Chapter 13 Localization and function of the 2Fe-2S outer mitochondrial membrane protein mitoNEET

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.

A novel link between Fic (filamentation induced by cAMP)-mediated adenylylation/AMPylation and the unfolded protein response

The maintenance of endoplasmic reticulum (ER) homeostasis is a critical aspect of determining cell fate and requires a properly functioning unfolded protein response (UPR). We have discovered a previously unknown role of a post-translational modification termed adenylylation/AMPylation in regulating signal transduction events during UPR induction. A family of enzymes, defined by the presence of a Fic (filamentation induced by cAMP) domain, catalyzes this adenylylation reaction. The human genome encodes a single Fic protein, called HYPE (Huntingtin yeast interacting protein E), with adenylyltransferase activity but unknown physiological target(s). Here, we demonstrate that HYPE localizes to the lumen of the endoplasmic reticulum via its hydrophobic N terminus and adenylylates the ER molecular chaperone, BiP, at Ser-365 and Thr-366. BiP functions as a sentinel for protein misfolding and maintains ER homeostasis. We found that adenylylation enhances BiP's ATPase activity, which is required for refolding misfolded proteins while coping with ER stress. Accordingly, HYPE expression levels increase upon stress. Furthermore, siRNA-mediated knockdown of HYPE prevents the induction of an unfolded protein response. Thus, we identify HYPE as a new UPR regulator and provide the first functional data for Fic-mediated adenylylation in mammalian signaling.

Mössbauer study and modeling of iron import and trafficking in human jurkat cells

The Fe content of Jurkat cells grown on transferrin-bound iron (TBI) and Fe(III) citrate (FC) was characterized using Mössbauer, electron paramagnetic resonance, and UV-vis spectroscopies, as well as electron and inductively coupled plasma mass spectrometry. Isolated mitochondria were similarly characterized. Fe-limited cells contained ~100 μM essential Fe, mainly as mitochondrial Fe and nonmitochondrial non-heme high-spin Fe(II). Cells replete with Fe also contained ferritin-bound Fe and Fe(III) oxyhydroxide nanoparticles. Only 400 ± 100 Fe ions were loaded per ferritin complex, regardless of the growth medium Fe concentration. Ferritin regulation thus appears to be more complex than is commonly assumed. The magnetic and structural properties of Jurkat nanoparticles differed from those of yeast mitochondria. They were smaller and may be located in the cytosol. The extent of nanoparticle formation scaled nonlinearly with the concentration of Fe in the medium. Nanoparticle formation was not strongly correlated with reactive oxygen species (ROS) damage. Cells could utilize nanoparticle Fe, converting such aggregates into essential Fe forms. Cells grown on galactose rather than glucose respired faster, grew slower, exhibited more ROS damage, and generally contained more nanoparticles. Cells grown with TBI rather than FC contained less Fe overall, more ferritin, and fewer nanoparticles. Cells in which the level of transferrin receptor expression was increased contained more ferritin Fe. Frataxin-deficient cells contained more nanoparticles than comparable wild-type cells. Data were analyzed by a chemically based mathematical model. Although simple, it captured essential features of Fe import, trafficking, and regulation. TBI import was highly regulated, but FC import was not. Nanoparticle formation was not regulated, but the rate was third-order in cytosolic Fe.

Mitochondrial membrane lipidome defines yeast longevity

Our studies revealed that lithocholic acid (LCA), a bile acid, is a potent anti-aging natural compound that in yeast cultured under longevity-extending caloric restriction (CR) conditions acts in synergy with CR to enable a significant further increase in chronological lifespan. Here, we investigate a mechanism underlying this robust longevity-extending effect of LCA under CR. We found that exogenously added LCA enters yeast cells, is sorted to mitochondria, resides mainly in the inner mitochondrial membrane, and also associates with the outer mitochondrial membrane. LCA elicits an age-related remodeling of glycerophospholipid synthesis and movement within both mitochondrial membranes, thereby causing substantial changes in mitochondrial membrane lipidome and triggering major changes in mitochondrial size, number and morphology. In synergy, these changes in the membrane lipidome and morphology of mitochondria alter the age-related chronology of mitochondrial respiration, membrane potential, ATP synthesis and reactive oxygen species homeostasis. The LCA-driven alterations in the age-related dynamics of these vital mitochondrial processes extend yeast longevity. In sum, our findings suggest a mechanism underlying the ability of LCA to delay chronological aging in yeast by accumulating in both mitochondrial membranes and altering their glycerophospholipid compositions. We concluded that mitochondrial membrane lipidome plays an essential role in defining yeast longevity.

