Diazoxide induces delayed pre-conditioning in cultured rat cortical neurons

Toll-like receptor 4-dependent innate immune responses are mediated by intracrine corticosteroids and activation of glycogen synthase kinase-3β in astrocytes

Reactive astrocytes are important pathophysiologically and synthesize neurosteroids. We observed that LPS increased immunoreactive TLR4 and key steroidogenic enzymes in cortical astrocytes of rats and investigated whether corticosteroids are produced and mediate astrocytic TLR4-dependent innate immune responses. We found that LPS increased steroidogenic acute regulatory protein (StAR) and StAR-dependent aldosterone production in purified astrocytes. Both increases were blocked by the TLR4 antagonist TAK242. LPS also increased 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) and corticosterone production, and both were prevented by TAK242 and by siRNAs against 11β-HSD1, StAR, or aldosterone synthase (CYP11B2). Knockdown of 11β-HSD1, StAR, or CYP11B2 or blocking either mineralocorticoid receptors (MR) or glucocorticoid receptors (GR) prevented dephosphorylation of p-Ser9GSK-3β, activation of NF-κB, and the GSK-3β-dependent increases of C3, IL-1β, and TNF-α caused by LPS. Exogenous aldosterone mimicked the MR- and GSK-3β-dependent pro-inflammatory effects of LPS in astrocytes, but corticosterone did not. Supernatants from astrocytes treated with LPS reduced MAP2 and viability of cultured neurons except when astrocytic StAR or MR was inhibited. In adrenalectomized rats, intracerebroventricular injection of LPS increased astrocytic TLR4, StAR, CYP11B2, and 11β-HSD1, NF-κB, C3 and IL-1β, decreased astrocytic p-Ser9GSK-3β in the cortex and was neurotoxic, except when spironolactone was co-injected, consistent with the in vitro results. LPS also activated NF-κB in some NeuN+ and CD11b+ cells in the cortex, and these effects were prevented by spironolactone. We conclude that intracrine aldosterone may be involved in the TLR4-dependent innate immune responses of astrocytes and can trigger paracrine effects by activating astrocytic MR/GSK-3β/NF-κB signaling.

Mechanism of action and neuroprotective role of nicorandil in ischemic stroke

Nicorandil is a dual mechanism anti-anginal agent that acts as a nitric oxide (NO) donor and a potassium (K+) channel opener. Recent studies have evaluated the effect of nicorandil on ischemic stroke. Neurons have a low tolerance to hypoxia and therefore the brain tissue is significantly vulnerable to ischemia. Current approved treatments for ischemic stroke are tissue plasminogen activators and clot retrieval methods. The narrow therapeutic time window and lack of efficacy in restoring the dying neurons urge researchers to develop an alternative approach. In the terminal stages of anoxia, K+ channels induce hyperpolarization in various types of neuronal cells, leading to decreased neuronal activity and the preservation of the brain's energy. Nicorandil can open these K+ channels and sustain the hyperpolarization phase, which may have a neuroprotective effect during hypoxia. Additionally, we review how nicorandil can improve overall stroke outcomes through its anti-inflammatory, anti-oxidative, and edema-reducing effects. One of the major components evaluated in stroke patients is blood pressure. Studies have demonstrated that the effect of nicorandil on blood pressure is related to both its K+ channel opening and NO donating mechanisms. Since both hypertension and hypotension need correction before stroke intervention, it's crucial to consider the role of nicorandil and its impact on blood pressure. Previously published studies indicate that the right dosage of nicorandil can improve cerebral blood flow without significant changes in hemodynamic profiles. In this review, we discuss how nicorandil may contribute to better stroke outcomes based on previously published literature and laboratory findings.

ATP Synthase K+- and H+-fluxes Drive ATP Synthesis and Enable Mitochondrial K+-"Uniporter" Function: II. Ion and ATP Synthase Flux Regulation

We demonstrated that ATP synthase serves the functions of a primary mitochondrial K⁺ "uniporter," i.e., the primary way for K⁺ to enter mitochondria. This K⁺ entry is proportional to ATP synthesis, regulating matrix volume and energy supply-vs-demand matching. We show that ATP synthase can be upregulated by endogenous survival-related proteins via IF₁. We identified a conserved BH3-like domain of IF₁ which overlaps its "minimal inhibitory domain" that binds to the β-subunit of F₁. Bcl-xL and Mcl-1 possess a BH3-binding-groove that can engage IF₁ and exert effects, requiring this interaction, comparable to diazoxide to augment ATP synthase's H⁺ and K⁺ flux and ATP synthesis. Bcl-xL and Mcl-1, but not Bcl-2, serve as endogenous regulatory ligands of ATP synthase via interaction with IF₁ at this BH3-like domain, to increase its chemo-mechanical efficiency, enabling its function as the recruitable mitochondrial K_ATP-channel that can limit ischemia-reperfusion injury. Using Bayesian phylogenetic analysis to examine potential bacterial IF₁-progenitors, we found that IF₁ is likely an ancient (∼2 Gya) Bcl-family member that evolved from primordial bacteria resident in eukaryotes, corresponding to their putative emergence as symbiotic mitochondria, and functioning to prevent their parasitic ATP consumption inside the host cell.

Kir6.2-deficient mice develop somatosensory dysfunction and axonal loss in the peripheral nerves

Glucose-responsive ATP-sensitive potassium channels (K_ATP) are expressed in a variety of tissues including nervous systems. The depolarization of the membrane potential induced by glucose may lead to hyperexcitability of neurons and induce excitotoxicity. However, the roles of K_ATP in the peripheral nervous system (PNS) are poorly understood. Here, we determine the roles of K_ATP in the PNS using K_ATP-deficient (Kir6.2-deficient) mice. We demonstrate that neurite outgrowth of dorsal root ganglion (DRG) neurons was reduced by channel closers sulfonylureas. However, a channel opener diazoxide elongated the neurite. K_ATP subunits were expressed in mouse DRG, and expression of certain subunits including Kir6.2 was increased in diabetic mice. In Kir6.2-deficient mice, the current perception threshold, thermal perception threshold, and sensory nerve conduction velocity were impaired. Electron microscopy revealed a reduction of unmyelinated and small myelinated fibers in the sural nerves. In conclusion, K_ATP may contribute to the development of peripheral neuropathy.

Discovery of Halogenated Benzothiadiazine Derivatives with Anticancer Activity*

Mitochondrial respiratory complex II (CII), also known as succinate dehydrogenase, plays a critical role in mitochondrial metabolism. Known but low potency CII inhibitors are selectively cytotoxic to cancer cells including the benzothiadiazine-based anti-hypoglycemic diazoxide. Herein, we study the structure-activity relationship of benzothiadiazine derivatives for CII inhibition and their effect on cancer cells for the first time. A 15-fold increase in CII inhibition was achieved over diazoxide, albeit with micromolar IC₅₀ values. Cytotoxicity evaluation of the novel derivatives resulted in the identification of compounds with much greater antineoplastic effect than diazoxide, the most potent of which possesses an IC₅₀ of 2.93±0.07 μM in a cellular model of triple-negative breast cancer, with high selectivity over nonmalignant cells and more than double the potency of the clinical agent 5-fluorouracil. No correlation between cytotoxicity and CII inhibition was found, thus indicating an as-yet-undefined mechanism of action of this scaffold. The derivatives described herein represent valuable hit compounds for therapeutic discovery in triple-negative breast cancer.

