol during
neuronal activity (Fig. 6). The main glial acid extruder, which also plays a prominent role in the brain
pathology, is the Na+-H+ exchanger (NHE1/SLC9A1) (Deitmer & Rose, 1996; Chesler, 2003; Obara et
al., 2008; Zhao et al., 2016; Begum et al., 2018). NHE1 couples the inwardly directed Na+ gradient to
the export of H+ and its activity is thus decreased following increases in the intracellular Na+,
resulting in cellular acidification (Bondarenko et al., 2005). In addition, several transporters for HCO3-
This article is protected by copyright. All rights reserved.
11
are expressed by astrocytes. These include the above-mentioned NBCe1 (Brune et al., 1994;
Bevensee et al., 1997; Schrodl-Haussel et al., 2015), which is usually inwardly directed, but may
readily reverse in response to an intracellular alkalosis and thereby serve as acid loader
(Theparambil & Deitmer, 2015). Moreover, astrocytes have been reported to express Na+-dependent
and Na+-independent Cl-/HCO3--exchangers, the functional parameters of which are yet to be
characterised, as well as a vacuolar-type H+-ATPase (Deitmer & Rose, 1996; Chesler, 2003; Obara et
al., 2008). Protons extrusion is also mediated by export of lactate through monocarboxylate
transporters, MCTs (Pierre & Pellerin, 2005).
pH changes in astroglia and their signalling functions
Neuronal activity in the vertebrate brain is accompanied by substantial and complex shifts in pH in
all involved compartments: while neurones acidify and astrocytes alkalinise, the extracellular space
undergoes a biphasic alkaline-acid shift (Chesler & Kraig, 1989; Chesler, 2003; Deitmer & Rose, 2010;
Sinning & Hubner, 2013). The alkalinisation of astrocytes in response to excitatory neuronal activity
has been attributed to the accompanying rise in extracellular K+ resulting in a depolarisation of
astrocytes and a concomitant activation of inward NBCe1 activity (Brune et al., 1994; Pappas &
Ransom, 1994; Bevensee et al., 1997).
The so-called “depolarisation-induced alkalinisation” of astrocytes effectively results in an
extracellular acidification (Chesler & Kraig, 1989). Because a decrease in extracellular pH dampens
neuronal activity (Sinning & Hubner, 2013), this was long viewed as a mechanism by which
astrocytes might modulate the excitability of neurones (Ransom, 1992, 2000). An exception is found
in areas of the brainstem that are responsible for sensing CO2 and pH and control breathing patterns
(Marina et al., 2018). Here, a CO2-induced acidification was accompanied by activation of astrocyte
NBCe1, resulting in secondary activation of NCX, an increase in Ca2+ and release of ATP from
astrocytes, exciting respiratory circuits (Gourine et al., 2010; Kasymov et al., 2013; Turovsky et al.,
2016).
In forebrain astrocytes, several laboratories have now convincingly established that NBCe1 plays a
key role in neuro-metabolic coupling (Chatton et al., 2016). Activation of NBCe1 results in
stimulation of a soluble adenylyl cyclase and production of cAMP in astrocytes, which enhances
glycolysis (Choi et al., 2012). Another major player, coupling astrocytic depolarisation-induced
alkalinisation (and NBCe1) to metabolism, is the phosphofructokinase, a key enzyme of glycolysis. It
is strongly pH-dependent and is efficiently stimulated by alkalinisation, resulting in an activation of
astrocyte glycolysis in response to increases in extracellular K+ (Bittner et al., 2011; Ruminot et al.,
2011). The pH-related increase in glycolysis was suggested to play a main role in the increased
production of lactate by astrocytes, driving the so-called “astrocyte-neurone lactate shuttle”
(Pellerin & Magistretti, 2012; Chatton et al., 2016). While this concept is still under debate (Dienel,
2014; Bak & Walls, 2018; Barros & Weber, 2018), a recent study found that NBCe1 also regulates the
This article is protected by copyright. All rights reserved.
12
NADH/NAD+ redox state of astrocytes, again confirming its central role in adapting astrocytic
metabolism to the energy requirements of active neurones (Kohler et al., 2018).
Recapitulation and Conclusion
Astroglial excitability and astroglial physiological responses are mediated and regulated by highly
coordinated changes in intracellular ion concentrations, which affect astroglial biochemistry (mostly
through Ca2+ and Cl-) and astroglial membrane transporters (mostly Na+ and K+). Ionic fluxes, which
form the backbone for this type of “cytosolic ionic excitability”, are mediated through membrane
channels and transporters which couple translocation of these ions. Notably, the NKA is central to
the transport of all major ions: it directly controls [Na2+]i and [K+]i and indirectly affects fluxes of Ca2+
(through NCX), of Cl- (through K+/Cl- and K+/Na+/Cl- co-transporters) and of H+ (through NHE and
NBC). The molecules responsible for ionic signalling are concentrated in astroglial perisynaptic
processes where they provide for coordination of the astrocytic homeostatic response with neuronal
activity.
The concept of astroglial Ca2+ excitability is well defined and intimate mechanisms have been
revealed. Molecular cascades responsible for signalling mediated by other ions are, in contrast, in
statu nascendi, and numerous fundamental properties of ionic intracellular dynamics are yet to be
described. In particular, the organisation of micro- and nano-domains for different ions has to be
characterised, and relevant molecular mechanisms revealed. Similarly, molecular targets and
physiological outcomes of signals mediated by different ions have to be elucidated and placed in
context of nerve tissue operation in physiological and pathological conditions.
Competing interests
The authors disclose any conflict of interest.
Author contributions
All authors have contributed to the conception and drafting of the text and figures. Moreover, all
authors contributed its revision and approved the final version of the manuscript. All authors qualify
for authorship, and all those who qualify for authorship are listed.
