Glutamate biosensor imaging reveals dysregulation of glutamatergic pathways in a model of developmental cortical malformation

Real-time imaging of glutamate transients in the extracellular space of acute human brain slices using a single-wavelength glutamate fluorescence nanosensor

Glutamate is the most important excitatory neurotransmitter in the brain. The ability to assess glutamate release and re-uptake with high spatial and temporal resolution is crucial to understand the involvement of this primary excitatory neurotransmitter in both normal brain function and different neurological disorders. Real-time imaging of glutamate transients by fluorescent nanosensors has been accomplished in rat brain slices. We performed for the first time single-wavelength glutamate nanosensor imaging in human cortical brain slices obtained from patients who underwent epilepsy surgery. The glutamate fluorescence nanosensor signals of the electrically stimulated human cortical brain slices showed steep intensity increase followed by an exponential decrease. The spatial distribution and the time course of the signal were in good agreement with the position of the stimulation electrode and the dynamics of the electrical stimulation, respectively. Pharmacological manipulation of glutamate release and reuptake was associated with corresponding changes in the glutamate fluorescence nanosensor signals. We demonstrated that the recently developed fluorescent nanosensors for glutamate allow to detect neuronal activity in acute human cortical brain slices with high spatiotemporal precision. Future application to tissue samples from different pathologies may provide new insights into pathophysiology without the limitations of an animal model.

In vivo KPT-350 treatment decreases cortical hyperexcitability following traumatic brain injury

PRIMARY OBJECTIVE

We tested whether KPT-350, a novel selective inhibitor of nuclear export, could attenuate cortical network hyperexcitability, a major risk factor for developing post-traumatic epilepsy (PTE) following traumatic brain injury (TBI).

RESEARCH DESIGN

All mice in this study underwent TBI and were subsequently treated with either KPT-350 or vehicle during the post-injury latent period. Half of the animal cohort was used for electrophysiology while the other half was used for immunohistochemical analysis.

METHODS AND PROCEDURES

TBI was induced using the controlled cortical impact (CCI) model. Cortical network activity was recorded by evoking field potentials from deep layers of the cortex and analyzed using Matlab software. Immunohistochemistry coupled with fluorescence microscopy and Image J analysis detected changes in neuronal and glial markers.

MAIN OUTCOMES AND RESULTS

KPT-350 attenuated TBI-associated epileptiform activity and restored disrupted input-output responses in cortical brain slices. In vivo KPT-350 treatment reduced the loss of parvalbumin-(+) GABAergic interneurons after CCI but did not significantly change CCI-induced loss of somatostatin-(+) GABAergic interneurons, nor did it reduce reactivity of GFAP and Iba1 glial markers.

CONCLUSION

KPT-350 can prevent cortical hyperexcitability and reduce the loss of parvalbumin-(+) GABAergic inhibitory neurons, making it a potential therapeutic option for preventing PTE.

Cortical Malformations: Lessons in Human Brain Development

Creating a functional cerebral cortex requires a series of complex and well-coordinated developmental steps. These steps have evolved across species with the emergence of cortical gyrification and coincided with more complex behaviors. The presence of diverse progenitor cells, a protracted timeline for neuronal migration and maturation, and diverse neuronal types are developmental features that have emerged in the gyrated cortex. These factors could explain how the human brain has expanded in size and complexity. However, their complex nature also renders new avenues of vulnerability by providing additional cell types that could contribute to disease and longer time windows that could impact the composition and organization of the cortical circuit. We aim to discuss the unique developmental steps observed in human corticogenesis and propose how disruption of these species-unique processes could lead to malformations of cortical development.

