Separate nuclear genes encode sarcomere-specific and ubiquitous human mitochondrial creatine kinase isoenzymes

Toward structural-omics of the bovine retinal pigment epithelium

The use of an integrated systems biology approach to investigate tissues and organs has been thought to be impracticable in the field of structural biology, where the techniques mainly focus on determining the structure of a particular biomacromolecule of interest. Here, we report the use of cryoelectron microscopy (cryo-EM) to define the composition of a raw bovine retinal pigment epithelium (RPE) lysate. From this sample, we simultaneously identify and solve cryo-EM structures of seven different RPE enzymes whose functions affect neurotransmitter recycling, iron metabolism, gluconeogenesis, glycolysis, axonal development, and energy homeostasis. Interestingly, dysfunction of these important proteins has been directly linked to several neurodegenerative disorders, including Huntington's disease, amyotrophic lateral sclerosis (ALS), Parkinson's disease, Alzheimer's disease, and schizophrenia. Our work underscores the importance of cryo-EM in facilitating tissue and organ proteomics at the atomic level.

Ckmt1 is Dispensable for Mitochondrial Bioenergetics Within White/Beige Adipose Tissue

Within brown adipose tissue (BAT), the brain isoform of creatine kinase (CKB) has been proposed to regulate the regeneration of ADP and phosphocreatine in a futile creatine cycle (FCC) that stimulates energy expenditure. However, the presence of FCC, and the specific creatine kinase isoforms regulating this theoretical model within white adipose tissue (WAT), remains to be fully elucidated. In the present study, creatine did not stimulate respiration in cultured adipocytes, isolated mitochondria or mouse permeabilized WAT. Additionally, while creatine kinase ubiquitous-type, mitochondrial (CKMT1) mRNA and protein were detected in human WAT, shRNA-mediated reductions in Ckmt1 did not decrease submaximal respiration in cultured adipocytes, and ablation of CKMT1 in mice did not alter energy expenditure, mitochondrial responses to pharmacological β3-adrenergic activation (CL 316, 243) or exacerbate the detrimental metabolic effects of consuming a high-fat diet. Taken together, these findings solidify CKMT1 as dispensable in the regulation of energy expenditure, and unlike in BAT, they do not support the presence of FCC within WAT.

Creatine metabolism in the uterus: potential implications for reproductive biology

Creatine is an amino acid derivative synthesized from arginine, glycine and methionine. It serves as the substrate for the creatine kinase system, which is vital for maintaining ATP levels in tissues with high and fluctuating energy demand. There exists evidence that the creatine kinase system operates in both the endometrial and myometrial layers of the uterus. While use and regulation of this system in the uterus are not well understood, it is likely to be important given uterine tissues undergo phases of increased energy demand during certain stages of the female reproductive cycle, pregnancy, and parturition. This review discusses known adaptations of creatine metabolism in the uterus during the reproductive cycle (both estrous and menstrual), pregnancy and parturition, highlighting possible links to fertility and the existing knowledge gaps. Specifically, we discuss the adaptations and regulation of uterine creatine metabolite levels, cell creatine transport, de novo creatine synthesis, and creatine kinase expression in the various layers and cell types of the uterus. Finally, we discuss the effects of dietary creatine on uterine metabolism. In summary, there is growing evidence that creatine metabolism is up-regulated in uterine tissues during phases where energy demand is increased. While it remains unclear how important these adaptations are in the maintenance of healthy uterine function, furthering our understanding of uterine creatine metabolism may uncover strategies to combat poor embryo implantation and failure to conceive, as well as enhancing uterine contractile performance during labor.

Creatine metabolism: energy homeostasis, immunity and cancer biology

Perturbations in metabolic processes are associated with diseases such as obesity, type 2 diabetes mellitus, certain infections and some cancers. A resurgence of interest in creatine biology is developing, with new insights into a diverse set of regulatory functions for creatine. This resurgence is primarily driven by technological advances in genetic engineering and metabolism as well as by the realization that this metabolite has key roles in cells beyond the muscle and brain. Herein, we highlight the latest advances in creatine biology in tissues and cell types that have historically received little attention in the field. In adipose tissue, creatine controls thermogenic respiration and loss of this metabolite impairs whole-body energy expenditure, leading to obesity. We also cover the various roles that creatine metabolism has in cancer cell survival and the function of the immune system. Renewed interest in this area has begun to showcase the therapeutic potential that lies in understanding how changes in creatine metabolism lead to metabolic disease.

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.

Resistance Artery Creatine Kinase mRNA and Blood Pressure in Humans

Hypertension remains the main risk factor for cardiovascular death. Environmental and biological factors are known to contribute to the condition, and circulating creatine kinase was reported to be the main predictor of blood pressure in the general population. This was proposed to be because of high resistance artery creatine kinase-BB rapidly regenerating ATP for vascular contractility. Therefore, we assessed whether creatine kinase isoenzyme mRNA levels in human resistance arteries are associated with blood pressure. We isolated resistance-sized arteries from omental fat donated by consecutive women undergoing uterine fibroid surgery. Blood pressure was measured in the sitting position. Vessels of 13 women were included, 6 normotensive and 7 hypertensive, mean age 42.9 years (SE, 1.6) and mean systolic/diastolic blood pressure, 144.8 (8.0)/86.5 (4.3) mm Hg. Arteriolar creatine kinase isoenzyme mRNA was assessed using quantitative real-time polymerase chain reaction. Normalized creatine kinase B mRNA copy numbers, ranging from 5.2 to 24.4 (mean, 15.0; SE, 1.9), showed a near-perfect correlation with diastolic blood pressure (correlation coefficient, 0.9; 95% confidence interval, 0.6–1.0) and were well correlated with systolic blood pressure, with a 90% relative increase in resistance artery creatine kinase B mRNA in hypertensives compared with normotensives, normalized copy numbers were, respectively, 19.3 (SE, 2.0) versus 10.1 (SE, 2.1), P =0.0045. To our knowledge, this is the first direct evidence suggesting that resistance artery creatine kinase mRNA expression levels concur with blood pressure levels, almost doubling with hypertension. These findings add to the evidence that creatine kinase might be involved in the vasculature’s pressor responses.

Increase in serum mitochondrial creatine kinase levels induced by tenofovir administration

Recently, 2 monoclonal antibodies that specifically inhibit mitochondrial creatine kinase (MtCK) activity have been developed. In this study, we measured the serum MtCK activity in HIV-1-infected individuals (n = 100) by employing a novel method using these antibodies. The mean serum MtCK activity in 44 patients treated with highly active antiretroviral therapy (HAART) including tenofovir disoproxil fumarate (TDF) was 16.0 IU/L. The MtCK activity was significantly higher in patients receiving TDF than in those receiving HAART without TDF (3.4 IU/L) or in naïve patients (6.9 IU/L) (Tukey-Kramer test, p < 0.0001 and p = 0.0029, respectively). The serum MtCK activity reached a plateau at 1 month after the initiation of TDF administration and decreased upon discontinuation. It showed no significant correlation with the trough plasma TDF concentration, serum creatinine level, or red blood cell count. The activity was elevated in 75% of the patients receiving TDF, and this elevation was specific to TDF; it was not observed with other anti-HIV drugs. In addition, our report emphasizes the careful interpretation of creatine kinase-MB (CK-MB) test results in patients receiving TDF because MtCK in serum could cause false-positive results on a conventional CK-MB test, which does not include MtCK-specific inhibitory antibodies.

