Octameric mitochondrial creatine kinase induces and stabilizes contact sites between the inner and outer membrane

Context-dependent roles for ubiquitous mitochondrial creatine kinase CKMT1 in breast cancer progression

Metabolic reprogramming is a hallmark of cancer, enabling cancer cells to rapidly proliferate, invade, and metastasize. We show that creatine levels in metastatic breast cancer cell lines and secondary metastatic tumors are driven by the ubiquitous mitochondrial creatine kinase (CKMT1). We discover that, while CKMT1 is highly expressed in primary tumors and promotes cell viability, it is downregulated in metastasis. We further show that CKMT1 downregulation, as seen in breast cancer metastasis, drives up mitochondrial reactive oxygen species (ROS) levels. CKMT1 downregulation contributes to the migratory and invasive potential of cells by ROS-induced upregulation of adhesion and degradative factors, which can be reversed by antioxidant treatment. Our study thus reconciles conflicting evidence about the roles of metabolites in the creatine metabolic pathway in breast cancer progression and reveals that tight, context-dependent regulation of CKMT1 expression facilitates cell viability, cell migration, and cell invasion, which are hallmarks of metastatic spread.

Mitochondrial Creatine Kinase Attenuates Pathologic Remodeling in Heart Failure

BACKGROUND

Abnormalities in cardiac energy metabolism occur in heart failure (HF) and contribute to contractile dysfunction, but their role, if any, in HF-related pathologic remodeling is much less established. CK (creatine kinase), the primary muscle energy reserve reaction which rapidly provides ATP at the myofibrils and regenerates mitochondrial ADP, is down-regulated in experimental and human HF. We tested the hypotheses that pathologic remodeling in human HF is related to impaired cardiac CK energy metabolism and that rescuing CK attenuates maladaptive hypertrophy in experimental HF.

METHODS

First, in 27 HF patients and 14 healthy subjects, we measured cardiac energetics and left ventricular remodeling using noninvasive magnetic resonance 31P spectroscopy and magnetic resonance imaging, respectively. Second, we tested the impact of metabolic rescue with cardiac-specific overexpression of either Ckmyofib (myofibrillar CK) or Ckmito (mitochondrial CK) on HF-related maladaptive hypertrophy in mice.

RESULTS

In people, pathologic left ventricular hypertrophy and dilatation correlate closely with reduced myocardial ATP levels and rates of ATP synthesis through CK. In mice, transverse aortic constriction-induced left ventricular hypertrophy and dilatation are attenuated by overexpression of CKmito, but not by overexpression of CKmyofib. CKmito overexpression also attenuates hypertrophy after chronic isoproterenol stimulation. CKmito lowers mitochondrial reactive oxygen species, tissue reactive oxygen species levels, and upregulates antioxidants and their promoters. When the CK capacity of CKmito-overexpressing mice is limited by creatine substrate depletion, the protection against pathologic remodeling is lost, suggesting the ADP regenerating capacity of the CKmito reaction rather than CK protein per se is critical in limiting adverse HF remodeling.

CONCLUSIONS

In the failing human heart, pathologic hypertrophy and adverse remodeling are closely related to deficits in ATP levels and in the CK energy reserve reaction. CKmito, sitting at the intersection of cardiac energetics and redox balance, plays a crucial role in attenuating pathologic remodeling in HF. Registration: URL: https://www.clinicaltrials.gov; Unique identifier: NCT00181259.

Creatine Supplementation for Patients with Inflammatory Bowel Diseases: A Scientific Rationale for a Clinical Trial

Based on theoretical considerations, experimental data with cells in vitro, animal studies in vivo, as well as a single case pilot study with one colitis patient, a consolidated hypothesis can be put forward, stating that “oral supplementation with creatine monohydrate (Cr), a pleiotropic cellular energy precursor, is likely to be effective in inducing a favorable response and/or remission in patients with inflammatory bowel diseases (IBD), like ulcerative colitis and/or Crohn’s disease”. A current pilot clinical trial that incorporates the use of oral Cr at a dose of 2 × 7 g per day, over an initial period of 2 months in conjunction with ongoing therapies (NCT02463305) will be informative for the proposed larger, more long-term Cr supplementation study of 2 × 3–5 g of Cr per day for a time of 3–6 months. This strategy should be insightful to the potential for Cr in reducing or alleviating the symptoms of IBD. Supplementation with chemically pure Cr, a natural nutritional supplement, is well tolerated not only by healthy subjects, but also by patients with diverse neuromuscular diseases. If the outcome of such a clinical pilot study with Cr as monotherapy or in conjunction with metformin were positive, oral Cr supplementation could then be used in the future as potentially useful adjuvant therapeutic intervention for patients with IBD, preferably together with standard medication used for treating patients with chronic ulcerative colitis and/or Crohn’s disease.

TAZ encodes tafazzin, a transacylase essential for cardiolipin formation and central to the etiology of Barth syndrome

Tafazzin, which is encoded by the TAZ gene, catalyzes transacylation to form mature cardiolipin and shows preference for the transfer of a linoleic acid (LA) group from phosphatidylcholine (PC) to monolysocardiolipin (MLCL) with influence from mitochondrial membrane curvature. The protein contains domains and motifs involved in targeting, anchoring, and an active site for transacylase activity. Tafazzin activity affects many aspects of mitochondrial structure and function, including that of the electron transport chain, fission-fusion, as well as apoptotic signaling. TAZ mutations are implicated in Barth syndrome, an underdiagnosed and devastating disease that primarily affects male pediatric patients with a broad spectrum of disease pathologies that impact the cardiovascular, neuromuscular, metabolic, and hematologic systems.

Over-expression of mitochondrial creatine kinase in the murine heart improves functional recovery and protects against injury following ischaemia-reperfusion

Aims

Mitochondrial creatine kinase (MtCK) couples ATP production via oxidative phosphorylation to phosphocreatine in the cytosol, which acts as a mobile energy store available for regeneration of ATP at times of high demand. We hypothesized that elevating MtCK would be beneficial in ischaemia-reperfusion (I/R) injury.

Methods and results

Mice were created over-expressing the sarcomeric MtCK gene with αMHC promoter at the Rosa26 locus (MtCK-OE) and compared with wild-type (WT) littermates. MtCK activity was 27% higher than WT, with no change in other CK isoenzymes or creatine levels. Electron microscopy confirmed normal mitochondrial cell density and mitochondrial localization of transgenic protein. Respiration in isolated mitochondria was unaltered and metabolomic analysis by 1 H-NMR suggests that cellular metabolism was not grossly affected by transgene expression. There were no significant differences in cardiac structure or function under baseline conditions by cine-MRI or LV haemodynamics. In Langendorff-perfused hearts subjected to 20 min ischaemia and 30 min reperfusion, MtCK-OE exhibited less ischaemic contracture, and improved functional recovery (Rate pressure product 58% above WT; P < 0.001). These hearts had reduced myocardial infarct size, which was confirmed in vivo: 55 ± 4% in WT vs. 29 ± 4% in MtCK-OE; P < 0.0001). Isolated cardiomyocytes from MtCK-OE hearts exhibited delayed opening of the mitochondrial permeability transition pore (mPTP) compared to WT, which was confirmed by reduced mitochondrial swelling in response to calcium. There was no detectable change in the structural integrity of the mitochondrial membrane.

Conclusions

Modest elevation of MtCK activity in the heart does not adversely affect cellular metabolism, mitochondrial or in vivo cardiac function, but modifies mPTP opening to protect against I/R injury and improve functional recovery. Our findings support MtCK as a prime therapeutic target in myocardial ischaemia.

