Kainate postconditioning restores LTP in ischemic hippocampal CA1: onset-dependent second pathophysiological stress

Ischemic tolerance - blessing or curse

Application of knowledge about ischemic tolerance to clinic requires the solid understanding of mechanism of creation of this phenomenon. This review summarizes research that has been carried out in many laboratories over a long period of time, but the main focus will be on own experimental research. The main emphasis is devoted to the possibility of preparing full tolerance in the donor's body and its transfer to the patient in the form of activated blood plasma. Such plasma could be administered as soon as the patient is transported to the hospital and would take effect immediately after administration to the patient's bloodstream. One chapter is also devoted to anticonditioning, i.e. the possibility of preventing the activation of tolerance. Anticonditioning could be used to treat oncologic patients. We expect that this method could increase effectiveness of cancer treatment. Cross-tolerance with a wide range of diverse stressors gives us the courage to assume that activated plasma can significantly help with a wide range of pathological events.

Kainate receptors: from synaptic activity to disease

Kainate receptors (KARs) are glutamate receptors that participate in the postsynaptic transmission of information and in the control of neuronal excitability, as well as presynaptically modulating the release of the neurotransmitters GABA and glutamate. These modulatory effects, general follow a biphasic pattern, with low KA concentrations provoking an increase in GABA and glutamate release, and higher concentrations mediating a decrease in the release of these neurotransmitters. In addition, KARs are involved in different forms of long- and short-term plasticity. Importantly, altered activity of these receptors has been implicated in different central nervous system diseases and disturbances. Here, we describe the pre- and postsynaptic actions of KARs, and the possible role of these receptors in disease, a field that has seen significant progress in recent years.

Effects of the Pentapeptide P33 on Memory and Synaptic Plasticity in APP/PS1 Transgenic Mice: A Novel Mechanism Presenting the Protein Fe65 as a Target

Regulated intramembrane proteolysis (RIP) of the amyloid precursor protein (APP) leads to the formation of fragments, among which the intracellular domain of APP (AICD) was also identified to be a causative of early pathological events. AICD-counteracting proteins, such as Fe65, may serve as alternative therapeutic targets of Alzheimer’s disease (AD). The detection of elevated levels of Fe65 in the brains of both human patients and APP transgenic mice may further strengthen the hypothesis that influencing the interaction between Fe65 and APP may have a beneficial effect on the course of AD. Based on a PXP motif, proven to bind to the WW domain of Fe65, a new pentapeptide was designed and tested. The impedimental effect of P33 on the production of beta amyloid (Aβ) (soluble fraction and aggregated plaques) and on the typical features of the AD pathology (decreased dendritic spine density, synaptic markers, elevated inflammatory reactions) was also demonstrated. Significant enhancements of both learning ability and memory function were observed in a Morris water maze paradigm. The results led us to formulate the theory that P33 acts by altering the conformation of Fe65 via binding to its WW domain, consequently hindering any interactions between Fe65 and key members involved in APP processing.

Neuroprotective effect of pharmacological postconditioning on cerebral ischaemia-reperfusion-induced injury in mice

OBJECTIVES

To investigate the mechanism of neuroprotection rendered via pharmacological postconditioning in cerebral ischaemia-reperfusion-induced injury in mice.

METHODS

Pharmacological postconditioning is strategy which either involves hindering deleterious pathway or inducing modest stress level which triggers intracellular defence pathway to sustain more vigorous insult leading to conditioning. Hence, in current research we explored the potentiality of CGS21680 (0.5 mg/kg; i.p), an adenosine A₂ A receptor agonist and PTEN inhibitor, SF1670 (3 mg/kg; i.p.) to trigger postconditioning after inducing cerebral global ischaemia (17 min) and reperfusion (24 h)-induced injury via occlusion of both carotid arteries. Mice were also given treatment with LY294002 (1.5 mg/kg; i.p.), a PI3K inhibitor and adenosine A₂ A receptor antagonist, Istradefylline (2 mg/kg; i.p.), to establish the precise mechanism of postconditioning. Various biochemical and behavioural parameters were assessed to examine the effect of pharmacological postconditioning.

KEY FINDINGS

Pharmacological postconditioning induced with CGS21680 and SF1670 attenuated the infarction along with improved behavioural and biochemical parameters in comparison with ischaemia-reperfusion control group. The outcome of postconditioning with CGS21680 and SF1670 was significantly reversed by LY294002 and Istradefylline, respectively.