Inflammatory Myopathy with Severe Tongue Atrophy in Pembroke Welsh Corgi Dogs

A disease characterized by tongue and facial muscle atrophy has been recognized sporadically among Pembroke Welsh Corgi (PWC) dogs in Japan. The present study describes the pathologic findings of this canine syndrome. Histopathologic examinations were performed in 2 dogs, including a case of muscular biopsy. Identification and characterization of autoantibodies were attempted by fluorescent antibody test (FAT) and Western blot (WB) by using sera from 7 PWC dogs with typical clinical features, 6 PWC dogs with other clinical signs, and 2 from other breeds with polymyositis. Clinically, the 7 affected PWC dogs exhibited dysphagia with severe tongue atrophy, facial muscular atrophy, and occasional walking difficulty. Histopathologic examinations of the 2 dogs with clinical symptoms revealed moderate to severe inflammatory lesions characterized by lymphohistiocytic infiltration and muscular atrophy in the tongue and/or femoral muscles. The tongue lesions were very severe and accompanied by diffuse fatty infiltration. There were no major lesions in the nervous tissues examined. By FAT, an autoantibody against the cross striation of skeletal muscle was detected in sera from 5 affected PWC dogs. By using WB analysis, the autoantibodies recognized a 42-kDa molecule in striated muscle but not in the nervous tissues. All of the findings indicated that the unique disease of PWC dogs might be generalized inflammatory myopathy, whereas the detailed etiology concerning the dominant involvement of tongue muscles and the role of the autoantibody in the canine disease remain to be clarified.

The creatine kinase phosphotransfer network: thermodynamic and kinetic considerations, the impact of the mitochondrial outer membrane and modelling approaches

In this review, we summarize the main structural and functional data on the role of the phosphocreatine (PCr)--creatine kinase (CK) pathway for compartmentalized energy transfer in cardiac cells. Mitochondrial creatine kinase, MtCK, fixed by cardiolipin molecules in the vicinity of the adenine nucleotide translocator, is a key enzyme in this pathway. Direct transfer of ATP and ADP between these proteins has been revealed both in experimental studies on the kinetics of the regulation of mitochondrial respiration and by mathematical modelling as a main mechanism of functional coupling of PCr production to oxidative phosphorylation. In cells in vivo or in permeabilized cells in situ, this coupling is reinforced by limited permeability of the outer membrane of the mitochondria for adenine nucleotides due to the contacts with cytoskeletal proteins. Due to these mechanisms, at least 80% of total energy is exported from mitochondria by PCr molecules. Mathematical modelling of intracellular diffusion and energy transfer shows that the main function of the PCr-CK pathway is to connect different pools (compartments) of ATP and, by this way, to overcome the local restrictions and diffusion limitation of adenine nucleotides due to the high degree of structural organization of cardiac cells.

Mitochondrial creatine kinase with atypical pI values detected in serum of a patient with ovarian hepatoid yolk sac tumor

Atypical mitochondrial creatine kinase (creatine N-phosphotransferase, CK, EC 2.7.3.2) was detected in the serum of a patient with carcinoma of germ cell origin, probably hepatoid yolk sac tumor. The pI of the oligomeric atypical mitochondrial CK (Mi-CK) was found at the acidic side compared to that of the typical ubiquitous Mi-CK (uMi-CK), while the molecular size of the atypical Mi-CK was similar to that of the typical uMi-CK. The pIs of the oligomeric and the dimeric atypical Mi-CKs became the same as those of the typical uMi-CK upon treatment with 2-mercaptoethanol. Therefore, the atypical Mi-CK was suggested to be an oxidized form of uMi-CK, and the oxidation might have occurred in the mitochondria because the oligomeric atypical Mi-CK had atypical pIs. The physicochemical characteristics of the oxidized uMi-CK were similar to those of the typical uMi-CK.

Role of creatine kinase isoenzymes on muscular and cardiorespiratory endurance: genetic and molecular evidence

The ability to perform well in activities that require muscular and cardiorespiratory endurance is a trait influenced, in a considerable part, by the genetic make-up of individuals. Early studies of performance and recent scans of the human genome have pointed at various candidate genes responsible for the heterogeneity of these phenotypes within the population. Among these are the genes for the various creatine kinase (CK) isoenzyme subunits. CK and phosphocreatine (PCr) form an important metabolic system for temporal and spatial energy buffering in cells with large variations in energy demand. The different CK isoenzyme subunits (CK-M and CK-B) are differentially expressed in the tissues of the body. Although CK-M is the predominant form in both skeletal and cardiac muscle, CK-B is expressed to a greater extent in heart than in skeletal muscle. Studies in humans and mice have shown that the expression of CK-B messenger RNA (mRNA) and the abundance and activity of the CK-MB dimer increase in response to cardiorespiratory endurance training. Increases in muscle tissue CK-B content can be energetically favourable because of its lower Michaelis constant (Km) for ADP. The activity of the mitochondrial isoform of CK (Scmit-CK) has also been significantly and positively correlated to oxidative capacity and to CK-MB activity in muscle. In mice where the CK-M gene has been knocked out, significant increases in fatigue resistance together with cellular adaptations increasing aerobic capacity have been observed. These observations have led to the notion that this enzyme may be responsible for fatigue under normal circumstances, most likely because of the local cell compartment increase in inorganic phosphate concentration. Studies where the Scmit-CK gene was knocked out have helped demonstrate that this isoenzyme is very important for the stimulation of aerobic respiration. Human studies of CK-M gene sequence variation have shown a significant association between a polymorphism, distinguished by the NcoI restriction enzyme, and an increase in cardiorespiratory endurance as indexed by maximal oxygen uptake following 20 weeks of training. In conclusion, there is now evidence at the tissue, cell and molecular level indicating that the CK-PCr system plays an important role in determining the phenotypes of muscular and cardiorespiratory endurance. It is envisioned that newer technologies will help determine how the genetic variability of these genes (and many others) impact on performance and health-related phenotypes.