Mitochondrial Creatine Kinase is Decreased in the Serum of Idiopathic Parkinson's Disease Patients

Mitochondrial creatine kinase (MtCK) is vital in the process of mitochondrial energy metabolism, and mitochondrial dysfunction has been implicated in the pathogenesis of Parkinson’s disease (PD). Therefore, we speculated that MtCK activity could be altered in the serum of PD patients. However, no studies to date have investigated this specific topic, so we sought to investigate the serum MtCK activities among a cohort of PD patients. 50 patients with PD and 30 age-matched controls were recruited for this study. Serum ubiquitous MtCK (uMtCK) and sarcomeric MtCK (sMtCK) activities were assayed using an immunoinhibition method. Correlations between serum uMtCK/sMtCK activities and clinical features/parameters were explored in the PD group. Our study revealed a significant decrease in the uMtCK activity in the PD group when compared with the control group. No significant difference was found in the serum sMtCK activity between the PD and control groups. There was a significant correlation between serum uMtCK activities and the disease progression rate, duration, and age at onset in PD patients. While no significant relationship was found between the serum uMtCK activities and the Hoehn & Yahr stage or main non-motor symptoms scale. There was a significant decrease in the uMtCK activity in the serum of PD patients, which was associated with the rate of disease progression, duration, and age at onset of disease. Therefore, uMtCK activity in serum offers a useful clue for identification of PD biomarkers.

Proteolipid domains form in biomimetic and cardiac mitochondrial vesicles and are regulated by cardiolipin concentration but not monolyso-cardiolipin

Cardiolipin (CL) is an anionic phospholipid mainly located in the inner mitochondrial membrane, where it helps regulate bioenergetics, membrane structure, and apoptosis. Localized, phase-segregated domains of CL are hypothesized to control mitochondrial inner membrane organization. However, the existence and underlying mechanisms regulating these mitochondrial domains are unclear. Here, we first isolated detergent-resistant cardiac mitochondrial membranes that have been reported to be CL-enriched domains. Experiments with different detergents yielded only nonspecific solubilization of mitochondrial phospholipids, suggesting that CL domains are not recoverable with detergents. Next, domain formation was investigated in biomimetic giant unilamellar vesicles (GUVs) and newly synthesized giant mitochondrial vesicles (GMVs) from mouse hearts. Confocal fluorescent imaging revealed that introduction of cytochrome c into membranes promotes macroscopic proteolipid domain formation associated with membrane morphological changes in both GUVs and GMVs. Domain organization was also investigated after lowering tetralinoleoyl-CL concentration and substitution with monolyso-CL, two common modifications observed in cardiac pathologies. Loss of tetralinoleoyl-CL decreased proteolipid domain formation in GUVs, because of a favorable Gibbs-free energy of lipid mixing, whereas addition of monolyso-CL had no effect on lipid mixing. Moreover, murine GMVs generated from cardiac acyl-CoA synthetase-1 knockouts, which have remodeled CL acyl chains, did not perturb proteolipid domains. Finally, lowering the tetralinoleoyl-CL content had a stronger influence on the oxidation status of cytochrome c than did incorporation of monolyso-CL. These results indicate that proteolipid domain formation in the cardiac mitochondrial inner membrane depends on tetralinoleoyl-CL concentration, driven by underlying lipid-mixing properties, but not the presence of monolyso-CL.

Mitochondrial Proteolipid Complexes of Creatine Kinase

Isoforms of creatine kinase (CK) generate and use phosphocreatine, a concentrated and highly diffusible cellular "high energy" intermediate, for the main purpose of energy buffering and transfer in order to maintain cellular energy homeostasis. The mitochondrial CK isoform (mtCK) localizes to the mitochondrial intermembrane and cristae space, where it assembles into peripherally membrane-bound, large cuboidal homooctamers. These are part of proteolipid complexes wherein mtCK directly interacts with cardiolipin and other anionic phospholipids, as well as with the VDAC channel in the outer membrane. This leads to a stabilization and cross-linking of inner and outer mitochondrial membrane, forming so-called contact sites. Also the adenine nucleotide translocator of the inner membrane can be recruited into these proteolipid complexes, probably mediated by cardiolipin. The complexes have functions mainly in energy transfer to the cytosol and stimulation of oxidative phosphorylation, but also in restraining formation of reactive oxygen species and apoptosis. In vitro evidence indicates a putative role of mtCK in mitochondrial phospholipid distribution, and most recently a role in thermogenesis has been proposed. This review summarizes the essential structural and functional data of these mtCK complexes and describes in more detail the more recent advances in phospholipid interaction, thermogenesis, cancer and evolution of mtCK.

The role of succinate and ROS in reperfusion injury - A critical appraisal

We critically assess the proposal that succinate-fuelled reverse electron flow (REF) drives mitochondrial matrix superoxide production from Complex I early in reperfusion, thus acting as a key mediator of ischemia/reperfusion (IR) injury. Real-time surface fluorescence measurements of NAD(P)H and flavoprotein redox state suggest that conditions are unfavourable for REF during early reperfusion. Furthermore, rapid loss of succinate accumulated during ischemia can be explained by its efflux rather than oxidation. Moreover, succinate accumulation during ischemia is not attenuated by ischemic preconditioning (IP) despite powerful cardioprotection. In addition, measurement of intracellular reactive oxygen species (ROS) during reperfusion using surface fluorescence and mitochondrial aconitase activity detected major increases in ROS only after mitochondrial permeability transition pore (mPTP) opening was first detected. We conclude that mPTP opening is probably triggered initially by factors other than ROS, including increased mitochondrial [Ca2+]. However, IP only attenuates [Ca2+] increases later in reperfusion, again after initial mPTP opening, implying that IP regulates mPTP opening through additional mechanisms. One such is mitochondria-bound hexokinase 2 (HK2) which dissociates from mitochondria during ischemia in control hearts but not those subject to IP. Indeed, there is a strong correlation between the extent of HK2 loss from mitochondria during ischemia and infarct size on subsequent reperfusion. Mechanisms linking HK2 dissociation to mPTP sensitisation remain to be fully established but several related processes have been implicated including VDAC1 oligomerisation, the stability of contact sites between the inner and outer membranes, cristae morphology, Bcl-2 family members and mitochondrial fission proteins such as Drp1.

Cardiac mitochondrial dynamics: miR-mediated regulation during cardiac injury

Mitochondrial integrity is indispensable for cardiac health. With the advent of modern imaging technologies, mitochondrial motility and dynamics within the cell are extensively studied. Terminally differentiated and well-structured cardiomyocytes depict little mitochondrial division and fusion, questioning the contribution of mitochondrial fusion proteins (Mitofusin 1/2 and Optic Atrophy 1 protein) and fission factors (Dynamin-like protein 1 and mitochondrial fission 1 protein) in cardiomyocyte homeostasis. Emerging evidences suggest that alterations in mitochondrial morphology from globular, elongated network to punctate fragmented disconnected structures are a pathological response to ensuing cardiac stress and cardiomyocyte cell death, bringing forth the following question, "what maintains this balance between fusion and fission?" The answer hinges upon the classical "junk" DNA: microRNAs, the endogenous non-coding RNAs. Because of their essential role in numerous signaling pathways, microRNAs are considered to play major roles in the pathogenesis of various diseases. Mitochondria are not exempted from microRNA-mediated regulation. This review defines the importance of mitochondrial structural integrity and the microRNA-mitochondrial dynamics tandem, an imminent dimension of the cardiac homeostasis network. Elucidating their coordinated interaction could spur RNA-based therapeutics for resuscitating functional mitochondrial population during cardiovascular disorders.

