Contribution of a mitochondrial pathway to excitotoxic neuronal necrosis

In the fast lane: Receptor trafficking during status epilepticus

Status epilepticus (SE) remains a significant cause of morbidity and mortality and often is refractory to standard first-line treatments. A rapid loss of synaptic inhibition and development of pharmacoresistance to benzodiazepines (BZDs) occurs early during SE, while NMDA and AMPA receptor antagonists remain effective treatments after BZDs have failed. Multimodal and subunit-selective receptor trafficking within minutes to an hour of SE involves GABA-A, NMDA, and AMPA receptors and contributes to shifts in the number and subunit composition of surface receptors with differential impacts on the physiology, pharmacology, and strength of GABAergic and glutamatergic currents at synaptic and extrasynaptic sites. During the first hour of SE, synaptic GABA-A receptors containing γ2 subunits move to the cell interior while extrasynaptic GABA-A receptors with δ subunits are preserved. Conversely, NMDA receptors containing N2B subunits are increased at synaptic and extrasynaptic sites, and homomeric GluA1 ("GluA2-lacking") calcium permeant AMPA receptor surface expression also is increased. Molecular mechanisms, largely driven by NMDA receptor or calcium permeant AMPA receptor activation early during circuit hyperactivity, regulate subunit-specific interactions with proteins involved with synaptic scaffolding, adaptin-AP2/clathrin-dependent endocytosis, endoplasmic reticulum (ER) retention, and endosomal recycling. Reviewed here is how SE-induced shifts in receptor subunit composition and surface representation increase the excitatory to inhibitory imbalance that sustains seizures and fuels excitotoxicity contributing to chronic sequela such as "spontaneous recurrent seizures" (SRS). A role for early multimodal therapy is suggested both for treatment of SE and for prevention of long-term comorbidities.

Availability of neuregulin-1beta1 protects neurons in spinal cord injury and against glutamate toxicity through caspase dependent and independent mechanisms

Spinal cord injury (SCI) causes sensorimotor and autonomic impairment that partly reflects extensive, permanent loss of neurons at the epicenter and penumbra of the injury. Strategies aimed at enhancing neuronal protection are critical to attenuate neurodegeneration and improve neurological recovery after SCI. In rat SCI, we previously uncovered that the tissue levels of neuregulin-1beta 1 (Nrg-1β1) are acutely and persistently downregulated in the injured spinal cord. Nrg-1β1 is well-known for its critical roles in the development, maintenance and physiology of neurons and glia in the developing and adult spinal cord. However, despite this pivotal role, Nrg-1β1 specific effects and mechanisms of action on neuronal injury remain largely unknown in SCI. In the present study, using a clinically-relevant model of compressive/contusive SCI in rats and an in vitro model of glutamate toxicity in primary neurons, we demonstrate Nrg-1β1 provides early neuroprotection through attenuation of reactive oxygen species, lipid peroxidation, necrosis and apoptosis in acute and subacute stages of SCI. Mechanistically, availability of Nrg-1β1 following glutamate challenge protects neurons from caspase-dependent and independent cell death that is mediated by modulation of mitochondria associated apoptotic cascades and MAP kinase and AKT signaling pathways. Altogether, our work provides novel insights into the role and mechanisms of Nrg-1β1 in neuronal injury after SCI and introduces its potential as a new neuroprotective target for this debilitating neurological condition.

CDDO-Me Selectively Attenuates CA1 Neuronal Death Induced by Status Epilepticus via Facilitating Mitochondrial Fission Independent of LONP1

2-Cyano-3,12-dioxo-oleana-1,9(11)-dien-28-oic acid methyl ester (CDDO-Me) is a triterpenoid analogue of oleanolic acid that exhibits promising anti-cancer, anti-inflammatory, antioxidant and neuroprotective activities. In addition, CDDO-Me affects cellular differentiation and cell cycle arrest, and irreversibly inhibits Lon protease-1 (LONP1). In the present study, we evaluate the effects of CDDO-Me on mitochondrial dynamics and its downstream effectors in order to understand the underlying mechanism of the neuronal death following status epilepticus (SE, a prolonged seizure activity). CDDO-Me increased dynamin-related proteins 1 (DRP1)-serine 616 phosphorylation via activating extracellular-signal-regulated kinase 1/2 (ERK1/2) and c-Jun N-terminal kinase (JNK), but not protein kinase A (PKA) or protein phosphatases (PPs). In addition, CDDO-Me facilitated DRP1-mediated mitochondrial fissions, which selectively attenuated SE-induced CA1 neuronal death. Unlike CDDO-Me, LONP1 knockdown led to SE-induced massive degeneration of dentate granule cells, CA1 neurons and hilus interneurons without altering the expression and phosphorylation of DRP1, ERK1/2, JNK and PP2B. LONP1 knockdown could not inhibit SE-induced mitochondrial elongation in CA1 neurons. Co-treatment of CDDO-Me with LONP1 siRNA ameliorated only CA1 neuronal death, concomitant with abrogation of mitochondrial elongation induced by SE. Thus, our findings suggest that CDDO-Me may selectively attenuate SE-induced CA1 neuronal death by rescuing the abnormal mitochondrial machinery, independent of LONP1 activity.