Acute changes of myocardial creatine kinase gene expression under beta-adrenergic stimulation

Creatine kinase (CK) plays a crucial role in myocardial energy metabolism. Alterations in CK gene expression are found in hypertrophied and failing heart, but the mechanisms behind these changes are unclear. This study tests the hypothesis that increased adrenergic stimulation, which is observed in heart failure, induces changes of myocardial CK-activity, -isoenzyme distribution and -gene expression that are characteristic of the failing and hypertrophied heart. Isolated rat hearts were perfused (constant pressure of 80 mmHg) with red cell suspensions. Following a 20-min warm-up period, perfusion for 3 h with 10(-8) M (iso 3 h) or without (control 3 h) isoproterenol was started or experiments were immediately terminated (control 0 h). Left ventricular tissue was analyzed for total CK-activity, CK-isoenzyme distribution and, by use of quantitative RT-PCR, for B-CK, M-CK, mito-CK and GAPDH- (as internal standard) mRNA. After beta-adrenergic stimulation (iso 3 h) but not after control perfusion (control 3 h) a roughly threefold increase in B-CK mRNA levels and a decrease in M-CK mRNA levels by 18% was found. There were no significant differences among the three groups in total CK-activity and in distribution of CK-MM, CK-BB, CK-MB and mito-CK. Thus, beta-adrenergic stimulation induces a switch in CK gene expression from M-CK to B-CK, which is characteristic for the hypertrophied and failing heart. This may be interpreted as an adaptive mechanism making energy transduction via CK more efficient at times of increased metabolic demand.

Divergent enzyme kinetics and structural properties of the two human mitochondrial creatine kinase isoenzymes

The mitochondrial isoenzymes of creatine kinase (MtCK), ubiquitous uMtCK and sarcomeric sMtCK, are key enzymes of oxidative cellular energy metabolism and play an important role in human health and disease. Very little is known about uMtCK in general, or about sMtCK of human origin. Here we have heterologously expressed and purified both human MtCK isoenzymes to perform a biochemical, kinetic and structural characterization. Both isoenzymes occurred as octamers, which can dissociate into dimers. Distinct Stokes' radii of uMtCK and sMtCK in solution were indicative for conformational differences between these equally sized proteins. Both human MtCKs formed 2D-crystals on cardiolipin layers, which revealed further subtle differences in octamer structure and stability. Octameric human sMtCK displayed p4 symmetry with lattice parameters of 145 A, indicating a 'flattening' of the octamer on the phospholipid layer. pH optima and enzyme kinetic constants of the two human isoenzymes were significantly different. A pronounced substrate binding synergism (Kd > Km) was observed for all substrates, but was most pronounced in the forward reaction (PCr production) of uMtCK and led to a significantly lower Km for creatine (1.01 mM) and ATP (0.11 mM) as compared to sMtCK (creatine, 7.31 mM; ATP, 0.68 mM).

Creatine kinase isoenzymes specificities: histidine 65 in human CK-BB, a role in protein stability, not in catalysis

Creatine kinases (CK) play a prominent role in cell energy distribution through an energy shuttle between mitochondria and other organelles. Human brain CK was cloned and overexpressed in COS-7 cells. We then deleted His-65 and/or Pro-66 situated near the center of a flexible loop as shown by X-ray crystallography on mitochondrial and cytosolic CK. The DeltaH65 mutant had nearly the same affinity for its substrates as wild isoenzyme, but its stability was very low. Unlike DeltaH65, DeltaH65P66 had a eightfold decreased affinity for creatine phosphate and was unable to dephosphorylate cyclocreatine phosphate. Our results demonstrate that, despite an overall similar shape of the proteins, this loop accounts for some subtle differences in isoenzyme functions.

Crystal structure of human ubiquitous mitochondrial creatine kinase

Creatine kinase (CK), catalyzing the reversible trans-phosphorylation between ATP and creatine, plays a key role in the energy metabolism of cells with high and fluctuating energy requirements. We have solved the X-ray structure of octameric human ubiquitous mitochondrial CK (uMtCK) at 2.7 A resolution, representing the first human CK structure. The structure is very similar to the previously determined structure of sarcomeric mitochondrial CK (sMtCK). The cuboidal octamer has 422 point group symmetry with four dimers arranged along the fourfold axis and a central channel of approximately 20 A diameter, which extends through the whole octamer. Structural differences with respect to sMtCK are found in isoform-specific regions important for octamer formation and membrane binding. Octameric uMtCK is stabilized by numerous additional polar interactions between the N-termini of neighboring dimers, which extend into the central channel and form clamp-like structures, and by a pair of salt bridges in the hydrophobic interaction patch. The five C-terminal residues of uMtCK, carrying positive charges likely to be involved in phospholipid-binding, are poorly defined by electron density, indicating a more flexible region than the corresponding one in sMtCK. The structural differences between uMtCK and sMtCK are consistent with biochemical studies on octamer stability and membrane binding of the two isoforms.

A dimeric creatine kinase from a sponge: implications in terms of phosphagen kinase evolution

This study demonstrates conclusively that tissues of the sponge Tethya aurantia contain significant creatine kinase (CK) activity. This CK was purified and analyzed with respect to a number of physico-chemical properties. Size exclusion chromatography and denaturing gel electrophoresis analyses showed that this enzyme is dimeric. The sequences of several Lys-C endoproteinase peptides from Tethya CK are consistent with this enzyme being a member of the phosphagen kinase family and a true CK. CK in higher organisms exists in a variety of quaternary structure forms--dimer, octamer and large monomer consisting of a three contiguous CK domains. The present results indicate that CK evolved very early in metazoan evolution and that the dimeric structure preceded other subunit association forms.

Characterization of the human very-long-chain acyl-CoA dehydrogenase gene promoter region: a role for activator protein 2

Very-long-chain acyl-CoA dehydrogenase (VLCAD) is one of a family of nuclear-encoded enzymes that catalyze the initial step in mitochondrial fatty acid beta-oxidation (FAO). Previous studies have indicated that two other members of the AD gene family (medium-chain AD and long-chain AD) are controlled at the transcriptional level by nuclear hormone receptors. In this study, we have cloned and characterized the human VLCAD gene promoter region to identify cis-acting elements involved in its transcriptional control. VLCAD gene promoter-luciferase reporter (VLCAD-Luc) constructs were found to be transcriptionally active in a variety of mammalian cell lines and in primary rat cardiomyocytes when driven by varying lengths of the VLCAD promoter region. Removal of a 20-bp DNA segment of the proximal VLCAD gene promoter markedly reduced the transcriptional activity of VLCAD-Luc constructs. Gel mobility shift assays identified a DNA-binding activity in nuclear extracts prepared from human hepatoma G2 cells that interacted with the 20-bp regulatory region. Competition studies revealed that this DNA-binding activity could be abolished by a molar excess of unlabeled specific oligonucleotide as well as a DNA fragment containing an activator protein 2 (AP-2)-binding site but not by an unrelated nonspecific DNA fragment. These results provide an initial characterization of the human VLCAD gene promoter, identify AP-2 as a candidate activator of VLCAD gene transcription, and suggest that VLCAD gene transcription may be regulated by pathways distinct from that of other AD genes.

