JCN Open Access REVIEW pISSN 1738-6586 / eISSN 2005-5013 / J Clin Neurol 2017;13(1):1-9 / https://doi.org/10.3988/jcn.2017.13.1.1

Postischemic in Acute

Simone Vidalea Cerebral is caused by arterial occlusion due to a thrombus or an embolus. Such oc- Arturo Consolib a clusion induces multiple and concomitant pathophysiological processes that involve bioener- Marco Arnaboldi getic failure, acidosis, loss of homeostasis, , and disruption of the blood- c Domenico Consoli barrier. All of these mechanisms contribute to neuronal death, mainly via or necro- aDepartment of and Stroke sis. The immune system is involved in this process in the early phases after brain injury, which Unit, Sant’Anna Hospital, Como, Italy contributes to potential enlargement of the infarct size and involves the penumbra area. Where- bDepartment of Interventional as inflammation and the immune system both exert deleterious effects, they also contribute Neurovascular Unit, Careggi University to brain protection by stimulating a preconditioning status and to the concomitant repair of Hospital, Florence, Italy the injured parenchyma. This review describes the main phases of the inflammatory process cDepartment of Neurology, G. Jazzolino Hospital, Vibo Valentia, Italy occurring after arterial cerebral occlusion, with an emphasis on the role of single mediators. Key Wordszzischemic stroke, inflammation, immune response.

INTRODUCTION

Stroke is a leading cause of disability in adults that has a heavy social burden worldwide. This disease is the third highest cause of mortality, resulting in approximately six million deaths annually.1 Acute cerebral ischemia accounts for more than 80% of all and is due to brain arterial occlusion resulting from a thrombus or embolus. The pathophysiologi- cal processes following ischemic stroke are complex, involving bioenergetic failure, acidosis, loss of cell homeostasis, excitotoxicity, activation of neuronal and glial cells, and disruption of the blood-brain barrier (BBB) with infiltration of leukocytes.2 There is evidence that fac- tors of the immune system are involved in all stages of acute cerebral ischemia (Fig. 1).3 The ischemic brain promotes a potent suppressive effect on lymphoid organs via the autonomic nervous system, which increases the risk of the poststroke infections that are major deter- minants of morbidity and mortality.4 On the other hand, the innate immune system con- tributes to subsequent repair of the damaged cerebral tissue.5 In this review we describe the main phases of the inflammatory processes during the ear- ly postischemic period, with an emphasis on the role of single mediators.

INFLAMMATION, ENDOTHELIUM, AND CLOT FORMATION

Received September 12, 2016 A growing amount of attention is being paid to the mechanisms of clot formation, particu- Revised October 30, 2016 Accepted October 31, 2016 larly in the field of endovascular treatment of acute ischemic stroke. Although most of the Correspondence focus has been on intervention devices (with developments from first- to second-generation Simone Vidale, MD devices, thrombus aspiration, and balloon-occlusion guiding catheters), some research Department of Neurology and groups have studied the physiopathological mechanisms of clot formation. The relation be- Stroke Unit, Sant’Anna Hospital, Via Napoleona 60, 22100 Como, Italy tween inflammation and clot formation has been described previously, and it indicates that Tel +39-0315859282 cc This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Com- Fax +39-0315854989 mercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial E-mail [email protected] use, distribution, and reproduction in any medium, provided the original work is properly cited.

