Hypothermic preconditioning induces rapid tolerance to focal ischemic injury in the rat

Ischemic Tolerance—A Way to Reduce the Extent of Ischemia–Reperfusion Damage

Individual tissues have significantly different resistance to ischemia–reperfusion damage. There is still no adequate treatment for the consequences of ischemia–reperfusion damage. By utilizing ischemic tolerance, it is possible to achieve a significant reduction in the extent of the cell damage due to ischemia–reperfusion injury. Since ischemia–reperfusion damage usually occurs unexpectedly, the use of preconditioning is extremely limited. In contrast, postconditioning has wider possibilities for use in practice. In both cases, the activation of ischemic tolerance can also be achieved by the application of sublethal stress on a remote organ. Despite very encouraging and successful results in animal experiments, the clinical results have been disappointing so far. To avoid the factors that prevent the activation of ischemic tolerance, the solution has been to use blood plasma containing tolerance effectors. This plasma is taken from healthy donors in which, after exposure to two sublethal stresses within 48 h, effectors of ischemic tolerance occur in the plasma. Application of this activated plasma to recipient animals after the end of lethal ischemia prevents cell death and significantly reduces the consequences of ischemia–reperfusion damage. Until there is a clear chemical identification of the end products of ischemic tolerance, the simplest way of enhancing ischemic tolerance will be the preparation of activated plasma from young healthy donors with the possibility of its immediate use in recipients during the initial treatment.

Neuroprotection in aortic arch surgery: untold flaws and future directions

The current paradigm of brain protection in aortic surgery falls short of delivering good outcomes with minimal complications. A renewed understanding of neuroprotective methods and biomarkers to predict brain injury and aortic disease are crucial towards the development of more effective clinical management strategies. A review of current literature was carried out to identify current flaws in our approach to neuroprotection in aortic surgery. Emerging evidence surrounding neuroprotective strategies, biomarkers for brain injury, and biomarkers for predicting aortic disease are evaluated in terms of their impact for future therapeutic approaches. Current literature suggests that the prevailing methods of neuroprotection need renewal. Clinical outcomes associated with deep hypothermic circulatory arrest remain varied. Branch-first and endovascular approaches to aortic repair are particularly promising alternatives. The use of biomarkers to identify and manage brain injury, as well as to diagnose aortic disease in the nonacute and acute settings, would further help to improve our overall paradigm of neuroprotection in aortic surgery. Though much prospective research is still required, the outlook for neuroprotection in aortic surgery is promising. Adopting alternative surgical techniques and exploiting predictive novel biomarkers will help us to gradually eliminate the risk of brain damage in aortic surgery.

Review Cerebral Ischemic Tolerance and Preconditioning: Methods, Mechanisms, Clinical Applications, and Challenges

Stroke is one of the leading causes of morbidity and mortality worldwide, and it is increasing in prevalence. The limited therapeutic window and potential severe side effects prevent the widespread clinical application of the venous injection of thrombolytic tissue plasminogen activator and thrombectomy, which are regarded as the only approved treatments for acute ischemic stroke. Triggered by various types of mild stressors or stimuli, ischemic preconditioning (IPreC) induces adaptive endogenous tolerance to ischemia/reperfusion (I/R) injury by activating a multitude cascade of biomolecules, for example, proteins, enzymes, receptors, transcription factors, and others, which eventually lead to transcriptional regulation and epigenetic and genomic reprogramming. During the past 30 years, IPreC has been widely studied to confirm its neuroprotection against subsequent I/R injury, mainly including local ischemic preconditioning (LIPreC), remote ischemic preconditioning (RIPreC), and cross preconditioning. Although LIPreC has a strong neuroprotective effect, the clinical application of IPreC for subsequent cerebral ischemia is difficult. There are two main reasons for the above result: Cerebral ischemia is unpredictable, and LIPreC is also capable of inducing unexpected injury with only minor differences to durations or intensity. RIPreC and pharmacological preconditioning, an easy-to-use and non-invasive therapy, can be performed in a variety of clinical settings and appear to be more suitable for the clinical management of ischemic stroke. Hoping to advance our understanding of IPreC, this review mainly focuses on recent advances in IPreC in stroke management, its challenges, and the potential study directions.

Surgically-induced brain injury: where are we now?

Neurosurgical procedures cause inevitable brain damage from the multitude of surgical manipulations utilized. Incisions, retraction, thermal damage from electrocautery, and intraoperative hemorrhage cause immediate and long-term brain injuries that are directly linked to neurosurgical operations, and these types of injuries, collectively, have been termed surgical brain injury (SBI). For the past decade, a model developed to study the underlying brain pathologies resulting from SBI has provided insight on cellular mechanisms and potential therapeutic targets. This model, as seen in a rat, mouse, and rabbit, mimics a neurosurgical operation and causes commonly encountered post-operative complications such as brain edema, neuroinflammation, and hemorrhage. In this review, we elaborate on SBI and its clinical impact, the SBI animal models and their clinical relevance, the importance of applying therapeutics before neurosurgical procedures (i.e., preconditioning), and the new direction of applying venom-derived proteins to attenuate SBI.