Cloning, Escherichia coli expression, and phase-transition chromatography-based purification of recombinant rabbit heart mitochondrial creatine kinase

A cDNA clone of the mitochondrial sarcomeric creatine kinase cDNA was obtained by screening a rabbit heart library. This cDNA is characterized by a 1257-nucleotide open reading frame encoding a 419-amino-acid protein with a cleavable 39-amino-acid mitochondrial presequence (Accession No. AJ011334). This new member of the guanidino kinase family shows a high degree of sequence similarity with the other phosphagen kinases sequenced so far. The mature enzyme was efficiently expressed in Escherichia coli BL21(DE3) cells as a soluble octameric protein using the pET21 plasmid and purified by a three-step improved method including a final phase-transition chromatography.

Creatine kinase transcript accumulation: effect of nerve during muscle development

To determine the role of the nerve in regulating the accumulation of cytoplasmic creatine kinase (CK) mRNAs in hindleg muscles of the developing mouse, the lumbosacral spinal cords of 14-day gestation mice (E14) were laser ablated, and the accumulation of muscle CK (MCK) and brain CK (BCK) mRNAs was evaluated just prior to birth with in situ hybridization. Numbers of molecules of each of these transcripts/ng total RNA in the soleus and extensor digitorum longus (EDL) muscles were determined with competitive PCR and compared to transcripts found in innervated crural muscles. Data suggest that: 1) the level of BCK mRNA accumulation in innervated hindlimb muscles peaks at E16.5 and remains at fetal levels until the second month postnatal, when it falls to the level found in the adult. Given that MCK transcripts meet or exceed adult levels by day 28 postnatal, the "down-regulation" of the BCK gene and the "up-regulation" of the MCK gene are not tightly coupled; 2) the developmental switch from BCK to MCK, as the dominant cytoplasmic CK mRNA, occurs in innervated and aneural leg muscles between E14 and E16.5, indicating this switch is not nerve dependent; 3) the absence of innervation has no effect on BCK mRNA accumulation. MCK transcripts/ng total RNA continue to increase in aneural muscle throughout the late fetal period, but from E16.5-E19.5 the MCK transcript levels in aneural muscles become progressively lower than in age-matched innervated muscles. Thus, the accumulation of the muscle specific cytoplasmic CK, but not BCK, transcripts is affected by the absence of innervation during the fetal period. Dev Dyn 1999;215:285-296.

Structural and functional implications of the amino acid sequences of dimeric, cytoplasmic and octameric mitochondrial creatine kinases from a protostome invertebrate

The cDNA and deduced amino-acid sequences for dimeric and octameric isoforms of creatine kinase (CK) from a protostome, the polychaete Chaetopterus variopedatus, were elucidated and then analysed in the context of available vertebrate CK sequences and the recently determined crystal structure of chicken sarcomeric mitochondrial CK (MiCK). As protostomes last shared a common ancestor with vertebrates roughly 700 million years ago, observed conserved residues may serve to confirm or reject contemporary hypotheses about the roles of particular amino acids in functional/structural processes such as dimer/octamer formation and membrane binding. The isolated cDNA from the dimeric CK consisted of 1463 nucleotides with an open reading frame of 1116 nucleotides encoding a 372-amino-acid protein having a calculated molecular mass of 41.85 kDa. The percentage identity of C. variopedatus dimeric CK to vertebrate CK is as high as 69%. The octameric MiCK cDNA is composed of 1703 nucleotides with an open reading frame of 1227 nucleotides. The first 102 nucleotides of the open reading frame encode a 34-amino-acid leader peptide whereas the mature protein is composed of 375 amino acids with a calculated molecular mass of 42.17 kDa. The percentage identity of C. variopedatus MiCK to vertebrate CK is as high as 71%. This similarity is also evident in residues purported to be important in the structure and function of dimeric and octameric CK: (a) presence of seven basic amino acids in the C-terminal end thought to be important in binding of MiCK to membranes; (b) presence of a lysine residue (Lys110 in chicken MiCK) also thought to be involved in membrane binding; and (c) presence of a conserved tryptophan thought to be important in dimer stabilization which is present in all dimeric and octameric guanidino kinases. However, C. variopedatus MiCK lacks the N-terminal heptapeptide present in chicken MiCK, which is thought to mediate octamer stabilization. In contrast with vertebrate MiCK, polychaete octamers are very stable indicating that dimer binding into octamers may be mediated by additional and/or other residues. Phylogenetic analyses showed that both octamer and dimer evolved very early in the CK lineage, well before the divergence of deuterostomes and protostomes. These results indicate that the octamer is a primitive feature of CK rather than being a derived and advanced character.

Characterization of isoforms of human mitochondrial creatine kinase by isoelectric focusing

High enzyme activity of mitochondrial creatine kinase (creatine-N-phosphotransferase, mCK, EC 2.7.3.2) was detected in serum from a patient with advanced carcinoma of the rectum and its isoforms were characterized by means of isoelectric focusing (IEF). Three forms of mCK, membrane-bound (pI 6.9-7.0), octameric (pI 7.0-7.9) and dimeric (pI 6.7, 6.8, 6.9 and 7.0), were detected in the fresh serum. These three forms of mCK were converted to five dimeric isoforms, and these were characterized as one reduced form (pI 7.0) and four oxidized (pI 6.6, 6.7, 6.8 and 6.9) forms upon treatment with urea, hydrogen peroxide or 2-mercaptoethanol (2-ME). The C-terminal of the mCKs was concluded to be a lysine residue because the mCKs treated with carboxypeptidase B migrated to positions closer to the anode than did those not treated with carboxypeptidase B. Therefore, four bands were concluded to represent one reduced-delysined isoform (pI 6.4) and three oxidized-delysined isoforms (pI 6.1, 6.2 and 6.3). The broad octameric mCK band disappeared and a narrow band focused at pI 6.8-6.9 appeared upon probable delysination of the mCKs. Thus, the number of lysine residues at the C-terminal of the octamer was concluded to be variable due to variable catalysis by carboxypeptidase N in the plasma. mCKs seemed to be inactivated during conversion from a membrane-bound form to dimeric oxidized-delysined forms via the octameric, dimeric reduced and oxidized forms.

Octamer formation and coupling of cardiac sarcomeric mitochondrial creatine kinase are mediated by charged N-terminal residues

Mitochondrial creatine kinases form octameric structures composed of four active and stable dimers. Octamer formation has been postulated to occur via interaction of the charged amino acids in the N-terminal peptide of the mature enzyme. We altered codons for charged amino acids in the N-terminal region of mature sarcomeric mitochondrial creatine kinase (sMtCK) to those encoding neutral amino acids. Transfection of normal sMtCK cDNA or those with the mutations R42G, E43G/H45G, and K46G into rat neonatal cardiomyocytes resulted in enzymatically active sMtCK expression in all. After hypoosmotic treatment of isolated mitochondria, mitochondrial inner membrane-associated and soluble sMtCK from the intermembranous space were measured. The R42G and E43G/H45G double mutation caused destabilization of the octameric structure of sMtCK and a profound reduction in binding of sMtCK to the inner mitochondrial membrane. The other mutant sMtCK proteins had modest reductions in binding. Creatine-stimulated respiration was markedly reduced in mitochondria isolated from cells transfected with the R42G mutant cDNA as compared with those transfected with normal sMtCK cDNA. We conclude that neutralization of charges in N-terminal peptide resulted in destabilization of octamer structure of sMtCK. Thus, charged amino acids at the N-terminal moiety of mature sMtCK are essential for octamer formation, binding of sMtCK with inner mitochondrial membrane, and coupling of sMtCK to oxidative phosphorylation.

