Increased membrane Ca2+ permeability drives astrocytic Ca2+ dynamics during neuronal stimulation at excitatory synapses

Astrocytes are intricately involved in the activity of neural circuits; however, their basic physiology of interacting with nearby neurons is not well established. Using two-photon imaging of neurons and astrocytes during higher frequency stimulation of hippocampal CA3-CA1 Schaffer collateral (Scc) excitatory synapses, we could show that increasing levels of released glutamate accelerated local astrocytic Ca2+ elevation. However, blockage of glutamate transporters did not abolish this astrocytic Ca2+ response, suggesting that astrocytic Ca2+ elevation is indirectly associated with an uptake of extracellular glutamate. However, during the astrocytic glutamate uptake, the Na+ /Ca2+ exchanger (NCX) reverse mode was activated, and mediated extracellular Ca2+ entry, thereby triggering the internal release of Ca2+ . In addition, extracellular Ca2+ entry via membrane P2X receptors further facilitated astrocytic Ca2+ elevation via ATP binding. These findings suggest a novel mechanism of activity induced Ca2+ permeability increases of astrocytic membranes, which drives astrocytic responses during neuronal stimulation of CA3-CA1 Scc excitatory synapses.

Distinct effects of AMPAR subunit depletion on spatial memory

Pharmacological studies established a role for AMPARs in the mammalian forebrain in spatial memory performance. Here we generated global GluA1/3 double knockout mice (Gria1/3-/-) and conditional knockouts lacking GluA1 and GluA3 AMPAR subunits specifically from principal cells across the forebrain (Gria1/3ΔFb). In both models, loss of GluA1 and GluA3 resulted in reduced hippocampal GluA2 and increased levels of the NMDAR subunit GluN2A. Electrically-evoked AMPAR-mediated EPSPs were greatly diminished, and there was an absence of tetanus-induced LTP. Gria1/3-/- mice showed premature mortality. Gria1/3ΔFb mice were viable, and their memory performance could be analyzed. In the Morris water maze (MWM), Gria1/3ΔFb mice showed profound long-term memory deficits, in marked contrast to the normal MWM learning previously seen in single Gria1-/- and Gria3-/- knockout mice. Our results suggest a redundancy of function within the pool of available ionotropic glutamate receptors for long-term spatial memory performance.

BubbleDrive, a low-volume incubation chamber for acute brain slices

Acute brain slices are a common and useful preparation in experimental neuroscience. A wide range of incubation chambers for brain slices exists but only a few are designed with very low volumes of the bath solution in mind. Such chambers are necessary when high-cost chemicals are to be added to the solution or when small amounts of substances released by the slice are to be collected for analysis. The principal challenge in designing a very low-volume incubation chamber is maintaining good oxygenation and flow without mechanically disturbing or damaging the slices. We designed and validated BubbleDrive, a 3D-printed incubation chamber with a minimum volume of 1.5 mL which can hold up to three coronal mouse slices from one hemisphere. It employs the carbogen gas bubbles to drive the flow circulation in a consistent and reproducible manner, and without disturbing the brain slices. The BubbleDrive design and construction were successfully validated by comparison to a conventional large-volume incubation chamber in several experimental designs involving measurements of extracellular diffusion parameters, the electrophysiology of neuronal and astrocytic networks, and the effectiveness of slice incubation with hyaluronidase enzyme.

Impaired astrocytic Ca2+ signaling in awake-behaving Alzheimer's disease transgenic mice

Increased astrocytic Ca2+ signaling has been shown in Alzheimer's disease mouse models, but to date no reports have characterized behaviorally induced astrocytic Ca2+ signaling in such mice. Here, we employ an event-based algorithm to assess astrocytic Ca2+ signals in the neocortex of awake-behaving tg-ArcSwe mice and non-transgenic wildtype littermates while monitoring pupil responses and behavior. We demonstrate an attenuated astrocytic Ca2+ response to locomotion and an uncoupling of pupil responses and astrocytic Ca2+ signaling in 15-month-old plaque-bearing mice. Using the genetically encoded fluorescent norepinephrine sensor GRABNE, we demonstrate a reduced norepinephrine signaling during spontaneous running and startle responses in the transgenic mice, providing a possible mechanistic underpinning of the observed reduced astrocytic Ca2+ responses. Our data points to a dysfunction in the norepinephrine-astrocyte Ca2+ activity axis, which may account for some of the cognitive deficits observed in Alzheimer's disease.

