Identification of a novel mitochondrial protein ("mitoNEET") cross-linked specifically by a thiazolidinedione photoprobe

Structures of PPARγ complexed with lobeglitazone and pioglitazone reveal key determinants for the recognition of antidiabetic drugs

Peroxisome proliferator-activator receptor (PPAR) γ is a nuclear hormone receptor that regulates glucose homeostasis, lipid metabolism, and adipocyte function. PPARγ is a target for thiazolidinedione (TZD) class of drugs which are widely used for the treatment of type 2 diabetes. Recently, lobeglitazone was developed as a highly effective TZD with reduced side effects by Chong Kun Dang Pharmaceuticals. To identify the structural determinants for the high potency of lobeglitazone as a PPARγ agonist, we determined the crystal structures of the PPARγ ligand binding domain (LBD) in complex with lobeglitazone and pioglitazone at 1.7 and 1.8 Å resolutions, respectively. Comparison of ligand-bound PPARγ structures revealed that the binding modes of TZDs are well conserved. The TZD head group forms hydrogen bonds with the polar residues in the AF-2 pocket and helix 12, stabilizing the active conformation of the LBD. The unique p-methoxyphenoxy group of lobeglitazone makes additional hydrophobic contacts with the Ω-pocket. Docking analysis using the structures of TZD-bound PPARγ suggested that lobeglitazone displays 12 times higher affinity to PPARγ compared to rosiglitazone and pioglitazone. This structural difference correlates with the enhanced affinity and the low effective dose of lobeglitazone compared to the other TZDs.

Insulin Sensitizing Pharmacology of Thiazolidinediones Correlates with Mitochondrial Gene Expression rather than Activation of PPARγ

Insulin sensitizing thiazolidinediones (TZDs) are generally considered to work as agonists for the nuclear receptor peroxisome proliferative activated receptor-gamma (PPARγ). However, TZDs also have acute, non-genomic metabolic effects and it is unclear which actions are responsible for the beneficial pharmacology of these compounds. We have taken advantage of an analog, based on the metabolism of pioglitazone, which has much reduced ability to activate PPARγ. This analog (PNU-91325) was compared to rosiglitazone, the most potent PPARγ activator approved for human use, in a variety of studies both in vitro and in vivo. The data demonstrate that PNU-91325 is indeed much less effective than rosiglitazone at activating PPARγ both in vitro and in vivo. In contrast, both compounds bound similarly to a mitochondrial binding site and acutely activated PI-3 kinase-directed phosphorylation of AKT, an action that was not affected by elimination of PPARγ activation. The two compounds were then compared in vivo in both normal C57 mice and diabetic KKAy mice to determine whether their pharmacology correlated with biomarkers of PPARγ activation or with the expression of other gene transcripts. As expected from previous studies, both compounds improved insulin sensitivity in the diabetic mice, and this occurred in spite of the fact that there was little increase in expression of the classic PPARγ target biomarker adipocyte binding protein-2 (aP2) with PNU-91325 under these conditions. An examination of transcriptional profiling of key target tissues from mice treated for one week with both compounds demonstrated that the relative pharmacology of the two thiazolidinediones correlated best with an increased expression of an array of mitochondrial proteins and with expression of PPARγ coactivator 1-alpha (PGC1α), the master regulator of mitochondrial biogenesis. Thus, important pharmacology of the insulin sensitizing TZDs may involve acute actions, perhaps on the mitochondria, that are independent of direct activation of the nuclear receptor PPARγ. These findings suggest a potential alternative route to the discovery of novel insulin sensitizing drugs.

Understanding the molecular basis for multiple mitochondrial dysfunctions syndrome 1 (MMDS1): impact of a disease-causing Gly189Arg substitution on NFU1

Iron-sulfur (Fe/S) cluster-containing proteins constitute one of the largest protein classes, with highly varied function. Consequently, the biosynthesis of Fe/S clusters is evolutionarily conserved and mutations in intermediate Fe/S cluster scaffold proteins can cause disease, including multiple mitochondrial dysfunctions syndrome (MMDS). Herein, we have characterized the impact of defects occurring in the MMDS1 disease state that result from a point mutation (p.Gly189Arg) near the active site of NFU1, an Fe/S scaffold protein. In vitro investigation into the structure-function relationship of the Gly189Arg derivative, along with two other variants, reveals that substitution at position 189 triggers structural changes that increase flexibility, decrease stability, and alter the monomer-dimer equilibrium toward monomer, thereby impairing the ability of the Gly189X derivatives to receive an Fe/S cluster from physiologically relevant sources.

Upregulation of CISD2 augments ROS homeostasis and contributes to tumorigenesis and poor prognosis of lung adenocarcinoma

CISD2 is a redox-sensitive gene critical for normal development and mitochondrial integrity. CISD2 was known to have aberrant expression in several types of human cancers. However, its relation with lung cancer is still not clear. In this study we found CISD2 mRNA was significantly upregulated in lung adenocarcinoma (ADC) samples, compared with their adjacent normal counterparts, and was correlated with tumor stage, grade, and prognosis based on analysis of clinical specimens-derived expression data in public domain and our validation assay. Cell based assay indicated that CISD2 expression regulated accumulation of reactive oxygen species (ROS), polarization of mitochondrial membrane potential, as well as cell viability, apoptosis, invasiveness, and tumorigenicity. In addition, CISD2 expression was found significantly correlated with stress response/redox signaling genes such as EGR1 and GPX3, while such correlations were also found valid in many public domain data. Taken together, upregulation of CISD2 is involved in an increased antioxidant capacity in response to elevated ROS levels during the formation and progression of lung ADC. The molecular mechanism underlying how CISD2 regulates ROS homeostasis and augments malignancy of lung cancer warrants further investigations.

Combined Biochemical, Biophysical, and Cellular Methods to Study Fe-S Cluster Transfer and Cytosolic Aconitase Repair by MitoNEET

MitoNEET is the first identified Fe-S protein anchored to mammalian outer mitochondrial membranes with the vast majority of the protein polypeptide located in the cytosol, including its [2Fe-2S] cluster-binding domain. The coordination of the cluster is unusual and involves three cysteines and one histidine. MitoNEET is capable of transferring its redox-active Fe-S cluster to a bacterial apo-ferredoxin in vitro even under aerobic conditions, unlike other Fe-S transfer proteins such as ISCU. This specificity suggests its possible involvement in Fe-S repair after oxidative and/or nitrosative stress. Recently, we identified cytosolic aconitase/iron regulatory protein 1 (IRP1) as the first physiological protein acceptor of the mitoNEET Fe-S cluster in an Fe-S repair process. This chapter describes methods to study in vitro mitoNEET Fe-S cluster transfer/repair to a bacterial ferredoxin used as a model aporeceptor and in a more comprehensive manner to cytosolic aconitase/IRP1 after a nitrosative stress using in vitro, in cellulo, and in vivo methods.

