Creatine metabolism and the consequences of creatine depletion in muscle

Currently, considerable research activities are focussing on biochemical, physiological and pathological aspects of the creatine kinase (CK) — phosphorylcreatine (PCr) — creatine (Cr) system (for reviews see [1, 2]), but only little effort is directed towards a thorough investigation of Cr metabolism as a whole. However, a detailed knowledge of Cr metabolism is essential for a deeper understanding of bioenergetics in general and, for example, of the effects of muscular dystrophies, atrophies, CK deficiencies (e.g. in transgenic animals) or Cr analogues on the energy metabolism of the tissues involved. Therefore, the present article provides a short overview on the reactions and enzymes involved in Cr biosynthesis and degradation, on the organization and regulation of Cr metabolism within the body, as well as on the metabolic consequences of 3-guanidinopropionate (GPA) feeding which is known to induce a Cr deficiency in muscle. In addition, the phenotype of muscles depleted of Cr and PCr by GPA feeding is put into context with recent investigations on the muscle phenotype of gene knockout mice deficient in the cytosolic muscle-type M-CK.Abbreviations Cr creatine - Crn creatinine - PCr phosphorylcreatine - CK creatine kinase - M-CK cytosolic muscle type CK isoenzyme - Mi-CK mitochondrial CK isoenzyme - AGAT L-arginine: glycine amidinotransferase - GAMT S-adenosylmethionine: guanidinoacetate methyltransferase - Arg arginine - Met methionine - GPA guanidinopropionate=-guanidinopropionate - PGPA phosphorylated GPA - GBA 3-guanidinobutyrate=-guanidinobutyrate - CPEO chronic progressive external ophthalmoplegia     

Sarcotubular development in dystrophic skeletal muscle cells

Summary Dilations of the sarcotubular system and misaligned myofilaments have been reported as early indicators of muscular dystrophy in skeletal muscle. Since the developing tubular component is believed instrumental in initial myofilament alignment during myogenesis, tubular development is evaluated using normal and dystrophic chick embryo skeletal muscle and cultures of normal and dystrophic embryonic pectoral muscle incubated in the presence of horse spleen ferritin. Comparisons of the findings show that periodic tubules are absent from dystrophic somitic muscle and that invaginating tubules from the sarcolemma are found in fewer, randomly located areas of dystrophic pectoral muscle cells. The results indicate that the tubular component is not involved in the bizarre vesiculations seen in mature dystrophic muscle, however, the malalignment of dystrophic myofilaments is probably the result of the poorer development of the T system in this muscle.     

Creatine supplementation in health and disease. Effects of chronic creatine ingestion in vivo: Down-regulation of the expression of creatine transporter isoforms in skeletal muscle

Interest in creatine (Cr) as a nutritional supplement and ergogenic aid for athletes has surged over recent years. After cellular uptake, Cr is phosphorylated to phosphocreatine (PCr) by the creatine kinase (CK) reaction using ATP. At subcellular sites with high energy requirements, e.g. at the myofibrillar apparatus during muscle contraction, CK catalyzes the transphosphorylation of PCr to ADP to regenerate ATP, thus preventing a depletion of ATP levels. PCr is thus available as an immediate energy source, serving not only as an energy buffer but also as an energy transport vehicle. Ingestion of creatine increases intramuscular Cr, as well as PCr concentrations, and leads to exercise enhancement, especially in sprint performance. Additional benefits of Cr supplementation have also been noticed for high-intensity long-endurance tasks, e.g. shortening of recovery periods after physical exercise.The present article summarizes recent findings on the influence of Cr supplementation on energy metabolism, and introduces the Cr transporter protein (CreaT), responsible for uptake of Cr into cells, as one of the key-players for the multi-faceted regulation of cellular Cr homeostasis. Furthermore, it is suggested that patients with disturbances in Cr metabolism or with different neuro-muscular diseases may benefit from Cr supplementation as an adjuvant therapy to relieve or delay the onset of symptoms. Although it is still unclear how Cr biosynthesis and transport are regulated in health and disease, so far there are no reports of harmful side effects of Cr loading in humans. However, in this study, we report that chronic Cr supplementation in rats down-regulates in vivo the expression of the CreaT. In addition, we describe the presence of CreaT isoforms most likely generated by alternative splicing.     

Abnormal collagen synthesis in skeletal muscle of dystrophic chicken

Specific molecular properties of skeletal muscle collagens from normal and dystrophic chickens have been compared. When dystrophy develops in skeletal muscle tissue there was an increase in the amount of total collagen and an increased proportion of Type III collagen in the tissue. The results from the cross-link study as well as the analysis of the solubility of collagen showed that skeletal muscle of dystrophic chicken produces more immature collagen fibers compared to normal chicken. These findings strongly indicate an important role of collagen in the pathogenesis of the extensive connective tissue prolipheration characteristic of muscular dystrophies.     