Voltage-independent GluN2A-type NMDA receptor Ca2+ signaling promotes audiogenic seizures, attentional and cognitive deficits in mice

The NMDA receptor-mediated Ca2+ signaling during simultaneous pre- and postsynaptic activity is critically involved in synaptic plasticity and thus has a key role in the nervous system. In GRIN2-variant patients alterations of this coincidence detection provoked complex clinical phenotypes, ranging from reduced muscle strength to epileptic seizures and intellectual disability. By using our gene-targeted mouse line (Grin2aN615S), we show that voltage-independent glutamate-gated signaling of GluN2A-containing NMDA receptors is associated with NMDAR-dependent audiogenic seizures due to hyperexcitable midbrain circuits. In contrast, the NMDAR antagonist MK-801-induced c-Fos expression is reduced in the hippocampus. Likewise, the synchronization of theta- and gamma oscillatory activity is lowered during exploration, demonstrating reduced hippocampal activity. This is associated with exploratory hyperactivity and aberrantly increased and dysregulated levels of attention that can interfere with associative learning, in particular when relevant cues and reward outcomes are disconnected in space and time. Together, our findings provide (i) experimental evidence that the inherent voltage-dependent Ca2+ signaling of NMDA receptors is essential for maintaining appropriate responses to sensory stimuli and (ii) a mechanistic explanation for the neurological manifestations seen in the NMDAR-related human disorders with GRIN2 variant-meidiated intellectual disability and focal epilepsy.

The Glutamine Transporter Slc38a1 Regulates GABAergic Neurotransmission and Synaptic Plasticity

GABA signaling sustains fundamental brain functions, from nervous system development to the synchronization of population activity and synaptic plasticity. Despite these pivotal features, molecular determinants underscoring the rapid and cell-autonomous replenishment of the vesicular neurotransmitter GABA and its impact on synaptic plasticity remain elusive. Here, we show that genetic disruption of the glutamine transporter Slc38a1 in mice hampers GABA synthesis, modifies synaptic vesicle morphology in GABAergic presynapses and impairs critical period plasticity. We demonstrate that Slc38a1-mediated glutamine transport regulates vesicular GABA content, induces high-frequency membrane oscillations and shapes cortical processing and plasticity. Taken together, this work shows that Slc38a1 is not merely a transporter accumulating glutamine for metabolic purposes, but a key component regulating several neuronal functions.

Optic Atrophy 1 Controls Human Neuronal Development by Preventing Aberrant Nuclear DNA Methylation

Optic atrophy 1 (OPA1), a GTPase at the inner mitochondrial membrane involved in regulating mitochondrial fusion, stability, and energy output, is known to be crucial for neural development: Opa1 heterozygous mice show abnormal brain development, and inactivating mutations in OPA1 are linked to human neurological disorders. Here, we used genetically modified human embryonic and patient-derived induced pluripotent stem cells and reveal that OPA1 haploinsufficiency leads to aberrant nuclear DNA methylation and significantly alters the transcriptional circuitry in neural progenitor cells (NPCs). For instance, expression of the forkhead box G1 transcription factor, which is needed for GABAergic neuronal development, is repressed in OPA1+/− NPCs. Supporting this finding, OPA1+/− NPCs cannot give rise to GABAergic interneurons, whereas formation of glutamatergic neurons is not affected. Taken together, our data reveal that OPA1 controls nuclear DNA methylation and expression of key transcription factors needed for proper neural cell specification.