Simultaneous triple therapy for the treatment of status epilepticus

Early maladaptive internalization of synaptic GABA_A receptors (GABA_AR) and externalization of NMDA receptors (NMDAR) may explain the time-dependent loss of potency of standard anti-epileptic drugs (AED) in refractory status epilepticus (SE). We hypothesized that correcting the effects of changes in GABA_AR and NMDAR would terminate SE, even when treatment is delayed 40 minutes. SE was induced in adult Sprague-Dawley rats with a high dose of lithium and pilocarpine. The GABA_AR agonist midazolam, the NMDAR antagonist ketamine and the AED valproate were injected 40 min after SE onset in combination or as monotherapy. The midazolam-ketamine-valproate combination was more efficient than triple-dose midazolam, ketamine or valproate monotherapy or higher-dose dual therapy in reducing several parameters of SE severity. Triple therapy also reduced SE-induced acute neuronal injury and spatial memory deficits. In addition, simultaneous triple therapy was more efficient than sequential triple therapy: giving the three drugs simultaneously was more efficient at stopping seizures than the standard practice of giving them sequentially. Furthermore, midazolam-ketamine-valproate therapy suppressed seizures far better than the midazolam-fosphenytoin-valproate therapy, which follows evidence-based AES guidelines. These results show that a treatment aimed at correcting maladaptive GABA_AR and NMDAR trafficking can reduce the severity of SE and its long-term consequences.

Down-regulation of Pin1 in Temporal Lobe Epilepsy Patients and Mouse Model

Peptidyl-prolyl cis-trans isomerase NIMA-interacting 1 (Pin1) is a unique PPIase belonging to the parvulin family, and it isomerizes peptide bond between phospho-(Ser/Thr) and Pro. Pin1 has been linked to the pathogenesis of various human diseases; however, its exact biological functions remain unclear. The aim of the present study is to explore the expression pattern of Pin1 in patients with refractory epilepsy and in a chronic pilocarpine-induced epileptic mouse model. Using Western blot, immunofluorescence and immunoprecipitation analysis, we found that Pin1 protein was mainly distributed in neurons, demonstrated by colocalization with the dendritic marker, MAP2. However, the expression of Pin1 decreased remarkably in epileptic patients and experimental mice. Furthermore, the reciprocal coimmunoprecipitation analysis showed that Pin1 interacted with NR2A and NR2B-containing NMDA receptors not AMPA receptors in epileptic mouse models. Our results are the first to indicate that the expression of Pin1 in epileptic brain tissue could play important roles in epilepsy.

NR4A1 Knockdown Suppresses Seizure Activity by Regulating Surface Expression of NR2B

Nuclear receptor subfamily 4 group A member 1 (NR4A1), a downstream target of CREB that is a key regulator of epileptogenesis, has been implicated in a variety of biological processes and was previously identified as a seizure-associated molecule. However, the relationship between NR4A1 and epileptogenesis remains unclear. Here, we showed that NR4A1 protein was predominantly expressed in neurons and up-regulated in patients with epilepsy as well as pilocarpine-induced mouse epileptic models. NR4A1 knockdown by lentivirus transfection (lenti-shNR4A1) alleviated seizure severity and prolonged onset latency in mouse models. Moreover, reciprocal coimmunoprecipitation of NR4A1 and NR2B demonstrated their interaction. Furthermore, the expression of p-NR2B (Tyr1472) in epileptic mice and the expression of NR2B in the postsynaptic density (PSD) were significantly reduced in the lenti-shNR4A1 group, indicating that NR4A1 knockdown partly decreased surface NR2B by promoting NR2B internalization. These results are the first to indicate that the expression of NR4A1 in epileptic brain tissues may provide new insights into the molecular mechanisms underlying epilepsy.

