Inflammatory mediators and stroke: new opportunities for novel therapeutics

Cerebral ischemia: gene activation, neuronal injury, and the protective role of antioxidants

A large number of gene products appear after an ischemic insult making it difficult to decipher which genes are involved in tissue injury. Reactive oxygen species (ROS) can influence gene expression and have a role in the events that lead to neuronal death. In global cerebral ischemia the oxidative responsive transcription factor, NF-kappa B, is persistently activated in neurons that are destined to die. There are several potential routes through which NF-kappa B can act to induce neuronal death, including production of death proteins and an aborted attempt to reenter the cell cycle. NF-kappa B is only transiently activated in neurons that survive. Persistent NF-kappa B activation can be blocked by antioxidants, which suggests that the neuroprotective effect of antioxidants may be due to inhibiting activation of NF-kappa B.

Hypertonic mannitol loading of NF-kappaB transcription factor decoys in human brain microvascular endothelial cells blocks upregulation of ICAM-1

BACKGROUND AND PURPOSE

An acute inflammatory response exacerbates tissue injury during acute ischemic stroke. The transcription factor nuclear factor (NF)-kappaB plays a key role in endothelial cell activation and the inflammatory response. Targeted genetic disruption of NF-kappaB activation in cerebral endothelial cells may be protective in stroke. We determined whether a NF-kappaB transcription factor decoy (TFD) could block intercellular adhesion molecule (ICAM)-1 upregulation, an indicator of endothelial cell activation.

METHODS

We modeled ischemia-reperfusion in vitro by exposing cultured human brain microvascular endothelial cells (HBMEC) to tumor necrosis factor (TNF)-alpha and conditions of hypoxia-reoxygenation (H/R). Mannitol was used to load phosphothiorated oligonucleotides containing 3 copies of the kappaB binding sequences (TFDs) into cultured HBMEC. An NF-kappaB TFD, a mutated NF-kappaB TFD, and a scrambled TFD were studied for their effect on ICAM-1 mRNA levels and surface ICAM-1 by ELISA.

RESULTS

Hyperosmolar loading with mannitol permitted rapid transfection of TFD into endothelial cell nuclei. The NF-kappaB TFD but not the mutated or scrambled TFD competed with a kappaB sequence for binding to nuclear extracts from HBMEC exposed to TNF-alpha. The NF-kappaB TFD blocked the TNF-alpha-induced and H/R-induced increase in ICAM-1 mRNA levels and the upregulation of surface ICAM-1.

CONCLUSIONS

Mannitol delivers phosphothiorated oligonucleotides into cultured HBMEC. An NF-kappaB decoy blocks both TNF-alpha-induced and H/R-induced ICAM-1 upregulation in HBMEC. Targeted genetic disruption of endothelial NF-kappaB activation may be of benefit in acute ischemic stroke.

Benign focal ischemic preconditioning induces neuronal Hsp70 and prolonged astrogliosis with expression of Hsp27

We have established a focal preconditioning (PC) paradigm that produces significant and prolonged ischemic tolerance (IT) of the brain to subsequent permanent middle cerebral artery occlusion (MCAO). PC using 10 min of MCAO induces brain tolerance at 1-7 days of reperfusion that requires active protein synthesis. The protective protein(s) involved are unknown. In these studies the increased transcription and translation of the inducible 70-kDa heat shock protein (Hsp70) and the 27-kDa heat shock protein (Hsp27), and astrogliosis/glial fibrillary acidic protein (GFAP) were determined by Northern analysis and immunohistochemistry following PC. Cellular localization of proteins was determined by double labeling. PC produced no brain injury but did increase Hsp70 mRNA transiently at 6 h and increased Hsp27 mRNA later at 24 h for at least 5 days. Protein expression induced by PC exhibited a similar profile. Hsp70 protein was primarily expressed in neurons from 1 to 5 days post-PC throughout the PC cortex. Hsp27 protein expression was initiated later for a much longer period of time. A remarkable astroglyosis was verified with increased astrocytic Hsp27 from 1 to 7 days after PC. Gliosis with increased Hsp27 in the PC cortex was still present but reduced 4 weeks after PC. Therefore, PC that results in brain tolerance/neuroprotection increases neuronal Hsp70 in the PC cortex and activated astrocytic Hsp27 in the PC cortex in a temporal fashion associated with developing IT. The short duration of benign ischemia (PC) that produces IT produces a robust, long-lived cellular and protein synthetic response that extends throughout the entire cortex (i.e. well beyond the MCA perfusion territory). The resulting IT is associated with changes in astrocyte-activation that might provide increased support and protection from injury. Although both Hsp70 and Hsp27 may participate in the neuroprotection/brain tolerance induced by PC, the temporal expression patterns of these proteins indicate that they are not solely responsible for the tolerance to brain injury.

