Spatiotemporal dynamics of epileptiform propagations: imaging of human brain slices

Pharmacological evaluation of E2730, a novel selective uncompetitive GAT1 inhibitor, on epileptiform activities in resected brain tissues from human focal cortical dysplasia ex vivo

Focal cortical dysplasia (FCD) is an important etiology of focal epilepsy in children and adults. However, only a few preclinical models sufficiently reproduce the characteristic histopathologic features of FCD. To improve the success rate of clinical trials for antiseizure medications (ASMs) in patients with FCD, more human-relevant preclinical models are needed, and epileptic foci resected from patients are a powerful tool for this purpose. Here, we conducted ex vivo studies using epileptic foci resected from patients with FCD type II to evaluate the pharmacologic effects of the ASM candidate E2730, a selective uncompetitive inhibitor of γ-aminobutyric acid transporter 1. We used the same ex vivo assay system to assess carbamazepine (CBZ), an ASM often prescribed for focal epilepsy, as a reference. At the higher dose tested (200 µM), both E2730 and CBZ suppressed spontaneous epileptiform activities almost completely. At the lower dose (100 µM), CBZ reduced the area of brain tissue showing epileptiform activity, whereas E2730 significantly decreased the number of epileptiforms. These findings suggest that E2730-both as a single agent and in combination with CBZ-merits evaluation in clinical trials involving patients with FCD.

Introduction: What Are Dendritic Spines?

Dendritic spines are cellular specializations that greatly increase the connectivity of neurons and modulate the "weight" of most postsynaptic excitatory potentials. Spines are found in very diverse animal species providing neural networks with a high integrative and computational possibility and plasticity, enabling the perception of sensorial stimuli and the elaboration of a myriad of behavioral displays, including emotional processing, memory, and learning. Humans have trillions of spines in the cerebral cortex, and these spines in a continuum of shapes and sizes can integrate the features that differ our brain from other species. In this chapter, we describe (1) the discovery of these small neuronal protrusions and the search for the biological meaning of dendritic spines; (2) the heterogeneity of shapes and sizes of spines, whose structure and composition are associated with the fine-tuning of synaptic processing in each nervous area, as well as the findings that support the role of dendritic spines in increasing the wiring of neural circuits and their functions; and (3) within the intraspine microenvironment, the integration and activation of signaling biochemical pathways, the compartmentalization of molecules or their spreading outside the spine, and the biophysical properties that can affect parent dendrites. We also provide (4) examples of plasticity involving dendritic spines and neural circuits relevant to species survival and comment on (5) current research advancements and challenges in this exciting research field.

Meningoencephalocele in the Lateral Sphenoid Sinus Showing Malformation of Cortical Development: A Case Report

Meningoencephalocele in the lateral sphenoid sinus (SS) has been determined to be a rare entity often detected by cerebrospinal fluid (CSF) rhinorrhea. To date, the pathology of meningoencephalocele in the lateral SS has remained to be unclear in many cases. In this study, we report on a case of a 72-year-old woman with an arteriovenous malformation who presented with CSF rhinorrhea. Radiologic investigations revealed a left temporal meningoencephalocele in the lateral SS. We removed the meningoencephalocele and performed skull base repair, after which the CSF rhinorrhea resolved. Pathological examination showed congenital cortical abnormalities with dysmorphic neurons in various shapes and acquired chronic tissue alterations including fibrillary gliosis and scattered Rosenthal fibers. These findings may further aid in understanding the etiopathogenesis of meningoencephalocele in the lateral SS.

Visualization of cortical activation in human brain by flavoprotein fluorescence imaging

OBJECTIVE

To develop an innovative brain mapping and neuromonitoring method during neurosurgery, the authors set out to establish intraoperative flavoprotein fluorescence imaging (iFFI) to directly visualize cortical activations in human brain. The significance of iFFI was analyzed by comparison with intraoperative perfusion-dependent imaging (iPDI), which is considered the conventional optical imaging, and by performing animal experiments.

METHODS

Seven patients with intracerebral tumors were examined by iFFI and iPDI following craniotomy, using a single operative microscope equipped with a laser light source for iFFI and xenon lamp for iPDI. Images were captured by the same charge-coupled device camera. Responses to bipolar stimulation at selected points on the cortical surface were analyzed off-line, and relative signal changes were visualized by overlaying pseudocolor intensity maps onto cortical photographs. Signal changes exceeding 3 SDs from baseline were defined as significant. The authors also performed FFI and PDI on 10 mice using similar settings, and then compared signal patterns to intraoperative studies.

