The creatine kinase system is essential for optimal refill of the sarcoplasmic reticulum Ca2+ store in skeletal muscle

Performance of TMRM and Mitotrackers in mitochondrial morphofunctional analysis of primary human skin fibroblasts

Mitochondrial membrane potential (Δψ) and morphology are considered key readouts of mitochondrial functional state. This morphofunction can be studied using fluorescent dyes ("probes") like tetramethylrhodamine methyl ester (TMRM) and Mitotrackers (MTs). Although these dyes are broadly used, information comparing their performance in mitochondrial morphology quantification and Δψ-sensitivity in the same cell model is still scarce. Here we applied epifluorescence microscopy of primary human skin fibroblasts to evaluate TMRM, Mitotracker Red CMXros (CMXros), Mitotracker Red CMH2Xros (CMH2Xros), Mitotracker Green FM (MG) and Mitotracker Deep Red FM (MDR). All probes were suited for automated quantification of mitochondrial morphology parameters when Δψ was normal, although they did not deliver quantitatively identical results. The mitochondrial localization of TMRM and MTs was differentially sensitive to carbonyl cyanide-4-phenylhydrazone (FCCP)-induced Δψ depolarization, decreasing in the order: TMRM ≫ CHM2Xros = CMXros = MDR > MG. To study the effect of reversible Δψ changes, the impact of photo-induced Δψ "flickering" was studied in cells co-stained with TMRM and MG. During a flickering event, individual mitochondria displayed subsequent TMRM release and uptake, whereas this phenomenon was not observed for MG. Spatiotemporal and computational analysis of the flickering event provided evidence that TMRM redistributes between adjacent mitochondria by a mechanism dependent on Δψ and TMRM concentration. In summary, this study demonstrates that: (1) TMRM and MTs are suited for automated mitochondrial morphology quantification, (2) numerical data obtained with different probes is not identical, and (3) all probes are sensitive to FCCP-induced Δψ depolarization, with TMRM and MG displaying the highest and lowest sensitivity, respectively. We conclude that TMRM is better suited for integrated analysis of Δψ and mitochondrial morphology than the tested MTs under conditions that Δψ is not substantially depolarized.

Methionine can reduce the sublethal risk of Chlorantraniliprole to honeybees (Apis mellifera L.): Based on metabolomics analysis

Bees, essential for pollination in agriculture and global economic growth. However, the great wax moth (Galleria mellonella, GWM), a Lepidopteran insect, poses a substantial threat to bee colonies, contributing to a global decline in bee populations. Chlorantraniliprole (CH) is one of the primary insecticide used to control GWM due to its efficacy and low toxicity to bees. To improve beekeeping safety and reduce the risk of GWM developing resistance to prolonged use of CH, we investigated the potential of combining methionine (MET) which has been found to have insecticidal activity against certain Lepidoptera pests, with chlorantraniliprole for use in the apiculture industry. This study assessed the combined effects of MET and CH on GWM and honeybees by employing the maximum concentration of MET (1 %, w/w), previously reported as safe for honeybees, and the practical concentration of CH (1 mg/kg) for GWM control. The results revealed limited acute lethal toxicity of MET to GWM and honeybees, whereas the combined chronic exposure of MET and CH (MIX) led to significant synergistic lethal effects on GWM mortality. Nevertheless, the protective effect of MET on honeybees exposed to CH was significant under chronic exposure. Potential mechanisms underlying the synergistic actions of MET and CH may stem from MET-induced protection of the "Cysteine and methionine" and the "Glycine, serine, and threonine" metabolism pathways. Furthermore, immune stress mitigation was also observed in honeybee immune-related gene transcripts treated by the combination of MET and CH under both acute and chronic exposure. The effects of MET on CH activity in GWM and honeybees are likely due to metabolic regulation. This study suggests the potential of developing MET as a promising biopesticide or protective agent in the future.

SMDT1 variants impair EMRE-mediated mitochondrial calcium uptake in patients with muscle involvement

Ionic calcium (Ca2+) is a key messenger in signal transduction and its mitochondrial uptake plays an important role in cell physiology. This uptake is mediated by the mitochondrial Ca2+ uniporter (MCU), which is regulated by EMRE (essential MCU regulator) encoded by the SMDT1 (single-pass membrane protein with aspartate rich tail 1) gene. This work presents the genetic, clinical and cellular characterization of two patients harbouring SMDT1 variants and presenting with muscle problems. Analysis of patient fibroblasts and complementation experiments demonstrated that these variants lead to absence of EMRE protein, induce MCU subcomplex formation and impair mitochondrial Ca2+ uptake. However, the activity of oxidative phosphorylation enzymes, mitochondrial morphology and membrane potential, as well as routine/ATP-linked respiration were not affected. We hypothesize that the muscle-related symptoms in the SMDT1 patients result from aberrant mitochondrial Ca2+ uptake.

Evaluation of cinnamaldehyde derivatives as potential protective agents against oxidative-stress induced myotube atrophy using chemical, biological and computational analysis

Skeletal muscle atrophy, associated with increased morbidity, mortality and poor quality of life, is a metabolic disorder with no FDA approved drug. Oxidative stress is one of the key mediators of atrophy that influences various cell signaling molecules. The goal of this study is to identify potential antioxidant agents that could be used to treat atrophy. In this study in vitro and in situ screening of different cinnamaldehyde (CNA) derivatives for their antioxidant effects was done along with computational analysis to understand the relationship between their chemical structure and biological activity. Data show that 2-hydroxycinnamaldehyde (2HCNA) worked better than other CNA analogues at physiological pH, while 4-Fluoro-2-methoxycinnamaldehyde (4FoCNA) showed the maximum antioxidant activity under acidic conditions. However, these derivatives (2HCNA and 4FoCNA) were found to be toxic to the cultured myotubes (mature myofiber) under both physiological and pathophysiological conditions. Immunofluorescence, bright-field microscopic and biochemical studies conducted using live C2C12 cells showed that pre-incubation with other CNA analogues i.e. 2-methoxycinnamaldehyde (2MeCNA) and 2-benzyloxycinnamaldehyde (2BzCNA) not only maintained the normal morphology of myotubes but also protected them from H2O2-induced atrophy. These compounds (2MeCNA and 2BzCNA) showed higher stability and antioxidant potential, as indicated by computer simulation data analyzed by Density Functional Theory (DFT) based molecular modeling. Overall, the chemical, biological, and computational studies reveal the therapeutic potential of CNA analogues (BzCNA and MeCNA) against oxidative-stress induced muscle atrophy in C2C12 cells.

Metabolic Basis of Creatine in Health and Disease: A Bioinformatics-Assisted Review

Creatine (Cr) is a ubiquitous molecule that is synthesized mainly in the liver, kidneys, and pancreas. Most of the Cr pool is found in tissues with high-energy demands. Cr enters target cells through a specific symporter called Na⁺/Cl⁻-dependent Cr transporter (CRT). Once within cells, creatine kinase (CK) catalyzes the reversible transphosphorylation reaction between [Mg²⁺:ATP⁴⁻]²⁻ and Cr to produce phosphocreatine (PCr) and [Mg²⁺:ADP³⁻]⁻. We aimed to perform a comprehensive and bioinformatics-assisted review of the most recent research findings regarding Cr metabolism. Specifically, several public databases, repositories, and bioinformatics tools were utilized for this endeavor. Topics of biological complexity ranging from structural biology to cellular dynamics were addressed herein. In this sense, we sought to address certain pre-specified questions including: (i) What happens when creatine is transported into cells? (ii) How is the CK/PCr system involved in cellular bioenergetics? (iii) How is the CK/PCr system compartmentalized throughout the cell? (iv) What is the role of creatine amongst different tissues? and (v) What is the basis of creatine transport? Under the cellular allostasis paradigm, the CK/PCr system is physiologically essential for life (cell survival, growth, proliferation, differentiation, and migration/motility) by providing an evolutionary advantage for rapid, local, and temporal support of energy- and mechanical-dependent processes. Thus, we suggest the CK/PCr system acts as a dynamic biosensor based on chemo-mechanical energy transduction, which might explain why dysregulation in Cr metabolism contributes to a wide range of diseases besides the mitigating effect that Cr supplementation may have in some of these disease states.

