Evolving Paradigms for Neuroprotection: Molecular Identification of Ischemic Penumbra

Involvement of Ceramide Metabolism in Cerebral Ischemia

Ischemic stroke, caused by the interruption of blood flow to the brain and subsequent neuronal death, represents one of the main causes of disability in worldwide. Although reperfusion therapies have shown efficacy in a limited number of patients with acute ischemic stroke, neuroprotective drugs and recovery strategies have been widely assessed, but none of them have been successful in clinical practice. Therefore, the search for new therapeutic approaches is still necessary. Sphingolipids consist of a family of lipidic molecules with both structural and cell signaling functions. Regulation of sphingolipid metabolism is crucial for cell fate and homeostasis in the body. Different works have emphasized the implication of its metabolism in different pathologies, such as diabetes, cancer, neurodegeneration, or atherosclerosis. Other studies have shown its implication in the risk of suffering a stroke and its progression. This review will highlight the implications of sphingolipid metabolism enzymes in acute ischemic stroke.

Blood Biomarkers for Stroke Diagnosis and Management

Biomarkers are objective indicators used to assess normal or pathological processes, evaluate responses to treatment and predict outcomes. Many blood biomarkers already guide decision-making in clinical practice. In stroke, the number of candidate biomarkers is constantly increasing. These biomarkers include proteins, ribonucleic acids, lipids or metabolites. Although biomarkers have the potential to improve the diagnosis and the management of patients with stroke, there is currently no marker that has demonstrated sufficient sensitivity, specificity, rapidity, precision, and cost-effectiveness to be used in the routine management of stroke, thus highlighting the need for additional work. A better standardization of clinical, laboratory and statistical procedures between centers is indispensable to optimize biomarker performance. This review focuses on blood biomarkers that have shown promise for translation into clinical practice and describes some newly reported markers that could add to routine stroke care. Avenues for the discovery of new stroke biomarkers and future research are discussed. The description of the biomarkers is organized according to their expected application in clinical practice: diagnosis, treatment decision, and outcome prediction.

Oxidative Stress: Love and Hate History in Central Nervous System

Molecular oxygen is essential for aerobic organisms in order to synthesize large amounts of energy during the process of oxidative phosphorylation and it is harnessed in the form of adenosine triphosphate, the chemical energy of the cell. Oxygen is toxic for anaerobic organisms but it is also less obvious that oxygen is poisonous to aerobic organisms at higher concentrations of oxygen. For instance, oxygen toxicity is a condition resulting from the harmful effects of breathing molecular oxygen at increased partial pressures. Reactive oxygen species (ROS) are chemically reactive molecules containing oxygen that are formed as a natural byproduct of the normal metabolism of oxygen and have important roles in cell signaling and homeostasis. However, in pathological conditions ROS levels can increase dramatically. This may result in significant damage to cell structures. Living organisms have been adapted to the ROS in two ways: they can mitigate the unwanted effects through removal by the antioxidant systems and can advantageously use them as messengers in cell signaling and regulation of body functions. Some other physiological functions of ROS include the regulation of vascular tone, detection, and adaptation to hypoxia. In this review, we describe the mechanisms of oxidative damage and its relationship with the most highly studied neurodegenerative diseases.