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OPA1 haploinsufficiency impairs formation of DLX1/2-positive GABAergic neurons

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Reduced OPA1 levels significantly alter the transcriptional circuitry in neural cells

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Expression of the pioneer factor FOXG1 is decreased in OPA1+/− neural progenitor cells

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Impaired FOXG1 expression correlates with increased CpG methylation at its promoter

Ca2+ Signals in Astrocytes Facilitate Spread of Epileptiform Activity

Epileptic seizures are associated with increased astrocytic Ca²⁺ signaling, but the fine spatiotemporal kinetics of the ictal astrocyte–neuron interplay remains elusive. By using 2-photon imaging of awake head-fixed mice with chronic hippocampal windows we demonstrate that astrocytic Ca²⁺ signals precede neuronal Ca²⁺ elevations during the initial bout of kainate-induced seizures. On average, astrocytic Ca²⁺ elevations preceded neuronal activity in CA1 by about 8 s. In subsequent bouts of epileptic seizures, astrocytes and neurons were activated simultaneously. The initial astrocytic Ca²⁺ elevation was abolished in mice lacking the type 2 inositol-1,4,5-trisphosphate-receptor (Itpr2^−/−). Furthermore, we found that Itpr2^−/− mice exhibited 60% less epileptiform activity compared with wild-type mice when assessed by telemetric EEG monitoring. In both genotypes we also demonstrate that spreading depression waves may play a part in seizure termination. Our findings imply a role for astrocytic Ca²⁺ signals in ictogenesis.

Astroglial endfeet exhibit distinct Ca2+ signals during hypoosmotic conditions

Astrocytic endfeet cover the brain surface and form a sheath around the cerebral vasculature. An emerging concept is that endfeet control blood-brain water transport and drainage of interstitial fluid and waste along paravascular pathways. Little is known about the signaling mechanisms that regulate endfoot volume and hence the width of these drainage pathways. Here, we used the genetically encoded fluorescent Ca²⁺ indicator GCaMP6f to study Ca²⁺ signaling within astrocytic somata, processes, and endfeet in response to an osmotic challenge known to induce cell swelling. Acute cortical slices were subjected to artificial cerebrospinal fluid with 20% reduction in osmolarity while GCaMP6f fluorescence was imaged with two-photon microscopy. Ca²⁺ signals induced by hypoosmotic conditions were observed in all astrocytic compartments except the soma. The Ca²⁺ response was most prominent in subpial and perivascular endfeet and included spikes with single peaks, plateau-type elevations, and rapid oscillations, the latter restricted to subpial endfeet. Genetic removal of the type 2 inositol 1,4,5-triphosphate receptor (IP3R2) severely suppressed the Ca²⁺ responses in endfeet but failed to affect brain water accumulation in vivo after water intoxication. Furthermore, the increase in endfoot Ca²⁺ spike rate during hypoosmotic conditions was attenuated in mutant mice lacking the aquaporin-4 anchoring molecule dystrophin and after blockage of transient receptor potential vanilloid 4 channels. We conclude that the characteristics and underpinning of Ca²⁺ responses to hypoosmotic stress differ within the astrocytic territory and that IP3R2 is essential for the Ca²⁺ signals only in subpial and perivascular endfeet.

Aquaporin-4-independent volume dynamics of astroglial endfeet during cortical spreading depression

Cortical spreading depression (CSD) is a slowly propagating wave of depolarization of gray matter. This phenomenon is believed to underlie the migraine aura and similar waves of depolarization may exacerbate injury in a number of neurological disease states. CSD is characterized by massive ion dyshomeostasis, cell swelling, and multiphasic blood flow changes. Recently, it was shown that CSD is associated with a closure of the paravascular space (PVS), a proposed exit route for brain interstitial fluid and solutes, including excitatory and inflammatory substances that increase in the wake of CSD. The PVS closure was hypothesized to rely on swelling of astrocytic endfeet due to their high expression of aquaporin-4 (AQP4) water channels. We investigated whether CSD is associated with swelling of endfeet around penetrating arterioles in the cortex of living mice. Endfoot cross-sectional area was assessed by two-photon microscopy of mice expressing enhanced green fluorescent protein in astrocytes and related to the degree of arteriolar constriction. In anesthetized mice CSD triggered pronounced endfoot swelling that was short-lasting and coincided with the initial arteriolar constriction. Mice lacking AQP4 displayed volume changes of similar magnitude. CSD-induced endfoot swelling and arteriolar constriction also occurred in awake mice, albeit with faster kinetics than in anesthetized mice. We conclude that swelling of astrocytic endfeet is a robust event in CSD. The early onset and magnitude of the endfoot swelling is such that it may significantly delay perivascular drainage of interstitial solutes in neurological conditions where CSD plays a pathophysiological role.