Tonic Activation of GluN2C/GluN2D-Containing NMDA Receptors by Ambient Glutamate Facilitates Cortical Interneuron Maturation

Developing cortical GABAergic interneurons rely on genetic programs, neuronal activity, and environmental cues to construct inhibitory circuits during early postnatal development. Disruption of these events can cause long-term changes in cortical inhibition and may be involved in neurological disorders associated with inhibitory circuit dysfunction. We hypothesized that tonic glutamate signaling in the neonatal cortex contributes to, and is necessary for, the maturation of cortical interneurons. To test this hypothesis, we used mice of both sexes to quantify extracellular glutamate concentrations in the cortex during development, measure ambient glutamate-mediated activation of developing cortical interneurons, and manipulate tonic glutamate signaling using subtype-specific NMDA receptor antagonists in vitro and in vivo We report that ambient glutamate levels are high (≈100 nm) in the neonatal cortex and decrease (to ≈50 nm) during the first weeks of life, coincident with increases in astrocytic glutamate uptake. Consistent with elevated ambient glutamate, putative parvalbumin-positive interneurons in the cortex (identified using G42:GAD1-eGFP reporter mice) exhibit a transient, tonic NMDA current at the end of the first postnatal week. GluN2C/GluN2D-containing NMDA receptors mediate the majority of this current and contribute to the resting membrane potential and intrinsic properties of developing putative parvalbumin interneurons. Pharmacological blockade of GluN2C/GluN2D-containing NMDA receptors in vivo during the period of tonic interneuron activation, but not later, leads to lasting decreases in interneuron morphological complexity and causes deficits in cortical inhibition later in life. These results demonstrate that dynamic ambient glutamate signaling contributes to cortical interneuron maturation via tonic activation of GluN2C/GluN2D-containing NMDA receptors.SIGNIFICANCE STATEMENT Inhibitory GABAergic interneurons make up 20% of cortical neurons and are critical to controlling cortical network activity. Dysfunction of cortical inhibition is associated with multiple neurological disorders, including epilepsy. Establishing inhibitory cortical networks requires in utero proliferation, differentiation, and migration of immature GABAergic interneurons, and subsequent postnatal morphological maturation and circuit integration. Here, we demonstrate that ambient glutamate provides tonic activation of immature, putative parvalbumin-positive GABAergic interneurons in the neonatal cortex via high-affinity NMDA receptors. When this activation is blocked, GABAergic interneuron maturation is disrupted, and cortical networks exhibit lasting abnormal hyperexcitability. We conclude that temporally precise activation of developing cortical interneurons by ambient glutamate is critically important for establishing normal cortical inhibition.

α2δ-1 Signaling Drives Cell Death, Synaptogenesis, Circuit Reorganization, and Gabapentin-Mediated Neuroprotection in a Model of Insult-Induced Cortical Malformation

Developmental cortical malformations (DCMs) result from pre- and perinatal insults, as well as genetic mutations. Hypoxia, viral infection, and traumatic injury are the most common environmental causes of DCMs, and are associated with the subsyndromes focal polymicrogyria and focal cortical dysplasia (FCD) Type IIId, both of which have a high incidence of epilepsy. Understanding the molecular signals that lead to the formation of a hyperexcitable network in DCMs is critical to devising novel treatment strategies. In a previous study using the freeze-lesion (FL) murine model of DCM, we found that levels of thrombospondin (TSP) and the calcium channel auxiliary subunit α2δ-1 were elevated. TSP binds to α2δ-1 to drive the formation of excitatory synapses during development, suggesting that overactivation of this pathway may lead to exuberant excitatory synaptogenesis and network hyperexcitability seen in DCMs. In that study, antagonizing TSP/α2δ-1 signaling using the drug gabapentin (GBP) reduced many FL-induced pathologies. Here, we used mice with a genetic deletion of α2δ-1 to determine how α2δ-1 contributes to cell death, elevated excitatory synapse number, and in vitro network function after FL and to examine the molecular specificity of GBP's effects. We identified a critical role for α2δ-1 in FL-induced pathologies and in mediating the neuroprotective effects of GBP. Interestingly, genetic deletion of α2δ-1 did not eliminate GBP's effects on synaptogenesis, suggesting that GBP can have α2δ-1-independent effects. Taken together these studies suggests that inhibiting α2δ-1 signaling may have therapeutic promise to reduce cell death and network reorganization associated with insult-induced DCMs.