INJECTED CITICOLINE IMPROVES IMPAIRMENT AND DISABILITY DURING ACUTE PHASE TREATMENT IN ISCHEMIC STROKE PATIENTS

Treatment strategy of ischemic stroke is to reduce the extent of the damage and rescue neurons from death in the early days of ischemic events. Recombinant Tissue-Plasminogen Activator (r-TPA) is the only recommended therapy, but their use is very limited. Citicoline is a neuroprotectant with a therapeutic effect on several stages of the ischemic cascade. However, its use is still being debated. The purpose of this study was to analyze the use of supplementation citicoline injection in patients with acute ischemic stroke in relations to differences in changes in the level of interference (impairment), rate limitation (disability) and the level of obstruction (handicap) between the group receiving supplementation of citicoline injection 2x500 mg iv and the group without supplementation during acute phase treatment. This study was a prospective cohort study using experimental design in patients with acute ischemic stroke who met the inclusion and exclusion criteria with or without supplementation citicoline between January - April 2015 in the National Stroke Hospital, Bukittinggi. Rate of interference was assessed with NIHSS, level of limitations with Barthel Index, and level of obstruction with modified Rankin Scale. Assessment was done 2 times, before and after the treatment. Statistical methods used in this study were Wilcoxon signed rank test, paired T-test and Mann-Whitney test. This study was conducted on 50 subjects divided into 2 groups, a control group without supplementation and group treated with injected citicoline of 2x500 mg iv. Demographic and baseline characteristics did not differ between groups. There were differences in level of interference changes. Mean decrease in control group was 0.96 ± 1.74 NIHSS, while that in treatment group was 2.84 ± 1.46 NIHSS (p <0.05). There were differences in changes in the level of limitations. Mean increase of Barthel Index in control group 9.60 ± 11.17 and in treatment group 20.40 ± 13.99 (p <0.05). However, changes in the level obstacle showed no difference. In conclusion, citicoline injection supplementation in patients with ischemic stroke during acute phase treatment showed improvement differences in changes in the level of distraction (impairment) and the rate limitations (disability), but showed no difference in changes in the level of obstruction (handycaps).

Human endothelial progenitor cells rescue cortical neurons from oxygen-glucose deprivation induced death

BACKGROUND AND AIM

Cerebral ischemia is characterized by both acute and delayed neuronal injuries. Neuro-protection is a major issue that should be properly addressed from a pharmacological point of view, and cell-based treatment approaches are of interest due to their potential pleiotropic effects. Endothelial progenitor cells have the advantage of being mobilized from the bone marrow into the circulation, but have been less studied than other stem cells, such as mesenchymal stem cells. Therefore, the comparison between human endothelial progenitor cells (hEPC) and human mesenchymal progenitor cells (hMSC) in terms of efficacy in rescuing neurons from cell death after transitory ischemia is the aim of the current study, in the effort to address further directions.

MATERIALS AND METHODS

In vitro model of oxygen-glucose deprivation (OGD) on a primary culture of rodent cortical neurons was set up with different durations of exposure: 1, 2 and 3hrs with assessment of neuron survival. The 2hrs OGD was chosen for the subsequent experiments. After 2hrs OGD neurons were either placed in indirect co-culture with hMSC or hEPC or cultured in hMSC or hEPC conditioned medium and cell viability was evaluated by MTT assay.

RESULTS

At day 2 after 2hrs OGD exposure, mean neuronal survival was 47.9±24.2%. In contrast, after treatment with hEPC and hMSC indirect co-culture was 74.1±27.3%; and 69.4±18.8%, respectively. In contrast, treatment with conditioned medium did not provide any advantage in terms of survival to OGD neurons

CONCLUSION

The study shows the efficacy of hEPC in indirect co-culture to rescue neurons from cell death after OGD, comparable to that of hMSC. hEPC deserve further studies given their potential interest for ischemia.

Passiflora actinia hydroalcoholic extract and its major constituent, isovitexin, are neuroprotective against glutamate-induced cell damage in mice hippocampal slices

OBJECTIVES

To investigate whether Passiflora actinia hydroalcoholic extract and its major constituent, isovitexin, protect mice hippocampal brain slices from glutamate-induced neurotoxicity.

METHODS

Neuroprotective effect of the extract against glutamate-induced excitotoxicity (10 mm) was evaluated through cell viability of hippocampal slices. The extract or its flavonoids were directly applied to hippocampal slices and then subjected to glutamate-induced toxicity. Alternatively, hippocampal slices from extract-treated mice were also subjected to the same toxicity protocol.

KEY FINDINGS

Mice supplementation with the extract protected hippocampal slices from in-vitro neurotoxicity. When directly applied to hippocampal slices, the extract showed a higher neuroprotective potential than a commercial dry extract of Passiflora incarnata, which was related to P. actinia extract which had higher isovitexin and total flavonoid content expressed as isovitexin. Isovitexin, but not apigenin, induced a similar neuroprotective response when applied alone, at a concentration equivalent to that found in the extract.

CONCLUSIONS

This study highlights new neuropharmacological activity of the Passiflora genus, suggesting that it can act as modulator of the glutamatergic system. The search for improved pharmacotherapies with novel mechanisms of action has been shown of great importance for the treatment of resistant neurological and psychiatric disorders.

