Yuxiang Han

Role of mitochondrial fission in neuronal injury in pilocarpine-induced epileptic rats

Mitochondrial fission has been reported to be involved in oxidative stress, apoptosis and many neurological diseases. However, the role of mitochondrial fission in seizures, which could induce oxidative stress and neuronal loss, remains unknown. In this study, we used pilocarpine to elicit seizures in rats. Meanwhile, we used mitochondrial division inhibitor 1 (mdivi-1), a selective inhibitor of mitochondrial fission protein dynamin-related protein1 (Drp1), to suppress mitochondrial fission in epileptic model of rats in vivo. We found that mitochondrial fission was increased after seizures and the inhibition of mitochondrial fission by mdivi-1 significantly attenuated oxidative stress and reduced neuronal loss after seizures, shown by the decreased 8-hydroxy deoxyguanosine (8-oHdG) content, the increased superoxide dismutase (SOD) activity, the reduced expression of cytochrome c and caspase3 and the increased surviving neurons in the hippocampus. These results indicated that mitochondrial fission is up-regulated after seizures and the inhibition of mitochondrial fission is protective against neuronal injury in seizures, the underlying mechanism may be through the mitochondria/reactive oxygen species (ROS)/cytochrome c pathway.

Mitochondrial DNA damage and the involvement of antioxidant defense and repair system in hippocampi of rats with chronic seizures.

In this study, we demonstrated a decreased level of mitochondrial DNA (mtDNA) with a large number of oxidized bases in hippocampi of rats with epilepsy induced by pilocarpine. In order to verify the underlying mechanism of mtDNA impairment, we detected the response of antioxidant defense system and mitochondrial base excision repair (mtBER) pathway. Superoxide dismutase2 (SOD-2) and glutathione (GSH) were significantly decreased in the experimental group, manifesting a decreased capacity of scavenging free radicals. Mitochondrial base excision repair (mtBER) pathway, which is the main repair pathway for the removal of oxidative base modifications, displayed unbalanced expression in epileptic group. DNA polymeraseγ (polγ) increased, while apurinic/apyrimidinic endonuclease (APE1), one of mtBER initiators, decreased in mitochondria in the chronic phase of epileptogenesis. In conclusion, mtDNA was impaired during chronic recurrent seizures, whereas the endogenous antioxidants and the mtBER pathway failed to respond to the elevated mtDNA damage.

Role of PI3K/Akt in diazoxide preconditioning against rat hippocampal neuronal death in pilocarpine-induced seizures.

Diazoxide (DZ), a highly selective opener of the mitochondrial ATP-sensitive potassium (mitoK(ATP)) channel, has neuroprotective effects. However, the mechanism of DZ protecting hippocampal neurons against cell death in pilocarpine-induced seizures is unknown. In this study, we investigated DZ attenuating neuronal loss caused by pilocarpine-induced seizures in rat hippocampus. DZ inhibited seizure-induced change in phospho-Akt expression, translocation of apoptosis-inducing factor (AIF), release of cytochrome c (CytC) and caspase-3 activation, which could be abolished by preincubation with 5-hydroxydecanoic acid, an inhibitor of mitoK(ATP). In addition, wortmannin, an inhibitor of phosphatidylinositol-3-kinase (PI3K), attenuated the translocation of AIF, CytC release and caspase-3 activation after seizures. DZ could reduce neuronal death induced by seizures in hippocampus by suppressing the translocation of AIF, CytC release and the activation of caspase-3 via the PI3K/Akt pathway.

Poly(ADP-ribose) polymerase inhibition protects epileptic hippocampal neurons from apoptosis via suppressing Akt-mediated apoptosis-inducing factor translocation in vitro

Inhibition of poly(ADP-ribose) polymerase (PARP) has been proposed to have a neuroprotective effect on hippocampal neurons in animal models of epilepsy. However, the mechanisms of PARP-mediated epileptic neuron apoptosis in vitro are still not thoroughly understood. Therefore, we investigated the effect of PARP inhibition and the underlying mechanisms in the hippocampal neuronal culture model of acquired epilepsy which is generally accepted as the neuronal culture model of spontaneous seizure discharge in vitro. As a result, PARP was activated and the administration of 3,4-dihydro-5-[4-(1-piperidinyl)butoxy]-1(2H)-isoquinolinone (DPQ), an inhibitor of PARP, significantly decreased the percentage of neuron apoptosis induced by Mg(2+)-free treatment. Western blot and confocal laser scanning microscopy (CLSM) analysis showed that DPQ increased the phosphorylation of Akt and attenuated mitochondria-nucleus translocation of the apoptosis-inducing factor (AIF). Furthermore, wortmannin, an inhibitor of PI-3K, inhibited the translocation of AIF to the nucleus. The results of the present study demonstrated that the inhibition of PARP might have therapeutic value in seizure-induced hippocampal neuron damage in vitro via suppressing Akt-mediated AIF translocation.

