Danielle N. Doll

Cytokines: their role in stroke and potential use as biomarkers and therapeutic targets.

Inflammatory mechanisms both in the periphery and in the CNS are important in the pathophysiologic processes occurring after the onset of ischemic stroke (IS). Cytokines are key players in the inflammatory mechanism and contribute to the progression of ischemic damage. This literature review focuses on the effects of inflammation on ischemic stroke, and the role pro-inflammatory and anti-inflammatory cytokines play on deleterious or beneficial stroke outcome. The discovery of biomarkers and novel therapeutics for stroke has been the focus of extensive research recently; thus, understanding the roles of pro-inflammatory and anti-inflammatory cytokines that are up-regulated during stroke will help us further understand how inflammation contributes to the progression of ischemic damage and provide potential targets for novel therapeutics and biomarkers for diagnosis and prognosis of stroke.

Rapid mitochondrial dysfunction mediates TNF-alpha-induced neurotoxicity

Tumor necrosis factor alpha (TNF‐α) is known to exacerbate ischemic brain injury; however, the mechanism is unknown. Previous studies have evaluated the effects of TNF‐α on neurons with long exposures to high doses of TNF‐α, which is not pathophysiologically relevant. We characterized the rapid effects of TNF‐α on basal respiration, ATP production, and maximal respiration using pathophysiologically relevant, post‐stroke concentrations of TNF‐α. We observed a reduction in mitochondrial function as early as 1.5 h after exposure to low doses of TNF‐α, followed by a decrease in cell viability in HT‐22 cells and primary neurons. Subsequently, we used the HT‐22 cell line to determine the mechanism by which TNF‐α causes a rapid and profound reduction in mitochondrial function. Pre‐treating with TNF‐R1 antibody, but not TNF‐R2 antibody, ameliorated the neurotoxic effects of TNF‐α, indicating that TNF‐α exerts its neurotoxic effects through TNF‐R1. We observed an increase in caspase 8 activity and a decrease in mitochondrial membrane potential after exposure to TNF‐α which resulted in a release of cytochrome c from the mitochondria into the cytosol. These novel findings indicate for the first time that an acute exposure to pathophysiologically relevant concentrations of TNF‐α has neurotoxic effects mediated by a rapid impairment of mitochondrial function.

Mitochondrial Crisis in Cerebrovascular Endothelial Cells Opens the Blood–Brain Barrier

Background and Purpose— The blood–brain barrier (BBB) is a selectively permeable cerebrovascular endothelial barrier that maintains homeostasis between the periphery and the central nervous system. BBB disruption is a consequence of ischemic stroke and BBB permeability can be altered by infection/inflammation, but the complex cellular and molecular changes that result in this BBB alteration need to be elucidated to determine mechanisms. Methods— Infection mimic (lipopolysaccharide) challenge on infarct volume, BBB permeability, infiltrated neutrophils, and functional outcomes after murine transient middle cerebral artery occlusion in vivo; mitochondrial evaluation of cerebrovascular endothelial cells challenged by lipopolysaccharide in vitro; pharmacological inhibition of mitochondria on BBB permeability in vitro and in vivo; the effects of mitochondrial inhibitor on BBB permeability, infarct volume, and functional outcomes after transient middle cerebral artery occlusion. Results— We report here that lipopolysaccharide worsens ischemic stroke outcome and increases BBB permeability after transient middle cerebral artery occlusion in mice. Furthermore, we elucidate a novel mechanism that compromised mitochondrial function accounts for increased BBB permeability as evidenced by: lipopolysaccharide-induced reductions in oxidative phosphorylation and subunit expression of respiratory chain complexes in cerebrovascular endothelial cells, a compromised BBB permeability induced by pharmacological inhibition of mitochondrial function in cerebrovascular endothelial cells in vitro and in an in vivo animal model, and worsened stroke outcomes in transient middle cerebral artery occlusion mice after inhibition of mitochondrial function. Conclusions— We concluded that mitochondria are key players in BBB permeability. These novel findings suggest a potential new therapeutic strategy for ischemic stroke by endothelial cell mitochondrial regulation.