Funding
Work in the laboratory of CRR is supported by the Deutsche Forschungsgemeinschaft (DFG, Ro
2327/8-2 and 13-1).
Acknowledgements
We are grateful to Lisa Felix, Institute of Neurobiology, Heinrich Heine University Düsseldorf, for
proofreading the manuscript.
This article is protected by copyright. All rights reserved.
13
References
Augustin V, Bold C, Wadle SL, Langer J, Jabs R, Philippot C, Weingarten DJ, Rose CR, Steinhauser C &
Stephan J. (2016). Functional anisotropic panglial networks in the lateral superior olive. Glia
64, 1892-1911.
Bak LK & Walls AB. (2018). CrossTalk opposing view: lack of evidence supporting an astrocyte-to-
neuron lactate shuttle coupling neuronal activity to glucose utilisation in the brain. J Physiol
596, 351-353.
Ballanyi K, Grafe P & ten Bruggencate G. (1987). Ion activities and potassium uptake mechanisms of
glial cells in guinea-pig olfactory cortex slices. J Physiol 382, 159-174.
Barros LF & Weber B. (2018). CrossTalk proposal: an important astrocyte-to-neuron lactate shuttle
couples neuronal activity to glucose utilisation in the brain. J Physiol 596, 347-350.
Barry WH. (2006). Na"Fuzzy space": does it exist, and is it important in ischemic injury? J Cardiovasc
Electrophysiol 17 Suppl 1, S43-S46.
Bazargani N & Attwell D. (2016). Astrocyte calcium signaling: the third wave. Nat Neurosci 19, 182-
189.
Begum G, Song S, Wang S, Zhao H, Bhuiyan MIH, Li E, Nepomuceno R, Ye Q, Sun M, Calderon MJ,
Stolz DB, St Croix C, Watkins SC, Chen Y, He P, Shull GE & Sun D. (2018). Selective knockout
of astrocytic Na+ /H+ exchanger isoform 1 reduces astrogliosis, BBB damage, infarction, and
improves neurological function after ischemic stroke. Glia 66, 126-144.
Bekar LK & Walz W. (2002). Intracellular chloride modulates A-type potassium currents in astrocytes.
Glia 39, 207-216.
Benfenati V, Amiry-Moghaddam M, Caprini M, Mylonakou MN, Rapisarda C, Ottersen OP & Ferroni
S. (2007a). Expression and functional characterization of transient receptor potential
vanilloid-related channel 4 (TRPV4) in rat cortical astrocytes. Neuroscience 148, 876-892.
Benfenati V, Nicchia GP, Svelto M, Rapisarda C, Frigeri A & Ferroni S. (2007b). Functional down-
regulation of volume-regulated anion channels in AQP4 knockdown cultured rat cortical
astrocytes. J Neurochem 100, 87-104.
Benjamin AM. (1987). Influence of Na+, K+, and Ca2+ on glutamine synthesis and distribution in rat
brain cortex slices: a possible linkage of glutamine synthetase with cerebral transport
processes and energetics in the astrocytes. J Neurochem 48, 1157-1164.
Bennay M, Langer J, Meier SD, Kafitz KW & Rose CR. (2008). Sodium signals in cerebellar Purkinje
neurons and Bergmann glial cells evoked by glutamatergic synaptic transmission. Glia 56,
1138-1149.
Bernardinelli Y, Magistretti PJ & Chatton JY. (2004). Astrocytes generate Na+-mediated metabolic
waves. Proc Natl Acad Sci U S A 101, 14937-14942.
This article is protected by copyright. All rights reserved.
14
Bevensee MO, Apkon M & Boron WF. (1997). Intracellular pH regulation in cultured astrocytes from
rat hippocampus. II. Electrogenic Na/HCO3 cotransport. J Gen Physiol 110, 467-483.
Bhattacharjee A & Kaczmarek LK. (2005). For K+ channels, Na+ is the new Ca2+. Trends Neurosci 28,
422-428.
Bindocci E, Savtchouk I, Liaudet N, Becker D, Carriero G & Volterra A. (2017). Three-dimensional Ca2+
imaging advances understanding of astrocyte biology. Science 356.
Bittner CX, Valdebenito R, Ruminot I, Loaiza A, Larenas V, Sotelo-Hitschfeld T, Moldenhauer H, San
Martin A, Gutierrez R, Zambrano M & Barros LF. (2011). Fast and reversible stimulation of
astrocytic glycolysis by K+ and a delayed and persistent effect of glutamate. J Neurosci 31,
4709-4713.
Blanz J, Schweizer M, Auberson M, Maier H, Muenscher A, Hubner CA & Jentsch TJ. (2007).
Leukoencephalopathy upon disruption of the chloride channel ClC-2. J Neurosci 27, 6581-
6589.
Blumenstein Y, Maximyuk OP, Lozovaya N, Yatsenko NM, Kanevsky N, Krishtal O & Dascal N. (2004).
Intracellular Na+ inhibits voltage-dependent N-type Ca2+ channels by a G protein subunit-
dependent mechanism. J Physiol 556, 121-134.
Bondarenko A, Svichar N & Chesler M. (2005). Role of Na+-H+ and Na+-Ca2+ exchange in hypoxia-
related acute astrocyte death. Glia 49, 143-152.
Breslin K, Wade JJ, Wong-Lin K, Harkin J, Flanagan B, Van Zalinge H, Hall S, Walker M, Verkhratsky A
& McDaid L. (2018). Potassium and sodium microdomains in thin astroglial processes: A
computational model study. PLoS Comput Biol 14, e1006151.
Brune T, Fetzer S, Backus KH & Deitmer JW. (1994). Evidence for electrogenic sodium-bicarbonate
cotransport in cultured rat cerebellar astrocytes. Pflugers Arch 429, 64-71.