Preconditioning hippocampal slices with hypothermia promotes rapid tolerance to hypoxic depolarization and swelling: Mediation by erythropoietin

We suggested previously that hippocampal slices were protected from hypoxic depolarization and swelling by preincubating them at room temperature (Kreisman et al., 2000). We postulated that hypothermic preconditioning induced tolerance in our slices, which protected against hypoxic depolarization and swelling. Control hippocampal slices were incubated at 34-35 °C for two hours and the response to 10 min of severe hypoxia was compared to slices which were preconditioned for two hours at room temperature (22-23 °C) prior to warming to 34-35 °C. Recordings of the extracellular DC potential provided an index of tissue depolarization and changes in tissue light transmittance provided an index of swelling. Hypothermic preconditioning significantly reduced hypoxia-induced swelling, particularly in CA3 and the dentate inner blade. Since erythropoietin (EPO) had been shown to mediate hypoxic preconditioning, we tested whether EPO also mediated hypothermic preconditioning in our slices. Recombinant rat EPO (1-10 micromolar) mitigated hypoxia-induced swelling and depolarization in dentate inner blade of unconditioned slices in a dose-dependent manner. We also blocked the protective effects of hypothermic preconditioning on hypoxic depolarization and swelling in the inner blade of the dentate gyrus by administering soluble EPO receptor in the bath and treating slices with wortmannin to block phosphorylation of PI3 kinase, a critical step in the activation of the downstream neuroprotectant, Akt. These results suggest that EPO mediates tolerance to hypoxic depolarization and swelling induced by hypothermic preconditioning. They also emphasize that various preincubation protocols used in experiments with hippocampal slices may differentially affect basal electrophysiological and metabolic properties of those slices.

Models and methods for conditioning the ischemic brain

BACKGROUND

In the last decades the need to find new neuroprotective targets has addressed the researchers to investigate the endogenous molecular mechanisms that brain activates when exposed to a conditioning stimulus. Indeed, conditioning is an adaptive biological process activated by those interventions able to confer resistance to a deleterious brain event through the exposure to a sub-threshold insult. Specifically, preconditioning and postconditioning are realized when the conditioning stimulus is applied before or after, respectively, the harmul ischemia.

AIMS AND RESULTS

The present review will describe the most common methods to induce brain conditioning, with particular regards to surgical, physical exercise, temperature-induced and pharmacological approaches. It has been well recognized that when the subliminal stimulus is delivered after the ischemic insult, the achieved neuroprotection is comparable to that observed in models of ischemic preconditioning. In addition, subjecting the brain to both preconditioning as well as postconditioning did not cause greater protection than each treatment alone.

CONCLUSIONS

The last decades have provided fascinating insights into the mechanisms and potential application of strategies to induce brain conditioning. Since the identification of intrinsic cell-survival pathways should provide more direct opportunities for translational neuroprotection trials, an accurate examination of the different models of preconditioning and postconditioning is mandatory before starting any new project.

New roles of reactive astrocytes in the brain; an organizer of cerebral ischemia

The brain consists of neurons and much higher number of glial cells. They communicate each other, by which they control brain functions. The brain is highly vulnerable to several insults such as ischemia, but has a self-protective and self-repairing mechanisms against these. Ischemic tolerance or preconditioning is an endogenous neuroprotective phenomenon, where a mild non-lethal ischemic episode can induce resistance to a subsequent severe ischemic injury in the brain. Because of its neuroprotective effects against cerebral ischemia or stroke, ischemic tolerance has been widely studied. However, almost all studies have been performed from the viewpoint of neurons. Glial cells are structurally in close association with synapses. Recent studies have uncovered the active roles of astrocytes in modulating synaptic connectivity, such as synapse formation, elimination and maturation, during development or pathology. However, glia-mediated ischemic tolerance and/or neuronal repairing have received only limited attention. We and others have demonstrated that glial cells, especially astrocytes, play a pivotal role in regulation of induction of ischemic tolerance as well as repairing/remodeling of neuronal networks by phagocytosis. Here, we review our current understanding of (1) glial-mediated ischemic tolerance and (2) glia-mediated repairing/remodeling of the penumbra neuronal networks, and highlight their mechanisms as well as their potential benefits, problems, and therapeutic application.