Sulforaphane preconditioning of the Nrf2/HO-1 defense pathway protects the cerebral vasculature against blood-brain barrier disruption and neurological deficits in stroke

Disruption of the blood-brain barrier (BBB) and cerebral edema are the major pathogenic mechanisms leading to neurological dysfunction and death after ischemic stroke. The brain protects itself against infarction via activation of endogenous antioxidant defense mechanisms, and we here report the first evidence that sulforaphane-mediated preactivation of nuclear factor erythroid 2-related factor 2 (Nrf2) and its downstream target heme oxygenase-1 (HO-1) in the cerebral vasculature protects the brain against stroke. To induce ischemic stroke, Sprague-Dawley rats were subjected to 70 min middle cerebral artery occlusion (MCAo) followed by 4, 24, or 72 h reperfusion. Nrf2 and HO-1 protein expression was upregulated in cerebral microvessels of peri-infarct regions after 4-72 h, with HO-1 preferentially associated with perivascular astrocytes rather than the cerebrovascular endothelium. In naïve rats, treatment with sulforaphane increased Nrf2 expression in cerebral microvessels after 24h. Upregulation of Nrf2 by sulforaphane treatment prior to transient MCAo (1h) was associated with increased HO-1 expression in perivascular astrocytes in peri-infarct regions and cerebral endothelium in the infarct core. BBB disruption, lesion progression, as analyzed by MRI, and neurological deficits were reduced by sulforaphane pretreatment. As sulforaphane pretreatment led to a moderate increase in peroxynitrite generation, we suggest that hormetic preconditioning underlies sulforaphane-mediated protection against stroke. In conclusion, we propose that pharmacological or dietary interventions aimed to precondition the brain via activation of the Nrf2 defense pathway in the cerebral microvasculature provide a novel therapeutic approach for preventing BBB breakdown and neurological dysfunction in stroke.

Relationships between brain and body temperature, clinical and imaging outcomes after ischemic stroke

Pyrexia soon after stroke is associated with severe stroke and poor functional outcome. Few studies have assessed brain temperature after stroke in patients, so little is known of its associations with body temperature, stroke severity, or outcome. We measured temperatures in ischemic and normal-appearing brain using (1)H-magnetic resonance spectroscopy and its correlations with body (tympanic) temperature measured four-hourly, infarct growth by 5 days, early neurologic (National Institute of Health Stroke Scale, NIHSS) and late functional outcome (death or dependency). Among 40 patients (mean age 73 years, median NIHSS 7, imaged at median 17 hours), temperature in ischemic brain was higher than in normal-appearing brain on admission (38.6°C-core, 37.9°C-contralateral hemisphere, P=0.03) but both were equally elevated by 5 days; both were higher than tympanic temperature. Ischemic lesion temperature was not associated with NIHSS or 3-month functional outcome; in contrast, higher contralateral normal-appearing brain temperature was associated with worse NIHSS, infarct expansion and poor functional outcome, similar to associations for tympanic temperature. We conclude that brain temperature is higher than body temperature; that elevated temperature in ischemic brain reflects a local tissue response to ischemia, whereas pyrexia reflects the systemic response to stroke, occurs later, and is associated with adverse outcomes.

Blood biomarkers of ischemic stroke

This review provides a summary of the protein and RNA biomarkers that have been studied for the diagnosis and assessment of ischemic stroke. Many of the biomarkers identified relate to the pathophysiology of ischemic stroke, including ischemia of CNS tissue, acute thrombosis and inflammatory response. These biomarkers are summarized by their intended clinical application in ischemic stroke including diagnosis, prediction of stroke severity and outcome, and stratification of patients for stroke therapy. Among the biomarkers discussed are recent whole genome studies using RNA expression profiles to diagnose ischemic stroke and stroke etiology. Though many candidate blood based biomarkers for ischemic stroke have been identified, none are currently used in clinical practice. With further well designed study and careful validation, the development of blood biomarkers to improve the care of patients with ischemic stroke may be achieved.

