Oxidative Stress as a Potential Mechanism Underlying Membrane Hyperexcitability in Neurodegenerative Diseases

Neurodegenerative diseases are characterized by gradually progressive, selective loss of anatomically or physiologically related neuronal systems that produce brain damage from which there is no recovery. Despite the differences in clinical manifestations and neuronal vulnerability, the pathological processes appear to be similar, suggesting common neurodegenerative pathways. It is well known that oxidative stress and the production of reactive oxygen radicals plays a key role in neuronal cell damage. It has been proposed that this stress, among other mechanisms, could contribute to neuronal degeneration and might be one of the factors triggering the development of these pathologies. Another common feature in most neurodegenerative diseases is neuron hyperexcitability, an aberrant electrical activity. This review, focusing mainly on primary motor cortex pyramidal neurons, critically evaluates the idea that oxidative stress and inflammation may be involved in neurodegeneration via their capacity to increase membrane excitability.

Age-Dependent Vulnerability to Oxidative Stress of Postnatal Rat Pyramidal Motor Cortex Neurons

Oxidative stress is one of the main proposed mechanisms involved in neuronal degeneration. To evaluate the consequences of oxidative stress on motor cortex pyramidal neurons during postnatal development, rats were classified into three groups: Newborn (P2-P7); infantile (P11-P15); and young adult (P20-P40). Oxidative stress was induced by 10 µM of cumene hydroperoxide (CH) application. In newborn rats, using the whole cell patch-clamp technique in brain slices, no significant modifications in membrane excitability were found. In infantile rats, the input resistance increased and rheobase decreased due to the blockage of GABAergic tonic conductance. Lipid peroxidation induced by CH resulted in a noticeable increase in protein-bound 4-hidroxynonenal in homogenates in only infantile and young adult rat slices. Interestingly, homogenates of newborn rat brain slices showed the highest capacity to respond to oxidative stress by dramatically increasing their glutathione and free thiol content. This increase correlated with a time-dependent increase in the glutathione reductase activity, suggesting a greater buffering capacity of newborn rats to resist oxidative stress. Furthermore, pre-treatment of the slices with glutathione monoethyl ester acted as a neuroprotector in pyramidal neurons of infantile rats. We conclude that during maturation, the vulnerability to oxidative stress in rat motor neurons increases with age.

Factors affecting expression and transcription of uncoupling protein 2 gene

Previous studies suggest a negative relationship between hepatic oxidative stress and productivity in beef cattle. Uncoupling protein 2 (UCP2) is involved in the disappearance of reactive oxygen species, suggesting the defensive role of UCP2 against oxidative stress. The present study examined the relationship between oxidative stress and expression levels of UCP2/Ucp2 in cultured human and mouse liver-derived cells. We also explored factors regulating bovine Ucp2 transcription. As oxidative stress inducers, hydrogen peroxide, ethanol, and cumene hydroperoxide (CmHP) were used. Expression levels of hemoxygenase 1 (HMOX1), a representative gene induced by oxidative stress, were not affected by any oxidative stress inducers in HepG2 human liver-derived cells. The levels of UCP2 mRNA were also unaffected by the oxidative stress inducers. Treatment with CmHP increased expression of Hmox1 in Hepa1-6 mouse liver-derived cells, but Ucp2 expression was not changed. Stimulus screening for regulator of transcription (SSRT) revealed that expression of p50 or p65, transcription factors conferring response to oxidative stress, did not stimulate bovine Ucp2 transcrition in HepG2 cells. SSRT also showed 11 molecules that induced Ucp2 transcription more than 4-fold; among them, endoplasmic reticulum (ER) stress-related transcription factors such as XBP1, c-JUN, JUNB, and C/EBPβ were identified. However, treatment with ER stress inducers did not increase Ucp2 expression in HepG2 and Hepa1-6 cells. The present results suggest that 1) neither oxidative stress nor ER stress induces Ucp2 expression in liver-derived cells, and 2) Ucp2 transcription is stimulated by several transcription factors.