Differential activation of MAPK/ERK and p38/SAPK in neurones and glia following focal cerebral ischaemia in the rat

Two relatively well characterised kinase signalling pathways are those involving MAPK/ERK and p38/SAPK2, that are known to be activated in vitro by various factors known to increase following stroke, such as glutamate, IL-1 and TNF. The present study was designed to investigate the activation and cellular distribution of phosphorylated-ERK1/2, -p38 and the transcription factor CREB following focal cerebral ischaemia using phosphospecific antibodies. Up to 24 h following transient MCAO (90 min) and 6 h following permanent MCAO, phospho-ERK1/2 staining was markedly increased within the cytoplasm of neuronal perikarya in 'penumbral-like' regions. In contrast, phospho-p38 immunostaining was markedly increased in cells with astrocyte-like morphology in both 'core' and 'penumbral-like' regions. Phospho-p38 staining was also detected in some neurones within 'penumbral-like' regions up to 24 h following transient MCAO. CREB activation was confined to neurones in 'penumbral-like' regions. Increased phospho-p38 immunoreactivity was detected in astrocyte-like cells present in the subcortical white matter ipsilateral to the occluded MCAO, while phospho-CREB and -ERK1/2 staining was localised to cells with the morphological appearance of oligodendrocytes. This study demonstrates phosphorylation, indicative of activation, of both the MAPK and p38 pathways following transient and permanent MCAO. However, each pathway shows a distinct cellular and spatial distribution within ischaemic tissue. Together these data indicate that neuroprotection offered by agents directed towards the ERK1/2 pathway may act directly through protection of neurones and oligodendrocytes, while those directed towards the p38 pathway kinase signalling pathways may be indirectly via inhibition of cytokines and other mediators involved in the brains response to injury.

What animal models have taught us about the treatment of acute stroke and brain protection

Stroke research has progressed in leaps and bounds in the past decades. A driving force is the increasing availability of new research tools in this field (eg, animal stroke models). Animal stroke models have been extensively applied to advance our understanding of the mechanisms of ischemic brain injury and to develop novel therapeutic strategies for reducing brain damage after a stroke. Animal stroke models have been useful in characterizing the molecular cascades of injury processes. These "injury pathways" are also the targets of therapeutic interventions. The major achievements made in the past 2 decades applying animal stroke models include 1) the identification of the mediator role of excitotoxin and oxygen free radicals in ischemic brain injury; 2) the confirmation of apoptosis as a major mechanism of ischemic cell death; 3) the characterization of postischemic gene expression; 4) the delineation of postischemic inflammatory reaction; 5) the application of transgenic mice to confirm the roles of purported mediators in ischemic brain injury; 6) development of novel magnetic resonance imaging sequences for early noninvasive detection of ischemic brain lesions; and, 7) the development of novel therapeutic strategies based on preclinical findings derived from animal stroke models.