Therapeutic potential of ATP-sensitive potassium channels in Parkinson's disease

In the past decade, there was an increasing interest in the therapeutic potential targeting ATP-sensitive potassium (K_ATP) channels for an effective treatment of Parkinson's disease (PD). K_ATP channels are widely expressed in the central nervous system and were reported to mediate the degeneration and death of nigral dopamine neurons in the pathogenesis of PD. This review aims to address the pivotal roles of K_ATP channels played in the mechanisms underlying PD pathogenesis, and provide possible directions for further research from different perspectives, such as the vulnerability of dopamine neurons in the substantia nigra, neurotransmitter releasing, iron metabolism in the brain, α-synuclein secretion and mitochondrial dysfunction, which are off critical importance in the investigation of K_ATP channels-targeted precise therapeutic interventions for PD.

Ions, the Movement of Water and the Apoptotic Volume Decrease

The movement of water across the cell membrane is a natural biological process that occurs during growth, cell division, and cell death. Many cells are known to regulate changes in their cell volume through inherent compensatory regulatory mechanisms. Cells can sense an increase or decrease in their cell volume, and compensate through mechanisms known as a regulatory volume increase (RVI) or decrease (RVD) response, respectively. The transport of sodium, potassium along with other ions and osmolytes allows the movement of water in and out of the cell. These compensatory volume regulatory mechanisms maintain a cell at near constant volume. A hallmark of the physiological cell death process known as apoptosis is the loss of cell volume or cell shrinkage. This loss of cell volume is in stark contrast to what occurs during the accidental cell death process known as necrosis. During necrosis, cells swell or gain water, eventually resulting in cell lysis. Thus, whether a cell gains or loses water after injury is a defining feature of the specific mode of cell death. Cell shrinkage or the loss of cell volume during apoptosis has been termed apoptotic volume decrease or AVD. Over the years, this distinguishing feature of apoptosis has been largely ignored and thought to be a passive occurrence or simply a consequence of the cell death process. However, studies on AVD have defined an underlying movement of ions that result in not only the loss of cell volume, but also the activation and execution of the apoptotic process. This review explores the role ions play in controlling not only the movement of water, but the regulation of apoptosis. We will focus on what is known about specific ion channels and transporters identified to be involved in AVD, and how the movement of ions and water change the intracellular environment leading to stages of cell shrinkage and associated apoptotic characteristics. Finally, we will discuss these concepts as they apply to different cell types such as neurons, cardiomyocytes, and corneal epithelial cells.

Ischemic Postconditioning Reduces NMDA Receptor Currents Through the Opening of the Mitochondrial Permeability Transition Pore and KATP Channel in Mouse Neurons

Ischemic postconditioning (PostC) is known to reduce cerebral ischemia/reperfusion (I/R) injury; however, whether the opening of mitochondrial ATP-dependent potassium (mito-K_ATP) channels and mitochondrial permeability transition pore (mPTP) cause the depolarization of the mitochondrial membrane that remains unknown. We examined the involvement of the mito-K_ATP channel and the mPTP in the PostC mechanism. Ischemic PostC consisted of three cycles of 15 s reperfusion and 15 s re-ischemia, and was started 30 s after the 7.5 min ischemic load. We recorded N-methyl-D-aspartate receptors (NMDAR)-mediated currents and measured cytosolic Ca²⁺ concentrations, and mitochondrial membrane potentials in mouse hippocampal pyramidal neurons. Both ischemic PostC and the application of a mito-K_ATP channel opener, diazoxide, reduced NMDAR-mediated currents, and suppressed cytosolic Ca²⁺ elevations during the early reperfusion period. An mPTP blocker, cyclosporine A, abolished the reducing effect of PostC on NMDAR currents. Furthermore, both ischemic PostC and the application of diazoxide potentiated the depolarization of the mitochondrial membrane potential. These results indicate that ischemic PostC suppresses Ca²⁺ influx into the cytoplasm by reducing NMDAR-mediated currents through mPTP opening. The present study suggests that depolarization of the mitochondrial membrane potential by opening of the mito-K_ATP channel is essential to the mechanism of PostC in neuroprotection against anoxic injury.

SK channel-mediated metabolic escape to glycolysis inhibits ferroptosis and supports stress resistance in C. elegans

Metabolic flexibility is an essential characteristic of eukaryotic cells in order to adapt to physiological and environmental changes. Especially in mammalian cells, the metabolic switch from mitochondrial respiration to aerobic glycolysis provides flexibility to sustain cellular energy in pathophysiological conditions. For example, attenuation of mitochondrial respiration and/or metabolic shifts to glycolysis result in a metabolic rewiring that provide beneficial effects in neurodegenerative processes. Ferroptosis, a non-apoptotic form of cell death triggered by an impaired redox balance is gaining attention in the field of neurodegeneration. We showed recently that activation of small-conductance calcium-activated K+ (SK) channels modulated mitochondrial respiration and protected neuronal cells from oxidative death. Here, we investigated whether SK channel activation with CyPPA induces a glycolytic shift thereby increasing resilience of neuronal cells against ferroptosis, induced by erastin in vitro and in the nematode C. elegans exposed to mitochondrial poisons in vivo. High-resolution respirometry and extracellular flux analysis revealed that CyPPA, a positive modulator of SK channels, slightly reduced mitochondrial complex I activity, while increasing glycolysis and lactate production. Concomitantly, CyPPA rescued the neuronal cells from ferroptosis, while scavenging mitochondrial ROS and inhibiting glycolysis reduced its protection. Furthermore, SK channel activation increased survival of C. elegans challenged with mitochondrial toxins. Our findings shed light on metabolic mechanisms promoted through SK channel activation through mitohormesis, which enhances neuronal resilience against ferroptosis in vitro and promotes longevity in vivo.

Ischemic postconditioning prevents surge of presynaptic glutamate release by activating mitochondrial ATP-dependent potassium channels in the mouse hippocampus

A mild ischemic load applied after a lethal ischemic insult reduces the subsequent ischemia–reperfusion injury, and is called ischemic postconditioning (PostC). We studied the effect of ischemic PostC on synaptic glutamate release using a whole-cell patch-clamp technique. We recorded spontaneous excitatory post-synaptic currents (sEPSCs) from CA1 pyramidal cells in mouse hippocampal slices. The ischemic load was perfusion of artificial cerebrospinal fluid (ACSF) equilibrated with mixed gas (95% N₂ and 5% CO₂). The ischemic load was applied for 7.5 min, followed by ischemic PostC 30 s later, consisting of three cycles of 15 s of reperfusion and 15 s of re-ischemia. We found that a surging increase in sEPSCs frequency occurred during the immediate-early reperfusion period after the ischemic insult. We found a significant positive correlation between cumulative sEPSCs and the number of dead CA1 neurons (r = 0.70; p = 0.02). Ischemic PostC significantly suppressed this surge of sEPSCs. The mitochondrial K_ATP (mito-K_ATP) channel opener, diazoxide, also suppressed the surge of sEPSCs when applied for 15 min immediately after the ischemic load. The mito-K_ATP channel blocker, 5-hydroxydecanoate (5-HD), significantly attenuated the suppressive effect of both ischemic PostC and diazoxide application on the surge of sEPSCs. These results suggest that the opening of mito-K_ATP channels is involved in the suppressive effect of ischemic PostC on synaptic glutamate release and protection against neuronal death. We hypothesize that activation of mito-K_ATP channels prevents mitochondrial malfunction and breaks mutual facilitatory coupling between glutamate release and Ca²⁺ entry at presynaptic sites.