Blood levels of glutamate oxaloacetate transaminase are more strongly associated with good outcome in acute ischaemic stroke than glutamate pyruvate transaminase levels

Ischaemic stroke is associated with an excessive release of glutamate in brain. GOT (glutamate-oxaloacetate transaminase) and GPT (glutamate-pyruvate transaminase) are two enzymes that are able to metabolize blood glutamate facilitating the lowering of extracellular levels of brain glutamate. Our aim was to study the association between blood levels of both enzymes and stroke outcome in patients with acute ischaemic stroke. We prospectively studied 365 patients with first ischaemic stroke<12 h. Glutamate, GOT and GPT levels were determined in blood samples obtained at admission. We considered functional outcome at 3 months [good outcome: mRS (modified Rankin Scale)≤2; poor outcome mRS >2], END (early neurological deterioration) in the first 72 h [increment ≥4 points in NIHSS (National Institutes of Health Stroke Scale)] and infarct volume [CT (computed tomography) at 36-72 h] as end points. We have found an inverse correlation between GOT and GPT levels and blood glutamate levels. Patients with poor outcome showed lower levels of GOT (11.9±8.2 compared with 22.7±10.2 m-units/ml, P<0.0001) and GPT (19.5±14.3 compared with 24.7±20.3 m-units/ml; P=0.004). A negative correlation has been found between GOT (Pearson coefficient=-0.477, P<0.0001) and GPT (Pearson coefficient=-0.116; P=0.027) levels and infarct volume. Patients with END showed higher levels of blood glutamate (381.7±97.9 compared with 237.6±114.0 μmol/l, P<0.0001) and lower levels of GOT (10.8±6.7 compared with 18.1±10.8 m-units/ml; P<0.0001). This clinical study shows an association between high blood GOT and GPT levels and good outcome in ischaemic stroke patients, this association being stronger for GOT than GPT levels.

Neuroprotection by glutamate oxaloacetate transaminase in ischemic stroke: an experimental study

As ischemic stroke is associated with an excessive release of glutamate into the neuronal extracellular space, a decrease in blood glutamate levels could provide a mechanism to remove it from the brain tissue, by increasing the brain-blood gradient. In this regard, the ability of glutamate oxaloacetate transaminase (GOT) to metabolize glutamate in blood could represent a potential neuroprotective tool for ischemic stroke. This study aimed to determine the neuroprotective effects of GOT in an animal model of cerebral ischemia by means of a middle cerebral arterial occlusion (MCAO) following the Stroke Therapy Academic Industry Roundtable (STAIR) group guidelines. In this animal model, oxaloacetate-mediated GOT activation inhibited the increase of blood and cerebral glutamate after MCAO. This effect is reflected in a reduction of infarct size, smaller edema volume, and lower sensorimotor deficits with respect to controls. Magnetic resonance spectroscopy confirmed that the increase of glutamate levels in the brain parenchyma after MCAO is inhibited after oxaloacetate-mediated GOT activation. These findings show the capacity of the GOT to remove glutamate from the brain by means of blood glutamate degradation, and suggest the applicability of this enzyme as an efficient and novel neuroprotective tool against ischemic stroke.

High blood glutamate oxaloacetate transaminase levels are associated with good functional outcome in acute ischemic stroke

The capacity of the blood enzyme glutamate oxaloacetate transaminase (GOT) to remove glutamate from the brain by means of blood glutamate degradation has been shown in experimental models to be an efficient and novel neuroprotective tool against ischemic stroke; however, the beneficial effects of this enzyme should be tested in patients with stroke to validate these results. This study aims to investigate the association of GOT levels in blood with clinical outcome in patients with acute ischemic stroke. In two clinical independent studies, we found that patients with poor outcome show higher glutamate and lower GOT levels in blood at the time of admission. Lower GOT levels and higher glutamate levels were independently associated with poorer functional outcome at 3 months and higher infarct volume. These findings show a clear association between high blood glutamate levels and worse outcome and vice versa for GOT, presumably explained by the capacity of this enzyme to metabolize blood glutamate.

Targeting the Ischemic Penumbra

Background—

In the last 3 decades, and from a therapeutic point of view, the classical concept of ischemic penumbra based on hemodynamic and electrophysiological parameters has loosened up the rigidity of therapeutic windows in acute stroke management. Thirty years later, the ischemic penumbra is an evolved concept that presents more applications. Thus, the ischemic penumbra is a diagnostic target, allowing the extension of therapeutic windows; it is also a biochemical target, in which an intermittent bioenergetic compromise takes place, and it is a target for brain plasticity, neuroprotection, and neurorepair.

