Myofibrillar or mitochondrial creatine kinase deficiency alone does not impair mouse diaphragm isotonic function

Proteomic Analysis of Human Brown Adipose Tissue Reveals Utilization of Coupled and Uncoupled Energy Expenditure Pathways

Human brown adipose tissue (BAT) has become an attractive target to combat the current epidemical spread of obesity and its associated co-morbidities. Currently, information on its functional role is primarily derived from rodent studies. Here, we present the first comparative proteotype analysis of primary human brown adipose tissue versus adjacent white adipose tissue, which reveals significant quantitative differences in protein abundances and in turn differential functional capabilities. The majority of the 318 proteins with increased abundance in BAT are associated with mitochondrial metabolism and confirm the increased oxidative capacity. In addition to uncoupling protein 1 (UCP1), the main functional effector for uncoupled respiration, we also detected the mitochondrial creatine kinases (CKMT1A/B, CKMT2), as effective modulators of ATP synthase coupled respiration, to be exclusively expressed in BAT. The abundant expression and utilization of both energy expenditure pathways in parallel highlights the complex functional involvement of BAT in human physiology.

In the absence of phosphate shuttling, exercise reveals the in vivo importance of creatine-independent mitochondrial ADP transport

The transport of cytosolic adenosine diphosphate (ADP) into the mitochondria is a major control point in metabolic homeostasis, as ADP concentrations directly affect glycolytic flux and oxidative phosphorylation rates within mitochondria. A large contributor to the efficiency of this process is thought to involve phosphocreatine (PCr)/Creatine (Cr) shuttling through mitochondrial creatine kinase (Mi-CK), whereas the biological importance of alterations in Cr-independent ADP transport during exercise remains unknown. Therefore, we utilized an Mi-CK knockout (KO) model to determine whether in vivo Cr-independent mechanisms are biologically important for sustaining energy homeostasis during exercise. Ablating Mi-CK did not alter exercise tolerance, as the time to volitional fatigue was similar between wild-type (WT) and KO mice at various exercise intensities. In addition, skeletal muscle metabolic profiles after exercise, including glycogen, PCr/Cr ratios, free ADP/adenosine monophosphate (AMP), and lactate, were similar between genotypes. While these data suggest that the absence of PCr/Cr shuttling is not detrimental to maintaining energy homeostasis during exercise, KO mice displayed a dramatic increase in Cr-independent mitochondrial ADP sensitivity after exercise. Specifically, whereas mitochondrial ADP sensitivity decreased with exercise in WT mice, in stark contrast, exercise increased mitochondrial Cr-independent ADP sensitivity in KO mice. As a result, the apparent ADP Km was 50% lower in KO mice after exercise, suggesting that in vivo activation of voltage-dependent anion channel (VDAC)/adenine nucleotide translocase (ANT) can support mitochondrial ADP transport. Altogether, we provide insight that Cr-independent ADP transport mechanisms are biologically important for regulating ADP sensitivity during exercise, while highlighting complex regulation and the plasticity of the VDAC/ANT axis to support adenosine triphosphate demand.

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.

Modulation of the actomyosin interaction during fatigue of skeletal muscle

Fatigue of skeletal muscle involves many systems beginning with the central nervous system and ending with the contractile machinery. This review concentrates on those factors that directly affect the actomyosin interaction: the build-up of metabolites; myosin phosphorylation; and oxidation of the myofibrillar proteins by free radicals. The decrease in [ATP] and increase in [ADP] appear to play little role in modulating function. The increase in phosphate inhibits tension. The decrease in pH, long thought to be a major factor, is now known to play a more minor role. Myosin phosphorylation potentiates the force achieved in a twitch, and a further role in inhibiting velocity is proposed. Protein oxidation can both potentiate and inhibit the actomyosin interaction. It is concluded that these factors, taken together, do not fully explain the inhibition of the actomyosin interaction observed in living fibers, and thus additional modulators of this interaction remain to be discovered.