RESULTS

Signals acquired by iFFI exhibited biphasic spatiotemporal changes consisting of an early positive signal peak (F1) and a delayed negative signal peak (F2). In contrast, iPDI signals exhibited only 1 negative peak (P1) that was significantly delayed compared to F1 (p < 0.02) and roughly in phase with F2. Compared to F2 and P1, F1 was of significantly lower amplitude (p < 0.02) and located closer to the bipolar stimulus center (p < 0.03), whereas F2 and P1 were more widespread, irregular, and partially overlapping. In mice, the spatiotemporal characteristics of FFI and PDI resembled those of iFFI and iPDI, but the early positive signal was more robust than F1.

CONCLUSIONS

This is the first report in humans of successful intraoperative visualization of cortical activations by using iFFI, which showed rapid evoked cortical activity prior to perfusion-dependent signal changes. Further technical improvements can lead to establishment of iFFI as a real-time intraoperative tool.

Reactive astrocytes contribute to epileptogenesis in patients with cavernous angioma

OBJECTIVE

Patients with cavernous angioma (CA) often suffer from severe epilepsy, and surgical resection is often performed to attenuate these epileptic seizures. Several studies have suggested that surgical removal of the surrounding hemosiderin-pigmented tissues adjacent to CA achieves better seizure control than restricted lesionectomy. Pathological examination of the resected foci reveals not only hemosiderin pigmentation but also various degrees of inflammatory change, such as hemosiderin-laden macrophages, gliosis and fibrosis. However, there is some controversy regarding the epileptogenic potential of these regions due to the uncertain nature of the mechanisms contributing to these histopathological changes.

METHODS

To investigate the correlations between neuron hyperexcitability and evident pathological changes, we performed ex vivo flavoprotein fluorescence imaging using surgically resected epileptogenic foci surrounding CA. The mirror surfaces of the tissues used for the physiological experiment were also subjected to morphological examination.

RESULTS

Hemosiderin-laden macrophages and many gemistocytic astrocytes were observed in the area adjacent to CA, where horizontal spreading excitations were detected significantly more frequently. Outside these areas, we found fine granular iron deposits and only a few fibrillary astrocytes, and weakly propagating excitations were detected. Furthermore, areas of enhanced activation were more clearly correlated with the glial proliferation index than with iron deposition.

CONCLUSION

These results suggest that the epileptogenesis in patients with CA may be based on a biological process, such as alteration of glial function, rather than direct chemical reactions involving iron deposition.

Pathophysiological Characteristics Associated With Epileptogenesis in Human Hippocampal Sclerosis

Mesial temporal lobe epilepsy (MTLE) is the most frequent focal epileptic syndrome in adults, and the majority of seizures originate primarily from the hippocampus. The resected hippocampal tissue often shows severe neuronal loss, a condition referred to as hippocampal sclerosis (HS). In order to understand hippocampal epileptogenesis in MTLE, it seems important to clarify any discrepancies between the clinical and pathological features of affected patients. Here we investigated epileptiform activities ex vivo using living hippocampal tissue taken from patients with MTLE. Flavoprotein fluorescence imaging and local field potential recordings revealed that epileptiform activities developed from the subiculum. Moreover, physiological and morphological experiments revealed possible impairment of K⁺ clearance in the subiculum affected by HS. Stimulation of mossy fibers induced recurrent trans-synaptic activity in the granule cell layer of the dentate gyrus, suggesting that mossy fiber sprouting in HS also contributes to the epileptogenic mechanism. These results indicate that pathophysiological alterations involving the subiculum and dentate gyrus could be responsible for epileptogenesis in patients with MTLE.

Ca2+ -permeable AMPA receptors associated with epileptogenesis of hypothalamic hamartoma

Hypothalamic hamartoma (HH), composed of neurons and glia without apparent cytologic abnormalities, is a rare developmental malformation in humans. Patients with HH often have characteristic medically refractory gelastic seizures, and intrinsic epileptogenesis within the lesions has been speculated. Herein we provide evidence to suggest that in HH neurons, Ca²⁺ permeability through α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors is aberrantly elevated. In needle biopsy specimens of HH tissue, field potential recordings demonstrated spontaneous epileptiform activities similar to those observed in other etiologically distinct epileptogenic tissues. In HH, however, these activities were clearly abolished by application of Joro Spider Toxin (JSTX), a specific inhibitor of the Ca²⁺ -permeable AMPA receptor. Consistent with these physiologic findings, the neuronal nuclei showed disappearance of adenosine deaminase acting on RNA 2 (ADAR2) immunoreactivity. Furthermore, examination of glutamate receptor 2 (GluA2) messenger RNA (mRNA) revealed that editing efficiency at the glutamine/arginine site was significantly low. These results suggest that neurons in HH may bear Ca²⁺ -permeable AMPA receptors due to dislocation of ADAR2.