Modulation of cellular energetics by galactose and pioglitazone

The Warburg effect is ameliorated by culturing transformed cells in the presence of galactose instead of glucose as the primary carbon source. However, metabolic consequences may occur in addition to sensitizing the cells to mitochondrial toxins. The screening of pharmaceutical agents against transformed cells while using galactose must therefore be carefully evaluated. Pioglitazone is employed in clinical applications to treat type-2 diabetes but clearly has other off-target effects. Human hepatocellular carcinoma cells (HepG2) were cultured in glucose or galactose-containing medium to investigate the role of pioglitazone on cellular bioenergetics by calorimetry and respirometry. Compared with cells cultured in 10 mM glucose, HepG2 cells cultured in the presence of 10 mM galactose showed decreased metabolic activity as measured by cellular heat flow. Interestingly, cellular heat flow increased after the addition of pioglitazone for cells cultured in glucose, but not for cells cultured in galactose. Our calorimetric data indicated that a reduction in cellular capacity for glycolysis was the mechanism responsible for the increase in sensitivity to pioglitazone, and possibly to mitochondrial toxins in general, for cells cultured in galactose. Furthermore, oxygen consumption rates were decreased after the addition of pioglitazone to cells grown in glucose but remained unchanged for cells grown in the presence of galactose. We have demonstrated that pioglitazone induces a reduction in mitochondrial activity that is partially compensated via an increase in glycolysis in the presence of glucose.

Level of dietary energy and 2,4-thiazolidinedione alter molecular and systemic biomarkers of inflammation and liver function in Holstein cows

Background

The objective of the study was to evaluate the effect of overfeeding a moderate energy diet and a 2,4-thiazolidinedione (TZD) injection on blood and hepatic tissue biomarkers of lipid metabolism, oxidative stress, and inflammation as it relates to insulin sensitivity.

Results

Fourteen dry non-pregnant cows were fed a control (CON) diet to meet 100% of NRC requirements for 3 wk, after which half of the cows were assigned to a moderate-energy diet (OVE) and half of the cows continued on CON for 6 wk. All cows received an intravenous injection of 4 mg TZD/kg of body weight (BW) daily from 2 wk after initiation of dietary treatments and for 2 additional week. Compared with CON cows and before TZD treatment, the OVE cows had lower concentration of total protein, urea and albumin over time. The concentration of cholesterol and tocopherol was greater after 2 wk of TZD regardless of diet. Before and after TZD, the OVE cows had greater concentrations of AST/GOT, while concentrations of paraoxonase, total protein, globulin, myeloperoxidase, and haptoglobin were lower compared with CON cows. Regardless of diet, TZD administration increased the concentration of ceruloplasmin, ROMt, cholesterol, tocopherol, total protein, globulin, myeloperoxidase and beta-carotene. In contrast, the concentration of haptoglobin decreased at the end of TZD injection regardless of diet. Prior to TZD injection, the mRNA expression of PC, ANGPTL4, FGF21, INSR, ACOX1, and PPARD in liver of OVE cows was lower compared with CON cows. In contrast, the expression of HMGCS2 was greater in OVE compared with CON cows. After 1 wk of TZD administration the expression of IRS1 decreased regardless of diet; whereas, expression of INSR increased after 2 wk of TZD injection. Cows fed OVE had lower overall expression of TNF, INSR, PC, ACOX1, FGF21, and PPARD but greater HMGCS2 expression. These differences were most evident before and after 1 wk of TZD injection, and by 2 wk of TZD differences in expression for most genes disappeared.

Conclusions

Based on molecular and blood data, administration of TZD enhanced some aspects of insulin sensitivity while causing contradictory results in terms of inflammation and oxidative stress. The bovine liver is TZD-responsive and level of dietary energy can modify the effects of TZD. Because insulin sensitizers have been proposed as useful tools to manage dairy cows during the transition period, further studies are required to investigate the potential hepatotoxicity effect of TZD (or similar compounds) in dairy cattle.

Electronic supplementary material

The online version of this article (doi:10.1186/s40104-017-0196-y) contains supplementary material, which is available to authorized users.

MitoNEET-dependent formation of intermitochondrial junctions

MitoNEET (mNEET) is a dimeric mitochondrial outer membrane protein implicated in many facets of human pathophysiology, notably diabetes and cancer, but its molecular function remains poorly characterized. In this study, we generated and analyzed mNEET KO cells and found that in these cells the mitochondrial network was disturbed. Analysis of 3D-EM reconstructions and of thin sections revealed that genetic inactivation of mNEET did not affect the size of mitochondria but that the frequency of intermitochondrial junctions was reduced. Loss of mNEET decreased cellular respiration, because of a reduction in the total cellular mitochondrial volume, suggesting that intermitochondrial contacts stabilize individual mitochondria. Reexpression of mNEET in mNEET KO cells restored the WT morphology of the mitochondrial network, and reexpression of a mutant mNEET resistant to oxidative stress increased in addition the resistance of the mitochondrial network to H₂O₂-induced fragmentation. Finally, overexpression of mNEET increased strongly intermitochondrial contacts and resulted in the clustering of mitochondria. Our results suggest that mNEET plays a specific role in the formation of intermitochondrial junctions and thus participates in the adaptation of cells to physiological changes and to the control of mitochondrial homeostasis.

Pioglitazone treatment following spinal cord injury maintains acute mitochondrial integrity and increases chronic tissue sparing and functional recovery

Pioglitazone is an FDA-approved PPAR-γ agonist drug used to treat diabetes, and it has demonstrated neuroprotective effects in multiple models of central nervous system (CNS) injury. Acute treatment after spinal cord injury (SCI) in rats is reported to suppress neuroinflammation, rescue injured tissues, and improve locomotor recovery. In the current study, we additionally assessed the protective efficacy of pioglitazone treatment on acute mitochondrial respiration, as well as functional and anatomical recovery after contusion SCI in adult male C57BL/6 mice. Mice received either vehicle or pioglitazone (10mg/kg) at either 15min or 3h after injury (75kdyn at T9) followed by a booster at 24h post-injury. At 25h, mitochondria were isolated from spinal cord segments centered on the injury epicenters and assessed for their respiratory capacity. Results showed significantly compromised mitochondrial respiration 25h following SCI, but pioglitazone treatment that was initiated either at 15min or 3h post-injury significantly maintained mitochondrial respiration rates near sham levels. A second cohort of injured mice received pioglitazone at 15min post injury, then once a day for 5days post-injury to assess locomotor recovery and tissue sparing over 4weeks. Compared to vehicle, pioglitazone treatment resulted in significantly greater recovery of hind-limb function over time, as determined by serial locomotor BMS assessments and both terminal BMS subscores and gridwalk performance. Such improvements correlated with significantly increased grey and white matter tissue sparing, although pioglitazone treatment did not abrogate long-term injury-induced inflammatory microglia/macrophage responses. In sum, pioglitazone significantly increased functional neuroprotection that was associated with remarkable maintenance of acute mitochondrial bioenergetics after traumatic SCI. This sets the stage for dose-response and delayed administration studies to maximize pioglitazone's efficacy for SCI while elucidating the precise role that mitochondria play in governing its neuroprotection; the ultimate goal to develop novel therapeutics that specifically target mitochondrial dysfunction.