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.

VDAC1 functions in Ca2+ homeostasis and cell life and death in health and disease

In the outer mitochondrial membrane (OMM), the voltage-dependent anion channel 1 (VDAC1) serves as a mitochondrial gatekeeper, controlling the metabolic and energy cross-talk between mitochondria and the rest of the cell. VDAC1 also functions in cellular Ca²⁺ homeostasis by transporting Ca²⁺ in and out of mitochondria. VDAC1 has also been recognized as a key protein in mitochondria-mediated apoptosis, contributing to the release of apoptotic proteins located in the inter-membranal space (IMS) and regulating apoptosis via association with pro- and anti-apoptotic members of the Bcl-2 family of proteins and hexokinase. VDAC1 is highly Ca²⁺-permeable, transporting Ca²⁺ to the IMS and thus modulating Ca²⁺ access to Ca²⁺ transporters in the inner mitochondrial membrane. Intra-mitochondrial Ca²⁺ controls energy metabolism via modulating critical enzymes in the tricarboxylic acid cycle and in fatty acid oxidation. Ca²⁺ also determines cell sensitivity to apoptotic stimuli and promotes the release of pro-apoptotic proteins. However, the precise mechanism by which intracellular Ca²⁺ mediates apoptosis is not known. Here, the roles of VDAC1 in mitochondrial Ca²⁺ homeostasis are presented while emphasizing a new proposed mechanism for the mode of action of pro-apoptotic drugs. This view, proposing that Ca²⁺-dependent enhancement of VDAC1 expression levels is a major mechanism by which apoptotic stimuli induce apoptosis, position VDAC1 oligomerization at a molecular focal point in apoptosis regulation. The interactions of VDAC1 with many proteins involved in Ca²⁺ homeostasis or regulated by Ca²⁺, as well as VDAC-mediated control of cell life and death and the association of VDAC with disease, are also presented.

Regulation of brain-type creatine kinase by AMP-activated protein kinase: interaction, phosphorylation and ER localization

AMP-activated protein kinase (AMPK) and cytosolic brain-type creatine kinase (BCK) cooperate under energy stress to compensate for loss of adenosine triphosphate (ATP) by either stimulating ATP-generating and inhibiting ATP-consuming pathways, or by direct ATP regeneration from phosphocreatine, respectively. Here we report on AMPK-dependent phosphorylation of BCK from different species identified by in vitro screening for AMPK substrates in mouse brain. Mass spectrometry, protein sequencing, and site-directed mutagenesis identified Ser6 as a relevant residue with one site phosphorylated per BCK dimer. Yeast two-hybrid analysis revealed interaction of active AMPK specifically with non-phosphorylated BCK. Pharmacological activation of AMPK mimicking energy stress led to BCK phosphorylation in astrocytes and fibroblasts, as evidenced with a highly specific phospho-Ser6 antibody. BCK phosphorylation at Ser6 did not affect its enzymatic activity, but led to the appearance of the phosphorylated enzyme at the endoplasmic reticulum (ER), close to the ER calcium pump, a location known for muscle-type cytosolic creatine kinase (CK) to support Ca²⁺-pumping.

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.

Gill transcriptome response to changes in environmental calcium in the green spotted puffer fish

Background

Calcium ion is tightly regulated in body fluids and for euryhaline fish, which are exposed to rapid changes in environmental [Ca²⁺], homeostasis is especially challenging. The gill is the main organ of active calcium uptake and therefore plays a crucial role in the maintenance of calcium ion homeostasis. To study the molecular basis of the short-term responses to changing calcium availability, the whole gill transcriptome obtained by Super Serial Analysis of Gene Expression (SuperSAGE) of the euryhaline teleost green spotted puffer fish, Tetraodon nigroviridis, exposed to water with altered [Ca²⁺] was analysed.

Results

Transfer of T. nigroviridis from 10 ppt water salinity containing 2.9 mM Ca²⁺ to high (10 mM Ca²⁺ ) and low (0.01 mM Ca²⁺) calcium water of similar salinity for 2-12 h resulted in 1,339 differentially expressed SuperSAGE tags (26-bp transcript identifiers) in gills. Of these 869 tags (65%) were mapped to T. nigroviridis cDNAs or genomic DNA and 497 (57%) were assigned to known proteins. Thirteen percent of the genes matched multiple tags indicating alternative RNA transcripts. The main enriched gene ontology groups belong to Ca²⁺ signaling/homeostasis but also muscle contraction, cytoskeleton, energy production/homeostasis and tissue remodeling. K-means clustering identified co-expressed transcripts with distinct patterns in response to water [Ca²⁺] and exposure time.

Conclusions

The generated transcript expression patterns provide a framework of novel water calcium-responsive genes in the gill during the initial response after transfer to different [Ca²⁺]. This molecular response entails initial perception of alterations, activation of signaling networks and effectors and suggests active remodeling of cytoskeletal proteins during the initial acclimation process. Genes related to energy production and energy homeostasis are also up-regulated, probably reflecting the increased energetic needs of the acclimation response. This study is the first genome-wide transcriptome analysis of fish gills and is an important resource for future research on the short-term mechanisms involved in the gill acclimation responses to environmental Ca²⁺ changes and osmoregulation.

Ins(1,4,5)P(3) facilitates ATP accumulation via phosphocreatine/creatine kinase in the endoplasmic reticulum extracted from MDCK cells

So far, the content and accumulation of ATP in isolated endoplasmic reticulum (ER) are little understood. First, we confirmed using electron microscopic and Western blotting techniques that the samples extracted from MDCK cells are endoplasmic reticulum (ER). The amounts of ATP in the extracted ER were measured from the filtrate after a spinning down of ultrafiltration spin column packed with ER. When the ER sample (5mug) after 3days freezing was suspended in intracellular medium (ICM), 0.1% Triton X and ultrapure water (UPW), ATP amounts from the ER with UPW were the highest and over 10 times compared with that from the control with ICM, indicating that UPW is the most effective tool in destroying the ER membrane. After a 10-min-incubation with ICM containing phosphocreatine (PCr)/creatine kinase (CK) of the fresh ER. ATP amounts in the filtrate obtained by spinning down were not changed from that in the control (no PCr/CK). However, ATP amounts in the filtrate from the second spinning down of the ER (treated with PCr/CK) suspended in UPW became over 10-fold compared with the control. When 1muM inositol(1,4,5)trisphosphate (Ins(1,4,5)P(3)) was added in the incubation medium (ICM with PCr/CK), ATP amounts from the filtrate after the second spinning down were further enhanced around three times. This enhancement was almost canceled by Ca(2+)-removal from ICM and by adding thapsigargin, a Ca(2+)-ATPase inhibitor, but not by 2-APB and heparin, Ins(1,4,5)P(3) receptor antagonists. Administration of 500muM adenosine to the incubation medium (with PCr/CK) failed to enhance the accumulation of ATP in the ER. These findings suggest that the ER originally contains ATP and ATP accumulation in the ER is promoted by PCr/CK and Ins(1,4,5)P(3).

