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 GRAB_NE, 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.

Neurodegenerative conditions such as Parkinson’s or Alzheimer’s disease are characterized by neurons dying and being damaged. Yet neurons are only one type of brain actors; astrocytes, for example, are star-shaped ‘companion’ cells that have recently emerged as being able to fine-tune neuronal communication. In particular, they can respond to norepinephrine, a signaling molecule that acts to prepare the brain and body for action. This activation results, for instance, in astrocytes releasing chemicals that can act on neurons.

Certain cognitive symptoms associated with Alzheimer’s disease could be due to a lack of norepinephrine. In parallel, studies in anaesthetized mice have shown perturbed astrocyte signaling in a model of the condition. Disrupted norepinephrine-triggered astrocyte signaling could therefore be implicated in the symptoms of the disease. Experiments in awake mice are needed to investigate this link, especially as anesthesia is known to disrupt the activity of astrocytes.

To explore this question, Åbjørsbråten, Skaaraas et al. conducted experiments in naturally behaving mice expressing mutations found in patients with early-onset Alzheimer’s disease. These mice develop hallmarks of the disorder. Compared to their healthy counterparts, these animals had reduced astrocyte signaling when running or being startled. Similarly, a fluorescent molecular marker for norepinephrine demonstrated less signaling in the modified mice compared to healthy ones.

Over 55 million individuals currently live with Alzheimer’s disease. The results by Åbjørsbråten, Skaaraas et al. suggest that astrocyte–norepinephrine communication may be implicated in the condition, an avenue of research that could potentially lead to developing new treatments.

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.

Abstract P3-03-02: Na+,HCO3--cotransport is the major mechanism of cellular acid extrusion in human and murine breast cancer

High metabolism and insufficient blood supply are characteristics of cancer tissue, which in combination with biochemical changes favor glycolytic metabolism and result in prominent intracellular acid production. Although extracellular pH at the core of malignant tumors is as low as one unit below normal, intracellular pH (pHi) in tumor cells is typically normal or even slightly alkaline. Thus, cancer cells must possess efficient mechanisms of acid extrusion to eliminate the excess acid load.

We investigated the role of the Na+,HCO3–cotransporter NBCn1 (SLC4A7), which in recent genome-wide association studies has been linked to human breast cancer. Based on immunohistochemistry of tumor slices and immunoblotting of enzymatically isolated epithelial organoids, we found that NBCn1 expression is upregulated in human and murine primary breast carcinomas and metastases compared to normal breast tissue. The upregulation of NBCn1 was of similar or greater magnitude than that observed for the Na+/H+-exchanger NHE1, which has previously been implicated in cell migration, proliferation and malignancy. Measurements of pHi from slices of human and murine breast cancers and from malignant and normal breast epithelial organoids showed that Na+,HCO3–cotransport is the major mechanism of acid extrusion in the near-physiological pHi range. Na+/H+-exchange was important for acid extrusion only at very low pHi values. We furthermore found that Na+,HCO3–cotransport activity was substantially greater in malignant compared to normal breast epithelial organoids of both human and murine origin, while no apparent difference in Na+/H+-exchange activity was detected between cancer and normal breast tissue. Steady-state pHi was higher in the breast cancer tissue compared to normal breast epithelium in the presence of CO2/HCO3- but not in its nominal absence.

We propose that NBCn1 plays a major role for cellular acid extrusion and pHi regulation in human and murine breast cancer. The upregulated expression of NBCn1 and the functional importance of Na+,HCO3–cotransport for pHi regulation support a causative role for NBCn1 in breast cancer development or progression.

Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P3-03-02.

Dissecting spatial knowledge from spatial choice by hippocampal NMDA receptor deletion

Hippocampal NMDA receptors (NMDARs) and NMDAR-dependent synaptic plasticity are widely considered crucial substrates of long-term spatial memory, although their precise role remains uncertain. Here we show that Grin1(ΔDGCA1) mice, lacking GluN1 and hence NMDARs in all dentate gyrus and dorsal CA1 principal cells, acquired the spatial reference memory water maze task as well as controls, despite impairments on the spatial reference memory radial maze task. When we ran a spatial discrimination water maze task using two visually identical beacons, Grin1(ΔDGCA1) mice were impaired at using spatial information to inhibit selecting the decoy beacon, despite knowing the platform's actual spatial location. This failure could suffice to impair radial maze performance despite spatial memory itself being normal. Thus, these hippocampal NMDARs are not essential for encoding or storing long-term, associative spatial memories. Instead, we demonstrate an important function of the hippocampus in using spatial knowledge to select between alternative responses that arise from competing or overlapping memories.

