Molecular Mechanisms of Ischemia–Reperfusion Injury in Brain: Pivotal Role of the Mitochondrial Membrane Potential in Reactive Oxygen Species Generation

Stroke and circulatory arrest cause interferences in blood flow to the brain that result in considerable tissue damage. The primary method to reduce or prevent neurologic damage to patients suffering from brain ischemia is prompt restoration of blood flow to the ischemic tissue. However, paradoxically, restoration of blood flow causes additional damage and exacerbates neurocognitive deficits among patients who suffer a brain ischemic event. Mitochondria play a critical role in reperfusion injury by producing excessive reactive oxygen species (ROS) thereby damaging cellular components, and initiating cell death. In this review, we summarize our current understanding of the mechanisms of mitochondrial ROS generation during reperfusion, and specifically, the role the mitochondrial membrane potential plays in the pathology of cerebral ischemia/reperfusion. Additionally, we propose a temporal model of ROS generation in which posttranslational modifications of key oxidative phosphorylation (OxPhos) proteins caused by ischemia induce a hyperactive state upon reintroduction of oxygen. Hyperactive OxPhos generates high mitochondrial membrane potentials, a condition known to generate excessive ROS. Such a state would lead to a “burst” of ROS upon reperfusion, thereby causing structural and functional damage to the mitochondria and inducing cell death signaling that eventually culminate in tissue damage. Finally, we propose that strategies aimed at modulating this maladaptive hyperpolarization of the mitochondrial membrane potential may be a novel therapeutic intervention and present specific studies demonstrating the cytoprotective effect of this treatment modality.     

A new penumbra: transitioning from injury into repair after stroke

The penumbra is an area of brain tissue that is damaged but not yet dead after focal ischemia. The existence of a penumbra implies that therapeutic salvage is theoretically possible after stroke. The first decade of penumbral science investigated the ischemic regulation of electrophysiology, cerebral blood flow and metabolism. The second decade advanced our understanding of molecular mechanisms that mediate penumbral cell death. And the third decade saw the rapid development of clinical neuroimaging tools that are now increasingly applied in stroke patients. But how can we look ahead as we move into the fourth decade of penumbra research? This author speculates that a paradigm shift is needed. Most molecular targets for therapy have biphasic roles in stroke pathophysiology. During the acute phase, these targets mediate injury. During the recovery phase, the same mediators contribute to neurovascular remodeling. It is this boundary zone that comprises the new penumbra, and future investigations should dissect where, when and how damaged brain makes the transition from injury into repair.     

Cellular neuroprotective mechanisms in cerebral ischemia: Bcl-2 family proteins and protection of mitochondrial function

Mitochondria are central to brain cell response to ischemia, with critical roles in generation of ATP, production of free radicals, and regulation of apoptotic cell death. Changes in the permeability of the outer mitochondrial membrane to regulators of apoptosis can control ischemic cell death and this permeability is directly controlled by the Bcl-2 family of proteins. The Bcl-2 family regulate apoptosis by several mechanisms including affecting the formation of apoptotic protein-conducting pores in the outer mitochondrial membrane. The anti-apoptotic protein Bcl-2 improves neuron survival following various insults, and is protective even when administered after stroke onset in a rat model of focal ischemia. Despite intense study, the precise molecular mechanisms underlying protection by the anti-apoptotic members of the Bcl-2 family are not completely understood. This review focuses on the mechanisms by which Bcl-2 family members control the permeability of the mitochondrial membrane and influence other aspects of mitochondrial function after brain ischemia, concluding with discussion of the potential use of Bcl-2 for the treatment of cerebral ischemia.     

What have genetically engineered mice taught us about ischemic injury?

Stroke, is the third leading cause of death and disability in the Western world. Stroke refers to set of ischemic conditions resulting from the occlusion or hemorrhage of blood vessels supplying the brain. Loss of blood flow to the brain results in neuronal injury due to both oxygen and nutrient deprivation and the activation of injurious signal cascades. Ultimately cerebral ischemia results in death and dysfunction of brain cells, and neurological deficits that reflect the location and size of the compromised brain area. Injury due to ischemic stroke occurs by a highly choreographed series of complex spatial and temporal events that evolve over hours to days. These events involve complex interactions between fundamental cell injury mechanisms including excitotoxicity and ionic imbalance, oxidative and nitrosative stress, apoptotic-like cell death and inflammatory responses. Genetically engineered mice have been valuable tools to probe putative mechanisms of neuronal death and uncover potential strategies that might render neurons resistant to ischemic injury. Findings from experimental stroke studies in genetically engineered animals are discussed.     