Altered α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor function and expression in hippocampus in a rat model of attention-deficit/hyperactivity disorder (ADHD)

Glutamatergic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) carry the bulk of excitatory synaptic transmission. Their modulation plays key roles in synaptic plasticity, which underlies hippocampal learning and memory. A dysfunctional glutamatergic system may negatively affect learning abilities and underlie symptoms of attention-deficit/hyperactivity disorder (ADHD). The aim of this study was to investigate whether the expression and function of AMPARs were altered in ADHD. We recorded AMPAR mediated synaptic transmission at hippocampal excitatory synapses and quantified immunogold labelling density of AMPAR subunits GluA1 and GluA2/3 in a rat model for ADHD; the spontaneously hypertensive rat (SHR). Electrophysiological recordings showed significantly reduced AMPAR mediated synaptic transmission at the CA3-to-CA1 pyramidal cell synapses in stratum radiatum and stratum oriens in SHRs compared to control rats. Electronmicroscopic immunogold quantifications did not show any statistically significant changes in labelling densities of the GluA1 subunit of the AMPAR on dendritic spines in stratum radiatum or in stratum oriens. However, there was a significant increase of the GluA2/3 subunit intracellularly in stratum oriens in SHR compared to control, interpreted as a compensatory effect. The proportion of synapses lacking AMPAR subunit labelling was the same in the two genotypes. In addition, electronmicroscopic examination of tissue morphology showed the density of this type of synapse (i.e., asymmetric synapses on spines), and the average size of the synaptic membranes, to be the same. AMPAR dysfunction, possibly involving molecular changes, in hippocampus may in part reflect altered learning in individuals with ADHD.

Omega-3 fatty acids regulate plasticity in distinct hippocampal glutamatergic synapses

Dietary omega-3 fatty acids accumulate and are actively retained in central nervous system membranes, mainly in synapses, dendrites and photoreceptors. Despite this selective enrichment, their impact on synaptic function and plasticity has not been fully determined at the molecular level. In this study, we explored the impact of omega-3 fatty acid deficiency on synaptic function in the hippocampus. Dietary omega-3 fatty acid deficiency for 5 months after weaning led to a 65% reduction in the concentration of docosahexaenoic acid in whole brain synaptosomal phospholipids with no impact on global dopaminergic or serotonergic turnover. We observed reduced concentrations of glutamate receptor subunits, including GluA1, GluA2 and NR2B, and synaptic vesicle proteins synaptophysin and synaptotagmin 1 in hippocampal synaptosomes of omega-3 fatty acid-deficient mice as compared to the omega-3 fatty acid rich group. In contrast, an increased concentration of neuronal inositol 1,4,5-trisphosphate-receptor (IP₃ -R) was observed in the deficient group. Furthermore, omega-3 fatty acid deficiency reduced the long-term potentiation (LTP) in stratum oriens of the hippocampal CA1 area, but not in stratum radiatum. Thus, omega-3 fatty acids seem to have specific effects in distinct subsets of glutamatergic synapses, suggesting specific molecular interactions in addition to altering plasma membrane properties on a more global scale.

Presynaptic increase in IP3 receptor type 1 concentration in the early phase of hippocampal synaptic plasticity

The inositol 1,4,5-trisphosphate receptor (IP₃R) subtype IP₃R1 is highly enriched in the brain, including hippocampal neurons. It plays an important function in regulating intracellular calcium concentrations. Residing on the smooth endoplasmic reticulum (sER), the IP₃R1 mobilizes calcium into the cytosol upon binding the intracellular signaling molecule IP₃, whose concentration is increased by stimulating certain metabotropic glutamate receptors. Increased calcium may mediate synaptic changes occurring during long-term plasticity, which includes molecular mechanisms underlying memory encoding. The exact synaptic localization of IP₃R1 in the central nervous system (CNS) remains unclear. We hypothesized that IP₃R1, in addition to its known expression in soma and dendritic shafts of hippocampal CA1 pyramidal neurons, also may be present in postsynaptic spines. Moreover, we hypothesized that IP₃R1 may be present in presynaptic terminals as well, given the importance of calcium in regulating presynaptic neurotransmitter exocytosis. To test these two hypotheses, we used IP₃R1 immunocytochemistry at the light and electron microscopical levels in the CA1 area of the hippocampus. Furthermore, we hypothesized that induction of long-term potentiation (LTP) would be accompanied by an increase in synaptic IP₃R1 concentrations, thereby facilitating synaptic mechanisms of long term plasticity. To investigate this, we used quantitative immunogold electron microscopy to determine possible changes in IP₃R1 concentration in sub-synaptic compartments before and five minutes after high frequency tetanizations. Firstly, our data confirm localization of IP₃R1 in both presynaptic terminals and postsynaptic spines. Secondly, the concentration of IP₃R1 after tetanization was significantly increased in the presynaptic compartment, suggesting a presynaptic role of IP₃R1 in early phases of synaptic plasticity. It is therefore possible that IP₃R1 is involved in modulating neurotransmitter release by regulating calcium homeostasis presynaptically.

