Acidotoxicity in brain ischaemia

Detection of tissue pH with quantitative chemical exchange saturation transfer magnetic resonance imaging

Chemical exchange saturation transfer (CEST) magnetic resonance imaging (MRI) has emerged as a novel means for sensitive detection of dilute labile protons and chemical exchange rates. By sensitizing to pH-dependent chemical exchange, CEST MRI has shown promising results in monitoring tissue statuses such as pH changes in disorders like acute stroke, tumor, and acute kidney injury. This article briefly reviews the basic principles for CEST imaging and quantitative measures, from the simplistic asymmetry analysis to multipool Lorentzian decoupling and quasi-steady-state reconstruction. In particular, the advantages and limitations of commonly used quantitative approaches for CEST applications are discussed.

Inhibition of intracellular proton-sensitive Ca2+-permeable TRPV3 channels protects against ischemic brain injury

Ischemic brain stroke is pathologically characterized by tissue acidosis, sustained calcium entry and progressive cell death. Previous studies focusing on antagonizing N-methyl-d-aspartate (NMDA) receptors have failed to translate any clinical benefits, suggesting a non-NMDA mechanism involved in the sustained injury after stroke. Here, we report that inhibition of intracellular proton-sensitive Ca²⁺-permeable transient receptor potential vanilloid 3 (TRPV3) channel protects against cerebral ischemia/reperfusion (I/R) injury. TRPV3 expression is upregulated in mice subjected to cerebral I/R injury. Silencing of TRPV3 reduces intrinsic neuronal excitability, excitatory synaptic transmissions, and also attenuates cerebral I/R injury in mouse model of transient middle cerebral artery occlusion (tMCAO). Conversely, overexpressing or re-expressing TRPV3 increases neuronal excitability, excitatory synaptic transmissions and aggravates cerebral I/R injury. Furthermore, specific inhibition of TRPV3 by natural forsythoside B decreases neural excitability and attenuates cerebral I/R injury. Taken together, our findings for the first time reveal a causative role of neuronal TRPV3 channel in progressive cell death after stroke, and blocking overactive TRPV3 channel may provide therapeutic potential for ischemic brain injury.

Acid-sensing ion channel 1a is involved in ischaemia/reperfusion induced kidney injury by increasing renal epithelia cell apoptosis

Acidic microenvironment is commonly observed in ischaemic tissue. In the kidney, extracellular pH dropped from 7.4 to 6.5 within 10 minutes initiation of ischaemia. Acid‐sensing ion channels (ASICs) can be activated by pH drops from 7.4 to 7.0 or lower and permeates to Ca²⁺entrance. Thus, activation of ASIC1a can mediate the intracellular Ca²⁺ accumulation and play crucial roles in apoptosis of cells. However, the role of ASICs in renal ischaemic injury is unclear. The aim of the present study was to test the hypothesis that ischaemia increases renal epithelia cell apoptosis through ASIC1a‐mediated calcium entry. The results show that ASIC1a distributed in the proximal tubule with higher level in the renal tubule ischaemic injury both in vivo and in vitro. In vivo, Injection of ASIC1a inhibitor PcTx‐1 previous to ischaemia/reperfusion (I/R) operation attenuated renal ischaemic injury. In vitro, HK‐2 cells were pre‐treated with PcTx‐1 before hypoxia, the intracellular concentration of Ca²⁺, mitochondrial transmembrane potential (∆ψm) and apoptosis was measured. Blocking ASIC1a attenuated I/R induced Ca²⁺ overflow, loss of ∆ψm and apoptosis in HK‐2 cells. The results revealed that ASIC1a localized in the proximal tubular and contributed to I/R induced kidney injury. Consequently, targeting the ASIC1a may prove to be a novel strategy for AKI patients.

More sensitivity of cortical GABAergic neurons than glutamatergic neurons in response to acidosis

Acidosis impairs brain functions. Neuron-specific mechanisms underlying acidosis-induced brain dysfunction remain elusive. We studied the sensitivity of cortical GABAergic neurons and glutamatergic neurons to acidosis by whole-cell recording in brain slices. The acidification to the neurons was induced by perfusing artificial cerebral spinal fluid with lower pH. This acidification impairs excitability and synaptic transmission in the glutamatergic and GABAergic neurons. Acidosis impairs spiking capacity in the GABAergic neurons more than in the glutamatergic neurons. Acidosis also strengthens glutamatergic synaptic transmission and attenuates GABAergic synaptic transmission on the GABAergic neurons more than the glutamatergic neurons, which results in the functional impairment of these GABAergic neurons. This acidosis-induced dysfunction predominantly in the cortical GABAergic neurons drives the homeostasis of neuronal networks toward overexcitation and exacerbates neuronal impairment.

