Mitochondrial fission and fusion in secondary brain damage after CNS insults

May the force be with you: Transfer of healthy mitochondria from stem cells to stroke cells

Stroke is a major cause of death and disability in the United States and around the world with limited therapeutic option. Here, we discuss the critical role of mitochondria in stem cell-mediated rescue of stroke brain by highlighting the concept that deleting the mitochondria from stem cells abolishes the cells' regenerative potency. The application of innovative approaches entailing generation of mitochondria-voided stem cells as well as pharmacological inhibition of mitochondrial function may elucidate the mechanism underlying transfer of healthy mitochondria to ischemic cells, thereby providing key insights in the pathology and treatment of stroke and other brain disorders plagued with mitochondrial dysfunctions.

Dynamin-related protein 1: A critical protein in the pathogenesis of neural system dysfunctions and neurodegenerative diseases

Mitochondria play a key role in the maintenance of neuronal function by continuously providing energy. Here, we will give a detailed review about the recent developments in regards to dynamin-related protein 1 (Drp1) induced unbalanced mitochondrial dynamics, excessive mitochondrial division, and neuronal injury in neural system dysfunctions and neurodegenerative diseases, including the Drp1 knockout induced mice embryonic death, the dysfunction of the Drp1-dependent mitochondrial division induced neuronal cell apoptosis and impaired neuronal axonal transportation, the abnormal interaction between Drp1 and amyloid β (Aβ) in Alzheimer's disease (AD), the mutant Huntingtin (Htt) in Huntington's disease (HD), and the Drp1-associated pathogenesis of other neurodegenerative diseases such as Parkinson's disease (PD) and amyotrophic lateral sclerosis (ALS). Drp1 is required for mitochondrial division determining the size, shape, distribution, and remodeling as well as maintaining of mitochondrial integrity in mammalian cells. In addition, increasing reports indicate that the Drp1 is involved in some cellular events of neuronal cells causing some neural system dysfunctions and neurodegenerative diseases, including impaired mitochondrial dynamics, apoptosis, and several posttranslational modification induced increased mitochondrial divisions. Recent studies also revealed that the Drp1 can interact with Aβ, phosphorylated τ, and mutant Htt affecting the mitochondrial shape, size, distribution, axonal transportation, and energy production in the AD and HD neuronal cells. These changes can affect the health of mitochondria and the function of synapses causing neuronal injury and eventually leading to the dysfunction of memory, cognitive impairment, resting tremor, posture instability, involuntary movements, and progressive muscle atrophy and paralysis in patients.

Pathways to neurodegeneration: lessons learnt from unbiased genetic screens in Drosophila

Neurodegenerative diseases are a complex set of disorders that are known to be caused by environmental as well as genetic factors. In the recent past, mutations in a large number of genes have been identified that are linked to several neurodegenerative diseases. The pathogenic mechanisms in most of these disorders are unknown. Recently, studies of genes that are linked to neurodegeneration in Drosophila, the fruit flies, have contributed significantly to our understanding of mechanisms of neuroprotection and degeneration. In this review, we focus on forward genetic screens in Drosophila that helped in identification of novel genes and pathogenic mechanisms linked to neurodegeneration. We also discuss identification of four novel pathways that contribute to neurodegeneration upon mitochondrial dysfunction.

Synaptic Mitochondria are More Susceptible to Traumatic Brain Injury-induced Oxidative Damage and Respiratory Dysfunction than Non-synaptic Mitochondria

Traumatic brain injury (TBI) results in mitochondrial dysfunction and induction of lipid peroxidation (LP). Lipid peroxidation-derived neurotoxic aldehydes such as 4-HNE and acrolein bind to mitochondrial proteins, inducing additional oxidative damage and further exacerbating mitochondrial dysfunction and LP. Mitochondria are heterogeneous, consisting of both synaptic and non-synaptic populations. Synaptic mitochondria are reported to be more vulnerable to injury; however, this is the first study to characterize the temporal profile of synaptic and non-synaptic mitochondria following TBI, including investigation of respiratory dysfunction and oxidative damage to mitochondrial proteins between 3 and 120 h following injury. These results indicate that synaptic mitochondria are indeed the more vulnerable population, showing both more rapid and severe impairments than non-synaptic mitochondria. By 24 h, synaptic respiration is significantly impaired compared to synaptic sham, whereas non-synaptic respiration does not decline significantly until 48 h. Decreases in respiration are associated with increases in oxidative damage to synaptic and non-synaptic mitochondrial proteins at 48 h and 72 h, respectively. These results indicate that the therapeutic window for mitochondria-targeted pharmacological neuroprotectants to prevent respiratory dysfunction is shorter for the more vulnerable synaptic mitochondria than for the non-synaptic population.