Butenko O, Dzamba D, Benesova J, Honsa P, Benfenati V, Rusnakova V, Ferroni S & Anderova M.
(2012). The increased activity of TRPV4 channel in the astrocytes of the adult rat
hippocampus after cerebral hypoxia/ischemia. PLoS One 7, e39959.
Capuani C, Melone M, Tottene A, Bragina L, Crivellaro G, Santello M, Casari G, Conti F & Pietrobon D.
(2016). Defective glutamate and K+ clearance by cortical astrocytes in familial hemiplegic
migraine type 2. EMBO Mol Med 8, 967-986.
Chatton JY, Magistretti PJ & Barros LF. (2016). Sodium signaling and astrocyte energy metabolism.
Glia 64, 1667-1676.
Chatton JY, Marquet P & Magistretti PJ. (2000). A quantitative analysis of L-glutamate-regulated Na+
dynamics in mouse cortical astrocytes: implications for cellular bioenergetics. Eur J Neurosci
12, 3843-3853.
Chen H & Sun D. (2005). The role of Na-K-Cl co-transporter in cerebral ischemia. Neurol Res 27, 280-
286.
This article is protected by copyright. All rights reserved.
15
Chesler M. (2003). Regulation and modulation of pH in the brain. Physiol Rev 83, 1183-1221.
Chesler M & Kraig RP. (1989). Intracellular pH transients of mammalian astrocytes. J Neurosci 9,
2011-2019.
Chever O, Djukic B, McCarthy KD & Amzica F. (2010). Implication of Kir4.1 channel in excess
potassium clearance: an in vivo study on anesthetized glial-conditional Kir4.1 knock-out
mice. J Neurosci 30, 15769-15777.
Choi HB, Gordon GR, Zhou N, Tai C, Rungta RL, Martinez J, Milner TA, Ryu JK, McLarnon JG,
Tresguerres M, Levin LR, Buck J & MacVicar BA. (2012). Metabolic communication between
astrocytes and neurons via bicarbonate-responsive soluble adenylyl cyclase. Neuron 75,
1094-1104.
Cholet N, Pellerin L, Magistretti PJ & Hamel E. (2002). Similar perisynaptic glial localization for the
Na+,K+-ATPase 2 subunit and the glutamate transporters GLAST and GLT-1 in the rat
somatosensory cortex. Cereb Cortex 12, 515-525.
Clayton GH, Owens GC, Wolff JS & Smith RL. (1998). Ontogeny of cation-Cl- cotransporter expression
in rat neocortex. Brain Res Dev Brain Res 109, 281-292.
Danbolt NC. (2001). Glutamate uptake. Prog Neurobiol 65, 1-105.
Deitmer JW. (2002). A role for CO2 and bicarbonate transporters in metabolic exchanges in the brain.
J Neurochem 80, 721-726.
Deitmer JW & Rose CR. (1996). pH regulation and proton signalling by glial cells. Prog Neurobiol 48,
73-103.
Deitmer JW & Rose CR. (2010). Ion changes and signalling in perisynaptic glia. Brain Res Rev 63, 113-
129.
Dienel GA. (2014). Lactate shuttling and lactate use as fuel after traumatic brain injury: metabolic
considerations. J Cereb Blood Flow Metab 34, 1736-1748.
Doly S, Fischer J, Salio C & Conrath M. (2004). The vanilloid receptor-1 is expressed in rat spinal
dorsal horn astrocytes. Neurosci Lett 357, 123-126.
Donnan F. (1911). Theorie der Membrangleichgewichte und Membranpotentiale bei Vorhandensein
von nicht dialysierenden Elektrolyten. Ein Beitrag zur physikalisch-chemischen Physiologie.
Zeitschrift Für Elektrochemie Angew Phys Chem 17, 572-581.
Egawa K, Yamada J, Furukawa T, Yanagawa Y & Fukuda A. (2013). Cl- homeodynamics in gap
junction-coupled astrocytic networks on activation of GABAergic synapses. J Physiol 591,
3901-3917.
Eulenburg V, Armsen W, Betz H & Gomeza J. (2005). Glycine transporters: essential regulators of
neurotransmission. Trends Biochem Sci 30, 325-333.
This article is protected by copyright. All rights reserved.
16
Eulenburg V & Gomeza J. (2010). Neurotransmitter transporters expressed in glial cells as regulators
of synapse function. Brain Res Rev 63, 103-112.
Eulenburg V, Retiounskaia M, Papadopoulos T, Gomeza J & Betz H. (2010). Glial glycine transporter 1
function is essential for early postnatal survival but dispensable in adult mice. Glia 58, 1066-
1073.
Golovina V, Song H, James P, Lingrel J & Blaustein M. (2003a). Regulation of Ca2+ signaling by Na+
pump 2 subunit expression. Ann N Y Acad Sci 986, 509-513.
Golovina VA, Song H, James PF, Lingrel JB & Blaustein MP. (2003b). Na+ pump 2-subunit expression
modulates Ca2+ signaling. Am J Physiol Cell Physiol 284, C475-486.
Gourine AV, Kasymov V, Marina N, Tang F, Figueiredo MF, Lane S, Teschemacher AG, Spyer KM,
Deisseroth K & Kasparov S. (2010). Astrocytes control breathing through pH-dependent
release of ATP. Science 329, 571-575.
Grosche J, Kettenmann H & Reichenbach A. (2002). Bergmann glial cells form distinct morphological
structures to interact with cerebellar neurons. J Neurosci Res 68, 138-149.
Habela CW, Ernest NJ, Swindall AF & Sontheimer H. (2009). Chloride accumulation drives volume
dynamics underlying cell proliferation and migration. J Neurophysiol 101, 750-757.