Adenosine monophosphate-activated protein kinase and peroxisome proliferator-activated receptor gamma coactivator 1α signaling provides neuroprotection in status epilepticus in rats.

Status epilepticus (SE) can cause severe neuronal loss and oxidative damage. Peroxisome proliferator-activated receptor gamma coactivator 1α (PGC-1α) can be neuroprotective by inducing the antioxidant system, so we evaluated the role of PGC-1α in SE. The expression of PGC-1α and one of its target genes, uncoupling protein 2 (UCP2), was upregulated after SE, which may represent an endogenous neuroprotective mechanism. Furthermore, pretreatment with an adenosine monophosphate-activated protein kinase (AMPK) inhibitor significantly attenuated both AMPK and PGC-1α activation, followed by downregulation of UCP2 and enhanced oxidative stress and hippocampal neuronal damage. AMPK/PGC-1α may be neuroprotective in SE-induced brain damage, at least in part via UCP2.

Mitochondrial base excision repair pathway failed to respond to status epilepticus induced by pilocarpine

Oxidative damage to mitochondrial DNA (mtDNA) has been implicated as an important mechanism underlying mitochondrial deficiency in epileptic seizures. In focusing on the role of the DNA repair pathway, we determined the response of the mitochondrial base excision repair (mtBER) pathway in pilocarpine-induced status epilepticus (SE) in hippocampi of male Wistar rats. The expression of 8-oxoguanine DNA glycosylase (OGG1) and polymerase gamma (polgamma) was decreased at both the cellular mRNA and mitochondrial protein levels at 3, 9 and 25h after the onset of SE. The mRNA and protein levels of APE1 were maintained, but the mitochondrial protein level decreased at 3 and 9h and recovered at 25h. Therefore, the mtBER pathway failed to respond to SE induced by pilocarpine. The failure of mitochondrial import might be an important factor responsible for the lowered mtBER enzymes in mitochondria. We hypothesize that the down-regulation of mtBER enzymes may aggravate mtDNA damage and mitochondrial deficiency after the onset of SE.

Alpha2-adrenergic receptors in spiral ganglion neurons may mediate protective effects of brimonidine and yohimbine against glutamate and hydrogen peroxide toxicity

Brimonidine, an alpha2-adrenergic receptor (α(2)-AR) agonist, is thought to be neuroprotective in some types of neurons via the activation of α(2)-AR. However, it is still unknown whether the α(2)-ARs exist in cochlear spiral ganglion neurons (SGNs). The authors aimed to demonstrate the presence and localization of α(2)-ARs in rat-cultured SGNs and to investigate the effect of brimonidine on glutamate- and hydrogen peroxide (H(2)O(2))-induced damage in the primary-cultured rat SGNs. The expression of α(2)-ARs was determined by reverse transcription-polymerase chain reaction, Western blot analysis and immunofluorescence. Then SGNs were exposed to glutamate or H(2)O(2) respectively with or without brimonidine. Cell viability was measured by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide assay. Apoptosis was determined by acridine orange and Hoechst 33342/propidium iodide double staining. The protein expressions of α(2)-ARs, Bax, Bcl-2, Caspase-9, Caspase-3, p-ERK1/2, iNOS, and artemin were determined by Western blot respectively. The cell viability was markedly reduced after exposure of glutamate (1mM) or H(2)O(2) (300 μM) to SGNs. Treatment with brimonidine protected SGNs against glutamate- or H(2)O(2)-induced cell damage, enhanced SGNs survival, decreased the elevation of Bax, Caspase-9, Caspase-3, p-ERK1/2, and artemin triggered by glutamate or H(2)O(2), and altered the expressions of Bcl-2 and iNOS. These protective effects of brimonidine can be reversed by yohimbine. Overall, the study describes the localization of α(2)-ARs in rat-cultured SGNs and indicates that brimonidine, which may work directly via interaction with α(2)-ARs, attenuates glutamate- and H(2)O(2)-induced damage in SGNs by Caspase-dependent modes as well as Caspase-independent modes.

