Mitochondria-specific Protein Research Articles

A system for inducible mitochondria-specific protein degradation in vivo

Targeted protein degradation systems developed for eukaryotes employ cytoplasmic machineries to perform proteolysis. This has prevented mitochondria-specific analysis of proteins that localize to multiple locations, for example, the mitochondria and the nucleus. Here, we present an inducible mitochondria-specific protein degradation system in Saccharomyces cerevisiae based on the Mesoplasma florum Lon (mf-Lon) protease and its corresponding ssrA tag (called PDT). We show that mitochondrially targeted mf-Lon protease efficiently and selectively degrades a PDT-tagged reporter protein localized to the mitochondrial matrix. The degradation can be induced by depleting adenine from the medium, and tuned by altering the promoter strength of the MF-LON gene. We furthermore demonstrate that mf-Lon specifically degrades endogenous, PDT-tagged mitochondrial proteins. Finally, we show that mf-Lon-dependent PDT degradation can also be achieved in human mitochondria. In summary, this system provides an efficient tool to selectively analyze the mitochondrial function of dually localized proteins.

Ovarian cancer cells regulate their mitochondrial content and high mitochondrial content is associated with a poor prognosis

Most cancer patients ultimately die from the consequences of distant metastases. As metastasis formation consumes energy mitochondria play an important role during this process as they are the most important cellular organelle to synthesise the energy rich substrate ATP, which provides the necessary energy to enable distant metastasis formation. However, mitochondria are also important for the execution of apoptosis, a process which limits metastasis formation. We therefore wanted to investigate the mitochondrial content in ovarian cancer cells and link its presence to the patient’s prognosis in order to analyse which of the two opposing functions of mitochondria dominates during the malignant progression of ovarian cancer. Monoclonal antibodies directed against different mitochondrial specific proteins, namely heat shock proteins 60 (HSP60), fumarase and succinic dehydrogenase, were used in immunohistochemistry in preliminary experiments to identify the antibody most suited to detect mitochondria in ovarian cancer cells in clinical tissue samples. The clearest staining pattern, which even delineated individual mitochondria, was seen with the anti-HSP60 antibody, which was used for the subsequent clinical study staining primary ovarian cancers (n = 155), borderline tumours (n = 24) and recurrent ovarian cancers (n = 26). The staining results were semi-quantitatively scored into three groups according to their mitochondrial content: low (n = 26), intermediate (n = 50) and high (n = 84). Survival analysis showed that high mitochondrial content correlated with a statistically significant overall reduced survival rate In addition to the clinical tissue samples, mitochondrial content was analysed in ovarian cancer cells grown in vitro (cell lines: OVCAR8, SKOV3, OVCAR3 and COV644) and in vivo in severe combined immunodeficiency (SCID) mice.In in vivo grown SKOV3 and OVCAR8 cells, the number of mitochondria positive cells was markedly down-regulated compared to the in vitro grown cells indicating that mitochondrial number is subject to regulatory processes. As high mitochondrial content is associated with a poor prognosis, the provision of high energy substrates by the mitochondria seems to be more important for metastasis formation than the inhibition of apoptotic cell death, which is also mediated by mitochondria. In vivo and in vitro grown human ovarian cancer cells showed that the mitochondrial content is highly adaptable to the growth condition of the cancer cells.

Structure of the mitoribosomal small subunit with streptomycin reveals Fe-S clusters and physiological molecules.