Astrocytes and ischemic tolerance

A mild non-lethal ischemic episode can induce resistance to a subsequent severe ischemic injury in the brain. This phenomenon is termed ischemic tolerance or ischemic preconditioning, and is an endogenous mechanism that can provide robust neuroprotection. Because of its neuroprotective effects against cerebral ischemia or stroke, ischemic tolerance has been widely studied. However, almost all studies have been performed from the viewpoint of neurons. Accumulating evidence suggests that glial cells have various roles in regulation of brain function, including modulation of synaptic transmission, neuronal excitation, and neuronal structure. In addition, astrocytes are closely related to homeostasis, stability of brain function, and protection of neurons. However, glial cells have received only limited attention with regard to ischemic tolerance. Cross-ischemic preconditioning is a phenomenon whereby non-ischemic preconditioning such as mechanical, thermal, and chemical treatment can induce ischemic tolerance. Of these, chemical treatments that affect the immune system can strongly induce ischemic tolerance, suggesting that glial cells may have important roles in this process. Indeed, we and others have demonstrated that glial cells, especially astrocytes, play a pivotal role in the induction of ischemic tolerance. This glial-mediated ischemic tolerance provides a robust and long-lasting neuroprotection against ischemic injury. In this review, we discuss the mechanisms underlying glial-mediated ischemic tolerance, as well as its potential benefits, problems, and therapeutic application.

Ischemic Tolerance of the Brain and Spinal Cord: A Review

Ischemic tolerance is an endogenous neuroprotective phenomenon induced by sublethal ischemia. Ischemic preconditioning (IPC), the first discovered form of ischemic tolerance, is widely seen in many species and in various organs including the brain and the spinal cord. Ischemic tolerance of the spinal cord is less familiar among neurosurgeons, although it has been reported from the viewpoint of preventing ischemic spinal cord injury during aortic surgery. It is important for neurosurgeons to have opportunities to see patients with spinal cord ischemia, and to understand ischemic tolerance of the spinal cord as well as the brain. IPC has a strong neuroprotective effect in animal models of ischemia; however, clinical application of IPC for ischemic brain and spinal diseases is difficult because they cannot be predicted. In addition, one drawback of preconditioning stimuli is that they are also capable of producing injury with only minor changes to their intensity or duration. Numerous methods to induce ischemic tolerance have been discovered that vary in their timing and the site at which short-term ischemia occurs. These methods include ischemic postconditioning (IPoC), remote ischemic preconditioning (RIPC), remote ischemic perconditioning (RIPerC) and remote ischemic postconditioning (RIPoC), which has had a great impact on clinical approaches to treatment of ischemic brain and spinal cord injury. Especially RIPerC and RIPoC to induce spinal cord tolerance are considered clinically useful, however the evidence supporting these methods is currently insufficient; further experimental or clinical research in this area is thus necessary.

Non-linear actions of physiological agents: Finite disarrangements elicit fitness benefits

Finite disarrangements of important (vital) physiological agents and nutrients can induce plethora of beneficial effects, exceeding mere attenuation of the specific stress. Such response to disrupted homeostasis appears to be universally conserved among species. The underlying mechanism of improved fitness and longevity, when physiological agents act outside their normal range is similar to hormesis, a phenomenon whereby toxins elicit beneficial effects at low doses. Due to similarity with such non-linear response to toxins described with J-shaped curve, we have coined a new term “mirror J-shaped curves” for non-linear response to finite disarrangement of physiological agents. Examples from the clinical trials and basic research are provided, along with the unifying mechanisms that tie classical non-linear response to toxins with the non-linear response to physiological agents (glucose, oxygen, osmolarity, thermal energy, calcium, body mass, calorie intake and exercise). Reactive oxygen species and cytosolic calcium seem to be common triggers of signaling pathways that result in these beneficial effects. Awareness of such phenomena and exploring underlying mechanisms can help physicians in their everyday practice. It can also benefit researchers when designing studies and interpreting growing number of scientific data showing non-linear responses to physiological agents.

Cerebral Ischemic Preconditioning: the Road So Far…

Cerebral preconditioning constitutes the brain's adaptation to lethal ischemia when first exposed to mild doses of a subtoxic stressor. The phenomenon of preconditioning has been largely studied in the heart, and data from in vivo and in vitro models from past 2-3 decades have provided sufficient evidence that similar machinery exists in the brain as well. Since preconditioning results in a transient protective phenotype labeled as ischemic tolerance, it can open many doors in the medical warfare against stroke, a debilitating cerebrovascular disorder that kills or cripples thousands of people worldwide every year. Preconditioning can be induced by a variety of stimuli from hypoxia to pharmacological anesthetics, and each, in turn, induces tolerance by activating a multitude of proteins, enzymes, receptors, transcription factors, and other biomolecules eventually leading to genomic reprogramming. The intracellular signaling pathways and molecular cascades behind preconditioning are extensively being investigated, and several first-rate papers have come out in the last few years centered on the topic of cerebral ischemic tolerance. However, translating the experimental knowledge into the clinical scaffold still evades practicality and faces several challenges. Of the various preconditioning strategies, remote ischemic preconditioning and pharmacological preconditioning appears to be more clinically relevant for the management of ischemic stroke. In this review, we discuss current developments in the field of cerebral preconditioning and then examine the potential of various preconditioning agents to confer neuroprotection in the brain.

