Jesús M. Pradillo

Toll-Like Receptor 4 Is Involved in Brain Damage and Inflammation After Experimental Stroke

Background— Stroke is the second to third leading cause of death. Toll-like receptor 4 (TLR4) is a signaling receptor in innate immunity that is a specific immunologic response to systemic bacterial infection and cerebral injury. The role of TLR4 in brain ischemia has not been examined yet. We have therefore investigated whether cerebral ischemia and inflammation produced by permanent occlusion of the middle cerebral artery differ in mice that lack a functional TLR4 signaling pathway. Methods and Results— Permanent occlusion of the middle cerebral artery was performed on 2 strains of TLR4-deficient mice (C3H/HeJ and C57BL/10ScNJ) and respective controls (C3H/HeN and C57BL/10ScSn). Stroke outcome was evaluated by determination of infarct volume and assessment of neurological scores. Brains were collected 24 hours and 7 days after stroke. When compared with control mice, TLR4-deficient mice had lower infarct volumes and better outcomes in neurological and behavioral tests. Mice that lacked TLR4 had minor expression of stroke-induced interferon regulatory factor-1, inducible nitric oxide synthase, and cyclooxygenase-2, mediators implicated in brain damage. The levels of interferon-β and of the lipid peroxidation marker malondialdehyde were also lower in brains from TLR4-deficient mice than in those from control mice. In addition, the expression of matrix metalloproteinase-9, which is induced and mediates brain damage, was also reduced in TLR4-deficient mice after experimental stroke. Conclusions— TLR4-deficient mice have minor infarctions and less inflammatory response after an ischemic insult. These data demonstrate that TLR4 signaling and innate immunity are involved in brain damage and in inflammation triggered by ischemic injury.

Toll-Like Receptor 4 Is Involved in Subacute Stress–Induced Neuroinflammation and in the Worsening of Experimental Stroke

Background and Purpose— Psychological stress causes an inflammatory response in the brain and is able to exacerbate brain damage caused by experimental stroke. We previously reported that subacute immobilization stress in mice worsens stroke outcome through mechanisms that involve inflammatory mechanisms, such as accumulation of oxidative/nitrosative mediators and expression of inducible nitric oxide synthase and cyclooxygenase-2 in the brain. Some of these inflammatory mediators could be regulated by innate immunity, the activation of which takes place in the brain and produces an inflammatory response mediated by toll-like receptors (TLRs). Recently, we described the implications of TLR4 in ischemic injury, but the role of TLR4 in stress has not yet been examined. We therefore investigated whether inflammation produced by immobilization stress differs in mice that lack a functional TLR4 signaling pathway. Methods— We used an experimental paradigm consisting of the exposure of mice to repeated immobilization sessions (1 hour daily for 7 days) before permanent middle cerebral artery occlusion. Results— We found that TLR4-deficient mice subjected to subacute stress had a better behavioral condition compared with normal mice (C3H/HeN) and that this effect was associated with a minor inflammatory response (cyclooxygenase-2 and inducible nitric oxide synthase expression) and lipid peroxidation (malondialdehyde levels) in brain tissue. Furthermore, previous exposure to stress was followed by a smaller infarct volume after permanent middle cerebral artery occlusion in TLR4-deficient mice than in mice that express TLR4 normally. Conclusions— Our results indicate that TLR4 is involved in the inflammatory response after subacute stress and its exacerbating effect on stroke. These data implicate the effects of innate immunity on inflammation and damage in the brain after stroke.

Silent Information Regulator 1 Protects the Brain Against Cerebral Ischemic Damage