The mitoribosome regulates cellular energy production, and its dysfunction is associated with aging. Inhibition of the mitoribosome can be caused by off-target binding of antimicrobial drugs and was shown to be coupled with a bilateral decreased visual acuity. Previously, we reported mitochondria-specific protein aspects of the mitoribosome, and in this article we present a 2.4-Å resolution structure of the small subunit in a complex with the anti-tuberculosis drug streptomycin that reveals roles of non-protein components. We found iron-sulfur clusters that are coordinated by different mitoribosomal proteins, nicotinamide adenine dinucleotide (NAD) associated with rRNA insertion, and posttranslational modifications. This is the first evidence of inter-protein coordination of iron-sulfur, and the finding of iron-sulfur clusters and NAD as fundamental building blocks of the mitoribosome directly links to mitochondrial disease and aging. We also report details of streptomycin interactions, suggesting that the mitoribosome-bound streptomycin is likely to be in hydrated gem-diol form and can be subjected to other modifications by the cellular milieu. The presented approach of adding antibiotics to cultured cells can be used to define their native structures in a bound form under more physiological conditions, and since streptomycin is a widely used drug for treatment, the newly resolved features can serve as determinants for targeting.

Functional analyses of mitoribosome 54S subunit devoid of mitochondria-specific protein sequences.

In Saccharomyces cerevisiae, mitoribosomes are composed of a 54S large subunit (mtLSU) and a 37S small subunit (mtSSU). The two subunits altogether contain 73 mitoribosome proteins (MRPs) and two ribosomal RNAs (rRNAs). Although mitoribosomes preserve some similarities with their bacterial counterparts, they have significantly diverged by acquiring new proteins, protein extensions, and new RNA segments, adapting the mitoribosome to the synthesis of highly hydrophobic membrane proteins. In this study, we investigated the functional relevance of mitochondria-specific protein extensions at the C-terminus (C) or N-terminus (N) present in 19 proteins of the mtLSU. The studied mitochondria-specific extensions consist of long tails and loops extending from globular domains that mainly interact with mitochondria-specific proteins and 21S rRNA moieties extensions. The expression of variants devoid of extensions in uL4 (C), uL5 (N), uL13 (N), uL13 (C), uL16 (C), bL17 (N), bL17 (C), bL21 (24), uL22 (N), uL23 (N), uL23 (C), uL24 (C), bL27 (C), bL28 (N), bL28 (C), uL29 (N), uL29 (C), uL30 (C), bL31 (C), and bL32 (C) did not rescue the mitochondrial protein synthesis capacities and respiratory growth of the respective null mutants. On the contrary, the truncated form of the mitoribosome exit tunnel protein uL24 (N) yields a partially functional mitoribosome. Also, the removal of mitochondria-specific sequences from uL1 (N), uL3 (N), uL16 (N), bL9 (N), bL19 (C), uL29 (C), and bL31 (N) did not affect the mitoribosome function and respiratory growth. The collection of mutants described here provides new means to study and evaluate defective assembly modules in the mitoribosome biogenesis process.

Prohibitin Overexpression Promotes Opa1 Cleavage, Reduces ROS and Improves Respiration Post Hypoxia

Prohibitin is a multifunctional protein highly localized to mitochondria. Prohibitin 1 and 2 can make large oligomeric complexes that are associated with cristae formation and localization of mitochondrial signaling cascades including Opa1, an important mediator of mitochondrial inner membrane fusion, cristae formation, and respiration. These studies were performed to determine any beneficial effects of PHB2 on Opa1, ROS, and mitochondrial function following hypoxic injury. H9c2 cells were infected with adenoviral PHB2 or empty virus for 48 hours. ROS was measured using a mitochondrial matrix-targeted roGFP imaged intermittently in a gas and temperature-controlled imaging plate reader for 1 hour baseline, 3 hours hypoxia, 1 hour reoxygenation. Respiration was measured using a Seahorse Analyzer. PHB2 infection caused significant increase in both PHB2 and PHB1 expression. PHB2 overexpression did not significantly change total Opa1 levels, but did cause a significant increase in the short form of Opa1 as well as an increase in short:long ratio. In addition, PHB2 overexpression was associated with an increase in mitochondrial specific proteins HSP60 (matrix), VDAC (outer membrane) and electron transport chain protein subunits normalized to total cellular proteins indicating a general increase in mitochondrial protein expression and/or biogenesis. In vitro hypoxia (3h) and reperfusion (1 hour) caused a significant increase in ROS production during reoxygenation that was attenuated by PHB2 overexpression (32.8 +/- 1.8 vs 49.6 +/-2.4, % oxidized roGFP, P<.01) PHB2 overexpression also significantly increased mitochondrial basal, maximal and ATP-linked respiration following 3 hours hypoxia and reoxygenation. Overexpression of PHB2 is associated with increased Opa1 cleavage, increased ETC and mitochondrial proteins, but decreased ROS generation and improved respiration post-hypoxia. Therefore, strategies to increase prohibitin expression may be cytoprotective against hypoxic injury.