Conserved hydrogen bonding networks of MitoNEET tune Fe-S cluster binding and structural stability

While its biological function remains unclear, the three-cysteine, one-histidine ligated human [2Fe-2S] cluster containing protein mitoNEET is of interest because of its interaction with the anti-diabetes drug pioglitazone. The mitoNEET [2Fe-2S] cluster demonstrates proton-coupled electron transfer (PCET) and marked cluster instability, which have both been linked to the single His ligand. Highly conserved hydrogen bonding networks, which include the His-87 ligand, exist around the [2Fe-2S] cluster. Through a series of site-directed mutations, PCET of the cluster has been examined, demonstrating that multiple sites of protonation exist in addition to the His ligand, which can influence redox potential. The mutations also demonstrate that while replacement of the His ligand with cysteine results in a stable cluster, the removal of Lys-55 also greatly stabilizes the cluster. We have also noted for the first time that the oxidation state of the cluster controls stability: the reduced cluster is stable, while the oxidized one is much more labile. Finally, it is shown that upon cluster loss the mitoNEET protein structure becomes less stable, while upon in vitro reconstitution, both the cluster and the secondary structure are recovered. Recently, two other proteins have been identified with a three-Cys(sulfur), one-His motif, IscR and Grx3/4-Fra2, both of which are sensors of iron and redox homeostatsis. These results lead to a model in which mitoNEET could sense the cellular oxidation state and proton concentration and respond through cluster loss and unfolding.

The mitochondrial complex I activity is reduced in cells with impaired cystic fibrosis transmembrane conductance regulator (CFTR) function

Cystic fibrosis (CF) is a frequent and lethal autosomal recessive disease. It results from different possible mutations in the CFTR gene, which encodes the CFTR chloride channel. We have previously studied the differential expression of genes in CF and CF corrected cell lines, and found a reduced expression of MTND4 in CF cells. MTND4 is a mitochondrial gene encoding the MTND4 subunit of the mitochondrial Complex I (mCx-I). Since this subunit is essential for the assembly and activity of mCx-I, we have now studied whether the activity of this complex was also affected in CF cells. By using Blue Native-PAGE, the in-gel activity (IGA) of the mCx-I was found reduced in CFDE and IB3-1 cells (CF cell lines) compared with CFDE/6RepCFTR and S9 cells, respectively (CFDE and IB3-1 cells ectopically expressing wild-type CFTR). Moreover, colon carcinoma T84 and Caco-2 cells, which express wt-CFTR, either treated with CFTR inhibitors (glibenclamide, CFTR(inh)-172 or GlyH101) or transfected with a CFTR-specific shRNAi, showed a significant reduction on the IGA of mCx-I. The reduction of the mCx-I activity caused by CFTR inhibition under physiological or pathological conditions may have a profound impact on mitochondrial functions of CF and non-CF cells.