Discovery of Novel Insulin Sensitizers: Promising Approaches and Targets

Insulin resistance is the undisputed root cause of type 2 diabetes mellitus (T2DM). There is currently an unmet demand for safe and effective insulin sensitizers, owing to the restricted prescription or removal from market of certain approved insulin sensitizers, such as thiazolidinediones (TZDs), because of safety concerns. Effective insulin sensitizers without TZD-like side effects will therefore be invaluable to diabetic patients. The specific focus on peroxisome proliferator-activated receptor γ- (PPARγ-) based agents in the past decades may have impeded the search for novel and safer insulin sensitizers. This review discusses possible directions and promising strategies for future research and development of novel insulin sensitizers and describes the potential targets of these agents. Direct PPARγ agonists, selective PPARγ modulators (sPPARγMs), PPARγ-sparing compounds (including ligands of the mitochondrial target of TZDs), agents that target the downstream effectors of PPARγ, along with agents, such as heat shock protein (HSP) inducers, 5'-adenosine monophosphate-activated protein kinase (AMPK) activators, 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) selective inhibitors, biguanides, and chloroquines, which may be safer than traditional TZDs, have been described. This minireview thus aims to provide fresh perspectives for the development of a new generation of safe insulin sensitizers.

The mitochondrial outer membrane protein mitoNEET is a redox enzyme catalyzing electron transfer from FMNH2 to oxygen or ubiquinone

Increasing evidence suggests that mitoNEET, a target of the type II diabetes drug pioglitazone, is a key regulator of energy metabolism in mitochondria. MitoNEET is anchored to the mitochondrial outer membrane via its N-terminal α helix domain and hosts a redox-active [2Fe-2S] cluster in its C-terminal cytosolic region. The mechanism by which mitoNEET regulates energy metabolism in mitochondria, however, is not fully understood. Previous studies have shown that mitoNEET specifically interacts with the reduced flavin mononucleotide (FMNH₂) and that FMNH₂ can quickly reduce the mitoNEET [2Fe-2S] clusters. Here we report that the reduced mitoNEET [2Fe-2S] clusters can be readily oxidized by oxygen. In the presence of FMN, NADH, and flavin reductase, which reduces FMN to FMNH₂ using NADH as the electron donor, mitoNEET mediates oxidation of NADH with a concomitant reduction of oxygen. Ubiquinone-2, an analog of ubiquinone-10, can also oxidize the reduced mitoNEET [2Fe-2S] clusters under anaerobic or aerobic conditions. Compared with oxygen, ubiquinone-2 is more efficient in oxidizing the mitoNEET [2Fe-2S] clusters, suggesting that ubiquinone could be an intrinsic electron acceptor of the reduced mitoNEET [2Fe-2S] clusters in mitochondria. Pioglitazone or its analog NL-1 appears to inhibit the electron transfer activity of mitoNEET by forming a unique complex with mitoNEET and FMNH₂ The results suggest that mitoNEET is a redox enzyme that may promote oxidation of NADH to facilitate enhanced glycolysis in the cytosol and that pioglitazone may regulate energy metabolism in mitochondria by inhibiting the electron transfer activity of mitoNEET.

Modification of the Orthosteric PPARγ Covalent Antagonist Scaffold Yields an Improved Dual-Site Allosteric Inhibitor

GW9662 and T0070907 are widely used commercially available irreversible antagonists of peroxisome proliferator-activated receptor gamma (PPARγ). These antagonists covalently modify Cys285 located in an orthosteric ligand-binding pocket embedded in the PPARγ ligand-binding domain and are used to block binding of other ligands. However, we recently identified an alternate/allosteric ligand-binding site in the PPARγ LBD to which ligand binding is not inhibited by these orthosteric covalent antagonists. Here, we developed a series of analogs based on the orthosteric covalent antagonist scaffold with the goal of inhibiting both orthosteric and allosteric cellular activation of PPARγ by MRL20, an orthosteric agonist that also binds to an allosteric site. Our efforts resulted in the identification of SR16832 (compound 22), which functions as a dual-site covalent inhibitor of PPARγ transcription by PPARγ-binding ligands. Molecular modeling, protein NMR spectroscopy structural analysis, and biochemical assays indicate the inhibition of allosteric activation occurs in part through expansion of the 2-chloro-5-nitrobenzamidyl orthosteric covalent antagonist toward the allosteric site, weakening of allosteric ligand binding affinity, and inducing conformational changes not competent for cellular PPARγ activation. Furthermore, SR16832 better inhibits binding of rosiglitazone, a thiazolidinedione (TZD) that weakly activates PPARγ when cotreated with orthosteric covalent antagonists, and may better inhibit binding of endogenous PPARγ ligands such as docosahexaenoic acid (DHA) compared to orthosteric covalent antagonists. Compounds such as SR16832 may be useful chemical tools to use as a dual-site bitopic orthosteric and allosteric covalent inhibitor of ligand binding to PPARγ.

Interactions between mitoNEET and NAF-1 in cells

The NEET proteins mitoNEET (mNT) and nutrient-deprivation autophagy factor-1 (NAF-1) are required for cancer cell proliferation and resistance to oxidative stress. NAF-1 and mNT are also implicated in a number of other human pathologies including diabetes, neurodegeneration and cardiovascular disease, as well as in development, differentiation and aging. Previous studies suggested that mNT and NAF-1 could function in the same pathway in mammalian cells, preventing the over-accumulation of iron and reactive oxygen species (ROS) in mitochondria. Nevertheless, it is unknown whether these two proteins directly interact in cells, and how they mediate their function. Here we demonstrate, using yeast two-hybrid, in vivo bimolecular fluorescence complementation (BiFC), direct coupling analysis (DCA), RNA-sequencing, ROS and iron imaging, and single and double shRNA lines with suppressed mNT, NAF-1 and mNT/NAF-1 expression, that mNT and NAF-1 directly interact in mammalian cells and could function in the same cellular pathway. We further show using an in vitro cluster transfer assay that mNT can transfer its clusters to NAF-1. Our study highlights the possibility that mNT and NAF-1 function as part of an iron-sulfur (2Fe-2S) cluster relay to maintain the levels of iron and Fe-S clusters under control in the mitochondria of mammalian cells, thereby preventing the activation of apoptosis and/or autophagy and supporting cellular proliferation.

Phylogenetic analysis of eukaryotic NEET proteins uncovers a link between a key gene duplication event and the evolution of vertebrates

NEET proteins belong to a unique family of iron-sulfur proteins in which the 2Fe-2S cluster is coordinated by a CDGSH domain that is followed by the “NEET” motif. They are involved in the regulation of iron and reactive oxygen metabolism, and have been associated with the progression of diabetes, cancer, aging and neurodegenerative diseases. Despite their important biological functions, the evolution and diversification of eukaryotic NEET proteins are largely unknown. Here we used the three members of the human NEET protein family (CISD1, mitoNEET; CISD2, NAF-1 or Miner 1; and CISD3, Miner2) as our guides to conduct a phylogenetic analysis of eukaryotic NEET proteins and their evolution. Our findings identified the slime mold Dictyostelium discoideum’s CISD proteins as the closest to the ancient archetype of eukaryotic NEET proteins. We further identified CISD3 homologs in fungi that were previously reported not to contain any NEET proteins, and revealed that plants lack homolog(s) of CISD3. Furthermore, our study suggests that the mammalian NEET proteins, mitoNEET (CISD1) and NAF-1 (CISD2), emerged via gene duplication around the origin of vertebrates. Our findings provide new insights into the classification and expansion of the NEET protein family, as well as offer clues to the diverged functions of the human mitoNEET and NAF-1 proteins.