A possible postsynaptic role for SNAP-25 in hippocampal synapses

The SNARE protein SNAP-25 is well documented as regulator of presynaptic vesicle exocytosis. Increasing evidence suggests roles for SNARE proteins in postsynaptic trafficking of glutamate receptors as a basic mechanism in synaptic plasticity. Despite these indications, detailed quantitative subsynaptic localization studies of SNAP-25 have never been performed. Here, we provide novel electron microscopic data of SNAP-25 localization in postsynaptic spines. In addition to its expected presynaptic localization, we show that the protein is also present in the postsynaptic density (PSD), the postsynaptic lateral membrane and on small vesicles in the postsynaptic cytoplasm. We further investigated possible changes in synaptic SNAP-25 protein expression after hippocampal long-term potentiation (LTP). Quantitative analysis of immunogold-labeled electron microscopy sections did not show statistically significant changes of SNAP-25 gold particle densities 1 h after LTP induction, indicating that local trafficking of SNAP-25 does not play a role in the early phases of LTP. However, the strong expression of SNAP-25 in postsynaptic plasma membranes suggests a function of the protein in postsynaptic vesicle exocytosis and a possible role in hippocampal synaptic plasticity.

Somatic Accumulation of GluA1-AMPA Receptors Leads to Selective Cognitive Impairments in Mice

The GluA1 subunit of the L-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) plays a crucial, but highly selective, role in cognitive function. Here we analyzed AMPAR expression, AMPAR distribution and spatial learning in mice (Gria1^R/R ), expressing the "trafficking compromised" GluA1(Q600R) point mutation. Our analysis revealed somatic accumulation and reduction of GluA1(Q600R) and GluA2, but only slightly reduced CA1 synaptic localization in hippocampi of adult Gria1^R/R mice. These immunohistological changes were accompanied by a strong reduction of somatic AMPAR currents in CA1, and a reduction of plasticity (short-term and long-term potentiation, STP and LTP, respectively) in the CA1 subfield following tetanic and theta-burst stimulation. Nevertheless, spatial reference memory acquisition in the Morris water-maze and on an appetitive Y-maze task was unaffected in Gria1^R/R mice. In contrast, spatial working/short-term memory during both spontaneous and rewarded alternation tasks was dramatically impaired. These findings identify the GluA1(Q600R) mutation as a loss of function mutation that provides independent evidence for the selective role of GluA1 in the expression of short-term memory.

Different Forms of AMPA Receptor Mediated LTP and Their Correlation to the Spatial Working Memory Formation

Spatial working memory (SWM) and the classical, tetanus-induced long-term potentiation (LTP) at hippocampal CA3/CA1 synapses are dependent on L-α-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors (AMPARs) containing GluA1 subunits as demonstrated by knockout mice lacking GluA1. In GluA1 knockout mice LTP and SWM deficits could be partially recovered by transgenic re-installation of full-length GluA1 in principle forebrain neurons. Here we partially restored hippocampal LTP in GluA1-deficient mice by forebrain-specific depletion of the GluA2 gene, by the activation of a hypomorphic GluA2(Q) allele and by transgenic expression of PDZ-site truncated GFP-GluA1(TG). In none of these three mouse lines, the partial LTP recovery improved the SWM performance of GluA1-deficient mice suggesting a specific function of intact GluA1/2 receptors and the GluA1 intracellular carboxyl-terminus in SWM and its associated behavior.