Global SUMOylation is a molecular mechanism underlying hypothermia-induced ischemic tolerance

The molecular mechanisms underlying hypothermic neuroprotection have yet to be fully elucidated. Herein we demonstrate that global SUMOylation, a form of post-translational modification with the Small Ubiquitin-like MOdifer, participates in the multimodal molecular induction of hypothermia-induced ischemic tolerance. Mild (32°C) to moderate (28°C) hypothermic treatment(s) during OGD (oxygen-glucose-deprivation) or ROG (restoration of oxygen/glucose) increased global SUMO-conjugation levels and protected cells (both SHSY5Y and E18 rat cortical neurons) from OGD and ROG-induced cell death. Hypothermic exposure either before or after permanent middle cerebral artery occlusion (pMCAO) surgery in wild type mice increased global SUMO-conjugation levels in the brain and in so doing protected these animals from pMCAO-induced ischemic damage. Of note, hypothermic exposure did not provide an additional increase in protection from pMCAO-induced ischemic brain damage in Ubc9 transgenic (Ubc9 Tg) mice, which overexpress the sole E2 SUMO conjugating enzyme and thereby display elevated basal levels of global SUMOylation under normothermic conditions. Such evidence suggests that increases in global SUMOylation are critical and may account for a substantial part of the observed increase in cellular tolerance to brain ischemia caused via hypothermia.

Immune mechanisms in cerebral ischemic tolerance

Stressor-induced tolerance is a central mechanism in the response of bacteria, plants, and animals to potentially harmful environmental challenges. This response is characterized by immediate changes in cellular metabolism and by the delayed transcriptional activation or inhibition of genetic programs that are not generally stressor specific (cross-tolerance). These programs are aimed at countering the deleterious effects of the stressor. While induction of this response (preconditioning) can be established at the cellular level, activation of systemic networks is essential for the protection to occur throughout the organs of the body. This is best signified by the phenomenon of remote ischemic preconditioning, whereby application of ischemic stress to one tissue or organ induces ischemic tolerance (IT) in remote organs through humoral, cellular and neural signaling. The immune system is an essential component in cerebral IT acting simultaneously both as mediator and target. This dichotomy is based on the fact that activation of inflammatory pathways is necessary to establish IT and that IT can be, in part, attributed to a subdued immune activation after index ischemia. Here we describe the components of the immune system required for induction of IT and review the mechanisms by which a reprogrammed immune response contributes to the neuroprotection observed after preconditioning. Learning how local and systemic immune factors participate in endogenous neuroprotection could lead to the development of new stroke therapies.

A method for hypothermia-induction and maintenance allows precise body and brain temperature control in mice

The benefits as well as mechanisms of hypothermia in brain injuries are actively studied at the bench and in the clinic. However, methods used in controlling hypothermia vary among laboratories, and usually brain temperatures are not monitored directly in animals due to the need for an invasive procedure. Here we show a method, water immersion technique, which we developed recently to regulate body temperature in mice during hypothermia process. This method significantly reduced the temperature variation around target temperature. Importantly, this method demonstrated a parallel and consistent relationship between rectal temperature and brain temperature (the brain temperature was consistently 0.5C higher than rectal temperature) throughout hypothermia maintenance. This technique may be well adapted to hypothermia studies in mice and other rodents, especially to the assessment and regulation of brain temperature during studies.

Moderate hypothermia induces marked increase in levels and nuclear accumulation of SUMO2/3-conjugated proteins in neurons

Deep hypothermia protects the brain from ischemic damage and is therefore used during major cardiovascular surgeries requiring cardiopulmonary bypass and a period of circulatory arrest. Here, we demonstrated that small ubiquitin-like modifier (SUMO1-3) conjugation is markedly activated in the brain during deep to moderate hypothermia. Animals were subjected to normothermic (37°C) or deep to moderate (18°C, 24°C, 30°C) hypothermic cardiopulmonary bypass, and the effects of hypothermia on SUMO conjugation were evaluated by Western blot and immunohistochemistry. Exposure to moderate 30°C hypothermia was sufficient to markedly increase levels and nuclear accumulation of SUMO2/3-conjugated proteins in these cells. Deep hypothermia induced nuclear translocation of the SUMO-conjugating enzyme Ubc9, suggesting that the increase in nuclear levels of SUMO2/3-conjugated proteins observed in brains of hypothermic animals is an active process. Exposure of primary neuronal cultures to deep hypothermia induced only a moderate rise in levels of SUMO2/3-conjugated proteins. This suggests that neurons in vivo have a higher capacity than neurons in vitro to activate this endogenous potentially neuroprotective pathway upon exposure to hypothermia. Identifying proteins that are SUMO2/3 conjugated during hypothermia could help to design new strategies for preventive and therapeutic interventions to make neurons more resistant to a transient interruption of blood supply.