Neuropathophysiology of Brain Injury

Every year in the United States, millions of individuals incur ischemic brain injury from stroke, cardiac arrest, or traumatic brain injury. These acquired brain injuries can lead to death or long-term neurologic and neuropsychological impairments. The mechanisms of ischemic and traumatic brain injury that lead to these deficiencies result from a complex interplay of interdependent molecular pathways, including excitotoxicity, acidotoxicity, ionic imbalance, oxidative stress, inflammation, and apoptosis. This article reviews several mechanisms of brain injury and discusses recent developments. Although much is known from animal models of injury, it has been difficult to translate these effects to humans.

Acidosis-Induced Dysfunction of Cortical GABAergic Neurons through Astrocyte-Related Excitotoxicity

BACKGROUND

Acidosis impairs cognitions and behaviors presumably by acidification-induced changes in neuronal metabolism. Cortical GABAergic neurons are vulnerable to pathological factors and their injury leads to brain dysfunction. How acidosis induces GABAergic neuron injury remains elusive. As the glia cells and neurons interact each other, we intend to examine the role of the astrocytes in acidosis-induced GABAergic neuron injury.

RESULTS

Experiments were done at GABAergic cells and astrocytes in mouse cortical slices. To identify astrocytic involvement in acidosis-induced impairment, we induced the acidification in single GABAergic neuron by infusing proton intracellularly or in both neurons and astrocytes by using proton extracellularly. Compared the effects of intracellular acidification and extracellular acidification on GABAergic neurons, we found that their active intrinsic properties and synaptic outputs appeared more severely impaired in extracellular acidosis than intracellular acidosis. Meanwhile, extracellular acidosis deteriorated glutamate transporter currents on the astrocytes and upregulated excitatory synaptic transmission on the GABAergic neurons. Moreover, the antagonists of glutamate NMDA-/AMPA-receptors partially reverse extracellular acidosis-induced injury in the GABAergic neurons.

CONCLUSION

Our studies suggest that acidosis leads to the dysfunction of cortical GABAergic neurons by astrocyte-mediated excitotoxicity, in addition to their metabolic changes as indicated previously.

Does closure of acid-sensing ion channels reduce ischemia/reperfusion injury in the rat brain?

Acidosis is a common characteristic of brain damage. Because studies have shown that permeable Ca²⁺-acid-sensing ion channels can mediate the toxic effects of calcium ions, they have become new targets against pain and various intracranial diseases. However, the mechanism associated with expression of these channels remains unclear. This study sought to observe the expression characteristics of permeable Ca²⁺-acid-sensing ion channels during different reperfusion inflows in rats after cerebral ischemia. The rat models were randomly divided into three groups: adaptive ischemia/reperfusion group, one-time ischemia/reperfusion group, and severe cerebral ischemic injury group. Western blot assays and immunofluorescence staining results exhibited that when compared with the one-time ischemia/reperfusion group, acid-sensing ion channel 3 and Bcl-x/l expression decreased in the adaptive ischemia/reperfusion group. Calmodulin expression was lowest in the adaptive ischemia/reperfusion group. Following adaptive reperfusion, common carotid artery flow was close to normal, and the pH value improved. Results verified that adaptive reperfusion following cerebral ischemia can suppress acid-sensing ion channel 3 expression, significantly reduce Ca²⁺ influx, inhibit calcium overload, and diminish Ca²⁺ toxicity. The effects of adaptive ischemia/reperfusion on suppressing cell apoptosis and relieving brain damage were better than that of one-time ischemia/reperfusion.

Plasmin-dependent modulation of the blood-brain barrier: a major consideration during tPA-induced thrombolysis?

Plasmin, the principal downstream product of tissue-type plasminogen activator (tPA), is known for its potent fibrin-degrading capacity but is also recognized for many non-fibrinolytic activities. Curiously, plasmin has not been conclusively linked to blood-brain barrier (BBB) disruption during recombinant tPA (rtPA)-induced thrombolysis in ischemic stroke. This is surprising given the substantial involvement of tPA in the modulation of BBB permeability and the co-existence of tPA and plasminogen in both blood and brain throughout the ischemic event. Here, we review the work that argues a role for plasmin together with endogenous tPA or rtPA in BBB alteration, presenting the overall controversy around the topic yet creating a rational case for an involvement of plasmin in this process.