Who and how in the regulation of mitochondrial cristae shape and function

Mitochondrial adaptation to different physiological conditions highly relies on the regulation of mitochondrial ultrastructure, particularly at the level of cristae compartment. Cristae represent the membrane hub where most of the respiratory complexes embed to account for OXPHOS and energy production in the form of adenosine triphosphate (ATP). Changes in cristae number and shape define the respiratory capacity as well as cell viability. The identification of key regulators of cristae morphology and the understanding of their contribution to the mitochondrial ultrastructure and function have become an strategic goal to understand mitochondrial disorders and to exploit as therapeutic targets. This review summarizes the known regulators of cristae ultrastructure and discusses their contribution and implications for mitochondrial dysfunction.

Isolation of Contact Sites Between Inner and Outer Mitochondrial Membranes

Mitochondria are essential organelles of all eukaryotic cells. They perform a plethora of important metabolic functions and have a highly complex architecture that differs drastically between different cells and tissues. Mitochondria are delimited from the cytosol by the mitochondrial envelope that consists of the outer membrane and the inner membrane. The inner membrane is subdivided into the inner boundary membrane that runs parallel to the outer membrane and the crista membrane. Both sections of the inner membrane are linked by crista junctions. A further important architectural element of mitochondria are the contact sites between outer membrane and inner membrane. These sites were observed a long time ago by classical electron microscopy, but their molecular structure was identified only recently when it was recognized that proteins of crista junctions and proteins of the outer membrane are responsible for these strong contacts. Mitochondrial function is severely affected when contact sites are disturbed. This underlines the notion that mitochondrial architecture and function are intimately connected. In the following a method is described to generate and to isolate membrane vesicles from isolated yeast mitochondria that contain these contact sites.

Cardiolipin in Central Nervous System Physiology and Pathology

Cardiolipin, an anionic phospholipid found primarily in the inner mitochondrial membrane, has many well-defined roles within the peripheral tissues, including the maintenance of mitochondrial membrane fluidity and the regulation of mitochondrial functions. Within the central nervous system (CNS), cardiolipin is found within both neuronal and non-neuronal glial cells, where it regulates metabolic processes, supports mitochondrial functions, and promotes brain cell viability. Furthermore, cardiolipin has been shown to act as an elimination signal and participate in programmed cell death by apoptosis of both neurons and glia. Since cardiolipin is associated with regulating brain homeostasis, the modification of its structure, or even a decrease in the overall levels of cardiolipin, can result in mitochondrial dysfunction, which is a characteristic feature of many diseases. In this review, we outline the various functions of cardiolipin within the cells of the CNS, including neurons, astrocytes, microglia, and oligodendrocytes. In addition, we discuss the role cardiolipin may play in the pathogenesis of the neurodegenerative disorders Alzheimer's disease and Parkinson's disease, as well as traumatic brain injury.

Mg(++) requirement for MtHK binding, and Mg(++) stabilization of mitochondrial membranes via activation of MtHK & MtCK and promotion of mitochondrial permeability transition pore closure: A hypothesis on mechanisms underlying Mg(++)'s antioxidant and cytoprotective effects

Evidence points to magnesium's antioxidant, anti-necrotic, and anti-apoptotic effects in cardio- and neuroprotection. With magnesium being involved in over 300 biochemical reactions, the mechanisms underlying its cytoprotective and antioxidant effects have remained elusive. The profound anti-apoptotic, anabolic, and antioxidant effects of mitochondrion bound hexokinase (MtHk), and the anti-apoptotic, anti-necrotic, and antioxidant functions of mitochondrial creatine kinase (MtCK) have been established over the past few decades. As powerful regulators of the mitochondrial permeability transition pore (PTP), MtHK and MtCK promote anti-apoptosis and anti-necrosis by stabilizing mitochondrial outer and inner membranes. In this article, it is proposed that magnesium is essentially and directly involved in mitochondrial membrane stabilization via (i) Mg(++) ion requirement for the binding of mitochondrial hexokinase (ii) Mg(++)'s allosteric activation of mitochondrial bound hexokinase, and stimulation of mitochondrial bound creatine kinase activities, and (iii) Mg(++) inhibition of PTP opening by Ca(++) ions. These effects of Mg(++) ions are indirectly supplanted by the stimulatory effect of magnesium on the Akt kinase survival pathway. The "Magnesium/Calcium Yin Yang Hypothesis" proposes here that because of the antagonistic effects of Ca(++) and Mg(++) ions in the presence of high Ca(++) ion concentration at MtHK, MtCK, and PTP, magnesium supplementation may provide cytoprotective effects in the treatment of some degenerative diseases and cytopathies with high intracellular [Ca(++)]/ [Mg(++)] ratio at these sites, whether of genetic, developmental, drug induced, ischemic, immune based, toxic, or infectious etiology.

Purification and stability of octameric mitochondrial creatine kinase isoform from herring (Clupea harengus) organ of vision

Creatine kinases (CKs) constitute a large family of isoenzymes that are involved in intracellular energy homeostasis. In cells with high and fluctuating energy requirements ATP level is maintained via phosphocreatine hydrolysis catalyzed by creatine kinase. In contrast to invertebrates and higher vertebrates, in poikilothermic vertebrates the adaptations for the regulation of energy metabolism by changes in the oligomeric state of CK isoforms are not well known. The present study aimed at identification of herring eye CK isoforms and focuses on factors affecting the CK-octamer stability. In addition to the CK octamer, three different dimeric isoforms of CK were detected by cellulose acetate native electrophoresis. Destabilization of octamer was studied in the presence of TSAC substrates and about 50% of octamers dissociated into dimers within 24h. Moreover, we found that the increase of temperature from 4 °C to 30 °C caused rapid inactivation of dimers in TSAC-treated samples but did not affect octameric structures. In a thermostability assay we demonstrated that octamers retain their activity even at 50 °C. Our results indicate that destabilization of the octameric structure can lead to loss of enzyme activity at higher temperatures (above 30 °C). Furthermore, our results based on N-terminal sequence analysis suggest that probably the mitochondrial s-type CK, rather than u-type, is predominantly expressed in herring eye. In conclusion the existence of four various CK isoforms in one organ may reflect complex regulation of energy metabolism in the phototransduction process in teleost fishes.

The role of hexokinase in cardioprotection - mechanism and potential for translation

Mitochondrial permeability transition pore (mPTP) opening plays a critical role in cardiac reperfusion injury and its prevention is cardioprotective. Tumour cell mitochondria usually have high levels of hexokinase isoform 2 (HK2) bound to their outer mitochondrial membranes (OMM) and HK2 binding to heart mitochondria has also been implicated in resistance to reperfusion injury. HK2 dissociates from heart mitochondria during ischaemia, and the extent of this correlates with the infarct size on reperfusion. Here we review the mechanisms and regulations of HK2 binding to mitochondria and how this inhibits mPTP opening and consequent reperfusion injury. Major determinants of HK2 dissociation are the elevated glucose‐6‐phosphate concentrations and decreased pH in ischaemia. These are modulated by the myriad of signalling pathways implicated in preconditioning protocols as a result of a decrease in pre‐ischaemic glycogen content. Loss of mitochondrial HK2 during ischaemia is associated with permeabilization of the OMM to cytochrome c, which leads to greater reactive oxygen species production and mPTP opening during reperfusion. Potential interactions between HK2 and OMM proteins associated with mitochondrial fission (e.g. Drp1) and apoptosis (B‐cell lymphoma 2 family members) in these processes are examined. Also considered is the role of HK2 binding in stabilizing contact sites between the OMM and the inner membrane. Breakage of these during ischaemia is proposed to facilitate cytochrome c loss during ischaemia while increasing mPTP opening and compromising cellular bioenergetics during reperfusion. We end by highlighting the many unanswered questions and discussing the potential of modulating mitochondrial HK2 binding as a pharmacological target.