CONCLUSIONS

The neuroprotective effects of CGS21680 and SF1670 postconditioning on cerebral ischaemia-reperfusion injury may be due to PI3K/Akt pathway activation.

Ketamine destabilizes growth of dendritic spines in developing hippocampal neurons in vitro via a Rho‑dependent mechanism

The safety of anesthetics on the developing brain has caused concern. Ketamine, an N-methyl-D-aspartate receptor antagonist, is widely used as a general pediatric anesthetic. Recent studies suggested that ketamine alters the plasticity of dendritic spines in the developing brain and may be an important contributing factor to learning and cognitive impairment. However, the underlying molecular mechanism remains poorly understood. Therefore, the aim of the present study was to investigate the effect of ketamine on the plasticity of dendritic spines in cultured hippocampal neurons and the potential underlying mechanisms. After 5 days in vitro, rat hippocampal neurons were exposed to different concentrations (100, 300 and 500 µM) of ketamine for 6 h. Ketamine decreased the number and length of dendritic spines in a dose-dependent manner. Ketamine at a concentration of 300 µM caused an upregulation of transforming protein RhoA (RhoA) and Rho-associated kinase (ROCK) protein. These effects were inhibited by the ROCK inhibitor Y27632. These results suggested that ketamine induces loss and shortening of dendritic spines in hippocampal neurons via activation of the RhoA/ROCK signaling pathway.

Cardiac Arrest Induces Ischemic Long-Term Potentiation of Hippocampal CA1 Neurons That Occludes Physiological Long-Term Potentiation

Ischemic long-term potentiation (iLTP) is a form of synaptic plasticity that occurs in acute brain slices following oxygen-glucose deprivation. In vitro, iLTP can occlude physiological LTP (pLTP) through saturation of plasticity mechanisms. We used our murine cardiac arrest and cardiopulmonary resuscitation (CA/CPR) model to produce global brain ischemia and assess whether iLTP is induced in vivo, contributing to the functionally relevant impairment of pLTP. Adult male mice were subjected to CA/CPR, and slice electrophysiology was performed in the hippocampal CA1 region 7 or 30 days later. We observed increased miniature excitatory postsynaptic current amplitudes, suggesting a potentiation of postsynaptic AMPA receptor function after CA/CPR. We also observed increased phosphorylated GluR1 in the postsynaptic density of hippocampi after CA/CPR. These data support the in vivo induction of ischemia-induced plasticity. Application of a low-frequency stimulus (LFS) to CA1 inputs reduced excitatory postsynaptic potentials in slices from mice subjected to CA/CPR, while having no effects in sham controls. These results are consistent with a reversal, or depotentiation, of iLTP. Further, depotentiation with LFS partially restored induction of pLTP with theta burst stimulation. These data provide evidence for iLTP following in vivo ischemia, which occludes pLTP and likely contributes to network disruptions that underlie memory impairments.

Dihydropyridine Derivatives Modulate Heat Shock Responses and have a Neuroprotective Effect in a Transgenic Mouse Model of Alzheimer's Disease

Heat shock proteins (Hsps) have chaperone activity and play a pivotal role in the homeostasis of proteins by preventing misfolding, by clearing aggregated and damaged proteins from cells, and by maintaining proteins in an active state. Alzheimer’s disease (AD) is thought to be caused by amyloid-β peptide that triggers tau hyperphosphorylation, which is neurotoxic. Although proteostasis capacity declines with age and facilitates the manifestation of neurodegenerative diseases such as AD, the upregulation of chaperones improves prognosis. Our research goal is to identify potent Hsp co-inducers that enhance protein homeostasis for the treatment of AD, especially 1,4-dihydropyridine derivatives optimized for their ability to modulate cellular stress responses. Based on favorable toxicological data and Hsp co-inducing activity, LA1011 was selected for the in vivo analysis of its neuroprotective effect in the APPxPS1 mouse model of AD. Here, we report that 6 months of LA1011 administration effectively improved the spatial learning and memory functions in wild type mice and eliminated neurodegeneration in double mutant mice. Furthermore, Hsp co-inducer therapy preserves the number of neurons, increases dendritic spine density, and reduces tau pathology and amyloid plaque formation in transgenic AD mice. In conclusion, the Hsp co-inducer LA1011 is neuroprotective and therefore is a potential pharmaceutical candidate for the therapy of neurodegenerative diseases, particularly AD.

