The role of cardiac mitochondria in myocardial ischemia/reperfusion injury

Cardiovascular disease (CVD) exerts economic and humanitarian costs that are unparalleled by any other disease. Of the many etiologies of CVD, myocardial infarction accounts for over 50% of the associated mortality and anything that can decrease the extent of infarction could drastically impact the burden of CVD. The purpose of this work was to further our understanding of the role of cardiac mitochondria in ischemia/reperfusion injury. Herein I found that physiologic (exercise) and pharmacologic (Bendavia) interventions that lessen the oxidative burden during ischemia/reperfusion have the potential to limit myocardial infarction. Under conditions of oxidative stress, animals who received short term exercise (Ex) were better able to maintain the glutathione couple in a reduced state, likely through an increase in glutathione reductase (GR) activity. This phenotypic change was associated with decreased reactive oxygen species (ROS) accumulation and a lower incidence of fatal ventricular arrhythmias. Furthermore, I found that ROS generated within the cytosol, and not the mitochondria, during bouts of Ex are important signaling molecules that increase GR activity and this increased activity may be responsible for the cardioprotective effects observed with Ex. Finally, I found that treatment with the mitochondrially-targeted peptide Bendavia was successful at lowering infarct size in isolated guinea pig hearts, due to an ability to decrease ROS accumulation and maintain mitochondrial energetics. Taken together, these studies suggest that therapies aimed at decreasing mitochondrial ROS and/or maintaining mitochondrial energetics during ischemia/reperfusion may have significant clinical impact.  Ph.D

Reduction of Ischemia/Reperfusion Injury With Bendavia, a Mitochondria-Targeting Cytoprotective Peptide

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The genetic architecture of the human cerebral cortex

The cerebral cortex underlies our complex cognitive capabilities, yet little is known about the specific genetic loci that influence human cortical structure. To identify genetic variants that affect cortical structure, we conducted a genome-wide association meta-analysis of brain magnetic resonance imaging data from 51,665 individuals. We analyzed the surface area and average thickness of the whole cortex and 34 regions with known functional specializations. We identified 199 significant loci and found significant enrichment for loci influencing total surface area within regulatory elements that are active during prenatal cortical development, supporting the radial unit hypothesis. Loci that affect regional surface area cluster near genes in Wnt signaling pathways, which influence progenitor expansion and areal identity. Variation in cortical structure is genetically correlated with cognitive function, Parkinson's disease, insomnia, depression, neuroticism, and attention deficit hyperactivity disorder

The role of cardiac mitochondria in myocardial ischemia/reperfusion injury

Cardiovascular disease (CVD) exerts economic and humanitarian costs that are unparalleled by any other disease. Of the many etiologies of CVD myocardial infarction accounts for over 50% of the associated mortality and anything that can decrease the extent of infarction could drastically impact the burden of CVD. The purpose of this work was to further our understanding of the role of cardiac mitochondria in ischemia/reperfusion injury. Herein I found that physiologic (exercise) and pharmacologic (Bendavia) interventions that lessen the oxidative burden during ischemia/reperfusion have the potential to limit myocardial infarction. Under conditions of oxidative stress animals who received short term exercise (Ex) were better able to maintain the glutathione couple in a reduced state likely through an increase in glutathione reductase (GR) activity. This phenotypic change was associated with decreased reactive oxygen species (ROS) accumulation and a lower incidence of fatal ventricular arrhythmias. Furthermore I found that ROS generated within the cytosol and not the mitochondria during bouts of Ex are important signaling molecules that increase GR activity and this increased activity may be responsible for the cardioprotective effects observed with Ex. Finally I found that treatment with the mitochondrially-targeted peptide Bendavia was successful at lowering infarct size in isolated guinea pig hearts due to an ability to decrease ROS accumulation and maintain mitochondrial energetics. Taken together these studies suggest that therapies aimed at decreasing mitochondrial ROS and/or maintaining mitochondrial energetics during ischemia/reperfusion may have significant clinical impact.