The isoenzyme-diagnostic regions of muscle-type creatine kinase, the M-260 and M-300 box, are not responsible for its binding to the myofibrillar M-band

Muscle-type creatine kinase is known for its unique interaction with the myofibrillar M-band, but the molecular origin for this structural relationship is not well understood. A systematic sequence comparison between the highly homologous cytosolic isoforms, muscle-type and brain-type creatine kinase, yielded two isoenzyme-specific regions in the muscle-type creatine kinases, the M-260 box (residues 258-270) and the M-300 box (residues 300-315). These particular regions were conspicuous for the specific interaction of this CK isoenzyme, but not of brain-type creatine kinase, with the sarcomeric M-band. In situ diffusion assays with fluorescently labeled native, as well as mutated muscle-type creatine kinase variants, were used to study by laser confocal microscopy their association with the M-band of chemically skinned muscle fibers. Neither a set of charge mutants of the M-260 box and/or the M-300 box nor a hybrid construct of both isoforms with the entire C-terminal region derived from the brain-type isoform showed any significant alteration in the in situ M-band-binding properties when compared to the wild-type form of muscle-type creatine kinase. This indicates that in the intact protein of muscle type creatine kinase, these C-terminal isoenzyme-specific regions are not important for M-band interaction and that the actual M-band interaction domain(s) lay mostly within the N-terminal half of the molecule. The highly conserved motives (M-260 box and M-300 box) may serve an isoenzyme-specific purpose yet to be identified.

Elements regulating cardiomyocyte expression of the human sarcomeric mitochondrial creatine kinase gene in transgenic mice

Sarcomeric mitochondrial creatine kinase (sMtCK) is one component of a multiprotein, high energy channeling complex consisting of porin, mitochondrial creatine kinase, and adenine nucleotide translocase. To study the transcriptional mechanisms specifying sMtCK gene expression to the heart, transgenic mice were created carrying the 5'-flanking sequences of the human sMtCK gene ligated upstream of the human growth hormone (hGH) reporter gene. RNA blot hybridization demonstrated that the human sMtCK sequence, -485 to +6 base pair (bp), did not activate reporter gene expression to a detectable level. However, the human sMtCK sequence, -921 to +6 bp, expressed the hGH reporter gene at a high level in heart and skeletal muscle and at a very low level in esophagus and kidney, and it did not express the hGH gene in other organs tested (brain, lung, liver, spleen, bladder, uterus, and stomach). In situ hybridization revealed that reporter gene transcription was specified to cardiac and skeletal myocytes, recapitulating precisely the expression pattern of the endogenous gene. Sequence analysis identified several consensus binding sites between -921 and -757 bp, including four GATT motifs, one E box, and one MEF2 site. Further analysis of a third transgenic mouse strain demonstrated that the human sMtCK sequence, -757 to +6 bp, did not direct detectable expression of the hGH reporter gene. We conclude that this 160-bp genomic sequence, from -921 to -757 bp, is necessary in specifying expression of the human sMtCK gene to the oxidative and highly metabolically active heart tissue.

Muscle-specific creatine kinase gene polymorphism and VO2max in the HERITAGE Family Study

This study examined the association between a DNA polymorphism in the muscle-specific creatine kinase (CKMM) gene and VO2max in the sedentary state, as well as its response (deltaVO2max) to a standardized 20-wk endurance training program. The subjects were 160 biologically unrelated Caucasian parents (80 women, 80 men) and 80 biologically unrelated adult offspring of the HERITAGE Family Study. The CKMM polymorphism was detected by PCR and digestion with the NcoI restriction enzyme. VO2max was measured during maximal cycle ergometer tests. VO2max was 2119 +/- 45 mL x min(-1) (mean +/- SE) or 26 +/- 0.4 mL x kg(-1) x min(-1). Both sexes had a significant (P < 0.05) increase in the deltaVO2max (women = 283 +/- 20 mL x min[-1] and men = 363 +/- 25 mL x min[-1]). Allele and genotype frequencies were not significantly different (P > 0.05) between sexes. Age and sex adjusted VO2max was significantly (P = 0.007) associated with the CKMM genotype in the parents, whereas no association (P > 0.05) was observed in the offspring. DeltaVO2max values adjusted for age, sex, VO2max, and body mass were characterized by genotype differences in both parents (P = 0.0004) and offspring (P = 0.0025). A significantly (P < 0.05) lower deltaVO2max to endurance training was detected in both parents and offspring homozygotes for the rare allele. The genotype accounted for at least 9% of the variance in deltaVO2max. These results indicate that the NcoI polymorphism in the 3' untranslated region of the muscle-specific creatine kinase gene is associated with the deltaVO2max to endurance training.

Evolution of the creative kinases. The chicken acidic type mitochondrial creatine kinase gene as the first nonmammalian gene

In both mammals and birds, the creatine kinase (CK) family consists of four types of genes: cytosolic brain type (B-CK); cytosolic muscle type (M-CK); mitochondrial ubiquitous, acidic type (Mia-CK); and mitochondrial sarcomeric, basic type (Mib-CK). We report here the cloning of the chicken Mia-CK cDNA and its gene. Amino acid sequences of the mature chicken Mi-CK proteins show about 90% identity to the homologous mammalian isoforms. The leader peptides, however, which are isoenzyme-specifically conserved among the mammalian Mi-CKs, are quite different in the chicken with amino acid identity values compared with the mammalian leader peptides of 38.5-51.3%. The chicken Mia-CK gene spans about 7.6 kilobases and contains 9 exons. The region around exon 1 shows a peculiar base composition, with more than 80% GC, and has the characteristics of a CpG island. The upstream sequences lack TATA or CCAAT boxes and display further properties of housekeeping genes. Several transcription factor binding sites known from mammalian Mi-CK genes are absent from the chicken gene. Although the promoter structure suggests a ubiquitous range of expression, analysis of Mia-CK transcripts in chicken tissues shows a restricted pattern and therefore does not fulfill all criteria of a housekeeping enzyme.