Background and Purpose— Sirtuin 1 (SIRT1) is a member of NAD+-dependent protein deacetylases implicated in a wide range of cellular functions and has beneficial properties in pathologies including ischemia/reperfusion processes and neurodegeneration. However, no direct evidence has been reported on the direct implication of SIRT1 in ischemic stroke. The aim of this study was to establish the role of SIRT1 in stroke using an experimental model in mice. Methods— Wild-type and Sirt1−/− mice were subjected to permanent focal ischemia by permanent ligature. In another set of experiments, wild-type mice were treated intraperitoneally with vehicle, activator 3 (SIRT1 activator, 10 mg/kg), or sirtinol (SIRT1 inhibitor, 10 mg/kg) for 10 minutes, 24 hours, and 40 hours after ischemia. Brains were removed 48 hours after ischemia for determining the infarct volume. Neurological outcome was evaluated using the modified neurological severity score. Results— Exposure to middle cerebral artery occlusion increased SIRT1 expression in neurons of the ipsilesional mouse brain cortex. Treatment of mice with activator 3 reduced infarct volume, whereas sirtinol increased ischemic injury. Sirt1−/− mice displayed larger infarct volumes after ischemia than their wild-type counterparts. In addition, SIRT1 inhibition/deletion was concomitant with increased acetylation of p53 and nuclear factor &kgr;B (p65). Conclusions— These results support the idea that SIRT1 plays an important role in neuroprotection against brain ischemia by deacetylation and subsequent inhibition of p53-induced and nuclear factor &kgr;B-induced inflammatory and apoptotic pathways.

Toll-like receptor 4 is involved in neuroprotection afforded by ischemic preconditioning

It has been demonstrated that a short ischemic event (ischemic preconditioning, IPC) results in a subsequent resistance to severe ischemia (ischemic tolerance, IT). We have recently demonstrated the role of innate immunity and in particular of toll‐like receptor (TLR) 4 in brain ischemia. Several evidences suggest that TLR4 might also be involved in IT. Therefore, we have now used an in vivo model of IPC to investigate whether TLR4 is involved in IT. A 6‐min temporary bilateral common carotid arteries occlusion was used for focal IPC and it was performed on TLR4‐deficient mice (C57BL/10ScNJ) and animals that express TLR4 normally (C57BL/10ScSn). To assess the ability of IPC to induce IT, permanent middle cerebral artery occlusion was performed 48 h after IPC. Stroke outcome was evaluated by determination of infarct volume and assessment of neurological scores. IPC caused neuroprotection as shown by a reduction in infarct volume and better outcome in mice expressing TLR4 normally. TLR4‐deficient mice showed less IPC‐induced neuroprotection than wild‐type animals. Western blot analysis of tumor necrosis factor alpha (TNF‐α), inducible nitric oxide synthase (iNOS) and cyclooxygenase‐2 (COX‐2) showed an up‐regulation in the expression of these proteins in both substrains of mice measured 18, 24 and 48 h after IPC, being higher in mice with TLR4. Similarly, nuclear factor‐kappa B (NF‐κB) activation was observed 18, 24 and 48 h after IPC, being more intense in TLR4‐expressing mice. These data demonstrate that TLR4 signalling is involved in brain tolerance as shown by the difference in the percentage of neuroprotection produced by IPC between ScSn and ScNJ (60% vs. 18%). The higher expression of TNF‐α, iNOS and cyclooxygenase‐2 and NF‐κB activation in mice expressing TLR4 is likely to participate in this endogenous neuroprotective effect.

Synthesis of Lipoxin A4 by 5-Lipoxygenase Mediates PPARγ-Dependent, Neuroprotective Effects of Rosiglitazone in Experimental Stroke

Peroxisome proliferator-activated receptors gamma (PPARγ) are nuclear receptors with essential roles as transcriptional regulators of glucose and lipid homeostasis. PPARγ are also potent anti-inflammatory receptors, a property that contributes to the neuroprotective effects of PPARγ agonists in experimental stroke. The mechanism of these beneficial actions, however, is not fully elucidated. Therefore, we have explored further the actions of the PPARγ agonist rosiglitazone in experimental stroke induced by permanent middle cerebral artery occlusion (MCAO) in rodents. Rosiglitazone induced brain 5-lipoxygenase (5-LO) expression in ischemic rat brain, concomitantly with neuroprotection. Rosiglitazone also increased cerebral lipoxin A4 (LXA4) levels and inhibited MCAO-induced production of leukotriene B4 (LTB4). Furthermore, pharmacological inhibition and/or genetic deletion of 5-LO inhibited rosiglitazone-induced neuroprotection and downregulation of inflammatory gene expression, LXA4 synthesis and PPARγ transcriptional activity in rodents. Finally, LXA4 caused neuroprotection, which was partly inhibited by the PPARγ antagonist T0070907, and increased PPARγ transcriptional activity in isolated nuclei, showing for the first time that LXA4 has PPARγ agonistic actions. Altogether, our data illustrate that some effects of rosiglitazone are attributable to de novo synthesis of 5-LO, able to induce a switch from the synthesis of proinflammatory LTB4 to the synthesis of the proresolving LXA4. Our study suggests novel lines of study such as the interest of lipoxin-like anti-inflammatory drugs or the use of these molecules as prognostic and/or diagnostic markers for pathologies in which inflammation is involved, such as stroke.