Linked Articles

This article is part of a themed section on Conditioning the Heart – Pathways to Translation. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue‐8

The mitochondrial permeability transition: a current perspective on its identity and role in ischaemia/reperfusion injury

The mitochondrial permeability transition pore (MPTP) is a non-specific pore that opens in the inner mitochondrial membrane (IMM) when matrix [Ca(2+)] is high, especially when accompanied by oxidative stress, high [Pi] and adenine nucleotide depletion. Such conditions occur during ischaemia and subsequent reperfusion, when MPTP opening is known to occur and cause irreversible damage to the heart. Matrix cyclophilin D facilitates MPTP opening and is the target of its inhibition by cyclosporin A that is cardioprotective. Less certainty exists over the composition of the pore itself, with structural and/or regulatory roles proposed for the adenine nucleotide translocase, the phosphate carrier and the FoF1 ATP synthase. Here we critically review the supporting data for the role of each and suggest that they may interact with each other through their bound cardiolipin to form the ATP synthasome. We propose that under conditions favouring MPTP opening, calcium-triggered conformational changes in these proteins may perturb the interface between them generating the pore. Proteins associated with the outer mitochondrial membrane (OMM), such as members of the Bcl-2 family and hexokinase (HK), whilst not directly involved in pore formation, may regulate MPTP opening through interactions between OMM and IMM proteins at "contact sites". Recent evidence suggests that cardioprotective protocols such as preconditioning inhibit MPTP opening at reperfusion by preventing the loss of mitochondrial bound HK2 that stabilises these contact sites. Contact site breakage both sensitises the MPTP to [Ca(2+)] and facilitates cytochrome c loss from the intermembrane space leading to greater ROS production and further MPTP opening. This article is part of a Special Issue entitled "Mitochondria: From Basic Mitochondrial Biology to Cardiovascular Disease".

Hearts of some Antarctic fishes lack mitochondrial creatine kinase

Creatine kinase (CK; EC 2.7.3.2) functions as a spatial and temporal energy buffer, dampening fluctuations in ATP levels as ATP supply and demand change. There are four CK isoforms in mammals, two cytosolic isoforms (muscle [M-CK] and brain [B-CK]), and two mitochondrial isoforms (ubiquitous [uMtCK] and sarcomeric [sMtCK]). Mammalian oxidative muscle couples expression of sMtCK with M-CK, creating an energy shuttle between mitochondria and myofibrils. We hypothesized that the expression pattern and activity of CK would differ between hearts of red- and white-blooded Antarctic notothenioid fishes due to their striking differences in cardiac ultrastructure. Hearts of white-blooded icefishes (family Channichthyidae) have significantly higher mitochondrial densities compared to red-blooded species, decreasing the diffusion distance for ATP between mitochondria and myofibrils and potentially minimizing the need for CK. The distribution of CK isoforms was evaluated using western blotting and maximal activity of CK was measured in mitochondrial and cytosolic fractions and tissue homogenates of heart ventricles of red- and white-blooded notothenioids. Transcript abundance of sMtCK and M-CK was also quantified. Overall, CK activity is similar between hearts of red- and white-blooded notothenioids but hearts of icefishes lack MtCK and have higher activities of M-CK in the cytosol compared to red-blooded fishes. The absence of MtCK may compromise cardiac function under stressful conditions when ATP supply becomes limiting.

CKMT1 regulates the mitochondrial permeability transition pore in a process that provides evidence for alternative forms of the complex

The permeability transition pore (PT-pore) mediates cell death through the dissipation of the mitochondrial membrane potential (ΔΨm). Because the exact composition of the PT-pore is controversial, it is crucial to investigate the actual molecular constituents and regulators of this complex. We found that mitochondrial creatine kinase-1 (CKMT1) is a universal and functionally necessary gatekeeper of the PT-pore, as its depletion induces mitochondrial depolarization and apoptotic cell death. This can be inhibited efficiently by bongkrekic acid, a compound that is widely used to inhibit the PT-pore. However, when the 'classical' PT-pore subunits cyclophilin D and VDAC1 are pharmacologically inhibited or their expression levels reduced, mitochondrial depolarization by CKMT1 depletion remains unaffected. At later stages of drug-induced apoptosis, CKMT1 levels are reduced, suggesting that CKMT1 downregulation acts to reinforce the commitment of cells to apoptosis. A novel high-molecular-mass CKMT1 complex that is distinct from the known CKMT1 octamer disintegrates upon treatment with cytotoxic drugs, concomitant with mitochondrial depolarization. Our study provides evidence that CKMT1 is a key regulator of the PT-pore through a complex that is distinct from the classical PT-pore.

Unchanged mitochondrial organization and compartmentation of high-energy phosphates in creatine-deficient GAMT-/- mouse hearts

Disruption of the creatine kinase (CK) system in hearts of CK-deficient mice leads to changes in the ultrastructure and regulation of mitochondrial respiration. We expected to see similar changes in creatine-deficient mice, which lack the enzyme guanidinoacetate methyltransferase (GAMT) to produce creatine. The aim of this study was to characterize the changes in cardiomyocyte mitochondrial organization, regulation of respiration, and intracellular compartmentation associated with GAMT deficiency. Three-dimensional mitochondrial organization was assessed by confocal microscopy. On populations of permeabilized cardiomyocytes, we recorded ADP and ATP kinetics of respiration, competition between mitochondria and pyruvate kinase for ADP produced by ATPases, ADP kinetics of endogenous pyruvate kinase, and ATP kinetics of ATPases. These data were analyzed by mathematical models to estimate intracellular compartmentation. Quantitative analysis of morphological and kinetic data as well as derived model fits showed no difference between GAMT-deficient and wild-type mice. We conclude that inactivation of the CK system by GAMT deficiency does not alter mitochondrial organization and intracellular compartmentation in relaxed cardiomyocytes. Thus, our results suggest that the healthy heart is able to preserve cardiac function at a basal level in the absence of CK-facilitated energy transfer without compromising intracellular organization and the regulation of mitochondrial energy homeostasis. This raises questions on the importance of the CK system as a spatial energy buffer in unstressed cardiomyocytes.

Progressive stages of mitochondrial destruction caused by cell toxic bile salts

The cell-toxic bile salt glycochenodeoxycholic acid (GCDCA) and taurochenodeoxycholic acid (TCDCA) are responsible for hepatocyte demise in cholestatic liver diseases, while tauroursodeoxycholic acid (TUDCA) is regarded hepatoprotective. We demonstrate the direct mitochondrio-toxicity of bile salts which deplete the mitochondrial membrane potential and induce the mitochondrial permeability transition (MPT). The bile salt mediated mechanistic mode of destruction significantly differs from that of calcium, the prototype MPT inducer. Cell-toxic bile salts initially bind to the mitochondrial outer membrane. Subsequently, the structure of the inner boundary membrane disintegrates. And it is only thereafter that the MPT is induced. This progressive destruction occurs in a dose- and time-dependent way. We demonstrate that GCDCA and TCDCA, but not TUDCA, preferentially permeabilize liposomes containing the mitochondrial membrane protein ANT, a process resembling the MPT induction in whole mitochondria. This suggests that ANT is one decisive target for toxic bile salts. To our knowledge this is the first report unraveling the consecutive steps leading to mitochondrial destruction by cell-toxic bile salts.