Translational Application of Measuring Mitochondrial Functions in Blood Cells Obtained from Patients with Acute Poisoning

It is conservatively estimated that 5,000 deaths per year and 20,000 injuries in the USA are due to poisonings caused by chemical exposures (e.g., carbon monoxide, cyanide, hydrogen sulfide, phosphides) that are cellular inhibitors. These chemical agents result in mitochondrial inhibition resulting in cardiac arrest and/or shock. These cellular inhibitors have multi-organ effects, but cardiovascular collapse is the primary cause of death marked by hypotension, lactic acidosis, and cardiac arrest. The mitochondria play a central role in cellular metabolism where oxygen consumption through the electron transport system is tightly coupled to ATP production and regulated by metabolic demands. There has been increasing use of human blood cells such as peripheral blood mononuclear cells and platelets, as surrogate markers of mitochondrial function in organs due to acute care illnesses. We demonstrate the clinical applicability of measuring mitochondrial bioenergetic and dynamic function in blood cells obtained from patients with acute poisoning using carbon monoxide poisoning as an illustration of our technique. Our methods have potential application to guide therapy and gauge severity of disease in poisoning related to cellular inhibitors of public health concern.

Electroacupuncture preconditioning protects against focal cerebral ischemia/reperfusion injury via suppression of dynamin-related protein 1

Electroacupuncture preconditioning at acupoint Baihui (GV20) can reduce focal cerebral ischemia/reperfusion injury. However, the precise protective mechanism remains unknown. Mitochondrial fission mediated by dynamin-related protein 1 (Drp1) can trigger neuronal apoptosis following cerebral ischemia/reperfusion injury. Herein, we examined the hypothesis that electroacupuncture pretreatment can regulate Drp1, and thus inhibit mitochondrial fission to provide cerebral protection. Rat models of focal cerebral ischemia/reperfusion injury were established by middle cerebral artery occlusion at 24 hours after 5 consecutive days of preconditioning with electroacupuncture at GV20 (depth 2 mm, intensity 1 mA, frequency 2/15 Hz, for 30 minutes, once a day). Neurological function was assessed using the Longa neurological deficit score. Pathological changes in the ischemic penumbra on the injury side were assessed by hematoxylin-eosin staining. Cellular apoptosis in the ischemic penumbra on the injury side was assessed by terminal deoxyribonucleotidyl transferase-mediated dUTP-digoxigenin nick end labeling staining. Mitochondrial ultrastructure in the ischemic penumbra on the injury side was assessed by transmission electron microscopy. Drp1 and cytochrome c expression in the ischemic penumbra on the injury side were assessed by western blot assay. Results showed that electroacupuncture preconditioning decreased expression of total and mitochondrial Drp1, decreased expression of total and cytosolic cytochrome c, maintained mitochondrial morphology and reduced the proportion of apoptotic cells in the ischemic penumbra on the injury side, with associated improvements in neurological function. These data suggest that electroacupuncture preconditioning-induced neuronal protection involves inhibition of the expression and translocation of Drp1.

Mitochondrial dynamics and preconditioning in white matter

Mechanisms of ischemic preconditioning have been extensively studied in gray matter. However, an ischemic episode affects both the gray matter (GM) and white matter (WM) portions of the brain. Inhibition of mitochondrial fission is one of the mechanisms of preconditioning neuronal cell bodies against ischemia. Although axons are anatomical extensions of neuronal cell bodies, injury mechanisms differ between GM and WM. Indeed, axonal dysfunction is responsible for much of the disability associated with clinical deficits observed after stroke; however, the signaling process underlying preconditioning remains unexplored in axons. Using mouse optic nerve, which is a pure isolated WM tract, we show that mitochondria in myelinated axons undergo rapid and profuse fission during oxygen glucose deprivation (OGD) that is mediated by translocation of cytoplasmic Dynamin Related Protein-1 (Drp-1) to mitochondria. OGD-induced mitochondrial fission correlates with reduced mitochondrial motility and loss of axon function. Mitochondrial fragmentation and loss of motility become permanent during the recovery period. Inhibiting mitochondrial fission by administering mitochondrial division inhibitor-1 (Mdivi-1) during OGD preserves mitochondrial shape and motility and promotes axon function recovery. In contrast, preconditioning WM by applying Mdivi-1 only before OGD fails to conserve mitochondrial shape or motility and fails to benefit axon function. Our findings suggest that inhibition of mitochondrial fission during ischemia promotes axon function recovery, but is not sufficient to precondition WM against ischemia. These results raise caution in that approaches to preconditioning neuronal cell bodies may not successfully translate into functional improvement following ischemia.