Apoptosis of Cortical Neurons in Ischemic Insult

Acute brain ischemia is accompanied by the intense apoptotic and/or necrotic death of cortical neurons. This review deals with the molecular mechanisms underlying apoptosis, in particular those activated in progressive cerebral ischemic insult. We analyze the data of experimental studies and clinical findings that confirm the principal role of caspase-dependent cell death resulting from acute disorder of the brain circulation. The prospects for the use of apoptosis inhibitors in neurological practice for prevention or minimization of cerebral ischemic injury and reduction of neuronal degeneration within a penumbral zone are discussed.     

Nitric oxide and cerebral ischemic preconditioning

Nitric oxide (NO) is an important mediator of cerebral blood flow and metabolism. As a vasodilator, NO regulates cerebral blood flow, and couples regional brain perfusion with metabolic activity. Following cerebral ischemia, NO levels rise significantly due to activation of neuronal nitric oxide synthase by NMDA receptor mediated calcium entry. Depending on its tissue and enzymatic source, NO may be protective or toxic. This article reviews the effects of NO following cerebral ischemia, the signaling pathways through which NO acts, and its potential roles in cerebral ischemic preconditioning.     

Neuroprotection: lessons from hibernators

Mammals that hibernate experience extreme metabolic states and body temperatures as they transition between euthermia, a state resembling typical warm blooded mammals, and prolonged torpor, a state of suspended animation where the brain receives as low as 10% of normal cerebral blood flow. Transitions into and out of torpor are more physiologically challenging than the extreme metabolic suppression and cold body temperatures of torpor per se. Mammals that hibernate show unprecedented capacities to tolerate cerebral ischemia, a decrease in blood flow to the brain caused by stroke, cardiac arrest or brain trauma. While cerebral ischemia often leads to death or disability in humans and most other mammals, hibernating mammals suffer no ill effects when blood flow to the brain is dramatically decreased during torpor or experimentally induced during euthermia. These animals, as adults, also display rapid and pronounced synaptic flexibility where synapses retract during torpor and rapidly re-emerge upon arousal. A variety of coordinated adaptations contribute to tolerance of cerebral ischemia in these animals. In this review we discuss adaptations in heterothermic mammals that may suggest novel therapeutic targets and strategies to protect the human brain against cerebral ischemic damage and neurodegenerative disease.     

Molecular Mechanisms of Apoptosis in Cerebral Ischemia: Multiple Neuroprotective Opportunities

Cerebral ischemia/reperfusion (I/R) injury triggers multiple and distinct but overlapping cell signaling pathways, which may lead to cell survival or cell damage. There is overwhelming evidence to suggest that besides necrosis, apoptosis do contributes significantly to the cell death subsequent to I/R injury. Both extrinsic and intrinsic apoptotic pathways play a vital role, and upon initiation, these pathways recruit downstream apoptotic molecules to execute cell death. Caspases and Bcl-2 family members appear to be crucial in regulating multiple apoptotic cell death pathways initiated during I/R. Similarly, inhibitor of apoptosis family of proteins (IAPs), mitogen-activated protein kinases, and newly identified apoptogenic molecules, like second mitochondrial-activated factor/direct IAP-binding protein with low pI (Smac/Diablo), omi/high-temperature requirement serine protease A2 (Omi/HtrA2), X-linked mammalian inhibitor of apoptosis protein-associated factor 1, and apoptosis-inducing factor, have emerged as potent regulators of cellular apoptotic/antiapoptotic machinery. All instances of cell survival/death mechanisms triggered during I/R are multifaceted and interlinked, which ultimately decide the fate of brain cells. Moreover, apoptotic cross-talk between major subcellular organelles suggests that therapeutic strategies should be optimally directed at multiple targets/mechanisms for better therapeutic outcome. Based on the current knowledge, this review briefly focuses I/R injury-induced multiple mechanisms of apoptosis, involving key apoptotic regulators and their emerging roles in orchestrating cell death programme. In addition, we have also highlighted the role of autophagy in modulating cell survival/death during cerebral ischemia. Furthermore, an attempt has been made to provide an encouraging outlook on emerging therapeutic approaches for cerebral ischemia. Venkata Prasuja Nakka and Anchal Gusain equally contributed to this work.     