Hage TA & Salkoff L. (2012). Sodium-activated potassium channels are functionally coupled to
persistent sodium currents. J Neurosci 32, 2714-2721.
Hajek I, Subbarao KV & Hertz L. (1996). Acute and chronic effects of potassium and noradrenaline on
Na+, K+-ATPase activity in cultured mouse neurons and astrocytes. Neurochem Int 28, 335-
342.
Heja L, Nyitrai G, Kekesi O, Dobolyi A, Szabo P, Fiath R, Ulbert I, Pal-Szenthe B, Palkovits M & Kardos
J. (2012). Astrocytes convert network excitation to tonic inhibition of neurons. BMC Biol 10,
26.
Hertz L. (1965). Possible role of neuroglia: a potassium-mediated neuronal--neuroglial--neuronal
impulse transmission system. Nature 206, 1091-1094.
Hertz L, Song D, Xu J, Peng L & Gibbs ME. (2015). Role of the astrocytic Na+, K+-ATPase in K+
homeostasis in brain: K+ uptake, signaling pathways and substrate utilization. Neurochem
Res 40, 2505-2516.
Hodgkin AL & Huxley AF. (1952). A quantitative description of membrane current and its application
to conduction and excitation in nerve. J Physiol 117, 500-544.
Illarionava NB, Brismar H, Aperia A & Gunnarson E. (2014). Role of Na,K-ATPase 1 and 2 isoforms
in the support of astrocyte glutamate uptake. PLoS One 9, e98469.
Jackson JG, Robinson MB. 2018. Regulation of mitochondrial dynamics in astrocytes: Mechanisms,
consequences, and unknowns. Glia 66, 1213-1234.
This article is protected by copyright. All rights reserved.
17
Jayakumar AR, Valdes V & Norenberg MD. (2011). The Na-K-Cl cotransporter in the brain edema of
acute liver failure. J Hepatol 54, 272-278.
Jen JC, Wan J, Palos TP, Howard BD & Baloh RW. (2005). Mutation in the glutamate transporter
EAAT1 causes episodic ataxia, hemiplegia, and seizures. Neurology 65, 529-534.
Kasymov V, Larina O, Castaldo C, Marina N, Patrushev M, Kasparov S & Gourine AV. (2013).
Differential sensitivity of brainstem versus cortical astrocytes to changes in pH reveals
functional regional specialization of astroglia. J Neurosci 33, 435-441.
Katritch V, Fenalti G, Abola EE, Roth BL, Cherezov V & Stevens RC. (2014). Allosteric sodium in class A
GPCR signaling. Trends Biochem Sci 39, 233-244.
Kavanaugh MP, Arriza JL, North RA & Amara SG. (1992). Electrogenic uptake of -aminobutyric acid
by a cloned transporter expressed in Xenopus oocytes. J Biol Chem 267, 22007-22009.
Kazmierczak J, Kempe S & Kremer B. (2013). Calcium in the early evolution of living systems: A
biohistorical approach Curr Organic Chem 17, 1738-1750.
Kelly T & Rose CR. (2010). Ammonium influx pathways into astrocytes and neurones of hippocampal
slices. J Neurochem 115, 1123-1136.
Kettenmann H, Backus KH & Schachner M. (1987). -Aminobutyric acid opens Cl- channels in cultured
astrocytes. Brain Res 404, 1-9.
Kettenmann H & Schachner M. (1985). Pharmacological properties of gamma-aminobutyric acid-,
glutamate-, and aspartate-induced depolarizations in cultured astrocytes. J Neurosci 5, 3295-
3301.
Kimelberg HK. (1981). Active accumulation and exchange transport of chloride in astroglial cells in
culture. Biochim Biophys Acta 646, 179-184.
Kimelberg HK, Macvicar BA & Sontheimer H. (2006). Anion channels in astrocytes: biophysics,
pharmacology, and function. Glia 54, 747-757.
Kirischuk S, Kettenmann H & Verkhratsky A. (1997). Na+/Ca2+ exchanger modulates kainate-triggered
Ca2+ signaling in Bergmann glial cells in situ. FASEB J 11, 566-572.
Kirischuk S, Kettenmann H & Verkhratsky A. (2007). Membrane currents and cytoplasmic sodium
transients generated by glutamate transport in Bergmann glial cells. Pflugers Arch 454, 245-
252.
Kirischuk S, Parpura V & Verkhratsky A. (2012). Sodium dynamics: another key to astroglial
excitability? Trends Neurosci 35, 497-506.
Kofuji P & Newman EA. (2004). Potassium buffering in the central nervous system. Neuroscience 129,
1045-1056.
Kohler S, Winkler U, Sicker M & Hirrlinger J. (2018). NBCe1 mediates the regulation of the
NADH/NAD+ redox state in cortical astrocytes by neuronal signals. Glia in press.
This article is protected by copyright. All rights reserved.
18
Kucheryavykh YV, Antonov SM, Shuba YM, Rivera Y, Inyushin MY, Veh RW, Verkhratsky A, NIichols
CG, Eaton MJ & Skatchkov SN. (2012). Sodium accumulated in glia during glutamate
transport increases polyamine dependent block of Kir4.1 channels. Programme No 23605/
C15 2012 Society for Neuroscience New Orleans, LA: Society for Neuroscience.
Langer J, Gerkau NJ, Derouiche A, Kleinhans C, Moshrefi-Ravasdjani B, Fredrich M, Kafitz KW, Seifert
G, Steinhauser C & Rose CR. (2017). Rapid sodium signaling couples glutamate uptake to
breakdown of ATP in perivascular astrocyte endfeet. Glia 65, 293-308.
Langer J & Rose CR. (2009). Synaptically induced sodium signals in hippocampal astrocytes in situ. J
Physiol 587, 5859-5877.
