Reactive gamma-ketoaldehydes formed via the isoprostane pathway disrupt mitochondrial respiration and calcium homeostasis

Isolevuglandins Promote Mitochondrial Dysfunction and Electrophysiologic Abnormalities in Atrial Cardiomyocytes

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia, yet the cellular and molecular mechanisms underlying the AF substrate remain unclear. Isolevuglandins (IsoLGs) are highly reactive lipid dicarbonyl products that mediate oxidative stress-related injury. In murine hypertension, the lipid dicarbonyl scavenger 2-hydroxybenzylamine (2-HOBA) reduced IsoLGs and AF susceptibility. We hypothesized that IsoLGs mediate detrimental pathophysiologic effects in atrial cardiomyocytes that promote the AF substrate. Using Seahorse XFp extracellular flux analysis and a luminescence assay, IsoLG exposure suppressed intracellular ATP production in atrial HL-1 cardiomyocytes. IsoLGs caused mitochondrial dysfunction, with reduced mitochondrial membrane potential, increased mitochondrial reactive oxygen species (ROS) with protein carbonylation, and mitochondrial DNA damage. Moreover, they generated cytosolic preamyloid oligomers previously shown to cause similar detrimental effects in atrial cells. In mouse atrial and HL-1 cells, patch clamp experiments demonstrated that IsoLGs rapidly altered action potentials (AP), implying a direct effect independent of oligomer formation by reducing the maximum Phase 0 upstroke slope and shortening AP duration due to ionic current modifications. IsoLG-mediated mitochondrial and electrophysiologic abnormalities were blunted or totally prevented by 2-HOBA. These findings identify IsoLGs as novel mediators of oxidative stress-dependent atrial pathophysiology and support the investigation of dicarbonyl scavengers as a novel therapeutic approach to prevent AF.

The MT1 receptor as the target of ramelteon neuroprotection in ischemic stroke

Stroke is the leading cause of death and disability worldwide. Novel and effective therapies for ischemic stroke are urgently needed. Here, we report that melatonin receptor 1A (MT1) agonist ramelteon is a neuroprotective drug candidate as demonstrated by comprehensive experimental models of ischemic stroke, including a middle cerebral artery occlusion (MCAO) mouse model of cerebral ischemia in vivo, organotypic hippocampal slice cultures ex vivo, and cultured neurons in vitro; the neuroprotective effects of ramelteon are diminished in MT1-knockout (KO) mice and MT1-KO cultured neurons. For the first time, we report that the MT1 receptor is significantly depleted in the brain of MCAO mice, and ramelteon treatment significantly recovers the brain MT1 losses in MCAO mice, which is further explained by the Connectivity Map L1000 bioinformatic analysis that shows gene-expression signatures of MCAO mice are negatively connected to melatonin receptor agonist like Ramelteon. We demonstrate that ramelteon improves the cerebral blood flow signals in ischemic stroke that is potentially mediated, at least, partly by mechanisms of activating endothelial nitric oxide synthase. Our results also show that the neuroprotection of ramelteon counteracts reactive oxygen species-induced oxidative stress and activates the nuclear factor erythroid 2-related factor 2/heme oxygenase-1 pathway. Ramelteon inhibits the mitochondrial and autophagic death pathways in MCAO mice and cultured neurons, consistent with gene set enrichment analysis from a bioinformatics perspective angle. Our data suggest that Ramelteon is a potential neuroprotective drug candidate, and MT1 is the neuroprotective target for ischemic stroke, which provides new insights into stroke therapy. MT1-KO mice and cultured neurons may provide animal and cellular models of accelerated ischemic damage and neuronal cell death.

An umpolung strategy for chemically selective intermolecular cross-enolate-type coupling of N-alkenoxypyridinium salts with aldehydes

An efficient method for the synthesis of 1,4-ketoaldehydes via the cross-coupling of N-alkenoxyheteroarenium salts and primary aldehydes is developed. This method provides a broad substrate scope and excellent functional group compatibility. The utility of this method is demonstrated via the diverse transformations of heterocyclic compounds and cycloheptanone, as well as the late-stage functionalization of biorelevant molecules.

A Paal-Knorr agent for chemoproteomic profiling of targets of isoketals in cells

Natural systems produce various γ-dicarbonyl-bearing compounds that can covalently modify lysine in protein targets via the classic Paal-Knorr reaction. Among them is a unique class of lipid-derived electrophiles - isoketals that exhibit high chemical reactivity and critical biological functions. However, their target selectivity and profiles in complex proteomes remain unknown. Here we report a Paal-Knorr agent, 4-oxonon-8-ynal (herein termed ONAyne), for surveying the reactivity and selectivity of the γ-dicarbonyl warhead in biological systems. Using an unbiased open-search strategy, we demonstrated the lysine specificity of ONAyne on a proteome-wide scale and characterized six probe-derived modifications, including the initial pyrrole adduct and its oxidative products (i.e., lactam and hydroxylactam adducts), an enlactam adduct from dehydration of hydroxylactam, and two chemotypes formed in the presence of endogenous formaldehyde (i.e., fulvene and aldehyde adducts). Furthermore, combined with quantitative chemoproteomics in a competitive format, ONAyne permitted global, in situ, and site-specific profiling of targeted lysine residues of two specific isomers of isoketals, levuglandin (LG) D2 and E2. The functional analyses reveal that LG-derived adduction drives inhibition of malate dehydrogenase MDH2 and exhibits a crosstalk with two epigenetic marks on histone H2B in macrophages. Our approach should be broadly useful for target profiling of bioactive γ-dicarbonyls in diverse biological contexts.