Complete brain-type creatine kinase deficiency in mice blocks seizure activity and affects intracellular calcium kinetics

PURPOSE

Brain-type creatine kinase (CK-B) and ubiquitous mitochondrial creatine kinase (UbCKmit) act as components of local phosphocreatine ATP shuttles that help in the compartmentalization and maintenance of pools of high-energy phosphate molecules in both neurons and glial cells. We investigated the role of these brain-type creatine kinases during extreme energy-demanding conditions in vivo (generalized tonic-clonic seizures) and in vitro.

METHODS

The physiologic response of wild-types and mice lacking both CK-B and UbCKmit (CK--/--mice) to pentylenetetrazole (PTZ)-induced seizures was measured using electroencephalography (EEG) recordings and behavioral monitoring. In vitro intracellular Ca(2+) kinetics in hippocampal granule neurons were monitored upon single and repetitive depolarizations.

RESULTS

PTZ induced in only a few CK--/-- mice PTZ seizure-like behavior, but in all wild-types a full-blown seizure. EEG analysis showed that preseizure jerking was associated with high-amplitude discharges. Wild-type EEG recordings showed continuous runs of rhythmic 4-6 Hz activity, whereas no rhythmic EEG activities were observed in the few CK--/-- mice that developed a behavioral seizure. All other CK--/-- mice displayed a sudden postictal depression without any development of a generalized seizure. Hippocampal granule neurons of CK--/-- mice displayed a higher Ca(2+) removal speed following repetitive KCl-induced depolarizations.

DISCUSSION

Deficiency for creatine kinase is affecting brain energy metabolism and will likely contribute to the disturbance of seizure development. Because CK--/-- hippocampal neurons exhibited an increase in Ca(2+) removal rate of elevated intracellular levels, we conclude that altered Ca(2+) clearance in CK--/-- neurons could play a role in the abnormal EEG and seizure activity.

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.

Local ATP generation by brain-type creatine kinase (CK-B) facilitates cell motility

Background

Creatine Kinases (CK) catalyze the reversible transfer of high-energy phosphate groups between ATP and phosphocreatine, thereby playing a storage and distribution role in cellular energetics. Brain-type CK (CK-B) deficiency is coupled to loss of function in neural cell circuits, altered bone-remodeling by osteoclasts and complement-mediated phagocytotic activity of macrophages, processes sharing dependency on actomyosin dynamics.

Methodology/Principal Findings

Here, we provide evidence for direct coupling between CK-B and actomyosin activities in cortical microdomains of astrocytes and fibroblasts during spreading and migration. CK-B transiently accumulates in membrane ruffles and ablation of CK-B activity affects spreading and migration performance. Complementation experiments in CK-B-deficient fibroblasts, using new strategies to force protein relocalization from cytosol to cortical sites at membranes, confirmed the contribution of compartmentalized CK-B to cell morphogenetic dynamics.

Conclusion/Significance

Our results provide evidence that local cytoskeletal dynamics during cell motility is coupled to on-site availability of ATP generated by CK-B.

Bacterial conjugation in the cytoplasm of mouse cells

Intracellular pathogenic organisms such as salmonellae and shigellae are able to evade the effects of many antibiotics because the drugs are not able to penetrate the plasma membrane. In addition, these bacteria may be able to transfer genes within cells while protected from the action of drugs. The primary mode by which virulence and antibiotic resistance genes are spread is bacterial conjugation. Salmonellae have been shown to be competent for conjugation in the vacuoles of cultured mammalian cells. We now show that the conjugation machinery is also functional in the mammalian cytosol. Specially constructed Escherichia coli strains expressing Shigella flexneri plasmid and chromosomal virulence factors for escape from vacuoles and synthesizing the invasin protein from Yersinia pseudotuberculosis to enhance cellular entry were able to enter 3T3 cells and escape from the phagocytic vacuole. One bacterial strain (the donor) of each pair to be introduced sequentially into mammalian cells had a conjugative plasmid. We found that this plasmid could be transferred at high frequency. Conjugation in the cytoplasm of cells may well be a general phenomenon.

Creatine kinase B deficient neurons exhibit an increased fraction of motile mitochondria

BACKGROUND

Neurons require an elaborate system of intracellular transport to distribute cargo throughout axonal and dendritic projections. Active anterograde and retrograde transport of mitochondria serves in local energy distribution, but at the same time also requires input of ATP. Here we studied whether brain-type creatine kinase (CK-B), a key enzyme for high-energy phosphoryl transfer between ATP and CrP in brain, has an intermediary role in the reciprocal coordination between mitochondrial motility and energy distribution. Therefore, we analysed the impact of brain-type creatine kinase (CK-B) deficiency on transport activity and velocity of mitochondria in primary murine neurons and made a comparison to the fate of amyloid precursor protein (APP) cargo in these cells, using live cell imaging.

RESULTS

Comparison of average and maximum transport velocities and global transport activity showed that CK-B deficiency had no effect on speed of movement of mitochondria or APP cargo, but that the fraction of motile mitochondria was significantly increased by 36% in neurons derived from CK-B knockout mice. The percentage of motile APP vesicles was not altered.

CONCLUSION

CK-B activity does not directly couple to motor protein activity but cells without the enzyme increase the number of motile mitochondria, possibly as an adaptational strategy aimed to enhance mitochondrial distribution versatility in order to compensate for loss of efficiency in the cellular network for ATP distribution.

Superoxide production is inversely related to complex I activity in inherited complex I deficiency

Deficiency of NADH:ubiquinone oxidoreductase or complex I (CI) is the most common cause of disorders of the oxidative phosphorylation system in humans. Using life cell imaging and blue-native electrophoresis we quantitatively compared superoxide production and CI amount and activity in cultured skin fibroblasts of 7 healthy control subjects and 21 children with inherited isolated CI deficiency. Thirteen children had a disease causing mutation in one of the nuclear-encoded CI subunits, whereas in the remainder the genetic cause of the disease is not yet established. Superoxide production was significantly increased in all but two of the patient cell lines. An inverse relationship with the amount and residual activity of CI was observed. In agreement with this finding, rotenone, a potent inhibitor of CI activity, dose-dependently increased superoxide production in healthy control cells. Also in this case an inverse relationship with the residual activity of CI was observed. In sharp contrast, however, rotenone did not decrease the amount of CI. The data presented show that superoxide production is increased in inherited CI deficiency and that this increase is primarily a consequence of the reduction in cellular CI activity and not of a further leakage of electrons from mutationally malformed complexes.

Comparative effects of a low-carbohydrate diet and exercise plus a low-carbohydrate diet on muscle sarcoplasmic reticulum responses in males

We employed a glycogen-depleting session of exercise followed by a low-carbohydrate (CHO) diet to investigate modifications that occur in muscle sarcoplasmic reticulum (SR) Ca2+-cycling properties compared with low-CHO diet alone. SR properties were assessed in nine untrained males [peak aerobic power (V̇o2 peak) = 43.6 ± 2.6 (SE) ml·kg−1·min−1] during prolonged cycle exercise to fatigue performed at ∼58% V̇o2 peakafter 4 days of low-CHO diet (Lo CHO) and after glycogen-depleting exercise plus 4 days of low-CHO (Ex+Lo CHO). Compared with Lo CHO, Ex+Lo CHO resulted in 12% lower ( P < 0.05) resting maximal Ca2+-ATPase activity ( Vmax= 174 ± 12 vs. 153 ± 10 μmol·g protein−1·min−1) and smaller reduction in Vmaxinduced during exercise. A similar effect was observed for Ca2+uptake. The Hill coefficient, defined as slope of the relationship between cytosolic free Ca2+concentration and Ca2+-ATPase activity, was higher ( P < 0.05) at rest (2.07 ± 0.15 vs. 1.90 ± 0.10) with Ex+Lo CHO, an effect that persisted throughout the exercise. The coupling ratio, defined as the ratio of Ca2+uptake to Vmax, was 23–30% elevated ( P < 0.05) at rest and during the first 60 min of exercise with Ex+Lo CHO. The ∼27 and 34% reductions ( P < 0.05) in phase 1 and phase 2 Ca2+release, respectively, observed during exercise with Lo CHO were not altered by Ex+Lo CHO. These results indicate that when prolonged exercise precedes a short-term Lo CHO diet, Ca2+sequestration properties and efficiency are improved compared with those during Lo CHO alone.