Copyright © 2017 Korean Neurological Association 1 JCN Inflammation in Acute Ischemic Stroke some cytokines (e.g., the RANTES)6 are responsible for the EARLY POSTISCHEMIA TIME: activation of a or, indirectly, supports THE ISCHEMIC CASCADE the concept that infections/inflammation promote athero- genesis and that some endothelial modifications that can The ischemic cascade is represented by a complex series of lead to a prothrombotic status.7 Several research groups are interlinked molecular and cellular mechanisms that contrib- currently focusing on the clot structure, with many findings ute to ischemic cell death via or apoptosis (Fig. 2). supporting the concept of an inflammation-induced process. The primary insult after arterial occlusion is hypoperfusion, In particular, fibrinogen is susceptible to oxidation, and which dramatically reduces the availability of both glucose chronic exposure to supported by inflamma- and oxygen in the brain, with particular vulnerability to isch- tion may lead to prothrombotic alterations in fibrin forma- emic injury being evident in specific regions: the caudate tion and architecture. Previous studies concerning air pollu- body, putamen, insular ribbon, paracentral lobule, precen- tion have shown that particulate matter contributes to tral, and middle and inferior frontal gyri.13 This situation modulation of the fibrin structure.8-11 Furthermore, some in- contributes to bioenergetic failure by stopping or slowing ad- teresting studies that have used electron microscopy to inves- enosine triphosphate (ATP) production.14,15 A few minutes tigate the clot surface (in ) highlight the after an arterial occlusion, an ionic imbalance occurs with less-investigated issue of the relation between the clot and the abnormal influx of Na+ and efflux of K+, contributing to a the endothelium.12 widespread anoxic depolarization in the membranes of neu- rons and glial cells.16 The increased influx of Na+ into neu- rons causes the osmotic transport of water into cells that leads to cytotoxic edema, cell lysis, and necrosis. A recent

Recovery Phase Acute Phase

Clearance of debris

DAMPs release

Angiogenesis Necrotic Purines VEGF TNF-a Inflammation TGF-b IL-1b IGF-1 Macrophage IL-23 activation

IL-20 IL-17

Anti-inflammatory activation T-cells

Days/weeks Minutes/hours

Fig. 1. Postischemic inflammation. Necrotic neurons release damage-associated molecular patterns (DAMPs), and these molecules activate mac- rophages via pattern-recognition receptors and inflammasomes. The activated macrophages contribute to enhance the inflammatory process via the release of proinflammatory cytokines and recruiting T-cells that contribute to maintain inflammation by interleukin (IL)-17. At several days -af ter the acute injury, the cellular elements of the innate immune system change to an anti-inflammatory phenotype, contributing to inhibit the in- flammation (dashed lines). In particular, anti-inflammatory cytokines (e.g., IL-10) are released. During this phase, the postischemic inflammation is resolved by the clearance of debris as well as angiogenesis supported by the release of growth factors. IGF: insulin-like growth factor, TGF, trans- forming growth factor, TNF: tumor necrosis factor, VEGF: vascular endothelial growth factor.

2 J Clin Neurol 2017;13(1):1-9 Vidale S et al. JCN neuroimaging study using 23Na MRI and quantitative histo- inhibits the formation of free radicals and ROS that contrib- chemical K+ staining revealed heterogeneity in the rate of Na ute to necrotic or apoptotic cell death.23,24 The depolarization concentration increase and in the K+ distribution within the of other neurons produces a further Ca2+ influx and addi- ischemic core.17 The reduced ATP production following Na/ tional glutamate release, leading to local amplification of the K imbalance (due to Na/K ATPase) also contributes to re- ischemic damage.25 Contemporary with those processes, the duce the reuptake of glutamate, which is the main excitatory persistence of arterial occlusion contributes to a critical re- 18 . This condition overstimulates the gluta- duction of pO2 and a concomitant increase in pCO2. In the mate receptors so as to influence the Ca2+ influx, producing a case of hypercapnia, the tissue pH could fall to around 6.6 or series of nuclear and cytoplasmic events (excitotoxicity) that lower if severe ischemia and tissue hypoxia occur; in the last lead to mitochondrial failure and apoptosis.19,20 At the same situation, anaerobic glycolysis leads to accumula- time, the Ca2+ influx triggers the activation of catabolic en- tion with signs of irreversible injury identifiable in the cell zymes with the production of arachidonic acid, and increases morphology.26 The acidosis state increases necrosis and cell the formation of (ROS), mainly in death via a mechanism called acidotoxicity and mediated by neurons rather than astrocytes.21 Ca2+-permeable acid-sensing channels.27-29 Other delete- Previous studies found that mitochondrial failure could be rious effects of acidosis influence the synthesis and degrada- predicted from the K+ concentration. Indeed, the mitochon- tion of cellular constituents, the mitochondrial function, the drial ATP-dependent K+ (mitoKATP) channel plays a critical cell volume control, the postischemic flow, and the stimula- role in the neuroprotective action, contributing to the so- tion of ROS production, all conditions that occur also in the called ischemic pre- and postconditioning states.22 The open- ischemic penumbra.30 Acidosis and ROS contribute to trig- ing of mitoKATP channels attenuates the Ca2+ overload and ger a subsequent and concomitant phase represented by the