Time-dependent effects of hypothermia on microglial activation and migration

Background

Therapeutic hypothermia is one of the neuroprotective strategies that improve neurological outcomes after brain damage in ischemic stroke and traumatic brain injury. Microglial cells become activated following brain injury and play an important role in neuroinflammation and subsequent brain damage. The aim of this study was to determine the time-dependent effects of hypothermia on microglial cell activation and migration, which are accompanied by neuroinflammation.

Methods

Microglial cells in culture were subjected to mild (33 °C) or moderate (29 °C) hypothermic conditions before, during, or after lipopolysaccharide (LPS) or hypoxic stimulation, and the production of nitric oxide (NO), proinflammatory cytokines, reactive oxygen species, and neurotoxicity was evaluated. Effects of hypothermia on microglial migration were also determined in in vitro as well as in vivo settings.

Results

Early-, co-, and delayed-hypothermic treatments inhibited microglial production of inflammatory mediators to varying degrees: early treatment was the most efficient, and delayed treatment showed time-dependent effects. Delayed hypothermia also suppressed the mRNA levels of proinflammatory cytokines and iNOS, and attenuated microglial neurotoxicity in microglia-neuron co-cultures. Furthermore, delayed hypothermia reduced microglial migration in the Boyden chamber assay and wound healing assay. In a stab injury model, delayed local hypothermia reduced migration of microglia toward the injury site in the rat brain.

Conclusion

Taken together, our results indicate that delayed hypothermia is sufficient to attenuate microglial activation and migration, and provide the basis of determining the optimal time window for therapeutic hypothermia. Delayed hypothermia may be neuroprotective by inhibiting microglia-mediated neuroinflammation, indicating the therapeutic potential of post-injury hypothermia for patients with brain damages exhibiting some of the inflammatory components.

Downregulation of miR-199a may play a role in 3-nitropropionic acid induced ischemic tolerance in rat brain

MicroRNAs (miR) are single-stranded short RNA molecules that regulate gene expression by degradation or translational repression of mRNA. It has been reported that the downregulation of miR-199a plays an important role in cardiac ischemic tolerance. We examined the expression of miR-199a after 3-nitropropionic acid (3-NPA) preconditioning in rat brain. 3-NPA (20mg/kg), an irreversible inhibitor of succinate dehydrogenase, was injected intraperitoneally to induce ischemic tolerance in rats. For comparison, the control group received intraperitoneal injections of vehicle (0.9% sodium chloride). Quantitative RT-PCR assay was applied to detect the expression of miR-199a. Luciferase reporter assays and Western blotting were used to verify the target genes of miR-199a. In cortex and striatum, miR-199a was downregulated at two separate time intervals (the 2nd and 4th day), while in the hippocampus, it was downregulated on the 2nd day after 3-NPA preconditioning. The maximum reduction of miR-199a was 66.3% in striatum (4th day), 54.9% in hippocampus (2nd day), and 27.6% in cortex (2nd day). The level of sirt1 protein, a putative target of miR-199a and a known mediator of neuroprotective effect in brain ischemic tolerance, decreased significantly in hippocampal neurons by overexpression of miR-199a, while it increased with knockdown of miR-199a. Taking these results together, we hypothesize miR-199a may have a role in the formation of cerebral ischemic tolerance.

Therapeutic hypothermia: benefits, mechanisms and potential clinical applications in neurological, cardiac and kidney injury

Therapeutic hypothermia involves the controlled reduction of core temperature to attenuate the secondary organ damage which occurs following a primary injury. Clinicians have been increasingly using therapeutic hypothermia to prevent or ameliorate various types of neurological injury and more recently for some forms of cardiac injury. In addition, some recent evidence suggests that therapeutic hypothermia may also provide benefit following acute kidney injury. In this review we will examine the potential mechanisms of action and current clinical evidence surrounding the use of therapeutic hypothermia. We will discuss the ideal methodological attributes of future studies using hypothermia to optimise outcomes following organ injury, in particular neurological injury. We will assess the importance of target hypothermic temperature, time to achieve target temperature, duration of cooling, and re-warming rate on outcomes following neurological injury to gain insights into important factors which may also influence the success of hypothermia in other organ injuries, such as the heart and the kidney. Finally, we will examine the potential of therapeutic hypothermia as a future kidney protective therapy.