Decreased Reactive Oxygen Species Buffering Capacity May Underlie Arrhythmia Susceptibility in the Scn1b-/- Mouse Model of Dravet Syndrome

Dravet syndrome (DS) is a severe pediatric-onset epilepsy disorder that mostly arises from loss-of-function mutations in voltage-gated sodium channel genes. Patients with DS have an elevated risk (~17%) of Sudden Unexpected Death in Epilepsy (SUDEP), a fatal complication of seizure disorders. While the exact physiological mechanisms underlying SUDEP remain unclear, cardiac arrhythmias have been heavily implicated. Previous work indicates that reactive oxygen species (ROS) accumulation during oxidative stress, a condition where ROS generation exceeds antioxidant capacity, can lead to mitochondrial instability and cardiac arrhythmias. The purpose of this investigation was to determine how ROS scavenging and antioxidant levels may be altered in the Scn1b-/- mouse model of DS. In the heart, the primary antioxidant pathway is the glutathione (GSH) system. This study tested the hypothesis that Scn1b-/- (KO) mice are more susceptible to arrhythmias due to a decreased ability to buffer ROS accumulations during prolonged oxidative stress through the GSH system. First, qPCR analysis was conducted on hearts from KO and Scn1b+/+ (WT) mice to determine differences in gene expression of key enzymes in the GSH system, glutathione peroxidase (Gpx) and glutathione reductase (Gsr). Gpx is responsible for the reduction of H2O2 to H2O. Gsr maintains GSH in the reduced state. In P17 (after seizure onset) KO mice, Gpx expression was decreased (0.27-fold; p = 0.03) but Gsr remained unchanged. The enzymatic activity of GPx and GR were also measured. Counter to the qPCR data, the results suggested that there are no differences in enzyme activity in KO mice. To determine if there were differences at the cellular level, isolated cardiac cells were subjected to a prolonged oxidative challenge. Fluorescence microscopy was used to assess changes in the signal of CM-DCF, a fluorescent ROS indicator, following the addition of 80 mM diamide to cells. Diamide causes oxidative stress by depleting intracellular stores of GSH. Cells isolated from KO mice subject to prolonged oxidative stress die at much faster rates than cells isolated from WT mice. In addition, death occurs at earlier timepoints (as early as 1 min). The mean time to death in KO cells was significantly shorter (p = 0.03) and occurred on average 4.25 minutes faster than in WT cells. In addition, CM-DCF signals intensified shortly before cell death, indicating ROS accumulation precedes cell death. Finally, to test for arrhythmia susceptibility at the whole organ level, isolated hearts were perfused with diamide and scored for arrhythmia susceptibility and severity. Over the first 15 and 30 minutes of perfusion KO hearts had significantly higher scores. In conclusion, the results of this study indicate that hearts from Scn1b-/- mice have a decreased capability to handle ROS accumulations during prolonged oxidative stress. This may be the result of deficits in the GSH system that compromise ROS scavenging in the heart

Heart Failure and Diabetes: Role of ATM

Heart failure is a leading cause of death in the United States. Diabetes, also known as diabetes mellitus (DM), exponentially increases the risk of heart failure. The increase in oxidative stress and metabolic dysfunction caused by DM can lead to DNA damage and the development of diabetic cardiomyopathy. Ataxia telangiectasia mutated kinase (ATM) is a DNA damage response protein with a primary nuclear function to regulate cell cycle progression in response to double-strand DNA breaks, acts as a redox sensor, and facilitates DNA repair. ATM deficiency associates with the development of insulin resistance and DM. Consequently, patients with Ataxia telangiectasia, a rare autosomal recessive disorder, have an increased risk of developing heart failure. The main objective of this review is to summarize the shared metabolic and cardiac abnormalities associated with DM and ATM deficiency, with a focus on the development of heart failure

Circadian Effects of Acebutolol Administration in the Scn1b-/- Dravet Syndrome Mouse Model