Rapid surface accumulation of NMDA receptors increases glutamatergic excitation during status epilepticus

After 1h of lithium-pilocarpine status epilepticus (SE), immunocytochemical labeling of NMDA receptor NR1 subunits reveals relocation of subunits from the interior to the cell surface of dentate gyrus granule cells and CA3 pyramidal cells. Simultaneously, an increase in NMDA-miniature excitatory postsynaptic currents (mEPSC) as well as an increase in NMDA receptor-mediated tonic currents is observed in hippocampal slices after SE. Mean-variance analysis of NMDA-mEPSCs estimates that the number of functional postsynaptic NMDA receptors per synapse increases 38% during SE, and antagonism by ifenprodil suggests that an increase in the surface representation of NR2B-containing NMDA receptors is responsible for the augmentation of both the phasic and tonic excitatory currents with SE. These results provide a potential mechanism for an enhancement of glutamatergic excitation that maintains SE and may contribute to excitotoxic injury during SE. Therapies that directly antagonize NMDA receptors may be a useful therapeutic strategy during refractory SE.

Pranlukast attenuates hydrogen peroxide-induced necrosis in endothelial cells by inhibiting oxygen reactive species-mediated collapse of mitochondrial membrane potential

OBJECTIVE

Recently, we reported that pranlukast, an antagonist of cysteinyl leukotriene receptor 1, attenuates ischemic injury in endothelial cells by decreasing reactive oxygen species (ROS) production and inhibiting nuclear factor-κB activation in a leukotriene-independent manner. In this study, we investigated the effect of pranlukast on oxidative stress injury induced by hydrogen peroxide (H2O2) in EA.hy926 cells, a human endothelial cell line, and the possible mechanisms.

METHODS AND RESULTS

We found that H2O2 reduced cell viability and increased lactate dehydrogenase release in a concentration- and time-dependent manner. Necrosis was the main death mode, and the necrotic rate increased 32% after exposure to 220 μM H2O2 for 4 hours. Pretreatment with pranlukast significantly ameliorated the reduced viability and the increased lactate dehydrogenase release and necrosis after exposure to H2O2. We next examined the mechanisms underlying the antinecrotic effects of pranlukast. The results showed that pranlukast attenuated excessive ROS production and ameliorated the reduced superoxide dismuase and glutathione peroxidase activity in EA.hy926 cells exposed to H2O2. Pranlukast also inhibited the collapse of mitochondrial membrane potential (MMP) induced by H2O2. Inhibition of ROS production by N-acetyl-l-cysteine, a powerful antioxidant, reduced MMP collapse and necrosis. Inhibition of MMP collapse by cyclosporine A, a mitochondrial permeability transition inhibitor, attenuated necrosis but failed to reduce ROS production. In addition, we found no expression of 5-lipoxygenase in EA.hy926 cells and zileuton, a 5-lipoxygenase inhibitor, did not affect the cellular injury induced by H2O2.

CONCLUSION

Pranlukast protects endothelial cells from H2O2-induced necrosis by inhibiting ROS-mediated collapse of mitochondrial membrane potential, and this is leukotriene-independent.

Distinct caspase pathways mediate necrosis and apoptosis in subpopulations of hippocampal neurons after status epilepticus

Status epilepticus in the immature brain induces neuronal injury in the hippocampal formation, but the mode and mechanism of death are poorly understood. Our laboratory has recently investigated the role of caspase-3, -8, and -9 in neuronal injury, using a lithium-pilocarpine model of status epilepticus in 2-week-old rat pups. Our results showed that dying neurons in the dentate gyrus and CA1-subiculum area do not share the same mechanism of death. In CA1-subiculum, caspase-8 upregulation preceded caspase-3 activation in morphologically necrotic neurons. The pan-caspase inhibitor Q-VD-OPH reduced CA1 damage, showing that caspases contribute to status epilepticus-induced necrosis. In the dentate gyrus, dying neurons were caspase-9 and -3 immunoreactive and morphologically apoptotic. It is not clear why the same seizures cause different types of cell death in neurons that are connected in series along the same hippocampal circuit, but the apoptotic dentate neurons express doublecortin, and do not express calbindin-D28k, suggesting that their immaturity may be a factor in producing an apoptotic mode of death.

Vulnerability of postnatal hippocampal neurons to seizures varies regionally with their maturational stage

The mechanism of status epilepticus-induced neuronal death in the immature brain is not fully understood. In the present study, we examined the contribution of caspases in our lithium-pilocarpine model of status epilepticus in 14 days old rat pups. In CA1, upregulation of caspase-8, but not caspase-9, preceded caspase-3 activation in morphologically necrotic cells. Pretreatment with a pan-caspase inhibitor provided neuroprotection, showing that caspase activation was not an epiphenomenon but contributed to neuronal necrosis. By contrast, upregulation of active caspase-9 and caspase-3, but not caspase-8, was detected in apoptotic dentate gyrus neurons, which were immunoreactive for doublecortin and calbindin-negative, two features of immature neurons. These results suggest that, in cells which are aligned in series as parts of the same excitatory hippocampal circuit, the same seizures induce neuronal death through different mechanisms. The regional level of neuronal maturity may be a determining factor in the execution of a specific death program.