Max Dolder

The role of creatine kinase in inhibition of mitochondrial permeability transition.

Cyclosporin A sensitive swelling of mitochondria isolated from control mouse livers and from the livers of transgenic mice expressing human ubiquitous mitochondrial creatine kinase occurred in the presence of both 40 μM calcium and 5 μM atractyloside which was accompanied by a 2.5‐fold increase over state 4 respiration rates. Creatine and cyclocreatine inhibited the latter only in transgenic liver mitochondria. Protein complexes isolated from detergent solubilised rat brain extracts, containing octameric mitochondrial creatine kinase, porin and the adenine nucleotide translocator, were reconstituted into malate loaded lipid vesicles. Dimerisation of creatine kinase in the complexes and exposure of the reconstituted complexes to 200 μM calcium induced a cyclosporin A sensitive malate release. No malate release occurred with complexes containing octameric creatine kinase under the same conditions.

Inhibition of the Mitochondrial Permeability Transition by Creatine Kinase Substrates REQUIREMENT FOR MICROCOMPARTMENTATION

Mitochondria from transgenic mice, expressing enzymatically active mitochondrial creatine kinase in liver, were analyzed for opening of the permeability transition pore in the absence and presence of creatine kinase substrates but with no external adenine nucleotides added. In mitochondria from these transgenic mice, cyclosporin A-inhibited pore opening was delayed by creatine or cyclocreatine but not by β-guanidinopropionic acid. This observation correlated with the ability of these substrates to stimulate state 3 respiration in the presence of extramitochondrial ATP. The dependence of transition pore opening on calcium and magnesium concentration was studied in the presence and absence of creatine. If mitochondrial creatine kinase activity decreased (i.e. by omitting magnesium from the medium), protection of permeability transition pore opening by creatine or cyclocreatine was no longer seen. Likewise, when creatine kinase was added externally to liver mitochondria from wild-type mice that do not express mitochondrial creatine kinase in liver, no protective effect on pore opening by creatine and its analog was observed. All these findings indicate that mitochondrial creatine kinase activity located within the intermembrane and intercristae space, in conjunction with its tight functional coupling to oxidative phosphorylation, via the adenine nucleotide translocase, can modulate mitochondrial permeability transition in the presence of creatine. These results are of relevance for the design of creatine analogs for cell protection as potential adjuvant therapeutic tools against neurodegenerative diseases.

RECONSTITUTED ADENINE NUCLEOTIDE TRANSLOCASE FORMS A CHANNEL FOR SMALL MOLECULES COMPARABLE TO THE MITOCHONDRIAL PERMEABILITY TRANSITION PORE

Highly purified adenylate translocase (ANT) from rat heart mitochondria was functionally reconstituted as ATP/ADP exchange carrier in asolectin/cardiolipin vesicles. The ANT preparations used were free of porin, cyclophilin D, and Bax as analysed immunologically and by activity measurements. After pre‐loading the ANT‐containing proteoliposomes with ATP, malate or AMP, a gradual release of the trapped compounds by increasing the external Ca2+ concentrations could be demonstrated. N‐Methyl‐Val‐4‐cyclosporin did not inhibit the Ca2+ dependent release of internal substances from ANT liposomes. This inhibitor was found to be specific for the mitochondrial permeability transition pore (MTP) in intact mitochondria or reconstituted MTP‐like protein complexes (e.g. hexokinase, porin, ANT complex). However, ADP in concentrations >20 μM inhibited the liberation of internal compounds, while in contrast, atractyloside (30 μM) and HgCl2 (5 μM) both induced permeability of the ANT‐containing liposomes resulting in a release of trapped substances. These results strongly suggest that ANT itself is capable to adopt a pore‐like structure under conditions known to induce the permeability transition in mitochondria.