Effects of prior exercise and a low-carbohydrate diet on muscle sarcoplasmic reticulum function during cycling in women

The effects of exercise and diet on sarcoplasmic reticulum Ca(2+)-cycling properties in female vastus lateralis muscle were investigated in two groups of women following four different conditions. The conditions were 4 days of a low-carbohydrate (Lo CHO) and glycogen-depleting exercise plus a Lo CHO diet (Ex + Lo CHO) (experiment 2) and 4 days of normal CHO (Norm CHO) and glycogen-depleting exercise plus Norm CHO (Ex + Norm CHO) (experiment 1). Peak aerobic power (Vo2peak)) was 38.1 +/- 1.4 (SE); n = 9 and 35.6 +/- 1.4 ml.kg(-1).min(-1); n = 9, respectively. Sarcoplasmic reticulum properties measured in vitro in homogenates (micromol.g protein(-1).min(-1)) indicated exercise-induced reductions (P < 0.05) in maximal Ca(2+)-ATPase activity (0 > 30, 60 min > fatigue), Ca(2+) uptake (0 > 30 > 60 min, fatigue), and Ca(2+) release, both phase 1 (0, 30 > 60 min, fatigue) and phase 2 (0 > 30, 60 min, fatigue; 30 min > fatigue) in Norm CHO. Exercise was without effect in altering the Hill slope (n(H)), defined as the slope of relationship between Ca(2+)-ATPase activity and Ca(2+) concentration. No differences were observed between Norm CHO and Ex+Norm CHO. Compared with Norm CHO, Lo CHO resulted in a lower (P < 0.05) Ca(2+) uptake, phase 1 Ca(2+) release (30 min), and n(H). Ex + Lo CHO resulted in a greater (P < 0.05) Ca(2+) uptake and n(H) compared with Lo CHO. The results demonstrate that Lo CHO alone can disrupt SR Ca(2+) cycling and that, with the exception of Ca(2+) release, a glycogen-depleting session of exercise before Lo CHO can reverse the effects.

Simultaneous quantitative measurement and automated analysis of mitochondrial morphology, mass, potential, and motility in living human skin fibroblasts

BACKGROUND

Understanding the interdependence of mitochondrial and cellular functioning in health and disease requires detailed knowledge about the coupling between mitochondrial structure, motility, and function. Currently, no rapid approach is available for simultaneous quantification of these parameters in single living cells.

METHODS

Human skin fibroblasts were pulse-loaded with the mitochondria-selective fluorescent cation rhodamine 123. Next, mitochondria were visualized using video-rate (30 Hz) confocal microscopy and real-time image averaging. To highlight the mitochondria, the acquired images were binarized using a novel image processing strategy.

RESULTS

Our approach enabled rapid and simultaneous quantification of mitochondrial morphology, mass, potential, and motility. It was found that acute inhibition of mitochondrial complex I (NADH:ubiquinone oxidoreductase) by means of rotenone transiently reduced mitochondrial branching, area, and potential. In contrast, mitochondrial motility was permanently reduced.

CONCLUSIONS

We present and validate a novel approach for rapid, unbiased, and simultaneous quantification of multiple mitochondrial parameters in living cells. Because this method is automated, large numbers of cells can be analyzed in a short period of time.

Mitochondrial creatine kinase in human health and disease

Mitochondrial creatine kinase (MtCK), together with cytosolic creatine kinase isoenzymes and the highly diffusible CK reaction product, phosphocreatine, provide a temporal and spatial energy buffer to maintain cellular energy homeostasis. Mitochondrial proteolipid complexes containing MtCK form microcompartments that are involved in channeling energy in form of phosphocreatine rather than ATP into the cytosol. Under situations of compromised cellular energy state, which are often linked to ischemia, oxidative stress and calcium overload, two characteristics of mitochondrial creatine kinase are particularly relevant: its exquisite susceptibility to oxidative modifications and the compensatory up-regulation of its gene expression, in some cases leading to accumulation of crystalline MtCK inclusion bodies in mitochondria that are the clinical hallmarks for mitochondrial cytopathies. Both of these events may either impair or reinforce, respectively, the functions of mitochondrial MtCK complexes in cellular energy supply and protection of mitochondria form the so-called permeability transition leading to apoptosis or necrosis.

Simultaneous quantification of oxidative stress and cell spreading using 5-(and-6)-chloromethyl-2′,7′-dichlorofluorescein

BACKGROUND

Mitochondrial dysfunction may lead to increased oxidative stress and consequent changes in cell spreading. Here, we describe and validate a novel method for simultaneous quantification of these two parameters.

METHODS

Human skin fibroblasts were loaded with 5-(and-6)-chloromethyl-2',7'-dichlorodihydrofluorescein (CM-H(2)DCF), and its oxidative conversion into CM-DCF was monitored as a function of time by video-rate confocal microscopy and real-time image averaging. Cell size was determined after binarization of the acquired images.

RESULTS

At the lowest practical laser output, CM-DCF formation occurred with zero order kinetics, indicating that [CM-H(2)DCF] was not rate-limiting and that the rate of [CM-DCF] formation (V(CM-DCF)) was a function of the cellular oxidant level. Analysis of fibroblasts of a healthy control subject and a patient with a deficiency of NADH:ubiquinone oxidoreductase, the first complex of the oxidative phosphorylation system, revealed a significant increase in cellular oxidant level in the latter cells that was, however, not accompanied by a change in cell spreading. Conversely, chronic treatment with 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox), a derivative of vitamin E, markedly decreased the oxidant level and cell spreading in both control and patient fibroblasts.

CONCLUSIONS

We present a reliable method for simultaneous quantification of oxidant levels and cell spreading in living cells.

Decreased agonist-stimulated mitochondrial ATP production caused by a pathological reduction in endoplasmic reticulum calcium content in human complex I deficiency

Although a large number of mutations causing malfunction of complex I (NADH:ubiquinone oxidoreductase) of the OXPHOS system is now known, their cell biological consequences remain obscure. We previously showed that the bradykinin (Bk)-induced increase in mitochondrial [ATP] ([ATP](M)) is significantly reduced in primary skin fibroblasts from a patient with an isolated complex I deficiency. The present work addresses the mechanism(s) underlying this impaired response. Luminometry of fibroblasts from 6 healthy subjects and 14 genetically characterized patients expressing mitochondria targeted luciferase revealed that the Bk-induced increase in [ATP](M) was significantly, but to a variable degree, decreased in 10 patients. The same variation was observed for the increases in mitochondrial [Ca(2+)] ([Ca(2+)](M)), measured with mitochondria targeted aequorin, and cytosolic [Ca(2+)] ([Ca(2+)](C)), measured with fura-2, and for the Ca(2+) content of the endoplasmic reticulum (ER), calculated from the increase in [Ca(2+)](C) evoked by thapsigargin, an inhibitor of the ER Ca(2+) ATPase. Regression analysis revealed that the increase in [ATP](M) was directly proportional to the increases in [Ca(2+)](C) and [Ca(2+)](M) and to the ER Ca(2+) content. Our findings provide evidence that a pathological reduction in ER Ca(2+) content is the direct cause of the impaired Bk-induced increase in [ATP](M) in human complex I deficiency.