Rapamycin ameliorates dystrophic phenotype in mdx mouse skeletal muscle

Duchenne muscular dystrophy (DMD) is an X-linked, lethal, degenerative disease that results from mutations in the dystrophin gene, causing necrosis and inflammation in skeletal muscle tissue. Treatments that reduce muscle fiber destruction and immune cell infiltration can ameliorate DMD pathology. We treated the mdx mouse, a model for DMD, with the immunosuppressant drug rapamycin (RAPA) both locally and systemically to examine its effects on dystrophic mdx muscles. We observed a significant reduction of muscle fiber necrosis in treated mdx mouse tibialis anterior (TA) and diaphragm (Dia) muscles 6 wks post-treatment. This effect was associated with a significant reduction in infiltration of effector CD4(+) and CD8(+) T cells in skeletal muscle tissue, while Foxp3(+) regulatory T cells were preserved. Because RAPA exerts its effects through the mammalian target of RAPA (mTOR), we studied the activation of mTOR in mdx TA and Dia with and without RAPA treatment. Surprisingly, mTOR activation levels in mdx TA were not different from control C57BL/10 (B10). However, mTOR activation was different in Dia between mdx and B10; mTOR activation levels did not rise between 6 and 12 wks of age in mdx Dia muscle, whereas a rise in mTOR activation level was observed in B10 Dia muscle. Furthermore, mdx Dia, but not TA, muscle mTOR activation was responsive to RAPA treatment.     

Localization of creatine kinase isoenzymes in myofibrils. I. Chicken skeletal muscle

Purified, repeatedly washed, skeletal muscle myofibrils contain approx. 0.2 U of creatine kinase (CK) activity (equivalent to 2.5 micrograms CK) per milligram dry weight; this firmly bound CK activity is estimated to represent 3-5% of the total cellular CK. It had been shown previously that the myofibrillar CK, which can be quantitatively extracted at low ionic strength and purified to homogeneity, is very similar, if not identical, to the bulk MM-CK. It is shown that the two protein preparations also have the same peptide pattern after cyanogen bromide fractionation and very similar specific activities, confirming their identity. The earlier demonstration that the bound CK is specifically located at the M-lines of isolated myofibrils has been confirmed by immunofluorescence. Antibodies directed against purified MM- and BB-CK were used in the indirect fluorescent antibody technique to study the specificity of myofibril binding sites for different forms of CK. With myofibrils from adult muscle, which has only MM-CK, as well as from early developmental stages in which BB-CK is the predominant isoenzyme, M-type CK was localized exclusively at the M-line, while greater or lesser amounts of B-type CK were found at the Z-line. The data provide strong evidence that the MM-CK at the M-lines in skeletal myofibrils is not adventitiously bound but is rather an integral element in the M-line structure. The amount of CK bound is reasonably consistent with the earlier proposal that the CK molecules might be the transverse M-bridges and appears to be sufficient to regenerate all of the ATP hydrolyzed during muscle contraction.     

Accelerated protein turnover in the skeletal muscle of dystrophic mice

The excretion of 3-methylhistidine increased in the urine of dystrophic mice C57BL/6J. The content of 3-methylhistidine residue decreased in the muscle proteins of dystrophic mice, but not in other organs. Methylated proteins in the skeletal muscle, actin and myosin, were partially purified from the dystrophic and control muscles. The amount of 3-methylhistidine residue in unit weight of the actin and myosin preparations was normal in dystrophic muscle. These three facts indicate that the turnover rates of actin and myosin are increased in the muscle of the dystrophic mice.     

Properties of creatine kinase from skeletal muscle mitochondria

Creatine kinase from pigeon breast muscle was obtained in a homogeneous (as evidenced from polyacrylamide gel SDS electrophoresis) state. The molecular mass of the enzyme monomer is 43,000. Ultracentrifugation in a sucrose density gradient and gel filtration revealed that the enzyme is present in solution as a mixture of two major forms, i.e., octamer and dimer, which differ in their activity. The decrease of ionic strength from 0.25 to 0.02 results in reversible dissociation of the octameric form. A temperature rise from 5 degrees to 20 degrees C or the nature of monovalent anions (e.g., Cl-, CH3COO-, NO3-) and cations (K+, Na+) present in the medium do not influence the distribution of oligomeric forms. At pH 6.0 the major form is represented by the octamer; its dissociation is caused by an increase of pH. The octamer dissociation occurs in a mixture of substrates of the creatine kinase reaction in the presence of Mg2+; no such dissociation is observed in the absence of Mg2+ and in the presence of each of the reaction substrates. The non-interacting pair of substrates--ADP and creatine--causes the dissociation of the octamer in the presence of nitrate ions but not acetate. It is concluded that the dissociating effect of substrates is due to the conformational changes of subunits during catalysis. At physiological concentrations of nucleotide substrates the degree of octamer dissociation depends on the ratio of creatine phosphate and creatine concentrations, as well as on the presence of chlorine and phosphate ions. A qualitative estimation of the rate of pH- and substrate-dependent dissociation of creatine kinase octamer revealed that under the given experimental conditions the pH-dependent dissociation is completed within hours, whereas the substrate-dependent one--within seconds or minutes. According to its properties, mitochondrial creatine kinase from pigeon breast muscle is close to its bovine heart counterpart; the observed differences were found to be quantitative.