Fast, non-competitive and reversible inhibition of NMDA-activated currents by 2-BFI confers neuroprotection

Excessive activation of the N-methyl-D-aspartic acid (NMDA) type glutamate receptors (NMDARs) causes excitotoxicity, a process important in stroke-induced neuronal death. Drugs that inhibit NMDA receptor-mediated [Ca(2+)]i influx are potential leads for development to treat excitotoxicity-induced brain damage. Our previous studies showed that 2-(2-benzofu-ranyl)-2-imidazoline (2-BFI), an immidazoline receptor ligand, dose-dependently protects rodent brains from cerebral ischemia injury. However, the molecular mechanisms remain unclear. In this study, we found that 2-BFI transiently and reversibly inhibits NMDA, but not AMPA currents, in a dose-dependent manner in cultured rat cortical neurons. The mechanism of 2-BFI inhibition of NMDAR is through a noncompetitive fashion with a faster on (Kon = 2.19±0.33×10(-9) M(-1) sec(-1)) and off rate (Koff = 0.67±0.02 sec(-1)) than those of memantine, a gold standard for therapeutic inhibition NMDAR-induced excitotoxicity. 2-BFI also transiently and reversibly blocked NMDA receptor-mediated calcium entry to cultured neurons and provided long-term neuroprotection against NMDA toxicity in vitro. Collectively, these studies demonstrated a potential mechanism of 2-BFI-mediated neuroprotection and indicated that 2-BFI is an excellent candidate for repositioning as a drug for stroke treatment.

Metabolic multianalyte microphysiometry reveals extracellular acidosis is an essential mediator of neuronal preconditioning

Metabolic adaptation to stress is a crucial yet poorly understood phenomenon, particularly in the central nervous system (CNS). The ability to identify essential metabolic events which predict neuronal fate in response to injury is critical to developing predictive markers of outcome, for interpreting CNS spectroscopic imaging, and for providing a richer understanding of the relevance of clinical indices of stress which are routinely collected. In this work, real-time multianalyte microphysiometry was used to dynamically assess multiple markers of aerobic and anaerobic respiration through simultaneous electrochemical measurement of extracellular glucose, lactate, oxygen, and acid. Pure neuronal cultures and mixed cultures of neurons and glia were compared following a 90 min exposure to aglycemia. This stress was cytotoxic to neurons yet resulted in no appreciable increase in cell death in age-matched mixed cultures. The metabolic profile of the cultures was similar in that aglycemia resulted in decreases in extracellular acidification and lactate release in both pure neurons and mixed cultures. However, oxygen consumption was only diminished in the neuron enriched cultures. The differences became more pronounced when cells were returned to glucose-containing media upon which extracellular acidification and oxygen consumption never returned to baseline in cells fated to die. Taken together, these data suggest that lactate release is not predictive of neuronal survival. Moreover, they reveal a previously unappreciated relationship of astrocytes in maintaining oxygen uptake and a correlation between metabolic recovery of neurons and extracellular acidification.

An impairment of cortical GABAergic neurons is involved in alkalosis-induced brain dysfunctions

Acid-base imbalance leads to pathological cognition and behaviors in the clinical practices. In the comparison with acidosis, the cellular mechanisms underlying alkalosis-induced brain dysfunction remain unclear. By using electrophysiological approach, we investigated the influences of high extracellular pH environment on cortical GABAergic neurons in terms of their responsiveness to synaptic inputs and their ability to produce action potentials. Artificial cerebral spinal fluid in high pH impairs excitatory synaptic transmission and spike initiation in cortical GABAergic neurons. The alkalosis-induced dysfunction of GABAergic neurons is associated with the decrease of receptor responsiveness and the increases of spike refractory periods and threshold potentials. Our studies reveal that alkalosis impairs cortical GABAergic neurons and subsequently deteriorate brain functions. The molecular targets for alkalosis action include glutamate receptor-channels and voltage-gated sodium channels on GABAergic neurons.

Quercetin in hypoxia-induced oxidative stress: novel target for neuroprotection

Oxidative stress in the central nervous system is one of the key players for neurodegeneration. Thus, antioxidants could play important roles in treating several neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, and aging-related brain disorders. This review is focused on the new developments in oxidative stress-induced neurodegeneration. Further, based on our own investigations, new roles of quercetin, an antioxidant compound in hypoxia and ischemia induced neuroprotection in relation to suppression of oxidative stress, improvement in behavioral function, reduction in infarct volume, brain swelling, and cellular injury in both in vivo and in vitro models are discussed. Our new findings clearly suggest that antioxidant compounds have potential role in therapeutic strategies to treat neurodegenerative diseases in clinical settings.