Qualitative Characterization of the Rat Liver Mitochondrial Lipidome Using All Ion Fragmentation on an Exactive Benchtop Orbitrap MS

Untargeted lipidomics profiling by liquid chromatography -mass spectrometry (LC-MS) allows researchers to observe the occurrences of lipids in a biological sample without showing intentional bias to any specific class of lipids and allows retrospective reanalysis of data collected. Typically, and in the specific method described, a general extraction method followed by LC separation is used to achieve nonspecific class coverage of the lipidome prior to high resolution accurate mass (HRAM) MS detection . Here we describe a workflow including the isolation of mitochondria from liver tissue, followed by mitochondrial lipid extraction and the LC-MS conditions used for data acquisition. We also highlight how, in this method, all ion fragmentation can be used to identify species of lower abundances, often missed by data dependent fragmentation techniques. Here we describe the isolation of mitochondria from liver tissue, followed by mitochondrial lipid extraction and the LC-MS conditions used for data acquisition.

Isolevuglandins (isoLGs) as toxic lipid peroxidation byproducts and their pathogenetic role in human diseases

Lipid peroxidation results in generation of a variety of lipid hydroperoxides and other highly reactive species that covalently modify proteins, nucleic acids, and other lipids, thus resulting in lipotoxicity. Although biological relevance of 4-hydroxynonenal (4-HNE) and malondialdehyde (MDA) is well studied, the existing data on the role of isolevuglandins (isoLGs) in pathology are insufficient. Therefore, the objective of the present study was to review the existing data on biological effects of isoLG and isoLG adducts and their role in multiple diseases. Sixty four highly reactive levuglandin-like γ-ketoaldehyde (γ-KA, or isoketals, IsoK, or isolevuglandins, IsoLG) regio- and stereo-isomers are formed as products of arachidonic acid oxidation. IsoLGs react covalently with lysyl residues of proteins to form a stable adduct and intramolecular aminal, bispyrrole, and trispyrrole cross-links. Phosphatidylethanolamine was also shown to be the target for isoLG binding as compared to proteins and DNA. Free IsoLGs are not detectable in vivo, although isolevuglandin adduction to amino acid residues of particular proteins may be evaluated with liquid chromatography-tandem mass spectrometry. Adducts formed were shown to play a significant role in the development and maintenance of oxidative stress, endoplasmic reticulum stress, mitochondrial dysfunction, and inflammation. These, and more specific molecular pathways, link isoLG and isoLG-adduct formation to develop a variety of pathologies, including cardiovascular diseases (atherosclerosis, hypertension, heart failure), obesity and diabetes, cancer, neurodegeneration, eye diseases (retinal degeneration and glaucoma), as well as ageing. Hypothetically, isoLGs and isoLG adduct formation may be considered as the potential target for treatment of oxidative stress-related diseases.

Mitochondrial Isolevuglandins Contribute to Vascular Oxidative Stress and Mitochondria-Targeted Scavenger of Isolevuglandins Reduces Mitochondrial Dysfunction and Hypertension

Hypertension remains a major health problem in Western Societies, and blood pressure is poorly controlled in a third of patients despite use of multiple drugs. Mitochondrial dysfunction contributes to hypertension, and mitochondria-targeted agents can potentially improve treatment of hypertension. We have proposed that mitochondrial oxidative stress produces reactive dicarbonyl lipid peroxidation products, isolevuglandins, and that scavenging of mitochondrial isolevuglandins improves vascular function and reduces hypertension. To test this hypothesis, we have studied the accumulation of mitochondrial isolevuglandins-protein adducts in patients with essential hypertension and Ang II (angiotensin II) model of hypertension using mass spectrometry and Western blot analysis. The therapeutic potential of targeting mitochondrial isolevuglandins was tested by the novel mitochondria-targeted isolevuglandin scavenger, mito2HOBA. Mitochondrial isolevuglandins in arterioles from hypertensive patients were 250% greater than in arterioles from normotensive subjects, and ex vivo mito2HOBA treatment of arterioles from hypertensive subjects increased deacetylation of a key mitochondrial antioxidant, SOD2 (superoxide dismutase 2). In human aortic endothelial cells stimulated with Ang II plus TNF (tumor necrosis factor)-α, mito2HOBA reduced mitochondrial superoxide and cardiolipin oxidation, a specific marker of mitochondrial oxidative stress. In Ang II-infused mice, mito2HOBA diminished mitochondrial isolevuglandins-protein adducts, raised Sirt3 (sirtuin 3) mitochondrial deacetylase activity, reduced vascular superoxide, increased endothelial nitric oxide, improved endothelium-dependent relaxation, and attenuated hypertension. Mito2HOBA preserved mitochondrial respiration, protected ATP production, and reduced mitochondrial permeability pore opening in Ang II-infused mice. These data support the role of mitochondrial isolevuglandins in endothelial dysfunction and hypertension. We conclude that scavenging of mitochondrial isolevuglandins may have therapeutic potential in treatment of vascular dysfunction and hypertension.