Effects of lipopolysaccharide priming on acute ischemic brain injury

BACKGROUND AND PURPOSE

Infection has been implicated as a stroke risk factor. Activation and infiltration of polymorphonuclear neutrophils (PMNs) after cerebral ischemia may contribute to ischemic brain injury. This study was conducted to investigate how enhanced postischemic PMN infiltration by lipopolysaccharide (LPS) altered the acute ischemic outcomes.

METHODS

LPS (0.05 mg/kg SC) or vehicle was given to Long-Evans male rats 24 hours before ischemia. Focal cerebral ischemia was induced by temporary ligation of the right middle cerebral artery and both common carotid arteries for 45 minutes. Animals were killed 6 and 24 hours after reperfusion to determine the extent of PMN infiltration (myeloperoxidase assay), brain edema (wet-dry weight method), and vascular injury (fluorescein isothiocyanate-conjugated dextran extravasation). The infarct volumes were measured on the basis of TTC stain 24 hours after ischemia.

RESULTS

LPS had little effect on body temperature or peripheral white count but substantially enhanced PMN infiltration into the ischemic right middle cerebral artery cortex on the basis of myeloperoxidase activity (6 hours: control, 0 U/g; LPS, 0.186+/-0. 025 U/g; 24 hours: control, 0.185+/-0.025 U/g; LPS, 0.290+/-0.040 U/g; P<0.001) and morphological studies. The extent of vascular injury defined by the extravasation of fluorescein isothiocyanate-conjugated dextran into the ischemic tissue (6 hours: control, 3.11+/-0.41 microliter/mg protein; LPS, 0.48+/-0.16 microliter/mg protein; 24 hours: control, 1.77+/-0.23 microliter/mg protein; LPS, 0. 90+/-0.19 microliter/mg protein; P<0.001) and brain edema determined by the brain water content (6 hours: control, 84.77+/-1.63%; LPS, 82. 09+/-1.25%; 24 hours: control, 89.40+/-0.43%; LPS, 87.88+/-0.58%; P<0.01) were paradoxically reduced by LPS priming. LPS-primed rats also had smaller infarct volumes (control, 135+/-5 mm(3); LPS, 108+/-12 mm(3); P<0.05).

CONCLUSIONS

Enhanced postischemic PMN infiltration is anticipated to facilitate ischemic brain injury. Contrary to this expectation, results from the present study suggest that an increase in postischemic PMN infiltration after LPS priming was not detrimental. These findings challenge the notion that postischemic PMN infiltration is uniformly deleterious.

Inflammatory mediators and stroke: new opportunities for novel therapeutics

Contrary to previous dogmas, it is now well established that brain cells can produce cytokines and chemokines, and can express adhesion molecules that enable an in situ inflammatory reaction. The accumulation of neutrophils early after brain injury is believed to contribute to the degree of brain tissue loss. Support for this hypothesis has been drawn from many studies where neutrophil-depletion blockade of endothelial-leukocyte interactions has been achieved by various techniques. The inflammation reaction is an attractive pharmacologic opportunity, considering its rapid initiation and progression over many hours after stroke and its contribution to evolution of tissue injury. While the expression of inflammatory cytokines that may contribute to ischemic injury has been repeatedly demonstrated, cytokines may also provide "neuroprotection" in certain conditions by promoting growth, repair, and ultimately, enhanced functional recovery. Significant additional basic work is required to understand the dynamic, complex, and time-dependent destructive and protective processes associated with inflammation mediators produced after brain injury. The realization that brain ischemia and trauma elicit robust inflammation in the brain provides fertile ground for discovery of novel therapeutic agents for stroke and neurotrauma. Inhibition of the mitogen-activated protein kinase (MAPK) cascade via cytokine suppressive anti-inflammatory drugs, which block p38 MAPK and hence the production of interleukin-1 and tumor necrosis factor-alpha, are most promising new opportunities. However, spatial and temporal considerations need to be exercised to elucidate the best opportunities for selective inhibitors for specific inflammatory mediators.