Astrocyte membrane properties are altered in a rat model of developmental cortical malformation but single-cell astrocytic glutamate uptake is robust

Developmental cortical malformations (DCMs) are linked with severe epilepsy and are caused by both genetic and environmental insults. DCMs include several neurological diseases, such as focal cortical dysplasia, polymicrogyria, schizencephaly, and others. Human studies have implicated astrocyte reactivity and dysfunction in the pathophysiology of DCMs, but their specific role is unknown. As astrocytes powerfully regulate glutamate neurotransmission, and glutamate levels are known to be increased in human epileptic foci, understanding the role of astrocytes in the pathological sequelae of DCMs is extremely important. Additionally, recent studies examining astrocyte glutamate uptake in DCMs have reported conflicting results, adding confusion to the field. In this study we utilized the freeze lesion (FL) model of DCM, which is known to induce reactive astrocytosis and cause significant changes in astrocyte morphology, proliferation, and distribution. Using whole-cell patch clamp recording from astrocytes, we recorded both UV-uncaging and synaptically evoked glutamate transporter currents (TCs), widely accepted assays of functional glutamate transport by astrocytes. With this approach, we set out to test the hypothesis that astrocyte membrane properties and glutamate transport were disrupted in this model of DCM. Though we found that the developmental maturation of astrocyte membrane resistance was disrupted by FL, glutamate uptake by individual astrocytes was robust throughout FL development. Interestingly, using an immunolabeling approach, we observed spatial and developmental differences in excitatory amino acid transporter (EAAT) expression in FL cortex. Spatially specific differences in EAAT2 (GLT-1) and EAAT1 (GLAST) expression suggest that the relative contribution of each EAAT to astrocytic glutamate uptake may be altered in FL cortex. Lastly, we carefully analyzed the amplitudes and onset times of both synaptically- and UV uncaging-evoked TCs. We found that in the FL cortex, synaptically-evoked, but not UV uncaging-evoked TCs, were larger in amplitude. Additionally, we found that the amount of electrical stimulation required to evoke a synaptic TC was significantly reduced in the FL cortex. Both of these findings are consistent with increased excitatory input to the FL cortex, but not with changes in how individual astrocytes remove glutamate. Taken together, our results demonstrate that the maturation of astrocyte membrane resistance, local distribution of glutamate transporters, and glutamatergic input to the cortex are altered in the FL model, but that single-cell astrocytic glutamate uptake is robust.

WONOEP appraisal: molecular and cellular imaging in epilepsy

Great advancements have been made in understanding the basic mechanisms of ictogenesis using single-cell electrophysiology (e.g., patch clamp, sharp electrode), large-scale electrophysiology (e.g., electroencephalography [EEG], field potential recording), and large-scale imaging (magnetic resonance imaging [MRI], positron emission tomography [PET], calcium imaging of acetoxymethyl ester [AM] dye-loaded tissue). Until recently, it has been challenging to study experimentally how population rhythms emerge from cellular activity. Newly developed optical imaging technologies hold promise for bridging this gap by making it possible to simultaneously record the many cellular elements that comprise a neural circuit. Furthermore, easily accessible genetic technologies for targeting expression of fluorescent protein-based indicators make it possible to study, in animal models of epilepsy, epileptogenic changes to neural circuits over long periods. In this review, we summarize some of the latest imaging tools (fluorescent probes, gene delivery methods, and microscopy techniques) that can lead to the advancement of cell- and circuit-level understanding of epilepsy, which in turn may inform and improve development of next generation antiepileptic and antiepileptogenic drugs.

Functional changes in glutamate transporters and astrocyte biophysical properties in a rodent model of focal cortical dysplasia

Cortical dysplasia is associated with intractable epilepsy and developmental delay in young children. Recent work with the rat freeze-induced focal cortical dysplasia (FCD) model has demonstrated that hyperexcitability in the dysplastic cortex is due in part to higher levels of extracellular glutamate. Astrocyte glutamate transporters play a pivotal role in cortical maintaining extracellular glutamate concentrations. Here we examined the function of astrocytic glutamate transporters in a FCD model in rats. Neocortical freeze lesions were made in postnatal day (PN) 1 rat pups and whole cell electrophysiological recordings and biochemical studies were performed at PN 21–28. Synaptically evoked glutamate transporter currents in astrocytes showed a near 10-fold reduction in amplitude compared to sham operated controls. Astrocyte glutamate transporter currents from lesioned animals were also significantly reduced when challenged exogenously applied glutamate. Reduced astrocytic glutamate transport clearance contributed to increased NMDA receptor-mediated current decay kinetics in lesioned animals. The electrophysiological profile of astrocytes in the lesion group was also markedly changed compared to sham operated animals. Control astrocytes demonstrate large-amplitude linear leak currents in response to voltage-steps whereas astrocytes in lesioned animals demonstrated significantly smaller voltage-activated inward and outward currents. Significant decreases in astrocyte resting membrane potential and increases in input resistance were observed in lesioned animals. However, Western blotting, immunohistochemistry and quantitative PCR demonstrated no differences in the expression of the astrocytic glutamate transporter GLT-1 in lesioned animals relative to controls. These data suggest that, in the absence of changes in protein or mRNA expression levels, functional changes in astrocytic glutamate transporters contribute to neuronal hyperexcitability in the FCD model.