Mechanisms of action, physiological effects, and complications of hypothermia

BACKGROUND

Mild to moderate hypothermia (32-35 degrees C) is the first treatment with proven efficacy for postischemic neurological injury. In recent years important insights have been gained into the mechanisms underlying hypothermia's protective effects; in addition, physiological and pathophysiological changes associated with cooling have become better understood.

OBJECTIVE

To discuss hypothermia's mechanisms of action, to review (patho)physiological changes associated with cooling, and to discuss potential side effects.

DESIGN

Review article.

INTERVENTIONS

None.

MAIN RESULTS

A myriad of destructive processes unfold in injured tissue following ischemia-reperfusion. These include excitotoxicty, neuroinflammation, apoptosis, free radical production, seizure activity, blood-brain barrier disruption, blood vessel leakage, cerebral thermopooling, and numerous others. The severity of this destructive cascade determines whether injured cells will survive or die. Hypothermia can inhibit or mitigate all of these mechanisms, while stimulating protective systems such as early gene activation. Hypothermia is also effective in mitigating intracranial hypertension and reducing brain edema. Side effects include immunosuppression with increased infection risk, cold diuresis and hypovolemia, electrolyte disorders, insulin resistance, impaired drug clearance, and mild coagulopathy. Targeted interventions are required to effectively manage these side effects. Hypothermia does not decrease myocardial contractility or induce hypotension if hypovolemia is corrected, and preliminary evidence suggests that it can be safely used in patients with cardiac shock. Cardiac output will decrease due to hypothermia-induced bradycardia, but given that metabolic rate also decreases the balance between supply and demand, is usually maintained or improved. In contrast to deep hypothermia (<or=30 degrees C), moderate hypothermia does not induce arrhythmias; indeed, the evidence suggests that arrhythmias can be prevented and/or more easily treated under hypothermic conditions.

CONCLUSIONS

Therapeutic hypothermia is a highly promising treatment, but the potential side effects need to be properly managed particularly if prolonged treatment periods are required. Understanding the underlying mechanisms, awareness of physiological changes associated with cooling, and prevention of potential side effects are all key factors for its effective clinical usage.

Deep hypothermia markedly activates the small ubiquitin-like modifier conjugation pathway; implications for the fate of cells exposed to transient deep hypothermic cardiopulmonary bypass

Various cardiovascular operations are performed during conditions of deep hypothermic circulatory arrest. Here we investigated the effects of deep hypothermia on the small ubiquitin-like modifier (SUMO) conjugation pathway using a clinically relevant animal model of deep hypothermic cardiopulmonary bypass (DHCPB). Deep hypothermic cardiopulmonary bypass induced a marked activation of the SUMO conjugation pathway and triggered a nuclear translocation of SUMO2/3-conjugated proteins. Furthermore, DHCBP significantly modified gene expression. Activation of the SUMO conjugation pathway is believed to protect neurons from damage caused by low blood flow. This pathway may, therefore, play a key role in defining the outcome of cells exposed to DHCPB.

Hypothermic preconditioning of endothelial cells attenuates cold-induced injury by a ferritin-dependent process

Hypothermia for myocardial protection or storage of vascular grafts may damage the endothelium and impair vascular function upon reperfusion/rewarming. Catalytic iron pools and oxidative stress are important mediators of cold-induced endothelial injury. Because endothelial cells are highly adaptive, we hypothesized that hypothermic preconditioning (HPC) protects cells at 0 degrees C by a heme oxygenase-1 (HO-1) and ferritin-dependent mechanism. Storage of human coronary artery endothelial cells at 0 degrees C caused the release of lactate dehydrogenase, increases in bleomycin-detectible iron (BDI), and increases in the ratio of oxidized/reduced glutathione, signifying oxidative stress. Hypoxia increased injury at 0 degrees C but did not increase BDI or oxidative stress further. HPC at 25 degrees C for 15-72 h attenuated these changes by an amount achievable by pretreating cells with 10-20 microM deferoxamine, an iron chelator, and protected cell viability. Treating cells with hemin chloride at 37 degrees C transiently increased intracellular heme, HO-1, BDI, and ferritin. Elevated heme/iron sensitized cells to 0 degrees C but ferritin was protective. HPC increased iron maximally after 2 h at 25 degrees C and ferritin levels peaked after 15 h. HO-1 was not induced. When HPC-mediated increases in ferritin were blocked by deferoxamine, protection at 0 degrees C was diminished. We conclude that HPC-mediated endothelial protection from hypothermic injury is an iron- and ferritin-dependent process.