A Genomic Profile of the Immune Response to Stroke With Implications for Stroke Recovery

Objectives: The objectives of this study were to determine the change in gene expression between two time points following stroke and to identify biomarkers of stroke recovery through gene expression profiling and pathway analysis. Methods: Peripheral blood was collected from 34 ischemic stroke patients (confirmed by magnetic resonance imaging) ≥18 years of age, within 24 hr of symptom onset and 24–48 hr later, and from healthy controls. The Modified Rankin Scale (MRS) was used to determine 30-day recovery. Total RNA was extracted from whole blood in Paxgene RNA tubes, amplified, and hybridized to Illumina HumanRef-8v2 bead chips. Gene expression was compared in a univariate manner between stroke patients at both time points and good versus bad outcome using t-test in GeneSpring. Inflation of Type 1 error was corrected by false discovery rate (FDR), and Ingenuity Systems Pathway analysis (IPA) was performed. A secondary validation cohort was recruited from a local hospital. Results: Three genes were significantly downregulated over time (LY96, IL8, and SDPR; FDR corrected p < .05). This finding was confirmed in a validation cohort of stroke patients (n = 8). IPA revealed cytotoxic T-lymphocyte antigen 4 (CTLA4) signaling was the most significant pathway present in the peripheral whole blood of stroke patients 24–48 hr after onset. When controlling for age and National Institutes of Health Stroke Scale score, high baseline expression of TLR2 and TLR4 significantly predicted worse scores on the MRS. Conclusion: CTLA4 signaling is a novel pathway for the study of stroke-induced immune suppression. Markers of immune dysfunction early after stroke may prove useful for identifying patients with increased risk of poor recovery.

Mitochondrial Impairment in Cerebrovascular Endothelial Cells is Involved in the Correlation between Body Temperature and Stroke Severity.

Stroke is the second leading cause of death worldwide. The prognostic influence of body temperature on acute stroke in patients has been recently reported; however, hypothermia has confounded experimental results in animal stroke models. This work aimed to investigate how body temperature could prognose stroke severity as well as reveal a possible mitochondrial mechanism in the association of body temperature and stroke severity. Lipopolysaccharide (LPS) compromises mitochondrial oxidative phosphorylation in cerebrovascular endothelial cells (CVECs) and worsens murine experimental stroke. In this study, we report that LPS (0.1 mg/kg) exacerbates stroke infarction and neurological deficits, in the mean time LPS causes temporary hypothermia in the hyperacute stage during 6 hours post-stroke. Lower body temperature is associated with worse infarction and higher neurological deficit score in the LPS-stroke study. However, warming of the LPS-stroke mice compromises animal survival. Furthermore, a high dose of LPS (2 mg/kg) worsens neurological deficits, but causes persistent severe hypothermia that conceals the LPS exacerbation of stroke infarction. Mitochondrial respiratory chain complex I inhibitor, rotenone, replicates the data profile of the LPS-stroke study. Moreover, we have confirmed that rotenone compromises mitochondrial oxidative phosphorylation in CVECs. Lastly, the pooled data analyses of a large sample size (n=353) demonstrate that stroke mice have lower body temperature compared to sham mice within 6 hours post-surgery; the body temperature is significantly correlated with stroke outcomes; linear regression shows that lower body temperature is significantly associated with higher neurological scores and larger infarct volume. We conclude that post-stroke body temperature predicts stroke severity and mitochondrial impairment in CVECs plays a pivotal role in this hypothermic response. These novel findings suggest that body temperature is prognostic for stroke severity in experimental stroke animal models and may have translational significance for clinical stroke patients - targeting endothelial mitochondria may be a clinically useful approach for stroke therapy.

TNF-ñ and Beyond: Rapid Mitochondrial Dysfunction Mediates TNF-ñ-InducedNeurotoxicity

This short communication describes our research which demonstrates that TNF-α causes a rapid decline in mitochondrial function, leading to neuronal cell death. As such, this neurotoxic proinflammatory cytokine may play a role in brain damage from stroke and neurodegeneration in chronic conditions such as Alzheimer’s disease (AD) and Parkinson’s disease. We have extended this initial observation by demonstrating that TNF-α stimulates a microRNA (miR-34a) which we have shown reduces five key proteins in the mitochondrial electron transport chain through base-pair complementarity. miR-34a is increased in affected brain regions of Alzheimer’s patients and transgenic AD mouse models. We have further shown that oligomeric amyloid beta 42 (oAβ42) stimulates miR-34a. Collectively, these data suggest that TNF-α, oAβ42, and miR-34a participate in a vicious cycle, resulting in mitochondrial dysfunction, which is critical to the neuropathology of AD.