Scavenging Reactive Lipids to Prevent Oxidative Injury

Oxidative injury due to elevated levels of reactive oxygen species is implicated in cardiovascular diseases, Alzheimer's disease, lung and liver diseases, and many cancers. Antioxidant therapies have generally been ineffective at treating these diseases, potentially due to ineffective doses but also due to interference with critical host defense and signaling processes. Therefore, alternative strategies to prevent oxidative injury are needed. Elevated levels of reactive oxygen species induce lipid peroxidation, generating reactive lipid dicarbonyls. These lipid oxidation products may be the most salient mediators of oxidative injury, as they cause cellular and organ dysfunction by adducting to proteins, lipids, and DNA. Small-molecule compounds have been developed in the past decade to selectively and effectively scavenge these reactive lipid dicarbonyls. This review outlines evidence supporting the role of lipid dicarbonyls in disease pathogenesis, as well as preclinical data supporting the efficacy of novel dicarbonyl scavengers in treating or preventing disease.

Highly Reactive Isolevuglandins Promote Atrial Fibrillation Caused by Hypertension

Oxidative damage is implicated in atrial fibrillation (AF), but antioxidants are ineffective therapeutically. The authors tested the hypothesis that highly reactive lipid dicarbonyl metabolites, or isolevuglandins (IsoLGs), are principal drivers of AF during hypertension. In a hypertensive murine model and stretched atriomyocytes, the dicarbonyl scavenger 2-hydroxybenzylamine (2-HOBA) prevented IsoLG adducts and preamyloid oligomers (PAOs), and AF susceptibility, whereas the ineffective analog 4-hydroxybenzylamine (4-HOBA) had minimal effect. Natriuretic peptides generated cytotoxic oligomers, a process accelerated by IsoLGs, contributing to atrial PAO formation. These findings support the concept of pre-emptively scavenging reactive downstream oxidative stress mediators as a potential therapeutic approach to prevent AF.

Targeting of reactive isolevuglandins in mitochondrial dysfunction and inflammation

Inflammation is a major cause of morbidity and mortality in Western societies. Despite use of multiple drugs, both chronic and acute inflammation still represent major health burdens. Inflammation produces highly reactive dicarbonyl lipid peroxidation products such as isolevuglandins which covalently modify and cross-link proteins via lysine residues. Mitochondrial dysfunction has been associated with inflammation; however, its molecular mechanisms and pathophysiological role are still obscure. We hypothesized that inflammation-induced isolevuglandins contribute to mitochondrial dysfunction and mortality. To test this hypothesis, we have (a) investigated the mitochondrial dysfunction in response to synthetic 15-E2-isolevuglandin (IsoLG) and its adducts; (b) developed a new mitochondria-targeted scavenger of isolevuglandins by conjugating 2-hydroxybenzylamine to the lipophilic cation triphenylphosphonium, (4-(4-aminomethyl)-3-hydroxyphenoxy)butyl)-triphenylphosphonium (mito2HOBA); (c) tested if mito2HOBA protects from mitochondrial dysfunction and mortality using a lipopolysaccharide model of inflammation. Acute exposure to either IsoLG or IsoLG adducts with lysine, ethanolamine or phosphatidylethanolamine inhibits mitochondrial respiration and attenuates Complex I activity. Complex II function was much more resistant to IsoLG. We confirmed that mito2HOBA markedly accumulates in isolated mitochondria and it is highly reactive with IsoLGs. To test the role of mitochondrial IsoLGs, we studied the therapeutic potential of mito2HOBA in lipopolysaccharide mouse model of sepsis. Mito2HOBA supplementation in drinking water (0.1 g/L) to lipopolysaccharide treated mice increased survival by 3-fold, improved complex I-mediated respiration, and histopathological analyses supported mito2HOBA-mediated protection of renal cortex from cell injury. These data support the role of mitochondrial IsoLG in mitochondrial dysfunction and inflammation. We conclude that reducing mitochondrial IsoLGs may be a promising therapeutic target in inflammation and conditions associated with mitochondrial oxidative stress and dysfunction.

Measuring Mitochondrial Dysfunction Caused by Soluble α-Synuclein Oligomers

Accumulation of misfolded αSyn and mitochondrial dysfunction are central features of Parkinson's disease. Growing evidence points to a relationship between these two phenomena as oligomeric α-synuclein (αSyn) can interact with mitochondria and impair their function. Standardization of methods to prepare αSyn oligomers and isolate functional mitochondria will facilitate efforts to expand upon early findings. Here we present detailed protocols for preparing soluble αSyn oligomers; for isolating functional mitochondria from mouse tissue; and for simultaneously measuring several aspects of mitochondrial physiology. These protocols will benefit future studies aimed at characterizing the mitotoxicity of αSyn species isolated from the brains of synucleinopathy patients as well as efforts to identify small molecules and genetic or environmental alterations that prevent αSyn-induced mitochondrial dysfunction.

Isolevuglandins and cardiovascular disease

Isolevuglandins are 4-ketoaldehydes formed by peroxidation of arachidonic acid. Isolevuglandins react rapidly with primary amines including the lysyl residues of proteins to form irreversible covalent modifications. This review highlights evidence for the potential role of isolevuglandin modification in the disease processes, especially atherosclerosis, and some of the tools including small molecule dicarbonyl scavengers utilized to assess their contributions to disease.

Directed C–C bond cleavage of a cyclopropane intermediate generated fromN-tosylhydrazones and stable enaminones: expedient synthesis of functionalized 1,4-ketoaldehydes

An efficient method to construct functionalized 1,4-ketoaldehydes bearing all-carbon α-quaternary centersviaregioselective C–C bond activation has been described.