Lipopolysaccharide induces early tolerance to excitotoxicity via nitric oxide and cGMP

BACKGROUND AND PURPOSE

Sublethal injury induces tolerance to a subsequent lethal insult, a phenomenon termed preconditioning (PC). PC occurs within hours (early tolerance) or days (delayed tolerance) after the inducing stimulus. In the brain, delayed tolerance has been studied extensively, but very little is known about early tolerance. We investigated whether the proinflammatory agent lipopolysaccharide (LPS), a well-established inducer of delayed tolerance, can also induce early tolerance and, if so, whether nitric oxide (NO) is involved in its mechanisms.

METHODS

In C57BL/6 mice, LPS was administered and N-methyl-D-aspartate (NMDA) was microinjected into the neocortex 30 minutes to 24 hours later. Lesion volume was assessed 24 hours after NMDA administration in thionine-stained sections.

RESULTS

LPS reduced NMDA lesions when administered 1 hour (-25+/-1%; P<0.05, n=5 per group) or 24 hours (-25+/-4%; P<0.05, n=5 per group) before NMDA application. LPS administration 30 minutes or 2 to 4 hours before NMDA administration was not neuroprotective (P>0.05). The protection at 1 hour was independent of protein synthesis and was blocked by inhibition of neuronal NO synthase or soluble guanylyl cyclase. Furthermore, early protection was not observed in neuronal or endothelial NO synthase-null mice, but it was present in inducible NO synthase-null mice.

CONCLUSIONS

The data demonstrate that LPS induces both early and late tolerance. At variance with delayed tolerance, which depends on inducible NO synthase and peroxynitrite, early tolerance is mediated by endothelial and neuronal NO through production of cGMP. The findings suggest that LPS can trigger signaling between endothelial cells and neurons, leading to NO production and cGMP-dependent neuroprotection.

Temperature preconditioning of isolated rat hearts--a potent cardioprotective mechanism involving a reduction in oxidative stress and inhibition of the mitochondrial permeability transition pore

We investigate whether temperature preconditioning (TP), induced by short-term hypothermic perfusion and rewarming, may protect hearts against ischaemic/reperfusion injury like ischaemic preconditioning (IP). Isolated rat hearts were perfused for 40 min, followed by 25 min global ischaemia and 60 min reperfusion (37 degrees C). During pre-ischaemia, IP hearts underwent three cycles of 2 min global ischaemia and 3 min reperfusion at 37 degrees C, whereas TP hearts received three cycles of 2 min hypothermic perfusion (26 degrees C) interspersed by 3 min normothermic perfusion. Other hearts received a single 6 min hypothermic perfusion (SHP) before ischaemia. Both IP and TP protocols increased levels of high energy phosphates in the pre-ischaemic heart. During reperfusion, TP improved haemodynamic recovery, decreased arrhythmias and reduced necrotic damage (lactate dehydrogenase release) more than IP or SHP. Measurements of tissue NAD+ levels and calcium-induced swelling of mitochondria isolated at 3 min reperfusion were consistent with greater inhibition of the mitochondrial permeability transition at reperfusion by TP than IP; this correlated with decreased protein carbonylation, a surrogate marker for oxidative stress. TP increased protein kinase Cepsilon (PKCepsilon) translocation to the particulate fraction and pretreatment with chelerythrine (PKC inhibitor) blocked the protective effect of TP. TP also increased phosphorylation of AMP-activated protein kinase (AMPK) after 5 min index ischaemia, but not before ischaemia. Compound C (AMPK inhibitor) partially blocked cardioprotection by TP, suggesting that both PKC and AMPK may mediate the effects of TP. The presence of N-(2-mercaptopropionyl) glycine during TP also abolished cardioprotection, indicating an involvement of free radicals in the signalling mechanism.

Early excitability changes in a novel acute model of transient focal ischemia and reperfusion in the in vitro isolated guinea pig brain

The study of the early events that characterize cerebral ischemia is limited in available experimental models. The study of neurophysiological network changes that occur in brain tissue during the early minutes that follow focal ischemia induction is restricted in the in vivo condition. Very simplified systems, such as in vitro brain slices and in isolated neurons, have been utilized for this type of studies. We describe here a new model of transient focal ischemia and reperfusion developed in the isolated guinea pig brain, maintained in vitro by arterial perfusion with a complex saline solution without blood cells. In this preparation, that combines the advantage of an in vitro preparation with the functional preservation of both vascular and neuronal compartments, the arteries of the Willis circle are directly accessible by visual control. To induce transitory focal ischemia, one medial cerebral artery (MCA) was transiently tied for 30 min, while brain activity was recorded with multiple electrodes positioned in brain areas within and outside MCA territory. Anoxic depression in ischemic areas propagated to the surrounding tissue and was associated with the abolition of evoked responses due to both functional impairment of afferent olfactory input and tissue depression. Recovery of evoked responses was obtained after MCA reperfusion. The spatial distribution of hypoxic depressions was characterized and was correlated with the extension of brain damage, defined by immunohistochemical analysis with antibodies against microtubule-associated protein (MAP-2). We propose that the present model can be utilized to analyze brain activity changes that occur in early stages of focal brain ischemia and reperfusion.