Isotonic fatigue in laminin alpha2-deficient dy/dy dystrophic mouse diaphragm

Laminin alpha2 deficiency causes approximately 50% of human congenital muscular dystrophies. Muscle in the corresponding dy/dy mouse model has reduced force but increased fatigue resistance during isometric contractions. To determine whether a similar pattern of alterations is present during isotonic contractions, dy/dy diaphragm was studied in vitro. During 20% load, dystrophic diaphragm had significantly reduced shortening, shortening velocity, work and power deficits, which persisted during the fatigue-inducing stimulation. In contrast, during 40% load, isotonic contractile performance of diseased muscle was impaired only mildly and only for some contractile parameters. At both loads, rate of isotonic fatigue when expressed relative to initial contractile values was similar for dystrophic and normal muscle, or in some instances slightly higher for dystrophic muscle. Therefore, fatigue resistance is considerably impaired during isotonic contractions relative to that reported previously for isometric contractions. This has important implications for increased susceptibility to respiratory failure in laminin alpha2-deficient muscular dystrophy.

Proteome analysis of skeletal muscle from obese and morbidly obese women

Obesity-related diseases such as the metabolic syndrome and type 2 diabetes originate, in part, from the progressive metabolic deterioration of skeletal muscle. A preliminary proteomic survey of rectus abdominus muscle detected a statistically significant increase in adenylate kinase (AK)1, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and aldolase A in obese/overweight and morbidly obese women relative to lean control subjects. AK1 is essential for the maintenance of cellular energy charge, and GAPDH and aldolase A are well known glycolytic enzymes. We found that muscle AK1 protein and enzymatic activity increased 2.9 and 90%, respectively, in obese women and 9.25 and 100%, respectively, in morbidly obese women. The total enzymatic activity of creatine kinase, which also regulates energy metabolism in muscle, was shown to increase 30% in obese/overweight women only. We propose that increased protein and enzymatic activity of AK1 is representative of a compensatory glycolytic drift to counteract reduced muscle mitochondrial function with the progression of obesity. This hypothesis is supported by increased abundance of the glycolytic enzymes GAPDH and aldolase A in obese and morbidly obese muscle. In summary, proteome analysis of muscle has helped us better describe the molecular etiology of obesity-related disease.

Creatine kinase-deficient hearts exhibit increased susceptibility to ischemia-reperfusion injury and impaired calcium homeostasis

The creatine kinase (CK) system is involved in the rapid transport of high-energy phosphates from the mitochondria to the sites of maximal energy requirements such as myofibrils and sarcolemmal ion pumps. Hearts of mice with a combined knockout of cytosolic M-CK and mitochondrial CK (M/Mito-CK−/−) show unchanged basal left ventricular (LV) performance but reduced myocardial high-energy phosphate concentrations. Moreover, skeletal muscle from M/Mito-CK−/−mice demonstrates altered Ca2+homeostasis. Our hypothesis was that in CK-deficient hearts, a cardiac phenotype can be unmasked during acute stress conditions and that susceptibility to ischemia-reperfusion injury is increased because of altered Ca2+homeostasis. We simultaneously studied LV performance and myocardial Ca2+metabolism in isolated, perfused hearts of M/Mito-CK−/−( n = 6) and wild-type (WT, n = 8) mice during baseline, 20 min of no-flow ischemia, and recovery. Whereas LV performance was not different during baseline conditions, LV contracture during ischemia developed significantly earlier (408 ± 72 vs. 678 ± 54 s) and to a greater extent (50 ± 2 vs. 36 ± 3 mmHg) in M/Mito-CK−/−mice. During reperfusion, recovery of diastolic function was impaired (LV end-diastolic pressure: 22 ± 3 vs. 10 ± 2 mmHg), whereas recovery of systolic performance was delayed, in M/Mito-CK−/−mice. In parallel, Ca2+transients were similar during baseline conditions; however, M/Mito-CK−/−mice showed a greater increase in diastolic Ca2+concentration ([Ca2+]) during ischemia (237 ± 54% vs. 167 ± 25% of basal [Ca2+]) compared with WT mice. In conclusion, CK-deficient hearts show an increased susceptibility of LV performance and Ca2+homeostasis to ischemic injury, associated with a blunted postischemic recovery. This demonstrates a key function of an intact CK system for maintenance of Ca2+homeostasis and LV mechanics under metabolic stress conditions.