Mitochondrial dynamics in the adult cardiomyocytes: which roles for a highly specialized cell?

Mitochondrial dynamics is a recent topic of research in the field of cardiac physiology. The study of mechanisms involved in the morphological changes and in the mobility of mitochondria is legitimate since the adult cardiomyocytes possess numerous mitochondria which occupy at least 30% of cell volume. However, architectural constraints exist in the cardiomyocyte that limit mitochondrial movements and communication between adjacent mitochondria. Still, the proteins involved in mitochondrial fusion and fission are highly expressed in these cells and could be involved in different processes important for the cardiac function. For example, they are required for mitochondrial biogenesis to synthesize new mitochondria and for the quality-control of the organelles. They are also involved in inner membrane organization and may play a role in apoptosis. More generally, change in mitochondrial morphology can have consequences in the functioning of the respiratory chain, in the regulation of the mitochondrial permeability transition pore (MPTP), and in the interactions with other organelles. Furthermore, the proteins involved in fusion and fission of mitochondria are altered in cardiac pathologies such as ischemia/reperfusion or heart failure (HF), and appear to be valuable targets for pharmacological therapies. Thus, mitochondrial dynamics deserves particular attention in cardiac research. The present review draws up a report of our knowledge on these phenomena.

Cardiolipin-mediated cellular signaling

This review focuses on recent studies showing that cardiolipin (CL), a unique mitochondrial phospholipid, regulates many cellular functions and signaling pathways, both inside and outside the mitochondria. Inside the mitochondria, CL is a critical target of mitochondrial generated reactive oxygen species (ROS) and regulates signaling events related to apoptosis and aging. CL deficiency causes perturbation of signaling pathways outside the mitochondria, including the PKC-Slt2 cell integrity pathway and the high osmolarity glycerol (HOG) pathway, and is a key player in the cross-talk between the mitochondria and the vacuole. Understanding these connections may shed light on the pathology of Barth syndrome, a disorder of CL remodeling.

Yeast mitochondrial interactosome model: metabolon membrane proteins complex involved in the channeling of ADP/ATP

The existence of a mitochondrial interactosome (MI) has been currently well established in mammalian cells but the exact composition of this super-complex is not precisely known, and its organization seems to be different from that in yeast. One major difference is the absence of mitochondrial creatine kinase (MtCK) in yeast, unlike that described in the organization model of MI, especially in cardiac, skeletal muscle and brain cells. The aim of this review is to provide a detailed description of different partner proteins involved in the synergistic ADP/ATP transport across the mitochondrial membranes in the yeast Saccharomyces cerevisiae and to propose a new mitochondrial interactosome model. The ADP/ATP (Aacp) and inorganic phosphate (PiC) carriers as well as the VDAC (or mitochondrial porin) catalyze the import and export of ADP, ATP and Pi across the mitochondrial membranes. Aacp and PiC, which appear to be associated with the ATP synthase, consist of two nanomotors (F₀, F₁) under specific conditions and form ATP synthasome. Identification and characterization of such a complex were described for the first time by Pedersen and co-workers in 2003.

The creatine kinase system and pleiotropic effects of creatine

The pleiotropic effects of creatine (Cr) are based mostly on the functions of the enzyme creatine kinase (CK) and its high-energy product phosphocreatine (PCr). Multidisciplinary studies have established molecular, cellular, organ and somatic functions of the CK/PCr system, in particular for cells and tissues with high and intermittent energy fluctuations. These studies include tissue-specific expression and subcellular localization of CK isoforms, high-resolution molecular structures and structure–function relationships, transgenic CK abrogation and reverse genetic approaches. Three energy-related physiological principles emerge, namely that the CK/PCr systems functions as (a) an immediately available temporal energy buffer, (b) a spatial energy buffer or intracellular energy transport system (the CK/PCr energy shuttle or circuit) and (c) a metabolic regulator. The CK/PCr energy shuttle connects sites of ATP production (glycolysis and mitochondrial oxidative phosphorylation) with subcellular sites of ATP utilization (ATPases). Thus, diffusion limitations of ADP and ATP are overcome by PCr/Cr shuttling, as most clearly seen in polar cells such as spermatozoa, retina photoreceptor cells and sensory hair bundles of the inner ear. The CK/PCr system relies on the close exchange of substrates and products between CK isoforms and ATP-generating or -consuming processes. Mitochondrial CK in the mitochondrial outer compartment, for example, is tightly coupled to ATP export via adenine nucleotide transporter or carrier (ANT) and thus ATP-synthesis and respiratory chain activity, releasing PCr into the cytosol. This coupling also reduces formation of reactive oxygen species (ROS) and inhibits mitochondrial permeability transition, an early event in apoptosis. Cr itself may also act as a direct and/or indirect anti-oxidant, while PCr can interact with and protect cellular membranes. Collectively, these factors may well explain the beneficial effects of Cr supplementation. The stimulating effects of Cr for muscle and bone growth and maintenance, and especially in neuroprotection, are now recognized and the first clinical studies are underway. Novel socio-economically relevant applications of Cr supplementation are emerging, e.g. for senior people, intensive care units and dialysis patients, who are notoriously Cr-depleted. Also, Cr will likely be beneficial for the healthy development of premature infants, who after separation from the placenta depend on external Cr. Cr supplementation of pregnant and lactating women, as well as of babies and infants are likely to be of benefit for child development. Last but not least, Cr harbours a global ecological potential as an additive for animal feed, replacing meat- and fish meal for animal (poultry and swine) and fish aqua farming. This may help to alleviate human starvation and at the same time prevent over-fishing of oceans.

Enzymatic characteristics of creatine kinase system under conditions of cerebral circulatory disorders

Changes in the catalytic properties of cytoplasmic and mitochondrial creatine kinase were studied at various periods of cerebral circulatory disorders. Cerebral ischemia was accompanied by activation of free radical processes, changes in the dimer/octamer ratio for mitochondrial creatine kinase, and appearance of new catalytic properties of isoenzymes.

Mitochondrial therapy for Parkinson's disease: neuroprotective pharmaconutrition may be disease-modifying

Progressive destruction of neurons that produce dopamine in the basal ganglia of the brain, particularly the substantia nigra, is a hallmark of Parkinson's disease. The syndrome of the Parkinsonian phenotype is caused by many etiologies, involving multiple contributing mechanisms. Characteristic findings are pathologic inclusions called Lewy bodies, which are protein aggregates inside nerve cells. Environmental insults are linked with the disease, and a number of associated genes have also been identified. Neuroinflammation, microglia activation, oxidative stress, and mitochondrial dysfunction are central processes producing nerve damage. In addition, protein misfolding, driven by accumulation and condensation of α-synuclein, compounded by inadequate elimination of defective protein through the ubiquitin- proteasome system, promote apoptosis. Current pharmacologic therapy is palliative rather than disease- modifying, and typically becomes unsatisfactory over time. Coenzyme Q10 and creatine, two agents involved in energy production, may be disease-modifying, and able to produce sufficient beneficial pathophysiologic changes in preclinical studies to warrant large studies now in progress. Use of long-chain omega-3 fatty acids and vitamin D in PD are also topics of current interest.