Mitochondrial depolarization stimulates vascular repair-relevant functions of CD34+ cells via reactive oxygen species-induced nitric oxide generation

BACKGROUND AND PURPOSE

CD34⁺ haematopoietic stem/progenitor cells have revascularization potential and are now being tested for the treatment of ischaemic vascular diseases in clinical trials. We tested the hypothesis that mitochondrial depolarization stimulates the reparative functions of CD34⁺ cells.

EXPERIMENTAL APPROACH

Peripheral blood was obtained from healthy individuals (n = 63), and mononuclear cells (MNCs) were separated. MNCs were enriched for lineage negative cells, followed by isolation of CD34⁺ cells. Vascular repair-relevant functions of CD34⁺ cells, proliferation and migration, were evaluated in the presence and absence of diazoxide. Mitochondrial membrane potential, ROS and NO levels were evaluated by flow cytometry by using JC-1, mitoSOX and DAF-FM respectively.

KEY RESULTS

Diazoxide stimulated the proliferation and migration of CD34⁺ cells that were comparable to the responses induced by stromal-derived factor-1α (SDF) or VEGF. Effects of diazoxide were blocked by either 5-hydroxydecanoate (5HD), a selective mitochondrial ATP-sensitive potassium channel (mitoK_ATP ) inhibitor, or by L-NAME. Diazoxide induced mitochondrial depolarization, and NO and cGMP generation that were 5HD-sensitive. The generation of NO and cGMP by diazoxide was blocked by an endothelial NOS (eNOS)-selective inhibitor, NIO, but not by a neuronal (n)NOS-selective inhibitor, N^ω -propyl-L-arginine (NPA). A Ca²⁺ chelator, BAPTA, Akt inhibitor, triciribine, or PI3K inhibitor, LY294002, inhibited the NO release induced by diazoxide. Phosphorylation of eNOS at Ser¹¹⁷⁷ and dephosphorylation at Thr⁴⁹⁵ were increased. Diazoxide-induced ROS generation and phosphorylation of eNOS at Ser¹¹⁷⁷ were reduced by NPA.

CONCLUSION AND IMPLICATIONS

Diazoxide stimulates vascular repair-relevant functions of CD34⁺ cells via the mitoK_ATP -dependent release of NO and ROS.

LINKED ARTICLES

This article is part of a themed section on Mitochondrial Pharmacology: Featured Mechanisms and Approaches for Therapy Translation. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.22/issuetoc.

The role of KATP channels in cerebral ischemic stroke and diabetes

ATP-sensitive potassium (KATP) channels are ubiquitously expressed on the plasma membrane of cells in multiple organs, including the heart, pancreas and brain. KATP channels play important roles in controlling and regulating cellular functions in response to metabolic state, which are inhibited by ATP and activated by Mg-ADP, allowing the cell to couple cellular metabolic state (ATP/ADP ratio) to electrical activity of the cell membrane. KATP channels mediate insulin secretion in pancreatic islet beta cells, and controlling vascular tone. Under pathophysiological conditions, KATP channels play cytoprotective role in cardiac myocytes and neurons during ischemia and/or hypoxia. KATP channel is a hetero-octameric complex, consisting of four pore-forming Kir6.x and four regulatory sulfonylurea receptor SURx subunits. These subunits are differentially expressed in various cell types, thus determining the sensitivity of the cells to specific channel modifiers. Sulfonylurea class of antidiabetic drugs blocks KATP channels, which are neuroprotective in stroke, can be one of the high stoke risk factors for diabetic patients. In this review, we discussed the potential effects of KATP channel blockers when used under pathological conditions related to diabetics and cerebral ischemic stroke.

Connexin 43 and ATP-sensitive potassium channels crosstalk: a missing link in hypoxia/ischemia stress

Connexin 43 (Cx43) is a gap junction protein expressed in various tissues and organs of vertebrates. Besides functioning as a gap junction, Cx43 also regulates diverse cellular processes like cell growth and differentiation, cell migration, cell survival, etc. Cx43 is critical for normal cardiac functioning and is therefore abundantly expressed in cardiomyocytes. On the other hand, ATP-sensitive potassium (K_ATP) channels are metabolic sensors converting metabolic changes into electrical activity. These channels are important in maintaining the neurotransmitter release, smooth muscle relaxation, cardiac action potential repolarization, normal physiology of cellular repolarization, insulin secretion and immune function. Cx43 and K_ATP channels are part of the same signaling pathway, regulating cell survival during stress conditions and ischemia/hypoxia preconditioning. However, the underlying molecular mechanism for their combined role in ischemia/hypoxia preconditioning is largely unknown. The current review focuses on understanding the molecular mechanism responsible for the coordinated role of Cx43 and K_ATP channel protein in protecting cardiomyocytes against ischemia/hypoxia stress.

Diazoxide protects rat vascular endothelial cells against hypoxia and cold-induced damage

The present study aimed to examine the effects of hypoxia and cold on vascular endothelial cells (VECs), as well as the protective ability of novel VECs-protective drugs against these injuries. A rat model simulating exposure to hypoxia and cold at high altitude environments was established. Based on these animal experiments, rat aortic VECs were established as injury models and exposed to hypoxia and/or adrenaline (ADR) in vitro. The results revealed that hypoxia significantly altered the levels of nitric oxide and vascular endothelial growth factor, while the cold temperature significantly increased the release of ADR and noradrenaline. Exposure to hypoxia combined with cold temperature significantly affected all these indices. In vitro experiments demonstrated that hypoxia, ADR (which was used to simulate cold in the animal experiments) and the combination of the two factors resulted in damage to the VECs and endothelial dysfunction. In addition, the results also showed that diazoxide, a highly selective mitoK_ATP opener, protected VECs against these injuries. In conclusion, hypoxia and cold temperature induced endothelial cell dysfunction and endocrine disorders, respectively. Improving endothelial function using diazoxide may be an effective therapeutic strategy in patients with altitude-associated disorders. However, the potential for clinical application requires further study.

Role of Mitochondria in Cerebral Vascular Function: Energy Production, Cellular Protection, and Regulation of Vascular Tone

Mitochondria not only produce energy in the form of ATP to support the activities of cells comprising the neurovascular unit, but mitochondrial events, such as depolarization and/or ROS release, also initiate signaling events which protect the endothelium and neurons against lethal stresses via pre-/postconditioning as well as promote changes in cerebral vascular tone. Mitochondrial depolarization in vascular smooth muscle (VSM), via pharmacological activation of the ATP-dependent potassium channels on the inner mitochondrial membrane (mitoKATP channels), leads to vasorelaxation through generation of calcium sparks by the sarcoplasmic reticulum and subsequent downstream signaling mechanisms. Increased release of ROS by mitochondria has similar effects. Relaxation of VSM can also be indirectly achieved via actions of nitric oxide (NO) and other vasoactive agents produced by endothelium, perivascular and parenchymal nerves, and astroglia following mitochondrial activation. Additionally, NO production following mitochondrial activation is involved in neuronal preconditioning. Cerebral arteries from female rats have greater mitochondrial mass and respiration and enhanced cerebral arterial dilation to mitochondrial activators. Preexisting chronic conditions such as insulin resistance and/or diabetes impair mitoKATP channel relaxation of cerebral arteries and preconditioning. Surprisingly, mitoKATP channel function after transient ischemia appears to be retained in the endothelium of large cerebral arteries despite generalized cerebral vascular dysfunction. Thus, mitochondrial mechanisms may represent the elusive signaling link between metabolic rate and blood flow as well as mediators of vascular change according to physiological status. Mitochondrial mechanisms are an important, but underutilized target for improving vascular function and decreasing brain injury in stroke patients. © 2016 American Physiological Society. Compr Physiol 6:1529-1548, 2016.