Dravet syndrome (DS) is a severe form of pediatric epilepsy with characterizations of pharmacoresistant seizures and developmental delay. A rarer variant of the DS model is caused by heterozygous loss-of-function mutations in SCN1B, which is essential in regulating sodium channel gating, expression, localization, and the firing of action potentials. Mutations in SCN1B result in severe seizures as well as a higher risk of Sudden Unexpected Death in EPilepsy (SUDEP). Factors underlying SUDEP are poorly understood, although cardiac arrythmias have been implicated. Acebutolol (ACE) is a common beta-blocker used in the treatment of arrhythmias and hypertension. We hypothesized that treating mice with ACE will decrease cardiac arrhythmias and the incidence of SUDEP, prolonging lifespan of Scn1b null mice. Wild-type (WT) and null (KO) mice were given daily injections of 10 mg/kg ACE or saline starting at postnatal day 15 (after typical seizure onset) either during the day (09:00) or at night (21:00). ECG was recorded daily including a baseline and a 20-minute post injection measurement to analyze the long-term and acute effects of treatment. 20 minutes following ACE injection, KO mice displayed a significant reduction in heart rate compared to WT (38% vs. 11%). Interestingly, HR the day prior to death consistently dropped ~50% (average 414 bpm to 193 bpm) in our saline group; this was prevented in KO animals treated with ACE (421 bpm). A modest, but significant, increase in survival curves in our KO animals was observed compared to saline treated mice for those given injections during the day (a 2 day increase in median survival). In addition, in this group, the onset of animal death was delayed. Surprisingly, in the mice injected during the night hours, there was a trend towards a decrease in lifespan. From these findings there is a notable hypersensitivity to ACE in this DS model. Leading up to death, we believe it is possible ACE assisted in decreased cardiovascular deficits that could lead to SUDEP and contributed to the modest increased lifespan. While we are still seeing death in the ACE treated group because of unnoticed, prolonged seizures, or other mechanisms of SUDEP. In addition, our results demonstrate the importance of timing in delivery of drugs targeted at SUDEP. Further work includes testing this hypothesis by adding 24 hour drug delivery or an anti-epileptic drug to see if lifespan is further affected is warranted

In a mouse model of Dravet Syndrome, mitochondrial dysfunction may contribute to SUDEP.

Dravet syndrome (DS) is a severe, pediatric-onset epilepsy disorder linked to loss-of-function mutations in the sodium channel gene SCN1B. DS patients have a high risk of Sudden Unexpected Death in Epilepsy (SUDEP). Cardiac arrhythmias have been implicated as a potential cause underlying SUDEP. An exact pathway for how mutations in SCN1B leads to arrhythmia in DS is unclear. One cellular component linked to regulation of cardiac homeostasis are mitochondria, known as “the powerhouse of the cell” due to their ability to produce cellular energy (ATP) via the electron transport chain (ETC). The ETC is a major producer of reactive oxygen species (ROS). Typically, ROS are buffered by cellular antioxidants, to prevent oxidative stress, an imbalance of ROS that can lead to cell damage. Our previous work indicates that cardiac arrhythmias may result from mitochondrial instability and imbalances between ROS production and buffering. We analyzed whether Scn1b-/-mice are susceptible to arrhythmias due to altered mitochondrial ATP generation, ROS production, and compromised cellular antioxidant defenses. We isolated cardiac mitochondria from postnatal day (P) 15-20 KO and Scn1b+/+ (WT) mice. To assess mitochondrial ATP and ROS production, high-resolution respirometry (O2k, Oroboros) was used to measure mitochondrial O2 and H2O2 flux. We used a substrate-uncoupler inhibitor (SUIT) protocol to elucidate flux under different ETC pathways, including Complex I- and II-linked respiration. As a next step, we evaluated expression of superoxide dismutase (Sod) proteins associated with mitochondrial antioxidant defenses, including Cu/Zn-Sod (Sod1) and Mn-Sod (Sod2) in hearts from KO and WT mice pre- (P10) and post- (P17) seizure development. After addition of substrates supporting Complex-II linked respiration (succinate, ADP) there were no differences in O2 flux between mitochondria isolated from KO and WT hearts. Upon further addition of pyruvate to mitochondria to stimulate Complex I, O2 flux was significantly reduced (p \u3c 0.0001) in mitochondria from KO mice, when compared to WT. Moreover, upon titration of rotenone (a Complex I inhibitor) its negative effect on O2 flux was not as substantial in KO mitochondria as in WT, suggesting that mitochondria from KO have deficits in Complex-I linked respiration. Furthermore, we detected significant differences in ROS production by mitochondria isolated from KO animals. Under conditions of reverse electron flow (succinate as substrate), a state where ROS production is highest, H2O2 flux was elevated significantly (p = 0.048) in mitochondria isolated from KO mice, compared to those isolated from WT. During our analysis of Sod expression, we found that Sod1 (p = 0.01) and Sod2 (p = 0.01) expression is significantly decreased at P17 in KO hearts compared to WT. Overall, our results suggest imbalances between mitochondrial activity and antioxidant defenses, which may underlie increased arrhythmia susceptibility in KO mice