Aquaporin-4 regulates extracellular space volume dynamics during high-frequency synaptic stimulation: a gene deletion study in mouse hippocampus

Little is known about the physiological roles of aquaporin-4 (AQP4) in the central nervous system. AQP4 water channels are concentrated in endfeet membranes of astrocytes but also localize to the fine astrocytic processes that abut central synapses. Based on its pattern of expression, we predicted that AQP4 could be involved in controlling water fluxes and changes in extracellular space (ECS) volume that are associated with activation of excitatory pathways. Here, we show that deletion of Aqp4 accentuated the shrinkage of the ECS that occurred in the mouse hippocampal CA1 region during activation of Schaffer collateral/commissural fibers. This effect was found in the stratum radiatum (where perisynaptic astrocytic processes abound) but not in the pyramidal cell layer (where astrocytic processes constitute but a minor volume fraction). For both genotypes the ECS shrinkage was most pronounced in the pyramidal cell layer. Our data attribute a physiological role to AQP4 and indicate that this water channel regulates extracellular volume dynamics in the mammalian brain.

P4-01-17: TIMP-1 Over-Expression Confers Resistance of MCF-7 Breast Cancer Cells to Fulvestrant

Background

Endocrine resistance represents a major challenge in the management of estrogen receptor (ER) positive breast cancer. Currently no predictive biomarkers for endocrine resistance in ERpositive breast cancer patients are in clinical use.

In a clinical study, patients with metastatic breast cancer and high levels of serum Tissue Inhibitor of Metalloproteinases-1 (TIMP-1) had less benefit from endocrine therapy than patients with a lower level of serum TIMP-1 [1].

Therefore, we evaluated the association between TIMP-1 and response to endocrine therapy using an in vitro approach.

We have previously presented initial results on TIMP-1 and response to endocrine therapy [2].

Materials and Methods: MCF-7 cells were stably transfected with pcDNA3.1(Hyg)-TIMP-1 plasmid, and a panel of 11 subclones with different expression levels of TIMP-1 was generated. TIMP-1 expression levels were confirmed using enzyme-linked immunosorbent assay (ELISA).

Four subclones with high or low TIMP-1 expression were analyzed for the growth response to estrogen, 4-hydroxytamoxifen and fulvestrant. These four subclones were analyzed for protein expression by western blotting. All subclones were analyzed by whole human genome oligo microarrays 4×44K for determination of gene expression levels. Data were analyzed using the limma R/Bioconductor package. Paired-end Solexa sequencing was applied to selected subclones with high and low TIMP-1 levels to identify transcriptomic changes.

Results: High expression of TIMP-1 was associated with resistance to fulvestrant, whereas growth response to either estrogen or 4-hydroxytamoxifen was independent of TIMP-1 expression levels. High expression of TIMP-1 protein and mRNA was associated with undetectable levels of progesterone receptor (PgR) protein and mRNA whereas ER protein and mRNA levels were unaffected by TIMP-1. To characterize the potential role of TIMP-1 in estrogen signaling we analyzed the expression of reported estrogen-responsive genes and no general change was observed. We identified genes that correlated positively or negatively to TIMP-1 expression. Among the identified genes was PgR, which is a direct target for ER.

Conclusion: Our data suggest that a high expression of TIMP-1 in vitro is associated with resistance to fulvestrant but not to 4-hydroxytamoxifen. Estrogen-regulated genes are not generally affected by changes in TIMP-1 expression levels and therefore TIMP-1 appears to affect endocrine resistance through other mechanisms than globally regulating ER signaling. However, high expression of TIMP-1 is associated with loss of PgR and this may be related to the resistance towards fulvestrant.

References:

[1] Lipton, A et al: Serum TIMP-1 and Response to the Aromatase Inhibitor Letrozole Versus Tamoxifen in Metastatic Breast Cancer. J Clin Oncol; 2008; 26;(16); 2653–8

[2] Effect of TIMP-1 Overexpression on Endocrine Sensitivity of MCF-7 ER-positive Human Breast Cancer Cells In Vitro. Cancer Res; 2009; 69(24 suppl); abstract nr 2029

Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P4-01-17.