Chemical Conditioning as an Approach to Ischemic Stroke Tolerance: Mitochondria as the Target

It is well established that the brain can be prepared to resist or tolerate ischemic stroke injury, and mitochondrion is a major target for this tolerance. The preparation of ischemic stroke tolerance can be achieved by three major approaches: ischemic conditioning, hypoxic conditioning and chemical conditioning. In each conditioning approach, there are often two strategies that can be used to achieve the conditioning effects, namely preconditioning (Pre-C) and postconditioning (Post-C). In this review, we focus on chemical conditioning of mitochondrial proteins as targets for neuroprotection against ischemic stroke injury. Mitochondrial targets covered include complexes I, II, IV, the ATP-sensitive potassium channel (mitoKATP), adenine dinucleotide translocase (ANT) and the mitochondrial permeability transition pore (mPTP). While numerous mitochondrial proteins have not been evaluated in the context of chemical conditioning and ischemic stroke tolerance, the paradigms and approaches reviewed in this article should provide general guidelines on testing those mitochondrial components that have not been investigated. A deep understanding of mitochondria as the target of chemical conditioning for ischemic stroke tolerance should provide valuable insights into strategies for fighting ischemic stroke, a leading cause of death in the world.

Depolarization of mitochondria in neurons promotes activation of nitric oxide synthase and generation of nitric oxide

The diverse signaling events following mitochondrial depolarization in neurons are not clear. We examined for the first time the effects of mitochondrial depolarization on mitochondrial function, intracellular calcium, neuronal nitric oxide synthase (nNOS) activation, and nitric oxide (NO) production in cultured neurons and perivascular nerves. Cultured rat primary cortical neurons were studied on 7-10 days in vitro, and endothelium-denuded cerebral arteries of adult Sprague-Dawley rats were studied ex vivo. Diazoxide and BMS-191095 (BMS), activators of mitochondrial KATP channels, depolarized mitochondria in cultured neurons and increased cytosolic calcium levels. However, the mitochondrial oxygen consumption rate was unaffected by mitochondrial depolarization. In addition, diazoxide and BMS not only increased the nNOS phosphorylation at positive regulatory serine 1417 but also decreased nNOS phosphorylation at negative regulatory serine 847. Furthermore, diazoxide and BMS increased NO production in cultured neurons measured with both fluorescence microscopy and electron spin resonance spectroscopy, which was sensitive to inhibition by the selective nNOS inhibitor 7-nitroindazole (7-NI). Diazoxide also protected cultured neurons against oxygen-glucose deprivation, which was blocked by NOS inhibition and rescued by NO donors. Finally, BMS induced vasodilation of endothelium denuded, freshly isolated cerebral arteries that was diminished by 7-NI and tetrodotoxin. Thus pharmacological depolarization of mitochondria promotes activation of nNOS leading to generation of NO in cultured neurons and endothelium-denuded arteries. Mitochondrial-induced NO production leads to increased cellular resistance to lethal stress by cultured neurons and to vasodilation of denuded cerebral arteries.

Effect of Bradykinin Postconditioning on Ischemic and Toxic Brain Damage

Brain damage caused by ischemia or toxic agents leads in selectively vulnerable regions to apoptosis-like delayed neuronal death and can result in irreversible damage. Selectively vulnerable neurons of the CA1 area of hippocampus are particularly sensitive to ischemic damage. We investigated the effects of bradykinin (BR) postconditioning on cerebral ischemic and toxic injury. Transient forebrain ischemia was induced by four-vessel occlusion for 10 min and toxic injury was induced by trimethyltin (TMT, 8 µg/kg i.p.). BR as a postconditioner at a dose of 150 µg/kg was applied intraperitoneally 48 h after ischemia or TMT intoxication. Experimental animals were divided into groups according to the length of survival (short—3 and 7 days, and long—28 days survival) and according to the applied ischemic or toxic injury. Glutamate concentration was lowered in both CA1 and dentate gyrus areas of hippocampus after the application of BR postconditioning in both ischemic and toxic brain damage. The number of degenerated neurons in the hippocampal CA1 region was significantly lower in BR-treated ischemic and toxic groups compared to vehicle group. The behavioral test used in our experiments confirms also the memory improvement in conditioned animals. The rats’ ability to form spatial maps and learn was preserved, which is visible from our Barnes maze results. By using the methods of delayed postconditioning is possible to stimulate the endogenous protective mechanisms of the organism and induce the neuroprotective effect. In this study we demonstrated that BR postconditioning, if applied before the onset of irreversible neurodegenerative changes, induced neuroprotection against ischemic or toxic injury.

Diversity of mitochondria-dependent dilator mechanisms in vascular smooth muscle of cerebral arteries from normal and insulin-resistant rats

Mitochondrial depolarization following ATP-sensitive potassium (mitoKATP) channel activation has been shown to induce cerebral vasodilation by generation of mitochondrial reactive oxygen species (ROS), which sequentially promotes frequency of calcium sparks and activation of large conductance calcium-activated potassium channels (BKCa) in vascular smooth muscle (VSM). We previously demonstrated that cerebrovascular insulin resistance accompanies aging and obesity. It is unclear whether mitochondrial depolarization without the ROS generation enhances calcium sparks and vasodilation in phenotypically normal [Sprague Dawley (SD); Zucker lean (ZL)] and insulin-resistant [Zucker obese (ZO)] rats. We compared the mechanisms underlying the vasodilation to ROS-dependent (diazoxide) and ROS-independent [BMS-191095 (BMS)] mitoKATP channel activators in normal and ZO rats. Arterial diameter studies from SD, ZL, and ZO rats showed that BMS as well as diazoxide induced vasodilation in endothelium-denuded cerebral arteries. In normal rats, BMS-induced vasodilation was mediated by mitochondrial depolarization and calcium sparks generation in VSM and was reduced by inhibition of BKCa channels. However, unlike diazoxide-induced vasodilation, scavenging of ROS had no effect on BMS-induced vasodilation. Electron spin resonance spectroscopy confirmed that diazoxide but not BMS promoted vascular ROS generation. BMS- as well as diazoxide-induced vasodilation, mitochondrial depolarization, and calcium spark generation were diminished in cerebral arteries from ZO rats. Thus pharmacological depolarization of VSM mitochondria by BMS promotes ROS-independent vasodilation via generation of calcium sparks and activation of BKCa channels. Diminished generation of calcium sparks and reduced vasodilation in ZO arteries in response to BMS and diazoxide provide new insights into mechanisms of cerebrovascular dysfunction in insulin resistance.

