Redox control of human mitochondrial outer membrane protein MitoNEET [2Fe-2S] clusters by biological thiols and hydrogen peroxide

Loss of the mitochondrial protein mitoNEET in mice is associated with cognitive impairments and increased neuroinflammation

BACKGROUND

Mitochondrial dysfunction is implicated in several neurodegenerative diseases associated with memory and cognitive deficits, including Alzheimer's disease. Changes in bioenergetic function results in reactive oxygen species, oxidative damage and consequently neuroinflammation, which contributes to neuronal cell loss.

OBJECTIVE

In this study, we evaluated the impact of the loss of the redox active [2Fe-2S] mitochondrial-associated protein mitoNEET (CISD1) on neuroinflammation and cognition using an age-appropriate preclinical model. While associations between neuroinflammation and poor cognitive impacts have been shown in recent work, little has been done to assess whether loss of mitoNEET is associated with changes in neuroinflammatory markers or negative cognitive-behavioral outcomes.

METHODS

Using 9-11-month-old mitoNEET knockout (CISD1-/-) and wild-type mice, we conducted a battery of cognitive tests to assess the impact of mitoNEET loss on performance. We then histologically evaluated the effect of absence of mitoNEET on markers of neuroinflammation in the aged brain.

RESULTS

We found loss of mitoNEET in mice was associated with a significant reduction in willingness to explore within an open field and impaired short-term spatial working memory in the Y-maze. We also found a significant reduction in novel object recognition memory that was gene-dependent and accompanied by reduced c-fos expression in hippocampus and cortical regions.

CONCLUSIONS

Our findings indicate that mitoNEET loss is significantly associated with impairments in cognitive-behavioral and neuroinflammatory outcomes; specifically, learning and memory, anxiety-like behaviors, neuroinflammation, and neural activation. This is the first study to demonstrate cognitive-associated behavioral deficits with neuroinflammation in the mitoNEET knockout mouse model.

Regulations of mitoNEET by the key redox homeostasis molecule glutathione

Human mitoNEET (mNT) and CISD2 are two NEET proteins characterized by an atypical [2Fe-2S] cluster coordination involving three cysteines and one histidine. They act as redox switches with an active state linked to the oxidation of their cluster. In the present study, we show that reduced glutathione but also free thiol-containing molecules such as β-mercaptoethanol can induce a loss of the mNT cluster under aerobic conditions, while CISD2 cluster appears more resistant. This disassembly occurs through a radical-based mechanism as previously observed with the bacterial SoxR. Interestingly, adding cysteine prevents glutathione-induced cluster loss. At low pH, glutathione can bind mNT in the vicinity of the cluster. These results suggest a potential new regulation mechanism of mNT activity by glutathione, an essential actor of the intracellular redox state.

Biochemical and cellular characterization of the CISD3 protein: Molecular bases of cluster release and destabilizing effects of nitric oxide

The NEET proteins, an important family of iron-sulfur (Fe-S) proteins, have generated a strong interest due to their involvement in diverse diseases such as cancer, diabetes, and neurodegenerative disorders. Among the human NEET proteins, CISD3 has been the least studied, and its functional role is still largely unknown. We have investigated the biochemical features of CISD3 at the atomic and in cellulo levels upon challenge with different stress conditions i.e., iron deficiency, exposure to hydrogen peroxide, and nitric oxide. The redox and cellular stability properties of the protein agree on a predominance of reduced form of CISD3 in the cells. Upon the addition of iron chelators, CISD3 loses its Fe-S clusters and becomes unstructured, and its cellular level drastically decreases. Chemical shift perturbation measurements suggest that, upon cluster oxidation, the protein undergoes a conformational change at the C-terminal CDGSH domain, which determines the instability of the oxidized state. This redox-associated conformational change may be the source of cooperative electron transfer via the two [Fe2S2] clusters in CISD3, which displays a single sharp voltammetric signal at -31 mV versus SHE. Oxidized CISD3 is particularly sensitive to the presence of hydrogen peroxide in vitro, whereas only the reduced form is able to bind nitric oxide. Paramagnetic NMR provides clear evidence that, upon NO binding, the cluster is disassembled but iron ions are still bound to the protein. Accordingly, in cellulo CISD3 is unaffected by oxidative stress induced by hydrogen peroxide but it becomes highly unstable in response to nitric oxide treatment.

MitoNEET Provides Cardioprotection via Reducing Oxidative Damage and Conserving Mitochondrial Function

Cardiometabolic diseases exert a significant health impact, leading to a considerable economic burden globally. The metabolic syndrome, characterized by a well-defined cluster of clinical parameters, is closely linked to an elevated risk of cardiovascular disease. Current treatment strategies often focus on addressing individual aspects of metabolic syndrome. We propose that exploring novel therapeutic approaches that simultaneously target multiple facets may prove more effective in alleviating the burden of cardiometabolic disease. There is a growing body of evidence suggesting that mitochondria can serve as a pivotal target for the development of therapeutics aimed at resolving both metabolic and vascular dysfunction. MitoNEET was identified as a binding target for the thiazolidinedione (TZD) class of antidiabetic drugs and is now recognized for its role in regulating various crucial cellular processes. Indeed, mitoNEET has demonstrated promising potential as a therapeutic target in various chronic diseases, encompassing cardiovascular and metabolic diseases. In this review, we present a thorough overview of the molecular mechanisms of mitoNEET, with an emphasis on their implications for cardiometabolic diseases in more recent years. Furthermore, we explore the potential impact of these findings on the development of novel therapeutic strategies and discuss potential directions for future research.

Ancient complexes of iron and sulfur modulate oncogenes and oncometabolism

Inorganic complexes of iron and sulfur, that is, iron-sulfur [FeS] clusters, have played a fundamental role in life on Earth since the prebiotic period. These clusters were involved in elementary reactions leading to the emergence of life and, since then, gained function in processes, such as respiration, replication, transcription, and the immune response. We discuss how three [FeS] proteins involved in the innate immune response play a role in oncogene expression/function and oncometabolism. Our analysis highlights the importance of future research into understanding the [FeS] clusters' roles in cancer progression and proliferation. The outcomes of these studies will help identify new targets and develop new anticancer therapeutics.

