Kainate-induced zinc translocation from presynaptic terminals causes neuronal and astroglial cell death and mRNA loss of BDNF receptors in the hippocampal formation and amygdala

A companion to the preclinical common data elements and case report forms for neuropathology studies in epilepsy research. A report of the TASK3 WG2 Neuropathology Working Group of the ILAE/AES Joint Translational Task Force

The International League Against Epilepsy/American Epilepsy Society (ILAE/AES) Joint Translational Task Force initiated the TASK3 working group to create common data elements (CDEs) for various aspects of preclinical epilepsy research studies, which could help improve the standardization of experimental designs. This article addresses neuropathological changes associated with seizures and epilepsy in rodent models of epilepsy. We discuss CDEs for histopathological parameters for neurodegeneration, changes in astrocyte morphology and function, mechanisms of inflammation, and changes in the blood-brain barrier and myelin/oligodendrocytes resulting from recurrent seizures in rats and mice. We provide detailed CDE tables and case report forms (CRFs), and with this companion manuscript, we discuss the rationale and methodological aspects of individual neuropathological examinations. The CDEs, CRFs, and companion paper are available to all researchers, and their use will benefit the harmonization and comparability of translational preclinical epilepsy research. The ultimate hope is to facilitate the development of rational therapy concepts for treating epilepsies, seizures, and comorbidities and the development of biomarkers assessing the pathological state of the disease.

CDDO-Me Attenuates Clasmatodendrosis in CA1 Astrocyte by Inhibiting HSP25-AKT Mediated DRP1-S637 Phosphorylation in Chronic Epilepsy Rats

Clasmatodendrosis is one of the irreversible astroglial degeneration, which is involved in seizure duration and its progression in the epileptic hippocampus. Although sustained heat shock protein 25 (HSP25) induction leads to this autophagic astroglial death, dysregulation of mitochondrial dynamics (aberrant mitochondrial elongation) is also involved in the pathogenesis in clasmatodendrosis. However, the underlying molecular mechanisms of accumulation of elongated mitochondria in clasmatodendritic astrocytes are elusive. In the present study, we found that clasmatodendritic astrocytes showed up-regulations of HSP25 expression, AKT serine (S) 473 and dynamin-related protein 1 (DRP1) S637 phosphorylations in the hippocampus of chronic epilepsy rats. 2-Cyano-3,12-dioxo-oleana-1,9(11)-dien-28-oic acid methyl ester (CDDO-Me; bardoxolone methyl or RTA 402) abrogated abnormal mitochondrial elongation by reducing HSP25 upregulation, AKT S473- and DRP1 S637 phosphorylations. Furthermore, HSP25 siRNA and 3-chloroacetyl-indole (3CAI, an AKT inhibitor) abolished AKT-DRP1-mediated mitochondrial elongation and attenuated clasmatodendrosis in CA1 astrocytes. These findings indicate that HSP25-AKT-mediated DRP1 S637 hyper-phosphorylation may lead to aberrant mitochondrial elongation, which may result in autophagic astroglial degeneration. Therefore, our findings suggest that the dysregulation of HSP25-AKT-DRP1-mediated mitochondrial dynamics may play an important role in clasmatodendrosis, which would have implications for the development of novel therapies against various neurological diseases related to astroglial degeneration.

The role of astrocytes in epileptic disorders

Epilepsy affects about 1% of the population and approximately 30% of epileptic patients are resistant to current antiepileptic drugs. As a hallmark in epileptic tissue, many of the epileptic patients show changes in glia morphology and function. There are characteristic changes in different types of glia in different epilepsy models. Some of these changes such as astrogliosis are enough to provoke epileptic seizures. Astrogliosis is well known in mesial temporal lobe epilepsy (MTLE), the most common form of refractory epilepsy. A better understanding of astrocytes alterations could lead to novel and efficient pharmacological approaches for epilepsy. In this review, we present the alterations of astrocyte morphology and function and present some instances of targeting astrocytes in seizure and epilepsy.

Cell death of hippocampal CA1 astrocytes during early epileptogenesis

OBJECTIVE

Growing evidence suggests that dysfunctional astrocytes are crucial players in the development of mesial temporal lobe epilepsy (MTLE). Using a mouse model closely recapitulating key alterations of chronic human MTLE with hippocampal sclerosis, here we asked whether death of astrocytes contributes to the initiation of the disease and investigated potential underlying molecular mechanisms.