Arterial Cell adhesion Membrane Leukocyte occlusion molecules infiltration Inflammation degradation expression

Hypoperfusion Acidosis Activation COX2

H2O BBB disruption Anaerobic glycolisis accumulation

Production cytokines (IL-6, TNF-a) NO synthase NO Oxidative stress Na/K-ATPase pump failure activation

Mithocondrial BAD, BAX Apoptosis failure activation Cell membrane depolarization

Intracellular Na, Ca Free radicals

Excitotoxicity glutamate release Arachidonic acid production

Activation Cytotoxic Necrosis Activation of kinase and proteinases edema NMDA/AMPA

Cell lysis

Fig. 2. Cerebral ischemic cascade. AMPA: α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, BAD: Bcl-2-associated death promoter, BBB: blood-brain barrier, COX: cyclo-oxigenase, IL: interleukin, NMDA: N-methyl-D-aspartate, TNF: tumor necrosis factor.

www.thejcn.com 3 JCN Inflammation in Acute Ischemic Stroke activation of innate immunity and involving both resident astrocytes, and neurons [involving matrix metalloproteinases cells (microglia) and circulating cells. (MMPs) or ROS], and allowing the extravasation of proteins and endothelial cells along with the activation of macro- INFLAMMATORY AND INNATE phages and mast cells via the ischemia and reperfusion mech- IMMUNITY ACTIVATIONS anisms.37 This extravasation is also supported by a break- down of the BBB−secondary to the pericyte death−leading Inflammation in the ischemic brain: to a long-lasting decrease in capillary blood flow.38 cell infiltration and damage A few minutes after arterial occlusion and the associated in- Postischemic inflammation begins in the vascular compart- tra- and extracellular modifications, the acute local damage is ment immediately after arterial occlusion. The production of detected also by pattern-recognition receptors (PRR) (includ- ROS leads to an increase in the procoagulant state involving ing Toll-like receptors) that respond to microbial structures the activation of complement, platelet, and endothelial (pathogen-associated molecular patterns) and host-derived cells.31,32 The increased activity of cyclooxygenase-2 in in- danger signals (damage-associated molecular patterns).39-41 flammatory cells and neurons may lead to tissue damage due These molecules can be released by stressed cells, such as dur- to excessive ROS production and toxic prostanoids.33,34 ROS ing the ischemic cascade. There is new evidence that the PRR contribute also to reduce the availability of NO, leading to of neurons and glial cells can play a fundamental role in acti- platelet aggregation and the adhesion of leukocytes, which vating intracellular signaling pathways so as to enhance the aggravate the ischemic damage.35 There is evidence of iNOS proinflammatory expression of different genes (Fig. 3).42,43 This (the inducible isoform of NO) being a critical effector and mechanism activates the immune system elements, resulting amplifier of tissue damage related to postischemic inflamma- in mast cells releasing vasoactive mediators (e.g., histamine), tion (Table 1).36 The oxidative stress and inflammatory medi- proteases, and tumor necrosis factor (TNF), while macro- ators affect the permeability of the BBB, impairing the so- phages release proinflammatory cytokines.44 called neurovascular unit that includes endothelial cells, Adhesion-molecule P- and E-selectins (Table 2) and inter-