PINK1 and Parkin regulate IP3R-mediated ER calcium release

Although defects in intracellular calcium homeostasis are known to play a role in the pathogenesis of Parkinson's disease (PD), the underlying molecular mechanisms remain unclear. Here, we show that loss of PTEN-induced kinase 1 (PINK1) and Parkin leads to dysregulation of inositol 1,4,5-trisphosphate receptor (IP3R) activity, robustly increasing ER calcium release. In addition, we identify that CDGSH iron sulfur domain 1 (CISD1, also known as mitoNEET) functions downstream of Parkin to directly control IP3R. Both genetic and pharmacologic suppression of CISD1 and its Drosophila homolog CISD (also known as Dosmit) restore the increased ER calcium release in PINK1 and Parkin null mammalian cells and flies, respectively, demonstrating the evolutionarily conserved regulatory mechanism of intracellular calcium homeostasis by the PINK1-Parkin pathway. More importantly, suppression of CISD in PINK1 and Parkin null flies rescues PD-related phenotypes including defective locomotor activity and dopaminergic neuronal degeneration. Based on these data, we propose that the regulation of ER calcium release by PINK1 and Parkin through CISD1 and IP3R is a feasible target for treating PD pathogenesis.

Biochemical Control of the Mitochondrial Protein MitoNEET by Biological Thiols and Lipid-derived Electrophiles

MitoNEET is a mitochondrial [2Fe-2S] protein known for its involvement in cellular metabolism, iron regulation, and oxidative stress. The protein has been associated with diseases ranging from diabetes to Parkinson's disease which has prompted development of compounds designed to selectively target mitoNEET. Unfortunately, drug development is limited due to a lack of understanding on the mechanistic level how mitoNEET integrates into pathophysiological processes. In particular, biological compounds that govern mitoNEET function are still ill defined. We demonstrate an oxygen-dependent reaction with biological thiols catalyzed by mitoNEET. Furthermore, we observed that formation of a covalently linked mitoNEET homodimer is controlled by both thiols and lipid-derived electrophiles. Finally, we demonstrate that reduced glutathione (L-GSH) regulates the reactivity of two lipid-derived biomarkers of oxidative stress, 4-HNE and 4-ONE, towards mitoNEET. We find that exposure to L-GSH prior to treatment with either of the electrophilic aldehydes prevents the formation of the covalently linked mitoNEET dimer. Meanwhile, addition of L-GSH after electrophile treatment recovers mitoNEET from the 4-HNE induced modification but not from the modification induced by 4-ONE. Our results collectively suggest that the thiol-electrophile redox balance governing ferroptotic cell death also controls mitoNEET's state at multiple biochemical levels. These results indicate a possible role for mitoNEET in thiol-mediated oxidative stress and may inform about development of probes designed to modulate mitoNEET activity to improve pathophysiological states.

The Intriguing mitoNEET: Functional and Spectroscopic Properties of a Unique [2Fe-2S] Cluster Coordination Geometry

Despite the number of cellular and pathological mitoNEET-related processes, very few details are known about the mechanism of action of the protein. The recently discovered existence of a link between NEET proteins and cancer pave the way to consider mitoNEET and its Fe-S clusters as suitable targets to inhibit cancer cell proliferation. Here, we will review the variety of spectroscopic techniques that have been applied to study mitoNEET in an attempt to explain the drastic difference in clusters stability and reactivity observed for the two redox states, and to elucidate the cellular function of the protein. In particular, the extensive NMR assignment and the characterization of first coordination sphere provide a molecular fingerprint helpful to assist the design of drugs able to impair cellular processes or to directly participate in redox reactions or protein-protein recognition mechanisms.

Ferroptosis Signaling in Pancreatic β-Cells: Novel Insights & Therapeutic Targeting

Metabolic stress impairs pancreatic β-cell survival and function in diabetes. Although the pathophysiology of metabolic stress is complex, aberrant tissue damage and β-cell death are brought on by an imbalance in redox equilibrium due to insufficient levels of endogenous antioxidant expression in β-cells. The vulnerability of β-cells to oxidative damage caused by iron accumulation has been linked to contributory β-cell ferroptotic-like malfunction under diabetogenic settings. Here, we take into account recent findings on how iron metabolism contributes to the deregulation of the redox response in diabetic conditions as well as the ferroptotic-like malfunction in the pancreatic β-cells, which may offer insights for deciphering the pathomechanisms and formulating plans for the treatment or prevention of metabolic stress brought on by β-cell failure.

MitoNEET's Reactivity of Lys55 toward Pyridoxal Phosphate Demonstrates its Activity as a Transaminase Enzyme

MitoNEET is a [2Fe-2S] redox active mitochondrial protein belonging to the CDGSH iron-sulfur domain (CISD) family of proteins. MitoNEET has been implicated as a potential target for drug development to treat various disorders, including type-2 diabetes, cancer, and Parkinson's disease. However, the specific cellular function(s) for mitoNEET still remains to be fully elucidated, and this presents a significant roadblock in rational drug development. Here, we show that mitoNEET binds the enzymatic cofactor pyridoxal phosphate (PLP) specifically at only one of its 11 lysine residues, Lys55. Lys55 is part of the soluble portion of the protein and is in a hydrogen-bonding network with the histidine residue that ligates the [2Fe-2S] cluster. In the presence of mitoNEET, PLP catalyzes the transamination reaction of the amino acid cysteine and the alpha-keto acid 2-oxoglutarate to form 3-mercaptopyruvate and glutamate. This work identifies, for the first time, mitoNEET as an enzyme with cysteine transaminase activity.

Nitric oxide reversibly binds the reduced [2Fe-2S] cluster in mitochondrial outer membrane protein mitoNEET and inhibits its electron transfer activity

MitoNEET is a mitochondrial outer membrane protein that regulates energy metabolism, iron homeostasis, and production of reactive oxygen species in cells. Aberrant expression of mitoNEET in tissues has been linked to type II diabetes, neurodegenerative diseases, and several types of cancer. Structurally, the N-terminal domain of mitoNEET has a single transmembrane alpha helix that anchors the protein to mitochondrial outer membrane. The C-terminal cytosolic domain of mitoNEET hosts a redox active [2Fe-2S] cluster via an unusual ligand arrangement of three cysteine and one histidine residues. Here we report that the reduced [2Fe-2S] cluster in the C-terminal cytosolic domain of mitoNEET (mitoNEET45-108) is able to bind nitric oxide (NO) without disruption of the cluster. Importantly, binding of NO at the reduced [2Fe-2S] cluster effectively inhibits the redox transition of the cluster in mitoNEET45-108. While the NO-bound [2Fe-2S] cluster in mitoNEET45-108 is stable, light excitation releases NO from the NO-bound [2Fe-2S] cluster and restores the redox transition activity of the cluster in mitoNEET45-108. The results suggest that NO may regulate the electron transfer activity of mitoNEET in mitochondrial outer membrane via reversible binding to its reduced [2Fe-2S] cluster.