Inhibitor of protein kinases 1-(4-chlorobenzyl)-3-chloro-4-(3-trifluoromethylphenylamino)-1Н-pyrrole-2,5-dione induces DNA damage and apoptosis in human colon carcinoma cells

Background. The heterocyclic scaffolds are in the list of key structural blocks used at synthesis of novel biologically active compounds. Materials and Methods. The present study addressed the evaluation of the mecha­nisms of the DNA damaging and pro-apoptotic actions in vitro of the maleimide derivative 1-(4-chlorobenzyl)-3-chloro-4-(3-trifluoromethylphenylamino)-1Н-pyrrole-2,5-dione (MI-1) targeting human colon carcinoma cells of HCT116 line. The Western-blot analysis was used to study changes in apoptosis-associated proteins, DNA comet assay under alkaline conditions was applied for evaluation of the DNA-damaging events, and Barton’s assay with diphenylamine was applied for measuring the level of DNA fragmentation in human colon carcinoma cells treated with MI-1 compound. Results. The results of the Western-blot analysis demonstrated that MI-1 induced the apoptosis in HCT116 cells via mitochondria-dependent pathway. It activated caspase 3 via its cleavage in the treated human colon carcinoma cells. Besides, MI-1 increased the content of mitochondria-specific proteins: endonuclease G (EndoG) and the pro-apoptotic cytosolic protein protease-activating factor 1 (Apaf1). At the same time, MI-1 reduced the level of the anti-apoptotic Bcl-2 protein in HCT116 cells. The DNA comet analysis under alkaline conditions of the targeted human colon carcinoma cells of HCT116 line demonstrated that MI-1 induced DNA single-strand breaks in line with the olive tail moment of 13.2. The results of the colorimetric diphenylamine assay in HCT116 cells have shown that cell treatment with MI-1 increased the content of fragmented DNA to 14.2 %. Conclusions. The anti-proliferative action of MI-1 in human colon carcinoma cells of HCT116 line is associated with apoptosis induction via mitochondria-dependent pathway, as well as the DNA damage through single-strand breaks and DNA fragmentation. These data suggest that the 1-(4-chlorobenzyl)-3-chloro-4-(3-trifluoromethylpheny­l­amino)-1Н-pyrrole-2,5-dione (MI-1) might be a promising agent for suppression of growth of colon tumor cells. Keywords: 1Н-pyrrole-2,5-diones, apoptosis, Western-blot assay, comet assay, single-strand breaks, Barton’s assay, DNA fragmentation

PDE5 inhibition rescues mitochondrial dysfunction and angiogenic responses induced by Akt3 inhibition by promotion of PRC expression