Amplitude modulation of nuclear Ca2+ signals in human skeletal myotubes: A possible role for nuclear Ca2+ buffering

Video-rate confocal microscopy of Indo-1-loaded human skeletal myotubes was used to assess the relationship between the changes in sarcoplasmic ([Ca(2+)](S)) and nuclear ([Ca(2+)](N)) Ca(2+) concentration during low- and high-frequency electrostimulation. A single stimulus of 10 ms duration transiently increased [Ca(2+)] in both compartments with the same time of onset. Rate and amplitude of the [Ca(2+)] rise were significantly lower in the nucleus (4.0- and 2.5-fold, respectively). Similarly, [Ca(2+)](N) decayed more slowly than [Ca(2+)](S) (mono-exponential time constants of 6.1 and 2.5 s, respectively). After return of [Ca(2+)] to the prestimulatory level, a train of 10 stimuli was applied at a frequency of 1 Hz. The amplitude of the first [Ca(2+)](S) transient was 25% lower than that of the preceding single transient. Thereafter, [Ca(2+)](S) increased stepwise to a maximum that equalled that of the single transient. Similarly, the amplitude of the first [Ca(2+)](N) transient was 20% lower than that of the preceding single transient. In contrast to [Ca(2+)](S), [Ca(2+)](N) then increased to a maximum that was 2.3-fold higher than that of the single transient and equalled that of [Ca(2+)](S). In the nucleus, and to a lesser extent in the sarcoplasm, [Ca(2+)] decreased faster at the end of the stimulus train than after the preceding single stimulus (time constants of 3.3 and 2.1 s, respectively). To gain insight into the molecular principles underlying the shaping of the nuclear Ca(2+) signal, a 3-D mathematical model was constructed. Intriguingly, quantitative modelling required the inclusion of a satiable nuclear Ca(2+) buffer. Alterations in the concentration of this putative buffer had dramatic effects on the kinetics of the nuclear Ca(2+) signal. This finding unveils a possible mechanism by which the skeletal muscle can adapt to changes in physiological demand.

Impaired voluntary running capacity of creatine kinase-deficient mice

The creatine kinase system (CK) is important for energy delivery in skeletal and cardiac muscles. The two main isoforms of this enzyme, cytosolic MM-CK and mitochondrial mi-CK, are expressed in a developmental and muscle-type specific manner. Mice deficient in one or both of these isoforms are viable and fertile but exhibit profound functional, metabolic and structural muscle remodelling that primarily affects fast skeletal muscles, which show an increased contribution of oxidative metabolism to contractile function. However, the consequences of these alterations in terms of physical capabilities have not yet been characterized. Consequently, we compared the voluntary exercise capacity of 9-month-old male wild-type (WT), M-CK knockout (M-CK(-/-)), and M-CK and mi-CK double knockout (CK(-/-)) mice, using cages equipped with running wheels. Exercise performance, calculated by total distance covered and by work done during the training period, was more than 10-fold lower in CK(-/-) mice than controls, with M-CK(-/-) mice exhibiting intermediate performance. Similarly, the mean distance run per activation was lower in M-CK(-/-) and even lower in CK(-/-) mice. However, the maximal running speed (V(max)) was lower only for CK(-/-) mice. This was accompanied by severe skeletal muscle mass decrease in CK(-/-) mice, with signs of histological damage that included enlarged interstitial areas, aggregations of mononuclear cells in the interstitium, heterogeneity of myofibre size and the presence of very small fibres. No overt sign of cardiac dysfunction was observed by magnetic resonance imaging during dobutamine stimulation. These results show that metabolic failure induced by CK deficiency profoundly affects the ability of mice to engage in chronic bouts of endurance running exercise and that this decrease in performance is also associated with muscle wasting.

Targeting cellular energy production in neurological disorders

The concepts of energy dysregulation and oxidative stress and their complicated interdependence have rapidly evolved to assume primary importance in understanding the pathophysiology of numerous neurological disorders. Therefore, neuroprotective strategies addressing specific bioenergetic defects hold particular promise in the treatment of these conditions (i.e., amyotrophic lateral sclerosis, Huntington's disease, Parkinson's disease, Friedreich's ataxia, mitochondrial cytopathies and other neuromuscular diseases), all of which, to some extent, share 'the final common pathway' leading to cell death through either necrosis or apoptosis. Compounds such as creatine monohydrate and coenzyme Q(10) offer substantial neuroprotection against ischaemia, trauma, oxidative damage and neurotoxins. Miscellaneous agents, including alpha-lipoic acid, beta-OH-beta-methylbutyrate, riboflavin and nicotinamide, have also been shown to improve various metabolic parameters in brain and/or muscle. This review will highlight the biological function of each of the above mentioned compounds followed by a discussion of their utility in animal models and human neurological disease. The balance of this work will be comprised of discussions on the therapeutic applications of creatine and coenzyme Q(10).

Effects of acute creatine kinase inhibition on metabolism and tension development in isolated single myocytes

This study investigated the effects of acute creatine kinase (CK) inhibition (CKi) on contractile performance, cytosolic Ca2+concentration ([Ca2+]c), and intracellular Po2(Pi[Formula: see text]) in Xenopus laevis isolated myocytes during a 2-min bout of isometric tetanic contractions (0.33-Hz frequency). Peak tension was similar between trials during the first contraction but was significantly ( P < 0.05) attenuated for all subsequent contractions in CKivs. control (Con). The fall in Pi[Formula: see text](ΔPi[Formula: see text]) from resting values was significantly greater in Con (26.0 ± 2.2 Torr) compared with CKi(17.8 ± 1.8 Torr). However, the ratios of Con to CKiend-peak tension (1.53 ± 0.11) and ΔPi[Formula: see text](1.49 ± 0.11) were similar, suggesting an unaltered aerobic cost of contractions. Additionally, the mean response time (MRT) of ΔPi[Formula: see text]was significantly faster in CKivs. Con during both the onset (31.8 ± 5.5 vs. 49.3 ± 5.7 s; P < 0.05) and cessation (21.2 ± 4.1 vs. 68.0 ± 3.2 s; P < 0.001) of contractions. These data demonstrate that initial phosphocreatine hydrolysis in single skeletal muscle fibers is crucial for maintenance of sarcoplasmic reticulum Ca2+release and peak tension during a bout of repetitive tetanic contractions. Furthermore, as Pi[Formula: see text]fell more rapidly at contraction onset in CKicompared with Con, these data suggest that CK activity temporally buffers the initial ATP-to-ADP concentration ratio at the transition to an augmented energetic demand, thereby slowing the initial mitochondrial activation by mitigating the energetic control signal (i.e., ADP concentration, phosphorylation potential, etc.) between sites of ATP supply and demand.