D4 dopamine receptors modulate NR2B NMDA receptors and LTP in stratum oriens of hippocampal CA1

Dopamine plays an important role in synaptic plasticity and learning and is involved in the pathogenesis of various neurological and psychiatric disorders. Here, we reveal staining of dopaminergic fibers in stratum oriens of the mouse hippocampal CA1 region, a finding that is consistent with earlier reports. Furthermore, we examined the effect of dopamine agonists on NMDAR-dependent early long-term potentiation (LTP) (40 min) during γ-aminobutyric acid (GABA)(A)-mediated blockade. LTP of the AMPA component was strongly reduced in stratum oriens but barely affected in stratum radiatum. This layer-specific effect was caused by D4 receptor activation, which augmented the inactivation of synaptic NMDAR-mediated currents (NMDA EPSCs) during LTP induction through a Ca(2+)-dependent G-protein-independent mechanism. A similar dopaminergic modulation of both NMDA EPSCs and LTP was also observed in mice constitutively lacking NR2A but was absent in mice lacking NR2B in principal forebrain neurons. Together, these experiments strongly indicate that dopaminergic modulation of early LTP in stratum oriens occurs through NMDARs containing NR2B subunits via D4Rs. Thus, a dopamine hyperfunction in stratum oriens may result in NMDAR hypofunction that could affect both normal and pathological conditions.

Evidence that compromised K+ spatial buffering contributes to the epileptogenic effect of mutations in the human Kir4.1 gene (KCNJ10)

Mutations in the human Kir4.1 potassium channel gene (KCNJ10) are associated with epilepsy. Using a mouse model with glia-specific deletion of Kcnj10, we have explored the mechanistic underpinning of the epilepsy phenotype. The gene deletion was shown to delay K(+) clearance after synaptic activation in stratum radiatum of hippocampal slices. The activity-dependent changes in extracellular space volume did not differ between Kcnj10 mutant and wild-type mice, indicating that the Kcnj10 gene product Kir4.1 mediates osmotically neutral K(+) clearance. Combined, our K(+) and extracellular volume recordings indicate that compromised K(+) spatial buffering in brain underlies the epilepsy phenotype associated with human KCNJ10 mutations.

Synapsin-dependent vesicle recruitment modulated by forskolin, phorbol ester and ca in mouse excitatory hippocampal synapses

Repeated release of transmitter from presynaptic elements depends on stimulus-induced Ca²⁺ influx together with recruitment and priming of synaptic vesicles from different vesicle pools. We have compared three different manipulations of synaptic strength, all of which are known to increase short-term synaptic efficacy through presynaptic mechanisms, in the glutamatergic CA3-to-CA1 stratum radiatum synapse in the mouse hippocampal slice preparation. Synaptic responses elicited from the readily releasable vesicle pool during low-frequency synaptic activation (0.1 Hz) were significantly enhanced by both the adenylate cyclase activator forskolin, the priming activator β-phorbol-12,13-dibutyrate (PDBu) and 4 mM [Ca²⁺]_o′ whereas during 20 Hz stimulation, the same manipulations reduced the time needed to reach the peak and increased the magnitude of the resulting frequency facilitation. In contrast, paired-pulse facilitations were unchanged in the presence of forskolin, decreased by 4 mM [Ca²⁺]_o and essentially abolished by PDBu. The subsequent delayed response enhancement (DRE) responses, elicited during continuous 20 Hz stimulations and mediated by recruited vesicles, were enhanced by forskolin, essentially unchanged by PDBu and slightly decreased by 4 mM [Ca²⁺]_o· Similar experiments done on slices devoid of the vesicle-associated synapsin I and II proteins indicated that synapsin I/II-induced enhancements of vesicle recruitment were restricted to Ca²⁺-induced frequency facilitations and forskolin-induced enhancements of the early DRE phase, whereas the proteins had minor effects during PDBu-treatment and represented constraints on late Ca²⁺-induced responses. The data indicate that in these glutamatergic synapses, the comparable enhancements of single synaptic responses induced by these biochemical mechanisms can be transformed during prolonged synaptic stimulation into highly distinct short-term plasticity patterns, which are partly dependent on synapsins I/II.