The critical roles of mitophagy in cerebral ischemia

Mitochondria play a key role in various cell processes including ATP production, Ca²⁺ homeostasis, reactive oxygen species (ROS) generation, and apoptosis. The selective removal of impaired mitochondria by autophagosome is known as mitophagy. Cerebral ischemia is a common form of stroke caused by insufficient blood supply to the brain. Emerging evidence suggests that mitophagy plays important roles in the pathophysiological process of cerebral ischemia. This review focuses on the relationship between ischemic brain injury and mitophagy. Based on the latest research, it describes how the signaling pathways of mitophagy appear to be involved in cerebral ischemia.     

多胺及其代谢产物对脑缺血的影响与治疗

多胺(Polyamines)是直链多价阳离子碱性胺,包括腐胺(putrescine,PUT),精胺(spermine,SPM),精脒(spermidine,SPD)等。广泛存在于各种组织细胞内,是一种代谢调控物质,在细胞的增殖分化中起着重要作用。脑梗死是成人致残、致死的最常见疾病之一。研究表明,脑缺血后,多胺及其代谢产物增加,能引起梗死面积的扩大及缺血半暗带神经细胞的坏死。其潜在机制尚不明确,可能与缺血后多胺代谢产生腐胺,3-氨基丙醛(3-amidopropanal 3-AP),过氧化氢及丙烯醛等的活性物质有关,它们参与开放钙离子通道,破坏血脑屏障,形成血管源性脑水肿及缺血再灌注性神经性损伤等病理过程。而抑制多胺代谢可有效地缓解缺血后多胺及其代谢产物增加引起的神经损伤。本文就多胺及代谢产物对脑缺血的神经毒性作用及药物抑制多胺代谢治疗脑梗死做一综述。     

Neurogenic Neuroprotection

1. Stimulation of the rostral-ventromedial pole of the cerebellar fastigial nucleus exerts powerful effects on systemic and cerebral circulation.2. Excitation of fibers passing through the fastigial nucleus evokes sympathoactivation and increases in arterial pressure.3. Increase in cerebral blood flow evoked by excitation of fibers passing through the FN is mediated by intrinsic brain mechanisms independently of metabolism.4. Excitation of the fastigial nucleus neurons in contrast decreases arterial pressure and cerebral blood flow. The latter probably is secondary to the suppression of brain metabolism.5. Excitation of the fastigial nucleus neurons significantly decreases damaging effects of focal and global ischemia on the brain.6. The fastigial nucleus-evoked neuroprotection can be conditioned: 1-h stimulation protects the brain for up to 3 weeks.7. Other brain structures such as subthalamic cerebrovasodilator area and dorsal periaqueductal gray matter also produce long-lasting brain salvage when stimulated.8. More than one mechanism may account for neurogenic neuroprotection.9. Early neuroprotection, which develops immediately after the stimulation, involves opening of potassium channels.10. Delayed long-lasting neuroprotection may involve changes in genes expression resulting in suppression of inflammatory reaction and apoptotic cascade.11. It is conceivable that intrinsic neuroprotective system exists within the brain, which renders the brain more tolerant to adverse stimuli when activated.12. Knowledge of the mechanisms of neurogenic neuroprotection will allow developing new neuroprotective approaches.     