Expression of brain-type creatine kinase and ubiquitous mitochondrial creatine kinase in the fetal rat brain: evidence for a nuclear energy shuttle

To test the hypothesis that embryonic brain cells utilize a creatine phosphate energy shuttle, we examined the pattern of creatine kinase (CK) isoform expression and localization in the fetal rat brain. Moderate levels of CK activity are present at embryonic day 14 (7 U/mg protein) and decrease slightly until 3 days postpartum followed by a rapid, fourfold up-regulation to adult levels by 1 month (18 U/mg protein). In parallel with changes in enzyme activity, there is a biphasic and coordinate pattern of expression of brain-type CK (BCK) and ubiquitous mitochondrial CK (uMtCK) determined by nondenaturing electrophoresis and immunoblot analysis. The localization of CK isoforms was examined by immunocytochemistry, and, during the fetal period, BCK and uMtCK immunoreactivity was detected throughout the central and peripheral nervous system, especially in neuroepithelial regions of the cerebral vesicles and spinal cord. In large cells within the olfactory neuroepithelium and ventral spinal cord, differential compartmentation of CK isoforms was evident, with BCK localized primarily in cell nuclei, whereas uMtCK immunoreactivity was present in the cell body (but not within nuclei). In olfactory bulb neuroepithelium, both isoforms were expressed in the middle zone of the germinal layer associated with DNA synthesis. In embryonic skeletal and cardiac muscle, which also express BCK, the same compartmentation of BCK was seen, with BCK localized primarily in the cell nucleus of cardiac and skeletal myoblasts. These results demonstrate a coordinate pattern of expression and compartmentation of BCK and uMtCK isoforms in the fetal brain that, in some cells, provides the anatomic basis for a nuclear energy shuttle.

Mouse ubiquitous mitochondrial creatine kinase: gene organization and consequences from inactivation in mouse embryonic stem cells

Individual members of the creatine kinase isoenzyme family (CK; EC 2.7.3.2), which play a prominent role in energy homeostasis, are encoded by four separate nuclear genes. We have isolated and characterized the complete mouse UbCKmit gene, the product of which is ubiquitously expressed and is located in the intermembrane space of mitochondria. Transcription of this gene is initiated at multiple adjacent positions and the region immediately upstream of these sites shares many features with genes encoding housekeeping proteins. These include a high G/C content, absence of TATA and CCAAT motifs, and presence of SP1 and AP2 recognition sequences. In addition, a binding site for HIP1, hormone-responsive elements, and three Mt-motifs, known as boxes shared between nuclear genes encoding mitochondrial proteins, were identified. To study the functional role of the UbCKmit protein, we have inactivated both UbCKmit alleles in mouse embryonic stem (ES) cells. UbCKmit-deficient cells, obtained by consecutive rounds of gene targeting using homologous recombination and drug selection-driven gene conversion events, show no obvious growth disadvantage or abnormal differentiation potential. Activities of mitochondrial cytochrome c oxidase and citrate synthase, as well as the rate of pyruvate oxidation, showed values equal to wild-type cells, indicating a normal aerobic metabolism. Mitochondria of in vivo differentiated knock-out cells were structurally intact, as demonstrated by electron microscopy. Approaches to study the role of the UbCKmit gene further are discussed.

Developmental expression of sarcomeric and ubiquitous mitochondrial creatine kinase is tissue-specific

Creatine kinase (CK) isoenzymes play prominent roles in myocardial energy metabolism. Two nuclear genes encode mitochondrial creatine kinase (MtCK), are tissue-specific in their expression, and are thus designated as sarcomeric MtCK (sMtCK) and ubiquitous MtCK (uMtCK). Quantitative analysis of the mRNA expression of both MtCKs in developing rat tissues demonstrates tissue-specific developmental regulation. sMtCK mRNA in heart is undetectable prenatally but is dramatically upregulated by 28 d postnatally. sMtCK mRNA in skeletal muscle is also extremely low prenatally but is markedly upregulated at birth and doubles by 28 d postnatally. uMtCK mRNA expression is present at low levels in fetal brain and intestine. Brain uMtCK mRNA continues to rise from -4 d prenatally until 28 d postnatally (6-fold increase), but intestinal uMtCK mRNA increases immediately prior to birth, falls, and is upregulated again at 28 d (20-fold). uMtCK mRNA is undetectable in fetal skeletal muscle or heart, but increases to low levels in skeletal muscle at birth and remains at this level into adulthood. uMtCK is not detectable in heart, lung, testes, or liver at any stage examined. We conclude that sMtCK and uMtCK are developmentally regulated in a tissue-specific manner. Unlike cytosolic muscle CK and brain CK, there is no isoenzyme switch between sMtCK and uMtCK in the developing animal. Our results suggest that specific trans-acting factors regulate the different developmental and tissue-specific expression of the MtCK genes.

Transcription of the brain creatine kinase gene in glial cells is modulated by cyclic AMP-dependent protein kinase

The brain creatine kinase (CKB) gene is expressed in a variety of tissues with highest expression seen in the brain. We have previously shown in primary rat brain cell cultures that CKB mRNA levels are high in oligodendrocytes and astrocytes and low in neurons (Molloy et al.: J Neurochem 59:1925-1932, 1992). In this report we show that treatment of human U87 glioblastoma cells with forskolin and IBMX, to elevate intracellular cAMP, induces expression of CKB mRNA from the transiently transfected rat CKB gene by 14-fold and also increases expression from the endogenous human CKB gene. This induction of CKB mRNA i) is due to increased transcription; ii) occurs rapidly (with maximal induction after 6 hr; iii) requires the activity of protein kinase A (PKA), but iv) does not require de novo protein synthesis and, in fact, is superinduced in the presence of cycloheximide. Given the role of oligodendrocytes in the energy-demanding process of myelination and of astrocytes in ion transport, these results have physiological significance, since they suggest that changes in cellular energy requirements in the brain during events, such as glial cell differentiation and increased neuronal activity, may in part be met by a cAMP-mediated modulation of CKB gene expression. Of particular importance is the possible modulation of CKB gene expression during myelinogenesis, since oligodendrocyte differentiation has been shown previously to be stimulated by increases in cAMP.

Compartmentation of brain-type creatine kinase and ubiquitous mitochondrial creatine kinase in neurons: evidence for a creatine phosphate energy shuttle in adult rat brain

Multiple isoforms of creatine kinase (CK) are expressed in specific cell types as part of an energy delivery or shuttle system. To test the hypothesis that neurons utilize a creatine phosphate energy shuttle, we examined the pattern of CK isoform expression and localization in adult rat brain. Two isoforms of CK are present in brain extracts, "brain-type," or BCK, and the ubiquitous form of the mitochondrial CK (uMtCK), as detected by enzyme activity following nondenaturing electrophoresis and by Western blotting following denaturing electrophoresis. In formalin-fixed and paraffin-embedded sections of rat brain, uMtCK immunostaining is detected in the somata of all Golgi type I neurons in the cerebellum, pontine reticular formation, red nucleus, hippocampus, and cerebral cortex. Immunostaining for uMtCK appears throughout the cell body but not in nuclei. BCK immunostaining is also present in somata of Golgi type I neurons in the cerebellum, red nucleus, and pons and is distributed throughout the cell body and within nuclei. BCK immunostaining also appears in neuronal processes and is concentrated in the molecular layers of the cerebellum and the hippocampus and in cortical pyramidal cell dendrites. These results demonstrate a coordinate pattern of expression and compartmentation of BCK and uMtCK isoforms in neurons, which provides an anatomic basis for the transfer of metabolic energy via a creatine phosphate energy shuttle.