Anti-androgenic, anti-oxidant and anti-apoptotic effects of the aqueous and methanol extracts of Pterorhachis zenkeri (Meliaceae): Evidence from in vivo and in vitro studies

The aim of this study was to evaluate the effects of Pterorhachis zenkeri (Meliaceae) on sex organ growth in immature male rats and, oxidative stress and apoptosis markers in CCL-97 (R2C) Leydig cells. For the in vivo studies, 70 immature male Wistar rats (n = 10/group) were treated for 2 or 4 weeks with: distilled water (10 ml/kg, per os) plus soya oil (1 ml/kg, sc), bicalutamide (10 mg/kg, per os), aqueous or methanol extract of P. zenkeri (10 mg/kg or 62 mg/kg, per os) or testosterone propionate (3 mg/kg, sc). After each treatment period, body and sexual organ weights, plasmatic testosterone, total proteins and total cholesterol levels were measured. In the in vitro test, the effects of the methanol extract of P. zenkeri on cell viability, apoptosis, reactive oxygen species (ROS) production, intracellular calcium release and caspases 3/9 were assessed using CCL-97 Leydig cells. Pterorhachis zenkeri extracts decreased sex organ weights, plasmatic testosterone and protein levels in rats. In the in vitro studies, P. zenkeri inhibited apoptosis, ROS production, calcium release and caspase 3/9 activities. These results suggest that P. zenkeri has anti-androgenic, anti-oxidant and anti-apoptotic activities with methanol extract being the most active and could be an effective alternative for the management of androgen-related diseases.

Mycobacterium tuberculosis WhiB3 maintains redox homeostasis and survival in response to reactive oxygen and nitrogen species

Mycobacterium tuberculosis (Mtb) survives under oxidatively and nitosatively hostile niches inside host phagocytes. In other bacteria, adaptation to these stresses is dependent upon the redox sensitive two component systems (e.g., ArcAB) and transcription factors (e.g., FNR/SoxR). However, these factors are absent in Mtb. Therefore, it is not completely understood how Mtb maintains survival and redox balance in response to reactive oxygen species (ROS) and reactive nitrogen species (RNS). Here, we present evidences that a 4Fe-4S-cofactor containing redox-sensitive transcription factor (WhiB3) is exploited by Mtb to adapt under ROS and RNS stress. We show that MtbΔwhiB3 is acutely sensitive to oxidants and to nitrosative agents. Using a genetic biosensor of cytoplasmic redox state (Mrx1-roGFP2) of Mtb, we show that WhiB3 facilitates recovery from ROS (cumene hydroperoxide and hydrogen peroxide) and RNS (acidified nitrite and peroxynitrite). Also, MtbΔwhiB3 displayed reduced survival inside RAW 264.7 macrophages. Consistent with the role of WhiB3 in modulating host-pathogen interaction, we discovered that WhiB3 coordinates the formation of early human granulomas during interaction of Mtb with human peripheral blood mononuclear cells (PBMCs). Altogether, our study provides empirical proof that WhiB3 is required to mitigate redox stress induced by ROS and RNS, which may be important to activate host/bacterial pathways required for the granuloma development and maintenance.

The neurotoxic effects of hydrogen peroxide and copper in Retzius nerve cells of the leech Haemopis sanguisuga

Oxidative stress and the generation of reactive oxygen species (ROS) play an important role in cellular damage. Electrophysiological analyses have shown that membrane transport proteins are susceptible to ROS. In the present study, oxidative stress was induced in Retzius nerve cells of the leechHaemopis sanguisugaby bath application of 1 mM of hydrogen peroxide (H2O2) and 0.02 mM of copper (Cu) for 20 min. The H2O2/Cu(II) produced considerable changes in the electrical properties of the Retzius nerve cells. Intracellular recording of the resting membrane potential revealed that the neuronal membrane was depolarized in the presence of H2O2/Cu(II). We found that the amplitude of action potentials decreased, while the duration augmented in a progressive way along the drug exposure time. The combined application of H2O2and Cu(II) caused an initial excitation followed by depression of the spontaneous electrical activity. Voltage-clamp recordings revealed a second effect of the oxidant, a powerful inhibition of the outward potassium channels responsible for the repolarization of action potentials. The neurotoxic effect of H2O2/Cu(II) on the spontaneous spike electrogenesis and outward K(+)current of Retzius nerve cells was reduced in the presence of hydroxyl radical scavengers, dimethylthiourea and dimethyl sulfoxide, but not mannitol. This study provides evidence for the oxidative modification of outward potassium channels in Retzius nerve cells. The oxidative mechanism of the H2O2/Cu(II) system action on the electrical properties of Retzius neurons proposed in this study might have a wider significance, referring not only to leeches but also to mammalian neurons.