Cholesterol and peroxidized cardiolipin in mitochondrial membrane properties, permeabilization and cell death

Mitochondria are known to actively regulate cell death with the final phenotype of demise being determined by the metabolic and energetic status of the cell. Mitochondrial membrane permeabilization (MMP) is a critical event in cell death, as it regulates the degree of mitochondrial dysfunction and the release of intermembrane proteins that function in the activation and assembly of caspases. In addition to the crucial role of proapoptotic members of the Bcl-2 family, the lipid composition of the mitochondrial membranes is increasingly recognized to modulate MMP and hence cell death. The unphysiological accumulation of cholesterol in mitochondrial membranes regulates their physical properties, facilitating or impairing MMP during Bax and death ligand-induced cell death depending on the level of mitochondrial GSH (mGSH), which in turn regulates the oxidation status of cardiolipin. Cholesterol-mediated mGSH depletion stimulates TNF-induced reactive oxygen species and subsequent cardiolipin peroxidation, which destabilizes the lipid bilayer and potentiates Bax-induced membrane permeabilization. These data suggest that the balance of mitochondrial cholesterol to peroxidized cardiolipin regulates mitochondrial membrane properties and permeabilization, emerging as a rheostat in cell death.

The unique mitochondrial form and function of Antarctic channichthyid icefishes

Antarctic icefishes of the family Channichthyidae are the only vertebrate animals that as adults do not express the circulating oxygen-binding protein hemoglobin (Hb). Six of the 16 family members also lack the intracellular oxygen-binding protein myoglobin (Mb) in the ventricle of their hearts and all lack Mb in oxidative skeletal muscle. The loss of Hb has led to substantial remodeling in the cardiovascular system of icefishes to facilitate adequate oxygenation of tissues. One of the more curious adaptations to the loss of Hb and Mb is an increase in mitochondrial density in cardiac myocytes and oxidative skeletal muscle fibers. The proliferation of mitochondria in the aerobic musculature of icefishes does not arise through a canonical pathway of mitochondrial biogenesis. Rather, the biosynthesis of mitochondrial phospholipids is up-regulated independently of the synthesis of proteins and mitochondrial DNA, and newly-synthesized phospholipids are targeted primarily to the outer-mitochondrial membrane. Consequently, icefish mitochondria have a higher lipid-to-protein ratio compared to those from red-blooded species. Elevated levels of nitric oxide in the blood plasma of icefishes, compared to red-blooded notothenioids, may mediate alterations in mitochondrial density and architecture. Modifications in mitochondrial structure minimally impact state III respiration rates but may significantly enhance intracellular diffusion of oxygen. The rate of oxygen diffusion is greater within the hydrocarbon core of membrane lipids compared to the aqueous cytosol and impeded only by proteins within the lipid bilayer. Thus, the proliferation of icefish's mitochondrial membranes provides an optimal conduit for the intracellular diffusion of oxygen and compensates for the loss of Hb and Mb. Currently little is known about how mitochondrial phospholipid synthesis is regulated and integrated into mitochondrial biogenesis. The unique architecture of the oxidative muscle cells of icefishes highlights the need for further studies in this area.

Flow of energy in the outer retina in darkness and in light

Structural features of neurons create challenges for effective production and distribution of essential metabolic energy. We investigated how metabolic energy is distributed between cellular compartments in photoreceptors. In avascular retinas, aerobic production of energy occurs only in mitochondria that are located centrally within the photoreceptor. Our findings indicate that metabolic energy flows from these central mitochondria as phosphocreatine toward the photoreceptor's synaptic terminal in darkness. In light, it flows in the opposite direction as ATP toward the outer segment. Consistent with this model, inhibition of creatine kinase in avascular retinas blocks synaptic transmission without influencing outer segment activity. Our findings also reveal how vascularization of neuronal tissue can influence the strategies neurons use for energy management. In vascularized retinas, mitochondria in the synaptic terminals of photoreceptors make neurotransmission less dependent on creatine kinase. Thus, vasculature of the tissue and the intracellular distribution of mitochondria can play key roles in setting the strategy for energy distribution in neurons.

Different proteolipid protein mutants exhibit unique metabolic defects

PMD (Pelizaeus–Merzbacher disease), a CNS (central nervous system) disease characterized by shortened lifespan and severe neural dysfunction, is caused by mutations of the PLP1 (X-linked myelin proteolipid protein) gene. The majority of human PLP1 mutations are caused by duplications; almost all others are caused by missense mutations. The cellular events leading to the phenotype are unknown. The same mutations in non-humans make them ideal models to study the mechanisms that cause neurological sequelae. In the present study we show that mice with Plp1 duplications (Plp1tg) have major mitochondrial deficits with a 50% reduction in ATP, a drastically reduced mitochondrial membrane potential and increased numbers of mitochondria. In contrast, the jp (jimpy) mouse with a Plp1 missense mutation exhibits normal mitochondrial function. We show that PLP in the Plp1tg mice and in Plp1-transfected cells is targeted to mitochondria. PLP has motifs permissive for insertion into mitochondria and deletions near its N-terminus prevent its co-localization to mitochondria. These novel data show that Plp1 missense mutations and duplications of the native Plp1 gene initiate uniquely different cellular responses.

Mitochondria and energetic depression in cell pathophysiology

Mitochondrial dysfunction is a hallmark of almost all diseases. Acquired or inherited mutations of the mitochondrial genome DNA may give rise to mitochondrial diseases. Another class of disorders, in which mitochondrial impairments are initiated by extramitochondrial factors, includes neurodegenerative diseases and syndromes resulting from typical pathological processes, such as hypoxia/ischemia, inflammation, intoxications, and carcinogenesis. Both classes of diseases lead to cellular energetic depression (CED), which is characterized by decreased cytosolic phosphorylation potential that suppresses the cell's ability to do work and control the intracellular Ca(2+) homeostasis and its redox state. If progressing, CED leads to cell death, whose type is linked to the functional status of the mitochondria. In the case of limited deterioration, when some amounts of ATP can still be generated due to oxidative phosphorylation (OXPHOS), mitochondria launch the apoptotic cell death program by release of cytochrome c. Following pronounced CED, cytoplasmic ATP levels fall below the thresholds required for processing the ATP-dependent apoptotic cascade and the cell dies from necrosis. Both types of death can be grouped together as a mitochondrial cell death (MCD). However, there exist multiple adaptive reactions aimed at protecting cells against CED. In this context, a metabolic shift characterized by suppression of OXPHOS combined with activation of aerobic glycolysis as the main pathway for ATP synthesis (Warburg effect) is of central importance. Whereas this type of adaptation is sufficiently effective to avoid CED and to control the cellular redox state, thereby ensuring the cell survival, it also favors the avoidance of apoptotic cell death. This scenario may underlie uncontrolled cellular proliferation and growth, eventually resulting in carcinogenesis.

Cardiolipin acts as a mitochondrial signalling platform to launch apoptosis

Cardiolipin (CL) is a unique anionic phospholipid specific to the mitochondria. CL influences the activity of electron transport chain enzyme complexes as well as members of the Bcl-2 family. Interactions between Bcl-2 family members and other pro-apoptotic enzymes have been shown to be crucial for the transduction of the apoptotic signalling cascades during programmed cell death. Targeting of tBid to the mitochondria, which is necessary for Bax/Bak oligomerization and cristae remodelling, is dependent on the exposure of CL at contact sites between the inner and outer mitochondrial membranes. Also, the mobilization of cytochrome c, another key apoptotic event, is tightly regulated by the oxidative state of cardiolipin. Moreover, CL has been shown to be essential for translocation and autoprocessing of caspase-8 on the mitochondria after death receptor stimulation. Deficiencies in CL inhibit the formation of tBid and prevent apoptosis by removing an essential activation platform for the autoprocessing of caspase-8. It is now apparent that CL acts as a crucial signalling platform from which it orchestrates apoptosis by integrating signals from a variety of death inducing proteins.