Compartmentation of ATP synthesis and utilization in smooth muscle: roles of aerobic glycolysis and creatine kinase

The phosphocreatine content of smooth muscle is of similar magnitude to ATP. Thus the function of the creatine kinase system in this tissue cannot simply be regarded as an energy buffer. Thus an understanding of its role in smooth muscle behavior can point to CK function in other systems. From our perspective CK function in smooth muscle is one example of a more general phenomenon, that of the co-localization of ATP synthesis and utilization. In an interesting and analogous fashion distinct glycolytic cascades are also localized in regions of the cell with specialized energy requirements. Similar to CK, glycolytic enzymes are known to be localized on thin filaments, sarcoplasmic reticulum and plasma membrane. In this chapter we will describe the relations between glycolysis and smooth muscle function and compare and contrast to that of the CK system. Our goal is to more fully understand the significance of the compartmentation of distinct pathways for ATP synthesis with specific functions in smooth muscle. This organization of metabolism and function seen most clearly in smooth muscle is likely representative of many other cell types.

Approaching the multifaceted nature of energy metabolism: inactivation of the cytosolic creatine kinases via homologous recombination in mouse embryonic stem cells

To study the physiological role of the creatine kinase/phosphocreatine (CK/PCr) system in cells and tissues with a high and fluctuating energy demand we have concentrated on the site-directed inactivation of the B- and M-CK genes encoding the cytosolic CK protein subunits. In our approach we used homologous recombination in mouse embryonic stem (ES) cells from strain 129/Sv. Using targeting constructs based on strain 129/Sv isogenic DNA we managed to ablate the essential exons of the B-CK and M-CK genes at reasonably high frequencies. ES clones with fully disrupted B-CK and two types of M-CK gene mutations, a null (M-CK-) and leaky (M-CK1) mutation, were used to generate chimaeric mutant mice via injection in strain C57BL/6 derived blastocysts. Chimaeras with the B-CK null mutation have no overt abnormalities but failed to transmit the mutation to their offspring. For the M-CK- and M-CK1 mutations successful transmission was achieved and heterozygous and homozygous mutant mice were bred. Animals deficient in MM-CK are phenotypically normal but lack muscular burst activity. Fluxes through the CK reaction in skeletal muscle are highly impaired and fast fibres show adaptation in cellular architecture and storage of glycogen. Mice homozygous for the leaky M-CK allele, which have 3-fold reduced MM-CK activity, show normal fast fibres but CK fluxes and burst activity are still not restored to wildtype levels.

Expression of the mitochondrial creatine kinase genes

Mitochondrial Creatine Kinase (MtCK) is responsible for the transfer of high energy phosphate from mitochondria to the cytosolic carrier, creatine, and exists in mammals as two isoenzymes encoded by separate genes. In rats and humans, sarcomere-specific MtCK (sMtCK) is expressed only in skeletal and heart muscle, and has 87% nucleotide identity across the 1257 bp coding region. The ubiquitous isoenzyme of MtCK (uMtCK) is expressed in many tissues with highest levels in brain, gut, and kidney, and has 92% nucleotide identity between the 1254 bp coding regions of rat and human. Both genes are highly regulated developmentally in a tissue-specific manner. There is virtually no expression of sMtCK mRNA prior to birth. Unlike cytosolic muscle CK (MCK) and brain CK (BCK), there is no developmental isoenzyme switch between the MtCKs. Cell culture models representing the tissue-specific expression of either sMtCK or uMtCK are available, but there are no adequate developmental models to examine their regulation. Several animal models are available to examine the coordinate regulation of the CK gene family and include 1) Cardiac Stress by coarctation (sMtCK, BCK, and MCK), 2) Uterus and placenta during pregnancy (uMtCK and BCK), and 3) Diabetes and mitochondrial myopathy (sMtCK, BCK, and MCK). We report the details of these findings, and discuss the coordinate regulation of the genes necessary for high-energy transduction.

Creatine kinase in non-muscle tissues and cells

Over the past years, a concept for creatine kinase function, the 'PCr-circuit' model, has evolved. Based on this concept, multiple functions for the CK/PCr-system have been proposed, such as an energy buffering function, regulatory functions, as well as an energy transport function, mostly based on studies with muscle. While the temporal energy buffering and metabolic regulatory roles of CK are widely accepted, the spatial buffering or energy transport function, that is, the shuttling of PCr and Cr between sites of energy utilization and energy demand, is still being debated. There is, however, much circumstantial evidence, that supports the latter role of CK including the distinct, isoenzyme-specific subcellular localization of CK isoenzymes, the isolation and characterization of functionally coupled in vitro microcompartments of CK with a variety of cellular ATPases, and the observed functional coupling of mitochondrial oxidative phosphorylation with mitochondrial CK. New insight concerning the functions of the CK/PCr-system has been gained from recent M-CK null-mutant transgenic mice and by the investigation of CK localization and function in certain highly specialized non-muscle tissues and cells, such as electrocytes, retina photoreceptor cells, brain cells, kidney, salt glands, myometrium, placenta, pancreas, thymus, thyroid, intestinal brush-border epithelial cells, endothelial cells, cartilage and bone cells, macrophages, blood platelets, tumor and cancer cells. Studies with electric organ, including in vivo 31P-NMR, clearly reveal the buffer function of the CK/PCr-system in electrocytes and additionally corroborate a direct functional coupling of membrane-bound CK to the Na+/K(+)-ATPase. On the other hand, experiments with live sperm and recent in vivo 31P-NMR measurements on brain provide convincing evidence for the transport function of the CK/PCr-system. We report on new findings concerning the isoenzyme-specific cellular localization and subcellular compartmentation of CK isoenzymes in photoreceptor cells, in glial and neuronal cells of the cerebellum and in spermatozoa. Finally, the regulation of CK expression by hormones is discussed, and new developments concerning a connection of CK with malignancy and cancer are illuminated. Most interesting in this respect is the observed upregulation of CK expression by adenoviral oncogenes.

Sequence homology and structure predictions of the creatine kinase isoenzymes

Comparisons of the protein sequences and gene structures of the known creatine kinase isoenzymes and other guanidino kinases revealed high homology and were used to determine the evolutionary relationships of the various guanidino kinases. A 'CK framework' is defined, consisting of the most conserved sequence blocks, and 'diagnostic boxes' are identified which are characteristic for anyone creatine kinase isoenzyme (e.g. for vertebrate B-CK) and which may serve to distinguish this isoenzyme from all others (e.g. from M-CKs and Mi-CKs). Comparison of the guanidino kinases by near-UV and far-UV circular dichroism further indicates pronounced conservation of secondary structure as well as of aromatic amino acids that are involved in catalysis.

