Electrical stimulation modulates IGF binding protein transcript levels in C2C12 myotubes

Electrical stimulation (ES) of skeletal muscle can produce changes in metabolic enzyme and contractile protein gene expression resulting in fast-to-slow phenotypic changes. The molecular mechanism by which ES induces changes in phenotype is not entirely understood but recent reports have demonstrated that the calcineurin/NF-AT signalling pathway is involved. IGF-1 is also capable of inducing changes in phenotype through the same calcineurin/NF-AT pathway but little is known of the direct effect of ES on the IGF system. In this study, we examined the effects of ES on the expression of igf-1, igf-2 and the six igfbp genes in the C2C12 muscle cell line. Results showed that ES induced a change in phenotype that was accompanied by downregulation of igf-2 and upregulation of igfbp-4 mRNA levels. However, ES did not significantly alter the transcription of igf-1, igfbp-2, igfbp-5 and igfbp-6 genes. This study demonstrates that ES of muscle cells in vitro not only directly modulates the gene expression of contractile proteins but also modulates proteins that are part of the IGF regulatory system, in particular IGFBP-4.

Carnosine reacts with protein carbonyl groups: another possible role for the anti-ageing peptide?

Carnosine (beta-alanyl-L-histidine) can delay senescence and provoke cellular rejuvenation in cultured human fibroblasts. The mechanisms by which such a simple molecule induces these effects is not known despite carnosine's well documented anti-oxidant and oxygen free-radical scavenging activities. Carbonyl groups are generated on proteins post-synthetically by the action of reactive oxygen species and glycating agents and their accumulation is a major biochemical manifestation of ageing. We suggest that, in addition to the prophylactic actions of carnosine, it may also directly participate in the inactivation/disposal of aged proteins possibly by direct reaction with the carbonyl groups on proteins. The possible fates of these 'carnosinylated' proteins including the formation of inert lipofuscin, proteolysis via the proteasome system and exocytosis following interaction with receptors are also discussed. The proposal may point to a hitherto unrecognised mechanism by which cells/organisms normally defend themselves against protein carbonyls.

Carnosine, the anti-ageing, anti-oxidant dipeptide, may react with protein carbonyl groups

Carnosine (beta-alanyl-L-histidine) is a physiological dipeptide which can delay ageing and rejuvenate senescent cultured human fibroblasts. Carnosine's anti-oxidant, free radical- and metal ion-scavenging activities cannot adequately explain these effects. Previous studies showed that carnosine reacts with small carbonyl compounds (aldehydes and ketones) and protects macromolecules against their cross-linking actions. Ageing is associated with accumulation of carbonyl groups on proteins. We consider here whether carnosine reacts with protein carbonyl groups. Our evidence indicates that carnosine can react non-enzymically with protein carbonyl groups, a process termed 'carnosinylation'. We propose that similar reactions could occur in cultured fibroblasts and in vivo. A preliminary experiment suggesting that carnosine is effective in vivo is presented; it suppressed diabetes-associated increase in blood pressure in fructose-fed rats, an observation consistent with carnosine's anti-glycating actions. We speculate that: (i) carnosine's apparent anti-ageing actions result, partly, from its ability to react with carbonyl groups on glycated/oxidised proteins and other molecules; (ii) this reaction, termed 'carnosinylation,' inhibits cross-linking of glycoxidised proteins to normal macromolecules; and (iii) carnosinylation could affect the fate of glycoxidised polypeptides.

Phenotypic expression of IGF binding protein transcripts in muscle, in vitro and in vivo

The actions of the insulin-like growth factors (IGF-I and IGF-II) which are essential components of skeletal muscle growth are mediated via their receptors and modulated by six binding proteins (IGFBPs). We studied IGFBP transcripts in C2C12 cell cultures and in adult control and denervated gastrocnemius muscle. IGFBP-2, -4, -5, and -6 were detected in C2C12 cells. IGFBP-6 mRNA levels remained unchanged, IGFBP-2 levels decreased and IGFBP-4 and -5 increased over 1, 5, and 9 days after serum reduction. In a range of adult muscles studied, IGFBP-4 mRNA levels were similar, IGFBP-5 was present at different levels in slow and fast muscles and IGFBP-6 had the lowest expression in the tibialis anterior. Denervation resulted in dramatic up-regulation of IGFBP-4 and -5 transcripts but there was no change in IGFBP-6. These results suggest that either lack of neural input and/or mechanical loading, both of which contribute to muscle atrophy, affect IGFBP expression.