Akt3 regulates mitochondrial content in endothelial cells through the inhibition of PGC-1α nuclear localization and is also required for angiogenesis. However, whether there is a direct link between mitochondrial function and angiogenesis is unknown. Here we show that Akt3 depletion in primary endothelial cells results in decreased uncoupled oxygen consumption, increased fission, decreased membrane potential, and increased expression of the mitochondria-specific protein chaperones, HSP60 and HSP10, suggesting that Akt3 is required for mitochondrial homeostasis. Direct inhibition of mitochondrial homeostasis by the model oxidant paraquat results in decreased angiogenesis, showing a direct link between angiogenesis and mitochondrial function. Next, in exploring functional links to PGC-1α, the master regulator of mitochondrial biogenesis, we searched for compounds that induce this process. We found that, sildenafil, a phosphodiesterase 5 inhibitor, induced mitochondrial biogenesis as measured by increased uncoupled oxygen consumption, mitochondrial DNA content, and voltage-dependent anion channel protein expression. Sildenafil rescued the effects on mitochondria by Akt3 depletion or pharmacological inhibition and promoted angiogenesis, further supporting that mitochondrial homeostasis is required for angiogenesis. Sildenafil also induces the expression of PGC-1 family member PRC and can compensate for PGC-1α activity during mitochondrial stress by an Akt3-independent mechanism. The induction of PRC by sildenafil depends upon cAMP and the transcription factor CREB. Thus, PRC can functionally substitute during Akt3 depletion for absent PGC-1α activity to restore mitochondrial homeostasis and promote angiogenesis. These findings show that mitochondrial homeostasis as controlled by the PGC family of transcriptional activators is required for angiogenic responses.

Structural basis of mitochondrial translation.

Translation of mitochondrial messenger RNA (mt-mRNA) is performed by distinct mitoribosomes comprising at least 36 mitochondria-specific proteins. How these mitoribosomal proteins assist in the binding of mt-mRNA and to what extent they are involved in the translocation of transfer RNA (mt-tRNA) is unclear. To visualize the process of translation in human mitochondria, we report ~3.0 Å resolution structure of the human mitoribosome, including the L7/L12 stalk, and eight structures of its functional complexes with mt-mRNA, mt-tRNAs, recycling factor and additional trans factors. The study reveals a transacting protein module LRPPRC-SLIRP that delivers mt-mRNA to the mitoribosomal small subunit through a dedicated platform formed by the mitochondria-specific protein mS39. Mitoribosomal proteins of the large subunit mL40, mL48, and mL64 coordinate translocation of mt-tRNA. The comparison between those structures shows dynamic interactions between the mitoribosome and its ligands, suggesting a sequential mechanism of conformational changes.

Ciliate mitoribosome illuminates evolutionary steps of mitochondrial translation.

To understand the steps involved in the evolution of translation, we used Tetrahymena thermophila, a ciliate with high coding capacity of the mitochondrial genome, as the model organism and characterized its mitochondrial ribosome (mitoribosome) using cryo-EM. The structure of the mitoribosome reveals an assembly of 94-ribosomal proteins and four-rRNAs with an additional protein mass of ~700 kDa on the small subunit, while the large subunit lacks 5S rRNA. The structure also shows that the small subunit head is constrained, tRNA binding sites are formed by mitochondria-specific protein elements, conserved protein bS1 is excluded, and bacterial RNA polymerase binding site is blocked. We provide evidence for anintrinsic protein targeting system through visualization of mitochondria-specific mL105 by the exit tunnel that would facilitate the recruitment of a nascent polypeptide. Functional protein uS3m is encoded by three complementary genes from the nucleus and mitochondrion, establishing a link between genetic drift and mitochondrial translation. Finally, we reannotated nine open reading frames in the mitochondrial genome that code for mitoribosomal proteins.

Distinct pre-initiation steps in human mitochondrial translation

Translation initiation in human mitochondria relies upon specialized mitoribosomes and initiation factors, mtIF2 and mtIF3, which have diverged from their bacterial counterparts. Here we report two distinct mitochondrial pre-initiation assembly steps involving those factors. Single-particle cryo-EM revealed that in the first step, interactions between mitochondria-specific protein mS37 and mtIF3 keep the small mitoribosomal subunit in a conformation favorable for a subsequent accommodation of mtIF2 in the second step. Combination with fluorescence cross-correlation spectroscopy analyses suggests that mtIF3 promotes complex assembly without mRNA or initiator tRNA binding, where exclusion is achieved by the N-terminal and C-terminal domains of mtIF3. Finally, the association of large mitoribosomal subunit is required for initiator tRNA and leaderless mRNA recruitment to form a stable initiation complex. These data reveal fundamental aspects of mammalian protein synthesis that are specific to mitochondria.