Changes in creatine kinase M localization in acute ischemic myocardial cells. Immunoelectron microscopic studies

To clarify the changes in creatine kinase M localization along with the progress of myocardial ischemia, immunoelectron microscopic studies were performed using rabbit anti-canine creatine kinase M Fab'-horseradish peroxidase conjugate in 21 dogs. Myocardial ischemia was induced by occlusion of the left anterior descending coronary artery for 15 (n = 5), 30 (n = 5), 60 (n = 5), or 180 (n = 4) minutes. Two dogs were used as normal controls. As we have already demonstrated, most creatine kinase M in normal myocardial cells was localized over the entire A band in association with the thick filament, suggesting that creatine kinase in this region (A-band creatine kinase) was the enzyme coupled with myosin ATPase. After 15 minutes of ischemia, creatine kinase M showed only minimal changes in its location, indicating that A-band creatine kinase still has the ability to couple with myosin ATPase (reversible injury). However, after 30 minutes of ischemia, A-band creatine kinase diffused markedly to the I band (transitional phase), and after 60 minutes of ischemia, it leaked out to extracellular spaces (irreversible injury). After 180 minutes of ischemia, most A-band creatine kinase disappeared from the myocardial cells (coagulation necrosis). These features of creatine kinase M localization seemed to reflect each stage of ischemic cell injury. We conclude that myocardial ischemia results in a dissociation of creatine kinase molecules from the thick filament, which leads the energy transport system to destruction.

Expression of the rat brain creatine kinase gene in C6 glioma cells

We have recently shown that while brain creatine kinase (CKB) mRNA was detectable in RNA from cultured primary rat brain neurons, CKB mRNA was about 15-fold higher in primary astrocytes and 17-fold higher in oligodendrocytes (Molloy et al., J Neurochem 59:1925-1932, 1992). To begin to understand the molecular mechanisms responsible for brain glial cells containing the highest levels of CKB mRNA in the body, we have examined the expression of rat CKB mRNA in established C6 glioma cells. RNase-protection analysis showed the endogenous CKB mRNA levels in exponentially growing C6 were high and measured 50% of that in total RNA from rat brain lysate and 60% of that in cultured primary astrocytes and oligodendrocytes. The 5' and 3' ends of CKB mRNA in C6 were mapped to the same nucleotides as CKB mRNA from rat brain, indicating that the sites of in vivo transcription initiation and termination/polyadenylation of CKB mRNA in C6 are the same as in total rat brain RNA. The level of CKB enzyme activity in C6 whole cell lysates was among the highest of the glial cell lines which we measured. All creatine kinase enzyme activity present in C6 was found in the dimeric CKB isoform (BB), which is characteristic of CKB expression in the brain. A 2.9 kb gene fragment containing the basal CKB promoter and far-upstream 5' sequences was cloned upstream of the chloramphenicol acetyltransferase (CAT) gene and transfected into C6 cells. CAT activity was readily detectable in C6 and mapping of the 5' end of the CAT mRNA showed that transcription was directed from the correct initiation site. Since we found C6 cells were difficult to transfect, conditions were established which both maximized transfection efficiency and maintained normal C6 cell morphology. These results should permit the future identification of the nuclear trans-acting factors and the cognate cis-acting regulatory elements responsible for high CKB mRNA expression in brain glial cells.

Rat brain creatine kinase messenger RNA levels are high in primary cultures of brain astrocytes and oligodendrocytes and low in neurons

Rat brain creatine kinase (CKB) gene expression is highest in the brain but is also detectable at lower levels in some other tissues. In the brain, the CKB enzyme is thought to be involved in the regeneration of ATP necessary for transport of ions and neurotransmitters. To understand the molecular events that lead to high CKB expression in the brain, we have determined the steady-state levels of CKB mRNA in homogeneous cultures of primary rat brain astrocytes, oligodendrocytes, and neurons. Northern blot analysis showed that whereas the 1.4-kb CKB mRNA was detectable in neurons, the level was about 17-fold higher in oligodendrocytes and 15-fold higher in astrocytes. The blots were hybridized with a CKB-specific 32P-antisense RNA probe, complementary to the 3' untranslated sequence of CKB, which hybridizes to CKB mRNA but not CKM mRNA. Also, the 5' and 3' ends of CKB mRNA from the glial cells were mapped, using exon-specific antisense probes in the RNase-protection assay, and were found to be the same in astrocytes and oligodendrocytes. This indicated that (a) the site of in vivo transcription initiation in astrocytes and oligodendrocytes was directed exclusively by the downstream, nonconcensus TTAA sequence at -25 bp in the CKB promoter that is also utilized by all other cell types that express CKB and (b) the 3' end of mature CKB mRNA was the same in astrocytes and oligodendrocytes. In addition, there was no detectable alternate splicing in exon 1, 2, or 8 of CKB mRNA in rat astrocytes and oligodendrocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

Selective reduction of creatine kinase subunit mRNAs in striated muscle of diabetic rats

Creatine kinase (CK) is important for energy transfer and is composed of mitochondrial (mitCK), muscle (MCK), and brain (BCK) subunits, each being the product of separate nuclear genes. The concentrations of MCK and BCK mRNAs have been shown to decrease in streptozotocin-hypoinsulinemic rat hearts, and in this report, we examined in detail the diabetic effect on CK gene expression in cardiac muscle and in two types of skeletal muscle. The level of sarcomeric mitCK mRNA was not altered in the diabetic myocardium, but was reduced by 86 and 67% in diabetic slow-twitch soleus muscle and fast-twitch extensor digitorum longus (EDL) muscle, respectively. MCK mRNA was also lowered in diabetic soleus muscle by 56%, while it remained at control levels in diabetic EDL. In both skeletal muscles, at either state, BCK mRNA was not detectable. There was a 33% decrease in total CK activity in diabetic cardiac and soleus muscle, but not in EDL. Diabetes thus exerts a widespread, muscle type-dependent adverse effect on CK expression that we found to be insulin therapy revertible. This study adds to our understanding of defective energy transduction in diabetic muscle.

Cytochromes c1 and b2 are sorted to the intermembrane space of yeast mitochondria by a stop-transfer mechanism

The pathway by which cytochromes c1 and b2 reach the mitochondrial intermembrane space has been controversial. According to the "conservative sorting" hypothesis, these proteins are first imported across both outer and inner membranes into the matrix, and then are retranslocated across the inner membrane. Our data argue against this model: import intermediates of cytochromes c1 and b2 were found only outside the inner membrane; maturation of these proteins was independent of the matrix-localized hsp60 chaperone; and dihydrofolate reductase linked to the presequence of either cytochrome was imported to the intermembrane space in the absence of ATP. We conclude that cytochromes c1 and b2 are sorted by a mechanism in which translocation through the inner membrane is arrested by a "stop-transfer" signal in the presequence. The arrested intermediates may be associated with a proteinaceous channel in the inner membrane.