The mechanistic target of rapamycin (mTOR) pathway and S6 Kinase mediate diazoxide preconditioning in primary rat cortical neurons

We examined the role of the mechanistic target of rapamycin (mTOR) pathway in delayed diazoxide (DZ)-induced preconditioning of cultured rat primary cortical neurons. Neurons were treated for 3 days with 500 μM DZ or feeding medium and then exposed to 3 h of continuous normoxia in Dulbecco's modified eagle medium with glucose or with 3 h of oxygen-glucose deprivation (OGD) followed by normoxia and feeding medium. The OGD decreased viability by 50%, depolarized mitochondria, and reduced mitochondrial respiration, whereas DZ treatment improved viability and mitochondrial respiration, and suppressed reactive oxygen species production, but did not restore mitochondrial membrane potential after OGD. Neuroprotection by DZ was associated with increased phosphorylation of protein kinase B (Akt), mTOR, and the major mTOR downstream substrate, S6 Kinase (S6K). The mTOR inhibitors rapamycin and Torin-1, as well as S6K-targeted siRNA abolished the protective effects of DZ. The effects of DZ on mitochondrial membrane potential and reactive oxygen species production were not affected by rapamycin. Preconditioning with DZ also changed mitochondrial and non-mitochondrial oxygen consumption rates. We conclude that in addition to reducing reactive oxygen species (ROS) production and mitochondrial membrane depolarization, DZ protects against OGD by activation of the Akt-mTOR-S6K pathway and by changes in mitochondrial respiration. Ischemic strokes have limited therapeutic options. Diazoxide (DZ) preconditioning can reduce neuronal damage. Using oxygen-glucose deprivation (OGD), we studied Akt/mTOR/S6K signaling and mitochondrial respiration in neuronal preconditioning. We found DZ protects neurons against OGD via the Akt/mTOR/S6K pathway and alters the mitochondrial and non-mitochondrial oxygen consumption rate. This suggests that the Akt/mTOR/S6k pathway and mitochondria are novel stroke targets.

Mitochondria: a multimodal hub of hypoxia tolerance

Decreased oxygen availability impairs cellular energy production and, without a coordinated and matched decrease in energy consumption, cellular and whole organism death rapidly ensues. Of particular interest are mechanisms that protect brain from low oxygen injury, as this organ is not only the most sensitive to hypoxia, but must also remain active and functional during low oxygen stress. As a result of natural selective pressures, some species have evolved molecular and physiological mechanisms to tolerate prolonged hypoxia with no apparent detriment. Among these mechanisms are a handful of responses that are essential for hypoxia tolerance, including (i) sensors that detect changes in oxygen availability and initiate protective responses; (ii) mechanisms of energy conservation; (iii) maintenance of basic brain function; and (iv) avoidance of catastrophic cell death cascades. As the study of hypoxia-tolerant brain progresses, it is becoming increasingly apparent that mitochondria play a central role in regulating all of these critical mechanisms. Furthermore, modulation of mitochondrial function to mimic endogenous neuroprotective mechanisms found in hypoxia-tolerant species confers protection against otherwise lethal hypoxic stresses in hypoxia-intolerant organs and organisms. Therefore, lessons gleaned from the investigation of endogenous mechanisms of hypoxia tolerance in hypoxia-tolerant organisms may provide insight into clinical pathologies related to low oxygen stress.

Mitochondrial reactive oxygen species: a double edged sword in ischemia/reperfusion vs preconditioning

Reductions in the blood supply produce considerable injury if the duration of ischemia is prolonged. Paradoxically, restoration of perfusion to ischemic organs can exacerbate tissue damage and extend the size of an evolving infarct. Being highly metabolic organs, the heart and brain are particularly vulnerable to the deleterious effects of ischemia/reperfusion (I/R). While the pathogenetic mechanisms contributing to I/R-induced tissue injury and infarction are multifactorial, the relative importance of each contributing factor remains unclear. However, an emerging body of evidence indicates that the generation of reactive oxygen species (ROS) by mitochondria plays a critical role in damaging cellular components and initiating cell death. In this review, we summarize our current understanding of the mechanisms whereby mitochondrial ROS generation occurs in I/R and contributes to myocardial infarction and stroke. In addition, mitochondrial ROS have been shown to participate in preconditioning by several pharmacologic agents that target potassium channels (e.g., ATP-sensitive potassium (mKATP) channels or large conductance, calcium-activated potassium (mBKCa) channels) to activate cell survival programs that render tissues and organs more resistant to the deleterious effects of I/R. Finally, we review novel therapeutic approaches that selectively target mROS production to reduce postischemic tissue injury, which may prove efficacious in limiting myocardial dysfunction and infarction and abrogating neurocognitive deficits and neuronal cell death in stroke.

Mitochondrial mechanisms in cerebral vascular control: shared signaling pathways with preconditioning

Mitochondrial-initiated events protect the neurovascular unit against lethal stress via a process called preconditioning, which independently promotes changes in cerebrovascular tone through shared signaling pathways. Activation of adenosine triphosphate (ATP)-dependent potassium channels on the inner mitochondrial membrane (mitoKATP channels) is a specific and dependable way to induce protection of neurons, astroglia, and cerebral vascular endothelium. Through the opening of mitoKATP channels, mitochondrial depolarization leads to activation of protein kinases and transient increases in cytosolic calcium (Ca(2+)) levels that activate terminal mechanisms that protect the neurovascular unit against lethal stress. The release of reactive oxygen species from mitochondria has similar protective effects. Signaling elements of the preconditioning pathways also are involved in the regulation of vascular tone. Activation of mitoKATP channels in cerebral arteries causes vasodilation, with cell-specific contributions from the endothelium, vascular smooth muscles, and nerves. Preexisting chronic conditions, such as insulin resistance and/or diabetes, prevent preconditioning and impair relaxation to mitochondrial-centered responses in cerebral arteries. Surprisingly, mitochondrial activation after anoxic or ischemic stress appears to protect cerebral vascular endothelium and promotes the restoration of blood flow; therefore, mitochondria may represent an important, but underutilized target in attenuating vascular dysfunction and brain injury in stroke patients.

Preconditioning provides neuroprotection in models of CNS disease: paradigms and clinical significance

Preconditioning is a phenomenon in which brief episodes of a sublethal insult induce robust protection against subsequent lethal injuries. Preconditioning has been observed in multiple organisms and can occur in the brain as well as other tissues. Extensive animal studies suggest that the brain can be preconditioned to resist acute injuries, such as ischemic stroke, neonatal hypoxia/ischemia, surgical brain injury, trauma, and agents that are used in models of neurodegenerative diseases, such as Parkinson's disease and Alzheimer's disease. Effective preconditioning stimuli are numerous and diverse, ranging from transient ischemia, hypoxia, hyperbaric oxygen, hypothermia and hyperthermia, to exposure to neurotoxins and pharmacological agents. The phenomenon of "cross-tolerance," in which a sublethal stress protects against a different type of injury, suggests that different preconditioning stimuli may confer protection against a wide range of injuries. Research conducted over the past few decades indicates that brain preconditioning is complex, involving multiple effectors such as metabolic inhibition, activation of extra- and intracellular defense mechanisms, a shift in the neuronal excitatory/inhibitory balance, and reduction in inflammatory sequelae. An improved understanding of brain preconditioning should help us identify innovative therapeutic strategies that prevent or at least reduce neuronal damage in susceptible patients. In this review, we focus on the experimental evidence of preconditioning in the brain and systematically survey the models used to develop paradigms for neuroprotection, and then discuss the clinical potential of brain preconditioning.