Status epilepticus stimulates peroxisome proliferator-activated receptor γ coactivator 1-α/mitochondrial antioxidant system pathway by a nitric oxide-dependent mechanism.

Peroxisome proliferator-activated receptor (PPAR) γ coactivator 1-α (PGC-1α) is a transcriptional coactivator identified as an upstream regulator of lipid catabolism, mitochondrial number and function. PGC-1α protects neurons against oxidative damage by inducing several members of the mitochondrial antioxidant system such as superoxide dismutase 2 (SOD2) and uncoupling protein 2 (UCP2). Its role in seizure-induced oxidative stress has not been studied. Here we showed that pilocarpine-induced status epilepticus (SE) stimulates the PGC-1α/mitochondrial antioxidant system signaling pathway in the rat hippocampus. Because nitric oxide (NO) is the key factor of mitochondrial biogenesis through the transcriptional induction of PGC-1α, we investigated whether NO is involved in activation of the PGC-1α/mitochondrial antioxidant system after SE. Treatment with the NO synthase (NOS) inhibitor N(G)-nitro-l-argininemethyl ester (l-NAME) attenuated the increased expression of the PGC-1α/mitochondrial antioxidant system after SE and enhanced oxidative stress. These results suggest that SE can induce the PGC-1α/mitochondrial antioxidant system signaling pathway, which may represent a protective mechanism against SE-induced oxidative stress. Furthermore, NO may positively regulate the mitochondrial antioxidant system by inducing PGC-1α in pilocarpine-induced SE.

Impaired mitochondrial biogenesis in hippocampi of rats with chronic seizures.

Mitochondrial dysfunction has been suggested to be a contributing factor of epilepsy, but the underlying mechanisms are not completely explored. Mitochondrial biogenesis is involved in regulation of mitochondrial content, morphology, and function. In the current study, we show mitochondrial biogenesis severely impaired in hippocampi of rats with chronic seizures induced by pilocarpine, as evidenced by decreased mitochondrial DNA (mtDNA) content and decreased mtDNA-encoded protein level. Furthermore, we show mtDNA transcription and replication reduced in rats with chronic seizures. These defects were independent of downregulation of mitochondrial biogenesis-related factors, such as peroxisome proliferator-activated receptor gamma coactivator-1α, nuclear respiratory factor-1, and mitochondrial transcription factor A (Tfam), but depended on reduced Tfam-DNA binding activity. The present study suggests novel mechanisms for mitochondrial dysfunction during chronic seizures.

Subtypes evaluation of motor dysfunction in Parkinson’s disease using neuromelanin-sensitive magnetic resonance imaging

Parkinsons disease (PD) is characterized by the loss of neuromelanin (NM)-containing neurons in the substantia nigra pars compacta (SNc), and it is divided into two motor subtypes: the postural instability gait difficulty (PIGD) and the tremor dominant (TD) subtypes. With NM-sensitive Magnetic Resonance Imaging (NM-MRI), investigators have been able to accurately detect signal attenuation in SNc of PD; however, the difference of NM loss between PIGD and TD subtypes is still unclear. Thus, the aim of this study was to evaluate the differences in NM-MRI between PD motor subtypes. PD patients were classified into PIGD (n=14) and TD groups (n=9); 20 age and sex matched controls were recruited. We compared the signal intensity contrast ratios in medial and lateral regions of the SNc using NM-MRI in PIGD, TD, and controls, respectively. Remarkable signal attenuation was observed in the lateral part of SNc in PD when compared with the controls, and we were able to detect more severe signal attenuation in the medial part of SNc in PIGD patients in comparison with that in the TD group. Also, the medial part of SNc, ipsilateral to the most clinically affected side, showed the highest power to discriminate the PD motor subtypes (AUC, 81%; sensitivity, 71.4%; specificity, 77.8%). Our results indicated a potential diagnostic value of NM-MRI to discriminate the PD motor subtypes, providing new evidence for the neuropathology-based differences between the two subtypes.