Magnetic resonance characterization of ischemic tissue metabolism

Magnetic resonance imaging (MRI) and spectroscopy (MRS) are versatile diagnostic techniques capable of characterizing the complex stroke pathophysiology, and hold great promise for guiding stroke treatment. Particularly, tissue viability and salvageability are closely associated with its metabolic status. Upon ischemia, ischemic tissue metabolism is disrupted including altered metabolism of glucose and oxygen, elevated lactate production/accumulation, tissue acidification and eventually, adenosine triphosphate (ATP) depletion and energy failure. Whereas metabolism impairment during ischemic stroke is complex, it may be monitored non-invasively with magnetic resonance (MR)-based techniques. Our current article provides a concise overview of stroke pathology, conventional and emerging imaging and spectroscopy techniques, and data analysis tools for characterizing ischemic tissue damage.

Apomyoglobin mutants with single point mutations at val10 can form amyloid structures at permissive temperature

Formation of amyloid-like protein aggregates in human organs and tissues underlies many serious diseases, therefore being in the focus of numerous biochemical, medical, and molecular biological studies. So far, formation of amyloids by globular proteins has been studied mostly under conditions that strongly destabilized their native structure. Here we present our results obtained at permissive temperature by thioflavin T fluorescence, far UV CD, IR spectroscopy, and electron microscopy. We used apomyoglobin and its mutants with Ala or Phe substituted for Val10 that are structurally close to wild type apomyoglobin. It is shown that at permissive temperature the ability of the protein to form amyloids depends on the extent of its structural destabilization, but not on hydrophobicity of the substituting residue. A possible difference between amyloids formed by strongly destabilized proteins and those yielded by proteins with a slightly fluctuating native structure, as well as the stroke and infarction effect on the ability of proteins to form amyloid structures, are discussed.

Neuroprotective effect of acid-sensing ion channel inhibitor psalmotoxin-1 after hypoxia-ischemia in newborn piglet striatum

Na+,Ca2+-permeable acid-sensing ion channel 1a (ASIC1a) is involved in the pathophysiologic process of adult focal brain ischemia. However, little is known about its role in the pathogenesis of global cerebral ischemia or newborn hypoxia-ischemia (H-I). Here, using a newborn piglet model of asphyxia-induced cardiac arrest, we investigated the effect of ASIC1a-specific blocker psalmotoxin-1 on neuronal injury. During asphyxia and the first 30min of recovery, brain tissue pH fell below 7.0, the approximate activation pH of ASIC1a. Psalmotoxin-1 injection at 20min before hypoxia, but not at 20min of recovery, partially protected the striatonigral and striatopallidal neurons in putamen. Psalmotoxin-1 pretreatment largely attenuated the increased protein kinase A-dependent phosphorylation of DARPP-32 and N-methyl-d-aspartate (NMDA) receptor NR1 subunit and decreased nitrative and oxidative damage to proteins at 3h of recovery. Pretreatment with NMDA receptor antagonist MK-801 also provided partial neuroprotection in putamen, and combined pretreatment with psalmotoxin-1 and MK-801 yielded additive neuroprotection. These results indicate that ASIC1a activation contributes to neuronal death in newborn putamen after H-I through mechanisms that may involve protein kinase A-dependent phosphorylation of NMDA receptor and nitrative and oxidative stress.

The role of ASIC1a in neuroprotection elicited by quercetin in focal cerebral ischemia

One of the major instigators to neuronal cell death and brain damage following cerebral ischemia is calcium dysregulation. The intracellular calcium overload resulting from glutamate excitotoxicity is considered a major determinant for neuronal loss during cerebral ischemia. Moreover, ASIC1a activation due to acidosis also promotes intracellular calcium overload during ischemic insult. Interestingly, ASIC1a was found to be inhibited by some flavonoids which carry an anti-inflammatory property particularly quercetin, which could be exploited in hypoxic conditions like cerebral ischemia. This encourages us to investigate the neuroprotective effect of quercetin besides its possible downstream signaling mechanism in focal cerebral ischemia. The treatment of quercetin 30min before ischemia and 4h after reperfusion shows significant protection from ischemic injury as noticed by reduction in cerebral infarct volume and neurobehavioral deficit. In addition to earlier calcium dependent rise in the levels of nitrite and MDA exhibited marked reduction (P<0.01) in their levels when given quercetin pretreatment in ischemic brain regions. The quercetin treatment also reduced the spectrin break down products (SBDP) caused by ischemic activation of calcium dependent protease calpain. In ex-vivo study, it was also observed that quercetin inhibited the acid mediated intracellular calcium levels in rat brain synaptoneurosomes. These studies suggest the neuroprotective role of quercetin in focal cerebral ischemia by regulation of ASIC1a.