Abstract P6-01-09: HER4 Is Downregulated in Lymphnode Metastases Compared to the Paired Primary Breast Carcinoma

Introduction: The Human Epidermal Growth Factor Receptor 4 (HER4) of the EGF receptor family has been characterized in both normal and malignant human breast tissue and HER4 overexpression has been shown to predict prolonged survival compared to HER4 receptor negative disease.

In our study we investigated the HER4 expression in normal breast tissue, primary breast carcinoma and in ipsilateral metastatic axillary lymphnodes at the time of primary breast cancer surgery.

Material and methods: Paired tissue samples from normal breast tissue and primary breast carcinomas were obtained from 169 patients. Out of these a third sample was obtained from 66 patients with metastatic lymphnodes. The primary tumour specimens were sampled uniformly at random. The mRNA expression of HER4 was quantified with real time RT-PCR and expressed relative to the householdgene (HMBS) in arbitrary units (arb.u.).

Results: The mRNA expression of HER4 was significantly higher in breast carcinoma with a mean of 2.26 arb.u. [95% c.i.:1.87 to 2.65 arb.u.] than in the paired sample of normal breast tissue with a mean of 0.82 arb.u. [95% c.i.: 0.47 to 1.16 arb.u.] (p=0.0001). The mRNA expression of HER4 was also significantly higher in breast carcinoma than in the corresponding lymphnode mean 0.75 arb. u. [95% c.i.: 0.43 to 1.07 arb.u.] (p=0.015). There was no significant difference observed in mRNA expression of HER4 between the metastatic lymphnode and the corresponding normal breast tissue (P>0.05). When HER4 expression was compared between the primary tumours that had metastasised and those that had not, there was no significant difference (P>0.05). Conclusion:

The HER4 expression was high in the primary tumour as compared to normal breast tissue and the corresponding lymphnode. In view of previously published relations between a low expression of HER4 and a poor prognosis our results warrant further studies in order to evaluate whether suppression of HER4 in tumour cells could be involved in lymphogenic metastatic spread.

Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P6-01-09.

The spontaneously hypertensive rat model of ADHD--the importance of selecting the appropriate reference strain

Although several molecular and genetic manipulations may produce hyperactive animals, hyperactivity alone is insufficient for the animal to qualify as a model of ADHD. Based on a wider range of criteria - behavioral, genetic and neurobiological - the spontaneously hypertensive rat (SHR) obtained from Charles River, Germany (SHR/NCrl) at present constitutes the best validated animal model of ADHD combined subtype (ADHD-C), and the Wistar Kyoto substrain obtained from Harlan, UK (WKY/NHsd) is its most appropriate control. Although other rat strains may behave like WKY/NHsd rats, genetic results indicate significant differences when compared to the WKY/NHsd substrain, making them less suitable controls for the SHR/NCrl. The use of WKY/NCrl, outbred Wistar, Sprague Dawley or other rat strains as controls for SHRs may produce spurious neurobiological differences. Consequently, data may be misinterpreted if insufficient care is taken in the selection of the control group. It appears likely that the use of different control strains may underlie some of the discrepancies in results and interpretations in studies involving the SHR and WKY. Finally, we argue that WKY rats obtained from Charles River, Germany (WKY/NCrl) provide a promising model for the predominantly inattentive subtype of ADHD (ADHD-PI); in this case also the WKY/NHsd substrain should be used as control.

Synapsin-dependent development of glutamatergic synaptic vesicles and presynaptic plasticity in postnatal mouse brain

Inactivation of the genes encoding the neuronal, synaptic vesicle-associated proteins synapsin I and II leads to severe reductions in the number of synaptic vesicles in the CNS. We here define the postnatal developmental period during which the synapsin I and/or II proteins modulate synaptic vesicle number and function in excitatory glutamatergic synapses in mouse brain. In wild-type mice, brain levels of both synapsin I and synapsin IIb showed developmental increases during synaptogenesis from postnatal days 5-20, while synapsin IIa showed a protracted increase during postnatal days 20-30. The vesicular glutamate transporters (VGLUT) 1 and VGLUT2 showed synapsin-independent development during postnatal days 5-10, following which significant reductions were seen when synapsin-deficient brains were compared with wild-type brains following postnatal day 20. A similar, synapsin-dependent developmental profile of vesicular glutamate uptake occurred during the same age periods. Physiological analysis of the development of excitatory glutamatergic synapses, performed in the CA1 stratum radiatum of the hippocampus from the two genotypes, showed that both the synapsin-dependent part of the frequency facilitation and the synapsin-dependent delayed response enhancement were restricted to the period after postnatal day 10. Our data demonstrate that while both synaptic vesicle number and presynaptic short-term plasticity are essentially independent of synapsin I and II prior to postnatal day 10, maturation and function of excitatory synapses appear to be strongly dependent on synapsin I and II from postnatal day 20.