Abstract P5-05-03: Upregulated purinergic signaling enhances cell proliferation in human and murine breast carcinomas

The composition of the extracellular tumor microenvironment differs from that of most other tissues and is thought to provide cancer cells with a growth and survival advantage compared to normal cells. In solid tumors, the extracellular concentration of ATP can be elevated to ~100 µm and extracellular pH can be as low as 6.5. In the current project, we investigate the consequences of purinergic signaling in human and murine breast carcinomas: we study intracellular Ca2+ signals and associated changes in cell proliferation during stimulation with extracellular nucleotides.

We employ biopsies of human and murine primary breast carcinomas and compare them with matched normal breast tissue. Human biopsies are obtained with written informed consent from patients undergoing breast conserving surgery at Aarhus University Hospital or Regional Hospital Randers in Denmark. Murine biopsies are from mice overexpressing unactivated ErbB2 specifically in the breast tissue. We isolate epithelial organoids (~150 µm diameter) from tissue biopsies by partial digestion with collagenase III. Organoids loaded with the Ca2+-sensitive fluorophore Fura-2 are studied by fluorescence microscopy. In separate experiments, cell proliferation is quantified by detecting newly synthesized DNA using immunofluorescence imaging of organoids incubated with the thymidine analogue bromodeoxyuridine (BrdU).

We find that intracellular Ca2+ responses during stimulation with extracellular ATP are elevated 2- to 10-fold in breast carcinomas from mice and humans, respectively, compared to matched normal breast tissue. We observe similar differences between breast cancer tissue and normal breast tissue in response to stimulation with the P2Y2/P2Y4-agonist UTP, whereas virtually no rise in the intracellular concentration of Ca2+ is observed in response to the P2X7-agonist 3'-O-(4-benzoyl)benzoyl-ATP. Application of cyclopiazonic acid – an inhibitor of the sarcoplasmic/endoplasmic reticulum Ca2+-ATPase – also cause exaggerated intracellular Ca2+ responses in breast cancer compared to normal breast tissue. Consistent with the elevated Ca2+ responses, stimulation with 100 µm ATP or 100 µm UTP increases the rate of cell proliferation (i.e., fraction of BrdU-positive cells) by ~2-fold in the breast cancer tissue.

In conclusion, we find that purinergic signaling is upregulated in human and murine breast carcinomas compared to normal breast tissue. Activation of purinergic receptors – most likely P2Y2 and/or P2Y4 – enhances cell proliferation in breast cancer tissue. We propose that the high ATP levels in the tumor microenvironment promote breast cancer development or progression and that the associated signaling pathways represent promising targets for therapy.

Citation Format: Mele M, McWhan K, Henningsen M, Vahl P, Jensen V, Johansen T, Pedersen H, Christiansen P, Bødtkjer E. Upregulated purinergic signaling enhances cell proliferation in human and murine breast carcinomas [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P5-05-03.

Deletion of Aquaporin-4 Curtails Extracellular Glutamate Elevation in Cortical Spreading Depression in Awake Mice

Cortical spreading depression (CSD) is a phenomenon that challenges the homeostatic mechanisms on which normal brain function so critically depends. Analyzing the sequence of events in CSD holds the potential of providing new insight in the physiological processes underlying normal brain function as well as the pathophysiology of neurological conditions characterized by ionic dyshomeostasis. Here, we have studied the sequential progression of CSD in awake wild-type mice and in mice lacking aquaporin-4 (AQP4) or inositol 1,4,5-triphosphate type 2 receptor (IP3R2). By the use of a novel combination of genetically encoded sensors that a novel combination - an unprecedented temporal and spatial resolution, we show that CSD leads to brisk Ca2+ signals in astrocytes and that the duration of these Ca2+ signals is shortened in the absence of AQP4 but not in the absence of IP3R2. The decrease of the astrocytic, AQP4-dependent Ca2+ signals, coincides in time and space with a decrease in the duration of extracellular glutamate overflow but not with the initial peak of the glutamate release suggesting that in CSD, extracellular glutamate accumulation is extended through AQP4-dependent glutamate release from astrocytes. The present data point to a salient glial contribution to CSD and identify AQP4 as a new target for therapy.