DAMPs Inflammasome TLR NLRP and 3

Pro-caspase 1 Caspase 1

Transcriptional mediators Pro-IL-1 IL-1 Pro-IL-18 IL-18

Pro-inflammatory gene expression

Nucleus

Fig. 3. DAMP receptors and signaling pathways. Cells detect DAMPs via DAMP receptors in two ways: (1) activation of a type of pattern-recogni- tion receptor [Toll-like receptor (TLR)] and (2) activation of inflammasomes. The first mechanism involves proinflammatory factors being released by the nuclear gene expression mediated by transcriptional mediators activated by TLR. The second mechanism involves the activation of cas- pase-1 leading to the clivation of the proinflammatory cytokines IL-1 and IL-18, converting them into their activated forms. DAMP: damage-asso- ciated molecular pattern, IL: interleukin, NLRP: nod-like receptor pyrin.

4 J Clin Neurol 2017;13(1):1-9 Vidale S et al. JCN cellular adhesion molecule-1 are expressed on the membrane nucleotides from injured cells.47 These cytokines contribute after their intracellular translocation and with the rapid gen- to leukocyte infiltration in the damaged tissue, and they acti- eration of proinflammatory signals.45,46 The adhesion recep- vate the presentation of antigens between dendritic cells and tors mediate interactions between adhesion molecules and T-cells.48 T-cells lead to tissue damage by innate immunity, integrins, contributing to an initial rolling mechanism of leu- through interferon-gamma and ROS. T-cells activated by IL- kocytes and leading to adhesion to the endothelium and to a 23 released from microglia and macrophages produce IL-17, subsequent transmigration into the brain parenchyma (leu- which worsens the acute ischemic cerebral injury.49 This un- kocyte infiltration). Following ischemia, these cells rapidly balanced interplay between the immune and sympathetic release proinflammatory mediators into the area, which pro- nervous systems contributes to an early down-regulation of motes ischemic injury via different pathways: 1) the cerebral systemic cellular immune responses that leads to a functional no-reflow phenomenon by impeding the flow of red blood deactivating of monocytes, T-helper cells, and invariant nat- cells, 2) increased production of ROS and proteases at the ural-killer T-cells.50 endothelium surface, 3) platelet aggregation by leukotriene, Previous studies have demonstrated the predictive role of prostaglandin, or eicosanoid production due to activation of certain acute immune and stress biomarkers on clinical out- leukocyte phospholipases, and 4) deleterious activity of pro- comes: copeptin and mid-regional proatrial natriuretic pep- inflammatory cytokines in the penumbra area. In particular, tide.51,52 Even if the immune system is activated with recruit- during the acute phase of , microglia and ing elements in the focal ischemic area, specific injured macrophages release interleukin (IL)-1b, IL-6, and IL-18 cerebral