Model of the MitoNEET [2Fe-2S] Cluster Shows Proton Coupled Electron Transfer

MitoNEET is an outer membrane protein whose exact function remains unclear, though a role of this protein in redox and iron sensing as well as in controlling maximum mitochondrial respiratory rates has been discussed. It was shown to contain a redox active and acid labile [2Fe-2S] cluster which is ligated by one histidine and three cysteine residues. Herein we present the first synthetic analogue with biomimetic {SN/S2} ligation which could be structurally characterized in its diferric form, 52-. In addition to being a high fidelity structural model for the biological cofactor, the complex is shown to mediate proton coupled electron transfer (PCET) at the {SN} ligated site, pointing at a potential functional role of the enzyme's unique His ligand. Full PCET thermodynamic square schemes for the mitoNEET model 52- and a related homoleptic {SN/SN} capped [2Fe-2S] cluster 42- are established, and kinetics of PCET reactivity are investigated by double-mixing stopped-flow experiments for both complexes. While the N-H bond dissociation free energy (BDFE) of 5H2- (230 ± 4 kJ mol-1) and the free energy ΔG°PCET for the reaction with TEMPO (-48.4 kJ mol-1) are very similar to values for the homoleptic cluster 4H2- (232 ± 4 kJ mol-1, -46.3 kJ mol-1) the latter is found to react significantly faster than the mitoNEET model (data for 5H2-: k = 135 ± 27 M-1 s-1, ΔH‡ = 17.6 ± 3.0 kJ mol-1, ΔS‡ = -143 ± 11 J mol-1 K-1, and ΔG‡ = 59.8 kJ mol-1 at 293 K). Comparison of the PCET efficiency of these clusters emphasizes the relevance of reorganization energy in this process.

Pioglitazone inhibits mitochondrial pyruvate metabolism and glucose production in hepatocytes

Pioglitazone is used globally for the treatment of type 2 diabetes mellitus (T2DM) and is one of the most effective therapies for improving glucose homeostasis and insulin resistance in T2DM patients. However, its mechanism of action in the tissues and pathways that regulate glucose metabolism are incompletely defined. Here we investigated the direct effects of pioglitazone on hepatocellular pyruvate metabolism and the dependency of these observations on the purported regulators of mitochondrial pyruvate transport, MPC1 and MPC2. In cultured H4IIE hepatocytes, pioglitazone inhibited [2-¹⁴ C]-pyruvate oxidation and pyruvate-driven oxygen consumption and, in mitochondria isolated from both hepatocytes and human skeletal muscle, pioglitazone selectively and dose-dependently inhibited pyruvate-driven ATP synthesis. Pioglitazone also suppressed hepatocellular glucose production (HGP), without influencing the mRNA expression of key HGP regulatory genes. Targeted siRNA silencing of MPC1 and 2 caused a modest inhibition of pyruvate oxidation and pyruvate-driven ATP synthesis, but did not alter pyruvate-driven HGP and, importantly, it did not influence the actions of pioglitazone on either pathway. In summary, these findings outline a novel mode of action of pioglitazone relevant to the pathogenesis of T2DM and suggest that targeting pyruvate metabolism may lead to the development of effective new T2DM therapies.

Binding of Nitric Oxide in CDGSH-type [2Fe-2S] Clusters of the Human Mitochondrial Protein Miner2

Iron-sulfur proteins are among the primary targets of nitric oxide in cells. Previous studies have shown that iron-sulfur clusters hosted by cysteine residues in proteins are readily disrupted by nitric oxide forming a protein-bound dinitrosyl iron complex, thiolate-bridged di-iron tetranitrosyl complex, or octanitrosyl cluster. Here we report that human mitochondrial protein Miner2 [2Fe-2S] clusters can bind nitric oxide without disruption of the clusters. Miner2 is a member of a new CDGSH iron-sulfur protein family that also includes two mitochondrial proteins: the type II diabetes-related mitoNEET and the Wolfram syndrome 2-linked Miner1. Miner2 contains two CDGSH motifs, and each CDGSH motif hosts a [2Fe-2S] cluster via three cysteine and one histidine residues. Binding of nitric oxide in the reduced Miner2 [2Fe-2S] clusters produces a major absorption peak at 422 nm without releasing iron or sulfide from the clusters. The EPR measurements and mass spectrometry analyses further reveal that nitric oxide binds to the reduced [2Fe-2S] clusters in Miner2, with each cluster binding one nitric oxide. Although the [2Fe-2S] cluster in purified human mitoNEET and Miner1 fails to bind nitric oxide, a single mutation of Asp-96 to Val in mitoNEET or Asp-123 to Val in Miner1 facilitates nitric oxide binding in the [2Fe-2S] cluster, indicating that a subtle change of protein structure may switch mitoNEET and Miner1 to bind nitric oxide. The results suggest that binding of nitric oxide in the CDGSH-type [2Fe-2S] clusters in mitochondrial protein Miner2 may represent a new nitric oxide signaling mode in cells.

Flavin nucleotides act as electron shuttles mediating reduction of the [2Fe-2S] clusters in mitochondrial outer membrane protein mitoNEET

MitoNEET, a primary target of type II diabetes drug pioglitazone, has an essential role in regulating energy metabolism, iron homeostasis, and production of reactive oxygen species in mitochondria. Structurally, mitoNEET is anchored to the mitochondrial outer membrane via its N-terminal transmembrane α-helix. The C-terminal cytosolic domain of mitoNEET hosts a redox active [2Fe-2S] cluster via three cysteine and one histidine residues. Here we report that the reduced flavin nucleotides can rapidly reduce the mitoNEET [2Fe-2S] clusters under anaerobic or aerobic conditions. In the presence of NADH and flavin reductase, 1 molecule of flavin nucleotide is sufficient to reduce about 100 molecules of the mitoNEET [2Fe-2S] clusters in 4min under aerobic conditions. The electron paramagnetic resonance (EPR) measurements show that flavin mononucleotide (FMN), but not flavin adenine dinucleotide (FAD), has a specific interaction with mitoNEET. Molecular docking models further reveal that flavin mononucleotide binds mitoNEET at the region between the N-terminal transmembrane α-helix and the [2Fe-2S] cluster binding domain. The closest distance between the [2Fe-2S] cluster and the bound flavin mononucleotide in mitoNEET is about 10Å, which could facilitate rapid electron transfer from the reduced flavin nucleotide to the [2Fe-2S] cluster in mitoNEET. The results suggest that flavin nucleotides may act as electron shuttles to reduce the mitoNEET [2Fe-2S] clusters and regulate mitochondrial functions in human cells.

Protective Effect of PPARγ Agonists on Cerebellar Tissues Oxidative Damage in Hypothyroid Rats

The aim of the current study was to investigate the effects of peroxisome proliferator-activated receptor gamma (PPARγ) agonists on cerebellar tissues oxidative damage in hypothyroid rats. The animals included seven groups: group I (control), the animals received drinking water; group II, the animals received 0.05% propylthiouracil (PTU) in drinking water; besides PTU, the animals in groups III, IV, V, VI, and VII, were injected with 20 mg/kg vitamin E (Vit E), 10 or 20 mg/kg pioglitazone, and 2 or 4 mg/kg rosiglitazone, respectively. The animals were deeply anesthetized and the cerebellar tissues were removed for biochemical measurements. PTU administration reduced thiol content, superoxide dismutase (SOD), and catalase (CAT) activities in the cerebellar tissues while increasing malondialdehyde (MDA) and nitric oxide (NO) metabolites. Vit E, pioglitazone, and rosiglitazone increased thiol, SOD, and CAT in the cerebellar tissues while reducing MDA and NO metabolites. The results of present study showed that, similar to Vit E, both rosiglitazone and pioglitazone as PPARγ agonists exerted protective effects against cerebellar tissues oxidative damage in hypothyroid rats.