Subcellular expression and neuroprotective effects of SK channels in human dopaminergic neurons

Small-conductance Ca(2+)-activated K(+) channel activation is an emerging therapeutic approach for treatment of neurological diseases, including stroke, amyotrophic lateral sclerosis and schizophrenia. Our previous studies showed that activation of SK channels exerted neuroprotective effects through inhibition of NMDAR-mediated excitotoxicity. In this study, we tested the therapeutic potential of SK channel activation of NS309 (25 μM) in cultured human postmitotic dopaminergic neurons in vitro conditionally immortalized and differentiated from human fetal mesencephalic cells. Quantitative RT-PCR and western blotting analysis showed that differentiated dopaminergic neurons expressed low levels of SK2 channels and high levels of SK1 and SK3 channels. Further, protein analysis of subcellular fractions revealed expression of SK2 channel subtype in mitochondrial-enriched fraction. Mitochondrial complex I inhibitor rotenone (0.5 μM) disrupted the dendritic network of human dopaminergic neurons and induced neuronal death. SK channel activation reduced mitochondrial membrane potential, while it preserved the dendritic network, cell viability and ATP levels after rotenone challenge. Mitochondrial dysfunction and delayed dopaminergic cell death were prevented by increasing and/or stabilizing SK channel activity. Overall, our findings show that activation of SK channels provides protective effects in human dopaminergic neurons, likely via activation of both membrane and mitochondrial SK channels. Thus, SK channels are promising therapeutic targets for neurodegenerative disorders such as Parkinson's disease, where dopaminergic cell loss is associated with progression of the disease.

Spermine attenuates the preconditioning of diazoxide against transient focal cerebral ischemia in rats

It is known that mitochondrial ATP-sensitive potassium channels (mitoKATP) play a significant role in protecting cerebral function from ischemia-reperfusion injury, which is related with a decrease in the mitochondrial matrix calcium. However, the effect of mitochondrial calcium uniporter (MCU) on diazoxide-induced cerebral protection is still indistinct. The purpose of the present paper is to further observe the relationship between mitoKATP and MCU, and to probe the mechanism. Adult male Wistar rats were randomly divided into five groups: the Sham group, the I-R group, the Dzx+I-R group, the Dzx+Sper+I-R group, and the Sper+I-R group. Rats not in the Sham group were exposed to 2-hour ischemia followed by 24-hour reperfusion. Diazoxide and spermine were administrated 30 minutes before ischemia or 10 minutes before reperfusion, respectively. After 24-hour reperfusion, animals were given neurological performance tests, overdosed with general anesthesia, and then their brains were excised for infarct volume, pathological changes, and apoptosis analysis. The beneficial effects of diazoxide (improved neurological deficits, decreased infarct volume, and apoptosis, evidenced by the decreased expression of cytochrome c and Bax) were significantly neutralized by spermine. The results of the present work suggest that diazoxide-induced cerebral protection against ischemia-reperfusion injury is mediated by spermine through apoptotic pathway.

Is there a place for cerebral preconditioning in the clinic?

Preconditioning (PC) describes a phenomenon whereby a sub-injury inducing stress can protect against a later injurious stress. Great strides have been made in identifying the mechanisms of PC-induced protection in animal models of brain injury. While these may help elucidate potential therapeutic targets, there are questions over the clinical utility of cerebral PC, primarily because of questions over the need to give the PC stimulus prior to the injury, narrow therapeutic windows and safety. The object of this review is to address the question of whether there may indeed be a clinical use for cerebral PC and to discuss the deficiencies in our knowledge of PC that may hamper such clinical translation.

Parvalbumin is overexpressed in the late phase of pharmacological preconditioning in skeletal muscle

Pharmacological preconditioning (PPC) with mitochondrial ATP-sensitive K(+) channel openers such as diazoxide, provides protection against ischemia in cardiac muscle, skeletal muscle, and other tissues. Effects on Ca(2+) homeostasis during the late phase of PPC have been described in cardiomyocytes, but no information is available regarding intracellular Ca(2+) changes in skeletal muscle fibers during late PPC. Intracellular Ca(2+) signals were measured in single fibers of adult mouse skeletal muscle, with fluorescent probes, 48 h after the administration of diazoxide. Parvalbumin levels in the myofibers were quantitated by Western blot. Diazoxide induction of late PPC was confirmed by partial protection of muscles from peroxide-induced damage. Late PPC was associated with a significant decrease in the duration of Ca(2+) signals during single twitches and tetanus with no changes in peak values. This effect was prevented by the reactive oxygen species (ROS) scavenger tiron. Late PPC was accompanied by a 30% increase in parvalbumin levels, and this effect was also blocked by tiron. Our data show, for the first time, a role of parvalbumin in late PPC in skeletal muscle.

Mitochondrial dynamics associated with oxygen-glucose deprivation in rat primary neuronal cultures

Our objective was to investigate the mitochondrial dynamics following oxygen-glucose deprivation (OGD) in cultured rat cortical neurons. We documented changes in morphology, protein expression, and DNA levels in mitochondria following OGD and examined the roles of mitochondrial fission [dynamin-related protein 1 (Drp1), fission protein-1 (Fis1)] and fusion [mitofusin-1 (Mfn1), mitofusin-2 (Mfn2), and optic atrophy-1 protein (OPA1)] proteins on mitochondrial biogenesis and morphogenesis. We tested the effects of two Drp1 blockers [15-deoxy-Δ12,14-Prostaglandin J2 (PGJ2) and Mitochondrial Division Inhibitor (Mdivi-1)] on mitochondrial dynamics and cell survival. One hour of OGD had minimal effects on neuronal viability but mitochondria appeared condensed. Three hours of OGD caused a 60% decrease in neuronal viability accompanied by a transition from primarily normal/tubular and lesser number of rounded mitochondria during normoxia to either poorly labeled or small and large rounded mitochondria. The percentage of rounded mitochondria remained the same. The mitochondrial voltage-dependent anion channel, Complex V, and mitoDNA levels increased after OGD associated with a dramatic reduction in Drp1 expression, less reduction in Mfn2 expression, an increase in Mfn1 expression, with no changes in either OPA1 or Fis1. Although PGJ2 increased polymerization of Drp1, it did not reduce cell death or alter mitochondrial morphology following OGD and Mdivi-1 did not protect neurons against OGD. In summary, mitochondrial biogenesis and maintained fusion occurred in neurons along with mitochondrial fission following OGD; thus Mfn1 but not Drp1 may be a major regulator of these processes.

Depolarization of mitochondria in endothelial cells promotes cerebral artery vasodilation by activation of nitric oxide synthase

OBJECTIVE

Mitochondrial depolarization after ATP-sensitive potassium channel activation has been shown to induce cerebral vasodilation by the generation of calcium sparks in smooth muscle. It is unclear, however, whether mitochondrial depolarization in endothelial cells is capable of promoting vasodilation by releasing vasoactive factors. Therefore, we studied the effect of endothelial mitochondrial depolarization by mitochondrial ATP-sensitive potassium channel activators, BMS-191095 (BMS) and diazoxide, on endothelium-dependent vasodilation.

APPROACH AND RESULTS

Diameter studies in isolated rat cerebral arteries showed BMS- and diazoxide-induced vasodilations that were diminished by endothelial denudation. Mitochondrial depolarization-induced vasodilation was reduced by inhibition of mitochondrial ATP-sensitive potassium channels, phosphoinositide-3 kinase, or nitric oxide synthase. Scavenging of reactive oxygen species, however, diminished vasodilation induced by diazoxide, but not by BMS. Fluorescence studies in cultured rat brain microvascular endothelial cells showed that BMS elicited mitochondrial depolarization and enhanced nitric oxide production; diazoxide exhibited largely similar effects, but unlike BMS, increased mitochondrial reactive oxygen species production. Measurements of intracellular calcium ([Ca(2+)]i) in cultured rat brain microvascular endothelial cells and arteries showed that both diazoxide and BMS increased endothelial [Ca(2+)]i. Western blot analyses revealed increased phosphorylation of protein kinase B and endothelial nitric oxide synthase (eNOS) by BMS and diazoxide. Increased phosphorylation of eNOS by diazoxide was abolished by phosphoinositide-3 kinase inhibition. Electron spin resonance spectroscopy confirmed vascular nitric oxide generation in response to diazoxide and BMS.