Matrix metalloproteinase-9 involvement in the structural plasticity of dendritic spines

Dendritic spines are the locus for excitatory synaptic transmission in the brain and thus play a major role in neuronal plasticity. The ability to alter synaptic connections includes volumetric changes in dendritic spines that are driven by scaffolds created by the extracellular matrix (ECM). Here, we review the effects of the proteolytic activity of ECM proteases in physiological and pathological structural plasticity. We use matrix metalloproteinase-9 (MMP-9) as an example of an ECM modifier that has recently emerged as a key molecule in regulating the morphology and dysmorphology of dendritic spines that underlie synaptic plasticity and neurological disorders, respectively. We summarize the influence of MMP-9 on the dynamic remodeling of the ECM via the cleavage of extracellular substrates. We discuss its role in the formation, modification, and maintenance of dendritic spines in learning and memory. Finally, we review research that implicates MMP-9 in aberrant synaptic plasticity and spine dysmorphology in neurological disorders, with a focus on morphological abnormalities of dendritic protrusions that are associated with epilepsy.

Prenatal deletion of the RNA-binding protein HuD disrupts postnatal cortical circuit maturation and behavior

The proper functions of cortical circuits are dependent upon both appropriate neuronal subtype specification and their maturation to receive appropriate signaling. These events establish a balanced circuit that is important for learning, memory, emotion, and complex motor behaviors. Recent research points to mRNA metabolism as a key regulator of this development and maturation process. Hu antigen D (HuD), an RNA-binding protein, has been implicated in the establishment of neuronal identity and neurite outgrowth in vitro. Therefore, we investigated the role of HuD loss of function on neuron specification and dendritogenesis in vivo using a mouse model. We found that loss of HuD early in development results in a defective early dendritic overgrowth phase and pervasive deficits in neuron specification in the lower neocortical layers and defects in dendritogenesis in the CA3 region of the hippocampus. Subsequent behavioral analysis revealed a deficit in performance of a hippocampus-dependent task: the Morris water maze. Further, HuD knock-out (KO) mice exhibited lower levels of anxiety than their wild-type counterparts and were overall less active. Last, we found that HuD KO mice are more susceptible to auditory-induced seizures, often resulting in death. Our findings suggest that HuD is necessary for the establishment of neocortical and hippocampal circuitry and is critical for their function.

Surgical pathologic features of cerebral cortical lesions taken from 600 patients with intractable epilepsy

OBJECTIVES

To determine the scope of histopathological variation in lesions responsible for epileptogenesis, I retrospectively reviewed the features of surgical specimens taken consecutively from 600 patients with intractable epilepsy.

METHODS

The patients were divided into three groups on the basis of age at seizure onset: 94 patients with infantile onset (before 1year of age), 307 patients with juvenile onset (between 1 and 12years of age), and 199 patients with adolescent/adult onset (at 13years of age or beyond). Histological and immunohistochemical evaluations of the surgical specimens were performed.

RESULTS

In the infant group, seizure duration was significantly shorter than in the other groups, and malformations caused by abnormalities of cortical development, including focal cortical dysplasia (FCD) type IIa/b, tuberous sclerosis, hemimegalencephaly, and polymicrogyria were predominant, whereas in the juvenile and adolescent/adult groups, other lesions such as hippocampal sclerosis (HS), tumors, FCD type I, and vascular lesions were frequently observed. For patients with HS, seizure duration in the juvenile group was significantly longer than in the adolescent/adult group. FCD type IIIa was noted in nearly half of patient with HS in both juvenile and adolescent/adult groups. The causative tumors included dysembryoplastic neuroepithelial tumors, gangliogliomas, astrocytomas, and other glioneuronal and glial tumors.

CONCLUSION

Various histopathological entities and types, showing clear predominance depending on the age at seizure onset, were observed in patients with epilepsy. These features appear to provide information on the pathomechanisms of the lesions and their clinical relevance in affected patients.

Remodeling of dendrites and spines in the C1q knockout model of genetic epilepsy

PURPOSE

To determine whether developmental synaptic pruning defects in epileptic C1q-knockout (KO) mice are accompanied by postsynaptic abnormalities in dendrites and/or spines.

METHODS

Immunofluorescence staining was performed on biocytin-filled layer Vb pyramidal neurons in sensorimotor cortex. Basal dendritic arbors and their spines were reconstructed with NEUROLUCIDA software, and their morphologic characteristics were quantitated in Neuroexplorer.