Langer J, Stephan J, Theis M & Rose CR. (2012). Gap junctions mediate intercellular spread of sodium
between hippocampal astrocytes in situ. Glia 60, 239-252.
Larsen BR, Assentoft M, Cotrina ML, Hua SZ, Nedergaard M, Kaila K, Voipio J & MacAulay N. (2014).
Contributions of the Na+/K+-ATPase, NKCC1, and Kir4.1 to hippocampal K+ clearance and
volume responses. Glia 62, 608-622.
Larsen BR, Stoica A & MacAulay N. (2016). Managing Brain Extracellular K+ during Neuronal Activity:
The Physiological Role of the Na+/K+-ATPase Subunit Isoforms. Front Physiol 7, 141.
Levy LM, Warr O & Attwell D. (1998). Stoichiometry of the glial glutamate transporter GLT-1
expressed inducibly in a Chinese hamster ovary cell line selected for low endogenous Na+-
dependent glutamate uptake. J Neurosci 18, 9620-9628.
Ma BF, Xie MJ & Zhou M. (2012). Bicarbonate efflux via GABAA receptors depolarizes membrane
potential and inhibits two-pore domain potassium channels of astrocytes in rat hippocampal
slices. Glia 60, 1761-1772.
Macaulay N & Zeuthen T. (2012). Glial K+ clearance and cell swelling: key roles for cotransporters and
pumps. Neurochem Res 37, 2299-2309.
Machtens JP, Kortzak D, Lansche C, Leinenweber A, Kilian P, Begemann B, Zachariae U, Ewers D, de
Groot BL, Briones R & Fahlke C. (2015). Mechanisms of anion conduction by coupled
glutamate transporters. Cell 160, 542-553.
Malarkey EB, Ni Y & Parpura V. (2008). Ca2+ entry through TRPC1 channels contributes to
intracellular Ca2+ dynamics and consequent glutamate release from rat astrocytes. Glia 56,
821-835.
Marina N, Turovsky E, Christie IN, Hosford PS, Hadjihambi A, Korsak A, Ang R, Mastitskaya S,
Sheikhbahaei S, Theparambil SM & Gourine AV. (2018). Brain metabolic sensing and
metabolic signaling at the level of an astrocyte. Glia 66, 1185-1199.
Minelli A, Castaldo P, Gobbi P, Salucci S, Magi S & Amoroso S. (2007). Cellular and subcellular
localization of Na+-Ca2+ exchanger protein isoforms, NCX1, NCX2, and NCX3 in cerebral
cortex and hippocampus of adult rat. Cell Calcium 41, 221-234.
This article is protected by copyright. All rights reserved.
19
Moshrefi-Ravasdjani B, Hammel EL, Kafitz KW & Rose CR. (2017). Astrocyte Sodium Signalling and
Panglial Spread of Sodium Signals in Brain White Matter. Neurochem Res 42, 2505-2518.
Narahashi T, Moore JW & Scott WR. (1964). Tetrodotoxin Blockage of Sodium Conductance Increase
in Lobster Giant Axons. J Gen Physiol 47, 965-974.
Noda M & Hiyama TY. (2015). The Nax Channel: What It Is and What It Does. Neuroscientist 21, 399-
412.
Obara M, Szeliga M & Albrecht J. (2008). Regulation of pH in the mammalian central nervous system
under normal and pathological conditions: facts and hypotheses. Neurochem Int 52, 905-
919.
Ochoa-de la Paz LD, Lezama R, Toscano B & Pasantes-Morales H. (2005). Mechanisms of chloride
influx during KCl-induced swelling in the chicken retina. Pflugers Arch 449, 526-536.
Ono Y, Ojima K, Torii F, Takaya E, Doi N, Nakagawa K, Hata S, Abe K & Sorimachi H. (2010). Skeletal
muscle-specific calpain is an intracellular Na+-dependent protease. J Biol Chem 285, 22986-
22998.
Orkand RK, Nicholls JG & Kuffler SW. (1966). Effect of nerve impulses on the membrane potential of
glial cells in the central nervous system of amphibia. J Neurophysiol 29, 788-806.
Orlov SN & Hamet P. (2006). Intracellular monovalent ions as second messengers. J Membr Biol 210,
161-172.
Paluzzi S, Alloisio S, Zappettini S, Milanese M, Raiteri L, Nobile M & Bonanno G. (2007). Adult
astroglia is competent for Na+/Ca2+ exchanger-operated exocytotic glutamate release
triggered by mild depolarization. J Neurochem 103, 1196-1207.
Pappalardo LW, Black JA & Waxman SG. (2016). Sodium channels in astroglia and microglia. Glia 64,
1628-1645.
Pappas CA & Ransom BR. (1994). Depolarization-induced alkalinization (DIA) in rat hippocampal
astrocytes. J Neurophysiol 72, 2816-2826.
Park H, Han KS, Oh SJ, Jo S, Woo J, Yoon BE & Lee CJ. (2013). High glutamate permeability and distal
localization of Best1 channel in CA1 hippocampal astrocyte. Mol Brain 6, 54.
Park H, Oh SJ, Han KS, Woo DH, Park H, Mannaioni G, Traynelis SF & Lee CJ. (2009). Bestrophin-1
encodes for the Ca2+-activated anion channel in hippocampal astrocytes. J Neurosci 29,
13063-13073.
Parkerson KA & Sontheimer H. (2004). Biophysical and pharmacological characterization of
hypotonically activated chloride currents in cortical astrocytes. Glia 46, 419-436.
Parpura V & Verkhratsky A. (2012). Homeostatic function of astrocytes: Ca2+ and Na+ signalling.
Transl Neurosci 3, 334-344.