Arsenite renal apoptotic effects in chickens co-aggravated by oxidative stress and inflammatory response

The kidney is the most crucial site for the excretion of arsenic and its metabolites.

YiQiFuMai Powder Injection Protects against Ischemic Stroke via Inhibiting Neuronal Apoptosis and PKCδ/Drp1-Mediated Excessive Mitochondrial Fission

YiQiFuMai (YQFM) powder injection has been reported to be used in cardiovascular and nervous system diseases with marked efficacy. However, as a treatment against diseases characterized by hypoxia, lassitude, and asthenia, the effects and underlying mechanisms of YQFM in neuronal mitochondrial function and dynamics have not been fully elucidated. Here, we demonstrated that YQFM inhibited mitochondrial apoptosis and activation of dynamin-related protein 1 (Drp1) in cerebral ischemia-injured rats, producing a significant improvement in cerebral infarction and neurological score. YQFM also attenuated oxidative stress-induced mitochondrial dysfunction and apoptosis through increasing ATP level and mitochondria membrane potential (Δψm), inhibiting ROS production, and regulating Bcl-2 family protein levels in primary cultured neurons. Moreover, YQFM inhibited excessive mitochondrial fission, Drp1 phosphorylation, and translocation from cytoplasm to mitochondria induced by oxidative stress. We provided the first evidence that YQFM inhibited the activation, association, and translocation of PKCδ and Drp1 upon oxidative stress. Taken together, we demonstrate that YQFM ameliorates ischemic stroke-induced neuronal apoptosis through inhibiting mitochondrial dysfunction and PKCδ/Drp1-mediated excessive mitochondrial fission. These findings not only put new insights into the unique neuroprotective properties of YQFM associated with the regulation of mitochondrial function but also expand our understanding of the underlying mechanisms of ischemic stroke.

How AMPK and PKA Interplay to Regulate Mitochondrial Function and Survival in Models of Ischemia and Diabetes

Adenosine monophosphate-activated protein kinase (AMPK) is a conserved, redox-activated master regulator of cell metabolism. In the presence of oxidative stress, AMPK promotes cytoprotection by enhancing the conservation of energy by suppressing protein translation and by stimulating autophagy. AMPK interplays with protein kinase A (PKA) to regulate oxidative stress, mitochondrial function, and cell survival. AMPK and dual-specificity A-kinase anchoring protein 1 (D-AKAP1), a mitochondrial-directed scaffold of PKA, interact to regulate mitochondrial function and oxidative stress in cardiac and endothelial cells. Ischemia and diabetes, a chronic disease that increases the onset of cardiovascular diseases, suppress the cardioprotective effects of AMPK and PKA. Here, we review the molecular mechanisms by which AMPK and D-AKAP1/PKA interplay to regulate mitochondrial function, oxidative stress, and signaling pathways that prime endothelial cells, cardiac cells, and neurons for cytoprotection against oxidative stress. We discuss recent literature showing how temporal dynamics and localization of activated AMPK and PKA holoenzymes play a crucial role in governing cellular bioenergetics and cell survival in models of ischemia, cardiovascular diseases, and diabetes. Finally, we propose therapeutic strategies that tout localized PKA and AMPK signaling to reverse mitochondrial dysfunction, oxidative stress, and death of neurons and cardiac and endothelial cells during ischemia and diabetes.