METHODS

Antibody staining was combined with confocal imaging and semiquantitative real-time polymerase chain reaction analysis to identify markers of different cellular death mechanisms between 4 h and 3 days after epilepsy induction.

RESULTS

Four hours after kainate-mediated induction of status epilepticus (SE), we found a significant reduction in the density of astrocytes in the CA1 stratum radiatum (SR) of the ipsilateral hippocampus. This reduction was transient, as within the next 3 days, astrocyte cell numbers recovered to the initial values, which was accompanied by enhanced proliferation. Four hours after SE induction, a small proportion of astrocytes in the ipsilateral CA1 SR expressed autophagy-related genes and proteins, whereas we did not find astrocytes positive for cleaved caspase 3 or terminal deoxynucleotide transferase-mediated deoxyuridine triphosphate nick-end labeling, ruling out apoptosis-related astrocytic death. Importantly, at the same early time point post-SE, many astrocytes in the ipsilateral CA1 SR showed strong expression of genes encoding pro-necroptosis factors, including receptor-interacting protein kinase 3 (RIPK3) and mixed lineage kinase domain-like protein (MLKL). Phosphorylation of MLKL (pMLKL), formation of necrosome complexes composed of RIPK3 and pMLKL, and translocation of pMLKL to the nucleus and to the plasma membrane were often observed in astrocytes of the ipsilateral hippocampus 4 h post-SE.

SIGNIFICANCE

The present study revealed that astrocytes die shortly after induction of SE. Our expression data and immunohistochemistry suggest that necroptosis and autophagy contribute to astrocytic death. These findings help to better understand how dysfunctional and pathological remodeling of astrocytes contributes to the initiation of temporal lobe epilepsy.

CDDO-Me Attenuates Astroglial Autophagy via Nrf2-, ERK1/2-SP1- and Src-CK2-PTEN-PI3K/AKT-Mediated Signaling Pathways in the Hippocampus of Chronic Epilepsy Rats

Clasmatodendrosis is an autophagic astroglial death showing extensive swollen cell bodies with vacuoles and disintegrated/beaded processes. This astroglial degeneration is closely relevant to the synchronous epileptiform discharges. However, the underlying molecular mechanisms and the roles of clasmatodendrosis in spontaneous seizure activity are still unknown. The 2-cyano-3,12-dioxo-oleana-1,9(11)-dien-28-oic acid methyl ester (CDDO-Me; RTA 402) is one of the activators for nuclear factor-erythroid 2-related factor 2 (Nrf2) that is a redox-sensitive transcription factor. In the present study, we explored the effects of CDDO-Me on clasmatodendrosis in chronic epilepsy rats, which could prevent epilepsy-related complications. In the present study, clasmatodendritic astrocytes showed reduced Nrf2 expression and its nuclear accumulation, which were restored by CDDO-Me. CDDO-Me also abrogated heat shock protein 25 (HSP25) upregulation in clasmatodendritic astrocytes by regulating extracellular signal-related kinases 1/2 (ERK1/2)-specificity protein 1 (SP1)- and Src-casein kinase 2 (CK2)-phosphatase and tensin homolog deleted on chromosome 10 (PTEN)-phosphatidylinositol-3-kinase (PI3K)-AKT-glycogen synthase kinase 3β (GSK3β)-bax-interacting factor 1 (Bif-1)-mediated signaling pathways in chronic epilepsy rats. In addition, CDDO-Me ameliorated spontaneous seizure duration, but not seizure frequency and behavioral seizure severity. Therefore, our findings suggest that clasmatodendrosis may affect seizure duration in chronic epilepsy rats, and that CDDO-Me may attenuate autophagic astroglial degeneration by regulating various signaling pathways.

CDDO-Me Distinctly Regulates Regional Specific Astroglial Responses to Status Epilepticus via ERK1/2-Nrf2, PTEN-PI3K-AKT and NFκB Signaling Pathways