Pellerin L & Magistretti PJ. (2012). Sweet sixteen for ANLS. J Cereb Blood Flow Metab 32, 1152-1166.
This article is protected by copyright. All rights reserved.
20
Pierre K & Pellerin L. (2005). Monocarboxylate transporters in the central nervous system:
distribution, regulation and function. J Neurochem 94, 1-14.
Plattner H & Verkhratsky A. (2015). The ancient roots of calcium signalling evolutionary tree. Cell
Calcium 57, 123-132.
Plattner H & Verkhratsky A. (2016). Inseparable tandem: evolution chooses ATP and Ca2+ to control
life, death and cellular signalling. Philos Trans R Soc Lond B Biol Sci 371.
Plattner H & Verkhratsky A. (2018). The remembrance of the things past: Conserved signalling
pathways link protozoa to mammalian nervous system. Cell Calcium 73, 25-39.
Plotkin MD, Kaplan MR, Peterson LN, Gullans SR, Hebert SC & Delpire E. (1997). Expression of the
Na+-K+-2Cl- cotransporter BSC2 in the nervous system. Am J Physiol 272, C173-183.
Ransom BR. (1992). Glial modulation of neural excitability mediated by extracellular pH: a
hypothesis. Prog Brain Res 94, 37-46.
Ransom BR. (2000). Glial modulation of neural excitability mediated by extracellular pH: a hypothesis
revisited. Prog Brain Res 125, 217-228.
Ransom CB, O'Neal JT & Sontheimer H. (2001). Volume-activated chloride currents contribute to the
resting conductance and invasive migration of human glioma cells. J Neurosci 21, 7674-7683.
Reichenbach A, Derouiche A & Kirchhoff F. (2010). Morphology and dynamics of perisynaptic glia.
Brain Res Rev 63, 11-25.
Reyes RC, Verkhratsky A & Parpura V. (2012). Plasmalemmal Na+/Ca2+ exchanger modulates Ca2+-
dependent exocytotic release of glutamate from rat cortical astrocytes. ASN Neuro 4, 00075.
Reyes RC, Verkhratsky A & Parpura V. (2013). TRPC1-mediated Ca2+ and Na+ signalling in astroglia:
differential filtering of extracellular cations. Cell Calcium 54, 120-125.
Rimmele TS & Chatton JY. (2014). A novel optical intracellular imaging approach for potassium
dynamics in astrocytes. PLoS One 9, e109243.
Rishal I, Keren-Raifman T, Yakubovich D, Ivanina T, Dessauer CW, Slepak VZ & Dascal N. (2003). Na+
promotes the dissociation between G GDP and G, activating G protein-gated K+ channels.
J Biol Chem 278, 3840-3845.
Rojas H, Colina C, Ramos M, Benaim G, Jaffe EH, Caputo C & DiPolo R. (2007). Na+ entry via
glutamate transporter activates the reverse Na+/Ca2+ exchange and triggers Cai2+-induced
Ca2+ release in rat cerebellar Type-1 astrocytes. J Neurochem 100, 1188-1202.
Rose CR & Chatton JY. (2016). Astrocyte sodium signaling and neuro-metabolic coupling in the brain.
Neuroscience 323, 121-134.
Rose CR & Karus C. (2013). Two sides of the same coin: sodium homeostasis and signaling in
astrocytes under physiological and pathophysiological conditions. Glia 61, 1191-1205.
This article is protected by copyright. All rights reserved.
21
Rose CR & Ransom BR. (1996a). Intracellular sodium homeostasis in rat hippocampal astrocytes. J
Physiol 491 ( Pt 2), 291-305.
Rose CR & Ransom BR. (1996b). Mechanisms of H+ and Na+ changes induced by glutamate, kainate,
and D-aspartate in rat hippocampal astrocytes. J Neurosci 16, 5393-5404.
Rose CR & Verkhratsky A. (2016). Principles of sodium homeostasis and sodium signalling in
astroglia. Glia 64, 1611-1627.
Rose CR, Ziemens D, Untiet V & Fahlke C. (2018). Molecular and cellular physiology of sodium-
dependent glutamate transporters. Brain Res Bull 136, 3-16.
Rose EM, Koo JC, Antflick JE, Ahmed SM, Angers S & Hampson DR. (2009). Glutamate transporter
coupling to Na,K-ATPase. J Neurosci 29, 8143-8155.
Ruminot I, Gutierrez R, Pena-Munzenmayer G, Anazco C, Sotelo-Hitschfeld T, Lerchundi R, Niemeyer
MI, Shull GE & Barros LF. (2011). NBCe1 mediates the acute stimulation of astrocytic
glycolysis by extracellular K+. J Neurosci 31, 14264-14271.
Rusakov DA. (2015). Disentangling calcium-driven astrocyte physiology. Nat Rev Neurosci 16, 226-
233.
Schrodl-Haussel M, Theparambil SM, Deitmer JW & Roussa E. (2015). Regulation of functional
expression of the electrogenic sodium bicarbonate cotransporter 1, NBCe1 (SLC4A4), in
mouse astrocytes. Glia 63, 1226-1239.
Shigetomi E, Bushong EA, Haustein MD, Tong X, Jackson-Weaver O, Kracun S, Xu J, Sofroniew MV,
Ellisman MH & Khakh BS. (2013). Imaging calcium microdomains within entire astrocyte
territories and endfeet with GCaMPs expressed using adeno-associated viruses. J Gen Physiol
141, 633-647.
Shigetomi E, Patel S & Khakh BS. (2016). Probing the complexities of astrocyte calcium signaling.
Trends Cell Biol 26, 300-312.
Shigetomi E, Tong X, Kwan KY, Corey DP & Khakh BS. (2012). TRPA1 channels regulate astrocyte
resting calcium and inhibitory synapse efficacy through GAT-3. Nat Neurosci 15, 70-80.