Molecular docking and mechanisms of fusaric acid induced mitochondrial sirtuin aberrations in glycolytically and oxidatively poised human hepatocellular carcinoma (HepG2) cells

Fusaric acid (FA) is a ubiquitous yet neglected mycotoxin. The toxicity of FA is associated with mitochondrial dysfunction and oxidative stress. Sirtuins (SIRTs) are key mediators of cell stress responses through deacetylation of antioxidant, mitochondrial maintenance and energy metabolism proteins. Dietary bioactive compounds have profound effects on SIRT activity, however little is known regarding common foodborne toxins and SIRTs. In this study the interaction of FA with mitochondrial SIRTs – SIRT3 and SIRT5, were firstly studied by molecular docking. Thereafter we substantiated the in silico findings by investigating the effect of FA on expression profiles of SIRT3 and SIRT5, and transcriptional and post-transcriptional regulators, PGC-1α and miRNA-30c using western blots and qPCR in vitro. FA was predicted to bind to the active site of SIRT3 and SIRT5 having implications for biological activity. Furthermore, protein expression of SIRT3 and SIRT5 was down-regulated despite elevated mRNA levels. Further experimentation revealed post-transcriptional regulation of both SIRTs as evidenced by elevated miRNA-30c despite induction of PGC-1α. This study highlights the potential of a diet contaminated with FA to dysregulate mitochondrial specific proteins that can lead to initiation and progression of sirtuin related diseases including cancer and insulin resistance.

An Overview on ATP Dependent and Independent Proteases Including an Anterograde to Retrograde Control on Mitochondrial Function; Focus on Diabetes and Diabetic Complications.

Mitochondria are the central power stations of the cell involved with a myriad of cell signalling pathways that contribute for whole health status of the cell. It is a well known fact that not only mitochondrial genome encodes for mitochondrial proteins but there are several other mitochondrial specific proteins encoded by nuclear genome which regulate plethora of cell catabolic and anabolic process. Anterograde pathways include nuclear gene encoded proteins and their specific transport into the mitochondria and regulation of mitochondrial homeostasis. The retrograde pathways include crosstalk between the mitochondria and cytoplasmic proteins. Indeed, ATP dependent and independent proteases are identified to be very critical in balancing anterograde to retrograde signalling and vice versa to maintain the cell viability or cell death. Different experimental studies conducted on silencing the genes of these proteases have shown embryonic lethality, cancer cells death, increased hepatic glucose output, insulin tolerance, increased protein exclusion bodies, mitochondrial dysfunction, and defect in mitochondrial biogenesis, increased inflammation, Apoptosis etc. These experimental studies included from eubacteria to eukaryotes. Hence, many lines of theories proposed these proteases are conservative from eubacteria to eukaryotes. However, the regulation of these proteases at gene level is not clearly understood and still research is warranted. In this review, we articulated the origin and regulation of these proteases and the cross talk between the nucleus and mitochondria vice versa, and highlighted the role of these proteases in diabetes and diabetic complications in human diseases.

Reduced cytochrome oxidase activity and increased protein tyrosine phosphorylation of mitochondria-rich fractions of buffalo (Bubalus bubalis) spermatozoa after a cycle of freezing and thawing.