Biophysical investigation of the ironome of human jurkat cells and mitochondria

The speciation of iron in intact human Jurkat leukemic cells and their isolated mitochondria was assessed using biophysical methods. Large-scale cultures were grown in medium enriched with (57)Fe citrate. Mitochondria were isolated anaerobically to prevent oxidation of iron centers. 5 K Mössbauer spectra of cells were dominated by a sextet due to ferritin. They also exhibited an intense central quadrupole doublet due to S = 0 [Fe(4)S(4)](2+) clusters and low-spin (LS) Fe(II) heme centers. Spectra of isolated mitochondria were largely devoid of ferritin but contained the central doublet and features arising from what appear to be Fe(III) oxyhydroxide (phosphate) nanoparticles. Spectra from both cells and mitochondria contained a low-intensity doublet from non-heme high-spin (NHHS) Fe(II) species. A portion of these species may constitute the "labile iron pool" (LIP) proposed in cellular Fe trafficking. Such species might engage in Fenton chemistry to generate reactive oxygen species. Electron paramagnetic resonance spectra of cells and mitochondria exhibited signals from reduced Fe/S clusters, and HS Fe(III) heme and non-heme species. The basal heme redox state of mitochondria within cells was reduced; this redox poise was unaltered during the anaerobic isolation of the organelle. Contributions from heme a, b, and c centers were quantified using electronic absorption spectroscopy. Metal concentrations in cells and mitochondria were measured using inductively coupled plasma mass spectrometry. Results were collectively assessed to estimate the concentrations of various Fe-containing species in mitochondria and whole cells - the first "ironome" profile of a human cell.

Complexes of the outer mitochondrial membrane protein mitoNEET with resveratrol-3-sulfate

Binding of the thiazolidinedione antidiabetic drug pioglitazone led to the discovery of a novel outer mitochondrial membrane protein of unknown function called mitoNEET. The protein is homodimeric and contains a uniquely ligated two iron-two sulfur cluster in each of its two cytosolic domains. Electrospray ionization mass spectrometry was employed to characterize solutions of the soluble cytosolic domain (amino acids 32--108) of the protein. Ions characteristic of dimers containing the cofactors were readily detected under native conditions. mitoNEET responded to exposure to solutions at low pH by dissociation to give monomers that retained the cofactor, followed by dissociation of the cofactor in a concerted fashion. mitoNEET formed complexes with resveratrol-3-sulfate, one of the primary metabolites of the natural product resveratrol. Resveratrol itself showed no tendency to interact with mitoNEET. The formation of complexes was evident in both electrospray ionization mass spectrometry and isothermal titration calorimetry measurements. Up to eight molecules of the compound associated with the dimeric form of the protein in a sequential fashion. Dissociation constants determined by micorcalorimetry were in the range 5-16 μM for the various binding sites. The only other known naturally occurring binding partner for mitoNEET at present is NADPH. It is very interesting that the iron-sulfur cluster containing protein interacts with two potentially redox active substances at the surface of mitochondria. These findings provide a new direction for research into two poorly understood, yet biomedically relevant, species.

Evaluation of basic mitochondrial functions using rat tissue homogenates

The primary attempt in diagnostic and experimental studies of numerous pathological states associated with mitochondrial dysfunction is a precise evaluation of changes in function, content and structure of mitochondrial OXPHOS system. The analysis of rat heart, liver, brain and kidney by oxygraphy, enzyme activities, membrane potential, and BN/SDS-PAGE western blotting demonstrated that tissue homogenates can substitute for isolated mitochondria, providing comparable qualitative mitochondrial parameters. The use of homogenate avoids the loss of the majority of mitochondria during their isolation. Only 50-100mg of the tissue is required for the complex OXPHOS analysis, i.e. five times less as compared with isolated mitochondria.

Mitochondrial dysfunction in NnaD mutant flies and Purkinje cell degeneration mice reveals a role for Nna proteins in neuronal bioenergetics

The Purkinje cell degeneration (pcd) mouse is a recessive model of neurodegeneration, involving cerebellum and retina. Purkinje cell death in pcd is dramatic, as >99% of Purkinje neurons are lost in 3 weeks. Loss of function of Nna1 causes pcd, and Nna1 is a highly conserved zinc carboxypeptidase. To determine the basis of pcd, we implemented a two-pronged approach, combining characterization of loss-of-function phenotypes of the Drosophila Nna1 ortholog (NnaD) with proteomics analysis of pcd mice. Reduced NnaD function yielded larval lethality, with survivors displaying phenotypes that mirror disease in pcd. Quantitative proteomics revealed expression alterations for glycolytic and oxidative phosphorylation enzymes. Nna proteins localize to mitochondria, loss of NnaD/Nna1 produces mitochondrial abnormalities, and pcd mice display altered proteolytic processing of Nna1 interacting proteins. Our studies indicate that Nna1 loss of function results in altered bioenergetics and mitochondrial dysfunction.