Simard JM, Kent TA, Chen M, Tarasov KV & Gerzanich V. (2007). Brain oedema in focal ischaemia:
molecular pathophysiology and theoretical implications. Lancet Neurol 6, 258-268.
Sinning A & Hubner CA. (2013). Minireview: pH and synaptic transmission. FEBS Lett 587, 1923-1928.
Stoica A, Larsen BR, Assentoft M, Holm R, Holt LM, Vilhardt F, Vilsen B, Lykke-Hartmann K, Olsen ML
& MacAulay N. (2017). The 22 isoform combination dominates the astrocytic Na+/K+ -
ATPase activity and is rendered nonfunctional by the 2.G301R familial hemiplegic migraine
type 2-associated mutation. Glia 65, 1777-1793.
Theparambil SM & Deitmer JW. (2015). High effective cytosolic H+ buffering in mouse cortical
astrocytes attributable to fast bicarbonate transport. Glia 63, 1581-1594.
This article is protected by copyright. All rights reserved.
22
Thrane AS, Rangroo Thrane V & Nedergaard M. (2014). Drowning stars: reassessing the role of
astrocytes in brain edema. Trends Neurosci 37, 620-628.
Todd, A.C., M.C. Marx, S.R. Hulme, S. Broer, and B. Billups. (2017). SNAT3-mediated glutamine
transport in perisynaptic astrocytes in situ is regulated by intracellular sodium. Glia 65, 900-
916.
Turovsky E, Theparambil SM, Kasymov V, Deitmer JW, Del Arroyo AG, Ackland GL, Corneveaux JJ,
Allen AN, Huentelman MJ, Kasparov S, Marina N & Gourine AV. (2016). Mechanisms of
CO2/H+ Sensitivity of Astrocytes. J Neurosci 36, 10750-10758.
Unichenko P, Dvorzhak A & Kirischuk S. (2013). Transporter-mediated replacement of extracellular
glutamate for GABA in the developing murine neocortex. Eur J Neurosci 38, 3580-3588.
Untiet V, Kovermann P, Gerkau NJ, Gensch T, Rose CR & Fahlke C. (2017). Glutamate transporter-
associated anion channels adjust intracellular chloride concentrations during glial
maturation. Glia 65, 388-400.
Vandenberg RJ, Huang S & Ryan RM. (2008). Slips, leaks and channels in glutamate transporters.
Channels (Austin) 2, 51-58.
Verkhratsky A. (2010). Physiology of neuronal-glial networking. Neurochem Int 57, 332-343.
Verkhratsky A & Nedergaard M. (2014). Astroglial cradle in the life of the synapse. Philos Trans R Soc
Lond B Biol Sci 369, 20130595.
Verkhratsky A & Nedergaard M. (2018). Physiology of Astroglia. Physiol Rev 98, 239-389.
Verkhratsky A, Nedergaard M & Hertz L. (2015). Why are astrocytes important? Neurochem Res 40,
389-401.
Verkhratsky A, Noda M, Parpura V & Kirischuk S. (2013). Sodium fluxes and astroglial function. Adv
Exp Med Biol 961, 295-305.
Verkhratsky A, Orkand RK & Kettenmann H. (1998). Glial calcium: homeostasis and signaling
function. Physiol Rev 78, 99-141.
Verkhratsky A & Parpura V. (2014). Store-operated calcium entry in neuroglia. Neurosci Bull 30, 125-
133.
Verkhratsky A, Rodriguez JJ & Parpura V. (2012). Calcium signalling in astroglia. Mol Cell Endocrinol
353, 45-56.
Verkhratsky A, Trebak M, Perocchi F, Khananshvili D & Sekler I. (2018). Crosslink between calcium
and sodium signalling. Exp Physiol 103, 157-169.
Volterra A, Liaudet N & Savtchouk I. (2014). Astrocyte Ca2+ signalling: an unexpected complexity. Nat
Rev Neurosci 15, 327-335.
This article is protected by copyright. All rights reserved.
23
Wendt-Gallitelli MF, Voigt T & Isenberg G. (1993). Microheterogeneity of subsarcolemmal sodium
gradients. Electron probe microanalysis in guinea-pig ventricular myocytes. J Physiol 472, 33-
44.
Wilson CS & Mongin AA. (2018). The signaling role for chloride in the bidirectional communication
between neurons and astrocytes. Neurosci Lett.
Winter N, Kovermann P & Fahlke C. (2012). A point mutation associated with episodic ataxia 6
increases glutamate transporter anion currents. Brain 135, 3416-3425.
Zhao H, Carney KE, Falgoust L, Pan JW, Sun D & Zhang Z. (2016). Emerging roles of Na+/H+ exchangers
in epilepsy and developmental brain disorders. Prog Neurobiol 138-140, 19-35.
Zhou Y & Danbolt NC. (2013). GABA and glutamate transporters in brain. Front Endocrinol (Lausanne)
4, 165.
Figure legends
Figure 1. Molecules mediating Na+ fluxes in the plasmalemma of astrocyte.
Influx of Na+ occurs though (i) Na+-permeable channels which include ionotropic receptors (AMPAR,
MNDAR, P2XR - AMPA, NMDA glutamate receptors and ionotropic purinoceptors); channels of the
transient receptor potential (TRP) family (TRPC1/4/5 channels that operate as a part of store-
operated Ca2+ entry and hence generate Na+ influx in response to the depletion of endoplasmic
reticulum Ca2+ stores; as well as TRPA and TRPV channels); voltage-dependent Nav channels and
[Na+]o-activated Nax channels; (ii) through Na+-dependent SLC transporters that include excitatory
amino acid transporters EAAT1,2, GABA transporters GAT 1,3, glycine transporters GlyT,
noradrenaline transporters NET or concentrative adenosine transporters CNT2/3. The main pathway
for Na+ exit is provided by Na+-K+ pump, NKA. The Na+-Ca2+ exchanger NCX fluctuates between
forward and reverse mode and couples Na+ and Ca2+ signalling.