The motility and fertility of mammalian spermatozoa are compromised when they are cryopreserved. Sperm mitochondrial proteins play a vital role in conferring motility. However, the effects of cryopreservation on mitochondria-specific proteins remain primarily unexplored in domestic animals, including buffaloes, so the present study aimed to evaluate this issue. Mitochondria were isolated from both non-cryopreserved and cryopreserved buffalo spermatozoa by sonication followed by sucrose density gradient ultracentrifugation. The purity of the mitochondrial preparation was assessed by cytochrome oxidase assay and electron microscopy. Mitochondria separated from cryopreserved buffalo spermatozoa were associated with significantly lower (P ≤ 0.05) cytochrome oxidase activity as compared with non-cryopreserved spermatozoa. The intensities of two low-molecular-mass mitochondrial proteins (30.1 kDa and 26.1 kDa) were significantly reduced as compared with the non-cryopreserved group. In addition, in cryopreserved buffalo sperm mitochondria, the intensities of three tyrosine phosphorylated proteins (126.6, 106.7 and 26 kDa) increased significantly compared with the non-cryopreserved group. Of these, tyrosine phosphorylation of the 26-kDa mitochondrial protein of cryopreserved sperm was very intense and unique because it could not be detected in the mitochondria of non-cryopreserved sperm. Thus, the study confirmed that both cytochrome oxidase activity and the proteins of buffalo sperm mitochondria undergo significant cryogenic changes in terms of quantity and quality after a cycle of freezing and thawing and this may be one of the important causes of reduced post-thaw motility and fertility of cryopreserved buffalo spermatozoa.

SMALL MOLECULE PROMOTES MITOCHONDRIAL BIOGENESIS BY UPREGULATING PGC1α IN SH-SY5Y CELLS AND PRIMARY NEURONAL CELLS

Pathogenesis of neurodegenerative diseases such as Parkinson's disease (PD), Huntington's disease, Alzheimer's disease and amyotrophic lateral sclerosis are proposed by mitochondrial abnormalities. Under pathological and physiological conditions regulatory mechanisms of mitochondrial biogenesis is unknown. Transcriptional regulatory network for mitochondrial biogenesis from peroxisome proliferator-activated receptor coactivator-1 (PGC-1) family plays a central role. Mitochondrial biogenesis proteins was studied by western blot and immunofluorescence, while as measurement of reactive oxygen species and mitochondrial biogenesis was studied by flow cytometer and integrity of mitochondrial was studied by electron microscope. IIIM 533 enhances mitochondrial biogenesis through upregulation of PGC-1α which further enhances the activities of NRF1 and NRF2 in differentiated SHSY-5Y cells and murine primary neurons. Furthermore, activation of NRF1 and NRF2 leads to the activation of tfam, mitochondrial specific proteins mitochondrial DNA replication and transcription. After mechanistically confirming the induction of mitochondrial biogenesis by IIIM 533, we further examined its effect on disturbances in mitochondrial dynamics during amyloid beta neurotoxicity. Surprisingly SHSY-5Y cells and murine primary neurons pre-treated with IIIM 533 attenuated mitochondrial damage by activating PGC-1α. In order to demonstrate the involvement of IIIM 533, knock down of PGC-1α increased amyloid beta induced mitochondrial damage. IIIM 533 also rescued SHSY-5Y cells and murine primary neurons from mitochondrial membrane potential loss, reduced elevated ROS and increased cell viability against amyloid beta insult. Based on these data IIIM 533 can be further investigated as a potential anti Alzheimer therapeutic lead.

Unaltered Fatty Acid Uptake, Utilization, and Mitochondrial Respiration in Right Atrial Appendage of Type 2 Diabetic Human Heart