Laser-scanning astrocyte mapping reveals increased glutamate-responsive domain size and disrupted maturation of glutamate uptake following neonatal cortical freeze-lesion

Astrocytic uptake of glutamate shapes extracellular neurotransmitter dynamics, receptor activation, and synaptogenesis. During development, glutamate transport becomes more robust. How neonatal brain insult affects the functional maturation of glutamate transport remains unanswered. Neonatal brain insult can lead to developmental delays, cognitive losses, and epilepsy; the disruption of glutamate transport is known to cause changes in synaptogenesis, receptor activation, and seizure. Using the neonatal freeze-lesion (FL) model, we have investigated how insult affects the maturation of astrocytic glutamate transport. As lesioning occurs on the day of birth, a time when astrocytes are still functionally immature, this model is ideal for identifying changes in astrocyte maturation following insult. Reactive astrocytosis, astrocyte proliferation, and in vitro hyperexcitability are known to occur in this model. To probe astrocyte glutamate transport with better spatial precision we have developed a novel technique, Laser Scanning Astrocyte Mapping (LSAM), which combines glutamate transport current (TC) recording from astrocytes with laser scanning glutamate photolysis. LSAM allows us to identify the area from which a single astrocyte can transport glutamate and to quantify spatial heterogeneity in the rate of glutamate clearance kinetics within that domain. Using LSAM, we report that cortical astrocytes have an increased glutamate-responsive area following FL and that TCs have faster decay times in distal, as compared to proximal processes. Furthermore, the developmental shift from GLAST- to GLT-1-dominated clearance is disrupted following FL. These findings introduce a novel method to probe astrocyte glutamate uptake and show that neonatal cortical FL disrupts the functional maturation of cortical astrocytes.

Gabapentin attenuates hyperexcitability in the freeze-lesion model of developmental cortical malformation

Developmental cortical malformations are associated with a high incidence of drug-resistant epilepsy. The underlying epileptogenic mechanisms, however, are poorly understood. In rodents, cortical malformations can be modeled using neonatal freeze-lesion (FL), which has been shown to cause in vitro cortical hyperexcitability. Here, we investigated the therapeutic potential of gabapentin, a clinically used anticonvulsant and analgesic, in preventing FL-induced in vitro and in vivo hyperexcitability. Gabapentin has been shown to disrupt the interaction of thrombospondin (TSP) with α2δ-1, an auxiliary calcium channel subunit. TSP/α2δ-1 signaling has been shown to drive the formation of excitatory synapses during cortical development and following injury. Gabapentin has been reported to have neuroprotective and anti-epileptogenic effects in other models associated with increased TSP expression and reactive astrocytosis. We found that both TSP and α2δ-1 were transiently upregulated following neonatal FL. We therefore designed a one-week GBP treatment paradigm to block TSP/α2δ-1 signaling during the period of their upregulation. GBP treatment prevented epileptiform activity following FL, as assessed by both glutamate biosensor imaging and field potential recording. GBP also attenuated FL-induced increases in mEPSC frequency at both P7 and 28. Additionally, GBP treated animals had decreased in vivo kainic acid (KA)-induced seizure activity. Taken together these results suggest gabapentin treatment immediately after FL can prevent the formation of a hyperexcitable network and may have therapeutic potential to minimize epileptogenic processes associated with developmental cortical malformations.