2-Cyano-3,12-dioxo-oleana-1,9(11)-dien-28-oic acid methyl ester (CDDO-Me) is a triterpenoid analogue of oleanolic acid. CDDO-Me shows anti-inflammatory and neuroprotective effects. Furthermore, CDDO-Me has antioxidant properties, since it activates nuclear factor-erythroid 2-related factor 2 (Nrf2), which is a key player of redox homeostasis. In the present study, we evaluated whether CDDO-Me affects astroglial responses to status epilepticus (SE, a prolonged seizure activity) in the rat hippocampus in order to understand the underlying mechanisms of reactive astrogliosis and astroglial apoptosis. Under physiological conditions, CDDO-Me increased Nrf2 expression in the hippocampus without altering activities (phosphorylations) of phosphatase and tensin homolog deleted on chromosome 10 (PTEN), phosphatidylinositol-3-kinase (PI3K), and AKT. CDDO-Me did not affect seizure activity in response to pilocarpine. However, CDDO-Me ameliorated reduced astroglial Nrf2 expression in the CA1 region and the molecular layer of the dentate gyrus (ML), and attenuated reactive astrogliosis and ML astroglial apoptosis following SE. In CA1 astrocytes, CDDO-Me inhibited the PI3K/AKT pathway by activating PTEN. In contrast, CDDO-ME resulted in extracellular signal-related kinases 1/2 (ERK1/2)-mediated Nrf2 upregulation in ML astrocytes. Furthermore, CDDO-Me decreased nuclear factor-κB (NFκB) phosphorylation in both CA1 and ML astrocytes. Therefore, our findings suggest that CDDO-Me may attenuate SE-induced reactive astrogliosis and astroglial apoptosis via regulation of ERK1/2-Nrf2, PTEN-PI3K-AKT, and NFκB signaling pathways.

CDK5 inhibitors prevent astroglial apoptosis and reactive astrogliosis by regulating PKA and DRP1 phosphorylations in the rat hippocampus

Status epilepticus (SE) results in the unique pattern of dynamin-related protein 1 (DRP1)-mediated mitochondrial dynamics, which is associated with astroglial apoptosis and reactive astrogliosis in the regional-specific pattern representing the differential astroglial properties. However, less defined are the epiphenomena/upstream effecters for DRP1 phosphorylation in this process. Since cyclin-dependent kinase 5 (CDK5) is involved in reactive astrogliosis, CDK5 is one of the possible upstream regulators for DRP1 phosphorylation. In the present study, both olomoucine and roscovitine (CDK5 inhibitors) effectively ameliorated SE-induced astroglial apoptosis in the dentate gyrus without changed seizure susceptibility. In addition, they inhibited reactive astrogliosis in the CA1 region independent of neuronal death induced by SE. These effects of CDK5 inhibitors were relevant to abrogation of altered DRP1 phosphorylation ratio and mitochondrial length induced by SE. CDK5 inhibitors also negatively regulated protein kinase A (PKA) activity in astrocytes. Therefore, our findings suggest that CDK5 inhibitors may mitigate astroglial apoptosis and reactive astrogliosis accompanied by modulations of DRP1-mediated mitochondrial dynamics.

The Differential DRP1 Phosphorylation and Mitochondrial Dynamics in the Regional Specific Astroglial Death Induced by Status Epilepticus

The response and susceptibility to astroglial degenerations are relevant to the distinctive properties of astrocytes in a hemodynamic-independent manner following status epilepticus (SE). Since impaired mitochondrial fission plays an important role in mitosis, apoptosis and programmed necrosis, we investigated whether the unique pattern of mitochondrial dynamics is involved in the characteristics of astroglial death induced by SE. In the present study, SE induced astroglial apoptosis in the molecular layer of the dentate gyrus, accompanied by decreased mitochondrial length. In contrast, clasmatodendritic (autophagic) astrocytes in the CA1 region showed mitochondrial elongation induced by SE. Mdivi-1 (an inhibitor of mitochondrial fission) effectively attenuated astroglial apoptosis, but WY14643 (an enhancer of mitochondrial fission) aggravated it. In addition, Mdivi-1 accelerated clasmatodendritic changes in astrocytes. These regional specific mitochondrial dynamics in astrocytes were closely correlated with dynamin-related protein 1 (DRP1; a mitochondrial fission protein) phosphorylation, not optic atrophy 1 (OPA1; a mitochondrial fusion protein) expression. To the best of our knowledge, the present data demonstrate for the first time the novel role of DRP1-mediated mitochondrial fission in astroglial loss. Thus, the present findings suggest that the differential astroglial mitochondrial dynamics may participate in the distinct characteristics of astroglial death induced by SE.

Modulation of the chelatable Zn pool in the brain by diethyldithiocarbamate is associated with behavioral impairment in adult zebrafish

DEDTC leads to a buildup of DEDTC in the brain with consequent chelation of reactive Zn and behavioral impairment of zebrafish.