Distinguishing the Protonation State of the Histidine Ligand to the Oxidized Iron-Sulfur Cluster from the MitoNEET Family of Proteins

The iron-sulfur cluster located in the recently discovered human mitoNEET protein (and related proteins) is structurally similar to the more well-known ferredoxin and Rieske clusters. Although its biological function is uncertain, the iron-sulfur cluster in mitoNEET has been proposed to undergo proton-coupled electron transfer involving the histidine ligand to the cluster. The cluster is also released from the protein at low pH. This contribution reports density functional calculations to model the structures, vibrations, and Heisenberg coupling constants (J) for high-spin (HS), broken symmetry (BS) singlet, and extended broken symmetry (EBS) singlet states of the oxidized iron-sulfur cluster from mitoNEET. This work suggests that J values or ¹⁵ N isotopic frequency shifts may provide methods for determining experimentally whether the histidine ligand to the oxidized iron-sulfur cluster in human mitoNEET and mitoNEET-related proteins is protonated or deprotonated.

PPARγ as a therapeutic target to rescue mitochondrial function in neurological disease

There is increasing evidence for the involvement of mitochondrial dysfunction and oxidative stress in the pathogenesis of many of the major neurodegenerative and neuroinflammatory diseases, suggesting that mitochondrial and antioxidant pathways may represent potential novel therapeutic targets. Recent years have seen a rapidly growing interest in the use of therapeutic strategies that can limit the defects in, or even to restore, mitochondrial function while reducing free radical generation. The peroxisome proliferation-activated receptor gamma (PPARγ), a ligand-activated transcription factor, has a wide spectrum of biological functions, regulating mitochondrial function, mitochondrial turnover, energy metabolism, antioxidant defence and redox balance, immune responses and fatty acid oxidation. In this review, we explore the evidence for potential beneficial effects of PPARγ agonists in a number of neurological disorders, including Parkinson's disease, Alzheimer's disease, Amyotrophic lateral sclerosis and Huntington's disease, ischaemia, autoimmune encephalomyelitis and neuropathic pain. We discuss the mechanisms underlying those beneficial effects in particular in relation to mitochondrial function, antioxidant defence, cell death and inflammation, and suggest that the PPARγ agonists show significant promise as therapeutic agents in otherwise intractable neurological disease.

MicroRNA-127 targeting of mitoNEET inhibits neurite outgrowth, induces cell apoptosis and contributes to physiological dysfunction after spinal cord transection

Neuroregeneration and apoptosis are two important pathophysiologic changes after spinal cord injury (SCI), but their underlying mechanisms remain unclear. MicroRNAs (miRNAs) play a crucial role in the regulation of neuroregeneration and neuronal apoptosis, research areas that have been greatly expanded in recent years. Here, using miRNA arrays to profile miRNA transcriptomes, we demonstrated that miR-127-3p was significantly down-regulated after spinal cord transection (SCT). Then, bioinformatics analyses and experimental detection showed that miR-127-3p exhibited specific effects on the regulation of neurite outgrowth and the induction of neuronal apoptosis by regulating the expression of the mitochondrial membrane protein mitoNEET. Moreover, knockdown of MitoNEET leaded to neuronal loss and apoptosis in primary cultured spinal neurons. This study therefore revealed that miR-127-3p, which targets mitoNEET, plays a vital role in regulating neurite outgrowth and neuronal apoptosis after SCT. Thus, modificatioin of the mitoNEET expression, such as mitoNEET activition may provide a new strategy for the treatment of SCI in preclinical trials.

Insulin resistance and Parkinson's disease: A new target for disease modification?

There is growing evidence that patients with Type 2 diabetes have an increased risk of developing Parkinson's disease and share similar dysregulated pathways suggesting common underlying pathological mechanisms. Historically insulin was thought solely to be a peripherally acting hormone responsible for glucose homeostasis and energy metabolism. However accumulating evidence indicates insulin can cross the blood-brain-barrier and influence a multitude of processes in the brain including regulating neuronal survival and growth, dopaminergic transmission, maintenance of synapses and pathways involved in cognition. In conjunction, there is growing evidence that a process analogous to peripheral insulin resistance occurs in the brains of Parkinson's disease patients, even in those without diabetes. This raises the possibility that defective insulin signalling pathways may contribute to the development of the pathological features of Parkinson's disease, and thereby suggests that the insulin signalling pathway may potentially be a novel target for disease modification. Given these growing links between PD and Type 2 diabetes it is perhaps not unsurprising that drugs used the treatment of T2DM are amongst the most promising treatments currently being prioritised for repositioning as possible novel treatments for PD and several clinical trials are under way. In this review, we will examine the underlying cellular links between insulin resistance and the pathogenesis of PD and then we will assess current and future pharmacological strategies being developed to restore neuronal insulin signalling as a potential strategy for slowing neurodegeneration in Parkinson's disease.

Identification of small molecules that bind to the mitochondrial protein mitoNEET

MitoNEET (CISD1) is a 2Fe-2S iron-sulfur cluster protein belonging to the zinc-finger protein family. Recently mitoNEET has been shown to be a major role player in the mitochondrial function associated with metabolic type diseases such as obesity and cancers. The anti-diabetic drug pioglitazone and rosiglitazone were the first identified ligands to mitoNEET. Since little is known about structural requirements for ligand binding to mitoNEET, we screened a small set of compounds to gain insight into these requirements. We found that the thiazolidinedione (TZD) warhead as seen in rosiglitazone was not an absolutely necessity for binding to mitoNEET. These results will aid in the development of novel compounds that can be used to treat mitochondrial dysfunction seen in several diseases.

MitoNEET Deficiency Alleviates Experimental Alcoholic Steatohepatitis in Mice by Stimulating Endocrine Adiponectin-Fgf15 Axis

MitoNEET (mNT) (CDGSH iron-sulfur domain-containing protein 1 or CISD1) is an outer mitochondrial membrane protein that donates 2Fe-2S clusters to apo-acceptor proteins. In the present study, using a global mNT knock-out (mNTKO) mouse model, we investigated the in vivo functional role of mNT in the development of alcoholic steatohepatitis. Experimental alcoholic steatohepatitis was achieved by pair feeding wild-type (WT) and mNTKO mice with Lieber-DeCarli ethanol-containing diets for 4 weeks. Strikingly, chronically ethanol-fed mNTKO mice were completely resistant to ethanol-induced steatohepatitis as revealed by dramatically reduced hepatic triglycerides, decreased hepatic cholesterol level, diminished liver inflammatory response, and normalized serum ALT levels. Mechanistic studies demonstrated that ethanol administration to mNTKO mice induced two pivotal endocrine hormones, namely, adipose-derived adiponectin and gut-derived fibroblast growth factor 15 (Fgf15). The elevation in circulating levels of adiponectin and Fgf15 led to normalized hepatic and serum levels of bile acids, limited hepatic accumulation of toxic bile, attenuated inflammation, and amelioration of liver injury in the ethanol-fed mNTKO mice. Other potential mechanisms such as reduced oxidative stress, activated Sirt1 signaling, and diminished NF-κB activity also contribute to hepatic improvement in the ethanol-fed mNTKO mice. In conclusion, the present study identified adiponectin and Fgf15 as pivotal adipose-gut-liver metabolic coordinators in mediating the protective action of mNT deficiency against development of alcoholic steatohepatitis in mice. Our findings may help to establish mNT as a novel therapeutic target and pharmacological inhibition of mNT may be beneficial for the prevention and treatment of human alcoholic steatohepatitis.