Excitatory and inhibitory synaptic connectivity to layer V fast-spiking interneurons in the freeze lesion model of cortical microgyria

A variety of major developmental cortical malformations are closely associated with clinically intractable epilepsy. Pathophysiological aspects of one such disorder, human polymicrogyria, can be modeled by making neocortical freeze lesions (FL) in neonatal rodents, resulting in the formation of microgyri. Previous studies showed enhanced excitatory and inhibitory synaptic transmission and connectivity in cortical layer V pyramidal neurons in the paramicrogyral cortex. In young adult transgenic mice that express green fluorescent protein (GFP) specifically in parvalbumin positive fast-spiking (FS) interneurons, we used laser scanning photostimulation (LSPS) of caged glutamate to map excitatory and inhibitory synaptic connectivity onto FS interneurons in layer V of paramicrogyral cortex in control and FL groups. The proportion of uncaging sites from which excitatory postsynaptic currents (EPSCs) could be evoked (hotspot ratio) increased slightly but significantly in FS cells of the FL vs. control cortex, while the mean amplitude of LSPS-evoked EPSCs at hotspots did not change. In contrast, the hotspot ratio of inhibitory postsynaptic currents (IPSCs) was significantly decreased in FS neurons of the FL cortex. These alterations in synaptic inputs onto FS interneurons may result in an enhanced inhibitory output. We conclude that alterations in synaptic connectivity to cortical layer V FS interneurons do not contribute to hyperexcitability of the FL model. Instead, the enhanced inhibitory output from these neurons may partially offset an earlier demonstrated increase in synaptic excitation of pyramidal cells and thereby maintain a relative balance between excitation and inhibition in the affected cortical circuitry.

A local glutamate-glutamine cycle sustains synaptic excitatory transmitter release

Biochemical studies suggest that excitatory neurons are metabolically coupled with astrocytes to generate glutamate for release. However, the extent to which glutamatergic neurotransmission depends on this process remains controversial because direct electrophysiological evidence is lacking. The distance between cell bodies and axon terminals predicts that glutamine-glutamate cycle is synaptically localized. Hence, we investigated isolated nerve terminals in brain slices by transecting hippocampal Schaffer collaterals and cortical layer I axons. Stimulating with alternating periods of high frequency (20 Hz) and rest (0.2 Hz), we identified an activity-dependent reduction in synaptic efficacy that correlated with reduced glutamate release. This was enhanced by inhibition of astrocytic glutamine synthetase and reversed or prevented by exogenous glutamine. Importantly, this activity dependence was also revealed with an in-vivo-derived natural stimulus both at network and cellular levels. These data provide direct electrophysiological evidence that an astrocyte-dependent glutamate-glutamine cycle is required to maintain active neurotransmission at excitatory terminals.

Traumatic Brain Injury Increases Cortical Glutamate Network Activity by Compromising GABAergic Control

Traumatic brain injury (TBI) is a major risk factor for developing pharmaco-resistant epilepsy. Although disruptions in brain circuitry are associated with TBI, the precise mechanisms by which brain injury leads to epileptiform network activity is unknown. Using controlled cortical impact (CCI) as a model of TBI, we examined how cortical excitability and glutamatergic signaling was altered following injury. We optically mapped cortical glutamate signaling using FRET-based glutamate biosensors, while simultaneously recording cortical field potentials in acute brain slices 2-4 weeks following CCI. Cortical electrical stimulation evoked polyphasic, epileptiform field potentials and disrupted the input-output relationship in deep layers of CCI-injured cortex. High-speed glutamate biosensor imaging showed that glutamate signaling was significantly increased in the injured cortex. Elevated glutamate responses correlated with epileptiform activity, were highest directly adjacent to the injury, and spread via deep cortical layers. Immunoreactivity for markers of GABAergic interneurons were significantly decreased throughout CCI cortex. Lastly, spontaneous inhibitory postsynaptic current frequency decreased and spontaneous excitatory postsynaptic current increased after CCI injury. Our results suggest that specific cortical neuronal microcircuits may initiate and facilitate the spread of epileptiform activity following TBI. Increased glutamatergic signaling due to loss of GABAergic control may provide a mechanism by which TBI can give rise to post-traumatic epilepsy.