CONCLUSIONS

Pharmacological depolarization of endothelial mitochondria promotes activation of eNOS by dual pathways involving increased [Ca(2+)]i as well as by phosphoinositide-3 kinase-protein kinase B-induced eNOS phosphorylation. Both mitochondrial reactive oxygen species-dependent and -independent mechanisms mediate activation of eNOS by endothelial mitochondrial depolarization.

Mitochondrial ATP-sensitive potassium channels enhance angiotensin-induced oxidative damage and dopaminergic neuron degeneration. Relevance for aging-associated susceptibility to Parkinson's disease

Recent studies have shown that renin-angiotensin system overactivation is involved in the aging process in several tissues as well as in longevity and aging-related degenerative diseases by increasing oxidative damage and inflammation. We have recently shown that angiotensin II enhances dopaminergic degeneration by increasing levels of reactive oxygen species and neuroinflammation, and that there is an aging-related increase in angiotensin II activity in the substantia nigra in rats, which may constitute a major factor in the increased risk of Parkinson's disease with aging. The mechanisms involved in the above mentioned effects and particularly a potential angiotensin-mitochondria interaction have not been clarified. The present study revealed that activation of mitochondrial ATP-sensitive potassium channels [mitoK(ATP)] may play a major role in the angiotensin II-induced effects on aging and neurodegeneration. Inhibition of mitoK(ATP) channels with 5-hydroxydecanoic acid inhibited the increase in dopaminergic cell death induced by angiotensin II, as well as the increase in superoxide/superoxide-derived reactive oxygen species levels and the angiotensin II-induced decrease in the mitochondrial inner membrane potential in cultured dopaminergic neurons. The present study provides data for considering brain renin-angiotensin system and mitoK(ATP) channels as potential targets for protective therapy in aging-associated diseases such as Parkinson's disease.

Mitochondrial ERK plays a key role in δ-opioid receptor neuroprotection against acute mitochondrial dysfunction

It is well established that stimulating delta-opioid receptor (DOR) with its specific agonists elicits neuroprotection against hypoxia/ischemia. Mitochondrial dysfunction plays a key role in hypoxic neuronal injury, but the effects of DOR activation on mitochondrial dysfunction in neurons are poorly elucidated. In this investigation, we studied the effects of [D-Ala2, D-Leu5] enkephalin (DADLE), a potent DOR agonist, on acute mitochondrial dysfunction and ensuing cell damage induced by sodium azide in primary rat cortical neuronal cultures, and explored possible mechanisms underlying. Here, we show that DADLE reverses NaN(3)-induced acute mitochondrial dysfunction by selectively activating DOR, mainly including mitochondrial membrane depolarization, mitochondrial Ca(2+) overload and reactive oxygen species generation. DOR stimulation also inhibits cytochrome c release and caspase-3 activation, and attenuates neuronal death caused by acute NaN(3) insults. Furthermore, DOR activation with DADLE protects neurons from acute NaN(3) insults mainly through PKC-ERK pathway, and mitochondrial ERK activation is especially required for DOR neuroprotection against acute mitochondrial dysfunction.

Diazoxide decreases ischemia-reperfusion injury in a rat model of lung transplantation

BACKGROUND

Ischemia-reperfusion injury (IRI) is a significant factor contributing to primary graft failure in lung transplantation. Given a pivotal role of mitochondria in IRI-related molecular events, the effects of diazoxide, a selective opener of mitochondrial adenosine-5'-triphosphate (ATP)-sensitive potassium channels (mitoK(ATP)), on IRI were investigated in a rat model of lung transplantation.

METHODS

The 108 rats were randomly assigned to 5 groups; a sham-operated, 2 control, and 2 experimental groups that received either diazoxide alone or a combination of diazoxide with 5-hydroxydecanoate sodium salt. Lung injuries were assessed by multiple parameters at 2 hours or 24 hours after reperfusion, including oxygenation index, wet/dry weight ratio of transplanted lungs, lung morphology, as well as measurements of myeloperoxidase, malondialdehyde, total antioxidant capacity, tumor necrosis factor-α, and interleukin-6.

RESULTS

Compared with the sham group, the 2 control groups revealed significant changes among most parameters of lung injury measured at either 2 hours or 24 hours after reperfusion. The extent of the changes was dramatically reduced by the administration of diazoxide. Importantly, the protective effect of diazoxide was almost completely reversed by co-administration of 5-hydroxydecanoate sodium salt, a selective blocker of mitoK(ATP).

CONCLUSIONS

These data provide evidence for substantial protective effects of diazoxide in an in vivo rat lung IRI model. Pharmacological modulation of mitoK(ATP) may be a potential strategy to reduce IRI-induced primary graft failure in lung transplantation.

Postconditioning in focal cerebral ischemia: role of the mitochondrial ATP-dependent potassium channel

INTRODUCTION

Ischemic postconditioning (IpostC) has been described in both heart and brain. The first aim of this study was to evaluate the effects of IpostC on brain infarct size and neurological function in the middle cerebral artery occlusion (MCAO) model. The second aim was to determine the involvement of the mitochondrial potassium ATP-dependent channel (mitoK(ATP)) opening and its capacity to improve mitochondrial dysfunction induced by ischemia-reperfusion.

METHODS

Wistar rats were subjected to 60min MCAO followed by 24-h reperfusion. Postconditioning was performed by 3 cycles of 30-s occlusion-reperfusion at the onset of reperfusion. Three behavioral tests were performed following 24h of reperfusion. Involvement of mitoK(ATP) was determined by the modulation of IpostC effects by 5-hydroxydecanoate (5-HD) and diazoxide. Mitochondrial function after 24h of reperfusion on isolated mitochondria was assessed through mitochondrial oxygen consumption, mitochondrial membrane potential and calcium retention capacity to evaluate impact of IpostC on mitochondrial permeability transition pore (MPTP) opening.

RESULTS

IpostC resulted in a 40% decrease in infarct size and improved neurological outcome. These effects were lost when IpostC was delayed by 5min. The administration of diazoxide resulted in a 60% in infarct size. The beneficial effects of IpostC and diazoxide were blocked by 5-HD. Furthermore, 5-HD also blocked the inhibition of MPTP opening by IpostC and diazoxide. The hyperpolarization induced by ischemia-reperfusion was corrected by IpostC without any effect on oxidative phosphorylation.

CONCLUSION

Our results confirm ischemic postconditioning-induced neuroprotection. They also support the involvement of mitoK(ATP) opening and its role in inhibiting the opening of MTPT induced by postconditioning.