Synapsin- and actin-dependent frequency enhancement in mouse hippocampal mossy fiber synapses

The synapsin proteins have different roles in excitatory and inhibitory synaptic terminals. We demonstrate a differential role between types of excitatory terminals. Structural and functional aspects of the hippocampal mossy fiber (MF) synapses were studied in wild-type (WT) mice and in synapsin double-knockout mice (DKO). A severe reduction in the number of synaptic vesicles situated more than 100 nm away from the presynaptic membrane active zone was found in the synapsin DKO animals. The ultrastructural level gave concomitant reduction in F-actin immunoreactivity observed at the periactive endocytic zone of the MF terminals. Frequency facilitation was normal in synapsin DKO mice at low firing rates (approximately 0.1 Hz) but was impaired at firing rates within the physiological range (approximately 2 Hz). Synapses made by associational/commissural fibers showed comparatively small frequency facilitation at the same frequencies. Synapsin-dependent facilitation in MF synapses of WT mice was attenuated by blocking F-actin polymerization with cytochalasin B in hippocampal slices. Synapsin III, selectively seen in MF synapses, is enriched specifically in the area adjacent to the synaptic cleft. This may underlie the ability of synapsin III to promote synaptic depression, contributing to the reduced frequency facilitation observed in the absence of synapsins I and II.

N-methyl-D-aspartate receptor subunit dysfunction at hippocampal glutamatergic synapses in an animal model of attention-deficit/hyperactivity disorder

Attention-deficit/hyperactivity disorder (ADHD) is the most common neurobehavioural disorder among children. ADHD children are hyperactive, impulsive and have problems with sustained attention. These cardinal features are also present in the best validated animal model of ADHD, the spontaneously hypertensive rat (SHR), which is derived from the Wistar Kyoto rat (WKY). Current theories of ADHD relate symptom development to factors that alter learning. N-methyl-D-aspartate receptor (NMDAR) dependent long term changes in synaptic efficacy in the mammalian CNS are thought to represent underlying cellular mechanisms for some forms of learning. We therefore hypothesized that synaptic abnormality in excitatory, glutamatergic synaptic transmission might contribute to the altered behavior in SHRs. We studied physiological and anatomical aspects of hippocampal CA3-to-CA1 synapses in age-matched SHR and WKY (controls). Electrophysiological analysis of these synapses showed reduced synaptic transmission (reduced field excitatory postsynaptic potential for a defined fiber volley size) in SHR, whereas short-term forms of synaptic plasticity, like paired-pulse facilitation, frequency facilitation, and delayed response enhancement were comparable in the two genotypes, and long-term potentiation (LTP) of synaptic transmission was of similar magnitude. However, LTP in SHR was significantly reduced (by 50%) by the NR2B specific blocker CP-101,606 (10 microM), whereas the blocker had no effect on LTP magnitude in the control rats. This indicates that the SHR has a functional predominance of NR2B, a feature characteristic of early developmental stages in these synapses. Quantitative immunofluorescence and electron microscopic postembedding immunogold cytochemistry of the three major NMDAR subunits (NR1, NR2A; and NR2B) in stratum radiatum spine synapses revealed no differences between SHR and WKY. The results indicate that functional impairments in glutamatergic synaptic transmission may be one of the underlying mechanisms leading to the abnormal behavior in SHR, and possibly in human ADHD.