sites contribute to different down-regulating respons- from the transcriptional intracellular pathways activated by es being exhibited by the autonomous nervous system. In particular, involvement of the right frontoparietal cortex, in- Table 1. Mediators of amplification of postischemic inflammation sula, or brainstem could lead to increased cerebral inflamma- 53 Mediator Type Producing cell tion and concomitant systemic immunosuppression. This ICAM1 Adhesion molecule EC, Leuk, PVM, MG, AG process is characterized by the increased apoptosis of lym- VCAM-1 Adhesion molecule EC, Leuk, PVM, MG, AG phocytes, suppression of peripheral cytokine release, and fi- P-selectin Adhesion molecule EC, Leuk, PVM, MG, AG Table 2. Mediators of initial postischemic inflammation E-selectin Adhesion molecule EC, Leuk, PVM, MG, AG Mediator Type Producing cell Mac-1 Adhesion molecule EC, Leuk, PVM, MG, AG P-selectin Adhesion molecule EC, PLT VLA-1 Adhesion molecule EC, Leuk, PVM, MG, AG IL-1b Cytokine MG, PVM, MC IL-1 Cytokine EC, neurons, PVM, MG, AG IL-1a Cytokine PLT IL-6 Cytokine EC, neurons, PVM, MG, AG TNF-a Cytokine MC IL-10 Cytokine EC, neurons, PVM, MG, AG CCL5 Chemokine IL-17 Cytokine EC, neurons, PVM, MG, AG CXCL4 Chemokine IL-20 Cytokine EC, neurons, PVM, MG, AG CXCL7 Chemokine PLT TNF Cytokine EC, neurons, PVM, MG, AG CX3CL1 Chemokine Neurons CCL2 Chemokine EC, neurons, PVM, MG, AG Elastase Protease CCL3 Chemokine EC, neurons, PVM, MG, AG MMP8 Protease CCL5 Chemokine EC, neurons, PVM, MG, AG MMP9 Protease CXCL2/3 Chemokine EC, neurons, PVM, MG, AG MT6-MMP Protease Leuk CXCL8 Chemokine EC, neurons, PVM, MG, AG Clotting factors Protease Plasma MMP2 Protease Circ, EC, AG, neurons Complement Protease Plasma, EC, MMP9 Protease Circ, EC, AG, neurons Prostanoids Small molecule EC, PLT, MG, neurons Complement Protease Circ, EC, AG, neurons Leukotrienes Small molecule EC, PLT, MG, neurons iNOS Other mediator MG, Leuk, EC ATP Small molecule Plasma, neurons COX-2 Other mediator Neurons, MG, Leuk, EC Free radicals Small molecule EC, PLT, Leuk, PVM, MG, Modified from Iadecola et al.Nat Med 2011;17:796-808, with permis- sion of Springer Nature.45 neurons AG: astroglia, CCL: chemokine ligand, Circ: plasma, COX: cyclo-oxige- Modified from Iadecola et al.Nat Med 2011;17:796-808, with permis- nase, CX: d- chemokine ligand, EC: endothelial cell, ICAM: intercellular sion of Springer Nature.45 adhesion molecule, IL: interleukin, iNOS: inducible isoform of NO, Mac: ATP: , CCL: chemokine ligand, CX: d-chemokine macrophage antigen, MG: microglia, MMP: matrix metalloproteinase, ligand, EC: endothelial cell, IL: interleukin, Leuk: leukocytes, MC: mast PVM: perivascular macrophages, TNF: tumor necrosis factor, VCAM: cells, MG: microglia, MMP: matrix metalloproteinase, PLT: platelets, vascular cell adhesion molecule, VLA: very late antigen. PVM: perivascular macrophages, TNF: tumor necrosis factor.