Carnosine reacts with a glycated protein

Oxidation and glycation induce formation of carbonyl (CO) groups in proteins, a characteristic of cellular aging. The dipeptide carnosine (beta-alanyl-L-histidine) is often found in long-lived mammalian tissues at relatively high concentrations (up to 20 mM). Previous studies show that carnosine reacts with low-molecular-weight aldehydes and ketones. We examine here the ability of carnosine to react with ovalbumin CO groups generated by treatment of the protein with methylglyoxal (MG). Incubation of MG-treated protein with carnosine accelerated a slow decline in CO groups as measured by dinitrophenylhydrazine reactivity. Incubation of [(14)C]-carnosine with MG-treated ovalbumin resulted in a radiolabeled precipitate on addition of trichloroacetic acid (TCA); this was not observed with control, untreated protein. The presence of lysine or N-(alpha)-acetylglycyl-lysine methyl ester caused a decrease in the TCA-precipitable radiolabel. Carnosine also inhibited cross-linking of the MG-treated ovalbumin to lysine and normal, untreated alpha-crystallin. We conclude that carnosine can react with protein CO groups (termed "carnosinylation") and thereby modulate their deleterious interaction with other polypeptides. It is proposed that, should similar reactions occur intracellularly, then carnosine's known "anti-aging" actions might, at least partially, be explained by the dipeptide facilitating the inactivation/removal of deleterious proteins bearing carbonyl groups.

A possible new role for the anti-ageing peptide carnosine

The naturally occurring dipeptide carnosine (beta-alanyl-L-histidine) is found in surprisingly large amounts in long-lived tissues and can delay ageing in cultured human fibroblasts. Carnosine has been regarded largely as an anti-oxidant and free radical scavenger. More recently, an anti-glycating potential has been discovered whereby carnosine can react with low-molecular-weight compounds that bear carbonyl groups (aldehydes and ketones). Carbonyl groups, arising mostly from the attack of reactive oxygen species and low-molecular-weight aldehydes and ketones, accumulate on proteins during ageing. Here we propose, with supporting evidence, that carnosine can react with protein carbonyl groups to produce protein-carbonyl-carnosine adducts ('carnosinylated' proteins). The various possible cellular fates of the carnosinylated proteins are discussed. These proposals may help explain anti-ageing actions of carnosine and its presence in non-mitotic cells of long-lived mammals.

The LIM-domain protein FHL1 (SLIM 1) exhibits functional regulation in skeletal muscle

The LIM domain protein FHL1 (SLIM 1) transcript is preferentially expressed in postnatal skeletal muscle but almost nothing is known about its function in this tissue. In this study we have examined the expression of the FHL1 transcript at the cellular level by in situ hybridisation. Muscle fibers exist as a number of discrete subpopulations or "types" which are differentiated by their contractile and metabolic properties. It was observed that the FHL1 transcript was not fiber-type specific but was however more abundant in oxidative fibers. Muscle atrophy induced by disuse caused a significant decline in the expression of the transcript but atrophy induced by short-term denervation did not. Hypertrophy of skeletal muscle induced by passive stretch was associated with an up-regulation of the FHL1 transcript. These data are consistent that FHL1 is involved in synthetic processes within the muscle fibre.

UVR modulates the steady-state levels of skin collagen transcripts in hairless mice

Exposure to solar UV radiation (UVR) leads to changes in the extracellular matrix of the dermis, which is largely composed of collagen and elastin. Collagen and elastin proteins and their corresponding mRNA were assessed in dorsal skins of hairless mice exposed to 0.64 J/cm2 UVR (295-400 nm), 5 days per week, over a 12 week period. A 48% increase in skin-fold thickness was accompanied by increased elastic tissue deposition and more compressed collagen bundles as assessed histologically. Collagen I mRNA levels were similar to those in control skins at 1, 2, 3 and 6 weeks of UVR and less than control levels at 9 and 12 weeks. Collagen III mRNA levels were elevated after 1 week of UVR, remained elevated for a further 2 weeks and then returned to control levels at weeks 6, 9 and 12 when changes are occurring in collagen I transcripts. There was no evidence of corresponding changes in collagen I and III protein levels assessed using electrophoretic techniques. These results suggest that damage to the extracellular matrix, consequent on UVR, is associated with some pretranslational events. Elastin mRNA levels were unaffected by UVR, suggesting that elastic tissue hyperplasia is a posttranscriptional phenomenon.

Two myogenic regulatory factor transcripts exhibit muscle-specific responses to disuse and passive stretch in adult rats

Levels of myogenic regulatory factor (MRF) transcripts are altered in a muscle-specific manner in response to hind limb immobilisation of adult male rats, for a 2 day period, in either a lengthened or shortened position which result in passive stretch or disuse atrophy respectively. Myogenin transcript levels were dramatically elevated in the stretched plantaris but not soleus, whereas the MRF4 transcript was significantly elevated in soleus but not plantaris. Levels of myogenin mRNA were unaffected by disuse in either muscle and MRF4 was markedly lower in plantaris in response to disuse.