Hypersensitive and Circadian Effects of Acebutolol Administration in Scn1b-/- Mice

Title: Hypersensitive and circadian effects of acebutolol administration in Scn1b-/- mice. Rationale: Dravet syndrome (DS) is a severe form of pediatric epilepsy with characterizations of pharmacoresistant seizures and developmental delay. A rarer variant of the DS model is caused by homozygous loss-of-function mutations in SCN1B, which is essential in regulating sodium channel gating, expression, localization, and the firing of action potentials. Mutations in SCN1B result in severe seizures as well as a higher risk of Sudden Unexpected Death in EPilepsy (SUDEP). Factors underlying SUDEP are poorly understood, although cardiac arrhythmias have been implicated. Acebutolol (ACE) is a common beta-blocker used in the treatment of arrhythmias and hypertension. We hypothesized that treating mice with ACE will decrease cardiac arrhythmias and the incidence of SUDEP, prolonging lifespan of Scn1b null mice. Methods: Wild-type (WT) and null (KO) mice were given daily injections of 10 mg/kg ACE or saline starting at postnatal day 15 (after typical seizure onset) either during the day (09:00) or at night (21:00). In the day group, ECG was recorded daily from P13 until animal death. Starting at P15 mice were recorded both pre- and post- injection to analyze the long-term and acute effects of treatment. Results: A modest, but significant, increase in survival curves in our KO animals was observed compared to saline treated mice for those given injections during the day (a 2 day increase in median survival). In addition, in this group, the onset of animal death was delayed. To investigate the timing of drug delivery, a subset of mice was given injections at night. In this group there was actually a decrease in lifespan, with an earlier onset of death compared to saline treated mice. On a daily basis from P13, the heart rate (HR) of KO mice was significantly lower than WT but remained steady until the day prior to animal death. HR the day prior to death consistently dropped ~50% (average 414 bpm to 193 bpm) in our saline group; this was prevented in KO animals treated with ACE (421 bpm). Analysis of acute recordings following ACE administration showed that KO mice had a significantly larger reduction in heart rate compared to WT (38% vs. 11%). Further analysis of heart rate variability in these recordings demonstrated that RMSSD (a measure of vagal control of the heart) was reduced in KO mice, with differences in both baseline and following ACE administration. Conclusions: Leading up to death, we believe it is possible ACE assisted in decreased cardiovascular deficits that could lead to SUDEP and contributed to the modestly increased lifespan. In addition, our results demonstrate the importance of timing in delivery of drugs targeted at SUDEP. Finally, these results suggest that there is a possible hypersensitivity to beta-adrenergic blockade in Scn1b-/- mice. Funding: This work was supported by a grant from the Research Development Committee at East Tennessee State University and NIH grant R21NS116647 to C.R.F

Teriflunomide Treatment Exacerbates Cardiac Ischemia Reperfusion Injury in Isolated Rat Hearts

PURPOSE: Previous work suggests that Dihydroorotate dehydrogenase (DHODH) inhibition via teriflunomide (TERI) may provide protection in multiple disease models. To date, little is known about the effect of TERI on the heart. This study was performed to assess the potential effects of TERI on cardiac ischemia reperfusion injury. METHODS: Male and female rat hearts were subjected to global ischemia (25 min) and reperfusion (120 min) on a Langendorff apparatus. Hearts were given either DMSO (VEH) or teriflunomide (TERI) for 5 min prior to induction of ischemia and during the reperfusion period. Left ventricular pressure, ECG, coronary flow, and infarct size were determined using established methods. Mitochondrial respiration was assessed via respirometry. RESULTS: Perfusion of hearts with TERI led to no acute effects in any values measured across 500 pM-50 nM doses. However, following ischemia-reperfusion injury, we found that 50 nM TERI-treated hearts had an increase in myocardial infarction (p \u3c 0.001). In 50 nM TERI-treated hearts, we also observed a marked increase in the severity of contracture (p \u3c 0.001) at an earlier time-point (p = 0.004), as well as reductions in coronary flow (p = 0.037), left ventricular pressure development (p = 0.025), and the rate-pressure product (p = 0.008). No differences in mitochondrial respiration were observed with 50 nM TERI treatment (p = 0.24-0.87). CONCLUSION: This study suggests that treatment with TERI leads to more negative outcomes following cardiac ischemia reperfusion, and administration of TERI to at-risk populations should receive special considerations