Previous studies on diabetic human heart have shown decreased utilization of fatty acid (FA)/carbohydrate substrates. We hypothesized that increased mitochondrial dysfunction with concomitant decrease in fatty acid utilization would be found in diabetic hearts of patients from the Indian subcontinent that would explain their increased propensity of cardiovascular complications. We have compared mitochondrial respiration in atrial appendage tissue from nondiabetic and diabetic human subjects undergoing elective coronary artery bypass graft (CABG) surgery using high-resolution respirometry with FA and carbohydrate substrate combinations. In the presence of FA, diabetic mitochondria showed unimpaired complex I and electron transferring flavoprotein (ETF) mediated respiration while significantly lower oxygen consumption was noted when succinate was added, indicating the possible derangement of complex II respiration. Unlike other human studies, we report no significant reduction in FA- mediated mitochondrial respiration. However, the carbohydrate protocol indicated impairment of complex I mediated respiration. There was no significant change in the expression of OXPHOS proteins. Unchanged levels of FA uptake proteins like CD36 and FABP, FA oxidation enzyme, long chain acyl CoA deydrogenase (LCAD), regulators of FA metabolism like sirtuins and nuclear receptors, PPARα and transcription coactivators, PGC1α/β were observed in diabetic heart. The acetylation status of mitochondrial specific proteins involved in fatty acid oxidation (acetylated LCAD and PGC1α/β) also showed no significant changes. Diabetic patients showed equal expression of the antioxidant enzymes and other markers of oxidative stress. The unimpaired FA uptake, oxidation and mitochondrial respiration indicates absence of overt mitochondrial dysfunction in type 2 diabetic human heart. Disclosure N. Rj: None. R. Sr: None. V.V. Pillai: None. J. K.: None. S. Gopala: None.

An off-on fluorescent probe for the detection of mitochondria-specific protein persulfidation.

Protein persulfidation is a newly defined oxidative posttranslational modification and plays important roles in many biological processes. Detection of protein persulfidation in living systems is urgently needed to advance the study of H2S/H2Sn-based signalling and cellular redox regulation. Here, we developed a novel off-on fluorescent probe for the detection of persulfidation using a chemical sensor, HQO-SSH, in biological systems. HQO-SSH features fast reaction, good selectivity and high sensitivity. Due to the distinctive features of HQO-SSH, this probe was successfully applied to image protein persulfidation changes in pulmonary cells. We also demonstrated that the probe is suitable for imaging protein persulfidation in lung tissues. In addition, confocal imaging with this method revealed that sulfur mustard, a commonly used chemical warfare agent, decreased mitochondrial protein persulfidation in living lung cells and tissues. Due to these results, this probe holds great promise for exploring the role of protein persulfidation in a variety of pathophysiological conditions.

Mitochondrial ribosome bL34 mutants present diminished translation of cytochrome c oxidase subunits.

Saccharomyces cerevisiae mitoribosomes are specialized in the translation of a few number of highly hydrophobic membrane proteins, components of the oxidative phosphorylation system. Mitochondrial characteristics, such as the membrane system and its redox state driven mitoribosomes evolution through great diversion from their bacterial and cytosolic counterparts. Therefore, mitoribosome presents a considerable number of mitochondrial-specific proteins, as well as new protein extensions. In this work we characterize temperature sensitive mutants of the subunit bL34 present in the 54S large subunit. Although bL34 has bacterial homologs, in yeast it has a long 65 aminoacids mitochondrial N-terminal addressing sequence, here we demonstrate that it can be replaced by the mitochondrial addressing sequence of Neurospora crassa ATP9 gene. The bL34 temperature sensitive mutants present lowered translation of mitochondrial COX1 and COX3, which resulted in reduced cytochrome c oxidase activity and respiratory growth deficiency. The sedimentation properties of bL34 in sucrose gradients suggest that similarly to its bacterial homolog, bL34 is also a later participant in the process of mitoribosome biogenesis.

Structure of the Human Mitochondrial Ribosome Studied In Situ by Cryoelectron Tomography

Mitochondria maintain their own genome and its corresponding protein synthesis machine, the mitochondrial ribosome (mitoribosome). Mitoribosomes primarily synthesize highly hydrophobic proteins ofthe inner mitochondrial membrane. Recent studiesrevealed the complete structure of the isolated mammalian mitoribosome, but its mode of membrane association remained hypothetical. In this study, we used cryoelectron tomography to visualize human mitoribosomes in isolated mitochondria. The subtomogram average of the membrane-associated human mitoribosome reveals a single major contact site with the inner membrane, mediated by the mitochondria-specific protein mL45. A second rRNA-mediated contact site that is present in yeast is absent in humans, resulting in a more variable association of the human mitoribosome with the inner membrane. Despite extensive structural differences of mammalian and fungal mitoribosomal structure, the principal organization of peptide exit tunnel and the mL45 homolog remains invariant, presumably to align the mitoribosome with the membrane-embedded insertion machinery.