Studies for Improving a Rat Model of Alzheimer's Disease: Icv Administration of Well-Characterized β-Amyloid 1-42 Oligomers Induce Dysfunction in Spatial Memory

During the past 15 years, several genetically altered mouse models of human Alzheimer’s disease (AD) have been developed. These costly models have greatly facilitated the evaluation of novel therapeutic approaches. Injecting synthetic β-amyloid (Aβ) 1-42 species into different parts of the brain of non-transgenic rodents frequently provided unreliable results, owing to a lack of a genuine characterization of the administered Aβ aggregates. Previously, we have published a new rat AD-model in which protofibrillar-fibrillar Aβ1-42 was administered into rat entorhinal cortex (Sipos 2007). In order to develop a more reliable model, we have injected well-characterized toxic soluble Aβ1-42 species (oligomers, protofibrils and fibrils) intracerebroventricularly (icv) into rat brain. Studies of the distribution of fluorescent-labeled Aβ1-42 in the brain showed that soluble Aβ-species diffused into all parts of the rat brain. After seven days, the Aβ-treated animals showed a significant decrease of spatial memory in Morris water maze test and impairment of synaptic plasticity (LTP) measured in acute hippocampal slices. The results of histological studies (decreased number of viable neurons, increased tau levels and decreased number of dendritic spines) also supported that icv administration of well-characterized toxic soluble Aβ species into rat brain provides a reliable rat AD-model.

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.

The Role of NMDA Receptors in the Development of Brain Resistance through Pre- and Postconditioning

Brain tolerance or resistance can be achieved by interventions before and after injury through potential toxic agents used in low stimulus or dose. For brain diseases, the neuroprotection paradigm desires an attenuation of the resulting motor, cognitive, emotional, or memory deficits following the insult. Preconditioning is a well-established experimental and clinical translational strategy with great beneficial effects, but limited applications. NMDA receptors have been reported as protagonists in the adjacent cellular mechanisms contributing to the development of brain tolerance. Postconditioning has recently emerged as a new neuroprotective strategy, which has shown interesting results when applied immediately, i.e. several hours to days, after a stroke event. Investigations using chemical postconditioning are still incipient, but nevertheless represent an interesting and promising clinical strategy. In the present review pre- and postconditioning are discussed as neuroprotective paradigms and the focus of our attention lies on the participation of NMDA receptors proteins in the processes related to neuroprotection.

Low dose Hsp90 inhibitor 17AAG protects neural progenitor cells from ischemia induced death

Stress adaptation effect provides cell protection against ischemia induced apoptosis. Whether this mechanism prevents other types of cell death in stroke is not well studied. This is an important question for regenerative medicine to treat stroke since other types of cell death such as necrosis are also prominent in the stroke brain apart from apoptosis. We report here that treatment with 17-N-Allylamino-17-demethoxygeldanamycin (17AAG), an Hsp90 inhibitor, protected neural progenitor cells (NPCs) against oxygen glucose deprivation (OGD) induced cell death in a dose dependent fashion. Cell death assays indicated that 17AAG not only ameliorated apoptosis, but also necrosis mediated by OGD. This NPC protection was confirmed by exposing cells to oxidative stress, a major stress signal prevalent in the stroke brain. Mechanistic studies demonstrated that 17AAG activated PI3K/Akt and MAPK cell protective pathways. More interestingly, these two pathways were activated in vivo by 17AAG and 17AAG treatment reduced infarct volume in a middle cerebral artery occlusion (MCAO) stroke model. These data suggest that 17AAG protects cells against major cell death pathways and thus might be used as a pharmacological conditioning agent for regenerative medicine for stroke.

Biological networks in ischemic tolerance - rethinking the approach to clinical conditioning

The adaptive response (conditioning) to environmental stressors evokes evolutionarily conserved programs in uni- and multicellular organisms that result in increased fitness and resistance to stressor induced injury. Although the concept of conditioning has been around for a while, its translation into clinical therapies targeting neurovascular diseases has only recently begun. The slow pace of clinical adoption might be partially explained by our poor understanding of underpinning mechanisms and of the complex responses of the organism to the stressor. At the 2(nd) Translational Preconditioning Meeting participants engaged in an intense discussion addressing whether the time has come to more aggressively implement clinical conditioning protocols in the treatment of cerebrovascular diseases or whether it would be better to wait until preclinical data would help to minimize clinical empiricism. This review addresses the complex involvement of biological networks in establishing ischemic tolerance at the organism level using two clinically promising conditioning modalities, namely remote ischemic preconditioning, and per- or post-conditioning, as examples.