www.thejcn.com 5 JCN Inflammation in Acute Ischemic Stroke nally the inhibition of T-helper-1 cells and alteration of the An increased activity of inflammasomes is also associated T-helper-1/T-helper-2 ratio.54 The stroke-induced immuno- with neuron and glial cell death. Experimental studies and depression contributes to increasing the risk of infection (in- human data suggest that the NLRP1 and NLRP3 inflamma- fluenced also by comorbidities such as age, diabetes mellitus, somes in brain cells could activate pro-caspase-1 (cleaving to and atrial fibrillation) and consequently a poor functional caspase-1) to produce IL-1b and IL-18, which are proinflam- outcome. matory cytokines, and lead to a particular type of cell death called pyroptosis (Fig. 3).61 In this way, the inflammasomes The activity of microglia contribute to activating and supporting the innate immunity The microglia represent the resident immune cells of the but also to worsening the tissue injury. central nervous system and can be activated by local and sys- temic infections, neurodegenerative conditions, and injury. RESOLUTION OF INFLAMMATION Microglia are able to modify their morphology from a rest- AND REPAIR OF TISSUE ing (ramified) to an active (amoeboid) state.55 Microglia re- spond rapidly to ischemic stroke and other injuring condi- The inflammatory postischemia process is self-limiting, and tions. By entering the ischemic core within 60 minutes after its resolution is mediated by numerous factors that suppress the induction of focal ischemia without reperfusion, microg- the immune activity (Table 3). The termination of inflamma- lia significantly increase the number of their processes, while tion triggers structural and functional reorganization of the 24 hours later they are reduced in both number and distance injured brain. The first mechanism involved in this phase is from the insult. Previous studies have detected various forms the removal of dead cells, which is performed by microglia of activated microglia.56 M1 is the proinflammatory type and and infiltrating macrophages, mainly comprising phago- is able to release TNF-a, IL-1b, IL-18, and IL-6. On the other cytes.59,62 The principal factors driving these cells to the isch- hand, the M2 type is the healing cell involved in neuropro- emic site are purines released from injured cells and chemo- tection and repair, producing transforming growth factor kines. Immunoglobulins directed against antigens of the (TGF)-b, nerve growth factor, and IL-4.57 The M1 type is ob- central nervous system may also promote the release of IL- served for the first 24 hours in the ischemic core, and it in- creases in number over the first 2 weeks after the ischemic Table 3. Mediators of resolution of postischemic inflammation injury.58 The M2 type has been found at the end of the first Mediator Type Producing cell 24 hours, entering the area during the first week before de- BDNF Growth factor EC, Macr, AG, PVM, neurons clining in number.55 EPO Growth factor EC, Macr, AG, PVM, neurons Recent studies have provided evidence of microglia activa- FGF Growth factor EC, Macr, AG, PVM, neurons tion in the penumbra area. Indeed, the pattern of activity dif- G-CSF Growth factor EC, Macr, AG, PVM, neurons fers between the peri-infarct zone microglia and the ischemic IGF-1 Growth factor EC, Macr, AG, PVM, neurons core. During the first week, the M2 type was found to pre- NGF Growth factor EC, Macr, AG, PVM, neurons dominate in the ischemic core while the M1 type predomi- VEGF Growth factor EC, Macr, AG, PVM, neurons nated in the peri-infarct zone.59 These observations suggest TGF-b Cytokine MG, Macr, AG that the peri-infarct region is dominated by proinflammatory IL-10 Cytokine MG, Macr, AG and activated microglia whose abundance increases during IL-17 Cytokine MG, Macr, AG the first days after ischemic injury.60 The spatial distribution of IL-23 Cytokine MG, Macr, AG the microglia phenotypes changes over time, which suggests MMP9 Protease AG, neurons the enlargement of injured and damaged cerebral tissue. Complement Protease Circ, EC, AG, neurons Prostaglandin Small molecule The role of inflammasomes Lipoxin Small molecule The above-mentioned inflammatory response leads to the Docosanoid Small molecule production of proinflammatory cytokines and neuronal and Modified from Iadecola et al.Nat Med 2011;17:796-808, with permis- sion of Springer Nature.45 glial cell death mediated by large intracellular multiprotein AG: astroglia, BDNF: brain-derived neurotrophic factor, Circ: plasma, complexes called inflammasomes.45 Recent studies have EC: endothelial cell, EPO: erythropoietin, FGF: fibroblast growth factor, demonstrated that the nod-like receptor pyrin (NLRP) and G-CSF: granulocyte colony-stimulating factor, IGF: insulin-like growth factor, IL: interleukin, Macr: macrophages, MG: microglia, MMP: matrix NLRP3 inflammasomes in neurons and glial cells allow de- metalloproteinase, NGF: nerve growth factor, PVM: perivascular mac- tection of cellular damage and mediation of the inflammato- rophages, TGF: transforming growth factor, VEGF: vascular endothelial ry responses to aseptic tissue injury during cerebral ischemia.42 growth factor.