Reactive gamma-ketoaldehydes as novel activators of hepatic stellate cells in vitro

AIMS

Products of lipid oxidation, such as 4-hydroxynonenal (4-HNE), are key activators of hepatic stellate cells (HSC) to a pro-fibrogenic phenotype. Isolevuglandins (IsoLG) are a family of acyclic γ-ketoaldehydes formed through oxidation of arachidonic acid or as by-products of the cyclooxygenase pathway. IsoLGs are highly reactive aldehydes which are efficient at forming protein adducts and cross-links at concentrations 100-fold lower than 4-hydroxynonenal. Since the contribution of IsoLGs to liver injury has not been studied, we synthesized 15-E₂-IsoLG and used it to investigate whether IsoLG could induce activation of HSC.

RESULTS

Primary human HSC were exposed to 15-E₂-IsoLG for up to 48h. Exposure to 5μM 15-E₂-IsoLG in HSCs promoted cytotoxicity and apoptosis. At non-cytotoxic doses (50 pM-500nM) 15-E₂-IsoLG promoted HSC activation, indicated by increased expression of α-SMA, sustained activation of ERK and JNK signaling pathways, and increased mRNA and/or protein expression of cytokines and chemokines, which was blocked by inhibitors of JNK and NF-kB. In addition, IsoLG promoted formation of reactive oxygen species, and induced an early activation of ER stress, followed by autophagy. Inhibition of autophagy partially reduced the pro-inflammatory effects of IsoLG, suggesting that it might serve as a cytoprotective response.

INNOVATION

This study is the first to describe the biological effects of IsoLG in primary HSC, the main drivers of hepatic fibrosis.

CONCLUSIONS

IsoLGs represent a newly identified class of activators of HSC in vitro, which are biologically active at concentrations as low as 500 pM, and are particularly effective at promoting a pro-inflammatory response and autophagy.

Scavenging of highly reactive gamma-ketoaldehydes attenuates cognitive dysfunction associated with epileptogenesis

Cognitive dysfunction is a major comorbidity of the epilepsies; however, treatments targeting seizure-associated cognitive dysfunction, particularly deficits in learning and memory are not available. Isoketals and neuroketals, collectively known as gamma-ketoaldehydes are formed via the non-enzymatic, free radical catalyzed oxidation of arachidonic acid and docosahexaenoic acid, respectively. They are attractive candidates for oxidative protein damage and resultant cognitive dysfunction due to their formation within the plasma membrane and their high proclivity to form cytotoxic adducts on protein lysine residues. We tested the hypothesis that gamma-ketoaldehydes mechanistically contribute to seizure-associated memory impairment using a specific gamma-ketoaldehyde scavenger, salicylamine in the kainic acid and pilocarpine rat models of temporal lobe epilepsy. We show that gamma-ketoaldehydes are increased following epileptogenic injury in hippocampus and perirhinal cortex, two brain regions imperative for learning and memory. Treatment with an orally bioavailable, brain permeable scavenger, salicylamine attenuated 1) spatial memory deficits 2) reference memory deficits and 3) neuronal loss and astrogliosis in two mechanistically distinct models of epilepsy without affecting the epileptogenic injury or the development of chronic epilepsy. We have previously demonstrated that reactive oxygen species and the lipid peroxidation biomarkers, F₂-isoprostanes are produced following status epilepticus. However, which reactive species specifically mediate oxidative damage to cellular macromolecules remains at large. We provide novel data suggesting that memory impairment occurs via gamma-ketoaldehyde production in two models of epilepsy and that treatment with a gamma-ketoaldehyde scavenger can protect vulnerable neurons. This work suggests a novel target and therapy to treat seizure-induced memory deficits in epilepsy.

Accumulation of isolevuglandin-modified protein in normal and fibrotic lung

Protein lysine modification by γ-ketoaldehyde isomers derived from arachidonic acid, termed isolevuglandins (IsoLGs), is emerging as a mechanistic link between pathogenic reactive oxygen species and disease progression. However, the questions of whether covalent modification of proteins by IsoLGs are subject to genetic regulation and the identity of IsoLG-modified proteins remain unclear. Herein we show that Nrf2 and Nox2 are key regulators of IsoLG modification in pulmonary tissue and report on the identity of proteins analyzed by LC-MS following immunoaffinity purification of IsoLG-modified proteins. Gene ontology analysis revealed that proteins in numerous cellular pathways are susceptible to IsoLG modification. Although cells tolerate basal levels of modification, exceeding them induces apoptosis. We found prominent modification in a murine model of radiation-induced pulmonary fibrosis and in idiopathic pulmonary fibrosis, two diseases considered to be promoted by gene-regulated oxidant stress. Based on these results we hypothesize that IsoLG modification is a hitherto unrecognized sequelae that contributes to radiation-induced pulmonary injury and IPF.

Decreased Bioenergetic Health Index in monocytes isolated from the pericardial fluid and blood of post-operative cardiac surgery patients

Translational bioenergetics requires the measurement of mitochondrial function in clinically relevant samples and the integration of the data in a form that can be applied to personalized medicine. In the present study, we show the application of the measurement of the Bioenergetic Health Index (BHI) to cardiac surgery patients.