TNFR1 upregulation mediates tolerance after brain ischemic preconditioning

A short ischemic event (ischemic preconditioning (IPC)) can result in subsequent resistance to severe ischemic injury (ischemic tolerance (IT)). The expression and neuroprotective role of tumor necrosis factor (TNF-α) have been described in models of IPC and we have showed the participation of its processing enzyme, the TNF-α convertase enzyme (TACE) in this process. We have now decided to explore the expression and localization of TNF receptors (TNFR) as well as other signalling mechanisms involved in IT. A period of 10 mins of temporary middle cerebral artery occlusion (tMCAO) was used for focal IPC. To evaluate the ability of IPC to produce IT, permanent MCAO was performed 48 hours after IPC. Ischemic preconditioning produced a reduction in infarct volume, as we showed previously. Ischemic preconditioning caused upregulation of neuronal TNFR1 that was reduced by the selective TACE inhibitor BB1101. Intracerebral administration of TNFR1 antisense oligodeoxynucleotide, which caused a reduction in TNFR1 expression, inhibited the IPC-induced protective effect, showing that TNFR1 upregulation is implicated in IT. Moreover, treatment with BB1101, TNFR1 antisense and lactacystin—a specific proteasome inhibitor—blocked IPC-induced NF-κB. Immunohistochemical studies showed the expression of TACE and TNFR1 in neurons. In summary, these data show that IPC produces neuronal upregulation of TACE and TNFR1, and that the pathway TACE/TNF-α/TNFR1/NF-κB is involved in IT.

3,4‐Methylenedioxymethamphetamine increases interleukin‐1β levels and activates microglia in rat brain: studies on the relationship with acute hyperthermia and 5‐HT depletion

3,4‐Methylenedioxymethamphetamine (MDMA) administration to rats produces acute hyperthermia and 5‐HT release. Interleukin‐1β (IL‐1β) is a pro‐inflammatory pyrogen produced by activated microglia in the brain. We examined the effect of a neurotoxic dose of MDMA on IL‐1β concentration and glial activation and their relationship with acute hyperthermia and 5‐HT depletion. MDMA, given to rats housed at 22°C, increased IL‐1β levels in hypothalamus and cortex from 1 to 6 h and [3H]‐(1‐(2‐chlorophenyl)‐N‐methyl‐N‐(1‐methylpropyl)3‐isoquinolinecarboxamide) binding between 3 and 48 h. Increased immunoreactivity to OX‐42 was also detected. Rats became hyperthermic immediately after MDMA and up to at least 12 h later. The IL‐1 receptor antagonist did not modify MDMA‐induced hyperthermia indicating that IL‐1β release is a consequence, not the cause, of the rise in body temperature. When MDMA was given to rats housed at 4°C, hyperthermia was abolished and the IL‐1β increase significantly reduced. The MDMA‐induced acute 5‐HT depletion was prevented by fluoxetine coadministration but the IL‐1β increase and hyperthermia were unaffected. Therefore, the rise in IL‐1β is not related to the acute 5‐HT release but is linked to the hyperthermia. Contrary to IL‐1β levels, microglial activation is not significantly modified when hyperthermia is prevented, suggesting that it might be a process not dependent on the hyperthermic response induced by MDMA.