Mitochondrial NUDIX hydrolases: A metabolic link between NAD catabolism, GTP and mitochondrial dynamics

NAD+ catabolism and mitochondrial dynamics are important parts of normal mitochondrial function and are both reported to be disrupted in aging, neurodegenerative diseases, and acute brain injury. While both processes have been extensively studied there has been little reported on how the mechanisms of these two processes are linked. This review focuses on how downstream NAD+ catabolism via NUDIX hydrolases affects mitochondrial dynamics under pathologic conditions. Additionally, several potential targets in mitochondrial dysfunction and fragmentation are discussed, including the roles of mitochondrial poly(ADP-ribose) polymerase 1(mtPARP1), AMPK, AMP, and intra-mitochondrial GTP metabolism. Mitochondrial and cytosolic NUDIX hydrolases (NUDT9α and NUDT9β) can affect mitochondrial and cellular AMP levels by hydrolyzing ADP- ribose (ADPr) and subsequently altering the levels of GTP and ATP. Poly (ADP-ribose) polymerase 1 (PARP1) is activated after DNA damage, which depletes NAD+ pools and results in the PARylation of nuclear and mitochondrial proteins. In the mitochondria, ADP-ribosyl hydrolase-3 (ARH3) hydrolyzes PAR to ADPr, while NUDT9α metabolizes ADPr to AMP. Elevated AMP levels have been reported to reduce mitochondrial ATP production by inhibiting the adenine nucleotide translocase (ANT), allosterically activating AMPK by altering the cellular AMP: ATP ratio, and by depleting mitochondrial GTP pools by being phosphorylated by adenylate kinase 3 (AK3), which uses GTP as a phosphate donor. Recently, activated AMPK was reported to phosphorylate mitochondria fission factor (MFF), which increases Drp1 localization to the mitochondria and promotes mitochondrial fission. Moreover, the increased AK3 activity could deplete mitochondrial GTP pools and possibly inhibit normal activity of GTP-dependent fusion enzymes, thus altering mitochondrial dynamics.

To mdivi-1 or not to mdivi-1: Is that the question?

The fission/division and fusion of mitochondria are fundamental aspects of mitochondrial biology. The balance of fission and fusion sets the length of mitochondria in cells to serve their physiological requirements. The fission of mitochondria is markedly induced in many disease states and in response to cellular injury, resulting in the fragmentation of mitochondria into dysfunctional units. The mechanism that drives fission is dependent on the dynamin related protein 1 (Drp1) GTPase. mdivi-1 is a quinazolinone originally described as a selective inhibitor of Drp1, over other dynamin family members, and reported to inhibit mitochondrial fission. A recent study has challenged the activity of mdivi-1 as an inhibitor of Drp1. This study raises serious issues regarding the interpretation of data addressing the effects of mdivi-1 as reflective of the inhibition of Drp1 and thus fission. This commentary considers the evidence for and against mdivi-1 as an inhibitor of Drp1 and presents the following considerations; (1) the activity of mdivi-1 toward Drp1 GTPase activity requires further biochemical investigation, (2) as there is a large body of literature using mdivi-1 in vitro with effects as predicted for inhibition of Drp1 and mitochondrial fission, reviewed herein, the evidence is in favor of mdivi-1's originally described bioactivity, and (3) until the issue is resolved, experimental interpretations for the effects of mdivi-1 on inhibition of fission in cell and tissue experiments warrants stringent positive controls directly addressing the effects of mdivi-1 on fission. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 77: 1260-1268, 2017.

Circular RNA Expression Profiles Alter Significantly in Mouse Brain After Transient Focal Ischemia

BACKGROUND AND PURPOSE

Circular RNAs (circRNAs) are a novel class of noncoding RNAs formed from many protein-coding genes by backsplicing. Although their physiological functions are not yet completely defined, they are thought to control transcription, translation, and microRNA levels. We investigated whether stroke changes the circRNAs expression profile in the mouse brain.

METHODS

Male C57BL/6J mice were subjected to transient middle cerebral artery occlusion, and circRNA expression profile was evaluated in the penumbral cortex at 6, 12, and 24 hours of reperfusion using circRNA microarrays and real-time PCR. Bioinformatics analysis was conducted to identify microRNA binding sites, transcription factor binding, and gene ontology of circRNAs altered after ischemia.

RESULTS

One thousand three-hundred twenty circRNAs were expressed at detectable levels mostly from exonic (1064) regions of the genes in the cerebral cortex of sham animals. Of those, 283 were altered (>2-fold) at least at one of the reperfusion time points, whereas 16 were altered at all 3 time points of reperfusion after transient middle cerebral artery occlusion compared with sham. Postischemic changes in circRNAs identified by microarray analysis were confirmed by real-time PCR. Bioinformatics showed that these 16 circRNAs contain binding sites for many microRNAs. Promoter analysis showed that the circRNAs altered after stroke might be controlled by a set of transcription factors. The major biological and molecular functions controlled by circRNAs altered after transient middle cerebral artery occlusion are biological regulation, metabolic process, cell communication, and binding to proteins, ions, and nucleic acids.