Removal of aquaporin-4 from glial and ependymal membranes causes brain water accumulation

There is a constitutive production of water in brain. The efflux routes of this excess water remain to be identified. We used basal brain water content as a proxy for the capacity of water exit routes. Basal brain water content was increased in mice with a complete loss of aquaporin-4 (AQP4) water channels (global Aqp4^-/- mice), but not in mice with a selective removal of perivascular AQP4 or in a novel mouse line with a selective deletion of ependymal AQP4 (Foxj1-Cre:Aqp4^flox/flox mice). Unique for the global Aqp4^-/- mice is the loss of the AQP4 pool subjacent to the pial membrane. Our data suggest that water accumulates in brain when subpial AQP4 is missing, pointing to a critical role of this pool of water channels in brain water exit.

Dynamics of Ionic Shifts in Cortical Spreading Depression

Cortical spreading depression is a slowly propagating wave of near-complete depolarization of brain cells followed by temporary suppression of neuronal activity. Accumulating evidence indicates that cortical spreading depression underlies the migraine aura and that similar waves promote tissue damage in stroke, trauma, and hemorrhage. Cortical spreading depression is characterized by neuronal swelling, profound elevation of extracellular potassium and glutamate, multiphasic blood flow changes, and drop in tissue oxygen tension. The slow speed of the cortical spreading depression wave implies that it is mediated by diffusion of a chemical substance, yet the identity of this substance and the pathway it follows are unknown. Intercellular spread between gap junction-coupled neurons or glial cells and interstitial diffusion of K(+) or glutamate have been proposed. Here we use extracellular direct current potential recordings, K(+)-sensitive microelectrodes, and 2-photon imaging with ultrasensitive Ca(2+) and glutamate fluorescent probes to elucidate the spatiotemporal dynamics of ionic shifts associated with the propagation of cortical spreading depression in the visual cortex of adult living mice. Our data argue against intercellular spread of Ca(2+) carrying the cortical spreading depression wavefront and are in favor of interstitial K(+) diffusion, rather than glutamate diffusion, as the leading event in cortical spreading depression.

Augmentation of Ca(2+) signaling in astrocytic endfeet in the latent phase of temporal lobe epilepsy

Astrocytic endfeet are specialized cell compartments whose important homeostatic roles depend on their enrichment of water and ion channels anchored by the dystrophin associated protein complex (DAPC). This protein complex is known to disassemble in patients with mesial temporal lobe epilepsy and in the latent phase of experimental epilepsies. The mechanistic underpinning of this disassembly is an obvious target of future therapies, but remains unresolved. Here we show in a kainate model of temporal lobe epilepsy that astrocytic endfeet display an enhanced stimulation-evoked Ca(2+) signal that outlast the Ca(2+) signal in the cell bodies. While the amplitude of this Ca(2+) signal is reduced following group I/II metabotropic receptor (mGluR) blockade, the duration is sustained. Based on previous studies it has been hypothesized that the molecular disassembly in astrocytic endfeet is caused by dystrophin cleavage mediated by Ca(2+) dependent proteases. Using a newly developed genetically encoded Ca(2+) sensor, the present study bolsters this hypothesis by demonstrating long-lasting, enhanced stimulation-evoked Ca(2+) signals in astrocytic endfeet.

Deletion of aquaporin-4 increases extracellular K(+) concentration during synaptic stimulation in mouse hippocampus

The coupling between the water channel aquaporin-4 (AQP4) and K⁺ transport has attracted much interest. In this study, we assessed the effect of Aqp4 deletion on activity-induced [K⁺]_o changes in acute slices from hippocampus and corpus callosum of adult mice. We show that Aqp4 deletion has a layer-specific effect on [K⁺]_o that precisely mirrors the known effect on extracellular volume dynamics. In CA1, the peak [K⁺]_o in stratum radiatum during 20 Hz stimulation of Schaffer collateral/commissural fibers was significantly higher in Aqp4^−/− mice than in wild types, whereas no differences were observed throughout the [K⁺]_o recovery phase. In stratum pyramidale and corpus callosum, neither peak [K⁺]_o nor post-stimulus [K⁺]_o recovery was affected by Aqp4 deletion. Our data suggest that AQP4 modulates [K⁺]_o during synaptic stimulation through its effect on extracellular space volume.