MitoNEET-Parkin Effects in Pancreatic α- and β-Cells, Cellular Survival, and Intrainsular Cross Talk

Mitochondrial metabolism plays an integral role in glucose-stimulated insulin secretion (GSIS) in β-cells. In addition, the diabetogenic role of glucagon released from α-cells plays a major role in the etiology of both type 1 and type 2 diabetes because unopposed hyperglucagonemia is a pertinent contributor to diabetic hyperglycemia. Titrating expression levels of the mitochondrial protein mitoNEET is a powerful approach to fine-tune mitochondrial capacity of cells. Mechanistically, β-cell-specific mitoNEET induction causes hyperglycemia and glucose intolerance due to activation of a Parkin-dependent mitophagic pathway, leading to the formation of vacuoles and uniquely structured mitophagosomes. Induction of mitoNEET in α-cells leads to fasting-induced hypoglycemia and hypersecretion of insulin during GSIS. MitoNEET-challenged α-cells exert potent antiapoptotic effects on β-cells and prevent cellular dysfunction associated with mitoNEET overexpression in β-cells. These observations identify that reduced mitochondrial function in α-cells exerts potently protective effects on β-cells, preserving β-cell viability and mass.

Iron Regulation of Pancreatic Beta-Cell Functions and Oxidative Stress

Dietary advice is the cornerstone in first-line treatment of metabolic diseases. Nutritional interventions directed at these clinical conditions mainly aim to (a) improve insulin resistance by reducing energy-dense macronutrient intake to obtain weight loss and (b) reduce fluctuations in insulin secretion through avoidance of rapidly absorbable carbohydrates. However, even in the majority of motivated patients selected for clinical trials, massive efforts using this approach have failed to achieve lasting efficacy. Less attention has been given to the role of micronutrients in metabolic diseases. Here, we review the evidence that highlights (a) the importance of iron in pancreatic beta-cell function and dysfunction in diabetes and (b) the integrative pathophysiological effects of tissue iron levels in the interactions among the beta cell, gut microbiome, hypothalamus, innate and adaptive immune systems, and insulin-sensitive tissues. We propose that clinical trials are warranted to clarify the impact of dietary or pharmacological iron reduction on the development of metabolic disorders.

Redox Control of the Human Iron-Sulfur Repair Protein MitoNEET Activity via Its Iron-Sulfur Cluster

Human mitoNEET (mNT) is the first identified Fe-S protein of the mammalian outer mitochondrial membrane. Recently, mNT has been implicated in cytosolic Fe-S repair of a key regulator of cellular iron homeostasis. Here, we aimed to decipher the mechanism by which mNT triggers its Fe-S repair capacity. By using tightly controlled reactions combined with complementary spectroscopic approaches, we have determined the differential roles played by both the redox state of the mNT cluster and dioxygen in cluster transfer and protein stability. We unambiguously demonstrated that only the oxidized state of the mNT cluster triggers cluster transfer to a generic acceptor protein and that dioxygen is neither required for the cluster transfer reaction nor does it affect the transfer rate. In the absence of apo-acceptors, a large fraction of the oxidized holo-mNT form is converted back to reduced holo-mNT under low oxygen tension. Reduced holo-mNT, which holds a [2Fe-2S](+)with a global protein fold similar to that of the oxidized form is, by contrast, resistant in losing its cluster or in transferring it. Our findings thus demonstrate that mNT uses an iron-based redox switch mechanism to regulate the transfer of its cluster. The oxidized state is the "active state," which reacts promptly to initiate Fe-S transfer independently of dioxygen, whereas the reduced state is a "dormant form." Finally, we propose that the redox-sensing function of mNT is a key component of the cellular adaptive response to help stress-sensitive Fe-S proteins recover from oxidative injury.

Mitochondrial specific therapeutic targets following brain injury

Traumatic brain injury is a complicated disease to treat due to the complex multi-factorial secondary injury cascade that is initiated following the initial impact. This secondary injury cascade causes nonmechanical tissue damage, which is where therapeutic interventions may be efficacious for intervention. One therapeutic target that has shown much promise following brain injury are mitochondria. Mitochondria are complex organelles found within the cell. At a superficial level, mitochondria are known to produce the energy substrate used within the cell called ATP. However, their importance to overall cellular homeostasis is even larger than their production of ATP. These organelles are necessary for calcium cycling, ROS production and play a role in the initiation of cell death pathways. When mitochondria become dysfunctional, they can become dysregulated leading to a loss of cellular homeostasis and eventual cell death. Within this review there will be a deep discussion into mitochondrial bioenergetics followed by a brief discussion into traumatic brain injury and how mitochondria play an integral role in the neuropathological sequelae following an injury. The review will conclude with a discussion pertaining to the therapeutic approaches currently being studied to ameliorate mitochondrial dysfunction following brain injury. This article is part of a Special Issue entitled SI:Brain injury and recovery.

His-87 ligand in mitoNEET is crucial for the transfer of iron sulfur clusters from mitochondria to cytosolic aconitase

MitoNEET is the first identified iron sulfur protein that located in the mitochondrial outer membrane. We showed that knockdown of mitoNEET did not affect the iron sulfur protein expression in mitochondria and cytoplasm, but significantly reduced the cytosolic aconitase activity. The reduction of aconitase activity was rescued by transfection of wild type mitoNEET, but not by mitoNEET mutants H87C and H87S. Our results confirm the observation that mitoNEET is important in transferring the iron sulfur clusters to the cytosolic aconitase in living cells and the His-87 ligand in mitoNEET plays important role in this process.

CISD1 in association with obesity-associated dysfunctional adipogenesis in human visceral adipose tissue

OBJECTIVE

To investigate CISD1 mRNA and protein in human adipose tissue in association with obesity and adipogenesis.

METHODS

Subcutaneous (SAT) and visceral (VAT) adipose tissue CISD1 gene expression (real-time PCR) and protein (Western blot) levels were investigated in human adipose tissue and during human adipocyte differentiation.

RESULTS

SAT and VAT CISD1 mRNA and protein levels were significantly decreased in subjects with obesity and negatively correlated with BMI after controlling for age and sex. In participants with morbid obesity, VAT CISD1 gene expression was positively correlated with insulin sensitivity (r = 0.47, P = 0.01), and bariatric surgery-induced weight loss led to increased SAT CISD1 mRNA levels. In both VAT and SAT, CISD1 gene expression was significantly associated with SIRT1, ISCA2, and mitochondrial biogenesis-related (PPARGC1A, TFAM, and MT-CO3) and browning-related (PRDM16 and UCP1) gene expression. In addition, VAT CISD1 gene expression was significantly associated with adipogenic and iron metabolism-related genes. Importantly, these correlations were replicated in a second cohort. At the cellular level, CISD1 gene expression increased during human adipocyte differentiation in correlation with adipogenic genes (r > 0.60, P < 0.005).

CONCLUSIONS

These data suggest a possible role of CISD1 in obesity-associated dysfunctional adipogenesis in human VAT.