Inverse metabolic engineering with phosphagen kinase systems improves the cellular energy state

Inverse metabolic engineering attempts to identify or construct desired phenotypes of applied interest to endow them on appropriate host organisms. A particular desirable phenotype is the ATP homeostasis exhibited by animal cells with high and variable ATP turnover through temporal and spatial energy buffering. This buffering is achieved by phosphagen kinase systems that consist of a specific kinase and its cognate phosphagen, which functions as a large pool of 'high-energy phosphates' that are used to replenish ATP during periods of high energetic demand. This review discusses recent advances and potentials of inverse metabolic engineering of cell types that do not normally contain such systems--bacteria, yeast, plants, and liver--with creatine or arginine kinase systems. Examples are discussed that illustrate how microbial metabolism can be tailored for large-scale industrial processes with imperfect mixing and how the liver can be protected from metabolic insults or stimulated for better regeneration.

Superoxide scavengers augment contractile but not energetic responses to hypoxia in rat diaphragm

Acute exposure to severe hypoxia depresses contractile function and induces adaptations in skeletal muscle that are only partially understood. Previous studies have demonstrated that antioxidants (AOXs) given during hypoxia partially protect contractile function, but this has not been a universal finding. This study confirms that specific AOXs, known to act primarily as superoxide scavengers, protect contractile function in severe hypoxia. Furthermore, the hypothesis is tested that the mechanism of protection involves preservation of high-energy phosphates (ATP, creatine phosphate) and reductions of P(i). Rat diaphragm muscle strips were treated with AOXs and subjected to 30 min of hypoxia. Contractile function was examined by using twitch and tetanic stimulations and the degree of elevation in passive force occurring during hypoxia (contracture). High-energy phosphates were measured at the end of 30-min hypoxia exposure. Treatment with the superoxide scavengers 4,5-dihydroxy-1,3-benzenedisulfonic acid (Tiron, 10 mM) or Mn(III)tetrakis(1-methyl-4-pyridyl) porphyrin pentachloride (50 microM) suppressed contracture during hypoxia and protected maximum tetanic force. N-acetylcysteine (10 or 18 mM) had no influence on tetanic force production. Contracture during hypoxia without AOXs was also shown to be dependent on the extracellular Ca(2+) concentration. Although hypoxia resulted in only small reductions in ATP concentration, creatine phosphate concentration was decreased to approximately 10% of control. There were no consistent influences of the AOX treatments on high-energy phosphates during hypoxia. The results demonstrate that superoxide scavengers can protect contractile function and reduce contracture in hypoxia through a mechanism that does not involve preservation of high-energy phosphates.

The role of creatine in the management of amyotrophic lateral sclerosis and other neurodegenerative disorders

Creatine is consumed in the diet and endogenously synthesised in the body. Over the past decade, the ergogenic benefits of synthetic creatine monohydrate have made it a popular dietary supplement, particularly among athletes. The anabolic properties of creatine also offer hope for the treatment of diseases characterised by weakness and muscle atrophy. Moreover, because of its cellular mechanisms of action, creatine offers potential benefits for diseases involving mitochondrial dysfunction. Recent data also support the hypothesis that creatine may have a neuroprotective effect. Amyotrophic lateral sclerosis (ALS) is characterised by progressive degeneration of motor neurons, resulting in weakening and atrophy of skeletal muscles. In patients with this condition, creatine offers potential benefits in terms of facilitating residual muscle contractility as well as improving neuronal function. It may also help stabilise mitochondrial dysfunction, which plays a key role in the pathogenesis of ALS. Indeed, the likely multifactorial aetiology of ALS means the combined pharmacodynamic properties of creatine offer promise for the treatment of this condition. Evidence from available animal models of ALS supports the utility of treatment with creatine in this setting. Limited data available in other neuromuscular and neurodegenerative diseases further support the potential benefit of creatine monohydrate in ALS. However, few randomised, controlled trials have been conducted. To date, two clinical trials of creatine monohydrate in ALS have been completed without demonstration of significant improvements in overall survival or a composite measure of muscle strength. These trials have also posed unanswered questions about the optimal dosage of creatine and its beneficial effects on muscle fatigue, a measure distinct from muscle strength. A large, multicentre, clinical trial is currently underway to further investigate the efficacy of creatine monohydrate in ALS and address these unresolved issues. Evidence to date shows that creatine supplementation has a good safety profile and is well tolerated by ALS patients. The purpose of this article is to provide a short, balanced review of the literature concerning creatine monohydrate in the treatment of ALS and related neurodegenerative diseases. The pharmacokinetics and rationale for the use of creatine are described along with available evidence from animal models and clinical trials for ALS and related neurodegenerative or neuromuscular diseases.

Structural and functional adaptations of striated muscles to CK deficiency

In adult mammalian muscle cells, energy consuming processes are mainly localized to the sarcolemma, sarcoplasmic reticulum (SR) and myofibrillar compartments, while energy production occurs within mitochondria or glycolytic complexes. Due to the restricted diffusion of adenine nucleotides near the active sites of ATPases involved in contractile activity and calcium homeostasis, there are multiple local systems that can locally rephosphorylate ADP and provide ATP. The creatine kinase (CK) system, with specific isoenzymes localized within each compartment, efficiently controls local adenylate pools and links energy production and utilization. However, mice lacking one or both of the MM-CK and mi-CK isoforms (CK-/-) are viable and develop almost normal cardiac and skeletal muscle function under the conditions of moderate workload, suggesting adaptations or other mechanisms that may ensure efficient energy transfer. While fixed CK is essentially important, other systems could also be involved as well, such as bound glycolytic enzymes or adenylate kinase. We have shown that, additionally, a direct functional interplay exists between mitochondria and sarcoplasmic reticulum, or between mitochondria and myofilaments in muscle cells, that catalyzes direct energy and signal transfer between organelles. In cardiac cells of CK-/- mice, marked cytoarchitectural modifications were observed, and direct adenine nucleotide channeling between mitochondria and organelles was very effective to rescue SR and myofilament functions. In fast skeletal muscles, increased oxidative capacity also indicates compensatory mechanisms. In mutant mice, mitochondrial capacity increases and a direct energy channeling occurs between mitochondria on one hand and ATP consuming sites on the other. However, these systems appear to be insufficient to fully compensate for the lack of CK at high workload. It can be concluded that local rephosphorylation of ADP is a crucial regulatory point in highly differentiated and organized muscle cells to ensure contractile diversity and efficiency and that the CK system is important to control energy fluxes and energy homeostasis.

Neuroprotective mechanisms of creatine occur in the absence of mitochondrial creatine kinase

There is substantial evidence that creatine administration exerts neuroprotective effects both in vitro and in vivo. The precise mechanisms for these neuroprotective effects however are as yet unclear. We investigated whether creatine administration could exert neuroprotective effects in mice deficient in ubiquitous mitochondrial creatine kinase (UbMi-CK). UbMi-CK-deficient mice showed increased sensitivity to 1-methyl-1, 2, 3, 6-tetrahydropyridine (MPTP)-induced dopamine depletion and loss of tyrosine hydroxylase (TH) stained neurons. Isolated mitochondria from these mice showed no alterations in calcium retention, oxygen utilization, membrane potential, or swelling in response to a calcium challenge. Creatine administration significantly increased brain concentrations of both creatine and PCr in the UbMi-CK knockout mice. Creatine administration to the UbMi-CK-deficient mice exerted significant neuroprotective effects against MPTP toxicity that were comparable in magnitude to those seen in wild-type mice. These results suggest that the neuroprotective effects of creatine are not mediated by an effect on UbMi-CK to inhibit the mitochondrial permeability transition, and are more likely to be mediated by maintenance of appropriate ATP/ADP and PCr/Cr levels.