Calcineurin is essential for depolarization-induced nuclear translocation and tyrosine phosphorylation of PYK2 in neurons

Proline-rich tyrosine kinase 2 (PYK2) is a non-receptor tyrosine kinase expressed in many cell types and enriched in neurons. PYK2 is a cytoplasmic enzyme activated by increases in cytosolic free Ca(2+) through an unknown mechanism. We report that depolarization or electrical stimulation of hippocampal slices induced a rapid and transient nuclear accumulation of PYK2. Depolarization of cultured neurons or PC12 cells also triggered a Ca(2+)-dependent nuclear accumulation of PYK2, much more pronounced than that induced by blockade of nuclear export with leptomycin B. Src-family kinase activity, PYK2 autophosphorylation and kinase activity were not required for its nuclear translocation. Depolarization induced a slight decrease in PYK2 apparent molecular mass, compatible with a Ca(2+)-activated dephosphorylation. Pretreatment of PC12 cells with inhibitors of calcineurin (protein phosphatase 2B), cyclosporin A and FK506, prevented depolarization-induced nuclear translocation and tyrosine phosphorylation of PYK2. Transfection with dominant-negative and constitutively active calcineurin-A confirmed the role of calcineurin in the regulation of PYK2 tyrosine phosphorylation and nuclear accumulation. Our results show that depolarization independently induces nuclear translocation and tyrosine phosphorylation of PYK2, and that both responses require calcineurin activation. We suggest that PYK2 exerts some of its actions in the nucleus and that the effects of calcineurin inhibitors may involve PYK2 inhibition.

A delayed response enhancement during hippocampal presynaptic plasticity in mice

High frequency afferent stimulation of chemical synapses often induces short-term increases in synaptic efficacy, due to increased release probability and/or increased supply of readily releasable synaptic vesicles. This may be followed by synaptic depression, often caused by vesicle depletion. We here describe an additional, novel type of delayed and transient response enhancement phase which occurred during prolonged stimulation at 5-20 Hz frequency of excitatory glutamatergic synapses in slices from the adult mouse CA1 hippocampal region. This second enhancement phase, which was most clearly defined at physiological temperatures and essentially absent at 24 degrees C, was dependent on the presence of F-actin filaments and synapsins I and/or II, and could not be ascribed to changes in presynaptic action potentials, inhibitory neurotransmission or glutamate receptor desensitization. Time course studies showed that the delayed response phase interrupted the synaptic decay 3-4 s after stimulus train initiation and continued, when examined at 5-10 Hz frequencies, for approximately 75 stimuli before decay. The novel response enhancement, probably deriving from a restricted pool of synaptic vesicles, may allow maintenance of synaptic efficacy during prolonged periods of excitatory synaptic activity.

Impaired spatial working memory but spared spatial reference memory following functional loss of NMDA receptors in the dentate gyrus

Novel spatially restricted genetic manipulations can be used to assess contributions made by synaptic plasticity to learning and memory, not just selectively within the hippocampus, but even within specific hippocampal subfields. Here we generated genetically modified mice (NR1^ΔDG mice) exhibiting complete loss of the NR1 subunit of the N-methyl-d-aspartate receptor specifically in the granule cells of the dentate gyrus. There was no evidence of any reduction in NR1 subunit levels in any of the other hippocampal subfields, or elsewhere in the brain. NR1^ΔDG mice displayed severely impaired long-term potentiation (LTP) in both medial and lateral perforant path inputs to the dentate gyrus, whereas LTP was unchanged in CA3-to-CA1 cell synapses in hippocampal slices. Behavioural assessment of NR1^ΔDG mice revealed a spatial working memory impairment on a three-from-six radial arm maze task despite normal hippocampus-dependent spatial reference memory acquisition and performance of the same task. This behavioural phenotype resembles that of NR1^ΔCA3 mice but differs from that of NR1^ΔCA1 mice which do show a spatial reference memory deficit, consistent with the idea of subfield-specific contributions to hippocampal information processing. Furthermore, this pattern of selective functional loss and sparing is the same as previously observed with the global GluR-A l-α-amino-3-hydroxy-5-methyl-4-isoxazelopropionate receptor subunit knockout, a mutation which blocks the expression of hippocampal LTP. The present results show that dissociations between spatial working memory and spatial reference memory can be induced by disrupting synaptic plasticity specifically and exclusively within the dentate gyrus subfield of the hippocampal formation.