The pathway for coenzyme M biosynthesis in bacteria

Mercaptoethane sulfonate or coenzyme M (CoM) is the smallest known organic cofactor and is most commonly associated with the methane-forming step in all methanogenic archaea but is also associated with the anaerobic oxidation of methane to CO₂ in anaerobic methanotrophic archaea and the oxidation of short-chain alkanes in Syntrophoarchaeum species. It has also been found in a small number of bacteria capable of the metabolism of small organics. Although many of the steps for CoM biosynthesis in methanogenic archaea have been elucidated, a complete pathway for the biosynthesis of CoM in archaea or bacteria has not been reported. Here, we present the complete CoM biosynthesis pathway in bacteria, revealing distinct chemical steps relative to CoM biosynthesis in methanogenic archaea. The existence of different pathways represents a profound instance of convergent evolution. The five-step pathway involves the addition of sulfite, the elimination of phosphate, decarboxylation, thiolation, and the reduction to affect the sequential conversion of phosphoenolpyruvate to CoM. The salient features of the pathway demonstrate reactivities for members of large aspartase/fumarase and pyridoxal 5'-phosphate-dependent enzyme families.

The Cluster Transfer Function of AtNEET Supports the Ferredoxin-Thioredoxin Network of Plant Cells

NEET proteins are conserved 2Fe-2S proteins that regulate the levels of iron and reactive oxygen species in plant and mammalian cells. Previous studies of seedlings with constitutive expression of AtNEET, or its dominant-negative variant H89C (impaired in 2Fe-2S cluster transfer), revealed that disrupting AtNEET function causes oxidative stress, chloroplast iron overload, activation of iron-deficiency responses, and cell death. Because disrupting AtNEET function is deleterious to plants, we developed an inducible expression system to study AtNEET function in mature plants using a time-course proteomics approach. Here, we report that the suppression of AtNEET cluster transfer function results in drastic changes in the expression of different members of the ferredoxin (Fd), Fd-thioredoxin (TRX) reductase (FTR), and TRX network of Arabidopsis, as well as in cytosolic cluster assembly proteins. In addition, the expression of Yellow Stripe-Like 6 (YSL6), involved in iron export from chloroplasts was elevated. Taken together, our findings reveal new roles for AtNEET in supporting the Fd-TFR-TRX network of plants, iron mobilization from the chloroplast, and cytosolic 2Fe-2S cluster assembly. In addition, we show that the AtNEET function is linked to the expression of glutathione peroxidases (GPXs), which play a key role in the regulation of ferroptosis and redox balance in different organisms.

Electron transfer activity of the nanodisc-bound mitochondrial outer membrane protein mitoNEET

MitoNEET is the first iron-sulfur protein found in mitochondrial outer membrane. Abnormal expression of mitoNEET in cells has been linked to several types of cancer, type II diabetes, and neurodegenerative diseases. Structurally, mitoNEET is anchored to mitochondrial outer membrane via its N-terminal single transmembrane alpha helix. The C-terminal cytosolic domain of mitoNEET binds a [2Fe-2S] cluster via three cysteine and one histidine residues. It has been shown that mitoNEET has a crucial role in energy metabolism, iron homeostasis, and free radical production in cells. However, the exact function of mitoNEET remains elusive. Previously, we reported that the C-terminal soluble domain of mitoNEET has a specific binding site for flavin mononucleotide (FMN) and can transfer electrons from FMNH2 to oxygen or ubiquinone-2 via its [2Fe-2S] cluster. Here we have constructed a hybrid protein using the N-terminal transmembrane domain of Escherichia coli YneM and the C-terminal soluble domain of human mitoNEET and assembled the hybrid protein YneM-mitoNEET into phospholipid nanodiscs. The results show that the [2Fe-S] clusters in the nanodisc-bound YneM-mitoNEET can be rapidly reduced by FMNH2 which is reduced by flavin reductase using NADH as the electron donor. Addition of lumichrome, a FMN analog, effectively inhibits the FMNH2-mediated reduction of the [2Fe-2S] clusters in the nanodisc-bound YneM-mitoNEET. The reduced [2Fe-2S] clusters in the nanodisc-bound YneM-mitoNEET are quickly oxidized by oxygen under aerobic conditions or by ubiquinone-10 in the nanodiscs under anaerobic conditions. Because NADH oxidation is required for cellular glycolytic activity, we propose that the mitochondrial outer membrane protein mitoNEET may promote glycolysis by transferring electrons from FMNH2 to oxygen or ubiquinone-10 in mitochondria.

Multiple Poses and Thermodynamics of Ligands Targeting Protein Surfaces: The Case of Furosemide Binding to mitoNEET in Aqueous Solution

Human NEET proteins, such as NAF-1 and mitoNEET, are homodimeric, redox iron-sulfur proteins characterized by triple cysteine and one histidine-coordinated [2Fe-2S] cluster. They exist in an oxidized and reduced state. Abnormal release of the cluster is implicated in a variety of diseases, including cancer and neurodegeneration. The computer-aided and structure-based design of ligands affecting cluster release is of paramount importance from a pharmaceutical perspective. Unfortunately, experimental structural information so far is limited to only one ligand/protein complex. This is the X-ray structure of furosemide bound to oxidized mitoNEET. Here we employ an enhanced sampling approach, Localized Volume-based Metadynamics, developed by some of us, to identify binding poses of furosemide to human mitoNEET protein in solution. The binding modes show a high variability within the same shallow binding pocket on the protein surface identified in the X-ray structure. Among the different binding conformations, one of them is in agreement with the crystal structure’s one. This conformation might have been overstabilized in the latter because of the presence of crystal packing interactions, absent in solution. The calculated binding affinity is compatible with experimental data. Our protocol can be used in a straightforward manner in drug design campaigns targeting this pharmaceutically important family of proteins.

An anti-diabetic drug targets NEET (CISD) proteins through destabilization of their [2Fe-2S] clusters

Elevated levels of mitochondrial iron and reactive oxygen species (ROS) accompany the progression of diabetes, negatively impacting insulin production and secretion from pancreatic cells. In search for a tool to reduce mitochondrial iron and ROS levels, we arrived at a molecule that destabilizes the [2Fe-2S] clusters of NEET proteins (M1). Treatment of db/db diabetic mice with M1 improved hyperglycemia, without the weight gain observed with alternative treatments such as rosiglitazone. The molecular interactions of M1 with the NEET proteins mNT and NAF-1 were determined by X-crystallography. The possibility of controlling diabetes by molecules that destabilize the [2Fe-2S] clusters of NEET proteins, thereby reducing iron-mediated oxidative stress, opens a new route for managing metabolic aberration such as in diabetes.