Iron metabolism and regulation by neutrophil gelatinase-associated lipocalin in cardiomyopathy

Neutrophil gelatinase-associated lipocalin (NGAL) has recently become established as an important contributor to the pathophysiology of cardiovascular disease. Accordingly, it is now viewed as an attractive candidate as a biomarker for various disease states, and in particular has recently become regarded as one of the best diagnostic biomarkers available for acute kidney injury. Nevertheless, the precise physiological effects of NGAL on the heart and the significance of their alterations during the development of heart failure are only now beginning to be characterized. Furthermore, the mechanisms via which NGAL mediates its effects are unclear because there is no conventional receptor signalling pathway. Instead, previous work suggests that regulation of iron metabolism could represent an important mechanism of NGAL action, with wide-ranging consequences spanning metabolic and cardiovascular diseases to host defence against bacterial infection. In the present review, we summarize rapidly emerging evidence for the role of NGAL in regulating heart failure. In particular, we focus on iron transport as a mechanism of NGAL action and discuss this in the context of the existing strong associations between iron overload and iron deficiency with cardiomyopathy.

Cancer-Related NEET Proteins Transfer 2Fe-2S Clusters to Anamorsin, a Protein Required for Cytosolic Iron-Sulfur Cluster Biogenesis

Iron-sulfur cluster biogenesis is executed by distinct protein assembly systems. Mammals have two systems, the mitochondrial Fe-S cluster assembly system (ISC) and the cytosolic assembly system (CIA), that are connected by an unknown mechanism. The human members of the NEET family of 2Fe-2S proteins, nutrient-deprivation autophagy factor-1 (NAF-1) and mitoNEET (mNT), are located at the interface between the mitochondria and the cytosol. These proteins have been implicated in cancer cell proliferation, and they can transfer their 2Fe-2S clusters to a standard apo-acceptor protein. Here we report the first physiological 2Fe-2S cluster acceptor for both NEET proteins as human Anamorsin (also known as cytokine induced apoptosis inhibitor-1; CIAPIN-1). Anamorsin is an electron transfer protein containing two iron-sulfur cluster-binding sites that is required for cytosolic Fe-S cluster assembly. We show, using UV-Vis spectroscopy, that both NAF-1 and mNT can transfer their 2Fe-2S clusters to apo-Anamorsin with second order rate constants similar to those of other known human 2Fe-2S transfer proteins. A direct protein-protein interaction of the NEET proteins with apo-Anamorsin was detected using biolayer interferometry. Furthermore, electrospray mass spectrometry of holo-Anamorsin prepared by cluster transfer shows that it receives both of its 2Fe-2S clusters from the NEETs. We propose that mNT and NAF-1 can provide parallel routes connecting the mitochondrial ISC system and the CIA. 2Fe-2S clusters assembled in the mitochondria are received by NEET proteins and when needed transferred to Anamorsin, activating the CIA.

Loss of Mitochondrial Pyruvate Carrier 2 in the Liver Leads to Defects in Gluconeogenesis and Compensation via Pyruvate-Alanine Cycling

Pyruvate transport across the inner mitochondrial membrane is believed to be a prerequisite for gluconeogenesis in hepatocytes, which is important for the maintenance of normoglycemia during prolonged food deprivation but also contributes to hyperglycemia in diabetes. To determine the requirement for mitochondrial pyruvate import in gluconeogenesis, mice with liver-specific deletion of mitochondrial pyruvate carrier 2 (LS-Mpc2(-/-)) were generated. Loss of MPC2 impaired, but did not completely abolish, hepatocyte conversion of labeled pyruvate to TCA cycle intermediates and glucose. Unbiased metabolomic analyses of livers from fasted LS-Mpc2(-/-) mice suggested that alterations in amino acid metabolism, including pyruvate-alanine cycling, might compensate for the loss of MPC2. Indeed, inhibition of pyruvate-alanine transamination further reduced mitochondrial pyruvate metabolism and glucose production by LS-Mpc2(-/-) hepatocytes. These data demonstrate an important role for MPC2 in controlling hepatic gluconeogenesis and illuminate a compensatory mechanism for circumventing a block in mitochondrial pyruvate import.

Reduction of mitochondrial protein mitoNEET [2Fe-2S] clusters by human glutathione reductase

The human mitochondrial outer membrane protein mitoNEET is a newly discovered target of the type 2 diabetes drug pioglitazone. Structurally, mitoNEET is a homodimer with each monomer containing an N-terminal transmembrane α helix tethered to the mitochondrial outer membrane and a C-terminal cytosolic domain hosting a redox-active [2Fe-2S] cluster. Genetic studies have shown that mitoNEET has a central role in regulating energy metabolism in mitochondria. However, the specific function of mitoNEET remains largely elusive. Here we find that the mitoNEET [2Fe-2S] clusters can be efficiently reduced by Escherichia coli thioredoxin reductase and glutathione reductase in an NADPH-dependent reaction. Purified human glutathione reductase has the same activity as E. coli thioredoxin reductase and glutathione reductase to reduce the mitoNEET [2Fe-2S] clusters. However, rat thioredoxin reductase, a human thioredoxin reductase homolog that contains selenocysteine in the catalytic center, has very little or no activity to reduce the mitoNEET [2Fe-2S] clusters. N-ethylmaleimide, a potent thiol modifier, completely inhibits human glutathione reductase from reducing the mitoNEET [2Fe-2S] clusters, indicating that the redox-active disulfide in the catalytic center of human glutathione reductase may be directly involved in reducing the mitoNEET [2Fe-2S] clusters. Additional studies reveal that the reduced mitoNEET [2Fe-2S] clusters in mouse heart cell extracts can be reversibly oxidized by hydrogen peroxide without disruption of the clusters, suggesting that the mitoNEET [2Fe-2S] clusters may undergo redox transition to regulate energy metabolism in mitochondria in response to oxidative signals.

Novel thiazolidinedione mitoNEET ligand-1 acutely improves cardiac stem cell survival under oxidative stress

Ischemic heart disease (IHD) is a leading cause of death worldwide, and regenerative therapies through exogenous stem cell delivery hold promising potential. One limitation of such therapies is the vulnerability of stem cells to the oxidative environment associated with IHD. Accordingly, manipulation of stem cell mitochondrial metabolism may be an effective strategy to improve survival of stem cells under oxidative stress. MitoNEET is a redox-sensitive, mitochondrial target of thiazolidinediones (TZDs), and influences cellular oxidative capacity. Pharmacological targeting of mitoNEET with the novel TZD, mitoNEET Ligand-1 (NL-1), improved cardiac stem cell (CSC) survival compared to vehicle (0.1% DMSO) during in vitro oxidative stress (H2O2). 10 μM NL-1 also reduced CSC maximal oxygen consumption rate (OCR) compared to vehicle. Following treatment with dexamethasone, CSC maximal OCR increased compared to baseline, but NL-1 prevented this effect. Smooth muscle α-actin expression increased significantly in CSC following differentiation compared to baseline, irrespective of NL-1 treatment. When CSCs were treated with glucose oxidase for 7 days, NL-1 significantly improved cell survival compared to vehicle (trypan blue exclusion). NL-1 treatment of cells isolated from mitoNEET knockout mice did not increase CSC survival with H2O2 treatment. Following intramyocardial injection of CSCs into Zucker obese fatty rats, NL-1 significantly improved CSC survival after 24 h, but not after 10 days. These data suggest that pharmacological targeting of mitoNEET with TZDs may acutely protect stem cells following transplantation into an oxidative environment. Continued treatment or manipulation of mitochondrial metabolism may be necessary to produce long-term benefits related to stem cell therapies.