Increasing small conductance Ca2+-activated potassium channel activity reverses ischemia-induced impairment of long-term potentiation

Global cerebral ischemia following cardiac arrest and cardiopulmonary resuscitation (CA/CPR) causes injury to hippocampal CA1 pyramidal neurons and impairs cognition. Small conductance Ca(2+)-activated potassium channels type 2 (SK2), expressed in CA1 pyramidal neurons, have been implicated as potential protective targets. Here we showed that, in mice, hippocampal long-term potentiation (LTP) was impaired as early as 3 h after recovery from CA/CPR and LTP remained impaired for at least 30 days. Treatment with the SK2 channel agonist 1-Ethyl-2-benzimidazolinone (1-EBIO) at 30 min after CA provided sustained protection from plasticity deficits, with LTP being maintained at control levels at 30 days after recovery from CA/CPR. Minimal changes in glutamate release probability were observed at delayed times after CA/CPR, implicating post-synaptic mechanisms. Real-time quantitative reverse transcriptase-polymerase chain reaction indicated that CA/CPR did not cause a loss of N-methyl-D-aspartate (NMDA) receptor mRNA at 7 or 30 days after CA/CPR. Similarly, no change in synaptic NMDA receptor protein levels was observed at 7 or 30 days after CA/CPR. Further, patch-clamp experiments demonstrated no change in functional synaptic NMDA receptors at 7 or 30 days after CA/CPR. Electrophysiology recordings showed that synaptic SK channel activity was reduced for the duration of experiments performed (up to 30 days) and that, surprisingly, treatment with 1-EBIO did not prevent the CA/CPR-induced loss of synaptic SK channel function. We concluded that CA/CPR caused alterations in post-synaptic signaling that were prevented by treatment with the SK2 agonist 1-EBIO, indicating that activators of SK2 channels may be useful therapeutic agents to prevent ischemic injury and cognitive impairments.

Delayed remote ischemic postconditioning protects against transient cerebral ischemia/reperfusion as well as kainate-induced injury in rats

To test the appropriateness of using delayed remote ischemic postconditioning against damage caused to the hippocampus by ischemia or apoptosis inducing intoxication, we chose 10-min normothermic ischemia induced by four-vessel occlusion or kainate injection (8 mg/kg i.p.) in rats. Ischemia alone caused the number of degenerated CA1 neurons after 7 days lasting reperfusion to be significantly (p<0.001) increased by 72.77%. Delayed remote ischemic postconditioning lasting 20 min was able to prevent massive increase in the neurodegeneration. The group with 10 min of ischemia and postconditioning after 2 days of reperfusion had only 15.87% increase in the number of apoptotic neurons. Seven days after kainic acid injection the number of surviving neurons was 42.8% (p<0.001), but the portion of surviving pyramidal cells in the postconditioning group is more than 98%. Our data show that remote postconditioning, performed with 20 min of tourniquet ischemia applied to the hind limb, is a simple method able to effectively stop the onset of neurodegeneration and prevent occurrence of massive muscle cell necrosis, even when used 2 days after the end of the adverse event. Surviving neurons retained a substantial part of their learning and memory ability.

Simultaneous changes of spatial memory and spine density after intrahippocampal administration of fibrillar aβ1-42 to the rat brain

Several animal models of Alzheimer's disease have been used in laboratory experiments. Intrahippocampal injection of fibrillar amyloid-beta (fAβ) peptide represents one of the most frequently used models, mimicking Aβ deposits in the brain. In our experiment synthetic fAβ_1–42 peptide was administered to rat hippocampus. The effect of the Aβ peptide on spatial memory and dendritic spine density was studied. The fAβ_1–42-treated rats showed decreased spatial learning ability measured in Morris water maze (MWM). Simultaneously, fAβ_1–42 caused a significant reduction of the dendritic spine density in the rat hippocampus CA1 region. The decrease of learning ability and the loss of spine density were in good correlation. Our results prove that both methods (MWM and dendritic spine density measurement) are suitable for studying Aβ-triggered neurodegeneration processes.