The Intriguing Role of Iron-Sulfur Clusters in the CIAPIN1 Protein Family

Iron-sulfur (Fe/S) clusters are protein cofactors that play a crucial role in essential cellular functions. Their ability to rapidly exchange electrons with several redox active acceptors makes them an efficient system for fulfilling diverse cellular needs. They include the formation of a relay for long-range electron transfer in enzymes, the biosynthesis of small molecules required for several metabolic pathways and the sensing of cellular levels of reactive oxygen or nitrogen species to activate appropriate cellular responses. An emerging family of iron-sulfur cluster binding proteins is CIAPIN1, which is characterized by a C-terminal domain of about 100 residues. This domain contains two highly conserved cysteine-rich motifs, which are both involved in Fe/S cluster binding. The CIAPIN1 proteins have been described so far to be involved in electron transfer pathways, providing electrons required for the biosynthesis of important protein cofactors, such as Fe/S clusters and the diferric-tyrosyl radical, as well as in the regulation of cell death. Here, we have first investigated the occurrence of CIAPIN1 proteins in different organisms spanning the entire tree of life. Then, we discussed the function of this family of proteins, focusing specifically on the role that the Fe/S clusters play. Finally, we describe the nature of the Fe/S clusters bound to CIAPIN1 proteins and which are the cellular pathways inserting the Fe/S clusters in the two cysteine-rich motifs.

A VDAC1-mediated NEET protein chain transfers [2Fe-2S] clusters between the mitochondria and the cytosol and impacts mitochondrial dynamics

Here we address the important question of cross-talk between the mitochondria and cytosol. We show that the inner mitochondrial protein, MiNT, interacts with a protein on the outer mitochondrial membrane (mNT). This interaction occurs within the major outer membrane protein VDAC1. Inside the inner space of VDAC1, MiNT transfers its [2Fe-2S] clusters to mNT, which was shown to be a [2Fe-2S] cluster donor protein that donates its cluster(s) to apo-acceptor proteins residing in the cytosol. Hence, we suggest a pathway for transferring [2Fe-2S] clusters from inside the mitochondria to the cytosol.

Mitochondrial inner NEET (MiNT) and the outer mitochondrial membrane (OMM) mitoNEET (mNT) proteins belong to the NEET protein family. This family plays a key role in mitochondrial labile iron and reactive oxygen species (ROS) homeostasis. NEET proteins contain labile [2Fe-2S] clusters which can be transferred to apo-acceptor proteins. In eukaryotes, the biogenesis of [2Fe-2S] clusters occurs within the mitochondria by the iron–sulfur cluster (ISC) system; the clusters are then transferred to [2Fe-2S] proteins within the mitochondria or exported to cytosolic proteins and the cytosolic iron–sulfur cluster assembly (CIA) system. The last step of export of the [2Fe-2S] is not yet fully characterized. Here we show that MiNT interacts with voltage-dependent anion channel 1 (VDAC1), a major OMM protein that connects the intermembrane space with the cytosol and participates in regulating the levels of different ions including mitochondrial labile iron (mLI). We further show that VDAC1 is mediating the interaction between MiNT and mNT, in which MiNT transfers its [2Fe-2S] clusters from inside the mitochondria to mNT that is facing the cytosol. This MiNT–VDAC1–mNT interaction is shown both experimentally and by computational calculations. Additionally, we show that modifying MiNT expression in breast cancer cells affects the dynamics of mitochondrial structure and morphology, mitochondrial function, and breast cancer tumor growth. Our findings reveal a pathway for the transfer of [2Fe-2S] clusters, which are assembled inside the mitochondria, to the cytosol.

Loss of the redox mitochondrial protein mitoNEET leads to mitochondrial dysfunction in B-cell acute lymphoblastic leukemia

B-cell acute lymphoblastic leukemia (ALL) affects both pediatric and adult patients. Chemotherapy resistant tumor cells that contribute to minimal residual disease (MRD) underlie relapse and poor clinical outcomes in a sub-set of patients. Targeting mitochondrial oxidative phosphorylation (OXPHOS) in the treatment of refractory leukemic cells is a potential novel approach to sensitizing tumor cells to existing standard of care therapeutic agents. In the current study, we have expanded our previous investigation of the mitoNEET ligand NL-1 in the treatment of ALL to interrogate the functional role of the mitochondrial outer membrane protein mitoNEET in B-cell ALL. Knockout (KO) of mitoNEET (gene: CISD1) in REH leukemic cells led to changes in mitochondrial ultra-structure and function. REH cells have significantly reduced OXPHOS capacity in the KO cells coincident with reduction in electron flow and increased reactive oxygen species. In addition, we found a decrease in lipid content in KO cells, as compared to the vector control cells was observed. Lastly, the KO of mitoNEET was associated with decreased proliferation as compared to control cells when exposed to the standard of care agent cytarabine (Ara-C). Taken together, these observations suggest that mitoNEET is essential for optimal function of mitochondria in B-cell ALL and may represent a novel anti-leukemic drug target for treatment of minimal residual disease.

The two redox states of the human NEET proteins' [2Fe-2S] clusters

The NEET proteins constitute a unique class of [2Fe-2S] proteins. The metal ions bind to three cysteines and one histidine. The proteins' clusters exist in two redox states; the oxidized protein (containing two FeIII ions) can transfer the cluster to apo-acceptor protein(s), while the reduced form (containing one ferrous ion) remains bound to the protein frame. Here, we perform in silico and in vitro studies on human NEET proteins in both reduced and oxidized forms. Quantum chemical calculations on all available human NEET proteins structures suggest that reducing the cluster weakens the Fe-NHis and Fe-SCys bonds, similar to what is seen in other Fe-S proteins (e.g., ferredoxin and Rieske protein). We further show that the extra electron in the [2Fe-2S]+ clusters of one of the NEET proteins (mNT) is localized on the His-bound iron ion, consistently with our previous spectroscopic studies. Kinetic measurements demonstrate that the mNT [2Fe-2S]+ is released only by an increase in temperature. Thus, the reduced state of human NEET proteins [2Fe-2S] cluster is kinetically inert. This previously unrecognized kinetic inertness of the reduced state, along with the reactivity of the oxidized state, is unique across all [2Fe-2S] proteins. Finally, using a coevolutionary analysis, along with molecular dynamics simulations, we provide insight on the observed allostery between the loop L2 and the cluster region. Specifically, we show that W75, R76, K78, K79, F82 and G85 in the latter region share similar allosteric characteristics in both redox states.

Intracellular targeting of Cisd2/Miner1 to the endoplasmic reticulum

Background

Cisd1 and Cisd2 proteins share very similar structures with an N-terminal membrane-anchoring domain and a C-terminal cytosolic domain containing an iron-cluster binding domain and ending with a C-terminal KKxx sequence. Despite sharing a similar structure, Cisd1 and Cisd2 are anchored to different compartments: mitochondria for Cisd1 and endoplasmic reticulum for Cisd2. The aim of this study was to identify the protein motifs targeting Cisd2 to the ER and ensuring its retention in this compartment.