Monitoring the bioenergetics of leucocytes is now emerging as an important approach in translational research to detect mitochondrial dysfunction in blood or other patient samples. Using the mitochondrial stress test, which involves the sequential addition of mitochondrial inhibitors to adherent leucocytes, we have calculated a single value, the Bioenergetic Health Index (BHI), which represents the mitochondrial function in cells isolated from patients. In the present report, we assess the BHI of monocytes isolated from the post-operative blood and post-operative pericardial fluid (PO-PCF) from patients undergoing cardiac surgery. Analysis of the bioenergetics of monocytes isolated from patients’ PO-PCF revealed a profound decrease in mitochondrial function compared with monocytes isolated from their blood or from healthy controls. Further, patient blood monocytes showed no significant difference in the individual energetic parameters from the mitochondrial stress test but, when integrated into the BHI evaluation, there was a significant decrease in BHI compared with healthy control monocytes. These data support the utility of BHI measurements in integrating the individual parameters from the mitochondrial stress test into a single value. Supporting our previous finding that the PO-PCF is pro-oxidant, we found that exposure of rat cardiomyocytes to PO-PCF caused a significant loss of mitochondrial membrane potential and increased reactive oxygen species (ROS). These findings support the hypothesis that integrated measures of bioenergetic health could have prognostic and diagnostic value in translational bioenergetics.

Mass spectrometry detection of isolevuglandin adduction to specific protein residues

The aging process seems to be associated with oxidative stress and hence increased production of lipid peroxidation products, including isolevuglandins (isoLGs). The latter are highly reactive γ-ketoaldehydes which can form covalent adducts with primary amino groups of enzymes and proteins and alter the properties of these biomolecules. Yet little is currently known about amino acid-containing compounds affected by isoLG modification in different age-related pathological processes. To facilitate the detection of these biomolecules, we developed a strategy in which the purified enzyme (or protein) of interest is first treated with authentic isoLG in vitro to evaluate whether it contains reactive lysine residues prone to modification with isoLGs. The data obtained serve as a basis for making the "GO/NO GO" decision as to whether to pursue a further search of this isoLG modification in a biological sample. In this chapter, we describe the conditions for the in vitro isoLG modification assay and how to use mass spectrometry to identify the isoLG-modified peptides and amino acid residues. Our studies were carried out on cytochrome P450 27A1, an important metabolic enzyme, and utilized iso[4]levuglandin E2 as a prototypical isoLG. The isoLG-treated cytochrome P450 was subjected to proteolysis followed by liquid chromatography-tandem mass spectrometry for peptide separation and analysis by Mascot, a proteomics search engine, for the presence of modified peptides. The developed protocol could be applied to characterization of other enzymes/proteins and other types of unconventional posttranslational protein modification.

Isolevuglandin adducts in disease

SIGNIFICANCE

A diverse family of lipid-derived levulinaldehydes, isolevuglandins (isoLGs), is produced by rearrangement of endoperoxide intermediates generated through both cyclooxygenase (COX) and free radical-induced cyclooxygenation of polyunsaturated fatty acids and their phospholipid esters. The formation and reactions of isoLGs with other biomolecules has been linked to alcoholic liver disease, Alzheimer's disease, age-related macular degeneration, atherosclerosis, cardiac arythmias, cancer, end-stage renal disease, glaucoma, inflammation of allergies and infection, mitochondrial dysfunction, multiple sclerosis, and thrombosis. This review chronicles progress in understanding the chemistry of isoLGs, detecting their production in vivo and understanding their biological consequences.

CRITICAL ISSUES

IsoLGs have never been isolated from biological sources, because they form adducts with primary amino groups of other biomolecules within seconds. Chemical synthesis enabled investigation of isoLG chemistry and detection of isoLG adducts present in vivo.

RECENT ADVANCES

The first peptide mapping and sequencing of an isoLG-modified protein present in human retina identified the modification of a specific lysyl residue of the sterol C27-hydroxylase Cyp27A1. This residue is preferentially modified by iso[4]LGE2 in vitro, causing loss of function. Adduction of less than one equivalent of isoLG can induce COX-associated oligomerization of the amyloid peptide Aβ1-42. Adduction of isoLGE2 to phosphatidylethanolamines causes gain of function, converting them into proinflammatory isoLGE2-PE agonists that foster monocyte adhesion to endothelial cells.

FUTURE DIRECTIONS

Among the remaining questions on the biochemistry of isoLGs are the dependence of biological activity on isoLG isomer structure, the structures and mechanism of isoLG-derived protein-protein and DNA-protein cross-link formation, and its biological consequences.

Qualitative characterization of the rat liver mitochondrial lipidome using all ion fragmentation on an Exactive benchtop Orbitrap MS

Untargeted lipidomics profiling by liquid chromatography-mass spectrometry (LC-MS) allows researchers to observe the occurrences of lipids in a biological sample without showing intentional bias to any specific class of lipids and allows retrospective reanalysis of data collected. Typically, and in the specific method described, a general extraction method followed by LC separation is used to achieve nonspecific class coverage of the lipidome prior to high-resolution accurate mass (HRAM) MS detection. Here we describe a workflow including the isolation of mitochondria from liver tissue, followed by mitochondrial lipid extraction and the LC-MS conditions used for data acquisition. We also highlight how, in this method, all-ion fragmentation can be used to identify species of lower abundances, often missed by data-dependent fragmentation techniques. Here we describe the isolation of mitochondria from liver tissue, followed by mitochondrial lipid extraction and the LC-MS conditions used for data acquisition.