Mitochondrial kinases and their molecular interaction with cardiolipin

Mitochondrial isoforms of creatine kinase (MtCK) and nucleoside diphosphate kinase (NDPK-D) are not phylogenetically related but share functionally important properties. They both use mitochondrially generated ATP with the ultimate goal of maintaining proper nucleotide pools, are located in the intermembrane/cristae space, have symmetrical oligomeric structures, and show high affinity binding to anionic phospholipids, in particular cardiolipin. The structural basis and functional consequences of the cardiolipin interaction have been studied and are discussed in detail in this review. They mainly result in a functional interaction of MtCK and NDPK-D with inner membrane adenylate translocator, probably by forming proteolipid complexes. These interactions allow for privileged exchange of metabolites (channeling) that ultimately regulate mitochondrial respiration. Further functions of the MtCK/membrane interaction include formation of cardiolipin membrane patches, stabilization of mitochondria and a role in apoptotic signaling, as well as in case of both kinases, a role in facilitating lipid transfer between two membranes. Finally, disturbed cardiolipin interactions of MtCK, NDPK-D and other proteins like cytochrome c and truncated Bid are discussed more generally in the context of apoptosis and necrosis.

Mitochondrial creatine kinase binding to phospholipid monolayers induces cardiolipin segregation

It is well established that the octameric mitochondrial form of creatine kinase (mtCK) binds to the outer face of the inner mitochondrial membrane mainly via electrostatic interactions with cardiolipin (CL). However, little is known about the consequences of these interactions on membrane and protein levels. Brewster angle microscopy investigations provide, for the first time to our knowledge, images indicating that mtCK binding induced cluster formation on CL monolayers. The thickness of the clusters (10-12 nm) corresponds to the theoretical height of the mtCK-CL complex. Protein insertion into a condensed CL film, together with monolayer stabilization after protein addition, was observed by means of differential capacity measurements. Polarization modulation infrared reflection-absorption spectroscopy showed that the mean orientation of alpha-helices within the protein shifted upon CL binding from 30 degrees to 45 degrees with respect to the interface plane, demonstrating protein domain movements. A comparison of data obtained with CL and phosphatidylcholine/phosphatidylethanolamine/CL (2:1:1) monolayers indicates that mtCK is able to selectively recruit CL molecules within the mixed monolayer, consolidating and changing the morphology of the interfacial film. Therefore, CL-rich domains induced by mtCK binding could modulate mitochondrial inner membrane morphology into a raft-like organization and influence essential steps of mitochondria-mediated apoptosis.

Morphology modifications in negatively charged lipid monolayers upon mitochondrial creatine kinase binding

Mitochondrial creatine kinase (mtCK) may participate to membrane organization at the mitochondrial level by modulating lipid state and fluidity. The effect of the protein on lipid phase behaviour of different acyl chain length phosphatidylglycerol monolayers was analyzed from pressure-area isotherms and from the compressional modulus variation with respect to the surface pressure. Monolayer morphology was visualized by Brewster angle microscopy. No condensation effect was visible on dimyristoylphosphatidylglycerol (DMPG). For the other PG monolayers tested, dipalmitoylphosphatidylglycerol (DPPG) and distearoylphosphatidylglycerol (DSPG), mtCK facilitated the formation of a liquid condensed phase. The effect depended on the surface pressure at which transition phase occurred. The effect of mtCK was more pronounced for tetramyristoylcardiolipin (TMCL) monolayers, as liquid condensed regions appeared 10 mN/m below the transition phase of the pure TMCL monolayer. The observed domains were circular and rather uniform, indicating a stabilization of the condensed phase. The same effect, namely an overall condensation of the monolayer with formation of circular domains, was observed upon protein injection beneath TMCL monolayers in different condensation states at constant area. MtCK ability to induce and stabilize a LC phase on monolayers could have important consequences in membrane organization and emphasize its structural role at mitochondrial level.

The nucleoside diphosphate kinase D (NM23-H4) binds the inner mitochondrial membrane with high affinity to cardiolipin and couples nucleotide transfer with respiration

Nucleoside diphosphate kinase (NDPK/Nm23), responsible for intracellular di- and triphosphonucleoside homeostasis, plays multiple roles in cellular energetics, signaling, proliferation, differentiation and tumor invasion. The only human NDPK with a mitochondrial targeting sequence is NDPK-D, the NME4 gene product, which is a peripheral protein of mitochondrial membranes. Subfractionation of rat liver and HEK 293 cell mitochondria revealed that NDPK-D is essentially bound to the inner membrane. Surface plasmon resonance analysis of the interaction using recombinant NDPK-D and model liposomes showed that NDPK-D interacts electrostatically with anionic phospholipids, with highest affinity observed for cardiolipin. Mutation of the central arginine (Arg-90) in a surface-exposed basic RRK motif unique to NDPK-D strongly reduced interaction with anionic phospholipids. Due to its symmetrical hexameric structure, NDPK-D was able to cross-link anionic phospholipid-containing liposomes, suggesting that NDPK-D could promote intermembrane contacts. Latency assays with isolated mitochondria and antibody binding to mitoplasts indicated a dual orientation for NDPK-D. In HeLa cells, stable expression of wild type but not of the R90D mutant led to membrane-bound enzyme in vivo. Respiration was significantly stimulated by the NDPK substrate TDP in mitochondria containing wild-type NDPK-D, but not in those expressing the R90D mutant, which is catalytically equally active. This indicates local ADP regeneration in the mitochondrial intermembrane space and a tight functional coupling of NDPK-D with oxidative phosphorylation that depends on its membrane-bound state.

In vivo magnetic resonance spectroscopy of transgenic mice with altered expression of guanidinoacetate methyltransferase and creatine kinase isoenzymes

Mice with an under- or over-expression of enzymes catalyzing phosphoryl transfer in high-energy supplying reactions are particulary attractive for in vivo magnetic resonance spectroscopy (MRS) studies as substrates of these enzymes are visible in MR spectra. This chapter reviews results of in vivo MRS studies on transgenic mice with alterations in the expression of the enzymes creatine kinase and guanidinoacetate methyltransferase. The particular metabolic consequences of these enzyme deficiencies in skeletal muscle, brain, heart and liver are addressed. An overview is given of metabolite levels determined by in vivo MRS in skeletal muscle and brain of wild-type and transgenic mice. MRS studies on mice lacking guanidinoacetate methyltransferase have demonstrated metabolic changes comparable to those found in the deficiency of this enzyme in humans, which are (partly) reversible upon creatine feeding. Apart from being a model for a creatine deficiency syndrome, these mice are also of interest to study fundamental aspects of the biological role of creatine. MRS studies on transgenic mice lacking creatine kinase isoenzymes have contributed significantly to the view that the creatine kinase reaction together with other enzymatic steps involved in high-energy phosphate transfer builds a large metabolic energy network, which is highly versatile and can dynamically adapt to genotoxic or physiological challenges.