Summary of Review—

In this work, we review how the concept of ischemic penumbra has been evolving from its purely electrophysiological/ hemodynamic based definition to the wider metabolic–cellular–therapeutic concept that is managed today by neuroscientists.

Functional and molecular interactions between aquaporins and Na,K-ATPase

The water channel aquaporin 4 (AQP4) is abundantly expressed in astrocytes and provides a mechanism by which water permeability of the plasma membrane can be regulated. Astrocytes play a key role in the clearance of both potassium (K(+)) and glutamate released during neuronal activity. Emerging evidence suggests that AQP4 facilitates K(+) clearance by astrocytes and contributes to recovery of neuronal excitability. Here we report that AQP4 can assemble with its regulator metabotropic glutamate receptor 5 (mGluR5) and with Na,K-ATPase; the enzyme responsible for active K(+) transport and for establishing the electrochemical gradient across the cell plasma membrane. We have, by use of pull down assays in rat brain tissue, identified the segment in the AQP4 NH(2)-terminus containing the amino acid residues 23-32 as the site for interaction with Na,K-ATPase catalytic subunit and with mGluR5. Mutagenesis studies revealed that the AQP4 amino acids K27 and W30 are of key importance for interaction with both Na,K-ATPase and mGluR5. To confirm that interaction also occurs within intact cells, we have performed fluorescence resonance energy transfer (FRET) studies in primary astrocytes derived from rat striatum. The results indicate close proximity of wild type AQP4 and Na,K-ATPase in the plasma membrane of rat astrocytes. FRET efficiencies observed with the mutants AQP4 K27A and AQP4 W30A were significantly lower, highlighting the importance of these residues for the interaction between AQP4 and Na,K-ATPase. We conclude that AQP4/Na,K-ATPase/mGluR5 can form a macromolecular complex/transporting microdomain in astrocytes. This complex may be of functional importance for the regulation of water and K(+) homeostasis in the brain, as well as for neuron-astrocyte metabolic crosstalk.

The molecular mechanisms of cell death in the course of transient ischemia are differentiated in evolutionary distinguished brain structures

There is large body of evidence suggesting distinct susceptibility to ischemia in various brain regions. However, the reason for this remains unexplained. Comparative studies of programmed cell death (PCD) pathways indicate their differentiated evolutional origin. The caspase-independent pathway is regarded as an older, whereas the caspase-dependent--as more advanced. In our study we address the question of whether there are any characteristic differences in the activation and course of PCD in phylogenetically and morphologically distinguished brain structures after transient focal ischemia. Using Western blot, we studied changes in expression of caspases: 3, 8, 9, and AIF in the frontoparietal neocortex, archicortex (CA1 and CA2 sectors of the hippocampus) and striatum, during reperfusion after 1 h occlusion of the middle cerebral artery. The caspase and AIF expression were differentiated between the studied structures. The activation of only the caspase-dependent pathway was observed in the neocortex. In the archicortex and striatum both caspase-dependent and caspase-independent pathways were activated, although in the latter the extrinsic apoptotic pathway was not activated. In summary, it is conceivable that structures of different evolutionary origin undergo cell-death processes with the participation of phylogenetically distinguished mechanisms. The previously reported unequal susceptibility to ischemia may co-exist with activation of different cell death pathways.

Brain angiogenesis in developmental and pathological processes: neurovascular injury and angiogenic recovery after stroke

Pathophysiologic responses in brain after stroke are highly complex. Thus far, a singular focus on saving neurons alone has not revealed any clinically effective neuroprotectants. To address this limitation, the concept of a neurovascular unit was developed. Within this conceptual framework, brain function and dysfunction are manifested at the level of cell-cell signaling between neuronal, glial and vascular elements. For stroke, coordinated responses at the neurovascular interface will mediate acute as well as chronic events in ischemic and hemorrhagic brain tissue. In this minireview, we briefly survey two representative examples of neurovascular responses in stroke. During the early acute phase of neurovascular injury, blood-brain barrier perturbations should predominate with key roles for various matrix proteases. During the delayed phase, brain angiogenesis may provide the critical neurovascular substrates for neuronal remodeling. In this minireview, we propose the hypothesis that the biphasic nature of neurovascular responses represents an endogenous attempt by damaged parenchyma to trigger brain angiogenesis and repair. This phenomenon may allow acute deleterious signals to transition into beneficial effects during stroke recovery. Understanding how neurovascular signals and substrates make the transition from initial injury to angiogenic recovery will be important if we are to find new therapeutic approaches for stroke.