Time and dose dependent effects of oxidative stress induced by cumene hydroperoxide in neuronal excitability of rat motor cortex neurons

It has been claimed that oxidative stress and the production of reactive oxygen radicals can contribute to neuron degeneration and might be one factor in the development of different neurological diseases. In our study, we have attempted to clarify how oxidative damage induces dose dependent changes in functional membrane properties of neurons by means of whole cell patch clamp techniques in brain slices from young adult rats. Our research demonstrates physiological changes in membrane properties of pyramidal motor cortex neurons exposed to 3 concentrations of cumene hydroperoxide (CH; 1, 10 and 100μM) during 30min. Results show that oxidative stress induced by CH evokes important changes, in a concentration and time dependent manner, in the neuronal excitability of motor cortex neurons of the rat: (i) Low concentration of the drug (1μM) already blocks inward rectifications (sag) and decreases action potential amplitude and gain, a drug concentration which has no effects on other neuronal populations, (ii) 10μM of CH depresses the excitability of pyramidal motor cortex neurons by decreasing input resistance, amplitude of the action potential, and gain and maximum frequency of the repetitive firing discharge, and (iii) 100μM completely blocks the capability to produce repetitive discharge of action potentials in all cells. Both larger drug concentrations and/or longer times of exposure to CH narrow the current working range. This happens because of the increase in the rheobase, and the reduction of the cancelation current. The effects caused by oxidative stress, including those produced by the level of lipid peroxidation, are practically irreversible and, this, therefore, indicates that neuroprotective agents should be administered at the first symptoms of alterations to membrane properties. In fact, the pre-treatment with melatonin, acting as an antioxidant, prevented the lipid peroxidation and the physiological changes induced by CH. Larger cells (as estimated by their cell capacitance) were also more susceptible to oxidative stress. Our results provide previously unavailable observations that large size and high sensitivity to oxidative stress (even at low concentrations) make pyramidal neurons of the motor cortex, in particular corticofugal neurons, more susceptible to cell death when compared with other neuronal populations. These results could also shed some light on explaining the causes behind diseases such as Amyotrophic Lateral Sclerosis.

Oxidative stress induced by cumene hydroperoxide evokes changes in neuronal excitability of rat motor cortex neurons

Oxidative stress and the production of reactive oxygen radicals play a key role in neuronal cell damage. This paper describes an in vitro study that explores the neuronal responses to oxidative stress focusing on changes in neuronal excitability and functional membrane properties. This study was carried out in pyramidal cells of the motor cortex by applying whole-cell patch-clamp techniques on brain slices from young adult rats. Oxygen-derived free radical formation was induced by bath application of 10μM cumene hydroperoxide (CH) for 30min. CH produced marked changes in the electrophysiological properties of neurons (n=30). Resting membrane potential became progressively depolarized, as well as depolarization voltage, with no variations in voltage threshold. Membrane resistance showed a biphasic behavior, increasing after 5min of drug exposure and then it started to decrease, even under control values, after 15 and 30min. At the same time, changes in membrane resistance produced compensatory variations in the rheobase. The amplitude of the action potentials diminished and the duration increased progressively over time. Some of the neurons under study also lost their ability to discharge action potentials in a repetitive way. Most of the neurons, however, kept their repetitive discharge even though their maximum frequency and gain decreased. Furthermore, cancelation of the repetitive firing discharge took place at intensities that decreased with time of exposure to CH, which resulted in a narrower working range. We can conclude that oxidative stress compromises both neuronal excitability and the capability of generating action potentials, and so this type of neuronal functional failure could precede the neuronal death characteristics of many neurodegenerative diseases.