Seong-Il Yeo

Astroglial loss and edema formation in the rat piriform cortex and hippocampus following pilocarpine‐induced status epilepticus

In the present study we analyzed aquaporin‐4 (AQP4) immunoreactivity in the piriform cortex (PC) and the hippocampus of pilocarpine‐induced rat epilepsy model to elucidate the roles of AQP4 in brain edema following status epilepticus (SE). In non‐SE‐induced animals, AQP4 immunoreactivity was diffusely detected in the PC and the hippocampus. AQP4 immunoreactivity was mainly observed in the endfeet of astrocytes. Following SE the AQP4‐deleted area was clearly detected in the PC, not in the hippocampus. Decreases in dystrophin and α‐syntrophin immunoreactivities were followed by reduction in AQP4 immunoreactivity. These alterations were accompanied by the development of vasogenic edema and the astroglial loss in the PC. In addition, acetazolamide (an AQP4 inhibitor) treatment exacerbated vasogenic edema and astroglial loss both in the PC and in the hippocampus. These findings suggest that SE may induce impairments of astroglial AQP4 functions via disruption of the dystrophin/α‐syntrophin complex that worsen vasogenic edema. Subsequently, vasogenic edema results in extensive astroglial loss that may aggravate vasogenic edema. J. Comp. Neurol. 518:4612–4628, 2010.

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.

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.

p65/RelA-Ser529 NF-κB Subunit Phosphorylation Induces Autophagic Astroglial Death (Clasmatodendrosis) Following Status Epilepticus

Clasmatodendrosis is an irreversible astroglial degenerative change, which includes extensive swelling and vacuolization of cell bodies and disintegrated and beaded processes. This study was designed to elucidate whether clasmatodendrosis may be one of the autophagy-related degeneration of astrocytes. In this study, clasmatodendritic astrocytes were observed only in the stratum radiatum in the CA1 region. Vacuoles in clasmatodendritic astrocytes showed LAMP-1 immunoreactivity. In addition, both LC3-II and Beclin-1 expression were detected in most of clasmatodendritic astrocytes as well as a few non-vacuolized astrocytes. Clasmatodendritic astrocytes also showed p65/RelA-Ser529 phosphorylation in the nuclei. The neutralization of TNF-α by sTNFp55R infusion reduced clasmatodendritic astrocytes with nuclear p65/RelA-Ser529 phosphorylation. Therefore, these findings suggest that clasmatodendrosis may be autophagic astroglial death in response to epileptic seizures through TNF-α-mediated p65/RelA-Ser529 phosphorylation.

Up-regulation of endothelial endothelin-1 expression prior to vasogenic edema formation in the rat piriform cortex following status epilepticus.

Endothelin-1 (ET-1) is one of potential factors to induce vasogenic edema formation, since exogenous ET-1 treatment decreases aquaporin 4 (AQP4) expression and increases chemokines induction. To identify the role of endogenous ET-1 in vasogenic edema formation, we examined the correlation between endogenous ET-1 expression and vasogenic edema formation in the pirifom cortex following status epilepticus (SE). In the present study, SMI-71 (a brain-blood barrier marker) immunoreactivity was significantly reduced in blood vessels at 1 day after SE when vasogenic edema and neuronal damage were observed. ET-1 expression was up-regulated in endothelial cells prior to reduction in SMI-71 immunoreactivity. Furthermore, ET-1 expressing endothelial cells showed the absence of SMI-71 immunoreactivity. Increase in ET-1 expression was followed by reduced AQP4 immunoreactivity prior to vasogenic edema formation. Only a few microglia showed monocyte chemotactic protein-1 (a chemokine induced by ET-1) outside vasogenic edema lesion. Taken together, our findings suggest that endothelial ET-1 expression may contribute to SE-induced vasogenic edema formation via brain-blood barrier disruption at AQP4/MCP-1 independent manners.

ReLA/P65-serine 536 nuclear factor-kappa B phosphorylation is related to vulnerability to status epilepticus in the rat hippocampus.

Although nuclear factor-kappa B (NF-κB) is essential for neuron survival and its activation may protect neuron against oxidative-stresses or ischemia-induced neurodegeneration, NF-κB activation can contribute to inflammatory reaction and apoptotic cell death after brain injury and stroke. However, there are little data concerning the specific pattern of NF-κB phosphorylations in neuronal damage/survival induced by status epilepticus (SE). In the present study, NF-κB phosphorylation showed the cellular specific pattern in responses to SE. p52-S865, p52-Ser869, p65-Ser276, p65-Ser311, p65-Ser468, and p65-Ser529 NF-κB phosphorylation was significantly decreased in the CA1 and CA3 pyramidal cells vulnerable to SE, although neuronal specific nuclear antigen immunoreactivity was strongly detected. In contrast, p65-Ser536 NF-κB phosphorylation was enhanced in these neurons accompanied by TUNEL- and Fluoro-Jade B 244signals. These findings serve as the first comprehensive description of the cellular specific distribution of NF-κB phosphorylation in response to pilocarpine-induced SE in the rat hippocampus, and suggest that enhancement in p65-Ser536 NF-κB phosphorylation may be closely relevant to neuronal vulnerability to SE, while others may be involved in neuronal survival.