The nucleoside diphosphate kinase D (NM23-H4) binds the inner mitochondrial membrane with high affinity to cardiolipin and couples nucleotide transfer with respiration

Nucleoside diphosphate kinase (NDPK/Nm23), responsible for intracellular di- and triphosphonucleoside homeostasis, plays multiple roles in cellular energetics, signaling, proliferation, differentiation and tumor invasion. The only human NDPK with a mitochondrial targeting sequence is NDPK-D, the NME4 gene product, which is a peripheral protein of mitochondrial membranes. Subfractionation of rat liver and HEK 293 cell mitochondria revealed that NDPK-D is essentially bound to the inner membrane. Surface plasmon resonance analysis of the interaction using recombinant NDPK-D and model liposomes showed that NDPK-D interacts electrostatically with anionic phospholipids, with highest affinity observed for cardiolipin. Mutation of the central arginine (Arg-90) in a surface-exposed basic RRK motif unique to NDPK-D strongly reduced interaction with anionic phospholipids. Due to its symmetrical hexameric structure, NDPK-D was able to cross-link anionic phospholipid-containing liposomes, suggesting that NDPK-D could promote intermembrane contacts. Latency assays with isolated mitochondria and antibody binding to mitoplasts indicated a dual orientation for NDPK-D. In HeLa cells, stable expression of wild type but not of the R90D mutant led to membrane-bound enzyme in vivo. Respiration was significantly stimulated by the NDPK substrate TDP in mitochondria containing wild-type NDPK-D, but not in those expressing the R90D mutant, which is catalytically equally active. This indicates local ADP regeneration in the mitochondrial intermembrane space and a tight functional coupling of NDPK-D with oxidative phosphorylation that depends on its membrane-bound state.

Proteomic analysis of rat brain mitochondria following exposure to dopamine quinone: implications for Parkinson disease

Oxidative stress and mitochondrial dysfunction have been linked to dopaminergic neuron degeneration in Parkinson disease. We have previously shown that dopamine oxidation leads to selective dopaminergic terminal degeneration in vivo and alters mitochondrial function in vitro. In this study, we utilized 2-D difference in-gel electrophoresis to assess changes in the mitochondrial proteome following in vitro exposure to reactive dopamine quinone. A subset of proteins exhibit decreased fluorescence labeling following dopamine oxidation, suggesting a rapid loss of specific proteins. Amongst these proteins are mitochondrial creatine kinase, mitofilin, mortalin, the 75 kDa subunit of NADH dehydrogenase, and superoxide dismutase 2. Western blot analyses for mitochondrial creatine kinase and mitofilin confirmed significant losses in isolated brain mitochondria exposed to dopamine quinone and PC12 cells exposed to dopamine. These results suggest that specific mitochondrial proteins are uniquely susceptible to changes in abundance following dopamine oxidation, and carry implications for mitochondrial stability in Parkinson disease neurodegeneration.

Reduced creatine-stimulated respiration in doxorubicin challenged mitochondria: particular sensitivity of the heart

Doxorubicin (DXR) belongs to the most efficient anticancer drugs. However, its use is limited by a risk of cardiotoxicity, which is not completely understood. Recently, we have shown that DXR impairs essential properties of purified mitochondrial creatine kinase (MtCK), with cardiac isoenzyme (sMtCK) being particularly sensitive. In this study we assessed the effects of DXR on respiration of isolated structurally and functionally intact heart mitochondria, containing sMtCK, in the presence and absence of externally added creatine (Cr), and compared these effects with the response of brain mitochondria expressing uMtCK, the ubiquitous, non-muscle MtCK isoenzyme. DXR impaired respiration of isolated heart mitochondria already after short-term exposure (minutes), affecting both ADP- and Cr-stimulated respiration. During a first short time span (minutes to 1 h), detachment of MtCK from membranes occurred, while a decrease of MtCK activity related to oxidative damage was only observed after longer exposure (several hours). The early inhibition of Cr-stimulated respiration, in addition to impairment of components of the respiratory chain involves a partial disturbance of functional coupling between MtCK and ANT, likely due to interaction of DXR with cardiolipin leading to competitive inhibition of MtCK/membrane binding. The relevance of these findings for the regulation of mitochondrial energy production in the heart, as well as the obvious differences of DXR action in the heart as compared to brain tissue, is discussed.

Inhibition of cytosolic and mitochondrial creatine kinase by siRNA in HaCaT- and HeLaS3-cells affects cell viability and mitochondrial morphology

The creatine kinase (CK) system is essential for cellular energetics in tissues or cells with high and fluctuating energy requirements. Creatine itself is known to protect cells from stress-induced injury. By using an siRNA approach to silence the CK isoenzymes in human keratinocyte HaCaT cells, expressing low levels of cytoplasmic CK and high levels of mitochondrial CK, as well as HeLa cancer cells, expressing high levels of cytoplasmic CK and low levels of mitochondrial CK, we successfully lowered the respective CK expression levels and studied the effects of either abolishing cytosolic brain-type BB-CK or ubiquitous mitochondrial uMi-CK in these cells. In both cell lines, targeting the dominant CK isoform by the respective siRNAs had the strongest effect on overall CK activity. However, irrespective of the expression level in both cell lines, inhibition of the mitochondrial CK isoform generally caused the strongest decline in cell viability and cell proliferation. These findings are congruent with electron microscopic data showing substantial alteration of mitochondrial morphology as well as mitochondrial membrane topology after targeting uMi-CK in both cell lines. Only for the rate of apoptosis, it was the least expressed CK present in each of the cell lines whose inhibition led to the highest proportion of apoptotic cells, i.e., downregulation of uMi-CK in case of HeLaS3 and BB-CK in case of HaCaT cells. We conclude from these data that a major phenotype is linked to reduction of mitochondrial CK alone or in combination with cytosolic CK, and that this effect is independent of the relative expression levels of Mi-CK in the cell type considered. The mitochondrial CK isoform appears to play the most crucial role in maintaining cell viability by stabilizing contact sites between inner and outer mitochondrial membranes and maintaining local metabolite channeling, thus avoiding transition pore opening which eventually results in activation of caspase cell-death pathways.

In vivo magnetic resonance spectroscopy of transgenic mouse models with altered high-energy phosphoryl transfer metabolism

Studies of transgenic mice provide powerful means to investigate the in vivo biological significance of gene products. Mice with an under- or overexpression of enzymes involved in high-energy phosphoryl transfer (approximately P) are particulary attractive for in vivo MR spectroscopy studies as the substrates of these enzymes are metabolites that are visible in MR spectra. This review provides a brief overview of the strategies used for generation and study of genetically altered mice and introduces the reader to some practical aspects of in vivo MRS studies on mice. The major part of the paper reviews results of in vivo MRS studies on transgenic mice with alterations in the expression of enzymes involved in approximately P metabolism, such as creatine kinase, adenylate kinase and guanidinoacetate methyl transferase. The particular metabolic consequences of these enzyme deficiencies in skeletal muscle, brain, heart and liver are addressed. Additionally, the use of approximately P systems as markers of gene expression by MRS, such as after viral transduction of genes, is described. Finally, a compilation of tissue levels of metabolites in skeletal muscle, heart and brain of wild-type and transgenic mice, as determined by in vivo MRS, is given. During the last decade, transgenic MRS studies have contributed significantly to our understanding of the physiological role of phosphotransfer enzymes, and to the view that these enzymes together build a much larger metabolic energy network that is highly versatile and can dynamically adapt to intrinsic genotoxic and extrinsic physiological challenges.