6 J Clin Neurol 2017;13(1):1-9 Vidale S et al. JCN

10 and TGF-b, which contribute to suppressing the immune tenance.69 In particular, Hsp70 leads to neuronal survival, process and to inhibiting the expression of adhesion mole- mitochondrial stabilization, and cell-death blocking, mainly cules and the production of proinflammatory cytokines (Fig. by facilitating the activation of transcription factor and NF- 1).63 These pleiotropic immunoregulatory cytokines can fa- kB.70,71 NF-kB is affected also by the phosphorylation of Akt cilitate tissue repair after promoting the resolution of inflam- secondary to the activation of phosphoinositide-3-kinase72 mation, and they exert cytoprotective effects on the surviving This is also able to attenuate cellular apoptosis in the cells in the ischemic area.35 Concomitant growth factors re- neurovascular unit, but it is inhibited after ischemia or reper- leased by inflammatory cells, neurons, and astrocytes64 sup- fusion so as to induce cell death.73 Several studies have indi- port cell sprouting, neurogenesis, and angiogenesis as well as cated that any stimulus that modifies brain function appears matrix reorganization after ischemic injury. Insulin-like to increase the resistance of the brain to further injuries, in- growth factor-1 is a critical factor in the sprouting of neurons cluding to different types of injury.74 The results of some after cerebral ischemia, while the reactivity of astrocytes is studies involving patients with previous transient ischemic mandatory for the functional recovery of damaged tissue.65 attacks suggest that brain preconditioning and tolerance oc- Concomitant actions of vascular endothelial growth factor cur because these patients have a more favorable clinical out- and neutrophil MMPs are required in angiogenesis, support- come and smaller infarcts.75,76 ing the need for the combined activity of inflammatory cells and astrocytes.66 CONCLUSIONS NEURONAL PRECONDITIONING AND Inflammatory responses during the acute stage of ischemic INFLAMMATORY MEDIATORS stroke have effects ranging from deleterious to beneficial. A first reaction is negative due to cytotoxicity, which leads to The brain is the most metabolically active organ in the body, tissue damage by cellular death. At the same time, inflamma- consuming about 25% of total glucose and oxygen, and so a tion exerts protective effects by stimulating a preconditioning high oxidative stress is generated by its metabolism. Neurons status that preserves brain tissue by adapting the brain itself would therefore be expected to respond actively to stress, after an insult. Finally, inflammatory mediators contribute to have low capacities for replacement, and strong potential autolimit the pathological process and to repair the injured strategies for protection and repair. While most of the under- parenchyma by remodeling the tissue. lying neuroprotective and repairing pathways are still under Future observations will contribute to a better understanding investigation, this adaptive response to an insult by the acti- of other inflammatory mechanisms involved in these early vating intracellular signals is well known, and called precon- stages of ischemic stroke, and this information will potential- ditioning.67 Preconditioning occurs in two distinct phases. ly lead to the development of effective neuroprotective The first happens early and is associated with posttransla- agents. tional modifications of proteins. This period lasts from min- utes to 1–2 hours, and involves certain protective proteins SEARCH STRATEGY AND (mainly protein chaperones) being rapidly released from SELECTION CRITERIA stressed cells. The second phase requires the synthesis of new proteins. During this phase, which lasts several days, the pro- References for use in this review were identified by searching inflammatory cytokines IL-1b and TNF activate intracellular the PubMed and EMBASE databases for articles published signaling pathways that lead to a tolerant state by mediating between 1976 and June 2016, as well as references in relevant the stress responses. In particular, ROS and cytokines are articles. The search terms used were “inflammation,” “im- able to activate Ras, Raf, and kinases of the cellular mem- mune system,” “immunity,” “brain ischemia,” “cerebral isch- brane, with the subsequent intracellular transcription and emia,” “stroke,” “cerebral occlusion,” and “cerebrovascular production of survival proteins. These last mechanisms con- disease.” There were no language restrictions. The final refer- tribute to preserving the endothelium via vascular protection ence list was generated based on the relevance to the topics and angiogenesis [mitogen-activated protein kinase p38 and covered in this review. extracellular signal-regulated kinase 1/2 (ERK 1/2)], the neu- Conflicts of Interest rons via and neurogenesis [ERK 1/2 and The authors have no financial conflicts of interest. nuclear factor kB (NF-kB)], and the glia via an anti-inflam- matory action (serine/threonine kinase).68 Other proteins, called heat-shock proteins (Hsp), contribute to cellular main-

www.thejcn.com 7 JCN Inflammation in Acute Ischemic Stroke

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