Acetyl-L-carnitine normalizes the impaired long-term potentiation and spine density in a rat model of global ischemia

As a consequence of an ischemic episode, energy production is disturbed, leading to neuronal cell death. Despite intensive research, the quest for promising neuroprotective drugs has largely failed, not only because of ineffectiveness, but also because of serious side-effects and dosing difficulties. Acetyl-l-carnitine (ALC) is an essential nutrient which plays a key role in energy metabolism by transporting fatty acids into mitochondria for β-oxidation. It is an endogenous compound and can be used at high dose without toxicity in research into ischemia. Its neuroprotective properties have been reported in many studies, but its potential action on long-term potentiation (LTP) and dendritic spine density has not been described to date. The aim of the present study was an evaluation of the possible protective effect of ALC after ischemic insults inflicted on hippocampal synaptic plasticity in a 2-vessel occlusion (2VO) model in rats. For electrophysiological measurements, LTP was tested on hippocampal slices. The Golgi-Cox staining technique was used to determine spine density. 2VO resulted in a decreased, unstable LTP and a significant loss of dendritic spines. ALC administered after 2VO was not protective, but as pretreatment prior to 2VO it restored LTP nearly to the control level. This finding paralleled the histological analysis: ALC pretreatment resulted in the reappearance of dendritic spines on the CA1 pyramidal cells. Our data demonstrate that ALC administration can restore hippocampal function and spine density. ALC probably acts by enhancing the aerobic metabolic pathway, which is inhibited during and following ischemic attacks.

Delayed post-conditioning reduces post-ischemic glutamate level and improves protein synthesis in brain

In the clinic delayed post-conditioning would represent an attractive strategy for the survival of vulnerable neurons after an ischemic event. In this paper we studied the impact of ischemia and delayed post-conditioning on blood and brain tissue concentrations of glutamate and protein synthesis. We designed two groups of animals for analysis of brain tissues and blood after global ischemia and post-conditioning, and one for analysis of blood glutamate after transient focal ischemia. Our results showed elevated blood glutamate in two models of transient brain ischemia and decreases in blood glutamate to control in the first 20min of post-conditioning recirculation followed by a consecutive drop of about 20.5% on the first day. Similarly, we recorded reduced protein synthesis in hippocampus and cortex 2 and 3days after ischemia. However, increased glutamate was registered only in the hippocampus. Post-conditioning improves protein synthesis in CA1 and dentate gyrus and, surprisingly, leads to 50% reduction in glutamate in whole hippocampus and cortex. In conclusion, ischemia leads to meaningful elevation of blood and tissue glutamate. Post-conditioning activates mechanisms resulting in rapid elimination of glutamate from brain tissue and/or in the circulatory system that could otherwise impede brain-to-blood glutamate efflux mechanisms. Moreover, post-conditioning induces protein synthesis renewing in ischemia affected tissues that could also contribute to elimination of excitotoxicity. In addition, the potential of glutamate for monitoring the progress of ischemia and efficacy of therapy was shown.

From rapid to delayed and remote postconditioning: the evolving concept of ischemic postconditioning in brain ischemia

Ischemic postconditioning is a concept originally defined to contrast with that of ischemic preconditioning. While both preconditioning and postconditioning confer a neuroprotective effect on brain ischemia, preconditioning is a sublethal insult performed in advance of brain ischemia, and postconditioning, which conventionally refers to a series of brief occlusions and reperfusions of the blood vessels, is conducted after ischemia/reperfusion. In this article, we first briefly review the history of preconditioning, including the experimentation that initially uncovered its neuroprotective effects and later revealed its underlying mechanisms-of-action. We then discuss how preconditioning research evolved into that of postconditioning--a concept that now represents a broad range of stimuli or triggers, including delayed postconditioning, pharmacological postconditioning, remote postconditioning--and its underlying protective mechanisms involving the Akt, MAPK, PKC and K(ATP) channel cell-signaling pathways. Because the concept of postconditioning is so closely associated with that of preconditioning, and both share some common protective mechanisms, we also discuss whether a combination of preconditioning and postconditioning offers greater protection than preconditioning or postconditioning alone.