Results

We used new recombinant antibodies to localize Cisd1 and Cisd2 proteins, as well as various protein chimeras. Cisd2 is targeted to the ER by its N-terminal sequence. It is then retained in the ER by the combined action of a C-terminal COPI-binding KKxx ER retrieval motif, and of an ER-targeting transmembrane domain. As previously reported for Cisd1, Cisd2 can alter the morphology of the compartment in which it accumulates.

Conclusion

Although they share a very similar structure, Cisd1 and Cisd2 use largely different intracellular targeting motifs to reach their target compartment (mitochondria and endoplasmic reticulum, respectively).

Supplementary Information

The online version contains supplementary material available at 10.1186/s12860-021-00387-1.

The Mitochondrial mitoNEET Ligand NL-1 Is Protective in a Murine Model of Transient Cerebral Ischemic Stroke

PURPOSE

Therapeutic strategies to treat ischemic stroke are limited due to the heterogeneity of cerebral ischemic injury and the mechanisms that contribute to the cell death. Since oxidative stress is one of the primary mechanisms that cause brain injury post-stroke, we hypothesized that therapeutic targets that modulate mitochondrial function could protect against reperfusion-injury after cerebral ischemia, with the focus here on a mitochondrial protein, mitoNEET, that modulates cellular bioenergetics.

METHOD

In this study, we evaluated the pharmacology of the mitoNEET ligand NL-1 in an in vivo therapeutic role for NL-1 in a C57Bl/6 murine model of ischemic stroke.

RESULTS

NL-1 decreased hydrogen peroxide production with an IC50 of 5.95 μM in neuronal cells (N2A). The in vivo activity of NL-1 was evaluated in a murine 1 h transient middle cerebral artery occlusion (t-MCAO) model of ischemic stroke. We found that mice treated with NL-1 (10 mg/kg, i.p.) at time of reperfusion and allowed to recover for 24 h showed a 43% reduction in infarct volume and 68% reduction in edema compared to sham-injured mice. Additionally, we found that when NL-1 was administered 15 min post-t-MCAO, the ischemia volume was reduced by 41%, and stroke-associated edema by 63%.

CONCLUSION

As support of our hypothesis, as expected, NL-1 failed to reduce stroke infarct in a permanent photothrombotic occlusion model of stroke. This report demonstrates the potential therapeutic benefits of using mitoNEET ligands like NL-1 as novel mitoceuticals for treating reperfusion-injury with cerebral stroke.

The long-standing relationship between paramagnetic NMR and iron-sulfur proteins: the mitoNEET example. An old method for new stories or the other way around?

Paramagnetic NMR spectroscopy and iron-sulfur (Fe-S) proteins have maintained a synergic relationship for decades. Indeed, the hyperfine shifts with their temperature dependencies and the relaxation rates of nuclei of cluster-bound residues have been extensively used as a fingerprint of the type and of the oxidation state of the Fe-S cluster within the protein frame. The identification of NMR signals from residues surrounding the metal cofactor is crucial for understanding the structure-function relationship in Fe-S proteins, but it is generally impaired in standard NMR experiments by paramagnetic relaxation enhancement due to the presence of the paramagnetic cluster(s). On the other hand, the availability of systems of different sizes and stabilities has, over the years, stimulated NMR spectroscopists to exploit iron-sulfur proteins as paradigmatic cases to develop experiments, models, and protocols. Here, the cluster-binding properties of human mitoNEET have been investigated by 1D and 2D 1 H diamagnetic and paramagnetic NMR, in its oxidized and reduced states. The NMR spectra of both oxidation states of mitoNEET appeared to be significantly different from those reported for previously investigated [ Fe 2 S 2 ] 2 + / + proteins. The protocol we have developed in this work conjugates spectroscopic information arising from "classical" paramagnetic NMR with an extended mapping of the signals of residues around the cluster which can be taken, even before the sequence-specific assignment is accomplished, as a fingerprint of the protein region constituting the functional site of the protein. We show how the combined use of 1D NOE experiments, 13 C direct-detected experiments, and double- and triple-resonance experiments tailored using R1 - and/or R2 -based filters significantly reduces the "blind" sphere of the protein around the paramagnetic cluster. This approach provided a detailed description of the unique electronic properties of mitoNEET, which are responsible for its biological function. Indeed, the NMR properties suggested that the specific electronic structure of the cluster possibly drives the functional properties of different [ Fe 2 S 2 ] proteins.

New Insights of the NEET Protein CISD2 Reveals Distinct Features Compared to Its Close Mitochondrial Homolog mitoNEET

Human CISD2 and mitoNEET are two NEET proteins anchored in the endoplasmic reticulum and mitochondria membranes respectively, with an Fe–S containing domain stretching out in the cytosol. Their cytosolic domains are close in sequence and structure. In the present study, combining cellular and biochemical approaches, we compared both proteins in order to possibly identify specific roles and mechanisms of action in the cell. We show that both proteins exhibit a high intrinsic stability and a sensitivity of their cluster to oxygen. In contrast, they differ in according to expression profiles in tissues and intracellular half-life. The stability of their Fe–S cluster and its ability to be transferred in vitro are affected differently by pH variations in a physiological and pathological range for cytosolic pH. Finally, we question a possible role for CISD2 in cellular Fe–S cluster trafficking. In conclusion, our work highlights unexpected major differences in the cellular and biochemical features between these two structurally close NEET proteins.

Exploring the FMN binding site in the mitochondrial outer membrane protein mitoNEET

MitoNEET is a mitochondrial outer membrane protein that hosts a redox active [2Fe-2S] cluster in the C-terminal cytosolic domain. Increasing evidence has shown that mitoNEET has an essential role in regulating energy metabolism in human cells. Previously, we reported that the [2Fe-2S] clusters in mitoNEET can be reduced by the reduced flavin mononucleotide (FMNH₂) and oxidized by oxygen or ubiquinone-2, suggesting that mitoNEET may act as a novel redox enzyme catalyzing electron transfer from FMNH₂ to oxygen or ubiquinone. Here, we explore the FMN binding site in mitoNEET by using FMN analogs and find that lumiflavin, like FMN, at nanomolar concentrations can mediate the redox transition of the mitoNEET [2Fe-2S] clusters in the presence of flavin reductase and NADH (100 μM) under aerobic conditions. The electron paramagnetic resonance (EPR) measurements show that both FMN and lumiflavin can dramatically change the EPR spectrum of the reduced mitoNEET [2Fe-2S] clusters and form a covalently bound complex with mitoNEET under blue light exposure, suggesting that FMN/lumiflavin has specific interactions with the [2Fe-2S] clusters in mitoNEET. In contrast, lumichrome, another FMN analog, fails to mediate the redox transition of the mitoNEET [2Fe-2S] clusters and has no effect on the EPR spectrum of the reduced mitoNEET [2Fe-2S] clusters under blue light exposure. Instead, lumichrome can effectively inhibit the FMNH₂-mediated reduction of the mitoNEET [2Fe-2S] clusters, indicating that lumichrome may act as a potential inhibitor to block the electron transfer activity of mitoNEET.