The role of NMDAR subtypes and charge transfer during hippocampal LTP induction

Activation of NMDA receptors (NMDARs) is a requirement for persistent synaptic alterations, such as long-term potentiation of synaptic transmission (LTP). NMDARs are composed of NR1 and NR2 subunits, and NR2 subunit-dependent gating properties of NMDAR subtypes cause dramatic differences in the timing of charge transfer. These postsynaptic temporal profiles are further influenced by the frequency of synaptic activation. Here, we investigated in the CA1 region of hippocampal slices from P28 mice, whether particular NMDAR subtypes are recruited based on NR2 subunit-specific gating following different induction protocols. For high frequency afferent stimulation (HFS), we found that genetic impairment of NR2A or pharmacological block of NR2A- or NR2B-type NMDARs can reduce field LTP. In contrast, when pairing low frequency synaptic stimulation with postsynaptic depolarization (LFS pairing) in single CA1 neurons, pharmacological antagonism of either subtype modestly reduced the charge transfer during LFS pairing without reducing the LTP magnitude. These results indicate that HFS-triggered LTP is induced by more than one NMDAR subtype, whereas a single subtype is sufficient during LFS pairing. Analysis of charge transfer during LFS pairing in 13 different conditions revealed a threshold for LTP induction, which was independent of the NR2 antagonist tested. Thus, at least for LFS pairing, the amount of charge transfer, and thus Ca2+ influx, during LTP induction is a factor more critical than the participation of a particular NMDAR subtype.

Massive localized lipolymphedema pseudotumor in a morbidly obese patient

We describe a 31 year old man with a massive localized tumor-like lipolymphedema, a puzzling entity that afflicts the morbidly obese. The 281 kg man presented with a growing ulcerated bleeding mass located on his proximal medial thigh and suspicious for sarcoma. After en bloc resection of the 28.2 kg edematous mass, no evidence of neoplasm was found, only prominent lymphatic vessel dilation and edema with large quantities of unremarkable adipose and connective tissue. The lesion conformed to the diagnostic criteria for massive localized lipolymphedema (MLL) pseudotumor.

Forebrain-specific glutamate receptor B deletion impairs spatial memory but not hippocampal field long-term potentiation

We demonstrate the fundamental importance of glutamate receptor B (GluR-B) containing AMPA receptors in hippocampal function by analyzing mice with conditional GluR-B deficiency in postnatal forebrain principal neurons (GluR-B(deltaFb)). These mice are as adults sufficiently robust to permit comparative cellular, physiological, and behavioral studies. GluR-B loss induced moderate long-term changes in the hippocampus of GluR-B(deltaFb) mice. Parvalbumin-expressing interneurons in the dentate gyrus and the pyramidal cells in CA3 were decreased in number, and neurogenesis in the subgranular zone was diminished. Excitatory synaptic CA3-to-CA1 transmission was reduced, although synaptic excitability, as quantified by the lowered threshold for population spike initiation, was increased compared with control mice. These changes did not alter CA3-to-CA1 long-term potentiation (LTP), which in magnitude was similar to LTP in control mice. The altered hippocampal circuitry, however, affected spatial learning in GluR-B(deltaFb) mice. The primary source for the observed changes is most likely the AMPA receptor-mediated Ca2+ signaling that appears after GluR-B depletion, because we observed similar alterations in GluR-B(QFb) mice in which the expression of Ca2+-permeable AMPA receptors in principal neurons was induced by postnatal activation of a Q/R-site editing-deficient GluR-B allele.

Lack of NMDA receptor subtype selectivity for hippocampal long-term potentiation

NMDA receptor (NMDAR) 2A (NR2A)- and NR2B-type NMDARs coexist in synapses of CA1 pyramidal cells. Recent studies using pharmacological blockade of NMDAR subtypes proposed that the NR2A type is responsible for inducing long-term potentiation (LTP), whereas the NR2B type induces long-term depression (LTD). This contrasts with the finding in genetically modified mice that NR2B-type NMDARs induce LTP when NR2A signaling is absent or impaired, although compensatory mechanisms might have contributed to this result. We therefore assessed the contribution of the two NMDAR subtypes to LTP in mouse hippocampal slices by different induction protocols and in the presence of NMDAR antagonists, including the NR2A-type blocker NVP-AAM077, for which an optimal concentration for subtype selectivity was determined on recombinant and native NMDARs. Partial blockade of NMDA EPSCs by 40%, either by preferentially antagonizing NR2A- or NR2B-type NMDARs or by the nonselective antagonist D-AP-5, did not impair LTP, demonstrating that hippocampal LTP induction can be generated by either NMDAR subtype.