Omega-3 polyunsaturated fatty acids and oxygenated metabolism in atherothrombosis

Numerous epidemiological studies and clinical trials have reported the health benefits of omega-3 polyunsaturated fatty acids (PUFA), including a lower risk of coronary heart diseases. This review mainly focuses on the effects of alpha-linolenic (ALA), eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids on some risk factors associated with atherothrombosis, including platelet activation, plasma lipid concentrations and oxidative modification of low-density lipoproteins (LDL). Special focus is given to the effects of marine PUFA on the formation of eicosanoids and docosanoids, and to the bioactive properties of some oxygenated metabolites of omega-3 PUFA produced by cyclooxygenases and lipoxygenases. The antioxidant effects of marine omega-3 PUFA at low concentrations and the pro-oxidant effects of DHA at high concentrations on the redox status of platelets and LDL are highlighted. Non enzymatic peroxidation end-products deriving from omega-3 PUFA such as hydroxy-hexenals, neuroketals and EPA-derived isoprostanes are also considered in relation to atherosclerosis. This article is part of a Special Issue entitled "Oxygenated metabolism of PUFA: analysis and biological relevance".

Soluble, prefibrillar α-synuclein oligomers promote complex I-dependent, Ca2+-induced mitochondrial dysfunction

α-Synuclein (αSyn) aggregation and mitochondrial dysfunction both contribute to the pathogenesis of Parkinson disease (PD). Although recent studies have suggested that mitochondrial association of αSyn may disrupt mitochondrial function, it is unclear what aggregation state of αSyn is most damaging to mitochondria and what conditions promote or inhibit the effect of toxic αSyn species. Because the neuronal populations most vulnerable in PD are characterized by large cytosolic Ca(2+) oscillations that burden mitochondria, we examined mitochondrial Ca(2+) stress in an in vitro system comprising isolated mitochondria and purified recombinant human αSyn in various aggregation states. Using fluorimetry to simultaneously measure four mitochondrial parameters, we observed that soluble, prefibrillar αSyn oligomers, but not monomeric or fibrillar αSyn, decreased the retention time of exogenously added Ca(2+), promoted Ca(2+)-induced mitochondrial swelling and depolarization, and accelerated cytochrome c release. Inhibition of the permeability transition pore rescued these αSyn-induced changes in mitochondrial parameters. Interestingly, the mitotoxic effects of αSyn were specifically dependent upon both electron flow through complex I and mitochondrial uptake of exogenous Ca(2+). Our results suggest that soluble prefibrillar αSyn oligomers recapitulate several mitochondrial phenotypes previously observed in animal and cell models of PD: complex I dysfunction, altered membrane potential, disrupted Ca(2+) homeostasis, and enhanced cytochrome c release. These data reveal how the association of oligomeric αSyn with mitochondria can be detrimental to the function of cells with high Ca(2+)-handling requirements.

N-acetyl-serotonin offers neuroprotection through inhibiting mitochondrial death pathways and autophagic activation in experimental models of ischemic injury

N-acetylserotonin (NAS) is an immediate precursor of melatonin, which we have reported is neuroprotective against ischemic injury. Here we test whether NAS is a potential neuroprotective agent in experimental models of ischemic injury. We demonstrate that NAS inhibits cell death induced by oxygen-glucose deprivation or H2O2 in primary cerebrocortical neurons and primary hippocampal neurons in vitro, and organotypic hippocampal slice cultures ex vivo and reduces hypoxia/ischemia injury in the middle cerebral artery occlusion mouse model of cerebral ischemia in vivo. We find that NAS is neuroprotective by inhibiting the mitochondrial cell death pathway and the autophagic cell death pathway. The neuroprotective effects of NAS may result from the influence of mitochondrial permeability transition pore opening, mitochondrial fragmentation, and inhibition of the subsequent release of apoptogenic factors cytochrome c, Smac, and apoptosis-inducing factor from mitochondria to cytoplasm, and activation of caspase-3, -9, as well as the suppression of the activation of autophagy under stress conditions by increasing LC3-II and Beclin-1 levels and decreasing p62 level. However, NAS, unlike melatonin, does not provide neuroprotection through the activation of melatonin receptor 1A. We demonstrate that NAS reaches the brain subsequent to intraperitoneal injection using liquid chromatography/mass spectrometry analysis. Given that it occurs naturally and has low toxicity, NAS, like melatonin, has potential as a novel therapy for ischemic injury.

N-acetyl-serotonin offers neuroprotection through inhibiting mitochondrial death pathways and autophagic activation in experimental models of ischemic injury

N-acetylserotonin (NAS) is an immediate precursor of melatonin, which we have reported is neuroprotective against ischemic injury. Here we test whether NAS is a potential neuroprotective agent in experimental models of ischemic injury. We demonstrate that NAS inhibits cell death induced by oxygen-glucose deprivation or H2O2 in primary cerebrocortical neurons and primary hippocampal neurons in vitro, and organotypic hippocampal slice cultures ex vivo and reduces hypoxia/ischemia injury in the middle cerebral artery occlusion mouse model of cerebral ischemia in vivo. We find that NAS is neuroprotective by inhibiting the mitochondrial cell death pathway and the autophagic cell death pathway. The neuroprotective effects of NAS may result from the influence of mitochondrial permeability transition pore opening, mitochondrial fragmentation, and inhibition of the subsequent release of apoptogenic factors cytochrome c, Smac, and apoptosis-inducing factor from mitochondria to cytoplasm, and activation of caspase-3, -9, as well as the suppression of the activation of autophagy under stress conditions by increasing LC3-II and Beclin-1 levels and decreasing p62 level. However, NAS, unlike melatonin, does not provide neuroprotection through the activation of melatonin receptor 1A. We demonstrate that NAS reaches the brain subsequent to intraperitoneal injection using liquid chromatography/mass spectrometry analysis. Given that it occurs naturally and has low toxicity, NAS, like melatonin, has potential as a novel therapy for ischemic injury.