Nanoparticle formulation and in vitro efficacy testing of the mitoNEET ligand NL-1 for drug delivery in a brain endothelial model of ischemic reperfusion-injury

Ischemic reperfusion injury after a stroke is a leading cause of mortality and disability due to neuronal loss and tissue damage. Mitochondrial dysfunction plays a major role in the reperfusion-injury sequelae, and offers an attractive drug target. Mitochondrial derived reactive oxygen species (ROS) and resultant apoptotic cascade are among the primary mechanisms of neuronal death following ischemia and reperfusion injury. Here we optimized a nanoparticle formulation for the mitoNEET ligand NL-1, to target mitochondrial dysfunction post ischemic reperfusion (IR) injury. NL-1, a hydrophobic drug, was formulated using PLGA polymers with a particle size and entrapment efficiency of 123.9 ± 17.1 nm and 59.7 ± 10.1%, respectively. The formulation was characterized for physical state of NL-1, in vitro release, uptake and nanoparticle localization. A near complete uptake of nanoparticles was found to occur by three hours, with the process being energy-dependent and occurring via caveolar mediated endocytosis. The fluorescent nanoparticles were found to localize in the cytoplasm of the endothelial cells. An in vitro oxygen glucose deprivation (OGD) model to mimic IR was employed for in vitro efficacy testing in murine brain vascular endothelium cells (bEND.3 cells). Efficacy studies showed that both NL-1 and the nanoparticles loaded with NL-1 had a protective activity against peroxide generation, and displayed improved cellular viability, as seen via reduction in cellular apoptosis. Finally, PLGA nanoparticles were found to have a non-toxic profile in vitro, and were found to be safe for intravenous administration. This study lays the preliminary work for potential use of mitoNEET as a target and NL-1 as a therapeutic for the treatment of cerebral ischemia and reperfusion injury.

Redox-dependent gating of VDAC by mitoNEET

This work demonstrates that the outer mitochondrial-anchored [2Fe-2S] mitoNEET is able to bind within the central cavity of the voltage-dependent anion channel (VDAC) and regulate its gating in a redox-dependent manner. These findings have implications for ferroptosis, apoptosis, and iron metabolism by linking VDAC function, mitoNEET, and the redox environment of the cell. Furthermore, these findings introduce a potential player to the many mechanisms that may alter VDAC’s governance in times of homeostasis or strife.

MitoNEET is an outer mitochondrial membrane protein essential for sensing and regulation of iron and reactive oxygen species (ROS) homeostasis. It is a key player in multiple human maladies including diabetes, cancer, neurodegeneration, and Parkinson’s diseases. In healthy cells, mitoNEET receives its clusters from the mitochondrion and transfers them to acceptor proteins in a process that could be altered by drugs or during illness. Here, we report that mitoNEET regulates the outer-mitochondrial membrane (OMM) protein voltage-dependent anion channel 1 (VDAC1). VDAC1 is a crucial player in the cross talk between the mitochondria and the cytosol. VDAC proteins function to regulate metabolites, ions, ROS, and fatty acid transport, as well as function as a “governator” sentry for the transport of metabolites and ions between the cytosol and the mitochondria. We find that the redox-sensitive [2Fe-2S] cluster protein mitoNEET gates VDAC1 when mitoNEET is oxidized. Addition of the VDAC inhibitor 4,4′-diisothiocyanatostilbene-2,2′-disulfonate (DIDS) prevents both mitoNEET binding in vitro and mitoNEET-dependent mitochondrial iron accumulation in situ. We find that the DIDS inhibitor does not alter the redox state of MitoNEET. Taken together, our data indicate that mitoNEET regulates VDAC in a redox-dependent manner in cells, closing the pore and likely disrupting VDAC’s flow of metabolites.

NEET Proteins: A New Link Between Iron Metabolism, Reactive Oxygen Species, and Cancer

SIGNIFICANCE

Cancer cells accumulate high levels of iron and reactive oxygen species (ROS) to promote their high metabolic activity and proliferation rate. However, high levels of iron and ROS can also lead to enhanced oxidative stress and the activation of cell death pathways such as apoptosis and ferroptosis. This has led to the proposal that different drugs that target iron and/or ROS metabolism could be used as anticancer drugs. However, due to the complex role iron and ROS play in cells, the majority of these drugs yielded mixed results, highlighting a critical need to identify new players in the regulation of iron and ROS homeostasis in cancer cells. Recent Advances: NEET proteins belong to a newly discovered class of iron-sulfur proteins (2Fe-2S) required for the regulation of iron and ROS homeostasis in cells. Recent studies revealed that the NEET proteins NAF-1 (CISD2) and mitoNEET (CISD1) play a critical role in promoting the proliferation of cancer cells, supporting tumor growth and metastasis. Moreover, the function of NEET proteins in cancer cells was found to be dependent of the degree of lability of their 2Fe-2S clusters.

CRITICAL ISSUES

NEET proteins could represent a key regulatory link between the maintenance of high iron and ROS in cancer cells, the activation of cell death and survival pathways, and cellular proliferation.

FUTURE DIRECTIONS

Because the function of NEET proteins depends on the lability of their clusters, drugs that target the 2Fe2S clusters of NEET proteins could be used as promising anticancer drugs.