CONCLUSIONS

This is a first study that shows that stroke alters the expression of circRNAs with possible functional implication to poststroke pathophysiology.

Extracellular Mitochondria in Cerebrospinal Fluid and Neurological Recovery After Subarachnoid Hemorrhage

BACKGROUND AND PURPOSE

Recent studies suggest that extracellular mitochondria may be involved in the pathophysiology of stroke. In this study, we assessed the functional relevance of endogenous extracellular mitochondria in cerebrospinal fluid (CSF) in rats and humans after subarachnoid hemorrhage (SAH).

METHODS

A standard rat model of SAH was used, where an intraluminal suture was used to perforate a cerebral artery, thus leading to blood extravasation into subarachnoid space. At 24 and 72 hours after SAH, neurological outcomes were measured, and the standard JC1 (5,5',6,6'-tetrachloro-1,1',3,3'-tetraethyl-benzimidazolylcarbocyanineiodide) assay was used to quantify mitochondrial membrane potentials in the CSF. To further support the rat model experiments, CSF samples were obtained from 41 patients with SAH and 27 control subjects. Mitochondrial membrane potentials were measured with the JC1 assay, and correlations with clinical outcomes were assessed at 3 months.

RESULTS

In the standard rat model of SAH, extracellular mitochondria was detected in CSF at 24 and 72 hours after injury. JC1 assays demonstrated that mitochondrial membrane potentials in CSF were decreased after SAH compared with sham-operated controls. In human CSF samples, extracellular mitochondria were also detected, and JC1 levels were also reduced after SAH. Furthermore, higher mitochondrial membrane potentials in the CSF were correlated with good clinical recovery at 3 months after SAH onset.

CONCLUSIONS

This proof-of-concept study suggests that extracellular mitochondria may provide a biomarker-like glimpse into brain integrity and recovery after injury.

Haploinsufficiency in the mitochondrial protein CHCHD4 reduces brain injury in a mouse model of neonatal hypoxia-ischemia

Mitochondria contribute to neonatal hypoxic-ischemic brain injury by releasing potentially toxic proteins into the cytosol. CHCHD4 is a mitochondrial intermembrane space protein that plays a major role in the import of intermembrane proteins and physically interacts with apoptosis-inducing factor (AIF). The purpose of this study was to investigate the impact of CHCHD4 haploinsufficiency on mitochondrial function and brain injury after cerebral hypoxia-ischemia (HI) in neonatal mice. CHCHD4+/- and wild-type littermate mouse pups were subjected to unilateral cerebral HI on postnatal day 9. CHCHD4 haploinsufficiency reduced insult-related AIF and superoxide dismutase 2 release from the mitochondria and reduced neuronal cell death. The total brain injury volume was reduced by 21.5% at 3 days and by 31.3% at 4 weeks after HI in CHCHD4+/- mice. However, CHCHD4 haploinsufficiency had no influence on mitochondrial biogenesis, fusion, or fission; neural stem cell proliferation; or neural progenitor cell differentiation. There were no significant changes in the expression or distribution of p53 protein or p53 pathway-related genes under physiological conditions or after HI. These results suggest that CHCHD4 haploinsufficiency afforded persistent neuroprotection related to reduced release of mitochondrial intermembrane space proteins. The CHCHD4-dependent import pathway might thus be a potential therapeutic target for preventing or treating neonatal brain injury.

AMPK: keeping the (power)house in order?

Metabolically energetic organs, such as the brain, require a reliable source of ATP, the majority of which is provided by oxidative phosphorylation in the mitochondrial matrix. Maintaining mitochondrial integrity is therefore of paramount importance in highly specialized cells such as neurons. Beyond acting as cellular 'power stations' and initiators of apoptosis, neuronal mitochondria are highly mobile, transported to pre- and post-synaptic sites for rapid, localized ATP production, serve to buffer physiological and pathological calcium and contribute to dendritic arborization. Given such roles, it is perhaps unsurprising that recent studies implicate AMP-activated protein kinase (AMPK), a cellular energy-sensitive metabolic regulator, in triggering mitochondrial fission, potentially balancing mitochondrial dynamics, biogenesis and mitophagy.