Lipid peroxidation generates biologically active phospholipids including oxidatively N-modified phospholipids

Peroxidation of membranes and lipoproteins converts "inert" phospholipids into a plethora of oxidatively modified phospholipids (oxPL) that can act as signaling molecules. In this review, we will discuss four major classes of oxPL: mildly oxygenated phospholipids, phospholipids with oxidatively truncated acyl chains, phospholipids with cyclized acyl chains, and phospholipids that have been oxidatively N-modified on their headgroups by reactive lipid species. For each class of oxPL we will review the chemical mechanisms of their formation, the evidence for their formation in biological samples, the biological activities and signaling pathways associated with them, and the catabolic pathways for their elimination. We will end by briefly highlighting some of the critical questions that remain about the role of oxPL in physiology and disease.

Pretreatment with pyridoxamine mitigates isolevuglandin-associated retinal effects in mice exposed to bright light

The benefits of antioxidant therapy for treating age-related macular degeneration, a devastating retinal disease, are limited. Perhaps species other than reactive oxygen intermediates should be considered as therapeutic targets. These could be lipid peroxidation products, including isolevuglandins (isoLGs), prototypical and extraordinarily reactive γ-ketoaldehydes that avidly bind to proteins, phospholipids, and DNA and modulate the properties of these biomolecules. We found isoLG adducts in aged human retina but not in the retina of mice kept under dim lighting. Hence, to test whether scavenging of isoLGs could complement or supplant antioxidant therapy, we exposed mice to bright light and found that this insult leads to retinal isoLG-adduct formation. We then pretreated mice with pyridoxamine, a B6 vitamer and efficient scavenger of γ-ketoaldehydes, and found that the levels of retinal isoLG adducts are decreased, and morphological changes in photoreceptor mitochondria are not as pronounced as in untreated animals. Our study demonstrates that preventing the damage to biomolecules by lipid peroxidation products, a novel concept in vision research, is a viable strategy to combat oxidative stress in the retina.

Posttranslational modification by an isolevuglandin diminishes activity of the mitochondrial cytochrome P450 27A1

Posttranslational modification by isolevuglandins (isoLGs), arachidonate oxidation products, is an important yet understudied process associated with altered protein properties. This type of modification is detected in cytochrome P450 27A1 (CYP27A1), a multifunction enzyme expressed in almost every cell and involved in the metabolism of cholesterol and other sterols. Previously, the CYP27A1 Lys(358)-isoLG adduct was found in human retina afflicted with age-related macular degeneration. Yet, the effect of Lys(358) modification on enzyme activity was not investigated. Herein, we characterized catalytic properties of Lys(358) as well as Lys(476) CYP27A1 mutants before and after isoLG treatment and quantified the extent of modification by multiple reaction monitoring. The K358R mutant was less susceptible to isoLG-induced loss of catalytic activity than the wild type (WT), whereas the K476R mutant was nearly as vulnerable as the WT. Both mutants showed less isoLG modification than WT. Thus, modification of Lys(358), a residue involved in redox partner interactions, is the major contributor to isoLG-associated loss of CYP27A1 activity. Our data show the specificity of isoLG modification, provide direct evidence that isoLG adduction impairs enzyme activity, and support our hypothesis that isoLG modification in the retina is detrimental to CYP27A1 enzyme activity, potentially disrupting cholesterol homeostasis.

Qualitative Characterization of the Rat Liver Mitochondrial Lipidome using LC-MS Profiling and High Energy Collisional Dissociation (HCD) All Ion Fragmentation

Lipids play multiple roles essential for proper mitochondrial function, from their involvement in membrane structure and fluidity, cellular energy storage, and signaling. Lipids are also major targets for reactive species, and their peroxidation byproducts themselves mediate further damage. Thousands of lipid species, from multiple classes and categories, are involved in these processes, suggesting lipid quantitative and structural analysis can help provide a better understanding of mitochondrial physiological status. Due to the diversity of lipids that contribute to and reflect mitochondrial function, analytical methods should ideally cover a wide range of lipid classes, and yield both quantitative and structural information. We developed a high resolution LC-MS method that is able to monitor the major lipid classes found in biospecimens (ie. biofluids, cells and tissues) with relative quantitation in an efficient, sensitive, and robust manner while also characterizing individual lipid side-chains, by all ion HCD fragmentation and chromatographic alignment. This method was used to profile the liver mitochondrial lipids from 192 rats undergoing a dietary macronutrient study in which changes in mitochondria function are related to changes in the major fat and glycemic index component of each diet. A total of 381 unique lipids, spanning 5 of the major LIPID MAPS defined categories, including fatty acyls, glycerophospholipids, glycerolipids, sphingolipids and prenols, were identified in mitochondria using the non-targeted LC-MS analysis in both positive and negative mode. The intention of this report is to show the breadth of this non-targeted LC-MS profiling method with regards to its ability to profile, identify and characterize the mitochondrial lipidome and the details of this will be discussed.

Role of mitochondria in programmed cell death mediated by arachidonic acid-derived eicosanoids

Arachidonic acid-derived eicosanoids from cyclooxygenases, lipoxygenases, and cytochrome P450 are important lipid mediators involved in numerous homeostatic and pathophysiological processes. Most eicosanoids act primarily on their respective cell surface G-protein coupled receptors to elicit downstream signaling in an autocrine and paracrine fashion. Emerging evidence indicates that these hormones are also critical in apoptosis in a cell/tissue specific manner. In this review, we summarize the formation of eicosanoids and their roles as mediators in apoptosis, specifically on the roles of mitochondria in mediating these events and the signaling pathways involved. The biological relevance of eicosanoid-mediated apoptosis is also discussed.