高血糖加重脑缺血损伤机制的研究现状

脑缺血是引起人类死亡的一个重要原因,由于其发病的分子机制十分复杂,各种因子作用相互影响,且多数因子的作用同时存在损伤和保护两种机制,使得脑缺血的研究充满了困难。目前众多研究都证实高血糖对缺血脑组织有损害作用,并可能导致局部或广泛缺血后预后更差。本文依据近几年的实验,重点阐述了五种最新的高血糖加重脑缺血过程和预后损伤的机制假说,包括高血糖通过引起过量谷氨酸释放导致的Ca2^＋大量内流造成损伤、高血糖状态下造成氧化应激从而产生各种自由基对神经元造成损伤、炎症因子相关的损伤、高血糖相关的血液灌流的减少以及高血糖造成脑内酸中毒从而引起损伤。期望这些对机制的探讨能够上加深广大医药研究人员对高血糖加重脑缺血损伤的认识,帮助找到新的药物作用靶点和治疗手段,启发新的研究思路。     

Transforming growth factor-beta: a neuroprotective factor in cerebral ischemia

Transforming growth factor-β (TGF-β) has diverse and multiple roles throughout the body. This review focuses on the evidence supporting its functions in the central nervous system, with a particular emphasis on its purported role in cerebral ischemia. Numerous studies have documented that TGF-β1 levels are enhanced in the brain following cerebral ischemia. As evidence that such an upregulation is beneficial, agonist studies have demonstrated that TGF-β1 reduces neuronal cell death and infarct size following middle cerebral artery occlusion (MCAO), while conversely, antagonist studies have shown increased neuronal cell death and infarct size after MCAO. These studies suggest that TGF-β1 has a neuroprotective role in cerebral ischemia. Recent work with adenoviral-mediated overexpression of TGF-β1 in vivo in mice has further implicated a neuroprotective role for TGF-β1 in cerebral ischemia, as evidenced by a reduction in neuronal cell death, infarct size, and neurological outcome. Additionally, numerous in vitro studies have documented the neuroprotective ability of TGF-β1 in neurons from a variety of species, including rats, mice, chicks, and humans. Of significant interest, TGF-β1 was shown to be protective against a wide variety of death-inducing agents/insults, including hypoxia/ischemia, glutamate excitotoxicity, β-amyloid, oxidative damage, and human immunodeficiency virus. The mechanism of TGF-β1-mediated neuroprotection remains to be resolved, but early evidence suggests that TGF-β1 regulates the expression and ratio of apoptotic (Bad) and antiapoptotic proteins (Bcl-2, Bcl-x₁), creating an environment favorable for cell survival of death-inducing insults. Taken as a whole, these results suggest that TGF-β1 is an important neuroprotective factor that can reduce damage from a wide-array of death-inducing agents/insults in vitro, as well as exert protection of the brain during cerebral ischemia. The authors’ research is supported by research grants (HD-28964 and AG-17186 to DWB) from the National Institutes of Health, NICHD, and NIA.     

Mitochondrial respiratory chain and free radical generation in stroke

Being the second most common cause of death in the industrial countries and one of the major causes of death and disability, stroke has a great effect on public health and is the neurological disease which accounts for the largest number of hospitalizations. In order to develop new treatments, biochemical mechanisms involved in brain damage have been investigated. Among them, oxidant species generated during stroke have been implicated as critical mediators of neuronal injury in this condition, although neuroprotective roles have also been demonstrated. This review is focused on the role of the mitochondrial respiratory chain as both source and target of reactive oxygen and nitrogen species such as nitric oxide, superoxide and peroxynitrite produced in cerebral ischemia. The neuroprotective role of antioxidants or other molecules acting on the mitochondrial respiratory chain and ATP synthesis in the setting of cerebral ischemia is discussed.     

Up-Regulation of TREK-2 Potassium Channels in Cultured Astrocytes Requires De Novo Protein Synthesis: Relevance to Localization of TREK-2 Channels in Astrocytes after Transient Cerebral Ischemia