This article is protected by copyright. All rights reserved.
24
Figure 2. Molecular targets of Na+ signalling in astroglia.
Abbreviations: ASCT2 - alanine-serine-cysteine transporter 2; ASIC - acid sensing ion channels; CNT2,
concentrative nucleoside transporters; EAAT - excitatory amino acid transporters; ENaC - epithelial
sodium channels; GAT - GABA transporters; GS - glutamine synthetase, GlyT1-glycine transporter.
iGluRs - ionotropic glutamate receptors; Nax - Na+ channels activated by extracellular Na+; NAAT -
Na+-dependent ascorbic acid transporter; NBC - Na+/HCO3- (sodium-bicarbonate) co-transporter;
NCX - Na+/Ca2+ exchanger; NCLX - mitochondrial Na+/Ca2+ exchanger; NHE - Na+/H+ exchanger; NKCC1
- Na+/K+/Cl- co-transporter, NET - norepinephrine transporter; MCT1 - monocarboxylase transporter
1; P2XRs - ionotropic purinoceptors; SN1,2 - sodium-coupled neutral amino acid transporters which
underlie exit of glutamine; TRP - transient receptor potential channels; ROS - reactive oxygen
species; VRAC - volume-regulated anion channels.
See text for further explanation.
Modified from (Verkhratsky & Nedergaard, 2018).
This article is protected by copyright. All rights reserved.
25
Figure 3. Molecular pathways of Cl- fluxes in astroglia
Transmembrane Cl- fluxes are mediated by (i) ionotropic GABAA and glycine (Gly) receptors, (ii) by
anionic channels, including volume regulated anion channels VRAC, chloride channels ClC1,2,3 and
bestrophine1(Bes1) channels; (iii) Cl- may diffuse through plasmalemma via Cl- channels associated
with (but not thermodynamically coupled to) glutamate transporters EAAT1,2; (iv) Cl- flux is
thermodynamically coupled to the transport of GABA by GAT1,3 transporters, which may operate in
forward and reverse modes; (v) Cl- efflux is mediated by K+-Cl- co-transporter and (vi) Cl-
accumulation is associated with the activity of Na+-K+-Cl- co-transporter NKCC1.
Figure 4. Targets of Cl- intracellular signalling
The signalling role of Cl- is defined by multiple intracellular targets. Regulation of cell volume is
mediated by volume regulating anion channel (VRAC), hence morphological plasticity and cell
migration. As signalling molecule, Cl- activates K+ channels and TRPM7 channels, cSLO-2 and NBCe1.
Intracellular signalling cascades as for example WNK kinases are regulated by Cl- changes.
Programmed cell death is accompanied by apoptotic volume decrease and thus Cl- efflux.
Furthermore, regulation of cell proliferation and differentiation are targets of Cl- signalling.
This article is protected by copyright. All rights reserved.
26
Figure 5. Molecular pathways of K+ fluxes in astroglia.
Potassium fluxes are mediated by (i) K+ channels of which the most abundant is inward rectifying
Kir4.1 channel; (ii) by K+-Cl- co-transporter (iii) Na+-K+-Cl- co-transporter NKCC1 and by (iv) Na+-K+
pump, NKA
Figure 6. Molecular pathways of H+ fluxes in astroglia.
Abbreviations: EAAT1,2 - Excitatory amino acid transporters 1, 2; NHE - Na+ H+ exchanger, PMCA -
plasmalemmal Ca2+ ATPase; VH+ pump - vacuolar H+ pump localised in plasmalemma; MCT -
monocarboxylate transporter; NBC - Na+-bicarbonate exchanger.
Modified from (Verkhratsky & Nedergaard, 2018).
This article is protected by copyright. All rights reserved.
27
NAME: Alexei Verkhratsky
BIOGRAPHICAL NOTE:
Professor Alexei Verkhratsky, PhD, D.Sc, Member of Academia Europaea (2003), Member of the
German National Academy of Sciences Leopoldina (2013), Member of Real Academia Nacional de
Farmacia of Spain (2012), member of Polish Academy of Sciences (2017); member of The Dana
Alliance for Brain Initiatives (2012), is an internationally recognised scholar in the field of cellular
neurophysiology. His research is concentrated on the mechanisms of inter- and intracellular
signalling in the CNS, being especially focused on two main types of neural cells, on neurones and
neuroglia. He made important contributions to understanding the chemical and electrical
transmission in reciprocal neuronal-glial communications and on the role of intracellular Ca2+ signals
in the integrative processes in the nervous system. Many of his studies are dedicated to
investigations of cellular mechanisms of neurodegeneration. He was the first to perform intracellular
Ca2+ recordings in old neurones in isolation and in situ, which provided direct experimental support
for “Ca2+ hypothesis of neuronal ageing”. In recent years he studies glial ageing and gliopathology in
age-related brain diseases, including Alzheimer disease as well as in neuropsychiatric diseases. He
authored a pioneering hypothesis of astroglial atrophy as a general mechanism of cognitive brain
disorders including neurodegenerative and psychiatric diseases.
Scientometry: Prof Verkhratsky authored and edited 12 books and published ~ 400 papers and
chapters. His papers were cited >21000 times, H-index 79 (Scopus, 11/2018).
He was among 30 Most cited European neuroscientists in 2016
http://www.labtimes.org/labtimes/ranking/2016_01/index2.lasso
This article is protected by copyright. All rights reserved.
28
This article is protected by copyright. All rights reserved.
29
Recommended