A combination of methods for the preparation of highly intact mitoplasts from beef heart mitochondria

Unraveling the mechanisms of cardiolipin function: The role of oxidative polymerization of unsaturated acyl chains

Cardiolipin is a unique phospholipid of the inner mitochondrial membrane (IMM) as well as in bacteria. It performs several vital functions such as resisting osmotic rupture and stabilizing the supramolecular structure of large membrane proteins, like ATP synthases and respirasomes. The process of cardiolipin biosynthesis results in the production of immature cardiolipin. A subsequent step is required for its maturation when its acyl groups are replaced with unsaturated acyl chains, primarily linoleic acid. Linoleic acid is the major fatty acid of cardiolipin across all organs and tissues, except for the brain. Linoleic acid is not synthesized by mammalian cells. It has the unique ability to undergo oxidative polymerization at a moderately accelerated rate compared to other unsaturated fatty acids. This property can enable cardiolipin to form covalently bonded net-like structures essential for maintaining the complex geometry of the IMM and gluing the quaternary structure of large IMM protein complexes. Unlike triglycerides, phospholipids possess only two covalently linked acyl chains, which constrain their capacity to develop robust and complicated structures through oxidative polymerization of unsaturated acyl chains. Cardiolipin, on the other hand, has four fatty acids at its disposal to form covalently bonded polymer structures. Despite its significance, the oxidative polymerization of cardiolipin has been overlooked due to the negative perception surrounding biological oxidation and methodological difficulties. Here, we discuss an intriguing hypothesis that oxidative polymerization of cardiolipin is essential for the structure and function of cardiolipin in the IMM in physiological conditions. In addition, we highlight current challenges associated with the identification and characterization of oxidative polymerization of cardiolipin in vivo. Altogether, the study provides a better understanding of the structural and functional role of cardiolipin in mitochondria.

Measuring Mitochondrial Function in Permeabilized Cells Using the Seahorse XF Analyzer or a Clark-Type Oxygen Electrode

Measurements of mitochondrial respiration in intact cells can help define metabolism and its dysregulation in fields such as cancer, metabolic disease, immunology, and neurodegeneration. Although cells can be offered various substrates in the assay medium, many cell types can oxidize stored pools of energy substrates. A general bioenergetic profile can therefore be obtained using intact cells, but the inability to control substrate provision to the mitochondria can restrict an in-depth, mechanistic understanding. Mitochondria can be isolated from intact cells, but the yield and quality of the end product is often poor and prone to subselection during isolation. Plasma membrane permeabilization of cells provides a solution to this challenge, allowing experimental control of the medium surrounding the mitochondria. This unit describes techniques to measure respiration in permeabilized adherent cells using a Seahorse XF Analyzer or permeabilized suspended cells in a Hansatech Oxygraph.

Probes of the mitochondrial cAMP-dependent protein kinase

The development of a fluorescent assay to detect activity of the mitochondrial cAMP-dependent protein kinase (PKA) is described. A peptide-based sensor was utilized to quantify the relative amount of PKA activity present in each compartment of the mitochondria (the outer membrane, the intermembrane space, and the matrix). In the process of validating this assay, we discovered that PKA activity is regulated by the protease calpain. Upon exposure of bovine heart mitochondria to digitonin, Ca(2+), and a variety of electron transport chain inhibitors, the regulatory subunits of the PKA holoenzyme (R2C2) are digested, releasing active catalytic subunits. This proteolysis is attenuated by calpain inhibitor I (ALLN). This article is part of a Special Issue entitled: Inhibitors of Protein Kinases (2012).

Suborganelle sensing of mitochondrial cAMP-dependent protein kinase activity

A fluorescent sensor of protein kinase activity has been developed and used to characterize the compartmentalized location of cAMP-dependent protein kinase activity in mitochondria. The sensor functions via a phosphorylation-induced release of a quencher from a peptide-based substrate, producing a 150-fold enhancement in fluorescence. The quenching phenomenon transpires via interaction of the quencher with Arg residues positioned on the peptide substrate. Although the cAMP-dependent protein kinase is known to be present in mitochondria, the relative amount of enzyme positioned in the major compartments (outer membrane, intermembrane space, and the matrix) of the organelle is unclear. The fluorescent sensor developed in this study was used to reveal the relative matrix/intermembrane space/outer membrane (85:6:9) distribution of PKA in bovine heart mitochondria.