Mitochondrial Creatine Kinase in Contact Sites: Interaction with Porin and Adenine Nucleotide Translocase, Role in Permeability Transition and Sensitivity to Oxidative Damage

The creatine/phosphocreatine circuit provides an efficient energy buffering and transport system in a variety of cells with high and fluctuating energy requirements. It connects sites of energy production (mitochondria, glycolysis) with sites of energy consumption (various cellular ATPases). The cellular creatine/phosphocreatine pool is linked to the ATP/ADP pool by the action of different isoforms of creatine kinase located at distinct subcellular compartments. Octameric mitochondrial creatine kinase (MtCK), together with porin and adenine nucleotide translocase, forms a microcompartment at contact sites between inner and outer mitochondrial membranes and facilitates the production and export into the cytosol of phosphocreatine. MtCK is probably in direct protein-protein contact with outer membrane porin, whereas interaction with inner membrane adenine nucleotide translocase is rather mediated by acidic phopholipids (like cardiolipin) present in significant amounts in the inner membrane. Octamer-dimer transitions of MtCK as well as different creatine kinase substrates have a profound influence on controlling mitochondrial permeability transition (MPT). Inactivation by reactive oxygen species of MtCK and destabilization of its octameric structure are factors that contribute to impairment of energy homeostasis and facilitated opening of the MPT pore, which eventually lead to tissue damage during periods of ischemia/reperfusion.

Mitochondrial Creatine Kinase and Mitochondrial Outer Membrane Porin Show a Direct Interaction That Is Modulated by Calcium

Mitochondrial creatine kinase (MtCK) co-localizes with mitochondrial porin (voltage-dependent anion channel) and adenine nucleotide translocator in mitochondrial contact sites. A specific, direct protein-protein interaction between MtCK and mitochondrial porin was demonstrated using surface plasmon resonance spectroscopy. This interaction was independent of the immobilized binding partner (porin reconstituted in liposomes or MtCK) or the analyzed isoform (chicken sarcomeric MtCK or human ubiquitous MtCK, human recombinant porin, or purified bovine porin). Increased ionic strength reduced the binding of MtCK to porin, suggesting predominantly ionic interactions. By contrast, micromolar concentrations of Ca2+ increased the amount of bound MtCK, indicating a physiological regulation of complex formation. No interaction of MtCK with reconstituted adenine nucleotide translocator was detectable in our experimental setup. The relevance of these findings for structure and function of mitochondrial contact sites is discussed.

Creatine kinase: an enzyme with a central role in cellular energy metabolism.

The enzyme creatine kinase (CK), catalyzing the reversible transfer of the N-phosphoryl group from phosphocreatine (PCr) to ADP to regenerate ATP, plays a key role in the energy homeostasis of cells with intermittently high, fluctuating energy requirements, e.g. in skeletal and cardiac muscle, neurons, photoreceptors, spermatozoa and electrocytes. Cytosolic CK isoenzyme(s) (MM-, MBand BB-CK) are always coexpressed in a tissue-specific fashion together with a mitochondrial isoform. Using biochemical fractionation and in situ localization, one was able to show that the CK isoenzymes, earlier considered to be strictly soluble, are in fact compartmentalized subcellularly and coupled functionally and/or structurally either to sites of energy production (glycolysis and mitochondria) or energy consumption (cellular ATPases, such as the actomyosin ATPase and SR-Ca 2+-ATPase), thus forming an intricate, highly regulated energy distribution network, the PCr-circuit or PCr-shuttle (Fig. 1, [1]). This non-equilibrium energy transport model has been challenged, based upon global 31p-NMR experiments, measuring CK-mediated flux in muscles at different work-loads [2,3]. The conclusions reached were that the CK system is in equilibrium with the substrates and behaves like a solution of well-mixed enzymes, that effects of compartmentation were negligible with respect to total cellular bioenergetics and that thermodynamic characteristics of the cytosol could be predicted as if the CK metabolites were freely mixing in solution.