Skeletal muscle sarcoplasmic reticulum glycogen status influences Ca2+ uptake supported by endogenously synthesized ATP

The purpose of this investigation was to determine whether there is a link between sarcoplasmic reticulum (SR) glycogen status and SR Ca2+ handling. In this investigation, skeletal muscle SR was purified from female Sprague-Dawley rats (200-250 g). Glycogen was extracted from the SR purified from one hindlimb, whereas the SR purified from the contralateral limb served as control. Before removal of the tissue, the animals were anesthetized with an intraperitoneal injection of ketamine (80 mg/kg) and xylazine (10 mg/kg). Both alpha-amylase treatment (AM) and removal of EDTA from the homogenization and storage buffers reduced the amount of glycogen associated with the SR (P < 0.05). AM treatment reduced the glycogen phosphorylase content of SR (P < 0.05). In contrast, creatine kinase (CK) and pyruvate kinase (PK) contents were increased after both glycogen extraction protocols (P < 0.05). Under exogenous ATP conditions, both AM and EDTA-free (EF) treatments resulted in an increase in Ca2+-stimulated ATPase activity when normalized to sarco(endo)plasmic reticulum calcium-ATPase (SERCA) content (P < 0.05). CK and PK-supported SR Ca2+ uptake was decreased (P < 0.05) in the AM group when normalized to SERCA and CK or SERCA and PK content, respectively. AM was more effective than the EF for extracting glycogen associated with purified SR. Glycogen extraction alters the yield of purified SR proteins and must be taken into account when investigating SR calcium handling. Removal of glycogen from purified SR causes a change in Ca2+-handling properties as measured by ATPase and uptake activities.

Upregulation of Ca2+ removal in human skeletal muscle: a possible role for Ca2+-dependent priming of mitochondrial ATP synthesis

In muscle, ATP is required for the powerstroke of the myosin head, the detachment of actin and myosin filaments, and the reuptake of Ca2+ into the sarcoplasmic reticulum. During contraction-relaxation, large amounts of ATP are consumed at the sites of action of the myosin-ATPase and sarcoplasmic reticulum Ca2+-ATPase. The present study addresses the consequences of a reduction in mitochondrial ATP production capacity on sarcoplasmic Ca2+ handling. To this end, myotubes were cultured from patient quadriceps with a biochemically defined decrease in the maximal rate of mitochondrial ATP production and were loaded with indo 1 for imaging of sarcoplasmic Ca2+ changes in real time by confocal microscopy. Myotubes were field-stimulated with 10-ms pulses of 16 V to evoke transient rises in sarcoplasmic Ca2+ concentration ([Ca2+]S). Three single pulses, two pulse trains (1 Hz), and one single pulse were applied in succession to mimic changing workloads. Control myotubes displayed [Ca2+]S transients with an amplitude that was independent of the strength of the stimulus. Intriguingly, the rate of sarcoplasmic Ca2+ removal (CRR) was significantly upregulated during the second and subsequent transients. In myotubes with a reduced mitochondrial ATP production capacity, the amplitude of the [Ca2+]S transients was markedly increased at higher stimulus intensities. Moreover, upregulation of the CRR was significantly decreased compared with control. Taken together, these results are in good agreement with a tight coupling between mitochondrial ATP production and sarcoplasmic Ca2+ handling. Moreover, they support the existence of a relatively long-lasting mitochondrial memory for sarcoplasmic [Ca2+] rises. This memory, which manifested itself as an increase in CRR upon recurrent stimulation, was impaired in patient myotubes with a reduced mitochondrial ATP production capacity.

Metabolic factors contributing to altered Ca2+ regulation in skeletal muscle fatigue

AIM

Skeletal muscle fatigue is characterized by a failure to maintain force production or power output during intense exercise. Many recent studies on isolated fibres have used brief repetitive tetanic contractions to mimic fatigue resulting from intensive exercise and to investigate the underlying cellular mechanisms. Such studies have shown that characteristic changes in Ca2+ regulation occur during fatiguing stimulation. This includes prolongation of the 'Ca2+-tails' which follow each period of tetanic stimulation and a progressive rise in resting [Ca2+]. More importantly, the final stage of fatigue is associated with a rapid decrease in tetanic [Ca2+]i and force. These fatigue-induced changes in sarcoplasmic reticulum (SR) Ca2+ regulation are temporally associated with alterations in the intracellular levels of phosphate metabolites and a causal relationship has often been proposed. The aim of this review is to evaluate the evidence linking changes in the levels of phosphate metabolites and altered Ca2+ regulation during fatigue.

RESULTS

The following current hypotheses will be discussed: (1) the early changes in Ca2+ regulation reflect alterations in the intracellular levels of phosphate metabolites, (2) inhibition of the SR Ca2+ release mechanism (e.g. caused by ATP depletion and increased [Mg2+]) contributes to the decrease in tetanic [Ca2+]i during the final stages of fatigue and (iii) delayed entry of inorganic phosphate ions (Pi) into the SR, followed by precipitation of calcium phosphate (Ca-Pi), can explain the fatigue-induced decrease in tetanic [Ca2+]i.

CONCLUSION

There is strong evidence that changes in phosphate metabolite levels contribute to early changes in SR Ca2+ regulation during fatigue and that inhibition of the SR Ca2+ release mechanism can partially explain the rapid decrease in tetanic [Ca2+]i during the final stages of fatigue. While precipitation of Ca-Pi may occur within the SR during fatigue, there is currently insufficient evidence to establish whether this contributes to the late decline in tetanic [Ca2+]i.

Knockout of Kir6.2 negates ischemic preconditioning-induced protection of myocardial energetics

Although ischemic preconditioning induces bioenergetic tolerance and thereby remodels energy metabolism that is crucial for postischemic recovery of the heart, the molecular components associated with preservation of cellular energy production, transfer, and utilization are not fully understood. Here myocardial bioenergetic dynamics were assessed by (18)O-assisted (31)P-NMR spectroscopy in control or preconditioned hearts from wild-type (WT) or Kir6.2-knockout (Kir6.2-KO) mice that lack metabolism-sensing sarcolemmal ATP-sensitive K(+) (K(ATP)) channels. In WT vs. Kir6.2-KO hearts, preconditioning induced a significantly higher total ATP turnover (232 +/- 20 vs. 155 +/- 15 nmol x mg protein(-1) x min(-1)), ATP synthesis rate (58 +/- 3 vs. 46 +/- 3% (18)O labeling of gamma-ATP), and ATP consumption rate (51 +/- 4 vs. 31 +/- 4% (18)O labeling of P(i)) after ischemia-reperfusion. Moreover, preconditioning preserved cardiac creatine kinase-catalyzed phosphotransfer in WT (234 +/- 26 nmol x mg protein(-1) x min(-1)) but not Kir6.2-KO (133 +/- 18 nmol x mg protein(-1) x min(-1)) hearts. In contrast with WT hearts, preconditioning failed to preserve contractile recovery in Kir6.2-KO hearts, as tight coupling between postischemic performance and high-energy phosphoryl transfer was compromised in the K(ATP)-channel-deficient myocardium. Thus intact K(ATP) channels are integral in ischemic preconditioning-induced protection of cellular energetic dynamics and associated cardiac performance.