Regulation of cerebral vasculature in normal and ischemic brain

We outline the mechanisms currently thought to be responsible for controlling cerebral blood flow (CBF) in the physiologic state and during ischemia, focusing on the arterial pial and penetrating microcirculation. Initially, we categorize the cerebral circulation and then review the vascular anatomy. We draw attention to a number of unique features of the cerebral vasculature, which are relevant to the microcirculatory response during ischemia: arterial histology, species differences, collateral flow, the venous drainage, the blood-brain barrier, astrocytes and vascular nerves. The physiology of the arterial microcirculation is then assessed. Lastly, we review the changes during ischemia which impact on the microcirculation. Further understanding of the normal cerebrovascular anatomy and physiology as well as the pathophysiology of ischemia will allow the rational development of a pharmacologic therapy for human stroke and brain injury.

Applicability of biomarkers in ischemic stroke

Cerebral ischemia results in the activation of a cascade of molecular events as a result of which several substances with the potential characteristics of biomarkers are released into the peripheral blood. Although still in the research phase, the analysis of these biomarkers in the serum has proved to be useful for stroke diagnosis, as well as for the prediction of the evolution of the ischemic lesion and the clinical prognosis. In fact, the feasibility and applicability of a panel of biomarkers for the diagnosis of stroke has recently been tested. Biomarkers of excitotoxicity, inflammation and oxidative stress have been demonstrated as being useful in the prediction of ischemic lesion enlargement and secondary neurological deterioration. On the other hand, biomarkers of endothelial damage have been shown to be especially helpful in the prediction of hemorrhagic transformation of the ischemic lesion, both spontaneously and after the administration of thrombolytic therapy, as well as in the prediction of brain edema with the secondary development of malignant middle-cerebral-artery infarction. Moreover, coagulation and fibrinolytic-cascade markers have been reported as being correlated with the recanalization rate after the administration of thrombolysis, and they might therefore be useful in estimating the effectiveness of thrombolytic therapy. However, for these biomarkers to become applicable to routine clinical practice, faster tests to perform the analyses are required and further studies must be undertaken to validate and generalize the results.

Translational research in stroke: Taking advances in the pathophysiology and treatment of stroke from the experimental setting to clinical trials

Many advances have occurred regarding an increased understanding of the basic pathophysiology of ischemic brain injury that could lead to enhanced therapy for this disorder. Among the more important basic science advances are enhanced knowledge of the components of the ischemic cascade, the phenomenon of ischemic preconditioning, the potential relevance of hibernation, studies on gene expression in ischemic tissue, and imaging identification of the ischemic penumbra. The large number of unsuccessful prior clinical trials with a wide range of purported acute stroke therapies has provided many insights and lessons regarding how to perform better trials in the future. Translating these basic science and clinical trial design advances into effective and safe therapies will require increased interaction and cooperation between basic scientists and clinical researchers.

State-of-the-Art Imaging of Acute Stroke

Stroke is a leading cause of mortality and morbidity in the developed world. The goals of an imaging evaluation for acute stroke are to establish a diagnosis as early as possible and to obtain accurate information about the intracranial vasculature and brain perfusion for guidance in selecting the appropriate therapy. A comprehensive evaluation may be performed with a combination of computed tomography (CT) or magnetic resonance (MR) imaging techniques. Unenhanced CT can be performed quickly, can help identify early signs of stroke, and can help rule out hemorrhage. CT angiography and CT perfusion imaging, respectively, can depict intravascular thrombi and salvageable tissue indicated by a penumbra. These examinations are easy to perform on most helical CT scanners and are increasingly used in stroke imaging protocols to decide whether intervention is necessary. While acute infarcts may be seen early on conventional MR images, diffusion-weighted MR imaging is more sensitive for detection of hyperacute ischemia. Gradient-echo MR sequences can be helpful for detecting a hemorrhage. The status of neck and intracranial vessels can be evaluated with MR angiography, and a mismatch between findings on diffusion and perfusion MR images may be used to predict the presence of a penumbra. The information obtained by combining various imaging techniques may help differentiate patients who do not need intravenous or intraarterial therapy from those who do, and may alter clinical outcomes.