Mitophagy and mitochondrial biogenesis in atrial tissue of patients undergoing heart surgery with cardiopulmonary bypass.

Mitophagy occurs during ischemia/reperfusion (I/R) and limits oxidative stress and injury. Mitochondrial turnover was assessed in patients undergoing cardiac surgery involving cardiopulmonary bypass (CPB). Paired biopsies of right atrial appendage before initiation and after weaning from CPB were processed for protein analysis, mitochondrial DNA/nuclear DNA ratio (mtDNA:nucDNA ratio), mtDNA damage, mRNA, and polysome profiling. Mitophagy in the post-CPB samples was evidenced by decreased levels of mitophagy adapters NDP52 and optineurin in whole tissue lysate, decreased Opa1 long form, and translocation of Parkin to the mitochondrial fraction. PCR analysis of mtDNA comparing amplification of short vs. long segments of mtDNA revealed increased damage following cardiac surgery. Surprisingly, a marked increase in several mitochondria-specific protein markers and mtDNA:nucDNA ratio was observed, consistent with increased mitochondrial biogenesis. mRNA analysis suggested that mitochondrial biogenesis was traniscription independent and likely driven by increased translation of existing mRNAs. These findings demonstrate in humans that both mitophagy and mitochondrial biogenesis occur during cardiac surgery involving CPB. We suggest that mitophagy is balanced by mitochondrial biogenesis during I/R stress experienced during surgery. Mitigating mtDNA damage and elucidating mechanisms regulating mitochondrial turnover will lead to interventions to improve outcome after I/R in the setting of heart disease.

SIRT3 mediates decrease of oxidative damage and prevention of ageing in porcine fetal fibroblasts

AimsSirtuin 3 (SIRT3) is a mitochondria-specific protein required for the deacetylation of metabolic enzymes and the action of oxidative phosphorylation by acting as a nicotinamide adenine dinucleotide (NAD+)-dependent deacetylase. SIRT3 increases oxidative stress resistance and prevents mitochondrial decay associated with ageing in response to caloric restriction. However, the effects of SIRT3 on oxidative damage and ageing are not well understood. We investigated the physiological functions of porcine SIRT3 on the damage and ageing in porcine fetal fibroblasts (PFFs). Main methodsOverexpression and knockdown of SIRT3 were confirmed by quantitative real-time polymerase chain reaction (qRT-PCR) and western blot analysis, respectively. All cells were treated with three different stress reagents 12-o-tetradecanoylphorbol-13-acetate (TPA), methanesulfonic acid methylester (MMS), and tert-butylhydroperoxide (t-BHP), respectively, and then examined by flow cytometry following JC-1 (5, 5′, 6, 6′-tetrachloro-1, 1′, 3, 3′-tetraethylbenzimidazol-carbocyanine iodide) staining. Key findingsSIRT3 overexpression enhanced the ability of superoxide dismutase 2 (SOD2) to reduce cellular reactive oxygen species (ROS), which further decreased the damage to the membranes and the organelles of the cells, especially to mitochondria. It inhibited the initial decrease of mitochondrial membrane potential, and prevented the decrease of adenosine triphosphate (ATP) production and activity of Nampt. In contrast, SIRT3 knockdown reduced the ability of SOD2 to increase cellular ROS which was directly correlated with stress-induced oxidative damage and ageing in PFFs. SignificanceOur findings identify one function of SIRT3 in PFFs was to dampen cytotoxicity, and, therefore, to decrease oxidative damage and attenuate ageing possibly by enhancing the activity of SOD2.