P2X7 receptor differentially modulates astroglial apoptosis and clasmatodendrosis in the rat brain following status epilepticus

Recently, it has been reported that astroglial loss/dysfunction plays a role in epileptogenesis. In addition, astroglial loss is accompanied by up-regulation of P2X7 receptor expression in microglia. Therefore, we investigated whether P2X7 receptor is involved in astroglial damages induced by status epilepticus (SE). In the present study, astroglial loss showed the regional-specific manner and the differential responses to P2X7 receptor functions. Both OxATP and brilliant blue G (P2X7 receptor antagonists) infusion prevented apoptotic astroglial loss in the molecular layer of the dentate gyrus and the frontoparietal cortex, while it promoted clasmatodendrosis in the CA1 region as compared to saline treatment. In contrast, BzATP (a P2X7 receptor agonist) treatment exacerbated apoptotic astroglial loss in the molecular layer of the dentate gyrus and the frontoparietal cortex, but alleviated SE-induced astroglial swelling in the CA1 region. Astroglial loss in the piriform cortex was not affected by P2X7 receptor agonist- or antagonist-infusion. These findings suggest that P2X7 receptor function differently modulates SE-induced astroglial loss in distinct brain regions.

Differential expressions of aquaporin subtypes in astroglia in the hippocampus of chronic epileptic rats

In order to elucidate the roles of aquaporins (AQPs) in astroglial responses, we investigated AQP expressions in the experimental epileptic hippocampus. In control animals, AQP1 protein expression was restricted to the ventricular-facing surface of the choroid plexus. AQP4 was expressed in astrocyte foot processes near blood vessels and in ependymal and pial surfaces in contact with cerebrospinal fluid. AQP9 protein has been detected in cells lining the cerebral ventricles, and in astrocytes. Six to eight weeks after status epilepticus (SE), AQP1 expression was mainly, but not all, detected in vacuolized astrocytes, which were localized in the stratum radiatum of the CA1 region. AQP4 was negligible in vacuolized CA1 astrocytes, although AQP4 immunoreactivity in non-vacuolized astrocytes was increased as compared to control level. AQP9 expression was shown to be mainly induced in non-vacuolized CA1 astrocytes. Therefore, our findings suggest that AQP subunits may play differential roles in various astroglial responses (including astroglial swelling and astroglial loss) in the chronic epileptic hippocampus.

Spatiotemporal characteristics of astroglial death in the rat hippocampo-entorhinal complex following pilocarpine-induced status epilepticus

Recently we reported that astroglial loss and subsequent gliogenesis in the dentate gyrus play a role in epileptogenesis following pilocarpine-induced status epilepticus (SE). In the present study we investigated whether astroglial damages in the hippocampo-entorhinal complex following SE are relevant to pathological or electrophysiological properties of temporal lobe epilepsy. Astroglial loss/damage was observed in the entorhinal cortex and the CA1 region at 4 weeks and 8 weeks after SE, respectively. These astroglial responses in the hippocampo-entorhinal cortex were accompanied by hyperexcitability of the CA1 region (impairment of paired-pulse inhibition and increase in excitability ratio). Unlike the dentate gyrus and the entorhinal cortex, CA1 astroglial damage was protected by conventional anti-epileptic drugs. alpha-Aminoadipic acid (a specific astroglial toxin) infusion into the entorhinal cortex induced astroglial damage and changed the electrophysiological properties in the CA1 region. Astroglial regeneration in the dentate gyrus and the stratum oriens of the CA1 region was found to originate from gliogenesis, while that in the entorhinal cortex and stratum radiatum of the CA1 region originated from in situ proliferation. These findings suggest that regional specific astroglial death/regeneration patterns may play an important role in the pathogenesis of temporal lobe epilepsy.

Zinc and 4-hydroxy-2-nonenal mediate lysosomal membrane permeabilization induced by H2O2 in cultured hippocampal neurons