Moderate cerebral venous congestion induces rapid cerebral protection via adenosine A1 receptor activation

Stroke is a devastating complication in cardiovascular surgery, and neuronal damage is worsened by intracranial pressure elevation caused by cerebral venous circulatory disturbances (CVCD). However, we have previously reported that CVCD before cerebral ischemia decreases the infarct area. In the present study, focal cerebral ischemia was induced in spontaneously hypertensive rats by filament insertion through the carotid artery. Rats were divided into the following four groups: sham-operated, mild or severe venous congestion (VC), and DPCPX. The DPCPX group received the adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) prior to mild VC. Behavior, infarct volume, edema and S-100 protein were evaluated among the four groups. The infarct volume rates in mild VC and severe VC groups were significantly less than that in sham-operated and DPCPX groups. However, the mortality of the severe VC group worsened in a time-dependent manner. We observed a significant decrease in edema in the mild VC group compared to the DPCPX group. Behavioral scores also indicated that the mild VC group had fewer neurological deficits than the other three groups, including the DPCPX group. We were able to induce rapid cerebral protection via adenosine A1 receptor activation by administering an appropriate degree of VC prior to cerebral ischemia produced by middle cerebral artery occlusion. Our work suggests possible mechanisms by which such effective VC may lead to cerebral protection and adenosine A1 receptor activation.

Serum S-100b protein as a biomarker for the assessment of neuroprotectants

The study of biomarkers associated with stroke has proved to be of considerable utility. The astroglial protein S-100b is a candidate marker for cerebral tissue damage. We used a rat embolic model produced by injection of microspheres to demonstrate that serum S-100b is a useful biochemical marker for ischemic brain injury. Serum S-100b levels were significantly increased following microsphere injection, which was closely correlated with the development of brain edema. We found that structurally and mechanistically independent neuroprotective agents, such as 3-[2-[4-(3-chloro-2-methylphenylmethyl)-1-piperazinyl]ethyl]-5,6-dimethoxy-1-(4-imidazolylmethyl)-1H-indazole dihydrochloride 3.5 hydrate (DY-9760e), a novel calmodulin antagonist, and the N-methyl-d-aspartate (NMDA) receptor antagonist MK-801, are capable of attenuating increased serum S-100b levels and brain edema. In contrast, the hyperosmolar agent glycerol, which has no direct neuroprotective action, had little effect on serum S-100b levels, despite a significant decrease in brain water content. These results suggest that lowering of serum S-100b is mediated by neuroprotection against ischemic brain injury. Thus, serum S-100b reflects the extent of brain damage following cerebral ischemia and serves as a useful biomarker for the assessment of neuroprotectants.

Putative endogenous mediators of preconditioning-induced ischemic tolerance in rat brain identified by genomic and proteomic analysis

In brain, a brief ischemic episode induces protection against a subsequent severe ischemic insult. This phenomenon is known as preconditioning-induced neural ischemic tolerance. An understanding of the molecular mechanisms leading to preconditioning helps in identifying potential therapeutic targets for preventing the post-stroke brain damage. The present study conducted the genomic and proteomic analysis of adult rat brain as a function of time following preconditioning induced by a 10-min transient middle cerebral artery (MCA) occlusion. GeneChip analysis showed induction of 40 putative neuroprotective transcripts between 3 to 72 h after preconditioning. These included heat-shock proteins, heme oxygenases, metallothioneins, signal transduction mediators, transcription factors, ion channels and apoptosis/plasticity-related transcripts. Real-time PCR confirmed the GeneChip data for the transcripts up-regulated after preconditioning. Two-dimensional gel electrophoresis combined with MALDI-TOF analysis showed increased expression of HSP70, HSP27, HSP90, guanylyl cyclase, muskelin, platelet activating factor receptor and beta-actin at 24 h after preconditioning. HSP70 protein induction after preconditioning was localized in the cortical pyramidal neurons. The infarct volume induced by focal ischemia (1-h MCA occlusion) was significantly smaller (by 38 +/- 7%, p < 0.05) in rats subjected to preconditioning 3 days before the insult. Preconditioning also prevented several gene expression changes induced by focal ischemia.