Excitotoxicity due to glutamate receptor over-activation is one of the key mediators of neuronal death after an ischemic insult. Therefore, a major function of astrocytes is to maintain low extracellular levels of glutamate. The ability of astrocytic glutamate transporters to regulate the extracellular glutamate concentration depends upon the hyperpolarized membrane potential of astrocytes conferred by the presence of K⁺ channels in their membranes. We have previously shown that TREK-2 potassium channels in cultured astrocytes are up-regulated by ischemia and may support glutamate clearance by astrocytes during ischemia. Thus, herein we determine the mechanism leading to this up-regulation and assess the localization of TREK-2 channels in astrocytes after transient middle cerebral artery occlusion. By using a cell surface biotinylation assay we confirmed that functional TREK-2 protein is up-regulated in the astrocytic membrane after ischemic conditions. Using real time RT-PCR, we determined that the levels of TREK-2 mRNA were not increased in response to ischemic conditions. By using Western blot and a variety of protein synthesis inhibitors, we demonstrated that the increase of TREK-2 protein expression requires De novo protein synthesis, while protein degradation pathways do not contribute to TREK-2 up-regulation after ischemic conditions. Immunohistochemical studies revealed TREK-2 localization in astrocytes together with increased expression of the selective glial marker, glial fibrillary acidic protein, in brain 24 hours after transient middle cerebral occlusion. Our data indicate that functional TREK-2 channels are up-regulated in the astrocytic membrane during ischemia through a mechanism requiring De novo protein synthesis. This study provides important information about the mechanisms underlying TREK-2 regulation, which has profound implications in neurological diseases such as ischemia where astrocytes play an important role.     

Mitochondrial and apoptotic neuronal death signaling pathways in cerebral ischemia

Mitochondria play important roles as the powerhouse of the cell. After cerebral ischemia, mitochondria overproduce reactive oxygen species (ROS), which have been thoroughly studied with the use of superoxide dismutase transgenic or knockout animals. ROS directly damage lipids, proteins, and nucleic acids in the cell. Moreover, ROS activate various molecular signaling pathways. Apoptosis-related signals return to mitochondria, then mitochondria induce cell death through the release of pro-apoptotic proteins such as cytochrome c or apoptosis-inducing factor. Although the mechanisms of cell death after cerebral ischemia remain unclear, mitochondria obviously play a role by activating signaling pathways through ROS production and by regulating mitochondria-dependent apoptosis pathways.     

The role of free radicals in cerebral hypoxia and ischemia

This review focuses on the effects that ischemia and hypoxia have on the cerebral cortex and the cerebellum during different periods of life. The acute interruption or reduction of cerebral blood flow, that can be induced by several factors and clinical pathologies, reduces available oxygen to the nervous system and this causes either focal or global brain damage, with characteristic biochemical and molecular alterations that can result in permanent or transitory neurological sequelae or even death. Under these circumstances, an increase in the activity of different isoforms of nitric oxide synthase occurs and nitric oxide is produced. This excess of nitric oxide reacts with cellular proteins yielding nitrotyrosine, thus contributing to cerebral damage. This phenomenon has been studied at different stages of perinatal and postnatal development, including aging animals. Both the duration and the intensity of the ischemic injury were evaluated. In all cases there is overproduction of nitric oxide in ischemia, which may represent an effort to reestablish normal blood flow. Unfortunately, in many cases this response becomes excessive and it triggers a cascade of free-radical reactions, leading to modifications of cerebral plasticity and overt injury.     

Biochemical and Molecular Characteristics of the Brain with Developing Cerebral Infarction

1. We review the biochemical and molecular changes in brain with developing cerebral infarction, based on recent findings in experimental focal cerebral ischemia.2. Occlusion of a cerebral artery produces focal ischemia with a gradual decline of blood flow, differentiating a severely ischemic core where infarct develops rapidly and an area peripheral to the core where the blood flow reduction is moderate (called penumbra). Neuronal injury in the penumbra is essentially reversible but only for several hours. The penumbra area tolerates a longer duration of ischemia than the core and may be salvageable by pharmacological agents such as glutamate antagonists or prompt reperfusion.3. Upon reperfusion, brain cells alter their genomic properties so that protein synthesis becomes restricted to a small number of proteins such as stress proteins. Induction of the stress response is considered to be a rescue program to help to mitigate neuronal injury and to endow the cells with resistance to subsequent ischemic stress. The challenge now is to determine how the neuroprotection conferred by prior sublethal ischemia is achieved so that rational strategies can be developed to detect and manipulate gene expression in brain cells vulnerable to ischemia.4. Expansion of infarction may be caused by an apoptotic mechanism. Investigation of apoptosis may also help in designing novel molecular strategies to prevent ischemic cell death.5. Ischemia/reperfusion injury is accompanied by inflammatory reactions induced by neutrophils and monocytes/macrophages infiltrated and accumulated in ischemic areas. When the role of the inflammatory/immune systems in ischemic brain injury is revealed, new therapeutic targets and agents will emerge to complement and synergize with pharmacological intervention directed against glutamate and Ca²⁺ neurotoxicity.     