Mitochondria: the missing link between preconditioning and neuroprotection

The quote "what does not kill you makes you stronger" perfectly describes the preconditioning phenomenon - a paradigm that affords robust brain tolerance in the face of neurodegenerative insults. Over the last few decades, many attempts have been made to identify the molecular mechanisms involved in preconditioning-induced protective responses, and recent data suggests that many of these mechanisms converge on the mitochondria, positing mitochondria as master regulators of preconditioning-triggered endogenous neuroprotection. In this review, we critically discuss evidence for the involvement of mitochondria within the preconditioning paradigm. We will highlight the crucial targets and mediators by which mitochondria are integrated into neuroprotective signaling pathways that underlie preconditioning, putting focus on mitochondrial respiratory chain and mitochondrial reactive oxygen species, mitochondrial ATP-sensitive potassium channels, mitochondrial permeability transition pore, uncoupling proteins, and mitochondrial antioxidant enzyme manganese superoxide dismutase. We also discuss the role of mitochondria in the induction of hypoxia-inducible factor-1, a transcription factor engaged in preconditioning-mediated neuroprotective effects. The identification of intrinsic mitochondrial mechanisms involved in preconditioning will provide new insights which can be translated into potential pharmacological interventions aimed at counteracting neurodegeneration.

Mitochondrial preconditioning: a potential neuroprotective strategy

Mitochondria have long been known as the powerhouse of the cell. However, these organelles are also pivotal players in neuronal cell death. Mitochondrial dysfunction is a prominent feature of chronic brain disorders, including Alzheimer's disease (AD) and Parkinson's disease (PD), and cerebral ischemic stroke. Data derived from morphologic, biochemical, and molecular genetic studies indicate that mitochondria constitute a convergence point for neurodegeneration. Conversely, mitochondria have also been implicated in the neuroprotective signaling processes of preconditioning. Despite the precise molecular mechanisms underlying preconditioning-induced brain tolerance are still unclear, mitochondrial reactive oxygen species generation and mitochondrial ATP-sensitive potassium channels activation have been shown to be involved in the preconditioning phenomenon. This review intends to discuss how mitochondrial malfunction contributes to the onset and progression of cerebral ischemic stroke and AD and PD, two major neurodegenerative disorders. The role of mitochondrial mechanisms involved in the preconditioning-mediated neuroprotective events will be also discussed. Mitochondrial targeted preconditioning may represent a promising therapeutic weapon to fight neurodegeneration.

Regulation of AMPA receptor currents by mitochondrial ATP-sensitive K+ channels in anoxic turtle neurons

Mammalian neurons rapidly undergo excitotoxic cell death during anoxia, whereas neurons from the anoxia-tolerant painted turtle survive without oxygen for hours and offer a unique model to study mechanisms to reduce the severity of cerebral stroke. An anoxia-mediated decrease in whole cell N-methyl-D-aspartate receptor and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) currents are an important part of the turtle's natural defense. Here we investigate the role of mitochondrial ATP-sensitive K(+) (mK(ATP)) channels in the regulation of AMPAR. Whole cell AMPAR currents were stable over 90 min of normoxic recording; however, anoxia resulted in a 52% decrease in AMPAR currents. Pharmacological activation of mK(ATP) channels with diazoxide or levcromakalim resulted in a 46% decrease in normoxic AMPAR currents and the decrease was abolished with application of the antagonists 5-hydroxydecanoic acid and glibenclamide, whereas mK(ATP) antagonists blocked the anoxia-mediated decrease. Mitochondrial K(Ca) channel modulators responded similarly. The Ca(2+)-uniporter antagonist ruthenium red reduced AMPAR currents by 38% and was blocked with the agonist spermine. The calcium chelator BAPTA in the recording electrode during anoxia or diazoxide perfusion also abolished the reduction in AMPAR currents. We conclude that the mK(ATP) channel is involved in the anoxia-mediated down-regulation of AMPAR activity during anoxia and that it is a common mechanism to reduce glutamatergic excitability.

Mitochondrial complex II prevents hypoxic but not calcium- and proapoptotic Bcl-2 protein-induced mitochondrial membrane potential loss

Mitochondrial membrane potential loss has severe bioenergetic consequences and contributes to many human diseases including myocardial infarction, stroke, cancer, and neurodegeneration. However, despite its prominence and importance in cellular energy production, the basic mechanism whereby the mitochondrial membrane potential is established remains unclear. Our studies elucidate that complex II-driven electron flow is the primary means by which the mitochondrial membrane is polarized under hypoxic conditions and that lack of the complex II substrate succinate resulted in reversible membrane potential loss that could be restored rapidly by succinate supplementation. Inhibition of mitochondrial complex I and F(0)F(1)-ATP synthase induced mitochondrial depolarization that was independent of the mitochondrial permeability transition pore, Bcl-2 (B-cell lymphoma 2) family proteins, or high amplitude swelling and could not be reversed by succinate. Importantly, succinate metabolism under hypoxic conditions restores membrane potential and ATP levels. Furthermore, a reliance on complex II-mediated electron flow allows cells from mitochondrial disease patients devoid of a functional complex I to maintain a mitochondrial membrane potential that conveys both a mitochondrial structure and the ability to sequester agonist-induced calcium similar to that of normal cells. This finding is important as it sets the stage for complex II functional preservation as an attractive therapy to maintain mitochondrial function during hypoxia.

The phosphatidylinositol-3 kinase/Akt pathway mediates geranylgeranylacetone-induced neuroprotection against cerebral infarction in rats

Previous studies demonstrated the cytoprotective effect of geranylgeranylacetone (GGA), a heat shock protein inducer, against ischemic insult. Phosphatidylinositol-3 kinase/Akt (PI3K/Akt) is thought to be an important factor that mediates neuroprotection. However, the signaling pathways in the brain in vivo after oral GGA administration remain unclear. We measured and compared infarction volumes to investigate the effect of GGA on cerebral infarction induced by permanent middle cerebral artery occlusion in rats. We evaluated the effects of pretreatment with 5-hydroxydecanoate (5HD), a specific mitochondrial ATP-sensitive potassium (mitoK(ATP)) channel inhibitor; diazoxide (DZX), a selective mitoK(ATP) channel opener and wortmannin (Wort), a specific PI3K inhibitor of GGA-induced neuroprotection against infarction volumes. To clarify the relationship between PI3K/Akt activation and neuroprotection, we used immunoblot analysis to determine the amount of p-Akt proteins present after GGA administration with or without Wort treatment. Neuroprotective effects of GGA (pretreatment with a single oral GGA dose (800 mg/kg) 48 h before ischemia) were prevented by 5HD, DZX and Wort pretreatment, which indicates that the selective mitoK(ATP) channel and the PI3K/Akt pathway may mediate GGA-dependent protection. Oral GGA-induced p-Akt and GGA pretreatment enhanced ischemia-induced p-Akt, both of which were prevented by Wort pretreatment. These results suggest that a single oral dose of GGA induces p-Akt and that GGA plays an important role in neuroprotection against cerebral ischemia through the mitoK(ATP) channel opening.

Novel neuroprotective strategies in ischemic retinal lesions

Retinal ischemia can be effectively modeled by permanent bilateral common carotid artery occlusion, which leads to chronic hypoperfusion-induced degeneration in the entire rat retina. The complex pathways leading to retinal cell death offer a complex approach of neuroprotective strategies. In the present review we summarize recent findings with different neuroprotective candidate molecules. We describe the protective effects of intravitreal treatment with: (i) urocortin 2; (ii) a mitochondrial ATP-sensitive K⁺ channel opener, diazoxide; (iii) a neurotrophic factor, pituitary adenylate cyclase activating polypeptide; and (iv) a novel poly(ADP-ribose) polymerase inhibitor (HO3089). The retinoprotective effects are demonstrated with morphological description and effects on apoptotic pathways using molecular biological techniques.