Lysosomal membrane permeabilization (LMP) is implicated in cancer cell death. However, its role and mechanism of action in neuronal death remain to be established. In the present study, we investigate the function of cellular zinc in oxidative stress-induced LMP using hippocampal neurons. Live-cell confocal microscopy with FluoZin-3 fluorescence showed that H(2)O(2) exposure induced vesicles containing labile zinc in hippocampal neurons. Double staining with LysoTracker or MitoTracker disclosed that the majority of the zinc-containing vesicles were lysosomes and not mitochondria. H(2)O(2) additionally augmented the 4-hydroxy-2-nonenal (HNE) adduct level in lysosomes. Intracellular zinc chelation with TPEN [tetrakis(2-pyridylmethyl)ethylenediamine] completely blocked both HNE accumulation and neuronal death. Interestingly, within 1 h after the onset of H(2)O(2) exposure, some of zinc-loaded vesicles lost their zinc signals. Consistent with the characteristics of LMP, a lysosomal enzyme, cathepsin D, was released into the cytosol, and cathepsin inhibitors partially rescued neuronal death. We further examined the possibility that HNE or zinc mediates H(2)O(2)-triggered LMP. Similar to H(2)O(2), exposure to HNE or zinc triggered lysosomal zinc accumulation and LMP. Moreover, isolated lysosomes underwent LMP when exposed to HNE or zinc, but not H(2)O(2), supporting the direct mediation of LMP by HNE and/or zinc. The appearance of zinc-containing vesicles and the increases in levels of cathepsin D and HNE, were also observed in hippocampal neurons of rats after kainate seizures. Thus, under oxidative stress, neuronal lysosomes accumulate zinc and HNE, and eventually undergo LMP, which may constitute a key mechanism of oxidative neuronal death.

Astrocytes are a specific immunological target in Rasmussen's encephalitis

OBJECTIVE

The current histopathological criteria of Rasmussen's encephalitis (RE) include the presence of T-cell-dominated inflammation, microglial activation, neuronal loss, and astrocytic activation. An in vitro study, however, suggested glutamate receptor 3 (GluR3) antibody-mediated astrocytic loss. Therefore, we investigated astrocytic apoptosis and loss in situ.

METHODS

Histochemical, immunohistochemical, terminal deoxynucleotidyltransferase-mediated biotin-dUTP nick end labeling and in situ hybridization techniques were applied to paraffin sections of 20 RE cases, 6 healthy control subjects, and 6 paraneoplastic encephalomyelitis, 10 Ammon's horn sclerosis, and 5 focal cortical dysplasia cases.

RESULTS

Astrocytic apoptosis and subsequent loss of these cells is a specific feature of RE. Such lesions are not found in the control groups. In RE, astrocytic apoptosis and loss was present both in cortical and in white matter areas. Astrocytes in these tissues showed major histocompatibility complex class I expression. Furthermore, granzyme-B(+) lymphocytes were found in close apposition to astrocytes bordering astrocyte-deficient lesions. Granzyme-B(+) granules in these lymphocytes were polarized and faced the astrocytic membranes. No evidence was found for an antibody-mediated destruction.

INTERPRETATION

We suggest a specific attack by cytotoxic T lymphocytes as a possible mechanism responsible for astrocytic degeneration in RE. The loss of astrocytes might play a role in neuronal dysfunction, seizure induction, and enhancement of neuronal cell death.

Degeneration and proliferation of astrocytes in the mouse dentate gyrus after pilocarpine-induced status epilepticus

Astrocytes are relatively resistant to injury compared to neurons and oligodendrocytes. Here, we report transient region-specific loss of astrocytes in mice early after pilocarpine-induced status epilepticus (SE). In the dentate hilus, immunoreactivity for glial acidic fibrillary protein (GFAP) was decreased, and the number of healthy appearing GFAP- or S100beta-positive cells was significantly reduced (> or =65%) 1 and 3 days after pilocarpine-induced SE. Many remaining GFAP-positive cells were shrunken, and 1 day after SE electron microscopy revealed numerous electron-dense degenerating astrocyte processes and degenerating glial somata in the hilus. Degeneration of GFAP-expressing cells may be linked to hilar neuronal death, because we did not observe loss of astrocytes after kainate-induced SE, after which hilar neurons remained intact. Ten days after SE, hilar GFAP immunoreactivity had returned, partially from GFAP-positive cells in the hilus. Unlike control mice, many GFAP-positive hilar processes originated from cell bodies located in the subgranular zone (SGZ). To investigate whether proliferation contributes to hilar repopulation, we injected 5-bromo-2'-deoxyuridine (BrdU) 3 days after SE. Five hours later and up to 31 days after SE, many BrdU/GFAP colabeled cells were found in the hilus and the SGZ, some with hilar processes, indicating that proliferation in both areas contributes to generation of hilar astrocytes and astrocyte processes. In contrast to pilocarpine-induced SE in mice, astrocyte degeneration was not found after pilocarpine-induced SE in rats. These findings demonstrate astrocyte degeneration in the mouse dentate hilus specifically in the mouse pilocarpine epilepsy model, followed by astrogenesis leading to hilar repopulation.