KEY FINDINGS

Seven to nine completely filled pyramidal neurons were analyzed from the wild-type (WT) and C1q KO groups. Compared to WT controls, KO mice showed significant structural modifications in their basal dendrites including (1) higher density of dendritic spines (0.60 ± 0.03/μm vs. 0.49 ± 0.03/μm dendritic length in WT, p < 0.05); (2) remarkably increased occurrence of thin spines (0.26 ± 0.02/μm vs. 0.14 ± 0.02/μm dendritic length in control, p < 0.01); (3) longer dendritic length (2,680 ± 159 μm vs. 2,119 ± 108 μm in control); and (4) increased branching (22.6 ± 1.9 vs. 16.2 ± 1.3 in WT at 80 μm from soma center, p < 0.05; 12.4 ± 1.4 vs. 8.2 ± 0.6 in WT at 120 μm from soma center, respectively, p < 0.05). Dual immunolabeling demonstrated the expression of putative glutamate receptor 2 (GluR2) on some thin spines. These dendritic alterations are likely postsynaptic structural consequences of failure of synaptic pruning in the C1q KO mice.

SIGNIFICANCE

Failure to prune excessive excitatory synapses in C1q KO mice is a likely mechanism underlying abnormalities in postsynaptic dendrites, including increased branching and alterations in spine type and density. It is also possible that seizure activity contributes to these abnormalities. These structural abnormalities, together with increased numbers of excitatory synapses, likely contribute to epileptogenesis in C1q KO mice.

Significance of horizontal propagation of synchronized activities in human epileptic neocortex investigated by optical imaging and immunohistological study

To characterize the physiological condition of human epileptic neocortex, we employed flavoprotein fluorescence imaging (FFI), an optical imaging method which detects intrinsic signals accompanying neural activation, and immunohistologically studied human cortical specimens. The experimented materials were cortical tissues surrounding various intracerebral lesions obtained from 5 patients with epilepsy (epileptic patients: EPs) and 5 without epilepsy (non-epileptic patients: NEPs). These tissues were immersed in oxygenated artificial cerebrospinal fluid immediately after removal in the operating room. Signal changes of FFI in the cortical layers subjected to electrical stimulation were observed under bicuculline methiodide perfusion. Immunohistological staining for parvalbumin (PV), calbindin, and calretinin were performed on the same specimens to evaluate expressions of calcium-binding protein positive cells. The FFI study showed the characteristic cortical propagation pattern of elicited activities horizontally along the cortical layers in EPs but not in NEPs. The propagated area with more than 0.5% signal changes was significantly larger in EPs than in NEPs (p=0.008). Only the expression of PV positive neurons was significantly lower in EPs than in NEPs (p=0.006). The propagated area on FFI and the decrease in PV positive neurons correlated significantly (R=-0.78, p=0.04). The present study visualized the unique horizontal propagation of signal changes on FFI and demonstrated a correlation of this propagation with immunohistological decreases in PV positive neurons in human epileptic cortex. Further investigations may elucidate the mechanism of hyper-excitability and hyper-synchronization in epileptic cortical tissue itself.

Periventricular nodular heterotopia functionally couples with the overlying hippocampus

Patients with periventricular nodular heterotopia (PVNH) often have severe epilepsy. However, it is unclear how the heterotopia contributes to epileptogenesis. Recently, electrophysiologic studies using intraoperative depth electrodes have indicated that interaction between the heterotopia and overlying cortex is crucial for seizure onset. We performed an in vitro physiologic study using slices of resected brain from a 22-year-old man with PVNH, who manifested medically refractory mesial temporal lobe epilepsy. Preoperative evaluation indicated that the right mesial temporal structure and PVNH were the epileptogenic focus. The resected tissue was immediately immersed in cold artificial cerebrospinal fluid, and then slices of the brain tissue including the heterotopic nodules and overlying hippocampus were prepared. We electrically stimulated the incubated slices, and the elicited neural activities were analyzed as changes in the flavoprotein fluorescence signals. When we stimulated either the heterotopic nodule or the overlying hippocampus, clear functional coupling of neural activities between these structures was observed. The coupling response evoked by stimulation of the subiculum and developing within the heterotopic nodule was enhanced by application of bicuculline. Therefore, activities of the hippocampus and the nodule are closely correlated.

Dendritic spine pathology in epilepsy: cause or consequence?

Abnormalities in dendritic spines have commonly been observed in brain specimens from epilepsy patients and animal models of epilepsy. However, the functional implications and clinical consequences of this dendritic pathology for epilepsy are uncertain. Dendritic spine abnormalities may promote hyperexcitable circuits and seizures in some types of epilepsy, especially in specific genetic syndromes with documented dendritic pathology, but in these cases it is difficult to differentiate their effects on seizures versus other comorbidities, such as cognitive deficits and autism. In other situations, seizures themselves may cause damage to dendrites and dendritic spines and this seizure-induced brain injury may then contribute to progressive epileptogenesis, memory problems and other neurological deficits in epilepsy patients. The mechanistic basis of dendritic spine abnormalities in epilepsy has begun to be elucidated and suggests novel therapeutic strategies for treating epilepsy and its complications.