Electron transfer kinetics of the mitochondrial outer membrane protein mitoNEET

Increasing evidence suggests that the mitochondrial outer membrane protein mitoNEET is a key regulator of energy metabolism, iron homeostasis, and production of reactive oxygen species in mitochondria. Previously, we reported that mitoNEET is a redox enzyme that catalyzes electron transfer from the reduced flavin mononucleotide (FMNH₂) to oxygen or ubiquinone via its unique [2Fe-2S] clusters. Here, we explore the reduction and oxidation kinetics of the mitoNEET [2Fe-2S] clusters under anaerobic and aerobic conditions. We find that the mitoNEET [2Fe-2S] clusters are rapidly reduced by a catalytic amount of FMNH₂ which is reduced by flavin reductase and an equivalent amount of NADH under anaerobic conditions. When the reduced mitoNEET [2Fe-2S] clusters are exposed to air, the [2Fe-2S] clusters are slowly oxidized by oxygen at a rate constant of about 6.0 M^-1 s^-1. Compared with oxygen, ubiquinone-2 has a much higher activity to oxidize the reduced mitoNEET [2Fe-2S] clusters at a rate constant of about 3.0 × 10³ M^-1 s^-1 under anaerobic conditions. Under aerobic conditions, the mitoNEET [2Fe-2S] clusters can still be reduced by FMNH₂ in the presence of flavin reductase and excess NADH. However, when NADH is completely consumed, the reduced mitoNEET [2Fe-2S] clusters are gradually oxidized by oxygen. Addition of ubiquinone-2 also rapidly oxidizes the pre-reduced mitoNEET [2Fe-2S] clusters and effectively prevents the FMNH₂-mediated reduction of the mitoNEET [2Fe-2S] clusters under aerobic conditions. The results suggest that ubiquinone may act as an intrinsic oxidant of the reduced mitoNEET [2Fe-2S] clusters in mitochondria under aerobic and anaerobic conditions.

EPR spectroscopy of complex biological iron-sulfur systems

From the very first discovery of biological iron-sulfur clusters with EPR, the spectroscopy has been used to study not only purified proteins but also complex systems such as respiratory complexes, membrane particles and, later, whole cells. In recent times, the emphasis of iron-sulfur biochemistry has moved from characterization of individual proteins to the systems biology of iron-sulfur biosynthesis, regulation, degradation, and implications for human health. Although this move would suggest a blossoming of System-EPR as a specific, non-invasive monitor of Fe/S (dys)homeostasis in whole cells, a review of the literature reveals limited success possibly due to technical difficulties in adherence to EPR spectroscopic and biochemical standards. In an attempt to boost application of System-EPR the required boundary conditions and their practical applications are explicitly and comprehensively formulated.

Phylogenetic analysis of the CDGSH iron-sulfur binding domain reveals its ancient origin

The iron-sulfur (2Fe-2S) binding motif CDGSH appears in many important plant and animal proteins that regulate iron and reactive oxygen metabolism. In human it is found in CISD1-3 proteins involved in diabetes, obesity, cancer, aging, cardiovascular disease and neurodegeneration. Despite the important biological role of the CDGSH domain, its origin, evolution and diversification, are largely unknown. Here, we report that: (1) the CDGSH domain appeared early in evolution, perhaps linked to the heavy use of iron-sulfur driven metabolism by early organisms; (2) a CISD3-like protein with two CDGSH domains on the same polypeptide appears to represent the ancient archetype of CDGSH proteins; (3) the origin of the human CISD3 protein is linked to the mitochondrial endosymbiotic event; (4) the CISD1/2 type proteins that contain only one CDGSH domain, but function as homodimers, originated after the divergence of bacteria and archaea/eukaryotes from their common ancestor; and (5) the human CISD1 and CISD2 proteins diverged about 650–720 million years ago, and CISD3 and CISD1/2 share their descent from an ancestral CISD about 1–1.1 billion years ago. Our findings reveal that the CDGSH domain is ancient in its origin and shed light on the complex evolutionary path of modern CDGSH proteins.

The unique fold and lability of the [2Fe-2S] clusters of NEET proteins mediate their key functions in health and disease

NEET proteins comprise a new class of [2Fe-2S] cluster proteins. In human, three genes encode for NEET proteins: cisd1 encodes mitoNEET (mNT), cisd2 encodes the Nutrient-deprivation autophagy factor-1 (NAF-1) and cisd3 encodes MiNT (Miner2). These recently discovered proteins play key roles in many processes related to normal metabolism and disease. Indeed, NEET proteins are involved in iron, Fe-S, and reactive oxygen homeostasis in cells and play an important role in regulating apoptosis and autophagy. mNT and NAF-1 are homodimeric and reside on the outer mitochondrial membrane. NAF-1 also resides in the membranes of the ER associated mitochondrial membranes (MAM) and the ER. MiNT is a monomer with distinct asymmetry in the molecular surfaces surrounding the clusters. Unlike its paralogs mNT and NAF-1, it resides within the mitochondria. NAF-1 and mNT share similar backbone folds to the plant homodimeric NEET protein (At-NEET), while MiNT's backbone fold resembles a bacterial MiNT protein. Despite the variation of amino acid composition among these proteins, all NEET proteins retained their unique CDGSH domain harboring their unique 3Cys:1His [2Fe-2S] cluster coordination through evolution. The coordinating exposed His was shown to convey the lability to the NEET proteins' [2Fe-2S] clusters. In this minireview, we discuss the NEET fold and its structural elements. Special attention is given to the unique lability of the NEETs' [2Fe-2S] cluster and the implication of the latter to the NEET proteins' cellular and systemic function in health and disease.

Glutathione, Glutaredoxins, and Iron

SIGNIFICANCE

Glutathione (GSH) is the most abundant cellular low-molecular-weight thiol in the majority of organisms in all kingdoms of life. Therefore, functions of GSH and disturbed regulation of its concentration are associated with numerous physiological and pathological situations. Recent Advances: The function of GSH as redox buffer or antioxidant is increasingly being questioned. New functions, especially functions connected to the cellular iron homeostasis, were elucidated. Via the formation of iron complexes, GSH is an important player in all aspects of iron metabolism: sensing and regulation of iron levels, iron trafficking, and biosynthesis of iron cofactors. The variety of GSH coordinated iron complexes and their functions with a special focus on FeS-glutaredoxins are summarized in this review. Interestingly, GSH analogues that function as major low-molecular-weight thiols in organisms lacking GSH resemble the functions in iron homeostasis.

CRITICAL ISSUES

Since these iron-related functions are most likely also connected to thiol redox chemistry, it is difficult to distinguish between mechanisms related to either redox or iron metabolisms.

FUTURE DIRECTIONS

The ability of GSH to coordinate iron in different complexes with or without proteins needs further investigation. The discovery of new Fe-GSH complexes and their physiological functions will significantly advance our understanding of cellular iron homeostasis. Antioxid. Redox Signal. 27, 1235-1251.