The roles of fractalkine/CX3CR1 system in neuronal death following pilocarpine-induced status epilepticus

Although fractalkine is one of chemokines involved in mediation of neuronal/microglial interaction, it is not known whether fractalkine/CX3CR1-mediated pathogenesis occurs in the rat brain following epileptogenic insults. In order to elucidate the roles of the fractalkine/CX3CR1 system in microglial activation and neurodegeneration induced by status epilepticus (SE), we investigated changes in fractalkine/CX3CR1 system within the rat hippocampus following SE. In non-SE induced animals, fractalkine and CX3CR1 immunoreactivity was detected in neurons and microglia, respectively. Following SE, fractalkine immunoreactivity was transiently increased in neurons and astrocytes. CX3CR1 immunoreactivity was also transiently detected in neurons (particularly in CA1 pyramidal cells). Intracerebroventricular infusions of recombinant rat fractalkine aggravated SE-induced neuronal damage, while fractalkine IgG or CX3CR1 IgG infusion alleviated it, compared to saline-infused animals. These findings suggest that fractalkine/CX3CR1 system may play an important role in SE-induced neuronal damages via neuron-microglial interactions.

Pyridoxal-5′-phosphate phosphatase/chronophin inhibits long-term potentiation induction in the rat dentate gyrus

Pyridoxal‐5′‐phosphate (PLP)‐phosphatase/chronophin (PLPP/CIN) directly dephosphorylates actin‐depolymerizing factor (ADF)/cofilin as well as PLP. Although PLPP/CIN plays a role in the regulation of F‐actin and vitamin B6 metabolism, there is no direct evidence to support a correlation between PLPP/CIN and F‐actin polymerization during long‐term potentiation (LTP) induction. In this study, we investigated whether the expression of PLPP/CIN is altered following LTP induction, and whether Tat‐PLPP/CIN transduction affects LTP induction in the rat dentate gyrus (DG). PLPP/CIN immunoreactivity was markedly decreased in dentate granule cells after the induction of LTP. Tat‐PLPP/CIN transduction (20 and 200 μg/kg) decreased the efficiency of high frequency stimulus‐induced potentiation of populations spike amplitude as compared to saline or Tat‐protein‐treated animals. The PLPP/CIN protein level showed an inverse correlation with phosphorylated ADF/cofilin levels and F‐actin content. These findings suggest that PLPP/CIN‐mediated actin dynamics may play an important role in the changes of morphological properties (dendritic spine reorganization) of the hippocampus in LTP.

F-actin depolymerization accelerates clasmatodendrosis via activation of lysosome-derived autophagic astroglial death.

Clasmatodendrosis is an irreversible astroglial degenerative change, which includes extensive swelling and vacuolization of cell bodies and disintegrated and beaded processes. Since alteration in F-actin level influences on the formation of vacuoles/vesicles during exocytosis/endocytosis in astrocytes, we investigated whether F-actin polymerization involves clasmatodendrosis in the rat hippocampus following status epilepticus (SE). In the present study, vacuoles in clasmatodendrotic astrocytes showed LAMP-1 and LC3-II (a marker for autophagy) immunoreactivity. These findings reveal that clasmatodendrosis may be lysosome-derived autophagic astroglial death. Jasplakinolide (an F-actin stabilizer) infusion significantly decreased the size and the number of medium/large-sized vacuoles in each clasmatodendritic astrocyte accompanied by enhancement of phalloidin signals, as compared to vehicle-infusion. In contrast, latrunculin A (an F-actin-depolymerizing agent) infusion increased the size and the number of medium/large-sized vacuoles, which were dissociated adjacent to cell membrane. Therefore, our findings suggest that F-actin stabilization may inhibit lysosome-derived autophagic astroglial death during clasmatodendrosis.

Potential Roles of D-Serine and Serine Racemase in Experimental Temporal Lobe Epilepsy

To confirm the roles of D‐serinergic gliotransmission in epilepsy, we investigated the relationship between spatiotemporally specific glial responses and the D‐serine/serine racemase system in mesial temporal structures following status epilepticus (SE). In control animals, D‐serine and serine racemase immunoreactivities were detected mainly in astrocytes. After SE, D‐serine and serine racemase immunoreactivities were increased in astrocytes. Double‐immunofluorescence study revealed that up‐regulation of serine racemase immunoreactivity was relevant not to D‐serine immunoreactivity but to nestin or vimentin immunoreactivity. Neither D‐serine nor serine racemase was found in naïve or reactive microglia. In addition, phosphorylated N‐methyl‐D‐aspartate (NMDA) receptor subunit 1 (pNR1‐Ser896) immunoreactivity in the hippocampus was increased compared with controls. Increased D‐serine immunoreactivity showed direct correlation with the phosphorylation of Ser896 of NR1. Given the findings of our previous study, these findings suggest that D‐serine and serine racemase in astrocytes may play roles in neuronal hyperexcitability via a cooperative activation of NMDA receptors. Furthermore, serine racemase may be involved in migration and differentiation of immature astrocytes, which is relevant to reactive astrogliosis.