Dietary Omega-3 Fatty Acids Do Not Change Resistance of Rat Brain or Liver Mitochondria to Ca(2+) and/or Prooxidants

Omega-3 polyunsaturated fatty acids (n-3 PUFAs) block apoptotic neuronal cell death and are strongly neuroprotective in acute and chronic neurodegeneration. Theoretical considerations, indirect data, and consideration of parsimony lead to the hypothesis that modulation of mitochondrial pathway(s) underlies at least some of the neuroprotective effects of n-3 PUFAs. We therefore systematically tested this hypothesis on healthy male FBFN1 rats fed for four weeks with isocaloric, 10% fat-containing diets supplemented with 1, 3, or 10% fish oil (FO). High resolution mass spectrometric analysis confirmed expected diet-driven increases in docosahexaenoic acid (DHA, 22:6, n-3) and eicosapentaenoic acid (EPA, 20:5, n-3) in sera, liver and nonsynaptosomal brain mitochondria. We further evaluated the resistance of brain and liver mitochondria to Ca(2+) overload and prooxidants. Under these conditions, neither mitochondrial resistance to Ca(2+) overload and prooxidants nor mitochondrial physiology is altered by diet, despite the expected incorporation of DHA and EPA in mitochondrial membranes and plasma. Collectively, the data eliminate one of the previously proposed mechanism(s) that n-3 PUFA induced augmentation of mitochondrial resistance to the oxidant/calcium-driven dysfunction. These data furthermore allow us to define a specific series of follow-up experiments to test related hypotheses about the effect of n-3 PUFAs on brain mitochondria.

Comparison of in-gel protein separation techniques commonly used for fractionation in mass spectrometry-based proteomic profiling

Fractionation of complex samples at the cellular, subcellular, protein, or peptide level is an indispensable strategy to improve the sensitivity in mass spectrometry-based proteomic profiling. This study revisits, evaluates, and compares the most common gel-based protein separation techniques i.e. 1D SDS-PAGE, 1D preparative SDS-PAGE, IEF-IPG, and 2D-PAGE in their performance as fractionation approaches in nano LC-ESI-MS/MS analysis of a mixture of protein standards and mitochondrial extracts isolated from rat liver. This work demonstrates that all the above techniques provide complementary protein identification results, but 1D SDS-PAGE and IEF-IPG had the highest number of identifications. The IEF-IPG technique resulted in the highest average number of detected peptides per protein. The 2D-PAGE was evaluated as a protein fractionation approach. This work shows that the recovery of proteins and resulting proteolytic digests is highly dependent on the total volume of the gel matrix. The performed comparison of the fractionation techniques demonstrates the potential of a combination of orthogonal 1D SDS-PAGE and IEF-IPG for the improved sensitivity of profiling without significant decrease in throughput.

Origins of metabolic complications in obesity: ectopic fat accumulation. The importance of the qualitative aspect of lipotoxicity

PURPOSE OF REVIEW

This study highlights two aspects of the concept of lipotoxicity. First, the metabolic consequences following ectopic fat accumulation are not only determined by the amount of lipid accumulated, but also the quality of lipid species. Second, the existence of allostatic mechanisms operating at cellular and tissue levels, which counterbalance the negative effects of lipid overload.

RECENT FINDINGS

The development of lipidomics has allowed the isolation and identification of a wide range of lipid species. Some are highly reactive and capable of inducing undesirable toxic effects. Here we focus on recent information related to pathways involved in the production of these reactive lipid species, their sites of generation and tropism for specific organelles and the molecular mechanisms through which they exert toxic effects. We describe how cell membranes and the lipid species forming their bilayer constitute the main platform from which reactive lipid species are generated. We propose that strategies aimed at maintaining membrane lipid homeostasis are fundamental to preventing the initiation of metabolically relevant lipotoxicity.

SUMMARY

It is essential to understand the qualitative component of lipid species involved in cellular toxicity and the molecular mechanisms mediating these toxic effects to identify new therapeutic targets.

Isolation of functional mitochondria from rat kidney and skeletal muscle without manual homogenization

Isolation of functional and intact mitochondria from solid tissue is crucial for studies that focus on the elucidation of normal mitochondrial physiology and/or mitochondrial dysfunction in conditions such as aging, diabetes, and cancer. There is growing recognition of the importance of mitochondria both as targets for drug development and as off-target mediators of drug side effects. Unfortunately, mitochondrial isolation from tissue is generally carried out using homogenizer-based methods that require extensive operator experience to obtain reproducible high-quality preparations. These methods limit dissemination, impede scale-up, and contribute to difficulties in reproducing experimental results over time and across laboratories. Here we describe semiautomated methods to disrupt tissue using kidney and muscle mitochondria preparations as exemplars. These methods use the Barocycler, the PCT Shredder, or both. The PCT Shredder is a mechanical grinder that quickly breaks up tissue without significant risk of overhomogenization. Mitochondria isolated using the PCT Shredder are shown to be comparable to controls. The Barocycler generates controlled pressure pulses that can be adjusted to lyse cells and release organelles. The mitochondria subjected to pressure cycling-mediated tissue disruption are shown to retain functionality, enabling combinations of the PCT Shredder and the Barocycler to be used to purify mitochondrial preparations.