Anamorsin/Ndor1 Complex Reduces [2Fe-2S]-MitoNEET via a Transient Protein-Protein Interaction

Human mitoNEET is a homodimeric protein anchored to the outer mitochondrial membrane and has a C-terminal [2Fe-2S] binding domain located in the cytosol. Recently, human mitoNEET has been shown to be implicated in Fe/S cluster repair of cytosolic iron regulatory protein 1 (IRP1), a key regulator of cellular iron homeostasis in mammalian cells. The Fe/S cluster repair function of mitoNEET is based on an Fe/S redox switch mechanism: under normal cellular conditions, reduced [2Fe-2S]⁺-mitoNEET is present and is inactive as an Fe/S cluster transfer protein; under conditions of oxidative cellular stress, the clusters of mitoNEET become oxidized, and the formed [2Fe-2S]²⁺-mitoNEET species reacts promptly to initiate Fe/S cluster transfer to IRP1, recycling the cytosolic apo-IRP1 into holo-aconitase. Until now, no clear data have been available on which is the system that reduces the mitoNEET clusters back once oxidative stress is not present anymore. In the present work, we used UV-vis and NMR spectroscopies to investigate the electron transfer process between mitoNEET and the cytosolic electron-donor Ndor1/anamorsin complex, a component of the cytosolic iron-sulfur protein assembly (CIA) machinery. The [2Fe-2S] clusters of mitoNEET are reduced via the formation of a transient complex that brings the [2Fe-2S] clusters of mitoNEET close to the redox-active [2Fe-2S] cluster of anamorsin. Our data provide in vitro evidence of a possible direct link between the CIA machinery and the mitoNEET cluster transfer repair pathway. This link might contribute to recovery of CIA machinery efficiency to mature cytosolic and nuclear Fe/S proteins.

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.

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.

Iron-Sulfur Clusters in Mitochondrial Metabolism: Multifaceted Roles of a Simple Cofactor

Iron-sulfur metabolism is essential for cellular function and is a key process in mitochondria. In this review, we focus on the structure and assembly of mitochondrial iron-sulfur clusters and their roles in various metabolic processes that occur in mitochondria. Iron-sulfur clusters are crucial in mitochondrial respiration, in which they are required for the assembly, stability, and function of respiratory complexes I, II, and III. They also serve important functions in the citric acid cycle, DNA metabolism, and apoptosis. Whereas the identification of iron-sulfur containing proteins and their roles in numerous aspects of cellular function has been a long-standing research area, that in mitochondria is comparatively recent, and it is likely that their roles within mitochondria have been only partially revealed. We review the status of the field and provide examples of other cellular iron-sulfur proteins to highlight their multifarious roles.

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.

Mitochondrial drug targets in neurodegenerative diseases

Growing evidence suggests that mitochondrial dysfunction is the main culprit in neurodegenerative diseases. Given the fact that mitochondria participate in diverse cellular processes, including energetics, metabolism, and death, the consequences of mitochondrial dysfunction in neuronal cells are inevitable. In fact, new strategies targeting mitochondrial dysfunction are emerging as potential alternatives to current treatment options for neurodegenerative diseases. In this review, we focus on mitochondrial proteins that are directly associated with mitochondrial dysfunction. We also examine recently identified small molecule modulators of these mitochondrial targets and assess their potential in research and therapeutic applications.

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.

Production of the Marine Carotenoid Astaxanthin by Metabolically Engineered Corynebacterium glutamicum

Astaxanthin, a red C40 carotenoid, is one of the most abundant marine carotenoids. It is currently used as a food and feed additive in a hundred-ton scale and is furthermore an attractive component for pharmaceutical and cosmetic applications with antioxidant activities. Corynebacterium glutamicum, which naturally synthesizes the yellow C50 carotenoid decaprenoxanthin, is an industrially relevant microorganism used in the million-ton amino acid production. In this work, engineering of a genome-reduced C. glutamicum with optimized precursor supply for astaxanthin production is described. This involved expression of heterologous genes encoding for lycopene cyclase CrtY, β-carotene ketolase CrtW, and hydroxylase CrtZ. For balanced expression of crtW and crtZ their translation initiation rates were varied in a systematic approach using different ribosome binding sites, spacing, and translational start codons. Furthermore, β-carotene ketolases and hydroxylases from different marine bacteria were tested with regard to efficient astaxanthin production in C. glutamicum. In shaking flasks, the C. glutamicum strains developed here overproduced astaxanthin with volumetric productivities up to 0.4 mg·L⁻¹·h⁻¹ which are competitive with current algae-based production. Since C. glutamicum can grow to high cell densities of up to 100 g cell dry weight (CDW)·L⁻¹, the recombinant strains developed here are a starting point for astaxanthin production by C. glutamicum.

Cleavage and polyadenylation specificity factor 30: An RNA-binding zinc-finger protein with an unexpected 2Fe-2S cluster

Cleavage and polyadenylation specificity factor 30 (CPSF30) is a key protein involved in pre-mRNA processing. CPSF30 contains five Cys3His domains (annotated as "zinc-finger" domains). Using inductively coupled plasma mass spectrometry, X-ray absorption spectroscopy, and UV-visible spectroscopy, we report that CPSF30 is isolated with iron, in addition to zinc. Iron is present in CPSF30 as a 2Fe-2S cluster and uses one of the Cys3His domains; 2Fe-2S clusters with a Cys3His ligand set are rare and notably have also been identified in MitoNEET, a protein that was also annotated as a zinc finger. These findings support a role for iron in some zinc-finger proteins. Using electrophoretic mobility shift assays and fluorescence anisotropy, we report that CPSF30 selectively recognizes the AU-rich hexamer (AAUAAA) sequence present in pre-mRNA, providing the first molecular-based evidence to our knowledge for CPSF30/RNA binding. Removal of zinc, or both zinc and iron, abrogates binding, whereas removal of just iron significantly lessens binding. From these data we propose a model for RNA recognition that involves a metal-dependent cooperative binding mechanism.

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.

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.

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.

Design, synthesis and evaluation of Fe-S targeted adenosine 5'-phosphosulfate reductase inhibitors

Adenosine 5'-phosphosulfate reductase (APR) is an iron-sulfur enzyme that is vital for survival of Mycobacterium tuberculosis during dormancy and is an attractive target for the treatment of latent tuberculosis (TB) infection. The 4Fe-4S cluster is coordinated to APR by sulfur atoms of four cysteine residues, is proximal to substrate, adenosine 5'-phopsphosulfate (APS), and is essential for catalytic activity. Herein, we present an approach for the development of a new class of APR inhibitors. As an initial step, we have employed an improved solid-phase chemistry method to prepare a series of N(6)-substituted adenosine analogues and their 5'-phosphates as well as adenosine 5'-phosphate diesters bearing different Fe and S binding groups, such as thiols or carboxylic and hydroxamic acid moieties. Evaluation of the resulting compounds indicates a clearly defined spacing requirement between the Fe-S targeting group and adenosine scaffold and that smaller Fe-S targeting groups are better tolerated. Molecular docking analysis suggests that the S atom of the most potent inhibitor may establish a favorable interaction with an S atom in the cluster. In summary, this study showcases an improved solid-phase method that expedites the preparation of adenosine and related 5'-phosphate derivatives and presents a unique Fe-S targeting strategy for the development of APR inhibitors.