A chronic treatment with CDP-choline improves functional recovery and increases neuronal plasticity after experimental stroke

Chronic impairment of forelimb and digit movement is a common problem after stroke that is resistant to therapy. Although in the last years some studies have been performed to increase the efficacy of rehabilitative experience and training, the pharmacological approaches in this context remain poorly developed. We decided to study the effect of a chronic treatment with CDP-choline, a safe and well-tolerated drug that is known to stabilize membranes, on functional outcome and neuromorphological changes after stroke. To assess the functional recovery we have performed the staircase reaching test and the elevated body swing test (EBST), for studying sensorimotor integration and asymmetrical motor function respectively. The treatment with CDP-choline, initiated 24 h after the middle cerebral artery occlusion (MCAO) and maintained during 28 days, improved the functional outcome in both the staircase test (MCAO+CDP=87.0+/-6.6% pellets eaten vs. MCAO+SAL=40.0+/-4.5%; p<0.05) and the EBST (MCAO+CDP=70.0+/-6.8% vs. MCAO+SAL=88.0+/-5.4%; contralateral swing p<0.05). In addition, to study potential neuronal substrates of the improved function, we examined the dendritic morphology of layer V pyramidal cells in the undamaged motor cortex using a Golgi-Cox procedure. The animals treated with CDP-choline showed enhanced dendritic complexity and spine density compared with saline group. Our results suggest that a chronic treatment with CDP-choline initiated 24 h after the insult is able to increase the neuronal plasticity within noninjured and functionally connected brain regions as well as to promote functional recovery.

Neurorepair versus neuroprotection in stroke.

Stroke is the second to third leading cause of death and the main cause of severe, long-term disability in adults. However, treatment is almost reduced to fibrinolysis, a therapy useful in a low percentage of patients. Given that the immediate treatment for stroke is often unfeasible in the clinical setting, the need for new therapy strategies is imperative. After stroke, the remaining impairment in functions essential for routine activities, such as movement programming and execution, sensorimotor integration, language and other cognitive functions have a deep and life-long impact on the quality of life. An interesting point is that a slow but consistent recovery can be observed in the clinical practice over a period of weeks and months. Whereas the recovery in the first few days likely results from edema resolution and/or from reperfusion of the ischemic penumbra, a large part of the recovery afterwards is due mainly to brain plasticity, by which some regions of the brain assume the functions previously performed by the damaged areas. Neurogenesis and angiogenesis are other possible mechanisms of recovery after stroke. An understanding of the mechanisms underlying functional recovery may shed light on strategies for neurorepair, an alternative with a wide therapeutic window when compared with neuroprotective strategies.

Rosiglitazone-induced CD36 up-regulation resolves inflammation by PPARγ and 5-LO-dependent pathways

PPARγ‐achieved neuroprotection in experimental stroke has been explained by the inhibition of inflammatory genes, an action in which 5‐LO, Alox5, is involved. In addition, PPARγ is known to promote the expression of CD36, a scavenger receptor that binds lipoproteins and mediates bacterial recognition and also phagocytosis. As phagocytic clearance of neutrophils is a requisite for resolution of the inflammatory response, PPARγ‐induced CD36 expression might help to limit inflammatory tissue injury in stroke, an effect in which 5‐LO might also be involved. Homogenates, sections, and cellular suspensions were prepared from brains of WT and Alox5−/− mice exposed to distal pMCAO. BMMs were obtained from Lys‐M Cre+ PPARγf/f and Lys‐M Cre− PPARγf/f mice. Stereological counting of double‐immunofluorescence‐labeled brain sections and FACS analysis of cell suspensions was performed. In vivo and in vitro phagocytosis of neutrophils by microglia/macrophages was analyzed. PPARγ activation with RSG induced CD36 expression in resident microglia. This process was mediated by the 5‐LO gene, which is induced in neurons by PPARγ activation and at least by one of its products—LXA4—which induced CD36 independently of PPARγ. Moreover, CD36 expression helped resolution of inflammation through phagocytosis, concomitantly to neuroprotection. Based on these findings, in addition to a direct modulation by PPARγ, we propose in brain a paracrine model by which products generated by neuronal 5‐LO, such as LXA4, increase the microglial expression of CD36 and promote tissue repair in pathologies with an inflammatory component, such as stroke.