Disturbed Ca2+ kinetics in N-deacetylase/N-sulfotransferase-1 defective myotubes

The biosynthesis of heparan sulfate, present on the cell surface and in the basal lamina surrounding cells, is a multistep process in which each step is mediated by a specific enzyme. The initial modification of the precursor polysaccharide, N-deacetylation followed by N-sulfation of selected N-acetyl-D-glucosamine residues, is catalyzed by the enzyme glucosaminyl N-deacetylase/N-sulfotransferase (NDST). This event is a key step that regulates the overall sulfate content of the polysaccharide. Here, we report on the effects of NDST deficiency on Ca2+ kinetics in myotubes from NDST-1- and NDST-2-deficient mice, indicating a novel role for heparan sulfate in skeletal muscle physiology. Immunostaining for specific heparan sulfate epitopes showed major changes in the heparan sulfate composition in skeletal muscle tissue derived from NDST-1-/- mice and NDST-/- cultured myotubes. Biochemical analysis indicates a relative decrease in both N-sulfation and 2-O-sulfation of skeletal muscle heparan sulfate. The core protein of heparan sulfate proteoglycan perlecan was not affected, as judged by immunohistochemistry. Also, acetylcholine receptor clustering and the occurrence of other ion channels involved in excitation-contraction coupling were not altered. In NDST-2-/- mice and heterozygous mice no changes in heparan sulfate composition were observed. Using high-speed UV confocal laser scanning microscopy, aberrant Ca2+ kinetics were observed in NDST-1-/- myotubes, but not in NDST-2-/- or heterozygous myotubes. Electrically induced Ca2+ spikes had significantly lower amplitudes, and a reduced removal rate of cytosolic Ca2+, indicating the importance of heparan sulfate in muscle Ca2+ kinetics.

From energy store to energy flux: a study in creatine kinase-deficient fast skeletal muscle

Fast-twitch skeletal muscle of mice deficient in cytosolic and mitochondrial creatine kinase isoforms (CK-/-) lack burst activity but can sustain prolonged contractile activity, suggesting that adaptive mechanisms can regulate local adenine nucleotide turnover. We investigated whether direct energy and signal channeling between mitochondria and sarcoplasmic reticulum (SR) or myofilaments may exist that compensate for the lack of CK isoenzymes. Oxidative capacity of fast-twitch muscle was increased twofold in CK-/- mice. Energy cross talk between organelles was studied in muscle fibers with permeabilized sarcolemma. Energy supply to SR was estimated by analyzing the tension transient induced by caffeine and energy supply to myofilaments was estimated by the relaxation of rigor tension, both under different conditions of energy supply. In normal mice, ATP directly produced by mitochondria was not able to sustain calcium uptake and to relax rigor tension as efficiently as ATP produced by bound CK. However, in CK-/- mice, mitochondria ability to provide ATP for calcium uptake and relaxation of rigor tension was dramatically enhanced, suggesting a direct ATP/ADP channeling between sites of energy production mitochondria) and energy utilization in CK-/- mice. These results demonstrate two possible patterns of energy transport in muscle cells: energy store with phosphocreatine and energy flux through mitochondria.

Sarcolipin overexpression in rat slow twitch muscle inhibits sarcoplasmic reticulum Ca2+ uptake and impairs contractile function

Sarcolipin (SLN) is an inhibitor of sarco(endo)plasmic reticulum Ca(2+)-ATPases (SERCAs) in vitro, but its function in vivo has not been defined. NF-SLN cDNA (SLN tagged N-terminally with a FLAG epitope) was introduced into rat soleus muscle in one hindlimb by plasmid injection and electrotransfer. Western blotting showed expression and co-immunoprecipitation showed physical interaction between NF-SLN and SERCA2a. Contractile properties and SERCA2a function were assessed and compared with vector-injected contralateral soleus muscles. NF-SLN reduced both peak twitch force (P(t)) (123.9 +/- 12.5 versus 69.8 +/- 8.9 millinewtons) and tetanic force (P(o)) (562.3 +/- 51.0 versus 300.7 +/- 56.9 millinewtons) and reduced both twitch and tetanic rates of contraction (+dF/dt) and relaxation (-dF/dt) significantly. Repetitive stimulation (750-ms trains at 50 Hz once every 2 s for 3 min) showed that NF-SLN increased susceptibility to fatigue. These changes in contractile function were observed in the absence of endogenous phospholamban, and NF-SLN had no effect on either SERCA2a or SERCA1a expression levels. NF-SLN also decreased maximal Ca(2+) transport activity at pCa 5 by 31% with no significant change in apparent Ca(2+) affinity (6.36 +/- 0.07 versus 6.39 +/- 0.08 pCa units). These results show that NF-SLN expression impairs muscle contractile function by inhibiting SERCA function and diminishing sarcoplasmic reticulum Ca(2+) stores.

Functional properties and [Ca(2+)](i) metabolism of creatine kinase--KO mice myocardium

One major function of the creatine kinase system is to maintain energy demand of myofibrillar contraction processes. Loss of the CK-system led to adaptations in skeletal muscle. To analyze the impact on myocardial function contractile parameters and intracellular calcium metabolism of transgenic mice lacking mitochondrial CK (ScCKmit(-/-)) alone or both mitochondrial and cytoplasmic ScCK (CK(-/-)) were investigated compared to wild type at various workload conditions using isolated intact muscle fibers. Force development at baseline conditions, force-frequency relationship (60-600/min), and rapid frequency switch (60-600/min) were unaltered in myocardium of transgenic mice compared to wild type. Intracellular calcium metabolism revealed unchanged amplitude of the intracellular calcium transients (ICT), refilling of the sarcoplasmic reticulum (calcium reuptake, post-rest behavior) in the ScCKmit(-/-) and CK(-/-) mice. The results demonstrate the effectiveness of myocardial energy-recruiting compensatory mechanisms at baseline as well as under stress conditions in CK depleted myocardium of transgenic mice.

Gene expression profiling of the aging mouse cardiac myocytes

Heart disease remains the most frequent cause of death in the general population with increasing incidence in the elderly population. The pathologic failure of the aging heart may be related to structural and functional alterations in cardiac muscle cells. However, the molecular mechanisms underlying the aging-related decline in cardiac muscle function are largely unknown. To provide the first analysis of cardiac aging at the level of gene expression, we established and compared cDNA libraries from apparently healthy young and aged mouse ventricular cardiac muscle cells. We report the identification of genes that exhibit aging-related changes of mRNA levels. Aging expression profiles in aged hearts indicate decreased cellular adaptation and protection against stress-induced injury together with the development of contractile dysfunction. The data suggest reduced activity of the mitochondrial electron transport system and reduced levels of cardiac-specific transcription regulators. The cardiomyocyte aging profile of gene expression displays similarities with known heart disorders. Genes whose mRNA levels change with aging in cardiomyocytes might profoundly affect pathological changes in the heart.

Coupling of cell energetics with membrane metabolic sensing. Integrative signaling through creatine kinase phosphotransfer disrupted by M-CK gene knock-out

Transduction of metabolic signals is essential in preserving cellular homeostasis. Yet, principles governing integration and synchronization of membrane metabolic sensors with cell metabolism remain elusive. Here, analysis of cellular nucleotide fluxes and nucleotide-dependent gating of the ATP-sensitive K+ (K(ATP)) channel, a prototypic metabolic sensor, revealed a diffusional barrier within the submembrane space, preventing direct reception of cytosolic signals. Creatine kinase phosphotransfer, captured by 18O-assisted 31P NMR, coordinated tightly with ATP turnover, reflecting the cellular energetic status. The dynamics of high energy phosphoryl transfer through the creatine kinase relay permitted a high fidelity transmission of energetic signals into the submembrane compartment synchronizing K(ATP) channel activity with cell metabolism. Knock-out of the creatine kinase M-CK gene disrupted signal delivery to K(ATP) channels and generated a cellular phenotype with increased electrical vulnerability. Thus, in the compartmentalized cell environment, phosphotransfer systems shunt diffusional barriers and secure regimented signal transduction integrating metabolic sensors with the cellular energetic network.