Quercetin protects against cerebral ischemia/reperfusion and oxygen glucose deprivation/reoxygenation neurotoxicity

Beyond nutrition effect, quercetin is applied as a complement or an alternative for promoting human health and treating diseases. However, its complicated neuroprotective mechanisms have not yet been fully elucidated. This study provides evidence of an alternative target for quercetin, and sheds light on the mechanisms of its neuroprotection against cerebral ischemia/reperfusion (I/R) injury in Sprague–Dawley rats. Oral pretreatment using quercetin has alleviated cerebral I/R-induced neurological deficits, brain infarction, blood–brain barrier disruption, oxidative stress, TNF-α and IL-1β mRNA expression, along with apoptotic caspase 3 activity. The neuroprotective, anti-oxidative, anti-inflammatory, and anti-apoptotic effects of quercetin were replicated in rat hippocampal slice cultures and neuron/glia cultures which suffered from oxygen–glucose deprivation and reoxygenation (OGDR). Biochemical studies revealed a reduction of extracellular signal-regulated kinase (ERK) and Akt phosphorylation, along with an increase in protein tyrosine and serine/threonine phosphatase activity in cerebral I/R rat cortical tissues and OGDR hippocampal slice and neuron/glia cultures. Quercetin alleviated the changes in ERK/Akt phosphorylation and protein phosphatase activities. Inhibition of ERK or Akt alone was enough to cause apoptotic cell death and cytotoxicity in hippocampal slice cultures and neuron/glia cultures, while activators of ERK or Akt alleviated OGDR-induced cytotoxicity. Taken together, our results demonstrate that quercetin alleviated the increment of protein tyrosine and serine/threonine phosphatase activity, along with the reduction of ERK and Akt phosphorylation, which may play pivotal roles in the expansion of brain injury after cerebral I/R.     

Neuronal Ca2+-activated K+ channels limit brain infarction and promote survival

Neuronal calcium-activated potassium channels of the BK type are activated by membrane depolarization and intracellular Ca(2+) ions. It has been suggested that these channels may play a key neuroprotective role during and after brain ischemia, but this hypothesis has so far not been tested by selective BK-channel manipulations in vivo. To elucidate the in vivo contribution of neuronal BK channels in acute focal cerebral ischemia, we performed middle cerebral artery occlusion (MCAO) in mice lacking BK channels (homozygous mice lacking the BK channel alpha subunit, BK(-/-)). MCAO was performed in BK(-/-) and WT mice for 90 minutes followed by a 7-hour-reperfusion period. Coronal 1 mm thick sections were stained with 2,3,5-triphenyltetrazolium chloride to reveal the infarction area. We found that transient focal cerebral ischemia by MCAO produced larger infarct volume, more severe neurological deficits, and higher post-ischemic mortality in BK(-/-) mice compared to WT littermates. However, the regional cerebral blood flow was not significantly different between genotypes as measured by Laser Doppler (LD) flowmetry pre-ischemically, intra-ischemically, and post-ischemically, suggesting that the different impact of MCAO in BK(-/-) vs. WT was not due to vascular BK channels. Furthermore, when NMDA was injected intracerebrally in non-ischemic mice, NMDA-induced neurotoxicity was found to be larger in BK(-/-) mice compared to WT. Whole-cell patch clamp recordings from CA1 pyramidal cells in organotypic hippocampal slice cultures revealed that BK channels contribute to rapid action potential repolarization, as previously found in acute slices. When these cultures were exposed to ischemia-like conditions this induced significantly more neuronal death in BK(-/-) than in WT cultures. These results indicate that neuronal BK channels are important for protection against ischemic brain damage.