Cortical GABAergic neurons and cerebellar Purkinje cells respond to ischemia-pathogenic factors differently

GABAergic neurons in the central nervous system are vulnerable to hazard situations, such as ischemia and toxic substances, under which their dysfunction results in neuronal excitotoxicity and subsequently cell death. How ischemia-related pathogenic factors influence the functions of different GABAergic neurons remains to be documented. We investigated this issue at cortical GABAergic neurons and cerebellar Purkinje cells in brain slices by whole-cell recordings. Our results demonstrate that ischemia, cellular Ca(2+)-overload and acidosis lower the spike capacity of cortical GABAergic neurons, but elevate that of cerebellar Purkinje cells. These changes of spike encoding at two types of GABAergic cells are associated with the different effects of three factors on spike refractory periods and threshold potentials, which are mediated by voltage-gated sodium channels. Mechanisms underlying such differences are discussed.

Modes of Neuronal Calcium Entry and Homeostasis following Cerebral Ischemia

One of the major instigators leading to neuronal cell death and brain damage following cerebral ischemia is calcium dysregulation. The neuron's inability to maintain calcium homeostasis is believed to be a result of increased calcium influx and impaired calcium extrusion across the plasma membrane. The need to better understand the cellular and biochemical mechanisms of calcium dysregulation contributing to neuronal loss following stroke/cerebral ischemia is essential for the development of new treatments in order to reduce ischemic brain injury. The aim of this paper is to provide a concise overview of the various calcium influx pathways in response to ischemia and how neuronal cells attempts to overcome this calcium overload.

Ca2+ and acidosis synergistically lead to the dysfunction of cortical GABAergic neurons during ischemia

Cell death in cerebral ischemia is presumably initiated by neural excitotoxicity resulted from the dysfunction of inhibitory neurons in early stage. Molecular processes underlying the ischemic injury of inhibitory neurons remain to be elusive, which we investigated by biochemical manipulations with cellular imaging and patch clamp at GFP-labeled GABAergic cells in cortical slices. Ischemia induces Ca(2+) elevation, acidosis and dysfunction in GABAergic cells. An elevation of cytoplasmic Ca(2+) or H(+) impairs the encoding of action potentials in these neurons. The effects of Ca(2+) and H(+) are additive in nature and occlude ischemic outcomes. Ischemia impairs spike production through prolonging spike refractory periods and raising threshold potentials. Therefore, calcium toxicity and acidosis during ischemia synergistically impair the dynamics of sodium channels and function of cortical GABAergic neurons, which lead to neural excitotoxicity. Our results also suggest that the cocktail therapeutics is needed to prevent neuronal death from ischemia.

Bioenergetics of cerebral ischemia: a cellular perspective

In cerebral ischemia survival of neurons, astrocytes, oligodendrocytes and endothelial cells is threatened during energy deprivation and/or following re-supply of oxygen and glucose. After a brief summary of characteristics of different cells types, emphasizing the dependence of all on oxidative metabolism, the bioenergetics of focal and global ischemia is discussed, distinguishing between events during energy deprivation and subsequent recovery attempt after re-circulation. Gray and white matter ischemia are described separately, and distinctions are made between mature and immature brains. Next comes a description of bioenergetics in individual cell types in culture during oxygen/glucose deprivation or exposure to metabolic inhibitors and following re-establishment of normal aerated conditions. Due to their expression of NMDA and non-NMDA receptors neurons and oligodendrocytes are exquisitely sensitive to excitotoxicity by glutamate, which reaches high extracellular concentrations in ischemic brain for several reasons, including failing astrocytic uptake. Excitotoxicity kills brain cells by energetic exhaustion (due to Na(+) extrusion after channel-mediated entry) combined with mitochondrial Ca(2+)-mediated injury and formation of reactive oxygen species. Many (but not all) astrocytes survive energy deprivation for extended periods, but after return to aerated conditions they are vulnerable to mitochondrial damage by cytoplasmic/mitochondrial Ca(2+) overload and to NAD(+) deficiency. Ca(2+) overload is established by reversal of Na(+)/Ca(2+) exchangers following Na(+) accumulation during Na(+)-K(+)-Cl(-) cotransporter stimulation or pH regulation, compensating for excessive acid production. NAD(+) deficiency inhibits glycolysis and eventually oxidative metabolism, secondary to poly(ADP-ribose)polymerase (PARP) activity following DNA damage. Hyperglycemia can be beneficial for neurons but increases astrocytic death due to enhanced acidosis.