Phosphocreatine kinetics at the onset of contractions in skeletal muscle of MM creatine kinase knockout mice

Phosphocreatine (PCr) depletion during isometric twitch stimulation at 5 Hz was measured by (31)P-NMR spectroscopy in gastrocnemius muscles of pentobarbital-anesthetized MM creatine kinase knockout (MMKO) vs. wild-type C57B (WT) mice. PCr depletion after 2 s of stimulation, estimated from the difference between spectra gated to times 200 ms and 140 s after 2-s bursts of contractions, was 2.2 +/- 0.6% of initial PCr in MMKO muscle vs. 9.7 +/- 1.6% in WT muscles (mean +/- SE, n = 7, P < 0.001). Initial PCr/ATP ratio and intracellular pH were not significantly different between groups, and there was no detectable change in intracellular pH or ATP in either group after 2 s. The initial difference in net PCr depletion was maintained during the first minute of continuous 5-Hz stimulation. However, there was no significant difference in the quasi-steady-state PCr level approached after 80 s (MMKO 36.1 +/- 3.5 vs. WT 35.5 +/- 4.4% of initial PCr; n = 5-6). A kinetic model of ATPase, creatine kinase, and adenylate kinase fluxes during stimulation was consistent with the observed PCr depletion in MMKO muscle after 2 s only if ADP-stimulated oxidative phosphorylation was included in the model. Taken together, the results suggest that cytoplasmic ADP more rapidly increases and oxidative phosphorylation is more rapidly activated at the onset of contractions in MMKO compared with WT muscles.

Health implications of creatine: can oral creatine supplementation protect against neurological and atherosclerotic disease?

Major achievements made over the last several years have highlighted the important roles of creatine and the creatine kinase reaction in health and disease. Inborn errors of metabolism have been identified in the three main steps involved in creatine metabolism: arginine:glycine amidinotransferase (AGAT), S-adenosyl-L-methionine:N-guanidinoacetate methyltransferase (GAMT), and the creatine transporter. All these diseases are characterized by a lack of creatine and phosphorylcreatine in the brain, and by (severe) mental retardation. Similarly, knockout mice lacking the brain cytosolic and mitochondrial isoenzymes of creatine kinase displayed a slightly increased creatine concentration, but no phosphorylcreatine in the brain. These mice revealed decreased weight gain and reduced life expectancy, disturbed fat metabolism, behavioral abnormalities and impaired learning capacity. Oral creatine supplementation improved the clinical symptoms in both AGAT and GAMT deficiency, but not in creatine transporter deficiency. In addition, creatine supplementation displayed neuroprotective effects in several animal models of neurological disease, such as Huntington's disease, Parkinson's disease, or amyotrophic lateral sclerosis. All these findings pinpoint to a close correlation between the functional capacity of the creatine kinase/phosphorylcreatine/creatine system and proper brain function. They also offer a starting-point for novel means of delaying neurodegenerative disease, and/or for strengthening memory function and intellectual capabilities.Finally, creatine biosynthesis has been postulated as a major effector of homocysteine concentration in the plasma, which has been identified as an independent graded risk factor for atherosclerotic disease. By decreasing homocysteine production, oral creatine supplementation may, thus, also lower the risk for developing, e.g., coronary heart disease or cerebrovascular disease. Although compelling, these results require further confirmation in clinical studies in humans, together with a thorough evaluation of the safety of oral creatine supplementation.

Changes in the mitochondrial proteome from mouse hearts deficient in creatine kinase

Creatine kinase (CK) is an abundant enzyme, important for maintenance of high-energy phosphate homeostasis in many tissues including heart. Double-knockout CK (DbKO-CK) mice missing both the muscle (MM) and sarcomeric mitochondrial (ScMit) isoforms of CK have recently been studied. Despite a large change in skeletal muscle function in DbKO-CK mice, there is little functional change in the heart. To investigate whether there are specific changes in cardiac mitochondrial proteins associated with the loss of MM- and ScMit-CK isoforms, we have used difference gel electrophoresis (DIGE) to compare mitochondrial proteins from wild-type and DbKO-CK mice. Mass spectrometry fingerprinting was used to identify 40 spots as known mitochondrial proteins. We have discovered that the loss of MM- and ScMit-CK isoforms did not cause large scale changes in heart mitochondrial proteins. The loss of ScMit-CK was readily detected in the DbKO-CK samples. We have also detected a large decrease in the precursor form of aconitase. Furthermore, two mitochondrial protein differences have been found in the parent mouse strains of the DbKO-CK mice.