Advanced paramagnetic resonance spectroscopies of iron-sulfur proteins: Electron nuclear double resonance (ENDOR) and electron spin echo envelope modulation (ESEEM)

The advanced electron paramagnetic resonance (EPR) techniques, electron nuclear double resonance (ENDOR) and electron spin echo envelope modulation (ESEEM) spectroscopies, provide unique insights into the structure, coordination chemistry, and biochemical mechanism of nature's widely distributed iron-sulfur cluster (FeS) proteins. This review describes the ENDOR and ESEEM techniques and then provides a series of case studies on their application to a wide variety of FeS proteins including ferredoxins, nitrogenase, and radical SAM enzymes. This article is part of a Special Issue entitled: Fe/S proteins: Analysis, structure, function, biogenesis and diseases.

Physiological levels of resveratrol metabolites are ineffective as anti-leukemia agents against Jurkat leukemia cells

Dietary resveratrol is metabolically transformed in vivo by the intestine and liver to produce resveratrol glucuronides and sulfates in humans. Little is known about the anticancer activities of these metabolic products. The majority of in vitro studies have investigated effects of resveratrol aglycone at supraphysiological levels. Physiological levels of resveratrol-3-O-glucuronide, resveratrol-4'-O-glucuronide, and resveratrol-3-O-sulfate, the major in vivo metabolites of dietary resveratrol, were evaluated as anticancer agents against Jurkat T leukemia cells. Propidium iodide was use to measure cell death and changes in cell cycle, and the mitochondrial membrane dye JC-1 was used to measure changes in mitochondrial membrane potential by flow cytometry. PKH67 was used to evaluate changes in proliferation of the cells by flow cytometry. Jurkat cells were exposed to 0, 2.5, 5, 10, 15, and 20 μM of each resveratrol metabolite, which are concentrations achievable in vivo. None of the resveratrol metabolites were able to kill Jurkat T leukemia cells or alter cell cycle or proliferation at these concentrations. Only resveratrol-3-O-sulfate induced depolarization of mitochondrial membranes but without induction of cell death. These results suggest that the in vivo transformation of resveratrol to these glucuronide and sulfate metabolites renders these agents ineffective against T leukemia cells.

Photoaffinity labeling in target- and binding-site identification

Photoaffinity labeling (PAL) using a chemical probe to covalently bind its target in response to activation by light has become a frequently used tool in drug discovery for identifying new drug targets and molecular interactions, and for probing the location and structure of binding sites. Methods to identify the specific target proteins of hit molecules from phenotypic screens are highly valuable in early drug discovery. In this review, we summarize the principles of PAL including probe design and experimental techniques for in vitro and live cell investigations. We emphasize the need to optimize and validate probes and highlight examples of the successful application of PAL across multiple disease areas.

A novel MitoNEET ligand, TT01001, improves diabetes and ameliorates mitochondrial function in db/db mice

The mitochondrial outer membrane protein mitoNEET is a binding protein of the insulin sensitizer pioglitazone (5-[[4-[2-(5-ethylpyridin-2-yl)ethoxy]phenyl]methyl]-1,3-thiazolidine-2,4-dione) and is considered a novel target for the treatment of type II diabetes. Several small-molecule compounds have been identified as mitoNEET ligands using structure-based design or virtual docking studies. However, there are no reports about their therapeutic potential in animal models. Recently, we synthesized a novel small molecule, TT01001 [ethyl-4-(3-(3,5-dichlorophenyl)thioureido)piperidine-1-carboxylate], designed on the basis of pioglitazone structure. In this study, we assessed the pharmacological properties of TT01001 in both in vitro and in vivo studies. We found that TT01001 bound to mitoNEET without peroxisome proliferator-activated receptor-γ activation effect. In type II diabetes model db/db mice, TT01001 improved hyperglycemia, hyperlipidemia, and glucose intolerance, and its efficacy was equivalent to that of pioglitazone, without the pioglitazone-associated weight gain. Mitochondrial complex II + III activity of the skeletal muscle was significantly increased in db/db mice. We found that TT01001 significantly suppressed the elevated activity of the complex II + III. These results suggest that TT01001 improved type II diabetes without causing weight gain and ameliorated mitochondrial function of db/db mice. This is the first study that demonstrates the effects of a mitoNEET ligand on glucose metabolism and mitochondrial function in an animal disease model. These findings support targeting mitoNEET as a potential therapeutic approach for the treatment of type II diabetes.

The ongoing pursuit of neuroprotective therapies in Parkinson disease

Many agents developed for neuroprotective treatment of Parkinson disease (PD) have shown great promise in the laboratory, but none have translated to positive results in patients with PD. Potential neuroprotective drugs, such as ubiquinone, creatine and PYM50028, have failed to show any clinical benefits in recent high-profile clinical trials. This 'failure to translate' is likely to be related primarily to our incomplete understanding of the pathogenic mechanisms underlying PD, and excessive reliance on data from toxin-based animal models to judge which agents should be selected for clinical trials. Restricted resources inevitably mean that difficult compromises must be made in terms of trial design, and reliable estimation of efficacy is further hampered by the absence of validated biomarkers of disease progression. Drug development in PD dementia has been mostly unsuccessful; however, emerging biochemical, genetic and pathological evidence suggests a link between tau and amyloid-β deposition and cognitive decline in PD, potentially opening up new possibilities for therapeutic intervention. This Review discusses the most important 'druggable' disease mechanisms in PD, as well as the most-promising drugs that are being evaluated for their potential efficiency in treatment of motor and cognitive impairments in PD.

PPAR gamma activation is neuroprotective in a Drosophila model of ALS based on TDP-43

Amyotrophic Lateral Sclerosis (ALS) is a progressive neuromuscular disease for which there is no cure. We have previously developed a Drosophila model of ALS based on TDP-43 that recapitulates several aspects of disease pathophysiology. Using this model, we designed a drug screening strategy based on the pupal lethality phenotype induced by TDP-43 when expressed in motor neurons. In screening 1200 FDA-approved compounds, we identified the PPARγ agonist pioglitazone as neuroprotective in Drosophila. Here, we show that pioglitazone can rescue TDP-43-dependent locomotor dysfunction in motor neurons and glia but not in muscles. Testing additional models of ALS, we find that pioglitazone is also neuroprotective when FUS, but not SOD1, is expressed in motor neurons. Interestingly, survival analyses of TDP or FUS models show no increase in lifespan, which is consistent with recent clinical trials. Using a pharmacogenetic approach, we show that the predicted Drosophila PPARγ homologs, E75 and E78, are in vivo targets of pioglitazone. Finally, using a global metabolomic approach, we identify a set of metabolites that pioglitazone can restore in the context of TDP-43 expression in motor neurons. Taken together, our data provide evidence that modulating PPARγ activity, although not effective at improving lifespan, provides a molecular target for mitigating locomotor dysfunction in TDP-43 and FUS but not SOD1 models of ALS in Drosophila. Furthermore, our data also identify several 'biomarkers' of the disease that may be useful in developing therapeutics and in future clinical trials.