Synapsin-regulated synaptic transmission from readily releasable synaptic vesicles in excitatory hippocampal synapses in mice

The effects of synapsin proteins on synaptic transmission from vesicles in the readily releasable vesicle pool have been examined by comparing excitatory synaptic transmission in hippocampal slices from mice devoid of synapsins I and II and from wild-type control animals. Application of stimulus trains at variable frequencies to the CA3-to-CA1 pyramidal cell synapse suggested that, in both genotypes, synaptic responses obtained within 2 s stimulation originated from readily releasable vesicles. Detailed analysis of the responses during this period indicated that stimulus trains at 2-20 Hz enhanced all early synaptic responses in the CA3-to-CA1 pyramidal cell synapse, but depressed all early responses in the medial perforant path-to-granule cell synapse. The synapsin-dependent part of these responses, i.e. the difference between the results obtained in the transgene and the wild-type preparations, showed that in the former synapse, the presence of synapsins I and II minimized the early responses at 2 Hz, but enhanced the early responses at 20 Hz, while in the latter synapse, the presence of synapsins I and II enhanced all responses at both stimulation frequencies. The results indicate that synapsins I and II are necessary for full expression of both enhancing and decreasing modulatory effects on synaptic transmission originating from the readily releasable vesicles in these excitatory synapses.

Frequency-dependent impairment of hippocampal LTP from NMDA receptors with reduced calcium permeability

Changes in postsynaptic Ca2+ levels are essential for alterations in synaptic strength. At hippocampal CA3-to-CA1 synapses, the Ca2+ elevations required for LTP induction are typically mediated by NMDA receptor (NMDAR) channels but a contribution of NMDAR-independent Ca2+ sources has been implicated. Here, we tested the sensitivity of different protocols modifying synaptic strength to reduced NMDAR-mediated Ca2+ influx by employing mice genetically programmed to express in forebrain principal neurons an NR1 form that curtails Ca2+ permeability. Reduced NMDAR-mediated Ca2+ influx did not facilitate synaptic depression in CA1 neurons of these genetically modified mice. However, we observed that LTP could not be induced by pairing low frequency synaptic stimulation (LFS pairing) with postsynaptic depolarization, a protocol that induced robust LTP in wild-type mice. By contrast to LFS pairing, similar LTP levels were generated in both genotypes when postsynaptic depolarization was paired with high frequency synaptic stimulation (HFS). This indicates that the postsynaptic Ca2+ elevation also reached threshold during HFS in the mutant, probably due to summation of NMDAR-mediated Ca2+ influx. However, only in wild-type mice did repeated HFS further enhance LTP. All tested forms of LTP were blocked by the NMDAR antagonist D-AP5. Collectively, our results indicate that only NMDAR-dependent Ca2+ sources (NMDARs and Ca2+-dependent Ca2+ release from intracellular stores) mediate LFS pairing-evoked LTP. Moreover, LTP induced by the first HFS stimulus train required lower Ca2+ levels than the additional LTP obtained by repeated trains.

A genetic switch for epilepsy in adult mice

Premature death from seizures afflicts gene-targeted mice expressing the Q/R site-unedited glutamate receptor subunit GluR-B(Q) of AMPA receptors in central neurons. Early seizure-related death has now been circumvented by a genetic switch that restricts GluR-B(Q) expression to forebrain principal neurons from postnatal stages onward, prominently in hippocampus and striatum and less so in cortex and amygdala. When switched on, functional receptor incorporation of GluR-B(Q) could be demonstrated by imaging evoked AMPA channel-mediated spinous Ca2+ transients in CA1 pyramidal cells. Sustained GluR-B(Q) expression in adult mice led to smaller excitatory postsynaptic responses in the CA1 region with unchanged presynaptic fiber excitability. Notably, despite the smaller excitatory response, the CA1 cells exhibited a reduced population spike threshold, which might underlie the spontaneous manifestations of epilepsy, including myocloni and generalized seizures with limbic components, observed by synchronous video monitoring and electroencephalographic recordings. No neuropathological symptoms developed when GluR-B(Q) expression was restricted to only hippocampal neurons. Our results show that seizure susceptibility is triggered by GluR-B(Q) expression also in the adult brain and that circuit hyperexcitability is not an immediate consequence of GluR-B(Q) but requires yet unknown downstream events, likely to be induced by non-Hebbian plasticity from Ca2+-permeable AMPA channels in principal neurons.