The micelle to vesicle transition of lipids and detergents in the presence of a membrane protein: towards a rationale for 2D crystallization

The assembly of two‐dimensional membrane protein crystals in the presence of lipids was analyzed with quasielastic light scattering and electron microscopy. Mixtures of detergent‐solubilized lipids and/or proteins were submitted to slow or rapid dilution while measuring the hydrodynamic radii of the aggregates. Lipids alone exhibited λ‐shaped dilution curves with intermediate rod‐shaped particles that converted into small vesicles. Depending on the protein‐protein and protein‐lipid interactions, detergent‐solubilized protein‐lipid mixtures showed a sharp transition from micelles to large, densely packed proteoliposomes. Electron microscopy revealed that formation of crystals occurred shortly after this phase transition.

Oligomeric state and membrane binding behaviour of creatine kinase isoenzymes: implications for cellular function and mitochondrial structure.

The membrane binding properties of cytosolic and mitochondrial creatine kinase isoenzymes are reviewed in this article. Differences between both dimeric and octameric mitochondrial creatine kinase (Mi-CK) attached to membranes and the unbound form are elaborated with respect to possible biological function. The formation of crystalline mitochondrial inclusions under pathological conditions and its possible origin in the membrane attachment capabilities of Mi-CK are discussed. Finally, the implications of these results on mitochondrial energy transduction and structure are presented.

Cholesterol in Negatively Charged Lipid Bilayers Modulates the Effect of the Antimicrobial Protein Granulysin

The release of granulysin, a 9-kDa cationic protein, from lysosomal granules of cytotoxic T lymphocytes and natural killer cells plays an important role in host defense against microbial pathogens. Granulysin is endocytosed by the infected target cell via lipid rafts and kills subsequently intracellular bacteria. The mechanism by which granulysin binds to eukaryotic and prokaryotic cells but lyses only the latter is not well understood. We have studied the effect of granulysin on large unilamellar vesicles (LUVs) and supported bilayers with prokaryotic and eukaryotic lipid mixtures or model membranes with various lipid compositions and charges. Binding of granulysin to bilayers with negative charges, as typically found in bacteria and lipid rafts of eukaryotic cells, was shown by immunoblotting. Fluorescence release assays using LUV revealed an increase in permeability of prokaryotic, negatively charged and lipid raft-like bilayers devoid of cholesterol. Changes in permeability of these bilayers could be correlated to defects of various sizes penetrating supported bilayers as shown by atomic force microscopy. Based on these results, we conclude that granulysin causes defects in negatively charged cholesterol-free membranes, a membrane composition typically found in bacteria. In contrast, granulysin is able to bind to lipid rafts in eukaryotic cell membranes, where it is taken up by the endocytotic pathway, leaving the cell intact.

Compositional heterogeneity reflects partial dehydration in three-dimensional crystals of bacteriorhodopsin.

Absorption, fluorescence and excitation spectra of three-dimensional bacteriorhodopsin crystals harvested from a lipidic cubic phase are presented. The combination of the spectroscopic experiments performed at room temperature, controlled pH and full external hydration reveals the presence of three distinct protein species. Besides the well-known form observed in purple membrane, we find two other species with a relative contribution of up to 30%. As the spectra are similar to those of dehydrated or deionized membranes containing bacteriorhodopsin, we suggest that amino acid residues, located in the vicinity of the retinal chromophore, have changed their protonation state. We propose partial dehydration during crystallization and/or room temperature conditions as the main source of this heterogeneity. This assignment is supported by an experiment showing interconversion of the species upon intentional dehydration and by crystallographic data, which have indicated an in-plane unit cell in 3D crystals comparable to that of dehydrated bacteriorhodopsin membranes. Full hydration of the proteins after the water-withdrawing crystallization process is hampered. We suggest that this hindered water diffusion originates mainly from a closure of hydrophobic crystal surfaces by lipid bilayers. The present spectroscopic work complements the crystallographic data, due to its ability to determine quantitatively compositional heterogeneity resulting from proteins in different protonation states.