Alterations in the mRNA levels of two metabolic enzymes in rat skeletal muscle during stretch-induced hypertrophy and disuse atrophy

Changes in carbonic anhydrase III (CAIII: normally predominant in slow type 1 fibres) and phosphoglucoisomerase (PGI: normally predominant in fast type 2 fibres) mRNAs were studied in rat slow postural soleus and fast phasic plantaris muscles which had been immobilised in shortened (disuse) and lengthened (passive stretch) positions for 2 and 5 days. Our results provide evidence that limb immobilisation in both positions affects the expression of these metabolic enzymes. Muscle disuse resulted in considerable loss of CAIII mRNA in soleus but not in plantaris, whereas, PGI mRNA levels were unaffected in soleus but declined in plantaris after 2 days. Passive stretch caused an increase in CAIII mRNA in soleus muscles after 2 days, although this was not maintained after 5 days when a decrease was observed, and an increase in plantaris muscles after 5 days. In contrast, PGI mRNA declined in both muscles. These results indicate that immobilisation of muscles in the shortened and lengthened positions affects the levels of transcripts of these soluble enzymes in different ways and these effects are muscle specific.

Reciprocal changes in myosin isoform mRNAs of rabbit skeletal muscle in response to the initiation and cessation of chronic electrical stimulation

Changes in myosin heavy chain (MHC) mRNAs were studied in rabbit fast-twitch muscles during continuous electrical stimulation at 10 Hz for periods up to 3 weeks, and during the first 12 days of the recovery process that followed cessation of 6 weeks' stimulation. Two cDNA probes were used to detect MHC mRNAs specific to fast- and slow-twitch skeletal muscle in RNase protection assays and Northern- and slot-blot analyses. The isolation and base sequence of one of these probes, corresponding to the MHC gene expressed in soleus (slow-twitch), is described. At an early stage of the response to stimulation, fast MHC mRNA was replaced by slow MHC mRNA. During recovery, this process occurred in reverse but took longer. The time course of recovery was slightly faster in tibialis anterior than in extensor digitorum longus. The changes in mRNAs during both stimulation and recovery reflected changes in the corresponding muscle proteins.

A non-methylated CpG-rich island associated with the human muscle-specific carbonic anhydrase III gene

A cluster of CpG dinucleotides immediately upstream from exon 1 in the muscle-specific carbonic anhydrase III gene (CAIII) resembles the 'HpaII tiny fragment' (HTF) islands characteristic of mammalian 'housekeeping' genes. Since this CAIII gene shows tissue-specific expression we have carried out a detailed examination of methylation status within the CpG cluster using a polyacrylamide gel/electroblot procedure to extend the range of conventional Southern blotting. None of the clustered CpGs are methylated in DNA from muscle or other somatic tissues or in DNA from spermatozoa although flanking CpGs are methylated. Comparison with a candidate HTF island from the more ubiquitously expressed carbonic anhydrase II gene (CAII) shows that the CAII CpG cluster is markedly more CpG-rich than that from the strictly tissue-specific CAIII gene.

Changes in skeletal muscle gene transcription induced by chronic stimulation

The mRNA products of four genes, carbonic anhydrase III (CAIII), fast myosin heavy chain (MHCf), actin and glyceraldehyde-3-phosphate dehydrogenase (GAPDH), were assayed by Northern and slot-blot analysis in rabbit tibialis anterior and extensor digitorum longus muscles that were subjected to electrical stimulation for periods of up to 21 days. Marked changes in mRNA levels were seen for all four genes. The mRNA for CAIII, which is specific for type 1 fibers, rose significantly, whereas the MHCf mRNA fell markedly in the stimulated muscles. Changes in GAPDH mRNA were consistent with a reduced dependence on anaerobic glycolysis as an energy source. Actin mRNA levels were noticeably depressed in the early stages of stimulation. Thus for several classes of muscle protein, the response to chronic low-frequency stimulation appears to involve changes at the level of gene transcription.

Human muscle carbonic anhydrase: gene structure and DNA methylation patterns in fetal and adult tissues

We report the isolation and analysis of genomic clones comprising the entire gene coding for the human muscle carbonic anhydrase, CAIII. The gene spans 10.3 kb and has a seven-exon/six-intron structure. A noncanonical TATA box, a CCAAT motif, and two CCGCCC elements are present in the sequences upstream of exon 1. Although the expression of CAIII shows strict tissue specificity, the gene exhibits a number of features normally associated with housekeeping enzymes. For example, there is 48% homology with a 25-bp consensus sequence between the TATA box and the cap site and there is a CpG-rich island spanning a 469-bp sequence near to the origin of transcription. Methylation studies suggest that some CCGG sites clustered in the CpG-rich island are undermethylated in DNA from fetal and adult muscle and in other tissues irrespective of CAIII expression. In contrast, several nonclustered CCGG sites show a methylation pattern that correlates with gene expression. However DNA from differentiated type II adult muscle fibers is undermethylated at these sites even though CAIII is not expressed.