Mitochondrial superoxide anion production and release into intermembrane space

The topological distribution of ubiquinone in the mitochondrial respiratory chain suggests that both ubiquinone pools may participate in O2.- production and, hence, are vectorially released into the matrix and intermembrane space. Mitoplasts, obtained by either digitonin or hypotonic KCl treatment, are a suitable experimental model for measuring O2.- in the intermembrane space. The use of membrane-impermeable spin-broadening agents strengthens the notion that part of the O2.- generated by the respiratory chain may be released into the intermembrane space. This, together with the putative occurrence of a Cu, Zn-superoxide dismutase in this compartment may account for part of H2O2 released by mitochondria and contributing to a cytosolic steady-state level of this species in cytosol.

Heterogeneous nuclear ribonucleoprotein K protein associates with multiple mitochondrial transcripts within the organelle

Heterogeneous nuclear ribonucleoprotein K (hnRNP K) protein interacts with a subset of cellular RNAs. We used K protein as a bait in the yeast three-hybrid screen to identify RNAs that bind K protein in vivo. A large number of K protein-binding RNA clones were identified from a human hybrid RNA library. These sequences consisted of C-rich patches and were G-poor. Unexpectedly, several of the RNA clones were encoded by the mitochondrial genome. In a subsequent three-hybrid screen of a hybrid RNA library generated from a mouse liver mitochondrial genome, K protein bound RNA sequences encoded by different loci spanning nearly the entire mitochondrial genome. Western blot analysis of extracts from mitochondria and mitochondrial fractions showed that K protein is localized within mitoplasts. Reverse transcriptase PCR of RNA co-immunoprecipitated with K protein from lysates of isolated mitochondria showed that K protein is associated with several processed mitochondrial transcripts. In contrast, in the same assay, the polycistronic nascent mtRNA bound K protein weakly or not at all. Results of this study suggest that K protein acts within functional modules that are responsible for expression of genes in mitochondria.

Cyclic adenosine monophosphate-dependent phosphorylation of mammalian mitochondrial proteins: enzyme and substrate characterization and functional role

A study is presented on cyclic adenosine monophosphate- (cAMP-) dependent phosphorylation of mammalian mitochondrial proteins. Immunodetection with specific antibodies reveals the presence of the catalytic and the regulatory subunits of cAMP-dependent protein kinase (PKA) in the inner membrane and matrix of bovine heart mitochondria. The mitochondrial cAMP-dependent protein kinase phosphorylates mitochondrial proteins of 29, 18, and 6.5 kDa. With added histone as substrate, PKA exhibits affinities for ATP and cAMP and pH optimum comparable to those of the cytosolic PKA. Among the mitochondrial proteins phosphorylated by PKA, one is the nuclear-encoded (NDUFS4 gene) 18 kDa subunit of complex I, which has phosphorylation consensus sites in the C terminus and in the presequence. cAMP promotes phosphorylation of the 18 kDa subunit of complex I in myoblasts in culture and in their isolated mitoplast fraction. In both cases cAMP-dependent phosphorylation of the 18 kDa subunit of complex I is accompanied by enhancement of the activity of the complex. These results, and the finding of mutations in the NDUFS4 gene in patients with complex I deficiency, provide evidence showing that cAMP-dependent phosphorylation of the 18 kDa subunit of complex I plays a major role in the control of the mitochondrial respiratory activity.

Arachidonic acid interaction with the mitochondrial electron transport chain promotes reactive oxygen species generation

A study has been carried out on the interaction of arachidonic acid and other long chain free fatty acids with bovine heart mitochondria. It is shown that arachidonic acid causes an uncoupling effect under state 4 respiration of intact mitochondria as well as a marked inhibition of uncoupled respiration. While, under our conditions, the uncoupling effect is independent of the fatty acid species considered, the inhibition is stronger for unsaturated acids. Experiments carried out with mitochondrial particles indicated that the arachidonic acid dependent decrease of the respiratory activity is caused by a selective inhibition of Complex I and III. It is also shown that arachidonic acid causes a remarkable increase of hydrogen peroxide production when added to mitochondria respiring with either pyruvate+malate or succinate as substrate. The production of reactive oxygen species (ROS) at the coupling site II was almost double than that at site I. The results obtained are discussed with regard to the impairment of the mitochondrial respiratory activity as occurring during the heart ischemia/reperfusion process.