Comparative studies of nuclear phosphoproteins from four different muscle tissues and liver

1. Nuclei isolated from red and white skeletal muscle, heart, uterus and liver of rabbit were incubated with [lambda-32P]ATP and the incorporation of 32P into nuclear protein was examined. 2. Liver nuclear proteins were phosphorylated to a much greater extent than those of muscle. Particularly low levels of phosphorylation were observed in white greater extent than those of muscle. Particularly low levels of phosphorylation were observed in white muscle nuclei. 3. Most of the phosphoproteins were common to all the tissues but significant quantitative differences and a limited number of qualitative differences were evident, particularly between liver and the muscle tissues. 4. Many phosphoproteins detected were minor components of the total nuclear protein.

The modification of cholinesterase activity by 5,5'-dithiobis-(2-nitrobenzoic acid) included in the coupled spectrophotometric assay. Evidence for a non-catalytic substrate-binding site

1. Compared with the acetylcholinesterase assay carried out in the absence of a dithiol, the presence of 5,5'-dithiobis-(2-nitrobenzoic acid) caused marked activation, 6,6'-dithiodinicotinic acid and 2,2'-dithiobis-(5-nitropyridine) less so and 2,2'-dithiodipyridine (aldrithiol-2) had no effect at all. Measurements are further complicated in that the 5-thio-2-nitrobenzoate ion also appears to interact with the enzyme, resulting in slightly lowered absorbance values. 2. Acetylthiocholine competes for the 5,5'-dithiobis-(2-nitrobenzoic acid)-binding site so that activation is essentially eliminated by saturating concentrations of substrate. The presence of the dithiol decreases the K(m) value of acetylthiocholine. 3. Similar results were obtained with pseudocholinesterase. However, with butyrylthiocholine clear activation was still observed under V(max.) conditions in addition to K(m) being lowered. 4. All the data yielded Hill coefficients of 1 and analysis of the results leads to the conclusion that activation results from the dithiol being bound to a site on the subunit that is actively catalysing ester hydrolysis. 5. The use of aldrithiol-2 is recommended for kinetic work where absolute quantitative measurements are required.

Kinetic studies on the two common inherited forms of human erythrocyte adenylate kinase

1. The kinetic properties of two genetic variants of human erythrocyte adenylate kinase were studied at limiting concentrations of both ADP and MgADP(-) in the forward direction and at limiting concentrations of both AMP and MgATP(2-) in the reverse direction. 2. Primary reciprocal plots rule out the possibility of a Ping Pong mechanism for both forms of the enzyme. 3. Analysis of the kinetic data by an appropriate computer program gave the following K(m) values for the type 1 enzyme: AMP, 0.33mm+/-0.1; MgATP(2-), 0.95mm+/-0.13; ADP, 0.12mm+/-0.03; MgADP(-), 0.22mm+/-0.04. Values for the type 2 enzyme were: AMP, 0.27mm+/-0.03; MgATP(2-), 0.40mm+/-0.05; ADP, 0.08mm+/-0.07; MgADP(-), 0.20mm+/-0.04. 4. Product inhibition studies were done by studying the reverse reaction. With ADP as product inhibitor competitive inhibition patterns were obtained with AMP and/or MgATP(2-) as variable substrate. Similar results were obtained for product inhibition by MgADP(-) with AMP as variable substrate. The results are consistent with a Rapid Equilibrium Random mechanism. 5. Secondary plots of slope versus product concentration were linear. The data were fitted to the appropriate equation and analysed by computer to give values for the product inhibition constants. 6. Differences between the values of certain kinetic constants for the two forms of the enzyme were observed.

Partial purification and properties of the two common inherited forms of human erythrocyte adenylate kinase

1. The partial purification of adenylate kinase, types 1 and 2, from human erythrocytes is described. 2. Gel chromatography of both forms of the enzyme gave estimates of the molecular weights in the range 20000-23000. 3. Studies on crude haemolysates at various pH values indicated that the type 2 enzyme was less stable than the type 1. Heat denaturation studies on the partially purified enzymes confirmed these findings. 4. Measurements of rates of inhibition by iodoacetate and iodoacetamide showed that the type 2 enzyme reacts more readily than the type 1 enzyme with both reagents. 5. The effect of temperature on the initial velocity of ADP formation was measured at a single concentration of both AMP and MgATP(2-). The two forms of the enzyme responded differently to increasing temperature.